We recently discovered a fundamentally new reaction with broad implications for the redox chemistry of O2 in water.1 Previous work showed that at pH values from 7 to 2, one-electron reduced Keggin heteropolytungstates cluster-anions reduce O2 to O2•– via outer-sphere electron transfer (ET).2 Using the one-electron-reduced anion, α-PW12O404-, (11e), we recently documented the emergence at lower pH values of a concerted (simultaneous) multi-site proton-electron transfer (CPET) reaction that converts O2 directly to HO2• in a single elementary step. CPET3-6 reactions have broad implications to chemistry and biology. CPET to O2, especially in water, could assist in designing more efficient fuel cells and alternative energy processes.
In the new CPET reaction,1 the transferred proton comes from H+ in bulk water, and the reaction is first order in all three components, 11e, O2 and H+. This raises mechanistic questions never before encountered in CPET reactions. In addressing these, we show that the emergence of CPET at large [H+] values is facilitated by the unique nature of the excess proton in water. Moreover, two iso-structural one-electron-reduced heteropolytungstates, α-SiW12O405-, (21e) and α-AlW12O406-, (31e), whose ET reactions with O2 are more exergonic, were also examined. From this, the relationship between Gibbs free energies of ET versus CPET were determined, and found consistent with expectations from theoretically derived models.
Ongoing research on the generality of the CPET reaction will also be presented. Namely, CPET to O2 in water is also observed when hydronium ion is replaced by a series of protic organic acids. Work on additional electron donors (not POMs, but similar to 11e in critical respects) is currently in progress and will also be discussed.