Highly Selective Electrocatalytic CO2 Reduction by [Pt(dmpe)2]2+ through Kinetic and Thermodynamic Control

Circumventing Kinetic Barriers to Metal Hydride Formation with Metal-Ligand Cooperativity

We report the two-electron, one-proton mechanism of cobalt hydride formation for the conversion of [CoIIICp(PPh2NBn2)(CH3CN)]2+ to [HCoIIICp(PPh2NBn2)]+. This complex catalytically converts CO2 to formate under CO2 reduction conditions, with hydride formation as a key elementary step. Through a combination of electrochemical measurements, digital simulations, theoretical calculations, and additional mechanistic and thermochemical studies, we outline the explicit role of the PPh2NBn2 ligand in the proton-coupled electron transfer (PCET) reactivity that leads to hydride formation. We reveal three unique PCET mechanisms, and we show that the amine on the PPh2NBn2 ligand serves as a kinetically accessible protonation site en route to the thermodynamically favored cobalt hydride. Cyclic voltammograms recorded with proton sources that span a wide range of pKa values show four distinct regimes where the mechanism changes as a function of acid strength, acid concentration, and timescale between electrochemical steps. Peak shift analysis was used to determine proton transfer rate constants where applicable. This work highlights the astute choices that must be made when designing catalytic systems, including the basicity and kinetic accessibility of protonation sites, acid strength, acid concentration, and timescale between electron transfer steps, to maximize catalyst stability and efficiency.

Understanding Ligand Effects on Bielectronic Transitions: Chemo- and Electroreduction of Rhodium Bis(Diphosphine) Complexes to Low Oxidation States

Rhodium complexes in the -I and 0 oxidation states are of great potential interest in catalytic applications. In contrast to their rhodium +I congeners, however, the structural and electronic parameters governing their access and stability are far less understood. Herein, we investigate the two-electron reduction of a parameterized series of bis(diphosphine) Rh complexes [Rh(dxpy)2]NTf2 (x=P-substituent, y=alkanediyl bridging P atoms). Through (electro)reductions from the RhI parents, Rh-I d10-complexes were obtained and characterized spectroscopically, including 103Rh NMR data. The reductive steps convolute with structural rearrangements from square planar to tetrahedral coordination. We found that the extent of these reorganisations defines whether the first E0(RhI/0) and second E0(Rh0/-I) reduction potentials are normally ordered, leading to monoelectronic stepwise transitions, or inverted, giving bielectronic events. Reductionist approaches based on Hammett parameters or the P-Rh-P bite angles provide only partial correlations with the redox potentials. However, we identified the C-O stretch of analogue diphosphine complexes as an expedient computational parameter that enables these correlations through both electronic and geometric features, even in a predictive manner. Gaining control over two-electron reduction behaviors through rationalized ligand effects has potential impact beyond Rh complexes, for molecular and enzymatic metal sites commonly exhibiting bielectronic transitions.

Electrocatalytic CO2 reduction to formate by a cobalt phosphino-thiolate complex

Electrochemical conversion of CO2 to value-added products serves as an attractive method to store renewable energy as energy-dense fuels. Selectivity in this type of conversion can be limited, often leading to the formation of side products such as H2. The activity of a cobalt phosphino-thiolate complex ([Co(triphos)(bdt)]+) towards the selective reduction of CO2 to formate is explored in this report. In the presence of H2O, selective production of formate (as high as 94%) is observed at overpotentials of 750 mV, displaying negligible current degradation during long-term electrolysis experiments ranging as long as 24 hours. Chemical reduction studies of [Co(triphos)(bdt)]+ indicates deligation of the apical phosphine moiety is likely before catalysis. Computational and experimental results suggest a metal-hydride pathway, indicating an ECEC based mechanism.

Bioinspired Binickel Catalyst for Carbon Dioxide Reduction: The Importance of Metal-ligand Cooperation

Catalyst design for the efficient CO2 reduction reaction (CO2RR) remains a crucial challenge for the conversion of CO2 to fuels. Natural Ni-Fe carbon monoxide dehydrogenase (NiFe-CODH) achieves reversible conversion of CO2 and CO at nearly thermodynamic equilibrium potential, which provides a template for developing CO2RR catalysts. However, compared with the natural enzyme, most biomimetic synthetic Ni-Fe complexes exhibit negligible CO2RR catalytic activities, which emphasizes the significance of effective bimetallic cooperation for CO2 activation. Enlightened by bimetallic synergy, we herein report a dinickel complex, NiIINiII(bphpp)(AcO)2 (where NiNi(bphpp) is derived from H2bphpp = 2,9-bis(5-tert-butyl-2-hydroxy-3-pyridylphenyl)-1,10-phenanthroline) for electrocatalytic reduction of CO2 to CO, which exhibits a remarkable reactivity approximately 5 times higher than that of the mononuclear Ni catalyst. Electrochemical and computational studies have revealed that the redox-active phenanthroline moiety effectively modulates the electron injection and transfer akin to the [Fe3S4] cluster in NiFe-CODH, and the secondary Ni site facilitates the C-O bond activation and cleavage through electron mediation and Lewis acid characteristics. Our work underscores the significant role of bimetallic cooperation in CO2 reduction catalysis and provides valuable guidance for the rational design of CO2RR catalysts.

Further Understanding the Roles of Solvent, Brønsted Acids, and Hydrogen Bonding in Iron Porphyrin-Mediated Carbon Dioxide Reduction

Improving our understanding of how molecules and materials mediate the electrochemical reduction of carbon dioxide (CO2) to upgraded products is of great interest as a means to address climate change. A leading class of molecules that can facilitate the electrochemical conversion of CO2 to carbon monoxide (CO) is iron porphyrins. These molecules can have high rate constants for CO2-to-CO conversion; they are robust, and they rely on abundant and inexpensive synthetic building blocks. Important foundational work has been conducted using chloroiron 5,10,15,20-tetraphenylporphyrin (FeTPPCl) in N,N-dimethylformamide (DMF) solvent. A related and recent report points out that the corresponding perchlorate complex, FeTPPClO4, can have superior function due to its solubility in other organic solvents. However, the importance of hydrogen bonding and solvent effects was not discussed. Herein, we present a detailed kinetic study of the triflate (CF3SO3-) complex of FeTPP in DMF and in MeCN using a range of phenol Brønsted acid additives. We also detected the formation of Fe(III)TPP-phenolate complexes using cyclic voltammetry experiments. Importantly, our new analysis of apparent rate constants with different added phenols allows for a modification to the established mechanistic model for CO2-to-CO conversion. Critically, our improved model accounts for hydrogen bonding and solvent effects by using simple hydrogen bond acidity and basicity descriptors. We use this augmented model to rationalize function in other reported porphyrin systems and to make predictions about operational conditions that can enhance the CO2 reduction chemistry.

Proton transfer kinetics of transition metal hydride complexes and implications for fuel-forming reactions

Proton transfer reactions involving transition metal hydride complexes are prevalent in a number of catalytic fuel-forming reactions, where the proton transfer kinetics to or from the metal center can have significant impacts on the efficiency, selectivity, and stability associated with the catalytic cycle. This review correlates the often slow proton transfer rate constants of transition metal hydride complexes to their electronic and structural descriptors and provides perspective on how to exploit these parameters to control proton transfer kinetics to and from the metal center. A toolbox of techniques for experimental determination of proton transfer rate constants is discussed, and case studies where proton transfer rate constant determination informs fuel-forming reactions are highlighted. Opportunities for extending proton transfer kinetic measurements to additional systems are presented, and the importance of synergizing the thermodynamics and kinetics of proton transfer involving transition metal hydride complexes is emphasized.

Mechanism and Selectivity of Electrochemical Reduction of CO2 on Metalloporphyrin Catalysts from DFT Studies

Electrochemical reduction of CO₂ to value-added chemicals has been hindered by poor product selectivity and competition from hydrogen evolution reactions. This study aims to unravel the origin of the product selectivity and competitive hydrogen evolution reaction on [MP]⁰ catalysts (M = Fe, Co, Rh and Ir; P is porphyrin ligand) by analyzing the mechanism of CO₂ reduction and H₂ formation based on the results of density functional theory calculations. Reduction of CO₂ to CO and HCOO^- proceeds via the formation of carboxylate adduct ([MP-COOH]⁰ and ([MP-COOH]^-) and metal-hydride [MP-H]^-, respectively. Competing proton reduction to gaseous hydrogen shares the [MP-H]^- intermediate. Our results show that the pK_a of [MP-H]⁰ can be used as an indicator of the CO or HCOO^-/H₂ preference. Furthermore, an ergoneutral pH has been determined and used to determine the minimum pH at which selective CO₂ reduction to HCOO^- becomes favorable over the H₂ production. These analyses allow us to understand the product selectivity of CO₂ reduction on [FeP]⁰, [CoP]⁰, [RhP]⁰ and [IrP]⁰; [FeP]⁰ and [CoP]⁰ are selective for CO whereas [RhP]⁰ and [IrP]⁰ are selective for HCOO^- while suppressing H₂ formation. These descriptors should be applicable to other catalysts in an aqueous medium.

Biomimetic Metal-Free Hydride Donor Catalysts for CO2 Reduction

The catalytic reduction of carbon dioxide to fuels and value-added chemicals is of significance for the development of carbon recycling technologies. One of the main challenges associated with catalytic CO₂ reduction is product selectivity: the formation of carbon monoxide, molecular hydrogen, formate, methanol, and other products occurs with similar thermodynamic driving forces, making it difficult to selectively reduce CO₂ to the target product. Significant scientific effort has been aimed at the development of catalysts that can suppress the undesired hydrogen evolution reaction and direct the reaction toward the selective formation of the desired products, which are easy to handle and store. Inspired by natural photosynthesis, where the CO₂ reduction is achieved using NADPH cofactors in the Calvin cycle, we explore biomimetic metal-free hydride donors as catalysts for the selective reduction of CO₂ to formate. Here, we outline our recent findings on the thermodynamic and kinetic parameters that control the hydride transfer from metal-free hydrides to CO₂. By experimentally measuring and theoretically calculating the thermodynamic hydricities of a range of metal-free hydride donors, we derive structural and electronic factors that affect their hydride-donating abilities. Two dominant factors that contribute to the stronger hydride donors are identified to be (i) the stabilization of the positive charge formed upon HT via aromatization or by the presence of electron-donating groups and (ii) the destabilization of hydride donors through the anomeric effect or in the presence of significant structural constrains in the hydride molecule. Hydride donors with appropriate thermodynamic hydricities were reacted with CO₂, and the formation of the formate ion (the first reduction step in CO₂ reduction to methanol) was confirmed experimentally, providing an important proof of principle that organocatalytic CO₂ reduction is feasible. The kinetics of hydride transfer to CO₂ were found to be slow, and the sluggish kinetics were assigned in part to the large self-exchange reorganization energy associated with the organic hydrides in the DMSO solvent. Finally, we outline our approaches to the closure of the catalytic cycle via the electrochemical and photochemical regeneration of the hydride (R-H) from the conjugate hydride acceptors (R⁺). We illustrate how proton-coupled electron transfer can be efficiently utilized not only to lower the electrochemical potential at which the hydride regeneration takes place but also to suppress the unwanted dimerization that neutral radical intermediates tend to undergo. Overall, this account provides a summary of important milestones achieved in organocatalytic CO₂ reduction and provides insights into the future research directions needed for the discovery of inexpensive catalysts for carbon recycling.

Activating the Fe(I) State of Iron Porphyrinoid with Second-Sphere Proton Transfer Residues for Selective Reduction of CO2 to HCOOH via Fe(III/II)-COOH Intermediate(s)

The ability to tune the selectivity of CO₂ reduction by first-row transition metal-based complexes via the inclusion of second-sphere effects heralds exciting and sought-after possibilities. On the basis of the mechanistic understanding of CO₂ reduction by iron porphyrins developed by trapping and characterizing the intermediates involved ( J. Am. Chem. Soc. 2015, 137, 11214), a porphyrinoid ligand is envisaged to switch the selectivity of the iron porphyrins by reducing CO₂ from CO to HCOOH as well as lower the overpotential to the process. The results show that the iron porphyrinoid designed can catalyze the reduction of CO₂ to HCOOH using water as the proton source with 97% yield with no detectable H₂ or CO. The iron porphyrinoid can activate CO₂ in its Fe(I) state resulting in very low overpotential for CO₂ reduction in contrast to all reported iron porphyrins, which can reduce CO₂ in their Fe(0) state. Intermediates involved in CO₂ reduction, Fe(III)-COOH and a Fe(II)-COOH, are identified with in situ FTIR-SEC and subsequently chemically generated and characterized using FTIR, resonance Raman, and Mössbauer spectroscopy. The mechanism of the reaction helps elucidate a key role played by a closely placed proton transfer residue in aiding CO₂ binding to Fe(I), stabilizing the intermediates, and determining the fate of a rate-determining Fe(II)-COOH intermediate.

Electrochemical reduction of CO2 to CO and HCOO- using metal-cyclam complex catalysts: predicting selectivity and limiting potential from DFT

Sustainable fuel production from CO₂ through electrocatalytic reduction is promising but challenging due to high overpotential and poor product selectivity. Herein, we computed the reaction free energies of electrocatalytic reduction of CO₂ to CO and HCOO^- using the density functional theory method and screened transition metal(M)-cyclam(L) complexes as molecular catalysts for CO₂ reduction. Our results showed that pK_a of the proton adduct formed by the protonation of the reduced metal center can be used as a descriptor to select the operating pH of the solution to steer the reaction toward either the CO or hydride cycle. Among the complexes, [LNi]²⁺ and [LPd]²⁺ catalyze the reactions by following the CO cycle and are the CO selective catalysts in the pH ranges 1.81-7.31 and 6.10 and higher, respectively. Among the complexes that catalyze the reactions by following the hydride cycle, [LMo]²⁺ and [LW]³⁺ are HCOO^- selective catalysts and have low limiting potentials of -1.33 V and -1.54 V, respectively. Other complexes, including [LRh]²⁺, [LIr]²⁺, [LW]²⁺, [LCo]²⁺, and [LTc]²⁺ catalyze the reactions resulting in either HCOO^- from CO₂ reduction or H₂ from proton reduction; however, HCOO^- formation is always thermodynamically more favorable. Notably, [LMo]²⁺, [LW]³⁺, [LW]²⁺ and [LCo]²⁺ have limiting potentials less negative than -1.6 V and are based on Earth-abundant elements, making them attractive for practical application.

Transition Metal Complexes as Catalysts for the Electroconversion of CO2 : An Organometallic Perspective

The electrocatalytic transformation of carbon dioxide has been a topic of interest in the field of CO2 utilization for a long time. Recently, the area has seen increasing dynamics as an alternative strategy to catalytic hydrogenation for CO2 reduction. While many studies focus on the direct electron transfer to the CO2 molecule at the electrode material, molecular transition metal complexes in solution offer the possibility to act as catalysts for the electron transfer. C1 compounds such as carbon monoxide, formate, and methanol are often targeted as the main products, but more elaborate transformations are also possible within the coordination sphere of the metal center. This perspective article will cover selected examples to illustrate and categorize the currently favored mechanisms for the electrochemically induced transformation of CO2 promoted by homogeneous transition metal complexes. The insights will be corroborated with the concepts and elementary steps of organometallic catalysis to derive potential strategies to broaden the molecular diversity of possible products.

Kinetics of Hydride Transfer from Catalytic Metal-Free Hydride Donors to CO2

Selective reduction of CO₂ to formate represents an ongoing challenge in photoelectrocatalysis. To provide mechanistic insights, we investigate the kinetics of hydride transfer (HT) from a series of metal-free hydride donors to CO₂. The observed dependence of experimental and calculated HT barriers on the thermodynamic driving force was modeled by using the Marcus hydride transfer formalism to obtain the insights into the effect of reorganization energies on the reaction kinetics. Our results indicate that even if the most ideal hydride donor were discovered, the HT to CO₂ would exhibit sluggish kinetics (<100 turnovers per second at -0.1 eV driving force), indicating that the conventional HT may not be an appropriate mechanism for solar conversion of CO₂ to formate. We propose that the conventional HT mechanism should not be considered for CO₂ reduction catalysis and argue that the orthogonal HT mechanism, previously proposed to address thermodynamic limitations of this reaction, may also lead to lower kinetic barriers for CO₂ reduction to formate.

Kinetic and mechanistic analysis of a synthetic reversible CO2/HCO2- electrocatalyst

[Pt(depe)2](PF6)2 electrocatalyzes the reversible conversion between CO2 and HCO2- with high selectivity and low overpotential but low rates. A comprehensive kinetic analysis indicates the rate determining step for CO2 reduction is the reactivity of a Pt hydride intermediate to produce HCO2-. To accelerate catalysis, the use of cationic and hydrogen-bond donor additives are explored.

Thermodynamic and kinetic hydricity of transition metal hydrides

This review of thermodynamic and kinetic hydricity provides conceptual overviews, tutorials on how to determine hydricity both experimentally and computationally, and salient case studies.