Ligand steals spotlight from metal to orchestrate hydrogen production

In comparison to green plants, we humans severely underuse the sun’s energy. Although the high costs of solar photovoltaics have been an early barrier to widespread utilization, the lack of efficient solar energy storage mechanisms has greatly hindered the adoption of this sustainable energy resource. The development of sunlight-to-fuel technologies, with the goal of mimicking the process of photosynthesis on an industrial scale, has been a major and growing global research endeavor over the last decade (1, 2). Key to these technologies is the transformation of abundant but energy-poor feedstocks (like water and carbon dioxide) into energy-rich fuels (like hydrogen and methanol). Transition metal catalysts help orchestrate the proton-coupled electron transfer processes that underpin fuel synthesis, such as the production of hydrogen (3–6). 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.

The nickel metalloporphyrin electrocatalysts studied in this work, along with their cobalt analogs, are known to mediate hydrogen evolution (8, 9). The covalently linked xanthene moiety with an appended carboxylic acid in the “hangman” porphyrin ligand provides a pathway for intramolecular proton transfer (Fig. 1). These and similar architectures that place a proton relay in the secondary coordination sphere of the transition metal catalyst have been demonstrated to enhance proton transfer processes in catalytic hydrogen production cycles (10, 11). Recent theoretical study on the cobalt hangman complex identified a cobalt phlorin intermediate formed via intramolecular proton transfer from the pendant carboxylic acid upon reduction of the complex, although in the presence of a stronger acid, direct protonation of the metal center to form a cobalt–hydride complex occurs (12, 13). This divergent reactivity highlights how careful tuning of thermochemical parameters can be used to promote and control reaction pathways involving proton and electron transfers.

Fig. 1.

Upon the two sequential one-electron reductions of the nickel(II) hangman porphyrin, intramolecular proton transfer from the pendant carboxylic acid to the meso carbon of the ligand is promoted to form the phlorin species. The first reduction is metal based, and the second reduction is ligand based; however, protonation of the ligand proceeds via a proton-coupled electron transfer reaction in which an electron is intramolecularly transferred from the nickel center to the ligand to form a formally nickel(II) phlorin. Figure adapted from ref. 7.

Now Solis et al. reveal that the key intermediate in hydrogen evolution for the nickel hangman complex is not a nickel hydride, but an organo-hydride, thus demonstrating the versatile reactivity of phlorin intermediates in catalysis. The first reduction of the Ni(II) hangman complex produces a formally Ni(I) species, whereas the second reduction leads to a Ni(I) porphyrin radical. This second reduction promotes an intramolecular proton transfer reaction from the pendant carboxylic acid. However, where does the proton go? Most traditional catalytic schemes suggest the proton simply transfers directly to the metal atom to form a metal hydride. Excitingly, unconventional reactivity is revealed for this nickel species through a combined density functional theory and experimental approach. First, the meso carbon center of the porphyrin ligand accepts the proton to yield an anionic phlorin species, the presence of which is supported by direct spectroelectrochemical observation of the phlorin intermediate (Fig. 1). Second, the formation of the anionic phlorin species is proposed to proceed through an intramolecular proton-coupled electron transfer reaction. The details of this reaction are intriguing—the intramolecular proton transfer from the carboxylic acid to the meso carbon is accompanied by an intramolecular electron transfer from the nickel center to the ligand to form a Ni(II) anionic phlorin complex with a pendant carboxylate. The anionic phlorin is effectively storing two electrons and one proton (a hydride equivalent) while the metal center is simply standing by.

In the presence of a stronger acid than the pendant carboxylic acid, the Ni(I) species can react directly before further reduction, here forming a neutral phlorin species as opposed to a metal hydride. Like the protonation of the Ni(I) porphyrin radical by the pendant carboxylic acid, reaction of the Ni(I) species with a strong acid is proposed to proceed via a proton-coupled electron transfer in which an intramolecular electron transfer from the Ni(I) center to the porphyrin accompanies the proton transfer from the external strong acid to the meso carbon center of the porphyrin.

As noted above, the nickel phlorin species produced via these proton-coupled electron transfer reactions are effectively storing formal hydride equivalents. Upon further ligand-based reduction, both the neutral and anionic phlorin species react with either weak or strong acids, respectively, to release hydrogen. In both of these reactions, the C–H bond of the phlorin can be likened to the metal–hydride bond in a more conventional hydrogen release mechanism. In the context of typical hydrogen evolution reactions for molecular electrocatalysts (5), this reactivity is perceived as unusual. Only a handful of examples of ligand-based reactivity in hydrogen evolution have been reported, and all involve heteroatom-based protonation sites, such as the molybdenum–sulfur dimers reported by Rakowski DuBois and coworkers (14) that mediate proton/electron chemistry via hydrosulfido intermediates. Carbon-based reactivity in hydrogen production has not previously been recognized, which is surprising when one considers that nature is legendary for using reductive dearomitization/carbon-centered protonation reactivity to generate organo-hydrides (15, 16). For instance, the reducing equivalents generated upon charge separation in Photosystem II are stored in NADPH, and this organo-hydride provides electrons for the reduction of CO₂ to carbohydrates in the Calvin cycle (17).

It is becoming clear that ligand “noninnocence” can take many forms. Redox-active ligands have received much attention for helping to mediate multielectron redox transformations in both enzymes and molecular catalysts (18). Similarly, ligands containing pendant proton relays have been shown to facilitate proton transfer processes in natural and artificial systems (10). Ligands capable of directly participating in proton-coupled electron transfer, however, have received much less attention in the context of fuel-forming reactions. Although the reactivity proposed by Solis et al. is indeed unusual, maybe it should not come as such a surprise, given the importance of proton-coupled electron transfer pathways in energy conversion reactions, that ligands able to juggle both protons and electrons can facilitate transformations like hydrogen production. Although the prevailing viewpoint has been that the metal center(s) dominate catalytic substrate rearrangements, perhaps the ligand-based reactivity revealed for these nickel porphyrin species can relegate the metal center to simply tuning the ligand energetics, storing redox equivalents, and influencing ligand conformations—much as how ligands are traditionally viewed. Conceivably, we may even be able to design catalysts in which the multielectron redox activity of both transition metals and ligands are exploited in concert to facilitate complex transformations like the six-electron, six-proton reduction of CO₂ to methanol. Ultimately, this finding of unusual ligand-based reactivity reveals new mechanisms for molecular-based hydrogen evolution, opening the door for a new paradigm of catalysts that exploit ligand hydride storage ability.