Octahedral adducts of dichlorosilane with substituted pyridines: Synthesis, reactivity and a comparison of their structures and Si-29 NMR chemical shifts

摘要

H2SiCl2 and substituted pyridines (Rpy) form adducts of the type all-trans-SiH2Cl2 center dot 2 Rpy. Pyridines with substituents in the 4- (CH3, C2H5, H2C=CH, (CH3)(3)C, (CH3)(2)N) and 3-positions (Br) give the colourless solids 1a-f. The reaction with pyrazine results in the first 1:2 adduct (2) of H2SiCl2 with an electron-deficient heteroaromatic compound. Treatment of 1d and 1e with CHCl3 yields the ionic complexes [SiH2(Rpy)(4)]Cl-2 center dot 6CHCl(3) (Rpy = 4-methylpyridine (3d) and 4-ethylpyridine (3e)). All products are investigated by single-crystal X-ray diffraction and Si-29 CP/MAS NMR spectroscopy. The Si atoms are found to be situated on centres of symmetry (inversion, rotation), and the Si-N distances vary between 193.3 pm for 1c (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 SiH2 units, except for the ethyl-substituted derivative 1e, 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 Si-29 NMR chemical shifts (delta(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 1e is an exception and shows the highest shielding. The bonding situation at the Si atoms and the Si-29 NMR tensor components are analysed by quantum chemical methods at the density functional theory level. The natural bond orbital analysis indicates polar covalent Si-H bonds and very polar Si-Cl bonds, with the highest bond polarisation being observed for the Si-N 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.