Metal-Assisted Ligand-Centered Electrocatalytic Hydrogen Evolution upon Reduction of a Bis(thiosemicarbazonato)Cu(II) Complex

Abstract

In this study, we report the electrocatalytic behavior of the neutral, monomeric Cu(II) complex of diacetyl-bis(N-4-methyl-3-thiosemicarbazonato), CuL, for metal-assisted ligand-centered hydrogen evolution in acetonitrile and dimethylformamide. CuL displays a maximum turnover frequency (TOF) of 10 000 s in acetonitrile and 5100 s in dimethylformamide at an overpotential of 0.80 and 0.76 V, respectively. The rate law is first-order in catalyst and second-order in proton concentration. Gas analysis from controlled potential electrolysis confirms CuL as an electrocatalyst to produce H with a minimum Faradaic efficiency of 81% and turnover numbers as high as 73 while showing no sign of degradation over 23 h. The H evolution reaction (HER) was probed using deuterated acid, demonstrating a kinetic isotope effect of 7.54. A proton inventory study suggests one proton is involved in the rate-determining step. Catalytic intermediates were identified using H NMR, X-ray photoelectron, and UV-visible spectroscopies. All catalytic intermediates in the proposed mechanism were successfully optimized using density functional theory calculations with the B3LYP functional and the 6-311g(d,p) basis set and support the proposed mechanism.

Electrocatalytic H2 Evolution by the Co‐Mabiq Complex Requires Tempering of the Redox‐Active Ligand

“…Consequently, the ligands can act as reservoirs for hydrogen atoms or hydrides during the catalytic cycle. The H 2 evolution pathways of, e. g., pincer, thiosemicarbazone, and thiolate complexes are known to involve the redox‐active scaffold. Proton relay sites and redox‐active ligands are proposed to significantly enhance the kinetics of H 2 formation and decrease the HER overpotentials of metal complexes .…”

“…The H 2 evolution pathways of, e. g., pincer, thiosemicarbazone, and thiolate complexes are known to involve the redox‐active scaffold. Proton relay sites and redox‐active ligands are proposed to significantly enhance the kinetics of H 2 formation and decrease the HER overpotentials of metal complexes . However, a redox non‐innocent coordination environment also may result in non‐productive pathways .…”

Hydrogen Evolution Reactions Catalyzed by a Bis(thiosemicarbazone) Cobalt Complex: An Experimental and Theoretical Study

“…[6,[11][12][13][14] Metal complexes based on thiosemicarbazone ligands are now emerging asan ew class of electrocatalyst for HER. [14][15][16][17] These complexes presents ome interesting features that are relevant for electrocatalytic protonreduction:the thiosemicarbazone ligand has already been shown to be redox active, [14,[18][19][20][21] whereas the presence of S-donors and severalNatoms allows for protonation of the ligand and can serve as proton relays. [5,22] Based on these two features, we can define three types of HER reactivity:1 )ligand-assistedm etal-centered reactivity, [14] 2) ligand-centered reactivity, [16] and 3) metal-assisted ligand-centered reactivity.…”

“…[5,22] Based on these two features, we can define three types of HER reactivity:1 )ligand-assistedm etal-centered reactivity, [14] 2) ligand-centered reactivity, [16] and 3) metal-assisted ligand-centered reactivity. [17] Af irst example of an on-transition-metal complex with ab is(thiosemicarbazone) ligand capable of HER wasr eported by Grapperhaus and co-workers. [16] This mononuclearz inc complex functioned as ah omogeneous electrocatalyst for proton reduction with am aximum turnover frequency (TOF)o f1 170 s À1 .The reactivity was shown to be exclusively ligand-centereda voiding traditional metal-hydride intermediates.…”

Molecular Electrocatalysts for the Hydrogen Evolution Reaction: Input from Quantum Chemistry

“…[49] It is worth mentioning that it is possible for the metal to assist in the catalytic cycle, acting as ar edox site, but the protonation sites are away from it, which requires intramolecular ET acrosst he ligand.W ed iscussed above af ew examples involving bis-thiosemicarbazone. [50] [50] Their study involved the use of DFT( B3LYP/6-311g(d,p)) calculationst oi nvestigate the energetics associated with the stability of various intermediates ford istinct spin states and protonation sites.…”