Structure‐Functional Analysis of Hydrogen Production Catalyzed by Molecular Cobalt Complexes with Pentadentate Ligands in Aqueous Solutions

Abstract

Hydrogen production from sunlight and water represents one promising solution to resolve the environmental problems caused by the consumption of fossil fuels and to meet the increasing global energy demands. Catalysts based on transition metal complexes have been extensively studied for electro-and photocatalytic production of hydrogen. Among the reported catalysts, molecular cobalt complexes have received

Two routes to hydrogen evolution for a Co-polypyridyl complex with two open sites

“…There are also considerable uncertainties associated with these calculations because of the changes in total system charge when adding or removing Cl − ligands. Still, the reaction sequence with ligand exchange in Co II and loss of water after reduction to Co I , is in agreement with detailed studies of the early reaction steps [14,38,62]. Even if there is no Cl − bound in Co I (3) the binding affinity increases after protonation.…”

“…Understanding the reaction pathway is necessary for rational design of more stable and efficient catalysts. This is highlighted by several studies that show how small changes in the ligand affect catalytic performance, although sometimes with seemingly contradictory results [13,40,45,46,53,55,[59][60][61][62][63][64]. As an example, adding an electron withdrawing −CF 3 group in Co(bpy) 2 PyMe depresses catalytic activity [46], while it increases activity in Co-pentapyridyl complexes [13].…”

Two Novel Dinuclear Cobalt Polypyridyl Complexes in Electro‐ and Photocatalysis for Hydrogen Production: Cooperativity Increases Performance

“…In photocatalytic water splitting, cobalt polypyridyl complexes are potent catalysts for proton reduction [1–8] . While exhibiting higher stabilities compared to other molecular water reduction catalysts (WRCs), [9] they display higher overpotentials decreasing the efficiency of light‐induced proton reduction.…”

“…In photocatalytic water splitting, cobalt polypyridyl complexes are potent catalysts for proton reduction. [1][2][3][4][5][6][7][8] While exhibiting higher stabilities compared to other molecular water reduction catalysts (WRCs), [9] they display higher overpotentials decreasing the efficiency of light-induced proton reduction. Due to the low basicity of pyridyl-coordinated cobalt, several subsequent metal-and ligand-based reductions are required, preceding the first protonation, except when very strong acids are applied.…”

Structural and Electronic Influences on Rates of Tertpyridine−Amine CoIII−H Formation During Catalytic H2 Evolution in an Aqueous Environment

“…[12] In particular, Co complexes have emerged as a particularly attractive class of hydrogen evolving catalysts due to their high solubility in water, relatively high stability, relatively low overpotential, and high turnover efficiency. [4,13,[15][16][17][18] The ability to function in aqueous environments is particularly attractive, as many catalysts based on earth abundant metals are either insoluble or unstable in aqueous environments. [9,12] Although photocatalytic hydrogen evolution has been observed with simple Co tris-(2,2'-bipyridine)-type complexes with [Ru(α-diimine) 3 ] 2 + as sensitizers and tertiary amines as sacrificial electron donors, [19,20] the reaction mechanism shows [Co I (bpy) 2 (OH 2 )] + as an active catalyst formed by a photoinduced electron transfer and the loss of a bpy ligand.…”