Recent findings and future directions in photosynthetic hydrogen evolution using polypyridine cobalt complexes

Abstract

The production of hydrogen gas using water as the molecular substrate currently represents one of the most challenging and appealing reaction schemes in the field of artificial photosynthesis (AP), i.e., the conversion of solar energy into fuels. In order to be efficient, this process requires a suitable combination of a light-harvesting sensitizer, an electron donor, and a hydrogen-evolving catalyst (HEC). In the last few years, cobalt polypyridine complexes have been discovered to be competent molecular catalysts for the hydrogen evolution reaction (HER), showing enhanced efficiency and stability with respect to previously reported molecular species. This perspective collects information about all relevant cobalt polypyridine complexes employed for the HER in aqueous solution under light-driven conditions in the presence of Ru(bpy)₃²⁺ (where bpy = 2,2'-bipyridine) as the photosensitizer and ascorbate as the electron donor, trying to highlight promising chemical motifs and aiming towards efficient catalytic activity in order to stimulate further efforts to design molecular catalysts for hydrogen generation and allow their profitable implementation in devices. As a final step, a few suggestions for the benchmarking of HECs employed under light-driven conditions are introduced.

Scheme 1. Possible mechanisms for photochemical hydrogen evolution involving cobalt polypyridine complexes (the potential contribution of the polypyridine ligand to protonated intermediates has been neglected). Oxidative (red box) and reductive (blue box) quenching routes are shown. Abbreviations: PS = photosensitizer, D = electron donor.

Fig. 1. The molecular structures of the Ru(bpy)₃²⁺ photosensitizer and the ascorbate sacrificial electron donor used in combination with cobalt polypyridine complexes for light-driven hydrogen evolution studies.

Fig. 2. An example of the kinetic trace obtained from a typical photochemical experiment with the identification of two experimental quantities, r and n_max, which can be used (according to eqn (2)–(4)) to determine the relevant figures-of-merit required for a meaningful comparison of the photosynthetic activities of different cobalt polypyridine complexes in combination with Ru(bpy)₃²⁺ as the sensitizer and ascorbate as the electron donor.

Fig. 3. The molecular structures of tetradentate polypyridine complexes used in photochemical hydrogen evolution experiments with Ru(bpy)₃²⁺ and ascorbate.

Fig. 4. The molecular structures of pentadentate polypyridine complexes used in photochemical hydrogen evolution experiments with Ru(bpy)₃²⁺ and ascorbate.

Fig. 5. The molecular structures of cobalt catalysts with amino–polypyridyl ligands used in photochemical hydrogen evolution experiments with Ru(bpy)₃²⁺ and ascorbate.

Fig. 6. The molecular structures of cobalt catalysts with hexadentate ligands used in photochemical hydrogen evolution experiments with Ru(bpy)₃²⁺ and ascorbate.