Synthesis, cytotoxicity and antibacterial studies of p-methoxybenzyl- substituted and benzyl-substituted n-heterocyclic carbene-silver complexes

Article abstract of DOI:10.1002/ejic.200900889

p-Methoxybenzyl-substituted and benzyl-substituted Nheterocycllc carbene (NHC) [(3a-c) and (6a-c)] precursors were synthesised from, the reaction of 1H-imidazole (la), 4,5dichloro-1H-imidazole (1b), and 1H-benzimidazole (1c) with p-methoxybenzyl bromide (

Full text of DOI:10.1002/ejic.200900889

FULL PAPER
DOI: 10.1002/ejic.200900889
Synthesis, Cytotoxicity and Antibacterial Studies of
p-Methoxybenzyl-Substituted and Benzyl-Substituted N-Heterocyclic
Carbene–Silver Complexes
Siddappa Patil,[a] James Claffey,[a] Anthony Deally,[a] Megan Hogan,[a] Brendan Gleeson,[a]
Luis Miguel Menéndez Méndez,[a] Helge Müller-Bunz,[a] Francesca Paradisi,[a] and
Matthias Tacke*[a]
Keywords: Anticancer drug / Antibacterial drug / Silver / NHC / Caki-1 / Staphylococcus aureus / Carbenes
p-Methoxybenzyl-substituted and benzyl-substituted N-
heterocyclic carbene (NHC) [(3a–c) and (6a–c)] precursors
were synthesised from the reaction of 1H-imidazole (1a), 4,5-
dichloro-1H-imidazole (1b), and 1H-benzimidazole (1c) with
p-methoxybenzyl bromide (2) and benzyl bromide (5). These
NHC precursors were then treated with silver(I) acetate to
yield the NHC–silver complexes [1,3-bis(4-methoxybenzyl)-
imidazol-2-ylidene]silver(I) acetate (4a), [4,5-dichloro-1,3-
3c, four NHC–silver complexes 4c and 7a–c were character-
ised by single-crystal X-ray diffraction method. The prelimi-
nary antibacterial activity of all the compounds was studied
against Gram-negative bacteria Escherichia coli, and Gram-
positive bacteria Staphylococcus aureus using the Kirby–
Bauer disk-diffusion method. Almost all the NHC–silver com-
plexes have shown high antibacterial activity compared to
the NHC precursors. In addition, the NHC–silver complexes
had their cytotoxicity investigated through MTT-based pre-
liminary in vitro testing on the Caki-1 cell lines in order to
determine their IC50 values. NHC–silver complexes 4a–c and
7a–c were found to have IC50 values of 7.3 (+/–6), 12.7
bis(4-methoxybenzyl)imidazol-2-ylidene]silver(I)
acetate
(4b), [1,3-bis(4-methoxybenzyl)benzimidazol-2-ylidene]-
silver(I) acetate (4c), (1,3-dibenzylimidazol-2-ylidene)silver(I)
acetate (7a), (1,3-dibenzyl-4,5-dichloroimidazol-2-ylidene)-
silver(I) acetate (7b), and (1,3-dibenzylbenzimidazol-2-ylid-
ene)silver(I) acetate (7c), respectively. The NHC precursor
(+/–3), 25.2 (+/–5), 2.5 (+/–3), 10.8 (+/–4) and 12.5 (+/–4) µ
M
respectively on the Caki-1 cell line.
makes bis[(p-methoxybenzyl)cyclopentadienyl]titanium(IV)
dichloride (Titanocene Y)[8] available, which exhibits an
IC50 value of 21 µ when tested on the LLC-PK cell line,
while the corresponding Vanadocene Y[9] shows an im-
proved IC50 value of 3.0 µ. Because this p-methoxybenzyl-
substitution proved so successful in the case of the metallo-
cene chemistry, it will now be applied to NHC–silver com-
plexes.
Within this paper, we present the synthesis, preliminary
cytotoxicity and antibacterial studies of a series of six new
p-methoxybenzyl-substituted and benzyl-substituted NHC–
silver acetate derivatives. These compounds were tested on
the human cancerous renal-cell line Caki-1 as well as on
the Gram-positive bacteria Staphylococcus aureus and the
Gram-negative bacteria Escherichia coli.
Introduction
N-Heterocyclic carbenes (NHCs), which are usually de-
rived from imidazolium halides, are electronically stabilised
cyclic carbene ligands unfolding a new chapter of organo-
metallic chemistry.[1,2] Quite quickly NHCs have been devel-
oped into promising “spectator” ligands for catalysis, but
more recently NHCs found an application in NHC–silver
complexes exhibiting antimicrobial activity, in particular for
the possible treatment of cystic fibrosis (CF) and chronic
lung infections[3–5] and maybe in the treatment of cancer.[6]
This possible anticancer activity attracted our interest,
since we investigate novel organometallic anticancer drugs.
By starting from fulvenes via reductive dimerisation with
titanium dichloride, carbolithiation or hydridolithiation of
the fulvene followed by transmetallation with titanium tet-
rachloride in the latter two cases these syntheses lead to
anti-proliferative titanocene derivatives.[7] Hydridolithiation
of 6-anisylfulvene and subsequent reaction with TiCl4
Results and Discussion
Syntheses
[a] Conway Institute of Biomolecular and Biomedical Research,
Centre for Synthesis and Chemical Biology (CSCB), UCD
School of Chemistry and Chemical Biology, University College
Dublin,
The synthetic pathway for p-methoxybenzyl-substituted
and benzyl-substituted N-heterocyclic carbenes as ligand
precursors and their corresponding silver complexes de-
scribed in this work is outlined in Schemes 1, 2 and 3. We
Belfield, Dublin 4, Ireland
E-mail: matthias.tacke@ucd.ie
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Synthesis and Studies of NHC–Silver Complexes
did not follow the synthetic procedure of 1,3-dibenzylimid- IR, UV/Vis, mass spectroscopy, and elemental analysis. Ad-
azolium bromide (6a), and 1,3-dibenzylbenzimidazolium ditionally the solid-state structure of 3c was determined by
bromide (6c), published in literature[10–12] instead we syn- single-crystal X-ray diffraction method.
1
thesised according to our new and milder procedure. On
The H NMR spectra of all precursors 3a–c and 6a–c
the other hand, the synthesis of 1,3-dibenzyl-4,5-dichlo- show a characteristic downfield shift in the range δ = 10.12–
roimidazolium bromide (6b) was carried out according to a 11.85 ppm for the NCHN proton attributable to the posi-
published literature procedure.[13] The NHCs precursors tive charge of the molecule.[14–17] In addition, their identit-
1,3-bis(p-methoxybenzyl)imidazolium bromide (3a), 4,5- ies have also been confirmed by a base peak for the [M+ –
dichloro-1,3-bis(p-methoxybenzyl)imidazolium
(3b), 1,3-bis(p-methoxybenzyl)benzimidazolium bromide
(3c), 1,3-dibenzylimidazolium bromide (6a) and 1,3-dibenz- imidazol-2-ylidene]silver(I) acetate (4a), [4,5-dichloro-1,3-
ylbenzimidazolium bromide (6c) were prepared by stirring bis(4-methoxybenzyl)imidazol-2-ylidene]silver(I) acetate
bromide Br] fragments in their positive mode ESI mass spectra.
The NHC–silver complexes [1,3-bis(4-methoxybenzyl)-
1H-imidazole (1a), 4,5-dichloro-1H-imidazole (1b), and (4b), [1,3-bis(4-methoxybenzyl)benzimidazol-2-ylidene]sil-
1H-benzimidazole (1c) with 2 equiv. of alkyl halides [p- ver(I) acetate (4c), (1,3-dibenzylimidazol-2-ylidene)silver(I)
methoxybenzyl bromide (2) and benzyl bromide (5)] in the acetate (7a), (1,3-dibenzyl-4,5-dichloroimidazol-2-ylidene)-
presence of K2CO3 as a base in CH3CN at room tempera- silver(I) acetate (7b), and (1,3-dibenzylbenzimidazol-2-ylid-
ture for 2–3 d with moderate to good 40% to 91% yields. ene)silver(I) acetate (7c) were synthesized by the reaction of
4,5-Dichloro-1-benzylimidazole (5Ј) was prepared in a 3a–c and 6a–c with 2 equiv. of silver acetate in CH2Cl2. The
straightforward way by deprotonation of 4,5-dichloroimid- reaction mixture was stirred for 1–2 d at room temperature
azole with K2CO3 in CH3CN and further alkylation with in dark to afford the NHC–silver acetate complexes as off-
benzyl bromide. Subsequent benzylation of the obtained white solid in 62% to 87% yield. The complexes were fully
product in toluene with another equivalent of the benzyl characterized by 1H, 13C NMR, IR, UV/Vis, mass spec-
bromide yielded 69.0% of the 1,3-dibenzyl-4,5-dichloro- troscopy, and elemental analysis. Furthermore, the solid-
imidazolium bromide (6b) as an air stable white powder. All state structures of 4c and 7a–c were analysed by single-crys-
1
the precursors were fully characterized by H, 13C NMR, tal X-ray diffraction method. The absence of a downfield
Scheme 1. General reaction scheme for the synthesis of p-methoxybenzyl-substituted N-heterocyclic carbenes 3a–c and their correspond-
ing N-heterocyclic carbene–silver complexes 4a–c.
Scheme 2. General reaction scheme for the synthesis of benzyl-substituted N-heterocyclic carbenes 6a and 6c and their corresponding N-
heterocyclic silver-carbene complexes 7a and 7c.
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M. Tacke et al.
FULL PAPER
Scheme 3. General reaction scheme for the synthesis of benzyl-substituted N-heterocyclic carbene 6b and its N-heterocyclic carbene–
silver complex 7b.
NCHN signals and presence of new signals at 2.08– for the carbonyl and methyl carbon atoms of the acetate
1.79 ppm for the acetate protons in all the 1H NMR spectra group of complexes 4a–c and 7a–c in the range 168.6–166.1
for 4a–c and 7a–c, however, O2CCH3 indicates a successful and 23.7–21.0 ppm, respectively, showed the formation of
complex formation. The 13C NMR resonances of the car- the NHC–silver complexes.[3,5] Furthermore, positive mode
bene carbon atoms in complexes 4a–c and 7a–c occur in ESI mass spectra of all six NHC–silver complexes (4a–c
the range δ = 178.9–175.1 ppm respectively. These signals and 7a–c) are dominated by [M+ – O2CCH3] fragment
are shifted downfield compared to the corresponding pre- peaks arising from the loss of one acetate ligand.
cursors of 3a–c and 6a–c carbene carbon atoms resonance
Structural Discussion
at the range 130.8–143.1 ppm, respectively, which further
demonstrates the formation of expected NHC–silver acetate
The crystal structures of the compounds 3c, 4c, and 7a–
complexes. Also appearance of the 13C NMR resonances c have been determined. The molecular structures of the
Figure 1. X-ray diffraction structure of 3c; hydrogen bond linked moiety; thermal ellipsoids are drawn on the 50% probability level.
1022 Eur. J. Inorg. Chem. 2010, 1020–1031
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Synthesis and Studies of NHC–Silver Complexes
compounds 3c, 4c, and 7a–c are depicted in Figures 1, 2, 3,
4, 5, and 6. The crystal data and refinement details for all
five compounds are found in Table 1, whereas selected bond
lengths and bond angles are displayed in Table 2.
The X-ray structure of 3c reveals that the benzimidazol-
ium core is planar. In the five-membered ring (NCNCC),
the bond lengths and angles are in good agreement with the
bond lengths found in the similar compound 1,3-diiso-
propylbenzimidazolium bromide reported in literature (see
Figure 4. X-ray diffraction structure of 7a; molecule; thermal ellip-
soids are drawn on the 50% probability level.
Figure 5. X-ray diffraction structure of 7b; molecule; thermal ellip-
soids are drawn on the 50% probability level.
Table 2).[18] Hydrogen bonds connect two cations, two
anions and two water molecules to isolated dimeric moieties
(Figure 1).
Figure 2. X-ray diffraction structure of 4c; molecule; thermal ellip-
soids are drawn on the 50% probability level.
Figure 3. Hydrogen-bonded chain of 4c running along [1 1 0].
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M. Tacke et al.
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data.[19–23] Comparing precursor 3c with the corresponding
complex 4c (see Table 2), one finds a slight increase of both
the C(9)–N distances. The biggest difference between the
silver complexes reported here is the behaviour of the ace-
tate groups: In 4c, one oxygen of the anion connects
strongly to one silver and weakly to another, resulting in a
binuclear species around an inversion centre [with an Ag–
Ag distance of 3.81(1) Å] (Figure 2). The second oxygen is
3.2 Å away from the silver atom, clearly too far for any
interaction. Instead, it acts as a hydrogen-bond acceptor for
two adjacent water molecules. The remaining proton of
each water molecule acts as hydrogen-bond donor to an-
other complex dimer. That leads to hydrogen-bonded
chains as depicted in Figure 3. The compounds 7a to 7c are
mononuclear complexes. In 7a (Figure 4), the second oxy-
gen atom of the acetate is now 2.81 Å away from the silver,
which is significantly closer than in 4c but in our opinion
still too far to call it a bond. In 7b (Figure 5) and 7c (Fig-
ure 6), however, O(2) is close enough to the silver atom to
do so (Table 2). This is further supported by the significant
deviation of the C(8)–Ag–O(1) angle from 180° (Table 2).
Figure 6. X-ray diffraction structure of 7c; molecule; thermal ellip-
soids are drawn on the 50% probability level.
Also, in all the silver complexes reported here, bond
lengths and angles in and directly around the NHC core
agree very well among each other and with literature
Table 1. Crystal data and structure refinement for 3c, 4c and 7a–c.
Identification code
3c
4c
7a
7b
7c
Empirical formula
Molecular formula
C23H25BrN2O3
C50H54Ag2N4O10
C19H19AgN2O2
C19H19AgN2O2
C19H17AgCl2N2O2
C19H17AgCl2N2O2
C23H21AgN2O2
C23H21AgN2O2
[C23H23N2O2]+ [Br]–· C50H50Ag2N4O8·
H2O
2H2O
Formula weight
Crystal system
Space group
457.36
1086.71
triclinic
P1 (#2)
415.23
484.12
465.29
monoclinic
P21/c (#14)
a = 7.2090(5) Å
b = 22.4949(16) Å
c = 25.7455(18) Å
α = 90°
monoclinic
C2/c (#15)
a = 19.8904(15) Å
b = 10.8442(8) Å
c = 15.8836(12) Å
α = 90°
monoclinic
P21/c (#14)
a = 7.4711(8) Å
b = 21.285(2) Å
c = 12.4037(13) Å
α = 90°
triclinic
¯
¯
P1 (#2)
Unit cell dimensions
a = 10.6490(13) Å
b = 10.7944(14) Å
c = 11.7540(15) Å
α = 96.122(3)°
β = 99.845(3)°
γ = 118.625(3)°
1140.4(2) Å3
1
a = 8.0255(8) Å
b = 11.1776(11) Å
c = 11.8084(11) Å
α = 85.681(2)°
β = 81.621(2)°
γ = 73.819(2)°
1005.79(17) Å3
2
β = 95.146(2)°
γ = 90°
β = 90.791(1)°
γ = 90°
β = 102.327(2)°
γ = 90°
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
4158.2(5) Å3
8
3425.7(4) Å3
8
1927.0(4) Å3
4
1.461 Mg/m3
2.004 mm–1
1888
1.582 Mg/m3
1.610 Mg/m3
1.190 mm–1
1680
1.669 Mg/m3
1.339 mm–1
968
1.536 Mg/m3
1.022 mm–1
472
0.924 mm–1
556
Crystal size
Theta range (data coll.)
Index ranges
0.60ϫ0.20ϫ0.15 mm 0.60ϫ0.50ϫ0.30 mm 0.80ϫ0.60ϫ0.10 mm 0.50ϫ0.10ϫ0.05 mm 0.40ϫ0.30ϫ0.20 mm
1.81 to 26.40°
–9ՅhՅ8
–28ՅkՅ28
–32ՅlՅ32
36494
1.80 to 26.45°
–13ՅhՅ13
–13ՅkՅ13
–14ՅlՅ14
10305
2.05 to 29.00°
–27ՅhՅ27
–14ՅkՅ14
–21ՅlՅ21
35864
1.91 to 24.10°
–8ՅhՅ8
–24ՅkՅ24
–14ՅlՅ14
14050
1.74 to 26.38°
–10ՅhՅ10
–13ՅkՅ13
–14ՅlՅ14
17951
Reflections collected
Independent reflections
R(int)
8466
4685
4552
3064
4112
0.0312
0.0404
0.0229
0.0199
0.0203
Completeness to θmax.
Max./min. transmission
Data/restraints/param.
Goodness-of-fit on F2
Final R indices
[IϾ2σ(I)]
R indices
99.4%
99.5%
99.9%
99.9%
99.9%
0.7531/0.4885
8466/0/551
1.054
R1 = 0.0322
wR2 = 0.0777
R1 = 0.0400
wR2 = 0.0809
0.578/–0.237 eÅ–3
0.7691/0.5975
4685/2/307
1.005
R1 = 0.0415
wR2 = 0.0969
R1 = 0.0504
wR2 = 0.1017
1.336/–0.425 eÅ–3
0.8903/0.6467
4552/0/218
1.062
R1 = 0.0211
wR2 = 0.0535
R1 = 0.0223
wR2 = 0.0542
0.622/–0.402 eÅ–3
0.9361/0.7583
3064/0/237
1.048
R1 = 0.0411
wR2 = 0.0895
R1 = 0.0474
wR2 = 0.0934
0.823/–1.047 eÅ–3
0.8216/0.7261
4112/0/256
1.215
R1 = 0.0418
wR2 = 0.0922
R1 = 0.0455
wR2 = 0.0952
1.698/–2.355 eÅ–3
(all data)
Largest diff. peak/hole
1024
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Synthesis and Studies of NHC–Silver Complexes
Table 2. Selected bond lengths [Å] and angles (°) for compounds 3c, 4c and 7a–c.[a]
Bond lengths [Å]
3c
4c
Bond lengths [Å]
7a
7b
7c
N(1)–C(9)
N(1)–C(10)
C(9)–N(2)
N(2)–C(15)
C(10)–C(15)
C(9)–Ag
1.328(3)
1.395(2)
1.328(3)
1.394(2)
1.393(3)
1.348(4)
1.392(4)
1.360(4)
1.388(4)
1.384(5)
2.072(3)
2.612(3)
2.121(2)
C(8)–N(2)
N(1)–C(8)
N(1)–C(9)
N(2)–C(10)
C(9)–C(10)
C(8)–Ag
1.353(2)
1.356(2)
1.387(2)
1.388(2)
1.352(2)
2.065(1)
2.131(1)
1.349(5)
1.347(5)
1.376(5)
1.372(5)
1.331(5)
2.064(4)
2.277(4)
2.502(5)
1.238(6)
1.225(6)
1.688(4)
1.691(4)
1.355(4)
1.356(4)
1.393(4)
1.393(4)
1.380(5)
2.059(3)
2.250(3)
2.475(3)
Ag–O(3)
Ag–O(3)#1
Ag–O(1)
Ag–O(2)
O(1)–C(18)
O(2)–C(18)
C(9)–Cl(1)
C(10)–Cl(2)
O(1)–C(22)
O(2)–C(22)
1.282(2)
1.244(2)
1.252(5)
1.225(5)
Bond angles [°]
3c
4c
Bond angles [°]
7a
7b
7c
N(1)–C(9)–N(2)
C(9)–N(1)–C(10)
C(9)–N(2)–C(15)
C(10)–C(15)–N(2) 106.5(2)
C(15)–C(10)–N(1) 106.3(2)
C(9)–Ag–O(3)
110.4(2)
108.4(2)
108.3(2)
105.5(3)
111.1(3)
111.2(3)
105.9(3)
106.3(3)
112.9(1)
170.1(1)
73.1(1)
N(2)–C(8)–N(1)
C(8)–N(1)–C(9)
C(8)–N(2)–C(10)
C(10)–C(9)–N(1)
C(9)–C(10)–N(2)
C(8)–Ag–O(1)
104.3(1)
111.5(1)
111.3(1)
106.3(1)
106.6(1)
172.76(5)
105.0(3)
110.2(3)
110.6(3)
107.3(3)
106.9(3)
150.3(2)
155.2(1)
97.2(3)
105.8(3)
110.7(3)
110.9(3)
106.5(3)
106.1(3)
164.2(1)
141.6(1)
C(9)–Ag–O(3)#1
O(3)#1–Ag–O(3)
C(8)–Ag–O(2)
C(18)–O(1)–Ag
O(1)–Ag–O(2)
C(18)–O(2)–Ag
O(2)–C(18)–O(1)
C(22)–O(1)–Ag
C(22)–O(2)–Ag
O(2)–C(22)–O(1)
107.64(9)
53.4(1)
86.8(4)
122.3(5)
54.1(1)
97.2(3)
87.3(2)
121.4(4)
[a] Symmetry operations: #1 –x + 1, –y + 1, –z + 1.
Biological Evaluation
The biological activities of the silver acetate, NHC pre-
cursors and their corresponding silver complexes are sum-
marized in Figures 7, 8, 9, and 10. Almost no antibacterial
activity was observed for silver acetate and compound 7c
against the both Gram-positive bacteria Staphylococcus au-
reus and Gram-negative bacteria Escherichia coli using the
Kirby–Bauer disk-diffusion method. Low to medium ac-
tivity was observed with the compounds 3a, 3b, 4c, 6b and
7b against Gram-positive bacteria Staphylococcus aureus
whereas the 3c, 4a, 4b, 6a, 6c and 7a compounds have
shown high activity. In case of Gram-negative bacteria Es-
cherichia coli 3b, 3c, 4b, 4c, 6a, 6b and 6c compounds
showed medium activity whereas the 3a, 4a, 7a and 7b com- Figure 7. Area of clearance on Staphylococcus aureus (Gram +ve)
by 3a–c, 4a–c and AgOAc.
pounds showed high activity. The solvent used to prepare
the stock solutions (DMSO) played no role in growth inhi-
bition on the same bacteria as previously reported.[24]
Thus, on the basis of the above observation it can be precursors and complexes concentration increases, the anti-
stated that (i) the complexes 4c and 7c are less active against microbial activity becomes higher and (iv) It was observed
the both Gram-positive bacteria Staphylococcus aureus and that, compared to the precursors and silver acetate, the
Gram-negative bacteria Escherichia coli than the free pre- complexes exhibited enhanced antibacterial activity, which
cursors 3c and 6c. This is probably due to the poor solubil- is due to the synergistic effect that increases the lipophilicity
ity of the complexes 4c and 7c in DMSO. (ii) On compari- of the complexes. Chelation decreases the polarity of the
son with the complexes 4c and 7c, the complexes 4a, 4b, 7a metal ion, which further leads to the enhanced lipophilici-
and 7b have shown enhanced activity. The enhanced activity ties of the complex. Because the microorganism cell is sur-
of the complexes 4a, 4b, 7a and 7b is due to the fact that rounded by a lipid membrane, which favours the passage of
easy solubility in DMSO. (iii) It was concluded that, as the lipid soluble materials, increased lipophilicities allows the
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M. Tacke et al.
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Cytotoxicity Studies
The in vitro cytotoxicity of compounds 4a–c and 7a–c
were determined by MTT-based assays[26] involving a 48 h
drug-exposure period, followed by 24 h of recovery time.
Compounds were tested for their activity on the human
cancerous renal-cell line Caki-1 and the results are shown
in Figure 11 and 12, respectively. p-Methoxybenzyl-substi-
tuted NHC–silver acetate complexes 4a–c, which contains
1H-imidazole, 4,5-dichloro-1H-imidazole and 1H-benz-
imidazole groups, has IC50 values 7.3, 12.7 and 25.2 µm,
respectively. Compound 4a has an approximately twofold
and threefold increase in magnitude when compared with
4b and 4c, respectively, and in comparison to cisplatin (IC50
value = 3.3 µ) it represents an approximately twofold de-
crease in magnitude. Benzyl-substituted NHC–silver acetate
complexes 7a–c, which also contains 1H-imidazole, 4,5-
dichloro-1H-imidazole and 1H-benzimidazole groups, has
IC50 values of 2.5, 10.8 and 12.5 µ, respectively. Com-
pound 7a is the most promising candidate in this paper be-
cause of highest IC50 value and has an approximately three-
fold and fourfold increase in magnitude when compared
with 7b and 7c, respectively. In comparison with cisplatin,
7a has an approximately equal in magnitude in terms of the
IC50 value.
Figure 8. Area of clearance on Escherichia coli (Gram -ve) by 3a–
c, 4a–c and AgOAc.
Figure 9. Area of clearance on Staphylococcus aureus (Gram +ve)
by 6a–c and 7a–c.
Figure 11. Cytotoxicity curves from typical MTT assays showing
the effect of compounds 4a–c on the viability of Caki-1 cells.
Figure 10. Area of clearance on Escherichia coli (Gram -ve) by 6a–
c and 7a–c.
In comparison to p-methoxybenzyl-substituted NHC–sil-
ver acetate complexes to benzyl-substituted NHC–silver
acetate complexes IC50 values are almost same in magni-
penetration of complex into and through the membrane tude because of their solubility factor. Both p-methoxyben-
and deactivates the active enzyme sites of the microorga- zyl-substituted NHC–silver acetate complexes and benzyl-
nisms.[25]
substituted NHC–silver acetate complexes are easily soluble
In comparison with the known reported NHC precursors in DMSO solvent and all compounds are stable in saline
and NHC–silver complexes from the literature[3,5] the NHC solution with respect to silver chloride precipitation. It was
precursors 3a–c and 6a–c and their corresponding NHC– also observed that, compared to known reported NHC–sil-
silver complexes 4a–c and 7a–c have almost same antibacte- ver complexes from the literature,[6] the NHC–silver com-
rial activity.
plexes 4a–c and 7a–c have almost same cytotoxicity activity.
1026
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 1020–1031

Synthesis and Studies of NHC–Silver Complexes
methane and 50% methanol. MS spectra were obtained in the ES+
(electron-spray positive ionisation) mode for compounds 3a–c, 4a–
c, 6a–c and 7a–c. CHN analysis was done with an Exeter Analyti-
cal CE-440 Elemental Analyser. Ag was estimated by spectrophoto-
metric method (Atomic absorption spectra 55B Varian), while Cl
and Br were determined in mercurimetric titrations. X-ray diffrac-
tion data for compounds 3c, 4c and 7a–c were collected using Mo-
Kα radiation and a Bruker SMART APEX CCD area detector dif-
fractometer. A full sphere of reciprocal space was scanned by phi-
omega scans. Pseudo-empirical absorption correction based on re-
dundant reflections was performed by the program SADABS.[27]
The structures were solved by direct methods using SHELXS-97[28]
and refined by full-matrix least-squares on F2 for all data using
SHELXL-97.[28] Hydrogen atoms involved in hydrogen bonding
were located in the difference Fourier map and allowed to refine
freely. In 4c the O–H bond lengths in the water molecules were
restrained to be 0.84 Å, and the thermal displacement parameters
of the water protons were fixed to be 1.5 times the equivalent ther-
mal displacement parameter of the oxygen atom. All other hydro-
gen atoms were added at calculated positions and refined using a
riding model. Their isotropic temperature factors were fixed to 1.2
times (1.5 times for methyl groups) the equivalent isotropic dis-
placement parameters of the parent carbon atom. Anisotropic ther-
mal displacement parameters were used for all non-hydrogen
atoms. Suitable crystals of 3c, 4c and 7a–c for X-ray studies were
grown from the slow evaporation of acetone, CH2Cl2 and methanol
solutions respectively at room temperature.
Figure 12. Cytotoxicity curves from typical MTT assays showing
the effect of compounds 7a–c on the viability of Caki-1 cells.
Conclusions and Outlook
A series of six new p-methoxybenzyl-substituted and
CCDC-742733 (for 3c), -742734 (for 4c), -742735 (for 7a), -742736
benzyl-substituted N-heterocyclic carbene silver acetate de- (for 7b) and -742737 (7c) respectively, contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from the Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
rivatives 4a–c and 7a–c were synthesised through the reac-
tion of appropriately p-methoxybenzyl-substituted and
benzyl-substituted N-heterocyclic carbenes 3a–c and 6a–c
with silver acetate. Almost all the complexes have shown
high antibacterial activity compared to the precursors and
Syntheses: The synthesis of 1,3-dibenzyl-4,5-dichloroimidazolium
bromide (6b) was carried out accordingly to literature procedure[13]
it is also clear that, as the precursors and complexes concen- where as the syntheses of 1,3-dibenzylimidazolium bromide (6a)
and 1,3-dibenzylbenzimidazolium bromide (6c) were carried out
according to our new procedure instead of literature pro-
cedures.[10–12]
tration increases, the antibacterial activity becomes higher.
Complexes 4a–c and 7a–c yielded antitumor IC50 values of
7.3, 12.7, 25.2, 2.5, 10.8 and 12.5 µ, respectively, on the
Caki-1 cell line. The complex 7a however gave a superior
IC50 value of 2.5 µ. Further work is currently underway
1,3-Bis(4-methoxybenzyl)imidazolium Bromide (3a): 1H-Imidazole
(0.79 g, 11.68 mmol) and K2CO3 (2.42 g, 17.52 mmol) were stirred
in order to improve these values by performing formulation for 15 min in 60 mL of dry acetonitrile. p-Methoxybenzyl bromide
(3.40 mL, 23.36 mmol) was added in one portion and stirring was
continued at room temperature for further 2 d. After the solvent
was removed under reduced pressure 140 mL of water were added.
The aqueous phase was extracted with CH2Cl2 (4ϫ40 mL). Or-
ganic phases were combined and dried with magnesium sulfate. The
solvent was evaporated off under reduced pressure. The crude prod-
uct was washed first with pentane and then with ether. The resulted
white solid dried under vacuum to yield (1.85 g, 4.75 mmol, 40.9%
yield) 3a. 1H NMR (CDCl3, 400 MHz): δ = 10.77 (s, 1 H, NCHN),
7.42 (d, J = 8.6 Hz, 4 H, CHBenzyl), 7.14 (d, J = 1.5 Hz, 2 H,
CHImid), 6.90 (d, J = 8.7 Hz, 4 H, CHBenzyl), 5.47 (s, 4 H, CH2),
3.79 (s, 6 H, OCH3) ppm. 13C NMR (CDCl3, 100 MHz, proton-
decoupled): δ = 159.4, 135.9, 129.7, 123.5, 120.3, 113.8 (NCN,
CImid, CBenzyl), 54.3 (OCH3), 52.0 (CH2) ppm. IR absorptions
experiments to improve solubility of these NHC–silver ace-
tate complexes, which should allow for in vivo testing of 7a
in the nearby future.
Experimental Section
General: If not indicated otherwise, all manipulations were carried
out without taking precautions to exclude air and moisture. All
solvents were used as received. 1H-Imidazole, 4,5-dichloro-1H-
imidazole, 1H-benzimidazole, p-methoxybenzyl bromide, benzyl
bromide, silver acetate and K2CO3 were procured commercially
from Sigma–Aldrich Chemical Company and were used without
further purification. NMR spectra were measured with a Varian
400 MHz spectrometer. Chemical shifts are reported in ppm and
are referenced to TMS. IR spectra were recorded with a Perkin–
Elmer Paragon 1000 FT-IR Spectrometer employing a KBr disc.
UV/Vis spectra were recorded with a Unicam UV4 Spectrometer.
Electron spray mass spectrometry (MS) was performed with a qua-
drupole tandem mass spectrometer (Quattro Micro, Micromass/
Water’s Corp., USA), using solutions made up in 50% dichloro-
(KBr): ν = 3100 (w), 3080 (w), 2998 (m), 2969 (w), 2934 (w), 2838
˜
(m), 1612 (m), 1558 (m), 1515 (s), 1463 (m), 1439 (w), 1305 (m),
1253 (s), 1183 (m), 1151 (s), 1034 (m), 860 (w), 821 (w) cm–1. UV/
Vis (CH3OH): λ (ε) = 204 (22507), 228 (23746), 274 (3654), 281
(3482) nm. MS (m/z, QMS-MS/MS): 309.42 [M+
– Br].
C19H21BrN2O2 (389.29): calcd. C 58.62, H 5.44, N 7.20, Br 20.53;
found C 57.94, H 5.42, N 7.03, Br 19.82.
Eur. J. Inorg. Chem. 2010, 1020–1031
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1027

M. Tacke et al.
FULL PAPER
4,5-Dichloro-1,3-bis(4-methoxybenzyl)imidazolium Bromide (3b):
4,5-Dichloro-1H-imidazole (1.59 g, 11.68 mmol) and K2CO3
(2.42 g, 17.52 mmol) were stirred for 15 min in 60 mL of dry aceto-
nitrile. p-Methoxybenzyl bromide (3.40 mL, 23.36 mmol) was
added in one portion and stirring was continued at room tempera-
ture for further 2 d. After the solvent was removed under reduced
pressure 140 mL of water were added. The aqueous phase was ex-
tracted with CH2Cl2 (4ϫ40 mL). Organic phases were combined
and dried with magnesium sulfate. The solvent was evaporated off
under reduced pressure. The resulted yellow resude was saturated
with diethyl ether (10 mL) to get a fine yellow solid (2.56 g,
5.58 mmol, 47.8% yield) 3b. 1H NMR (CDCl3, 400 MHz): δ =
11.77 (s, 1 H, NCHN), 7.55 (d, J = 8.5 Hz, 4 H, CHBenzyl), 6.91
(d, J = 8.5 Hz, 4 H, CHBenzyl), 5.53 (s, 4 H, CH2), 3.79 (s, 6 H,
OCH3) ppm. 13C NMR (CDCl3, 100 MHz, proton-decoupled): δ
= 160.6, 130.8, 129.0, 123.4, 118.9, 114.8 (NCN,CCl,CBenzyl), 55.3
ppm. IR absorptions (KBr): ν = 3123 (w), 3006 (w), 2963 (w), 2836
˜
(m), 1611 (m), 1560 (s), 1514 (s), 1440 (w), 1410 (s), 1345 (w), 1305
(m), 1252 (s), 1178 (m), 1145 (m), 1030 (m), 962 (w), 925 (w), 823
(m), 769 (m) cm–1. UV/Vis (CH3OH): λ (ε) = 204 (25860), 229
(20349), 275 (2679), 282 (2341) nm. MS (m/z, QMS-MS/MS):
416.23 [M+ – O2CCH3]. C21H23AgN2O4 (475.28): calcd. C 53.06,
H 4.88, N 5.89, Ag 22.69; found C 52.63, H 4.92, N 5.60, Ag 19.28.
[4,5-Dichloro-1,3-bis(4-methoxybenzyl)imidazol-2-ylidene]silver(I)
Acetate (4b): 4,5-Dichloro-1,3-bis(4-methoxybenzyl)imidazolium
bromide (0.91 g, 2.0 mmol) was dissolved in CH2Cl2 (60 mL) and
silver acetate (0.66 g, 4.0 mmol) was added. The mixture was stirred
at room temperature for 2 d. Light was excluded by covering the
flask by aluminium foil. The yellow silver bromide suspension was
filtered to give a light yellow colored solution. The volatile compo-
nents were removed in vacuo to produce a light yellow sticky solid.
The sticky solid was washed with hexane (3ϫ20 mL) and dried
under reduced pressure for 4 h to yield a light yellow crystalline
solid (0.79 g, 1.45 mmol, 72.7% yield) 4b. 1H NMR (CDCl3,
400 MHz): δ = 7.34 (d, J = 8.7 Hz, 4 H, CHBenzyl), 6.88 (d, J =
8.7 Hz, 4 H, CHBenzyl), 5.28 (s, 4 H, CH2), 3.79 (s, 6 H, OCH3),
2.07 (s, 3 H, CH3) ppm. 13C NMR (CDCl3, 100 MHz, proton-
decoupled): δ = 176.1 (NCN), 167.6 (C=O), 158.9, 128.5, 125.2,
116.7, 113.4 (CCl, CBenzyl), 54.3 (OCH3), 53.3 (CH2), 21.0 (CH3)
(OCH ), 52.2 (CH ) ppm. IR absorptions (KBr): ν = 3020 (m),
˜
3
2
2965 (w), 2940 (w), 2861 (w), 1614 (m), 1584 (m), 1546 (m), 1518
(s), 1458 (m), 1427 (m), 1403 (w), 1338 (m), 1308 (m), 1294 (m),
1255 (s), 1179 (m), 1143 (m), 1029 (m), 942 (w), 815 (m) cm–1. UV/
Vis (CH3OH): λ (ε) = 202 (23016), 228 (17075), 272 (2381), 283
(1891) nm. MS (m/z, QMS-MS/MS): 378.28 [M+
– Br].
C19H19BrCl2N2O2 (458.18): calcd. C 49.81, H 4.18, N 6.11, Br
17.44, Cl 15.48; found C 51.28, H 4.25, N 6.25, Br 16.89, Cl 16.85.
ppm. IR absorptions (KBr): ν = 2999 (w), 2960 (w), 2935 (w), 2837
˜
1,3-Bis(4-methoxybenzyl)benzimidazolium Bromide (3c): 1H-Benz-
imidazole (1.37 g, 11.68 mmol) and K2CO3 (2.42 g, 17.52 mmol)
were stirred for 15 min in 60 mL of dry acetonitrile. p-Methoxyben-
zyl bromide (3.40 mL, 23.36 mmol) was added in one portion and
stirring was continued at room temperature for further 3 d. After
the solvent was removed under reduced pressure 150 mL of water
were added. The precipitate was filtered, washed with diethyl ether
(3ϫ20 mL) and dried in suction at room temperature for 4 h. A
fine white powder was obtained and recrystallised from acetone to
give white crystalline product (2.25 g, 4.91 mmol, 42.9% yield) 3c.
1H NMR (CDCl3, 400 MHz): δ = 11.85 (s, 1 H, NCHN), 7.60–
7.57 (m, 2 H, CHBenzimid), 7.51–7.49 (m, 6 H, CHBenzimid, CHBenzyl),
6.89 (d, J = 8.4 Hz, 4 H, CHBenzyl), 5.78 (s, 4 H, CH2), 3.78 (s, 6
H, OCH3) ppm. 13C NMR (CDCl3, 100 MHz, proton-decoupled):
δ = 159.2, 141.8, 130.32, 129.0, 126.0, 123.4, 113.7, 112.7 (NCN,
(w), 1611 (m), 1582 (m), 1513 (s), 1439 (m), 1406 (w), 1385 (w),
1332 (m), 1305 (m), 1253 (s), 1177 (m), 1114 (m), 1032 (m), 920
(w), 810 (m), 718 (w) cm–1. UV/Vis (CH3OH): λ (ε) = 204 (34418),
228 (30891), 276 (4252), 286 (2753) nm. MS (m/z, QMS-MS/MS):
485.24 [M+ – O2CCH3]. C21H21AgCl2N2O4 (544.21): calcd. C
46.34, H 3.89, N 5.14, Cl 13.02, Ag 19.82; found C 46.87, H 4.06,
N 4.96, Cl 13.14, Ag 19.56.
[1,3-Bis(4-methoxybenzyl)benzimidazol-2-ylidene]silver(I) Acetate
(4c): A mixture of 1,3-bis(4-methoxybenzyl)benzimidazolium bro-
mide (0.91 g, 2.0 mmol) and silver acetate (0.66 g, 4.0 mmol) in
CH2Cl2 (60 mL) was stirred at room temperature for 3 d. Light was
excluded by covering the flask by Al foil. The reaction mixture was
filtered to remove a yellow precipitate presumably AgBr and the
volatile components were removed in vacuo to yield a white solid
1
(0.86 g, 1.58 mmol, 79.6% yield) 4c. H NMR (CDCl3, 400 MHz):
CBenzimid, CBenzyl), 54.3 (OCH3), 50.1 (CH2) ppm. IR absorptions
δ = 7.30–7.28 (m, 2 H, CHBenzimid), 7.23–7.18 (m, 6 H, CHBenzimid
,
(KBr): ν = 3114 (w), 3059 (w), 2966 (w), 2834 (w), 1611 (m), 1564
˜
CHBenzyl), 6.77 (d, J = 8.4 Hz, 4 H, CHBenzyl), 5.48 (s, 4 H, CH2),
3.69 (s, 6 H, OCH3), 2.03 (s, 3 H, CH3) ppm. 13C NMR (CDCl3,
100 MHz, proton-decoupled): δ = 178.9 (NCN), 168.3 (C=O),
159.6, 133.9, 128.8, 127.0, 124.0, 114.4, 112.0 (CBenzimid, CBenzyl),
55.2 (OCH3), 53.1 (CH2), 22.7 (CH3) ppm. IR absorptions (KBr):
(s), 1513 (s), 1477 (w), 1441 (m), 1401 (m), 1342 (w), 1303 (m),
1253 (s), 1176 (m), 1027 (m), 924 (w), 822 (m), 799 (m), 743 (m),
709 (w) cm–1. UV/Vis (CH3OH): λ (ε) = 202 (23982), 226 (13108),
276 (4639), 278 (4554) ppm. MS (m/z, QMS-MS/MS): 359.90 [M+ –
Br – H2O]. C23H25BrN2O3 (457.37): calcd. C 60.40, H 5.51, N 6.13,
Br 17.47; found C 61.89, H 5.30, N 6.08, Br 17.71.
ν = 3448 (s), 2955 (m), 2834 (w), 1614 (m), 1557 (m), 1516 (s), 1464
˜
(m), 1439 (w), 1384 (w), 1301 (m), 1259 (s), 1181 (m), 1114 (w),
1092 (w), 1029 (m), 914 (w), 879 (w), 809 (m), 754 (m), 701 (w)
cm–1. UV/Vis (CH3OH): λ (ε) = 207 (28063), 223 (28049), 277
[1,3-Bis(4-methoxybenzyl)imidazol-2-ylidene]silver(I) Acetate (4a):
1,3-Bis(4-methoxybenzyl)imidazolium bromide (0.77 g, 2.0 mmol)
was dissolved in CH2Cl2 (60 mL) and silver acetate (0.66 g,
4 mmol) was added. The mixture was stirred at room temperature
for 2 d. Light was excluded by covering the flask with aluminium
foil. The yellow precipitate of AgBr was filtered and discarded. The
volume of the reaction mixture was reduced under reduced pressure
to 5 mL. Hexane (30 mL) was added and kept in refrigerator for
one week. The fine white precipitate was filtered and washed with
20 mL of hexane and dried under reduced pressure to yield a off
(11104), 285 (9306) nm. MS (m/z, QMS-MS/MS): 466.76 [M+
–
H2O – O2CCH3]. C25H27AgN2O5 (543.36): calcd. C 55.25, H 5.01,
N 5.15, Ag 19.85; found C 55.77, H 5.08, N 5.01, Ag 20.28.
1,3-Dibenzylimidazolium Bromide (6a): 1H-Imidazole (0.79 g,
11.7 mmol) and K2CO3 (2.42 g, 17.5 mmol) were stirred for 15 min
in 60 mL of acetonitrile. Benzyl bromide (2.77 mL, 23.36 mmol)
was added in one portion and stirring was continued at room tem-
white solid (0.83 g, 1.74 mmol, 87.4% yield) 4a. 1H NMR (CDCl3, perature for further 2 d. After the solvent was removed under re-
400 MHz): δ = 7.24 (d, J = 8.6 Hz, 4 H, CHBenzyl), 6.91–6.85 (m, duced pressure 140 mL of water were added. The aqueous phase
6 H, CHImid, CHBenzyl), 5.21 (s, 4 H, CH2), 3.79 (s, 6 H, OCH3),
2.08 (s, 3 H, CH3) ppm. 13C NMR (CDCl3, 100 MHz, proton-
decoupled): δ = 177.8 (NCN), 167.9 (C=O), 159.6, 129.5, 127.5,
was extracted with CH2Cl2 (4ϫ40 mL). Organic phases were com-
bined and dried with magnesium sulfate. The solvent was evapo-
rated off under reduced pressure. The white solid was washed with
121.0, 114.4 (CImid, CBenzyl), 55.4 (OCH3), 55.2 (CH2), 22.6 (CH3) diethyl ether to remove unreacted 1-benzylimidazole and dried in
1028 Eur. J. Inorg. Chem. 2010, 1020–1031
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Synthesis and Studies of NHC–Silver Complexes
vacuo to yield 6a (3.50 g, 10.62 mmol, 91.0% yield). 1H NMR
(CDCl3, 400 MHz): δ = 10.79 (s, 1 H, NCHN), 7.49–7.45 (m, 4 H,
CHBenzyl), 7.38–7.35 (m, 6 H, CHBenzyl), 7.26 (d, J = 1.5 Hz, 2 H,
(m, 2 H, CHBenzimid), 7.53–7.51 (m, 4 H, CHBenzyl), 7.43–7.34 (m,
6 H, CHBenzyl), 5.79 (s, 4 H, CH2) ppm. 13C NMR ([D6]DMSO,
125 MHz, proton-decoupled): δ = 143.1, 134.3, 131.5, 129.4, 129.2,
CHImid), 5.56 (s, 4 H, CH2) ppm. 13C NMR (CDCl3, 100 MHz, 128.7, 127.2, 114.4 (NCN, CBenzimid, CBenzyl), 50.4 (CH2) ppm. IR
proton-decoupled): δ = 137.0, 132.7, 129.5, 129.4, 129.0, 121.8
absorptions (KBr): ν = 3452 (s), 3123 (w), 3057 (m), 2924 (m), 2853
˜
(NCN, CImid, CBenzyl), 53.4 (CH ) ppm. IR absorptions (KBr): ν = (w), 1605 (m), 1558 (s), 1476 (m), 1457 (s), 1427 (s), 1373 (s), 1337
˜
2
3453 (s), 3124 (w), 3068 (m), 2929 (w), 2847 (w), 1628 (w), 1559 (w), 1190 (m), 1081 (w), 1016 (m), 913 (w), 822 (w), 756 (s), 739
(s), 1496 (m), 1456 (s), 1383 (w), 1261 (w), 1208 (w), 1150 (s), 1078 (m), 703 (s) cm–1. UV/Vis (CH3OH): λ (ε) = 207 (26158), 256
(w), 1029 (w), 874 (w), 822 (w), 720 (s) cm–1. UV/Vis (CH3OH): λ (6902), 262 (7042), 270 (7469), 278 (6021) nm. MS (m/z, QMS-MS/
(ε) = 204 (20788), 268 (14373) nm. MS (m/z, QMS-MS/MS): 249.47
[M+ – Br]. C17H17BrN2 (329.26): calcd. C 62.02, H 5.20, N 8.51,
Br 24.26; found C 62.25, H 5.29, N 9.16, Br 22.21.
MS): 299.49 [M+ – Br]. C21H19BrN2 (379.29): calcd. C 66.50, H
5.05, N 7.39, Br 21.07; found C 65.92, H 5.03, N 6.79, Br 19.74.
(1,3-Dibenzylimidazol-2-ylidene)silver(I) Acetate (7a): 1,3-Dibenz-
ylimidazolium bromide (0.329 g, 1.0 mmol) was dissolved in
CH2Cl2 (50 mL) and silver acetate (0.333 g, 2.0 mmol) was added.
The mixture was stirred at room temperature in the absence of light
for 1 d. The yellow silver bromide suspension was filtered to give a
4,5-Dichloro-1-benzylimidazole (5Ј): 4,5-Dichloro-1H-imidazole
(0.80 g, 5.84 mmol) and K2CO3 (1.21 g, 8.76 mmol) were stirred for
10 min in 30 mL of acetonitrile. Benzyl bromide (0.69 mL,
5.84 mmol) was added in one portion and stirring was continued
at room temperature for further 2 d. After the solvent was removed colorless solution. The volume of the reaction was concentrated in
under reduced pressure 70 mL of water were added. The aqueous
phase was extracted with CH2Cl2 (4ϫ20 mL). Organic phases were
combined and dried with sodium sulfate. 5Ј was obtained as orange
vacuo. Addition of diethyl ether gave an off-white precipitate which
was isolated by filtration, washed with diethyl ether and dried in
vacuo to yield 7a (0.300 g, 0.72 mmol, 72.2% yield). 1H NMR
(CDCl3, 400 MHz): δ = 7.39–7.27 (m, 10 H, CHBenzyl), 6.91 (s, 2
solid after solvent removal under reduced pressure (1.24 g,
1
5.46 mmol, 93.0% yield). H NMR (CDCl3, 400 MHz): δ = 7.40– H, CHImid), 5.30 (s, 4 H, CH2), 2.08 (s, 3 H, CH3) ppm. 13C NMR
7.33 (m, 4 H, NCHN, CHBenzyl), 7.19–7.17 (m, 2 H, CHBenzyl), (CDCl3, 125 MHz, proton-decoupled): δ = 177.0 (NCN), 166.6
5.08 (s, 2 H, CH2) ppm. 13C NMR (CDCl3, 100 MHz, proton-
(C=O), 134.4, 128.0, 127.6, 126.2, 120.3 (CImid, CBenzyl), 54.9
decoupled): δ = 134.4, 134.3, 129.1, 128.6, 127.3, 126.4, 113.8 (CH ), 21.7 (CH ) ppm. IR absorptions (KBr): ν = 3428 (s), 3092
˜
2
3
(NCN, CBenzyl, CCl), 49.7 (CH ) ppm. IR absorptions (KBr): ν =
(w), 3030 (w), 2965 (w), 2930 (w), 1637 (w), 1560 (s), 1496 (m),
1454 (m), 1413 (s), 1350 (w), 1262 (w), 1229 (w), 1150 (m), 1078
˜
2
3122 (m), 3032 (w), 2962 (w), 1519 (s), 1496 (w), 1484 (s), 1456
(m), 1437 (s), 1389 (m), 1346 (m), 1254 (s), 1180 (m), 1115 (m), 976 (m), 924 (w), 880 (w), 806 (w), 728 (s), 709 (w) cm–1. UV/Vis
(m), 807 (s), 722 (s) cm–1. UV/Vis (CH3OH): λ (ε) = 208 (9035),
(CH3OH): λ (ε) = 205 (27199), 209 (27265), 258 (3043), 269 (2988)
O2CCH3].
C19H19AgN2O2 (415.27): calcd. C 54.95, H 4.61, N 6.74, Ag 25.97;
227 (3280), 258 (413), 260 (399), 269 (352) nm. MS (m/z, QMS- nm. MS (m/z, QMS-MS/MS): 356.22 [M+
–
MS/MS): 227.37 [M + H+]. C10H8Cl2N2 (226.01): calcd. C 52.89,
H 3.55, N 12.34, Cl 31.22; found C 51.40, H 3.83, N 11.36, Cl
30.79.
found C 54.51, H 4.68, N 6.52, Ag 26.20.
(1,3-Dibenzyl-4,5-dichloroimidazol-2-ylidene)silver(I) Acetate (7b):
1,3-Dibenzyl-4,5-dichloroimidazolium bromide (0.199 g, 0.5 mmol)
was dissolved in CH2Cl2 (25 mL) and silver acetate (0.166 g,
1.0 mmol) was added. The mixture was stirred at room temperature
in the dark for 1 d. The yellow silver bromide suspension was fil-
tered to give a colorless solution. The volatile components were
removed in vacuo to produce off white sticky solid. The solid was
first washed with hexane and then with diethyl ether and dried
1,3-Dibenzyl-4,5-dichloroimidazolium Bromide (6b): Benzyl bromide
(0.79 mL, 6.6 mmol) was added in one portion to a stirred suspen-
sion of 1-benzyl-4,5-dichloroimidazole (0.50 g, 2.2 mmol) in 5 mL
of toluene. The mixture was stirred for 40 h at room temperature.
Afterwards the solvent was removed under reduced pressure. The
residue was washed with THF (4ϫ5 mL) and the title compound
6b was isolated as a white solid (0.609 g, 1.52 mmol, 69.0% yield).
1H NMR (CDCl3, 400 MHz): δ = 11.79 (s, 1 H, NCHN), 7.57– under reduced pressure for 2 h to yield (0.152 g, 0.31 mmol, 62.7%
7.55 (m, 4 H, CHBenzyl), 7.41–7.34 (m, 6 H, CHBenzyl), 5.66 (s, 4 H, yield) 7b. 1H NMR (CDCl3, 400 MHz): δ = 7.39–7.32 (m, 10 H,
CH2) ppm. 13C NMR (CDCl3, 100 MHz, proton-decoupled): δ =
137.8, 131.5, 129.6, 129.4, 128.9, 119.3 (NCN, CBenzyl, CCl), 52.7
CHBenzyl), 5.38 (s, 4 H, CH2), 2.06 (s, 3 H, CH3) ppm. 13C NMR
(CDCl3, 100 MHz, proton-decoupled): δ = 177.5 (NCN), 168.6
(C=O), 134.1, 129.1, 128.8, 127.7, 118.0 (CBenzyl, CCl), 54.8 (CH2),
(CH ) ppm. IR absorptions (KBr): ν = 3098 (m), 3012 (s), 2861
˜
2
(m), 1585 (s), 1550 (s), 1498 (s), 1456 (s), 1405 (m), 1343 (s), 1305
(m), 1205 (s), 1180 (s), 1146 (s), 1077 (m), 1031 (m), 901 (w), 768
(s), 731 (s), 710 (s) cm–1. UV/Vis (CH3OH): λ (ε) = 204 (24368),
22.0 (CH ) ppm. IR absorptions (KBr): ν = 3424 (s), 3063 (w),
˜
3
3029 (w), 2960 (w), 2926 (w), 2855 (w), 1556 (s), 1496 (m), 1454
(w), 1411 (s), 1345 (m), 1260 (m), 1207 (m), 1078 (w), 1028 (w),
928 (w), 804 (w), 744 (w), 714 (w) cm–1. UV/Vis (CH3OH): λ (ε) =
207 (29415), 246 (9196), 286 (2907) nm. MS (m/z, QMS-MS/MS):
425.23 [M+ – O2CCH3]. C19H17AgCl2N2O2 (484.15): calcd. C
47.13, H 3.54, N 5.78, Cl 14.64, Ag 22.28; found C 46.26, H 3.61,
N 5.66, Cl 14.50, Ag 21.54.
261 (428), 268 (234) nm. MS (m/z, QMS-MS/MS): 318.22 [M+
–
Br]. C17H15BrCl2N2 (398.12): calcd. C 51.29, H 3.80, N 7.04, Br
20.07, Cl 17.81; found C 51.10, H 3.68, N 6.95, Br 20.54, Cl 18.03.
1,3-Dibenzylbenzimidazolium Bromide (6c): 1H-Benzimidazole
(1.37 g, 11.68 mmol) and K2CO3 (2.42 g, 17.52 mmol) were stirred
for 15 min in 60 mL of acetonitrile. Benzyl bromide (2.77 mL,
23.36 mmol) was added in one portion and stirring was continued
(1,3-Dibenzylbenzimidazol-2-ylidene)silver(I) Acetate (7c): 1,3-Di-
benzylbenzimidazolium bromide (0.189 g, 0.50 mmol) was dis-
at room temperature for further 2 d. After the solvent was removed solved in CH2Cl2 (25 mL) and silver acetate (0.166 g, 1.0 mmol)
under reduced pressure 150 mL of water were added. The white
precipitate was filtered, washed with diethyl ether (2ϫ25 mL) to
remove unreacted 1-benzylbenzimidazole and dried in suction at
room temperature for 4 h to afforded the product (3.20 g,
8.43 mmol, 72.2% yield) 6c. 1H NMR ([D6]DMSO, 400 MHz): δ
= 10.12 (s, 1 H, NCHN), 7.97–7.93 (m, 2 H, CHBenzimid), 7.63–7.59
was added. The mixture was stirred at room temperature in the
dark for 1 d. The yellow silver bromide suspension was filtered to
give a colorless solution. The volatile components were removed in
vacuo to produce off-white sticky solid. The solid was washed with
hexane and dried under reduced pressure for 1 h to yield (0.153 g,
0.32 mmol, 65.7% yield) 7c. 1H NMR ([D6]DMSO, 400 MHz): δ
Eur. J. Inorg. Chem. 2010, 1020–1031
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Tacke et al.
FULL PAPER
= 7.69–7.65 (m, 2 H, CHBenzimid), 7.41 (d, J = 7.0 Hz, 4 H,
(Multilabel HTS Counter) Plate Reader was used to measure ab-
CHBenzimid, CHBenzyl), 7.35–7.24 (m, 8 H, CHBenzyl), 5.73 (s, 4 H, sorbance at 540 nm. Cell viability was expressed as a percentage of
CH2), 1.79 (s, 3 H, CH3) ppm. 13C NMR ([D6]DMSO, 100 MHz,
proton-decoupled): δ = 175.7 (NCN), 166.1 (C=O), 136.6, 133.8,
the absorbance recorded for control wells. The values used for the
dose response curves represent the values obtained from four con-
129.2, 128.4, 127.9, 124.4, 112.8 (CBenzimid, CBenzyl), 52.4 (CH2), sistent MTT-based assays for each compound tested.
23.7 (CH ) ppm. IR absorptions (KBr): ν = 3434 (s), 3063 (w),
˜
3
2925 (w), 1565 (s), 1496 (m), 1477 (m), 1401 (s), 1261 (w), 1185
(w), 1096 (w), 1078 (w), 1026 (m), 925 (w), 821 (w), 746 (s), 705
(m) cm–1. UV/Vis (CH3OH): λ (ε) = 207 (30176), 247 (5368), 276
Acknowledgments
(7706), 285 (7598). MS (m/z, QMS-MS/MS): 406.28 [M+
–
The authors thank the Irish Research Council for Science Engi-
neering and Technology (IRCSET) for funding through a postdoc-
toral fellowship for Dr. Siddappa Patil.
O2CCH3]. C23H21AgN2O2 (465.33): calcd. C 59.36, H 4.55, N 6.02,
Ag 23.18; found C 59.19, H 4.58, N 5.91, Ag 23.21.
Antibacterial Studies: Preliminary antibacterial activity of p-meth-
oxybenzyl-substituted and benzyl-substituted N-heterocyclic carb-
enes and their corresponding silver complexes were screened simul-
taneously with silver acetate against two bacterial strains. The test
organisms included Staphylococcus aureus (SA) (NCTC 7447) as a
gram-positive bacteria and Escherichia coli as gram-negative bacte-
ria.
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Received: September 8, 2009
Published Online: February 3, 2010
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