Co(II)4Gd(III)6 phosphonate grid and cage as molecular refrigerants

Article abstract of DOI:10.1016/j.ica.2015.12.013

The study on structure-property relationships of polynuclear 3d-Gd clusters are very essential in the fields of magnetic refrigerants. We designed two new bulky phosphonic acids: (3-(9H-carbazol-9-yl)propyl)phosphonic acid (CarbpPO₃H₂) and (2-(9-methyl-9H-fluoren-9-yl)ethyl)phosphonic acid (FlumePO₃H₂). Based on these ligands, two clusters [Co(II)₄Gd(III)₆(CarbpPO₃)₆(^tBuCO₂)₁₄(^tBuCO₂H)(H₂O)₃]^.9CH₃CN^.CH₂Cl₂ (1) and [Co(II)₄Gd(III)₆(FlumePO₃)₃(^tBuCO₂)₁₄(μ₃-OH)₆(H₂O)₂(CH₃CN)]^.6CH₃CN (2) were synthesized under ambient condition. Compound 1 has a [3 × 3] grid-like structure, while compound 2 has a rare helmet-like cage structure. The magnetocaloric effect (MCE) have been tuned from 22.83 to 29.06 J kg^-1 K^-1 as the molecular structure changes from grid to cage.

Full text of DOI:10.1016/j.ica.2015.12.013

Inorganica Chimica Acta 442 (2016) 195–199
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Co(II)4Gd(III)6 phosphonate grid and cage as molecular refrigerants
⇑
Xiaoyan Tang, Qixuan Zhong, Ji Xu, Haiqing Li, Sinong Xu, Xueyuan Cui, Bo Wei ,
Yunsheng Ma , Rongxin Yuan
⇑
⇑
Key Laboratory of Advanced Functional Materials, School of Chemistry & Materials Engineering, Changshu Institute of Technology, Changshu 215500, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 October 2015
Received in revised form 29 November 2015
Accepted 1 December 2015
Available online 23 December 2015
The study on structure–property relationships of polynuclear 3d-Gd clusters are very essential in the
fields of magnetic refrigerants. We designed two new bulky phosphonic acids: (3-(9H-carbazol-
9-yl)propyl)phosphonic acid (CarbpPO3H2) and (2-(9-methyl-9H-fluoren-9-yl)ethyl)phosphonic acid
(FlumePO3H2). Based on these ligands, two clusters [Co(II)4Gd(III)6(CarbpPO3)6(tBuCO2)14(tBuCO2H)
(H2O)3].9CH3CN.CH2Cl2 (1) and [Co(II)4Gd(III)6(FlumePO3)3(tBuCO2)14 3-OH)6(H2O)2(CH3CN)].6CH3CN
(l
(2) were synthesized under ambient condition. Compound 1 has a [3 Â 3] grid-like structure, while
compound 2 has a rare helmet-like cage structure. The magnetocaloric effect (MCE) have been tuned
from 22.83 to 29.06 J kgÀ1 KÀ1 as the molecular structure changes from grid to cage.
Ó 2015 Elsevier B.V. All rights reserved.
Keywords:
Co–Gd
Cage
Phosphonate
Molecular refrigerant
1. Introduction
we designed the carbazolyl and fluorenyl phosphonate ligands to
construct Co–Gd clusters to tune MCE. Fortunately, we obtained
Molecular clusters have received a great deal attention due to
their potential applications in magnetic science such as single-
molecule magnets (SMMs) and magnetic refrigerants [1,2]. The
latter are based on the magnetocaloric effect (MCE), where an adi-
abatic demagnetization process leads to cooling [3]. Since the
observation of MCE in Mn12 and Fe8 clusters, molecule-based
magnetic refrigerants have been a hot research topic and rapidly
developing for their synthetic and functional tunability [4].
However, to increase the MCE of clusters is still a great challenge.
Phosphonic acid of the general formula RPO3H2 (R = alkyl or
aryl) have been proved to be ideal for the construction of molecular
clusters [5]. Interestingly, the 3d-4f phosphonate clusters could
function as magnetic refrigerants [6]. To date, the strategy focusing
on metal ions has been reported to increase the MCE of the clus-
ters. Such as, utilizing ferromagnetic interactions between metal
ions and incorporation of the negligible magnetic anisotropy of
Gd(III) ion to build Ni–Gd clusters [7]. Employing both isotropic
and higher spins of Mn(II) and Gd(III) ions to build Mn–Gd clusters
[8]. Increasing the percentage of the Gd(III) ions in the Co–Gd clus-
ters to increase the MCE of the clusters [9].
two Co4Gd6 clusters with distinct structures and molecular weight,
thus we think this could provide us a platform to study the light
molecular weight on the tuning of MCE.
We deem that reducing the ratio of organic ligands in a cluster
with different inorganic components would be feasible to increase
the MCE [10]. As for we have found the R groups in RPO3H2 play a
key role for the formation of the phosphonate clusters [11], then
⇑
Corresponding authors.
E-mail addresses: myschem@hotmail.com (Y. Ma), yuanrx@cslg.edu.cn
Scheme 1. Coordination modes of the phosphonates. Labeled with Harris Notation
(R. Yuan).
[12].
http://dx.doi.org/10.1016/j.ica.2015.12.013
0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

196
X. Tang et al. / Inorganica Chimica Acta 442 (2016) 195–199
Table 1
2. Experimental section
2.1. Materials and methods
Crystallographic data for compounds 1 and 2.
1
2
Formula
M
Crystal system
Space group
a (Å)
C
184H254Cl2Co4Gd6N15O51P6
C132H202Co4Gd6N7O45P3
3879.14
[Co2(l
-OH)2(tBuCO2)4(tBuCO2H)4] Co2 [13], [Co9(tBuCO2H)4
4927.96
triclinic
P1
(O)3(OH)3(tBuCO2)12] Co9 [14], and [Gd2(tBuCO2)6(tBuCO2H)6] Gd2
[9] were prepared according to the literature methods. (3-(9H-car-
bazol-9-yl)propyl)phosphonic acid (CarbpPO3H2) and (2-(9-
methyl-9H-fluoren-9-yl)ethyl)phosphonic acid (FlumePO3H2)
were prepared as described in ESI. All other chemicals and solvents
were commercially purchased and used as received. Elemental
analyses (EA) were performed on a PE 240C elemental analyzer.
The IR spectra were recorded on a NICOLET 380 spectrometer with
pressed KBr pellets. All the magnetic studies were performed on
microcrystalline state. The magnetic susceptibilities were mea-
sured on a Quantum Design MPMS SQUID-XL7 magnetometer.
Diamagnetic corrections were made for both the sample holder
and the compounds estimated from Pascal’s constants [15].
triclinic
ꢀ
ꢀ
P1
19.539(4)
19.715(4)
31.817(6)
84.43(3)
87.87(3)
61.18(3)
10687(4)
2
1.531
2.281
4974
148937/37411
17.5286(19)
17.7822(18)
29.359(3) A
88.414(2)
87.751(2)
69.2580(10)
8550.5(15)
2
1.507
2.767
3880
116983/29837
b (Å)
c Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (M g mÀ3
)
l
(mmÀ1
)
F(000)
Total/unique
reflections
Rint
0.0789
1.060
0.0382
0.980
Goodness-of-fit
(GOF) on F2
a
R1, wR2 [I > 2
r
(I)]
0.0609, 0.1813
0.0937, 0.2302
3.345, À2.270
0.0685, 0.1896
0.0880, 0.2199
2.798, À1.608
2.1.1. Synthesis of [Co(II)4Gd(III)6(CarbpPO3)6(tBuCO2)14(tBuCO2H)
(H2O)3].9CH3CN.CH2Cl2 (1)
R1, wR2 (all data)
max, (
(e ÅÀ3
(D
q)
Dq)
min
)
Compound 1 was obtained by mixing Co2 (0.0949 g, 0.1 mmol),
Gd2 (0.1149 g, 0.075 mmol) and CarbpPO3H2 (0.0289 g, 0.1 mmol)
in CH3CN/CH2Cl2 (1:1) (16 mL) and stirred at room temperature
for 12 h. The resulting solid was filtered and the purple solution
were kept in a vial for ca. two weeks. Purple crystals were collected
by filtration, yield: 0.044 g, 36% (based on Gd2). EA for 1,
a
R1
=
R
||Fo| À |Fc|/
R
|Fo|; wR2 = {
R
w(F2o À F2c)2/
R .
w(F2o)2}1/2
3. Results and discussion
C
184H254Cl2Co4Gd6N15O51P6: C, 44.84; H, 5.20; N, 4.26. Found: C,
44.69; H, 5.01; N, 4.17%. IR (cmÀ1, KBr): 2962.2(m), 1540.5(vs),
1485.5(vs), 1426.3(vs), 1231.1(s), 1181.4(s), 1005.7(s), 748.6(m),
722.4(m), 612.6(m).
3.1. Synthesis
We have used [Gd2(tBuCO2)6(tBuCO2H)6] Gd2 and two kinds of
cobalt starting materials, the dimetallic [Co2(
-OH)2(tBuCO2)4
(tBuCO2H)4] Co2 and nonametallic cobalt(II) [Co9(tBuCO2H)4(O)3
(OH)3(tBuCO2)12
Co9, to react with carbazolyl and fluorenyl
l
]
2.1.2. Synthesis of [Co(II)4Gd(III)6(FlumePO3)3(tBuCO2)14
(l3-OH)6(H2O)2
phosphonate ligands to produce two kinds of cluster under ambi-
ent conditions. Solvothermal conditions have been tried, however,
gave dark-pink solution with some flocculation. If Gd(NO3)3Á6H2O
and Co9 were used in place of Gd2 and Co2 in the same reaction
that gives compound 1, pale-pink block crystals can be obtained.
We have measured this crystal data for many times, however, the
structure cannot be fully solved due to the highly disordered
organic groups. Compound 2 can also be obtained if Co2 was used
as the starting material in place of Co9, however, the yield is low
and the crystals were very fine. The formation of different types
of clusters for 1 and 2 may due to the steric hindrance of the
bulky organic groups. To our knowledge, clusters 1 and 2 are rare
examples of metal phosphonate clusters bearing bulky organic R
groups [17].
(CH3CN)]. 6CH3CN (2)
Compound
2
was obtained by mixing Co9 (0.1125 g,
0.05 mmol), Gd2 (0.1149 g, 0.075 mmol) and FlumePO3H2
(0.0288 g, 0.1 mmol) in CH3CN/CH2Cl2 (1:1) (16 mL) and stirred
at room temperature for 24 h. The resulting solid was filtered
and the purple solution were kept in a vial for ca. one week. Purple
crystals were collected by filtration, yield: 0.048 g, 49% (based on
Gd2). EA for 2, C132H202Co4Gd6N7O45P3: C, 40.87; H, 5.25; N, 2.53.
Found: C, 40.65; H, 5.06; N, 2.38%. IR (cmÀ1, KBr): 2966.9(m),
1628.1(m), 1545.7(s), 1483.4(m), 1420.9(s), 1225.8(m), 1173.7
(m), 1013.5(m), 732.8(m), 616.4(m).
2.2. X-ray Crystallography
Single crystals of dimensions 0.35 Â 0.34 Â 0.32 mm3 for 1,
0.34 Â 0.33 Â 0.31 mm3 for 2 were used for structural determina-
tions on a Bruker APEX-II diffractometer using graphite monochro-
3.2. Structural description
X-ray single crystal diffraction reveals that compound 1 fea-
tures a grid-like structure with one Gd(III) ion surrounded by other
metal centers due to the bridging phosphonates (Fig. 1). All the
cobalt centers are divalent, which have been verified by the Bond
Valence Sum (BVS) calculation (Table S1). Take the edge Co(II)
dimer as a single node, the resulting topology is a [3 Â 3] grid. In
the heterometallic core, two isolated corner cobalt ions are four-
coordinated with tetrahedron geometry, while cobalt sites in the
Co(II) dimer are five-coordinated with a square-pyramidal geome-
matized Mo Ka radiation (k = 0.71073 Å) at room temperature. Cell
parameters were refined by using the program Bruker SAINT on all
observed reflections. The collected data were reduced by using the
program Bruker SAINT, and an absorption correction (multi-scan)
was applied. The reflection data were also corrected for Lorentz
and polarization effects. The structures were solved by direct
methods and refined on F2 by full matrix least squares using SHELXTL
[16]. All of the non-hydrogen atoms were located from the Fourier
maps, and were refined anisotropically. All H atoms were refined
isotropically, with the isotropic vibration parameters related to
the non-H atom to which they are bonded. The crystallographic
and refinement details are listed in Table 1.
try (s range from 0.14 to 0.37) [18]. The Co–O and Gd–O bond
lengths range from 1.914 to 2.219 Å and 2.207 to 2.752 Å, respec-
tively, which are consistent with that observed in the Co–Gd clus-
ters [19]. The nearest metal–metal contacts of GdÁ Á ÁGd, CoÁ Á ÁGd

X. Tang et al. / Inorganica Chimica Acta 442 (2016) 195–199
197
Fig. 1. (a) The structure of 1 in the crystal. Color codes: blue, cobalt; purple,
lanthanide; green, phosphorous; yellow, oxygen; gray, carbon; hydrogen atoms and
lattice solvent molecules are omitted for clarity. (b) The comparison of the core of 1
(left) and the [3 Â 3] {Co4Gd6} grid (right) by taking the cobalt dimer as a node. (For
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
Fig. 2. (a) The structure of 2 in the crystal. Color codes are the same to compound 1.
Lattice solvent molecules are omitted for clarity. (b) Helmet-like core of 2. (For
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
and CoÁ Á ÁCo contacts are 3.92, 3.76 and 3.10 Å, respectively. The
phosphonates show 5.222, 4.221 and 3.211 coordination modes
in 1. Fourteen deprotonated pivalates are surrounded in 2.11 mode
in the periphery (see Scheme 1). There exists a protonated pivalic
acid coordinating with Gd atom in 1.10 mode. The inorganic core
was surrounded by the strongly hydrophilic carbazolyl and
tert-butyl groups from phosphonates and pivalates. It should be
noted that this grid-like core structure is similar to that in
[Mn4Gd6(O3PCH2Ph)6(HO2CtBu)13(O2CMe)(HO2CtBu)(OH2)2(MeCN)2]
and [Co4Dy6(O3PCH2Ph)6(O2CtBu)14(HO2CtBu)(MeCN)(H2O)2] with
PhCH2PO3H2 and HO2CtBu ligands [8,9]. Fig. S1 shows the packing
diagram of compound 1 along the a-axis. Clearly, the decanuclear
molecules are stacked in the lattice, forming vacancies where the
Valence Sum (BVS) calculation (Table S2). Two phosphonates in
the cage show 5.222 mode to coordinate with Co and Gd atoms,
while the third one shows 4.221 mode to coordinate with four
Gd atoms. Fourteen deprotonated pivalates are surrounded in the
periphery in 2.11 or 1.11 modes. The Co–O and Gd–O bond lengths
lie in the ranges of 1.934–2.156 Å and 2.207–2.752 Å, respectively.
The nearest GdÁ Á ÁGd, CoÁ Á ÁGd and CoÁ Á ÁCo separations are 3.95,
3.70 and 3.12 Å, respectively, which are similar to that in
compound 1. The exterior of the cage consists of fluorenyl and
tert-butyl groups from the ligands, leading to a hydrophobic exte-
rior. The molecules are stacked in the lattice, forming vacancies to
fill the guest molecules (Fig. S2).
lattice molecules reside. The
p–p stacking interaction is found
between the molecules with centroid–centroid distance between
the carbazolyl groups of 4.46 Å [20].
Compound 2 has a helmet-like cage structure due to the bridg-
3.3. Magnetic properties
ing phosphonates, pivalates and l3-OH groups (Fig. 2). The helmet
can be described as one cobalt atom locates on the top position,
two Gd2O2 rhombuses sit on the earmuff sites with each connected
with another cobalt atom, one CoGdO2 rhombus lies on the visor
site, and one Gd atom locates on the back of the head. Earmuffs
and visor are joined together by three phosphonate groups, while
The magnetic behavior of 1 and 2 were studied on polycrys-
talline samples using a SQUID magnetometer (Fig. 3). At room
temperature the
v
T values (64.5 cm3 K molÀ1 for 1, 62.7 cm3
K molÀ1 for 2) are larger than the calculated spin-only value
(54.75 cm3 K molÀ1 for four S = 3/2 and six S = 7/2 centers, g = 2).
The differences are due to orbital contributions from Co(II) ions
the top, earmuffs and visor are connected by
l3-OH groups. The
cage contains two four-coordinate cobalt ions with tetrahedral
coordination geometries. While, the last two cobalt sites are six
and five-coordinate with octahedral and square-pyramidal geome-
tries. All the cobalt centers are divalent according to the Bond
[15]. Upon cooling, for 1, the
temperature down to about 50 K, before decreasing more rapidly.
At 1.8 K the
T is 43.0 cm3 K molÀ1, indicating paramagnetic states
are still populated due to the very weak exchange involving
vT decreases slowly with decreasing
v

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X. Tang et al. / Inorganica Chimica Acta 442 (2016) 195–199
Fig. 4. Experimental
DSm for 1 (a) and 2 (b) at various fields and temperatures.
Fig. 3. The
v
T versus T plot of 1 (a) and 2 (b) under 0.1 T dc field.
Lines are guides to the eye.
4. Conclusions
4f-ions. The quench of the orbital-contribution also contributes to
the decrease of T with lowing the temperature because of the
tetra- and pentacoordinated Co(II). For 2, T remains nearly
unchanged down to 16 K, this reflects a very weak magnetic
interaction between the metal centers. Upon cooling, T decreases
v
The preparation of grid and helmet-like {Co(II)4Gd(III)6} clusters
by employing bulky phosphonate ligands was performed. The
results show that both starting metal sources and bulky car-
bazolyl/fluorenyl ligands are responsible for the isolation of these
clusters. The helmet-like {Co(II)4Gd(III)6} shows light molecular
weight compared with the grid-like one, which is advantageous
to magnetic refrigerants. The observation of a larger MCE in 2
means our strategy of tuning MCE by designing the cluster with
light molecular weight is practicable. In addition, our studies on
Co–Gd clusters bearing carbazolyl/fluorenyl phosphonate ligands
should be advantageous since 3d ions and a wide variety of ligands
are available. Further studies on Mn–Gd clusters for magnetic
refrigeration are underway based on the carbazolyl/fluorenyl phos-
phonate ligands.
v
v
sharply to 46.6 cm3 K molÀ1 at 1.8 K, indicating a paramagnetic
state still present. Magnetization measurements were performed
in the range of 0–7 T at 2 K (Figs. S3 and S4). The M values show
a steady increase with increasing field to 55.4 and 55.3 Nb at 7 T
without achieving saturation. The quantitative analysis was not
performed given the complexity of the structures and the orbital
degenerate nature of the cobalt ions.
The magnetocaloric properties were investigated. The magnetic
entropy changes for changing applied field were calculated indi-
rectly from the magnetization behavior as a function of applied
field and temperature (Fig. 4) using the standard relationship
D
S = ʃ[oM(T, H)/oT]HdH [21]. For 1, the À
DS versus T plots increase
Acknowledgements
gradually from 10 to 2 K, reaching a maximum of 22.83 J kgÀ1 KÀ1
at 7 T. The observed value is close to that reported for {Co4Gd6} [9].
There is no sign of a downturn, which means the maximum should
come at a lower temperature for the investigated applied field
This work is supported by NSFC (Nos. 20101018, 21201025) and
NSF of Jiangsu Provence (Nos. BK2012643, BK2012210,
12KJA150001, 14KJA150001).
changes. For 2, the À
DS versus T plots reach a maximum of
29.06 J kgÀ1 KÀ1 at 2 K. This entropy change is slightly higher than
the reported (28.6 J kgÀ1 KÀ1) of the {Co6Gd8} cage [9]. Take a
Appendix A. Supplementary material
comparison of entropy exchanges of 1,
2 and the reported
{Co(II)4Gd(III)6} cluster, we can draw a conclusion that the larger
MCE exhibited by compound 2 is own to its light molecular
weight.
CCDC 1417642 and 1417643 contain the supplementary crys-
tallographic data for compounds 1 and 2. These data can be
obtained free of charge from The Cambridge Crystallographic Data

X. Tang et al. / Inorganica Chimica Acta 442 (2016) 195–199
199
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