Switching between penta- and hexacoordination with salen-silicon-complexes

The reaction between phenyltrichlorosilane and the tetradentate ligands o-HO-C6H4–C(CH3)double bond; length as m-dashN–(CH2)n–Ndouble bond; length as m-dashC(CH3)-o-C6H4-OH (n = 2, 3, 4), supported by an amine base, yields pentacoordinate silicon complexes (C6H5)Si-[o-O-C6H4–C(CH3)double bond; length as m-dashN–(CH2)n–N–C(double bond; length as m-dashCH2)-o-C6H4-O] with enamine functionalized ligands. This reaction pattern can be transferred onto various ligands of 2-iminomethylphenolate-type. The resulting pentacoordinate silicon complexes react with a variety of Brønsted acids HY to yield hexacoordinate salen silicon complexes (C6H5)(Y)Si-[o-O-C6H4–C(CH3)double bond; length as m-dashN–(CH2)n–Ndouble bond; length as m-dashC(CH3)-o-C6H4-O] (Y = benzoate, picrate, 8-oxyquinolinate, 2-oxy-1,4-naphthoquinonate, p-tert-butylphenolate, (5-phenyltetrazol)-2-ide, fluoride, tetrafluoroborate). Hexacoordination of their Si-atoms was confirmed by 29Si NMR spectroscopy and, in some cases, by X-ray crystal structure analysis. Examples for similarities and differences in the coordination behavior of the silicon atom and its heavier congeners (Ge, Sn) in the salen-type coordination sphere as well as data regarding the nucleophilicity of some of these novel enamine complexes are presented.