The use of a cis-dioxomolybdenum(VI) dinuclear complex with quadradentate 1,4-benzenediylbis(benzyldithiocarbamate)(2−) as model compound for the active site of oxo transfer molybdoenzymes: Reactivity, kinetics, and catalysis

Abstract

Dinuclear cis-dioxomolybdenum(VI) complex [{MoO2(Bz2Benzenediyldtc)}2] coordinated by a quadradentate dithiocarbamate (Bz2Benzenediyldtc2−=1,4-benzenediylbis(benzyldithiocarbamate)(2−)) has been prepared and characterized by elemental analysis, 13C NMR, IR and UV–vis spectroscopy. The kinetics of the oxygen atom transfer between [{MoO2(Bz2Benzenediyldtc)}2] and PPh3 was studied spectrophotometrically in CH2Cl2 medium at 520nm and four different temperatures, 288, 293, 298 and 303K, respectively. The reaction follows second order kinetics with the rate constant k=0.163(2)M−1S−1 and its increasingly strong absorption at 520nm clearly indicate the formation of a μ-oxo molybdenum(V) species as a product. Despite the steric restrictions imposed by the ligand structure to prevent the formation of Mo(V) species, experimental evidence confirms its interference during the process. The product can then be formulated as [MoO2(Bz2Benzenediyldtc)2Mo2O3(Bz2Benzenediyldtc)2MoO2] which has one μ-oxomolybdenum(V) moiety. An Eyring plot allows the activation parameters ΔH‡=64.2(1)kJmol−1 and ΔS‡=−45.1(6)JK−1mol−1 to be determined from the temperature dependence of the rate constant, suggesting an associative transition state for the oxo transfer reaction. Catalytic oxygen atom transfer reaction from DMSO to PPh3 was also followed by monitoring the chemical shift changes in 31P NMR spectroscopy. The substrate oxidation process follows a well-defined catalytic cycle capable of 100% conversion for the reaction of PPh3 and DMSO without intervention of Mo(V) formation during about 36h.