1. A 2D coordination polymer based on Co3-SBU showing
spin-canting ferromagnetic behaviour†
Sajal Khatua, Soumyabrata Goswami, Srinivasulu Parshamoni, Himanshu Sekhar Jena
and Sanjit Konar*
A 2D MOF with the secondary building unit [Co3(CTC)6(Py)2(OH)2] is synthesized using the sodium
salt of a flexible ligand, cis,cis-cyclohexane-1,3,5-tricarboxylate (Na-CTC), and a cobalt trimer
([Co3O(CH3COO)6(Py)3]$ClO4). Structural analysis shows that the complex crystallizes in the
monoclinic space group P21/n and forms a 2D framework with channel sizes of 9.0 Â 4.2 ˚A2
.
Magnetic characterization shows spin-canting ferromagnetic behaviour at low temperatures.
Frequency dependency for in-phase (c 0
M) and out-of-phase (c00
M
) signals at low temperatures is
observed from the AC measurements. The gas adsorption behaviour suggests selectivity towards
CO2 over N2, and demonstrates enhancement of CO2 uptake (19–33 cm3
gÀ1
) on lowering the
temperature (273–195 K).
Introduction
Metal Organic Frameworks (MOFs) are a new class of functional
materials that have attracted substantial attention from scien-
tists because of their ability to mimic, and even improve, solid
state properties like gas storage, separation, drug delivery,
catalysis, luminescence, magnetism etc.1–3
In this respect,
designing MOFs that combine more than one property is a
major synthetic challenge for chemists. Moreover, synthesis of
multinuclear transition metal clusters using suitable multi-
dentate carboxylate ligands, is also a subject of interest for the
following reasons: (i) they can engender magnetic ordering in
coordination networks and prompt individual molecules to
behave as magnets at low temperature, so called Single Molecule
Magnets (SMMs);4
(ii) they can efficiently catalyze different
organic reactions;5
(iii) they also can be used as precursors for
the synthesis of nanosized magnetic oxides6
etc. In this context
the creation of porous magnets from an extended polynuclear
cluster is a long-sought academic goal, since magnetism and
porosity could be simultaneously achieved by proper choice or
design of the ligands used. In this work we have chosen a exible
ligand 1,3,5-cyclohexane tricarboxylic acid (H3CTC), which could
facilitate strong magnetic interactions between the metal
centres via carboxylate bridging, as well as induce signicant gas
adsorption due to the presence of polar functional groups.
Although CTC ligands have been well explored in the fabrication
of MOFs7
with specic topologies and interesting properties,
cobalt complexes with CTC ligands are scarce in the literature.8
We have selected cobalt because of its large single-ion anisotropy
(making it a good candidate for single-molecule and single-chain
magnets) and its exibility in adopting different coordination
modes.9
Variable temperature magnetic susceptibility measure-
ments show antiferromagnetic coupling between the adjacent
Co2+
centres at higher temperatures and spin-canting ferro-
magnetic behaviour at lower temperatures. AC susceptibility
measurements show prominent frequency dependent
in-phase (c0
M) and out-of-phase (c00
M
) signals and conrm the
presence of long range ordering. In addition, the gas sorption
behaviour of 1 shows selectivity towards CO2 adsorption over
N2, and demonstrates the enhancement of CO2 uptake (19–
33 cm3
gÀ1
) on lowering the temperature (273–195 K).
Results and discussion
Complex 1 was synthesized via a hydrothermal method using a
cobalt trimer ([Co3O(CH3COO)6(Py)3]$ClO4)10
and Na-CTC in a
1 : 3 ratio in 6 mL H2O. TG analysis of 1 reveals a weight loss of
18.77% (calc. 18.74%) over the temperature range of 140
C to
333
C, which can be assigned to the loss of coordinated pyri-
dine molecules from the framework (Fig. S1†). The weight loss
of 44% (calc. 47%) over the temperature range 400–505
C
corresponds to the decomposition of CTC ligands in the
framework followed by framework collapse. From the TG prole
it is concluded that the framework is quite robust in nature and
stable up to 330
C. The bulk phase powder X-ray diffraction
pattern (Fig. S2†) of the complex is in good agreement with the
simulated pattern, based on the single crystal structure data,
indicating phase purity in the synthesized product.
Department of Chemistry, IISER Bhopal, Bhopal 462023, MP, India. E-mail: skonar@
iiserb.ac.in; Fax: +91-755-4092392; Tel: +91-755-4092336
† Electronic supplementary information (ESI) available: Additional structures,
PXRD, additional magnetic data, gas adsorption and TGA plots provided. CCDC
941541. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3ra44331k
Cite this: DOI: 10.1039/c3ra44331k
Received 13th August 2013
Accepted 24th September 2013
DOI: 10.1039/c3ra44331k
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2. Structural description
Single crystal X-ray analysis of 1 reveals that it crystallizes in the
monoclinic space group P21/n. The asymmetric unit comprises
of two six-coordinated Co2+
ions, one tri-anionic CTC ligand,
one coordinated Py molecule and one coordinated hydroxyl
group as shown in Fig. 1. The crystal data and structure
renement details are summarized in Table 1. From the core
structure it can be seen that the Co1 and Co2 centres are non-
equivalent in terms of their coordination environment. The Co1
centre is coordinated by four oxygen atoms from three different
CTC ligands, one from the hydroxyl group and one nitrogen
atom (N1) from the Py ligand, whereas the Co2 centre is bound
to six CTC ligands through six oxygen atoms. The cyclohexane
ring of the CTC ligand adopts the most stable chair conforma-
tion, with the carboxylic substituents in the equatorial posi-
tions. From a topological perspective, each CTC ligand is linked
to three di-nuclear Co2+
centres which further bridge to another
symmetry equivalent Co2+
centre through a bridging oxygen
atom in a carboxylate ligand which results in a tri-nuclear cobalt
cluster. Thus, three pseudo-octahedral Co2+
centres form a
linear array (Co1–Co2–Co1A) with distances between Co1–Co2
(Co2–Co1A) and Co1–Co1A of 3.109(3) ˚A and 6.218(5) ˚A
respectively (Table S1†).8a
The central Co2 is situated on a pseudo-inversion centre.
Each Co3 cluster, [Co3(CTC)6(Py)2(OH)2] acts as a secondary
building unit (SBU) as shown in Fig. 2. The trinuclear cobalt
core is surrounded by six CTC ligands, two Py molecules and
two hydroxyl groups (Fig. 3). Each CTC ligand binds to three Co2
centres with a distorted trigonal geometry (Fig. 4). Overall,
complex 1 displays a 2D framework (Fig. 5). The structure can
also be described as a 2D framework, which is a combination of
two 1D chains. The trinuclear cobalt clusters are rst linked by
two of the three carboxylate groups on the CTC ligands and then
interconnected through the remaining carboxylate groups
forming channels with the dimensions 9.0 Â 4.2 ˚A2
along the
a-axis. Careful analysis of the framework demonstrates a tetra-
hedral arrangement of the cyclohexane rings of the ligands
(Fig. S3†).
Since in complex 1 each trinuclear cobalt centre is connected
with six CTC ligands it can be regarded as a six-connected node,
whereas each CTC ligand connected with three trinuclear cobalt
Fig. 1 Ball and stick representation of the asymmetric unit of 1; colour code:
blue N, gray C, red O and pink Co.
Table 1 Data collection and structure refinement parameters for complex 1a
Formula C28H30Co3N2O14
Mw (g molÀ1
) 795.33
Crystal size (mm) 0.42 Â 0.38 Â 0.28
Crystal system Monoclinic
Space group P21/n
T (K) 296
Wavelength (˚A) 0.71073
a (˚A) 11.6092(7)
b (˚A) 9.0436(5)
c (˚A) 14.3380(8)
b (
) 105.713(3)
V (˚A3
) 1449.08(15)
Z 2
rcalcd (g cmÀ3
) 1.823
m(MoKa) (mmÀ1
) 1.776
F(000) 810.0
Tmax, Tmin 0.608, 0.479
h, k, l range À12 # h # 15
À5 # k # 12
À19 # l # 19
Collected reections 4041
Independent reections 3091
Goodness-of-t (GOF) on F2
0.951
R1, wR2 (I 2sI) 0.0299, 0.1090
R1, wR2 (all data) 0.0324, 0.1113
Largest difference in peak and hole (e ˚AÀ3
) 0.669/À0.537
CCDC Number 941541
a
R1 ¼
P
||Fo| À |Fc||/
P
|Fo| wR2 ¼ |
P
w(|Fo|2
À |Fc|2
)|/
P
|w(Fo)2
|1/2
.
Fig. 2 Coordination environment of the trinuclear cobalt cluster (Co3-SBU)
found in 1; colour code same as in Fig. 1.
Fig. 3 Core structures of the trinuclear Co3 units, showing the different modes of
connection via one-atom (O) and three-atom (O–C–O) bridges.
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3. centres can be considered as a three connected node. Hence the
aforementioned 2D framework is constructed from six- and
three-connected nodes. Furthermore, three pairs of oxygen
atoms from each CTC ligand bridge to three different cobalt
clusters, in three different bridging modes (Fig. 6): in pair I, the
oxygen atoms exhibit both a m1 bridging mode (O3) and a m1,1
bridging mode (O4) (2.21, according to Harris notation11
), in
pair II both are in the m1 bridging mode (O5 and O6) (2.11) and
in pair III one is in a m1,1 bridging mode (O2) and the other is
free (O1) (2.20) (Fig. 6). The free oxygen atom in each CTC ligand
exhibits a strong, charge-assisted H-bonding interaction with a
coordinated hydroxyl group (O7–H7/O1 ¼ 2.782(2) ˚A; O7/
O1 ¼ 2.824(2) ˚A) in a helical fashion (Fig. S4†). The H-bonded
interactions give the complex its 3D features.
Magnetic property study
Previously, Pavlishchuk et al. reported a similar type of cobalt
trimer [Co3(PhCOO)6(Py)2] complex, but its magnetic properties
were not investigated.12
We have carried out magnetic
measurements on a well-powdered crystalline sample using a
SQUID-VSM magnetometer. The temperature dependence of
the molar magnetic susceptibility (cM) of complex 1 was
measured in the temperature range 1.8–300 K at 0.1 T. The cMT
vs. T plot (Fig. 7) shows a room temperature cMT value of 8.14
cm3
molÀ1
K, which is greater than the expected spin-only value
(3 Â 1.87 ¼ 5.61 cm3
molÀ1
K) for three high-spin Co2+
ions with
S ¼ 1/2 and g ¼ 2.0.14a
This might be due to the orbital contri-
bution typical for the 4
T1g ground state of octahedral high-spin
Co2+
complexes.13
As the temperature decreases from room
temperature, cMT declines very slowly until a value of 7.52 cm3
molÀ1
K is reached at 43.26 K, then increases sharply at low
temperatures to a value of 10.32 cm3
molÀ1
K at 5.26 K, aer
which an abrupt drop in the curve is observed. The gradual
decline in the cMT value below 300 K is due to an antiferro-
magnetic (AF) interaction between the neighbouring Co2+
centres, which is then overcome by an appreciable ferromag-
netic (FM) interaction down to a temperature of 5.26 K. The
sudden drop in the cMT value at even lower temperatures may
be due to a zero-eld splitting (ZFS) effect.14
The 1/cM data for
complex 1, within the temperature range of 1.8–300 K, were
tted to the Curie–Weiss equation, [cM ¼ C/(T À q)], which
afforded the Curie constant, C ¼ 7.96 cm3
molÀ1
K, and Weiss
constant, q ¼ À1.47 K (Fig. 8). The negative value of q indicates
the presence of an antiferromagnetic (AF) interaction between
Co2+
ions. For an antiferromagnetic system, the exhibition of a
signicant ferromagnetic interaction in the lower temperature
Fig. 4 Illustration of the distorted trigonal arrangements of the three Co2
centres bridged by a single CTC ligand.
Fig. 5 Perspective view of the 2D framework found in 1.
Fig. 6 Illustration of the bridging mode of each pair of oxygen atoms found in
complex 1, with Harris notations.
Fig. 7 cMT vs. T curve measured at 0.1 T. The red solid line shows the best fit to
the given equation. The inset curve presents the magnetization plot.
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4. region can be ascribed to spin-canting behaviour.15
Within the
trimeric unit, the magnetic axes (axial) of the coordination
octahedrons are not parallel (Fig. 9), which suggests that the
spin orientations are also not perfectly parallel. This is likely to
be the probable reason for the spin-canting behaviour.
To t the experimental data from 300 to 5 K a non-critical
scaling theory with the following simple phenomenological
equation was used14b,16
cMT ¼ A exp(ÀE1/kT) + B exp(ÀE2/kT)
where A + B equals the high-temperature Curie constant and E1
(0) represents the “activation energy” corresponding to the
spin–orbit coupling, responsible for the initial high-tempera-
ture decay of cMT. The low-temperature behaviour of cMT is
denoted by the second term E2, which represents ferromagne-
tism. The best t to the above equation gives E1/k ¼ 2.58 cmÀ1
,
E2/k ¼ 0.14 cmÀ1
and (A + B) ¼ 7.88 cm3
K molÀ1
, which is
consistent with that obtained from the Curie–Weiss law in the
high-temperature range (C z 7.96 cm3
K molÀ1
).
The isothermal magnetization curve (M/NmB vs. H plot, Fig. 7
inset) shows a saturated value of 5.12NmB at 2 K and 7 T, which
is almost half the saturation value of 9NmB for the three inde-
pendent S ¼ 3/2 spins with g ¼ 2, indicating canted-antiferro-
magnetic behaviour, but it is higher than the value of 3NmB
expected for three antiferromagnetically coupled Co2+
ions, due
to spin-canted ferromagnetic ordering with the nearest-neigh-
bouring centres via carboxyl groups.17
As the isothermal magnetization plots did not superimpose
(Fig. 10), indicating anisotropy, the AC magnetization dynamics
were also studied as a function of temperature (1.8–10 K) and
frequency (n ¼ 1–782 Hz), in zero and higher DC elds and an
3.5 Oe AC eld. Both the in-phase (c0
M) and out-of-phase (c00
M
) AC
signals clearly show frequency dependency below 3.7 K (Fig. 11)
and conrm the presence of long range magnetic ordering.
Gas adsorption property
To evaluate the porosity of complex 1, adsorption experiments
were performed on the activated samples at 77 K with N2 and at
298 K, 273 K and 195 K with CO2. The adsorption isotherm for
N2 (Fig. 12) shows limited uptake (6 cm3
gÀ1
) in the low relative
pressure region and the isotherm can be categorised as Type
II18c
according to the IUPAC classication,18
indicating that a
typical physisorption process is occurring. The BET and Lang-
muir surface areas calculated from the adsorption and
desorption isotherms were 7.52 m2
gÀ1
and 14.07 m2
gÀ1
respectively. The uptake of CO2 is negligible at 298 K (Fig. S5†).
At 273 K the CO2 uptake is 19 cm3
gÀ1
and it increases to a better
value of 33 cm3
gÀ1
at 195 K (Fig. 12) signifying that the trend in
Fig. 8 1/cM vs. T plot for complex 1. The solid red line represents the best fitting
according to the Curie–Weiss equation.
Fig. 9 Schematic representation of the inclination of the magnetic axes of the
terminal cobalt ions from the basal plane passing through the central cobalt ion.
Fig. 10 M/NmB vs. H/T plots for complex 1 measured at 2–8 K.
Fig. 11 c0
0
M vs. T and c0
M vs. T (in inset) plots measured in 3.5 G AC field and 1800
Oe DC field.
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5. CO2 uptake increases as the temperature is lowered. This may
be because CO2, with its large quadruple moment and polariz-
ability, interacts with small and permanent hydrophobic pores,
preventing deep diffusion into the pores. The CO2 isotherms
did not show any signicant hysteresis during adsorption and
desorption, indicating that no expansion of the pore volume
had occurred, and can be identied as Type I.18d
Thus, complex
1 adsorbs CO2 selectively over N2.
Conclusions
We have reported for the rst time a linear Co3 cluster as a SBU
in the construction of a 2D MOF using a cobalt trimer and a
CTC ligand. The magnetic behaviour is antiferromagnetic at
high temperatures and spin-canted ferromagnetic behaviour
is observed at low temperatures. The frequency dependency of
the in-phase and out-of-phase AC magnetic susceptibility was
also demonstrated for the complex. From the adsorption
isotherms it can be seen that the complex has selectivity
towards CO2 over N2. Hence the complex can be claimed to be
a smart material showing interesting magnetic and adsorption
properties.
Experimental section
General information
All the reagents and solvents were commercially available and
were used as supplied without further purication. The cobalt
trimer, ([Co3O(CH3COO)6(Py)3]$ClO4), was prepared using a
literature method10
and cyclohexane tricarboxylic acid (CTC)
was obtained from the Sigma Aldrich Chemical Co. The
elemental analysis (C, H and N) was carried out using a Ele-
mentar Micro vario Cube Elemental Analyzer. FT-IR spectra
(4000–400 cmÀ1
) were measured from samples on KBr pellets
with a Perkin Elmer Spectrum BX spectrometer. Powder X-ray
diffraction (PXRD) data were collected using a PANalytical
EMPYREAN instrument using Cu-Ka radiation. Magnetic
measurements were performed using a Quantum Design VSM
SQUID magnetometer. The measured values were corrected for
the experimentally measured contribution of the sample
holder, while the derived susceptibilities were corrected for the
diamagnetism of the samples, estimated from Pascal's tables.19
Synthesis of 1 (C28H30Co3N2O14)
In a typical hydrothermal synthesis, a mixture of cobalt trimer
([Co3O(CH3COO)6(Py)3]$ClO4), as prepared according to the
literature method (0.1 mmol, 86 mg) and Na-CTC (0.3 mmol, 85
mg) were added to 6 mL H2O and stirred for 1 h. The nal
mixture was transferred to a Teon-lined stainless steel (23 mL)
vessel and kept under autogenous pressure at 100
C for ve
days. Aer that, it was le to cool slowly to room temperature,
which afforded dark pink colored crystals at the bottom of the
vessel. The crystals were separated by ltration, washed with
water and air-dried. Yield: 54 mg (68%). Anal. Calc. for
C28H30Co3N2O14: C, 42.28; H, 3.80; N, 3.50%; found: C, 42.27; H,
3.79; N, 3.48%. FT-IR (KBr pellet, 4000–400 cmÀ1
): n (COOÀ
)
1607 cmÀ1
(asym.), 1216 cmÀ1
(sym.) and n (C]N) 1576 cmÀ1
.
X-ray crystallographic analysis
Single crystal X-ray data were collected on a Bruker APEX-II CCD
diffractometer equipped with graphite monochromatized
Mo-Ka radiation (l ¼ 0.71073 ˚A) at 296 K using 4 and u scans.
The structures were solved using direct methods followed by
full matrix least square renements against F2
(all data HKLF 4
format) using SHELXTL.20,21
Anisotropic renement was used
for all non-hydrogen atoms. Hydrogen atoms were placed in
appropriate calculated positions.
Gas adsorption measurements
N2 and CO2 adsorption studies were performed using a BEL-
SORP MAX (BEL JAPAN) volumetric adsorption analyser. All the
gases used were Ultra-pure research grade (99.99%). Prior to
the measurements a known weight of adsorbent powder sample
(120 mg) was activated for 12 h under a 10À2
kPa continuous
vacuum using a Bel Prepvac II pump and purged with N2 on
cooling.
Acknowledgements
SK, SG and SP thank IISER Bhopal for the PhD fellowships. HSJ
thanks IISER Bhopal for Post-Doctoral fellowship. SK acknowl-
edges DST and CSIR, Government of India (DST Project no.
SR/FT/CS-016/2010 and CSIR Project no. 01/(2473)/11/EMR-II)
and IISER Bhopal for generous nancial and infrastructural
support.
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