Ru Metal Center Research Articles

Addition of Imidazolium-Based Ionic Liquid to Improve Methanol Production in Polyamine-Assisted CO2 Capture and Conversion Systems Using Pincer Catalysts.

Ionic liquids have been studied as CO2 capture agents. However, they are rarely used in combined CO2 capture and conversion processes. Utilizing imidazolium-based ionic liquids, the conversion of CO2 to methanol was greatly improved in polyamine assisted systems catalyzed by homogeneous pincer catalysts with Ru and Mn metal centers. Among the ionic liquids tested, [BMIM]OAc was found to perform the best under the given reaction conditions. Among the polyamine tested, pentaethylenehexamine (PEHA) led to the highest conversion rates. Ru-Macho and Ru-Macho-BH were the most active catalysts. Direct air capture utilizing PEHA as the capture material was also demonstrated and produced an 86 % conversion of the captured CO2 to methanol in the presence of [BMIM]OAc.

Design, Synthesis, and Characterization of Organometallic BODIPY-Ru(II) Dyads: Redox and Photophysical Properties with Singlet Oxygen Generation Capability†.

A series of Ru(II)-acetylide complexes (Ru1, Ru2, and Ru1m) with alkynyl-functionalized borondipyrromethene (BODIPY) conjugates were designed by varying the position of the linker that connects the BODIPY unit to the Ru(II) metal center through acetylide linkage at either the 2-(Ru1) and 2,6-(Ru2) or the meso-phenyl (Ru1m) position of the BODIPY scaffold. The Ru(II) organometallic complexes were characterized by various spectroscopic methods, including nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, CHN, and high-resolution mass spectrometry (HRMS) analyses. The Ru(II)-BODIPY conjugates exhibit fascinating electrochemical and photophysical properties. All BODIPY-Ru(II) complexes exhibit strong absorption (εmax = 29,000-72,000 M-1 cm-1) in the visible region (λmax = 502-709 nm). Fluorescence is almost quenched for Ru1 and Ru2, whereas Ru1m shows the residual fluorescence of the corresponding BODIPY core at 517 nm. The application of the BODIPY-Ru(II) dyads as nonporphyrin-based triplet photosensitizers was explored by a method involving the singlet oxygen (1O2)-mediated photo-oxidation of diphenylisobenzofuran. Effective π-conjugation between the BODIPY chromophore and Ru(II) center in the case of Ru1 and Ru2 was found to be necessary to improve intersystem crossing (ISC) and hence the 1O2-sensitizing ability. In addition, electrochemical studies indicate electronic interplay between the metal center and the redox-active BODIPY in the BODIPY-Ru(II) dyads.

Impact of Molecular Orientation on Lateral and Interfacial Electron Transfer at Oxide Interfaces.

Molecular dyes, called sensitizers, with a cis-[Ru(LL)(dcb)(NCS)2] structure, where dcb is 4,4'-(CO2H)2-2,2'-bipyridine and LL is dcb or a different diimine ligand, are among the most optimal for application in dye-sensitized solar cells (DSSCs). Herein, a series of five sensitizers, three bearing two dcb ligands and two bearing one dcb ligand, were anchored to mesoporous thin films of conducting tin-doped indium oxide (ITO) or semiconducting TiO2 nanocrystallites. The number of dcb ligands impacts the surface orientation of the sensitizer; density functional theory (DFT) calculations revealed an ∼1.6 Å smaller distance between the oxide surface and the Ru metal center for sensitizers with two dcb ligands. Interfacial electron transfer kinetics from the oxide material to the oxidized sensitizer were measured as a function of the thermodynamic driving force. Analysis of the kinetic data with Marcus-Gerischer theory indicated that the electron coupling matrix element, Hab, was sensitive to distance and ranged from Hab = 0.23 to 0.70 cm-1, indicative of nonadiabatic electron transfer. The reorganization energies, λ, were also sensitive to the sensitizer location within the electric double layer and were smaller, with one exception, for sensitizers bearing two dcb ligands λ = 0.40-0.55 eV relative to those with one λ = 0.63-0.66 eV, in agreement with dielectric continuum theory. Electron transfer from the oxide to the photoexcited sensitizer was observed when the diimine ligand was more easily reduced than the dcb ligand. Lateral self-exchange "hole hopping" electron transfer between surface-anchored sensitizers was found to be absent for sensitizers with two dcb ligands, while those with only one were found to hop with rates similar to those previously reported in the literature, khh = 47-89 μs-1. Collectively, the kinetic data and analysis reveal that interfacial kinetics are highly sensitive to the surface orientation and sensitizers bearing two dcb ligands are most optimal for practical applications of DSSCs.

Highly efficient hydrogenation of furfural to valuable products catalysed by Rh (III), Pd (II), Ni (II), Fe (II) and Ru (II) half‐sandwich picolinamide and the SBA‐15 supported molecular complexes as pre‐catalysts

Novel palladium, rhodium, iron, ruthenium and nickel half‐sandwich picolinamide complexes have been prepared and characterised in detail using high‐resolution electrospray ionisation mass spectrometry (HR‐ESI‐MS), multi‐dimensional nuclear magnetic resonance spectroscopy, elemental analysis (EA), Fourier transform‐infrared spectroscopy (FT‐IR), and single crystal X‐ray diffraction (SCXRD). All the systems proved to be active with good catalytic performance for the hydrogenation of furfural. Among the pre‐catalyst,C2(Rh metal centre) displayed complete conversion (100%). Pre‐catalystC5(Ru metal centre) also resulted in excellent selectivity to EL (51%) and FA (49%). All the pre‐catalysts with early transition metal centres (Ni, Fe) gave selectivity towards FA (100%) under optimal conditions. Possible pathways for the formation of catalytically active have been proposed based on in situ NMR studies obtained usingC2. Successful modification of molecular complexes (C2andC3) on SBA‐15 resulted in supported molecular complexes (C2@SBA‐15andC3@SBA‐15). Furthermore, the supported molecular catalysts were characterised using inductively coupled plasma‐optical emission spectroscopy (ICP‐OES), Fourier‐transform infrared spectroscopy (FT‐IR), powder X‐ray diffraction (PXRD), BET surface area, selected area electron diffraction (SAED), X‐ray photoelectron spectroscopy (XPS) and high‐resolution transmission electron microscopy (HR‐TEM). The supported molecular complexC2@SBA‐15was recycled five times (with consistent activity and selectivity).

Enhancing aromatic hydrocarbon formation via catalytic depolymerization of lignin waste over Ru/WOx/N-C catalyst

Direct converting Kraft lignin into valuable chemicals by catalytic hydrogenolysis is a promising method to develop biomass valorization. In our research, the multifunctional porous carbon material catalyst was prepared for lignin depolymerization reaction. Through the introduction of ammonium oxalate as both foaming agent and nitrogen source increased the carbon materials surface area (608.74 m2/g), which improved the stability and dispersion of Ru and WOx nanoparticles. Based on the catalyst characterization and liquid product composition analysis, the lignin could quickly achieve hydrodeoxygenation process over the connected multi-stage pore structure assisted with Ru metal center and WOx oxophilic sites. Especially, the 96.89 wt% bio-oil yield, 36.85 wt% monomer yield and 20.85 wt% arenes yield were achieved over a porous 5Ru/30WOx/N-C catalyst. Under the optimal reaction condition, the higher heating values of bio-oil could increase to 35.68 MJ/kg. This research provided a new method for catalyst design towards highly efficient and selective hydrogenolysis of lignin to useful aromatic chemicals.

Ruthenium(II) Phosphine/Picolylamine Dichloride Complexes Hydrogenation and DFT Calculations

Treating [Ru(PPh3)3Cl2] with the amine 2-picolylamine (Picam) ligand in a 1:1 molar ratio, the Ru(II) complex trans-Ru(PPh3)2(Picam)Cl2 (1) is obtained in methylene chloride and can be isolated as a pure solid compound. The single-crystal structure of 1 was determined by X-ray crystallography. The geometry at the Ru metal center is a distorted octahedral environment with a trans arrangement of the two chlorides. A trans effect of the bond lengths was observed within the structure. Similarly, treating [Ru(PPh3)3Cl2] with 1:1:1 molar ratios of 2-picolylamine (Picam) and 1,1′-bis(diphenylphosphine)ferrocene (DPPF) ligands yielded the Ru(II) complex trans-Ru(DPPF)(Picam)Cl2 (2). In identical conditions, the homogeneous hydrogen transfer catalytic reactivity of complexes 1 and 2 has been tested in a basic 2-propanol solution and they indicate different catalytic activity. It was discovered that monodentate and bidentate phosphine ligands of Ru(II) complexes, as well as cis- and trans-chloro configuration display different catalytic properties from our experimental data, in agreement with literature data. Based on DFT calculations, the relative molecular catalytic reactivity of all available experimental data is understood from the relative calculated molecular energy.

Selenocysteine of thioredoxin reductase as the primary target for the antitumor metallodrugs: A computational point of view

Selenocysteine (Sec) has been recognized as a soft metal-coordinating residue that dues its relevance to the important role played in the catalytic activity of thioredoxin reductase (TrxR), a well-characterized anticancer target. Thus, several computational studies have been devoted to the metalation of this Se-based residue and several metal scaffolds have been proposed as potential Sec-binders, i.e. TrxR inhibitors. Here, we reported an overview of the chemical features of Sec, as well as of typical Sec-binding metal complexes, and of the computational studies carried on metallodrugs that primary target TrxR, by specifically posing our attention to antitumour metallodrugs based on Au(I), Au(III), Ag(I), Pt(II), Pt(IV), Ru(II), Ru(III), and Rh(III) metal centers. Moreover, we provided a computational insight on the available structural data and highlighted how an optimal articulation of computational methods and modeling of the metallodrug-target system may expand our understanding of the targeting of TrxR active site by metallic scaffolds.

Synthesis, characterization and application of Ru(II) complexes containing pyridil ligands for dye-sensitized solar cells

Abstract In this research, a series of Ru(II) complexes, ([Ru(1-7)(ina)(NCS)2] (1-7=5-[6-(5-mercapto-1,3,4-oxadiazol-2-yl)pyridin- 2-yl]-1,3,4-oxadiazole-2-thiol’s, ina=isonicotinic acid) were synthesized and characterized using different spectroscopic and analytic techniques, such as NMR, UV, IR, CV and CHN. Also, the new complexes were used in dye-sensitized solar cells (DSSC) as sensitizers. Current-voltage characteristics showed that the modifications of ligands clearly affected DSSC yield. Additionally, DFT calculations were performed and showed locations of frontier molecular orbitals of the complexes. While the locations of HOMO and HOMO – 1 orbitals are on Ru(II) metal center and SCN− ligands, the location of LUMO and LUMO + 1 orbitals are on the 1-7 ligands.

Development of highly potent Arene-Ru (II)-ninhydrin complexes for inhibition of cancer cell growth

In this communication, two new Ru(II)-arene-ninhydrin complexes [(η6-p-cymene)RuCl(k2-N,O-2-(1,3-dioxo-1,3-dihydro-2H-inden-2-ylidene)hydrazine-1-carboxamide (Ru-1) and [(η6-p-cymene)RuCl(k2-N,S-2-(1,3-dioxo-1,3-dihydro-2H-inden-2-ylidene)hydrazine-1-carbothioamide (Ru-2) have been prepared and analyzed by different analytical and spectroscopic techniques. The studies show that Ru(II) complexes have “piano stool” coordination geometry, comprises of one π-bonded arene centroid, two σ-bonded nitrogen atoms and one chlorine atom from ninhydrin thiosemicarbazole and Ru(II) metal centre respectively. The molar conductivity study of these complexes in DMSO:water (9:1, v/v) media reveals that complexes are ionic in nature. The photo-physical properties of Ru-1 and Ru-2 with calf thymus deoxyribonucleic acid (CT-DNA) and bovine serum albumin (BSA) were explored. The intrinsic binding constant (Kb) of Ru-1 and Ru-2 with DNA/BSA was found to be 4.5 × 105 M−1/1.3 × 105 M−1 and 2.1 × 105 M−1/1.2 × 105 M−1 respectively. The competitive displacement of ethidium bromide (EtBr) from DNA in the presence of Ru-1 and Ru-2 quantified by quenching constant and viscosity studies. This high binding ability is due to groove binding or intercalation of Ru(II) complexes with CT-DNA which is well supported with in-silico studies. The effect of the Ru-1 and Ru-2 were examined on two different cancer cell lines (HeLa and MDA-MB-231) using the MTT assay. The obtained results show that Ru-1 and Ru-2 exhibits high potency and selectivity with HeLa (IC50 = 9.5 × 10−6 mol/L) and MDA-MB-231(IC50 = 14.7 × 10−6 mol/L) cell lines respectively.

The rate of substitution from η6-arene ruthenium(II) complexes

The rate and mechanism of substitution in the Ru(II) complexes (C1–C6) by thiourea nucleophiles was studied at pH 2 and rate constants measured as a function of nucleophile concentrations and temperature using spectrometric methods. There is increased electron density at the Ru metal atom of C2 as a result of inductive donation by substituents on the arene ligand, making it less positive and therefore less reactive than C1. For the complexes C3–C6 bearing the 2,2′-bipyridyl ligand, the aqua ligands are located trans to the arene ligands, and hence, their reactivity increases in accordance to the number and type of alkyl substituents on the η6-arene ligands which donate inductively into the π-molecular orbitals, causing increased trans labialisation of the coordinated aquo co-ligand. Compared to the reactivity of triaquo complex (C1), the auxiliary bipyridyl ligand of (C3) complex lowers the rate of substitution for the later complex by a factor of about 100, possibly due to its steric hindrance at the Ru(II) metal centre. The significantly negative activation entropies and positive activation enthalpies suggest an associative mode of substitution. The reactivity of the nucleophiles follow the order DMTU > TU > TMTU.

Naphthoquinones of natural origin: Aqueous chemistry and coordination to half-sandwich organometallic cations

Half-sandwich organometallic complexes featuring Ru(II), Os(II) and Rh(III) metal centers and naturally occurring bidentate 2-hydroxy-[1,4]-naphthoquinone ligands (lawsone and phthiocol) have been synthesized and characterized in both solid state and solution phase by analytical, spectroscopic, electrochemical and single crystal X-ray diffraction techniques. Comparative studies revealed the influence of the respective metal center (Ru, Os, Rh), leaving group (Cl, Br, I) and arene (p-cymene, toluene, pentamethylcyclopentadienyl), as well as the naphthoquinone ligand on the structural properties and solution speciation. Additionally, cytotoxicity was tested in SW480, CH1/PA-1 and A549 human cancer cell lines showing a broad range of IC50 values.

Benzoate Cyclometalation Enables Oxidative Addition of Haloarenes at a Ru(II) Center.

The first Ru(II)-catalyzed arylation of substrates without a directing group was recently developed. Remarkably, this process only worked in the presence of a benzoate additive, found to be crucial for the oxidative addition step at Ru(II). However, the exact mode of action of the benzoate was unknown. Herein, we disclose a mechanistic study that elucidates the key role of the benzoate salt in the C–H arylation of fluoroarenes with aryl halides. Through a combination of rationally designed stoichiometric experiments and DFT studies, we demonstrate that the aryl–Ru(II) species arising from initial C–H activation of the fluoroarene undergoes cyclometalation with the benzoate to generate an anionic Ru(II) intermediate. The enhanced lability of this intermediate, coupled with the electron-rich anionic Ru(II) metal center renders the oxidative addition of the aryl halide accessible. The role of an additional (NMe4)OC(CF3)3 additive in facilitating the overall arylation process is also shown to be linked to a shift in the C–H pre-equilibrium associated with benzoate cyclometalation.

Ter-Ionic Complex that Forms a Bond Upon Visible Light Absorption.

A "ter-ionic complex" composed of a tetracationic Ru(II) complex and two iodide ions was found to yield a covalent I-I bond upon visible light excitation in acetone solution. 1H NMR, visible absorption and DFT studies revealed that one iodide was associated with a ligand while the other was closer to the Ru metal center. Standard Stern-Volmer quenching of the excited state by iodide revealed upward curvature with a novel saturation at high concentrations. The data were fully consistent with a mechanism in which the Ru metal center in the excited state accepts an electron from iodide to form an iodine atom and, within 70 ns, that atom reacts with the iodide associated with the ligand to yield I2•-. This rapid formation of an I-I bond was facilitated by the supramolecular assembly of the three reactant ions necessary for this ter-ionic reaction that is relevant to solar fuel production.

Intramolecular Electronic Coupling Enhances Lateral Electron Transfer across Semiconductor Interfaces

The control of lateral electron-transfer reactions is important for many solar energy conversion strategies. Herein, four compounds with two redox-active groups, a bis(tridentate) cyclometalated RuII metal center and a substituted triphenylamine (TPA) donor separated by an organic bridge, were anchored to TiO2 surfaces to facilitate study of lateral intermolecular electron transfer. An important finding was that the TPA+/0 diffusion coefficients were about 1.6 times larger when the bridge promoted intramolecular electronic coupling between the Ru metal center and TPA. Under conditions where TPA+ was able to oxidize the RuII center or intramolecular electronic coupling was large, the RuIII/II electron transfer was facilitated by TPA+/0 transport. These findings indicate that synergistic interactions between redox-active groups can be tailored to control electron transfer at the molecular level across metal oxide surfaces.

Mechanistic Considerations on Homogeneously Catalyzed Formic Acid Dehydrogenation

The dehydrogenation of formic acid permits the production of hydrogen virtually free of carbon monoxide, which is a key requisite for its use in fuel cells. Moreover, HCOOH is a suitable hydrogen carrier, better in several crucial characteristics than other liquid organic hydrogen carriers, namely, low toxicity, high hydrogen content and recyclability (via CO2 hydrogenation). The most successfully employed catalysts are those based on Ir, Ru, Rh, and Fe metal centers. The catalytic cycles through which homogeneous metal complexes operate show a wide mechanistic diversity. The nature of catalytic cycle depends mainly on the nuclearity of the active species, the type of CO2 formation step (β‐hydride elimination, hydride abstraction, or outer‐sphere mechanism), and whether the rate‐limiting step relates to CO2 or H2 formation. Different outer‐sphere interactions have been proposed in the literature in order to explain the success of homogeneous catalyst, which seems to be common ground for most of the catalysts so far reported.

Electrochemistry of Ruthenium Bis(imino)pyridine Compounds: Evidence for an ECE Mechanism and Isolation of Mono and Dicationic Complexes.

The electrochemical properties of a series of ruthenium complexes bearing the redox noninnocent bis(imino)pyridine ligand were investigated with cyclic voltammetry. An unexpected electrochemical feature in one of the complexes indicated a possible electrochemical-chemical-electrochemical (ECE) process. Further investigation into the oxidation mechanism included the synthesis and isolation of cationic and dicationic analogues of the complex. Density functional theory calculations suggested that a solvent interaction facilitated the ECE mechanism, so a solvent adduct was also synthesized and isolated. Calculations of this series of compounds were used to investigate the stabilizing role of the ligands over various oxidation states. As the overall charge of the complex was varied from anion to dication, a relatively constant Ru metal center oxidation state was found over all four states probed. This result suggests the strong involvement of the coligands in the redox behavior of the complexes investigated.

Chloride Oxidation by Ruthenium Excited-States in Solution.

Photodriven HCl splitting to produce solar fuels is an important goal that requires strong photo-oxidants capable of chloride oxidation. In a molecular approach toward this goal, three ruthenium compounds with 2,2'-bipyrazine backbones were found to oxidize chloride ions in acetone solution. Nanosecond transient absorption measurements provide compelling evidence for excited-state electron transfer from chloride to the Ru metal center with rate constants in excess of 1010 M-1 s-1. The Cl atom product was trapped with an olefin. This reactivity was promoted through pre-organization of ground-state precursors in ion pairs. Chloride oxidation with a tetra-cationic ruthenium complex was most favorable, as the dicationic complexes were susceptible to photochemical ligand loss. Marcus analysis afforded an estimate of the chlorine formal reduction potential E°(Cl•/-) = 1.87 V vs NHE that is at least 300 meV more favorable than the accepted values in water.

2,2'-Bipyridyl-Embedded Cycloparaphenylenes as a General Strategy To Investigate Nanohoop-Based Coordination Complexes.

Because of their unique cyclic architectures, tunable electronic properties, and supramolecular chemistries, cycloparaphenylenes (CPPs) have the potential to act as a new class of ligands for coordination cages, metal-organic frameworks, and small-molecule transition-metal complexes. However, currently there is no general strategy to coordinate the cyclic framework to a variety of metal centers. We report here a general and scalable synthetic strategy to embed 2,2'-bipyridine units into the backbone of CPPs. We use this approach to synthesize a 2,2'-bipyridine-embedded [8]CPP, which we show can successfully coordinate to both Pd(II) and Ru(II) metal centers. The resulting coordination complexes, a Pd(II)-nanohoop dimer and a bis(bipyridyl)ruthenium(II)-functionalized nanohoop, show unique solid-state and photophysical properties. This work provides a proof of concept for a general strategy to use nanohoops and their derivatives as a new class of ligands.

Heteroleptic tris-chelate ruthenium(II) complexes of N,N-disubstituted-N′-acylthioureas: Synthesis, structural studies, cytotoxic activity and confocal microscopy studies

Ruthenium complexes have been assessed as anti-tumor agents against cancer cells. In this project, new heteroleptic ruthenium(II) complexes with general formulae [Ru(L)(bipy)(dppb)](PF6) (where L=N,N-disubstituted-N′-acylthiourea, bipy=2,2′-bipyridine and dppb=1,4-bis(diphenylphosphino)butane) were synthesized and characterized by elemental analysis, IR and NMR (1H and 31P{1H}) spectroscopies, molar conductivity measurements and single crystal X-ray diffractometry. The IR and NMR data suggest the coordination of the ligands to the Ru(II) metal center through the thiocarbonyl and carbonyl groups. The structures of the new complexes were further studied by X-ray crystallography, which confirmed the coordination of the ligands with the metal through the sulfur and oxygen atoms, leading to the formation of distorted octahedral complexes. The N,N-disubstituted-N′-acylthioureas and their complexes were screened with respect to their in vitro cytotoxicity. All compounds exhibited considerable antiproliferative activity against MCF-7 (human breast tumor cells ATCC HTB-26), DU-145 (human prostate tumor cells ATCC HTB-26), and relatively low toxicity against fibroblast L929 cells (health cell line from mouse ATCC CCL-1). A preliminary study regarding the mechanism of action of these compounds by confocal microscopy shows alterations of the actin filaments leading to modifications in cytoskeletal supporting the cell death and that the cell nucleus is not main target of these complexes.

A New Kaempferol‐based Ru(II) Coordination Complex, Ru(kaem)Cl(DMSO)3: Structure and Absorption–Emission Spectroscopy Study

Recent interest in developing a new anticancer drug with low side effects has led to the study of the combination of two new anticancer drugs. Although both kaempferol (kaem) and Ru‐based metal complexes have not been proven as effective drugs, their unique anticancer activities with reduced side effects have drawn our attention to the need for further studies on their potential in anticancer application. Herein, we report the synthesis, characterization, structure, and spectroscopic properties of a kaem‐based Ru (II) complex, RuCl(kaem)(DMSO)3 (1). Because of the presence of a catechol‐like functional group in its dihydropyran ring, kaem can strongly bind to the Ru(II) metal center in a basic medium. The molecular structure of the complex was characterized by spectroscopic studies and X‐ray crystal structure analysis. In addition, the complex forms a molecular dimer as a result of the cooperative effect of H‐bonding and π–π stacking interactions. Moreover, the molecular dimer forms a ladder‐like one‐dimensional network structure by water mediated H‐bonding that further extended into a three‐dimensional packing structure. UV–Vis spectroscopy studies of the complex demonstrated the appearance of a strong metal to ligand charge transfer (MLCT) band in the visible region with strong fluorescence emission derived from the MLCT. Further studies are now in progress to demonstrate synergetic anticancer activity.