A Catalytic Cycle Related to Molybdenum Enzymes Containing [Mo^VIO₂]²⁺ Oxidized Active Sites

Interconversion of mononuclear cis-dioxo−Mo(VI) and oxo−Mo(V,IV) complexes of the hydrotris(3,5-dimethylpyrazol-1-yl)borate ligand (L) by one-electron and two-electron reactions is described. In the coordinating solvent pyridine (py), LMo^VIO₂(SPh) is reduced by cobaltocene in one-electron steps to stable LMo^IVO(SPh)(py). The compound LMo^IVO(SPh)(py)·0.6MeOH crystallizes in orthorhombic space group Pbca, with a = 13.790(2) Å, b = 15.266(2) Å, c = 27.807(5), V = 5853(3) ų, and Z = 8. The complex exhibits a distorted octahedral structure with a facially tridentate ligand L and mutually cis oxo [MoO = 1.667(5) Å], pyridine [Mo−N = 2.184(5) Å], and thiolate [Mo−S = 2.390(3) Å] ligands. This and other LMo^IVO(SR)(py) (R = Ph, CH₂Ph, CHMe₂) complexes are also obtained from LMo^VIO₂(SR) via two-electron oxygen atom transfer reactions involving tertiary phosphines in pyridine. In dry solvents, the oxo−Mo(IV) complexes are oxidized by ferrocenium ion to the EPR-active cations [LMo^VO(SR)(py)]⁺ which are hydrolyzed rapidly in wet solvents to LMo^VO(OH)(SR). More generally, the complexes LMo^VIO₂X (X = Cl, Br, NCS, OPh, SPh, SCH₂Ph, SCHMe₂) react with PPh₃ at room temperature to yield OPPh₃ and unstable, coordinatively-unsaturated intermediates LMo^IVOX. The latter are oxidized back to LMo^VIO₂X by Me₂SO or can be trapped in a number of ways, depending on available ligands. For example, the complexes LMo^IVOX(solvent) are detected in coordinating solvents, LMo^VOClX in chlorinated solvents, LMo^VO(OMe)X in MeOH, and [LMo^VO]₂(μ-O) in dry toluene. However, in wet weakly-coordinating solvents, LMo^VO(OH)X complexes are produced cleanly and can be oxidized quantitatively to LMo^VIO₂X. Consequently, LMo^VIO₂X complexes are catalysts for the oxidation of PPh₃ by O₂ in the presence of H₂O. Oxygen isotope tracing shows that H₂O rather than O₂ is the source of the oxygen atom which is transferred to PPh₃. This is the first model system which displays the full cycle proposed for oxidizing molybdoenzymes featuring [Mo^VIO₂]²⁺ resting states.