Strictly linear trinuclear Dy-Ca/Mg-Dy single-molecule magnets: The impact of long-range f-f ferromagnetic interactions on suppressing quantum tunnelling of magnetization leading to slow magnetic relaxation

Article abstract of DOI:10.1039/c7dt01395g

The synthesis, structural characterization and magnetic properties of four heterometallic complexes with formulas Ln₂M(OQ)₈ [Ln(iii) = Dy, M(ii) = Ca (1), Mg (2); Ln(iii) = Er, M(ii) = Ca (3), Mg (4); HOQ = 8-hydroxyquinoline] are reported. Complexes display a perfectly linear arrangement of three metal ions involving two terminal Ln(iii) ions and one central alkaline earth M(ii) ion, and they are bridged by three phenolato oxygen atoms from three 8-hydroxyquinoline ligands. Direct-current (dc) magnetic susceptibility studies show that there exists a long-range ferromagnetic (FM) interaction between two f-electronic centers in Dy-based complexes 1 and 2 albeit they are separated by diamagnetic alkaline earth ions, and the FM interaction has been successfully reproduced by ab initio calculations. Alternating current (ac) magnetic susceptibility measurements indicate that complexes 1 and 2 exhibit significant single-molecule magnet (SMM) behaviors. This is mostly attributed to the parallel magnetic axes of individual Dy(iii) sites aligning closely to the Dy-M-Dy direction, and the linearly extending magnetic susceptibility tensors of each ion lead to the FM interaction. The dc and ac susceptibility studies of the intramolecularly diluted complexes reveal that the f-f coupling suppresses the QTMs significantly in the absence of the external magnetic field.

Full text of DOI:10.1039/c7dt01395g

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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Strictly linear trinuclear Dy-Ca/Mg-Dy single-molecule magnets:
the impact of long-range f-f ferromagnetic interactions on
suppressing quantum tunnelling of magnetization leading to slow
magnetic relaxation
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Wan-Ying Zhang,a Yong-Mei Tian,a Hong-Feng Li,a Peng Chen,a Yi-Quan Zhang,b,* Peng-Fei
Yana,*and Wen-Bin Sun,a,*
www.rsc.org/
The synthesis, structural characterization and magnetic properties of four heterometallic complexes with formulas
Ln2M(OQ)8 [Ln(III) = Dy, M(II) = Ca (1), Mg (2); Ln(III) = Er, M(II) = Ca (3), Mg (4); HOQ = 8-hydroxyquinoline] are reported.
Complexes display a perfectly linear arrangement of three metal ions involving two terminal Ln(III) ions and one central
alkaline earth M(II) ion, they are bridged by three phenolato oxygen atoms from three 8-hydroxyquinoline ligands. Direct-
current (dc) magnetic susceptibility studies show that there exist long-range ferromagnetic (FM) interaction between two
f-electronic centers in Dy-based complexes 1 and 2 albeit they are separated by diamagnetic alkaline earth ions, and the
FM ineraction has been successfully reproduced by ab initio calculations. Alternating current (ac) magnetic susceptibility
measurements indicate that complexes 1 and 2 exhibit significant single-molecule magnet (SMM) behaviors. This is mostly
attributed to the parallel magnetic axes of individual Dy(III) sites aligning closely to the Dy-M-Dy direction, and the linearly
extending magnetic susceptibility tensors of each ions leads to the FM interaction. The dc and ac susceptibility studies of
the intramolecularly diluted complexes reveal that the f-f coupling suppress the QTMs significantly in the absence of the
external magnetic field.
significant magnetic moment [i.e., Dy(III), Tb(III), Ho(III), Er(III)].
In spite of the attractiveness of lanthanide ion complexes in
single-molecule magnets are increasing,3 it was also to be
noted that fast quantum tunnelling of magnetization (QTM)
Introduction
Since the first single-molecule magnet (SMM), Mn12Ac, was
discovered in the 1990s,1 many magnetic molecules exhibiting
a slow relaxation of magnetization have been synthesized and
magnetically characterized because of their possible
applications for high-density information storage, molecular
spintronic and quantum computing devices at a molecular
level.2 High-spin ground state (ST) and uniaxial (Ising) negative
magnetic anisotropy (D) are two essential factors to obtain an
SMM with a high relaxation barrier (Ueff) and blocking
temperature (TB). The lanthanide ions are regarded as the
ideal candidates for constructing SMMs due to the fact that
some of them have the strong intrinsic spin-orbit coupling,
large unquenched orbital angular momenta and consequently
relaxation processes generally induced by dipolar interactions,
transverse anisotropy or hyperfine interactions prevails in
nearly all SMM systems, which has been a serious factor to
impede developing SMMs with enhanced performance.4
Obviously, considering both the local high axial symmetry and
whole spin is an effective strategy to improve the magnetic
anisotropy.5 Moreover, the appropriate intramolecular
couplings have been proved to suppress the QTM
significantly.6 In view of this, the dinuclear SMM systems have
the preponderant properties and are promising to enhance the
SMM performance.7 However it is challenging to control the
anisotropies of individual ions to ensure high local axial
symmetry, let alone preserving a parallel spin configuration
and producing a FM interaction between them. Generally, the
parallel orientation of individual magnetic easy axis closes
more to the direction of two spin center and more pronounced
magnetic anisotropy might be produced. Besides, a few
techniques are also effective to overcome the QTM relaxation
processes, one of which is to dope the superparamagnetic
carriers into the diamagnetic matrix to eliminate dipolar
interactions8 and consequently prevent the occurrence of
QTM. In addition, the intramolecular coupling of heteronuclear
large intrinsic magnetic anisotropy besides carrying
a
a.
Key Laboratory of Functional Inorganic Material Chemistry Ministry of Education,
Heilongjiang University, Harbin 150080, P. R. China. E-mail: wenbinsun@126.com
and yanpf@vip.sina.com
b.Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology,
Nanjing Normal University, Nanjing 210023, P. R. China. E-mail:
zhangyiquan@njnu.edu.cn
†
magnetic data and the methods/results for ab initio calculation for . CCDC
Electronic Supplementary Information (ESI) available: Additional structural,
1-4
1542700-1542703. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/x0xx00000x
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complexes involving transition metal (TM) and lanthanide (4f)
Single-crystal X-ray analyses reveal that the linear trinuclear
), [Dy2Mg(OQ)8] ( ), [Er2Ca(OQ)8] (
) are isostructural, except for the present
usually seen due to the intrinsic shielded electronic structure of two CHCl3 molecules as lattice solvent in , and one
of lanthanide elements. In the most of heteronuclear SMMs diethyl ether molecule in complex , and there are two
involving 3d/4f dinuclear [TM(II) = Mn, Co, Ni, and Cu] and/or molecules in one unit for complex (Fig. 1 and Fig. S1-S3†,
trinuclear [TM(II) = Co and Ni] complexes, the weak JTM-Ln ESI). Complexes and crystallize in the triclinic space
values are usually observed. These lead to the declined SMM group Pī, but complex crystallize in the monoclinic space
ions have a crucial impact on the dynamic magnetic relaxation complexes [Dy2Ca(OQ)8] (
of the heteronuclear TM/4f SMM.9 The weak coupling was and [Er2Mg(OQ)8] (
1
2
3)
4
2-4
1
3
1,
2
4
3
performance in association with the narrow separations of the group P21/c (Table S1†, ESI). All centrosymmetric structures
low-lying split sublevels, and which consequently result in a possess two peripheral rare earth atoms and are symmetry-
smaller energy barrier for magnetization flipping.10 In this related around the central alkaline earth atom, which is
regard, an effective strategy to optimize the SMM properties positioned on the inversion centre (Fig. 1). Each rare earth
of the 3d/4f aggregates has been proposed through getting rid atom was chelated by four bidentated N and O atoms of one
of the weak TM(II)-Ln(III) interactions that likely split the terminal HOQ and three bridged HOQ. The lanthanide ions are
ground sublevels of the Ln(III) ion and replacing the bridged to the central alkaline earth metal through the
paramagnetic TM ion by
a
diamagnetic one.8b,10e,11,12 phenoxyl oxygen atoms of HOQ, and the alkaline earth metal is
Meanwhile, the diamagnetic metallic ion also favor diluting the surrounded by the oxygen atoms. The local symmetries of the
superparamagnetic host, in which the TM ions were used eight-coordinated lanthanide atoms and six-coordinated
mostly. Recently, we develop a series of interesting f-s-f type alkaline earth metal were analysed by the continuous-shape-
heterometallic complexes, in which the alkaline earth (M) was measures (CShMs) method using the SHAPE software,14 which
sandwiched between two lanthanide ions by 8- allowed us to quantify the degree of distortion of the
hydroxyquinoline (HOQ) bridge, and consequently leads to a coordination sphere of real complexes (S value equals 0,
series of strictly linear trinuclear Ln-M-Ln SMMs. It’s worth corresponds to the perfect polyhedron). The relative large S
noting that, there are very few reports of 3d-4f complexes values (Table S2†, ESI) indicate the coordination environment
constructed by only 8-hydroxyquinoline.13 Their structures, of the Ln(III) ions in
however, display lanthanide-centred linear TM-Ln-TM indicated that the Ln(III) ions possess biaugmented trigonal
arrangement, in which magnetic properties are not studied. prism (C2v) with value of 1.929 for triangular
Herein, we present of the synthesis, structure and magnetic dodecahedron (D2d) with S value of 1.468 for , and the Ln(III)
properties of four Ln-M-Ln complexes Ln2M(OQ)8 [Ln(III) = Dy, ions lie among the ideal biaugmented trigonal prism (C2v) and
M(II) = Ca ( ), Mg ( ); Ln(III) = Er, M(II) = Ca ( ), Mg ( ); HOQ = triangular dodecahedron (D2d) with S value 2.437 and 2.276 for
8-hydroxyquinoline]. Magnetic investigations reveal that there and respectively. The central alkaline earth atoms in all
is a weak ferromagnetic coupling between Dy(III) ions in complexes in a distorted octahedral arrangement with S
complexes and albeit they are separated by the alkaline value 4.265, 2.011, 4.273 and 2.008 respectively (Fig. S4-S7†,
1-4 are in a low symmetry. SHAPE analysis
a
S
1,
3
1
2
3
4
2
4
1-4
1
2
earth ions, and significant single-molecule magnet (SMM) ESI).
behaviors were observed and more importantly the QTM
relaxation processes were suppressed efficiently in these
systems.
Results and Discussion
Synthesis and Structures. A series of amorphous precursor
material of Ln(OQ)3 and M(OQ)2 derivatives were prepared by
aqueous reaction of Na(OQ) with LnCl3 (Ln = Dy and Er) and
MCl2 (M = Ca and Mg) and they were thoroughly dried before
use in syntheses. All heterometallic complexes were prepared
by rearrangement reactions between appropriate Ln(OQ)3 and
M(OQ)2 derivatives in trichloromethane (Scheme 1).
Fig. 1 The crystal structure of complex 1 (hydrogen atoms are omitted for clarity).
Bond length data (Table S3†, ESI) show a somewhat variable
response to the decrease in ionic radii (0.38 Å) from Ca(II) to
Mg(II). Thus, for the average Ln-Obr (br = bridging) bond length,
the decrease (2.340 to 2.332 Å for
for to ) is as expected, whereas the elongation of the
shorter Ln-Oter (ter = terminal) bond length is larger (2.251 to
2.267 Å for to , and 2.237 to 2.258 Å for to ). Three Ln-N
1 to 2, and 2.323 to 2.315 Å
3
4
1
2
3
4
bonds are associated with bridging ligands and one with a
terminal ligand, the latter being marginally shorter than the
average of the former. In these molecules, the M-Ln
Scheme 1 Synthesis of complexes 1-4.
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complexes, the Er-based complexes,
decrement of the χMT values upon decreasing the temperature
the nonbonding Ln···M···Ln distance, approximately 0.47 Å from 50 to 2 K. The ferromagnetic interaction between
3 and 4, show monotonic
separations are 3.4239(1) Å, 3.1902(3) Å, 3.3871(2) Å and
3.1668(2) Å for , respectively. The decreased tendency of
1-4
from
contraction’s. The three metal centers are arranged in a linear
fashion with an intermetallic Ln-M-Ln angle of 180.0 in and
1 to 2, and 0.44 Å from 3 to 4, follows the alkaline earth
lanthanide(III) ions is not usually observed because the nearly
insulated orbital nature of lanthanide ions generally
communicated hardly with each other, let alone they are
separated by the magnetically silent alkaline earth in the
present work. Furthermore, strong magnetic anisotropy as
well as thermal population of low-lying excited states generally
tend to mask these weak interactions. The M versus H plots for
the Dy and Er-based complexes show a relatively rapid
increase in the magnetization at low field to reach almost
1-4
finally resulting in a strictly linear f-s-f moiety. In addition, the
shortest Ln···Ln distances between neighboring [LnLn] units
are 9.5237(7) Å, 9.3629(6) Å, 8.7803(5) Å and 9.2600(3) Å in
respectively. In addition, the Dy-M-Dy units of complexes
and are “stacked” along the crystallographic a-axis in a way
1-
4
1
2
depicted in Fig. S8-S9† (ESI), respectively. The π-bond
interactions are found in both complexes and the length of the
saturation for magnetic fields of 7 T (Fig. S10-S13†, ESI). The
π-bonds are 3.865 Å and 3.664 Å for 1 and 2, respectively.
observed saturation values for the Dy and Er-based complexes
are rather lower than the calculated ones, which is due to
crystal-field effects leading to significant magnetic anisotropy
and/or low-lying excited states.
To explore the intramolecular magnetic interactions of
these dinuclear complexes, the magnetic measurement for the
Gd(III) analogues (complex 1Gd and 2Gd) were carried out (Fig.
2). The χMT product for the Gd(III) complexes remain almost
constant from room temperature to 2K, as excepted for such
an isotropic ion. The room-temperature χMT values of
complexes 1Gd and 2Gd are 16.54 and 16.56 cm3·K·mol-1
respectively, which are consistent with the expected value of
15.76 for two uncoupled Gd ions (8S7/2, S = 7/2, L = 0). The
magnetic properties were analyzed by spin Hamiltonian H = -
JSGd1·SGd2, where J is the exchange coupling constant and SGd1
and SGd2 are the spin operator. Least-squares fitting of the
experimental data with Equation (S1) leads to J = 0.086 cm-1
Fig. 2 Calculated (black solid line for 1, red solid line for 2, magenta solid line for 1Gd
and dark yellow solid line for 2Gd) and experimental data of magnetic susceptibility of
complexes 1-4, 1Gd and 2Gd. The intermolecular interactions zJ’ of 1 and 2 were all set
and
(χMT)obs]2/Σ[(χMT)obs]2) for 1Gd , J = 0.103 cm-1 and g = 2.0 (R =
Magnetic Properties. The direct current (dc) magnetic 2.71×10-3) for 2Gd
indicating the weak ferromagnetic
properties of complexes were investigated under a 1000 interaction between the Gd(III) centers. While the negative
Oe field in the temperature range 2-300 K (Fig. 2). Field deviation of the χMT at low temperature most likely due to the
dependence of the magnetization for complexes between non-negligible zero-field splitting (ZFS) caused by the low
2 and 8 K are shown in Fig. S10-S13†. At room temperature, symmetry of the Gd(III) ion.15
the χMT values of complexes and are 28.2 and 27.3
cm3·K·mol-1, and are 22.7 and 22.5 cm3·K·mol-1,
and
g = 2.0 (R = -
1.95×10-3, defined as Σ[(χMT)calc
to 0.00 cm−1
.
,
1-4
1-4
1
2
3
4
respectively, which are in good agreement with the theoretical
values (28.34 cm3·K·mol-1 and 22.94 cm3·K·mol-1) for two non-
interacting Dy(III) ions (6H15/2, S = 5/2, L = 5, g = 4/3, C = 14.17
cm3·K·mol-1) and Er(III) ions (4I15/2, g = 6/5, J = 15/2, C = 11.47
cm3·K·mol-1). Upon decreasing the temperature, the χMT
products of
decrease slightly to reach 26.8 and 26.5 cm3·K·mol-1 at about
12 K, respectively. The latter gradual drop in χMT for and , is
1 and 2 nearly remain constant until 50 K and then
Fig. 3 Temperature dependence of the out-of-phase (χ’’) ac susceptibility of complex 1
(left) and complex 2 (right) in the frequency range 1-1000 Hz under 0 Oe.
1
2
most likely due to depopulation of the Stark sublevels and/or
significant magnetic anisotropy present in Dy(III) systems. It is
worth noting that below 12 K a sharp increase can be observed
and the χMT reach a maximum value of 28.1 and 28.4
Alternating current (ac) susceptibility measurements on
and were performed in the temperature range 2-20 K under
zero dc field at frequencies between 1 and 1000 Hz. They
displayed significant ac peaks under zero field at
temperature range between 3 K and 14 K, which is indicative
of the prevalent SMM behavior in these two systems (Fig. 3
and Fig. S14†, ESI).
1
2
a
cm3·K·mol-1 at 2.5 K for
1 and 2 respectively. Such an increase
can be attributed to weak intramolecular ferromagnetic
interactions between the Dy(III) ions. Contrary to the Dy-based
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Fig.
4 Frequency dependence of the out-of-phase (χ’’) ac susceptibility in the
temperature range 2-15 K of complex 1 (left) and complex 2 (right) under 0 Oe. Inset:
Plot of ln(τ) versus 1/T under 0 Oe for 1 and 2, respectively, with the red line that
represent pure Arrhenius fitting at the high-temperature linear region.
For
1 and 2, the out-of-phase (χ′′) component of the ac
susceptibility (Fig. 3) clearly exhibits a temperature dependent
full peak with one maximum of 11 K and 14 K respectively.
However, the increasing χ′′ below 3 K in 2 indicates the
Fig. 5 Plot of ln(τ) versus 1/T at 1500 Oe field for 1 and 2. The blue lines represent the
fitting of the frequency-dependent data by Equation 1 for 1 and 2. The red lines
represent pure Arrhenius fitting at the high-temperature linear region for 1 and 2.
intervention of QTM, which generally indicates the presence of
QTM in lanthanide SMMs systems. Owing to the Kramers
nature of Dy(III) ion, at zero field, dipole-dipole and hyperfine
interactions should be responsible for the mixing of the two
Kramers ground states that allows the zero-field quantum
tunnelling dynamics of the magnetization. Additionally, a
frequency dependence of the maximum associated only with a
single relaxation process appears clearly on the plot of the
variation of χ′′ versus the frequency of the oscillating field
It is possible to shortcut the QTM by applying a static dc
field. The temperature and frequency dependent ac
susceptibility were measured under an applied 1500 Oe field
(Fig. S17-S20†, ESI). As expected, after applying an external
field the disappearance of the upward χ’’ under low
temperatures reveal that the QTM was suppressed efficiently.
Clear single relaxation processes are still detected in
1 and 2
between 1 and 1000 Hz (Fig. 4 and Fig. S15†, ESI) for
1 and 2.
The single relaxation mode agrees with the presence of a
unique crystallographic Dy(III) ion in the centrosymmetric
complexes. The relaxation time was extracted and an
Arrhenius plot was constructed to study the temperature
dependence of the magnetic relaxation. By fitting the data to
the Arrhenius law τ = τ0exp(Ueff/kBT), the anisotropic effective
energy barrier (Ueff) and the pre-exponential factor (τ0) can be
under an applied 1500 Oe field. It is noteworthy that the plots
of ln(τ) versus 1/T under 1500 Oe field still exhibit obvious
curvature which indicates that perhaps another relaxation
pathway is also operative (Fig. 5). The presence of multiple
relaxation processes is possible as reported in a few SIMs.15 In
view of this, we fitted the magnetic data with the equation
considering the spin-lattice relaxation occurring through both
Raman and Orbach processes.19 The fitting results are
collected in Table S6† (ESI).
obtained with Ueff/kB = 56.9 K (τ0 = 1.1×10-6 s) for
= 53.9 K (τ0 = 3.7×10-6 s) for
from the high-temperature
1 and Ueff/kB
2
regime of the relaxation where it is thermally induced (Fig. 4).
The single relaxation processes can be clearly identified in
1/τ = CT n +τ0 -1exp(-Ueff/kBT)
(1)
the graphical representation of χ′′ vs χ′ (Cole-Cole plot16) for
1
The first and second terms correspond to the Raman and
Orbach processes, respectively. In general, n = 9 is rational for
Kramers ions, but when both the acoustic and optical phonons
are considered depending on the structure of energy levels, n
values between 1 and 6 are reasonable.20 Equation 1 affords
and
2 in a certain temperature range under zero field
conditions (Fig S16†, the fitting results are given in Table S4-
S5†, ESI). For temperatures higher than 4 K, symmetric
semicircles are obtained. The data can be fitted using a
generalized Debye model17 (Fig. S16† and Table S4-S5†, ESI).
Ueff/kB = 67.0 K, τ0 = 2.9 ×10-6 s for complex
K, τ0 = 1.7 ×10-5 s for complex
under 1500 Oe dc field,
1 and Ueff/kB = 41.0
2
The α parameters for
1 and 2, indicating deviation from the
respectively (Fig. 5 and Table S6†, ESI). Moreover, using the
pure Arrhenius law to fit the high-temperature region (11-14 K
pure Debye model, are in a range of 0.04-0.2 for these
temperature range that increase significantly as the
temperature decreases. These low degree of distribution verify
a single relaxation mode with a narrow distribution of
for
s for complex
respectively under 1500 Oe dc field (Fig. 5).
To contrast with Dy(III) complexes and
independent ac susceptibility signals of Er-based complexes
and under zero dc field were observed (Fig. S21-S22†, ESI).
1
and 9.5-11 K for
2
) show that Ueff/kB = 59.6 K, τ0 = 8.5×10-7
1
and Ueff/kB = 53.6 K, τ0 = 3.8×10-6 s for complex
relaxation times in complexes 1 .
and 2 18
2
1
2, the temperature
3
4
Therefore, ac susceptibility measurements were performed
under optimum static dc field of 3500 and 5000 Oe for
complexes
3 and 4, respectively (Fig. S23-S24†, ESI). The
optimum field was selected by determining which field was
able to slow the frequency dependent χ’’ maxima to the
slowest relaxation rate. After applying an external field, the
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out-of-phase (χ’’) susceptibilities show the temperature
dependence behaviors with tails and no full peak, which
clearly indicates the negligible field-induced slow relaxation of
magnetization associated with SMM behavior of
S23-S24†, ESI).
3 and 4 (Fig.
To further investigate magnetic anisotropy of dysprosium
ions, ab initio calculation was used as an efficient and powerful
method. Complete-active-space self-consistent field (CASSCF)
calculations on one type of individual Dy(III) fragment on the Fig. 6 Orientations of the local main magnetic axes of the ground doublets on Dy(III)
ions of 1 (left) and 2 (right).
basis of X-ray determined geometries of complexes 1-2 have
been carried out with MOLCAS 8.021 and SINGLE_ANISO22
programs (see Supporting Information for details). The lowest
This type of trinuclear complexes is also exhibit the f-f
interactions between two Ln(III) centers located at a distance
of longer than 6.8 Å albeit a magnetically silent alkaline earth
lies in the center of two Dy(III) ions.25 The ferromagnetic
spin-orbit energies and the corresponding
complexes are shown in Table S7† (ESI). From Table S7†,
the calculated gz values of the Dy(III) fragments of are both
g tensors of
1-2
1-2
interactions between Dy(III) ions observed for the complexes
and can be attributed to the magnetic susceptibility tensors
for Dy(III) centers of and are longitudinally extending on
the line of Ln-M-Ln as discussed in previous report.26 On the
contrary, those for the Er-based systems are likely laterally
extending and leading to the non-ferromagnetic interactions
1
close to 20, which shows that the Dy(III)-Dy(III) exchange
interactions for each of them can be approximately regarded
as the Ising type.
2
1
2
Table 1. Fitted exchange coupling constants Jexch, the calculated dipole-dipole
interactions Jdipolar and the total J between Dy(III) ions in 1-2 )
(cm−1
1
2
between two Er(III) centers in complexes
In order to probe the intermolecular magnetic interactions
which may contribute to the χMT values of and , the χMT
data of and diluted with the corresponding diamagnetic Y2
3 and 4.
Jdipolar
Jexch
J
0.98
6.25
7.23
1.24
3.63
4.87
1
2
1
2
congener complexes (black circles) were also obtained. For
extracting the intramolecular f-f interactions, magnetic data of
the complex having two magnetic centers must be subtracted
The program POLY_ANISO21 (Fig. 2) was used to fit the by those having one magnetic center with a practically
magnetic susceptibilities of complexes
parameters from Table 1.
All parameters from Table 1 were calculated with respect to (red circles), which can be obtained only by the reaction using
1-
2
using the exchange identical molecular structure. For this purpose, the
intramolecularly diluted samples 1b and 2b were prepared
the pseudospin Ŝ = 1/2 of the Dy(III) ions. For two complexes, a mixture of Dy(OQ)3 and Ca(OQ)2 or Mg(OQ)2 with an excess
the total coupling parameters J (dipolar and exchange) were amount of Y(OQ)3 so that the formation of dilanthanide
included into fit the magnetic susceptibilities. The calculated can be suppressed. As depicted in Fig. 7, the corresponding
and experimental χMT versus T plots of complexes are χMT values of diluted samples are close to that expected for
1 or 2
1-2
shown in Fig. 2, where the fits are all close to the experimental one or two free Dy(III) ions (14.17 and 28.34 cm3·K·mol-1,
data.23 The observed weak ferromagnetic coupling is in respectively) at 300 K. Upon decreasing the temperature from
agreement with expectation. From Table 1, the Dy(III)-Dy(III) 300 to 2 K, the direct current (dc) susceptibility measurements
interactions in two complexes within Lines model24 are all show monotonic decrement of the χMT values, which are due
ferromagnetic. The main magnetic axes on two Dy(III) were to the depopulation of the excited mJ sublevels of the Dy(III)
indicated in Fig. 6 where the magnetic axes on Dy(III) for ion, and/or antiferromagnetic interactions between the Dy(III)
complexes
1
and
2
align on the same straight line. It may be centers. It is obvious that the diluted samples exhibit entirely
and display significant SMM different results with those of the pristine samples, indicating
the reason that complexes
1
2
properties. We also gave the exchange energies and the main that the intermolecular magnetic interactions should not be
values of the gz for the lowest two exchange doublets of two neglected. Furthermore, the ΔχMT values defined as χM(1a)T-
complexes in Table S8† (ESI) where the gz values of the ground 2χM(1b)T are also plotted (green circles). As a consequence,
exchange states for all complexes are all close to 40, which the ΔχMT value is nearly zero above 50 K and a marked
confirms that the Dy(III)-Dy(III) couplings are both elevation of the ΔχMT value was observed upon lowering the
ferromagnetic.
temperature. Accordingly, the positive ΔχMT value of 2.38 and
2.64 cm3·K·mol-1 at
indicate that the presence of
ferromagnetic interaction between two ions in and
2
K
1
2.
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ARTICLE
Journal Name
was presented, in which the long-distance magnetic
interaction between the two dysprosium ions separated
effectively suppresses the QTM.33 In the present case,
interestingly, the long-range ferromagnetic f-f interactions
between two f-electronic systems are observed in Dy-based
complexes, in which two Dy(III) ions are separated via a
magnetically silent cadmium ion with a distance of longer than
6.8 Å. Importantly, the presence of ferromagnetic
intramolecular coupling plays a crucial role in suppressing QTM
and significant slow magnetic relaxation was observed in the
Fig. 7 Plots of χM(1a)/T (black circles) and χM(1b)/T (red circles) for 1 (left). Plots of
χ
M(2a)/T (black circles) and χM(2b)/T (red circles) for 2 (left). The differences between
the intermolecularly and intramolecularly diluted samples, ΔχMT, are show by green
circles for and respectively.
Dy-based complexes 1 and 2. This is most likely due to not only
the parallel arrangement of magnetic axes of individual Dy(III),
but also their parallel tendency to the direction of Dy---Dy. In
addition, the special strictly linear configuration favor the
communication between the longitudinally extending
magnetic susceptibility tensors of the two Dy(III) ions, which
likely leads to the ferromagnetic intramolecular interaction.
The present work provides a simple and easy operating
strategy to build f-s-f type linear SMM by pure 8-
hydroxyquinoline ligands without any other auxiliary ligands, in
which the magnetic axis of individual spin are tended to
parallel with each other and their colligated direction. And this
work will attract further research interests towards designing
and synthesizing novel s-4f multinuclear SMMs.
1
2
In order to probe the impact of f-f interactions on the slow
relaxation of the magnetization and the quantum tunnelling
effects occurring in these SMMs, alternating-current (ac)
measurements were performed on all of the diluted samples in
the temperature range 2-20
K under zero dc field at
frequencies between 1 and 1000 Hz. The temperature and
frequency dependent ac susceptibility signals were observed
below 15 K for the complexes 1a
ESI). Contrast to nondiluted Dy2 samples, a peak maximum
was observed at ~11 K and ~13 K for all diluted samples of
and respectively, which indicates their same origin of slow
, 2a, 1b and 2b (Fig. S25-S32†,
1
2
magnetic relaxation, but below 6 K the tail of a temperature
dependent peak signal was observed for diluted samples (Fig.
S25-S26 and S29-S30†, ESI). Such a tail of a peak is usually
indicative of QTM through the spin-reversal barrier via
degenerate ±MJ energy levels.27,28,29 The noticeable different
magnetic relaxation behaviors in the lower temperature range
indicate that the magnetic relaxation processes are more
suppressed for undiluted than diluted samples in the lower
temperature range at Hdc = 0. These observations are clearly
demonstrated also in the plots of χ’’ against f at Hdc = 0 (Fig.
S27-S28 and S31-S32†, ESI). Therefore, undiluted samples
show slower magnetic relaxations than diluted samples due to
Experimental Section
All chemicals and solvents were obtained from commercial
sources and used as received without further purification.
Elemental (C, H, O and N) analyses were performed on a
Perkin-Elmer 2400 analyzer. The elements Ca/Mg, Dy and Y
were quantitatively analysed by Inductively Coupled Plasma-
Atomic Emission Spectrometer (ICP-AES) using a Leeman
PROFILE SPEC spectrometer. The magnetic susceptibilities of
complexes 1-4 were measured using a Quantum Design VSM
superconducting quantum interference device (SQUID)
magnetometer. Data were corrected for the diamagnetism of
suppression of the possible relaxation processes by the the samples using Pascal constants and the sample holder by
measurement.
Precursors
metal
8-hydroxyquinolate
presence of intermolecular f-f interactions. Contrary to the
intramolecular f-f interactions, therefore, it is conjectured that
the intermolecular f-f interactions more promote the magnetic
relaxations, which cannot be neglected for the undiluted
samples especially.
complexes were prepared by mixing aqueous solutions of the
metal chloride (either as purchased or from dissolution of the
metal oxide in hydrochloric acid) and a stoichiometric amount
of Na(OQ) (prepared by stoichiometric reaction of HOQ with
NaOH in EtOH and subsequent concentration of the product),
filtration and subsequent drying of the precipitated
complexes.34 Compared with the reported method that
Conclusions
powdered M(OQ)2 and Ln(OQ)3 reacted in
a 1,2,4,5-
tetramethylbenzene (TMB) flux at elevated temperature
through pseudo solid-state rearrangement,35 a simple and easy
operating strategy was offered to build linear trinuclear Ln-M-
Ln complexes in this case.
Four novel linear heterometallic complexes with an [Ln-M-Ln]
core [Ln(III) = Dy and Er, M(II) = Ca and Mg] have been
synthesized, in which M(II) and Dy(III) ions are bridged by 8-
hydroxyquinolate oxygen atoms. Compared with general
3d/4f/3d30and 4f/3d/4f31 linear trinuclear complexes, reports
about s/4f/s32 and 4f/s/4f33 trinuclear SMMs are scarce. In the
former case, the diamagnetic metal Mg(II) ion was used to
enhance the SMM properties by suppressing quantum
tunneling as well as increasing the energy gap between ground
state and excited state.32 As for the latter case, a novel
macrocyclic ligand-based sandwich-type Dy-Ca-Dy complex
X-ray crystallographic Studies of
complexes were collected on a Xcalibur, Eos, Gemini
diffractometer with Mo Kα radiation (λ = 0.71073 Å). were
collected at 293(2) K, while 4 was collected at 150.0(1) K. The
1-4. Crystal data for
1-4
1-3
structures of complexes 1-4 were solved by direct methods
and refined on F2 by full-matrix least-squares using the ShelXL-
2014 program.36 All non-hydrogen atoms were refined with
isomorphous displacement parameters. In the case of
1 and 3,
6 | J. Name., 2012, 00, 1-3
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Journal Name
ether molecules have been treated as a diffuse contribution to
ARTICLE
Synthesis of [Gd2Ca(OQ)8]
(
1Gd). Complex 1Gd was
the overall scattering without specific atom positions by synthesized according to the same procedure as for complex
squeeze/platon.37 All crystallographic data for complexes
except that Gd(OQ)3 (0.295 g, 0.5 mmol) was used instead of
are summarized in Table S1† (ESI), and selected bond lengths Dy(OQ)3. Yield: 0.308 g (81.65%). Elemental analysis (wt%)
for complexes are tabulated in Table S3† (ESI). calcd. for C76H58CaGd2N8O9 (1581.88): C 57.65, H 3.67, N 7.08;
Powder X-ray Crystallography. Unfortunately, the single found: C 57.72, H 3.83, N 7.10.
crystals of 1Gd and 2Gd were not obtained, as an Synthesis of [Gd2Mg(OQ)8] (2Gd). Complex 2Gd was
alternative, the powder X-ray diffraction measurements have synthesized according to the same procedure as for complex
been performed on 1Gd and 2Gd and confirmed that the except that Gd(OQ)3 (0.295 g, 0.5 mmol) and Mg(OQ)2 (0.078
frame structures were maintained albeit existence of a slightly g, 0.25 mmol) were used instead of Dy(OQ)3 and Ca(OQ)2,
different pattern compared with complexes and respectively. Yield: 0.306 g (82.05%). Elemental analysis (wt%)
1,
1-4
1-4
5, 6,
1
,
5, 6,
1
2
respectively (Fig. S33-S34†, ESI). Powder X-ray data were calcd. for C74H50Gd2MgN8O8Cl6 (1730.73): C 51.30, H 2.89, N
recorded at room temperature on a Bruker D8 diffractometer 6.47; found: C 51.37, H 2.98, N 6.50.
using Cu Kα (λ = 1.5406 Å) radiation. The accelerating voltage Procedure for the synthesis of diluted samples of complexes
and the applied current were 40 kV and 20 mA, respectively. and , which were labelled as 1a 2a (intermolecular) and 1b
Synthesis of [Dy2Ca(OQ)8] ( ). A mixture of Dy(OQ)3 (0.297 g, 2b (intramolecular): A mixture of
0.5 mmol) and Ca(OQ)2 (0.082 g, 0.25 mmol) in chloroform (25 diamagnetic Y2 congener or
1
2
,
,
1
1
or
2
and corresponding
5
6
(0.1 mmol in total, in a 1:9
mL) was stirred at room temperature for 8 h, during which moral ratio) are stirred followed by reprecipitation to yield the
time an exquisite yellow suspension was formed. The solid was diluted 1a and 2a samples. The diluted complexes 1b and 2b
collected by filtration and the pure crystals qualified for XRD were prepared by following the same method as that for
1 and
were acquired by allowing diethyl ether diffusing slowly into 2 but here we have used Dy(OQ)3/Y(OQ)3 (1.0 mmol in total, in
the chloroform (CHCl3) solution of products in a sealed a 1:9 moral ratio) instead of 1.0 mmol of Dy(OQ)3. Specific
container after one week. Crystal yield: 0.018 g (4.75%). details of each reaction and the characterization data of the
Elemental analysis (wt%) calcd. for C76H58CaDy2N8O9 (1592.38): products obtained are given below.
C 57.27, H 3.64, N 7.08; found: C 56.97, H 3.29, N 7.40.
Synthesis of [Dy2Mg(OQ)8] ( ). Complex was synthesized A slurry of complex
according to the same procedure as for complex , except that (0.09 mmol, 0.123 g) in CHCl3 followed by Et2O diffusion led to
Synthesis of intermolecular dilution samples of complex
1 (1a):
2
2
1
(0.01 mmol, 0.0152 g) with complex 5
1
Mg(OQ)2 (0.078 g, 0.25 mmol) was used instead of Ca(OQ)2. the formation of intermolecular dilution samples in 79 % yield.
Crystal yield: 0.013 g (3.47%).Elemental analysis (wt%) calcd. The ICP analyses reveal that the ratios of Dy(III):Ca(II):Y(III) is
for C74H50Dy2MgN8O8Cl6 (1741.23): C 51.00, H 2.87, N 6.43; 1:4.72:8.46.
found: C 51.09, H 2.98, N 6.45.
Synthesis of [Er2Ca(OQ)8] (
according to the same procedure as for complex
Synthesis of intermolecular dilution samples of complex
was synthesized A slurry of complex (0.01 mmol, 0.0174 g) with complex
, except that (0.09 mmol, 0.143 g) in CHCl3 followed by Et2O diffusion led to
2 (2a):
3
). Complex
3
2
6
1
Er(OQ)3 (0.299 g, 0.5 mmol) was used instead of Dy(OQ)3. the formation of intermolecular dilution samples in 76% yield.
Crystal yield: 0.014 g (3.67%). Elemental analysis (wt%) calcd. The ICP analyses reveal that the ratios of Dy(III):Mg(II):Y(III) is
for C74H50CaEr2N8O8Cl6 (1766.52): C 50.27, H 2.83, N 6.34; 1:4.79:8.61.
found: C 50.34, H 2.96, N 6.40.
Synthesis of intramolecular dilution samples of complex
was synthesized A mixture of Dy(OQ)3 (0.0297 g, 0.05 mmol), Y(OQ)3 (0.234 g,
, except that 0.45 mmol) and Ca(OQ)2 (0.082 g, 0.25 mmol) in chloroform
1 (1b):
Synthesis of [Er2Mg(OQ)8] (4). Complex 4
according to the same procedure as for complex
1
Er(OQ)3 (0.299 g, 0.5 mmol) and Mg(OQ)2 (0.078 g, 0.25 mmol) (25 mL) was stirred at room temperature for 8 h, during which
were used instead of Dy(OQ)3 and Ca(OQ)2, respectively. time an exquisite yellow suspension was formed. The yellow
Crystal yield: 0.011 g (2.92%). Elemental analysis (wt%) calcd. powder yield: 0.253 g (73.0%). The ICP analyses reveal that the
for C74H50Er2MgN8O8Cl6 (1750.75): C 50.72, H 2.86, N 6.40; ratios of Dy(III):Ca(II):Y(III) is 1:4.50:8.06. Elemental analysis
found: C 50.80, H 2.97, N 6.45.
Synthesis of [Y2Ca(OQ)8] (
according to the same procedure as for complex
Y(OQ)3 (0.260 g, 0.5 mmol) was used instead of Dy(OQ)3. Yield: A mixture of Dy(OQ)3 (0.0297 g, 0.05 mmol), Y(OQ)3 (0.234 g,
0.285 (83.14%). Elemental analysis (wt%) calcd. for 0.45 mmol) and Mg(OQ)2 (0.078 g, 0.25 mmol) in chloroform
C76H58CaY2N8O9 (1445.23): C 63.10, H 4.01, N 7.75; found: C (25 mL) was stirred at room temperature for 8 h, during which
(wt%) calcd. for C76H58CaY1.8Dy0.2N8O9(1459.96): C 62.47, H
was synthesized 3.97, N 7.67; found: C 62.41, H 3.59, N 8.10.
, except that Synthesis of intramolecular dilution samples of complex
5
). Complex 5
1
2 (2b):
g
63.14, H 3.59, N 8.20.
time an exquisite yellow suspension was formed. The yellow
Synthesis of [Y2Mg(OQ)8] (
6
). Complex
6
was synthesized powder yield: 0.270 g (79.1%). The ICP analyses reveal that the
according to the same procedure as for complex 1, except that ratios of Dy(III):Mg(II):Y(III) is 1:4.35:7.74. Elemental analysis
Y(OQ)3 (0.260 g, 0.5 mmol) and Mg(OQ)2 (0.078 g, 0.25 mmol) (wt%) calcd. for C74H50Y1.8Dy0.2MgN8O8Cl6 (1608.89): C 55.19, H
were used instead of Dy(OQ)3 and Ca(OQ)2, respectively. Yield: 3.11, N 6.96; found: C 55.26, H 3.23, N 6.99.
0.287
g (85.0%). Elemental analysis (wt%) calcd. for
C74H50Y2MgN8O8Cl6 (1594.08): C 55.76, H 3.16, N 7.03; found: C
55.83, H 3.29, N 7.08.
Acknowledgements
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This work was supported by the NSFC (no. 21572048,
21102039, 51302068, 21272061, 51102081, and 21302045),
the Educational Commission of Heilongjiang Province
(1254G045,
12541639) and Heilongjiang University
(JCL201605). Natural Science Foundation of Jiangsu Province of
China (BK20151542) and the Priority Academic Program
Development of Jiangsu Higher Education Institutions.
8
9
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