A series of 3D metal organic frameworks based on [24-MC-6] metallacrown clusters: Structure, magnetic and luminescence properties

Article abstract of DOI:10.1039/c4dt01593b

Four isostructural metal organic frameworks (MOFs), namely [Co ₆(HipO)₆]·6H₂O (1), [Mn ₆(HipO)₆]·6H₂O (2), [Cd ₆(HipO)₆]·6H₂O (3) and [Zn ₆(HipO)₆]·7H₂O (4) (H₃ipO = 2-hydroxyisophthalic acid), were synthesized and structurally characterized. They have a 3D (4,6)-connected framework based on [24-MC-6] metallacrown clusters ([24-MC-6]-based MOFs). The arrangements of the 24-MC-6 metallacrown SBUs show a regular change indicated by the orientation of their symmetry axes, resulting in a special dense packing mode different from other [24-MC-6]-based MOFs. The analysis of SQUID measurements reveal that compound 1 displays the dominant antiferromagnetic exchanges in 300-10 K between the adjacent Co(ii) ions and a ferromagnetic-like behavior at lower temperatures, whereas compound 2 shows an antiferromagnetic interaction between the adjacent Mn(ii) ions. Compound 1 exhibits a magnetocaloric effect (MCE) with the resulting entropy change (-ΔS_m) of 15.20 J kg^-1 K^-1 for ΔH = 50 kG at 6 K, which is the highest value among the cobalt-based MOFs with MCE reported so far. The luminescence properties of compounds 3 and 4 were studied, both of them exhibit photoluminescence in the solid state at room temperature which can be ascribed to intraligand π→π* transitions. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt01593b

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A series of 3D metal organic frameworks based
on [24-MC-6] metallacrown clusters: structure,
magnetic and luminescence properties†
Cite this: Dalton Trans., 2014, 43,
12989
a,b
b
b
b
a
Kai Wang, Hua-Hong Zou, Zi-Lu Chen, Zhong Zhang, Wei-Yin Sun and
b,c
Fu-Pei Liang*
6 6 2 6 6 2
Four isostructural metal organic frameworks (MOFs), namely [Co (HipO) ]·6H O (1), [Mn (HipO) ]·6H O
(
2), [Cd (HipO) ]·6H O (3) and [Zn (HipO) ]·7H O (4) (H ipO = 2-hydroxyisophthalic acid), were syn-
6
6
2
6
6
2
3
thesized and structurally characterized. They have a 3D (4,6)-connected framework based on [24-MC-6]
metallacrown clusters ([24-MC-6]-based MOFs). The arrangements of the 24-MC-6 metallacrown SBUs
show a regular change indicated by the orientation of their symmetry axes, resulting in a special dense
packing mode different from other [24-MC-6]-based MOFs. The analysis of SQUID measurements reveal
that compound 1 displays the dominant antiferromagnetic exchanges in 300–10 K between the adjacent
Co(II) ions and a ferromagnetic-like behavior at lower temperatures, whereas compound 2 shows an anti-
ferromagnetic interaction between the adjacent Mn(II) ions. Compound 1 exhibits a magnetocaloric effect
(MCE) with the resulting entropy change (−ΔS ) of 15.20 J kg− K−1 for ΔH = 50 kG at 6 K, which is the
1
Received 29th May 2014,
Accepted 2nd July 2014
m
highest value among the cobalt-based MOFs with MCE reported so far. The luminescence properties of
compounds 3 and 4 were studied, both of them exhibit photoluminescence in the solid state at room
temperature which can be ascribed to intraligand π→π* transitions.
DOI: 10.1039/c4dt01593b
www.rsc.org/dalton
this strategy, the resulting MOFs may combine the novel archi-
tectures with interesting properties appearing in their SBUs.
Introduction
3
With intriguing structural topologies and potential appli- Furthermore, the organic components of the SBUs can also be
cations in the areas of catalysis, ion exchange, gas separation functionalized, providing an effective platform for the further
4
and storage, luminescence and magnetism, metal organic development of functional molecular materials. However,
frameworks (MOFs) remain an attracting research focus in compared with the common approach using the designed
1
coordination chemistry and materials science. To design and linkers and preselected metal centers as nodes for the
synthesize novel functional MOFs, the construction of cluster- rationalization of the network topology, the cluster-based SBUs
based MOFs using polynuclear clusters as the secondary build- are often generated in situ in different reactions. Thus the
ing units (SBUs) by rational choices of organic ligands is design and synthesis of cluster-based MOFs through this
2
demonstrated to be a powerful synthetic strategy. Through strategy is a rather complex process and it still represents a
5
challenge in coordination chemistry.
Metallacrowns are an interesting class of polynuclear clus-
a
ters that draw on the crown ether analogy for its definition and
Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry,
6
School of Chemistry and Chemical Engineering, Nanjing National Laboratory of
Microstructures, Nanjing University, Nanjing, 210093, China
nomenclature. Since Pecoraro and Lah reported the first
7
metallacrown structure in 1989, metallacrowns with an assort-
b
State Key Laboratory Cultivation Base for the Chemistry and Molecular Engineering
ment of functions including bioactivity, molecular recognition,
catalysis, mimics of surface science, liquid crystals and single-
of Medicinal Resources, School of Chemistry and Pharmacy, Guangxi Normal
University, Guilin, 541004, China. E-mail: fliangoffice@yahoo.com
6
c
molecule magnets have been synthesized. Although it is not
College of Chemistry and Bioengineering, Guilin University of Technology, Guilin,
5
†
41004, China
so easy to control the formation of metallacrown-type building
blocks, they were regarded as excellent SBU candidates to tune
the structural topology and function of MOFs. The reported
examples tend to use primary building units to construct
the metallacrown SBUs, which were linked via other bridging
Electronic supplementary information (ESI) available: Crystal data and struc-
ture refinement, selected bond lengths, X-ray structural data in CIF format, IR
spectra, powder X-ray patterns, thermogravimetric curves and other additional
pictures and data for compounds 1–4. CCDC 997667–997670. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c4dt01593b
8
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groups to form different coordination networks. For example,
the metallacrown SBUs of the 2D [12-MCCu -4]-based MOFs
Experimental
General materials and methods
II
were constructed with (S)-β-phenylalanine hydroxamic acid
8
a
and linked via nitrate anions, the metallacrown SBUs of the All reagents were used as received without further purification.
III
−1
3
D [18-MCMn -6]-based MOFs were constructed with N-substi- IR spectra were recorded in the range of 4000–400 cm on a
tuted salicylhydrazidate and linked via 1,2-bis(4-pyridyl)ethane Perkin-Elmer Spectrum One FT/IR spectrometer using a KBr
8
b–d
or 1,2-bis(pyridyl)ethane,
[
the metallacrown SBUs of the 3D pellet. Elemental analyses for C, H and N were carried out on a
24-MCCd -12]-based MOFs were constructed with diphenic Model 2400 II, Perkin-Elmer elemental analyzer. The ESI mass
acid, isonicotinic acid and linked via 1,2-di(4-pyridyl)ethylene, spectra were acquired using a Bruker Daltonics HCT mass
II
8
e
etc. For the [24-MC-6]-based MOFs, the metallacrown SBUs in spectrometer. The powder X-ray diffraction (PXRD) data were
the reported examples of Co, Mn, Fe, Ni, and Mg were both collected using a Rigaku D/max 2500v/pc diffractometer with
constructed and linked with some multitopic carboxylate Cu-Kα radiation (λ = 1.5418 Å). The thermal analysis was
9
a–c
ligands such as 1,3,5-benzenetricarboxylate,
2-amino-1,4- performed on a Pyris Diamond TG/DTA. The crushed single-
9
d
9e
benzenedicarboxylate and (E)-3-(pyridin-3-yl)acrylate, while crystal sample was heated up to 850 °C in N at a heating rate
2
the formate groups were used as auxiliary bridging ligands of 10 °C min− . The luminescence spectra for the solid
1
in the SBU skeletons. There was only one heterometallic samples were measured at 293 K on a FL3-PTCSPC spectro-
[
24-MCZn-6]-based MOF reported to be constructed without photometer with a xenon lamp as the light source. Magnetic
9
f
any auxiliary ligand. Up to now, the examples and topological susceptibility measurements were performed in the tempera-
nets of [24-MC-6]-based MOFs remain limited, and their pro- ture range of 2–300 K, using a Quantum Design MPMS
perties were scarcely explored.
SQUIDXL-5 magnetometer equipped with a 5 T magnet. The
On the other hand, great interest has emerged over the last diamagnetic corrections for these complexes were estimated
few years in the investigations on molecule-based magnetic using Pascal’s constants, and magnetic data were corrected for
cryocooling materials not only for their environmentally diamagnetic contributions of the sample holder.
friendly, energy-efficient superiority, but also for their high
X-ray structure determination
magnetocaloric effect (MCE), synthetic tunability, and easily
1
0
tailored functionality. Since the first report on {Fe } cage All the data for compounds 1–4 were collected with a Bruker
1
4
1
1
molecular nanomagnets with MCE, the development of this SMART CCD instrument by using graphite monochromatic
field has moved from the discrete clusters to extended struc- Mo-Kα radiation (λ = 0.71073 Å). The data were collected at
tures. As there is no need to prevent aggregations of clusters 160(2) K. Absorption effects were corrected by semi-empirical
with bulky ligands, the extended structure would be advan- methods. The structures were solved by direct methods with the
14a
tageous for obtaining higher metal/ligand mass ratio and program SHELXS-97
and were refined by full-matrix least-
1
0,12
14b
increased magnetic density for large MCE.
Nevertheless, squares methods using Olex2.
The non-hydrogen atoms
most of the current studies on MCE based on extended struc- were refined anisotropically. The aromatic hydrogen atoms
tures are focused on gadolinium(III) MOFs, the investigation were placed in calculated positions and refined using a riding
1
3
on 3d transition metal MOFs is extremely sparse. As a model, but those of phenolic groups were located in the last
matter of fact, 3d transition metal ions could generate a final difference Fourier map. The hydrogen atoms bonded to
better free-ion MCE, and the spin frustration resulted from the disordered water molecules were not discernible from the
competing magnetic exchange-couplings between 3d tran- last final difference Fourier maps and consequently were not
sition metal ions could also be beneficial to the large included in the structure refinement. The final cycle of full-
1
0b,11
MCE.
interesting and worth exploring the MCE of 3d transition tions and variable parameters. A summary of crystal data and
metal MOFs. relevant refinement parameters, selected bond lengths and
These special advantages suggest that it would be matrix least-squares refinement was based on observed reflec-
In this contribution, we report the synthesis and structural bond angles for compounds 1–4 are given in Tables S1–S5 in
characterization of four novel isostructural [24-MC-6]-based the ESI.† The CCDC numbers of 1, 2, 3 and 4 are 997667,
MOFs with a 3D (4,6)-connected pcu net, namely [Co
O (1), [Mn (HipO) ]·6H O (2), [Cd (HipO) ]·6H O (3) and
Zn (HipO) ]·7H O (4) (H ipO = 2-hydroxyisophthalic acid).
6 6
(HipO) ]· 997668, 997669 and 997670, respectively.
6
[
H
2
6
6
2
6
6
2
Syntheses of the ligand
6
6
2
3
The magnetic characterization reveals that compound 1 shows The synthetic routes for the ligand H
3
ipO are presented in
dominant antiferromagnetic interactions between adjacent Scheme S1 in the ESI.† Starting from 2,6-dimethylanisole, the
Co(II) ions and a ferromagnetic-like behavior at lower tempera- intermediate 2-methoxyisophthalic acid was prepared accord-
1
5
tures, whereas compound 2 shows overall antiferromagnetic ing to the reference. Subsequently, a suspension of 2-methoxy-
interactions between Mn(II) ions. Especially, compound 1 exhi- isophthalic acid (15 g, 76.5 mmol) in 33% HBr–AcOH
bits an interesting MCE with the highest −ΔSm value among (150 ml) was heated to 120 °C with vigorous stirring until the
the cobalt-based MOFs reported so far. In addition, the photo- gas evolution subsided. The homogeneous orange mixture was
luminescence of compounds 3 and 4 are presented and cooled to room temperature and the precipitates appeared.
discussed.
The precipitates were separated via filtration, washed with cold
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2
water and then recrystallized from H O–MeOH, affording
H ipO as white powder in 92% yield. M.p. 243–244 °C.
3
1
H NMR (500 MHz, CD
3
OD): δ = 8.11 (d, 2H, J = 7.8), 7.00 (t,
1
H, J = 7.7). Elemental analysis (%) calcd: C, 52.76; H, 3.32.
−
Found: C, 52.49; H, 3.23. ESI-MS m/z: 180.80 [M − H] .
Syntheses of complexes 1–4
3 2 2
A mixture of M(NO ) ·6H O (0.10 mmol, M = Co, Mn, Cd, Zn),
H ipO (0.10 mmol) and 3 drops of pyridine in 1.2 mL H O–
3
2
MeOH (V/V = 1/1, 1.2 mL of H
2
O for 4) was sealed in a Pyrex Fig. 1 (a) [24-MC-6] metallacrown SBU for compound 1. (b) Simplified
2−
6
view of the {Co } chair structure. (c) Coordination modes of HipO
tube and heated to 100 °C for 72 h followed by cooling to
−
1
ligands. Color scheme: black for C, red for O, and purple for Co.
room temperature at a rate of 0.5 °C min . The block-shaped
crystals obtained were filtered and washed with water, to give
compounds 1–4.
HipO − ligands, to form a distorted octahedral geometry with
2
2
+
[
Co (HipO) ]·6H O (1). Yield: 45% (based on Co ). Elemen- the Co–O bond distances in the range of 2.021(3)–2.166(3) Å
6
6
2
tal analysis (%) calcd: C, 37.38; H, 2.35. Found: C, 37.44; H, and the O–Co–O angles ranging from 79.31(10)° to 97.18(10)°
−
1
2
2
7
.22. IR (KBr disk, cm ) selected bands: 3431 (m), 3149 (w), and 170.62(11)° to 174.74(10)°. Similar to other [24-MC-6]-
360 (w), 1603 (vs), 1481 (s), 1450 (s), 1383 (vs), 1296 (m), based MOF reported previously, six Co(II) ions in the SBU of
9
c
57 (m), 726 (w), 612 (w), 455 (m).
Mn (HipO) ·6H (2). Yield: 43% (based on Mn ). HipO
Elemental analysis (%) calcd: C, 37.97; H, 2.39. Found: (Fig. 1c), which is very different from those found in the
C, 37.79; H, 2.17. IR (KBr disk, cm ) selected bands: 3419 (m), reported complexes where the phenolic oxygen atom of the
165 (w), 2364 (vw), 1602 (vs), 1482 (s), 1449 (s), 1374 (vs), ipO ligand bridges two metal ions in a η -mode to form a
compound 1 presents a chair-like conformation (Fig. 1b). All
2
+
2−
1
1
1
2
1
[
6
6
2
O
ligands adopt η :η :η :η :η :μ coordination mode
5
−
1
3
−
2
3
1
2
−
II
II 16
293 (m), 757 (m), 715 (w), 609 (w), 450 (m).
Cd (HipO) ]·6H O (3). Yield: 36% (based on Cd ). Elemen- result, the μ -briding HipO ligands in compound 1 help to
tal analysis (%) calcd: C, 30.94; H, 1.95. Found: C, 31.15; H, construct the [24-MC-6] metallacrown SBUs, as well as to link
planar dinuclear core of [M (ipO) ] (M = V , Cu ). As a
2 2
2
+
2−
[
6
6
2
5
−1
1
2
7
.82. IR (KBr disk, cm ) selected bands: 3432 (m), 3178 (w), them along the crystallographic a-, b- and c-axes (Fig. 2) into
360 (vw), 1659 (vs), 1553 (s), 1447 (s), 1386 (vs), 1289 (m), the final 3D [24-MC-6]-based MOF without auxiliary ligands
62 (m), 704 (w), 629 (w), 450 (m).
Zn (HipO) ]·7H O (4). Yield: 41% (based on Zn ). Elemen- heterometallic example was constructed without any auxiliary
(Fig. 3). Among the reported [24-MC-6]-based MOFs, only one
2
+
[
6
6
2
9
f
tal analysis (%) calcd: C, 36.05; H, 2.40. Found: C, 36.18; H, ligand, while others were constructed with formate groups as
−
1
9a–e
2
2
7
.22. IR (KBr disk, cm ) selected bands: 3427 (w), 3093 (w), auxiliary ligands in their SBU skeletons.
Compared with
360 (vw), 1602 (vs), 1481 (s), 1450 (s), 1374 (vs), 1295 (m), these examples with auxiliary ligands, the successful construc-
57 (m), 720 (m), 641 (w), 455 (m).
tion of the [24-MC-6]-based MOFs without auxiliary ligands
in this work was attributed to the carboxylate groups of the
Results and discussion
Crystal structure
Compound 1–4 were produced from the solvothermal/hydro-
thermal reaction of M(NO ) ·6H O (M = Co, Mn, Cd, Zn) with
3
2
2
3
H ipO. Single-crystal X-ray crystallography determinations
reveal that they are isostructurally crystallized in a cubic space
group with hexanuclear cobalt(II) metal clusters as the SBUs.
Here compound 1 is taken as the example to present and
discuss their structures in detail. As shown in Fig. 1a, six Co(II)
ions in a SBU are linked via twelve carboxylate groups from
twelve ligands, in which the adjacent Co(II) ions are triple-
1
1
bridged through one carboxylate group via η :η :μ
2
-mode,
1
2
another carboxylate group via η :η :μ -mode and one oxygen
atom from the third carboxylate group with η :η :μ -mode. The
3
3
1
2
triple-bridge mode is repeated among the six Co(II) ions, result-
ing in the formation of a [24-MC-6] metallacrown SBU consist-
ing of a [Co–O–C–O] repeating unit with a nonbonding Co⋯Co
distance of 3.459 Å. Each Co(II) ion is coordinated by five
Fig. 2 Portion view of the coordination details of compound 1 showing
twelve ligands around each [24-MC-6] metallacrown SBU, ligands with
green, blue and yellow ochre benzene ring expand in crystallographic
carboxylic oxygen atoms and one phenolic oxygen atom from a-, b- and c-axes, respectively.
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Fig. 5 Schematic representation of the (4,6)-connected network of
compound 1.
whole 3D framework of compound 1 could be rationalized as a
uninodal (4,6)-connected pcu topology net with the Schläfli
1
2
3
symbol of {4 ·6 } as shown in Fig. 5.
Fig. 3 3D framework of compound 1 viewed from the a-axis direction.
Thermal analysis and PXRD
Thermogravimetric analysis (TGA) was carried out to examine
H ipO ligands playing the role of the auxiliary ligand of
3
the thermal stability of compounds 1–4 (Fig. S2 in the ESI†).
formate. Because of this, the arrangements of the [24-MC-6]
metallacrown SBUs in the crystal cell of compound 1 show a
regular change indicated by the orientation of their symmetry
axes. As shown in Fig. 4, the symmetry axes of the two SBUs
locating in the diagonal vertices of cubic crystal cell show the
same orientation, which are different from those of the other
six SBUs. It results in a novel arrangement mode of the SBUs
different from those in the other [24-MC-6]-based MOFs
reported in the literature. From the viewpoint of structural top-
ology, if the [24-MC-6] metallacrown SBU is viewed as a 6-con-
nected node which is linked to six nearest neighbours, the
The crushed single-crystal sample was heated up to 850 °C in
1
N
2
at a heating rate of 10 °C min− . The TGA studies reveal
that the first weight loss of 6.5% between 40 and 160 °C for
compound 1 corresponds to the loss of free lattice water (calc.
7
6
2
.0%). The pyrolysis of ligands occurs at 350 °C and ends at
00 °C with the final residue probably being CoO (found,
8.3%; calc., 29.4%). Compounds 2 and 3 undergo a weight
loss of 6.9% (calc. 7.1%) in the temperature region of
0–170 °C and 6.1% (calc. 6.0%) in the region of 40–250 °C,
4
respectively, corresponding to the loss of their free lattice
water molecules. For compound 4, the free lattice water mole-
cules lost in the temperature range of 40–350 °C with a weight
loss of 6.5% (calc. 7.9%). The followed decomposition of the
frameworks occurs above 310 °C for compounds 2 and 3, and
350 °C for compound 4 with a complicated course of weight
losses which is not complete even when the temperature
reached 850 °C. The above thermal behaviours are attributed
to the structural features and the TGA results of compounds
1
–4 agree well with their formula, respectively. Furthermore,
the PXRD experimental and computer-simulated patterns of
the corresponding complexes are shown in Fig. S3 in the ESI.†
The PXRD patterns of the bulk samples match their simulated
patterns from the single-crystal structures, demonstrating the
phase purity.
Magnetic properties
The variable-temperature magnetic susceptibility of com-
pounds 1 and 2 were measured in a temperature range from
3
00 to 2 K under 1 kOe applied field. As shown in Fig. 6, the
3
m
χ T value of compound 1 at room temperature is 29.74 cm
K mol , which is much higher than that expected for six non-
interacting Co(II) ions with octahedral coordination geometry
(χ T = 11.25 cm mol K, S = 3/2 ions and g = 2) due to the
m
spin–orbital coupling of Co(II) ions. Upon cooling, the χmT
−
1
3
−1
Fig. 4 The packing mode of [24-MC-6] metallacrown SBUs in a cubic
crystal cell of compound 1.
1
7
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Fig. 6
m m
χ , χ T vs. T curves for compound 1 recorded under a 1000 Oe
Fig. 8
χ
m
, χ
m
T vs. T curves for compound 2 recorded under a 1000 Oe
applied field.
applied field.
value decreases gradually and reaches
a
minimum of indicating the absence of any long-range magnetic ordering in
3
−1
1
5.62 cm mol K at 10 K, then abruptly increases to a peak compound 1. The AC susceptibility was further performed
3
−1
of 16.56 cm mol K at 4.0 K followed by a drop down to under the DC field HDC = 0 Oe and the AC field HAC = 2.5 Oe
3
−1
1
5.59 cm mol K at 2 K. In the higher temperature range of from 2 to 8 K (Fig. S6 in the ESI†). The in-phase signal (χ′) is
0–300 K, the reciprocal molar magnetic susceptibility data frequency-independent and a peak appears at 2.2 K, whereas
5
obey the Curie–Weiss law with the Weiss constant θ = −51.99 K the out-of-phase signal (χ″) is not observed.
3
−1
and the Curie constant C = 34.42 cm K mol (Fig. S4 in
For compound 2, the χ T value at room temperature is
m
3
−1
the ESI†), suggesting dominant antiferromagnetic interactions 16.68 cm K mol , which is smaller than the spin-only value
between Co(II) ions, while the contribution of spin–orbital expected for six non-interacting Mn (II) ions with octahedral
coupling and zero splitting should not be ignored. The type coordination geometry (χ
1
8
3
−1
T = 26.25 cm K mol , S = 5/2 ions
m
of magnetic behavior suggested by the steep increase of χ
m
T
and g = 2). Upon lowering of the temperature, the χ
m
T value
3
−1
below 10 K should be related to the ferromagnetic interactions decreases gradually to 13.63 cm mol K at 100 K and then
3
−1
between the effective spins S′ of Co(II) ions, which overcomes monotonously decreases to attain a value of 0.41 cm mol
K
i
−
1
the effect of spin–orbital coupling and compensates the at 2 K (Fig. 8). The fit of the χ
m
versus T curve agrees with the
T. The magnetization curve (M–H) is shown in Curie–Weiss law in the temperature range of 60–300 K with a
Fig. 7, at 2 K, the magnetization value increases rapidly from Weiss temperature θ = −38.21 K and a Curie constant C =
1
9
decrease of χ
m
3
−1
the very beginning, then reaches a maximum value of 15.29Nβ 18.82 cm K mol (Fig. S4 in the ESI†). The negative Weiss
at the highest field (5 kOe), which is approaching the expected constant value reveals the existence of antiferromagnetic coup-
saturation value of 18.0Nβ for six “isolated” Co(II) ions with S = ling between the Mn(II) ions of compound 2. This magnetic
3
/2 ions and g = 2. To determine whether the magnetic order- behaviour of compound 2 is similar to that of compound 1 in
ing occurs at low temperature, the temperature dependencies the high temperature range, but is different from that of com-
of field-cooled (FC) and zero-field cooled (ZFC) magnetization pound 1 in the low temperature range where ferromagnetic
were performed under a field of 20 Oe (Fig. S5 in the ESI†). No interactions between the Co(II) ions are observed. This dissimi-
divergence between the FC and ZFC data was observed, larity arises probably from the difference of the spin–orbital
coupling and magnetic anisotropy of Co(II) and Mn(II) ions
with different electronic structures.
Considering its higher metal/ligand ratio and magnetic
density stemming from the special dense packing mode of
[24-MC-6] metallacrown SBUs, the MCE of compound 1 was
investigated. The isothermal magnetic entropy changes −ΔS
m
can be described as Maxwell relations by integrating over the
Ð
2
0
magnetic field change, ΔS
m
= [∂M(T,H)/∂T]
H
dH. As shown in
Fig. 9, the maximum value of −ΔS
m
of compound 1 is 15.20 J
for ΔH = 50 kG at 6 K. To the best of our knowledge,
−
1
−1
kg
K
the MCE investigation on cobalt-based MOFs is extremely rare.
There were only two examples of cobalt-based MOFs where
their MCE were studied. One shows a −ΔS value of 13.23 J
m
−
1
−1
13a
kg
K
for ΔH = 80 kG at 4 K,
and the other has a −ΔS
and a ΔTad value of 1.5 K for μ
Compared with these two examples, compound 1
m
−
1
−1
value of 2.4 J kg
50 kG.
K
0
ΔH =
Fig. 7 The field-dependent magnetization plots (M/Nβ vs. H) plots of
compound 1.
1
3b
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Conclusions
In summary, we have successfully synthesized and character-
ized four novel isostructural 3D [24-MC-6]-based MOFs with a
(
4,6)-connected net. These MOFs were constructed without any
auxiliary ligand and exhibit a dense packing mode which is
different from the other [24-MC-6]-based MOFs reported in the
literature. Magnetic studies reveal that compound 1 shows
dominant antiferromagnetic interactions (in 300–50 K) and
ferromagnetic-like behaviors (at lower temperatures), while
antiferromagnetic exchanges between the Mn(II) ions are
observed in compound 2. Compound 1 also exhibits a MCE
Fig. 9 Experimental –ΔS
compound 1 at various fields and temperatures.
m
obtained from the magnetization data of with the highest −ΔS
m
value among the cobalt-based MOFs
reported so far. Both 3 and 4 exhibit photoluminescence in
the solid state at room temperature which can be ascribed to
intraligand π→π* transitions. This work provides not only new
shows the highest value of −ΔSm for cobalt-based MOFs examples of metallacrown cluster-based MOFs with novel
reported so far.
structures and properties, but also new insights into the
design of molecule-based magnetic cryocooling materials from
transition metal-based MOFs.
Luminescenee properties
The luminescence properties of compounds 3 and 4 were
investigated in the solid state at room temperature. The
Acknowledgements
emission spectra of compounds 3, 4 and the ligand (H ipO)
3
are depicted in Fig. 10. Upon excitation at 335 nm and
This work was supported by the National Natural Science
Foundation of China (21271050, 20971029), Program for New
Century Excellent Talents in University (NCET-10-0095) and
Guangxi Natural Science Foundation (2012GXNSFBA053018,
3
36 nm, compounds 3 and 4 exhibit strong photoluminescene
at room temperature with maximum emission peaks at 406
and 402 nm, respectively. The maximum emission peak of the
3
free ligand (H ipO) is at 447 nm, which is excited at 347 nm.
2
013GXNSFGA019008).
1
0
Considering the d closed shell electronic configuration of Zn(II)
and Cd(II) ions, the emissions of both compounds 3 and 4 are
neither the metal-to-ligand charge transfer (MLCT) nor the
2
1
ligand-to-metal charge transfer (LMCT). The emission peaks Notes and references
for compounds 3 and 4 could be originated from the intra-
2
2
1 (a) N. Stock and S. Biswas, Chem. Rev., 2012, 112, 933;
b) J. P. Zhang, Y. B. Zhang, J. B. Lin and X. M. Chen, Chem.
ligand π→π* transitions. Compared with the maximum emis-
sion peak at 447 nm of the free ligand, those of compounds 3
and 4 show a blue shift. It is presumably attributed to the
(
Rev., 2012, 112, 1001; (c) Y. F. Zeng, X. Hu, F. C. Liu and
X. H. Bu, Chem. Soc. Rev., 2009, 38, 469.
(a) M. O’Keeffe and O. M. Yaghi, Chem. Rev., 2012, 112,
675; (b) S. M. Cohen, Chem. Rev., 2012, 112, 970;
2
2
coordination of ligands to metal centers, which effectively
2
3
4
5
6
increases the rigidity and asymmetry of the ligands and the
2
3
thus reduced the loss of energy by radiationless decay.
(
1
c) T. R. Cook, Y. R. Zheng and P. J. Stang, Chem. Rev., 2013,
13, 734.
(a) X. J. Li, F. L. Jiang, M. Y. Wu, L. Chen, J. J. Qian,
K. Zhou, D. Q. Yuan and M. C. Hong, Inorg. Chem., 2014,
53, 1032; (b) E. C. Yang, Y. Y. Zhang, Z. Y. Liu and
X. J. Zhao, Inorg. Chem., 2014, 53, 327.
(a) Y. Q. Lan, S. L. Li, H. L. Jiang and Q. Xu, Chem. – Eur. J.,
2012, 18, 8076; (b) S. D. Han, J. P. Zhao, Y. Q. Chen,
S. J. Liu, X. H. Miao, T. L. Hu and X. H. Bu, Cryst. Growth
Des., 2014, 14, 2.
(a) D. S. Li, J. Zhao, Y. P. Wu, B. Liu, L. Bai, K. Zou and
M. Du, Inorg. Chem., 2013, 52, 8091; (b) Y. W. Li, J. R. Li,
L. F. Wang, B. Y. Zhou, Q. Chen and X. H. Bu, J. Mater.
Chem. A, 2013, 1, 495.
G. Mezei, C. M. Zaleski and V. L. Pecoraro, Chem. Rev.,
2007, 107, 4933.
Fig. 10 Emission spectra of compounds 3 and 4 in the solid state at
room temperature.
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This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
7
8
(a) V. L. Pecoraro, Inorg. Chim. Acta, 1989, 155, 171;
b) M. S. Lah and V. L. Pecoraro, J. Am. Chem. Soc., 1989,
11, 7258.
T. Gaudisson and M. François, Inorg. Chem., 2012, 51,
2885; (c) C. B. Tian, R. P. Chen, C. He, W. J. Li, Q. Wei,
X. D. Zhang and S. W. Du, Chem. Commun., 2014, 50, 1915.
(
1
(a) J. J. Bodwin and V. L. Pecoraro, Inorg. Chem., 2000, 39, 14 (a) G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crys-
3
2
434; (b) M. Moon, I. Kim and M. S. Lah, Inorg. Chem.,
000, 39, 2710; (c) D. Moon, J. Song, B. J. Kim, B. J. Suh
tallogr., 2008, 64, 112; (b) O. V. Dolomanov, L. J. Bourthis,
R. L. Gildea, J. A. K. Howard and H. J. Puschmann, Appl.
Crystallogr., 2009, 42, 339.
and M. S. Lah, Inorg. Chem., 2004, 43, 8230; (d) D. Moon
and M. S. Lah, Inorg. Chem., 2005, 44, 1934; (e) R. H. Wang, 15 J. Zhu, X. Z. Wang, Y. Q. Chen, X. K. Jiang, X. Z. Chen and
M. C. Hong, J. H. Luo, R. Cao and J. B. Weng, Chem.
Commun., 2003, 1018.
Z. T. Li, J. Org. Chem., 2004, 69, 6221.
16 (a) E. Solari, A. Klose and C. Floriani, Polyhedron, 1996, 15,
4103; (b) Y. Z. Zheng, M. L. Tong and X. M. Chen, New
J. Chem., 2004, 28, 1412; (c) S. Y. Yang, L. S. Long,
R. B. Huang, L. S. Zheng and S. W. Ng, Inorg. Chim. Acta,
2005, 358, 1882; (d) Z. B. Han, Y. K. He, C. H. Ge, J. Ribas
and L. Xu, Dalton Trans., 2007, 3020; (e) Y. Z. Zheng,
Y. B. Zhang, M. L. Tong, W. Xue and X. M. Chen, Dalton
Trans., 2009, 1396; (f) M. A. Nadeem, A. W. Thornton,
M. R. Hill and J. A. Stride, Dalton Trans., 2011, 40, 3398.
9
(a) J. H. He, Y. T. Zhang, Q. H. Pan, J. H. Yu and R. R. Xu,
Microporous Mesoporous Mater., 2006, 90, 145; (b) Y. Fu,
J. Su, S. H. Yang, G. B. Li, F. H. Liao, M. Xiong and
J. H. Lin, Inorg. Chim. Acta, 2010, 363, 645; (c) T. Z. Zhang,
Z. M. Zhang, Y. Lu and E. B. Wang, J. Coord. Chem., 2012,
6
5, 48; (d) L. Han, L. Qin, X. Z. Yan, L. P. Xu, J. L. Sun,
L. Yu, H. B. Chen and X. D. Zou, Cryst. Growth Des., 2013,
3, 1807; (e) J. P. Zhao, W. C. Song, R. Zhao, Q. Yang,
1
B. W. Hu and X. H. Bu, Cryst. Growth Des., 2013, 13, 2858; 17 (a) F. Luo, Y. X. Che and J. M. Zheng, Cryst. Growth Des.,
(
f) X. Meng, X. Z. Song, S. Y. Song, G. C. Yang, M. Zhu,
Z. M. Hao, S. N. Zhao and H. J. Zhang, Chem. Commun.,
013, 49, 8483.
0 (a) M. Evangelisti and E. K. Brechin, Dalton Trans., 2010,
9, 4672; (b) Y. Z. Zheng, G. J. Zhou, Z. P. Zheng and
2009, 9, 1066; (b) Y. G. Huang, D. Q. Yuan, L. Pan,
F. L. Jiang, M. Y. Wu, X. D. Zhang, W. Wei, Q. Gao,
J. Y. Lee, J. Li and M. C. Hong, Inorg. Chem., 2007, 46, 9609;
(c) O. Kahn, Molecular Magnetism, Wiley-VCH, New York,
1993.
2
1
3
R. E. P. Winpenny, Chem. Soc. Rev., 2014, 43, 1462; 18 (a) H. P. Jia, W. Li, Z. F. Ju and J. Zhang, Chem. Commun.,
(
c) T. N. Hooper, J. Schnack, S. Piligkos, M. Evangelisti and
E. K. Brechin, Angew. Chem., Int. Ed., 2012, 51, 4633;
d) Y. Z. Zheng, M. Evangelisti, F. Tuna and
R. E. P. Winpenny, J. Am. Chem. Soc., 2012, 134, 1057;
2008, 371; (b) J. Li, B. Li, P. Huang, H. Y. Shi, R. B. Huang,
L. S. Zheng and J. Tao, Inorg. Chem., 2013, 52, 11573;
(c) J. Y. Zou, W. Shi, N. Xu, L. L. Li, J. K. Tang, H. L. Gao,
J. Z. Cui and P. Cheng, Chem. Commun., 2013, 49, 8226.
(
(e) E. Cremades, S. Gomez-Coca, D. Aravena, S. Alvarez and 19 Q. Li, J. Qian, C. Tian, P. Lin, Z. He, N. Wang, J. Shen,
E. Ruiz, J. Am. Chem. Soc., 2012, 134, 10532;
f) J. W. Sharples and D. Collison, Polyhedron, 2013, 54, 91.
H. Zhang, T. Chu, D. Yuan, Y. Yang, L. Xue and S. W. Du,
Dalton Trans., 2014, 43, 3238.
(
1
1 D. M. Low, L. F. Jones, A. Bell, E. K. Brechin, T. Mallah, 20 (a) R. Sessoli, Angew. Chem., Int. Ed., 2012, 51, 43;
E. Rivière, S. J. Teat and E. J. L. McInnes, Angew. Chem., Int.
Ed., 2003, 42, 3781.
(b) M. Evangelisti, F. Luis, L. J. deJongh and M. J. Affronte,
J. Mater. Chem., 2006, 16, 2534.
1
2 (a) P. F. Shi, Y. Z. Zheng, X. Q. Zhao, G. Xiong, B. Zhao, 21 A. Barbieri, G. Accorsi and N. Armaroli, Chem. Commun.,
F. F. Wam and P. Cheng, Chem. – Eur. J., 2012, 18, 15086; 2008, 2185.
b) M. Wu, F. Jiang, X. Kong, D. Yuan, L. Long, S. A. Al- 22 (a) C. S. Liu, J. J. Wang, Z. Chang, L. F. Yan and X. H. Bu,
(
Thabaiti and M. Hong, Chem. Sci., 2013, 4, 3104;
CrystEngComm, 2010, 12, 1833; (b) X. L. Wang, C. Qin,
E. B. Wang and Z. M. Su, Chem. – Eur. J., 2006, 12, 2680.
(c) Y. C. Chen, F. S. Guo, Y. Z. Zheng, J. L. Liu, J. D. Leng,
R. Tarasenko, M. Orendáč, J. Prokleška, V. Sechovský and 23 (a) S. L. Zheng, J. H. Yang, X. L. Yu, X. M. Chen and
M. L. Tong, Chem. – Eur. J., 2013, 19, 14876.
3 (a) C. B. Tian, Z. J. Lin and S. W. Du, Cryst. Growth Des.,
W. T. Wong, Inorg. Chem., 2004, 43, 830; (b) S. Sanda,
S. Parshamoni, A. Adhikary and S. Konar, Cryst. Growth
Des., 2013, 13, 5442.
1
2
013, 13, 3746; (b) R. Sibille, T. Mazet, B. Malaman,
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