Heterometallic compounds: Rare linear SMMs with divalent manganese ions

Article abstract of DOI:10.1039/c4dt03713h

The reaction of Mn(OAc)₂·4H₂O and Ln(NO₃)₃·6H₂O with N-(2-aminopropyl)-2-hydroxybenzamide and salicylic aldehyde in methanol/methylene dichloride produces yellow crystals of Ln₂Mn(C₇

Full text of DOI:10.1039/c4dt03713h

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Transactions
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PAPER
III
II
III
[
Ln –Mn –Ln ] heterometallic compounds: rare
linear SMMs with divalent manganese ions†
Cite this: Dalton Trans., 2015, 44,
3430
a
a,b
a
a
b
Xiao-Lei Li,‡ Fan-Yong Min,‡ Chao Wang, Shuang-Yan Lin, Zhiliang Liu* and
a
Jinkui Tang*
The reaction of Mn(OAc)
salicylic aldehyde in methanol/methylene dichloride produces yellow crystals of Ln
Gd (1), Tb (2), Dy (3), Ho (4) and Er (5)), in the presence of triethylamine. Three metal ions are connected by
2
·4H
2
O and Ln(NO
3
)
3
2
·6H O with N-(2-aminopropyl)-2-hydroxybenzamide and
2
7 5 2 8
Mn(C H O ) (Ln =
III
II
III
2
six μ -phenolate oxygen atoms of six salicylic aldehyde ligands, resulting in perfect linear [Ln –Mn –Ln ]
structures. Magnetic studies of these complexes have been performed and AC susceptibility measurements
show the presence of a temperature-dependent out-of-phase ac signal for complexes 2 and 3 indicating
III
2
II
single-molecule magnet (SMM) behavior. The Dy
Mn compound shows double relaxation pathways
III
which may originate from the single ion behavior of individual Dy ions and the weak coupling between
III
II
Received 4th December 2014,
Accepted 25th December 2014
eff
Dy and Mn ions, respectively. The U of 92.4(2) K is a relatively high value among 3d–4f SMMs. Moreover,
complexes 2 and 3 represent the first linear Mn–Ln SMMs containing only divalent manganese ions as far as
DOI: 10.1039/c4dt03713h
we know. The result suggests the positive effects of magnetic coupling to enhance their SMM behavior,
presenting a promising strategy for constructing efficient heterometallic SMMs.
www.rsc.org/dalton
5
and uniaxial anisotropy. Thus, plenty of examples of clusters
Introduction
III
incorporating Mn ions with improved properties were found,
6
The research on design, synthesis and magnetic properties of including molecules with the highest spin states and blocking
7
polynuclear clusters of paramagnetic metal ions has long been temperatures seen so far for clusters as well as the highest
8
a focus of coordination chemists due to their novel topologies barrier height. However, the empirical finding that D is inver-
1
and potential applications in high-density data storage,
sely proportional to S suggests that increasing the cluster size
2
3
quantum computation, and magnetic refrigeration, since the to merely augment the spin to very high values may not necess-
9
discovery of the very first example of a molecule displaying the arily be the best strategy to ensure SMM behavior at times.
phenomenon of SMMs, namely the Mn OAc molecule first One of such examples is [Mn ] with a S = 83/2 record ground
1
2
19
4
6
isolated by Lis. The properties of SMMs are derived from the spin state reported by Powell’s group, in which no SMM pro-
combination of a large ground-state spin (S) and a significant perties are observed because of the nearly absent magnetic an-
negative magneto-anisotropy (D) which leads to an energy isotropy. On the other hand, the SMM behavior was triggered
barrier (Ueff = S |D|) to the reorientation of the spin of the by deliberately replacing the central Mn ion with the higher
ground state. As a result, such a molecule can act as a single- anisotropic Dy ion. Thus, inspired by this prospected incor-
2
II
III
10
domain nano-magnet below a certain blocking temperature poration, in the quest for new SMMs synthetic efforts have
5
f,11
(
TB). Initially, much emphasis has been placed on incorporat- recently been focused not only on the pure 3d
and 4f clus-
1
2
13
ing the transition-metal ions into such clusters especially con- ters, but also on heterometallic 3d–4f and even 3d–5f
taining the high spin Mn ion which contributes a large spin clusters.
The first mixed 3d–4f SMM, Cu Tb , was reported in
III
14
2
2
1
3d
2004,
6 6
and the first Mn/Ln SMM was a Mn Dy complex
reported in the same year. In the subsequent several years, a
few of other Mn–Ln clusters have been reported with varied
It is worth noting that most of the Mn–Ln
SMMs reported involve the Mn ions, and high nuclearity
Mn –Ln and Mn /Mn –Ln clusters are well-known. Never-
theless, oligonuclear Mn–Ln compounds containing only Mn
a
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of
15
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China.
E-mail: tang@ciac.ac.cn
3
a,b,10,16
b
nuclearities.
College of Chemistry and Chemical Engineering, Inner Mongolia University,
III
Hohhot 010021, P. R. China
†
Electronic supplementary information (ESI) available: Table of selected bond
III
III
III
II
III
distances (Å) and angles (°) and crystal packing of complex 3, magnetic measure-
ments (Fig. S1–S14). CCDC 1019625–1019629. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c4dt03713h
II
1
6g,k,s,x,17
ions are quite rare.
Moreover, only a few linear and
quasi linear Mn -4f complexes have been reported up to now
II
‡
Both authors contributed equally to this work.
3430 | Dalton Trans., 2015, 44, 3430–3438
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and none of them shows a prominent out-of-phase ac suscepti- diffractometer with graphite-monochromated Mo-Kα radiation
1
7f,18
bility signal, typical of SMM behavior.
Actually, relatively (λ = 0.71073 Å). Data processing was accomplished with the
III
II
isotropic ions such as Cr and Mn can also be used to SAINT processing program. The structure was solved by direct
2
assemble 3d–4f clusters that behave as SMMs often with methods and refined on F by full-matrix least squares using
enhanced properties compared to the pure 3d analo- SHELXTL97. The locations of the heaviest atoms (Mn and
To date, the heterometallic complex with a Ln) were easily determined, and the O and C atoms were sub-
2
Fe Dy ] core recently reported by Tong et al. holds the sequently determined from the difference Fourier maps. The
2
2
1
3b,16r,y
gues.
II
III
[
highest barrier of 459 K to magnetization reversal for 3d–4f non-H atoms were refined anisotropically. The H atoms were
1
9
SMMs. Here we report a new family of trinuclear heterome- introduced in calculated positions and refined with fixed geo-
III
II
III
II
III
tallic Ln –Mn clusters with a Ln –Mn –Ln linear topology. metry with respect to their carrier atoms. The crystallographic
As revealed by magnetic studies, complexes 2 and 3 behave as data and refinement details are given in Table 1. CCDC
SMMs in which complex 3 displays relatively high energy bar- numbers 1019625 (1), 1019626 (2), 1019627 (3), 1019628 (4)
riers (Ueff) for 3d–4f complexes, reaching 92.4(2) K. As far as and 1019629 (5) contain the supplementary crystallographic
we know, complexes 2 and 3 are the first linear Mn–Ln SMMs data for this paper.
containing only divalent manganese ions, developing further
A similar procedure was employed to prepare complexes
the idea to enhance the SMM properties through introduction 1–5 and hence only the synthesis of complex 1 is described
of isotropic high spin 3d ions in the 3d–4f system.
here
in
detail.
N-(2-Aminopropyl)-2-hydroxybenzamide
(
0.15 mmol, 44.7 mg) was dissolved in a mixture of methanol
and methylene dichloride (1 : 2, 15 ml), and then salicylic alde-
hyde (0.15 mmol) was added to the mixture. The reaction
Experimental section
General
mixture was stirred for
0.2 mmol, 49 mg) and Dy(NO
to the above mixture successively, and finally triethylamine
0.1 mmol, 14 µL) was added. The resulting solution was
5
min. Then, Mn(OAc)
2 2
·4H O
(
3
)
3
·6H O (0.2 mmol) were added
2
All starting materials were of analytical reagent grade and were
used as received without further purification. N-(2-Amino-
propyl)-2-hydroxybenzamide was prepared according to a pro-
(
stirred for another 3 h, followed by filtration and then allowed
for slow evaporation of the solution without being perturbed.
Yellow block single crystals, suitable for X-ray diffraction analy-
sis were obtained after one week. The crystals were collected by
filtration, washed with the mother solution and dried in air.
The characterization data for these complexes are given below.
2
0
cedure reported previously
by condensation of phenyl
salicylate and 1,2-diaminopropane in 1 : 1 molar ratio and
refluxed in isopropanol for 30 min, then cooled to room temp-
erature and filtered, washed with ethyl ether to give the
product for standby. All reactions were carried out under
aerobic conditions. Elemental analysis for C, H, and N was
carried out on a Perkin-Elmer 2400 analyzer. Fourier transform
infrared (FTIR) spectra were recorded with a Vertex 70 FTIR
2 2 2
Gd Mn (1): Yield: 25 mg (9.35% based on Mn(OAc) ·4H O).
Elemental analysis calcd for C H Gd MnO : C, 50.21; H,
5
6
40
2
16
2
.99. Found: C, 49.65; H, 3.12.
Tb Mn (2): Yield: 20 mg (7.5% based on Mn(OAc)
Elemental analysis calcd for C H Tb MnO : C, 50.09; H,
spectrophotometer
(
using
the
reflectance
technique
2
2 2
·4H O).
4000–300 cm− ). Samples were prepared as KBr disks. Mag-
1
5
6
40
2
16
netic susceptibility measurements were performed in the
temperature range 2–300 K using a Quantum Design MPMS
XL-7 SQUID magnetometer equipped with a 7 T magnet. The
experimental magnetic susceptibility data are corrected for the
diamagnetism estimated from Pascal’s constants and contri-
butions of the sample holder calibration. For magnetization
experiments, the temperature was set from 2.0 to 5.0 K and the
field was varied from 0 to 7 T. Alternating-current (ac) suscepti-
bility measurements were taken of the powdered 1–5 to deter-
mine the in-phase and out-of-phase components of the
magnetic susceptibility. The data were collected by decreasing
the temperature from 30.0 to 2.0 K, without an applied exter-
nal direct-current (dc) field and a drive frequency of 3.0 Oe,
with frequencies between 1 and 1500 Hz.
2
.98. Found: C, 49.56; H, 3.12.
Dy Mn (3): Yield: 22 mg (8.15% based on Mn(OAc)
Elemental analysis calcd for C H Dy MnO : C, 49.82; H,
2
2 2
·4H O).
5
6
40
2
16
2
.96. Found: C, 49.06; H, 3.16.
Ho Mn (4): Yield: 18 mg (6.65% based on Mn(OAc)
Elemental analysis calcd for C H Ho MnO : C, 49.64; H,
2
2 2
·4H O).
2
1
5
6
40
2
16
2
.95. Found: C, 48.98; H, 3.08.
Er Mn (5): Yield: 22 mg (8.2% based on Mn(OAc)
Elemental analysis calcd for C H Er MnO : C, 49.47; H,
2
2 2
·4H O).
5
6
40
2
16
2
.94. Found: C, 49.16; H, 3.09.
Complexes 1–5 were synthesized by the reaction of
Mn(OAc) ·4H O and Ln(NO ) ·6H O with N-(2-aminopropyl)-2-
2
2
3 3
2
23
hydroxybenzamide and salicylic aldehyde
in methanol/
methylene dichloride (5 ml/15 ml), in the presence of triethyl-
amine. It is worth noting that the N-(2-aminopropyl)-2-hydro-
xybenzamide ligands, although not incorporated in the final
X-ray crystallography
Suitable single crystals with dimensions of 0.26 × 0.20 × 0.14, structure, may play a crucial role to some extent in the for-
0
0
.20 × 0.16 × 0.08, 0.14 × 0.10 × 0.08, 0.20 × 0.18 × 0.10 and mation of the heterometallic structures since complexes 1–5
.20 × 0.16 × 0.06 mm for 1–5 were selected for single-crystal cannot be obtained without it. Actually, the reaction of N-(2-
3
X-ray diffraction analysis. Crystallographic data were collected aminopropyl)-2-hydroxybenzamide
with
salicylaldehyde
at a temperature of 293 K for 1–5 on a Bruker Apex II CCD should give an imine–amide ligand able to coordinate metal
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Table 1 Crystal data and structural refinement parameters
1
2
3
4
5
Formula
C
56
H
40Gd
2
MnO16
C
56
H
40Tb
2
MnO16
C
56
H
40Dy
2
MnO16
C
56
H
40Ho
2
MnO16
56 2
C H40Er MnO16
1358.34
−
1
F
w
(g mol
)
1338.32
1341.68
1348.82
1353.68
Cryst syst
Triclinic
Triclinic
Triclinic
Triclinic
Triclinic
Space group
Cryst size (mm)
T (K)
Pˉ1
Pˉ1
Pˉ1
Pˉ1
Pˉ1
0.26 × 0.20 × 0.14
293(2)
0.20 × 0.16 × 0.08
293(2)
0.14 × 0.10 × 0.08
293(2)
0.20 × 0.18 × 0.10
293(2)
0.20 × 0.16 × 0.06
293(2)
a (Å)
b (Å)
c (Å)
α [°]
10.057(4)
11.090(4)
12.102(4)
95.619(6)
113.535(5)
93.643(6)
1223.7(8)
Yellow
10.0559(16)
11.0995(18)
12.0981(19)
95.703(3)
113.502(2)
93.670(3)
1224.2(3)
Yellow
10.0369(11)
11.0999(12)
12.0690(13)
95.808(2)
113.305(2)
93.734(2)
1220.4(2)
Yellow
10.020(3)
11.088(3)
12.067(3)
95.722(4)
113.233(4)
93.782(5)
1217.7(6)
Yellow
10.051(2)
11.113(2)
12.086(3)
95.695(4)
113.292(4)
93.752(4)
1225.7(4)
Yellow
β [°]
γ [°]
V (Å )
3
Cryst color
−
3
ρ
c
(g cm
)
1.816
3.008
657
1.820
3.186
659
1.835
3.360
661
1.846
3.549
663
1.840
3.721
665
−
1
μ (mm
F(000)
)
θ for data collection (°)
Collected reflns
Indep reflns
1.85–25.10
6313
4316
1.85–26.14
6847
4801
1.86–25.00
6201
4244
1.85–25.09
6296
4293
1.85–25.10
6313
4323
R
int
0.0375
0.0208
0.0285
0.0375
0.0347
R [I > 2σ(I)]
wR (all data)
GOF
Largest diff peak and hole (e Å−
0.0472
0.0853
0.992
0.886 and −0.942
0.0333
0.0658
1.027
0.629 and −0.629
0.0393
0.0764
1.005
0.799 and −0.539
0.0459
0.0783
1.002
1.005 and −0.757
0.0458
0.0935
1.026
0.916 and −0.809
2
3
)
2
4
ions under aerobic conditions. However, the excess of metal
ions might protonate the free amine function, thus preventing
the formation of the imine–amide ligand and eventually the
addition of a small amount of triethylamine goes in the same
way.
Results and discussion
Single crystal X-ray diffraction studies indicate that all five
compounds crystallize in the triclinic space group P ˉ1 with Z =
2
, and they are isomorphous, which can be seen from their Fig. 1 (a) The molecular structures of 3; (b) core of a Dy
2
Mn complex in
3
; (c) three planes generated by metal ions and bridging oxygens.
identical IR spectra (Fig. S2, ESI†). In the following, the mole-
cular structure of 1 will be described in detail as a representa-
tive example to illustrate the mutual structural features of the
five complexes. The partially labeled molecular structure of
complex 1 and the salicylic aldehyde ligand are presented in
Fig. 1a and Scheme 1, respectively. A summary of important
selected interatomic distances and angles of complexes 1–5
are listed in Tables S1–S5.†
Hydrogen atoms are omitted for clarity. Color scheme: blue, Dy; yellow,
Mn; red, O; black, C. Symmetry transformations used to generate equi-
valent atoms: #1 −x + 1, −y, −z + 1.
The heterometallic complex 1 contains a perfectly linear
2
+
arrayed trinuclear [Dy
2
Mn(μ
2
-O)
6
]
core (Fig. 1b), which is con-
nected by six μ -phenolate oxygen atoms as a result of only one
2
coordination mode of six singly deprotonated salicylic alde-
III
hyde ligands. The two Dy ions are located on the terminal
Scheme 1 Structure of the salicylic aldehyde ligand and its two specific
coordinate modes.
II
sides and Mn ions in the middle of the Dy
2
Mn straight line.
The symmetrical bridging atoms (O1, O1′; O3, O3′; O5, O5′)
and three metal ions generate three planes (Fig. 1c), further-
more the bridging action of the phenolate oxygen atoms shape screw propeller type configuration, in which eight salicylic
up into three successive polyhedra with collective faces aldehyde ligands coordinate with two types of modes: six
1
2
defined by O1/O3/O5 and O1′/O3′/O5′ (Fig. 2a).
μ–η :η bridging ligands and two chelating ligands.
III
As clearly viewed in Fig. 3, the overall arrangement of the
Each Dy ion is all eight coordinated by eight oxygen
ligand framework around the three metal ions shape up into a atoms from three salicylic aldehyde ligands with μ–η :η and
1
2
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Fig. 4 Temperature-dependent of the χT product under 1000 Oe (with
χ defined as the dc magnetic susceptibility equal to M/H and normalised
Fig. 2 (a) Polyhedral representation of the face sharing successive poly-
III
hedral of 3; (b) distorted square-antiprism geometry around (left) Dy
II
per [Ln Mn] unit).
ion, distorted octahedron geometry around (right) the Mn ion.
2
complexes, the χ
M
T values decrease gradually until 100 K as
the temperature decreases, and then drop rapidly at low temp-
3
−1
eratures reaching minimum values of 18.95 cm mol K,
3
−1
3
−1
3
−1
2
3.80 cm mol K, 23.58 cm mol K and 11.23 cm mol
K
at 8 K, 10 K, 9 K and 2 K for 2–5, respectively. The overall be-
havior indicates the thermal depopulation of Stark sublevels
resulting from the splitting of the free-ion ground states of
III 25–26
Ln .
Because of the presence of large spin–orbital coup-
ling from the 4f ions in compounds 2–5, fitting the data to
II
II
II
III
III
III
obtain the Mn –Mn , Mn –Ln , and Ln –Ln coupling con-
stants is not feasible.
III
II
For complex 1, which contains the isotropic Gd and Mn
Fig. 3 The linear arrangement of the metal ions in 3 and overall screw
propeller arrangement of the ligand framework around the three metal ions with no spin–orbit contribution in the ground state, the
ions.
behavior is indicative of dominant antiferromagnetic inter-
actions (AF) within the cluster, but with a non-zero spin
ground state. The isotropic Heisenberg spin Hamiltonian
one with chelating coordination mode, yielding a distorted
square-antiprism geometry (Fig. 2b, left). The Dy–O bond
lengths are in normal ranges of 2.241(4)–2.393(4) Å. The Mn
III
describing the exchange interactions within a linear [Gd –
II
III
Mn –Gd ] topology is given by eqn (1)
II
ion (Fig. 2b, right) is surrounded by six bridging phenolic
oxygens forming a distorted octahedron coordination geome-
try with Mn–O bond lengths range from 2.158(4) Å to 2.241(4)
Å. Crystal packing structures along a, b and c axis of complex 3
Ĥ ¼ ꢀ 2JGd–MnðŜGd1ŜMn þ ŜGd2ŜMnÞ ꢀ 2JGd1Gd2ŜGd1ŜGd2
þ gβHðŜGd1 þ ŜGd2 þ ŜMnÞ; gGd ¼ gMn
ð1Þ
are depicted in Fig. S1.† The shortest intermolecular Mn⋯Mn, where J
Dy⋯Dy and Dy⋯Mn distance of 10.037(6), 7.563(3) and between the Gd –Mn and Gd –Gd pairs. The fit (solid line
1 2
and J represent exchange coupling constants
III
II
III
III
.304(3) Å, respectively, does not necessarily preclude any in Fig. 4) gave J
1
= −0.35(2) cm− , J
= −0.11(3) cm , and g =
1
2
−1
9
intermolecular exchange interactions.
1.98. Therefore AF interactions dominate and lead to a global
decrease of the χ T product at high temperature. It is worth
M
III
II
noting that ferromagnetic interactions between Gd and Mn
ions were observed in previously reported Mn –Gd –Mn
Magnetic properties
II
III
II
1
8c
III
II
The temperature dependence of the dc magnetic susceptibility complex.
Although the Gd and Mn ions in both com-
for randomly oriented polycrystalline samples of 1–5 were plexes are triply bridged by phenoxo groups from the ligands,
measured in an applied magnetic field of 1 kOe over the temp- the Mn–O–Gd angles and Mn⋯Gd distances are dramatically
erature range 2–300 K. The plots of χ T versus T, where χ is different. The Mn–O–Gd angles range from 89.75–92.54° and
M
M
the molar magnetic susceptibility, are displayed in Fig. 4. The the Mn⋯Gd distance is 3.27 Å for our complex whereas those
II
II
M
observed χ T values at 300 K are 26.17, 30.93, 29.93, and for Mn –Gd–Mn complex are 92.96–94.21° and 3.334 Å,
3
−1
2
4.59 cm mol K for 2–5, respectively, which are all slightly respectively. This discrepancy is likely to affect magnetic coup-
lower than the expected value of 27.99, 32.71, 32.52, and ling between metal centers and is responsible for the different
3
−1
III
2
7.27 cm mol K in turn for uncoupled two Ln ions and magnetic properties observed. As seen from Fig. S3,† except
II
25
one Mn ion (as summarized in Table 2). For all the four for 1 which manifests a well-defined S = 9/2 spin ground state
T
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a
Table 2 Direct current magnetic data for complexes 1–5
2
S+1
b
b
b
c
Complex
Gd Mn
L
J
Expected χT at 300 K
Experimental χT at 300 K
Experimental χT at 2 K
Magnetization at 2 K and 7 T
8
7
6
5
4
2
S
F
H
I
I
7/2
20.14
27.99
32.71
32.52
27.27
19.24
26.17
30.93
29.93
24.59
11.02
17.21
24.18
26.66
11.23
13.14
13.56
14.30
17.58
13.56
Tb
Dy
2
Mn
Mn
Mn
Mn
6
2
15/2
Ho
2
8
Er
2
15/2
a
II
b
3
−1
c
B
Taking g = 2 for Mn ion. Unit: cm mol K. Unit: μ .
thermally populated below 3 K, no obvious saturation is
observed at low temperature region for 2–5, indicating a lack
of well-defined ground state.
The temperature dependence of the magnetic suscepti-
bilities for complexes 2–4 shows different thermal evolution in
the low temperature region. For compounds 2 and 3, on lower-
ing the temperature, the χT products first decrease to 18.95
3
−1
and 23.80 cm mol K around 10 K, and then increase up to
3
−1
1
9.02 and 25.48 cm mol K at 6 and 3.5 K, respectively, as a
result of ferromagnetic interactions within the clusters. After
3
−1
that they lower again sharply to 17.20 and 24.17 cm mol
K
at 2 K respectively, as expected owing to the magnetic an-
isotropy and/or weak intermolecular antiferromagnetic inter-
actions. However, for compound 4, upon lowering the
temperature, the χT product exhibits a minimum at 9 K reach-
3
−1
3
−1
ing 23.58 cm mol K and then increases to 26.66 cm mol
K at 2 K, without a further decrease as observed in complexes
2
and 3.
The field dependence of the magnetization performed at
.9–5 K for each of the compounds 1–5 shows a relatively rapid
1
increase of the magnetization at low fields, followed by an
approximate continuous increase without clear saturation up
to 70 kOe, where it reaches 13.14, 14.06, 14.29, 17.58 and
Fig. 5 Frequency dependence of the in-phase (χ’) and out-of-phase
χ’’) ac susceptibility signals for 3 under zero-dc field. The solid lines are
a guide for the eyes.
(
1
B
3.56μ for 1–5, respectively (Fig. S4–S6, ESI†). The M vs. H/T
plots for 2–5 are not superimposed on a single master curve,
which are indicative of the presence of magnetic anisotropy
and/or lack of a well-defined ground state suggesting the pres-
ence of low-lying excited states that might be populated when
a dc-field is applied. Moreover, it is worth noting that isotropic
III
II
Gd
2 B
Mn (1) shows an inflection point M which reaches 8μ
on the M versus H around 15 kOe in good agreement with an
ST = 9/2 ground state, corresponding to a saturation value of
III
II
AF interactions between Gd and Mn ions. After that the plot
followed a gradual increase above 50 kOe, indicating that the
antiferromagnetic interactions are overcome by the applied dc
field.
In view of the magnetic anisotropy presenting in com-
pounds 2–5, the temperature-dependent ac susceptibility
measurements were performed to probe the signs of slow
dynamics in these molecular systems. The ac susceptibilities
measured in the temperature range 1.9–16 K with a 3.0 Oe ac
field oscillating at 1–1500 Hz are shown in Fig. 5 and Fig. S9–
S11.† Complexes 2 and 3 containing Tb and Dy ions while only a weak out-of-phase ac signal is observed both
display strong temperature and frequency-dependent maxima without and with 2000 Oe external applied dc fields at 1.9 K
in both χ′ and χ″ below 5 and 16 K, respectively (Fig. 5 and 6), for complex 4.
Fig. 6 Out-of-phase (χ’’) ac susceptibility signals for 2 under zero-dc
field. The dash lines are the best fits obtained with the generalized
Debye model. All data have been well simulated with small α values of
29
less than 0.13.
III
III
3434 | Dalton Trans., 2015, 44, 3430–3438
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The maximum in χ″(ν) gradually shifts to lower frequencies crossover at around T− = 0.1 K−1, which implies the presence
as the temperature is lowered, confirming slow magnetization of dual relaxation pathways. Above 11 K (Fig. 7, bottom), the
relaxation and SMM behavior of complexes 2 and 3 under zero anisotropy barrier for the relaxation process can be fitted by
dc field. Semicircular Cole–Cole plots of χ″ vs. χ′ were obtained the Arrhenius law to be 92.4(2) K with the pre-exponential
1
−
8
at fixed temperatures in the range 1.9–2.6 K and 1.9–11 K for 2 factor of τ = 5.37 × 10 s. It is worth noting that this Ueff is
0
and 3, respectively, and were fitted by a generalized Debye among the high values for 3d–4f SMMs, which is comparable
2
7
III
II
III
model giving α < 0.13 and α < 0.31 for 2 and 3, respectively to that of [Mn
6 3 2 2 2
O Tb ] and [Co Dy ] cores with Ueff of
30
(
Fig. 7, top, and Fig. S13, ESI†). The relatively high values of 103 K and 118 K, respectively, and lower than that of the
the α parameter for 3 imply that more than one relaxation [Fe
process is occurring, which is consistent with the unsymme- 3d–4f SMMs. The other relaxation process was estimated to
trical Cole–Cole plot and the overlapping maxima χ″(T) curves have U /k = 15.4(4) K and a pre-exponential factor of τ = 2.99
Fig. S10, ESI†).
II
III
2
Dy ] core which holds the highest barrier of 459 K for
2
8
19
eff
B
0
−
5
(
× 10 . It is worth stressing that at the higher temperatures the
The height of the effective energy barrier (Ueff), which pre- relaxation is probably dominated by the single ion behavior of
III
vents the thermal relaxation of the magnetization, can also be individual Dy ions, while at the lower temperature region the
III
II
determined by ac magnetic susceptibility. From these data, weak coupling between Dy and Mn ions becomes dominant
thermally induced relaxation can be fitted using the Arrhenius and the molecule formed shows small splitting of ground and
law (τ = τ exp(U /k T), where τ is the relaxation rate, k is the first excited sublevels resulting in a low barrier for spin
0
eff
B
B
3
0,31
Boltzmann constant, and τ
0
is a pre-exponential factor). For flipping.
complex 2, the relaxation time τ follows a thermally-activated
mechanism and the plot of ln τ versus 1/T is linear (Fig. S14,
ESI†). Fitting the data to the Arrhenius law produced an
Conclusions
effective energy barrier to magnetization reversal of Ueff/k =
B
−
8
1
8.79(4) K with the pre-exponential factor of τ = 2.56 × 10
0
In summary, we have successfully obtained a new family of tri-
nuclear heterometallic Ln
s. However, for complex 3, the ln τ versus 1/T plot shows a
III
II
2
Mn (Ln = Gd (1), Tb (2), Dy (3),
Ho (4) and Er (5)) complexes with a linear type structural top-
ology through the reactions between relevant 3d, 4f metal salts
III
and salicylic aldehyde ligands. The two Ln ions are located
II
in the peripheral sides and the Mn ion in the middle of the
III
II
Ln 2Mn straight line. Dc magnetic susceptibility measure-
ments show intramolecular AF interactions for complex 1. Ac
magnetic susceptibilities for complexes 2 and 3 exhibit slow
relaxation of the magnetization, thus complexes 2 and 3
II
III
become new members of the family of Mn –Ln SMMs.
Furthermore, complex 3 displays dual relaxation pathways with
a higher energy barrier (Ueff) of 92.4(2) K, a relatively high
value for 3d–4f SMMs furthering the prospect in enhancing
SMM properties through the incorporation of an isotropic
high spin 3d ion in the 3d–4f system. Moreover, besides the
III
III
contribution of strong magnetic anisotropy Tb and Dy , the
III
II
III
perfectly linear [Ln –Mn –Ln ] array might be also respon-
sible for the enhancement of the SMMs properties. Thus the
SMM property of the present Ln–Mn–Ln heterotrinuclear com-
plexes was a property characteristic of both anisotropic Ln
ions and weak coupling between Ln and Mn ions.
III
III
II
18c32
III
II
Attempts to synthesize and characterize other Ln
2
M
(M =
transition metal) clusters are under way to further explore the
origin of magnetic relaxation and magnetic interactions
between the spin carriers.
Fig. 7 Top: Cole–Cole plots measured below 11 K and zero-dc field for
Acknowledgements
3
; the solid lines are the best fits to the experimental data. As seen from
the fit above, the asymmetric Cole–Cole plots at low temperatures
We thank the National Natural Science Foundation of China
(grants 21331003, 21221061, and 21371166) for financial
support.
cannot be fitted well, indicating more than one relaxation process.
Bottom: Magnetization relaxation time, ln τ versus T−1 plot for 3 under
zero-dc field. The solid line is fitted with the Arrhenius law (see text).
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