Insights into the nature of Mo(V) species in solution: Modeling catalytic cycles for molybdenum enzymes

The tris(pyrazolyl)borate and related tripodal N-donor ligands originally developed by Trofimenko stabilize mononuclear compounds containing MoVIO2, MoVIO, MoVO, and MoIVO units and effectively inhibit their polynucleation in organic solvents. Dioxo-Mo(VI) complexes of the type LMoO2(SPh), where L = hydrotris(3,5-dimethylpyrazol-1-yl)borate (Tp∗), hydrotris(3-isopropylpyrazol-1-yl)borate (TpiPr), and hydrotris(3,5-dimethyl-1,2,4-triazol-1-yl)borate (Tz) and related derivatives are the only model systems that mimic the complete reaction sequence of sulfite oxidase, in which oxygen from water is ultimately incorporated into product. The quasi-reversible, one-electron reduction of Tp∗MoO2(SPh) in acetonitrile exhibits a positive potential shift upon addition of a hydroxylic proton donor, and the magnitude of the shift correlates with the acidity of the proton donor. These reductions produce two Mo(V) species, [Tp∗MoVO2(SPh)]− and Tp∗MoVO(OH)(SPh), that are related by protonation. Measurement of the relative amounts of these two Mo(V) species by EPR spectroscopy enabled the pKa of the MoV(OH) unit in acetonitrile to be determined and showed it to be several pKa units smaller than that for water in acetonitrile. Similar electrochemical-EPR experiments for TpiPrMoO2(SPh) indicated that the pKa for its MoV(OH) unit was ∼1.7 units smaller than that for Tp∗MoVO(OH)(SPh). Density functional theory calculations also predict a smaller pKa for TpiPrMoVO(OH)(SPh) compared to Tp∗MoVO(OH)(SPh). Analysis of these results indicates that coupled electron–proton transfer (CEPT) is thermodynamically favored over the indirect process of metal reduction followed by protonation. The crystal structure of TpiPrMoO2(SPh) is also presented.