Surprising diversity of non-classical silicon–hydrogen interactions in half-sandwich complexes of Nb and Ta: M–H ⋯ Si–Cl interligand hypervalent interaction (IHI) versus stretched and unstretched β-Si–H⋯M agostic bonding

Abstract

Reaction of the niobium diphosphine compound [NbCp(NAr)(PMe₃)₂] (Ar = 2,6-C₆H₃Prⁱ₂) with HSiMe₂Cl gives the formally d² silylamido derivative [NbCp{η³-N(Ar)SiMe₂-H}Cl(PMe₃)] 6. X-Ray diffraction and NMR studies of this compound show that it has a stretched β-agostic Si–H → Nb interaction. Reaction of the related precursor [NbCp(NAr′)(PMe₃)₂] (Ar′ = 2,6-C₆H₃Me₂) with HSiMe₂Cl gives an isomeric structure [NbCp{η³-N(Ar′)SiMe₂-H}(PMe₃)(Cl)] 7 differing from 6 in that the phosphine rather than chloride lies trans to the co-ordinated Si–H bond. A preliminary X-ray study and large ¹J(Si–H) coupling constant of 116 Hz suggest that this compound is best described as an unstretched β-agostic (Si–H⋯M) d² silylamide complex. Reaction of the tantalum diphosphine compound [TaCp(NAr)(PMe₃)₂] with HSiMe₂Cl affords the d⁰ silylhydride derivative [TaCp(NAr)(H)(SiMe₂Cl)(PMe₃)] 8 which, according to an X-ray diffraction study and NMR data, has an interligand hypervalent interaction (IHI) between the silyl and hydride ligands. Reactions of 6 and 8 with Me₃SiX (X = I, OTf) lead to the corresponding iodido and triflate derivatives [NbCp{η³-N(Ar)SiMe₂-H}X(PMe₃)] (X = OTf 11 or I 12) and [TaCp(NAr)(H)(SiMe₂X)(PMe₃)] (X = OTf 14 or I 15). Reaction of 8 with AgOTf gives [TaCp(NAr)(PMe₃)₂Cl]OTf 13, the crystal structure of which has been determined. Density functional theory calculations on models of the compounds 6 and 7 showed that the experimental geometries are only correctly reproduced when the phosphine ligands are adequately modelled. The extent of oxidative addition of the Si–H bond to the metal in 6 mainly depends on the basicity of the phosphine ligand. With PH₃ in place of PMe₃ the calculated structures are better described as silanimine-hydrido derivatives. The formation of isomeric type 6versus7 is determined by an interplay of the steric and electronic effects of the ligand environment.