Linear 3d-4f compounds: synthesis, structure, and determination of the d-f magnetic interaction

Article abstract of DOI:10.1039/c5dt01382h

A family of [RE₂M₂L₂(BA)₆] (RE = Gd, Tb, Dy, Y; M = Co, Ni, Cu, Zn; L = 1,2-bis(2-hydroxy-3-methoxybenzylidene)hydrazine; BA = benzoic acid) complexes were synthesized and structurally and magnetically characterized. The magnetic interactions between spin centers were systematically investigated. The interactions of M-M (M = Co, Ni and Cu) and Gd-Ni/Cu were extracted from the fitting of the experimental data of Y₂M₂ (M = Co, Ni and Cu) and Gd₂M₂ (M = Ni and Cu) derivatives, respectively. Furthermore, the Ln-M interactions were qualitatively analyzed by deducting the M-M coupling and the contribution of the crystal-field effects of the Ln^III ion from the total magnetic susceptibility of Ln₂M₂ (Ln = Dy and Tb) compounds. In addition, complex [Dy₂Zn₂L₂(BA)₆] shows field induced single molecular magnet behavior profiting from the isolated anisotropic dysprosium ions.

Full text of DOI:10.1039/c5dt01382h

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DOI: 10.1039/C5DT01382H
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ARTICLE
Linear 3d‐4f compounds: Synthesis, structure, and determination
of the d‐f magnetic interaction
Jianfeng Wu,a,b Lang Zhao,a Peng Zhang,a,b Li Zhang,a,b Mei Guoa,b and Jinkui Tang*a
Received 00th January 20xx,
Accepted 00th January 20xx
A family of [RE2M2L2(BA)6] (RE = Gd, Tb, Dy, Y; M = Co, Ni, Cu, Zn; L = 1,2‐bis(2‐hydroxy‐3‐methoxybenzylidene)hydrazine;
BA = benzoic acid) complexes were synthesized and structurally and magnetically characterized. The magnetic interactions
between spin centers were systematically investigated. The interactions of M‐M (M = Co, Ni and Cu) and Gd‐Ni/Cu were
extracted from the fitting of the experimental data of Y2M2 (M = Co, Ni and Cu) and Gd2M2 (M = Ni and Cu) derivatives,
respectively. Furthermore, the Ln‐M interactions were qualitative analyzed by deducting the M‐M coupling and the
contribution of the crystal‐field effects of the LnIII ion from the total magnetic susceptibility of Ln2M2 (Ln = Dy and Tb)
compounds. In addition, complex [Dy2Zn2L2(BA)6] shows field induced single molecular magnet behavior profiting from the
isolated anisotropic dysprosium ions.
DOI: 10.1039/x0xx00000x
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factor. In 2003, the [Ln(Pc)2] complex,15 which was reported as
the first lanthanide SMMs with energy barriers of Ueff = 230 K,
brings lanthanide based complexes into the leading candidate
Introduction
Since the report in 1993 of the first single molecular magnet
(SMM),1 which exhibits magnetism at molecular level,
enormous attention have been focalized around constructing
these promising new materials for potential application in
high‐density data storage.2‐5 Towards this end, an elementary
standpoint should be borne in mind theoretically that is the
effective barrier (Ueff) for overturning the magnetic moment
must be large, thus the magnetization could be maintained
after removing the magnetic field. Generally speaking, SMM
behavior arises from the large spin ground state (S) and
uniaxial magnetic anisotropy (|D|).3 The energy barrier Ueff can
be defined as S2|D| or (S2‐1/4)|D| for integer or half integer
spins, respectively. The early researches were mainly focused
on increasing S by introducing strong ferromagnetically
coupled metal ions.6‐12 Due to the open‐shell orbital structure
of subgroup elements, the 3d molecular clusters have been
to construct SMMs for future applications. Up to date, the best
SMMs performing the largest energy barrier and blocking
temperature are still terbium‐phthalocyanine derivatives,15, 16
3‐
18
N2 radical‐bridged terbium complex17,
and polymetallic
Dy@[Y4K2] complex.19
Besides the pure lanthanide SMMs, heterometallic 3d‐4f
compounds are indispensable in understanding and improving
SMM behaviors. 3d‐4f clusters have turned into a new class of
SMMs since the tetranuclear [CuIILTbIII(hfac)2]2 was reported
showing SMM behavior in the early 2000's.20 After that, the
23
24‐26
27, 28
Dy4Mn11,21 GdCo2,22,
Dy2Co2
and Dy2Cr2
clusters
showing significant SMM behaviors were reported.
Particularly, the butterfly type Dy2Cr2 cluster,27 reported by
Murray, highlights that the exchange interactions between DyIII
and CrIII ions can significantly reduce quantum tunnelling of
the magnetization (QTM), thus leading to considerably longer
relaxation times. Recently, Tong and coworkers obtained a
largely studied, such as V4,13 Mn12,1 Fe4 et al. However, the
14
fact that the Mn19 possessing a large spin of 83/2 did not show
SMM behavior due to the lack of anisotropy suggests that
enlarging S is no longer an effective way to enhance energy
barrier. It is the anisotropy barrier that claimed to be the key
unique FeII‐DyIII‐FeII SMM with record anisotropy barrier for
30
3d‐4f SMMs of 319 cm‐1.29,
While, the slow magnetic
relaxation of this complex does not come from exchange
interaction, it is the Ising‐type anisotropy of DyIII center which
is modulated mainly by the quasi‐D5h local symmetry that
freezes the magnetic moments at low temperatures. It is still
necessary to design and synthesize heterometallic, especially
polynuclear clusters in order to better understand the
relationship between the magnetic coupling and the SMM
behavior. Herein fifteen linear heterometallic compounds of
a.State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China.
E‐mail: tang@ciac.ac.cn.
b.University of Chinese Academy of Sciences, Beijing, 100039, P. R. China.
† Electronic Supplementary Information (ESI) available: IR spectra of compounds
12 are given in Fig. S1 and S2. Elemental analysis results of 15 are listed in Table
S1. Crystallographic data for complexes 10
Table S2. Selected bond lengths and angles of complexes
Tables S3. The packing of complex
compounds 15 are presented in Fig. S4‐S8. The temperature‐dependent ac
1‐
1
11
‐
1‐
2,
4‐
5,
7‐
8,
‐
,
,
9
13‐
15 are listed in formula
[RE2M2L2(BA)6]
(L
=
1,2‐bis(2‐hydroxy‐3‐
3
6,
and 12 are listed in
methoxybenzylidene) hydrazine), with two 3d metal ions in the
middle and two lanthanide ions on each side, were synthesized
and structurally and magnetically characterized. The M‐M and
3
are given in Fig. S3. The M vs H plots for
1‐
susceptibility of complexes
DOI: 10.1039/x0xx00000x
2, 3, 6, 8, 9 and 11 are presented in Fig. S9‐S12. See
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Ln‐M interactions were analyzed on the basis of the
experimental magnetic data. Among them, the [Dy2Zn2L2(BA)6]
complex exhibits field induced SMM behavior.
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DOI: 10.1039/C5DT01382H
Experiment Section
Materials and general procedures. All chemicals and solvents
were commercially obtained and used without any further
purification. Ligand 1,2‐bis(2‐hydroxy‐3‐methoxybenzylidene)
hydrazine (H2L) was prepared according to a previously
reported method.31 Elemental analysis for C, H, and N were
carried out on a Perkin‐Elmer 2400 analyzer (Table S1). IR
spectra (Fig. S1 and S2) were recorded with a Perkin‐Elmer
Fourier transform infrared spectrophotometer using the
reflectance technique (4000‐300 cm‐1) with samples prepared
as KBr disks.
Fig. 1 Coordination mode of the ligand (a), space filling (b) and
Ball‐Stick representations (c) of complex 3 with purple, celeste,
Table 1 Crystallographic data for complexes
3
,
6
,
9
and 12
gray, blue and red spheres representing Dy, Co, C, N, and O,
respectively; hydrogen atoms and solvents have been omitted
for clarity.
compounds
chemical
formula
3
6
9
12
C76H70Dy2
N4O24Zn2
1879.10
293(2)
C2/c
C78H74Co2
Dy2N4O24
C76H70Cu2
Dy2N4O26
1915.44
296(2)
C2/c
C78H76Dy2
Ni2N4O25
1911.84
FW, (g∙mol‐1) 1894.27
Magnetic
measurements.
Magnetic
susceptibility
measurements were recorded on a Quantum Design MPMS‐
XL7 SQUID magnetometer equipped with a 7 T magnet. The
variable‐temperature magnetization was measured in the
temperature range of 1.9−300 K with an external magneꢀc
field of 1000 Oe. The dynamics of the magnetization were
investigated from the ac susceptibility measurements in the
zero static fields and a 3.0 Oe ac oscillating field. Diamagnetic
corrections were made with the Pascal’s constants33 for all the
constituent atoms as well as the contributions of the sample
holder.
Preparation of ligand H2L. The Schiff base ligand H2L was
prepared as previously reported.31 The reaction of o‐vanillin
(15.2 g, 0.1 mol) and hydrazine hydrate (3.26 ml, 0.05 mol) in
ethanol (100 ml) was stirred and refluxed for 10 h. After
cooling, the solution was reduced to one‐third of its volume
with rotary evaporator. The resulting precipitate was filtered
off and dried in vacuum, yielding H2L as yellow powder (13.0 g,
87%). IR (KBr, cm‐1 ): 3438 (br), 1629 (S), 1469 (S), 1255 (m),
1080 (w), 964 (w), 734 (S). Elemental analysis (%) calcd: C,
64.85; H, 6.53; N, 11.34; found: C, 64.74; H, 6.67; N, 11.37.
Synthesis of complexes [Ln2Co2L2(BA)6(MeOH)2]∙2MeOH∙xH2O (Ln
= Gd (1), Tb (2), Dy (3)): Ln(BA)3∙6H2O (0.1 mmol) was dissolved
in 15 ml/5 ml CH3OH/CH3CN, followed by the addition of H2L
(0.15 mmol) and Co(OAc)2∙6H2O (0.1 mmol). Then
triethylamine (0.2 mmol, 2.8 ml) was added, and the mixture
was stirred for 3 h. Red brown block‐shaped single crystals,
suitable for X‐ray diffraction analysis, were isolated after 7
days.
T (K)
293(2)
C2/c
296(2)
C2/c
space group
Z
4
4
4
4
a, Å
24.10(5)
16.90(3)
22.71(6)
24.03(5)
16.83(3)
22.73(5)
23.59(6)
16.69(4)
23.14(8)
119.9(4)
7904(4)
0.71073
3864
23.87(3)
16.89(3)
22.92(4)
120.6(9)
7960(2)
0.71073
3832
b, Å
c, Å
β, °
V, Å3
120.9 (9) 121.1(3)
7935(3)
0.71073
3792
7874(3)
0.71073
3832
μ, mm‐1
F(000)
ρcalcd, g∙cm‐3
1.586
1.611
1.622
1.598
R1a), wR2 (I ≥ 0.0742,
0.0561,
0.1375
0.1145,
0.1761
1.011
0.0688,
0.1639
0.1203,
0.2009
1.036
0.0761,
0.2067
0.1241,
0.2494
1.059
2 σ (I))b)
R1, wR2 (all 0.2030,
0.1796
data)
GOF
0.2502
0.964
a) R1 = ∑||Fo|‐|Fc||/∑|Fo|, b) wR2 = [∑w(Fo ‐Fc2)2/∑w(Fo ) ]
2
2 2 1/2
X‐ray crystal structure determinations. Single‐crystal X‐ray data
of the complexes were collected on a Bruker Apex II CCD
diffractometer equipped with graphite‐monochromatized Mo‐
Kα radiation (λ = 0.71073 Å) at 293(2) K. The structures were
solved by direct methods and refined by full‐matrix least‐
squares methods on F2 using SHELXTL‐97.32 All non‐hydrogen
atoms were determined from the difference Fourier maps and
refined anisotropically. Hydrogen atoms were introduced in
calculated positions and refined with fixed geometry with
respect to their carrier atoms. Crystallographic data are listed
in Tables 1 and S2. CCDC 1057880‐1057888, 1057890‐1057894
and 1063586 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
Synthesis of complexes [Ln2Ni2L2(BA)6(MeOH)2]∙2MeOH∙xH2O (Ln =
Gd (4), Tb (5), Dy (6)): Ln(BA)3∙6H2O (0.1 mmol) was dissolved in
20 ml CH3OH, followed by the addition of H2L (0.15 mmol) and
Ni(OAc)2∙6H2O (0.1 mmol). Then triethylamine (0.2 mmol,
0.028 ml) was added, and the mixture was stirred for 3 h.
the
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
2 | J. Name., 2012, 00, 1‐3
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Dark‐green block‐shaped single crystals, suitable for X‐ray showing distort octahedral symmetry. However, VthieweA3rtdiclemOenltinael
DOI: 10.1039/C5DT01382H
ions in compounds 7‐12 and 15 are five coordinated due to the
diffraction analysis, were isolated after 5 days.
Synthesis of complexes [Ln2Cu2L2(BA)6]∙xMeOH∙yH2O (Ln = Gd (7), characteristic coordination abilities of CuII and ZnII ions. The
Tb (8), Dy (9)): Ln(BA)3∙6H2O (0.1 mmol) was dissolved in 20 ml Ln/Y ion and the MII ion are linked by two phenol oxygen
CH3OH, followed by the addition of H2L (0.15 mmol) and atoms (O2 and O4) from L ligand with Ln‐O‐M angles of
Cu(OAc)2∙6H2O (0.1 mmol). Then triethylamine (0.2 mmol, 104.05° and the Ln/Y‐M distance is as long as 3.43 Å and the
0.028 ml) was added, and the mixture was stirred for 3 h. M‐M distance reaches 3.82 Å. In such long distance, the
Brown block‐shaped single crystals, suitable for X‐ray magnetic interactions might be weak, which could not
diffraction analysis, were isolated after 3 days.
compose strong intramolecular magnetic coupling. The four
Synthesis of complexes [Ln2Zn2L2(BA)6]∙xMeOH∙yH2O (Ln = Gd (10), ions in the structure are almost linearly aligned with Ln/Y‐M‐M
Tb (11), Dy (12)): Ln(BA)3∙6H2O (0.1 mmol) was dissolved in 10 angle of 173.39°. The packing of complex 3 are given in Fig. S3.
ml/10 ml CH3OH/CH3CN, followed by the addition of H2L (0.15
mmol) and Zn(OAc)2∙6H2O (0.1 mmol). Then triethylamine (0.2
mmol, 0.028 ml) was added, and the mixture was stirred for 3
h. Yellow block‐shaped single crystals, suitable for X‐ray
diffraction analysis, were isolated after 5 days.
Synthesis of complexes [Y2M2L2(BA)6(MeOH)2]∙2MeOH∙xH2O (M =
Co (13), Ni (14)) and [Y2Cu2L2(BA)6]∙4H2O (15): Y(BA)3∙6H2O (0.1
mmol) was dissolved in 15 ml/5 ml CH3OH/CH3CN, followed by
the addition of H2L (0.15 mmol) and M(OAc)2∙6H2O (0.1 mmol).
Then triethylamine (0.2 mmol, 0.028 ml) was added, and the
mixture was stirred for 3 h. Block‐shaped single crystals,
suitable for X‐ray diffraction analysis, were isolated after 7
days.
Result and Discussion
Crystal structures. The reactions of Ln/Y(BA)3∙6H2O (Ln = Gd,
Tb, Dy) and M(OAc)2∙2H2O (M = Co, Ni, Cu, Zn) with H2L in the
presence of triethylamine afforded a series of heterometallic
compounds. Details for the structure solution and refinement
are summarized in Tables 1 and S2, and selected bond
distances and angles are listed in Table S3. Single‐crystal X‐ray
studies reveal that all compounds are isostructural and
crystallize in monoclinic space group C2/c with Z = 4. The cell
unit is composed of two MII ions (M = Co, Ni), two Ln/Y ions,
two L ligands, six benzoic acid anions and two methanol. The
structure can be described as a “double‐helix”: two twisted
ligands screw along the four linear arranged metal ions (Fig. 1)
with two 3d metal ions in the middle and two lanthanide ions
on each side. Just because of the twisting of the ligands, four
coordination pockets are formed along the ligands: two of
which in the middle are small and suitable to accommodate 3d
metal ions, the other two on each side are larger so that can
clamp lanthanide ions. The “double‐helix” is further stabilized
by six benzoic acid ligands: four of which are chelated to two
terminal lanthanide ions, the other two bridge each lanthanide
ion to the central 3d metal pair with common syn‐syn bridging
mode. A closer look at the coordination sphere of Ln/Y ions
reveals that the ions are coordinated by nine oxygen atoms
with distorted mono‐capped square‐antiprismatic geometry.
The coordination sphere of MII (M = Co, Ni) ions located in two
N2O3 pockets formed by the donor atoms from H2L ligands and
the bridging benzoic acid is completed by two methanol,
Fig. 2 Temperature dependence of χMT product at 1 kOe for
(top), 10 15 (bottom).
Magnetic properties. The static magnetic susceptibilities of
complexes 15 (Fig. 2) were collected in the temperature
1‐
9
‐
1
‐
range of 2‐300 K under an applied magnetic field of 1000 Oe,
and the magnetization (M) data were collected in the field
range of 0‐7 T below 5 K. The corresponding room
temperature χMT products for compounds [RE2Cu2], [RE2Ni2]
and [Ln2Zn2] approach to the expected value for the uncoupled
3d metal ions (g = 2, S = 1 for NiII and g = 2, S = 1/2 for CuII) and
lanthanide ions (g = 2, S = 7/2 for GdIII; g = 3/2, J = 13/2 for
TbIII; g = 4/3, J = 15/2 for DyIII), which indicates the free‐ion
approximation applies.34 While, the room temperature χMT for
[RE2Co2] are larger than that calculated by a spin‐only equation
with two isolated CoII ions (g = 2, S = 3/2) and lanthanide ions,
suggesting the significant orbital contribution of CoII center.
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With lowering the temperature, however, the χMT values show crystal field, then increase slightly at low temperature and
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3
‐1
DOI: 10.1039/C5DT01382H
different tendency, which are dependent on 3d ion. Herein, we reach the maximum of 22.5, 27.1 and 29.2 cm ∙K∙mol before
classify the compounds based on 3d ions to describe the decreasing again and reaching 18.8, 25.0 and 26.2 cm3∙K∙mol‐1
magnetic properties.
for 1, 2 and 3 at 2 K, respectively. The increase of magnetic
susceptibilities suggests weak ferromagnetic interactions
between the paramagnetic centers. The common tendency of
χMT for each compound with different lanthanide ions are
indicative of similar magnetic properties, which might be
manipulated by CoII
center. The field‐dependent
magnetizations for all compounds are shown in Fig. S4‐S8. For
, the magnetization rises abruptly at low fields and reaches
2
1‐3
19.1, 18.8 and 17.6 μB at 7 T, without saturation (Fig. S4). This
suggests the presence of magnetic anisotropy and/or the
considerable crystal‐field effects.35
With the replacement of CoII ions with NiII and CuII ions,
analogues compounds of [Ln2Ni2] (4‐6) and [Ln2Cu2] (7‐9) are
isolated. The temperature dependence of χMT products at
room temperature are almost in good agreement with the
expected value for 3d metal ions and lanthanide ions. With
decreasing temperature from 300 to 2 K, the χMT products
decrease gradually before sharp drops at low temperatures.
The sudden drops of magnetic susceptibilities suggest that
magnetic couplings within these complexes are dominated by
antiferromagnetic interactions between the paramagnetic
centers. In contrasted with compounds
1
‐
3, the absence of the
increase of χMT products in compounds
4‐9
suggests
inappreciable ferromagnetic coupling within the spin centers.
The field‐dependent magnetization approximate to the
expected values for two isolated 3d metal ions and two
lanthanide ions, without saturation (Fig. S5‐S6), suggesting the
presence of magnetic anisotropy and/or the considerable
crystal‐field effects.35
In view of the temperature dependent χMT products of 1‐9,
the magnetic interactions of M‐M and Ln‐M are still unclear. In
order to study the magnetic coupling within the spin center,
compounds [Ln2Zn2] (10
contains diamagnetic ZnII or YIII ions, were prepared. The
temperature dependence of χMT products for [Ln2Zn2] (10 12
‐12) and [Y2M2] (13‐15), each of which
‐
)
does not remarkably change in the temperature ranges of 2‐
300 K, suggesting weak or none interactions between the
lanthanide ions. This is coincidence with the large Ln‐Ln
distance of 10.64 Å. Within such long distance, significant
magnetic coupling cannot be induced. For the case of [Y2M2],
M‐M interactions can be easily defined by the temperature
dependent χMT products, profiting from the absence of
paramagnetic lanthanide ions. Upon cooling, the χMT products
Fig. 3 Extracted temperature dependence of ∆χMT products for
{[Ln2M2] ‐ [Y2M2]} (top), {[Ln2M2] ‐ [Ln2Zn2]} (middle) and
{[Ln2M2] ‐ [Ln2Zn2] ‐ [Y2M2]} (bottom).
of compounds [Y2M2] (13‐15) decrease slowly, indicating the
weak antiferromagnetic interaction between the metal ions. In
order to evaluate the coupling parameters, a simple isotropic
For complexes
1‐3
, with two CoII ions settled between



Heisenberg form expression34 of
are applied on
lanthanide ions, the temperature dependent χMT products
approach 20.8, 28.0 and 34.2 cm3∙K∙mol‐1 at room temperature,
respectively. These values are larger than that calculated by
spin‐only CoII ions (g = 2, S = 3/2) and lanthanide ions
indicating the significant orbital contribution of CoII center.
Upon cooling, the χMT products decrease gradually, which
results from Stark level splitting at the lanthanide ions in weak
H  J SA SB
compounds 13‐15 with S = 1/2 for [Y2Cu2], S = 1 for [Y2Ni2], and
S = 3/2 for [Y2Co2]. The fits for [Y2Cu2] and [Y2Ni2] give J = ‐
28.32(9) cm‐1, g = 2.08(4), with R = 7.11(6)×10‐4 and J = ‐5.48(1)
cm‐1, g = 2.18(9), with R = 3.01×10‐3. However, it is
unsuccessful for [Y2Co2] using isotropic model, because the
distortion of the octahedral symmetry for CoII center is not
4 | J. Name., 2012, 00, 1‐3
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In order to qualitative analysis the magnetic interaction of
DOI: 10.1039/C5DT01382H
Ln‐M, further investigations based on the static magnetic
susceptibility are performed. The nature of the lanthanide ions
donating to the total magnetic susceptibility, in our case, can
be wiped out by the general procedure43 that represents the
large enough to induce total quenching of the orbital moment
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4
of the T1g ground state of CoII ions.36 In this situation, Lines’
model36‐38 was used to analysis of the magnetic susceptibility
data on the basis of the Hamiltonian as shown by equation (1)
where SA,Z and SB,Z are the component of SA and SB along Z axis
and LA,Z and LB,Z are the component of orbital number along Z
axis.
temperature dependent difference ΔχMT
= χMT[Ln2M2] ‐
χMT[Ln2Zn2], where the intrinsic magnetic behavior of the
lanthanide ions as found within the [Ln2Zn2] complexes is
subtracted from the one measured for the [M2Ln2] complexes.
The results obtained by this procedure for the [Ln2Co2],
[Ln2Ni2] and [Ln2Cu2] are shown in Fig. 3 (middle). The ΔχMT
values for [Ln2Co2] are equal to the corresponding values of
compounds [Y2Co2] until 20 K, below which an upturn is
observed with a maximum value of 6.1, 5.45 and 7.26
cm3∙K∙mol‐1 before decreasing again and reaching minimum
value of 3.93, 4.25 and 3.6 cm3∙K∙mol‐1, for compounds
[Dy2Co2], [Tb2Co2] and [Gd2Co2], at 2 K. The upturning of ΔχMT
reveals that the Ln‐Co interaction is mainly ferromagnetic (the
antiferromagnetic Co‐Co coupling causes the decrease of
ΔχMT). However, the ΔχMT values for [Ln2Ni2] and [Ln2Cu2]
perform steadily decrease and sudden drop at low
temperature, reaching the minimum values of ‐3.9, ‐9.89, ‐7.8,
‐13.3, ‐16.6 and ‐7.3 cm3∙K∙mol‐1 for [Dy2Ni2], [Tb2Ni2], [Gd2Ni2],
[Dy2Cu2], [Tb2Cu2] and [Gd2Cu2], respectively. The sudden
drops and the negative ΔχMT values reveal overall
antiferromagnetic interactions between magnetic centers.
Unfortunately, we could not conclude that the Ln‐Ni and Ln‐Cu
interactions are antiferromagnetic, because the negative J
parameters of Ni‐Ni and Cu‐Cu interaction were obtained in
compounds [Y2Ni2] and [Y2Cu2] by the MAGPACK program.
Only if the contributions of the Ni‐Ni and Cu‐Cu, χMT[Y2Ni2] and
χMT[Y2Cu2], are subtracted from the ΔχMT, the magnetic
interactions between metal ions and lanthanide ions can be
clearly revealed. Herein, we use the expression of ΔχMT =
χMT[Ln2M2] ‐ χMT[Ln2Zn2] ‐ χMT[Y2M2] to evaluate the Ln‐M coupling
donation to the magnetic susceptibility. The ΔχMT values for
the [Ln2M2] system (Fig. 3 bottom) are zero in high
temperature. Upon cooling, the ΔχMT increase with maximum
value of 1.86, 1.29 and 2.12 cm3∙K∙mol‐1 before drop to zero
again for compound [Dy2Co2], [Tb2Co2] and [Gd2Co2],
respectively, confirming the ferromagnetic Ln‐Co interaction.
The ΔχMT vs T plots of [Ln2Ni2] and [Ln2Cu2] give a sudden drop
and reach the minimum value of ‐6.77, ‐8.43, ‐3.77, ‐1.68, ‐
3.93 and ‐4.67 cm3∙K∙mol‐1 for [Dy2Ni2], [Tb2Ni2], [Gd2Ni2],
[Dy2Cu2], [Tb2Cu2] and [Gd2Cu2], respectively. The obvious
negative ΔχMT prove that antiferromagnetic interactions are
dominated between lanthanide ions and NiII, and CuII ions.
However, further quantitative evaluation of the Dy‐M coupling
J values through ab initio calculations is definitely needed.44,45





 




 




3


H  J S A SB   H 2 S A.Z  SB.Z

 LA.Z  LB.Z
(1)


Z


2


The molar susceptibility is given by Equation (2), with a
temperature‐dependent g factor g(T) extracted from Lines’
equations for an octahedral cobalt(II) dimer (take no account
of diamagnetism Y) exhibiting isotropic magnetic exchange:38
M  2N2 g T 2 kT 3 exp 25J 9kT 1
(2)
 








The best fit of magnetic susceptibility data gives
antiferromagnetic interaction parameter of J = ‐1.34 cm‐1,
orbital‐reduction factor κ = 0.8 (typical values for a six‐
coordinate high‐spin CoII ion are in the range of 0.70‐0.95,39, 40
and the agreement factor R2 = 0.97(7).
)
In order to evaluate the strength of exchange coupling
between lanthanides and 3d metal ions, the magnetic
susceptibility data of the compounds [Gd2Cu2] and [Gd2Ni2]
were fitted on the basis of the following simplified spin
Hamiltonian:











H  J1 (SGd1 SM1)  (SM1A SGd1A )  J2 (SM1 SM1A
)
(3)


The best fit was obtained with parameters of J1 = ‐11.2 cm‐1, J2 = ‐
35.2 cm‐1 for [Gd2Cu2] and J1 = ‐17.4 cm‐1, J2 = ‐2.2 cm‐1 for [Gd2Ni2],
respectively. The negative J1 confirms the antiferromagnetic
interactions between GdIII ions and 3d metal ions. This type
antiferromagnetic interaction is common in most 3d‐4f compounds
such as the [CuII GdIIIn] compound.41 The negative J2 values, which
5
are consistent with the results of compounds [Y2Cu2] and [Y2Ni2],
prove the antiferromagnetic interaction between the 3d metal ions.
However, the magnetic interaction of Dy‐M, Tb‐M and Gd‐Co could
not be extracted with MAGPACK program42 because of the
anisotropic DyIII and TbIII and the zero field splitting effect of CoII
ions.
Fig. 4 Temperature‐dependent (left) and frequency‐dependent
(right) ac susceptibility for 12, under 600 Oe dc field.
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T), the Raman relaxation process (
typically), and the Orbach process (


Tn, n = 4, 5, 7, or 9
View Article Online
eDxOpI:(U10e.f1f0/k39BT/C))5.DT013T8h2He
49‐52
plots were fitted using equation as shown below:
1
1
U




 AH 2T  CT n 01 exp
(4)
eff



kBT
obs QTM
While, the best fit only involves quantum tunneling pathway
and Orbach process, and gives Ueff = 14.69 K with pre‐
exponential factors (τ0) of 3.15×10‐5 and τQTM = 3.68×10‐3. We
attribute the quantum tunneling pathway to the coexistence
of tunneling pathways and dipole‐dipole interaction.49
It is well‐known that magnetic coupling, ferro or antiferro,
would enhance the SMM behavior since it could suppress
tunneling. However, the field induced SMM behavior displayed
by compound [Dy2Zn2] is unexpected since the absence of the
magnetic coupling as a result of the long intermolecular
Dy∙∙∙Dy distance (10.408 Å). Such SMM behavior might arise
directly from the anisotropy of DyIII ions. Herein, each Dy
center behaves isolated relaxation process in high
temperature region. The present results demonstrate that the
magnetic coupling, ferro or antiferro, does not always have
positive influence on the magnetic properties within 3d‐4f
SMMs.
Fig. 5 Cole‐Cole plots of 12 under 600 Oe dc field in the
temperature range of 1.9‐10 K. The red lines are the best fits
with an extended Debye model.
Conclusions
In summary, fifteen new linear tetranuclear Ln‐M2‐Ln
complexes have been isolated by utilizing Schiff base ligands.
The M‐M couplings were confirmed as antiferromagnetic in
compounds [Y2M2] analogues. The Gd‐M interactions for the
[Gd2M2] family were evaluated, as well, to be
antiferromagnetic. In the case of complexes [Dy2M2] and
[Tb2M2], the Ln‐M interactions were identified indirectly by a
generalprocedure, where the Ln‐Co exchange interactions are
ferromagnetic, while the Ln‐Ni and Ln‐Cu show
antiferromagnetic coupling. In addition, compound [Dy2Zn2]
Fig. 6 τ vs T‐1 plot for 12 under 600 Oe dc field in the
temperature range of 3‐9 K. The red line is fitted with the
Arrhenius law.
AC susceptibility measurements also have been performed
on [Dy2M2] and [Tb2M2]. However, no temperature‐ and
frequency‐dependent in‐phase (χ') and out‐of phase (χ'')
shows field induced SMMs behavior, which is as
a
consequence of anisotropy of DyIII in the absence of magnetic
coupling from 3d ions.
susceptibilities peaks are detected for compounds
2, 3, 5, 6, 8,
9
and 11 (Fig. S9‐S12), even under the application of additional
dc field, revealing only the onset of slow relaxation of the
magnetization. While, the AC susceptibility for compound 12
shows field induced single magnets behavior with
temperature‐dependent peaks (Fig. 4), at 3 K with 600 Oe dc
field. More interestingly, frequency‐dependent out‐of‐phase
magnetic susceptibility signals exhibit a new peak tail at 1.9 K,
when decreasing the frequency below 10 Hz (Fig. 4). This
behavior indicates multiple processes may participate in the
magnetic relaxation. In order to study the magnetic process,
Debye model were adopted. The best fits of Cole‐Cole plots
(Fig. 5), which show semi‐cycle shape within the temperature
Acknowledgements
We thank the National Natural Science Foundation of China
(Grants 21371166, 21331003, and 21221061) for financial
support.
Notes and references
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