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; CpR = Cp′, {C₅H₄(SiMe₃)} or Cp′′, {C5H3(SiMe3)2-1,3}; X = Cl, C3H5). The separate salt metathesis reactions of [M(CpR)2(Cl)2] (M = Zr or Hf, CpR = Cp′; M = Hf or Th, CpR = 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(CpR)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).