Cation-dependent structural diversity of zinc(II), calcium(II) mono- and binuclear complexes of aryl-imidazo-1,10-phenanthroline derivatives

Article abstract of DOI:10.1016/j.ica.2016.02.029

The syntheses of azacrown-containing aryl-imidazophenanthroline by reaction of 1,10-phenanthroline-5,6-dion with 4-(1.4.7.10-tetraoxa-13-azacyclopentadecan-13-yl)benzaldehyde producing the ligand 2 in high yield is described. Examination of their Zn(II), Ca(II) and mixed complexes by UV/Vis and NMR spectroscopy suggests electronic communication between the two metal centers of ligand 2. Stability constants, determined by spectrophotometric titration, indicate that the Zn(II) possesses higher affinity toward phenanthroline residue than Ca(II) and does not bind with azacrown-ether residue. This selectivity allows to prepare a mixed Zn(II)/Ca(II) complex.

Full text of DOI:10.1016/j.ica.2016.02.029

Inorganica Chimica Acta 445 (2016) 103–109
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Cation-dependent structural diversity of zinc(II), calcium(II) mono- and
binuclear complexes of aryl-imidazo-1,10-phenanthroline derivatives
Yulia Sotnikova a, Elena Lukovskaya a, Alla Bobylyova a, Yury Fedorov a,b, Valentin Novikov b,
Alexander Peregudov b, Aleksander Anisimov a, Anthony D’Aléo c, Frédéric Fages c, Olga Fedorova a,b,
⇑
a Department of Chemistry, M. V. Lomonosov Moscow State University, Leninskie Gory, 119992 Moscow, Russia
b A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilova Str., 119991 Moscow, Russia
c Aix Marseille Université, CNRS, CINaM UMR 7325, Campus de Luminy, Case 913, Marseille 13288, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The syntheses of azacrown-containing aryl-imidazophenanthroline by reaction of 1,10-phenanthroline-
5,6-dion with 4-(1.4.7.10-tetraoxa-13-azacyclopentadecan-13-yl)benzaldehyde producing the ligand 2
in high yield is described. Examination of their Zn(II), Ca(II) and mixed complexes by UV/Vis and NMR
spectroscopy suggests electronic communication between the two metal centers of ligand 2. Stability
constants, determined by spectrophotometric titration, indicate that the Zn(II) possesses higher affinity
toward phenanthroline residue than Ca(II) and does not bind with azacrown-ether residue. This selectiv-
ity allows to prepare a mixed Zn(II)/Ca(II) complex.
Received 8 September 2015
Received in revised form 15 February 2016
Accepted 17 February 2016
Available online 24 February 2016
Keywords:
Macrocyclic ligands
Crown-ether
Ó 2016 Published by Elsevier B.V.
Zinc(II) complex
Calcium(II) complex
1. Introduction
still remain rare. Thus, a Zn2+ complex of the hybrid of terpyridine
with 2-phenylimidazo[4,5-f][1,10]phenanthroline acts as a multi-
The design and synthesis of the organometallic systems that
exhibit pre-designed functionality on the supramolecular organi-
zation are important in physics, chemistry and materials science.
Precise control on the nanoscale will allow the development of
the devices as well as the development of materials with new,
potentially controllable properties, e.g. catalytic, magnetic and
sensing [1–8].
1H-imidazo[4,5-f][1,10]phenanthroline (abbreviated as IPh) as
N-donor ligand possesses several interesting structural character-
istics appropriate for coordination with different types of metal
ions (Zn2+, Mn2+ [9], Cd2+ [10], Co2+ [11], Pt2+ [12], lanthanides
(III) (Ln = Eu, Dy, Er, Tb, Sm, Yb) [13], Ru2+ [14])). Imidazophenan-
throline complexes are suitable for the various applications, such
as fluorescent probes [15,16], DNA binding [17–25], sensing ele-
ments for cations and anions [9,26]. Imidazo[4,5-f][1,10]phenan-
throline derivatives and their metal complexes are candidates for
the third-order NLO materials in virtue of their high harmonic gen-
eration and rapid response [27–30]. However, reports on
self-assembly with dinuclear imidazophenanthroline complexes
functional optical sensor for multiple analytes (cations and anions)
[9]. It was previously reported that bimetallic complex Re–IPh–Ir
that demonstrates the ability to react with CO with appreciable
changes in spectroscopic measurement reveal an electronic com-
munication between the two metal centers [31]. Two polypyridyl
ligands containing macrocyclic unit benzo-12-crown-4 or
benzo-15-crown-5, and their mononuclear IPh–Ru(II) complexes
have been prepared. These complexes exhibit a modest binding
ability to Li+ or Na+ [32].
Our research in this area is directed towards studies of binu-
clear complexes containing two types of metal ions, namely, tran-
sition Zn2+ cation and alkaline earth Ca2+ cation. Metal cations such
as Ca2+ and Zn2+ play vital roles in many biologic processes and
tracking their homeostasis in living systems via a suitable tech-
nique is of great significance to clarify their biological effects
[33]. The phenyl-aza-15-crown-5 ether IPh (2 in Scheme 1) was
used as a ditopic ligand. Imidazo[4,5-f][1,10]phenanthroline is well
suited as bridging ligand having bidentate coordination center
suitable for binding with Zn2+ [9]. Phenylazacrown ether forms
complex effectively with Ca2+ cation [34]. At the same time, the
ability of Ca2+ to interact with IPh has not been discussed earlier.
For study that is more detailed the model compound 1 has been
also prepared (see Scheme 1). Our study was to investigate the
crown-containing IPh fragment, in order to study the interplay of
⇑
Corresponding author at: Department of Chemistry, M. V. Lomonosov Moscow
State University, Leninskie Gory, 119992 Moscow, Russia. Tel.: +7 (495)939 24 48;
fax: +7 (495) 932 85 68.
E-mail address: fedorova@ineos.ac.ru (O. Fedorova).
http://dx.doi.org/10.1016/j.ica.2016.02.029
0020-1693/Ó 2016 Published by Elsevier B.V.

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Y. Sotnikova et al. / Inorganica Chimica Acta 445 (2016) 103–109
unbalancing factor alpha 2.0, relaxation delay 5.0 s. Signal attenu-
NH OAc,
4
ation was achieved by increasing the gradient strength from 5% to
80% as defined by the pulse sequence in 32 steps with 32 scans,
each with the maximum gradient strength of 0.27 T/m.
O
+
N
N
N
N
N
CH COOH ice-cold
3
R
R
boil, 4-8h.
O
N
H
O
The rows of quasi-2D diffusion dataset were phased and base-
line corrected. Pseudo-2D DOSY spectra were obtained using a
standard fitting procedure of Bruker Topspin 2.1 software. The
actual diffusion coefficients were determined using T1/T2 analysis
module of the Bruker Topspin 2.1 software. As the signals corre-
sponding to the protons of crown fragments were characterized
by the highest signal-to-noise ratio, their intensities were used
for the computation of the signal decay curves. Viscosities of solu-
tions were obtained by comparing the measured diffusion coeffi-
cient of the residual solvent signal with the known value
(4.14 ꢁ 10ꢂ9 m2sꢂ1 for CD3CN [38]). The hydrodynamic radii and
volumes were calculated from viscosity-corrected diffusion coeffi-
cients via Stokes–Einstein relation.
O
O
1: 85%
2: 46%
N
R = H (1),
(2)
O
O
Scheme 1. Synthetic scheme of 1, 2 dye preparing.
Ca2+ and Zn2+ complexation and also to assess whether this moiety
would be a suitable building block towards the synthesis of mixed
cation systems.
2. Experimental
UV–Vis spectra recorded with a Varian Cary 50 spectropho-
tometer, FLUORAT-02-Panorama fluorescence spectrophotometer,
and Avantes spectrophotometer.
2.1. Materials
Anhydrous MeCN, Zn(ClO4)2ꢀ6H2O, Ca(ClO4)2ꢀ4H2O (Aldrich),
70% HClO4 (Sigma–Aldrich) were used as received. For study of
ligands 1 and 2 protonation 0.01 M HClO4 was prepared by diluting
of 70% HClO4 with MeCN (total 0.04% water in MeCN–water mix-
ture). 2-Phenyl-1H-imidazo[4,5-f][1,10]phenantroline (1) was pre-
pared as described [35,36].
2.4. Equilibrium constant determination
Complex formation of ligands with Zn(ClO4)2, Ca(ClO4)2 in ace-
tonitrile at 20 1 °C was studied by spectrophotometric titration.
The ratio of Me(ClO4)2 to ligand was varied by adding aliquots of
a solution of known concentrations Me(ClO4)2 to a solution of
ligand alone of the known concentration. The UV–Vis absorption
spectrum of each solution was recorded and the stability constants
of the complexes were determined using the SPECFIT/32TM program
[39], a program designed to extract equilibrium constants from
spectrophotometric titration data. SpecFit starts with an assumed
complex formation scheme and uses a least-squares approach to
derive the spectra of the complexes and the stability constants.
Typically, 12–15 spectra were taken for calculations.
2.2. Syntheses of 2-[4-(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-
yl)phenyl]-1H-imidazo[4,5-f][1,10]phenanthroline (2)
To a solution of 4-(1.4.7.10-tetraoxa-13-azacyclopentadecan-
13-yl)benzaldehyde (70 mg, 0.216 mmol) in ice-cold acetic acid
(5 ml) 1,10-phenanthroline-5,6-dion (45.5 mg, 0.216 mmol) and
ammonium acetate (350 mg, 4.55 mmol) were added. The mixture
was refluxed for 8 h. After cooling to room temperature, water
solution of ammonium acetate (5 M) was added until pH = 8. Pre-
cipitated solid was filtered, washed with water and diethyl ether.
Drying in vacuo at 75 °C for 1 h yielded 51.5 mg (0.1 mmol, 46%)
of a brown powder, m.p. 130–133 °C. 1H NMR (DMSO-d6, d; ppm,
J/Hz): 3.52 (s, 4H), 3.57 (m, 8H), 3.70 (m, 8H) (CH2O), 6.85 (d, 2H,
Those equilibriums were used to model the data:
þZn
þZn
*
*
*
L þ Zn )
L3Zn )
L2Zn )
LZn
K31
K21
K11
½L3Znꢃ
½L2Znꢃ
½LZnꢃ
3J = 8.36), 8.08 (d, 2H, J = 8.37) H(Ph), 7.81 (m, 2H, H(30,60)), 8.90
3
K31
¼
K21
¼
K11
¼
3
2
½Lꢃ ꢄ ½Znꢃ
(d, 2H, 3J = 7.00), 9.00 (m, 2H) H(20,40,50,70). 13C NMR (APT)
(DMSO-d6, d, ppm): 52.41, 68.30, 69.51, 69.99, 70.77
[CH2O],111.70, 123.55, 128.07, 129.91, 147.79 [CH], 117.45,
143.67, 148.98, 152.21, 172.84 [Cquat]. Anal. Calc. for C29H31N5O4:
C, 67.82; H, 6.08, N, 13.64. Found: C, 67.90; H, 6.11; N, 13.68%.
ESI-mass m/z (I, (%)): 514 [M]+ (100).
½Lꢃ ꢄ ½Znꢃ
½Lꢃ ꢄ ½Znꢃ
þCa
½12Caꢃ
½1Caꢃ
*
*
1þCa )
2 þ Ca )
12Ca )
1Ca K21
¼
K11
¼
2
K21
K11
½1ꢃꢄ½Caꢃ
½1ꢃ ꢄ½Caꢃ
þCa
þCa
*
*
*
22Ca )
2Ca )
2Ca2
K21
K11
K12
2.3. Analytical and physical measurements
½22Caꢃ
½2Caꢃ
½2ꢃ ꢄ ½Caꢃ
½2Ca2ꢃ
K21
¼
K11
¼
K12
¼
2
2
Mass spectra were recorded using Agilent 1100 Series LC/MSD
trap interface operated in positive-ion mode. Direct infusion of
the analyzed solution was used. Optimum flow rate of the gas
dryer was 11 l/m. The dry gas temperature was 150–250 °C. Gas
pressure was 60 psi. Overload on the nebulizer tip was 2–3 kV.
1H NMR spectra were recorded on a Bruker Avance 600 spec-
trometer (working frequency 600.13 MHz). Chemical shifts were
measured with an accuracy of 0.01 ppm, and a measurement error
of spin–spin coupling constants was 0.1 Hz. The measurements
were done using the CD3CN signal as an internal reference
(1.94 ppm at 295 K). Data acquisition and processing were per-
formed with Topspin 2.1 software (Bruker).
½2ꢃ ꢄ ½Caꢃ
½2ꢃ ꢄ ½Caꢃ
L¼ligands 1;2:
UV–Vis spectra recorded with
spectrophotometer.
a
Varian Cary 50
Steady-state fluorescence spectra were recorded with a Fluo-
rolog-3 (Model FL3-22) fluorescence spectrophotometer at
20 1 °C. Fluorescence quantum yields of the ligands and their
complexes were determined in air-saturated acetonitrile at
20 1 °C relative to quinine sulfate in 0.5 M H2SO4 as standard
(uF = 0.546) with excitation at 315 nm for ligand 1, 360 nm for
ligand 2 and its complex with Zn2+ and 320 nm for mixed Zn–Ca
complex of ligand 2.
DOSY experiments were performed on
a Bruker Avance
Fluorescence quantum yields were calculated using
spectrometer at 25 °C using the DOSY-ONESHOT pulse sequence
[37]. The following experimental parameters of the pulse sequence
were used: diffusion time 0.2 s, gradient pulse duration 1 ms,
h
i
ꢂDx
2
2
s
ux
¼
usðAx=AsÞ ð1 ꢂ 10ꢂD Þ=ð1 ꢂ 10 Þ ðnx =ns Þ

Y. Sotnikova et al. / Inorganica Chimica Acta 445 (2016) 103–109
105
where us is the fluorescence quantum yield of the standard, A is the
integrated area under the emission spectrum, D is the absorbance at
the wavelength of excitation, and n is the refractive index of the sol-
Upon the addition of 0 to 1 equivalent of perchloric acid absorption
spectra demonstrates up to 7 isosbestic points that indicates the
formation of a single-type strong complex (see Fig. S6a in SI).
The first protonation induces an enhancement of the low-energy
band, which would correspond to an increase of intramolecular
charge transfer in the protonation species. Such effect can been
rationalized by considering that the first protonation of phenan-
throline residue and the strength of formed complex is due to coor-
dination of the proton with both N-atoms of phenanthroline,
which increases the electron acceptor character of the phenanthro-
line unit. Upon the addition of 1 to 40 equivalents of acid absorp-
tion spectra demonstrates another set of isosbestic points that
indicates transition of monoprotonated complex to diprotonated
through the protonation of imidazolium part (see Fig. S6b in SI).
Ligand 2 can coordinate three protons (see Scheme 2 and
Figs. S7, S8 in SI). Based on the position of long wavelength bands
of 2ꢀH+ (kmax = 347 nm) and 2ꢀ(H+)2 (kmax = 310 nm) the coordina-
tion of protons with phenanthroline and imidazole units can be
suggested. At the large amount of HClO4, the formation of 2ꢀ(H+)3
(kmax = 280 nm) was observed. The position of its long wavelength
band is shifted to the short region. This fact points on the coordina-
tion of the third proton with N-atom of azacrown ether what
switches off the lone pair of the azacrown ether nitrogen atom
from conjugation with the chromophore.
vent. Subscripts
respectively.
x and s refer to unknown and standard,
3. Results and discussion
Imidazo[4,5-f][1,10]phenanthroline based ligands 1 and 2 are
easily prepared by condensation of 1,10-phenanthroline-5,6-dione
and corresponding aldehydes in the presence of ammonium acet-
ate in ice-cold acetic acid. The structure of prepared compounds
1, 2 was confirmed by 1H, 13C, COSY, NMR spectroscopy, elemental
analysis and mass-spectrometry methods (see Experimental part
and Figs. S1–S5 in Supporting Information (SI)).
Electronic absorption spectra of phenanthroline derivatives 1
and 2 in acetonitrile (MeCN) showed an intense lowest energy
charge-transfer absorption band in the UV-visible region (Fig. 1).
The position of this band depends on the electronic nature of the
substituent at position 4 of the aryl moiety. The reason for the sub-
stantial red shift in the investigated compound 2, functionalized
with donor group, kmax 342 nm, relative to that of unsubstituted
phenanthroline 1 (kmax = 274 and 333 nm) is the strong conjuga-
tive electron donor effect of the azacrown substituent at the para
position of the phenyl ring.
Firstly, we analyzed the protonation of the compounds 1 and 2
that could help in the understanding of the binding process with
metal ions. To define the protonation order of nitrogen atoms in
ligands 1 and 2 we studied the protonation with HClO4 by spec-
trophotometric titration method. The addition of the increasing
amount of HClO4 causes the appearance of 1ꢀH+ (kmax = 308 nm)
followed by 1ꢀ(H+)2 (kmax = 290 nm) (see Figs. 2 and S6a, b in SI).
The analysis of 1H NMR spectra of the ligand 2 in the presence of
increasing amount of HClO4 showed the significant changes of the
position of imidazophenanthroline proton signals even at small
amount of HClO4 (see Fig. S9a, b in SI). This confirms that firstly
the protonation of imidazophenanthroline residues occurs.
Detailed investigations were carried out to understand the
coordinating behaviors of 1 and 2 with Zn2+. In literature, there
are a few examples of Zn2+complexes with IPh. The synthetic
procedure of obtaining of [Zn2(Hcpip)2(ox)]nꢀ2nH2O and
[Zn4(Hcpip)2(cpip)2(ox)]nꢀ5nH2O, where H2cpip = 2-(2-carboxy-
phenyl)imidazo[4,5-f][1,10]phenanthroline, H2ox = oxalic acid,
was described in [40]. The complex was formed only after boiling
of metal–ligand water solution for 8 h. An imidazophenanthroline
scaffold with a boronic acid unit, viz. 3-(1H-imidazo[4,5-f][1,10]
phenanthrolin-2-yl)phenylboronic acid, is found that IPh induces
fluorogenic responses when Zn(II) ion coordinates with phenan-
throline residue [41]. The observed complex has the composition
1:1. In mixture THF-MeOH ZnCl2 forms exclusively the 2 ligands
to 1 metal ion complex with IPh [42]. Nevertheless, ligand does
not exhibit any zinc-corresponding emission. Furthermore, the
complex is not stable in the presence of DMSO or water. In [9]
the composition of IPh–Zn2+ complex in DMF–MeCN mixture has
not been determined.
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1
2
200
250
300
350
400
450
Wavelength, nm
In our experiments, we would like to understand how the com-
plex formation between 1, 2 and Zn(ClO4)2 in MeCN solution at
room temperature occurs. The UV–Vis absorption spectra of 1, 2
in the presence of Zn(ClO4)2 in MeCN are displayed in Fig. 3, Table 1
and Figs. S10–S17 in SI.
Fig. 1. The absorption spectra of ligands 1 and 2 (1.67 ꢁ 10ꢂ5 M) in MeCN solution.
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2
Upon addition of Zn2+, the band at 276 nm of 1 showed a sub-
stantial increase of intensity with absorption tail extending to
about 410 nm, indicating that Zn2+ has a strong interaction with
1 (Figs. 3 and S10–S12 in SI). The analysis of the spectral data with
SpecFit32 program showed that depending on Zn2+ amount the
formation of three types of complexes 13ꢀZn2+, 12ꢀZn2+, 1ꢀZn2+
occurs (Scheme 3). The calculated stability constants are presented
in Table 1. We proposed the formation of all types of complexes
through the coordination of metal ions with phenanthroline resi-
due. According the known literature data cited in the introduction
of the paper, phenanthroline is the preferable place for metal ion
coordinated with IPh.
3
1
220
270
320
370
Wavelength, nm
Fig. 2. The absorption spectra of ligand
(1.67 ꢁ 10ꢂ5 M) in MeCN.
1
(1), 1ꢀH+ (2) and 1ꢀ(H+)2 (3)
As optical investigation showed (Figs. S13–S16 in SI), the pro-
cess of interaction of Zn2+ with ligand 2 is similar to what we

106
Y. Sotnikova et al. / Inorganica Chimica Acta 445 (2016) 103–109
+H
+H
O
O
O
O
+H
N
N
N
N
N
N
O
O
N
HClO4
O
O
HClO4
N
O
O
HClO4
H+
N
H+
N
N
H+
N
H
N
H
N
H
N
O
O
-
-
3ClO4
-
2ClO4
ClO4
2.
+
2.H+, λmax = 347nm
2.
+
(H )3, λmax = 280nm
(H )2, λmax = 310nm
Scheme 2. Protonation of dye 2 in reaction with HClO4.
0.8
corresponding metal ions. To supplement the data of routine 1D
1H NMR measurements, the diffusion-ordered spectroscopy
(DOSY) [44] was employed to obtain diffusion coefficients of the
complexes in solution. As the direct interpretation of the diffusion
coefficients is notoriously troublesome, especially for charged
species, we used the approach [45] based on the comparison of
effective hydrodynamic volumes of the complexes with that of
the free ligand. Note that in most cases, we observed the dynamic
equilibrium in solution, and the corresponding line broadening
dramatically decreased the accuracy of our measurements.
Although the resulting data provide valuable complementary
information on the composition of the complexes, no quantitative
characteristics (such as equilibrium constant values, etc.) can be
reliably obtained.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2
1
250
300
350
400
450
Wavelength, nm
Fig. 3. Electronic absorption spectra of: (1) 1; (2) 1ꢀZn2+. Initial concentration of 1
was C1 = 1.67 ꢁ 10ꢂ5 M, concentration of zinc perchlorate was varied in the interval
0–0.005 M. Solvent–acetonitrile, T = 294 K.
The changes in the 1H NMR spectra of the ligand 2 following the
addition of ZnClO4 solution are consistent with the successive for-
mation of several complexes of different compositions (Fig. 4).
Only signals of aromatic protons were shifted upon the complex
formation, while the signals of a crown fragment remained con-
stant, which confirms very low affinity of Zn2+ ions to the crown
ether moiety. Although the exchange-induced broadening of the
signals in the aromatic part of the spectra prevents the full assign-
ment of all the signals observed, the data obtained suggest the
formation of two distinctly different complexes. The addition of
the first portion of Zn2+ solution leads to the shift of signals corre-
sponding to the protons of phenanthroline moiety, signaling the
observed for 1. It is known that azacrown ether does not bind
heavy and transition metal ions [43]. This is why in our experiment
we did not see the complex formation of Zn2+ with azacrown ether
unit. Moreover, the calculated electronic absorption spectra of both
(1)3Zn and (2)3Zn in MeCN show an increase of the charge transfer
band at low energy, which indicates that complexations does
involve the nitrogen atoms of the phenathroline unit, as observed
in the case of the first protonation.
The formation of zinc complexes of the ligand 2 was followed
by NMR titration of the ligand in CD3CN solution by the
Table 1
Stability constants (LogK) of complexes 1 and 2 with metal cations in MeCN (293 K).
2+
Ligand
LogK of Zn2+-complexes (L: Zn
)
LogK of Ca2+-complexes (L:Ca2+
)
LogK of Zn2+–Ca2+–complex (L:Zn2+:Ca2+
LogK212
)
LogK11
LogK21
LogK31
LogK21
LogK11
LogK12
1
2
6.0 0.17
6.0 0.32
12.7 0.21
13.9 0.24
18.7 0.27
20.5 0.31
11.1 0.19
12.8 0.45
5.4 0.14
5.5 0.29
–
–
>20
8.4 0.29
Ca2+
O
O
Ca2+
N
N
N
N
O
N
N
O
Ca2+
O
Zn
2+
N
N
O
N
H
N
H
O
O
O
O
O
O
R =
N
R
N
NH
N
N
N
N
Zn(ClO )
N
N
N
N
N
N
4 2
N
N
N
Zn(ClO )
2+
4
2
2+
R
2+
Zn
Zn
R
Zn
R
R
N
N
H
N
N
H
N
H
N
H
N
N
HN
O
O
O
O
R = H,
N
R
Scheme 3. The suggested structures of Zn2+ and Zn2+–Ca2+ complexes.

Y. Sotnikova et al. / Inorganica Chimica Acta 445 (2016) 103–109
107
70000
60000
50000
40000
30000
20000
10000
2
4
3
1
0
200
300
400
500
600
Wavelength, nm
Fig. 6. Electronic absorption spectra of the solution of dye 2 with increasing
concentration of calcium perchlorate: (1) 2; (2) 22ꢀCa2+; (3) 2ꢀCa2+; (4) 2ꢀ(Ca2+)2.
Initial concentration of dye C2 = 1.67 ꢁ 10ꢂ5 M, concentration of calcium perchlo-
rate vary in the interval 0–0.005 M. Solvent–acetonitrile, T = 294.
of 2 and Ca2+ can contain complexes with both macrocyclic and
heterocyclic types of coordination. In the presence of large amount
of Ca2+ cations the substantial hypsochromic shift was observed
indicating that complex in which Ca2+ ions is placed in macrocyclic
unit is dominant (complex 2ꢀ(Ca2+)2) (Fig. 6, Scheme 4).
Fig. 4. Dependence of 1H NMR spectra of 1.33 ꢁ 10ꢂ3 M CD3CN solution of the
ligand 2 and zinc perchlorate on the molar Zn/ligand ratio. The low-field region of
the spectra is scaled to ensure the visibility of the signals assigned to amine protons.
The titration of the ligand 2 by Ca2+ ions resulted in changes in
chemical shifts in the aromatic and crown fragment regions of the
1H NMR spectra, implying the possible binding of calcium ions to
both these moieties. Unfortunately, even at 335 K the exchange
was slow enough to significantly broaden the signals, which,
together with relatively low signal-to-noise ratio, prevented the
assignment of the signals and obtaining the meaningful diffusion
data. However, it is clear that in contrast to zinc, which shows
the affinity to the phenanthroline binding site, calcium ions bind
to both the phenanthroline and crown ether fragments, but the
affinity is significantly lower than that for the phenanthroline/zinc
pair.
formation of ZnL3 complex that is in equilibrium with the excess of
the free ligand. However, when a molar ligand-to-zinc ratio is 3:1,
the second set of signals emerges, suggesting the formation of the
second, more stable complex ZnL2. The latter is further stabilized
with increasing concentration of zinc ions. Spectra do not change
significantly if the molar ligand-to-zinc ratio increases beyond
2:1, and there was no indication of the presence of a ZnL complex
even in the case of the ten-fold excess of Zn2+
.
The consecutive formation of two different complexes also fol-
lows from the diffusion data. The initial addition of zinc ions signif-
icantly increases the complex-to-free ligand volume ratio (Fig. 5)
owing to the formation of a bulky ZnL3 complex. Further addition
of zinc ions reduces this volume ratio because of the formation
of a more compact ZnL2 complex.
We studied the possibility to obtain the complex containing
two different metal ions. According to the data in Table 1, Zn2+
forms complexes with ligand 2 that are more stable than those
of Ca2+. Thus, we can suggest that complexes with Zn2+ should
In opposite to Zn2+, Ca2+ can bind with macrocyclic azacrown
ether units [34]. The addition of calcium perchlorate to the acetoni-
trile solution of 1 or 2 causes firstly the bathochromic shift of long
wavelength band at 274 nm up to 294 nm for 1 and 2 nm shift of
342 nm band for 2 (Figs. 6 and S17–S23 in SI). The obtained spec-
trophotometric data fit well with the formation of 2 (or 1)2ꢀCa2+, 2
(or 1)ꢀCa2+ complexes (Scheme 4, Table 1). Obviously, that solution
not be dissociated upon the addition of Ca2+ ions. So as in Zn2+
–
complex macrocyclic coordination place is free, Ca2+ ions can inter-
act with azacrown ether. For this purpose, firstly we prepared
22ꢀZn2+ complex by taking into account the obtained spectrophoto-
metric data. The addition of Ca2+ perchlorate to 22ꢀZn2+ complex
causes the hypsochromic shift of the absorption band that points
out on Ca2+ coordination with crown ether part (see Fig. 7). The
spectrophotometric analysis showed the formation of mixture
complex 22ꢀZn2+ꢀ(Ca2+)2.
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
The effect of simultaneous presence of zinc and calcium ions in
solution on the composition of the resulting complexes was stud-
ied by the titration of a mixture of the ligand 2 with two equimolar
amount of zinc ions, where predominance of a 22ꢀZn2+ complex
was expected. The addition of calcium ions leads to a significant
shift of the signals assigned to the crown fragment (Fig. S24 in
SI), while in the aromatic region of the spectra only the signals of
phenyl protons adjacent to the crown moiety were shifted.
This, together with a larger affinity of zinc ions to phenanthro-
line binding site in comparison to calcium ions, suggests the grad-
ual occupation of both crown moieties by calcium ions and the
formation of the complex ZnL2Ca2. This result is in a good agree-
ment with the diffusion data: the addition of calcium ions steadily
increases the effective hydrodynamic volume of the complex
(Fig. 8).
0.0
0.5
1.0
9.5
10.0
Zn / Ligand ratio
Both ligands 1 and 2 demonstrate the fluorescence with closed
values of quantum yield. The analysis of the fluorescence showed
Fig. 5. Dependence of the ratio of effective hydrodynamic volumes of the
complexes to that of the free ligand 2 on the molar Zn/ligand ratio.

108
Y. Sotnikova et al. / Inorganica Chimica Acta 445 (2016) 103–109
O
O
O
O
O
O
O
N
N
N
N
N
N
N
N
O
O
N
N
O
O
O
O
N
O
O
N
Ca(ClO4)2
Ca(ClO4)2
Ca2+
Ca
N
N
N
Ca2+
2+ N
Ca 2+
N
H
N
H
N
H
N
H
O
Ca(ClO4)2
O
N
O
N
2+ N
Ca
O
N
H
N
O
Scheme 4. The structures of Ca2+ complexes.
0.25
0.2
3
1
2
0.15
0.1
0.05
0
Scheme 5. Optimized structure of ligand 2 calculated by MOPAC 7 program.
250
300
350
400
450
500
Wavelength, nm
ligand showed its plane structure (Scheme 5) providing the easy
communication between the end groups of the ligand. In addition,
the plane structure of 2 points out that the reason of fluorescence
quenching could be LMCT but not the photoinduced electron trans-
fer (PET) process.
Fig. 7. Electronic absorption spectra of the solution of: (1) 2; (2) 22ꢀZn2+
;
(3) 22ꢀZn2+ꢀ(Ca2+)2. Initial concentration of 2 was C2 = 1.67 ꢁ 10ꢂ5 M, concentration
of calcium perchlorate was varied in the interval 0–0.005 M. Solvent–acetonitrile,
T = 294.
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
4. Conclusion
The analysis of the obtained data allows making the following
conclusions. This research showed that in crown-containing
phenanthroline derivative 2 the phenanthroline residue is the
preferable place for binding as heavy Zn2+ as well as alkaline earth
Ca2+ metal ions. According our observation imidazole nitrogen-
atom does not participate in complex formation. As demonstrated
the NMR analysis at high concentration of ligand 2 the strong com-
plex 22ꢀCa2+ forms through the coordination of Ca2+ with phenan-
throline residue. At equimolar 2 and Ca2+ ratio, complex 2ꢀCa2+
was observed which is presented the mixture of two complexes
possessing different structures. The first complex forms through
the coordination of Ca2+ with phenanthroline heterocyclic part, in
the other one Ca2+ is located in crown ether moiety.
0
1
2
3
4
5
6
7
8
9
10
49 50
Ca2+ / Ligand ratio
Fig. 8. Changes in the effective hydrodynamic volumes’ ratio for the mixed
zinc/calcium complexes upon titration of equimolar 7.13 ꢁ 10ꢂ3 M acetonitrile-d3
solution of the ligand 2 and ZnClO4 by calcium perchlorate.
The nature of metal ion influences on the structures of the
formed complexes. Thus, at the large ligand concentration in case
of Zn2+the formation of 23ꢀZn2+ was found, whereas, for Ca2+ we
observed the complex 22ꢀCa2+. The composition of complexes
changes as 23ꢀZn2+ ? 22ꢀZn2+ ? 2ꢀZn2+ and 22ꢀCa2+ ? 2ꢀCa2+ ? 2ꢀ
(Ca2+)2 upon increase of metal ion concentration. Interestingly, that
coordination of Ca2+ by 22ꢀZn2+ causes the gradual occupation of
both the crown moieties by calcium ions resulting in the
that addition of metal ions to crown free ligand causes the shift of
the fluorescence band to longer wavelengths (Table 2, Fig. S27 in
SI). The quantum yield of fluorescence of ligand 1 slightly changes
upon complex formation. In contrary, practically full fluorescence
quenching additionally to bathochromic shift was found upon
complex formation of crown-containing phenanthroline ligand 2
with both Zn2+ and Ca+ cations (Table 2). The phenomenon could
be connected with ligand–metal charge transfer (LMCT) between
electron donor azacrown ether part and metal ions bound with
phenanthroline residue. Indeed, the optimized geometry of free
22ꢀZn2+ꢀ(Ca2+
) complex.
2
Plane ligand 2 structure provides long chromophoric molecule
reached out from phenanthroline residue up to N-atom of the
macrocyclic part. Due to the structure, it is possible to monitor
the interaction of the metal with heterocyclic or macrocyclic parts
Table 2
Quantum yields of fluorescence of ligands 1, 2 and their complexes with Zn2+ and Ca2+ cations, MeCN.
Compound
1
1ꢀCa2+
0.17
13ꢀZn2+
0.09
2
2ꢀCa2+
0.002
23ꢀZn2+
0.6 ꢁ 10ꢂ3
22ꢀZn2+ꢀ(Ca2+
0.68 ꢁ 10ꢂ3
)
2
Quantum yield, u
0.11
0.1

Y. Sotnikova et al. / Inorganica Chimica Acta 445 (2016) 103–109
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residue causes the bathochromic shift in absorption and fluores-
cence spectra. In opposite, the binding of metal ion by macrocyclic
moiety results in the hypsochromic spectral shift.
Molecular binuclear complexes of the metal–ligand–metal type
in their mixed nature are useful systems to study the basic aspects
of electron, energy, charge transfer reactions. The understanding of
the synthetic chemistry associated with the preparation of highly-
conjugated, polymetallic metal complexes and the availability of
spectroscopic and computational methods which permit detailed
assessments of electronic structure provide a growing capacity to
identify and rationalize the intricacies of conductance measure-
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Acknowledgments
A.P. thanks RFBR grants 15-03-04695 (NMR study of ligands
and complexes), A.A. thanks RFBR-CNRS 14-03-93105 (synthesis
and optical study of ligands and complexes), V.N. thanks RSF grant
14-13-00724 (diffusion studies).
Appendix A. Supplementary material
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the online version, at http://dx.doi.org/10.1016/j.ica.2016.02.029.
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