Renewable Formate from C-H Bond Formation with CO: 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 CO drive our efforts to discover ways of converting excess CO 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 CO requires a fundamental understanding of how to generate metal hydrides that selectively donate H to CO, rather than recombining with H to liberate H. Our work with a unique series of water-soluble and -stable, low-valent iron electrocatalysts offers mechanistic and thermochemical insights into formate production from CO. Of particular interest are the nitride- and carbide-containing clusters: [FeN(CO)] and its derivatives and [FeC(CO)]. In both aqueous and mixed solvent conditions, [FeN(CO)] forms a reduced hydride intermediate, [H-FeN(CO)], through stepwise electron and proton transfers. This hydride selectively reacts with CO 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 [FeN(CO)] facilitates proton transfer to the active site, successfully intercepting the hydride intermediate before it reacts with CO; only H is observed in this case. In contrast, isoelectronic [FeC(CO)] features a concerted proton-electron transfer mechanism to form [H-FeC(CO)], which is selective for H production even in the presence of CO, 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-FeN(CO)] and its derivative [H-FeN(CO)(PPh)] have hydricities modest enough to avoid H production but strong enough to make formate. [H-FeC(CO)] 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 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 where C-H bond formation may be favored in water or MeCN, respectively.