Justine P. Roth, Johns Hopkins University
Illuminating Mechanisms of Water Oxidation
This ACS-PRF New Directions (50046-ND3) award has allowed research in a new area devoted to illuminating chemical principles of photosynthetic and artificial photosynthetic water oxidation. Present studies focus on homogenous catalysts containing one or two redox-active transition metals. The objective is to understand mechanisms that use water and sunlight to produce “solar hydrogen”, a clean-burning and sustainable fuel. With this knowledge, new and improved homogenous and possibly heterogeneous catalysts may be deliverable. Solar hydrogen could effectively replace fossil fuels with minimal development of new technologies for storage and transport. Solar hydrogen is beneficial because it does not release carbon dioxide into the atmosphere and, thereby, contribute global warming.
During the funding period supported by the ND grant, combined experimental and computational methods were developed for using natural abundance oxygen isotope effects to probe structures and mechanisms involved in water oxidation. Specifically, the calculation of competitive oxygen-18 kinetic isotope effects (O-18 KIEs) from vibrations derived using density functional theory (DFT) calculations has implicated O–O bond formation in the turnover-limiting step of catalysis. This work has resulted in unique training experiences for students at all levels. Three peer-reviewed papers which acknowledge the ND grant have been published and two others are in preparation. Two book chapters have been published, illustrating the diversity of problems that can be addressed using the above-mentioned approach. A complete publication list is available at http://www.jhu.edu/chem/roth/publications.html.
(i) “Oxygen Isotope Effects as Structural and Mechanistic Probes in Inorganic Oxidation Chemistry.” D. C. Ashley, D. W. Brinkley, J. P. Roth* Inorg. Chem., 2010, 49, 3661-3675
(ii).“Oxygen Kinetic Isotope Effects upon Catalytic Water Oxidation by a Monomeric Ruthenium Complex.” A.M. Angeles-Boza, J. P. Roth* Inorg. Chem. 2012, 51, 4722−4729.
(iii). “Studies of the Di-Iron(VI) Intermediate in Ferrate-Dependent Oxygen Evolution from Water.” R. Sarma, A.M. Angeles-Boza, D.W. Brinkley, J.P. Roth* J. Am. Chem. Soc. 2012, 134, 15371-86.
The method for measuring O-18 KIEs on water oxidation by structurally defined inorganic complexes is in the published articles. In the first citation, the rationale for using natural abundance O-18 KIEs to study O–O bond-making and bond-breaking under catalytic conditions is described. Evidence is presented that the transition state for O–O heterolysis in a heme peroxidase is related to the transition state for O–O bond formation by the principle of microscopic reversibility. Calculations of oxygen-18 equilibrium isotope effects (EIEs) and O-18 KIEs from vibrational frequencies are described. In the second article, chemical and photochemical methods for measuring O-18 KIEs on water oxidation are outlined. The activity of a monomeric ruthenium polypyridyl complex as a water oxidation catalyst is described. The turnover-limiting step is shown to vary in response to changing the thermodynamics associated with electron transfer or proton-coupled electron transfer to the sacrificial oxidant. The proposed kinetic mechanism has since been corroborated computationally in a forthcoming collaborative manuscript entitled “Combined Experimental and Theoretical Investigations of Competitive O-18 KIEs on Water Oxidation by Homogenous Ruthenium Catalysts,” by Roth et al. Such comparisons of monomeric to dimeric catalysts is critical to understanding multi-site reactivity. In the third article, the kinetics and mechanism of water oxidation by an iron(VI) oxo (or ferryl) is scrutinized. Rapid-mixing stopped-flow kinetics afforded pH and pD profiles of observed rate constants. The reaction was second order with respect to K2FeO4 and the solvent kinetic isotope effects found to approach unity. These results, implicating water oxidation via a di-iron(VI) species, were corroborated by X-band EPR.
Computational analyses of O-18 KIEs, using a previously calibrated methodology for reactions of O2 and H2O2, shed light on the experimentation. Transition states for monomeric and dimeric forms of ferrate in various spin states were considered. The findings revealed that oxo-coupling transition states are associated with large O-18 KIEs (≥ 1.03); whereas water attack transition states are associated with smaller more variable O-18 KIEs (from 1.00 to < 1.02). The aggregate results are consistent with water oxidation within a μ-oxo-bridged di-iron(VI) intermediate. The structure resembles that found in the stable dichromate anion (Cr2O72-) as well as pyrophosphate (P2O72-) and pyrosulfate (S2O72-). Studies of the ferrate model system are un-obscured by oxidative steps under catalytic conditions allowing O–O bond-formation to be examined in isolation. Importantly, the measured O-18 KIEs, second order dependence on ferrate and absence of a significant solvent kinetic isotope effect are in good agreement with an oxo-coupling transition state. Such mechanisms are predicted to have the lowest energy barriers, in contrast to reactions of mono-ferrate and reactions involving water attack upon di-ferrate. The findings lay a foundation for using O-18 KIEs to evaluate water oxidation mechanisms. In a forthcoming manuscript, O-18 KIEs are measured and calculated for catalytic reactions.
Efforts are underway to develop catalysts using the relatively abundant and environmentally benign first row transition metals. We are particularly interested in homogenous and possibly heterogeneous reactions which use light to produce solar hydrogen and/or hydrogen atom equivalents from water. In summary, the ACS-PRF 50046-ND3 grant has allowed the development of a new methodology for studying water oxidation mechanisms. The approach holds promise for future studies of the naturally occurring photosystem II as well as for stable heterogeneous catalysts with a band gap in the visible region. Collaborations are in place to refine DFT approaches, which presently afford O-18 KIEs with precisions of ± 0.005 using the Gaussian09 suite of programs. The error in calculated isotope effects is less than or comparable to the experimental error; however, errors in the computed free energy barriers are significantly larger. Collaborations with experts are in place to improve this aspect of the DFT analysis so that calculated O-18 KIEs have improved predictive capabilities. Future experimentation will target multi-site catalysts, where new types transition states may facilitate water oxidation by lowering barriers to O–O bond formation. Integration of theory and experiment is needed to make the rational design of catalysts with low activation barriers/ “over-potentials” possible.