Synthesis, spectral and electrochemical properties of Al(III) and Zn(II) complexes with flavonoids

Article abstract of DOI:10.1016/j.saa.2004.07.029

The synthesis, electrochemical and spectral (UV-vis, ¹H NMR, IR, fluorescence) properties as well as thermal behaviors of Al(III) and Zn(II) complexes with the flavonoids quercetin (H₂L¹), rutin (H₂L²) and galangin (HL³) are presented. The complexes may be formulated as [Al₂(L¹)(H ₂O)₈]Cl₄, [Al₃(L²) ₂(H₂O)₁₂]Cl₅, [Al(L ³)(H₂O)₄]Cl₂, [Zn₂(L ¹)(H₂O)₄]Cl₂, [Zn₃(L ²)₂(H₂O)₆]Cl₂ and [Zn(L³)(H₂O)₂]Cl. The higher fluorescence intensities of the complexes related to the free flavonoids, are attributed to the coordination of the ligands to the small, highly charged Al(III) and Zn(II) ions. The coordination effectively increases the rigidity of the ligand structure and increases the fluorescence quantum yield by reducing the probability of non-radiative energy dissipation process. Antioxidant activities of the compounds were also investigated under an electrochemical point of view. The cyclic voltammetric data show a considerable decrease of the oxidation potentials of the complexes related to that of the free flavonoids. Thus, the flavonoid-metal complexes are more effective antioxidants than the free flavonoids.

Full text of DOI:10.1016/j.saa.2004.07.029

Spectrochimica Acta Part A 61 (2005) 1985–1990
Synthesis, spectral and electrochemical properties of
Al(III) and Zn(II) complexes with flavonoids
Rubens F.V. de Souza, Wagner F. De Giovani∗
Departamento de Quimica, Faculdade de Filosofia Cieˆncias e Letras de Ribeira˜o Preto, Universidade de Sa˜o Paulo,
Av. Bandeirantes 3900, Ribeira˜o Preto 14040-901, SP, Brazil
Received 27 May 2004; accepted 27 July 2004
Abstract
1
The synthesis, electrochemical and spectral (UV–vis, H NMR, IR, fluorescence) properties as well as thermal behaviors of Al(III) and
Zn(II) complexes with the flavonoids quercetin (H2L1), rutin (H2L2) and galangin (HL3) are presented. The complexes may be formulated
as [Al2(L1)(H2O)8]Cl4, [Al3(L2)2(H2O)12]Cl5, [Al(L3)(H2O)4]Cl2, [Zn2(L1)(H2O)4]Cl2, [Zn3(L2)2(H2O)6]Cl2 and [Zn(L3)(H2O)2]Cl. The
higher fluorescence intensities of the complexes related to the free flavonoids, are attributed to the coordination of the ligands to the small,
highly charged Al(III) and Zn(II) ions. The coordination effectively increases the rigidity of the ligand structure and increases the fluorescence
quantum yield by reducing the probability of non-radiative energy dissipation process. Antioxidant activities of the compounds were also
investigated under an electrochemical point of view. The cyclic voltammetric data show a considerable decrease of the oxidation potentials
of the complexes related to that of the free flavonoids. Thus, the flavonoid–metal complexes are more effective antioxidants than the free
flavonoids.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Flavonoids; Al(III) and Zn(II) complexes; Spectral and electrochemical properties
1. Introduction
flavonoids. We have recently reported the synthesis and the
antioxidant properties of complexes of flavonoids with Cu(II)
and Fe(II) [11,12]. Due to the importance of metal chela-
tion in the antioxidant behavior of flavonoids and continuing
our interest in flavonoid–metal complexes, we report here the
synthesis and structural investigation of the Al(III) and Zn(II)
complexes with quercetin (H2L1), rutin (H2L2) and galan-
gin (HL3). Elemental, thermogravimetric analyses, atomic
Flavonoids are phenolic compounds widely distributed in
plants with potential beneficial effects for human health. The
basic structures (Fig. 1) consist of two aromatic rings (noted
A and B) linked through three carbons that usually form
an oxygenated heterocycle (C ring). In recent years, these
molecules have attracted the attention of many researchers
because flavonoids display a remarkable array of biologi-
cal and pharmacological activities as antioxidant, antiinflam-
matory, antimicrobial, anticancer, cardiovascular protection,
etc [1–4]. In particular, the antioxidant activity is basically
a function of their abilities to act as free radical scavenger
[5–7]. Furthermore, flavonoids are effective metal ions chela-
tors [8,9]. Metal chelation is usually considered as another
mechanism of the antioxidant activity of flavonoids [9,10].
The interaction of flavonoids with metal ions may change
the antioxidant properties and some biological effects of the
1
absorption, molar conductance, UV–vis, IR, H NMR, flu-
orescence spectroscopies and electrochemistry were used to
characterize the compounds.
2. Experimental
2.1. Materials
All reagents and solvents were analytical reagent grade.
AlCl3·6H2O, ZnCl2, quercetin, rutin and galangin were pur-
chased from Aldrich Chemical Co.
∗
Corresponding author. Tel.: +55 16 602 3810; fax: +55 16 633 8151.
E-mail address: wfdgiova@usp.br (W.F. De Giovani).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2004.07.029

1986
R.F.V. de Souza, W.F. De Giovani / Spectrochimica Acta Part A 61 (2005) 1985–1990
Electrolyses were performed at fixed applied potentials at 25
1 ◦C, using a glassy carbon plate working electrode.
2.3. Preparation of the metal complexes
Job’s method [13] was used to determinate, in methanol,
the stoichiometric ratios for the reactions between the
flavonoids and the metal ions. The solutions were prepared
by mixing solutions of both components with equal molar
concentrations (1.1 × 10−4 M) in ratios varying from 1:9 to
9:1. The complexes were prepared by mixing, in methanol,
adequate stoichiometric amounts of the flavonoids and the
corresponding salts. In all preparations, pale yellow precip-
itates were formed immediately when the cooled mixtures
were poured into H2O. The crystals were washed thrice with
1:3 EtOH/H2O mixtures and then several times with H2O and
dried under vacuum.
Fig. 1. Common structures of the flavone subclass and the division of bands
I and II related to the UV–vis absorption spectra.
2.2. Instruments
3. Results and discussion
Elemental analyses were performed using a CHNS-O CE
Instruments model EA 1110 elemental analyzer. Aluminum
and zinc were determined in a model V-85 B, Braun ICP-
AES atomic emission spectrometer. Molar conductances, in
MeOH, were determined in a DSS-11A Metrohm conduc-
tivity meter. 1H NMR spectra, in DMSO-d6, were obtained
on a RMN Bruker DR X 4009.4T spectrometer. IR spectra
were recorded on a Nicolet FTIR 5ZDX spectrometer using
KBr pellets. UV–vis spectra, in MeOH, were registered on
a Hewlett–Packard 8453 Diode Array UV–vis spectropho-
tometer using standard 1.00 cm quartz cells. The ε values
represent the average obtained from three different solutions.
The fluorescence spectra were recorded on a CD-900 Edin-
burgh spectrofluorimeter using a quartz cell (1 cm × 1 cm
cross-section). Thermogravimetric analyses were performed
by a TA Instruments SDT 2960-Simultaneous DTA-TGA-
Thermal Analysis, between room temperature and 800 ◦C at
a heating rate of 10 ◦C min−1. The cyclic voltammograms
were obtained on a 273A EG&G Potentiostat/Galvanostat,
using a one-compartment electrochemical cell equipped with
a glassy carbon-working electrode, a platinum wire auxiliary
electrode and a Ag/AgCl reference electrode, in MeOH (1
× 10−3 M solutions) with LiClO4 as supporting electrolyte.
The pale yellow complexes are soluble in MeOH, EtOH
and DMSO, slightly soluble in Me2CO, but scarcely soluble
in H2O and CCl4. Table 1 shows the analytical and molar
conductance data for the complexes. The molar conductance
values suggest that the chloride ions are outside the coordi-
nation sphere of the complexes. The analytical data are con-
sistent with the stoichiometries obtained previously by Job’s
method.
3.1. IR spectra
IR spectra of the ligands and the complexes present ev-
idence of coordination between the transition metal ions
and flavonoids. The data are summarized in Table 2. IR
spectra of the compounds were dominated by strong ab-
sorptions of the ligands in the carbonyl region. Specifi-
cally, in the free flavonoids these bands, ␯(C O), appear at
1658–1682 cm−1. For all quercetin and galangin complexes,
thebandswereshiftedtolowerfrequenciescomparedtothose
of free flavonoids. The ∼50 cm−1 displacements suggest that
the coordination of quercetin and galangin involves the oxy-
1
gen atoms of the carbonyl groups. The H NMR data indi-
cate preferential metal coordination to the deprotonated hy-
Table 1
Analytical and molar conductance data for the complexes
Complexes
Molecular formula
Molecular weight
Anal. found (calcd.) (%)
Molar conductance ꢀM
(ꢁ−1 cm2 mol−1
)
(g mol−1
)
C
H
M
[Al2(L1)(H2O)8]Cl4
[Zn2(L1)(H2O)4]Cl2
[Al3(L2)2(H2O)12]Cl5
[Zn3(L2)2(H2O)6]Cl2
[Al(L3)(H2O)4]Cl2
[Zn(L3)(H2O)2]Cl
Al2C15H24O15Cl4
Zn2C52H72O36Cl2
Al3C52H72O44Cl5
Zn3C52H60O38Cl2
AlC15H17O9Cl2
ZnC15H13O7Cl
640.12
573.98
1659.34
1560.38
439.19
406.10
28.35 (27.14)
31.45 (31.39)
37.82 (37.64)
40.12 (40.03)
40.93 (41.02)
44.49 (44.36)
3.72 (3.78)
2.77 (2.81)
4.41 (4.37)
3.92 (3.88)
3.94 (3.91)
3.24 (3.21)
8.49 (8.43)
22.92 (22.78)
4.84 (4.88)
12.54 (12.58)
6.18 (6.14)
16.24 (16.11)
547
197
625
217
202
96

R.F.V. de Souza, W.F. De Giovani / Spectrochimica Acta Part A 61 (2005) 1985–1990
1987
Table 2
IR dataa (cm−1) for ligands and complexes
Compound
␯(C O)
␯(C C)
␯(C
O
C)
␯(O H)
␯(M O)
H2L1
1668 s
1590 s
1618 s
1682 s
1677 s
1680 s
1658 s
1601 s
1613 s
1589 s
1588 s
1586 s
1590 s
1588 s
1589 s
1590 s
1590 s
1588 s
1260 s
1258 s
1259 s
1257 s
1255 s
1257 s
1260 s
1258 s
1258 s
3409–3144 b, m
3674–2357 b, m
3633–2978 b, m
4140–2580 b, m
3725–2415 b, m
3710–2491 b, m
3607–3084 b, m
3706–2590 b, m
3681–2395 b, m
–
[Al2(L1)(H2O)8]Cl4
[Zn2(L1)(H2O)4]Cl2
H2L2
612 w
606 w
–
645 w
628 w
–
[Al3(L2)2(H2O)12]Cl5
[Zn3(L2)2(H2O)6]Cl2
HL3
[Al(L3)(H2O)4]Cl2
[Zn(L3)(H2O)2]Cl
a
626 w
617 w
b: Broad; m: medium; s: strong; w: weak.
droxyl groups at 3-OH group. This fact was confirmed by the
dissociation constants measured potentiometrically for both
flavonoids; the pKa values for 3-OH group is ∼8.7; the higher
proton acidity at the carbon 3 position correlates well with its
easy deprotonation and the enhanced metal-binding ability.
For the rutin complexes, no shifts of the carbonyl bands were
observed, presumably because the 3-OH group of the rutin
is blocked by the sugar moiety (disaccharide glucorhamno-
side). It can be noted for all complexes that: (a) ␯(C O C)
and ␯ring(C C) frequencies are not shifted upon complexa-
tion indicating that the ring oxygen (position 1, Fig. 1) is not
involved in the complexation, (b) ␯(M O) frequencies appear
in the 606–645 cm−1 range, (c) ␯(O H) frequencies appear
as broad bands (3725−2357 cm−1) indicating the presence
of water. The presence of coordinated water is also supported
by thermal analyses.
explained by the extension of the conjugated system with the
complexation. The UV–vis spectra give significant informa-
tion about the coordination sites of flavonoids, for example,
the interactions of Al(III) and Zn(II) ions with quercetin at
2:1 metal:flavonoid ratio produced bathochromic shifts in the
absorbances of both bands (Fig. 2). As the 3-hydroxy group
has a more acidic proton, the 3-OH and 4-oxo groups are the
first sites to be involved in the complexation process [15].
The 3ꢀ,4ꢀ-dihydroxy groups bind a second metal ion. The 5-
OH group is not involved due to lesser proton acidity and the
steric hindrance caused by the first complexation.
3.3. 1H NMR spectra
The 1H NMR data for the complexes, as well as those for
free ligands are listed below.
Quercetin: δ 12.50 (s, 1H, 5-OH), 10.76 (s, 1H, 7-OH),
9.60 (s, 1H, 3-OH), 9.39 (s, 1H, 4ꢀ-OH), 9.31 (s, 1H, 3ꢀ-OH),
3.2. UV–vis spectra
ꢀ
ꢀ
ꢀ
ꢀ
7.68 (d, JH2 /H6 = 1.8 Hz, 1H, 2 -H), 7.55 (dd, 1H, 6 -H), 6.89
ꢀ
ꢀ
ꢀ
(d, JH5 /H6 = 8.5 Hz, 1H, 5 -H), 6.43 (d, JH8/H6 = 1.5 Hz, 1H,
8-H), 6.22 (d, 1H, 6-H). [Al2(L1)(H2O)8]Cl2: δ 12.46 (s, 1H,
5-OH), 10.75 (s, 1H, 7-OH), 9.24 (s, 4ꢀ-OH), 9.20 (s, 1H,
The flavonoids exhibit two major absorption bands in
the ultraviolet/visible region (Fig. 1). The absorptions in
the 320–385 nm range correspond to the B ring portion
(cinnamoyl system, band I), and the absorptions in the
240–280 nm range correspond to the A ring portion (ben-
zoyl system, band II) [14]. The spectra are related to the
␲ → ␲* transitions within the aromatic ring of the ligand
molecules. In comparison with flavonoids absorption spec-
tra, those of the complexes are shifted to the long-wavelength
region as shown in Table 3. Such bathochromic shift can be
3ꢀ-OH), 7.60 (d, JH2 /H6 = 2.1 Hz, 1H, 2ꢀ-H), 7.46 (dd, 1H,
ꢀ
ꢀ
6ꢀ-H), 6.80 (d, JH5 /H6 = 8.1 Hz, 1H, 5ꢀ-H), 6.40 (d, JH8/H6
ꢀ
ꢀ
Table 3
UV–vis data for ligands and complexes
Compound
λmax (nm), (ε in M−1 cm−1
)
Band I
Band II
H2L1
372 (15,700)
440 (6200)
427 (9900)
357 (18,260)
387 (74,00)
409 (17,500)
360 (16,800)
415 (7300)
256 (14,600)
270 (7100)
265 (9800)
256 (22,000)
263 (10,900)
272 (17,800)
267 (24,400)
273 (9000)
[Al2(L1)(H2O)8]Cl4
[Zn2(L1)(H2O)4]Cl2
H2L2
[Al3(L2)2(H2O)12]Cl5
[Zn3(L2)2(H2O)6]Cl2
HL3
[Al(L3)(H2O)4]Cl2
[Zn(L3)(H2O)2]Cl
Fig. 2. UV–vis spectra of the quercetin and metal–quercetin complexes in
methanol (50 ␮M).
420 (10,300)
271 (10,800)

1988
R.F.V. de Souza, W.F. De Giovani / Spectrochimica Acta Part A 61 (2005) 1985–1990
Table 4
Thermal analytical data for ligands and complexes
Compound
Dehydration temperature (◦C)
Decomposition temperature (◦C)
T1
T2
Weight loss (%)
T3
T4
Total weight loss (%)
H2L1
–
–
–
393
303
285
398
347
355
265
307
316
638
732
655
678
791
828
471
692
724
100.00
83.90
71.46
100.00
90.65
84.24
100.00
88.31
79.96
[Al2(L1)(H2O)8]Cl4
[Zn2(L1)(H2O)4]Cl2
H2L2
121
126
–
156
158
–
208
197
–
210
215
–
22.65
12.63
–
13.15
7.01
–
[Al3(L2)2(H2O)12]Cl5
[Zn3(L2)2(H2O)6]Cl2
HL3
[Al(L3)(H2O)4]Cl2
[Zn(L3)(H2O)2]Cl
132
133
189
188
16.54
8.98
T1–T2: Temperature range corresponding to complex dehydration; T3–T4: temperature range corresponding to complex decomposition.
= 1.8 Hz, 1H, 8-H), 6.22 (d, 1H, 6-H). [Zn2(L1)(H2O)4]Cl2:
the rutin complexes indicating the losses of the OH protons
due to complexation.
δ 12.47 (s, 1H, 5-OH), 10.74 (s, 1H, 7-OH), 9.24 (s, 4ꢀ-OH),
9.21 (s, 1H, 3ꢀ-OH), 7.62 (d, JH2 /H6 = 2.0 Hz, 1H, 2ꢀ-H),
ꢀ
ꢀ
7.45 (dd, 1H, 6ꢀ-H), 6.82 (d, JH5 /H6 = 8.3 Hz, 1H, 5ꢀ-H),
6.41 (d, JH8/H6 = 1.9 Hz, 1H, 8-H), 6.22 (d, 1H, 6-H).
Rutin: δ 12.63 (s, 1H, 5-OH), 10.82 (s, 1H, 7-OH), 9.69 (s,
ꢀ
ꢀ
3.4. Thermal analysis
The TG-DTA curves of the complexes exhibited two
stages of mass loss, the mass loss with a small endothermic
peak in first stage (dehydration) and the one in the second
stagewithanexothermicpeak(decomposition). Thermaldata
for the dehydration and decomposition of the complexes are
given in Table 4. These data show that all complexes contain
coordinated water. The mass loss in the first stage indicates
the number of water molecules. The complexes decompose
causing ca. 80% weight loss. The final products are the metal
oxides.
1H, 4ꢀ-OH), 9.28 (s, 1H, 3ꢀ-OH), 7.49 (d, JH2 /H6 = 1.9 Hz,
ꢀ
ꢀ
1H, 2ꢀ-H), 7.52 (d, JH5 /H6 = 8.8 Hz, 1H, 5ꢀ-H), 7.85 (dd,
1H, 6ꢀ-H), 6.41 (d, JH8/H6 = 1.5 Hz, 1H, 8-H), 6.24 (d, 1H,
6-H). [Al3(L2)2(H2O)12]Cl2: δ 12.60 (s, 1H, 5-OH), 9.58 (s,
ꢀ
ꢀ
4ꢀ-OH), 9.23 (s, 1H, 3ꢀ-OH), 7.41 (d, JH2 /H6 = 1.5 Hz, 1H,
ꢀ
ꢀ
2ꢀ-H), 7.37 (d, JH5 /H6 = 8.5 Hz, 1H, 5ꢀ-H), 6.78 (dd, 1H,
6ꢀ-H), 6.38 (d, JH8/H6 = 1.9 Hz, 1H, 8-H), 6.19 (d, 1H, 6-H).
[Zn3(L2)2(H2O)6]Cl2: δ12.59 (s, 1H, 5-OH), 9.56 (s, 4ꢀ-OH),
ꢀ
ꢀ
9.25 (s, 1H, 3ꢀ-OH), 7.40 (d, JH2 /H6 = 1.5 Hz, 1H, 2 -H), 7.36
ꢀ
ꢀ
ꢀ
(d, JH5 /H6 = 8.4 Hz, 1H, 5ꢀ-H),6.79 (dd, 1H, 6ꢀ-H), 6.37 (d,
JH8/H6 = 1.8 Hz, 1H, 8-H), 6.20 (d, 1H, 6-H).
ꢀ
ꢀ
3.5. Fluorescence spectroscopy
Galangin: δ 12.52 (s, 1H, 5-OH), 10.79 (s, 1H, 7-OH),
9.61 (s, 1H, 3-OH), 8.32 (m, 2H, 2ꢀ, 6ꢀ-H), 7.64 (m, 2H, 3ꢀ,
5ꢀ-H), 7.55 (m, 1H, 4ꢀ-H), 6.45 (d, JH8/H6 = 1.5 Hz, 1H, 8-H),
6.23 (d, 1H, 6-H). [Al(L3)(H2O)4]Cl2: δ 12.49 (s, 1H, 5-OH),
10.76 (s, 1H, 7-OH), 8.29 (m, 2H, 2ꢀ, 6ꢀ-H), 7.61 (m, 2H, 3ꢀ,
5ꢀ-H), 7.52 (m, 1H, 4ꢀ-H),), 6.43 (d, JH8/H6 = 1.5 Hz, 1H,
8-H), 6.24 (d, 1H, 6-H). [Zn(L3)(H2O)2]Cl: δ 12.48 (s, 1H,
5-OH), 10.75 (s, 1H, 7-OH), 8.30 (m, 2H, 2ꢀ, 6ꢀ-H), 7.62 (m,
2H, 3ꢀ, 5ꢀ-H), 7.50 (m, 1H, 4ꢀ-H), 6.44 (d, JH8/H6 = 1.7 Hz,
1H, 8-H), 6.25 (d, 1H, 6-H).
The fluorescence data of the ligands and complexes are
listed in Table 5. Emission spectra for quercetin and quercetin
complexes are shown in Fig. 3. The fluorescence data show
that ligands themselves exhibit fluorescences. The emissions
of the complexes are due to the fluorescences from the in-
traligand emission excited states. The enhanced fluorescence
intensity of the complexes is attributed to the coordination
of the ligand to the small, highly charged Al(III) and Zn(II)
The protons signal of the complexes are shifted to lower
frequencies relative to the free flavonoids. This is probably
due to the increase of the conjugation caused by the effect
of coordination when the complex is formed, increasing the
planarityoftheflavonoidmolecules. Thisisalsoinagreement
with the considerable bathochromic shifts observed in band
I of the compounds. The 1H NMR spectra also allowed some
conclusions about the chelation sites: quercetin and rutin are
able to chelate metal ions via 3ꢀ and 4ꢀ phenolic groups; upon
complexation, the metal ion displaces one hydrogen from
Table 5
Fluorescence data for ligands and complexes
Compoundsa
Emission
wavelength (nm)
Relative fluorescence
intensity
H2L1
510
478
484
525
495
505
535
502
512
36.4
80.1
60.5
43.2
74.4
55.1
40.3
60.4
57.5
[Al2(L1)(H2O)8]Cl4
[Zn2(L1)(H2O)4]Cl2
H2L2
[Al3(L2)2(H2O)12]Cl5
[Zn3(L2)2(H2O)6]Cl2
HL3
1
the flavonoid [11]. The H NMR spectra of the complexes
[Al(L3)(H2O)4]Cl2
[Zn(L3)(H2O)2]Cl
a
also reveal that the resonances of the hydrogens of the 3-OH
groups are not present in quercetin and galangin complexes
and that the hydrogens of the 7-OH groups are not present in
Excitation wavelength: 450 nm.

R.F.V. de Souza, W.F. De Giovani / Spectrochimica Acta Part A 61 (2005) 1985–1990
1989
Fig. 4. Cyclic voltammograms of the galangin and metal–galangin com-
plexes (1 mM); 0.1 M LiClO4/MeOH; scan rate 0.1 V s−1
.
Fig. 3. The emission spectra of the quercetin and metal–quercetin complexes
in methanol (50 ␮M) at room temperature.
ment of the fluorescence signal upon chelation of flavonoids
with a nonparamagnetic metal (e.g., Al(III) and Zn(II)) is
related to the inhibition of the excited state intramolecu-
lar proton transfer (ESPT) process between hydroxyl and
metal ions that effectively increases the rigidity of the lig-
and structure and increases the fluorescence quantum yield
by reducing the probability of non-radiative energy dissipa-
tion process [16]. Another origin plausible for the enhance-
Fig. 5. Proposed structures of the complexes: (A) quercetin complexes, M = Al(III) and Zn(II), x = n = 4 for Al(III) and x = n = 2 for Zn(II); (B) rutin complexes,
M = Al(III) and Zn(II), x = 4, n = 5 for Al(III) and x = n = 2 for Zn(II); and (C) galangin complexes, M = Al(III) and Zn(II), x = 4, n = 2 for Al(III) and x = 2, n
= 1 for Zn(II).

1990
R.F.V. de Souza, W.F. De Giovani / Spectrochimica Acta Part A 61 (2005) 1985–1990
Table 6
The postulated structural arrangements of the synthesized
flavonoid complexes, based on the experimental data, are
shown in Fig. 5.
Electrochemical data for ligands and complexes
Compound
Epa vs. Ag/AgCl (mV)
H2L1
631 and 934
549 and 823
573 and 853
873
693
738
856
696
715
[Al2(L1)(H2O)8]Cl4
[Zn2(L1)(H2O)4]Cl2
H2L2
4. Conclusion
[Al3(L2)2(H2O)12]Cl5
[Zn3(L2)2(H2O)6]Cl2
HL3
This work contributes to a better elucidation of the chela-
tion chemistry of flavonoids with Al(III) and Zn(II) ions.
The spectral results indicate that the chelation sites are: for
quercetin 3-OH, 4-oxo and 3ꢀ,4ꢀ-dihydroxy groups, for rutin
3ꢀ,4ꢀ-dihydroxy and 7-OH groups, for galangin 3-OH, 4-
oxo groups. The results obtained have also demonstrated
that the chelation influences the electrochemical properties
of flavonoids. Al(III) and Zn(II) ions increase the flavonoids
antioxidant activities as shown by the lower complexes oxi-
dation potentials relative to those of free flavonoids.
[Al(L3)(H2O)4]Cl2
[Zn(L3)(H2O)2]Cl
4-oxo groups of the C ring (Fig. 1) [17,18]. It is worth to
note that for some flavonoid ligands, the fluorescence in-
tensity for Al(III) complexes is higher than for Zn(II) com-
plexes. Moreover, the emission wavelengths of all complexes
are shifted to lower wavelengths (20–33 nm) related to the
free ligands, under the same excitation wavelength; this blue
shift suggests that the electronic field of the metal ions de-
creases the energy level difference of the highest occupied
molecular orbital and the lowest unoccupied orbital of the
flavonoids.
Acknowledgments
˜
The authors thank Fundac¸ao de Amparo a Pesquisa do
˜
Estado de Sao Paulo (FAPESP) for financial support.
3.6. Electrochemical properties of the compounds
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The cyclic voltammograms of the ligands and complexes
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voltammetric oxidation peak potential for flavonoids
and complexes [11,12]. The lower the potential, the
higher the antioxidant activity. Thus, Al(III) complexes
are more effective antioxidants than the Zn(II) com-
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