Nickel phlorin intermediate formed by proton-coupled electron transfer in hydrogen evolution mechanism

Abstract

The development of more effective energy conversion processes is critical for global energy sustainability. The design of molecular electrocatalysts for the hydrogen evolution reaction is an important component of these efforts. Proton-coupled electron transfer (PCET) reactions, in which electron transfer is coupled to proton transfer, play an important role in these processes and can be enhanced by incorporating proton relays into the molecular electrocatalysts. Herein nickel porphyrin electrocatalysts with and without an internal proton relay are investigated to elucidate the hydrogen evolution mechanisms and thereby enable the design of more effective catalysts. Density functional theory calculations indicate that electrochemical reduction leads to dearomatization of the porphyrin conjugated system, thereby favoring protonation at the meso carbon of the porphyrin ring to produce a phlorin intermediate. A key step in the proposed mechanisms is a thermodynamically favorable PCET reaction composed of intramolecular electron transfer from the nickel to the porphyrin and proton transfer from a carboxylic acid hanging group or an external acid to the meso carbon of the porphyrin. The C-H bond of the active phlorin acts similarly to the more traditional metal-hydride by reacting with acid to produce H 2 . Support for the theoretically predicted mechanism is provided by the agreement between simulated and experimental cyclic voltammograms in weak and strong acid and by the detection of a phlorin intermediate through spectroelectrochemical measurements. These results suggest that phlorin species have the potential to perform unique chemistry that could prove useful in designing more effective electrocatalysts.electrocatalysis | metalloporphyrin | proton transfer | dearomatization D irect solar-to-fuel processes are important components of global energy sustainability efforts (1, 2). Such processes include the hydrogen evolution reaction (HER), oxidation of water to oxygen, and reduction of CO 2 to hydrocarbons (3, 4). Protoncoupled electron transfer (PCET), which is generally defined in terms of coupling between electron transfer (ET) and proton transfer (PT) reactions, is essential to all of these processes. PCET can be classified as occurring via either a sequential or a concerted mechanism (5, 6). The mechanism is determined to be sequential rather than concerted if a stable intermediate associated with initial ET or PT can be identified. This distinction is not rigorous, however, because the identification of a stable intermediate may depend on the experimental approach or the level of theory, as well as the lifetime of the intermediate. Regardless of the specific mechanism, the coupling of ET and PT plays a significant role in a wide range of energy conversion processes (7-11). Moreover, the coupling of ET and PT can be enhanced by incorporating proton relays into molecular catalysts, exploiting the proximal positioning of the proton donor and acceptor (12-16). Recognition and characterization of successful PCET ...

Proton–hydride tautomerism in hydrogen evolution catalysis

“…Hydrogen evolution catalysts have been discovered recently in which the metal center and associated ligands cooperate in unexpected ways. A case in point features ligand-centered protonation and subsequent hydride-like reactivity of a nascent C-H bond in a nickel phlorin system (16), although a nickel hydride was not implicated in the cycle for hydrogen evolution. In other work of note, Hull et al (17) reported dehydrogenation of HCO 2 H promoted by proton-responsive hydroxybipyridine-ligated catalysts, and Lacy et al (18) showed that ligand-centered protonation of a cobalt complex could lead to hydrogen evolution.…”

Catalysts by Design: The Power of Theory

“…Despite initial skepticism about this theoretical prediction, the phlorin intermediate was subsequently isolated and characterized spectroelectrochemically using phenol as the acid, which was strong enough to form the phlorin intermediate but not strong enough to produce hydrogen. 22 These types of theoretical predictions that are later tested experimentally are an important component of theoretical catalyst design.…”

Ligand steals spotlight from metal to orchestrate hydrogen production

“…Transition metal hydride intermediates are almost always invoked as key species during hydrogen evolution, with the metal at the center of the reactivity docking both protons and electrons. Reporting in PNAS, Solis et al (7) now show that the ligand can play a role similar to the metal center, with a C-H bond in a phlorin reacting like a metal hydride to release hydrogen.…”