Rates of water exchange for two cobalt(II) heteropolyoxotungstate compounds in aqueous solution

摘要

Polyoxometalate ions are used as ligands in water-oxidation processes related to solar energy production. An important step in these reactions is the association and dissociation of water from the catalytic sites, the rates of which are unknown. Here we report the exchange rates of water ligated to Co ~(II) atoms in two polyoxotungstate sandwich molecules using the ~(17) O-NMR-based Swift-Connick method. The compounds were the [Co _4(H_2O)_2(B-α-PW_9O _(34))_2]~(10-) and the larger αββ α-[Co_4(H_2O)_2(P_2W _(15)O_(56))_2]~(16-) ions, each with two water molecules bound trans to one another in a Co~(II) sandwich between the tungstate ligands. The clusters, in both solid and solution state, were characterized by a range of methods, including NMR, EPR, FT-IR, UV-Vis, and EXAFS spectroscopy, ESI-MS, single-crystal X-ray crystallography, and potentiometry. For [Co_4(H_2O)_2(B-α-PW _9O_(34))_2]~(10-) at pH 5.4, we estimate: k~(298)=1.5(5)±0.3à- 10~6 s~(-1), ΔH~≠=39.8±0.4 kJmol~(-1), ΔS ~≠=+7.1±1.2 Jmol~(-1)K~(-1) and ΔV~≠=5.6 ±1.6 cm~3mol~(-1). For the Wells-Dawson sandwich cluster (αββα-[Co4_(H _2O)_2(P_2W_(15)O_(56)) _2]~(16-)) at pH 5.54, we find: k~(298)=1.6(2) ±0.3à- 10~6 s~(-1), ΔH~≠=27. 6±0.4 kJmol~(-1) ΔS~≠=-33±1.3 Jmol ~(-1)K~(-1) and ΔV~≠=2.2±1.4 cm ~3mol~(-1) at pH 5.2. The molecules are clearly stable and monospecific in slightly acidic solutions, but dissociate in strongly acidic solutions. This dissociation is detectable by EPR spectroscopy as S=3/2 Co ~(II) species (such as the [Co(H_2O)_6]~(2+) monomer ion) and by the significant reduction of the Co-Co vector in the XAS spectra.