Asymmetric sandwich-type polyoxoanions. Synthesis, characterization, and X-ray crystal structures of diferric complexes [(TMFe2III)-Fe-II(P2W15O56)((P2TM2W13O52)-W-II)](16-), TM = Cu or Co

Reaction of the diferric sandwich-type polyoxometalate (NaOH2)(2)Fe-2(III)( P2W15O56)(2)(16-) (1) with excess aqueous Cu(II) or Co(II) yields a new typ e of d-electron-metal substituted polyoxometalate, [(TMFe2III)-Fe-II(P2W15O 56) ((P2TM2W13O52)-W-II)](16-), TM = Cu (2), Co (3), respectively. The stru cture of the sodium salt of 2 (Na2), determined by single-crystal X-ray dif fraction analysis (a = 13.44,13(9) Angstrom, b = 21.2590(15) Angstrom, c = 25.5207(18) Angstrom, alpha = 80.475(2)degrees, beta = 85.555(2)degrees, ga mma = 89.563(2)degrees, triclinic, P (1) over bar, R1 = 5.42%, based on 430 97 independent reflections), consists of a defect Fe2Cu central unit sandwi ched between two different trivacant Wells-Dawson-type units, P2W15 and P2C u2W13, where the latter unit has two octahedral Cu(II) ions substituted for two adjacent belt W(VI) atoms. The CuO5OH2 octahedron in the central unit shows pronounced Jahn-Teller distortion. A low-resolution X-ray structure o f Na3 is included in the Supporting Information. UV-visible, infrared, P-31 NMR, cyclic voltammetric, and elemental analysis data are all consistent w ith the structure determined from the X-ray analysis. Cyclic voltammograms of 2 and 3 exhibit multiple electron-transfer processes under ambient condi tions, and copper or cobalt incorporation into the framework of I results i n a substantial pertubation of the electrochemical properties of the polyox otungstate framework. The tetra-ii-butylammonium salts of 2 and 3 (readily prepared by metathesis) are stable and effective catalysts for the oxidatio n of some alkenes with high yields based on H2O2.