Tetranuclear lanthanide (III) complexes containing dimeric subunits: Single-molecule magnet behavior for the Dy4 analogue

Article abstract of DOI:10.1021/ic401652f

The reaction of the lanthanide(III) salts [Dy(III), Tb(III), and Gd (III)] with a hetero donor chelating ligand N′-(2-hydroxy-3-methoxybenzylidene)- 6-(hydroxymethyl) picolinohydrazide (LH₃) and pivalic acid (PivH) in the presence of tetra-n-bu

Full text of DOI:10.1021/ic401652f

Article
pubs.acs.org/IC
Tetranuclear Lanthanide (III) Complexes Containing Dimeric
Subunits: Single-Molecule Magnet Behavior for the Dy4 Analogue
Vadapalli Chandrasekhar,*,†,‡ Sourav Das,† Atanu Dey,† Sakiat Hossain,† and Jean-Pascal Sutter*,§,∥
†Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
‡Centre for Interdisciplinary Sciences, Tata Institute of Fundamental Research, 21 Brundavan Colony, Narsingi, Hyderabad 500075,
India
§Laboratoire de Chimie de Coordination (LCC), CNRS, 205, route de Narbonne, F-31077 Toulouse, France
∥UPS, INPT, LCC, Universite
́
de Toulouse, F-31077 Toulouse, France
S
* Supporting Information
ABSTRACT: The reaction of the lanthanide(III) salts [Dy(III),
Tb(III), and Gd (III)] with a hetero donor chelating ligand N′-
(2-hydroxy-3-methoxybenzylidene)-6-(hydroxymethyl) picolino-
hydrazide (LH3) and pivalic acid (PivH) in the presence of tetra-
n-butylammonium hydroxide (TBAH) afforded the tetranuclear
Ln(III) coordination compounds, [Ln4(LH)2(LH2)2(μ2-
η1η1Piv)2(η1Piv)4]·2CHCl3 [Ln = Dy(1), Tb(2), and Gd(3)].
The molecular structure of these complexes reveals that the
tetranuclear derivatives are composed of two dinuclear subunits
which are interconnected through the coordination action of the
picolinoyl hydrazine ligand. Within each subunit two different
types of Ln(III) ions are present. One of these is eight-coordinate
in a distorted triangular dodecahedral geometry while the other is
nine-coordinate in a distorted spherical capped square antiprism geometry. Alternating current (ac) susceptibility measurements
of complex 1 reveal a frequency- and temperature-dependent two step out-of-phase signals under 1kOe DC field which is
characteristic of a single-molecule magnet (SMM) behavior. Analysis of the magnetic data afforded the anisotropic barriers and
relaxation times: Δ/kB = 62.6 K, τ0 = 8.7 × 10−7 s; Δ/kB = 26.3 K, τ0 = 1.26 × 10−6 s for the slow and fast relaxations respectively.
8,24c−e
of them containing dysprosium(III) such as DyIII
,
INTRODUCTION
2
■
10,4d
11
12
13
14
15
DyIII3,9 DyIII
,
DyIII
,
5
DyIII
,
6
DyIII
,
7
DyIII
,
DyIII
,
9
4
8
Research on lanthanide complexes has undergone a remarkable
resurgence in the recent past. This is due to their potential
applications in catalysis,1 luminescence,2 magnetic resonance
imaging (MRI), or bioprobes for immunoassays.3 In addition,
considerable attention is also being bestowed on these
compounds for investigating their magnetic properties such as
single molecule magnet (SMM) behavior.4 As is now
reasonably well understood, SMM behavior is of molecular
origin and can be seen in molecules that possess a high ground-
state spin along with an intrinsic magnetic anisotropy.5 Many
lanthanide ions, particularly, Dy(III), Tb(III), or Er(III) seem
to be suitable for this purpose. Because of this, either
homometallic lanthanide complexes containing these ions or
heterometallic 3d/4f complexes containing a combination of
these lanthanide ions with appropriate transition metal ions
have been receiving attention.6,7 In particular, in recent years,
there has been an upsurge of 4f-based polynuclear complexes
for the development of higher energy-barrier SMMs as a result
of the significant magnetic anisotropy of lanthanide ions arising
from their large, unquenched orbital angular momentum.7 A
survey of literature reveals that homometallic lanthanide (III)-
based SMMs are being increasingly reported in literature, most
and so forth. Some examples of SMMs involving Tb(III),16
Ho(III),17 Er(III),18 and Yb(III)18b,19 are also known.
Modulation of the nuclearity among polynuclear Ln(III)
complexes is a synthetic challenge and has to be addressed
by a proper design of the ligand systems. Recently our group
has successfully assembled a variety of 3d-4f SMMs by using a
phosphorus supported tris-hydrazone ligand.20 However, the
limitations of this ligand prompted us to look for an alternative
design. By choosing a compartmental hydrazine ligand, which is
not anchored on phosphorus, we were able to synthesize
homometallic tetranuclear Dy(III) and Ho(III) complexes.10m
Inspired by this, we extended this strategy by modifying the
ligand to N′-(2-hydroxy-3-methoxybenzylidene)-6-(hydroxy-
methyl) picolinohydrazide (LH3) and also used an ancillary
pivalate ligand. Accordingly, herein, we are reporting the
synthesis of a new family of [Ln4(LH)2(LH2)2(μ2-η1η1Piv)2-
(η1Piv)4]·2CHCl3 complexes [LnIII = Dy(1), Tb(2), and
Received: June 28, 2013
© XXXX American Chemical Society
A
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Table 1. Crystal Data and Structure Refinement Parameters of 1−3
1
2
3
formula
C92H110Cl6Dy4N12O28
2694.62
C92H110Cl6Tb4N12O28
2680.30
C92H110Cl6Gd4N12O28
2673.62
M/g
crystal system
space group
a/Å
orthorhombic
Pbca
orthorhombic
Pbca
orthorhombic
Pbca
24.962(5)
25.0177(9)
15.6762(6)
27.8190(11)
90
25.250(4)
b/Å
15.241(5)
15.664(2)
c/Å
27.609(5)
27.906(4)
α = β = γ (deg)
V/Å3
90
90
10504(4)
10910.1(7)
4
11037(3)
Z
4
4
ρc/g cm−3
μ/mm−1
F(000)
1.704
1.632
1.609
3.044
2.784
2.593
5344
5328
5312
cryst size (mm3)
θ range (deg)
limiting indices
0.052 × 0.036 × 0.019
4.16 to 25.03
−29 ≤ h ≤ 18
−17 ≤ k ≤ 18
−28 ≤ l ≤ 32
52537
0.053 × 0.031 × 0.017
4.14 to 25.03
−29 ≤ h ≤ 22
−13 ≤ k ≤ 18
−26 ≤ l ≤ 33
44285
0.081 × 0.053 × 0.035
4.11 to 25.03
−22 ≤ h ≤ 30
−18 ≤ k ≤ 18
−33 ≤ l ≤ 33
72114
reflns collected
independent reflns
9229 [R(int) =0.0738]
99.5
9564 [R(int) =0.0594]
99.4
9707 [R(int) =0.1517]
99.5
completeness to θ (%)
data/restraints/params
goodness-of-fit on F2
final R indices [I > 2θ(I)]
9229/4/657
1.049
9564/22/657
1.051
9707/4/657
0.998
R1 = 0.0464
wR2 = 0.1128
R1 = 0.0679
wR2 = 0.1257
2.377 and −1.312
R1 = 0.0402
wR2 = 0.0858
R1 = 0.0774
wR2 = 0.1039
1.110 and −1.003
R1 = 0.0492
wR2 = 0.0887
R1 = 0.1179
wR2 = 0.1085
0.991 and −0.861
R indices (all data)
largest diff. peak and hole(e·Å−3
)
X-ray Crystallography. The crystal data for the compounds have
been collected on a Bruker SMART CCD diffractometer (Mo Kα
radiation, λ = 0.71073 Å). The program SMART22a was used for
collecting frames of data, indexing reflections, and determining lattice
parameters, SAINT22a for integration of the intensity of reflections and
scaling, SADABS22b for absorption correction, and SHELXTL22c,d for
space group and structure determination and least-squares refinements
on F2. All the structures were solved by direct methods using the
program SHELXS-9722e and refined by full-matrix least-squares
methods against F2 with SHELXL-97.22e Hydrogen atoms were fixed
at calculated positions and their positions were refined by a riding
model. All the non-hydrogen atoms were refined with anisotropic
displacement parameters. The crystallographic figures have been
generated using Diamond 3.1e software.22f The crystal data and the
cell parameters for compounds 1−3 are summarized in Table 1.
CCDC-945440 (1), CCDC-945441 (2), and CCDC-945442 (3)
contain crystallographic data for this paper. This data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Gd(3)]. Detailed magnetic studies of these complexes are also
reported.
EXPERIMENTAL SECTION
■
Solvents and other general reagents used in this work were purified
according to standard procedures.21 Pyridine-2,6-dicarboxylic acid,
sodium borohydride, Dy(NO3)3·5H2O, Tb(NO3)3·5H2O, and Gd-
(NO3)3·6H2O were obtained from Sigma Aldrich Chemical Co. and
were used as received. Hydrazine hydrate (80%) and sodium sulfate
(anhydrous) were obtained from SD. Fine Chemicals, Mumbai, India,
and were used as such.
Instrumentation. Melting points were measured using a JSGW
melting point apparatus and are uncorrected. IR spectra were recorded
as KBr pellets on a Bruker Vector 22 FT IR spectrophotometer
operating at 400−4000 cm−1. Elemental analyses of the compounds
were obtained from Thermoquest CE instruments CHNS-O, EA/110
model. Electrospray ionization mass spectrometry (ESI-MS) spectra
were recorded on a Micromass Quattro II triple quadrupole mass
1
spectrometer. H NMR spectra were recorded in CDCl3, CD3OD or
SYNTHESIS
CD3)2SO solutions on a JEOL JNM LAMBDA 400 model
spectrometer operating at 400.0 MHz; Chemical shifts are reported
in parts per million (ppm) and are referenced with respect to internal
tetramethylsilane (1H).
■
Methyl 6-(Hydroxymethyl) Picolinate. The preparation
of the title compound was carried out by a modification of a
previously published method.23 NaBH4 (2.03 g, 53.85 mmol,
1.5 equiv) was added in small portions over a period of 1 h to a
stirred suspension of dimethylpyridine-2,6-dicarboxylate (6.00
g, 35.9 mmol) in MeOH (150 mL) at 0 °C. This mixture was
stirred at room temperature for another 3 h and then MeOH
was removed in a rotary evaporator. A saturated NaHCO3
aqueous solution (200 mL) was added to the residue and the
resulting aqueous solution was extracted with CHCl3 (5 × 100
mL). The combined organic layers were dried (anhydrous
Na2SO4), filtered, and concentrated in vacuo to dryness. The
Magnetic Measurements. Magnetic measurements were carried
out with a Quantum Design MPMS 5S SQUID susceptometer in the
temperature domain 2−300 K. The measurements were performed on
crushed crystals from freshly isolated samples to avoid solvent loss.
The powders were mixed with grease and put in gelatin capsules. The
magnetic susceptibilities were measured in an applied field of 1000 Oe.
The molar susceptibility (χM) was corrected for sample holder and for
the diamagnetic contribution of all the atoms by using Pascal’s tables.
AC susceptibility was measured with an oscillating ac field of 3 Oe
using frequencies between 1 to 1500 Hz.
B
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resulting crude residue was purified by column chromatography
(n-hexane/EtOAc; 1:1, then 1:2) giving the desired product
(4.22 g, 82.17%) as a white solid. Anal. Calcd. for C8H9NO3: C,
57.5; H, 5.4; N 8.4%. Found: C, 56.9; H, 5.3; N 8.3%. Mp: 88.0
0.23 mmol), TBAH (0.62 mL, 0.57 mmol), PivH (0.036 g, 0.34
mmol). Yield: 0.065 g, 41.2% (based on Dy). Mp: 200 °C (d).
IR (KBr)(cm−1): 3447(b), 3169(w), 2955(s), 1651(s),
1612(s), 1574(s), 1480(s), 1420(s), 1388(s), 1357(s),
1303(s), 1216(w), 1110(w), 1066(s), 1020(w), 997(w),
818(w), 791(s). Anal. Calcd for C92H110N12O28Cl6Dy4
(2694.63): C, 41.01; H, 4.11; N, 6.26. Found: C, 40.86; H,
3.92; N, 6.42.
1
°C; H NMR (400 MHz, CDCl3): δ = 7.95 (d, 1H, pyr-H),
7.79 (t, 1H, pyr-H), 7.55 (d, 1H, pyr-H), 4.83 (s, 2H,
methylene-H), 4.31 (s, 1H, -OH), 3.92 (s, 3H, methyl-H). IR
(cm−1): 1740 ν(CO); 1591 ν(CN)py. ESI-MS (m/z): (M
+H).168.0658.
[Tb4(LH)2(LH2)2(μ2-η1η1Piv)2(η1Piv)4]·2CHCl3 (2). Quanti-
ties: LH3 (0.072 g, 0.23 mmol), Tb(NO3)3·5H2O (0.103 g, 0.23
mmol), TBAH (0.62 mL, 0.57 mmol), PivH (0.036 g, 0.34
mmol). Yield: 0.072 g, 46.8% (based on Tb). Mp: 200 °C (d).
IR (KBr) (cm−1): 3417(b), 3168(w), 2955(s), 1650(s),
1612(s), 1575(s), 1481(s), 1420(s), 1389(s), 1316(s),
1303(s), 1215(w), 1109(w), 1069(s), 1019(w), 999(w),
819(w), 790(s). Anal.Calcd for C92H110N12O28Cl6Tb4
(2680.34): C, 41.23; H, 4.14; N, 6.27. Found: C, 40.81; H,
3.89; N, 6.38.
6-(Hydroxymethyl) Picolinohydrazide. A methanolic
solution of methyl 6-(hydroxymethyl) picolinate (2.00 g,
11.96 mmol) was added dropwise to a stirred solution of
hydrazine hydrate (3 mL, 59.82 mmol, 5 equiv) in methanol
(60 mL) at room temperature. After the addition was over, the
reaction mixture was heated under reflux for 2 h and
subsequently allowed to come to room temperature before
being kept in a refrigerator at 5 °C. A needle-shaped crystalline
product was isolated which was suction-filtered, washed with a
small amount of cold methanol, and air-dried. Yield 1.87 g
(93.5%). Anal. Calcd. for C7H9N3O2: C, 50.29; H, 5.43; N,
[Gd4(LH)2(LH2)2(μ2-η1η1Piv)2(η1Piv)4]·2CHCl3 (3). Quanti-
ties: LH3 (0.072 g, 0.23 mmol), Gd(NO3)3·6H2O (0.104 g,
0.23 mmol), TBAH (0.62 mL, 0.57 mmol), PivH (0.036 g, 0.34
mmol). Yield: 0.065 g, 42.3% (based on Tb). Mp: 200 °C (d).
IR (KBr) cm−1: 3425(b), 3166(w), 2955(s), 1651(s), 1612(s),
1574(s), 1481(s), 1420(s), 1386(s), 1355(s), 1308(s),
1215(w), 1112(w), 1067(s), 1017(w), 997(w), 818(w),
790(s). Anal.Calcd for C92H110N12O28Cl6Gd4 (2673.62): C,
41.33; H, 4.15; N, 6.29. Found: C, 41.62; H, 4.05; N, 6.52.
1
25.14. Found: C, 50.02; H, 5.16; N 24.86. Mp: 110 °C. H
NMR (400 MHz, CD3OD): δ = 7.92 (d, 2H, pyr-H), 7.58 (t,
1H, pyr-H), 4.72 (s, 2H, methylene-H). IR(cm−1): 3407 and
3303 ν(N−H); 1655 ν(CO); 1571 ν(CN)py. ESI-MS (m/
z): (M+H). 168.0772
N′-(2-hydroxy-3-methoxybenzylidene)-6-(hydroxy-
methyl) Picolinohydrazide (LH3). A 20 mL methanolic
solution of o-vanillin (1.00 g, 6.50 mmol) was added dropwise
to a stirred suspension of 6-(hydroxymethyl) picolinohydrazide
(1.09 g, 6.50 mmol) in methanol (30 mL) at room
temperature. The reaction mixture was heated under reflux
for 3 h and allowed to come to room temperature where it was
kept for 12 h without stirring. A precipitate formed was
collected through filtration and washed with diethyl ether. Yield
1.82 g (92.9%). Anal. Calcd. for C15H15N3O4: C, 59.79; H,
5.02; N, 13.95. Found: C, 60.04; H, 4.98; N, 13.95. Mp: 90 °C.
1H NMR (400 MHz, [(CD3)2SO]): δ = 12.17 (s, 1H, phenolic-
OH), 10.94 (s, 1H, NH), 8.82 (s, 1H, imine-H), 8.02 (t, 1H,
pyr-H), 7.96 (d, 1H, pyr-H), 7.69 (d, 1H, pyr-H), 7.11 (d, 1H,
benz-H), 7.02 (d, 1H, benz-H), 6.86 (t, 1H, benz-H), 5.50 (s,
1H, -OH), 4.68(s, 2H, methylene-H), 3.78 (s, 3H, methoxy-H).
IR (cm−1): 3416 ν(O−H); 3283 ν(N−H); 1672 ν(CO);
1609 ν(CN)imine; 1571 ν(CN)py. ESI-MS (m/z): (M+H).
302.1148.
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization. Based on
recent literature, it appears that multidentate Schiff base ligands
are very effective in enabling the synthesis of polynuclear
heterometallic 3d/4f6c,e,f, i,j, 20 and homometallic
4f4d,8a,d,e,9c,10c,d,i,m,14d,24 complexes. Among these ligands,
aroylhydrazone-based Schiff base ligands are particularly
interesting in view of the presence of keto−enol tautomerism
as well as the flexibility arising out of the conformational
isomerism resulting from the C−C bond rotation.12c,24d Tang
and co- workers have utilized the feature of keto−enol
tautomerism to assemble and modulate the magnetic dynamics
of Dy2 complexes (Figure 1a). Although the beneficial feature
of the conformational flexibility of the ligand was exploited to
prepare the Dy2 complex (Figure 1b), because of the lack of
additional binding sites, cluster expansion does not occur in this
case. On the other hand this group has synthesized a Dy6
cluster by using a o-vanilloylhydrazone ligand, {(N′-(amino-
Tetranuclear Metal Complexes 1−3. The following
general protocol was utilized for the preparation of the
tetranuclear metal complexes. To a stirred solution of LH3 in
methanol (30 mL), Ln(NO3)3·xH2O (for 1, 2, x = 5; 3, x = 6)
was added. After this, tetra-n-butyl ammonium hydroxide
(TBAH) (in 25% MeOH) was added dropwise and stirred for
15 min. Then, pivalic acid (PivH) was added. The reaction
mixture was heated under reflux for 6 h, affording a clear yellow
solution, allowed to come to room temperature, filtered, and
the filtrate stripped off its solvent in vacuum affording a solid
residue which was washed with diethyl ether and dried. The
residue was dissolved in chloroform/acetonitrile (3:1) and kept
for crystallization under slow evaporation conditions at room
temperature. After 7−10 days, pure crystalline material suitable
for X-ray diffraction was isolated. Specific quantities of the
reactants involved in each reaction, yields of the products, and
their characterization data are given below.
Figure 1. Some Dy2 complexes reported in the literature, prepared
through the use ligands possessing the feature of Keto−Enol
tautomerism. (a) A complex where the enol form of the ligand is
involved in binding. (b) Utilization of keto−enol tautomerism and
conformational isomerism in the formation of the dinuclear
complex.24d
[Dy4(LH)2(LH2)2(μ2-η1η1Piv)2(η1Piv)4]·2CHCl3 (1). Quanti-
ties: LH3 (0.072 g, 0.23 mmol), Dy(NO3)3·5H2O (0.104 g,
C
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Figure 2. Dy6 complex known in the literature whose assembly involved the use of both keto−enol tautomerism as well as conformational isomerism
of the ligand.12c
Scheme 1. Detailed Outline of the Synthesis of the Ligand (LH3)
(pyridin-2-yl)methylene)-o-vanilloylhydrazine)} where both
the keto−enol tautomerism as well as conformational isomer-
ism plays a vital role to link pairs of Dy3 aggregate to extend the
cluster size (Figure 2).12c
Scheme 2. Synthesis of the Homometallic Ln4 Complexes 1−
3
In this context it may be mentioned that Thompson and co-
workers have utilized the feature of keto−enol tautomerism and
conformational isomerism of ligands for preparing polynuclear
complexes containing 3d-metal ions.25
With this background, we maneuvered the picolinoyl
hydrazone part of the ligand by introducing a pendant
−CH2OH arm at the sixth position of the pyridine ring
which generates an extra coordination pocket there by
increasing the chances of increasing the nuclearity of the
complex (Scheme 1).
Accordingly, the reaction of LH3 with Ln(III) salts along
with pivalic acid in the presence of tetra-n-butylammonium
hydroxide afforded the tetranuclear complexes,
[Ln4(LH)2(LH2)2(μ2-η1η1Piv)2(η1Piv)4]·2CHCl3 [Ln = Dy(1),
Tb(2) and Gd(3)] (Scheme 2). Single-crystal X-ray diffraction
studies revealed that compounds 1−3 are isomorphous. They
crystallize in the orthorhombic space group, Pbca with Z = 4. In
view of their structural similarity only the structure of 1 will be
described herein as a representative example. The structural
diagrams of 2 and 3 are given in the Supporting Information,
Figures S1 and S2. The detailed bond parameters for 1−3 are
given in Supporting Information, Tables S1−S3.
A perspective view of the molecular structure of 1 is
represented in Figure 3a. This reveals that the tetra nuclear
complex contains two symmetrically related dinuclear subunits
that are linked to each other by the intervention of two [LH2]−
ligands (Figure 3b). The asymmetric unit contains one-half of
the molecules (Figure 3c). The two DyIII ions within a given
subunit are held together by phenolate and enolate oxygen
atoms O7 and O2 respectively (Dy1−O7 = 2.362(4), Dy1−O2
D
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μ2-η1,η1 manner (Dy1−O9 = 2.304(5), Dy(2)−O(10) =
2.331(4) Å). To the best of our knowledge, this type of
topology exhibited by 1 is new and has not been found among
tetranuclear 3d or 4f complexes.
An interesting aspect of the assembly of the dinuclear subunit
is that the ligand LH3, as anticipated, functions in two different
ways. In one way (Scheme 3, Ligand A) the ligand is present in
−
the keto form [LH2] and possesses 3 types of coordination
pockets. While pockets II and III are utilized to hold the
dinuclear unit, pocket III extends the cluster size. The other
form of the ligand (Scheme 3, Ligand B) is present in the
enolate form [LH]2− and possesses two coordination pockets.
Both of these are utilized for holding the two metal centers in
the dinuclear part (Scheme 4). The coordination modes of
these two forms of the ligand are depicted in Scheme 4. The
Dy···Dy separation in the dinuclear unit is 3.851(9) Å.
Thus, to reiterate, the ligand utilizes both the phenomenon
of the keto−enol tautomerism as well as its conformational
flexibility (as a result of C−C rotation between C21−C22 or
C6−C7 bonds) in the formation of 1. A comparison of the
metric parameters found in the current instance along with a
literature precedent is given in Table 2. The extended
nuclearity of 1 arises from the presence of the additional
−CH2OH binding site. Interestingly, this site functions in its
neutral mode and does not get deprotonated under the reaction
conditions.
As mentioned above, for these complexes, the two Ln(III)
are octa- (7O, 1N, Figure 4a) and nona-coordinated (6O, 3N,
Figure 4b). The evaluation of the polyhedral shapes of the two
Ln(III) centers was ascertained by continuous shape measure-
ment analysis that was carried out with SHAPE to ascertain the
actual shape.27 The eight-coordinated Ln1 center was found to
exhibit a distorted coordination polyhedron that significantly
deviates from ideal geometries and is somewhere among
triangular dodecahedron (DD), square antiprism (SA) and
biaugmented trigonal prism (BTP) (see Supporting Informa-
tion, Table S4). For the nine-coordinated Ln2 center the
geometry around the metal is distorted spherical capped square
antiprism (Supporting Information, Table S4). The variation of
the coordination geometry around the two Dy(III) centers has
implications in their magnetic properties, particularly, the
magnetic relaxation under ac susceptibility conditions as
discussed below.
It may be mentioned, that in contrast to the current instance,
where the two dinuclear sub units are connected to each other
affording an overall tetranuclear ensemble, we have recently
reported a compact tetranuclear complex by the use of a
different ligand; the hydrazide motif is similar in the latter and
in the ligand used in the current instance (Figure 5).
The X-ray crystal structure of 1 reveals the presence of intra-
and intermolecular hydrogen-bonding interactions (see Sup-
porting Information, Table S5) to generate a one-dimensional
supramolecular architecture with a zigzag arrangement of the
molecules (see Supporting Information, Figure S3).
IR Spectroscopy. A detailed comparison of the IR data of
the free ligand and the metal complexes is presented in Table 3
which reflects the coordination modes of the ligand in its two
different forms (Ligand A and Ligand B). The IR spectrum of
the ligand exhibits a band at 3283 cm−1 due to the ν(N−H)
stretching frequency, signifying its keto form in the solid state.
This band is seen in all the complexes (1−3) around 3170
cm−1. The ν(CO) of the free ligand is observed at 1672 cm−1;
in the metal complexes this is shifted to a lower energy value
Figure 3. (a) Molecular structure of 1. Hydrogen atoms (non-
hydrogen bonded) and lattice chloroform molecules are omitted for
clarity. To distinguish the keto and enol forms of the ligand a different
color code for carbon has been chosen: pink color for the enol form
and sky blue for the keto form. (b) A view of the pair of Dy2 cores
covalently linked by two [LH2]− ligands. (c) Asymmetric unit of
complex 1 (solvent molecule and hydrogen atoms are omitted for
clarity).
= 2.343(4), Dy2−O7 = 2.355(4), Dy2−O2 = 2.377(4) Å). In
addition, the pivalate ligand functions as a bridging ligand in a
E
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Scheme 3. Conformational Isomerism of the Ligand LH3 Showing Different Coordination Pockets
Scheme 4. Coordination Modes of the LH3 with Various
Deprotonated Forms Indicated by the Harris Notation26
Figure 4. (a) Eight coordinate distorted geometry around the Dy1 ion.
(b) Nine coordinate distorted coordination around the Dy2 ion.
(∼1650 cm−1). The ν(CN) vibration mode at 1609 cm−1 in the
spectrum of the free ligand remains unchanged in the
complexes. However, the IR spectra of 1−3 reveals, a broad
and strong peak around 1574 cm−1. This is attributed to the
formation of the new CN bond upon the deprotonation of
the free N−H group of the ligand.
bridge; the formylpyridine moiety linking two Gd dimers is not
expected to mediate efficiently an exchange coupling. Therefore
modeling was performed by considering two magnetically
independent Gd2 units. Best fit to experimental data yielded
Magnetic Properties. The temperature dependence of
χMT (χM stands for the molar magnetic susceptibility) for 1−3
between 2 and 300 K are plotted in Figure 6. For each
compound, the values found at 300 K (i.e., 56.3, 47.0, and 31.1
cm3 mol−1 K respectively for 1−3) are in agreement with that
anticipated for four Ln(III) ions in the absence of exchange
J
GdGd = −0.12 0.07 cm−1 (based on H = −JSGd1·SGd2) and g =
2.0 0.06 (Figure 6). The strength found is in the order of the
values usually found for related species.29 It is indicative for
very weak antiferromagnetic interactions taking place in these
Ln4 compounds. This is confirmed by the field dependence of
the magnetization for 3 (See Supporting Information, Figure
S4) that reaches 27.3 μB for 5 T, a value close to the anticipated
saturation value Ms = 28 μB. We expect a nonmagnetic ground
state for the Gd4 compound; however, according to the very
weak antiferromagnetic interaction (−0.12 cm−1) this state is
reached only for temperatures below 0.5 K. At 2 K (the lowest
T we can investigate) the excited states (i.e., the paramagnetic
regime) still strongly contribute to the magnetic susceptibility,
which accounts for the large χMT value observed.
interactions (i.e., 56.7 (Dy4), 47.2 (Tb4), and 31.5 cm3 mol−1
K
(Gd4)) and decreases as T is lowered with a more rapid fall
below 100 K. For 1 and 2 this behavior originates from the
crystal field effects and possible exchange interactions28 while
for the Gd derivative only antiferromagnetic interactions can be
involved. To evaluate the strength of this interaction, an
analysis of the experimental behavior for 3 has been performed.
Based on the structural features for this compound, the most
likely exchange pathway between the Gd ions is through the O-
Table 2. Comparison of Bond Lengths (Å) of the Keto-Enol Forms of the Ligands between Complexes Reported Here and
Those Known from the Literature
bond length
C−O
N−C
C−O
N−C
complex
reference
(ketoform)
(ketoform)
(enolform)
(enolform)
Dy4(LH)2(LH2)2(μ2η1η1Piv)2(η1Piv)4·2CHCl3 LH3 = N′-(2-hydroxy-3-methoxybenzylidene)-
1.235(8)
1.346(9)
1.291(8)
1.295(8)
this
work
6-(hydroxymethyl) picolinohydrazide
[Dy2(Hovph)(ovph)(NO3)2(H2O)4]·NO3·2CH3OH·3H2O H2ovph = (2-hydroxy-3-
methoxybenzylidene) picolinohydrazide
1.240(10)
1.323(11)
1.297(10)
1.315(10) 24d
F
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Figure 5. Ln4 complexes reported recently using a Schiff-base formed
between the condensation of 6-(hydroxymethyl) picolinohydrazide
and 8-hydroxyquinoline-2-carbaldehyde.10m
Table 3. IR Data (cm−1) of the Ligand and the Complexes
1−3
LH3
1
2
3
ν(CN)
ν(CO)
ν(N−H)
1609
1672
3283
1613, 1574
1652
1612,1574
1650
1613,1572
1650
3169
3168
3169
Figure 7. AC susceptibility data for 1 recorded with DC field of 1 kOe.
(top) Temperature dependence of χM′ and χM″ as a function of the
frequency. (bottom) Plot of ln τ = f(1/TB) and best fit (straight lines)
of the Arrhenius law to the data point yielding Δ/kB = 26.3 K and τ0 =
1.3 × 10−6 s for the LT relaxation and Δ/kB = 62.6 K and τ0 = 8.7 ×
10−7 s for the HT relaxation, see text.
weak, exchange interaction among them, they behave
independently and exhibit each a characteristic relaxation of
the magnetization. Such a behavior has been well documented
in recent years8f and was also found for a related Dy4
compound we have reported recently.10m The analysis of the
Argand plots5a (see Supporting Information, Figure S6) suggest
a single relaxation process for the relation observed in the 9−15
K temperature range. For lower temperatures this is no longer
the case because of the concomitant contributions of the two
relaxation processes, hence leading to a wider distribution for
the relaxation time.
□
●
Figure 6. Temperature dependence of χMT for 1 ( , Dy4), 2 ( , Tb4)
○
and 3 ( , Gd4). Calculated behavior (solid line) for JGdGd = −0.12
0.07 cm−1 and g = 2.0 0.06 (see text).
AC susceptibility data revealed the occurrence of a slow
relaxing magnetization for 1. Below 20 K, an out-of-phase
component, χM″, is observed and both χM′ and χM″ are
frequency dependent. The behavior recorded with a DC field of
1 kOe is depicted in Figure 7; the external field was applied to
reduce the QTM at low temperature, but this does not affect
the maxima for χM′ and χM″ found at higher temperatures (see
Supporting Information, Figure S5). A salient feature of these
curves are the occurrence of two peaks with maxima for χM″ at
5.7 and 12.8 K for 1450 Hz, revealing that two relaxation
processes take place for this compound. Such a behavior can be
ascribed to the presence of two Ln sites with different
coordination spheres in 1. Because of the absence, or too
An evaluation of the effective energy barriers for the
magnetization reversal and relaxation times associated to the
lower (LT) and higher temperature (HT) signals have been
deduced from the plot of lnτ = f(1/TB) where τ is the
relaxation time for a given frequency, i.e. (2πν)−1, and TB the
blocking temperature (i.e., the T of the maxima of χM″ for this
frequency). Fitting the Arrhenius law τ = τ0 eΔ/k T to the data
B
points (Figure 7) lead to Δ/kB = 26.3 K and τ0 = 1.3 × 10−6
s
s
for the LT relaxation, and Δ/kB = 62.6 K and τ0 = 8.7 × 10−7
for the HT relaxation. These values are in agreement with a
SMM behavior for 1.
The energy barriers obtained show the strong effect of the
coordination sphere of the Dy ion on Δ/kB. Following the
G
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Article
tentative assignment we have discussed in a previous case,10m it
is likely that the larger barrier for reversal of magnetization is
due to the octacoordinated Dy centers. Theoretical inves-
tigations devoted to anisotropic Ln ions suggest that the
magnetization blocking barrier for these ions is very versatile;
coordination geometry, ligand strengths, and their positioning
in the coordination sphere as well as intercenter interactions
play a prominent role.30
deposited with the Cambridge Crystallographic Data Center,
CCDC Nos. 945440−945442. Copies of this information may
be obtained free of charge from The Director, CCDC, 12
Union Road, Cambridge, CB2 1EZ, U.K. (fax +44−1223−
336033; e-mail deposit@ccdc.cam.ac.uk; http://www.ccdc.cam.
ac.uk).
AUTHOR INFORMATION
Corresponding Authors
*E-mail: vc@iitk.ac.in (V.C.).
*E-mail: sutter@lcc-toulouse.fr (J.-P.S.).
■
The energy barrier for magnetization reversal of Δ/kB = 62.6
K obtained here is fairly high as compared to the majority of
Ln-based SMMs or SIMs. It is also larger by 8 K to the energy
barrier obtained for octacoordinated DyIII in a distorted
dodecahedral geometry with a very similar ligand set [the
hydrazide part is common while the aldehyde part is 8-
hydroxyquinoline-2-carbaldehyde (Figure 5)]. It is interesting
to compare the magnetic properties observed in the current
instance with that of the tetranuclear rhombus-shaped
dysprosium analogue, that was previously reported by us.10m
The latter also showed two frequency-dependent peaks in the
χM″ vs temperature curves, indicating the occurrence of two
relaxation processes that lead to two energy barriers (16.8 and
54.2 K) and time constants (τ0 = 1.3 × 10−6 s, τ0 = 7.2 × 10−7
s).10m This was related to the presence of two distinct
geometrical sites (seven- and eight coordinated) for Dy(III)
ions in rhombus Dy4 complex. In the current instance also,
although the complex is made up of two pairs of dimers, two
energy barriers (26.3 and 62.6 K) and time constants (τ0 = 1.4
× 10−6 s, τ0 = 8.7 × 10−7 s) are obtained from their ac
susceptibility studies. In both of these instances distinct
coordination geometries, we believe, is responsible for the
observation of two relaxation processes. These results are
indicative of the importance of the geometrical features around
the lanthanide ions in determining and differentiating their
magnetic properties.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
V.C. is thankful to the Department of Science and Technology
for a J. C. Bose fellowship. S.D., A.D., and S.H. thank the
Council of Scientific and Industrial Research, India, for Senior
Research Fellowships.
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CONCLUSION
■
To summarize, we report the synthesis and structural
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ASSOCIATED CONTENT
* Supporting Information
■
S
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