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MOW-n»: ...;1. ....1. 1.- 1.1.19.9 .1 . inn-”.1... ... 1? 41%. . 3....11.m.11. ...11. 1 .thufm, .1141... ,hw............r .....m.. . 1 . . . ., . ...trfiwafinugflr .. .1 v1; 1 1.1.3.. . 1 1 ..l . . .. ....» 1 1n. .llo ‘l‘ . ‘ . ‘. v‘l-pm ;' 1-1-1111. v.1..- 11. o»... . I .1 .51... . ‘..u!. '411 THESlS This is to certify that the dissertation entitled LAYERED METAL PHOSPHONATES AS TEMPLATES FOR THE ASSEMBLY OF ORDERED MOLECULAR SYSTEMS presented by Mark R. Torgerson has been accepted towards fulfillment of the requirements for Ph.D. Chemistry degree in / MW Major professor Date 4mm \Cfib MS U is an Affirmative Action/Equal Opportunity Institution 042771 ICHIGAN IIIIIIIIIIIIIIIIIIIIII willWilli“!!!lmiimmimlmi 3 01579 5978 LIBRARY L - Michigan State ? University . PLACE Ii RETURN BOX to mow this chockom from your record. TO AVOID FINES Mum on or More data duo. DATE DUE DATE DUE DATE DUE F usu ioAnNflnnativoActiaVEqni Opportutfllylnotituion ml LAYERED METAL PHOSPHONATES AS TEMPLATES FOR THE ASSEMBLY or ORDERED MOLECULAR SYSTEMS By Mark R. Torgerson A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 1996 ABSTRACT LAYERED METAL PHOSPHONATES AS TEMPLATES FOR THE ASSEMBLY OF ORDERED MOLECULAR SYSTEMS By Mark R. Torgerson Self-assembly is a method Nature has developed to bring molecules together into larger systems. DNA double helices, enzyme~substrate complexes and cell membranes are formed by self-assembly interactions. Chemists use self-assembly methods to bring molecules together in solution or at interfaces to create supramolecular complexes and designer surfaces that display physical or chemical properties different than the component molecules. Self-assembly has been used to construct catalysts, sensors and optically active materials. Layered metal phosphonates (LMPRS) are two dimensional materials that are able to assemble molecules into ordered arrays. LMPRs consist of stable inorganic layers that separate or collapse to accommodate a wide variety of organic species that are covalently bonded to the phosphorus of the layer or intercalated as guest molecules. To date, little application has been made of the self-assembling power of these adapting and modifiable materials. The work presented in this dissertation capitalizes on the versatility of LMPRs to design and build well-ordered molecular arrays with specific functions. The first design was developed to control the photophysical properties of aromatic molecules within LMPRs. For this design, vanadyl phosphonates layers are used to place naphthalene units into ordered arrays. The intercalation of guest molecules or modifications to the organophosphonate pendant can be used to adjust the relative positioning of the interlayer naphthalenes. Controlling the ring-ring overlap of the naphthalene molecules also controls the fluorescence emission characteristics displayed by these aromatics as a result of excimer formation. In the second application, LMPRs are incorporated into the design of a light-to- energy conversion catalyst based on a novel photochemical reaction discovered by our group. The catalyst is based on a quadrupiy bonded dimolybdenum core which has been shown to produce a multielectron excited state leading to a two electron oxidative addition reaction at the bimetallic core. Incorporation of this photocenter into a redox active LMPR support may establish a method to reductively eliminate the substrate fi'agments and regenerate the active site. The synthetic feasibility of this system is demonstrated by incorporating a dimolybdenum center into layered zirconium phosphonate. For Beatrice, Quentin, Xavier and all the others who kept me going. iv ACKNOWLEDGMENTS It is obvious that this publication would not have been possible without a lifetime of support and encouragement from family, friends and teachers. I am indebted to all of my family members who have contributed toward this effort. My grandfather, Henry Nietfeld, for teaching me the power of knowledge and education. My parents, Rodger and Gen, for believing in me. And my wife, Beatrice, for her coaching and motivating me throughout this endeavor. I also thank all the friends, coworkers and faculty who have inspired, assisted, and taught me during my studies at Michigan State. Professor Nocera’s ideas and knowledge along with his trust in me to explore new areas of chemistry are especially appreciated. In the process of completing the work for this dissertation I have learned many lessons. Surprisingly, the most valuable lessons I have learned have little to do with chemistry. Dan Nocera has taught me the importance of being a good teacher, mentor and communicator. And the members of the Nocera Group, past and present, through their diversity in origin and personality have taught me more about life than I could fit in this book. You have all aided in making my years of graduate school enriching as well as enjoyable. Finally, I would like to thank the people of the MSU Chemistry Department who keep the “big machine” working: the computer and instrumentation specialists; the secretarial staff; and technicians from the electronics, glass and machine shops. Your help is truly appreciated. TABLE OF CONTENTS LIST OF TABLES ................................................................................................. ix LIST OF FIGURES ............................................................................................... x CHAPTER I INTRODUCTION .................................................................................................. 1 A. Background ............................................................................................... l B. Self-Assembly Methods to Molecular Order ............................................... 3 1. Supramolecular Assemblies ................................................................ 3 2. Bilipid Vessicles: Synthetic Membranes ............................................ 10 3. Langmuir-Blodgett Films and Self-Assembled Monolayers ................ l4 4. Layered Guest-Host Materials: Metal Phosphates and Phosphonates . 16 a. Synthesis, Structure and Properties of Layered Metal Phosphates and Phosphonates .................................................. 18 i. Zirconium Phosphates ....................................................... 18 ii. Zirconium Phosphonates .................................................. 25 iii. Vanadium Phosphates ..................................................... 28 iv. Vanadyl Phosphonates ..................................................... 29 b. Examples of Controlled Molecular Self-Assembly in Layered Metal Phosphates and Phosphonates ........................... 37 c. Phosphonic Acid Synthesis: Opportunity for Variations on a Theme ................................................................................... 39 C. Thesis Outline .......................................................................................... 42 1. Assembly of Aromatic Molecules Within Layered Metal Phosphonates: Control and Switching of Optical Properties ............... 43 2. Controlled Assembly of a Potential Two-Electron Photocatalyst for Applications in Light to Energy Conversion ...................................... 44 CHAPTER II EXPERIMENTAL METHODS ............................................................................. 46 A. Instrumental Setups ................................................................................. 46 1. Powder X—Ray Diffraction ................................................................ 46 2. UV-Vis Spectroscopy ....................................................................... 47 a. Absorption Spectroscopy ......................................................... 47 b. Solid State Diffuse Reflectance ................................................ 48 3. Fluorescence Spectroscopy .............................................................. 48 4. Magnetic Susceptibility .................................................................... 49 5. ESCA ............................................................................................... 49 B. Synthesis of Vanadyl Phosphonates Materials ........................................... 50 1. Synthesis of V(O)O 3PR-I-I20-R' OH Layers ...................................... 50 2. Alcohol Exchange Reactions of V(O)O 3’PR-H200R OH ................... 51 C. Synthesis of Zirconium Phosphonate Materials ......................................... 53 1. Synthesis of Zr(O PR) ..................................................................... 53 2. Synthesis of Zr(O C.322PCli2CH2CO 'M")2 from Zr(O3 PCH2 c COH) (M=Na, NH +) ...................................... 54 3.1ntercalation of imolyg denum Complexes into Zirconium Phosphonates ................................................................................... 56 D. Phosphonic Acids ..................................................................................... 57 1. Alkyl Phosphonic Acids ..................................................................... 57 a. Michaelis-Becker Reaction ....................................................... 58 b. NaP(O)(EtO)2 Addition to Activated Double Bonds ................ 6O 2. Aryl Phosphonic Acid Synthesis ........................................................ 61 a. Modified Arbuzov .................................................................... 61 b. Grignard Synthesis .................................................................... 62 c. Nucleophilic Aromatic Substitution ........................................... 63 E. Synthesis of Quadruply Bonded Dirnolybdenum Complexes ....................... 64 1. Mo (CH3 COO)4 ............................................................................... 6S 2.(NIi4 )4 M02 C18- 0 ......................................................................... 6S 3. M02 Cl 4[(C2H3) ............................................................................ 66 4 Mo: (:1 (c ch): ............................................................................ 66 5. Mo2 2(CH3 C O)2 (CH3 CN)6 (BF ) and Mo2 2(CH3 COO)2 (CH: CN): (PF6 4)2 .................................................... 67 6. Mo 23(CH 3CN)8(BF )4 3and Mo2((.z 1&CN)8(P.176)4 .............................. 67 F. Computer-Aided Molecu ar Design and isualization ................................ 68 1. Atom Substitutions .......................................................................... 68 2. Adjustments to Fractal Atomic Coordinates and Unit Cell ................. 69 3. Molecular Dynamics and Energy Minimizations ................................ 70 CHAPTER III CONTROL OF NAPHTHALENE EXCIMER FORMATION IN LAYERED VANADYL PHOSPHONATES ............................................................................ 72 A. Background .............................................................................................. 72 l. Intercalation of Guests into LMPR Materials .................................... 73 a. Intercalations Driven by Guest-Pendent Interactions ................. 76 b. Intercalation Driven by Guest-Layer Interactions ...................... 77 2. Structural Aspects of Vanadyl Phosphonates ..................................... 78 3. Molecular Positioning within Vanadyl Phosphonate Layers: Control of Aromatic Excimer Formation ....................................................... 82 B. Results ...................................................................................................... 87 1. Feasibility of Concept ........................................................................ 87 a. Molecular Modeling of Naphthalene Units In Vanadyl Phosphonate Layers ................................................................. 87 vii b. Optical Properties of Vanadyl Ion and Naphthalene ................... 91 2. Synthesis of Vanadyl Phosphonate Layers ........................................ 96 3. Naphthalene-Modified Layered Vanadyl Phosphonates .................... 101 a. Studies of V(O)O3PR°H20-b-C10H2CH20H ......................... 101 b. Studies of V(O)O PR- O-bcC10H2CH2CH20H ................. 115 c. V(O)O3PC10H7- O-R H ................................................... 119 (1. Studies of V(O)O3PCH2CH2C10H7-H20-ROH ..................... 134 C. Overview ................................................................................................ 136 CHAPTER IV THE INCORPORATION OF DIMOLYBDENUM CENTERS INTO LAYERED ZIRCONIUM PHOSPHONATES ....................................................................... 138 A Background ............................................................................................ 138 B. Results .................................................................................................. 154 1. Reactions of Zr(O3PCH2CH2COOH)2 with Quadruply Bonded Dimolybdenum Compounds .......................................................... 154 2. Synthesis of Zr(O3PCH2CH2COOM)2 for (M = NH 4+, Na+) ......... 155 3. Reactions of Dimolybdenum Complexes with . Zr(O3PC CH2COOM)2 for (M = NH4+, Na+) ........................... 156 4. Synthesis 0 Zirconium (Pyridyl)Phosphonates .............................. 164 C. Conclusions .......................................................................................... 16S LIST OF REFERENCES ..................................................................................... 168 viii LIST OF TABLES Table 1. BR and MP. of alkylphosphonic acids and esters ................................. 59 Table 2. Formula, structure, magnetism and emission data for V(O)03PC10H7.H20.ROHX ................................................................ 133 Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 LIST OF FIGURES (a) Fischer’s model of steric fit and (b) Koshland’s model of induced fit for enzyme - substrate interactions ...................................................... 5 The supramolecular catalyst (11) positions the nucleophilic amine (b) and the displacement site with the halide leaving group (c) in close proximity to promote an 8N2 reaction .................................................... 9 Schematic representations of (a) bilipid vesicle, (b) Langmuir-Blodgett film of a carboxylic acid adsorbed onto a glass substrate and (c) a thio- alkane self-assembled monolayer bound to a gold surface ....................... 12 Generalized structure of a-zirconium phosphate. (a) The strongly bound metal-oxygen-phosphorus plane (hydroxyl groups of the hydrogen-phosphates were omitted for clarity). (b) The acidic interlayer region of Zr(O3POH)2 (each box represent a metal-oxygen- phosphorus layer) .................................................................................. 20 The interlayer distance of amine-intercalated zirconium phosphate as a function of the number of carbon atoms in the intercalated alkyl amines (slope'=2.07 A / carbon). The inset shows the bilipid-layered arrangement of the organic fragments predicted by a slope of ~2.0 A / carbon ..................................................................................... 23 The idealized structure Of ZI‘(O3PC6H5)2 .............................................. 27 The interdigitated nature of vanadyl phosphonates with the ”dihydrate” structure and the position of intercalated alcohol molecules .................... 32 The inorganic metal-oxygen-phosphorus plane of vanadyl phosphonate dihydrate ............................................................................................... 36 Common synthetic routes to phosphonate diesters which can be hydrolyzed to the corresponding phosphonic acids ................................ 41 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Figure 20 Figure 21 (a) Intercalation driven by interaction of the guest (swiggle) and the phosphonate pendant (oval) in an organic bilayer structure. (b) Intercalation driven by interaction of the guest (swiggle) and an active site on the inorganic layer ...................................................................... 75 a-c Inorganic plane of vanadyl phosphonate dihydrate ........................... 80 Orbital description of the electronic absorption and emission of: (a) isolated naphthalene and (b) naphthalene excimer .................................. 85 The distance organic fragments attached to the surface (x') is dependant on the distance between attachment sites on the surface (x) and the tilt angle (9) of the organic fi'om perpendicular to the layer by x' = x cos9 ............................................................................................. 90 The a—c plane of the organic interlayer region filled with naphthalene units after minimization by a molecular dynamics calculation to determine the orientation and distrubution of naphthalene molecules held within vanadyl phosphonate fiameworks ........................................ 93 Molar extinction coemcients, a, of (a) naphthalene and (b) vanadyl ion as a fi1nction of wavelength ............................................................. 95 Powder x-ray difiaction patterns for compounds of the formula V(O)O3PR-H20-C6H5CH20H for R=(a) propyl, (b) butyl, (c) pentyl, (d) hexyl, (e) heptyl .............................................................................. 98 d-Space of V(O)O3PR-H20-C6H5CH20H plotted as a fimction of the number of carbon atoms in R ............................................................... 100 Schematic of how the environment about the naphthalene units in V(O)O3PR-H20-b-C10H7CH20H can be changed when the length of R varied .......................................................................................... 103 Powder x-ray difli'action patterns for compounds of the formula V(O)O3PR-H20-b-C10H7CH20H for R= (a) methyl, (b) ethyl, (c) butyl, (d) pentyl, (e) hexyl, (f) heptyl, (g) octyl, (h) decyl ................ 106 d-Space of V(O)O3PR-H20-C10H7CH20H plotted as a function of the number of carbon atoms in R ......................................................... 108 UV-visible diffuse reflectance spectra of naphthalene containing layers with the formula V(O)O3PR-H20-b-CloH-,CH20H where R=(a) methyl, (b) ethyl, (c) butyl .................................................................. 110 Figure 22 Figure 23 Figure 24 Figure 25 Figure 26 Figure 27 Figure 28 Figure 29 Figure 30 Figure 31 Figure 32 Figure 33 Figure 34 Figure 35 Solid state fluorescence emission of V(O)O3PR°H20-B-C10H7CH20H for R=(a) methyl, (b) ethyl, (c) butyl .................................................. 113 Rotations about the V—O bond which coordinates the C10H7CHZOH alcohol to the vanadium can move rings attached to the same layer close enough to produce n-stacking arrangements and form excimers...117 (a) Rotation about the CHz-Np bond of 2-(2-naphthalene)ethanol intercalated in VPR leads to a 10% change in the cross-sectional area (x —> x') and a 30% change in the interlayer distance (y —> y') occupied by the alcohol; (b) rotation of the same bond in naphthalene- methanol intercalated within VPR [as used in the previous section] results no change in the cross-sectional area (x) or interlayer distance (y) . ........................................................................................ 121 Powder x-ray diffraction data for (a) alcohol free V(O)O3P- B-C10H7'H20 and alcohol intercalated V(O)O3P-B-C10H7‘H20'- ROH for R= (b) ethyl, (c)butyl, (d) hexyl, (e) octyl .............................. 124 d-Space of V(O)O3P-B-C10H7-H20-ROH plotted as a fiinction of the number of carbon atoms in R .............................................................. 126 Solid state fluorescence emission of vanadyl naphthylphosphonates: (a) [fee of alcohol, and (b) intercalated with ethanol ............................. 129 Magnetic Susceptibility of vanadyl naphthylphosphonates intercalated with: no alcohol (0), ethanol (+), hexanol (A), and octanol (x) ............... 132 Vectorially arranged light harvesting center and e" donor-acceptor triad within a zeolite-L channel ............................................................ 141 Generalized quadruply bonded dimolybdenum compound .................... 144 Orbital basis of the quadruply bonding of dimolybdenum compounds 146 Edge-sharing bioctahedral product resulting from the reaction of a dimolybdenum compound CH3I ........................................................... 148 A layered integrated photochemical system ........................................ 151 Powder x-ray diffaction patterns from (a) M02 intercalated (c) Zr(O3P-CH2CH2COONH4)2 ......................................................... 160 Difi‘use reflectance UV-visible spectrum of the dimolybdenum intercalated zirconium phosphonate layer (11m = 530 nm) .................. 162 xii CHAPTER I INTRODUCTION A . Background The design and synthesis of molecules, molecular assemblies and materials to perform the functions of what has traditionally been accomplished by mechanical or electronic devices has become a major area of chemistry. Chemists are beginning to explore the potential of individual molecules,l discrete molecular assemblies,2 and materials in the design of devices such as sensors,3 switches,4 and logic gates.5 These chemistry-based devices are intended to improve the sensitivity, increase the speed or decrease the size of electronic devices currently available. The basic concept behind chemistry-based devices, especially sensors, is that the exact physical and chemical properties of a particular molecular fragment be dependent on the immediate environment about that fragment.3 A functional unit on a sensor must respond to changes in the micro-environment of the molecule by displaying a new set of chemical or physical properties when the environment is altered. And, conversely, a detectable change in a chemical or physical property would indicate a change in the 2 environment about the active functional group within a sensor. Actuators and logic gates must be sensitive to a stimulus like an optical input and display a modified characteristic.“ Catalysis is another field that requires exacting intermolecular interactions. For effective catalysis to occur, the environment of a substrate species must be controlled at an active site. Effective catalysis, such as that carried out by enzymes in biology, must provide both high turnover rates and high specificity. Traditionally, synthetic catalysts have been discovered by trial and error methods to produce catalytic activity by reaction sequences that are determined after the fact.6 Now, like the methods used for device synthesis, various methods are being used to direct the synthesis of catalysts into pre- designed assemblies to achieve pre-planned reactions with improved efficiency and selectivity.7 The synthesis of single molecules that are able to serve as molecule-specific sensors, optical based switches and catalysts require extensive design and multi-step synthesis.8 As for solid phase devices or catalysts, the prediction of how molecules will crystallize requires crystal engineering — a field still in it's infancy9 and, as of yet, it is not possible to pre-determine the crystalline state of any given molecule.10 Therefore, chemists have started taking short cuts to circumvent the need to carry out long, multi- step synthetic pathways or predict crystalline structures in order to design devices and sensors. Chemists have made inroads into the realms of biology by mimicking the self- assembly techniques used by Nature to fabricate intricate molecular assemblies and well- ordered structures with highly specific functions. Nature has utilized many types of strong bonding and weak intermolecular interactions for the self-assembly of stable units such as: biological membranesl 1; double- stranded DNA12 and transcription13 processes; enzyme-substrate and antibody-antigen . . 14 . . . 15 complexes; 1on-speC1fic pumps; and structural matenals l1ke l1gament, tendon and 3 bone.16 Nature has created these organized structures and processes by taking advantage of intermolecular interactions such as hydrogen bonding; ion-ion, ion-dipole and dipole- dipole interactions; metal coordination; hydrophilic-hydrophobic effects; and size and shape selectivity. By utilizing the molecular self-assembly techniques found in Nature, chemists can now act as molecular architects and atomistic engineers to assemble molecular building blocks into complex but stable associations to produce devicesn’13 and catalysts.19 The following sections review some of the popular methods chemists have used to assemble molecules in a controlled fashion through the use of: receptor-substrate (supramolecular) assemblies; colloids, vesicles and micelles; interfacial phenomena; and porous or layered materials to produce highly ordered architectures. B . Self-Assembly Methods to Molecular Order 1 . Supramolecular Assemblies The assembly of two or more molecules that are held together by non—covalent interactions have been termed “supramolecular” to indicate that the complex is “above” or “beyond” a simple, discrete molecule.20 Supramolecular chemists have now mimicked most of the above-mentioned self assembly techniques utilized by Nature to construct synthetic molecular assemblies. The classic analogies used to describe enzyme-substrate interactions such as Fischer’s “Lock and Key” explanation of steric mm (Figure 1a) and Koshland’s theory of induced fit22 (Figure 1b) have been used to explain the shape-, size- and electrostatic-specificities of supramolecular chemistry.23 Similar to enzyme— substrate interactions, the interactions necessary to produce supramolecular assemblies Figure l. (a) Fischer’s model of steric fit and (b) Koshland’s model of induced fit for enzyme - substrate interactions , Figure l. 6 rely on self-assembly through molecular recognition. The factors involved with two or more molecules “recognizing” each other is dependent on the factors outlined by Lehn24 who transferred the biochemical concepts of molecular recognition into mainstream chemistry.25 Lehn states that, "Molecular recognition is more than a general complexation or association. Self-assembly through molecular recognition requires that there is specificity for a given pair or set of molecules and combines: steric complementarity (size & shape specificity); interactional complementarity (complementary electrostatic interactions); and multiple interactions to produce an overall strong bond."26 Initial studies of synthetic molecular recognition and supramolecular chemistry consisted of examining the selective binding of polyethers,27 crown ethers28 and cryptands29 to specific alkali metal ions. The selective binding of alkali metal ions and the stability of the ion-macrocycle assembly is dependent on the fit of the substrate ion in the hole of the macrocyclic receptor.30 The naturally occurring parallel to this molecular recognition is the selective binding of alkali metal ions by the macrocycle valinomycin31 and the selectivity of the sodium-potassium pump in cellular membranes.32 Supramolecular chemistry has evolved quickly over the last 25 years and now involves the use of more complex receptor-substrate pairs and diverse intermolecular interactions to control chemical reactivity and physical characteristics. The organization of the relative positions and orientations of two or more molecules to produce a desired function is clearly demonstrated by the enzyme-mimetic catalysis applications of supramolecular chemistry. Lehn has created a supramolecular complex using a tetracarboxylic acid-modified crown ether receptor bound to an 0-acetylhydroxyl-ammonium ion,33 (CH3COONH3T), substrate. The ammonium protons of the substrate coordinate to the oxygen atoms of the crown ether. The complex is so stable that the O-acetylhydroxylammonium (pKa = 2.15) stays protonated even after the pH is raised to 7.0, which is basic enough 7 to hydrolyze the O-acetylhydroxylamine to hydroxylamine and acetate ion. Without the crown ether catalyst, or when K+ ion is present to competitively displace the substrate from the crown ether, hydrolysis is slower and acetylhydroxamic acid, (CH3CONHOH), is produced as a side product. In this mechanism, the coordination of the three hydrogen atoms of the protonated amine to the oxygen atoms on the macrocyclic receptor served to prevent deprotonation of the nitrogen atom, which activated the hydrolysis while also preventing the rearrangement of the oxygen atom that could otherwise insert into a N—H bond to form acetylhydroxarnic acid as a side product. In a second example of supramolecular based catalysis, Kelly and co-workers34 have designed a template that acts as a synthetic enzyme for the catalysis of an 8N2 reaction by orienting two different substrate molecules so that the nucleophile on one molecule is directed toward the leaving group on the second molecule (Figure 2). The selective three-point hydrogen bonding of each substrate molecule not only directs the orientation of each molecule but also masks other nucleophiles on the two substrate molecules to promote specificity. Consequently, the reaction rate of this 8N2 reaction is increased and the side reactions are reduced through a pre-designed supramolecular enzymatic process. Similar templating schemes have also been used to catalyze Diels- Alder reactions35 and Rebek36 has developed methods to catalyze the synthesis of more catalyst in a molecular replication scheme. In addition to ion-dipole and hydrogen bonding interactions to assemble supramolecular complexes, interactions such as dipole-dipole, hydrophilic-hydrophobic effects, size exclusion and metal ion coordination have all been used to build supramolecular assemblies. The use of solution-based supramolecular chemistry to assemble complexes that control the orientations and distances between two or more functional units to manipulate the physical or chemical properties of those functional can be quite challenging. This is because the functional units on the two interacting molecules Figure 2. The supramolecular catalyst (a) positions the nucleophilic amine (b) and the displacement site with the halide leaving group (c) in close proximity to promote an 8N2 reaction. Figure 2. 10 must initiate a bimolecular interaction (molecular recognition) before the new complex can form. The controlled assembly of molecules into ordered geometries can be simplified by utilizing molecular level physical effects associated with a wide variety of interfaces and the structural phenomena these interfaces can induce. The structures resulting from liquid-liquid (vesicles) and liquid solid interactions (Langmuir—Blodgett films and self- assembled monolayers) shown in Figure 3 have all been used to force molecules into predictable arrangements with respect to intermolecular orientations and distances. Molecular-level knowledge of these interfacial structures can be used to design a specific intermolecular interaction and affect the physical or chemical characteristics of molecules encased in these architectures. 2. Bilipid Vesicles: Synthetic Membranes Amphiphilic polar-lipid molecules have long been known to associate and form various self-assembled structures in solution. Kunitake has defined numerous structures formed by polar-lipid molecules in aqueous solutions including globules, rods, tubes, lamella and vesicles.37 The most widely used self-assembly structures formed by amphiphilic molecules in water are micelles38 and bilipid vesicles.39 The bilipid vesicle, a structure similar to cell membranes, has proven to be the most useful in producing a high level of long-range order. Vesicles are formed by the close-packed tail-to-tail arrangement of amphiphilic molecules consisting of an ionic or polar head group and a lipid tail section.40 In most cases, the amphiphiles are based on dialkyldimethylammonium halides or sodium dialkyl phosphate salts dissolved in water. Kunitake has developed a variety of amphiphilic molecules to form bilipid vesicles in water with characteristics that are pre-designed to display a desired physical property 11 Figure 3. Schematic representations of (a) bilipid vesicle, (b) Langmuir- Blodgett film of a carboxylic acid adsorbed onto a glass substrate and (c) a thioalkane self-assembled monolayer bound to a gold surface. 12 82.2%.)“ «0962.8: 82.250... “53.32 «22.2% “KENS: «232$ (gt/”szos. 8.2.2.75“ “gas: Figure 3. 5,5,95,53,38? S ./'-'-'-'-'-'—7 C. 13 upon the formation of a vesicle. These designed assemblies have proposed uses in applications such as sensors,41 electronics,42 magnetism43 and catalysis.44 For example, when the tail of the amphiphile is modified with an azobenzene chromophore, the azobenzene units in the lipid tail align and form n-stacking interactions that produce a red-shift in the electronic absorption spectrum. Diluting the chromophore- modified amphiphiles with alkyl—tailed amphiphiles disperses the chromophores and moves the peak of maximum absorbance to the normal position for unaligned azobenzenes”. Vesicles containing mixtures of chromophore-modified amphiphiles and simple alkyl amphiphiles display a temperature dependence on the absorption spectra that indicates the formation of chromophore aggregates at low temperatures and become disrupted at higher temperatures. Magnetic properties can also be conferred on a material by the induced alignment of magnetic centers integrated into the polar lipid molecules which make up the bilipid vesicles.46 In one transition metal-containing bilipid layer, copper(II) cyclam was used as the ionic head-group. The formation of the vesicle positions the copper ions into organized arrays to produced strong antiparamagnetic interactions between the aligned metal centers. Clearly a wide variety of effects and controlled molecular interactions can be obtained through the use of organic bilayer assemblies. The disadvantage of these solution-based assemblies are the dynamics of the media. The forces holding each molecule in place to form a structure are very weak hydrophilic-hydrophobic interactions and the specific structural type and arrangement of individual molecules can be altered by changes in temperature, solvent polarity, amphiphile concentration and the degree of . . 47 mlxrng. 14 3. Langmuir-Blodgett Films and Self-Assembled Monolayers Solid state surfaces may serve as a support to assemble stable arrays of polar-lipid molecules.48 Monolayers, bilayers and multilayers have been formed on the surfaces of substrate materials through the technique of Langmuir and Blodgett.49 Langmuir— Blodgett (LB) films are closely packed arrays of polar-lipid organic molecules on a solid surface where polar head-groups are hydrogen bonded to a glass surface in a hexagonal pattern and the lipid tails are directed away from the surface.50 Additional layers of the same or different polar-lipid molecules may be added to the film surface in a repeating tail-to-tail-head-to-head arrangement.51 Through the study of LB films, the organization and geometries of the assembled molecules have been determined by various mass,52 acoustic53, optical54 and x-rays5 methods. Applications of LB films generally involve the incorporation of an active species into the lipid tail of the amphiphilic monomer units and allowing those active species come into alignment as the film forms. The normal crystal packing forces of the crystalline state of the monomer are overcome by the interaction with a surface to form an LB film which forces the monomers into an artificial alignment.56 The induced alignment of molecules in LB films has been used to design systems that take advantage of this organization and include: non-linear optical, pyroelectric, and photochromic effects.57 Although the amphiphiles are bound to a solid surface, the interaction is only physical and the stability of the film is limited. One widely used method for increasing the stability of LB films is to incorporate polymers into the lipid tail so that the amphiphiles can be linked together and increase the stability of the film. Amphiphiles can be Strapped together either before film formation58 or by integrating a monomer unit into the organic tail59 of the film-forming molecules and initiating a polymerization of the formed layer. Additionally, a binding agent may be added to a pre-formed film to tie the tail groups together.60 These methods have not proven to be entirely satisfactory because polymerization leads to 15 structural defects61 and leaves behind some unpolymerized monomer. The general instability of LB films has led to the growing use of self-assembled monolayers in place of LB films.62 Self-assembled monolayers (SAMs) form similar structures to LB films63 except that the polar lipid molecules are bound to the substrate surface by stronger chemical adsorption64 compared to the weaker physical adsorption of molecules forming LB films. Assemblies consisting of n-alkyltrichlorosilanes chemisorbed to silica surfaces65 or various alkyl-sulfur compounds chemisorbed to gold surfaces66 yield consistently- formed close-packed monolayer arrangements of substrate molecules with a fairly high degree of stability.“ The order induced by the self-assembly of molecules onto gold or silica surfaces have produced monolayer catalysts,68 photoresist/ photolithography surfacesf’9 sensors70 and nonlinear optical materials.71 Self-assembled monolayers provide an excellent method to control the orientation of tethered donor-acceptor molecules to produce non-linear optical (NLO) effects. The crux of non-linear optical materials has been to orient molecules which form highly polarized intramolecular charge transfer states into non-centrosymmetric patterns. Non- centrosymmetric geometries of hyperpolarizable molecules can produce second-order (frequency doubling) NLO effect.72 Porphyrins,73 azo-dyes74 and aniline-n-tethered- pyridine molecules75 have all successfully produced SAMs that display NLO activity. The use of self-assembled monolayers for the positioning of molecules to produce non-linear optical properties is necessary for the bulk positioning of molecules into vectorially similar orientations. The electron transfer, in this case, is perpendicular to the surface, actually down the longitudinal axis of the bound molecules. Interactions of a given molecule with the surface or neighboring molecules is only required to keep the 16 bulk position constant - similar to a large handful of pencils where the center pencils are held in the same position as the peripheral pencils only through contact. The order created by monolayer formation can also be used to control interactions between neighboring molecules. One set of studies that depends on the relative position and direct interaction with neighboring molecules is the control of optical properties of aromatic molecules. Optical properties of aromatic molecules can be altered by creating or disrupting n-stacking geometries of the aromatic rings. n-Stacking geometries produce a red-shift in the emission and, in some cases, absorption spectra of aromatic molecules. Most aromatic molecules crystallize in a herring-bone geometry with no 1t-1t overlap. However, through the use of surface adsorption, amphiphiles containing aromatic rings can be placed into tr-stacking geometries and display altered emission . . 76.77.78 characteristics. 4 . Layered Guest-Host Materials: Metal Phosphates and Phosphonates Organic-modified two-dimensional layered materials79 are self-supporting solid state analogs of bilayer vesicles, LB films and SAMs. Layered materials consist of two- dimensional polymeric arrays of strongly bonded atoms and are held together by hydrogen bonding80 or weaker van der Waals forces“. What makes these materials interesting is that the interlayer surfaces of these crystalline materials are both available for reaction or modification through intercalation. The intercalation (insertion) of a molecule or ion between the weakly bound sheets leads to isotactic expansion of the inorganic layered host to accommodate the new guest species.82 A variety of chemistries have been used to drive these intercalation reactions including acid-base, reduction-oxidation, ion exchange, metal-ligand coordination and hydrogen bonding.83 17 Some organic molecules can also act as guest species and insert into certain layered hosts. Amines and alcohols are the most commonly intercalated organic species due to the ability of amines to undergo acid-base reactions, and both alcohol and amines to form hydrogen bonds or coordinate to metal ions. The polar head groups of intercalated organic molecules are positioned at specific sites on the surface of the layered material and lipid tail of the guest molecule is positioned in a fully extended geometry away from the layer similar to those seen in studies involving bilayer vesicles, LB films and SAMS. Of the available layered materials, layered metal phosphates (LMPS) and phosphonates (LMPRS) have proven to be an exceptionally large and diverse class of materials which are well suited for the controlled placement of substrate species. Layered metal phosphates and phosphonates are extremely stable two-dimensional materials consisting of metal atoms bound together by a network of O—P-O bridges. The O—P—O bridges can be from phosphate (PO43‘ ), hydrogen phosphate (HPO42‘ ), dihydrogen phosphate (H2PO4' ), phosphite (PO33’ ), hydrogenphosphite (HPO32') or phosphonate (RPO32' )84 groups. Diversity in the exact structure and chemistry of the inorganic matrix is increased through the large number of metal ions that have been shown to produce layered phosphate materials including magnesium, calcium,85 vanadium,86 niobium,87 iron,88 copper,89 cobalt and zinc90 as well as all of the Group 4 and 14 metals and some lanthanide metals.91 Of the many known layered metal phosphates and phosphonates reported in the literature, only those of vanadium and Group 4 and 14 metals have undergone extensive studies of both structure and intercalation chemistry. The following sections review the structural and intercalation chemistry of zirconium and vanadium phosphates as well as the organophosphonates of these metals. 18 3. Synthesis, Structure and Properties of Layered Metal Phosphates and Phosphonates i . Zirconium Phosphates There are two polymorphs of the layered metal phosphates of Groups 4 and 14 metals and each metal of these groups can form both structural types. The two structural isomers have the same general formula, M(HPO4)2, however, the nature of the bonding in the (It—structure is better described as metal bis(hydrogenphosphate), [M(O3POH)2]92 whereas the 'y—structure has recently been determined to be a metal phosphate dihydrogenphosphate, [M(PO4)(02P(OH)2)].93 The Ot-structure is made by the precipitation of an acid salt of the tetravalent metal ion, M“, with phosphoric acid.94 Prolonged refluxing95 and catalytic amounts of hydrofluoric acid96 aid to increase the crystallinity of the otherwise gelatinous metal phosphate precipitates.97 The less common y—structure is obtained through the precipitation of the tetravalent metal ion with monobasic sodium phosphate (NaH2P04) in hydrochloric acid solvent.98 The y—form initially precipitates as the monosodium salt of the layered material, [M(P04)(NaHPO4)], which is converted to the fully-protonated form by washing the material in acid to yield M(P04)(H2PO4). The exact crystal structure of the y—structural type has yet to be determined due to the general difficulty of growing crystals of layered materials coupled with the crystal-damaging solid state ion exchange reaction which is required as a final step.99 The crystal structure of a—ZrP has been determined and the idealized structure is represented in Figure 4.100 The intercalation chemistry of zirconium phosphate (ZrP) and other Group 4 and 14 metal analogs have been dominated by ion exchange and heavily explored by Clearfieldml and Alberti.102 Lithium, sodium and potassium ions all exchange in acid 19 Figure 4. Generalized structure of a-zirconium phosphate. (a) The strongly bound metal-oxygen-phosphorus plane (hydroxyl groups of the hydrogenphosphates were omitted for clarity). (b) The acidic interlayer region of Zr(O3POH)2 (each box represent a metal-oxygen-phosphorus layer). N ..$ [6... ...e r5... 2 Z \ u I \ t I O u 0 O .m. 0 P. nu QP: nu ..o. _ .o.. ..o. . ..o $36 33% \ul \ul 0 u 0 O m 0 0 LI. 0 hp: 0.. _ . 0.. ..o. . . o. ... {\an ~s~aloo \.l \-l O o O 0 o /P\ O /P\ OH OH 9H OH OH 6H Figure 4. 21 media to form various compositions of alkali metal exchanged ZrP materials. ZrP can also act as a solid acid and has the ability to undergo Bronstead acid-base reactions where by the acidic protons of the hydrogenphosphates are exchanged with the cations of an aqueous base. Acid-base reactions may be used to intercalate ions that are too large to fit into the pores which allow access to the interlayer region. Additionally, thermodynamic barriers like large heats of hydration for certain ions can be overcome by acid-base chemistry to promote exchange with protons. In most cases the intercalated metal ion may be removed by re-protonating the phosphate with acidm Slight expansions of the interlayer distances are noted for metal ion exchanged materials. The number of ions, charge of the ion and the waters of hydration about each ion all detemline the interlayer expansion needed to accommodate the guest ion.104 Organic amines may also insert into acidic ZrP layers. The Lewis basic amines coordinate to the protons of the layer and, to some extent, ionize the solid state acid to form interlayer alkyl ammonium phosphates. The inorganic ZrP layers separate to accommodate the alkyl chains of the organic amines. The exact size of the organic tail group of the guest amine determines the distance the inorganic planes must move. The interlayer distance of laminar materials can easily and accurately be measured by powder x-ray diffraction. Important structural information may be obtained by plotting the interlayer distance of the amine-intercalated layered material as a function of the number of carbon atoms on the alkyl chain (Figure 5). This plot forms a straight line with the slope of 2.0 A/CHZ. This slope is more than the distance occupied by each carbon in a fully-extended polymethylene chain ( 1.27 A/CHZ) when standard bond angles and bond lengths are used.105 To account for the large slope, a bilipid-like tail-to-tail arrangement of amine groups must exist between each inorganic layer. A bilayer arrangement could result in slopes up to 2.54 AICHZ if fully-extended alkyl chains from adjacent layers were placed perpendicular to each layer. A measured slope of 2.0 A/CHz indicates a bilayered 22 Figure 5. The interlayer distance of amine-intercalated zirconium phosphate as a function of the number of carbon atoms in the intercalated alkyl amines (slope=2.07A/carbon). The inset shows the bilipid-layered arrangement of the organic fragments predicted by a slope of ~2.0A/carbon. 23 d-Space I A I I I I I I J I I 0012345678910 Carbon Atoms in Amine Figure 5. 24 arrangement of alkyl chains which are fully-extended and angled ~54° from the inorganic plane. The amine head groups are separated by approximately 5.2 A (Figure 4a) in a hexagonal arrangement corresponding to the positioning of the acidic phosphate groups for a 1:1 phosphate/alkylamine material. Alcohol molecules may also be intercalated into ZrP and have an organic chain length dependence of the interlayer space similar to amine-intercalated ZrP. Alcohol molecules are weakly held in the interlayer region and are stable only as a dispersion in the alcohol solvent.106 From the alcohol exchanged ZrP, other polar solvents such as acetonitrile, acetylacetone and methyl acetone may be inserted into ZrP with the corresponding removal of the alcohol.107 The materials produced by exchanging out alcohol molecules with secondary polar solvents are only stable as swelled materials in the presence of the specific polar solvent. Drying of the ZrP material leads to removal of the intercalated solvent molecule. Organic guests can also be covalently anchored to the host layer through two methods reported in the literature. In the first method, pre-existing zirconium phosphate is exposed to ethylene oxide.108 The epoxide ring inserts between the acidic proton and the oxygen atom on the phosphate in a ring-opening addition reaction to convert the zirconium hydrogenphosphate into hydroxyethylphosphate, Zr(O3POCH2CI-I20H)2. Additional equivalents of ethylene oxide may be added to the hydroxyethylphosphate ester at increased temperature and pressure in a similar ring-opening reaction to produce poly(ethylene oxide) pendants on the phosphate. Accordingly, the inorganic planes of the ZrP must separate to accommodate the new appendages on the phosphate. Reaction of ZrP with epoxides is obviously limited to modifications of the interlayer by poly(ethylene oxide) or poly(propylene oxide) of various chain lengths. Phosphoric acid monoesters (11203POR) have also been used as the phosphorus source in the synthesis of covalently- bonded organic-modified zirconium phosphate materials but the acidic conditions of the 25 layer synthesis tend to hydrolyze the monoesters, to some degree, to phosphoric acid yielding plain ZIP.109 ii. Zirconium Phosphonates A general method to prepare pure and stable layered zirconium phosphates with organic species covalently bonded to the layer is through the use of phosphonic acids, (H203PR), as the phosphorus source.110 These materials are known as zirconium phosphonates (ZrPR) with a general formula Zr(O3PR)2 where R is an organic radical attached to the inorganic layer through an exceptionally strong phosphorus-carbon covalent bond.1 11 The pendants from each surface form an organic bilayer conformation similar to the amine-intercalated ZrP materials. The idealized structure of ZrPR R=Phenyl “2 Alberti concluded that the is presented in Figure 6 as described by Alberti. connectivity of the atoms forming the inorganic layer of the organic modified ZrPR are identical to that of the a—zirconium phosphate. There are only two limitations to the types of organic pendants to be used in ZrPR layered materials. The first limitation is that the phosphonate pendant cannot be susceptible to hydrolysis.113 The acidic solution and long refluxing times needed to produce crystalline ZrPR layered materials tend to hydrolyze esters, amides, nitriles and phenylethers as well as other organic species which are obviously sensitive to water or acid. The second restriction on the pendant organic of the phosphonate is that the cross— sectional area of the pendant cannot be greater than the area available to each phosphorus atom. Excessively large pendant groups can distort the inorganic framework and lead to either altered bonding of the layered metal—oxygen—phosphorus array to form egg-crate inorganic layers114 or spherical metal phosphonates may result.115 26 Figure 6. The idealized structure of Zr(O3PC6H5)2. 27 7. O 7. 6 \ . __ . .x.\q. _____ 0‘. __ _. __ e e I e e ZPOC O®oe Figure 6. 28 Like zirconium phosphates, the interlayer regions of layered phosphonates are available for ion exchange or organic reaction116 depending on the types of functional groups which make up the organic phosphonate pendant. For example, Clearfield has developed ion exchange materials based ZrPRs with sulfonic acid modified pendants117 and others have reacted carboxylate modified pendants with amines and strong bases. iii. Vanadium Phosphates A large number of structures of layered vanadium phosphates (VP) with a variety of compositions are known. The variety of structures of VPs is due to the fact that vanadium is stable as V3”:1 18 V4+1 19 and V5“120 in layered phosphates, all of which can form layered phosphate materials. 121 Additional structural motifs of VPs are afforded by changes in the degree of hydration at the vanadium atom. Finally, variations at the phosphorus atom by protonation or organic modification offer even more structural variations. Vanadium phosphates can undergo acid-base intercalation if acidic hydrogen- phosphate building blocks are present and accessible just like ZrP materials.122 The vanadium atom of some VP materials can also participate in intercalation processes to complement add to the versatility of intercalation pathways for these materials. The vanadium atom can participate in intercalation reactions in two ways. First, polar molecules with lone pairs of electrons on a terminal atom may attach to an empty coordination site on the vanadium atom in a direct intercalation.123 Second, the many stable oxidation states of vanadium can be capitalized on by utilizing reductive intercalation of cations.124 In reductive intercalation, a reducing agent donates an electron to a fully oxidized V5+ of the layer to produce V“, usually in the form of vanadyl ion, (V=O)2+. The reduction of the vanadium produces an overall negative charge on the 29 layer which is neutralized by the uptake (reductive intercalation) of a positive ion from . 125 . . . 126 . . . solution. Bimetalhc 10118 and even mtact coordination complexes127 have also been added to layered vanadium phosphates through reductive intercalation. The facile reduction of V5+ to V4+ can be achieved with a reductant as mild as iodide ion (Equation 128 l). VO(PO4) + NaI —> VO(PO4)Na + £12 (1) The intercalated metal ions coordinate to the oxygen atoms of the phosphate groups of the . . . 129,130 inorganic layer or to mterlayer water molecules. iv. Vanadyl Phosphonates Covalently appended organophosphonates of vanadium (VPR) also exist as layered materials. These materials are made by a reduction reaction whereby V205 is converted to vanadyl ion (V =0)2+ by the oxidation of a primary alcohol in the presence a mineral acid catalyst.'31 The (V=O)2+ ion then precipitates with the phosphonic acid to form a layered metal phosphonate of the formula V(O)O3PR°2H20 or, more commonly with one water being displaced by a solvent alcohol molecule132 as V(O)O3PR-H200R’CH20H. Equation 2 shows how the alcohol serves as the solvent, the reducing agent and an intercalant in this synthesis. R’CHZOIL évzos + H203PR V(O)O3PR-H20-R’CH20H + R’CHO (2) 30 There are several interesting features about the structure of vanadyl phosphonates. First, the 1:1 ratio of metal to phosphorus means that there is a larger area available for each organic pendant of VPRs compared to that of ZrPRs which have two phosphonates for each metal center, Zr(O3PR)2. The framework of the vanadyl phosphonate layers is further opened by the presence of two water molecules (or one water and one alcohol molecule) which occupy coordination sites on the octahedral vanadium metals leaving only three sites available for coordination by phosphonate oxygen atoms. The three phosphonate oxygen atoms and one strongly coordinated water molecule occupy the equatorial positions on the vanadium while the vanadyl oxygen atom and a weakly bound water molecule occupy the two axial sites to give the formula, V(O)O3PR°H20(eq)°H20(ax). The axial water molecule can be removed thermally with an isotactic contraction of the inorganic layers. Additionally, if only trace amounts of water are present and the synthesis of the vanadyl phosphonate is carried out in a primary alcohol, the axial site trans- to the vanadyl oxygen atom is occupied by an alcohol molecule [V(O)O3PR°H20(eq)°R’OH(ax)]. The dispersed arrangement of the organic phosphonates of VPR layers results in an interdigitated packing arrangement of the organic fragments known as an organic monolayer and is illustrated in Figure 7. For interdigitated VPR materials, a plot of the number of carbon atoms in the phosphonate pendant versus the interlayer space results in a slope of ~1.0 A/CH2 for a given intercalated alcohol. Conversely, the slope is also 1.0 A/CH2 if the phosphonate pendant is held constant and different sized alcohol molecules are intercalated. In both cases, the measured slopes are exactly half of the slope of 2.0 A/CHZ reported for a similar experiment with ZrPRs and amine-intercalated ZrP materials which form an organic bilayer. The use of bulky phosphonate pendants in the synthesis of vanadyl phosphonates results in either interlayer distances inconsistent with the length of the phosphonate 31 Figure 7. The interdigitated nature of vanadyl phosphonates with the "dihydrate” structure and the position of intercalated alcohol molecules. 33 pendant or layered materials with more than one phase.133 An unusually large interlayer distance measured for a given organic pendant or the absence of an intercalated alcohol molecule could be the result of an organic pendant that is too large for the cross-sectional area available with in VPR. Oversized pendants may either be unable to form an interdigitated structure or mask the vacant coordination site on the vanadium atom normally coordinated by a solvent alcohol molecule. Single-phase VPROR’OH materials most often result when one organic fragment (R or R’) is aromatic and the other is aliphatic (R’ or R). One aromatic species and one aliphatic species cleanly fill the cross- sectional area provided on each inorganic surface. A second major structural type of vanadium phosphonates has been synthesized by hydrothermal methods and structurally characterized by x-ray crystallography. The formula of this second structure is V(O)O3PR°H20.134 Even though this formula is identical to the formula of the de-alcoholated form mentioned above, the bonding of the inorganic layer is different and the changes to the inorganic layer lead to changes in the packing of organic fragments in the interlayer region. In both structures, each vanadium is equatorially bound by three oxygen atoms from three different phosphonate groups and one water molecule. The difference between the two structures lies in the bonding at the axial sites of the vanadium. In the first structural type discussed, known as the “dihydrate”, one axial position is occupied by a double-bonded oxygen atom an the other axial site is filled by either a water molecule or an alcohol molecule. In this second structure, known as the “monohydrate”, one axial site is occupied by a doubly bonded oxygen atom, but the second axial site is filled by a vanadyl oxygen atom from a neighboring vanadium center. This coordination of a nearby vanadyl oxygen creates a serpentine chain of alternating double and single bonded of vanadium and oxygen atoms (---V=O---~-V=O---). The organic interlayer region is affected by the formation of the puckered chain in the inorganic layer because the space available to each phosphorus atom 34 is reduced. Also, the alcohol molecules can no longer coordinate to the vanadium atoms because the Lewis acid site on the vanadium is occupied by the vanadyl oxygen atoms from a neighboring metal. A visual inspection of the inorganic planes of the two structural types of vanadyl phosphonates shown in Figure 8 would lead one to believe that the two bonding motifs should be interconvertible but this is not the case. Attempts to break the weak single bond of the ---V=O------V=O°-- chains by hydration have been unsuccessful presumably due to the large crystal packing forces and the steric problems with accessing the inorganic layer which is protected by a close-packed sheet of organic phosphonate pendants. Removal of the axial water or alcohol molecules from the dihydrate structure does not cause rearrangement to the monohydrate in Figure 8, but instead causes the interlayer space to collapse. The vacant coordination sites created by the removal of water or alcohol from the dihydrate cannot be accessed by neighboring oxygen atoms because the vanadium centers are too far apart and held in that distal geometry by a closely-packed array of interdigitated organic phosphonate pendants. Thus the two structural types of VPRs are not interconvertible. The literature of VPR intercalation chemistry has focused on the affinity of VPR dihydrates to reversibly intercalate alcohol molecules which coordinate to the Lewis acid sites of the vanadium centers created by the thermal removal of the axially bonded water or alcohol molecule. Dehydrated or de-alcoholated VPRs can re-intercalate many different primary alcohols to fill the empty coordination sites on the vanadium. A primary alcohol is required for the intercalation because the geometries of the V-O-P framework. The unoccupied coordination site is near the center of the layer with two O3PR fragments restricting access to the coordination site. Thus, secondary and tertiary alcohols are sterically hindered by the neighboring phosphonates and cannot engage the vanadium atom to form pure phases of intercalation complexes. 35 Figure 8. The inorganic metal-oxygen-phosphorus plane of vanadyl phosphonate dihydrate. 36 .me °°8° Figure 8. 37 b. Examples of Controlled Molecular Self-Assembly in Layered Metal Phosphates and Phosphonate: A few examples that utilize the ordering of organic molecules by LMP and LMPR materials do exist. To date, applications have been limited to the research groups of Thompson, Mallouk and Jacobson and Johnson. Thompson135 has intercalated propargylamine into ZrP materials. The propargylamine-intercalated ZrP was then heated or photolyzed to initiate the polymerization of the intercalated molecules. The polymerization of these acetylenic amines was identified by the development of a copper color in the otherwise white powder. Additionally, the polymerization was verified by IR, 13C and 31P MAS NMR and several thermal techniques. Propargyl groups are normally difficult to polymerize136 in solution but seem to react readily when the unsaturated units are placed into alignment. Thompson’s work mimicked work previously done by Day137 and Tieke138 who both used layered metal halides as templates for the polymerization of intercalated monomeric units. Mallouk has used the templating effect of ZrP materials for the construction of an amine intercalated ZrP material with potential applications as chromatography stationary phases for chiral separations.139 The basis for the design of the stereo selective material was the series of chiral selector molecules developed by PirkleI40 that are able to selectively bind to one stereoisomer of a racemic mixture of specific analyte molecules. Mallouk appended one of Pirkle’s chiral selector molecules, [(S)-(+)-N-(3.5- dinitrobenzoy1)-L-leucine], with a cationic quaternary amine and introduced this selector into ZrP by reaction of the Lewis base site of the selector with the acidic hydrogen phosphate layer. The ammonium ion head becomes oriented toward the layer leaving the hydrogen-bonding and u-stacking sites on the selector molecule free for interaction by analyte molecules. The bulk of the quaternary ammonium head only allows intercalation 38 of approximately 0.5 mmole per gram of ZrP. This low degree of ion exchange leaves the quaternary ammonium selector molecules widely dispersed on the ZrP surfaces. Thus, the widely dispersed organic molecules form an interdigitated organic monolayer between the inorganic host layers with an interlayer spacing of only 19 A. The ZrP intercalated with an optically active host has been shown to separate racemic mixtures of the complementary Pirkle analyte molecule, [(R , S) - 2 - C10H7NHC*H(CH3)C(O)C(O)OCH2CH3]. The S-(-)-isomer of the racemic analyte mixture selectively complexes with the quaternary ammonium selector molecule. The complexation is driven by the formation of a charge transfer salt with the selector molecule being the electron acceptor and the analyte acting as the electron donor. The inorganic layers swell to an interlayer distance of 30 A to accommodate the charge transfer complex-forming analyte molecules. The enantiomeric excesses measured for Mallouk’s system approaches 90% of the theoretical maximum141 for the concentrations tested. Jacobson and Johnson have shown vanadyl phosphonates to be analogous to a supramolecular material because the layered solid can be used for molecular recognition.142 The alcohol intercalation ability of vanadyl phosphonate dihydrates is limited to primary alcohols and therefore it is able to selectively remove normal-butanol form mixtures containing secondary-butanol and tertiary-butanol. It has also been found that LMPs can recognize the difference between similarly functionalized substrates with an odd versus even number of alkyl carbon atoms.143 These same three research groups have developed other applications for layered metal phosphates and phOSphonates and include: structural fabrication materials;I44 purported viologen-based artificial photosynthesis;145 templates for the production of quantum-sized metal particles through the reduction of intercalated ions;146 and platforms 39 . . . . 147 . for the construction of non-linear optical materials and vectonal charge transfer “wires”148 when coupled with self-assembled monolayer technology. 149‘150 c . Phosphonic Acid Synthesis: Opportunity for Variations on a Theme A vast library of phosphonic acids exist and many of these can be used as pendants on layered metal phosphonates.151 A variety of synthetic schemes have been used for the synthesis of phosphonic acids. Most attempts to add elemental phosphorus or phosphorus oxides to organic groups are of low yield and little synthetic importance. The most common methods to produce phosphonic acids (H203PR) is by the hydrolysis of phosphonate diesters, R’203PR. Phosphonate diesters are most commonly made by the Michaelis-Arbuzov reaction.152 This reaction can produce primary and secondary alkylphosphonate diesters by simply heating a mixture of a trialkylphosphite with the alkylhalide which will become the pendant of the phosphonate. Arylhalides can be used in this same reaction to produce arylphosphonates with the help of catalysts such as copper(I),153 nickel(II)154 and palladium(0).155 Michaelis-Becker reactions,156 addition to activated double bonds,157 and Grignard reactions provide additional reaction pathways to phosphonic acid synthesis and are outlined in Figure 9. The robust covalent P-C bond of the phosphonate can withstand many reaction conditions so it is also possible, and quite feasible, to synthesize phosphonates through modifications to the organic pendant of an existing phosphonic acid or phosphonate diester. Hydrogenation,158 dehydrohalogenation,159 esterification,160 and oxidation or reduction161 reactions162 have all successfully been used to modify the organic pendant of phosphonic acids and esters. Phosphonate dialkylesters are converted to the corresponding diprotic phosphonic acid by acid or base hydrolysis.I63 If the 40 Figure 9. Common synthetic routes to phosphonate diesters which can be hydrolyzed to the corresponding phosphonic acids. 41 Arbuzov: Br P(OEt)3 + 1,44" Modified Arbuzo v: Br P(OEI)3 + Michaelis-Becker: P~OE Bf Na‘ '0’ ‘OEtt + 14"“ Micheal Addition: .3“ C’ 8+ / ~OEt + ”0 ‘OEt A Grignard: 0 II P~ Bf MgBr GB + ,‘ Modification of "R": o I I P- 3 CB - EtBr i \ NiBrz - EtBr - NaBr NaOEt - Mng 95mm... Reduction Halogenation etc. Figure 9 42 functionalities on the pendant are particularly sensitive to acid or base, the dialkyl phosphonate may be converted to trimethylsilyl diesters164 followed by methanolysis165 to give the corresponding phosphonic acid. C . Thesis Outline Layered metal phosphates and phosphonates clearly provide excellent frameworks upon which designed molecular assemblies can be constructed. LMPs and LMPRs have an added advantage over other self-assembling materials because the structure of a pre- existing layered material can be altered in a controlled manner through the intercalation of a guest molecule to change the micro-environment of the interlayer space. The controlled assembly and controlled alterations possible within LMP and LMPR materials lead one to believe that these frameworks could be the basis for the design and construction of devices and catalysts. Applications involving LMPs and LMPRs to date only take advantage of the templating benefits of these materials to assemble a single final structure involving the positions of the phosphonate pendants and/or the intercalated guest species. A method is developed in this thesis to explore the change induced by the mechanical expansion of the layers resulting from intercalation with possible applications in the production of devices. Additionally, layered phosphonates will be examined as templates for a photoactive species in the controlled fabrication of a photocatalytic material. 43 l . Assembly of Aromatic Molecules Within Layered Metal Phosphonates: Control and Switching of Optical Properties In Chapter III, layered vanadyl phosphonates are used as supports for the construction of arrays of naphthalene molecules. The interdigitated arrangement of the organic pendant groups and intercalated alcohol molecules of layered vanadyl phosphonate dihydrate structures will be used to control the relative positioning of interlayer naphthalene molecules. The relative position of a naphthalene molecule relative to another naphthalene can significantly alter expression of naphthalene’s photophysical properties. The face-to-face overlap of aromatic molecules can lead to the formation of excimer (excited state dimers). The formation of an aromatic excimer leads to a red-shift of the fluorescence emission compared to an isolated naphthalene. Two methods are possible to control the local environment about the naphthalene in interdigitated vanadyl phosphonate layers. First, naphthalene can be introduced into a layer as an intercalated alcohol and the relative positions of the (naphthalene)alcohol molecules within the layer can be “tuned” by utilizing phosphonate pendants of differing lengths. Conversely, the naphthalene can be appended directly to the phosphonate pendant of the layer and the overlap of the rings can then be adjusted by intercalating alcohols of different lengths. Thus, the presence or absence of an optically clear aliphatic alcohol can greatly influence the emission behavior of naphthalene. The intercalation- induced disruption or alignment of the naphthalene units can be though of as the “on” and “off” states of a sensor which indicate the presence or absence of a substrate molecule. The on or off state can be determined by the specific optical characteristics displayed by the naphthalene rings within the layered support. 44 2 . Controlled Assembly of a Potential Two-Electron Photocatalyst for Applications in Light to Energy Conversion In Chapter IV, a method to incorporate a multiply bonded bimetallic photochemical center into a layer metal phosphonate host is presented. The basis of this effort to couple transition metal photochemistry with solid state chemistry is to integrate a novel photochemical center into a solid state support capable of regenerating the photocenter and prevent a back reaction of the photoproducts. The assembled architecture may then be able to serve as a catalyst for efficient light to energy conversion. The novel photocenter is a quadruply bonded dimolybdenum complex166 capable of producing a multielectron excited state which undergoes a two electron photoreaction.167 With a single photon being able to move more than one electron at a time, the need to separate electron-hole pairs with vectorially arranged electron donor and acceptor reaction sequences is eliminated. Layered metal phosphates and phosphonates fulfill all of the prerequisites to act as a solid support for dimolybdenum photocenters and produce a catalyst. The layered materials are stable to acidic and organic solvents similar to the environments needed for dimolybdenum compounds. Additionally, the intercalation behavior of layered phosphates should allow substrate molecules to access the photocenter. Finally, the redox activity of some layered metal phosphates should allow for the layer to serve as a source of electrons to regenerate the oxidized bimetallic core and create a catalytic system. Currently, three plans exist to introduce the dimolybdenum core into layered metal phosphate materials. '68 One method has already proven successful and works by totally solvating the dimolybdenum center and reductively intercalating the metal center into a redox active layer. However, the coordination of the dimolybdenum center in this case can only consist of phosphate ions from the layer. A second method for the insertion of dimolybdenum which does allow for alternative ligand sets about the metal consists of an 45 acid-base intercalation of an intact dimolybdenum complex. For this case, a Lewis basic site is designed into one or more ligands attached to the metal center. The basic ligand and the attached metal then intercalate as a single unit. This method is being explored by Mr. Eric Saari from our group. The final method and the method which will be presented in Chapter IV is a hybrid of the two previous options. In this case a solvated dimolybdenum core is again used but the host layer is not a pure metal phosphate but an organic modified phosphonate. The distal end of the organic pendants of the phosphonates can be modified with ligating functional groups known to promote two- electron photochemistry of the dimolybdenum core. The photochemistry of the dimolybdenum center, and the design and synthesis of dimolybdenum-incorporated layered metal phosphonates will be presented. CHAPTER II EXPERIMENTAL METHODS A . Instrument Setups l . Powder X-Ray Diffraction The powder x-ray diffraction data was recorded on a Rotaflex system from Rigaku with a Bragg-Brentano geometry. The Cu-Ka line was obtained from a rotating Copper anode (45 kV, 100 mA) and directed toward the sample chamber using a l/6° divergence slit and a 1/6° receiving slit. The diffracted x-ray beam was further refined by a curved graphite single crystal monochromator (1.05° scatter slit and l/6° monochromator receiving slit), which was set for detection of the secondary x-ray diffraction line. The compounds were mounted in glass sample holders available from Rigaku. The glass sample holder consisted of a 1.5 cm x 3 cm plate of glass 3 mm thick with a recess ground into the surface of the slide to pack the sample. The recess in the slides used was 1 cm x 1 cm in area and ground to a depth of 0.2 mm. The powdered sample was placed in the recess of the sample holder and packed tightly into place by 46 47 pressing and sliding a clean glass microscope slide over a small mound of sample placed on the recessed area of the sample holder. The resulting data were recorded and processed using the manufacturer-provided DMAXB software on an IBM personal computer under the MS-DOS operating system. 2. UV-Vis Spectroscopy a. Absorption Spectroscopy The electronic absorption spectra of solution samples were recorded on either a Cary 2300 Spectrophotometer from Varian or a Cary 17 instrument modified with a computer controlled data acquisition system by On-Line Instruments (OLIN) Corporation of Bogart, GA (USA). Air or moisture sensitive samples were prepared in a nitrogen- or argon-purged glovebag and the sample cell was capped with a poly(fluoroethylene) stopper before being transferred to the nitrogen-purged spectrophotometer. In all cases, quartz sample cells with a 1 cm path length were used. Plots of molar extinction coefficients as a function of wavelength were made by the following procedure. The absorption response of the Cary 17 instrument was calibrated with a gadolinium oxide standard of known absorbances. Absorption spectra were then collected at three different sample concentrations, 1 x 10'3 M, 1 X 10'4 M and 1 x 10'5 M and each data set was then converted from absorbance versus wavelength to 8 versus wavelength by Beer’s Law. The three data sets were then averaged and plotted. 48 b. Solid State / Diffuse Reflectance The electronic spectra of solid state samples were recorded on a Shimadzu UV- 3101PC UV-Vis-NIR Scanning Spectrophotometer equipped with an ISR-3100 UV-Vis- NIR Integrating Sphere Attachment for diffuse reflectance. Diffuse reflectance spectra are reported as a percentage of reflectance referenced to a white background of BaSO4. Each sample was prepared by spreading a thin layer of sample onto the surface of a BaSO4 (W ako Pure Industries) cake, which was pressed into the standard sample holder for the ISR-3100 diffuse reflectance attachment as described by the manufacturer. The spectrometer was controlled by an AST personal computer under the MS Windows operating system with software provided by Shimadzu. 3. Fluorescence Spectroscopy Steady-state luminescence spectra were recorded by using a high-resolution emission spectrometer constructed at Michigan State University and previously described elsewhere.169 Mercury lines from the 200 Watt Hg—Xe excitation light were selected by the double monochromator of the emission spectrometer in conjunction with an either a 253.7 nm or a 313 nm interference filter. The solid samples were packed into the recess of the glass sample holder described earlier for powder x-ray diffraction and placed at an angle approximately 20° to 30° degrees from the incident radiation to prevent direct reflection of excitation light off of the sample and into the detection optics, which are 90° from the excitation radiation. Luminescence from the sample was directed through the detector-side single monochromator and onto a dry ice-cooled PMT (R1104, Hamamatsu). Slit widths for the excitation and emission monochromators were 5 mm/ 5 mm and 3 mm/ 3 mm, respectively. The entrance slit of the emission monochromator 49 was equipped with either a 280 nm or 335 nm cutoff filter for 254 nm or 313 nm excitation light respectively. 4 . Magnetic Susceptibility Magnetic susceptibilities were measured on a SHE 800 series variable-temperature SQUID magnetometer controlled by an IBM-PC microcomputer. A known quantity of layered compound was placed in a small pouch constructed by heat sealing polyethylene film. Data collection was then made by scanning from 300 K to 3 K and recording a data point every degree between 300 K and 150 K and every half degree below 150 K. Ten readings were made at each temperature and the average of those ten measurements was recorded as a data point. 5 . ESCA The analyses of elements in layered materials were performed with a Perkin Elmer PHI 4500 ESCA System, which is housed in the Composite Materials and Structure Center at Michigan State University. The x-ray source was monochromated Al or Mg K3 lines (Al(Ka) = 1486.6 eV, 600 W/ 15 kV; Mg(Ka) = 1253.6 eV, 400 W/ 15 kV). Samples to be analyzed were pressed onto double-sided tape adhered to the sample stage of the instrument, which was placed at an oblique angle to the incident x-ray beam (65° for the Al source and 45° for the Mg source) under ultra-high vacuum (1078-10-9 torr). The sample stage was positioned so that the signal intensity for oxygen (binding energy of 531 eV) was maximized because oxygen was sure to be in each sample. Elements were identified by survey scans from binding energies of 1200 eV to 0 eV. The strongest 50 photoelectron line from each element was scanned separately to obtain relative atomic concentrations by integrating the area of the peak and multiplying an appropriate response factor. All data manipulation was accomplished on an Apollo Workstation using XPS- ESCA software provided by Perkin-Elmer. B . Synthesis of Vanadyl Phosphonate Materials 1 . Synthesis of V(O)O3PR-H200R’OH Layers The vanadyl phosphonates were prepared as derivatives of the so called dihydrate form of VPR, which is known to intercalate alcohols and develop aninterdigitated arrangement of interlayer organic pendants. The synthesis of V(O)O3PR-HZO°R’OH (R ’OH = alcohol or water) is are based on the original procedure reported by Jacobson and Johnson.170 Vanadyl alkylphosphonates. Vanadium (V) oxide (Aldrich) (0.273 g, 1.5 mmole) was finely ground with an agate mortar and pestle to produce a silky microcrystalline material. The finely divided V205 (0.273 g, 1.5 mmole) was placed in a 50 ml round bottom flask. A 10% excess of phosphonic acid (3.3 mmole) was dissolved into 30 ml of benzyl alcohol (C6H5CH20H). The phosphonic acid solution and 0.9 ml of 1 M HCl(aq) were added to the flask containing the vanadium oxide. The two-phase mixture containing V205 solid and phosphonic acid/alcohol solution was then heated to 85 °C on a thermostat-controlled stirring hot plate. The two-phase mixture of orange-brown vanadium oxide in phosphonic acid solution was slowly converted to a new two-phase mixture with the solid product being light blue to green vanadyl phosphonate. The reaction mixture was heated with stirring until no noticeable amount of 51 V205 could be seen in the solid mass when the stirring was periodically stopped. Generally, the reaction required 24 to 48 hours to complete as determined by visual means. The blue product was then filtered into a medium frit and the filtered cake of vanadyl phosphonate was washed three or more times with benzyl alcohol (the alcohol solvent used for this particular reaction) and three times with diethyl ether. The microcrystalline product was aspirated on the frit until dry and powdery when stirred. The final product, V(O)O3PR¢C6H5CH20H-H20, was analyzed by powder x-ray diffraction. If a peak corresponding to V205 starting material was noted, the product was ground and placed in a flask containing the filtrate from the reaction mixture and concentrated with 0.5 mole of the appropriate phosphonic acid. This mixture was then heated for an additional 24 to 48 hours to remove any remaining V205. Vanadyl arylphosphonates. The procedure used to make vanadyl arylphosphonates was identical to that of the vanadyl alkylphosphonates except that 30 ml of a primary alkyl alcohol was used in place of benzyl alcohol. Reactions in alkyl alcohol solvents required 3 to 7 days to go to completion. After filtering the vanadyl arylphosphonate products, the alkyl alcohol used as the synthesis solvent was used to wash the product followed by washes with diethyl ether. As with any reaction involving a solid state reactant, the particle size and mixing efficiency determine the exact rate of reaction and thus reaction time. In general, the reactions using benzyl alcohol solvent and finely ground vanadium (V) oxide required 24 hours to completely remove the vanadium oxide. 2. Alcohol Exchange Reactions of V(O)O3PR-HZO°R’OH The alcohol molecule of crystallization, R’OH, in V(O)O3PR0HZO0R’OH, referred to as the “intercalated” alcohol, can be selectively removed to induce an isotactic 52 collapse of the metal phosphonate layers to produce a material with a reduced d-space and a vacant coordination site on the vanadium atom.171 The vacant coordination site can be refilled by water or a new alcohol molecule to produce a new material with an expanded d-spacing. Two different methods have been used to remove the alcohol of crystallization. A new alcohol is introduced into the layer by simply submerging the alcohol-free solid into a neat or diluted sample of the desired alcohol. The solvent for diluting or dissolving the new alcohol must be inert to the vanadium layer. Two different approaches (Methods A and B) were employed to remove the alcohol from a layered vanadyl phosphonate thereby permitting V(O)O3PR0HZO°C6H5CHZOH to be converted to V(O)O3PR-HZO¢B-C10H7CHZOH. In Method A, benzyl alcohol intercalated vanadyl alkylphosphonates were synthesized as discribed in Section B.1 of this Chapter. The dry benzyl alcohol-intercalated material was placed into a round bottom flask and heated to 110 °C under vacuum until the alcohol is removed, as determined by powder x—ray diffraction. This method, reported by Jacobson and Johnson,172 leads to inconsistent results due to uneven heat transfer to the solid. Heating the dry vanadyl phosphonate under vacuum is expected to yield an alcohol-free material but often results in either no change in the x-ray powder diffraction patterns of the product or a multiphase product with a mixture of low angle reflections caused by the non-reversible loss of the equatorial water molecule on the vanadium atom. Method B was generally more successful and led to more consistent results in the removal of the intercalated alcohol molecule. Method B was carried out by adding 0.50 grams of vanadyl phosphonate to a 100 ml flask containing 50 ml of dodecane. The flask is slowly purged with argon and heated to 110 °C for 2 to 4 hours with stirring. The alcohol-free solid was filtered while still hot and washed once with petroleum ether while on the glass filter frit. The solid was then quickly transferred to a second flask and tightly 53 capped to prevent atmospheric water from intercalating into the material and occupying the newly vacated coordination site on the vanadium atom. Alcohol Intercalation. All alcohol intercalation reactions were carried out using a minimum of a ten-fold excess of alcohol. A l M solution of B-C10H7CHZOH in dry toluene (30 ml) was added to a flask containing the alcohol-free vanadyl phosphonate (3 moles). This two-phase mixture was. then stirred at room temperature for a minimum of 48 hours. The B-C10H7CHZOH-intercalated products were analyzed by powder x-ray diffraction, UV-visible diffuse reflectance spectroscopy and fluorescence emission spectroscopy. Similar alcohol-exchange reactions were carried out by replacing benzyl alcohol with a—C10H7CH20H and B—C10H7CH2CHZOH. C . Synthesis of Zirconium Phosphonate Materials 1 . Synthesis of Zr(O3PR)2 Zirconium phosphonates were synthesized according to the methods developed by Alberti.173 All of phosphonic acids with essentially straight chains or linearly arranged aromatic groups lead to the same (it—zirconium phosphate structure where the phosphonate organic pendant is directed into the interlayer region in a fully extended geometry. Generalized Zirconium Phosphonates. One equivalent of zirconyl chloride octahydrate (ZrOC1208HzO) or zirconium chloride (ZrCl4) was dissolved in a minimum amount of water in a round bottom flask. The desired phosphonic acid (2.2 equiv.) was dissolved in a second flask with a minimum amount of water. If the phosphonic acid seemed insoluble in water, then the water was heated until the phosphonic acid dissolved. ‘5. The phosphonic acid solution was added with stirring to the zirconium solution which resulted in the immediate formation of a white, gelatinous precipitate. Water was then added to the precipitate until the slurry could easily be stirred with a magnetic stir bar and stirring plate. The flask was then equipped with a condenser and heating mantle and brought to reflux. Once boiling, concentrated HFM) was added to the solution dropwise until a noticeable amount of solid was dissolved. Approximately 0.5 ml of 48% HFm) per 0.1 mole of zirconium salt was needed to effect a noticeable change in the amount of zirconium phosphonate gel present in the flask. The reaction mixture was left to reflux over night, cooled and filtered into a medium-pore glass frit. The crystalline material was washed on the frit by filling the funnel three times with each of water, ethanol and diethyl ether. Care was taken to not let the filtered cake of Zr(O3PR)2 dry and crack during the washings, to thereby ensuring the most efficient washings. After the final washing with ether, the crystalline material was allowed to dry on the aspirated frit. The resulting materials were analyzed by powder x-ray diffraction. 2. Synthesis of Zr(O3PCH2CH2COZ" W); from Zr(O3PCH2CH2COOH)2 (M: Na+, NH4+) Zirconium phosphonates with carboxylic acid pendant groups act as weak acid ion exchange materials. Attempts to exchange the acid proton of the carboxylate pendant require basic conditions to break up the interlayer hydrogen bonding before a new ion may be introduced into the layer. Literature methods usually utilize basic metal oxides or hydroxides to simultaneously break up the hydrogen bonding with an acid-base reaction and introduce the counter cation ion into the layer. The acid-base reaction of acidic layers is diffusion controlled and often leads to a buildup of base. This buildup of base leads to decomposition of acid-stable zirconium phosphate and phosphonate salts as the pH 55 climbs above 8 to produce polymeric zirconium hydroxides and oxides. Thus, neutralization reactions of this type often lead to decreased crystallinity — noted by a marked decrease in intensity and a broadening of peaks in the powder x-ray diffraction patterns. To decrease the peak broadening brought on by excess base, a new method to intercalate sodium ions was developed. Ammonium ion exchanged material was produced by the acid-base reaction of anhydrous ammonia with the acidic to produce the ammonium ion within the layer similar to the method developed by Thompson.174 Sodium Ion Exchange of Zr(O3PCH2CH2COOH)2. Dry and finely ground zirconium carboxyethylphosphonate (5 g., 13.5 mole) and a magnetic stir bar were added to a 250 ml flask. A solution of sodium bicarbonate (150 ml, 0.1 M) was then added to the zirconium materials. The mixture evolved a large amount of bubbles over a period of 1 hour. The resulting powder was then filtered on a medium frit and washed with a large amount of water followed by washings with ethanol and ether. The product was analyzed by powder x-ray diffraction. Ammonium Ion Exchange of Zr(O3PCH2CH2COOH)2. Zirconium carboxyethylphosphonate (5 g., 13.5 mmole) was placed in a 100 ml round bottom flask equipped with a mineral oil bubbler and gas inlet. Anhydrous ammonia gas (Matheson) was then passed over the solid zirconium phosphonate with stirring. The reaction flask became very hot as the solid state acid and gaseous base reacted. Ammonia was diffused over the solid for 0.5 hours after the heat from the reaction had dissipated. The resulting material was characterized with powder x-ray diffraction. 56 3 . Intercalation of Dimolybdenum Complexes into Zirconium Phosphonates To ensure a maximum “loading” of the ZrPR with dimolybdenum centers, an excess of dimolybdenum was always used in the reaction mixtures so that ten equivalents of dimolybdenum were present for each pair of interlayer ligands. Thus, a 20:1 ratio of dimolybdenum to phosphonate (equal to 10:1 of dimolybdenum to zirconium phosphonate) was used in all cases. The sodium or ammonium ion exchanged form of zirconium carboxyethylphosphonate was placed in a three neck flask and heated under vacuum to a temperature of 60 °C for a period of 5 hours to remove any water that may be present in the material. A dimolybdenum compound from Table 1 was dissolved into 30 ml of rigorously dry and degassed solvent under an argon atmosphere. The solution of dimolybdenum compound was then transferred to the flask containing the ion exchanged zirconium phosphonate. This two-phase mixture was then stirred until the white zirconium phosphonate developed a consistent color. The coloration of the zirconium phosphonate is the result of intercalation of a dimolybdenum center. The quadruply bonded molybdenum-molybdenum (MoA-Mo) intercalated zirconium phosphonate materials were analyzed by UV-visible diffuse reflectance spectroscopy, powder x-ray diffraction and ESCA. Attempts to increase the accuracy of the metal ratios was sought by analyzing samples by ICP. However, it is not possible to make homogeneous solutions of the molybdenum-zirconium-phosphonate hybrid material because molybdenum precipitats formed in base and the zirconium layers are insoluble in all acids except HF, a solvent which most ICP instruments cannot tolerate. For these reasons, ICP results were not accurate and are not reported. ‘57 D . Phosphonic Acids The following phosphonic acids and phosphonate esters were available commercially from the corresponding source(s) and used as received: methylphosphonic acid (Aldrich, Lancaster), ethylphosphonic acid (Aldrich, Lancaster), propylphosphonic acid (Aldrich), butylphosphonic acid (Aldrich, Lancaster), carboxymethylphosphonic acid (Lancaster), 2-carboxyethylphosphonic acid (Lancaster), 3-carboxypropyl- phosphonic acid (Aldrich, Lancaster), l-naphthylmethylphosphonic acid (Lancaster), arsonilic acid (Aldrich). It was found that the methyl-, ethyl- and propylphosphonic acids are extremely hygroscopic and should be handled accordingly to avoid errors in weighing — a small amount of water will not hamper the synthesis of the layered metal phosphonate products. Phosphonic acids not available commercially were prepared by the methods described belowm. In all cases, phosphonic acids were prepared by synthesizing the corresponding diethyl, dibutyl or di-isopropyl phosphonate ester followed by hydrolysis to the desired phosphonic acid in 12 M hydrochloric acid. 1 . Alkyl Phosphonic Acid Synthesis The n-alkyl phosphonic acids that were not available commercially, i.e. C5 - C12 and 2-(2-naphthyl)ethylphosphonic acid, were prepared by Michaelis-Becker reactions according to the procedures of Kosolapoff.176 Additionally, activated double bonds can be used to make alkyl phosphonic acids by a Michael-like addition. In both reactions, dibutylphosphite sodium salts are the phosphorus source and displace the bromide of alkylhalides or add to the double bond in a Michael-like addition. 58 a. Michaelis-Becker Reaction Sodium metal (1.15 g., 50 mole) was cut, washed with hexane, weighed and quickly placed in a 250 ml three-neck round-bottom flask, which was purged with N; or Argon gas and equipped with an addition funnel, reflux condenser and magnetic stir bar. 150 ml of dry hexane was added to the flask and dibutylphosphite (9.7 grams, 50 mole) in an addition funnel. The hexane solvent was heated to reflux with an oil bath on a hotplate equipped with a magnetic stirrer. Once the solvent was at reflux, dibutylphosphite was added dropwise over twenty to thirty minutes. The reaction flask was heated with stirring for three hours or until all of the sodium metal was consumed in the reaction. Any sodium metal remaining after three hours was removed with a spatula. The addition funnel was replaced with a clean addition funnel and filled with 50 mole of alkylhalide. The alkylhalide was added to the refluxing hexane/sodium dibutylphosphite solution dropwise over thirty minutes. Sodium halide began to precipitate fifteen minutes after alkylhalide addition began. The reaction was left at reflux with stirring for five hours after alkylhalide addition was complete. At this point, the reaction pot was cooled and the organic mixture was washed thoroughly with water to remove the precipitated sodium bromide. The hexane solvent was removed by rotary evaporation and the oily phosphonate product was purified by vacuum distillation according to Table 2. The pure alkylphosphonate dibutyl ester was hydrolyzed to the corresponding alkylphosphonic acid in 100 ml of refluxing 12 M hydrochloric acid over night. The high molecular weight phosphonic acids (C3 and above) crystallized upon cooling of the hydrolysis mixture and the resulting solids were separated by filtration and purified by recrystallization from hot hexane. Lower molecular weight phosphonic acids were purified by evaporating the resulting hydrolysis mixture (HCl(aq), butanol and butylchloride) to dryness and recrystallizing the resulting crude alkylphosphonic acid 59 Table 1. BF. and M.P. of alkylphosphonic acids and esters. Alkylphosphonic RBr B.P. of M.P. acid Reactant RP(O)(OBu)2 of RPO3H2 (°C) (°C / mm Hg) measured. literature measured. literature CH3(CH2)4PO3H2 Pentyl— 122 l 7 167 / 17 119-120 121 CH3(CH2)5PO3H2 Hexyl— 124 / 5 182/ 20 106 104-106 CH3(CH2)6PO3H2 Heptyl— 136/ 1 188/ 17 102-103 103.5 CH3(CH2)7PO3H2 Octyl- 151 / 2 147 / 2 97-99 995-100 CH3(CH2)9PO3H2 Decyl— 148 /0.5 161 I 1 98-99 100.5-101 CH3(CH2)1 1PO3H2 Dodecyl- 176/ 1 196/ 3 94 94.5-95.5 C10H7(CH2)2PO3H2 B—Naphthylethyl- Not n/a 147-149 n/a Distilled 60 from hot hexane. All melting points were 1151 °C of literature values177 and all resonances in the NMR were attributable to product or recrystallization solvents. b. NaP(O)(EtO)2 Addition to Activated Double Bonds Sodium diethylphosphite undergoes a Michael-like addition to activated double bonds to form phosphonate esters with an ethylene linkage between the activating group and the phosphorus atom. Unlike the non-polar solvent conditions required for the Michealis-Becker reaction above, this reactions is carried out in an alcohol solvent with a small amount of sodium alkoxide catalyst. The following protocol is the generalized procedure for the preparation of 2-carboxyethylphosphonic acid, 2-(4-pyridyl)ethylphosphonic acid, and 2-cyanoethylphosphonate diethyl ester from ethylacrylate, 4-vinylpyridine and acrylonitrile respectively. 2-(4-pyridyl)-ethylphosphonic acid.178 A three neck flask equipped with a reflux condenser and an addition funnel was charged with diethylphosphite (27.6 grams, 0.2 mole) and 4-vinylpyridine (Aldrich) (21 grams, 0.2 mole). The reaction mixture was stirred magnetically for 10 to 15 minutes to initiate a reaction, which was noted by an increase in the temperature of the reaction mixture. The temperature of the mixture was controlled with a water bath to maintain a temperature in the range of 50 to 70 °C. When the reaction mixture returned to room temperature it was washed with water (5 x 50 ml), dried with MgSO4 and distilled under vacuum to give the pure ester, diethyl 2-(4- pyridyl)ethylphosphonate. The ester was hydrolyzed to the corresponding phosphonic acid by refluxing in 50 ml of 12 M HCl overnight. The 2-(4-pyridyl) ethylphosphonic acid was isolated by evaporating the hydrolysis mixture and recrystallizing the crude materials from hot ethanol to yield pure 2-(4-pyridyl) ethylphosphonic acid as the hydrochloride salt (M.P. 139-140°C). 61 2. Aryl Phosphonic Acid Synthesis a . Modified Arbuzov The Modified Arbuzov reaction is similar to the standard Michaelis-Arbuzov Reaction used to prepare alkylphosphonate esters but the presence of a free radical catalyst allows a trialkylphosphite to substitute a halide atom directly attached to an aromatic ring to produce an arylphosphonate dialkylester. Common catalysts for the Modified Arbuzov Reaction include: copperng, Ni(II) salts,180 palladium metal or_ platinum(0) phosphinesm, or Pd(II) or Pt(II) salts. Tav's report, which utilized anhydrous nickel(II) bromide as a catalyst, was deemed the most economical — providing a reasonable yield at reasonable cost. B-Naphthylphosphonic acid. B-Naphthylbromide (5 grams, 24 mole) and anhydrous NiBr2 (Aldrich) (0.31 grams) were added to a 25 ml 3-neck round bottom flask equipped with a distillation head and an addition funnel and held under an argon atmosphere. The reaction pot was heated to 130 °C on an oil bath. Triethylphosphite (5 grams, 30 mole) was placed into an addition funnel and added dropwise to the heated naphthylbromide. The mixture immediately turned purple with the addition of triethylphosphite. The purple color subsided after approximately 2 hours of heating and the yellow color of the NiBrz catalyst returned. The mixture was heated for an additional 2 hours after the purple color disappeared to assure complete reaction. The reaction was then cooled and mixed with 100 ml of 5% NaHCO3(aq) solution. The two-phase mixture was stirred vigorously for 1 hour to remove the Ni2+ salts from the organic phase. The organic phase was then distilled under vacuum where the fraction collected at 155 - 165°C / 0.5 torr yielded approximately 2 ml of diethyl B-naphthylphosphonate. The phosphonate ester was then hydrolyzed in 12 M hydrochloric acid for 16 hours. The 62 hydrolysis mixture was evaporated to complete dryness with a rotary evaporator and the solid material was recrystallized from hot ethanol to produce 0.8 grams of B—naphthylphosphonic acid (M.P. 176°C). b . Grignard Synthesis The free radical mechanism of the Modified Arbuzov reaction lead to some speculation as to the exact substitution of the phosphonate group on the naphthalene ring. The substitution could not easily be determined by NMR because the additional splitting introduced into the spectrum by the phosphorus atom. The singlet in the aromatic region, which is characteristic of B—substituted naphthalenes, was not easily identifiable due to splitting caused by the spin 1,2 phosphorus atom directly attached to the ring and splitting caused by resonance forms of proximally and distally placed ethyl groups of the phosphonate ester. Because of the possibility of rearrangement and the fact that the substitution could not conclusively be determined by NMR, the synthesis of B— naphthylphosphonic acid was repeated utilizing a Grignard reaction to eliminate the chance of rearrangement. A Grignard reaction is not a high yield reaction for the production of phosphonate esters because a second and even a third equivalent of Grignard (RMgBr) reagent may continue to add to the product phosphonate ester, [RP(O)(OR’)2], to produce phosphinate esters, [R2P(O)OR’], and even phosphine oxides [R3P(O)]. Multiple substitutions can be kept to a minimum by slowly adding the Grignard Reagent to a large excess of phosphorus reagent. Although the reaction is of low yield, a Grignard reaction will produce only B-substituted naphthalenes from B- naphthylbrornide and is thus a good method to verify the substitution of the product produced by the Modified Arbuzov Reaction from above. 63 The Grignard reagent, fl-bromomagnesiurrmaphthalene, was prepared in ether by the customary methods from fi-bromonaphthalene (Aldrich) in anhydrous diethyl ether. The naphthalene Grignard was then transferred to an addition funnel where it was added dropwise over a period of 0.5 hour to an excess of diethylchlorophosphate [(EtO)2P(O)Cl] (Aldrich) in 50 ml of ether. The reaction was allowed to stir for one hour after the transfer was complete. The mixture was then gently heated to reflux for an additional 0.5 hour and allowed to cool to room temperature. Ice chips were then added to the reaction pot to quench any excess Grignard and convert the remaining (EtO)2P(O)Cl to the diethyl ester of phosphoric acid. The aqueous phase of the mixture was removed and the organic phase was washed three times with 50 ml of 5% NaHCO3(aq) to remove the phosphoric acid ester. The ether was evaporated and the resulting oil was distilled under vacuum to yield 10% of diethyl ester of B-naphthylphosphonic acid. This product produced the same NMR spectrum, melting point and ion distribution by mass spectrometry as the product from the Modified Arbuzov reaction when B—naphthylbromide was used as the starting material. c. Nucleophilic Aromatic Substitution The nitrogen atom of pyridine acts to deactivate the aromatic ring similar to a nitro group on benzene. The pyridine ring can be additionally deactivated by converting the pyridine ring to a pyridinium oxide or N-alkylpyridinium ion. The pyridinium ion may be easily substituted by nucleophilic attack at the positions ortho- to the nitrogen atom. If the ortho- positions are already substituted or sufficiently hindered, substitution may take place at the para- position. 4-pyridylphosphonic acid is produced by this method. For 4- pyridylphosphonic acid, the nitrogen is quaternized with the bulky triphenylpyridinium 64 ion to simultaneously deactivate the pyridine ring and to sterically block the ortho- positions from substitution. 4—pyridy1phosphonic acid.182 Quaternization of pyridine was achieved by adding pyridine (10 ml) to a stirred solution of triphenylcarbonium tetrafluoroborate (13.8 g) in CHZCIZ (250 ml). The mixture was allowed to stand overnight at 5 °C. The resulting crystals of triphenylmethylpyridinium tetrafluoroborate (~10 grams) were filtered. N ucleophilic aromatic substitution of the pyridine was then carried out by suspending the triphenylmethylpyridinium tetrafluoroborate crystals in cold benzene (150 ml) and adding a solution of 15 ml of diethylphosphite in which 0.80 grams of sodium metal had been previously reacted. The mixture was then heated to reflux for 2 hours, cooled and quenched with 50 ml of water. The organic phase was extracted with 3 M HCl to recover the basic product. A clear oil was obtained after the aqueous HCl was removed by rotary evaporation and that oil was distilled under vacuum, (100-105 °C /0.05 torr) to give clear, viscous diethyl pyridylphosphonate. The ester was then hydrolyzed in 100 ml of 6 M HCl(aq) over a period of 16 hours to yield 4-pyridylphosphonic acid upon evaporation of the aqueous solvent (M.P. 118°C). The product was analyzed by 13C NMR and all resonances matched reported values. E. Synthesis of Quadruply Bonded Dimolybdenum Complexes The syntheses of the quadruply-bonded dimolybdenum compounds were carried out under standard Schlenk-line techniques. The solvents for the synthesis and spectroscopy of all molybdenum-containing compounds were rigorously dried and deoxygenated according to standard methods.183 All reagents were used as received except as noted. All of the dimolybdenum compounds reported here have been 65 previously reported in the literature and thus were analyzed only by UV—visible absorption spectroscopy. 1 . M02(CH3COO)4 Dimolybdenum acetate is the starting material for the synthesis for the dimolybdenum compounds used in this study. Mo(CO)6 (10.0 grams, 37.9 mmole) (Aldrich), 250 ml of o-dichlorobenzene, 30 ml of glacial acetic acid, 10 ml of acetic anhydride and 10 ml of hexane were placed into a 500 ml round—bottom flask. The flask was fitted with a reflux condenser and a gas inlet adapter and purged with argon. The solution was then heated to reflux for 16-24 hours. During the period of reflux, the reaction flask was monitored for the buildup of Mo(CO)6 in the reflux condenser. Mo(CO)6 was removed from the condenser by pouring dry hexane down the condenser as needed.184 Bright yellow crystals of dimolybdenum acetate fall out of the yellowish- green solution over the course of the reaction. The flask was cooled to room temperature and the radiant yellow crystals were isolated by filtration. The product was washed three times with ~10 ml of fresh, anhydrous diethyl ether while still on the aspirated frit and . 185 then dried under vacuum and stored under vacuum. 2 . (NH4)4M02C18'H20 The ammonium salt of dimolybdenum octachloride was prepared by adding 6 grams of M02(CH3COZ)4 to a solution consisting of 4 grams of NH4C1 (76 mole) in 200 ml of 12 M hydrochloric acid. The yellow dimolybdenum acetate is immediately converted to the reddish-purple M02084“ in the hydrochloric acid. The product 66 precipitated as reddish-purple crystals after 1 hour of stirring. The crystals were filtered and washed while still on the aspirated frit with three 25 ml portions of diethyl ether and . 186 dried under vacuum. 3 . MOZCI4[(CH3)2S]4 The ammonium salt of dimolybdenum octachloride (3.00 grams, 4.84 mole) was added to a degassed solution of dimethylsulfide (10 ml) and dry methanol (10 ml). The mixture was stirred overnight and the resulting pale greenish-blue crystals were filtered and washed with methanol on the aspirated glass frit. The product was dried 187 under vacuum and stored under vacuum. 4 . M02Cl4(CH3CN)4 Dimolybdenum tetrachlorotetra(dimethylsulfide) (1.0 grams, 1.72 mole) was placed into a flask and purged with argon. Dry and degassed acetonitrile (10 ml) was then added to the septum-covered flask with a syringe. The mixture was stirred for 16 hours. The product was isolated by vacuum evaporation of the solvent and the displaced dimethylsulfide ligands.188 67 5 . [M02(CH3COO)2(CH3CN)6](BF4)2 and [M02(CH3COO)2(CH3CN)6](PF6)2 Two of the four acetate ligands of dimolybdenum tetraacetate are esterified and thus eliminated by the powerful alkylating reagent, triethyl oxonium tetrafluoroborate. Mild conditions and a minimum of esterifying reagent are used to prevent esterification of all acetate groups. To a stirred suspension of dimolybdenum acetate (0.428 grams, 1.0 male) in 30 ml of CHZCIZ and 1 ml of CH3CN was added (CH3CH2)3O+ BF4' as a 1 M solution in CH2C12 (Aldrich). The yellow crystals of dimolybdenum acetate slowly went into solution as bright red M02(CH3COO)2(CH3CN)42+ ions. Stirring was stopped and the reaction vessel was left overnight upon which time large red crystals precipitated.139 The PF6' salt was made in a similar method as the BF4‘ salt except (CH3CH2)3O+ PF6' (Aldrich) was used as the alkylating reagent prepared as a 1 M solution in CH2C12. Both compounds have similar characteristics and absorption spectra, xmax = 530 nm (111. 530 nm). 6 . [M02(CH3CN)3](BF4)4 and [M02(CH3CN)3](PF6)4 All four of the acetate ligands of M02(CH3COO)4 were esterified and eliminated from the dimolybdenum core by the alkylating reagent, (CH3CH2)3OPF6, under reflux conditions. The vacant coordination sites on the dimolybdenum core are filled by N—coordination of acetonitrile (CH3CN) solvent molecules to give the completely solvated dimolybdenum ion. Dimolybdenum acetate (1.00 gram, 2.34 mole) was added to a . flask containing 30 ml of dry and degassed acetonitrile. To this slurry was added 30 ml of (CH3CH2)3OPF6 (Aldrich) as a 1 M solution in dry and degassed CH2C12. The mixture was refluxed for 10 days to ensure complete removal of all acetate ligands. The 68 product precipitated as royal blue microcrystals throughout the course of the reaction. [M02(CH3CN)8](PF6)4 was recovered by filtration and washed with CH2C12. The product was dried for a short period under vacuum and stored under vacuum. 190 F . Computer-Aided Molecular Design and Visualization The InsightII molecular modeling software package from Biosym was run on a Silicon Graphics Indig02 EX with an upgraded 150 MHz MIPS R4000 central processing unit under the IRIX 5.2 operating system from Silicon Graphics. The InsightII software suite from Biosym was used to visualize layered structures which do not readily form crystals but have been indexed by powder x-ray diffraction to known structures. Additionally, the organization of the organic fragments bound to the layers can be approximated through the use of molecular dynamics calculations available under the Discover module of the Biosym suite. The vanadyl phosphonate materials are reported to have the same metal-oxygen- phosphorus bonding of the mineral newberyite, MgHPO4-3H20.19l Computer models of the vanadyl phosphonates were created by using the fractional atomic coordinates of newberyite192 with the following atom substitutions, changes in fractal atomic coordinates and changes to the unit cell. 1 . Atom Substitutions Newberyite consists of a two-dimensional array of magnesium atoms octahedrally coordinated by six oxygen atoms from three water molecules and three different phosphate groups. Three oxygen atoms of the hydrogenphosphate groups coordinate to 69 three different magnesium atoms while the fourth oxygen atom is directed towards adjacent layers and holds an acidic proton. The atom substitutions consist of replacing the two trans- water molecules with one oxygen-coordinated alcohol molecule and one doubly-bonded oxygen atom to make a vanadyl ion, (V=O)2+. Finally, the phosphate is turned into a phosphonate by eliminating the acidic hydrogen atom and replacing the oxygen atom which held the proton with a carbon atom to form the P-C bond of a phosphonate. This metal-oxygen-phosphorus framework, which lies on the a-c plane, is now the basis for all vanadyl phosphonate structures with various sizes of alcohol molecules and organic phosphonate pendants and, therefore, various interlayer distances. 2 . Adjustments to Fractal Atomic Coordinates and the Unit Cell The materials to be modeled were first synthesized and powder x-ray diffraction patterns were made to determine the interlayer distances. The experimentally measured d- space was then used to determine the unique b—cell dimension for the lattice. The array of metal atoms extend in the a- and c-directions of the cell and all have a y-fractal coordinate value of 0.25. Therefore, there are two complete layers per unit cell. A model of a compound with a larger interlayer space cannot simply be made by changing the length of the b-axis because this would only stretch all of the interatom distances. To prevent any distortions of the metal-oxygen-phosphorus layer, the y-coordinate values of all atoms must be operated on by the function in Equation 3 to get a new layer with identical inter- atom distances and sufficient room for larger alcohol molecules and phosphonate pendant fragments. f(y-axis) = [(y-axis - 0.25) / (d-spacenew / d—space )] + 0.25 (3) CUITCI'II 70 If d-spacene is larger than the current d-space, this function reduced the W y-distance between atoms on a basis of fractional atomic coordinates so that when the larger b-cell dimension is used, the new layer will have identical inter-atom distances in real space. 3 . Molecular Dynamics Energy Minimizations Molecular dynamics energy minimizations were carried out to predict possible geometries of the naphthalene units within the interlayer region of vanadyl phosphonate. The geometries of the naphthalenes and inter—naphthalene distances are important in determining the possibility of achieving rt-stacking arrangements of the naphthalenes so that excimers may form when vanadyl phosphonates are used as templates. The structures formed from the newberyite structure with appropriate atom substitutions and adjustments to the fractal atom coordinates were made for the structures examined. The interlayer distance in each case was obtained from the powder x-ray diffraction data for each compound. A crystal lattice was then constructed using the Solids Builder module from Biosym from the modified set of fractal atomic coordinates. The lattice used for all measurements consisted of two complete inorganic layers with an area of three by three unit cells in the a- and c-directions. Atoms generated by this procedure, which did not form bonds to the bulk of the layer, were deleted. The crystal lattice was then converted to a molecule with the Solids Adjustment module by removing all symmetry from the structure. The remaining molecule consisting of only the V—O—P inorganic layer was then appended with appropriate organic fragments with the Builder module. Because the inorganic atoms of the polymeric two-dimensional inorganic plane have inappropriate coordination numbers at the perimeter of the structure, the forcefields ‘7'1 supplied with the Discover module could not accommodate a minimization that included the inorganic layer. Therefore, all atoms of the inorganic layer were deleted from the structure to leave only appropriately spaced organic fragments. The atom that bound the organic fragment was adjusted for hybridization and fixed into position with the Discover module. The ESFF forcefield was then used to minimize the molecules as if they were bound to the interior surfaces of a layered template. The minimization was allowed to proceed until the derivative of the energy change was less than 0.01 kcal. Several variations of the initial geometries of naphthalene units were produced to determine if consistent global minima were achieved. CHAPTER III CONTROL OF NAPHTHALENE EXCIMER FORMATION IN LAYERED VANADYL PHOSPHONATES A . Background Layered metal phosphonates (LMPRs) offer an ideal method to produce ordered arrays of molecules similar to membrane-mimetic structures. Many applications have been developed to take advantage of the molecular order induced by the formation of vesicles,193 Langmuir-Blodgett (LB) films and self-assembled monolayers (SAMs)194 but a surprisingly small amount of work has been done to utilize the molecular order created in LMPR materials.‘95 To date, most studies concerning LMPR materials have been restricted to their synthesis and basic intercalation properties. Not withstanding, LMPRs are self supporting and inherently more stable than LB films and SAMs. When these properties are combined with the intercalation of guest molecules in LMPRs, an ordered array of molecules can be assembled within the structure. The object of this Chapter is to mechanically separate the layers by intercalation while controlling the guest- 72 73 guest interlayer interactions on the order of a bond length. Angstrom-scale control of the interlayer dimensions available in LMPR materials has obvious potential for regiospecific synthesis and in the development of molecular based sensors and devices based on the interlayer environmental changes induced by intercalation. 1 . Intercalation of Guests into LMPR Materials The type of interlayer organic species and the particular interactions driving the intercalation determines the impact that the mechanical separation of the layers plays toward altering chemical or physical properties of the interlayer organic species. The intercalation of guest molecules into layered metal phosphonates can occur by two different pathways. The first pathway is driven by the interaction of a guest molecule with a reactive group at the terminal end of the organic phosphonate pendant. LMPR Intercalation driven by interaction at the tail-end of the phosphonate pendant is shown schematically in Figure 10a. This type of intercalation leads to the formation of organic bilayer arrangements like the one observed for the zirconium phosphonate structure presented in Figure 6‘96 Here, each surface of each layer is covered with a close- packing array of organic pendants. In the case of zirconium phosphonate, the phosphonate pendants are in a hexagonal pattern and separated by only 5.2 A. The bilayer of close-packing organic pendants prevents intercalated species from accessing any other part of the layered structure. The second type of intercalation is driven by an interaction of the guest molecule with a site on the surface of the inorganic layer (Figure 10b). For the guest molecule to establish an interaction with the layer, the phosphonate pendants must be dispersed widely enough to allow the guest molecules to access the inorganic framework. The first reported LMPRs to allow guest molecules to directly access the inorganic layer are vanadyl phosphonates with the so—called dihydrate 74 Figure 10. (a) Intercalation driven by interaction of the guest (swiggle) and the phosphonate pendant (oval) in an organic bilayer structure. (b) Intercalation driven by interaction of the guest (swiggle) and an active site on the inorganic layer. 76 structure. 197 The phosphonate pendants of vanadyl phosphonate dihydrates are separated from four neighboring phosphonates by 5 A along one axis and 10 A along the other axis198 providing almost double the surface area to each phosphonate pendant when compared to the zirconium phosphonate structure (50 A2 vs. 27.5 A2).199 a . Intercalations Driven by Guest-Pendant Interactions To achieve an intercalation based on interactions of the guest molecule with the phosphonate pendant, an interaction between the guest and pendant must be strong enough to force the layers apart. This can occur for pendants possessing polar functionality at their tail—ends. ZrPR materials, where R is functionalized with an (lo-carboxylic or sulfonic acid group, have been shown to undergo acid-base reactivity with amines200 and ion exchange reactions with a variety of ions201 to produce intercalated materials With expanded interlayer distances. Additionally, Thompson has shown that functional groups on the end of phosphonate pendants can undergo organic reaction chemistry by converting a carboxylic acid pendant group to an acid chloride and then to esters or amides within the layer.202 When the intercalation is driven by an interaction of the guest molecule with the terminal end of the phosphonate pendant, the only interlayer changes to result are a disruption of the original tail-to-tail association of the pendants from adjacent layers. This form of interdigitation generally increases the overall interlayer distance. Notice that in Figure 10a, the molecules surrounding the pendant before and after intercalation are essentially unchanged. Because of this, the physical properties displayed by the materials do not change significantly and cannot easily be used in the design of sensors or devices. 77 b. Intercalation Driven by Guest-Layer Interactions Interaction of the guest species with the surface of the inorganic layer can produce significant changes to the environment immediately surrounding a pendant. When the phosphonate pendants are placed far enough apart to allow access to the layer, the organic phosphonates are also able to access the adjacent layer to form an interdigitated arrangement of the organic pendants. Figure 10b clearly shows the drastic changes of the environment at the lateral faces of the phosphonate pendant resulting from the intercalation of a guest molecule. Before intercalation, the pendants are deeply interdigitated; each pendant is surrounded by pendants from the adjacent layer. After intercalation, pendants from the adjacent layer are pushed away and each organic phosphonate becomes surrounded by the newly intercalated guest molecules. The ability to control the local environment along the entire length of the phosphonate pendants facilitates the control of physical properties displayed by the pendants. Similarly, the physical properties of the intercalated molecule can also be tuned by utilizing LMPRs with differing sized or functionalized organic groups to precisely adjust the positions of the intercalated guest molecules. Intercalation reactions driven by guest-layer interactions have been shown to occur in several classes of layered metal phosphonates including vanadium (vanadyl),203 copper204 and zinc.205 The active site on the layer of each of these materials is created by the thermal removal of a solvent molecule of crystallization to produce a coordinatively unsaturated site on the metal atom. Polar guest molecules have then been shown to intercalate into these materials and occupy the empty coordination site on the metal of the layer. In the case of vanadium, primary alcohols serve as the guest molecules and alkyl amines preferentially intercalate into copper and zinc phosphonates. 78 2 . Structural Aspects of Vanadyl Phosphonates Three discrete structural types of vanadyl phosphonates have been identified to this point with possibilities of several othersmé. The two vanadyl phosphonates polymorphs known as the monohydrate207 and methylphosphonate208 form organic bilayers similar to the well-known zirconium phosphonate. The third structural type known as the dihydrate is the polymorph which does form the desired organic monolayer arrangement of phosphonate pendants, which is necessary to allow guest molecules to access the inorganic layer. The inorganic plane of vanadyl phosphonate dihydrate (Figure 11) consists of octahedrally coordinated vanadium atoms surrounded by three phosphonate oxygen atoms from three different phosphonate groups. Each phosphonate oxygen is attached to an equatorial site on the vanadium. The fourth equatorial position is occupied by a water of crystallization which is fairly strongly coordinated. The two axial sites are occupied by a vanadyl oxygen atom and a second water of hydration which is weakly bound to the vanadium. It was pointed out in Chapter I that the dihydrate material does not usually contain two water molecules but most commonly has one water molecule and one alcohol molecule coordinated to the vanadium. The alcohol molecule occurs as a solvent molecule of crystallization and is weakly coordinated to the site trans to the vanadyl oxygen atom. The weakly coordinated alcohol molecule can be removed thermally without any bonding or structural rearrangements of the inorganic plane. The only structural change associated with alcohol removal is manifested in a collapse of the interlayer distance. The vacated coordination site on the metal resulting from the removal of an alcohol can be re-occupied by the intercalation of water or a primary alcohol molecule. As new molecules are intercalated to fill the vacant coordination site on the vanadium, the interlayer distance readjusts to accommodate the new guest molecules. 79 Figure 11. a-c inorganic plane of vanadyl phosphonate dihydrate. 80 Figure 11. 81 In the synthesis of vanadyl phosphonate dihydrate, no cross contamination from the other polymorphic phases occurs. This is apparently due to the fact that the other polymorphic phases are synthesized by hydrothermal methods whereas the dihydrate is made at standard pressures with only mild heating. It has also been shown that thermal treatment of these materials does not lead to interconversion between the three structural types. The bonding motif of the inorganic plane of the methylphosphonate contains four- membered V-O-V—O rings and is significantly different than the other two structures. An initial examination of the monohydrate and dihydrate may appear as though the two structures are interconvertable. This is not the case however because a closer examination of the layers shows that no two metal atoms of the dihydrate form bond to the same two phosphonates and only extended 12-member (-V-O-P-O-V-O-)3 rings are present. In the monohydrate form, 8 member rings form because two vanadium atoms share two phosphonate groups. The exact crystal structure of the so-called dihydrate has not been determined by single crystal methods but the unit cell has been indexed209 and the connectivity of the metal, oxygen and phosphorus atoms of the inorganic layer have been determined to be similar to the connectivity of the mineral newberyite (Mg11P04-3H20),2‘° except that one water of the newberyite is replaced by a vanadyl oxygen atom and the protonated oxygen on the phosphorus is replaced by the organic pendant of the phosphonate. The vanadyl phosphonate dihydrate structure is shown in Figure 11 and has an orthorhombic cell with in-plane cell constants of a = 10.03 and c = 9.77 A. The interlayer distance is variable and dependent on the size of the phosphonate pendant and intercalated alcohol molecules. 82 3 . Molecular Positioning within Vanadyl Phosphonate Layers: Control of Aromatic Excimer Formation There are two structural features of vanadyl phosphonates that allow these frameworks to be used to control the lateral overlap of molecules within the layer. First, there are two different types of organic fragments available within layered vanadyl phosphonate dihydrate. Second, the organic fragments within the layers form an interdigitated configuration. From the formula of vanadyl phosphonate dihydrates, V(O)O3PR°HZO°R’OH, it can be seen that a probe molecule could be attached to one organic site, R or R’, and the interlayer distance could be controlled by varying the length of the other organic site, R’ or R, respectively. The interdigitated nature of the phosphonate pendants dictates that a change to the interlayer distance will result in a change in the lateral overlap of adjacent probe molecules. Thus the local environment of the probe molecule is dependent on the interlayer separation, which can be controlled by the size of the complementary organic functionality. The ability to tune the lateral overlap between molecules lends itself perfectly to the control of the optical properties of aromatic molecules. This is because the optical properties displayed by aromatic molecules can change significantly when the conjugated rings are placed into co-facial rt-stacking geometries compared to non-n-stacking geometries. In vanadyl phosphonates the n—stacking geometries can be attained when the probe molecules are precisely aligned in an interdigitated geometry. The optical properties can then be altered by replacing the complementary organic group with a longer or shorter molecule to disrupt the n-stacking geometry of the interlayer aromatic molecules. Fluorescence studies involving probes which form excimers is a common tool211 to measure structural parameters and homogeneity of monomer units in polymerszn, Langmuir-Blodgett films and self-assembled monolayers213 as well as the distributions 83 and locations of aromatic guest molecules in guest-host complexes such as cyclodextr’ins,214 intercalated DNA215 and zeolites.216 We will utilize excimer formation here to examine the suitability of layered vanadyl phosphonates as supports to position naphthalene units so that the fluorescence properties of these aromatic molecules can be controlled. An excimer is an excited state dimer that forms as a result of a bonding interaction between an excited state and a ground state molecule. Figure 12 presents an orbital energy diagram, which explains both the electronic and bonding factors that stabilize the excited dimer as well as the optical absorption and emission characteristics of an excimer. In Figure 12a, the up arrow represents the it -) 1t* excitation and the down arrow shows the corresponding 1t* —> 1: emission of an isolated naphthalene molecule. However, when two naphthalene molecules are able to form an overlapping face-to-face configuration, an excited state naphthalene can form a bonding interaction with a ground state naphthalene. The half-filled 1t and 1t* orbitals form the excited state naphthalene can form plus and minus linear combinations with the full 1: and empty 1t* orbitals of the ground state naphthalene to form the four new orbitals of an excimer (Figure 12b). The creation of new excimer based orbitals results in one stabilized (Whither orbital holding two electrons and one destabilized (rt1t)"‘dimer orbital holding one electron. The interactions of the two 1t* orbitals is similar and produces a stabilized (1t"‘1t"‘)di,,,,er orbital with one electron and a destabilized (1t""il:"‘)*dimer orbital which is empty. The net result is that three electrons become stabilized and only one electron becomes destabilized compared to the two isolated molecules. Thus, the two naphthalene molecules are held together by a net bond order of one to create the excited state dimer. However, an excimer is only stable for the lifetime of the excited state. Upon decay to the ground state, the dimer falls apart because the bond order between two ground state naphthalene molecules is zero. 84 Figure 12. Orbital description of the electronic absorption and emission of: (a) isolated naphthalene and (b) naphthalene excimer. Energy 85 Figure 12. 86 From Figure 12b, it can be seen that the excimer clearly forms only after the absorption of a photon to produce the excited state. Therefore, excimers cannot be identified by absorption experiments. Excimers can only be identified by a shift of the dimer's emission to a lower energy. The emission is red-shifted because the molecular 1t* orbital, in which the excited electron resided, was stabilized at a lower energy as the character of the orbital became excimer based. Thus excimers can be identified by the lower energy (red-shifted) emission of the excimer compared to the isolated molecule. There is typically no fine structure in the emission of an excimer because the potential energy surface of the excimer is soft. As the excited state electron decays out of the stabilized excimer-based 1t*-orbital, the bond order between the two naphthalene molecules goes to zero. As the bond order goes to zero, a multitude of possible electronic excited state configurations are created with energy extremes ranging from that of the excimer to that of the isolated molecule. The lack of a well-defined minimum along the intermolecular coordinate eliminates the possibility of fine structure. Excimers do not form in the crystalline states of many aromatic compounds because crystal packing geometries precludes dimer formation. Face-to-face configurations of aromatic molecules in crystals can by prevented by the formation of a herring-bone arrangement of molecules whereby the two faces of each ring are directed towards the edges of two neighboring aromatics. Also, the distance between the aromatic rings in some crystals may be outside the 3.0 to 3.7 A range necessary to form an excimer. 87 B . Results 1 . Feasibility of Concept a. Molecular Modeling of Naphthalene Units In Vanadyl Phosphonate Layers. The concept of adjusting the interlayer distance of vanadyl phosphonates to control the environment about a probe molecule has already been discussed. However, the distances between the interdigitated aromatic rings must examined to demonstrate that rt-stacking geometries can be attained and that the distances between the aromatic rings is in the domain known to produce excimers. The distances between the attachment sites (vanadium for naphthalene alcohols and phosphorus for naphthylphosphonates) can be determined by generating the lattice of vanadyl phosphonate dihydrate from the fractional atomic coordinates of newberyite” 7 and using the cell parameters indexed for vanadyl phosphonate. Because the vanadium . and phosphorus atoms of VPR layers are equally distributed in the inorganic plane, the phosphorus-phosphorus distances will be used for the following discussion and the same dimensions apply to the vanadium sites. The phosphorus atoms direct their organic pendants either above or below the inorganic plane. The distribution of upwardly and downwardly directed pendants is not in an alternating checkerboard-like pattern but in a "com-row" arrangement leading to two different phosphorus-phosphorus distances for phosphorus atoms which direct their pendant to the same side of the layer. The first P—P distance is between phosphorus atoms within the same “row” which is 5.1 A. The second P-P distance is for 88 phosphorus atoms in adjacent rows which is 10.0 A. Because a jump is made across a row for the 10.0 A measurement, it must be recognized that a pendant from a neighboring layer will "interdigitate" into the inter-row area and therefore the P—P distance measured between rows should be divided in half (5.0 A). Attachment sites spaced at 5.0 and 5.1 A suggest that excimer formation within VPRs is not possible because both values are outside the optimal excimer forming range of 3.0 to 3.7 A. However, Ulman218 has shown that the distance between attachment sites at a surface does not reflect the actual separation of the tail portions of amphiphilic molecules appended to a surface as in LB films and SAMs. Tinting an amphiphile's tail to some angle less than 90° moves the pendants closer together (Figure 13a). The distance between pendants (x’) can be determined by the formula x’= x cosO where x is the distance between attachment sites on the surface and 6 is the tilt from perpendicular. It has already been shown that the phosphonate pendants of VPRs are not perpendicular to the layer but tilted 54° from the layer (6 = 36°).219 At 9 = 36°, the theoretical pendant-pendant distance in VPRs is 4.0 A, only 0.3 A from the excimer forming range. Ulman also pointed out that if attachment sites at a surface are separated too far, the probe functionalized tails will come into close contact near the base of adjacent pendants after tilting and probe-probe interactions will not occur. However, if the pendants are from adjacent layers, the problem of keeping active sites together after tilting can be overcome (Figure 13b) because the layers can alter the naphthalene-naphthalene distance by making adjustments to the interlayer distance or glide across the surface of the neighboring layers similar to the way the hook and loop layers of VelcroTM become firmly interlaced. Molecular modeling was implemented to verify that the rings within the layers can come into close enough contact to form excimers and if ring-ring overlapping geometries do form. The inorganic layers for these models were based on the structure of newberyite and necessary atom substitutions and adjustments to the unit cell parameters 89 Figure 13. The distance organic fragments attached to the surface (x') is dependant on the distance between attachment sites on the surface (x) and the tilt angle (0) of the organic from perpendicular to the layer by x' = x cosO. 91 were made to create the inorganic layer of VPR as described in Chapter II.F. Naphthalene rings were then appended to the layer at either the vanadium atom or at the phosphorus and molecular dynamics (ESFF forcefield: Biosym, Discover3 software) were used to bring the tethered naphthalene units to a minimum energy. The view down the interlayer b-axis in Figure 14 shows the distribution and orientation of aromatic rings produced after the minimization of an interdigitated array of naphthalenes from a VPR template. After minimization, the naphthalene rings toward the middle of the a-c plane (away from boundary effects) developed face-to—face distances in the range of 3.3 to 3.6 A and the angles between the planar aromatic molecules are well below 20°, the maximum angle at which excimers may form. b . Optical Properties of Vanadyl Ion and Naphthalene Along with the structural requirements necessary to promote the formation of excimers, the optical properties of the material must also be accommodating to the design of the system. In other words, the V-O—P inorganic support cannot interfere with the absorption or the emission the light from the naphthalene chromophore. In Figure 15 the electronic absorption spectra of both the vanadyl ion and naphthalene are presented in terms of the molar extinction coefficient (8) as a function of wavelength. This data indicates that the absorption of excitation light by naphthalene is 30 to 3000 times greater than that absorbed by the vanadyl ion in the regions from 254 to 313 nm. Also, the vanadyl ion has an extinction coefficient less than 15 for all regions of the emission of naphthalene and nearly zero in the region of the naphthalene excimer emission near 400 nm. The blue color of the vanadyl phosphonate layer is the result of a d-d transition of the d1 V4+ metal center of the V=02+ vanadyl ion. The absorption maximum of the 92 Figure 14. The a-c plane of the organic interlayer region filled with naphthalene units after minimization by a molecular dynamics calculation to determine the orientation and distrubution of naphthalene molecules held within vanadyl phosphonate frameworks. Figure 14. 94 Figure 15. Molar extinction coefficients, a, of (a) naphthalene and (b) vanadyl ion as a function of wavelength. 95 _M_L_._e_._r_.L._ 0 o o 0 0 O 0 0 O O 0 0 O 4 3 2 1 E06580 cozoczxm 5.0.2 350 300 250 k/nm Figure 15. 96 vanadyl ion is in the near infrared region (~750 nm) but the structure of this broad peak trails into the visible region which leads to the blue color of this ion. 2 . Synthesis of Vanadyl Phosphonate Layers In all cases, the same general synthetic scheme is used to produce the vanadyl phosphonate layers whereby V205 solid is finely ground and added to an alcohol solution of the appropriate phosphonic acid, (H203PR). A catalytic amount of aqueous l M HCl is then added to the mixture and the two-phase mixture is heated with stirring for one to three days. As the solid yellow—brown V205 is consumed in the reaction, blue vanadyl phosphonate solid fills the reaction flask and is recovered by filtration. Synthetic details are available in Chapter H. The non-naphthalene-containing series of compounds V(O)O3PR°H200- C6H5CH20H were previously reported by Jacobson and Johnson220 and prepared to serve as a reference for comparison of all other naphthalene containing materials prepared in this chapter. The single, narrow, and strong low angle peak in the powder x-ray diffraction patterns (Figure 16) produced by each of these materials indicate that they consist of a single phase and are well ordered layers. A plot of the interlayer distance as a function of the number of alkyl carbon atoms of the pendant results in a straight line with a slope of 1.01 A/CH2 and an intercept of 11.7 A (Figure 17). The slope of 1.01 A/CH2 assures an interdigitate motif for the interlayer organic species. An interdigitated motif is verified for each new series of compounds because our group has discovered some inconsistencies in the structures of vanadyl phosphonates which lead to a non- interdigitated organic bilayer arrangement of the organic functionalities.221 The bilayer 97 Figure 16. Powder x-ray diffraction patterns for compounds of the formula V(O)O3PR-H20-C6H5CHZOH for R=(a) propyl, (b) butyl, (c) pentyl, (d) hexyl, (e) heptyl. Counts 98 5 10 15 20 26 Degrees Figure 16. 99 Figure 17. d-Space of V(O)O3PR-H200C6H5CHZOH plotted as a function of the number of carbon atoms in R. 100 30 25 20 15 d-Space I A 10 y =1.01x + 11.72 R: 0.996 TIIIIIIUII ITIIIIIIIIII'IIIU I l l I I l l I I 12 3 4 5 6 7 8 910 Carbons Atoms on R for V(0)03PR-H,O-C6H5C H20 H 0 Figure 17. 101 motif is identified by slopes for this type of plot in the range of 1.5 to 2.5 A/CHZ. The bilayer materials tend to form when excess water or acid catalyst is added to the reaction mixture during synthesis or if the reaction mixture is overheated at any time. The benzyl alcohol intercalated VPRs made in this section are the starting materials for the synthesis of the naphthalene(alcohol)-intercalated materials in the following sections. 3 . Naphthalene-Modified Layered Vanadyl Phosphonates Four series of compounds will be developed, synthesized and analyzed for their ability to control the position and overlap of the naphthalene rings in the interlayer region of vanadyl phosphonates. In the first two series of compounds, the naphthalene is appended to the intercalated alcohol molecule. The interlayer distance is controlled by utilizing alkylphosphonate pendants of varying lengths. In the second two series of compounds, the naphthalene unit is covalently tethered to the phosphorus atom of the layer to produce (naphthalene)phosphonates. In these compounds, the interlayer distance is adjusted by the intercalation of alkyl alcohols of various lengths. a. Studies 0f V(O)O3PR'H20°B-C10H7CH20H The schematic of the method by which V(O)O3PR0HzO-B-C10H7CHZOH compounds will control the overlap of naphthalene units is illustrated in Figure 18. Depending on the size of the alkylphosphonate spacer, the layers will be pried apart or allowed to collapse together. In theory, the naphthalene rings should be in alignment for 102 Figure 18. Schematic of how the environment about the naphthalene units in V(O)O3PR°H200[3-C10H7CH20H can be changed when the length of R varied. 103 -::§¥/7$AP;7${_ - L H% A? H% PhoLsotmoerlale ‘% \‘E ’fiifi-EEEH R \.:: ° fig fix ‘73’ \.::' l / Figure 18. 104 very short phosphonic acid pendants and form excimers that could be seen by the red- shifted emission. As the length of the pendant is increased, the layers and naphthalene rings should pull apart to prevent the aromatic rings from overlapping and lead to only monomeric emission. This series of compounds, V(O)O3PR°HzO-B-C10H7CHZOH (R=alkyl), a.k.a. VPR'NpMeOH, has been made by both methods outlined in Chapter II.B.2. The first method is a direct synthesis in a solvent of 2—naphthalenemethanol (NpMeOH). The second method utilizes the benzyl alcohol intercalated materials from the previous chapter and consists of an alcohol exchange process. Both methods produced the desired NpMeOH intercalated product. Because a small peak, which can be attributed to NpMeOH, always appears to some extent in the powder x-ray diffraction patterns of the products made in NpMeOH solvent (M.P. 80 °C), the alcohol exchange method was used to prepare all products for which photochemical data were to be collected. The powder x-ray diffraction data for the products of NpMeOH intercalated VPR R = alkyl are shown in Figure 19. Like the benzyl alcohol precursor material, a series of well ordered one-phase materials is observed from this data. The materials prove to maintain the interdigitated interlayer after the exchange of benzyl alcohol for NpMeOH by the d-space versus number of alkyl carbons plot in Figure 20: slope = 1.01; intercept = 13.86. The hashed line in Figure 20 is the data for the benzyl alcohol intercalated starting materials from Figure 17. The electronic spectra of all compounds in this series were collected by diffuse reflectance and clearly show well defined absorptions that are characteristic of naphthalene at ~220 nm (‘13,, band) and ~280 nm (’1.a band) (Figure 21). A definite red-shift of the 1La band is noted for this series. For R = methyl, Max = 276 nm and as the layers are separated by longer phosphonate pendants, km, shifts up to 288 nm for the butyl derivative and remains in the region of 285 to 288 nm for all higher 105 Figure 19. Powder x-ray diffraction patterns for compounds of the formula V(O)O3PR°HzO°B-C 10H7CHZOH for R: (a) methyl, (b) ethyl, (c) butyl, (d) pentyl, (e) hexyl, (f) heptyl, (g) octyl, (h) decyl. Counts 106 Figure 19. _JL A A A .. J .... I a. _JL 1. i .. _JL J A .. J A A .1. _JL 4 A c. A A b. A A JL 8. 1 . l 1 i . 1 . . . 1 . 5 1O 15 20 Degrees 29 107 Figure 20. d-Space of V(O)O3PR-HZO-C10H7CH20H plotted as a function of the number of carbon atoms in R. 108 30 25 y=1.01x+13.86 R: 0.998 20 IIIIIIUITIIIIII d-Space l A 15 , ..D. E] [O 10E t 5.” o. 1 I I 1 1 l 1 l I O 1 2 3 4 5 6 7 8 9 Carbon Atoms in F1 for V(0)03PR-H,OoNpMeOH Figure 20. 10 109 Figure 21. UV-visible diffuse reflectance spectra of naphthalene containing layers with the formula V(O)O3PR-HZO¢B-C10H7CHZOH where R=(a) methyl, (b) ethyl, (c) butyl. 110 3560.3: @5320:— Iv T 352034 @5322: 300 350 400 450 500 A/nm 250 200 Figure 21. 111 alkyl-spaced analogs. This effect has been seen in other ordered assemblies of naphthalene such as Langmuir-Blodgett films222 and self-assembled monolayers,223 and has been attributed to changes in the dielectric constant of the environment immediately surrounding the naphthalene fragments. Matsuki compared the absorption spectrum of naphthalene in hydrocarbon solvent (low dielectric environment) to the red-shifted absorption spectrum of naphthalene assembled in LB films. The LB films are thought to have a higher dielectric constant immediately surrounding the naphthalenes because the structure of the LB film causes each naphthalene to be surrounded by other aromatics which have a higher dielectric constant than the aliphatic hydrocarbon solvents to which the spectra were compared. For the present case, the methyl derivative may in fact allow naphthalene groups from adjacent layers to interdigitate too deeply and prevent the alignment of the naphthalene units. “Over-interdigitation" of the naphthalenes may be the reason why the absorption spectrum indicates that the micro-environment about the naphthalenes have a lower dielectric constant about them than the naphthalenes of the ethyl and butyl spaced derivatives. The red-shift of the 1La band may also be due to the formation of ground state naphthalene dimer complexes.224 Dimer complexes are described as the forced ground state overlap of orbitals from two different molecules as a result of geometric constraints. Unlike excimers which cannot be seen by absorption, dimer complexes display a red-shifted absorption (as well as emission). This dimer complex model would also indicate that the methyl derivative does not have n—n: ring overlap of aromatic rings whereas the red-shifted ethyl and butyl derivatives do have forced ground state lt—lt ring overlap. The shoulder on the low-energy side of the diffuse reflectance spectrum of the ethyl derivative in Figure 21 may be the result of an especially strong forced overlap of ground state orbitals. The fluorescence emission data collected for these solid state samples is presented in Figure 22. Clearly two types of emissions are present for these compounds. The 112 Figure 22. Solid state fluorescence emission of V(O)O3PR-HZO°B- C10H7CH20H for R=(a) methyl, (b) ethyl, (c) butyl. 113 3x 3.22:. :o_mm_Em ’ 350 450 500 550 600 400 A/nm Figure 22. 114 methyl derivative displays a strong and highly structured spectrum at 378 run while the ethyl and butyl derivatives display excimer like emission with km, = 410 nm. This data complements the electronic absorption data inasmuch as excimers do not form in the methyl-spaced derivative, which is assumed to be interdigitated too far and does not form p-stacking structures. Additionally, the ethyl- and butyl-spaced derivatives do produce excimers as seen by the fluorescence emission data and this confirms the results of the electronic absorption data which concluded that naphthalene units are in close contact with other naphthalenes by both the dielectric model and the ground state dimer complex model. As mentioned earlier, the initial design of this system predicted that short alkyl spacers would place the naphthalene rings in an overlapped configuration and excimers would form. And, as the length of the alkylphosphonate spacer was increased, the inorganic layers would be placed further apart which would prevent the naphthalene rings from aligning and prohibit the formation of excimers. These predictions have obviously has not proven to be true. Surprisingly, the methyl-spaced material does not form excimers. By coupling the results of the absorption and emission data, one could conclude that the -OI-I—CH2- tether, which links the naphthalene(alcohol) to the inorganic layer is long enough and the methylphosphonate spacer is short enough so as to allow the naphthalene rings to interdigitate too deeply and prevent the formation of excimers. The ethyl-spaced material did produce excimers and may have even produced ground state dimer complexes. The overlap seen in the ethyl-spaced materials could be explained by taking into account the fact that the two-atom ethyl spacer which pushes the naphthalene away from the layer is equal to the two-atom oxygen-carbon tether holding the naphthalene to the layer. It was also expected that the formation of excimers would be disrupted as the layers were forced apart by longer and longer alkylphosphonate pendants. This is not the case however. Excimers continue to form for all alkyl 115 phosphonates longer than ethyl as well. It is obvious that the excimers cannot be a result of the overlap of naphthalenes attached to adjacent layers for alkyl spacers as long as decyl. Therefore, at some point the formation of excimers changes from the overlap of naphthalenes bound to adjacent layers to the overlap of naphthalene rings bound to the same layer. The structure of the vanadyl phosphonate does allow for intralayer naphthalene overlap to occur because the naphthalene rings are separated by only 5.11 A within the so-called rows of vanadium atoms (the site at which the naphthalenemethanols attach to the layer).225 Figure 23 shows how the free rotation about the V—O bond of the alcohol coordination and the CHz-Np bond in the linkage holding the naphthalene to the layer can place the naphthalene rings into excimer forming geometries based purely on bond angle and bond length arguments using standard values for C-0 and C-C bond distances226 and 109° and 120° for the bond angles of sp3 and sp2 hybridized atoms respectively. To prevent the overlap of naphthalene rings that are bound to the same layer, the linkage of the naphthalene to the layer must be less flexible. If the naphthalene is intercalated as an alcohol there is no way to prevent rotation about the vanadium oxygen bond which coordinates the alcohol to the layer. b. Studies of V(O)O3PR-Hzo-B-C10H7CH2CH20H In the previous section, 2-naphthalenemethanol was intercalated into various vanadyl alkylphosphonates. The structure of the interlayer region, particularly the relative positions of the naphthalene rings, was elucidated by x-ray diffraction, electronic absorption and fluorescence emission data. Although the system did not work exactly as designed, there was a “switching” effect for the formation of excimers when the length of 116 Figure 23. Rotations about the V-O bond which coordinates the C10H7CHZOH alcohol to the vanadium can move rings attached to the same layer close enough to produce p-stacking arrangements and form excimers. H —\'/ I o a HO===>% Rotation u—sA—w 117 Bond H‘—< 3.7A Figure 23. 118 the short methylphosphonate pendant (no excimer) was lengthened to the ethylphosphonate pendant (excimer). The V(O)O3PR°H200[3-C10H7CH2CHZOH series of compounds should delay the point at which the formation of excimer starts. In the previous case a —O—CH2 linkage held the naphthalene to the layer and the switch between no excimer formation and excimer formation occurred when the alkylphosphonate spacer was changed from methyl to ethyl. In the current series, the length of the tether holding the naphthalene to the layer is increased by one methylene to —O—CH2-CH2— in an attempt to alter the point at which the no—excimer to excimer transition occurs as the length of the alkylphosphonate pendant is increased. The attempted synthesis of this series of compound were made by the same method as in the previous section with 2-(2-naphthalene)ethanol serving as the intercalated alcohol in place of naphthalenemethanol. The results of the powder x-ray diffraction data for this series of compounds quickly indicated the additional methylene group between the naphthalene and hydroxyl fragments resulted in a major structural change in the interlayer region. The powder xrd patterns indicate the layers are definitely not one-phase and in some cases were probably not interdigitated. Also, when the alkyl phosphonate pendant was over five carbons long, a significant amount of alcohol free material would remain. Pure, one—phase 2-(2-naphthalene)ethanol intercalated VPR could not be produced. The orientation of the naphthalene group imposed by the number of carbon atoms in the chain which holds the naphthalene to the layer has been reported in VPR materials when the naphthalene is part of the phosphonate.227 From this series of compounds, it is apparent that the number of carbon atoms linking a naphthalene unit to the vanadium has a similar effect. The odd number of heavy atoms in the —O-CH2-CH2- chain, which links the naphthalene to the layer, can lead to a large number of rotational isomers. These isomers can produce geometries with vastly different demands on the interlayer space 119 which must accommodate them. The geometric effects of rotational isomerism can clearly be seen in Figure 24a which shows how rotation about the bond between the naphthalene and the first CH2 can change the cross-sectional area (x —-) x’) and interlayer distance (y —> y’) required by 2-(2-naphthalene)ethanol. Also notice in Figure 24a that the intercalated alcohol can cut into the space needed by the phosphonate pendant. For comparison, the same Np-CH2 bond is rotated on a naphthalenemethanol molecule bound within VPR in Figure 24b. Rotation about the Np-CHz bond of naphthalenemethanol produces no change in the cross-sectional area or the interlayer distance occupied by the intercalated molecule. The single phase materials produced by naphthalenemethanol intercalation into VPR can be seen in Figure 19. Because of the inconsistent structural results as determined by powder x-ray diffraction, no Optical studies of these compounds were carried out. c. V(O)O3PC10H7-H200ROH When naphthalene rings are introduced into layered vanadyl phosphonate as naphthalene—appended alcohol molecules, the flexibility afforded by the alcohol which links the naphthalene to the inorganic framework can allow naphthalene molecules appended to the same layer to form face-to-face contacts and produce excimers. Intralayer excimer formation defeats the design our system which is based on the formation of excimers by aromatic rings attached to adjacent layers. To prevent the possibility of rings on the same layer from overlapping a rigid linkage was proposed to inhibit bond rotations from moving naphthalene rings into a rt-stacking geometry with rings attached to the same layer. It was concluded that the naphthalene could not be introduced as an alcohol because primary alcohols allow free rotation about the 120 Figure 24. (a) Rotation about the CHz-Np bond of 2-(2- naphthalene)ethanol intercalated in VPR leads to a 10% change in the cross- sectional area (x —> x’) and a 30% change in the interlayer distance (y -) y’) occupied by the alcohol; (b) rotation of the same bond in naphthalenemethanol intercalated within VPR [as used in the previous section] results no change in the cross-sectional area (x) or interlayer distance (y). 121 J p....------ (Y) (Y) ‘1 -----ll- Figure 24. 1‘22 vanadium—oxygen bond to allow the O-CH2 bond to sweep out a circular area with a radius of more than 1.4 A based on a carbon-carbon bond length of 1.54 A (i.e. 1.4 A = 1.54 A sin109°). The V(O)O3PC10H7.HZO°ROH series of compounds was designed to eliminate the formation of rotational isomers and corresponding intralayer excimer formation by directly attaching the naphthalene ring to the phosphorus atom on the inorganic layer. Direct attachment of the naphthalene to the layer was achieved by utilizing B- naphthalene phosphonic acid as the naphthalene source. In this scheme, n-alkyl alcohols are used as the spacer groups to control the interlayer distance and, therefore, the overlap of the naphthalene groups bound to neighboring layers. Naphthalene phosphonic acid was synthesized by both a modified Arbuzov reaction (i.e. with a free-radical catalyst) and a Grignard reaction as outlined in Chapter II.D. Each alcohol-intercalated VPR (R = C10H7) was made directly by using the alcohol to be intercalated as the synthesis solvent. The general formula for this series of materials is V(O)O3PC10H7°H200ROH with R = ethyl, butyl, hexyl and octyl. One additional compound which is related to this series is the alcohol-free material V(O)O3PC10H7'HZO made by heating either the ethanol or butanol intercalated derivatives. The alcohol-free vanadyl naphthylphosphonate represents the deepest possible interdigitation of the naphthalene rings whereas the octanol-intercalated derivative should have an interlayer space large enough to prevent any possible overlap of naphthalenes from adjacent layers. The powder x—ray diffraction patterns for this series indicates well-ordered single- phase materials (Figure 25). The pendant naphthalenes covalently bound to the phosphorus atoms do form the desired interdigitated arrangement as indicated by the slope of 0.98 A/CH2 from the plot in Figure 26. 123 Figure 25. Powder x-ray diffraction data for (a) alcohol free V(O)O3P-B- C10H7°H20 and alcohol intercalated V(O)O3P-B-CloH7-HZO°ROH for =(b) ethyl, (c) butyl, (d) hexyl, (e) octyl. 124 Counts Ir it Ill—LJlllIlIlllllllllll 5 10 15 20 26 Degrees Figure 25. 25 125 Figure 26. d-Space of V(O)O3P-B-C10H7°HZO°ROH plotted as a function of the number of carbon atoms in R. 126 30 25 20 IlIlleIllI'IIll 15 d-Space I A ..I. O y = 0.98x + 12.84 H. 0.999 O 01 O lllllfilrllt 2 3 4 5 6 7 8 Carbon Atoms in ROH for V(O)03P C1 0H7- H20~ROH Figure 26. 9 10 127 The electronic absorption data measured by diffuse reflectance for the alcohol-free (1L3 lmax = 285 nm) and other alcohol-intercalated vanadyl naphthylphosphonates (lLa Am” = 276-278 nm) gives an initial indication that these materials are functioning as they were designed. A red-shift of the 1La band for the alcohol free material indicates naphthalene-naphthalene overlap. This red-shift can be the result of two different but agreeing explanations involving steric constrains and dielectric environment as presented in Section B.3.b above. Using either explanation for the red-shift, the 1La transition of the alcohol-free naphthylphosphonate indicates each naphthalene is surrounded by other naphthalenes and, as the layers are forced apart, the absorption corresponding to the 1La transition shifts back to that of free naphthalene. The solid state fluorescence emission data shows that the deeply interdigitated, alcohol-free material, displays purely excimer emission to indicate the rings are overlapped. The alcohol-intercalated materials emit at shorter wavelengths, indicating that the intercalated alcohol pushes the layers apart and disrupt the stacking of the naphthalene rings. The fluorescence emission spectra of the alcohol-free and ethanol-intercalated vanadyl naphthylphosphonates are shown in Figure 27. All alcohols longer than ethanol produce emissions similar to the ethanol-intercalated derivative which does not form excimers. The vanadyl naphthylphosphonates display very weak emissions compared to the emissions of the naphthalenemethanol intercalated VPR materials. One explanation for the weak emission may be quenching of the excited state by the single electron of the V“ ion form the inorganic framework. The electron of the (l1 vanadium in the vanadyl ion resides in the dxy orbital. When the naphthalene ring was intercalated as an alcohol, the alcohol coordinated to the z-axis of the vanadium, orthogonal to the magnetic dxy orbital and no quenching was observed. In vanadyl naphthylphosphonate, however, an aromatic carbon from the naphthalene ring is attached directly to the phosphorus atom. 128 Figure 27. Solid state fluorescence emission of vanadyl naphthylphosphonates: (a) free of alcohol, and (b) intercalated with ethanol. Emission Intensity 129 350 400 450 500 A/nm Figure 27. 550 130 The phosphorus atom has been shown to be a part of the superexchange pathway of antiferromagnetic interactions of V4+ centers containing V-O—P—O—V linkages.228 Therefore it is possible that a similar exchange occurs through the phosphorus atom in vanadyl naphthylphosphonates and may lead to quenching of the naphthalene excited state. The magnetic susceptibility data in Figure 28 and in Table 2 does indicate that there is an antiferromagnetic exchange between vanadium centers. If the phosphorus atom is part of the superexchange pathway, the low emission intensity may be a result of quenching from the magnetic layer. A second explanation for the low emission intensity of vanadyl naphthylphosphonates could be a result of contamination by V511 V205 is a strong UV absorber and is the major chromophore of yellow, UV-absorbing glass.229 Vanadium(V) oxide is a starting material in the synthesis of all the layered vanadyl phosphonates reported here and has previously been reported to be difficult to completely eliminate from some vanadyl phosphonates. Close inspection of the powder x-ray diffraction patterns does indicate some V205 is present by the weak but characteristic reflections observed near 29: 20° but the amount of V205 present cannot be quantitated by xrd. Attempts to remove the V205 contaminant were made by heating the contaminated powder in a solution of alcohol and additional naphthylphosphonic acid; but these methods were not completely successful. Strong absorption of the UV excitation light or emission light by V205 will greatly reduce the observed emission intensity from naphthalene. Moving the interlayer space into the range between that of the alcohol-free and the ethanol intercalated derivative would lead to an even finer determination of the ideal distance at which excimer formation is allowed or prevented. However, attempts to make the methanol-intercalated derivative of vanadyl-naphthylphosphonate have not been successful. Jacobson and Johnson have had similarly disappointing results and they have attributed the difficulty in obtaining methanol-intercalated vanadyl phosphonates to the 131 Figure 28. Magnetic Susceptibility of vanadyl naphthylphosphonates intercalated with: no alcohol (0), ethanol (+), hexanol (A), and octanol (x). x" I emu mol' ‘ 0.04 0.03 0.02 0.01 0.00 132 I I I I I I I 0 5 10 15 20 25 30 35 40 Temperature I K Figure 28. 133 Table 2. Formula, structure, magnetism and emission data for V(O)O3PC10H7 ' H20 ' ROH): Formula & Magnetic Emission Structure Data Data ROH X d-Space IA ..., / K 52: _Q / K J / K A...“ / nm None 0 12.10 3.8 1.961 -0.86 405 (excimer) Ethanol 0.70 14.54 3.8 1.961 -2.53 -0.86 377 (monomer) Butanol 0.70 17.40 3.8 1.961 -1.84 -0.88 378 (monomer) Hexanol 0.70 18.45 3.8 1.960 -2.20 -0.90 377 (monomer Octanol 0.70 20.83 3.8 1.961 -2.60 -O.87 379 (monomer) 134 possibility of methanol replacing the water molecule coordinated to the equatorial site on the vanadium atom. Our attempts to make the methanol-intercalated derivatives lead to products with multiple-phases, one of which has an interlayer spacing of 13.6 A. This is in the expected range for methanol intercalated materials considering that the d-space of the alcohol-free material is 12.3A and the ethanol-intercalated materials is 14.6 A. Further manipulation of the synthetic conditions or alcohol exchange process may lead to single-phase methanol intercalated vanadyl phosphonates. d. Studies or V(0)o3PcHzcnzcmH7-Hzo-Ron Because it has been shown that it is not possible to make a pure, one-phase material of the methanol-intercalated vanadyl naphthylphosphonate, a fairly large d-space differential exists between the alcohol-free, excimer-forming material and ethanol- intercalated, non-excimer forming material. To better define the exact interlayer distance at which excimer formation is allowed and prevented, a modified system was developed to tether the naphthalene units to the layer with an ethylene linkage. This linkage would allow the naphthalene fragments to interdigitate too deeply and prevent the formation of excimers when no alcohol was present. Thus an alcohol would be necessary to jack the layers apart to align the naphthalene rings. The need for some alcohol to be present to form excimers has two advantages - one for a fundamental and one for a practical reason. For academic purposes, a tethered naphthalene setup would allow an alcohol to be intercalated and still have the naphthalene units interdigitated too deeply to form excimers. The layers could then be pushed apart at an angstrom scale by changing the intercalated alcohol by a single methylene group until the naphthalene units are brought into alignment. A tethered naphthalene system may allow for the development of a better 135 model which represents the distance dependencies and degree of overlap to form an excimer. On a practical stand point, tethering the naphthalene to the layer could lead to the formation of excimers for a specific intercalated alcohol. The issue of specificity is important in the field of chemical sensors. When the naphthalene is directly attached to the inorganic layer by an aromatic carbon, all intercalated alcohols serve to switch excimer-forming alcohol-free materials to materials which cannot form excimers. When a tether links the naphthalene to the layer, only a specific alcohol (or specific range of alcohols) may exactly align the naphthalene rings and switch the non-excimer-forming, alcohol-flee material to an excimer forming alcohol-intercalated material. The phosphonic acid which is required to synthesize the naphthalene-tethered materials is 2-(2-naphthyl)ethylphosphonic acid. This phosphonic acid is made by converting 2-(2-naphthyl)ethanol (Aldrich) to the corresponding bromide. The 2-(2- naphthyl)ethylbromide is then converted to 2-(2-naphthyl)ethyl diethylphosphonate ester by a Michaelis-Becker reaction and the diester is then hydrolyzed to the diacid in strong mineral acid as outlined in the previous chapter. The standard method was used to produce the alcohol-intercalated materials with the general formula V(O)O3PCH2CH2C10H7-H200ROH from 2-(2-naphthyl)- ethylphosphonic acid and V205 in alcohol solvent. Each of the various alcohol- intercalated materials were synthesized directly in the alcohol to be intercalated where R = ethyl, butyl, hexyl and octyl. Analysis of this series by powder x-ray diffraction indicates that the materials are pure and show only one strong 001 line to indicate one pure layered phase. A plot of the interlayer distance as a function of alcohol length indicates the expected interdigitated arrangement of the organic fragments by yielding a slope of 1.16 A/CHZ. 1:36 The fluorescence emission data collected for this series of compounds reflects problems observed previously for the compounds of the formula V(O)O3PR-H200NpMeOH in Section B.3.a above. Namely, all members of the V(O)O3PCH2CH2C10H7°HZO°ROH series direct the naphthalene units to form excimers independent of the length of the alcohol spacer. For the materials in Section B.3.a above, the consistent formation of excimers was attributed to the flexibility of the V—O—CHer linkage between the naphthalene and the layer. Free rotation about the V-O bond allowed the naphthalene fragments on the same side of a given layer to come into close enough contact to produce excimers as shown previously in Figure 23. In the present series, a similar two-atom linkage holds the naphthalene unit to the layer. This time the linkage is CHz—CHZ and is bonded to the phosphorus atom but the effect is the same. All alcohol-intercalated V(O)O3PCH2CH2C10H7-H200ROH materials, even the octanol- intercalated analog, allow naphthalene units to come into face-to-face contact and form excimers. A fact that reconfirms the need to design these systems with rigid linkages between the naphthalene and the layer. C . Overview From the above sets of compounds, it has been shown that angstrom-scale control of the formation of excimers can be achieved in some cases. The naphthalenemethanol- intercalated vanadyl alkylphosphonate series from Section B.3a displayed a definite switch from excimer-preventing for vanadyl methylphosphonate to excimer-inducing for vanadyl ethylphosphonate. Likewise, the series of vanadyl naphthylphosphonates also displayed a dependence of interlayer distance on the formation of excimers. The fully interdigitated, alcohol-free vanadyl naphthylphosphonate placed the rings in close 137 proximity to allow the formation of excimers and all alcohol intercalated materials pushed the rings apart to prevent excimer formation. Some design difficulties do exist in the systems presented. A major difficulty is rotational isomerism of alkyl chains and coordinated alcohols used to append the naphthalenes to the inorganic layers. Rotational isomerism allows naphthalene rings attached to the same side of a single layer to achieve a face-to-face overlap and form excimers. The control of molecular geometries offered by layered vanadyl phosphonate supports apparently cannot restrict all degrees of freedom and a static and ordered interlayer of organic pieces cannot be assumed even though the interlayer distances can be accurately and consistently measured at the sub-angstrom scale. The greatest opportunity lies in those materials with the naphthalene appended to the phosphonate unit. This is because the fluorescence of these materials can be switched by the intercalation of a non-photoactive alkyl alcohol. Thus a sensor for the detection of alcohols can be built from vanadyl naphthylphosphonate materials. To make the sensor specific for a given alcohol, a rigid spacer must be place between the naphthalene unit and the phosphorus atom for reasons discussed in Section B.3.d of this Chapter. Therefore, avenues should be explored to synthesize phosphonic acids with rigid spacers between the phosphorus atom and the naphthalene unit to allow the development of alcohol specific sensing materials. CHAPTER IV THE INCORPORATION OF DIMOLYBDENUM CENTERS INTO LAYERED ZIRCONIUM PHOSPHONATES A . Background Controlled structure and reactivity is a major concern in the development of catalysts.230 The structural factors in the design of a catalytic system naturally become more demanding as the complexity of the reaction sequence increases. Photosynthesis is an excellent example of a complex reaction sequence dependent on a support structure. The bilipid membrane of the chloroplast serves as a framework to support the reaction centers of photosynthesis and these reaction centers also utilize the membrane to separate the photogenerated electron and hole. The cascade of reaction centers of photosynthesis are directed in a vectorial manner to maintain spatial separation of the complementary oxidizing hole and reducing electron pair. The process is ultimately manifested in the multielectron chemistry of water oxidation at the oxygen-evolving complex and proton reduction in Photosystem 1.231 Several major schemes have been used to mimic photosynthesis and achieve light to energy conversion with varying degrees of efficiency utilizing molecular, 138 139 supramolecular as well as solid state architectures to overcome the obstacles of storing solar energy as chemical energy. The design of most photocatalysts involve the same general reaction schemes as photosynthesis where by an electron-hole pair are created at a light harvesting center and some system is set up to separate the pair and draw them away from the photocenter to prevent recombination. Electron-hole pair separation is generally achieve by the use of electron donors (D) and acceptors (A) to spatially divided the pair 232 and Wasielewski's233 molecular-based from the photocenter (P). Gust and Moore's DPA triads permit the photogenerated hole localized on a light harvesting center to be trapped by the following sequence of charge-separating intramolecular electron transfer events: DP*A -) DP+A‘ -> D*PA‘. Although this photogenerated charge separated state may persist into the microsecond range, the quantum yield can be relatively low.234 In an effort to achieve better charge separation, the covalently bonded molecular approach to charge separation has been complemented by the vectorial arrangement of D, P and A electron transfer components within organized architectures such as micelles,235 polymers,236 and solid state substrates.237 For example, one particularly intriguing approach is that of Mallouk238 who has concatenated a metalloporphyrin or Ru(bpy)32+ photosensitizer to a methylviologen charge transfer/Pt catalyst self-assembly organized within the channels of zeolite-L (Figure 29). The zeolite microstructure is a spatially organized triad (sacrificial electron donor, photosensitizer, electron transfer carrier). All of the artificial photosynthetic systems presented to this point are based on the same process of moving a single excited electron with the aid of specifically placed donors and acceptors to prevent back reaction. The electrons are then assembled at a site at which a multielectron reaction can ensue. A photoreaction developed in our group may make it possible to achieve light to energy conversion catalysts which can circumvent the need to design complex electron- hole separation procedures.239 The essence of this alternative method is based on a 140 Figure 29. Vectorially arranged light harvesting center and e“ donor- acceptor triad within a zeolite—L channel. 141 “ac N—RU\2-=-N CH, H2 2H+ Figure 29. 142 bimetallic reaction center that allows a single photon to generate a multielectronic excited state and exists as a zwitterion rather than the customary radical.240 The formation of an excited state zwitterion by a single photon means that a multi-electron catalytic site is created instantaneously and that no effort need be made to separate and store electron-hole pairs. Furthermore, no unstable, odd-electron intermediates are present in the reaction pathway and the two electron excited state of the zwitterionic center can possibly transfer in unison. The unusual reaction center that generates the zwitterion upon excitation is a quadruply-bonded dimolybdenum center (Moi-Mo). The Maj-Mo center consists of two square planar molybdenum(II) ions that come together in an eclipsed geometry as shown in Figure 30. The orbital description of the bonding interactions for a quadruply bonded bimetallic core is presented in Figure 31. The four bonds between the two molybdenum atoms are formed by one o-bond (dzz-dzz), two rt-bonds (dxz-dxz and dyz- dyz) and the face-to-face overlap of the (1,,y orbitals on each metal to form one 5-bond. The formation of the 5-bond by overlap of (1,,y orbitals dictates that the two square planar metal centers be eclipsed. The lowest energy excited state of the dimolybdenum cores is the 565* transition with an absorption energy in the visible region. The specific reaction sequence which has led to the development of the two- electron exited state chemistry is the asymmetric oxidative addition of a substrate molecule to one metal of dimolybdenum core. Upon excitation of the MolLMoII core, the electrons from the 8-bond forming d,‘y orbitals form a zwitterionic Mom-MoI species which can be thought of as M‘-"-M'. The two electrons on the reduced metal participate m core in in an oxidative addition reaction of methyl iodide to produce the Mom-Mo some fashion similar to the schematic in Figure 32. Thus a single photon absorbed by the MoILMoII can lead to a net two electron transfer, in this case, a two electron oxidation of 143 Figure 30. Generalized quadruply bonded dimolybdenum compound. 144 A P P ' l a.....Cl LVC' EM 01/ I CI/ I P P V Figure 30. 145 Figure 31. Orbital basis of the quadruply bonding of dimolybdenum compounds. 146 Figure 31. dYdez dzz 147 Figure 32. Edge-sharing bioctahedral product resulting from the reaction of a dimolybdenum compound CH3I. -0- Clad," ..uCL. I I cHlf‘cF']: -v- Figure 32. 149 the bimetallic core (MolLMoII -) Mom-Mom) caused by an oxidative addition of some substrate molecule to the excited state mixed-valence core. Oxidative addition of a substrate molecule to the multielectron Moi-Mo photocenter must be coupled to a reductive elimination reaction to regenerate the Mon-MoII bimetallic center for the process to be catalytic. Reductive elimination, in this case, is not merely a reverse reaction to yield the Mo-‘I-Mo dimer and the starting substrate. Reductive elimination in this case could conceivably convert a substrate molecule, A-B into two non-interacting species A’ and B". By incorporating the dimolybdenum site into an appropriate support, an efficient reductive elimination process may make it possible to build a catalytic system. The generalized catalytic assemblage which we have designed is known as a layered integrated photochemical system. The premise is to support the bimetallic photocenter in a layered material and allow substrate molecules to diffuse or intercalate into the material and oxidatively add to the dimolybdenum complex similar to what has been shown occur in solution. The layered framework then serves to regenerate the active site of the catalyst by supplying electrons to the McA-Mo center from redox active metals which are a part of the layer. The electron transfer from metal atoms of the layered support to guest molecules or ions is well know for reduction-oxidation intercalation reactions of layered materials.241 The addition of electrons from the layer to the MoiMo center leads to reductive elimination of the substrate fragments from the bimetallic center. A complementary reduction-oxidation reaction at the surface of the layer by a sacrificial electron donor can serve to replenish the metal atoms of the layer with electrons and the catalytic cycle can continue (Figure 33). The design and synthesis of the layered integrated photochemical system will be based on the self assembly of layered metal phosphonates and the reactivity of the assembled layered platform to incorporate the photochemical center. Layered metal 150 Figure 33. A layered integrated photochemical system. 151 1%va no“ r/ 1". my Mo-Mo MEMO-M005 oxned ,/ a"; / hv Figure 33. 152 phosphates and phosphonates are ideal hosts which satisfy all of the requirements of a support for the layered integrated photochemical system including redox activity, acid stability and intercalation reactive. Layered vanadium and niobium phosphates have already proven to accommodate the intercalation of dimolybdenum centers but do not allow for the control of the ligand set around the photoactive core. For simple layered metal phosphates, the only coordination sites available to an intercalated dimolybdenum core is phosphate ion from the layer. Because the ligand set about the Maj-Mo core is vital in promoting the formation of the zwitterionic rather than the biradical excited state, methods must be implemented to control the local environment about the intercalated bimetallic center. Two methods have be developed which utilize layered metal phosphates as structural templates and allow for the control of the ligand set about the interlayer Mo-iMo core. The first method is based on the intercalation of an intact dimolybdenum complex with a pre-assembled ligand set. Lewis basic sites integrated are integrated into the ligands and acid base intercalation is used to place the pre-assembled dimolybdenum complex into the layer. This method is being explored by Mr. Eric Saari of our group and has had some initial success. The second method to introduce a McA-Mo core into a metal-oxygen-phosphorus layer while controlling the ligand set about the core is through the use of layered metal phosphonates (LMPRs). LMPRs offer a versatile method to modify the interlayer “ligands” available to the intercalated dimolybdenum core by utilizing various phosphonate pendants in the synthesis of the layered metal phosphonate. An extensive synthesis library of phosphonic acids is available and can be used to develop layered metal phosphonates with specific phosphonate pendants to act as ligands for dimolybdenum centers intercalated into the layer. The remainder of this Chapter deals with the design, synthesis and analysis of MolLMoII intercalated layered metal phosphonates. 153 The synthetic feasibility of the layered integrated photochemical system must be proven for layered metal phosphonates as compatible supports for the incorporation of Maj-Mo centers. Because only the construction of the layered metal phosphonate- MoA-Mo complex is being tested, the need for redox active metals in the layer was ignored and zirconium phosphonates were chosen as the layered material support for initial studies. Zirconium phosphonates are a good material to use for initial synthetic based studies because it offers a white background with no magnetic ions and a well known structure which is unaffected by the incorporation of a broad range of secondary functional groups on the organic phosphonate pendant. Secondary functional groups such as alkyl- or aryl-alcohols (phenols), carboxylic acids, esters, amides, acid chlorides, aldehydes, halides, thiols and sulfonic acids have all consistently produced zirconium phosphonates with the well-known a—zirconium phosphate structure. The synthetic schemes to be used to incorporate the dimolybdenum guest into the layered zirconium phosphonate host are all currently based on the intercalation of weakly ligated dimolybdenum compounds into zirconium phosphonate layers which contain secondary organic functionalities known to coordinate to dimolybdenum cores. The pendant of choice for these studies is (-CH2CH2COOH) produced by 2-carboxy- ethylphosphonic acid. Because the first carbon-phosphorus bond is nearly perpendicular to the inorganic zirconium layer, the two carbon —CH2CH2- tether allows the carboxylic acid to be similarly directed perpendicular to the layer. The use of an even number of methylene groups to tether the carboxylate group to the layer must be used so that the carboxylate is directed toward the interlayer region and can serve to ligate the intercalated molybdenum. 154 B . Results 1 . Reactions of Zr(O3PCH2CH2COOH)2 with Quadruply Bonded Dimolybdenum Compounds Zirconium carboxyethylphosphonate Zr(O3PCH2CH2COOH)2 was synthesized by literature methods of precipitating an aqueous solution of a Zr“ salt with two equivalents of phosphonic acid. The gelatinous precipitate was then slightly digested by the addition of 5% aqueous hydrofluoric acid until a noticeable amount of solid had dissolved. The mixture was then refluxed with stirring to produce a white micro- crystalline powdered material. Powder x-ray diffraction of Zr(O3PCH2CH2COOH)2 yields a sharp 001 peak corresponding to an interlayer distance of 12.6 A. Details of the synthesis are presented in Chapter 2. Ion exchange was to be the bases for the driving force for the introduction of the dimolybdenum core into the protonated carboxylic acid- appended zirconium phosphonate. Attempts exchange the dimolybdenum core into the carboxylic acid modified layer failed - even when the [Mail-Mo]4+ core was only ligated by weakly coordinating acetonitrile solvent molecules as in the ion M02(CH3CN)84+. Reaction of zirconium carboxyethylphosphonate with Mo-‘I-Mo complexes led to no change of color or interlayer distance. A report of the interlayer hydrogen bonding in carboxylic acid appended layered zirconium phosphonates led us to realize why intercalation by ion exchange did not proceed as planned.242 Thompson realized the difficulty of accessing the carboxylic acid pendants when attempting to produce the acid chloride of the carboxylic acid pendant by reaction with thinly chloride.243 The protonated acid was not at all reactive to SOClz and was reported to be due to the large interlayer bonding energy formed for these materials by the formation of carboxylic acid dimers. We have concluded that interlayer hydrogen 155 bonding is also the cause for the inert behavior of Zr(O3PCH2CH2COOH)2 toward Mo-4-Mo complexes. 2. Synthesis of Zr(O3PCH2CH2COOM)2 for (M = NHf, Na+) Thompson disrupted the interlayer hydrogen bonding by an acid-base reaction of the carboxylic acid pendants with anhydrous ammonia to form an interlayer ammonium carboxylate salt. The anhydrous ammonia was able to expand the layers, disrupt the hydrogen bonding and allow access and reactivity with the carboxylate pendant. We repeated the process developed by Thompson to produce ammonium intercalated carboxylate materials and have also explored the production of sodium carboxylates. We have used these carboxylate salt-zirconium phosphonate layers as the starting materials for all remaining intercalation and ion exchange reactions of dimolybdenum compounds. The ammonium carboxylate salt was made by Thompson’s method of passing anhydrous ammonia gas over the solid zirconium carboxyethylphosphonate. This highly exothermic reaction was highly efficient and produced a noticeable expansion of the volume of the solid material. The apparent increase in volume of the solid materials was probably not attributable to the increase in interlayer distance but instead due to the fragmentation of zirconium phosphonate aggregates into a micro-powder as a result of this exothermic reaction. Two other methods were also used to disrupt the interlayer hydrogen bonding of the carboxylic acid pendants and both involve the production of the sodium carboxylate salt. The first method utilized sodium hydroxide but did not lead to satisfactory results because of the high pH created by the addition of the strong base. Even dropwise 156 addition of sodium hydroxide solution created high pH of the solution until the base could diffuse into the layer and be neutralized by the protons of the pendant carboxylic acid. Zirconium phosphonates and phosphates are susceptible to attack by base at pH > 8 and leads to the conversion of zirconia or various degrees of hydroxylated zirconium and glassy sodium zirconate. In any case, the intensity of the diffraction patterns associated with zirconium phosphonate layer are decreased and a noisy baseline of the diffraction pattern develops which is characteristic of the semicrystalline zirconium hydroxide hydrates. The second method to produce sodium salts of the carboxylic acid pendant is by reaction of the carboxylic acid with aqueous sodium bicarbonate. Sodium carbonate eliminates the possibility of producing high pH conditions while providing a good means of producing the sodium salt. Although sodium bicarbonate is not a strong base, the reaction of N aHCO3 with weak acids like the carboxylate pendant are irreversible due to the liberation of carbon dioxide from the reaction mixture as the neutralization proceeds. 3 . Reactions of Dimolybdenum Complexes with Zr(O3PCH2CH2COOM)2 for (M = NHf, Na") The expanded interlayer distance and decreased interlayer hydrogen bonding achieved by deprotonation of the carboxylic acid pendant was thought to lead to easy insertion of MoA-Mo centers into the zirconium layers. However, only two of the dimolybdenum complexes listed in Table 1 lead to the noticeable insertion of a dimolybdenum center into zirconium carboxyethylphosphonate. Several reasons have been proposed for the reluctance of the various dimolybdenum centers to insert into the zirconium carboxyethylphosphonate. For the neutral dimolybdenum complexes, the net charge of the molybdenum center would 157 become negative if the easily displaced neutral dimethylsulfide, acetonitrile or pyridine ligands of M02C14(Me28)4, M02CI4(CH3CN)4 and M02Cl4(py)4244 respectively were replaced by the carboxylate of the layer. The intercalated molybdenum core would have a ligand set consisting of M02Cl4(-CH2COZ)2 where -CH2C02 represents the carboxylate of the layer. With this ligand set, the net charge of the molybdenum center would be negative two and interlayer counter cations must be present to balance the charge. In the case of the dipositive ions of dimolybdenum, M02(CH3C02)2(CH3CN)42+245 in the form of a BF4‘ or PF6’ salt, the expected dimolybdenum centers would be neutral upon coordination to two carboxylates form adjacent layers and have a coordination of M02(CH3C02)2(-CH2COZ)2. However, Cotton has shown by single crystal x-ray structural determination of the BF4‘ salt that the carboxylates of M02(CI~I3COZ)2(CH3CN)42+ are in a cis- geometry and therefore it is not possible to produce a structure consisting of trans- pendant carboxylates from the cis- M02(CH3C02)2(CH3CN)42+. Reaction of the fully solvated M02(CH3CN)3(BF4)4246 or M02(CH3CN)8(PF6)4 with Zr(O3PCH2CH2C02Na)2 result in no reaction as well. This lack of reaction can be explained by the fact that as the exchange takes place, NaBF4 or NaPF6 must exit the layer as part of the metathesis reaction of the ion exchange. However, neither NaBF4 or N aPF6 are soluble in the acetonitrile solvent used for the reaction an may precipitate at the pores which allow access to the interlayer region. A traffic jam may occur and prevent a significant proportion of dimolybdenum complex from entering the layer. The only compatible pairs of compounds which produce an acetonitrile soluble salt as a secondary product are M02(CH3CN)3(PF6)4 or M02(CH3CN)8(BF4)4 and Zr(O3PCH2CH2COzNH4)2. The resulting ammonium hexafluorophosphate, and ammonium tetrafluoroborate are soluble in acetonitrile and this solubility may be the key in allowing a dimolybdenum complex to intercalate into the layer. The product of the 158 reactions of both forms of M02(CH3CN)84+ with Zr(O3PCH2CH2COzNH4)2 in acetonitrile solvent is a pink powder. The powder x-ray analysis of the pink material (Figure 34) shows a 1.2 A decrease of the interlayer distance in the MoiMo intercalated material compared to the ammonium-intercalated starting material (14.6 A —> 13.4 A). This decreased interlayer distance is what would be predicted by a pillaring reaction whereby the molybdenum atoms bond to the carboxylates from adjacent layers. The contraction of the layers induced by molybdenum pillaring brings the interlayer distance between the maximum created by ammonium intercalation and the minimum of the hydrogen bonded, protonated carboxylic acid layer (12.6 A). The distance between two oxygen atoms of a carboxylic acid dimer is 3.0 A and the molybdenum to oxygen distance is 2.0 A for most molybdenum carboxylates. Therefore, the difference between the hydrogen bonded protonated carboxylate layer and the dimolybdenum intercalated dicarboxylated layer should be 1.0 A [3 A vs. (2 x 2 A = 4 A)]. The difference between the d-space of the hydrogen bonded layer protonated layer and the molybdenum intercalated layer is 0.8 A, very close to the differences between the reported distances. The electronic absorption spectrum of the pink material obtained by diffuse reflectance shows an absorption profile similar to that of the red M02(CH3C02)(CH3CN)42+ reported by Cotton247 with a Am, = 530 nm. The diffuse reflectance spectrum of the red powder resulting from the treatment of Zr(O3PCH2CH2COzNH4)2 with M02(CH3CN)8(PF6)4 is presented in Figure 35. Because the absorption spectrum of the pink powder is similar to that of M02(CH3C02)(CH3CN)42+ and the likely coordination by carboxylate within the layer, a dicarboxylate coordination geometry for the intercalated MoA-Mo is assumed. Additionally, the two carboxylates are believed to be from adjacent layers due to the evidence obtained by x-ray powder diffraction data which shows a contraction of the 159 Figure 34. Powder x-ray diffaction patterns from (a) M02 intercalated Zr(O3PCH2CH2COO’)2, (b) Zr(O3PCH2CH2COOH)2 and (C) ZI'(O3PCH2CH2COONH4)2. 160 Intensity IIIIlIIIlllllIllIlIIIIIIIIIl 5 10 15 20 25 30 Degrees 29 Figure 34. 161 Figure 35. Diffuse reflectance UV-visible spectrum of the dimolybdenum intercalated zirconium phosphonate layer 0cm, = 530 nm). 162 Ecomm T 225?? T oocouuozom 9:39.02. I. T 5:902? @5323. 250 300 350 400 450 500 550 600 650 l/nm Figure 35. 163 interlayer distance. The remaining four coordination sites on the dimolybdenum core are assumed to contain acetonitrile because no other coordinating ligands are present within the layer or solvent. The second lowest energy absorption of the pink powder may be a result of either the 1t—)1t* absorption transition of M02(-CH2C02)2(CH3CN)42+ or it may be the result of the lowest energy transition (6—98“) of M02(-CH2C02)4. The molecular version of the tetracarboxylate, M02(CH3C02)4 is yellow in color with a km“ for the 5—95" absorption in the same spectral region. In solution, the 1t—m* transition of molecular M02(CH3COZ)2(CH3CN)42+ is much weaker than the 8—)5“ transition . It was though that the origin of the strong second lowest transition could be ascertained by measuring the solid state diffuse reflectance spectrum of M02(CH3C02)2(CH3CN)42+ If the intensity difference was a result of physical state (solution verses solid state) the peak would be attributed to the n—m* transition of Mo2(-CH2C02)2(CH3CN)42+. 1f the intensity remained the same in solution as in the solid state, the fairly strong absorption at 445 nm would be attributed to the presence of M02(-CH2C02)4. Attempts to measure the solid state spectrum of M02(CH3C02)2(CH3CN)42+ failed miserably because of the high air—and moisture sensitivity of these samples. Additionally, diffuse reflectance requires that the sample be mounted on a reflective BaSO4 substrate. As soon as the air- and moisture-sensitive samples were mounted onto the BaSO4 substrate, the sample decomposed to a brown powder and eventually a blue oil or glass. Elemental analysis of the pink compound were carried out by ESCA to determine the ratio of zirconium to molybdenum and indicate a 1:2 ratio of molybdenum to zirconium. A maximum of two molybdenum atoms (one dimer) per pair of phosphonate pendants is possible assuming only that the structural data gleaned from the powder x-ray diffraction data is correct, i.e. a carboxylate pendant from adjacent layers are coordinated to each dimolybdenum center. This maximum ratio of Mo:Zr was determined by the fact 164 that the formula of the layer is one zirconium per two phosphorus atoms (and phosphonate pendants), and two phosphonate pendants are coordinated to each dimolybdenum core. However this maximum ratio of molybdenum to zirconium based purely on the fact that two pendant carboxylates are attached to each dimolybdenum core. An in-depth examination of this system indicates that the charge of the intercalated molybdenum center must also be take into account when calculating maximum loading of molybdenum into the layer. In the current model for the ligation about the [Mo-Q-Mo]4+ core there exists two negatively charged carboxylates form adjacent layers and four neutral acetonitriles. Thus a net two-plus charge exists on each dimolybdenum core. The only available anion to compensate the positive charge on the metal centers is carboxylate of the layer. Therefore, two fiee carboxylates (not bound to a dimolybdenum core) must be present for each intercalated dimolybdenum core to balance the positive charge. When overall charge is taken into account, the maximum ratio of molybdenum atoms to zirconium atoms can be half of the two to one ration calculated earlier to yield a maximum of one molybdenum per zirconium. From ESCA molybdenum to zirconium ratio was determined to be 1:2 which is exactly half of the maximum 1:1 (or 2:2) ratio possible. Therefore, 50% of the carboxylate sites available to dimolybdenum cores are occupied by Mo-A-Mo in the reaction Of M02(CH3CN)8(PF6)4 With ZI(O3PCH2CH2C02NH4)2. 4 . Synthesis of Zirconium (Pyridyl)Phosphonates Zirconium phosphonates with pendants other than carboxylate have been made for reaction with dimolybdenum centers. Specifically, zirconium 4-pyridylphosphonate and zirconium 2—(4-pyridyl)ethylphosphonate. In both cases, the lone pair of the pyridine is directed toward the interlayer region which is theoretically able to coordinate to an 165 intercalated dimolybdenum center. However, because pyridine is a fairly good Lewis base and the layered zirconium phosphonate is made in a strong mineral acid solvent, HCl, the pyridine is converted to a hydrochloride salt during the synthesis of the layer. The formation of an intralayer hydrochloride salt presumably induces a high degree of hydration of the interlayer region and prevents the formation of crystalline materials presuming a layered structure forms at all. It may occur that the phosphonic acid is autoprotonating the pyridines to some degree and when a 2:1 ratio of phosphonate to zirconium are combined, not enough protonated phosphonic acid is present to fill all coordination sites of the zirconium thus producing a low crystallinity glass or gel. Attempts to synthesize more crystalline materials by using a higher ratio of pyridine- containing phosphonic acid also produced glassy materials. For the ethylpyridine phosphonate layer of zirconium the material is a clear glassy or gelatinous polymer which cannot be filtered on paper or glass flit. Drying the pyridine phosphonate layer on a glass slide leads to optically clear coatings on the glass which may have applications in sol-gel chemistry. No attempts to combine dimolybdenum compounds into the non-crystalline pyridine(phosphonates) have been made. C . Conclusions It has been shown that dimolybdenum compounds can be intercalated into zirconium phosphonate layers and that the nature of the bonding about the dimolybdenum center can be deduced by optical absorption spectroscopy based on the library of known dimolybdenum compounds. The synthetic feasibility of the M02/ZrPR system has been demonstrated. 166 To move on and design a working layered integrated photochemical system some additional design features must be examined. First, the dimolybdenum center must be introduced into a redox active host and electron transfer between Mos-Mo and the host layer must be demonstrated. Layered tin or titanium phosphonates form the same layered structure as zirconium but these metals are redox active. Titanium (IV) can undergo a one electron reduction to Titanium(III) and tin can interconvert between the 4+ and 2+ oxidation states with a two electron reductions and oxidations. Additionally, manganese, vanadium and cobalt phosphonates are known and each of these metals has several common oxidation states that can easily be accessed. Second, the carboxylate pendant from this study must be altered to some pendant capable of promoting multielectron photochemistry at the Mos-Mo active site. The ligands which promote multielectron chemistry of the MoA-Mo cores consists of four halides and two 2-pyridones or bis- phosphines that bridge across the two metals and are positioned trans- to each other. Recently, alkyl248 and aryl249 phosphine pendants have been placed on phosphonate esters. The alkyl phosphine(phosphonate) is quit reactive towards water and is not suitable for use in synthesizing a layered metal phosphonate because the layers must all be made in aqueous solution. The known aryl phosphine(phosphonate) is a monophosphine and therefore cannot bridge the two metal atoms. Aryl-diphosphine(phosphonates) could be made by similar chemistry but the steric bulk of aryl diphosphines may not fit into the surface area allowed to it by the host layer. The final alternative is using the 2-pyridone ligand to bridge the metals and ligate at the nitrogen and oxygen atoms. The pyridine ring must be anchored to the layer at the position para- to the nitrogen for the nitrogen and oxygen atoms to be properly oriented. Only one synthesis of para-substituted (pyridine)phosphonates is known in the literature consists of the nucleophilic aromatic substitution presented in Chapter II.D.25o By this method, the two ortho- positions must be occupied to prevent attack of the phosphite anion from attacking at these locations. 167 Masking the ortho- locations with methyl and methoxy will force the attack at the 4- position and hydrolysis of the methoxy ether may produce the desired product. However, the pyridine must be converted to an N—oxide as an intermediate step to deactivate the ring to allow nucleophilic substitution. However, N—oxides of 2- alkoxyethers of pyridine have been shown to be very unstable an alternative approach must be made.251 One possibility may be the conversion of an existing phosphonate ester into the 2-pyridone by a ring closing reaction of triethyl-4»phosphonocrotonate with urea or ammonia and an aldehyde in a modified Hantzsch Reaction252 to give an ester of the pyridinol which could by hydrolyzed to the pyridone. Finally, once the two molybdenum atoms are inserted into the zirconium phosphonate layer, the layer essentially becomes pillared and no expansion or contraction of the interlayer distance can occur to accommodate the intercalation of new molecules. Intercalation is vital to the introduction of substrate molecules however and a method must be developed which still allows substrate to enter the layer after the dimolybdenum complex has been inserted. 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