Structural Characterization and Unusual Reactivity of Oxosulfido-Mo(V) Compounds: Implications for the Structure and Electronic Description of the Very Rapid Form of Xanthine Oxidase

The first structural characterization of a mononuclear, EPR-active, oxosulfido-Mo­(V) compound related to the very rapid form of xanthine oxidase (VR-XnO) is reported. The compound, [CoCp₂]­[Tp^<i>i</i>PrMo^VOS­(2-OC₆H₄CO₂Et)] [Cp = cyclopentadienyl; Tp^<i>i</i>Pr = hydrotris­(3-isopropylpyrazol-1-yl)­borate], exhibits a distorted octahedral geometry with MoO and Mo⎓S distances of 1.761(5) and 2.215(2) Å, respectively, and an OMo⎓S angle of 107.33(14)°. Significantly, the Mo^V⎓S distance is much shorter than the value of 2.36 Å reported for oxosulfido-Mo­(V) compounds (Singh, R.; et al. Inorg. Chem. 1989, 28, 8) but close to the range established for VR-XnO by protein crystallography. The methyl and phenyl esters were also prepared but the latter is highly reactive and undergoes an intramolecular, radical-based cyclization/elimination reaction to form [CoCp₂]­[Tp^<i>i</i>PrMo^IVO­{2-OC₆H₄C­(O)­S-κ<i>O</i>,κ<i>S</i>}]. This study provides the first definitive measurement of the Mo^V⎓S bond distance in an unambiguously characterized oxosulfido-Mo­(V) compound and supports the presence of a short (ca. 2.22 Å) Mo⎓S bond in VR-XnO. It also demonstrates that the Mo^V⎓S moiety participates in radical-based reactions that are facilitated by the facile redox interplay of Mo and S and by substrates susceptible to radical eliminations