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 H-2. Our work with a unique series of water-soluble and-stable, lowvalent 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 H-2 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 H-2 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 H-2 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 H-2 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 ma(-1) where C-H bond formation may be favored in water or MeCN, respectively.