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

Abstract

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).