5
2
B. Cristóvão, Z. Hnatejko / Journal of Molecular Structure 1088 (2015) 50–55
Results and discussion
Infrared spectra
2
The selected FTIR data of the free Schiff base ligand H L and its
lanthanide(III) complexes are listed in Table 2. The infrared spectra
of the complexes are compared with this of the free ligand in order
to determine the coordination sites that may involve in chelation.
There are some guide peaks, in the spectra of the ligand, which are
of good help for achieving this goal. The position and/or the inten-
sities of these peaks are expected to be changed upon chelation.
In the FTIR spectrum the band for the weak OAH stretching
ꢂ1
vibration at ca. 2800 cm in the ligand is replaced by the medium
ꢂ1
band with the maximum at around 3416–3436 cm in the com-
+
plexes, due to the NAH vibration of C@N AH moiety [9,14,26].
The broadness of the band indicates that this hydrogen atom is still
involved in intramolecular H-bonding with the phenolic oxygen.
The presence of this band in the spectra of complexes 1–5 confirms
proposed structure for 1–5 (Scheme 1). Additionally, the H NMR
6
spectral data of Ln(III) complexes recorded in DMSO-d solution
3 3 2
Fig. 1. TG, DTG and DTA curves of [Gd(NO ) H L] (1).
1
similar way. As an example TG/DTG and DSC curves of the Gd(III)
complex 1 in the air atmosphere are presented in the Fig. 1. The
recorded TG curves for 1–5 are stable, showing a TG plateau up to
at least 230–250 °C. It may indicate the absence of water and other
solvent molecules in the coordination sphere of the studied coordi-
nation compounds. As the temperature is increased, the TG curves
of 1–5 exhibit mass loss in a wide temperature range: 250–860 °C
1), 230–750 °C (2), 240–780 °C (3), 240–820 °C (4) and 240–
70 °C (5), respectively, which are accompanied with exothermic
peaks in the DSC curves at 290 °C, 510 °C and 810 °C (1), 310 °C,
30 °C and 740 °C (2), 300 °C, 520 °C and 730 °C (3), 310 °C, 530 °C
and 740 °C (4), 310 °C, 530 °C and 740 °C (5), respectively, that
may be due to the decomposition of the ligands. The decomposition
process of the mononuclear compounds is intricate and it is difficult
to distinguish intermediate solid products. The solid residues
obtained during thermal decomposition of the Schiff base complex-
and compared with this obtained for the free Schiff base further
substantiate the mode of coordination suggested by FTIR studies.
1
The H NMR data indicate that in the complexes of Ln(III) the Schiff
base ligand is coordinated to the metal ion(III) without deprotona-
tion. A singlet peak observed at 13.78 ppm (free Schiff base) is
assigned to phenolic hydroxyl (OH) protons and this peak is also
occurred in lanthanide(III) compounds (13.81 ppm Gd, 13.93 ppm
Tb, 13.88 ppm Dy, 13.84 ppm Ho and 13.79 ppm Er).
The DMSO is polar aprotic solvent and Schiff base is present in
the enol-imine form with strong intramolecular OHꢁ ꢁ ꢁN@C hydro-
gen bond [7]. For this reason in the DMSO solution of free ligand
and its complexes we do not observe hydrogen atom located on
azomethine nitrogen. Observation of a proton migration in similar-
ly related species has been reported by Binnemans et al. [26],
Costes et al. [27] and Xie et al. [28]. The azomethine C@N stretch-
(
8
5
2 3 4 7 2 3 2 3 2 3
es are suitable metal oxides: Gd O , Tb O , Dy O , Ho O and Er O
[
10,11,15]. The mass losses calculated from TG curves are in the
ing vibration,
m (C@N), in the complexes is shifted to higher
ꢂ1
ꢂ1
range of 76.40–78.80 % (the theoretical values are 76.69–78.50 %).
The final products were calculated from TG curves and experimen-
tally verified by their X-ray diffraction patterns.
wavenumber (by 12–16 cm to about 1648–1652 cm ) in com-
parison to the same transition in the ligand (1636 cm ) [14,26].
ꢂ1
It is consistent to the proposal that the wavenumber of C@N with
respect to a free ligand shifts toward higher wavenumber after the
coordination of the ligand to a metal ion only through the phenolic
oxygen atoms [9,14,26,29,30]. In contrast, the Schiff base copper(II)
complexes in which the deprotonated ligand coordinates via the
phenolic oxygen and via the azomethine nitrogen to the metal(II),
Magnetic properties
In lanthanides, 4f orbitals are efficiently shielded and the influ-
ence of the neighboring groups on the magnetic properties is less
evident than in 3d paramagnetic compounds. The magnetic prop-
erties of these ions are governed by strong orbital angular momen-
the
indicating less double-bond character in the C@N bond [14,31]. In
the FTIR spectra of the free ligand the phenolic (CAO) stretching
m (C@N) stretching vibration is shifted to lower wavenumbers,
2
S+1
m
tum contribution and the spin-orbit coupling that splits the
L
ꢂ1
n
2S+1
vibration band is observed at around 1252 cm while in all com-
multiplets of the 4f ions into
J
L J-states. Usually, for most of
L free ion ground state is well
J
ꢂ1
2S+1
plexes, this band appears at lower frequencies 1212 cm , confirm-
the trivalent rare-earths ions, the
ing the involvement of the phenolic group in the complex
separated from the excited ones, thus at room and at lower tem-
ꢂ1
formation [9,14,26]. Four new bands near 1500 cm
(m
4
), 1308–
peratures the ground state is the only thermally populated ones.
ꢂ1
ꢂ1
ꢂ1
2S+1
1
2 cm
(m
1
), 1092 cm
(m
2
) and 852–856 cm
(m
6
), respectively
As lanthanide ions are placed in a crystal field, the
J
L states
can be assigned to vibrations of the coordinated nitrate ions. The
are split into Stark components (up to 2J + 1, n = even; J + 1/2,
n = odd). The magnetic behavior will then be governed by the ther-
mal population of these set of levels, preventing the application of
difference between the
4 1
m and the m peak positions is ca.
ꢂ1
1
80 cm , which is typical for bidentate nitrate groups (monoden-
tate nitrate groups display a much smaller splitting) [9,14,26,29–
a spin-only Hamiltonian for quantitative investigations of
ꢂ1
3
2]. The bands at ca. 424–428 cm are due to
m
(MAO) stretching
exchange interactions between lanthanide ions. Only the Gd(III)
ion exhibits a quenched orbital momentum which can be therefore
treated as a pure spin ground state [12,13,33].
vibrations [10,11,32].
The magnetic behaviors of complexes 1–5 are reported in Figs. 2–
4. Temperature-dependent of molar susceptibility measurements
Thermal analysis
of powdered samples of [Gd(NO
[Dy(NO L] (3), [Ho(NO L] (4) and [Er(NO
carried out in an applied magnetic field of 0.1 T over the tem-
perature range 1.8–300 K. The data are presented as plots of
versus T, in Fig. 2, where is the molar magnetic susceptibility
3
)
3
H
2
L] (1), [Tb(NO
3
)
3
H
2
L] (2),
The thermal behavior of the new Schiff base complexes 1–5 were
investigated by thermogravimetric analysis (TG), differential ther-
mogravimetric analysis (DTG) and differential scanning calorimetry
3
)
3
H
2
3
)
H
3 2
) H L] (5) were
3 3 2
M
v T
(
DSC). The investigated Schiff base complexes decompose in the
v
M
Cristóv?o, Beata
