Electrocatalytic Hydrogen Evolution by Cobalt Complexes with a Redox Non-Innocent Polypyridine Ligand

Abstract

Novel cobalt and zinc complexes with the tetradentate ppq (8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2′-yl)quinoline) ligand have been synthesized and fully characterized. Electrochemical measurements have shown that the formal monovalent complex [Co(ppq)(PPh 3 )] + (2) undergoes two stepwise ligand-based electroreductions in DMF, affording a [Co(ppq)-DMF] −1 species. Theoretical calculations have described the electronic structure of [Co(ppq)DMF] −1 as a low-spin Co(II) center coupling with a triple-reduced ppq radical ligand. In the presence of triethylammonium as the proton donor, the cobalt complex efficiently drives electrocatalytic hydrogen evolution with a maximum turnover frequency of thousands per second. A mechanistic investigation proposes an EECC H 2 -evolving pathway, where the second ligand-based redox process (E), generating the [Co(ppq)DMF] −1 intermediate, initiates proton reduction, and the second proton transfer process (C) is the rate-determining step. This work provides a unique example for understanding the role of redoxactive ligands in electrocatalytic H 2 evolution by transition metal sites.

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

“…For fully conjugated ligand systems (such as cyano‐methylene bridged bipyridyls), one single reduction was reported in this region, whereas electronically separated moieties within the ligand, as in R1 , C1 , and C2 , lead to several subsequent reductions [35] . Analyses of the CVs of R1 with the software package DigiElch [38–47] at varying scan rates confirm these potentials, and gives a diffusion coefficient of 2.8×10 −6 cm 2 s −1 (Figure SI5 and Table SI3), in line with literature reports for similar compounds [10,48,49] …”

“…Clearly, [Co II (dmgH) 2 py] is unrivalled in terms of overpotential. Interestingly, protonation with the relatively weak acid [HNEt 3 ] + occurs at Co I for the latter, as opposed to the cobalt polypyridyl catalysts investigated here, or elsewhere [10,11,49] . This, in turn, explains the substantial difference in overpotential, and might be related to the spin state of the respective Co I intermediate: Tong and co‐workers recently demonstrated a high‐spin S=1 state for a similar Co I intermediate, derived from a S=3/2 Co II [49] .…”

“…Analyses of the CVs of R1 with the software package DigiElch [38][39][40][41][42][43][44][45][46][47] at varying scan rates confirm these potentials, and gives a diffusion coefficient of 2.8 × 10 À 6 cm 2 s À 1 (Figure SI5 and Table SI3), in line with literature reports for similar compounds. [10,48,49] The bipyridyl-bridged C1 (red trace) exhibits one reversible redox peak with a half wave potential of E 1/2 = À 1.13 V, followed by two broader peaks at À 1.84 and À 1.98 V. Comparison with R1 reveals an anodic shift for the first reduction by 245 mV, a significantly smaller peak separation of 48 mV (as compared to 71 mV for R1), and an approximately doubled peak current. The latter two are clear indications for a two-electron reduction in C1, as opposed to a one-electron process in R1.…”

“…Interestingly, protonation with the relatively weak acid [HNEt 3 ] + occurs at Co I for the latter, as opposed to the cobalt polypyridyl catalysts investigated here, or elsewhere. [10,11,49] This, in turn, explains the substantial difference in overpotential, and might be related to the spin state of the respective Co I intermediate: Tong and co-workers recently demonstrated a high-spin S = 1 state for a similar Co I intermediate, derived from a S = 3/2 Co II . [49] These results coincide with a recent study on R1 and a related cobalt polypyridyl, evidencing the importance of the Co I singlet state for protonation.…”

Through‐Space Electrostatic Effects of Positively Charged Substituents on the Hydrogen Evolution Reaction

“…Molecular complexes have the benefits for studying structure‐function relationships [26–29] . Recent efforts have led to the identification of a variety of molecular complexes, made of earth‐abundant transition metals, as active HER electrocatalysts [30–51] . By studying these complexes, it is learned that introducing proton relays [31,52,53] and 2 nd sphere redox‐active groups [54] can boost electrocatalytic HER.…”