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Download (6Mb) A Thesis Submitted for the Degree of PhD at the University of Warwick Permanent WRAP URL: http://wrap.warwick.ac.uk/138089 Copyright and reuse: This thesis is made available online and is protected by original copyright. Please scroll down to view the document itself. Please refer to the repository record for this item for information to help you to cite it. Our policy information is available from the repository home page. For more information, please contact the WRAP Team at: [email protected] warwick.ac.uk/lib-publications pn - prr Bonding In Organosilicon Compounds by Maher Y. El-Sheikh A dissertation submitted to the University of Warwick for the degree of Doctor of Philosophy. t O January, 1975. ACKNOWLEDGEMENT This Thesis is the result of approximately two years work, between October, 1972 and December, 1974, and is the climax of my career as a student. It is all entirely original work, except where references are made. I would like to thank Professor V.M. Clark for the use of the laboratory space and other facilities in the Department. I am in great debt to Dr. J. M. Brown, and I would like to take this opportunity to express my most sincere gratitude for all his help, not only as my research adviser, but also as a good friend to whom I could take any problems concerning my life here in England. I would like to acknowledge the help given by Mr. K. Holden for making the glass apparatus required for this work, and Dr. D. Couch for running the nmr spectra on the Bruker nmr machine. Thanks to the European Research Office of the U.S. A. Army for financing this work; all my colleagues in the Department of Molecular Sciences at the University of Warwick, and special thanks to Mrs. Hazel Phillips for typing this Thesis at such short notice. CONTENTS CHAPTER 1 (Introduction)................................................................ 1 (A) Theoretical Aspects of - pn Bonding in Organosilicon Compounds 1 (B) Experimental Evidence for pn - pn Bonding in Organosilicon Compounds 8 (C) On pr - p17-Bonding in Other Heavy Elements 19 (D) Object of the Present Investigation 23 CHAPTER 2 (Nucleophilic Reactions of p-Haloalkylsilanes) .. 24 (A) On the Solvolysis of «-Haloalkylsilanes 24 (B) On the Reactions of Trialkylsilane and Related Compounds 29 (C) Reactions of Chloromethyldimethylsilane with Lithium t- Butoxide in Pyridine 34 (D) A Possible Mechanism for the Reaction Between Chloromethyldimethylsilane and Li OBuC in Pyridine 50 (E ) product Analysis by Mass Spectrometry 56 (F) Experimental * 62 (G) Preparation of Solutions for Kinetic Analyses 75 CHAPTER 3 (An Attempt to Synthesise Silabenzene)............................ 79 (A) g-Elimination of 1,1-Dichloro-1-Sila-2, 4-Cyclohexadiene 82 (B) Reactions of 1-Sila-2, 4-Cyclohexadiene with Ir(I) and Rh(I) Complexes (C) Catalytic Dehydrogenation of 1-Sila-2, 4-Cyclohexadiene 93 (D) Experimental gg R EFER E N C E S........................................................................................ 120 CHAPTER 1 INTRODUCTION (A) Theoretical Aspects of pn-pn Bonding in Organo silicon Compounds Although chemists have known for a long time that compounds containing a carbon carbon double bond are stable, the situation is totally different on going from carbon to silicon. Up to the time of writing, there was n" known isolable species containing a formal double or triple carbon-silicon bond. Theoretical chemists have discussed 2prr -3prr bonding in organosilicon compounds. In 1948, Pitzer^ explained the preference of second row atoms for singly-bonded structures. The atoms of the second and subsequent rows all have inner electron shells which are larger (with respect to both size and number of electrons) than those of the first row. Consequently X-X single bond interatomic distances (Table (I)-1) are all larger for second-row atoms because of the increased repulsive forces between the inner shell electrons of one and the bonding orbital of the other. If a multiple bond is to be formed the py or pz orbitals must approach sufficiently closely to form the tt- component of the multiple bond by overlapping in a "sideways" fashion, and effective achieve­ ment of this is prevented by the repulsive effect of the large inner shells. 3 Mulliken examined Pitzer's concepts quantitatively by evaluating the overlap integral S*, which gives a measure of the electron density between the nuclei under consideration. Table (I)-2 gives calculated values of the over­ lap integral (S) for 2p - 3p orbitals. * S = J* ^ • dv, where ^ and are the atomic orbitals which overlap to form the bond considered, and dv is a small volume element. Table (I)-1 . Single-Bond Interatomic Distances, A°. c - c N - N 0-0 F - F 1.54 1.47 1.47 1.42 Si - Si P - P S - S Cl - Cl 2.34 2.20 2.08 1.98 Ge - Ge As - As Se - Se Br - Br 2.44 2.50 2.34 2.28 Sn - Sn Sb - Sb Te - Te I - I 2.80 2.86 2.76 2.67 2 Data from Pauling . Table (I)-2. Calculated Overlap Integral at Appropriate Distances. C-C OC Si-Si Si=Si Intemuclear Distance (1) 1.54 1.34 2.32 2.14 * R/2r 1.18 1.03 0.91 0.84 S(^)cr-^a) 0.33 0.32 - - S(3po-3p'a) - - 0.37 0.35 S(2pr-^jrr ) - 0.27 - - S0{tt -3prr ) “ 0.33 (R = bond distance( r is the maximum in the radial probability curve, 5= ——— ) r 1 + r 2 2 1 ' If an appropriate value of the parameter § was chosen, the value h r S(2pft - 2 p n ) was found to be less than S(3jjr-3jr)# On repeating this calculation at constant S, one can obtain the expected results of low values of S(3prt-3prr) compared to S(2pn-%rr) as shown from Table (I)-3. Table (I)-3. Calculated Overlap Integral at Constant (I). Bond ? 0 .8 1.0 1.2 S(2po-2pj) 0.21 0.32 0.33 S(3pff-3pa) 0.31 0.39 0.33 S(2prT-2l)T) 0.43 0.29 0.18 S0ÌV-3PTT) 0.37 0.22 0.12 At 5= 1.0, the C-C*-overlaps were found to be greater than Si-Si rr-overlap (0.29 and 0.22 respectively). However, when the ct-overlaps were compared at 5 = 1.0 die third-row overlap were found to be greater than the corresponding second-row overlaps. This led Mulliken to calculate that third-row elements form relatively stronger o-bonds (as compared ton-bonds) than second-row elements. The reluctance for n-bonding in compounds of heavy elements is thus attributed to relatively stronger a-bonds available on polymerisation. 4 In another paper by Mulliken , he pointed out that partial double bond character due to (drt-prr) bonding in compounds of second row elements would improve their stability relative to a situation in which only prr-pr bonding were permitted. In order to study the problem of pn-pn bonding in organosilicon compounds in more detail one must have an idea of die energetics ofn -bonding. From the kinetic data of Flowers and Gusel#nikov^^on the pyrolysis of 1,1 -dimethylsilacyclobutane(9) and from similar work by Davidson and Thomson on trimethylsilane (see experimen­ tal part of the Introduction), Walsh^ estimated a value of 138 + 22 kj/moles for both 3 1f 1-dimethylsilaethylene (1) and 1-methylsilaethylene (2). The independence of the value for n-bonding energy on the substituent, is in agreement with the situation for olefins which also show little change in bond energy on changing g substituent : . The value estimated by Walsh is not very different from a much 9 lower estimate of 117 kj/mol based on the dissociation energy of Si = C . The estimated value for tt-bonding in organosilicon compounds (~- 5 the values for olefins) does not imply instability with respect to a unimolecular decomposition leaction but rather the very reactive behaviour of silaethylene in bimolecular reactions with other molecules. H2C= SiMe2 H2C = SiMeH H2Si = CH2 H„Si = SiH„ 2 2 d) (2) (3) (4) The factors responsible for the instability of prr-pn bonding in organosilicon compounds have been investigated thoroughly by C urtis^, using extended Huckel (EHMO) calculations on the molecules (3) and (4). The calculation showed that n C = Si is exceedingly polar and behaves like a carba'ion-siliconium ion combination. Energy mismatching between carbon and silicon p orbitals was suggested to be an important factor in the instability of silaethylene intermediates, the relatively high overlap integral of the Si= Si n-bond (Table (l)-4) suggesting that compounds con­ taining such bonds might be amenable to isolation. Table (I)-4. Overlap Integrals pn-pn and pn-drr Bonded atoms. Orbitals S Orbitals S 2prr - 2prr 0.270 3prr - 3prr 0.228 2prr - 3pn 0.182 3prr - 3dn 0.448 2prr - 3dn 0.362 3dn - 3drr 0.303 if 1 The high value of dn -pn suggests that its overall contribution to prr - prr bonding in silaethylene and disilaethylene can be substantial in fact the result for silaethylene overlap population is 0.327 and is composed of pn - pn and pn - dn contributions. Fig.(I)-1. 2p-3'p 2p-3d 0.130 + 0.197 = 0.327 pr-pt prT-drr Fig. (I)-1 Contribution ofp*-pand p^-d*hybrid to the silaethylene overlap population. The overlap population for disilaethylene was broken down into p*-p» p-d" and rT-cTcontributions^. Pt-pr prr-drr dT-dn 0.277 +0.348 + 0.038= 0.663 Fig. (I)-2 schematically shows the form of the molecular orbitals and their relative energies as calculated by the EHMO (Extended bfuckel Molecular Orbital) method.
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