Ru Center Research Articles

Ruthenium(η⁶,η¹-arene-CH₂-NHC) Catalysts for Direct Arylation of 2-Phenylpyridine with (Hetero)Aryl Chlorides in Water.

A series of new benzimidazolium halides were synthesized in good yields as unsymmetrical N-heterocyclic carbene (NHC) precursors containing the N–CH2–arene group. The benzimidazolium halides were readily converted into ruthenium(II)–NHC complexes with the general formula [RuCl2(η6,η1–arene–CH2–NHC)]. The structures of all new compounds were characterized by 1H NMR (Nuclear Magnetic Resonance), 13C NMR, FT-IR (Fourier Transform Infrared) spectroscopy and elemental analysis techniques. The single crystal structure of one benzimidazole ruthenium complex, 2b, was determined. The complex is best thought of as containing an octahedrally coordinated Ru center with the arene residue occupying three sites, the remaining sites being occupied by a (carbene)C–Ru bond and two Ru–Cl bonds. The catalytic activity of [RuCl2(η6,η1–arene–CH2–NHC)] complexes was evaluated in the direct (hetero)arylation of 2-phenylpyridine with (hetero)aryl chlorides in water as the nontoxic reaction medium. These results show that catalysts 2a and 2b were the best for monoarylation with simple phenyl and tolyl chlorides. For functional aryl chlorides, 2d, 2e, and 2c appeared to be the most efficient.

Probing α-RuCl_{3} Beyond Magnetic Order: Effects of Temperature and Magnetic Field.

Recent studies have brought α-RuCl_{3} to the forefront of experimental searches for materials realizing Kitaev spin-liquid physics. This material exhibits strongly anisotropic exchange interactions afforded by the spin-orbit coupling of the 4d Ru centers. We investigate the dynamical response at finite temperature and magnetic field for a realistic model of the magnetic interactions in α-RuCl_{3}. These regimes are thought to host unconventional paramagnetic states that emerge from the suppression of magnetic order. Using exact diagonalization calculations of the quantum model complemented by semiclassical analysis, we find a very rich evolution of the spin dynamics as the applied field suppresses the zigzag order and stabilizes a quantum paramagnetic state that is adiabatically connected to the fully polarized state at high fields. At finite temperature, we observe large redistributions of spectral weight that can be attributed to the anisotropic frustration of the model. These results are compared to recent experiments and provide a road map for further studies of these regimes.

3-Aminoquinazoline–phosphine ligands and their ruthenium(II) complexes: application in catalytic hydrogenation and transfer hydrogenation reactions

3-Aminoquinazolinone–phosphine proligands (5a–e) and their Ru(II) complexes (6a–e) were prepared and characterized by NMR (1H, 13C, 31P{1H}), FTIR and microanalysis. The 3-aminoquinazolinone–phosphine ligands were found to coordinate with the Ru(II) center via their phosphorus and nitrogen atoms. The Ru(II) complexes were applied as catalysts for the hydrogenation and transfer hydrogenation of prochiral ketones. The results showed that these complexes are efficient transfer hydrogenation catalysts.

Mixed-ligand (triphenylphosphine)ruthenium complexes of diphenylcarborane by ligand manipulation and an asymmetric, bimolecular “symbiotic” cluster

Deprotonation of [exo-5,6,10-{RuCl(PPh3)2}-5,6,10-(μ-H)3-nido-7,8-C2B9H9] (1a) and its C,C-diphenyl analogue (1b) affords the anions [3,3-(PPh3)2-3-Cl-closo-3,1,2-RuC2B9H11]− (2a) and [1,2-Ph2-3,3-(PPh3)2-3-Cl-closo-3,1,2-RuC2B9H9]− (2b), respectively. Dehalogenation of 2b in the presence of one equivalent of tBuNC yields [1,2-Ph2-3,3-(tBuNC)2-3-PPh3-pseudocloso-3,1,2-RuC2B9H9] (3), and if 2b is dehalogenated in the presence of CO a very similar compound, [1,2-Ph2-3,3-(CO)2-3-PPh3-pseudocloso-3,1,2-RuC2B9H9] (4), is formed. Treatment of 2b with Ag+ in the absence of a donor ligand affords, amongst a number of products, the compound [{1,2-Ph2-pseudocloso-3,1,2-RuC2B9H9}{1′,8′-Ph2-closo-2′,1′,8′-RuC2B9H9}] (5) in which two different {RuPh2C2B9H9} units both achieve electronic and coordinative saturation at their Ru centres by η6-coordination of a Ph ring on the other cluster in a symbiotic manner. Compounds 1b, 3, 4 and 5 were fully characterised, including by crystallographic studies.

Geometric Isomerism and Redox Properties of Ruthenium(II/III) Complexes with 3-Hydroxypicolinamide

Two geometric isomers of [RuCl2(PPh3)2(O–N)]-type complex were prepared and characterized by X-ray crystallography (PPh3 = triphenylphosphine, O–N = Hpia/pia−, Hpia = 3-hydroxypicolinamide). The electrochemical properties of the complexes upon protonation/deprotonation were investigated. The redox potential of the Ru center was remarkably shifted by isomerization. The UV-vis and electrochemical measurements suggested that slow isomerization from trans(P,Cl),trans(P,N) geometry to trans(P,P) geometry induced the oxidation and deprotonation of the complex.

P4VP-RuII(bda) polyelectrolyte-metal complex as water oxidation catalyst: on the unique slow-diffusion and multi-charge effects of the polyelectrolyte ligand.

In this work, we analyze the catalytic mechanism of P4VP–RuII(bda) polyelectrolyte–metal complex (PMC) as a water oxidation catalyst and elucidate how the unique slow diffusion and multi-charge properties of the polyelectrolyte ligand dominate the catalytic process. Four poly(4-vinyl pyridine)–Ru(bda) (P4VP–Ru) PMCs with different chain lengths and controlled Ru loading amounts were prepared and used as catalysts for catalytic water oxidation. These catalysts present excellent catalytic performance with turnover numbers (TON) from ∼1200 to ∼1700 because of the good hydration properties. Surprisingly, the combined catalysis kinetics and kinetic isotope effect (KIE) studies for P4VP–Ru PMCs confirm the single-site water nucleophilic attack (WNA) mechanism in catalysis, rather than the interaction between two metal oxide units (I2M). A combination of dynamic light scattering characterization, zeta-potential measurement and molecular dynamics simulation reveals that the slow diffusion and multi-charge properties of the polyelectrolyte ligand are responsible for the observed mechanism difference between the P4VP–Ru PMC system and small-molecule multi-nuclear system, though the two systems actually own a similar structural feature (flexible linkages between Ru centers). Our experimental and simulation results highlight the fact that though the existence of flexible linkages between Ru centers could provide large conformation entropy for the occurrence of Ru-dimerization in small-molecule and neutral polymer systems, the entropy elasticity could not overcome the electrostatic interaction energy in the PMC system. Clearly, this work unambiguously clarified why both intra-chain and inter-chain Ru-dimerization (I2M) are prohibited for the PMC system from a perspective of macromolecular chemistry and physics.

P-C reductive elimination in Ru(ii) complexes to convert triarylphosphine ligands into five- or six-membered phosphacycles.

Rare examples of P-C reductive elimination in ruthenium complexes to generate phosphonium salts are presented. Triarylphosphines are converted into benzophospholium or phosphaphenalenium ligands via cyclometalation and 1,2-insertion of an alkyne followed by P-C reductive elimination. The intermediate in each step was successfully characterized using NMR and X-ray diffraction studies.

Computational mechanistic study of Ru-catalyzed CO2 reduction by pinacolborane revealing the σ-π coupling mechanism for CO2 decarbonylation.

It has been reported that RuH2(η2-H2)2(PCy3)2 (1) could mediate CO2 reduction by pinacolborane (HBpin), affording pinBOBpin (7), pinBOCH3 (8), pinBOCHO (9), pinBOCH2OBpin (10), and an unprecedented C2 species pinBOCH2OCHO (11), which meanwhile is converted to the Ru complexes, including the transient 3 (RuH(κ2-O2CH)(CO)(PCy3)2) and 5 (RuH{(μ-H)2Bpin}(CO)(PCy3)2), and the persistent 4 (RuH(κ2-O2CH)(CO)2(PCy3)2) and 6 (RuH2(CO)2(PCy3)2). To gain an insight into the catalysis, a DFT study was carried out. The study identified the key active catalyst to be the hydride 13 (RuH2(CO)(PCy3)2) and characterized the mechanisms leading to the experimentally observed species (3-11). By investigating the experimental system, we learned a new mechanism called σ-π coupling for CO2 decarbonylation. Under this mechanism, CO2 and HBpin first co-coordinate to the Ru center of 13, then σ-π coupling takes place, forming a B-O bond between CO2 and HBpin, Ru-H and Ru-C bonds, and simultaneously breaking the H-Bpin bond, followed by -OBpin group migration to the Ru center, completing the CO2 decarbonylation. An interesting feature regarding the Ru catalysis was the involvement of η1-Hη1-H → η2-H2 and η1-Hη1-Bpin → η2-HBpin reductions, which facilitated the oxidative H-Bpin addition or the coordination mode change of CO2 from η1-O to η2-CO for CO2 activation or σ-π coupling. The facilitation effects could be attributed to the reductions enhancing the electron donations from the Ru center to the antibonding orbitals of the activating bonds.

Synthesis, characterization and biological evaluation of six highly cytotoxic ruthenium(ii) complexes with 4'-substituted-2,2':6',2''-terpyridine.

Herein, six ruthenium(ii) terpyridine complexes, i.e. [RuCl2(4-EtN-Phtpy)(DMSO)] (Ru1), [RuCl2(4-MeO-Phtpy)(DMSO)] (Ru2), [RuCl2(2-MeO-Phtpy)(DMSO)] (Ru3), [RuCl2(3-MeO-Phtpy)(DMSO)] (Ru4), [RuCl2(1-Bip-Phtpy)(DMSO)] (Ru5), and [RuCl2(1-Pyr-Phtpy)(DMSO)] (Ru6) with 4'-(4-diethylaminophenyl)-2,2':6',2''-terpyridine (4-EtN-Phtpy), 4'-(4-methoxyphenyl)-2,2':6',2''-terpyridine (4-MeO-Phtpy), 4'-(2-methoxyphenyl)-2,2':6',2''-terpyridine (2-MeO-Phtpy), 4'-(3-methoxyphenyl)-2,2':6',2''-terpyridine (3-MeO-Phtpy), 4'-(1-biphenylene)-2,2':6',2''-terpyridine (1-Bip-Phtpy), and 4'-(1-pyrene)-2,2':6',2''-terpyridine (1-Pyr-Phtpy), respectively, were synthesized and fully characterized. The MTT assay demonstrates that the in vitro anticancer activity of Ru1 is higher than that of Ru2-Ru6 and more selective for Hep-G2 cells than for normal HL-7702 cells. In addition, various biological assays show that Ru1 and Ru6, especially the Ru1 complex, are telomerase inhibitors targeting c-myc G4 DNA and also cause apoptosis of Hep-G2 cells. With the same Ru center, the in vitro antitumor activity and cellular uptake ability of the 4-EtN-Phtpy and 1-Bip-Phtpy ligands follow the order 4-EtN-Phtpy > 1-Bip-Phtpy.

An integrated immobilization strategy manipulates dual active centers to boost enantioselective tandem reactions.

Manipulation of dual active centers via an integrated immobilization strategy can overcome the restriction of homogeneous catalysis in a sequential organic transformation. Herein, by utilizing hollow-shell-structured silica, hydrogen-bonding immobilization via a ship-in-a-bottle synthesis locks a Pd(carbene) center in a nanocage and covalent-bonding immobilization tethers chiral Ru(diamine) centers within the nanochannels, constructing a hetero-bifunctional catalyst. The benefit of this dual center manipulation enables a challenging Suzuki coupling-asymmetric transfer hydrogenation tandem reaction, and the advantage of this process provides various chiral biarylols with enhanced reactivity and enantioselectivity.

Unexpected arene ligand exchange results in the oxidation of an organoruthenium anticancer agent: the first X-ray structure of a protein-Ru(carbene) adduct.

The first X-ray structures of adducts formed between a RuII(N-heterocyclic carbene)(η6-p-cymene) compound and a protein are reported. Coordination to the protein induced the cleavage of the cymene ligand and EPR spectroscopy demonstrated the oxidation of the Ru centre.

Redox-Dependent Metal-Metal Bonding in Trinuclear Metal Chains: Probing the Transition from Covalent Bonding to Exchange Coupling.

The synthesis and physical properties of two new cationic tri-metallic chains, [(PEt3 )3 RuCl3 M'Cl3 Ru(PEt3 )3 ]1+ , M'=Rh and Ir are reported. These are isostructural with a previously reported 17-electron all-ruthenium analogue, but replacing a d5 RuIII ion in the central position with d6 RhIII /IrIII has a significant impact on the nature of the metal-metal interactions. All three materials have been characterized electrochemically at the 18-, 17- and 16-electron levels. X-ray crystallography and spectroelectrochemistry, complemented by electronic structure analysis at the DFT and CASPT2 levels, indicate that whilst the presence of a RuIII ion in the center of the chain allows multi-center covalent bonding to develop, a closed-shell RhIII /IrIII ion pushes the system towards the exchange-coupled limit, where the outer Ru centers are only weakly interacting. This family of three isostructural compounds reveals how changes in metal composition can have subtle effects on physical properties of systems that lie close to the localized/delocalized borderline.

Supported Ru Metalloporphyrins for Electrocatalytic CO2 Conversion

AbstractThis paper reports for the first time a computational analysis of the redox properties of graphene‐supported Ru‐porphyrins as potential catalytic materials for electrochemical CO2 reduction. Density functional theory reveals that such catalytic ensembles can efficiently activate both CO2 and CH4 molecules indicating their generic utility as C1‐functionalization catalysts. The charge transfer from the graphene surface to the catalytic Ru centers influences the thermodynamic stability of the key reaction intermediates and therefore determines the selectivity of the electrochemical process. The electrochemical reduction of CO2 can yield CO or methane, depending on the applied potential and reaction conditions. Calculations also identified alternative paths towards methanol and formic acid.

Ruthenium Carbon-Rich Group as a Redox-Switchable Metal Coupling Unit in Linear Trinuclear Complexes.

The preparation and properties of novel ruthenium carbon-rich complexes [(Ph-C≡C-)2-nRu(dppe)2(-C≡C-bipyM(hfac)2)n] (n = 1, 2; M = CuII, MnII; bipy = 2,2'-bipyridin-5-yl) characterized by single-crystal X-ray diffraction and designed for molecular magnetism are reported. With the help of EPR spectroscopy, we show that the neutral ruthenium system sets up a magnetic coupling between two remote paramagnetic CuII units. More specifically, these copper compounds are unique examples of bimetallic and linear heterotrimetallic compounds for which a complete rationalization of the magnetic interactions could be made for exceptionally long distances between the spin carriers (8.3 Å between adjacent Cu and Ru centers, 16.6 Å between external Cu centers) and compared at two different redox states. Surprisingly, oxidation of the ruthenium redox-active metal coupling unit (MCU), which introduces an additional spin unit on the carbon-rich part, leads to weaker magnetic interactions. In contrast, in the simpler parent complexes bearing only one paramagnetic metal unit [Ph-C≡C-Ru(dppe)2-C≡C-bipyCu(hfac)2], one-electron oxidation of the ruthenium bis(acetylide) unit generates an interaction between the Cu and Ru spin carriers of magnitude comparable to that observed between the two far apart Cu ions in the above corresponding neutral trimetallic system. Evaluation and rationalization of this coupling with theoretical tools are in rational agreement with experiments for such complex systems.

Cyclometalated Ru(II)‐NHC Complexes as Effective Catalysts for Transfer Hydrogenation: Influence of Wingtip Group on Catalytic Outcome

AbstractCyclometalated N‐heterocyclic carbene (NHC) based ruthenium benzene complexes of general formula [(η6‐C6H6)Ru(C C)Cl] (C C=1‐butyl‐3‐phenylimidazol‐2‐ylidene 1 a, 1‐isopropyl‐3‐phenylimidazol‐2‐ylidene 1 b and 1‐benzyl‐3‐phenylimidazol‐2‐ylidene 1 c) have been synthesised via C−H activation of 1‐alkyl‐3‐phenylimidazolium salts with [(η6‐C6H6)RuCl2]2 under silver transmetallation condition. All the complexes have been characterized by elemental analysis and spectral methods. The solid‐state structures of the complexes 2 a‐c have been established by a X‐ray single crystal diffraction study which revealed that NHC ligands behaved as mono anionic bidentate C C donors to the Ru(II) center via carbenic carbon and cyclometalated carbon. The newly synthesised ruthenium(II) NHC complexes 2 a‐c have been exploited as potential homogeneous catalysts to the transfer hydrogenation reactions for a broad scope of ketones to alcohols using 2‐propanol as a hydrogen source. The titled complexes worked well with low catalyst loading of 0.2 mol% with maximum conversion of 100% under optimized condition with reference to base, temperature, time, catalyst loading and substrate scope. Besides, the influence of the nature of wingtip group of the NHC ligands on the transfer hydrogenation reaction has also been reported to demonstrate the efficacy of catalyst.

C–H Activation of π-Arene Ruthenium Complexes

We present a C–H activation protocol for aromatic compounds that overcomes the current limitations of the need for a directing group or covalently bound activating groups, by exploiting the increase in C–H acidity of aromatic compounds on π-coordination to a Ru(II) center. The increased acidity facilitates catalytic concerted metalation–deprotonation and subsequent arylation reactions. We present the development and optimization of the C–H activation protocol and show the applicability of the reaction to a range of aromatic substrates, including the simplest of substrates (benzene). Furthermore, we demonstrate the recyclability of the activating Ru(II) fragment using photolysis and give a mechanistic study, which provides strong evidence that this reaction occurs via a silver-mediated C–H bond activation. This is the first time Ru complexes have been shown to allow C–H activation of arenes by a π-coordination mechanism.

Generation of Stable Ruthenium(IV) Ketimido Complexes by Oxidative Addition of Oxime Esters to Ruthenium(II): Reactivity Studies Based on Electronic Properties of the Ru-N Bond.

The reaction of an oxime ester with [Ru(PPh3 )3 X2 ] proceeded smoothly at room temperature to afford a stable RuIV ketimido complex as oxidative adduct. The structure of the complex was unambiguously determined by X-ray crystallographic analysis, which showed an almost linear Ru-N-C array. The electronic properties of the nitrogen atom were estimated by DFT calculations, and the results suggested double-bond character of the Ru-N bond. Kinetic studies and consideration of the substituent effect on the oxime ester led to the proposal of a reaction mechanism involving oxidative addition, which could proceed by N,O-chelating coordination to the Ru center prior to N-O bond cleavage. The obtained Ru ketimido complex could be transformed into a ruthenacycle by C-H activation by a concerted metalation-deprotonation mechanism in dichloromethane/methanol. Ru ketimido complexes with a tethered alkyne or alkene moiety underwent chloroamination of unsaturated C-C bonds followed by C-H activation, which resulted in the formation of a ruthenacycle. Considering the LUMO of an isolated Ru ketimido complex, the chloroamination should proceed by a synchronous 1,3-dipolar cycloaddition-type mechanism. Insight into the character and reactivity of Ru ketimido complexes will be helpful for developments in the catalytic transformation of oxime esters.

Inverting the Swelling Trends in Modular Self‐Oscillating Gels Crosslinked by Redox‐Active Metal Bipyridine Complexes

AbstractThe developing field of active, stimuli‐responsive materials is in need for new dynamic architectures that may offer unprecedented chemomechanical switching mechanisms. Toward this goal, syntheses of polymerizable bipyridine ligands, bis(4‐vinylbenzyl)[2,2′‐bipyridine]‐4,4′‐dicarboxylate and N4,N4′‐bis(4‐vinylphenyl)‐2,2′‐bipyridine‐4,4′‐dicarboxamide, and a number of redox‐active Ruthenium(II) and Iron(II) complexes with them are reported. Detailed characterizations by NMR, Fourier transform infrared spectroscopy, high‐resolution mass‐spectrometry, X‐ray, and cyclic voltammetry show that the topology of these molecules allows them to serve as both comonomers and crosslinkers in polymerization reactions. Electronic properties of the ligands are tunable by choosing carboxylate‐ or carboxamido‐linkages between the core and the vinylaryl moieties, leading to a library of Ru and Fe complexes with the M(III)/M(II) standard redox potentials suitable for catalyzing self‐oscillating Belousov–Zhabotinskii (BZ) reaction. New poly(N‐isopropylacrylamide)‐based redox‐responsive functional gels containing hydrophilic comonomers, which have been prepared using representative Ru bpy complexes as both a crosslinker and redox‐active catalyst, exhibit a unique feature: their swelling/contraction mode switches its dependence on the oxidation state of the Ru center, upon changing the ratio of comonomers in the hybrid gel network. The BZ self‐oscillations of such crosslinked hydrogels have been observed and quantified for both supported film and free‐standing gel samples, demonstrating their potential as chemomechanically active modules for new functional materials.

Synthesis, characterization and properties of oxo-bridged diruthenium(III) complexes with thiocyanato and cyanato ligands

The oxo-bridged diruthenium(III) complexes with thiocyanato and cyanato ligands, trans(μ-O,L)-[Ru2(μ-O)(μ-CH3COO)2(bpy)2(L)2] (bpy=2,2′-bipyridine; L=SCN− (1); OCN− (2)), have been synthesized, characterized, and their properties studied. X-ray single crystallography revealed that the thiocyanato ligands of 1 are N-bonded to the Ru centers at the trans sites to the oxo-bridge. The OCN− ligand in 2 is also N-bonded based on the infrared spectroscopy. The cyclic voltammograms of 1 and 2 in dimethyl sulfoxide (DMSO) solutions of 0.1M (n-Bu)4NPF6 showed two redox couples at around −0.5V and +0.5Vvs Ag/AgCl, which were assigned to the processes Ru2III,III/Ru2III,II and Ru2IV,III/Ru2III,III, respectively. The electronic absorption spectra of 1 and 2 in CH3CN solutions showed strong absorption bands at 625 and 637nm, respectively. The bands showed a remarkable blueshift in H2O to 578 and 581nm as a consequence of the solvent interactions at the free end of the thiocyanato and cyanato ligands, respectively. The theoretical calculations of both 1 and 2 and also their coordination isomers (N-bonded and S-bonded for the thiocyanate complex, and N-bonded and O-bonded for the cyanate complex) revealed that the N-bonded isomers are energetically more stable. Both the SCN− and OCN− ligands are primarily good σ donors to the Ru2O core. The contributions of SCN− to the dπ(Ru)–pπ(μ-O) conjugated system are particularly significant (>20%). The strong visible absorption bands of both 1 and 2 were attributed to mainly the metal-to-metal charge transfer (MMCT) transitions with some contributions from the ligand (SCN−, OCN−)-to-ligand (μ-O) charge transfer (LLCT).

β‐Diketiminato–Germylene Complexes of Carbonyl Ruthenium: Synthesis, Structure, and Reactivity

The reaction of the β‐diketiminatochlorogermylene L(Cl)Ge {1, L = HC[C(Me)N‐2,6‐iPr2C6H3]2} with Ru3(CO)12 in molar ratios of 3:1 or 6:1 led to the Ge2Ru complex [L(Cl)Ge]2Ru(CO)3 (3). A speculative GeRu intermediate, L(Cl)GeRu(CO)4, was detected when the reaction was monitored by 1H NMR spectroscopy at elevated temperatures. The formation of this intermediate was supported by similar monitoring of the reaction of L(PhC≡C)Ge (2) with Ru3(CO)12 by 1H NMR spectroscopy and the isolation of a mixture of L(PhC≡C)GeRu(CO)4 (4) and [L(R)Ge]2Ru(CO)3 (5) from this reaction in a molar ratio of 3:1 as well as the clean formation of 5 from such a reaction in a 6:1 molar ratio. Complex 3 reacted with LiHBEt3 through an unusual Cl/Et metathesis to give L(Et)GeRu(CO)4 (6). However, the reaction of 3 with LiN(SiMe3)2 resulted in the base‐assisted dehydrochlorination involving one methyl group of each L ligand backbone and the chlorine atom at the Ge center to yield [L′Ge]2Ru(CO)3 {7, L′ = HC[C(Me)N‐2,6‐iPr2C6H3][C(CH2)N‐2,6‐iPr2C6H3]}. The further reaction of 7 with a stoichiometric amount of H2O afforded L(HO)GeRu(CO)4 (8) through a 1,4‐dipolar addition across the L′Ge functionality. In addition, the reaction of 3 with I2 occurred through an oxidative addition at the Ru center to generate L(Cl)GeRuI2(CO)3 (10).