A Smorgasbord of 17 Cobalt Complexes Active for Photocatalytic Hydrogen Evolution

Abstract

Seventeen cobalt complexes-eleven dinuclear cobalt(II) complexes and three tetranuclear cobalt complexes (two mixed valent) of ditopic ligands, with varying N-donor aromatic bridging moieties and pendant pyridine side arms, as well as three mononuclear cobalt(II) complexes of Schiff base macrocyclic ligands-have been screened for photocatalytic hydrogen evolution reaction (HER) activity. All 17 complexes are active catalysts for the HER, in both DMF and aqueous solution, in tandem with the [Ru(bpy) ] (bpy=2,2'-bipyridine) photosensitiser. All are benchmarked to the literature standard [Co (dmgH) (py)Cl] (dmg=dimethylglyoxime, py=pyridine) under identical conditions. Two families of dinuclear cobalt(II) complexes of bis-tetradentate ligands that provide a triazole bridging moiety and mononuclear cobalt(II) complexes of tetradentate Schiff base macrocycles were found to be the most active catalysts, outperforming [Co (dmgH) (py)Cl] by two- to three-fold. Within these two families, the use of shorter alkyl linkers between the N donors, and hence, smaller chelate ring sizes, was found to significantly enhance catalytic performance, whereas the variation of peripheral functional groups was found to have little effect. This last point will be convenient for subsequent surface immobilisation studies.

Multielectron Redox Chemistry of Transition Metal Complexes Supported by a Non‐Innocent N3P2 Ligand: Synthesis, Characterization, and Catalytic Properties

“…Despite this difference, the L N3P2 ‐based complexes are promising candidates for HER catalysts for several reasons. Firstly, the coordination environment provided by L N3P2 is consistent with the design principles recently formulated by Brooker and co‐workers for cobalt HER catalysts with Schiff‐based ligands . This study noted the improved performance of five‐coordinate complexes (relative to 4C analogs) and the advantage of five‐membered chelate rings, which is fully consistent with our previous findings.…”

“…Starting with the well‐studied cobaloxime‐based systems, the large majority of cobalt catalysts for the hydrogen evolution reaction (HER) have employed ligands with only N ‐donors . Examples include catalysts with tetra‐ or pentadentate polypyridyl ligands and those featuring tetradentate Schiff‐base macrocycles , . While the development of HER catalysts with CoN 4 and CoN 5 structures has proven fruitful, there are potential advantages in using ligand scaffolds that also incorporate non‐nitrogenous donors.…”

Computational Approaches to Photoelectrode Design through Molecular Functionalization for Enhanced Photoelectrochemical Water Splitting

“…However, many questions remain about the various factors that affect the efficiency of the photoelectrode/molecular catalyst interface such as band‐edge alignment between the photoelectrode and molecular catalyst and between the molecular catalyst and the electrolyte, stable immobilization of the molecular catalyst to the photoelectrode surface, the absence of deleterious surface states, and facile interfacial charge transfer . Therefore, the systematic experimental search for photoelectrode/molecular catalyst systems for water splitting is still a relatively nascent field, even though there are a few comparative studies of experimental catalytic activities and mechanisms of commonly used molecular catalysts . This provides ample opportunity for the development of computational approaches to design photoelectrode/molecular catalyst systems.…”

“…However, strategies to obtain widely applicable design rules must be developed to avoid detailed first‐principles simulations for every conceivable system. This is especially important because of the large variety of molecular catalysts, ligands, and anchor groups for immobilization that are investigated in experiments …”

“…The significant computational cost of first‐principles MD, hybrid functional DFT, and MBPT calculations should prompt efforts to formulate widely applicable design rules or heuristics. This may be accelerated by the availability of experimental measurements on a wide variety of photoelectrode/molecular catalyst systems, allowing easy benchmarking and validation of computational methods.…”