Deprotonation of eta(2)(4e)-bonded alkynes as a pathway to sigma,eta(3e)-prop-2-ynyl, eta(5)-pentadienyl and eta(4)-trans-1,3-diene substituted molybdenum complexes

Addition of Li[N(SiMe(3))(2)] to the halogenobis(alkyne) complexes [MoBr(eta(2)-MeC(2)R)(2)(eta-C5H5)] (R = Me-or Ph) resulted in a dehydrohalogenation reaction and formation of the sigma,eta(2)(3e)-prop-2-ynyl/eta(2)(4e)-alkyne-substituted complexes [Mo{sigma,eta(2)(3e)-CH(2)C(2)R}{eta(2)(4e)-MeC(2)R}(eta-C5H5)] (R = Ph 7 or Me). The structure when R = Ph has been confirmed by a single-crystal X-ray diffraction study. The eta(2)(4e)-bonded alkyne and sigma,eta(2)(3e)-prop-2-ynyl ligands lie in planes essentially parallel to that of the eta-C5H5 ligand, with a C(1)-C(2)-C(3) angle:and Md-C(1), Mo-C(2) and Mo-C(3) bond lengths for the prop-2-ynyl fragment of 146.1(6)degrees, 2.278(6), 2.164(5) and 2.105(5) Angstrom respectively. The nature of the M-sigma,eta(2)-C3H3 bonding in complex 7 and in the related species [Mo{sigma, eta(2)(3e)-CH(2)C2H}(CO)(2)(eta(6)-C(6)Me(6))][BF4] and [ZrMe{sigma,eta(2)(3e)-CH(2)C(2)Ph}(eta-C5H5)(2)] has been examined using a comparative standard extended-Huckel molecular orbital (EHMO) study. Reprotonation of the new complexes with CF3CO2H affords [Mo(O2CCF3) (eta(2)-MeC(2)R)(2)(eta-C5H5)] (R = Ph or Me), and it is suggested in agreement with an EHMO charge-distribution calculation that protonation occurs initially on an alkyne contact carbon followed by transfer of the hydrogen via the metal to the prop-2-ynyl ligand. An attempt to extend the deprotonation reaction to the X-ray crystallographically identified eta(2)-alkene/eta(2)(4e)-alkyne-substituted complex [Mo{eta(2)(4e)-MeC(2)Ph}(dpps)(eta-C5H5][BF4] (dpps = o-diphenylphosphinostyrene) resulted in an unexpected reaction. Treatment with Li[N(SiMe(3))(2)] afforded the eta(5)-pentadienyl complex [Mo{eta(2),eta(3)(5e)-CH2CHC(Ph)CH=CHC(6)H(4)PPh(2)-o}(eta-C(5)H5)], which was structurally characterised by single-crystal X-ray crystallography. The pentadienyl ligand is wrapped around:the molybdenum atom resulting in a dihedral angle of 122 degrees for C(27)-C(26)-C(25)-C(24). The bond length C(25)-C(26) of 1.484(11) Angstrom indicates there is little interaction between the eta(3) and eta(2) pi systems. Reaction of this complex with HBF4 . Et(2)O resulted in the formation of a cationic trans (twisted) eta(4)-1,3-diene substituted complex stabilised by an Mo(mu-H)C interaction. A single-crystal X-ray diffraction study confirmed these features. The trans-1,3-diene carbons adopt a twisted, non-planar arrangement with a dihedral angle of 123 degrees. The agostic interaction is not displaced by carbon monoxide.