Non-Metals Chelated by Redox Active Chelators as Electrocatalysts for Hydrogen Evolution Reaction (HER)
Authors: Kumar Sachin , Gross Zeev 
Published: February 2024

Global energy needs the development of non-polluting and renewable sources. Hydrogen is most attractive as such, it has high energy density of 140 MJ/Kj and water as a byproduct.1 Despite the “simplicity” of the H-H bond, a catalyst is required to make its formation process efficient and economic.2 Although platinum group metal (PGM) are the best catalysts for the Hydrogen Evolution Reaction (HER) to date, they are rare and expensive.3 Conventional HER catalysis employs a transition metal with the ability to accommodate several oxidation states for the two-electron hydride transfer associated with hydrogen evolution. Emerging interest is being shown in the utilization of redox-active ligands that open alternative HER pathways, such as ligand-assisted metal-centered,4 ligand-centered,5and metal-assisted ligand-centered reactivity.6

Antimony corroles7 and two boron subphthalocyanines8 are reported as HER electrocatalysts. For that, two new corroles with electron donating and electron withdrawing meso-substituents were fully characterized X-ray crystallography, NMR, electronic spectra, and electrochemistry. All the complexes were studied under homogeneous conditions (CH3CN solvent and TFA as proton source), which revealed that catalysis relies on ligand reduction, in contrast with ordinary transition metal complexes. It revealed that the electron withdrawing substituents are best for homogeneous catalysis and the most electron rich substituents are best in heterogeneous catalysis in terms of onset potential and faradaic efficiency due to secondary sphere.

References:

(1) Beyene, B. B.; Hung, C. H. Recent Progress on Metalloporphyrin-Based Hydrogen Evolution Catalysis. Coord. Chem. Rev.
2020, 410, 213234. https://doi.org/10.1016/j.ccr.2020.213234.

(2) Bullock, R. M.; Appel, A. M.; Helm, M. L. Production of Hydrogen by Electrocatalysis: Making the H-H Bond by Combining Protons and Hydrides. Chem. Commun.
2014, 50 (24), 3125–3143. https://doi.org/10.1039/c3cc46135a.

(3) Karunadasa, H. I.; Chang, C. J.; Long, J. R. A Molecular Molybdenum-Oxo Catalyst for Generating Hydrogen from Water. Nature
2010, 464 (7293), 1329–1333. https://doi.org/10.1038/nature08969.

(4) Jurss, J. W.; Khnayzer, R. S.; Panetier, J. A.; El Roz, K. A.; Nichols, E. M.; Head-Gordon, M.; Long, J. R.; Castellano, F. N.; Chang, C. J. Bioinspired Design of Redox-Active Ligands for Multielectron Catalysis: Effects of Positioning Pyrazine Reservoirs on Cobalt for Electro- and Photocatalytic Generation of Hydrogen from Water. Chem. Sci.
2015, 6 (8), 4954–4972. https://doi.org/10.1039/c5sc01414j.

(5) Haddad, A. Z.; Garabato, B. D.; Kozlowski, P. M.; Buchanan, R. M.; Grapperhaus, C. A. Beyond Metal-Hydrides: Non-Transition-Metal and Metal-Free Ligand-Centered Electrocatalytic Hydrogen Evolution and Hydrogen Oxidation. J. Am. Chem. Soc.
2016, 138 (25), 7844–7847. https://doi.org/10.1021/jacs.6b04441.

(6) Sherbow, T. J.; Fettinger, J. C.; Berben, L. A. Control of Ligand PKa Values Tunes the Electrocatalytic Dihydrogen Evolution Mechanism in a Redox-Active Aluminum(III) Complex. Inorg. Chem.
2017, 56 (15), 8651–8660. https://doi.org/10.1021/acs.inorgchem.7b00230.

(7) Luobeznova, I.; Raizman, M.; Goldberg, I.; Gross, Z. Synthesis and Full Characterization of Molybdenum and Antimony Corroles and Utilization of the Latter Complexes as Very Efficient Catalysts for Highly Selective Aerobic Oxygenation Reactions. Inorg. Chem.
2006, 45 (1), 386–394. https://doi.org/10.1021/ic051483g.

(8) Mizrahi, A.; Bukuroshi, E.; Vestfrid, J.; Bender, T. P.; Gross, Z. Axial/Peripheral Chloride/Fluoride-Substituted Boron Subphthalocyanines as Electron Acceptors. Inorg. Chem.
2020, 59 (5), 2641–2645. https://doi.org/10.1021/acs.inorgchem.9b03529.