RuII Complexes Research Articles

Photocatalytic Substrate Oxidation Catalyzed by a Ruthenium(II) Complex with a Phenazine Moiety as the Active Site Using Dioxygen as a Terminal Oxidant.

We have developed photocatalytic oxidation of aromatic substrates using O2 as a terminal oxidant to afford only 2e--oxidized products without the reductive activation of O2 in acidic water under visible-light irradiation. A RuII complex (1) bearing a pyrazine moiety as the active site in tetrapyrido[3,2-a:2',3'-c:3″,2″-h:2‴,3‴-j]phenazine (tpphz) as a ligand was employed as a photocatalyst. The active species for the photocatalysis was revealed to be not complex 1 itself but the protonated form, 1-H+, protonated at the vacant diimine site of tpphz. Upon photoexcitation in the presence of an organic substrate, 1-H+ was converted to the corresponding dihydro-intermediate (2-H+), where the pyrazine moiety of the ligand received 2e- and 2H+ from the substrate. 2-H+ was facilely oxidized by O2 to recover 1-H+. Consequently, an oxidation product of the substrate and H2O2 derived from dioxygen reduction were obtained; however, the H2O2 formed was also used for oxidation of 2-H+. In the oxidation of benzyl alcohol to benzaldehyde, the turnover number reached 240 for 10 h, and the quantum yield was determined to be 4.0%. The absence of ring-opening products in the oxidation of cyclobutanol suggests that the catalytic reaction proceeds through a mechanism involving formal hydride transfer. Mechanistic studies revealed that the photocatalytic substrate oxidation by 1-H+ was achieved in neither the lowest singlet excited state nor triplet excited state (S1 or T1) but in the second lowest singlet excited state (S2), i.e., 1(π-π*)* of the tpphz ligand. Thus, the photocatalytic substrate oxidation by 1-H+ can be categorized into unusual anti-Kasha photocatalysis.

Enabling Visible Light Sensitization of YbIII, NdIII and ErIII in Dimeric LnIII/GaIII Metallacrowns through Functionalization with RuII Complexes for NIR-II Multiplex Imaging.

Multiplex imaging in the second near-infrared window (NIR-II, 1000-1700 nm) provides exciting opportunities for more precise understanding of biological processes and more accurate diagnosis of diseases by enabling real-time acquisition of images with improved contrast and spatial resolution in deeper tissues. Today, the number of imaging agents suitable for this modality remains very scarce. In this work, we have synthesized and fully characterized, including theoretical calculations, a series of dimeric LnIII/GaIII metallacrowns bearing RuII polypyridyl complexes, LnRu-3 (Ln=YIII, YbIII, NdIII, ErIII). Relaxed structures of YRu-3 in the ground and the excited electronic states have been calculated using dispersion-corrected density functional theory methods. Detailed photophysical studies of LnRu-3 have demonstrated that characteristic emission signals of YbIII, NdIII and ErIII in the NIR-II range can be sensitized upon excitation in the visible range through RuII-centered metal-to-ligand charge transfer (MLCT) states. We have also showed that these NIR-II signals are unambiguously detected in an imaging experiment using capillaries and biological tissue-mimicking phantoms. This work opens unprecedented perspectives for NIR-II multiplex imaging using LnIII-based molecular compounds.

In Vitro and In Vivo Studies on a Mononuclear Ruthenium Complex Reveals It is a Highly Effective, Fast-Acting, Broad-Spectrum Antimicrobial in Physiologically Relevant Conditions.

The crystal structure of a previously reported antimicrobial RuII complex that targets bacterial DNA is presented. Studies utilizing clinical isolates of Gram-negative bacteria that cause catheter-associated urinary tract infection, (CA)UTI, in media that model urine and plasma reveal that good antimicrobial activity is maintained in all conditions tested. Experiments with a series of Staphylococcus aureus clinical isolates show that, unlike the majority of previously reported RuII-based antimicrobial leads, the compound retains its potent activity even in MRSA strains. Furthermore, experiments using bacteria in early exponential growth and at different pHs reveal that the compound also retains its activity across a range of conditions that are relevant to those encountered in clinical settings. Combinatorial studies involving cotreatment with conventional antibiotics or a previously reported analogous dinuclear RuII complex showed no antagonistic effects. In fact, although all combinations show distinct additive antibacterial activity, in one case, this effect approaches synergy. It was found that the Galleria Mellonella model organism infected with a multidrug resistant strain of the ESKAPE pathogen Acinetobacter baumannii could be successfully treated and totally cleared within 48 h after a single dose of the lead complex with no detectable deleterious effect to the host.

Synthesis, characterization, and investigation of catalytic properties of RuII complexes with Schiff bases based on 2-hydroxybenzaldehyde and 5-bromo-2-hydroxybenzaldehyde

Pursuing our recent interest concerning the catalytic activity of half-sandwich RuII complexes containing Schiff base ligands (L1 and L2), in this study new Schiff bases and their half-sandwich RuII complexes were prepared. The structures of these compounds were confirmed by a variety of spectral and physical methods such as microanalysis, FT-IR, electronic spectra, proton and carbon NMR, TGA and mass spectra. The catalytic activities of the RuII complexes toward the transfer hydrogenation of ketones to their corresponding alcohols were investigated using i-PrOH as a hydrogen source and solvent. The complexes showed high catalytic activities at catalyst concentrations of 0.01 mmol and were able to carry out transfer hydrogenation (TH) to a wide range of ketones in the presence of KOH. In general, Schiff bases recognition, the coordination chemistry, and the ketone substrates were found to influence the catalytic performance of these complexes.

Predictable electronic tuning of FeII and RuII complexes via choice of azine: correlation of ligand pKa with Epa(MIII/II) of complex.

Five new mononuclear ruthenium(II) tris-ligated complexes have been synthesised, varying through the choice of azine in the family of 3-azinyl-4-(4-methylphenyl)-5-phenyl-4H-1,2,4-triazole ligands (Lazine): [Ru(Lpyridine)](PF6)2 (1), [Ru(Lpyridazine)](PF6)2 (2), [Ru(L4-pyrimidine)](PF6)2 (3), [Ru(Lpyrazine)](PF6)2 (4), [Ru(L2-pyrimidine)](PF6)2 (5). Three of them, 1·2MeCN·Et2O, 3·2MeCN·Et2O and 4·2MeCN, have been structurally characterised, confirming the presence of the meridional isomer, as was previously reported for the FeII analogues. Cyclic voltammetry studies, in dry CH3CN vs. Ag/0.01 M AgNO3, show that all five RuII complexes undergo a reversible RuIII/RuII process, with the midpoint potential (Em) increasing from 0.87 to 1.18 V as the azine is changed: pyridine < pyridazine < 2-pyrimidine < 4-pyrimidine < pyrazine. A strong inverse linear correlation (R2 = 0.98) is found between the RuIII/RuII redox potential and the calculated HOMO orbital energies, which is consistent with the expectation that it is easier to oxidise (lower Em) a metal ion with a higher HOMO orbital energy. The same trend was reported earlier for the family of analogous FeII complexes, albeit at lower values of Em in all cases. In addition, the ionisation potentials of the RuII complexes, as well as those of the other group 8 analogues (FeII and OsII), showed a linear relationship with Epa. As the MIII/II redox potentials of a family of complexes has been previously reported to correlate with ligand pKa values, a computational protocol to calculate, in silico, the pKa of the Lazine family of ligands was developed. A strong linear relationship was found between the readily calculated pKa of the Lazine ligand and the Epa of the MII complex, for all three families of complexes (R2 = 0.98).

RuII and RuIII complexes with 2,6-di-tert-butylphenol ligands: synthesis, electrochemical behaviour, antioxidant properties and antiproliferative activity

New ruthenium(ii) and ruthenium(iii) complexes with 2,6-di-tert-butylphenol as the antioxidant moiety were synthesized, and their antioxidant and antiproliferative activities were evaluated. Electrochemical behaviour and the potential of these compounds to act as inhibitors of lipid peroxidation in biological systems were explored.

Cytotoxic Imidazolyl-Mesalazine Ester-Based Ru(II) Complexes Reduce Expression of Stemness Genes and Induce Differentiation of Oral Squamous Cell Carcinoma.

The aggressiveness and recurrence of cancer is linked to cancer stem cells (CSCs), but drugs targeting CSCs may not succeed in the clinic due to the lack of a distinct CSC subpopulation. Clinical Pt(II) drugs can increase stemness. We screened 15 RuII or IrIII complexes with mesalazine or 3-aminobenzoate Schiff bases of the general formulas [Ru(p-cym)L]+, [Ru(p-cym)L], and [Ir(Cp*)L]+ (L = L1-L9) and found three complexes (2, 12, and 13) that are active against oral squamous cell carcinoma (OSCC) CSCs. There is a putative oncogenic role of transcription factors (viz. NOTCH1, SOX2, c-MYC) to enhance the stemness. Our work shows that imidazolyl-mesalazine ester-based RuII complexes inhibit growth of CSC-enriched OSCC 3D spheroids at low micromolar doses (2 μM). Complexes 2, 12, and 13 reduce stemness gene expression and induce differentiation markers (Involucrin, CK10) in OSCC 3D cultures. The imidazolyl-mesalazine ester-based RuII complex 13 shows the strongest effect. Downregulating c-MYC suggests that RuII complexes may target c-MYC-driven cancers.

Self-Photosensitizing Dinuclear Ruthenium Catalyst for CO2 Reduction to CO.

The promise of artificial photosynthesis to solve environmental and energy issues such as global warming and the depletion of fossil fuels has inspired intensive research into photocatalytic systems for CO2 reduction to produce value-added chemicals such as CO and CH3OH. Among the photocatalytic systems for CO2 reduction, self-photosensitizing catalysts, bearing the functions of both photosensitization and catalysis, have attracted considerable attention recently, as such catalysts do not depend on the efficiency of electron transfer from the photosensitizer to the catalyst. Here, we have synthesized and characterized a dinuclear RuII complex bearing two molecules of a tripodal hexadentate ligand as chelating and linking ligands by X-ray crystallography to establish the structure explicitly and have used various spectroscopic and electrochemical methods to elucidate the photoredox characteristics. The dinuclear complex has been revealed to act as a self-photosensitizing catalyst, which acts not only as a photosensitizer but also as a catalyst for CO2 reduction. The dinuclear RuII complex is highly durable and performs efficient and selective CO2 reduction to produce CO with a turnover number of 2400 for 26 h. The quantum yield of the CO formation is also very high─19.7%─and the catalysis is efficient, even at a low concentration (∼1.5%) of CO2.

Enhancement of Reactivity of a RuIV-Oxo Complex in Oxygen-Atom-Transfer Catalysis by Hydrogen-Bonding with Amide Moieties in the Second Coordination Sphere.

We have synthesized and characterized a RuII-OH2 complex (2), which has a pentadentate ligand with two pivalamide groups as bulky hydrogen-bonding (HB) moieties in the second coordination sphere (SCS). Complex 2 exhibits a coordination equilibrium through the coordination of one of the pivalamide oxygens to the Ru center in water, affording a η6-coordinated complex, 3. A detailed thermodynamic analysis of the coordination equilibrium revealed that the formation of 3 from 2 is entropy-driven owing to the dissociation of the axial aqua ligand in 2. Complex 2 was oxidized by a CeIV salt to produce the corresponding RuIII(OH) complex (5), which was characterized crystallographically. In the crystal structure of 5, hydrogen bonds are formed among the NH groups of the pivalamide moieties and the oxygen atom of the hydroxo ligand. Further 1e--oxidation of 5 yields the corresponding RuIV(O) complex, 6, which has intramolecular HB of the oxo ligand with two amide N-H protons. Additionally, the RuIII(OH) complex, 5, exhibits disproportionation to the corresponding RuIV(O) complex, 6, and a mixture of the RuII complexes, 2 and 3, in an acidic aqueous solution. We investigated the oxidation of a phenol derivative using complex 6 as the active species and clarified the switch of the reaction mechanism from hydrogen-atom transfer at pH 2.5 to electron transfer, followed by proton transfer at pH 1.0. Additionally, the intramolecular HB in 6 exerts enhancing effects on oxygen-atom transfer reactions from 6 to alkenes such as cyclohexene and its water-soluble derivative to afford the corresponding epoxides, relative to the corresponding RuIV(O) complex (6') lacking the HB moieties in the SCS.

Synthesis of ruthenium-based metallo-covalent organic frameworks for efficient photocatalytic ring opening of α-ketoepoxides under visible light

Two-dimensional (2D) metallo-covalent organic frameworks (MCOFs) have been regarded as an interesting new class of photoactive materials due to their superior thermal and chemical stability. Herein, we reported a heterogeneous metallaphotocatalyst Ru-COF which was prepared by using an sp2 carbon-conjugated covalent organic framework containing bipyridine as the support for a RuII complex. Under visible-light irradiation, the Ru-COF was used as the photocatalyst that catalyzed the ring-opening reaction of α-ketoepoxides to give various target products with good functional group tolerance in moderate to high yields at room temperature. Importantly, compared with Ru(bpy)3Cl2, Ru-COF shows admirable reusability and higher catalytic activity. Furthermore, radical ions captured by the control experiments provided direct evidence for the mechanism of the visible light photoredox reaction.

'Click' Derived 1,2,3-Triazolium Salts Featuring a 4-Halo-phenyl Group: Precursors for Heterobimetallic Complex.

The 'click' derived 1,2,3-triazolium salts [L1/L2-H]I (1-(4-iodo/bromo-phenyl)-1H-1,2,3-triazolium salt) featuring two distinct coordination sites, one via oxidative addition and other via classical deprotonation cum metalation, were designed and explored towards the synthesis of various mono- and bimetallic complexes. Accordingly, a series of well-characterized 1,2,3-triazol-5-ylidene coordinated cyclometalated transition metal complexes (1-3) were obtained. However, these complexes were found to be ineffective in accessing the desired heterobimetallic complexes. Nevertheless, same ligand systems readily underwent oxidative addition with Pd0 metal precursors, to give the PdII complexes 4/5, and the considerable change and reversal in chemical shift values of the triazole C4/C5-backbone protons (C4-H being downfield shifted than that of C5-H) of these oxidatively added PdII complexes were detected. The PdII complex 5 with chelating DPPE was finally successfully applied for the synthesis of the heterobimetallic complex 6 (PdII-RuII) and 2D NMR analyses, as well as DFT calculations supported the formation of the rare C4/C5-unprotected 1,2,3-triazol-4-ylidene coordinated RuII-complex.

Synthesis and Transition Metal Complexes of ((Phenylphosphanediyl) bis(ethyne‐2,1‐diyl))bis(diphenyl‐phosphane), [PhP{C≡CPPh2}2

AbstractThis paper describes the synthesis of a new acetylene‐bridged triphosphine, ((phenylphosphanediyl)bis‐(ethyne‐2,1‐diyl))bis(diphenylphosphane) [PhP{C≡CPPh2}2] (2) and its coordination complexes of RuII, RhIII, CuI, PdII and PtII. The reaction of diethynylphenylphosphine [PhP{C≡CH}2] (1) with two equivalents of LiHMDS followed by the addition of PPh2Cl resulted in 2. Treatment of 2 with [Ru(η6‐p‐cymene)Cl2]2 and [RhCp*Cl2]2 in 1 : 1.5 molar ratios produced trinuclear complexes [{Ru(η6‐p‐cymene)Cl2}3{μ3‐PPh(C≡CPPh2)2}] (3) and [(RhCp*Cl2)3{μ3‐PPh(C≡CPPh2)2}] (4). The molecular structure of 3 was confirmed by single crystal X‐ray analysis. Treatment of 2 with M(COD)Cl2 (M=Pd, Pt) in 2 : 3 ratio afforded open book type complexes [(MCl2)3{μ3‐PPh(C≡CPPh2)}2] (5 M=Pd, 6 M=Pt). The reaction of 2 with CuI in 1 : 2 ratio afforded 1‐D coordination polymer [{Cu2(μ2‐I)2(CH3CN)}{PPh(C≡CPPh2)2}3{CuI}2] (7) containing [(CuI)2{μ2‐(P−C≡C−P)3}] cylindrical units.

Electrically Amplified Circularly Polarized Luminescence by a Chiral Anion Strategy

AbstractThe development of circularly polarized electroluminescence (CPEL) is currently hampered by the high difficulty and cost in the syntheses of suitable chiral materials and the notorious chirality diminishment issue in electrical devices. Herein, diastereomeric IrIII and RuII complexes with chiral (±)‐camphorsulfonate counteranions are readily synthesized and used as the active materials in circularly polarized light‐emitting electrochemical cells to generate promising CPELs. The addition of the chiral ionic liquid (±)‐1‐butyl‐3‐methylimidazole camphorsulfonate into the active layer significantly improves the device performance and the electroluminescence dissymmetry factors (≈10−3), in stark contrast to the very weak circularly polarized photoluminescence of the spin‐coated films of these diastereomeric complexes. Control experiments with enantiopure IrIII complexes suggest that the chiral anions play a dominant role in the electrically‐induced amplification of CPELs.

Electrically Amplified Circularly Polarized Luminescence by a Chiral Anion Strategy.

The development of circularly polarized electroluminescence (CPEL) is currently hampered by the high difficulty and cost in the syntheses of suitable chiral materials and the notorious chirality diminishment issue in electrical devices. Herein, diastereomeric IrIII and RuII complexes with chiral (±)-camphorsulfonate counteranions are readily synthesized and used as the active materials in circularly polarized light-emitting electrochemical cells to generate promising CPELs. The addition of the chiral ionic liquid (±)-1-butyl-3-methylimidazole camphorsulfonate into the active layer significantly improves the device performance and the electroluminescence dissymmetry factors (≈10-3 ), in stark contrast to the very weak circularly polarized photoluminescence of the spin-coated films of these diastereomeric complexes. Control experiments with enantiopure IrIII complexes suggest that the chiral anions play a dominant role in the electrically-induced amplification of CPELs.

Incorporation of Switchable Inorganic Building Blocks into Heterometallic Coordination Polymers

A new terpyridine-based ligand with a pyridyl-1,2,4-triazoyl bidentate pocket “out the back” has been used to synthesize monometallic CoII, FeII, and RuII complexes where the octahedral metal is bound in a N6 bis-terpyridine coordination sphere. All four complexes ([MII(Lpyrtz-tpy)2](X)2 where MII = FeII, RuII, CoII with X = BF4–, or MII = CoII with X = PF6–) were structurally characterized confirming the expected bis-terpyridine coordination sphere. The monometallic complexes all show reversible MII/III redox processes, while the CoII complexes also undergo a slow spin crossover over the temperature range 50 K – 350 K. Utilizing the free triazole-pyridine bidentate pocket we show the ability of these complexes to act as supramolecular building blocks able to form 1D and 2D coordination polymers, {[MIIMI(Lpyrtz-tpy)2(MeCN)m]X3}n, with MI being CuI or AgI metal ions, MII = CoII, FeII, or RuII, X = BF4 or PF6, and m = 0 or 1, all five of which were structurally characterized.

Generation of Ru(III)-hypochlorite with resemblance to the heme-dependent haloperoxidase enzyme.

The reaction of [(Me/BnTPEN)RuII(NCCH3)]2+ (BnTPEN = N1-benzyl-N1,N2,N2-tris(pyridine-2-ylmethyl)ethane-1,2-diamine and MeTPEN = N1-methyl-N1,N2,N2-tris(pyridine-2-ylmethyl)ethane-1,2-diamine) with mCPBA in the presence of chloride ions in CH3CN : H2O generated a novel (Me/BnTPEN)RuIII-OCl species at room temperature. This hypochlorite adduct could also be obtained by the direct reaction of NaOCl and HClO4 with (L)RuII complexes. The current study mimics the synthesis of a metal hypochlorite adduct in a similar fashion as in the heme-dependent haloperoxidase enzyme. As an electrophilic oxidant, the ruthenium hypochlorite adduct catalyzes hydrogen atom abstraction reactions of phenols and their derivatives.

Correction: A photoactive lysosome targeting RuII complex downregulates stemness genes in oral squamous cell carcinoma

Correction for ‘A photoactive lysosome targeting RuII complex downregulates stemness genes in oral squamous cell carcinoma’ by Souryadip Roy et al., Inorg. Chem. Front., 2022, 9, 5840–5852, https://doi.org/10.1039/D2QI01079H.

Tuning the Photocatalytic Performance of Ruthenium(II) Polypyridine Complexes Via Ligand Modification for Visible-Light-Induced Phosphorylation of Tertiary Aliphatic Amines.

Tuning the redox potential of commonly available photocatalyst to improve the catalytic performance or expand its scope for challenging synthetic conversions is an ongoing demand in synthetic chemistry. Herein, the excited state properties and redox potential of commercially available [Ru(bpy)3 ]2+ photocatalyst were tuned by modifying the structure of the bipyridine ligands with electron-donating/withdrawing units. The visible-light-mediated photoredox phosphorylation of tertiary aliphatic amines was demonstrated under mild conditions. A series of cross-dehydrogenative coupling reactions were performed employing the RuII complexes as photocatalyst giving the corresponding α-aminophosphinoxides and α-aminophosphonates via carbon-phosphorus (C-P) bond formation.

Heterobimetallic Gold/Ruthenium Complexes Synthesized via Post-functionalization and Applied in Dual Photoredox Gold Catalysis.

The synthesis of heterobimetallic AuI /RuII complexes of the general formula syn- and anti-[{AuCl}(L1∩L2){Ru(bpy)2 }][PF6 ]2 is reported. The ditopic bridging ligand L1∩L2 refers to a P,N hybrid ligand composed of phosphine and bipyridine substructures, which was obtained via a post-functionalization strategy based on Diels-Alder reaction between a phosphole and a maleimide moiety. It was found that the stereochemistry at the phosphorus atom of the resulting 7-phosphanorbornene backbone can be controlled by executing the metal coordination and the cycloaddition reaction in a different order. All precursors, as well as the mono- and multimetallic complexes, were isolated and fully characterized by various spectroscopic methods such as NMR, IR, and UV-vis spectroscopy as well as cyclic voltammetry. Photophysical measurements show efficient phosphorescence for the investigated monometallic complex anti-[(L1∩L2){Ru(bpy)2 }][PF6 ]2 and the bimetallic analogue syn-[{AuCl}(L1∩L2){Ru(bpy)2 }][PF6 ]2 , thus indicating a small influence of the {AuCl} fragment on the photoluminescence properties. The heterobimetallic AuI /RuII complexes syn- and anti-[{AuCl}(L1∩L2){Ru(bpy)2 }][PF6 ]2 are both active catalysts in the P-arylation of aryldiazonium salts promoted by visible light with H-phosphonate affording arylphosphonates in yields of up to 91 %. Both dinuclear complexes outperform their monometallic counterparts.

Proton Conduction at High Temperature in High-Symmetry Hydrogen-Bonded Molecular Crystals of RuIII Complexes with Six Imidazole-Imidazolate Ligands.

Invited for the cover of this issue is mainly the group of Makoto Tadokoro and co-workers at Tokyo University of Science. Other co-workers are Masaki Itoh, Ryota Nishimura, Kensuke Sekiguchi (TUS students), Dr. Norihisa Hoshino (Tohoku Univ.), Dr. Hajime Kamebuchi (Nihon Univ.), Dr. Jun Miyazaki (Tokyo Denki Univ.), Prof. Motohiro Mizuno (Kanazawa Univ.) and Prof. Tomoyuki Akutagawa (Tohoku Univ.). The image depicts on two mechanisms of proton transport rotations of the proton-conductive starburst molecule [RuIII (HIm)3 (Im)3 ]. Read the full text of the article at 10.1002/chem.202201397.