Multiiron polyoxoanions. Synthesis, characterization, X-ray crystal structure, and catalytic H2O2-based alkene oxidation by [(n-C4H9)(4)N](6)[Fe-4(III)(H2O)(2)(PW9O34)(2)]

A tetranuclear ferric Keggin sandwich-type heteropolyanion has been synthesized by the reaction of the lacunary species Delta-Na8H[PW9O34] with FeCl2 followed by O-2 oxidation in nonaqueous media. The structure of [(n-C4H9)(4)N](6) [Fe-4(III)(H2O)(2)(PW9O34)(2)]. 4CH(3)CN . 2CH(2)Cl(2) . 2H(2)O (TBA-1) was determined by single-crystal X-ray diffraction (orthorhombic, Pbca; R = 0.0693 for 14 963 reflections with F-O > 4 sigma(F-O)). The compound was further characterized by infrared and UV-visible spectroscopy, electrochemistry, magnetic susceptibility, FAB mass spectrometry (FAB-MS), and elemental analyses. Five lines of evidence are consistent with the Fe-4(III) oxidation state: (i) valence sum calculations from the X-ray structure (ca. 2.86 +/- 0.07 per Fe); (ii) the rest potential from cyclic voltammetry; (iii) charge balance requirements; (iv) titration with Ce-IV(SO4)(2); and (v) oxidation by O-2. In contrast to the tetranuclear ferric Wells-Dawson-derived sandwich complex, [Fe-4(III)(H2O)(2)(P2W15O56)(2)](12-), TBA-1 can only be prepared from a ferrous precursor. TBA-1 is a catalyst for the oxidation of hydrocarbons with H2O2 The selectivity for epoxide is low (10-50%). Unlike most other transition-metal-substituted polyoxometalates (TMSPs), TBA-1 is very stable under turnover conditions in aqueous H2O2 (no detectable changes in the IR or NMR spectra after 48 h). The rate of H2O2 disproportionation catalyzed by TBA-1 is relatively low (the ratio of rates, disproportionation to oxidation, is ca. 3:2).