Molecular design represents an exciting platform to refine mechanistic details of electrocatalytic water oxidation and explore new perspectives. In the growing number of publications some general trends seem to be outlined concerning the operation mechanisms, with the help of experimental and theoretical approaches that have been broadly applied in the case of bioinorganic systems. In this review we focus on bio-inspired Cu-containing complexes that are classified according to the proposed mechanistic pathways and the related experimental evidence, strongly linked to the applied ligand architecture. In addition, we devote special attention to features of molecular compounds, which have been exploited in the efficient fabrication of catalytically active thin films.

Given the rising socioeconomic issues of fossil fuels, efficient artificial photosynthesis would be an important milestone toward a sustainable world. A key step of photosynthesis is the catalytic photooxidation of water by photosystem II, which has a mean lifetime of 30 min under full sunlight. Since the efficiency of photosystem II is controlled by redox-active tyrosine–histidine pairs that regulate the light-induced flow of charges, research has recently focused on the utilization of redox-active ligands in artificial systems. Here we review the molecular catalysis of water oxidation with emphasis on redox cooperation modes between ligands and metal centers. Molecular systems involving redox-active ligands could achieve up to 100% efficiency with respect to oxygen production, overpotential of 200–300 mV and turnover frequency above 100 s−1, which is comparable to the natural process. Nonetheless, molecular catalysts are often prone to degradation of the organic ligand. The oxidative activation of ligands can contribute to the water oxidation reactivity of a metal–ligand complex, or lead to controlled catalyst film formation. We discuss the design of functional analogs to the tyrosine–histidine pair that for the most part rely on abundant elements and exploit redox-active molecular moieties to assist the catalytic centers. We highlight analogies with the cooperation between the natural oxygen-evolving complex and the redox-active tyrosine–histidine pairs found in photosystem II.

The imidazole ring of histidine (His) and the methylcarboxylate function of aspartic acid (Asp) are crucial for crucial for catalysis, but also acts as inhibitor anion according to cyclic voltammetry observations. The system is highly tolerant to the presence of chloride, which is a feature of practical importance in efficient water oxidation catalysis.

Pincer ligands occupy three coplanar sites at metal centers and often support both stability and reactivity. The five-coordinate [FeIIICl2(tia-BAI)] complex (tia-BAI− = 1,3-bis(2’-thiazolylimino)isoindolinate(−)) was considered as a potential pre-catalyst for water oxidation providing the active form via the exchange of chloride ligands to water molecules. The tia-BAI− pincer ligand renders water-insolubility to the Fe–(tia-BAI) assembly, but it tolerates the presence of water in acetone and produces electrocatalytic current in cyclic voltammetry associated with molecular water oxidation catalysis. Upon addition of water to [FeIIICl2(tia-BAI)] in acetone the changes in the Fe3+/2+ redox transition and the UV-visible spectra could be associated with solvent-dependent equilibria between the aqua and chloride complex forms. Immobilization of the complex from methanol on indium-tin-oxide (ITO) electrode by means of drop-casting resulted in water oxidation catalysis in borate buffer. The O2 detected by gas chromatography upon electrolysis at pH 8.3 indicates >80% Faraday efficiency by a TON > 193. The investigation of the complex/ITO assembly by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) before and after electrolysis, and re-dissolution tests suggest that an immobilized molecular catalyst is responsible for catalysis and de-activation occurs by depletion of the metal.