Oxo-molybdenum and oxo-tungsten complexes of Schiff bases relevant to molybdoenzymes

Abstract

A series of octahedral dioxomolybdenum(VI) complexes of the type [MoO₂L₂] {L = 4-Ar-pent-2-en-ol; L_i−Pr2Ph with Ar = 2,6-diisopropylphenyl (1); L_Me2Ph with Ar = 2,6-dimethylphenyl (2), L_MePh with Ar = 2-methylphenyl (3) and with Ar = phenyl (4)} and dioxotungsten(VI) compounds [WO₂L₂] {L_i-Pr2Ph (5); L_Me2Ph (6) and L_MePh (7)} with Schiff bases have been synthesized as models for oxotransferases. Spectroscopic characterization in solution shows with the sterically encumbered ligands L_i-Pr2Ph and L_Me2Ph isomerically pure products whereas the ligand with only one substituent in ortho position at the aromatic ring L_MePh revealed a dynamic mixture of three isomers as confirmed by variable temperature NMR spectroscopy. Single crystal X-ray diffraction analyses of compounds 1, 2 and 4 showed them to be in the N,N-trans conformation consistent with the larger steric demand at nitrogen. Oxygen atom transfer (OAT) properties towards trimethylphosphine were investigated leading to the isolation of two mononuclear molybdenum(IV) compounds [MoO(PMe₃)(L_Me2Ph)₂] (8) and [MoO(PMe₃)(L_MePh)₂] (9) as confirmed by spectroscopic and crystallographic means. The kinetics of OAT between complex [MoO₂(L_Me2Ph)₂] (2) and PMe₃ was investigated by UV/Vis spectroscopy under pseudo-first-order conditions revealing single-step reactions with Eyring values of ΔH^≠ = +60.79 kJ mol⁻¹ and ΔS^≠ = −112 J mol⁻¹K⁻¹ and a first-order dependence of phosphine consistent with a slow nucleophilic attack of the phosphine showing the octahedral geometries of this system to be unfavorable for OAT. Compound 1 showed no OAT reactivity towards PMe₃ emphasizing the influence of sterical properties. Furthermore, the reactivity of the reduced compounds [MoO(PMe₃)(L_Me2Ph)₂] (8) and [MoO(PMe₃)(L_MePh)₂] (9) towards molecular oxygen was investigated leading, in the case of 8, to the substitution of PMe₃ by O₂ under formation of the peroxo compound [MoO(O₂)(L_Me2Ph)₂] (10). In contrast, the analogous reaction employing 9 led to oxidation forming the dioxo compound [MoO₂(L_MePh)₂] (3).