Synthesis and reactivity studies of model complexes for molybdopterin-dependent enzymes

The molybdenum cofactor (Moco)-containing enzymes are divided into three cl asses that are named after prototypical members of each family, viz. sulfit e oxidase, DMSO reductase and xanthine oxidase. Functional or structural mo dels have been prepared for these three prototypical enzymes: (i) The compl ex [MoO2(mnt)(2)](2-) (mnt(2-) = 1,2-dicyanoethylenedithiolate) has been fo und to be able to oxidize hydrogen sulfite to HSO4- and is thus a functiona l model of sulfite oxidase. Kinetic and computational studies indicate that the reaction proceeds via attack of the substrate at one of the oxo ligand s of the complex, rather than at the metal. (ii) The coordination geometrie s of the mono-ore [Mo(VI) (O-Ser) (S-2)(2)] entity (S-2 = dithiolene moiety of molybdopterin) found in the crystal structure of R. sphaeroides DMSO re ductase and the corresponding des-oxo Mo(IV) unit have been reproduced in t he complexes [M(VI)O(OSiR3) (bdt)(2)] and [M(VI)O(OSiR3) (bdt)(2)] (M = Mo, W; bdt = benzene dithiolate). (iii) A facile route has been developed for t he preparation of complexes containing a cis-Mo(VI)OS molybdenum ore, sulfi do moiety similar to that detected in the oxidized form of xanthine oxidase . (C) 2000 Elsevier Science Inc. All rights reserved.