Reactions of co-ordinated ligands. Part 33. Mononuclear η²-vinyl complexes: synthesis, structure, and reactivity

Abstract

Treatment of the four-electron alkyne cation [Mo(η2-PhC2Ph){P(OMe)3} 2(η-C5H5)][BF4] with K[BHBus3] affords the η2-vinyl or metallacyclopropene complex [Mo{=C(Ph)CHPh}{P(OMe)3}2(η-C5H 5)] (2). The related complexes [Mo{=C(But)CHPh}{P(OMe)3}2(η-C 5H5)] (3),[Mo{=C(But)CH(C6H4Me-4)}{P(OMe) 3}2(η-C5H5)] (4), [Mo{=C(Pri)CH(C6H4Me-4)}{P(OMe) 3}2(η-C5H5)] (5), and [Mo{=C(Me)CPh2}{P(OMe)3}2(η-C 5H5)] (6) are obtained on reaction of the corresponding lithium diarylcuprate with the respective alkyne cation [Mo(η2-R1C2R2){P(OMe) 3}2(η-C5H5)][BF4] (R1 = But, R2 = H; R1 = Pri, R2 = H; R1 = Me, R2 = Ph). The structures of (2), (3), and (6) have been determined by single-crystal X-ray diffraction studies. The molecules show close similarities; each has a molybdenum atom to which a vinyl moiety is co-ordinated via one short and one long Mo-C bond. These molecules may be described either as η2(3 e)-vinyl or metallacyclopropene complexes. The orientation of the C2 vinyl group relative to the Mo{P(OMe)3}2(η-C5H5) fragment is discussed in terms of the torsion angles. In (2) and (3), Cα of the vinyl group lies closer to the plane of the η-C5H5, ligand than does Cβ, whereas in (6) the reverse orientation is observed. The solution n.m.r. spectra of (2) and (3) have distinct 31P environments, whereas in (6) the phosphorus environments are equivalent. This is discussed in terms of a rotational movement, an extended Hückel molecular orbital analysis suggesting that the orientation and fluxional behaviour of the η2-vinyl ligands parallel those of the related alkyne complexes. Reaction of [Mo(η2-PhC2CH2Ph){P(OMe)3} 2(η-C5H5)][BF4] with K[BHBus3] affords a separable mixture of isomeric complexes [Mo{=C(CH2Ph)CHPh}{P(OMe)3}2(η-C 5H5)], which differ only in the orientation of the η2-vinyl moiety. The complex (6) slowly rearranges in solution to an η3-allylic complex this being explained in terms of an η2 to σ change in the bonding mode of the vinyl ligand. A similar transformation is suggested to explain the formation of η3-allylic complexes on reaction of [Mo(η2-RC2H){P(OMe)3} 2(η-C5H5)][BF4] (R = But or Pri) with LiCuMe2. Extension of the dimethyl- or diphenyl-cuprate reactions to the cations [Mo(η2- R1C2R2){P(OMe)3} 2(η-C5H5)][BF4] provides evidence for competing reaction pathways involving either direct attack on the metal centre or on a co-ordinated alkyne carbon. The regioselectivity of the latter reaction is discussed in terms of steric and electronic effects.