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Deprotonation of η2(4e)-Bonded Alkynes as a Pathway to ?,η2(3e)-Prop-2-ynyl, η5-Pentadienyl and η4-trans-1,3-Diene Substituted Molybdenum Complexes

DOI: 10.1039/DT9950003975

Source and publish data:

Journal of the Chemical Society, Dalton Transactions p. 3975 - 3986 (1995)

Update date:2022-08-03

Topics:

Authors:

Carfagna, CarlaCarfagna, Carla

Deeth, Robert J.Deeth, Robert J.

Green, MichaelGreen, Michael

Mahon, Mary F.Mahon, Mary F.

McInnes, Jacqueline M.McInnes, Jacqueline M.

et al.et al.

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Article abstract of DOI:10.1039/DT9950003975

Addition of Li to the halogenobis(alkyne) complexes 2-MeC2R)2(η-C5H5)> (R = Me or Ph) resulted in a dehydrohalogenation reaction and formation of the ?,η2(3e)-prop-2-ynyl/η2(4e)-alkyne-substituted complexes 2(3e)-CH2C2R)(η2(4e)-MeC2R)(η-C5H5)> (R = Ph 7 or Me).The structure when R = Ph has been confirmed by a single-crystal X-ray diffraction study.The η2(4e)-bonded alkyne and ?,η2(3e)-prop-2-ynyl ligands lie in planes essentially parallel to that of the η-C5H5 ligand, with a C(1)-C(2)-C(3) angle and Mo-C(1), Mo-C(2) and Mo-C(3) bond lengths for the prop-2-ynyl fragment of 146.1(6) deg, 2.278(6), 2.164(5) and 2.105(5) Angstroem respectively.The nature of the M-?,η2-C3H3 bonding in complex 7 and in the related species 2(3e)-CH2C2H)(CO)2(η6-C6Me6)> and 2(3e)-CH2C2Ph)(η-C5H5)2> has been examined using a comparative standard extended-Hueckel molecular orbital (EHMO) study.Reprotonation of the new complexes with CF3CO2H affords 2-MeC2R)2(η-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 η2-alkene/η2(4e)-alkyne-substituted complex 2(4e)-MeC2Ph)(dpps)(η-C5H5)> (dpps = o-diphenylphosphinostyrene) resulted in an unexpected reaction.Treatment with Li afforded the η5-pentadienyl complex 2,η3(5e)-CH2CHC(Ph)CH=CHC6H4PPh2-o)(η-C5H5)> 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 deg for C(27)-C(26)-C(25)-C(24).The bond length C(25)-C(26) of 1.484(11) Angstroem indicates there is little interaction between the η3 and η2 ? systems.Reaction of this complex with HBF4*Et2O resulted in the formation of a cationic trans (twisted) η4-1,3-diene substituted complex stabilised by an Mo(μ-H) 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 deg.The agostic interaction is not displaced by carbon monoxide.

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