Synthesis, crystal structures and magnetic behavior of Ni II-4f-NiII compounds

Article abstract of DOI:10.1016/j.poly.2012.05.045

We report here synthesis, crystal and molecular structures as well as magnetic measurement results for new heterotrinuclear Schiff base complexes having a general formula [Ni₂Ln(L)₂(CH₃COO) ₂(MeOH)₂]N

Full text of DOI:10.1016/j.poly.2012.05.045

Polyhedron 43 (2012) 47–54
Contents lists available at SciVerse ScienceDirect
Polyhedron
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Synthesis, crystal structures and magnetic behavior of NiII–4f–NiII compounds
b
c
Beata Cristóvão a, , Julia Kłak , Barbara Miroslaw
⇑
a Department of General and Coordination Chemistry, Maria Curie-Sklodowska University, Maria Curie-Sklodowska sq. 2, 20-031 Lublin, Poland
b Faculty of Chemistry, University of Wrocław, F. Joliot Curie 14, 50-383 Wrocław, Poland
c Department of Crystallography, Faculty of Chemistry, Maria Curie-Sklodowska University, Maria Curie-Sklodowska sq. 3, 20-031 Lublin, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
We report here synthesis, crystal and molecular structures as well as magnetic measurement results for
new heterotrinuclear Schiff base complexes having a general formula [Ni2Ln(L)2(CH3COO)2(MeOH)2]-
NO3ꢀ4H2O (where Ln = LaIII (1) and NdIII (2) and L = C19H18N2O4Br2 = N,N0-bis(5-bromo-3-methoxysalicy-
lidene)-1,3-diaminopropanato) and the mononuclear [NiL(H2O)2] (3) complex. The NiII/4f compounds are
isostructural and crystallize in the monoclinic space group P21/n. The lanthanide(III) ion lies on the center
of symmetry and is ten-coordinated forming pentagonal antiprism. Compound 3 crystallizes in ortho-
rhombic space group Pnma with the molecule lying on the mirror plane. The complex 1 indicated a weak
antiferromagnetic interaction between the terminal NiII ions. The magnetic properties of 2 revealed that
the NiII–NdIII interaction is antiferromagnetic. The magnetic data for 3 show dominant zero-field splitting
effect of NiII ions.
Received 22 April 2012
Accepted 30 May 2012
Available online 19 June 2012
Keywords:
Heterotrinucelar complexes
Nickel
Lanthanides
Schiff base
Crystal structure
Magnetic properties
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
found to be simple paramagnetic systems, NiII–LnIII–NiII (LnIII
=
GdIII, TbIII) compounds exhibit the ferromagnetic coupling, whereas
the complex NiII–DyIII–NiII indicates a single-molecule magnet
behavior [3].
The studies on NiII–4f heteronuclear compounds are mainly fo-
cused on research their crystal structures and magnetic features
[1–17]. The magnetic properties of heterotrinuclear NiII–LnIII–NiII
complexes (LnIII from LaIII to LuIII) were studied by Shiga et al.
and they indicated that the NiII–LnIII interaction is weakly antifer-
romagnetic for LnIII = CeIII, PrIII and NdIII, and ferromagnetic for
LnIII = GdIII, TbIII, DyIII, HoIII and ErIII. Moreover, their magnetic stud-
ies of NiII–LnIII–NiII compounds with diamagnetic rare earth(III)
ions (LaIII and LuIII) showed an antiferromagnetic interaction be-
tween the terminal NiII ions [1]. Pasatoiu et al. investigated a series
of heterodinuclear NiII–LnIII Schiff base complexes (LnIII from LaIII to
ErIII) and stated that there are antiferromagnetic interactions be-
tween NiII and LnIII ions for LnIII from the beginning of the 4f series
(CeIII, PrIII, NdIII, SmIII), while ferromagnetic occured from GdIII to-
ward the end of the 4f series [2]. Bayly, et al.’s magnetic studies
of heterotrinuclear NiII–LnIII–NiII compounds also showed the fer-
romagnetic exchange between NiII and GdIII, TbIII, DyIII, HoIII or ErIII
ions [5]. Kahn et al. observed with oxamato bridged NiII–LnIII com-
plexes that NiII–LnIII interaction is antiferromagnetic for LnIII = CeIII,
PrIII, NdIII, ErIII and ferromagnetic for LnIII = GdIII, TbIII, DyIII and per-
haps HoIII [8]. Chandrasekhar et al. reported the magnetic behavior
of the NiII–LnIII–NiII complexes (LnIII from LaIII to ErIII), the NiII–
LnIII–NiII (LnIII = LaIII, CeIII, PrIII, NdIII, SmIII, EuIII, HoIII and ErIII) were
In comparison to the magnetic studies of CuII–4f compounds
[2,18,19] the magnetic properties of NiII–4f complexes have been
much less investigated because many of these complexes contain
a square planar NiII ion which is diamagnetic [7,20–23].
We have undertaken study on heteronuclear NiII–4f complexes
in order to obtain more information on the crystal structures and
magnetic properties of NiII–LnIII Schiff base compounds. In the
present paper we report the synthesis, structural characterization
and magnetism of the heterotrinuclear [Ni2La(L)2(CH3COO)2
(MeOH)2]NO3ꢀ4H2O (1) and [Ni2Nd(L)2(CH3COO)2(MeOH)2]NO3ꢀ
4H2O (2) and the mononuclear [NiL(H2O)2] (3) complexes.
2. Experimental
2.1. Synthesis
2.1.1. Materials
The reagents and solvents i.e. 1,3-diaminopropane, 5-bromo-2-
hydroxy-3-methoxybenzaldehyde, Ni(CH3COO)2ꢀ4H2O, La(NO3)3ꢀ
6H2O, Nd(NO3)3ꢀ6H2O and methanol used for synthesis were com-
mercially available from Aldrich Chemical Company and Polish
Chemical Reagents. They were used as received without further
purification.
⇑
Corresponding author.
E-mail address: beata.cristovao@poczta.umcs.lublin.pl (B. Cristóvão).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.05.045

48
B. Cristóvão et al. / Polyhedron 43 (2012) 47–54
2.1.2. N,N0-bis(5-bromo-3-methoxysalicylidene)propylene-1,3-
diamine, H2L
The Schiff base (N,N0-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 reported procedure [18,19].
with the palladium rod sample. Corrections are based on subtract-
ing the sample – holder signal and contribution
the Pascal’s constants [24].
vD estimated from
2.3. X-ray crystal structure determination
Crystal data for 1, 2 and 3 were collected on Oxford Diffraction
Xcalibur CCD diffractometer with the graphite-monochromatized
2.1.3. [Ni2La(L)2(CH3COO)2(MeOH)2]NO3ꢀ4H2O (1)
Mo K
a
radiation (k = 0.71073 Å) at the temperatures of 100(2) K
scan
To a stirred solution of the Schiff base H2L (0.4 mmol, 0.1999 g)
in MeOH (30 mL) was added Ni(CH3COO)2ꢀ4H2O (0.4 mmol,
0.0995 g) in MeOH (10 mL). As result the brown solid appeared.
The obtained mixture was stirred for about 30 min, after this time
the solution of La(NO3)3ꢀ6H2O (0.2 mmol, 0.0866 g) in MeOH
(5 mL) was added. The brown precipitate dissolved and solution
turned pale-green. The resulting solution was left to stir for about
30 min, during which time it did not change color. The solution
was then filtered and allowed to evaporate slowly at low temper-
ature in the fridge (277 K). Pale-green crystals of 1 formed in three
weeks. Yield: 97 mg/31%. Anal. Calc. (1568.84): C, 33.68; H, 3.70; N,
4.46; Ni, 7.48; La, 8.85. Found: C, 33.47; H, 3.46; N, 4.31; Ni, 7.57;
La, 8.35%. FTIR bands (KBr, cmꢁ1): 3424m, 2932w, 2848w, 1644s,
1572w, 1468s, 1440m, 1384s, 1292s, 1236s, 1216m, 1104w,
1012w, 966w, 852w, 780m, 760w, 692w, 628w, 576w, 540w,
460m.
(1, 2) and 300(2) K (3). Data sets were collected using the
x
technique, with an angular scan width of 1.0°. The programs Cry-
sAlis CCD and CrysAlis Red [25] were used for data collection, cell
refinement and data reduction. Analytical absorption correction
based on the indexing of crystal faces was applied for 1–3 [26].
The structures were solved by direct methods using SHELXS-97
and refined by the full-matrix least-squares on F2 using the SHEL-
XL-97 [27] (both operating under WinGX [28]). Non-hydrogen
atoms with except of disordered nitrate N and O atoms and one
water molecule in 1 and 2 were refined with anisotropic displace-
ment parameters. The Nd ion in 1 and 2 lies on the center of inver-
sion with sof = 0.5. The nitrate ion is disordered over two positions
by the center of inversion (sof’s = 0.5). Because of disorder in nitrate
molecule the bond length restraints were applied by DFIX instruc-
tions to 1.23(1) Å and 2.13(1) Å for N–O and O–O distances, respec-
tively. In 3 the mirror plane bisects the molecule through Ni1, C9,
O3 and O4 atoms (sof’s = 0.5). The C-bound H atoms were included
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 O-bound H atoms were located in a difference Fourier map and
refined using a riding model with O–H distances of 0.82–0.85 Å
and Uiso(H) = 1.5 Ueq(O). A summary of data collection conditions
and the crystal structure refinement parameters are given in Table
1. The molecular plots were drawn with ORTEP3 for Windows [29]
and Diamond [30].
2.1.4. [Ni2Nd(L)2(CH3COO)2(MeOH)2]NO3ꢀ4H2O (2)
Complex 2 was synthesized according to the procedure fol-
lowed for 1 (Nd(NO3)3ꢀ6H2O, 0.2 mmol, 0.0877 g). Yield: 102 mg/
32%. Anal. Calc. (1574.17): C, 33.57; H, 3.68; N, 4.55; Ni, 7.46; Nd,
9.16. Found: C, 33.12; H, 3.36; N, 4.97; Ni, 7.10; Nd, 8.98%. FTIR
bands (KBr, cmꢁ1): 3424m, 2932w, 2848w 1640s, 1572w, 1468s,
1440m, 1384s, 1292s, 1236s, 1216m, 1104w, 1012w, 966w,
852w, 780m, 760w, 692w, 628w, 576w, 540w, 460m.
2.1.5. [NiL(H2O)2] (3)
Complex 3 was obtained by the treatment of Ni(CH3COO)2ꢀ4H2O
(0.2 mmol, 0.0498 g) in MeOH (10 mL) with H2L (0.2 mmol,
0.0995 g) in 10 mL of hot MeOH (about 318 K). The reaction mix-
tures were stirred for about 30 min, cooled and the resulting
brown precipitates were filtered off. The obtained solution was al-
lowed to evaporate slowly at low temperature in the fridge (277 K).
Pale-green crystals 3 formed over several days. Yield: 88 mg/74%.
Anal. Calc. (592.88): C, 38.59; H, 3.38; N, 4.74; Ni, 9.93. Found: C,
38.12; H, 3.23; N, 4.56; Ni, 9.49%. FTIR bands (KBr, cmꢁ1):
3464m, 2932w, 2844w 1624s, 1540w, 1468s, 1432m, 1384s,
1292s, 1240s, 1216m, 1112m, 1012w, 972w, 828m, 784m, 768m,
688m, 628w, 576w.
3. Results and discussion
3.1. Description of crystal and molecular structure of the trinuclear
complexes 1 and 2
The trinuclear Schiff base complexes [Ni2Ln(L)2(CH3COO)2
(MeOH)2]NO3ꢀ4H2O (where Ln = LaIII (1) and NdIII (2) and
L = C19H18N2O4Br2 = N,N0-bis(5-bromo-3-methoxysalicylidene)-
1,3-diaminopropanato) are isostructural and crystallize in the
monoclinic space group P21/n. Their schematic diagram is given in
Fig. 1. The atom numbering scheme of 1 is shown in Fig. 2 (analogues
for compound 2). Details of the single crystal X-ray diffraction data
collection and refinement are summarized in Table 1. The selected
bond lengths and angles are given in Table 2. These trinuclear com-
pounds consist of a cationic [Ni2Nd(L)2(CH3COO)2(MeOH)2]+ unit,
one uncoordinated nitrate ion and four water molecules.
The lanthanide(III) ion lies on the center of inversion and is ten-
coordinated forming pentagonal antiprism (Fig. 3). It is surrounded
by two Schiff base ligands. Comparing to our previously reported
trinuclear complexes of CuII–LnIII–CuII (where LnIII = CeIII, PrIII and
NdIII) with pentacoordinated CuII ions [19], the Schiff base mole-
cules in heteronuclear NiII compounds are more flat and their rel-
ative position may be described as partly overlapped instead of
propeller-like structure (Fig. 4). The organic ligand molecule is less
distorted in NiII–4f–NiII complexes, thus the disorder of bridge car-
bon atoms is no longer observed. The dihedral angles between two
phenyl rings in the Schiff base molecule are as follows (in °): 40 in
1 (NiII–LaIII–NiII) and 38 in 2 (NiII–NdIII–NiII) whereas in CuII–4f–CuII
compounds they are 51 (CuII–CeIII–CuII), 49 (CuII–PrIII–CuII) and 52
(CuII–NdIII–CuII), respectively.
2.2. Methods
The contents of carbon, hydrogen and nitrogen in the analysed
compounds were determined by elemental analysis using a CHN
2400 Perkin Elmer analyser.
The contents of nickel and lanthanides were established using
ED XRF spectrophotometer (Canberra–Packard).
The FTIR spectra of complexes were recorded over the range of
4000–400 cmꢁ1 using M–80 spectrophotometer (Carl Zeiss Jena).
Samples for FTIR spectra measurements were prepared as KBr
discs.
The magnetization of the (1), (2) and (3) powdered samples was
measured over the temperature range 1.8–300 K using a Quantum
Design SQUID – based MPMSXL–5-type magnetometer. The super-
conducting magnet was generally operated at a field strength rang-
ing from 0 to 5 T. Measurements sample of compounds were made
at magnetic field 0.5 T. The SQUID magnetometer was calibrated

B. Cristóvão et al. / Polyhedron 43 (2012) 47–54
49
Table 1
Crystallographic data for crystals 1, 2 and 3.
1
2
3
Empirical formula
Formula weight
T (K)
C
44H58Br4N5O21Ni2La
C44H58Br4N5O21Ni2Nd
1574.17
100(2)
C19H22Br2N2O6Ni
592.88
300(2)
1568.84
100(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/n
monoclinic
P21/n
orthorhombic
Pnma
7.583(2)
24.312(5)
11.619(4)
90
13.682(1)
15.461(1)
14.831(1)
117.11(1)
2792.6(4)
2
13.706(1)
15.525(1)
14.761(1)
117.34(1)
2790.1(4)
2
b (°)
V (Å3)
2142(1)
4
Z
Crystal form/colour
Crystal size (mm)
block/green
0.35 ꢂ 0.25 ꢂ 0.20
1.866
block/green
0.38 ꢂ 0.20 ꢂ 0.18
1.874
block/green
0.10 ꢂ 0.10 ꢂ 0.08
1.838
Dcalc (g cmꢁ3
)
l
(mmꢁ1
)
4.359
4.528
4.675
Absorption correction
h range (°)
Reflections collected/
analytical
2.74–25.24
22196/5046 (0.0387)
analytical
2.76–25.31
11001/5098 (0.0211)
analytical
3.06–25.24
7000/1983 (0.0991)
unique (Rint
Data/restraints/parameters 5046 / 7 / 342
)
5068 / 7 / 342
1.056
1983 / 0 / 143
1.031
Goodness-of-fit (GOF) on
1.067
F2
R1, wR2 [I > 2
R1, wR2 (all data)
qmin (e Åꢁ3
r
(I)]
0.0521, 0.1462
0.0760, 0.1568
0.0436, 0.1171
0.0582, 0.1221
0.0830, 0.2079
0.1569, 0.2352
D
qmax
,
D
)
1.633 (0.65 Å from H5B) and ꢁ1.330 (1.06 Å 2.358 (1.01 Å from H5A) and ꢁ1.105 (0.96 Å 2.044 (1.41 Å from O4) and ꢁ1.591 (0.92 Å
from Br1) from Br1) from H3)
The rare earth(III) ion supplements its coordination environ-
ment by two acetate ions. Each pair of the NiII and LnIII metal ions
are linked by two bridging phenoxo oxygen atoms (O1 and O2) of
the Schiff base ligand and one bidentate bridging carboxylate
group (O6C20O7) of the acetate ion.
expected due to their differences in ionic size and are observed in
related 3d–4f compounds [2,3,7]. The Ln1–O bond lengths depend
on the nature of the oxygen atoms and increase in sequence ace-
tate < phenolato < methoxo (Table 2).
The dihedral angles (a) between the (O1Ni1O2) and (O1La1/
Nickel(II) ion occupies the N2O2 compartment of the Schiff base.
It has distorted octahedral geometry with methanol molecule and
acetate anion (bridging to LnIII) in apical positions. The NiII ion lies
in the mean N1N2O1O2 plane of the Schiff base ligand with devi-
ation being only 0.045 and 0.042 Å for 1 and 2, respectively. While
in CuII–CeIII–CuII, CuII–PrIII–CuII and CuII–NdIII–CuII complexes the
corresponding deviations for CuII ions are higher, viz. 0.148, 0.144
and 0.154 Å, respectively [19].
Nd1O2) planes are ca. 23° and 26°, respectively (Table 2).
The intramolecular separations Ni1...Ln, and Ni1...Ni1i (symme-
try code (i) ꢁx, ꢁy, ꢁz) being ca. 3.6/3.3 and 7.2/7.0 Å in 1 and 2,
respectively (Table 2) are within the normal ranges of values for
polynuclear NiII–LnIII complexes [2,3,7]. Intermolecular M...M
The M–O bond distances of NiII and LaIII/NdIII with bridging phe-
nolate and carboxylate groups are significantly different (Ni–O ca.
2.0 Å, and La/Nd–O ca. 2.5 Å). Such differences in bond distance are
Fig. 2. Molecular structure and atom numbering scheme of complex 1, displace-
ment ellipsoids were drawn at 50% probability level, hydrogen spheres were drawn
with an arbitrary radius. The nitrate ion and water molecules in the outer sphere
were omitted for clarity. Molecule lies on the center of inversion thus only half of
molecule is symmetry independent and was numbered in the picture.
Fig. 1. Diagram of molecular structure of complexes 1 and 2 (Ln = LaIII for 1, NdIII for
2).

50
B. Cristóvão et al. / Polyhedron 43 (2012) 47–54
Table 2
Selected geometric parameters (Å, °) in the coordination environments of metal
centers.
Bond lengths
1
2
Bond lengths
3
Ln1–O1
Ln1–O2
Ln1–O3
Ln1–O4
Ln1–O7
Ni1–N1
Ni1–N2
Ni1–O1
Ni1–O2
Ni1–O6
Ni1–O8
Ni1–Ln1
Ni1...Ni1i
2.514(4)
2.521(4)
2.851(4)
2.902(4)
2.477(6)
2.041(5)
2.032(5)
2.036(4)
2.047(4)
2.067(5)
2.158(5)
3.607(4)
7.214(4)
2.450(4)
2.450(4)
2.857(4)
2.924(3)
2.401(4)
2.037(4)
2.028(5)
2.024(4)
2.037(4)
2.070(4)
2.163(4)
3.254(4)
7.049(4)
Ni1–O1
Ni1–O3
Ni1–O4
Ni1–N1
2.006(7)
2.125(9)
2.13(1)
2.063(9)
Fig. 3. A view of coordination polyhedra in 1.
Angles
1
2
Angles
3
Ni1–O1–Ln1
Ni1–O2–Ln1
O1–Ln1–O2
N2–Ni1–N1
N2–Ni1–O1
N2–Ni1–O2
N1–Ni1–O2
O1–Ni1–N1
O1–Ni1–O2
O1–Ni1–O6
O2–Ni1–O6
N1–Ni1–O6
N2–Ni1–O6
N1–Ni1–O8
N2–Ni1–O8
O1–Ni1–O8
O2–Ni1–O8
O6–Ni1–O8
O1–Ln1–O3
O2–Ln1–O3
O7–Ln1–O3
O1–Ln1–O4
O2–Ln1–O4
O3–Ln1–O4
O7–Ln1–O1
O7–Ln1–O2
O7–Ln1–O4
a
104.4(2)
103.8(2)
64.0(2)
96.5(2)
172.8(2)
91.4(2)
171.6(2)
90.4(2)
81.6(2)
94.9(2)
95.4(2)
87.8(2)
87.9(2)
90.1(2)
95.2(2)
82.2(2)
86.4(1)
176.5(2)
57.7(1)
111.6(1)
116.3(2)
112.8(1)
57.0(1)
119.8(1)
76.0(2)
78.1(2)
117.2(2)
23(2)
103.6(2)
103.2(1)
64.9(1)
96.4(2)
172.5(2)
91.9(2)
171.2(2)
90.9(2)
80.8(1)
93.8(2)
95.9(2)
87.3(2)
88.6(2)
89.4(2)
96.1(2)
82.0(2)
86.7(1)
174.6(2)
57.7(1)
112.0(1)
117.2(1)
113.6(1)
57.1(1)
119.3(1)
78.0(1)
80.4(1)
118.2(1)
26(2)
O1–Ni1–O1ii
O1–Ni1–N1
N1–Ni1–N1ii
O1–Ni1–O3
N1–Ni1–O3
O1–Ni1–O4
N1–Ni1–O4
O3–Ni1–O4
85.9(4)
88.7(3)
96.6(5)
91.2(3)
88.5(3)
91.3(3)
89.2(3)
176.5(5)
Fig. 4. A view showing partly overlapping two Schiff base units in complex 1.
Table 3
Hydrogen bond parameters (Å, °) in 1, 2 and 3.
D–HꢀꢀꢀA
D–H
HꢀꢀꢀA
DꢀꢀꢀA
<DHA
Complex
O5–H5A. . .O12
O5–H5B. . .O7i
O8–H8. . .O5
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.82
2.10
1.85
1.91
1.94
2.02
1.99
1.89
1.98
2.12
2.24
2.858(7)
2.682(6)
2.682(6)
2.708(7)
2.782(7)
2.819(6)
2.722(7)
2.826(7)
2.87(1)
148.5(4)
166.4(3)
172.7(4)
148.5(4)
149.2(4)
166.1(4)
165.2(4)
173.5(4)
145(1)
1
1
1
1
2
2
2
2
3
3
O12–H12B. . .O9ii
O5–H5A. . .O12
O5–H5B. . .O7i
O8–H8. . .O5
a
– dihedral angle between the O1Ni1O2 and O1Ln1O2 planes.
i ii
Symmetry codes: ꢁx, ꢁy, ꢁz; x, ꢁy + 1/2, z.
O12–H12B. . .O9ii
O3–H3. . .O1iii
O4–H4. . .O2iv
separations in the NiII–LnIII–NiII structures indicate well separation
of the trinuclear 3d–4f–3d cores (Table 2).
2.94(1)
143(1)
Because of special position of molecule in the unit cell two NiII
and one LnIII ions are arranged in a line, with a Ni1–Ln1–Ni1i (sym-
metry code (i): ꢁx, ꢁy, ꢁz) angle equal to 180° (Table 2). In the out-
er coordination sphere between nitrate ions and water molecules
forms extensive network of hydrogen bonds (Table 3).
i
Symmetry codes: ꢁx, ꢁy, ꢁz; iix + 1, y, z; iiix + 1/2, ꢁy + 1/2, ꢁz + 1/2, vix ꢁ 1/2,
y ꢁ 1/2, ꢁz + 1/2.
3.3. Magnetic properties
The magnetic susceptibility of NiII–LaIII–NiII (1) has been mea-
sured in the temperature range of 1.8–300 K in a 0.5 T applied mag-
netic field. The data obtained for complex are represented in Fig. 7.
The central ion in 1 is diamagnetic (LaIII, f0, S = 0), therefore the mag-
netization in this sample results only from the two NiII (d8, S = 1)
ions. The NiII ions are octahedrally coordinated and are expected
to exhibit spin-only moments modified by the effects of second-or-
der spin-orbit coupling, which leads to a g value in excess of 2 [31].
3.2. Description of crystal and molecular structure of the complex 3
The complex 3 [NiL(H2O)2] is built of one-nuclear units. A view
of the complex 1 is shown in Fig. 5. Details of the single crystal
X-ray diffraction data collection and refinement are summarized
in Table 1 and the principal bond distances and angles are listed
in Table 2. The mirror plane bisects molecule through C9, Ni1, O3
and O4 atoms. The NiII ion has octahedral coordination and occu-
pies the N2O2 cavity of the Schiff base ligand. The larger compart-
ment is occupied by water molecules which connect the
mononuclear units by O–H. . .O hydrogen bonds into one-dimen-
sional columns lying parallel to the a axis (Table 3, Fig. 6). Between
At the room temperature, the
2.32 cm3 molꢁ1 K and it is consistent with only two S = 1 nickel(II)
centers (the expected with
mT value is 2.42 cm3 mol–1
vmT product is equal to
v
K
gNi = 2.20). The moment decreases only marginally from 300 to
25 K, below which a rather sharp drop to 0.767 cm3 mol–1 K at
1.8 K due to the intramolecular interaction and/or the effect of
zero-field splitting are observed. In this complex, the intramolecu-
aromatic rings there are
p. . .p stacking interactions with separa-
tion between best planes being equal to 3.57 Å.

B. Cristóvão et al. / Polyhedron 43 (2012) 47–54
51
Fig. 5. Ortep view of molecule 3 showing octahedral coordination of Ni ion. Atoms Ni1, C9, O3 and O4 lie on the mirror plane and only half of the molecule is symmetrically
independent.
Fig. 6. View along a direction. Stacking columns in 3.
lar Ni. . .Ni distance is certainly large and may preclude any signifi-
cant magnetic interactions between the nickel centers in the het-
erotrinuclear compound. The thermal dependence of
characteristic of a small antiferromagnetic interaction since no
maximum appears in the mT vs T curve until 1.8 K. This apparent
vmT is
v
weak antiferromagnetic interaction may be mediated by the dia-
magnetic lanthanum(III) center. The examples in the literature
[1,3–5,32] and previously reported by us heterotrinuclear complex,
CuII–LaIII–CuII [19] show, that the diamagnetic lanthanum(III)
center are capable of mediating magnetic exchange between the
metal(II) in heteronuclear compounds.
The magnetization curve for 1 measured at 2 K was reproduced
with the Brillouin function for two centers of SNi = 1 (Fig. 8) and
clearly confirm our previous assumption.
Magnetic analyses were carried out using the magnetic suscep-
tibility expression (Eq. (1)) based on the isotropic Heisenberg mod-
~
~
el (H ¼ ꢁ2JSNi1 ꢀ SNi2) [33]:
ꢀ
ꢁ
2Ng2b2
5 expð6J=kTÞ þ expð2J=kTÞ
5 expð6J=kTÞ þ 3 expð2J=kTÞ þ 1
-1
Fig. 7. Temperature dependence of experimental
vmT (s) and vm (d) vs T for
vm
¼
þ Na
ð1Þ
kT
complex 1. The solid line is the calculated curve derived from Eq. (1).

52
B. Cristóvão et al. / Polyhedron 43 (2012) 47–54
by lowering the temperature to 0.933 cm3 mol–1 K in 1.8 K. This
decrease could be caused by crystal field effect, as well as cooper-
ative antiferromagnetic interactions of NiII–NdIII pairs. The suscep-
tibility data obey the Curie–Weiss law with the Curie constant of
C = 3.71 cm3 mol–1 K and the Weiss constant of h = ꢁ7.4 K. Nega-
tive value of Weiss constant could also confirm the antiferromag-
netic exchange coupling between the metal ions. Previously, we
reported heterotrinuclear complex, CuII–NdIII–CuII [19] and mag-
netic investigations also reveal a antiferromagnetic interaction be-
tween the CuII and LnIII centres. These results are consistent with
the empirical studies concerning heterometallic 3d–4f compounds,
in which the 4f ions display a spin–orbit coupling [1,5,23,34–42]. A
final conclusion on the nature of the exchange interaction in com-
pounds cannot be drawn, since the magnetic properties of this type
of systems are not only due to the coupling between the 3d and 4f
metal ions, but also to the thermal depopulation of the Stark sub-
levels of the 4f ions.
To confirm the nature of the ground state of 2, we investigated
the variation of the magnetization, M, with respect to the field, at
2 K. The results are shown in Fig. 10, where molar magnetization
M is expressed in lB units. The compound does not reach the sat-
uration in the applied field range and magnetization in 5 T is equal
Fig. 8. Field dependence of the magnetization for complex 1. The solid line is the
Brillouin function for two independent S = 1 of NiII–LaIII–NiII unit.
where b is the Bohr magneton, g is Lande factor, k is Boltzman’s con-
stant, J is exchange parameter and N is the temperature-indepen-
a
dent paramagnetism of NiII ion. For simplicity, other small magnetic
contributions (intermolecular interaction, single ion zero-field split-
ting, etc.) are included in parameter J. The best least-squares fit to
the experimental data in whole temperature range (solid line in
only to 3.52 lB.. This value is much lower than the simulated val-
ues calculated with the Brillouin function for two NiII ions and one
NdIII ion, which do not interact. This further indicates that the com-
plex behaves as an antiferromagnet. Unfortunately, the quantita-
tive description of the magnetic properties of heterometallic
complexes containing lanthanide(III) ions is not an easy task be-
cause of the ligand-field effect and spin-orbit coupling of the LnIII
ion [23,36].
Fig. 7) is obtained with J = ꢁ0.75 cmꢁ1, g = 2.18 and N = 320 ꢂ 10ꢁ6
a
as indicated by the solid curve in Figure. The agreement factor R is
equal 5.29 ꢂ 10ꢁ4 (R =
R
(
v
expT ꢁ
v R(v
calcT)2/ expT)2). The exchange
value of complex
2
is in good agreement with the J value
(ꢁ0.63 cmꢁl) of the similar trinuclear complex [Ni2La(L)2(NO3)3
(H2O)4]H2O [1]. This result strongly suggests that weak antiferro-
magnetic intramolecular Ni. . .Ni interactions in the trinuclear unit
are present and are partially or totally responsible for the decrease
The magnetic properties of the complex 3 was determined over
the temperature range of 1.8–300 K. Plots of magnetic susceptibil-
ity vmꢁ1 and
v
mT product vs T are given in Fig. 11. The value
vmT of
at 300 K equals 1.19 cm3 molꢁ1 K and remains constant down to
20 K. The value of vmT in room temperature is larger than that ex-
of
vmT product at low temperatures. Nevertheless, it is worth add-
pected for the spin-only formula for a mononuclear NiII complex
ing that the given values of J should be taken with caution as even
the presence of a very weak anisotropy of NiII can enhance artifi-
cially its estimation.
(S = 1), indicating that a relevant orbital contribution is involved.
The
vmT product decreases significantly at lower temperatures,
attaining a value of 0.362 cm3 molꢁ1 K at 1.8 K. The decrease of
The magnetic properties of the heterotrinuclear complex, NiII–
NdIII–NiII (2), was also determined over the teꢁm1perature range of
the values
vmT in the lowest temperature range is mainly due to
zero-field splitting effect of NiII ions and probably weak antiferro-
1.8–300 K. Plots of magnetic susceptibility vm and
vmT product
magnetic intermolecular interactions between the NiII ions.
vs T are given in Fig. 9. The experimental vmT values at room tem-
perature (3.71 cm3 mol–1 K) approximately correspond to the cal-
Since the decrease vmT in low temperature range can be attrib-
À
Á
uted the zero-field splitting (ZFS) of NiII ions, we used the Hamilto-
culated ones
v
mT ¼ ðNb2=3kÞ½2gN2 iSNiðSNi þ 1Þ þ g2LnJLnðJLn þ 1Þꢃ
nian (Eq. (2)) for an isolated NiII ion with S = 1:
for uncoupled metal ions (3.64 cm3 mol–1 K). The
v
mT decreases
Fig. 10. Field dependence of the magnetization for complex 2. The solid line is the
Fig. 9. Temperature dependence of experimental
complex 2.
v
mT (s) and vm (d) vs T for
Brillouin function curve for a two independent S = 1 and one S = 3 2 of NiII–NdIII–NiII
/
-1
unit.

B. Cristóvão et al. / Polyhedron 43 (2012) 47–54
53
cated as the solid curve in Fig. 11. The criterion used in determina-
tion of the best fit was based on minimization of the sum of squares
of the deviation, R =
R
(v
expT ꢁ
v
calcT)2/
R(
v
expT)2 (R = 2.15 ꢂ 10ꢁ4).
A small, negative value of zJ’ obtained from calculations indicates
occurrence of weak antiferromagnetic interactions between the
nickel(II) ions in crystal lattice. Calculated zero-field splitting
parameter of examined complex 3 equals 5.89 cmꢁ1 is typical for
large group of monomeric complexes of the nickel(II), which indi-
cates values of the parameter in the range 3–6 cmꢁ1 [45–48], how-
ever are known examples much lower values [49–51]. Nevertheless,
the calculated D parameter is larger than zJ’ and suggests that in the
compound studied the zero-field splitting effect of the NiII ions is
dominant.
Fig. 12 presents the relation M = f(H) for complex 3. As the mag-
netic field increase, the M vs H curve indicates linear relation up to
5 T with value of magnetization 1.38
ple even at 5 Tesla and 2 K is below the saturation value of 2
l
B Magnetization of the sam-
B ex-
l
-1
Fig. 11. Temperature dependence of experimental
vmT (s) and vm (d) vs T for
pected for a S = 1 ground state with g = 2 in the absence of zero-
field splitting. The shape of the curve formed by the experimental
points of 3 does not follow the Brillouin function [52]. The experi-
mental values of magnetizations are lower than the simulated val-
ues calculated with the Brillouin function for isolated nickel(II) ion
and demonstrate the role played by zero-field splitting effect for
the NiII ions and/or weak intermolecular antiferromagnetic
interaction.
complex 3. The solid line is the calculated curve derived from Eqs. (3)–(6).
4. Conclusions
The size of 3d cation and its coordination preferences are crucial
for geometry of trinuclear 3d–4f–3d complex resulting in different
relative position of two Schiff base ligands and slightly shorter dis-
tances between 3d and 4f ions for NiII–LnIII–NiII (where LnIII = LaIII
(1) and NdIII (2)) in comparison to previously reported by us heter-
onuclear CuII–LnIII–CuII (where LnIII = CeIII, PrIII and NdIII) com-
pounds. In presented crystals 1 and 2 the metal centers of
nickel(II) and lantadnide(III) ions are bridged by two Schiff base’s
phenolato oxygens and additionally by the acetate ions.
The magnetic properties obtained for NiII–LaIII–NiII (1) show
weak antiferromagnetic exchange coupling between the terminal
nickel(II) ions through the diamagnetic lanthanum(III) center.
The measurement of variable-temperature magnetic susceptibility
reveals antiferromagnetic interaction between spin carriers in the
heterotrinuclear complex NiII–NdIII–NiII (2). The magnetic data for
the mononuclear NiII (3) compound indicates zero field splitting ef-
fect of the NiII ion.
Fig. 12. Field dependence of the magnetization for complex 3. The solid line is the
Brillouin function curve for non coupled ions, the g–factor is taken 2.00 and S = 1.
ꢂ
ꢃ
1
3
2
^
^
H ¼ D Sz ꢁ SðS þ 1Þ þ gbHS
ð2Þ
Developing the Hamiltonian (Eq. (2)) we can deduce the follow-
ing expressions (Eqs. (3)–(5)) [43] for the magnetic susceptibility
corrected by the factor predicted from the molecular field (Eq.
(6)) [44]:
Appendix A. Supplementary data
vjj þ 2v?
CCDC 876079, 876080 and 876081 contains the supplementary
crystallographic data for 1, 2 and 3. 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.
vm
vjj
¼
ð3Þ
ð4Þ
3
2Ng2jjb2
kT 1 þ 2 expðꢁD=kTÞ
expðꢁD=kTÞ
¼
2Ng2?b2 1 ꢁ expðꢁD=kTÞ
v?
¼
ð5Þ
ð6Þ
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