**5. Cytotoxic activities**

The anticancer activities of benzimidazole salts **2a-j** and Ag(I) complexes **3a-j** were investigated against breast cancer MCF-7, MDA-MB-231 cells. The results are listed in **Table 3**. The cytotoxicity of **3i** and **3f** was significantly higher against MCF7 cells as shown in **Table 3** with IC50 values of 0.68 and 0.6 mg/ml, respectively, than its activity against MDA-MB-231 cells. Additionally, compound


#### *Carbene*

*Values are mean value ± standard deviation of three different replicates. The concentration was 30 mg, NT: not tested, NA: not active.*

#### **Table 3.**

*Anticancer activities of synthesized benzimidazoles salts 2a-j and Ag(I) complexes 3a-j [27–31].*

**3j** exhibited cytotoxicity towards MCF7 and MDA-MB-231 cells equal to 2.3 and 3.4 mg/ml. whereas compounds **2a** and **2d** were not active against MCF7 and MDA-MB-231. The compounds **2f-j** had showed IC50 values higher than 100 mg/ml.

On the other hand, benzimidazolium salts (**4a-4j**) have been synthesized following our previous work [56, 57] (**Figure 5)**. The 1 H NMR spectra of the benzimidazolium salts (**4a-j**) showed an acid proton H2 which appeared as a typical singlet at 12.02, 11.80, 12.02, 11.77, 11.61, 11.79, 12.15, 12.27, 11.46 and 11.26 ppm, respectively.

The protons of the aromatic group on benzimidazolium salts (**4a-4j**) were identified in the range of 6.30–8.02 ppm. The H2' protons of the isobutyl group were seen as heptate in the range between 2.25 and 2.44 ppm. The signals resonated between 0.98 and 1.04 are assigned to protons of isobutyl group Hab on benzimidazolium salts (**4a-4j**). Further evidence for the formation of benzimidazolium salts (**4a-4j**) is provided by the peak of C2 of the carbons as typical singlets in the range 144.1–144.5 ppm. The 13C NMR spectra showed also aromatic carbons of benzimidazolium salts (**4a**-**4j**) in the range of 105.8–153.8 ppm. The terminal carbons Cab of the isobutyl group of all benzimidazolium salts (**4a-4j**) showed peaks in the

*Novel N-Heterocyclic Carbene Silver (I) Complexes: Synthesis, Structural Characterization… DOI: http://dx.doi.org/10.5772/intechopen.101950*

**Figure 5.** *Synthesis of benzimidazoles salts 4a-j.*

region 19.3–19.9 ppm. While the carbons C2' of the isobutyl group were identified between 28.6–28.9 ppm. These values are consistent with those in the corresponding literature [58].

The synthesis of Ag(I) complexes was performed in the absence of light. The reaction is carried out between benzimidazolium salt with 1 equiv. Ag2O in dichloromethane at room temperature. The Ag(I) complex was produced as a crystalline solid (**Figure 6**). The reaction was monitored by 1H NMR spectroscopy in δ-CDCl3 and demonstrated that the benzimidazolium salts were fully converted to silver complexes in moderate yields (72–93%).

The Ag(I) complexes are stable in air and moisture with high solubility in polar solvents. The formation of the silver carbene complexes was proved by the absence of an NCHN proton peak in their 1H NMR spectra, which confirms the complete conversion to Ag(I) complexes (**5a-5j**).

The successful formation of the silver carbene complexes was also indicated by the presence of the characteristic carbon (NCHN) signals in the bottom region of the field in comparison with those of the corresponding benzimidazolium salts (**4a-4j**). For example, it was observed at 186.7 ppm for complex **5j**. However, the rest of the carbon signal for the rest of the complexes was not observed. These values are in agreement with reported by Asekunowo et al. [59, 60] who have reported the synthesis of a series of monocarbon silver halides [R2NHC]-AgCl and demonstrated the effect of halide ions and solvent on the structural formulas of Ag(I) complexes. In addition, the formation of the Ag(I) complexes (**5a-5j**) was verified by the IR

**Figure 6.** *Synthesis of Ag-NHC 4a-j.*

spectra, which showed vibrations of the CN bond at 1567, 1583, 1450, 1467, 1433, 1437, 1450, 1433, 1600 cm−1, respectively.
