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^R)₂{Si(SiMe₃)₃}(X)] (M = Hf, Th; Cp^R = Cp', {C₅H₄(SiMe₃)} or Cp'', {C₅H₃(SiMe₃)₂-1,3}; X = Cl, C₃H₅). The separate salt metathesis reactions of [M(Cp^R)₂(Cl)₂] (M = Zr or Hf, Cp^R = Cp'; M = Hf or Th, Cp^R = Cp'') with equimolar K{Si(SiMe₃)₃} gave the respective mono-silanide complexes [M(Cp')₂{Si(SiMe₃)₃}(Cl)] (M = Zr, 1; Hf, 2), [Hf(Cp'')(Cp'){Si(SiMe₃)₃}(Cl)] (3) and [Th(Cp'')₂{Si(SiMe₃)₃}(Cl)] (4), with only a trace amount of 3 presumably formed via silatropic and sigmatropic shifts; the synthesis of 1 from [Zr(Cp')₂(Cl)₂] and Li{Si(SiMe₃)₃} has been reported previously. The salt elimination reaction of 2 with one equivalent of allylmagnesium chloride gave [Hf(Cp')₂{Si(SiMe₃)₃}(η³-C₃H₅)] (5), whilst the corresponding reaction of 2 with equimolar benzyl potassium yielded [Hf(Cp')₂(CH₂Ph)₂] (6) together with a mixture of other products, with elimination of both KCl and K{Si(SiMe₃)₃}. Attempts to prepare isolated [M(Cp^R)₂{Si(SiMe₃)₃}]⁺ cations from 4 or 5 by standard abstraction methodologies were unsuccessful. The reduction of 4 with KC₈ gave the known Th(III) complex, [Th(Cp'')₃]. Complexes 2-6 were characterised by single crystal XRD, whilst 2, 4 and 5 were additionally characterised by ¹H, ¹³C{¹H} and ²⁹Si{¹H} 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).