Bio-inspired mechanistic insights into CO2 reduction

Molecular Cobalt Complexes with Pendant Amines for Selective Electrocatalytic Reduction of Carbon Dioxide to Formic Acid

We report here on a new series of CO₂-reducing molecular catalysts based on Earth-abundant elements that are very selective for the production of formic acid in dimethylformamide (DMF)/water mixtures (Faradaic efficiency of 90 ± 10%) at moderate overpotentials (500-700 mV in DMF measured at the middle of the catalytic wave). The [CpCo(P^R₂N^R'₂)I]⁺ compounds contain diphosphine ligands, P^R₂N^R'₂, with two pendant amine residues that act as proton relays during CO₂-reduction catalysis and tune their activity. Four different P^R₂N^R'₂ ligands with cyclohexyl or phenyl substituents on phosphorus and benzyl or phenyl substituents on nitrogen were employed, and the compound with the most electron-donating phosphine ligand and the most basic amine functions performs best among the series, with turnover frequency >1000 s^-1. State-of-the-art benchmarking of catalytic performances ranks this new class of cobalt-based complexes among the most promising CO₂-to-formic acid reducing catalysts developed to date; addressing the stability issues would allow further improvement. Mechanistic studies and density functional theory simulations confirmed the role of amine groups for stabilizing key intermediates through hydrogen bonding with water molecules during hydride transfer from the Co center to the CO₂ molecule.

A bio-inspired zinc finger analogue anchored in 2D hexagonal mesoporous silica for room temperature CO2activation via a hydrogenocarbonate route

A room temperature hydrogenocarbonate intermediate in CO₂activation by a carbonic anhydrase active site analog inserted in the nanopores of mesostructured porous silicas.

CO2 Reduction Catalyzed by Nitrogenase: Pathways to Formate, Carbon Monoxide, and Methane

The reduction of N2 to NH3 by Mo-dependent nitrogenase at its active-site metal cluster FeMo-cofactor utilizes reductive elimination of Fe-bound hydrides with obligatory loss of H2 to activate the enzyme for binding/reduction of N2. Earlier work showed that wild-type nitrogenase and a nitrogenase with amino acid substitutions in the MoFe protein near FeMo-cofactor can catalytically reduce CO2 by two or eight electrons/protons to carbon monoxide (CO) and methane (CH4) at low rates. Here, it is demonstrated that nitrogenase preferentially reduces CO2 by two electrons/protons to formate (HCOO(-)) at rates >10 times higher than rates of CO2 reduction to CO and CH4. Quantum mechanical calculations on the doubly reduced FeMo-cofactor with a Fe-bound hydride and S-bound proton (E2(2H) state) favor a direct reaction of CO2 with the hydride ("direct hydride transfer" reaction pathway), with facile hydride transfer to CO2 yielding formate. In contrast, a significant barrier is observed for reaction of Fe-bound CO2 with the hydride ("associative" reaction pathway), which leads to CO and CH4. Remarkably, in the direct hydride transfer pathway, the Fe-H behaves as a hydridic hydrogen, whereas in the associative pathway it acts as a protic hydrogen. MoFe proteins with amino acid substitutions near FeMo-cofactor (α-70(Val→Ala), α-195(His→Gln)) are found to significantly alter the distribution of products between formate and CO/CH4.

Reduction of Carbon Dioxide by a Molybdenum-Containing Formate Dehydrogenase: A Kinetic and Mechanistic Study

Carbon dioxide accumulation is a major concern for the ecosystems, but its abundance and low cost make it an interesting source for the production of chemical feedstocks and fuels. However, the thermodynamic and kinetic stability of the carbon dioxide molecule makes its activation a challenging task. Studying the chemistry used by nature to functionalize carbon dioxide should be helpful for the development of new efficient (bio)catalysts for atmospheric carbon dioxide utilization. In this work, the ability of Desulfovibrio desulfuricans formate dehydrogenase (Dd FDH) to reduce carbon dioxide was kinetically and mechanistically characterized. The Dd FDH is suggested to be purified in an inactive form that has to be activated through a reduction-dependent mechanism. A kinetic model of a hysteretic enzyme is proposed to interpret and predict the progress curves of the Dd FDH-catalyzed reactions (initial lag phase and subsequent faster phase). Once activated, Dd FDH is able to efficiently catalyze, not only the formate oxidation (kcat of 543 s(-1), Km of 57.1 μM), but also the carbon dioxide reduction (kcat of 46.6 s(-1), Km of 15.7 μM), in an overall reaction that is thermodynamically and kinetically reversible. Noteworthy, both Dd FDH-catalyzed formate oxidation and carbon dioxide reduction are completely inactivated by cyanide. Current FDH reaction mechanistic proposals are discussed and a different mechanism is here suggested: formate oxidation and carbon dioxide reduction are proposed to proceed through hydride transfer and the sulfo group of the oxidized and reduced molybdenum center, Mo(6+)═S and Mo(4+)-SH, are suggested to be the direct hydride acceptor and donor, respectively.

Toward Rational Design of 3d Transition Metal Catalysts for CO2 Hydrogenation Based on Insights into Hydricity-Controlled Rate-Determining Steps

Carbon dioxide functionalization attracts much interest due to the current environmental and energy challenges. Our earlier work (Mondal, B.; Neese, F.; Ye, S. Inorg. Chem. 2015, 54, 7192-7198) demonstrated that CO2 hydrogenation mediated by base metal catalysts [M(H)(η(2)-H2)(PP3(Ph))](n+) (M = Co(III) and Fe(II), n = 1, 2; PP3(Ph) = tris(2-(diphenylphosphino)phenyl)phosphine) features discrete rate-determining steps (RDSs). Specifically, the reaction with [Co(III)(H)(η(2)-H2)(PP3(Ph))](2+) passes through a hydride-transfer RDS, whereas the conversion with [Fe(II)(H)(η(2)-H2)(PP3(Ph))](+) traverses a H2-splitting RDS. More importantly, we found that the nature and barrier of the RDS likely correlate with the hydride affinity or hydricity of the dihydride intermediate [M(H)2(PP3(Ph))]((n-1)+) generated by H2-splitting. In the present contribution, following this notion we design a series of potential Fe(II) and Co(III) catalysts, for which the respective dihydride species possess differential hydricities, and computationally investigated their reactivity toward CO2 hydrogenation. Our results reveal that lowering the hydrictiy of [Co(III)(H)2(PP3(Ph))](+) by introducing anionic anchors in PP3(Ph) dramatically decreases the hydride-transfer RDS barrier, as shown for the enhanced reactivity of [Co(H)(η(2)-H2)(CP3(Ph))](+) and [Co(H)(η(2)-H2)(SiP3(Ph))](+) (CP3(Ph) = tris(2-(diphenylphosphino)phenyl)methyl, SiP3(Ph) = tris(2-(diphenylphosphino)phenyl)silyl), while the same ligand modification increases the H2-splitting RDS barriers for [Fe(H)(η(2)-H2)(CP3(Ph))] and [Fe(H)(η(2)-H2)(SiP3(Ph))] relative to that for [Fe(H)(η(2)-H2)(PP3(Ph))](+). Conversely, upon increasing the hydricity of [Fe(II)(H)2(PP3(Ph))] by adding an electron-withdrawing group to PP3(Ph), the transformation with [Fe(H)(η(2)-H2)(PP3(PhNO2))](+) (PP3(PhNO2) = tris(2-(diphenylphosphino)-4-nitrophenyl)phosphine) is predicted to encounter a lower barrier for H2-splitting and a higher barrier for hydride transfer than those for [Fe(H)(η(2)-H2)(PP3(Ph))](+). Thus, we have shown that hydricity can be used as a guide to direct the rational design and development of more efficient catalysts.

Concurrent electrochemical CO2 reduction to HCOOH and methylene blue removal on metal electrodes

Experimental setup for CO₂ reduction and MB removal.

A review of harvesting clean fuels from enzymatic CO2 reduction

This review has summarised single enzyme, multi enzymatic and semiconducting nanomaterial integrated enzymatic systems for CO₂ conversion to clean fuels.

Catalytic Reduction of CO2 by Renewable Organohydrides

Dihydropyridines are renewable organohydride reducing agents for the catalytic reduction of CO2 to MeOH. Here we discuss various aspects of this important reduction. A centerpiece, which illustrates various general principles, is our theoretical catalytic mechanism for CO2 reduction by successive hydride transfers (HTs) and proton transfers (PTs) from the dihydropyridine PyH2 obtained by 1H(+)/1e(-)/1H(+)/1e(-) reductions of pyridine. The Py/PyH2 redox couple is analogous to NADP(+)/NADPH in that both are driven to effect HTs by rearomatization. High-energy radical intermediates and their associated high barriers/overpotentials are avoided because HT involves a 2e(-) reduction. A HT-PT sequence dictates that the reduced intermediates be protonated prior to further reduction for ultimate MeOH formation; these protonations are aided by biased cathodes that significantly lower the local pH. In contrast, cathodes that efficiently reduce H(+) such as Pt and Pd produce H2 and create a high interfacial pH, both obstructing dihydropyridine production and formate protonation and thus ultimately CO2 reduction by HTPTs. The role of water molecule proton relays is discussed. Finally, we suggest future CO2 reduction strategies by organic (photo)catalysts.

Control in the Rate-Determining Step Provides a Promising Strategy To Develop New Catalysts for CO2 Hydrogenation: A Local Pair Natural Orbital Coupled Cluster Theory Study

The development of efficient catalysts with base metals for CO2 hydrogenation has always been a major thrust of interest. A series of experimental and theoretical work has revealed that the catalytic cycle typically involves two key steps, namely, base-promoted heterolytic H2 splitting and hydride transfer to CO2, either of which can be the rate-determining step (RDS) of the entire reaction. To explore the determining factor for the nature of RDS, we present herein a comparative mechanistic investigation on CO2 hydrogenation mediated by [M(H)(η(2)-H2)(PP3(Ph))](n+) (M = Fe(II), Ru(II), and Co(III); PP3(Ph) = tris(2-(diphenylphosphino)phenyl)phosphine) type complexes. In order to construct reliable free energy profiles, we used highly correlated wave function based ab initio methods of the coupled cluster type alongside the standard density functional theory. Our calculations demonstrate that the hydricity of the metal-hydride intermediate generated by H2 splitting dictates the nature of the RDS for the Fe(II) and Co(III) systems, while the RDS for the Ru(II) catalyst appears to be ambiguous. CO2 hydrogenation catalyzed by the Fe(II) complex that possesses moderate hydricity traverses an H2-splitting RDS, whereas the RDS for the high-hydricity Co(III) species is found to be the hydride transfer. Thus, our findings suggest that hydricity can be used as a practical guide in future catalyst design. Enhancing the electron-accepting ability of low-hydricity catalysts is likely to improve their catalytic performance, while increasing the electron-donating ability of high-hydricity complexes may speed up CO2 conversion. Moreover, we also established the active roles of base NEt3 in directing the heterolytic H2 splitting and assisting product release through the formation of an acid-base complex.

Bioinspired Tungsten Dithiolene Catalysts for Hydrogen Evolution: A Combined Electrochemical, Photochemical, and Computational Study

Bis(dithiolene)tungsten complexes, W(VI)O2 (L = dithiolene)2 and W(IV)O (L = dithiolene)2, which mimic the active site of formate dehydrogenases, have been characterized by cyclic voltammetry and controlled potential electrolysis in acetonitrile. They are shown to be able to catalyze the electroreduction of protons into hydrogen in acidic organic media, with good Faradaic yields (75-95%) and good activity (rate constants of 100 s(-1)), with relatively high overpotentials (700 mV). They also catalyze proton reduction into hydrogen upon visible light irradiation, in combination with [Ru(bipyridine)3](2+) as a photosensitizer and ascorbic acid as a sacrificial electron donor. On the basis of detailed DFT calculations, a reaction mechanism is proposed in which the starting W(VI)O2 (L = dithiolene)2 complex acts as a precatalyst and hydrogen is further formed from a key reduced W-hydroxo-hydride intermediate.

Sulfido and cysteine ligation changes at the molybdenum cofactor during substrate conversion by formate dehydrogenase (FDH) from Rhodobacter capsulatus

Formate dehydrogenase (FDH) enzymes are attractive catalysts for potential carbon dioxide conversion applications. The FDH from Rhodobacter capsulatus (RcFDH) binds a bis-molybdopterin-guanine-dinucleotide (bis-MGD) cofactor, facilitating reversible formate (HCOO(-)) to CO2 oxidation. We characterized the molecular structure of the active site of wildtype RcFDH and protein variants using X-ray absorption spectroscopy (XAS) at the Mo K-edge. This approach has revealed concomitant binding of a sulfido ligand (Mo=S) and a conserved cysteine residue (S(Cys386)) to Mo(VI) in the active oxidized molybdenum cofactor (Moco), retention of such a coordination motif at Mo(V) in a chemically reduced enzyme, and replacement of only the S(Cys386) ligand by an oxygen of formate upon Mo(IV) formation. The lack of a Mo=S bond in RcFDH expressed in the absence of FdsC implies specific metal sulfuration by this bis-MGD binding chaperone. This process still functioned in the Cys386Ser variant, showing no Mo-S(Cys386) ligand, but retaining a Mo=S bond. The C386S variant and the protein expressed without FdsC were inactive in formate oxidation, supporting that both Mo-ligands are essential for catalysis. Low-pH inhibition of RcFDH was attributed to protonation at the conserved His387, supported by the enhanced activity of the His387Met variant at low pH, whereas inactive cofactor species showed sulfido-to-oxo group exchange at the Mo ion. Our results support that the sulfido and S(Cys386) ligands at Mo and a hydrogen-bonded network including His387 are crucial for positioning, deprotonation, and oxidation of formate during the reaction cycle of RcFDH.