Synthesis and characterization of ortho-xylyl linked bis-benzimidazolium salts (Part-II)

Article abstract of DOI:10.14233/ajchem.2013.13509

A number of N-alkylbenzimidazoles were synthesized by reactions of benzimidazole with alkyl halides (PentBr, HexBr, OctBr, DecBr). The subsequent treatment of the resulting N-alkylbenzimidazoles with 1,2-bis(bromomethylene) benzene afforded corresponding bisbenzimidazolium salts. All the compounds were characterized by spectroscopic techniques (NMR and FT-IR) and microanalysis. Molecular structures of selected compounds were established through single crystal X-ray diffraction studies.

Full text of DOI:10.14233/ajchem.2013.13509

Asian Journal of Chemistry; Vol. 25, No. 6 (2013), 3049-3054
http://dx.doi.org/10.14233/ajchem.2013.13509
Synthesis and Characterization of ortho-Xylyl Linked Bis-benzimidazolium Salts (Part-II)
*
ROSENANI A. HAQUE and MUHAMMAD ADNAN IQBAL
School of Chemical Sciences, Universiti Sains Malaysia, 11800 USM, Pulau Pinang, Malaysia
*
Corresponding author: E-mail: rosenani@usm.my
(
Received: 24 January 2012;
Accepted: 12 December 2012)
AJC-12516
A number of N-alkylbenzimidazoles were synthesized by reactions of benzimidazole with alkyl halides (PentBr, HexBr, OctBr, DecBr).
The subsequent treatment of the resulting N-alkylbenzimidazoles with 1,2-bis(bromomethylene) benzene afforded corresponding bis-
benzimidazolium salts.All the compounds were characterized by spectroscopic techniques (NMR and FT-IR) and microanalysis. Molecular
structures of selected compounds were established through single crystal X-ray diffraction studies.
Key Words: Benzimidazolium salts, N-Heterocyclic carbenes, Crystal structures.
and mixture was stirred for 0.5 h at RT (25-27 ºC) and respective
alkyl halide (1 equivalent) was added dropwise under vigorous
stirring. After 2 h, the mixture was poured into 200-300 mL
of water and extracted with chloroform (6 mL × 25 mL), the
combined extract was filtered through 5 plies of Whatman
filter papers in order to dry the extract. This process of filtration
was repeated twice to collect crystal clear solution of desired
compound which was finally evaporated under reduced
pressure. Desired compounds were obtained as thick yellowish
fluids and characterized by spectroscopic (FT-IR and NMR)
techniques before further use.
INTRODUCTION
N-Heterocyclic carbenes (NHCs) have become universal
1
ligands in organometallic and inorganic coordination chemistry
2
as their metal complexes have widespread biological and
-5
6
-12
catalytic applications. Our group is actively working on the
synthesis and characterization of imidazole and benzimidazole
based azolium salts and their metal (Ag, Pd, V, Hg) complexes
with biological and catalytic applications. The recent work in
our group comprises the synthesis of series of xylyl (ortho,
meta and para) linked bis-benzimidazolium salts with alkyl
substitutions (ethyl-decyl) and their metal (Ag and Hg) comp-
lexes. We reported a part of ortho-xylyl linked bis-benzimida-
N-Pentylbenzimidazole (I): Thick yellowish fluid.Yield
16
.21 g (92.37 %). For characterization refer .
5
13
14
zolium salts and selected crystal structures from ortho series.
Herein we report the remaining part of this series with few
more crystal structures.
N-Hexylbenzimidazole (II): Thick yellowish fluid.Yield
1
5
.72 g (94.24 %). H NMR (500 MHz, DMSO-d
6
): δ 0.77
), 1.72 (2H,
2
-N, J = 7.00 Hz),
(
3H, t, CH
3
, J = 7.00 Hz), 1.19 (6H, br.m, 3 × CH
2
Pent, CH , J = 7.00 Hz), 4.14 (2H, t, CH
2
EXPERIMENTAL
7.17 (2H, m, Ar 2 × CH), 7.47 (1H, d, Ar 1 × CH, J = 7.5 Hz),
7.65 (1H, d, Ar 1 × CH, J = 8.00 Hz), 8.12 (1H, s, NCHN);
The FT-IR spectra were recorded on a Perkin-Elmer
system 2000 spectrometer in KBr (for solids) and on thallium
13
1
C{ H NMR}(125.1 MHz, DMSO-d
25.67, 29.20, 30.58 (4 × CH ), 44.01 (R-CH
119.29, 121.08, 121.93, 133.60 (Ar-C) 143.50 (NCHN, d, J =
6
): 13.54 (CH
3
), 21.82,
1
13
bromide disks (for liquids). The H and C NMR spectra were
obtained on a Bruker DPX-500 instrument from solutions in
2
2
-N) 109.86,
-
1
DMSO-d
6
. The melting points were determined using Stuart
8.75 Hz). FT-IR (KBr, νmax, cm ): 3434 (Caliph-Nbenzimi); 3077,
3056 (C-Harom); 2957, 2931, 2859 (C-Haliph), 1615 (Carom-Carom);
1496, 1459, 1286, 1365 (Carom-Nbenzimi).
Scientific SMP-1 (UK). The CHN microanalyses were carried
out by using a Perkin Elmer 2400 LS Series CHN/S analyzer.
Synthesis of N-substituted benzimidazoles (I-IV): All
the N-substituted benzimidazole compounds (I-IV) were pre-
N-Octylbenzimidazole (III): Thick yellowish fluid.Yield
1
5.72 g (93.44 %). H NMR (500 MHz, DMSO-d
6
): δ 0.84
),
-N, J =
15
pared according to the method developed by Starikova et al. ,
(3H, t, CH
3
, J = 7.00 Hz), 1.17-1.23 (10H, br.m, 5 × CH
2
with minor modifications. In general, potassium hydroxide
(1.5 equivalent) was added to a solution of benzimidazole (1
equivalent) in DMSO (30-40 mL for 0.01-0.02 M of reactants)
2
1.75 (2H, Pent, CH , J = 7.00 Hz), 4.18 (2H, t, CH
2
7.00 Hz), 7.21 (2H, m, Ar 2 × CH), 7.52 (2H, d, Ar 1 × CH,
J = 8.00 Hz), 7.68 (1H, d, Ar 1 × CH, J = 8.00 Hz), 8.19 (1H,

3
050 Haque et al.
Asian J. Chem.
1
3
1
s, NCHN); C{ H NMR}(125.1 MHz, DMSO-d
CH ), 21.92, 26.03, 28.39 (CH ), 28.46 (2 × CH , d, J = 8.75
Hz), 29.28, 31.06 (CH ), 43.98 (R-CH -N), 109.96, 119.32,
21.08, 121.92, 133.68 (Ar-C) 143.64 (NCHN, d, J = 17.51
6
): 13.65
(R-CH -N), 47.74 (Ar-CH
2
2
-N), 114.11 (Ar-C, d, J = 30 Hz),
126.68 (Ar-C, d, J = 12.51 Hz), 128.66, 129.19, (Ar-C, s),
(
3
2
2
2
2
131.13 (Ar-C, d, J = 106 Hz), 131.90, 132. 08 (Ar-C) and
-1
1
142.46 (NCHN). FT-IR (KBr, νmax, cm ): 3435, 3386 (Caliph-
-1
Hz). FT-IR (KBr, νmax, cm ): 3422 (Caliph-Nbenzimi); 3056 (C-
arom); 2956, 2929, 2857 (C-Haliph), 1615 (Carom-Carom); 1496,
N
benzimi); 3111, 3029 (C-Harom); 2997, 2958, 2931, 2858 (C-
aliph), 1631, 1609, 1559 (Carom-Carom); 1192, 1429, 1446, 1485
H
H
1459, 1286, 1365 (Carom-Nbenzimi).
45 4 2
(Carom-Nbenzimi). Anal. calcd. (%) for: C34H N OBr : C, 59.57;
N-Decylbenzimidazole (IV): Thick yellowish fluid.Yield
H, 6.62; N, 8.17. Found (%): C, 59.44; H, 6.43; N, 8.17.
3,3'-(1,2-Phenylenebis(methylene))bis(1-octyl-benzimi-
dazolium)dibromide monohydrate (VII.2Br): White crys-
1
7
.14 g (92.37 %). H NMR (500 MHz, DMSO-d
6
): δ 0.822
),
-N, J =
.00 Hz), 7.20 (2H, m, Ar 2 × CH), 7.54 (1H, d, Ar 1 × CH,
(
3H, t, CH
3
, J = 7.00 Hz), 1.17 - 1.21 (16H, br.m, 8 × CH
2
1
1
7
.75 (2H, Pent, CH , J = 7.00 Hz), 4.19 (2H, t, CH
2
2
talline powder. Yield 7.25 g (97.76 %), m.p. 198-201 ºC. H
NMR (500 MHz, DMSO-d
Hz), 1.21 - 1.36 (20H, br.m, 10 × CH
CH , J = 7.50 Hz), 4.54 (4H, t, 2 × CH
(4H, s, 2 × Ar-CH
6 3
): δ 0.82 (6H, t, 2 × CH , J = 7.00
J = 8.00 Hz), 7.65 (1H, d, Ar 1 × CH, J = 8.00 Hz), 8.19 (1H,
2
), 1.91 (4H, Pent, 2 ×
1
3
1
s, NCHN); C{ H NMR}(125.1 MHz, DMSO-d
CH ), 21.98, 26.03, 28.44 (CH ), 28.57 (2 × CH , d, J = 16.26
Hz), 28.82 (2 × CH , d, J = 1.25 Hz), 29.29 (CH ), 43.98
R-CH -N), 110.05, 119.33, 121.12, 121.95, 133.71 (Ar-C)
6
): 13.73
2
2
-N, J = 7.50 Hz), 6.22
(
3
2
2
2
-N), 7.22 (2H, q, Ar 2 × CH, J = 3.50 Hz),
2
2
7.39 (2H, q, Ar 2 × CH, J = 3.50 Hz), 7.62 (2H, t, Ar 2 × CH,
J = 7.75 Hz), 7.68 (2H, t, Ar 2 × CH, J = 7.75 Hz), 8.07 (2H,
d, Ar 2 × CH, J = 8.50 Hz), 8.18 (2H, d, Ar 2 × CH, J = 8.50
(
2
-1
1
43.74 (NCHN, d, J = 31.27 Hz). FT-IR (KBr, νmax, cm ):
13
1
3
428 (Caliph-Nbenzimi); 3055 (C-Harom); 2929, 2855 (C-Haliph), 1615
Hz), 10.08 (2H, s, 2 × NCHN); C{ H NMR} (125.1 MHz,
DMSO-d ): 13.83 (CH ), 21.96, 25.36, 25.74 (3 × CH ), 28.44
(3 × CH , t, JC5,6 = 10 Hz, JC6,7 = 6.25 Hz ), 46.89 (R-CH -N),
47.73 (Ar-CH -N), 114.12 (Ar-C, d, J = 30 Hz), 126.68
(C
arom-Carom); 1496, 1459, 1286, 1366 (Carom-Nbenzimi).
6
3
2
Synthesis of bis-benzimidazolium salts (V-VIII): The
2
2
synthesis of ligands (V-VIII) was carried out by same method
2
13
as we reported previously . All the bis-benzimidazolium salts
were collected as powderous material directly from reaction
medium and dried under vacuum for 24 h. The physical
appearances, yields and instrumental characterizations are
mentioned under respective headings.
(Ar-C, d, J = 11.25 Hz), 128.65, 129.19, (Ar-C), 131.15 (Ar-
C, d, J = 10 Hz), 132.10 (Ar-C) and 142.49 (NCHN). FT-IR
-1
(KBr, νmax, cm ): 3439, 3388 (Caliph-Nbenzimi); 3107, 3024 (C-
arom); 2994, 2928, 2856 (C-Haliph); 1609, 1557 (C-Haliph); 1198,
1429, 1446, 1483 (Carom-Nbenzimi). Anal. calcd. (%) for:
OBr : C, 61.54; H, 7.20; N, 7.55. Found (%): C, 61.63;
H, 7.25; N, 7.60.
3,3'-(1,2-Phenylenebis(methylene))bis(1-decyl-
H
3
,3'-(1,2-Phenylenebis(methylene))bis(1-pentyl-
C H
38 54
N
4
2
benzimidazolium) dibromide monohydrate (V.2Br):White
1
powder. Yield 0.55 g (16.81 %), m.p. 212-216 ºC. H NMR
(
500 MHz, DMSO-d
.31-1.36 (8H, m, 4 × CH
Hz), 4.50 (4H, t, 2 × N-CH
N-CH -Ar), 7.20 (2H, q, Ar 2 × CH, J = 3.5 Hz), 7.41 (2H, q,
Ar 2 × CH, J = 3.50 Hz), 7.65 (2H, t, Ar 2 × CH, J = 7.5 Hz),
.71 (2H, t, Ar 2 × CH, J = 7.50 Hz), 7.98 (2H, d, Ar 2 × CH,
J = 8.0 Hz), 8.16 (2H, d, Ar 2 × CH, J = 8.50 Hz), 9.93 (2H, s,
6
): 0.87 (6H, t, 2 × CH
), 1.91 (4H, pent, 2 × CH
-R, J = 7.50 Hz), 6.10 (4H, s, 2 ×
3
, J = 7.00 Hz),
benzimidazolium)dibromide monohydrate (VIII.2Br):
1
White powder. Yield 5.65 g (70.89 %), m.p. 148-150 ºC. H
1
2
2
, J = 7.5
2
NMR (500 MHz, DMSO-d
Hz), 1.22-1.34 (28H, br.m, 14 × CH
CH , J = 7.50 Hz), 4.51 (4H, t, 2 × CH
(4H, s, 2 × Ar-CH -N), 7.21 (2H, q, Ar 2 × CH, J = 3.50 Hz),
6
): δ 0.83 (6H, t, 2 × CH
), 1.90 (4H, Pent, 2 ×
-N, J = 7.50 Hz), 6.16
3
, J = 7.25
2
2
2
2
7
2
7.40 (2H, q, Ar 2 × CH, J = 3.50 Hz), 7.64 (2H, t, Ar 2 × CH,
J = 7.75 Hz), 7.69 (2H, t, Ar 2 × CH, J = 7.75 Hz), 8.03 (2H,
d, Ar 2 × CH, J = 8.50 Hz), 8.16 (2H, d, Ar 2 × CH, J = 8.00
13
1
2
× NCHN); C{ H NMR}(125.1 MHz, DMSO-d
CH ), 21.57 (CH ), 28.44 (2 × CH , d, JC3,4 = 43.78 Hz), 46.85
R-CH -N), 47.57 (Ar-CH -N), 114.01 (Ar-C, d, J = 16.26
6
): 13.72
(
(
3
2
2
13
1
2
2
Hz), 10.09 (2H, s, 2 × NCHN); C{ H NMR}(125.1 MHz,
DMSO-d ): 13.86 (CH ), 22.00, 25.76, (CH ), 28.53 (3 × CH
t, JC4,5 = 12.50 Hz, JC5,6 = 7.50 Hz ), 28.82 (2 × CH
2.50 Hz), 39.20 (CH ), 46.88 (R-CH -N), 47.66 (Ar-CH
Hz), 126.78 (Ar-C, d, J = 11.25 Hz), 128.66, 129.30, (Ar-C),
31.12 (Ar-C, d, J = 16.26 Hz), 132.01 (Ar-C) and 142.59
6
3
2
2
,
1
2
, d, JC7,8
=
-1
(NCHN). FT-IR (KBr, νmax, cm ): 3446, 3380 (Caliph-Nbenzimi);
2
2
2
-N),
3
117, 3028 (C-Harom); 2991, 2953, 2933, 2860 (C-Haliph), 1633,
607, 1560 (Carom-Carom); 1196, 1429, 1459, 1485 (Carom-Nbenzimi).
114.06 (Ar-C, d, J = 23.76 Hz), 126.72 (Ar-C, d, J = 10 Hz),
128.67, 129.24, (Ar-C), 131.20 (Ar-C, d, J = 12.5 Hz), 132.06
1
-1
Anal. calcd. (%) for: C32
Found (%): C, 58.10; H, 6.11; N, 8.40.
,3'-(1,2-Phenylenebis(methylene))bis(1-hexyl-
H
42
4 2
N OBr : C, 58.45; H, 6.29; N, 8.52.
(Ar-C) and 142.53 (NCHN). FT-IR (KBr, νmax, cm ): 3395
(Caliph-Nbenzimi); 3124, 3025 (C-Harom); 2923, 2853 (C-Haliph);
1609, 1561 (C-Haliph); 1204, 1429, 1457, 1481 (Carom-Nbenzimi).
3
benzimidazolium)dibromide monohydrate (VI.2Br):White
1
powder. Yield 5.35 g (78.10 %), m.p. 242-244 ºC. H NMR
62 4 2
Anal. calcd. (%) for: C42H N OBr : C, 63.23; H, 7.71; N, 7.02.
Found (%): C, 61.95; H, 7.51; N, 6.81.
(
500 MHz, DMSO-d
6
): δ 0.82 (6H, t, 2 × CH
.23-1.36 (12H, br.m, 6 × CH ), 1.90 (4H, Pent, 2 × CH
.5 Hz), 4.54 (4H, t, 2 × CH -N, J = 7.5 Hz), 6.24 (4H, s, 2 ×
-N), 7.23 (2H, q, Ar 2 × CH, J = 3.5 Hz), 7.39 (2H, q,
Ar 2 × CH, J = 3.50 Hz), 7.62 (2H, t, Ar 2 × CH, J = 7.5 Hz),
.68 (2H, t, Ar 2 × CH, J = 8.00 Hz), 8.07 (2H, d, Ar 2 × CH,
J = 8.5 Hz), 8.17 (2H, d, Ar 2 × CH, J = 8.00 Hz), 10.10 (2H,
3
, J = 7.00 Hz),
RESULTS AND DISCUSSION
1
7
2
2
, J =
2
Most of the naturally occurring and synthetically available
imidazolium or benzimidazolium derivatives are of the non-
bridged variety. It was therefore of great interest to investigate
the addition of an alkyl spacer of 1,2-dimethylene benzene to
such non-bridged systems, to be able to extend the application
of direct electrophilic substitution of long-chain and branched
alkyl chain methodology.
Ar-CH
2
7
13
1
s, 2 × NCHN); C{ H NMR}(125.1 MHz, DMSO-d ): 13.72
6
(CH ), 21.18, 25.36, 28.45, 30.56, 39. 02 (5 × CH ), 46.89
2
3

Vol. 25, No. 6 (2013)
Synthesis and Characterization of ortho-Xylyl Linked Bis-benzimidazolium Salts (Part-II) 3051
Syntheses: The reaction of two equivalents of N-alkyl
benzimidazole with 1,2-bis(bromomethylene)benzene in 1,4-
dioxane at 100 ºC for 24 h afforded the product in 72-99 %
yield with an exception of V.2Br (yield 16.85 %).
the bis-benzimidazolium salts as halides. The synthesis can
also be carried out in acetonitrile, methanol etc., but collection
of bis-benzimidazolium salts as halide in pure form will prob-
6
ably be difficult and anion conversion to PF by metathesis
14,17,18
We have already reported
the synthesis of a variety
route will probably become compulsory to achieve the purity.
FT-IR spectra of the compounds: IR spectrum of all the
compounds has been analyzed in the form of KBr pellets of
compounds (for solids) and thallium bromide disks (for liquids)
of medium- and long-chain-derivatized mono- and bis- imida-
zolium and benzimidazolium salts by our simple, high-yielding
and general synthetic method. We previously employed ethyl
bromide, propyl bromide, i-propyl bromide, butyl bromide,
-1
over the scan range 4000-400 cm . Representative IR spectra
of the compounds and the functional assignments are shown
in Figs. 1 and 2. It is of much importance to study the spectral
features in both near and mid IR spectra, for their strong corre-
lation to vibrational structures of the molecules. For all synthe-
sized compounds, two strong and sharp stretching vibrations
13
benzyl chloride and heptyl bromide as the electrophiles and
now show that the use of pentyl bromide, hexyl bromide, octyl
bromide and decyl bromide is also successful. The method
adopted for the synthesis of title compounds is selective for
the 1-position heterocycle and the two-step process is essen-
tially quantitative. Indeed, upon treating alkylbenzimidazole
-1
(3439-3380 cm ) appeared for the tertiary nitrogen of benzimi-
dazolium ring in the observed spectra. The pure modes of the
C-H stretching vibrational bands in both, alkyl benzimidazoles
and bis-benzimidazolium salts appeared at around 3000-2900
(
I-IV) with o-xylyl dibromide [1,2-bis(bromomethylene)-
benzene], respective bis-benzimidazolium salt was observed,
to give V.2Br-VIII.2Br in almost quantitative yield. All
prepared compounds are completely soluble in polar solvents
like DMSO, DMF, methanol and insoluble in common non-
polar to slightly polar organic solvents like benzene, diethyl
ether, n-hexane, toluene, etc.
-1
cm . This variation in the range is due to presence of C-H
3
(sp -s) stretching of alkyl chains and methylene (N-CH
2
-Ar)
group. A strong and sharp intense band observed in the range
-1
1450-1400 cm ascribed to the stretching modes of vibrations of
19
Furthermore the use of 1,4-dioxane as reaction medium
for the synthesis of bis-benzimidazolium salts is highly
recommended because of its inert nature and moderate boiling
point (101 ºC). By using 1,4-dioxane, bis-benzimidazolium
salts can be collected directly as solids from the reaction
medium by filtration and can be purified by washing of fresh
benzimidazole ring due to the presence of -HC=N- module .
It may be concluded that the reduction in the intensity of this
band in benzimidazolium salts is probably caused by the conju-
gation of C=N bond with the benzimidazole ring and due to
N-alkylation, where alkyl group acts as electron donating
entity. In benzimidazole, the modes due to the ring vibrations
-1
1,4-dioxane (5 mL × 3 mL). It is important to note that solubility
are characteristically strong near 1400 and 1460 cm absor-
20
of these salts in 1,4-dioxane increases with the increase in
chain length of alkyl substitutions due to decrease in polarity,
for example compound VIII.2Br (decyl substituted) is comp-
letely soluble in hot 1,4-dioxane and was collected as solid by
cooling the reaction medium to RT (overnight) in crystalline
form whereas V-VI.2Br (pentyl and hexyl substituted) are
insoluble/partially soluble in boiling 1,4-dioxane. In conclusion
using inert solvent like 1,4-dioxane is a good choice to collect
ptions . The other ring vibrations are intense bands at around
-1
1050 and 1220 cm .
FT-NMR spectra of the compounds: FT-NMR spectrum
6
of all the compounds has been analyzed in DMSO-d over the
1
scan range 0-16 δ ppm for H NMR and 0-200 δ ppm for C
13
1
NMR studies. H NMR spectra of all the salts evidenced a
sharp singlet in the range 9.93-10.10 δ ppm ascribed to the
benzimidazolium ring (NCHN) acidic proton. This signal for
R
(i) KOH, DMSO, RT
Stirring for 0.5 h
N
NH
N
N
+
R
Br
+ KBr + H O
2
(
ii) Alkyl Halide at RT
h stirring
2
I (R = pentyl), II (hexyl), III (octyl), IV (decyl)
Step-I
R
N
N
Reflux in dioxane
80-100 ºC), 24 h
N
N
2
+
(
+
+
N
N
-
Br Br
2Br
R
R
V (R = pentyl), VI (hexyl), VII (octyl), VIII (decyl)
Step-II
Scheme-I: Synthesis of N-alkyl benzimidazoles (I-IV) and 3,3'-(1,2-phenylenebis(methylene))bis(1-alkyl-benzimidazolium) bromides (V-VIII)

3
052 Haque et al.
Asian J. Chem.
a
b
4
4.9
a
b
c
N-hexylbenzimidazole (II)
40
35
30
25
20
15
10
5
0
5
N-octylbenzimidazole (III)
N-decylbenzimidazole (IV)
c
N-alkylation
(tert amine)
C-N stretch
-
C=C-H stretch in
benzene ring
-
-
10
4
000
3000
2000
1500
1
1000
400
–
cm
Fig. 1. FT-IR overlay spectrum of N-alkylated benzimidazoles (II, III and IV)
8
7
6
5
4
3
2
1
0
a
a
R = hexyl (VI)
b
c
0
0
0
0
0
0
0
0
R = octyl (VII)
R = decyl (VIII)
R = pentyl (V)
c
b
d
d
N
N
N
Br–
N
2
N-alkylation
tert amine
with resonance)
R
R
(
-C=C-H stretch in
benzene ring
C
N
4
000
3000
2000
1500
1
1000
400
–
cm
Fig. 2. FT-IR overlay spectrum of xylyl linked bis-benzimidazolium salts (V, VI, VII and VIII)
imidazole based salts was observed usually in up field region,
In addition it is evident that N-C-Ar carbon, in all cases,
is more deshielded as compare to N-C-R carbon due to being
sandwiched by two electron withdrawing groups because
N-C-R group is attached to electron withdrawing and donor
groups (Fig. 3). In future, such deshielding of N-C-Ar carbon
will probably allow the substitution of protons with functional
groups which may open new ways of cage like metal complexes.
18
due to the absence of an electron withdrawing phenyl group .
Resonances of the aromatic protons of benzimidazole as well
as spacer were observed in the range 7.19-8.18 δ ppm as doublet
of doublets, triplet of triplets and multiplets with comparable
coupling constants. The signals caused by the methylene (N-
2
CH -Ar) group, which connects xylyl unit with benzimidazolium
units, displays sharp singlets in the range 6.10-6.23 δ ppm.
Finally, the resonance of alkyl protons appeared in the range
0
.80-4.55 δ ppm with comparable coupling constants.
Similarly, the structural assortments of the salts were
N-C-R
is less deshielded
due to being sandwiched
between e- donor and
13
further confirmed by the C NMR spectral technique. The
spectrum of all the salts evidenced a distinguished peak in the
most down field range 142.49-142.59 δ ppm ascribed to the
benzimidazole ring carbon (NCN). Resonances of aromatic
carbons were found in the comparable region around 113.80-
N
N
withdrawing groups
N
N
N-C-Ar
R
is more deshielded due to
being sandwiched between
e- withdrwing groups
R
132.50 δ ppm.Also, the methylene carbon (N-C-Ar) and alkyl
chain carbon resonances were observed in the chemical shift
regions 47.57-47.74 and 13.50-46.88 δ ppm, respectively
(
Fig. 3. Relative deshielding of methylene groups, attached directly to
1 13
benzimidazolium units in H and C NMR
Table-1).

Vol. 25, No. 6 (2013)
Synthesis and Characterization of ortho-Xylyl Linked Bis-benzimidazolium Salts (Part-II) 3053
TABLE-1
CHARACTERISTIC SIGNALS OF BIS-BENZIMIDAZOLIUM SALTS IN NMR SPECTROSCOPY
1
HNMR (δ ppm)
R
N-CH -R
2
N-CH -Ar
2
NCHN
9.93
Butyl
4.50
6.10
Pentyl
4.53
6.23
10.10
N-CH -R
2
N
N
N
N
Heptyl
Nonyl
4.53
4.51
6.22
6.16
10.08
10.09
N-CH -Ar
2
H
H
R
R
NCHN
13
CNMR (δ ppm)
R
N-C-R
46.87
N-C-Ar
47.57
NCN
Butyl
142.59
Pentyl
Heptyl
46.89
46.89
47.74
47.73
142.46
142.49
N-C-R
N
N
N
N
N-C-Ar
H
R
R
Nonyl
46.88
47.66
142.53
NCN
Crystallography: The molecular structures with ethyl,
propyl, heptyl and octyl substitutions from this series have
molecule so molecular names of respective compounds have
been assigned as monohydrate in this work.
14,21,22
successfully been solved
showed one/half water molecule attached through H-bonding
Figs. 4 and 5). Such compounds also showed a sharp peak at
. Each of the solved structure
Conclusion
A series a bis-benzimidazolium salts was prepared in two
steps. First, the preparation of N-alkyl benzimidazoles by the
reactions of benzimidazole with appropriate alkyl halide in
presence of potassium hydroxide. Second, the subsequent treat-
ment of respective N-alkyl benzimidazole derivatives with 1,2-
dibromomethylbenzene in 1,4-dioxane at refluxing temperature
afforded the title compounds in good yields. Crystal structures
of selected compounds from this series have successfully been
(
1
3
.36-3.44 δ ppm in H NMR spectroscopy, indicating the
hydrated water molecule (see additional information). The
CHN analysis also supported the presence of water in each
1
4,21
solved . Further studies need to be conducted to prepare
the respective transition metal-NHC complexes of these salts.
ACKNOWLEDGEMENTS
One of the authors (RAH) thanks Universiti Sains Malaysia
USM) for the RU grants (1001/PKIMIA/811157) and (1001/
(
PKIMIA/823082). MuhammadAdnan Iqbal is grateful to (IPS)
USM for financial support [fellowship: USM.IPS/JWT/1/19
(JLD 6)] and funds [P-KM0018/10(R)-308/AIPS/415401] for
a research attachment to University of Western Australia
Fig. 4. Single crystal structure of an o-xylyl linked bis-benzimidazolium
salt with propyl substitutions, showing a hydrated molecule of
2
1
water
(
UWA), Perth, Australia.
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