A new dioxotetraamine ligand derived from binicotinic acid: Synthesis, coordination, and fluorescence behaviour towards divalent transition metal ions

Article abstract of DOI:10.1007/s00706-009-0235-2

A novel bipyridyl-based fluorescent system, N,N'-bis(2-aminoethyl)-2,2'- bipyridine-3,3'-dicarboxamide (BABD), with two different coordination sites, bidentate bipyridyl and tetradentate dioxotetraamine, has been synthesized, and characterized by elemental analysis and spectroscopic (UV-Vis, IR, : ¹H NMR, and ¹³CNMR) methods. The lowest energy molecular geometry of BABD was obtained by empirical then quantum mechanical treatment. The binding ability of BABD with H⁺, Co(II), Ni(II), Cu(II), and Zn(II) ions was investigated in aqueous 0.1 M KCl at 25 ± 1 °C by potentiometric methods. Four protonation constants were determined for BABD, and were used as input data to evaluate the formation constants of the metal complexes. The coordination behaviour of BABD in solution indicated that at low pH the bipyridyl unit coordinates to all metal ions, but at higher pH (>7.0) coordination of the dioxotetraamine unit occurs with the Cu(II) and Ni(II) ions only. The 3D-model structure of the metal complex was predicted by semi-empirical calculation using the AM1/d Hamiltonian. Fluorimetric titrations indicate that at pH 9 BABD exhibits fluorescence enhancement with increasing concentration of Cu(II) and Ni(II) ions, but at pH 7.2 fluorescence enhancement is observed only in the presence of Cu(II) ions. No remarkable effect on the fluorescence of BABD was observed in the presence of other biologically relevant metal ions, for example Ni(II), Fe(II), Mn(II), Co(II), Zn(II), Mg(II), Ca(II), and Hg(II). Springer-Verlag 2009.

Full text of DOI:10.1007/s00706-009-0235-2

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Monatsh Chem (2010) 141:157–168
DOI 10.1007/s00706-009-0235-2
ORIGINAL PAPER
A new dioxotetraamine ligand derived from binicotinic acid:
synthesis, coordination, and fluorescence behaviour towards
divalent transition metal ions
•
Bikram Kishore Kanungo Minati Baral
•
•
Rati Kanta Bera Suban Kumar Sahoo
Received: 17 September 2008 / Accepted: 27 November 2009 / Published online: 9 January 2010
Ó Springer-Verlag 2009
Abstract A novel bipyridyl-based fluorescent system,
N,N0-bis(2-aminoethyl)-2,20-bipyridine-3,30-dicarboxamide
(BABD), with two different coordination sites, bidentate
bipyridyl and tetradentate dioxotetraamine, has been syn-
thesized, and characterized by elemental analysis and
spectroscopic (UV–Vis, IR, 1H NMR, and 13C NMR)
methods. The lowest energy molecular geometry of BABD
was obtained by empirical then quantum mechanical
treatment. The binding ability of BABD with H?, Co(II),
Ni(II), Cu(II), and Zn(II) ions was investigated in aqueous
0.1 M KCl at 25 1 °C by potentiometric methods. Four
protonation constants were determined for BABD, and
were used as input data to evaluate the formation constants
of the metal complexes. The coordination behaviour of
BABD in solution indicated that at low pH the bipyridyl
unit coordinates to all metal ions, but at higher pH ([7.0)
coordination of the dioxotetraamine unit occurs with the
Cu(II) and Ni(II) ions only. The 3D-model structure of the
metal complex was predicted by semi-empirical calculation
using the AM1/d Hamiltonian. Fluorimetric titrations
indicate that at pH 9 BABD exhibits fluorescence
enhancement with increasing concentration of Cu(II) and
Ni(II) ions, but at pH 7.2 fluorescence enhancement is
observed only in the presence of Cu(II) ions. No remark-
able effect on the fluorescence of BABD was observed in
the presence of other biologically relevant metal ions,
for example Ni(II), Fe(II), Mn(II), Co(II), Zn(II), Mg(II),
Ca(II), and Hg(II).
Keywords Semi-empirical calculations ꢀ Sensor ꢀ
Metal complexes ꢀ Copper ꢀ Stability constants
Introduction
In recent years there has been burgeoning interest in the
design of chelating agents which can selectively bind metal
ions of interest [1–4]. The design of such chelates received
widespread stimulation in the late 1970s [5, 6] after the
development of supramolecular chemistry and the pio-
neering work of Pedersen, Cram, and Lehn. Whenever such
a chelate is suitably linked to a light-emitting group
(fluorophore) through a spacer, it produces a distinct fluo-
rescence signal on chelation with the metal ion (analyte).
This signal is helpful for both qualitative and quantitative
estimation of the target metal ion for which the chelate
is designed. During analyte detection, the fluorosensor
undergoes two processes, i.e. molecular recognition and
signal transduction, which can be understood schematically
from Scheme 1 representing an ON–OFF fluorosensor. In
an ON–OFF fluorosensor, complete fluorescence of the
fluorophore is observed when no analyte is bound to the
receptor. Once the analyte is recognized by the receptor,
fluorescence is quenched, presumably because of either
electron-transfer (eT) or energy-transfer (ET) processes
[7, 8]. Besides quenching, fluorescence enhancement of the
fluorophore is also implemented in the design of fluoro-
sensors [9–14].
B. K. Kanungo (&) ꢀ R. K. Bera
Department of Chemistry, Sant Longowal Institute of
Engineering and Technology, Longowal 148 106, Punjab, India
e-mail: b.kanungo@vsnl.com
M. Baral
Department of Chemistry, National Institute of Technology,
Kurukshetra 136 119, Haryana, India
S. K. Sahoo
Department of Chemistry, S V National Institute of Technology,
Surat, India
123

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158
B. K. Kanungo et al.
square-planar N4 complex [16]. Analysis of the solid-state
structures of their metal complexes reveals that the
metal–N(amide) bond length is somewhat shorter than the
corresponding metal–N(amine) bond length [17, 18]. This
effect is ascribed mainly to the strong r-donor properties of
the deprotonated amide groups and helps, to some extent,
to provide extra stabilization [19]. Solution studies indicate
that the coordination of dioxo-amides is pH dependent. In
acidic media, the ligand coordinates only through amine
nitrogen atoms whereas in basic or slightly acidic media,
amide N-atoms and amine N-atoms coordinate upon
deprotonation as anions (Fig. 1a) [18, 20]. The pH-
dependent, selective, and efficient metal ion-binding abil-
ities of dioxotetraamines stimulated the idea of designing
molecular devices such as cation-selective fluorosensors
(Fig. 1a) [21–29], and multi-component systems in which a
metal ion can be translocated with the change in pH
(Fig. 1b) [30–34]. The importance of dioxotetraamines is
also related with their:
Scheme 1
In designing fluorescent metal ion sensors, the receptor
chosen must have selective and efficient metal ion-binding
ability. Dioxotetraamine ligands are widely implemented
in the design of various molecular devices because of their
fascinating coordination behaviour [15]. In dioxotetraam-
ines, the metal ion coordinates through two deprotonated
amide N-atoms and two amine N-atoms to form a
1
2
dual structural features of tetraamine and oligopeptide;
potential to stabilize anomalously high oxidation states
(e.g. Ni(III) and Cu(III)) in aqueous media [35–37]; and
Fig. 1 a Schematic
representation of metal ion
coordination in
dioxotetraamines, and some
cation-selective fluorescent
sensors (eT electron transfer,
ET energy transfer);
b dioxotetraamine-based ditopic
ligands designed for pH-driven
translocation of divalent
(a)
eT/ET
NH2
hν
hν
O
O
-
NH
NH
NH2
NH2
N
N
M2+, 2OH
2H+
M
-
O
O
hν'
NH2
transition metal ions
O
O
C
N
Ru
N
N
N
N
N
N
N
C
Ru
Cl
=
C
O
Luminophores
(b)
O
O
O
N
N
NH HN
NH HN
N
HN
A
HN
NH
NH
B
R
N
O
O
NH
NH
NH
NH
N
N
N
CH2
CH2
=
R
N
O
123

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A new dioxotetraamine ligand derived from binicotinic acid
159
3
implementation as model compounds to elucidate the
reactivity of superoxide dismutase (SOD) etc. [38].
Results and discussion
Synthesis of the ligand
Bipyridyl and its derivatives are known to be excellent
chelators for transition metal ions and have been
exploited in the development of supramolecular devices
[39–42].
The ligand BABD was synthesized as shown in Scheme 2
by reacting an ester of binicotinic acid with freshly distilled
ethylenediamine. It was characterized by elemental analysis
and use of a variety of spectroscopic (UV–Vis, IR, 1H NMR,
and 13C NMR) methods. The ligand is soluble in water,
methanol, and ethanol but insoluble in the common organic
solvents chloroform, dichloromethane, and THF.
Keeping the above facts in view, herein we have intro-
duced a new water-soluble chelator N,N0-bis(2-amino-
ethyl)-2,20-bipyridine-3,30-dicarboxamide (BABD) with
two different coordination sites, i.e. bipyridyl and dioxo-
tetraamine units. The coordination behaviour of BABD
with the metal ions Co(II), Ni(II), Cu(II), and Zn(II) has
been investigated by pH–potentiometric methods at an
ionic strength of 0.1 M KCl at 25 1 °C. The formation
constants of the various complexes formed in solution and
their selectivity towards the metal ions are reported. The
3D-model structures of the metal complexes depicted in
solution were predicted by means of molecular modelling
calculations. Fluorimetric titration was carried out in
presence of various biologically relevant metal ions in
order to explore BABD as a cation-selective fluorescence
sensor.
The ligand in aqueous medium has two peaks at 212 and
261 nm attributed to p ? p* transitions associated with
the chromophoric bipyridyl ring. The calculated electronic
spectrum of BABD obtained from a PM3 (RHF) optimized
structure by employing the semi-empirical ZINDO method
showed peaks with kmax at 236 and 280 nm. The optimized
structure of BABD and the HOMO and LUMO molecular
orbital diagrams at 280 nm are shown in Fig. 2.
The infrared spectra of BABD contained a band
at 3,350 cm-1, characteristic of mN–H. A broad band
at 3,426 cm-1 was observed for mN–H (amine) and
Scheme 2
N
OH
N
O
N
NaOH
O
HO
N
KMnO4
H+
C2H5OH
H
N
N
O
NH2
OC2H5
N
O
H2N NH2
H2N
N
O
N
O
H
H5C2O
N
Fig. 2 HOMO (a) and LUMO
(b) molecular orbital diagrams
of the ligand BABD at
kmax = 280 nm obtained by
applying the semi-empirical
PM3/ZINDO method
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160
B. K. Kanungo et al.
presumably also due to the mN–H (2° amide). The charac-
teristic amide I and II mC=O bands were observed at 1,652
and 1,578 cm-1, respectively. The band 1,578 cm-1 is
possibly also due to the bending vibration of the amine
group (dNH). The amide III mC–N and dN–H for BABD were
observed at 1,242 and 796 cm-1, respectively.
½LHnꢂ
LHnꢁ1 þ Hþ ꢀ LHn Kn ¼
ð1Þ
½LHnꢁ1ꢂ½Hþꢂ
The first two protonation constants of BABD are similar
to that obtained for dioxotetraamine (log K = 9.6 and 8.3
[21]), and can be assigned to the protonation of two amine
groups, whereas the other two values are because of the
protonation of bipyridyl N-atoms of BABD (for a non-
substituted bipyridyl log K = 4.34 and 1.46 [43]). This is
again supported by the fact that bipyridyl nitrogens are
generally characterized by far lower basicity than amine
nitrogens. The amide group requires strongly basic
conditions for deprotonation, thus it was not possible to
determine the protonation constants of the amide groups
under the experimental conditions used. The two
protonation constants of the ligand assigned to bipyridyl
nitrogens, with Dlog K = 0.86, can be attributed to the
change in the electronic environment of the ligand after
uptake of one proton.
1
The H NMR spectrum of the ligand showed three dd
signals at 7.5, 8.0, and 8.6 ppm, characteristic of aromatic
protons. Two triplets centred at 3.1 and 2.5 ppm are
assigned for the methylene groups linked to the amide and
amine groups, respectively. No peak was observed for
amine and amide protons, which may be because of deu-
terium exchange. The 13C NMR spectrum of BABD
contained peaks at 39.65 and 42.21 ppm for CH2 groups
attached to the amine and amide groups, whereas the peaks
at 124.49, 131.38, 137.14, 150.04, and 154.16 ppm are
attributed to aromatic carbons. The peak at 169.72 ppm
corresponds to the amide carbonyl carbon.
The pH dependent species distribution of BABD
(Fig. 4) indicates that the ligand exists in different pro-
tonated forms between pH *2 and *11. Completion of
the deprotonation of the bipyridyl groups was observed in
steps, forming LH3 and LH2 species at pH \ *6. As the
pH increases, two amine nitrogen atoms are deprotonated
stepwise to form LH and the neutral species L, which is
predominant above pH 9.5.
Ligand protonation constants
The protonation constants of the ligand were determined by
potentiometric and spectrophotometric methods. For the
potentiometric method the acidified ligand solution was
titrated against standard KOH at an ionic strength of 0.1 M
KCl and 25 1 °C in aqueous medium. Analysis of
potentiometric curve (Fig. 3) of the ligand BABD with the
computer software Hyperquad 2000 gave four protonation
constants: 8.89 0.04, 8.41 0.03, 3.54 0.09, and
2.68 0.07.
Spectrophotometric titration of the ligand BABD was
also carried out in aqueous solution in acidic and basic
media to explain the deprotonation process. The spectra
were recorded in the region 200–380 nm, and in the pH
range 2.54–9.83. The state of equilibrium between the
protonated and deprotonated ligand can be examined
The equilibrium reactions for these protonation con-
stants are given by the equations:
Fig. 3 Potentiometric titration curves of BABD in the absence and
presence of the metal ions Co(II), Ni(II), Cu(II), and Zn(II) in 1:1
ligand–metal molar ratio, where ‘‘a’’ is moles of base added per mole
of ligand present
Fig. 4 pH-dependent species distribution curve for BABD
123