[Fe]-Hydrogenase-Inspired Proton-Shuttle Installation in a Molecular Cobalt Complex for High-Efficiency H₂ Evolution Reaction

Abstract

Developing efficient H 2 -evolution artificial electrocatalysts often takes inspiration from the active sites of [FeFe]/[NiFe]-hydrogenases to incorporate smartly designed amine/thiolato-based proton-shuttle motifs. Interestingly, [Fe]-hydrogenase (the third type of hydrogenases) consists of an Fe-bound adjacent pyridinol/pyridinone proton shuttle, which assists in reversible heterolytic H 2 -cleavage during its operation. However, so far, this type of proton-shuttle functionality has not been exploited to develop H 2 -evolution electrocatalysts. We reckoned that installing such a proton-shuttle motif in a suitable redox-active transitionmetal-based complex can facilitate electrocatalytic H 2 generation via easy hydride/proton coupling, through proper positioning of the electrochemically generated metal-hydride and the "protonated" pyridinol form of the proton shuttle. To demonstrate this idea, a rationally designed Co-based complex Co− NHC U, containing an anionic uracil nucleobase, connected with a strong sigma-donor N-heterocyclic carbene (NHC) ligand, has been developed, wherein the anionic uracil motif offers a pyridinone-type proton-responsive site adjacent to the Co center. Gratifyingly, Co− NHC U acts as a highefficiency H 2 -generation electrocatalyst with variable proton sources, achieving maximum turnover frequency (TOF max ) values in the range of 5000−13 700 s −1 (from cyclic voltammetry) or ∼2200−7360 s −1 (from controlled potential electrolysis) with Faradaic efficiencies of 93%−98%, at the overpotential range of 0.50−0.78 V. This system delivered the highest TOF max values achieved to date by any cobalt electrocatalyst in a nonaqueous medium: 10 200−13 700 s −1 with CF 3 COOH and Et 3 NHBF 4 as proton sources. This work could provide insights into the future design principles of homogeneous HER catalysts via capitalizing the power of pyridinone/pyridinol-based proton shuttle motifs coupled with Earth-abundant transition metals.

Bioinspired Diiron Complex with Proton Shuttling and Redox-Active Ligand for Electrocatalytic Hydrogen Evolution

“…For [Fe III −(μ-O)−Fe III ] , we observed a moderate KIE value along with first-order dependency of acid concentration in a nonsaturating acid regime. Similar results were also observed by Choudhury et al, which follow rate-limiting intramolecular H + /H – coupling toward H 2 evolution . Our case can be explained by intramolecular coupling of a bridged O–H proton and Fe–H hydride.…”

“…The fact that there is no anodic shift in the Fe III Fe III /Fe II Fe II cathodic wave suggests that AcOH and HDBU + are unable to protonate owing to their higher p K a values than [Fe III −(μ-O)−Fe III ] . Recently, Choudhury and co-workers reported a similar observation when titrating a cobalt uracil-based complex with AcOH . The protonation/deprotonation of the proton-responsive pyridinone group was governed by the p K a values of the bulk substrate.…”

Metal- versus ligand-centered reactivity of a cobalt-phenylenediamide complex with electrophiles

“…Several previous reports have demonstrated the electrochemical properties of half-sandwich [CpCo] complexes, which typically show reversible or quasi-reversible features associated with the Co III /Co II and Co II /Co I redox couples. 38–47 However, the redox potentials for 2 are more positive by comparison, despite the presence of the formally anionic CF 3 ligand; for example, [CpCo(bpy)(MeCN)] 2+ (bpy = 2,2′-bipyridine) undergoes reduction at −0.40 and −1.15 V vs. Fc +/0 in MeCN. 38,43 This deviation suggests that one or both reduction processes for 2 may not be localized to the metal center.…”