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

Abstract

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.

Directing the reactivity of metal hydrides for selective CO 2 reduction

“…Consistent with the free energy associated with H 2 and HCO 2 − formation (Eqs. 4 and 7, respectively) the only two molecular catalysts with >90% selectivity for formate generation function optimally at modest to high pK a /pH conditions, with decreasing selectivity at lower proton activities (16,20). Yet, both maintain fairly high selectivity under more acidic conditions, indicating that the product distribution is also under kinetic control (64).…”

“…Eq. 6 has been applied to rationalize or predict the activity of the iron-based CO 2 reduction electrocatalyst by Berben and coworkers (20) and CO 2 hydrogenation catalysts (45-50) as well as formate oxidation electrocatalysts (51). Transition metal hydricity values lower than that of formate will result in exergonic hydride transfer to CO 2 .…”

“…Understanding the reactivity of metal hydrides is key to controlling the bifurcating reaction pathways that ultimately determine selectivity. Most selective catalysts for CO 2 reduction utilize kinetic inhibition (or a high-transition state barrier) to minimize H 2 evolution.In this report, we utilize a thermodynamic approach to describe the reactivity of metal hydrides with H + and CO 2 by applying known free energy relationships (14,(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28). We depict our findings in a diagram (Fig.…”

Iron Porphyrin Allows Fast and Selective Electrocatalytic Conversion of CO2 to CO in a Flow Cell

“…Regarding the catalysts employed, molecular complexes have been widely investigated for the carbon dioxide reduction reaction (CO 2 RR) due to the possibility of fine tuning the ligand structure (steric and electronic effects, as well as second coordination sphere effects) . Earth abundant metal‐based catalysts have been shown to efficiently catalyze the production of CO and formate in organic solvents and in water, even if examples in pure aqueous solutions are less numerous. In the latter case, the catalysts are typically dispersed into thin conductive films usually made of carbon nanomaterials, such as carbon black or carbon nanotubes .…”

Metal Clusters on Semiconductor Surfaces and Application in Catalysis with a Focus on Au and Ru

“…For the case of clusters in solution, experiments have been performed on using nonsupported ligand‐stabilized clusters as homogenous catalysts. For example, recent publications have reported the use of [Fe 4 N(CO) 12 ] − and [Fe 4 N(CO) 11 PPh 3 ] − clusters in solution as highly selective homogeneous catalysts for CH bond formation with CO 2 to produce formate, an environmentally relevant reaction for converting excess CO 2 into chemical fuels. This homogeneous catalysis is not unique to Fe clusters, and Ru as well as other clusters have also been used to catalyze reactions in solution …”