Molecular Ruthenium Research Articles

The electron-donating effect of pyridinic N sites in covalent triazine frameworks on the molecular ruthenium catalyzing CO2 hydrogenation to formate

In this study, molecular ruthenium (Ru3+) was immobilized onto pyridine-functionalized covalent triazine frameworks (CTFs) to catalyze the hydrogenation of CO2 to formate efficiently. The energy barrier for this process on N3-Ru, coordinated by both pyridinic N and triazinic-N, was found to be 29.91 kcal/mol, significantly lower than the 38.07 kcal/mol on N2-Ru, coordinated only by triazinic-N. Increasing the content of pyridine ring in the catalyst (RuCl3@CTF-1<RuCl3@CTF-2< RuCl3@CTF-3) led to a higher ratio of N3-Ru to N2-Ru. The higher the N3-Ru content, the denser the electron cloud density of Ru species, favoring Ru-H nucleophilic attack on C atoms in CO2, as confirmed by XPS. RuCl3@CTF-3, with the highest content of N3-Ru structure, exhibited the best performance (TOF=1274 h−1 at 120 °C, 6 Mpa) and the lowest apparent activation energy (Ea=36.53 KJ·mol−1) in CO2 hydrogenation, consistent with DFT calculations.

Immobilised Ruthenium Complexes for the Electrooxidation of 5-Hydroxymethylfurfural.

Abundantly available biomass-based platform chemicals, including 5-hydroxymethylfurfural (HMF), are essential stepping stones in steering the chemical industry away from fossil fuels. The efficient catalytic oxidation of HMF to its diacid derivative, 2,5-furandicarboxylic acid (FDCA), is a promising research area with potential applications in the polymer industry. Currently, the most encouraging approaches are based on solid-state catalysts and are often conducted in basic aqueous media, conditions where HMF oxidation competes with its decomposition. Efficient molecular catalysts are practically unknown for this reaction. In this study, we report on the synthesis and electrocatalysis of surface-bound molecular ruthenium complexes for the transformation of HMF to FDCA under acidic conditions. Catalyst immobilisation on mesoporous indium tin oxide electrodes is achieved through the incorporation of phosphonic acid anchoring groups. Screening experiments with HMF and further reaction intermediates revealed the catalytic route and bottlenecks in the catalytic synthesis of FDCA. Utilising these immobilised electrocatalysts, FDCA yields of up to 85 % and faradaic efficiencies of 91 % were achieved, without any indication of substrate decomposition. Surface analysis by X-ray photoelectron spectroscopy (XPS) post-electrocatalysis unveiled the desorption of the catalyst from the electrode surface as a limiting factor in terms of catalytic performance.

A solid xantphos macroligand based on porous organic polymers for the catalytic hydrogenation of CO2

A xanthphos-based porous organic polymer enables the support of a molecular ruthenium complex as solid catalyst for the hydrogenation of CO2 to formic acid as renewable platform chemical.

Molecular Ruthenium Cyclopentadienone Bifunctional Catalysts for the Conversion of Sugar Platforms to Hydrogen.

Molecular ruthenium cyclopentadienone complexes were employed for the first time as pre-catalysts in the homogeneously catalysed Aqueous Phase Reforming (APR) of glucose. Shvo's complex resulted the best pre-catalyst (loading 2 mol%) with H2 yields up to 28.9% at 150 °C. Studies of the final mixture allowed to identify the catalyst's resting state as a mononuclear dicarbonyl complex in the extracted organic fraction. In situ NMR experiments and HPLC analyses on the aqueous fraction gave awareness of the presence of sorbitol, fructose, 5-hydroxymethylfurfural and furfural as final fate or intermediates in the transformations under APR conditions. These results were summarized in a proposed mechanism, with particular emphasis on the steps where hydrogen was obtained as the product. Benzoquinone positively affected the catalyst activation when employed as an equimolar additive.

Proton-Coupled Electron Transfer at the Surface of Polyoxovanadate-Alkoxide Clusters.

ConspectusProton-coupled electron transfer (PCET) is a fundamental process involved in all areas of chemistry, with relevance to biological transformations, catalysis, and emergent energy storage and conversion technologies. Early observations of PCET were reported by Meyer and co-workers in 1981 while investigating the proton dependence of reduction of a molecular ruthenium oxo complex. Since that time, this conceptual framework has grown to encompass an enormous scope of charge transfer and compensation reactions. In this Account, we will discuss ongoing efforts in the Matson Laboratory to understand the fundamental thermodynamics and kinetics of PCET processes at the surface of a series of Lindqvist-type polyoxovanadate clusters. This project aims to provide atomistic resolution of net H atom uptake and transfer at the surfaces of transition-metal oxide materials.First, we discuss our efforts aimed at understanding PCET at metal oxide surfaces using the Lindqvist-type polyoxovanadate-alkoxide (POV-alkoxide) cluster [nBu4N]2[V6O13(TRIOLNO2)2]. These clusters reversibly bind H atom equivalents at bridging oxide sites, mirroring the proposed uptake and release of e-/H+ pairs at transition-metal oxide surfaces. Summarized results include the measurement of bond dissociation free energies of surface hydroxide moieties (BDFE(O-H)) as well as mechanistic analyses that verify concerted proton electron transfer as the operative pathway for PCET at the surface of POV-alkoxide clusters.Next, we discuss net proton and H atom uptake at the surface of reduced variants of the Lindqvist-type POV-alkoxide cluster, [V6O7(OR)12]n (R = Me, Et; n = -2, -1, 0, + 1). In the case of these low-valent POV-alkoxide clusters, nucleophilic bridging sites are kinetically inhibited by functionalization of the cluster surface with organic ligands. This molecular modification enables site-selectivity in proton and H atom uptake to terminal oxide sites. The impact of reaction site and cluster electronics on reaction driving force of PCET is explored, with core electron density playing a critical role in dictating thermodynamics of H atom uptake and transfer. Additional work described here contrasts the kinetics of PCET at terminal oxide sites to the reactivity observed at bridging oxides in POV-alkoxide clusters.Overall, this Account summarizes our foundational knowledge regarding the assessment of PCET reactivity at the surfaces of molecular metal oxides. Drawing analogies between POV-alkoxide clusters and nanoscopic metal oxide materials provide design principles for the advancement of materials applications with atomic precision. These complexes are additionally highlighted as tunable redox mediators in their own right; our studies demonstrate how cluster surface reactivities can be optimized by modifying electronic structure and surface functionalities.

Single‐Site Molecular Ruthenium(II) Water‐Oxidation Catalysts Grafted into a Polymer‐Modified Surface for Improved Stability and Efficiency

AbstractWater‐oxidation is a mechanistic bottleneck in artificial photosynthesis that is critical for achieving sustainable energy‐to‐fuel conversion. Grafting molecular water‐oxidation catalyst assemblies on the surface of a polymer can engender better stability and higher efficiency. Herein, we present a hybrid construct which consists of a molecular single‐site RuCat1 encapsulated into a UV cross‐linked polymer assembly on an F‐doped tin oxide (FTO). Incorporation of high‐density surface‐binding sites (COO− groups) resulted in a high surface loading of the RuCat1 and surface stability as shown from ATR‐FTIR, UV‐vis and cyclic voltammetry data. Furthermore, we found that the grafted RuCat1 into the polymer brush layers promotes pH‐dependent electrocatalytic water‐oxidation with a turnover frequency of 0.45 s−1 and an estimated 88 % faradaic efficiency during control potential electrolysis at pH 8.5. The general applicability of this approach is illustrated by screening a series of molecular single‐site catalysts which points to the importance of electrostatic interactions as well as hydrogen bonding in the functionalization of the electrode.

Block copolymer micelles as efficient colloidal photosensitizers in the light-driven hydrogen evolution reaction

We herein demonstrate the use of block copolymer micelles as highly efficient colloidal photosensitizers in light-driven HER.

Engineering a molecular ruthenium catalyst into three-dimensional metal covalent organic frameworks for efficient water oxidation.

The water oxidation reaction plays an important role in clean energy conversion, utilization, and storage, but mimicking the oxygen-evolving complex of photosystem II for designing active and stable water oxidation catalysts (WOCs) is still an appealing challenge. Here, we innovatively engineered a molecular ruthenium WOC as a metal complex building unit to construct a series of three-dimensional metal covalent organic frameworks (3D MCOFs) for realizing efficient oxidation catalysis. The resultant MCOFs possessed rare 3D interlocking structures with inclined interpenetration of two-dimensional covalent rhombic nets, and the Ru sites were periodically arranged in the crystalline porous frameworks. Impressively, these MCOFs showed excellent performance towards water oxidation (the O2 evolution rate is as high as 2830 nmol g-1 s-1) via the water nucleophilic attack pathway. Besides, the MCOFs were also reactive for oxidizing organic substrates. This work highlights the potential of MCOFs as a designable platform in integrating molecular catalysts for various applications.

Real-Time Polymer Viscosity-Catalytic Activity Relationships on the Microscale.

Polymer growth induces physical changes to catalyst microenvironments. Here, these physical changes are quantified in real time and are found to influence microscale chemical catalysis and the polymerization rate. By developing a method to "peer into" optically transparent living-polymer particles, simultaneous imaging of both viscosity changes and chemical activity was achieved for the first time with high spatiotemporal resolution through a combination of fluorescence intensity microscopy and fluorescence lifetime imaging microscopy techniques. Specifically, an increase in microenvironment viscosity led to a corresponding local decrease in the catalytic molecular ruthenium ring-opening metathesis polymerization rate, plausibly by restricting diffusional access to active catalytic centers. Consistent with this diffusional-access model, these viscosity changes were found to be monomer-dependent, showing larger changes in microenvironment viscosity in cross-linked polydicyclopentadiene compared to non-crosslinked polynorbornene. The sensitivity and high spatial resolution of the imaging technique revealed significant variations in microviscosities between different particles and subparticle regions. These revealed spatial heterogeneities would not be observable through alternative ensemble analytical techniques that provide sample-averaged measurements. The observed spatial heterogeneities provide a physical mechanism for variation in catalytic chemical activity on the microscale that may accumulate and lead to nonhomogeneous polymer properties on the bulk scale.

Cover Feature: Electrocatalytic CO2 Reduction by Molecular Ruthenium Complexes with Polypyridyl Ligands (Chem. Asian J. 14/2022)

A molecular catalyst toward the CO2 reduction reaction (CO2RR) was searched for in the ‘curious garden’ of polypyridine ruthenium complexes. Two series of ruthenium complexes (eight examples) with various polypyridyl ligands were prepared and assessed for electrocatalytic CO2RR. In wet acetonitrile medium, the complexes bearing a 2,2′:6′,2′′:6′′,2′′′-quaterpyridine ligand scaffold effectively catalyze the CO2 to CO conversion. A maximum turnover frequency of 1084.1 s−1 and a Faradaic efficiency of 66% were achieved under the optimized electrocatalytic conditions. More information can be found in the Research Article by Lianpeng Tong et al.

Superresolved Motions of Single Molecular Catalysts during Polymerization Show Wide Distributions.

The motion of single molecular ruthenium catalysts during and after single turnover events of ring-opening metathesis polymerization is imaged through single-molecule superresolution tracking with a positional accuracy of ±32 nm. This tracking is achieved through the real-time incorporation of spectrally tagged monomer units into active polymer chain ends during living polymerization; thus, by design, only active-catalyst motion is detected and imaged, without convolution by inactive catalysts. The catalysts show diverse individualistic diffusive behaviors with respect to time that persist for up to 20 s. Catalysts occupy three mobility populations: quasi-stationary (23%), intermediate (53%, 65 nm), and large (24%, 145 nm) step sizes. Differences in catalyst mobility populations also exist between individual aggregates (p < 0.001). Such differential motion indicates widely different local catalyst microenvironments during the catalytic turnover. These mobility differences are uniquely observable through single-catalyst microscopy and are not measurable through traditional ensemble analytical techniques for characterizing the behavior of molecular catalysts, such as nuclear magnetic resonance spectroscopy. The measured distributions of active molecular catalyst motions would not be readily predictable through modeling or first-principles, and the range likely impacts individual catalyst turnover rate and selectivity. This range plausibly contributes to property distributions observable in bulk polymers, such as molecular weight polydispersity (e.g., 1.9 in this system), leading to a revised understanding of the mechanistic, microscale origins of macroscale polymer properties.

Electrocatalytic CO2 Reduction by Molecular Ruthenium Complexes with Polypyridyl Ligands.

Two series of ruthenium complexes with various polypyridyl ligands have been prepared. One series of complexes (5 examples) are featured with tetradentate polypyridyl ligands and two acetonitrile molecules at the axial positions of the coordination sphere; the other series (3 examples) include combinations of a tridentate polypyridyl ligand, one 2,2'-bipyridine (bpy) or two picolines, and one acetonitrile ligand. All these complexes were fully characterized by their NMR spectra as well as X-ray single crystal structures. Their electronic absorption and redox data were measured and reported. Of the 8 complexes, three candidates effectively catalyze electrochemical CO2 reduction reaction (CO2 RR) in wet acetonitrile medium, generating CO as the major product. All these three catalytically active complexes contain a 2,2':6',2'':6'',2'''-quaterpyridine (qpy) ligand scaffold. A maximum turnover frequency (TOFmax ) of>1000 s-1 was achieved for the electrocatalytic CO2 reduction at a modest overpotential. On the basis of electrochemical and spectroelectrochemical evidences, the CO2 substrate was proposed to bind with the ruthenium center at the two-electron reduced state of the complex and then entered the catalytic cycle.

Transforming Escherichia coli Proteomembranes into Artificial Chloroplasts Using Molecular Photocatalysis.

During the light‐dependent reaction of photosynthesis, green plants couple photoinduced cascades of redox reactions with transmembrane proton translocations to generate reducing equivalents and chemical energy in the form of NADPH (nicotinamide adenine dinucleotide phosphate) and ATP (adenosine triphosphate), respectively. We mimic these basic processes by combining molecular ruthenium polypyridine‐based photocatalysts and inverted vesicles derived from Escherichia coli. Upon irradiation with visible light, the interplay of photocatalytic nicotinamide reduction and enzymatic membrane‐located respiration leads to the simultaneous formation of two biologically active cofactors, NADH (nicotinamide adenine dinucleotide) and ATP, respectively. This inorganic‐biologic hybrid system thus emulates the cofactor delivering function of an active chloroplast.

Exploiting consecutive photoinduced electron transfer (ConPET) in CO2 photoreduction.

The consecutive photoinduced electron transfer (ConPET) process of 1,2,3,5-Tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) in CO2 photoreduction to achieve powerful reducing species has been disclosed by activating a bis(terpyridine)ruthenium(II) complex bearing a high overpotential for selective light-driven reduction of CO2 to CO in homogeneous solution.

Remarkable stability of a molecular ruthenium complex in PEM water electrolysis.

The dinuclear Ru diazadiene olefin complex, [Ru2(OTf)(μ-H)(Me2dad)(dbcot)2], is an active catalyst for hydrogen evolution in a Polymer Exchange Membrane (PEM) water electrolyser. When supported on high surface area carbon black and at 80 °C, [Ru2(OTf)(μ-H)(Me2dad)(dbcot)2]@C evolves hydrogen at the cathode of a PEM electrolysis cell (400 mA cm−2, 1.9 V). A remarkable turn over frequency (TOF) of 7800 molH2 molcatalyst−1 h−1 is maintained over 7 days of operation. A series of model reactions in homogeneous media and in electrochemical half cells, combined with DFT calculations, are used to rationalize the hydrogen evolution mechanism promoted by [Ru2(OTf)(μ-H)(Me2dad)(dbcot)2].

The impact of secondary coordination sphere engineering on water oxidation reactivity catalysed by molecular ruthenium complexes: a next-generation approach to develop advanced catalysts.

Water oxidation is the bottleneck for producing hydrogen from the water-splitting reaction. Developing efficient water oxidation catalysts (WOCs) has recently been of paramount interest. Ruthenium-based WOCs have gained much attention due to their enriched redox property, robust nature, and superior catalytic performances compared to other transition metal-based molecular catalysts. The performance of a catalyst is highly dependent on the design of the ligand framework. In nature, the secondary coordination sphere around the active site of a metalloenzyme plays a vital role in catalysis. This principle has been employed in the recent development of efficient catalysts. With the aid of secondary interactions, some landmark Ru-based WOCs, producing remarkable turnover frequencies (TOFs) in the order of 104 s-1, have been developed. In this account, we have discussed the underlying chemistry related to the effect of secondary interactions (such as hydrogen-bonding, π-π stacking, electrostatic interaction, hydrophobic-hydrophilic environment, etc.) on the kinetics of the water oxidation reaction catalysed by molecular Ru-complexes. The use of secondary interactions (such as π-π and C-H⋯π) in anchoring the molecular catalyst onto the solid conducting surface has also been discussed. We aim to provide a brief overview of the positive impact of outer-sphere engineering on water oxidation reactivity, which may offer guidelines for developing the next generation of advanced catalysts.

Electrochemical ruthenium-catalysed C–H activation in water through heterogenization of a molecular catalyst

The synthesis and characterisation of an immobilised molecular ruthenium catalyst for the selective oxidation of organic substrates in fully aqueous media with high selectivity over the competing oxygen evolution reaction is reported.

Acceptorless Dehydrogenation of Methanol to Carbon Monoxide and Hydrogen using Molecular Catalysts.

The acceptorless dehydrogenation of methanol to carbon monoxide and hydrogen was investigated using homogeneous molecular complexes. Complexes of ruthenium and manganese comprising the MACHO ligand framework showed promising activities for this reaction. The molecular ruthenium complex [RuH(CO)(BH4)(HN(C2H4PPh2)2)] (Ru‐MACHO‐BH) achieved up to 3150 turnovers for carbon monoxide and 9230 turnovers for hydrogen formation at 150 °C reaching pressures up to 12 bar when the decomposition was carried out in a closed vessel. Control experiments affirmed that the metal complex mediates the initial fast dehydrogenation of methanol to formaldehyde and methyl formate followed by subsequent slow decarbonylation. Depending on the catalyst and reaction conditions, the CO/H2 ratio in the gas mixture thus varies over a broad range from almost pure hydrogen to the stoichiometric limit of 1:2.

Acceptorless Dehydrogenation of Methanol to Carbon Monoxide and Hydrogen using Molecular Catalysts

AbstractThe acceptorless dehydrogenation of methanol to carbon monoxide and hydrogen was investigated using homogeneous molecular complexes. Complexes of ruthenium and manganese comprising the MACHO ligand framework showed promising activities for this reaction. The molecular ruthenium complex [RuH(CO)(BH4)(HN(C2H4PPh2)2)] (Ru‐MACHO‐BH) achieved up to 3150 turnovers for carbon monoxide and 9230 turnovers for hydrogen formation at 150 °C reaching pressures up to 12 bar when the decomposition was carried out in a closed vessel. Control experiments affirmed that the metal complex mediates the initial fast dehydrogenation of methanol to formaldehyde and methyl formate followed by subsequent slow decarbonylation. Depending on the catalyst and reaction conditions, the CO/H2 ratio in the gas mixture thus varies over a broad range from almost pure hydrogen to the stoichiometric limit of 1:2.

Efficient workflow for the investigation of the catalytic cycle of water oxidation catalysts: Combining GFN-xTB and density functional theory.

Photocatalytic water oxidation remains the bottleneck in many artificial photosynthesis devices. The efficiency of this challenging process is inherently linked to the thermodynamic and electronic properties of the chromophore and the water oxidation catalyst (WOC). Computational investigations can facilitate the search for favorable chromophore‐catalyst combinations. However, this remains a demanding task due to the requirements on the computational method that should be able to correctly describe different spin and oxidation states of the transition metal, the influence of solvation and the different rates of the charge transfer and water oxidation processes. To determine a suitable method with favorable cost/accuracy ratios, the full catalytic cycle of a molecular ruthenium based WOC is investigated using different computational methods, including density functional theory (DFT) with different functionals (GGA, Hybrid, Double Hybrid) as well as the semi‐empirical tight binding approach GFN‐xTB. A workflow with low computational cost is proposed that combines GFN‐xTB and DFT and provides reliable results. GFN‐xTB geometries and frequencies combined with single‐point DFT energies give free energy changes along the catalytic cycle that closely follow the full DFT results and show satisfactory agreement with experiment, while significantly decreasing the computational cost. This workflow allows for cost efficient determination of energetic, thermodynamic and dynamic properties of WOCs.