The synthesis, spectroscopic properties and X-ray structure of Zn(II) complexes with amino derivatives of chromone

Article abstract of DOI:10.1016/j.poly.2011.01.033

Three new Zn(II) complexes containing the ligands 5-amino-8-methyl-4H- chromen-4-one (1), 6- or 7-amino-2-phenyl-4H-chromen-4-one (2, 3) were prepared. The new synthesised compounds were characterised by IR, ¹H NMR and MS spectroscopy. The crystal structure of complex 4 was determined with the use X-ray diffraction. The Zn(II) centre of 4 is linked by two chlorido and two N-bound aminochromone ligands, 1, in a strongly distorted tetrahedral configuration with the dissymetric point group C₂. The protonation constants of the ligands 1, 2 and 3 corresponded to 3.68, 3.88 and 6.83, respectively. The stability constants of the Zn(II) complexes were calculated from the potentiometric titration data. The complexes were found to have the formulae ML and ML₂ for ligands 1 and 2, and ML for ligand 3. Fluorescence spectroscopic properties were also studied; the strongest fluorescence in solution was exhibited by complex 6.

Full text of DOI:10.1016/j.poly.2011.01.033

Polyhedron 30 (2011) 1177–1184
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
The synthesis, spectroscopic properties and X-ray structure of Zn(II) complexes
with amino derivatives of chromone
Bogumiła Kupcewicz a, Magdalena Grazul b, Ingo-Peter Lorenz c, Peter Mayer c, Elzbieta Budzisz a,b,
⇑
a Department of Inorganic and Analytical Chemistry, Faculty of Pharmacy, Collegium Medicum in Bydgoszcz, UMK Torun, Sklodowskiej-Curie St. 9, 85-094 Bydgoszcz, Poland
b Department of Cosmetic Raw Materials Chemistry, Faculty of Pharmacy, Medical University of Lodz, Muszynskiego St. 1, 90-151 Lodz, Poland
c Department of Chemistry, Ludwig-Maximilians-University, Butenandtstr. 5-13 (D), D-81377 Munich, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 November 2010
Accepted 20 January 2011
Three new Zn(II) complexes containing the ligands 5-amino-8-methyl-4H-chromen-4-one (1), 6- or
7-amino-2-phenyl-4H-chromen-4-one (2, 3) were prepared. The new synthesised compounds were char-
acterised by IR, 1H NMR and MS spectroscopy. The crystal structure of complex 4 was determined with
the use X-ray diffraction. The Zn(II) centre of 4 is linked by two chlorido and two N-bound aminochro-
mone ligands, 1, in a strongly distorted tetrahedral configuration with the dissymetric point group C2.
The protonation constants of the ligands 1, 2 and 3 corresponded to 3.68, 3.88 and 6.83, respectively.
The stability constants of the Zn(II) complexes were calculated from the potentiometric titration data.
The complexes were found to have the formulae ML and ML2 for ligands 1 and 2, and ML for ligand 3.
Fluorescence spectroscopic properties were also studied; the strongest fluorescence in solution was
exhibited by complex 6.
Keywords:
Synthesis of Zn(II) complexes
Physico-chemical properties
Chromones, Flavones
Stability constant
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
for the growth and development in all forms of life and has bene-
ficial therapeutic and preventative effects on infectious diseases.
For a long time, we have been synthesising and investigating
the X-ray structures, cytotoxic effect and lipophilicity of metal
ion complexes [1–4]. Special effort has been focused on chromone
Zn(II) ions play an important role as radioprotective agents, tu-
mour photosensitizers and antidiabetic insulin-mimetic agents
[18]. The coordination number and geometry of the compounds
are therefore dependant on the size and charge of the ligand. In
many complexes the Zn(II) cation prefers a tetrahedral coordina-
tion sphere [19]. Fluorescent imaging has been proven to be the
most suitable technique for Zn(II) in vivo monitoring [20].
In the present work we have conducted the synthesis of three
Zn(II) complexes with chromone or flavone derivatives, and char-
acterised them using IR, 1H NMR and MS spectroscopy. The X-ray
crystal structure of one complex was determined. The coordination
tendencies of the ligands towards the Zn(II) ions in solution were
studied with potentiometric and spectrophotometric methods.
We have been exploring the influence of the position of the elec-
tron donating group NH2 in the molecule on the coordination
behaviour of the ligands towards the Zn(II) centre. Moreover the
ability of the ligands to enhance fluorescence through complexa-
tion may give the opportunity for the photochemical applications
of these complexes.
(benzo-c-pyrone) derivatives as ligands. Chromone and its deriva-
tive flavone (2-phenyl-4H-chromen-2-one) are of great interest
due to their chemical properties in particular, with their reactivity
towards nucleophilic reagents containing nitrogen, oxygen, sul-
phur and carbon [5]. Their pharmacological activity [6], such as
anticancer [7–9] antioxidant [10], antimitotic [11], antiinflamatory
[12] and many others [13,14] makes both molecules attractive for
further backbone derivatisation and screening as novel therapeutic
agents. Most of flavonoids are good metal chelators which can che-
late many metal ions to form different complexes due to their con-
jugated system. It has been extensively reported that the
interaction of some metals with flavonoids contributes to their
antioxidant activity [15]. In recent years, metal ion complexes have
played a very important role in medicine, pharmacy and diagnos-
tics [16]. Zn(II) is a one of the most important metal ions in the
human body. This ion is found at the active sites of over 200 types
of enzymes, most of which catalyse the hydrolysis of esters (car-
boxylic or phosphate) and amides [17]. Moreover, Zn(II) is essential
2. Experimental
2.1. Materials and methods
⇑
Corresponding author at: Department of Cosmetic Raw Materials Chemistry,
Faculty of Pharmacy, Medical University of Lodz, Muszynskiego St. 1, 90-151 Lodz,
Poland. Tel.: +48 42 677 91 25; fax: +48 42 678 83 98.
All substances were used without further purification. Chloro-
form-d and DMSO-d6 solvents for NMR spectroscopy were
E-mail address: elzbieta.budzisz@umed.lodz.pl (E. Budzisz).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.01.033

1178
B. Kupcewicz et al. / Polyhedron 30 (2011) 1177–1184
obtained from Dr. Glaser AG Basel. Solvents for the synthesis (ace-
tonitrile, diethyl ether) were reagent grade or better and were
dried according to standard protocols [21]. The melting points
were determined using an Electrothermal Buechi540 apparatus
and they are uncorrected. The IR spectra were recorded on a Pye-
Unicam 200G spectrophotometer in KBr discs. The 1H NMR spectra
were registered at 300 MHz on a Varian Mercury spectrometer. The
MS-FAB data were determined on Finnigan Matt 95 mass spec-
trometer (NBA, Cs+ gun operating at 13 keV). For the new com-
pounds satisfactory elemental analyses (0.4% of the calculated
values) were obtained using a Perkin–Elmer PE 2400 CHNS ana-
lyser. 5-Amino-8-methyl-4H-chromen-4-one [22] was prepared
as described in [23].
with diethyl ether and dried in air. Yield: 93 mg (37%), mp:
248.7–250.0 °C. FT IR (KBr cmꢀ1):
(C–NH2) 3345, 3228; (C„N)
2200; (C@O) 1629; (C@C) 1607, 1580, 1517;
(M–N) 485. 1H
m
m
m
m
m
NMR (300 MHz, DMSO-d6, 25 °C) d (ppm): 2.07 (s, 3H, CH3CN),
3
6.33 (s, 2H, NH2), 6.62 (s, 1HB), 6.68 (d, 1HD, JHH = 8.73 Hz),
3
3
6.675, 6.682 (dd, 1H5’, JHH = 8.73 Hz, JHH = 8.53 Hz), 7.52, 7.545
3
3
3
0
0
(dd, 1H3 , JHH = 5.55 Hz, JHH = 3.76 Hz), 7.556 (d, 1H2 , JHH
=
5.33 Hz), 7.58 (d, 1H6 , JHH = 3.76 Hz), 7.69 (d, 1HC, 3JHH = 8.73 Hz),
3
0
3
3
0
7.998 (s, 1HF), 8.00, 8.011 (dd, 1H4 , JHH = 7.73 Hz, JHH = 5.36 Hz).
MS-FAB (m/z): 612 (1%, M++2H), 577.8 [ZnL2Cl+], 371.3 (ZnLCl2),
366.2 (ZnLCl+), 238.2 (100%, L). Anal. Calc. for C30H22N2O4-
ZnCl2ꢁCH3CN (651.77 g/mol) for 6: C, 58.96; H, 3.87; N, 6.44.
Found: C, 58.75; H, 3.61; N, 6.12%.
2.2. Synthesis of bis(5-amino-8-methyl-4H-chromen-4-one)-N5,N05-
dichlorido-zinc(II) (Zn(L1)2Cl2) (4)
2.5. Potentiometric studies
Potentiometric measurements were carried out in 1,4-dioxane/
water (10:90) at 25 °C in a constant-temperature circulating water
jacketed titration cell. Titrations were performed with an Alpha Ti-
tro-plus (Schott) equipped with a microcombination glass elec-
trode and an Ag/AgCl reference electrode (BluLine 16pH, Schott).
The electrode was calibrated in terms of [H+] by titrating HNO3
solutions with standard NaOH solutions and the pKw value
(13.77) was calculated by means of Gran’s method. Carbonate-free
standard NaOH solution (0.01 mol dm3) was used as the titrant.
The total concentration of each ligand ranged from 2.20 ꢂ 10ꢀ3
to 2.60 ꢂ 10ꢀ3 mol dmꢀ3. All the experiments were performed at
constant ionic strength (0.1 mol dmꢀ3 NaClO4). Each titration was
repeated (80–100 data points per titration) four times and the data
were processed with the use of the HYPERQUAD2008 [24] program to
compute the protonation constants. The distribution diagrams
were plotted by the program HYSS 2006.
The stability constant (bpqr) of a complex MpLqHr refers to the
formation of that complex from the participating species, i.e. the
reaction: pM + qL + rH = MpHqLr (charges are omitted for simplic-
ity). The indexes p, q and r are stoichiometric numbers; p and q
are positive or zero value. R is a positive integer for protonated spe-
cies, zero for neutral species and negative for hydroxo or deproto-
nated species.
A solution of ZnCl2 (0.021 g, 0.15 mmol) in acetonitrile (10 ml)
was added dropwise to a solution of the ligand 1 (0.052 g,
0.03 mmol) in acetonitrile (15 ml). The reaction mixture was stirred
at room temperature for 24 h. The solvent was removed under re-
duced pressure at room temperature to one-half of the initial vol-
ume. After 24 h, the precipitated yellow solid was filtered off,
washed with diethyl ether and dried in the air. Yellow crystals of
4 suitable for X-ray diffraction were obtained after a few days by
recrystallising the precipitated product from an acetonitrile solu-
tion. Yield: 41 mg (56.24%), mp: 191.5–192.8 °C. FT IR (KBr cmꢀ1):
m
(C–NH2) 3175, 3078;
m(C@O) 1641; m(C@C) 1621, 1553, 1487;
m
(M–N) 482. 1H NMR (300 MHz, DMSO-d6, 25 °C) d (ppm): 2.17 (s,
3H, CH3), 6.11 (d, 1HB, 3JHH = 5.77 Hz), 6.44 (d, 1HD, 3JHH = 8.33 Hz),
3
3
7.2 (d, 1HE, JHH = 8.33 Hz), 7.21 (s, 2H, NH2), 8.09 (d, 1HA, JHH
=
5.75 Hz). MS-FAB (m/z): 452 (ZnL2Cl+), 276 (ZnLCl+), 176 (100%, L).
Anal. Calc. for C20H18N2O4ZnCl2ꢁ0.5H20 (495.66 g/mol) for 4: C,
48.46; H, 3.86; N, 5.65. Found: C, 48.40; H, 3.39; N, 5.62%.
2.3. Synthesis of acetonitrile-bis(6-amino-2-phenyl-4H-chromen-4-
one)-N6,N06-dichlorido-zinc(II) (Zn(L2)2Cl2) (5)
Zn(II) chloride ZnCl2 (56.2 mg, 0.41 mmol) was dissolved in
10 ml acetonitrile and then added dropwise to a stirred solution
of the ligand 6-amino-2-phenyl-4H-chromen-4-one (2) (195.69
mg, 0.82 mmol) in acetonitrile (20 ml) at room temperature. The
mixture was stirred for 10 min and a yellow precipitate was ob-
tained. The solid was filtered off, washed with diethyl ether and
dried in vacuo and over P2O5. Yield: 176.4 mg (70.44%), mp:
2.6. Spectroscopic measurements
Fluorescence measurements were performed on a Shimadzu
RF5301 spectrofluorimeter equipped with a 150 W Xenon lamp
and 10 mm quartz cells. The absorption spectra were recorded on
Shimadzu UV 2450 double-beam spectrophotometer equipped
with 10 mm quartz cells at 25 °C and an ionic strength of
0.1 mol dmꢀ3 (NaClO4).
263.9–265.1 °C. FT IR (KBr cmꢀ1) (selected bands):
m(C–NH2)
3281, 3117;
m(C@O) 1627; m(C@C) 1612, 1573, 1487; m(M–N)
481. 1H NMR (300 MHz, DMSO-d6, 25 °C) d (ppm): 5.52 (s, 2H,
3
0
NH2), 6.87 (s, 1HB), 7.07 (d, 1HE, JHH = 8.73 Hz), 7.086 (d, 1H6 ,
3
3
3
0
JHH = 14.88 Hz), 7.493, 7.53 (dd, 1H5 , JHH = 8.73 Hz, JHH
=
2.7. X-ray structure determination of 4
3
0
13.09 Hz), 7.534 (d, 1HF, JHH = 8.93 Hz), 7.55, 7.57 (dd, 1H3 ,
3JHH = 1.78 Hz, JHH = 1.98 Hz), 8.039 (s, 1HC), 8.052, 8.041 (dd,
3
The data were collected at 200 K by means of an Oxford Diffrac-
3
3
0
1H4 , JHH = 5.36 Hz, JHH = 7.74 Hz). MS-FAB (m/z): 612 (5%,
M++2H), 577.6 (ZnL2Cl+), 304 (ZnL2+), 238.2 (100%, L). Anal. Calc.
for C30H22N2O4ZnCl2 (610.72 g/mol) for 5: C, 58.99; H, 3.63; N,
4.58. Found: C, 58.57; H, 3.49; N, 4.71%.
tion XCalibur diffractometer with graphite-monochromated Mo K
a
radiation. The data have been corrected by a numerical absorption
correction with XRED [25]. The structure has been solved with the
direct method using SIR97 [26]. All non-hydrogen atoms were re-
fined anisotropically, and the hydrogen atoms were generated in
idealised positions, riding on their parent atoms [27].
2.4. Synthesis of acetonitrile-bis(7-amino-2-phenyl-4H-chromen-4-
one)-N7,N07-dichlorido-zinc(II) (Zn(L3)2Cl2CH3CN) (6)
To a solution of the ligand 3 (0.176 g, 0.74 mmol) in acetonitrile
(20 ml) was added dropwise a solution of ZnCl2 (0.51 g, 0.37 mmol)
in acetonitrile (10 ml). The reaction mixture was stirred at room
temperature for 24 h. The solvent was removed under reduced
pressure at room temperature to one-half of the initial volume.
After 3 h, a yellow solid precipitated and was filtered off, washed
3. Results and discussion
3.1. Chemistry
The complexes 4–6 were prepared by mixing ZnCl2 with 5-ami-
no-8-methyl-4H-chromen-4-one (1), 6-amino-2-phenyl-4H-chro-

B. Kupcewicz et al. / Polyhedron 30 (2011) 1177–1184
1179
CH3
NH2
O
O
O
O
O
O
NH2
NH2
3
1
2
ZnCl2 / AcCN
O
CH3
O
O
O
O
O
x AcCN
NH2
Zn
NH2
Zn
O
Ph
O
O
NH2
NH2
NH2
Zn
O
O
Cl
Cl
Cl
Cl
CH3
NH2
Cl
6
O
4
Cl
5
Scheme 1. Synthesis of the Zn(II) complexes with aminochromone derivatives.
men-4-one (2) or 7-amino-2-phenyl-4H-chromen-4-one (3) in a
1:2 (M:L) molar ratio. The reactions were carried out in acetonitrile
solution. Three new Zn(II) complexes 4–6 (see Scheme 1) of the
aminochromones derivatives 1–3 have been obtained in good
yield.
Table 2
IR data of the ligands and their complexes.
Compound
m(NH2)
m
(C@O)
maromat
m(M–N)
L1
4
L2
5
L3
6
3432, 3324
3175, 3078
3342, 3238
3281, 3117
3460, 3293
3345, 3228
1648
1641
1642
1627
1630
1629
1625, 1588, 1569
1621, 1553, 1487
1581, 1565, 1487
1612, 1573, 1487
1600, 1586, 1498
1607, 1580, 1517
–
482
–
481
–
485
3.2. MS-FAB
All the compounds have been investigated by positive/negative
mass spectrometric measurements, giving valuable structural
information. The major structurally informative molecular ions
and fragment ions in the positive-ion mode of compounds 4–6
are presented in Table 1. The molecular peaks relating to com-
plexes 5 and 6 were observed at 612 [M+2H]+ in the FAB-MS spec-
tra. For complex 4, we have not observed a molecular peak, but a
strong peak at m/z 452 was attributed to [MꢀCl]+. The FAB mass
spectrum of 6, recorded in the positive ion mode, contains a peak
at m/z 654 due to the [M+2H]+ ion and at m/z 577.8 due to MCl+.
In addition, a predominant peak corresponding to the ligands 2
and 3 at m/z 238.2 [100%, L] was observed.
were observed in comparison with those of ligands 1–3. The IR
spectra of the ligands 1, 2 and 3 (see Table 2) show prominent
stretching vibrations at 3432, 3342 and 3345 cmꢀ1 respectively,
which were assigned to the asymmetric
m(NH2) and at 3324,
3238 and 3228 cmꢀ1 which were assigned to the symmetric
m
(NH2). These bands shift to lower energies (by 257 and 246
cmꢀ1 for compound 4; 61 and 121 cmꢀ1 for compound 5; 52 and
41 cmꢀ1 for compound 6, respectively), showing that coordination
of the amino group has occurred. The strong m(C@O) bands, occur-
ring at 1641, 1627 and 1629 cmꢀ1 for 4, 5 and 6 respectively, are
shifted only slightly compared to the free ligands (1648, 1642
and 1630 cmꢀ1). Several absorptions in the 1588–1411 cmꢀ1 re-
gion can be assigned to aromatic vibrations. Ligands coordination
The data presented above are in good agreement with the re-
sults obtained from the elemental analysis and therefore confirm
the proposed structure of investigated complexes by X-ray
spectroscopy.
to the metal centre is substantiated by bands appearing for m(M–
N) at 482, 481 and 485 cmꢀ1 for 4–6, respectively [28].
3.3. Infrared spectra
3.4. 1H NMR spectra
Some characteristic IR vibrations for the complexes are given in
Table 2. Significant differences in the IR spectra of complexes 4–6
The 1H NMR spectra of the complexes and their ligands were re-
corded after dissolution in d6-DMSO, with TMS as an internal stan-
dard. Careful analysis of the 1H NMR spectral data is summarised in
Table 3. This data has been used to get information regarding the
coordination mode of the ligands with the metal ion for complexes
4–6. For complexes 4–6, the resonances of the amino group pro-
tons at positions 5, 6 and 7 were found at 7.23, 5.52 and
6.33 ppm respectively and they were shifted by 0.02, 001 and
0.03 ppm from their positions in the free ligands due to the coordi-
Table 1
MS data (m/z) of the complexes 4–6.
Compound Molecular
ions
Fragment ions
4
5
6
–
612
612
452 (ZnL2Cl+), 276 (ZnLCl+), 176 (100%, L)
577.6 (ZnL2Cl+), 304 (ZnL2+), 238.2 (100%, L)
577.8 [ZnL2Cl+], 371.3 [ZnLCl2], 366.2 [ZnLCl+],
238.2 [100%, L]

1180
B. Kupcewicz et al. / Polyhedron 30 (2011) 1177–1184
Table 3
1H NMR data of the ligands and their Zn(II) complexes
3'
3'
2'
2'
4'
5'
4'
5'
H
H
H
H
F
CH3
F
H2N
O
O
H
O
O
H
H
O
O
H
A
E
E
6'
6'
H
D
H
B
H2N
H
H
B
B
D
C
NH2
C
1
2
3
.
HA
HB
HC
HD
HE
HF
NH2
3
3
3
3
L1
4
L2
5
L3
6
8.08, JHH = 5.75
6.12, JHH = 5.77
–
–
6.44, JHH = 8.33
7.20, JHH = 8.33
–
–
7.21
7.23
5.53
5.52
6.36
6.33
3
3
3
3
8.09, JHH = 5.75
6.11, JHH = 5.77
6.44, JHH = 8.33
7.21, JHH = 8.33
3
3
–
–
–
–
6.87
6.87
6.62
6.62
8.04
8.039
7.70, JHH = 8.53
–
–
7.20, JHH = 8.73
7.53, JHH = 8.93
7.54, JHH = 8.93
8.01
7.998
3
3
7.20, JHH = 8.73
3
3
6.68, JHH = 8.73
–
–
3
3
7.69, JHH = 8.73
6.68, JHH = 8.73
nation of the amino nitrogen atom with the metal centre. As is seen
in Fig. 1, the 1H NMR spectra of ligands 1, 2 and 3 are almost indis-
tinguishable from those of complexes 4–6. The aromatic protons in
all the spectra are observed around 6.11–8.08 ppm as singlets or
doublets.
Zn(II)-ligand 3 systems, precipitation occurred above pH 5.5, so
only the stability constant for M:L = 1:1 could be calculated.
Many authors have found positive linear correlations between
the ligand basicity and the stability constants of complexes
[30,31], but high pKa values do not always ensure more stable com-
plexes [32]. In Fig. 3, the first stepwise stability constants for the
Zn(II) complexes 4–6 are represented as a negative linear function
of the basicity of ligands. This linear relationship is exclusively
obeyed by similar ligands with the same electron-donor atom rel-
ative to both the metal ion and the proton [30].
3.5. pH-metric and spectrophotometric measurements
The protonation constants of the ligands, which may be consid-
ered as monoprotic bases, were determined by potentiometric and
spectrophotometric methods. The proton dissociation constants of
the ligands 1, 2 and 3 (Table 4) corresponded to 3.68, 3.88 and 6.83,
respectively and were attributed to the deprotonation of the pro-
tonated amino group. The basicities of the ligands 1 and 2 are three
orders of magnitude smaller than the basicity of ligand 3 due to the
electron withdrawing effect of the C@O group. The basicity of ami-
no derivatives of chromone depends on the electron density of the
nitrogen atom. In the ligands strong intramolecular electron do-
nor–acceptor interactions are possible. The most important inter-
action is hyperconjugation related to electron donation from the
N atom of the NH2 group to the anti-bonding acceptor r⁄(C–C) of
the phenyl ring. Intramolecular hyperconjugative interactions are
The influence other than ligand basicity factors on the ⁄stability
of the complexes could be illustrated by the values of log b1 (Eqs.
(1)–(3)).
2þ
½ZnðH2OÞ ꢃ þ HLþ $ ½ZnðH2OÞ Lꢃ2þ þ H3Oþ
ð1Þ
ð2Þ
ð3Þ
6
5
2þ
½ZnðH2OÞ Lꢃ ½H3Oþꢃ
bꢄ ¼
5
2þ
½ZnðH2OÞ ꢃ ½HLþꢃ
6
logꢄb ¼ log b ꢀ pKa
1
1
Similar to log b1, the log ⁄b1 values significant decrease with
increasing basicity of the ligands. The negative slope of the dashed
line (Fig. 3) is close unity (ꢀ1.15), which means that the stability of
the complexes is weakly affected by a contribution of
ing to the M–L bonds. The log b1 values for chromone derivatives
p
M ? L bond-
⁄
formed by the orbital overlap between
p
(C–C) and p⁄(C–C) bond
orbitals. Intramolecular charge transfer (ICT) resulting from hyper-
conjugation causes stabilization of the molecule. For 6-aminoflav-
one (2), a strong interaction between the ring oxygen atom and the
neighbouring r⁄(C–C) orbitals has been observed [29]. A stronger
hyperconjugative interaction could be expected to occur in the
7-aminoflavone molecule.
The pKa values from the potentiometric study for ligands 1 and
2 were in good agreement with those obtained from the spectro-
photometric titration. Because the ionizable –NH2 group was
remoted from the chromophore, the change in absorbance of
7-amino-2-phenyl-4H-chromen-4-one (ligand 3) was very low,
thus the protonation constant computation with the use spectro-
photometric titration data was not possible.
decrease in the order: 1 > 2 >> 3. This can be caused by a reduction
of the
p-acceptor properties of the ligands resulting from an in-
crease in electron density on the donor N-atom of the NH2 group
[30].
The spectrophotometric titration was carried out under the
same conditions as the potentiometric titration, except for the con-
centration of the ligands (10 times lower in the spectrophotomet-
ric measurements). Representative species-distribution diagrams
for the Zn(II)-1, Zn(II)-2 and Zn(II)-3 systems are shown in Fig. 4.
From Fig. 4 it can be seen that the protonated form (LH) of the li-
gands 1 and 2 exist in the acidic medium at pH <2 and under pH
4 for ligand 3. The complexation process for both 5-amino-8-
methyl-4H-chromen-4-one and 6-amino-2-phenyl-4H-chromen-
4-one began at low pH due to the formation of ZnL and ZnL2 spe-
cies. The complex ML2 was a predominant species over pH 4.5–5.0.
The spectrophotometric titration of the ligands 1 and 2 is shown
in Figs. 5 and 6, respectively. The spectra were recorded in the re-
gion 220–500 nm and between pH 2 and 10. The absorption spec-
tra of the ligand 1 with the pH <5.0 have two characteristic peaks,
with the maxima at 300 and 370 nm. The absorption band at
The equilibrium constant values for the complexes formed be-
tween Zn(II) and the ligands were studied in the 1:1 and 1:2 me-
tal–ligand systems (Fig.
2 for ligand 1 and its complexes).
Analyses of the potentiometric curves with the use of the program
HYPERQUAD allowed the determination of Zn(II) complex species in
equilibrium and their formation constants. The best fit models in-
cluded two complexes (ML1 and ML2) for ligands 1 and 2. For the

B. Kupcewicz et al. / Polyhedron 30 (2011) 1177–1184
1181
Table 4
The values of the overall protonation constants (pKa) of the ligands and the stability
constants (log b) of the Zn(II) complexes from potentiometric titrations.
Ligand
pKa (HL)
Log b1 (ML)
Log b2 (ML2)
Log ⁄b1
1
2
3
3.68(7)
3.88(6)
6.83(8)
6.83(3)
6.75(8)
3.26(6)
9.71(10)
9.47(12)
–
3.15
2.87
ꢀ3.57
Values in parentheses are the standard deviations in the last significant digit.
Fig. 2. Potentiometric titration curves of ligand 1 in the absence and presence of
Zn(II) ions.
Fig. 3. Stability constants log b1 (solid line) and log ⁄b1 (dashed line) of the Zn(II)
complexes as function of the pKa values of the ligands.
The UV–Vis spectra of ligand 2 changed significantly in pH
range 2.8–5.0, and over pH 5.0 no change was observed. The
unprotonated form of ligand 2 has three bands, with the maxima
at 280, 315 and 370 nm. During the protonation process (decreas-
ing pH) the band at 280 nm showed a bathochromic shift to
300 nm coupled with decreasing absorbance.
3.6. Fluorescensce spectra
The fluorescence properties of ligands 1–3 and their complexes
with Zn(II) (4–6) were measured in the polar solvents 10% 1,4-
dioxane in water and methanol at room temperature. Ligand 1
and complex 4 did not show any fluorescence features, whereas
6-aminoflavone (2), 7-aminoflavone (3) and their complexes with
Zn(II) ions exhibited fluorescence in both solvents. Both complexes
5 and 6 exhibited stronger fluorescence than their respective li-
gands. For the Zn(II) complexes, no emissions originating from me-
tal-centred excited states were expected, because the Zn(II) ion is
difficult to oxidise or reduce due to its stable d10 configuration
[33,34]. Thus excited states after metal ion complexation are typi-
cally ligand centred in nature. In the absence of any Zn(II) ions
Fig. 1. 1H NMR spectra of the ligands and their Zn(II) complexes.
300 nm was associated with the protonated form of the ligand and
this peak gradually decreased during the titration and disappeared
at pH >5.0. In the range pH 5.0–8.1 no changes were observed in
the absorption spectra.

1182
B. Kupcewicz et al. / Polyhedron 30 (2011) 1177–1184
100
80
60
40
20
0
100
100
80
60
40
20
0
c
a
b
LH
80
60
L
ZnL
L
2
LH
ZnL
2
40
20
0
ZnL
ZnL
L
ZnL
LH
2
4
6
8
3
5
pH
7
2
4
6
8
pH
pH
Fig. 4. Species distribution curves for a 1:2 metal to ligand molar ratio of (a) Zn-1, (b) Zn-2 and (c) Zn-3 systems at 25 °C and I = 0.1 mol dmꢀ3 in 10% 1,4-dioxane in water.
Fig. 7. The excitation and emission spectra of 7-amino-2-phenyl-4H-chromen-
4-one (3) (solid line) and its complex (dashed line) with Zn(II) ions (6) in 10% 1,4-
Fig. 5. Absorption spectra for 5-amino-8-methyl-4H-chromen-4-one at various pH
(2.844–8.172) conditions, c = 2.10 ꢂ 10ꢀ4 mol dmꢀ3 in 10% 1,4-dioxane in water,
dioxane in water; [L] = 8.688ꢂ10ꢀ3 mol dmꢀ3, [ML2Cl2] = 6.55 ꢂ 10ꢀ4 mol dmꢀ3
.
I = 0.1 mol dmꢀ3
.
nation of the ligand with the zinc atom caused a significant
enhancement of the fluorescence. As can be seen in Fig. 6, the com-
plex with Zn(II) showed the same level of fluorescence with a 10
times lower concentration than that of the free ligand. The excita-
tion and emission wavelengths were the same for compounds 3
and 6 in methanol solution, and the fluorescence enhancement
was comparable to that in the dioxane-water solution.
The fluorescence spectra of ligand 2 (6-aminoflavone) and the
complex with Zn(II) (5) were recorded in 10% dioxane in water
solution and in methanol, on excitation at 445 and 438 nm for
the ligand and complex, respectively. A blue shift (7 nm) of the
excitation band was observed, but the emission band maximum
was at the same wavelength (555 nm) for both the ligand and
the complex.
6-Aminoflavone (2) emitted fluorescence at a wavelength red-
shifted by about 50 nm in relation to 7-aminoflavone (3). This sug-
gests that the position of the amine group in the ligand molecules
strongly affects their fluorescence properties. As in coumarins [35]
and quinolinones [36], a donor substituent (e.g. methoxy, NH2) in
position 6 has a more pronounced bathochromic effect than that
in position 7.
Fig. 6. Absorption spectra of 6-amino-2-phenyl-4H-chromen-4-one at various pH
(2.756–10.079) conditions, c = 2.62 ꢂ 10ꢀ4 mol dmꢀ3 in 10% 1,4-dioxane in water,
I = 0.1 mol dmꢀ3
.
photo-induced electron transfer (PET) due to the presence of a lone
pair of electrons on the donor atom may cause quenching of the li-
gand fluorescence [20]. The chelation of the ligand to Zn(II) pre-
vents the PET process and increases the rigidity of the ligand,
and hence increases the fluorescence intensity.
The excitation and emission spectra of ligand 3 (7-aminoflav-
one) and complex 6 in dioxane-water solution are shown in
Fig. 7. The absorption band of ligand 3 at 418 nm shifted to
410 nm for complex 6 (a blue-shift of 10 nm). Both compounds
emitted fluorescence at the same wavelength, 505 nm. The coordi-
3.7. X-ray structure analysis of 4
The molecular structure of complex 4 was determined by X-ray
structure analysis. The corresponding details are summarised in
Table 5 and selected geometrical parameters are gathered in Table
6. As shown in Fig. 8, the zinc centre of 4 is linked by two chlorido
and two N-bound aminochromone ligands (1) in a strongly dis-
torted tetrahedral configuration, with the dissymmetric point
group C2. Thus, the bond angle Cl–Zn–Cla of 117.17(3)° is much lar-

B. Kupcewicz et al. / Polyhedron 30 (2011) 1177–1184
1183
Table 5
Crystallographic data of Zn(II) complex 4.
Net formula
C20H18Cl2N2O4Zn
486.66
Mr (g molꢀ1
)
Crystal size (mm)
T (K)
0.35 ꢂ 0.12 ꢂ 0.12
200(2)
Radiation
Diffractometer
Crystal system
Space group
a (Å)
Mo Ka
‘Oxford XCalibur’
monoclinic
C2/c
12.8390(7)
17.6268(10)
8.8660(6)
90
b (Å)
c (Å)
a
(°)
b (°)
102.184(6)
90
1961.3(2)
4
1.64816(17)
1.555
c
(°)
V (Å3)
Z
Dcalc (g cmꢀ3
)
l
(mmꢀ1
)
Absorption correction
Transmission factor range
Reflections measured
Rint
Numerical
0.6306–0.8903
5171
0.0359
Mean
r(I)/I
0.0747
h Range
3.99–26.37
1313
0.0284, 0
Constr
1989
133
Observed reflections
x, y (weighting scheme)
Hydrogen refinement
Reflections in refinement
Parameters
Fig. 8. An ORTEP drawing of the molecular structure of 4 with the atom numbering
and 30% probability ellipsoids. The non-labelled atoms of the C2-symmetric
molecule are generated by the symmetry operation x + 1, y, ꢀz + 3/2.
Restraints
0
mely from each other. The bond lengths Zn–Cl (2.2382(6) Å) and
Zn–N (2.048(2) Å) are pairwise equal. The amino chromone
ligand is bent off the Zn–N axis by the angle Zn–N–C1 of
120.10(13)°. Whereas the benzoic ring shows relatively equal C–
C distances (1.389(3) Å), the quinoide ring exhibits two different
sets of values (C6–C7 and C7–C8 1.472(3) and 1.432(3) Å), both
contrary to the very short one of C8–C9 1.327(3) Å. The longest
C–C bond is given by the single bond C4–C10 (1.503(3) Å). In com-
parison to the exocyclic C7–O1 carbonyl bond (1.237(3) Å), the two
cyclic ones C5–O2 and C9–O2 (1.379(3) and 1.352(3) Å) are signif-
icantly longer. Both oxygen atoms O1 and O2 lie within the plane
of the chromone ligand, as the torsion angles indicate. One strong
intramolecular hydrogen bond (N–H11–O1) as well as two weaker
intermolecular ones (N–H12–Clb and C10–H10b–O1c) were found.
R(Fobs
)
0.0282
0.0618
0.914
0.001
0.381
ꢀ0.364
Rw(F2)
S
Shift/errormax
Maximum electron density (e Åꢀ3
)
)
Minimum electron density (e Åꢀ3
Table 6
Selected structural parameters of complex 4.
Bond lengths (Å)
Zn–Cl
Zn–N
O1–C7
O2–C5
O2–C9
N–C1
Bond angles (°)
Cl–Zn–N
2.2382(6)
2.048(2)
1.237(3)
1.379(3)
1.352(3)
1.445(3)
0.92
115.20(5)
Cl–Zn–Cla
Cla–Zn–N
N–Zn–Na
Cla–Zn–Na
Zn–N–C1
Zn–N–H
117.17(3)
100.00(5)
109.80(8)
115.20(5)
120.10(13)
107
4. Conclusions
N–H
C1–C2
C1–C6
C2–C3
C3–C4
C4–C10
C4–C5
C5–C6
C6–C7
C7–C8
C8–C9
1.378(3)
1.403(3)
1.381(3)
1.375(3)
1.503(3)
1.394(3)
1.404(3)
1.472(3)
1.432(3)
1.327(3)
C1–N–N
107
In this paper we have conducted the synthesis and investigated
the spectroscopic properties and X-ray structure of Zn(II) com-
plexes. The structure of the complexes was confirmed by spectral
and elemental analyses. All the ligands created neutral complexes
of the general type ML2Cl2.
The results of potentiometric studies indicate that both the ML
and ML2 Zn(II) complexes with ligands 1 and 2 exhibited similar
stability in polar (10% 1,4-dioxane in water) solution. Ligand 3 only
formed one complex (ML) which had a lower stability than the
respective complexes of ligands 1 and 2. The formation of a precip-
itate above pH 5.5 prevented the formation of the ML2 complex.
The fluorescence properties of ligands 1–3 and their Zn(II) com-
plexes 4–6 were measured in polar solvents at room temperature.
The most significant fluorescence enhancement after Zn(II) com-
plexation was exhibited by 7-aminoflavone (complex 6).
O2–C5–C4
O2–C9–C8
C5–O2–C9
C6–C7–C8
O1–C7–C6
O1–C7–C8
114.8(2)
124.2(2)
118.3(2)
114.5(2)
123.1(2)
122.4(2)
Torsion angles (°)
Zn–N–C1–C2
Zn–N–C1–C6
N–C1–C6–C5
N–C1–C6–C7
ꢀ69.7(2)
110.2(2)
ꢀ179.2(2)
1.0(3)
C2–C1–C6–C7
C2–C1–C6–C5
C9–O2–C5–C4
C9–O2–C5–C6
ꢀ179.2(2)
0.6(3)
ꢀ179.0(2)
0.2(3)
Hydrogen bonds (Å, °)
D–Hꢁ ꢁ ꢁA
D–H
Hꢁ ꢁ ꢁA
1.94
2.36
2.55
Dꢁ ꢁ ꢁA
D–Hꢁ ꢁ ꢁA
141
172
N–H11ꢁ ꢁ ꢁO1
N–H12ꢁ ꢁ ꢁClb
C10–H10bꢁ ꢁ ꢁ01c
0.92
0.92
0.98
2.717(3)
3.272(2)
3.496(3)
162
Acknowledgements
Symmetry code: (a) 1 ꢀ x, y, 3/2 ꢀ z, (b) 1 ꢀ x, 1 ꢀ y, 1 ꢀ z, (c) ½ + x, ½ ꢀ y, ꢀ½ + z.
´
Financial support from Medical University of Łódz (grant No.
503/3-066-02/503-01 to E. Budzisz), the European Social Fund
and the National Budget within the framework of Measure 2.6 of
the Integrated Regional Operational Program connected with the
ger that that of N–Zn–Na (109.80(8)°). Both the mixed angles, how-
ever, N–Zn–Cl (115.20(5)°) and Na–Zn–Cl (100.00(5)°) differ extre-

1184
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for complex 4. These data can be obtained free of charge via http://
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