Kinetics of the Trans Effect in Ruthenium Complexes Provide Insight into the Factors That Control Activity and Stability in CO2 Electroreduction

Breaking a Molecular Scaling Relationship Using an Iron-Iron Fused Porphyrin Electrocatalyst for Oxygen Reduction

The design of efficient electrocatalysts is limited by scaling relationships governing trade-offs between thermodynamic and kinetic performance metrics. This ″iron law″ of electrocatalysis arises from synthetic design strategies, where structural alterations to a catalyst must balance nucleophilic versus electrophilic character. Efforts to circumvent this fundamental impasse have focused on bioinspired applications of extended coordination spheres and charged sites proximal to a catalytic center. Herein, we report evidence for breaking a molecular scaling relationship involving electrocatalysis of the oxygen reduction reaction (ORR) by leveraging ligand design. We achieve this using a binuclear catalyst (a diiron porphyrin), featuring a macrocyclic ligand with extended electronic conjugation. This ligand motif delocalizes electrons across the molecular scaffold, improving the catalyst's nucleophilic and electrophilic character. As a result, our binuclear catalyst exhibits low overpotential and high catalytic turnover frequency, breaking the traditional trade-off between these two metrics.

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.

Ruthenium(II) Complexes of a Xanthene-Spanned Dicarbene Ligand

Octahedral ruthenium(II) complexes of a xanthene-di(N-heterocyclic carbene) ancillary ligand (XdC) have been prepared and structurally characterized. Examples catalyze the transfer hydrogenation of ketones {[Ru(CO)I2(C,O,C'-XdC)] (1) and [Ru(CO)(MeCN)2(C,O,C'-XdC)]2+ (22+)} and the selective electrochemical reduction of CO2 to CO {[Ru(N,N'-bpy)(CO)(C,O,C'-XdC)]2+ (32+) at 0.40 V overpotential in MeCN-H2O (1 M)}. The reaction of 1 with KBEt3H afforded isomers of [(C,C'-XdC)Ru(μ-H)(H)]2 dimers, which are stable to reductive elimination of the XdC ligand, thereby suggesting similar (XdC)Rh(coligand)(H)x species may be viable intermediates in catalyses. The electrochemical reduction of CO2 involves a double reduction of 32+ to 3••, which has been characterized by IR-SEC and DFT calculations. The DFT calculations suggest the Ru-Oxanth bond breaks in 3••, opening a metal site for CO2 binding with selectivity over protons enabled by the diffuse nature of the HOMO delocalized over the metal and the bipyridine and carbonyl coligands. The results point to the promise of metal complexes of flexible and hemilabile xanthene-(NHC)2 ancillary ligands in catalysis.

Electrocatalytic properties of a novel ruthenium(II) terpyridine-based complex towards CO2 reduction

The electrocatalytic properties of Ru complexes are of great technological interest given their potential application in reactions such water splitting and CO₂ reduction. In this work, a novel terpyridine-based Ru(II) complex, [RuCl(trpy)(acpy)], trpy = 2,2':6',2''-terpyridine, acpy^- = 2-pyridylacetate was synthesized and its spectroscopic, electrochemical and catalytic properties were explored in detail. In dry acetonitrile, the complex exhibits two reduction peaks at -1.95 V and -2.20 V vs. Fc/Fc⁺, attributed to consecutive 1 e^- reduction. Under CO₂ atmosphere, a catalytic wave is observed (E_onset = 2.1 V vs. Fc/Fc⁺), with CO as the main reduction product. Bulk electrolysis reveals a turnover number (TON) of 12 (k_obs = 1.5 s^-1). In the presence of 1% water, an improvement in the catalytic activity is observed (TON_CO = 21 and k_obs = 2.0 s^-1) and, additionally, formate was also detected (TON_HCOO = 7). Spectroelectrochemical experiments allowed the identification of a metallocarboxylate (Ru-COO^-) intermediate under anhydrous conditions, while in water, the partial labilization of the acpy^- ligand was observed in the course of the catalytic cycle. The experimental data was combined with DFT calculations, allowing the proposal of a catalytic cycle. The results establish important relationships between selectivity, ligand structure and reaction conditions.

Sulphur vs NH Group: Effects on the CO2 Electroreduction Capability of Phenylenediamine-Cp Cobalt Complexes

The cobalt complex (I) with cyclopentadienyl and 2-aminothiophenolate ligands was investigated as a homogeneous catalyst for electrochemical CO₂ reduction. By comparing its behavior with an analogous complex with the phenylenediamine (II), the effect of sulfur atom as a substituent has been evaluated. As a result, a positive shift of the reduction potential and the reversibility of the corresponding redox process have been observed, also suggesting a higher stability of the compound with sulfur. Under anhydrous conditions, complex I showed a higher current enhancement in the presence of CO₂ (9.41) in comparison with II (4.12). Moreover, the presence of only one -NH group in I explained the difference in the observed increases on the catalytic activity toward CO₂ due to the presence of water, with current enhancements of 22.73 and 24.40 for I and II, respectively. DFT calculations confirmed the effect of sulfur on the lowering of the energy of the frontier orbitals of I, highlighted by electrochemical measurements. Furthermore, the condensed Fukui function f ^- values agreed very well with the current enhancement observed in the absence of water.

Dual electronic effects achieving a high-performance Ni(II) pincer catalyst for CO2 photoreduction in a noble-metal-free system

A carbazolide-bis(NHC) Ni^II catalyst (1; NHC, N-heterocyclic carbene) for selective CO₂ photoreduction was designed herein by a one-stone-two-birds strategy. The extended π-conjugation and the strong σ/π electron-donation characteristics (two birds) of the carbazolide fragment (one stone) lead to significantly enhanced activity for photoreduction of CO₂ to CO. The turnover number (TON) and turnover frequency (TOF) of 1 were ninefold and eightfold higher than those of the reported pyridinol-bis(NHC) Ni^II complex at the same catalyst concentration using an identical Ir photosensitizer, respectively, with a selectivity of ∼100%. More importantly, an organic dye was applied to displace the Ir photosensitizer to develop a noble-metal-free photocatalytic system, which maintained excellent performance and obtained an outstanding quantum yield of 11.2%. Detailed investigations combining experimental and computational studies revealed the catalytic mechanism, which highlights the potential of the one-stone-two-birds effect.

Visible light-driven CO2 reduction with a Ru polypyridyl complex bearing an N-heterocyclic carbene moiety

A novel Ru polypyridyl complex with an N-heterocyclic carbene ligand was successfully synthesised and characterised. The complex exhibited an intense absorption band in the visible-light region derived from the strong electron-donating character of the carbene ligand, and efficiently catalysed the visible light-driven CO₂ reduction with the reaction rate of 36.7 h^-1.

Cis and trans platinum(II) N-heterocyclic carbene isomers: synthesis, characterization and biological activity

The synthesis of cis and trans geometrical isomers of platinum(II) complexes with a symmetric N-heterocyclic carbene ligand (MeNHC) is reported. These complexes were obtained from 1,3-dimethylimidazolium-2-carboxylate, a masked NHC precursor....

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.

Coordination-driven discrete metallo-supramolecular assembly for rapid and selective photochemical CO2 reduction in aqueous solution

A discrete metallo-supramolecular assembly composed of six iron(ii) cations and twelve redox-active terpyridine fragments has been developed for the highly efficient visible-light-driven reduction of CO2 to CO with a TON of 14 956 and 99.6% selectivity in the presence of an organic thermally activated delayed fluorescence (TADF) photosensitizer 4CzIPN in aqueous solution. The photochemical system proceeds rapidly with a turnover frequency (TOF) of 276 min-1. It is demonstrated that the redox-active terpyridine fragments in the assembly are reduced by the photosensitizer which could further act as an electron reservoir for CO2 reduction, resulting in the highly efficient reduction of CO2. This work shows that discrete metallo-supramolecular assemblies could be used for robust photochemical CO2 reduction.

Dye-Sensitized Nonstoichiometric Strontium Titanate Core-Shell Photocathodes for Photoelectrosynthesis Applications

A core-shell approach that utilizes a high-surface-area conducting core and an outer semiconductor shell is exploited here to prepare p-type dye-sensitized solar energy cells that operate with a minimal applied bias. Photocathodes were prepared by coating thin films of nanocrystalline indium tin oxide with a 0.8 nm Al₂O₃ seeding layer, followed by the chemical growth of nonstoichiometric strontium titanate. Films were annealed and sensitized with either a porphyrin chromophore or a chromophore-catalyst molecular assembly consisting of the porphyrin covalently tethered to the ruthenium complex. The sensitized photoelectrodes produced cathodic photocurrents of up to -315 μA/cm² under simulated sunlight (AM1.5G, 100 mW/cm²) in aqueous media, pH 5. The photocurrent was increased by the addition of regenerative hole donors to the system, consistent with slow interfacial recombination kinetics, an important property of p-type dye-sensitized electrodes.