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Nature of the Oxomolybdenum−Thiolate π-Bond:  Implications for Mo−S Bonding in Sulfite Oxidase and Xanthine Oxidase

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journal contribution
posted on 08.03.2004, 00:00 by Rebecca L. McNaughton, Matthew E. Helton, Michele Mader Cosper, John H. Enemark, Martin L. Kirk
The electronic structure of cis,trans-(L-N2S2)MoO(X) (where L-N2S2 = N,N‘-dimethyl-N,N‘-bis(2-mercaptophenyl)ethylenediamine and X = Cl, SCH2C6H5, SC6H4−OCH3, or SC6H4CF3) has been probed by electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopies to determine the nature of oxomolybdenum−thiolate bonding in complexes possessing three equatorial sulfur ligands. One of the phenyl mercaptide sulfur donors of the tetradentate L-N2S2 chelating ligand, denoted S180, coordinates to molybdenum in the equatorial plane such that the O⋮Mo−S180−Cphenyl dihedral angle is ∼180°, resulting in a highly covalent π-bonding interaction between an S180 p orbital and the molybdenum dxy orbital. This highly covalent bonding scheme is the origin of an intense low-energy S → Mo dxy bonding-to-antibonding LMCT transition (Emax ∼16 000 cm-1, ε ∼ 4000 M-1 cm-1). Spectroscopically calibrated bonding calculations performed at the DFT level of theory reveal that S180 contributes ∼22% to the HOMO, which is predominantly a π antibonding molecular orbital between Mo dxy and the S180 p orbital oriented in the same plane. The second sulfur donor of the L-N2S2 ligand is essentially nonbonding with Mo dxy due to an O⋮Mo−S−Cphenyl dihedral angle of ∼90°. Because the formal Mo dxy orbital is the electroactive or redox orbital, these Mo dxy−S 3p interactions are important with respect to defining key covalency contributions to the reduction potential in monooxomolybdenum thiolates, including the one- and two-electron reduced forms of sulfite oxidase. Interestingly, the highly covalent Mo−S180 π bonding interaction observed in these complexes is analogous to the well-known Cu−SCys π bond in type 1 blue copper proteins, which display electronic absorption and resonance Raman spectra that are remarkably similar to these monooxomolybdenum thiolate complexes. Finally, the presence of a covalent Mo−S π interaction oriented orthogonal to the MO⋮O bond is discussed with respect to electron-transfer regeneration in sulfite oxidase and MoSsulfido bonding in xanthine oxidase.