A New Interpretation for Oxygen Atom-Transfer Reactions for the Berg-Holm Oxo-Molybdenum Enzyme Model: Evidence for a Highly Active Oxygen Atom Transfer Acceptor

Abstract

In 1984, a synthetic model system for certain molybdenum oxotransferase enzymes was reported. These reports claimed that an oxygen atom could be extracted from a designed dioxomolybdenum(VI) complex to produce a monoxomolybdenum(IV) complex without the formation of an oxo-bridged molybdenum(V) binuclear species. The reduced product was shown to accept oxygen atoms from substrates such as dimethylsulfoxide with substrate saturation kinetics. Fifteen years later, it was demonstrated that the reduced product was, in fact, the oxo-bridged molybdenum(V) binuclear species. Here, it is shown that the kinetic data can be reinterpreted in terms of rate-limiting disproportionation of the oxo-bridged molybdenum(V) binuclear species to form a highly reactive monoxomolybdenum(IV) complex. The second order rate constant for oxygen atom transfer from dimethyl sulfoxide to this complex is more than 100,000 times higher than those reported for other monoxomolybdenum(IV) complexes. The five-coordinate molybdenum sites in the dioxomolybdenum(VI) and presumed monoxomolybdenum(IV) complexes are quite similar to those observed for eukaryotic nitrate reductase enzymes and this model system shows relatively rapid reduction of nitrate through a similar mechanistic scheme.