Structural and property characterization of two new charge-transfer salts based on Keggin ions and ferrocene

Two new charge-transfer salts based on ferrocene and Keggin-type anions, [NBu4]6 [Fe(C5H5)2][HAsMo12O40]2 (1) and [NBu4]2[Fe(C5H5)2]2[HSiMo12O40]·2CH3CN (2), were synthesized and their structures were determined by X-ray diffraction analysis. There are different stoichiometries (Keggin anion: ferrocene cation) in 1 and 2, namely, 2:1 and 1:2. There exist complex interactions between the polyanions, and the framework oxygens of anions and the cyclopentadienyl rings of ferrocene. One of two crystallographic asymmetric Keggin ions in 1 is surrounded by four parallel ferrocene cations with the closest distance of framework oxygens of anions and the cyclopentadienyl rings of ferrocene moiety of 3.16 Ǻ. Each anion in 2 is surrounded by six parallel ferrocene cations with the close distance of framework oxygens of anion and the cyclopentadienyl rings of ferrocene moiety from 3.197 to 3.401 Ǻ. The bond valence sum of Mo atoms in compounds 1 and 2 is in the range of 6.234–6.623 and much larger than 6, which suggests that there is more covalent ingredient between Mo–O in the change-transfer salts owing to the delocalization of one unpaired electron in the reduced Keggin ion. The UV–visible spectra, EPR and CV properties of the salts were also discussed.