Probing the Noninnocent π-Bonding Influence of N-Carboxyamidoquinolate Ligands on the Light Harvesting and Redox Properties of Ruthenium Polypyridyl Complexes

Non-innocent ligand flavone and curcumin inspired ruthenium photosensitizers for solar energy conversion

A series of ruthenium photosensitizers incorporating a β-diketonate non-innocent ligand were synthesized, characterized, and implemented in dye-sensitized solar cells. Electrochemical studies exhibited well behaved reversible oxidations and reductions for all β-diketonate complexes. The acac- and Ph2acac- based photosensitizers possess limited delocalization across the ligand π*-manifold, which is significant for exhibition of respectable power conversion efficiencies in a dye-sensitized solar cell (DSC) device. As the π-orbital network was extended on the flavone and curcumin inspired NILs, increased molar absorptivity was observed, however this ultimately proved detrimental to DSC performance consistent with exhibition of negligible photocurrent.

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 Ru^II 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.

Luminescent cis-Bis(bipyridyl)ruthenium(II) Complexes with 1,2-Azolylamidino Ligands: Photophysical, Electrochemical Studies, and Photocatalytic Oxidation of Thioethers

New 1,2-azolylamidino complexes cis-[Ru(bipy)₂(NH═C(R)az*-κ²N,N)](OTf)₂ (R = Me, Ph; az* = pz, indz, dmpz) are synthesized via chloride abstraction after a subsequent base-catalyzed coupling of a nitrile with the previously coordinated 1,2-azole. The synthetic procedure allows the easy obtainment of complexes having different electronic and steric 1,2-azoylamidino ligands. All of the compounds have been characterized by ¹H, ¹³C, and ¹⁵N NMR and IR spectroscopy and by monocrystal X-ray diffraction. Photophysical studies support their phosphorescence, whereas their electrochemistry reveals reversible Ru^II/Ru^III oxidations between +1.13 and +1.25 V (vs SCE). The complexes have been successfully used as catalysts in the photooxidation of different thioethers, the complex cis-[Ru(bipy)₂(NH═C(Me)dmpz-κ²N,N)]²⁺ showing better catalytic performance in comparison to that of [Ru(bipy)₃]²⁺. Moreover, the significant catalytic performance of the dimethylpyrazolylamidino complex is applied to the preparation of the drug modafinil, which is obtained using ambient oxygen as an oxidant. Finally, mechanistic assays suggest that the oxidation reaction follows a photoredox route via oxygen radical anion formation.

(1,2-Azole)bis(bipyridyl)ruthenium(II) Complexes: Electrochemistry, Luminescent Properties, And Electro- And Photocatalysts for CO2 Reduction

New cis-(1,2-azole)-aquo bis(2,2'-bipyridyl)ruthenium(II) (1,2-azole (az*H) = pzH (pyrazole), dmpzH (3,5-dimethylpyrazole), and indzH (indazole)) complexes are synthesized via chlorido abstraction from cis-[Ru(bipy)₂Cl(az*H)]OTf. The latter are obtained from cis-[Ru(bipy)₂Cl₂] after the subsequent coordination of the 1,2-azole. All the compounds are characterized by ¹H, ¹³C, ¹⁵N NMR spectroscopy as well as IR spectroscopy. Two chlorido complexes (pzH and indzH) and two aquo complexes (indzH and dmpzH) are also characterized by X-ray diffraction. Photophysical and electrochemical studies were carried out on all the complexes. The photophysical data support the phosphorescence of the complexes. The electrochemical behavior of all the complexes in an Ar atmosphere indicate that the oxidation processes assigned to Ru(II) → Ru(III) occurs at higher potentials in the aquo complexes. The reduction processes under Ar lead to several waves, indicating that the complexes undergo successive electron-transfer reductions that are centered in the bipy ligands. The first electron reduction is reversible. The electrochemical behavior in CO₂ media is consistent with CO₂ electrocatalyzed reduction, where the values of the catalytic activity [i_cat(CO₂)/i_p(Ar)] ranged from 2.9 to 10.8. Controlled potential electrolysis of the chlorido and aquo complexes affords CO and formic acid, with the latter as the major product after 2 h. Photocatalytic experiments in MeCN with [Ru(bipy)₃]Cl₂ as the photosensitizer and TEOA as the electron donor, which were irradiated with >300 nm light for 24 h, led to CO and HCOOH as the main reduction products, achieving a combined turnover number (TON_CO+HCOO^-) as high as 107 for 2c after 24 h of irradiation.

Oxygen Reduction Assisted by the Concert of Redox Activity and Proton Relay in a Cu(II) Complex

A copper complex, [Cu(dpaq)](ClO₄) (1), of a monoanionic pentadentate amidate ligand (dpaq) has been isolated and characterized to study its efficacy toward electrocatalytic reduction of oxygen in neutral aqueous medium. The Cu(II) mononuclear complex, poised in a distorted trigonal bipyramidal structure, reduces oxygen at an onset potential of 0.50 V vs RHE. Kinetics study by hydrodynamic voltammetry and chronoamperometry suggests a stepwise mechanism for sequential reduction of O₂ to H₂O₂ to H₂O at a single-site Cu-catalyst. The foot-of-the-wave analysis records a turnover frequency of 5.65 × 10² s^-1. At pH 7.0, complex 1 undergoes a quasi-reversible mixed metal-ligand-based reduction and triggers the reduction of dioxygen to water. Electrochemical studies in tandem with quantum chemical investigation, conducted at different redox states, portray the active participation of ligand in completing the process of proton-coupled electron transfer internally. The protonated carboxamido moiety acts as a proton relay, while the quinoline-based orbital supplies the necessary redox equivalent for the conversion of complex 1 to Cu(II)-hydroperoxo species. Thus, a suitable combination of redox non-innocence and proton shuttling functionality in the ligand makes it an effective electron-proton-transfer mediator and subsequently assists the process of oxygen reduction.

Iodine-Catalyzed Synthesis of N,N'-Chelate Organoboron Aminoquinolate

We disclose a novel method for the synthesis of fluorescent N,N'-chelate organoboron compounds in high efficiency by treatment of aminoquinolates with NaBAr₄/R'COOH in the presence of an iodine catalyst. These compounds display high air and thermal stability. A possible catalytic mechanism based on the results of control experiments has been proposed. Fluorescence quantum yield of 3b is up to 0.79 in dichloromethane.

Non-emissive RuII Polypyridyl Complexes as Efficient and Selective Photosensitizers for the Photooxidation of Benzylamines

Five new Ru^II polypyridyl complexes bearing N-(arylsulfonyl)-8-amidoquinolate ligands and three of their biscyclometalated Ir^III congeners have been prepared and employed as photocatalysts (PCs) in the photooxidation of benzylamines with O₂ . In particular, the new Ru^II complexes do not exhibit photoluminescence, rather they harvest visible light efficiently and are very stable in solution under irradiation with blue light. Their non-emissive behavior has been related to the low electrochemical energy gaps and rationalized on the basis of theoretical calculations (DFT analysis) that predict low S₀ ←T₁ energy values. Moreover, the Ru^II complexes, despite being non-emissive, display excellent activities in the selective photocatalytic transformation of benzylamines into the corresponding imines. The presence of an electron-withdrawing group (-CF3) on the arene ring of the N-(arylsulfonyl)-8-amidoquinolate ligand improves the photocatalytic activity of the corresponding photocatalyst. Furthermore, all the experimental evidence, including transient absorption spectroscopy measurements suggest that singlet oxygen is the actual oxidant. The Ir^III analogues are considerably more photosensitive and consequently less efficient photosensitizers (PSs).

Light activation of cyclometalated ruthenium complexes drives towards caspase 3 dependent apoptosis in gastric cancer cells

Polypyridyl ruthenium complexes have been intensively investigated for their remarkable antiproliferative properties and some are currently being tested in clinical trials. Here, we investigated the impact of illumination on the biological properties of a series of new cyclometalated ruthenium compounds with increased π-conjugation. We determined that various of these complexes display a bivalent biological activity as they are highly cytotoxic by themselves in absence of light while their cytotoxicity can significantly be elevated towards an IC₅₀ in the nanomolar range upon illumination. In particular, we showed that these complexes are particularly active (IC₅₀ < 1 μM) on two gastric cancer cell lines (AGS, KATO III) that are resistant towards cisplatin (IC₅₀ > 25 μM). As expected, light activation leads to increased production of singlet oxygen species in vitro and accumulation of reactive oxygen species in vivo. Importantly, we established that light exposure shifts the mode of action of the complexes towards activation of a caspase 3-dependent apoptosis that correlates with increased DNA damage. Altogether, this study characterizes novel ruthenium complexes with dual activity that can be tuned towards different mode of action in order to bypass cancer cell resistance mechanisms.

Synthesis of α-amino nitriles through one-pot selective Ru-photocatalyzed oxidative cyanation of amines

A new family of Ru(ii) photocatalysts exhibits excellent performance in the efficient and eco-friendly one-pot preparation of α-amino nitriles from primary and secondary amines.

Critical Overview of the Use of Ru(II) Polypyridyl Complexes as Photosensitizers in One-Photon and Two-Photon Photodynamic Therapy

Photodynamic Therapy (PDT) is an emerging technique to treat certain types of cancer, bacterial, fungal, and viral infections, and skin diseases. In past years, different research groups developed new ruthenium-containing photosensitizers (PSs) with tuned photophysical and biological properties to better fit the requirements of PDT. In this Account, we report and discuss the latest results in this research area, emphasizing particularly our own research. For example, inspired by the DNA intercalating complex [Ru(bpy)₂(dppz)]²⁺ (bpy = 2,2'-bipyridine; dppz = (dipyrido[3,2-a:2',3'-c]phenazine), a series of ruthenium complexes bearing differently functionalized dppz ligands were synthesized to target DNA. The introduction of the substituents on the dppz ligand did not reduce much the affinity of the complexes to DNA but highly affected their cellular uptake. The most effective complex in this series, [Ru(bpy)₂(dppz-7-OMe)]²⁺, showed IC₅₀ values in the low micromolar range against several types of cancer cells upon light irradiation and, importantly, a high phototoxic index (PI) of >150. This value is comparable to or even better than several PSs used in clinics under comparable experimental conditions. This compound was found to localize in the nucleus and to induce DNA damage in HeLa cells upon light irradiation. Interestingly, cells in the mitotic phase were found to be more affected and to have a different mechanism of cell death (apoptosis) upon light irradiation than those in the interphase (paraptosis). To take advantage of that, the PS was combined with a cell cycle inhibitor to synchronize cells in the mitotic phase, further improving the phototoxicity by a factor of 3.6. In addition, our group recently demonstrated that [Ru(bphen)₂(benzene-1,2-dislufinate)] (bphen = 4,7-diphenyl-1,10-phenanthroline) localizes in mitochondria and has an IC₅₀ value of 0.62 μM with a PI of over 80 in HeLa cells upon light irradiation at 420 nm. Interestingly, this complex was also found to efficiently kill Gram-positive Staphylococcus aureus under light irradiation. Antimicrobial PDT (aPDT) is another field of research where Ru(II) polypyridyl complexes can play an interesting role to fight antibiotics resistance. [Ru(dqpCO₂Me)(ptpy)]²⁺ (dqpCO₂Me = 4-methylcarboxy-2,6-di(quinolin-8-yl)pyridine), ptpy = 4'-phenyl-2,2':6',2″-terpyridine) is additionally efficient against Gram-negative Escherichia coli. The efficacy of positively charged Ru(II) PSs is related to their affinity to the negatively charged membrane of Gram-negative bacteria. A drawback of many Ru(II) polypyridyl PSs is their low absorption in the biological optical window (600-900 nm) where light penetration depth into tissue is the highest. The lowest energy transition in the UV/Vis spectra of Ru(II) polypyridyl complexes is usually a metal-to-ligand charge-transfer band. To shift the absorption into this range, tuning of the ligand system, for example, by extending π-systems, has been described in the literature. Another approach to make excitation in the optical biological window possible is Two-Photon Absorption (2PA). High photon density is needed and usually confocal laser beams are used for excitation. In collaboration with the Chao group, a series of homoleptic Ru(II) complexes bearing tertiary alkyl ammonium substituted bipyridine ligands with two photon cross sections between 185 and 250 GM at around 800 nm was tested in vitro. They showed IC₅₀ values in the micromolar range and PIs between 103 and 313. The highly positive-charged complexes were found to enter the cell via endocytosis and to target lysosomes. Studies on 3D tumor cell spheroids, a model closer to real tumors than commonly used 2D cell monolayers, were also performed. It could be demonstrated that 2P-PDT treatment with 800 nm laser irradiation was significantly more effective than that with 450 nm laser irradiation.