Transfer Hydrogenation and Antiproliferative Activity of Tethered Half-Sandwich Organoruthenium Catalysts

In-cell Catalysis by Tethered Organo-Osmium Complexes Generates Selectivity for Breast Cancer Cells

Anticancer agents that exhibit catalytic mechanisms of action offer a unique multi-targeting strategy to overcome drug resistance. Nonetheless, many in-cell catalysts in development are hindered by deactivation by endogenous nucleophiles. We have synthesised a highly potent, stable Os-based 16-electron half-sandwich ('piano stool') catalyst by introducing a permanent covalent tether between the arene and chelated diamine ligand. This catalyst exhibits antiproliferative activity comparable to the clinical drug cisplatin towards triple-negative breast cancer cells and can overcome tamoxifen resistance. Speciation experiments revealed Os to be almost exclusively albumin-bound in the extracellular medium, while cellular accumulation studies identified an energy-dependent, protein-mediated Os accumulation pathway, consistent with albumin-mediated uptake. Importantly, the tethered Os complex was active for in-cell transfer hydrogenation catalysis, initiated by co-administration of a non-toxic dose of sodium formate as a source of hydride, indicating that the Os catalyst is delivered to the cytosol of cancer cells intact. The mechanism of action involves the generation of reactive oxygen species (ROS), thus exploiting the inherent redox vulnerability of cancer cells, accompanied by selectivity for cancerous cells over non-tumorigenic cells.

Naphthalene Diimide-Functionalized Half-Sandwich Ru(II) Complexes as Mitochondria-Targeted Anticancer and Antimetastatic Agents

In this work, four naphthalene diimide (NDI)-functionalized half-sandwich Ru(II) complexes Ru1-Ru4 bearing the general formula [(η⁶-arene)Ru^II(N^N)Cl]PF₆, where arene = benzene (bn), p-cymene (p-cym), 1,3,5-trimethylbenzene (tmb), and hexamethylbenzene (hmb), have been synthesized and characterized. By introducing the NDI unit into the N,N-chelating ligand of these half-sandwich complexes, the poor luminescent half-sandwich complexes are endowed with excellent emission performance. Besides, modification on the arene ligand of arene-Ru(II) complexes can influence the electron density of the metal center, resulting in great changes in the kinetic properties, catalytic activities in the oxidative conversion of NADH to NAD⁺, and biological activities of these compounds. Particularly, Ru4 exhibits the highest reactivity and the strongest inhibitory activity against the growth of three tested cancer cell lines. Further study revealed that Ru4 can enter cells quickly in an energy-dependent manner and preferentially accumulate in the mitochondria of MDA-MB-231 cells, inducing cell apoptosis via reactive oxygen species overproduction and mitochondrial dysfunction. Significantly, Ru4 can effectively inhibit the cell migration and invasion. Overall, the complexation with NDI and modification on the arene ligand endowed the half-sandwich Ru(II) complexes with improved spectroscopic properties and anticancer activities, highlighting their potential applications for cancer treatment.

New Triazolyl N^N Bidentate Rh(III), Ir(III), Ru(II) and Os(II) Complexes: Synthesis and Characterization, Probing Possible Relations between Cytotoxicity with Transfer Hydrogenation Efficacy and Interaction with Model Biomolecules

Cisplatin and other metallodrugs have realised great success in clinical chemotherapeutic applications as anticancer drugs. However, severe toxicity to healthy cells and non-selectivity to cancer cells remains a challenge, warranting the further search for alternative agents. Herein, we report the anticancer potential of a series of complexes of the general formula [MCl(p-cym)(k2-N^N-L)]+ X− and [MCl(Cp*)(k2-N^N-L)]+ X−, where M is the metal centre (Ru(II), Os(II), Rh(III) or Ir(III)), L = 1-benzyl-4-pyridinyl-1-H-1,2,3-triazole for L1 and 1-picolyl-4-pyridinyl-1-H-1,2,3-triazole for L2 and X− = Cl−, BF4−, BPh4−. When evaluated for activity against some cancerous and non-cancerous cell lines (namely, HeLa, HEK293, A549 and MT4 cancer cells and the normal healthy kidney cells (BHK21)), most of the compounds displayed poor cytotoxicities against cancer cells except for complexes C2 ([RuCl(p-cym)(k2-N^N-L1)]+ BPh4−, EC50 = 9−16 µM and SI = 14), C7 ([RuCl(p-cym)(k2-N^N-L2)]+ BPh4−, EC50 = 17−53 µM and SI = 4) and C11 ([IrCl(Cp*)(k2-N^N-L2)]+ BF4−, EC50 < 5 µM and SI > 10). Selected complexes C1 ([RuCl(p-cym)(k2-N^N-L1)]+ BF4−), C5 ([IrCl(Cp*)(k2-N^N-L1)]+ BF4−) and C11 showed significant interactions with model biomolecules such as guanosine-5′-monophosphate (5′-GMP), bovine serum albumin (BSA) and amino acids under physiological conditions, possibly through carbenylation and N-coordination with 5′-GMP, N-coordination with L-Histidine and L-proline. While the compounds showed good activities in reducing pyruvate to lactate, there was no direct correlation between catalytic transfer hydrogenation of pyruvate and the observed cytotoxic activities. As observed in this work, the marked influence of single atom replacement in ligand may provide a pivotal approach to improving the cytotoxicity and fine-tuning the selectivity to cancer cells.

Effect of cysteine thiols on the catalytic and anticancer activity of Ru(II) sulfonyl-ethylenediamine complexes

We have synthesized a series of novel substituted sulfonyl ethylenediamine (en) Ru^II arene complexes 1-8 of [(η⁶-arene)Ru(R¹-SO₂-EnBz)X], where the arene is benzene, HO(CH₂)₂O-phenyl or biphenyl (biph), X = Cl or I, and R¹ is phenyl, 4-Me-phenyl, 4-NO₂-phenyl or dansyl. The 'piano-stool' structure of complex 3, [(η⁶-biph)Ru(4-Me-phenyl-SO₂-EnBz)I], was confirmed by X-ray crystallography. The values of their aqua adducts were determined to be high (9.1 to 9.7). Complexes 1-8 have antiproliferative activity against human A2780 ovarian, and A549 lung cancer cells with IC₅₀ values ranging from 4.1 to >50 μM, although, remarkably, complex 7 [(η⁶-biph)Ru(phenyl-SO₂-EnBz)Cl] was inactive towards A2780 cells, but as potent as the clinical drug cisplatin towards A549 cells. All these complexes also showed catalytic activity in transfer hydrogenation (TH) of NAD⁺ to NADH with sodium formate as hydride donor, with TOFs in the range of 2.5-9.7 h^-1. The complexes reacted rapidly with the thiols glutathione (GSH) and N-acetyl-L-cysteine (NAC), forming dinuclear bridged complexes [(η⁶-biph)₂Ru₂(GS)₃]^2- or [(η⁶-biph)₂Ru₂(NAC-H)₃]^2-, with the liberation of the diamine ligand which was detected by LC-MS. In addition, the switching on of fluorescence for complex 8 in aqueous solution confirmed release of the chelated DsEnBz ligand in reactions with these thiols. Reactions with GSH hampered the catalytic TH of NAD⁺ to NADH due to the decomposition of the complexes. Co-administration to cells of complex 2 [(η⁶-biph)Ru(4-Me-phenyl-SO₂-EnBz)Cl] with L-buthionine sulfoximine (L-BSO), an inhibitor of GSH synthesis, partially restored the anticancer activity towards A2780 ovarian cancer cells. Complex 2 caused a concentration-dependent G1 phase cell cycle arrest, and induced a significant level of reactive oxygen species (ROS) in A2780 human ovarian cancer cells. The amount of induced ROS decreased with increase in GSH concentration, perhaps due to the formation of the dinuclear Ru-SG complex.

Density functional theory investigation of Ru(II) and Os(II) asymmetric transfer hydrogenation catalysts

Transition metal ions have a unique ability to organise and control the steric and electronic effects around a substrate in the active site of a catalyst. We consider half-sandwich Ru(II) (Noyori-type) and Os(II) sulfonyldiamine 16-electron active catalysts [Ru/Os(η⁶-p-cymene)(TsDPEN-H₂)], where TsDPEN is N-tosyl-1,2-diphenylethylenediamine containing S,S or R,R chiral centres, which catalyse the highly efficient asymmetric transfer hydrogenation of aromatic ketones to chiral alcohols using formic acid as a hydride source. We discuss the recognition of the prochiral ketone acetophenone by the catalyst, the protonation of a ligand NH and transfer of hydride from formate to the metal, subsequent transfer of hydride to one enantiotopic face of the ketone, followed by proton transfer from metal-bound NH₂, and regeneration of the catalyst. Our DFT calculations illustrate the role of the two chiral carbons on the N,N-chelated sulfonyldiamine ligand, the axial chirality of the π-bonded p-cymene arene, and the chirality of the metal centre. We discuss new features of the mechanism, including how a change in metal chirality of the hydride intermediate dramatically switches p-cymene coordination from η⁶ to η². Moreover, the calculations suggest a step-wise mechanism involving substrate docking to the bound amine NH₂ followed by hydride transfer prior to protonation of the O-atom of acetophenone and release of the enantio-pure alcohol. This implies that formation and stability of the M-H hydride intermediate is highly dependent on the presence of the protonated amine ligand. The Os(II) catalyst is more stable than the Ru(II) analogue, and these studies illustrate the subtle differences in mechanistic behaviour between these 4d⁶ and 5d⁶ second-row and third-row transition metal congeners in group 8 of the periodic table.

μ-Oxo-bridged iron(iii) complexes for the selective reduction of aromatic ketones catalyzed through base promoted in situ nanoparticle formation

Selective TH of aromatic ketones catalyzed by μ-oxo-diiron complexes and in situ NP formation in the presence of a base gave excellent yields (∼99–100%).

Anticancer half-sandwich Ir(iii) complex and its interaction with various biomolecules and their mixtures – a case study with ascorbic acid

An anticancer azo bond-containing half-sandwich Ir(iii) complex oxidizes ascorbate to dehydroascorbate, and ascorbate recovers in the presence of reduced glutathione.

Synthesis, Structure and Cytotoxicity of N,N and N,O-Coordinated RuII Complexes of 3-Aminobenzoate Schiff Bases against Triple-negative Breast Cancer

Half-sandwich Ru^II complexes, [(YZ)Ru^II (η⁶ -arene)(X)]+, (YZ=chelating bidentate ligand, X=halide), with N,N and N,O coordination (1-9) show significant antiproliferative activity against the metastatic triple-negative breast carcinoma (MDA-MB-231). 3-aminobenzoic acid or its methyl ester is used in all the ligands while varying the aldehyde for N,N and N,O coordination. In the N,N coordinated complex the coordinated halide(X) is varied for enhancing stability in solution (X=Cl, I). Rapid aquation and halide exchange of the pyridine analogues, 2 and 3, in solution are a major bane towards their antiproliferative activity. Presence of free -COOH group (1 and 4) make complexes hydrophilic and reduces toxicity. The imidazolyl 3-aminobenzoate based N,N coordinated 5 and 6 display better solution stability and efficient antiproliferative activity (IC₅₀ ca. 2.3-2.5 μM) compared to the pyridine based 2 and 3 (IC₅₀ >100 μM) or the N,O coordinated complexes (7-9) (IC₅₀ ca. 7-10 μM). The iodido coordinated, 6, is resistant towards aquation and halide exchange. The N,O coordinated 7-9 underwent instantaneous aquation at pH 7.4 generating monoaquated complexes stable for at least 6 h. Complexes 5 and 6, bind to 9-ethylguanine (9-EtG) showing propensity to interact with DNA bases. The complexes may kill via apoptosis as displayed from the study of 8. The change in coordination mode and the aldehyde affected the solution stability, antiproliferative activity and mechanistic pathways. The N,N coordinated (5 and 6) exhibit arrest in the G2/M phase while the N,O coordinated 8 showed arrest in the G0/G1 phase.

Development of novel ruthenium(II)-arene complexes displaying potent anticancer effects in glioblastoma cells

Glioblastomas (GBs) are highly aggressive and malignant brain tumors, which are highly resistant to conventional multimodal treatments, leading to their abysmal prognosis. Herein, we designed two organometallic half-sandwich Ru(ii)-η6-p-cymene complexes containing Schiff bases derived from 3-aminoquinoline and 2-hydroxy-benzaldehyde (L1) and 2-hydroxy-naphthaldehyde (L2), namely [Ru(η6-p-cymene)(L1)Cl] (1) and [Ru(η6-p-cymene)(L2)Cl] (2), respectively, and studied their activity on GB cells. Both complexes were structurally characterized using single-crystal X-ray diffraction, which exhibited their half-sandwich three-legged piano-stool geometry. Furthermore, we studied their physicochemical behavior, solution speciation, aquation kinetics, and photo-substitution reactions using various spectroscopic methods. The complexes exhibited a moderate binding affinity with calf-thymus (CT)-DNA (Kb ∼ 105 M-1). The complexes effectively interacted with human serum albumin (HSA) (K ∼ 105 M-1) with preferential tryptophan binding, as determined via synchronous fluorescence studies. The in vitro studies showed their significant antiproliferative activity against an aggressive human GB cell line, LN-229 (IC50 = 22.8 μM), with moderate selectivity relative to normal mouse fibroblast L929 cells. Notably, [Ru(η6-p-cymene)(L1)Cl] (1) exhibited a higher selectivity index (S.I.) than [Ru(η6-p-cymene)(L2)Cl] (2) and cisplatin. We evaluated the clonogenic potential of the GB cells using a colony formation assay in the presence of complex 1. Excitingly, it showed ∼75% inhibition of the clonogenic potential of GB cells at the IC50 concentration. Complex 1 also effectively lowered the migratory potential of the GB cells, as assessed by the wound healing assay. The studied compound led to the apoptosis of GB cells, as evidenced by nuclear condensation, blebbing, and enhanced caspase 3/7 activity, and thus has anticipated utility in the treatment of GBs using photochemotherapy.

Metal-Based Catalytic Drug Development for Next-Generation Cancer Therapy

Considering the high increase in mortality caused by cancer in recent years, cancer drugs with novel mechanisms of anticancer action are urgently needed to overcome the drawbacks of platinum-based chemotherapeutics. Recently, in the area of metal-based cancer drug development research, the concept of catalytic cancer drugs has been introduced with organometallic Ru^II , Os^II , Rh^III and Ir^III complexes. These complexes are reported as catalysts for many important biological transformations in cancer cells such as nicotinamide adenine dinucleotide (NAD(P)H) oxidation to NAD⁺ , reduction of NAD⁺ to NADH, and reduction of pyruvate to lactate. These unnatural intracellular transformations with catalytic and nontoxic doses of metal complexes are known to severely perturb several important biochemical pathways and could be the antecedent of next-generation catalytic cancer drug development. In this concept, we delineate the prospects of such recently reported organometallic Ru^II , Os^II , Rh^III and Ir^III complexes as future catalytic cancer drugs. This new approach has the potential to deliver new cancer drug candidates.

Ru-TsDPEN catalysts and derivatives in asymmetric transfer hydrogenation reactions

This review summarizes current developments, novel synthetic routes for Ruthenium tethered chiral catalyst, and its derivatives along with its application in asymmetric synthesis. The review also covers derivatization in tethering unit, modification in N-monofunctionalized ligand as well as ligation of other ligand with Ru metal in chiral catalyst. Apparently, the effect of a modified tethered catalyst in the enantioselective synthesis of chiral products as well as in synthetic chemistry is also discussed in detail.

Piano-Stool Ruthenium(II) Complexes with Delayed Cytotoxic Activity: Origin of the Lag Time

We have recently reported a series of piano-stool ruthenium(II) complexes of the general formula [RuCl₂(η⁶-arene)(P(1-pyrenyl)R²R³)] showing excellent cytotoxic activities (particularly when R² = R³ = methyl). In the present study, new members of this family of compounds have been prepared with the objective to investigate the effect of the steric hindrance of a bulky phosphane ligand, namely diisopropyl(1-pyrenyl)phosphane (L), on exchange reactions involving the coordinated halides (X = Cl, I). Two η⁶-arene rings were used, i.e. η⁶-methyl benzoate (mba) and η⁶-p-cymene (p-cym), and four complexes were synthesized, namely [RuCl₂(mba)(L)] (1_Cl2^iPr), [RuI₂(mba)(L)] (1_I2^iPr), [RuCl₂(p-cym)(L)] (2_Cl2^iPr), and [RuI₂(p-cym)(L)] (2_I2^iPr). Unexpectedly, all of the complexes exhibited poor cytotoxic activities after 24 h of incubation with cells, in contrast to the related compounds previously reported. However, it was observed that aged DMSO solutions of 2_I2^iPr (from 2 to 7 days) exhibited better activities in comparison to freshly prepared solutions and that the activity improved over “aging” time. Thorough studies were therefore performed to uncover the origin of this lag time in the cytotoxicity efficiency. The data achieved clearly demonstrated that compounds 2_I2^iPr and 2_Cl2^iPr were undergoing a series of transformation reactions in DMSO (with higher rates for the iodido complex 2_I2^iPr), ultimately generating cyclometalated species through a mechanism involving DMSO as a coordinated proton abstractor. The cyclometalated complexes detected in solution were subsequently prepared; hence, pure [RuCl(p-cym)(κ²C-diisopropyl(1-pyrenyl)phosphane)] (3_Cl^iPr), [RuI(p-cym)(κ²C-diisopropyl(1-pyrenyl)phosphane)] (3_I^iPr), and [Ru(p-cym)(κS-dmso)(κ²C-diisopropyl(1-pyrenyl)phosphane)]PF₆ (3_dmso^iPr) were synthesized and fully characterized. Remarkably, 3_Cl^iPr, 3_I^iPr, and 3_dmso^iPr are all very efficient cytotoxic agents, exhibiting slightly better activities in comparison to the chlorido noncyclometalated complexes [RuCl₂(η⁶-arene)(P(1-pyrenyl)R²R³)] described in an earlier report. For comparison purposes, the iodido compounds [RuI₂(mba)(dimethyl(1-pyrenyl)phosphane)] (1_I2^Me) and [RuI₂(p-cym)(dimethyl(1-pyrenyl)phosphane)] (2_I2^Me), bearing the less hindered dimethyl(1-pyrenyl)phosphane ligand, have also been prepared. The cytotoxic and chemical behaviors of 1_I2^Me and 1_I2^Me were comparable to those of their chlorido counterparts reported previously.

DMSO gradually converts half-sandwich, 1-pyrenyl-containing ruthenium(II) complexes into cyclometalated species showing notable cytotoxic properties.

Transfer hydrogenation catalysis in cells

Hydrogenation reactions in biology are usually carried out by enzymes with nicotinamide adenine dinucleotide (NAD(P)H) or flavin mononucleotide (FAMH2)/flavinadenine dinucleotide (FADH2) as cofactors and hydride sources. Industrial scale chemical transfer hydrogenation uses small molecules such as formic acid or alcohols (e.g. propanol) as hydride sources and transition metal complexes as catalysts. We focus here on organometallic half-sandwich RuII and OsII η6-arene complexes and RhIII and IrIII η5-Cp x complexes which catalyse hydrogenation of biomolecules such as pyruvate and quinones in aqueous media, and generate biologically important species such as H2 and H2O2. Organometallic catalysts can achieve enantioselectivity, and moreover can be active in living cells, which is surprising on account of the variety of poisons present. Such catalysts can induce reductive stress using formate as hydride source or oxidative stress by accepting hydride from NAD(P)H. In some cases, photocatalytic redox reactions can be induced by light absorption at metal or flavin centres. These artificial transformations can interfere in biochemical pathways in unusual ways, and are the basis for the design of metallodrugs with novel mechanisms of action.

DNA targeting half sandwich Ru(II)-p-cymene-N^N complexes as cancer cell imaging and terminating agents: influence of regioisomers in cytotoxicity

For diagnosing and annihilating cancer in the human body, herein, we have adopted a one pot convenient synthetic protocol to synthesize a library of half sandwich Ru(ii)-p-cymene-N^N complexes under continuous sonication and isolated their regioisomers by preparative thin layer chromatography followed by justification of stability using DFT. The present work has resulted in a library of ruthenium arene complexes and their isolated regioisomers following environmentally benign green processes and their screening of anticancer activity in terms of cytotoxicity and selectivity against cancer cell lines where [(η6-p-cymene)RuCl{2-(5,6-dichloro-1H-benzo[d]imidazole-2-yl)quinolone}] (11j) has been elicited to be significantly more potent as well as selective in Caco-2 and HeLa cell lines than the normal HEK-293 cell line compared to cisplatin and it has even shown marked cytotoxicity against the more aggressive HT-29 colorectal cancer cell line being capable of producing oxidative stress or arresting the cell cycle. Moreover, these types of Ru(ii)-arene complexes exhibited excellent binding efficacy with DNA and the compounds [(η6-p-cymene)RuCl{5-chloro-2-(6-(4-chlorophenyl)pyridin-2-yl)benzo[d]thiazole}]PF6 (8l4), [(η6-p-cymene)Ru-2-(6-(benzofuran-2-yl)pyridin-2-yl)-5-chlorobenzo[d]thiazole (8l9) and [(η6-p-cymene)RuCl{2-(6-nitro-1H-benzo[d]imidazol-2-yl)quinolone}]Cl (11f') and might be applied for cancer theranostic treatment due to their good fluorescence properties and remarkable potency.

Recent Advances in the Biological Investigation of Organometallic Platinum-Group Metal (Ir, Ru, Rh, Os, Pd, Pt) Complexes as Antimalarial Agents

In the face of the recent pandemic and emergence of infectious diseases of viral origin, research on parasitic diseases such as malaria continues to remain critical and innovative methods are required to target the rising widespread resistance that renders conventional therapies unusable. The prolific use of auxiliary metallo-fragments has augmented the search for novel drug regimens in an attempt to combat rising resistance. The development of organometallic compounds (those containing metal-carbon bonds) as antimalarial drugs has been exemplified by the clinical development of ferroquine in the nascent field of Bioorganometallic Chemistry. With their inherent physicochemical properties, organometallic complexes can modulate the discipline of chemical biology by proffering different modes of action and targeting various enzymes. With the beneficiation of platinum group metals (PGMs) in mind, this review aims to describe recent studies on the antimalarial activity of PGM-based organometallic complexes. This review does not provide an exhaustive coverage of the literature but focusses on recent advances of bioorganometallic antimalarial drug leads, including a brief mention of recent trends comprising interactions with biomolecules such as heme and intracellular catalysis. This resource can be used in parallel with complementary reviews on metal-based complexes tested against malaria.

Activation of the Ir-N(pyridine) Bond in Half-Sandwich Tethered Iridium(III) Complexes

We present four new organometallic half-sandwich iridium(III) complexes of formula [Ir(η⁵:κ¹-C₅Me₄CH₂py)(N,N)](PF₆)₂, bearing a N,N-chelating ligand [ethylenediamine (en), 1; 1,3-diaminopropane (dap), 2; 2,2'-bipyridine (bipy), 3; 1,10-phenanthroline (phen), 4]; and a derivatized cyclopentadienyl ligand, C₅Me₄CH₂C₅H₄N, which forms an additional five-membered chelate. The latter is hemilabile, and the Ir-N(py) bond can be reversibly cleaved by various stimuli. The four complexes are unreactive toward hydrolysis at pH 7. Interestingly, 1 and 2 react with hydrochloric acid and formate, and speciation between closed and open tether complexes can be followed by ¹H NMR spectroscopy. Complex 1 binds to nucleobase guanine (9-ethylguanine, 9-EtG), yet interaction to calf-thymus DNA was not observed. New X-ray structures of closed tether complexes 1-4 and open tether complexes [Ir(η⁵-C₅Me₄CH₂pyH)(en)Cl](PF₆)₂ (1·HCl) and [Ir(η⁵-C₅Me₄CH₂py)(en)H]PF₆ (1·hyd) have been determined. Hydride capture is efficient for 1 and 2. The kinetics of Ir-H bond formation and hydride transfer in a model organic molecule have been investigated, revealing a strong dependence on the temperature. Coincubation of complex 1 with nontoxic concentrations of sodium formate decreases the IC₅₀ value in MCF7 breast cancer cells, indicating the possibility of intracellular activation of the Ir-N(py) tether bond to generate cytotoxic activity via iridium-mediated transfer hydrogenation.

Synthesis, Characterisation and In Vitro Anticancer Activity of Catalytically Active Indole-Based Half-Sandwich Complexes

The synthesis, characterisation and evaluation of the in vitro cytotoxicity of four indole-based half-sandwich metal complexes towards two ovarian cancer cell lines (A2780 and A2780cisR) and one normal prostate cell line (PNT2) are presented herein. Although capable of inducing catalytic oxidation of NADH and able to reduce NAD+ with high turnover frequencies, in cells and in the presence of sodium formate, these complexes also strongly interact with biomolecules such as glutathione. This work highlights that efficient out-of-cells catalytic activity might lead to higher reactivity towards biomolecules, thus inhibiting the in-cells catalytic processes.

Can a Nonorganometallic Ruthenium(II) Polypyridylamine Complex Catalyze Hydride Transfer? Mechanistic Insight from Solution Kinetics on the Reduction of Coenzyme NAD+ by Formate

Application of organometallic ruthenium(II) arene complexes has been successful for the modulation of cellular redox processes via their interaction with species such as formate to control the NAD⁺/NADH balance in cells. Here we present the first evidence that similar effects can be reached with the application of a nonorganometallic ruthenium(II) polypyridyl complex. Kinetic studies performed demonstrate the ability of [Ru^II(terpy)(en)(H₂O/EtOH)]²⁺ in water/ethanol (1:9, v/v) solution, where terpy = 2,2':6',2″-terpyridine and en = ethylenediamine, to catalyze the reduction of the NAD⁺ coenzyme to NADH in the presence of formate as hydride transfer source. In this case, terpy instead of arene is responsible for the labilization of coordinated solvent. The suggested catalytic cycle begins with the fast anation of the [Ru^II(terpy)(en)(H₂O/EtOH)]²⁺ complex by formate. This is followed by the rate-determining formate-catalyzed decarboxylation of the generated ruthenium(II) formato complex to form [Ru^II(terpy)(en)H]⁺. Rapid hydride transfer to NAD⁺ from [Ru^II(terpy)(en)H]⁺ to form NADH and to regenerate the starting ruthenium(II) solvato complex, closes the overall catalytic cycle.

Ruthenium(II) Phosphine/Mercapto Complexes: Their in Vitro Cytotoxicity Evaluation and Actions as Inhibitors of Topoisomerase and Proteasome Acting as Possible Triggers of Cell Death Induction

In this paper, a series of new ruthenium complexes of the general formula [Ru(NS)(dpphpy)(dppb)]PF₆ (Ru1-Ru3), where dpphpy = diphenyl-2-pyridylphosphine, NS ligands = 2-thiazoline-2-thiol (tzdt, Ru1), 2-mercaptopyrimidine (pySm, Ru2), and 4,6-diamino-2-mercaptopyrimidine (damp, Ru3), and dppb = 1,4-bis(diphenylphosphino)butane, were synthesized and characterized by elemental analysis, spectroscopic techniques (IR, UV/visible, and 1D and 2D NMR), and X-ray diffraction. In the characterization, the correlation between the phosphorus atoms and their respective aromatic hydrogen atoms of the compounds in the assignment stands outs, by ¹H-³¹P HMBC experiments. The compounds show anticancer activities against A549 (lung) and MDA-MB-231 (breast) cancer cell lines, higher than the clinical drug cisplatin. All of the complexes are more cytotoxic against the cancer cell lines than against the MRC-5 (lung) and MCF-10A (breast) nontumorigenic human cell lines. For A549 tumor cells, cell cycle analysis upon treatment with Ru2 showed that it inhibits the mitotic phase because arrest was observed in the Sub-G1 phase. Additionally, the compound induces cell death by an apoptotic pathway in a dose-dependent manner, according to annexin V-PE assay. The multitargeted character of the compounds was investigated, and the biomolecules were DNA, topoisomerase IB, and proteasome, as well as the fundamental biomolecule in the pharmacokinetics of drugs, human serum albumin. The experimental results indicate that the complexes do not target DNA in the cells. At low concentrations, the compounds showed the ability to partially inhibit the catalytic activity of topoisomerase IB in the process of relaxation of the DNA plasmid. Among the complexes assayed in cultured cells, complex Ru3 was able to diminish the proteasomal chymotrypsin-like activity to a greater extent.

Cytotoxicity of ruthenium-N,N-disubstituted-N'-acylthioureas complexes

Five new complexes with general formula [Ru(L_n)(PP)(bipy)]PF₆, where L_n = N,N'-dimethyl-N-Acyl thiourea, and P-P: 1,2-bis(diphenylphosphino)ethane (dppe) or 1,4-bis(diphenylphosphino)butane (dppb)) were synthesized and characterized by elemental analysis, molar conductivity, cyclic voltammetry, IR, NMR (¹H, ¹³C{¹H} and ³¹P{¹H}), and single crystal X-ray diffractometry. The cytotoxicity of compounds against lung and breast tumor cell lines was significant, where two complexes, [Ru(L₃)(bipy)(dppe)]PF₆ (3) and [Ru(L₃)(bipy)(dppb)]PF₆ (6), were selected to evaluate changes in morphology, inhibition of migration and cell death in the MDA-MB-231 lineage. The complexes caused alterations in the cell morphology and were able to inhibit cell migration at the concentrations evaluated, induce the cell cycle arrested in the Sub-G1 phase, and induced cell death by apoptosis. All the complexes presented interaction with HSA, and the interaction studies with DNA suggested weak interactions, probably by the minor groove.

Characterization of a Mixed-Valence Ru(II)/Ru(III) Ion-Pair Complex. Unexpected High-Frequency Electron Paramagnetic Resonance Evidence for Ru(III)-Ru(III) Dimer Coupling

In this contribution, we report the synthesis and full characterization of the first mixed-valence Ru(II)/Ru(III) ion-pair complex, [Ru^II(bipy)₂(pic)]⁺[cis-Ru^IIICl₂(pic)₂]^-, in the solid state and in aqueous solution, where bipy = 2,2'-bipyridine and pic^- = picolinate. In addition, unexpected high-frequency electron paramagnetic resonance evidence for interactions between two neighboring Ru(III) ions, resulting in a triplet state, S = 1, was found.

Inorganic reaction mechanisms. A personal journey

This review covers highlights of the work performed in the van Eldik group on inorganic reaction mechanisms over the past two decades in the form of a personal journey. Topics that are covered include, from NO to HNO chemistry, peroxide activation in model porphyrin and enzymatic systems, the wonder-world of Ru^III(edta) chemistry, redox chemistry of Ru(iii) complexes, Ru(ii) polypyridyl complexes and their application, relevant physicochemical properties and reaction mechanisms in ionic liquids, and mechanistic insight from computational chemistry. In each of these sections, typical examples of mechanistic studies are presented in reference to related work reported in the literature.

Ruthenium(II)-Arene Thiocarboxylates: Identification of a Stable Dimer Selectively Cytotoxic to Invasive Breast Cancer Cells

Ru^II -arene complexes provide a versatile scaffold for novel anticancer drugs. Seven new Ru^II -arene-thiocarboxylato dimers were synthesized and characterized. Three of the complexes (2 a, b and 5) showed promising antiproliferative activities in MDA-MB-231 (human invasive breast cancer) cells, and were further tested in a panel of fifteen cancerous and noncancerous cell lines. Complex 5 showed moderate but remarkably selective activity in MDA-MB-231 cells (IC₅₀ =39±4 μm Ru). Real-time proliferation studies showed that 5 induced apoptosis in MDA-MB-231 cells but had no effect in A549 (human lung cancer, epithelial) cells. By contrast, 2 a and b showed moderate antiproliferative activity, but no apoptosis, in either cell line. Selective cytotoxicity of 5 in aggressive, mesenchymal-like MDA-MB-231 cells over many common epithelial cancer cell lines (including noninvasive breast cancer MCF-7) makes it an attractive lead compound for the development of specifically antimetastatic Ru complexes with low systemic toxicity.

Effect of N,N Coordination and RuII Halide Bond in Enhancing Selective Toxicity of a Tyramine-Based RuII (p-Cymene) Complex

Ruthenium compounds are promising anticancer candidates owing to their lower side-effects and encouraging activities against resistant tumors. Half-sandwich piano-stool type Ru^II compounds of general formula [(L)Ru^II(η⁶-arene)(X)]⁺ (L = chelating bidentate ligand, X = halide) have exhibited significant therapeutic potential against cisplatin-resistant tumor cell lines. In Ru^II (p-cymene) based complexes, the change of the halide leaving group has led to several interesting features, viz., hydrolytic stability, resistance toward thiols, and alteration in pathways of action. Tyramine is a naturally occurring monoamine which acts as a catecholamine precursor in humans. We synthesized a family of N,N and N,O coordinated Ru^II (p-cymene) complexes, [(L)Ru^II(η⁶-arene)(X)]⁺ (1-4), with tyramine and varied the halide (X = Cl, I) to investigate the difference in reactivity. Our studies showed that complex 2 bearing N,N coordination with an iodido leaving group shows selective in vitro cytotoxicity against the pancreatic cancer cell line MIA PaCa-2 (IC₅₀ ca. 5 μM) but is less toxic to triple-negative breast cancer (MDA-MB-231), hepatocellular carcinoma (Hep G2), and the normal human foreskin fibroblasts (HFF-1). Complex 2 displays stability toward hydrolysis and does not bind with glutathione, as confirmed by ¹H NMR and ESI-HRMS experiments. The inert nature of 2 leads to enhancement of cytotoxicity (IC₅₀ = 5.3 ± 1 μM) upon increasing the cellular treatment time from 48 to 72 h.

Design, synthesis, characterization and evaluation of the anticancer activity of water-soluble half-sandwich ruthenium(ii) arene halido complexes

Synthesis, crystal structure determination, DFT studies, experimental and theoretical evaluation of DNA/BSA interactions and cytotoxicity studies of three piano-stool Ru(ii)(p-cymene)chloride complexes (1–3) are presented herein.

Catalytic recycling of NAD(P)H

A large number of industrially relevant enzymes depend upon dihydronicotinamide adenine dinucleotide (NADH) and dihydronicotinamide adenine dinucleotide phosphate (NADPH) cofactors, which are too expensive to be added in stoichiometric amounts. Existing NAD(P)H-recycling systems suffer from low activity, or the generation of side products. This review focuses on NAD(P)H cofactor regeneration catalyzed by transition metal complexes such as rhodium, ruthenium and iridium complexes using cheap reducing agents such as hydrogen (H₂) and ethanol, which have attracted increasing attention as sustainable energy carriers. The catalytic mechanisms for the regioselective reduction of NAD(P)⁺ are discussed with emphasis on identification of catalytically active intermediates such as transition metal hydride complexes. Applications of NAD(P)H-recycling systems to develop artificial photosynthesis are also discussed.

Genetic Engineering of an Artificial Metalloenzyme for Transfer Hydrogenation of a Self-Immolative Substrate in Escherichia coli's Periplasm

Artificial metalloenzymes (ArMs), which combine an abiotic metal cofactor with a protein scaffold, catalyze various synthetically useful transformations. To complement the natural enzymes' repertoire, effective optimization protocols to improve ArM's performance are required. Here we report on our efforts to optimize the activity of an artificial transfer hydrogenase (ATHase) using Escherichia coli whole cells. For this purpose, we rely on a self-immolative quinolinium substrate which, upon reduction, releases fluorescent umbelliferone, thus allowing efficient screening. Introduction of a loop in the immediate proximity of the Ir-cofactor afforded an ArM with up to 5-fold increase in transfer hydrogenation activity compared to the wild-type ATHase using purified mutants.

Accessible Bifunctional Oxy-Tethered Ruthenium(II) Catalysts for Asymmetric Transfer Hydrogenation

A concise synthesis of new oxy-tethered ruthenium complexes effective for the asymmetric transfer hydrogenation of aromatic ketones is described. The oxy-tether was constructed via a defluorinative etherification arising from an intramolecular nucleophilic substitution of a perfluorinated phenylsulfonyl substituent. The obtained tethered complexes exhibited desirable catalytic activity and selectivity, especially in the asymmetric transfer hydrogenation of functionalized aromatic ketones. The robustness and rigidity of the tether contribute to their superior catalytic performance relative to the nontethered prototype complex.

Anticancer Activity and Catalytic Potential of Ruthenium(II)-Arene Complexes with N,O-Donor Ligands

The special ability of organometallic complexes to catalyze various transformations might offer new effective mechanisms for the treatment of cancer. Studies that report both the biological properties and the ability of metallic complexes to promote therapeutically relevant catalytic reactions are limited. Herein, we report the anticancer activity and catalytic potential of some ruthenium(II)-arene complexes bearing bidentate Schiff base ligands (2a and 2b) and their reduced analogues (5a and 5b, respectively). In comparison to their Schiff base counterparts 2a and 2b, we demonstrate that amine complexes 5a and 5b display (i) a higher in vitro antiproliferative activity on different human cancer cell lines, (ii) a lower rate of hydrolysis, and (iii) an improved initial catalytic rate for the reduction of NAD⁺ to NADH. In contrast to their imine analogues 2a and 2b, we also show that amine complexes 5a and 5b induce the generation of intracellular reactive oxygen species (ROS) in MCF-7 breast cancer cells. Our results highlight the impact that a simple ligand modification such as the reduction of an imine moiety can have on both the catalytic and biological activities of metal complexes. Moreover, the ruthenium complexes reported here display some antiproliferative activity against T47D breast cancer cells, known for their cis-platin resistance.