Preparation of carbonyl, halogeno, and alkyl niobocene complexes and their reactions with oxygen. Crystal structures of [Nb(η-C5H4SiMe3)2Cl2], [Nb(η-C5H4SiMe3)2(O)Me], and [Nb(η-C5H4SiMe3)2(C6F5)(CO)]

Abstract

The complexes [Nb(cp)₂Cl₂][cp =η-C₅H₅, η-C₅H₄SiMe₃, or η-C₅H₃(SiMe₃)₂-1,3] are reduced with Na–Hg in the presence of CO to give the carbonylniobium(III) complexes [Nb(cp)₂Cl(CO)]. The alkylation of these compounds using LiR, MgRX, or MgR₂ leads to the alkyl derivatives [Nb(cp)₂R(CO)](cp =η-C₅H₅ or η-C₅H₄SiMe₃; R = Me, CH₂CMe₃, CH₂SiMe₃, CH₂Ph, or C₆F₅). These alkyl carbonyl niobium(III) complexes react with oxygen to produce an unresolved mixture of compounds when cp =η-C₅H₅ whereas the complex [Nb(cp)₂(O₂)Me] is obtained in the reaction of [Nb(cp)₂Me(PMe₃)](cp =η-C₅H₄SiMe₃) with oxygen. Similar reactions of [Nb(cp)₂Cl(CO)] with O₂ lead to the oxo-halogenoniobium(V) derivatives [Nb(cp)₂(O)Cl][cp =η-C₅H₅, η-C₅H₄SiMe₃, or η-C₅H₃(SiMe₃)₂-1,3]. However, the corresponding peroxo-chloro complex [Nb(cp)₂(O₂)Cl](cp =η-C₅H₄SiMe₃) results from the reaction with oxygen of the chloro(alkyl isocyanide) niobium(III) complex and can be preparated alternatively by reaction of the dichloride with H₂O₂. The oxo-chloro niobium(V) complex [Nb(η-C₅H₄SiMe₃)₂(O)Cl] can be easily alkylated in reactions with LiR (R = Me, Buⁿ, CH₂CMe₃, or CH₂SiMe₃). All these complexes were characterised by i.r., ¹H and ¹³C n.m.r. spectroscopy and the chloro-carbonyl niobium(III) and oxo niobium(V) complexes were studied by cyclic voltammetry. The crystal structures of [Nb(η-C₅H₄SiMe₃)₂Cl₂](2), [Nb(η-C₅H₄SiMe₃)₂(C₆F₅)(CO)](2f), and [Nb(η-C₅H₄SiMe₃)₂(O)Me](2h) have been determined by X-ray diffraction methods. Compound (2) crystallizes in the triclinic space group P1, with a= 12.680(5), b= 12.923(5), c= 6.806(3)Å, α= 97.82(2), β= 104.18(3), γ= 67.79(2)°, and Z= 2; (2h) crystallizes in the monoclinic space group P2₁/n, with a= 8.842(4), b= 37.108(10), c= 6.331(3)Å, β= 108.03(4)°, and Z= 4; (2f) crystallizes in the triclinic space group P, with a= 9.781(4), b= 14.440(6), c= 8.765(4)Å, α= 90.84(2), β= 85.86(2), γ= 93.90(2)°, and Z= 2. All the structures were solved from diffractometer data by Patterson and Fourier methods and refined by full-matrix least-squares methods to R= 0.0377 for 2 889 observed reflections [(2)], R= 0.0441 for 2 269 observed reflections [(2f)], and R= 0.0385 for 1 466 observed reflections [(2h)]. All structures contain a niobium atom bonded to two cyclopentadienyl rings in a η⁵ fashion; the co-ordination of the metal is completed by two Cl atoms in (2), by a terminal carbonyl group and a carbon atom from the C₆F₅ ligand in (2f), and by an oxygen atom and a carbon atom from a methyl group in (2h).