Mononuclear Ruthenium Research Articles

Selective Oxidative Scission of Olefins to Aldehydes Enabled by Ruthenium Complexes Containing Salicylaldimine Ligands

ABSTRACTTreatment of Ru3(CO)12 with salicylaldimine ligands [ArNCH(3‐tBuC6H3OH) [ArC6H5 (L1H); 4‐OMeC6H4 (L2H); 4‐MeC6H4 (L3H); 2,4,6‐Me3C6H2 (L4H); 4‐ClC6H4 (L5H); 4‐BrC6H4 (L6H)] in refluxing toluene afforded six bis‐ligand mononuclear ruthenium carbonyl complexes [ArNCH(3‐tBuC6H3O)]2Ru(CO)2 1a–1f in good yields. All of the Ru(II) complexes were characterized using IR, NMR, and elemental analysis. The molecular structures of complexes 1a, 1b, 1e, and 1f were further confirmed through X‐ray diffraction analysis. Additionally, the catalytic performance of these complexes was investigated in the oxidation of olefins to aldehydes, using TBHP as the oxidant in refluxing trichloromethane. With 1.0 mol% catalyst loading, these ruthenium complexes displayed high reactivity and good functional‐group compatibility, selectivity providing the corresponding oxidation product aldehydes in 73%–94% yields.

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.

Management of Triplet States in Modified Mononuclear Ruthenium(II) Complexes for Enhanced Photocatalysis.

Controlling the interplay between relaxation and charge/energy transfer processes in the excited states of photocatalysts is crucial for the performance of artificial photosynthesis. Metal-to-ligand charge-transfer triplet states (3MLCT*) of ruthenium(II) complexes are broadly implemented for photocatalysis, but an effective means of managing the triplets for enhanced photocatalysis has been lacking. Herein, We proposed a strategy to considerably prolong the triplet excited-state lifetime by decorating a ruthenium(II) phosphine complex (RuP-1) with pendent polyaromatic hydrocarbons (PAHs). Systematic studies demonstrate that in RuP-4 decorated with anthracene, sub-picosecond electron transfer from anthracene to 3MLCT* leads to a charge-separated state that can mediate the formation of the intra-ligand triplet state (3IL) of anthracene, resulting in an exceptionally long excited-state up to several milliseconds. This triplet management strategy enables impressive photocatalytic reduction of CO2 to CO with a turnover number (TON) of 404, an optimized quantum yield of 43 % and 100 % selectivity, which is the highest reported performance for mononuclear photocatalysts without additional photosensitizers. RuP-4 also catalyzes photochemical hydrogen generation under argon. This work opens up an avenue for regulating the excited-state charge/energy flow for the development of long-lived 3IL multi-functional mononuclear photocatalysts to boost artificial photosynthesis.

Management of Triplet States in Modified Mononuclear Ruthenium(II) Complexes for Enhanced Photocatalysis

AbstractControlling the interplay between relaxation and charge/energy transfer processes in the excited states of photocatalysts is crucial for the performance of artificial photosynthesis. Metal‐to‐ligand charge‐transfer triplet states (3MLCT*) of ruthenium(II) complexes are broadly implemented for photocatalysis, but an effective means of managing the triplets for enhanced photocatalysis has been lacking. Herein, We proposed a strategy to considerably prolong the triplet excited‐state lifetime by decorating a ruthenium(II) phosphine complex (RuP‐1) with pendent polyaromatic hydrocarbons (PAHs). Systematic studies demonstrate that in RuP‐4 decorated with anthracene, sub‐picosecond electron transfer from anthracene to 3MLCT* leads to a charge‐separated state that can mediate the formation of the intra‐ligand triplet state (3IL) of anthracene, resulting in an exceptionally long excited‐state up to several milliseconds. This triplet management strategy enables impressive photocatalytic reduction of CO2 to CO with a turnover number (TON) of 404, an optimized quantum yield of 43 % and 100 % selectivity, which is the highest reported performance for mononuclear photocatalysts without additional photosensitizers. RuP‐4 also catalyzes photochemical hydrogen generation under argon. This work opens up an avenue for regulating the excited‐state charge/energy flow for the development of long‐lived 3IL multi‐functional mononuclear photocatalysts to boost artificial photosynthesis.

Pyrazine-Coordinated Dinuclear and Mononuclear Ruthenium Complexes Formed via the Conversion of the Triply Chlorido-Bridged Diruthenium(II) Complex.

The pyrazine-coordinated dinuclear and mononuclear ruthenium complexes were synthesized through the framework conversion reactions of the triply chlorido-bridged diruthenium(II) complex [{RuII,II(bbpma)}2(μ-Cl)3]+ (bbpma; benzylbis(2-pyridylmethyl)amine, [1]+) in the presence of pyrazine, which could function as the simple molecular multinucleation ligand of metal compounds. A reduction reaction of fac-[RuIIICl3(bbpma)] with zinc in the presence of hydrochloric acid afforded [1]+ in solution, and the following addition of pyrazine (1 equiv) in the solution led to the formation of a singly pyrazine (pz)-bridged diruthenium complex, [{RuII,II(μ-Cl2ZnCl2)(bbpma)}2(μ-pz)] ([2(II,II)(ZnCl2)2]). The stoichiometric two-electron oxidation of [2(II,II)(ZnCl2)2] was successfully proceeded, and a Ru(III)-Ru(III) species, [{RuIII,IIICl2(bbpma)}2(μ-pz)](PF6)2 ([III,III](PF6)2), was isolated. The reaction of [1]+ with excess amounts of pyrazine without hydrochloric acid afforded mononuclear Ru(III) and Ru(II) complexes containing one or two pyrazine, fac-[RuClm(pz)3-m(bbpma)]+ (m = 2; [3]+; m = 1; [4]+). The details of the electrochemical and spectroscopic properties of [1]+-[4]+ in organic and aqueous solutions were discussed.

Dinitrogen-bridged ruthenium(II) complexes featuring dithiocarbamate ligands

In quest of low molecular weight metal-thiolate complexes as model complexes for nitrogenase, four binuclear ruthenium(II) dinitrogen-bridged complexes featuring dithiocarbamate ligands, [{(Me3tacn)Ru(κ2-S2CNR2)}2(µ-N2)][PF6]2 (Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane; 1, R = Me; 2, R = Et; 3, R = nPr; 4, R = iPr), were synthesized by reduction of [(Me3tacn)RuIII(κ2-S2CNR2)Cl]PF6 with zinc powder in a nitrogen atmosphere. A 16-electron mononuclear ruthenium(II) species [(Me3tacn)Ru(κ2-S2CNnPr2)]PF6 (3′) was isolated during preparation of 3, which may help elucidate the formation of complexes 1-4. Molecular structures of complexes 1, 2, 4 and 3′ were established by single crystal X-ray diffraction analysis. Moreover, complexes 1-4 were characterized by infrared, 1H NMR, UV–vis, fluorescence, Raman (solid state and solution state) spectroscopies and mass spectrometry, and their electrochemical properties were also investigated. Density functional theory (DFT) calculation was performed on complex 2 to study its Lewis structure and explain the UV–vis absorption spectra. Structural and DFT analyses demonstrate that N2 activation is accompanied by electron redistribution of the dithiocarbamate ligands.

Tuning of the photophysical and electrochemical properties of ruthenium(II) phthalocyaninates by variation of axial ligands

A series of mononuclear ruthenium(II) tetra-tert-butyl-phthalocyaninates bearing axial N-donor ligands [tBu4PcRu]L2 (L = trimethylamine, pyrazine, 4,4′-bipyridine and N-methyl-4,4′-bipyridinium iodide) as well as the binuclear complex [tBu4PcRu]2(BiPy)3 were synthesized starting from the complex with axially coordinated carbonyl group [tBu4PcRu](CO). All compounds have been characterized by NMR, UV-Vis and cyclic voltammetry. The latter allowed the determination of oxidation and reduction potentials, HOMO-LUMO gaps and revealed the possibility of electropolymerization of BiPy-containing complexes in clear contrast to complexes containing another bidentate ligand – pyrazine. Comparative electrochemical studies of the mono- and binuclear complexes [tBu4PcRu](BiPy)2 and [tBu4PcRu]2(BiPy)3 revealed that the phthalocyanine rings are not conjugated in the binuclear species. The remarkable dependence of singlet oxygen generation from axial ligands in ruthenium phthalocyaninates was observed: the complexes with carbonyl group [tBu4PcRu](CO) and with pyrazine molecules [tBu4PcRu](Pyz)2 show higher 1O2 quantum yields, whereas the complex with axially coordinated molecules of trimethylamine [tBu4PcRu](NMe3)2 and quaternized BiPy ligands [tBu4PcRu](BiPy-Me+)2 have the lowest ability to generate singlet oxygen. The revealed influence of axial ligands on key physicochemical properties paves the way for the design of new ruthenium phthalocyaninates with potential optoelectronic and biomedical applications.

Synthesis, spectral characterization and biological evaluation of diosgenin coordinated ruthenium-arene complexes

A set of mononuclear ruthenium complexes, i.e. [Ru(Dg)(C6H6)Cl2] (RuBDg), [Ru(Dg)((p-cymene)Cl2] (RuCDg) were synthesized using a phytosteroid diosgenin (Dg) as ligand. The formulations and characterizations of the complexes were achieved through elemental analyses, infrared, electronic absorption, electron paramagnetic resonances, mass spectrometric analysis, and molar conductance studies. Ruthenium ion is octahedrally coordinated to the oxygen of diosgenin, one arene moiety and two chlorides. An assessment of the antioxidant activities of the compounds indicated better scavenging ability against ABTS•+, DPPH•, O2−, NO• and OH− radicals. The metal complexes showed activity towards free radicals compared to the parent diosgenin. The anticancer activities of diosgenin and the ruthenium complexes were evaluated by MTT assay against the human lung cancer cell line (A549) and cervical carcinoma cell line (HeLa), with cisplatin as a reference. RuCDg exhibited potent ability to kill A549 and HeLa cancer cells with IC50 values of 5.8 ± 0.5 µM and 5.5 ± 0.6 µM, respectively. Similarly, RuBDg showed activity towards A549 and HeLa cancer cells with IC50 values of 16 ± 1 µM and 12 ± 1 µM, respectively. These values are better than that of free diosgenin (A549 − 48 ± 2 µM and HeLa − 43 ± 1 µM). The morphological changes of the HeLa and A549 cells with the complexes, studied by AO-EB and DAPI staining methods, suggested cell death through apoptosis.

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).

Synthesis, characterization, and anticancer activity of some mononuclear ruthenium(II) compounds

Mononuclear Ru-based compounds cis-RuII(dppm)2Cl2 (1) and trans-RuII(dppe)2Cl2 (4) were synthesized by treatment of RuIICl2(DMSO)4 with dppm or dppe, respectively (dppm = bis(diphenylphosphino) methane, dppe = 1,2-bis(diphenylphosphino) ethane, DMSO = dimethyl sulfoxide). Mononuclear Ru-based compounds trans-RuII(dppm)2Cl2 (2) and cis-RuII(dppe)2Cl2 (3) were obtained by reaction of RuIICl2(PPh3)3 with dppm or dppe, separately. These two pair of cis–trans isomers, cis/trans-RuII(dppm)2Cl2 and cis/trans-RuII(dppe)2Cl2, were characterized by electronic absorption spectra, infrared, elemental analysis, 1H NMR, 13C NMR, 31P NMR, and single-crystal X-ray diffraction analysis. Both the crystal structure data and electronic absorption spectra indicate that the cis-isomer is a more thermodynamically stable structure and has lower energy than the trans-isomer. The luminescence spectroscopic study and relative cell viability of 1–4 on HeLa cancer cells has been investigated.

Enhanced Photo-Electrochemical Responses through Photo-Responsive Ruthenium Complexes on ITO Nanoparticle Surface

The mononuclear ruthenium 1-Cl and dinuclear ruthenium 2-Cl complexes undergo a photo-induced ligand exchange in water, affording the corresponding 1-H2O and 2-H2O complexes. The use of indium tin oxide nanoparticles (nanoITOs) to explore the photo-electrochemistry of the in situ-generated 1-H2O and 2-H2O in solution revealed greater photocurrents produced by these two complexes when compared with an experiment using a buffer only. Interestingly, the high photocurrent shown by the dinuclear complex 2-H2O was accompanied by the deposition of its higher oxidation state (H2O)RuII–RuIII(OH), as evidenced with cyclic voltammetry, SEM and XPS. The IPCE and spectro-electrochemistry studies supported by TD-DFT calculations revealed the visible light harvesting ability of 1-H2O and 2-H2O in solution and the subsequent electron injection into the conduction band of the nanoITOs, enhanced in 2-H2O via a plausible chelating effect.

Mononuclear ruthenium (II) complex covalently anchored on melem and g-C3N4 as efficient heterogeneous catalysts for chemical water oxidation

Ru-melem and Ru-C3N4 were synthesized by a simple and facile strategy to construct a novel covalently anchoring by introducing easily synthesized amide bond as a bridge connecting the Ru-terpy and melem or g-C3N4, respectively. The covalent anchoring of Ru complex on melem or C3N4 not only makes these materials exhibit water oxidation activity under CeIV-driven (CeIV = Ce(NH4)2(NO3)6) reaction condition, but also makes the obtained heterogeneous catalysts show higher catalytic activity than the corresponding homogeneous catalysts, which reveals that the covalent anchoring strategy of Ru complex is beneficial to improve the catalytic activity of homogeneous Ru catalysts. The synthetic method of hybrid catalysts offers an insightful strategy for enhancing water oxidation activity of molecular catalysts.

PH dependent spectrophotometric study, cytotoxicity and antimicrobial activity of mononuclear ruthenium(II) polypyridine complexes

pH dependent spectrophotometric study, cytotoxicity and antimicrobial activity of mononuclear ruthenium(II) polypyridine complexes

Assembled Porphyrin‐Based Multinuclear Ruthenium(II)‐NNNN Complex Catalysts for Transfer Hydrogenation of Ketones

AbstractMultinuclear porphyrin‐based ruthenium(II)‐NNNN complexes were efficiently assembled by means of coordinatively unsaturated 16‐electron mononuclear ruthenium(II)‐pyrazolyl‐imidazolyl‐pyridine complex, zinc(II) meso‐tetra(4‐pyridyl)‐porphyrin (ZnTPyP), and 4,4′‐linked bipyridines. The resultant multinuclear (Ru4 and Ru8) porphyrin‐based ruthenium(II)‐NNNN complexes exhibited exceptionally high catalytic activity at as low as 0.008 mol % Ru loading for the transfer hydrogenation reaction of ketones in refluxing 2‐propanol, reaching up to 99 % yields and 5.7×106 h−1 TOFs.

Ruthenium Complexes of Rigid, Dianionic, Tetradentate N‐Donor Ligands and their Potential as Catalysts for Water Oxidation

AbstractTwo mononuclear ruthenium(II) complexes based on dianionic {N4} ligands and with axial pyridines have been prepared and characterized crystallographically (1) or by 2D NMR spectroscopy using residual dipolar couplings (2). The {N4} ligands provide a constrained equatorial coordination with one large N−Ru−N angle, and additional non‐coordinating N atoms in case of 2. Their redox properties have been investigated (spectro)electrochemically, and their potential to serve as water oxidation catalysts has been probed using cerium ammonium nitrate (CAN) at pH 1.0. Complex 1 undergoes rapid degradation, likely via ligand oxidation, whereas 2 is more rugged and exhibits 80 % efficiency in the CeIV‐driven water oxidation, with a high initial turnover frequency (TOFi) of 3.07×10−2 s−1 (at 100 equiv. CAN). The initial rate of O2 evolution exhibits 1st order dependence on catalyst concentration, suggesting a water nucleophilic attack mechanism. Repeated addition of CAN and control experiments show that high ionic strength conditions (both NO3− and CeIII) significantly decrease the TOF.

Distinctive Aspects in Aquation, Proton-Coupled Redox, and Photoisomerization Reactions between Geometric Isomers of Mononuclear Ruthenium Complexes with a Large-π-Conjugated Tetradentate Ligand.

Geometric isomers of mononuclear ruthenium(II) complexes, distal-/proximal-[Ru(tpy)(dpda)Cl]+ (d-/p-RuCl, tpy = 2,2':6',2″-terpyridine, dpda = 2,7-bis(2-pyridyl)-1,8-diazaanthracene), were newly synthesized to comprehensively investigate the geometric and electronic structures and distinctive aspects in various reactions between isomers. The ultraviolet (UV)-visible absorption spectra of d-/p-RuCl isomers show intense bands for metal-to-ligand charge transfer (MLCT) at close wavelengths of 576 and 573 nm, respectively. However, time-dependent density functional theory (TD-DFT) calculations suggest that the MLCT transition of d-RuCl involves mainly single transitions to the π* orbital of the dpda ligand in contrast to mixing of the π* orbitals of the dpda and tpy ligands for p-RuCl. The aquation reaction (1.5 × 10-3 s-1) of p-RuCl to yield proximal-[Ru(tpy)(dpda)(OH2)]2+ (p-RuH2O) is faster than that (5.3 × 10-6 s-1) of d-RuCl in D2O/CD3OD (4:1 v/v) by three orders of magnitude, which resulted from the longer Ru-Cl bond by 0.017 Å and the distorted angle (100.2(3)°) of Cl-Ru-N (a nitrogen of dpda, being on a tpy plane) due to the steric repulsion between Cl and dpda for p-RuCl. Electrochemical measurements showed that d-RuH2O undergoes a 2-step oxidation reaction of 1H+-coupled 1e- processes of RuII-OH2/RuIII-OH and RuIII-OH/RuIV═O at pH 1-9, whereas p-RuH2O undergoes a 1-step oxidation reaction of a 2H+-coupled 2e- process of RuII-OH2/RuIV═O in the pH range of pH 1-10. The irreversible photoisomerization from d-RuH2O to p-RuH2O was observed in aqueous solution with an internal quantum yield (Φ) of 5.4 × 10-3% at 520 nm, which is lower compared with Φ = 1.1-2.1% of mononuclear Ru(II) aquo complexes with similar bidentate ligands instead of dpda by three orders of magnitude. This is possibly ascribed to the faster nonradiative decay rate from the excited 3MLCT state to the ground state for d-RuH2O due to the lower π* level of dpda ligands according to the energy-gap law: the rate decreases exponentially with the increasing energy gap.

Ferromagnetic Coupling and Single-Ion Magnet Phenomenon in Mononuclear Ruthenium(III) Complexes Based on Guanine Nucleobase

Two mononuclear RuIII complexes of formula trans-[RuCl4(Hgua)(dmso)]·2H2O (1) and trans-[RuCl4(Hgua)(gua)]·3H2O (2) [Hgua = protonated guanine (gua), dmso = dimethyl sulfoxide] have been synthesized and characterized magnetostructurally. 1 and 2 crystallize in the monoclinic system with space groups P21/n and Pc, respectively. Each RuIII ion in 1 and 2 is bonded to four chloride ions and one (1) or two (2) nitrogen atoms from guanine molecules and one sulfur atom (1) of a dmso solvent molecule, generating axially compressed octahedral geometries in both cases. In their crystal packing, the RuIII complexes are connected through an extended network of N-H⋯Cl hydrogen bonds and π⋯Cl intermolecular interactions, forming novel supramolecular structures of this paramagnetic 4d ion. Variable-temperature dc magnetic susceptibility data were obtained from polycrystalline samples of 1 and 2 and their plots show a different magnetic behavior. While 1 is a ferromagnetic compound at low temperature, 2 exhibits a behavior of magnetically isolated mononuclear RuIII complexes with S = 1/2. The study on ac magnetic susceptibility data reveal slow relaxation of the magnetization, when external dc fields are applied, only for 2. These results highlight the presence of field-induced single-ion magnet (SIM) behavior for this mononuclear guanine-based RuIII complex.

Development of Mononuclear (Arene) Ruthenium Complexes as Anticancer Agents: A Review

The OUSL Journal is a peer-reviewed open access journal published by the Open University of Sri Lanka (OUSL). It is the first scholarly journal in Sri Lanka focusing on research in the field of Open and Distance Learning (ODL), and is being published since 1997. The journal provides a platform to publish research and pedagogy relating to teaching and learning at the University level, and in particular to Open and Distance Learning (ODL). It also provides a forum for research and review articles related in the areas of Agriculture, Natural Sciences, Education, Engineering, Humanities and Social Sciences and Health Sciences.

A Highly Durable, Self-Photosensitized Mononuclear Ruthenium Catalyst for CO2 Reduction

AbstractA novel mononuclear ruthenium (Ru) complex bearing a PNNP-type tetradentate ligand is introduced here as a self-photosensitized catalyst for the reduction of carbon dioxide (CO2). When the pre-activation of the Ru complex by reaction with a base was carried out, an induction period of catalyst almost disappeared and the catalyst turnover numbers (TONs) over a reaction time of 144 h reached 307 and 489 for carbon monoxide (CO) and for formic acid (HCO2H), respectively. The complex has a long lifespan as a dual photosensitizer and reduction catalyst, due to the sterically bulky and structurally robust (PNNP)Ru framework. Isotope-labeling experiments under 13CO2 atmosphere indicate that CO and HCO2H were both produced from CO2.

Dangling Carboxylic Group That Participates in O-O Bond Formation Reaction to Promote Water Oxidation Catalyzed by a Ruthenium Complex: Experimental Evidence of an Oxide Relay Pathway.

Two mononuclear ruthenium(II) complexes of the types [Ru(trpy)(HL1)(OH2)]2+ (1Aq) and [Ru(trpy)(L2-κ-N2O)] (2) [where trpy = 2,2':6',2″-terpyridine, HL1 = 2-(2-pyridyl)benzimidazole, H2L2 = 2-(pyridin-2-yl)-1H-benzo[d]imidazole-4-carboxylic acid] have been synthesized and thoroughly characterized by analytical and spectroscopic [UV-vis, NMR, high-resolution mass spectrometry, and IR] techniques. Complex 1Aq has been further characterized by X-ray crystallography. In an acidic aqueous medium (pH 1), complex 2 undergoes carboxylate/water exchange readily to form an aqua-ligated complex, [Ru(trpy)(H2L2-κ-N2)(OH2)]2+ (2Aq), having a dangling carboxylic group. This exchange phenomenon has been followed by IR, 1H NMR, and UV-vis spectroscopic techniques. Electrochemical analyses of 1Aq and 2Aq (Pourbaix diagram) suggest the generation of a formal RuV═O species that can potentially promote the oxidation of water. A comparative study of the water oxidation activity catalyzed by 1Aq and 2Aq is reported here to see the effect of a dangling carboxylic group in the catalytic performance. Complex 2Aq shows an enormously higher rate of reaction than 1Aq. The pendant carboxylic group in 2Aq participates in an intramolecular O-O bond formation reaction with the reactive formal RuV═O unit to form a percarboxylate intermediate and provides an electron-deficient carbon center where water nucleophilic attack takes place. The isotope labeling experiment using 18O-labeled water verifies the attack of water at the carbon center of the carboxylic group rather than a direct attack at the oxo of the formal RuV═O unit. The present work provides experimental evidence of the uncommon functionality of the carboxylic group, the oxide relay, in molecular water oxidation chemistry.