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]­[TpiPrMoVOS­(2-OC6H4CO2Et)] [Cp = cyclopentadienyl; TpiPr = 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]­[TpiPrMoIVO­{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.