Reactions of co-ordinated ligands. Part 39. The synthesis of carbyne complexes from alkyne molybdenum and tungsten cations; formation and crystal structures of [Mo(CCH2But){P(OMe)3}2(η-C5H5)] and [Mo{C(SiMe3)CH2}{P(OMe)3}2(η5-C9H7)]

Abstract

Thermolysis of the σ-vinyl complexes [M{σ-(E)-CHCHBu^t}{P(OMe)₃}₃(η⁵-C_mH_n)](M = Mo or W; m=n= 5; m= 9, n= 7), formed by reaction of hydride-anion donors with the cations [M(η²-Bu^tC₂H){P(OMe)₃}₂(η⁵-C_mH_n)][BF₄], leads to formation of the carbyne complexes [M(CCH₂Bu^t){P(OMe)₃}₂(η⁵-C_mH_n)]. The η-cyclopentadienylmolybdenum carbyne complex was structurally characterised by X-ray crystallography. The molecule adopts a ‘three-legged piano-stool’ geometry with one of the legs being a MoC triple bond [1.796(2)Å]. A similar reaction (–78 °C) between [Mo(η²-PrⁱC₂H){P(OMe)₃}₂(η-C₅H₅)][BF₄] and K[BHBu^s₃] in the presence of P(OMe)₃ afforded the η³-allyl complex [Mo(η³-CH₂ CH CMe₂){P(OMe)₃}₂(η-C₅H₅)], whereas the same cation with NaBH₄–P(OMe)₃ gave the carbyne complex [Mo(CCH₂Prⁱ){P(OMe)₃}₂(η-C₅H₅)]. Possible mechanisms for these carbyne-forming reactions are discussed. In contrast, reaction of the trimethylsilyl-substituted cation [Mo(η²-Me₃SiC₂H){P(OMe)₃}₂(η⁵-C₉H₇)][BF₄] with K[BHBu^s₃] or Li(CuPh₂) affords the η²-vinyl complexes [[graphic omitted]HR}{P(OMe)₃}₂(η⁵-C₉H₇)](R = H or Ph). The phenyl-substituted η²-vinyl is also formed on reaction of [Mo(η²-Me₃SiC₂Ph){P(OMe)₃}₂(η⁵-C₉H₇)][BF₄] with K[BHBu^s₃], the corresponding reaction with Li(CuPh₂) affording [[graphic omited]Ph₂}{P(OMe)₃}₂(η⁵-C₉H₇)]. The η²-vinyl species [graphic omitted]HR}(R = H or Ph) thermally rearrange to the carbyne complexes [Mo(CCH₂SiMe₃){P(OMe)₃}₂(η⁵-C₉H₇)] and [Mo{CCH(Ph)SiMe₃}{P(OMe)₃}₂(η⁵-C₉H₇)]. These trimethylsilyl-substituted carbynes are desilylated in NaF–H₂O–MeCN to form [Mo(CCH₃){P(OMe)₃}₂(η⁵-C₉H₇)] and [Mo(CCH₂Ph){P(OMe)₃}₂(η⁵-C₉H₇)]. The molecular geometry of [[graphic omitted]H₂}{P(OMe)₃}₂(η⁵-C₉H₇)] was established by X-ray crystallography. The η²-vinyl C–C bond lies nearly parallel to the Mo–P(1) vector the P(1)–Mo–C(8)–C(7) torsion angle being 13.8(2)°, with the carbene carbon C(8) nearer than C(7) to the plane of the indenyl ligand. The Mo–C(8) bond length is 1.955(3)Å, whereas Mo–C(7) is 2.258(4)Å.