MULTIIRON POLYOXOANIONS - SYNTHESES, CHARACTERIZATION, X-RAY CRYSTAL-STRUCTURES, AND CATALYSIS OF H2O2-BASED HYDROCARBON OXIDATIONS BY [FE-4(III)(H2O)(2)(P2W15O56)(2)](12-)

The reaction of [P2W15O56](12-) and aqueous Fe3+ forms two ferric poly tungstophosphate derivatives that have been isolated, purified, and ch aracterized by X-ray crystallography and several physical methods. At pH similar to 3 and [Na+]=2 M, needles of Na-12[Fe-4(III)(H2O)(2)(P2W1 5O56)(2)].58H(2)O, Na-1, form, in which the conventional ion packing a rrangement in solid heteropoly acids is observed: each polyanion, [Fe- 4(III)(H2O)(2)(P2W15O56)(2)](12-) (1), is well separated by the cation s (Na+). At pH similar to 1 and [Na+]=1 hi, however, elongated prisms of H3Na9[Fe-4(III)(H2O)(2)-(P2W15O56)(2)].62H(2)O, (Na-l)(n), form. (N a-l), is a novel one-dimensional inorganic polymer composed of 1 linke d via ion pairing with strongly associated hydrated sodium cations. Bo th Na-1 and (Na-1)(n) display identical characteristics in H2O solutio n. Seven lines of evidence are consistent with the Fe-4(III) oxidation state in 1: (1) the synthesis itself (only Fe3+ used); (2) bond-lengt h-based valence sum calculations based on the two X-ray structures; (3 ) the electrochemical behavior; (4) the charge requirements and elemen tal analyses; (5) no reaction between 1 and Ce4+; (6) the lack of a Ja hn-Teller distortion; and (7) the infrared spectrum relative to those of the literature [M-4(H2O)(2)(P2W15O56)(2)](16-) sandwich compounds, where M=a divalent first-row transition metal ion in all cases. The te tra-n-butylammonium salt of 1, TEA-1, exhibits catalytic activity for oxidation of alkenes by H2O2. The catalytic effect is substantial. For example, in the reaction of cyclohexene + H:O:, rate(with TBA-l)/rate (without TEA-1)(-d[cyclohexene]/dt) is similar to 25. Alkene oxidation is characterized predominantly by allylic attack on aliphatic substra tes and oxidative cleavage of stilbenes. The polyanion, 1, is quite st able under catalytic conditions (no decomposition of 1 was detected by P-31 NMR after a solution of 1, alkene, and 0,25 M H2O2 was incubated for 48 h).