RuII Center Research Articles

PH-Activatable Ruthenium(II) Fluorescein Salphen Schiff Base Photosensitizers for Theranostic Applications.

Ruthenium(II) polypyridyl complexes exhibit a lack of selectivity toward cancer tissues despite extensive studies as photosensitizers for photodynamic therapy (PDT). Here, we report pH-activatable RuII photosensitizers for molecularly targeted PDT by exploiting the higher acidity of tumoral tissue. The fluorescein moiety, well known for its high pH sensitivity, was connected to a RuII center to yield novel photosensitizers for pH-sensitive 1O2 photogeneration. Their ability to photosensitize molecular dioxygen was studied at various pHs and revealed a drastic enhancement from 0.07 to 0.66 of the 1O2 quantum yield under acidic conditions (pH 7.5 to pH 5.5). Their photocytotoxicity against U2OS osteosarcoma cells was also investigated at pH 5.5 and 7.5 through IC50 determination. A strong enhancement of the photocytotoxicity reaching 930 nM was observed at pH 5.5, which showed the potential of such photosensitizers for pH-activatable PDT.

A Near‐Infrared Light‐Activated Photocage Based on a Ruthenium Complex for Cancer Phototherapy

AbstractConventional photocages only respond to short wavelength light, which is a significant obstacle to developing efficient phototherapy in vivo. The development of photocages activated by near‐infrared (NIR) light at wavelengths from 700 to 950 nm is important for in vivo studies but remains challenging. Herein, we describe the synthesis of a photocage based on a ruthenium (Ru) complex with NIR light‐triggered photocleavage reaction. The commercial anticancer drug, tetrahydrocurcumin (THC), was coordinated to the RuII center to create the Ru‐based photocage that is readily responsive to NIR light at 760 nm. The photocage inherited the anticancer properties of THC. As a proof‐of‐concept, we further engineered a self‐assembled photocage‐based nanoparticle system with amphiphilic block copolymers. Upon exposure to NIR light at 760 nm, the Ru complex‐based photocages were released from the polymeric nanoparticles and efficiently inhibited tumor proliferation in vivo.

The Pincer Ligand Supported Ruthenium Catalysts for Acetylene Hydrochlorination: Molecular Mechanisms from Theoretical Insights

Pincer ligand supported RuII chloride complexes may be used for acetylene hydrochlorination as non-mercury molecular catalysts. Based on theoretical calculations, the catalytic mechanism and the interaction between catalysts and reactants has been evaluated, indicating that the (pincer)RuIICl2 platform supports electrophilic proton-ruthenation of C2H2. Energy decomposition studies further illustrate the electron-rich property of the RuII center, which can increase the negative charge of C2H2 via 4d-electron backdonation. Thus, the electrophilic reaction mechanism is favored due to lower energetic barriers. By improving the electron-donating ability of ligands, this lowering of energetic barriers can be enhanced. Therefore, non-mercury catalysts for acetylene hydrochlorination with milder reaction conditions and higher catalytic activity can be designed.

Integrating Light-Harvesting Ruthenium(II)-based Units into Three-Dimensional Metal Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution.

Three metal covalent organic frameworks (MCOFs), namely RuCOF-ETTA, RuCOF-TPB and RuCOF-ETTBA, were synthesized by incorporating the photosensitive RuII tris(2,2'-bipyridine) unit into the skeleton. Interestingly, each RuCOF contains three isostructural covalent organic frameworks that interlock together with the RuII centers serving as point of registry. The covalently linked network coupling with uniformly distributed RuII units allowed the RuCOFs to exhibit superior chemical stability, strong light-harvesting ability, and high photocatalytic activity toward hydrogen evolution (20 308 μmol g-1 h-1 ). This work illustrates the potential of developing versatile MCOFs-based photocatalysts from functionalized metal complex building unit and further enriches the MCOFs family.

Integrating Light‐Harvesting Ruthenium(II)‐based Units into Three‐Dimensional Metal Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution

AbstractThree metal covalent organic frameworks (MCOFs), namely RuCOF‐ETTA, RuCOF‐TPB and RuCOF‐ETTBA, were synthesized by incorporating the photosensitive RuIItris(2,2′‐bipyridine) unit into the skeleton. Interestingly, each RuCOF contains three isostructural covalent organic frameworks that interlock together with the RuIIcenters serving as point of registry. The covalently linked network coupling with uniformly distributed RuIIunits allowed the RuCOFs to exhibit superior chemical stability, strong light‐harvesting ability, and high photocatalytic activity toward hydrogen evolution (20 308 μmol g−1 h−1). This work illustrates the potential of developing versatile MCOFs‐based photocatalysts from functionalized metal complex building unit and further enriches the MCOFs family.

Bioinspired Heterobimetallic Photocatalyst (RuIIchrom-FeIIIcat) for Visible-Light-Driven C-H Oxidation of Organic Substrates via Dioxygen Activation.

We report a bioinspired heterobimetallic photocatalyst RuIIchrom-FeIIIcat and its relevant applications toward visible-light-driven C-H bond oxidation of a series of hydrocarbons using O2 as the O-atom source. The RuII center absorbs visible light near 460 nm and triggers a cascade of electrons to FeIII to afford a catalytically active high-valent FeIV═O species. The in situ formed FeIV═O has been employed for several high-impact oxidation reactions in the presence of triethanolamine (TEOA) as the sacrificial electron donor.

Isolation and Identification of Pseudo Seven-Coordinate Ru(III) Intermediate Completing the Catalytic Cycle of Ru-bda Type of Water Oxidation Catalysts

Isolation and Identification of Pseudo Seven-Coordinate Ru(III) Intermediate Completing the Catalytic Cycle of Ru-bda Type of Water Oxidation Catalysts

Enhanced Electrochemical Performance of Aprotic Li‐CO2 Batteries with a Ruthenium‐Complex‐Based Mobile Catalyst

AbstractLi‐CO2 batteries are regarded as next‐generation high‐energy‐density electrochemical devices. However, the greatest challenge arises from the formation of the discharge product, Li2CO3, which would accumulate and deactivate heterogenous catalysts to cause huge polarization. Herein, Ru(bpy)3Cl2 was employed as a solution‐phase catalyst for Li‐CO2 batteries and proved to be the most effective one screened so far. Spectroscopy and electrochemical analyses elucidate that the RuII center could interact with both CO2 and amorphous Li2C2O4 intermediate, thus promoting electroreduction process and delaying carbonate transformation. As a result, the charge potential is reduced to 3.86 V and over 60 discharge/charge cycles are achieved with a fixed capacity of 1000 mAh g−1 at a current density of 300 mA g−1. Our work provides a new avenue to improve the electrochemical performance of Li‐CO2 batteries with efficient mobile catalysts.

Enhanced Electrochemical Performance of Aprotic Li‐CO2 Batteries with a Ruthenium‐Complex‐Based Mobile Catalyst

Li-CO2 batteries are regarded as next-generation high-energy-density electrochemical devices. However, the greatest challenge arises from the formation of the discharge product, Li2 CO3 , which would accumulate and deactivate heterogenous catalysts to cause huge polarization. Herein, Ru(bpy)3 Cl2 was employed as a solution-phase catalyst for Li-CO2 batteries and proved to be the most effective one screened so far. Spectroscopy and electrochemical analyses elucidate that the RuII center could interact with both CO2 and amorphous Li2 C2 O4 intermediate, thus promoting electroreduction process and delaying carbonate transformation. As a result, the charge potential is reduced to 3.86 V and over 60 discharge/charge cycles are achieved with a fixed capacity of 1000 mAh g-1 at a current density of 300 mA g-1 . Our work provides a new avenue to improve the electrochemical performance of Li-CO2 batteries with efficient mobile catalysts.

Revisiting Dinuclear Ruthenium Water Oxidation Catalysts: Effect of Bridging Ligand Architecture on Catalytic Activity.

An attractive catalytic pathway for the conversion of water to oxygen would involve two metal oxide centers combining in a constructive sense to make O═O. This prospect makes the study of certain dinuclear transition metal complexes particularly attractive. In this work, we describe the design and synthesis of two symmetrical bis-tridentate polypyridine ligands 6 and 12 that bind two RuII centers at a separation of 3.6 Å in 7 and 5.7 Å in 13. In the presence of CeIV at pH = 1, these systems oxidize water with the system having the more proximal metals being more reactive. In the case of the more proximal metal centers, the bridging ligand is a 3,6-disubstituted pyridazine which, under the influence of CeIV, cleaves into two [Ru(bpc)(pic)2CH3CN]+ fragments (14) which then function as the actual catalyst (bpc = 2,2'-bipyridine-6-carboxylate, pic = 4-methylpyridine). The second dinuclear catalyst contains a central pyrimidine ring which is less sensitive to oxidative decay and hence less reactive. Caution is advised in the use of CeIV as a sacrificial electron acceptor due to unexpected oxidative decay of the catalyst.

Ruthenium-Induced Cyclization of Heteroatom-Functionalized Alkynes: Progress, Challenges and Perspectives.

Metal-induced cyclization of functionalized alkynes represents one of the most general approaches to prepare organic heterocycles. Although RuII centers are well-established to promote alkyne to vinylidene rearrangements and many RuII -mediated alkyne cyclizations have been rationalized to be the results of post-vinylidene transformations, recent discoveries indicate that RuII centers can serve as electrophiles and induce alkyne cyclizations without vinylidene intermediacy. In this Minireview, an overview of the RuII -induced cyclization of heteroatom-functionalized alkynes in the last decade is provided, with an emphasis on the discoveries and validations of the unconventional "non-vinylidene-involving" pathways.

Ruthenium [NNN] and [NCN]-type pincer complexes with phosphine coligands: synthesis, structures and catalytic applications

A series of ruthenium [NNN]- or [NCN]-type complexes (3–7) bearing PPh3 ancillary ligands have been synthesized from pyridine- or phenylene-bridged bis(triazoles) 1 and 2. In the case of [NNN]-pincer complex 3, an unusual and unexpected cis-orientation adopted by two sterically demanding PPh3 ligands was observed, and such configuration proved to be unchanged in solution for a long time. By contrast and as expected, the two phosphines are found to be trans to each other in the case of [NCN]-type pincer complex 4, but an oxidation of RuII center to RuIII occurred. Complex cis-3 underwent ligand exchanges leading to the formations of diphosphine derivatives 5 and 6. As a representative, cis-3 was treated with the base in isopropanol affording a mixture of Ru–hydrido complexes with various phosphine binding modes, one of which (trans-7) bearing two trans-standing phosphines has been successfully isolated and fully characterized. The catalytic performances of all newly synthesized Ru complexes have been examined and compared in transfer hydrogenations of ketones and enones, in which mono-phosphine complexes proved to be significantly superior to their diphosphine counterparts. The catalytic process proved to involve Ru–H key intermediates, but the trans-oriented Ru–H species is unlikely to be the main catalytic contributor. In particular, the best performer cis-3 exhibits high chemoselectivity in certain cases catalyzing α,β-unsaturated ketones, whose behavior is quite different compared to most precedents.

Excited-State Dipole Moments of Homoleptic [Ru(bpy')3]2+ Complexes Measured by Stark Spectroscopy.

The visible absorption and Stark spectra of five [Ru(4,4'-(R)2-2,2'-bipyridine)3](PF6)2 and [Ru(bipyrazine)3](PF6)2 complexes, where R = CH3O-, tert-butyl-, CH3-, H-, or CF3-, were obtained in butyronitrile glasses at 77 K as a function of an applied electric field in the 0.2-0.8 MV/cm range. Analysis of the metal-to-ligand charge-transfer (MLCT) absorption and Stark spectra with the Liptay treatment revealed dramatic light-induced dipole moment changes, [Formula: see text]. The application of a two-state model to the [Formula: see text] values provided metal-ligand electronic coupling, HDA = 3100-4500 cm-1. The ground state of these complexes has no net dipole moment and with the RuII center as the point of reference, the dipole moment changes were reasonably assigned to the dipole present in the initially formed MLCT excited state. Further, the excited-state dipole moment was sensitive to the presence of electron-donating (MeO-, tert-butyl-, CH3-) or -withdrawing (CF3-) substituents and was correlated with the substituent Hammett parameters. Hence, the data show for the first time that substituents on the bipyridine ligands, which are often introduced to tune formal reduction potentials, can also induce significant changes in the excited-state dipole, behavior that should be taken into consideration for artificial photosynthesis applications.

The Important Role of Coordination Geometry on Photophysical Properties of Blue-Green Emitting Ruthenium(II) Diisocyano Complexes Bearing 2-Benzoxazol-2-ylphenolate.

A series of blue-green emitting RuII diisocyano complexes containing 2-benzoxazol-2-ylphenolate (PBO) have been prepared. The complexes were isolated under varied reaction conditions in two isomeric forms, i.e., trans,trans,trans- (1) and cis,trans,cis- (2), with varied ligand coordination geometry above the RuII center. The photoluminescence of the isomeric complexes has been compared and tuned by the systematic variation of the electronic properties of the isocyanides. The cis,trans,cis- isomers exhibit structureless emission in the blue-green region (471-517 nm) upon excitation at λex > 400 nm in dichloromethane solution at room temperature. Both isomeric forms show similarly structured greenish emission at 499-523 nm on excitation at λex > 355 nm in a methanol/ethanol (4:1) glassy medium at 77 K. On careful comparison with the corresponding absorption and electrochemical data, it is suggested that the solution emission of the cis,trans,cis- isomers (2) at room temperature is originated from the metal-to-ligand charge transfer (MLCT), while a ligand-centered (LC) parentage is assigned for the emission in a glassy state for both isomeric forms. In line with the above experimental results, DFT calculation demonstrates the change in the nature and relative energy of the HOMOs and LUMOs with respect to the varied ligand coordination geometry and π-accepting ability of the isocyanides.

Sequence and Structure of Peptoid Oligomers Can Tune the Photoluminescence of an Embedded Ruthenium Dye.

The understanding of structure-function relationships within synthetic biomimetic systems is a fundamental challenge in chemistry. Herein we report the direct correlation between the structure of short peptoid ligands-N-substituted glycine oligomers incorporating 2,2'-bipyridine groups-varied in their monomer sequence, and the photoluminescence of RuII centers coordinated by these ligands. Based on circular dichroism and fluorescence spectroscopy we demonstrate that while helical peptoids do not affect the fluorescence of the embedded RuII chromophore, unstructured peptoids lead to its significant decay. Transmittance electron microscopy (TEM) revealed significant differences in the arrangements of metal-bound helical versus unstructured peptoids, suggesting that only the latter can have through-space interactions with the ruthenium dye leading to its quenching. High-resolution TEM enabled the remarkable direct imaging of singular ruthenium-bound peptoids and bundles, supporting our explanation for structure-depended quenching. Moreover, this correlation allowed us to fine-tune the luminescence properties of the complexes simply by modifying the sequence of their peptoid ligands. Finally, we also describe the chiral properties of these Ru-peptoids and demonstrate that remote chiral induction from the peptoids backbone to the ruthenium center is only possible when the peptoids are both chiral and helical.

Phosphonium-Ring-Fused Bicyclic Metallafuran Complexes of Ruthenium and Osmium.

Metallafuran complexes with a fused five-membered phosphonium ring were synthesized from reactions between terminal ynones HC≡C(C=O)R and cis-[Ru/Os(dppm)2 Cl2 ] (dppm=1,1-bis(diphenylphosphino)methane). A metal-vinylidene-involving pathway was found to be an energetically feasible formation mechanism for these complexes. These phosphonium-containing metallafurans, like many phosphonium-functionalized drugs, have the ability to induce mitochondrial dysfunction. They also exhibit stronger cytotoxicity against several human cancer cell lines in comparison with their metal precursors and the classic anticancer drug cisplatin. Overall, this work provides structural and mechanistic insights for the rational design of functional metallacycles via activation of alkynes by RuII and OsII centers.

Structural Investigation into the Threading Intercalation of a Chiral Dinuclear Ruthenium(II) Polypyridyl Complex through a B-DNA Oligonucleotide.

Herein we report the separation of the three stereoisomers of the DNA light-switch compound [{Ru(bpy)2}2(tpphz)]4+ (tpphz = tetrapyrido[3,2-a:2',3'-c:3″,2″-h:2‴,3‴-j]phenazine) by column chromatography and the characterization of each stereoisomer by X-ray crystallography. The interaction of these compounds with a DNA octanucleotide d(GCATATCG).d(CGATATGC) has been studied using NMR techniques. Selective deuteration of the bipyridyl rings was needed to provide sufficient spectral resolution to characterize structures. NMR-derived structures for these complexes show a threading intercalation binding mode with slow and chirality-dependent rates. This represents the first solution structure of an intercalated bis-ruthenium ligand. Intriguingly, we find that the binding site selectivity is dependent on the nature of the stereoisomer employed, with Λ RuII centers showing a better intercalation fit.

Intramolecular Electronic Coupling Enhances Lateral Electron Transfer across Semiconductor Interfaces

The control of lateral electron-transfer reactions is important for many solar energy conversion strategies. Herein, four compounds with two redox-active groups, a bis(tridentate) cyclometalated RuII metal center and a substituted triphenylamine (TPA) donor separated by an organic bridge, were anchored to TiO2 surfaces to facilitate study of lateral intermolecular electron transfer. An important finding was that the TPA+/0 diffusion coefficients were about 1.6 times larger when the bridge promoted intramolecular electronic coupling between the Ru metal center and TPA. Under conditions where TPA+ was able to oxidize the RuII center or intramolecular electronic coupling was large, the RuIII/II electron transfer was facilitated by TPA+/0 transport. These findings indicate that synergistic interactions between redox-active groups can be tailored to control electron transfer at the molecular level across metal oxide surfaces.

Visible Light Driven Bromide Oxidation and Ligand Substitution Photochemistry of a Ru Diimine Complex.

The complex [Ru(deeb)(bpz)2]2+ (RuBPZ2+, deeb = 4,4'-diethylester-2,2'-bipyridine, bpz = 2,2'-bipyrazine) forms a single ion pair with bromide, [RuBPZ2+, Br-]+, with Keq = 8400 ± 200 M-1 in acetone. The RuBPZ2+ displayed photoluminescence (PL) at room temperature with a lifetime of 1.75 μs. The addition of bromide to a RuBPZ2+ acetone solution led to significant PL quenching and Stern-Volmer plots showed upward curvature. Time-resolved PL measurements identified two excited state quenching pathways, static and dynamic, which were operative toward [RuBPZ2+, Br-]+ and free RuBPZ2+, respectively. The single ion-pair [RuBPZ2+, Br-]+* had a lifetime of 45 ± 5 ns, consistent with an electron transfer rate constant, ket = (2.2 ± 0.3) × 107 s-1. In contrast, RuBPZ2+* was dynamically quenched by bromide with a quenching rate constant, kq = (8.1 ± 0.1) × 1010 M-1 s-1. Nanosecond transient absorption revealed that both the static and dynamic pathways yielded RuBPZ+ and Br2•- products that underwent recombination to regenerate the ground state with a second-order rate constant, kcr = (2.3 ± 0.5) × 1010 M-1 s-1. Kinetic analysis revealed that RuBPZ+ was a primary photoproduct, while Br2•- was secondary product formed by the reaction of a Br• with Br-, k = (1.1 ± 0.2) × 1010 M-1 s-1. Marcus theory afforded an estimate of the formal reduction potential for E0(Br•/-) in acetone, 1.42 V vs NHE. A 1H NMR analysis indicated that the ion-paired bromide was preferentially situated close to the RuII center. Prolonged steady state photolysis of RuBPZ2+ and bromide yielded two ligand-substituted photoproducts, cis- and trans-Ru(deeb)(bpz)Br2. A photochemical intermediate, proposed to be [Ru(deeb)(bpz)(κ1-bpz)(Br)]+, was found to absorb a second photon to yield cis- and trans-Ru(deeb)(bpz)Br2 photoproducts.

Crystal Structure of a Heterometallic Luminophore: The RuII Complex of a Ferrocenyl-Terpyridine with a Flexible Linkage

Crystal structure determination on the mixed ligand complex [Ru(tpy)(L1)](ClO4)2 (tpy = 2,2′:6′,2″-terpyridine; L1 = 4′-(ferrocenylmethylaminomethylphenyl)terpyridine) shows the ferrocenyl group to be located as remotely as possible from the RuII centre. This may explain the fact that emission from the RuII centre is detectable even at room temperature in solution.