Multiiron Polyoxoanions. Synthesis, Characterization, X-ray Crystal Structure, and Catalytic H(2)O(2)-Based Alkene...

A tetranuclear ferric Keggin sandwich-type heteropolyanion has been synthesized by the reaction of the lacunary species Delta-Na(8)H[PW(9)O(34)] with FeCl(2) followed by O(2) oxidation in nonaqueous media. The structure of [(n-C(4)H(9))(4)N](6)[Fe(III)(4)(H(2)O)(2)(PW(9)O(34))(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) > 4sigma(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(III)(4) 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)(SO(4))(2); and (v) oxidation by O(2). In contrast to the tetranuclear ferric Wells-Dawson-derived sandwich complex, [Fe(III)(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](12-), TBA-1 can only be prepared from a ferrous precursor. TBA-1 is a catalyst for the oxidation of hydrocarbons with H(2)O(2). 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 H(2)O(2) (no detectable changes in the IR or NMR spectra after 48 h). The rate of H(2)O(2) disproportionation catalyzed by TBA-1 is relatively low (the ratio of rates, disproportionation to oxidation, is ca. 3:2).