Theoretical Studies on Models for the Oxo-Transfer Reaction of Dioxomolybdenum Enzymes.

Patterned after synthetic model systems for dioxomolybdenum enzymes, our theoretical model system produces an energy profile and structures for the various species and oxidation states in the catalytic cycle. A key step in this cycle is the oxo-transfer reaction. Here, our substrate, PMe(3), approaches [Mo(VI)O(2)](2+) at an O-Mo-O-P dihedral angle of 90 degrees, i.e. perpendicular to the MoO(2) plane, crosses over a barrier of 14 kcal/mol, and rotates to an O-Mo-O-P dihedral angle of 0 degrees to form an intermediate, [Mo(IV)O(OPMe(3))](2+), which is 69 kcal/mol more stable than the reactants. The direction of the substrate's attack leaves the two d electrons of this Mo(IV) system in an orbital which is delta with respect to the remaining spectator Mo-O bond, a configuration which allows this O to form a formal triple Mo-O bond. The displacement of the product, OPR(3), by water, H(2)O, proceeds via an associative mechanism with a barrier of only 19 kcal/mol. In our model, [Mo(IV)O(OH(2))](2+) then reacts with [Mo(VI)O(2)](2+) to form [Mo(V)O(OH)](2+), a process which is exothermic by 14 kcal/mol. The addition of O(2) then oxidizes [Mo(V)O(OH)](2+) to [Mo(VI)O(2)](2+) to complete our model catalytic cycle.