New mononuclear CuII and tetranuclear CuII 2-LaIII2 Schiff base complexes - Physicochemical properties

Article abstract of DOI:10.1016/j.poly.2013.06.023

The new mononuclear inclusion compound [CuLc(H₂O)] (1) and heterotetranuclear [Cu₂La₂(L)₂(NO ₃)₆ (EtOH)₂] (2) (L= C₁₉H ₁₈N₂O₄Br_2<

Full text of DOI:10.1016/j.poly.2013.06.023

Accepted Manuscript
New mononuclear CuII and tetranuclear CuII –LaIII Schiff base complexes –
2
2
Physicochemical properties
Beata Cristóvão, Barbara Miroslaw, Julia Kłak
PII:
S0277-5387(13)00474-9
DOI:
http://dx.doi.org/10.1016/j.poly.2013.06.023
POLY 10198
Reference:
To appear in:
Polyhedron
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Accepted Date:
28 March 2013
8 June 2013
Please cite this article as: B. Cristóvão, B. Miroslaw, J. Kłak, New mononuclear CuII and tetranuclear CuII –LaIII
2
Schiff base complexes – Physicochemical properties, Polyhedron (2013), doi: http://dx.doi.org/10.1016/j.poly.
2
2013.06.023
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New mononuclear CuII and tetranuclear CuII –LaIII Schiff base complexes –
2
2
physicochemical properties
Beata Cristꢀ vãoa*, Barbara Miroslawb, Julia Kłakc
aDepartment of General and Coordination Chemistry, Maria Curie-Sklodowska University,
Maria Curie-Sklodowska sq. 2, 20-031 Lublin, Poland
bDepartment of Crystallography, Faculty of Chemistry, Maria Curie-Sklodowska University,
Maria Curie-Sklodowska sq. 3, 20-031 Lublin, Poland
cFaculty of Chemistry, University of Wrocław, F. Joliot Curie 14, 50-383 Wrocław, Poland
*beata.cristovao@poczta.umcs.lublin.pl
Abstract
The new mononuclear inclusion compound [CuL(H2O)] (1) and heterotetranuclear
[Cu2La2(L)2(NO3)6(EtOH)2] (2) (L = C19H18N2O4Br2) complex, derived from the hexadentate
Schiff base compartmental ligand N,N’-bis(5-bromo-3-methoxysalicylidene)propylene-1,3-
diamine, have been synthesized and characterized by elemental analysis, XRF, FTIR, EPR
and TG/DSC techniques. Their molecular structures have been confirmed by single crystal X-
ray diffraction studies. Compound 1 crystallizes in the triclinic P-1 space group with the
Cu(II) ion occupying the N2O2 cavity and having a water molecule encapsulated by O–H…O
hydrogen bonds in the O2O2 compartment. The tetranuclear complex 2 crystallizes in the
monoclinic space group P21/c with only half of the molecule in the asymmetric unit. The
inner N2O2 salen-type cavities are occupied by Cu(II) ions, whereas the La(III) ions are
present in the open, larger O2O2 space of the Schiff base ligands. The two symmetrically
independent heterodinuclear fragments of the molecule are bridged via nitrato ions, linking
Cu(II) and La(III) ions Magnetic studies for the complexes indicated a weak
antiferromagnetic interaction between the copper(II) ions in the crystal lattice of 1 and in the
tetranuclear unit of 2.
Keywords: N,O-donors ligand, Compartmental Schiff base, Heteronuclear complex, Inclusion
compound, Water encapsulation,
1

1. Introduction
Complexes of 3d and 3d4f metal ions with Schiff bases as the main ligands have attracted
special attention in view of their numerous applications in bioinorganic chemistry,
magnetochemistry, separation processes, luminescence, environmental chemistry,
electrochemistry and catalysis [1-8]. Bicompartmental Schiff bases, obtained by the
condensation of 3-methoxy/ethoxysalicyladehyde with diamines, have two cavities of
different dimensions. Thus, these ligands can bind with one or two metal centers, enabling the
successful synthesis of homo- and/or heteronuclear metal complexes with interesting
stereochemistries. For heteronuclear 3d-4f compounds, the 3d metal ion occupies the inner
N2O2 cavity, whereas all four oxygen atoms of the capacious outer O2O2 cavity coordinate
to the 4f center due to the oxophilicity and the larger ionic radius of the lanthanide ions [5-
7,9-13]. The ionic radii of the 3d metal ions are smaller compared to 4f metal ions. Therefore,
in the 3d coordination compounds the O2O2 compartment of the Schiff base ligand is not a
suitable center for coordination of the 3d metal ion and it is expected that these oxygen atoms
will not coordinate to a second 3d metal center to form a 3d-3d compounds. The empty cavity
may be satisfied by molecules of water (or other solvents), resulting in the formation of an
inclusion product [14–20]. This is the first report on the crystal structure of a copper(II)
complex of N,N’-bis(5-bromo-3-methoxysalicylidene)propylene-1,3-diamine with an
encapsulated water molecule in the O2O2 compartment. In the literature, there are reports on
two types of structures of analogues Schiff base complexes with water molecules involved in
hydrogen bonds with the oxygen atoms of the O2O2 Schiff base ligand cavity. In the first type
of supramolecular structure, the water molecules are completely encapsulated in the O2O2
compartment, usually forming four short intermolecular contacts with all the O atoms. The
transition metal ion usually has a coordination number of 4, with an almost planar
coordination environment. It is worth noting that water inclusion compounds are formed with
ethoxy [17–23] as well as with methoxy derivatives of Schiff base ligands [24-28]. The
second type of structure is a “pseudo”-encapsulation. In such structures, the water molecules
forming hydrogen bonds with the O2O2 atoms are in fact coordinated to the metal center
present in the N2O2 cavity from a neighboring molecule. There are known examples of such
coordination compounds with Cu(II) [29], Ni(II) [30], Fe(III) [31] and Mn(II/III) [32] metal
ions.
The present investigation concerns the synthesis, molecular and crystal structures, magnetic,
spectral and thermal properties of the supramolecular compound [CuL(H2O)] (1) and the
2

coordination compound [Cu2La2(L)2(NO3)6(EtOH)2] (2) derived from N,N’-bis(5-bromo-3-
methoxysalicylidene)propylene-1,3-diamine (Scheme 1). In our earlier paper [7] we described
the synthesis and physicochemical properties of a heterotrinuclear CuIILaIIICuII compound
(the dinuclear cationic complex having nitrates as the counterion), also with N,N’-bis(5-
bromo-3-methoxysalicylidene)propylene-1,3-diamine (H2L) as a ligand. It is worth noticing
that by using the same substrates (H2L, copper(II) acetate monohydrate and lanthanide nitrate
hexahydrate), but changing the ratio of the reagents and the conditions of the synthesis, we
II
have now obtained the tetranuclear compound Cu2 La2III (2) (an inert complex).
2. Experimental
2.1. Synthesis
2.1. 1. Materials
The reagents and solvents, i.e. 1,3-diaminopropane, 5-bromo-2-hydroxy-3-
methoxybenzaldehyde, Cu(CH3COO)2H2O, La(NO3)36H2O, ethanol, methanol and DMF,
used for synthesis were commercially available from Aldrich Chemical Company and Polish
Chemical Reagents. They were used as received without further purification.
2.1.2. N,N’-bis(5-bromo-3-methoxysalicylidene)propylene-1,3-diamine, H2L
The Schiff base (N,N’-bis(5-bromo-3-methoxysalicylidene)propylene-1,3-diamine,
C19H20Br2N2O4), abbreviated as H2L, was obtained by the 2:1 condensation of 5-bromo-2-
hydroxy-3-methoxybenzaldehyde and 1,3-diaminopropane in methanol according to the
procedure reported by Maneiro et al. [6].
2.1.3. [CuL(H2O)] (1)
Complex 1 was obtained by the treatment of Cu(CH3COO)2H2O (0.2 mmol, 0.0399g) in
MeOH (10mL) with H2L (0.2 mmol, 0.0998 g) in 10mL of hot MeOH (about 318 K). As a
result, a brown solid appeared. The obtained mixture was stirred for about 30 min, after this
time the brown solid was collected by filtration. Recrystallization of this product from
dimethylformamide yielded diffraction quality brown crystals.
3

Yield: 84 mg/72 %. Anal. Calc. (579.73): C, 39.35; H, 3.45; N, 4.83; Cu, 10.97%. Found: C,
39.47; H, 3.34; N, 4.51; Cu, 10.62%. FTIR (KBr, cm-1): 3564w, 3516w, 2928w, 2828w,
1612s, 1536m, 1460s, 1320s, 1236s, 1076m, 984w, 848m, 788m, 688m, 572w, 488w, 348w.
2.1.4. [Cu2La2(L)2(NO3)6(EtOH)2] (2)
A mixture of the Schiff base H2L (0.050 g, 0.10 mmol) and Cu(CH3COO)2H2O
(0.0199g, 0.10 mmol) in 15 ml of EtOH was stirred and refluxed for 30 min. A solution of
La(NO3)36H2O (0.0433 g, 0.10 mmol) in 10 ml EtOH was added and the mixture was
refluxed for 15 min and filtered. Diethyl ether was allowed to diffuse slowly into this solution
at room temperature and green single crystals were obtained in three weeks.
Yield: 72 mg/80 %. Anal. Calc. (1865.44): C, 27.05; H, 2.57; N, 7.51; Cu, 6.81; La ,
14.90 %. Found: C, 27.40; H, 2.56; N, 7.36; Cu, 6.50; La 14.60%. FTIR (KBr, cm-1): 3420m,
2928w, 1628s, 1468s, 1384s, 1296s, 1236m, 1076w, 1040w, 840w, 792w, 696w, 568w,
468w.
2.2. Methods
The carbon, hydrogen and nitrogen contents in the analysed compounds were
determined by elemental analysis using a CHN 2400 Perkin Elmer analyser. The copper and
lanthanum contents were established using an ED XRF spectrophotometer
(CanberraPackard).
The FTIR spectra of the complexes were recorded over the range 4000-200 -1 using
cm
an M-80 spectrophotometer (Carl Zeiss Jena). Samples for FTIR spectra measurements were
prepared as KBr discs.
The magnetization of powdered samples of the complexes was measured over the
temperature range 1.8-300 K using a Quantum Design SQUID MPMSXL–5 magnetometer.
The superconducting magnet was generally operated at a field strength ranging from 0 to 5 T.
Measurements were made at a magnetic field of 0.5 T. The SQUID magnetometer was
calibrated with a palladium rod sample. Corrections are based on subtracting the sample
holder signal and a contribution χD estimated from Pascal’s constants [33].
EPR spectra were obtained using a Bruker Elexsys E 500 spectrometer equipped with
an NMR teslameter (ER 036TM) and frequency counter (E 41 FC) at the X-band at room
temperature and at about 77 K.
4

Thermal analyses of the complexes and the Schiff base ligand were carried out by
thermogravimetric (TG) and differential scanning calorimetry (DSC) methods using a
SETSYS 16/18 analyser (Setaram). The experiments were carried out under an air flow in the
temperature range 20-700 C at a heating rate of 10 C min-1. The samples (5.01 mg (1), 7.78
mg (2) and 5.27 mg (Schiff base ligand)) were heated in Al2O3 crucibles.
The X-ray powder diffraction patterns of the products of the decomposition process
were taken on a HZG-4 (Carl–Zeiss, Jena) diffractometer using Ni filtered CuK radiation.
The measurements were made within the range 2 = 4-80  by means of the Bragg-Brentano
method.
2.3. X-ray crystal structure determination
The X-ray diffraction intensity data for crystals 1 and 2 were collected on an Oxford
Diffraction Xcalibur CCD diffractometer with graphite-monochromatized MoK radiation (λ
= 0.71073 Å) at 100(2) K. Data sets were collected using the ω scan technique, with an
angular scan width of 1.0°. The programs CrysAlis CCD and CrysAlis Red [34] were used for
data collection, cell refinement and data reduction. Empirical absorption correction using
spherical harmonics was made for 1 [35] and analytical absorption correction based on the
indexing of crystal faces was applied for 2 [36]. The structures were solved by direct methods
using SHELXS-97 and refined by the full-matrix least-squares on F2 using SHELXL-97 [37]
(both operating under WinGX [38]). The non-hydrogen atoms were refined with anisotropic
displacement parameters. The water H atoms in 1 were found in the Fourier difference map
and refined isotropically. The ethanol O-bound H atom in 2 was located in the difference
Fourier map and refined using a riding model with O-H distances of 0.84 Å and Uiso(H) = 1.2
Ueq(O). The other hydrogen atoms were placed in calculated positions and treated as riding
atoms: C-H = 0.93 - 0.99 Å, with Uiso(H) = 1.2 Ueq(C) or = 1.5 Ueq(C) for methyl H atoms.
The tetranuclear molecule in 2 lies on a center of inversion, with only half of the
molecule being symmetrically independent. Bond length restraint was applied by DFIX
instructions to 2.00(2) Å for the C20–C21 distance in the ethanol molecule in 2.
A summary of the data collection conditions and the crystal structure refinement parameters
are given in Table 1. The molecular plots were drawn with ORTEP3 for Windows [39].
3. Results and discussion
5

3.1. Infrared spectra
The solid state FTIR spectra of the Schiff base (H2L) and its complexes were recorded and
compared in order to get some information about the binding of the Schiff base ligand to the
Cu(II) and La(III) ions (Figs 1 and 2). The FTIR spectrum of compound 1 (Fig. 1) exhibits
two sharp bands at 3564 and 3516 cm-1 due to the OH stretching vibrations of the water
molecules and one medium intensity band at 1536 cm-1 corresponding to the bending
vibrational mode (H2O) of the water molecules. In hydrated crystals, the asymmetric and
symmetric (OH) stretching vibrations are spread over a considerable range of energies and
are normally observed as a broad band. However, because of the encapsulation, the motion of
the water molecule in 1 is restricted and the asymmetric and symmetric stretching bands
appear separately (Fig. 1) [14-20]. The crystal structure analysis of 1 revealed that the water
molecule is encapsulated in the O2O2 compartment through strong O–H…O hydrogen bonds
with the phenoxo and methoxy oxygen atoms.
In the FTIR spectrum of 2, the broad medium strong band with a maximum around 3420 cm-1
may be assigned to the OH stretching vibrations of ethanol molecules, (OH) [5,7,40,41]. The
weak bands at around 2940 and 2840 cm-1 in the spectra of the ligand and complexes may be
attributed to the asymmetric and symmetric stretching vibrations of the (CH3) group from
the methoxy substituent [8,42]. Four bands near 1468 (4), 1296 (1), 1040 (2) and 840 cm-1
(6) for 2 may be assigned to the coordinated nitrate groups [43-45]. The strong band
attributed to (C=N) stretching vibrations of the imine nitrogen, present in the free ligand at
1636 cm-1, is shifted to lower wavenumbers in 1 and 2, at 1612 and 1628 cm-1, respectively
(Figs 1 and 2), confirming the coordination of the nitrogen atom from the azomethine group
to the copper(II) ion. This coordination mode is expected to reduce the electron density in the
azomethine fragment as a result of the withdrawal of electron density from the nitrogen atom,
and as such lowers the C=N absorption frequency [3,5,8,16,41]. A strong band observed at
1253 cm-1 in the spectrum of the free Schiff base has been assigned to the phenolic stretching
vibration (C–O). On complexation, this band is shifted to a lower frequency of 1236 cm-1,
indicating coordination through the phenolic oxygen atom [3,4,46]. The occurrence of two
bands at ca. 572 and 448 cm-1 in 1 and at ca. 568 and 468 cm-1 in 2, and their absence in the
spectrum of the free Schiff base ligand, give further evidences for M–O and M–N interactions
6

[16,40,46]. The infrared spectroscopic results are in accordance with the X-ray crystal
structure data for 1 and 2.
3.2. Description of the crystal and molecular structure of complex 1
The molecular structure of the mononuclear Cu(II) inclusion compound [CuL(H2O)] (1)
(where L = C19H18N2O4Br2) is shown in Figure 3. It crystallizes in the triclinic P-1 space
group. Details of the relevant data collection and refinement are summarized in Table 1.
Selected bond lengths and angles are listed in Table 2. In this compound the metal center has
a coordination number of 4 and occupies the N2O2 cavity of the salen-type Schiff base
ligand. The bond lengths and angles around the Cu(II) ion indicate large deviations from
planarity for the coordination polyhedron. The dihedral angle between the (O1Cu1N1) and
(O2Cu1N2) planes is 31(1)º. A similar structure was observed in related compounds
[11,17,20]. The water molecule in the crystal of 1 is hydrogen bonded to the oxygen atoms
inside the O2O2 compartment of the Schiff base ligand. The donoracceptor distances
between the water molecule and the phenoxo and methoxy oxygen atoms are in the range
2.823(4) to 3.236(5) Å (Table 3), indicating that the hydrogen bonds can be considered as
moderately strong.
3.3. Description of the crystal and molecular structure of complex 2
The tetranuclear complex [Cu2La2(L)2(NO3)6(EtOH)2] (2) (where L = C19H18N2O4Br2)
crystallizes in the monoclinic P21/c space group with two symmetrically dependent fragments
related to each other by the inversion center. Its crystal structure, along with the atomic
numbering scheme, is shown in Figure 4. Details of the relevant data collection and
refinement are summarized in Table 1. Selected bond lengths and angles are given in Table 2.
The molecule of 2 is composed of two moieties linked by two nitrato groups, both acting as
monodentate ligands towards the copper ion from one unit and towards a lanthanum ion from
the other unit. The Cu(II) ion adopts a square-based 4+1 coordination mode, the equatorial
N2O2 donors being afforded by the Schiff base ligand, while the axial position is occupied by
an oxygen atom of bridging nitrate ion. The four atoms of the N2O2 site are almost coplanar
and are displaced from the mean plane by less than 0.038 Å. The copper(II) center is 0.113 Å
above the mean N2O2 coordination plane. As observed in related compounds, the CuN/O
7

bond lengths are within the usual range of values observed for copper coordinated to Schiff
base ligands [11,17,20].
The La(III) ions are deca-coordinated. In addition to the phenolate oxygen atoms, the
coordination sphere contains two oxygen atoms of the OMe side arms of the Schiff base, five
oxygen atoms of the two bidentate (2-chelating) and one bridging nitrato ions, and one
oxygen atom of the ethanol molecule. The dihedral angle () between the (O1Cu1O2) and
(O1Ln1O2) planes is c.a. 24(1)º (Table 2). The LaO bond lengths depend on the nature of
the oxygen atoms. The distances increase in sequence:
phenolato < ethanol < nitrato < methoxo (from 2.526(3) to 2.646(3) Å).
i
The intramolecular separations Cu1...La1 and Cu1...La1i (symmetry code: -x, -y, -z+1),
being ca. 3.6 and 6.4 Å, respectively (Table 2), are within the normal range of values for
polynuclear CuLn complexes [9,45]. The ethanol O8 atom is involved as a donor in the
hydrogen bond with the nitrate O9 atom. The methyl groups interact via C–H…O hydrogen
bonds with the nitrato O6 oxygen atom. In the crystal structure, weak C–H…O hydrogen
bonds are observed between the tetranuclear units (Table 3). Additionally, the presence of Br
atoms in the crystal structure results in the formation of intermolecular contacts between the
O9 and Br2 atoms, with a separation shorter than the sum of the van der Waals radii (3.297(2)
Å).
3.4. Magnetic properties
The magnetic susceptibilities of [CuL(H2O)] (1) and [Cu2La2(L)2(NO3)6(EtOH)2] (2)
have been measured in the temperature range 1.8–300 K in a 0.5 T applied magnetic field. As
shown in the Figure 5, at the room temperature the χmT product of 1 is equal to 0.835 cm3 mol-
1
K and it is consistent with only one unpaired electron in magnetically diluted copper(II)
complexes [47]. The value of the product χmT is constant with temperature lowering and
reaches 0.427 cm3 mol-1 K at 1.8 K. The decrease in the value of χmT in the low-temperature
range is caused by the occurrence of weak antiferromagnetic interactions between the CuII
ions in the crystal lattice. The susceptibility data obey the Curie-Weiss law over the
temperature range 1.8–300 K with a Curie constant (C) of 0.391 cm3 K mol-1 and a Weiss
constant (θ) of - 1.1 K. A small, negative value of the Weiss constant could also confirm a
small antiferromagnetic exchange coupling between the metal ions in the crystal lattice.
8

From a magnetic point of view, compound 1 will be considered as a mononuclear
compound. In such a situation, the magnetic data were fitted using the susceptibility equation
for S = 1/2 (eq. 1). To elucidate the significance of the exchange between the copper(II) ions in
the crystal lattice, a molecular field correction term was also included, eq. (2) [47,48].
N 2g2
3kT
(1)
(2)
M 
S(S 1)
M
Mcorr

2zJ'
N 2 g2
1
 M
where N is Avogadro’s number, g – the spectroscopic splitting factor, β – the Bohr magneton,
k – Boltzmann constant, zJ’ – the intermolecular exchange parameter and z is the number of
the nearest neigbours of the CuII center. The least-squares fit of the experimental data using
these equations were limited to the temperature range from 1.8 K to 300 K, and leads to zJ’ =
- 2.98(1) cm-1 and g = 2.07(1), as indicated by the solid curve in Figure 5. The agreement
factor R is equal to 5.52×10-4. The criterion used in the determination of the best fit was based
on minimization of the sum of the squares of the deviation: R = Σ(χexpT – χcalcT)2/Σ(χexpT)2.
The result of the calculations may be indicative of weak interactions between the copper(II)
centers, transmitted through a system of hydrogen bonds in the crystal lattice. This fact is
expected in mononuclear magnetically diluted CuII compounds [49].
To confirm the nature of the ground state of 1, we investigated the variation of the
magnetization, M, with respect to the field, at 2 K. The results are shown in Figure 6, where
the molar magnetization M is expressed in µB units. As the magnetic field increases, the M vs.
H curve is linear over the whole field range and indicates a value of the magnetization close
0.7 µB at 5 T. This value is somewhat lower than the simulated value calculated with the
Brillouin function for one unpaired electron for the CuII ion.
As depicted in Figure 7 at the room temperature, the χmT product of 2 is equal to 0.790
cm3 mol-1 K. Obviously, this value is equal to the expected value originating from two
uncorrelated CuII (d8, S = 1) ions, since no contribution is expected from the non-magnetic
LaIII ions (f 0, S = 0). In this compound, the intramolecular Cu…Cu distance is certainly large
and may preclude any significant magnetic interactions between the copper centers in the
tetranuclear unit. However, below 10 K, a discernible drop in the χmT values to 0.668 cm3 K
mol-1 at 1.8 K was also noted. The thermal dependence of χmT is characteristic of a small
9

antiferromagnetic interaction since no maximum appears in the χm vs T curve until 1.8 K. This
apparent weak antiferromagnetic interaction may be mediated by the diamagnetic
lanthanide(III) center. The examples in the literature [7,50] show that diamagnetic
lanthanide(III) centers are capable of mediating magnetic exchange between copper(II)
centres in a molecule.
This conclusion is fully confirmed by the magnetization versus field curve of 2 at 2 K
(Fig. 6). This measurement shows a saturation value of ca. 2 µB, which is the expected value
1
for two independent S = /2 systems with g = 2, and confirm our previous assumption. To
estimate the magnitude of the interaction in 2, we used the following equation for the
magnetic susceptibility for two exchange coupled copper(II) ions [51]. The data have been
 
H  2JS1  S
fitted to a model which assumes the exchange Hamiltonian to be
susceptibility is then predicted to be:
2 . The
2N 2 g2
3kT
1




(3)
m 
1 exp

 2J / kT



3
where the symbols have their usual meaning. Least-squares fitting of the experimental data
over the whole temperature range leads to J = - 0.37(1) cm-1, g = 2.08(2), as indicated by the
solid curve in Figure 6. The agreement factor R is equal to 1.29×10-5 (R = Σ(χexpT –
χcalcT)2/Σ(χexpT)2). The nearest Cu…Cu distance in the tetranuclear unit is 7.730 Å. This fact
causes very poor overlap of magnetic orbitals, resulting in very weak antiferromagnetic
coupling. Since the Cu···Cu distances in the examined compounds are all rather long, it is to
be expected that the coupling between the electrons of the CuII ions in the systems will be
weak.
3.5. EPR spectra
The EPR spectra of a polycrystalline sample of complex 1 at room temperature and 77
K resemble monomeric CuII species, with well-resolved hyperfine features and g|| > g┴ > ge
(g|| =2.22 and g┴ =2.05) (Fig. 8). The spectral features are characteristic of a square-planar
geometry and a dx2_ 2 ground state for the copper(II) center [53,54].
y
The polycrystalline EPR spectra of 2, at both room temperature and liquid nitrogen
temperature (77 K), show rhombic spectra with g1 = 2.02, g2 = 2.10 and g3 = 2.21, consistent
10

2_ 2
y
with a dx
ground state for the square pyramidal geometry of the CuN2O2(O) chromophore
[55]. Moreover, in low field part of the spectra the ΔMS = 2 transition is observed, which
confirms the presence of weak magnetic exchange interactions between the CuII centers [56–
59], as detected by the direct magnetic measurements (Fig. 9).
The EPR results are in accord with the X-ray structural data obtained for complexes 1
and 2, respectively.
3.6. Thermal analysis
The thermal behaviors of complexes 1 and 2 were characterized in air atmosphere
using thermogravimetric analysis (TG), differential thermogravimetric analysis (DTG) and
differential scanning calorimetry (DSC) (Figs 10 and 11). The complexes are stable at room
temperature. As shown in Figure 10, on heating over the temperature range 80–100 C, the
change in mass (2.90%) estimated from the TG curve recorded for 1 corresponds to the loss of
one water molecule (the theoretical value is 3.10 %). The recorded DSC curve reveals the first
endothermic peak with a maximum at around 90 C. The anhydrous complex is stable up to
290 C. The second endothermic peak recorded on the DSC curve, with a maximum at around
220 C and without changing the mass on the TG, may indicate the melting of complex 1. The
next step for the decomposition of 1 is a gradually loss of mass within the higher temperature
range 290-650 C. It is probably connected with the decomposition of Schiff base ligand,
which is accompanied by strong exothermic peaks, seen on the DSC curve. The final product
of the decomposition of 1 is CuO, which is formed at around 660 C. The corresponding mass
loss calculated from the TG curve is 14.10 % (the theoretical value is 13.70 %).
Complex 2 (Fig. 11) is stable on heating up to 150 C. The first mass loss recorded on
the TG curve at a higher temperatures (160-180 C) is equal to 5.30%, which is connected
with the release of two molecules of coordinated ethanol molecules (the theoretical value is
4.90%). This process is accompanied by an endothermic peak, seen on the DSC curve. As the
temperature is increased, the TG curve of the heteronuclear compound exhibits a mass loss
over a wide temperature range, 290-700 C, which is accompanied with exothermic peaks
seen on DSC curves at around 300 and 530 C. This may be due to the decomposition of the
Schiff base ligand (Figure 11 (inside)). The free Schiff base, on heating in the temperature
range 20-700 C, first melts. This process is confirmed by the recorded DSC and TG curves.
The DSC curve shows an endothermic peak with a maximum at about 130 C which is
11

connected with no mass loss on the TG curve. At higher temperature, the TG curve exhibits a
mass loss in the range 220-700 C, which is accompanied by exothermic peaks seen on the
DSC curves, with maxima at around 280 and 550 C, connected with the burning of the Schiff
base ligand. The decomposition process of CuII LaIII2 compounds is intricate and it is difficult
2
to distinguish the intermediate solid products. The solid residue obtained during the thermal
decomposition of complex 2 is a mixture of La2CuO4 and CuO, which is in accord with results
presented by Blake et al [60]. The residual mixture percentage, determined from the TG
curve, is 25.80%, and this value is fairly coincident with the theoretical one of 26.00%. The
final products of the thermal decomposition of 1 and 2 were experimentally verified by phase
analysis on the basis of the obtained X-ray diffraction powder patterns.
Conclusion
Starting from the same Schiff base ligand molecule we have synthesized two
coordination compounds with different architectures. The host-guest supramolecular
compound [CuL(H2O)] (1) and the novel heterotetranuclear coordination compound
[Cu2La2(L)2(NO3)6(EtOH)2] (2) were obtained. The water molecule found in the crystal
structure of 1 is not coordinated to the metal center, but is encapsulated through O–H…O
hydrogen bonds to the phenolate and methoxy oxygen atoms as a guest molecule in the O2O2
compartment of the Schiff base ligand. This interesting inclusion property is observed for
other transition metal Schiff base complexes. However, little is still known about the factors
that determine the formation of host-guest compounds instead of compounds with “pseudo-
encapsulated” water molecules that are coordinated to the transition metal center and interact
additionally with the oxygen atoms from the outer O2O2 empty cavity of a neighboring
molecule. The magnetic susceptibility measurements, as well as the relationship between the
magnetization and magnetic field strength in 1 revealed weak antiferromagnetic interactions
between the magnetic CuII centers in the crystal lattice at low temperature.
The inclusion product with the water molecule in the O2O2 compartment is not
formed when the lanthanide ions are present in the solution during the synthesis.
Determination of the crystal structure of 2 shows that this heterotetranuclear compound
consists of two symmetrically dependent moieties with two Schiff base molecules behaving as
dianionic hexadentate chelating ligands with two N2O2 and O2O2 donor sites. The Cu(II) and
La(III) ions in both parts of the molecule are linked via two nitrato bridging ligands. A weak
12

antiferromagnetic interaction is observed in the tetranuclear compound 2, between the
copper(II) centers and mediated by the diamagnetic lanthanum(III) center.
Appendix A. Supplementary data
CCDC 931234 and 931235 contain the supplementary crystallographic data for 2 and 1,
respectively.
These
data
can
be
obtained
free
of
charge
via
http://ww.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail
deposit@ccdc.cam.ac.uk.
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Captions for illustration
Scheme 1. Chemical structure of the Schiff base ligand H2L.
Fig. 1. FTIR spectra of complex 1 and the free ligand H2L (inside: part of the FTIR spectra of
complex 1 and the H2L ligand demonstrating the stretching vibrations of the encapsulated
water molecule in 1).
Fig. 2. FTIR spectra of the complex 2 and the ligand H2L.
Fig. 3. Ortep view with the atom numbering scheme of the mononuclear copper complex with
the encapsulated water molecule in the crystal of 1; the displacement ellipsoids are drawn at
the 50% probability level, hydrogen spheres are drawn with an arbitrary radius.
Fig. 4. Molecular structure of the tetranuclear Cu2La2 complex 2; only half of the molecule is
symmetrically independent and it is numbered in the picture; the displacement ellipsoids are
drawn at the 50% probability level, hydrogen atoms are omitted for clarity.
Fig. 5. Temperature dependence of the experimental mT (○) and m (●) vs T for complex 1.
The solid line is the calculated curve derived from equations (1) and (2).
Fig.6. Field dependence of the magnetization for complex 1 (●) and 2 (■) at 2 K. The dashed
line is the Brillouin function for an independent S = 1 system; the solid line is the Brillouin
function curve for two independent S = 1/2 of the CuII LaIII2 unit.
2
Fig. 7. Temperature dependence of the experimental mT (□) and m (■) vs T for complex 2.
The solid line is the calculated curve derived from equation (3).
Fig. 8. EPR (X-band) spectrum of powdered compound 1 at room temperature.
Fig. 9. EPR (X-band) spectrum of powdered compound 2 at room temperature. The inset
shows the forbidden line ΔMS = 2.
Fig. 10. TG, DTG and DSC curves of 1 in an air atmosphere.
Fig. 11. TG, DTG and DSC curves of 2 (inside TG, DTG and DSC curves of the Schiff base
ligand) in an air atmosphere.
16

Table 1
Crystallographic data for crystals 1 and 2.
1
2
Empirical formula
Formula weight
Temp. (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C19H20Br2N2O5Cu
579.73
100(2)
triclinic
P-1
9.406(1)
9.796(1)
11.508(2)
93.34(1)
111.21(1)
94.44(1)
981.3(2)
2; 1
C42H48Br4N10O28Cu2La2
1865.44
100(2)
monoclinic
P21/c
11.683(1)
11.475(1)
21.576(1)
90
90.14(1)
90
2892.5(4)
2; 0.5
γ (°)
V (Å3)
Z; Z’
Crystal form/colour
Crystal size (mm)
Dcalc (g cm-3)
µ (mm-1)
needle/brown
0.40 x 0.20 x 0.10
1.962
plate/green
0.40 x 0.35 x 0.25
2.142
5.220
5.032
Absorption correction
θ range (°)
Reflns coll./unique (Rint) 5785/3545 (0.0402)
Data/restr./parameters
GooF on F2
multiscan
2.70 – 25.24
analytical
3.12 – 25.24
10169/5218 (0.0156)
5218 / 1 / 397
1.069
3545 / 0 / 270
0.815
R1, wR2 [I>2σ(I)]
R1, wR2 (all data)
Δρmax, Δρmin (e Å-3)
0.0327, 0.0599
0.0579, 0.0629
0.616 and -0.586
0.0291, 0.0699
0.0339, 0.0712
1.506, -0.781
Table 2
Selected geometric parameters (Å, °) in the coordination
environments of the metal centers.
1
bond lengths
N1-Cu1
N2-Cu1
O1-Cu1
O2-Cu1
angles
1.989(3) O1-Cu1-O2 86.7(1)
1.936(4) O1-Cu1-N1 92.9(1)
1.908(3) O1-Cu1-N2 156.0(1)
1.919(3) N2-Cu1-O2 92.3(1)
O2-Cu1-N1 158.2(1)
N2-Cu1-N1 96.6(1)
2
bond lengths
N1-Cu1
N2-Cu1
angles
1.987(3) O1-Cu1-O2
1.963(3) O1-Cu1-N1
78.8(1)
92.5(1)
O1-Cu1
1.949(3) O1-Cu1-N2 167.4(1)