Mononuclear Ruthenium Research Articles

Water‐Soluble Half‐Sandwich RuII–Arene Complexes: Synthesis, Structure, Electrochemistry, DFT Studies, and Aqueous Phase Hydroformylation of 1‐Octene

AbstractWater‐soluble mononuclear ruthenium complexes containing monodentate P‐donor ligands [1,3,5‐triaza‐7‐phosphaadamantane (PTA) and P(OMe)3], together with a mononuclear bidentate salicylaldimine ruthenium–arene complex, were synthesized from a water‐soluble dimeric ruthenium(II)–arene precursor. These complexes were used as catalyst precursors in the aqueous biphasic hydroformylation of 1‐octene. Hydroformylation reactions with these Ru‐arene complexes showed that the complexes are active hydroformylation catalyst precursors in water. The catalyst precursors convert 1‐octene into a range of products, which include aldehydes and alcohols, as well as isomerization and hydrogenation products. The less basic the ligand, the more it favoured the production of aldehydes. The relative basicity of the ligands P(OMe)3, PTA and the N,O‐chelating ligand was elucidated by cyclic voltammetry and supported by DFT calculations. The catalysts show good recyclability without a significant decrease in 1‐octene conversion. All of the catalysts could be reused three times.

Evaluation of DNA-binding, DNA cleavage, antioxidant and cytotoxic activity of mononuclear ruthenium(II) carbonyl complexes of benzaldehyde 4-phenyl-3-thiosemicarbazones

Two 4-phenyl-3-thiosemicarbazone ligands, (E)-2-(2-chlorobenzylidene)-N-phenylhydrazinecarbothioamide (HL1) and (E)-2-(2-nitrobenzylidene)-N-phenylhydrazinecarbothioamide (HL2), and its ruthenium(II) complexes were synthesized and characterized by physico-chemical and spectroscopic methods. The Schiff bases act as bidentate, monobasic chelating ligands with S and N as the donor sites and are preferably found in the thiol form in all the complexes studied. The molecular structure of HL1 and HL2 were determined by single crystal X-ray diffraction method. DNA binding of the compounds was investigated by absorption spectroscopy which indicated that the compounds bind to DNA via intercalation. The oxidative cleavage of the complexes with CT-DNA inferred that the effects of cleavage are dose dependent. Antioxidant study of the ligands and complexes showed significant antioxidant activity against DPPH radical. In addition, the in vitro cytotoxicity of the ligands and complexes assayed against HeLa and MCF-7 cell lines showed higher cytotoxic activity with the lower IC50 values indicating their efficiency in killing the cancer cells even at low concentrations.

An Electrochemical and Raman Spectroscopy Study of the Surface Behaviour of Mononuclear Ruthenium and Osmium Polypyridyl Complexes Based on Pyridyl‐ and Thiophene‐Based Linkers

AbstractThe utility of a thiophene anchor unit as an alternative for thiols in the immobilisation of ruthenium and osmium complexes on gold and platinum is examined with special attention focused on the relative contributions of physi‐ and chemisorption of the complexes and the chemical stability of the thiophene anchoring unit. The redox and spectroscopic properties of the ruthenium(II) and osmium(II) complexes are described in solution and the effect of surface immobilisation examined through a combined electrochemical and surface‐enhanced Raman spectroscopic study. A key finding is that although the thiophene unit is involved in surface anchoring it also undergoes chemical reactions with the gold surface as demonstrated by Raman spectroscopy.

Mechanisms of Photoisomerization and Water-Oxidation Catalysis of Mononuclear Ruthenium(II) Monoaquo Complexes

A ligation of Ru(tpy)Cl3 (tpy = 2,2':6',2"-terpyridine) with 2-(2-pyridyl)-1,8-naphthyridine) (pynp) in the presence of LiCl gave distal-[Ru(tpy)(pynp)Cl](+) (d-1Cl) selectively, whereas the ligation gave proximal-[Ru(tpy)(pynp)OH2](2+) (p-1H2O) selectively in the absence of halide ions. (The proximal/distal isomers were defined by the structural configuration between the 1,8-naphthyridine moiety and the aquo or chloro ligand.) An aquation reaction of d-1Cl quantitatively afforded distal-[Ru(tpy)(pynp)OH2](2+) (d-1H2O) in water, and d-1H2O is quantitatively photoisomerized to p-1H2O. The mechanism of the photoisomerization was investigated by transient absorption spectroscopy and quantum chemical calculations. The temperature dependence of the transient absorption spectral change suggests existence of the thermally activated process from the (3)MLCT state with the activation energy (ΔE = 49 kJ mol(-1)), which is close to that (41.7 kJ mol(-1)) of the overall photoisomerization reaction. However, quantum chemical calculations suggest another activation process involving the conformational change of the pentacoordinated distal structure to the proximal structure. Quantum chemical calculations provide redox potentials and pK(a) values for proton-coupled electron transfer reactions from Ru(II)-OH2 to Ru(IV)═O in good agreement with experiments and provide an explanation for mechanistic differences between d-1H2O and p-1H2O with respect to water oxidation. The calculations show that water nucleophilic attack (WNA) on d-[Ru(V)-O](3+) (the ruthenyl oxo species derived from d-1H2O, calculated ΔG(‡) of 87.9 kJ/mol) is favored over p-[Ru(V)-O](3+) (calculated ΔG(‡) of 104.6 kJ/mol) for O-O bond formation. Examination of the lowest unoccupied molecular orbitals in d- and p-[Ru(V)-O](3+) indicates that more orbital amplitude is concentrated on the [Ru-O] unit in the case of d-[Ru(V)-O](3+) than in the case of p-[Ru(V)-O](3+), where some of the amplitude is instead delocalized over the pynp ligand, making this isomer less electrophilic.

Synthesis and Characterization of an Azido-Bridged Dinuclear Ruthenium(II) Polypyridylamine Complex Forming a Mixed-Valence State

We have synthesized a mononuclear ruthenium(II) azido complex (1) and a dinuclear ruthenium(II) μ-azido complex (2) having N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine (N4Py) as a pentadentate ancillary ligand. In the crystal structure of 2, intramolecular π-π stacking was found between the pyridine rings of the two different N4Py ligands, contributing to stabilize the dinuclear μ-azido structure. π donation from the HOMO π* orbital of the μ-azido ligand to the Ru-N(pyr) bond increases the bond order between the terminal and central N atoms in the μ-azido ligand to strengthen the N-N bonds of the μ-azido ligand. The μ-azido complex 2 was revealed to exhibit a stepwise oxidation behavior in CH3CN to afford a Ru(II)-μ-azido-Ru(III) mixed-valence (MV) state upon one-electron oxidation. The MV state of one-electron-oxidized 2 was categorized in the Robin-Day class II with the electronic coupling constant (H(ab)) of 570 cm(-1).

What Factors Control O2 Binding and Release Thermodynamics in Mononuclear Ruthenium Water Oxidation Catalysts? A Theoretical Exploration

Mononuclear Ru-based water oxidation catalysts (WOCs) constitute an important class of WOCs for water splitting. This work constitutes a systematic study of Ru-O2 complexes of mononuclear ruthenium WOCs, with a focus on the thermodynamics of water-assisted O2 release in various electronic states and conformations. Our extensive DFT study reveals several factors that affect the O2 release thermodynamics: (1) steric effect from the ligand sphere of Ru; (2) trans effect of ligands trans to O2; (3) oxygen cis coordinating effect; (4) carbon coordinating effect; and (5) Ru coordination strength. Some of these effects could selectively stabilize/destabilize some states/conformations of the Ru-O2 complexes relative to Ru-OH2 complexes, and affect thereby the O2 release thermodynamics. The identification and rationalization of factors for O2 release thermodynamics, as in this work, could be helpful toward a better understanding of this final step of the ruthenium-catalyzed water oxidation.

Computational comparison of stepwise oxidation and O–O bond formation in mononuclear ruthenium water oxidation catalysts

A computational comparison of key steps along proposed water oxidation pathways for eleven distinct mononuclear ruthenium catalysts is presented. Free energies are determined via density functional calculations and analyzed for catalyst structural stability, energetic comparisons to experimentally measured potentials in highly acidic conditions, and other features. A single feature for optimal catalyst design does not emerge; different steps along the oxidation cycle are impacted in subtle and sometimes divergent ways depending on the nature of the ligand modification on the catalyst. The results reinforce the notion observed in previous experimental studies – in such multi-step mechanisms with complex catalysts, a careful balance between energetic optimization and catalyst stability at multiple steps along the cycle may be critical for practical performance considerations. Whereas comparing catalysts at an individual step is likely to provide an incomplete picture, behavior of chemically modified catalysts can be distinguished computationally across multiple mechanistic steps.

Mononuclear Ruthenium–Water Oxidation Catalysts: Discerning between Electronic and Hydrogen-Bonding Effects

New mononuculear complexes of the general formula [Ru(trpy)(n,n'-F2-bpy)X](m+) [n = n' = 5, X = Cl (3(+)) and H2O (5(2+)); n = n' = 6, X = Cl (4(+)) and H2O (6(2+)); trpy is 2,2':6':2"-terpyridine] have been prepared and thoroughly characterized. The 5,5'- and 6,6'-F2-bpy ligands allow one to exert a remote electronic perturbation to the ruthenium metal center, which affects the combination of species involved in the catalytic cycle. Additionally, 6,6'-F2-bpy also allows through-space interaction with the Ru-O moiety of the complex via hydrogen interaction, which also affects the stability of the different species involved in the catalytic cycle. The combination of both effects has a strong impact on the kinetics of the catalytic process, as observed through manometric monitoring.

Synthesis and structural characterization of asymmetric mononuclear ruthenium(II) complexes derived from 2-(1,2,3-thiadiazol-4-yl)pyridine and azoimine ligands

Five mononuclear ruthenium complexes of the general type trans-[RuII(L)(Azo)Cl2] {(Azo=C6H5NNC(COCH3)NC6H4Y, Y=H (1), CH3 (2), OCH3 (3), Cl (4), Br (5)} and L is 2-(1,2,3-thiadiazol-4-yl)pyridine have been synthesized and their structures investigated by elemental analyses, spectroscopic (IR, UV/–Vis, and NMR) and electrochemical (cyclic voltammetry) techniques. In addition, complex (3) has been further characterized by X-ray diffraction analysis. The absorption spectrum of 3 in acetonitrile has been modeled by time-dependent density functional theory (TD-DFT) using mixed basis set, LanL2DZ/6–-31+g(d,p), in acetonitrile as a solvent.

Newly Designed Resorcinolate Binding for Ru(II)– and Re(I)–Polypyridyl Complexes on Oleic Acid Capped TiO2 in Nonaqueous Solvent: Prolonged Charge Separation and Substantial Thermalized 3MLCT Injection

Femtosecond pump–probe spectroscopic studies on a series of newly synthesized resorcinol-based Ru(II) and Re(I) complexes on oleic acid capped TiO2 nanoparticles have been carried out in chloroform, and the results are compared with those of the catechol analogues. The ruthenium complex shows biexponential injection; the second component arises due to injection from the thermally equilibrated 3MLCT states as a result of a weaker strength of the resorcinolate binding. Also, in comparison with catechol binding, as a result of a greater diffusion of the injected electrons into TiO2, the back electron transfer (BET) is slowed down significantly for the ruthenium complex. These are distinctive observations for any mononuclear ruthenium–polypyridyl–enediol complex reported thus far. However, the rhenium complex educes single exponential ultrafast injection (<120 fs) because of apparent injection in a high density of states and shows the most prominent results with ∼50% slowdown in the charge recombination rate ...

The synthesis and antiparasitic activity of aryl- and ferrocenyl-derived thiosemicarbazone ruthenium(ii)–arene complexes

A series of aryl-functionalized and ferrocenyl monothiosemicarbazone compounds (L1-L4) were synthesized in moderate yields via a general Schiff-base condensation reaction. The thiosemicarbazone (TSC) ligands were reacted with the ruthenium dimer [Ru(Ar)(μ-Cl)Cl](2) (Ar = benzene; p-cymene) to yield a series of cationic mononuclear ruthenium(II)-arene thiosemicarbazone complexes of the general type [Ru(Cl)(TSC)(Ar)]Cl (1-8). The thiosemicarbazone ligands act as bidentate chelating ligands that coordinate to the ruthenium(ii) ion via the imine nitrogen and the thione sulfur atoms. The thiosemicarbazone ligands, as well as their metal complexes, were characterized by NMR, IR spectroscopy and ESI(+)-mass spectrometry. The molecular structure of the mononuclear ruthenium(II)-arene thiosemicarbazone complex (6) was determined by single-crystal X-ray diffraction analysis. The ruthenium(II)-arene thiosemicarbazone complexes were further evaluated for their in vitro antiparasitic activities against the Plasmodium falciparum chloroquine-sensitive (NF54) and chloroquine-resistant (Dd2) strains, as well as the G3 strain of Trichomonas vaginalis.

The preparation, characterisation and electronic structures of 2,4-pentadiynylnitrile (cyanobutadiynyl) complexes

Convenient preparative routes to mononuclear ruthenium complexes containing the 2,4-pentadiynylnitrile, or cyanobutadiynyl, ligand are described. The electronic properties of the [C(5)N](-) ligand are closely related to those of not only the cyanide ([CN](-)) and 2-propynylnitrile or cyanoacetylide ([C≡CC≡N](-)) ligands, but also those of the isoelectronic polyynyl ([{C≡C}(n)R](-)) ligands.

Structure and Electronic Configurations of the Intermediates of Water Oxidation in a Highly Active and Robust Molecular Ruthenium Catalyst

Photosynthesis is one of the most important chemical processes on our planet. Splitting water into O2 and H2 using sunlight is a clean and sustainable proposition for addressing energy and environmental problems encountered in our society. Mimicking this reaction in a manmade device will allow for sunlight-to-energy conversion with water providing electrons and protons for production of chemical fuels. About 30 years ago Meyer and coworkers reported the first ruthenium-based catalyst for water oxidation, known as the “blue dimer”. This catalyst may be considered as an artificial analog of the oxygen-evolving complex (OEC) in Photosystem II (PS II). Recently it has been demonstrated that single-site Ru and Ir catalysts are also active in water oxidation. These single-site catalysts are attractive model compounds for both experimental and theoretical studies of mechanism of water oxidation and show improved catalytic activity compared to “blue dimer”. A better understanding of this mechanism and identification of the rate-limiting steps could pave the way to light-driven generation of molecular hydrogen by water splitting.A mononuclear ruthenium complex [Ru(bda)(pic)2] (pic=4-picoline) was found to show high catalytic activity as well as high chemical stability. EPR, Raman and XAS characterization of the electronic structure and molecular geometry of Ru(III)-OH2,Ru(IV)=O and Ru(IV)-OO-Ru(IV) are reported in the single site water oxidizing complex. Formation of metal bound peroxides as the result of O-O coupling has been implicated in the mechanism of catalytic water oxidation by Photosystem II oxygen evolving complex (OEC) and in Ru-based catalysts. However, such intermediates were never isolated and their structural and electronic characterization has not been reported. We believe that the intermediates described here are direct products of the O-O bond formation step in the studied catalyst.

Antimicrobial and Antimalarial Activity of Novel Synthetic Mononuclear Ruthenium(II) Compounds

AbstractThe present work was aimed that the two Ruthenium compounds namely, [Ru(A)2(B)]Cl2, where A = 1,10‐phenanthroline; B = 2‐NO2‐phenyl thiosemicarbazone (Compound R1)/2‐OH‐phenyl thiosemicarbazone (Compound R2) have been tested for antibacterial activity at the concentrations of 1 mg/mL against various Gram‐Positive organisms (Lactobacillus, Staphylococcus pyrogenes, Bacillus subtilis, Staphylococcus aureus &amp; Bacillus megatarium) and Gram‐Negative organisms (Pseudomonas aeruginosa, Escherichia coli, Proteus vulgaris, Enterobacter aerogenes, Salmonella paratyphi, Klebsiella pneumonia &amp; Proteus mirabilis). The compounds were also tested for antifungal activity against Aspergillus clavatus, Aspergillus niger, Colletotrichum &amp; Penicillium notatum by using agar diffusion assay and antimalarial activity against Plasmodium falciparum (Strain 3D7) using MTT assay. The results concluded that the compound R1 exhibited significant antibacterial activity than R2 against Gram‐Negative bacteria with zones of inhibition ranging from 15‐20 mm. and mild antibacterial activity against Gram‐Positive bacteria in comparison to tetracycline, streptomycin and rifampicin. These complexes were found to have moderate antifungal activity with no activity was however observed against Aspergillus niger. The compound, R1 exhibited antimalarial activity at 10 μg/mL, whereas R2 did not show antimalarial activity upto 50 μg/mL. Sensitivity to the compounds was greatest in the gram‐negative bacteria, followed by the gram‐positive bacteria and fungi.

Novel Mononuclear Ruthenium(II) Compounds in Cancer Therapy

The present study was conducted to evaluate in vivo anticancer activity of two novel mononuclear ruthenium(II) compounds, namely Ru(1,10-phenanthroline)2(2-nitro phenyl thiosemicarbazone)Cl2 (Compound R1) and Ru (1,10-phenanthroline)2(2-hydroxy phenyl thiosemicarbazone)Cl2 (Compound R2) against Ehrlich ascites carcinoma (EAC) mice and in vitro cytotoxic activity against IEC-6 (small intestine) cell lines and Artemia salina nauplii using MTT [(3-(4,5-dimethylthiazol-2yl)-2,5-diphenyltetrazolium bromide)] and BLT [brine shrimp lethality] assays respectively. The test ruthenium compounds at the doses 2 and 4 mg/kg body weight showed promising biological activity, especially in decreasing tumor volume, viable ascites cell counts and body weights. These compounds prolonged the life span (% ILS), mean survival time (MST) of mice bearing-EAC tumor. The results for in vitro cytotoxicity against IEC-6 cells showed the ruthenium compound R2 to have significant cytotoxic activity with a IC50 value of 20.0 μg/mL than R1 (IC50=78.8 μg/mL) in the MTT assay and the LC50 values of R1 and R2 compounds were found to be 38.3 and 43.8 μg/mL respectively in the BLT assay. The biochemical and histopathological results revealed that there was no significant hepatotoxicity and nephrotoxicity associated with the ruthenium administration to mice.

Highly efficient and robust molecular ruthenium catalysts for water oxidation

Water oxidation catalysts are essential components of light-driven water splitting systems, which could convert water to H(2) driven by solar radiation (H(2)O + hν → 1/2O(2) + H(2)). The oxidation of water (H(2)O → 1/2O(2) + 2H(+) + 2e(-)) provides protons and electrons for the production of dihydrogen (2H(+) + 2e(-) → H(2)), a clean-burning and high-capacity energy carrier. One of the obstacles now is the lack of effective and robust water oxidation catalysts. Aiming at developing robust molecular Ru-bda (H(2)bda = 2,2'-bipyridine-6,6'-dicarboxylic acid) water oxidation catalysts, we carried out density functional theory studies, correlated the robustness of catalysts against hydration with the highest occupied molecular orbital levels of a set of ligands, and successfully directed the synthesis of robust Ru-bda water oxidation catalysts. A series of mononuclear ruthenium complexes [Ru(bda)L(2)] (L = pyridazine, pyrimidine, and phthalazine) were subsequently synthesized and shown to effectively catalyze Ce(IV)-driven [Ce(IV) = Ce(NH(4))(2)(NO(3))(6)] water oxidation with high oxygen production rates up to 286 s(-1) and high turnover numbers up to 55,400.

In Vitro Antioxidant Activity of Some Novel Synthetic Mononuclear Ruthenium (II) Compounds

In Vitro Antioxidant Activity of Some Novel Synthetic Mononuclear Ruthenium (II) Compounds

Synthesis, spectroscopic characterization, antineoplastic, in vitro-cytotoxic, and antibacterial activities of mononuclear ruthenium(II) complexes

The synthesis, antineoplastic, cytotoxic, and antibacterial activities of Ru(II) complexes derived from quinazoline and thiosemicarbazone ligands are reported. These complexes have been prepared and characterized by UV-Vis, IR, 1H-NMR, FAB-mass spectroscopy, and elemental analysis. The ligands and resulting complexes were subjected to in vivo antineoplastic activity against a transplantable murine tumor cell line Ehrlich ascites carcinoma (EAC) and in vitro cytotoxic activity against human cancer cell line Molt 4/C8, CEM, and murine tumor cell line L 1210. The ruthenium complexes show promising biological activity especially in decreasing tumor volume and viable ascitic cell counts. These complexes prolonged the life span of mice bearing EAC tumors by 10–52%. In vitro evaluation of these ruthenium complexes revealed cytotoxic activity from 0.29 to 2.9 µmol L−1 against Molt 4/C8, 0.22 to 2.1 µmol L−1 against CEM and 0.42 to 4.7 µmol L−1 against L1210 cell proliferation, depending on the nature of the compound. The metal complexes are more active than the parent ligand and exhibit mild to moderate antibacterial activity.

Water Oxidation Catalysis: Influence of Anionic Ligands upon the Redox Properties and Catalytic Performance of Mononuclear Ruthenium Complexes

Aiming at highly efficient molecular catalysts for water oxidation, a mononuclear ruthenium complex Ru(II)(hqc)(pic)(3) (1; H(2)hqc = 8-hydroxyquinoline-2-carboxylic acid and pic = 4-picoline) containing negatively charged carboxylate and phenolate donor groups has been designed and synthesized. As a comparison, two reference complexes, Ru(II)(pdc)(pic)(3) (2; H(2)pdc = 2,6-pyridine-dicarboxylic acid) and Ru(II)(tpy)(pic)(3) (3; tpy = 2,2':6',2"-terpyridine), have also been prepared. All three complexes are fully characterized by NMR, mass spectrometry (MS), and X-ray crystallography. Complex 1 showed a high efficiency toward catalytic water oxidation either driven by chemical oxidant (Ce(IV) in a pH 1 solution) with a initial turnover number of 0.32 s(-1), which is several orders of magnitude higher than that of related mononuclear ruthenium catalysts reported in the literature, or driven by visible light in a three-component system with [Ru(bpy)(3)](2+) types of photosensitizers. Electrospray ionization MS results revealed that at the Ru(III) state complex 1 undergoes ligand exchange of 4-picoline with water, forming the authentic water oxidation catalyst in situ. Density functional theory (DFT) was employed to explain how anionic ligands (hqc and pdc) facilitate the 4-picoline dissociation compared with a neutral ligand (tpy). Electrochemical measurements show that complex 1 has a much lower E(Ru(III)/Ru(II)) than that of reference complex 2 because of the introduction of a phenolate ligand. DFT was further used to study the influence of anionic ligands upon the redox properties of mononuclear aquaruthenium species, which are postulated to be involved in the catalysis cycle of water oxidation.

A novel triazidoruthenium(iii) building block for the construction of polynuclear compounds

Reaction of [Ru(VI)(N)(sap)Cl] with excess NaN(3) affords a novel paramagnetic triazidoruthenium(III) complex [Ru(III)(sap)(N(3))(3)](2-), which is isolated as a PPh(4)(+) salt (1). Reaction of 1 with Ni(2+) and Co(2+) ions produce two isostructural hexanuclear [Ru(4)M(2)] compounds, [Ru(IV)(4)M(II)(2)(μ(3)-OMe)(2)(μ-OMe)(2)(μ-N)(2)(μ-N(3))(2)(μ-O(phenoxy))(2)(sap)(4) (MeOH)(4)] (M = Ni 2 or Co 3). The molecular structures of 1-3 have been determined by X-ray crystallography. 1 is a mononuclear ruthenium(III) compound where three azide ligands are bonded to ruthenium in a meridional fashion, while compounds 2 and 3 are isostructural hexanuclear compounds containing a defective face-sharing dicubane-like core with two missing vertexes. Variable-temperature dc magnetic susceptibility studies have been carried out for 2 and 3. These data indicate that there are four diamagnetic Ru(IV) ions in 2 and 3 and there is ferromagnetic interaction between the two Ni(2+) in 2 and Co(2+) in 3 via the methoxy bridges.