Synthesis, characterization, electrochemistry and oxo-transfer kinetics of oxomolybdenum-(VI), -(V) and -(IV) complexes with ONS donors

Abstract

cis-Dioxomolybdenum(VI) complexes of the type [MoO₂L][H₂L =S-benzyl 3-(2-hydroxyphenyl)methylenedithiocarbazate or its derivative having a 5-methyl substituent in the salicyl phenyl ring] are MoO ⋯ Mo bridged oligomers. In the presence of donors [D = Me₂CO, pyridine, dimethylformamide (dmf) or Me₂SO] they are converted into monomeric [MoO₂L(D)]. When a 5-Cl or 5-Br substituent occurs in the salicyl phenyl ring oligomers are not formed, but besides [MoO₂L(D)] also [MoO₂L(MeOH)] are obtained. All these complexes undergo oxo transfer to PPh₃ at room temperature, furnishing oxomolybdenum(IV) derivatives, [MoO(L)], and PPh₃O. The complexes [MoO(L)] in dmf accepts an oxygen atom from Me₂SO, affording [MoO₂L(D)] and Me₂S. The ions [MoOX₅]^2–(X = Cl or Br) react with the ligands H₂L furnishing thiolato-bridged dimers, [Mo₂O₂X₂L₂], which show sub-normal magnetic moments and EPR spectra typical of an antiferromagnetic material. The MoO₂²⁺ complexes undergo irreversible electrochemical reduction furnishing oxomolybdenum(V) derivatives at potentials commensurate with the nature of substituent in the salicyl group of the ligand. The molybdenum-(V) and -(IV) complexes also show interesting electrochemical reductive as well as oxidative responses centred on the metal. The oxo transfer from [MoO₂L](unsubstituted salicyl ring) to PPh₃ occurs in a second-order process with rate constant 1.32 × 10^–2 dm³ mol^–1 s^–1 at 30 °C. The oxo transfer from Me₂SO to the MoO²⁺ core is much faster. The reaction between PPh₃ and Me₂SO furnishing PPh₃O and Me₂S becomes highly facile in the presence of [MoO₂L] as catalyst.