Deprotonation of η2(4e)-bonded alkynes as a pathway to σ,η2(3e)-prop-2-ynyl, η5-pentadienyl and η4-trans-1,3-diene substituted molybdenum complexes

Abstract

Addition of Li[N(SiMe₃)₂] to the halogenobis(alkyne) complexes [MoBr(η²-MeC₂R)₂(η-C₅H₅)](R = Me or Ph) resulted in a dehydrohalogenation reaction and formation of the σ,η²(3e)-prop-2-ynyl/η²(4e)-alkyne-substituted complexes [Mo{σ,η²(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 σ,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)°, 2.278(6), 2.164(5) and 2.105(5)Å respectively. The nature of the M–σ,η²-C₃H₃ bonding in complex 7 and in the related species [Mo{σ,η²(3e)CH₂C₂H}(CO)₂(η⁶-C₆Me₆)][BF₄] and [ZrMe{σ,η²(3e)-CH₂C₂Ph}(η-C₅H₅)₂] has been examined using a comparative standard extended-Hückel molecular orbital (EHMO) study. Reprotonation of the new complexes with CF₃CO₂H affords [Mo(O₂CCF₃)(η²-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 [Mo{η²(4e) MeC₂Ph}(dpps)(η-C₅H₅)][BF₄](dpps =o-diphenylphosphinostyrene) resulted in an unexpected reaction. Treatment with Li[N(SiMe₃)₂]afforded the η⁵-pentadienyl complex [Mo{η²,η³(5e)-CH₂CHC(Ph)CHCHC₆H₄PPh₂-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° for C(27)–C(26)–C(25)–C(24). The bond length C(25)–C(26) of 1.484(11)Å indicates there is little interaction between the η³ and η²π 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) 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°. The agostic interaction is not displaced by carbon monoxide.