Chemistry

Protons Coming and Going

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Science  05 Oct 2012:
Vol. 338, Issue 6103, pp. 19
DOI: 10.1126/science.338.6103.19-b

Photosynthesis channels light into chemical energy by splitting water. Klauss et al. explored the sequence of steps whereby the manganese cluster in plants and algae extracts four protons and four electrons from two water molecules, forming O2 in the process. By applying a photothermal beam deflection technique to probe the relative timing of electron and proton transfers, the authors uncovered two deprotonation steps, which taken together with prior studies establish that the water-splitting cycle proceeds through strictly alternating extraction of electrons and protons.

Many synthetic systems focus on forming hydrogen from water-derived protons, rather than replicating the natural CO2 functionalization cycle. Marinescu et al. probed the mechanism for one such catalyst that relies on redox-active cobalt. Using nuclear magnetic resonance spectroscopy, they tracked the rate of disappearance of a Co(III)-H intermediate, uncovering a bimolecular decay inhibited by increasing proton (acid) concentration. Analysis of these results together with cyclic voltammetry data and numerical simulations showed that the dominant mechanism involved a one-electron reduction of the intermediate by unprotonated Co(I) before H2 formation, with a slower concurrent direct reaction involving two Co(III)-H complexes.

Proc. Natl. Acad. Sci. U.S.A. 109, 10.1073/pnas.1206266109; 10.1073/pnas.1213442109 (2012).

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