A discrete dysprosium trigonal prism showing single-molecule magnet behaviour

Article abstract of DOI:10.1002/chem.201102547

A magnetic personality: A unique dysprosium trigonal prism with a new polydentate Schiff-base ligand behaves as a single molecule magnet (SMM), which may stimulate further investigations into the relaxation dynamics of SMMs with different topologies (see

Full text of DOI:10.1002/chem.201102547

DOI: 10.1002/chem.201102547
A Discrete Dysprosium Trigonal Prism Showing Single-Molecule Magnet
Behaviour
Haiquan Tian,[a, b] Min Wang,[c] Lang Zhao,[a] Yun-Nan Guo,[a] Yang Guo,[a]
Jinkui Tang,*[a] and Zhiliang Liu[b]
The synthesis of new single-molecule magnets (SMMs)
based on appealing topologies and fascinating magnetic be-
haviours remains an exciting challenge. In recent years,
many synthetic chemists and physicists have employed sev-
eral lanthanide (Ln) ions (TbIII, DyIII, HoIII and ErIII) and
bridging ligands to synthesise novel SMMs[1] and have stud-
ied in detail their unique magnetic properties at the molecu-
lar level (e.g., SMM behaviour,[2] such as slow relaxation
and quantum tunnelling of the magnetisation[3]). Ln ions are
incorporated into these systems because they not only con-
tribute high spins but also introduce anisotropy to the mole-
cule as a result of the nature of the f-electron shell. Among
the Ln family, the DyIII ions seem to have indisputable
virtue in this respect, as we and others have recently demon-
strated.[4] A number of pure DyIII-containing compounds ex-
hibiting various topologies based on a linear,[5] triangular,[4a]
planar,[6] cubane,[7] pyramid,[8] wheel,[9] or a disc,[10] have
been described in the literature. It is worth noting that the
interesting magnetic behaviour, which has been increasingly
identified in dysprosium compounds can be attributed to the
magnetic anisotropy of the molecules. This anisotropy de-
pends not only on the individual anisotropies of the metal
ions but also on the relative orientation of the local axes. In
this respect, the design of a new dysprosium cluster with a
new topology might provide unique opportunity to probe
the relaxation dynamics of polynuclear complexes, thus en-
riching the structure correlation to magnetic properties of
dysprosium family. In addition, selection of a suitable bridg-
ing ligand is crucial for assembling a new lanthanide cluster
with interesting magnetic properties, due to the difficulty in
promoting magnetic interactions between the lanthanide
ions through overlap of bridging ligand orbitals with the
“contracted” 4 f orbitals of the Ln ions.[4 g] With these issues
in mind, we designed a new
multidentate ligand, namely,
(E)-N’-(2-hyborxy-3-methoxy-
benzylidene)pyrazine-2-carbo-
hydrazide (H2L) (see Scheme 1)
and isolated a dysprosium trigo-
nal prism that behaves as
SMM. To our knowledge, a dys-
prosium SMM with trigonal
prism geometry has not been
reported.
Scheme 1. Structural formulae
for
(E)-N’-(2-hyborxy-3-me-
thoxybenzylidene)pyrazine-2-
carbohydrazide, H2L.
The selection of hydrazone ligand, formed by reaction be-
tween the o-vanillin aldehyde and pyrazine-2-carbohydra-
zide, is especially useful to construct polynuclear dysprosium
complexes because of the following considerations: 1) This
type of N, O-donor linear ligand has a strong coordination
ability for lanthanide ions with suitable relative positions.
2) Similar ligands have successfully synthesised several
structurally and magnetically interesting dysprosium
SMMs.[1b,4c,h] Herein, we report the structure and magnetic
properties of a unique dysprosium trigonal prism SMM as-
sembled from this new ligand, H2L.
The reaction of Dy
MeOH/EtOH/CH2Cl2 (1:1:2 ratio), in the presence of trie-
thylamine (6 equiv), produces yellow crystals of [Dy6A(OAc)3-
(m3-CO3)2(L)5(HL)(MeOH)2]·4H2O·5MeOH·EtOH (1). The
ACHTUGNTREN(NUNG OAc)3·6H2O with H2L (1:1 ratio) in
CHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
molecular structure was determined by single-crystal X-ray
diffraction and is depicted in Figure 1 (top) and the crystal
data are summarised in the Supporting Information
(Table S1). Compound 1 crystallises in the monoclinic space
group P21/c and can be described as having a unique trigo-
nal prism architecture. Each triangular face of the trigonal
prism is an approximate equilateral triangle based on the
Dy···Dy interatomic distances ranging from 4.8009(8) to
[a] H. Tian, Dr. L. Zhao, Y.-N. Guo, Dr. Y. Guo, Prof. Dr. J. Tang
State Key Laboratory of Rare Earth Resource Utilization
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences, Renmin Street 5625
Changchun 130022 (P. R. China)
Fax : (+86)431-85262878
E-mail: tang@ciac.jl.cn
4.9105(8) ꢀ and angles from 59.114(11)8 to 61.151(11)8;
2À
moreover, a CO3 is encapsulated in the DyIII plane and li-
[b] H. Tian, Z. Liu
3
College of Chemistry and Chemical Engineering
Inner Mongolia University, Hohhot 010021 (P. R. China)
gated by its oxygen atoms at the midpoints of the sides of
the triangle formed by the DyIII ions (see the Supporting In-
formation, Scheme S1a); in doing so the C···C distance of
[c] M. Wang
The Laboratory of Advanced Materials, Fudan University
Shanghai, 200438 (P. R. China)
2À
two central CO3 ligands is 4.4085(8) ꢀ and the perpendic-
ular distance between the two triangular faces is 3.8245(8) ꢀ.
It is interesting to note that the carbonate adopts a less
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102547.
442
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Chem. Eur. J. 2012, 18, 442 – 445

COMMUNICATION
quiring a dianion in these sites. In particular, IR stretching
bands characteristic of the coordinated carbonate are ob-
served at n˜ =1558 and 1333 cmÀ1 (see Figure S4 in the Sup-
porting Information).[12] Nitrate was not present in the reac-
tion at any stage, therefore it appears that atmospheric CO2
has been incorporated into the structure, thus templating
further aggregation and growth of the cluster.[13]
Direct-current (dc) magnetic susceptibility studies of 1
have been carried out in an applied magnetic field of 1 kOe
in the temperature range 300–2 K. The plot of cMT versus T,
in which cM is the molar magnetic susceptibility, is shown in
Figure 2. At 300 K, the cMT value of 85.78 cm3 KmolÀ1 is
Figure 1. Top: the molecular structure of 1, showing one of the two iso-
mers. Solvents, counter ions and hydrogen atoms are omitted for clarity.
Bottom: the Dy6 core.
common hexadentate m3-fashion to bridge three metal cen-
tres.[11] The parallelisms of the two triangular faces are
linked through bridging carboxy oxygen atoms of six hydra-
zone ligands, stacked on the top of each triangular face, re-
sulting in the hexanuclear trigonal prism (see Figure 1,
bottom). All of the DyIII ions are eight-coordinate, display-
ing distorted dodecahedral geometry. Only one coordination
and bridging mode (h1:h1:h2:h1:m2-fashion) can be observed
for the unique polydentate Schiff-base ligand (see the Sup-
porting Information, Scheme S1b). Two different coordina-
tion modes of acetate groups (syn h1-fashion and syn–syn m2-
h1:h1-fashion) coexist in the crystal structure (see the Sup-
porting Information, Scheme S1c). Two methanol molecules
occupy the remaining coordination sites of the Dy2 and Dy4
metal centres. Additionally, the structure consists of two iso-
mers in the unit cell and four non-coordinated water mole-
cules, five methanol molecules and one ethanol molecule of
crystallisation are located in the crystal lattice, as depicted
in the Supporting Information (Figures S1 and S2). Strong
intermolecular hydrogen-bonding interactions produce a
one-dimensional supramolecular chain of the molecules (see
the Supporting Information, Figure S3). The shortest inter-
molecular Dy···Dy distance is 8.8153(8) ꢀ.
Figure 2. Top: temperature dependence of the cMT product at 1 kOe.
Bottom: M versus H/T plots at different temperatures below 5 K.
very close to the expected value of 85.02 cm3 KmolÀ1 if the
6
six DyIII ions (S=5/2, L=5, H15/2, g=4/3) were non-inter-
acting. The cMT product gradually decreases with the tem-
perature to reach a minimum of 66.93 cm3 KmolÀ1 at about
2 K, which is mainly ascribed to the progressive depopula-
tion of excited Stark sublevels.[14] The shoulder observed
around 20 K might suggest a competition between ligand
field effect and possible weak ferromagnetic interactions be-
tween the DyIII ions.
The M versus H/T (Figure 2, bottom) data at different
temperatures show non-superposition plots and a rapid in-
crease of the magnetisation at low magnetic fields, which
eventually reaches the value of 40.58 mB at 1.9 K and 7 T
without any sign of saturation. This value is lower than the
expected saturation value of 60 mB (for six noninteracting
DyIII ions). This difference is most likely due to significant
The assignment of bridging carbonate was made based on
careful consideration of the diffraction data, which indicated
the presence of a trigonal fragment, and charge balance re-
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443

J. Tang et al.
anisotropy and the crystal-field effect at the DyIII ion that
3 K, a temperature-independent relaxation regime is ob-
served with a characteristic time of 0.07461 s. This behaviour
is expected for a SMM when the quantum tunnelling of the
magnetisation becomes dominant (i.e., faster than the ther-
mal-activated relaxation).[1b,17] Above 3 K, the relaxation be-
comes thermal-activated (Arrhenius-like behaviour). An es-
timation of the anisotropic barrier around 56 K can be ob-
tained through fitting t values above 8 K utilising an Arrhe-
nius law, and the pre-exponential factor (t0) is 6.6ꢂ10À6 s for
1.
6
eliminates the 16-fold degeneracy of the H15/2 ground
state.[4a,15] The non-superposition of the M versus H/T data
on a single master curve and the high-field non-saturation
suggests the presence of a significant magnetic anisotropy
and/or low lying excited states in compound 1.
To investigate the dynamics of the magnetisation, the
temperature and frequency dependences of the ac-suscepti-
bility were undertaken under zero-dc field (Figure 3 and S5
Although such ac signals were observed above 1.9 K, no
hysteresis was detected in the M versus H data obtained
using a traditional SQUID magnetometer (see the Support-
ing Information, Figure S9). Therefore, to probe further the
relaxation dynamics of 1, hysteresis measurements were per-
formed on a SQUID-VSM in the temperature range 1.8–5 K
with a higher sweep rate (70 mTsÀ1). Below 5 K, hysteresis
loops were clearly observed, which were strongly tempera-
ture dependent down to 1.8 K (Figure 4). The coercivities of
the hysteresis loops increase with decreasing temperature, as
expected for an SMM. This behaviour confirms further the
SMM nature of 1.[4f]
Figure 3. Frequency dependence of the in-phase (top) and out-of-phase
Figure 4. Hysteresis loops for 1 (powder sample) at different tempera-
ac susceptibility (bottom) of 1 below 26.0 K, under zero-dc field.
tures and a constant field sweep rate of 0.07 TsÀ1
.
(see the Supporting Information)) for 1. They both reveal
the presence of slow relaxation of the magnetisation typical
of a SMM behaviour. It is noteworthy that the temperature
dependence of in-phase and out-of-phase ac susceptibilities
do not go to zero after the maxima for low frequencies (see
the Supporting Information, Figure S5), as for most 3d
SMMs, might be indicative of a fast relaxation process that
becomes dominant in the lower temperature region.[2d,16]
The temperature dependence of the ac-susceptibility mea-
surement on the gadolinium analogue (see the Supporting
Information and Figure S6 for details) under zero-dc field
does not exhibit any out-of-phase ac signals (Figure S7), cor-
roborating further that the magnetic relaxation phenomena
basically originates from an intrinsic molecular properties of
the compound, rather than intermolecular interactions and
long-range magnetic order. The relaxation time was extract-
ed from the frequency-dependent data between 1.9 and
26 K and the Arrhenius plot obtained from these data is
given in Figure S8 (see the Supporting Information). Below
In conclusion, a novel hexanuclear DyIII SMM with a par-
ticular trigonal prismatic topology has been synthesised
from a new H2L ligand. It should be noted that the rare m3-
2À
fashion CO3 ligands derived from the capture of atmos-
pheric CO2 play a crucial role for assembling the peculiar
triangles of the trigonal prism. This Dy6 complex behaves as
a SMM with an anisotropic barrier of 56 K, and a quantum
regime of relaxation below 3 K. In addition, hysteresis loops
can be observed below 5 K. We believe this unique trigonal
prismatic SMM will not only enrich the polynuclear dyspro-
sium SMMs family but also stimulate further research into
the relaxation dynamics of SMMs with different topologies
based on versatile multidentate ligands.
Experimental Section
Synthesis of the complex 1: A solution of Dy
ACHTUNGTREN(UNGN OAc)3·6H2O (53.3 mg,
0.15 mmol) and the H2L (40.5 mg, 0.15 mmol) in CH3OH/CH3CH2OH/
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Chem. Eur. J. 2012, 18, 442 – 445

Single-Molecule Magnet Behaviour
COMMUNICATION
CH2Cl2 (15 mL, 1:1:2 v/v) was stirred with triethylamine (0.6 mmol) for
8 h. The resultant yellow solution was left unperturbed to allow the slow
evaporation of the solvent. Yellow single crystals, suitable for X-ray dif-
fraction analysis, were formed after one week. Yield: 16 mg (43%, based
on metal salt). IR (KBr): n˜ =3051(w), 2930(w), 1605(vs), 1558(s),
1475(vs), 1446(vs), 1416(m), 1373(w), 1333(vs), 1240(s), 1152(m),
1107(w), 1036(s), 971(w), 870(w), 742(s), 645(w), 589(w), 430(m) cmÀ1; el-
emental analysis calcd (%) for C183H190Dy12N48O74: C, 34.20, H, 2.49, N,
12.20: found C, 34.13, H, 2.37, N, 12.05.
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Magnetic measurements were performed in the temperature range 2–
300 K using Quantum Design MPMS-XL SQUID and 1.9–5.0 K using
SQUID-VSM magnetometers, respectively. The diamagnetic corrections
for the compounds were estimated using Pascalꢃs constants, and magnetic
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Acknowledgements
This work was supported by the National Natural Science Foundation of
China (20871113, 91022009 and 20921002).
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Keywords: dysprosium trigonal prism
magnetic properties · Schiff bases · slow magnetic relaxation
·
hydrazones
·
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Received: August 17, 2011
Published online: December 12, 2011
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