New dicyclopentadienyl phosphine, phosphite, and acetylene niobium(III) complexes. Crystal structure of [Nb(η-C5H4SiMe3)2Cl(PhCCPh)]

Abstract

The reduction of the niobocene complex [Nb(η-C₅H₄SiMe₃)₂Cl₂](1) with one equivalent of sodium amalgam (10%) in the presence of π-acid ligands yields [Nb(η-C₅H₄SiMe₃)₂Cl(L)][L = PMe₃(2), PMe₂Ph (3), P(OMe)₃(4), P(OEt)₃(5), PhCCPh (6), HCCPh (7), or HCCH (8)]. Reactions of (2) and (3) with LiMe give the new alkyl complexes [Nb(η-C₅H₄SiMe₃)₂Me(L)][L = PMe₃(9) or PMe₂Ph (10)]. The structural characterization of all these compounds has been carried out by i.r. and n.m.r. spectroscopy and the oxidation potentials of the niobium(III) complexes have been evaluated by cyclic voltammetric measurements. The X-ray crystal structure of (6) has been determined. It crystallizes in the orthorhombic space group P2₁2₁2₁ with a= 15.388(1), b= 15.148(1), c= 12.397(1)Å, and Z= 4. The structure was solved by Patterson and Fourier methods and refined by full-matrix least-squares methods to R= 0.053 for 2 695 observed reflections. The structure obtained was consistent with that expected on the basis of spectroscopic observations. The molecule represents a wedge-like sandwich, with an angle between both closely planar cyclopentadienyl rings of 52.10(42)°, probably due to the repulsions between the –SiMe₃ substituents of the cyclopentadienyl rings. The ligand PhCCPh behaves as a two-electron donor as indicated by the distance C(1)–C(2) 1.27(1)Å, and angles C(2)–C(1)–C(11) 143.3(8)° and C(1)–C(2)–C(21) 141.1(9)°; these values confirm the π back-donation effect.