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

Abstract

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 Mo IV + Me 2 SO ⇄ Mo VI O + Me 2 S. 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 [Mo IV O(S 2 C 2 R 2 ) 2 ] 2 - complexes is reported. Compounds of the type [Mo(S 2 C 2 R 2 ) 2 (R‘NC) 2 ], including [Mo(bdt) 2 (MeNC) 2 ] ( 13 ), were prepared by reacting Na 2 (S 2 C 2 R 2 ) and R‘NC with [MoCl 4 (MeCN) 2 ] and were shown to be identical (R‘ = Me) to byproducts in the synthesis of [Mo IV O(S 2 C 2 R 2 ) 2 ] 2 - . In one such reaction, the Fischer carbene complex [Mo(Me 4 bdt) 2 (MeNC)(CMe 4 bdt)] ( 16 ) was isolated. Silylation of [Mo IV O(bdt) 2 ] 2 - affords [Mo IV (bdt) 2 (OSiBu t Ph 2 )] - ( 8 ); an analogous reaction of [MoO 2 (bdt) 2 ] 2 - yields [Mo VI O(bdt) 2 (OSiBu t Ph 2 )] - ( 11 ). The structures of square pyramidal 8 and cis-octahedral 11 reveal them to be minimal unconstrained representations of the des-oxo [Mo IV (S 2 pd) 2 (O·Ser)] and mono-oxo [Mo VI O(S 2 pd) 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 (PMePh 2 ) 2 ] ( 17 ) was formed. This compound was also prepared independently from [MoCl 4 (MeCN) 2 ] and the phosphine. The compounds 13, 16, and 17 form an isoelectronic set with idealized trigonal prismatic ( C 2 v ) 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−), S 2 pd = pterin dithiolene(2−).)