A pyridylideneamide ligand with variable donor properties owing to a pronounced zwitterionic and a neutral diene-type resonance structure was used as a dynamic ligand at a Cp* iridium center to facilitate water oxidation catalysis, a reaction that requires the stabilization of a variety of different iridium oxidation states and that is key for developing an efficient solar fuel device. The ligand imparts high activity (nearly three-fold increase of turnover frequency compared to benchmark systems), and exceptionally high turnover numbers, which indicate a robust catalytic cycle and little catalyst degradation.

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.

We report the photocatalytic dehydrogenation of alcohols with a Rh catalyst and I⁻ in an acidic medium. The catalyst screening of a structurally diverse family of Rh complexes found [Rh^III(dtbbpy)₂(OSO₂CF₃)₂](CF₃SO₃) (Rh7; dtbbpy=4,4′-di-tert-butyl-2,2′-bipyridine) to be the best catalyst. All components (Rh7, HI, alcohol, and light) were found to be essential for the dehydrogenation to occur. Under optimal conditions, generation of both H₂ and acetone (for isopropyl alcohol) was observed, which increased linearly for 4 days with no significant decrease in activity. The proposed mechanism involves the formation of [Rh^III(dtbbpy)₂(I)₂]⁺ [Rh7(a)] followed by the generation of photoinduced I₃⁻ via a ligand (I⁻) to metal (Rh^III) charge transfer. The resulting Rh^I species reduces H⁺ and I₃⁻ oxidizes the alcohol, which completes the catalytic cycle. Supporting evidence for the catalytic mechanism was obtained by using UV/Vis spectroscopy, electrospray ionization mass spectrometry, cyclic voltammetry and time-dependent DFT calculations.

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 IrC 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.