Hydrogen Bonding in Silanols and their Adducts A Thesis presented by Lisa Dawn Cother In partial fulfilment of the requirements for the award of the degrees of Doctor of Philosophy of the University of London and Diploma of Imperial College Department of Chemistry Imperial College London SW7 2AY June 1998 Abstract ABSTRACT The work described in this thesis concentrates mainly on the formation of hydrogen bonded adducts of silanols with nitrogen and oxygen containing molecules. Hydrogen bonded adducts of triphenylsilanol, Ph3SiOH, and tetraphenyldisiloxane-1,3-diol, (HOPh2Si)20, with a range of amines, azacrowns, alcohols, ethers and crown ethers have been prepared and characterised by infrared (IR), multinuclear solution NMR and solid state 29Si CPMAS NMR spectroscopy, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The adducts [(Me3S03CSiPh2OH}2.TMEDA, (HOPh2Si)20. { {}10Ph2SiOSiPh20} [Et3N11] } , [(HOPh2Si)20]4.(Et2NH)2, (HOPh2S i)2 0 . dioxane, Ph3SiOH.tris(2-aminoethyl)amine, (Ph3SiOH)2.piperazine, (Ph3SiOH)2.TMEDA and (Ph3SiOH)2.18-crown-6.(H20)2 have also been characterised by X-ray crystallography which reveals a range of hydrogen bonded arrangements. Attempts were also made at preparing, and characterising by X-ray crystallography, unusual or interesting silanols The X-ray structure of tBu2Si(H)OH, a rare example of a stable hydridosilane, is reported. In infrared spectroscopy, the shift in OH streching frequency, AD, between a 'free' SiOH and one involved in hydrogen bonding to a suitable base is proportional to the enthalpy of hydrogen bonding, AH. This relationship was used to assess the relative hydrogen bonding capabilities of a variety of silanols towards bases in solution before adduct formation in the solid state was,attempted. By this method the hydrogen bonding interactions of Ph3SiOH with relatively weak hydrogen bond acceptor molecules such as silyl ethers, siloxanes and alkenes were studied since these species are important in industrial reaction mixtures. The low temperature X-ray structure of the cyclic siloxane (Me2SiO)5 was also determined and the relationship previously proposed between Si-O-Si bond angle and basicity further investigated. Variable temperature infrared spectroscopic studies also enabled the equilibrium constants and enthalpy of hydrogen bonding to be determined quantitatively for Ph3SiOH with a series of ethers. The application of 170 NMR spectroscopy to the study of organosilicon compounds is reviewed and 170 NMR studies carried out on a variety of silanols. The occurance of significant exchange between the OH groups in Ph3SiOH and 170 labelled water was also revealed. 2 Dedication To My Parents 3 Acknowledgements ACKNOWLEDGEMENTS Firstly, I would like to thank my supervisor, Dr. Paul Lickiss, for all his help, encouragement and advice throughout the course of this work. I would also like to thank my industrial supervisors, Dr. Sian Rees and Dr. Richard Taylor (Dow Corning Corporation), for their advice and support, and donations of samples. Thanks must also go to the following people for their help and invaluable services: Paul Hammerton for his time and patience setting up the 170 NMR experiment, as well as helpful discussions and advice on other NMR aspects of this work, also Simeon Bones for setting up the 170 NMR experiment at Dow Corning, Dr. Abil Aliev (University College, London) for the solid state 29Si CPMAS NMR spectra, Prof. David Williams, Dr. Andrew White and Dr. Ian Baxter (Imperial College), and Prof. Mary Mc Partlin and Dr. Nick Choi (University of North London) for X-ray crystallographic studies, Dr. Simon Parsons (Edinburgh University) for low temperature X-ray crystallographic studies, Simon Turner for the loan of many items of equipment, John Barton and Geoff Tucker for mass spectrometry, Hilary O'Callaghan for microanalyses, Dr. Alan Bailey for Raman spectra, and Izoldi Bezougli and Phil Blakeman for themogravimetric analyses. I would like to give a big thanks to the past and present members of the Lickiss research group for providing help, friendship and lots of entertainment over the years: Dipti Shah, Colin Smith, Mark Bronstrup, Phindile Masangane, Guilaine Veneziani and Chris Yates. These thanks are also extended to Carole Dupuy for preliminary research into this project before I started and for her generous hospitality whilst attending the XIth International Symposium on Organosilicon Chemistry in Montpellier. I would also like to thank everyone on the sixth floor for making it an enjoyable and fun place to work. In particular, thanks go to Brent, Nige, Big Steve, Mine and Colin (GBY group), and Phil Dyer, Little Steve, Crispin and Phil B. (DMPM group). Additional thanks go to Phil Dyer for many helpful discussions and practical advice. Many thanks are also due to Edie for brightening up the mornings and her endless kindness and generosity. Thanks go to everyone I met in R & D at Dow Corning, Barry, for their help and for making my time spent there very enjoyable, including the Doonans for their warm hospitality and excellent food. Outside college, I would especially like to thank Sarah Lancaster for being a great friend and flatmate over the last three years and for providing me with such luxurious accomodation. These thanks are also extended to her parents, Mike and Gill, and her sister, Karen, for everything that they have done for me, and for the fun I had. I would also like to thank Roxy, Betty, Alex and Laura for many entertaining evenings out and 4 Acknowledgements dim sum lunches, and Andy for all his patience and encouragement, especially whilst writing up. The EPSRC and Dow Corning Corporation are thanked for financial support via the CASE sheme. Last, but not least, I would like to thank my parents for their continuous support and encouragement throughout my studies. 5 Contents CONTENTS Title Page 1 Abstract 2 Dedication 3 Acknowledgements 4 Contents 6 List of Figures 11 List of Tables 15 List of Schemes 17 List of Abbreviations 18 Chapter 1 Introduction 1.1 General introduction 22 1.1.1 The silanol group in nature 22 1.1.2 The silanol group in industry 24 1.1.3 The degradation of polydimethylsiloxanes (silicones) to silanols in the environment 25 1.2 Synthesis of silanols 27 1.3 Acidity and basicity of silanols 29 1.3.1 Acidity 29 1.3.2 Basicity 30 1.4 Hydrogen bonding interactions of silanols 31 1.4.1 The hydrogen bond 31 1.4.2 Infrared spectroscopy in the study of hydrogen bonding 32 1.4.2.1 Correlation between Aus and the enthalpy of hydrogen bond formation, AH 34 1.4.3 Infrared spectroscopic studies of the hydrogen bonding interactions of silanols in solution 35 1.4.4 X-ray crystallographic studies of the hydrogen bonded structures formed by silanols in the solid state 39 1.4.4.1 Compounds containing one SiOH group, silanols 39 1.4.4.2 Compounds containing an Si(OH)2 group, silanediols, or two Si-OH groups, oc,co-siloxanediols 42 1.4.4.3 Compounds containing an Si(OH)3 group, silanetriols 43 1.4.4.4 Compounds containing both Si-OH and other functional groups 44 6 Contents 1.4.5 Hydrogen bonded adducts of silanols with other molecules in the 48 solid state and their X-ray structures Chapter 2 Infrared spectroscopic studies of the hydrogen bonding interactions of silanols in solution 2.1 Introduction 57 2.2 The relative propensity of a series of silanols towards hydrogen bonding interactions with suitable bases 58 2.3 Infrared spectroscopic studies of the hydrogen bonding interactions between mixtures of silanols in solution 64 2.4 Qualitative infrared spectroscopic studies of the hydrogen bonding interactions between Ph3SiOH and other suitable molecules 65 2.4.1 With silyl ethers and siloxanes 65 2.4.1.1 Low temperature crystal structure of D5 72 2.4.2 With alkenes 75 2.5 Determination of thermodynamic data for hydrogen bonded adducts of Ph3SiOH by infrared spectroscopy 78 2.5.1 Method 78 2.5.2 Thermodynamic data for hydrogen bonded adducts of Ph3SiOH with ethers 82 2.5.3 Thermodynamic data for hydrogen bonded adducts of Ph3SiOH with amines 87 Chapter 3 Hydrogen bonded adducts of silanols 3.1 Introduction 89 3.2 General methods for the preparation of adducts of silanols 90 3.3 Identification and characterisation of adducts of silanols 92 3.4 Adducts of (HOPh2S020 97 3.4.1 With amines 98 3.4.1.1 Primary amines 98 3.4.1.2 Secondary amines 99 3.4.1.2.1 Crystal structure of RHOPh2Si)2014.(Et2NH)2 104 3.4.1.3 Tertiary amines 107 3.4.1.3.1 Crystal structure of (HOPh2S020. ( [HOPh2SiOSiPh20][Et3N11] } 113 3.4.1.4 With nitrogen heterocycles 115 3.4.2 With alcohols 118 3.4.3 With ethers 119 Contents 3.4.3.1 Crystal structure of (HOPh2Si)20.1,4-dioxane 120 3.4.4 With crown ethers 123 3.4.5 With amino acids 123 3.4.6 With mixtures of amines and oxygen containing molecules 124 3.4.7 With phosphines 125 3.5 Adducts of Ph3SiOH 126 3.5.1 With amines 126 3.5.1.1 Primary amines 126 3.5.1.1.1 Crystal structure of Ph3SiOH.tris(2- aminoethyl)amine 127 3.5.1.2 Secondary amines 129 3.5.1.2.1 Crystal structure of (Ph3SiOH)2.piperazine 130 3.5.1.3 Tertiary amines 130 3.5.1.3.1 Crystal structure of (Ph3SiOH)2.TMEDA 135 3.5.1.4 With nitrogen heterocycles 135 3.5.2 With azacrowns 135 3.5.3 With crown ethers 140 3.5.3.1 Crystal structure of (Ph3SiOH)2.18-crown-6.(H20)2 146 3.5.4 With azacrown ethers 149 3.5.5 With amine hydrochlorides 151 3.5.6 With carboxylic acids 152 3.5.7 With phosphines 152 3.6 Adducts of other silanols 153 3.6.1 Adducts of Ph2Si(OH)2 153 3.6.2 Adducts of HO(SiPh2O)3H 154 3.6.3 Adducts of meso-(HOMePhSi)20 and