Artificial photosynthesis carries promise to deliver abundant clean energy for the needs of a growing population. Deep mechanistic understanding is required to achieve rational design of fast and durable water oxidation catalysts. Here we provided first evidence for a new mechanism of the OO bond formation via radical coupling of the oxidized metaloxo of radicaloid character (Ru IV O) and ligand based radical ([ligand-NO] +• cation radical). OO bond formation is facilitated via spin alignment and takes place via a virtually barrier less pathway inside the single metal complex. In situ reactive intermediate conversion was monitored by mass spectrometry, resonance Raman (RR) and EPR. Computational analysis have shown that the formation of [ligand-NO] +• happens at a lower overpotential than the formation of the [Ru V O(ligand)] 3+ intermediate. Overall, the presented paradigm for OO bond formation opens new opportunities for rational catalyst design. Communication pubs.acs.org/JACS

The biological generation of oxygen by the oxygen-evolving complex (OEC) in photosystem II (PS II) is one of nature's most important reactions. The OEC is a Mn 4 Ca cluster that has multiple Mn−O−Mn and Mn−O−Ca bridges and binds four water molecules. Previously, binding of an additional oxygen was detected in the S 2 to S 3 transition. Here we demonstrate that early binding of the substrate oxygen to the five-coordinate Mn 1 center in the S 2 state is likely responsible for the S 2 high-spin EPR signal. Substrate binding in the Mn 1 −OH form explains the prevalence of the high-spin S 2 state at higher pH and its low-temperature conversion into the S 3 state. The given interpretation was confirmed by X-ray absorption spectroscopic measurements, DFT, and broken symmetry DFT calculations of structures and magnetic properties. Structural, electronic, and spectroscopic properties of the high-spin S 2 state model are provided and compared with the available S 3 state models. New interpretation of the high-spin S 2 state opens opportunity for analysis of factors controlling the oxygen substrate binding in PS II.

The realization of artificial photosynthesis requires the design of fast and durable water oxidation catalysts that can be incorporated into future sunlight-to-chemical-fuels assemblies. Here we report on the simple and readily synthetically accessible cis-[Ru(bpy)(5,5′-dcbpy)(H 2 O) 2 ] 2+ (1) (bpy = 2,2′-bipyridine, 5,5′dcbpy = 2,2′-bipyridine-5,5′-dicarboxylic acid) water oxidation catalyst. Its O 2 evolution has first-order dependence on the catalyst following the water nucleophilic attack mechanism. The cis-[Ru V O,−OH(bpy)(5,5′-dcbpy)] 2+ intermediate was detected in the reaction mixtures by X-ray absorption and resonance Raman spectroscopy in agreement with the proposed mechanism and DFT calculations. To avoid a deactivating dimerization, the catalyst was postsynthetically incorporated into the UIO-67 metal−organic framework with retention of water oxidation activity. Similar results were obtained via the incorporation of cis-[Ru(bpy)(5,5′-dcbpy)]Cl 2 , a chemical precursor of 1, to UIO-67 followed by hydrolysis. Postsynthetic doping of the UIO-67 thin film on a FTO glass electrode with 1 leads to an increase of the catalytic current by two orders of magnitude. Thus, [Ru(bpy)(5,5′-dcbpy)(H 2 O) 2 ] 2+ −UIO-67 is a promising building block for integrated photosynthetic systems.

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Artificial photosynthesis could promise abundant, carbon-neutral energy, but implementation is currently limited by a lack of control over the multi-electron catalysis of water oxidation. Discoveries of the most active catalysts still rely heavily on serendipity. [Ru­(tpy)­(bpy)­(H2O)]2+ (1; bpy = 2,2′-bipyridine, tpy = 2,2′;6′,2″-terpyridine) is representative of the largest known class of water oxidation catalysts. We undertook an extensive spectroscopic analysis of the prototypical 1 water oxidation catalyst and its fastest known analog [Ru­(EtO-tpy)­(bpy)­(H2O)]2+ (2), capable of 10 times faster water oxidation, to investigate the mechanism of action and factors controlling catalytic activity. EPR and resonance Raman spectroscopy did not detect the proposed [RuVO] intermediate in 1 and 2 but indicated the possible formation of N-oxides. A lag phase was observed prior to O2 evolution, suggesting catalyst modification before the onset of catalysis. The reactive intermediate [Ru­(tpy)­(bpy-NO)­(H2O)]2+ (1-NO; bpy-NO = 2,2′-bipyridine-N-oxide) proposed by combined spectroscopic and DFT analysis was de novo synthesized and demonstrated 100-fold greater catalytic activity than 1. Thus, in situ transient formation of small amounts of the Ru complex with N-oxide ligands can significantly activate single-site Ru-based catalysts. Furthermore, the rate of O2 evolution was found to correlate with the redox potential of the ligand. This observation might assist with rational design of new catalysts.