Mark R. Chance

High-valent manganese in polyoxotungstates—1. Manganese(IV) keggin derivatives

Abstract Aqueous and non-aqueous soluble salts of manganese(IV)-containing Keggin polyoxotungstates [(XO 4 )W 11 Mn IV O 36 H x ] n − (X ue5f8 Si, B, and Zn) have been prepared by chemical and electrolytic oxidation of corresponding manganese(II) or manganese(III) anions, and have been characterized by magnetic susceptibilities, ESR and X-ray absorption near-edge spectroscopy (XANES), in addition to conventional elemental and spectroscopic analyses. The crystal structure of dark brown K 7 [(ZnO 4 )W 11 Mn IV O 36 H] · 19H 2 O ( I ) was determined by X-ray diffraction and reveals a disordered Keggin anion. Manganese EXAFS investigation of I and the corresponding manganese(II) complex, which contains a terminal Mn II ue5f8OH 2 group, strongly indicates that I contains Mn IV ue5f8OH ( r Mn ue5f8O ≈ 1.82 A) rather than Mn IV ue5fbO.

Formation of a square-planar Co(I) B12 intermediate. Implications for enzyme catalysis.

X-ray edge and extended x-ray absorption fine structure (EXAFS) techniques provide powerful tools for analysis of local molecular structure of complexes in solution. We present EXAFS results for Co(I) B12 that demonstrate a four-coordinate (distorted) square-planar configuration. Comparison of EXAFS solutions for Co(I) and Co(II) B12 (collected previously; Sagi et al. 1990. J. Am. Chem. Soc. 112:8639-8644) suggest that modulation of the Co-N bond to the axial 5,6-dimethylbenzimidazole (DMB), in the absence of changes in Co-N (equatorial) bond distances, may be a key mechanism in promoting homolytic versus heterolytic cleavage. As Co-C bond homolysis occurs, the Co-N (DMB) bond becomes stronger. However, for heterolytic cleavage to occur, earlier electrochemical studies (D. Lexa and J. M. Saveant. 1976. J. Am. Chem. Soc. 98:2652-2658) and recent studies of methylcobalamin-dependent Clostridium thermoaceticum (Ragsdale et al. 1987. J. Biol. Chem. 262:14289-14297) suggest that removal of the DMB ligand (before Co-C bond cleavage) favors formation of the four-coordinate square-planar Co(I) species while inhibiting formation of the five-coordinate Co(II) B12 complex. This paper presents the first direct evidence that formation of the Co(I) B12 intermediate must involve breaking of the Co-N (DMB) bond.

Temperature dependent coordination effects in base-off adenosyl and methylcobalamin by X-ray edge spectroscopy

Examination of the role of base-off cobalamin species (where the 5,6-dimethylbenzimidazole ligand coordinated to cobalt is detached by protonation of the imidazole nitrogen) in differentiation between homolytic and heterolytic cobalt-carbon bond cleavage mechanisms is a primary step in better understanding B12-dependent enzyme catalysis. X-ray absorption edge spectroscopy provides the first direct structural evidence of five-coordination in base-off adenosyl- and base-off methylcobalamin complexes at room temperature. Integration of 1s-3d pre-edge transitions of the base-off species reveals the dependence of coordination number on temperature. Gradual increases in 1s-3d transition intensities, as the temperature is increased from 180 K to 298 K, reflect a change in the coordination number from six (where a water molecule is presumed to occupy the coordination site vacated by the 5,6-dimethylbenzimidazole ligand) to primarily five-coordinate. Base-off configurations that strengthen the Co-C bond may be both decreasing the tendency for homolytic cleavage while increasing the tendency for hetrolytic Co(I) B12 formation.