1-(Pyridin-2-ylmethyl)-2-(3-(1-(pyridin-2-ylmethyl)benzimidazol-2-yl) propyl) benzimidazole and its copper(II) complex as a new fluorescent sensor for dopamine (4-(2-aminoethyl)benzene-1,2-diol)

Article abstract of DOI:10.1016/j.inoche.2013.02.015

A new ligand 1-(pyridin-2-ylmethyl)-2-(3-(1-(pyridin-2-ylmethyl) benzimidazol-2-yl) propyl) benzimidazole (L) and its Cu(II) complex (1) have been synthesized and characterized spectroscopically and structurally. The Cu(II) ion is coordinated by two nitrogen atoms of benzimidazole groups, two oxygen atoms of the nitrate anions and one oxygen atom of a water molecule forming distorted trigonal bipyramidal geometry. The ligand and its complex have been utilized as a fluorescent sensor for 4-(2-aminoethyl)benzene-1,2-diol. A plot of F₀/F - F₀ vs 1/Conc (4-(2-aminoethyl)benzene-1,2- diol) at a selected wavelength of 306 nm with (L) that shows a straight line behavior, supports the validity of the assumption of 1:1 complex formation and the association constant of (L) with 4-(2-aminoethyl)benzene-1,2-diol is calculated to be 9868 M^-1. Sensor (L) is found to be selective for 4-(2-aminoethyl)benzene-1,2-diol over aromatic amines, phenols, amino catechol (L-3,4-dihydroxyphenylalanine) and 4,6-ditertiarybutyl benzene-1,2-diol.

Full text of DOI:10.1016/j.inoche.2013.02.015

Inorganic Chemistry Communications 31 (2013) 37–43
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
1-(Pyridin-2-ylmethyl)-2-(3-(1-(pyridin-2-ylmethyl)benzimidazol-2-yl) propyl)
benzimidazole and its copper(II) complex as a new fluorescent sensor for
dopamine (4-(2-aminoethyl)benzene-1,2-diol)
⁎
Raghvi Khattar, Pavan Mathur
Department of Chemistry, University of Delhi, Delhi, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 July 2012
Accepted 18 February 2013
Available online 24 February 2013
A new ligand 1-(pyridin-2-ylmethyl)-2-(3-(1-(pyridin-2-ylmethyl)benzimidazol-2-yl) propyl) benzimidazole (L)
and its Cu(II) complex (1) have been synthesized and characterized spectroscopically and structurally. The Cu(II)
ion is coordinated by two nitrogen atoms of benzimidazole groups, two oxygen atoms of the nitrate anions and one
oxygen atom of a water molecule forming distorted trigonal bipyramidal geometry. The ligand and its complex
have been utilized as a fluorescent sensor for 4-(2-aminoethyl)benzene-1,2-diol. A plot of F0/F−F0 vs 1/Conc
(4-(2-aminoethyl)benzene-1,2-diol) at a selected wavelength of 306 nm with (L) that shows a straight line behav-
ior, supports the validity of the assumption of 1:1 complex formation and the association constant of (L) with
4-(2-aminoethyl)benzene-1,2-diol is calculated to be 9868 M−1. Sensor (L) is found to be selective for 4-(2-
aminoethyl)benzene-1,2-diol over aromatic amines, phenols, amino catechol (L-3,4-dihydroxyphenylalanine)
and 4,6-ditertiarybutyl benzene-1,2-diol.
Keywords:
Benzimidazole
Copper complex
Structure
Fluorescent sensor
© 2013 Elsevier B.V. All rights reserved.
The development and use of fluorescent chemosensors have in-
creased over the recent years [1]. The ability of chemosensors to bind
to neurotransmitters, in particular catecholamines has been of great
interest. Catecholamines, such as 4-(2-aminoethyl)benzene-1,2-diol,
4-(1-hydroxy-2-(methylamino)ethyl)benzene-1,2-diol, 4-(2-amino-1-
hydroxyethyl)benzene-1,2-diol and L-3,4-dihydroxyphenylalanine are
involved in a variety of central nervous system functions. The lowered
concentration of 4-(2-aminoethyl)benzene-1,2-diol acting as a neu-
rotransmitter in the brain is known to lead to several neurodegenera-
tive diseases, like alzheimer, Wilson and Parkinson [2–4]. Besides
the Tyrosinase mediated oxidation of L-3,4-dihydroxyphenylalanine
[5] non enzymatic oxidation of 4-(2-aminoethyl)benzene-1,2-diol
is reported to be accelerated by nteraction of transition metal ions
[6]. Metal ion sensing has also been reported using new efficient
chemosensors [7]. Recently Schiff-base ligand derivatives of neuro-
transmitter 4-(2-aminoethyl)benzene-1,2-diol, bearing an imidazole,
methylimidazole or hydroxynaphthaldehyde units have been reported
for the selective detection of Al3+ [8]. However there are only few
reports regarding fluorescence sensing of neurotransmitters 4-(2-
aminoethyl)benzene-1,2-diol and L-3,4-dihydroxyphenylalanine [9–12].
A phenylboronic acid derivative of a well-known dye (Lucifer
yellow) recognizes L-3,4-dihydroxyphenylalanine through a combina-
tion of reversible esterification, charge transfer, and electrostatic inter-
actions. The selective recognition event is signaled by a drop in the
emission intensity of the fluorescent chemosensor [10]. In a more recent
study Raymo presented a diazapyrene-based 4-(2-aminoethyl)benzene-
1,2-diol chemosensor attached to silica particles [13] and glass and his
coworker reported coumarin aldehyde as a selective chemosensor for
4-(2-aminoethyl)benzene-1,2-diol and 4-(2-amino-1-hydroxyethyl)
benzene-1,2-diol [11]. Chemosensors, anthracene fluorophore bearing
boronic acid and aldehyde group for 4-(2-aminoethyl)benzene-1,2-diol
recognition have also been reported [9,12].
In the present paper, we have developed a new benzimidazole
fluorophore 1-(pyridin-2-ylmethyl)-2-(3-(1-(pyridin-2-ylmethyl)
benzimidazol-2-yl)propyl)benzimidazole (L) (Scheme 1) [14] and
its Cu(II) complex (1) [15]. They have been characterized by single
crystal X-ray crystallographic measurement [16]. The purpose of
including the N-pyridyl group is to understand its impact on the
fluorescence properties of the ligating moiety. Further modification
of the steric properties of the ligand would also affect the geometry
of the bound Cu(II) ion. The present ligating system has been uti-
lized as a selective fluorescent chemosensor for 4-(2-aminoethyl)
benzene-1, 2-diol.
All chemicals glutaric acid (Aldrich), benzene-1, 2-diamine,
2-(chloromethyl)pyridine and spectroscopic solvents were obtained
from commercial sources and used without further purification. 2-(3-
(benzimidazol-2-yl)propyl)benzimidazole (GAB) was prepared as de-
scribed earlier [17,18].
1H NMR spectrum of the free ligand (L) [Fig. 1.1 Supplementary
data] shows signal at 8.4 ppm and is assigned to the H's adjacent to
the N atom of the pyridyl group. A singlet is also observed at 5.6 ppm
which is assigned to the CH2 protons linked to pyridine ring. A triplet
in the range 2.9–3.0 ppm and a quintet in the range 2.2–2.3 ppm are
⁎
Corresponding author. Tel.: +91 1127667725(1381); fax: +91 1127666605.
E-mail address: pavanmat@yahoo.co.in (P. Mathur).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.02.015

38
R. Khattar, P. Mathur / Inorganic Chemistry Communications 31 (2013) 37–43
N
N
H2
C
H2
C
H2
C
+
N
N
H
N
H
CH2
Cl
2-(3-(benzimidazol-2-yl)propyl)benzimidazole
1 mol (GAB)
2-(chloromethyl)pyridine
2.5 mols
70°C
K2CO3
DMF
72 Hrs
N
N
N
H2
C
H2
C
H2
C
N
CH2
H2C
N
N
1-(pyridin-2-ylmethyl)-2-(3-(1-(pyridin-2-ylmethyl)benzimidazol-2-yl) propyl)benzimidazole
Scheme 1. Synthesis of 1-(pyridin-2-ylmethyl)-2-(3-(1-(pyridin-2ylmethyl)benzimidazol-2-yl)propyl)benzimidazole (L).
observed and are assigned to CH2 protons present in the aliphatic chain.
Multiplets are observed in the range 7.1–7.8 ppm and are assigned to
the protons of the benzimidazole ring and pyridine ring. 13C NMR spec-
trum of the ligand [Fig. 1.2 Supplementary data] shows signals at 28 and
26 ppm and is assigned to methylene carbons of the aliphatic chain. Sig-
nals at 156.5 and 149 ppm are attributed to carbons adjacent to the
N-atom of the pyridine ring. Benzene ring and pyridine ring carbons
appeared in the range (119–124 ppm) and (137–144 ppm) [19].
The free ligand (L) has characteristic IR bands at 1616 and 1463 cm1.
These are assigned to pyridyl (mainly υC_N stretch) and benzimidazole
νC_N\C_C stretching frequencies, respectively. Small shift upon com-
plexation is attributed to the binding of imine nitrogen to copper(II).
I.R data of ligand and its complex is being reported [14,15]. A broad
band in the region 3350–470 cm− in the ligand and its complex is
assigned to the νO\H stretching vibrations. In the nitrate complex
three bands at 1011, 1336 and 1435 cm−1 are assigned to monodentate
nitrate bound to copper(II), separation between the two highest peak is
around 100 cm−1 [20].
an Ag/AgNO3 electrode reference electrode were used for the measure-
ment. The reversible one electron Fc+/Fc couple in the above solvent
system has an E1/2 of 59.5 mV vs. Ag/AgNO3 electrode. The complex dis-
plays a quasi reversible redox waves due to the Cu(II)/Cu(I) reduction
process with E1/2 value −96.3 mV [Fig. 1.3 Supplementary data]. The
E1/2 value of this complex is relatively more cathodic when compared
to benzimidazole and diamide benzimidazole bound Cu(II) complexes
[6d,21] indicating that this will not undergo any redox reaction with
4-(2-aminoethyl)benzene-1,2-diol. This is also revealed in the fluores-
cence titration of this complex with 4-(2-aminoethyl)benzene-1,2-diol
where no new band is observed due to oxidation of 4-(2-aminoethyl)
benzene-1,2-diol to quinone.
The X-band EPR spectra of Cu(II) complex (1) have been recorded
in DMSO at liquid nitrogen temperature. The spectra typically indi-
2
2
cates a dx −y ground state (g( >g/ >2.0024). The g( and g/ are
found to be ~2.29 and ~2.01 with an A( value of 128 G. The complex
shows four g( components and a broadening of g/ component
[Fig. 1.4 Supplementary data]. Such a broadening of the g/ component
is indicative of the lowered symmetry [22]. As fluorescence experi-
ments were carried out in solution state, therefore it was important
to find out if there are geometric changes of the copper(II) complex
from the solid state to the solution state. EPR of the copper(II) complex
was recorded for this purpose and indeed shows that there are
changes in the geometry, the distorted trigonal bipyramidal geometry
of the solid state changes to distorted six coordinate tetragonal geom-
etry in DMF, as suggested by the g(, g/ and A( parameters. Thus it is
this tetragonal copper(II) species which is showing fluorescence
emission spectra in the titration experiment carried out with 4-(2-
aminoethyl)benzene-1,2-diol.
The electronic spectra of ligand (L) and the complex were recorded
in HPLC grade DMF [14,15]. Three peaks in the range 265–285 nm
are observed in the free ligand and its complex are assigned to
the intraligand π–π* transition of benzimidazole and pyridyl moiety
present in the ligating system. The Cu(II) complex (1) displays a
broad d–d band at 800 nm assigned to the overlapping transition
2
2
2
2
d
xz,yz →dx −y and dxy→dx −y
.
The cyclic voltammogram of the Cu(II) complex (1) was recorded in
3:2 DMSO:CH3CN with tert-butyl ammonium peroxide (TBAP) 0.1 M
as supporting electrolyte. A three electrode configuration composed of
Pt working electrode 3.1 mm2 area, a Pt wire counter electrode, and

R. Khattar, P. Mathur / Inorganic Chemistry Communications 31 (2013) 37–43
39
The ORTEP diagram of 1-(pyridin-2-ylmethyl)-2-(3-(1-(pyridin-
2-ylmethyl)benzimidazol-2-yl)propyl)benzimidazole (L) with atom
numbering scheme is shown in (Fig. 1). A total of 19,709 reflections
were measured, of which 4809 were unique and 2181 were consid-
ered as observed (I>2σ (I)]. It crystallizes in the monoclinic system
with space group P21/n. All H atoms were positioned geometrically
with C–H distances ranging from 0.95 to 0.99 Å. The ligand is
bidentate with two benzimidazole N-donar atoms. The bond lengths
of C\N imidazole are in the range 1.308–1.385 Å and bond angles
N\C\N of imidazole ring are 110.6(3) and 111.5(3)° and are compa-
rable to the similar ligand 1,3-bis(1-benzyl-1H-benzimidazol-2-yl)-
2-oxapropane reported earlier with C\N imidazole bond lengths
in the range 1.31–1.37 Å and N\C\N bond angles that are 113.39°
and 113.75 [23].
forming the two largest angles. Thus geometric parameter τ is ap-
plicable to five co-ordinated structures as an index of the degree
of trigonality within the structural continuum between trigonal
bipyramidal (τ=1) and square pyramidal (τ=0).
A
ð1Þ
B
M
C
E
D
The ORTEP diagram of Cu(II) complex (1) and atom numbering
scheme is shown in (Fig. 2). The complex was dissolved in HPLC
grade methanol, and slow evaporation at room temperature over a
period of 48 h resulted in the formation of green needle shape crystal.
A total of 21,455 reflections were measured, of which 5193 were
unique and 3314 were considered as observed (I>2σ (I)]. It crystal-
lizes in the monoclinic system with space group P21/n. Cu(II) ion
is penta-coordinated by two N atoms of benzimidazole (N1, N4),
two oxygen atoms of nitrate group (O2, O5) and O(1) atom of solvent
molecule H2O. One uncoordinated methanol molecule is present
in the molecular structure. The Cu\N bond distances of 2.279(2) Å
(Cu\N1) and 2.0058(18) Å (Cu\N4) are in the range found for
similar benzimidazole ligated compounds [24]. (Cu\O1), (Cu\O2)
and (Cu\O5) bond distances are 1.9470(18), 1.9729(17) and
2.0110(15) Å. Bond angles C(17)\N(4)\Cu(1) and C(7)\N(1)\
Cu(1) are 129.28(17)° and 129.25(16)°. In five co-ordinate systems
the actual geometry of the complex can be described by a structural
index parameter τ. Five co-ordinate system such as (1) ideally square
pyramidal geometry is associated with α=β=180°, for A as the axial
ligand (β is the greater of the basal angle BMC). For perfectly trigonal
bipyramidal geometry α becomes 120° and BMC the principal axis
180°. The penta co-ordination geometries can be characterized by the
value of (β-α) which is 0° for C4v and 60° for a D3 h co-ordination
geometry. The geometric parameter τ (Addison parameter) is thus
defined as τ=(β-α)/60°. According to this model, out of the equatorial
ligands (A, D, E), A is chosen such that the Cu\A bond length is longer
than the Cu\D/E and A should not be any one of the four donar atoms
The complex has τ=0.372, where α is the angle between
O5\Cu\N4 (156.97(8)°) and β is the angle between O2\Cu\O1
(179.34(9)°) indicating a highly distorted trigonal bipyramidal ge-
ometry. This type of distorted geometries are reported earlier with
τ=0.39 [25]. The equatorial bond angles deviates from the expected
value of 120° N1\Cu (1)\O5=93.38(7)°, O5\Cu(1)\N4=
156.97(8)° and N1\Cu (1)\N4=109.65(8)°. The deviation in equa-
torial bond angles suggests remarkable asymmetry within the trigo-
nal plane. The sum of the equatorial bond angles is 360° indicating
that Cu metal center resides in the equatorial plane.
The fluorescence spectra of the ligand 2-(3-(benzimidazol-2-yl)
propyl)benzimidazole (GAB), 1-(pyridin-2-ylmethyl)-2-(3-(1-(pyridin-
2-ylmethyl)benzimidazol-2-yl)propyl) benzimidazole (L) and its
copper(II) complex (1) in DMF are shown in (Fig. 3). Quantum yield
Φ were calculated by comparison of the spectra with that of anthracene
(Φ=0.292) taking the area under the total emission [26]. Emission
spectra of (L) and its complex were recorded at slit width 5.0 but the
intensity of the unsubstituted ligand (GAB) is out of the range at slit
width 5.0, so emission spectrum is recorded at 2.5 slit width, still its
intensity is higher than (L). This suggests that fluorescence intensity
is quenched due to N-substitution of (GAB). The fluorescence spectra
of the copper(II) complex (1) in DMF shows only a very slight shift in
the emission spectra but the intensity of the complex is quenched as
compared to the parent ligand showing complexation with imine nitro-
gens of the benzimidazole moiety. This change is reflected in the quan-
tum yield. The fluorescence quantum yield of copper(II) complex (1) is
found to be lower (Φ=0.02) than that of the parent ligand (Φ=0.06).
Fig. 1. Ortep diagram of ligand (L) drawn in 30% thermal probability ellipsoids showing atomic numbering schemes.

40
R. Khattar, P. Mathur / Inorganic Chemistry Communications 31 (2013) 37–43
Fig. 2. Ortep diagram of Cu(II) complex (1) drawn in 30% thermal probability ellipsoids showing atomic numbering schemes. Selected bond lengths [Å] and bond angles [°]:
Cu(1)\O(1) 1.9470(18), Cu(1)\O(2) 1.9729(17), Cu(1)\N(4) 2.0058(18), Cu(1)\O(5) 2.0110(15), O(1)\Cu(1)\O(2) 179.34(9), O(1)\Cu(1)\N(4) 87.24(8), O(2)\Cu(1)\N(4)
92.44(7), O(1)\Cu(1)\O(5) 90.64(8), O(2)\Cu(1)\O(5) 89.44(7), N(4)\Cu(1)\O(5)156.97(8), O(1)\Cu(1)\N(1) 95.84(8), O(2)\Cu(1)\N(1) 84.81(8), N(4)\Cu(1)\N(1)
109.65(8), O(5)\Cu(1)\N(1) 93.38(7), Cu(1)\O(1)\H(101)120.7(17), Cu(1)\O(1)\H(102) 118(2), H(101)\O(1)\H(102) 118(3) and N(7)\O(2)\Cu(1) 112.01(17).
The fluorescence quantum yield of the present ligating system
is similar to that reported for bis-benzimidazole type ligand but
quite higher than for some of the nitrogen containing fluorophores,
implying that the lone pair of the benzimidazole nitrogen is only
weakly involved in photo induced electron transfer (PET) to the aro-
matic fluorophore [26].
4-(2-Aminoethyl)benzene-1,2-diol has been utilized to evaluate the
fluorescent properties of 1-(pyridin-2-ylmethyl)-2-(3-(1-(pyridin-2-
ylmethyl)benzimidazol-2-yl)propyl)benzimidazole (L) and its copper(II)
complex (1). Sensor (L) displayed an emission at 297 nm when excited
at 277 nm in DMF and on addition of 4-(2-aminoethyl)benzene-1,2-diol
the monomer fluorescence intensity decreases at 297 nm and growth
of a new band at 306 nm is observed (Fig. 4) which is probably
due to the formation of a host guest complex between ligand and 4-(2-
aminoethyl)benzene-1,2-diol accompanied by an isoemissive point at
300 nm. The band at 306 nm is a real band with no role of solvent
molecule DMF and is attributed to ligand on addition of 4-(2-
aminoethyl)benzene-1,2-diol. This is confirmed when similar experi-
ment is carried out with an unsubstituted ligand 2-(3-(benzimidazol-
2-yl)propyl)benzimidazole (GAB) [Fig. 1.5 Supplementary data] in the
same solvent DMF, where only fluorescence quenching has been ob-
served and no new band formation is found. A small band at 380 nm is
observed in sensor (L) which is absent in (GAB) is due to fluorophore
pyridine moiety present in (L). This is further proved by recording
the emission spectrum at slit width 5.0 of 2-(chloromethyl)pyridine
which exhibits emission at 380 nm [Fig. 1.6 Supplementary data].
Sensor (L) is also selective for 4-(2-aminoethyl)benzene-1,2-diol over
aromatic amines and phenols because it responds differently to
4-(2-aminoethyl)benzene-1,2-diol. This is proved when similar fluo-
rescence experiments were conducted with 2,3-dimethyl phenol and
2-phenylethanamine [Fig. 1.7, 1.8 Supplementary data], fluorescence
emission intensity of (L) is not affected on addition of 2,3-dimethyl
1.0
0.5
0.4
0.3
0.2
0.1
0.0
0.9
(GAB)
0.8
0.7
0.6
0.5
(L)
0.4
0.3
0.2
(1)
0.1
0.0
300
320
340
360
380
400
420
440
300
320
340
360
380
400
420
440
Wavelength (nm)
Wavelength (nm)
Fig. 4. Fluorescence titration of ligand (L), c=6 μM with 4-(2-aminoethyl)benzene-
1,2-diol in DMF with increasing 4-(2-aminoethyl)benzene-1,2-diol concentration
(4.8–47.4 μM).
Fig. 3. Emission spectra of (GAB), c=6 μM at slit width 2.5, ligand (L), c=6 μM at slit
width 5.0 and Cu(II) complex (1) in DMF at slit width 5.0.

R. Khattar, P. Mathur / Inorganic Chemistry Communications 31 (2013) 37–43
41
0.5
0.4
0.3
0.2
0.1
0.0
(1) Ligand (L)
(2) (4-(2-aminoethyl)benzene-1,2-diol)
(3) DTBC
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(6)
(4) L-3,4-dihydroxyphenylalanine
(5) 2,3-dimethyl phenol
(6)2-phenylethanamine
(2)
(3)
(1)
(4)
(5)
300
320
340
360
380
400
420
440
Wavelength (nm)
306
Fig. 5. Fluorescence titration of ligand (L), c=6 μM with L-3,4-dihydroxyphenylalanine
in DMF with increasing L-3,4-dihydroxyphenylalanine concentration (4.8–47.4 μM).
Fig. 7. Normalized fluorescence of sensor (L) with the addition of 8 equivalents
of 4-(2-aminoethyl)benzene-1,2-diol, 4,6-tertiarybutyl benzene1,2-diol(DTBC), L-3,4-
dihydroxyphenylalanine, 2,3-dimethyl phenol and 2-phenylethanamine in DMF.
phenol while intensity increases on addition of 2-phenylethanamine. This
suggests that our sensor (L) is selective to the compounds bearing both
the amino and phenolic moiety, as is the case with 4-(2-aminoethyl)
benzene-1,2-diol. Further sensor (L) is specific and selective for
4-(2-aminoethyl)benzene-1,2-diol in comparison to similar compounds
like L-3,4-dihydroxyphenylalanine and 4,6-ditertiarybutylbenzene-1,2-
diol (DTBC). Similarly fluorescence titration experiments of sensor
(L) were conducted with L-3,4-dihydroxyphenylalanine and DTBC
(Figs. 5 and 6), no significant change in the fluorescence emission
intensity of (L) is observed. In conclusion results are presented as
histograms at 306 nm (Fig. 7) in order to highlight the selectivity of
sensor (L) towards 4-(2-aminoethyl)benzene-1,2-diol over L-3,4-
dihydroxyphenylalanine, DTBC and 2,3-dimethyl phenol. However,
intensity increases on adding 2-phenylethanamine but no new band
is observed at 306 nm.
constant is higher than that reported with other ligating systems. For
e.g. association constant of selective sensor, coumarin aldehyde for
4-(2-aminoethyl)benzene-1,2-diol and 4-(2-amino-1-hydroxyethyl)
benzene-1,2-diol is 3400 and 6500 M−1 and the association con-
stants of anthracene fluorophore bearing boronic acid and alde-
hyde group with 4-(2-aminoethyl)benzene-1,2-diol, 4-(1-hydroxy-2-
(methylamino)ethyl)benzene-1,2-diol and catechol are 5720, 5050
and 2010 M−1 [11,12]. This shows that sensor (L) exhibits strong
binding affinity towards 4-(2-aminoethyl)benzene-1,2-diol.
It has been well established that nonenzymatic auto oxidation of
4-(2-aminoethyl)benzene-1,2-diol can occur in vitro into polymeric
material which resembles neuromelanin. Such an autooxidation is
known to be accelerated by interaction with transition-metal ions.
Since copper is a trace metal and acts as a redox catalyst for the oxida-
tion of 4-(2-aminoethyl)benzene-1,2-diol to quinones [6d], therefore
titrations were performed using Cu(II) complex (1) as a catalyst with
4-(2-aminoethyl)benzene-1,2-diol in order to check the sensitivity of
ligand in the complex form and also any redox activity of copper(II)
metal ion with this ligand. A different pattern is observed (Fig. 8) in
the fluorescence emission spectrum of copper complex; monomer
Fig. 1.9 of the Supplementary data shows the plot of F0/F−F0 vs
1/Conc (4-(2-aminoethyl)benzene-1,2-diol) at a selected wavelength
of 306 nm with (L). Straight line behavior supports the validity of
the assumption of 1:1 complex formation and the association con-
stant of (L) with 4-(2-aminoethyl)benzene-1,2-diol is calculated to
be 9868 M−1 from the slope of the line [27]. This value of association
0.5
0.4
0.3
0.2
0.1
0.0
0.25
0.20
0.15
0.10
0.05
0.00
300
320
340
360
380
400
420
440
300
320
340
360
380
400
420
440
Wavelength (nm)
Wavelength (nm)
Fig. 8. Fluorescence titration of Cu (II) complex (1) (c=6 μM) with 4-(2-aminoethyl)
benzene-1,2-diol in DMF with increasing 4-(2-aminoethyl)benzene-1,2-diol concen-
tration (4.8–47.4 μM).
Fig. 6. Fluorescence titration of ligand (L), c=6 μM with 4,6-ditertiarybutyl
benzene1,2-diol (DTBC) in DMF with increasing DTBC concentration (4.8–47.4 μM).

42
R. Khattar, P. Mathur / Inorganic Chemistry Communications 31 (2013) 37–43
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intensity broadens and shifts to a higher wavelength on addition of
4-(2-aminoethyl)benzene-1,2-diol accompanied by the formation of
a shoulder at 322 nm attributed to weak interaction of Cu(II) complex
with 4-(2-aminoethyl)benzene-1,2-diol. This shows that when Cu(II)
ion binds with the ligand or when ligand is used in complex form as a
sensor, the sensing ability of the ligating moiety of (L) towards
4-(2-aminoethyl)benzene-1,2-diol gets weakened thereby restricting
the formation of host guest complex at 306 nm. Association constant
of complex (1) at 322 nm is found to be lower by 6844 M−1 than
the parent ligand (L) [Fig. 2.0 Supplementary data], suggesting a
weaker association complex formation in comparison to the parent
ligand (L).
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Conclusion
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benzene-1,2-diol derivatives as fluorescent chemosensors for metal ions: a CHEF
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of L-3,4-dihydroxyphenylalanine, Org. Lett. 6 (2004) 3107–3109.
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for 4-(2-aminoethyl)benzene-1,2-diol and 4-(2-amino-1-hydroxyethyl)benzene-
1,2-diol, Org. Lett. 6 (2004) 3727–3730.
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1,2-diol with immobilized fluorescent probes, Org. Lett. 4 (2002) 3183–3185.
[14] Preparation of the ligand 1-(pyridin-2-ylmethyl)-2-(3-(1-(pyridin-2-ylmethyl)
benzimidazol-2-yl)propyl)benzimidazole (L): A solution of 2-(3-(benzimidazol-
2-yl)propyl)benzimidazole (GAB) (1 g,3.62mmols) in DMF was stirred for
3 h with K2CO3 (1.25 g,9.05mmols) at 70 °C. When turbidity appeared,
2-(chloromethyl)pyridine (1.48 g, 9.05mmols) was added and the solution was
stirred for 72 h continuously at 70 °C. Subsequently the solvent was stripped
off on a rotatory evaporator and the residue was treated with small amount
of distilled water to obtain a brown colored crude product. The product was
recrystallised from hot acetonitrile and water (2:1) and a light brownish crystal-
line powder was obtained. Yield: 55%. 1H NMR:δH (400 MHz, d6 DMSO, Me4Si):
2.98–3.04(4H, t, CH2), 2.24–2.33 (2H, q, CH2), 5.55 (4H, s, NCH2), 7.07–7.13
(2H, m, benz),7.16–7.20 (2H, m, pyr), 7.37–7.41(2H, m, benz), 7.50–7.54 (2H,
m, pyr), 7.61–7.65 (2H, m, pyr), 8.37–8.40 (2H, d, pyr); 13C NMR: (400 MHz;
d6 DMSO):156.5,155, 149,142.5,137.6,136,123.8,121, 122,119,49,28, 26; Anal
Calcd for C31 H33 N7 O2:C 69.5, H 6.1, N 18.3, Found: C 69.2, H 6.2, N18.1; U.V
data: λmax(DMF)/nm [logε/dm3 mol−1 cm−1]=269 [4.13], 277 [4.14] and 284
[4.12]; νmax/cm−1 3366 (OH), 1463 (C_N\C_C) and 1616 (C_N).
[15] Preparation of Cu(II) complex (1): A methanolic solution (5 ml) of Cu(NO3)2.3H2O
(52.7 mg, 0.22 mmol) was added to a methanolic solution (15 ml) of the ligand
(100 mg, 0.22 mmol). The resulting green colored solution was stirred for
30 min's. A light green colored product was formed which was centrifuged, washed
with small amounts of cold methanol and dried over P2O5. Yield: 45%. Anal Calcd for
We have developed a new fluorescent sensor (L), which is found
to be selective for 4-(2-aminoethyl)benzene-1,2-diol over aromatic
amines and phenols. Sensor (L) is also specific and selective to
4-(2-aminoethyl)benzene-1,2-diol in comparison to other catechol
amines. This has been demonstrated by conducting fluorescent titration
experiments of sensor (L) with the amine (2-phenylethanamine), phenol
(2,3-dimethyl phenol) catechol amine (L-3,4-dihydroxyphenylalanine)
and 4,6-ditertiarybutyl benzene1,2-diol (DTBC). No significant change
in the fluorescence emission intensity of (L) is observed as is the
case with 4-(2-aminoethyl)benzene-1,2-diol where it responds dif-
ferently, growth of a new band at 306 nm is observed due to the forma-
tion of a host guest complex between ligand and 4-(2-aminoethyl)
benzene-1,2-diol accompanied by an isoemissive point at 300 nm. Asso-
ciation constant of (L) with 4-(2-aminoethyl)benzene-1,2-diol is found
to be higher by 9868 M−1 than the other reported ligating systems.
Furthermore it is found that when a similar titration experiment was
conducted with the ligand in complex form (1), the sensing ability of
the ligating moiety of (L) towards 4-(2-aminoethyl)benzene-1,2-diol
gets weakened thereby restricting the formation of host guest complex
at 306 nm. Association constant of complex (1) is found to be lower by
6844 M−1 than the parent ligand (L), suggesting a weaker association
complex formation in comparison to the parent ligand (L).
Acknowledgment
We gratefully acknowledge the financial support from the Univer-
sity of Delhi, Delhi, India for the special grant.
Appendix A. Supplementary material
Crystallographic data for the structural analysis have been deposited
with the Cambridge Crystallographic Data Centre (CCDC of Ligand
(L) and Cu(II) complex (1) is 779380 and 779381 respectively). Supple-
mentary data associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.inoche.2013.02.015.
C
λ
30 H32 Cu N8 O8:C 51.7, H 4.6, N16.1, Found: C 52.0, H 4.4, N16.3; U.V Visible data:
max(DMF)/nm [logε/dm3 mol−1 cm−1]=268 [4.16], 277 [4.12], 284 [4.06], 800
[1.75];
max/cm−1 3432 (OH), 1456 (C_N\C_C), 1594 (C_N), 1011, 1336
ν
(O\N\O (symm)) and1435 (O\N\O(asym))
[16] X-ray diffraction data were collected on an Oxford Diffraction Xcalibur CCD dif-
fractometer with graphite monochromated (Mo–Kα radiation, λ=0.71073 A°),
temperature of 298(2) K at IIT Bombay. The data were corrected for Lorentz
and polarization effects. Multi-scan absorption correction was applied. The data
reduction was performed using the CrysAlis software package [28]. The structure
was solved by direct methods using SHELXS-97 and refined by full-matrix
least-squares method on F2 (SHELXL-97) [29]. All calculations were carried out
using the Wings package of the crystallographic programs [30]. For the molecular
graphics, the programs ORTEP-3 [31] and Mercury were used [32]. Crystal data of
ligand (L):C31 H31 N7 O2, Mr=533.63, T= 150(2) K, λ=0.71073 Å, Monoclinic,
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