Enhancement of Magnetocaloric Effect through Fixation of Carbon Dioxide: Molecular Assembly from Ln4 to Ln4 Cluster Pairs

Article abstract of DOI:10.1021/acs.inorgchem.7b00094

A series 1.Ln of tetranuclear lanthanide clusters [Ln₄(μ₄-O)L₂(PhCOO)₆]·solvent (Ln = Gd (1.Gd), Dy (1.Dy), Ho (1.Ho)) and octanuclear lanthanide Ln₄ cluster pairs 2.Ln [Ln₈(μ₃-O

Full text of DOI:10.1021/acs.inorgchem.7b00094

Article
pubs.acs.org/IC
Enhancement of Magnetocaloric Effect through Fixation of Carbon
Dioxide: Molecular Assembly from Ln to Ln Cluster Pairs
4
4
†
,‡
†
†
†,‡
,†
Jianfeng Wu, Xiao-Lei Li, Lang Zhao, Mei Guo, and Jinkui Tang*
†State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, P. R. China
‡
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
*
S Supporting Information
ABSTRACT: A series 1.Ln of tetranuclear lanthanide clusters
Ln (μ -O)L (PhCOO) ]·solvent (Ln = Gd (1.Gd), Dy (1.Dy),
[
4
4
2
6
Ho (1.Ho)) and octanuclear lanthanide Ln cluster pairs 2.Ln
4
[
(
Ln (μ −OH) (CO ) L (PhCOO) ]·solvent (Ln = Gd (2.Gd), Dy
8
3
4
3 2
4
8
2.Dy), Tb (2.Tb)) were assembled by using a bi-Schiff-based
ligand H L and characterized structurally and magnetically.
2
Interestingly, the octanuclear Ln cluster pairs 2.Ln are proposed
4
to be assembled from the tetranuclear clusters 1.Ln through the
uptake of CO from air in a more basic media. X-ray structural
2
analyses approved the possible evolution mechanism. Magnetic
studies reveal the coexistence of ferro- and anti-ferromagnetic interaction in 1.Gd and 2.Gd by simulating the direct-current
2−
magnetic susceptibility and indicate the CO3 bridges produce weak ferromagnetic interaction in 2.Gd rather than anti-
ferromagnetic interaction by benzoate bridges in 1.Gd. The magnitude of the magnetocaloric effect has been examined and
shows that complex 2.Gd exhibits larger magnetocaloric effect than 1.Gd, which could be probably ascribed to the weak
2
−
ferromagnetic interaction produced by the CO3 bridges.
INTRODUCTION
promising candidates to assemble SMMs with high effective
energy barrier and blocking temperature.
■
34−38
Lanthanide-based coordination clusters displaying interesting
magnetic properties have attracted increasing attention of
physicists, chemists, and material scientists over the past two
decades not only because of the beauty of their fascinating
structures but also due to the potential applications in
functional materials, including magnetism and electricity.
Among these elements, heavy rare earths are particularly
attractive and have been proposed to be used as magnetic
Attempts to seek efficient synthetic strategies to assemble
high-nuclear lanthanide-based clusters have been made to
obtain remarkable MCE, SMMs, and SMTs. In the past two
decades, hundreds of polynuclear clusters showing fantastic
molecular magnetic property have been reported. For example,
the [Ln ] nanocages show the largest MCE with −ΔS = 66.5
1
−7
6
0
m
−1
−1 39
J·kg ·K , [Dy K ] represents the best polynuclear SMM
with energy barrier of 842 K, and the coupled Dy triangles
4
2
40
3
refrigeration, high-density data storage, and quantum computer8,
maximize the toroidal moments by strong couplings via a μ -
4
inspiring the researches of magnetocaloric effect (MCE),
2−
41−43
9
,10
O
ion.
As part of our research interests, we have focused
single-molecule magnets (SMMs),
and single-molecule
1
1−13
on the coordination system based on Schiff-base ligands, by
using which we have obtained a series of pure 4f and 3d-4f
heterometallic polynuclear clusters showing fantastic magnetic
toroics (SMTs).
Generally speaking, molecules featuring
large MCE should possess a large-spin ground state with
negligible magnetic anisotropy, a large magnetic density, and
dominant intramolecular ferromagnetic exchange interac-
36,44−52
properties.
In these systems, the Schiff-base ligands are
1
4−17
stable in strongly basic media; thus, CO was usually absorbed
2
tion.
Therefore, the isotropic gadolinium ions with high
53−55
from air and produced carbonate ligands,
resulting in the
spin state (S = 7/2) is proposed to construct multinuclear
construction of high-nuclear clusters and cluster pairs.
Herein, we successfully synthesized a series of tetranuclear
clusters, which would be a promising candidate to perform
1
8
magnificent MCE. In the past few years, lots of high-nuclear
III
lanthanide clusters of [Ln (μ -O)L (PhCOO) ]·solvent (Ln =
clusters based on Gd and/or transition metal have been
4 4 2 6
19−26
Gd (1.Gd), Dy (1.Dy), Ho (1.Ho)) by applying a bi-Schiff-
reported showing remarkable MCE,
in most of which the
Gd ions bond with small ligands, such as NO3− and CO3
III
2−
,
based ligand H L (Scheme 1). When the reactions were
2
performed in a more basic media relevant octanuclear
resulting in a large-spin state to guarantee a high magnetic
1
4,16,19−21,27,28
l a n t h a n i d e L n 4 c l u s t e r p a i r s o f [ L n ( μ −
density.
For an SMM or SMT, large-spin state
8
3
and high axial anisotropy in proper ligand field are needed for
1
3,29−33
blocking the magnetic moment in ground state.
Thus,
Received: January 12, 2017
III
III
III
III
the anisotropic Dy , Tb , Ho , and Er ions are the most
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b00094
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
diffraction analysis were collected after one week. Yield in 40−60%.
Scheme 1. Schematic Drawing of the Binding Modes of
56
Chemical formulas and IR spectra: 2.Gd, [Gd (μ −
Ligand H L Indicated by the Harris Notation
8
3
2
O H) ( C O ) L ( P h C O O ) ] · 2H O; 2 . D y , [D y (μ −
4
3
2
4
8
2
8
3
−
1
OH) (CO ) L (PhCOO) ]·4CH OH·2H O, Selected IR (cm ):
4
3
2
4
8
3
2
3
1
1
7
655.05(w), 2900.72(br), 1611.72(br), 1564.99(m), 1537.46(m),
466.53(s), 1403.32(s) 1339.19(w), 1295.74(w), 1249.66(w),
216.78(m), 1075.41(w), 998.75(w), 968.25(w), 853.52(w),
19.65(m), 682.36(m), 652.47(w); 2.Tb, [Tb (μ −
8
3
−
1
OH) (CO ) L (PhCOO) ]·2CH CN, selected IR (cm ):
4
3
2
4
8
3
3
1
1
7
652.56(w), 2900.98(br), 1609.89(br), 1567.80(m), 1536.87(m),
465.55(s), 1402.65(s) 1337.72(w), 1294.87(w), 1248.96(w),
215.33(m), 1169.41(w), 998.76(w), 968.39(w), 852.75(w),
20.32(m), 681.94(m), 652.31(w).
Crystallography. Single-crystal X-ray data of the titled complexes
were collected on a Bruker Apex II CCD diffractometer equipped with
graphite-monochromatized Mo Kα radiation (λ = 0.710 73 Å) at
2
93(2) K. The structures were solved by direct methods and refined
2
58
by full-matrix least-squares methods on F using SHELXTL-2014.
OH) (CO ) L (PhCOO) ]·solvent (Ln = Gd (2.Gd), Dy
All non-hydrogen atoms in the whole structure were refined with
anisotropic displacement parameters. Hydrogen atoms were intro-
duced in calculated positions and refined with fixed geometry with
respect to their carrier atoms. The large solvent voids were refined
using solvent masking routine in the Olex2 package. Crystallographic
data are listed in Table S2. Additional crystallographic information is
available in the Supporting Information.
4
3 2
4
8
(
2.Dy), Tb (2.Tb)) were obtained. X-ray structural analyses
revealed the possible evolution mechanism of 2.Ln. Magnetic
studies indicated that the gadolinium analogues 1.Gd and 2.Gd
exhibit significant cryogenic MCE, and the dysprosium
derivatives 1.Dy and 2.Dy display slow relaxation of magnet-
ization. Magnetic interaction analyses suggested the introduc-
tion of CO32 bridges probably enhanced the cryogenic MCE
in 2.Gd.
5
9
Magnetic Measurements. Magnetic susceptibility measurements
were recorded on a Quantum Design MPMS-XL7 SQUID magneto-
meter equipped with a 7 T magnet. Direct-current (dc) magnetic
susceptibility measurements were performed on polycrystalline
samples of 1.Ln and 2.Ln in the temperature range of 2−300 K, in
an applied field of 1000 Oe. The variable-temperature magnetization
was measured in the temperature range of 1.9−300 K with an external
magnetic field of 1000 Oe. The dynamics of the magnetization was
investigated from the alternating-current (ac) susceptibility measure-
ments in the zero static fields and a 3.0 Oe ac oscillating field.
Diamagnetic corrections were made with the Pascal’s constants for all
the constituent atoms as well as the contributions of the sample
−
EXPERIMENTAL SECTION
■
General Synthetic Considerations. All chemicals and solvents
were commercially obtained and used as received without any further
purification. FTIR spectra were measured using a Nicolet 6700 Flex
FTIR spectrometer equipped with smart iTR attenuated total
reflectance (ATR) sampling accessory in the range from 500 to
−1
4
000 cm (Figure S1). Elemental analyses for C, H, and N were
performed on a PerkinElmer 2400 analyzer (Table S1). Ligand H L
2
5
7
60
was synthesized by the same procedure as described previously. In
holder.
short, H L was obtained by the condensation of 3-methoxysalicylalde-
2
hyde and 1,2-diaminopropane in the molar ratio of 2:1 in 50 mL of
ethanol under heating at 80 °C for 12 h. Yield in 96%.
RESULTS AND DISCUSSION
■
Syntheses of 1.Ln. Ln(PhCOO) ·6H O (0.15 mmol) was added
Crystal Structures of 1.Ln. The reactions of H L with
3
2
2
to a solution of H L (0.15 mmol) in a 20 mL mixture of methanol and
2
Ln(PhCOO) ·6H O and triethylamine in the mole ratio 3:3:8
3
2
acetonitrile (1:3, v/v), and then triethylamine (0.4 mmol) was added.
The resultant solution was stirred for 3 h and subsequently filtered.
The filtrate was exposed to air to allow the slow evaporation of the
solvent. Yellow crystals of 1.Ln suitable for X-ray diffraction analysis
were collected after one week. Yield in 65−75%. Chemical formulas
and IR spectra: 1.Gd, [Gd (μ -O)L (PhCOO) ]·H O; 1.Dy, [Dy (μ -
in the mixture of methanol and acetonitrile produced
complexes 1.Ln, [Ln (μ -O)L (PhCOO) ]·solvent (Ln = Gd
4
4
2
6
(
1.Gd), Dy (1.Dy), Ho (1.Ho)). Full structure determinations
were performed for these complexes (Table S2). By contrasting
the structural cores, unit cells, and IR spectra of 1.Gd, 1.Dy,
and 1.Ho, we find that these complexes are isostructural,
differing only in the cocrystallized solvent molecules as
observed in their molecular formulas. Therefore, the structural
description of the complex 1.Gd will only be given here for the
sake of brevity. Single-crystal X-ray studies reveal that 1.Gd is
tetranuclear (Figures 1 and S2), which is similar to the complex
4
4
2
6
2
4
4
−1
O)L (PhCOO) ]·2CH OH·H O, selected IR (cm ): 3297.07(w),
2
6
3
2
3
1
1
9
1
2
1
1
6
059.73(br), 2941.06(br), 1629.07(m), 1594.98(s), 1541.38(s),
493.77(w), 1446.35(m), 1407.30(s), 1375.49(m), 1285.32(s),
238.94(w), 1221.94(s), 1168.98(w), 1070.40(m), 1024.30(w),
99.59(w), 965.66(w), 854.01(m), 712.18(s), 684.56(w), 625.04(w);
−1
.Ho, [Ho (μ -O)L (PhCOO) ], selected IR (cm ): 3061.84(w),
4
4
2
6
887.59(br), 1630.37(s), 1595.47(s), 1545.39(s), 1447.26(m),
407.93(s) 1376.03(m), 1286.89(s), 1238.81(w), 1221.66(s),
169.19(w), 1070.86(w), 967.29(w), 853.91(w), 711.93(m),
84.36(w), 625.32(w).
61
of [Dy (μ -O)L′ (C H COO) ] reported recently. The
4
4
2
6
5
6
asymmetric unit consists of the full neutral molecule: two
H L ligands, six-coordinated benzoate ligands, one μ -O
2
4
Syntheses of 2.Ln. Complexes 2.Ln were prepared by the similar
bridged Gd
4
core, and one cocrystallized water molecule in
procedure as 1.Ln with the extra addition of 0.2 mmol of
triethylamine. A small quantity of needle crystal was obtained in
lattice. Each ligand H L is completely deprotonated and
coordinates with three Gd ions in the binding mode of
2
III
2
0% yield. Alternatively, we performed the reaction under a
3
.1 2 2 1 1 1 , forming a crescent Gd plate (Scheme 1). The
1 12 23 3 2 2 3
continuous CO2 flow with a detailed procedure given as follows.
Ln(PhCOO) ·6H O (0.15 mmol) was added to a solution of H L
III
two crescent Gd plates share the two terminal Gd ions and
3
3
2
2
bond with the μ -O bridge, resulting in the construction of μ -
4
4
(
0.15 mmol) in a 20 mL mixture of methanol and acetonitrile (1:1, v/
O bridged Gd core (Figure 1). Six benzoate ligands finish the
4
v) under a continuous CO flow, and then triethylamine (0.6 mmol)
2
III
coordination sphere of Gd ions in three binding modes
was added. The resultant solution was stirred for 3 h and subsequently
filtered. The filtrate was exposed to air atmosphere to allow the slow
evaporation of the solvent. Yellow crystals of 2.Ln suitable for X-ray
(Figure 1) and provide six negative charges to balance the
III
positive Gd4 core. Herein, all the Gd ions are eight-
B
DOI: 10.1021/acs.inorgchem.7b00094
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Inorganic Chemistry
Article
Figure 2. Structure of 2.Gd (top) and the relevant bonding modes of
the ligands (bottom left) with parameter labels of C−O distance in the
carbonate ligand; solvents were omitted for clarity.
Figure 1. Structure of 1.Gd (top) and the relevant bonding modes of
the ligands (bottom); solvents were omitted for clarity.
coordinated and bound by the center μ -O bridge with Gd−O
4
bonds and Gd−O−Gd angles in the ranges of 2.31−2.36 Å and
9
7.9−125.9° (Table S3).
Crystal Structures of 2.Ln. Complexes 2.Ln, [Ln (μ −
8
3
OH) (CO ) L (PhCOO) ]·solvent (Ln = Gd (2.Gd), Dy
4
3 2
4
8
(
1
2.Dy), Tb (2.Tb)), were prepared by the similar procedure as
·Ln with the extra addition of 0.2 mmol of triethylamine.
Single-crystal X-ray studies reveal that complex 2.Gd
crystallizes in the triclinic space group P1 with Z = 1 (Figure
̅
S3) and complexes 2.Dy and 2.Tb crystallize in the monoclinic
space group P2(1)/n with Z = 2, while the structural cores, IR
spectra, and asymmetric units of these complexes are almost the
same, differing only in the cocrystallized solvent molecules.
Therefore, the structural description of the complex 2.Gd will
only be given here for the sake of brevity. Complex 2.Gd is
octanuclear Gd4 cluster pair with the asymmetric unit
Figure 3. Coordination motifs of the ligand L2 in complexes 1.Gd
−
(left) and 2.Gd (right).
tion motifs and orientations. As shown in Figure 3, the two
ligands in 1.Gd are in the same coordination motif. In the case
of 2.Gd, one ligand is almost in the same coordination motif
and orientation with 1.Gd, while the other rotates along the
Gd−O direction and breaks the Gd−O bonds between
consisting of one Gd cluster (half of the neutral molecule):
4
two H L ligands, four-coordinated benzoate ligands, two μ −
2
3
OH bridges, and one carbonate-bridged Gd core. In the Gd
4
4
cluster, both of the ligands H L are completely deprotonated
2
III
III
and coordinate to three Gd ions with different binding modes,
methoxyl groups and two Gd ions. Second, both clusters
one of which is in 3.1 2 2 1 1 1 mode and the other in
contain the same μ -O atom. In 1.Gd, the μ -O atom resides in
1
12 23
3
2
2
4
4
3
.1 2 2 1 1 1 mode (Scheme 1), forming two edge-shared
the center of Gd core and provides two negative charges, while
1
12 23
3
2
2
4
Gd plates (Figures 2 and 3). The two μ −OH bridges cap the
in 2.Gd the μ -O atom connects with CO group forming the
3
3
4
2
CO32 , bonding with the Gd ions in the concave face of the
−
III
Gd plates from the convex faces and the carbonate anion
3
III
2−
coordinates to the four Gd ions from the internal concave face
Gd plates. Herein, the formation of the CO3 would be
3
of the Gd plates in 3.1 4 1 mode (Figure 2). Two benzoate
ligands coordinate to the center Gd ions of each plate, and the
explained by the well-known CO uptake from air in basic
media. We suppose that complex 2.Gd possibly evolves from
1.Gd; that is, with the addition of extra base and the presence
3
1
1234
4
2
III
other two benzoate ligands bridge the Gd clusters, forming the
4
III
octanuclear Gd cluster pair. Herein, all the Gd ions in the
of CO in the reaction of 1.Gd, one of the benzoate ligands
4
2
III
center of the plates are nine-coordinated, and the Gd ions in
with syn−syn bridging mode is replaced by the μ −OH, and the
3
terminal are eight-coordinated. The Gd−O distances and Gd−
other is substituted by CO . As a result one of the H L ligands
2
2
III
O−Gd angles between Gd ions and the center O atom are in
rotates along the Gd−O direction and breaks the Gd−O bond
III
the ranges of 2.539−2.641 Å and 89.64−179.49°.
between methoxyl groups and Gd ions, and the coordinated
2−
2−
Further insight into the structures, we can find that
complexes 1.Gd and 2.Gd are closely related to each other.
CO reacts with μ -O forming the CO3 bridge. To probe
2 4
this assumption, the C−O distances within the carbonato
bridge are taken into account (Figure 2). The C−O (O
First, both of the Gd clusters in 1.Gd and 2.Gd contain two
4
μ4
μ4
2−
completely deprotonated H L ligands with different coordina-
represents the μ -O in the CO ) distance of 1.36 Å is longer
4 3
2
C
DOI: 10.1021/acs.inorgchem.7b00094
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Inorganic Chemistry
Article
than the C−O (Omc represent the two monodentate-
indicates the intramolecular magnetic interactions. We fitted
mc
2−
coordinated O atoms in the CO ) distances of 1.22 and
the χ T versus T in the temperature range of 150−300 K and
3
M
3
−1
1
.26 Å, indicating a relatively weak bonding between CO
gave χ = 0.0592 cm ·mol (Figure 4).
2
TIP
2
−
group and the μ -O . To probe this further, detailed
We then intend to investigate the magnetic interactions
within these complexes. Because of the lack of orbital
4
comparison on the benzoate ligands in 1.Gd and 2.Gd are
investigated. In complex 1.Gd, the benzoate ligands coordinate
with Gd1 and Gd2 in monodentate-coordinated modes (Figure
III
contribution to the magnetic moment of the Gd ions, the
magnetic interactions in 1.Gd and 2.Gd are easy to be analyzed.
The magnetic susceptibility data of complex 1.Gd can be fitted
1), which has the ability to chelate with neighboring cluster.
63
Indeed, the benzoate ligands coordinated with Gd1 and Gd2
bridge the neighboring Gd2 and Gd1, forming the cluster pair
of 2.Gd. The benzoate ligands coordinted with Gd3 and Gd4 in
complexes 1.Gd and 2.Gd are in almost same coordination
motifs and orientations (Figure 2), which provides the evidence
that the relevant orientations of the Gd3 and Gd4 did not
by using PHI package on the basis of the following simplified
spin Hamiltonian:
̂
̂
Ŝ + J2Ŝ Ŝ
+ J3Ŝ Ŝ
+ J4Ŝ
̂
S
H = −(J S
1
Gd1 Gd3
Gd1 Gd4
Gd1 Gd2
Gd3 Gd4
+
J Ŝ Ŝ
+ J6Ŝ Ŝ
)
5
Gd2 Gd4
Gd2 Gd3
(1)
change when ligand H L rotated along the Gd−O direction and
2
The best fit is obtained with parameters of J = −0.0438
−1 −1 −1 −1
1
constructed the cluster pair of 2.Gd.
cm , J = 0.0479 cm , J = −0.274 cm , J = −0.0620 cm ,
2
3
4
−
1
−1
Magnetic Properties. Direct-current (dc) magnetic
susceptibility measurements were performed on polycrystalline
samples of these complexes in the temperature range of 2−300
K, in applied fields of 1000 Oe. Magnetic susceptibility in the
J = −0.0421 cm , J = 0.0454 cm . The coexistence of
5
6
negative and positive J values confirms the mixed anti-
ferromagnetic and ferromagnetic magnetic interactions between
III
the Gd ions. To avoid the overparametrization problem, we
form of the χ T (χ = molar magnetic susceptibility) versus T
M
M
regard J and J , J and J the same (Figure 5), not only because
1
5
2
6
plots were investigated and shown in Figure 4. The room-
Figure 4. Temperature dependence of χ T product at 1 kOe, between
M
2
and 300 K for 1.Ln and 2.Ln.
Figure 5. Magnetic interaction models of 1.Gd (top) and 2.Gd
bottom).
(
3
temperature χ T product of 30.53, 55.69, and 53.96 cm ·K·
mol for complexes 1.Gd, 1.Dy, and 1.Ho are in good
M
−1
they are close to each other but also due to Gd1 and Gd2, Gd3
and Gd4 are in similar coordination environments with similar
distances (Gd1···Gd3 = Gd2···Gd4, Gd1···Gd4 = Gd2···Gd3,
see Table S3), and then fit the plot using the following
equation:
III
3
−1
agreement with the values for each Gd (7.88 cm ·K·mol ),
III
3
−1
III
3
−1
Dy (14.17 cm ·K·mol ), and Ho (14.07 cm ·K·mol ) ions,
indicating the free-ion approximation applies. When cooled, the
χ T product gradually decreases and reaches 8.55, 39.82, and
M
3
−1
3
8.07 cm ·K·mol at 2 K, respectively. The decrease of χMT
can be possibly attributed to the dominant anti-ferromagnetic
interaction. For complexes 2.Gd and 2.Dy, the χ T product at
̂
̂
Ŝ
̂
Ŝ
̂
Ŝ
̂
̂
S
H = −[J (S
+ S
) + J (S
+ S
)
1
Gd1 Gd3
Gd2 Gd4
2
Gd1 Gd4
Gd2 Gd3
M
+ J S
̂
Ŝ
+ J4S
̂
Ŝ
]
Gd1 Gd2
Gd3 Gd4
(2)
3
−1
3
room temperature is 63.01 and 108.31 cm ·K·mol , which is
slightly smaller than the expected value for each Gd and Dy
ion, and gradually decreases when decreasing the temperature,
reaching 23.17 and 32.46 cm ·K·mol at 2 K, respectively. The
case in complex 2.Tb is different; that is, the χ T product is
III
III
−1
−1
−1
giving J = −0.0432 cm , J = 0.0466 cm , J = −0.274 cm ,
4
1
2
3
−1
J = −0.0618 cm , which are in good agreement with the
values above. For complex 2.Gd, the magnetic interaction is
more complex due to the eight Gd ions would produce 14
3
−1
III
M
3
−1
1
10.45 cm ·K·mol at room temperature, which is much larger
exchange J values, which could not be fitted by PHI at once.
Therefore, we just evaluate the magnetic interaction within the
asymmetric unit of 2.Gd. In the asymmetric unit, Gd1 and Gd2,
Gd3 and Gd4 are in similar coordination environments with
similar Gd1···Gd4 and Gd2···Gd4, Gd1···Gd3 and Gd2···Gd3
distances (Table S3); thus, we regard J and J , J and J the
3
−1
III
than the value of 94.56 cm ·K·mol for eight Tb ions (11.82
cm ·K·mol for each Tb ) and decreases linearly until 150 K,
followed by a gradual decrease reaching 37.79 cm ·K·mol at 2
K. The linear decrease suggests the presence of temperature-
independent paramagnetism (χTIP), which arises from the
second-order Zeeman effect between the nonmagnetic ground
3
−1
III
3
−1
1
6
2
5
same (Figure 5), to avoid the overparametrization problem, and
6
2
state and higher magnetic states, and the final decrease
fit the χ T versus T plot using the following equation:
M
D
DOI: 10.1021/acs.inorgchem.7b00094
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Ŝ
Article
̂
̂
̂
̂
̂
̂
̂
Ŝ
H = −[J (S
+ S
S
) + J (S
S
+ S
)
1
Gd1 Gd4
Gd2 Gd4
2
Gd1 Gd3
Gd2 Gd3
J Ŝ Ŝ
̂
S
]
+
3
Gd1 Gd2
+ J4Ŝ
Gd3 Gd4
(3)
−
1
−1
−1
giving J = −0.261 cm , J = 0.0393 cm , J = −0.117 cm , J
1
2
3
4
−
1
=
0.245 cm . The positive J suggests ferromagnetic
4
2−
interaction between Gd3 and Gd4 through the CO3 bridge
in 2.Gd rather than anti-ferromagnetic interaction through the
benzoate bridge in 1.Gd. Generally speaking, the magnetic
interaction in 1.Gd and 2.Gd are comparable; therefore, we
could qualitatively analyze the magnetic interaction between
the Gd cluster pair in 2.Gd by subtracting twofold χ T of
4
M
1
.Gd from the χ T of 2.Gd to obtain the temperature-
M
dependent difference Δχ T = χ T
− 2χ T(1.Gd). The
M
M
(2.Gd)
M
upturn in Δχ T versus T plot suggests that the Gd cluster pair
M
4
is dominated by ferromagnetic interaction.
The field dependence of the magnetization for 1.Ln and 2.Ln
were performed at low-temperature region in the field range of
0−70 kOe (Figures S4−S9). The magnetization versus field
plots for 1.Dy, 1.Ho, 2.Dy, and 2.Tb reveal quick increase at
low-field region and slow increase at high-field region without
saturation until 70 kOe, indicating magnetic anisotropy possibly
presenting at the lanthanide ions. While the magnetizations for
1
.Gd and 2.Gd show steady increase in the whole field region,
especially at high temperature, reaching 27.7 and 53.5 Nβ for
an applied field of 70 kG and at 2 K, which are in good
agreement with the saturation values of 28 and 56 Nβ,
respectively. The relatively large magnetization values are in
favor of large magnetocaloric effect. Therefore, we use the
magnetization data to estimate the magnetic entropy change
ΔS by using the Maxwell relation ΔS = ∫ [∂M(T,H)/
Figure 6. Temperature dependences of the magnetic entropy change
(
−ΔS ) for 1.Gd (top) and 2.Gd (bottom), as obtained from the
m
magnetization data.
m
m
6
4
∂
T] dH. The −ΔS values reach maximum of 22.88 and
H
m
−1 −1
2
8.38 J·kg ·K at ΔH = 7 T for 1.Gd and 2.Gd, respectively
Figure 6). Assuming SGd = 7/2 for each uncorrelated Gd ion
III
(
at low temperature, the calculated −ΔS values based on −ΔS
m
m
=
nR ln(2S + 1) are 8.32R and 16.64R for 1.Gd and 2.Gd,
Gd
−1
−1
which correspond to the values of 33.41 and 36.29 J·kg ·K .
The theoretical values for both complexes are much larger than
the experimental values, which can be ascribed to the dominant
anti-ferromagnetic interactions in these complexes. The larger
MCE observed in 2.Gd than 1.Gd can be mainly attributed to
the large metal/ligand mass ratio of 0.49 for 2.Gd (0.44 for
1
.Gd), which helps to achieve a high spin density in this high-
nuclearity metal cluster pair. What’s more, the weak
ferromagnetic interactions between Gd3 and Gd4 induced by
the CO32 bridges, as well as the ferromagnetic interaction
−
between the Gd clusters, would also contribute to the MCE of
4
2
.Gd. Therefore, the absorption of CO not only produces the
2
transformation of the structure but also makes an enhancement
of the MCE.
Alternating current (ac) experiments were also performed at
low temperature under an oscillating field of 3.0 Oe to probe
the dynamics of the magnetic relaxation. However, no out-of-
phase (χ″) susceptibility is observed above 1.9 K for these
III
complexes except the Dy -based complexes 1.Dy and 2.Dy
(
2
Figure S10). Temperature-dependent χ″ signals for 1.Dy and
Figure 7. Frequency-dependent out-of-phase χ″ signals for 1.Dy and
2.Dy at indicated temperatures, under zero dc field.
.Dy are observed below 15 and 8 K, respectively (Figure 7),
suggesting slow magnetic relaxation behavior. When cooled, the
χ″ increase quickly without temperature-dependent χ″ peaks,
indicating the presence of fast quantum tunneling of magnet-
ization (QTM), which is a shortcut of the magnetic relaxation.
The SHAPE software is used to quantify the coordination
III
Dy ions in 1.Dy are eight-coordinated and reside in distorted
triangular dodecahedron coordination geometry (D ). For
complex 2.Dy, Dy1 and Dy2 are also eight-coordinated with
similar coordination geometry as the Dy ions in 1.Dy, while
Dy3 and Dy4 are nine-coordinated in distorted capped square
2
d
III
III
geometry of the Dy centers (Table S4) and reveals that all the
E
DOI: 10.1021/acs.inorgchem.7b00094
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
antiprism geometry (C ). The low and distorted coordination
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CONCLUSION
■
1
In conclusion, a series of tetranuclear lanthanide clusters of
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(
(
(
octanuclear lanthanide Ln cluster pair of 2.Ln (Ln = Gd
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(
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AUTHOR INFORMATION
■
5
Synthesis, Structure, SMM Behavior, and MCE Properties. Chem. -
Eur. J. 2015, 21, 16955−16967.
(
Corresponding Author
E-mail: tang@ciac.ac.cn.
*
18) Pyykko, P. Magically magnetic gadolinium. Nat. Chem. 2015, 7,
ORCID
6
80−680.
Jinkui Tang: 0000-0002-8600-7718
Notes
The authors declare no competing financial interest.
CCDC Nos. 1507070−1507075 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.
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ACKNOWLEDGMENTS
■
(
3
We thank the National Natural Science Foundation of China
Grant Nos. 21525103, 21371166, 21521092, and 21331003)
(
for financial support. J.T. gratefully acknowledges support of
K.; Srivastava, A. K.; Sanudo, E. C.; Moubaraki, B.; Murray, K. S.;
the Royal Society-Newton Advanced Fellowship (NA160075).
McInnes, E. J. L.; Affronte, M.; Shanmugam, M. Synthesis and
F
DOI: 10.1021/acs.inorgchem.7b00094
Inorg. Chem. XXXX, XXX, XXX−XXX

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