Comparison of group 4 and thorium M(IV) substituted cyclopentadienyl silanide complexes.

We report the synthesis and characterisation of a series of M(IV) substituted cyclopentadienyl hypersilanide complexes of the general formula [M(Cp){Si(SiMe)}(X)] (M = Hf, Th; Cp = Cp', {CH(SiMe)} or Cp'', {CH(SiMe)-1,3}; X = Cl, CH). The separate salt metathesis reactions of [M(Cp)(Cl)] (M = Zr or Hf, Cp = Cp'; M = Hf or Th, Cp = Cp'') with equimolar K{Si(SiMe)} gave the respective mono-silanide complexes [M(Cp'){Si(SiMe)}(Cl)] (M = Zr, 1; Hf, 2), [Hf(Cp'')(Cp'){Si(SiMe)}(Cl)] (3) and [Th(Cp''){Si(SiMe)}(Cl)] (4), with only a trace amount of 3 presumably formed silatropic and sigmatropic shifts; the synthesis of 1 from [Zr(Cp')(Cl)] and Li{Si(SiMe)} has been reported previously. The salt elimination reaction of 2 with one equivalent of allylmagnesium chloride gave [Hf(Cp'){Si(SiMe)}(η-CH)] (5), whilst the corresponding reaction of 2 with equimolar benzyl potassium yielded [Hf(Cp')(CHPh)] (6) together with a mixture of other products, with elimination of both KCl and K{Si(SiMe)}. Attempts to prepare isolated [M(Cp){Si(SiMe)}] cations from 4 or 5 by standard abstraction methodologies were unsuccessful. The reduction of 4 with KC gave the known Th(III) complex, [Th(Cp'')]. Complexes 2-6 were characterised by single crystal XRD, whilst 2, 4 and 5 were additionally characterised by H, C{H} and Si{H} NMR spectroscopy, ATR-IR spectroscopy and elemental analysis. In order to probe differences in M(IV)-Si bonds for d- and f-block metals we studied the electronic structures of 1-5 by density functional theory calculations, showing M-Si bonds of similar covalency for Zr(IV) and Hf(IV), and less covalent M-Si bonds for Th(IV).