Synthesis, spectral characterisation of 2-(5-methyl-1H-benzimidazol-2-yl)-4-bromo/nitro-phenols and their complexes with zinc(II) ion, and solvent effect on complexation

Article abstract of DOI:10.1016/j.saa.2005.05.020

2-(5-Methyl-1H-benzimidazol-2-yl)-4-bromo/nitro-phenols (HLBr and HLNO ₂) and their Zn(II) complexes with ZnX₂ (X = Cl, I, NO₃) were synthesized and characterized by elemental analysis, molar conductivity, IR, ¹

Full text of DOI:10.1016/j.saa.2005.05.020

Spectrochimica Acta Part A 63 (2006) 343–348
Synthesis, spectral characterisation of
2-(5-methyl-1H-benzimidazol-2-yl)-4-bromo/nitro-phenols and their
complexes with zinc(II) ion, and solvent effect on complexation
Aydın Tavman∗
Istanbul University, Faculty of Engineering, Department of Chemistry, 34320 Avcilar, Istanbul, Turkey
Received 23 March 2005; accepted 14 May 2005
Abstract
2-(5-Methyl-1H-benzimidazol-2-yl)-4-bromo/nitro-phenols (HLBr and HLNO2) and their Zn(II) complexes with ZnX2 (X = Cl, I, NO3)
1
were synthesized and characterized by elemental analysis, molar conductivity, IR, H and 13C NMR spectra. The OH proton appears near
the NH protons in the 1H NMR spectra of the ligands because of the strong intramolecular hydrogen bonding between the OH hydrogen and
the C N nitrogen atoms. The complexation is investigated in ethanol and isopropanol and it is observed that isopropanol is a better solvent
than ethanol for the complex forming. HLBr gives harder complexation reaction with Zn(II) according to HLNO2 because of the stronger
intramolecular hydrogen bonding in HLBr, and the both ligands react easier with Zn(NO3)2 than ZnCl2 and ZnI2. The Zn(II) complexes of
HLBr have 1:1 M:L ratio and ionic character, however, HLNO2 give a non-ionic complex that has 1:2 M:L ratio. In the complexes the phenolic
hydrogen is eliminated and a chelate structure is formed.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Zinc(II) complexes; 2-(5-Methyl-1H-benzimidazol-2-yl)-4-bromo/nitro-phenols; Solvent effect
1. Introduction
events has received increasing attention aiming at the char-
acterization of a large number of compounds.
It is known that solvent plays a considerable role in con-
trolling the dynamics of chemical reaction. In the literature,
the influence of the solvent on the structure was reported
[1–4]. Several reviews have underlined the importance of the
essential role played by solvents in many chemical reactions,
and especially in reactions involving charge transfer, with
particular relevance to chemistry and biochemistry [5–7].
The solvent plays a fundamental role in the relative sta-
bilization of different ground-state rotameric and tautomeric
structures, and determines the nature of the proton transfer
processes in the excited state [8]. Intra- or intermolecular
hydrogen bonding is very importance factor in the solvation,
reaction and stabilization of the compounds. In the last few
years, the study of ground state intramolecular proton transfer
According to Prieto et al., 2-(2-hydroxyphenyl)-1H-
benzimidazole (HBI) has different behaviour in water and
different solvents. In both water and ethanol, ground state
HBI exhibits conformational equilibrium between a cis-enol
form with an intramolecular hydrogen bond and a trans-
enol form that is hydrogen-bonded to the solvent; the ground
state keto tautomer is also present in water. The excited cis-
enol conformer always undergoes ultra-fast intramolecular
proton transfer to afford the excited keto tautomer (Fig. 1)
[5].
It is known that zinc(II) is essential for human health and
plays a key role in human metabolism. In this paper, the lig-
ands, 2-(5-methyl-1H-benzimidazol-2-yl)-4-bromo- (HLBr)
and nitro-phenols (HLNO2) (Fig. 2), and their complexes
with Zn(II) were synthesized and characterized. The com-
plexation ability of the ligands with Zn(II) ion in ethanol and
isopropanol is investigated, and the substituent, solvent and
Zn(II) salt effects on complexation is observed.
∗
Tel.: +90 212 473 70 70; fax: +90 212 473 70 26.
E-mail address: atavman@istanbul.edu.tr.
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.05.020

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A. Tavman / Spectrochimica Acta Part A 63 (2006) 343–348
Fig. 1. Different behaviours of 2-(2-hydroxyphenyl)-1H-benzimidazole in solvent.
obtained. The precipitate is filtered, washed with isopropanol
and ether and dried at 80 ◦C.
Zn(NO3)2 + HLBr + ipa → Zn(LBr)ipa(NO3) + HNO3
2.3.2. Zn2(LBr)2I2
HLBr (80 mg, 0.26 mmol) and ZnI2 (83 mg, 0.26 mmol) is
solved in isopropanol (20 ml) and the mixture refluxed with
stirring. After 7 h reaction a precipitate is obtained. The pre-
cipitate is filtered, washed with isopropanol and ether and
dried at 80 ◦C.
Fig. 2. Chemical structure of the ligands.
2. Experimental
2ZnI2 + 2HLBr → Zn2(LBr)2I2 + 2HI
2.1. Apparatus
2.3.3. Zn(LNO2)2
IR spectra were recorded in KBr disks on a Mattson 1000
HLNO2 (75 mg, 0.28 mmol) and ZnCl2·6H2O (69 mg,
0.28 mmol) or Zn(NO3)2·6H2O (83 mg, 0.28 mmol) is solved
in isopropanol (20 ml) and the mixture refluxed with stirring.
After 6 h (with ZnCl2) and 4 h reactions (with Zn(NO3)2) the
precipitate is filtered and washed with isopropanol and ether
and dried at 80 ◦C. The reaction equations are the following:
1
FT-IR spectrometer. H and 13C NMR spectra were run on
a Varian Unity Inova 500 NMR spectrometer. The residual
DMSO-d6 signal was also used as an internal reference. Ana-
lytical data were obtained with a Thermo Finnigan Flash EA
1112 analyser and Varian SpectrAA 220/SS atomic absorp-
tion spectrometer. The molar conductance of the compounds
was measured in DMSO on a WPA CMD 750 conductivity
meter. All the chemicals used were reagent grade.
Zn(NO3)2 + 2HLNO2 → Zn(LNO2)2 + 2HNO3
ZnCl2 + 2HLNO2 → Zn(LNO2)2 + 2HCl
2.2. Synthesis of the ligands
The ligands were prepared by the reaction of the addition
product of aldehydes (2-hydroxy-5-nitro/bromo benzalde-
hydes) with NaHSO3 and 4-methyl-1,2-phenylenediamine
[9,10]. 6 mmol of aldehyde (1.0 g 2-hydroxy-5-nitrobenz-
aldehydeor1.2 g2-hydroxy-5-nitrobenzaldehyde)and0.65 g
NaHSO3 (6 mmol) were stirred at room temperature in
ethanol (25 ml) and a precipitate is formed after 4 h reaction.
0.74 g 4-methyl-1,2-phenylenediamine (6 mmol) and 25 ml
DMF were added to this mixture. After 2 h reflux the solution
was poured into 10-fold water. The benzimidazole compound
precipitated was filtered, dried and crystallised from ethanol.
3. Results and discussion
3.1. General properties
The analytical data and some physico-chemical properties
of the ligands and the complexes are given in Table 1.
HLBr has good solubility and its complexes with Zn(II)
have moderate solubility in polar solvents such as ethanol.
However, HLNO2 and Zn(LNO2)2 have lower solubility than
the HLBr and its Zn(II) complexes. Reason of this is strong
intermolecular hydrogen bonding between nitro and OH, NH
groups of HLNO2 (NO2· · ·HO; NO2· · ·HN).
2.3. Synthesis of the complexes
Zn(II) complexes of HLBr are yellow colored compounds
whereas the ligand itself colourless. This shows that there
is a charge transfer transition from the ligand to Zn(II) ion
(L → M). Zn(LNO2)2 has 1:2 M:L ratio and non-ionic char-
acter. However, the HLBr complexes have 1:1 ratio and show
2.3.1. Zn(LBr)ipa(NO3)
HLBr (80 mg, 0.26 mmol) and Zn(NO3)2·6H2O (77 mg,
0.26 mmol) is solved in isopropanol (20 ml) and the mix-
ture refluxed with stirring. After 5 h reaction a precipitate is

A. Tavman / Spectrochimica Acta Part A 63 (2006) 343–348
345
Table 1
Analytical data and some properties of the ligands and the complexes
Compound
Found (calcd. %)
Yield (%)
m.p. (◦C)a
Λb
Color
C
H
N
Zn
HLBr C14H11BrN2O
Zn(LBr)ipa(NO3) C17H18BrN3O5Zn
Zn2(LBr)2I2 C28H22Br2I2N4O2Zn2
55.3 (55.5)
41.8 (41.7)
34.2 (33.9)
62.3 (62.4)
55.7 (55.9)
3.8 (3.7)
3.8 (3.7)
1.9 (2.2)
4.0 (4.1)
3.1 (3.3)
9.1 (9.2)
8.3 (8.6)
5.4 (5.6)
15.4 (15.6)
13.9 (14.0)
–
80
60
55
75
90
303
285
265
295–310
>350
0
62
25
0
Color-less
Yellow
Light yellow
Yellow
13.0 (13.3)
13.1 (13.2)
–
HLNO2 C14
H11N3O3
Zn(LNO2)2 C28H20N6O6Zn
11.2 (10.9)
2.0
Khaki-yellow
a
Decomposed.
b
Molar conductivity, measured in DMSO, ꢀ−1 cm2 mol−1, at 25 1 ◦C.
high molar conductivity in DMSO, namely they have ionic
character.
The methyl group at 4-position on the benzimidazole sec-
tion must affected the complexation too. Because the methyl
group has an electron deficient group and increase the elec-
tronegativity of the C N nitrogen. Therefore, the C N nitro-
gen shows stronger hydrogen bonding with the phenolic OH
hydrogen atom.
3.2. Solvent effect on complexation
HLBr did not react with ZnX2 (X = Cl, I, NO3) in both
ethanol and isopropanol for 15 h and, it did not react with
Zn(NO3)2 in ethanol for approximately 12 h. However, a
reaction is occurred between HLBr and Zn(II) ion for 5 h
(with Zn(NO3)2) and 7 h (with ZnI2) in isopropanol, respec-
tively.
3.3. IR spectra
The IR spectral data of the ligands and the complexes are
given in Table 2.
HLNO2 reacted with ZnCl2 and ZnI2 in isopropanol in
end of 6 h period and with Zn(NO3)2 end of 4 h. HLNO2 did
not reacted with Zn(II) for approximately 10 h in ethanol.
Ethanol is a more polar solvent according to isopropanol
and in ethanol the intramolecular and intermolecular hydro-
gen bonding is stronger. An interaction between the solvent
(ethanol) and the ligands may occur and the complex forming
is not observed because of the stronger hydrogen bonding and
The broad bands at around the 3450 cm−1 are assigned
to NH stretching vibrations in the IR spectra of the ligands
and the complexes. In the bromo derivative the strong and
broad band at 3272 cm−1 is attributed to the intramolecu-
lar hydrogen bonded ν(OH). In the nitro derivative ν(OH)
appears as a weaker band according to HLBr at 3203 cm−1
.
It is observed that the hydrogen bonding is weakened in the
complexes. The 3068 and 3072 cm−1 bands are belonging to
the aromatic CH groups in HLBr and HLNO2, respectively.
The weak band near 2920 cm−1 is due to the stretching vibra-
tions of the methyl group.
1
the solvent effect. This is supported by the H NMR spec-
tra of the ligands in CD3OD: the OH signals are not detected
becauseofthefactorsmentionedabove. However, theOHand
NH signals are detectable near 13 ppm in the 1H NMR spectra
in DMSO (NMR Spectra section). On the other hand, Zn(II)
ion itself is another factor for harder complexation, because
it is a weak Lewis acid because of a d-filler (d10) ion. Cu(II)
and Fe(III) ions are easily reacted with the ligands in alcohol.
The complexation of Cu(II) and Fe(III) will be evaluated in
another study). According these observations, it can be said
that isopropanol is a more suitable solvent than ethanol for
synthesis of the Zn(II) complexes, and HLBr gives harder
complexation reaction with Zn(II) according to HLNO2.
The 1632 and 1640 cm−1 medium bands are assigned to
the aromatic ν(C C) group for HLBr and HLNO2, respec-
tively. Also, the 1582 and 1607 cm−1 bands are belonging to
the ν(C N) groups, for bromo and nitro derivatives, respec-
tively. Inthecomplexes, the ν(C N)bandsshowconsiderable
changes shifting to the lower field. The strong peaks near
1490 cm−1 are due to the stretching vibration of the aro-
matic C C groups in the ligands and the complexes. The
medium and strong bands between 700 and 900 cm−1 are
assigned to the out-of-plane of CH bendings of the phenol
Table 2
IR spectral data of HLBr, HLNO2 and their Zn(II) complexes
Compound
HLBr
Frequencies, cm−1 (KBr disks)
3460 br, 3272 s, 3068 w, 2918 w, 1632 m, 1582 m, 1532 m, 1486 s, 1389 s, 1374 s, 1320 m, 1274 s, 1251 s, 1139 m, 974 m, 866 m,
800 s, 704 m, 634 m
Zn(LBr)ipa(NO3)
Zn2(LBr)2I2
HLNO2
3438 s,br, 3322 s, 3068 w, 2972 w, 2914 w, 1632 m, 1601 m, 1532 m, 1478 s, 1386 s, 1339 m, 1289 m, 1247 m, 1139 m, 1043 m,
997 m, 943 m, 827 m, 804 m, 700 m, 643 m
3265 s,br, 3053 m,br, 2914 w, 1621 m, 1605 m, 1543 m, 1478 s, 1370 m, 1293 m, 1247 s, 1139 m, 1100 m, 874 m, 804 s, 747 m,
708 m, 635 m
3441 m,br, 3203 m, 3072 m, 2922 w, 2610 w, 1851 w, 1640 m, 1607 m, 1555 s, 1497 s, 1479 s, 1320 s, 1304 s, 1293 s, 1158 m, 1235
m, 1135 s, 927 m, 881 m, 834 m, 808 m, 770 m, 739 m, 639 m, 612 m
Zn(LNO2)2
3303 s, 3083 w, 2918 w, 1636 m, 1605 s, 1563 m, 1493 s, 1312 s, 1128 s, 916 m, 831 m, 800 m, 750 m, 723 m, 646 m, 527 m

346
A. Tavman / Spectrochimica Acta Part A 63 (2006) 343–348
and the benzimidazole benzene rings in the spectra of the
compounds.
Two sharp bands are appeared at ca. 1320 and 1560 cm−1
for ν(NO2) in the IR spectra of HLNO2 and Zn(LNO2)2.
ThecoordinatedisopropanolbandsaredetectableintheIR
spectra of Zn(LBr)ipa(NO3). The 2972 cm−1 medium band
can be attributed to the ν(CH3) groups of isopropanol. The
strong and very broad band between 3500 and 3372 cm−1
is due to the combinational vibrational stretching of coordi-
nated OH and the NH groups. On the other hand, a shoulder
at 1463 cm−1 is probably due to the methylene groups of
coordinated isopropanol. A strong band at 1386 cm−1 in
Zn(LBr)ipa(NO3) complex is assigned to ν(NO3−). This
band is not detected in Zn2(LBr)2I2 and Zn(LNO2)2 com-
plexes. This observation supports the higher molar conduc-
tance and the ionic character of Zn(LBr)ipa(NO3) complex.
3.4. 1H and 13C NMR-spectra
The 1H- and 13C NMR spectral data are given in
Tables 3 and 4, respectively.
1
There are interesting details in the H NMR spectra of
the ligands. For example, proton H6ꢀ gives a broad singlet in
HLBr while it appears as a doublet in HLNO2. H7 shows a
doublet in HLBr, however it is in a multiplet in the spectra
of HLNO2. H4 gives a broad singlet in HLBr and a sharp
singlet in HLNO2. These observations exhibit that the HLBr
ligand has more acidic character than HLNO2. The acidic
character of HLBr may result from high electronegativity of
bromide and a strong cis-enol tautomer structure of HLBr.
Namely, the cis-enol tautomeric form of HLBr is more stable
than that of HLNO2.
The OH and NH protons appear as a broad singlet at
14.07 ppm in the spectra of HLNO2 in DMSO. However,
in the bromo derivative they give two broad singlets close to
each other at 13.13 and 13.33 ppm (Fig. 3). This observation
Fig. 3. 1H NMR signals of NH and OH protons in (a) HLBr, (b)
Zn(LBr)ipa(NO3), (c) HLNO2 and (d) Zn(LNO2)2.

A. Tavman / Spectrochimica Acta Part A 63 (2006) 343–348
347
Table 4
13C NMR spectral data
Compound
The benzimidazole carbons
The phenolic carbons
CH3
C2
C4
C5
C6
C7
C8+C9
C1ꢀ
C2ꢀ
C3ꢀ
C4ꢀ
C5ꢀ
C6ꢀ
HLBr
21.9
21.9
21.9
22.0
22.0
150.2
152.7
152.8
150.1
151.6
115.4
114.5
114.6
115.6
113.1
133.5
134.1
134.2
133.8
133.4
125.3
126.1
126.1
123.4
123.9
115.4
114.5
114.6
114.9
113.1
139.0
157.2
167.6
167.5
164.8
174.8
111.0
111.9
112.0
113.3
111.8
134.5
135.3
135.1
125.7
126.1
118.1
117.0
117.1
139.9
138.9
128.8
130.2
130.0
127.5
127.9
120.0
117.0
117.2
119.0
118.4
Zn(LBr)ipa(NO3)a
Zn2(LBr)2I2
HLNO2
138.3 140.6
138.2 140.5
135.6 136.9
134.2 134.8
Zn(LNO2)2
δC (ppm), in DMSO-d6.
a
The carbon of isopropanol signals: 26.2 and 62.7 ppm.
ation and a chelate structure is formed through the phenolic
oxygen and the C N nitrogen atoms, and, consequently, the
tautomeric equilibrium that make the NH proton fluxional is
removed and the NH proton appears as a sharper peak.
An isopropanol molecule that coordinated to Zn(II) ion is
detected in the NMR spectra of Zn(LBr)ipa(NO3) complex.
The NMR signals of the coordinated isopropanol molecule
are the following (in DMSO-d6): δH: 4.36 (s, 1H), 3.79
(septet, 1H), 1.05 (d, 6H) ppm; δC 26.2 and 62.7 ppm. The
non-coordinated solvent 2-propanol signals were observed
as below (in DMSO-d6): δH: 1.04 d, 3.01 s, 3.90 sept; δC:
25.4 and 64.9 ppm. These data are agreed with the elemental
analysis results (Table 1).
Fig. 4. Intramolecular hydrogen bonding in the ligands (cis-enol form).
showsthatthereisverystrongintramolecularhydrogenbond-
ing in the both ligands and agree with the literature (Fig. 4).
The OH proton does not appear in the complexes, only the
NH proton is detected. In the spectra of the complexes, NH
proton signal is sharpened according to the ligands (Fig. 3).
This exhibits that the OH hydrogen is eliminated on complex-
In the 13C NMR spectra of the complexes, the chemi-
cal shift of C1ꢀ (C OH) is approximately 10 ppm, e.g. from
Fig. 5. The proposal structures of the Zn(II) complexes.

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A. Tavman / Spectrochimica Acta Part A 63 (2006) 343–348
157.2 to 167.6 in Zn(LBr)ipa(NO3). This change is an impor-
tant indication for the elimination of the phenolic hydrogen
and coordination of the phenolic oxygen to the Zn(II) ion.
On the other hand, a considerable shift of the C N carbon
(C2) atom shows that the C N nitrogen atom coordinate to
the Zn(II) ion and a chelate structure is formed (Table 4). C8
and C9 carbon atoms appear in one signal in the spectra of
HLBr. However, they are observed as two signals separately
in the spectra of the complexes. This observation is the most
important evidence for the C N nitrogen atom coordination
(this shows that the tautomerism is stronger in HLBr accord-
ing to HLNO2). Because the tautomeric system is removed in
the benzimidazole part as a result of the C N nitrogen atom
coordination.
in Fig. 5 are proposed for the zinc(II) complexes of HLBr and
HLNO2 ligands.
Acknowledgments
This work was supported by the Research Fund of The
University of Istanbul. Project number: BYP-394/31032004.
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HLBr and HLNO2 ligands have strong intramolecular
hydrogen bonding in polar solvents. The intramolecular
hydrogen bonding is stronger in HLBr and consequently, its
complexationreactionwithzinc(II)isharderaccordingtothat
of HLNO2. HLBr and HLNO2 gave the different complexes
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