Simple polyoxometalate anions are known to be photoreduced in molecular solvents in the presence of 2-propanol or benzyl alcohol. The use of ionic liquids (ILs) as the solvent is now reported to also allow the photooxidation of water to be achieved. In particular, the photochemistry of the classic Dawson polyoxometalate salt K(6)[P(2)W(18)O(62)] has been studied in detail when water is present in the aprotic IL, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF(4)]) and the protic IL, diethanolamine hydrogen sulfate (DEAS). In these and other ILs, irradiation with white light (wavelength 275-750 nm) or UV light (wavelength 275-320 nm) leads to overall reduction of the [P(2)W(18)O(62)](6-) anion to [P(2)W(18)O(62)](7-) and concomitant oxidation of water to dioxygen and protons. The modified structure of bulk water present in ILs appears to facilitate its oxidation. Analogous results were obtained in aqueous solutions containing the protic IL as an electrolyte. The photoproducts (reduced polyoxometalate anion, dioxygen, and protons) were identified by, respectively, voltammetry, a Clark electrode, and monitoring of pH. The formal reversible potentials E(0)(F) for [P(2)W(18)O(62)](6-/7-/8-/9-/10-) couples are much more positive than in molecular solvents. The [P(2)W(18)O(62)](8-) and more reduced anions, if formed as intermediates, would efficiently reduce photoproducts H(+) or dioxygen to produce [P(2)W(18)O(62)](7-), rather than reform to [P(2)W(18)O(62)](6-). Thus, under photoirradiation conditions [P(2)W(18)O(62)](7-) acts as a kinetic sink so that in principle indirect splitting of water to produce dioxygen and dihydrogen can be achieved. The equivalent form of photooxidation does not occur in liquid water or in molecular solvents such as MeCN and MeCN/CH(2)Cl(2) containing added water, but does occur for solid K(6)[P(2)W(18)O(62)] in contact with water vapor.

DEAP ¼ diethanolamine hydrogen phosphate). Photochemical formation of reduced POMs at both thermodynamically stable and unstable water-IL interfaces led to their initial diffusion into the aqueous phase and subsequent extraction into the IL phase. The mass transport was monitored visually by color change and by steady-state voltammetry at microelectrodes placed near the interface and in the bulk solution phases. However, no diffusion into the organic phase was observed when ½P 2 W 18 O 62 6− was photo-reduced at the water-diethylether interface. In all cases, water acted as the electron donor to give the overall process:However, more highly reduced POM species are likely to be generated as intermediates. The rate of diffusion of photo-generated POM − was dependent on the initial concentration of oxidized POM and the viscosity of the IL (or mixed phase system produced in cases in which the interface is thermodynamically unstable). In the water-DEAS system, the evolution of dioxygen was monitored in situ in the aqueous phase by using a Clark-type oxygen sensor. Differences in the structures of bulk and interfacial water are implicated in the activation of water. An analogous series of reactions occurred upon irradiation of solid POM salts in the presence of water vapor.electrochemistry | water oxidation I t has been known for some time that photochemical reduction of polyoxometalate anions (POMs) in molecular solvents may occur in the presence of an efficient electron donor such as 2-propanol or benzyl alcohol (1-6). In recent studies, we have found that photoreduction of tetracyanoquinodimethane (TCNQ) to TCNQ − and ½P 2 W 18 O 62 6− to ½P 2 W 18 O 62 7− or more extensively reduced POMs occurs in "wet" ionic liquids (ILs) where water acts as an electron donor and is photo-oxidized to dioxygen (7,8). Intriguingly, these processes do not occur in wet molecular organic solvents or in neat water itself. The net reactions that describe the photo-irradiation of ½P 2 W 18 O 62 6− in wet 1-butyl-3-methylimidazolium tetrafluoroborate [Bmim][BF 4 ] (Fig. S1) are summarized in Eqs. 1 and 2:However, while ½P 2 W 18 O 62 7− is the detected product it is probable that more extensively reduced POMs are generated (8) as intermediates in the overall water oxidation reaction [Eq. 3]:[3]

The photochemistry of the Dawson-type [Bu(4)N](4)[S(2)Mo(18)O(62)] and [Bu(4)N](4)[S(2)W(18)O(62)] polyoxometalates in molecular solvents and [Bmim][BF(4)] and [Bmim][BF(6)] ionic liquids with water present as the electron donor is reported. Irradiation with UV (275-320 nm) or white (275-750 nm) light leads to reduction of the [S(2)Mo(18)O(62)](4-) anion and concomitant oxidation of water to give dioxygen and protons in all media examined. Oxygen gas also is rapidly evolved when solid [Bu(4)N](4)[S(2)Mo(18)O(62)] in contact with water is irradiated with light. In contrast, photoreduction of [S(2)W(18)O(62)](4-) is observed only in "wet" ionic liquids. Reactants and products associated with the photochemical reactions were monitored by a range of electrochemical methods. Substantial shifts in reversible potentials combined with modified structure of water introduced by water-IL interactions are hypothesised to facilitate photooxidation of water in ionic liquid environments. Intriguingly, whilst the polyoxotungstate is preferable as a photosensitizer, the molybdenum analogue is superior for photooxidation of water to dioxygen.

Ionic Liquid-Enhanced Photooxidation of Water Using the Polyoxometalate Anion [P2W18O62] 6as the Sensitizer. -The photochemistry of K6[P2W18O62] is studied in the presence of water and the aprotic ionic liquid (bmim)[BF4] and the protic ionic liquid diethanolamine hydrogen sulfate. In these and other ionic liquids, irradiation with white light (275-750 nm) or UV light (275-320 nm) leads to overall reduction of the [P 2 W 18 O 62 ] 6anion to [P 2 W 18 O 62 ] 7and concomitant oxidation of water to O 2 and protons. -(BERNARDINI, G.; ZHAO, C.; WEDD, A. G.; BOND*, A. M.; Inorg. Chem. 50 (2011) 13, 5899-5909, http://dx.

No abstract