Stabilization of oxo-metal bonding by the pi-conjugated system in dithiolate ligands: cis-Dioxotungsten(VI) bis(naphthalenedithiolato) and the related complexes as models for tungsten oxidoreductases

As a model of tungsten oxidoreductase, a dioxotungsten(VI) dithiolato complex, (NEt(4))(2)- [(WO2)-O-VI(ndt)(2)]. H2O (1) (ndt=2,3-naphthalenedithiolato), was synthesized. It has provided the detailed structural dimensions of mutual trans influence between oxo and thiolate. The complex, 1, crystallizes in space group P2(1)/n with a = 7.396(2), b = 17.581(2), c = 28.839(2) Angstrom, beta = 95.35(2), and Z = 2, and was refined to R = 2.3%. The short S-C distances of the thiolate trans to the W-VI = 0 groups indicate the presence of the partial double bonding at S-C (thiketone-like) which contributes to the stabilization of the dioxotungsten(VI) species by weakening x-interaction of the W-VI-S bonds trans to the W-VI = 0 groups. In the tungstopterin cofactor and the molybdopterin cofactor of the enzyme, the double bonding nature of S-C (thioketone-like) is expected to stabilize the oxo ligand of the tungsten and molybdenum ion.