Rhombus-shaped tetranuclear [Ln4] complexes [Ln = Dy(III) and Ho(III)]: Synthesis, structure, and SMM behavior

Article abstract of DOI:10.1021/ic302848k

The reaction of a new hexadentate Schiff base hydrazide ligand (LH ₃) with rare earth(III) chloride salts in the presence of triethylamine as the base afforded two planar tetranuclear neutral complexes: [{(LH)₂Dy₄}(μ₂

Full text of DOI:10.1021/ic302848k

Article
pubs.acs.org/IC
Rhombus-Shaped Tetranuclear [Ln4] Complexes [Ln = Dy(III) and
Ho(III)]: Synthesis, Structure, and SMM Behavior
Vadapalli Chandrasekhar,*,†,‡ Sakiat Hossain,† Sourav Das,† Sourav Biswas,† and Jean-Pascal Sutter*,§,∥
†Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India
‡Tata Institute of Fundamental Research, Centre for Interdisciplinary Sciences, 21 Brundavan Colony, Narsingi, Hyderabad-500075,
India
§CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, F-31077 Toulouse, France
∥Universite
́
de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France
S
* Supporting Information
ABSTRACT: The reaction of a new hexadentate Schiff base
hydrazide ligand (LH3) with rare earth(III) chloride salts in
the presence of triethylamine as the base afforded two planar
tetranuclear neutral complexes: [{(LH)2Dy4}(μ2-O)4]-
(H2O)8·2CH3OH·8H2O (1) and [{(LH)2Ho4}(μ2-O)4]-
(H2O)8·6CH3OH·4H2O (2). These neutral complexes possess
a structure in which all of the lanthanide ions and the donor
atoms of the ligand remain in a perfect plane. Each doubly
deprotonated ligand holds two Ln(III) ions in its two distinct
chelating coordination pockets to form [LH(Ln)2]4+ units. Two such units are connected by four [μ2-O]2− ligands to form a
planar tetranuclear assembly with an Ln(III)4 core that possesses a rhombus-shaped structure. Detailed static and dynamic
magnetic analysis of 1 and 2 revealed single-molecule magnet (SMM) behavior for complex 1. A peculiar feature of the χM″
versus temperature curve is that two peaks that are frequency-dependent are revealed, indicating the occurrence of two relaxation
processes that lead to two energy barriers (16.8 and 54.2 K) and time constants (τ0 = 1.4 × 10−6 s, τ0 = 7.2 × 10−7 s). This was
related to the presence of two distinct geometrical sites for Dy(III) in complex 1.
possess relatively high spins (Dy(III) = 15/2, Tb(III) = 6,
INTRODUCTION
■
Ho(III) = 8) and an intrinsic magnetic anisotropy. The latter is
the result of the splitting of the spectroscopic ground level by a
crystal field.13 In this context, Ishikawa’s double-decker
complex, [Pc2Tb]−, remains a spectacular example with a
high blocking temperature.14 Among the homometallic
lanthanide ion-containing systems, those containing Dy(III)
have received considerable attention, and complexes with a
nuclearity that varies from 1 to 11 [Dy1,15 Dy2,16Dy3,17
Dy412b,18Dy5,19 Dy6,20 Dy7,21 Dy8,22 Dy9,23 Dy10,24 Dy1125] are
known in the literature. Some of these compounds have been
shown to exhibit SMM behavior. We have been involved in
designing ligands, such as SP[N(Me)NCH−C6H3−2−OH−
3−OMe]3, that allow the assembly of heterometallic trinuclear
3d−4f compounds. Using a related ligand system[{NC(N-
(CH3 )2 )}2 {NP{N(CH3 )NCH−C6 H3 -(o-OH)(m-
OCH3)}2}] and [{N2P2(O2C12H8)2}{NP{N(CH3)NCH−
C6H3(o-OH)(m-OCH3)}2}] we could prepare heterometallic
dinuclear 3d−4f compounds.26 However, it was not possible to
access homometallic lanthanide(III) assemblies using either of
these ligands, and it was clear that a multi-site-coordinating,
multi-compartmental ligand was required. Compartmental
There has been vigorous research activity in recent years in
several laboratories around the world in the area of molecule-
based magnetic materials such as single-molecule and single-
chain magnets (SMMs and SCMs).1 This interest stems from
several points of view. The first aspect of interest is the exciting
potential of these new materials for use in exotic applications,
including data storage through quantum computation and
spintronics,2 and magnetic refrigeration.3 In this regard, the
deposition of SMMs on surfaces and their utilization in devices
are emerging areas of interest.2b,4 The second aspect of interest
in molecule-based magnetic materials is that these systems offer
the opportunity to study and to understand exotic physical
phenomenon such as quantum tunneling of the magnetization,5
finite size effects,1k,6 quantum phase interference,7 quantum
super positions,8 magnetic deflagration,9 and so forth. The third
point of interest, which is in the exclusive domain of chemists,
is to devise reliable synthesis strategies that allow the assembly
of multitudes of families of these new exotic materials.
Accordingly, several polynuclear transition metal ions,10
mixed transition−lanthanide metal ions (3d−4f),11 and
homometallic lanthanide ion aggregates12 have been prepared
and studied, and some of them have been shown to be SMMs.
In this series, compounds containing lanthanide ions, such as
Dy(III), Tb(III), and Ho(III), are of interest because they
Received: December 27, 2012
© XXXX American Chemical Society
A
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Table 1. Details of the Data Collection and Refinement Parameters for Complexes 1 and 2
formula
C36H64Dy4N8O28
1706.93
C40H72Ho4N8O28
1772.76
M/g
crystal system
space group
triclinic
triclinic
P1
̅
P1
̅
unit cell dimensions (Å, deg)
a = 10.552(5)
b = 11.547(5)
c = 11.927(5)
α = 104.585(5)
β = 91.465(5)
γ = 104.431(5)
1356.0(10)
a = 10.431(4)
b = 11.430(4)
c = 11.838(4)
α = 105.040(6)
β = 91.041(7)
γ = 104.007(7)
1317.6(8)
V/Å3
Z
1
1
ρc/g cm−3
μ/mm−1
2.090
2.186
5.540
6.037
F(000)
812
822
cryst size (mm3)
θ range (deg)
limiting indices
0.05 × 0.03 × 0.02
2.00 to 25.50
−12 ≤ h ≤ 12
−11 ≤ k ≤ 13
−14 ≤ l ≤ 14
7296
0.15 × 0.12 × 0.10
1.79 to 25.50
−12 ≤ h ≤ 7
−13 ≤ k ≤ 13
−14 ≤ l ≤ 14
7042
reflns collected
independent reflns
4945 [R(int) = 0.0247]
4809 [R(int) = 0.0482]
completeness to θ (%)
refinement method
97.9%
97.9%
full-matrix least-squares on F2
4945/14/360
1.062
full-matrix least-squares on F2
4809/114/351
1.052
data/restraints/params
goodness of fit on F2
final R indices [I > 2θ(I)]
R indices (all data)
R1 = 0.0703, wR2 = 0.2011
R1 = 0.0863, wR2 = 0.2365
3.858 and −2.766
R1 = 0.0855, wR2 = 0.2179
R1 = 0.1299, wR2 = 0.2797
3.733 and −3.028 e·Å−3
largest diff. peak and hole (e Å−3
)
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 the sample holder and for the diamagnetic
contribution of all the atoms using Pascal’s tables. The ac susceptibility
was measured with an oscillating ac field of 3 Oe in the frequency
range from 1 to 1500 Hz.
Synthesis. Preparation of 8-Hydroxyquinoline-2-carbaldehyde.
The following procedure was used for the preparation of the title
compound. This procedure is a modification of a previously published
synthesis method. In a two-necked round-bottomed flask, freshly
sublimed selenium dioxide (6.66 g, 60 mmol) was taken in 150 mL of
dioxane at 60 °C. To this, 2-methylquinolin-8-ol (5.2 g, 30 mmol)
dissolved in dioxane (200 mL) was added dropwise over a period of
2.5 h and was refluxed at 95 °C for 24 h. The reaction mixture was
cooled and filtered through Celite; dioxane was removed from the
filtrate under reduced pressure. The residue was purified by column
chromatography using silicagel (100 to 200 mesh) with 5:95 v/v
ethylacetate-n-hexane used as the eluant to give a light-yellowish
compound, 8-hydroxyquinoline-2-carbaldehyde. Yield = 4.15 g (80%).
Mp = 90 °C. Anal. Calcd for C10H7NO2: C, 69.36; H, 4.07; N, 8.09.
Found: C, 69.27; H, 4.13; N, 8.21. 1H NMR (CDCl3, δ) 7.28 (d, 1H),
7.42 (d, 1H), 7.61 (t, 1H), 8.04 (d, 1H), 8.31(d, 1H), 10.20 (s, 1H).
ESI-MS m/z (M + H): 174.06.
Schiff base ligands have been used in the preparation of
polynuclear lanthanide-containing compounds.17b,18c,27 In the
current study, our ligand design retained the hydrazine part of
the aforementioned phosphorus-supported ligands but varied
the remaining portion. Accordingly, we assembled (6-
hydroxymethyl)-N′-((8-hydroxyquinolin-2-yl)methylene)-
picolinohydrazide(LH3), and we used this to construct neutral
homotetranuclear Dy4 and Ho4 assemblies [{(LH)2Dy4}(μ2-
O)4](H2O)8·2CH3OH·8H2O (1) and [{(LH)2Ho4}(μ2-O)4]-
(H2O)8·6CH3OH·4H2O (2). Magnetic measurements on 1
were carried out and revealed it to be an SMM. These results
are discussed in this Article.
EXPERIMENTAL SECTION
■
Reagents and General Procedures. The common reagents and
the solvents that were used in this work were purified according to
standard procedures.28a,b Methyl-6-(hydroxymethyl)picolinate28c and
8-hydroxy-2-quinolinecarboxaldehyde29 were prepared by adapting
procedures that have been reported. Pyridine-2,6-dicarboxylic acid,
NaBH4, SeO2, 2-methyl-8-quinolinol, DyCl3·6H2O, and HoCl3·6H2O
(Sigma Aldrich) were used as received. Hydrazine hydrate was
obtained from Merck and was used as received.
Preparation of Methyl 6-(hydroxymethyl) Picolinate. The
preparation of methyl 6-(hydroxymethyl) picolinate was carried out
by a slightly modified procedure upon comparison to a previously
published synthesis method.c 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 (6.0 g, 35.9 mmol)
in methanol (150 mL) at 0 °C. This mixture was stirred at room
temperature for another 3 h, and then methanol 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 chloroform (5 × 100 mL). The combined organic layers
Instrumentation. Melting points were measured using a JSGW
apparatus and are uncorrected. Elemental analyses were carried out
using a Thermoquest CE instrument model EA/110 CHNS-O
elemental analyzer. 1H NMR was recorded on a JEOL-JNM LAMBDA
400 model NMR spectrometer in a CDCl3 solution. The chemical
shifts are referenced with respect to SiMe4. IR spectra were recorded in
KBr pellets on a Bruker Vector 22 FT IR spectrophotometer that was
operating from 400 to 4000 cm−1.
Magnetic measurements were carried out with a Quantum design
MPMS 5S SQUID susceptometer in the temperature range from 2 to
300 K. The measurements were performed on crushed crystals from
B
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Scheme 1. The Synthesis of the LH3 Ligand with Two Distinct Chelating Coordination Sites
were dried (NaSO4), filtered, and concentrated in vacuo to dryness.
The resulting crude residue was purified by column chromatography
(1:1 v/v n-hexane/EtOAc followed by 1:2 v/v of the same) and
afforded the desired product (4.22 g, 82.2%) 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,
(15 mL), affording a colorless solution. Two equiv of LnCl3·6H2O (Ln
= Dy or Ho) was added, and the color of the solution immediately
became deep red. The reaction mixture was stirred at room
temperature for 10 min, followed by the dropwise addition of 3
equiv of triethylamine. The reaction mixture was stirred at room
temperature for 20 h, filtered, and kept for crystallization. After about
15 days, the slow evaporation of the mother liquor resulted in block-
shaped deep red-colored crystals that were suitable for X-ray
crystallography. The quantity of the reactants used in each reaction
and the characterization data for compounds 1 and 2 are given below.
[{(LH)2Dy4}(μ2-O)4](H2O)8·2CH3OH·8H2O (1). DyCl3·6H2O (0.117
g, 0.31 mmol), LH3 (0.050 g, 0.155 mmol), and Et3N (0.047 g, 0.465
mmol). Yield = 72 mg (55%, based on Dy). Mp > 250 °C. IR (KBr)
(cm−1): 3643 (w), 3602 (w), 3361 (s), 1634 (w), 1590 (m), 1571 (w),
1546 (m), 1521 (s), 1498 (m), 1443 (w), 1425 (m), 1382 (m), 1324
(s), 1298 (m), 1103 (m), 1045 (w), 737 (m). Anal. Calcd for
C36H64Dy4N8O28 (1706.93): C, 25.33; H, 3.78; N, 6.56. Found: C,
25.07; H, 3.3; N, 6.96.
[{(LH)2Ho4}(μ2-O)4](H2O)8·6CH3OH·4H2O (2). HoCl3•6H2O (0.118
g, 0.31 mmol), LH3 (0.050 g, 0.155 mmol), and Et3N (0.047 g, 0.465
mmol). Yield = 70 mg (51%, based on Ho). Mp > 250 °C. IR (KBr)
(cm−1): 3643 (w), 3601 (w), 3361 (s), 1634 (w), 1590 (m), 1571 (w),
1543 (m), 1521 (s), 1498 (m), 1441 (w), 1426 (m), 1383 (m), 1324
(s), 1298 (m), 1103 (m), 1045 (w), 737 (m). Anal. Calcd for C40H72
Ho4N8O28 (1772.76): C, 27.10; H, 4.09; N, 6.32. Found: C, 26.37; H,
3.7; N, 6.66.
X-ray Crystallography. The crystal data and the cell parameters
for 1 and 2 are given in Table 1. Single crystals suitable for X-ray
analysis were grown by the slow evaporation of the mother liquor that
contained a mixture of methanol and dichloromethane. The X-ray
diffraction data for 1 and 2 were collected with a SMART CCD
diffractometer (Mo Kα radiation, λ = 0.71073 Å). The following
programs were used. SMART30 was used for collecting frames of data,
indexing reflections, and determining lattice parameters; SAINT30 was
used for the integration of the intensity of reflections and scaling;
SADABS31 was used for the absorption correction; SHELXTL32 was
used for the space group and structure determination and for the least-
squares refinement on F2. The structures were solved with direct
methods using the program SHELXS-9733 and refined by full-matrix
least-squares methods against F2 using the program SHELXL-97. The
hydrogen atoms were fixed at calculated positions, and their positions
1
8.3. Mp = 88 °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: 1740
ν(CO); 1591 ν(CN)py cm−1. ESI-MS m/z (M + H): 168.0658.
Preparation of 6-(Hydroxymethyl) Picolinohydrazide. A meth-
anolic solution (40 mL) 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. The reaction mixture was then heated under reflux for 2
h, cooled to room temperature, and kept in a refrigerator at 5 °C for
crystallization. A white needle-shaped crystalline product was obtained,
filtered under suction, washed with a small amount of cold methanol,
and air dried. Yield = 1.87 g (93.53%). Anal. Calcd for C7H9N3O2: C,
50.29; H, 5.43; N, 25.14. Found: C, 50.02; H, 5.16; N 24.86. Mp = 110
1
°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: 3407, 3303 ν(N−H);
1655 ν(CO); 1571 ν(CN)py cm−1. ESI-MS m/z (M + H):
168.0772.
Preparation of (6-Hydroxymethyl)-N′-((8-hydroxyquinolin-2-yl)-
methylene) Picolinohydrazide (LH3). A methanolic solution (50 mL)
of 8-hydroxyquinoline-2-carbaldehyde (1.13 g, 6.5 mmol) was added
dropwise to a stirred solution of 6-(hydroxymethyl) picolinohydrazide
(1.09 g, 6.5 mmol) in methanol (100 mL) in a 250 mL round-
bottomed flask at room temperature. The reaction mixture was then
heated under reflux for 3 h and slowly allowed to come to room
temperature. The precipitate that formed was filtered, washed with
diethyl ether, and dried. Yield = 1.57 g (75%). Anal. Calcd for
C17H14N4O3: C, 63.35; H, 4.38; N, 17.38. Found: C, 63.57; H, 4.67;
N, 17.77. Mp = 90 °C. 1H NMR (500 MHz, [(CD3)2SO], δ) 12.36 (s,
1H, phenolic−OH), 9.80 (s, 1H, NH), 8.82 (s, 1H, imine-H), 7.10−
8.33 (8H, Ar-H), 4.71 (s, 2H, −CH2). ESI-MS m/z (M + H): 323.33.
Preparation of the Tetranuclear Complexes [{(LH)2Dy4}(μ2-
O)4 ](H2 O)8 ·2CH3 OH·8H2 O (1) and [{(LH)2 Ho4}(μ2-O)4]
(H2O)8·6CH3OH·4H2O (2). The syntheses of 1 and 2 were carried
out with the following general procedure. LH3 (0.05g, 0.155 mmol)
was dissolved in a mixture of methanol (10 mL) and dichloromethane
C
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were refined with a riding model. All of the hydrogen atoms of the
water molecule that is coordinated to the lanthanide ions, all of the
hydrogen atoms of the noncoordinated water molecules of both
complexes 1 and 2, the hydrogen atoms of the one methanol (complex
2, solvent of crystallization), and the hydrogen atom on the oxygen
atom of the −CH2OH group (complex 2) could not be located; these
were included in the molecular formula directly. The figures were
generated using Diamond 3.1e software.34 Complexes 1 and 2
through a unique ONNONO donor action (Scheme 2). The
formation of the tetranuclear complexes is facilitated by the μ-
O bridges between the two pairs of lanthanide centers within
the two dinuclear subunits.
X-ray Crystal Structures of 1 and 2. Complexes 1 and 2
were crystallized in the triclinic crystal lattice system in the
centrosymmetric P1 space group with Z = 1. The asymmetric
̅
crystallized in the triclinic P1 space group. The asymmetric unit of
unit contains one-half of the molecule. 1 and 2 are isostructural
except for the variation in the number of lattice solvent
molecules. Therefore, only the molecular structure of 1 is
discussed herein. Some of the figures relating to the molecular
structure of 1 are given in the Supporting Information (Figures
S2, S3, and S4a,b). The details of the structure of 2 are also
presented in the Supporting Information (Figures S5−S7).
Selected bond distances of 1 are summarized in the caption of
Figure 1. The other bond parameters of 1 and 2 are given in the
Supporting Information (Tables S1 and 2).
̅
these complexes contains one-half of the molecule.
RESULTS AND DISCUSSION
■
Synthesis Aspects. The multidentate ligand (6-hydrox-
ymethyl)-N′-((8-hydroxyquinolin-2-yl)methylene)-
picolinohydrazide (LH3) was prepared in about 75% yield by
the condensation of 6-(hydroxymethyl) picolinohydrazide and
8-hydroxyquinoline-2-carbaldehyde (Scheme 1). The ESI-MS
of LH3 revealed a parent ion peak at m/z 323.33 (Supporting
Information, Figure S1).
The molecular structure of 1 is shown in Figure 1 and reveals
that it possesses two dinuclear subunits. Each subunit contains
two different types of Dy(III) ions and is built by the chelating
coordination action of the two parts of the [LH]2− ligand that
binds in its tautomeric form (Supporting Information, Scheme
S1). Thus, Dy1 is bound by a phenolate oxygen (O3), a
pyridinic nitrogen (N4), an imino nitrogen (N3), and an
enolate oxygen (O2) that arise from tautomerism during
coordination. In contrast, Dy2 is bound by an O2, a pyridinic
nitrogen N1, and a CH2OH (O1). The mode of coordination
of the ligand in 1 is shown in Supporting Information, Chart
S1.
LH3 can exist in different tautomeric forms, which upon
deprotonation can afford [LH]2− possessing (Supporting
Information, Figure S1) two distinct chelating coordination
pockets that were expected to accommodate two lanthanide
metal ions (Scheme 1). Accordingly, the reaction of LH3 with 2
equiv of LnCl3·6H2O [Ln = Dy(III) or Ho(III)] in the
presence of 3 equiv of triethylamine afforded the tetranuclear
complexes 1 and 2 in reasonable yields: [{(LH)2Ln4}(μ2-
O)4][H2O]8·xCH3OH·yH2O, where for 1, Ln = Dy(III), x = 2,
y = 8, and for 2, Ln = Ho(III), x = 6, y = 4 (Scheme 2). In these
complexes, each [LH]2− binds to the two lanthanide metal ions
The two subunits of 1 are joined to each other by oxido
bridges, resulting in the formation of two Dy2O2 four-
membered rings. Finally, all of the dysprosium centers have
two coordinated water molecules. Because of this collective
coordination behavior, Dy1 is eight-coordinate (2N, 6O). The
evaluation of the polyhedral shape of the two Dy(III) centers
was ascertained by continuous shape measures analysis that was
carried out with SHAPE.35 The geometry around Dy1 reveals
that its closest ideal geometry is dodecahedron. However, its
effective shape is distorted and is somewhere among triangular
dodecahedron (DD), snub disphenoid (SD), and bicapped
trigonal prism (BTP) (Supporting Information, Figure S4a). In
contrast, Dy2 is seven-coordinate (1N,6O). The shape for the
heptacoordinated Dy2 appears to be far from any reference
geometry. This is rather uncommon and may reflect the steric
constraint of the immediate coordination sphere that faces the
bridging OH of the adjacent Dy−Dy moiety (Supporting
Information, Figure S4b). An interesting aspect of the
molecular structure of 1 is that all four of the dysprosium
ions and all of the coordinating atoms of the [LH]2− ligand lie
in the same plane (Supporting Information, Figure S2). To the
best of our knowledge, this is a unique structural feature
observed among all of the Dy4 assemblies. The coordinated
water molecules lie nearly perpendicular to the plane discussed
above. Another interesting aspect of the structure of 1 is that 12
metal-containing ring systems (10 five-membered (a) Dy1−
O3−C17−C13−N4; (b) Dy1−N4−C9−C8−N3; (c) Dy1−
N3−N2−C7−O2; (d) Dy2−O2−C7−C6−N1; and (e) Dy2−
N1−C2−C1−O1, and two four-membered Dy2−O4−Dy2′−
O5) are generated as a result of the fusion of the two dinuclear
subunits through the two Dy2O2 bridges. Last, the four
dysprosium ions are present in the corners of a perfect rhombus
with the inter Dy−Dy distances being 3.79(13) Å (Supporting
Scheme 2. The Synthesis of the Tetranuclear Complexes 1
and 2
a
a
The solvents that were present in the crystallization of 1 and 2 are
not shown.
D
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Figure 1. Molecular structure of 1. The hydrogen atoms and the noncoordinated solvent molecules have been omitted for clarity. Selected bond
distances (Å): Dy1−O2, 2.381(8); Dy1−O3, 2.306(9); Dy1−O4, 2.293(13); Dy(1)−O(5) 2.29(2); Dy1−O6, 2.371(11); Dy1−O7, 2.416(10);
Dy1−N3, 2.562(10); Dy1−N4, 2.455(10); Dy2−O1, 2.352(9); Dy2−O2, 2.328(8); Dy2−O4#1, 2.258(14); Dy2−O5#1, 2.29(2); Dy2−O8,
2.468(12); Dy2−O9, 2.400(14); Dy2−N1, 2.537(10). Selected bone angles (deg): O4−Dy1−O5, 62.1(7); O4−Dy2′−O5, 62.03(5); O5#1−Dy2−
O4′#1, 62.03(5); O5#1−Dy1#1−O4′#1, 62.1(7)
Information, Figure S3), and the diagonal Dy−Dy distances are
6.664(2) and 3.636(2) Å.
Magnetic Properties. The temperature dependence of the
molar magnetic susceptibility, χM, has been investigated for 1
and 2 between 2 and 300 K, and the results are given as the
χMT versus T plot in Figure 2. For 1, the value of 53.4 cm3
Dy(III) and Ho(III) ions and results from the crystal field
effects.13 Therefore, it is difficult to conclude whether exchange
interactions are occurring between the Ln(III) centers.36
However, it is clear that even if such interactions are present
they are very weak. The field dependence of the magnetization
has been investigated in the range of 0 to 50 kOe at several
temperatures from 2 to 10 K. In Figure 2 (inset), we show the
behavior at 2 K, whereas that for the other T values are in
Supporting Information, Figures S8a,b, and S9a,b. A rapid
increase is observed for weak fields followed by a smoother but
continuous increase to reach, for 50 kOe magnetizations,
19.2μB and 18.7μB for 1 and 2, respectively. The saturation of
the magnetization is not reached, which is consistent with the
significant magnetic anisotropy of these Ln(III) centers and
possibly with low-lying excited states. This is supported by the
nonsuperposition of the M = f(H/T) curves (Supporting
Information, Figures S8c, S9c). Notice also that the M = f(H)
curves for 1 and 2 do not exhibit an S shape. This excludes the
occurrence of weak antiferromagnetic intra- or intermolecular
interactions.
The ac susceptibility data provided evidence of a slow-
relaxing magnetization for 1 but not for 2, even in the presence
of an applied field (Supporting Information, Figure S10). Below
20 K, the out-of-phase component, χM″, for 1 deviates from
zero and both χM′ and χM″ become frequency-dependent. A
peculiar feature of these curves is that they show two peaks that
are frequency-dependent (Figure 3) with maxima for χM″ at 3.9
and 10.7 K for 1490 Hz, revealing the occurrence of two
relaxation processes. This behavior has been described in recent
reports1l,12b,15a,b,18c,20a,37,38 and is attributed to the presence of
two Ln(III) sites in the crystal lattice. This applies to 1, which
possesses two Dy(III) units with distinct coordination spheres
and geometries. Because of the very weak coupling between the
Dy(III) centers, they may behave independently and are
characterized by their own temperature of blockage of
magnetization. The peaks of the χM″ signal can then be
associated with two relaxation times corresponding to each of
the two types of Dy(III) ions of 1. The analysis of the Argand
plots (Supporting Information, Figure S11) revealed that the α
Figure 2. Temperature dependence of χMT for 1 and 2. (Inset) Field
dependence of the magnetization (solid lines are a guide for the eye).
mol−1 K at 300 K is slightly lower than the value of 56.7 cm3
mol−1 K that was anticipated for four uncoupled Dy(III) ions
(6H15/2, S = 5/2, L = 5, J = 15/2, g = 4/3).13 As the temperature is
lowered, the χMT steadily decreases to reach a small plateau at
ca. 44.5 cm3 mol−1 K between 20 and 10 K before dropping
steeply to reach 29.6 cm3 mol−1 K at 2 K. For 2, a χMT of 53.0
cm3 mol−1 K was found at 300 K, which is slightly below the
theoretical value of 56.28 cm3 mol−1 K that was anticipated for
four noninteracting Ho(III) ions (5I8, S = 2, L = 6, J = 8, gJ =
5/4).38 The χMT continuously decreases when the temperature
is reduced with a more rapid fall below 30 K, reaching 21.0
cm3 mol−1 K at 2 K. Such overall behavior is characteristic of
E
dx.doi.org/10.1021/ic302848k | Inorg. Chem. XXXX, XXX, XXX−XXX

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Article
the present study may be related to the crystal field splitting
energy diagrams of these ions. For Dy(III), the energy between
the lowest mJ and the first-excited mJ states is usually larger than
for Ho(III).40 Moreover, Dy(III) is a Kramer ion and therefore
always has degenerate mJ states, which is not the case for
compounds containing Ho(III), except in some instances.13,41
Although many Dy(III)-based SMMs are known, very few
Ho(III) derivatives have been found that exhibit a slow
relaxation of their magnetization. The magnetic behavior of the
compounds reported in the present study is consistent with
such literature observations.
CONCLUSIONS
■
We have shown the successful design and assembly of a new
family of tetranuclear, neutral, homometallic LnIII complexes
4
that are characterized by a rhombus-shaped core topology. This
was achieved using a new unsymmetrical hexadentate Schiff
base hydrazide ligand. These centrosymmetric tetranuclear
compounds contain two nonequivalent Ln(III) ions in their
structures. Although the geometry around one type of Ln(III)
ion is that of a distorted dodecahedron, the other type of
Ln(III) is heptacoordinated and has a geometry that appears to
be far from any reference geometry. In addition, compounds 1
and 2 represent the first examples of a homometallic 4f SMM
family in which all of the donor atoms of the ligands and the
metal ions are in a single perfect plane. The magnetization
studies involving the ac susceptibility measurements revealed
that between compounds 1 and 2 only the former exhibits a
slow relaxation of magnetization below 20 K. A peculiar feature
of the χM″ versus temperature (T K) curve of this compound is
that it show two peaks that are frequency-dependent, revealing
the occurrence of two relaxation processes that lead to two
energy barriers (16.8 and 54.2 K) and time constants (τ0 = 1.4
× 10−6 s, τ0 = 7.2 × 10−7 s). This behavior is attributed to the
presence of two Dy(III) sites with distinct coordination spheres
and geometries. Because of the very weak coupling between the
Dy(III) centers, they behave independently and are charac-
terized by their own energies of blockage of magnetization.
Given the versatility of the LH3 ligand, we plan to modulate its
structural features and to study its influence on the formation of
other types of structurally diverse homonuclear 4f families.
Figure 3. Temperature dependence of χM′ and χM″ as a function of the
frequency for 1. (Inset) Plot of lnτ vs 1/TB and the straight line is a fit
to the data points yielding Δ/kB = 16.8 K and τ0 = 1.4 × 10−6 s for the
LT relaxation, and Δ/kB = 54.2 K and τ0 = 7.2 × 10−7 s for the HT
relaxation. See the text.
parameter is close to zero (a single relaxation process) for the
relaxation that was found in the temperature range from 8 to 12
K, whereas α increases for temperatures below 8 K in
agreement with a wide distribution of τ. The evaluation of
the effective energy barriers for magnetization reversal and
relaxation times for both the low-temperature (LT) and high-
temperature (HT) features have been deduced from the plot of
the respective blocking temperatures (i.e. the maximum of χM″
for a given frequency) as ln τ versus 1/TB, where τ = 1/(2πν) is
the corresponding relaxation time for a given frequency ν. The
straight lines in the inset of Figure 3 are the fits to the data
points and are the signature of the thermal activation that is
ASSOCIATED CONTENT
* Supporting Information
■
S
Different forms of the ligand under different pH conditions,
binding mode of the ligand LH3 in its [LH]2−, ESI-MS
spectrum of the ligand (LH3), planar arrangement of the four
Dy(III) ions along with the coordinating atoms of the ligand
LH2−, rhombus-shaped Dy4 core of 1, distorted dodecahedral
coordination environment around Dy1 in 1, coordination
environment around Dy2 in 1, molecular structure of 2, a view
of the tetranuclear core of 2, rhombus-shaped core of complex
2, M versus H curves for 1, M versus H curves for 2,
temperature dependence of χM′ and χM′′ as a function of the
frequency for 2, Cole−Cole plots for 1, important bond angle
and bond parameters for complex 1, important bond angle and
bond parameters for complex 2, and the crystallographic data in
CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
taking place following the Arrhenius law: τ = τ0 eΔ/k T. The
B
values obtained from the least-squares fitting are Δ/kB = 16.8 K
and τ0 = 1.4 × 10−6 s for the LT relaxation and Δ/kB = 54.2 K
and τ0 = 7.2 × 10−7 s for the HT relaxation. These values are in
agreement with the SMM behavior for 1. Interestingly, the
difference between the effective energy barriers is significant,
underlining the effect of the coordination of Dy(III). A
tentative assignment of the geometry for the higher blocking
temperature would be octacoordinated Dy1. Indeed, several
examples of Dy(III) SMMs with rather high energy barriers
have been found with eight-coordinated surroundings,12b,15a,38
whereas lower energy barriers have been observed for
heptacoordinated Dy(III) compounds.39 The difference in the
magnetic behavior of the Dy(III) and Ho(III) compounds in
AUTHOR INFORMATION
Corresponding Author
*E-mail: vc@iitk.ac.in (V.C.), sutter@lcc-toulouse.fr (J.-P.S.).
■
F
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Article
Abboud, K. A.; Christou, G. J. Am. Chem. Soc. 2004, 126, 15648.
(e) Li, M.; Lan, Y.; Ako, A. M.; Wernsdorfer, W.; Anson, C. E.; Buth,
G.; Powell, A. K.; Wang, Z.; Gao, S. Inorg. Chem. 2010, 49, 1158.
(7) (a) Lecren, L.; Wernsdorfer, W.; Li, Y.-G.; Vindigni, A.; Miyasaka,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
H.; Cler
Roubeau, O.; Coulon, C.; Li, Y.-G.; Goff, X. F. L.; Wernsdorfer, W.;
Miyasaka, H.; Clerac, R. J. Am. Chem. Soc. 2005, 127, 17353.
(c) Lecren, L.; Roubeau, O.; Li, Y.-G.; Goff, X. F. L.; Miyasaka, H.;
Richard, F.; Wernsdorfer, W.; Coulona, C.; Clerac, R. Dalton Trans.
2008, 755.
́
ac, R. J. Am. Chem. Soc. 2007, 129, 5045. (b) Lecren, L.;
V.C. thanks the Department of Science and Technology for a
J.C. Bose fellowship. S.H. and S.D. thank the Council of
Scientific and Industrial Research, India, for a senior research
fellowship. S.B thanks the Council of Scientific and Industrial
Research, India, for a junior research fellowship.
́
́
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2005, 95, 37203. (b) Lecren, L.; Wernsdorfer, W.; Li, Y.-G.; Roubeau,
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dx.doi.org/10.1021/ic302848k | Inorg. Chem. XXXX, XXX, XXX−XXX