Novel Nonsymmetrically p-Benzyl-Substituted (Benz)imidazole N -Heterocyclic Carbene-Silver(I) Acetate Complexes: Synthesis and Biological Evaluation

Nonsymmetrically substituted N-heterocyclic carbene (NHC) precursors 1a–d and 3a–d were synthesised by first reacting 1H(benz)imidazole with p-cyanobenzyl bromide to give 4-(1H-imidazole-1-ylmethyl)benzonitrile (1) and 4-(1H-benzimidazole-1ylmethyl)benzonitrile (3) and afterwards introducing benzyl bromide, 1-(bromomethyl)-4-methylbenzene, 1-(bromomethyl)-4methoxybenzene, and methyl 4-(bromomethyl)benzoate. The NHC-silver(I) acetate complexes (1-benzyl-3-(4-cyanobenzyl)-2,3dihydro-1H-imidazole-2-ylidene) silver(I) acetate (2a), (1-(4-cyanobenzyl)-3-(4-methylbenzyl)-2,3-dihydro-1H-imidazole-2ylidene) silver(I) acetate (2b), (1-(4-cyanobenzyl)-3-[4-(methoxycarbonyl)benzyl]-2,3-dihydro-1H-imidazole-2-ylidene) silver(I) acetate (2c), (1-benzyl-3-(4-cyanobenzyl)-2,3-dihydro-1H-benzimidazole-2-ylidene) silver(I) acetate (4a), (1-(4-cyanobenzyl)3-(4-methylbenzyl)-2,3-dihydro-1H-benzimidazole-2-ylidene) silver(I) acetate (4b), (1-(4-cyanobenzyl)-3-(4-methoxybenzyl)2,3-dihydro-1H-benzimidazole-2-ylidene) silver(I) acetate (4c), and (1-(4-cyanobenzyl)-3-[4-(methoxycarbonyl)benzyl]-2,3dihydro-1H-benzimidazole-2-ylidene) silver(I) acetate (4d) were yielded by reacting these NHC precursors with silver(I) acetate. The silver(I) acetate complex 4b was characterised by single crystal X-ray diffraction. Preliminary in vitro antibacterial studies against the Gram-positive bacteria Staphylococcus aureus and the Gram-negative bacteria Escherichia coli, using the Kirby-Bauer disc diffusion method, were carried out on the seven NHC-silver(I) acetate complexes 2a–c and 4a–d. Also the IC50 values of these seven complexes were determined by an MTT-based assay against the human renal cancer cell line Caki-1. The complexes 2a–c and 4a–c revealed the following IC50 values, respectively, 25 (±1), 15 (±2), 5.4 (±0.8), 16 (±2), 7.1 (±1), 20 (±4), and 14 (±1) μM.


Introduction
N-Heterocyclic carbenes (NHCs) are versatile ligands in silver complexes exhibiting antimicrobial activity, in particular for the possible treatment of pulmonary infections accompanying cystic fibrosis (CF) and chronic lung infections [1][2][3] and maybe in the treatment of cancer [4].Youngs' research group have reported antimicrobial activity of NHC-silver complexes derived from 1H-imidazole, 4,5dichloro-1H-imidazole and xanthines against a panel of highly resistant pathogens recovered from the respiratory tract of cystic fibrosis (CF) patients [1,3,5].Another important contribution by the Ghosh research group led to the synthesis and antimicrobial evaluation of NHC-silver complexes derived from 1-benzyl-3-tert-butylimidazole [6].In addition, in vitro and murine in vivo efficacy and toxicity studies of nebulised methylated caffeine-silver(I) complex (SCC1), for treatment of pulmonary infections [7].Recently, a larger number of known compounds were evaluated as potential antibiotics by Roland et al. [8].
Within this paper we present a new series of nonsymmetrically p-benzyl-substituted N-heterocyclic carbenesilver acetate complexes derived from imidazole and benzimidazole, their synthesis, cytotoxicity, and antibacterial studies.

Experimental
2.1.General.All reactions were carried out under aerobic conditions.All silver(I) acetate reactions were carried out under exclusion of light.1H-Imidazole, 1Hbenzimidazole, benzyl bromide, 4-methylbenzyl bromide, methyl 4-(bromomethyl)benzoate, 4-cyanobenzyl bromide, silver(I) acetate, and K 2 CO 3 were procured commercially from Sigma-Aldrich Chemical Company and were used without further purification.IR spectra were recorded on a Perkin-Elmer Paragon 1000 FT-IR spectrometer employing a KBr disc.NMR spectra were measured either on a Varian 300 MHz or 400 MHz spectrometer.All chemical shifts are reported in ppm and referenced to TMS.ESI MS was performed on a quadrupole tandem mass spectrometer (Quattro Micro, Micromass/Water's Corp., USA), using solutions in 100% MeOH.MS spectra were obtained in the ES+ (electron spray positive ionisation) mode for all compounds.CHN Analysis was carried out in an Exeter Analytical CE-440 elemental analyzer.Crystal Data was collected using an Agilent Technologies (former Oxford Diffraction) SuperNova diffractometer fitted with an Atlas detector.4b was measured with Mo-Kα (0.71073 Å) at 100 K.A four times redundant dataset was collected, assuming that the Friedel pairs are not equivalent.An analytical absorption correction based on the shape of the crystal was performed [20].The structure was solved by direct methods using SHELXS-97 [21] and refined by full matrix least-squares on F 2 for all data using SHELXL-97 [21].Hydrogen atoms were added at calculated positions and refined using a riding model.Their isotropic thermal displacement parameters were fixed to 1.2 (1.5 for methyl groups) times the equivalent ones of the parent atom.Anisotropic thermal displacement parameters were used for all nonhydrogen atoms.A suitable crystal of 4b was grown in a saturated solution of chloroform with slow infusion of pentane.Further details about the data collection are listed in Table 1.
CCDC 853680 (for 4b) contains the supplementary crystallographic data for this paper.These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/cgi-bin/catreq.cgi/.

Antibacterial Studies.
The silver(I) acetate complexes were screened in preliminary in vitro antibacterial tests against two bacterial strains.The test organisms included Staphylococcus aureus (SA) (NCTC 7447) as a Gram-positive bacteria and Escherichia coli (E.coli) as Gram-negative bacteria.
To assess the biological activity of compounds 2a-c and 4a-d, the qualitative Kirby-Bauer disk-diffusion method was applied [22].All bacteria were individually cultured from a single colony in sterile LB medium [23] overnight at 37 • C in an orbital shaker incubator.All the work carried out was performed under sterile conditions.
For each strain, 70 μL of culture was spread evenly on agar-LB medium.Four 5 mm diameter Whatman paper discs were placed evenly separated on each plate.Two stock solutions (9 : 1 DMSO : H 2 O) of every compound were prepared at 2.2 μM and 4.4 μM to be able to test the effect of different concentrations.Each plate was then tested with 5 μL and 7 μL of 2.2 μM solution and 5 μL and 10 μL for the 4.4 μM solution.The plates were covered and placed in an incubator at 37 • C for 24 h.The plates were then removed and the area of clearance, which is defined as the distance between the edge of the filter paper disc and the beginning of the bacterial growth, was measured for each sample in mm.

Cytotoxicity Studies.
Preliminary in vitro cell tests were performed on the human cancerous renal cell line Caki-1 in order to compare the cytotoxicity of the compounds presented in this paper.This cell line was chosen based on its regular and long-lasting growth behaviour, which is similar to the one shown in kidney carcinoma cells.The cells were obtained from the ATCC (American Tissue Cell Culture Collection) and maintained in Dulbecco's Modified Eagle medium containing 10% (v/v) FCS (fetal calf serum), 1% (v/v) penicillin streptomycin, and 1% (v/v) Lglutamine.Cells were seeded in 96-well plates containing 200 μL microtitre wells at a density of 5,000 cells/200 μL of medium and were incubated at 37 • C for 24 h to allow for exponential growth.Then the compounds used for the testing were dissolved in the minimal amount of DMSO (dimethylsulfoxide) possible and diluted with medium to obtain stock solutions of 5 × 10 −4 M in concentration and less than 0.7% of DMSO.The cells were then treated with varying concentrations of the compounds and incubated for 48 h at 37 • C.Then, the solutions were removed from the wells, and the cells were washed with PBS (phosphate buffer solution) and fresh medium was added to the wells.Following a recovery period of 24 h incubation at 37 • C, individual wells were treated with 200 μL of a solution of MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) in medium.The solution consisted of 22 mg of MTT in 40 mL of medium.The cells were incubated for 3 h at 37 • C. The medium was then removed, and the purple formazan crystals were dissolved in 200 μL DMSO per well.For all tests, cells with low passage numbers were used.A Wallac Victor (Multilabel HTS Counter) Plate Reader was used to measure absorbance at 540 nm.Cell viability was expressed as a percentage of the absorbance recorded for control wells.The values used for the dose response curves represent the values obtained from four consistent MTTbased assays for each compound tested.

Results and Discussion
In Schemes 1 and 2, the synthetic routes for the asymmetric substituted N-heterocyclic carbene ligand precursors as well as their corresponding silver(I) acetate complexes are given.The initial precursors 4-(1H-imidazole-1-ylmethyl)benzonitrile ( 1 precursors of 1a-d and 3a-d carbene carbons resonance at the range 144.3-137.9ppm, respectively, which further demonstrates the formation of expected NHC-silver(I) acetate complexes.Also the appearance of the 13 C NMR resonances for the carbonyl and methyl carbons of the acetate group of complexes 2a-c and 4a-d in the range 176.5-156.2and 23.3-21.1 ppm, respectively, showed the formation of the NHC-silver(I) acetate complexes [1,3].Furthermore, positive-mode ESI mass spectra of all six NHC-silver(I) acetate complexes (2a-c, 4a-d) are dominated by [M + -O 2 CCH 3 ] fragment peaks arising from the loss of one acetate ligand.
3.1.Structural Discussion.The crystal of 4b was formed in a saturated solution of chloroform with slow infusion of pentane.It crystallized in the monoclinic space group P2 1 /n (no.14) in absence of any solvent molecules.The crystal data and refinements are found in Table 1, whereas selected bond lengths and bond angles are compiled in Table 2.The X-ray structure shows that the benzimidazole ring is planar and the bond lengths and angles in the five-membered imidazole ring (NCNCC) are in good agreement with those in similar compounds reported earlier by our group [13][14][15][16].Complex 4b crystallises in two different species, which are shown in Figures 1 and 2.
The major species (95%, Figure 1) shows a nearly linear bond angle of 170.76 (7) • at the two-coordinated silver atom.
The bond distance Ag(1)-C(9) of 2.060 (2) Å agrees very well with previously reported examples of this compound class [13][14][15][16][17] and shows that the silver is strongly bonded to the carbene.The Ag(1)-O(1) bond length of 2.1088 (14) Å is quite short compared to the ones our group previously reported but is still within range of the corresponding ones in similar molecules (see e.g., [26,27]).This short distance suggests a predominantly covalent character of the Ag(1)-O(1) bond.
In comparison to this, the minor species (5%, Figure 2), where the silver atom is bonded to both oxygen atoms of the acetate group, the bond lengths to the oxygen atoms are much longer (Ag(2)-O(1): 2.408(6) Å, and Ag(2)-O(2): 2.554(5) Å).Furthermore the bond angle C(9)-Ag(2)-O( 1) is no longer linear with 135.9(4) • and the C(9)-Ag(2)-O(2) angle is found to be 164.9(3)• .This bonding pattern is characteristic for a predominantly ionic coordination of the acetate to the silver.Both of these modes have been observed earlier (see e.g., [17]).Having both of them in the same structure suggests a rather small energy difference between the two forms.

Antibacterial Testing.
Using the Kirby-Bauer disk diffusion method, the in vitro antibacterial activity of the NHC-silver(I) acetate complexes was tested and summarised in Figures 3 and 4 to prepare the complexes and the solvent (DMSO) used to prepare the stock solutions played no role in growth inhibition on the same bacteria as previously reported [17,28].Compounds 2b and 2c showed almost no antibacterial activity against the Gram-positive bacteria Staphylococcus aureus, and compound 4d showed the best activity against this bacterial strain, but, in comparison to previous reported NHC-silver(I) acetate complexes, the activity is more in a medium range.Hereby, an area of clearance of 0 mm is considered as no activity, areas of 1-4 mm as low, 5-9 as medium, and areas of clearance ≥10 mm as high activity.Low antibacterial activity was observed for complexes 2c and 4b against the Gram-negative bacteria Escherichia coli and all other complexes (2a, 2b, 4a, 4c, and 4d) exhibited only medium activity against this bacteria strain.It has been shown that there is no difference between the cytotoxic activities of the two compound classes, since all compounds show a good level of activity.The solubility of all compounds in DMSO was good, and they are stable in saline solution with respect to silver chloride precipitation.

Conclusion and Outlook
In summary, a series of seven nonsymmetrically p-cyanobenzyl-substituted NHC-silver(I) acetate complexes 2a-c and 4a-d were synthesised by reacting appropriate nonsymmetrically p-cyanobenzyl-substituted N-heterocyclic carbenes with silver(I) acetate.The preliminary antibacterial activity of the NHC-silver(I) acetate complexes was tested in vitro against two bacterial strains, where complex 4d showed superior activity against Staphylococcus aureus and complex 2b against Escherichia coli.Against the renal cancer cell line Caki-1, the NHC-silver(I) acetate complexes 2a-c and 4a-d yielded IC 50 values of 25 (±1), 15 (±2), 5.4 (±0.8), 16 (±2), 7.1 (±1), 20 (±4), and 14 (±1) μM, respectively.The complex 2c, however, gave a superior IC 50 value of 5.4 (±0.8) μM.Further work is currently underway to improve these results by varying the substituents on the imidazole ring in order to enhance stability and on the benzimidazole nitrogen atoms in order to improve biological activity.Overall, this should lead to enhanced solubility, stability, and activity in biological media and enable in vivo testing of a NHC-silver(I) acetate in the nearby future.

Scheme 2 :
Scheme 2: General reaction scheme for the synthesis of benzimidazole containing asymmetric substituted NHC precursors 3a-d and their corresponding NHC-silver(I) acetate complexes 4a-d.

Figure 1 :Figure 2 :Figure 3 :
Figure 1: X-ray diffraction structure of 4b showing the major occupied Ag position; thermal ellipsoids are drawn on the 50% probability level.

Table 1 :
Crystal data and structure refinement for 4b.