Vibrational spectra of the Ga(III) complexes with oxine and clioquinol

Article abstract of DOI:10.1016/j.saa.2011.05.053

The FTIR and FT-Raman spectra of the gallium(III) complexes of 8-hydroxyquinoline (oxine) and 5-chloro-7-iodo-8-hydroxyquinoline (clioquinol), were recorded and briefly discussed by comparison with the spectra of the uncoordinated ligands and with some related species.

Full text of DOI:10.1016/j.saa.2011.05.053

Spectrochimica Acta Part A 79 (2011) 1762–1765
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Vibrational spectra of the Ga(III) complexes with oxine and clioquinol
Claudia C. Wagnera, Ana C. González-Barób, Enrique J. Baranb,∗
a Departamento de Ingeniería Química, Facultad de Ingeniería, Universidad Nacional del Centro de la Provincia de Buenos Aires, 7400 Olavarría, Argentina
b Centro de Química Inorgánica (CEQUINOR/CONICET,UNLP), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, C. Correo 962, 1900-La Plata, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
The FTIR and FT-Raman spectra of the gallium(III) complexes of 8-hydroxyquinoline (oxine) and 5-chloro-
7-iodo-8-hydroxyquinoline (clioquinol), were recorded and briefly discussed by comparison with the
spectra of the uncoordinated ligands and with some related species.
Received 22 March 2011
Received in revised form 3 May 2011
Accepted 17 May 2011
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Gallium(III)
Oxine complex
Clioquinol complex
IR spectra
Raman spectra
1. Introduction
In order to advance in a better characterization of these novel
Ga(III) containing metallopharmaceuticals, we have now investi-
Gallium has been the second metal to show activity against
malignant tumors in humans soon after the establishment of plat-
inum drugs in routine clinical practice [1–4]. Gallium nitrate and
chloride solutions have shown moderately effectiveness against
soft-tissue tumors [1–4]. However, nowadays, the only approved
application of gallium nitrate remains its use for the treatment of
cancer-related hypercalcaemia [3,4].
Recently, the complexation of Ga(III) with suitable chelating
agents has been pursued to stabilize the cation against hydroly-
sis, the major impediment for good intestinal absorption, and to
facilitate membrane permeation. One of these complexes, namely
tris(8-hydoxyquinolinato)gallium(III) (usually known as KP46) is
currently being evaluated in clinical trials [2–5]. This complex
shows a high bioavailability in animal models when orally admin-
istered and is a more potent inhibitor of tumor cell proliferation in
vitro than the simple Ga(III) salts [3].
On the other hand, some other 8-hydroxyquinoline derivatives
have also been suggested as potentially adequate for the gener-
ation of therapeutically useful gallium(III) complexes [6]. Among
them, 5-chloro-7-iodo-8-hydroxyquinoline (clioquinol) appears as
particularly attractive. It is a now retired United States Pharma-
copoeia antibiotic and has recently attained renewed interest as an
useful chelating agent for the treatment of Alzheimer’s disease and
related neurological disorders [7,8].
gated the vibrational spectra of the 8-hydroxyquinoline complex
as well as those containing the derived 5-chloro-7-iodo ligand.
2. Experimental
For the synthesis of the oxine complex, 3.5 g (24.1 mmol) of
the ligand were dissolved in 150 mL of a 10% acetic acid solution
and 1.88 g (6.9 mmol) of powdered Ga(NO3)3 was slowly added at
room temperature. The solution was then stirred and heated at
80 ◦C under reflux. After a few minutes concentrated NH4OH was
added until a pH-value of 7.0 was reached. A yellow precipitate was
formed and the suspension was refluxed during one more hour.
The precipitate was filtered off, washed with hot water and dried
at 100 ◦C in a drying oven [6].
The clioquinol complex was obtained in the following way:
0.46 g (1.1 mmol) of Ga2(SO4)3 were dissolved in 200 mL of dis-
tilled water and the pH-value adjusted to 1.0 with concentrated
sulphuric acid. The resulting solution was heated up to 90 ◦C and
2.0 g (6.5 mmol) of clioquinol dissolved in 100 mL of acetone were
slowly added to the hot solution. The yellow suspension was cooled
to about 50 ◦C and then the precipitate was filtered off, washed
with 150 mL of a hot 1:1 acetone/water mixture and finally dried
at 100 ◦C in a drying oven [6].
In both cases the stoichiometry of the obtained complexes was
confirmed by elemental chemical analysis.
The infrared spectra in the spectral range between 4000
∗
and 400 cm−1 were recorded as KBr pellets with
Bruker-EQUINOX-55 spectrophotometer. A total of 60 scans were
a
FTIR-
Corresponding author. Tel.: +54 221 4259485.
E-mail address: baran@quimica.unlp.edu.ar (E.J. Baran).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.05.053

C.C. Wagner et al. / Spectrochimica Acta Part A 79 (2011) 1762–1765
1763
Fig. 1. Schematic representation of the structures of the two investigated
complexes: tris(8-hydroxyquinolinato)gallium(III) (left), tris(5-chloro-7-iodo-8-
hydroxiquinolinato)gallium(III) (right).
accumulated. Raman spectra, in the range 4000–100 cm−1 were
obtained with the FRA 106 Raman accessory of a Bruker IFS 66
instrument. A total of 60 scans were accumulated, using the 1064-
nm line of a solid state Nd:YAG laser for excitation. Spectral
resolution was 4 cm−1 in both measurements.
3. Results and discussion
3.1. Structural characteristics of the compounds
The crystal structure of tris(8-hydroxyquinolinato)gallium(III)
has been repeatedly investigated [9], whereas that of the corre-
sponding clioquinol complex has so far not been determined but
on the basis of the well-known general characteristics of this lig-
and it is clear that it behaves in a similar manner as oxine. In both
cases, and as shown in Fig. 1, the bidentate ligand interacts with the
metal center through its N-atom and the deprotonated O-atom. The
crystallographic studies of the oxine complex show that all Ga–O
bonds are somewhat shorter than the corresponding Ga–N bonds
[9].
Fig. 2. FTIR spectrum (above) and FT-Raman spectrum (below) of tris(8-
3.2. Infrared and Raman spectra of
tris(8-hydroxyquinolinato)gallium(III)
hydroxyquinolinato)gallium(III) in the spectral rage between 2000 and 400 cm−1
.
and of some other metallic complexes of this ligand [10–15] as well
as by the information provided by standard references [16,17], is
briefly commented as follows:
The FTIR and FT-Raman spectra of this complex, in the spec-
tral range between 2000 and 400 cm−1 are shown in Fig. 2 and
the proposed assignment is presented in Table 1. This assignment,
supported by results of previous studies of free 8-hydroxyquinoline
- The characteristic ꢀ(OH) stretching band, located at 3176 cm−1
in the IR spectrum of free oxine, disappears in the complex, due
to the deprotonation of the ligand. After deprotonation, the band
assigned to the ꢀ(C–O) vibration moves to lower energies in the
complex (from 1093 to 1056 cm−1 in the IR and from 1100 to
1059 cm−1 in the Raman spectrum). The respective out-of plane
deformational mode, found at 465 cm−1 in the IR spectrum of
the free ligand is also displaced to lower energies in the complex
(407 cm−1 in the IR and 403 cm−1 in the Raman spectrum).
- Most of the ring vibrations as well as the CH deformational modes
are scarcely affected by the coordination of Ga(III). Interestingly,
the strongest Raman line (1386 cm−1) is related to one of these
ring vibrations. Among these ring vibrations it was possible to
identify bands assignable to the ꢀ(C C) and ꢀ(C N) modes. The
first one also presents a relatively high intensity in the Raman
spectrum.
Table 1
Assignment of the IR and Raman spectra of tris(8-hydroxyquinolinato)gallium(III)
(band positions in cm−1).
Infrared
Raman
Assignment
3045 m
1598 sh
1579 s
3052 m
1605 sh
1588 s
1498 vw, 1470 vw, 1422 m
1386 vs, 1333 w
1284 w, 1214 vw
1178 vw, 1135 w, 1114vw
1059 m
ꢀ(CH)
ꢀ(C N)
ꢀ(C C)
ꢀ(C–C) ring
1497 s, 1463 vs, 1426 w
1379 vs, 1327 vs
1281 s, 1239 sh, 1227 s
1174 w, 1134 vw, 1112vs
1056 m
1034 m, 984 vw
958 vw, 914 w, 862 w
823 vs, 804 s
788 s, 741 vs
644 m, 628 s
593 vw, 576 w, 526 s
515 m, 500 w, 468 vw,
443 w
ꢀ(C–C) ring
ı(CH) in plane
ı(CH) in plane
ꢀ(C–O)
ı(CH) out of plane
ı(CH) out of plane
ı(CH) out of plane
Ring breathing
Ring deformations
Ring deformations
Ring deformations
Ring deformations
ı(CO) out of plane
958 vw, 912 w, 864 vw
822 vw, 800 w
751 s
622 vw
571 w, 524 s
499 m
- The expected OGa in-plane deformational mode is surely located
in the region in which an important number of bands assigned to
ı(CH) in-plane vibrations are found. In the case of the Al(oxine)3
complex this OAl band is found at 1282 cm−1 [13] whereas in the
corresponding V(III) complex it was observed at 1270 cm−1 [14].
443 vw
403 vw
407 m
vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder.

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C.C. Wagner et al. / Spectrochimica Acta Part A 79 (2011) 1762–1765
Table 2
Assignment of the IR and Raman spectra of tris(5-chloro-7-iodo-8-
hydroxyquinolinato)gallium(III) (band positions in cm−1).
Infrared
Raman
Assignment
3066 w
3070 w
ꢀ(CH)
1623 s
1602 sh
ꢀ(C N)
1554 s
1581 s
ꢀ(C C)
1484 vs, 1373 vs
1281 vw, 1273 m, 1221 m
1200 vw, 1137 w
1112 s,
1490 vw, 1373 vs
1280 w, 1250 vw, 1220 vw
1142 vw
ꢀ(CC) ring
ı(CH) in plane
ı(CH) in plane
ꢀ(C–O)
1110 vw
1049 m, 972 s, 880 m
855 m
812 m, 784 m, 752 vs
712 s
1055 w
857 vw
758 s
ı(CH) out of plane
ı(CH) out of plane
Ring breathing
ꢀ(C–Cl)
658 s, 604 w
583 m, 504 w
473 m
645 w, 620 w
585 m, 500 w
450 vw
Ring deformations
Ring deformations
ı(C–O)
418 sh, 405 m
400 vw
vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder.
assigned to the ꢀ(C–O) vibration moves to somewhat higher ener-
gies in the complex (from 1082 to 1112 cm−1 in the IR and from
1081 to 1110 cm−1 in the Raman spectrum). This behavior is sim-
ilar to that observed in the Cu(II) complex [18] but differs from
that commented above for the Ga(oxine)3 complex.
- The out-of plane ı(C–OH) deformational mode, found at 496 cm−1
in the IR spectrum of the free ligand, is displaced to lower energies
in the complex. The expected in plane deformational mode is also
located here between the ı(CH) in-plane vibrations.
- Most of the ring vibrations as well as the CH deformational modes
are also here scarcely affected after complex formation. Both, the
ꢀ(C N) and ꢀ(C C) bands, are found at similar energies as in the
free ligand (in the IR spectrum of free clioquinol these bands are
located at 1604 and 1576 cm−1, respectively).
- The strong IR band found at 712 cm−1 was assigned to the
ꢀ(C–Cl) vibration [17]. This modifies and corrects our previous
assignment for this carbon–halogen stretching mode [15,18]. The
respective ꢀ(C–I) could not be clearly identified, but it is surely
located together with the ring deformational modes in the spec-
tral range between 600 and 500 cm−1 [17].
Fig. 3. FTIR spectrum (above) and FT-Raman spectrum (below) of tris(5-chloro-
7-iodo-8-hydroxyquinolinato)gallium(III) in the spectral rage between 2000 and
400 cm−1
.
- As mentioned above, bands related to Ga–O and Ga–N vibrations
are surely located below 400 cm−1. In the present case no addi-
tional information about these bands could be obtained from the
low-energy side of the Raman spectrum.
In the free ligand, the ı(C–OH) in-plane deformational mode was
observed at 1290 cm−1 in the IR spectrum.
- Bands related to Ga–O and Ga–N stretching vibrations are surely
located below 400 cm−1. In the lower energy side of the Raman
spectrum (not shown in Fig. 2) a weak doublet with components
at 300 and 273 cm−1 is observed and can, eventually, be related to
these modes. Besides, and on the basis of the scarce spectroscopic
information available on the lower energy region, it is impossible
to make comments on the geometrical arrangement of the Ga–O
and Ga–N bonds.
Acknowledgements
This work has been supported by the Universidad Nacional de la
Plata, the Universidad Nacional del Centro de la Provincia de Buenos
Aires and the Consejo Nacional de Investigaciones Científicas y Téc-
nicas de la República Argentina, CONICET. The authors are members
of the Research Career from this organism.
3.3. Infrared and Raman spectra of
tris(5-chloro-7-iodo-8-hydroxiquinolinato) gallium(III)
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