ESR Studies of Reduced Molybdovanadophosphoric Heteropoly Acids. II. Homogeneous Reduction

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Abstract
ESR spectra of the chemically reduced molybdovanadophosphoric heteropoly acids (H3+xMo12−xVxPO40·nH2O, where x=1 and 2) were observed in the solution and glassy state in various organic solvents. On addition of equimolar amounts of reductants such as SnCl2·2H2O, HSCH2CH2SH, and CH3SCH3, a transient ESR signal(B) appeared with eight hfs lines referred to as V(IV), I=7⁄2, and with =1.96 and =−0.83×10−2cm−1. However, it disappeared within ca. 5 hr at 20°C and another signal(C) with =1.962 and =−1.00×10−2 cm−1 gradually increased in intensity. The hyperfine coupling constant of the transient species was thus unusually small as compared with that of the ordinary V(IV) species and the C species. The mechanistic pathways of the reduction were discussed. The splitting observed in the low-field parallel components of the B signal was satisfactorily explained by a “geometrical charge isomerism” characteristic of the structure of the heteropoly anion. The origin of the unusually small hfs constant was discussed in the light of its molecular structure.