**Abstract**

Nowadays, N-heterocyclic carbene-based silver-complexes Ag(I) have been widely used as an organometallic drug candidate in medicinal and pharmaceutical chemistry researches due to their low toxicity. Due to the success of Ag(I) complexes in biological applications, interest in the synthesis and applications of such compounds is increasing rapidly. Therefore, in this study, a series of unsymmetrical N,N-disubstituted benzimidazolium salts were synthesized as N-heterocyclic carbene (NHC) (2a-2j). The interaction of these benzimidazolium salts having their two nitrogen atoms substituted by bulky groups with Ag2O in DMF has been carried out to afford Ag(I) complexes and characterized by 1 H NMR, 13C NMR, FT-IR and elemental analyses. The antimicrobial activity of Ag(I) complexes was tested against some standard culture collections of Gram-negative, Gram-positive bacterial strains and Fungal strains, which are the most frequently isolated among the society and hospital-acquired infectious microorganisms as potential metallopharmaceutical agents. The Ag-NHC complexes showed effective antimicrobial activity against microorganisms with MIC values between 0.0024 and 1.25 mg/ml. Moreover, these Ag-NHC complexes exhibited significant antioxidant activities. In addition, of benzimidazoles salts 2,4 and Ag(I) complexes 3,5 were screened for their antitumor activity. The highest antitumor activity was observed for **3e** and **3d** Complexes.

**Keywords:** N-heterocyclic carbene, benzimidazolium salts, silver (I)-NHC complexes, antimicrobial, antioxidant and antitumor activities

#### **1. Introduction**

N-Heterocyclic carbenes (NHCs) are nitrogen-based heterocyclic compounds containing a divalent carbon atom. Previously, many researchers tried numerous synthetic methods to isolate the stable NHCs, but they were not successful until the first stable free-carbene was isolated in 1991 as a crystal solid by Arduengo and coworkers [1]. Since then, the number of studies in carbene chemistry has

increased considerably, and has become stable in research laboratories throughout the world. Today, NHCs are one of the important classes of ligands for coordination chemistry. NHCs have strong σ-donating but, weak π-accepting properties, which show excellent support to stabilize various oxidation states of transition-metal. Also, they can provide steric and electronic properties for the optimal design of transition-metal complexes [2–8]. The modification at the nitrogen atoms of the NHCs significantly influence the reactivity and binding affinity of the ligand; thus, NHCs make the strong metal-carbon bond with different metals. Transition-metal complexes of NHCs are used as strong-, reactive- and selective-catalysts in many chemical reactions. Initially, the metal-NHC complexes were used extensively as a catalyst in organic transformations such as C-C, C-heteroatom cross-couplings, and C-H functionalization [9–12]. Also, in recent years, transition metal-NHC complexes containing Au, Pd, Cu, Ru, Pt, Ag, Rh metals have been widely used in medicine and pharmacy as the potential metallopharmaceutical agents against AMR [13–16]. Although, most of the organometallic drug research has focused on platinum- and gold-containing compounds, carbene-based silver-compounds stand out in the class of organometallic drugs owing to their low toxicity, easy synthesis, stability and limited possibility of side effects. Ag(I) complexes possess several properties, ranging from antibacterial, anticancer, anti-inflammatory and antiseptic to antineoplastic activity [17]. Ag(I) complexes have been recently at the focal point with increased attention due to their usually strong antimicrobial and anticancer properties, and have more effective than other transition-metal complexes, and also, they have low toxicity for humans. Ag(I) complexes also promise to be agents capable of overcoming AMR and beating antibiotic resistant bacteria, fungi and parasites [18–34]. Heterocyclic molecules are an important family of organic chemistry with a wide range of applications [35]. Although, this family is generally known for its agrochemical and veterinary applications, it is also used as a corrosion inhibitor, sanitizer, and dyestuff [36]. Many heterocyclic molecules such as favipiravir have also important pharmaceutical applications [37, 38]. For example, ribavirin is an N-heterocyclic molecule that is used in the treatment of COVID-19 [39]. The reasonable results obtained from bioactivity studies have enabled them to be a family that is frequently used in pharmaceutical chemistry [40]. NHCs, which are known for their high catalytic activity, are easily synthesized and modified [41, 42]. NHC metal complexes have become a popular research area with the frequent usage of metals in drug molecules. In our previous works, we concluded that the presence of electron-donating and bulky substituents attached to the nitrogen of the carbene ligand increases the antimicrobial activity of the silver complexes. These exciting results have led us to further investigate the antimicrobial properties of silver-NHCs. In this regard, herein, we now report the synthesis of novel NHC salts and their Ag(I) complexes and investigate their antimicrobial, antioxidant and cytotoxic activities. All salts and complex structures were characterized by elemental analysis, Fourier transforms infrared (FTIR), 1H and 13C nuclear magnetic resonance (NMR) spectroscopies.
