Non‐Innocent ligand complexes of aluminum are described in this Concept article, beginning with a discussion of their synthesis, and then structural and electronic characterization. The main focus ...concerns the ability of the ligands in these complexes to mediate proton transfer reactions. As examples, aluminum–ligand cooperation in the activation of polar bonds is described, as is the importance of hydrogen bonding to stabilization of a transition state for β‐hydride ion. Taken together these reactions enable catalytic processes such as the dehydrogenation of formic acid.
Al in tandem with cooperative ligands: Aluminum complexes of non‐innocent ligands participate in various reactions where metal–ligand cooperation promotes proton transfer to and from substrates. The properties of these complexes are outlined, followed by a discussion of OH and NH bond activation and the catalysis of dehydrogenation reactions that can result from the bond activation step.
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 pK a values, to rationalize our observations that the carbide-containing iron carbonyl cluster Fe4C(CO)122– 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)122– predict that the free energy for transfer of hydride, in MeCN, from the intermediate HFe4C(CO)122– 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)122– 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.
Environmentally sustainable hydrogen‐evolving electrocatalysts are key in a renewable fuel economy, and ligand‐based proton and electron transfer could circumvent the need for precious metal ions in ...electrocatalytic H2 production. Herein, we show that electrocatalytic generation of H2 by a redox‐active ligand complex of Al3+ occurs at −1.16 V vs. SCE (500 mV overpotential).
Two in one: Proton and electron transfer by a complex comprising Al3+ and a redox‐active iminopyridine ligand promotes electrocatalytic H2 evolution. The Al3+ center brings the reduction potential of the ligand into an accessible range for low‐overpotential proton production. The proposed mechanism involves two protonation events at the ligand and a subsequent two‐electron reduction to liberate hydrogen (see figure).
Activation of N–H bonds by a molecular aluminum complex via metal–ligand cooperation is described. (PhI2P2–)AlH (1b), in which PhI2P2– is a tridentate bis(imino)pyridine ligand, reacts with anilines ...to give the N–H-activated products (PhHI2P–)AlH(NHAr) (2). When heated, 2 releases H2 and affords (PhI2P–)Al(NHAr) (3). Complex 1b catalyzes the dehydrogenative coupling of benzylamine to afford H2, NH3, and N-(phenylmethylene)benzenemethanamine.
Pre-equilibrium reaction kinetics enable the overall rate of a catalytic reaction to be orders of magnitude faster than the rate-determining step. Herein, we demonstrate how pre-equilibrium kinetics ...can be applied to breaking the linear free-energy relationship (LFER) for electrocatalysis, leading to rate enhancement 5 orders of magnitude and lowering of overpotential to approximately thermoneutral. This approach is applied to pre-equilibrium formation of a metal-hydride intermediate to achieve fast formate formation rates from CO2 reduction without loss of selectivity (i.e., H2 evolution). Fast pre-equilibrium metal-hydride formation, at 108 M–1 s–1, boosts the CO2 electroreduction to formate rate up to 296 s–1. Compared with molecular catalysts that have similar overpotential, this rate is enhanced by 5 orders of magnitude. As an alternative comparison, overpotential is lowered by ∼50 mV compared to catalysts with a similar rate. The principles elucidated here to obtain pre-equilibrium reaction kinetics via catalyst design are general. Design and development that builds on these principles should be possible in both molecular homogeneous and heterogeneous electrocatalysis.
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).
Conspectus 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 H2. Our work with a unique series of water-soluble and -stable, low-valent 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)122–. 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 H2 is observed in this case. In contrast, isoelectronic Fe4C(CO)122– features a concerted proton–electron transfer mechanism to form H–Fe4C(CO)122–, which is selective for H2 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 H2 production but strong enough to make formate. H–Fe4C(CO)122– 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 H2 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–1 where C–H bond formation may be favored in water or MeCN, respectively.
The impact of cationic and Lewis acidic functional groups installed in the primary or secondary coordination sphere (PCS or SCS) of an (electro)catalyst is known to vary depending on the precise ...positioning of those groups. However, it is difficult to systematically probe the effect of that position. In this report, we probe the effect of the functional group position and identity on the observed reduction potentials (E p,c) using substituted iron clusters, Fe4N(CO)11R n , where R = NO+, PPh2-CH2CH2-9BBN, (MePTA+)2, (MePTA+)4, and H+ and n = 0, −1, +1, or +3 (9-BBN is 9-borabicyclo(3.3.1)nonane; MePTA+ is 1-methyl-1-azonia-3,5-diaza-7-phosphaadamantane). The cationic NO+ and H+ ligands cause anodic shifts of 700 and 320 mV, respectively, in E p,c relative to unsubstituted Fe4N(CO)12−. Infrared absorption band data, νCO, suggests that some of the 700 mV shift by NO+ results from electronic changes to the cluster core. This contrasts with the effects of cationic MePTA+ and H+ which cause primarily electrostatic effects on E p,c. Lewis acidic 9-BBN in the SCS had almost no effect on E p,c.
Selective reactivity of an electrocatalytically generated catalyst–hydride intermediate toward the hydrogen evolution reaction (HER) or reduction of CO2 is key for a CO2 reduction electrocatalyst. ...Under appropriate conditions, Et4NFe4N(CO)12 (Et4N-1) is a catalyst for the HER or for CO2 conversion at −1.25 V vs SCE using a glassy carbon electrode.
An exploration of secondary coordination sphere (SCS) functional groups is presented with a focus on proton transport to a metal hydride active site for H2 formation and transport of CO2 so that ...formate can be obtained. In MeCN–H2O, pK a(AH) and steric bulk of the SCS groups are discussed along with their influence on each step in the mechanism for CO2 to formate catalysis and along with the influence of the proton source, which is MeCN–H2O or (MeCN)2H2O in MeCN–H2O (95:5) under N2 atmosphere. Under CO2, carbonic acid is also available. Catalysts containing various SCS groups were synthesized from Fe4N(CO)12− and have the form Fe4N(CO)11L− where L is Ph2P-SCS. Hydride formation rates are distinct under N2 versus CO2, and that variation is dependent on the size of the SCS group. Under CO2, larger SCS groups inhibit access of the MeCN–H2O 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 H2 evolution, whereas larger N,N-dimethylaniline supports selective formate formation by slowing the transport of large MeCN–H2O adducts, allowing hydride transfer to the smaller substrate CO2.
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