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−OSi 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−Ox 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 O2 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.