The Unperturbed Oxo−Sulfido Functional Group cis-MoVIOS Related to That in the Xanthine Oxidase Family of Molybdoenzymes:  Synthesis, Structural Characterization, and Reactivity Aspects

The oxo-sulfido functional group cis- Mo VI OS is essential to the activity of the xanthine oxidase family of enzymes but has proven elusive to synthesis in molecules containing no other four-electron ligands. A direct route to molecules containing this group has been achieved. The reaction system [MoO 2 (OSiPh 3 ) 2 ]/L in dichloromethane yields the complexes [Mo VI O 2 (OSiPh 3 ) 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 structure of crystallographically imposed C 2 symmetry ( 1, 5 ) with cis oxo and trans silyloxy ligands. The related reaction system [MoO 3 S] 2- /2Ph 3 SiCl/L in acetonitrile affords the complexes [Mo VI OS(OSiPh 3 ) 2 L] (L = phen ( 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, 1 H NMR, and X-ray structure determinations ( 6, 7 ), complexes 6−9 are shown to contain the cis- Mo VI OS group in molecules with the same overall stereochemistry as dioxo complexes 1−5 . The crystal structures of 6 and 7 exhibit O/S disorder, which was modeled in refinements with 50% site occupancies. The Mo O (1.607(5) ( 6 ), 1.645(5) ( 7 ) Å) and Mo S (2.257(3) ( 6 ), 2.203(2) ( 7 ) Å) bond distances obtained in this way are somewhat shorter and longer, respectively, than expected. Distances obtained by molybdenum EXAFS analysis using the GNXAS protocol for 6−9 (Mo O 1.71−1.72 Å; Mo S 2.18−2.19 Å) are considered more satisfactory and are in good agreement with EXAFS values for xanthine oxidase. Molybdenum K-edge data for 1 and 6−9 are reported. Reaction of 7 with Ph 3 P in dichloromethane results in sulfur abstraction and formation of [Mo V OCl(OSiPh 3 ) 2 (Me 4 phen)] ( 10 ), which has a distorted octahedral structure with cis O/Cl and cis silyloxy ligands. Sulfur rather than oxygen abstraction is favored by relative Mo O/Mo S bond strengths. Complexes 6−9 should allow exploration of the biologically significant cis -Mo VI OS group.