Water-Soluble Manganese Complex for Selective Electrocatalytic CO2 Reduction to CO

Photochargeable Mn-Based Metal-Organic Framework and Decoupled Photocatalysis

Designing multifunctional materials that mimic the light-dark decoupling of natural photosynthesis is a key challenge in the field of energy conversion. Herein, we introduce MnBr-253, a precious metal-free metal-organic framework (MOF) built on Al nodes, bipyridine linkers and MnBr(CO)3(bipyridine) complexes. Upon irradiation, MnBr-253 colloids demonstrate an electron photocharging capacity of ~42 C ⋅ g-1 MOF, with state-of-the-art photocharging rate (1.28 C ⋅ s-1 ⋅ g-1 MOF) and incident photon-to-electron conversion efficiency of ~9.4 % at 450 nm. Spectroscopic and computational studies support effective electron accumulation at the Mn complex while high porosity and Mn loading account for the notable electron storage performance. The charged MnBr-253 powders were successfully applied for hydrogen evolution under dark conditions thus emulating the light-decoupled reactivity of photosynthesis.

Highly Efficient Carbon Dioxide Electroreduction via DNA-Directed Catalyst Immobilization

Electrochemical reduction of carbon dioxide (CO2) is a promising route to up-convert this industrial byproduct. However, to perform this reaction with a small-molecule catalyst, the catalyst must be proximal to an electrode surface. Efforts to immobilize molecular catalysts on electrodes have been stymied by the need to optimize the immobilization chemistries on a case-by-case basis. Taking inspiration from nature, we applied DNA as a molecular-scale "Velcro" to investigate the tethering of three porphyrin-based catalysts to electrodes. This tethering strategy improved both the stability of the catalysts and their Faradaic efficiencies (FEs). DNA-catalyst conjugates were immobilized on screen-printed carbon and carbon paper electrodes via DNA hybridization with nearly 100% efficiency. Following immobilization, a higher catalyst stability at relevant potentials is observed. Additionally, lower overpotentials are required for the generation of carbon monoxide (CO). Finally, high FE for CO generation was observed with the DNA-immobilized catalysts as compared to the unmodified small-molecule systems, as high as 79.1% FE for CO at -0.95 V vs SHE using a DNA-tethered catalyst. This work demonstrates the potential of DNA "Velcro" as a powerful strategy for catalyst immobilization. Here, we demonstrated improved catalytic characteristics of molecular catalysts for CO2 valorization, but this strategy is anticipated to be generalizable to any reaction that proceeds in aqueous solutions.

Highly selective CO2 electrolysis in aqueous media by a water-soluble cobalt dimethyl-bipyridine complex

A water-soluble Co complex with dimethyl-bipyridine ligands reduced CO2 to CO electrochemically with almost 100% selectivity at -0.80 V vs. NHE in an aqueous medium (pH 6.8) without an organic solvent. The reaction overpotential was 270 mV. A possible CO formation mechanism was discussed based on experiments and calculations.

Photocatalytic Reduction of CO2 to CO in Aqueous Solution under Red-Light Irradiation by a Zn-Porphyrin-Sensitized Mn(I) Catalyst

This work demonstrates photocatalytic CO₂ reduction by a noble-metal-free photosensitizer-catalyst system in aqueous solution under red-light irradiation. A water-soluble Mn(I) tricarbonyl diimine complex, [MnBr(4,4′-{Et₂O₃PCH₂}₂-2,2′-bipyridyl)(CO)₃] (1), has been fully characterized, including single-crystal X-ray crystallography, and shown to reduce CO₂ to CO following photosensitization by tetra(N-methyl-4-pyridyl)porphyrin Zn(II) tetrachloride [Zn(TMPyP)]Cl₄ (2) under 625 nm irradiation. This is the first example of 2 employed as a photosensitizer for CO₂ reduction. The incorporation of −P(O)(OEt)₂ groups, decoupled from the core of the catalyst by a −CH₂– spacer, afforded water solubility without compromising the electronic properties of the catalyst. The photostability of the active Mn(I) catalyst over prolonged periods of irradiation with red light was confirmed by ¹H and ¹³C{¹H} NMR spectroscopy. This first report on Mn(I) species as a homogeneous photocatalyst, working in water and under red light, illustrates further future prospects of intrinsically photounstable Mn(I) complexes as solar-driven catalysts in an aqueous environment.

A Mn(I) bipyridyl tricarbonyl complex, where the diimine ligand is functionalized with water-solubilizing phosphonate ester groups, has been prepared and is shown to catalytically convert CO₂ to CO in aqueous solution following photosensitization from a water-soluble Zn(II) porphyrin under red-light irradiation.

Manganese Carbonyl Complexes as Selective Electrocatalysts for CO2 Reduction in Water and Organic Solvents

The electrochemical reduction of CO₂ provides a way to sustainably generate carbon-based fuels and feedstocks. Molecular CO₂ reduction electrocatalysts provide tunable reaction centers offering an approach to control the selectivity of catalysis. Manganese carbonyl complexes, based on [Mn(bpy)(CO)₃Br] and its derivatives (bpy = 2,2′-bipyridine), are particularly interesting due to their ease of synthesis and the use of a first-row earth-abundant transition metal. [Mn(bpy)(CO)₃Br] was first shown to be an active and selective catalyst for reducing CO₂ to CO in organic solvents in 2011. Since then, manganese carbonyl catalysts have been widely studied with numerous reports of their use as electrocatalysts and photocatalysts and studies of their mechanism.

This class of Mn catalysts only shows CO₂ reduction activity with the addition of weak Brønsted acids. Perhaps surprisingly, early reports showed increased turnover frequencies as the acid strength is increased without a loss in selectivity toward CO evolution. It may have been expected that the competing hydrogen evolution reaction could have led to lower selectivity. Inspired by these works we began to explore if the catalyst would work in protic solvents, namely, water, and to explore the pH range over which it can operate. Here we describe the early studies from our laboratory that first demonstrated the use of manganese carbonyl complexes in water and then go on to discuss wider developments on the use of these catalysts in water, highlighting their potential as catalysts for use in aqueous CO₂ electrolyzers.

Key to the excellent selectivity of these catalysts in the presence of Brønsted acids is a proton-assisted CO₂ binding mechanism, where for the acids widely studied, lower pK_a values actually favor CO₂ binding over Mn–H formation, a precursor to H₂ evolution. Here we discuss the wider literature before focusing on our own contributions in validating this previously proposed mechanism through the use of vibrational sum frequency generation (VSFG) spectroelectrochemistry. This allowed us to study [Mn(bpy)(CO)₃Br] while it is at, or near, the electrode surface, which provided a way to identify new catalytic intermediates and also confirm that proton-assisted CO₂ binding operates in both the “dimer” and primary (via [Mn(bpy)(CO)₃]⁻) pathways. Understanding the mechanism of how these highly selective catalysts operate is important as we propose that the Mn complexes will be valuable models to guide the development of new proton/acid tolerant CO₂ reduction catalysts.

Redox-mediated carbon monoxide release from a manganese carbonyl-implications for physiological CO delivery by CO releasing moieties

The dynamics of hydrogen peroxide reactions with metal carbonyls have received little attention. Given reports that therapeutic levels of carbon monoxide are released in hypoxic tumour cells upon manganese carbonyls reactions with endogenous H2O2, it is critical to assess the underlying CO release mechanism(s). In this context, a quantitative mechanistic investigation of the H2O2 oxidation of the water-soluble model complex fac-[Mn(CO)3(Br)(bpCO2)]2-, (A, bpCO2 2- = 2,2'-bipyridine-4,4'-dicarboxylate dianion) was undertaken under physiologically relevant conditions. Characterizing such pathways is essential to evaluating the viability of redox-mediated CO release as an anti-cancer strategy. The present experimental studies demonstrate that approximately 2.5 equivalents of CO are released upon H2O2 oxidation of A via pH-dependent kinetics that are first-order both in [A] and in [H2O2]. Density functional calculations were used to evaluate the key intermediates in the proposed reaction mechanisms. These pathways are discussed in terms of their relevance to physiological CO delivery by carbon monoxide releasing moieties.

Resilient Poly(α-hydroxy acids) with Improved Strength and Ductility via Scalable Stereosequence-Controlled Polymerization

Despite the degradability and biocompatibility of poly(α-hydroxy acids), their utility remains limited because their thermal and mechanical properties are inferior to those of commodity polyolefins, which can be attributed to the lack of side-chain functionality on the polyester backbone. Attempts to synthesize high-molecular-weight functionalized poly(α-hydroxy acids) from O-carboxyanhydrides have been hampered by scalability problems arising from the need for an external energy source such as light or electricity. Herein, we report an operationally simple, scalable method for the synthesis of stereoregular, high-molecular-weight (>200 kDa) functionalized poly(α-hydroxy acids) by means of controlled ring-opening polymerization of O-carboxyanhydrides mediated by a highly redox reactive manganese complex and a zinc-alkoxide. Mechanistic studies indicated that the ring-opening process likely proceeded via the Mn-mediated decarboxylation with alkoxy radical formation. Gradient copolymers produced directly by this method from mixtures of two O-carboxyanhydrides exhibited better ductility and toughness than their corresponding homopolymers and block copolymers, therefore highlighting the potential feasibility of functionalized poly(α-hydroxy acids) as ductile and resilient polymeric materials.

A Water-Soluble Sodium Pectate Complex with Copper as an Electrochemical Catalyst for Carbon Dioxide Reduction

A selective noble-metal-free molecular catalyst has emerged as a fruitful approach in the quest for designing efficient and stable catalytic materials for CO2 reduction. In this work, we report that a sodium pectate complex of copper (PG-NaCu) proved to be highly active in the electrocatalytic conversion of CO2 to CH4 in water. Stability and selectivity of conversion of CO2 to CH4 as a product at a glassy carbon electrode were discovered. The copper complex PG-NaCu was synthesized and characterized by physicochemical methods. The electrochemical CO2 reduction reaction (CO2RR) proceeds at -1.5 V vs. Ag/AgCl at ~10 mA/cm2 current densities in the presence of the catalyst. The current density decreases by less than 20% within 12 h of electrolysis (the main decrease occurs in the first 3 h of electrolysis in the presence of CO2). This copper pectate complex (PG-NaCu) combines the advantages of heterogeneous and homogeneous catalysts, the stability of heterogeneous solid materials and the performance (high activity and selectivity) of molecular catalysts.

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.

Enhancing Molecular Electrocatalysis of CO2 Reduction with Pressure-Tunable CO2 -Expanded Electrolytes

Electrochemical studies of CO₂ conversion by molecular catalysts are typically carried out in a narrow range of near-ambient CO₂ pressures wherein low CO₂ solubilities in the liquid phase can limit the rate of CO₂ reduction. In this study, five-fold rate enhancements are enabled by pairing CO₂ -expanded electrolytes (CXEs), a class of media that accommodate multimolar concentrations of CO₂ in organic solvents at modest pressures, with a homogeneous molecular electrocatalyst, [Re(CO)₃ (bpy)Cl] (1, bpy=2,2'-bipyridyl). Analysis of cyclic voltammetry data reveals pressure-tunable rate behavior, with first-order kinetics at moderate CO₂ pressures giving way to zero-order kinetics at higher pressures. The significant enhancement in the space-time yield of CO demonstrates that CXEs offer a simple yet powerful strategy for unlocking the intrinsic potential of molecular catalysts by mitigating CO₂ solubility limitations commonly encountered in conventional liquid electrolytes. Moreover, our findings reveal that 1, a workhorse molecular catalyst, performs with intrinsic kinetic behavior, which is competitive with fast enzymes under optimal conditions in CXEs.

Molecular Catalysts with Intramolecular Re-O Bond for Electrochemical Reduction of Carbon Dioxide

A new Re bipyridine-type complex, namely, fac-Re(pmbpy)(CO)₃Cl (pmbpy = 4-phenyl-6-(2-hydroxy-phenyl)-2,2′-bipyridine), 1, carrying a single OH moiety as local proton source, has been synthesized, and its electrochemical behavior under Ar and under CO₂ has been characterized. Two isomers of 1, namely, 1-cis characterized by the proximity of Cl to OH and 1-trans, are identified. The interconversion between 1-cis and 1-trans is clarified by DFT calculations, which reveal two transition states. The energetically lower pathway displays a non-negligible barrier of 75.5 kJ mol^–1. The 1e^– electrochemical reduction of 1 affords the neutral intermediate 1-OPh, formally derived by reductive deprotonation and loss of Cl^– from 1. 1-OPh, which exhibits an entropically favored intramolecular Re–O bond, has been isolated and characterized. The detailed electrochemical mechanism is demonstrated by combined chemical reactivity, spectroelectrochemistry, spectroscopic (IR and NMR), and computational (DFT) approaches. Comparison with previous Re and Mn derivatives carrying local proton sources highlights that the catalytic activity of Re complexes is more sensitive to the presence of local OH groups. Similar to Re-2OH (2OH = 4-phenyl-6-(phenyl-2,6-diol)-2,2′-bipyridine), 1 and Mn-1OH display a selective reduction of CO₂ to CO. In the case of the Re bipyridine-type complex, the formation of a relatively stable Re–O bond and a preference for phenolate-based reactivity with CO₂ slightly inhibit the electrocatalytic reduction of CO₂ to CO, resulting in a low TON value of 9, even in the presence of phenol as a proton source.

A new Re bipyridine-type complex, namely, fac-Re(pmbpy)(CO)₃Cl (pmbpy = 4-phenyl-6-(2-hydroxy-phenyl)-2,2′-bipyridine), 1, carrying a single OH moiety as local proton source, has been synthesized, and its electrochemical behavior under Ar and under CO₂ has been characterized. Two isomers of 1, namely, 1-cis characterized by the proximity of Cl to OH and 1-trans, are identified.

Ultrafast Spectroscopy of [Mn(CO)3] Complexes: Tuning the Kinetics of Light-Driven CO Release and Solvent Binding

Manganese tricarbonyl complexes are promising catalysts for CO₂ reduction, but complexes in this family are often photosensitive and decompose rapidly upon exposure to visible light. In this report, synthetic and photochemical studies probe the initial steps of light-driven speciation for Mn(CO)₃(^Rbpy)Br complexes bearing a range of 4,4'-disubstituted 2,2'-bipyridyl ligands (^Rbpy, where R = ^tBu, H, CF₃, NO₂). Transient absorption spectroscopy measurements for Mn(CO)₃(^Rbpy)Br coordination compounds with R = ^tBu, H, and CF₃ in acetonitrile reveal ultrafast loss of a CO ligand on the femtosecond time scale, followed by solvent coordination on the picosecond time scale. The Mn(CO)₃(^NO₂bpy)Br complex is unique among the four compounds in having a longer-lived excited state that does not undergo CO release or subsequent solvent coordination. The kinetics of photolysis and solvent coordination for light-sensitive complexes depend on the electronic properties of the disubstituted bipyridyl ligand. The results indicate that both metal-to-ligand charge-transfer (MLCT) and dissociative ligand-field (d-d) excited states play a role in the ultrafast photochemistry. Taken together, the findings suggest that more robust catalysts could be prepared with appropriately designed complexes that avoid crossing between the excited states that drive photochemical CO loss.

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.

In situ study of the low overpotential "dimer pathway" for electrocatalytic carbon dioxide reduction by manganese carbonyl complexes

The electrocatalytic reduction of CO2 using [fac-Mn(bpy)(CO)3Br] (bpy = 2,2'-bipyridine) and its derivatives has been the subject of numerous recent studies. However the mechanisms of catalysis are still debated. Here we carry out in situ vibrational sum-frequency generation (VSFG) spectroelectrochemistry to examine how this catalyst behaves at an electrode surface. In particular, a low overpotential pathway involving a dimeric manganese has been reported in several studies using substituted bipyridine ligands. Here, we find that the "dimer pathway" can also occur with the unsubstuituted bipyridine complexes. Specifically we can observe spectroscopic evidence of the interaction between [Mn2(bpy)2(CO)6] with CO2 in the presence of a suitable acid. Detailed VSFG studies of [Mn2(bpy)2(CO)6], including of the potential dependence of the surface ν(CO) mode, allow us to construct a model of how it accumulates and behaves at the electrode surface under potentiostatic control.