Renewable Formate from C-H Bond Formation with CO₂: Using Iron Carbonyl Clusters as Electrocatalysts

As a society, we are heavily dependent on nonrenewable petroleum-derived fuels and chemical feedstocks. Rapid depletion of these resources and the increasingly evident negative effects of excess atmospheric CO₂ drive our efforts to discover ways of converting excess CO₂ into energy dense chemical fuels through selective C-H bond formation and using renewable energy sources to supply electrons. In this way, a carbon-neutral fuel economy might be realized. To develop a molecular or heterogeneous catalyst for C-H bond formation with CO₂ requires a fundamental understanding of how to generate metal hydrides that selectively donate H^- to CO₂, rather than recombining with H⁺ to liberate H₂. Our work with a unique series of water-soluble and -stable, low-valent iron electrocatalysts offers mechanistic and thermochemical insights into formate production from CO₂. Of particular interest are the nitride- and carbide-containing clusters: [Fe₄N(CO)₁₂]^- and its derivatives and [Fe₄C(CO)₁₂]^2-. In both aqueous and mixed solvent conditions, [Fe₄N(CO)₁₂]^- forms a reduced hydride intermediate, [H-Fe₄N(CO)₁₂]^-, through stepwise electron and proton transfers. This hydride selectively reacts with CO₂ and generates formate with >95% efficiency. The mechanism for this transformation is supported by crystallographic, cyclic voltammetry, and spectroelectrochemical (SEC) evidence. Furthermore, installation of a proton shuttle onto [Fe₄N(CO)₁₂]^- facilitates proton transfer to the active site, successfully intercepting the hydride intermediate before it reacts with CO₂; only H₂ is observed in this case. In contrast, isoelectronic [Fe₄C(CO)₁₂]^2- features a concerted proton-electron transfer mechanism to form [H-Fe₄C(CO)₁₂]^2-, which is selective for H₂ production even in the presence of CO₂, in both aqueous and mixed solvent systems. Higher nuclearity clusters were also studied, and all are proton reduction electrocatalysts, but none promote C-H bond formation. Thermochemical insights into the disparate reactivities of these clusters were achieved through hydricity measurements using SEC. We found that only [H-Fe₄N(CO)₁₂]^- and its derivative [H-Fe₄N(CO)₁₁(PPh₃)]^- have hydricities modest enough to avoid H₂ production but strong enough to make formate. [H-Fe₄C(CO)₁₂]^2- is a stronger hydride donor, theoretically capable of making formate, but due to an overwhelming thermodynamic driving force and the increased electrostatic attraction between the more negative cluster and H⁺, only H₂ is observed experimentally. This illustrates the fundamental importance of controlling thermochemistry when designing new catalysts selective for C-H bond formation and establishes a hydricity range of 15.5-24.1 or 44-49 kcal mol^-1 where C-H bond formation may be favored in water or MeCN, respectively.