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), [MoO2LS] (1), [MoO2LSe] (2), [WO2LS] (3) and [WO2LSe] (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 PPh3, 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 PPh3 with NO3−.