Half-sandwich arene ruthenium complexes: synthetic strategies and relevance in catalysis

Biomolecular Interactions and Anticancer Mechanisms of Ru(II)-Arene Complexes of Cinnamaldehyde-Derived Thiosemicarbazone Ligands: Analysis Combining In Silico and In Vitro Approaches

Our study focuses on synthesizing and exploring the potential of three N-(4) substituted thiosemicarbazones derived from cinnamic aldehyde, alongside their Ru(II)-(η6 -p-cymene)/(η6-benzene) complexes. The synthesized compounds were comprehensively characterized using a range of analytical techniques, including FT-IR, UV-visible spectroscopy, NMR (1H, 13C), and HRMS. We investigated their electronic and physicochemical properties via density functional theory (DFT). X-ray crystal structures validated structural differences identified by DFT. Molecular docking predicted promising bioactivities, supported by experimental observations. Notably, docking with EGFR suggested an inhibitory potential against this cancer-related protein. Spectroscopic titrations revealed significant DNA/BSA binding affinities, particularly with DNA intercalation and BSA hydrophobic interactions. RuPCAM displayed the strongest binding affinity with DNA (Kb = 6.23 × 107 M-1) and BSA (Kb = 9.75 × 105 M-1). Assessed the cytotoxicity of the complexes on cervical cancer cells (HeLa), and breast cancer cells (MCF-7 and MDA-MB 231), revealing remarkable potency. Additionally, selectivity was assessed by examining MCF-10a normal cell lines. The active complexes were found to trigger apoptosis, a vital cellular process crucial for evaluating their potential as anticancer agents utilizing staining assays and flow cytometry analysis. Intriguingly, complexation with Ru(II)-arene precursors significantly amplified the bioactivity of thiosemicarbazones, unveiling promising avenues toward the creation of powerful anticancer agents.

Half-Sandwich Iridium Complexes: A Recyclable and Stable Catalyst for Dehydrogenation of Alcohols to Carboxylic Acids

A series of acylhydrazone-based N,N-chelate half-sandwich iridium complexes have been synthesized through a facile route in good yields. The dehydrogenation of a series of aromatic and aliphatic primary alcohols to corresponding carboxylic acids has been accomplished catalyzed by the prepared air stable iridium complexes under mild reaction conditions. Carboxylic acids were obtained in high yields under open flask condition with broad substrates and good tolerance to sensitive functional groups. Such a half-sandwich iridium catalyst system exhibited high catalytic activity and stability, and a high TOF of 316.7 h-1 could be achieved with a catalyst loading as low as 0.05 mol %. Furthermore, the sustainable catalyst could be reused at least five times without obviously losing its activity, highlighting its potential application in industry. Molecular structure of iridium complex 1 was confirmed by single-crystal X-ray analysis.

Photocatalytic Conversion of CO2 to Formate/CO by an (η6-para-Cymene)Ru(II) Half-Metallocene Catalyst: Influence of Additives and TiO2 Immobilization on the Catalytic Mechanism and Product Selectivity

The catalytic efficacy of the monobipyridyl (η6-para-Cymene)Ru(II) half-metallocene, [(p-Cym)Ru(bpy)Cl]+ was evaluated in both mixed homogeneous (dye + catalyst) and heterogeneous hybrid systems (dye/TiO2/Catalyst) for photochemical CO2 reduction. A series of homogeneous photolysis experiments revealed that the (p-Cym)Ru(II) catalyst engages in two competitive routes for CO2 reduction (CO2 to formate conversion via RuII-hydride vs CO2 to CO conversion through a RuII-COOH intermediate). The conversion activity and product selectivity were notably impacted by the pKa value and the concentration of the proton source added. When a more acidic TEOA additive was introduced, the half-metallocene Ru(II) catalyst leaned toward producing formate through the RuII-H mechanism, with a formate selectivity of 86%. On the other hand, in homogeneous catalysis with TFE additive, the CO2-to-formate conversion through RuII-H was less effective, yielding a more efficient CO2-to-CO conversion with a selectivity of >80% (TONformate of 140 and TONCO of 626 over 48 h). The preference between the two pathways was elucidated through an electrochemical mechanistic study, monitoring the fate of the metal-hydride intermediate. Compared to the homogeneous system, the TiO2-heterogenized (p-Cym)Ru(II) catalyst demonstrated enhanced and enduring performance, attaining TONs of 1000 for CO2-to-CO and 665 for CO2-to-formate.

Cyclometalated Half-Sandwich Ruthenium Complexes: Preparation, Structure, and Catalytic Synthesis of Phenolic Esters from Aryl Halides without Using External CO under Air

A series of cyclometalated C,N-chelate half-sandwich ruthenium complexes based on N-heterocyclic ligands were prepared through a simple route with good yields. These air- and moisture-stable cyclometalated ruthenium complexes showed excellent catalytic efficiency in phenoxy carbonylation of aryl halides with phenyl formate derivatives as a CO source and phenol as a coupling partner under air. Ester products were obtained with high yields at room temperature and without the need for an inert atmosphere. The excellent catalytic activity, broad substrate range, and mild reaction conditions made this catalytic system potential for industrial production. In addition, DFT study has been carried out to elaborate the possible mechanism of this Ru-catalyzed reaction.

Organometallic Ionic Liquids Containing Sandwich Complexes: Molecular Design, Physical Properties, and Chemical Reactivities

Ionic liquids (ILs) are salts with low melting points and are useful as electrolytes and solvents. We have developed ILs containing cationic metal complexes, which form a family of functional liquids that exhibit unique physical properties and chemical reactivities originating from metal complexes. Our study explores the liquid chemistry in the field of coordination chemistry, where solid-state chemistry is currently the main focus. This review describes the molecular design, physical properties, and reactivities of organometallic ILs containing sandwich or half-sandwich complexes. This paper mainly covers stimuli-responsive ILs, whose magnetic properties, solvent polarities, colors, or structures change by the application of external fields, such as light, heat, and magnetic fields, or by reaction with coordinating molecules.

Cinchona-Alkaloid-Derived NN Ligands and Achiral Phosphines for Iridium-Catalyzed Asymmetric Hydrogenation of Heteroaromatic and α-Chloroheteroaryl Ketones

A concise synthesis of cinchona-alkaloid-derived NN ligands bearing alkyl substituents on chiral nitrogen atoms was described. Iridium catalysts containing new chiral NN ligands and achiral phosphines were effective for the asymmetric hydrogenation of heteroaromatic ketones, which afforded corresponding alcohols in up to 99.9% ee. The same protocol was applicable to the asymmetric hydrogenation of α-chloroheteroaryl ketones. Most importantly, the gram-scale asymmetric hydrogenation of 2-acetylthiophene and 2-acetylfuran proceeded smoothly even under 1 MPa of H2.

Half-Sandwich Iridium Complexes with Hydrazone Ligands: Synthesis and Catalytic Activity in N-Alkylation of Anilines or Nitroarenes with Alcohols via Hydrogen Autotransfer

Here, we synthesize a series of hydrazone-based N,O-chelate half-sandwich iridium complexes through a facile route. All air-stable iridium complexes show high catalytic activity in N-alkylation of a broad scope of aniline derivatives and alcohols with liberating water as the sole byproduct. This reaction provides a smooth route to synthesize diverse monoalkylated amines in good to excellent yields at moderate temperature with a low catalyst loading. Moreover, the challenging N-alkylation process using nitroarene substrates as coupling partners is also carried out in this catalytic system. The mechanistic study shows that the present iridium catalysis process proceeds through a hydrogen borrowing mechanism. All iridium(III) complexes 1-4 are characterized by infrared (IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and elemental analysis.

N-Heterocyclic Carbene-Phosphinidenide Complexes as Hydroboration Catalysts

The reactions of the N-heterocyclic carbene-phosphinidene adducts (NHC)PSiMe₃ and (NHC)PH with the dinuclear ruthenium and osmium complexes [(η⁶-p-cymene)MCl₂]₂ (M = Ru, Os) afforded the half-sandwich complexes [(η⁶-p-cymene){(NHC)P}MCl] and [(η⁶-p-cymene){(NHC)PH}MCl₂] with two- and three-legged piano-stool geometries, respectively (NHC = IDipp, IMes; IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). The complexes were initially tested as precatalysts for the hydroboration of benzonitrile, and the most active species, the ruthenium complex [(η⁶-p-cymene){(IMes)P}RuCl], was further used for the efficient hydroboration of a wide range (ca. 50 substrates) of nitriles, carboxylic esters, and carboxamides in neat pinacolborane (HBpin) under comparatively mild reaction conditions (60-80 °C, 3-5 mol % catalyst loading). Preliminary mechanistic and kinetic studies are reported, and stoichiometric reactions with HBpin indicate the initial formation of the monohydride complex [(η⁶-p-cymene){(IMes)P}RuH] as the putative catalytically active species.

Synthesis, Characterization, and Biological Activity of Water-Soluble, Dual Anionic and Cationic Ruthenium-Arene Complexes Bearing Imidazol(in)ium-2-dithiocarboxylate Ligands

An efficient synthetic protocol was devised for the preparation of five cationic ruthenium-arene complexes bearing imidazol(in)ium-2-dithiocarboxylate ligands from the [RuCl₂(p-cymene)]₂ dimer and 2 equiv of an NHC·CS₂ zwitterion. The reactions proceeded cleanly and swiftly in dichloromethane at room temperature to afford the expected [RuCl(p-cymene)(S₂C·NHC)]Cl products in quantitative yields. When the [RuCl₂(p-cymene)]₂ dimer was reacted with only 1 equiv of a dithiolate betaine under the same experimental conditions, a set of five bimetallic compounds with the generic formula [RuCl(p-cymene)(S₂C·NHC)][RuCl₃(p-cymene)] was obtained in quantitative yields. These novel, dual anionic and cationic ruthenium-arene complexes were fully characterized by various analytical techniques. NMR titrations showed that the chelation of the dithiocarboxylate ligands to afford [RuCl(p-cymene)(S₂C·NHC)]⁺ cations was quantitative and irreversible. Conversely, the formation of the [RuCl₃(p-cymene)]^- anion was limited by an equilibrium, and this species readily dissociated into Cl^- anions and the [RuCl₂(p-cymene)]₂ dimer. The position of the equilibrium was strongly influenced by the nature of the solvent and was rather insensitive to the temperature. Two monometallic and two bimetallic complexes cocrystallized with water, and their molecular structures were solved by X-ray diffraction analysis. Crystallography revealed the existence of strong interactions between the azolium ring protons of the cationic complexes and neighboring donor groups from the anions or the solvent. The various compounds under investigation were highly soluble in water. They were all strongly cytotoxic against K562 cancer cells. Furthermore, with a selectivity index of 32.1, the [RuCl(p-cymene)(S₂C·SIDip)]Cl complex remarkably targeted the erythroleukemic cells vs mouse splenocytes.

Arene-Osmium(II) Complexes in Homogeneous Catalysis

Although the application of arene-osmium(II) complexes in homogeneous catalysis has been much less studied than that of their ruthenium analogues, different works have shown that, in some instances, a comparable or even superior effectiveness can be achieved with this particular class of compounds. This review article focuses on the catalytic applications of arene-osmium(II) complexes. Among others, transfer hydrogenation, hydrogenation, oxidation, and nitrile hydration reactions, as well as different C-C bond forming processes, are comprehensively discussed.

Synthesis, Structure, and Catalytic Hydrogenation Activity of [NO]-Chelate Half-Sandwich Iridium Complexes with Schiff Base Ligands

A series of N,O-coordinate iridium(III) complexes with a half-sandwich motif bearing Schiff base ligands for catalytic hydrogenation of nitro and carbonyl substrates have been synthesized. All iridium complexes showed efficient catalytic activity for the hydrogenation of ketones, aldehydes, and nitro-containing compounds using clean H₂ as reducing reagent. The iridium catalyst displayed the highest TON values of 960 and 950 in the hydrogenation of carbonyl and nitro substrates, respectively. Various types of substrates with different substituted groups afforded corresponding products in excellent yields. All N,O-coordinate iridium(III) complexes 1-4 were well characterized by IR, NMR, HRMS, and elemental analysis. The molecular structure of complex 1 was further characterized by single-crystal X-ray determination.

Half-Sandwich Ruthenium Complexes of Amide-Phosphine Based Ligands: H-Bonding Cavity Assisted Binding and Reduction of Nitro-substrates

We present synthesis and characterization of two half-sandwich Ru(II) complexes supported with amide-phosphine based ligands. These complexes presented a pyridine-2,6-dicarboxamide based pincer cavity, decorated with hydrogen bonds, that participated in the binding of nitro-substrates closer to the Ru(II) centers, which is further supported with binding and docking studies. These ruthenium complexes functioned as the noteworthy catalysts for the borohydride mediated reduction of assorted nitro-substrates. Mechanistic studies not only confirmed the intermediacy of [Ru-H] in the reduction but also asserted the involvement of several organic intermediates during the course of the catalysis. A similar Ru(II) complex that lacked pyridine-2,6-dicarboxamide based pincer cavity substantiated its unique role both in the substrate binding and the subsequent catalysis.

DFT Investigation of the η6 ⇌ η6-Inter-ring Haptotropic Rearrangement of the Group 8 Metals Complexes [(graphene)MCp]+ (M = Fe, Ru, Os)

Metalcyclopentadienyl complexes (MCp)⁺ (M = Fe, Ru, Os) bound to the large polyaromatic hydrogenated hydrocarbon (PAH) C₉₆H₂₄ used as a model for pristine graphene have been studied using a density functional theory (DFT) generalized gradient approximation (PBE functional) to reveal their structural features and dynamic behavior. The inter-ring haptotropic rearrangements (IRHRs) for these complexes were shown to occur via two transition states and one intermediate. The energy barriers of the η⁶ ⇌ η⁶ IRHRs of the (MCp)⁺ unit were found to be 30, 27, and 29 kcal/mol for M = Fe, Ru, and Os, respectively. These values are significantly lower than the values found previously for smaller PAHs. Both polar and nonpolar solvents were found not to affect significantly the energy barrier heights. Investigated transition metal complexes could be used in general as catalysts in the design of novel derivatives or materials with promising properties. Metalcyclopentadienyl complexes (MCp)⁺ of PAHs show catalytic properties mainly due to their structural details as well as their important characteristic of inter-ring haptotropic rearrangement. IRHRs take place usually by intramolecular mechanisms. During IRHRs, the ML_n organometallic groups (OMGs) undergo shifting along the PAH plane and could coordinate additional reagents, which is important for catalysis. Large PAHs such as graphene, fullerenes, and nanotubes possess intrinsic anticancer activity, and numerous arene complexes of Ru and Os have been proven to have anticancer properties as well. We suppose that coordinating Ru or Os to very large PAHs could synergistically increase the anticancer activity of resulting complexes.

1,3,4-Oxadiazole-functionalized α-amino-phosphonates as ligands for the ruthenium-catalyzed reduction of ketones

1,3,4-Oxadiazole-functionalized ruthenium catalysts for the reduction of ketone.

Evolution of strept(avidin)-based artificial metalloenzymes in organometallic catalysis

Artificial metalloenzymes have been recently established as efficient alternatives to traditional transition metal catalysts. The presence of a secondary coordination sphere in artificial metalloenzymes makes them advantageous over transition metal catalysts, which rely essentially on their first coordination sphere to exhibit their catalytic activity. Recent developments on streptavidin- and avidin-based artificial metalloenzymes have made them highly chemically and genetically evolved for selective organometallic transformations. In this review, we discuss the chemo-genetic optimization of streptavidin- and avidin-based artificial metalloenzymes for the enhancement of their catalytic activities towards a wide range of synthetic transformations. Considering the high impact in vivo applications of artificial metalloenzymes, their catalytic efficacies to promote abiological reactions in intracellular as well as periplasmic environment are also discussed. Overall, this review can provide an insight to readers regarding the design and systematic optimization of strept(avidin)-based artificial metalloenzymes for specific reactions.

Assembly of ruthenium complexes on double stranded DNA using mismatch binding ligands

Herein, we show two new DNA binding small molecules, NCD-RO and NCD-RC, and their ability to bind and selectively assemble ruthenium complexes on G-G mismatch DNA. This study used a naphthyridine carbamate dimer (NCD) as an efficient scaffold to assemble metal complexes in a controlled manner on dsDNA.