Cationic Ruthenium Research Articles

Role of ancillary ligands in selectivity towards acceptorless dehydrogenation versus dehydrogenative coupling of alcohols and amines catalyzed by cationic ruthenium(II)-CNC pincer complexes.

An unexpected reversal in catalytic activity for acceptorless dehydrogenative coupling compared to acceptorless alcohol dehydrogenation has been observed using a series of cationic Ru(II)-CNC pincer complexes with different ancillary ligands. In continuation of our study of cationic Ru(II)-CNC pincer complexes 1a-6a, new complexes with bulky N-wingtips [Ru(CNCiPr)(CO)(PPh3)Br]PF6 (1b), [Ru(CNCCy)(CO)(PPh3)Cl]PF6 (1c), [Ru(CNCCy)(CO)(PPh3)H]PF6 (2c), [Ru(CNCiPr)(PPh3)2Cl]PF6 (3b), [Ru(CNCCy)(PPh3)2Cl]PF6 (3c), [Ru(CNCiPr)(PPh3)2H]PF6 (4b), [Ru(CNCCy)(PPh3)2H]PF6 (4c), [Ru(CNCiPr)(DMSO)2Cl]PF6 (6b), and [Ru(CNCCy)(DMSO)2Cl]PF6 (6c) [CNCR = 2,6-bis(1-alkylimidazol-2-ylidene)-pyridine] have been synthesized and the catalytic activities of the new complexes have been compared with their N-methyl analogues for transfer hydrogenation of cyclohexanone and acceptorless dehydrogenation of benzyl alcohol. Furthermore, all complexes have been utilized as catalysts in the dehydrogenative coupling reaction of benzyl alcohol with amines. While the catalytic activities of the new complexes for transfer hydrogenation and acceptorless alcohol dehydrogenation were found to be in line with the previously observed trend based on the ancillary ligands (CO > COD > DMSO > PPh3), for the acceptorless dehydrogenative coupling reaction, complexes containing PPh3 and DMSO ligands performed better compared to complexes containing CO and COD ligands. Based on NMR and mass investigation of catalytic reactions, a plausible mechanism has been suggested to explain the difference in catalytic activity and its reversal during the dehydrogenative coupling reaction. Furthermore, the substrate scope for the dehydrogenative coupling reaction of benzyl alcohol with a wide range of amines has been explored, including synthesizing some pharmaceutically important imines. All new complexes have been characterized by various spectroscopic techniques, and the structures of 4b and 6b have been confirmed by the single-crystal X-ray diffraction technique.

Synthesis of a cationic ruthenium(ii) complex and its non-enzymatic glucose-sensing properties

A new cationic Ru(ii) complex was synthesized from N,N′-[1,1′-(pyridine-2,6-diyl)bis(ethan-1-yl-1-ylidene)]bis(2-ethyl-6-methylaniline) and [RuCl2(p-cymene)]2, with full characterization via NMR, FT-IR, single-crystal XRD, ESI-MS, UV-vis, CV and TA/DTA.

Pyrazole-pyridine-pyrazole (NNN) ruthenium(II) complex catalyzed acceptorless dehydrogenation of alcohols to aldehydes.

A new series of cationic ruthenium(II) complexes, [RuCl(PPh3)2(κ3-NNN-L1)]Cl (1), [RuH(PPh3)2(κ3-NNN-L2)]Cl (2), and [RuH(PPh3)2(κ3-NNN-L3)]Cl (3), bearing a 2,6-bis (1-R-5-methyl-pyrazol-3-yl) pyridine ligand (L1: R = H, L2: R = C6H4-p-CF3, L3: R = C6H4-p-OCH3), were synthesized and characterized with NMR, HRMS, and single-crystal X-ray diffraction. Their catalytic application was investigated in the acceptorless dehydrogenation (AD) of primary alcohols. Complex 1 outperformed 2 and 3 in terms of the selectivity towards the aldehydes, and provided the aldehydes with a yield of up to 99%, with good functional group tolerance under mild conditions. In addition, the only by-product of the reaction was dihydrogen, which can be collected as clean energy, and the reaction meets the requirements of environment-friendly chemistry. Complex 1 also proved to be a promising catalyst in the sequential AD/condensation reaction between the primary and secondary alcohols, affording α,β-unsaturated ketones in moderate to good selectivity.

Asymmetric hydrogenation of dibenzo-fused azepines with chiral cationic ruthenium diamine catalysts

Highly efficient asymmetric hydrogenation of dibenzo-fused azepines derivatives with chiral cationic Ru diamine catalysts has been developed, giving diverse chiral seven-membered N-heterocycles with excellent results (up to 99% yield and 99% ee).

Structural Investigation of the Anion Effect in Ruthenium(II) Arene Complexes Bearing Pyridyl-Triazole Ligands: Implications for Transfer Hydrogenation Catalysis

Cationic ruthenium(II) half-sandwich complexes bearing 1-octyl-4-pyridyl-1H-1,2,3-triazole ligands were found to display strong ion-pairing, the strength of which was dependent on the nature of the anionic counterion. A detailed spectroscopic analysis revealed that the ion-pairing process is also solvent-dependent. For the complex containing the 1-octyl-4-pyridyl-1H-1,2,3-triazole ligand and a tetraphenylborate counterion (1a), the spectral data reveal that the complex forms a collection of four ions (quadruple ions) in CHCl3. In contrast, one set of contact ion-pairs (CIPs) was observed in MeCN with the extent of ion-pairing being 88%. Analogous complexes in which the tetraphenylborate counterion is replaced by a hexafluorophosphate (2a) and tetrafluoroborate (3a) anion exist as CIPs (92 and 90%) and solvent-separated ion-pairs (55 and 56%) in CHCl3 and MeCN, respectively. Conductivity measurement results indicate that the extent of ion-pairing for 1a, 2a, and 3a in 2-PrOH is greater in comparison to the CIPs formed in CH2Cl2. Our results provide a rationale for the reactivity differences observed for 1a and 2a in transfer hydrogenation catalysis.

Synthesis of Cationic Sandwich-type Ruthenium Complexes with η6-Arene Ligands Bearing Long Alkyl Chains and Their Behavior in Water

Abstract Novel sandwich-type ruthenium complexes [Ru(η6-arene)(p-cymene)](BF4)2, in which the η6-arene ligands have long alkyl chains, were prepared. These complexes were dispersed in water to form corresponding micelles. Alkoxy moieties on the η6-arene ligands were reacted with OH− to afford the corresponding phenolic species. Thus, [Ru(p-undecyloxyanisole)(p-cymene)]2+ was converted to [Ru(η6-p-methoxyphenolate)(p-cymene)]+ through the unexpected demethylation by treatment with Na2CO3 with the micelle remaining. The micelle collapsed when similar ruthenium complex bearing dodecyl 2-(4-methoxyphenoxy)acetate as an arene ligand was treated with Na2CO3 aq.

Visible-Light-Driven Oxidation of Amines to Imines in Air Catalyzed by Polyoxometalate-Tris(bipyridine)ruthenium Hybrid Compounds.

The development of visible-light photocatalysts for the selective oxidative coupling of amines to imines is an area of great interest. Herein, four hybrid compounds based on polyoxometalate anions and tris(bipyridine)ruthenium cations, Ru(bpy)3[M6O19] (M = Mo, W) 1-2, [Ru(bpy)3]2[Mo8O26] 3, [Ru(bpy)3]2[W10O32] 4, are prepared and characterized by X-ray diffraction (single-crystal and powder), elemental analysis, energy-dispersive X-ray spectroscopy (EDS) analysis, infrared (IR) spectroscopy, and solid diffuse reflective spectroscopy. Single-crystal structural analysis indicates that polyoxometalate anions and tris(bipyridine)ruthenium cations interact with each other through extensive hydrogen bonds in these compounds. These hybrid species with strong visible-light-harvesting abilities and suitable photocatalytic energy potentials show excellent photocatalytic activity and selectivity for the oxidation of amines to imines at room temperature in air as an oxidant. Among them, compound 1 with the [Mo6O19]2- anion has the highest catalytic activity, which can swiftly convert >99.0% of benzylamine into N-benzylidenebenzylamine with a selectivity of 98.0% in 25 min illumination by a 10 W 445 nm light-emitting diode (LED). Its turnover frequency reaches 392 h-1, which is not only better than the homogeneous catalyst [Ru(bpy)3]Cl2 but also much superior to those achieved over most of reported heterogeneous catalysts. Moreover, it shows a wide generality for various aromatic amines, accompanied by the advantages of good recyclability and stability. The photocatalytic oxidation mechanism of amines to the corresponding imines over polyoxometalate-based hybrid compounds was fully investigated.

Synthesis, characterization and anticancer activities of cationic η6-p-cymene ruthenium(II) complexes containing phosphine and nitrogenous ligands

Ruthenium-based anticancer agents have created a center of attention in the field of inorganic medicinal chemistry. The first fully characterized cationic ruthenium(II)-arene complexes [Ru(η6-p-cymene) (PAr3)LNCl]+ with highly lipophilic PAr3 ligands where Ar = 3,5-((CH3)3C)2C6H3– (L1), 3,5-(CH3)2C6H3– (L2), 4-CH3O-3,5-(CH3)2C6H2– (L3) and 4-CH3O-C6H4– (L4) with N = 3-methylpyridine (1–4, respectively), or L4 and 4-methylpyridine (5), or L4 and CH3CN (6) were obtained (yields 67–91%) as solids stable to light and air. Electrical conductance indicates that all the complexes are 1:1 electrolytes in solution. Their composition and purity have been unambiguously established by single-crystal X-ray diffraction, NMR spectroscopy and elemental analysis. The coordination geometries are uniform for all six complexes and each structure consist of a unipositive complex cation bearing the phosphine ligands L1-L4 and LN = 3-methylpyridine, 4-methylpyridine or CH3CN attached to the organometallic fragment. The equivalent unit cell volumes per formula unit decrease with 1 > 3 > 2 > 4 > 5 > 6, accurately reflecting the decreasing sizes of the phosphines L1-L4, and a greater occupied volume for 3-methyl- vs. 4-methylpyridine, and the smallest volume contribution from CH3CN. Electrochemical studies showed mixed electrochemical mechanisms (EC/ECE) from partial substitution of p-cymene by CH3CN ligands from the solvent. A large electrochemical stability window (>2.2 V) for Ru(II) was observed extending beyond the physiological E° range. The complexes were cytotoxic against human cancer cell lines in vitro, and some complexes altered cell morphology.

A sensitive photoluminescent sensor based on highly charged monoruthenium(II) complexes for dopamine detection

A sensitive and selective photoluminescent sensor based on the highly charged monoruthenium(II) complex was designed to detect dopamine (DA) in aqueous samples. Two novel highly charged cationic ruthenium(II) complexes [Ru(bpy)2(bpy-N)]X4 (bpy = 2,2′-bipyridine, bpy-N = 4,4′-bis[N,N,N-triethyl-(methylamino)]-2,2′-bipyridine, X− = [PF6]− (1a) or Cl− (1b) and [Ru(bpy)(bpy-N)2]X6 (X− = [PF6]− (2a) or Cl−(2b)) can be assembled with anionic surfactant sodium dodecylbenzene sulfonate (SDBS), leading to an enhancement of photoluminescence intensity. Upon addition of DA to the system, the photoluminescence intensity of the assembled system was quenched due to the energy transfer effect. It exhibited a wide linear range (0.1–50 μM) and low detection limit (10 nM). The sensor demonstrated a high selectivity toward DA, especially in the presence of adrenaline (Adr) and norepinephrine (NE), whose structures are similar to DA in biological systems. With the merits of simple operation, obvious phenomenon and fast response speed, the sensor had a potential application prospect in human urine sample.

Ancillary ligand effects and microwave‐assisted enhancement on the catalytic performance of cationic ruthenium(II)‐CNC pincer complexes for acceptorless alcohol dehydrogenation

The application of cationic Ru(II)‐CNC pincer complexes [Ru(CNC)(CO)(PPh3)Cl]PF6 (1), [Ru(CNC)(PPh3)2Cl]PF6 (2), [Ru(CNC)(η2:η2‐COD)Cl]PF6 (4), and [Ru(CNC)(DMSO)2Cl]PF6 (5) in acceptorless dehydrogenation of alcohols is reported under conventional “oil‐bath” and microwave heating conditions. The effect of different ancillary ligands (CO, PPh3, COD, and DMSO) have been demonstrated during catalysis. Complexes with more π‐acid ligands CO and COD perform better compared with those with PPh3 and DMSO ligands. A plausible mechanism is proposed based on NMR and mass investigation during the catalysis. The catalytic performance matches with the order of trans effect among the four ancillary ligands studied in these complexes. Further, a significant reduction in reaction time, as well as reaction temperature, was observed for catalytic reactions under the microwave‐assisted reaction compared with the oil‐bath heating reaction.

New organoruthenium(II) complexes containing N, X-donor (X = O, S) heterocyclic chelators: Synthesis, spectral characterization, in vitro cytotoxicity and apoptosis investigation

Four Schiff base ligands namely ((1E)-1-(1-(6-chloro-2-oxo-2H-chromen-3-yl) ethylidene)semicarbazide) (HL1), ((1E)-1-(1-(6-chloro-2-oxo-2H-chromen-3-yl)ethylidene thiosemicarbazide) (HL2), ((1E)-1-(1-(7-hydroxy-2-oxo-2H-chromen-3-yl)ethylidene) semicarbazide) (HL3) and ((1E)-1-(1-(7-hydroxy-2-oxo-2H-chromen-3-yl)ethylidene) thiosemicarbazide) (HL4) have been prepared, and the direct reaction of these ligands with [RuCl2(η6-p-cymene)]2 yielded the new cationic ruthenium(II) arene complexes (C I - C IV). The compounds were characterized using various spectroscopic methods. Spectroscopic analysis revealed that the ligands coordinated to ruthenium ion in a neutral bidentate fashion via N & S/O atoms. The in vitro anticancer activity of the ligands and complexes have been screened using MTT assay with two cancer cell lines, namely MCF-7 (human breast cancer cells) and A549 (human lung carcinoma). Interestingly, the cytotoxic nature of the complexes is much more potent than the standard drug cisplatin and their parent ligands with low IC50 values. The non-toxic nature of the compounds has been confirmed with human umbilical vein endothelial cells HUVEC cells. The morphological changes of the MCF-7 and A549 cells with the complexes C I- C IV have been studied by AO-EB (Acridine Orange-Ethidium Bromide) and Hoechst staining methods, and these results revealed that the complexes induced cell death only through apoptosis. The cytotoxic nature of the compounds was found to be C IV > C III > C II > C I > cisplatin > ligands. Complex C IV showed better activity due to the electron-donating hydroxyl group present in the 7th position of the coumarin ring. These results highlighted the strong possibility to develop highly active ruthenium arene complexes as anticancer agents.

Interactions of Ag nanoparticles with humic acid present in surface water

The present paper reports aggregation behaviour of humic acid (HA) in the presence of silver nanoparticles. Aggregation behaviour has been studied from dynamic light scattering (DLS) measurement, in the presence of silver nanoparticle. Silver nanoparticle has been prepared through chemical route and characterized by plasmon resonance spectroscopy. HA used in the study has been characterized by UV–Vis and fluorescence study; its charged state has been evaluated from the study of its interaction with a cationic dye ruthenium bipyridine. It has been found that HA forms small–medium- and large-sized aggregates in the presence of silver nanoparticle as obtained from DLS diameter. The result has been explained in terms of Langmuir–Hinshelwood adsorption model. It has been proposed that hydrogen bonding and hydrophobic interaction play an important role in the formation of aggregates of HA.

Regulating the coordination environment of Ru single-atom catalysts and unravelling the reaction path of acetylene hydrochlorination

In this work, DFT calculations were used firstly to simulate the nitrogen coordinated metal single-atom catalysts (M-Nx SACs, M = Hg, Cu, Au, and Ru) to predict their catalytic activities in acetylene hydrochlorination. The DFT results showed that Ru-Nx SACs had the best catalytic performance among the four catalysts, and Ru-Nx SACs could effectively inhibit the reduction of ruthenium cation. To verify the DFT results, Ru-Nx SACs were fabricated by pyrolyzing MOFs in-situ spatially confined metal precursors. The N coordination environment could be controlled by changing the pyrolysis temperature. Catalytic performance tests indicated that low N coordination number (Ru–N2, Ru–N3) exhibited excellent catalytic activity and stability compared to RuCl3 catalyst. DFT calculations further revealed that Ru–N2 and Ru–N3 had a tendency to activate HCl at the first step of reaction, whereas Ru–N4 tended to activate C2H2. These findings will serve as a reference for the design and control of metal active sites.

Hydrogenative Cycloisomerization and Sigmatropic Rearrangement Reactions of Cationic Ruthenium Carbenes Formed by Catalytic Alkyne gem‐Hydrogenation

Abstractgem‐Hydrogenation of propargyl alcohol derivatives with [CpXRu(MeCN)3]PF6 (CpX=substituted cyclopentadienyl) as catalysts affords cationic pianostool ruthenium carbene complexes which are so electrophilic that they attack a tethered olefin to furnish cyclopentene products; cyclopropanation or metathesis do not compete with this novel transformation. If the transient carbenes carry appropriate propargylic substituents, however, they engage in ([2,3]‐sigmatropic) rearrangements to give enol esters (carbonates, carbamates, sulfonates) or alkenyl halides. Both pathways are unprecedented in the vast hydrogenation literature. The proposed mechanistic scenarios are in line with labeling experiments and spectroscopic data; most notably, PHIP NMR spectroscopy (PHIP=parahydrogen induced polarization) provides compelling evidence that the reactions are indeed triggered by highly unorthodox gem‐hydrogenation events.

Hydrogenative Cycloisomerization and Sigmatropic Rearrangement Reactions of Cationic Ruthenium Carbenes Formed by Catalytic Alkyne gem-Hydrogenation.

gem‐Hydrogenation of propargyl alcohol derivatives with [CpXRu(MeCN)3]PF6 (CpX=substituted cyclopentadienyl) as catalysts affords cationic pianostool ruthenium carbene complexes which are so electrophilic that they attack a tethered olefin to furnish cyclopentene products; cyclopropanation or metathesis do not compete with this novel transformation. If the transient carbenes carry appropriate propargylic substituents, however, they engage in ([2,3]‐sigmatropic) rearrangements to give enol esters (carbonates, carbamates, sulfonates) or alkenyl halides. Both pathways are unprecedented in the vast hydrogenation literature. The proposed mechanistic scenarios are in line with labeling experiments and spectroscopic data; most notably, PHIP NMR spectroscopy (PHIP=parahydrogen induced polarization) provides compelling evidence that the reactions are indeed triggered by highly unorthodox gem‐hydrogenation events.

Cationic half-sandwich ruthenium (II) complexes ligated by pyridyl-triazole ligands: Transfer hydrogenation and mechanistic studies

A series of novel cationic ruthenium half-sandwich complexes, bearing 1-substituted-4-pyridyl-1H-1,2,3-triazole ligands, were synthesized and fully characterized by a range of analytical techniques. The complexes were found to be efficient catalyst precursors for transfer hydrogenation reactions using ketones as substrates. We demonstrated that the complexes could hydrogenate acetophenone in excellent conversions (∼75 %) within 3 h, employing a low concentration of base of only 2 mol %. Extending the reaction time to 6 h gave near quantitative conversions for both catalysts employed. In addition to this, the transfer hydrogenation of acetophenone was also found to proceed even at low catalyst loadings (0.025–0.05 mol%) and low base concentrations (0.25–1.0 mol%). Under these conditions substrate conversion was moderate (22–60 %). The catalytic efficiency of the most effective catalyst was then evaluated in the transfer hydrogenation of a small library of aromatic and aliphatic ketones. It was shown that more challenging substrates such as benzophenone and 2-octanone could be hydrogenated to the corresponding secondary alcohols in conversions > 90 %. Finally, based on several experimental observations, a possible mechanism for the process is proposed.

Synthesis, Characterization, and Biological Activity of Water-Soluble, Dual Anionic and Cationic Ruthenium–Arene Complexes Bearing Imidazol(in)ium-2-dithiocarboxylate Ligands

An efficient synthetic protocol was devised for the preparation of five cationic ruthenium-arene complexes bearing imidazol(in)ium-2-dithiocarboxylate ligands from the [RuCl2(p-cymene)]2 dimer and 2 equiv of an NHC·CS2 zwitterion. The reactions proceeded cleanly and swiftly in dichloromethane at room temperature to afford the expected [RuCl(p-cymene)(S2C·NHC)]Cl products in quantitative yields. When the [RuCl2(p-cymene)]2 dimer was reacted with only 1 equiv of a dithiolate betaine under the same experimental conditions, a set of five bimetallic compounds with the generic formula [RuCl(p-cymene)(S2C·NHC)][RuCl3(p-cymene)] was obtained in quantitative yields. These novel, dual anionic and cationic ruthenium-arene complexes were fully characterized by various analytical techniques. NMR titrations showed that the chelation of the dithiocarboxylate ligands to afford [RuCl(p-cymene)(S2C·NHC)]+ cations was quantitative and irreversible. Conversely, the formation of the [RuCl3(p-cymene)]- anion was limited by an equilibrium, and this species readily dissociated into Cl- anions and the [RuCl2(p-cymene)]2 dimer. The position of the equilibrium was strongly influenced by the nature of the solvent and was rather insensitive to the temperature. Two monometallic and two bimetallic complexes cocrystallized with water, and their molecular structures were solved by X-ray diffraction analysis. Crystallography revealed the existence of strong interactions between the azolium ring protons of the cationic complexes and neighboring donor groups from the anions or the solvent. The various compounds under investigation were highly soluble in water. They were all strongly cytotoxic against K562 cancer cells. Furthermore, with a selectivity index of 32.1, the [RuCl(p-cymene)(S2C·SIDip)]Cl complex remarkably targeted the erythroleukemic cells vs mouse splenocytes.

Effect of Ru loading on Ru/CeO2 catalysts for CO2 methanation

The conversion of CO2 towards added valuable products is considered as a potential alternative to achieve the increase of the petrochemical industry while reducing the CO2 emissions. In the present work, the effect of Ru loading on CeO2 supports has been studied for the CO2 methanation reaction, and catalysts with different Ru loading in the 1–5 wt. % range have been prepared, characterized, and tested. The optimum Ru loading has been found to be 2.5 wt. %. Ruthenium cations are reduced at the lowest temperature for this optimum loading according to H2-TPR experiments (even at room temperature), and the highest proportion of ruthenium cations with strong interaction with ceria is achieved, as deduced from XPS. XRD characterization suggests partial insertion of ruthenium cations into the ceria lattice.In situ DRIFTS experiments evidenced that the balance between formation upon CO2 chemisorption and further hydrogenation of surface carbon intermediates is optimum for 2.5 wt. % Ru/CeO2. For low metal contents, the CO2 chemisorption is limited and no relevant, while as the metal content is increased, the hydrogenation of carbon species is less favourable. The 2.5 wt.% Ru/CeO2 catalyst comprises a balance between surface-carbon groups formation and further hydrogenation.

Cationic ruthenium(II)-NHC pincer complexes with hemilabile COD: Solid-state structural characterization and theoretical study of an η2-(E,Z)-COD ligand

Cationic Ru(II)-CNC pincer complexes (CNC = 2,6-bis(1-methylimidazol-2-ylidene)pyridine), [Ru(CNC)(η2:η2-COD)Cl]Cl (1a) (COD = 1,5-cyclooctadiene), [Ru(CNC)(η2:η2-COD)Cl]PF6 (1b), [Ru(CNC)(η2-COD)Cl2] (2), [Ru(CNC)(DMSO)2Cl]Cl (3a), and [Ru(CNC)(DMSO)2Cl]PF6 (3b) have been synthesised and characterised. Single-crystal x-ray diffraction studies of the complexes reveal that the COD ligand remains coordinated to the Ru in η2:η2- and η2-fashion in 1a/b and 2, respectively. The solid-state structure of complex 2 shows an (E,Z) configuration for the η2-COD ligand, instead of the expected (Z,Z) configuration, with the free alkene having an E-configuration. DFT study of isomeric forms of 2 with E,Z- and Z,Z-COD indicates that the complex with Z,Z-COD ligand is thermodynamically more stable and should be favoured in solution. The complexes were investigated for the transfer hydrogenation reaction with 2-propanol and appear to be efficient catalyst precursors with no sign of hydrogenation of the COD ligand during the catalysis.

Selective upgrading of biomass-derived benzylic ketones by (formic acid)–Pd/HPC–NH2 system with high efficiency under ambient conditions

Selective upgrading of biomass-derived benzylic ketones by (formic acid)–Pd/HPC–NH2 system with high efficiency under ambient conditions