Study of complexation between Cu2+ ions and ortho-azidobenzoic acid

Article abstract of DOI:10.1007/s11173-005-0073-7

An aqua complex of copper(II) o-azidobenzoate, [Cu(OH)ABA·2H ₂O]₂ (ABA is o-azidobenzoic acid), was synthesized in an aqueous solution and identified by IR and electronic absorption spectroscopy. In the complex, the azido group is not coordinated by Cu²⁺. When dissolved in dry organic solvents (DMF, DMSO, dioxane, and methanol), the complex undergoes dehydration to give a chelate complex (CC) containing the Cu²⁺-coordinated azido group as a result of the electron density redistribution at its N atoms. The IR spectrum of the chelate complex contains no absorption band at 2135 cm^-1 corresponding to the stretching vibrations of the azido group. The resulting CC is unstable in solutions and spontaneously decomposes with a release of molecular nitrogen. The interaction of a Cu²⁺ ion with o-azidobenzoic acid in dry organic solvents affords a CC similar to the complex obtained on the dissolution of [Cu(OH)ABA·2H₂O]₂ in dry solvents.

Full text of DOI:10.1007/s11173-005-0073-7

Russian Journal of Coordination Chemistry, Vol. 31, No. 3, 2005, pp. 181–184. Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 3, 2005, pp. 195–198.
Original Russian Text Copyright © 2005 by Budruev, Karyakina, Levina, Oleinik.
Study of Complexation between Cu2+ Ions
and ortho-Azidobenzoic Acid
A. V. Budruev, L. N. Karyakina, O. P. Levina, and A. V. Oleinik
Lobachevsky State University, Nizhni Novgorod, Russia
Received March 18, 2004
Abstract—An aqua complex of copper(II) o-azidobenzoate, [Cu(OH)ABA · 2H2O]2 (ABA is Ó-azidobenzoic
acid), was synthesized in an aqueous solution and identified by IR and electronic absorption spectroscopy. In
the complex, the azido group is not coordinated by Cu2+. When dissolved in dry organic solvents (DMF, DMSO,
dioxane, and methanol), the complex undergoes dehydration to give a chelate complex (CC) containing the
Cu2+-coordinated azido group as a result of the electron density redistribution at its N atoms. The IR spectrum
of the chelate complex contains no absorption band at 2135 cm–1 corresponding to the stretching vibrations of
the azido group. The resulting CC is unstable in solutions and spontaneously decomposes with a release of
molecular nitrogen. The interaction of a Cu2+ ion with Ó-azidobenzoic acid in dry organic solvents affords a CC
similar to the complex obtained on the dissolution of [Cu(OH)ABA · 2H2O]2 in dry solvents.
Complexation between various N-containing room temperature and kept for about 2 h. White crystals
organic compounds and transition metal ions is deter-
mined by the possibility of the nitrogen atom to
donate its lone electron pair for the formation of a
bond to metal ions. There are a great number of com-
plexes of nitriles, Schiff bases, aromatic azo com-
pounds, and amines that form chelate complexes with
metal ions [1–4].
were filtered off and recrystallized from boiling light
petroleum–benzene (5 : 2 v/v). The yield of ABA was
1.57 g (65%), Tm = 144°C.
For C7H5N3O2
anal. calcd. (%):
Found (%):
C, 51.54;
C, 51.53;
H, 3.09.
H, 3.07.
Like other N-containing groups, the azido group can
form complexes [5–7] involving the lone electron pairs
at its first and third N atoms [8]. The specific character
of the interaction of the azido group with metal ions can
explain the mechanism of the catalyzed decomposition
of organic azides [8] and can be used to obtain various
heterocyclic compounds. That is the reason why the
synthesis of transition metal complexes with organic
azides and the study of the properties of the azido group
in them are of topical interest.
Synthesis
of
potassium
o-azidobenzoate
ë6ç4(N3)COOK (I). A solution of KOH (0.336 g) in
20 ml of ethanol was added to a solution of ABA (1 g)
in 30 ml of ethanol. The ethanol was removed in a water
bath in vacuo (30 mmHg) until crystallization started.
The resulting white crystals of salt I were filtered off.
Synthesis of [ëu(éç)ÄBA · 2ç2é]2 (II). A hot
solution of compound I (1.2 g) in 100 ml of water was
added at 75°C to a solution of CuSO4 · 5ç2é (1.5 g) in
50 ml of water. The resulting light green crystals of com-
plex II were filtered off and recrystallized from THF.
This study was devoted to the complexation
between Cu2+ ions and Ó-azidobenzoic acid (ABA) in
water and organic solvents.
For C14H18N6O10Cu2
anal. calcd. (%):
Found (%):
C, 28.09;
C, 28.15;
H, 3.01;
H, 2.95;
Cu, 21.40.
Cu, 21.50.
EXPERIMENTAL
Synthesis of ABA. A cooled solution of NaNO2
(1.08 g) in 10 ml of water was added dropwise at 5°C
to a vigorously stirred mixture of anthranilic acid (2.1 g,
0.15 mol), water (15 ml), and concentrated HCl
(17 ml). The solution was stirred for another 15 min
and a solution of NaN3 (1.08 g) in 10 ml of water was
Synthesis of the chelate complex (CC). A solution
of compound I (0.6 g) in 20 ml of an organic solvent
(DMF, DMSO, dioxane, or methanol) was added to a
solution of ëu(CH3COO)2 · ç2é (0.3 g) in 30 ml of the
added dropwise. The resulting mixture was warmed to same solvent. The reaction mixture was left in the dark
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182
BUDRUEV et al.
Transmittance
1
2
3
4
4000
3200
2400
1600
800 ν, cm–1
Fig. 1. IR spectra of (1) ABA, (2) salt I, (3) complex II, and (4) CC.
for a day and then chloroform (120 ml) was added. The determined by the concentration of éç– ions in aque-
resulting violet crystals of a CC were filtered off.
ous solution and can vary from normal to basic salts.
The number of molecules of crystallization water in
complex II was calculated from the volume of nitrogen
evolved upon its complete photochemical decomposi-
tion. Because the photolysis of aromatic azides is
accompanied by a release of molecular nitrogen to give
primary amines and azo compounds (a mole of nitrogen
per mole of basic copper(II) Ó-azidobenzoate), the
molecular mass of the complex calculated from the vol-
ume of the evolved nitrogen suggests its dihydrate for-
mula Cu(OH)ABA · 2ç2é.
The absence of a signal in the EPR spectrum of
complex II indicates, as shown in [10], an exchange
interaction between two copper centers resulting in a
binuclear associate. Therefore, the molecular formula
of complex II can be written as [Cu(OH)ABA · 2H2O]2.
The formation of a copper(II) complex with ABA is
also evident from IR spectroscopic data. For instance,
the νas(COOH) bands in the 1500–1700 cm–1 range are
shifted from 1680 and 1650 cm–1 in ABA to 1575 and
1550 cm–1 in salt I and to 1590 and 1550 cm–1 in com-
plex II (Fig. 1). This indicates the presence of a carbox-
ylate ion, thus confirming the formation of complex II.
For C14H10N6O6Cu2
anal. calcd. (%):
Found (%):
C, 34.57;
C, 34.73;
H, 2.06;
H, 2.52;
Cu, 26.34.
Cu, 25.32.
Electronic absorption spectra were recorded on an
SF-46 spectrophotometer (quartz cells, l = 1 cm) in the
220–800 nm range. IR spectra were recorded on a
Specord IR75 spectrophotometer in the 400–4000 cm–1
range (Nujol). The EPR spectrum was recorded on a
Bruker ER-200D-SRC spectrometer. The gas evolved
upon the dissolution of complex II in dry DMF was ana-
lyzed by gas chromatography with a Tsvet 102 instru-
ment. Photodecomposition of complex II was carried
out in a quartz tube exposed to the radiation from a
DRK-120 quartz mercury lamp. The number of azido
groups in the molecule of complex II was determined
from the volume of the evolved nitrogen.
RESULTS AND DISCUSSION
Since complex II is insoluble in water, its composi-
In the IR spectra of salt I and complex II, the νs(N3)
tion was determined by isomolar series method from and νas(N3) bands retain the same positions (at 2135
the weight of the precipitate formed in the reaction of and 1290 cm–1, respectively) as in the spectrum of free
CuSO4 · 5ç2é with salt I. It was found that complex II ABA; this indicates that the azido group is not coordi-
contains one mole of salt I per mole of copper sulfate; nated by the Cu2+ ion. The presence of water of crystal-
i.e., the product is basic copper(II) Ó-azidobenzoate lization in complex II was confirmed by a δ(OH) band
Cu(OH)ABA · nç2é. According to [9], the composi- appearing at 1625 cm–1, which is absent from the IR
tion of copper(II) complexes with organic acids is spectra of ABA and salt I. The broad ν(OH) band at
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 3 2005

STUDY OF COMPLEXATION BETWEEN Cu2+ IONS
183
A
A
1
2
0.3
1.0
3
1
0.2
0.2
0.1
0.1
0.8
0.6
0.4
3
2
0
20
40
60
80
τ, min
4
0.2
0
Fig. 3. Kinetics of the formation of the CC in DMF with the
water content c = (1) 0, (2) 1, and (3) 10%.
w
220
320
420
520
620
720
λ, nm
(Fig. 2, curve 4). The electronic absorption spectra of
the CC obtained in dry solvents are insensitive to the
nature of an organic solvent and identical with the elec-
tronic absorption spectrum of the CC obtained from
complex II.
Fig. 2. Electronic absorption spectra of (1) ABA, (2) salt I,
and (3) complex II in methanol and (4) CC in DMF; c =
5 × 10 mol l .
–5
–1
A comparison of the electronic absorption spectra of
complex II and the CC (Fig. 2, curves 2, 3, and 4)
showed that the absorption band in the range of the
d−d* transition of copper is shifted from 700 to 570 nm.
Two new absorption bands at 320 and 365 nm can be
assigned to intraligand π–π* and n–π* transitions in
the CC.
3250 to 3500 cm–1 is due to the presence of both water
of crystallization and hydroxy ions in complex II.
The electronic absorption spectra of ABA, salt I,
and complex II in aqueous methanol were also exam-
ined. The spectrum of complex II contains a band at
700 nm (Fig. 2, curve 3) characteristic of the d–d* tran-
sition in organic copper(II) salts [11]. In the range of
intraligand transitions (< 350 nm), ABA, salt I, and
complex II absorb at 254 nm (Fig. 2, curves 1–3). This
suggests that the electronic structure of the organic
anion remains unchanged and that the azido group is
not coordinated by the Cu2+ ion. Thus, although the
azido group is spatially close to Cu2+ in complex II, it
remains uncoordinated because the vacant coordination
sites of Cu2+ are occupied by molecules of crystalliza-
tion water. According to the literature data and the
results of our study, complex II and other binuclear
copper(II) carboxylates [9, 10] can be represented by
the following structural formula:
To elucidate the role of water in the conversion of
complex II into the CC, complex II was dissolved in
DMF containing different amounts of water (from 0 to
10 vol %). The curves of changing optical density of
solutions of complex II in DMF at 365 nm with time
indicate that an increase in the water content of DMF
inhibits the formation of the CC (Fig. 3).
Since the rate of formation of the CC depends on the
water content in the solvent, one can assume that the
dissolution of complex II in dry solvents is accompa-
nied by its dehydration. This is the case with those sol-
vents that can themselves form complexes with water
molecules. A similar dehydration of copper salt
hydrates was described in [12].
OH
The IR spectrum of the CC (Fig. 1, curve 4) shows
neither an absorption band at 1625 cm–1 for the OH
group of crystallization water, which confirms the
dehydration of complex II, nor a νas(N3) band at
2100 cm–1. In dry DMF, the formation of the CC from
complex II is completed in ~20 min without nitrogen
evolution, which suggests the retention of three N
atoms in the ligand. Apparently, the dehydration of
H2O
O
H O
O2H2
Cu
O
O
O
Cu
N3
N3
H2O
H O
OH
2
2
OH
When dissolved in dry organic solvents, complex II complex II makes it possible for the ëu2+ ion to coordi-
turns from green to dark violet as for a complex nate the azido group with the CC formation and to
obtained in the reaction of Cu(CH3COO)2 · 2ç2é with change the azido group electronic configuration in such
salt I in dry DMF, DMSO, dioxane, or methanol a manner that it no longer absorbs at 2100 cm–1.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 3 2005

184
BUDRUEV et al.
This work was supported by the Russian Foundation
for Basic Research, project no. 02-03-33021.
V, ml
8
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6
4
2
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ACKNOWLEDGMENTS
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