No abstract

Iridium complexes of Cp* and mesoionic carbene ligands were synthesized and evaluated as potential water oxidation catalysts using cerium ammonium nitrate as a chemical oxidant. Performance was evaluated by turnover frequency at 50% conversion and by absolute turnover number, and the most promising precatalysts were subjected to further study. Molecular turnover frequencies varied from 190 to 451 per hour with a maximum turnover number of 38,000. While the rate of oxygen evolution depends linearly on iridium concentration, 10 concurrent spectroscopic and manometric monitoring of stoichiometric additions of oxidant suggests oxygen evolution occurs as two sequential first order reactions. Under the conditions herein, the oxygen evolving species appears to be well defined and molecular based on the kinetic effects of careful ligand design, reproducibility, and the absence of persistent dynamic light scattering signals. Outside of these conditions, the complex mechanism is highly dependent on reaction conditions. While confident characterization of the catalytically active species is difficult, especially under high-turnover conditions, this work indicates IrOx is not essential for the 15 formation of catalytically active water oxidation species.

Metalation of a C2‐methylated pyridylimidazolium salt with [IrCp*Cl2]2 affords either an ylidic complex, resulting from C(sp3)H bond activation of the C2‐bound CH3 group if the metalation is performed in the presence of a base, such as AgO2 or Na2CO3, or a mesoionic complex via cyclometalation and thermally induced heterocyclic C(sp2)H bond activation, if the reaction is performed in the absence of a base. Similar cyclometalation and complex formation via C(sp2)H bond activation is observed when the heterocyclic ligand precursor consists of the analogous pyridyltriazolium salt, that is, when the metal bonding at the C2 position is blocked by a nitrogen rather than a methyl substituent. Despite the strongly mesoionic character of both the imidazolylidene and the triazolylidene, the former reacts rapidly with D+ and undergoes isotope exchange at the heterocyclic C5 position, whereas the triazolylidene ligand is stable and only undergoes H/D exchange under basic conditions, where the imidazolylidene is essentially unreactive. The high stability of the IrC bond in aqueous solution over a broad pH range was exploited in catalytic water oxidation and silane oxidation. The catalytic hydrosilylation of ketones proceeds with turnover frequencies as high as 6 000 h−1 with both the imidazolylidene and the triazolylidene system, whereas water oxidation is enhanced by the stronger donor properties of the imidazol‐4‐ylidene ligands and is more than three times faster than with the triazolylidene analogue.

Two new diiridium–triazolylidene complexes were prepared as bimetallic analogues of established mononuclear water oxidation catalysts. Both complexes are efficient catalyst precursors in the presence of cerium ammonium nitrate (CAN) as sacrificial oxidant. Up to 20000:1 ratios of CAN/complex, the turnover limitation is the availability of CAN and not the catalyst stability. The water oxidation activity of the bimetallic complexes is not better than the monometallic species at 0.6 mM catalyst concentration. Under dilute conditions (0.03 mM), the bimetallic complexes double their activity, whereas the monometallic complexes show an opposite trend and display markedly reduced rates, thereby suggesting a benefit of the close proximity of two metal centers in this low concentration regime. The high dependence of catalyst activity on reaction conditions indicates that caution is required when catalysts are compared by their turnover frequencies only.