Non-symmetrically substituted N-heterocyclic carbene–Ag(I) complexes of benzimidazol-2-ylidenes: Synthesis, crystal structures, anticancer activity and transmetallation studies

Cyanopropyl functionalized benzimidazolium salts and their silver N-heterocyclic carbene complexes: Synthesis, antimicrobial activity, and theoretical analysis

The reaction of N-substituted benzimidazole with 4-bromobutyronitrile gives the corresponding benzimidazolium salts as N-heterocyclic carbene (NHC) precursors. Silver(I) carbene complexes are synthesized by the reaction of the corresponding benzimidazolium salts with Ag₂ O in dichloromethane. These new NHC precursors and Ag-NHC complexes were characterized by spectroscopy techniques and also screened for their antibacterial activities against the standard bacterial strains Escherichia coli, Pseudomonas aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae, Staphylococcus aureus, methicillin-resistant Staphylococcus aureus, and Enterococcus faecalis, and the standard fungal strains Candida albicans and Candida glabrata, and promising results were achieved. The compounds were also analyzed by density functional theory (DFT)/time-dependent DFT and docking methods.

A study of structure-activity relationship and anion-controlled quinolinyl Ag(I) complexes as antimicrobial and antioxidant agents as well as their interaction with macromolecules

In this communication, we feature the synthesis and in-depth characterization of a series of silver(I) complexes obtained from the complexation of quinolin-4-yl Schiff base ligands ((E)-2-((quinolin-4-ylmethylene)amino)phenol L_a, 2-(quinolin-4-yl)benzo[d]thiazole L_b, (E)-N-(2-fluorophenyl)-1-(quinolin-4-yl)methanimine L_c, (E)-N-(4-chlorophenyl)-1-(quinolin-4-yl)methanimine L_d, (E)-1-(quinolin-4-yl)-N-(p-tolyl)methanimine L_e, (E)-1-(quinolin-4-yl)-N-(thiophen-2-ylmethyl)methanimine L_f) and three different silver(I) anions (nitrate, perchlorate and triflate). Structurally, the complexes adopted different coordination geometries, which included distorted linear or distorted tetrahedral geometry. The complexes were evaluated in vitro for their potential antibacterial and antioxidant activities. In addition, their interactions with calf thymus-DNA (CT-DNA) and bovine serum albumin (BSA) were evaluated. All the complexes had a wide spectrum of effective antibacterial activity against gram-positive and gram-negative bacterial and good antioxidant properties. The interactions of the complexes with CT-DNA and BSA were observed to occur either through intercalation or through a minor groove binder, while the interaction of the complexes with BSA reveals that some of the complexes can strongly quench the fluorescence of BSA through the static mechanism. The molecular docking studies of the complexes were also done to further elucidate the modes of interaction with CT-DNA and BSA.

Coinage Metal N-Heterocyclic Carbene Complexes: Recent Synthetic Strategies and Medicinal Applications

New weapons are constantly needed in the fight against cancer. The discovery of cisplatin as an anticancer drug prompted the search for new metal complexes. The successful history of cisplatin motivated chemists to develop a plethora of metal-based molecules. Among them, metal-N-heterocyclic carbene (NHC) complexes have gained significant attention because of their suitable qualities for efficient drug design. The enhanced applications of coinage metal-NHC complexes have encouraged a gradually increasing number of studies in the fields of medicinal chemistry that benefit from the fascinating chemical properties of these complexes. This review aims to present recent developments in synthetic strategies and medicinal applications of copper, silver and gold complexes supported by NHC ligands.

Synthesis, characterization, crystal structure, α-glycosidase, and acetylcholinesterase inhibitory properties of 1,3-disubstituted benzimidazolium salts

Chloro-/fluorobenzyl-substituted benzimidazolium salts were synthesized from the reaction of 4-fluorobenzyl/2-chloro-4-fluorobenzyl-substituted benzimidazole and chlorinated aromatic hydrocarbons. They were characterized using various spectroscopic techniques (Fourier-transform infrared and nuclear magnetic resonance) and elemental analysis. In addition, the crystal structures of the complexes 1a -d and 2b were determined by single-crystal X-ray diffraction methods. These compounds were crystallized in the triclinic crystal system with a P-1 space group. The crystal packing of all complexes is dominated by O-H⋯Cl hydrogen bonds, which link the water molecules and chloride anions, forming a chloride-water tetrameric cluster. These synthesized salts were found to be effective inhibitors for α-glycosidase and acetylcholinesterase (AChE), with K_i values ranging from 45.77 ± 6.83 to 102.61 ± 11.56 µM for α-glycosidase and 0.94 ± 0.14 to 10.24 ± 1.58 µM for AChE. AChE converts acetylcholine into choline and acetic acid, thus causing the return of a cholinergic neuron to its resting state. Discovering AChE and α-glycosidase inhibitors is one of the important ways to develop new drugs for the treatment of Alzheimer's disease and diabetes.

Synthesis of N-Aryl- and N-alkyl-Substituted Imidazolium Silver Complexes: Cytotoxic Screening by Using Human Cell Lines Modelling Acute Myeloid Leukaemia

A series of N-aryl- and N-alkyl substituted imidazoles has been synthesised and complexed with Ag⁺ to obtain silver-NHC complexes of the form [Ag(NHC)₂ ]X. These silver-NHC complexes were tested in vitro against the human cell lines HL-60 and MOLM-13, which both model acute myeloid leukaemia (AML). A substantial difference in cytotoxicity was revealed varying in the range 13-4 μM and 22-9 μM for HL-60 and MOLM-13, respectively. Furthermore, this study revealed that when an alkyl group is installed on the imidazole scaffold, its position substantially influences the cytotoxicity of the corresponding silver NHC complex.

Recent Developments in the Medicinal Applications of Silver-NHC Complexes and Imidazolium Salts

Because of their great structural diversity and multitude of chemical properties, N-heterocyclic carbenes (NHCs) have been utilized in a variety of capacities. Most recently, NHCs have been utilized as carrier molecules for many transition metals in medicinal chemistry. Specifically, Ag(I)-NHCs have been investigated as potent antibacterial agents and chemotherapeutics and have shown great efficacy in both in vitro and in vivo studies. Ag(I)-NHC compounds have been shown to be effective against a wide range of both Gram-positive and Gram-negative bacterial strains. Many compounds have also shown great efficacy as antitumor agents demonstrating comparable or better antitumor activity than standard chemotherapeutics such as cisplatin and 5-fluorouracil. While these compounds have shown great promise, clinical use has remained an unattained goal. Current research has been focused upon synthesis of novel Ag(I)-NHC compounds and further investigations of their antibacterial and antitumor activity. This review will focus on recent advances of Ag(I)-NHCs in medicinal applications.

A comparative insight into the bioactivity of mono- and binuclear silver(I)-N-heterocyclic carbene complexes: synthesis, lipophilicity and substituent effect

Silver(I)-N-heterocyclic carbene (Ag(I)-NHC) complexes of mononuclear and binuclear species, synthesised by our group and others in recent years, offer a fascinating outlook on their bioactivity. These complexes advance a range of adaptable structural patterns, leading to intra-specific variation in anticancer and antimicrobial activities. This study therefore reviews the synthesis, structural analysis and bioactivity of Ag complexes derived from mononuclear-NHC complexes either with coordinating or non-coordinating anions and binuclear NHC complexes. Specifically, the effect of stability, chain lengths and lipophilicity on the biological activity of recently reported Ag(I)-NHC complexes is reviewed. These complexes can be further explored as novel antibacterial and anticancer drugs in the nearest future.

Mono-, bi- and multi-nuclear silver complexes constructed from bis(benzimidazole)-2-oxapropane ligands and methacrylate: syntheses, crystal structures, DNA-binding properties and antioxidant activities

Three mono-, bi- and multi-nuclear Ag(i) complexes have been synthesized and characterized systematically. The complexes bind to DNA in a intercalate mode. Complex2has the potential ability to scavenge hydroxyl radicals inin vitrostudies.

Synthesis, crystal structures, and in vitro anticancer properties of new N-heterocyclic carbene (NHC) silver(i)- and gold(i)/(iii)-complexes: a rare example of silver(i)–NHC complex involved in redox transmetallation

A series of cationic, linearly coordinated silver(i)– and gold(i/iii)–NHC complexes of (benz)imidazol-2-ylidene ligands was prepared and successfully characterized.

Macrophage and colon tumor cells as targets for a binuclear silver(I) N-heterocyclic carbene complex, an anti-inflammatory and apoptosis mediator

Chronic inflammation intensifies the risk for malignant neoplasm, indicating that curbing inflammation could be a valid strategy to prevent or cure cancer. Cancer and inflammation are inter-related diseases and many anti-inflammatory agents are also used in chemotherapy. Earlier, we have reported a series of novel ligands and respective binuclear Ag(I)-NHC complexes (NHC=N-heterocyclic carbene) with potential anticancer activity. In the present study, a newly synthesized salt (II) and respective Ag(I)-NHC complex (III) of comparable molecular framework were prepared for a further detailed study. Preliminarily, II and III were screened against HCT-116 and PC-3 cells, wherein III showed better results than II. Both the compounds showed negligible toxicity against normal CCD-18Co cells. In FAM-FLICA caspase assay, III remarkably induced caspase-3/7 in HCT-116 cells most probably by tumor necrosis factor-alpha (TNF-α) independent intrinsic pathway and significantly inhibited in vitro synthesis of cytokines, interleukin-1 (IL-1) and TNF-α in human macrophages (U937 cells). In a cell-free system, both the compounds inhibited cyclooxygenase (COX) activities, with III being more selective towards COX-2. The results revealed that III has strong antiproliferative property selectively against colorectal tumor cells which could be attributed to its pro-apoptotic and anti-inflammatory abilities.

Recent developments of metal N-heterocyclic carbenes as anticancer agents

Metal based anticancer drugs have demonstrated their crucial role in preventing all types of cancers whereas their effectiveness is selective with respect to the cancer cells rather than the normal cells. Recently metal N-heterocyclic carbenes have established their selective performance for cancer cells excluding normal healthy cells based on which they are widely utilised for targeting cancer cells specifically which leads to cell death or cell growth inhibition. This is mainly due to their ionic character which helps them to localise in cancer cells with the help of enhanced expression of Organic Cation Transporters (OCT). Also their unique mechanism of action involving DNA binding, less recognizable by DNA repair machinery, mitochondria targeting gives them a new area for anticancer drug development. This review summarises the medicinal as well as pharmacological approach to the anticancer properties of metal NHC complexes.

Potential of silver against human colon cancer: (synthesis, characterization and crystal structures of xylyl (Ortho, meta, &Para) linked bis-benzimidazolium salts and Ag(I)-NHC complexes: In vitro anticancer studies)

BACKGROUND

Since the first successful synthesis of Ag(I)-N-heterocyclic carbene complex in 1993, this class of compounds has been extensively used for transmetallation reactions where the direct synthesis using other metal ions was either difficult or impossible. Initially, silver(I)-NHC complexes were tested for their catalytic potential but could not get fame because of lower potential compare to other competent compounds in this field; however, these compounds proved to have vital antimicrobial activities. These encouraging biomedical applications further convinced researchers to test these compounds against cancer. The current work has been carried out with this aim.

RESULTS

N-ipropylbenzimidazole was synthesized by reaction of benzimidazole with ipropyl bromide. The subsequent treatment of the resulting N-alkylbenzimidazole with ortho/meta/para-(bromomethylene) benzene afforded corresponding bis-benzimidazolium bromides (5-7). The counter anion (Br-) of each salt was replaced by hexaflourophosphate (PF6-) for the ease of handling and further purification (8-10). Each salt (Ligand), in halide form, was further allowed to react with Ag2O with stirring at room temperature for a period of two days to synthesize dinuclear Ag(I)-NHC complexes (11-13). All synthesized compounds were characterized by spectroscopic techniques and microanalysis. Molecular structures of compounds 5, 9 &10 were established through single crystal x-ray diffraction technique. All the compounds were assessed for their anti-proliferation test on human colorectal cancer cell line (HCT 116). Results showed that the ligands (5-10) showed mild to negligible cytotoxicity on HCT 116 cells whereas respective silver complexes (11-13) exhibited dose dependent cytotoxicity towards the colon cancer cells with IC50 ranges between 9.7 to 44.5 μM. Interestingly, the complex 13 having para-xylyl spacer was found the most active (IC50 9.7 μM) that verifies our previously reported results.

CONCLUSIONS

All the bis-benzimidazolium salts (8-10) were found inactive whereas after bonding with silver cations, the Ag(I)-NHC complexes (11-13) showed a dose dependent cytotoxic activity. This proved that silver practice an important role in death of cancer cells. Also, the N-alkyl/aryl substitutions and ortho/metal/para xylyl units regulate the cytotoxicity.