Octahedral Adducts of Dichlorosilane with Substituted Pyridines: Synthesis, Reactivity and a Comparison of Their Structures and 29Si NMR Chemical Shifts
✍ Scribed by Gerrit W. Fester; Jörg Wagler; Erica Brendler; Uwe Böhme; Gerhard Roewer; Edwin Kroke
- Publisher
- John Wiley and Sons
- Year
- 2008
- Tongue
- English
- Weight
- 607 KB
- Volume
- 14
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Abstract
H~2~SiCl~2~ and substituted pyridines (Rpy) form adducts of the type all‐trans‐SiH~2~Cl~2~⋅2 Rpy. Pyridines with substituents in the 4‐ (CH~3~, C~2~H~5~, H~2~CCH, (CH~3~)~3~C, (CH~3~)~2~N) and 3‐positions (Br) give the colourless solids 1 a–f. The reaction with pyrazine results in the first 1:2 adduct (2) of H~2~SiCl~2~ with an electron‐deficient heteroaromatic compound. Treatment of 1 d and 1 e with CHCl~3~ yields the ionic complexes [SiH~2~(Rpy)~4~]Cl~2~⋅6 CHCl~3~ (Rpy=4‐methylpyridine (3 d) and 4‐ethylpyridine (3 e)). All products are investigated by single‐crystal X‐ray diffraction and ^29^Si CP/MAS NMR spectroscopy. The Si atoms are found to be situated on centres of symmetry (inversion, rotation), and the SiN distances vary between 193.3 pm for 1 c (4‐(dimethylamino)pyridine complex) and 197.3 pm for 2. Interestingly, the pyridine moieties are coplanar and nearly in an eclipsed position with respect to the SiH~2~ units, except for the ethyl‐substituted derivative 1 e, which shows a more staggered conformation in the solid state. Calculation of the energy profile for the rotation of one pyridine ring indicates two minima that are separated by only 1.2 kJ mol^−1^ and a maximum barrier of 12.5 kJ mol^−1^. The ^29^Si NMR chemical shifts (δ~iso~) range from −145.2 to −152.2 ppm and correlate with the electron density at the Si atoms, in other words with the +I and +M effects of the substituents. Again, compound 1 e is an exception and shows the highest shielding. The bonding situation at the Si atoms and the ^29^Si NMR tensor components are analysed by quantum chemical methods at the density functional theory level. The natural bond orbital analysis indicates polar covalent SiH bonds and very polar SiCl bonds, with the highest bond polarisation being observed for the SiN interaction, which must be considered a donor–acceptor interaction. An analysis of the topological properties of the electron distribution (AIM) suggests a Lewis structure, thereby supporting this bonding situation.