Redistribution and fluxionality in heteropolyoxoperoxo complexes: [PO4{M2O2(μ-O2)2(O2)2}2]3− with M = Mo and/or W

Abstract

When peroxotetramolybdophosphate, [(n-C₄H₉)₄N]₃[PO₄{Mo₂O₂(μ-O₂)₂(O₂)₂}₂], denoted (NBu₄)₃PMo₄, and its tungsten(VI) analogue, (NBu₄)₃PW₄, are mixed in acetonitrile at room temperature, redistribution occurs with the formation of three mixed-addenda species [PO₄{Mo_4−xW_xO₂₀}]³⁻ (x = 1–3). The temperature dependence of the phosphorus-31 NMR spectra of a 1 ∶ 1 mixture and of the pure salts, (NBu₄)₃PMo₄ or (NBu₄)₃PW₄, shows that {MO(O₂)₂} species are in chemical exchange, as are the {MO_p} units of certain heteropolyacids (e.g. H₃[PMo₁₂O₄₀]·aq and H₃[PW₁₂O₄₀]·aq). However, there is no chemical exchange between free phosphate and {MO(O₂)₂} species in these systems; but there is fluxional behaviour involving PMo₂W₂, PMo₄ and PW₄. This is attributed to the rapid equilibrium between isomers (PMo₂W₂) and to equilibrium between anionic structures with tridentate (μ-η²:η¹-O₂²⁻) and bidentate (η²-O₂²⁻) modes of coordination for the two peroxo groups of the {M₂O₂(μ-O₂)₂(O₂)₂} moieties.