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 iPrMoVOS(2-OC6H4CO2Et)] [Cp = cyclopentadienyl; Tp iPr = 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 MoV⎓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 iPrMoIVO{2-OC6H4C(O)S-κO,κS}]. This study provides the first definitive measurement of the MoV⎓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 MoV⎓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.