Ether and coumarin–functionalized (benz)imidazolium salts and their silver(I)–N–heterocyclic carbene complexes: Synthesis, characterization, crystal structures and antimicrobial studies

Article abstract of DOI:10.1016/j.jorganchem.2017.11.005

To explore the impact of different coumarin substituted N–heterocyclic carbene (NHC) ligand backbones on the biological applications of corresponding silver(I) complexes, a series of structurally related ether–functionalized imidazolium (3–5) and benzimidazolium (6–8) hexafluorophosphate salts bearing 6–methylcoumarin, 6–chlorocoumarin and 5,6–benzannulated coumarin substituents, have been reported. These salts have been employed to react with silver(I) oxide at mild reaction conditions to obtain corresponding ionic, bis–NHC coordinated silver(I) hexafluorophosphate complexes (9–14) in excellent yields following in situ deprotonation method. Further, the bromide counterparts of the salts have been treated with silver(I) oxide in dichloromethane to afford neutral mono–NHC coordinated bromido silver(I) complexes (15–20). Both, azolium salts and their silver(I)–NHC complexes, have been thoroughly characterized by ¹H and ¹³C NMR, ATR–IR and elemental analyses. The structure of a benzannulated coumarin substituted imidazolium hexafluorophosphate salt 5 and a silver complex 12 of benzimidazole–based NHC bearing methylcoumarin substituent have been studied through single crystal X–ray diffraction technique. In the case of complex 12, the metal center lies at the inversion center adopting linear coordination geometry with anti– arrangement of the NHC ligands. Feeble π–π stacking interactions between adjacent coumarin rings have been evident in the extended complex structure along with the hydrogen bonding interactions. In the preliminary antibacterial evaluations, silver complexes displayed promising activity with the MIC values in the range 8–64 μg/mL against Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Salmonella typhi, while azolium salts displayed almost no activity.

Full text of DOI:10.1016/j.jorganchem.2017.11.005

Accepted Manuscript
Ether and coumarin–functionalized (benz)imidazolium salts and their silver(I)–N–
heterocyclic carbene complexes: Synthesis, characterization, crystal structures and
antimicrobial studies
Gautam Achar, Purvika Agarwal, K.N. Brinda, Jan Grzegorz Małecki, Rangappa S.
Keri, Srinivasa Budagumpi
PII:
S0022-328X(17)30637-X
10.1016/j.jorganchem.2017.11.005
JOM 20170
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 29 August 2017
Revised Date: 2 November 2017
Accepted Date: 5 November 2017
Please cite this article as: G. Achar, P. Agarwal, K.N. Brinda, J.G. Małecki, R.S. Keri, S. Budagumpi,
Ether and coumarin–functionalized (benz)imidazolium salts and their silver(I)–N–heterocyclic carbene
complexes: Synthesis, characterization, crystal structures and antimicrobial studies, Journal of
Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.11.005.
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ACCEPTED MANUSCRIPT
Ether and coumarin–functionalized (benz)imidazolium salts and their
silver(I)–N–heterocyclic carbene complexes: Synthesis, characterization,
crystal structures and antimicrobial studies
Gautam Achara,†, Purvika Agarwala,†, Brinda K. N.a, Jan Grzegorz Małeckib, Rangappa S.
Keria and Srinivasa Budagumpia*
a Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Kanakapura,
Ramanagaram, Bangalore 562112, India.
b Department of Crystallography, Institute of Chemistry, Silesian University, Szkolna 9, 40–
006 Katowice, Poland.
* Corresponding author:
Dr. Srinivasa Budagumpi, PhD
Assistant Professor and Group Leader
Centre for Nano and Material Sciences
Jain University, Jain Global Campus
Bangalore 562112, Karnataka, India
H/P: +91 9008375705
E–mail:
b.srinivasa@jainuniversity.ac.in
dr.sinuvb@gmail.com
†: These authors contributed equally.
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Abstract
To explore the impact of different coumarin substituted N–heterocyclic carbene (NHC)
ligand backbones on the biological applications of corresponding silver(I) complexes, a
series of structurally related ether–functionalized imidazolium (3–5) and benzimidazolium
(6–8) hexafluorophosphate salts bearing 6–methylcoumarin, 6–chlorocoumarin and 5,6–
benzannulated coumarin substituents, have been reported. These salts have been employed to
react with silver(I) oxide at mild reaction conditions to obtain corresponding ionic, bis–NHC
coordinated silver(I) hexafluorophosphate complexes (9–14) in excellent yields following in
situ deprotonation method. Further, the bromide counterparts of the salts have been treated
with silver(I) oxide in dichloromethane to afford neutral mono–NHC coordinated bromido
silver(I) complexes (15–20). Both, azolium salts and their silver(I)–NHC complexes, have
1
13
been thoroughly characterized by H and C NMR, ATR–IR and elemental analyses. The
structure of a benzannulated coumarin substituted imidazolium hexafluorophosphate salt 5
and a silver complex 12 of benzimidazole–based NHC bearing methylcoumarin substituent
have been studied through single crystal X–ray diffraction technique. In the case of complex
12, the metal center lies at the inversion center adopting linear coordination geometry with
anti– arrangement of the NHC ligands. Feeble π–π stacking interactions between adjacent
coumarin rings have been evident in the extended complex structure along with the hydrogen
bonding interactions. In the preliminary antibacterial evaluations, silver complexes displayed
promising activity with the MIC values in the range 8–64 µg/mL against Staphylococcus
aureus, Bacillus subtilis, Escherichia coli and Salmonella typhi, while azolium salts
displayed almost no activity.
Key words: Antibacterial agent; Azolium salt; Benzannulated coumarin; N–heterocyclic
carbene; Silver(I)–NHC complex; X–ray diffraction.
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1. Introduction
Appropriately substituted azolium salts have been the common precursors of N–heterocyclic
carbene (NHC) ligands ever since the first isolation of a stable, free NHC from an
imidazolium salt by Arduengo et al. in 1991 [1]. NHCs have cherished organometallic
chemistry in many aspects and became the pick of ligands for the synthesis of numerous
valued chemicals because of their strong σ–donor ability, easy access, inexpensiveness and
non–air–sensitive nature [2]. NHCs can efficiently induce desired electronic and steric
properties around the metal atoms in the NHC–metal complexes predominantly through
functionalization of either the azole backbone or hydrocarbyl/heterocyclic substituents bound
to N–atoms. The research for new transition metal–NHC complexes for various applications
has mainly been dominated by early and group X metals as efficient catalytic systems [3].
Nevertheless, over the past few years, coinage metal NHC complexes have been proposed
for biological applications as antimicrobial and anticancer agents [4]. To date, most
applications of transition metal–NHC complexes are concentrated on various types of
homogeneous catalysis.
By far the most widespread and efficacious class of biologically relevant coinage metal–
NHC complexes that have been studied so far are principally based on silver(I) system.
Silver derivatives have been at the focal point with increased attention due to their usually
strong antibacterial and anticancer potentials. Recently, Tacke and co–workers reported a
series of NHC silver(I) acetate complexes as metal-based drugs against microbial strains and
cancer cells [5]. Among others, 1,3-dibenzyl-4,5-diphenylimidazol-2-ylidene) silver(I)
acetate displayed some promising antimicrobial and anticancer potentials. Despite the large
archive of coinage metal–NHC complexes available, there is a comparatively less number of
silver complexes derived from O–functionalized NHC ligands that have been explored. The
high medicinal values of silver(I) gives concomitantly less toxicity and high cytotoxic
activity against normal cells and bacterial strains or cancer cells, respectively. Importantly,
the facile functionalization of the NHC ligand scaffold with a biocompatible heterocyclic
system such as coumarin [6], employed for these complexes can lead to improved biological
applications. In particular, substituted coumarin derivatives have been extensively studied
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mainly due to their well–known biological properties [7] ranging from antimicrobial to
anticancer potentials which are due to their structural characteristic that lead to the possible
establishment of desired interactions with the biomacromolecules. The possible structural
modifications particularly at positions 6– and 7– of benzene ring fused to the α–pyrone found
able to control their conforming biological applications [8].
The main goal in the development of new bioactive silver complexes bearing coumarin–
tethered (benz)imidazole–based NHC ligands is to access a class of derivatives that show
appropriate stability under physiological conditions. Conversely, ether functionality has been
considered to understand the donor behavior of etherial oxygen to the metal center [9]. In
fact, the core azole and coumarin derivatives individually found stable and highly active for
biological applications, and it is thought that their combination along with silver would lead
to form even better candidates encompassing promising antibacterial potentials. More
recently, we reported a series of similar silver complexes of coumarin–tethered NHCs that
found stable in solution upon exposing to sunlight for more than a week time [10]. On the
other hand, our very recent studies have shown that a similar set of sterically–modulated
silver complexes [11] displayed almost comparable antibacterial activities against E. coli
with that of standard drug, ampicillin. Herein, as a continuation of our work on the
biologically active silver complexes, especially those derived from functionalized NHC
ligands, we report synthesis, structural characterization and antibacterial potentials of a series
of new ether and coumarin substituted (benz)imidazolium salts and their silver(I)–NHC
complexes.
2. Results and discussion
2.1. Synthesis of (benz)imidazolium salts and silver(I)–NHC complexes
In comparison with those N,N’–dialkyl imidazole and benzimidazole–based azolium salts as
NHC precursors, the silver complexes of O–functionalized NHC ligands for biological
applications have been reported less frequently. The N,N’–disubstituted (benz)imidazolium
salts were synthesized by first preparing their corresponding ethers (1 and 2) following base
assisted N–alkylation pathway, an aromatic electrophilic substitution reaction between the
azole and n–bromoalkyl ether. In these reactions, the azoles were treated with excess
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potassium hydroxide in dimethyl sulphoxide at elevated temperature and consequent reaction
with the bromoalkyl ether resulted in the formation of desired ether substituted azoles, 1 and
2, in good yields. Subsequently, these ether–substituted azoles were treated with 4–
bromomethyl–6–methylcoumarin/4–bromomethyl–6–chlorolcoumarin/4–bromomethyl–5,6–
benzocoumarin in 1,4–dioxane at refluxing temperature for 24 h to afford corresponding N–
ether–N’–coumarin substituted (benz)imidazolium bromide salts in good yields.
Subsequently, the hexafluorophosphate derivatives, 3–8, were then synthesized by following
the salt metathesis reactions of the corresponding bromide salts. These salts were prepared by
treating water/methanolic (1:9 v/v) solution of potassium hexafluorophosphate with the
stirring methanolic solution of bromide salts for 4 h at room temperature as shown in Scheme
1. Completeness of the reactions was monitored by TLC technique. The products were
isolated by filtration and washed with fresh cold methanol and water to remove unreacted
potassium hexafluorophosphate and characterized by standard spectral and analytical
techniques. Both ether–substituted imidazolium and benzimidazolium analogues with non–
coordinating anions show significant solubility in polar solvents such as DMSO, DMF,
acetonitrile and sparingly soluble in DCM and insoluble in non–polar solvents.
Numerous silver(I) complexes have been reported to display different architectures due to the
presence of functionalized NHC ligands at varied reaction conditions. The synthesis of
silver(I) complexes with NHC ligands following in situ deprotonation of corresponding
azolium salts is a well–established method [12]. Especially, azolium salts with non–
coordinating anions such as hexafluorophosphate and tetrafluoroborate proved to be suitable
candidates due to their exceptional air, moisture and light stability. The title silver(I)–NHC
complexes were prepared by in situ deprotonation of azolium salts, the most widely used
pathway. The ether substituted azolium hexafluorophosphate salts, 3–8, were treated with
silver(I) oxide in acetonitrile at 45oC for 24 h under dark; the desired complexes were
obtained as off–white, light–sensitive solids in 90–95% isolated yields after recrystallization
from acetonitrile/diethyl ether mixtures (Scheme 2). The complexes 9–14 were readily
soluble in polar organic solvents such as DMSO, DMF, acetonitrile, acetone and sparingly
soluble in DCM and insoluble in non–polar organic solvents. These new compounds were
1
fully characterized by H and {1H}13C NMR and ATR–IR spectroscopic and elemental
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analyses and conductivity studies. The structure of representative complexes, 9 and 12 were
unambiguously established following single crystal X–diffraction method.
To investigate the implication of a bromido ligand on the structural and biological
characteristics of silver(I) complexes, a set of neutral silver(I) complexes having general
molecular formula [NHC–Ag–Br] has been explored. The reactions of the ether substituted
(benz)imidazolium bromide salts with silver(I) oxide in dichloromethane for 24 h at room
temperature under dark yielded the desired ether–functionalized NHC coordinated silver(I)
bromido complexes, 15–20, as light–sensitive solids depicted in Scheme 3. These complexes
were purified by repeated precipitation in methanol using diethyl ether to yield stable beige
colored solids in good to moderate yields. Melting points of these complexes found relatively
lesser than that of bis–NHC coordinated silver complexes, which could be presumably due to
the easier elimination of bromide as hydrogen bromide, while this is not the case with the
latter complexes. Like, bis–NHC coordinated complexes, NHC silver(I) bromido complexes
are soluble in high polar organic solvents and insoluble in less polar solvents such as diethyl
ether and n–pentane. All these bromido–coordinated complexes were fully characterized
1
through classical techniques such as H and {1H}13C NMR and ATR–IR spectroscopy and
elemental analysis. Despite various attempts to grow suitable single crystals of these
complexes for structure determination through X–ray diffraction technique, all our efforts
ended up giving either amorphous solids or multiple crystals that are not suitable for desired
analysis.
2.2. Spectral characterization
The formation of the desired coumarin and ether–functionalized (benz)imidazolium salts and
their silver(I) mono– and bis–carbene complexes was confirmed by NMR spectroscopy in
d6–DMSO, and the NMR data of complexes together with their carbene precursors are
presented in experimental section. For the bis–carbene complexes, the two NHC groups are
chemically equivalent as are the coumarin and ether substituents of the azole rings are same.
1
In the H NMR spectra of the azolium salts, a characteristic singlet proton resonance peak
centered at δ 9.28–9.82 ppm was observed, which is attributed to the resonance of C2
(NCHN) proton. Similarly, coumarin methyl, methoxyethyl and C4–coumarin methylene
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proton resonances were observed at around δ 2.3, 3.0–4.7 and 6.0 ppm, respectively, and are
in well agreement with the NMR data of analogous 4–substituted coumarin and alkoxy
derivatives [13]. Further, two sharp singlet peaks were observed at around δ 6.0 and 7.5 ppm
in the cases of salts 3, 4, 6 and 7, which have been attributed to the resonances of coumarin
C3 and C5 protons, respectively and at δ 6.0 ppm in the cases of salts 5 and 8 for former
proton resonances. Coumarin, imidazole and benzimidazole ring aromatic proton resonances
have been observed in the range δ 6.0–8.2 ppm, which are in good agreement with the
13
similar azolium analogues reported earlier by us [14] and others [15]. In their C NMR
spectra, salts displayed a characteristic peak at around δ 137.5–137.7 and 143.6–143.7 ppm
for imidazolium and benzimidazolium salts, respectively; ascribed to the resonance of C2
carbon (NCHN). Furthermore, three characteristic carbon resonance signals were observed at
around δ 148, 152 and 159 ppm assigned to the resonances of coumarin ring carbons nuclei.
Finally, two sets of distinguished carbon resonance peaks were observed at around δ 51–71
and 112–145 ppm, ascribed to the resonances of aliphatic methoxyethyl and aromatic carbon
nuclei, respectively. Observed 13C NMR spectral data and their corresponding interpretations
are in good agreement with the reported azolium analogues, which cannot act as a bidentate
chelating NHC ligand [16].
1
As expected, in the H NMR spectra of mono– and bis–carbene silver complexes 9–20,
singlet peaks centered at δ 9.28–9.82 ppm representing their C2 proton (NCHN) resonances
of azolium salts 3–8 were notably absent. This observation confirmed the successful
formation of desired silver complexes through deprotonation of azolium salts. It is worth
13
mentioning that in the corresponding C NMR spectra of the complexes, resonances of
carbene carbon atom coordinating with silver center were observed at around δ 191.7 and
192.4 ppm for complexes 13 and 14, respectively; however, in the cases of rest of the
complexes, this resonance was not observed, which is due to the bulky nature of NHC
ligands. In all the cases, a distinguished carbon resonance peak centered at δ 137.6 and 143.6
ppm for imidazolium and benzimidazolium salts, respectively representing their C2 carbon
(NCHN) resonance of salts was particularly absent. These two observations collectively
confirmed the formation of desired silver complexes. Unsurprisingly, the other proton and
carbon resonances are appeared in the complex spectra with no or minor differences with that
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of corresponding salts. All the carbon NMR spectral observations and their interpretations
are in line with the similar compounds reported earlier [17].
Azolium salts, 3–8, and their mono– and bis–carbene coordinated silver complexes, 9–20,
were studied for their structure using ATR–IR spectroscopy over the range 600–4000 cm–1.
Upon proper examination of the IR spectra of the silver complexes 9–14, in comparison with
those of the free azolium salts, the most significant observation was the appearance of azole
stretching vibrations to the lesser energy region. Further, no significant changes were
observed in the stretching vibrations of lactonic carbonyl module in all the complexes,
indicating its non–involvement in the coordination with the metal center. It is worth
mentioning that in several reports such a characteristic observation was attributed to the non–
participation of the lactone carbonyl module in complex formation [18]; nevertheless, it has
also been reported for numerous 3–/4–substituted coumarin derivatives that the lactone
carbonyl was involved in coordination [19], which provides structural diversity to the
resulting complexes. It is highly difficult to distinguish mono– and bis–carbene complexes
on the basis of their IR spectra. In all the cases, however, sharp high intensity bands without
significant change in their position were observed at around 2860, 2935 cm–1 and 980 cm–1,
ascribed to the stretching vibrations of aliphatic/aromatic C–H and C–O modules,
respectively.
2.3. Elemental analyses and molar conductivity studies
In addition, the azolium salts and their mono– and bis–carbene coordinated silver(I)
complexes are further characterized by CHN microelemental analyses and the data obtained
is in well agreement with the calculated percentages of these elements with an acceptable
limit of variation with ±0.4 % in all the cases.
Room–temperature, solution conductivities of the azolium salts and bis–carbene and mono–
carbene coordinated silver(I) complexes are provided in the experimental part. These molar
conductivity values were determined for compounds 3–20 in dichloromethane at 10–3 M
concentration. By their nature, these conductivity measurements are influenced by the
electrostatic force of attraction between azolium cations and hexafluorophosphate anions.
The azolium salts displayed a molar conductivity value in the range 141–170 S cm2 mol–1,
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which is well above the observed molar conductivity values reported for non–complex 1:1
electrolytes such as tetraalkylammonium halides/perchlorates [20]. However, molar
conductivity values of bis–carbene coordinated silver(I) complexes fall into the range
expected for weak 1:1 electrolytes [21], 43–60 S cm2 mol–1 in dichloromethane, which could
be presumably due to the strong interactions operating between NHC–silver(I) cations and
hexafluorophosphate anions, which is further evidenced by the single crystal X–ray
diffraction studies. As expected, mono NHC coordinated silver complexes displayed almost
no molar conductivity, and therefore it can be concluded that these complexes are neutral in
nature, which further confirms the coordination of bromido ligand with the silver center.
2.4. Single crystal X–ray diffraction studies
To obtain the solid–state bonding and special connectivity information between azolium or
bis–NHC silver complex cations and hexafluorophosphate anions, a representative
benzannulated imidazolium hexafluorophosphate salt 5 and a benzimidazole complex 12
were structurally elucidated using single crystal X–ray diffraction technique. Suitable X–ray
quality crystals of 5 and 12 were obtained through the diffusion of diethyl ether into the
acetonitrile solution of 5 and 12, which allowed us to get insights on the solid–state structure
of the compounds.
The crystallographic and refinement data for compounds 5 and 12 are shown in Table 1. The
molecular structure of imidazolium salt 5 is depicted in Figure 1. Imperative bond distances
and angles for salt 5 are tabulated in Table 2. Salt 5 is a well ordered structure, crystallized in
the triclinic space group, P–1, having one 5,6–benzannulated coumarin substituted
imidazolium cation as well as one hexafluorophosphate anion in the asymmetric unit without
any co–crystallized solvent or water molecule. The 5,6–benzannulated coumarin and
imidazole rings are almost planar and aromatic with C–C, C–O and C–N bond distances in
the range 1.357(2)–1.465(3) Å. The dihedral angle between the planes of these two aromatic
systems is 114.3(2)o, and the ether wingtip is intact with the salt structure, attached to other
nitrogen atom of the imidazole ring with a bond angle of 110.9(2)o for N2–C4–C5 having no
disorders. The internal bond angle at imidazole ring C1 and benzannulated coumarin O2
atom are 108.4(2)o and 122.4(1)o for N1–C1–N2 and C10–O2–C11, respectively. Further, the
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bond distance for exocyclic C10–O3 of coumarin and O1–C6 of ether are 1.213(3) and
1.422(2) Å, respectively. These bond distances and angles are in well agreement with the
similar previously reported azolium [22] and substituted coumarin derivatives [23]. In the
extended crystal structure of the salt 5, the imidazolium cations are assembled in a head–to–
tail overlapping manner as shown in Figure S1 (Supporting Information), due to which the
interactions between the benzannulated coumarin rings of adjacent cations stack along the c–
axis. This stacking arrangement of the imidazolium cations lead to the formation of feeble π–
π stacking interactions between coumarin rings parallel to the bc–plane with the interaction
distance of 3.508 Å. Alongside, two predominant intramolecular (OCH3---F2 and C1H---
O1) and one intermolecular (C16H---F3) hydrogen bonding interactions have been observed
in the crystal structure with the interaction bond distance of 2.947, 2.634 and 2.803 Å,
respectively. These hydrogen bonding interactions and many short distance interactions
stabilize the salt and in turn formed 3–dimensional networks between imidazolium cations
and hexafluorophosphate anions.
On the other hand, complex 12 crystallized in the triclinic space group, P–1, with the
silver(I) at the inversion center. An asymmetric unit of complex 12 is composed of half a
molecule of the complex cation and half a molecule of the hexafluorophosphate anion. The
hexafluorophosphate anion displayed positional disorder over two sets, which has been
satisfactorily refined. The molecular structure of complex 12 is depicted in Figure 2 with
thermal ellipsoids drawn at the 50% probability level. Pertinent bond distances and angles for
complex 12 are tabulated in Table 3. The ion pair portrayed in the figure bearing bis–NHC
coordinated silver(I) as cationic fragment and the hexafluorophosphate as the counter anionic
fragment (not shown). In the complex, silver(I) center adopted a perfectly linear coordination
geometry through the bond angle of 180o for C1–Ag1–C1A with an antiparallel eclipsed
arrangement of NHC ligands. The bond angle at carbene carbon center in benzimidazole
ring, 106.0(2)o for N1–C1–N2, found significantly declined due to the coordination of
carbene carbon with the silver center [24], while the bond angle at coumarin heterocyclic
oxygen atom, 121.7(7)o for C11–O1–C12, found almost unaffected compared with its
structural analogous salt 5. As foreseen, the two coumaryl substituents projected away from
the coordination sphere, and hence the steric repulsion between the two NHC ligands has
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been reduced. Interestingly, two types of distinct intermolecular bonding interactions have
been observed between Ag1–H19, Ag1–H19A and Ag1–H8, Ag1–H8A with the interaction
distances of 2.978 and 3.007 Å, respectively that are slightly higher than that of the sum of
the Van Der Walls radii of silver and hydrogen (2.92 Å). Like salt 5, the dihedral angle
between the planes of benzimidazole and coumarin rings in the case of complex 12 is
113.7(2)o for N2–C8–C9 and exocyclic lactonic carbonyl bond distance is 1.208(3) Å.
Observed bond angles and bond distances are well comparable with the reported complexes
bearing non–coordinating coumarin substituents [25]. Further, the methoxyethyl chain is
intact with the structure, making a bond angle of 112.6(2)o for N1–C19–C20 with the azole
without any disorder. In the extended crystal structure of the complex (Figure S2, Supporting
Information), an intra–cationic hydrogen bonding interaction between ethereal oxygen and
ethyl hydrogen atoms (2.603 Å for O3---H19) and two types of prominent interionic
hydrogen bonding interactions between methoxy hydrogen and phenyl ring hydrogen atoms
with hexafluorophosphate anion have been observed (3.185 Å for C21H---F3 and 2.853 Å
for C13H---F1). Further, the benzimidazolium cations aggregate in a staggered orientation,
leading to the formation of weak π–π stacking interactions between the benzimidazole rings
of adjacent molecules parallel to b–axis with an interaction bond distance of 3.695 Å.
Together with hydrogen bonding interactions, π–π stacking and other short distance
interactions operated in the complex crystal to form the 3–dimensional networks between
benzimidazolium cations and hexafluorophosphate anions.
2.4. Antimicrobial studies
The existence of multidrug–resistant bacterial strains of S. aureus and P. aeruginosa to well–
known penicillin and sulfonamide drugs opened an avenue for the metal–based drugs as
newer alternatives. Recently, metallocene derivatives of iron and ruthenium were reported
for their antibacterial properties by high level of inhibition of DD–carboxypeptidase 64–575, a
penicillin binding protein through the formation of a covalent acyl−enzyme complex [26]. In
this perspective, silver(I) complexes of both functionalized and non–functionalized NHC
ligands have been proved as a class of important candidates possessing promising
antibacterial and anticancer potentials [27]. These potentials of complexes are majorly
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dependent on the nature of the wingtip substitutes of NHC ligands that in turn lead to the
formation of diverse structural motifs [5,28]. In the present study, we selected a series of
structurally varied analogues of coumarin and ether functionalized (benz)imidazolium salts
and corresponding two series of structurally different silver(I) NHC complexes to explore
their antibacterial potentials against Gram positive bacterial strains, S. aureus (ATCC
29213), B. subtilis (ATCC 6633), and Gram negative bacterial strains, E. coli (ATCC
25922), S. typhi (ATCC 19214) following broth microdilution method using ampicillin as a
standard drug.
The MIC (minimum inhibitory concentration) values for both (benz)imidazolium salts (3–8)
and corresponding bis–carbene (9–14) and mono–carbene (15–20) coordinated silver(I)
complexes are summarized in Table 4. All the microbial experiments were carried out in
triplicates and at least any two concurrent results are discussed. The solvent, DMSO, used to
prepare the stock solutions of the test samples 9–20 played no role in growth inhibition of the
bacterial strains. In line with our previous observations [10,11,14,29], both the types of
azolium salts found inactive against all the bacterial strains tested in the working
concentration range of 0.5–128 µg/mL. However, bis– and mono–carbene coordinated silver
complexes displayed poor antimicrobial activity against Gram positive bacteria, B. subtilis,
with a MIC value of 128 µg/mL. This observation could be attributed to the poor activity of
azolium salts. Meanwhile, former complexes evidenced a promising activity against S.
aureus with a MIC of 16 µg/mL, while latter complexes displayed an activity with a MIC
value in the range 16–128 µg/mL. Nevertheless, the antimicrobial activity of bis–NHC
complexes against Gram negative bacteria, E. coli, is found significant with a MIC value in
the range 8–16 µg/mL, while mono–NHC complexes displayed activity that is two dilutions
higher than corresponding bis–NHC complexes. Finally, all the complexes displayed poor
antimicrobial activity against S. typhi with a MIC value of 128 µg/mL, except complex 14,
which has shown a MIC of 64 µg/mL. Since the antimicrobial data of the complexes is a
preliminary study, it is difficult to predict the structure activity relationships.
3. Conclusions
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A series of ether and substituted coumarin functionalized (benz)imidazolium bromide and
hexafluorophosphate salts and their corresponding ionic bis–carbene coordinated silver(I)
hexafluorophosphate and neutral mono–carbene coordinated silver(I) bromido complexes are
reported. Salts were prepared in good to excellent yields by successive N–alkylation method
using 1–bromo–2–methoxyethane and substituted 4–bromomethyl coumarins as N–
alkylating agents with 1H–imidazole or 1H–benzimidazole followed by salt metathesis
reaction with KPF6. Difference in the counterions of the azolium salts and reaction
conditions resulted in the formation of two series of carbene coordinated silver complexes.
Salts with non–coordinating anion, hexafluorophosphate anion, treated with silver(I) oxide at
slightly elevated temperature in acetonitrile afforded ionic bis–carbene coordinated silver(I)
complexes with hexafluorophosphate as a counterion, while bromide salts treated with the
metal source in dichloromethane at room temperature resulted in the formation of neutral
mono–carbene coordinated silver(I) bromido complexes in excellent yields. All salts and
1
complexes were characterized by conventional spectroscopic techniques such as H and
{1H}13C NMR and ATR–IR spectroscopies and CHN microelemental analyses. Furthermore,
the structure of a representative benzo–coumarin substituted imidazolium salt and a
benzimidazole–based bis–carbene silver(I) hexafluorophosphate complex were
unambiguously studied by single crystal X–ray diffraction method. In the preliminary
antibacterial studies against two Gram positive and two Gram negative bacterial strains,
silver complexes were found active with MIC values in the range 8–64 µg/mL, while all the
salts displayed almost no activity against all the bacteria tested.
4. Experimental
4.1. General considerations
All solvents and chemicals used in the present investigation were obtained from commercial
sources and used as received unless otherwise indicated. Starting chemicals such as 1H–
imidazole, 1H–benzimidazole, 1–bromo–2–methoxyethane, ethyl acetoacetate, bromine, 4–
methylphenol, 4–chlorophenol, 2–naphthol, concentrated sulphuric acid, potassium
hexafluorophosphate, silver(I) oxide, nutrient agar and nutrient broth were purchased from
commercial sources. N–alkylated azoles such as, 1–(2–methoxyethyl)–1H–imidazole (1) and
13

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1–(2’–methoxyethyl)–1H–benzimidazole (2) [30] and coumarin derivatives such as, 4–
bromomethyl–6–methylcoumarin, 4–bromomethyl–6–chlorocoumarin and 4–bromomethyl–
5,6–benzocoumarin [31] were synthesized following literature procedures with slight
modifications. The preparations of (benz)imidazolium bromide/hexafluorophosphate salts
and corresponding mono– and bis–carbene silver(I) complexes were performed without
using sophisticated Schlenk technique and dry box. Thin layer chromatography was carried
out on Merck 1.05554 aluminum sheets precoated with silica gel 60 F254, and the desired
products spots were witnessed with UV light at 254 nm or in an iodine chamber. 1H and 13C
NMR spectra of all the new derivatives were collected in d6–DMSO solvent either on Bruker
AVANCE III 400 MHz or Bruker 300 MHz NMR spectrometer. Chemical shifts (δ ppm) are
given with solvent peaks as the internal references, and the signals are labeled as singlet (s),
doublet (d), triplet (t), and multiplet (m). The melting points were assessed using a Stuart
Scientific (UK) instrument with an accuracy of ±0.3 oC. CHN microelemental analyses were
performed using a Perkin Elmer 2400 Series II CHN/S microanalyzer. Infrared spectra were
recorded on a Bruker ECO–ATR spectrophotometer in the range 600–4000 cm–1.
4.2. X–ray crystallography
For the crystals of salt 5: The X–ray intensity data were collected at a temperature of 296(2)
K on a Bruker Proteum2 CCD diffractometer equipped with an X–ray generator operating at
45 kV and 10 mA, using CuKα radiation of wavelength 1.54178 Å. The structure was solved
by direct methods and refined by full–matrix least squares method on F2 using SHELXS and
SHELXL programs, while the geometrical calculations were carried out using the PLATON
program. The molecular graphic designs and packing diagrams of salt 5 for publication were
performed using OLEX2 and MERCURY software packages.
For the crystals of complex 12: The crystal was mounted in turn on a Gemini A Ultra Oxford
Diffraction automatic diffractometer equipped with a CCD detector, and used for data
collection. X–ray intensity data were collected with graphite monochromated MoKα
radiation (λ = 0.71073 Å) at a temperature of 295(2) K, with ω scan mode. Lorentz,
polarization and empirical absorption correction using spherical harmonics implemented in
SCALE3 ABSPACK scaling algorithm [32] were applied. All the non–hydrogen atoms were
14

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refined anisotropically using full–matrix, least–squares technique. All the hydrogen atoms
were found from difference Fourier synthesis after four cycles of anisotropic refinement, and
refined as “riding” on the adjacent carbon atom with individual isotropic temperature factor
equal 1.2 times the value of equivalent temperature factor of the parent atom. The Olex2 [33]
and SHELXS, SHELXL [34] programs were used for all the calculations. The geometrical
calculations were carried out using the PLATON program. The molecular graphic designs
and packing diagram for publication were performed using Olex2 and MERCURY software
packages.
4.3. Synthesis of 1–(2–methoxyethyl)–3–((6–methyl–2–oxo–2H–chromen–4–yl)methyl)
imidazolium bromide/hexafluorophosphate (3)
A solution of 1–(2–methoxyethyl)–1H–imidazole (1) (0.254 g, 2 mmol) in 35 mL of 1,4–
dioxane was added drop wise a solution of 4–(bromomethyl)–6–methyl–coumarin (0.506 g,
2 mmol) in 1,4–dioxane and stirred at room temperature for homogenization. The pale
yellow mixture was allowed to stir at reflux for 24 h based on TLC analysis. After the
stipulated time, the mixture was cooled to room temperature and the off–white solid obtained
was filtered and washed with fresh 1,4–dioxane (3 x 5 mL) to yield the corresponding
imidazolium bromide salt. Further, to a methanolic solution of so obtained bromide salt was
added a methanol–water solution (9:1 v/v) of potassium hexafluorophosphate at room
temperature and stirred for 4 h to precipitate the imidazolium hexafluorophosphate salt 3 as
an off–white solid through salt metathesis reaction. Crude salt 3 was filtered and washed
with fresh methanol and distilled water to remove any unreacted potassium
hexafluorophosphate and was purified by repeated precipitation in acetonitrile and diethyl
1
ethyl ether mixture. Yield: 65.2 %; M.P.: 140–141 °C. H NMR (300 MHz, d6–DMSO, 298
K): δ 2.49 (3H, s, CH3–coumarin), 3.27 (3H, s, CH3–O–CH2–CH2), 3.73–3.70 (2H, m, CH3–
O–CH2–CH2), 4.40 (2H, t, J = 8.8 Hz, CH3–O–CH2–CH2), 5.81 (2H, s, CH2–C4–coumarin),
5.98 (1H, s, C3H–coumarin), 7.40 (1H, d, J = 8.4 Hz, C4H–imidazole), 7.53 (1H, d, J = 8.6
Hz, C5H–imidazole), 7.66–7.60 (1H, m, ArCH–coumarin), 7.87–7.81 (2H, m, ArCH–
13
coumarin), 9.28 (1H, s, C2H–imidazole). C{1H} NMR (75 MHz, d6–DMSO, 298 K): δ
20.3 (CH3–coumarin), 48.9 (CH3–O–CH2–CH2), 49.0 (CH3–O–CH2–CH2), 50.0 (CH3–O–
15

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CH2–CH2), 69.3 (CH2–C4–coumarin), 123.3, 124.0 (C4, C5–imidazole), 113.4, 116.6, 123.0,
133.5, 134.1, 149.1, 151.0, 159.4 (ArC–coumarin), 137.5 (C2–imidazole). ATR–IR (in cm–
1): 2920, ~2860 ѵ(C–H, aliphatic and aromatic), 1703 ѵ(C=O, lactonic), 1568 ѵ(C=N,
imidazole), 1118 ѵ(C–O, coumarin). Anal. Calc. for C17H19N2O3PF6: C, 46.0; H, 4.3; N, 6.3.
Found: C, 46.3; H, 4.6; N, 6.5. Molar conductance (in CH2Cl2, 302 K): 150 S cm2 mol–1.
4.4. Synthesis of 1–(2–methoxyethyl)–3–((6–chloro–2–oxo–2H–chromen–4–yl)methyl)
imidazolium bromide/hexafluorophosphate (4)
Salt 4 was prepared following the procedure analogous to that of salt 3 with 4–
(bromomethyl)–6–chloro–coumarin (0.547 g, 2 mmol) instead of 4–(bromomethyl)–6–
1
methyl–coumarin. Off–white solid; Yield: 67.1 %; M.P.: 162 °C. H NMR (300 MHz, d6–
DMSO, 298 K): δ 3.26 (3H, s, CH3–O–CH2–CH2), 3.71 (2H, t, J = 8.8 Hz, CH3–O–CH2–
CH2), 4.39 (2H, t, J = 8.4 Hz, CH3–O–CH2–CH2), 5.80 (2H, s, CH2–C4–coumarin), 6.12
(1H, s, C3H–coumarin), 7.55 (1H, d, J = 8.7 Hz, C4H–imidazole), 7.77 (1H, d, J = 8.8 Hz,
C5H–imidazole), 7.85 (2H, d, J = 7.3 Hz, ArCH–coumarin), 7.96 (1H, s, ArCH–coumarin),
13
9.26 (1H, s, C2H–imidazole). C{1H} NMR (75 MHz, d6–DMSO, 298 K): δ 48.1 (CH3–O–
CH2–CH2), 49.0 (CH3–O–CH2–CH2), 58.0 (CH3–O–CH2–CH2), 69.3 (CH2–C4–coumarin),
114.9, 118.9, 122.9, 128.7, 132.3, 148.2, 151.6, 158.8 (ArC–coumarin), 123.4, 123.9 (C4,
C5–imidazole), 137.5 (C2–imidazole). ATR–IR (in cm–1): 2912, ~2860 ѵ(C–H, aliphatic and
aromatic), 1720 ѵ(C=O, lactonic), 1552 ѵ(C=N, imidazole), 1122 ѵ(C–O, coumarin). Anal.
Calc. for C16H16N2O3ClPF6: C, 41.4; H, 3.5; N, 6.0. Found: C, 41.0; H, 3.7; N, 6.1. Molar
conductance (in CH2Cl2, 302 K): 165 S cm2 mol–1.
4.5. Synthesis of 1–(2–methoxyethyl)–3–((5,6–benzo–2–oxo–2H–chromen–4–yl)methyl)
imidazolium bromide/hexafluorophosphate (5)
Salt 5 was prepared following the procedure analogous to that of salt 3 with 4–
(bromomethyl)–5,6–benzo–coumarin (0.578 g, 2 mmol) instead of 4–(bromomethyl)–6–
1
methyl–coumarin. Off–white solid; Yield: 83.0 %; M.P.: 160–162 °C. H NMR (300 MHz,
d6–DMSO, 298 K): δ 3.26 (3H, s, CH3–O–CH2–CH2), 3.70 (2H, t, J = 8.6 Hz, CH3–O–CH2–
CH2), 4.43 (2H, t, J = 8.8 Hz, CH3–O–CH2–CH2), 5.82 (1H, s, CH2–C4–coumarin), 6.35
(1H, s, C3H–coumarin), 7.79–7.64 (3H, m, ArCH–imidazole/coumarin), 7.88–7.83 (2H, m,
16

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ArCH–coumarin), 8.15 (1H, d, J = 8.1 Hz, ArCH–coumarin), 8.31 (1H, d, J = 9 Hz, ArCH–
coumarin), 8.37 (1H, d, J = 8.7 Hz, ArCH–coumarin), 9.25 (1H, s, C2H–imidazole).
13C{1H} NMR (75 MHz, d6–DMSO, 298 K): δ 49.0 (CH3–O–CH2–CH2), 52.4 (CH3–O–
CH2–CH2), 58.0 (CH3–O–CH2–CH2), 69.3 (CH2–C4–Coumarin), 112.5, 117.4, 123.0, 125.9,
128.4, 128.6, 129.9, 130.9, 134.6, 151.7, 154.2, 158.9 (ArC–benzo–coumarin), 123.4, 125.1
(C4, C5–imidazole), 137.7 (C2–imidazole). ATR–IR (in cm–1): 3059, 2965 ѵ(C–H, aliphatic
and aromatic), 1695 ѵ(C=O, lactonic), 1550 ѵ(C=N, imidazole), 1097 ѵ(C–O, coumarin).
Anal. Calc. for C20H19N2O3PF6: C, 50.0; H, 4.0; N, 5.8. Found: C, 50.5; H, 4.4; N, 6.0. Molar
conductance (in CH2Cl2, 302 K): 141 S cm2 mol–1.
4.6. Synthesis of 1–(2–methoxyethyl)–3–((6–methyl–2–oxo–2H–chromen–4–yl)methyl)
benzimidazolium bromide/hexafluorophosphate (6)
Salt 6 was prepared following the procedure analogous to that of salt 3 with 1–(2–
methoxyethyl)–1H–benzimidazole (2) (0.353 g, 2 mmol) instead of 1–(2–methoxyethyl)–
1H–imidazole (1). Yellow solid; Yield: 80.7 %; M.P.: 230–232 °C. 1H NMR (300 MHz, d6–
DMSO, 298 K): δ 2.44 (3H, s, CH3–coumarin), 3.28 (3H, s, CH3–O–CH2–CH2), 3.83 (2H, t,
J = 8.8 Hz, CH3–O–CH2–CH2), 4.77–4.73 (2H, m, CH3–O–CH2–CH2), 5.86 (1H, s, C3H–
coumarin), 6.19 (2H, s, CH2–C4–coumarin), 7.41 (1H, d, J = 8.4 Hz, ArCH–benzimidazole),
7.56 (1H, d, J = 8.4 Hz, ArCH–benzimidazole), 7.77–7.69 (3H, m, ArCH–
benzimidazole/coumarin), 8.04 (1H, d, J = 7.8 Hz, ArCH–coumarin), 8.18 (1H, d, J = 8.0
Hz, ArCH–coumarin), 9.82 (1H, s, C2H–benzimidazole). 13C{1H} NMR (75 MHz, d6–
DMSO, 298 K) : δ 20.3 (CH3–coumarin), 46.3 (CH3–O–CH2–CH2), 46.8 (CH3–O–CH2–
CH2), 58.1 (CH3–O–CH2–CH2), 68.7 (CH2–C4–coumarin), 112.2, 113.6, 114.1, 124.3, 126.9
(ArC–benzimidazole), 116.6, 126.7, 131.0, 131.4, 133.5, 133.9, 148.7, 151.0, 159.4 (ArC–
coumarin), 143.6 (C2–benzimidazole). ATR–IR (in cm–1): 2962, ~2890 ѵ(C–H, aliphatic and
aromatic), 1705 ѵ(C=O, lactonic), 1569 ѵ(C=N, benzimidazole), 1108 ѵ(C–O, coumarin).
Anal. Calc. for C21H21N2O3PF6: C, 51.0; H, 4.3; N, 5.7. Found: C, 51.2; H, 4.3; N, 5.8. Molar
conductance (in CH2Cl2, 302 K): 144 S cm2 mol–1.
4.7. Synthesis of 1–(2–methoxyethyl)–3–((6–chloro–2–oxo–2H–chromen–4–yl)methyl)
benzimidazolium bromide/hexafluorophosphate (7)
17

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Salt 7 was prepared following the procedure analogous to that of salt 3 with 1–(2–
methoxyethyl)–1H–benzimidazole (2) (0.353 g, 2 mmol) and 4–(bromomethyl)–6–chloro–
coumarin (0.547 g, 2 mmol) instead of 1–(2–methoxyethyl)–1H–imidazole (1) and 4–
(bromomethyl)–6–methyl–coumarin, respectively. Pale yellow solid; Yield: 98.5 %; M.P.:
1
236–237 °C. H NMR (300 MHz, d6–DMSO, 298 K): δ 3.28 (3H, s, CH3–O–CH2–CH2),
3.85–3.82 (2H, m, CH3–O–CH2–CH2), 4.76–4.73 (2H, m, CH3–O–CH2–CH2), 5.98 (1H, s,
C3H–coumarin), 6.19 (2H, s, CH2–C4–coumarin), 7.56 (1H, d, J = 9.0 Hz, ArCH–
benzimidazole) 7.82–7.70 (3H, m, ArCH–benzimidazole), 8.07–8.01 (2H, m, ArCH–
coumarin), 8.18 (1H, d, J = 8.1 Hz, ArCH–coumarin), 9.80 (1H, s, C2H–benzimidazole).
13C{1H} NMR (75 MHz, d6–DMSO, 298 K): δ 46.2 (CH3–O–CH2–CH2), 46.8 (CH3–O–
CH2–CH2), 58.1 (CH3–O–CH2–CH2), 68.7 (CH2–C4–coumarin), 113.4, 114.1, 118.2, 118.8,
124.3, 126.8 (ArC–benzimidazole), 113.6, 126.9, 128.6, 130.9, 131.5, 132.3, 147.9, 151.6,
158.8 (ArC–coumarin), 143.6 (C2–benzimidazole). ATR–IR (in cm–1): ~2930, 2820 ѵ(C–H,
aliphatic and aromatic), 1715 ѵ(C=O, lactonic), 1562 ѵ(C=N, benzimidazole), 1103 ѵ(C–O,
coumarin). Anal. Calc. for C20H18N2O3ClPF6: C, 46.7; H, 3.5; N, 5.4. Found: C, 46.7; H, 3.5;
N, 5.5. Molar conductance (in CH2Cl2, 302 K): 165 S cm2 mol–1.
4.8. Synthesis of 1–(2–methoxyethyl)–3–((5,6–benzo–2–oxo–2H–chromen–4–yl)methyl)
benzimidazolium bromide/hexafluorophosphate (8)
Salt 8 was prepared following the procedure analogous to that of salt 3 with 1–(2–
methoxyethyl)–1H–benzimidazole (2) (0.353 g, 2 mmol) and 4–(bromomethyl)–5,6–benzo–
coumarin (0.578 g, 2 mmol) instead of 1–(2–methoxyethyl)–1H–imidazole (1) and 4–
(bromomethyl)–6–methyl–coumarin, respectively. Pale yellow solid; Yield: 87.5 %; M.P.:
1
254–256 °C. H NMR (300 MHz, d6–DMSO, 298 K): δ 3.26 (3H, s, CH3–O–CH2–CH2),
3.84–3.81 (2H, m, CH3–O–CH2–CH2), 4.78–4.75 (2H, m, CH3–O–CH2–CH2), 5.81 (1H, s,
C3H–coumarin), 6.69 (2H, s, CH2–C4–coumarin), 7.80–7.69 (5H, m, ArCH–
benzimidazole/coumarin), 8.08 (1H, d, J = 8.1 Hz, ArCH–coumarin), 8.19 (2H, m, ArCH–
coumarin), 8.34 (1H, d, J = 9.0 Hz, ArCH–coumarin), 8.52 (1H, d, J = 8.7 Hz, ArCH–
coumarin), 9.76 (1H, s, C2H–benzimidazole). 13C{1H} NMR (75 MHz, d6–DMSO, 298 K): δ
46.3 (CH3–O–CH2–CH2), 48.5 (CH3–O–CH2–CH2), 58.1 (CH3–O–CH2–CH2), 68.7 (CH2–
18

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C4–coumarin), 111.7, 112.3, 114.2, 117.5, 125.6, 126.8 (ArC–benzimidazole), 114.0, 125.9,
127.0, 128.5, 128.7, 130.9, 131.0, 131.4, 134.7, 151.2, 154.3, 158.9 (ArC–benzo–coumarin),
143.7 (C2–benzimidazole). ATR–IR (in cm–1): ~2930, 2812 ѵ(C–H, aliphatic and aromatic),
1711 ѵ(C=O, lactonic), 1556 ѵ(C=N, benzimidazole), 1102 ѵ(C–O, coumarin). Anal. Calc.
for C24H21N2O3PF6: C, 54.4; H, 4.0; N, 5.3. Found: C, 54.5; H, 4.3; N, 5.6. Molar
conductance (in CH2Cl2, 302 K): 170 S cm2 mol–1.
4.9. Synthesis of bis{1–(2–methoxyethyl)–3–((6–methyl–2–oxo–2H–chromen–4–yl)methyl)
imidazol–2–ylidene}silver(I) hexafluorophosphate complex (9)
A suspension of imidazolium hexafluorophosphate salt 3 (0.445 g, 1 mmol) and silver(I)
oxide (0.138 g, 0.6 mmol) in acetonitrile was stirred at 45 °C for 24 h under the exclusion of
light (in a round bottom flask wrapped with aluminum foil). After the stipulated time, the
reaction mixture was filtered through a bed of cilite and filtrate was dried over anhydrous
magnesium sulfate. The solvent was removed in vacuo, and the resulted crude product was
washed with methanol (3 x 3 mL) and purified by repeated precipitation in acetonitrile–
diethyl ether mixture to afford light sensitive complex 9 as an off–white solid. Yield: 95.0 %;
1
M.P.: 112–114 °C. H NMR (400 MHz, d6–DMSO, 298 K): δ 2.31 (3H, s, CH3–coumarin),
3.09 (3H, s, CH3–O–CH2–CH2), 3.50 (2H, t, J = 8.4 Hz, CH3–O–CH2–CH2), 4.25 (2H, m,
CH3–O–CH2–CH2), 5.52 (1H, s, C3H–coumarin), 5.64 (2H, s, CH2–C4–coumarin), 7.24 (1H,
d, J = 8.4 Hz, C4H–imidazole), 7.43 (1H, d, J = 8.4 Hz, C5H–imidazole), 7.53–7.52 (3H, m,
13
ArCH–coumarin). C{1H} NMR (100 MHz, d6–DMSO, 298 K): δ 20.8 (CH3–coumarin),
50.8 (CH3–O–CH2–CH2), 51.3 (CH3–O–CH2–CH2), 58.4 (CH3–O–CH2–CH2), 71.6 (CH2–
C4–coumarin), 123.2, 124.3 (C4, C5–imidazole), 112.5, 116.9, 117.0, 133.7, 134.2, 151.3,
152.2, 159.7 (ArC–coumarin), C2–imidazole absent. ATR–IR (in cm–1): 2883, 2930 ѵ(C–H,
aliphatic and aromatic), 1720 ѵ(C=O, lactonic), 1572 ѵ(C=N, imidazole), 1113 ѵ(C–O,
coumarin). Anal. Calc. for C34H36N4O6AgPF6: C, 48.1; H, 4.3; N, 6.6. Found: C, 48.5; H, 4.3;
N, 6.9. Molar conductance (in CH2Cl2, 302 K): 58 S cm2 mol–1.
4.10. Synthesis of bis{1–(2–methoxyethyl)–3–((6–chloro–2–oxo–2H–chromen–4–yl)methyl)
imidazol–2–ylidene}silver(I) hexafluorophosphate complex (10)
19

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Complex 10 was prepared following the procedure analogous to that of bis–carbene
coordinated silver(I) complex 9 with imidazolium hexafluorophosphate salt 4 (0.464 g, 1
mmol) instead of imidazolium hexafluorophosphate salt 3. Off–white solid; Yield: 93.0 %;
M.P.: 82–84 °C. 1H NMR (400 MHz, d6–DMSO, 298 K): δ 3.12 (3H, s, CH3–O–CH2–CH2),
3.55 (2H, t, J = 8.8 Hz, CH3–O–CH2–CH2), 4.28 (2H, t, J = 8.8 Hz, CH3–O–CH2–CH2), 5.57
(1H, s, C3H–coumarin), 5.68 (2H, s, CH2–C4–coumarin), 7.41 (1H, d, J = 8.4 Hz, C4H–
imidazole), 7.54–7.49 (2H, m, ArCH–imidazole/coumarin), 7.67 (1H, d, J = 8.4 Hz, ArCH–
13
coumarin), 7.85–7.80 (1H, m, ArCH–coumarin). C{1H} NMR (100 MHz, d6–DMSO, 298
K): δ 50.7 (CH3–O–CH2–CH2), 51.3 (CH3–O–CH2–CH2), 58.5 (CH3–O–CH2–CH2), 71.7
(CH2–C4–coumarin), 123.5, 124.2 (C4, C5–imidazole), 113.5, 118.6, 119.2, 123.1, 129.0,
132.7, 151.5, 151.8, 159.1 (ArC–coumarin), C2–Imidazole absent. ATR–IR (in cm–1): 2962,
~3020 ѵ(C–H, aliphatic and aromatic), 1718 ѵ(C=O, lactonic), 1563 ѵ(C=N, imidazole),
1093 ѵ(C–O, coumarin). Anal. Calc. for C32H30N4O6AgCl2PF6: C, 43.2; H, 3.4; N, 6.3.
Found: C, 43.0; H, 3.3; N, 6.6. Molar conductance (in CH2Cl2, 302 K): 60 S cm2 mol–1.
4.11. Synthesis of bis{1–(2–methoxyethyl)–3–((5,6–benzo–2–oxo–2H–chromen–4–yl)methyl)
imidazol–2–ylidene}silver(I) hexafluorophosphate complex (11)
Complex 11 was prepared following the procedure analogous to that of bis–carbene
coordinated silver(I) complex 9 with imidazolium hexafluorophosphate salt 5 (0.481 g, 1
mmol) instead of imidazolium hexafluorophosphate salt 3. Off–white solid; Yield: 76.3 %;
M.P.: 86–88 °C. 1H NMR (400 MHz, d6–DMSO, 298 K): δ 3.17 (3H, s, CH3–O–CH2–CH2),
3.63–3.58 (2H, m, CH3–O–CH2–CH2), 4.38–4.34 (2H, m, CH3–O–CH2–CH2), 5.32 (1H, s,
C3H–coumarin), 6.13 (2H, s, CH2–C4–coumarin), 7.26 (1H, d, J = 8.4 Hz, ArCH–
imidazole), 7.42–7.34 (3H, m, ArCH–imidazole/coumarin), 7.56–7.53 (1H, m, ArCH–
coumarin), 7.88 (1H, d, J = 8.0 Hz, ArCH–coumarin), 7.99 (1H, d, J = 8.8 Hz, ArCH–
coumarin), 8.18 (1H, d, J = 8.0 Hz, ArCH–coumarin). 13C{1H} NMR (100 MHz, d6–DMSO,
298 K): δ 51.3 (CH3–O–CH2–CH2), 54.7 (CH3–O–CH2–CH2), 58.5 (CH3–O–CH2–CH2), 71.7
(CH2–C2–coumarin), 123.5, 125.7 (C4, C5–imidazole), 111.6, 112.5, 117.4, 117.9, 123.1,
128.4, 128.7, 129.9, 131.1, 134.6, 154.2, 154.7, 159.1 (ArC–benzo–coumarin), C2–
Imidazole absent. ATR–IR (in cm–1): 3015, 2943 ѵ(C–H, aliphatic and aromatic), 1724
20

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ѵ(C=O, lactonic), 1548 ѵ(C=N, imidazole), 1113 ѵ(C–O, coumarin). Anal. Calc. for
C40H36N4O6AgPF6: C, 52.1; H, 4.0; N, 6.1. Found: C, 52.5; H, 4.3; N, 5.9. Molar
conductance (in CH2Cl2, 302 K): 43 S cm2 mol–1.
4.12. Synthesis of bis{1–(2–methoxyethyl)–3–((6–methyl–2–oxo–2H–chromen–4–yl)methyl)
benzimidazol–2–ylidene}silver(I) hexafluorophosphate complex (12)
Complex 12 was prepared following the procedure analogous to that of bis–carbene
coordinated silver(I) complex 9 with benzimidazolium hexafluorophosphate salt 6 (0.495 g,
1 mmol) instead of imidazolium hexafluorophosphate salt 3. Off–white solid; Yield: 93.3 %;
1
M.P.: 212–214 °C. H NMR (400 MHz, d6–DMSO, 298 K): δ 2.38 (3H, s, CH3–coumarin),
3.02 (1H, s, CH3–O–CH2–CH2), 3.55–3.52 (2H, m, CH3–O–CH2–CH2 ) 4.67–4.65 (2H, m,
CH3–O–CH2–CH2), 5.35 (1H, s, C3H–coumarin), 6.13 (2H, s, CH2–C4–coumarin), 7.27 (1H,
d, J = 8.4 Hz, ArCH–benzimidazole), 7.48–7.39 (3H, m, ArCH–benzimidazole), 7.72 (2H, d,
J = 7.6 Hz, ArCH–coumarin), 7.77 (1H, d, J = 8.0 Hz, ArCH–coumarin). 13C{1H} NMR (100
MHz, d6–DMSO, 298 K): δ 20.9 (CH3–coumarin), 48.5 (CH3–O–CH2–CH2), 49.1 (CH3–O–
CH2–CH2), 58.6 (CH3–O–CH2–CH2), 71.0 (CH2–C4–coumarin), 112.5, 116.9, 117.1, 124.7,
124.9 (ArC–benzimidazole), 111.5, 113.0, 124.8, 133.8, 134.0, 134.2, 151.6, 159.6, 159.7
(ArC–coumarin), C2–benzimidazole absent. ATR–IR (in cm–1): 2962, ~2865 ѵ(C–H,
aliphatic and aromatic), 1715 ѵ(C=O, lactonic), 1567 ѵ(C=N, benzimidazole), 1107 ѵ(C–O,
coumarin). Anal. Calc. for C42H40N4O6AgPF6: C, 53.1; H, 4.2; N, 5.9. Found: C, 53.5; H, 4.3;
N, 5.6. Molar conductance (in CH2Cl2, 302 K): 48 S cm2 mol–1.
4.13. Synthesis of bis{1–(2–methoxyethyl)–3–((6–chloro–2–oxo–2H–chromen–4–yl)methyl)
benzimidazol–2–ylidene}silver(I) hexafluorophosphate complex (13)
Complex 13 was prepared following the procedure analogous to that of bis–carbene
coordinated silver(I) complex 9 with benzimidazolium hexafluorophosphate salt 7 (0.515 g,
1 mmol) instead of imidazolium hexafluorophosphate salt 3. Off–white solid; Yield: 91.2 %;
1
M.P.: 210–212 °C. H NMR (400 MHz, d6–DMSO, 298 K): δ 3.09 (3H, s, CH3–O–CH2–
CH2), 3.66–3.60 (2H, m, CH3–O–CH2–CH2), 4.69 (2H, t, J = 8.8 Hz, CH3–O–CH2–CH2),
5.37 (1H, s, C3H–coumarin), 6.15 (2H, s, CH2–C4–coumarin), 7.48–7.36 (3H, m, ArCH–
benzimidazole), 7.72 (2H, d, J = 8.8 Hz, ArCH–benzimidazole/coumarin), 7.88 (1H, d, J =
21

ACCEPTED MANUSCRIPT
8.0 Hz, ArCH–coumarin), 8.00 (1H, s, ArCH–coumarin). 13C{1H} NMR (100 MHz, d6–
DMSO, 298 K): δ 48.4 (CH3–O–CH2–CH2), 49.1 (CH3–O–CH2–CH2), 58.7 (CH3–O–CH2–
CH2), 71.0 (CH2–C4–coumarin), 112.4, 113.1, 118.8, 119.1, 124.7 (ArC–benzimidazole),
112.5, 127.6, 129.0, 132.7, 133.8, 134.1, 151.0, 151.9, 159.0 (ArC–coumarin), 191.7 (C2–
benzimidazole). ATR–IR (in cm–1): 2969, 2861 ѵ(C–H, aliphatic and aromatic), 1701
ѵ(C=O, lactonic), 1522 ѵ(C=N, benzimidazole), 1107 ѵ(C–O, coumarin). Anal. Calc. for
C40H34N4O6AgCl2PF6: C, 48.5; H, 3.5; N, 5.7. Found: C, 48.3; H, 3.6; N, 5.6. Molar
conductance (in CH2Cl2, 302 K): 44 S cm2 mol–1.
4.14. Synthesis of bis{1–(2–methoxyethyl)–3–((5,6–benzo–2–oxo–2H–chromen–4–yl)methyl)
benzimidazol–2–ylidene}silver(I) hexafluorophosphate complex (14)
Complex 14 was prepared following the procedure analogous to that of bis–carbene
coordinated silver(I) complex 9 with benzimidazolium hexafluorophosphate salt 8 (0.531 g,
1 mmol) instead of imidazolium hexafluorophosphate salt 3. Off–white solid; Yield: 78.7 %;
1
M.P.: 138–140 °C. H NMR (400 MHz, d6–DMSO, 298 K): δ 3.11 (3H, s, CH3–O–CH2–
CH2), 3.68–3.62 (2H, m, CH3–O–CH2–CH2), 4.72–4.67 (2H, m, CH3–O–CH2–CH2), 5.24
(1H, s, C3H–coumarin), 6.54 (2H, s, CH2–C4–coumarin), 7.36–7.31 (1H, m, ArCH–
benzimidazole), 7.47–7.40 (2H, m, ArCH–benzimidazole), 7.57–7.52 (2H, m, ArCH–
benzimidazole/coumarin), 7.64 (1H, d, J = 8.0 Hz, ArCH–coumarin), 7.88 (1H, d, J = 8.0
Hz, ArCH–coumarin), 8.01 (1H, m, ArCH–coumarin), 8.13 (1H, d, J = 8.2 Hz, ArCH–
13
coumarin). C{1H} NMR (100 MHz, d6–DMSO, 298 K): δ 49.2 (CH3–O–CH2–CH2), 52.7
(CH3–O–CH2–CH2), 58.7 (CH3–O–CH2–CH2), 71.0 (CH2–C4–coumarin), 112.8, 113.0,
117.6, 124.7, 126.0 (ArC–benzimidazole), 111.1, 126.3, 128.6, 129.1, 130.0, 131.3, 131.9,
133.6, 134.1, 134.9, 153.9, 154.5, 159.1 (ArC–benzo–coumarin), 192.4 (C2–benzimidazole).
ATR–IR (in cm–1): ~2930, 2807 ѵ(C–H, aliphatic and aromatic), 1724 ѵ(C=O, lactonic),
1549 ѵ(C=N, benzimidazole) 1113 ѵ(C–O, coumarin). Anal. Calc. for C48H40N4O6AgPF6: C,
56.4; H, 4.0; N, 5.5. Found: C, 56.5; H, 4.3; N, 5.7. Molar conductance (in CH2Cl2, 302 K):
48 S cm2 mol–1.
4.15. Synthesis of 1–(2–methoxyethyl)–3–((6–methyl–2–oxo–2H–chromen–4–yl)methyl)
imidazol–2–ylidenesilver(I) bromide complex (15)
22

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A suspension of imidazolium bromide salt, 1–(2–methoxyethyl)–3–(6–methylcoumarin–4–
yl)imidazolium bromide, (0.380 g, 1 mmol) and silver(I) oxide (0.253 g, 1.1 mmol) in
dichloromethane was stirred at room temperature for 24 h under the exclusion of light (in a
round bottom flask wrapped with aluminum foil). After the stipulated time, the reaction
mixture was filtered through a bed of cilite and filtrate was dried over anhydrous magnesium
sulfate. The solvent was removed in vacuo, and the resulted crude product was washed with
diethyl ether (3 x 3 mL) and purified by repeated precipitation using acetonitrile–diethyl
ether mixture to afford light sensitive complex 15 as a beige colored solid. Yield: 60.2 %;
1
M.P.: 80–82 °C. H NMR (400 MHz, d6–DMSO, 298 K): δ 2.35 (3H, s, CH3–coumarin),
3.12 (3H, s, CH3–O–CH2–CH2), 3.61 (2H, m, CH3–O–CH2–CH2), 4.28 (2H, t, J = 8.8 Hz,
CH3–O–CH2–CH2), 5.58 (1H, s, C3H–coumarin), 5.69 (2H, s, CH2–C4–coumarin), 7.22 (1H,
d, J = 8.2 Hz, C4H–imidazole), 7.44 (1H, d, J = 8.4 Hz, C5H–imidazole), 7.53–7.62 (3H, m,
13
ArCH–coumarin). C{1H} NMR (100 MHz, d6–DMSO, 298 K): δ 20.9 (CH3–coumarin),
50.5 (CH3–O–CH2–CH2), 51.4 (CH3–O–CH2–CH2), 58.4 (CH3–O–CH2–CH2), 71.5 (CH2–
C4–coumarin), 123.3, 124.5 (C4, C5–imidazole), 112.5, 116.4, 117.5, 132.7, 133.2, 151.5,
152.1, 159.9 (ArC–coumarin), C2–imidazole absent. ATR–IR (in cm–1): 2922, ~2885 ѵ(C–
H, aliphatic and aromatic), 1711 ѵ(C=O, lactonic), 1564 ѵ(C=N, imidazole), 1111 (C–O,
coumarin). Anal. Calc. for C17H18N2O3AgBr: C, 42.0; H, 3.7; N, 5.8. Found: C, 42.4; H, 3.4;
N, 5.6. Molar conductance (in CH2Cl2, 302 K): 0 S cm2 mol–1.
4.16. Synthesis of 1–(2–methoxyethyl)–3–((6–chloro–2–oxo–2H–chromen–4–yl)methyl)
imidazol–2–ylidenesilver(I) bromide complex (16)
Complex 16 was prepared following the procedure analogous to that of mono–carbene
coordinated silver(I) complex 15 with imidazolium bromide salt, 1–(2–methoxyethyl)–3–(6–
chlorocoumarin–4–yl)imidazolium bromide,
(0.400 g, 1 mmol) instead of 1–(2–
methoxyethyl)–3–(6–methylcoumarin–4–yl)imidazolium bromide. Beige colored solid;
Yield: 50.0 %; M.P.: 56–58 °C. 1H NMR (400 MHz, d6–DMSO, 298 K): δ 3.14 (3H, s, CH3–
O–CH2–CH2), 3.58 (2H, t, J = 8.6 Hz, CH3–O–CH2–CH2), 4.26 (2H, t, J = 8.8 Hz, CH3–O–
CH2–CH2), 5.60 (1H, s, C3H–Coumarin), 5.72 (2H, s, CH2–C4–Coumarin), 7.43 (1H, d, J =
8.4 Hz, C4H–Imidazole), 7.56 (2H, d, J = 8.4 Hz, C5H–Imidazole), 7.69 (2H, d, J = 8.2 Hz,
23

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ArCH–Coumarin), 7.88–7.82 (1H, m, ArCH–Coumarin). 13C{1H} NMR (100 MHz, d6–
DMSO, 298 K): δ 50.3 (CH3–O–CH2–CH2), 51.5 (CH3–O–CH2–CH2), 58.6 (CH3–O–CH2–
CH2), 71.7 (CH2–C4–Coumarin), 123.0, 123.6 (C4, C5–Imidazole), 113.5, 118.7, 119.3,
124.6, 129.1, 132.7, 151.6, 151.8, 159.3 (ArC–Coumarin), C2–Imidazole absent. ATR–IR
(in cm–1): 2923, ~2880 ѵ(C–H, aliphatic and aromatic), 1716 ѵ(C=O, lactonic), 1566 ѵ(C=N,
Imidazole), 1103 (C–O, Coumarin). Anal. Calc. for C16H15N2O3AgClBr: C, 37.9; H, 3.0; N,
5.5. Found: C, 37.7; H, 3.3; N, 5.7. Molar conductance (in CH2Cl2, 302 K): 02 S cm2 mol–1.
4.17. Synthesis of 1–(2–methoxyethyl)–3–((5,6–benzo–2–oxo–2H–chromen–4–yl)methyl)
imidazol–2–ylidenesilver(I) bromide complex (17)
Complex 17 was prepared following the procedure analogous to that of mono–carbene
coordinated silver(I) complex 15 with imidazolium bromide salt, 1–(2–methoxyethyl)–3–
(5,6–benzocoumarin–4–yl)imidazolium bromide, (0.416 g, 1 mmol) instead of 1–(2–
methoxyethyl)–3–(6–methylcoumarin–4–yl)imidazolium bromide. Beige colored solid;
1
Yield: 52.0 %; M.P.: 112–114 °C. H NMR (400 MHz, d6–DMSO, 298 K): δ 3.13 (3H, s,
CH3–O–CH2–CH2), 3.64–3.56 (2H, m, CH3–O–CH2–CH2), 4.39 (2H, d, J = 8.8 Hz, CH3–O–
CH2–CH2), 5.33 (1H, s, C3H–Coumarin), 6.15 (2H, s, CH2–C4–Coumarin), 7.29 (1H, d, J =
8.4 Hz, ArCH–Imidazole), 7.40–7.33 (3H, m, ArCH–Imidazole/Coumarin), 7.58–7.52 (1H,
m, ArCH–Coumarin), 7.83 (1H, d, J = 8.0 Hz, ArCH–Coumarin), 8.01 (1H, d, J = 8.2 Hz,
13
ArCH–Coumarin), 8.18 (1H, m, ArCH–Coumarin). C{1H} NMR (100 MHz, d6–DMSO,
298 K): δ 51.4 (CH3–O–CH2–CH2), 54.7 (CH3–O–CH2–CH2), 58.8 (CH3–O–CH2–CH2), 71.6
(CH2–C2–Coumarin), 123.4, 125.7 (C4, C5–Imidazole), 111.7, 112.7, 117.9, 123.1, 128.5,
128.7, 129.7, 131.4, 134.6, 151.3, 154.7, 159.6 (ArC–Benzo–coumarin), C2–Imidazole
absent. ATR–IR (in cm–1): 2926, 2866 ѵ(C–H, aliphatic and aromatic), 1715 ѵ(C=O,
lactonic), 1561 ѵ(C=N, Imidazole), 1109 (C–O, Coumarin). Anal. Calc. for
C20H18N2O3AgBr: C, 46.0; H, 3.5; N, 5.4. Found: C, 46.3; H, 3.4; N, 5.5. Molar conductance
(in CH2Cl2, 302 K): 02 S cm2 mol–1.
4.18. Synthesis of 1–(2–methoxyethyl)–3–((6–methyl–2–oxo–2H–chromen–4–yl)methyl)
benzimidazol–2–ylidenesilver(I) bromide complex (18)
24

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Complex 18 was prepared following the procedure analogous to that of mono–carbene
coordinated silver(I) complex 15 with benzimidazolium bromide salt, 1–(2–methoxyethyl)–
3–(6–methylcoumarin–4–yl)benzimidazolium bromide, (0.430 g, 1 mmol) instead of 1–(2–
methoxyethyl)–3–(6–methylcoumarin–4–yl)imidazolium bromide. Beige colored solid;
Yield: 58.3 %; M.P.: 90–92 °C. 1H NMR (400 MHz, d6–DMSO, 298 K): δ 2.33 (3H, s, CH3–
Coumarin), 3.04 (1H, s, CH3–O–CH2–CH2), 3.56 (2H, t, J = 8.2 Hz, CH3–O–CH2–CH2)
4.69–4.64 (2H, m, CH3–O–CH2–CH2), 5.37 (1H, s, C3H–Coumarin), 6.18 (2H, s, CH2–C4–
Coumarin), 7.24 (1H, d, J = 8.2 Hz, ArCH–Benzimidazole), 7.52–7.47 (3H, m, ArCH–
Benzimidazole), 7.70 (2H, d, J = 8.0 Hz, ArCH–Coumarin), 7.79 (1H, d, J = 8.0 Hz, ArCH–
Coumarin). 13C{1H} NMR (100 MHz, d6–DMSO, 298 K): δ 20.8 (CH3–Coumarin), 48.8
(CH3–O–CH2–CH2), 49.4 (CH3–O–CH2–CH2), 58.6 (CH3–O–CH2–CH2), 71.1 (CH2–C4–
Coumarin), 112.6, 116.9, 117.3, 124.3, 124.4, (ArC–Benzimidazole), 111.5, 113.1, 124.8,
133.7, 134.1, 134.2, 151.5, 159.8, 159.8 (ArC–Coumarin), C2–Benzimidazole absent. ATR–
IR (in cm–1): 2929, ~2830 ѵ(C–H, aliphatic and aromatic), 1710 ѵ(C=O, lactonic), 1564
ѵ(C=N, Imidazole), 1113 (C–O, Coumarin). Anal. Calc. for C21H20N2O3AgBr: C, 47.0; H,
3.8; N, 5.2. Found: C, 47.4; H, 4.0; N, 5.4. Molar conductance (in CH2Cl2, 302 K): 02 S cm2
mol–1.
4.19. Synthesis of 1–(2–methoxyethyl)–3–((6–chloro–2–oxo–2H–chromen–4–yl)methyl)
benzimidazol–2–ylidenesilver(I) bromide complex (19)
Complex 19 was prepared following the procedure analogous to that of mono–carbene
coordinated silver(I) complex 15 with benzimidazolium bromide salt, 1–(2–methoxyethyl)–
3–(6–chlorocoumarin–4–yl)benzimidazolium bromide, (0.450 g, 1 mmol) instead of 1–(2–
methoxyethyl)–3–(6–methylcoumarin–4–yl)imidazolium bromide. Beige colored solid;
1
Yield: 54.4 %; M.P.: 124–126 °C. H NMR (400 MHz, d6–DMSO, 298 K): δ 3.13 (3H, s,
CH3–O–CH2–CH2), 3.61–3.57 (2H, m, CH3–O–CH2–CH2), 4.72 (2H, t, J = 8.6 Hz, CH3–O–
CH2–CH2), 5.39 (1H, s, C3H–Coumarin), 6.16 (2H, s, CH2–C4–Coumarin), 7.48–7.42 (3H,
m, ArCH–Benzimidazole), 7.74 (2H, d, J = 8.8 Hz, ArCH–Benzimidazole/Coumarin), 7.89
13
(1H, d, J = 8.0 Hz, ArCH–Coumarin), 8.02 (1H, s, ArCH–Coumarin). C{1H} NMR (100
MHz, d6–DMSO, 298 K): δ 48.5 (CH3–O–CH2–CH2), 49.3 (CH3–O–CH2–CH2), 58.7 (CH3–
25

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O–CH2–CH2), 71.2 (CH2–C4–Coumarin), 112.4, 113.6, 118.3, 119.1, 124.3 (ArC–
Benzimidazole), 112.5, 127.7, 129.2, 131.3, 133.4, 134.1, 151.5, 151.9, 159.6 (ArC–
Coumarin), C2–Benzimidazole absent. ATR–IR (in cm–1): ~2920, 2869 ѵ(C–H, aliphatic and
aromatic), 1722 ѵ(C=O, lactonic), 1596 ѵ(C=N, Imidazole), 1110 (C–O, Coumarin). Anal.
Calc. for C20H17N2O3AgBrCl: C, 43.2; H, 3.1; N, 5.0. Found: C, 43.5; H, 3.2; N, 5.2. Molar
conductance (in CH2Cl2, 302 K): 0 S cm2 mol–1.
4.20. Synthesis of 1–(2–methoxyethyl)–3–((5,6–benzo–2–oxo–2H–chromen–4–yl)methyl)
benzimidazol–2–ylidenesilver(I) bromide complex (20)
Complex 20 was prepared following the procedure analogous to that of mono–carbene
coordinated silver(I) complex 15 with benzimidazolium bromide salt, 1–(2–methoxyethyl)–
3–(5,6–benzocoumarin–4–yl)benzimidazolium bromide, (0.466 g, 1 mmol) instead of 1–(2–
methoxyethyl)–3–(6–methylcoumarin–4–yl)imidazolium bromide. Beige colored solid;
1
Yield: 52.3 %; M.P.: 146–148 °C. H NMR (400 MHz, d6–DMSO, 298 K): δ 3.09 (3H, s,
CH3–O–CH2–CH2), 3.66 (2H, t, J = 8.2 Hz, CH3–O–CH2–CH2), 4.77–4.73 (2H, m, CH3–O–
CH2–CH2), 5.27 (1H, s, C3H–Coumarin), 6.54 (2H, s, CH2–C4–Coumarin), 7.38–7.34 (1H,
m, ArCH–Benzimidazole), 7.48–7.42 (2H, m, ArCH–Benzimidazole), 7.59–7.54 (2H, m,
ArCH–Benzimidazole/Coumarin), 7.66 (1H, d, J = 8.2 Hz, ArCH–Coumarin), 7.88 (1H, m,
ArCH–Coumarin), 8.02 (1H, m, ArCH–Coumarin), 8.14 (1H, d, J = 8.2 Hz, ArCH–
Coumarin), 8.41 (1H, m, ArCH–Coumarin). 13C{1H} NMR (100 MHz, d6–DMSO, 298 K): δ
49.5 (CH3–O–CH2–CH2), 52.7 (CH3–O–CH2–CH2), 58.3 (CH3–O–CH2–CH2), 71.1 (CH2–
C4–Coumarin), 112.7, 113.0, 117.9, 124.7, 126.3 (ArC–Benzimidazole), 111.3, 126.8, 128.6,
129.3, 131.5, 131.9, 133.8, 134.2, 134.9, 152.4, 154.5, 159.2 (ArC–Benzo–coumarin). ATR–
IR (in cm–1): ~2930, 2857 ѵ(C–H, aliphatic and aromatic), 1719 ѵ(C=O, lactonic), 1596
ѵ(C=N, Imidazole), 1112 (C–O, Coumarin). Anal. Calc. for C24H20N2O3AgBr: C, 50.4; H,
3.5; N, 4.9. Found: C, 50.5; H, 3.5; N, 4.6. Molar conductance (in CH2Cl2, 302 K): 02 S cm2
mol–1.
4.21. Determination of antibacterial potentials of azolium salts and silver complexes
The antibacterial potentials of the prepared azolium hexafluorophosphate salts (3–8) and
their mono– and bis–carbene silver(I) complexes (9–20) in terms of minimum inhibitory
26

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concentrations (MIC) were determined against both the type of bacteria following broth
dilution method. The stock solution (256 µg/mL) of the salts and complexes 3–20 was
prepared by using dimethylsulphoxide. A loop of the bacterial strain was suspended in
autoclaved distilled water and the turbidity of this solution was set to that of 0.5 McFarland
standard. The MIC of the salts and complexes was determined by serially diluting the test
compounds 3–20 to obtain a concentration range of 1–256 µg/mL which is further diluted by
adding 1 mL bacterial suspension to obtain a final concentration range from 0.5–128 µg/mL
and a final desired inoculum of 5×105 CFU mL–1. The control test tubes contained 10 %
DMSO mixture in nutrient broth along with the bacterial suspension. All the test tubes were
o
incubated at 37 C for 16 h. MIC was read as the lowest concentration of the complex that
inhibits visible bacterial growth.
Acknowledgments
S. B. thanks Science and Engineering Research Board–Department of Science and
Technology, New Delhi, India for the financial support through the Young Scientist Start Up
Research Grant, YSS/2014/000032. The authors are grateful to the Institution of Excellence,
Vijnana Bhavana, University of Mysore, India and Jain University, India, for providing the
single–crystal X–ray diffractometer (for salt 5) and spectral facilities, respectively.
Supplementary material
The crystallographic data for structural analysis have been deposited in the Cambridge
Crystallographic Data Center, CCDC 1570022 and 1570021 contain the supplementary
crystallographic data for the imidazolium salt 5 and silver complex 12, respectively. These
data can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Crystal packing diagrams for compounds 5 and 12 are
also available as Supporting Information.
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