Electrocatalytic CO2 reduction by a cobalt bis(pyridylmonoimine) complex: effect of acid concentration on catalyst activity and stability

Catalytic CO2 Reduction with Heptacoordinated Polypyridine Complexes: Switching the Selectivity via Metal Replacement

The discovery of molecular catalysts for the CO2 reduction reaction (CO2 RR) in the presence of water, which are both effective and selective towards the generation of carbon-based products, is a critical task. Herein we report the catalytic activity towards the CO2 RR in acetonitrile/water mixtures by a cobalt complex and its iron analog both featuring the same redox-active ligand and an unusual seven-coordination environment. Bulk electrolysis experiments show that the cobalt complex mainly yields formate (52 % selectivity at an applied potential of -2.0 V vs Fc+ /Fc and 1 % H2 O) or H2 (up to 86 % selectivity at higher applied bias and water content), while the iron complex always delivers CO as the major product (selectivity >74 %). The different catalytic behavior is further confirmed under photochemical conditions with the [Ru(bpy)3 ]2+ sensitizer (bpy=2,2'-bipyridine) and N,N-diisopropylethylamine as electron donor, where the cobalt complex leads to preferential H2 formation (up to 89 % selectivity), while the iron analog quantitatively generates CO (up to 88 % selectivity). This is ascribed to a preference towards a metal-hydride vs. a metal-carboxyl pathway for the cobalt and the iron complex, respectively, and highlights how metal replacement may effectively impact on the reactivity of transition metal complexes towards solar fuel formation.

Potential- and Buffer-Dependent Selectivity for the Conversion of CO2 to CO by a Cobalt Porphyrin-Peptide Electrocatalyst in Water

A semisynthetic electrocatalyst for carbon dioxide reduction to carbon monoxide in water is reported. Cobalt microperoxidase-11 (CoMP11-Ac) is shown to reduce CO₂ to CO with a turnover number of up to 32,000 and a selectivity of up to 88:5 CO:H₂. Higher selectivity for CO production is favored by a less cathodic applied potential and use of a higher pK _a buffer. A mechanistic hypothesis is presented in which avoiding the formation and protonation of a formal Co(I) species favors CO production. These results demonstrate how tuning reaction conditions impact reactivity toward CO₂ reduction for a biocatalyst previously developed for H₂ production.

Strategies for Breaking Molecular Scaling Relationships for the Electrochemical CO2 Reduction Reaction

The electrocatalytic CO2 reduction reaction (CO2RR) is a promising strategy for converting CO2 to fuels and value-added chemicals using renewable energy sources. Molecular electrocatalysts show promise for the selective conversion...

Enhancement of electrocatalytic abilities toward CO2 reduction by tethering redox-active metal complexes to the active site

Tethering metal complexes, like [Ru(bpy)₂Cl₂] (bpy = 2,2'-bipyridine), which are redox-active at low reduction potentials and have the ability to transfer electrons to another complex, to a [Ni(cyclen)]²⁺ electrocatalyst enhanced the reduction of CO₂ to CO at low overpotentials. The [Ni(cyclen)]²⁺ electrocatalyst was modified by tethering redox-active metal complexes via 4-methylpyridyl linkers. The redox-active metal complexes were reduced after CO₂ bound to the active site. In controlled potential electrolysis (CPE) experiments in 95 : 5 (v/v) CH₃CN/H₂O, [{([Ru]pic)₄cyclen}NiCl]⁵⁺ ([Ru]⁺ = {Ru(bpy)₂Cl}⁺; pic = 4-methylpyridyl) could be used to reduce CO₂ into CO at a turnover frequency (TOF) of 708 s^-1 with a faradaic efficiency (FE) of 80% at an onset potential of -1.60 V vs. NHE. At the same time, this electrocatalyst was active at an onset potential of -1.25 V vs. NHE, which is the reduction potential of one of the bpy ligands of the [Ru]⁺ moieties, with FE = 84% and TOF = 178 s^-1. When the electrocatalysis was performed using [bn₄cyclenNiCl]Cl (bn = benzyl) without tethered redox-active metal complexes, the TOF value was determined to be 8 s^-1 with FE = 77% at an onset potential of -1.45 V vs. NHE. The results show that tethering redox-active metal complexes significantly improves the electrocatalytic activities by lowering the potential needed to reduce CO₂.

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.

Ionic-caged heterometallic bismuth-platinum complex exhibiting electrocatalytic CO2 reduction

An air-stable heterometallic Bi-Pt complex with the formula [BiPt(SAc)5]n (1; SAc = thioacetate) was synthesized. The crystal structure, natural bond orbital (NBO) and local orbital locator (LOL) analyses, localized orbital bonding analysis (LOBA), and X-ray absorption fine structure (XAFS) measurements were used to confirm the existence of Bi-Pt bonding and an ionic cage of O atoms surrounding the Bi ion. From the cyclic voltammetry (CV) and controlled potential electrolysis (CPE) experiments, 1 in tetrahydrofuran reduced CO2 to CO, with a faradaic efficiency (FE) of 92% and a turnover frequency (TOF) of 8 s-1 after 30 min of CPE at -0.79 V vs. NHE. The proposed mechanism includes an energetically favored pathway via the ionic cage, which is supported by the results of DFT calculations and reflectance infrared spectroelectrochemistry data.

Electrocatalytic CO2 Reduction with a Half-Sandwich Cobalt Catalyst: Selectivity towards CO

We present herein a Cp*Co(III)-half-sandwich catalyst system for electrocatalytic CO₂ reduction in aqueous acetonitrile solution. In addition to an electron-donating Cp* ligand (Cp*=pentamethylcyclopentadienyl), the catalyst featured a proton-responsive pyridyl-benzimidazole-based N,N-bidentate ligand. Owing to the presence of a relatively electron-rich Co center, the reduced Co(I)-state was made prone to activate the electrophilic carbon center of CO₂ . At the same time, the proton-responsive benzimidazole scaffold was susceptible to facilitate proton-transfer during the subsequent reduction of CO₂ . The above factors rendered the present catalyst active toward producing CO as the major product over the other potential 2e/2H⁺ reduced product HCOOH, in contrast to the only known similar half-sandwich CpCo(III)-based CO₂ -reduction catalysts which produced HCOOH selectively. The system exhibited a Faradaic efficiency (FE) of about 70% while the overpotential for CO production was found to be 0.78 V, as determined by controlled-potential electrolysis.

Transition metal-based catalysts for the electrochemical CO2 reduction: from atoms and molecules to nanostructured materials

An overview of the main strategies for the rational design of transition metal-based catalysts for the electrochemical conversion of CO₂, ranging from molecular systems to single-atom and nanostructured catalysts.

Thermodynamic Considerations for Optimizing Selective CO2 Reduction by Molecular Catalysts

Energetically efficient electrocatalysts with high product selectivity are desirable targets for sustainable chemical fuel generation using renewable electricity. Recycling CO₂ by reduction to more energy dense products would support a carbon-neutral cycle that mitigates the intermittency of renewable energy sources. Conversion of CO₂ to more saturated products typically requires proton equivalents. Complications with product selectivity stem from competitive reactions between H⁺ or CO₂ at shared intermediates. We describe generalized catalytic cycles for H₂, CO, and HCO₂ ^- formation that are commonly proposed in inorganic molecular catalysts. Thermodynamic considerations and trends for the reactions of H⁺ or CO₂ at key intermediates are outlined. A quantitative understanding of intermediate catalytic steps is key to designing systems that display high selectivity while promoting energetically efficient catalysis by minimizing the overall energy landscape. For CO₂ reduction to CO, we describe how an enzymatic active site motif facilitates efficient and selective catalysis and highlight relevant examples from synthetic systems.

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

Substituents drive ligand rearrangements, giving dinuclear rather than mononuclear complexes, and tune CoII/III redox potential

Three new tetradentate imine ligands, HLHBr, HLClH and HLBrH (HLR1R2) were synthesised by 2 : 1 condensation of the appropriately n-halo substituted pyridine-2-carboxaldehyde (5-bromo-4a, 6-bromo-4b or 6-chloro-4c) with 1,3-diaminopropan-2-ol (5). Reactions of each of these three ligands with one equivalent of cobalt(ii) tetrafluoroborate resulted in the formation of three N4O2 coordinated cobalt(ii) complexes: the anticipated mononuclear complex [CoII(HLHBr)(MeOH)2](BF4)2 (1), and two unexpected dinuclear complexes, [CoII2(LBrH-BF2OMe)]2(BF4)2 (2) and [CoII2(LClH-BF2OMe)]2(BF4)2 (3). Dinuclear 2 and 3 result from complexation of cobalt(ii) to the ligands derived from the sterically demanding 6-halo substituted pyridine-2-carboxaldehydes (4b and 4c) undergoing rearrangement, reacting with MeOH and a BF4 anion, resulting in a pair of borate ester bridges between the two cobalt(ii) centres. A similar type of rearrangement is proposed for the PF6 analogues. Cyclic voltammetry in acetonitrile reveals that cobalt(ii) complexes 1-3 undergo a quasi-reversible oxidation: Em = 0.57, 0.38 and 0.29 V vs. 0.01 AgNO3/Ag, respectively. The observed Em value is tuned by the ligand, with the 6-chloro-substituent leading to the lowest Em value being observed for the corresponding cobalt complex, 3, rather than for either of the complexes of the n-bromo-substituted ligands (n = 6 or 5), 2 and 1.

Enhancement of electrocatalytic abilities for reducing carbon dioxide: functionalization with a redox-active ligand-coordinated metal complex

A binary system consisting of a ditopic planar pseudo-pincer ligand (qlca = quinoline-2-carbaldehyde (pyridine-2-carbonyl) hydrazone) coordinated to two metal centres affording [{Ru(bpy)2}(μ-qlca)NiCl2]Cl·4H2O·CH3OH (2) (bpy = 2,2'-bipyridine) is reported. The Ni2+ moiety acts as the electrocatalytic active site for CO2 reduction to CO. The turnover frequency (TOF) increased from 0.83 s-1 for [Ni(qlca)Cl2] (3) to 120 s-1 for 2, and the overpotential is 350 mV less than that for 3 due to the electronic influence of the {Ru(bpy)2}2+ moiety on the catalytic active site.