Structural and electrochemical studies of dimerization and rotational isomerization in multi-iron silicotungstates

A structural and electrochemical investigation of dimerization and Baker-Figgis (rotational) isomerization in the tri-fer-ric-substituted silicotungstates has been undertaken because these phenomena are important in a large number of polyoxometalates. A single-crystal X-ray diffraction analysis of K4Na7[( beta-SiFe3W9(OH)(3)O-34)(2)(OH)(3)] (beta 1) has been carried out [a = 12.9709(7) angstrom, b 38.720(2) angstrom; c = 21.4221(12) angstrom, orthorhombic, Pbcm, R-1 = 8.48 %, based on 13809 independent reflections]. The complex is isostructural with [(alpha-Si-Fe3W9(OH)(3)O-34)(2)(OH)(3)](11-) (alpha 1) except that the edge-shared W3O13 caps in each [SiFe3W9(OH)(3)O-34](4-) unit are rotated by 60 degrees. Electrochemical measurements, performed in a pH 5 acetate buffer, indicate a positive shift in the Fe-III-based peak potential (and no change for the W-VI-based potential) upon going from alpha 1 to its monomeric derivative [(alpha-Si(FeOH2)(3)W-9(OH)(3)O-34](4-) (alpha 2) (-0.484 +/- 0.005V and -0.474 +/- 0.005V, respectively). In contrast, the peak potentials of the Fe-III- and W-VI-based redox processes of beta 1 are both found at more negative values than its rotational isomer alpha 1. The absolute values of the reduction peak potential differences are 0.022 V for Fe-III and 0.162 V for W-VI. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).