31P-NMR Study of Reduction Mechanism of 12-Mo!ybdophosphoric Acid

Abstract

Chemical species of unreduced and reduced 12-molybdophosphoric acid (PMo12) and their reaction have been investigated in concentrated aqueous and 50% water-dioxane solutions using 31P-NMR and polarography. Phosphorus-31 chemical shifts of α-PMo12 and β-PMo12 in the range of the degree of reduction among 0 and 4 were obtained in 50% water-dioxane as follows (Fig. 13): α-PMo12 (0): -3.6ppm, β-PMo12(0): -3.3ppm, α-PMol2(II): -5.8ppm, β-PMo12(II): -6.6 ppm, α-PMo12(IV): -4.6 ppm, β-PMo12(IV).: -12. 9ppm. 81P-NMR study clarified the isomerization and disproportionation behavior of unreduced and reduced PMo12 in concentrated solutions (Scheme 1) and made it possible to estimate approximate half-life periods for each step (Table 1). Isomerization of β-PMo12(0) to α-PMo12(0) was immediate in water (Fig. 10) and rapid in 50% water-dioxane (half-life period: 5 h) (Fig. 7). The two-electron reduced product of α-PMo12(0) in 50% water-dioxane was α-PMo12(II) (Fig. 11), while that in water was a mixture of α-PMo12(0), β-PMo12(II) and β-PMo12(II) (Fig. 12). This is attributed to the isomerization of first-formed α-PMo12(II) to β-PMo12(II) which disproportionates quickly to α-PMo12(II) and β-PMo12(0), the latter of which is then reduced to β-PMo12(II) or isomerizes to α-PMo12(0) immediately. β-PMo12(IV) was the only stable species in four-electron reduced products of α-PMo12(0) both in water and in 50% water-dioxane (Fig. 1) since α-PMo12(IV) gradually isomerized to β-PMo12(IV) (Figs. 3, 4, 5). The following two routes are proposed for the reduction of α-PMol2(0) to β-PMo12(IV) in solutions. α-PMo12(0)→ α-PMo12(II)→ α-PMo12(IV) → β-PMo12(IV) (1) Strong reducing agents such as ascorbic acid in a sufficient amount take the former route (Eq. 1). However, even milder reducing agents incapable of reducing α-PMo12(II) can produce β-PMo12(IV) in water through the latter route (Eq. 7). © 1986, The Chemical Society of Japan. All rights reserved.