International Journal of Inorganic Chemistry
Volume 2014 (2014), Article ID 191054, 6 pages
http://dx.doi.org/10.1155/2014/191054

Research Article

Antimicrobial Studies of N-Heterocyclic Carbene Silver Complexes Containing Benzimidazol-2-ylidene Ligand

Yetkin Gök,¹ Yakup Sarı,¹ Senem Akkoç,² İlknur Özdemir,¹ and Selami Günal³

¹Department of Chemistry, Faculty of Science and Arts, Inönü University, 44280 Malatya, Turkey
²Department of Chemistry, Faculty of Science, Erciyes University, 38039 Kayseri, Turkey
³Department of Microbiology, Faculty of Medicine, Inönü University, 44280 Malatya, Turkey

Received 24 July 2014; Accepted 1 October 2014; Published 10 November 2014

Academic Editor: Hakan Arslan

Copyright © 2014 Yetkin Gök et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Seven novel 4-vinylbenzyl substituted N-heterocyclic carbene (NHC) silver complexes were synthesized from different benzimidazolium salts and silver (I) oxide in dichloromethane at room temperature. These new 4-vinylbenzyl substituted NHC silver complexes were characterized by spectroscopic (NMR, IR) and elemental analysis techniques. Using the agar dilution procedure, the antimicrobial activities of these synthesized new compounds were investigated against Gram (+)/(−) bacterial and fungal strains. These NHC silver complexes showed effective activities against Escherichia coli, Pseudomonas aeruginosa (Gram-negative bacterial strains), Enterococcus faecalis, Staphylococcus aureus (Gram-positive bacterial strains), and Candida tropicalis and Candida albicans (fungal strains).

1. Introduction

N-Heterocyclic carbenes (NHCs) are cyclic constructions which are generally derived from deprotonation of salts of ligands such as imidazolium, benzimidazolium, diazepinium, pyrimidium. Since isolation of first free carbene in 1991 [1], NHCs which are strong donor, low acceptor ability, and transition metal carbene complexes obtained by using carbene precursors have had a wide application area in organometallic chemistry [2–4]. Among various transition metal carbene complexes, NHC silver complexes have played a significant role in the development of N-heterocyclic carbene chemistry because of their structural diversity and their wide spread successful application as effective carbene transfer reagents in transmetallation reactions to make other NHC metal complexes, like nickel [5], copper [6], platinum [6], iridium [6], ruthenium [5], rhodium [6, 7], palladium [6–10], and gold [6, 10, 11] carbene complexes. Also, the prominent biological activity of many NHC silver complexes as antimicrobial and anticancer agents has been confirmed [12–18]. Therefore, many studies on silver complexes have been made by different research groups for many years [19–24].

Herein, we report the synthesis, characterization, and antibacterial and antifungal activity studies of seven novel symmetrically and unsymmetrically p-vinylbenzyl-substituted NHC silver complexes which were prepared starting from 1-(4-vinylbenzyl)-3-alkyl benzimidazolium salts and silver (I) oxide in dichloromethane. The characterization of the NHC silver complexes is consistent with the proposed formula. Using the agar dilution procedure, the antimicrobial activities of these compounds are investigated against Gram (+)/(−) bacterial and fungal strains. NHC silver complexes in the antimicrobial study were observed to have higher activity against fungal strains than against Gram-positive and Gram-negative bacterial strains.

2. Experimental

2.1. Materials and Methods

In this study, the synthesis of silver (I) complexes was pursued in a dark medium under an inert atmosphere. All solvents and reagents were commercially bought. The solvents used in the synthesis, diethyl ether over Na, and dichloromethane over P₄O₁₀ were distilled before they were used. The ¹H and ¹³C NMR spectra were measured on a Bruker AC300P FT spectrometer with dimethylsulfoxide [d₆] as solvent at 300.13 MHz (¹H) and 75.47 MHz (¹³C) at room temperature. All the chemical shifts (δ) were reported in ppm and referenced tetramethylsilane. Melting points (m.p.) were measured in open capillary tubes with an Electrothermal-9200 melting point apparatus. Elemental analyses (H, C, and N) were investigated by the TUBITAK analysis center.

Escherichia coli (ATCC 25922), Pseudomonas aeruginosa (ATCC 27853), Enterococcus faecalis (ATCC 29212), and Staphylococcus aureus (ATCC 29213) as bacterial strains were purchased from the American Type Culture Collection (Rockville, MD, USA). Candida tropicalis and Candida albicans as fungal strains were obtained from the Faculty of Medicine, Department of Microbiology, Ege University (Turkey). Bacterial strains were subcultured on Muller Hinton Broth (HiMedia Laboratories Pvt. Ltd., Mumbai, India) and fungal strains were also subcultured on RPMI 1640 Broth (Sigma-Aldrich Chemie GmbH Taufkirchen, Germany).

2.2. General Method for the Synthesis of 1-(4-Vinylbenzyl)-3-alkylbenzimidazol-2-ylidene Silver Complexes, 1a–g

1-(4-Vinylbenzyl)-3-alkylbenzimidazolium salts containing symmetrical and unsymmetrical groups were synthesized by reaction of 1-(4-vinylbenzyl)benzimidazole with various aryl halides in DMF according to the literature [25, 26]. Under an inert atmosphere, Ag₂O (0.5 mmol), 1-(4-vinylbenzyl)-3-alkyl benzimidazolium salt (1.0 mmol) and activated 4 Å molecular sieves were put in CH₂Cl₂ which was distilled prior to use over P₄O₁₀ (20 mL) and were stirred in darkness at ambient temperature for one day. The Schlenk-type flask used was covered with aluminum foil to avoid exposure to light. The resulting solution was filtered through celite and the solvent was removed under reduced pressure. The crude product was washed with diethyl ether which was distilled prior to use over Na ( mL) and was crystallized from dichloromethane/diethyl ether at room temperature.

2.2.1. Iyodo [1-(4-Vinylbenzyl)-3-methyl benzimidazol-2-ylidene]silver(I), 1a

1-(4-Vinylbenzyl)-3-methyl benzimidazolium salt (1.0 mmol) and Ag₂O (0.5 mmol) in dichloromethane (20 mL) were put in a Schlenk-type flask which was covered with aluminum foil. The resulting solution was stirred for 24 h. Then, it was filtered through celite and the solvent was removed under reduced pressure. The crude product was washed with diethyl ether and was crystallized from dichloromethane/diethyl ether at room temperature. Yield: 81%, m.p.: 146–148°C, FT-IR : 1445.13 cm⁻¹. ¹H NMR (300.13 MHz, DMSO-d₆), δ; 4.02 (s, 3 H, CH ₃); 5.23 and 5.81 (dd, 2 H, CH₂C₆H₄CH=CH ₂, J: 9.6 Hz); 5.69 (s, 2 H, CH ₂C₆H₄CH=CH₂); 6.65 (dd, 1 H, CH₂C₆H₄CH=CH₂, J: 11.4 Hz); 7.21–7.77 (m, 8 H, Ar–H). ¹³C NMR (75.47 MHz, DMSO-d₆), δ; 36.1 (CH₃); 52.0 (CH₂C₆H₄CH=CH₂); 112.5, 115.3, 115.6, 124.4, 124.5, 125.8, 126.3, 126.9, 127.4, 128.2, 129.6, 133.6, 134.6, 136.4, 136.6, 137.2, 137.4 and 138.0 (Ar–C and CH=CH₂); 189.0 (C –Ag). Anal. Calcd. for C₁₇H₁₆N₂AgI (481.95 g/mol): C, 42.27; H, 3.34; N, 5.80. Found: C, 42.35; H, 3.29; N, 5.76%.

2.2.2. Chloro[1-(4-vinylbenzyl)-3-benzyl benzimidazol-2-ylidene]silver(I), 1b

With a similar method to that used for compound 1a, compound 1b was prepared from 1-(4-vinylbenzyl)-3-benzyl benzimidazolium salt (1.0 mmol) and Ag₂O (0.5 mmol) in dichloromethane (20 mL). Yield: 76%, m.p.: 127–129°C, FT-IR : 1443.45 cm⁻¹. ¹H NMR (300.13 MHz, DMSO-d₆), δ; 5.29 and 5.77 (dd, 2 H, CH₂C₆H₄CH=CH ₂, J: 8.1 Hz); 5.71 (s, 2 H, CH ₂C₆H₄CH=CH₂); 5.67 (s, 2 H, CH ₂C₆H₅); 6.65 (dd, 1 H, CH₂C₆H₄CH=CH₂, J: 8.4 Hz); 7.16–7.43 (m, 13 H, Ar–H). ¹³C NMR (75.47 MHz, DMSO-d₆), δ; 53.4 (CH₂C₆H₅); 53.6 (CH₂C₆H₄CH=CH₂); 112.2, 114.8, 115.1, 124.5, 125.0, 126.3, 126.9, 127.2, 127.5, 128.6, 129.2, 129.4, 133.9, 134.2, 134.8, 135.2, 136.0, 136.1, 137.9 and 138.5 (Ar–C and CH=CH₂). Anal. Calcd. for C₂₃H₂₀N₂AgCl (466.04 g/mol): C, 59.06; H, 4.31; N, 5.99. Found: C, 59.01; H, 4.23; N, 5.97%.

2.2.3. Chloro[1-(4-vinylbenzyl)-3-(2-methylbenzyl)benzimidazol-2-ylidene]silver(I), 1c

With a similar method to that used for compound 1a, compound 1c was prepared from 1-(4-vinylbenzyl)-3-(2-methylbenzyl)benzimidazolium salt (1.0 mmol) and Ag₂O (0.5 mmol) in dichloromethane (20 mL). Yield: 86%, m.p.: 210-211°C, FT-IR : 1444.25 cm⁻¹. ¹H NMR (300.13 MHz, DMSO-d₆), δ; 2.42 (s, 3 H, CH₂C₆H₄CH ₃); 5.28 and 5.78 (dd, 2 H, CH₂C₆H₄CH=CH ₂, J: 11.1 Hz); 5.66 [s, 2 H, CH ₂C₆H(CH₃)₄]; 5.73 (s, 2 H, CH ₂C₆H₄CH=CH₂); 6.67 (dd, 1 H, CH₂C₆H₄CH=CH₂, J: 10.8 Hz); 7.01–7.53 (m, 12 H, Ar–H). ¹³C NMR (75.47 MHz, DMSO-d₆), δ; 19.7 (CH₂C₆H₄CH₃); 51.6 (CH₂C₆HCH₃); 53.5 (CH₂C₆H₄CH=CH₂); 112.2, 114.8, 124.5, 126.5, 126.7, 126.9, 127.4, 128.5, 131.0, 132.6, 133.8, 134.2, 135.5, 136.0, 137.9 and 143.4 (Ar–C and CH=CH₂). Anal. Calcd. for C₂₄H₂₂N₂AgCl (480.06 g/mol): C, 59.83; H, 4.60; N, 5.81. Found: C, 59.71; H, 4.69; N, 5.79%.

2.2.4. Chloro[1-(4-vinylbenzyl)-3-(4-methylbenzyl)benzimidazol-2-ylidene]silver(I), 1d

With a similar method to that used for compound 1a, compound 1d was prepared from 1-(4-vinylbenzyl)-3-(4-methylbenzyl)benzimidazolium salt (1.0 mmol) and Ag₂O (0.5 mmol) in dichloromethane (20 mL). Yield: 83%, m.p.: 216-217°C, FT-IR : 1439.17 cm⁻¹. ¹H NMR (300.13 MHz, DMSO-d₆), δ; 2.24 (s, 3 H, CH₂C₆H₄CH ₃); 5.25 and 5.83 (dd, 2 H, CH₂C₆H₄CH=CH ₂, J: 10.8 Hz); 5.71 (s, 2 H, CH ₂C₆H₄CH₃); 5.75 (s, 2 H, CH ₂C₆H₄CH=CH₂); 6.69 (dd, 1 H, CH₂C₆H₄CH=CH₂, J: 7.2 Hz); 7.01–8.18 (m, 12 H, Ar–H). ¹³C NMR (75.47 MHz, DMSO-d₆), δ; 21.13 (CH₂C₆H₄CH₃); 52.2 (CH₂C₆H₄CH₃); 52.1 (CH₂C₆H₄CH=CH₂); 114.8, 136.0, 112.9, 124.5, 126.5, 126.7, 126.9, 128.5, 131.0, 132.6, 133.8, 134.2, 135.5 and 137.9 (Ar–C and CH=CH₂). Anal. Calcd. for C₂₄H₂₂N₂AgCl (480.06 g/mol): C, 59.83; H, 4.60; N, 5.81. Found: C, 59.95; H, 4.66; N, 5.84%.

2.2.5. Chloro[1-(4-vinylbenzyl)-3-(2,4,6-trimethylbenzyl)benzimidazol-2-ylidene]silver(I), 1e

With a similar method to that used for compound 1a, compound 1e was prepared from 1-(4-vinylbenzyl)-3-(2,4,6-trimethylbenzyl)benzimidazolium salt (1.0 mmol) and Ag₂O (0.5 mmol) in dichloromethane (20 mL). Yield: 75%, m.p.: 230-231°C, FT-IR : 1403.38 cm⁻¹. ¹H NMR (300.13 MHz, DMSO-d₆), δ; 2.19 and 2.26 [s, 9 H, CH₂C₆H₂(CH ₃)₃]; 5.25 and 5.82 (dd, 2 H, CH₂C₆H₄CH=CH ₂, J: 10.8 Hz); 5.57 [s, 2 H, CH ₂C₆H₂(CH₃)₃]; 5.62 (s, 2 H, CH ₂C₆H₄CH=CH₂); 6.65 (dd, 1 H, CH₂C₆H₄CH=CH₂, J: 10.8 Hz); 6.93–8.10 (m, 10 H, Ar–H). ¹³C NMR (75.47 MHz, DMSO-d₆), δ; 19.8 and 21.1 [CH₂C₆H₂(CH₃)₃]; 52.1 [CH₂C₆H₂(CH₃)₃]; 52.2 (CH₂C₆H₄CH=CH₂); 112.9, 114.8, 124.5, 126.5, 126.7, 126.9, 128.5, 131.0, 132.6, 133.8, 134.2, 135.5, 136.0 and 137.9 (Ar–C ve CH=CH₂). Anal. Calcd. for C₂₆H₂₆N₂AgCl (508.09 g/mol): C, 61.25; H, 5.14; N, 5.49. Found: C, 61.33; H, 5.09; N, 5.45%.

2.2.6. Chloro[1-(4-vinylbenzyl)-3-(2,3,5,6-tetramethylbenzyl)benzimidazol-2-ylidene]silver(I), 1f

With a similar method to that used for compound 1a, compound 1f was prepared from 1-(4-vinylbenzyl)-3-(2,3,5,6-tetramethylbenzyl)benzimidazolium salt (1.0 mmol) and Ag₂O (0.5 mmol) in dichloromethane (20 mL). Yield: 73%, m.p.: 237-238°C, FT-IR : 1449.43 cm⁻¹. ¹H NMR (300.13 MHz, DMSO-d₆), δ; 2.10 and 2.19 [s, 12 H, CH₂C₆H(CH ₃)₄]; 5.23 and 5.81 (dd, 2 H, CH₂C₆H₄CH=CH ₂, J: 10.5 Hz); 5.60 [s, 2 H, CH ₂C₆H(CH₃)₄]; 5.76 (s, 2 H, CH ₂C₆H₄CH=CH₂); 6.66 (dd, 1 H, CH₂C₆H₄CH=CH₂, J: 10.5 Hz); 7.04–7.93 (m, 9 H, Ar–H). ¹³C NMR (75.47 MHz, DMSO-d₆), δ; 16.4 and 20.8 [CH₂C₆H(CH₃)₄]; 46.8 [CH₂C₆H(CH₃)₄]; 52.9 (CH₂C₆H₄CH=CH₂); 112.5, 112.9, 115.3, 124.6, 124.8, 126.9, 127.8, 131.8, 132.6, 133.3, 133.9, 134.7, 134.9, 136.2, 136.4 and 137.3 (Ar–C and CH=CH₂). Anal. Calcd. for C₂₇H₂₈N₂AgCl (522.11 g/mol): C, 61.91; H, 5.39; N, 5.35. Found: C, 61.82; H, 5.46; N, 5.28%.

2.2.7. Chloro[1,3-bis(4-vinylbenzyl)benzimidazol-2-ylidene]silver(I), 1g

With a similar method to that used in compound 1a, compound 1g was prepared from 1,3-bis(4-vinylbenzyl)benzimidazolium salt (1.0 mmol) and Ag₂O (0.5 mmol) in dichloromethane (20 mL). Yield: 79%, m.p.: 249-250°C, FT-IR : 1440.26 cm⁻¹. ¹H NMR (300.13 MHz, DMSO-d₆), δ; 5.25 and 5.83 (dd, 4 H, CH₂C₆H₄CH=CH ₂, J: 10.8 Hz); 5.76 (s, 4 H, CH ₂C₆H₄CH=CH₂); 6.69 (dd, 2 H, CH₂C₆H₄CH=CH₂, J: 10.8 Hz); 7.13–7.75 (m, 12 H, Ar–H). ¹³C NMR (75.47 MHz, DMSO-d₆), δ; 52.2 (CH₂C₆H₄CH=CH₂); 112.9, 115.3, 124.7, 127.0, 127.8, 128.1, 129.8, 133.9, 136.3, 136.5 and 137.4 (Ar–C and CH=CH₂). Anal. Calcd. for C₂₅H₂₂N₂AgCl (492.06 g/mol): C, 60.81; H, 4.49; N, 5.67. Found: C, 60.71; H, 4.55; N, 5.65%.

2.3. Antimicrobial Activity

The antimicrobial activity studies of the new synthesized NHC silver complexes were investigated according to the recommended agar dilution procedure [27, 28]. The minimal inhibitory concentrations (MICs) for each silver (I) complex were tested against standard bacterial strains (E. faecalis, S. aureus, P. aeruginosa, and E. coli) and fungal strains (C. tropicalis and C. albicans). Their turbidities matched that of a McFarland number 0.5 turbidity standard. The stock solutions of all silver complexes were prepared in dimethylsulfoxide. All dilutions were carried out using distilled water. The concentrations of the tested silver (I) complexes were 6.25, 12.5, 25, 50, 100, 200, 400, and 800 μg/mL. Fluconazole, ciprofloxacin, and ampicillin were used as the antifungal and antibacterial standard drugs. A loopful (0.01 mL) of the standardized inocula of the yeasts and bacteria (10⁶ CFUs/mL) was spread over the surface of agar plates. All the samples were inoculated after 16–20 h of incubation for bacteria and 48 h for yeasts. The lowest concentration of the NHC silver complexes that prevented visible growth was considered to be the MIC.

3. Results and Discussion

3.1. Synthesis and Characterization of N-Heterocyclic Carbene Silver Complexes, 1a–g

Different 1-(4-vinylbenzyl)-3-alkyl benzimidazolium salts as carbene precursors were synthesized [25, 26]. The intended silver (I) complexes 1a–g were obtained from the interaction of Ag₂O with benzimidazolium salts in CH₂Cl₂ at room temperature after 24 h as white solid crystals in 73–86% yields according to known methods (Scheme 1) [17, 24]. Their structures were characterized using spectroscopic and analytical techniques. The ¹H and ¹³C NMR spectra of these new complexes are consistent with the proposed formula. The spectra of these products in dimethylsulfoxide (DMSO-d₆) supported the formation of the silver complexes because of the loss of proton (NCHN) signal of the benzimidazolium salts. Compound 1a exhibited characteristic a carbenic carbon peak in the ¹³C NMR spectra as singlet at 189.0 ppm. The resonance for carbene carbon was not determined in the 1b–g complexes. This condition has been mentioned in the literature and has been given as a reason for the variable action of the NHC complexes [29–31]. The FT-IR data for NHC silver complexes exhibit a characteristic ν(C=N) band at 1445.13, 1443.45, 1444.25, 1439.17, 1403.38, 1449.43, and 1440.26 for 1a–g, respectively.

Scheme 1: Synthesis of 4-vinylbenzyl substituted NHC silver complexes 1a–g.

3.2. Antimicrobial Activity of N-(4-Vinylbenzyl) Substituted Silver Complexes, 1a–g

Using an agar dilution procedure, the antimicrobial activities of the synthesized silver (I) complexes were determined. Antimicrobial activities were found to be effective in the tested complexes (1a–g) against both fungi and bacteria strains with MIC values between 200 and 25 μgmL⁻¹. It was seen that these complexes are more effective against fungi strains than against bacteria strains. The obtained results are summarized in the Table 1. The silver carbene complexes showed effective activities against Escherichia coli, Pseudomonas aeruginosa (gram-negative bacterial strains), Enterococcus faecalis, Staphylococcus aureus (gram-positive bacterial strains), and Candida tropicalis and Candida albicans (fungal strains). Most of benzimidazolium salts containing both electron withdrawing and electron donating groups demonstrated antimicrobial activity. The 1b, 1e, and 1f silver carbene complexes which were containing benzyl, 2,4,6-trimethylbenzyl and 2,3,5,6-tetramethylbenzyl groups showed better antibacterial activity than the 1a, 1c, 1d, and 1g complexes which were including methyl, 2-methylbenzyl, 4-methylbenzyl, and 4-vinylbenzyl groups. Generally,results of this study showed that compound containing sterich effect on the nitrogen atom were more effective on antimicrobial activity. The NHC silver complexes including both symmetrical (1g) and unsymmetrical (1c, 1d) groups indicated the same activities against all bacteria and fungus. Also, these compounds (1c, 1d, and 1g) exhibited low activity for all bacteria and fungi strains. Among the NHC silver complexes, the 1a, 1b, 1e, and 1f complexes showed high activity against C. tropicalis at 25 μgmL⁻¹. As positive control, ampicillin, ciprofloxacin, and fluconazole were used. It is seen from the obtained data in this work that the substituents on the N-atoms play an important role in antimicrobial activity.

Table 1: MICs (g mL⁻¹) of silver-NHCs for test microorganisms.

Compound
E. coli
S. aureus
E. faecalis
P. aeruginosa
C. albicans
C. tropicalis

1a
50
100
100
100
25
25

1b
50
50
50
50
50
25

1c
200
200
200
200
200
200

1d
200
200
200
200
200
200

1e
50
50
50
50
25
25

1f
50
50
50
50
25
25

1g
200
200
200
200
200
200

Ampicillin
3.12
3.12
1.56
—
—
—

Ciprofloxacin
1.56
0.39
0.78
3.12
—
—

Fluconazole
—
—
—
—
3.12
3.12

4. Conclusions

The seven new 4-vinylbenzyl substituted benzimidazol-2-ylidene silver complexes were prepared by the reaction of different benzimidazolium salts with silver (I) oxide in dichloromethane and characterized using elemental analysis and spectroscopic techniques. Some of these complexes showed very good antibacterial activity, especially against Candida tropicalis and Candida albicans as the fungal strains. The obtained results imply that the tested silver complexes displayed a different effect against Escherichia coli, Pseudomonas aeruginosa (Gram-negative bacterial strains), Enterococcus faecalis, Staphylococcus aureus (Gram-positive bacterial strains), and Candida tropicalis and Candida albicans (fungal strains). 1b, 1e, and 1f compounds exhibited very good activity against all bacteria and fungi strains. Obtained results are helpful for the synthesis of NHC silver complexes possessing high antimicrobial activity.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

The authors would like to thank the Inonu University Research Fund (BAP 2011/25) for financial support.

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