Synthesis, crystal structure and Cu2+ recognition of novel azobenzene derivatives

Article abstract of DOI:10.3184/174751914X13893754559598

Three novel azobenzene derivatives have been synthesised from (E)-4,4′-bischloroformyl azobenzene and arylamines. The crystal structure of (E)-[{diazene-1,2-diylbis(4,1-phenylene)}bis{(3,5-dimethyl-1H-pyrazol-1-yl) methanone}] has been obtained The UV-Vis

Full text of DOI:10.3184/174751914X13893754559598

98 RESEARCH PAPER
FEBRUARY, 98–101
JOURNAL OF CHEMICAL RESEARCH 2014
Synthesis, crystal structure and Cu2+ recognition of novel azobenzene
derivatives
Wei Wanga, Mingzhu Lia, Xiaolong Yua, Qiang Zhanga, Xiaoyu Jianga, Junwu Wanga and Yan Gaob*
aSchool of Perfume and Aroma Technology, Shanghai Institute of Technology, Shanghai 200235, P.R. China
bDepartment of Chemical Engineering, University of Science and Technology Liaoning, Anshan, 114051, P.R. China
Three novel azobenzene derivatives have been synthesised from (E)-4,4′-bischloroformyl azobenzene and arylamines. The
crystal structure of (E)-[{diazene-1,2-diylbis(4,1-phenylene)}bis{(3,5-dimethyl-1H-pyrazol-1-yl)methanone}] has been obtained
The UV-Vis and fluorescence spectral data of (E)-[{diazene-1,2-diylbis(4,1-phenylene)}bis-{(1H-imidazol-1-yl)methanone}] show
that it can recognise selectively Cu2+ and a 1:2 stoichiometric complex is formed between the receptor and the cation.
Keywords: azobenzene, aromatic heterocyclic, X‑ray crystal structure, recognition, copper ion
Copper, an essential trace element for plants and animals, is the
third most abundant transition metal following zinc and iron
in human bodies and plays a crucial role in many fundamental
physiological processes in organisms.1 However, exposure to a
high level of copper even for a short period of time can cause
gastrointestinal disturbance, while long‑term exposure can
cause liver or kidney damage. Due to this Janus‑faced property
of copper ion in organisms, considerable attention has been
devoted to the design of efficient and selective fluorescence
chemosensors for Cu2+.2–7
Azobenzene derivatives, as ligands for transition metal ions,
have been widely employed in coordination chemistry and
supra‑molecular chemistry. Although there are a few reports
of azobenzene derivatives in selective detection of heavy
metal cations,8,9 there still is a need for alternative systems
to selectively recognise Cu2+ for environmental or biological
applications. We describe here a comprehensive study of
several azobenzene derivatives bearing aromatic heterocyclic
substituents (Scheme 1). Their metal‑binding properties toward
cations showed that compound 3a displays a highly selective
and sensitive response towards Cu2+ in DMF and H2O.
13C NMR, IR, and MS; the spectral data were in agreement with
the proposed structures.
Crystal data for compound 3b are shown in Table 1. Single
crystal X‑ray diffraction studies were performed on a Bruker
Smart 1000 CCD diffractometer with MoKα radiation
(λ=0.71073 Å). The structures were solved by direct methods
and semi‑empirical absorption corrections were applied. All
Table 1 Crystal data and structure refinement for compound 3b
Molecular formula
C24H22N6O2
Molecular weight
Temperature/K
Radiation λ
426.48
113(2)
0.71073
Crystal system
Space group
a/Å
Monoclinic
P2(1)/n
3.8560(3)
20.0439(17)
26.915(2)
b/Å
c/Å
V/Å3
2079.3(3)
Z
4
D calcd/g cm–3
Crystal size/mm
Crystal colour
Absorption coefficient/mm–1
Absorption correction Tmin and Tmax
F(000)
Reflections collected/unique
Range/indices (h, k, l)
θ limit (º)
No. of observed data, I>2σ (I)
No. of restraints
Goodness of fit on F2
R1, wR2 [I≥2σ (I)]
R1, wR2 (all data)
1.362
0.18×0.05×0.04
Colourless
0.091
0.9838 and 0.9964
896
Results and discussion
The target products were synthesised using a three‑step
procedure as shown in Scheme 1: (i) synthesis of (E)-4,4′-
bicarboxyl azobenzene 1 via the reduction reaction of
4-methylbenzoic acid; (ii) synthesis of (E)-4,4′-bischloroformyl
azobenzene 2 by chlorination reaction of 1 with thionyl chloride;
(iii) synthesis of azobenzene derivatives (3a–c) by nucleophilic
substitution reaction of 2 with different arylamines. The crude
products were purified by column chromatography and the
differently coloured, solid products were obtained. The target
compounds were characterised by elemental analysis, 1H NMR,
13446/3690 [R(int)=0.0550]
–4,4; –21,23; –32,32
1.51 to 25.02
3690
0
1.1816
0.0618, 0.1191
0.0768, 0.1261
NaOH/gluocose
SOCl2
HOOC
N
HOOC
ClOC
NO2
N
COOH
H2O, O2
1
H
R
N
ROC
N
Et3N/CH3CN
N
COCl
N
COR
3a-3c
2
SH
N
N
N
N
N
NH
N
(3a),
(3c)
(3b),
R=
N
CH3
Scheme 1 Synthesis of azobenzene derivatives 3a–c.
* Correspondent. E‑mail: gys20080901@126.com

JOURNAL OF CHEMICAL RESEARCH 2014 99
Fig. 1 ORTEP structure of compound 3b, showing 50% probability ellipsoids.
Fig. 2 A section of the crystal packing of the title compound, viewed down the a-axis. Dashed lines denote the intermolecular C18–H18···O1 hydrogen bonds.
H atoms were geometrically positioned and refined as riding
(C–H=0.95–0.98 Å) and allowed to ride on their parent atoms,
with Uiso (H)=1.2 or 1.5 Ueq (parent). Further details of the
structural analyses are summarised in Table 1. The molecular
structure of 3b and atom‑numbering scheme are shown in Fig. 1.
In the structure of compound 3b, the diphenyldiazene section
was almost coplanar with a mean derivation of 0.0293 Å.
At the end of the structure, two planar pyrazole rings (mean
derivations of 0.0035 Å for the ring C2/C3/C4/N1/N2 and
0.0005 Å for the ring C21/C22/C23/N5/N6, respectively) lie to
the opposite sides of the plane with a dihedral angle of 13.0°. In
the crystal (Fig. 2), inversion‑related molecules are linked by
pairs of intermolecular C18–H18···O1 hydrogen bonds, forming
a cyclic dimer with an R22(30) graph‑set motif, which further
stabilises the molecular structure.
(Ca2+, Cd2+, Cu2+, Hg2+, Mn2+, Ni2+, Pb2+, Zn2+, Mg2+, Na+, Al3+,
Fe3+, Ag+) by UV‑Vis spectroscopy. As shown in Fig. 3, the
maximum band of compound 3a occurs at 340 nm. When Cu2+
was added, the original peak was increased, whereas negligible
changes were observed with the other metal ions. Thus, it may
be concluded that compound 3a has special selectivity and
sensitivity to Cu2+.
The fluorescence enhancement effects of Cu2+ ions on
compound 3a in DMF/aqueous media were investigated as
shown in Fig. 4.
In the fluorescence spectrum, compound 3a exhibits a weak
fluorescence emission at 436 nm in aqueous DMF media. When
Cu2+ was added to compound 3a, the fluorescence intensity
was enhanced remarkably. An emission change of nine‑
fold was observed for Cu2+. The fluorescence enhancement
observed for 3a is attributed to the formation of the 3a–Cu2+
complex as a result of which the PET from sulfur atoms to the
The complexation properties of compound 3a were
investigated toward various heavy and transition metal cations

100 JOURNAL OF CHEMICAL RESEARCH 2014
imidazole moiety was suppressed, resulting in the fluorescent
enhancement.
The titration of compound 3a by Cu2+ with an excitation
at λex =393 nm, given in Fig. 5 as an example, exhibited an
increase of its emission intensity at 436 nm. According to the
titrations, we determined an association constant of compound
3a (Ka = 2.49 × 105 M–2) for Cu2+.
Fitting the changes in fluorescence spectra of compound 3a
with Cu2+ ions, we applied the method of continuous variation
(Job’s plot) to prove the complexation ratio between compound
3a and Cu2+ ions. As shown in Fig. 6, the maximum point at the
mole fraction of 0.67 indicates that the complexation ratio of
compound 3a and Cu2+ is 1:2.
In conclusion, we have synthesised three novel azobenzene
derivatives 3a–c with different arylamines. The structures
1
of target compounds were clearly identified by H NMR, 13C
NMR spectroscopy, elemental analysis and the structure of 3b
has been unambiguously confirmed by X‑ray crystallography.
The UV and fluorescence spectra data indicated that compound
3a could recognise Cu2+and a 1:2 stoichiometric complex was
formed between the receptors and the cation.
Fig. 3 UV-Vis spectra of compound 3a (10 μM) upon addition of various
ions (20 μM) in DMF and H2O (v/v=1/1).
Fig. 4 Fluorescence emission spectra of compound 3a (10 μM) upon
Fig. 6 Job’s plot for determining the stoichiometry of receptor 3a and Cu2+
ion in DMF and H2O (v/v=1/1). [I and I0 are the fluorescence intensity of 3a
in the presence and absence of Cu2+, respectively, the total concentration of
3a and Cu2+ ion was 0.1 mM (λex =393 nm)].
addition of Cu2+ (20 μM) in DMF and H2O (v/v=1/1).
Fig. 5 Fluorescence emission spectra of compound 3a (10 μM) for Cu2+ ion titration in a mixture of DMF/H2O (v/v=1/1)(λex =393 nm).

JOURNAL OF CHEMICAL RESEARCH 2014 101
(E)-4,4′-(Diazene-1,2-diyl)bis(N-(3-mercapto-5-methyl-4H-1,2,4-
triazol-4-yl)benzamide)(3c): Yield 73.8%; m.p. >300 °C; IR (KBr, cm–1):
3362, 3081, 2974, 2878, 1694, 1594, 1499, 1407, 1352, 1205, 854, 721; 1H
NMR (500 MHz, DMSO-d6): δ 13.787 (s, 1H, NH), 12.083 (s, 1H, NH),
8.124 (d, 4H, J=8.5, PhH), 8.027(d, 4H, J=8.5, PhH), 2.891 (s, 3H, CH3),
2.732 (s, 3H, CH3); 13C NMR (500 MHz, DMSO-d6): δC (ppm) 167.42,
154.83, 152.59, 133.47, 130.63, 122.85, 121.59; MS, m/z: 495(M+H+);
Anal. calcd for C20H18N10O2S2: C, 48.57; H, 3.67; N, 28.32; found: C,
48.52; H, 3.69; N, 28.35%.
Experimental
3-Methyl-4-amino-5-mercapto-1,2,4-triazole,10 3,5-dimethyl-1-H-
pyrazole,11 and (E)-4,4′-bicarboxyl azobenzene12 were prepared
according to literature procedures, other reactants and chemicals
1
were purchased from Aladdin. Solvents were of analytical grade. H
NMR and 13C NMR spectra were recorded at room temperature on a
Bruker Avance-500 NMR spectrometer. DMSO-d6 and CDCl3 were
used as solvents and tetramethylsilane (TMS) as internal standard.
Mass spectral data were obtained on an Agilent 1100 LC/MS. IR
spectra were obtained with a Perkin Elmer spectrophotometer.
Elemental analyses were made with a CHN Analyser (Thermo
Finnigan Company). UV‑Vis absorption spectra were recorded at
room temperature on a Lambda‑900 spectrometer. The fluorescence
emission spectra were measured on a LS-55 spectrometer.
We acknowledge the financial support of the Key Laboratory
Project (2008S127) and the Initial Fund for Young Teachers of
University of Science and Technology Liaoning (008131).
Synthesis of compounds 3a–c; general procedure
Received 29 September 2013; accepted 19 December 2013
Paper 1302208 doi: 10.3184/174751914X13893754559598
Published online: 5 February 2014
(E)-4,4′-Bischloroformyl azobenzene 2 (0.8 mmol) was added to a
mixture of corresponding arylamine (1.6 mmol), Et3N (1.6 mmol) in
CH3CN (25 mL) and the mixture was refluxed for 8 h. The mixture was
then cooled and filtered and the crude product was purified on a silica
gel column using petroleum ether/ethyl acetate (from 10/1 to 4/1, v/v).
The products were obtained as differently‑coloured solids.
(E)-(Diazene-1,2-diylbis(4,1-phenylene))bis-((1H-imidazol-1-yl)
methanone) (3a): Yield 69.2%; m.p. 243–245 °C; IR (KBr, cm–1): 3109,
2947, 2883, 1697, 1600, 1464, 1288, 789, 698; H NMR (500 MHz,
CDCl3): δ 8.181 (d, 4H, J=8.5, PhH), 8.030 (d, 4H, J=8.5, PhH), 7.847
(s, 2H, imidazole‑H), 7.111 (d, 4H, J=7.5, imidazole-H); 13C NMR
(500 MHz, CDCl3): δC (ppm) 167.21, 154.61, 135.55, 134.27, 131.11,
123.23, 122.00; MS, m/z: 371 (M+H+); Anal. calcd for C20H14N6O2: C,
64.86; H, 3.81; N, 22.69; found: C, 64.82; H, 3.84; N, 22.73%.
(E)-(Diazene-1,2-diylbis(4,1-phenylene))bis((3,5-dimethyl-1H-
pyrazol-1-yl)methanone) (3b): Yield 74.5%; m.p. 193–195 °C; IR (KBr,
cm–1): 3028, 2975, 2878, 1702, 1585, 1481, 1439, 1376, 1202, 855, 774;
1H NMR (500 MHz, CDCl3): δ 8.166 (d, 4H, J=8.0, PhH), 8.031 (d,
4H, J=8.0, PhH), 6.102 (s, 2H, pyrazole‑H), 2.666 (s, 6H, CH3), 2.268
(s, 6H, CH3); 13C NMR (500 MHz, CDCl3): δC (ppm) 167.63, 154.31,
152.52, 145.20, 135.75, 132.33, 122.34, 111.36, 14.32, 13.84; MS, m/z:
427(M+H+); Anal. calcd for C24H22N6O2: C, 67.59; H, 5.20; N, 19.71;
found: C, 67.54; H, 5.18; N, 19.75%.
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