Selectivity for HCO2– over H2 in the Electrochemical Catalytic Reduction of CO2 by (POCOP)IrH2

Quantum chemical package Jaguar: A survey of recent developments and unique features

This paper is dedicated to the quantum chemical package Jaguar, which is commercial software developed and distributed by Schrödinger, Inc. We discuss Jaguar's scientific features that are relevant to chemical research as well as describe those aspects of the program that are pertinent to the user interface, the organization of the computer code, and its maintenance and testing. Among the scientific topics that feature prominently in this paper are the quantum chemical methods grounded in the pseudospectral approach. A number of multistep workflows dependent on Jaguar are covered: prediction of protonation equilibria in aqueous solutions (particularly calculations of tautomeric stability and pKa), reactivity predictions based on automated transition state search, assembly of Boltzmann-averaged spectra such as vibrational and electronic circular dichroism, as well as nuclear magnetic resonance. Discussed also are quantum chemical calculations that are oriented toward materials science applications, in particular, prediction of properties of optoelectronic materials and organic semiconductors, and molecular catalyst design. The topic of treatment of conformations inevitably comes up in real world research projects and is considered as part of all the workflows mentioned above. In addition, we examine the role of machine learning methods in quantum chemical calculations performed by Jaguar, from auxiliary functions that return the approximate calculation runtime in a user interface, to prediction of actual molecular properties. The current work is second in a series of reviews of Jaguar, the first having been published more than ten years ago. Thus, this paper serves as a rare milestone on the path that is being traversed by Jaguar's development in more than thirty years of its existence.

Electrocatalytic Transfer Hydrogenation of 1-Octene with [(tBuPCP)Ir(H)(Cl)] and Water

Electrocatalytic hydrogenation of 1-octene as non-activated model substrate with neutral water as H-donor is reported, using [(tBuPCP)Ir(H)(Cl)] (1) as the catalyst, to form octane with high faradaic efficiency (FE) of 96 % and a kobs of 87 s-1. Cyclic voltammetry with 1 revealed that two subsequent reductions trigger the elimination of Cl- and afford the highly reactive anionic Ir(I) hydride complex [(tBuPCP)Ir(H)]- (2), a previously merely proposed intermediate for which we now report first experimental data by mass spectrometry. In absence of alkene, the stoichiometric electrolysis of 1 in THF with water selectively affords the Ir(III) dihydride complex [(tBuPCP)Ir(H)2] (3) in 88 % FE from the reaction of 2 with H2O. Complex 3 then hydrogenates the alkene in classical fashion. The presented electro-hydrogenation works with extremely high FE, because the iridium hydrides are water stable, which prevents H2 formation. Even in strongly alkaline conditions (Bu4NOH added), the electro-hydrogenation of 1-octene with 1 also proceeds cleanly (89 % FE), suggesting a highly robust process that may rely on H2O activation, reminiscent to transfer hydrogenation pathways, instead of classical H+ reduction. DFT calculations confirmed oxidative addition of H2O as a key step in this context.

Translating aqueous CO2 hydrogenation activity to electrocatalytic reduction with a homogeneous cobalt catalyst

A molecular cobalt CO₂ hydrogenation catalyst was explored for electrocatalytic CO₂ reduction under aqueous conditions. The resulting pH-dependent selectivity between H₂ and HCO₂^- is rationalized with thermodynamic analysis and stoichiometric experiments.

Formic Acid Generation from CO2 Reduction by MOF-253 Coordinated Transition Metal Complexes: A Computational Chemistry Perspective

The inclusion of transition metal elements within metal–organic frameworks (MOFs) is considered one of the most promising approaches for enhancing the catalytic capability of MOFs. In this study, MOF-253 containing bipyridine coordination sites is investigated for possible transition metal chelation, and a consequent possible CO2 reduction mechanism in the formation of formic acid. All transition metal elements of the third, fourth and fifth periods except hafnium and the lanthanide series are considered using density functional theory calculations. Two distinct types of CO2 reduction mechanisms are identified: (1) the five-coordination Pd center, which promotes formic acid generation via an intramolecular proton transfer pathway; (2) several four-coordination metal centers, including Mn, Pd, and Pt, which generate formic acid by means of heterolytic hydrogen activation. The MOF-253 environment is found to promote beneficial steric hindrance, and to constrain metal–ligand orientation, which consequently facilitates the formation of formic acid, particularly with the tetrahedral Mn center at high-spin electronic state.

Rhodium-Based Metal-Organic Polyhedra Assemblies for Selective CO2 Photoreduction

Heterogenization of molecular catalysts via their immobilization within extended structures often results in a lowering of their catalytic properties due to a change in their coordination sphere. Metal-organic polyhedra (MOP) are an emerging class of well-defined hybrid compounds with a high number of accessible metal sites organized around an inner cavity, making them appealing candidates for catalytic applications. Here, we demonstrate a design strategy that enhances the catalytic properties of dirhodium paddlewheels heterogenized within MOP (Rh-MOP) and their three-dimensional assembled supramolecular structures, which proved to be very efficient catalysts for the selective photochemical reduction of carbon dioxide to formic acid. Surprisingly, the catalytic activity per Rh atom is higher in the supramolecular structures than in its molecular sub-unit Rh-MOP or in the Rh-metal-organic framework (Rh-MOF) and yields turnover frequencies of up to 60 h^-1 and production rates of approx. 76 mmole formic acid per gram of the catalyst per hour, unprecedented in heterogeneous photocatalysis. The enhanced catalytic activity is investigated by X-ray photoelectron spectroscopy and electrochemical characterization, showing that self-assembly into supramolecular polymers increases the electron density on the active site, making the overall reaction thermodynamically more favorable. The catalyst can be recycled without loss of activity and with no change of its molecular structure as shown by pair distribution function analysis. These results demonstrate the high potential of MOP as catalysts for the photoreduction of CO₂ and open a new perspective for the electronic design of discrete molecular architectures with accessible metal sites for the production of solar fuels.

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.

Design of robust 2,2'-bipyridine ligand linkers for the stable immobilization of molecular catalysts on silicon(111) surfaces

The attachment of the 2,2'-bipyridine (bpy) moieties to the surface of planar silicon(111) (photo)electrodes was investigated using ab initio simulations performed on a new cluster model for methyl-terminated silicon. Density functional theory (B3LYP) with implicit solvation techniques indicated that adventitious chlorine atoms, when present in the organic linker backbone, led to instability at very negative potentials of the surface-modified electrode. In prior experimental work, chlorine atoms were present as a trace surface impurity due to required surface processing chemistry, and thus could plausibly result in the observed surface instability of the linker. Free energy calculations for the Cl-atom release process with model silyl-linker constructs revealed a modest barrier (14.9 kcal mol-1) that decreased as the electrode potential became more negative. A small library of new bpy-derived structures has additionally been explored computationally to identify strategies that could minimize chlorine-induced linker instability. Structures with fluorine substituents are predicted to be more stable than their chlorine analogues, whereas fully non-halogenated structures are predicted to exhibit the highest stability. The behavior of a hydrogen-evolving molecular catalyst Cp*Rh(bpy) (Cp* = pentamethylcyclopentadienyl) immobilized on a silicon(111) cluster was explored theoretically to evaluate differences between the homogeneous and surface-attached behavior of this species in a tautomerization reaction observed under reductive conditions for catalytic H2 evolution. The calculated free energy difference between the tautomers is small, hence the results suggest that use of reductively stable linkers can enable robust attachment of catalysts while maintaining chemical behavior on the electrode similar to that exhibited in homogeneous solution.

DFT Study on the Electrocatalytic Reduction of CO2 to CO by a Molecular Chromium Complex

A variety of molecular transition metal-based electrocatalysts for the reduction of carbon dioxide (CO₂) have been developed to explore the viability of utilization strategies for addressing its rising atmospheric concentrations and the corresponding effects of global warming. Concomitantly, this approach could also meet steadily increasing global energy demands for value-added carbon-based chemical feedstocks as nonrenewable petrochemical resources are consumed. Reports on the molecular electrocatalytic reduction of CO₂ mediated by chromium (Cr) complexes are scarce relative to other earth-abundant transition metals. Recently, our group reported a Cr complex that can efficiently catalyze the reduction of CO₂ to carbon monoxide (CO) at low overpotentials. Here, we present new mechanistic insight through a computational (density functional theory) study, exploring the origin of kinetic selectivity, relative energetic positioning of the intermediates, speciation with respect to solvent coordination and spin state, as well as the role of the redox-active bipyridine moiety. Importantly, these studies suggest that under certain reducing conditions, the formation of bicarbonate could become a competitive reaction pathway, informing new areas of interest for future experimental studies.

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.

CO2 Reduction: From Homogeneous to Heterogeneous Electrocatalysis

Due to increasing worldwide fossil fuel consumption, carbon dioxide levels have increased in the atmosphere with increasingly important impacts on the environment. Renewable and clean sources of energy have been proposed, including wind and solar, but they are intermittent and require efficient and scalable energy storage technologies. Electrochemical CO₂ reduction reaction (CO₂RR) provides a valuable approach in this area. It combines solar- or wind-generated electrical production with energy storage in the chemical bonds of carbon-based fuels. It can provide ways to integrate carbon capture, utilization, and storage in energy cycles while maintaining controlled levels of atmospheric CO₂. Electrochemistry allows for the utilization of an electrical input to drive chemical reactions. Because CO₂ is kinetically inert, highly active catalysts are required to decrease reaction barriers sufficiently so that reaction rates can be achieved that are sufficient for electrochemical CO₂ reduction. Given the reaction barriers associated with multiple electron-proton reduction of CO₂ to CO, formaldehyde (HC(O)H), formic acid, or formate (HC(O)OH, HC(O)O^-), or more highly reduced forms of carbon, there is also a demand for high selectivity in catalysis. Catalysts that have been explored include homogeneous catalysts in solution, catalysts immobilized on surfaces, and heterogeneous catalysts. In homogeneous catalysis, reduction occurs following diffusion of the catalyst to an electrode where multiple proton coupled electron transfer reduction occurs. Useful catalysts in this area are typically transition-metal complexes with organic ligands and electron transfer properties that utilize combinations of metal and ligand redox levels. As a way to limit the amount of catalyst, in device-like configurations, catalysts are added to the surfaces of conductive substrates by surface binding, in polymeric films, or on carbon electrode surfaces with molecular structures and electronic configurations related to catalysts in solution. Immobilized, homogeneous catalysts can suffer from performance losses and even decomposition during long-term CO₂ reduction cycles, but they are amenable to detailed mechanistic investigations. In parallel efforts, heterogeneous nanocatalysts have been explored in detail with the development of facile synthetic procedures that can offer highly active catalytic surface areas. Their high activity and stability have attracted a significant level of investigation, including possible exploitation for large-scale applications. However, translation of catalytic reactivity to the surface creates a new reactivity environment and complicates the elucidation of mechanistic details and identification of the active site in exploring reaction pathways. Here, the results of previous studies based on transition-metal complex catalysts for CO₂ electroreduction are summarized. Early studies showed that transition-metal complexes of Ru, Ir, Rh, and Os, with well-defined structures, are all capable of catalyzing CO₂ reduction to CO or formate. Derivatives of the complexes were surface attached to conducting electrodes by chemical bonding, noncovalent bonding, or polymerization. The concept of surface binding has also been extended to the preparation of surface area electrodes by the chemically controlled deposition of nanostructured catalysts such as nano tin, nano copper, and nano carbon, all of which have been shown to have high selectivities and activities toward CO₂ reduction. In our presentation, we end this Account with recent advances and a perspective about the application of electrocatalysis in carbon dioxide reduction.

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.

Structure–activity relationship study of half-sandwich metal complexes in aqueous transfer hydrogenation catalysis

A systematic structure–activity relationship study was performed to identify the factors that are important to enhancing the transfer hydrogenation efficiency of half-sandwich metal complexes.

Mapping free energy regimes in electrocatalytic reductions to screen transition metal-based catalysts

The free energy landscape of catalytic intermediates in the two-electron reduction of CO₂ and proton donors is mapped with density functional theory to screen catalyst candidates from a library of transition metals and ligands.

The free energy landscape of catalytic intermediates in the two-electron reduction of proton donors and/or CO₂ to H₂, CO and HCO₂^– is mapped with density functional theory to screen catalyst candidates from a library of different transition metals and ligands. The goal is to minimize the free energy corrugations between reactants, catalytic intermediates and each desired product, simultaneously screening against intermediates with low free energy that would be traps, and against necessary intermediates with high free energy. Catalysts are initially screened for those with: (a) standard state free energy of the metal hydride intermediate ergoneutral with HCO₂^–, which is the lowest energy product with weak proton donors, and (b) standard free energy of the metal carbonyl intermediate sufficiently high to avoid trapping. The design method is tested on a diverse range of ligands including cyclopentadienyl, polypyridyl, amino, phosphino and carbonyl ligands, around three earth-abundant d⁶ transition metal ions, Mn(i), Fe(ii) and Co(iii), using the BP86 density functional, the double-zeta 6-31+G* basis, LANL2DZ effective core potential on the metals and SMD polarizable continuum model for acetonitrile as solvent, which have previously provided chemically accurate values of several redox potentials, pK_a's and ligand exchange equilibria for transition metal complexes. Among the 36 complexes screened, an Fe(ii) center ligated to two bipyridines and a pyridine with a solvent-bound sixth coordination site for hydride formation from phenol as the proton donor is identified as a promising candidate for ergoneutral hydride formation without trapping by CO. The redox-active bipyridine ligands are predicted to provide near ergoneutral sites for accumulating the two electrons needed to form the hydride. To test the predictions, an Fe(ii) complex was prepared with the desired ligand environment using a pentadentate ligand to prevent ligand exchange. The synthesized complex was indeed found to be active towards electrocatalytic proton reduction as well as CO₂ reduction at the predicted redox potentials with no trapping by CO. However, contrary to the in silico predictions, we found electrochemical evidence of CO₂ binding after the first reduction leading to CO production. Mapping the free energies of key catalytic intermediates such as the metal hydride and metal carbonyl species by using density functional theory (DFT) serves as a first step in catalyst screening spanning large libraries of metals and ligands. In order to screen against all the intermediates in the catalytic pathway, such as reduced metal-bound CO₂ intermediates, further refinement and validation of the DFT methods are needed.

Unambiguous hydrogenation of CO2 by coinage-metal hydride anions: an intuitive idea based on in silico experiments

Coinage metal hydride anions, especially AgH⁻, can effectively and deterministically hydrogenate CO₂ to HCO₂⁻.

Directing the reactivity of metal hydrides for selective CO2 reduction

A critical challenge in electrocatalytic CO₂ reduction to renewable fuels is product selectivity. Desirable products of CO₂ reduction require proton equivalents, but key catalytic intermediates can also be competent for direct proton reduction to H₂ Understanding how to manage divergent reaction pathways at these shared intermediates is essential to achieving high selectivity. Both proton reduction to hydrogen and CO₂ reduction to formate generally proceed through a metal hydride intermediate. We apply thermodynamic relationships that describe the reactivity of metal hydrides with H⁺ and CO₂ to generate a thermodynamic product diagram, which outlines the free energy of product formation as a function of proton activity and hydricity (∆G_H-), or hydride donor strength. The diagram outlines a region of metal hydricity and proton activity in which CO₂ reduction is favorable and H⁺ reduction is suppressed. We apply our diagram to inform our selection of [Pt(dmpe)₂](PF₆)₂ as a potential catalyst, because the corresponding hydride [HPt(dmpe)₂]⁺ has the correct hydricity to access the region where selective CO₂ reduction is possible. We validate our choice experimentally; [Pt(dmpe)₂](PF₆)₂ is a highly selective electrocatalyst for CO₂ reduction to formate (>90% Faradaic efficiency) at an overpotential of less than 100 mV in acetonitrile with no evidence of catalyst degradation after electrolysis. Our report of a selective catalyst for CO₂ reduction illustrates how our thermodynamic diagrams can guide selective and efficient catalyst discovery.

Homogeneously Catalyzed Electroreduction of Carbon Dioxide-Methods, Mechanisms, and Catalysts

The utilization of CO₂ via electrochemical reduction constitutes a promising approach toward production of value-added chemicals or fuels using intermittent renewable energy sources. For this purpose, molecular electrocatalysts are frequently studied and the recent progress both in tuning of the catalytic properties and in mechanistic understanding is truly remarkable. While in earlier years research efforts were focused on complexes with rare metal centers such as Re, Ru, and Pd, the focus has recently shifted toward earth-abundant transition metals such as Mn, Fe, Co, and Ni. By application of appropriate ligands, these metals have been rendered more than competitive for CO₂ reduction compared to the heavier homologues. In addition, the important roles of the second and outer coordination spheres in the catalytic processes have become apparent, and metal-ligand cooperativity has recently become a well-established tool for further tuning of the catalytic behavior. Surprising advances have also been made with very simple organocatalysts, although the mechanisms behind their reactivity are not yet entirely understood. Herein, the developments of the last three decades in electrocatalytic CO₂ reduction with homogeneous catalysts are reviewed. A discussion of the underlying mechanistic principles is included along with a treatment of the experimental and computational techniques for mechanistic studies and catalyst benchmarking. Important catalyst families are discussed in detail with regard to mechanistic aspects, and recent advances in the field are highlighted.

Renewable Formate from C-H Bond Formation with CO2: Using Iron Carbonyl Clusters as Electrocatalysts

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)₁₂]^2-. 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)₁₂]^2- features a concerted proton-electron transfer mechanism to form [H-Fe₄C(CO)₁₂]^2-, 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)₁₂]^2- 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.

Aqueous Hydricity from Calculations of Reduction Potential and Acidity in Water

Hydricity, or hydride donating ability, is a thermodynamic value that helps define the reactivity of transition metal hydrides. To avoid some of the challenges of experimental hydricity measurements in water, a computational method for the determination of aqueous hydricity values has been developed. With a thermochemical cycle involving deprotonation of the metal hydride (pK_a), 2e^- oxidation of the metal (E°), and 2e^- reduction of the proton, hydricity values are provided along with other valuable thermodynamic information. The impact of empirical corrections (for example, calibrating reduction potentials with 2e^- organic versus 1e^- inorganic potentials) was assessed in the calculation of the reduction potentials, acidities, and hydricities of a series of iridium hydride complexes. Calculated hydricities are consistent with electronic trends and agree well with experimental values.