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(SiMe 3 ) 3 }(X)] (M = Hf, Th; Cp R = Cp', {C 5 H 4 (SiMe 3 )} or Cp'', {C 5 H 3 (SiMe 3 ) 2 -1,3}; X = Cl, C 3 H 5 ). 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(SiMe 3 ) 3 } gave the respective mono-silanide complexes [M(Cp') 2 {Si(SiMe 3 ) 3 }(Cl)] (M = Zr, 1; Hf, 2), [Hf(Cp'')(Cp'){Si(SiMe 3 ) 3 }(Cl)] (3) and [Th(Cp'') 2 {Si(SiMe 3 ) 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(SiMe 3 ) 3 } has been reported previously. The salt elimination reaction of 2 with one equivalent of allylmagnesium chloride gave [Hf(Cp') 2 {Si(SiMe 3 ) 3 }(η 3 -C 3 H 5 )] (5), whilst the corresponding reaction of 2 with equimolar benzyl potassium yielded [Hf(Cp') 2 (CH 2 Ph) 2 ] (6) together with a mixture of other products, with elimination of both KCl and K{Si(SiMe 3 ) 3 }. Attempts to prepare isolated [M(Cp R ) 2 {Si(SiMe 3 ) 3 }] + cations from 4 or 5 by standard abstraction methodologies were unsuccessful. The reduction of 4 with KC 8 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 1 H, 13 C{ 1 H} and 29 Si{ 1 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).