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Experimental Investigations and ab Initio Studies on Hexacoordinated Complexes of Dichlorosilane

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posted on 1998-09-24, 00:00 authored by Karl Hensen, Thorsten Stumpf, Michael Bolte, Christian Näther, Holger Fleischer
Dichlorosilane, SiH2Cl2, forms two different kinds of coordination compounds with pyridine and 3-picoline:  the simple adduct trans-SiH2Cl2(L)2 and the ionic complex [SiH2(L)4]Cl2·4CHCl3 (L = pyridine, 3-picoline). The adducts trans-SiH2Cl2(py)2, 1, and trans-SiH2Cl2(3pic)2, 2, form directly from the reaction of bis(dichlorosilyl)methylamine, NMe(SiHCl2)2 with pyridine (py), and 3-picoline (3pic). Reaction of 1 with an excess of pyridine in chloroform yielded [SiH2(py)4]Cl2·4CHCl3, 3. The molecular and crystal structures of 13 were investigated by single-crystal X-ray diffraction. The Si atoms of all three compounds are hexacoordinated and lie on centers of inversion. The basic structural parameters are the following:  1 (monoclinic, C2/c):  Si−N 196.9(1) pm, Si−Cl 228.8(1) pm; N−Si−Cl 90.0(1)°, C−N−C 119.0(2)o; Cl−Si−N−C 102.4(1)°. 2 (triclinic, P1̄):  Si−N 197.5(1) pm, Si−Cl 229.2(1) pm; N−Si−Cl 90.3(1)°, C−N−C 118.8(1)°; Cl−Si−N−C 101.2(1)°. 3 (monoclinic, C2/m):  Si−N 196.2(4) and 197.0(4) pm, Si···Cl 419.9 pm; N−Si−N 90.0°. All three complexes, neutral and cationic, exhibit very similar lengths of the dative Si−N bond. Hartree−Fock calculations with 3-21G(d) and 6-31G(d) basis sets and density functional calculations with the B3LYP functional and a 6-31G(d) basis set were employed for geometry optimizations of trans-SiH2Cl2(py)2 and [SiH2(py)4]2+. The optimized structures revealed similar Si−N bond lengths and geometric parameters of the pyridine moieties for both types of complexes as well. Single point energy calculations [B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d)], including corrections for the basis set superposition error, showed that the formation of 1 from SiH2Cl2 and pyridine in the gas phase is slightly endothermic (ΔH298 = 17.2 kJ mol-1) and endergonic (ΔG298 = 113.6 kJ mol-1). The significance of the Lewis acidity of SiH2Cl2 is thus questioned and intermolecular interactions (e.g. hydrogen bonds) are held responsible for the existence of 1 in the solid state.