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 H2. 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.

Keywords: dearomatization; electrocatalysis; metalloporphyrin; proton transfer.

Fig. 1.

Structures of nickel porphyrins [1-H] and [3]. Select carbon atoms of the porphyrin ring are labeled according to position, including meso carbons 5, 10, 15, and 20. Complex [2] is the analog of [1-H], where a bromine atom replaces the carboxylic acid substituent.

Fig. 2.

Depiction of the structures for the intramolecular PCET reaction for [1-H]. The labels show the formal oxidation state of the nickel center and the porphyrin (Por) or phlorin (Phl-H), as well as the protonation state of the hanging carboxylic acid (COOH). The IET arrow includes a spin flip of the unpaired electron on the nickel of the flat triplet to generate the flat open-shell singlet [1-H]^2–, followed by IET from the nickel to the porphyrin ring to produce the bent closed-shell singlet [1′-H]^2–. The IPT arrow includes proton transfer from the carboxylic acid to the meso carbon to produce the bent closed-shell singlet [1-H_C]^2–. The net PCET reaction is thermodynamically downhill by 18.4 kcal/mol.

Fig. 3.

Cyclic voltammetry of [1-H] and [3] in benzoic and tosic acids in 0.1 M TBAPF₆/acetonitrile electrolyte at a glassy carbon electrode with a scan rate of 0.1 V/s. (A) CVs of 0.3 mM [1-H] in the presence of 0 (black), 0.16 (red), 0.40 (green), 0.80 (dark blue), and 2.0 (light blue) mM benzoic acid. (B) CVs of 0.4 mM [1-H] in the presence of 0 (black), 0.40 (red), 1.0 (green), 2.0 (dark blue), 5.0 (light blue), and 10.0 (magenta) mM tosic acid. (C) CVs of 0.4 mM [3] in the presence of 0 (black), 0.20 (red), 0.40 (green), 0.80 (dark blue), and 2.0 (light blue) mM benzoic acid. (D) CVs of 0.4 mM [3] in the presence of 0 (black), 1.0 (green), 2.0 (dark blue), 5.0 (light blue), and 10.0 (magenta) mM tosic acid. Note that some CVs of [1-H] with benzoic acid (A) exhibit minor curve crossing, which is thought to be an artifact of background subtraction. No crossing is observed for the uncorrected CVs.

Fig. 4.

Free energy diagrams (Top) for H₂ production catalyzed by [1-H] with benzoic acid (C₆H₅COOH, pK_a = 20.7) and an applied potential of –1.8 V vs. Fc⁺/Fc and tosic acid (TsOH, pK_a = 8.0) with an applied potential of –1.37 V vs. Fc⁺/Fc (Inset). The chosen applied potentials, which define the zero for the free energy changes associated with reduction steps, correspond to the peaks of the catalytic waves in the CVs. Complete mechanistic cycle of proposed mechanisms (Bottom), starting from [1-H]⁰ (shown in green brackets). Reduction potentials are listed in volts vs. Fc⁺/Fc, and free energies are listed in kcal/mol. Proposed cycles in strong (red arrow) and weak (blue arrow) acid regimes begin with reduction to [1-H]^– (shown in purple brackets). With benzoic (weak) acid, additional reduction is required to form [1-H]^2–. The subsequent intramolecular PCET step is thermodynamically favorable and can occur either concertedly (dotted line) or sequentially via intramolecular ET from the nickel to the porphyrin to form the bent structure [1′-H]^2– followed by intramolecular PT to produce the phlorin [1-H_C]^2–. Protonation from benzoic acid at the carboxylate forms [1-HH_C]^–, which is subsequently reduced. H₂ is evolved from [1-HH_C]^2–, either via self-elimination to the deprotonated [1]^2– or by reaction with benzoic acid, forming [1-H]^–. With tosic (strong) acid, protonation of [1-H]^– yields the phlorin [1-HH_C]⁰, which is rapidly reduced at the operating potential. The phlorin formation involves an analogous PCET step as shown for the weak acid pathway, but it is not shown explicitly for the strong acid pathway. H₂ is evolved by reaction of [1-HH_C]^– with tosic acid, forming the neutral [1-H]⁰. Note that other branches leading to Ni chlorin, Ni bacteriochlorin, and Ni isobacteriochlorin species are not shown but may be thermodynamically favorable and possibly nonproductive toward H₂ catalysis.

Fig. 5.

Experimental (black curve) and simulated (dotted curves) CVs of (A) 0.34 mM [1-H] and (B) 0.30 mM [3]. The simulated curves correspond to the reaction in the absence of external acid (red curve), in the presence of 1 mM tosic acid (blue curve), or in the presence of 1 mM benzoic acid (green curve). The vertical lines correspond to the experimentally measured catalytic peak positions for tosic acid (blue line) or benzoic acid (green line). Parameters used for simulations are tabulated in SI Appendix, Tables S13 and S14.

Fig. 6.

(A) Absorption spectra of [3] acquired using thin-layer spectroelectrochemistry in 0.1 M TBAPF₆/acetonitrile electrolyte. Spectra taken before electrolysis (black), after electrolysis at –1.3 V vs. Fc⁺/Fc (red), after electrolysis at –1.9 V vs. Fc⁺/Fc in the absence of external acid (green), and after electrolysis at –1.9 V vs. Fc⁺/Fc in the presence of 10 mM phenol (blue). (B) CVs of 0.4 mM of [3] without acid (black curve) and in the presence of 1 mM phenol (red curve).