The True Nature of the Di-iron(III) γ-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.