Cobalt Complex of a Tetraamido Macrocyclic Ligand as a Precursor for Electrocatalytic Hydrogen Evolution

Abstract

Hydrogen (H2) is a clean fuel that can potentially store renewable energy and overcome some of the environmental problems that arise from fossil fuel consumption. One attractive approach is to produce H2 from water electrocatalytically using molecular complexes that can be systematically improved through ligand modifications. Here, we report cobalt and nickel complexes supported by tetraamido macrocyclic ligands (TAML), which consist of exclusively earth abundant elements. Although TAML systems are well established in high valent transition metal chemistry, little is known about their reactivity in reductive catalysis despite the electron-rich nature of the tetraanionic TAML. Thus, we explored the utility of these nucleophilic -ate complexes as potential electrocatalysts for H2 evolution using water as the proton source. Controlled potential electrolysis experiments were performed and the cobalt TAML variant exhibited catalytic H2 evolution activity in acetonitrile containing 1.0 M water, but was inactive in purely aqueous solutions. Further investigation revealed that cobalt metal nanoparticles were electrodeposited as the active catalyst for H2 evolution. We propose that these disparities in reactivity arise from the different number of water molecules coordinated to the cobalt center, with intermediate concentrations favoring a square pyramidal structure with labile ligands, whereas high concentrations of water result in a kinetically inert octahedral complex with no empty coordination sites.

Electrochemical Degradation of a Dicationic Rhenium Complex via Hoffman-Type Elimination

“…Notably, nonaromatic CN bonds, , C–S bonds, and S–Ni bonds , are prone to electrolytic decomposition and deposition of catalytically active materials. Additionally, metal ions such as cobalt, , even when bound to a particularly stable TAML ligand, and copper , have been identified as problematic metal ions for electrocatalytic applications. These degradation pathways are poorly understood and often ignored, as researchers move on to the next high-performing target.…”

Hydrogen Evolution Electrocatalysis with a Molecular Cobalt Bis(alkylimidazole)methane Complex in DMF: a Critical Activity Analysis

“…Literature analysis shows some precedence for the formation of deposits during non‐aqueous HER experiments using molecular cobalt complexes [12c,e,g,24a,25] . A dicationic tris(glyoximato) cobalt clathrochelate was reported to form a deposit during HER electrolysis in MeCN with NaClO 4 as supporting electrolyte salt and HClO 4 as sacrificial proton donor.…”

“…Literature analysis shows some precedence for the formation of deposits during non‐aqueous HER experiments using molecular cobalt complexes. [ 12c , 12e , 12g , 24a , 25 ] A dicationic tris(glyoximato) cobalt clathrochelate was reported to form a deposit during HER electrolysis in MeCN with NaClO 4 as supporting electrolyte salt and HClO 4 as sacrificial proton donor. In this case, energy‐dispersive X‐ray spectroscopy (EDX) analysis showed a relative ratio of atomic amounts adsorbed to the electrode of 63±7 % : 32.5±5 % cobalt/heteroatoms (F, O, N, B), which significantly deviates from our observations, particularly in terms of the amount of cobalt.…”

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

“…In several recently published articles [1690][1691][1692][1693][1694][1695][1696][1697][1698][1699] brilliant theoretical or experimental investigation of catalytic pathways and characterisation of intermediates, as well as highly-advanced structure-property relationships have been shown, which might guide future catalyst design of metal organic complexes. However, convincing activity and stability at least compatible with practical application, constitutes the vast exception.…”