C–H bond formation with CO2 to selectively form products such as formate (HCOO–) is an important step in harnessing CO2 emissions as a carbon-neutral or carbon-negative renewable energy source. In this report, we show that the iron carbonyl cluster, [Fe4N­(CO)12]−, is an electrocatalyst for the selective reduction of CO2 to formate in water (pH 5–13). With low applied overpotential (230–440 mV), formate is produced with a high current density of 4 mA cm–2 and 96% Faradaic efficiency. These metrics, combined with the long lifetime of the catalyst (>24 h), and the use of the Earth-abundant material iron, are advances in catalyst performance relative to previously reported homogeneous and heterogeneous formate-producing electrocatalysts. We further characterized the mechanism of catalysis by [Fe4N­(CO)12]− using cyclic voltammetry, and structurally characterized a key reaction intermediate, the reduced hydride [HFe4N­(CO)12]−. In addition, thermochemical measurements performed using infrared spectroelectrochemistry provided measures of the hydride donor ability (hydricity) for [HFe4N­(CO)12]− in both MeCN and aqueous buffered solution. These are 49 and 15 kcal mol–1, respectively, and show that the driving force for C–H bond formation with CO2 by [HFe4N­(CO)12]− is very different in the two solvents: +5 kcal mol–1 in MeCN (unfavorable) and −8.5 kcal mol–1 in water (favorable).

Aryl-substituted tetraimine complexes related to Co(dmgBF 2 ) 2 -(MeCN) 2 (dmg = dimethylglyoxime) were synthesized and are active for hydrogen evolution. Co(dmgBF 2 ) 2 (MeCN) 2 can be adsorbed to a glassy carbon electrode. The chemically modified electrode is active for hydrogen evolution in aqueous solution at pH o 4.5, with an overpotential of only 100 mV.Developing efficient hydrogen evolution catalysts composed of earth-abundant materials is of considerable current interest. 1 Recent work has established that cobalt tetraimine macrocyclic complexes such as Co(dmgBF 2 ) 2 (MeCN) 2 (dmg = dimethylglyoxime) (1) 2 can function as efficient electrocatalysts for the production of hydrogen from protons. [3][4][5] These systems are attractive because they operate at low overpotentials and at potentials that are quite positive relative to previously explored Co and Ni macrocycle systems. Nickelphosphine and cobalt systems pioneered by the DuBois lab are of similar interest. 6 Because the ultimate use of such electrocatalysts in a H 2 -evolving device will likely require grafting them to an electrode surface, we sought to ascertain whether these cobalt catalysts would still function once tethered to an electrode, and how their respective stabilities would be impacted. [7][8][9] Related surface studies have utilized a Nafion membrane to obtain a functional electrode-catalyst system. 10 Derivatization of tetraimine-based macrocycles with the form of 1 is not very versatile due to the metal-templated method used in the ligand's synthesis. We have therefore examined a ligand related to a known Schiff base/oxime tetraimine construct, 11 for which the ligand can be preformed before addition of cobalt. Herein, we report that these tetraimine cobalt complexes can be used to prepare chemically modified electrodes from indium tin oxide (ITO)-coated glass that are active for hydrogen evolution. We moreover establish that 1 is a substantially better hydrogen-evolving catalyst in aqueous solution once attached to a glassy carbon (GC) electrode.Access to various aryl-substituted tetraimine complexes related to 1 was achieved via a two-step synthesis involving initial isolation of the proton-linked glyoxime tetraimine followed by generation of the BF 2 -linked analogue (see Fig. 1 for labeling scheme and ESIw for synthetic details). 12 Solid-state structures were obtained for compounds 3-6 and 8 (Fig. S1).w

Selective conversion of formic acid to H2 and CO2 is catalysed by a molecular aluminum complex. Metal–ligand cooperative interactions stabilize a transition state for an outer-sphere β-hydride abstraction mechanism for catalysis.

Selective reactivity of an electrocatalytically generated catalyst-hydride intermediate toward the hydrogen evolution reaction (HER) or reduction of CO(2) is key for a CO(2) reduction electrocatalyst. Under appropriate conditions, Et(4)N[Fe(4)N(CO)(12)] (Et(4)N-1) is a catalyst for the HER or for CO(2) conversion at -1.25 V vs SCE using a glassy carbon electrode.

The design of electrocatalysts that will selectively transfer hydride equivalents to either H(+) or CO2 to afford H2 or formate is a long-standing goal in molecular electrocatalysis. In this Forum Article, we use experimentally determined thermochemical parameters, hydricity and pKa values, to rationalize our observations that the carbide-containing iron carbonyl cluster [Fe4C(CO)12](2-) reduces H(+) to H2 in the presence of CO2 in either acetonitrile (MeCN), MeCN with 5% water, or buffered water (pH 5-13), with no traces of formate or other carbon-containing products observed. Our previous work has shown that the closely related nitride-containing clusters [Fe4N(CO)12](-) and [Fe4N(CO)11(PPh3)](-) will also reduce H(+) to H2 in either MeCN with 5% water or buffered water (pH 5-13), but upon the addition of CO2, they selectively generate formate. The thermochemical measurements on [Fe4C(CO)12](2-) predict that the free energy for transfer of hydride, in MeCN, from the intermediate [HFe4C(CO)12](2-) to CO2 is thermoneutral and to H(+) is -32 kcal mol(-1). In water, these values are less than -19.2 and -8.6 kcal mol(-1), respectively. These results suggest that generation of both H2 and formate should be favorable in aqueous solution and that kinetic effects, such as a fast rate of H2 evolution, must influence the observed selectivity for generation of H2. The hydride-donating ability of [HFe4N(CO)12](-) is lower than that of [HFe4C(CO)12](2-) by 5 kcal mol(-1) in MeCN and by at least 3 kcal mol(-1) in water, and we speculate that this more modest reactivity provides the needed selectivity to obtain formate. We also discuss predictions that may guide future catalyst design.