N-Phenyl-substituted carbene precursors and their silver complexes: Synthesis, characterization and antimicrobial activities

Article abstract of DOI:10.1002/aoc.3116

A series of unsymmetrically substituted N-heterocyclic carbene (NHC) precursors (1a, 1b, 1c, 1d, 1e) were synthesized from the reaction of N-phenylbenzimidazole with various alkyl halides. These compounds were used to synthesize NHC-silver(I) complexes (2a, 2b, 2c, 2d, 2e). The five new 1-phenyl-3-alkylbenzimidazolium salts (1a, 1b, 1c, 1d, 1e) and their NHC-silver complexes (2a, 2b, 2c, 2d, 2e) were characterized by the ¹H NMR, ¹³C NMR and FT-IR spectroscopic methods and elemental analysis techniques. Also, the two NHC-silver complexes 2b and 2c were characterized by single-crystal X-ray crystallography, which confirmed the linear C - Ag - Cl arrangements. The antibacterial activities of the NHC precursor and NHC-silver complexes were tested against three Gram-positive bacterial strains (Bacillus subtilis, Listeria monocytogenes and Staphylococcus aureus) and three Gram-negative bacterial strains (Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa) using the microdilution broth method. The NHC-silver complexes showed higher antibacterial activity than the NHC precursors. In addition, silver complexes 2a, 2b, 2c, 2d showed high antibacterial activity against the Gram-positive bacteria L. monocytogenes and S. aureus compared to the standard, tetracycline. Copyright

Full text of DOI:10.1002/aoc.3116

Full Paper
Received: 2 September 2013
Revised: 21 November 2013
Accepted: 25 November 2013
Published online in Wiley Online Library: 30 January 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3116
N-Phenyl-substituted carbene precursors and
their silver complexes: synthesis, characterization
and antimicrobial activities
Yetkin Göka*, Senem Akkoçb*, Sevil Albayrakc, Mehmet Akkurtd
and Muhammad Nawaz Tahire
A series of unsymmetrically substituted N-heterocyclic carbene (NHC) precursors (1a–e) were synthesized from the reaction of
N-phenylbenzimidazole with various alkyl halides. These compounds were used to synthesize NHC–silver(I) complexes (2a–e).
The five new 1-phenyl-3-alkylbenzimidazolium salts (1a–e) and their NHC–silver complexes (2a–e) were characterized by the
1H NMR, 13C NMR and FT-IR spectroscopic methods and elemental analysis techniques. Also, the two NHC–silver complexes
2b and 2c were characterized by single-crystal X-ray crystallography, which confirmed the linear C―Ag―Cl arrangements.
The antibacterial activities of the NHC precursor and NHC–silver complexes were tested against three Gram-positive bacterial
strains (Bacillus subtilis, Listeria monocytogenes and Staphylococcus aureus) and three Gram-negative bacterial strains
(Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa) using the microdilution broth method. The NHC–silver
complexes showed higher antibacterial activity than the NHC precursors. In addition, silver complexes 2a–d showed high
antibacterial activity against the Gram-positive bacteria L. monocytogenes and S. aureus compared to the standard, tetracy-
cline. Copyright © 2014 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: N-heterocyclic carbene; benzimidazolium salt; silver complex; antimicrobial activity; X-ray diffraction
In this study, a new series of phenyl-substituted NHC precur-
sors and their NHC–silver complexes were synthesized and
Introduction
characterized by elemental analysis, FT-IR, NMR and X-ray
diffraction (for 2b and 2c) methods. The antibacterial activities
of the NHC precursors and NHC–silver complexes were tested
against three Gram-positive bacterial strains – Bacillus subtilis,
Listeria monocytogenes, Staphylococcus aureus – and three
Gram-negative bacterial strains – Escherichia coli, Klebsiella
pneumoniae, Pseudomonas aeruginosa – by using the microdilution
broth method.
N-Heterocyclic carbenes (NHCs) are stable singlet carbenes that can
act as excellent donor ligands towards 52 elements, except Sc, Tc
and Cd, in the periodic table.[1] Since the first report by Öfele in
1968, and the first isolation by Arduengo in 1991, NHCs have
attracted much attention.[2–10] NHC–metal complexes have higher
stability towards oxygen, heat and moisture than phosphine
metal-based complexes, which makes them quite attractive as
phosphine substitutes.[11] NHCs are most frequently prepared via
deprotonation of the corresponding azolium salts such as
benzimidazolium, tetrazolium, triazolium and imidazolium, and
have been extensively used as organocatalysts and ancillary ligands
in transition metal-catalysed reactions.[12–17] Using these ligands,
their metal complexes have shown high catalytic activity in metath-
esis, the creation of furan, polymerization, hydrosilylation, hydroge-
nation and coupling reactions.[18–33]
Recently, NHC–silver complexes have attracted particular inter-
est because of their wide use as ligand transfer agents to group
8–10 metals, catalysis, nanomaterials and for their different bio-
logical activity as antimitochondrial, antimicrobial and anticancer
agents.[1,34–50] Thus NHC–silver complexes have had a significant
role in the rapid development of NHC–metal complexes. One
important reason for this is that NHC–silver complexes are easily
synthesized by three different methods: (i) reaction of azolium
salts with silver bases such as Ag2CO3, AgOAc and Ag2O; (ii) reac-
tion of free NHC–carbene salts; and (iii) reaction of azolium salts
with silver salts under basic phase transfer conditions.[46,51,52]
Among these approaches, route (i) is more convenient and the
most frequently used.
* Correspondence to: Senem Akkoç, Department of Chemistry, Faculty of Sciences,
Erciyes University, 38039 Kayseri, Turkey. E-mail: senemakkoc@erciyes.edu.tr;
Yetkin Gök, Department of Chemistry, Faculty of Arts and Sciences, Inönü Univer-
sity, 44280, Malatya, Turkey. E-mail yetkin.gok@inonu.edu.tr
a Department of Chemistry, Faculty of Arts and Sciences, Inönü University,
44280, Malatya, Turkey
b Department of Chemistry, Faculty of Sciences, Erciyes University, 38039,
Kayseri, Turkey
c
Department of Biology, Faculty of Sciences, Erciyes University, 38039, Kayseri,
Turkey
d Department of Physics, Faculty of Sciences, Erciyes University, 38039, Kayseri,
Turkey
e Department of Physics, Sargodha University, Sargodha, Pakistan
Appl. Organometal. Chem. 2014, 28, 244–251
Copyright © 2014 John Wiley & Sons, Ltd.

N-Phenyl substituted carbene precursors and their silver complexes: sy
Experimental
Table 1. Crystallographic data and structure refinement for 2b and 2c
Compound
2b
2c
General Considerations
Empirical formula
Formula weight
Crystal system
Space group
C
25H26AgClN2
497.80
C23H22AgClN2O3
517.75
All reactions for the preparation of N-phenyl-substituted
benzimidazolium salts (1a–e) and their NHC–silver complexes
(2a–e) were carried out under argon in flame-dried glassware using
standard Schlenk techniques. Solvents were purified and degassed
by standard procedures. Chemicals were purchased from Aldrich
and Merck. All 1H and 13C NMR were performed in CDCl3 and
DMSO. The 1H and 13C NMR spectra were recorded using a Bruker
AC300P FT spectrometer operating at 300.13 MHz (1H) and
75.47 MHz (13C). Chemical shifts (δ) are given in ppm relative to
tetramethylsilane as an internal reference. Coupling constants (J)
Triclinic
Triclinic
¯
¯
P1
P1
Cell parameters (Å, °)
a
9.3934 (2)
9.9077 (2)
12.5491 (3)
74.355 (1)
81.372 (1)
85.213 (1)
1110.78 (4), 2
1.041
9.3399 (3)
10.4602 (3)
12.3177 (3)
67.483 (1)
79.146 (1)
77.227 (1)
1076.96 (5), 2
1.087
b
c
α
β
γ
1
are given in hertz (Hz). H NMR signals are labelled as singlet (s),
Volume (Å3), Z
Absorption
doublet (d), triplet (t) or multiplet (m). FT-IR spectra were recorded
on a Mattson 1000 spectrophotometer, with wavenumbers in
cmÀ1. Melting points were measured in glass capillary tubes with
an Electrothermal 9200 melting point apparatus. Elemental analy-
ses were performed using TUBITAK Microlab.
X-ray diffraction data for 2b and 2c were collected on a Bruker
Kappa APEXII CCD diffractometer with Mo-Kα (λ = 0.71073 Å) radiation
at room temperature. Lattice parameters were obtained by least-
squares fit to the optimized setting angles of the collected reflections
by means of APEX2.[53] Multi-scan numerical absorption correction
with SADABS was employed.[54] The molecular structures of 2b and
2c were solved by direct methods using the SHELXS-97 program[55]
and refined by full-matrix least-squares on F2 using SHELXL-97.[55]
For 2b and 2c, all H atoms were placed geometrically and
treated as riding with C―H = 0.93 Å (aromatic), 0.96 Å (methyl),
0.97 Å (methylene), with Uiso(H) = 1.2Ueq(methylene, methine) or
coefficient (mmÀ1
Crystal size (mm3)
)
0.20 × 0.22 × 0.30
1.488
0.25 × 0.16 × 0.14
1.597
Dx (Mg mÀ3
F(000)
)
508
524
θ range (°)
Number of
2.1–28.0
17 832
2.3–28.3
19 380
measured reflections
Number of
5 233
4 394
268
5 322
4 189
274
unique reflections
Number of reflections
with I > 2σ(I)
Number of
refined parameters
Goodness of fit on F2
R[F2 > 2σ(F2)], wR2
Residual density (e ÅÀ3
CCDC No.
1.03
1.03
Uiso(H) = 1.5Ueq(methyl). Details of the crystallographic data and
0.029, 0.075
À0.54, 0.42
891 112
0.033, 0.071
À0.39, 0.50
891 111
structure refinement for 2b and 2c are listed in Table 1.
)
Synthesis
(s, 1 H, NCHN). 13C NMR (75.47 MHz, CDCl3); δ: 16.9 [NCH2C6(CH3)
5-2,6]; 17.2 [NCH2C6(CH3)5-4,]; 17.3 [NCH2C6(CH3)5-3,5]; 49.4
[NCH2C6(CH3)5-2,3,4,5,6]; 113.4, 114.5, 125.0 and 131.7 (C6H4N2);
125.8, 130.7, 131.5 and 135.6 (NC6H5); 127.4, 133.0, 133.7 and
137.0 [NCH2C6(CH3)5]; 142.9 (NCHN). Anal. Calcd for C25H27N2Cl:
C, 76.80; H, 6.96; N, 7.17. Found: C, 76.69; H, 7.05; N, 7.13%.
General preparation of 1-phenyl-3-alkylbenzimidazolium salts, 1
To a solution of 1-phenylbenzimidazole (1.0 mmol) in DMF (4 ml),
alkyl halides (1.0 mmol) were added slowly at room temperature
and the resulting mixture was stirred at 80 °C for 12 h. Diethyl
ether (15 ml) was added to obtain a crystalline solid which was
filtered off. The solid was washed with diethyl ether (2 × 15 ml)
and dried under vacuum. The crude product was recrystallized
from ethyl alcohol/diethyl ether at room temperature. The crys-
tallized compounds were obtained as white, white, cream, white
and cream solids for NHC ligand precursors (1a–e), respectively.
1-Phenyl-3-(3,4,5-trimethoxybenzyl)benzimidazolium chloride, 1c. Yield
1
88%, m.p. 226–227 °C. ν(CN) = 1591.58 cmÀ1. H NMR (300.13 MHz,
CDCl3); δ: 3.46 [s, 6 H, NCH2C6H2(OCH3)3-3,5]; 3.56 [s, 3 H, NCH2C6H2
(OCH3)3-4]; 6.65 [s, 2 H, NCH2C6H2(OCH3)3-3,4,5]; 6.92 (s, 2 H,
NCH2C6H2(OCH3)3-3,4,5]; 7.31–7.65 (m, 11 H, C6H4 and C6H5); 10.21
(s, 1 H, NCHN). 13C NMR (75.47 MHz, CDCl3); δ: 53.7 [NCH2C6H2
(OCH3)3-3,5]; 58.7 [NCH2C6H2(OCH3)3-4]; 63.4 [NCH2C6H2(OCH3)3-
3,4,5]; 108.8, 116.1, 133.3 and 134.2, (C6H4N2); 130.2, 131.7, 132.9
and 133.6 (NC6H5); 127.3, 134,9, 135.2 and 139.9 [NCH2C6H2(OCH3)
3]; 155.6 (NCHN). Anal. Calcd for C23H23N2O3Cl: C, 67.23; H, 5.64; N,
6.82. Found: C, 67.35; H, 5.51; N, 6.79%.
1-Phenyl-3-(2,4,6-trimethylbenzyl)benzimidazolium chloride, 1a. Yield 86%; m.
p. 197–198 °C; ν(CN): 1548.74 cmÀ1. 1H NMR (300.13 MHz, CDCl3); δ: 2.31
[s, 3 H, NCH2C6H2(CH3)3-4]; 2.39 [s, 6 H, NCH2C6H2(CH3)3-2,6]; 6.23 [s,
2H, NCH2C6H2(CH3)3-2,4,6]; 6.93 [s, 2 H, NCH2C6H2(CH3)3-2,4,6];
7.16–7.87 (m, 9 H, C6H4 and C6H5); 11.72 (s, 1 H, NCHN). 13C NMR
(75.47 MHz, CDCl3); δ: 20.5 [NCH2C6H2(CH3)3-2,6]; 21.1 [NCH2C6H2
(CH3)3-4]; 48.2 [NCH2C6H2(CH3)3-2,4,6]; 113.5, 114.3, 127.4 and 131.5
(C6H4N2); 124.9, 130.1, 130.8 and 132.9 (NC6H5); 125.6, 128.5, 138.1
and 139.5 [NCH2C6H2(CH3)3]; 143.4 (NCHN). Anal. Calcd for C23H23N2Cl:
C, 76.12; H, 6.39; N, 7.72. Found: C, 76.23; H, 6.28; N, 7.69%.
1-Phenyl-3-naphthalenomethylbenzimidazolium chloride, 1d. Yield 82%,
m.p. 220–221 °C. ν(CN) = 1595.98 cmÀ1.1H NMR (300.13 MHz, CDCl3); δ:
6.65 (s, 2 H, NCH2C10H7); 7.28–7.89 (m, 16H, C6H4, C6H5 and
NCH2C10H7); 11.70 (s, 1 H, NCHN). 13C NMR (75.47 MHz, CDCl3); δ:
49.6 (NCH2C10H7); 122.8, 124.9, 125.4, 126.5, 127.7, 128.1, 128.4,
129.1, 130.2, 130.7, 130.9, 131.3, 131.7, 133.0 and 133.9 (C6H4N2,
NC6H5 and NCH2C10H7); 143.4 (NCHN). Anal. Calcd for C24H19N2Cl:
C, 77.72; H, 5.16; N, 7.55. Found: C, 77.81; H, 5.09; N, 7.50%.
1-Phenyl-3-(2,3,4,5,6-pentamethylbenzyl)benzimidazolium chloride, 1b.
Yield 83%, m.p. 215–216 °C.
ν .
(CN) = 1547.88 cmÀ1 1H NMR
(300.13 MHz, CDCl3); δ: 2.23 [s, 3 H, NCH2C6(CH3)5-4]; 2.27 [s, 6 H,
NCH2C6(CH3)5-2,6]; 2.37 [s, 6 H, NCH2C6(CH3)5-3,5]; 6.26 [s, 2 H,
NCH2C6(CH3)5-2,3,4,5,6]; 7.48–7.88 (m, 9 H, C6H4 and C6H5); 11.38
Appl. Organometal. Chem. 2014, 28, 244–251
Copyright © 2014 John Wiley & Sons, Ltd.
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Y. Gök et al.
1-Phenyl-3-(2-morpholinoethyl)benzimidazolium chloride, 1e. Yield 80%,
m.p. 148–150 °C. ν(CN) = 1552.82 cmÀ1. 1H NMR (300.13 MHz, CDCl3);
δ: 2.27 [t, J: 5.5Hz, 2 H, NCH2CH2NC4H8O]; 3.04 [t, J: 4.6Hz, 4 H,
NCH2CH2N(CH2CH2)2O]; 3.63 (t, J: 5.8 Hz, 2 H, NCH2CH2NC4H8O);
5.09 [t, J: 4.8Hz, 4 H, NCH2CH2N(CH2CH2)2O]; 7.34–7.92 (m, 9 H,
C6H4 and C6H5); 11.42 (s, 1 H, NCHN). 13C NMR (75.47 MHz, CDCl3);
δ: 44.7 [NCH2CH2N(CH2CH2)2O]; 56.6 [NCH2CH2N(CH2CH2)2O]; 53.6
[NCH2CH2N(CH2CH2)2O]; 66.9 [NCH2CH2N(CH2CH2)2O]; 110.5,
113.7, 130.1 and 131.1 (C6H4); 124.8, 127.5, 130.8 and 131.5 (C6H5);
143.4 (NCHN). Anal. Calcd for C19H22N3OCl: C, 66.37; H, 6.45; N,
12.22. Found: C, 66.29; H, 6.51; N, 12.27%.
6 H, NCH2C6H2(CH3)3-2,6]; 5.55 [s, 2 H, NCH2C6H2(CH3)3-2,4,6];
6.71 [s, 2 H, NCH2C6H2(CH3)3-2,4,6]; 7.45–8.26 (m, 9 H, C6H4
and C6H5). 13C NMR (75.47 MHz, DMSO); δ: 20.4 [NCH2C6H2
(CH3)3-2,6]; 21.1 [NCH2C6H2(CH3)3-4]; 55.3 [NCH2C6H2(CH3)3-
2,4,6]; 112.8, 125.0, 128.2 and 130.4 [C6H4N2]; 125.5, 129.8,
134.1 and 135.2 [NC6H5]; 126.7, 134.3, 137.9 and 138.5
[NCH2C6H2(CH3)3]; the carbenic carbon was not detected. Anal.
Calcd for C23H22N2AgCl: C, 58.81; H, 4.72; N, 5.96. Found: C,
58.90; H, 4.63; N, 5.88%.
[1-Phenyl-3-(2,3,4,5,6-pentamethylbenzyl)benzimidazol-2-ylidene]chlorosilver
(I), 2b. Yield 75%, m.p. 263–264 °C. ν(CN) = 1593.44 cmÀ1. 1H NMR
(300.13 MHz, DMSO); δ: 1.94 [s, 3 H, NCH2C6(CH3)5-4]; 2.17 [s, 12 H,
NCH2C6(CH3)5-2,3,5,6]; 5.62 [s, 2 H, NCH2C6(CH3)5-2,3,4,5,6]; 7.47–
8.20 (m, 9 H, C6H4 and C6H5). 13C NMR (75.47 MHz, DMSO); δ: 17.3
[NCH2C6(CH3)5-2,3,5,6]; 17.5 [NCH2C6(CH3)5-4]; 54.5 [NCH2C6(CH3)
5-2,3,4,5,6]; 112.4, 112.7, 125.5 and 130.4 [C6H4N2]; 126.6, 129.8,
130.3 and 133.4 [NC6H5]; 127.3, 133.5, 134.2 and 135.1 [NCH2C6
(CH3)5]; the carbenic carbon was not detected. Anal. Calcd for
C25H26N2AgCl: C, 60.32; H, 5.26; N, 5.63. Found: C, 60.19; H, 5.34;
N, 5.69%.
General preparation of N-phenyl-substituted NHC–silver complexes, 2
A solution of benzimidazolium salt (1.0 mmol), silver oxide
(0.5 mmol) and activated 4 molecular sieves in dichloromethane
(30 ml) was stirred at room temperature for 10 h in the dark.
The reaction mixture was filtered through celite and the solvent
removed under reduced pressure. The crude product was
recrystallized from dichloromethane–dimethyl ether at room
temperature. The crystallized compounds were obtained as very
light pink, white, white, very light cream and cream solid crystals
for NHC–silver complexes (2a–e), respectively.
[1-Phenyl-3-(3,4,5-trimethoxybenzyl)benzimidazol-2-ylidene]chlorosilver(I),
2c. Yield 73%, m.p. 267–269 °C. ν(CN) = 1594.06 cmÀ1.1H NMR
(300.13 MHz, DMSO); δ: 3.61 [s, 6 H, NCH2C6H2(OCH3)3-3,5]; 3.65
[s, 3 H, NCH2C6H2(OCH3)3-4]; 5.67 [s, 2 H, NCH2C6H2(OCH3)3-
3,4,5]; 6.97 [s, 2 H, NCH2C6H2(OCH3)3-3,4,5]; 7.24–8.00 (m, 9 H,
C6H4 and C6H5). 13C NMR (75.47 MHz,
[1-Phenyl-3-(2,4,6-trimethylbenzyl)benzimidazol-2-ylidene]chlorosilver(I),
2a. Yield 71%, m.p. 244–245 °C. ν(CN) = 1595.81 cmÀ1. 1H NMR
(300.13 MHz, DMSO); δ: 1.65 [s, 3 H, NCH2C6H2(CH3)3-4]; 2.52 [s,
DMSO); δ: 52.6 [NCH2C6H2(OCH3)3-3,5]; 56.4
[NCH2C6H2(OCH3)3-4];
60.5
[NCH2C6H2
(OCH3)3-3,4,5]; 106.1, 112.6, 131.8 and
134.2 (C6H4N2); 125.4, 126.5, 130.5 and
133.7 (NC6H5); 113.1, 137.8, 138.1 and
153.5 [NCH2C6H2(OCH3)3]; the carbenic car-
bon was not detected. Anal. Calcd for
C
23H22N2O3AgCl: C, 53.35; H, 4.28; N, 5.41.
Found: C, 53.24; H, 4.37; N, 5.43 %.
[1-Phenyl-3-naphthalenomethylbenzimidazol-2-ylidene]
chlorosilver(I), 2d. Yield 79%, m.p. 226–227 °C.
ν
(CN) = 1594.13 cmÀ1.1H NMR (300.13MHz,
DMSO); δ: 6.28 (s, 2H, NCH2C10H7); 7.07–7.76
(m, 16 H, C6H4, C6H5 and NCH2C10H7). 13C NMR
(75.47 MHz, DMSO); δ: 50.3 (NCH2C10H7); 123.6,
125.5, 126.0, 126.8, 127.3, 129.3, 129.9, 130.4,
130.7, 131.9, 133.8, 134.1, 134.3 and 138.1
(C6H4N2, NC6H5 and NCH2C10H7); the carbenic
carbon was not detected. Anal. Calcd for
C24H18N2AgCl: C, 60.34; H, 3.80; N, 5.86. Found:
C, 60.45; H, 3.69; N, 5.77%.
[1-Phenyl-3-(2-morpholinoethyl)benzimidazol-2-ylidene]
chlorosilver(I), 2e. Yield 80%, m.p. 134–135 °C.
ν
(CN) = 1594.48 cmÀ1 1H NMR (300.13MHz,
.
DMSO); δ: 2.44 [t, J: 5.0Hz, 4H, NCH2CH2N
(CH2CH2)2O]); 2.52 [t, J: 5.9 Hz, 2 H, NCH2CH2N
(CH2CH2)2O]; 2.79 [t, 2 H, J: 5.8Hz, NCH2CH2N
(CH2CH2)2O]; 4.63 [t, J: 5.3 Hz, 4 H, NCH2CH2N
(CH2CH2)2O]; 7.46–7.96 (m, 9 H, C6H4 and
C6H5). 13C NMR (75.47 MHz, DMSO); δ: 46.7
[NCH2CH2N(CH2CH2)2O]; 58.2 [NCH2CH2N
(CH2CH2)2O]; 54.0 [NCH2CH2N(CH2CH2)2O]; 66.6
[NCH2CH2N(CH2CH2)2O]; 112.5, 124.9, 129.8
and 133.8 (C6H4); 125.2, 126.6, 130.6, 130.3 and
Scheme 1. Synthesis of N-phenyl-substituted benzimidazolium salts as NHC precursors.
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Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 244–251

N-Phenyl substituted carbene precursors and their silver complexes: sy
138.2 (C6H5); 188.7 (2-Ccarbene). Anal. Calcd for C19H21N3OAgCl: C, 50.63;
H, 4.70; N, 9.32. Found: C, 50.51; H, 4.79; N, 9.35%.
Antibacterial Activity
The antibacterial activities of the synthesized compounds were
tested against B. subtilis (ATCC 6633), E. coli (ATCC 25922), K.
pneumoniae (FMC 5), L. monocytogenes (1/2B), Ps. aeruginosa
(ATCC 27853) and S. aureus (ATCC 25923). Antibacterial activity
was evaluated by determining the minimum inhibitory concen-
tration (MIC) using the microdilution broth method with some
modification, according to Ávila et al.[56] The compounds were
dissolved in dimethyl sulfoxide (DMSO) to obtain a stock solution
of 3200 μg lÀ1 concentration. This solution was then added to the
first two wells on a micro-titre plate and twofold dilutions were
made in Mueller Hinton broth (Oxoid, Basingstoke, UK) from well
two. The final concentration range was from 25 to 3200 μg lÀ1
.
One sample (100 μl) of each diluted solution and the samples
for both growth and sterility controls (containing sterile culture
medium and DMSO, and no antimicrobial agents) were distrib-
uted in a 96-well plate. Suspensions of bacteria were prepared
to contain approximately 105 cfu mlÀ1 and applied to
microtitration plates and incubated at 37 °C for 24 h. Bacterial
Figure 1. Molecular structure of 2b with displacement ellipsoids drawn
at the 30% probability level.
growth was detected by the addition of a
solution (2.5 mg mlÀ1) of 2,3,5-triphenyl
tetrazolium chloride (TTC) in 96% EtOH.
The MIC values (μg mlÀ1) of the tested sub-
stances, i.e. the lowest concentration at
which no growth occurred, were deter-
mined by the TTC colorimetric reaction
from yellow (absence of growth) to purple.
All experiments were performed in dupli-
cate. DMSO was found to have no effect
on the microorganisms for all the concen-
trations tested (data not shown). Tetracy-
cline (Sigma T3258-T6) was used as
positive control to assess the MIC values of
the reference strains.
Results and Discussion
Synthesis of N-Heterocyclic Carbene
Precursors and Silver Complexes
The synthetic routes for phenyl-
substituted N-heterocyclic carbene precur-
sors and their corresponding silver com-
plexes described in this study are given
in Schemes 1 and 2, respectively. The
1-phenyl-3-alkylbenzimidazolium salts (1a–e)
as N-phenyl-substituted NHC-precursors
were obtained by quaternization N-
phenylbenzimidazole with various alkyl ha-
lides in DMF (Scheme 1). The obtained salts
were stable to air and moisture both in the
solid state and in solution. The structures
of 1a–e were established by spectroscopic
1
data and elemental analysis. The H NMR
Scheme 2. Synthesis of NHC–silver complexes (2a–e).
spectra of the benzimidazolium salts further
Appl. Organometal. Chem. 2014, 28, 244–251
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Y. Gök et al.
supported the assigned structures; the resonance for C(2)―H was
observed as sharp singlets at 11.72, 11.38, 10.21, 11.70 and
11.42 ppm for 1a–e, respectively. 13C NMR chemical shifts were
consistent with the proposed structures; the imino carbons are typ-
ical singlets in the 1H-decoupled mode at 143.4, 142.9, 155.6, 143.4
and 143.4ppm for benzimidazolium salts (1a–e), respectively. The
FT-IR data clearly indicate the presence of ―C=N―with bands as-
cribed to ν(C= N) being observed at 1548.7, 1590.2, 1591.6, 1596.0
and 1593.36 cmÀ1 for benzimidazolium salts (1a–e), respectively.
The NHC–silver complexes (2a–e) were synthesized by treat-
ment of 1-phenyl-3-alkylbenzimidazolium salts with 0.5 equiv.
Ag2O in dichloromethane (Scheme 2). The reaction mixture was
stirred for 10 h at room temperature to afford the NHC–silver
complexes. NHC–silver complexes (2a–e) were soluble in haloge-
nated solvents and insoluble in non-polar solvents. The NHC–
silver complexes were characterized by spectroscopic methods
(1H NMR, 13C NMR and FT-IR) and elemental analysis. Further-
more, the solid-state structures of 2b and 2c were analysed by
single-crystal X-ray diffraction. 1H NMR and 13C NMR spectra
were consistent with the proposed formulae. The absence of a
downfield NCHN signal for silver complexes (2a–e) in the 1H
NMR spectra indicated a successful complex formation. Also in
the 13C NMR, the carbene carbon resonance observed in 1e at
Figure 2. Molecular structure of 2c with displacement ellipsoids drawn
at the 50% probability level.
Figure 3. View of the hydrogen bonding and packing of 2c down the a-axis. All hydrogen atoms not involved in hydrogen bonding are omitted for clarity.
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N-Phenyl substituted carbene precursors and their silver complexes: sy
Table 2. Hydrogen bond parameters for 2b and 2ca
The molecular structures of 2b and 2c are shown in Figs. 1 and
2; Figs. 1–3 were drawn using the PLATON program[58]
D—H (Å) H…A (Å)
D…A (Å) D—H…A (°)
In 2b and 2c, the benzimidazole groups are almost planar, with
maximum deviations of 0.062(2) Å for C1 and À0.017(2) Å for N1,
respectively. The terminal phenyl and benzene rings (which form
dihedral angles of 88.88(13)° for 2b and 76.32(14)° for 2c) make
dihedral angles of 63.48(10)° and 85.74(11)° for 2b and 55.23
(11)° and 74.89(11)° for 2c, with respect to the central benzimid-
azole ring system. The Ag―C and Ag―Cl single-bond lengths
are, respectively, 2.091(2) and 2.3301(8) Å for 2b and 2.092(2) and
2.3602(7) Å for 2c. The C―Ag–Cl bond angle is 176.28(6)° for 2b
and 167.60(6)° for 2c. The bond lengths and angles of 2b and 2c
are in agreement with those reported for similar compounds.[7a, 31]
In 2b, C―H…π interactions contribute to the stabilization of
the crystal structure (Table 2) and lead to a 3D supramolecular
architecture (see supporting information). In 2c, molecules are
linked by C―H…O hydrogen bonds forming C(11) motifs[59,60]
as chains parallel to the c-axis. Furthermore, a C―H…π interac-
tion and π–π stacking interactions (Table 2) help to stabilize the
3D crystal packing of 2c. Figure 3 shows a view of the crystal
packing for 2c.
For 2b
C9—H9…Cg2i
C14—H14B…Cg4ii
0.93
0.97
2.81
2.81
3.638(3)
3.620(3)
150
142
Symmetry codes: (i) 1 À x, Ày, 1 À z; (ii) 1 À x, Ày, Àz
For 2c
C2—H2…O2i
C22—H22B…Cg2ii
Cg1…Cg2iii
0.93
0.96
2.41
2.88
3.259(3)
151
133
0
3.601(3)
3.7093(14)
3.7089(15)
Cg2…Cg2iv
0
Symmetry codes: (i) x, y, À1 + z; (ii) À x, 1 À y, 1 À z; (iii) 1 À x, 1 À y,
Àz; (iv) 1 À x, 1 À y, Àz
aCg1 and Cg2 are the centroids of the N1/N2/C1/C6/C7 and C1–C6
rings, respectively.
143.4 ppm was shifted downfield in 2e, at 188.7 ppm, which is
characteristic for carbene metal complexes,[57] further demon-
strating the formation of expected NHC–silver complex. How-
ever, in the other complexes, resonances for carbene carbons
were not detected, which has also been mentioned in the liter-
ature and is likely due to the fluxional behaviour of NHC
complexes.[20,49–52] The FT-IR data for NHC–silver complexes
exhibited a characteristic ν(C=N) band at 1595.81, 1593.44,
1594.06, 1594.13 and 1594.48 for 2a–e, respectively.
Benzimidazolium salts at 48 h and silver complexes at 24 h
were obtained by Liu et al.[57] In our study, compounds were
achieved in reduced times (for benzimidazolium salts at 12 h
and for NHC–silver complexes at 10 h) and greater yields.
Antibacterial Evaluation
All the NHC precursors and NHC–silver complexes were
screened for antibacterial activities in vitro against three Gram-
positive bacterial strains – B. subtilis, L. monocytogenes, S. aureus
– and three Gram-negative bacterial strains – E. coli, K.
pneumoniae, P. aeruginosa – using the microdilution broth
method. Tetracycline was used as a positive control to assess
the MIC values of the strains. The MICs (the lowest concentra-
tions of compounds that completely inhibited bacteria growth)
of the NHC–precursors and NHC–silver complexes against the
six bacteria are presented in Table 3. A smaller MIC value corre-
sponds to higher activity. It can be seen from Table 3 that many
of these NHC precursors and NHC–silver complexes were effec-
tive against the six bacteria used in the biological experiments.
1a exhibited high antibacterial activity against P. aeruginosa
and B. subtilis, with an MIC as low as 50 μg mlÀ1 among the syn-
thesized NHC precursors. Similarly, 1b exhibited high
Structural Discussion
Suitable crystals for X-ray crystallography to determine the mo-
lecular structure of 2b and 2c were obtained in a saturated
dichloromethane solution with slow infusion of diethyl ether.
Table 3. MIC values of the synthesized NHC precursors (1a–1e) and NHC–silver complexes (2a–2e) against six bacteria
Compound
MIC (μg mlÀ1
)
Gram-negative
Gram-positive
E. coli
K. pneumoniae
P. aeruginosa
B. subtilis
L. monocytogenes
S. aureus
1a
400
400
1600
400
3200
50
200
400
3200
200
200
50
50
100
800
100
3200
50
50
50
200
200
400
400
3200
400
200
50
1b
1c
800
100
100
50
800
1d
100
1e
>3200
<50
<50
<50
50
2a
2b
50
25
25
50
100
100
100
200
125
2c
100
100
100
<7.8
50
50
100
50
2d
50
50
2e
100
<7.8
100
100
<7.8
100
Tetracycline
15.6
62.5
Appl. Organometal. Chem. 2014, 28, 244–251
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Y. Gök et al.
antibacterial activity against B. subtilis, with an MIC as low as
50 μg mlÀ1. 1e was effective against K. pneumoniae, B. subtilis
and S. aureus. However, 1e, which contained the alkyl group,
exhibited low antibacterial activities against the other three
bacteria (E. coli, P. aeruginosa and L. monocytogenes). 1c, which
contained three methoxy groups on the benzyl ring, had a lower
antibacterial effect against tested bacteria with 800μg mlÀ1 and
above. The similar MIC values obtained for 1a and 1b may be con-
sidered to be due to methyl substitution on the benzyl ring. As
shown in Table 3, the new NHC–silver complexes showed effective
activities against Gram-positive and Gram-negative bacteria with
regard to MIC values, between 25 and 200μg mlÀ1. According to
the results, 2a and 2b showed greater antibacterial activities
against the tested bacteria than the other synthesized silver com-
plexes. The best antibacterial activity was observed for complex 2b
against Gram-negative K. pneumoniae and P. aeruginosa (25 μgmlÀ1).
That also may be considered to be due to methyl substituted on
the benzyl ring. The presence of electron-donating or electron-
withdrawing substituents may be essential for antibacterial activity.
The silver complexes had stronger antibacterial effects than the
synthesized NHC precursors. Carbene precursor and silver com-
plex (1e and 2e) containing the morpholino group showed the
lowest antibacterial activities. Other compounds, except for 1e
and 2e, comprise aromatic substituents. The different activities
on both Gram-positive and Gram-negative bacteria may be
directly related to lipophilicity of the substituents of the tested
benzimidazolium salts and NHC–silver complexes. Thus
benzimidazolium precursors bearing 2,4,6-trimethylbenzyl and
2,3,4,5,6-pentamethylbenzyl groups were more active than the
other salts. For the NHC–silver complexes, those with 2,4,6-
trimethylbenzyl, 2,3,4,5,6-pentamethylbenzyl and naphthalenomethyl
groups were more active.
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