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[BHBu^s₃] affords the η²-vinyl or metallacyclopropene complex [[graphic omitted]HPh}{P(OMe)₃}₂(η-C₅H₅)](2). The related complexes [[graphic omitted]HPh}{P(OMe)₃}₂(η-C₅H₅)](3),[[graphic omitted]H(C₆H₄Me-4)}{P(OMe)₃}₂(η-C₅H₅)](4), [[graphic omitted]H(C₆H₄Me-4)}{P(OMe)₃}₂(η-C₅H₅)](5), and [[graphic omitted]Ph₂}{P(OMe)₃}₂(η-C₅H₅)](6) are obtained on reaction of the corresponding lithium diarylcuprate with the respective alkyne cation [Mo(η²-R¹C₂R²){P(OMe)₃}₂(η²-C₅H₅)][BF₄](R¹= Bu^t, 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(3e)-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), C_α of the vinyl group lies closer to the plane of the η-C₅H₅, ligand than does C_β, 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[BHBu^s₃] affords a separable mixture of isomeric complexes [[graphic omitted]HPh}{P(OMe)₃}₂(η-C₅H₅)], 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 = Bu^t or Prⁱ) with LiCuMe₂. Extension of the dimethyl- or diphenyl-cuprate reactions to the cations [Mo(η²- R¹C₂R²){P(OMe)₃}₂(η-₅H₅)][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.