The unperturberd oxo-sulfido functional group cis-Mo(VI)OS related to thatin the xanthine oxidase family of molybdoenzyme: Synthesis, structural characterization, and reactivity aspects

The oxo-sulfido functional group Cis-(MoOS)-O-VI is essential to the activi ty of the xanthine oxidase family of enzymes but has proven elusive to synt hesis in molecules containing no other four-electron ligands. A direct rout e to molecules containing this group has been achieved. The reaction system [MoO2(OSiPh3)(2)]/L in dichloromethane yields the complexes [(MoO2)-O-VI(O SiPh3)(2)L] (L = phen (1), Me(4)phen (2), 4,4'-Me(2)bpy (3), 5,5'-Me(2)bpy (4), 2 py (5)) (74-96%), which are shown to have a distorted octahedral str ucture of crystallographically imposed C-2 symmetry (1, 5) with cis oxo and trans silyloxy ligands. The related reaction system [MoO3S](2-)/2Ph(3)SiCl /L in acetonitrile affords the complexes [(MoOS)-O-VI(OSiPh3)(2)L] (L = phe n (6), Me(4)phen (7), 4,4'-Me(2)bpy (8), 5,5'-Me(2)bpy (9)) (36-69%). From the collective results of elemental analysis, mass spectrometry, H-1 NMR, a nd X-ray structure determinations (6, 7), complexes 6-9 are shown to contai n the cis-(MoOS)-O-VI group in molecules with the same overall stereochemis try as dioxo complexes 1-5. The crystal structures of 6 and 7 exhibit O/S d isorder, which was modeled in refinements with 50% site occupancies. The Mo =O (1.607(5) (6), 1.645(5) (7) Angstrom) and Mo=S (2.257(3) (6), 2.203(2) ( 7) Angstrom) bond distances obtained in this way are somewhat shorter and l onger, respectively, than expected. Distances obtained 6-9 (Mo=O 1.71-1.72 Angstrom; Mo=S 2.18-2.19 Angstrom) are considered more satisfactory and are in good agreement with EXAFS values for xanthine oxidase. Molybdenum K-edg e data for 1 and 6-9 are reported. Reaction of 7 with Ph3P in dichlorometha ne results in sulfur abstraction and formation of [(MoOCl)-O-V(OSiPh3)(2)(M e(4)phen)] (10), which has a distorted octahedral structure with cis O/Cl a nd cis silyloxy ligands. Sulfur rather than oxygen abstraction is favored b y relative Mo=O/Mo=S bond strengths. Complexes 6-9 should allow exploration of the biologically significant Cis-(MoOS)-O-VI group.