ROUTES TO SILSESQUIOXANES FUNCTIONALIZATION – CAPPING OF DDSQS FOR THE SYNTHESIS OF ASYMMETRIC POSS COMPOUNDS By Badru-Deen Barry A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of Chemistry - Master of Science 2019 ABSTRACT ROUTES TO SILSESQUIOXANES FUNCTIONALIZATION – CAPPING OF DDSQS FOR THE SYNTHESIS OF ASYMMETRIC POSS COMPOUNDS By Badru-Deen Barry Polyhedral oligomeric silsesquioxanes (POSS) have been explored for over 7 decades. These organic/inorganic nanostructures are extremely versatile compounds that are compartible with polymer systems, hence making them ideal components for the synthesis of new materials. Their properties may vary based on the organic peripheral unit, but they generally exhibit high thermal degradation temperatures, low dielectric constant and high degree of hardness as coating agents. Cubic silsesquioxanes and their double decker (DDSQ) counterparts have been the most useful because these compounds possess the architecture that accommodates functionalization. However, most of the progress recorded to date has been limited to the use of DDSQs to synthesize functionally symmetric POSS compound. These compounds do display the desired physicochemical properties, but their applications pose the challenge of tethering them with two dissimilar polymeric chains. In this thesis, efforts have been made to develop a route for the selective synthesis of asymmetric POSS compounds. The research utilized the versatility of protecting groups to effect selective functionalization of the DDSQ en route to the desired target. The asymmetric POSS cage has the potential to allow its dispersal into two dissimilar polymer matrices, generating a POSS/polymer composite that is unprecedented. This dissertation is dedicated to my parents, wife and two daughters. iii ACKNOWLEDGEMENTS The distance travelled this far is phenomenal and deserves gratifying reflections. The path was foggy, but the challenge was necessary and stimulating. This opportunity exposed me to a complete set of eminent scholars, research colleagues and friends that have positively influenced my life, especially from an academic and multicultural viewpoint. To this end, I must recognize the following people for their various roles in making this struggle a success and pleasant story. First and foremost, I will start with the outstanding contributions of my advisor, Dr. Robert Maleczka Jr. and collaborator, Dr. Andre Lee of the Department of Chemical Engineering, MSU, for their dedicated time, indulgence and emphasis for experimental details and results. Though their desire for quality work and adherence to standards has been very challenging, yet their comments and suggestions have made my dream come to fruition. Indeed, working with these fine gentlemen was a real stretch but an invocation that have helped to enormously grow my research skills and change my person. Next, I would like to acknowledge the immeasurable contributions of my committee, Dr. James Ned Jackson, Dr. Kevin D. Walker, and Dr. Aaron Odom. Whilst these gentlemen spent their precious time critiquing my findings all the time, they equally took the onus of proffering valuable suggestions that have irrefutably made my research objectives attainable. Their accessibility, backed with the smiles added to their faces, either on visits iv to discuss experimental results or otherwise, are moments that have left indelible marks in my life. I owe a great deal of appreciation to the United States Navy, the proud sponsor of my research. Their role is critical not only because they funded the project but also because they have introduced me to a new area of research field that I wish to make a career. To this wing, I say kudos. Technically, this dissertation would not have been complete without the assistance of three other gentlemen from the technical angle at the Department of Chemistry, MSU; Dr. Dan Holmes, Dr. Xie Li (both of the NMR facility) and Dr. Richard Staples (X-Ray Crystallographer). I am grateful to Dr. Holmes and Dr. Li for their willingness always to assist me in running my solid state and 2D NMR experiments and for always being there to answer to some of my questions. Dr. Staples’ involvement is noteworthy too. His interventions were not only limited to analyzing the crystal structures of my compounds but also went a long way to helping me grow crystals of substances that are otherwise very challenging to obtain crystals from. I have never been working alone in my lab and for which reason I cannot close this statement without recognizing the role played by my colleagues in the Maleczka group, including one collaborator from the Lee group of Chemical Engineering. My gratitude therefore goes to Jonathan Dannatt, Chachurika Jayasundara, Jose Montero, Fangyi v Shen, David F Vogelsang and Emmanuel Maloba for always being there with me. These revered ladies and gentlemen have been very much resourceful. In addition, I am gratified by the priceless roles played by all the secretaries both in the Graduate and Chairs’ offices. Heidi, Tiphani, Anna, Mary and Brenda have been influential right through from my application for admission into graduate school days onto date. They have (and are hitherto) keeping graduate students abreast with events and making sure that we are up-to-date with developments in the department and by extension, the university. I doff my heart to all of you. Most importantly, I cannot close this statement without recognizing two prominent people from whom my journey commenced. The first is Dr. Patrick Lukulay, a patriot Sierra Leonean who has being passionate about contributing to the growth of education in his country. As an alumnus of Michigan State University, Dr. Lukulay worked tirelessly to ensure that a collaboration existed between the University of Sierra Leone and MSU by opening the door of opportunity for graduate studies to deserving Sierra Leoneans. The other is Professor Thomas B. R. Yormah, an outstanding Sierra Leonean elite whose entire career has been devoted to improving education in Sierra Leone. Dr. Yormah would be always remembered for encouraging me to apply. I am particularly indebted to these two gentlemen who jointly made my career at Michigan State University realizable. Finally, I am thankful to my wife and two daughters for their motivation and courage in seeing me go through this struggle. They have always been with me in thick and thin. vi TABLE OF CONTENTS LIST OF TABLES ........................................................................................................... x LIST OF FIGURES......................................................................................................... xi LIST OF SCHEMES ..................................................................................................... xiii KEY TO ABBREVIATIONS ....................................................................................... xviii CHAPTER 1: Introduction ............................................................................................. 1 1.1 Background and Scope ....................................................................................... 1 1.2 Nanostructures and Nanocomposites ............................................................... 1 1.2.1 Nanostructures .............................................................................................. 2 1.2.2 Nanocomposites ............................................................................................ 2 1.3 Polyhedral Oligomeric Silsesquioxanes (POSS)/Polymer Composites .......... 3 1.4 Silsesquioxanes ................................................................................................... 5 1.5 Polyhedral Oligomeric Silsesquioxanes (POSS) ............................................... 9 1.6 Benefits of POSS Compounds .......................................................................... 13 1.7 Problem Identification, Research Justification, Research Aim and Objectives ................................................................................................................................... 17 1.7.1 Problem Identification ................................................................................. 17 1.7.2 Research Justification and Aims and Objectives ..................................... 18 1.8 Potentials for Asymmetric POSS Compounds (A/B Systems) ....................... 19 CHAPTER 2: Prior Strategies for the Synthesis of Completely and Incompletely Condensed POSS Cages ............................................................................................ 22 2.1 Introduction ........................................................................................................ 22 2.2 Synthetic Strategies for POSS Compounds .................................................... 22 2.2.1 Hydrolytic Condensation of Trifunctional Silane Monomers ................... 23 2.2.2 Selective Cleavage of the Silo-Ether Groups on the Inorganic Backbone ............................................................................................................................... 24 2.2.3 Co-condensation of Organosilicon Monomers and/or Oligomers of Different Structures and Composition ................................................................ 25 2.2.4 Condensation of Oligoorganylcyclosiloxanes .......................................... 26 2.3 Synthesis of Completely Condensed Silsesquioxane .................................... 27 2.3.1 Synthesis and Functionalization of Condensed T6 Cages ....................... 28 2.3.2 Synthesis of Condensed T12 POSS Compounds ...................................... 33 2.3.3 Synthesis of T8 Cages ................................................................................ 34 2.3.4 Incompletely Condensed POSS Cages ...................................................... 43 2.3.5 Cage Cleaving .............................................................................................. 47 vii 2.3.6 Symmetric Polyhedral Oligomeric Silsesquioxanes ................................ 53 CHAPTER 3 Asymmetric Polyhedral Oligomeric Silsesquioxanes ........................ 57 3.1 Mutifunctional Reagents ................................................................................... 57 3.2 Asymmetric Double-Decker Oligomeric Silsesquioxanes (DDSQs) from Prefunctionalized Divinyl-Capped DDSQs ............................................................. 57 3.3 Synthesis and Separation of Asymmetrically Capped Double-Decker Silsesquioxanes Mixtures ....................................................................................... 59 3.4 Attempts to the Selective Synthesis of Asymmetrically Capped DDSQ Cages ................................................................................................................................... 61 3.4.1 Masking and Demasking Techniques – A Tool for Asymmetric Synthesis ............................................................................................................................... 61 3.4.2 Protection of Hydroxyl and Amino Groups with Thiocarbonates ........... 61 3.4.3 Protection of Hydroxyl Groups with Silyl Reagents ................................. 62 3.5 Use of Solid Support/Immobilizing Agent in the Synthesis of Asymmetrically Capped DDSQs ........................................................................................................ 66 3.6 Selective Deprotection Techniques .................................................................. 69 3.6.1 Desilylation of Ph8DDSQ(OTMS)4 to Ph8DDSQ(OH)4 ................................ 69 3.6.2 Other TMS Deprotection Techniques ......................................................... 70 CHAPTER 4: Experimental Section ........................................................................... 72 4.1 Materials and Methods ...................................................................................... 72 4.1.1 Materials ....................................................................................................... 72 4.2 Analytical Techniques ....................................................................................... 72 4.2.1 NMR Spectroscopy ...................................................................................... 72 4.2.2 Mass Spectroscopy ..................................................................................... 73 4.2.3 Infrared Spectroscopy ................................................................................. 73 4.2.4 X-Ray Crystallography ................................................................................ 74 4.2.5 Melting Point Determination ....................................................................... 74 4.3 Experimental Procedures .............................................................................. 74 4.3.1 Synthesis of Cubic Octaphenylsilsesquioxane [OPS] from Phenyltrimethoxy-silane [PhSi(OMe)3] ............................................................... 74 4.4 General Procedure for the Synthesis of tetra(sodio)octaorganyl silsesquioxa-nolate, [DDSQ(ONa)4] ........................................................................ 75 4.4.1 Experimental Procedure - Synthesis of Ph8DDSQ(ONa)4 ......................... 76 4.4.2 Synthesis of Tetrakis(trimethylsilyl)octaphenyl Silsesquioxane [Ph8DDSQ(OTMS)4] ............................................................................................... 76 4.5 Synthesis of Symmetric POSS Cages .............................................................. 81 4.5.1 Corner-Capping of Double Decker Oligomeric Silsesquioxanes (DDSQs) ............................................................................................................................... 81 4.6 Synthesis of Asymmetric POSS cages by Monoprotection (One-Pot Approach) ................................................................................................................. 83 4.6.1 General Experimental Procedure ............................................................... 83 4.7 Tris- and Bis-protection of DDSQ(OM)4 with Trimethylsilylchloride ............. 84 4.7.1 General Procedure for Tris-Protection and Bis-Protection with TMSCl . 84 viii 4.8 Functionalization of Ph8DDSQ(OH)(OTMS)3 with MeHSiCl2 ........................... 90 4.9 Attempts at TMS Deprotection .......................................................................... 91 4.9.1 Desilylation of Ph8DDSQ(OSiMe3)4 50 to Ph8DDSQ(OH)4 10 .................... 91 4.9.2 Desilylation with Acidified MeOH in Ether ................................................. 92 CHAPTER 5 Results and Discussion ......................................................................... 94 5.1 Cubic OPS [Ph8T8 or T8] 31 ................................................................................ 94 5.1.1 Tetra(sodio)octaorganyl silsesquioxanolate, [DDSQ(ONa)4] and its TMS Derivatives ............................................................................................................ 94 5.2 Symmetric POSS Cages .................................................................................... 99 5.3 Selective Protection of DDSQ(OH)4 10 with TMSCl ....................................... 102 5.4 Attempted Removal of the TMS Protecting Group from Ph8DDSQ(OTMS)4 50 with MeSiCl3 and MeOH ......................................................................................... 109 CHAPTER 6: Conclusions and Future direction ..................................................... 114 6.1 Conclusions ..................................................................................................... 114 6.2 Future Directions ............................................................................................. 115 REFERENCES ............................................................................................................ 117 ix LIST OF TABLES Table 1: Crystallographic data for [Ph8DDSQ(OTMS)4] ............................................ 78 Table 2: Crystallographic Data for Tris-TMS Ph8DDSQ(OH) 84 ............................... 86 Table 3: Crystallographic Data for Bis-TMS Ph8DDSQ(OH)2 82 ............................... 89 Table 4: Substrate screening for optimal yield of Ph8DDSQ (OSiMe3)3(OH) 84 and Ph8DDSQ(OSiMe3)2(OH)2 82 ...................................................................................... 106 Table 5: Conditions screening for the desilylation of Ph8DDSQ(OSiMe3)4 50 to Ph8DDSQ(OH)4 10 ...................................................................................................... 110 Table 6: Conditions screening for optimal deprotection of Ph8DDSQ(OSiMe3)4 50 with acidified MeOH .................................................................................................. 112 x LIST OF FIGURES Figure 1: Major POSS/Polymer Architectures5 ........................................................... 4 Figure 2: Framework of Silsesquioxanes (ii) Incompletely Condensed .............................................................................................. 9 (i) Completely Condensed Figure 3: Polymer Architecture with POSS Blends via (14’) Grafting and (14’’) Copolymerization ........................................................................................................ 14 Figure 4: Structure of a typical POSS Polyimide ...................................................... 16 Figure 5: General Structure of a Two-Faced Janus Cube ........................................ 41 Figure 6: The tetraphenyltetravinyl, [PhSiO1.5]4[vinylSiO1.5], Janus from the functionalized Ph4 half cube sodium salt with vinyltrichlorosilane ........................ 43 Figure 7: Incompletely Condensed Silsesquioxanes, (7a) Double Decker, and (7b) Partially Opened Silsesquioxane ............................................................................... 43 Figure 8: Difunctional POSS cages obtained by the Selective Cleavage of One Si- O-Si Linkage ................................................................................................................ 48 Figure 9: Structures of Double Decker Oligomeric Silsesquioxanes (DDSQ) ....... 53 Figure 10: Structures of Symmetric POSS Compounds (59’) no isomer (59’’) cis/trans isomers ......................................................................................................... 54 Figure 11: Typical Structure of an Asymmetric POSS Compound, ‘A/B Cage’ ..... 57 Figure 12: Protected DDSQs with TMSCl .................................................................. 64 Figure 13:Schematic Representation of some Immobilization Techniques on a (Silica) Support; [89(a)] Grafting, [89(b)] Tethering .................................................. 67 Figure 14: Crystal Structure of [Ph8DDSQ(OTMS)4] 50 ............................................ 79 Figure 15: Crystal Structure of [Ph8DDSQ(OTMS)3(OH)] 84 .................................... 87 Figure 16: Crystal Structure of [Ph8DDSQ(OTMS)2(OH)2] 82 ................................... 90 Figure 17: Crystal Structure of [Ph8DDSQ(OTMS)4] ................................................. 95 Figure 18: 1H NMR of [i-Bu8DDSQ(OTMS)4] 85 .......................................................... 96 Figure 19: 29Si NMR of [i-Bu8DDSQ(OTMS)4] 85 ....................................................... 97 Figure 20: 1H NMR of [i-Bu8DDSQ(OH)4] 106 ............................................................ 98 xi Figure 21: 29Si NMR of [i-Bu8DDSQ(OH)4] 106 ........................................................... 99 Figure 22: 29Si NMR Spectrum Showing Signals for cis/trans Isomers of MeHSiDDSQSiMeH 61 ............................................................................................... 101 Figure 23: Ph8DDSQ (OSiMe3)2(OH)2 81 with the TMS groups on the same side . 105 Figure 24: Tris-TMS-DDSQ-OH [Ph8DDSQ(OSiMe3)3(OH)] Dimer .......................... 108 xii LIST OF SCHEMES Scheme 1: Hydrolytic Polycondensation of RSiX3 ..................................................... 5 Scheme 2: Polycondensation and Effects of Reaction Conditions on the Hydrolysis 9 ......................................................................................................................... 6 of RSiX3 Scheme 3: Synthesis of Six Methylpolycyclopolysiloxanes by Cohydrolysis and Thermal Rearrangement ............................................................................................. 10 Scheme 4: Acid and Base Catalyzed Routes to Incompletely Condensed Silsesquioxanes .......................................................................................................... 12 Scheme 5: Synthesis of (a) Completely Condensed POSS Cages and (b) Incompletely Condensed POSS Cages ..................................................................... 24 Scheme 6: General Scheme for the Synthesis of Incompletely Condensed POSS Cages via Acid- and Base-mediated Selective Cleavage ......................................... 25 Scheme 7: Synthesis of Monosubstituted POSS Cage from two Different Monomers ...................................................................................................................................... 25 Scheme of Oligoorganylcyclosiloxanes ....................................................................................... 26 Condensation Synthesis from the 8: of POSS Scheme 9: General Scheme for the Synthesis of T6R6 Species10 ........................... 29 Scheme 10: Synthesis of R6Si6O9 from RSiCl3 (R = c-C6H11)93 ................................. 30 Scheme 11: Synthesis of R6Si6O9 from RSiCl3 (R = c-C6H11) and DMSO solvent ... 30 Scheme 12: Synthesis of R6T6 from the Hydrolytic Condensation of Cyclopentyl Substituted Silanetriol (R = cyclopentyl)................................................................... 31 Scheme 13: Hydrolysis Cy6Si6O9 (Cy = c-C6H11) and Homologation to the Heptacyclohexyl Silsesquioxanetrisilanol ................................................................ 32 Scheme 14: Synthetic Scheme for T12-phenyl ......................................................... 33 Scheme 15: Proposed Reaction Pathway for T12-Phenyl Cage Silsesquioxanes . 34 Scheme 16: Base-promoted Synthesis of Cubic Octaphenylsilsesquioxane from Phenyltrichlorosilane .................................................................................................. 36 Scheme 17: Reaction Scheme for the Acid Catalyzed Synthesis of the Half-cage all- cis-Tetrasilanol ............................................................................................................ 36 xiii Scheme 18: Pathway for the Acid Catalyzed Hydrolysis of PhSiCl3\ ...................... 37 Scheme 19: Typical Reaction Scheme for the Synthesis of Completely Condensed T8 POSS from cis-Cyclic T4 Tetraol .......................................................................... 37 Scheme 20: Synthesis of octakis(4-bromophenyl)octasilsesquioxane from the Cyclic Tetrakis(4-bromophenyl) tetrasiloxanetetraol Sodium Salt ......................... 38 Scheme 21: Synthesis of T8 POSS cages from the Hydrolysis of Trialkoxysilanes Tetrabutylammonium Fluoride ................................................................................... 39 Scheme 22: General Pathways for the Formation of OPS and PPS ........................ 40 Scheme 23: Synthetic Route to Half-cage Tetraphenylsiloxane Tetrasodium Salt 41 Scheme 24: Synthesis of the Tetramethyltetra-phenyl (Me4Ph4) dimethoxy Half- cube .............................................................................................................................. 42 Scheme 25: Synthesis of [PhSiO1.5]4[MeSiO1.5] Janus Silsesquioxane Cube ........ 42 Scheme 26: Synthesis of cyclohexyl substituted silsesquioxane trisilanol, Cy7Si7O9(OH)3 from the fully condensed octacyclohexyl silsesquioxane .............. 44 Scheme 27: General Reaction Scheme for the hydrolytic condensation of cyclohexyltrichlorosilane (CySiCl3) in aqueous acetone ......................................... 45 Scheme 28: Corner-capping Octacyclohexyl silsesquioxane trisilanol with Trifunctional Monomers ............................................................................................. 46 Scheme 29: Synthesis and Functionalization of tetraphenylsilsesquioxanato tetrasodium salt........................................................................................................... 46 Scheme from Phenyltrimethoxysilane .............................................................................................. 47 Cubic Octaphenylsilsesquioxane of 30: Synthesis Scheme 31: Synthesis of heptaphenylsilsesquioxanato trisodium salt from Phenyltrimethoxysilane .............................................................................................. 47 Scheme 32: Synthesis of exo-Octacyclohexyl Octasilsesquioxane Difluoride ..... 49 Scheme 33: Hydrolysis of exo-octacyclohexyl octasilsesquioxane difluoride to the disilanol ........................................................................................................................ 49 Scheme 34: Synthetic scheme for endo-octacyclohexyl octasilsesquioxane disilanol from Cy8Si8O12 .............................................................................................. 50 Scheme 35: Functionalization of endo disilanol with MeHSiCl2 and subsequent Hydrosilylation ............................................................................................................ 50 xiv to endo-Cs- Scheme 36: Hydrolysis and Homologation of Cy6Si6O9 Cy6(vinyl)Si7O9(OH)3 .................................................................................................... 51 Scheme 37: Synthesis of (i-C4H9)8Si8O13(C6H5)2 from endo-(i-C4H9)8Si8O10(OH)2 ... 52 Scheme 38: Synthesis of Heptaphenyl Silsesquioxanetrisilanol from the Base- Catalyzed Hydrolysis of Condensed Ph8T8 ............................................................... 52 Scheme 39: Synthesis of Octaphenyl Silsesquioxanetetrasilanol from the Base- Catalyzed Hydrolysis of Condensed Ph8T8 ............................................................... 53 Scheme 40: Synthesis of DDSQ(Me)4 from Double-Decker Silsesquioxane Tetrasodium Salt DDSQ(ONa)4 ................................................................................... 54 Scheme 41: Synthesis of 9,19-di(hydro)octaphenyl double-decker silsesquioxane from DDSQ(OH)4 .......................................................................................................... 55 Scheme 42: Hydrosilylation of 9,19-di(hydro)octaphenyl Double-Decker Silsesquioxane with para-substituted Styrenes ....................................................... 55 Scheme 43: Cross Metathesis or Silylative Cross Coupling of 9,19- di(vinyl)octaphenyl Double-Decker Silsesquioxane with various Olefins and conjugated Arenes ...................................................................................................... 56 Scheme 44: Stereoselective Metathetic Transformations of Divinyl-functionalized DDSQ ............................................................................................................................ 58 Scheme 45: Synthetic Pathway for AA, AB and BB Capped POSS Cages ............ 60 Scheme 46: Monothiocarbonation of diols and diamines ....................................... 62 Scheme 47: General Reaction Scheme for the Monosilylation of Symmetric 1,n- diols .............................................................................................................................. 63 Scheme 48: Silylation of DDSQ(ONa)4 with Trimethylchlorosilane ........................ 65 Scheme 49: Monoprotection of hydroxyl groups with trialkylsilyl chloride .......... 66 Scheme 50: Protection of Hydroxyl Groups with Solid Surface Support/Immobilized Reagents ...................................................................................................................... 68 Scheme 51: Hydrosilylation of Propargylic Chloride with Red-Sil and Reaction of the Resulting Modified Red-Sil Surface with DDSQ(OH)4 ........................................ 69 Scheme 52: Desilylation of TMS-protected aniline DDSQ with Acidified Methanol ...................................................................................................................................... 70 Scheme 53: Desilylation of 3,13-di(trimethylsilyl)oxypropyl DDSQ with MeSiCl3/MeOH .............................................................................................................. 71 xv Scheme 54: General Synthetic Scheme of Cubic Octaphenylsilsesquioxne ......... 74 Scheme 55: General Reaction Scheme for the Synthesis of Double Decker Tetrasodium Salts [DDSQ(ONa)4] .............................................................................. 75 Scheme 56: Synthesis of Tetrakis(trimethylsilyl)octaphenyl Silsesquioxane [Ph8DDSQ(OTMS)4] from [Ph8DDSQ(ONa)4] .............................................................. 76 Scheme 57: Reaction scheme for the Synthesis of i-Bu8DDSQ(OTMS)4 from i- Bu8DDSQ(ONa)4 ........................................................................................................... 79 Scheme 58: Synthesis of i-Bu8DDSQ(OH)4 from i-Bu8DDSQ(ONa)4 ........................ 80 Scheme 59: The Synthesis of Tetramethyloctaphenyl Silsesquioxane, Me2SiPh8DDSQSiMe2 ................................................................................................... 82 Scheme 60: Synthesis of Dimethyldihydridooctaphenyl Silsesquioxane, MeHSiPh8DDSQSiMeH ................................................................................................ 82 Scheme 61: Attempted Monoprotection of DDSQ(ONa)4 with Me3SiCl as Protecting Group ........................................................................................................................... 83 Scheme 62: Condition Screening for the Optimal Production of Tris-(Me3Si)- Ph8DDSQ(OH) .............................................................................................................. 84 Scheme 63: Functionalization of Ph8DDSQ(OH)(OSiMe3)3 84 with MeHSiCl2......... 90 Scheme 64: Attempted Desilylation of Ph8DDSQ(OSiMe3)4 with MeOH and MeSiCl3 ...................................................................................................................................... 91 Scheme 65: Attempted Desilylation of Ph8DDSQ(OSiMe3)4 50 with Acidified MeOH ...................................................................................................................................... 92 Scheme 66: Synthetic Scheme of Cubic Octaphenyl Silsesquioxne (OPS) ........... 94 Scheme 67: Synthesis of Tetrakis(trimethylsilyl)octaphenyl Silsesquioxane [Ph8DDSQ(OTMS)4] ..................................................................................................... 95 Scheme 68: Corner-Capping of Octaphenyl Silsesquioxane Tetrasodium Salt [DDSQ(ONa)4] 49 ....................................................................................................... 100 Scheme 69: Synthesis of Asymmetric POSS Cage via Monoprotection .............. 102 Scheme 70: Monosilylation of Symmetric 1,n-diols ............................................... 103 Scheme 71: Attempted Monoprotection of DDSQ(ONa)4 with Me3SiCl................. 103 Scheme the Optimal Production of Ph8DDSQ(OH)(OSiMe3)3 ............................................................................................ 104 72: Condition Screening for xvi Scheme 73: Attempted Desilylation of Ph8DDSQ(OSiMe3)4 50.with MeOH and MeSiCl3 ....................................................................................................................... 110 Scheme 74: Attempted Desilylation of Ph8DDSQ(OSiMe3)4 50 with Acidified MeOH .................................................................................................................................... 112 xvii KEY TO ABBREVIATIONS APCI = Atmospheric Pressure Chemical Ionization BzTMAH = Benzyltrimethylammonium hydroxide DCM = Dichloromethane DDSQ = Double Decker Oligomeric Silsesquioxane DSC = Differential Scanning Calorimetry DPS = Diphenyl-functionalized Silsesquioxane HMBC = Heteronuclear Multiple Bond Correlation ICBM = Intercontinental Ballistic Missile LC/MS = Liquid Chromatography-Mass Spectrometry NASA = National Aeronautics and Space Administration NMR = Nuclear Magnetic Resonance Spectroscopy NOESY = Nuclear Overhauser Effect SpectroscopY MS = Mass Spectrometry MW = Molecular Weight OIBS = Octa-isobutyl Silsesquioxane OIBS-EDN = Octa-isobutyl Silsesquioxane Endo-disilanol OPS = Octaphenyl Silsesquioxane ORTEP = Oak Ridge Thermal Ellipsoid Plot PFG = Pulse-Field Gradient POSS = Polyhedral Oligomeric Silsesquioxane PRN = POSS Refined Nanocomposite xviii SBS = Styrene-Butadiene-Styrene T4 = Tetraorganylsilsesquioxane T6 = Hexaorganylsilsesquioxane T8 = Octaorganylsilsesquioxane T10 = Decaorganylsilsesquioxane T12 = Dodecaorganylsilsesquioxane TBAF = Tetrabutylammonium Fluoride THF = Tetrahydrofuran Tg = Glass Transition Temperature TGA = Thermogravimetric Analysis TMS = Tetramethylsilane TPS = Thermal Protection System xix CHAPTER 1: Introduction 1.1 Background and Scope This dissertation recounts my efforts toward developing protocols for the selective synthesis of asymmetric Polyhedral Oligomeric Silsesquioxanes (POSS). Double Decker Oligomeric Silsesquioxane tetraols [DDSQ(OH)4] are used as starting materials with different functionalizable chlorosilanes as capping agents. This chapter therefore gives a brief background on the constituents and structure of POSS compounds, their patterns of aggregation into polymers and the overall goals and relevance of the research. Special emphasis is also devoted to polyhedral oligomeric silsesquioxane chemistry, ranging from its genesis to synthesis and then applications as macromers for organic-inorganic hybrid polymers. 1.2 Nanostructures and Nanocomposites From a scientific viewpoint, materials are defined by both their macroscopic and microscopic structures. The creation of advanced materials is dependent on our understanding of their compositional makeup; precisely from the atomic level to an aggregation of such units into a macromer, and on and on. Interestingly today, materials on their own without blending do not satisfy the coveted requirements of low cost and high performance. With such a driven goal aimed at optimal functionalization, the modification of traditional materials via some sorts of scientific manipulations with nanostructures is therefore inevitable. An unambiguous path to achieving this is to understand materials from their nanoscopic structures and how they self-assemble into nanocomposite prior to blending with traditional materials. Building blocks with nanosize 1 dimensions have, for instance, made the assemblage of new polymeric materials with enhanced physical properties easy. A good starting point is therefore a brief comprehension of what nanoparticles and nanocomposites are. 1.2.1 Nanostructures According to R. W. Cahn in his book “The Coming of Materials Science”, a nanostructure material is any object with volume dimensions within the nanoscopic scale (1–100 nm).1 Technically, this scale refers to the molecular scale with the average material properties tending to fluctuate due to the behavior of its constituent particles. This often causes variations in the materials’ properties, hence making their surface-area effect (thickness, wideness, etc.) more prominent.2 Such materials can either be one-dimensional materials with surface thickness between 0.1-100 nm or nanotubes; two-dimensional tubes with diameters between 0.1-100 nm but longer length; or three-dimensional spherical nanoparticles with a 0.1-100 nm in every spatial direction. POSS molecules are ideal examples of environmentally friendly, non-volatile, and odorless nanostructures that exhibit enhanced performance over their non-hybrid counterparts like metals and ceramics. 1.2.2 Nanocomposites A nanocomposite on the other hand is a solid multiphase matrix bearing at least one nanoscale structure. Nanocomposites are often obtained as aggregates of nanostructures and used to fabricate new materials with flexible and enhanced physical properties.3 The properties of such composites are determined by both the bulk properties of the constituent components and the intricate interactions between different phases in them. Importantly, these nanofillers are lighter materials and require only small loadings 2 for property enhancements.4 Consequently, such materials often exhibit remarkable differences in their mechanical (strength, modulus, rigidity, oxidation resistance), electrical, thermal stability, viscosity, and optical properties compared to their nanostructural constituents. 1.3 Polyhedral Oligomeric Silsesquioxanes (POSS)/Polymer Composites Polymers are unique functional materials because of their macromolecular nature. They also occupy a unique role in industry often because of their light weight and ductility. Polymeric nanocomposites are themselves polymers that have been strengthened with dispersed nanosized materials in the polymeric matrix. Thus, with the aim to design polymers with improved strength and flexibility, the matrices are reinforced with materials like fiber or Polyhedral Oligomeric Silsesquioxanes (POSS) molecules to form composites with different physico-chemical properties from the original materials. Polyhedral Oligomeric Silsesquioxanes are a wide range of functional materials that exist as cage or cage-like oligomeric silsesquioxanes. They can be inert, mono- or multi-functional. This multifuntionality makes it easy for POSS molecules to interact with the polymer host in a three-dimensional fashion leading to the formation of a variety of hybrid composite shapes (Figure 1). Based on the nature of the vertex group, POSS molecules can either have a nonreactive (methyl, isobutyl, cyclopentyl, cyclohexyl, phenyl) or reactive (methacrylate, acrylate, styrene, norbornene, amine, epoxy, alcohol, and phenol) substituent that offers potentials for their dispersion through polymerization or grafting into a polymer chain or network.5 3 Figure 1: Major POSS/Polymer Architectures5 4 The presence of the organic peripherals makes POSS molecules compatible with many polymers. POSS reinforced nanocomposites (PRN) are therefore polymers bearing POSS in which POSS molecules are the dispersed (discontinuous phase) in the polymer matrix (continuous phase), giving composites with average property contributions from both the POSS molecule and its host organic polymer matrix.6–8 The resulting hybrid composite is generally a lightweight material with improved strength and performance. 1.4 Silsesquioxanes Silsesquioxanes are a diverse class of nano-scaled three-dimensional materials with the general formula (RSiO1.5)n, where R is a hydrogen atom or an organic group, and n ≥ 4. Structures are hybrid organic/inorganic nano-building blocks having amenable features for further chemical transformations (nano-building blocks) into nanocomposites. Etymologically, silsesquioxane is a three fragment coinage with the ‘sil’ denoting silicon, the ‘sesqui’ the latin word ‘semisque’ or three-halves, typifying a Si:O ratio of 1:1.5 in a molecule, and ‘oxane’ designating the oxygen atom.9 Generally, these compounds are obtained, among other methods, from the polycondensation of trifunctional organosilicon monomers, RSiX3, where R is a stable organic substituent and X, a highly reactive group like an alkoxide or a halogen (Scheme 1).10 Scheme 1: Hydrolytic Polycondensation of RSiX3 5 The synthesis is generally a consecutive, two-steps process generally involving two steps: Step one (1) is the rapid hydrolysis of the trifunctional chloro- or alkoxy-silane into the trisilanol followed by step two (2) which entails condensation processes that lead to different silsesquioxane frameworks (Scheme 2). Scheme 2: Polycondensation and Effects of Reaction Conditions on the Hydrolysis of RSiX3 9 The steps are quite complicated and even more so when there can be mutual competitive effects from the conditions for their formation. Thus, formation of various architectures is 6 condition dependent with factors such as, initial concentration of monomer and nature of R and X substituents, type of solvent, quantity of water added, temperature and catalyst being crucial. The effects of these factors on the reaction course has not been fully understood to date. This challenge has been ascribed by Voronkov et al. to the complexed polycondensation processes (Scheme 2) and the strong mutual effect of the factors above.9 However, over decades of research in the field, the following deductions have been reached: • Polycondensations conducted at higher temperatures favor formation of fully condensed cages. • Water is important for hydrolysis and cyclization. The concentration of the silanol is kept low by the slow addition of water.11 Whilst higher monomer concentration favors formation of silsesquioxane polymers, dilute solutions are biased toward intramolecular cyclization leading to formation of POSS compounds. • Right reaction solvent choice stabilizes the silanol groups by hydrogen-bonding of the Si-OH to the alcohol or ether function as well as to the remaining alkoxy groups.11 Thus polar solvents such as alcohols, can solvate the siloxane intermediates favoring polymer formation. Non-polar organic solvents, on the other hand, suppress intermolecular reactions in favor of intramolecular condensation for the formation of POSS compounds. • The nature of X in the precursor monomer influences the reaction kinetics. Thus, with X = Cl, OH the hydrolysis is much faster compared to cases where X = alkoxy and even more so, the reaction can be autocatalytic with the production of HCl as the condensation by-product. 7 • The pH of the solution in the reaction vessel is also crucial. A highly acidic medium will favor formation of oligosilsesquioxanes, whereas polymeric species dominate under basic environments.12 • The nature of the R substituent is also important. When R = hydrogen, or methyl the polycondensation is very fast in favor of the formation of silsesquioxane polymers. Thus, sterics and inductive effects are determining factors here. The R group to some extent influences both the solubility and yield of the silsesquioxane species. Silsesquioxanes that are less soluble precipitate easily and push the equilibrium process towards formation of intermediates that leads to this less soluble species.9,11 Thus, depending on the conditions for their syntheses, a variety of rigid frameworks ranging from amorphous structures, ladder-like, cyclic oligosiloxanes, and Polyhedral Oligomeric Silsesquioxanes (POSS) can be obtained (Scheme 2).10 The silica inorganic core provides the molecular rigidity, good thermal and mechanical properties of the molecules while the organic periphery accommodates functionalization that enhances material processability. Their properties may vary based on the peripheral organic functional group, but they generally exhibit high thermal decomposition temperature, low dielectric constant, and high degree of hardness as a coating agent. With over seventy (70) years of exploration of this chemistry, silsesquioxanes have found wide applications in polymers,13–15 catalysis,16 biomedical materials,17,18 and semiconducting materials for optoelectronic devices.19 To this end, classic organic/inorganic materials of the -Si-O-Si- framework have garnered much research interest because of the exceptional tailorable properties they exhibit. 8 1.5 Polyhedral Oligomeric Silsesquioxanes (POSS) Polyhedral oligomeric silsesquioxanes (POSS) are a framework of silicon-oxygen polyhedral core with an inert or reactive organic peripheral unit. They have the general formula [RSiO3/2]n, where n is an integer 6, 8, 10, 12, and 14. Generally, they are abbreviated as Tn, where n = number of Si on the cage. Such condensed oligomeric organosiliceous compounds are formed under various reaction conditions including solvent, concentration, catalyst, temperature, and the organic substituent in the starting silane.11,20 As part of the silsesquioxane family, POSS molecules have each Si bound to 1.5 O-atoms (sesqui-) and a hydrocarbon unit (-ane) giving rise to cages with different degrees of symmetry. A large variety of hexa-, octa-, deca- and dodeca-functional POSS compounds with R = H or organyl are known.21–23 Based on the conditions for their formation and hence the formula/structure of the compound, POSS molecules are broadly classified into two categories (Figures 2): (i) Completely Condensed Silsesquioxanes (ii) Incompletely Condensed Silsesquioxanes Figure 2: Framework of Silsesquioxanes Incompletely Condensed (i) Completely Condensed (ii) (i) Completely Condensed Silsesquoxanes 9 Figure 2 (cont’d) (ii) Incompletely condensed silsesquioxanes. In addition to displaying high thermal, mechanical, and chemical stabilities24,25 derived from the siloxane (Si–O–Si) frameworks, these three dimensional compounds have attracted enormous attention in the research fields of hybrid materials because of their diverse functional organic side chains (R groups) and the inert inorganic core that are remarkably compatible with organic materials.26 This makes them valuable building blocks in polymer chemistry for the synthesis of new materials including nano- composites27–29 and catalysts for polymerization30. Credit for the discovery of the completely condensed silsesquioxanes is given to Scott (of Dow Corning) in 1946 for the synthesis of methyl-substituted oligomeric silsesquioxanes.31 Scheme 3: Synthesis of Six Methylpolycyclopolysiloxanes by Cohydrolysis and Thermal Rearrangement 10 Though not fully characterized then, Scott in his pursuit to understand the chemistry of chlorosilanes, showed that six methylpolycyclopolysiloxanes could be obtained from the thermal rearrangement of the cohydrolysis product of dimethyldichlorosilane and methyltrichlorosilane. Thus, completely condensed methyl-substituted oligomeric silsesquioxanes with varied molecular formulae were obtained. Exact structures were not assigned to these compounds because of the number of several isomeric possible structures. However, the structures were later elucidated by Barry to have cubic and hexagonal prismatic shapes.32 While fabrication of other R-substituted silicon frameworks of the R8T8 structure (R = n- propyl, ethyl, and n-butyl) were initiated by Barry and Gilkey,33 Olsson in 1958 first reported the synthesis of octaphenyloctasilsesquioxane (Ph8T8) from the acid catalyzed hydrolysis of phenyltrichlorosilane (PhSiCl3) in methanol.34 The hexa- (T6), deca- (T10) and dodeca- (T12) silsesquioxane are now known.35,36 On the contrary, development of methodologies for the synthesis of incompletely condensed silsesquioxanes is credited to the Feher group.37–42 Feher recorded a remarkable feat in silsesquioxane chemistry with his success in synthesizing the incompletely condensed silsesquioxanes. Under both acid and base catalyzed conditions, selective cleavages of the fully condensed precursors have rendered various incompletely condensed silsesquioxane frameworks. (Scheme 4). 11 Scheme 4: Acid and Base Catalyzed Routes to Incompletely Condensed Silsesquioxanes Such incompletely condensed silsesquioxanes are outstanding precursors for a range of applications including the assemblage of novel silsesquioxane-containing polymer and other Si-O and Si-O-M models. For instance, POSS silanols possess the necessary architecture to trap catalysts, hence making them suitable for application as catalysts in industrial processes such as the epoxidation of olefins.43 12 1.6 Benefits of POSS Compounds POSS cages exhibit a myriad of desired properties that can be optimized for specific applications. Compared to traditional fillers, POSS molecules are environmentally benign and odorless and thus are non-volatile emitting compounds below 300 °C.44 A majority of research and applications in the field focuses on the integration of POSS molecules into polymeric structures via grafting, copolymerization and blending for the purpose of reinforcing the composite. Over the years, their incorporation into polymer matrices have been demonstrated to decrease material flammability and viscosity, and to enhance the mechanical properties, oxidative resistance and thermal stability of the polymer composites.45–47 The hybrid organic-inorganic framework promotes composite miscibility when integrated into a polymer matrix, making them superior tailorable molecules compared to other traditional fillers. Importantly also, polyhedral oligomeric silsesquioxanes (POSS) have been extensively utilized in space-survivable coatings, ablative and fire resistant materials.15,48,49 Their physical sizes make incorporating them into polymers reduce the chain mobility, and improve both the thermal and mechanical properties. Thus, completely condensed cages that are thermally stable are either chemically bound/grafted to the polymer backbone (14’, Figure 3) or incorporated via chemical modification of a preformed polymer where the POSS acts as pendant to the polymer backbone (14’’, Figure 3). 13 Figure 3: Polymer Architecture with POSS Blends via (14’) Grafting and (14’’) Copolymerization The various blending methods make modifications facile and offer access to different material designs, hence enabling light-weight materials for energy efficiency, aerospace, electronics and strong infrastructural applications to be assembled. Polymeric blocks linked to form block copolymers where chemically incompatible building blocks self- assemble into a number of ordered frameworks have been reported.50,51 Upon integration into polymeric materials, properties such as glass transition temperature (Tg), dielectric constant, and chemical resistance of the resulting structures are enhanced. For example, Lee and Haddad’s work on POSS–SBS polymers illustrated that the POSS not only enhances the Tg of the butadiene fraction as supposed, but also promotes the storage modulus at the melt temperature (Tm) of the styrene.141 In space exploration particularly, protective covers are needed as shields against harsh environments. To this end, and owing to the good thermal and physical properties of some polymers, these materials have been extensively utilized in spacecraft thermal blankets, solar arrays, and space inflatable structures.52,53 Worth mentioning is the fact that ordinary polymeric materials suffer from serious degradation in Low Earth Orbit (LEO). Various research groups have attributed this degradation to the high concentration 14 of atomic oxygen (AO) in this region and its reactivity with organic substances.54–59 For quite some time, organic polymers like kapton polyimide, have recorded extensive applications including its use as thermal blanket materials in spacecrafts, solar arrays and other space inflatable structures.60,61 In the LEO, this material, kapton polyimide (PI), is subject to extreme radiations and hence can be bombarded by high energy, charged particles that could lead to its degradation. AO, in this region, having translational energies above many reaction barriers, causes the degradation of the polymer. Thus, if spacecraft are to survive the ubiquitous aggressive conditions in space, the design of space-survivable materials is imperative. Lately, it has been observed that polymeric materials intercalated with POSS molecules make the matrix resistant to AO due to the higher Si-O bond energy (approx 6eV) which is above the collision energy of AO in LEO.62 With the presence of nanodispersed POSS molecules in the POSS-PI film (Figure 4), a protective silica layer is formed on the polyimide surface upon reaction of the film with AO. Sandra et al. have shown that exposure of a POSS-Polyimide film to AO causes erosion of the surface organic material and reaction of the AO with the POSS molecule to form a silica-based shielding layer. This is responsible for the observed AO resistance and the insignificant effects on modulus, glass transition temperature Tg, and coefficient of thermal expansion (α) in the resulting film.63 15 Figure 4: Structure of a typical POSS Polyimide In order to enhance resistance to AO therefore, polyimides are now coated with functionalized POSS molecules to make the Si-O bond of the resulting matrix unaffected by the exposure to AO. Thus when a POSS-Kapton encounters AO, the organic material degrades while the silicon and oxygen atoms in the matrix react with AO to passivate the polyimide surface; hence making the hybrid nature of POSS compounds a key feature for their use as ideal delivery systems as nano-dispersed silica that are unaffected by weather.64,65 Also, in the aerospace industry, polyimide compositions have been used for optical applications. Thus as membrane reflectors, the polyimide composites for such applications are prone to repeated and drastic heating and cooling cycles in orbit as a result of their exposure to interchanging periods of sunlight and shade.66 Since satellites do survive in orbit for as long as the materials used to make them remain functional, this thermal and chemical durability is of great value 16 As ablative materials, POSS compounds are used in thermal protection systems (TPS) as heat shields, especially in the aerospace industry.67,68 Ablative TPS are effective heat shields used in Intercontinental Ballistic Missile (ICBMs), the clustering of inertly cooled combustion chambers,69 and in payload and aerodynamic surfaces of all NASA planetary entry probes and vehicles against the severe thermal effects from hypersonic flight70 and and other propulsion devices.71 POSS/Polymer protective composites also find applications in the electronics and telecommunications industries as protective and stress buffer coatings. Thus the composite materials are often overlaid on electronic materials as in semiconductors, dielectric layers for multilayer integrated circuits and multi-chip modules, high temperature solder masks, passivating coatings on electronic devices, and the like.72,73 Due to their high dielectric strengths, high volume resistivity, and low dissipations, silsesquioxanes are used to laminate electrical materials through fiber- reinforced composites. Functionalized silsesquioxanes have also been used in light emitting diodes.73,74 1.7 Problem Identification, Research Justification, Research Aim and Objectives 1.7.1 Problem Identification Today, it is sufficiently evident that polymers and plastics are major components of a good number of commercial products and applications. However, besides the enormous benefits conferred by these materials, evaluation of recent progresses on the synthesis and applications of these materials have disclosed some drawbacks in relation to their resistance to oxidation and degradation at higher temperatures. Standard polymers, for 17 instance, are used as coatings of underlying materials, and to some extent, have been effective at minimizing oxidative degradation. However, over time, they often experience various effects ranging from cracking due to thermal and mechanical stresses, abrasion, and debris impact.75,76 Upon cracking, the protective surface is compromised, and the underlying polymeric material is prone to degradation from its exposure to the oxidizing environment. Prior to the use of POSS compounds for this purpose, polymer oxidative properties have been ameliorated to better tolerate harsh environments using certain fillers. Recent efforts to surmount these challenges have been the development of hybrid materials/plastics that can provide synergistic properties between those of traditional organic polymers and inorganic systems like ceramics. Such hybrid products would ideally exhibit properties of the organic polymer systems (i.e. processability, toughness, cost) and those of the core inorganic portions (i.e. thermal and oxidative stability). 1.7.2 Research Justification and Aims and Objectives Generally, polymeric materials are lightweight, ductile and easy to synthesize. Approaches that improve polymer properties while retaining their lightweight and ductile nature are known. However, compared to metals and ceramics, these materials exhibit some shortcomings including their low modulus and strength. In order to improve these mechanical properties, reinforcement enhancing structures/fillers such as fibres and POSS compounds are often added to the polymer matrix. These fillers have the ability to enhance the mechanical strength, heat and impact resistance, and also to abate the electrical conductivity and permeability due to exposure to oxygen and water vapor.77 18 Given their material toughness, light weight, and ease of processing, polyhedral oligomeric silsesquioxanes (POSS) are integral components in the materials and polymer industries for the synthesis of thermally and oxidatively stable materials. More so even, these materials have emerged as nonpareil with regards to processability, toughness and low cost. Thus work to advance our understanding on the prevailing and prospective properties (mechanical, thermal, flame retardation and viscoelastic properties) and applications of POSS nanocomposites, their morphology and polymer synthesis, is crucial to research chemists and materials engineers. To this end, the overarching goal of this research is to unravel routes for entry into functionalized asymmetric POSS cages using existing and new capping agents. The research issues to be addressed here include: (i) The development of cost-effective and efficient processing routes for entry into POSS cages. (ii) The recognition of polymerizable chlorosilanes and clustering of a novel and flexible precursor for the facile entry into asymmetric POSS compounds that are graftable. (iii) The development of protocol(s) for the selective synthesis of asymmetric systems (A/B POSS cages). 1.8 Potentials for Asymmetric POSS Compounds (A/B Systems) As illustrated in section 1.3 above, polyhedral oligomeric silsesquioxanes are an attractive class of compounds for both polymer and engineering purposes. Their unique nanoscaled structure coupled with their good solubility in organic solvents makes them tailorable and compatible materials with companion materials like polymers. As a result, POSS compounds are beneficial for the synthesis of hybrid materials with a wide range of 19 applications. Despite the enormous progress made so far on the study of symmetrical POSS cages, there is only scanty work reported in the literature for the synthesis and applications of the analogous products resulting from either the side-capping of incompletely condensed cages with two different polymerizable groups (A/B Systems) or the direct synthesis of such systems starting from traditional chloro- or alkoxy-silanes. To this end, ongoing explorations in our laboratories on this chemistry is devoted to uncovering techniques for the synthesis of novel POSS compounds of the asymmetric types. Additionally, efforts are also directed to the synthesis of new capping agents for the functionalization of double decker oligomeric Silsesquioxanes into asymmetric blocks with potential new features. Achieving such a feat will make the design of block copolymers with asymmetric systems more accessible as it provides a platform for capping agents of different nature and properties to be incorporated into polymer matrices. The challenge is to be able to fabricate high temperature nanocomposites without compromising the requisite properties of these POSS compounds. Thus, in the context of nanoparticle architecture, such bridging/capping might offer access to greater materials design from which a range of potential products that are oxidatively resistant, thermally stable, non-flammable and lightweight materials for energy efficiency, aerospace, and other applications could be reached. Compounds based on asymmetric (A/B) architecture from tetrafunctional POSS substrates have only been illusory or nascent since the discovery of silsesquioxane chemistry. On the contrary however, steady progress has been realized in the synthesis and applications of the symmetric analogues. With the recent spate of developments 20 unfolding in our laboratory on assembling such frameworks, it is envisioned that selective masking or protection of one or more of the functionalities on the cage could enable entry into our desired target. To this end, the advantages of mono- or multi-protection of organic functionalities via silylation, benzylation, benzoylation, borylation, etc. and ‘solid support’ chemistry are important paths for development. 21 CHAPTER 2: Prior Strategies for the Synthesis of Completely and Incompletely Condensed POSS Cages 2.1 Introduction There is quite extensive research that have been done on POSS chemistry, most of which has been dominated by the octafunctional oligomeric silsesquioxanes. This chapter is by no means exhaustive, but rather a survey of synthetic routes to polyhedral oligomeric silsesquioxanes. Transformations are herein elaborated for selected methodologies relevant to this project. Thus, focus is limited to synthetic techniques for the hexa-, octa-, deca, and dodeca-oligomeric silsesquioxanes. Importantly, because the past three decades have seen particular interest on the chemistry of POSS compounds of the T8 cage types, the synthese of the octaphenyl-, octacyclohexyl-, octacyclopentyl-, and octaisobutyl-silsesquioxanes are especially noteworthy. Thus, even though some mention is made of other caged architectures (T6, T10 and T12), this chapter discusses some techniques for assembling double deckers of both the fully and incompletely condensed cubic analogues of the T8 compounds. 2.2 Synthetic Strategies for POSS Compounds Polyhedral oligomeric silsesquioxanes are an important class of compounds comprising several Si-O-Si constituent rings linked in a finite three-dimensional pattern to form prismatic or cubic architectures. They can have various organic peripherals including aryls such as phenyl and benzyls, and aliphatics, vinyl, methacryl, amine and epoxy groups. Several synthetic routes have been documented in the literature for the synthesis 22 of these compounds, the most explored being those that follow the polycondensation of trifunctional silanes, RSiX3, under various reaction conditions. The R group is the organic functional group and the X, the condensable moiety. Broadly, below are the four (4) major approaches78 by which POSS compounds have been synthesized: (i) Hydrolytic condensation of trifunctional silane monomers (ii) Selective cleavage of the silyl-ether groups on the inorganic backbone (iii) Co-condensation of organosilicon monomers and/or oligomers of different structure and composition. (iv) Condensation of oligoorganylcyclosiloxanes. The first method, the most widely used approach, involves condensation of a precursor monomer and tandem cyclization to give the desired POSS molecule. The next two entail the selective breaking and rearrangement of Si-O-Si bond(s). The last approach differs from the first in the sense that the co-hydrolysis involves two different functional monomers. 2.2.1 Hydrolytic Condensation of Trifunctional Silane Monomers This approach is useful for the synthesis of homo-substituted POSS molecules where all the peripheral substituents are identical, as well as cage-like silsesquioxanes. The condensations are condition dependent79 and can afford both the fully condensed (Scheme 5a)77 silsesquuioxane and the incompletely condensed (Schemes 5b) counterpart37 The polycondensation of a trifunctional monomer, RSiX3 gives, in addition to the octaorganyl T8 POSS cage as the major product, side products such as the deca- T10 and dodeca- T12 derivatives (Scheme 5a).79,80 23 Scheme 5: Synthesis of (a) Completely Condensed POSS Cages and (b) Incompletely Condensed POSS Cages Scheme 5a: Synthesis of Completely Condensed POSS Cages Scheme 5b: Synthesis of Incompletely Condensed POSS Cages Techniques of the type in Scheme 5a above have also been achieved by substitution82,83 and corner-capping reactions.37,84 2.2.2 Selective Cleavage of the Silo-Ether Groups on the Inorganic Backbone Incompletely condensed silsesquioxanes bearing free silanol sites have been mostly achieved via this approach. For instance, Feher et al. have shown that treatment of cubic octameric polyhedral oligomeric silsesquioxanes (R8Si8O12) with both strong acids and strong bases can result in the selective cleavage of one or more Si-O-Si bond(s) to give R8Si8O11X2, R8Si8O9X4 and R7Si7O8X3 frameworks, which upon subsequent hydrolysis 24 can afford the respective silanols, R8Si8O11(OH)2, R8Si8O9(OH)4, R7Si7O9(OH)3 (Scheme 6).37,85,86,98,111 Scheme 6: General Scheme for the Synthesis of Incompletely Condensed POSS Cages via Acid- and Base-mediated Selective Cleavage 2.2.3 Co-condensation of Organosilicon Monomers and/or Oligomers of Different Structures and Composition POSS cages with more than one type of organic peripheral unit can be obtained from the co-hydrolysis of a mixture of trifunctional monomers, RSiX3 and R'SiX3 to give a statistical mixture of all possible combinations of R and R’ heterosubstituted oligosilsesquioxanes (Scheme 7).9,78 In this case, the initial reactivity of the starting trifunctional silanes is crucial but adjusting their ratio accordingly can lead to a considerable yield of the desired monosubstituted octamer. Scheme 7: Synthesis of Monosubstituted POSS Cage from two Different Monomers 25 2.2.4 Condensation of Oligoorganylcyclosiloxanes The condensation of oligoorganylcyclosiloxanes for the synthesis of completely condensed POSS compounds was first reported for the dehydrogenation of 1,3,5,7- tetramethylcyclotetrasiloxane with 1,3,5,7- tetraphenylcyclotetra-1,3,5,7-siloxane tetraol in ether under base catalysis (Scheme 8).87 In the specific case below, two cyclic siloxanes bearing different R substituents undergo condensation to afford a completely condensed POSS cage with alternating R groups. Synthesis Scheme Oligoorganylcyclosiloxanes 8: of POSS from the Condensation of In the subsequent sections, literature reports involving the aforementioned strategies for the synthesis of both completely and incompletely condensed POSS compounds are discussed. Several approaches that have been adopted to functionalize these organosilsesquioxanes are also reviewed. The octa-functionalized cages are particularly referenced as is evidenced in the increasing research attention in the past several decades due to their easy tailorability and hence, important applications in polymer chemistry and material science. 26 2.3 Synthesis of Completely Condensed Silsesquioxane Regardless of the tremendous progress realized to date, the chemistry of POSS compounds remains underexploited. Noteworthy however is the genesis of these compounds that dates far back to the pioneering work of Scott31,88 in 1946 for the synthesis of fully condensed silsesquioanes (Scheme 3). From this discovery, a remarkable growth in curiosity in the use of discrete molecular oligomeric silsesquioxanes often obtained from the hydrolytic condensation of trifunctional silanes ensued. Studies with c-C6H11SiCl3 and PhSiCl3 have also been strongly documented; all of which are illustrating that the condensations of trifunctional organosilicon compounds can result into a diverse range of products with general formula (RSiO3/2)n. Such product architectures include random polymeric networks of a ladder-like polymer89 resulting from intermolecular condensation, incompletely condensed structures and the fully condensed polyhedral products; that arise from intramolecular condensation (Scheme 5).90 In the recent past, material and polymer scientists/engineers have improved the thermo- oxidative stability of the polymers through the use of nanostructured materials and composites such as caged silsesquioxanes for both the insertion of graftable functional/reactive organic groups and the ultimate dispersion of such molecular materials into polymeric matrices. Importantly also, the percentage loading of POSS cages into a polymer matrix can directly influence the physical and mechanical properties, including strength, polymer modulus, interfacial properties and so on.91,92 Studies with c-C6H11SiCl3 and PhSiCl3 have also been strongly documented; all of which are illustrating that the condensations of trifunctional organosilicon compounds can result to a wide range of products of general formula (RSiO3/2)n. Such product architectures 27 include random polymeric networks of a ladder-like polymer89 resulting from intermolecular condensation, incompletely condensed structures and the fully condensed polyhedral products; a consequence of intramolecular condensation (Scheme 5).90 In the recent past, material and polymer scientists/engineers have realized the successful improvement of the thermo-oxidative stability of the polymers through the use of nanostructured materials and composites such as caged silsesquioxanes for both the insertion of graftable functional/reactive organic groups and the ultimate dispersion of such molecular materials into polymeric matrices. Importantly also, the percentage loadings of POSS cages into a polymer matrices can result in significant improvements on their physical and mechanical properties, including strength, polymer modulus, interfacial properties and so on.91,92 2.3.1 Synthesis and Functionalization of Condensed T6 Cages Silanetriols have particularly been shown to be suitable precursors to caged silsesquioxanes, giving rise to a number of different cages. Compared to T8R8, the synthesis of T6 compounds by this method has been limited to a relatively small substrate scope. However, like their T8 counterparts discussed in the sections that follow, T6 cages can be transformed to incompletely condensed cages that can be used as models for silica surfaces and supports for transition metal catalysts.38 Scheme 9 shows synthetic routes to T6R6 from either RSiCl3 or RSi(OMe)3 as precursor silanes.10 28 Scheme 9: General Scheme for the Synthesis of T6R6 Species10 It has been shown by the Vogt group34 that fully condensed, prismatic cyclohexyl silsesquioxane, (c-C6H11)6Si6O9 can be obtained from the hydrolytic condensation of the cyclohexyltrichlorosilane precursor in aqueous acetone and water (Scheme 10).93 29 Scheme 10: Synthesis of R6Si6O9 from RSiCl3 (R = c-C6H11)93 The resulting T6 POSS was very unstable due to the presence of the two strained six- membered rings which upon exposure to tetraalkylammonium hydroxide undergo ring openinig to form tetrasilanols. In a variation to the above however, a superior synthetic route for hexasilsesquioxane cages (T6) for a range of alkyl and aryl substituted trichlorosilanes was reported by Taylor et al.143 using a “nonaqueous” hydrolytic approach with dimethylsulfoxide (Scheme 11). The trichlorosilane was treated with a solution of DMSO in chloroform for 24 h to afford good yields of the hexasilsesquioxanes. Scheme 11: Synthesis of R6Si6O9 from RSiCl3 (R = c-C6H11) and DMSO solvent In 2017, the Pietschnig group reported the synthesis and complete characterization of fully condensed hexameric polyhedral oligomeric silsesquioxane (R6T6, where R = 30 cyclopentyl) from the hydrolytic condensation of cyclopentyl substituted silanetriol (Scheme 12).94 Scheme 12: Synthesis of R6T6 from the Hydrolytic Condensation of Cyclopentyl Substituted Silanetriol (R = cyclopentyl) The possibility that similar compounds bearing secondary alkyl substituents can be obtained via same route was proposed. 2.3.1.1 Base-catalyzed Cleavage and Homologation of T6 Oligosilsesquioxanes A wide variety of fully condensed silsesquioxane frameworks have been developed since their discovery by Scott in 1946.31 Remarkable progress has particularly been realized in the area of functionalizing the organic peripheral unit on the silicon backbone.41,95–97 However, regardless of the progress recorded on this chemistry so far, research on the selective cleavage of the inorganic Si-O-Si core had been limited. Lately, this has changed and a good deal of investigation is now on the selective cleavage of one or more Si–O–Si linkages prior to derivatization.41,98 The Feher group has made progress in this area and today condensed silsesquioxane cages can be transformed into novel compounds by cage breaking to give the incompletely condensed counterparts that can be functionalized or homologated. 31 Nguyen, et. al. described an approach for modeling POSS cages with the R6Si6O9 frameworks into precursor products usable in polymer synthesis and as ligands for transition-metal catalysts (Scheme 13).99 Thus Cy6Si6O9 (Cy = c-C6H11) was subjected to hydrolysis with aqueous NEt4OH in THF to afford endo-C2h-Cy6Si6O7(OH)4, which was further hydrolyzed (via CySi(OH)3 fragments) into the homologated endo-C3 Cy7Si7O9(OH)3. Scheme 13: Hydrolysis Cy6Si6O9 (Cy = c-C6H11) and Homologation to the Heptacyclohexyl Silsesquioxanetrisilanol POSS trisilanol ligands with reactive pendant groups possess the necessary structure to immobilize catalyst. For instance, titanium-silsesquioxane catalyst system have been used in ethylene polymerization.100 32 2.3.2 Synthesis of Condensed T12 POSS Compounds Little reported work is documented on methodologies that involve the direct synthesis of T12 cages. In 2012 however, Lee, et al.116 devised an effective route for the synthesis of caged dodecaphenylsilsesquioxane (T12-phenyl) via a base catalyzed hydrolysis of a phenyltrimethoxysilane precursor under ambient conditions. Prior to this development, the possibility of selectively synthesizing just a single isomer of any one cage silsesquioxane had been an unmet synthetic challenge.117,118 This challenge was attributed to a complex mixture of factors, including precursor monomer employed, type of catalyst and the reaction conditions. Importantly also, the hydrochloric acid produced under aqueous conditions makes controlling the hydrolysis– condensation equilibrium even more difficult. Thus, upon addition of phenyltrimethoxysilane (PTMS) onto a solution of potassium carbonate dissolved in distilled water and THF and stirred at room temperature for 14 days, a white precipitate of dodecaphenylsilsesquioxane was formed (Scheme 14). Scheme 14: Synthetic Scheme for T12-phenyl Mechanistic studies followed by 29Si NMR disclosed that a transient T1 species is the major intermediate in the condensations involved (Scheme 15). Unlike the T8 cage silsesquioxanes which several groups 145-149proposed to go through the more stable T2 33 tetraol intermediate species Si4O4R4(OH)4, kinetic studies conducted by the Hwang group116 shows the appearance of the intermediate T1 dimer, Si2OPh2(OH)4 at three days and its slow disappearance accompanied by the appearance of the two unique peaks on the 29Si NMR attributed to the fully condensed T12 cage structure. The T1 dimer was completely consumed in 14 days. Scheme 15: Proposed Reaction Pathway for T12-Phenyl Cage Silsesquioxanes Hwang and coworkers postulated that the bias toward T12 over T8, or T10, is a consequence of the relatively slow rate of condensation and rearrangement processes that resulted in the generation the most energetically favored product. 2.3.3 Synthesis of T8 Cages 2.3.3.1 Synthesis and Characterization of Completely Condensed POSS Cages Having established the primary contributions by Scott,31 Barry and Gilkey32 for the initial discovery and characterization of completely condensed octafunctional silsesquioxanes in the foregoing sections, additional research efforts on furthering this chemistry have ensued in a good number of research groups because of the three-dimensional nature and high symmetry of these compounds. A practical observation over the years had been the challenge to get appreciable yields for various synthetic routes to octafunctional silsesquixanes. With the R substituent and reaction conditions for their formation being 34 crucial, this difficulty has been ascribed to the complex hydrolysis and condensation kinetics and thermodynamics involved in their formation. With octaphenylsilsesquioxanes for instance, the task (overcoming poor yield) is even more daunting with the virtual insolubility of this compound in almost all organic solvents and the ability for it to undergo decomposition before melting. Consequently, while better yields are realized for some POSS synthesis under acid catalyzed conditions, others are obtained under base catalysis. Generally, the cubic silsesquioxanes are much easier to prepare owing to their relatively better solubility in organic solvents and hence the ease to purify them. Additionally, a wide range of functional groups have been introduced onto the cage, either by direct synthesis from the monomer or by cage-opening and functionalization of the resulting cubic structure.89,101,102 Among the various approaches to synthesize silsesquioxanes, the octafunctional analogues are mostly obtained from the hydrolysis of trifunctional chlorosilanes in suitable organic solvents. Both acid and base catalyzed route to the cubic silsesquioxanes have been reported with better yield often observed from the base catalyzed routes. While acid catalyzed hydrolysis favors linear polymers and branched polysiloxanes, base catalyzed hydrolysis89 (Scheme 16) is often biased toward cube and pseudo-ladder silsesquioxanes. 35 Scheme 16: Base-promoted Synthesis of Cubic Octaphenylsilsesquioxane from Phenyltrichlorosilane To date, there are no clear-cut procedures for the effective synthesis of the deca- and dodeca-silsesquioxanes. Mostly, they are obtained as minor products in the synthesis of the octafunctional analogues. A mechanistic study for the acid catalyzed synthesis of poly(phenylsilsesquioxane was documented by the Yamamoto group (Schemes 17, 18).103 Scheme 17: Reaction Scheme for the Acid Catalyzed Synthesis of the Half-cage all- cis-Tetrasilanol In that study, it was illustrated that phenyltrichlorosilane initially undergoes hydrolysis to afford the silantriol, which first condenses to form a dimer. The dimer undergoes further condensation via a linear tetramer and subsequent cyclization to yield the all-cis-tetramer (Scheme 18). 36 Scheme 18: Pathway for the Acid Catalyzed Hydrolysis of PhSiCl3\ This product can now undergo slow transformation in the solid state to give the fully condensed octaphenylsilsesquioxane species (Scheme 19). Scheme 19: Typical Reaction Scheme for the Synthesis of Completely Condensed T8 POSS from cis-Cyclic T4 Tetraol It is worth mentioning that the all-cis tetramer, cis(1,3,5,7-tetrahydroxy)-1,3,5,7- tetraphenylcyclooctasiloxane, had been isolated by several groups but the Feher team104 was the first to report its crystal structure. This half cage (all-cis tetramer) has been obtained by both acid and base explored routes. For instance, Shchegolikhina et al. 37 developed a base-catalyzed approach for the synthesis of the tetramer from stoichiometric amounts of NaOH in n-butyl alcohol. This base catalyzed approach is also proposed to go through similar species as shown in Scheme 15 above, but much better yields observed. To illustrate such a pathway, the octakis(4-bromo phenyl)octasilsesquioxane was obtained by the Hursthouse group105 in pure form from the cyclic tetra(4-bromo- phenyl) tetrasiloxanetetraol sodium salt (Scheme 20) prepared following the same literature procedure as reported for Scheme 15 above. Scheme 20: Synthesis of octakis(4-bromophenyl)octasilsesquioxane from the Cyclic Tetrakis(4-bromophenyl) tetrasiloxanetetraol Sodium Salt 38 Interestingly, modification of reaction conditions and use of different reagents and starting chloro- and alkoxy-silanes have seen the formation of a good number of octameric silsesquioxanes with potentially wider industrial applications. For instance, Hurstthouse described another variation for the synthesis of T8 POSS cages with improved yields from the treatment of trialkoxysilanes with a THF solution of tetrabutylammonium fluoride (Scheme 21).105 Scheme 21: Synthesis of T8 POSS cages from the Hydrolysis of Trialkoxysilanes Tetrabutylammonium Fluoride In a more general and efficient approach, functionalized T8 POSS cages, particularly the octaphenyl- silsesquioxanes were obtained in yields of up to 80% by the Laine group (Scheme 22).106 In addition to this product, the polymeric analogue, polyphenylsilsesquioxane was also obtained as a by-product. 39 Scheme 22: General Pathways for the Formation of OPS and PPS 2.3.3.2 Synthesis of Bifunctional Janus Cubes Janus structures are completely condensed silsesquioxanes bearing two different R peripherals with each tetrameric unit carrying a unique R corner substituent. The resulting structure from the condensation of two different half-cubes, cis-[RSi(ONa)O]4 and cis- [R’Si(ONa)O]4, is a fully condensed bifunctional silsesquioxane cage with two unique functional peripheral groups (Figure 5). Thus, the successful synthesis of various R- 40 substituted cis tetramers provides the platform to access two-faced condensed cubic silsesquioxanes, otherwise known as ‘Janus’ cubes.110 Figure 5: General Structure of a Two-Faced Janus Cube The bifunctional ‘Janus’ structures are designed to allow surface modification and/or thin films with nanometer-length control of individual layers.110 Additionally, these structures can accommodate deposition or layer-by-layer nano-coatings of hydrophobic/hydrophilic character and hence either serve as interfaces or control self-assembly wherever complementary or dissimilar chemical functional moieties are employed.110 The Laine group110 is credited for their tremendous efforts in developing routes to a variety of Janus cubes from various half cube tetraorganyl sodium salts. Starting with the fully condensed octaphenylsilsesquioxane, Asuncion et al.110 conducted a base-promoted hydrolysis to symmetrically cleave OPS and obtain the corresponding tetrasodium salt (Scheme 23). Scheme 23: Synthetic Route to Half-cage Tetraphenylsiloxane Tetrasodium Salt 41 The half-cube was characterized using NMR spectroscopy after converting the tetraphenyl tetrasodium salt into the tetramethyltetra-phenyl (Me4Ph4) dimethoxy half cube derivative [PhSi(O)O(Me)Si(OCH3)2]4 upon reaction with MeSiCl3 in methanol at room temperature (Scheme 24). Scheme 24: Synthesis of the Tetramethyltetra-phenyl (Me4Ph4) dimethoxy Half- cube Further reaction of the Me4Ph4 dimethoxy half cube via hydrolysis with H2O and concentrated HCl110 generated the fully condensed Me4Ph4 Janus cube in 85% yield (Scheme 25). Scheme 25: Synthesis of [PhSiO1.5]4[MeSiO1.5] Janus Silsesquioxane Cube The tetraphenyl tetrasodium salt half cage was also functionalized into the dimethoxy derivatives of different trichlorosilanes in methanol followed by acid hydrolysis to afford their corresponding Janus silsesquioxanes. The structure for a typical tetraphenyltetravinyl dimethoxy derivative, [PhSiO1.5]4[vinylSiO1.5] 46 obtained by the same process (Figure 6). 42 Figure 6: The tetraphenyltetravinyl, [PhSiO1.5]4[vinylSiO1.5], Janus from the functionalized Ph4 half cube sodium salt with vinyltrichlorosilane However, the precise oligomerization and hydrolysis mechanism for these structures is yet to be understood. 2.3.4 Incompletely Condensed POSS Cages Contrary to moving from monomeric chlorosilanes to condensed oligomers, fully condensed silsesquioxanes have also been ring-opened to offer a range of structures (Figure 7) that can be either double decker (Figure 7a) or other partial/corner ring- opened (Figure 7b) silsesquioxanes. Both acid and base catalyzed routes to incompletely condensed silsesquioxane cages have been reported. Figure 7: Incompletely Condensed Silsesquioxanes, (7a) Double Decker, and (7b) Partially Opened Silsesquioxane 43 Such functional silsesquioxanes have enormous potential as building blocks for the synthesis of nanosized inorganic-organic Si-O polyhedral clusters, hence making these structures potentially useful materials for a number of purposes including the ability to be dispersed into polymer matrices either as pendant groups or as beads on chain. Bifunctional silsesquioxanes are particularly useful because they provide the platforms for dispersing them into polymer chains as “string of beads”. However, this application has been limited by the relatively few synthetic routes known in the literature for the synthesis of these bifunctional precursors. The Feher group is noteworthy for discovering routes into a wide range of novel compounds by cage breaking. Progress in this direction have been recorded mostly for condensed octameric POSS molecules.111 For instance, Feher et al.42 obtained the cyclohexyl substituted silsesquioxane trisilanol, Cy7Si7O9(OH)3, 11 in 23% yield by refluxing a THF solution of cubic Cy8Si8O12 with 35% aqueous Et4NOH for 4 h followed by neutralization with dilute aqueous HCl (Scheme 26). Scheme 26: Synthesis of cyclohexyl substituted silsesquioxane trisilanol, Cy7Si7O9(OH)3 from the fully condensed octacyclohexyl silsesquioxane In the late 80’s also Feher and co-workers mimicked a protocol that was first developed by Brown and Vogt,100 to synthesize the cyclohexyl derivatives of the incompletely 44 condensed trisilanol [Cy7Si709(OH)3] 11, disilanols [Cy8Si8011(OH)2] 13 and a completely condensed hexacyclohexylsilsesquioxane [Cy6Si606] 24 from the kinetic hydrolytic condensation of monomeric cyclohexyltrichlorosilane (CySiCl3) in acetone/water medium (Scheme 27).112 Scheme 27: General Reaction Scheme for the hydrolytic condensation of cyclohexyltrichlorosilane (CySiCl3) in aqueous acetone Formation of the trisilanol results from the hydrolytic removal of one silicon atom from a stable octameric silsesquioxane, hence making it susceptible to reactions that can fill the vacant vertex. Functionalization of the trisilanol via corner-capping of the trisilanol silsesquioxane framework with trifunctional monomeric MeGeCl3, MeSnCl3, and CpZrCl3 afforded condensed strain-free products bearing seven silicon atoms with one much larger transition metal atom (Scheme 28).84,90,102,143,144 45 Scheme 28: Corner-capping Octacyclohexyl silsesquioxane trisilanol with Trifunctional Monomers In a complete variation to the above strategy, a more detailed approach showing the formation and reactivity of species obtained from the base-catalyzed hydrolysis of phenyltrialkoxysilanes in various reaction solvents was investigated by Kawakami and Lee.113 The duo noted interesting observations based on the feed ratio of the metallic hydroxide, MOH, to the trialkoxyphenylsilane unit. Thus, cyclic sym-cis (PhSiO2)4 tetraphenylsilsesquioxanetetraol tetrasodium salt was the major product when equimolar amounts of NaOH to the Si compound were subjected to refluxing water-i-propanol or water-i-butanol system (Scheme 29). Scheme 29: Synthesis and Functionalization of tetraphenylsilsesquioxanato tetrasodium salt On the contrary, when an excess of the NaOH was added to the silicon compound, cubic octaphenylsilsesquioxane was obtained as the major product (Scheme 30). 46 Scheme Phenyltrimethoxysilane 30: Synthesis of Cubic Octaphenylsilsesquioxane from The heptaphenylsilsesquioxanetriol trisodium salt was also obtained after 20 h reaction time and, at a prolonged reaction time involving the consumption of the trisilanol above, the double-decker octaphenyl silsesquioxanetetraol tetrasodium salt was formed in high yield (Scheme 31). Scheme 31: Synthesis of heptaphenylsilsesquioxanato trisodium salt from Phenyltrimethoxysilane 2.3.5 Cage Cleaving Cleavage of completely condensed silsesquioxanes can be effected to give various incompletely condensed counterparts. For instance, Feher et al.41 reported both the selective acid and base promoted cleavage of one Si-O-Si of fully condensed [RSiO3/2]n frameworks and the manipulation of functional groups for use as precursors in the synthesis of hybrid inorganic/organic materials. Also, Laine and Roll13 and Li and Kawakami125 independently demonstrated the formation of incompletely condensed octaphenylsilanol POSS from the hydrolysis of the completely condensed cage. In our 47 particular case aimed at desymmetrizing double decker oligomeric Silsesquioxanes, the work reported in the literature by Laine and Roll13 for the cleavage of completely condensed Ph8T8 was of great interest. 2.3.5.1 Acid-Catalyzed Approaches Under acid promoted approaches, Feher and coworkers42,90,95,98112 synthesized a variety of difunctionalized open caged silsesquioxanes from the selective cleavage of a single siloxane (Si-O-Si) linkage (Figure 8). Figure 8: Difunctional POSS cages obtained by the Selective Cleavage of One Si- O-Si Linkage According to their procedure, briefly refluxing a mixture of R8Si8O12 (R = cyclohexyl or cyclopentyl), excess of HBF4·OMe2 and BF3·OEt2 in CDCl3 or C6D6 can afford a C2v- symmetric difluoroPOSS framework resulting from the fissure of one Si–O–Si linkage of the condensed cage precursor (Scheme 32).108 48 Scheme 32: Synthesis of exo-Octacyclohexyl Octasilsesquioxane Difluoride Hydrolysis of this difluoride with Me3SnOH and aqueous HCl can proceed with retention of the stereochemistry at the silicon-bearing fluorine atom to afford the disilanol (Scheme 33). Scheme 33: Hydrolysis of exo-octacyclohexyl octasilsesquioxane difluoride to the disilanol Interestingly, this group demonstrated that the HBF4 is more important for the selective cleavage of one Si-O-Si linkage rather than acting as a fluoride source. This conclusion was reached when their trial with 5 equivalents of TfOH which lacks a fluoride, in benzene afforded the disilanol product in 30 minutes. However, the ditriflate resulting from TfOH is difficult to handle and undergoes spontaneous hydrolysis upon exposure to water to give the endo-disilanol. This endo disilanol was also obtained from the addition of H2O.Et2O and excess triethylamine to the ditriflate (Scheme 34). 49 Scheme 34: Synthetic scheme for endo-octacyclohexyl octasilsesquioxane disilanol from Cy8Si8O12 The structure of the endo disilanol was confirmed by capping with MeHSiCl2 in the presence of NEt3 followed by subsequent hydrosilylation with Kardstedt’s catalyst to give the functionalized structure A below (Scheme 35). Scheme 35: Functionalization of endo disilanol with MeHSiCl2 and subsequent Hydrosilylation 2.3.5.2 Base-Catalyzed Approaches Similar cages to those shown in Schemes 5-6 have also been obtained via base- catalyzed techniques. In addition, an interesting hydrolysis of condensed Cy6Si6O9 (Cy = 50 c-C6H11) framework followed by homologation to the heptacyclohexyl silsesquioxanetrisilanol was reported by Ziller et al.38 For instance, with a (vinyl)Si(OMe)3 monomer, Cy6Si6O9 (Cy = c-C6H11) undergoes hydrolysis and homologation in aqueous NEt4OH to afford trisilanol A’ (Scheme 36). Scheme 36: Hydrolysis and Homologation of Cy6Si6O9 Cy6(vinyl)Si7O9(OH)3 to endo-Cs- Upon exposure of Cy6Si6O9 24 with aqueous NEt4OH in THF for 1 h, two Si-O-Si linkages were selectively cleaved to give a tetrasilanol 27 with complete retention of stereochemistry at Si.39,100,114 This structure however undergoes decomposition into the Cy7Si7O9(OH)3 upon prolonged reaction time of several hours with a loss of the two silicon signals of tetrasilanol and appearance of the new Si signals characteristic of the trisilanol.38 Later, Li et al.47 developed a protocol for the synthesis of diphenyl-functionalized silsesquioxane (DPS) based on octa-isobutyl silsesquioxane endo-disilanol (OIBS-EDN) obtained from the fully condensed octa-isobutyl silsesquioxane (OIBS) following the selective Si-O-Si bond cleavage route reported by the Feher group and subsequent derivatization with the addition of diphenyldichlorosilane (Scheme 37).115 51 Scheme 37: Synthesis of (i-C4H9)8Si8O13(C6H5)2 from endo-(i-C4H9)8Si8O10(OH)2 In a somewhat different approach, a review assembled by Laine and Roll in 2011 disclosed various ways of generating a number of open-caged silsesquioxanes from their fully condensed precursors, particularly for the condensed octafunctional POSS cages.13 Thus varying the feed ratio of the metallic base to that of the PhSiO1.5 unit, a variety of open cage structures were achieved (Schemes 38-39). Scheme 38: Synthesis of Heptaphenyl Silsesquioxanetrisilanol from the Base- Catalyzed Hydrolysis of Condensed Ph8T8 52 Scheme 39: Synthesis of Octaphenyl Silsesquioxanetetrasilanol from the Base- Catalyzed Hydrolysis of Condensed Ph8T8 2.3.6 Symmetric Polyhedral Oligomeric Silsesquioxanes As stated earlier, double decker oligomeric silsesquioxanes (DDSQ) are a special case of open cage silsesquioxanes with two cyclosiloxane rings in parallel planes linked by two oxygen bridges (Figure 9). To date, only double deckers with phenyl moieties as organic substituents are known. Figure 9: Structures of Double Decker Oligomeric Silsesquioxanes (DDSQ) Functionalization of the open ends by capping with a suitable di- or tri-functional group results in the generation of a functionalized symmetric POSS compound. Based on the capping agents used, the resulting functionalized POSS can either be a single compound or one that displays geometric isomers (cis- and trans-isomers) (Figure 10). Interestingly, the cis and trans isomers display different physical properties, like melting point and solubility. 53 Figure 10: Structures of Symmetric POSS Compounds (59’) no isomer (59’’) cis/trans isomers Thus, the bridging of incompletely condensed double decker oligomeric silsesquioxanes with chlorosilanes is quite intriguing; and can lead to different product types based on the type of silylating agent. For instance, Kawakami and Lee obtained the teramethyldisilylDDSQ cage as the only product in quantitative yield upon treatment of the DDSQ(ONa)4 with dimethyldichlorosilane – a symmetrical capping agent (Scheme 40).113 Scheme 40: Synthesis of DDSQ(Me)4 from Double-Decker Silsesquioxane Tetrasodium Salt DDSQ(ONa)4 However, if the bridging dichlorosilane has two different substituents, the bridging reaction occurs to afford geometric isomers as described above. Such geometric isomers (cis and trans) are known for several capping agents. For instance, the Zheng group reported the synthesis of 9,19-di(hydro)-9,19-dimethyloctaphenyl double-decker silsesquioxane 54 monomer by treating the double decker tetrasilanol with MeHSiCl2 and Et3N (Scheme 41).129 The mixture was not separated into its pure cis/trans forms. Scheme 41: Synthesis of 9,19-di(hydro)octaphenyl double-decker silsesquioxane from DDSQ(OH)4 In 2017, the Marciniec group reported the cis/trans structures backed with the x-ray structures of 9,19-dihydro-9,19-dimethyloctaphenyl double-decker silsesquioxane (DDSQ-2SiH).109 This product was further subjected to hydrosilylation using various styrylarenes as comonomer (Scheme 42). Scheme 42: Hydrosilylation of 9,19-di(hydro)octaphenyl Double-Decker Silsesquioxane with para-substituted Styrenes In addition to the dihydrido difunctional double decker derivative, the divinyl counterparts were also obtained and similarly subjected to both hydrosilylation, cross-metathesis 55 (Scheme 43a) and silylative cross-coupling (Scheme 43b) to afford molecular and macromolecular modified products. Scheme 43: Cross Metathesis or Silylative Cross Coupling of 9,19- di(vinyl)octaphenyl Double-Decker Silsesquioxane with various Olefins and conjugated Arenes Thus, the nature of the substituents on the chlorosilane is crucial and may accommodate various transformations including hydrosilylation,109,126 Heck coupling, metathesis, etc109,134,135 reactions. 56 CHAPTER 3 Asymmetric Polyhedral Oligomeric Silsesquioxanes 3.1 Mutifunctional Reagents Multifunctional reagents are important synthons in organic synthesis. Asymmetric POSS compounds are known, but not those with a DDSQ architecture. Double decker oligomeric silsesquioxanes are highly symmetrical compounds bearing reactive functionalities. Desymmetrization by way of functionalizing these compounds with different capping agents has been daunting, particularly so with the multiple identical functional groups that are distributed in the structure. This has negatively impacted progress on developing functionalized silsesquioxanes that can be tethered to two dissimilar polymer chains with potentials to improve the physical and mechanical properties of the resulting composite. In recent times, however, some progress has been recorded in this direction. 3.2 Asymmetric Double-Decker Oligomeric Silsesquioxanes (DDSQs) from Prefunctionalized Divinyl-Capped DDSQs Asymmetric double-decker compounds are functionalized DDSQs that are side-capped with two different chlorosilanes or capping agents (Figure 11). Figure 11: Typical Structure of an Asymmetric POSS Compound, ‘A/B Cage’ 57 In 2018, the Marciniec group reported the synthesis of the first asymmetric double-decker oligomeric silsesquioxanes and illustrated how crucial the bridging group in a prefunctionalized symmetrically capped DDSQ can be in determining subsequent pathways for their transformation into more desirable products.134 In this report, Zak et. al.134 described a ruthenium-catalyzed metathesis between a prefunctionalized divinylDDSQ 64 and substituted styrenes for the synthesis of unsymmetrically functionalized DDSQ cages. However, the group ascribed the exclusive (E)-selective silylative coupling on the newly formed olefinic group to the high activity of the [RuHCl(CO)(NHC)(PCy3)]/CuCl catalytic system and the mild reaction conditions used (Scheme 44). Scheme 44: Stereoselective Metathetic Transformations of Divinyl-functionalized DDSQ 58 Even though this route reported an exclusive asymmetric product with high E-selectivity, yet the techniques is limited to the use of vinyl-capped precursors and styryl motifs to give the desymetrized product via metathesis. 3.3 Synthesis and Separation of Asymmetrically Capped Double- Decker Silsesquioxanes Mixtures In a rather different approach with research efforts in our lab, a colleague working on separating the cis- and trans-systems of capped T8 double deckers described a non- selective approach for the synthesis and isolation of asymmetric POSS compounds. Unlike the Marciniec team that reported an exclusive asymmetric product with no side products, the Lee group were able to obtain six different isomers that were cleanly separated and analyzed. This discovery has not only revealed and unraveled another route to asymmetric DDSQs but also contributed to several other documented routes for the separation of the cis and trans isomers which had only been achieved by fractional crystallization.107–109 In his specific case, Vogelsang et al.107 disclosed that functionalized DDSQs bearing different bridging chlorosilanes can be separated by nonpolar-LC based on polarity differences resulting from the different number of silanol groups in the final products. For this reason, the capping agents were carefully chosen to provide a product mixture with zero, one, and two hydroxyl groups that could afford the needed polarity differences to enable their clean separation. Trichlorosilane was therefore used as one capping agent in all cases against other different chlorosilanes. In addition to the cis/trans products present in the product mixture, an interesting class of novel capped cage, the asymmetric POSS cage was also obtained. Thus, the addition of an equimolar premixture 59 of Me(R1)SiCl2 and MeSiCl3 to a THF solution of DDSQ(OH)4 followed by Et3N, and stirring at room temperature for 4 h afforded a product mixture of cis/trans POSS cages and an asymmetric analogue (Scheme 45). Scheme 45: Synthetic Pathway for AA, AB and BB Capped POSS Cages The simultaneous introduction of the two different chlorosilanes to the stirring solution of DDSQ(OH)4 10 created competition at the capping sites (based on kinetics) that led to mixtures of three different products; two sets of symmetric stereoisomers and one stereoisomeric mixture asymmetrically capped DDSQ cages making a total of six products. However, this procedure is also limited to the use of a mixture of polar and non-polar capping agents for their effective separation and/or purification. 60 3.4 Attempts to the Selective Synthesis of Asymmetrically Capped DDSQ Cages The above discoveries in sections 3.2-3.3 and their limitations for asymmetric double deckers is intriguing, hence illustrating the need to unravel better synthetic routes and potential applications of this novel class of silsesquioxanes. The development of an approach to selectively synthesize these types of compounds is therefore imperative. The challenge might be surmountable with some manipulation of the chemistry of protecting groups. To this end, several protecting strategies including silylation, benzylation, benzoylation, borylation and other immobilizing agents (silicon supports) could be the envisaged path. If this is achievable for full protection of the DDSQ(OH)4, selective functionalization subsequent to cage closing would provide access to desymmetrization via silylation. 3.4.1 Masking and Demasking Techniques – A Tool for Asymmetric Synthesis Selectivity in multifunctional compounds is normally carried out using protection techniques. This allows some reactive functionalities in a molecule to be either regio- or chemo-selectively blocked temporarily while another reactive site can undergo transformation. The resulting compound is normally a stable protected substrate that allows deprotection (removal of the masking agent) with reagents that do not destroy the original scaffold or react with the regenerated functional group(s). 3.4.2 Protection of Hydroxyl and Amino Groups with Thiocarbonates Nagle et. al. demonstrated the selective monoprotection of diol, diamines and alcohol using cerium type bases.133 In his approach, mono dithiocarbamates were synthesized in 61 good yields from the coupling of various alcohols, amines and amino alcohols using alkyl bromides, CS2 Cs2CO3 and TBAI in DMF133(Scheme 46). Scheme 46: Monothiocarbonation of diols and diamines This protocol is amendable to multifunctional substrates as they illustrated in the selective bis-thiocarbonation of 1,2,6-hexanetriol by coupling with CS2 and p-methoxybenzyl chloride. 3.4.3 Protection of Hydroxyl Groups with Silyl Reagents Substrates bearing hydroxyl groups have also been protected using silyl reagents for the formation of silo-ether derivatives. Their formation and removal have been achieved under a wide variety of mild reaction conditions. They are readily cleaved by hydrolysis with MeOH, mild acids, mild bases and fluoride ion source.13,14,64,119,125,133 These techniques have been used in several synthetic strategies for the synthesis of various organic compounds and natural products. McDougal and coworkers,119 for instance demonstrated the selective monosilylation of symmetric 1,n-diols at room temperature and attributed the source of selectivity to the properties of the monosodium salt of the diol (Scheme 47). 62 Scheme 47: General Reaction Scheme for the Monosilylation of Symmetric 1,n- diols The rationale with this technique is to enable us to introduce a dichloroorganosilane on the free Si-ONa/Si-OH group after the TMS intermediate formation, then deprotection of the TMS group(s) and finally introduction of a second capping agent. Extension of these developments to silsesquioxane cages with the intention to assemble asymmetric A/B systems is yet to be reported. 3.4.3.1 Use of Silylating Agents as Protecting Groups on DDSQ(OH)4 Possible structures that could result from the protection of the hydroxyl groups with silylating agents are shown in Figure 12. 63 Figure 12: Protected DDSQs with TMSCl Structures 80, 82 and 83 are more likely to provide the requisite protection for the introduction of two different capping agents that could afford the desired asymmetric POSS compound. As an application of this technique to silsesquioxane chemistry, Kawakami and Lee113 have demonstrated that the hydroxyl functionalities of double decker oligomeric Silsesquioxanes can be efficiently masked with TMSCl to form a stable TMS-protected DDSQ (Scheme 48). 64 Scheme 48: Silylation of DDSQ(ONa)4 with Trimethylchlorosilane In our case, to selectively mask tetrafunctional substrate DDSQ silanol,64,119 it is envisioned that the masking agent should partner with the active reagent by selectively blocking one or more of the reactive functionalities while capping/closing manipulations are carried on the other end with a selected capping agent. The protected hydroxyl group could then be de-blocked followed by introduction of a different capping agent to complete the construct of the A/B cages (Scheme 49). 65 Scheme 49: Monoprotection of hydroxyl groups with trialkylsilyl chloride 3.5 Use of Solid Support/Immobilizing Agent in the Synthesis of Asymmetrically Capped DDSQs Silsesquioxanes could be blocked using solid surface support/immobilized reagents. Similar to the case with TMSCl, one side of the cage could be blocked using an immobilized reagent like ‘Red-Sil’123 or Merrifield resin.136 The rationale here is to covalently bind/immobilize one part of the reactant to a solid support, where owing to site isolation, only one side would react. Thus, the solid surface supports should provide anchorage to symmetrical cages using a linker while the open end of the molecule is closed with one equivalent of the organo dichlorosilane capping agent. 66 Such a support mechanism can be achieved through grafting (the covalent bond is anchored via the reaction of the reagent with silica surface hydroxyls/silanols to prevent leaching of the grafted moiety) [89(a), Figure 13], or in the case of alkoxysilanes, the use of a ligand as a linker [89(b), Figure 13] through a process called ‘tethering’.123,124 Figure 13:Schematic Representation of some Immobilization Techniques on a (Silica) Support; [89(a)] Grafting, [89(b)] Tethering Once the first capping agent has closed one end, the bond on the immobilized surface can be cleaved and a subsequent capping effected using a different dichlorosilane (Scheme 50). 67 Scheme 50: Protection of Hydroxyl Groups with Solid Surface Support/Immobilized Reagents Application of this technique to DDSQ(OH)4 10 was explored in our lab. Attanayake et al. achieved significant anchorage of the DDSQ(OH)4 to Red Sil but all attempts to detach the solid support by ozonolysis proved futile (Scheme 51).137 68 Scheme 51: Hydrosilylation of Propargylic Chloride with Red-Sil and Reaction of the Resulting Modified Red-Sil Surface with DDSQ(OH)4 3.6 Selective Deprotection Techniques 3.6.1 Desilylation of Ph8DDSQ(OTMS)4 to Ph8DDSQ(OH)4 Functionalized DDSQs and similar structures bearing TMS groups in remote parts of the architecture have been demasked using various techniques.127–129 Shoen, et. al.127 functionalized DDSQ(OH)4 with TMS-aniline protected chlorosilane and deprotected the aniline moiety without destroying the cage using acidified methanol (Scheme 52). 69 Scheme 52: Desilylation of TMS-protected aniline DDSQ with Acidified Methanol Thus, upon addition of H+/MeOH to an Et2O mixture of the prefunctionalized (N- trimethylsilyl)2-aniline-3-(dichloromethylsilane) DDSQ(OH)4 100 and stirring the mixture at 25 °C for 20 h, followed by workup, Schoen obtained the deprotected cis/trans forms of DDSQ(AP)(Me) 101. 3.6.2 Other TMS Deprotection Techniques Several other methods for the deprotection of silyl-ethers in organic synthesis have been reported in the literature.138 Most common of these techniques include the use of: MeSiCl3/MeOH, fluoride sources as TBAF, a mixture of AcOH/THF/H2O/HCl in various proportions,139 and so on. However, these approaches have so far been underexplored in silsesquioxane chemistry. 3.6.2.1 Desilylation of 3,13-di(trimethylsilyl)oxypropyl DDSQ for the synthesis of 3,13-dihydroxypropyl DDSQ Wei et al.,129 in 2015, reported the synthesis of 3,13-dihydroxypropyl DDSQ from the hydrosilylation reaction of a prefunctionalized DDSQ(H)(Me) 61 with a TMS protected allyl alcohol to give the intermediate 3,13-di(trimethylsilyl)oxypropyl DDSQ 104 which was ultimately deprotected with MeSiCl3 and MeOH (Scheme 53). 70 Scheme 53: Desilylation of 3,13-di(trimethylsilyl)oxypropyl DDSQ with MeSiCl3/MeOH Thus, the addition of MeOH followed by dropwise addition of MeSiCl3 to a stirring solution of 3,13-di(trimethylsilyl)oxypropyl DDSQ in dichloromethane afforded the desilylated cage in quantitative yield. This report was coincident with progresses recorded by a colleague in our lab, Attanayake,137 using the same reagents and approach as reported by the Zheng group. 71 CHAPTER 4: Experimental Section 4.1 Materials and Methods 4.1.1 Materials Except otherwise stated, all operations were done under an inert atmosphere using standard Schlenk techniques. Solvents such as tetrahydrofuran (THF) and toluene were freshly distilled prior to use. Distilled THF was obtained from a still of commercial THF distilled over benzophenone and sodium metal at a temperature of 50 oC under nitrogen. Distilled toluene was obtained from a still of commercial toluene distilled over calcium hydride at a temperature of 120 oC. Other solvents (dichloromethane, diethylether, chloroform, methanol and acetonitrile) were used as purchased without further purification. For a start, octaphenylsilsesquioxane [(C6H5SiO1.5)8 [OPS] was synthesized from phenyltrimethoxysilane [C6H5Si(OMe)3] following literature procedure by Laine and Roll,13 Later, all POSS compounds were obtained from Hybrid Plastics (Hattiesburg, MS), chlorosilanes and triethylamine were purchased from Sigma Aldrich. Triethylamine and chlorosilanes were freshly distilled over calcium hydride prior to use. 4.2 Analytical Techniques 4.2.1 NMR Spectroscopy All 1H-NMR, 13C-NMR and 29Si NMR were recorded on an Agilent DirectDrive2 500 MHz NMR spectrometer equipped with an OneProbe operating at 500 MHz for 1H NMR, 126 MHz for 13C NMR, and 99 MHz for 29Si NMR CDCl3 or THF-d and recorded at 25 ºC. 1H- NMR spectra were recorded with 8 scans, a relaxation delay of 1s, and a pulse angle of 72 45o and referenced to the residual protonated solvent in CDCl3 (7.24 ppm). 13C-NMR spectra were collected with 254 scans, a relaxation delay of 0.1 s, and a pulse angle 45°. 29Si NMR spectra were recorded with either 256 or 512 scans, a relaxation delay of 12 s and a pulse angle of 45°. All 2D experiments were acquired at 25 ºC on a Varian UNITY- Inova 600 spectrometer equipped with a 5 mm Pulsed-Field-Gradient (PFG) switchable broadband probe and operating at 599.80 MHz (1H) and 119.16 MHz (29Si). The 1H chemical shifts were referenced to that of residual protonated solvent in CDCl3 (7.24 ppm). 29Si{1H} NMR data were acquired using a recycle delay of 12 s with inverse-gated decoupling, a pulse angle set to 90° and referenced against the lock solvent using vendor supplied lock referencing. Also, 1H–29Si HMBC NOESY correlations were applied to confirm 29Si NMR and other assignments to compounds synthesized for the first time with the pulse angle set to 45°. Solid-state NMR spectra at 9.4 T (ν0 = 79.5 MHz (29Si) or 100.6 MHz (13C)) were recorded at ambient temperature (296 K) on a Bruker Avance 400 spectrometer equipped with a 4 mm MAS probe. All spectra were acquired using magic angle spinning (MAS) at spinning speeds between 9 and 14 kHz, and high power 1H decoupling was applied during acquisition. 4.2.2 Mass Spectroscopy APCI-mass spectra have been recorded on a Finnigan LCQ Deca (ThermoQuest) technologies with LC/MS/MS (quadrupole/time-of-flight) and Waters Xevo G2-XS UPLC/MS/MS inert XL MSD with SIS Direct Insertion Probe. 4.2.3 Infrared Spectroscopy IR spectra have been recorded using a JASCO-ATR PROONE FT/IR 6600 spectrometer. 73 4.2.4 X-Ray Crystallography X-ray diffraction measurements were performed on a Stoe IPDS2 or a Bruker-AXS SMART APEX 2 CCD diffractometer using graphite-monochromated Mo Kα radiation. The structures were solved using direct methods (SHELXL-97) and refined by full-matrix least-squares techniques against F2 (SHELXL-97). Cell parameters were retrieved using the SAINT (Bruker, V8.34A, after 2013) software and refined using SAINT (Bruker, V8.34A, after 2013) on 5941 reflections, 47 % of the observed reflections. Data reduction was performed using the SAINT (Bruker, V8.34A, after 2013) software which corrects for Lorentz polarization. 4.2.5 Melting Point Determination Melting points were measured with a Thomas HOOVER capillary uni-melt melting point apparatus and are uncorrected. 4.3 Experimental Procedures 4.3.1 Synthesis of Cubic Octaphenylsilsesquioxane [OPS] from Phenyltrimethoxy- silane [PhSi(OMe)3] Scheme 54: General Synthetic Scheme of Cubic Octaphenylsilsesquioxne 74 4.3.1.1 Experimental Procedure - Synthesis of Cubic Octaphenyl Silsesquioxane To an oven-dried, three-necked 250 mL round bottom flask equipped with magnetic stir bar and a cooling condenser, was added PhSi(OMe)3 1 (29 mmol, 5.74 g, 5.41 mL) and KOH (8.91 mmol, 0.50 g). The flask was placed under a nitrogen atmosphere and freshly distilled toluene (50 mL) added to the mixture with a syringe. The mixture was heated to reflux at 110 °C, and deionized water (1.0 mL) slowly added over a period of 30 minutes. The reaction mixture was refluxed for 60 h. The precipitated solid was filtered, washed with methanol (3 x 20 ml) and dried at 70 °C for 12 h to afford a white solid of the completely condensed octaphenyl silsesquioxane [Ph8T8] 2 in 2.39 g (64% yield) (Scheme 54).13,32,34,35 The resulting materials were characterized by 1H and 29Si (solid state) NMR spectroscopy. 1H NMR (500 MHz, CDCl3) δ 7.78 – 7.71 (m, 16H), 7.46 – 7.41 (m, 8H), 7.38 – 7.31 (m, 16H). 29Si NMR (99 MHz, CDCl3, TMS) δ: -79.15 (8Si, O3SiPh). mp > 350 ⁰C. Data are consistent with literature report.13 4.4 General Procedure for the Synthesis of tetra(sodio)octaorganyl silsesquioxa- nolate, [DDSQ(ONa)4] Scheme 55: General Reaction Scheme for the Synthesis of Double Decker Tetra- sodium Salts [DDSQ(ONa)4] 75 4.4.1 Experimental Procedure - Synthesis of Ph8DDSQ(ONa)4 To a 100 mL round bottom flask equipped with a stir bar was added Ph8T8 2 (4.13 g, 4 mmol) and NaOH (0.64 g, 4 equiv.). The reaction mixture was placed under nitrogen and 24 mL of iso-butanol added. Next 0.1 mL (2 equiv.) of water was added, and the mixture stirred at 90 ⁰C for 24 h. The resulting white precipitate was filtered, washed several times with 2-propanol and dried under vacuum at 70 ⁰C for 24 h. Product identification was done by derivatizing the sodium salt 49 to the Ph8T8 (OTMS)4 50 and characterized by 1H, 13C, 29Si NMR and MS. 4.4.2 Synthesis of Tetrakis(trimethylsilyl)octaphenyl Silsesquioxane [Ph8DDSQ(OTMS)4] 4.4.2.1 Experimental Procedure - Synthesis of [Ph8DDSQ(OTMS)4] Scheme 56: Synthesis of Tetrakis(trimethylsilyl)octaphenyl Silsesquioxane [Ph8DDSQ(OTMS)4] from [Ph8DDSQ(ONa)4] Ph8DDSQ(ONa)4 49 (2.32 g, 2 mmol) was charged into a 100 mL oven-dried round bottom flask equipped with a magnetic stirrer. The flask was purged with nitrogen gas for 30 minutes and THF (30 mL) added to it. Trimethylchlorosilane (42.4 mmol, 21.2 equiv.) was then added followed by dropwise addition of triethylamine (35.6 mmol, 5 mL) for 1 minute (Scheme 56). The reaction mixture was stirred at room temperature for 3 h. Deionized water (10 mL) was added to dissolve any NaCl produced and to hydrolyze unreacted 76 Me3SiCl.113 The organic layer was separated by means of a separating funnel. n-Hexane (10 mL) was added, and the organic layer repeatedly washed with deionized water. n- Hexane (50 mL) was added to this solution, and the solution left standing at -30 ⁰C in a refrigerator overnight. The precipitated solid was filtered off and dried at reduced pressure to afford 50, a white solid in 1.78 g (66%), mp. > 340 °C. The product was characterized by 1H, 29Si NMR, APCI+ QTof LCMS Measurements. 1H NMR (500 MHz, CDCl3) δ 7.62- 7.20 (40H, multiplet), 0.2 (s, 36H). 13C NMR (126 MHz, CDCl3) δ 134.24, 133.55, 131.75, 130.07, 129.69, 127.42, 1.88. 29Si{1H} NMR (99 MHz, CDCl3, TMS) δ 10.61 (4Si, OSiMe3), -75.99 (4Si, O3SiPh), 78.82 (4Si, O3SiPh). LC/MS QTof (m/z): 1375.28 [M + NH4]+, calc‘d: 1357.25. Fine crystals suitable for single X-ray crystallographic analysis were obtained by slow evaporation of a 10 wt% THF/i-PrOH solution of the compound according to the procedure described in the literature.113 ORTEP plot discloses that the compound is tetrakis(trimethylsilylated) ‘‘double-decker’’ octaphenylsilsesquioxane [Ph8T8(OTMS)4] 50 (Table 1) and that the crystal structure (Figure 14) provided has 2 molecules of THF per molecule of interest. 77 Table 1: Crystallographic data for [Ph8DDSQ(OTMS)4] 78 Figure 14: Crystal Structure of [Ph8DDSQ(OTMS)4] 50 4.4.2.2 Synthesis of i-Bu8DDSQ(OTMS)4 from i-Bu8DDSQ(ONa)4 Scheme 57: Reaction scheme for the Synthesis of i-Bu8DDSQ(OTMS)4 from i- Bu8DDSQ(ONa)4 Procedure Into a stirring solution of i-Bu8DDSQ(ONa)4 49’ (2 mmol, 2.0 g) obtained according to the procedure outlined in section 4.2.1 in THF (20 mL) under nitrogen, trimethylchlorosilane (42.4 mmol, 5.4 mL) was added followed by dropwise addition of triethylamine (35.6 79 mmol, 5 mL) for one minute. The solution was stirred at room temperature for 3 h (Scheme 57). Deionized water (10 mL) was then added to dissolve produced sodium chloride and to hydrolyze the unreacted trimethylchlorosilane. The organic layer was separated by means of a separatory funnel.113 Next, n-hexane (10 mL) was added, and the organic layer repeatedly washed with deionized water. The solvent was evaporated to give a colourless liquid 85 in 1.56 g (66%). The product was characterized by 1H, 13C, 29Si NMR measurements. 1H NMR (500 MHz, CDCl3) δ 1.83 (dp, J = 13.1, 6.7 Hz, 8H), 0.98 – 0.94 (m, 48H), 0.60 – 0.49 (m, 16H), 0.21 – 0.05 (m, 36H). 13C NMR (126 MHz, CDCl3) δ 26.38 – 25.53 (m), 24.93, 24.30, 24.25 – 23.49 (m), 22.48, 2.07 – 1.54 (m). 29Si NMR (99 MHz, CDCl3, TMS) δ: -8.70 (4Si, OSiMe3), -67.95 (4Si, O3SiiBu), -68.29 (4Si, O2SiOSiMe3). 4.4.2.3 Synthesis of i-Bu8DDSQ(OH)4 from i-Bu8DDSQ(ONa)4 Scheme 58: Synthesis of i-Bu8DDSQ(OH)4 from i-Bu8DDSQ(ONa)4 Procedure Acetic acid (0.6 g, 10 mmol) was added dropwise to a THF (20 mL) dispersion of i- Bu8DDSQ(ONa)4 49’ (2.0 g, 2 mmol) under nitrogen atmosphere according to the procedure described in the literature by Kawakami and Lee.113 The reaction mixture was stirred at room temperature for 1 h. The solution was then neutralized with a saturated aqueous sodium hydrogen carbonate (10 mL) and washed with deionized water (10 mL). 80 The organic layer was dried over anhydrous magnesium sulfate, filtered and the solvent evaporated by rotary evaporation. The resulting liquid was washed with acetonitrile to obtain a colorless oily product 106 in 1.06 g (58%). The product was characterized by 1H, 13C and 29Si NMR, APCI+ QTof LCMS Measurements. 1H NMR (500 MHz, CDCl3) δ 1.89 – 1.82 (m, 8H), 0.98 – 0.94 (m, 48H), 0.59 (tdd, J = 11.1, 8.4, 5.6 Hz, 16H). 13C NMR (126 MHz, CDCl3) δ 25.78, 25.74, 25.65, 23.93, 23.90, 23.09, 23.03, 22.78, 22.35, 14.08. 29Si NMR (99 MHz, CDCl3) δ -58.54 (4Si, i-BuSiO3), -68.52 (4Si, i-BuSiO2OH). 4.5 Synthesis of Symmetric POSS Cages 4.5.1 Corner-Capping of Double Decker Oligomeric Silsesquioxanes (DDSQs) 4.5.1.1 General Experimental Procedure for the Synthesis of Tetramethylocta- phenyl silsesquioxane [Me2SiPh8DDSQSiMe2] and Dimethyldihydridoocta- phenylsilsesquioxane, MeHSiPh8DDSQSiMeH from Ph8DDSQ(ONa)4 [Ph8DDSQ(ONa)4] 49 (2 mmol, 2.3 g) and THF (30 mL) were charged into a pre-dried flask bearing a magnetic stir bar. The flask was immersed into an ice bath, purged with highly pure nitrogen (nitrogen passed through a pad of silica) and stirred vigorously for 1 h. Next, a suitable capping agent (R1R2SiCl2, 2 equiv.) in 5 mL THF was added dropwise over a period of 30 minutes. The reaction mixture was stirred at 0 ⁰C for 4 h and at room temperature for 20 h. The insoluble solid was removed by filtration, and the solvent together with other volatiles by rotary evaporation to obtain a white solid. The solid was washed with MeOH (3 x 20 mL) and dried in vacuo at 40 ⁰C for 24 h (Schemes 59 and 60). Products in yields of 1.53 g (65%); mp. = 272 °C and 1.68 g, (72%), mp. = 254 °C for 81 [Me2SiPh8DDSQSiMe2] 60 and [MeHSiPh8DDSQSiMeH] 61 respectively were characterized by 1H, 13C, 29Si NMR and MS. Scheme 59: The Synthesis of Tetramethyloctaphenyl Silsesquioxane, Me2SiPh8DDSQSiMe2 Spectral data: 1H NMR (500 MHz, CDCl3) δ 7.56 – 7.17 (m, 40H), 0.30 (s, 12H). 13C NMR (126 MHz, CDCl3) δ 134.03, 130.35, 127.74. 29Si NMR (99 MHz, CDCl3) δ -16.54 (2Si, O2SiMe2), -78.60 (4Si, O3SiPh), -79.59 (4Si, O3SiPh). LC/MS QTof (m/z): 1182.11 Da [M + H]+, (calc‘d: 1181.82 Da). Scheme 60: Synthesis of Dimethyldihydridooctaphenyl Silsesquioxane, MeHSiPh8DDSQSiMeH Spectral data: 1H NMR (500 MHz, CDCl3) δ 7.59 – 7.16 (m, 40H), 5.00 (s, 2H), 0.38 (s, 6H). 13C NMR (126 MHz, CDCl3) δ 134.08 – 133.87 (m), 130.44, 127.84 – 127.59 (m), 0.62. 29Si NMR (99 MHz, CDCl3) δ -32.76 (2Si, MeHSiO2), -77.80 (4Si, PhSiO3), [-79.08 (1Si, s), -79.28 (2Si, s), -79.48 (1Si, s)] (cis/trans isomers). Spectral data are consistent with the structures of the compounds and the spectral data reported by Zheng et. al.126 82 4.6 Synthesis of Asymmetric POSS cages by Monoprotection (One-Pot Approach) Scheme 61: Attempted Monoprotection of DDSQ(ONa)4 with Me3SiCl as Protecting Group 4.6.1 General Experimental Procedure To a 100 mL pre-dried round bottom flask equipped with a stir bar, was added Ph8DDSQ(ONa)4 49 (2 mmol, 2.32 g) and THF (20 mL). The flask was purged with highly pure nitrogen (nitrogen passed through a pad of silica), and after vigorous stirring for 10 min, Me3SiCl (1 equiv., 0.22 g, 0.25 mL) was added dropwise (Scheme 61). The solution was stirred for a further 45 minutes at 25 ⁰C and then poured into Et2O (25 mL), washed with 10% K2CO3 (5 mL) and brine solution (5 mL).119 The mixture was then dried with anhydrous Na2SO4 and concentrated in vacuo. Attempts to further purify the resulting crude mixture were unsuccessful and no useful spectroscopic information could be obtained. 83 4.7 Tris- and Bis-protection of DDSQ(OM)4 with Trimethylsilylchloride Scheme 62: Condition Screening for the Optimal Production of Tris-(Me3Si)- Ph8DDSQ(OH) 4.7.1 General Procedure for Tris-Protection and Bis-Protection with TMSCl Into a 100 mL pre-dried round bottom flask equipped with a magnetic stirrer, was added Ph8DDSQ(ONa)4 49 (2 mmol, 2.32 g) or Ph8DDSQ(OH)4 10 (2 mmol, 2.14 g) and THF (20 mL). The flask was placed under nitrogen and the reaction mixture stirred vigorously for 10 minutes. Next, Me3SiCl (y equiv.) was added and stirring continued at 25 ⁰C for 45 min.119 The mixture was poured into Et2O (25 mL), washed with 10% K2CO3 (5 mL) and brine solution (5 mL), dried with anhydrous Na2SO4 and concentrated in vacuo. Products were purified by flash column chromatography (hexanes:ethylacetate = 30:1) and solvent evaporated to give two white solids (Table 2). The products were characterized individually by 1H, 29Si NMR, HMBC, NOESY, APCI+ QTof LCMS Measurements. Fine crystals suitable for single X-ray crystallographic analysis were obtained by slow evaporation of a 10 wt% THF/i-PrOH solution of both compounds.113 Ph8DDSQ8(SiMe3)3(OH) 84 Spectral data: 1H NMR (500 MHz, CDCl3) δ 7.45 – 7.06 (m, 40H), 4.99 (s, 1H), 0.13 (s, 9H), 0.11 (s, 9H), -0.01 (s, 9H). 13C NMR (126 MHz, CDCl3) δ 134.44, 134.38, 134.26, 84 134.35, 134.28, 134.18, 134.01, 127.64, 127.53, 127.51, 127.49, 1.90, 1.48. 29Si NMR (99 MHz, CDCl3) δ 15.60 (1Si, OSiMe3), 11.41 (1Si, OSiMe3), 11.27 (1Si, OSiMe3), -68.51 (1Si, O2Si(OH)Ph), -72.67 (1Si, O3SiPh), -76.11 (1Si, O3SiPh), -76.15 (1Si, O3SiPh), - 78.22 (2Si, O3SiPh), -79.29 (2Si, O3SiPh). 1H-29Si HMBC NMR (100 MHz, CDCl3) showing cross peaks. 1H-1H NOESY NMR (100 MHz, CDCl3). APCI+ QTof LCMS (m/z): [M + H]+ 1285.21 Da, (calc’d: 1285.21 Da). mp. 167 – 171 ⁰C. These data correlate with the crystal structure obtained for Tris-TMS Ph8T8(OH). Crytallograpic Data: ShelXT/ORTEP plot was determined by single-crystal X-ray diffraction for Crystal Data. C57H69O14Si11, MW = 1287.11, monoclinic, P21/c (No. 14), a = 21.347(3) Å, b = 14.0292(19) Å, c = 22.671(3) Å, β = 96.019(2)°, α = γ = 90°, V = 6752.0(16) Å3, T = 173(2) K, Z = 4, Z' = 1, µ(MoKα) = 0.270, 12519 reflections measured, 12519 unique (Rint = .) which were used in all calculations. The final wR2 was 0.1616 (all data) and R1 was 0.0688 (I > 2(I)). This data represents the crystallographic structure of tris-TMS protected cage (Me3Si)3Ph8DDSQ(OH) (Table 3) (Figure 15). 85 Table 2: Crystallographic Data for Tris-TMS Ph8DDSQ(OH) 84 86 Figure 15: Crystal Structure of [Ph8DDSQ(OTMS)3(OH)] 84 Ph8DDSQ(OH)2(OSiMe3)2 82 Spectral Data: 1H NMR (500 MHz, CDCl3) δ 7.66 – 7.16 (m, 40H), 4.75 (d, J = 10.9 Hz, 2H), 0.18 – 0.03 (m, 18H). 13C NMR (126 MHz, CDCl3) δ 134.41, 134.38, 134.30, 134.13, 134.06, 131.62, 130.59, 130.45, 130.38, 130.29, 127.74, 127.66, 127.58, 1.51. 29Si NMR (99 MHz, Chloroform-d) δ 16.07 (1Si, OSiMe3), 15.68 (1Si, OSiMe3), -69.06 (1Si, O2Si(OH)Ph), -69.28 (1Si, O2Si(OH)Ph), -73.23 (1Si, O3SiPh), -77.44 (1Si, O3SiPh), - 78.27 (1Si, O3SiPh), -79.26 (1Si, O3SiPh). APCI+ QTof LCMS (m/z): 1213.17 Da [M + H]+, (calc’d 1213.17 Da). m.p. 190 – 201 ⁰C. The 1H, 13C, 29Si and MS spectral data are consistent with X-ray crystal structure of Ph8DDSQ(OH)2(OSiMe3)2 82. 87 Crystallograpic Data: C54H60O14Si10, Mr = 1213.92, tetragonal, I-4 (No. 82), a = 29.6068(4) Å, b = 29.6068(4) Å, c = 14.0465(2) Å, α = β = γ = 90°, V = 12312.6(4) Å3, T = 173(2) K, Z = 8, Z' = 1, µ(CuKα) = 2.524, 46831 reflections measured, 11753 unique (Rint = 0.0968) which were used in all calculations. The final wR2 was 0.1849 (all data) and R1 was 0.0681 (I > 2(I)). (Table 3, Figure 16). 88 Table 3: Crystallographic Data for Bis-TMS Ph8DDSQ(OH)2 82 89 Figure 16: Crystal Structure of [Ph8DDSQ(OTMS)2(OH)2] 82 4.8 Functionalization of Ph8DDSQ(OH)(OTMS)3 with MeHSiCl2 Scheme 63: Functionalization of Ph8DDSQ(OH)(OSiMe3)3 84 with MeHSiCl2 Procedure [Ph8DDSQ(OSiMe3)3(OH)] 84 (1 mmol, 1.29 g) was charged into a pre-dried round bottom flask bearing a magnetic stir bar. THF (10 mL) was added to the flask. The flask was immersed into an ice bath and the reaction mixture stirred vigorously under argon for 1 h. 90 MeSi(H)Cl2 (1 equiv., 0.12 g, 0.11 mL) in 5 mL THF was added followed by dropwise addition of triethylamine (1 mmol, 0.14 mL) over a period of 30 minutes. The reaction was stirred at 0 ⁰C for 4 h and at room temperature for 20 h. The suspension was filtered through a glass frit and the filter cake washed with THF (3 x 5 mL). The filtrate was evaporated by rotary evaporation to afford a white solid. The crude product was purified by flash column chromatography (hexane: ethylacetate = 30:1) to afford 107 in 0.98 g (73%). 1H NMR (500 MHz, CDCl3) δ 7.44 – 7.07 (m, 40H), 4.99 (d, J = 0.8 Hz, 1H), 3.74 (s, 1H), 0.13 (s, 9H), 0.11 (s, 9H), 0.02 (s, 3H), -0.01 (s, 9H). 13C NMR (126 MHz, CDCl3) δ 134.43, 134.28, 134.16, 134.10, 134.02, 131.37, 131.15, 130.34, 130.27, 130.12, 130.08, 129.91, 129.81, 127.62, 127.59, 127.57, 127.54, 127.48, 1.87, 1.46. 29Si NMR (99 MHz, Chloroform-d) δ 15.61 (1Si, OSiMe3), 11.42 (1Si, OSiMe3), 11.28 (1Si, OSiMe3), -55.34 (1Si, -OSiMe(H)(OH)), -68.50 (1Si, O3SiPh), -76.10 (1Si, O3SiPh), -76.15 (1Si, O3SiPh), -78.21 (2Si, O3SiPh), -79.28 (2Si, O3SiPh). IR(KBr): 3491 (Ph-OH), 3073-3027 (C-H aromatic), 2968 (C-H aliphatic) 1594 - 1430 (Si-Ph), 1252 - 571 (Si-O-Si), cm1. APCI+ QTof LCMS (m/z): 1345.1748 Da [M + H]+, (calc’d: 1345.2102 Da). mp > 320 ⁰C. 4.9 Attempts at TMS Deprotection 4.9.1 Desilylation of Ph8DDSQ(OSiMe3)4 50 to Ph8DDSQ(OH)4 10 Scheme 64: Attempted Desilylation of Ph8DDSQ(OSiMe3)4 with MeOH and MeSiCl3 91 Procedure Ph8DDSQ(OSiMe3)4 50 (0.2 mmol, 0.27 g) and THF (5 mL) were added to a flask equipped with a magnetic stir bar. Methanol (4 equiv, 0.2 mmol, 2 mL) was added to the flask with vigorous stirring. Next, methyltrichlorosilane (1.5 equiv, 0.075 mmol, 0.045 g, 40 µL) dissolved in 60 µL THF was added dropwise over a period of 30 minutes using a syringe. The reaction mixture was stirred at room temperature for 5 h (Scheme 64). The excess methyltrichlorosilane and solvent were removed by rotary evaporation and the resulting solid product recrystallized from a mixture of THF/hexane (10/10 by volume).37 The crude product was dried under vacuum at 40 ⁰C for 24 h and analyzed by 29Si NMR. 29Si NMR (99 MHz, Chloroform-d) δ -38.68, -47.75, -47.97, -64.65, -78.50. See Screening results (Table 4) In a different approach from the conditions above, desilylation was also attempted using conditions as reported Lee et al (Scheme 56).127 4.9.2 Desilylation with Acidified MeOH in Ether Scheme 65: Attempted Desilylation of Ph8DDSQ(OSiMe3)4 50 with Acidified MeOH Procedure Ph8DDSQ(OSiMe3)4 50 (0.2 mmol, 0.27g) and THF (2 mL) [or Et2O (2 mL)] were added to a 50 mL round bottom flask equipped with a magnetic stir bar. The flask was placed 92 under nitrogen at room temperature. Next, 1 mL of acidified methanol (H+/MeOH) was added dropwise within 1 minute and the reaction mixture stirred at room temperature for 24 h. The heterogenous mixture was filtered and the retentate dried in an oven at 90 ⁰C for 5 h.108 The crude product was characterized by 29Si NMR (Table 5). 29Si NMR (99 MHz, Chloroform-d) δ 7.28, 3.30 [Appendix 38 (mineral acids). (acetic acid)]. 93 CHAPTER 5 Results and Discussion 5.1 Cubic OPS [Ph8T8 or T8] 31 Section 4.3.1.1 describes the transformation of commercial PhSi(OMe)3 into octaphenylT8; the compound that is used as starting material for the attempted synthesis of the asymmetric POSS compounds. Like the Laine group106 where PhSi(OEt)3 was used as starting material, hydrolysis and polycondensation of PhSi(OMe)3 under identical conditions afforded the cubic OPS, Ph8T8 31 target (Scheme 66). Spectral data were in good agreement with those of the Laine team. In addition to the fully condensed cage, polyphenylsilsesquioxane was also obtained as a byproduct in line with their observations. Scheme 66: Synthetic Scheme of Cubic Octaphenyl Silsesquioxne (OPS) 5.1.1 Tetra(sodio)octaorganyl silsesquioxanolate, [DDSQ(ONa)4] and its TMS Derivatives The selective cleavage of two Si-O-Si linkages of the synthesized OPS in Section 4.3.1.1 following the Kawakami route125 is illustrated in Sections 4.4.1 – 4.4.2. Hydrolysis of OPS with NaOH afforded [Ph8DDSQ(ONa)4] 49, which was characterized by conversion into 94 the TMS derivative (Scheme 67). [Ph8DDSQ(OSiMe3)4] 50 was isolated in gram scale as a white crystalline solid (66%), mp > 340 °C. Scheme 67: Synthesis of Tetrakis(trimethylsilyl)octaphenyl Silsesquioxane [Ph8DDSQ(OTMS)4] Spectral data discloses 36 methyl-protons and 40 phenyl-protons by 1H NMR and three kinds of Si atoms (each integrating to four (4)), characteristic of the structure of the target compound. This is consistent with the literature report13 and matched the crystal structure of the compound (Figure 17). Figure 17: Crystal Structure of [Ph8DDSQ(OTMS)4] Unlike [Ph8DDSQ(OTMS)4], 50 several attempts including silylation with TMSCl, PhHSiCl2, Ph2SiCl2, and stannation with SnCl4 to derivatize [i-Bu8DDSQ(ONa)4] 49’ into 95 crystalline organic products were unsuccessful. In all attempted cases, the products obtained were oily. However, the spectral data obtained for both [i-Bu8DDSQ(OTMS)4] 85 and [i-Bu8DDSQ(OH)4] 106 perfectly matched the respective structures (Figures 18–21). Figure 18: 1H NMR of [i-Bu8DDSQ(OTMS)4] 85 96 Figure 19: 29Si NMR of [i-Bu8DDSQ(OTMS)4] 85 97 Figure 20: 1H NMR of [i-Bu8DDSQ(OH)4] 106 98 Figure 21: 29Si NMR of [i-Bu8DDSQ(OH)4] 106 5.2 Symmetric POSS Cages The procedure for corner capping DDSQ(ONa)4 for the synthesis of symmetrically functionalized DDSQs is outlined (Scheme 68). Functionalization of the open ends with suitable di-functional chlorosilanes (RR’SiCl2) affords the symmetrically capped POSS cages. 99 Scheme 68: Corner-Capping of Octaphenyl Silsesquioxane Tetrasodium Salt [DDSQ(ONa)4] 49 With Me2SiCl2 (or any capping agent with two identical R groups on the chlorosilane), only a single product 60 was obtained. However, with two different R-substituents on the chlorosilane (e.g. MeHSiCl2), 61 was isolated as its geometric isomers (cis and trans). Clearly, unlike 60, the 29Si NMR spectrum for 61 shows further splitting of signals in the - 77 to -80 ppm range. Thus, the resonances in the -32.76 (2Si, MeHSiO2), -77.80 (4Si, PhSiO3) and -79.28 (4Si, PhSiO3) ppm are assigned to the trans isomer, and those at - 32.76 (2Si, MeHSiO2), -77.80 (4Si, PhSiO3), -79.08 (2Si, PhSiO3), and -79.48 (2Si, PhSiO3) ppm denote the silicon nuclei for the cis configuration (Figure 22). Thus, the cis- isomer exhibits two pairs of signals in a 1:1 ratio for the internal silicons of the tetracyclosiloxane rings linked by two oxygen atoms with unique resonances at -79.08 (2Si) and -79.48 (2Si). 100 Figure 22: 29Si NMR Spectrum Showing Signals for cis/trans Isomers of MeHSiDDSQSiMeH 61 These results are in good agreement with the chemical shifts reported by Wang et al.126 Me2SiPh8DDSQSiMe2 60 was isolated as a single white compound in 65% yield; mp = 272 °C and MeHSiPh8DDSQSiMeH 61 as a white geometric isomer (cis and trans) in 72% yield; mp = 254 °C. 101 5.3 Selective Protection of DDSQ(OH)4 10 with TMSCl Scheme 69: Synthesis of Asymmetric POSS Cage via Monoprotection In our attempts at synthesizing asymmetric POSS cages from the highly symmetrical DDSQ precursor, we turned to protection chemistry. As described in the literature, monosilylation and desilylation have been achieved for the protection of various organic functionalities like the OH group. McDougal and coworkers,119 for instance demonstrated the selective monosilylation of symmetric 1,n-diols at room temperature and attributed the source of selectivity to the properties of the monosodium salt of the diol (Scheme 70). These properties are based on the solubility of the sodium salt in the reaction solvent and the kinetics of silylation and proton transfer. The McDougal team claimed that the rate of silylation of the sodium salt precursor is faster than that for the proton transfer from the 102 alkoxide to the already silylated product. Thus, upon addition of the silylating agent the small amount of the sodium salt that dissolved in the solvent is immediately silylated. Scheme 70: Monosilylation of Symmetric 1,n-diols The starting materials for most asymmetric product are symmetrical precursors. Bifunctional products of type X CH2(CH2)nCH2Y can result from the selective protection of one hydroxyl group in (HO)CH2(CH2)nCH2(OH) (Scheme 70). Achieving selective protection from such precursors is therefore crucial; especially for cases where all the hydroxyl groups in the molecule are identical (all primary, or secondary or tertiary). In our attempt to extend this concept to our DDSQ(ONa)4 precursor, the procedure as outlined below was followed and reaction progress monitored by 29Si NMR (Scheme 71). Scheme 71: Attempted Monoprotection of DDSQ(ONa)4 with Me3SiCl The sodium salt precursor was obtained from the sodium hydroxide hydrolysis of OPS 2 (Sections 4.4.1 – 4.4.2). The voluminous white precipitate that resulted from this reaction was believed to be DDSQ(ONa)4 49. Silylation of 49 with sub-stoichiometric amounts will unavoidably afford a statitstical mixture of the unreacted tetrasodium salt, monosilylated, bis silylated, tris-silylated and tetra-silylated products. In this vein, selective silylation of 103 our substrate, DDSQ(ONa)4 49 with TMSCl was carried out with the hope of achieving some partial protections (Scheme 70). 29Si NMR of the crude product showed a complex mixture with no indication of the mono-protected product. Attempts to purify the products by either column chromatography or recrystallization/precipitation proved futile. This drove us to elaborate the protocol by setting up a series of reactions to examine whether increasing the amount of TMSCl could enable us to achieve some partial protections (Scheme 72). Scheme Ph8DDSQ(OH)(OSiMe3)3 72: Condition Screening for the Optimal Production of Results from the screening process are given in Table 4. Interestingly, NMR data indicates some multiple protection. From the crude material, both Ph8DDSQ8(OSiMe3)3(OH) 84 and Ph8DDSQ(OSiMe3)2(OH)2 82 were isolated in their pure forms by flash column chromatography. Surprisingly, the bis-protected cage with the two TMS groups on the same side of the molecule (Figure 23) was not observed. 104 Probably, compared to 81 (Figure 23), formation of the bis-TMS cage of type 82 is energetically the most favorable - the one that requires the least energy for formation. Figure 23: Ph8DDSQ (OSiMe3)2(OH)2 81 with the TMS groups on the same side 105 Table 4: Substrate screening for optimal yield of Ph8DDSQ (OSiMe3)3(OH) 84 and Ph8DDSQ(OSiMe3)2(OH)2 82 Reactions were run on a 2 mmol scale of starting materials, THF (20 mL), room temperature, 1 h. Percent yields are given in parenthesis. Maximal yields for both 82 and 84 (entry 4) were obtained with 4 equivalents Me3SiCl using DDSQ tetrasilanol as precursor. The X-ray crystallographic data for the tris-product (Table 3, Chapter 4) shows that the compound is the tris-protected cage, Ph8DDSQ(OTMS)3(OH) 84 (Figure 15). Within the framework of our design (Scheme 69), it is unlikely that 84 can get us to the target asymmetric DDSQs. This is so because any attempts to react the remaining silanols with chlorosilanes and subsequent deprotection can invariably afford symmetrically functionalized DDSQs. Nevertheless, the structure of 84 is still noteworthy as it has the potential to undergo further interesting transformations. 106 Figure 15: Crystal Structure of [Ph8DDSQ(OTMS)3(OH)] 84 The structure of 84 is not only novel but also very interesting. This is so because, so far, reports regarding general modification of POSS silanols and silnolates with chlorosilanes to give incompletely condensed silsesquioxane derivatives are scarce. The few available examples are limited to the complete substitution of the tetrasilanol-POSS, the trisilanol POSS or one or two such groups in trisilanol POSS (Figure 24). Figure 24: Structures of Some POSS Compounds with Silyl Protecting Groups With structures 82 and 84, introduction of certain moieties (eg. vinyl silyl moieties) on to the remaining hydroxyl groups can make them very versatile templates for further 107 chemical modifications. Thus, the silanol group in Ph8DDSQ(OSiMe3)3(OH) 84 is likely to undergo self-condensation to yield a tris-TMS-DDSQ(OH) cage dimer 110 with a new Si- O-Si linkage or surface functionalization via cross condensation with a silane coupling partner. Figure 24: Tris-TMS-DDSQ-OH [Ph8DDSQ(OSiMe3)3(OH)] Dimer Also, the acidity of the silanol group can allow this compound to form hydrogen bonds to organic compounds containing suitable hydrogen bonding sites. The free hydroxyl group can also be functionalized in ways that can open the opportunity to introducing a wide variety of functional groups into this type of organosilicon compounds. For instance, metal catalyzed O-silylation of the Si−OH in Ph8DDSQ(OSiMe3)3(OH) 84 with substituted- allylsilanes for the synthesis of functionalized POSS used in electronic devices. Similarly, the crystallographic data for the bis-product (Table 4, Chapter 5) shows that the compound is the bis-protected cage, Ph8DDSQ(OTMS)2(OH)2. 82 Interestingly, the crystal structure for this compound shows that the free hydroxyl groups are on alternating ends of the two cyclic cyclosiloxane rings (Figure 16). Like the tris-protected TMS cage, Ph8DDSQ(OTMS)2(OH)2 82 could also be an important industrial intermediate in silicone industries. However, unlike Ph8DDSQ(OTMS)3(OH) 84, the presence of two silianols in 108 Ph8DDSQ(OTMS)2(OH)2 82 makes it more multifunctional by providing the opportunity for it to undergo many more transformations. Thus, this compound can be potentially functionalized on the two ends with either the same or different functional groups. It can tether two polymer chains to give POSS-cage containing polymers that can lower the chain mobility of the polymer and also, surface modification via condensation or silylative coupling can, for instance, afford either the polymeric form or products that can undergo further functionalization. Their structure makes them ideal candidates for the synthesis of metalized polyhedral oligomeric silsesquioxane structures that are known to be very suitable for use as catalysts. Metalized POSS compounds have, for instance, been used as cure promoters, catalysts, and as alloying agents that can strengthen polymer microstructures, such as coils, domains, chains, and segments, at the molecular level.140 5.4 Attempted Removal of the TMS Protecting Group from Ph8DDSQ(OTMS)4 50 with MeSiCl3 and MeOH The TMS-masked DDSQ(OH)4 is predominantly composed of Si-O-Si linkages throughout its structure. This architecture has the potential of making the search for reagents and conditions that are selective for the removal of the TMS silicons a serious challenge. In our pursuit for ideal deprotection conditions, the tetrakis-TMS DDSQ cage 5a was chosen as a model substrate for two reasons, viz: Its structure is composed of Si- O-Si linkages, including the new bonds resulting from protection of its silanols with TMSCl and it is much easier to prepare than our preferred intermediate, tris-TMS-DDSQ(OH) 84 and bis-TMS-DDSQ(OH)2 82. However, attempts made with 50 following various literature procedures127–129 and in some case with some modifications, have been unsuccessful. All routes and conditions explored were either not efficient or, in most 109 cases, rupture the inner core siloxy ethers completely. For instance, in our initial attempts to desilylate 50 to 10 using MeSiCl3 and MeOH ruptured the cage compleltely (Scheme 73). Table 5 below displays some of our trial results. Scheme 73: Attempted Desilylation of Ph8DDSQ(OSiMe3)4 50.with MeOH and MeSiCl3 Table 5: Conditions screening for the desilylation of Ph8DDSQ(OSiMe3)4 50 to Ph8DDSQ(OH)4 10 Substrate: Ph8DDSQ(OSiMe3)4, CD = Cage Decomposed Reactions were followed by 29Si NMR spectroscopy. Signals in the -76 to -79 ppm region symbolic of the POSS cage and also the silanol peaks at the -68 ppm region for all conditions screened were absent. The crude 29Si NMR signal peaks in the -37 to -47 region symbolizes D-silicon – silicons bound to two oxygen atoms only. This result thus discloses a completely broken cage. 110 6.5 Attempted Removal of the TMS Protecting Group from Ph8DDSQ(OTMS)4 50 with H+/MeOH In a different approach from the conditions above, desilylation was also attempted using conditions reported by Schoen et. al.127 With Schoen, the precursor was a functionalized DDSQ which is structurally different from mine. While she desilylated a prefunctionalized N-TMS DDSQ (closed cage) with acidified MeOH, our trials were based on an open- caged TMS-functionalized double decker silsesquioxane. This structural difference might be the reason for the susceptibility of all the silo ether oxygens in 50 to attack by the acidified MeOH. Results from exploring her conditions to our system were therefore not surprising, albeit the use of other acids with different pka values (Scheme 74). Results of our trials are shown in Table 6. 111 Scheme 74: Attempted Desilylation of Ph8DDSQ(OSiMe3)4 50 with Acidified MeOH Table 6: Conditions screening for optimal deprotection of Ph8DDSQ(OSiMe3)4 50 with acidified MeOH Reactions were followed by Si-NMR, and like in Scheme 73, results also disclosed completely ruptured cages for the most part (entries 4 and 5). However, with acetic acid, the starting material was recovered (entries 1-3). The deprotection above was one that was applied by Shoen127 (Scheme 52) in removing the TMS group from her aniline-capped DDSQ. Interestingly, both her precursor (cis/trans functionalized DDSQ) and targets are completely different from ours. However, in addition to trying her conditions several other modifications were explored for amenability to our target. Thus, a number of acids with different pka values were investigated in pursuance 112 of appropriate conditions for deprotecting the tetrakis-TMS protected DDSQ (Scheme 74, Table 6). Results were similar to trial 1 for the most part. NMR data either showed a completely ruptured cage (Table 6, entries 4 and 5) or indicated no reactions (entries 1- 3). 113 CHAPTER 6: Conclusions and Future direction 6.1 Conclusions This dissertation explored opportunities to selectively assemble asymmetric POSS compounds starting from cubic octaphenyl silsesquioxane Ph8T8, 31 the double decker tetrasilanol Ph8DDSQ(OH)4 10 or its sodium salt Ph8DDSQ(ONa)4 49. Literature reports for the synthesis of symmetrically functionalized Ph8DDSQ(OH)4 10 or its sodium salt 49 were replicated. Synthesis of the cubic silsesquioxane from PhSi(OMe)3 and subsequent fission of Si-O-Si linkages to give the double decker product was also established. Importantly also, preliminary results from our efforts to synthesize other double decker silsesquioxanes including the i-Bu8DDSQ(OH)4 and Ph12DDSQ(OH)4 from their corresponding cubic precursors have been encouraging. However, desymmetrization of these molecules has been daunting to date. Like any other nascent discoveries, our approach stumbled into some difficulties, yet novel partial silylations were realized using protection techniques. From our proposed route, key potential intermediates were isolated and their crystal structures secured. This progress was constrained by the challenge to uncover the right reagents and conditions that could selectively and efficiently demask the protected DDSQ cage and hence provide the opportunity to introduce the second capping agent. Thus, our initial attempts to access an asymmetric DDSQ via the McDougals’ selective monoprotection failed for double decker silanol or silanolates as precursors. The platform for subsequent bridging upon monoprotection was not realized, likely due to the presence 114 of unprotected functionalities in the octaphenyl POSS cages or the plausible presence of the diprotected species on opposite ends of the cage. This presumption, and possibly other yet unidentified dynamics playing within the reaction vessel, should be further explored to reveal the cause for the failure in monoprotection. However, with some modifications on the technique, the tris-TMS-protected cage was synthesized and isolated. This development was inspiring as it would have provided entry into the asymmetric POSS cage only if deprotection had not crumbled our cage. Potentially though, the tris-TMS DDSQ(OH) 84 being reported for the first time could be utilized in forming particularly metal containing silsesquioxanes as new catalyst systems for both homogeneous and heterogeneous catalysis. This can not only broaden the metallasilsesquioxanes library but will also promote the commercialization of silsesquioxane-based catalytic materials for industrial applications. 6.2 Future Directions The progress recorded this far with the search for selective protocols for entry into asymmetric POSS compounds is noteworthy. A non-selective approach developed by a colleague in our lab has resulted in the generation of the asymmetric A/B POSS cages, albeit with other side products and yields that are no more than 50%. With such developments, a selective approach would ideally provide more scalable atom-efficient chemistry and afford better yields of functionalized asymmetric double deckers. Progress with the tris-protection route by way of being able to isolate and characterize various intermediates are encouraging. The concept surrounding the observed selectivity may be useful for entry into the desired target. This chemistry should therefore be further explored. However, this notwithstanding, and based on the current challenges faced so 115 far, prospective alternatives using other protecting groups and solid state supports described above are worthy of exploration. Focus should particularly be given to use of protecting groups that can result to the formation of other ethers that can exhibit a chemistry (at the capping sights) different from the Si-O-Si linkages that dominates silsesquioxanes architecture. Thus, formation of, for instance, Si-O-C, Si-O-B, Si-O-Sb, Si-O-Sn bonds as against Si-O-Si bonds should be given due considerations. Success on the foregoing could open a new area of research in silsesquioxane chemistry and will allow a great many new nanostructured compounds to be synthesized. With such anticipated glaring prospect, extension of the techniques to other variations of POSS cages including [Ph12T12], [Ph10T10], [i-Bu8T8] and other phenyl functionalized cages will invariably provide a basis for future exploration. This explains why exploration of this chemistry is to be encouraged to expand on the number of functionally useful double deckers for both research and engineering purposes. 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