Two new Dy3 triangles with trinuclear circular helicates and their single-molecule magnet behavior

Article abstract of DOI:10.1021/ic300371m

Self-assembly of polydentate Schiff base 2,6-diformyl-4-methylphenol di(benzoy1hydrazone) (H₃L), with dysprosium thiocyanate and sodium azide, affords two novel trinuclear triangular circular helicate dysprosium(III) complexes, [Dy₃(μ₃-OCH₃) ₂(HL)₃(SCN)] ·4CH₃OH·2CH ₃CN·2H₂O (1) or [Dy₃(μ₃- N₃)(μ₃-OH)(H₂L)₃(SCN) ₃](SCN) ·3CH₃OH·H₂O (2), depending on the presence or absence of base. Single-crystal X-ray analyses show that two μ₃-methoxy oxygens cap the Dy₃ triangle in complex 1 and that one μ₃-OH and one μ₃-N ₃^- cap the Dy₃ triangle of complex 2, representing the first example of a μ₃-N₃ ^--capped lanthanide complex reported to date. Ac susceptibility measurements reveal that multiple relaxation processes and the onset of slow magnetization relaxation occur for complex 1 and 2, respectively. Theoretical calculations are required to elucidate the underlying mechanism; however, the different magnetic anisotropy of the respective structures, which is dictated by the coordination environment of Dy^III ions and structural parameters of the triangles, is mostly responsible for the distinctive relaxation dynamics observed.

Full text of DOI:10.1021/ic300371m

Article
pubs.acs.org/IC
Two New Dy Triangles with Trinuclear Circular Helicates and Their
3
Single-Molecule Magnet Behavior
Shuang-Yan Lin,†,‡ Lang Zhao, Yun-Nan Guo, Peng Zhang,†,‡ Yang Guo, and Jinkui Tang*
†
†,‡
†
,†
†State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, People's Republic of China
‡
Graduate School of the Chinese Academy of Sciences, Beijing, 100039, People's Republic of China
*
S Supporting Information
ABSTRACT: Self-assembly of polydentate Schiff base 2,6-
diformyl-4-methylphenol di(benzoy1hydrazone) (H L), with
3
dysprosium thiocyanate and sodium azide, affords two novel
trinuclear triangular circular helicate dysprosium(III) com-
plexes, [Dy (μ -OCH ) (HL) (SCN)]·4CH OH·2CH -
3
3
3
2
3
3
3
CN·2H O (1) or [Dy (μ -N )(μ -OH)(H L) (SCN) ]-
2
3
3
3
3
2
3
3
(
SCN)·3CH OH·H O (2), depending on the presence or
3 2
absence of base. Single-crystal X-ray analyses show that two μ3-
methoxy oxygens cap the Dy triangle in complex 1 and that
3
−
one μ -OH and one μ -N cap the Dy triangle of complex 2,
3
3
3
3
−
representing the first example of a μ -N -capped lanthanide complex reported to date. Ac susceptibility measurements reveal
3
3
that multiple relaxation processes and the onset of slow magnetization relaxation occur for complex 1 and 2, respectively.
Theoretical calculations are required to elucidate the underlying mechanism; however, the different magnetic anisotropy of the
III
respective structures, which is dictated by the coordination environment of Dy ions and structural parameters of the triangles, is
mostly responsible for the distinctive relaxation dynamics observed.
INTRODUCTION
building blocks to create larger SMMs. Indeed, the opening up
and linkage of such highly anisotropic dysprosium triangles in
■
Complexes exhibiting single-molecule magnets (SMMs)
behavior, where relaxation and quantum tunneling of the
magnetization is molecule-based, have attracted increasing
6
d,9
different forms have become a hot topic
with the aim of
creating new SMMs and possibly advancing the prospects of
SMMs. Notably, the antiferromagnetical linkage of two Dy3
triangles has been found to give a spectacular increase in the
1
interest with the prospect of storing and processing magnetic
2
information at a molecular level. This interest has led to
temperature at which slowing of the magnetization is observed
intense activity to consistently synthesize new molecular
structures with a finite number of interacting magnetic centers
suitable for detailed magnetic study. As a large number of
SMMs emerged, understanding the factors determining the
relaxation dynamics in such molecules becomes of primary
importance with the goal of enhancing features such as the
relaxation energy barrier compared with those of the originally
studied examples.
6d
from 8 K to 25 K.
With this in mind, we focus our attention toward a systematic
survey of the structural types and characteristics of the resulting
Dy3 triangles through modifying the capping as well as
surrounding ligands. We have prepared two new Dy triangles,
3
i.e., [Dy (μ -OCH ) (HL) (SCN)]·4CH OH·2CH CN·2H O
3
3
3 2
3
3
3
2
(
(
1) and [Dy (μ -N )(μ -OH)(H L) (SCN) ]-
3 3 3 3 2 3 3
SCN)·3CH OH·H O (2), by using polydentate Schiff base
Recent advances have shown that the synthesis of molecules
3
2
III
3
III 1a,c
III
4
2,6-diformyl-4-methylphenol di(benzoy1hydrazone) (H L).
containing heavy lanthanide ions, such as Tb , Dy , Ho ,
3
III
5
The crystal structural analyses demonstrate that both structures
are triangular circular helicates. The triangle in 1 is capped by
two μ -methoxy oxygens, while that in 2 is capped by one μ -
and Er ions with their highly anisotropic magnetic moments,
is an important avenue to explore SMMs with higher
anisotropic barriers. In fact, these elements have been
responsible for many of the recent advances in SMMs, pushing
the frontiers to longer relaxation times and higher temperature
3
3
−
OH and one μ -N3 bridge. Magnetic studies reveal the
3
relaxation dynamics of 1 and 2 are drastically different. Indeed,
the two maxima χ″ are indicative of the operation of more than
one relaxation process in 1, while only a temperature-
dependent out-of-phase signal without peaks is observed in
complex 2. The distinct magnetic anisotropy reflected by the
3
d,6
regimes.
In addition to the well-established monometallic
4
a,5a,7
III
systems,
polynuclear Dy SMMs have become of
increasing interest since the observation of the unusual slow
relaxation behavior brought about by the toroidal arrangement
of local magnetization vectors in a Dy3 triangle. Its
8
unprecedented magnetic properties stimulate further inves-
Received: February 18, 2012
Published: April 17, 2012
tigation toward utilizing this highly anisotropic Dy triangle as
3
©
2012 American Chemical Society
10522
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Inorganic Chemistry
Article
different coordination environment of DyIII ions in respective
structures is most likely responsible for the significant magnetic
disparities observed in these complexes.
CN·2H O (1). A similar reaction, but in the absence of
2
KOH, produces another Dy3 cluster of formula [Dy (μ -
3
3
N )(μ -OH)(H L) (SCN) ](SCN)·3CH OH·H O (2). The
3
3
2
3
3
3
2
use of KOH might promote the formation of μ -OCH
3
3
EXPERIMENTAL SECTION
bridges; thus it is critical for the formation of complex 1.
Molecular structures of complexes 1 and 2 determined by
single-crystal X-ray diffraction are depicted in Figure 1.
Crystallographic data and refinement details are summarized
in Table 1.
■
General Procedures. All starting materials were of A.R. Grade and
were used as commercially obtained without further purification. 2,6-
Diformyl-4-methylphenol (DFMP) was prepared according to a
10
previously published method. The Schiff-base ligand 2,6-diformyl-
-methylphenol di(benzoy1hydrazone) was prepared by the in situ
4
condensation of 2,6-diformyl-4-methylphenol and benzoyl hydrazide
in a 1:2 ratio in methanol/acetonitrile. Elemental analyses for C, H,
and N were carried out on a Perkin-Elmer 2400 analyzer. Fourier
transform IR (FTIR) spectra were recorded with a Perkin-Elmer FTIR
−1
spectrophotometer using the reflectance technique (4000−300 cm ).
Samples were prepared as KBr disks. All magnetization data were
recorded on a Quantum Design MPMS-XL7 SQUID magnetometer
equipped with a 7 T magnet. The variable-temperature magnetization
was measured with an external magnetic field of 1000 Oe in the
temperature range 2−300 K. The experimental magnetic susceptibility
data are corrected for the diamagnetism estimated from Pascal’s tables
and sample holder calibration.
X-ray Crystallography. Single-crystal X-ray diffraction measure-
ments of the title complexes were carried out on a Bruker ApexII CCD
diffractometer with graphite-monochromated Mo Kα radiation (λ =
0
.71073 Å) at 185(2) K. The structures were solved by direct methods
2
and refined on F with full-matrix least-squares techniques using
11
SHELXS-97 and SHELXL-97 programs. The locations of Dy atoms
were easily determined, and S, O, N, and C atoms were subsequently
determined from the difference Fourier maps. Anisotropic thermal
parameters were assigned to all non-hydrogen atoms. The H atoms
were introduced in calculated positions and refined with a fixed
geometry with respect to their carrier atoms. CCDC 852631 (1) and
8
52632 (2) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
S y n t h e s i s o f [ D y 3 ( μ - O C H 3
)
( H L ) 3 -
3
2
(
SCN)]·4CH OH·2CH CN·2H O (1). The complex was prepared by
3 3 2
the reaction of Dy(SCN) ·6H O (104.6 mg, 0.24 mmol) with the H L
3
2
3
formed by the in situ condensation of DFMP (25.5 mg, 0.15 mmol)
and benzoyl hydrazide (41.7 mg, 0.3 mmol) in a 30 mL solution
mixture of MeOH and CH CN (1:1, v:v) in the presence of KOH (0.3
3
mmol) and NaN (13.5 mg, 0.2 mmol). The reaction mixture was
3
stirred at room temperature for 2 h, and the resultant solution was left
unperturbed. After a few days, orange, single crystals of complex 1
were obtained by slow evaporation of the solvent at room temperature.
Yield: 21 mg, (13%, based on the metal salt). Anal. (%) Calcd for
Dy C H N O S: C, 46.89, H, 4.23, N, 10.25. Found: C, 46.98, H,
3
80 86 15 17
−1
4
(
1
(
.03, N, 10.09. IR (KBr, cm ): 3176 (br), 3005 (w), 2853 (w), 2063
m), 1624 (s), 1552 (s), 1517 (s), 1439 (m), 1378 (s), 1315 (m),
231 (m), 1149 (w), 993 (w), 819 (w) 777 (m), 711 (s), 535 (w), 467
w).
Synthesis of [Dy (μ -N )(μ -OH)(H L) (SCN) ]-
3
3
3
3
2
3
3
(SCN)·3CH OH·H O (2). This complex was obtained by a similar
3 2
procedure to that described for 1, but without KOH. Yield: 28 mg,
16%, based on the metal salt). Analysis (%) Calcd for
Dy C H N O S : C, 43.64, H, 3.47, N, 12.72. Found: C, 43.28,
Figure 1. Structures of the Dy triangular unit in 1 (top) and 2
(
3
(bottom) with selective numbering scheme. Hydrogen atoms have
3
76 72 19 14 4
−1
been omitted for clarity.
H, 3.18, N, 12.45. IR (KBr, cm ): 3414 (br), 3188 (w), 3006 (w),
2
854 (w), 2069 (s), 1623 (s), 1573 (s), 1493 (w), 1445 (w), 1391
(
(
m), 1329 (m), 1313 (m), 1233 (m), 1083 (w), 991 (w), 902 (w), 821
w), 778 (m) 709 (m), 531 (w) 475 (w).
Complex 1 crystallizes in the monoclinic space group P2 /n.
1
The structure can be described as triangular circular helicate
12
architecture, which is twisted by three ligand strands along a
pseudo-3-fold axis defined by the three metal ions of Dy1, Dy2,
and Dy3. This triangular circular helicate structure is related to
the rarely reported circular polymetallic lanthanide-containing
RESULTS AND DISCUSSION
■
The reaction of H L with dysprosium(III) thiocyanate and
3
sodium azide in the presence of KOH in a mixture of methanol
and acetonitrile (v/v = 1:1) produces complex 1 with the
formula [Dy (μ -OCH ) (HL) (SCN)]·4CH OH·2CH -
13
helicates. It is noteworthy that the triangle is capped by two
μ -methoxy oxygens, instead of μ -hydroxo bridges as observed
3
3
3
2
3
3
3
3
3
1
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Inorganic Chemistry
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Table 1. Crystal Data and Structure Refinement for
the H L ligands imparts a coordination-induced chirality on
3
Complexes 1 and 2
each individual molecule, with the product crystallizing in an
optically inactive 50:50 racemic mixture. Thus, both left- (Λ)
and right-hand (Δ) configurations are clearly shown in both 1
1
2
empirical formula
fw (g/mol)
cryst syst
space group
cryst color
a (Å)
C H Dy N O S
C H Dy N O S
8
0
86
3
15 17
76 72
3
19 14
4
and 2 at the metal plane, as observed in triple-stranded
2049.20
2091.27
14
lanthanide helicate Ln complexes. The “meso” relation of the
2
monoclinic
trigonal
Λ and Δ configurations in the unit cell is clearly shown in
Figures 2, 3, and S2.
P2 /n
P3
̅
yellow
1
orange
15.0038(19)
30.470(4)
19.193(2)
94.460(2)
8747.7(19)
1.556
19.4317(4)
19.4317(4)
15.3514(7)
90
b (Å)
c (Å)
β (deg)
V (Å3)
ρcalcd (Mg/m3)
5020.0(3)
1.384
F(000)
4084
2070
Rint
0.0531
0.0617
R , wR [I > 2σ(I)]
0.0598, 0.1498
0.0740, 0.1620
1.113
0.0639, 0.2015
0.0958, 0.2303
1.069
1
2
R , wR (all data)
1
2
GOF
8a
6d,9a,b
in the prototype Dy3 and other Dy -related complexes.
3
Three edges of the triangle are bridged by three deprotonated
phenoxide oxygen atoms of the ligands (O1, O4, and O7).
Both Dy2 and Dy3 are coordinated by six O atoms and two N
atoms of the ligands, exhibiting a distorted dicapped trigonal-
prismatic coordinated arrangement, as shown in Figure S1. The
coordination arrangement of Dy1 is a distorted tricapped
trigonal-prismatic geometry filling the coordination sphere by
an additional thiocyanato-N. This coordination sphere results
in a less equilateral Dy triangle, with Dy···Dy distances of
3
Figure 2. (Top) Schematic representations of left- (Λ) and right-hand
3
.5762(6), 3.5838(6), and 3.5486(7) Å and Dy−Dy−Dy angles
(
Δ) configurations of the trinuclear circular-helical structure in
o
of 59.419(13)°, 60.397(11)°, and 60.184(11) . The two μ -
3
complex 1 with pseudo-C axis. (Bottom) Space-filling representations
3
OCH groups are displaced above and below the Dy plane by
of Λ and Δ helicates, showing the opposite helical arrangement of the
ligands. Hydrogens are omitted for clarity, and the ligand strands are
colored to emphasize the helicity.
3
3
1
.2320 and 1.2354 Å. The angles of μ -methoxy oxygen-bridged
3
o
metal atoms are in the range 95.46(18)−96.67(19) . The Dy−
N and Dy−O distances are in the ranges 2.438(7)−2.596(7)
and 2.273(6)−2.453(5) Å, respectively.
Inspection of the intermolecular interactions in complex 1
shows that two lattice methanol molecules as hydrogen-
Complex 2 crystallizes in the trigonal space group P3
̅
and has
a crystallographically centrosymmetric and therefore strictly
equilateral triangle Dy
bonding connectors join two Dy units via strong O15−
3
III
3
#4
#3
core. The structure shows three
H15a···N5 , O13−H13a···O15 , and N12−H12···O13 (sym-
metry code: #3, x+1/2, −y+1/2, z+1/2; #4, x−1, y, z−1; Figure
S3 and Table S1) hydrogen bonds; thus a 1D chain with a
zigzag arrangement of the molecules is afforded. Such a chain
disposes alternately Λ and Δ configurations of the molecules
with the shortest intrachain Dy−Dy distance of 11.5907(12) Å.
Obviously, the different enantiomers (Λ and Δ configurations)
alternate throughout the network, as shown in Figure S4.
Similarly, the packing arrangement for complex 2 also shows
alternating Λ and Δ configuration layers along the a, b, and c
axes with the shortest Dy···Dy distance of 9.5236(7) Å (Figure
S5).
mono-deprotonated H L ligands wrapped around a genuine 3-
3
fold axis defined by the three metal ions of Dy1, Dy1A
(
symmetry code A: −x+y+1, −x+1, z), and Dy1B (symmetry
code B: −y+1, x−y, z), giving rise to a trinuclear circular
helicate. It is interesting that the triangle of dysprosium centers
is capped by one μ -OH and one μ -N , which represents the
first example of a μ -N -bridged lanthanide complex reported
−
3
3
3
−
3
3
to date. Along each side of the triangle, a deprotonated
phenoxido group of the ligand bridges two metal centers as in
complex 1. The triangle in 2 is equilateral with a Dy···Dy
distance of 3.6314(8) Å. The environment of each Dy ion is
N O with a distorted tricapped trigonal-prismatic geometry
Magnetic Properties. The dc magnetic susceptibility
studies of 1 and 2 were carried out in an applied magnetic
field of 1000 Oe in the temperature range 300−2 K and plotted
4
5
−
(
Figure S1). The μ -OH and μ -N are displaced above and
3 3 3
below (1.1059 and 1.3573 Å) the Dy plane with Dy−O−Dy
3
o
o
angles of 99.991(1) and Dy−N−Dy angles of 93.269(1) . The
Dy−N and Dy−O distances are in the ranges 2.452(9)−
as χ T vs T (Figure 4). The two complexes show almost
M
identical behavior within the margin of error. The observed
3
−1
3
2
.590(8) and 2.315(6)−2.437(6) Å, respectively.
χ T value at 300 K is 42.63 cm K mol for 1 (42.92 cm K
M
−1
As shown above, the existence of KOH promotes the
mol for 2), which is in good agreement with the expected
value of 42.51 cm K mol for three uncoupled Dy ions (S =
5/2, L = 5, H1 , g = 4/3). Upon cooling, χ T gradually
3
−1
III
deprotonation of the CH OH, resulting in two μ -OCH caps
3
3
3
6
in complex 1. The ligands twisted along the central N−N
5/2
M
bonds wrap around the Dy plane, which gives rise to the
decreases until 50 K and then further decreases to reach a
3
3
−1
3
−1
circular helicate structures. Significantly, the configuration of
minimum of 21.19 cm K mol for 1 (14.85 cm K mol for
1
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Inorganic Chemistry
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Figure 3. (Top) Schematic representations of left- (Λ) and right-hand
(
Δ) configurations of the trinuclear circular-helical structure in
complex 2 with genuine C axis. (Bottom) Space-filling representations
3
of Λ and Δ helicates, showing the opposite helical arrangement of the
ligands. Hydrogens are omitted for clarity, and the ligand strands are
colored to emphasize the helicity.
Figure 5. Plots of the reduced magnetization M vs H/T in the field
range 0−70 kOe and temperature range 1.9−5.0 K. Inset: Field
dependence of the magnetization. (Top) for 1; (bottom) for 2.
might be populated when a field is applied. At 1.9 K, the M
versus H data of 1 exhibit slim butterfly-shaped hysteresis loops
(
Figure S6), without a remanence and a coercive field. This lack
is due to the slow sweep rate of the loop compared with the fast
17
zero-field relaxation, while the M vs H hysteresis effect at 1.9
K was not found for complex 2 due to relatively faster
relaxation process.
As shown above, χ T of both complexes at 2 K is quite far
M
away from zero, and the magnetization at 1.9 K does not show
any inflection in the low field; these tendencies indicate that
both complexes are not nonmagnetic ground state, significantly
Figure 4. Temperature dependence of the χ T product and χ (inset)
for 1 (open squares) and 2 (solid circles).
M
M
8a
different from the behavior of the prototype Dy3.
2
) at 2 K. The susceptibility (inset of Figure 4) gradually
Given the interesting magnetic behavior of the prototype Dy3
triangle, the dynamics of the magnetization were investigated
from the ac susceptibility measurements in the zero static field
and a 3.0 Oe ac oscillating field. As shown in Figures 6, 7 and
S7, an obvious temperature- and frenquency-dependent ac
signal is detected for 1, indicating slow relaxation of
magnetization expected for a single-molecule magnet. The
χ″(T) plot shows a broad shoulder between 2 and 8 K in the
range 100−1200 Hz and a tail of the peak below 2 K. The
nonvanishing of ac susceptibility at low temperatures is
indicative of quantum tunnelling of the magnetization often
increases with lowering the temperature and then dramatically
3
−1
3
−1
increases to 10.61 cm mol at 2 K for 1 (7.44 cm mol for
). The decline in χ T is likely due to a combination of the
2
M
III
15
progressive depopulation of Dy excited Stark sublevels
6
(Stark sublevels of the H
state) and possible exchange
1
5/2
interaction between the metal ions.
Magnetization (M) data for 1 and 2 were collected in the
field range 0−70 kOe below 5 K. The magnetization eventually
reaches a value of 17.7 μ for 1 (17.3 μ for 2) at 1.9 K and 70
B
B
kOe. This value is lower than the expected saturation value of
0 μ , but close to three uncorrelated Dy ions' magnetic
5c,18
3
seen in lanthanide SMMs.
Furthermore, the χ″(ν) plot
B
moments (3 × 5.23 μ ), which is likely due to crystal-field
clearly shows the occurrence of two distinct peaks in the out-of-
phase ac signals, which demonstrates the possible occurrence of
two relaxation processes. For instance, the χ″ versus frequency
plot at 1.9 K clearly evidences two peaks, which are centered at
B
16
effects and the low-lying excited states. Both the lack of
saturation of the M versus H data and the nonsuperimposition
of the M versus H/T data on a single master curve (Figure 5)
suggest the presence of magnetic anisotropy and/or the lack of
a well-defined ground state, where the low-lying excited states
10 and 570 Hz, respectively. In contrast, the prototype Dy
3
8a
shows a unique set of peaks.
1
0525
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Inorganic Chemistry
Article
exponential factors (τ ) of 4.4 × 10− and 8.9 × 10−5 s for the
5
0
FR (FR, fast relaxation phase corresponds to the high-
frequency peaks) and SR (SR, slow relaxation phase
corresponds to the low-frequency peaks), respectively (Figure
S9). Below 3 K the relaxation times for the FR deviate from
Arrhenius behavior, becoming temperature independent with a
τQTM value of 2 × 10− s. It should be noticed that τ are
4
0
−6
−11
relatively larger than the expected values around 10 −10
for SMMs. These values are obviously enhanced by the
presence of QTM.
s
Δχ1
+ (iωτ1)
Δχ2
1 + (iωτ2)
χAC(ω) = χ
s,tot
+
+
(1−α1)
(1−α2)
1
(1)
Figure 6. Temperature dependence of the out-of-phase (χ″) ac
susceptibility for 1. The solid lines are guides for the eyes.
The Cole−Cole plots (Figures 8 and S8) show two well-
separated relaxation phases at low temperature and an
asymmetric semicircle at high temperature (Figure S10). That
is, the Cole−Cole plots go through an evolution from FR to SR
at the frequency range of the test with increasing temperature.
The two separate relaxation processes are clearly observed at
1
1
.9 and 2.2 K (Figure S8), which can be nicely described by eq
with α parameters in the range 0.1−0.4.
For complex 2, an obvious temperature-dependent ac signal
(
Figures 9 and S11) is observed below 7 K, indicating the onset
Figure 7. Frequency dependence of the out-of-phase ac susceptibility
for 1 at H = 0 Oe. The solid lines indicate the fits to eq 1. The dotted
dc
lines are guides to the eye.
To better understand the nature of both types of dynamics,
the experimental χ (ω) curves between 1.9 and 5.0 K are
ac
1e
simulated by the sum of two modified Debye functions (eq 1)
and depicted as the χ′(ν), χ″(ν), and Cole−Cole plots in
Figures S7, 7, 8, and S8, respectively. The relaxation times (τ)
for each process derived from the two peaks of frequency-
dependence data are plotted as a function of 1/T. The effective
energy barriers obtained by modeling the behavior with the
Arrhenius law (τ = τ exp(U /kT)) are 4.4 and 8.9 K and pre-
Figure 9. Temperature dependence of the out-of-phase ac
susceptibility for 2 under a zero-dc field.
of slow magnetization (M) relaxation and, thus, probable SMM
behavior. Unluckily, slow relaxation of magnetization for this
complex is experimentally observed only over a short range of
temperature, and no maximum of χ″ is observed in the
temperature window technically available; thus we cannot
determine the energy barrier and corresponding relaxation
time. Alternatively, a method recently employed by Bartolome
0
eff
́
19
et al., assuming that there is only one characteristic relaxation
process of the Debye type with one energy barrier and one time
constant, can be used to evaluate roughly the energy barrier and
τ0 based on the following relation (eq 2):
ln(χ″/χ′) = ln(ωτ ) + E /k T
(2)
0
a
B
This method has been applied earlier in the determination of
20
15c
the Mn acetate and Dy complex. As shown in Figure 10,
12
2
by fitting the experimental χ″/χ′ data to eq 2, we extract an
estimate of the activation energy of ∼3 K and the characteristic
time of 10− s. A more precise result must wait for very low-
temperature measurements (T < 1 K) by using a micro-
SQUID.
6
Figure 8. Cole−Cole diagrams for 1 between 1.9 and 5.0 K at zero-dc
field, with the best fits to eq 1 given as solid lines.
1
0526
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Inorganic Chemistry
Article
CONCLUSIONS
■
III
3
Two novel triangular circular-helicate Dy complexes have
been synthesized using the versatile hydrazone ligand H L. The
3
helicates of the molecular unit are induced by the coordination
of the twisted diazine bridges from H L ligands. Unlike the μ -
3
3
8a
OH-capped prototype Dy triangle reported before, the Dy3
3
triangle of complex 1 is capped by two μ -methoxy oxygens,
3
while the triangle of complex 2 is capped by one μ -OH and
3
−
one μ -N3 linkage. As far as we know, this is the first example
of a μ -N -bridged lanthanide complex reported to date.
3
−
3
3
Magnetic properties reveal that two maxima χ″ in complex 1
are indicative of the operation of more than one relaxation
process in this compound, while only a temperature-dependent
out-of-phase signal without peaks is observed in complex 2.
The significant magnetic disparities may be associated with the
distinct anisotropic centers due to coordination-sphere differ-
ences of the metal centers. This presents an opportunity to
tune the magnetic properties of SMMs by modifying the
ligands and further adjusting the corresponding symmetry of
the ligand field. Theoretical studies are required to thoroughly
analyze the structural and magnetic property relationship of
Figure 10. Plot of natural logarithm of χ″/χ′ versus 1/T for 2. The
solid line represents the fitting results over the range 100−1200 Hz.
Although the dc magnetic susceptibilities of the two title Dy3
triangles are very similar, their dynamic behaviors are
dramatically different. It has been increasingly identified that
the relaxation of magnetization in most polynuclear lanthanide
clusters is presumably attributed to the large intrinsic magnetic
Dy triangles.
3
ASSOCIATED CONTENT
■
S
Supporting Information
18b,21
*
anisotropy of 4f ions,
due to very weak magnetic coupling
Tables of hydrogen bonds (Table S1) and figures of crystal
structures (Figures S1−S5) and magnetic measurements
between metal centers as a result of the efficient shielding of the
22
unpaired electrons in their 4f orbitals. It is also well-known
that magnetic relaxation is extremely sensitive to tiny
distortions of the coordination geometry. Therefore, the
difference in the relaxation dynamics of 1 and 2 is probably
(
Figures S6−S11). This material is available free of charge via
the Internet at http://pubs.acs.org.
III
AUTHOR INFORMATION
due to the distinct coordination environments around the Dy
■
ions, which are likely to affect the nature or directions of the
Corresponding Author
*E-mail: tang@ciac.jl.cn.
15c,18b,23
easy axes through the ligand fields.
Complex 1 contains two distinct metal centers, with nine-
coordinate Dy1 in a distorted tricapped trigonal-prismatic
arrangement and an eight-coordinate Dy2/Dy3 center in a
distorted bicapped trigonal-prismatic geometry. The multiple
relaxation processes observed are most likely associated with
these distinct anisotropic centers. While in complex 2 all three
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(Grants 91022009 and 20921002) for financial support.
III
Dy ions are the same nine-coordinate with a distorted
III
tricapped trigonal-prismatic geometry, in contrast, Dy ions in
8a
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■
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