TETRAHYDROPTERIN REACTIONS OF DIOXO-MOLYBDENUM(6+) COMPLEXES - DOES REDOX OCCUR

This report describes out continued investigation of reactions between tetrahydropterins and dioxomolybdenum complexes. We report the results of structural, reactivity, and theoretical experiments that indicate these reduced molybdenum-pterin complexes are better described as Mo(6+)-H(4)pterin rather than Mo(4+)-H(2)pterin as previously assigned. Both Mulliken charges calculated using the extended Huckel molecular orbital method and the bond valence sum method predict a formal molybdenum oxidation state midway between 5+ and 6+. The complexes Mo2O4Cl2(tetrahydro-6,7-dimethylpterin)(2), MoOCl3(tetrahydro-6,7-dimethylpterin), and MoOCl2(tetrahydro-6-(hydroxymethyl)pterin)(diethyldithiocarbaate) have been characterized by H-1 NMR, IR, UV/vis, and conductivity measurements. The X-ray crystal structure is reported for Mo2O4Cl2(tetrahydro-6,7-dimethylpterin)(2) . 4DMF. Inner coordination sphere bond distances indicate substantial electron density is donated from the pterin N(5) to Mo. These complexes undergo solvation and ligand substitution reactions. It is shown that solvation is associated with acid-base reactions at the tetrahydropterin ligands. The molybdenum-tetrahydropterin complexes show a variety of reactivities toward the oxidants O-2, 2,6-dichloroindophenol, and dimethyl sulfoxide. This study shows that tetrahydropterin has a high affinity to chelate Mo(6+) if one of the two oxo ligands is removed and that coordination to molybdenum stabilizes tetrahydropterins toward oxidation. Results from this study also suggest that dioxo-Mo(VI) coordination to tetrahydropterin is unlikely.