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.