Mn-carbonyl molecular catalysts containing a redox-active phenanthroline-5,6-dione for selective electro- and photoreduction of CO2 to CO or HCOOH

How to rationally design homogeneous catalysts for efficient CO2 electroreduction?

Electrified converting CO2 into valuable fuels and chemicals using a homogeneous electrochemical CO2 reduction (CO2ER) approach simplifies the operation, providing a potential option for decoupling energy harvesting and renewable chemical production. These merits benefit the scenarios where decentralization and intermittent power are key factors. This perspective aims to provide an overview of recent progress in homogeneous CO2ER. We introduce firstly the fundamentals chemistry of the homogeneous CO2ER, followed by a summary of the crucial factors and the important criteria broadly employed for evaluating the performance. We then highlight the recent advances in the most widely explored transition-metal coordinate complexes for the C1 and multicarbon (C2+) products from homogeneous CO2ER. Finally, we summarize the remaining challenges and opportunities for developing homogeneous electrocatalysts for efficient CO2ER. This perspective is expected to favor the rational design of efficient homogeneous electrocatalysts for selective CO2ER toward renewable fuels and feedstocks.

Electrocatalytic Reduction of CO2 Coupled with Organic Conversion to Selectively Synthesize High-Value Chemicals

The electrochemical process of coupling electrocatalytic CO₂ reduction and organic conversion reaction can effectively reduce the reaction overpotential and obtain value-added chemicals. Moreover, because of the diversity of substrates and the designability of coupling forms, more and more attention has been paid to this field. This review systematically summarizes the research progress of coupling electrolysis in recent years, (1) co-electrolysis of CO₂ and organics at the cathode to obtain specific products with high selectivity, (2) replacing traditional anodic oxygen evolution reaction (OER) with other valuable oxidation reactions to improve energy utilization efficiency and economic benefits of CO₂ conversion, (3) in an electrolytic cell without membrane, the cathode and anode jointly transform CO₂ and organics to redox products. We hope that the examples and insights on coupling electrolysis introduced in this review can inspire researchers to further explore and innovate in this direction.

Challenges and recent advancements in the transformation of CO2 into carboxylic acids: straightforward assembly with homogeneous 3d metals

Transformation of carbon dioxide (CO2) into valuable organic carboxylic acids is essential for maintaining sustainability. In this review, such CO2 thermo-, photo- and electrochemical transformations under 3d-transition metal catalysis are described from 2017 until 2022.

Manganese(I) tricarbonyl complexes as potential anticancer agents

The antiproliferative activity of [Mn(CO)₃(N^N)Br] (N^N = phendione 1, bipy 3) and of the two newly synthesized Mn complexes [Mn(CO)₃(acridine)(phendione)]OTf (2) and [Mn(CO)₃(di-triazole)Br] (4) has been evaluated by MTS against three tumor cell lines A2780 (ovarian carcinoma), HCT116 (colorectal carcinoma), HCT116doxR (colorectal carcinoma resistant to doxorubicin), and in human dermal fibroblasts. The antiproliferative assay showed a dose-dependent effect higher in complex 1 and 2 with a selectivity toward ovarian carcinoma cell line 21 times higher than in human fibroblasts. Exposure of A2780 cells to IC₅₀ concentrations of complex 1 and 2 led to an increase of reactive oxygen species that led to the activation of cell death mechanisms, namely via intrinsic apoptosis for 2 and autophagy and extrinsic apoptosis for 1. Both complexes do not target DNA or interfere with cell cycle progression but are able to potentiate cell migration and neovascularization (for 2) an indicative that their application might be directed for initial tumor stages to avoid tumor invasion and metastization and opening a new avenue for complex 2 application in regenerative medicine. Interestingly, both complexes do not show toxicity in both in vivo models (CAM and zebrafish).

Self-Assembled Manganese(I)-Based Selenolato-Bridged Tetranuclear Metallorectangles: Host-Guest Interaction, Anticancer, and CO-Releasing Studies

Supramolecular one-step self-assembly of dimanganese decacarbonyl, diaryl diselenide, and linear dipyridyl ligands (L = pyrazine (pz), 4,4'-bipyridine (bpy), and trans-1,2-bis(4-pyridyl)ethylene (bpe)) has resulted in the formation of selenolato-bridged manganese(I)-based metallorectangles. The synthesis of tetranuclear Mn(I)-based metallorectangles [{(CO)₃Mn(μ-SeR)₂Mn(CO)₃}₂(μ-L)₂] (1-6) was facilitated by the oxidative addition of diaryl diselenide to dimanganese decacarbonyl with the simultaneous coordination of linear bidentate pyridyl linker in an orthogonal fashion. Formation of metallorectangles 1-6 was ascertained using IR, UV-vis, NMR spectroscopic techniques, and elemental analyses. The molecular mass of compounds 2, 4, and 6 were determined by ESI-mass spectrometry. Solid-state structural elucidation of 2, 3, and 6 by single-crystal X-ray diffraction methods revealed a rectangular framework wherein selenolato-bridges and pyridyl ligands define the shorter and longer edges, respectively. Also, the guest binding capability of metallorectangles 3 and 5 with different aromatic guests was studied using UV-vis absorption and emission spectrophotometric titration methods that affirmed strong host-guest binding interactions. The formation of the host-guest complex between metallorectangle 3 and pyrene has been explicitly corroborated by the single-crystal X-ray structure of 3•pyrene. Moreover, select metallorectangles 1-4 and 6 were studied to explore their anticancer activity, while CO-releasing ability of metallorectangle 2 was further appraised using equine heart myoglobin assay.

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.

Development of photoactivable phenanthroline-based manganese(I) CO-Releasing molecules (PhotoCORMs) active against ESKAPE bacteria and bacterial biofilms

The synthesis and biological evaluation of a series of phenanthroline-based visible-light-activated manganese(I) carbon-monoxide-releasing molecules (PhotoCORMs) against ESKAPE bacteria and bacterial biofilms is reported. Four carbonyl compounds of general formula fac-[Mn(N^∧N)(CO)₃(L)] have been synthesized and characterized. Despite being thermally stable in the absence of light, these PhotoCORMs readily release CO upon blue (435-450 nm) LED light irradiation as confirmed by spectrophotometric CO releasing experiments (Mb Assay). The antibacterial activity of the four PhotoCORMs has been investigated against a panel of ESKAPE bacteria. The compounds 1-3 were found to be effective antibacterials at low concentrations against multidrug-resistant Klebsiella pneumoniae and Acinetobacter baumannii when photoactivated with blue-light. In addition, the PhotoCORMs 1-2 were found to inhibit the formation of Klebsiella pneumoniae and Acinetobacter baumannii bacterial biofilms at low concentrations (MIC = 4-8 μg/mL), turning out to be promising candidates to combat antimicrobial resistance. The antibacterial and biofilm inhibitory effect of the PhotoCORMs is plausibly due to the release of CO as well as the formation of phenanthroline photo-by-products as revealed by spectroscopy and microbiology experiments.

Effects of Chemically-Modified Polypyridyl Ligands on the Structural and Redox Properties of Tricarbonylmanganese(I) Complexes

Carbonyl complexes with manganese(I) as the central metal are very attractive catalysts. The introduction of redox-active ligands, such as quinones and methyl viologen analogs into these catalysts, would be expected to lead to superior catalyst performances, since they can function as excellent electron carriers. In this study, we synthesized four tricarbonylmanganese(I) complexes containing typical bidentate polypyridyl ligands, including 1,10-phenanthroline (phen) and 2,2′-bipyridine (bpy) frameworks bound to redox-active ortho-quinone/catechol or methyl viologen-like units. The molecular structures of the resulting complexes were determined by X-ray crystallography to clarify their steric features. As expected from the infrared (IR) data, three CO ligands for each complex were coordinated in the facial configuration around the central manganese(I) atom. Additionally, the structural parameters were found to differ significantly between the quinone/catechol units. Electrochemical analysis revealed some differences between them and their reference complexes, namely [MnBr(CO)₃(phen)] and [MnBr(CO)₃(bpy)]. Notably, interconversions induced by two-electron/two-proton transfers between the quinone and catechol units were observed in the phenanthroline-based complexes. This work indicated that the structural and redox properties in tricarbonylmanganese(I) complexes were significantly affected by chemically modified polypyridyl ligands. A better understanding of structures and redox behaviors of the present compounds would facilitate the design of new manganese complexes with enhanced properties.

Engineering Heterostructured Nanocatalysts for CO2 Transformation Reactions: Advances and Perspectives

As a conceivable route to achieving anthropological carbon looping, carbon capture and utilization (CCU) technologies employ waste CO₂ as an accessible C1 building block to generate upgraded chemicals or fuels, thereby simultaneously remedying environmental issues and energy crises. However, efficient CO₂ conversion is disfavored by both its thermodynamics and its kinetics. Heterostructured materials with well-controlled interfaces have great potential for enhanced catalytic performance in various CO₂ transformation reactions, owing to the synergistic effects among components, numerous interfacial and/or surface active sites, increased CO₂ adsorption capacity, promoted charge transfer efficiency, and unique physicochemical properties. This Review highlights the state of the art in typical heterostructures, such as core-shell, yolk-shell, Janus, hierarchical (branched and hollow), and 2D/2D layered structures, applied for CO₂ conversion with various energy inputs (radiation, electricity, heat). Fabrication methods of different heterostructures and structure-composition-performance relationships are also discussed concisely. Finally, a brief summary and prospective research directions are provided. The motivation of this Review is to offer instructive information on the applicability of inorganic heterostructures for CO₂ transformation reactions, and it is hoped that further enlightening studies in this field could emerge in the future.

Selective synthesis and crystal structures of manganese(I) complexes with a bi- or tridentate terpyridine ligand

The crystal structures of two manganese(I) complexes with a different coordination mode of the supporting ligand are reported: fac-bromido-tricarbon-yl(4'-phenyl-2,2':6',2''-terpyridine-κ2 N,N')manganese(I), [MnBr(C21H15N3)(CO)3], I, and cis-bromido-dicarbon-yl(4'-phenyl-2,2':6',2''-terpyridine-κ3 N,N',N'')manganese(I), [MnBr(C21H15N3)(CO)2], II. In both complexes, the manganese(I) atom is coordinated by terminal carbonyl ligands, a bromide ion, and a 4'-phenyl-2,2':6',2''-terpyridine ligand within a distorted octa-hedral environment. In I, the metal ion is facially coordinated by three carbonyl ligands and the terpyridine ligand binds in a bidentate fashion. The non-coordinating nitro-gen atom in the terpyridine ligand is positioned on the side opposite to the bromido ligand. In II, the metal ion is coordinated by two carbonyl ligands in a cis configuration and the terpyridine ligand binds in a tridentate fashion; notably, one carbonyl and the trans bromido ligand are mutually disordered over two positions. In I, the complex mol-ecules are linked by C-H⋯Br hydrogen bonds. In II, aromatic π-π contacts are present, as well as pairs of C-H⋯Br and C-H⋯O hydrogen bonds.

An Investigation of Electrocatalytic CO2 Reduction Using a Manganese Tricarbonyl Biquinoline Complex

The subject of this study [fac-Mn(bqn)(CO)₃(CH₃CN)]⁺ (bqn = 2,2′-biquinoline), is of particular interest because the bqn ligand exhibits both steric and electronic influence over the fundamental redox properties of the complex and, consequently, its related catalytic properties with respect to the activation of CO₂. While not a particularly efficient catalyst for CO₂ to CO conversion, in-situ generation and activity measurements of the [fac-Mn(bqn)(CO)₃]⁻ active catalyst allows for a better understanding of ligand design at the Mn center. By making direct comparisons to the related 2,2′-bipyridyl (bpy), 1,10-phenanthroline (phen), and 2,9-dimethyl-1,10-phenanthroline (dmphen) ligands via a combination of voltammetry, infrared spectroelectrochemistry, controlled potential electrolysis and computational analysis, the role of steric vs. electronic influences on the nucleophilicity of Mn-based CO₂ reduction electrocatalysts is discussed.

A cyanide-bridged di-manganese carbonyl complex that photochemically reduces CO2 to CO

A cyanide-bridged di-manganese complex, {[Mn(bpy)(CO)₃]₂(μ-CN)}⁺, is introduced as an efficient electrocatalyst and photochemically active for proton-assisted of CO₂ reduction to CO.