Heterometallic trinuclear 3d-4f-3d compounds based on a hexadentate Schiff base ligand

Article abstract of DOI:10.1016/j.poly.2013.10.019

The new heterometallic trinuclear complexes [Ni₂Ln(L) ₂(CH₃COO)₂(MeOH)₂]NO ₃·4H₂O (where Ln = Ce(III) 1, Pr(III) 2 and H ₂L = the Schiff base resulting from the condensation of 5-bromo-3-methoxysalicylaldehyde and 1,3-diaminopropane) were synthesized and characterized by elemental analysis, FTIR, TG/DSC, TG-FTIR, single crystal X-ray diffraction, magnetic, UV-Vis and luminescence studies. In the isostructural crystals of 1 and 2 the nickel(II) and lanthanide(III) ions are bridged by two Schiff base's phenolato oxygens and additionally by the acetate ions. The clathrate type of crystal architecture in reported complexes, results in their quite high stability in air and nitrogen atmospheres at room temperature despite the relatively large amount of non-coordinated water. The temperature dependence (1.8-300 K) of the magnetic susceptibility investigations and the field-dependent magnetization indicate crystal field effect, as well as, the presence of an antiferromagnetic interaction between spin carriers of Ni ^II and Ln^III pairs in the Ni^II-Ce ^III-Ni^II (1) and Ni^II-Pr^III-Ni ^II (2) compounds operating via oxygen atoms (J_NiCe = -1.1(4) cm^-1 (1) and J_NiPr = -1.3(8) cm^-1 (2)).

Full text of DOI:10.1016/j.poly.2013.10.019

Polyhedron 68 (2014) 180–190
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Heterometallic trinuclear 3d–4f–3d compounds based on a hexadentate
Schiff base ligand
Beata Cristóvão a, , Julia Kłak , Robert Pełka , Barbara Miroslaw , Zbigniew Hnatejko
⇑
b
c
d
e
a
Department of General and Coordination Chemistry, Maria Curie-Sklodowska University, Maria Curie-Sklodowska sq. 2, 20-031 Lublin, Poland
Faculty of Chemistry, University of Wrocław, F. Joliot Curie 14, 50-383 Wrocław, Poland
Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152, 31-342 Kraków, Poland
Department of Crystallography, Maria Curie-Sklodowska University, Maria Curie-Sklodowska sq. 3, 20-031 Lublin, Poland
b
c
d
e
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Pozna n´ , Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The new heterometallic trinuclear complexes [Ni Ln(L) (CH COO) (MeOH) ]NO ꢀ4H O (where
2
2
3
2
2
3
2
Received 10 June 2013
Accepted 15 October 2013
Available online 26 October 2013
2
Ln = Ce(III) 1, Pr(III) 2 and H L = the Schiff base resulting from the condensation of 5-bromo-3-methoxy-
salicylaldehyde and 1,3-diaminopropane) were synthesized and characterized by elemental analysis,
FTIR, TG/DSC, TG-FTIR, single crystal X-ray diffraction, magnetic, UV–Vis and luminescence studies. In
the isostructural crystals of 1 and 2 the nickel(II) and lanthanide(III) ions are bridged by two Schiff base’s
phenolato oxygens and additionally by the acetate ions. The clathrate type of crystal architecture in
reported complexes, results in their quite high stability in air and nitrogen atmospheres at room temper-
ature despite the relatively large amount of non-coordinated water. The temperature dependence
Keywords:
3d–4f Compound
Schiff base
Magnetism
Lanthanides
Luminescence
(1.8–300 K) of the magnetic susceptibility investigations and the field-dependent magnetization indicate
crystal field effect, as well as, the presence of an antiferromagnetic interaction between spin carriers of
II
III
II
III
II
II
III
II
Ni and Ln pairs in the Ni –Ce –Ni (1) and Ni –Pr –Ni (2) compounds operating via oxygen atoms
1
ꢁ1
(
J
NiCe = ꢁ1.1(4) cmꢁ (1) and JNiPr = ꢁ1.3(8) cm (2)).
Ó 2013 Elsevier Ltd. All rights reserved.
1
. Introduction
In recent years, there is growing interest in the synthesis, struc-
behavior of 3d–4f heteronuclear compounds from Schiff base li-
gands [1,3–11], there are relatively less reports on their thermal
and photophysical properties[1–3,12–15]. The results of thermal
analysis allowed us to confirm the presence of lattice and/or coor-
dination water molecules in the crystal structure of complexes and
to determine the endothermic and/or exothermic effects connected
with processes such as: desolvation, dehydration, melting and
decomposition. The near-infrared and photoluminescence (PL)
properties of lanthanide(III) (Pr , Nd , Eu , Tb , Er , Yb ) com-
plexes have been of special interest in recent years because these
ions exhibit long luminescent lifetime, large Stoke’s shifts and
characteristic line-like emission bands. The luminescence proper-
ties of homo- (4f) and heterometallic (3d–4f) complexes with
‘‘salen’’ style Schiff base ligands were studied by Jones at el.
[2,3,12–15] and they indicated that the photophysical properties
of lanthanide ions depend markedly on the environment surround-
ture and physicochemical properties of heteronuclear compounds
consisted of simultaneously 3d and 4f metal centers due to the
variety of potential applications of these complexes mainly in bio-
inorganic chemistry, magnetochemistry, separation processes,
luminescence, environmental chemistry, electrochemistry and
catalysis [1,2]. The salen-like hexadentate Schiff base derived
from 5-bromo-3-methoxysalicylaldehyde and 1,3-diaminopropane
3
+
3+
3+
3+
3+
3+
(
Scheme 1) contains a tetradentate inner core formed of two imino
nitrogen and two -phenoxo oxygen atoms and outer coordination
compartment that chelates through two -phenoxo and two meth-
l
l
oxy oxygen atoms. The methoxy groups from the ligands derived
from o-vanillin play an active role in holding rare earth cations
within heterometallic complexes. This ligand is particular for 3d/
4f chemistry because it is able to coordinate 3d and 4f metal ions
ing the metal center. Solvents containing O–H groups (i.e. H
2
O and
via N and O atoms in a different way. Lanthanide ions behave as
hard acids and they prefer oxygen to nitrogen donors, while 3d me-
tal ions can coordinate to both N and O donors [3,4]. In contrast to
the number reports concerning the synthesis and magnetic
CH OH) can efficiently quench the luminescence of lanthanide
3
ions. However, if the O–H fragment is replaced by O–D, this process
becomes much less efficient. As a continuation of our investigation
0
on 3d–4f multinuclear coordination compounds with N,N -bis
(
5-bromo-3-methoxysalicylidene)propylene-1,3-diamine in this
paper we describe the results of spectral, magnetic, thermal, lumi-
nescence (in the case of 2) studies and crystal structures of new
⇑
Corresponding author.
E-mail address: beata.cristovao@poczta.umcs.lublin.pl (B. Cristóvão).
0
277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.10.019

B. Cristóvão et al. / Polyhedron 68 (2014) 180–190
181
2
400 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
ꢁ1
4
000–400 cm using M-80 spectrophotometer (Carl Zeiss Jena).
Samples for FTIR spectra measurements were prepared as KBr
discs. The magnetization of the 1 and 2 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
with the palladium rod sample. Corrections are based on subtract-
2
Scheme 1. Schiff base ligand H L.
heteronuclear [Ni
Ni Pr(L) (CH COO)
2
Ce(L)
2
(CH
3
COO)
2
2
(MeOH) ]NO
3
ꢀ4H
2
O (1) and
ing the sample – holder signal and contribution
the Pascal’s constants [16] The UV–Vis spectra of Schiff base ligand
and compounds (methanol solution 2 ꢂ 10 M) were recorded in
0 ꢂ 10 mm quartz cells over the range 200–900 nm using GENE-
SYS 10S UV–Vis spectrophotometer. Emission spectra of the free
Schiff base ligand H L and complexes were recorded at concentra-
D
v estimated from
[
2
2
3
2
(MeOH)
2
]NO
3
2
ꢀ4H O (2) complexes.
.
ꢁ5
2
. Experimental
1
2.1. Materials
2
ꢁ5
tions of 2.5 ꢂ 10 M in methanol at room temperature. Excitation
and emission spectra were measured on a Hitachi 7000 spectroflu-
orimeter with excitation and emission slits of 5 nm. The emission
quantum yield for mononuclear Ni(II) complex was measured
using a relative method with anthracene as the standard [17]
and calculated from the equation [18].
The reagents and solvents i.e. Ni(CH
O, Pr(NO O, 1,3-diaminopropane, 5-bromo-2-hydroxy-3-
3
COO)
2
ꢀ4H
2
O, Ce(NO
3
)
3
ꢀ6H2-
3
)
3
ꢀ6H
2
methoxybenzaldehyde 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.
2
x s
I A n
x
/
¼ /s
2
2
2
.2. Synthesis
s x
I A n
s
0
.2.1. N,N -bis(5-bromo-3-methoxysalicylidene)propylene-1,3-
where: / is the quantum yield, subscript s stands for the reference
and x for the sample. A is the absorbance at the excitation wave-
length, n is the refractive index and I is the integrated emission
intensity. Thermal analyses of the prepared compounds were car-
ried out by the thermogravimetric (TG) and differential scanning
calorimetry (DSC) methods using the SETSYS 16/18 analyser (Seta-
ram). The experiments were carried out under air flow in the tem-
diamine, H
The synthesis of the Schiff base (N,N’-bis(5-bromo-3-methoxy-
salicylidene)propylene-1,3-diamine, C19 ) abbreviated
as H L was describe earlier [4].
2
L
2 2 4
H20Br N O
2
2
.2.2. [Ni
To a stirred solution of the Schiff base H
in hot methanol (30 ml) was added Ni(CH COO)
.0995 g) in hot MeOH (10 mL). As a result the brown solid ap-
peared. The obtained mixture was vigorous stirred for about
2 2
Ce(L) (CH
3 2 2
COO) (MeOH) ]NO
3
ꢀ4H
2
O (1)
L (0.4 mmol, 0.1999 g)
ꢀ4H O (0.4 mmol,
ꢁ1
perature range of 20–1000 °C at a heating rate of 10 °C min . The
2
2 3
samples (7,38 mg (1) and 7,85 mg (2)) were heated in Al O
crucibles.
The X-ray powder diffraction patterns of the products of
decomposition process were taken on a HZG-4 (Carl–Zeiss, Jena)
3
2
2
0
3
0
0 min, after this time the solution of Ce(NO
3
)
3
2
ꢀ6H O (0.2 mmol,
diffractometer using Ni filtered Cu K
a radiation. The measure-
.0868 g) in MeOH (5 mL) was added. Upon addition of lanthanide
ments were made within the range of 2h = 4–80° by means of
Bragg–Brentano method. The TG–FTIR coupled measurements
have been carried out using a Netzsch TG apparatus coupled with
a Bruker FTIR IFS66 spectrophotometer. The samples of about
nitrate the brown precipitate dissolved and the solution became
pale-green. The resulting solution was left to stir for about
3
0 min, during which time it did not change the color. Then the
solution was clarified by filtration and allowed to stand at low
temperature. Pale-green crystals were formed during three weeks.
Yield: 34%. Anal. Calc. (1570.13): C, 33.66; H, 3.69; N, 4.46; Ni, 7.48;
Ce, 8.92. Found: C, 33.12; H, 3.20; N, 4.80; Ni, 7.30; Ce, 8.70%. FTIR
1
0 mg were heated up to 800 °C (compounds) and 900 °C (Schiff
ꢁ1
base), respectively at a heating rate of 10°Cmin in flowing argon
atmosphere.
ꢁ1
bands (KBr, cm ): 3420m, 2932w, 2846w 1644s, 1570w, 1468s,
1
8
440m, 1384s, 1292s, 1236s, 1214m, 1104w, 1012w, 966w,
52w, 780m, 760w, 692w, 628w, 576w, 544w, 462m.
2.4. X-ray crystal structure determination
Intensity measurements were carried out at 295 K with Oxford
Diffraction Xcalibur CCD diffractometer with the graphite-mono-
2
.2.3. [Ni
Complex 2 was synthesized according to the procedure fol-
lowed for 1 (Pr(NO O, 0.2 mmol, 0.0870 g). Yield: 36%. Anal.
2
Pr(L)
2
(CH
3
COO)
2
(MeOH)
2
]NO
3
ꢀ4H
2
O (2)
chromatized MoK
lected using the
a
x
radiation (k = 0.71073 Å). Data sets were col-
scan technique, with an angular scan width of
3
)
3
ꢀ6H
2
Calc. (1570.92): C, 33.64; H, 3.69; N, 4.46; Ni, 7.47; Pr, 8.97. Found:
C, 33.25; H, 3.15; N, 4.65; Ni, 7.12; Pr, 8.49%. FTIR bands (KBr,
cm ): 3424m, 2932 w, 2848 w 1640s, 1572w, 1468s, 1440m,
1
7
1.0°. The programs CrysAlis CCD and CrysAlis Red [19] were used
for data collection, cell refinement and data reduction. Analytical
absorption correction based on the indexing of crystal faces was
applied [20]. The structures were solved by direct methods using
SHELXS-97 and refined by the full-matrix least-squares on F2 using
the SHELXL-97 [21]. The lanthanide ions in 1 and 2 lie on the center
of inversion 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 instructions to 1.23(1) and 2.13(1) Å for N–O and O–O
ꢁ1
384s, 1292s, 1236s, 1216m, 1100 w, 1012w, 966w, 852w,
80m, 760w, 692w, 628w, 578w, 540w, 460m.
2.3. Methods
The contents of carbon, hydrogen and nitrogen in the analysed
compounds were determined by elemental analysis using a CHN

1
82
B. Cristóvão et al. / Polyhedron 68 (2014) 180–190
distances, respectively. Non-hydrogen atoms with except of disor-
dered nitrate N and O atoms and one water molecule in 1 and 2
were refined with anisotropic displacement parameters. The C-
bound H atoms were positioned geometrically and the ‘riding’
model for the C–H bonds was used in the refinement 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 maps and refined
using a riding model with O–H distances of 0.82–0.85 Å and
Uiso(H) = 1.5 Ueq(O). The summary of experimental details and
the crystal structure refinement parameters are given in Table 1.
The molecular plots were drawn with ORTEP3 for Windows [22]
and Diamond [23].
3
. Results and discussion
3.1. Structural description
The two isostructural heterometallic trinuclear Schiff base com-
III
plexes [Ni
(
2
Ln(L)
2
(CH
1) and Pr (2) and L = C19
oxysalicylidene)-1,3-diaminopropanato) crystallize in the P2
space group. The atom numbering scheme of 1 is shown in Fig. 1
analogues for the compound 2). The selected bond lengths and an-
3
COO)
2
(MeOH)
2
]NO
3
ꢀ4H
2
O (where Ln = Ce
III
H
18
N
2
O
4
Br
2
= N,N’-bis(5-bromo-3-meth-
/n
1
(
gles are given in Table 2. In these compounds rare earth(III) ion
occupies the center of inversion coordinating ten oxygen atoms
(
eight O atoms from two hexadentate Schiff base ligands and two
Fig. 1. Molecular structure and atom numbering scheme of NiII–CeIII–NiII (1) at 20%
O atoms – both coming from two acetate ions acting as bidentate
bridging ligands) which form pentagonal antiprism (Figs. 1 and
probability, hydrogen atoms, nitrate ion and water molecules in the outer
coordination sphere are omitted for clarity. Molecule lies on the cennter of of
inversion thus only half of the molecule is symmetrically independent and was
numbered in the picture, symmetry code : ꢁx, ꢁy, ꢁz.
2). The nickel(II) ions are hexacoordinated. They are located in the
i
inner N cavities of the two bicompartmental Schiff base ligands
2 2
O
and their coordination polyhedra adopt a distorted octahedral
geometry (Figs. 1 and 2). The oxygen atoms of methanol molecules
III
II
and acetate anions (bridging to Ln ) occupy the axial sites. The Ni
depend on the nature of the oxygen atoms and increase in sequence
acetate < phenolato < methoxo (Table 2). Such differences in bond
distance are expected due to their differences in ionic size of 3d
and 4f metal ions and are observed in related 3d–4f compounds
[4,7,5]. The dihedral angles (a) between the (O1Ni1O2) and
(O1Ce1/Pr1O2) planes are 24.7° (1) and 25.8° (2), respectively
ions lie in the mean N1N2O1O2 plane of the Schiff base ligand with
deviation being only 0.034 and 0.039 Å for 1 and 2, respectively. As
shown in Figs. 1and 2 each pair of the Ni(II) and Ln(III) metal ions
are linked by two bridging phenoxo oxygen atoms (O1 and O2) of
the Schiff base ligand and additionally one bidentate bridging car-
boxylate group (O6C20O7) of the acetate ion. In both 1 and 2, the
average distances for the M–O bonds are significantly different
(Table 2). The intramolecular separations Ni1ꢀ ꢀ ꢀCe/Pr, and
i
Ni1ꢀ ꢀ ꢀNi1 (symmetry code (i) ꢁx, ꢁy, ꢁz) are ca. 3.5 and 7.1 Å,
(
Ni–O ca. 2.0 Å, and Ce/Pr—O ca. 2.5–2.9 Å). The Ln1–O bond lengths
respectively (Table 2) agree with the observed ranges of values
Table 1
Crystallographic data for crystals 1 and 2.
1
2
Empirical formula
Formula weight
T (K)
C
44
H
58Br
4
N
5
O
2
21Ni Ce
44 4 5 2
C H58Br N O21Ni Pr
1570.92
295(2)
1570.13
295(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P2 /n
monoclinic
P2 /n
1
1
13.815(1)
15.667(1)
14.885(1)
117.06(1)
2869.0(4)
2; 0.5
13.858(2)
15.654(1)
14.844(2)
116.98(2)
2872.0(8)
2; 0.5
b (°)3
V (A )
0
Z; Z
Crystal form/colour
Crystal size (mm)
plate/green
block/green
0.40 ꢂ 0.20 ꢂ 0.08
0.40 ꢂ 0.40 ꢂ 0.38
ꢁ
3
Dcalc (g cm
)
1.818
1.816
l
(mm-1
)
4.292
4.342
Absorption correction
h (°)
analytical
3.02–25.24
analytical
3.01–25.24
Reflections collected/unique (Rint
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
)
10987/5136 (0.0513)
5136/7/342
1.004
10882/5078 (0.0314)
5078/7/342
1.035
R
R
D
1
, wR
, wR
2
[I > 2
r
(I)]
0.0589, 0.1425
0.1026, 0.1708
0.964, ꢁ0.750
0.0519, 0.1292
0.0719, 0.1448
0.971, ꢁ0.753
1
2
(all data)
min (e A 3
ꢁ
)
q
max
,
D
q

B. Cristóvão et al. / Polyhedron 68 (2014) 180–190
183
Table 2
Selected geometric parameters (Å, °) in the coordination environments of metal
centers.
1
2
Bond lengths
Ln1–O1
Ln1–O2
Ln1–O3
Ln1–O4
Ln1–O7
Ni1–N1
Ni1–N2
Ni1–O1
Ni1–O2
Ni1–O6
Ni1–O8
Ni1–Ln1
2.507(5)
2.492(5)
2.900(5)
2.931(5)
2.434(6)
2.037(6)
2.014(6)
2.036(5)
2.062(5)
2.059(6)
2.170(6)
3.570(1)
7.141(2)
2.476(4)
2.462(4)
2.904(5)
2.933(4)
2.414(6)
2.043(5)
2.024(6)
2.027(4)
2.041(4)
2.065(5)
2.179(5)
3.538(1)
7.076(2)
i
Ni1ꢀ ꢀ ꢀNi1
i
symmetry code: ꢁx, ꢁy, ꢁz
Bond angles
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
103.1(2)
102.9(2)
65.1(2)
95.6(3)
173.6(2)
91.7(2)
172.2(2)
90.5(2)
82.0(2)
94.0(2)
95.5(2)
87.2(2)
88.0(3)
90.6(2)
95.4(2)
82.8(2)
86.3(1)
176.1(2)
56.9(2)
112.1(2)
116.6(2)
112.9(2)
56.7(1)
119.6(2)
77.5(2)
78.9(2)
117.6(2)
24.7(2
103.1(2)
103.2(2)
64.9(1)
96.3(2)
172.8(2)
91.7(2)
171.2(2)
90.7(2)
81.3(1)
94.0(2)
95.6(2)
87.4(2)
88.3(2)
90.1(2)
95.6(2)
82.4(2)
86.4(1)
175.6(2)
57.2(1)
112.1(1)
117.2(2)
112.9(1)
57.0(1)
119.1(1)
78.5(2)
79.7(2)
118.0(2)
25.8(2)
Fig. 2. The coordination environement of cations in 2.
the similar way. Heating the samples up to ca. 190 °C in air atmo-
sphere leads to the weight loss of 7.80% (calculated 8.66%) (1) and
7.60% (calculated 8.66%) (2) consistent with the lose simulta-
neously four lattice water molecules and two coordinated
molecules of methanol. This process is accompanied with endo-
thermic effect seen on the DSC curves As the temperature is
increased the recorded TG curves exhibit mass loss in a wide tem-
perature range 220–490 °C (1) 220–840 °C (2) which are accompa-
nied with exothermic peaks seen on DSC curves. That may be due
to the decomposition of the Schiff base ligand molecules. The
II
III
II
decomposition process of Ni –Ln –Ni compounds is intricate
and it is very difficult to distinguish intermediate solid products.
2
The residual mixed metal oxides NiO and CeO (1) and NiO and
Pr 11 (2), respectively percentages (21.22% (1), 20.35% (2)) calcu-
6
O
lated from TG curves were coincided fairly with the theoretical
values (20.47% (1), 19.51% (2)). The final products were experimen-
tally verified on the basis on their X-ray diffraction powder pat-
terns. The synthesized compounds are more stable in argon
atmosphere. As shown in Fig. 5 during heating in argon the first
change in mass (4.67% (1) and 4.70% (2)) estimated from TG curves
coincided fairly with the theoretical values recorded for 1 and 2
corresponds to the loss of four water molecules acting as solvent.
The dehydration process has been also confirmed by the TG-FTIR
analysis. The recorded FTIR spectrum (Fig. 6) at 102 °C shows char-
a
– dihedral angle between the Ni1O1O2 and Ln1O1O2 planes.
II
III
for polynuclear Ni –Ln complexes [4,7,5]. Intermolecular Mꢀ ꢀ ꢀM
ꢁ1
acteristic bands in the wavenumber ranges 4000–3400 cm and
II
III
II
separations in the Ni –Ln –Ni structures indicate well separation
of the trinuclear 3d–4f–3d cores. Because of the special position of
ꢁ1
2
060–1260 cm corresponding to stretching and deforming vibra-
2
tions of H O molecules, respectively [24,25]. During further heat-
II
III
the molecule in the unit cell two Ni and one Ln ions are arranged
ing the main volatiles emitted around 230 °C (the maximum
point of the DTG curve, Fig. 5) are identified generally as aromatic
and aliphatic compounds and nitric oxides. The recorded TG-FTIR
spectra at around 335 °C (the maximum point of the DTG curve,
i
in a line, with a Ni1–Ln1–Ni1 (symmetry code (i): ꢁx, ꢁy, ꢁz) angle
equal to 180°. Crystals of 1 and 2 are molecular structure with non-
coordinated water molecules and nitrato ions occupying the cages
in the crystal net formed by the molecules of trinuclear complexes
ꢁ1
Fig. 5) show the characteristic doublet bands at 2240–2400 cm
(
Fig. 3). Inside these cavities they are involved in extensive network
ꢁ1
and 669 cm , respectively assigned to stretching and deformation
of hydrogen bonds (Table 3). It is worth noticing, that the space in-
side each cage is large enough to enable some disorder of the guest
molecules, but the crystal structure of the host molecules is dense
enough to make the crystals air stable.
vibrations of carbon dioxide molecules, characteristic bands in the
ꢁ1
ꢁ1
wavenumber ranges 4000–3400 cm and 2060–1260 cm corre-
sponding to stretching and deforming vibrations of H
respectively, the bands at 2060–2240 cm characteristic for CO
2
O molecules,
ꢁ1
3
molecules and the bands indicating the presence of CH Br [25].
3.2. Thermal analysis
3.3. Magnetic properties
The thermal behavior of heterometallic trinuclear compounds
III
[
Ni
2
Ln(L)
2
(CH
3
COO)
2
(MeOH)
Br
2
]NO
3
ꢀ4H
2
O (where Ln = Ce (1) and
The magnetic properties of the heterotrinuclear complexes
Ni –Ce –Ni (1) and Ni –Pr –Ni (2) were determined over the
III
II
III
II
II
III
II
Pr (2) and L = C19
H
18
N
2
O
4
2
) were investigated in air and argon
atmosphere (Figs. 4 and 5). The isostructural complexes are stable
at room temperature and their decomposition processes proceed in
temperature range of 1.8–300 K. Figs. 7 and 8 show the plots of
the molar magnetic susceptibility in the form of the
vT product

1
84
B. Cristóvão et al. / Polyhedron 68 (2014) 180–190
Fig. 3. The cages ocupied by non-coordinated nitrato ions and water molecules (presented as van der Waals spheres) in the structure of 1.
Table 3
Hydrogen bond parameters (Å, °) in 1 and 2.
D–Hꢀ ꢀ ꢀA
D–H
Hꢀ ꢀ ꢀA
D–A
\DHA
1
O5–H5Aꢀ ꢀ ꢀO12
0.85
0.85
0.85
0.85
2.04
2.05
1.96
2.19
2.84(1)
2.86(1)
2.79(2)
3.03(1)
156(2)
161(1)
167(1)
166(3)
i
O5–H5Bꢀ ꢀ ꢀ07
O8–H8ꢀ ꢀ ꢀO5
O12–H12Bꢀ ꢀ ꢀO9
2
ii
ii
O5–H5Aꢀ ꢀ ꢀO12
0.85
0.85
0.85
0.85
2.04
2.05
1.96
2.19
2.84(5)
2.86(2)
2.79(2)
3.03(1)
156(2)
161(1)
167(1)
166(3)
i
O5–H5Bꢀ ꢀ ꢀ07
O8–H8ꢀ ꢀ ꢀO5
O12–H12Bꢀ ꢀ ꢀO9
i
ii
Symmetry codes: ꢁx, ꢁy, ꢁz; x + 1, y, z.
Fig. 5. TG, DTG and DSC curve of 2 in argon atmosphere.
3
ꢁ1
the value of 3.75 cm K mol at 300 K which is slightly lower than
3
ꢁ1
that of 3.91 cm K mol
from two Ni ions (SNi = 1, gNi = 2.15) and one Pr ion (J = 4,
comprising the free-ion contributions
II
III
J
g = 4/5). As a general trend, for the complexes of the paramagnetic
III
Ln ions of the first half of the series the
vT versus T curves are
characterized by a continuous decrease as the temperature is low-
ered. Our results are consistent with the empirical studies concern-
II
ing heterometallic Ni –4f compounds, in which the 4f ions display
a spin–orbit coupling [26]. Pasatoiu et al. studied a series of hete-
II
III
III
III
III
rodinuclear Ni –Ln Schiff base complexes (Ln from La to Er )
and stated that there are antiferromagnetic interactions between
Fig. 4. TG, DTG and DSC curve of 2 in air atmosphere.
II
III
III
III
Ni and Ln ions for Ln from the beginning of the 4f series (Ce ,
III
III
III
III
against temperature T for the powder samples of 1 and 2 (open
Pr , Nd , Sm ), while ferromagnetic occured from Gd toward the
end of the 4f series [26a]. Shiga et al. investigated the magnetic
II
III
II
squares). In the case of Ni –Ce –Ni (1) the corresponding curve
II
III
II
III
displays an abrupt increase in the temperature range 1.8–18 K
properties of heterotrinuclear Ni –Ln –Ni complexes (Ln from
La to Lu ) and they indicated that the Ni -Ln interaction is
weakly antiferromagnetic for Ln = Ce , Pr and Nd , and ferro-
3
ꢁ1
3
III
III
II
III
from the value of 1.28 cm K mol
to the value of 2.77 cm -
ꢁ
1
III
III
III
III
K mol , next it almost linearly increases reaching the value of
3
ꢁ1
ꢁ1
III
III
III
III
III
III
II
III
II
3
3
.18 cm K mol
at 300 K which is slightly lower than that of
magnetic for Ln = Gd , Tb , Dy , Ho and Er . The Ni –Ln –Ni
3
compounds with diamagnetic La and LuIII ions showed an antifer-
III
.23 cm K mol comprising the free-ion contributions from two
II
III
II
Ni ions (SNi = 1, gNi = 2.2) and one Ce ion (J = 5/2, g
J
= 6/7). Simi-
T versus T also
shows an abrupt increase in the temperature range 1.8–12 K from
romagnetic interaction between the terminal Ni ions [7a]. Kahn
et al.’s magnetic studies of oxamato bridged Ni –Ln compounds
II
III
II
II
III
larly is for Ni –Pr –Ni (2), the recorded curve
v
II
III
also showed that Ni -Ln interaction is antiferromagnetic for
3
ꢁ1
3
ꢁ1
III
III
III
III
III
III
III
III
the value of 0.90 cm K mol to the value of 2.41 cm Kꢀmol
,
Ln = Ce , Pr , Nd , Er and ferromagnetic for Ln = Gd , Tb ,
III
III
respectively and next it displays a more gradual increase reaching
Dy and probably Ho [26c].

B. Cristóvão et al. / Polyhedron 68 (2014) 180–190
185
Fig. 8. Temperature dependence of
T = 2 K.
v
T for 2. The inset: Isothermal magnetization at
Fig. 6. FTIR spectra of gaseous products of 2 decomposition in argon atmosphere.
Fig. 9. Temperature dependence of inverse susceptibility of 1. The red solid line
shows the Curie–Weiss fit. (Color online.)
Fig. 7. Temperature dependence of
T = 2 K.
vT for 1. The inset: Isothermal magnetization at
Fig. 10. Temperature dependence of inverse susceptibility of 2.The red solid line
shows the Curie–Weiss fit. (Color online.)
In our compounds the fit of the Curie–Weiss law (
to the inverse molar susceptibility in the temperature range
v
= C/(T ꢁ h))
3
ꢁ1
4
0–300 K (1) and 50–300 K (2) yielded C = 3.23(1) cm K mol ,
3
ꢁ1
PrIII (g = 4/5) and an unknown value of g , one arrives at the
and h = ꢁ9.9(8) K for 1 (Fig. 9) and C = 4.05(1) cm K mol and
h = ꢁ25.5(7) K for 2 (Fig. 10). The negative value of the Weiss con-
stant may indicate the presence of a weak antiferromagnetic inter-
J
Ni
estimate gNi ꢃ 2.20 (1) and g ꢃ 2.21 (2), respectively. In inside
Ni
of Figs. 7 and 8 the experimental data for the field dependence of
molecular interaction. Using the value of the Curie constant and
the magnetization at T = 2 K are shown (open circles). The corre-
sponding curves display a monotonic increase with increasing
III
assuming the free-ion value of the Landé factor of Ce (g
J
= 6/7)/

1
86
B. Cristóvão et al. / Polyhedron 68 (2014) 180–190
value of the magnetic field and reach the value of 3.79
case of 1) and 2.99 (in the case of 2) at H = 5 T. These values
are significantly lower than 6.17 (1) and 7.09 (2), respectively
l
B
(in the
ond term corresponds to the crystal field (CF) interaction for the
III
III
l
B
Ce /Pr ion, which is expressed in the framework of the extended
operator equivalent approach [31–34]. Following the notation by
Altshuler and Kozyrev [33], the crystal field part of the Hamiltonian
is written as
l
B
l
B
expected on the basis of the free-ion approximation calculated at
the same values of temperature and external magnetic field. This
III
III
II
indicates that the interactions between the Ce /Pr and Ni ions
as well as the crystal field effects play a crucial role in defining
k
X
X
q
k
q
k
H
CF
¼
B O
II
III
II
II
III
II
the magnetic properties of Ni –Ce –Ni (1) and Ni –Pr –Ni (2)
k¼2;4;6 q¼ꢁk
compounds at low temperatures.
q
k
q
k
The coefficients B are the parameters to be determined. The O
matrices are polynomials of the total angular momentum matrices
3
.3.1. Determination of crystal field parameters
The f-electrons in the majority of lanthanide complexes are
2
J , J , J , and J (and their definitions are given in the Table S1). The
z + ꢁ
q
operator equivalents Ok do not include the operator equivalent
considered to have properties close to those of isolated ions.
The energies of the ground- and excited multiplets are deter-
mined by the spin–orbit coupling in which the total angular
momentum J is a good quantum number. Fine structure of spec-
tral bands detected in the region from near-IR to UV is ascribed to
crystal-field splitting of the ground- and excited multiplets. Many
studies devoted to the multiplet structures of lanthanide f sys-
tems include only the cases for which sharp emission or absorp-
tion bands or both were available [27]. There is yet another
approach that can yield some information on the crystal-field
splitting, dispenses with the sources of spectroscopic information
and uses instead the output of magnetic measurements. Follow-
ing the ideas of Ishikawa et al.’s [28] we present an approach
based solely on magnetic data and use it to estimate the ex-
change interactions in the studied compounds. A similar approach
was already successfully applied in a series of compounds com-
prising the lanthanide ions [29]. The approach depends crucially
on the coordination number (CN) of the lanthanide ion. Crystallo-
k
coefficients or the radial factors hr i. Both factors are included in
q
k
the parameters B , which restricts their application to a single
J-manifold. The coefficients are conversed into another set of
q
k
k
parameters A hr i (more commonly used in the literature) using
the formula
q
k
q
k
k
B ¼ A hr ihJk kJi
a
k
where the last factor are the operator-equivalent coefficients.
They relate the angular momentum operators to the potential
operators and depend on the ion and the coupling scheme as-
sumed (e.g. L–S or intermediate). Their values tabulated for all
lanthanide ions in the L–S scheme can be found in [33]. For the
1
III
4f electronic configuration (Ce ) the coefficient hJka6kJi vanishes,
thus only crystal field parameters with k = 2 and 4 need be taken
into account.
The next component of the Hamiltonian H includes the zero-
ZFS
II
field splitting terms for the Ni ions. It is assumed that the rhombic
distortion is negligible. In the local frames of the Ni ions this con-
II
III
II
II
III
II
graphic analysis of isostructural Ni –Ce –Ni (1) and Ni –Pr –
Ni (2) compounds revealed that CN = 10 and the coordination
II
tribution reads
geometry is closest to that of a distorted pentagonal antiprism
h
i
h
i
^
2
^
2
(
PAP, D5d).
H
ZFS ¼ D
1
S
ꢁ 1=3SNiðSNi ꢁ 1Þ þ D
2
S
ꢁ 1=3SNiðSNi ꢁ 1Þ
Ni1z
Ni2z
The total angular momentum of the ground state in the triva-
II
III
II
Due to the inversion symmetry of the Ni –Ln –Ni (Ln = Ce, Pr)
lent cerium/praseodymium ion takes the minimal value, i.e.
J = |L–S| (for electronic configuration 4f1 and 4f2, respectively) in
molecule the zero field splitting parameters of both nickel ions are
assumed to be equal, i.e. D = D = D. The calculations were per-
formed in the local frame of the Ce /Pr ion (see the following
comments), so the contribution HZFS had to be duly transformed
to that frame.
the Russell–Saunders coupling scheme, i.e. J = 5/2 (for Ce(III)) and
J = 4 (for Pr(III)). The simulations were carried out using the
1
2
III
III
2J + 1 sublevels of the ground-state multiplet of the cerium/praseo-
dymium system. This can be accepted except the complexes con-
III
III
The last component of the total Hamiltonian H is to account for
the exchange coupling between the ions. The coupling should be
taining Eu and Sm . For these ions the energy separation
between the multiplet of the lowest energy and the first excited
multiplet is known to be weak [30]. As a result the low lying ex-
cited states may become thermally populated. In addition, the cou-
pling between the substates belonging to different multiplets of
the same ground term through the Zeeman perturbation must be
also taken into account.
E
taken between the corresponding spin operators. If we can neglect
mixing of the multiplets with different J quantum numbers, so that
only the matrix elements within the subspace of wave functions
corresponding to the ground term value of J are taken into account,
the spin operator S can be projected onto the total angular momen-
tum operator J. Equations L + S = J and L + 2S = g
projection is S = (g
ꢁ1)J. We further assume that the exchange cou-
pling between the Ni and Ce /Pr ions is isotropic so the Hamil-
tonian H reads
J
J imply that this
The Hamiltonian pertinent to the present system under external
0 Z E
magnetic field is H = H + HCF + HZFS + H . The first term accounts
J
II
III
III
III
III
II
for the Zeeman effect. For one Ce /Pr ion and two Ni ions it
reads
E
H
H
Z
Z
¼ bðg J þ g SNi1 þ gNi
S
Ni2Þ ꢀ H
Ce
Ni
H ¼ ꢁJ ðgCe ꢁ 1ÞJ ꢀ SNi1 ꢁ JNiCeðgCe ꢁ 1ÞJ ꢀ SNi2
E
NiCe
¼ bðg J þ g SNi1 þ gNi
SNi2Þ ꢀ H
Pr
Ni
E
H ¼ ꢁJNiPrðgPr ꢁ 1ÞJ ꢀ S ꢁ JNiPrðgPr ꢁ 1ÞJ ꢀ S
Ni1
Ni2
II
III
II
where b is the Bohr magneton and H denotes the external magnetic
field. The total magnetic moment operator
= ꢁb(gCeJ + gNi
Ni2) and Ni2), respectively is used in
Because the Ni –Ln –Ni (Ln = Ce, Pr) molecule is centrosym-
metric we assume a single exchange coupling, identical for both
l
Ni1
S +
II
III
III
g
Ni
S
l
= ꢁb(gPrJ + gNi
S
Ni1 + gNi
S
Ni -Ce /Pr pairs.
With an arbitrary choice of a set of Bk coefficients and a finite
external field, the Hamiltonian H is diagonalized. The determina-
q
the corresponding jJ; J i representation in its lanthanide part. SNij
z
(j = 1,2) denote the spin one operators of the nickel centers. The
0
II
spectroscopic tensor of the Ni centers gNi is assumed to be isotro-
pic with its principal value gNi relaxed during the fitting procedure,
whereas the components of gCe/gPr of which we assume to be diag-
onal and isotropic were fixed during the fitting procedure. The sec-
tion of a full set of eigenvalues and eigenfunctions permits the cal-
culation of the magnetic molar susceptibility together with the
isothermal magnetization. This is performed using the generalized
van Vleck formalism

B. Cristóvão et al. / Polyhedron 68 (2014) 180–190
187
xx yy
v , v , vzz. The observed molar magnetic susceptibility of powder
sample is
1
v
ꢀ ¼
ðvxx þ vyy þ vzzÞ
3
To calculate the magnetic moment per molecule measured on
powder sample one must carry out an appropriate averaging over
different orientations of the external magnetic field inducing the
moment. For a fixed magnitude H the magnetic field is parame-
trized with two spherical angles H(h
cosh ), where h ] (i = 1, . . ., N
= arccos [(iꢁ1)/N
(j = 1, . . ., N ). The calculation is performed for any orientation
i
,/
j
) = H(sin h
i
cos/
j
,sinh
i
j
sin/ ,-
i
i
h
h
) and /
j
= 2p(jꢁ1)/
N
/
/
i j
defined by the pair (h , / ) and finally the average is calculated as
the following mean
III
Fig. 11. The main local frame with origin at the Ce ion and two auxiliary local
frames of the Ni centers.
N
h
N
/
II
1
N N
h /
XX
ꢀ
M ¼
MðHÞ ꢀ Hðh ; /jÞ
i
i¼1 j¼1
In the calculations the averaging was carried out over 900 dif-
ferent orientations of the external magnetic field (N = 30, N = 30).
Table 4
h
/
Maximum absorption bands (kmax) and molar absorption coefficients (e) of the
Fitting was carried out with the help of a specially designed pro-
cedure prepared within Mathematica8.0 environment. Two test
functions to be minimized were used. The first test function was
the relative root-mean-square (r.m.s.) deviation from the mea-
studied compounds.
[104
M
ꢁ1
cm ])
ꢁ1
Compound
k
max [nm] (
e
a free H
2
L
207.5 (4.42) 236 (4.77) 293 (1.00); 351.5 (0.38); 425 (0.49)
204.5 (13.09) 202 (23.00) 274.5 (4.04); 365 (0.90)
205 (11.44) 240 (13.20) 283 (2.28) 359 (1.90)
II
b Ni
c Ni –PrIII–NiII
II
sured vT values:
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
uP
N
i¼1
v
2
u
t
½ð
v
v
ꢀ
T
Þ
ꢁ ð
v
T
Þ
2
ꢄ
i
i
theor
i
i
exp
2
ꢀD
ꢀ
Eꢀ 3
r
v
T
¼
P
;
ꢀ
ꢀ
2
N
v
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢁ
ꢂ
ðN ꢁ PÞ
ð
v
T Þ
X X D
ꢀ
ꢀ
ꢀ
ꢀ
Eꢀ
ꢀ
X
un;i
l
um;j
i¼1
i
i
exp
ꢀ
ꢀ
2
E
n
B
T
N
6
ꢀ
7
v
¼3k
4
ꢀ
un;i
l
un;j ꢀ ꢁ2k
B
T
5exp ꢁk
B
TZ
0
E
n
ꢁE
m
n;i
j
j;m–n
where N denotes the number of experimental points and P the
v
X
number of free parameters. The other test function one was the rel-
ative r.m.s. deviation from the measured M values defined as
follows
N
E
k
ðHÞ
M¼Z
hwkj
l
jw iexpðꢁ
Þ
T
k
H
k
B
k
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
u
t
P
2
P
P
N
j¼1
M
where Z
0
¼
nd
n
exp (ꢁE
n
/k
B
T), and Z
H
¼
exp (ꢁE
k
(H)/k
B
T). Here
in
(H)}
½Mjtheor ꢁ Mjexp
ꢄ
k
r
M
¼
P
;
/
n,i denote the d
n
-fold degenerate eigenfunctions with energy E
n
N
M
2
ðN
M
ꢁ PÞ
M
j¼1
jexp
the absence of magnetic field, whereas the eigensystem {
w
k
, E
k
was calculated assuming the nonzero external magnetic field H. The
three principal values of the magnetic susceptibility are denoted
where N
M
denotes the number of experimental magnetization
points. Both test functions were used independently. The fitting
procedure was continued until a consistent set of fitted parameters
was found. Although the lanthanide site is expected to be of low
symmetry (distorted PAP geometry) we assumed initially the
L (a), Ni –LnIII–NiII (b)
II
II
Fig. 12. Electronic absorption spectra of the free ligand H
and Ni (c) in methanol at 298 K.
2
Fig. 13. Excitation (left) spectrum of Ni (c) (kob = 343 nm) and emission (right)
II
II
III
II
II
2
spectra of H L (a), Ni –Ln –Ni (b) and Ni (c) (kex = 292 nm).

1
88
B. Cristóvão et al. / Polyhedron 68 (2014) 180–190
II
III
II
crystal field parameter set corresponding to the highest symmetry
The model system comprises a trinuclear complex Ni –Pr –Ni
0
0
D
5d involving only two nonzero parameters B and B (in the case
(2) exchange coupled through the oxygen bridges. Due to the
inversion symmetry we assume a single exchange coupling con-
stant JNiPr. The best fit to the experimental data was found for
the following set of parameters (the CF parameters, the zero-
field splitting parameter D, and the exchange coupling are
2
4
0
0
0
5
of cerium) and four nonzero parameters B , B , B , and B (in the
2
4
6
6
case of praseodymium). Otherwise the many-dimensional fits
where only powder data are available will be of poor reliability.
In the case of compound 2 the simulated susceptibility data re-
5
6
ꢁ1
0
2
2
vealed a very weak dependence on the crystal field parameter B ,
given in cm ): gNi ¼ 2:15ð2Þ; J
¼ ꢁ1:3ð8Þ; D ¼ 7:1ð4Þ; A hr i ¼
NiPr
6
0
4
4
0
6
therefore it was dropped from the calculations. The best fit was ob-
tained successfully with this restricted parameter set. Uncertainties
of the parameters were estimated in a standard way using the sec-
ond derivatives of the test function, which corresponds to the 100%
ꢁ310ð9Þ; A hr i ¼ 235ð11Þ; A hr i ¼ 80ð8Þ. The agreement factors
obtained for the susceptibility and magnetization data are
ꢁ4
ꢁ4
III
1.5 ꢂ 10 and 1.3 ꢂ 10 , respectively.
The spectrum of the Pr center consists of one singlet and four
tolerance deviation.
doublets. The lowest level is a singlet substate |0i (Table S3). The
first excited state comprises the |±1i states and the highest lying
doublet is built of |±2i substates. The population of the lowest sin-
glet at T = 2 K is practically 100%. It is only at T = 100 K that the
population of the first excited level (5.9%) stops being negligible.
The populations of the successive substates at T = 150 K are
81.4%, 16.3%, 1.3%, 0.6%, and 0.4%, respectively. This explains the
q
k
The crystal field parameters B as well as the zero field splitting
tensor do depend on the coordinate frame in which the Hamilto-
nian is calculated. Therefore one of the first tasks to be done was
to fix the local frame in which to do the calculations. Because of
the fact that the position of the Ce /Pr ion coincides with the
inversion center the origin of the local frame was fixed at that po-
sition. The coordination sphere of the Ce /Pr ion comprises ten
oxygen ions denoted in the cif file by symbols O1, O2, O3, O4,
and O7 (there are two ions related by inversion symmetry corre-
sponding to one symbol) forming two mutually parallel pentagons.
The z axis of the local frame was chosen to point perpendicular to
the plane comprising the center of gravity of the pentagonal frame-
work and minimizing the sum of the distances between the plane
and the ions. The x direction of the local frame was fixed as the pro-
III
III
III
III
steady decrease of the
sharp down-turn of the
cribed to the substantial zero-field splitting parameter of the Ni
centers. Solid lines in Fig. 8 show the best fit curves for the suscep-
tibility and isothermal magnetization.
v
T product on lowering temperature. The
T curve at lowest temperatures is as-
v
II
ꢁ
1
The values of the zero-field splitting parameter D = 6.3(4) cm
ꢁ
1
(1) and D = 7.1(4) cm (2), respectively are comparable to that
II
III
II
found in a trinuclear complex Ni –Gd –Ni [35] ([(LNi) Gd](NO ),
2
3
II
II
jection onto that plane of the Ni to Ni direction. The y axis was
determined so that the three axes x, y, and z form a right handed
triplet. The coordination sphere of each Ni ion forming distorted
L: triamine1,1,1-tris(aminomethyl)ethane) with octahedral coordi-
ꢁ
1
ꢁ1
nation of the nickel ions (4.5 cm ) [35a]. A lower value (1.5 cm
)
II
II
III
was detected in the heterodinuclear complex [Ni L)Gd (hfac)
2
pseudooctahedral geometry comprises six ions denoted by O1,
O2, N1, N2, O6, O8. The z axis of the corresponding local frame
was chosen as the direction of the vector joining O8 and O6 ions.
The x axis of that frame was assumed to be the direction perpen-
dicular to the z axis and pointing to the position of O1 ion. The y
axis was calculated so that the three x, y, and z axes form a right-
handed triple. Due to the inversion symmetry the local frames of
(EtOH)] (H L = 1,1,1-tris[(salicylideneamino)methyl]ethane, hfac =
hexafluoroacetylacetonate) [35b].
3
3.4. Electronic absorption spectra and luminescence properties
The photophysical properties of Schiff base ligand H L (a) and
2
II
III
II
its heterometallic trinuclear Ni –Pr –Ni (b) complex have been
examined in methanol solution at room temperature and com-
pared with those obtained for mononuclear [NiL(H O) ] (c) Schiff
II
thus defined local frames of the Ni ions are related by that sym-
III
metry. Fig. 11 shows the main local frame with origin at the Ce
2
2
II
center and both auxiliary local frames of the Ni centers.
base complex (the structure of this compound was reported by
us earlier [4]). The obtained results are summarized in Table 4
and Figs. 12 and 13. The absorption spectrum of the Schiff base li-
gand is characterized by three absorption bands in the region 200–
1
III
The ground state arising from the 4f configuration of the Ce ion
is 2F5/2. The dimension of its ground-state subspace is hence 6. The
theoretically predicted value of the Landé factor is 6/7(ꢃ0.86). The
II
III
II
⁄
⁄
model system comprises a trinuclear complex Ni –Ce –Ni (1) ex-
600 nm, which are assigned to p–p and n–p transition, respec-
change coupled through the oxygen bridges. Due to the inversion
tively. As shown in Table 4 and Fig. 12, the absorption spectra of
the metal complexes exhibit a quite similar absorption profiles.
The absorption band maxima of complexes are shifted to lower
symmetry we assume a single exchange coupling constant JNiCe
.
The best fit to the experimental data was found for the following
set of parameters (the CF parameters, the zero-field splitting param-
or higher wavelengths compared to free ligand H L, due to com-
2
ꢁ1
eter D, and the exchange coupling are given in cm ):
plexation. The molar absorption coefficients, e, of both bands of
0
2
2
0
4
4
II
III
II
gNi ¼ 2:23ð3Þ; J
¼ ꢁ1:1ð4Þ; D ¼ 6:3ð4Þ; A hr i ¼ ꢁ265ð10Þ; A hr i
complex Ni –Pr –Ni (b) are larger than those of the free ligand
NiCe
¼
291ð6Þ. The agreement factors obtained for the susceptibility and
H L (a) and mononuclear Ni(II) complex (c). The photoluminescent
2
ꢁ5
ꢁ4
magnetization data are 4.2ꢂ10 and 4.4ꢂ10 , respectively. The
lowest level is a Kramers doublet built of |±3/2i substates
behavior of all compounds were also studied. The excitation and
emission spectra are depicted in Fig. 13. The luminescence spec-
(
Table S2). The first excited state comprises the |±1/2i states and
trum of Schiff base ligand H L reveals two of high intensity emis-
2
⁄
⁄
the highest Kramers doublet is built of |±5/2i substates. The popula-
tion of the lowest Kramers doublet at T = 2 K is practically 100%. It is
only at T = 150 K that the population of the first excited level (1.2%)
stops being negligible. The populations of the successive Kramers
doublets at T = 200 K is 95.5%, 3.4%, and 1.1%, respectively. This ex-
sion bands at 344 and 465 nm (
p
–p
and
p
–n transitions). The
⁄
emission spectra of the complexes originate from
p–p
or ligand-
to-metal charge transfer transitions [36]. The emission band max-
ima of complexes are blue or red shifted compared to free ligand
H L. Emission maxima appear at 343, 512 and 335 nm respectively
2
II
III
II
plains the steady decrease of the
ture. The sharp down-turn of the
v
T product on lowering tempera-
for Ni(II) and Ni –Pr –Ni complexes, on UV irradiation
(kex = 292 nm). The emission band at 512 nm of (c) can be attribut-
able to the LMCT transition.
vT curve at lowest temperatures is
ascribed to the ‘‘switching-on’’ of the positive (spin damping) zero-
II
Metal cations such as Ni2+, Cu2+ and Co2+ usually quench the or-
field splitting of the Ni centers. Solid lines in Fig. 7 show the best fit
2+
curves for the susceptibility and isothermal magnetization.
ganic ligand luminescence [12b,37] in contrast to the Zn ion,
2
III
3+
The ground state arising from the 4f configuration of the Pr
where the effect is not present [12c,38]. In the case of Ln ions
3
ion is H
4
. The dimension of its ground-state subspace is hence 9.
an important factor for the effective emission of these ions is en-
3
+
The theoretically predicted value of the Landé factor is 4/5(=0.8).
ergy matching between the Ln emitting levels and the ligand

B. Cristóvão et al. / Polyhedron 68 (2014) 180–190
189
triplet excited state. The absorbed light by the ligands can be trans-
ferred to the lanthanide ion by the intramolecular energy transfer.
The luminescent properties of the lanthanide metal complexes are
strongly dependent on the efficiency of the organic ligand to ab-
sorb UV light, the efficiency of energy transfer from the organic li-
Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: de-
posit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2013.10.019.
3
+
3+
gand to Ln ions and the efficiency of Ln luminescence [39]. The
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