| In our attempts to synthesize new iron-containing Wells-Dawson polyoxometalates (POMs) for catalytic oxidation reactions, five new sandwich-type complexes have been prepared: alphaalphaalphaalpha-(NaOH2)2 FeIII2(P2W15O56 )216-, alphaalphabetaalpha-Cu II(OH2)FeIII2(P2W 15O56)(P2(CuII(OH2)) 2W13O52)16-, alphaalphabetaalpha-Co II(OH2)FeIII2-(P2 W15O56)(P2(CoII(OH 2))2W13O52)16-, alphaalphabetaalpha-(Fe IIIOH2)(NaOH2)FeIII2- (P2W15O56)214- , and alphabetabetaalpha-(NiIIOH2) 2FeIII2(P2W15O 56)214-. These complexes were characterized by X-ray crystallography, 31P NMR, IR, and elemental analysis. The complexes have been converted to tetra-n-butylammonium salts by a metathesis reaction and were used for the oxidation of alkenes by H2O2. alphaalphaalphaalpha-(NaOH2) 2FeIII2(P2W15O 56)216- and alphabetabetaalpha-(Ni IIOH2)2FeIII2(P 2W15O56)214- were the most effective catalysts, each giving more than 200 turnovers of epoxide (for the oxidation of cyclooctene) over a 40 hour period with more than 98% selectivity.;Reaction of alpha-P2W15O56 12- and Fe(III) in a saturated NaCl solution produces the first trisubstituted Wells-Dawson structure with three low-valent (non-d 0) metals, alpha-(FeIIICl)2(Fe IIIOH2)P2W15O59 11-. Dissolution of this species into 1 M NaBr gives the triaquated species alpha-(FeIIIOH2)3P2W 15O599-. Ionic strength values of 1 M or greater are necessary to avoid decomposition of these species to the conventional sandwich-type complex, alphabetabetaalpha-(Fe IIIOH2)2FeIII2(P 2W15O56)212-.;Finally, to further understand the role Baker-Figgis isomerization plays on the overall stability of POM structures (including the sandwich-type POMs), a quantitative kinetic, thermodynamic, and mechanistic study was undertaken which may also be a model for the related metal oxide systems. The isomerization of beta-P2W18O626- to alpha-P2W18O626-, followed by 31P NMR, has a bimolecular rate constant of 9.3 x 10-1 M-1 s-1 at 343K in pH = 4.24 acetate buffer. A mechanism is proposed involving the formation of a defect complex (missing one or more W centers) followed by isomerization and reconstitution of the parent structure. |
