Niobium tetrahalide complexes with neutral diphosphine ligands

Abstract

The reactions of NbCl₄ with diphosphine ligands o-C₆H₄(PMe₂)₂, Me₂PCH₂CH₂PMe₂ or Et₂PCH₂CH₂PEt₂ in a 1 : 2 molar ratio in MeCN solution produced eight-coordinate [NbCl₄(diphosphine)₂]. [NbBr₄(diphosphine)₂] (diphosphine = o-C₆H₄(PMe₂)₂ or Me₂PCH₂CH₂PMe₂) were made similarly from NbBr₄. X-ray crystal structures show that [NbCl₄{o-C₆H₄(PMe₂)₂}₂] has a dodecahedral geometry, but the complexes with dimethylene-backboned diphosphines are distorted square antiprisms. The Nb–P distances and <P–Nb–P angles are very similar in the two types, but Nb–Cl distances are ∼0.1 Å longer in the square antiprismatic complexes. These paramagnetic (d¹) complexes were also characterised by microanalysis, magnetic measurements, IR and UV-visible spectroscopy. Using a 1 : 1 molar ratio of NbCl₄ : diphosphine (diphosphine = Me₂PCH₂CH₂PMe₂, Et₂PCH₂CH₂PEt₂, Cy₂PCH₂CH₂PCy₂ and Ph₂PCH₂CH₂CH₂PPh₂) afforded [NbCl₄(diphosphine)], and [NbBr₄(Me₂PCH₂CH₂PMe₂)] was obtained similarly. These 1 : 1 complexes are unstable in solution, preventing X-ray crystallographic study, but based upon their diamagnetism, IR, UV-visible and ³¹P{¹H} NMR spectra, they are formulated as halide-bridged dimers [(diphosphine)X₂Nb(μ-X)₄NbX₂(diphosphine)], with single Nb–Nb bonds and chelating diphosphines. The Nb(IV) complexes are prone to hydrolysis and oxidation in solution and the structures of the Nb(V) complexes [NbBr₄(Me₂PCH₂CH₂PMe₂)₂][NbOBr₄(MeCN)] with a dodecahedral cation, and [{NbOCl₃{Et₂P(CH₂)₂PEt₂}}₂{μ-Et₂P(CH₂)₂PEt₂}] which contains seven-coordinate Nb(V) centres with a symmetrical diphosphine bridge, are reported. The structure of niobium tetrabromide, conveniently made from NbCl₄ and BBr₃, is a chain polymer with edge-linked NbBr₆ octahedra and alternating long and short Nb–Nb distances, the latter ascribed to Nb–Nb bonds.