Effects of substituents and n-donors on the energies of Si←N,Si←O,and Si=C bonds in hypervalent silenes |
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Authors: | V. G. Avakyan L. E. Gusel’nikov S. L. Gusel’nikov V. F. Sidorkin |
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Affiliation: | (1) A. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 29 Leninskii prosp., 119991 Moscow, Russian Federation;(2) Photochemistry Center of the Russian Academy of Sciences, 7a ul. Novatorov, 119421 Moscow, Russian Federation;(3) Department of Chemistry, M. V. Lomonosov Moscow State University, 1 Leninskie Gory, 119992 Moscow, Russian Federation;(4) A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, 1 ul. Favorskogo, 664033 Irkutsk, Russian Federation |
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Abstract: | The effect of the nature of substituents at sp2-hybridized silicon atom in the R2Si=CH2 (R = SiH3, H, Me, OH, Cl, F) molecules on the structure and energy characteristics of complexes of these molecules with ammonia, trimethylamine, and tetrahydrofuran was studied by the ab initio (MP4/6-311G(d)//MP2/6-31G(d)+ZPE) method. As the electronegativity, χ, of the substituent R increases, the coordination bond energies, D(Si← N(O)), increase from 4.7 to 25.9 kcal mol−1 for the complexes of R2Si=CH2 with NH3, from 10.6 to 37.1 kcal mol−1 for the complexes with Me3N, and from 5.0 to 22.2 kcal mol−1 for the complexes with THF. The n-donor ability changes as follows: THF ≤ NH3 < Me3N. The calculated barrier to hindered internal rotation about the silicon—carbon double bond was used as a measure of the Si=C π-bond energy. As χ increases, the rotational barriers decrease from 18.9 to 5.2 kcal mol−1 for the complexes with NH3 and from 16.9 to 5.7 kcal mol−1 for the complexes with Me3N. The lowering of rotational barriers occurs in parallel to the decrease in D π(Si=C) we have established earlier for free silenes. On the average, the D π(Si=C) energy decreases by ∼25 kcal mol−1 for NH3· R2Si=CH2 and Me3N·R2Si=CH2. The D(Si←N) values for the R2Si=CH2· 2Me3N complexes are 11.4 (R = H) and 24.3 kcal mol−1 (R = F). sp2-Hybridized silicon atom can form transannular coordination bonds in 1,1-bis[N-(dimethylamino)acetimidato]silene (6). The open form (I) of molecule 6 is 35.1 and 43.5 kcal mol−1 less stable than the cyclic (II, one transannular Si←N bond) and bicyclic (III, two transannular Si←N bonds) forms of this molecule, respectively. The D(Si←N) energy for structure III was estimated at 21.8 kcal mol−1. Dedicated to Academician N. S. Zefirov on the occasion of his 70th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1952–1961, September, 2005. |
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Keywords: | silenes hypervalent silenes complexes of silenes with ammonia, trimethylamine, and tetrahydrofuran tetracoordinate silenes, pentacoordinate silenes, n-donors (ammonia, trimethylamine, tetrahydrofuran) energies of intermolecular coordination bonds, energies of intramolecular transannular coordination bonds, rotational barriers, ab initio quantum-chemical calculations |
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