LAYER - LATTICE ‘SILICATES: r . A SUPPORT FOR A RHODIUM (I) -TR|PHENYLPHOSPH|NE' HOMOGENEOUS HYDROGENATION CATALYST ' 7 Dissertation for the Degree of Ph. D. MICHIGAN STATE UNIVERSITY JAMES FRANCIS HOFFMAN 1976 ' LIBRARY ‘1 C ti. Q ) é . ‘ . I .i .Ifko‘."' f? ." v t ‘ 1"' ' \J (’I‘ - .; _ ¢'- 4‘ "* l A w This is to certify that the thesis entitled LAYER-LATTICE SILICATES: A SUPPORT FOR A RHODIUM(I)- TRIPHENYLPHOSPHINE HOMOGENEOUS HYDROGENATION CATALYST presented by James Francis Hoffman has been accepted towards fulfillment of the requirements for Ph . D . degree in Chemistry Major professor 0-7 639 55‘5“ x :-'~“‘-’"" ""— ‘ ABSTRACT LAYER-LATTICE SILICATES: A SUPPORT FOR A RHODIUM(I)—TRIPHENYLPHOSPHINE HOMOGENEOUS HYDROGENATION CATALYST By James Francis Hoffman The research reported in this thesis demonstrates that smectites, a class of naturally occurring layer-lattice silicate minerals, afford an inorganic and crystalline ma— trix for supporting cationic homogeneous hydrogenation catalysts. A rhodium(I)-triphenylphosphine complex, Rh(PPh3)X+, which is a known hydrogenation catalyst precursor in solution, was introduced into the interlamellar space of hectorite. The rates at which the mineral supported com- plex catalyzes the hydrogenation of various unsaturated hydrocarbons were measured in methanol under ambient temper— ature and pressure. The product of the protonation of Rh2(CH COO)“, 3 Rh(CH3COO)4_xx+ (x = l or 2), was exchanged onto the sur- faces of hectorite. The amount of rhodium exchanged was dependent on the concentration of the green Rh2(CHBCOO)4_XX+ species used in the exchange reaction. For the catalytic . x+ studies, a 0.002M Rh2(CHBCOO)u_x solution was used to pre- pare the catalyst precursor. The partially exchanged mineral obtained with the 0.002M solution was found to contain 0.76% Rh, which corresponds to 7.4 mmole Rh/lOOg of the James Francis Hoffman mineral. The uv—visible spectrum of a mull of the mineral supported complex exhibits the same intense band that is found for the homogeneous complex at 258 nm. The ir spec- trum shows that acetate is coordinated to the rhodium com- plex. Upon the addition of triphenylphosphine to the green Rh2(CHBCOO)u_xx+-hectorite, an immediate color change to dark orange was observed. This orange mineral bound species, Rh(PPh3)xf—hectorite exhibited an uv-visible spectrum which contained the same features as Rh(PPh3)3BF4 in methanol. The infrared spectrum indicated the presence of triphenyl- phosphine, and the absence of coordinated acetate in the complex. Maximum catalytic activity was obtained with the min- eral supported catalyst at an initial PPh3 concentration of 4.6mfl. This concentration of PPh3 corresponded to a PPh :Rh ratio of 9:1. For the hydrogenation of 1M l-hexene, 3 a turnover number of 10.5 ml of HZ/min/mmole of Rh was ob— served. The mineral environment had an inhibitive effect on the reduction of olefins. A turnover number of 140 ml of H2/ min/mmole of Rh for 1M l-hexene was observed for the homo- geneous catalyst. For terminal olefins. a linear uptake of H2 was obtained, whereas for substituted olefins. a non— linear uptake of H2 was obtained. Internal olefins were not reduced with the mineral supported catalyst. Gas chrom- atographic analysis of the products of the hydrogenation of l-heptene showed that some isomerization to 2-heptene James Francis Hoffman occurred during the reaction. Both internal and terminal alkynes are hydrogenated by the mineral supported catalyst at a rate comparable to the rate with the homogeneous catalyst. l-Hexyne was reduced at a rate of 530 ml of HZ/min/mmole of Rh, under the same conditions as l-hexene. Analysis of the products of the re- action by gas chromatography showed that the hydrogenation of l—hexyne occurred in a stepwise manner, first with the re— duction to the olefin, then to the totally saturated hydro- carbon. The reduction of the olefin was insignificant as long as the alkyne concentration was large compared to the olefin concentration. 2—Hexyne was only reduced to the cor- responding olefin, at a rate comparable to l-hexyne. Similar trends in the relative rates of hydrogenation of both alkenes and alkynes indicated that the catalytic species on the mineral and in solution were chemically similar. It was not possible to establish the constitution of 31 the catalyst precursor, Rh(PPh3)x+, by P nmr spectroscopy because of the low solubility of the complex in methanol. However, a dimethylphenylphosphine Rh(I) complex, Rh(PPh(CH3)2)3BFu, was prepared by a reaction of the pro— tonated acetate complex with the phosphine ligand. The nmr study showed that the phosphine acted as the reducing agent X+ in the reduction of Rh2(CH3COO)4_x Based on the nmr data, PPh was assumed to be the reducing agent in the related 3 Rh(PPh3)k+ system. LAYER-LATTICE SILICATES: A SUPPORT FOR A RHODIUM(I)-TRIPHENYLPHOSPHINE HOMOGENEOUS HYDROGENATION CATALYST By James Francis Hoffman A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 1976 TO LIZ ii ACKNOWLEDGEMENTS I wish to thank Dr. Thomas J. Pinnavaia for the en- couragement, advice, and guidance he provided throughout this project. I also wish to express my appreciation to Dr. Carl H. Brubaker, Jr. for serving as my second reader and for all the suggestions on how to improve my writing skills. No words can express my love and appreciation for Liz, whose love and encouragement made all this possible. She suffered much loneliness, especially during the writing of this dissertation. She was always there with her love during all the happiness and depression one encounters in a pro- ject of this magnitude. I also wish to thank my parents, James and Ursula Hoffman, for their continual love and en- couragement. Finally, I wish to thank Michigan State University and the National Science Foundation for their financial support, which made this study possible. iii TABLE OF CONTENTS LIST OF TABLES . . . . . . . . . . . . . . LIST OF FIGURES I. INTRODUCTION . . . . . . . . . . . . . . . . . A. B C. D Research Objective Clay Minerals . . . . . . . . . . Chemistry of Rh2 (CH 3COO)“ Previous Studies of Supported Homogeneous Catalysts II.EXPERIMENTAL . . . . . . . . . . A. B. Reagents and Solvents . . . . . . Syntheses . . . . . . . . . . . . . . l. Methanol Adduct of Tetrakis-u-acetato- dirhodium(II) . . . . . . . . . . 2. Protonation of Rh2(CHBCOO)4-2CH3OH . 3. Exchanged Hectorites . . . . . . a. Rh 2(CHBCOO)4; +-Hectorite b. Rh(PPh3 )x +~Hectorite . . . . . . Physical Methodsx . . . . . . . . . . . . . 1. Gas Chromatography . . . . . . . . . 2. X—Ray Diffraction Studies . . . . . 3. UV-Visible Spectra . . . . . . 4. Infrared Spectra . . . . . . . . . 5. Proton NMR Studies . . . . . . . . 6. Phosphorus-31 NMR Studies . . . . . . Hydrogenation Studies . . . . . . . . . . iv Page vi vii ll 18 28 28 29 29 30 31 31 32 33 33 33 34 34 37 37 38 Page III. RESULTS AND DISCUSSION . . . . . . . . . . . . 44 A. Characterization of the Homogeneous Catalyst . . . . . . . . . . . . . . . . . A4 1. Solution Chemistry of Rh2(CH3COO)4_XX+ (x = 1,2) . . . . . . . . . 44 2. Solution Chemistry of Rh(PPh3)33F4 . . 51 B. Characterization of the Heterogeneous Catalyst . . . . . . . . . . . . . . . . . 65 C. Hydrogenation Studies . . . . . . . . . . 72 IV. CONCLUSIONS . . . . . . . . . . . . . . . . . 92 BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . 93 LIST OF TABLES Table Page I. UV-Visible Spectrometric Data . . . . . . . . 5h 2 31p andlgF NMR Data . . . . . . . . . . . . . 61 3. Rhodium Loading on Hectorite . . . . . . . . 65 4 Hydrogenation Rates of Alkenes in the Presence of Mineral Supported and Homogeneous Rh(I)-PPh3 Catalysts . . . . . . . . . . . . 77 5. Hydrogenation Rates of Alkynes in the Presence of Mineral Supported and Homogeneous Rh(I)-Pph3 catalYStS I o o I o o o o n o a O 85 6. Effect of Rhodium Loading on the Turnover Number . . . . . . . . . . . . . . . . . . . 91 vi Figure 1. LIST OF FIGURES Page (a) Octahedral-Tetrahedral Array of Smectites (b) Hexagonal Network of the Tetrahedral Layer 0 0 C C O C C C C O O O O O O O C I I 5 Layer Structure of Smectites Illustrating the Intracrystalline Space Which Contains the Exchangable Cations . . . . . . . . . 8 Infrared Spectra of Na+ —Hectorite, (a) After Treatment with Sodium Bisulfate to Remove Calcium Carbonate Impurity, (b) Before Treatment with Sodium Bisulfate . . . . . . 36 Schematic of Hydrogenation Apparatus . . . . 40 Reaction Flask with Adaptors for Connecting to the Hydrogenation Apparatus . . . . . . . 42 Possible Products of the Protonation of Rhodium(II) Acetate; (a) Rhodium(II) Acetate, (b) T§;§(acetato)dirhodium(II), (c) Trans- pi§(acetato)dirhodium(II), (d) gig-big- (acetato)dirhodium(II) . . . . . . . . . . . 47 1H NMR Spectrum of the Products of the Protonation of Rhodium(II) Acetate Performed in a Sealed NMR Tube . . . . . . . . . . . . 49 Visible Spectra Which Show the Progress of the Protonation of Rhodium(II) Acetate; (1) After 14 Hours, (2) After 23 Hours, (3) After 41 Hours, (4) After 112 Hours . . 53 vii Figure 10. 11. 12. 13. I4. 15. I6. 17. l8. 19. Page 1H NMR Spectra of (a) Products of the Pro- tonation of Rhodium(II) Acetate, (b) The Same Solution After the Addition of Triphenylphosphine . . . . . . . . . . 56 1P NMR Spectrum of Rh(PPh(CH3 )2 ) Been in Methanol . . . . . . . . . . . . . 60 Solution Sturucture of Rh(PPh(CH3 ) 2) 3BF“ . . 62 UV— Visible Spectrum of Rh(PPh3 ) 3BF: in Methanol . . . . . . . . . . . . . . . . . . 64 Infrared Spectra of Various Hectorites; (a) Sodium(I)-Hectorite After Treatment with Sodium Bisulfate, (b) The Same Mineral After Washing with Methanol, (c) ha (CH 3000),“ XX+- Hectorite, (d) Rh(PPh3 ); - Hectorite . . . . 68 UV- Visible Spectrum of Rh(PPh3 )x +-Hectorite. 7l Adsorption Curve of Triphenylphosphine on Na+-Hectorite . . . . . . . . . . . . . . . 75 Plot of the Hydrogenation Rate of l-Hexene . 80 Plot of the Hydrogenation Rate of Allyl Alcohol . . . . . . . . . . . . . . . . . . 83 Plot of the Hydrogenation Rate of l—Hexyne . 87 Plot of the Hydrogenation Rate of PrOpargyl Alcohol . . . . . . . . . . . . . . . . . . 9O viii I. INTRODUCTION A. Research Objective In recent years, there have been many reports which have dealt with the attachment of homogeneous catalysts to insoluble matrices as a means of combining most of the ad- vantages of homogeneous and heterogeneous catalysis. The advantages of homogeneous catalysis over heterogen- eous catalysis are:1 l. The catalysts usually exhibit higher selectivity. 2. The reactions are mechanistically easier to study, and occur under less stringent conditions. 3. All catalytic sites are accessible, therefore less catalyst is generally required. 4. The properties of the catalyst can be varied by changing the ligands: eg., the use of a chiral lig- and may lead to the formation of optically active products from prochiral substrates. The main disadvantages of homogeneous catalysis are the dif- ficulty of removing the products from the reaction mixture without loss of activity, and the ease at which the catalyst can be poisoned. With the exception of the Oxo-process, in- dustrial processes employ heterogeneous catalysts because it is generally not economically feasible to separate the homo- geneous catalyst from the products in solution. Supported homogeneous catalysts, besides combining the advantages of homogeneous and heterogeneous catalysis, give 1 2 rise to several other characteristics unique to these systems. The supported catalysts often lead to reaction selectivity based on size or polarity of the substrate. The selectivity is similiar to the effect that the surrounding protein exerts on the active site in an enzyme. Certain complexes, which are not catalytically active in solution because they undergo dimerization, may be activated when attached to a support which is sufficiently rigid to prevent the recom- bination of the monomeric units. There exists a class of layer-lattice silicate minerals generally known as smectites or "swelling clays", which are capable of bonding transition metal ions and organometallic cations in their interlamellar space. These minerals are structurely unique, and represent novel crystalline inor- ganic matrices for supports of homogeneous catalysts. Smec- ties are ubiquitous, relatively inexpensive, and usable as supports without pretreatment. The principal factor in the selection of a catalyst which is to be supported in the min- eral environment, is that the homogeneous complex must be .cationic. The objective of this research project was to inves- tigate the chemistry of a known homogeneous hydrogenation catalyst in solution and on the surfaces of the mineral. The hydrogenation reaction was chosen for study because the basic chemistry of this reaction is well investigated and there exist many known cationic transition metal complexes which function as homogeneous hydrogenation catalysts.2 The complex selected for study was a Rh(III) hydride derived from Rh2(CH3COO)u according to the method of Wilkinson and coworkers.3 I The two questions of main interest at the start of this thesis research were: 1. Will a homogeneous catalyst, when exchanged onto the silicate surfaces, retain its chemical constitution and catalytic activity? 2. Does the environment of the mineral modify the se- lectivity of the catalyst relative to its selec- tivity in solution? B. Clay Minerals The term ”clay", as used in geology and soil science, refers to any inorganic material with a particle size <2n.u However, the term "clay”, as used in this thesis, refers to the layer-lattice silicate minerals hectorite and montmoril- lonite. These minerals are composed of silicon, aluminum, oxygen, iron, alkali and alkaline earth cations. They are capable of adsorbing large amounts of water and a variety of other molecules. The adsorption process can occur in the region between the crystalline sheets and can cause one crystallo- graphic dimension to increase dramatically. These minerals are structurally similar to talc and perphyllite. The talc and pyrophyllite structures consist of two tetrahedral arrays of silica which sandwich an octahedral array of ahnnina as in pyrophyllite or of magnesia as in talc. These aluminosilicate sheets are stacked to form cry- stallites. The schematic in Figure la depicts the three- layer arrangement of the tetrahedral and octahedral holes in Figure l. (a) Octahedral—Tetrahedral Array of Smectites (b) Hexagonal Network of the Tetrahedral Layer (Adapted from reference 4) 6 the silicate sheets. The idealized structure has a silicon atom in the center of each tetrahedron and all the tips of the tetrahedra point toward the central octahedral layer. The base of the tetrahedrons are arranged to form an infinite hexagonal network (93,, Figure lb). The octahedral layer is formed from shared oxygen atoms of the tetrahedral layers and an occasional hydroxyl group, which is needed to balance the charge. The thickness of the three-layer sheet is 9.6A. When all the tetrahedral holes are filled with silicon atoms and all the octahedral holes are filled with magnesium atoms, the neutral unit, (Mg6)[SiBJOZO(OH)4’ represents the unit cell formula of talc, a trioctahedral mineral. If only two-thirds of the octahedral positions are occupied by aluminum atoms, the neutral dioctahedral unit pyrophyllite, (A14)[Si81020(0H)4’ is obtained. These minerals are held together by van der Waals forces, which give rise to the characteristic flakes of these minerals. Since these flakes are relatively large, greater than 2n, these minerals are classified as micas. Pyrophyllite and talc represent the parent members of the three-layer smectite minerals. The smectites differ from the micas by substitutions within the tetrahedral and octa- hedral layers. Cations of similar size but of lower charge are substituted into the aluminosilicate layers, which give rise to a net negative charge within these layers. This charge is balanced by cations of variable solvation which are located in the region between the anionic sheets (g§., Figure 2). In the naturally occurring minerals, these Figure 2. Layer Structure of Smectites Illustrating the Intracrystalline Space Which Contains the Exchangable Cations (Adapted from reference 4) 0 Oxygen @ Hydroxyls 0 Aluminum, magnesium ° and 0 Silicon, occasionally aluminum 9 interlayer ions are either sodium (Na+) or calcium (Ca2+). They can easily be replaced by transition metal cations or organic cations, which can dramaticly change the swelling and adsorption properties of the mineral. Several factors have been pointed out by van Olphen5 to have an effect on swelling: the solvation energy of the cations, the energy of adsorption of the solvent onto the surfaces, and the electrostatic energy between the charged species. Solvent polarity plays a role in determining whether cation solvation energy or surface solvation is the predominate force. The structure of hectorite is derived from that of talc. By an isomorphous substitution of Li+ for some Mg2+ in the octahedral positions, the idealized unit cell formula of hectorite, Na0.66(Mg5.34Lio.66)[SiBJOZO(OH)4’ is obtained. Montmorillonite has a similar substituion in the octahedral layer where Mg2+ replaces some Al3+ ions. The idealized unit cell formula of a typical montmorillonite is Na0.66(A13.34Mg0.66)[5i83020(on)4. Both of these minerals are capable of swelling to a large extent in the presence of water. Iron and other trace metals which occupy tetrahedral or octahedral sites in the silicate sheets are omitted from the idealized formulas. Vermiculite, a mineral related to the smectite class, has a larger negative charge in the silicate sheets. In contrast to hectorite and montmorillonite, the charge defi- ciency arises from the isomorphous substitution of Al3+ for some Si4+ in the tetrahedral layers. The result of these 10 changes is vermiculites have a reduced swelling capability. Under optimum hydration conditions, the mineral will swell to allow two monolayers of water in the intracrystalline space, whereas hectorite and montmorillonite will swell to accommodate multiple monolayers of water. In a water slurry, hectorite and montmorillonite will swell to approximately 100A so that under these conditions the sheets are totally separated. For each monolayer water adsorbed in the inter- layer, the basal spacing (OOl) increases by approximately 2.5A. This interlayer thickness remains constant at inter- grals of 2.5K over a large range of partial pressures of water, then increases abruptly. This stepwise change in interlayer thickness with increasing water uptake indicates that the intercrystalline water is well structured before the monolayer is complete. In addition to the interlamellar cations, exchangeable cations are located on edge sites, which arise from bond breakage along the edges of the mineral. Hydroxyl groups are attached to the silicons of the broken tetrahedral units to form Si-OH groups. These Si-OH groups are ionized sim- ilarly to silicic acid: Si-OH + OH. = 51-0" + H20 (1) which causes a negative charge on the lattice at high pH. The negative charge decreases as the pH is lowered. The edge sites can account for approximately 20% of the total cation exchange capacity (CEC) when the particle size is <2p. Clay minerals have been employed as catalysts for many 11 years. They were used by the petroleum industry as some of the first cracking catalysts. The cracking reactions were catalyzed by Lewis and Bronsted acid sites on the surfaces of the minerals. The early work in clay mineral catalysis has been reviewed by Grandquist. Recently, the clay minerals have been used as catalysts for other types of reactions. Fenn, gt gl.7 have reported the dimerization of anisole to 4,4'-dimethoxybiphenyl on Cu2+-hectorite over P205. The formation of porphyrins and porphyrin intermediates on various transition metal ion mont- morillonites has recently been reported.8 Also, Welty and Pinnavaia9 have reported the hydrogenation of l-hexene by using a rhodium exchanged hectorite. In all these cases, the catalytic reaction occurred in the interlamellar space of the mineral, and the metal ion was involed in the cat- alytic process. C. The Chemistry 2; Rh2(CH3000)” Although there exists a vast quantity of published re- search on 002+ (d7) chemistry, there is comparably little work published on its congener, Rh2+. Prior to 1962, no complexes of Rh(II) were known, but Rh(II) was postulated to exist as an intermediate in reactions of Rh(III). Chernyaev, fl al.10 were the first to synthesize a Rh(II) Species, IU12(CH3COO)4-2H20. This complex was investigated by X-ray Crystallographyll and was found to be isostructural to COp;per(II) acetate and chromium(II) acetate. These complexes Cofirtain. a metal-metal bond and four bridging acetate ligands. 12 The sixth coordination site is occupied by a solvent mole— cule, which can readily be displaced by another donor mole- cule. The sixth coordination position can also be vacant by removing the solvent molecule with vacuum and heat treatment. The earliest work in Rh(II) chemistry consisted of studying the properties of the different adducts of rhodium(II) ace- tate.12 The acetate ligands can be replaced by a variety of re- lated carboxylate ligands, eg., CF3C00-, CClBCOO_, and céHscoo', simply by allowing Rh2(CHBCOO)4 to react with an excess of the requisite carboxylic acid. By this method, 13 Cotton, gt 3;. were able to prepare Rh2(C6HSCOO)n which previously was only able to be prepared as a triphenylphos- phine adduct, Rh2(06HSCOO)#°2PPh3.3 A kinetics study of the exchange reaction between various carboxylate ligands and acetate showed that the first acetate bridge was difficult to remove, but once the first ligand was replaced, the second bridge was removed easily. The remaining two ligands were more difficult to remove than the first.14 Rhodium(II) acetate is an active catalyst for the hy- drogenation of terminal olefins in a variety of polar organic solvents.15’l6 This complex also catalyzes the hydrogenation of alkynes, but the rate of reduction is slower than for ol— efins. The catalytic reaction is known to occur at only one rhodium atom, and involves the heterolytic splitting of hy- drogen to form a monohydride. This hydride then reacts with the olefin. The catalyst is not poisoned by oxygen, instead it reduces 02 to water. 13 Interest in Rh(II) increased when Maspero and Taubel7 reported the simple dimeric complex, Rh2(H20)104+, in which bridging ligands were absent. They were able to reduce 2 Rh(III) with Cr + by an inner sphere one electron transfer: 2+ 2+ 4+ 2+ (2) 2Rh(H20)5Cl = Rh2(H20)lo + ZCrCl This aquo complex was stable under nitrogen at room temper- + ZCr ature. Ion exchange chromatography and conductometric ti- tration studies verified the presence of a +4 charge. No anion was found to precipitate the complex, and a salt has not been isolated. Ziolkowski and Taube.18 have pointed out that Rh2u+ in 2+ dilute solution was obtained by the reduction of RhCl with Cr2+ as long as the starting solution has a Cl':Rh 51: otherwise rhodium metal was formed. Also, ha4+ is obtained without formation of rhodium metal in a reaction between Rh(NH3)5012+ and Cr2+ in the presence of EDTA. The electron transfer reaction proceeded by an inner-sphere mechanism through a 01- of H20 bridge. After the transfer of an elec- tron, the Rh(H20)52+ moiety then dimerized to form ha4+ The oxidation of Rh2u+ to Rh(III) was discussed by (aq)' Ziolkowski.19 In the presence of dioxygen, a violet product, [Rh2(H20)1002]u+, was obtained. The author suggested that this complex contains a peroxo bridge between the rhodium atoms. When ha4+ was allowed to react with NO, a mono- meric complex, [Rh(H20)5N0]2+ was formed. Rhodium(II) acetate, which is a d7 complex with a metal-metal bond, is expected to be diamagnetic. However, l4 Rh2u+(aq) was found by Maspero and Taubel7 to be slightly paramagnetic. The paramagnetism was attributed to an equi- librium between the dimer and the monomer: Rh24+ = 2Rh2+ (3) The paramagnetism could be explained by 6-10% deviation of the dimer. Belova and Dergacheva20 have measured the mag- netic susceptibility of anhydrous rhodium(II) acetate and other acetato complexes of Rh(II). They found the susep- tibility to be constant, “eff = 0.52 t 0.05 BM, over the temperature range studied. It was concluded that the slight paramagentism was due to temperature-independant paramag- netism (TIP). This form of paramagnetism arises from coup- ling of the ground state with excited states of the complex when the complex is placed in a magnetic field. Taube has retracted his interpretation of the susceptibility of Rh24+, and now attributes the observed paramagnetism to TIP, not the equilibrium between the dimer and the'monomer.21 Legzdins, gt gt.3’22 reported the synthesis of ha4+ prepared in methanol by the protonation of Rh2(CH3000)4 with a strong non-complexing acid, gg., HBFu or CFBSOBH' Rh2(CH3C00)u-—Efi§%fi9»Rh24+ + CH3COOH + CHBCOOCH3 (4) They also were unable to isolate a stable salt. The pro- tonated species was reported to be stable in air. The com- plex was characterized by uv—visible spectroscopy and by the analysis of the uncoordinated acetate, as acetic acid and methyl acetate, which indicated that all four acetates were removed by the protonation. The electronic spectrum was 15 similar, but not identical, to the spectrum of ha l7 (aq) reported by Maspero and Taube. The protonated species could be reconverted to Rh2(CH3C00)# by the addition of ex- 23 cess sodium acetate. More recently, Wilson and Taube have reported that depending on the length of treatment, only one or two acetate ligands were removed by protonation. They were unable to reproduce the results of Legzdins, gt gt.3’22 This discrepancy will be discussed in greater detail later in this thesis. The electronic spectra of all the dimeric Rh(II) species are remarkably similar: they all exhibit two bands in the visible region, a low energy band in the range of 590- 650 nm, and a higher energy band between 400—450 nm. Two groups have studied the effects of different ligands on the visible absorption spectrum.24’25 Each group has derived a different M0 diagram, but their basic conclusions are similar. The high energy band (band II) is insensitive to changes in the axial ligands, whereas the low energy band (band I) is sensitive to changes in the axial ligands. The pronounced sensitivity of band I to changes in the axial ligands indicates that the transition involves the d 2 or- bital on rhodium. Most likely this orbital is the oie in which the transition terminates. Band II, however, is sensitive to the type of ligand coordinated in the equatorial plane. When the equatorial ligands are capable of biden- tate coordination, band II is approximately 445 nm, where— as with a ligand capable of only monodentate coordination, 16 the value dropped to approximately 405nm. 3 The Rh24+ species prepared by Legzdins, gt gt. is not an active hydrogenation catalyst in solution, though initial 9 have shown that Rh2u+ is an studies by Welty and Pinnavaia active hydrogenation catalyst when exchanged onto the sur- faces of HI-hectorite. The methanol solution of ha4+ changes color from green to orange upon the addition of a saturated methanol solution of triphenylphOphine. After some time, an orange solid pre— cipitates from solution. This solid gives a chemical anal- ysis which indicates that the complex has an empirical form- ula of Rh(PPh3)3BF4. Conductivity measurements in nitro- methane indicate that the complex is less than a 1:1 elec- tolyte. This study indicates that the complex exists as an associated ion pair with a fluorine from the BF4- ion coor— dinated to the metal. When less concentrated solutions are prepared, Rh(PPh3)3BF4 does not precipitate. Upon the ad- dition of hydrogen, the phosphine complex solution changes color from orange to light yellow, which indicates that an oxidative addition of H2 has occurred to form a hydride com- plex. No high field resonance for the metal hydride has been observed in the 1H nmr spectrum. Treatment of the orange Rh(PPh3)3BF4 solution with C0 yields a complex, Rh(CO)(PPh3)3+BF4', which is a 1:1 electrolyte in nitrometh— ane. Rh(PPh3)3BF4 is quantitatively converted to RhCl(PPh3)3 upon the addition of LiCl to the orange solution. The 17 reaction of Rh(PPh3)3BFu with an excess of LiX is a general method of preparing RhX(PPh3)3. The complex obtained from the reaction of Rh(PPh3)BFu and an excess of Li(CH3000), Rh(CH COO)(PPh3)3, is a catalyst for the hydrogenation of 3 alkenes and alkynes.2 The maximum rate of hydrogenation in the RhJH-PPh3 system is obtained when the PPhijh ratio is equal to two. The catalyst is poisoned by 02 and 00. Upon the addition of 4+ PPh3 to ha heterogeneous hydrogenation catalyst is obtained. exchanged onto a cation exchange resin, a The RhZLHZPPh3 system at a PPh:Rh ratio of 2 is also a catalyst for the hydroformylation of terminal olefins with a 1:1 mixture of H2 and CO at 450 torr and 35°C. This same system is an effective catalyst for the carbonylation of methanol to acetic acid at 100°C and 25 atm of CO.3 Recently Wilson and Taube21 have reported the synthesis of two Rh(II) dimers related to rhodium(II) acetate. These complexes, Rh2(003)42- and Rh2(804)42- were synthesized during attempts to prepare solutions of Rh24+(aq) in high concentrations. The treatment of Rh2(003)42- with a strong non-complexing acid yielded a cationic species, Rh2(HC03)22+, but no Rh24+was obtained. This species resembled the bis-u-acetatodirhodium cation the authors found by proton- ating Rh2(CHBCOO)u under the experimental conditions re- ported by Legzdins, gt gt.3 18 D. Previous Studies gt Supported Homogeneous Catalysts Most of the advantages of homogeneous and heterogeneous catalysis are combined by attaching homogeneous catalysts to inorganic (tg., silica gel, clay minerals) and organic (tg., polymers) matrices. Many research groups have become in- volved in the field of supported homogeneous catalysis in the hope of developing catalysts which might replace the tra- ditional heterogeneous catalysts for industrial processes, and in the hope of gaining a better insight into the reaction mechanisms of catalysis. The general means of preparing a supported homogeneous catalyst involves allowing the soluble catalyst to react with a functionalized support. The functional group is generally chemically similar to a ligand in the coordination sphere of the catalytically active metal atom. The re- actions investigated to date involve a variety of hydrocar- bon conversion reactions, which include hydrogenation, iso- merization, hydroformylation, hydrosilation, acetoxylation, polymerization, and oligomerization. The initial work in the field of supported homogeneous catalysis was reported in 1966, though there is some patent literature on the subject which dates back to 1940.27'28 Acres, gt gt.29 reported the impregnation of RhCl3 in eth- ylene glycol into the pores of silica gel. This complex was active for the isomerization of l-pentene. This pro- cedure of preparing supported catalysts is the same as the one used to prepare liquid phase gas chromatography columns. l9 Rony30 has reported the formation of a supported liquid phase catalyst (SLPC) for the hydroformylation of propylene on silica gel prepared with RhCl(CO)(PPh3)2 dissolved in butyl benzyl phthalate. The procedure for preparing the SLPC in both of these systems was to slurry the insoluble matrix in a volatile solvent and then add the homogeneous catalyst dissolved in sufficient nonvolatile solvent to just fill the pore volume of the solid support. Rony and Roth31 recently have published a further investigation of SLPC systems in which they have expanded the types of catalysts supported by this method to include hydrogenation and isomerization cat— alysts. They also have reported that these catalysts may be impregnated into the support without the use of a nonvolatile solvent. This type of supported catalyst was refered to as a solid supported catalyst. For the hydroformylation of pr0py1ene and the isomerization of l-butene, the SLPC was a more active catalyst when compared to the solid supported catalyst, but the Opposite was true for the hydrogenation catalyst. No explanation was given for these results except that for some reactions a solvent must be present in order to obtain maximum activity. 32 Manassen was one of the first to publish a report on binding transition metal catalysts to ion-exchange resins. By binding anionic and cationic metal salts or organometallic complexes of Group VIII metals to these resins, the author was able to examine catalytic carbonylation, hydroformylation, hydrogenation, isomerization, and transesterification. 20 The most studied form of heterogenizing a homogeneous catalyst has been to attach the soluble catalyst to an or- ganic polymer, particularly polystyrene cross-linked with divinylbenzene. Michalska and Webster,33 and Pittman and Evans34 have _thoroughly reviewed the literature of polymer supported catalysts through 1973. These articles also include some studies in which silica was used as a support for homogen— eous catalysts. In as much as this thesis deals with a rhodium hydrogenation catalyst, further discussion of other types of catalysts will not be included in this review un- less information concerning these catalysts is pertient to this study or a novel approach to the problem of supported homogeneous catalysis is presented. Much of the work pre- sented in the review articles is similar to the earliest work on this field except that a variety of catalysts and polymeric supports have been investigated. Grubbs and Kr01135 reported the attachment of a tran- sition metal catalyst, RhCl(PPh to a 2% cross—linked 3)3. polystyrene, which had been functionalized with a phosphine group. CH OH OCH C1 LiPPh P]-@ 3 2 2% P]—@-Cl——?> P]-<§>-CH2—PPh2 (5) SnClu The soluble catalyst was allowed to equilibrate with the polymer for 2-4 weeks. This supported catalyst was active for the hydrogenation of olefins, and the rate of reduction depended on the molecular size of the olefin. 21 Grubbs, 2: al.36 reported that as the polarity of the solvent increased, the catalyst became more selective for the re- duction of polar substrates over nonpolar olefins. The ab- solute activity of this catalyst decreased by a factor of 0.06 when attached to the polymeric support as compared to the homogeneous analog. Collman, gt gt.37 have prepared a slightly different linkage for attaching the catalyst to the polymer. Br n-BuLi PClPh P]-<:).___§; P]-<:>—Br —————9 P]-<:>-Li —————3§Efl-<§>-Pph2 FGBI'B THF (6) This likage is less basic that the one used by Grubbs and Kroll.35 When MCl(CO)(PPh (M = Rh, Ir) was attached to 3’2 the polymer, the authors showed that both phosphines in the complex were replaced by polymer attached phosphines. The polymer was mobile enough to bring two phosphines to close proximity of the metal for coordination. For a catalytic reaction to occur, a vacant coordination site on the metal atom must be made available for the bonding of the substrate. There exist many monomeric complexes which could exhibit catalytic activity, but in solution the monomers dimerize. By attaching a monomeric complex to a rigid polymer, the combination of the monomers might be pre- vented. By employing this method, Grubbs, Brubaker and oo- 38 workers were able to prepare a 20% cross-linked polysty— rene supported monomeric titanocene dichloride. This species was reduced by n-butyl lithium to yield supported monomeric titanocene, which normally will dimerize in 22 solution. This supported catalyst was six times more active for the hydrogenation of cyclohexene than the titanocene dimer in solution. Dumont, gt al.39 reported the preparation of a supported chiral rhodium complex, similar to the soluble Rh(I)-diop complexuo (diop = 2,3-O-isopropylidene-2,3-dihydroxy-l,4- bis(diphenylphosphino)butane) which is an asymmetric hydro- genation catalyst. The supported catalyst is much less active and gives lower optical yields than the homogeneous catalyst, but both the insoluble and soluble catlysts are equally efficient for the asymmetric hydrosilation of ke- tones. Bruner and BailarLLl have reported the synthesis of Pd(II) and Pt(II) dichloride complexes with polymeric di- phenylbenzylphosphine as a ligand in the complex. These complexes are capable of reducing soybean methyl ester pri- marily to the monoene in nonpolar solvents. The initial composition of soybean methyl ester is 14.2% saturate, 22.3% monoene, 56.2% diene, and 7.0% triene. When methanol is used as the solvent, more reduction to the saturate occurs than when the reaction is carried out in nonpolar solvents. A similarstudy, hydrogenation of polyenes to mono— enes, has been published in which RuC12(CO)2(PPh3)2 was used as the catalyst.)+2 Pittman, gt gt.43 have been able to attach this catalyst to a polymer. By using this supported catalyst, they obtained the same product yields and distri- butions that were obtained with the soluble catalyst. 23 In the same paper, Pittman, gt gt., reported that sup— ported Ni(C0)2(PPh3)2 was an effective cyclooligomerization catalyst, and that supported RhH(C0)(PPh3)3 was an effective hydrogenation catalyst. Also, the supported hydrogenation catalyst, the anchored analog of RhCl(PPh3)3, was examined in comparison to the soluble catalyst. When a finely di— vided (37-74p) 1% cross—linked polystyrene was used as a sup- port, the rate of hydrogenation was 0.8 times as fast as / RhCl(PPh3)3 under 24 atm of H and at 50°C. Grubbs, gt _;.3° 2 reported that the rate of hydrogenation with the insoluble catalyst was 0.06 times as fast as the homogeneous catalyst. An explanation for the difference between these two studies was that Grubbs used a less finely divided (74-149u) 2% cross- linked polymer, and and the reaction was performed under less stringent conditions (1 atm and 25°C). When Pittman carried out the catalytic reaction with the 1% cross—linked polystyrene at 25°C, the rate was 0.5 times as fast as with the homogeneous catalyst. Pittman and Smith#4 have reported the attachment of two catalytically active complexes to the same polymer, so that sequential multistep reactions can be catalyzed in the same system. They were able to anchor Ni(CO)2(PPh a cyclo— 3)2. oligomerization catalyst, and either RhCl(PPh3)3, a hydro- genation catalyst, RhH(CO)(PPh3)3, a hydroformylation cat— alyst, or RuClZ(CO)2(PPh3)2, a selective hydrogenation cat- alyst (ztgg ggptg), to the same polymer. With this system, the first step was to oligomerize butadiene to 24 4-vinylcyclohexene, l,5—cyclooctadiene, and 1,5,9— cyclo- dodecatriene, and then allow these products to interact with the second supported metal complex. It was impossible to compare the results of the supported catalysts to their homo- geneous analogs because the reactions were carried out under different conditions. . 45 Pittman and co-workers have reported that at PPh3:Ir ratios <5, the heterogeneous catalyst, derived from IrCl(CO)(PPh3)2, reduced l,5—cyclooctadiene to cyclooctene at a rate faster than the corresponding homogeneous catalyst. At higher PPh3:Ir ratios, the rates of hydrogenation for the homogeneous and heterogeneous catalysts were comparable. The supported catalyst also was capable of selectively hydro- genating 4-vinylcyclohexene to 4-ethylcyclohexene.46 IMuch of the earliest work in supported catalysis was done by researchers in the labs of British Petroleum, and published only in the patent literature.33 Recently this ”7'“9 It work has been published in the open literature. was found that a supported catalyst, prepared from a reaction between phosphinated polystyrene and Rh(acac)(CO)2, was active for the hydroformylation of l-hexene, and [RhCl(CBle)]2 anchored on,an identically prepared polymer was a catalyst for the hydrogenation of l-hexene and cyclo- hexene. For the hydrogenation reaction, more forcing con- ditions (50°C and l5atm of H2) were required than those re— ported by Grubbs and Kroll.35 The authors were also able to anchor Ni(08H12)2 to a phosphinated polystyrene which was 25 an oligomerization catalyst for butadiene. Diethyl aluminum ethoxide was required as a cocatalyst for this last reaction. All of these reactions were performed in a fixed bed reaction vessel with liquid feedstocks.47 Silica gel may also be functionalized with a variety of reagents, by using the available surface Si-OH groups. Two general procedures have been reported for anchoring of the 48,49 catalyst to silica. In the first method silica was treated with (2-diphenylphosphinoethyl)triethoxysilane, which yielded a functionalized silica: PhZPCHZCHZSi - (0)3 - silica The functionalized silica was then allowed to react with the catalyst precursor. The second method involved the synthesis of the catalyst with (2-diphenylphosphinoethyl)triethoxysi- lane used in place of triphenylphosphine. The product was then allowed to react with the silica. The advantage the second method has over the first was that the phosphine:metal ratio could be controlled to a greater extent. By these methods, phosphine complexes of rhodium, iridium, ruthenium, and platinum have been chemically bonded to silica. Com- plexes of the first three metals were prepared by the second method, and a platinum complex was prepared by the first. All of these complexes were active for the hydrogenation of 48,49 olefins and dienes. No rate data were presented, so a comparison between the supported catalyst and the analogous homogeneous catalyst was not possible. 50 Boucher and co-workers have anchored Rh(I) and Co(II) 26 phosphine complexes through silane linkages to silica. The silane linkages were formed by allowing trichlorosilylal- kylphOSphines, acyloxysilylphosphines, or alkyloxysilyl- phosphines to react with the surface Si-OH groups of silica. A variety of Rh(I) chloro complexes or Co(acac)2 were then allowed to react with the functionalized silica. These heter- ogeneous complexes were catalysts for the hydroformylation of propylene. The cobalt complex was less active than the rhodium species. In comparison to RhCl(PPh3)3, the anal- ogous heterogeneous species was 1.4 times more active than the soluble catalyst. Recently, Bayer and Schurigsl have reported the for- mation of a soluble supported catalyst prepared with a non— cross—linked polystyrene in a manner similar to the one used.by Collman, gt gt.37 The catalyst and products were separated easily by molecular weight differences. By this method the authors were able to prepare catalysts analogous to RhCl(PPh3)3 and RhH(C0)(PPh3)3 with polymeric phosphine ligands. The advantage this system has over the insoluble polymer catalysts is that as in solution all the active sites are equally accessible to the reactants. Parshall52 has reported a novel method of preparing a catalyst which is easily separated from the reaction pro— ducts. By dissolving PtCl2 in low—melting tetraalkylam- monium salts of SnCl3- and GeC13-, a catalytic system was obtained that was active for the hydrogenation , isomeri- zation, hydroformylation, and carbonylation of olefins. For 27 the hydrogenation of 1,5,9—cyclododecatriene, considerable selectivity to the monoene was obtained with this catalyst. The products were separated from the catalyst by distillation or decantation. The salts were found to act as stabilizing ligands, thus preventing the formation of platinum metal. II. EXPERIMENTAL A. Reagents and Solvents Hectorite and Wyoming montmorillonite were obtained from the Baroid Division of NL Industries as the spray—dried sodium form. The hectorite contained a CaCO3 impurity which was removed by reaction with HSOu-(see section on Infrared Spectra) before use in the spectroscopic studies. The CaCO3 was not removed from the mineral prior to use in the hydro- genation studies. RhCl '3H20, with a reported Rh assay of 3 42.82%, was purchased from Engelhard Industries. Triphenyl— phosphine, 3-butyn-l-ol, 3,3-dimethyl-l-butene, l-heptene, l,5-hexadiene, 2—hexene (gtg and ttggg mixture), phenyl acetylene, and propargyl chloride were purchased from Al- drich. Allyl phenyl ether, l-hexene, 1-hexyne, 2-hexyne, and 1-octyne were purchased from Pfatz and Bauer. Cyclo— hexene, 2-methyl-3-butyn-2-ol, propargyl alcohol, and 48% fluoroboric acid were purchased from J. T. Baker: and allyl alcohol was purchased from Fisher Scientific. Dimethylphenyl- phosphine was purchased from Research Organic/Inorganic Chemical Corporation. All unsaturated substrates were dis- tilled under argon prior to use, into a graduated reservoir before being added to the reaction flask. Triphenylphos— phine was used without further purification. A 0.05M 28 29 triphenylphosphine solution in methanol was prepared for use in the catalytic studies. Methanol, used in determining the activity of the min- eral-bound catalyst, was purchased from Matheson, Coleman and Bell. The water content in the methanol was reported as 0.2%. If methanol is obtained with less than 0.1% water, no catalytic reaction is observed. Also, when methanol is stored over CaH2 and distilled before use in the catalytic reaction, no activity is observed. All methanol is deaere- ated under a stream of dry nitrogen for 30 minutes before being stored in the dry box. All exchange reactions were carried out under oxygen-free conditions in a dry box with a dry nitrogen atmosphere. In all hydrogenation reactions, oxygen was rigorously excluded. Bottled hydrogen (Airco) was passed through a BTS catalyst at 120—130°C, and then through a bed of Aqua- sorg (Mallinkrodt Chemical Works ) before being passed through the hydrogenation apparatus. Bottled argon (Airco) and nitrogen (Airco) were used without treatment. Microanalyses were performed by Galbraith Laboratories, Inc., Knoxville, Tennessee. B. Syntheses l. Methanol Adduct gt Tetrakis:geacetatodirhodium(II) The procedure for the preparation of rhodium(II) ace- 52 tate was modified from literature methods. Rhodium tri- chloride trihydrate (5.0g) and sodium acetate trihydrate (10.0g) in glacial acetic acid (100ml) and absolute ethanol 30 (100 ml) were allowed to reflux for two hours. The initially red solution became green, and a green solid precipitated. After the solution was cooled to room temperature, the solid was collected by filtration on a glass frit. The crude pro- duct was dissolved in 600 ml of boiling methanol, and fil- tered. The solution was concentrated to 400 ml and refrig— erated at 0°C overnight. The methanol adduct was collected by filtration. The overall yield was 3.62g (75% based on RhClB'3H20). The infrared spectrum of the complex was iden- tical to the spectrum of anhydrous Rh2(CHBCOO)4 reported 54 in the literature with the exception of the bands which are due to coordinated methanol (v = 2980 and 2935 cm-l). The electronic spectrum of the complex in methanol (Omax = 586 and 445 nm) was also similar to the spectrum of the reported complex in ethanol (l = 590 and 446 nm).3 2. Protonation gt Rg2(CHBCOQ)Q-ZCH 9H 3 Protonation of rhodium(II) acetate by tetrafluoroboric acid was carried out in a water-methanol solution. The pro— X+ duct, Rh2(CHBCOO)4_x (x = 1 or 2), could not be precip- itated by any known anion. The stock solution, which was used in preparing the rhodium exchanged minerals, was pre— pared by placing l.2g of Rh2(CH3000)4-20H 0H (2.4 mmoles) in 3 300 ml of methanol and adding 1.32 ml of aqueous 48% tetra— fluoroboric acid (9.7 mmoles). The mixture was heated and stirred in an oil bath at 60°C, until the green solid dis- solved (approximately 2 days). The concentration of this green solution was 0.008M. The uv-visible spectrum of the 31 x+ protonated product, Rh2(CHBCOO)4_x (Xmax = 615 and 427 nm), was similar to the spectrum for Rh24+ reported by Legzdins, gt gt.3 (Xmax = 612 and 423 nm). 3. Exchanged Hectorites Hectorite was chosen as the primary mineral to be used as a support for the catalysts investigated in this work. This mineral has a reported cation exchange capacity (CBC) of 95.5 meg/100g,55 however an independant determination of the CEO by a conductometric titration of a CuZI-saturated hectorite with NaOH in a ethanol-water solution gave a value of 70 meq/lOOg.56 This value is in agreement with the one obtained by chemical analysis (2.05% Cu) of a Cu2+—saturated hectorite. Therefore, the CEC of the mineral was taken to be 70 meq/lOOg. a. EggLCHBCOO)4_xx+-Hectorite Ten milliliters of the stock 0.002M solution of Rh2(CHBCOO)1+_xx+ was allowed to react with 0.5g of hectorite. The slurry was stirred on a medium glass frit for 1-2 min— utes, then the solution was filtered off. Upon treatment with the rhodium complex, the white mineral became emerald green in color, while the color of the solution became less intense. The clay was then washed six times with methanol to remove excess Rh2(CH3COO)4_xx+, and the mineral was dried under N2. Chemical analysis (0.76% Rh, 7.4 mmoles/lOOg) in- dicated that Rh2(CH3C00)4_xX+ occupied approximately 12% of the exhange sites in the mineral. This was the general pro— cedure used for the catalytic studies, but the amount of 32 rhodium complex exchanged onto the mineral could be increased by prolonged contact between the solution and the mineral and by using a higher concentration of the rhodium solution. For all the catalytic studies, the Rh2(CH3000)4_xX+-hec- torite was prepared within a week of use because a reduced activity was observed after the mineral had aged longer than this period. (approximately 25% of the activity was lost) An analogously exchanged mineral, Rh2(06H5C00)4_xX+— hectorite was prepared with the product of the protonation of Rh2(06H5COO)4' Rhodium(II) benzoate was prepared ac- cording to literature methods.13 b. Rh(PPh3)X+-Hectorite A 0.2g sample of Rh2(CH3000)4_xx+-hectorite was allowed to react with varying amounts of a 0.05M PPh3 solution so that the PPh3th ratio was in the range of two to fourteen. The color of the mineral immediately changed color from Similar 57 green to dark orange upon the addition of PPhB. reactions were obtained when diphenylmethylphosphine and dimethylphenylphosphine were used in place of PPh3. This type of reaction is indicative of the formation of a Rh(I) species. The maximum rate of hydrogenation was found to occur with a PPh :Rh ratio of nine. The concentration of 3 the phosphine in the reaction mixture was 4.6gM. + . x When PPh3 was allowed to react With Rh2(06H COO)4_X - 5 hectorite, a reaction similar to the one with the acetate complex was observed. 33 C. Physical Methods 1. ggg Chromatggraphy Gas phase chromatography of liquid and vapor samples was performed with a Varian Associates Model 90-P single column chromatograph with a thermal conductivity detector. The output of the detector was recorded with a Sargent Model SR recorder. Olefin—alkane and alkyne-alkene-alkane separations were achieved by use of a 5' x %" stainless steel column packed with 3% SE-30 on diatomite CLQ 100/120. The column was operated at 30°C and at a flow rate of 30 ml/min. Helium was used as the carrier gas. A 0.6 m1 vapor sample or a 1.6 pl liquid sample was in- jected into the chromatograph to obtain optimum identi- fication of the components in the sample. 2. X-Ray Diffraction Studies Basal Spacings (001) for the rhodium exchanged mineral samples were determined by x—ray diffraction with a Phillips x-ray diffractometer with Ni-filtered Cu K(G) radiation. The minerals were deposited on microscope slides from a methanol slurry and the solvent was allowed to evaporate. Self—supporting film samples were also useful for determining 001 spacings. The films were placed on microscope slides in a small plastic bag with methanol added. The bag was then heat sealed before the basal spacings were measured. This procedure allowed the 001 spacings to be measured on a fully solvated and swelled minerals, and it prevented the sample 34 from coming in contact with air. The diffractions were mon- itored through 14° of 29. The peak positions in degrees of 29 were converted to d-spacings with a standard chart. 3. UV-Visible Spectra UV-Visible spectra of solutions were obtained in 1 cm path length matching quartz cells with either an Unicam Model SP 800B spectrOphotometer or a Cary 17 spectrophoto— meter. Electronic spectra of clay samples were recorded by use of a Cary 17 spectrophotometer. The solid samples were prepared by mulling the clay samples in mineral oil and by placing the mull between two Infrasil 2 quartz disks. In the reference beam was placed a mull sample of native hec— torite in order to reduce the effects of scattering. 4. tnfrared Spectra Infrared spectra were recorded by use of a Perkin-Elmer Model 457 grating spectrophotometer. The samples were pre- pared by mulling in Fluorolube (Hooker Chemical Co.) then placing the mulls between 081 plates. Some clay samples were examined as self-supporting films. The ir spectrum of native hectorite (gt. Figure 3b) exhibited a broad peak centered at 1440 cm'1 which was assigned to a CaCO3 impurity. This im- purity was removed by treating hectorite with NaHSOu-HZO. The mineral (4.0g) was washed twice with 0.1M sodium bisulphate solutions (100 ml each). The clay was rinsed with water and the wash solution was tested with a BaCl2 solution to insure the complete removal of 8042'. The min— eral was then washed with a 1.0M NaCl solution, rinsed, and 35 Infrared Spectra of Na+-Hectorite: (a) After Treatment with Sodium Bisulfate to Remove Calcium Carbonate Impurity, (b) Before Treatment with Sodium Bisulfate. Figure 3. 36 com 4 emo— oom— EU oowp _ oom— _ oom— _ 4 P m q Ht» 37 freeze-dried. This treatment completely removed the impur- ity band (gt. Figure 3a). 5. Proton NMR Studies Proton magnetic resonance spectra were obtained by use of a Varian Associates A56/60D analytical spectrometer which operates at a frequency of 60.000 MHz. The spectra were re- corded either in the normal mode, or in the case of very dilute samples, with the aid of a Varian Associates C-1024 time averaging computer. All spectra were recorded at field strengths well below the level required to produce saturation. Chemical shifts were measured by using tetramethylsilane as an internal standard, or by using sodium 3-trimethy1 silyl- 2,2,3,3-d4—propionate in D20 (5% w/v) as an external stan- dard. 6. Phosphorus-31 NMR Studies Phosphorus-31 nmr spectra were recorded on a Brfiker HFX-lO spectrometer modified for multinuclear measurements as descibed by Traficante, gt gt.58 and equiped with a Nicolet 1083 computer with 12K of memory, a Diablo disc memory unit, and a Nicolet 293 I/O controller. The spectra were obtained at a frequency of 36.442 MHz. All samples were dissolved in methanol with 30% hexa- fluorobenzene added to serve as an internal fluorine-19 lock. The spectra were measured over 5000 Hz (137 ppm) spectral width. Optimum signal—to-noise was obtained by using between 512 and 1024 scans per spectrum. Chemical shifts were mea- sured by using a 85% H3P04 solution as an external standard. 38 D. Hydrogenattgg Studies Hydrogenation rates were measured as the rate of hydrogen consumption at constant pressure and under am- bient conditions. A schematic of the hydrogenation appa— ratus is shown in Figure 4. The reaction flask was spec- ially designed with the help of Mr. Andrew Seer, a master- glassblower in this department, to prevent the splashing of the catalyst onto the walls of the flask (gt. Figure 5). The general procedure for obtaining hydrogenation rates at 25°C was to place 0.2g of Rh2(CH3C00)4_Xx+-hectorite into the reaction flask, and then to add sufficient deareated methanol so that after the addition of the required amount of 0.05M triphenylphosphine solution, and substrate, the total volume of the reaction mixture was 30 ml. After the addition of triphenylphosphine, the flask was removed from the dry box and connected to the hydrogenation apparatus. The system, with the exception of the reaction flask, was purged with hydrogen for a minimum of 10 minutes. The mer- cury levels in both the manometer and gas measuring tube were then manipulated to insure complete purge with H2. The manometer and gas measuring tube were isolated from the rest of the system. The manifold was then evacuated and filled with H2. This cyclic process was repeated three times, and then a similar procedure was applied to the reaction flask. The flask was opened to the rest of the system along with the manometer and gas measuring tube, and H2 was bub- bled through the stirred methanol slurry for two hours to 39 Figure 4. Schematic of Hydrogenation Apparatus a) Reaction Flask b) Condensor 0) Apparatus for the Distillation of the Substrates d) Manometer e) Gas Measuring Tube and Leveling Bulb 4o ESSOM> cam «Ni 1+ E550m> Ucm NI M Figure 5. ‘41 Reaction Flask with Adaptors for Connecting to the Hydrogenation Apparatus 43 allow time for the formation of the active hydride species. Finally, the substrate was distilled and added to the re- action mixture. The rate of H2 consumption was monitored over 15-20% of the reaction for alkenes, and 40-50% of the reaction for alkynes. III. RESULTS AND DISCUSSION A. Characterization gt the Homogeneous Catalyst 1. §glution Chemistry gt flzmhjcoCm-xf‘: (x = 1.2) When Legzdins, gt gt.3'22 reported the protonation of Rh2(CH3C00)4 with a strong non-complexing acid, HBFu, the authors believed that they had obtained a totally protonated species, Rh2u+, with all four of the acetate ligands re- moved from the coordination sphere. Analysis of free ace— tate, as acetic acid and methyl acetate, by glc methods in? dicated that all acetate ligands were lost upon protonation. They also reported that the ion prepared was not air sensi- tive. Rh2(CHBCOO)4 is Similar to M02(CH C00)“ and 3 Re2(CH3000)4012, which are complexes with strong metal-metal bonds. These latter two complexes are protonated by hydro- chloric acid to give M020183- and Re2C182_. ~Bridged acetate complexes of Cu(II) and Cr(II), when protonated with strong acid, give monomeric species. This behavior is indicitive ofa weak metal-metal bond in the complex. The stability of copper(II) acetate and chromium(II) acetate is due solely to the four bridging acetate ligands. 17 4+ Maspero and Taube also reported the formation of ha , 2+ prepared from the reduction of Rh(III) by Cr The elec- tronic spectrum of this ion differed from the one reported 44. 45 by Legzdins, gt gt.3 This ion was very air-sensitive. With both preparations, no counterions could be found to pre- cipitate a simple salt. 23 Wilson and Taube repeated the experiment of Legzdins, gt gt.3 and found that only one or two acetate ligands was removed from the coordination sphere by protonation. Their conclusions are based on ion-exchange chromatography. They were able to separate a species with a +1 charge, and another with a +2 charge. Figure 6 shows the probable products which were isolated by this experiment. In order to obtain more information on the products of the protonation of rhodium(II) acetate, the 1H nmr spec- trum of the reaction products was recorded. A 0.01M solution of Rh2(CH3000)4_xx+ was prepared in a manner identical to the procedure used in the catalytic studies. The pmr spec- trum (gt. Figure 7) of this dilute solution was recorded with the aid of a time averaging computer. The peak at 1.99 ppm has been assigned to the methyl protons of the acetate moiety in methyl acetate. Methyl acetate is the only form of uncoordinated acetate found in the product mixture. The equilibrium between acetic acid and methyl acetate in the acid catalyzed esterification of acetic acid is shifted com- pletely to the ester since there is a large excess of meth— anol present in the reaction mixture. + H A The methyl resonance for the ester group was observed at 3.61 ppm. The pmr spectrum of acetic acid (15 p1) in Figure 6. 46 Possible Products of the Protonation of Rhodium(II) Acetate: (a) Rhodium(II) Acetate, (b) Tris(acetato)dirhodium(II), (c) Trans-bis- (acetato)dirhodium(II), (d) Cis—bis(acetato)- dirhodium(II) 48 Figure 7. 1H NMR Spectrum of the Products of the Protonation of Rhodium(II) Acetate Performed in a Sealed NMR Tube 49 1.7 1.8 1.9 2.0 ppm 50 methanol (1 ml), with 5 pl of 48% aqueous HBFQ, exhibits two peaks of equal intensity, at 1.99 ppm and 3.61 ppm. These values agree with the reported values of 2.00 and 3.65 ppm for methyl acetate.59 When the protonation reaction is per- formed in 48% aqueous HBFu, with no methanol present, acetic acid is the only uncoordinated acetate product ob- served by pmr. In Figure 7, the other three resonances in the acetate region, with approximate relative intesities of 2:1:1, are assigned to the remaining coordinated acetate ligands. Planimetric integration of the peaks observed from the protonation reaction showed that the ratio of coordinated to uncoordinated acetate is 2.2:1. This value indicates that a mixture of tttg- and gtg-(acetato)dirhodium(II) exists in solution with the tttg-complex being the predom- inant species in the product mixture (3 tttg-:1 gtg-). If only the tttg-complex were present the ratio of the coordi- nated to uncoordinated acetate would be 3:1, whereas if the only the gtg-complex were formed, the ratio would be 2:2. The uv-visible spectrum for the protonated product mixture, along with the previously reported spectra for re- lated Rh(II) species are presented in Table 1. In the vis- ible region of the spectrum, all the species exhibit two ab- sorptions: band I at approximately 600 nm, and band II in the region of 400 to 450 nm. Band I is particularly sensitive to changes in the axial ligands, while band II is insensitive to changes in these ligands.24 Band II, however, seems to be sensitive to the presence of bridging ligands in the 51 coordination Sphere of the complex. Band I also is sensi- tive to the number of bridging acetate ligands. The values for the rhodium(II) acetate complexes range from 586 nm for the complex with four coordinated acetate ligands, to 648 nm for a complex without bridging acetate ligands. A batho- chromic shift in band I was observed as the protonation re- action proceeded. ‘At the end of the reaction the value for this band was intermediate to that reported for Rh2(CHBCOO)3+ and Rh2(CH3000)22+ (gt. Figure 8 and Table 1). Again it is resonable to postulate that both Rh2(CHBCOO)3+ and Rh2(CH3000)22+ exist in the product mixture. The nmr and electronic spectra show indeed that acetate ligands remain in the coordination sphere of the dirhodium(II) moiety after protonation of Rh2(CH3C00)4, as has been re- ported by Wilson and Taube.23 2. Solution Chemistry gt Rhipph3l3§34 The interaction of the green-colored Rh2(CH3C00)u__xx+ solution at a concentration of 8x10-3M with triphenylphos— phine yielded an orange solution which in the presence of hydrogen was an active catalyst for the hydrogenation of alkenes and alkynes, hydroformylation of alkenes, and car- bonylation of methanol to acetic acid and methyl acetate. The proton nmr spectra of the reactant solution and the pro- duct solution are shown in Figure 9. Spectrum A is similar to the spectrum presented in Figure 7, and B is the spectrum of the solution after the addition of PPh . Triphenylphos- 3 phine was added so that the PPh3:Rh ratio was equal to 3.5:1. Figure 8. 52 Visible Spectra Which Show the Progress of the Protonation of Rhodium(II) Acetate: (1) After 14 Hours, (2) After 23 Hours, (3) After 41 Hours, (4) After 112 Hours 53 CON. A85 A com m 1 000 \J/ u 00v 54 mm oesosommm As each cacomHSm m oesopomom Ao -HaseoanosHuane u memes as spasm mate as eossoesoa sac; As ma occohomom Am mowpa>flpmuompm smaoe ohm nonesssoumm QM mosam> AN ONmIHossseos ca name an: mom 9 +xx-:Aooommcostm Noses an omN Amaav NH: exomv one N see: as omN ASNHV No: exmmv mac +e rm omznaocmspoe CH :mmm mmm Amov mm: Ammv mam omowoomw .mchNwoq ease: as CNN .smN .scm lass MN: lass was e +NNAeecmmchss soaom ess omzaem ca mHN .omN .smomm mN: can t +mxooommostm Hocmnpoa as we: 90mm Host: o :H . m3: omwm 3 m m o m as NHN omN Acoav one oAHst mmm Aooo mov rm ON:IHossseos an ear: mmN .smomm AHNHV mNs Aamsv mac n +xwwsxooommostm mcowpwpcoo mpsmm sonpo mHH scam NH scam woaoogm memo oaseososeooam ospame>->: .H cease Figure 9. 1H NMR Spectra of (a) Products of the Protonation of Rhodium(II) Acetate, (b) The Same Solution After the Addition of Triphenylphosphine 56 I I I - I 2.0 L9 |.8 |.7 ppm 57 Spectrum B shows that upon the addition of the phosphine, all the resonances which are assigned to coordinated acetate disappear while the resonance which is assigned to methyl acetate increases. In order to obtain a better understanding of the rhodium— 31 phosphine interaction, a P nmr investigation was under- taken. The standard Rh2(CHBCOO)4_xx+ solution (8xlo'3m) was concentrated by rotoevaporation to 8x10-2M so that the phos- phorus signal would be observable with a resonable number of transients. When triphenylphosphine, tri-(p—tolyl)phosphine, or di- phenylmethylphosphine was added to the concentrated rhodium(II) solution so that the P:Rh ratio was equal to 3.5:1. an immediate precipitation of orange crystals oc— curred. A P:Rh ratio of 3.5 instead of 3 was used because it was likely that the phosphine was acting as the reducing agent for the reaction. Triphenylphosphine is known to be X+ the reducing agent in the reduction of Rh2(CH3COO)4_x to Rh(CH3C00)(PPh3)3 in the presence of excess lithium acetate 26 and triphenylphosphine. The phosphine is oxidized to the corresponding phosphine oxide. The precipitate from the + and dimethylphosphine was ana— . x reaction of Rh2(CHBCOO)4_x 1yzed and it was found to have the empirical formula, Rh(PPhZCH3)3BF4, which is similar to the related triphenyl— phosphine complex.3 When dimethylphenylphosphine was the phosphine used in the reaction, no precipitate was observed at a P:Rh ratio of 3.5. It was assumed that the species 58 formed was similar to the related PPh3 and PPh CH complexes. 2 3 The proton decoupled 31P nmr spectrum obtained from the . x product of the reaction between PPh(CH3)2 and Rh2(CH3C00)4_X is shown in Figure 10. It consists of a doublet at +5.9 ppm and a doublet of doublets centered at -5.2 ppm, and a sin- glet at -39.2 ppm. The latter line is assigned to the phos- phine oxide. The splitting of coordinated ligand resonances due to P-P coupling was not observed because the solution could not be cooled without precipitation of the complex. The larger splitting is due to lOBRh-31 P coupling, and the smaller splitting (26 Hz) arises from an unknown source. One possibility for the smaller splitting is coupling of the phosphorus in the tgggg position (of. Figure 11) to fluorine in the tetrafluoroborate ion. Previouly, an ion pair has been suggested to exist between the anion and the metal complex.3 However, the 19F nmr spectrum exhibits only a singlet for the BFu- ion. A doublet with a splitting of 26 Hz would be expected if the fluorine were coupled to a phosphorus. The resonance in the 19F nmr spectrum of the BFu- is shifted downfield by 2 ppm as compared to the signal due to NaBFu in methanol (Hexafluorobenzene was used as an internal standard.). A downfield shift would be expected if the tetrafluoroborate ion were associated with the rhodium complex. The absence of 103Rh-l9F coupling and 31P—19F coupling for BF4- suggest that the lifetime of the ion in the coordination shere of the metal must be short. The nmr data are compiled in Table 2. + 59 Figure 10. 31p NMR Spectrum of Rh(PPh(CH ))BF in Methanol 3 2 3 4 60 F _ qNI OOm. 61 Table 2. 31F and 19F NMR Data Compound 6 (ppm)a JRh-P (Hz) b c Rh(PPh(CH ) ) BF +5.9 d 135 3 2 3 4 -5.2 dd 84 0=PPh(CH3)2 -39.2 I PPh(CH3)2 +46.8 Rh(PPh(CH ) ) Cl d +5.5 112 3 2 3 3 we so Rh(PPh(CH3)2)3BF4e +152 NaBFu in methanol +154 Parts per million for 31F nmr were measured from 85% H3P04, and for fluorine-19 nmr from CFCl Phosphorus-31 data d = doublet, dd = doublet of doublets 3. Data for this complex were taken from reference 60. Fluorine-19 data 62 The nmr data are consistent with the solution spectrum shown in Figure 11. Rh X=BF4,CH30H Figure 11. Solution structure of Rh(PPh(CH3)2)3BF4 . Another source of the splitting of the signal for the unique phosphorus (Pb) in the 31P nmr could be that two dif- ferent RhL3X+ complexes exist in solution. The signals due to the phosphines (Pa) ttggg to each other in each of the complexes would have to be coincidental. The possibility of two complexes in solution seems unlikely, since the ratio of relative intensities in the phosphorus-31 spectrum for Pa and Pb is 2:1, and the line width of the signal assigned to Pa is narrow (1.5 Hz). In tgtg phosphine rhodium complexes, the coupling con- stant, JRh-P has been shown to be always larger for phos- phorus ttggg to each other than for phosphorus ligands gtg to the other phosphorus atoms and ttggg to a halogen.60’6l The uv-visible spectrum of Rh(PPh3)3BF4 is shown in Figure 12. The spectrum is dominated by a intense high energy band which consists of several peaks in the region of 250-275 nm. These peaks arise from transitions in tri- phenylphosphine.62 The spectrum also exhibits two inflections at 316 and 363 nm on the steeply rising high energy band. Figure 12. UV-Visible Spectrum of Rh(PPh3)33FLL in methanol 64 000 00m 65 B. Characterizattgg gt the Heterogeneous Catalyst It was found that Rh2(CH3C00)4_XX+ could be introduced into the exchange sites of Na+-hectorite by the treatment of the methanol-wet mineral with a methanol solution of Rh2(CH3C00)4_xX+ in the absence of air. The amount of the rhodium species exchanged onto the mineral could be con~ trolled by varying the concentration of the rhodium solution (gt. Table 3). Table 3. Rhodium Loading on Hectorite Initial Concentration Number of 07 of Rh2(CHBCOO)4_XX+ Washesa g °f Clay wtm Rh 8.0XlO-3m 3 0.5 1.51 8.0XlO-3M l 0.5 1.20 2.0X10-3_M_ l 0.5 0.7513 a) The methanol-wet mineral was washed with 10 m1 of rhodium solution for 1-2 minutes, the solution filtered off and the mineral rinsed six times with methanol. b) Average of four analyses (0 = £0.03) For the catalytic studies, one equivalent of native hectorite was slurried with 0.11 equivalents of the proton- ated rhodium species (2.0x10-3M) under oxygen-free conditions. The partially exchanged mineral had a rhodium content which corresponds to 12% of the CEC. Upon treatment with the green-colored Rh2(CHBCOO)u_XX+ 66 solution, the white mineral immediately changes color to green. When the 2.0x10_3M solution was used in the exchange reaction, most of the color in the solution was lost. The infrared spectra of two rhodium exchanged hector- ites are shown in Figure 13. Spectra A and B are shown for comparison. A is the spectrum of air-dried native hectorite, which has been treated with H304”. The clay was prepared as a self—supported film. After treatment with methanol, native hectorite exhibits a spectrum as shown in B. The spectrum of the emerald green mineral (gt. Figure 13C) ex— 1 hibits two peaks at 1575 and 1453 cm- which are assigned to the asymmetric and symmetric stretches of the carboxy— late group of the bidentate acetate 1igands.54’63 The Rh2(CH3C00)u_Xx+ ion on the mineral exhibits an intense band at 257 nm in the electronic spectrum. A similar band at 258 nm (e = 3.2x103) is observed in the solution spectrum. The bands in the visible region at 615 nm (e = 131) and 427 nm (S = 131) are not ovserved in the mineral bound species due to their low molar absorptivites and scattering effects of the mull sample. The infrared and electronic spectra indicate that the homogeneous cationic complex and the mineral bound species are similar. + and bio- An exchange reaction between Rh2(CH3C00)4_xx tite, attapulgite, talc, pyrophyllite, or kaolinite, minerals whose exhange capacity arises predominately from edge sites, did not lead to any observable binding of the ion.6L‘L Mont- morillonite, a smectite whose negative charge on the silicate 67 Figure 13. Infrared Spectra of Various Hectorites; (a) Sodium(I)-Hectorite After Treatment with Sodium Bisulfate, (b) The Same Mineral After Washing with Methanol, X+ . + (c) Rh2(CH3COO)4_X -Hectorlte, (d) Rh(PPh3)x - Hectorite %{T E3 1370 CM4 . 1575] E) 1453- '1485I h4 9 I . I . I . I '7 I I T I l I' 1\ 1m 1800 1600 1400 1200 Mom") 69 . . . . 2+ 3+ sheets arises from isomorphous substitution of Mg for A1 X-l- 2(CH3C00)4_X . is likely, therefore, that the complex is bound to the planar in the octahedral layer, readily binds Rh It surfaces of the mineral, and not on the external edge sites. Upon the addition of triphenylphosphine, the green colored mineral immediately changes color to a dark orange. A completely analogous reaction and color change occurs with the homogeneous catalyst (vide sgpra). The infrared spec— trum, shown in Figure 13D, exhibits two peaks at 1485 and 1439 cm.1 which are assigned to in plane deformations of the 65 phenyl rings bonded to phosphorus. Both bands which were assigned to coordinated acetate in ha(CH3C00)u_xx+-hectorite disappear upon the addition of PPh3. The uv-visible spectrum for Rh(PPh3)X+- hectorite is shown in Figure 14. This spectrum has several features which indicate again that the mineral bound species is sim- ilar to the homogeneous complex, Rh(PPh3)3BF4. The Spectrum is dominated by a high energy band which is poorly resovled into a series of peaks between 250-275 nm. Inflections are also observed at 315 and 367 nm on the high energy band. These same features are exhibited in the solution spectrum (Qt, Figure 12). Rhodium(II) benzoate, Rh2(C6H5C00)4, is protonated in methanol with HBFu in a manner similar to rhodium(II) acetate. The resulting Rh2(CéH5000)4_xX+ Species is also readily ex— changed onto the surfaces of hectorite. The product of the reaction between this mineral bound species and 70 Figure 14. UV-Visible Spectrum of Rh(PPh3)X+—Hectorite 71 00m 00m u - 72 triphenylphosphine is a dark orange Species whose infrared Spectrum contains the same peaks as Rh(PPh3)x+-hectorite prepared from the related protonated rhodium acetate species. The fact that both the protonated acetate and benzoate com- plexes are catalyst precursors indicates that the cataly- tically active species does not contain an acetate ligand. These supported rhodium phosphine complexes are both catas lysts for the hydrogenation of unsaturated hydrocarbons. C. Hydrogenation Studies Rh(PPh3)x+-hectorite functions as an effective catalyst for the hydrogenation of alkenes and alkynes in methanol after treatment of the orange mineral with hydrogen. The complex on the mineral undergoes an oxidative addition of hydrogen to give a light yellow colored mineral, presumably RhH2(PPh3)x+-hectorite. An analogous reaction occurs in the preparation of the homogeneous catalyst. Catalytic activity was found to be at a maximum for the mineral bound catalyst (wt% Rh = 0.76) when the PPh3 concen—- tration initially in the slurry was 4.6xlOI3M. This con- centration corresponds to a PPh3:Rh ratio of 9.5:1. The value of x in Rh(PPh3)x+-hectorite is not likely to be greater than three as in the analogous homogeneous complex, Rh(PPh BF“. No catalytic reaction is observed with the 3)3 supported catalyst when the initial phosphine concentration is less than 1.0xlo‘3m (PPh3:Rh = 2). In order to understand better the reason for the large excess of PPh needed for maximum catalytic activity, the 3 73 adsorption of triphenylphosphine on Na+-hectorite was studied. Various amounts of the phoSphine were added to a methanol Slurry of the mineral. After 5 minutes, an aliquot of the supernatant was removed. The absorbance of the super— natant at 260 nm was measured, and the amount of phosphine left in solution was calculated. The difference between the amount of phosphine initially added to the solution and the amount found in the supernatant is the quantity of PPh3 adsorbed on the mineral. By plotting the moles of phosphine initially in solution versus the moles adsorbed, an adsorption curve of PPh3 on native hectorite is obtained (gt. Figure 15). This figure Shows that at low concentrations of PPhB, little phosphine is adsorbed, so the amount of phosphine which reaches the rhodium(II) acetate complex will be small, pro— bably less than 2 moles per rhodium atom. With the homo- geneous catalyst, a PPh3:Rh ratio of two gave the maximum catalytic activity.3 AS the concentration of the phosphine is increased the fraction of total phosphine adsorbed in- creases abruptly, which indicates that the phosphine has penetrated the intercrystalline space. At the higher concen— trations of PPh3, the phosphine penetrates between the layers to react with the exchanged rhodium complex. Another function of PPh3 in the intralamellar space is that this large bulky molecule might act as a molecular prop, so that the reactants might have an easier access to the catalytic site. Initial rates over the first 8—10% of the reaction for 7L: Figure 15. Adsorption Curve of Triphenylphosphine on Na+- Hectorite 4 mole x IO PPh3 (adsorbedl/g 75 1.09 Not Hectorite 150 ml CHBOH mole x IO4 PPh3(initlol) 76 for the reduction of alkenes and alkynes at 25°C are pre- sented in Tables 4 and 5. In both tables, turnover numbers (m1 of HZ/min/mmole of Rh) are used to report the initial rates of hydrogenation. For comparison, the rates of hydro- genation of the substrates relative to l-hexene are also given in these tables. The relative rates for the homogen- eous catalyst are calculated from the data reported by Wilkinson and co-workers.3 Terminal olefins are reduced with a linear uptake of H2 to the corresponding alkane, with the exception of 3,3«di- methyl-lLbutene which does not undergo hydrogenation. Pro- ducts of these reductions were identified by gas chroma- tography. For the hydrogenation of 1—heptene, some 2-hep- tene was found in the product mixture. Some isomerization most likely occured in the hydrogenation of 1-hexene, but 2-hexene was not separated from the reaction mixture by gas chromatography. The retention times for 2-hexene and l—hex- ene were found to be nearly the same on the SE-30 column. Representative graphs of the hydrogenation rates of unsat- urated hydrocarbons are shown in Figures 16-19. Terminal olefins with other functional groups within the molecule, gg., —OR and -0H, are reduced with a non-linear H2 uptake. The non-linear uptake of H2 indicates that the re- duction of these olefins is a more complicated process than the reduction of terminal olefins. The reaction products were not identified for the hydrogenation of these olefins. Several possibilites are plausible as to why the reduction HE om n ossao> Hmpop 77 N Show cos n ma ao.a n mangoaog om: Sm.o spa; Hocmcpms u sco>aom Canaan mm.o\:m macs mtoaxmd.a t mm.o M 3m Sp; comm n owzpmquSoa as w.: n mmnmmu mSOflPHpcoo Am I N.H a.sH Hosooam asses e.a s.a o.mH torso ascend asses s.m s.m m.mm osoaeswcsIm.H I 0.0 .m.z ocoxosoaomo I o.o .m.z osoPSQIHIHSseoeaeIm.m m.o 0.0 .m.z Amcmup t mwov ozoxmnum I s.o . m.s crossosIH oo.H o.H m.oa ocoxogua opmm o>apmaom opmm o>flpmaom gm oHoS£\CH£\Nm HE D methpmpzm szamsmo msomzowosom thampmo popsommsm assoc“: mpmhamvmo msmmIAanm mSOQSTonom can sophommsm Hmuosflz mo monomosm ocp ca mosoxam mo snowmm Sofipmsowosuzm .d manna 78 .SUSPn macs Ca 3m oHoE£\:Hfi\Nm HE oaa on op eczem hopes: mo>ocuse Ac .m cosmnmmou CH p::o% memo Scum umpdaduamo pmhampmo msoocomoEoz Sow mmpmh m>apmaom An Ae.psocv s canoe 79 Figure 16. Plot of the Hydrogenation Rate of 1-Hexene Conditions: 0.2g of Rh2(CH3COO)4_xx+-hectorite (0.76% Rh) 2.8 ml of 0.05M PPh3 3.75 ml of 1-hexene total volume = 30 m1 80 00v REEL mEE. ON E... can 81 of these substituted olefins occurs with a non-linear H2 uptake. The substrate could irreversibly bind to the metal ion via the functional group, which would prevent further coordination of the olefin and hydrogen: or a side reaction could occur concurrently with the hydrogenation reaction. Polymerization and hydrogenolysis are examples of the type of reaction which modifies the substrate, therefore, the reaCtivity of the olefin to the catalyst would change. Rylander has reported that the allylic group is especially 66a susceptible to hydrogenolysis. Catalytic hydrogenolysis is defined as ”the cleavage of a molecule into fragments by hydrogen in the presence of a catalyst.""6681 Three possible reactions can occur in the hydrogenolysis of the allylic function: the splitting out of water from allylic alcohol with or without the subsequent reduction of the olefin, the reduction of the allylic group, and the formation of a ketone of aldehyde by a migration of the double bond.66b If both an aldehyde or a ketone were formed by the migration of the double bond, and the allylic function were reduced, a non- linear uptake of hydrogen would be observed. Internal olefins are not reduced by the mineral sup— ported catalyst to any observable extent. Internal olefins are hydrogenated at a much Slower rate than terminal ole- fins with a homogeneous catalyst.3 Internal olefins are reported to have a smaller association constant for coordi- 67 nation to the metal ion than terminal olefins. which could account for the reduced activity in solution. Steric factors 82 Figure 17. Plot of the Hydrogenation Rate of Allyl Alcohol Conditions: 0.2g of Rh2(CH3COO)4_xx+—hectorite (0.76% Rh) 2.8 m1 of 0.05M PPh3 2.0 ml of allyl alcohol total volume = 30 ml 83 00m AEEV «EC. 00. 84 are generally considered to effect the coordination ability of an olefin to bind to a metal. With the mineral supported catalyst, the olefin must react with the catalyst in a ster- ically restrictive environment which might magnify the steric effects. The trends in the relative rates of reaction for the supported and the homogeneous catalysts parallel each other, which means that similar species catalyze the reactions on the surface and in solution. The mineral environment, how- ever, exerts an inhibitive effect on the actual rate of hy- drogenation. The homogeneous catalyst reduces olefins at a rate an order of magnitude faster than the supported catalyst. Friedlander68 reported that olefins do not penetrate into the intralamellar space of smectites. This lack of pene- tration of the olefin into the intracrystalline space would lead to a concentration gradient between solution and the catalytic site. The concentration of the olefin at the catalytic site in the mineral would be less than 1.0M, the initial olefin concentration in solution. Terminal alkynes as well as internal alkynes are re- duced with a linear H2 uptake with the supported catalyst (cf. Figure 18). The hydrogenation reactions of these sub- strates were followed by gas chromatography. The chromato- graphic data Show that terminal alkynes are hydrogenated in a stepwise manner, first with reduction to the alkene, then to the alkane. A new molecule of alkyne will displace the alkene from the coordination Sphere of the metal as long as .SSSpm mans Cw gm oHoE£\Cfl£\mm HE cam on op agnoe amass: mo>ossse Ao .m oocoummou CH endow News Scum popmaonMO thHMPmO mzoo2oono: new hosts o>apmamm An .: manta SH newcomoum muons no mean one was mcoapwpcoo Am 85 oH.: km oom HoINIssPspImIHssposIN I o.m mm ocfluoano Hawsmmosm 0.H om mam Honooam szhmmonm I we own HoIHIcSesnIm I ma OMH encampmom ammosm I we owe oChpOOIH I on 0mm oshxmcIN H.m on 0mm ochxmnta nopmm o>wpmaom opmm o>flpmaom cm oHoE£\CH£\Nm HE pmzampmo msooSowoSo: vthmpmo poppommzm ammocfls mpmzampmo mnmmIAHvzm msoocoonom USN sophommsm Hapocwz mo ooflomoum onp Ca moczxa¢ go mopmm Cowpmcowopphm .m manna .m 86 Figure 18. Plot of the Hydrogenation Rate of l-Hexyne Coditions: 0.2g of Rh2(CH3000)4_XX+-hectorite (0.76% Rh) 2.8 ml of 0 05M PPh3 3.4 m1 of 1-hexyne total volume = 30 ml 87 Om OO 3;: sec. O¢ ON d 88 the alkyne is present in a large excess. AS the concentration of the alkyne decreases, the hydrogenation of the alkene be— comes more important. In the reduction of internal alkynes, no hydrogenation of the alkene was observed. Internal ole- fins were not reduced with this mineral supported catalyst (vide supra). Recently, a similar observation, 2—hexyne being selectively reduced to gtg-Z-hexene has been reported.69 The catalyst used in that study was a cationic homogeneous complex, Rh(NBD)(PPh3)2+. After the alkyne was totally re- duced to the alkene, the resulting olefin undergoes hydro- genation and isomerization. Substituted alkynes as is the case with substituted ole- fins undergo hydrogenation with a non-linear H2 uptake (gt. Figure 19). Again it seems likely that some side reaction is occurring at the same time as the hydrogenation reaction. Alkynes with an oxygen adjacent to the triple bond are known 66c to undergo hydrogenolysis to various extents. Water or an alcohol would be split from the alkyne to yield a simple alkyne. The substituted alkynes might also undergo dimer- ization or polymerization. RhCl(PPh3)3 is known to be an active catalyst for the polymerization of the substituted 7O alkynes studied in this project. Propargyl chloride poisons the catalyst by undergoing an oxidative addition reaction with the metal complex.70 The relative rates of hydrogenation for the supported and the homogeneous catalysts also follow parallel trends as is the case with the olefins. Similar catalytic species are 89 Figure 19. Plot of the Hydrogenation Rate of PrOpargyl Alcohol Conditions: 0.2g of Rh2(CH3COO)4_xx+—hectorite (0.76% Rh) 2.8 ml of 0.05M PPh3 1.75 ml of propargyl alcohol total volume = 30 m1 OON P AEEV met» 00. _ an x1 ON 91 involved in both the heterogeneous and homogeneous systems. The rate of reduction of alkynes is faster than that of olefins. The increased rate of hydrogenation means that the substrate is more readily accessible to the catalytic site than in the case of the olefins. The turnover numbers were found to be dependent on the amount of rhodium complex exchanged onto the mineral. If only a fraction of the complex is involved in the catalytic reaction, the turnover number is not an adequate represen- tation of the true activity of the complex. The data pre- sented in Table 6 illustrates how the turnover number changes with the rhodium content of the mineral. Table 6. Effect of Rhodium Loading on the Turnover Number wt% Rh Ratea (ml/min) T?§$§¥§£/fi§§f§§ 1.51 0.14 4.2 1.20 0.15 6.5 0.76 0.14 9.5 a) Rates are for the hydrogenation of l-hexene. Triphenyl- phosphine was added so that the initial concentration correSponded to a PPh3:Rh ratio of nine. The reaction occurs within the first 10-20 A of the intra- lamellar space of the mineral. At the higher loadings of rhodium, metal atoms are further into the crystal, and therefore, not accessible to the reactants. IV. CONCLUSIONS This investigation has shown that a homogeneous hydro- genation catalyst can be supported on hectorite, a naturally occurring crystalline silicate mineral, without loss of activity. Infrared and uv-visible spectra Show the mineral bound species is chemically Similar to the homogeneous com- plex. The selectivity of the complex, though, is affected by the mineral environment as Shown by the study of a va- riety of alkenes and alkynes. Terminal olefins are hydro- genated at a rate slower than with the homogeneous catalyst and internal olefins are not reduced to any appreciable ex- tent. The usefulness of the mineral supported catalyst is demonstrated by the hydrogenation of 2-hexyne, which is re- duced to 2-hexene only. No hexane was observed in the pro- duct mixture. 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