Ligand-controlled synthesis, reactivity and oxo-transfer kinetics of oxomolybdenum-(VI) and -(IV) complexes

Abstract

Trifunctional (ONS) dianionic Schiff-base ligands L^2–[H₂L =S-methyl 3-(2-hydroxyphenyl)methylene-dithiocarbazate or 5-R substituent derivatives (R = H, Me, Cl, Br or NO₂) or a naphthyl derivative], in sharp contrast to the S-benzyl analogues (H₂L′) form only MoO→Mo bridged oligomers [(MoO₂L)_n] in EtOH or MeOH, irrespective of the substituent R. However, these substituents R control the position of the ν(MoO→Mo) vibration, the Mo^VI–Mo^V redox couple, the ligand-to-metal charge-transfer transition, as well as the chemical shift of the azomethine proton signal and the asymmetric ν(MoO) vibration in solution, for both the polymeric complexes and, where relevant, the donor molecule (D) co-ordinated monomers [MoO₂L(D)][D = pyridine (py), dimethylformamide (dmf) or Me₂SO]. Reaction of [MoO₂L] with PPh₃ in CH₂Cl₂, MeOH or MeCN or in donor solvents D (dmf or py) produced oxomolybdenum(IV) derivatives, [MoO(L)] or [MoOL(D)], respectively. The kinetics of oxo transfer from MoO₂²⁺ to PPh₃ occurs in a second-order process. The rate constant of the oxo-transfer reaction from the polymer [(MoO₂L)_n](R = H) to the PPh₃ substrate is ≈ 10² times higher than that of the corresponding monomer [MoO₂L(D)]. Both [MoO(L)] and [MoOL(dmf)] react with the Me₂SO substrate in CH₂Cl₂ or dmf in a two-stage process. The first involves the equilibrium formation of a Me₂SO adduct while during the second stage an intramolecular oxo transfer occurs from Me₂SO to the MoO core via the elimination of Me₂S. The rate constant of the reverse oxo transfer (k_–1) is almost identical for both the polymer [MoO(L)] and monomer [MoOL(dmf)] but the equilibrium constant, K, for the formation of the Me₂SO complexed species is slightly higher for [MoOL(dmf)] than for [MoO(L)].