Hexacoordinate Silacyclobutane Dichelate Complexes: Structure, Properties, and Ligand Crossover

Molecular structures of various alkyldichlorosilanes in the solid state

A series of organodichlorosilanes RR'SiCl₂ (R,R' = (CH₂)₃; (CH₂)₄; (CH₂)₅; Me,Me; Me,H; Me,Cl) was studied by single-crystal X-ray diffraction analyses. At ambient temperature liquid chlorosilanes (melting points in the range of 180-220 K) were transferred into glass capillaries and crystallized in situ on a diffractometer. In the solid state structure these chlorosilanes are monomeric, even in the case of the sterically less demanding 1,1-dichlorosilacyclobutane (CH₂)₃SiCl₂. Interestingly, regardless the steric demand of the alkyl substituents, the dialkyldichlorosilanes exhibit essentially the same Cl-Si-Cl angle (for (CH₂)₃SiCl₂, (CH₂)₄SiCl₂, (CH₂)₅SiCl₂, Me₂SiCl₂: 106.08(3)°, 106.07(4)°/105.86(4)°, 106.91(2)° and 105.59(6)°, respectively). Replacement of one alkyl group by hydrogen has only a marginal influence on the Cl-Si-Cl angle (MeHSiCl₂ 106.31(3)°), whereas in MeSiCl₃ slightly wider Cl-Si-Cl angles are found (ranging between 107.04(11)° and 107.86(11)°), in accordance with VSEPR. Computational analyses, i.e., potential energy surface scans of the Cl-Si-Cl angle variation, of (CH₂)₃SiCl₂, Me₂SiCl₂, MeHSiCl₂ and H₂SiCl₂ reveal essentially identical energy profiles for the Cl-Si-Cl deformation in these four dichlorosilanes with basically superimposed curves for (CH₂)₃SiCl₂ and Me₂SiCl₂, whereas with increasing H-substitution the energetic minimum is shifted to a slightly wider Cl-Si-Cl angle. In the crystal packing only MeHSiCl₂ exhibits weak intermolecular SiCl van der Waals contacts, whereas the Si-Cl moieties of the other five chlorosilanes engage in intermolecular ClH and (for (CH₂)₃SiCl₂, (CH₂)₄SiCl₂ and MeSiCl₃) ClCl contacts.