Trapping an Iron(VI) Water Splitting Intermediate in Nonaqueous Media


Bryan M. Hunter,1 Niklas B. Thompson,1 Astrid M. Müller,1 ­­George R. Rossman,2 Michael G. Hill,3 Jay R. Winkler,1 and Harry B. Gray1

 1Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA

2Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

3Department of Chemistry, Occidental College, Los Angeles, CA 90041, USA


      The sustainable and economical conversion of solar energy into storable, transportable fuels by solar-driven water splitting is a grand challenge of 21st century chemistry. The mechanisms by which heterogeneous materials perform the anodic half-reaction, water oxidation, are not well understood. Here, we describe in-situ spectroscopic measurements in nonaqueous media designed to trap an exceptionally strong oxidant generated electrochemically from an iron-containing nickel layered double hydroxide ([NiFe]-LDH) material. Anodic polarization of this material in acetonitrile produces prominent infrared absorption features (840 and 856 cm-1) that are quenched by the addition of neutral or alkaline acetonitrile. These vibrational spectroscopic signatures along with an extremely narrow luminescence peak at 1633 nm indicate that the reactive intermediate is a cis­-dioxo-iron(VI) species. An absorption in the Mössbauer spectrum of the material, which disappears upon exposure to alkaline acetonitrile, is consistent with population of a high-valent iron-oxo species. Importantly, chemical trapping experiments reveal that addition of H2O to the polarized electrochemical cell produces hydrogen peroxide; and addition of HO generates oxygen. Re-polarization of the electrode restores the iron(VI) spectroscopic signatures, confirming that the high-valent oxo complex is active in the electrocatalytic water oxidation cycle. Tafel slopes of [NiFe]-LDH in 1%-100% 1 M aqueous KOH (59.1 ± 0.7 mV/decade) confirm that the conditions employed are mechanistically relevant to bulk water oxidation (S1). Our work demonstrates that in-situ spectroscopy in nonaqueous media offers a powerful new approach to the study of aqueous redox mechanisms.