Synthesis and Structures of Bis(dithiolene)tungsten(IV,VI) Thiolate and Selenolate Complexes: Approaches to the Active Sites of Molybdenum and Tungsten Formate Dehydrogenases

Formate dehydrogenases are molybdenum- or tungsten-containing enzymes that catalyze the oxidation of formate
to carbon dioxide. Among the significant characteristics of the mononuclear active sites are coordination of two
pyranopterindithiolene ligands and selenocysteinate to the metal in oxidation states IV−VI. The first detailed
investigation of the synthesis and structures of bis(dithiolene)tungsten selenolate and analogous thiolate complexes
of relevance to formate dehydrogenases has been undertaken. Some 17 complexes of the types [WIV(QR)(S2C2Me2)2]-,
[WVIO(QR)(S2C2Me2)2]-, and [WVIS(QR)(S2C2Me2)2]- (Q = S, Se; R = tert-butyl, 1-adamantyl) and the desoxo
species [WVI(SR)(OSiR‘3)(S2C2Me2)2] (R‘ = Me, Ph) were prepared. Ten structures of representative members of
these types were determined; WIV complexes are square-pyramidal and WVI complexes are six-coordinate, with
geometries intermediate between octahedral and trigonal-prismatic. Selenolate complexes are less stable than
similar thiolate species; decomposition products were identified as [WV2(μ2-Q)2(S2C2Me2)2]2- and [WIV,V2(μ2-Se)(S2C2Me2)4]-. The several [MoIV(QR)(S2C2Me2)2]- complexes prepared earlier and the tungsten compounds synthesized
in this work form a family of molecules whose overall stereochemistry and metric features are those expected in
the absence of protein structural constraints.