Replacement of Molybdenum by Tungsten in a Biomimetic Complex Leads to an Increase in Oxygen Atom Transfer Catalytic Activity.

Upon replacement of molybdenum by tungsten in DMSO reductase isolated from the Rhodobacteraceae family, the derived enzyme catalyzes DMSO reduction faster. To better understand this behavior, we synthesized two tungsten(VI) dioxido complexes [W VI O 2 L 2 ] with pyridine- (PyS) and pyrimidine-2-thiolate (PymS) ligands, isostructural to analogous molybdenum complexes we reported recently. Higher oxygen atom transfer (OAT) catalytic activity was observed with [WO 2 (PyS) 2 ] compared to the Mo species, independent of whether PMe 3 or PPh 3 was used as the oxygen acceptor. [W VI O 2 L 2 ] complexes undergo reduction with an excess of PMe 3 , yielding the tungsten(IV) oxido species [WOL 2 (PMe 3 ) 2 ], while with PPh 3 , no reactions are observed. Although OAT reactions from DMSO to phosphines are known for tungsten complexes, [WOL 2 (PMe 3 ) 2 ] are the first fully characterized phosphine-stabilized intermediates. By following the reaction of these reduced species with excess DMSO via UV-vis spectroscopy, we observed that tungsten compounds directly react to W VI O 2 complexes while the Mo analogues first form μ-oxo Mo(V) dimers [Mo 2 O 3 L 4 ]. Density functional theory calculations confirm that the oxygen atom abstraction from W VI O 2 is an endergonic process contrasting the respective reaction with molybdenum. Here, we suggest that depending on the sacrificial oxygen acceptor, the tungsten complex may participate in catalysis either via a redox reaction or as an electrophile.