Cis-trans isomerism modulates the magnetic relaxation of dysprosium single-molecule magnets

Article abstract of DOI:10.1039/c5sc04510j

Geometry and magnetic relaxation modulations in a series of mononuclear dysprosium complexes, [DyLz₂(o-vanilin)₂]·X·solvent (Lz = 6-pyridin-2-yl-[1,3,5]triazine-2,4-diamine; X = Br^- (1), NO₃^- (2), CF₃SO₃^- (3)), were realized by changing the nature of the counter-Anion. The Dy^III ions in all complexes are eight-coordinate and in approximate D_4d symmetry environments. The magnetic relaxation and anisotropy of these complexes were systematically investigated, both experimentally and from ab initio calculations. All complexes exhibit excellent single-molecule magnetic behavior. Remarkably, magneto-structural studies show that the rotation of the coordinating plane of the square-Antiprismatic environment in complex 2 induces a magnetic relaxation path through higher excited states, yielding a high anisotropy barrier of 615 K (696 K for a diluted sample). Additionally, obvious opening of the hysteresis loop is observed up to 7 K, which is the highest blocking temperature ever reported for dysprosium single-molecule magnets.

Full text of DOI:10.1039/c5sc04510j

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Cis–trans isomerism modulates the magnetic
relaxation of dysprosium single-molecule
Cite this: DOI: 10.1039/c5sc04510j
magnets†
ac
b
a
a
a
Jianfeng Wu, Julie Jung, Peng Zhang, Haixia Zhang, Jinkui Tang* and Boris Le
b
Guennic*
Geometry and magnetic relaxation modulations in a series of mononuclear dysprosium complexes,
]$X$solvent (Lz ¼ 6-pyridin-2-yl-[1,3,5]triazine-2,4-diamine; X ¼ Brꢀ (1), NO
(2),
2
(o-vanilin)
3ꢀ
2
3ꢀ
[
DyLz
III
CF
3
SO (3)), were realized by changing the nature of the counter-anion. The Dy ions in all complexes
are eight-coordinate and in approximate D4d symmetry environments. The magnetic relaxation and
anisotropy of these complexes were systematically investigated, both experimentally and from ab initio
calculations. All complexes exhibit excellent single-molecule magnetic behavior. Remarkably, magneto-
structural studies show that the rotation of the coordinating plane of the square-antiprismatic
environment in complex 2 induces a magnetic relaxation path through higher excited states, yielding
a high anisotropy barrier of 615 K (696 K for a diluted sample). Additionally, obvious opening of the
hysteresis loop is observed up to 7 K, which is the highest blocking temperature ever reported for
dysprosium single-molecule magnets.
Received 24th November 2015
Accepted 10th February 2016
DOI: 10.1039/c5sc04510j
www.rsc.org/chemicalscience
magnetization (QTM) concomitant with the intricate coordina-
Introduction
7
tion geometry. Recently, a NCN-pincer ligand dysprosium
8
In the quest of the ultimate miniaturization of magnets, fasci- complex and an equatorially coordinated erbium mononuclear
9
nating investigations have converged on lanthanide single- single-molecule magnet, reported by our group, represent
1
molecule magnets (SMMs) with promising applications in high- successful examples that simplifying the ligand eld facilitates
density information storage and molecular spintronics. Recent relaxations via higher excited states, thus opening new perspec-
2
results have shown the potential of the element dysprosium in tives for enhancing the anisotropy of excited doublets through
3
a,3c
the design of SMMs for future applications due to its doubly lowering the coordination number.
degenerate ground state and large intrinsic anisotropy. However,
3
Aside from low-coordinate systems, high symmetry cases,
large effective barriers of SMMs have been identied in terbium- such as D4d and D5h, have been widely investigated pre-
4
4a,10
phthalocyanine derivatives and an utmost blocking temperature viously,
in which QTM can be suppressed by tuning the local
3ꢀ
5
10i
was detected in a N
gesting terbium to be potentially superior to dysprosium in the ported to date, DyM
2
radical-bridged terbium complex, sug- symmetry. Among the D5h symmetry dysprosium SMMs re-
2
(M ¼ Zn, Fe) complexes represent the most
design of robust SMMs with high effective barriers and blocking successful enhancement of the magnetic blocking barrier, in
temperatures. Although a remarkable blocking temperature of which the axial crystal eld induces large anisotropic proper-
6
10i,10j
7
K was extracted from the polymetallic Dy@Y K complex, it is ties.
However, the situation becomes more complicated for
4
2
still a formidable challenge to push the frontiers to higher
temperature regimes, due to the fast quantum tunneling of polyoxometalate
families, where the lanthanide ions possess an almost perfect
D4d symmetry dysprosium complexes. For example, in the
4a,4c
10a
and phthalocyanine sandwich-type
11
D4d coordination environment, the SMM properties of
dysprosium complexes are less prominent when compared with
their terbium and erbium analogues, while the distorted D4d
coordination polyhedron in the b-diketonate series gives rise to
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
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Universit ´e de Rennes 1,
2
63 Avenue du General Leclerc, 35042 Rennes Cedex, France. E-mail: boris. strong Ising ground states, leading to signicant relaxation
leguennic@univ-rennes1.fr
10b,10g
blockages for dysprosium derivatives.
It seems that not
c
University of Chinese Academy of Sciences, Beijing, 100049, P. R. China
only the coordination geometry, but also the coordination
environment, such as the type of coordinating atoms, the
identity and nature of the ligand and cis–trans isomerism, could
inuence the relaxation behavior.
†
Electronic supplementary information (ESI) available: Experimental details and
additional gures (Tables S1–S6 and Fig. S1–S25). CCDC 1041137–1041139. For
ESI and crystallographic data in CIF or other electronic format see DOI:
1
0.1039/c5sc04510j
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3
.61%; N, 18.69%. For the synthesis of 2 and 3, Dy(NO
3 3 2
) $5H O
and Dy(CF SO ) $6H O were used to replace DyBr $6H O with
3
3 3
2
3
2
methanol (15 ml) as the solvent. Anal. calcd for 2 (C H DyN O ,
32
30
13 9
MW ¼ 903.19): C, 42.52%; H, 3.32%; N, 20.15%. Found: C,
1.93%; H, 3.51%; N, 20.26%. Anal. calcd for 3 (C33 30DyF
11S, MW ¼ 1022.26): C, 38.73%; H, 2.93%; N, 16.43%; S, 3.13%.
Found: C, 38.55%; H, 3.06%; N, 15.91%; S, 3.20%. Doping of
complex 2 was performed by adding Dy(NO $5H O and
Y(NO $5H O together (with ratios of 1 : 1, 1 : 20 and 1 : 50) in
the reaction. ICP OES analysis found the components of [Dy0.671
4
O
H
3 12
N -
3
)
3
2
Scheme 1 Schematic drawing of complexes 1, 3 (left) and 2 (right).
3 3
)
2
-
Y0.329Lz (o-vanilin) ]$NO , [Dy0.076Y0.924Lz (o-vanilin) ]$NO and
2
2
3
2
2
3
With this in mind, we intend to probe the effect of coordina- [Dy0.024
Y
0.976Lz
(o-vanilin)
]$NO
corresponding to the 2 : 1, 1 : 12
2
2
3
tion environments on the relaxation dynamics of lanthanide and 1 : 39 samples.
SMMs. Herein, a series of mononuclear dysprosium complexes of
the formula [DyLz
1,3,5]triazine-2,4-diamine; X ¼ Br (1), NO
with the metal ions in a distorted D coordination environment
2
(o-vanilin)
2
]$X$solvent (Lz ¼ 6-pyridin-2-yl- Crystallography
ꢀ ꢀ ꢀ
[
3
(2), CF
3
SO
3
(3)),
Single-crystal X-ray data of the title complexes were collected on
4
d
a Bruker Apex II CCD diffractometer equipped with graphite
(
Scheme 1), were synthesized and structurally and magnetically
˚
monochromated Mo-Ka radiation (l ¼ 0.71073 A) at 293(2) K.
characterized. Ab initio calculations were also performed in order
to rationalize the magnetic behavior of the above-mentioned
complexes. The change of the counter-anion results in great
differences in the coordination environment and dramatically
alters the relaxation behavior. Among these stable and simple
complexes, complex 2 exhibits slow magnetic relaxation at
temperatures approaching 50 K and the thermal energy barrier for
the reversal of magnetization reaches 696 K, which is the largest
observed yet for mononuclear dysprosium SMMs. Furthermore,
the opening of the hysteresis loop up to 7 K using the sweep rate
accessible with a conventional magnetometer is also remarkable.
The structures were solved by direct methods and rened by
2
13
full-matrix least-squares methods on F using SHELXTL-97. All
non-hydrogen atoms were determined from the difference
Fourier maps and rened anisotropically. Hydrogen atoms were
introduced in calculated positions and rened with xed
geometry with respect to their carrier atoms. Crystallographic
data are listed in Table S1.†
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 magnetic eld of 1000 Oe. The dynamics of the
magnetization were investigated from the ac susceptibility
measurements in the zero static elds and a 3.0 Oe ac oscil-
lating eld. Diamagnetic corrections were made with the Pas-
Experimental
All chemicals and solvents were commercially obtained and used
as received without any further purication. IR spectra were
recorded with a Perkin-Elmer Fourier transform infrared spec-
trophotometer. Elemental analysis for C, H, N and S was carried
out on a Perkin-Elmer 2400 analyzer. All doped samples were
analyzed using Inductively Coupled Plasma Optical Emission
Spectrometry (ICP OES). The ligand Lz (2,4-diamino-6-pyridyl-
,3,5-triazine) was prepared according to a previously published
method under ambient conditions. A mixture of 2-cyanopyr-
idine (0.1 mol), dicyandiamide (0.125 mol), potassium hydroxide
14
cal's constants for all the constituent atoms, as well as the
contributions of the sample holder.
Computational details
1
12
Wavefunction-based calculations were carried out on the X-ray
structures of 1, 2 and 3 using the SA-CASSCF/RASSI-SO
15
approach as implemented in MOLCAS 8.0, in which the rela-
tivistic effects are treated by means of the Douglas–Kroll
Hamiltonian in a two step scheme. First, the scalar terms are
included in the basis-set generation and are used to determine
the spin-free states in the Complete Active Space Self-Consistent
(
4
0.02 mol) and 1,2-dimethoxyethane (0.62 mol) was reuxed for
h. Aer cooling, the contents were poured into water, the
precipitate was collected by ltration and dried under vacuum. IR
ꢀ
1
(cm ): 3462 (m), 3393 (m), 3273 (b), 3130 (b), 1611 (m), 1536 (m),
1
394 (s), 1253 (m), 993 (m), 830 (m), 792 (s), 687 (w).
16
Field (CASSCF) method. Next, spin–orbit coupling is added
within the Restricted Active Space State-Interaction (RASSI-SO)
method, in which the spin-free states serve as basis states. The
Syntheses of complexes 1–3
17
All complexes were prepared by similar procedures; therefore only resulting energies and wave functions are nally used to
the synthesis of complex 1 is described here in detail. The reaction compute the magnetic properties (i.e. magnetization and 2K-
of DyBr $6H O (0.15 mmol) with Lz (0.1 mmol) and o-vanilin magnetic susceptibility curves, anisotropy tensors of the low-
3
2
(0.1 mmol) in the presence of triethylamine (0.1 mmol), with 5 : 5 energy states of the system, as well as the associated wave
ml methanol/acetonitrile as the media, produced yellow crystals of functions in term of M eigenstates) using the pseudo-spin S ¼
J
1
8
1
9
aer 7 days. Anal. calcd for 1 (C34
H
34BrDyN13
O
6.5, MW ¼ 1/2 approximation as dened in the SINGLE_ANISO routine.
71.15): C, 42.01%; H, 3.50%; N, 18.74%. Found: C, 42.12%; H, The Cholesky decomposition is used when computing
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19
bielectronic integrals. The active space of the CASSCF calcu- ligand rotation and considerable distortion from the D4d
III
lations consisted of the nine 4f electrons of the Dy ion span- symmetry, which is most likely induced by the change of
ning the seven 4f orbitals, i.e. CAS(9,7)SCF. The state-averaged counter-anions. In order to evaluate these differences, the
CASSCF calculations were performed for all the 21 sextet roots, a angle between the pseudo S axis and the Dy–L directions, the
8
all the 224 quadruplet roots and 300 out of the 490 doublet F space angle between the two Lz ligands, as well as the q angle
roots, due to soware limitations. In the RASSI-SO calculation, between the upper and lower mean planes, were investigated
the 21 sextet roots were allowed to mix through spin–orbit (Fig. 2 and S4†) and the values are listed in Table S3.† A rela-
ꢁ
coupling with the rst 128 quadruplet roots and the rst 107 tively small a angle is found in 2 (56.5 ) with respect to 1 and 3
ꢁ
doublet roots. All atoms were described by ANO-type basis sets (57.0 in both systems), indicating an axial extension of the
from the ANO-RCC library of MOLCAS. The following contrac- coordinating environment in 2. This can be explained by the
III
ꢀ
tions were used: [8s7p4d3f2g1h] for the Dy ion, [4s3p2d] for insertion of the NO3 counter-anion between the coordination
the four N and the four O atoms of the rst coordination sphere, planes in 2, while in 1 and 3 the counter-anions reside outside
[
[
3s2p1d] for the remaining N and O atoms and all C atoms, and these planes and relatively far from the molecular unit. For the
ꢁ
ꢁ
2s] for the H atoms.
F angle, small values are found in 1 and 3 (53.8 and 47.1 ,
respectively) and are characteristic of the two Lz ligands being
in the cis position relative to each other, while the much larger
ꢁ
value found in 2 (140.9 ) is characteristic of the two Lz ligands
Results and discussion
being in the trans position (Fig. 1). Finally, the q angles for 1, 2
The reaction of the dysprosium salt (0.15 mmol) with Lz (0.1
mmol) and o-vanilin (0.1 mmol) in the presence of triethyl-
amine (0.1 mmol), with 5 : 5 ml methanol/acetonitrile as the
reaction media leads to the formation of mononuclear dyspro-
sium complexes, [DyLz (o-vanilin) ]$X$solvent (Lz ¼ 6-pyridin-
ꢁ
ꢁ
ꢁ
and 3 are 6.1 , 4.8 and 2.6 , respectively, suggesting that p-
stacking interactions between the Lz ligands occur more pref-
erably in 3 than in 1, which is supported by the short inter-
˚
planar distance (2.61 A) and p-stacking distance between the
2
2
two triazinyl centers of the ligand Lz (3.56 A) found in 3
˚
ꢀ
ꢀ
ꢀ
3
2
(
-yl-[1,3,5]triazine-2,4-diamine; X ¼ Br (1), NO (2), CF SO
3
3
(Fig. S3†). With 2 being in the trans conguration, no p-stacking
can occur between the two Lz ligands due to the large F angle.
In 1 and 2, crystal packing is governed by both p-stacking
interactions between the Lz ligands of neighboring molecules,
and H-bonding between the triazine parts of the Lz ligands not
involved in p-stacking interactions. In 3, molecular units are
related only through p-stacking interactions between the Lz or
the o-vanilin ligands of neighboring molecules. The shortest
3)). The structures of 1, 2 and 3 are depicted in Fig. S1–S3.† In
III
4 4
all three complexes, the Dy ion is in a N O square-anti-
prismatic environment (Fig. 1 and Table S2†), where the four
nitrogen atoms come from the two Lz ligands, and the four
oxygen atoms come from the two o-vanilin ligands. The latter
III
ligands are arranged in planes in between which the Dy ion is
sandwiched. Each plane consists of one Lz and one o-vanilin
ligand, the relative orientation of which is mainly driven by
hydrogen bonding between the hydroxy oxygen of the o-vanilin
ligand and one hydrogen atom in the triazine part of the Lz
ligand (O/H distances are 2.07 A and 2.17 A for 1, 2.11 A and
2
˚
intermolecular Dy/Dy distances are 7.9, 7.7 and 7.5 A for 1, 2
and 3, respectively, suggesting the existence of weak dipolar
interactions. Such signicant changes to the structure upon
changing the counter-ions are rare in SMMs systems, but are
˚
˚
˚
˚
˚
.14 A for 2, and 2.05 A for 3). In all three complexes, the positive
most likely responsible for the alteration in the magnetic
ꢀ
ꢀ
charge is balanced by one counter-anion, namely Br in 1, NO
in 2 and CF SO3 in 3. Although 1, 2 and 3 have similar ligand
3
7b,7c,20
relaxation properties of these complexes
(see below).
ꢀ
3
sets, a closer look at the structural features reveals signicant
Magnetic properties
Direct-current (dc) magnetic susceptibilities were investigated
for 1–3 under a 1 kOe applied eld from 2 to 300 K (Fig. 3). The
3
ꢀ1
room-temperature c
M
T value of 14.1 cm K mol for 1 is close
3
ꢀ1
III
to the expected value of 14.17 cm K mol for the Dy single
3
ꢀ1
3
ꢀ1
M
ion. The c T values of 13.3 cm K mol and 13.1 cm K mol
for 2 and 3 are slightly smaller than the theoretical value, which
might be ascribed to the fact that the ground Russell–Saunders
multiplet, having been split by the crystal eld, is not equally
populated even at room temperature, since
increasing upon warming near room temperature. Upon
cooling, c T decreases slowly down to 4 K, 7 K and 3 K,
reaching, aer a sudden drop, values of 8.2, 5.5 and 9.3 cm K
M
c T keeps
21
M
3
ꢀ
1
mol at 2.0 K for 1, 2 and 3, respectively. This sudden drop is
22
indicative of magnetic blocking. At 1.9 K, the molar magneti-
zation (M) vs. T curves for 1–3 (Fig. S5–S7†) saturate rapidly at
about 5 m , which is consistent with a pure Ising ground state.
B
Ab initio calculations conrm the strong axiality of the ground
Fig. 1 Schematic drawing of absolute configurations with top views
(
[
right) for the cis (top) and trans (bottom) configurations of the
+
DyLz
2
(o-vanilin)
2
]
units.
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Fig. 2 The side view (top) and top view (bottom) of complexes 1 (a), 2 (b) and 3 (c) with pink, dark, blue and red spheres representing Dy, C, N, and
O, respectively; the grey planes (top) represent the upper and lower coordination planes and the F value (bottom) was defined as the space angle
between the two Lz ligands. The hydrogen atoms and solvents have been omitted for clarity.
1–1488 Hz under zero applied dc eld (Fig. S8–S10†). For 3, as
the temperature is lowered, the maximum peak in the out-of-
0
0
phase c signal is shied toward a lower frequency until 8 K,
beyond which the same frequency was maintained, conrming
the presence of the classical quantum regime (Fig. S10†).
Similar behavior was observed for 1 and 2 below 2 K and 4 K,
respectively, indicating slow relaxation of the magnetization
associated with SMM behavior. To evaluate the energy barrier,
relaxation times were extracted from the maxima of the out-of-
phase signal (Fig. S11–S13†). The Arrhenius ts yield effective
energy barriers of Ueff ¼ 221, 615 and 120 K, for 1, 2 and 3,
respectively. It is noteworthy that the energy barrier for complex
2
is the highest known for a mononuclear dysprosium-based
SMM. To reduce the dipole–dipole interactions between the
magnetic centers and slow down relaxation, diluted samples
were prepared. Magnetic dilution studies for 2 (Fig. 5) show
great enhancement of the magnetic relaxation, giving a relaxa-
tion time as long as 2.5 s. The extracted effective energy barrier
M
Fig. 3 Temperature dependence of c T products at 1 kOe, between 2
and 300 K for 1 (dark), 2 (red) and 3 (blue). Inset: plots of M–H for 1, 2
and 3 at 2 K. The solid lines correspond to ab initio calculations.
ꢀ
1
ꢀ11
reaches 696 K (484 cm ) with a s
0
¼ 5.7(5) ꢃ 10
s. The Cole–
0
0
0
states in 1–3 with large g
and 3, respectively) close to the expected value of 20.00 for tted to a generalized Debye model (Fig. S14–S16†). The values
a pure M
¼ ꢂ15/2 eigenstate. Wave-function analysis also of the a parameter are relatively large (a # 0.32, 0.34 and 0.23
conrms greater than 80% percentages of the M ¼ ꢂ15/2 for 1, 2 and 3, respectively), indicating a relatively wide distri-
Z
values (19.72, 19.80 and 19.68 for 1, 2 Cole plots of c versus c display semi-circular proles and are
24
J
J
eigenstate in 1–3.
bution of the relaxation times, and thus multiple pathways for
25
To investigate the SMM behavior of 1–3, alternating-current spin reversal. Thus, all plots were tted to multiple relaxation
26
(
ac) magnetic susceptibility measurements were also performed processes, requiring Orbach, Raman, and quantum-tunnel-
27
under zero dc elds (Fig. 4). Temperature-dependent in-phase ling processes. The obtained values of s
0
and Ueff are listed in
0
0
0
(
c ) and out-of-phase (c ) magnetic susceptibility signals at Table S4.†
1488 Hz for 1–3 exhibit peaks at 27 K, 42 K and 20 K, respec-
In order to investigate the blocking of magnetization,
tively. Upon cooling, a new tail peak appears below 2.5 K in the magnetic hysteresis measurements were performed on 1–3
0
00
c and c signals of 1 and 2, while a rapid increase is observed (Fig. 4 and S17†). The magnetic hysteresis of complex 1 displays
below 10 K for 3. This rapid increase in the low temperature a clear buttery shape hysteresis with openings up to 6 K at H s
region could be attributed to quantum tunneling effects at zero 0. Similar magnetic hysteresis was obtained for complex 3,
23
dc eld, which is very common in 4f SMMs. Frequency- though with a smaller opening (H s 0). In contrast to 1 and 3,
dependent susceptibility data were collected in the range of complex 2 displays distinct buttery shape hysteresis. Herein,
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0
Fig. 4 Temperature-dependent out-of-phase (c ) magnetic susceptibilities (top) and magnetic hysteresis (bottom) for 1 (a), 2 (b) and 3 (c) at
indicated temperatures.
groups results in systems with highly uniaxial magnetic
10f
anisotropies and high thermal barriers. Therefore, the appli-
cation of large aromatic aldehyde (o-vanilin) and triazine
analogues (Lz) is expected to signicantly inuence the energy
spectrum and magnetic anisotropy of the low-lying states of
III
Dy , and thus lead to efficient Dy-based SMMs, which is indeed
the case here. However, the different environment geometries
and subsequent various magnetic behaviors induced by the
change of counter-anions in our system with the same set of
ligands are remarkable. Notably, the energy barrier observed in
complex 2 is extraordinary larger than those in 1 and 3, which
might be ascribed to the rotation of the coordinating plane of
the square-antiprismatic environment.
ꢀ1
Fig. 5 Plots of s vs. T at Hdc ¼ 0 Oe for diluted samples with Dy : Y
ratios of 1 : 0, 2 : 1 and 1 : 12.2.
the hysteresis loops remain open until 7 K with a much larger
opening near zero-eld. More interestingly, non-coincidence
was observed in the M vs. H curves at H ¼ 0 (Fig. 4 inset), sug-
gesting a potential remnant magnetization and coercive eld. In
order to enlarge the opening gap, magnetic hysteresis
measurements were also conducted on the diluted samples. As
expected, signicant improvements were observed on the
diluted sample with a Dy : Y ratio of 1 : 39, with a clear opening
now observed up to 7 K at H ¼ 0, and remnant magnetization
and coercive eld of 0.75 m
Fig. 6 and S18–S22†).
All complexes presented herein show large energy barriers,
B
and 0.46 T, respectively, at 2.0 K
(
which is rare among the dysprosium-SMM family, and is most
III
likely due to the unique D environment at the Dy center, as
4
d
Fig. 6 Magnetic hysteresis for diluted sample of 2 (Dy : Y ¼ 1 : 39) with
28
III
well documented for b-diketonate Dy derivatives (Table S5†).
clear opening of the hysteresis loop under sweep rate accessible with
Additionally, it is known that the use of large aromatic auxiliary a conventional magnetometer. Inset: zoomed in hysteresis loop.
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To investigate this trend further, the transition moments
6
between all Kramer doublets from the H
ground multiplet
5/2
1
III
of Dy are computed from ab initio for all complexes (Fig. 7, 8
and S23†). In 1, the most probable pathway for relaxation is
found to go, at least, through the 3rd excited state, reaching an
ꢀ
1
energy barrier of approximately 430 cm (620 K). This value is
much higher than that obtained from ac measurements (221 K),
but it has to be kept in mind that this kind of calculation does
not account for all possible relaxation mechanisms (in partic-
ular, indirect mechanisms are not accounted for), and relies on
3a
several approximations. However, the latter pathway is sup-
ported by the magnetic anisotropy features of the low-lying
excited states. Indeed, the ground state has strongly axial
magnetic anisotropy with zero transversal components (Table
S6†). The same goes for the 1st and 2nd excited states, for which
the magnetic anisotropy is strongly axial (very small transversal
components), with small deviations in the orientation of the
associated magnetic easy-axis (Table S6†). On the contrary, the
3rd and higher excited states have magnetic anisotropies with
large transversal components, inducing quantum tunneling,
and thus short-cutting the direct relaxation process. In 3, the
situation is almost the same, except that large transversal
components already appear at the 2nd excited state (Table S6†),
through which calculations showed a non-zero probability of
transition. The associated energy barrier is approximately 300
ꢀ
1
cm (430 K). Here again, the computed value is much higher
than that from ac measurements (120 K), but still smaller than
that of 1, which is in good agreement with the experimental
tendency. Finally, for 2, calculations evidence a relaxation
pathway going through the 3rd excited state, leading to an
ꢀ
1
energy barrier of approx. 600 cm (860 K). This pathway is
supported by the high axiality of the magnetic anisotropy of the
three lower states and the small angular deviation between the
associated magnetic easy-axis directions, while for the 3rd
excited state, the transversal components become very large,
with a large deviation to the direction of the ground state
Fig. 8 The magnetization blocking barriers and relaxation pathways
with highest probability in complexes 1 (a), 2 (b) and 3 (c).
easy-axis (Table S6†). The tendency with respect to the values
obtained from ac measurements is respected for the whole
series, since the energy barrier in 2 is much larger than those of
1
and 3. In the end, it appears that magnetic relaxation goes
more or less through the same states (i.e. the 2nd or 3rd excited
states) in all complexes, suggesting that the main factor
responsible for the difference in energy barriers in 1–3 is the
6
total energy splitting of the H
ground multiplet, which itself
5/2
1
depends on the structural features of each complex, and most
likely on the cis or trans conguration of the Lz ligands. Indeed,
in the cis conguration, contrary to the trans conguration, p-
stacking interactions are operative and contribute towards
6
reducing the energy splitting of the H15/2 ground multiplet.
This explains why the splitting is much larger in 2 (trans
conguration) than in 1 and 3 (cis conguration). In more
detail, this also explains why the energy splitting of 1 is larger
than that of 3, since p-stacking interactions are much more
effective in 3 than in 1, thus having a larger stabilizing effect.
Fig. 7 Ab initio magnetic easy-axes (in various orientations) of the
ground states of 1 (top), 2 (middle) and 3 (bottom).
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Additionally, the axial extension in 2 might also, to some extent,
be responsible for the associated larger splitting, as well as the
exact C2 symmetry held by 3 (Fig. S24 and S25†) might be
responsible for further stabilization of its energy splitting with
respect to 1.
and R. Sessoli, J. Mater. Chem. C, 2013, 1, 2935–2942; (c)
C. R. Ganivet, B. Ballesteros, G. de la Torre,
J. M. Clemente-Juan, E. Coronado and T. Torres, Chem.–
Eur. J., 2013, 19, 1457–1465.
5 (a) J. D. Rinehart, M. Fang, W. J. Evans and J. R. Long, Nat.
Chem., 2011, 3, 538–542; (b) J. D. Rinehart, M. Fang,
W. J. Evans and J. R. Long, J. Am. Chem. Soc., 2011, 133,
Conclusions
14236–14239.
In conclusion, we have structurally isolated three dysprosium
6 (a) R. J. Blagg, L. Ungur, F. Tuna, J. Speak, P. Comar,
D. Collison, W. Wernsdorfer, E. J. L. McInnes,
L. F. Chibotaru and R. E. P. Winpenny, Nat. Chem., 2013, 5,
673–678; (b) R. J. Blagg, C. A. Muryn, E. J. L. McInnes,
F. Tuna and R. E. P. Winpenny, Angew. Chem., Int. Ed.,
2011, 50, 6530–6653.
7 (a) W. Wernsdorfer and R. Sessoli, Science, 1999, 284, 133–
135; (b) G. Cucinotta, M. Perfetti, J. Luzon, M. Etienne,
P.-E. Car, A. Caneschi, G. Calvez, K. Bernot and R. Sessoli,
Angew. Chem., Int. Ed., 2012, 51, 1606–1610; (c)
M. Murugesu, Nat. Chem., 2012, 4, 347–348.
8 Y.-N. Guo, L. Ungur, G. E. Granroth, A. K. Powell, C. Wu,
S. E. Nagler, J. Tang, L. F. Chibotaru and D. Cui, Sci. Rep.,
2014, 4, 5471.
9 P. Zhang, L. Zhang, C. Wang, S. Xue, S.-Y. Lin and J. Tang,
J. Am. Chem. Soc., 2014, 136, 4484–4487.
SMMs with similar square-antiprismatic environments (D4d
)
but discriminative counter-anions. The distortions in the D4d
symmetry environment led to large energy gaps between the
ground and rst excited states, and strong axial anisotropies for
complexes 1–3. Upon changing the counter-anion, signicant
differences in the structure and thus magnetic behavior were
observed: for instance, when going from 3 to 1, the p–p stacking
interactions between the Lz ligands lead to axial constriction in
the environment geometry, from which fast quantum tunneling
arises because the symmetry is lowered; proper rotation of one
coordinating plane, as well as relative axial extension in
complex 2 with respect to complex 3, allows magnetic relaxation
to pass through higher excited states, due to an increase in
molecular symmetry. Ab initio calculations substantiate the
diversity of the magnetic behaviors in complexes 1–3 and
suggest that the ground states are almost isolated, magnetically 10 (a) M. A. AlDamen, J. M. Clemente-Juan, E. Coronado,
speaking. Notably, the efficient magnetic relaxation pathways of
complex 2 probably go through the fourth and h Kramer
doublet states. This conforms a high anisotropy barrier of 696 K
C. Mart ´ı -Gastaldo and A. Gaita-Ari n˜ o, J. Am. Chem. Soc.,
2008, 130, 8874–8875; (b) S.-D. Jiang, B.-W. Wang, G. Su,
Z.-M. Wang and S. Gao, Angew. Chem., Int. Ed., 2010, 49,
7448–7451; (c) D.-P. Li, T.-W. Wang, C.-H. Li, D.-S. Liu,
Y.-Z. Li and X.-Z. You, Chem. Commun., 2010, 46, 2929–
2931; (d) Y. Bi, Y.-N. Guo, L. Zhao, Y. Guo, S.-Y. Lin,
S.-D. Jiang, J. Tang, B.-W. Wang and S. Gao, Chem.–Eur.
J., 2011, 17, 12476–12481; (e) G. J. Chen, C. Y. Gao,
J. L. Tian, J. Tang, W. Gu, X. Liu, S. P. Yan, D. Z. Liao and
P. Cheng, Dalton Trans., 2011, 40, 5579–5583; (f)
G.-J. Chen, Y.-N. Guo, J.-L. Tian, J. Tang, W. Gu, X. Liu,
S.-P. Yan, P. Cheng and D.-Z. Liao, Chem.–Eur. J., 2012,
18, 2484–2487; (g) T. T. da Cunha, J. Jung, M.-E. Boulon,
G. Campo, F. Pointillart, C. L. M. Pereira, B. Le Guennic,
O. Cador, K. Bernot, F. Pineider, S. Golhen and
L. Ouahab, J. Am. Chem. Soc., 2013, 135, 16332–16335; (h)
Y.-N. Guo, G.-F. Xu, W. Wernsdorfer, L. Ungur, Y. Guo,
J. Tang, H.-J. Zhang, L. F. Chibotaru and A. K. Powell, J.
Am. Chem. Soc., 2011, 133, 11948–11951; (i) J.-L. Liu,
Y.-C. Chen, Y.-Z. Zheng, W.-Q. Lin, L. Ungur,
W. Wernsdorfer, L. F. Chibotaru and M.-L. Tong, Chem.
Sci., 2013, 4, 3310–3316; (j) J.-L. Liu, J.-Y. Wu, Y.-C. Chen,
V. Mereacre, A. K. Powell, L. Ungur, L. F. Chibotaru,
X.-M. Chen and M.-L. Tong, Angew. Chem., Int. Ed., 2014,
53, 12966–12970.
ꢀ
1
(484 cm ) and magnetic blocking up to 7 K. This work offers
a new way to modulate the geometries of lanthanides in order to
facilitate magnetic relaxation climbing up to higher energy
levels.
Acknowledgements
We thank the National Natural Science Foundation of China
(

Grants 21525103, 21371166, 21521092 and 21331003) for
nancial support. B. L. G. and J. J. thank the French GENCI-
CINES center for high-performance computing resources
project x2015080649).
(
Notes and references
1
R. Sessoli, D. Gatteschi, A. Caneschi and M. A. Novak, Nature,
993, 365, 141–143.
(a) M. N. Leuenberger and D. Loss, Nature, 2001, 410, 789–
1
2
7
2
93; (b) A. Dei and D. Gatteschi, Angew. Chem., Int. Ed.,
011, 50, 11852–11858; (c) D. Gatteschi and R. Sessoli,
Angew. Chem., Int. Ed., 2003, 42, 268–297; (d) D. Gatteschi,
Adv. Mater., 1994, 6, 635–645.
3
4
(a) L. Ungur and L. F. Chibotaru, Phys. Chem. Chem. Phys., 11 (a) J. M. Clemente-Juan, E. Coronado and A. Gaita-Arino,
2
3
011, 13, 20086–20090; (b) D. Gatteschi, Nat. Chem., 2011,
, 830–830; (c) N. F. Chilton, C. Goodwin, D. P. Mills and
Chem. Soc. Rev., 2012, 41, 7464–7478; (b) L. Sorace,
C. Benelli and D. Gatteschi, Chem. Soc. Rev., 2011, 40,
3092–3104.
R. E. P. Winpenny, Chem. Commun., 2015, 51, 101–103.
(a) N. Ishikawa, M. Sugita, T. Ishikawa, S.-y. Koshihara and 12 F. H. Case and E. Ko, J. Am. Chem. Soc., 1959, 81, 905–906.
Y. Kaizu, J. Am. Chem. Soc., 2003, 125, 8694–8695; (b) 13 G. M. Sheldrick, SHELXS-97 Program for Crystal Structure
L. Malavolti, M. Mannini, P.-E. Car, G. Campo, F. Pineider
Solution, University of G ¨o ttingen, Germany, 1997.
This journal is © The Royal Society of Chemistry 2016
Chem. Sci.

View Article Online
Chemical Science
Edge Article
14 E. A. Boudreaux and L. N. Mulay, Theory and Applications of 22 K. R. Meihaus and J. R. Long, J. Am. Chem. Soc., 2013, 135,
Molecular Paramagnetism, John Wiley & Sons, New York,
976.
5 F. Aquilante, L. De Vico, N. Ferr ´e , G. Ghigo, P.- ˚a . Malmqvist,
17952–17957.
1
23 N. Ishikawa, M. Sugita and W. Wernsdorfer, Angew. Chem.,
Int. Ed., 2005, 44, 2931–2935.
1
P. Neogr ´a dy, T. B. Pedersen, M. Pito nˇ ´a k, M. Reiher, 24 (a) S. M. J. Aubin, Z. Sun, L. Pardi, J. Krzystek, K. Folting,
B. O. Roos, L. Serrano-Andr ´e s, M. Urban, V. Veryazov and
R. Lindh, J. Comput. Chem., 2010, 31, 224–247.
6 B. O. Roos, P. R. Taylor and P. E. M. Siegbahn, Chem. Phys.,
L.-C. Brunel, A. L. Rheingold, G. Christou and
D. N. Hendrickson, Inorg. Chem., 1999, 38, 5329–5340; (b)
K. S. Cole and R. H. Cole, J. Chem. Phys., 1941, 9, 341–351.
25 S. Demir, J. M. Zadrozny and J. R. Long, Chem.–Eur. J., 2014,
20, 9524–9529.
1
1
1
980, 48, 157–173.
˚
7 (a) P.-A. Malmqvist and B. O. Roos, Chem. Phys. Lett., 1989,
˚
1
55, 189–194; (b) P. A. Malmqvist, B. O. Roos and 26 (a) R. Orbach, Proc. R. Soc. London, Ser. A, 1961, 264, 485–495;
B. Schimmelpfennig, Chem. Phys. Lett., 2002, 357, 230–240.
(b) R. Orbach, Proc. R. Soc. London, Ser. A, 1961, 264, 458–484.
1
1
2
2
8 L. F. Chibotaru and L. Ungur, J. Chem. Phys., 2012, 137, 27 (a) J. M. Zadrozny, D. J. Xiao, M. Atanasov, G. J. Long,
0
64112.
F. Grandjean, F. Neese and J. R. Long, Nat. Chem., 2013, 5,
577–581; (b) K. R. Meihaus, S. G. Minasian, W. W. Lukens,
S. A. Kozimor, D. K. Shuh, T. Tyliszczak and J. R. Long, J.
Am. Chem. Soc., 2014, 136, 6056–6068.
˚
9 F. Aquilante, P.-A. Malmqvist, T. B. Pedersen, A. Ghosh and
B. O. Roos, J. Chem. Theory Comput., 2008, 4, 694–702.
0 F. Habib, G. Brunet, V. Vieru, I. Korobkov, L. F. Chibotaru
and M. Murugesu, J. Am. Chem. Soc., 2013, 135, 13242–13245. 28 F. Pointillart, K. Bernot, S. Golhen, B. Le Guennic,
1 S.-S. Liu, L. Xu, S.-D. Jiang, Y.-Q. Zhang, Y.-S. Meng, Z. Wang,
B.-W. Wang, W.-X. Zhang, Z. Xi and S. Gao, Inorg. Chem.,
T. Guizouarn, L. Ouahab and O. Cador, Angew. Chem., Int.
Ed., 2015, 54, 1504–1507.
2015, 54, 5162–5168.
Chem. Sci.
This journal is © The Royal Society of Chemistry 2016