Monodithiolene molybdenum(V, VI) complexes: a structural analogue of the oxidized active site of the sulfite oxidase enzyme family.

The active sites of the xanthine oxidase and sulfite oxidase enzyme families contain one pterin-dithiolene cofactor ligand bound to a molybdenum atom. Consequently, monodithiolene molybdenum complexes have been sought by exploratory synthesis for structural and reactivity studies. Reaction of [MoO(S(2)C(2)Me(2))(2)](1-) or [MoO(bdt)(2)](1-) with PhSeCl results in removal of one dithiolate ligand and formation of [MoOCl(2)(S(2)C(2)Me(2))](1-) (1) or [MoOCl(2)(bdt)](1-) (2), which undergoes ligand substitution reactions to form other monodithiolene complexes [MoO(2-AdS)(2)(S(2)C(2)Me(2))](1-) (3), [MoO(SR)(2)(bdt)](1-) (R = 2-Ad (4), 2,4,6-Pr(i)(3)C(6)H(2) (5)), and [MoOCl(SC(6)H(2)-2,4,6-Pr(i)(3))(bdt)](1-) (6) (Ad = 2-adamantyl, bdt = benzene-1,2-dithiolate). These complexes have square pyramidal structures with apical oxo ligands, exhibit rhombic EPR spectra, and 3-5 are electrochemically reducible to Mo(IV)O species. Complexes 1-6 constitute the first examples of five-coordinate monodithiolene Mo(V)O complexes; 6 approaches the proposed structure of the high-pH form of sulfite oxidase. Treatment of [MoO(2)(OSiPh(3))(2)] with Li(2)(bdt) in THF affords [MoO(2)(OSiPh(3))(bdt)](1-) (8). Reaction of 8 with 2,4,6-Pr(i)(3)C(6)H(2)SH in acetonitrile gives [MoO(2)(SC(6)H(2)-2,4,6-Pr(i)(3))(bdt)](1-) (9, 55%). Complexes 8 and 9 are square pyramidal with apical and basal oxo ligands. With one dithiolene and one thiolate ligand of a square pyramidal Mo(VI)O(2)S(3) coordination unit, 9 closely resembles the oxidized sites in sulfite oxidase and assimilatory nitrate reductase as deduced from crystallography (sulfite oxidase) and Mo EXAFS. The complex is the first structural analogue of the active sites in fully oxidized members of the sulfite oxidase family. This work provides a starting point for the development of both structural and reactivity analogues of members of this family.

Oxo/sulfidotungstate(VI) as precursors to W(VI)O2, W(VI)OS, and W(VI)S2 complexes and W(IV)-dithiolene chelate rings.

Synthetic models leading to oxosulfidotungsten(VI) groups and dithiolene chelate rings have been investigated. The heterogeneous reaction systems [WO4-nSn]2-/2Ph3SiCl/Me4phen (n = 0-2) in acetonitrile afford the complexes [WQ2(OSiPh3)2(Me4phen)] (1-3) in the indicated yields containing the groups W(VI)O2 (1; 86%), W(VI)O2 (2; 45%), and W(VI)S2 (3; 83%). In the crystalline state these complexes have imposed C2 symmetry, with cis-oxo/sulfido and trans-silyloxide ligands. 1H NMR spectra indicate that this stereochemistry is retained in solution. The colors of 2 (yellow, 367 nm) and 3 (orange, 451 nm) arise from LMCT absorptions at the indicated wavelengths. These results demonstrate that the silylation procedure previously introduced for the preparation of molecules with the Mo(VI)OS group (Thapper, et al. Inorg. Chem. 1999, 38, 4104) extends to tungsten. Methods for the formation of dithiolene chelate rings MS2C2R2 in reactions with sulfide-bound M = Mo or W precursors are summarized. In a known reaction type, 3 and activated acetylenes rapidly form [W(IV)(OSiPh3)2(Me4phen)(S2C2R2)] (R = CO2Me, 4, 83%, and Ph, 5, 98%). In a new reaction type not requiring the isolation of an intermediate, the systems [MO2S2]2-/2Ph3SiCl/Me4phen/PhC=CPh in acetonitrile afford 5 (68%) and [Mo(IV)(OSiPh3)2(Me4phen)(S2C2Ph2)] (6; 61%). Complexes 5 and 6 are isostructural, maintain the trans-silyloxide stereochemistry, and exhibit chelate ring dimensions indicative of ene- 1,2-dithiolate coordination. Reductions in the -1.4 to -1.7 V range are described as metal-centered. It remains to be seen whether the oxo/sulfidotungsten(VI) groups in 1-3 eventuate in the active sites of tungstoenzymes. (Me4phen = 3,4,7,8-tetramethyl-1,10-phenanthroline.)