Reactions of heteropolyanions in non-polar solvents. Part 3. Activation of dioxygen by manganese(II) centres in polytungstates. Oxidation of hindered phenols

Abstract

The heteropolyanions [XW₁₁O₃₉{Mn^II(OH₂)}]^n–[X = P(1), Si (2), Ge (3), or B (4)] and α₂-[P₂W₁₇O₆₁{Mn^II(OH₂)}]^8–(5) have been transferred into non-polar solvents (benzene, toluene) with the aid of tetraheptylammonium bromide or other phase-transfer agents. Following dehydration of their non-polar solutions, anions (2) and (3) show reactivity towards dioxygen. At temperatures below –35 [(2)] and ca. 22 °C [(3)] oxygenation produces a reversible colour change (λ_max 475, 585 nm); above these temperatures oxygenation results in irreversible oxidation to the manganese(III) heteropolyanions (λ_max 520 nm). The coloured oxygenated product is e.s.r.-inactive and is not formed in polar solvents nor in the presence of other polar solutes such as alcohols, pyridine, etc. Traces of water are, however, necessary for the formation of the colour, which intensifies as the temperature is lowered, and it is proposed that a weak dioxygen adduct is stabilised by a hydrogen-bonded water molecule. The rapid oxidation of anhydrous (2) by dioxygen was carried out in the presence of the spin trap 5,5-dimethyl-1-pyrroline N-oxide (dmpo). E.s.r. spectra indicate the transient formation of a polyanion–O₂–dmpo complex that decomposes to oxidised polyanion and dmpo–O₂H*. Solutions of anions (2), (3), and (5) catalyse the oxidation of 2,6- and 2,4,6-substituted phenols to benzoquinones or polyphenyl ethers, and e.s.r. evidence of polyanion–O₂–phenoxy radical complex formation is adduced. Comparisons are drawn with the chemistry of dioxygen complexes of cobalt(II). Anion (1) is neither oxygenated nor oxidised by O₂, and (4) is rapidly oxidised at –70 °C. Both behaviours are understandable in terms of the manganese(III)–manganese(II) redox potentials.