Non-symmetrically p-nitrobenzyl- and p-cyanobenzyl-substituted N-heterocyclic carbene-silver(I) complexes: synthesis, characterization and antibacterial studies

Article abstract of DOI:10.1080/00958972.2016.1276574

Various nitro/nitrile-functionalized benzimidazol-2-ylidene carbene complexes of silver(I) (7a–d and 11a–d) were synthesized by combination of 1-allyl/1-isopropyl/1-sec-butyl/1-isopentyl-3-(nitro/cyano-benzyl)-3H-benzimidazol-1-ium hexafluorophosphate (6a

Full text of DOI:10.1080/00958972.2016.1276574

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Non-symmetrically p-nitrobenzyl- and p-
cyanobenzyl-substituted N-heterocyclic carbene-
silver(I) complexes: Synthesis, characterization
and antibacterial studies
Subramanya Prasad T. V., Shahini C. R., Shivaputra A. Patil, Xiaojun Huang,
Alejandro Bugarin & Siddappa A. Patil
To cite this article: Subramanya Prasad T. V., Shahini C. R., Shivaputra A. Patil, Xiaojun
Huang, Alejandro Bugarin & Siddappa A. Patil (2016): Non-symmetrically p-nitrobenzyl-
and p-cyanobenzyl-substituted N-heterocyclic carbene-silver(I) complexes: Synthesis,
characterization and antibacterial studies, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2016.1276574
To link to this article: http://dx.doi.org/10.1080/00958972.2016.1276574
Accepted author version posted online: 23
Dec 2016.
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Date: 07 January 2017, At: 04:39

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2016.1276574
Non-symmetrically p-nitrobenzyl- and p-cyanobenzyl-substituted
N-heterocyclic carbene-silver(I) complexes: Synthesis, characterization
and antibacterial studies
SUBRAMANYA PRASAD T. V.†, SHAHINI C. R.†, SHIVAPUTRA A. PATIL‡, XIAOJUN HUANG§,
ALEJANDRO BUGARIN§ and SIDDAPPA A. PATIL*†
†
Centre for Nano and Material Sciences, Jain Global Campus, Jain University, Bangalore 562112,
Karnataka, INDIA
Pharmaceutical Sciences Department, College of Pharmacy, Rosalind Franklin University of Medicine
and Science, 3333 Green Bay Road, North Chicago, IL 60064, USA
Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, TX 76019, USA
‡
§
Various nitro/nitrile-functionalized benzimidazol-2-ylidene carbene complexes of silver(I) (7a-d
and 11a-d) were synthesized by combination of 1-allyl/1-isopropyl/1-sec-butyl/1-isopentyl-3-
(
nitro/cyano-benzyl)-3H-benzimidazol-1-ium hexafluorophosphate (6a-d and 10a-d) with
1
13
silver(I) oxide in acetonitrile. The compounds were characterized by H, C NMR, FT-IR, mass
spectrometry, and elemental analysis. Additionally, the in vitro antibacterial activity of the
N-heterocyclic carbene (NHC) precursors (6a-d and 10a-d) and their corresponding NHC-
silver(I) complexes (7a-d and 11a-d) were investigated against Gram-positive bacteria
Staphylococcus aureus and Gram-negative bacteria Escherichia coli using the qualitative Kirby-
Bauer disk diffusion method. All the NHC-silver(I) complexes exhibited medium-to-high
antibacterial activity with areas of clearance ranging from 12 mm to 21 mm, while the NHC
precursors were inactive against both strains of bacteria.
Keywords: NHCs; Silver complexes; Synthesis; Antibacterial; Staphylococcus aureus;
Escherichia coli
*Corresponding author. Email: p.siddappa@jainuniversity.ac.in
1

1
. Introduction
N-Heterocyclic carbenes (NHCs) were first isolated when Arduengo documented a deprotonation
of 1,3-bis(adamantyl)imidazolium chloride [1]. Ever since, NHCs experienced an exponential
increase in research and applications. Researchers have developed a variety of convenient routes
to prepare NHC-metal complexes with transition metals [2, 3]. NHC-metal complexes are known
particularly as catalysts [4-8] for many chemical transformations; later discoveries revealed that
NHC-silver and gold complexes can be used in medicinal applications [9-12].
Silver compounds have been used for medicinal applications for more than a hundred
years for water purification, wound care antiseptics and infections. Silver nitrate was commonly
th
th
used as an antimicrobial compound in the 17 and 18 centuries, before the discovery of
penicillin. It was not until bacteria resistance to other antibiotics were reported that silver
compounds regained importance. Antibacterial activity of silver compounds is attributed to
+
soluble silver ions, Ag . Silver metal and insoluble silver salts, such as silver chloride, do not
display antibacterial activity. The only side-effect of long-term silver treatment is the unpleasant
color change (to grey or blue) of patients’ skin. The effect called Argyria occurs due to
irreversible formation and deposition of silver sulfide in dermis and eyes.
Silver sulfadiazine (figure 1) reintroduced silver compounds as antimicrobials in 1968
and has remained an effective topical-burn treatment [13, 14]. Soon after the discovery of
N-heterocyclic carbenes, silver complexes with these ligands were synthesized for antimicrobial
applications [15]. Research groups have reported the antibacterial activities of various
p-nitrobenzyl- and p-cyanobenzyl-substituted NHC-silver(I) complexes against Gram-positive
bacteria Staphylococcus aureus and the Gram-negative bacteria Escherichia coli [16-25]. All the
reported NHC-silver complexes are light- and water-stable and can be synthesized in high yield
with high purity from inexpensive commercially available starting materials and show medium-
to-high antibacterial activities. These positive outcomes encouraged our research efforts on
diverse p-nitrobenzyl- and p-cyanobenzyl-substituted NHC-silver complexes as potential
antibacterial drug candidates.
We have focused on antibacterial studies of complexes [26]. Within this manuscript, we
present the synthesis, characterization and preliminary in vitro antibacterial studies of a series of
eight new non-symmetrically p-nitrobenzyl-substituted and p-cyanobenzyl-substituted
NHC-silver(I) complexes. These compounds were tested on the Gram-positive bacteria
2

Staphylococcus aureus and the Gram-negative bacteria Escherichia coli. During the course of
these studies, compounds 10b and 11b were reported by other researchers [17].
2
2
. Experimental
.1. General considerations
All reactions were carried out under aerobic conditions in oven-dried glassware with magnetic
stirring. Benzimidazole, p-nitrobenzyl bromide, p-cyanobenzylbromide, allyl bromide, isopropyl
bromide, 2-bromobutane, isopentyl bromide, potassium hexafluorophosphate, silver(I) oxide and
potassium hydroxide were procured commercially from Sigma-Aldrich chemical company and
used without purification. Heating was accomplished by either a heating mantle or silicone oil
bath. Reactions were monitored by thin-layer chromatography (TLC) performed on 0.25 mm
Merck TLC silica gel plates using UV light as a visualizing agent. Concentration under vacuum
refers to the removal of volatile solvent using a rotary evaporator attached to a dry diaphragm
pump (10-15 mm Hg) followed by pumping to a constant weight with an oil pump (<300
1
mTorr). H NMR spectra were recorded at 500 MHz and are reported relative to DMSO-d (δ
6
1
2
.50 ppm). H NMR coupling constants (J) are reported in Hertz (Hz) and multiplicities are
1
3
indicated as follows: s (singlet), d (doublet), t (triplet), m (multiplet). Proton-decoupled C
NMR spectra were recorded at 125 MHz and reported relative to DMSO-d (δ 39.5 ppm). IR
6
spectra were recorded on an Alpha-P Bruker FT-IR spectrometer. Elemental analyses of
compounds were done on a Euro Vector S.P.A, Euro EA 3000 CHNS elemental analyzer. Liquid
chromatography mass spectra (LCMS) were recorded on an Agilent technologies quadrupole
LC-MS system.
2
.2. Synthesis
General experimental procedure for synthesis of 1-alkyl-3-(4-nitrobenzyl)benzimidazolium
bromides (5a-d). Benzimidazole (1) (1.18 g, 10.0 mmol) and potassium hydroxide (0.84 g,
1
5.0 mmol) were stirred for 2 h in minimum DMSO at 100 °C. The temperature of the reaction
mixture was reduced to 40 °C and alkyl bromide (10.0 mmol) was added in one portion and
stirring was continued for a further 2 h. Afterwards the reaction mixture was cooled to room
temperature and ice-cold water (200 mL) was added. The precipitate was separated by
decantation, washed with water and dried under vacuum to get 1-alkylbenzimidazoles (3a-d).
3

The obtained 1-alkylbenzimidazoles (3a-d) were used in the next step without purification. The
mixture of 4-nitrobenzyl bromide (4) (2.16 g, 10.0 mmol) and 1-alkylbenzimidazole (10.0 mmol)
was stirred in dioxane (60 mL) at 100-110 °C for 24 h. Then the reaction mixture was cooled to
room temperature. The precipitate was filtered, washed with fresh dioxane (30 mL) and dried
under vacuum to get pure 1-alkyl-3-(4-nitrobenzyl)benzimidazolium bromides (5a-d).
1
-Allyl-3-(4-nitrobenzyl)benzimidazolium bromide (5a). Compound 5a was prepared from
allyl bromide (2a) (0.86 g, 10.0 mmol) and 1-allylbenzimidazole (3a) (1.58 g, 10.0 mmol)
according to the general procedure. Light yellow solid. Yield: 70%.
1
-Isopropyl-3-(4-nitrobenzyl)benzimidazolium bromide (5b). Compound 5b was prepared
from isopropyl bromide (2b) (0.93 g, 10.0 mmol) and 1-isopropylbenzimidazole (3b) (1.60 g,
1
0.0 mmol), according to the general procedure. White solid. Yield: 76%.
1-(Sec-butyl)-3-(4-nitrobenzyl)benzimidazolium bromide (5c). Compound 5c was prepared
from 2-bromobutane (2c) (1.08 g, 10.0 mmol) and 1-(sec-butyl)benzimidazole (3c) (1.74 g,
1
0.0 mmol), according to the general procedure. White solid. Yield: 80%.
1-Isopentyl-3-(4-nitrobenzyl)benzimidazolium bromide (5d). Compound 5d was prepared
from 1-isopentyl bromide (2d) (1.19 g, 10.0mmol) and 1-isopentylbenzimidazole (3d) (1.88 g,
0.0 mmol), according to the general procedure. Yellow solid. Yield: 86%.
1
General experimental procedure for synthesis of 1-alkyl-3-(4-nitrobenzyl)benzimidazolium
hexafluorophosphates (6a-d). The bromide salts (5a-d) (10.0 mmol) were directly converted
into their hexafluorophosphates counterpart by metathesis using KPF (2.76 g, 15.0 mmol) in a
6
mixture of methanol (45 mL) and water (5 mL). The resultant mixture was stirred for 4 h at room
temperature and poured into water (200 mL). The separated precipitate was filtered and washed
with water (100 mL) and dried under vacuum to obtain 6a-d in high purity.
1-Allyl-3-(4-nitrobenzyl)benzimidazolium hexafluorophosphate (6a). Compound 6a was
prepared from bromide salt (5a) (3.74 g, 10.0 mmol), according to the general procedure. White
1
solid. Yield: 68%. H NMR (DMSO-d , 500 MHz) δ (ppm): 10.10 (s, 1H, NCHN), 8.26-8.28 (m,
6
2
6
5
H, CHNitrobenzyl), 7.90-7.70 (m, 5H, CHNitrobenzyl+CHBenzimid), 7.11-7.13 (m, 1H, CHBenzimid), 6.33-
.35 (m, 1H, CH), 5.95-5.99 (m, 2H, CH ), 5.20-2.22 (m, 2H, CH ), 5.45-5.49 (m, 1H, CH),
2
2
1
3
.25-5.20 (m, 1H, CH). C NMR (DMSO-d
6
, 125 MHz) δ (ppm): 147.6, 142.9, 141.2, 131.2,
4

1
30.4, 129.4, 129.3, 127.3, 127.1, 123.9, 120.2, 113.9, 113.8
(
NCN+CBenzimid+CNitrobenzyl+CH+CH ), 49.3 (CH ), 49.1 (CH ). Mass data (m/z, LC-MS): 295.0
2
2
2
+
[M -PF
6
]. Anal. Calcd for C17
H
16
F
6
3 2
N O P: C, 46.48; H, 3.67; N, 9.57. Found: C, 46.45; H, 3.58;
-1
N, 9.80. IR (KBr, cm ) ν: 3145, 3020, 2944, 2850, 2759, 1640, 1550, 1365, 832.
-Isopropyl-3-(4-nitrobenzyl)benzimidazolium hexafluorophosphate (6b). Compound 6b
1
was prepared from bromide salt (5b) (3.76 g, 10.0 mmol), according to general the procedure.
1
White solid. Yield: 75%. H NMR (DMSO-d , 500 MHz) δ (ppm): 10.00 (s, 1H, NCHN), 8.26 (d,
6
J = 8.5 Hz, 2H, CHNitrobenzyl), 8.17 (d, J = 8.0 Hz, 1H, CHBenzimid), 7.87 (d, J = 8.0 Hz, 1H,
CHBenzimid), 7.77 (d, J = 8.5 Hz, 2H, CHNitrobenzyl), 7.63-7.71 (m, 2H, CHBenzimid), 5.93 (s, 2H,
13
CH ), 5.07-5.12 (m, 1H, CH), 1.68 (d, J = 6.5 Hz, 6H, CH ). C NMR (DMSO-d , 125 MHz) δ
2
3
6
(
(
[
ppm): 147.6, 141.5, 141.4, 131.0, 130.8, 129.3, 126.9, 126.7, 124.0, 114.3, 113.7
NCN+CBenzimid+CNitrobenzyl), 50.9 (CH), 49.2(CH ), 21.4 (CH ). Mass data (m/z, LC-MS): 297.0
2
3
+
M -PF ]. Anal. Calcd for C H F N O P: C, 46.27; H, 4.11; N, 9.52. Found: C, 46.21; H, 4.10;
6
17 18
6
3
2
-1
N, 9.49. IR (KBr, cm ) ν: 3140, 3019, 3112, 2941, 2883, 2849, 2760, 1645, 1557, 1360, 831.
-(Sec-butyl)-3-(4-nitrobenzyl)benzimidazolium hexafluorophosphate (6c). Compound 6c
1
was prepared from 5c (3.90 g, 10 mmol), according to the general procedure. White solid. Yield:
1
8
0%. H NMR (DMSO-d
6
, 500 MHz) δ (ppm): 10.06 (s, 1H, NCHN), 8.27 (d, J = 8.5 Hz, 2H,
CHNitrobenzyl), 8.19 (d, J = 7.5 Hz, 1H, CHBenzimid), 7.91 (d, J = 7.5 Hz, 1H, CHBenzimid), 7.76 (d, J
8.5 Hz, 2H, CHNitrobenzyl), 7.64-7.71 (m, 2H, CHBenzimid), 5.94 (s, 2H, CH ), 4.91-4.97 (m, 1H,
=
2
1
3
CH), 1.96-2.11 (m, 2H, CH ), 1.68 (d, J = 6.5 Hz, 3H, CH ), 0.90 (t, J = 7.5 Hz, 3H, CH ). C
2
3
3
NMR (DMSO-d , 125 MHz) δ (ppm): 147.6, 141.7, 141.4, 131.0, 129.5, 129.3, 127.0, 126.8,
6
1
3 2 3
24.0, 114.3, 113.8 (NCN+CBenzimid+CNitrobenzyl), 56.2 (CH), 49.3 (CH ), 28.4 (CH ), 19.2 (CH ),
+
1
0.0 (CH ). Mass data (m/z, LC-MS): 311.0 [M -PF ]. Anal. Calcd for C H F N O P: C, 47.48;
3
6
18 20
6
3
2
-1
H, 4.43; N, 9.23. Found: C, 47.45; H, 4.38; N, 9.21. IR (KBr, cm ) ν: 3139, 3021, 2942, 2880,
2
850, 2761, 1641, 1555, 1363, 830.
1-Isopentyl-3-(4-nitrobenzyl)benzimidazolium hexafluorophosphate (6d). Compound 6d was
prepared from 5d (4.04 g, 10.0 mmol), according to the general procedure. White solid. Yield:
1
8
2
5%. H NMR (DMSO-d
6
, 500 MHz) δ (ppm): 9.91 (s, 1H, NCHN), 8.21-8.24 (m,
H,CHBenzimid), 8.08 (d, J = 8.0 Hz, 1H, CHNitrobenzyl), 7.86 (d, J = 8.0 Hz, 1H, CHNitrobenzyl), 7.70
), 4.49 (t, J =
(
d, J = 8.5 Hz, 2H, CHNitrobenzyl), 7.59-7.67 (m, 2H, CHBenzimid), 5.89 (s, 2H, CH
2
5

7
.5 Hz, 2H, CH ), 1.80-1.85 (m, 2H, CH ), 1.61-1.68 (m, 1H, CH), 0.94 (d, J = 6.5 Hz, 6H, CH ).
2
2
3
1
3
C NMR (DMSO-d , 125 MHz) δ (ppm): 147.6, 142.8, 141.4, 131.3, 130.9, 129.4, 126.9, 126.8,
6
1
2
2 2 2
24.0, 114.0, 113.7 (NCN+CBenzimid+CNitrobenzyl), 49.1 (CH ), 45.4 (CH ), 36.9 (CH ), 25.2 (CH),
+
2.1 (CH ). Mass data (m/z, LC-MS): 325.0 [M -PF ]. Anal. Calcd for C H F N O P: C, 48.62;
3
6
19 22
6
3
2
-1
H, 4.72; N, 8.95. Found: C, 48.57; H, 4.70; N, 8.91. IR (KBr, cm ) ν: 3140, 3015, 2944, 2885,
2
849, 2760, 1640, 1547, 1350, 834.
General experimental procedure for synthesis of bis-(1-alkyl-3-(4-
nitrobenzyl)benzimidazolium)-silver(I) hexafluorophosphates (7a-d). 1-Alkyl-3-(4-
nitrobenzyl)benzimidazolium hexafluorophosphate (5.0 mmol) was dissolved in acetonitrile
(80 mL) and silver(I) oxide (2.31 g, 10.0 mmol) was added. The mixture was stirred at 45 °C for
1
d. Light was excluded by covering the flask with aluminum foil. The reaction mixture was
filtered through a pad of Celite and the resultant filtrate was concentrated to 5 mL under vacuum.
Fine grey precipitate was afforded by addition of diethylether (20 mL) to the filtrate. The
precipitate was filtered, washed with diethylether (20 mL) and dried by suction at room
temperature for 30 min to get pure (7a-d).
Bis-(1-allyl-3-(4-nitrobenzyl)benzimidazolium)-silver(I) hexafluorophosphate (7a).
Compound 7a was prepared from 1-allyl-3-(4-nitrobenzyl)benzimidazolium
hexafluorophosphate (6a) (2.19 g, 5.0 mmol), according to the general procedure. Grey solid.
1
Yield: 67%. H NMR (DMSO-d , 500 MHz) δ (ppm): 8.12-8.14 (m, 2H, CHNitrobenzyl), 7.71-7.72
6
(m, 2H, CHNitrobenzyl), 7.46-7.53 (m, 10H, CHNitrobenzyl+CHBenzimid), 7.05-7.06 (m, 2H, CHBenzimid),
1
3
6
1
1
.22-6.25 (m, 2H, CH), 5.92-5.96 (m, 8H, CH
2 2 6
), 5.17-5.30 (m, 4H, CH ). C NMR (DMSO-d ,
25 MHz) δ (ppm): 190.3, 147.0, 143.8, 133.7, 132.6, 129.6, 128.2, 128.1, 124.83, 123.8, 118.1,
12.5, 112.3 (NCN+CBenzimid+CNitrobenzyl+CH+CH
2
), 50.9 (CH
2
), 50.4 (CH
2
). Mass data (m/z, LC-
+
MS): 696.0[M] . Anal. Calcd for C H AgF N O P: C, 48.53; H, 3.83; N, 9.90; Ag, 12.82.
3
4
32
6
6
4
-1
Found: C, 48.52; H, 3.80; N, 9.89; Ag, 12.79. IR (KBr, cm ) ν: 3142, 3019, 2938, 2848, 2760,
828, 1641, 1551, 1350, 1242, 830.
1
Bis-(1-isopropyl-3-(4-nitrobenzyl)benzimidazolium)-silver(I) hexafluorophosphate (7b).
Compound 7b was prepared from 1-isopropyl-3-(4-nitrobenzyl)benzimidazolium
hexafluorophosphate (6b) (2.20 g, 5.0 mmol), according to the general procedure. Grey solid.
6

1
Yield: 71%. H NMR (DMSO-d , 500 MHz) δ (ppm): 8.13 (d, J = 9.0 Hz, 4H, CHNitrobenzyl), 7.98
6
(
d, J = 8.0 Hz, 2H, CHNitrobenzyl), 7.66 (d, J = 8.0 Hz, 2H, CHNitrobenzyl), 7.39-7.47 (m, 8H,
13
CHBenzimid), 5.92 (s, 4H, CH
2
), 5.09-5.15 (m, 2H, CH), 1.68 (d, J = 7.0 Hz, 12H, CH
3
). C NMR
(
DMSO-d , 125 MHz) δ (ppm): 188.0, 147.0, 144.0, 133.6, 132.5, 128.0, 124.4, 124.1, 123.8,
6
1
13.0, 112.3 (NCN+CBenzimid+CNitrobenzyl), 52.4 (CH), 51.2 (CH
2
), 22.4 (CH
3
). Mass data (m/z,
+
LC-MS): 700.0 [M] . Anal. Calcd for C H AgF N O P: C, 48.30; H, 4.29; N, 9.94; Ag, 12.76.
3
4
36
6
6
4
-1
Found: C, 48.29; H, 4.30; N, 9.92; Ag, 12.68. IR (KBr, cm ) ν: 3143, 3023, 2941, 2880, 2841,
757, 1826, 1637, 1551, 1363, 1267, 832.
2
Bis-(1-(sec-butyl)-3-(4-nitrobenzyl)benzimidazolium)-silver(I) hexafluorophosphate (7c).
Compound 7c was prepared from 1-(sec-butyl)-3-(4-nitrobenzyl)benzimidazolium
hexafluorophosphate (6c) (2.27 g, 5.0 mmol), according to the general procedure. Grey solid.
1
Yield: 77%. H NMR (DMSO-d , 500 MHz) δ (ppm): 8.14 (d, J = 9.0 Hz, 4H, CHNitrobenzyl), 7.97
6
(
d, J = 7.5 Hz, 2H, CHNitrobenzyl), 7.67 (d, J = 7.5 Hz, 2H, CHNitrobenzyl), 7.39-7.47 (m, 8H,
), 4.80-4.89 (m, 2H, CH), 1.96-2.03 (m, 4H, CH
), 1.67 (d, J =
.0 Hz, 6H, CH ), 0.75 (t, J = 7.5 Hz, 6H, CH ). C NMR (DMSO-d , 125 MHz) δ (ppm):
CHBenzimid), 5.93 (s, 4H, CH
2
2
1
3
7
1
3
3
6
88.2, 147.0, 144.1, 133.6, 132.9, 127.8, 124.4, 124.2, 123.9, 112.9, 112.3
(
NCN+CBenzimid+CNitrobenzyl), 58.1 (CH), 51.1 (CH ), 29.0 (CH ), 20.8 (CH ), 10.9 (CH ). Mass
3 2 3 3
+
data (m/z, LC-MS): 730.0 [M] . Anal. Calcd for C H AgF N O P: C, 49.50; H, 4.62; N, 9.62;
36
40
6
6
4
-1
Ag, 12.35. Found: C, 49.48; H, 4.63; N, 9.59; Ag, 12.32. IR (KBr, cm ) ν: 3142, 3039, 2881,
2
849, 2760, 1853, 1649, 1551, 1350, 1245, 830.
Bis-(1-isopentyl-3-(4-nitrobenzyl)benzimidazolium)-silver(I) hexafluorophosphate (7d).
Compound 7d was prepared from 1-isopentyl-3-(4-nitrobenzyl)benzimidazolium
hexafluorophosphate (6d) (2.34 g, 5.0 mmol), according to the general procedure. Grey solid.
1
Yield: 84%. H NMR (DMSO-d , 500 MHz) δ (ppm): 8.17 (d, J = 8.5 Hz, 4H, CHNitrobenzyl), 7.83
6
(d, J = 8.0 Hz, 2H, CHNitrobenzyl), 7.63 (d, J = 8.0 Hz, 2H, CHNitrobenzyl), 7.38-7.50 (m, 8H,
CHBenzimid), 5.91 (s, 4H, CH ), 4.51 (t, J = 7.5 Hz, 4H, CH ), 1.68-1.76 (m, 4H, CH ), 1.56-1.64
2
2
2
1
3
(m, 2H, CH), 0.91 (d, J = 6.5 Hz, 12H, CH
3
), 0.75 (t, J = 7.5 Hz, 6H). C NMR (DMSO-d
6
,
1
25 MHz) δ (ppm): 189.7, 171.5, 147.1, 144.1, 133.3, 133.2, 128.2, 124.2, 123.9, 112.3, 112.2
(
NCN+CBenzimid+CNitrobenzyl), 51.0 (CH ), 47.3 (CH ), 38.9 (CH ), 25.3 (CH), 22.4 (CH ). Mass
2 2 2 3
+
data (m/z, LC-MS): 758.0 [M] . Anal. Calcd for C H AgF N O P: C, 50.62; H, 4.92; N, 9.32;
38
44
6
6
4
7

-1
Ag, 11.96. Found: C, 50.61; H, 4.90; N, 9.31; Ag, 11.93. IR (KBr, cm ) ν: 3142, 3041, 29445,
883, 2850, 2759, 1878, 1638, 1550, 1348, 1244, 832.
2
General experimental procedure for synthesis of 1-alkyl-3-(4-
cyanobenzyl)benzimidazolium bromides (9a-d). The mixture of 4-cyanobenzyl bromide (8)
(
1.96 g, 10.0 mmol) and 1-alkylbenzimidazole (10.0 mmol) was stirred in dioxane (60 mL) at
00-110 °C for 24 h. Then the reaction mixture was cooled to room temperature. The isolated
precipitate was filtered, washed with fresh dioxane (30 mL) and dried under vacuum to get pure
1
1
-alkyl-3-(4-cyanobenzyl)benzimidazolium bromides (9a-d).
1
-Allyl-3-(4-cyanobenzyl)benzimidazolium bromide (9a). Compound 9a was prepared from
allyl bromide (2a) (0.86 g, 10.0 mmol) and 1-allylbenzimidazole (3a) (1.58 g, 10.0 mmol),
according to the general procedure. Light yellow solid. Yield: 72%.
1
-Isopropyl-3-(4-cyanobenzyl)benzimidazolium bromide (9b). Compound 9b was prepared
from isopropyl bromide (2b) (0.93 g, 10.0 mmol) and 1-isopropylbenzimidazole (3b) (1.60 g,
1
0.0 mmol), according to the general procedure. White semi solid. Yield: 74%.
1-(Sec-butyl)-3-(4-nitrobenzyl)benzimidazolium bromide (9c). Compound 9c was prepared
from 1-bromobutane (2c) (1.08 g, 10.0 mmol) and 1-(sec-butyl)benzimidazole (3c) (1.74 g,
1
0.0 mmol), according to general procedure. White solid. Yield: 83%.
1-Isopentyl-3-(4-cyanobenzyl)benzimidazoliumbromide (9d). Compound 9d was prepared
from 1-isopentyl bromide (2d) (1.19 g, 10.0 mmol) and 1-isopentylbenzimidazole (3d) (1.88 g,
0.0 mmol), according to the general procedure. White solid. Yield: 89%.
1
General experimental procedure for synthesis of 1-alkyl-3-(4-
cyanobenzyl)benzimidazolium hexafluorophosphates (10a-d). The bromide salts (9a-d)
(10.0 mmol) were directly converted into their hexafluorophosphate counterpart by metathesis
using KPF (2.76 g, 15.0 mmol) in a mixture of methanol (45 mL) and water (5 mL). The
6
resultant mixture was stirred for 4 h at room temperature and poured into water (200 mL). The
separated white precipitate was filtered and washed with water (100 mL) and dried under
vacuum to obtain (10a-d) in high purity.
8

1
-Allyl-3-(4-cyanobenzyl)benzimidazolium hexafluorophosphate (10a). Compound 10a was
prepared from 9a (3.54 g, 10.0 mmol), following the general procedure. Light yellow solid.
1
Yield: 76%. H NMR (500 MHz, DMSO-d
6
) δ (ppm): 10.08 (s, 1H, NCHN), 7.89-7.92 (m, 3H,
CHCyanobenzyl), 7.67-7.74 (m, 4H, CHBenzimid), 7.10-7.12 (m, 1H, CHCyanobenzyl), 6.31-6.34 (m, 1H,
CH), 5.89-5.92 (m, 2H, CH ), 5.90 (s, 2H, CH ), 5.50-5.41 (m, 1H, CH), 5.20-5.18 (m, 1H, CH).
2
2
1
3
C NMR (DMSO-d , 125 MHz) δ (ppm): 142.8, 139.2, 132.8, 131.2, 130.3, 129.3, 129.0, 127.2,
6
1
4
27.1, 120.2, 118.4, 113.9, 113.7, 111.4 (NCN+ CBenzimid+CCyanobenzyl+CH+CH ), 49.5 (CH ),
2
2
+
9.2 (CH
2
). Mass data (m/z, LC-MS): 275.0 [M -PF
6
]. Anal. Calcd for C18
H
16
F
6
N
3
P: C, 51.56;
-1
H, 3.85; N, 10.02. Found: C, 51.53; H, 3.88; N, 10.00. IR (KBr, cm ) ν: 3144, 3015, 2944, 2848,
2
760, 2251, 1645, 838.
1
-Isopropyl-3-(4-cyanobenzyl)benzimidazolium hexafluorophosphate (10b). Compound 10b
was prepared from 9b (3.56 g, 10.0 mmol), following the general procedure. White solid. Yield:
1
7
8%. H NMR (DMSO-d , 500 MHz) δ (ppm): 10.01 (s, 1H, NCHN), 8.16 (d, J = 8.0 Hz, 1H,
6
CHCyanobenzyl), 7.90 (d, J = 8.0 Hz, 2H, CHCyanobenzyl), 7.85 (d, J = 8.0 Hz, 1H, CHCyanobenzyl), 7.63-
7
.71 (m, 4H, CHBenzimid), 5.86 (s, 2H, CH ), 5.05-5.11 (m, 1H, CH), 1.67 (d, J = 6.5 Hz, 6H,
2
13
CH
3
). C NMR (DMSO-d
6
, 125 MHz) δ (ppm): 141.5, 139.5, 132.8, 131.0, 130.8, 128.9, 126.9,
1
26.7, 118.4, 114.3, 113.7 111.4 (NCN+CN+CBenzimid+CCyanobenzyl), 50.9 (CH), 49.2 (CH ), 21.4
2
+
(
CH ). Mass data (m/z, LC-MS): 277.0 [M -PF ]. Anal. Calcd for C H F N P: C, 51.31; H,
3
6
18 18
6
3
-1
4
.31; N, 9.97. Found: C, 51.28; H, 4.34; N, 9.93. IR (KBr, cm ) ν: 3145, 3113, 3022, 2943,
2
843, 2759, 2249, 1640, 835.
1
1
-(Sec-butyl)-3-(4-cyanobenzyl)benzimidazolium hexafluorophosphate (10c). Compound
0c was prepared from 9c (3.70 g, 10.0 mmol), following the general procedure. White solid.
1
Yield: 84%. H NMR (DMSO-d , 500 MHz) δ (ppm): 10.04 (s, 1H, NCHN), 8.18 (d, J = 8.0 Hz,
6
1
H, CHCyanobenzyl), 7.89-7.94 (m, 3H, CHCyanobenzyl), 7.64-7.72 (m, 4H, CHBenzimid), 5.91 (s, 2H,
CH ), 4.90-4.94 (m, 1H, CH), 2.00-2.04 (m, 2H, CH ), 1.67 (d, J = 6.5 Hz, 3H, CH ), 0.89 (t, J =
2
2
3
1
3
7
1
4
.5 Hz, 3H, CH ). C NMR (DMSO-d , 125 MHz) δ (ppm): 143.5, 141.6, 139.4, 132.9, 130.9,
3
6
27.0, 126.9, 126.7, 118.4, 114.3, 113.8, 111.47 (NCN+CN+CBenzimid+CCyanobenzyl), 56.2 (CH),
+
9.5 (CH ), 28.3 (CH ), 19.2 (CH ), 10.0 (CH ). Mass data (m/z, LC-MS): 291.0 [M -PF ]. Anal.
2
2
3
3
6
Calcd for C H F N P: C, 52.42; H, 4.63; N, 9.65. Found: C, 52.40; H, 4.67; N, 9.68. IR (KBr,
1
9
20
6
3
-1
cm ) ν: 3143, 3024, 2942, 2847, 2758, 2236, 1641, 840.
9

1-Isopentyl-3-(4-cyanobenzyl)benzimidazolium hexafluorophosphate (10d). Compound 10d
was prepared from 9d (3.84 g, 10.0 mmol), following the general procedure. White solid. Yield:
1
8
9%. H NMR (DMSO-d
6
, 500 MHz) δ (ppm): 9.92 (s, 1H, NCHN), 8.12 (d, J = 8.0 Hz, 1H,
CHCyanobenzyl), 7.88-7.92 (m, 3H, CHCyanobenzyl), 7.64-7.71 (m, 4H, CHBenzimid), 5.86 (s, 2H, CH2),
4
6
1
.52 (t,J= 7.5 Hz, 2H, CH
2
), 1.84-1.89 (m, 2H, CH
2
), 1.65-1.71 (m, 1H, CH), 0.98 (d, J = 6.5 Hz,
13
H, CH ). C NMR (DMSO-d , 125 MHz) δ (ppm): 142.7, 139.5, 132.9, 131.4, 130.9, 129.0,
3
6
26.9, 126.8, 118.4, 114.0, 113.7, 111.5 (NCN+CN+CBenzimid+CCyanobenzyl), 49.4 (CH ), 45.4
2
+
(
CH
2
), 36.9 (CH
2
), 25.2 (CH), 22.2 (CH
3
). Mass data (m/z, LC-MS): 305.0 [M -PF
6
]. Anal.
Calcd for C H F N P: C, 53.46; H, 4.93; N, 9.35. Found: C, 53.41; H, 4.92; N, 9.33. IR (KBr,
2
0
22
6
3
-1
cm ) ν: 3144, 3033, 2940, 2845, 2758, 2243, 1641, 835.
General experimental procedure for synthesis of bis-(1-alkyl-3-(4-
cyanobenzyl)benzimidazolium)-silver(I) hexafluorophosphates (11a-d). 1-Alkyl-3-(4-
cyanobenzyl)benzimidazolium hexafluorophosphate (5.0 mmol) was dissolved in acetonitrile
(80 mL) and silver(I) oxide (2.31 g, 10.0 mmol) was added. The mixture was stirred at room
temperature for 2 d. Light was excluded by covering the flask by aluminum foil. The reaction
mixture was filtered through a pad of Celite, and the resulted filtrate was concentrated to 5 mL
under vacuum. Addition of diethylether (20 mL) gave grey precipitate, which was isolated by
filtration, washed with diethylether (20 mL) and dried under vacuum to obtain pure (11a-d).
Bis-(1-allyl-3-(4-cyanobenzyl)benzimidazolium)-silver(I) hexafluorophosphate (11a).
Compound 11a was prepared from 1-allyl-3-(4-cyanobenzyl)benzimidazolium
hexafluorophosphate (10a) (2.09 g, 5.0 mmol), following the general procedure. Grey solid.
1
Yield: 76%. H NMR (DMSO-d , 500 MHz) δ (ppm): 7.78-7.80 (m, 2H, CHCyanobenzyl), 7.71-7.72
6
(m, 2H, CHCyanobenzyl), 7.44-7.53 (m, 10H, CHCyanobenzyl+CHBenzimid), 7.03-7.05 (m, 2H,
13
CHBenzimid), 6.20-6.23 (m, 2H, CH), 5.86-5.90 (m, 8H, CH ), 5.16-5.30 (m, 4H, CH ). C NMR
2
2
(DMSO-d
6
, 125 MHz) δ (ppm): 190.2, 141.8, 133.7, 132.6, 129.5, 127.9, 124.8, 124.7, 124.6,
1
18.4, 118.1, 112.5, 112.3, 110.8 (NCN+ CBenzimid+CCyanobenzyl+CH+CH ), 51.2 (CH ), 51.1
2
2
+
(
CH ). Mass data (m/z, LC-MS): 632.0 [M] . Anal. Calcd for C H AgF N P: C, 53.95; H, 4.02;
2
36 32
6
6
-1
N, 10.49; Ag, 13.46. Found: C, 53.92; H, 4.00; N, 10.45; Ag, 13.41. IR (KBr, cm ) ν: 3143,
3
020, 2939, 2845, 2759, 2244, 1843, 1643, 1244, 833.
10

Bis-(1-isopropyl-3-(4-cyanobenzyl)benzimidazolium)-silver(I) hexafluorophosphate (11b).
Compound 11b was prepared from 1-isopropyl-3-(4-cyanobenzyl)benzimidazolium
hexafluorophosphate (10b) (2.10 g, 5.0 mmol), following the general procedure. Grey solid.
1
Yield: 79%. H NMR (DMSO-d , 500 MHz) δ (ppm): 7.97-7.98 (m, 2H, CHCyanobenzyl), 7.77-7.83
6
(m, 4H, CHCyanobenzyl), 7.66-7.67 (m, 2H, CHCyanobenzyl), 7.39-7.44 (m, 8H, CHBenzimid), 5.80-5.86
1
3
(
m, 4H, CH ), 5.07-5.09 (m, 2H, CH), 1.65 (d, J = 76.5 Hz, 12H, CH ). C NMR (DMSO-d ,
2
3
6
1
1
25 MHz) δ (ppm): 187.8, 167.3, 142.0, 133.6, 132.4, 127.7, 126.8, 124.1, 118.4, 113.0, 112.2,
10.7 (NCN+CN+CBenzimid+CCyanobenzyl), 52.3 (CH), 51.4 (CH
2
), 22.3 (CH
3
). Mass data (m/z, LC-
+
MS): 660.0 [M] . Anal. Calcd for C H AgF N P: C, 53.68; H, 4.50; N, 10.43; Ag, 13.39.
3
6
36
6
6
-1
Found: C, 53.69; H, 4.51; N, 10.42; Ag, 13.35. IR (KBr, cm ) ν: 3142, 3056, 2941, 2847, 2761,
260, 1863, 1644, 1272, 840.
2
Bis-(1-(sec-butyl)-3-(4-cyanobenzyl)benzimidazolium)-silver(I) hexafluorophosphate (11c).
Compound 11c was prepared from 1-(sec-butyl)-3-(4-cyanobenzyl)benzimidazolium
hexafluorophosphate (10c) (2.17 g, 5.0 mmol) according to the general procedure. Grey solid.
1
Yield: 85%. H NMR (DMSO-d , 500 MHz) δ (ppm): 7.92-7.99 (m, 4H, CHCyanobenzyl), 7.62-7.85
6
2
(m, 4H, CHCyanobenzyl), 7.28-7.49 (m, 8H,CHBenzimid), 5.90 (s, 4H, CH ), 4.74-4.77 (m, 2H, CH),
1
3
2
1
1
.06-2.10 (m, 4H, CH ), 1.61-1.63 (m, 6H, CH ), 0.66-0.71 (m, 6H, CH ). C NMR (DMSO-d ,
2 3 3 6
25 MHz) δ (ppm): 171.5, 167.3, 142.0, 134.0, 132.7, 127.9, 127.0, 124.0, 118.4, 112.9, 112.3,
2 2 3
10.8 (NCN+CN+CBenzimid+CCyanobenzyl), 58.1 (CH), 51.3 (CH ), 28.9 (CH ), 20.8 (CH ), 10.8
+
(
CH ). Mass data (m/z, LC-MS): 690.0 [M] . Anal. Calcd for C H AgF N P: C, 54.75; H, 4.84;
3
38 40
6
6
-1
N, 10.08; Ag, 12.94. Found: C, 54.77; H, 4.83; N, Ag, 12.91, 10.01. IR (KBr, cm ) ν: 3141,
3
042, 2942, 2850, 2783, 2241, 1844, 1639, 838.
Bis-(1-isopentyl-3-(4-cyanobenzyl)benzimidazolium)-silver(I) hexafluorophosphate (11d).
Compound 11d was prepared from 1-isopentyl-3-(4-nitrobenzyl)benzimidazolium
hexafluorophosphate (10d) (2.24 g, 5.0 mmol), according to general the procedure. Grey solid.
1
Yield: 90%. H NMR (DMSO-d , 500 MHz) δ (ppm): 7.84 (d, J = 8.5 Hz, 2H, CHCyanobenzyl),
6
7
.79 (d, J = 8.5 Hz, 4H, CHCyanobenzyl), 7.64 (d, J = 8.0 Hz, 2H, CHCyanobenzyl), 7.39-7.48 (m, 8H,
CHBenzimid), 5.87 (s, 4H, CH ), 4.51 (t, J = 7.5 Hz, 4H, CH ), 1.72-1.76 (m, 4H, CH ), 1.57-1.63
2
2
2
1
3
(
m, 2H, CH), 0.91 (d, J = 6.5 Hz, 12H, CH ). C NMR (DMSO-d , 125 MHz) δ (ppm): 189.6,
3
6
1
42.1, 133.3, 132.9, 132.7, 129.0, 127.8, 124.3, 118.4, 112.3, 112.2, 110.8
(
NCN+CN+CBenzimid+CCyanobenzyl), 51.2 (CH ), 47.3 (CH ), 38.9 (CH ), 25.3 (CH), 22.3 (CH ).
2 2 2 3
11

+
Mass data (m/z, LC-MS): 718.0 [M] . Anal. Calcd for C H AgF N P: C, 55.76; H, 5.15; N,
9
4
0
44
6
6
-1
.75; Ag, 12.52. Found: C, 55.75; H, 5.10; N, 9.73; Ag, 12.50. IR (KBr, cm ) ν: 3140, 3046,
2
945, 2842, 2760, 2255, 1885, 1641, 1226, 830.
O N
2
O N
2
O N
2
N
Br
N
H
Br
4
N
N
KPF6
N
KOH, DMSO
00 ºC, 2 h
1
PF6
1
N
R
Dioxane
N
R
MeOH/H O
N
R
2
1
00 ºC, 24 h
R
Br
2a-d
3a-d
5a-d
6a-d
O N
2
2
a, 3a, 5a, 6a, 7a, R =
R
N
N
2
2
b, 3b, 5b, 6b, 7b, R =
Me
Me
Me
Me
Ag
PF6
Ag O
N
R
N
2
c, 3c, 5c, 6c, 7c, R
=
MeCN
45 ºC
Me
2d, 3d, 5d, 6d, 7d, R =
Me
7a-d
NO2
Scheme 1. General reaction scheme for synthesis of non-symmetrically p-nitrobenzyl-substituted
NHC precursors (6a-d) and their corresponding NHC-silver(I) complexes (7a-d).
NC
NC
NC
N
Br
N
H
Br
N
4
N
KPF6
N
KOH, DMSO
00 ºC, 2 h
1
PF6
1
N
R
Dioxane
N
R
MeOH/H O
N
R
2
1
00 ºC, 24 h
R
Br
2
a-d
3a-d
9a-d
10a-d
NC
2a, 3a, 9a, 10a, 11a, R =
R
N
N
2
b, 3b, 9b, 10b, 11b, R = Me
Me
Me
Ag
PF
Ag O
N
R
N
2
2c, 3c, 9c, 10c, 11c, R
=
6
Me
MeCN
Me
Me
45 ºC
2d, 3d, 9d, 10d, 11d, R =
11a-d
CN
Scheme 2. General reaction scheme for synthesis of non-symmetrically p-cyanobenzyl-
substituted NHC precursors (10a-d) and their corresponding NHC-silver(I) complexes (11a-d).
12

2
.3. Antibacterial studies
Initial in vitro antibacterial activity of non-symmetrically p-nitrobenzyl-substituted and
p-cyanobenzyl-substituted NHCs and their corresponding NHC-silver(I) complexes were
screened simultaneously with silver(I) oxide against two bacterial strains. The tested organisms
included S. aureus as a Gram-positive bacteria and E. coli as Gram-negative bacteria.
To assess the biological activity of 6a-d, 7a-d, 10a-d, 11a-d and silver(I) oxide, the
Kirby-Bauer disk diffusion method was applied [27]. All bacteria were individually cultured
from a single colony in sterile HB medium [28] overnight at 37 °C (orbital shaker incubator). All
the work carried out was performed under sterile conditions.
A 10 mL stock solution of each compound was prepared at 800 µg/mL. For each strain,
7
0 μL of culture were spread evenly on agar-MH medium. Five 5-mm-diameter paper discs were
placed at equal distances in each plate. First one being control, the remaining discs were loaded
with 3 µL, 6 µL, 9 µL and 12 µL of stock solution. The plates were covered and placed in an
incubator at 37 °C for 24 h. The plates were then removed and the zone of clearance (defined as
the diameter of inhibited bacterial growth around the filter paper) for each sample was measured
in millimeters.
3
3
. Results and discussion
.1. Synthesis and characterization
The synthetic pathway for non-symmetrically p-nitrobenzyl-substituted and p-cyanobenzyl-
substituted N-heterocyclic carbene-silver(I) complexes described in this work is outlined in
schemes 1 and 2. 1-Allylbenzimidazole (3a), 1-isopropylbenzimidazole (3b), 1-(sec-
butyl)benzimidazole (3c) and 1-isopentylbenzimidazole (3d) were prepared by stirring
benzimidazole (1) with one equivalent of alkyl halides [allyl bromide (2a), isopropyl bromide
(2b), 2-bromobutane (2c) and isopentyl bromide (2d)] in the presence of potassium hydroxide as
a base in DMSO at 100-110 °C for 4 h. The bromide salts (5a-d and 9a-d) were prepared by
reacting one equivalent of alkyl halides [4-nitrobenzyl bromide (4) and 4-cyanobenzyl bromide
(
8)] with 1-allylbenzimidazole (3a), 1-isopropylbenzimidazole (3b), 1-(sec-butyl)benzimidazole
3c) and 1-isopentylbenzimidazole (3d)] in dioxane at 100-110 °C for 24 h, with yields
(
fluctuating between 71%-89%. All the benzimidazolium hexafluorophosphate salts (6a-d and
0a-d) were prepared by stirring bromide salts (5a-d and 9a-d) with KPF in a mixture of
1
6
13

methanol and water at room temperature for 4 h as air and moisture stable white powders, with
moderate to good yields (68%-89%). The synthesized benzimidazolium hexafluorophosphate
salts (6a-d and 10a-d) were insoluble in common organic solvents such as hexane, pentane,
diethylether and toluene and soluble in DMSO, DMF, acetonitrile and acetone.
Structures of the benzimidazolium hexafluorophosphate salts (6a-d and 10a-d) were
1
13
1
confirmed by H, C NMR, FT-IR, MS spectrometry, and elemental analyses. H NMR spectra
of the NHC precursors (6a-d and 10a-d) show a characteristic downfield shift in the range δ =
1
0.10-9.91 ppm for the NCHN proton, indicating formation of benzimidazolium
hexafluorophosphate salts (6a-d and 10a-d) [29-31]. In the FT-IR spectra of NHC precursors
(6a-d) stretching vibrational bands of nitro were observed as medium-intensity bands at 1557-
-
1
-1
1
547 and 1365-1350 cm . The FT-IR spectra of 10a-d exhibit a strong band at ∼2250 cm
assigned to ν(C≡N). IR spectra of all the NHC precursors (6a-d and 10a-d) displayed two sharp
-1
bands of medium intensities at 3145-3123 and 2944-2941 cm , attributed to the aliphatic and
aromatic ν(C–H) stretching vibrations. Additionally, the benzimidazolium hexafluorophosphate
+
salts (6a-d and 10a-d) identities have been confirmed by a base peak for the [M -PF
6
] fragments
in their positive mode LRMS mass spectra. The results obtained from elemental analyses are in
agreement with the proposed structures of the benzimidazolium hexafluorophosphate salts (6a-d
and 10a-d).
The NHC-silver(I) complexes (7a-d and 11a-d) were synthesized by reaction of
hexafluorophosphate salts (6a-d and 10a-d) with two equivalents of silver(I) oxide in
acetonitrile. The reaction mixture was stirred for 2 d at room temperature in the dark to afford the
NHC-silver(I) complexes as grey solids in good yields (scheme 2).
1
13
The NHC-silver(I) complexes (7a-d and 11a-d) were characterized by H, C NMR,
FT-IR and mass spectrometry, and elemental analyses. The absence of a downfield NCHN signal
1
in the H NMR spectra for 7a-d and 11a-d indicates complex formation [25-33]. However, the
resonances of aromatic, benzylic methylene and aliphatic protons of NHC-silver(I) complexes
(7a-d and 11a-d) were observed with no or negligible changes from their respective
1
3
benzimidazolium hexafluorophosphate salts (6a-d and 10a-d). The C NMR resonances of the
carbene carbons in 7a-d and 11a-d occur in the δ range of 190.3-171.5 ppm. These signals are
shifted downfield compared to the corresponding precursor 6a-d and 10a-d carbene carbon
resonances at 147.6-141.5 ppm which further supports the formation of NHC-silver(I)
14

complexes. In FT-IR spectra of NHC-silver(I) complexes (7a-d) stretching vibrational bands of
-1
nitro were observed as medium intensity bands at 1551-1550 and 1363-1348 cm . FT-IR spectra
-1
of NHC-silver(I) complexes (11a-d) exhibit a strong band at ∼2250 cm assigned to ν(C≡N).
There is no difference in the ν(C≡N) stretching frequency of NHC-silver(I) complexes and their
respective benimidazolium salts, suggesting non-involvement of nitrile in coordination. The FT-
IR spectra of 7a-d and 11a-d displayed two sharp bands of medium intensity at 3142-3140 and
-1
2
944-2939 cm , attributed to the aliphatic and aromatic ν(C–H), respectively. These spectral
assignments are consistent with reported compounds [28, 30]. LRMS mass spectra of the eight
NHC-silver(I) complexes (7a-d and 11a-d) have been recorded and show molecular ion peaks
+
[
M] corresponding to the mass of the cationic complexes. Results obtained from the elemental
analyses are in agreement with the proposed structures of 7a-d and 11a-d.
3
.2. Biological evaluation
Preliminary in vitro antibacterial assay for the benzimidazolium hexafluorophosphate salts (6a-
d and 10a-10d) and their corresponding NHC-silver(I) complexes (7a-d and 11a-d) were
6
carried out by Kirby-Bauer disc diffusion against S. aureus, a gram-positive species and E. coli,
a gram-negative species. The observed in vitro antibacterial activities of 6a-d, 10a-d, 7a-d and
1
1a-d are tabulated in table 1. Silver(I) oxide used 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 [34].
Benzimidazolium hexafluorophosphate salts (6a-d and 10a-d) were inactive against both
bacterial strains. But, upon complexation with silver, 7a-d and 11a-d exhibited moderate to good
antibacterial activity with the diameter of the zone of inhibition ranging from 12 mm to 21 mm
for S. aureus and 13 mm to 20 mm for E. coli, indicating that silver is the active center and
responsible for the biological potential. Also, despite screening of different alkyl substitutions on
nitrogen of the benzimidazole core for antibacterial activity, irrespective of the functionalization
(
nitro and nitrile), none of the salts exhibited difference on the bioactivity. This proves that
neither the difference in functionalization, nor the nature of the alkyl substituent of the
heterocyclic moiety plays a significant role toward antibacterial potential, as both
functionalizations display comparable results. Also, as the range of activity is comparable for
both species, it can be established that the complexes are efficient antibacterial agents
15

irrespective of the cell wall physiology of the bacteria. Hence, it can be stated that the N-
substitutions alone cannot bring up the desired potential. The synergic effect of both the NHC
and silver is responsible for the overall behavior of the complex. The study also provides proof
that N-heterocyclic moiety has a huge role in the biological behavior when compared to non-
NHC based silver(I) complexes possessing biologically relevant moiety [35-38].
4
. Conclusions and outlook
Several new non-symmetrically p-nitrobenzyl- and p-cyanobenzyl-substituted NHC-silver(I)
complexes (7a-d and 11a-d) were synthesized through reaction of non-symmetrically
p-nitrobenzyl- and p-cyanobenzyl-substituted benzimidazolium hexafluorophosphate salts (6a-d
and 10a-d) with silver(I) oxide. The complexes displayed high antibacterial activity compared to
their NHC precursors against both Gram-positive bacteria S. aureus and Gram-negative bacteria
E. coli. It is also clear that, as the complex concentration increases, the antibacterial activity also
increases. Furthermore, enhanced antibacterial activity is linked to stable silver(I) complexes that
+
slowly release Ag ions by decomposition. Summarizing, it can be said that the NHC-silver(I)
complexes are easily accessible from inexpensive starting materials; give moderate-to-high
yields and the synthesis does not require harsh conditions and is not excessively sensitive
towards air and moisture.
Further work is underway to improve these values by performing formulation
experiments to improve solubility of these NHC-silver(I) complexes, which should allow for in
vivo testing in the nearby future. These new NHC-silver(I) complexes could be used for
transmetallation reactions, opening additional research venues.
Acknowledgements
The University of Texas at Arlington and the UTA Research Enhancement Program partially
funded this project. The authors also thank the DST-SERB (No. SERB/F/7013/2015-16) and
DST-Nano Mission (SR/NM/NS-20/2014(G)), India, for financial support.
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18

Figure 1. Structure of silver sulfadiazine.
19

Table 1. Area of clearance for 6a-d, 7a-d, 10a-d and 11a-d in mm.
Compound
#)
Concentration S. aureus (Gram positive)
E. coli (Gram negative)
(
(nM)
Zone of inhibition (mm)
Zone of inhibition (mm)
6
6
6
6
7
7
7
7
a
b
c
5.46
6±1
6±1
6±1
6±1
1
1
2
0.92
6.38
1.85
6±1
6±1
6±1
6±1
6±1
6±1
5.43
1
1
2
0.87
6.31
1.75
6±1
6±1
6±1
6±1
6±1
6±1
5.27
6±1
6±1
6±1
6±1
1
1
2
0.54
5.81
1.08
6±1
6±1
6±1
6±1
d
a
b
c
5.11
6±1
6±1
1
1
2
0.22
5.33
0.45
6±1
6±1
6±1
6±1
6±1
6±1
2.85
16±1
17±1
19±1
20±1
16±1
17±1
18±1
20±1
16±1
17±1
20±1
20±1
12±1
14±1
14±1
16±1
15±1
16±1
18±1
19±1
15±1
16±1
17±1
19±1
15±1
16±1
19±1
19±1
11±1
13±1
15±1
15±1
5
8
1
.71
.57
1.43
2.84
5
8
1
.69
.53
1.38
2.75
5
8
1
.50
.26
1.01
d
2.66
5
8
1
.33
.00
0.67
20

1
1
1
1
1
1
1
1
0a
0b
0c
0d
1a
1b
1c
1d
5.72
6±1
6±1
1
1
2
1.44
6±1
6±1
7.17
2.89
6±1
6±1
6±1
6±1
5.69
6±1
6±1
1
1
2
1.39
6±1
6±1
7.08
2.78
6±1
6±1
6±1
6±1
5.51
6±1
6±1
1
1
2
1.02
6.53
2.05
6±1
6±1
6±1
6±1
6±1
6±1
6±1
6±1
5.34
1
1
2
0.68
6±1
6±1
6.02
1.36
6±1
6±1
6±1
6±1
3.00
17±1
18±1
20±1
21±1
17±1
18±1
19±1
21±1
17±1
18±1
20±1
20±1
15±1
16±1
17±1
18±1
16±1
17±1
19±1
20±1
16±1
17±1
18±1
20±1
16±1
17±1
20±1
20±1
13±1
14±1
16±1
16±1
6
9
1
.00
.00
2.00
2.98
5
8
1
.97
.96
1.94
2.88
5
8
1
.77
.65
1.54
2.79
5
8
1
.58
.37
1.16
21

22