A series of Salen-type homodinuclear lanthanide complexes and their slow magnetic relaxation in Dy2 and Ho2

Article abstract of DOI:10.1002/aoc.5331

A series of homodinuclear lanthanide complexes, namely, [Ln₂(L)₂(MeOH)₂(NO₃)₂] [Ln = Gd (1), Tb (2), Dy (3), and Ho (4)], were synthesized by the reaction of Salen-type ligand, namely N, N′-bis(5-brom

Full text of DOI:10.1002/aoc.5331

Received: 23 August 2019
DOI: 10.1002/aoc.5331
Revised: 17 September 2019
Accepted: 20 September 2019
F U L L P A P E R
A series of Salen-type homodinuclear lanthanide complexes
and their slow magnetic relaxation in Dy2 and Ho2
Lun Zhang1 | Pei-Pei Yang1,2
Jin Tao3
| Ling-Fei Li1 | Yi-Ye Hu1 | Yu Gao1
|
1College of Chemistry and Materials
A
series of homodinuclear lanthanide complexes, namely, [Ln2(L)2
Science, Huaibei Normal University, 100
Dongshan Road, Huaibei, 235000, People's
Republic of China
2Anhui Key Laboratory of Energetic
Materials, Huaibei Normal University, 100
Dongshan Road, Huaibei, 235000, People's
Republic of China
3Jiangsu Key Laboratory for NSLSCS,
School of Physical Science and
Technology, Nanjing Normal University,
Nanjing, 210023, People's Republic of
China
(MeOH)2(NO3)2] [Ln = Gd (1), Tb (2), Dy (3), and Ho (4)], were synthesized
by the reaction of Salen-type ligand, namely N, N0-bis(5-bromosalicylidene)eth-
ane-1,2-diamine (H2L), with lanthanide salts in methanol and acetonitrile
solution. The two LnIII ions in 1–4 are linked by two Ophenoxo atoms of two L2−
ligands to build a dinuclear skeleton. The eight-coordinate LnIII center adopts
a triangular dodecahedron geometry of D2d symmetry. Theoretical calculations
revealed that antiferromagnetic interactions exist in those complexes. Dynamic
magnetic properties studies indicate that the Dy2 complex behaves as a single-
molecule magnet with an anisotropy barrier of Ueff ≈ 47.68 K and a pre-
exponential factor τ0 = 3.17 × 10−6 s under a zero applied field, whereas the
Ho2 complex exhibits a fast tunneling relaxation process that is rationalized
through ab initio calculations.
Pei-Pei Yang, College of Chemistry and
Materials Science, Huaibei Normal
University, 100 Dongshan Road, Huaibei,
235000, People's Republic of China.
Email: hbnuypp@163.com
K E Y W O R D S
Jin Tao, Jiangsu Key Laboratory for
NSLSCS, School of Physical Science and
Technology, Nanjing Normal University,
Nanjing 210023, People's Republic of
China.
antiferromagnetic interactions, homodinuclear lanthanide complexes, Salen-type ligand, single-
molecule magnet
Email: taojin1@njnu.edu.cn
Funding information
Foundation for Young Talents in College
of Anhui Province, Grant/Award
Numbers: no. gxyqZD2016412,
gxyqZD2016412; Anhui Provincial
Innovation Team of Design and
Application of Advanced Energetic
Materials, Grant/Award Number:
KJ2015TD003
1 | INTRODUCTION
complexes that show slow relaxation in the absence of an
external magnetic field and hysteresis loops because of
Single-molecule magnets (SMMs) have attracted much
attention because of their potential applications in high-
density information storage, magnetic refrigerators, and
quantum computing.[1] SMMs are discrete molecular
the presence of an energy barrier to the magnetization
reversal.[2] Since the discovery of mononuclear lantha-
nide complexes exhibiting fascinating properties,[3]
sustained efforts have been focused on developing
Appl Organometal Chem. 2020;e5331.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5331

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ZHANG ET AL.
lanthanide-based SMMs (Ln-SMMs).[4] In comparison
with transition elements, lanthanide ions provide large
magnetic moments and significant intrinsic magnetic
anisotropies profiting from a strong spin–orbit coupling
effect, making them ideal candidates for the construction
of high-performance SMMs.[5] Especially the DyIII ion,
arising from its bistable nature of the ground state
irrespective of the ligand field, significant magnetic
anisotropy, and large magnetic moment,[6] has indisput-
ably yielded the largest quality of strongly blocked
Ln-SMMs.
dinuclear complexes) have not been researched nearly as
extensively.[19] With the aim to extend the search for new
dinuclear lanthanide complexes, a Salen-type ligand,
namely,
N,N0-bis(5-bromosalicylidene)-ethane-
1,2-diamine (H2L), was chosen to react with lanthanide
salts. Thus, in this paper, we report the structures and
magnetic properties of a family of dinuclear LnIII com-
plexes obtained from the H2L ligand, namely,
[Ln2(L)2(MeOH)2(NO3)2], where Ln = Gd (1), Tb (2), Dy
(3), Ho (4). Magnetic studies indicate that there are anti-
ferromagnetic interactions between the Ln3+ ions in
those complexes. Complexes 3 and 4 exhibit a slow mag-
netic relaxation and complex 3 has an anisotropy barrier
Notably, lanthanide complexes of low nuclearity
(mononuclear in particular) should be better SMMs.[7]
Indeed, the dysprosium metallocene cation reported by
Layfield and co-workers very recently showed a new
record energy barrier (Ueff = 2217 K) and blocking tem-
perature (TB = 80 K), which represents the best SMM
reported to date.[8] Compared with mononuclear lantha-
nide complexes, dinuclear lanthanide complexes have
been regarded as the simplest molecular unit, which
allows to easily understand in-depth the nature and
strength of magnetic coupling between two spin carriers
and finally reveal the magnetic relaxation mechanism
affected by magnetic interaction. However, those have
been only little explored in lanthanide systems,[9–11] par-
ticularly in anisotropic lanthanides.[11] Moreover, the use
of ab initio calculation and electrostatic analysis to
explore the SMM properties of rare earth metal com-
plexes (such as anisotropic energy barriers [Ueff]) is also a
hot topic in molecular magnetics.[12] In a number of dil-
anthanide complexes, magnetic exchange interactions
have been also modelled by ab initio calculations.[13]
Clearly, it remains a significant challenge to establish
rational and effective guidelines for synthesizing din-
uclear Ln-SMMs, exploring the effect of f–f interactions
and uniaxial anisotropy on magnetic behavior. It has
been reported that the magnetic anisotropy for Ln-SMMs
is extremely sensitive to the ligand field effects and the
coordination geometry of the lanthanide metal ion.[14]
Therefore, choosing the suitable organic ligand is neces-
sary for the preparation of dinuclear Ln-SMMs.
According to the literature research, the ligands utilized
in the preparation of dinuclear lanthanide complexes are
comparatively few and limited to the Schiff bases
obtained from o-vanillin or its derivatives, with different
kinds of amines.[15–17] As a special Schiff base, the Salen
ligands, which are condensation products from
salicylaldehyde and diamine, exhibit good coordination
ability, and hence have been used for constructions of
functional coordination complexes. Compared with the
amount of effort on the 3d–4f heteronuclear complexes
from Salen-type Schiff base ligands,[18] the pure polynu-
clear lanthanide Salen complexes (especially for
of Ueff
≈ 47.68 K and a pre-exponential factor
τ0 = 3.17 × 10−6 s. The uniaxial magnetic anisotropies
and relaxation mechanism of complexes 3 and 4 were
investigated in depth by ab initio calculations.
2 | EXPERIMENTAL
2.1 | Syntheses and infrared spectra for
complexes 1–4
Triethylamine (0.0101 g, 0.1 mmol) was added dropwise
to a 10 mL solution of methanol and acetonitrile (1:1)
containing Ln(NO3)3ꢀ6H2O (0.1 mmol) and H2L
(0.0426 g, 0.1 mmol). After that, the mixture was stirred
at room temperature for 0.5 hr. The mixture was then
transferred to a 25-mL Teflon-lined autoclave, and heated
at 80 ꢁC in a precision thermostatic blast oven. After
5 days, it was cooled to 30 C at a rate of 5 ꢁC every 2 hr.
ꢁ
Yellow crystals suitable for X-ray diffraction were col-
lected and washed three to five times with cold MeOH in
a yield of 35%–40%. The complexes were characterized by
elemental analysis and Fourier-transform infrared (FT-
IR). The characteristic data of these complexes are as
follows:
[Gd2(L)2(MeOH)2(NO3)2] (1): Elemental analyses (%),
calculated: C 30.22, H 2.36, N 6.23. Found: C 30.20, H
2.37, N 6.22. IR (KBr) (cm−1): 3831 (m), 3743 (m), 3674
(m), 3390 (m), 2982 (m), 2354 (m), 1625 (w), 1464 (w),
1382 (w), 1314 (w), 1267(m), 1173 (m), 1014 (m), 828 (m),
691 (m), 635 (m), 440 (w).
[Tb2(L)2(MeOH)2(NO3)2] (2): Elemental analyses (%), cal-
culated: C 30.12, H 2.34, N 6.22. Found: C 30.13, H 2.36,
N 6.20. IR (KBr) (cm−1): 3851 (m), 3749 (m), 3648 (m),
3402 (m), 2926 (m), 2381 (m), 1626 (w), 1465 (w), 1383
(w), 1314 (w), 1268 (w), 1173 (m), 1012 (m), 827 (m),
690 (m), 634 (m), 442 (s).
[Dy2(L)2(MeOH)2(NO3)2] (3): Elemental analyses (%), cal-
culated: C 29.98, H 2.34, N 6.17. Found C 29.97, H 2.35,

SLOW MAGNETIC RELAXATION IN DY2 AND HO2
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N 6.17. IR (KBr) (cm−1): 3851 (m), 3749 (m), 3647 (m),
3419 (m), 2894 (s), 2359 (m), 1627 (w), 1466 (w), 1383
(w), 1314 (w), 1269 (m), 1174 (m), 1014 (m), 828 (m),
691 (m), 635 (s), 443 (s).
[Ho2(L)2(MeOH)2(NO3)2] (4): Elemental analyses (%),
calculated: C 29.87, H 2.35, N 6.14. Found: C 29.86, H
2.34, N 6.15. IR (KBr) (cm−1): 3853 (m), 3752 (m), 3650
(m), 3394 (m), 2942 (m), 2365 (m), 1628 (m), 1465 (m),
1383 (m), 1315 (m), 1268 (m), 1172 (m), 1012 (m),
827 (m), 688 (m), 635 (m), 439 (s).
2.2 | Materials and physical
measurements
All reagents used in the experiment were of analytical
reagent grade. All reagents were obtained from commer-
cial sources and used without further purification.
The Schiff base ligand, N, N0-bis(5-bromosalicylidene)eth-
ane-1,2-diamine (H2L) (Scheme 1a), was synthesized by
condensation of 5-bromosalicylaldehyde and eth-
ylenediamine in
a 2:1 aqueous ethanol solution,
according to the experimental methods mentioned in the
literature.[20] Elemental analyses (C, H, and N) were per-
formed using an Elementar Vario EL analyzer. IR spectra
were measured on a Bruker Tensor 27 spectrometer with
samples prepared as KBr disks. The magnetic properties
were measured on an MPMS SQUID VSM magnetome-
ter. Alternating current (AC) susceptibility measurements
were performed with an oscillating AC field of 2.0 Oe
and AC frequencies ranging from 1 to 1000 Hz.
2.3 | X-ray structure determinations
SCHEME 1 (a) Structure of the H2L ligand; (b) coordination
mode of the L2– ligand
The crystal data for complexes 1–4 were collected on a
Rigaku Saturn CCD diffractometer using graphite-
monochromated Cu-Kα radiation (λ = 1.54184 Å) at
293 K. Crystal field data gathering (ω scan) and
processing were carried out using the Crystal Clear Pack
(Cell Refinement, Data Simplification and Empirical
Absorption Correction).[21] All structures were solved by
direct methods and refined by full-matrix least-squares
methods of F2 using the shelxl97 package. All non-
hydrogen atoms were refined anisotropically.[22] The
positions of the rare earth metal atoms were easily deter-
mined, and the O, N, and C atoms were then determined
by the differential Fourier map. The hydrogen atoms
were introduced to calculated positions as riding on their
respective bonded atoms. Crystal data and structural
refinements are listed in Table URE S1. CCDC
(Cambridge Crystallographic Data Centre) 1904959 (1),
1904960 (2), 1904961 (3), 1904962 (4) contain all supple-
mentary crystallographic data of these four complexes.
3 | RESULTS AND DISCUSSION
3.1 | Syntheses and IR spectra
Ln (NO3)3ꢀ6H2O was reacted with H2L (Scheme 1a) and
triethylamine in a ratio of 1:1:1 in a solvent of MeOH and
MeCN to obtain a series of dinuclear lanthanide com-
plexes. As the normal temperature method does not give
us the desired product, we synthesized complexes 1–4 by
the solvothermal method. The IR spectrum of complexes
1–4 show all of the characteristic bands of the coordi-
nated ligand L2−. Because of ν(C=N) stretching, one such
protruding band appeared at about 1625 cm–1, and the
characteristic band caused by ν(C–O) appears at about
1269 cm–1. For NO3 anions, characteristic bands appear
−
around 1380 cm–1 as expected.[23] The phase purities of
the as-synthesized complexes 1–4 were demonstrated by

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ZHANG ET AL.
FIGURE 1 Molecular structure of complex
3. H atoms are omitted for clarity
X-ray powder diffraction. As shown in Figures S7–S10,
all experimental peaks are in agreement with those in the
simulated profile derived from single-crystal diffraction
data, which demonstrates the high phase purity of the
four complexes.
triangular dodecahedron geometry of D2d (as shown in
Figure 2). The intramolecular DyꢀꢀꢀDy distance is 3.816
(16) Å, suggesting the potential interaction between DyIII
ions. The shortest DyꢀꢀꢀDy distance between the adjacent
dinuclear molecules is 7.082(14) Å.
3.2 | Description of the structures
3.3 | Magnetic properties
The single-crystal X-ray diffraction studies confirmed that
complexes 1–4 are isostructural and crystallize in the tri-
clinic space group P1 (Table URE S1). The molecular
structure of complexes 1, 2, and 4 and coordination envi-
ronments for the LnIII ion in complexes 1, 2, and 4 are
shown in Figures URE S1–S6. Here, for the sake of con-
venience, the crystal structure of complex 3 is elaborated
as a representative. As shown in Figure 1, each asymmet-
ric unit of complex 3 contains two DyIII ions, two L2−
ligands, two nitrate ions, and two neutral methanol
ligands. The L2− ligand adopts the η1:η1:η1:η2:μ2 coordina-
tion mode (Scheme 1b). The two DyIII ions are bridged by
two Ophenoxo atoms (O1, O10) from two L2− ligands to
form a dimer. Each DyIII ion has an eight-coordinate
environment composed of two nitrogen atoms (N2 and
N3) and three oxygen atoms (O1, O10, and O2) from two
L2− ligands, two oxygen atoms (O3 and O4) from a
nitrate ion, and one oxygen atom (O6) from a methanol
molecule. The Dy–N bond lengths are in the range of
2.471(5)–2.932(6) Å, and the Dy–O bond distances are
2.166(11)–2.514(11) Å. The bond angles of O1–Dy1–O10,
O1–Dy10–O10 were 70.20(4)ꢁ (Table S2). These bond
lengths and angles are consistent with the reported
data.[24] To determine the local coordination environ-
ment symmetry of the eight-coordinated DyIII ions, the
continuous shape measure parameters of the DyIII cen-
ters were analyzed (Table S3) using the Shape 2.1
software,[25] which indicated a symmetry closest to a
3.3.1 | Static susceptibility
measurements
Variable-temperature (2–300 K) direct current
(DC) magnetic susceptibilities for all complexes were
measured on crystal samples under an applied DC
magnetic field of 1 kOe. After the data processing,
FIGURE 2 Coordination environment for the DyIII ion in
complex 3

SLOW MAGNETIC RELAXATION IN DY2 AND HO2
5 of 11
variable-temperature magnetic-susceptibility curves were
obtained, as shown in Figure 3. For complexes 1 and 2, at
300 K, the experimental χMT values of 15.79 and
23.53 cm3 K mol–1 are in good agreement with the
the χMT values of complexes 1–4 are drastically reduced.
Their χMT reach the deepest values of 6.93, 9.80, 11.58,
21.01 cm3 K mol–1 at 2 K, respectively. Given that
the anisotropy of GdIII is negligible, the behavior of
complex 1 is consistent with the presence of weak intra-
molecular antiferromagnetic exchange interactions.[27]
For complexes 2–4, such performance can be assigned to
antiferromagnetic coupling and/or the progressive
thermal depopulation of the excited Stark sublevels of
the LnIII ions.[28]
theoretical values of two uncoupled Gd3+ ions (8S7/2
,
S = 7/2, L = 0, g = 2, C = 15.76 cm3 K mol–1) and Tb3+
ions (7F6, S = 3, L = 3, g = 3/2, C = 23.64 cm3 K mol−1),
respectively. For complexes 3 and 4, at room tempera-
ture, the experimental χMT values are 27.30 and
27.92 cm3 K mol–1, which are slightly smaller than the
expected values of two uncoupled Dy3+ ions (6H15/2
,
The field-dependent magnetization (M) of complexes
1–4 were measured at a temperature of 2 K and the mag-
netic field strength was in the range of 0–70 KOe. As
shown in Figures 4 and S11, the saturation susceptibility
M value for the two Gd3+ ions (S = 7/2) in complex
1 was 13.78 Nβ, which is close to the expected saturation
value of 14 Nβ for two isolated Gd3+ ions. The
corresponding magnetization data can be well fitted via
the Brillouin function, as shown in Figure 4. The pres-
ence of the Brillouin function for magnetically uncoupled
GdIII ions is above the experimental magnetization curve
(S = 7/2, g = 2.0), further revealing the antiferromagnetic
exchange interactions in the dinuclear Gd complex.[29]
For anisotropic complexes 2–4, the field-dependent mag-
netization shows a rapid increase at low field followed by
a slow linear increase at high field with maxima of 9.62
Nβ (2), 10.39 Nβ (3), and 11.16 Nβ (4), under 7 T without
reaching the corresponding theoretical saturation values
(18 Nβ for two Tb3+ ions, 20 Nβ for two isolated Dy3+
ions, and 20 Nβ for two Ho3+ ions, respectively),[30]
which is indicative of the presence of significant mag-
netic anisotropy and/or low-lying excited states.
S = 5/2, L = 5, g = 4/3, C = 28.34 cm3 K mol−1), and Ho3
+ ions (5I8, S = 2, L = 6, g = 5/4, C = 28.14 cm3 K mol−1),
respectively.[26] Upon cooling, the χMT values of com-
plexes 1–4 gradually decrease to 14.64, 21.73,
24.77, 25.32 cm3 K mol–1 at 70 K. With further cooling,
FIGURE 3 (a) Plots of χMT versus T under a magnetic field of
1 kOe for complexes Gd2 and Ho2. (b) Plots of χMT versus T under a
magnetic field of 1 kOe for complexes Tb2 and Dy2. The red solid
line represents the result of ab initio calculations
FIGURE 4 Field dependence of magnetization at 2.0 K in the
field range of 0–70 kOe for complex 1. Brillouin functions for
magnetically uncoupled GdIII ions (red line)

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ZHANG ET AL.
3.3.2 | Dynamic magnetic properties
To further investigate the magnetic kinetics of complexes
2–4, the temperature and frequency dependence of the
AC susceptibility was measured under a zero DC field
with an alternating magnetic field of 2 Oe oscillating in
the range of 100–1000 Hz. At Hdc = 0 Oe, unfortunately,
complex 2 (Figure 5) does not exhibit frequency
dependence; instead, only complexes 3 (Figure 6) and
FIGURE 7 Temperature dependence of the out-of-phase (χ00)
AC susceptibilities of complex 4 under a zero DC field with an
oscillating field of 2 Oe in the frequency range of 100–1000 Hz
4 (Figure 7) exhibit strong temperature- and frequency-
dependent in-phase (χ') and out-of-phase (χ00) AC suscep-
tibility signals, confirming the SMM nature for complexes
3 and 4.[20] However, no χ00 signal peaks for complex
4 can be found in the available frequency range until
2.0 K, which may be attributed to the fast quantum
tunneling relaxation.
FIGURE 5 Temperature dependence of the in-phase (χ0) and
out-of-phase (χ00) AC susceptibilities of complex 2 under a zero DC
field with an oscillating field of 2 Oe in the frequency range of
100–1000 Hz
For complex 3, as shown in Figure 6, variable-
temperature χ0 and χ00 data are not only frequency depen-
dent (below 25 K) but also exhibit sharp maxima in the
ranges 14.25–23 K and 11.00–17.50 K, respectively.
Furthermore, the dynamics of the magnetization of com-
plex 3 have been studied by measuring its frequency
dependencies on the AC susceptibility under a zero DC
field. As shown in Figure 8, the in-phase (χ0) and out-of-
phase (χ00) signals of complex 3 show strong frequency
dependencies, suggesting the presence of slow magnetic
relaxation. The relaxation times (τ) extracted from the χ00
peaks for complex 3 at selected temperatures were used
to construct the Arrhenius plot[31] shown in Figure 9. For
complex 3, ln(τ) versus T–1 plots were fitted to provide an
anisotropy barrier of Ueff ≈ 47.68 K with a pre-
exponential factor (τ0) of 3.17 × 10−6 s (theoretical value
τ0 = 10−6–10−11 s).[32] Besides, the Cole–Cole plots based
on the frequency-dependent AC magnetic-susceptibility
measurement in the temperature range of 8.0–20 K
(Figure 10) reveal an asymmetrical semicircular shape.
These data can be fitted with generalized Debye model[33]
to give values of α parameter in the range of 0.09–0.23,
which indicates a relatively narrow width of relaxation
processes most likely due to a combination of quantum
FIGURE 6 Temperature dependence of the in-phase (χ0) and
out-of-phase (χ00) AC susceptibilities of complex 3 under a zero DC
field with an oscillating field of 2 Oe in the frequency range of 100–
1000 Hz

SLOW MAGNETIC RELAXATION IN DY2 AND HO2
7 of 11
FIGURE 10 Cole–Cole plot of complex 3 at temperatures
between 8.0 and 20 K. The solid lines are fits of the experimental
data using a generalized Debye model
FIGURE 8 Frequency dependence of the in-phase (χ M0) and
out-of-phase (χ M00) products under zero DC field for complex 3
COO)2]ꢀ0.5CH3OH, [Dy2(valdien)2](CF3COCHCOCF3)2],
[13a]. and [Dy2(valdien)2(NO3)2].[34b]
For complex 4, to suppress the stronger QTM, AC
measurements were performed at 2.0 K with 2000 Oe
applied DC fields in the frequency range of 100–1000 Hz
on complex 4 (Figure S12). Unfortunately, the out-of-
phase (χ00) AC susceptibility exhibits clear frequency
dependence, but still no frequency-dependence peaks
were found, which indicates that QTM is not suppressed.
Thus, profiles exclude the calculation of Ueff for complex
4. But here, it must be noted that complex 4 is a rare
phenoxo-O-bridged Ho2-Salen-type complex with slow
magnetic relaxation behavior. To the best of our knowl-
edge, only one case of a phenoxo-O-bridged Ho2 complex,
namely,
[Ho2L02(NO3)2(C2H5OH)2]0.5py,
where
H2L0 = 2-{[(2-hydroxy-3-methoxy benzyl)imino]methyl}
naphtha-len-1-ol[35] showed slow magnetic relaxation ).
FIGURE 9 Plot of ln(τ) versus T–1 fitting to the Arrhenius law
for complex 3
3.3.3 | Theoretical investigation
tunneling of magnetization (QTM) and thermally assisted
relaxation pathways.[13a, 34a]
Complete-active-space self-consistent field (CASSCF) cal-
culations on individual DyIII or HoIII fragment in com-
plexes 3 and 4 based on X-ray-determined geometries
have been carried out with MOLCAS 8.2[36] and SIN-
GLE_ANISO[37] programs. Binuclear complexes 3 and
4 with central symmetrical structure have one type of
magnetic center DyIII and HoIII ion, respectively. CAS-
SCF calculations on individual DyIII or HoIII fragment for
3 and 4 (see Figure 11 for the calculated model structures
of 3 and 4) based on single-crystal X-ray-determined
geometry have been carried out with MOLCAS 8.2[36]
program package. Each individual DyIII or HoIII fragment
To date, some Salen-type binuclear DyIII SMMs with
2
similar ligands have been reported. For comparison with
previously reported analogs, the relevant parameters are
given in Table 1. Complex 3 shows a comparatively
higher Ueff among the pure Salen-type dinuclear dyspro-
sium SMMs. Furthermore, replacing hydrogen atoms
with electron-withdrawing atoms on the terminal Salen-
type ligands could be a comparatively straightforward
way of attaining lager barriers (Ueff), such as complex 3,
[Dy2(valdien)2(ClCH2OO)2],
[Dy2(valdien)2(Cl2CH

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ZHANG ET AL.
TABLE 1 Differences in structural and magnetic parameters between complex 3 and the reported pure Salen-type Dy2 SMMs
Bond distance/Å
Molecular formula
Coordination mode
DyꢀꢀꢀDy
Ueff/K
Reference
[Dy2(L)2(MeOH)2(NO3)2
HL = N,N0-bis(5-bromosalicylidene)
ethane-1,2-diamine
N2O6
3.816
47.68
This work
[Dy(H2L)(OAc)2]2(PF6)2ꢀ4CH2Cl2 H2L = N,N0-bis(2-
hydroxy-3-methoxybenzylidene)-1,3
-propanediamine
O9
3.807
28
[26d]
[Dy(H2L)2(NO3)]2(PF6)4ꢀ4CH2Cl2ꢀ0.5C6H14 H2L1 =
O9, O10
N3O5
N3O5
N3O5
3.840
3.798
3.796
3.895
32.5
50
[26d]
[13a]
[13a]
[13a]
N,N0-bis(salicylidene)-1,3-propanediamine
[Dy2(valdien)2(ClCH2OO)2] Hvaldien = N1,N3-bis
(3-methoxysalicylidene)diethylenetriamine
[Dy2(valdien)2(Cl2CHCOO)2]ꢀ0.5CH2OH Hvaldien =
N1,N3-bis(3-methoxysalicylidene)diethylenetriamine
60
[Dy2(valdien)2(CH3COCHCOCH3]ꢀ2CH2Cl2 Hvaldien
= N1,N3-bis(3-methoxysalicylidene)
diethylenetriamine
16
[Dy2(valdien)2](CF3COCHCOCF3)2] H2valdien =
N1,N3-bis(3-methoxysalicylidene)diethylenetriamine
N3O5
N3O5
3.835
3.768
110
76
[13a]
[34b]
[Dy2(valdien)2(NO3)2 Hvaldien = N1,N3-bis
(3-methoxysalicylidene)diethylenetriamine
in 3 and 4 was calculated by keeping the experimentally
determined structure of the corresponding compound
while replacing the neighboring DyIII or HoIII ion by dia-
magnetic LuIII. The energy levels (cm−1), g (gx, gy, gz) ten-
sors, and the predominant mJ values of the lowest eight
or nine spin–orbit states of individual DyIII or HoIII frag-
ment for 3 and 4 are presented in Table S4. The mJ com-
ponents for the lowest two Kramers doublets (KDs) of
individual DyIII or HoIII fragment for 3 and 4 are shown
in Table S5, where the ground KD for DyIII is mostly
composed by mJ = 15/2, and its first excited state is
mostly composed by mJ = 13. However, the ground
non-KD for HoIII is mostly composed by mJ = 8, and
the first excited state is composed by several mJ states
severely. The corresponding magnetization blocking bar-
riers of individual DyIII or HoIII fragment for 3 and 4 are
shown in Figure 12, where the transversal magnetic
moment in the ground state of individual DyIII fragment
in 3 is 0.24 × 10−2 μB, and thus the QTM in its ground
KD could be suppressed at low temperature. The energy
differences between the ground and the first excited state
for individual DyIII fragment of complex 3 is 189 cm−1
(271.9 K), which is larger than its experimental energy
barrier of 47.68 K, which can be correlated with the QTM
effect and other relaxation process.[38] However, the
transversal magnetic moment in the ground state of indi-
vidual HoIII fragment for 4 is 0.27 μB, thus allowing a fast
QTM in its ground state, which is likely to show weak
SMM characteristics as also visible in our AC-
susceptibility studies (absence of clear maxima in the χ00
peak for complex 4). Although their magnetic anisot-
ropies mainly come from individual DyIII or HoIII ions,
the DyIII–DyIII or HoIII–HoIII interactions have some cer-
tain influence on their slow magnetic relaxation
processes.
The calculated ground gz values of individual DyIII or
HoIII fragment in complexes 3 and 4 are close to 20, which
shows that the DyIII–DyIII or HoIII–HoIII exchange inter-
action can be approximately regarded as the Ising type.
The program POLY_ANISO[37] was used to fit the mag-
netic susceptibilities of complexes 3 and 4 using the total
parameters from Table 2. For 1, the magnetic interaction
between two GdIII ions is isotropic. The program POLY_-
ANISO was also used to fit the total magnetic interaction
TABLE 2 Fitted exchange couplings J%exch, the calculated
dipole–dipole interactions J%dip, and the total constants J%
total
between magnetic center ions in 3 and 4 (cm−1), and the total
isotropic magnetic interaction parameter of Jtotal in 1 (cm−1). The
intermolecular interactions zJ´ of 1, 3, and 4 were fitted to −0.05,
−0.09, and − 0.01 cm−1, respectively
1
3
4
J%
J%
J%
0.85
0.73
dip
−3.00
−2.15
−2.08
−1.35
exch
total
Jtotal
−0.25

SLOW MAGNETIC RELAXATION IN DY2 AND HO2
9 of 11
parameter of J through comparison of the computed and
measured magnetic susceptibility.
modifying ligands and to investigate the magneto–
structural relationship in depth.
The parameters from Table 2were calculated with
respect to the pseudospin S% = 1/2 of the DyIII ion,
ACKNOWLEDGEMENTS
This work was supported by the Foundation for Young
Dy
the pseudospin S% = 1/2 of the HoIII ion, and the
Ho
spin SGd = 7/2 of the GdIII ion. For 3 and 4, the dipo-
Talents
in
College
of
Anhui
Province
lar magnetic coupling constants J%
were calculated
(no. gxyqZD2016412), the Anhui Provincial Innovation
Team of Design and Application of Advanced Energetic
Materials (KJ2015TD003).
dip
exactly, whereas the exchange coupling constants J%
exch
were fitted through comparison of the computed and
measured magnetic susceptibilities using the POLY_-
ANISO program. For 1, the total isotropic magnetic
interaction parameter of Jtotal was included to fit the
magnetic susceptibilities. The calculated and experimen-
tal χMT versus T plots of complexes 1, 3, and 4 are
shown in Figure 3, where their fits are close to the
experiments.[39] From Table 2, the DyIII–DyIII and
HoIII–HoIII interactions in 3 and 4 within the Lines
model[40] are both antiferromagnetic. We gave the
exchange energies, the energy differences between each
exchange doublet Δt, and the main values of the gz for
the lowest two exchange doublets of 1, 3, and 4 in
Table S6, where the gz values of the ground exchange
states for 1, 3, and 4 are all close to 0.000, which con-
firms that the GdIII–GdIII, DyIII–DyIII, and HoIII–HoIII
interactions for 1, 3, and 4 are all antiferromagnetic. In
addition, based on structural data, the shortest inter-
molecular LnIII–LnIII distance is about 7.0690(18),
7.0824(14), 7.0984(10) Å for complexes 1, 3, and 4,
respectively, which may produce intermolecular dipole–
dipole interactions. The main magnetic axes on two
DyIII or HoIII ions of 3 and 4 indicated in Figure S13
are both antiparallel ).
FUNDING INFORMATION
Foundation for Young Talents in College of Anhui Prov-
ince (no. gxyqZD2016412), Anhui Provincial Innovation
Team of Design and Application of Advanced Energetic
Materials (KJ2015TD003).
ORCID
Pei-Pei Yang https://orcid.org/0000-0002-7095-3195
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How to cite this article: Zhang L, Yang P-P,
Li L-F, Hu Y-Y, Gao Y, Tao J. A series of Salen-
type homodinuclear lanthanide complexes and
their slow magnetic relaxation in Dy2 and Ho2.
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Appl Organometal Chem. 2020;e5331. https://doi.
org/10.1002/aoc.5331
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