Molybdenum and tungsten complexes of bis(phenolate) ligands, O,X,O (X = S or Se): Synthesis, characterization and catalytic oxygen atom transfer properties

Novel molybdenum and tungsten complexes with tridentate bis(phenolate) ligands containing [O,X,O] donor atoms (X = S or Se), [MoO₂L ^S] (1), [MoO₂L^Se] (2), [WO₂L ^S] (3) and [WO₂L^Se] (4), were synthesized and characterized as functional models for molybdenum- and tungsten-dependent oxidoreductases. The catalytic oxo-transfer properties of 1-4 were investigated by oxo-transfer reactions from nitrate to PPh₃ with a ratio of complex (1-4):PPh₃:(Bu₄N)(NO₃^-) = 1:10:20 and from DMSO to PPh₃ with varied catalyst:PPh₃ ratios. For the oxygen atom transfer reaction from nitrate to PPh₃, the molybdenum compounds are catalytically more active than their tungsten analogues. Whereas for the oxygen atom transfer reaction from DMSO to PPh ₃, the tungsten compounds exhibit more efficient catalytic behavior than the molybdenum analogues. In all catalytic reactions, sulfur containing compounds have a higher catalytic ability than their selenium analogues. In comparison, all four compounds catalyze the oxygen atom transfer reaction from DMSO to PPh₃ much more effectively than the oxidation of PPh ₃ with NO₃^-.