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.

A proton shuttle in the second coordination sphere of [Fe4N(CO)12]– promotes H2 evolution over formate formation from CO2.

An exploration of secondary coordination sphere (SCS) functional groups is presented with a focus on proton transport to a metal hydride active site for H 2 formation and transport of CO 2 so that formate can be obtained. In MeCN−H 2 O, pK a (AH) and steric bulk of the SCS groups are discussed along with their influence on each step in the mechanism for CO 2 to formate catalysis and along with the influence of the proton source, which is MeCN−H 2 O or (MeCN) 2 H 2 O in MeCN−H 2 O (95:5) under N 2 atmosphere. Under CO 2 , carbonic acid is also available. Catalysts containing various SCS groups were synthesized from [Fe 4 N(CO) 12 ] − and have the form [Fe 4 N(CO) 11 L] − where L is Ph 2 P-SCS. Hydride formation rates are distinct under N 2 versus CO 2 , and that variation is dependent on the size of the SCS group. Under CO 2 , larger SCS groups inhibit access of the MeCN−H 2 O adducts to the active site and formate formation is observed, whereas smaller SCS groups allow transport of these adducts. This is best illustrated by catalysts containing the small SCS group pyridyl and the large SCS group N,N-dimethylaniline which both have the same pK a (AH) value. The smaller pyridyl group promotes selective H 2 evolution, whereas larger N,N-dimethylaniline supports selective formate formation by slowing the transport of large MeCN−H 2 O adducts, allowing hydride transfer to the smaller substrate CO 2 .

Fe 4 N(CO) 12 ] − is a first-row transition element electrocatalyst that selectively produces C−H bonds to give formate from CO 2 in water at −1.2 V vs SCE. We present a thermochemical analysis which probes the possibility that [H-Fe 4 N(CO) 12 ] 2− ((H-1) 2− ) is an intermediate in this process: we show that (H-1) 2− is accessible at −1.2 V vs SCE, but if it were formed, we predict that it would generate H 2 .[Fe 4 N(CO) 12 ] 3− and (H-1) 2− were interrogated spectroscopically, and the product of CO loss, [Fe 4 N(CO) 9 (μ-CO) 2 ] 3− , was synthesized and characterized. Ultimately, we demonstrate that (H-1) 2− is an unlikely participant in the catalytic transformation of CO 2 to formate.