Spectroscopy and Photophysics in Cyclometalated RuII-Bis(bipyridyl) Complexes

Metallopolymer strategy to explore hypoxic active narrow-bandgap photosensitizers for effective cancer photodynamic therapy

Practical photodynamic therapy calls for high-performance, less O2-dependent, long-wavelength-light-activated photosensitizers to suit the hypoxic tumor microenvironment. Iridium-based photosensitizers exhibit excellent photocatalytic performance, but the in vivo applications are hindered by conventional O2-dependent Type-II photochemistry and poor absorption. Here we show a general metallopolymerization strategy for engineering iridium complexes exhibiting Type-I photochemistry and enhancing absorption intensity in the blue to near-infrared region. Reactive oxygen species generation of metallopolymer Ir-P1, where the iridium atom is covalently coupled to the polymer backbone, is over 80 times higher than that of its mother polymer without iridium under 680 nm irradiation. This strategy also works effectively when the iridium atom is directly included (Ir-P2) in the polymer backbones, exhibiting wide generality. The metallopolymer nanoparticles exhibiting efficient O2•- generation are conjugated with integrin αvβ3 binding cRGD to achieve targeted photodynamic therapy.

Cyclometalated ruthenium (II) complexes induced HeLa cell apoptosis through intracellular reductive injury

The main challenge of cancer chemotherapy is the resistance of tumor cells to oxidative damage. Herein, we proposed a novel antitumor strategy: cyclic metal‑ruthenium (Ru) complexes mediate reductive damage to kill tumor cells. We designed and synthesized Ru(II) complexes with β-carboline as ligands: [Ru (phen)2(NO2-Ph-βC)](PF6) (RuβC-7) and [Ru(phen)2(1-Ph-βC)](PF6) (RuβC-8). In vitro experimental results showed that RuβC-7 and RuβC-8 can inhibit cell proliferation, promote mitochondrial abnormalities, and induce DNA damage. Interestingly, RuβC-7 with SOD activity could reduce intracellular reactive oxygen species (ROS) levels, while RuβC-8 has the opposite effect. Accordingly, this study identified the reductive damage mechanism of tumor apoptosis, and may provide a new ideas for the design of novel metal complexes.

A cycloruthenated 2-phenylimidazole: chromogenic sensor for nitrite in acidic buffer and fluoride in CH3CN

A cycloruthenated 2-phenylimidazole as a colorimetric sensor can detect fluoride in CH3CN by a great red-shift in absorption. However, it demonstrated sensitivity and selectivity to nitrite rather than fluoride in acidic buffer.

High Intrinsic Phosphorescence Efficiency and Density Functional Theory Modeling of Ru(II)-Bipyridine Complexes with π-Aromatic-Rich Cyclometalated Ligands: Attributions of Spin-Orbit Coupling Perturbation and Efficient Configurational Mixing of Singlet Excited States

A series of π-aromatic-rich cyclometalated ruthenium(II)-(2,2'-bipyridine) complexes ([Ru(bpy)₂(π_Ar-CM)]⁺) in which π_Ar-CM is diphenylpyrazine or 1-phenylisoquinoline were prepared. The [Ru(bpy)₂(π_Ar-CM)]⁺ complexes had remarkably high phosphorescence rate constants, k _RAD(p), and the intrinsic phosphorescence efficiencies (ι_em(p) = k _RAD(p)/(ν_em(p))³) of these complexes were found to be twice the magnitudes of simply constructed cyclometalated ruthenium(II) complexes ([Ru(bpy)₂(sc-CM)]⁺), where ν_em(p) is the phosphorescence frequency and sc-CM is 2-phenylpyridine, benzo[h]quinoline, or 2-phenylpyrimidine. Density functional theory (DFT) modeling of the [Ru(bpy)₂(CM)]⁺ complexes indicated numerous singlet metal-to-ligand charge transfers for ¹MLCT-(Ru-bpy) and ¹MLCT-(Ru-CM), excited states in the low-energy absorption band and ¹ππ*-(aromatic ligand) (¹ππ*-L_Ar) excited states in the high-energy band. DFT modeling of these complexes also indicated phosphorescence-emitting state (T_e) configurations with primary MLCT-(Ru-bpy) characteristics. The variation in ι_em(p) for the spin-forbidden T_e (³MLCT-(Ru-bpy)) excited state of the complex system that was examined in this study can be understood through the spin-orbit coupling (SOC)-mediated sum of intensity stealing (∑SOCM-IS) contribution from the primary intensity of the low-energy ¹MLCT states and second-order intensity perturbation from the significant configuration between the low-energy ¹MLCT and high-energy intense ¹ππ*-L_Ar states. In addition, the observation of unusually high ι_em(p) magnitudes for these [Ru(bpy)₂(π_Ar-CM)]⁺ complexes can be attributed to the values for both intensity factors in the ∑SOCM-IS formalism being individually greater than those for [Ru(bpy)₂(sc-CM)]⁺ ions.

Dissociation of Bipyridine and Coordination with Nitrosyl: Cyclometalated Ruthenium Nitrosyl Complex

A novel family of ruthenium nitrosyl complexes [Ru(bpy)(C^∧N)(MeCN)NO](PF₆)₂ (2a-2e, bpy = 2,2'-bipyridine, HC^∧N = 2-phenylpyridine and its derivatives) has been prepared by reacting cyclometalated ruthenium complexes [Ru(bpy)₂(C^∧N)][PF₆] (1a-1e) with NO⁺, which were comprehensively characterized by mass, IR, NMR, and UV-vis spectra as well as the single-crystal X-ray structure determinations. Herein, the coordination geometry of Ru atoms in 2a-2e is a distorted octahedron and {Ru^II-NO⁺}⁶ is present in these complexes. Theoretical calculations suggest that the reactions involving dissociation of one bipyridine and coordination with NO⁺ proceed spontaneously (ΔG < 0) and the transformation from 1a-1e to the intermediates is dominated by substituents (ΔG_RI varies from -1.19 to -1.53 eV), which influence the binding energy between Ru(II) and NO⁺ in complexes 2a-2e (-89.42 to -101.17 kcal/mol) and thus control the photorelease of NO on a certain scale. The weak absorption bands in the visible region could be attributed to the contribution of dπ(Ru^II) → π*(NO⁺), which were enhanced greatly under light, indicating the possible release of NO. The photoinduced NO, as well as singlet oxygen (¹O₂), was then confirmed by EPR spectra, and the amount of NO released from 2a-2e was estimated via Griess reagent assay. The cytotoxicity of these complexes with or without visible light irradiation was also investigated using an MTT assay.

Metalloimmunotherapy with Rhodium and Ruthenium Complexes: Targeting Tumor-Associated Macrophages

Tumor associated macrophages (TAMs) suppress the cancer immune response and are a key target for immunotherapy. The effects of ruthenium and rhodium complexes on TAMs have not been well characterized. To address this gap in the field, a panel of 22 dirhodium and ruthenium complexes were screened against three subtypes of macrophages, triple-negative breast cancer and normal breast tissue cells. Experiments were carried out in 2D and biomimetic 3D co-culture experiments with and without irradiation with blue light. Leads were identified with cell-type-specific toxicity toward macrophage subtypes, cancer cells, or both. Experiments with 3D spheroids revealed complexes that sensitized the tumor models to the chemotherapeutic doxorubicin. Cell surface exposure of calreticulin, a known facilitator of immunogenic cell death (ICD), was increased upon treatment, along with a concomitant reduction in the M2-subtype classifier arginase. Our findings lay a strong foundation for the future development of ruthenium- and rhodium-based chemotherapies targeting TAMs.

Cyclometalated Ru(II) β-carboline complexes induce cell cycle arrest and apoptosis in human HeLa cervical cancer cells via suppressing ERK and Akt signaling

Two new cyclometalated Ru(II)-β-carboline complexes, [Ru(dmb)₂(Cl-Ph-βC)](PF₆) (dmb = 4,4'-dimethyl-2,2'-bipyridine; Cl-Ph-βC = Cl-phenyl-9H-pyrido[3,4-b]indole; RuβC-3) and [Ru(bpy)₂(Cl-Ph-βC)](PF₆) (bpy = 2,2'-bipyridine; RuβC-4) were synthesized and characterized. The Ru(II) complexes display high cytotoxicity against HeLa cells, the stabilized human cervical cancer cell, with IC₅₀ values of 3.2 ± 0.4 μM (RuβC-3) and 4.1 ± 0.6 μM (RuβC-4), which were considerably lower than that of non-cyclometalated Ru(II)-β-carboline complex [Ru(bpy)₂(1-Py-βC)] (PF₆)₂ (61.2 ± 3.9 μM) by 19- and 15-folds, respectively. The mechanism studies indicated that both Ru(II) complexes could significantly inhibit HeLa cell migration and invasion, and effectively induce G0/G1 cell cycle arrest. The new Ru(II) complexes could also trigger apoptosis through activating caspase-3 and poly (ADP-ribose) polymerase (PARP), increasing the Bax/Bcl-2 ratio, enhancing reactive oxygen species (ROS) generation, decreasing mitochondrial membrane potential (MMP), and inducing cytochrome c release from mitochondria. Further research revealed that RuβC-3 could deactivate the ERK/Akt signaling pathway thus inhibiting HeLa cell invasion and migration, and inducing apoptosis. In addition, RuβC-3-induced apoptosis in HeLa cells was closely associated with the increase of intracellular ROS levels, which may act as upstream factors to regulate ERK and Akt pathways. More importantly, RuβC-3 exhibited low toxicity on both normal BEAS-2B cells in vitro and zebrafish embryos in vivo. Consequently, the developed Ru(II) complexes have great potential on developing novel low-toxic anticancer drugs.

Rational design of type I photosensitizers based on Ru(ii) complexes for effective photodynamic therapy under hypoxia

Photodynamic therapy (PDT) has been widely used in conjunction with molecular oxygen to cause cancer cell death. Hypoxia, the inherent property in solid tumors, is the obstacle during the process of PDT. It is urgent to develop PDT photosensitizers independent of the oxygen concentration. Herein, triphenylamine-modified Ru(ii) complexes have been used as photosensitizers to produce superoxide anions (O2-˙) and hydroxyl radicals (˙OH) through a type I photochemical process. Ru(ii) complexes with triphenylamine can provide a possibility to drive the reactive oxygen species production through low oxidation potential and good light-harvesting abilities. The investigation on light-mediated radical production showed that Ru4 could produce abundant ˙OH and O2-˙ compared to Ru1-Ru3 under hypoxic environments owing to the strong absorption. These radicals exhibit potent toxicity, which can damage the neighbouring biomolecules and cause the apoptosis of cancer cells. The PDT effect was evaluated in vitro under hypoxia, suggesting that Ru4 could maintain excellent performance in inducing a sharp decrease in the activity of cancer cells.

First cycloruthenation of 2-alkenylpyridines: synthesis, characterization and properties

Several cyclometalated ruthenium complexes 1-5 with 2-alkenylpyridines as C,N-chelating ligands were synthesized and then characterized by NMR, MS, IR and UV-Vis spectra. According to the single crystal of complex 2, it is evident that carbon from vinyl group is successfully bonded to Ru(ii) center. Moreover, the Ru-N bond trans to the Ru-C bond is elongated (2.127(5) Å), which is consistent with the strong trans effect of the carbon atom compared to that of the nitrogen atom. With different electron-donating groups linked to vinyl, these complexes exhibited regular changes in MLCT absorption bands, which were identified by UV-Vis and CV spectra in combination with DFT and TD-DFT. Interestingly, protonated intermediate species of these complexes in acidic solutions were tracked by the absorption changes and MS spectra, which displayed a possible protonation process of these complexes with the cleavage of Ru-C σ bonds.

Novel cyclometalated Ru(II) complexes containing isoquinoline ligands: Synthesis, characterization, cellular uptake and in vitro cytotoxicity

Two novel cyclometalated Ru(II) complexes containing isoquinoline ligand, [Ru(bpy)₂(1-Ph-IQ)](PF₆), (bpy = 2,2'-bipyridine; 1-Ph-IQ = 1-phenylisoquinoline; RuIQ-1) and [Ru(phen)₂(1-Ph-IQ)](PF₆) (phen = 1,10-phenanthroline; RuIQ-2) were found to show high cytotoxic activity against NCI-H460, A549, HeLa and MCF-7 cell lines. Notably, both of them exhibited IC₅₀ values that were an order of magnitude lower than those of clinical cisplatin and two structurally similar Ru(II)-isoquinoline complexes [Ru(bpy)₂(1-Py-IQ)](PF₆)₂ (Ru3) and [Ru(phen)₂(1-Py-IQ)](PF₆)₂ (Ru4) (1-Py-IQ = 1-pyridine-2-yl). The cellular uptake and intracellular localization displayed that the two cyclometalated Ru(II) complexes entered NCI-H460 cancer cells dominantly via endocytosis pathway, and preferentially distributed in the nucleus. Further investigations on the apoptosis-inducing mechanisms of RuIQ-1 and RuIQ-2 revealed that the two complexes could cause S, G2/M double-cycle arrest by regulating cell cycle related proteins. The two complexes also could reduce the mitochondrial membrane potential (MMP), promote the generation of intracellular ROS and trigger DNA damage, and then lead to apoptosis-mediated cell death. More importantly, RuIQ-2 exhibits low toxicity both towards normal HBE cells in vitro and zebrafish embryos in vivo. Accordingly, the developed complexes hold great potential to be developed as novel therapeutics for effective and low-toxic cancer treatment.

A highly sensitive and selective colorimetric probe based on a cycloruthenated complex: an Hg2+-promoted switch of thiophene coordination

A cyclometalated ruthenium complex [Ru(pthb)(bpy)2]+ (1, bpy = 2,2'-bipyridine, Hpthb = 3,3-dimethyl-2-(5-pyridylthiophen-2-yl)vinyl-benzo[e]indolium-1-propylsulfonate) could be converted from a C-coordinated structure to non-metallated species with N,S-bonded Hpthb upon treatment with mercury(ii) ions in water. Strikingly, the switch in the coordination mode resulted in a great absorption change along with a change in the solution color of 1 from dark red to light yellow. Therefore, 1 can be used as a colorimetric probe to detect mercury(ii) ions by the naked eye. Although the emission was not observed for 1 in water, it still demonstrated an appreciably low detection limit of 21 nM by using UV-Vis absorption spectroscopy, which was comparable with those of some probes determined by ratiometric fluorescence spectroscopy.

Low-Temperature Spectra and Density Functional Theory Modeling of Ru(II)-Bipyridine Complexes with Cyclometalated Ancillary Ligands: The Excited State Spin-Orbit Coupling Origin of Variations in Emission Efficiencies

The 77 K emission spectra of cyclometalated ruthenium(II)-2,2'-bipyridine (CM-Ru-bpy) chromophores are very similar to those of related Ru-bpy complexes with am(m)ine or diimmine ancillary ligands, and density functional theory (DFT) modeling confirms that the lowest energy triplet metal to ligand charge transfer (³MLCT) excited states of CM-Ru-bpy and related Ru-bpy complexes have very similar electronic configurations. However, the phosphorescence decay efficiencies of CM-Ru-bpy excited states are about twice those of the conventional Ru-bpy analogues. In contrast to the similar ³MLCT excited state electronic configurations of the two classes of complexes, the CM-Ru-bpy chromophores have much broader visible region MLCT absorptions resulting from several overlapping transitions, even at 87 K. The emitting excited-state emission efficiencies depend on spin-orbit coupling (SOC) mediated intensity stealing from singlet excited states, and this work explores the relationship between the phosphorescence efficiency and visible region absorption spectra of Ru-bpy ³MLCT excited states in the weak SOC limit. The intrinsic ³MLCT emission efficiency, ι_em, depends on mixing with singlet excited states whose Ru^III-dπ-orbital angular momenta differ from that of the emitting state. DFT modeling of the ¹MLCT excited-state electronic configurations that contribute significantly to the lowest energy absorption bands have Ru^III-dπ orbitals that differ from those of their emitting ³MLCT excited states. This leads to a very close relationship between ι_em and the lowest energy MLCT band absorptivities in Ru-bpy chromophores. Thus, the larger number of ¹MLCT transitions that contribute to the lowest energy absorption bands accounts for the enhanced phosphorescence efficiency of Ru-bpy complexes with cyclometalated ancillary ligands.

Photophysical Properties and Photobiological Activities of Ruthenium(II) Complexes Bearing π-Expansive Cyclometalating Ligands with Thienyl Groups

A new family of cyclometalated ruthenium(II) complexes [Ru(N^N)₂(C^N)]⁺ derived from the π-extended benzo[h]imidazo[4,5-f]quinolone ligand appended with thienyl groups (n = 1-4, compounds 1-4) was prepared and its members were characterized for their chemical, photophysical, and photobiological properties. The lipophilicities of 1-4, determined as octanol-water partition coefficients (log P_o/w), were positive and increased with the number of thienyl units. The absorption and emission bands of the C^N compounds were red-shifted by up to 200 nm relative to the analogous Ru(II) diimine systems. All of the complexes exhibited dual emission with the intraligand fluorescence (¹IL, C^N-based) shifting to lower energies with increasing n and the metal-to-ligand charge transfer phosphorescence (³MLCT, N^N-based) remaining unchanged. Compounds 1-3 exhibited excited state absorption (ESA) profiles consistent with lowest-lying ³MLCT states when probed by nanosecond transient absorption (TA) spectroscopy with 532 nm excitation and had contributions from ¹IL(C^N) states with 355 nm excitation. These assignments were supported by the lifetimes observed (<10 ns for the ¹IL states and around 20 ns for the ³MLCT states) as well as a noticeable ESA for 3 with 355 nm excitation that did not occur with 532 nm excitation. Compound 4 was the only member of the family with two ³MLCT emissive lifetimes (15, 110 ns), and the TA spectra collected with both 355 and 532 nm excitation was assigned to the ³IL state, which was corroborated by its 4-6 μs lifetime. The ESA for 4 had a rise time of approximately 10 ns and an initial decay of 110 ns, which suggests a possible ³MLCT-³IL excited state equilibrium that results in delayed emission from the ³MLCT state. Compound 4 was nontoxic toward human skin melanoma cells (SKMEL28) in the dark (EC₅₀ = >300 μM); 1-3 were cytotoxic and yielded EC₅₀ values between 1 and 20 μM. The photocytotoxicites with visible light ranged from 87 nM with a phototherapeutic index (PI) of 13 for 1 to approximately 1 μM (PI = >267) for 4. With red light, EC₅₀ values varied from 270 nM (PI = 21) for 3 to 12 μM for 4 (PI = >25). The larger PIs for 4, especially with visible light, were attributed to the much lower dark cytotoxicity for this compound. Because the dark cytotoxicity contributes substantially to the observed photocytotoxicity for 1-3, it was not possible to assess whether the ³IL state of 4 led to a much more potent phototoxic mechanism in the absence of dark toxicity. There was no stark contrast in cellular uptake and accumulation by laser scanning confocal and differential interference contrast microscopy to explain the large differences in dark toxicities between 1-3 and 4. Nevertheless, the study highlights a new family of Ru(II) C^N complexes where π-conjugation beyond a certain point results in low dark cytotoxicity with high photocytotoxicity, opposing the notion that cyclometalated Ru(II) systems are too toxic to be phototherapeutic agents.

Cyclometalated Ruthenium(II) Complexes Derived from α-Oligothiophenes as Highly Selective Cytotoxic or Photocytotoxic Agents

The photophysical and photobiological properties of a new class of cyclometalated ruthenium(II) compounds incorporating π-extended benzo[ h]imidazo[4,5- f]quinoline (IBQ) cyclometalating ligands (C^N) bearing thienyl rings ( n = 1-4, compounds 1-4) were investigated. Their octanol-water partition coefficients (log P_o/w) were positive and increased with n. Their absorption and emission energies were red-shifted substantially compared to the analogous Ru(II) diimine (N^N) complexes. They displayed C^N-based intraligand (IL) fluorescence and triplet excited-state absorption that shifted to longer wavelengths with increasing n and N^N-based metal-to-ligand charge transfer (MLCT) phosphorescence that was independent of n. Their photoluminescence lifetimes (τ_em) ranged from 4-10 ns for ¹IL states and 12-18 ns for ³MLCT states. Transient absorption lifetimes (τ_TA) were 5-8 μs with 355 nm excitation, ascribed to ³IL states that became inaccessible for 1-3 with 532 nm excitation (1-3, τ_TA = 16-17 ns); the ³IL of 4 only was accessible by lower energy excitation, τ_TA = 3.8 μs. Complex 4 was nontoxic (EC₅₀ > 300 μM) to SK-MEL-28 melanoma cells and CCD1064-Sk normal skin fibroblasts in the dark, while 3 was selectively cytotoxic to melanoma (EC₅₀= 5.1 μM) only. Compounds 1 and 2 were selective for melanoma cells in the dark, with submicromolar potencies (EC₅₀ = 350-500 nM) and selectivity factors (SFs) around 50. The photocytotoxicities of compounds 1-4 toward melanoma cells were similar, but only compounds 3 and 4 displayed significant phototherapeutic indices (PIs; 3, 43; 4, >1100). The larger cytotoxicities for compounds 1 and 2 were attributed to increased cellular uptake and nuclear accumulation, and possibly related to the DNA-aggregating properties of all four compounds as demonstrated by cell-free gel mobility-shift assays. Together, these results demonstrate a new class of thiophene-containing Ru(II) cyclometalated compounds that contain both highly selective chemotherapeutic agents and extremely potent photocytotoxic agents.

Excited-State Decay Pathways of Tris(bidentate) Cyclometalated Ruthenium(II) Compounds

The synthesis, electrochemistry, and photophysical characterization are reported for 11 tris(bidentate) cyclometalated ruthenium(II) compounds, [Ru(N^N)₂(C^N)]⁺. The electrochemical and photophysical properties were varied by the addition of substituents on the 2,2'-bipyridine, N^N, and 2-phenylpyridine, C^N, ligands with different electron-donating and -withdrawing groups. The systematic tuning of these properties offered a tremendous opportunity to investigate the origin of the rapid excited-state decay for these cyclometalated compounds and to probe the accessibility of the dissociative, ligand-field (LF) states from the metal-to-ligand charge-transfer (MLCT) excited state. The photoluminescence quantum yield for [Ru(N^N)₂(C^N)]⁺ increased from 0.0001 to 0.002 as more electron-withdrawing substituents were added to C^N. An analogous substituent dependence was observed for the excited-state lifetimes, τ_obs, which ranged from 3 to 40 ns in neat acetonitrile, significantly shorter than those for their [Ru(N^N)₃]²⁺ analogues. The excited-state decay for [Ru(N^N)₂(C^N)]⁺ was accelerated because of an increased vibronic overlap between the ground- and excited-state wavefunctions rather than an increased electronic coupling as revealed by a comparison of the Franck-Condon factors. The radiative (k_r) and non-radiative (k_nr) rate constants of excited-state decay were determined to be on the order of 10⁴ and 10⁷-10⁸ s^-1, respectively. For sets of [Ru(N^N)₂(C^N)]⁺ compounds functionalized with the same N^N ligand, k_nr scaled with excited-state energy in accordance with the energy gap law. Furthermore, an Arrhenius analysis of τ_obs for all of the compounds between 273 and 343 K was consistent with activated crossing into a single, fourth ³MLCT state under the conditions studied with preexponential factors on the order of 10⁸-10⁹ s^-1 and activation energies between 300 and 1000 cm^-1. This result provides compelling evidence that LF states are not significantly populated near room temperature unlike many ruthenium(II) polypyridyl compounds. On the basis of the underlying photophysics presented here for [Ru(N^N)₂(C^N)]⁺, molecules of this type represent a robust class of compounds with built-in design features that should greatly enhance the molecular photostability necessary for photochemical and photoelectrochemical applications.

Achieving efficient photodynamic therapy under both normoxia and hypoxia using cyclometalated Ru(ii) photosensitizer through type I photochemical process

Photodynamic therapy (PDT) through the generation of singlet oxygen utilizing photosensitizers (PSs) is significantly limited under hypoxic conditions in solid tumors. So it is meaningful to develop effective PSs which can maintain excellent therapeutic effects under hypoxia. Here we reported a coumarin-modified cyclometalated Ru(ii) photosensitizer (Ru2), which exhibits lower oxidation potential and stronger absorption in the visible region than the coumarin-free counterpart. The evaluation of the PDT effect was performed under both normoxia and hypoxia. The results showed that Ru2 has a better therapeutic effect than the coumarin-free counterpart in in vitro experiments. Especially under hypoxia, Ru2 still retained an excellent PDT effect, which can be attributed to the direct charge transfer between the excited PS and an adjacent substrate through a type I photochemical process, forming highly-oxidative hydroxyl radicals to damage tumor cells. The anti-tumor activity of Ru2 was further proven to be effective in tumor-bearing mice, and tumor growth was inhibited remarkably under PDT treatment.

Synthesis, characterization, cellular uptake and apoptosis-inducing properties of two highly cytotoxic cyclometalated ruthenium(II) β-carboline complexes

Two new cyclometalated Ru(II) complexes of the general formula [Ru(N-N)₂(1-Ph-βC)](PF₆), where N-N = 4,4'-dimethyl-2,2'-bipyridine (dmb, Ru1), 2,2'-bipyridine (bpy, Ru2), and 1-Ph-βC (1-phenyl-9H-pyrido[3,4-b]indole) is a β-carboline alkaloids derivatives, have been synthesized and characterized. The in vitro cytotoxicities, cellular uptake and localization, cell cycle arrest and apoptosis-inducing mechanisms of these complexes have been extensively explored by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, inductively coupled plasma mass spectrometry (ICP-MS), flow cytometry, comet assay, inverted fluorescence microscope as well as western blotting experimental techniques. Notably, Ru1 and Ru2 exhibit potent antiproliferative activities against selected human cancer cell lines with IC₅₀ values lower than those of cisplatin and other non-cyclometalated Ru(II) β-carboline complexes. The cellular uptake and localization exhibit that these complexes can accumulate in the cell nuclei. Further antitumor mechanism studies show that Ru1 and Ru2 can cause cell cycle arrest in the G0/G1 phase by regulating cell cycle relative proteins and induce apoptosis through mitochondrial dysfunction, reactive oxygen species (ROS) accumulation and ROS-mediated DNA damage.

Asymmetric Cyclometalated RuII Polypyridyl-Type Complexes with π-Extended Carbanionic Donor Sets

A series of novel cyclometalated Ru^II complexes were investigated featuring the tridentate dqp ligand platform (dqp is 2,6-di(quinolin-8-yl)pyridine), in order to utilize the octahedral coordination mode around the Ru center to modulate the electrochemical and photophysical properties. The heteroleptic complexes feature C₁ symmetry due to symmetry breaking by the peripheral five- or six-membered carbanionic chelate (phenyl, naphthyl, or anthracenyl units). The chelation mode is controlled by the steric effects and C-H activation selectivity of the ligand, which prompted the development of a general synthesis protocol. The optimized conditions to achieve high overall yields (55-75%) involve NaHCO₃ as the base and an simplified purification protocol: i.e., facile chromatographic separation using commercially available amino-functionalized silica applying nonaqueous salt-free conditions to omit the necessity of counterion exchange. The structural, photophysical, and electrochemical properties were studied in depth, and the results were corroborated by density functional theory (DFT) calculations. Steady state and time-resolved spectroscopy revealed red-shifted absorption (up to 750 nm) and weak IR emission (800-1000 nm) combined with prolonged emission lifetimes (up to 20 ns) in comparison to classical tpy-based (tpy is 2,2':6',2″-terpyridine) complexes. An enhanced stability was observed by blocking the reactive positions of the carbanionic ligand framework, while the reactive positions may be exploited for further functionalization.

Cycloruthenated complexes: pH-dependent reversible cyclometallation and reactions with nitrite at octahedral ruthenium centers

The pH-dependent reversible cyclometallation and reactions with nitrite in a near-aqueous system of a related set of cyclometallated ruthenium(ii) bipyridyl complexes were investigated in detail. These complexes are [Ru(ppy)(bpy)2]PF6 (, bpy = 2,2'-bipyridine, Hppy = 2-phenylpyridine), [Ru(thpy)(bpy)2]PF6 (, Hthpy = 2-(2-thienyl)pyridine), and [Ru(dfppy)(bpy)2]PF6 (, Hdfppy = 2-(2,4-difluorophenyl)pyridine). As expected, reversible UV-Vis spectra of these three complexes in near-aqueous solutions can be achieved by treating with acid or base, which indicates a pH-dependent reversible cyclometallation. However, the addition of nitrite to acidic solutions of these complexes disturbed the reversible pH-dependent processes. Unexpected ruthenium complexes in the aforementioned system were then isolated and characterized using FT-IR, MS, (1)H NMR, and UV-Vis spectra, indicating that two reaction modes occurred at the ruthenium(ii) centers: (1) the insertion of NO into the ruthenium-aryl bond to form a ruthenium C-nitroso complex; and (2) the coordination of NO with the entire dissociation of one bipyridine to form a {Ru-NO}(6) complex, which is the first example involving the cleavage of Ru-N(∧)N bonds in ruthenium bipyridyl complexes.

Excited state decay of cyclometalated polypyridine ruthenium complexes: insight from theory and experiment

This perspective article tackles the open question why cyclometalated polypyridine ruthenium(ii) complexes typically only emit very weakly at room temperature and delivers answers beyond the standard schemes involving ³MC and tunneling decay channels.

Sticking and patching: tuning and anchoring cyclometallated ruthenium(ii) complexes

In a series of [Ru(bpy)₂(C^N)][PF₆] complexes with a variety of functionalities in the cyclometallating ligand, the absorption response is enhanced considerably by the introduction of a 4-C₆H₄P(O)(OEt)₂ anchoring domain in the C^N pyridine ring.