New alkyne complexes of niobium(I)

Abstract

The reaction between the seven-co-ordinated niobium(I) complexes [NbX(CO)₄(dppe)](dppe = Ph₂PCH₂CH₂PPh₂, X = Br or I) or [NbX(CO)₃(PEt₃)₃](X = Cl, Br or I) and alkynes leads to the d⁴ niobium alkyne complexes [NbX(CO)₂(η²-RCCR)(dppe)](R = Ph) and [NbX(CO)₂(η²-RCCR)(PEt₃)₂](R = H, Me, Et or Ph). These complexes have been characterized by ¹H, ¹³C, ³¹P and ⁹³Nb NMR and IR spectroscopies. Low-field acetylenic proton and carbon resonances confirm the description of the alkyne as a four-electron donor. The ⁹³Nb NMR resonances exhibit the expected dependence of shielding on the nature of the halide ligand, i.e. Cl < Br < I. The crystal and molecular structures of [NbI(CO)₂(MeCCMe)(PEt₃)₂] have been determined: triclinic, space group P, a= 9.000(1), b= 16.906(2), c= 17.952(3)Å, α= 80.07(1), β= 78.59(1), γ= 75.50(1)°. The molecule has a slightly distorted octahedral geometry with the carbonyl and the phosphine ligands in trans positions. Average distances are: Nb–CO 2.11(5), Nb–C(alkyne) 2.09(9) and Nb–I 2.91(2)Å.