Tetranuclear Lanthanide Complexes Containing a Hydrazone-type Ligand. Dysprosium [2 × 2] Gridlike Single-Molecule Magnet and Toroic

Article abstract of DOI:10.1021/acs.inorgchem.6b02449

A multidentate hydrazone-type ligand (Z,Z)-bis(1-(pyridin-2-yl)-1-amino-methylidene)oxalohydrazide (H₂L) was utilized in the synthesis of three new isomorphous tetranuclear complexes of the general formula [Ln₄(HL)₄(H₂L)₂(NO₃)₄](NO₃)₄·4CH₃OH (Ln = Gd^III, 1, Tb^III, 2, Dy^III, 3) with the gridlike [2 × 2] topology. The analysis of the static magnetic data revealed weak anti-ferromagnetic interaction among lanthanide(III) atoms, whereas dynamic magnetic data led to the observation of the single-molecule magnet behavior in zero static magnetic field for the Dy₄ compound 3 with U_eff = 42.6 K and τ₀ = 1.50 × 10^-5 s. The theoretical CASSCF calculations supported also the presence of the net toroidal magnetic moment, which classifies compound 3 also as a single-molecule toroic.

Full text of DOI:10.1021/acs.inorgchem.6b02449

Article
pubs.acs.org/IC
Tetranuclear Lanthanide Complexes Containing a Hydrazone-type
Ligand. Dysprosium [2 × 2] Gridlike Single-Molecule Magnet and
Toroic
Alexey Gusev,*,† Radovan Herchel,‡ Ivan Nemec,‡ Victor Shul’gin,† Igor L. Eremenko,§
Kostantin Lyssenko,∥ Wolfgang Linert,⊥ and Zdenek Travnícek*,‡
̌
́
̌
†General and Physical Chemistry Department, Crimean Federal University, Simferopol, Crimea
‡Department of Inorganic Chemistry, RCPTM, Faculty of Science, Palacky University, Olomouc, Czech Republic
́
∥
§N. S. Kurnakov Institute of General and Inorganic Chemistry and A. N. Nesmeyanov Institute of Organoelement Compounds,
Russian Academy of Sciences, Moscow, Russia
⊥Institute of Applied Synthetic Chemistry, Vienna University of Technology, Vienna, Austria
S
* Supporting Information
ABSTRACT: A multidentate hydrazone-type ligand (Z,Z)-
bis(1-(pyridin-2-yl)-1-amino-methylidene)oxalohydrazide
(H2L) was utilized in the synthesis of three new isomorphous
tetranuclear complexes of the general formula
[Ln4(HL)4(H2L)2(NO3)4](NO3)4·4CH3OH (Ln = GdIII, 1,
TbIII, 2, DyIII, 3) with the gridlike [2 × 2] topology. The
analysis of the static magnetic data revealed weak anti-
ferromagnetic interaction among lanthanide(III) atoms,
whereas dynamic magnetic data led to the observation of the
single-molecule magnet behavior in zero static magnetic field
for the Dy4 compound 3 with Ueff = 42.6 K and τ0 = 1.50 ×
10−5 s. The theoretical CASSCF calculations supported also the presence of the net toroidal magnetic moment, which classifies
compound 3 also as a single-molecule toroic.
DyIII cluster with a [2 × 2] grid geometry showed the highest
INTRODUCTION
■
anisotropic barrier, U = 143 cm−1 (206 K), for the reported Dy4
The coordination compounds exhibiting slow relaxation of
SMMs.6 Furthermore, some polynuclear DyIII compounds with
magnetization on molecular level, so-called single-molecule
Ising-like magnetic anisotropy and vortex distributions of
magnets (SMMs), attract a lot of attention due to their
magnetic dipoles not only exhibit SMM properties but also
potential applications in high-density information storage,
possess the toroidal magnetic moment.7 This is regarded as the
quantum computing, and molecular spintronics.1 The magnetic
third kind of electromagnetic moment after the traditional
bistability of these molecular nanomagnets is associated with
the effective energy barrier (U) to reversal of the magnetization,
and the key property, which defines the height of this barrier, is
the uniaxial Ising-like magnetic anisotropy. It has been shown
polarization and magnetization, and such bistable molecules
with a toroidal magnetic state were called single-molecule
toroics (SMTs).8 So far, there is only one example of a
tetranuclear DyIII complex, [Dy4(μ3-OH)2(μ-OH)2(2,2-
bpt)4(NO3)4(EtOH)2] (2,2-bptH = 3,5-bis(pyridin-2-yl)-
1,2,4-triazole), exhibiting such properties.9
that lanthanides, especially DyIII, TbIII, and ErIII, are the most
promising components for observing large U and high blocking
temperature due to their large spin−orbit coupling and high
magnetic moment.2 Indeed, two novel DyIII compounds
published very recently established new records in blocking
temperature (T = 20 K) and in the effective energy barrier (Ueff
= 1025 K) in the class of SMMs.3
In this article, we report on structural and magnetic
properties of a new group of tetranuclear gridlike [2 × 2]
Ln4 complexes with the multidentate ligand (Z,Z)-bis(1-
(pyridin-2-yl)-1-amino-methylidene)oxalohydrazide (H2L;
Scheme 1). The H2L ligand was previously used in the
synthesis of 3d metal complexes such as dinuclear [Mn2(H2L)
(NO3)2(CH3OH)2(H2O)2](NO3)2·H2O, tetranuclear
[Fe4(H2L)4](ClO4)6·7.5H2O, [Co4(H2L)4]Br6·9H2O,
Furthermore, it has been shown on numerous examples
during the past decade that a very promising strategy for
obtaining high-barrier SMMs is a preparation of polynuclear
DyIII clusters.4 Thus, a number of Dy-containing compounds
exhibiting different topologies based on a dimeric, a triangular,
a defect-dicubane, or a square-pyramidal core, or a wheel, has
been described in the literature.5 Among them, a tetranuclear
Received: October 10, 2016
© XXXX American Chemical Society
A
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The magnetic data were corrected for sample holder signal and for
diamagnetic susceptibility. Measurements of alternating-current (AC)
susceptibility were performed in a 3.8 Oe with AC field oscillating at
various frequencies from 1 to 1500 Hz and with various DC fields
using an MPMS XL7 SQUID magnetometer
Scheme 1. Pathway Showing the Preparation of the H2L
Ligand and the Reported Complexes 1−3
X-ray Crystallography. X-ray diffraction experiments on the
selected crystal of 1 were performed on a Bruker SMART APEX II
automated diffractometer equipped with a CCD detector and a
monochromatic radiation source (Mo Kα radiation, λ = 0.710 73 Å).
The molecular structure was solved by direct methods using SHELX-
2014,11 and all the non-hydrogen atoms were refined anisotropically
by SHELXS-2014.11 All the hydrogen atoms were found in differential
Fourier maps, and their parameters were refined using a riding model
with Uiso(H) = 1.2 (CH, CH2, NH2, NH) or 1.5 (CH3) Ueq.
Nonroutine aspects of structure refinement: The electron density from
heavily disordered methanol molecules was removed by SQUEEZE
procedure.12
Synthesis. Preparation of H2L. Sodium metal (0.8 g) was added
carefully to 35 cm3 of methanol followed by addition of 2-pyridine
carbonitrile (10.9 g, 104 mmol). The solution rested for 30 min. Then,
oxalic dihydrazide (4.8 g, 0.040 mol) was added, and refluxing of
mixture for 10 h resulted in precipitation of a yellow powder. The
mixture was cooled to room temperature, and the product was filtered
off, washed with water, methanol, and diethyl ether, and dried in
vacuum. (found: C, 51.71; H, 4.17; N, 34.04%. C14H14N8O2 requires
C, 51.53; H, 4.31; N, 34.33%).
General Procedure for the Preparation of Complexes. Ln(NO3)3·
nH2O (Ln−DyIII, GdIII and TbIII (1.0 mmol) in methanol (10 cm3)
was added to a hot suspension of H2L (0.46 g, 1.5 mmol) in methanol
(10 cm3). The mixture was stirred while heated. The obtained yellow
solution was filtered and allowed to stand at room temperature. Yellow
crystals, suitable for structural analysis formed during the next day
(yield 37−53%).
[Dy4(HL)4(H2L)2(NO3)4](NO3)4·4MeOH. Found C 32.71; H 3.12; N
24.19%. C88H96N56O40Dy4 requires: C 32.74; H 2.99; N 24.29.
[Tb4(HL)4(H2L)2(NO3)4](NO3)4·4MeOH. Found C 32.06; H 2.73; N
24.92%. C88H96N56O40Tb4 requires: C 32.89; H 3.01; N 24.41
[Gd4(HL)4(H2L)2(NO3)4](NO3)4·4MeOH. Found C 32.26; H 3.08; N
24.63%. C88H96N56O40Gd4 requires: C 32.96; H 3.02; N 24.46
Theoretical Calculations. The post-Hartree−Fock calculations
for the mononuclear [Dy(HL)2(H2L)2(NO3)] fragment of 3 (Figure
S6) based on the experimental X-ray structure were performed with
the MOLCAS 8.0 program package.13 The active space of the
CASSCF calculations14 comprised of nine electrons in seven metal-
based f-orbitals (CAS(9,7). All 21 sextet, 224 quartet, and 490 doublet
states were considered in the CASSCF calculations by employing the
RASSCF method. The spin−orbit coupling based on atomic mean
field approximation (AMFI)15 was taken into account using RASSI-SO
[Co4(HL)4](NO3)6·4CH3OH·CH3CN·9H2O, [Ni4(H2L)6]-
(BF4)6F2·14H2O, or octanuclear {[Co4(H2L)4]2}[Co-
(NCS)4]2(NCS)4·8CH3OH·11H2O,10 but it has not been
used in lanthanide chemistry before. Herein, we report on the
crystal structure and detailed experimental and theoretical
investigations of magnetic properties of the selected lanthanide-
( I I I ) c o m p l e x e s w i t h t h e g e n e r a l f o r m u l a
[Ln4(HL)4(H2L)2(NO3)4](NO3)4·4CH3OH (Ln = GdIII, 1,
TbIII, 2, DyIII, 3).
EXPERIMENTAL SECTION
■
General Methods. Elemental analyses of C, H, and N were
performed with a Perkin-Elmer 240 C analyzer. The X-ray powder
diffraction patterns of all solid samples were recorded on a
MiniFlex600 (Rigaku) instrument equipped with the Bragg−Brentano
geometry, and with iron-filtered Cu Kα1,2 radiation. Direct-current
(DC) magnetic measurements were performed with MPMS XL7
SQUID magnetometer for 1 (T = 1.9−300 K at B = 0.1 T; B = 0−5 T
at T = 2 and 5 K) and with a PPMS Dynacool VSM magnetometer for
2−3 (T = 1.9−300 K at B = 0.1 T; B = 0−9 T at T = 2, 5, and 10 K).
Figure 1. Molecular structure of the [Ln4(HL)4(H2L)2(NO3)4]4+ complex cation of 3 with the omitted hydrogen atoms (left) (CCDC 1483169).
The detailed view on the core of 3 with ab initio computed orientation of the principal magnetization axes of the ground-state Kramers doublets
(middle). Static magnetic data for 1−3 (right). The lines represent the calculated data for 1 using eq 1, with the values of J = −0.013 cm−1, D =
−0.21 cm−1, g = 1.997.
B
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with 21 sextet, 128 quartet, and 130 doublet states. The relativistic
effects were treated with a Douglas−Kroll Hamiltonian.16 The
following basis sets were employed: Dy.ANO-RCC-VQZP for Dy
atom, O.ANO-RCC-VTZP for oxygen atoms directly bounded to Dy
atom, O.ANO-RCC-MB for other oxygen atoms, N.ANO-RCC-VTZP
for nitrogen atoms directly bounded to Dy atom, N.ANO-RCC-MB
for other nitrogen atoms, C.ANO-RCC-MB for carbon atoms, and
H.ANO-RCC-MB for hydrogen atoms.17 The resulting wave functions
and the energies of the molecular multiplets were used for the
calculation of the magnetic properties, g tensors of the lowest Kramers
doublet states, and blocking barrier using module SINGLE_ANISO
(Figures 1, 3, and S6 and Table 1).18 Afterward, the module
The reaction conditions needed to produce specifically the
LnIII clusters 1−3 were optimized using a trial-and-error
method, by changing the molar ratio of the reactants and
solvents. It has been found that the variation of components
ratio does not affect the composition of the resulting products
indicating thus on self-assembly character of the reaction. In all
the cases, the prepared complexes possess 2:3 Ln/H2L molar
ratio. The best yields for compounds 1−3 were obtained based
on the reaction of Ln(NO3)3·nH2O with H2L in methanol, and
the compounds were obtained as yellow crystals. The X-ray
powder studies (Figure S1) of 1−3 demonstrate that all the
complexes belong to an isostructural/isomorphous series.
Crystal Structure. The crystal structure of 3, as the
representative example of whole isostructural series, was
determined by a single-crystal X-ray analysis (Table S1). The
structure of the heteroleptic self-assembled Dy4 [2 × 2] grid is
shown in Figure 1. Each dysprosium atom is nine-coordinated
(with the coordination geometry close to spherical capped
square antiprism27) by one H2L (η3, tridentate chelate pocket
involving 2-pyridyl and amidrazone nitrogen atoms, with the
N2O donor set) ligand, by one HL− (η2, bidentate chelate
pocket, O2) ligand, and by the nitrato ligand (η2). One H2L
ligand is coordinated to the central atom by an oxygen atom in
a monodentate manner and provides bridging (μ-O) between
neighboring Dy atoms (Figure 1). Furthermore, this arrange-
ment provides intramolecular stabilization of the molecule by
two N−H···O hydrogen bonds (d(N···O) = 3.075(2) Å)
between the amine groups located on the H2L ligand and
oxygen atoms from the coordinated nitrato ligands. The Dy−O
bond lengths fall within the 2.329(7)−2.436(7) Å (for oxygen
atoms from the H2L and HL− ligands) and 2.498(10)−
2.523(8) Å ranges (for the coordinated nitrato ligand). Two
Dy−N bonds adopt lengths as follows: 2.427(10) and
2.517(11) Å (Table S2). The arrangement of four Dy atoms
is almost planar (average deviation of these atoms from the
best-fit mean plane through the Dy4 atoms is 0.008(2) Å. The
intramolecular Dy···Dy distances, going through covalent
bonding, are rather short (3.978 (3) Å).
Table 1. Energy Levels of Lowest Ligand Field Multiplets in
Zero Magnetic Field Derived from CASSCF/DKH2/
SINGLE_ANISO Calculations for the Molecular Fragment
[Dy(HL)2(H2L)2(NO3)] of Complex 3 with the Respective
g-Factors Derived for Each Kramers Doublet with Effective
Spin 1/2
a
energy (K)
gx
gy
gz
α
(deg)
0.0
307
386
482
589
653
730
801
0.005
0.26
0.45
4.97
0.38
1.36
0.37
0.29
0.007
0.45
1.28
6.06
3.64
3.72
0.64
1.01
19.86
17.82
13.77
9.38
32.3
30.5
28.2
66.6
78.7
73.9
94.4
10.62
14.29
16.46
18.10
a
α is an angle of gz of the excited state with respect to gz of ground
Kramers doublet.
POLY_ANISO was utilized to calculate magnetic properties and
energy levels for whole Dy4 complex [Dy4(HL)4(H2L)2(NO3)4]4+ of 3
employing S4 symmetry (Figures 4, 5, and S8).19,7g
The density functional theory (DFT) calculations were performed
with the program ORCA 3.0.20 The hybrid B3LYP functional21 was
used for the calculations of the isotropic exchange constants J
following Ruiz’s approach22 by comparing the energies of high-spin
(HS) and broken-symmetry (BS) spin states. The relativistic effects
were also included in the calculation with zero-order regular
approximation (ZORA)23,24 together with the scalar relativistic
contracted version of the basis functions: def2-TZVP for gadolinium
atoms, def2-TZVP(-f) for nitrogen and oxygen atoms, and def2-SVP
for carbon and hydrogen atoms.25
The calculations utilized the RI approximation with the
decontracted auxiliary def2-TZVP/J, def2-TZV/J, and def2-SVP/J
Coulomb fitting basis sets and the chain-of-spheres (RIJCOSX)
approximation to exact exchange as implemented in ORCA.26
Increased integration grids (Grid5 in ORCA convention) and tight
self-consistent field convergence criteria were used in all the
calculations. The isotropic exchange parameter J for the spin
Magnetic Properties. The static magnetic properties were
measured for all the compounds as temperature dependence
(1.9−300 K) of susceptibility and field dependence (0−7/9 T)
of magnetization (Figure 1, right). From the obtained results it
can be apparent that the room-temperature values of effective
magnetic moment (μeff/μB) are in a good agreement with the
expected values (the calculated values are in the parentheses):
1, 15.9 (15.9); 2, 19.4 (19.4); 20.8 (21.3). At low temperatures
(below 100 K μeff/μB) the decrease in μeff/μB value is observed,
and it is much more visible for 2 and 3 than for 1 due to
depopulation of Stark levels arising from the ground state (7F6
6
in 2 and H15/2 in 3). In 1, the small decrease in μeff/μB below
H a m i l t o n i a n u s e d i n e q
1
w a s c a l c u l a t e d f o r
30 K can be ascribed to a weak antiferromagnetic interaction
between the GdIII atoms and small single-ion anisotropy. Both
temperature- and field-dependent magnetic data were then
simultaneously analyzed utilizing the following spin Hamil-
tonian
[Gd4(HL)4(H2L)2(NO3)4]4+ molecular fragment, which was extracted
from experimental X-ray data of 3 by replacing Dy atoms with Gd
atoms. Then, the relationship J = (EBS13 − EHS)/224 was used to
quantify J as −0.0191 cm−1.
RESULTS AND DISCUSSION
■
3
4
2
2
Syntheses and General Characterization. The poly-
dentate ligand H2L shown in Scheme 1 reveales the potential to
bind to metals in a variety of different ways, with involvement
of 2-pyridyl moieties and/or the oxalylhydrazone ones. Except
for that, the ligand nominally has two ionizable protons, and
their role is to provide not only a scaffold for metal ion
coordination but also an overall charge balance.
̂
̂
̂
H = −J(∑ Si·Si+1 + S1·S4) + D ∑ Si,z − Si /3
i=1
i=1
̂
+ μBBgiSi,a
(1)
which resulted in these parameters: J = −0.013 cm−1, D =
−0.21 cm−1, g = 1.997 (Figure 1). The fitted J-value is also in
C
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good agreement with the calculated one J = −0.019 cm−1, as
supported by DFT calculations (see Experimental Section for
details).
The dynamic magnetic data were acquired for compounds 2
and 3. In the case of TbIII complex 2, the nonzero out-of-phase
susceptibility was detected at nonzero static magnetic field, but
the maxima were not observed until 1.9 K (Figures S2 and S3).
Therefore, AC susceptibility data were analyzed with the
simplified model28 based on eq 2
ters (α)Figure 2 and Table S3. This enabled us to construct
the Argand (Cole−Cole) plot and subsequently to estimate
reversal barrier U = 42.6 K with τ0 = 1.50 × 10−5 s (Figure 2)
using Arrhenius law. Alternatively, 1/τ = CTn + 1/τ0·exp(−U/
kT) was used incorporating also Raman relaxation process,
which resulted in U = 67.6 K, τ0 = 3.53 × 10−6 s, C = 64.9, and
n = 1.0. Such obtained U is comparable to other tetranuclear
gridlike DyIII SMMs (see Table S4). Nonetheless, it must be
stressed here that U reflects thermally activated Orbach
relaxation mechanism, and thus it limits the analysis of AC
susceptibility data only to the data having maxima in the
Argand diagram. That means that data measured between T =
1.9 and 13.93 K could be the source of underestimation of U,
because nonzero out-of-phase AC susceptibility signal is already
observed below 28 K. Thus, AC susceptibility data were also
analyzed with simplified model using eq 2, which resulted in
much higher magnetic moment reversal barrier, U = 217−270
K, and τ0 = 0.63−8.86 × 10−10 s (Figure S5).
ln(χ″/χ′) = ln(2πfτ0) + U/kT
(2)
where higher-temperature AC data for higher applied
frequencies (f) were analyzed as visualized in Figure S4,
which resulted in U = 9.2−13.2 K. However, DyIII complex 3
behaves as a single-molecule magnet (SMM) already at zero
static field, and the temperature-dependent experimental data
Theoretical Calculations. Ab initio CASSCF calculations
with MOLCAS 8.0 for the mononuclear [Dy-
(HL)2(H2L)2(NO3)] fragment of 3 and subsequent analysis
6
with Single_Aniso module revealed that H15/2 atomic term is
split into eight Kramers doublets (KD) (Figure 3 and Table 1).
Figure 3. Ab initio computed magnetization blocking barrier for
mononuclear moiety of 3. The thick blue/red bars indicate the
Kramer’s doublets as a function of magnetic moment. Green/orange
lines indicate the magnetization reversal mechanism. The cyan/
magenta lines show the possible pathway of the Orbach process. The
black lines represent the presence of QTM/TA-QTM between the
connecting pairs. The numbers provided on each arrow are the mean
absolute values for the corresponding matrix elements of the transition
magnetic moment, and for values larger than 0.1 an efficient relaxation
mechanism is expected.29
Figure 2. AC susceptibility data for DyIII complex 3 at the zero applied
external field BDC = 0.0 T. (top) In-phase χ′ and out-of-phase χ″ molar
susceptibilities. Lines serve as guides for the eyes. (middle) Frequency
dependence of χ′ and χ″ molar susceptibilities. Full points
experimental data, full linesfitted data using eq 3. (bottom) Argand
(Cole−Cole) plot and fit of resulting relaxation times to Arrhenius law
(red line) or to combination of Orbach and Raman relaxation
processes (blue line).
Each Kramers doublet was then analyzed with effective spin Seff
= 1/2, which resulted in effective g-tensor values (gx, gy, gz). It is
evident that ground state possesses a very large axial magnetic
anisotropy, gx = gy ≈ 0 and gz = 19.86 (Table 1). The first and
second excited states have also quite large axial magnetic
anisotropy (Table 1), but gx and gy components are already
nonzero and also we must note that easy axis of g-tensor of the
ground state is not perfectly collinear with the easy axes of g-
tensors of first and second excited states as visualized in Figure
S6. This is also reflected in Figure 3, where the values of the
transition magnetic moments between Kramers levels are
(Figure 2) were possible to analyze with the one-component
Debye model (ω = 2πf)
χ(ω) = χS + (χ − χS)/[1 + (iωτ)1−α
]
(3)
T
providing values of isothermal (χT) and adiabatic (χS)
susceptibilities, relaxation times (τ), and distribution parame-
D
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shown and this plot suggests that there is very low probability
of the quantum tunneling of the ground state and that the
relaxation through first excited states is most probable. This is
also in agreement with the experiment, where analysis of AC
susceptibility data resulted in the estimation of Ueff = 217−270
K (eq 2), which matches with the first excited state energy
separation value ΔE = 307 K. All these facts support the
observed zero-field SMM behavior of 3 and the relaxation
pathway through the first excited state.
moment in 3 (Figure 1), which means that this compound
belongs to the class of the single-molecule toroics.6 As a result,
there is a nonmagnetic ground state, where the crossing from
nonmagnetic to magnetic state is indicated at ∼0.5 T from
CASSCF calculations as plotted in Figure 5. This is also
Subsequently, the experimental DC magnetic data were
analyzed with the Poly_Aniso program utilizing S4 symmetry of
the molecule. Because of mutual orientation of the easy axes of
all Dy atoms in Dy4 unit as shown in Figure 1, there are
important antiferromagnetic interactions mostly of dipolar
origin between the magnetic dipoles of the respective Dy
atoms. However, also additional antiferromagnetic coupling Jex
= −0.03 cm−1 was applied to obtain even better agreement with
the experimental dataFigure 4. The impact of dipolar and
exchange interactions is demonstrated in Figure 4. Evidently,
the dipolar interactions are the main reason for the decrease of
the effective magnetic moment at low temperature (Figure 4).
Next outcome of these antiferromagnetic interactions within
the tetramer unit is that there is a net toroidal magnetic
Figure 5. Course of the exchange states with magnetic field
perpendicular to the plane of four Dy atoms of 3. The blue and red
lines correspond to dipolar and to dipolar + exchange coupling,
respectively, accompanied by energy crossing from nonmagnetic to
magnetic ground state indicated by blue and red arrows.
manifested in the experimental maxima of χmol versus T found
at T = 2.57 K (Figure 4) and of dMmol/dB versus B found at B
= 0.43 T (Figure S7). These characteristics were also recovered
from theoretical calculations using Poly_Aniso module (Figure
S8).
CONCLUSION
■
In conclusion, three novel tetranuclear 2 × 2 grid lanthanide
complexes [Ln4(HL)4(H2L)2(NO3)4](NO3)4·4CH3OH (Ln =
GdIII, 1, TbIII, 2, DyIII, 3) were prepared, and their magnetism
was thoroughly studied. In the case of the GdIII compound 1,
the spin Hamiltonian formalism is valid, and weak anti-
ferromagnetic exchange and relatively small magnetic aniso-
tropy were found (J = −0.013 cm−1 and D = −0.21 cm−1). The
TbIII compound 2 was found to behave as a field-induced SMM
with the estimated U = 9.2−13.2 K. From the magnetic point of
view, the most exciting is the DyIII compound 3, which behaves
as zero-field SMM and net toroidal SMT. The effective barrier
was estimated as U = 42.6 K up to U = 217−270 K depending
on the applied procedure. The CASSCF calculations, followed
by utilization of Single_Aniso and Poly_Aniso modules, were
crucial for the proper understanding of the magnetic properties
of this compound, both static and dynamic. Therefore, the
compound 3 enlarges a small class of single-molecule magnets
with net toroidal magnetic moment.
Figure 4. Magnetic data for 3. (top) Temperature dependence of the
effective magnetic moment and molar susceptibility measured at B =
0.1 T. (bottom) Isothermal magnetizations measured at T = 2, 5, and
10 K. Empty circles: experimental data. Full lines: calculated data using
module POLY_ANISO scaled with the factor equal to 0.953. Gray
linecalculation without any interactions, blue linecalculation with
all the possible dipolar interactions, red linecalculation with all the
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02449.
possible dipolar interactions and additional exchange interactions J12
J23 = J34 = J14 = −0.03 cm−1 applied.
=
E
DOI: 10.1021/acs.inorgchem.6b02449
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Experimental details, figures and tables referring to
Article
(6) Guo, P.-H.; Liu, J.; Wu, Z.-H.; Yan, H.; Chen, Y.-C.; Jia, J.-H.;
Tong, M.-L. Single-Molecule-Magnet Behavior in a [2 × 2] Grid DyIII4
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: galex0330@gmail.com. (A.G.)
*E-mail: zdenek.travnicek@upol.cz. (Z.T.)
ORCID
Radovan Herchel: 0000-0001-8262-4666
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
We acknowledge the financial support from the following
projects: LO1305 (C.Z.) and No. 3874 (R.U.). We also thank
Dr. L. Ungur for fruitful discussions.
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