The true nature of the Di-iron(III) gamma-Keggin structure in water: catalytic aerobic oxidation and chemistry of an unsymmetrical trimer

The complex [gamma(1,2)-SiW(10){Fe(OH(2))}(2)O(38)](6)(-) (1) has been reported to catalyze the much sought reductant-free selective O(2)-based epoxidation of alkenes (Nishiyama, Y.; Nakagawa, Y.; Mizuno, N. Angew. Chem. Int. Ed. 2001, 40, 3639-3641) in chlorocarbon-acetonitrile solution. The challenge of reproducing catalysis by 1 led us to examine this chemistry in detail. In H(2)O, a desirable solvent for catalysis, 1, does not exist in the proposed organic-medium form in which the two iron atoms are in the binding pocket defined by the equatorial oxygens and, importantly, by two oxygens bound to the central Si heteroatom. Instead, 1 in H(2)O initially forms an unusual trimer [{Fe(2)(OH)(3)(H(2)O)(2)}(3)(gamma-SiW(10)O(36))(3)](15)(-) (2). The X-ray structure of 2 shows that the Fe-O(Si) bonds are cleaved and new bonds (mu-hydroxo bridges) form between these Fe centers and those of the neighboring [gamma(1,2)-SiW(10)Fe(2)] units. Structural, physical, and computational evidence indicate that if the bonds between the d-electron center, M (Fe in the case of 1 and 2), and the terminal ligands on M are stronger than the M-O(x)() bonds, then the out-of-pocket form is more stable and is the one observed. Significantly, 2 in H(2)O forms an intermediate that catalyzes the effective aerobic oxidation of sulfur compounds (mercaptoethanol is oxidized to the corresponding disulfide by O(2) at ambient pressure and temperature). All experimental findings are consistent with dissociation of a gamma-SiW(10) Keggin unit from the trimer, 2, to form the catalytically active species.     

Computational insights into the mechanism of radical generation in B12-dependent methylmalonyl-CoA mutase

ONIOM calculations have provided novel insights into the mechanism of homolytic Co-C5' bond cleavage in the 5'-deoxyadenosylcobalamin cofactor catalyzed by methylmalonyl-CoA mutase. We have shown that it is a stepwise process in which conformational changes in the 5'-deoxyadenosine moiety precede the actual homolysis step. In the transition state structure for homolysis, the Co-C5' bond elongates by approximately 0.5 Angstroms from the value found in the substrate-bound reactant complex. The overall barrier to homolysis is approximately 10 kcal/mol, and the radical products are approximately 2.5 kcal/mol less stable than the initial ternary complex of enzyme, substrate, and cofactor. The movement of the deoxyadenosine moiety during the homolysis step positions the resulting 5'-deoxyadenosyl radical for the subsequent hydrogen atom transfer from the substrate, methylmalonyl-CoA.     

Computational studies of nucleophilic attack and protonation of electron-deficient benzoheterocycle triosmium clusters

The density functional theory method has been applied to gain insights into the regioselectivity of nucleophilic attack and protonation of electron-deficient benzoheterocycle triosmium clusters. We report our computational results on the reaction of the green 46-electron triosmium clusters Os3(CO)(9)(mu3-eta2-(LH))(mu-H) (L = benzoxazole, 1a; benzothiazole, 1b; dihydroquinoline, 1c; 1,3-dehydroindoline, 1d; 4H-3,1-benzoxazine, 1e) with hydride (H-) and proton (H+) in order to elucidate factors affecting the observed differences in the structure of the kinetic products of these reactions. Transition-state calculations for the interconversion of the anionic tautomers resulting from H- attack on the clusters 1a-e show that the activation energies of these anionic clusters are considerably lower than the previously reported barriers for related neutral clusters. Calculations also reveal that the structures of the kinetic products resulting from sequential H-/H+ attack are determined by the protonation process.