The true nature of the di-iron(III) γ-Keggin structure in water: Catalytic aerobic oxidation and chemistry of an unsymmetrical trimer

The complex [γ(1,2)-SiW₁₀{Fe(OH₂)} ₂O₃₈]⁶ (1) has been reported to catalyze the much sought reductant-free selective O₂-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 H₂O, 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 H₂O initially forms an unusual trimer [{Fe₂(OH)₃(H₂O)₂} ₃(γ-SiW₁₀O₃₆)₃]^15- (2). The X-ray structure of 2 shows that the Fe-O_Si bonds are cleaved and new bonds (μ-hydroxo bridges) form between these Fe centers and those of the neighboring [γ(1,2)-SiW₁₀Fe₂] 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 H₂O 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 γ-SiW₁₀ Keggin unit from the trimer, 2, to form the catalytically active species.