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‐ferric‐substituted silicotungstates has been undertaken because these phenomena are important in a large number of polyoxometalates. A single‐crystal X‐ray diffraction analysis of K 4 Na 7 [(β‐SiFe 3 W 9 (OH) 3 O 34 ) 2 (OH) 3 ] ( β 1 ) has been carried out [ a = 12.9709(7) Å, b = 38.720(2) Å, c = 21.4221(12) Å, orthorhombic, Pbcm , R 1 = 8.48 %, based on 13809 independent reflections]. The complex is isostructural with [(α‐SiFe 3 W 9 (OH) 3 O 34 ) 2 (OH) 3 ] 11– ( α 1 ) except that the edge‐shared W 3 O 13 caps in each [SiFe 3 W 9 (OH) 3 O 34 ] 4– unit are rotated by 60°. 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 α1 to its monomeric derivative [(α‐Si(FeOH 2 ) 3 W 9 (OH) 3 O 34 )] 4– ( α 2 ) (–0.484 ± 0.005 V and –0.474 ± 0.005 V, respectively). In contrast, the peak potentials of the Fe III ‐ and W VI ‐based redox processes of β1 are both found at more negative values than its rotational isomer α 1 . The absolute values of the reduction peak potential differences are 0.022 V for Fe III and 0.162 V for W VI . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)