Early-transition-metal ketene complexes: Synthesis, reactivity and structure of ketene complexes of bis(trimethylsilyl)niobocene, X-ray structure of [Nb(η 5-C 5H 4SiMe 3) 2Br(Ph 2CCO C,O)

Abstract

The “carbenoid-like” complex [Nb(η 5-C 5H 4SiMe 3) 2Br] 1a reacts with 1 equivalent of several ketenes, R 1R 2CCO, to give the niobium(V) complexes [Nb(η 5-C 5H 4SiMe 3) 2Br(R 1R 2CCO C,O)] ( 2a, R 1 = R 2 = Ph; 3a, R 1 = R 2 = Me; 4a, R 1 = Ph, R 2 = Me; 5a, R 1 = Ph, R 2 = Et) with the expected CO bonding mode found in several early-transition-metal moieties. The protonation of these complexes with 1 equivalent of H + (an ethereal solution of HBF 4) affords the acyl cationic niobocene [Nb(η 5-C 5H 4SiMe 3) 2Br(R 1R 2HCCO)] + ( 6a, R 1 = R 2 = Ph; 7a, R 1 = R 2 = Me; 8a, R 1 = Ph, R 2 = Me; 9a, R 1 = Ph, R 2 = Et). The ketene complexes 2a and Nb(η 5-C 5H 4SiMe 3) 2Cl(R 1R 2C CO C,O)] 2b (hereafter b refers to the chloro-complexes) undergo a two-electron reduction without transformation of the ketene moiety to give the same anionic niobium(III) species [Nb(η 5-C 5H 4SiMe 3) 2(η 2-(C,O)R 1R 2 CCO)] − 10 by an ECE process. The structure of 2a was determined by X-ray diffraction methods. The crystals are triclinic, space group P 1 with Z = 4 in a unit cell of dimensions a = 16.063(7), b = 19.108(8), c = 10.696(6) Å, α = 99.89(2), β = 94.64(2), γ = 111.95(2)°. The structure was solved from diffractometer data by Patterson and Fourier methods and refined by blocked full-matrix least-squares on the basis of 9706 observed reflections, to R and R w values of 0.0391 and 0.0567 respectively. The diphenylketene and the two Cp′ rings (Cp′ = C 5H 4SiMe 3) are η 2(CO) and η 5 respectively. The niobium atom is also bonded to a Br atom. If the centroids of the Cp′ rings and the midpoint of the CO ketene bond are considered, the Nb atom displays a distorted tetrahedral coordination.