Niobocene hydrides with functionalised silyl ligands. Tuning of interligand hypervalent interactions (IHI) M–H → Si–X

Abstract

This paper presents syntheses and X-ray structure determinations of a variety of mono- and bis-(silyl) niobocene hydride complexes with functionalised silyl ligands. Complexes of the type [NbCp₂(SiR₃)H₂] (SiR₃ = SiMe₂Cl (3), Si(OEt)₃, SiMe₂OEt (4)) can be prepared conveniently by heating the precursor [NbCp₂H₃] in the presence of the corresponding silane HSiR₃. As the electrondonating ability of the R group increases, the corresponding mono(silyl) derivatives [NbCp₂(SiR₃)H₂] are either unstable (SiR₃ = SiMe₃) or do not exist at all (SiR₃ = SiEt₃, Si(OCH₂CH₂)₃N). The previously described bis(silyl) complex [NbCp₂(SiMe₂H)₂H] easily reacts with half an equivalent of iodine, giving the functionalised derivative [NbCp₂(SiMe₂I)₂H] (10). A labelling experiment for the analogous reaction with bromine establishes that an electrophile attacks the more accessible Si–H bond rather than the more electron-rich Nb–H bond. Bis(silyl) complexes [NbCp₂(SiMe₂X)₂H] (X = Br (8), I (10), OTfl (11)) can be conveniently prepared by the reaction of [NbCp₂(SiMe₂Cl)₂H] (2) with the corresponding silane XSiMe₃. Reactions of 2 with fluorinating reagents such as [CPh₃][BF₄], BF₃·OEt₂ and AgBF₄ do not work well for the synthesis of the compound [NbCp₂(SiMe₂F)₂H]. Nucleophilic chlorine substitution in 2 under the action of lithium amides and alkoxides does not occur, whereas reaction with soft reagents such as MeLi, LiPPh₂ and LiSPh leads to the corresponding derivatives [NbCp₂(SiMe₃)₂H] and [NbCp₂(SiMe₂EPh_n)₂H] (E = S, n = 1 (14); E = P, n = 2 (15)). Methanolysis of 10 gives high yields of the derivative [NbCp₂(SiMe₂OMe)₂H] (18). The mono(silyl) precursor [NbCp₂(SiMe₂Cl)H₂] (3) reacts with LiOEt affording a mixture of [NbCp₂(SiMe₂OEt)H₂] and [NbCp₂H₃]. Reaction of 3 with LiPPh₂ gives high yields of the compound [NbCp₂(SiMe₂PPh₂)H₂] (17). Reduction of 3 with LiAlH₄ results in the dimeric compound [NbCp₂H₂Al(μ-H)₂]₂ (13). Carrying out this reaction in the presence of an amine leads to the formation of the complex [NbCp₂(SiMe₂H)H₂]. X-Ray structure determinations were performed for the compounds (10), (13), (17), and (18). Complex (10) has 5 centre–6 electron interligand hypervalent interactions (IHIs) I–Si⋯H⋯Si–I; the analogous IHI O–Si⋯H⋯Si–O in 18 is significantly weaker, whereas 17 has a classical structure. Complex 13 has a cenrosymmetric dimeric structure, with two five-co-ordinate aluminium centres bridged by hydride ligands.