Synthesis, Structures, and Reactivity of Bis(dithiolene)molybdenum(IV,VI) Complexes Related to the Active Sites of Molybdoenzymes

The existence of a universal pterin dithiolene cofactor ligand for the molybdenum and tungsten oxotransferases supports a biological significance of the fundamental chemistry of mono- and bis(dithiolene) complexes of these elements. Members of the dimethyl sulfoxide (DMSO) reductase family of enzymes contain two pterin dithiolene ligands; at least one enzyme functions by using the minimal reaction couple MoIV + Me2SO ⇄ MoVIO + Me2S. Accordingly, the synthesis, structures, and reactivity of bis(dithiolene)Mo(IV,VI) complexes of benzene-1,2-dithiolate and related ligands have been investigated. A convenient synthesis of square pyramidal [MoIVO(S2C2R2)2]2- complexes is reported. Compounds of the type [Mo(S2C2R2)2(R'NC)2], including [Mo(bdt)2(MeNC)2] (13), were prepared by reacting Na2(S2C2R2) and R'NC with [MoCl4(MeCN)2] and were shown to be identical (R' = Me) to byproducts in the synthesis of [MoIVO(S2C2R2)2]2-. In one such reaction, the Fischer carbene complex [Mo(Me4bdt)2(MeNC)(CMe4bdt)] (16) was isolated. Silylation of [MoIVO(bdt)2]2- affords [MoIV(bdt)2(OSiButPh2)]- (8); an analogous reaction of [MoO2(bdt)2]2- yields [MoVIO(bdt)2(OSiButPh2)]- (11). The structures of square pyramidal 8 and cis-octahedral 11 reveal them to be minimal unconstrained representations of the des-oxo [MoIV(S2pd)2(O·Ser)] and mono-oxo [MoVIO(S2pd)2(O·Ser)] sites, respectively, of Rhodobacter sphaeroides DMSO reductase. This description applies in the limit of symmetrical dithiolene coordination; silyloxide is a simulator of serinate binding. Complex 8 shows limited reactivity with sulfoxides, and 11 is unreactive toward sulfides. However, 11 is reduced to 8 by tertiary phosphines; with excess phosphine, [Mo(bdt)2(PMePh2)2] (17) was formed. This compound was also prepared independently from [MoCl4(MeCN)2] and the phosphine. The compounds 13, 16, and 17 form an isoelectronic set with idealized trigonal prismatic (C2v) stereochemistry. These results complement a parallel development of bis(dithiolene)W(IV,VI) complexes as active-site analogues of tungstoenzymes (Lorber, C.; Donahue, J. P.; Goddard, C. G.; Nordlander, E.; Holm, R. H. J. Am. Chem. Soc. 1998, 120, 8102). Certain comparisons between the properties of molybdenum and tungsten bis(dithiolenes) are offered. (bdt = benzene-1,2-dithiolate(2−), S2pd = pterin dithiolene(2−).)