Modifying the steric and electronic character within Re(I)-phenanthroline complexes for electrocatalytic CO 2 reduction

Fifty Shades of Phenanthroline: Synthesis Strategies to Functionalize 1,10-Phenanthroline in All Positions

1,10-Phenanthroline (phen) is one of the most popular ligands ever used in coordination chemistry due to its strong affinity for a wide range of metals with various oxidation states. Its polyaromatic structure provides robustness and rigidity, leading to intriguing features in numerous fields (luminescent coordination scaffolds, catalysis, supramolecular chemistry, sensors, theranostics, etc.). Importantly, phen offers eight distinct positions for functional groups to be attached, showcasing remarkable versatility for such a simple ligand. As a result, phen has become a landmark molecule for coordination chemists, serving as a must-use ligand and a versatile platform for designing polyfunctional arrays. The extensive use of substituted phenanthroline ligands with different metal ions has resulted in a diverse array of complexes tailored for numerous applications. For instance, these complexes have been utilized as sensitizers in dye-sensitized solar cells, as luminescent probes modified with antibodies for biomaterials, and in the creation of elegant supramolecular architectures like rotaxanes and catenanes, exemplified by Sauvage's Nobel Prize-winning work in 2016. In summary, phen has found applications in almost every facet of chemistry. An intriguing aspect of phen is the specific reactivity of each pair of carbon atoms ([2,9], [3,8], [4,7], and [5,6]), enabling the functionalization of each pair with different groups and leading to polyfunctional arrays. Furthermore, it is possible to differentiate each position in these pairs, resulting in non-symmetrical systems with tremendous versatility. In this Review, the authors aim to compile and categorize existing synthetic strategies for the stepwise polyfunctionalization of phen in various positions. This comprehensive toolbox will aid coordination chemists in designing virtually any polyfunctional ligand. The survey will encompass seminal work from the 1950s to the present day. The scope of the Review will be limited to 1,10-phenanthroline, excluding ligands with more intracyclic heteroatoms or fused aromatic cycles. Overall, the primary goal of this Review is to highlight both old and recent synthetic strategies that find applicability in the mentioned applications. By doing so, the authors hope to establish a first reference for phenanthroline synthesis, covering all possible positions on the backbone, and hope to inspire all concerned chemists to devise new strategies that have not yet been explored.

Rhenium Carbonyl Molecular Catalysts for CO2 Electroreduction: Effects on Catalysis of Bipyridine Substituents Mimicking Anchorage Functions to Modify Electrodes

Heterogenization of molecular catalysts on (photo)electrode surfaces is required to design devices performing processes enabling to store renewable energy in chemical bonds. Among the various strategies to immobilize molecular catalysts, direct chemical bonding to conductive surfaces presents some advantages because of the robustness of the linkage. When the catalyst is, as it is often the case, a transition metal complex, the anchoring group has to be connected to the complex through the ligands, and an important question is thus raised on the influence of this function on the redox and on the catalytic properties of the complex. Herein, we analyze the effect of conjugated and non conjugated substituents, structurally close to anchoring functions previously used to immobilize a rhenium carbonyl bipyridyl molecular catalyst for supported CO₂ electroreduction. We show that carboxylic ester groups, mimicking anchoring the catalyst via carboxylate binding to the surface, have a drastic effect on the catalytic activity of the complex toward CO₂ electroreduction. The reasons for such an effect are revealed via a combined spectro-electrochemical analysis showing that the reducing equivalents are mainly accumulated on the electron-withdrawing ester on the bipyridine ligand preventing the formation of the rhenium(0) center and its interaction with CO₂. Alternatively, alkyl-phosphonic ester substituents, not conjugated with the bpy ligand, mimicking anchoring the catalyst via phosphonate binding to the surface, allow preserving the catalytic activity of the complex.

A fundamental role of solvent polarity and remote substitution of 2-(4-R-phenyl)-1H-imidazo[4,5-f][1,10]phenanthroline framework in controlling of ground- and excited-state properties of Re(I) chromophores [ReCl(CO)3(R-C6H4-imphen)]

A series of Re(I) carbonyl chromophores with 1H-imidazo[4,5-f][1,10]phenanthroline (imphen) ligand functionalized with electron-donating amine groups attached to the imidazole ring via phenylene linkage was designed to investigate the impact of...

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.

β-Oxochlorin cobalt(II) complexes catalyze the electrochemical reduction of CO2

Inspired by the architecture of the macrocycle of heme d1, a series of synthetic mono-, di- and tri-β-oxo-substituted porphyrinoid cobalt(ii) complexes were evaluated as electrocatalytic CO2 reducers, identifying complexes of unusually high efficiencies in generating multi-electron reduction products, including CH4.

Methane Generation from CO2 with a Molecular Rhenium Catalyst

The atomic-level tunability of molecular structures is a compelling reason to develop homogeneous catalysts for challenging reactions such as the electrochemical reduction of carbon dioxide to valuable C₁-C_n products. Of particular interest is methane, the largest component of natural gas. Herein, we report a series of three isomeric rhenium tricarbonyl complexes coordinated by the asymmetric diimine ligands 2-(isoquinolin-1-yl)-4,5-dihydrooxazole (quin-1-oxa), 2-(quinolin-2-yl)-4,5-dihydrooxazole (quin-2-oxa), and 2-(isoquinolin-3-yl)-4,5-dihydrooxazole (quin-3-oxa) that catalyze the reduction of CO₂ to carbon monoxide and methane, albeit the latter with a low efficiency. To our knowledge, these complexes are the first examples of rhenium(I) catalysts capable of converting carbon dioxide into methane. Re(quin-1-oxa)(CO)₃Cl (1), Re(quin-2-oxa)(CO)₃Cl (2), and Re(quin-3-oxa)(CO)₃Cl (3) were characterized and studied using a variety of electrochemical and spectroscopic techniques. In bulk electrolysis experiments, the three complexes reduce CO₂ to CO and CH₄. When the controlled-potential electrolysis experiments are performed at -2.5 V (vs Fc^+/0) and in the presence of the Brønsted acid 2,2,2-trifluoroethanol, methane is produced with turnover numbers that range from 1.3 to 1.8. Isotope labeling experiments using ¹³CO₂ atmosphere produce ¹³CH₄ (m/z = 17) confirming that methane originates from CO₂ reduction. Theoretical calculations are performed to investigate the mechanistic aspects of the 8e^-/8H⁺ reduction of CO₂ to CH₄. A ligand-assisted pathway is proposed to be an efficient pathway in the formation of CH₄. Delocalization of the electron density on the (iso)quinoline moiety upon reduction stabilizes the key carbonyl intermediate leading to additional reactivity of this ligand. These results should aid the development of more robust catalytic systems that produce CH₄ from CO₂.

Towards better understanding of photophysical properties of rhenium(I) tricarbonyl complexes with terpy-like ligands

Series of Re(I) carbonyls complexes were designed and synthesized to explore the impact of the triimine skeleton and number of methoxy groups attached to aryl substituents on their optoelectronic and thermal properties. The chemical structures of the prepared complexes were confirmed by ¹H and ¹³C NMR spectroscopy, HR-MS, elemental anlsysis, and X-ray measurements. DSC measuremtns showed that they melted in the range of 198-325 °C. Some of them form stable molecular glasses with high glass transition temperatures (158-173 °C). Experimentally obtained optical properties were supported by DFT calculations. The UV-Vis spectra display a series of overlapping absorption bands in the range 200-350 nm, and much weaker broad band in the visible spectral region, due to intraligand and charge transfer transitions, respectively. All synthesized complexes were emissive in solution and in solid state as powder. Moreover, when applied in diodes, some of them exhibited ability for emission of light under external voltage with maximum of electroluminescence band located at 591-630 nm.

Photoluminescence enhancement of Re(i) carbonyl complexes bearing D-A and D-π-A ligands

Three Re(i) carbonyl complexes [ReCl(CO)3(Ln)] bearing 2,2'-bipyridine, 2,2':6',2''-terpyridine, and 1,10-phenanthroline functionalized with diphenylamine/or triphenylamine units (L1-L3) were synthesized to explore the impact of highly electron donating units appended to the imine ligand on the thermal and optoelectronic properties of Re(i) systems. Additionally, for comparison, the ligands L1-3 and parent complexes [ReCl(CO)3(bipy)], [ReCl(CO)3(phen)] and [ReCl(CO)3(terpy-κ2N)] were investigated. The thermal stability was evaluated by differential scanning calorimetry. The ground- and excited-state electronic properties of the Re(i) complexes were studied by cyclic voltammetry and differential pulse voltammetry, absorption and emission spectroscopy, as well as using density-functional theory (DFT). The majority of the compounds form amorphous molecular materials with high glass transition temperatures above 100 °C. Compared to the unsubstituted complexes [ReCl(CO)3(bipy)], [ReCl(CO)3(phen)] and [ReCl(CO)3(terpy-κ2N)], the HOMO-LUMO gap of the corresponding Re(i) systems bearing modified imine ligands is reduced, and the decrease in the value of the ΔEH-L is mainly caused by the increase in HOMO energy level. In relation to the parent complexes, all designed Re(i) carbonyls were found to show enhanced photoluminescence, both in solution and in solid state. The investigated ligands and complexes were also preliminarily tested as luminophores in light emitting diodes with the structures ITO/PEDOT:PSS/compound/Al and ITO/PEDOT:PSS/PVK:PBD:compound/Al. The pronounced effect of the ligand chemical structure on electroluminescence ability was clearly visible.