The search for selectivity

The 2H⁺/2e⁻ electroreduction of CO₂ can give either CO and H₂O or HCO₂H — products that make fuels when combined with alkenes (the Fischer–Tropsch process) or sugars, respectively. The question of selectivity adds to the challenge of developing an electrocatalyst that is robust and active at low overpotentials. One notable class of molecular electrocatalyst for CO₂ reduction is the metalloporphyrins, which can feature a redox-active metal with apical sites available to substrates. But the metal is not the only redox-active component nor is it the only possible substrate binding site. Writing in Journal of the American Chemical Society, Adrien Göttle and Marc Koper use density functional theory to evaluate metal- and ligand-centred reactivity pathways. Their thermochemical calculations enable us to rationalize why different metalloporphyrins operate through different pathways and give rise to different selectivities.

Credit: Image courtesy of M. T. M. Koper, Leiden University, Netherlands

Reducing a metalloporphyrin affords a complex of a low-valent and nucleophilic metal centre that can bind CO₂ at the electrophilic C atom to give a metallacarboxylate, with the η¹-CO₂^−/2− ligand undergoing double protonation to liberate H₂O and CO. Yet, the low-valent metal is not only Lewis basic but also Brønsted basic, and can instead react with H⁺ to give a hydrido. On reduction, this complex can donate H⁻ to CO₂, a pathway that gives HCO₂⁻ and eventually HCO₂H, at sufficiently low pH. A third pathway, identified in metalloporphyrin-catalyzed H₂ evolution, can also be operative. Here, the reduced metalloporphyrin is protonated not at the metal but at a meso-C atom to afford a phlorinato, which transfers H⁻ to CO₂ to give HCO₂⁻ in a similar way to the hydrido. Göttle and Koper knew of these pathways and wondered why porphyrinato complexes of middle first-row metals (such as Fe and Co) make CO, while those of late metals (such as Ni and Cu) are biased towards making HCO₂⁻. “The project was based on experimental findings in my group that the selectivity of immobilized metalloporphyrins in the electroreduction of CO₂ is very sensitive to the nature of the metal,” says Koper.

electroreduction of CO₂ is very sensitive to the nature of the metal

The team predicted ΔG values associated with porphinatometal complexes accepting H⁺ and e⁻ to give either a hydrido or the tautomeric phlorinato, as well as the situation in which CO₂ is added into the mix when one of two further products can form either a metallacarboxylate or an isomeric formate. Göttle and Koper used thermodynamic cycles and the computational hydrogen electrode method, thereby eschewing the need to use a value for G(H⁺). The duo reasoned that a thermodynamic preference for one pathway can hint at a kinetic preference because exergonic reactions typically have smaller activation barriers than endergonic reactions (the Brønsted–Evans–Polanyi relation).

The ΔG value for a porphinato converting into a phlorinato is independent of the metal present, an intuitive result for a ligand-centred pathway. Satisfyingly, the identity of the metal is influential when substrates land there. Metallacarboxylate and hydrido formation is more exergonic for Fe or Co than for Ni or Cu, with the former (more basic) metals giving lower ΔG values. The propensity for Fe and Co porphinatos to give metallacarboxylates is consistent with their high selectivity for CO evolution. Hydrido and phlorinato complexes give HCO₂⁻ (or H₂ at low pH), and formato intermediates are not involved at all. Alternatively, porphinatos can undergo net hydrogenation to give chlorinato intermediates that are unreactive to CO₂ but evolve H₂. Each intermediate forms through stepwise transfers, which contrast earlier studies predicting concerted proton-coupled electron transfers for heterogeneous catalysts, which operate through metallacarboxylate and formato species en route to CO and HCO₂H, respectively. For metalloporphyrins, “Future work will focus on finding further experimental evidence for the importance of the hydrido intermediate,” says Koper. Indeed, experiments on these simple metalloporphyrins will help us tease out the mechanisms at play and perhaps inform the design of more active species with desirable selectivity.