Design and in situ synthesis of a Cu-based porous framework featuring isolated double chain magnetic character

Article abstract of DOI:10.1039/c1cc13418c

A Cu-based double chain MOF was synthesized in hydrothermal conditions using an in situ ligand formation method. This double chain MOF shows temperature dependent ferromagnetic (long range ordering) and antiferromagnetic (intra double chain) coupling. The magnetic behaviour originating from the individual double chain remains almost same even after structural collapse due to the removal of guest water molecules.

Full text of DOI:10.1039/c1cc13418c

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Design and in situ synthesis of a Cu-based porous framework featuring
isolated double chain magnetic characterw
a
a
b
a
a
Chandan Dey,z Raja Das,z Binoy Krishna Saha, Pankaj Poddar* and Rahul Banerjee*
Received 9th June 2011, Accepted 22nd August 2011
DOI: 10.1039/c1cc13418c
A Cu-based double chain MOF was synthesized in hydrothermal
conditions using an in situ ligand formation method. This double
chain MOF shows temperature dependent ferromagnetic (long
range ordering) and antiferromagnetic (intra double chain)
coupling. The magnetic behaviour originating from the individual
double chain remains almost same even after structural collapse
due to the removal of guest water molecules.
Cu-DCM was synthesized in hydrothermal conditions
(Section S1, ESIw) using the mixture of Cu(NO ) ꢀxH O,
3
2
2
2 4
neocuproine, Na MoO and 1,3,5-BTC at 170 1C for 96 h
2 4
(Fig. 1a). The role of Na MoO is to catalyze the oxidation of
neocuproine. We expected that neocuproine after in situ oxi-
dation would become 2,9-dicarboxy phenanthroline. This link
may act as a bulky terminal ligand to prevent the growth of the
structure in two or three dimensions. Since all the coordinating
functionality of this link points inwards, the resulting network
should become 0D or 1D in nature. The usage of BTC was to
provide an opportunity for these zero/one dimensional units to
connect to each other in order to form isolated double or triple
chain magnets (Fig. S1, ESIw). As anticipated, the crystal structure
analysis of Cu-DCM reveals that the methyl functionality of
neocuproine was oxidized in situ and became a carboxylate
functionality. However, one of the two carboxylic acids was
Metal–organic frameworks (MOFs) represent a new class of
network solids that have great potential in specific applications
1
like separation, storage, catalysis and drug delivery. How-
ever, most of the research on MOFs has been concentrated on
2
gas storage and sequestration. It is only recently that the
molecular magnetism aspect of MOFs and its possible applications
3
in magnetic devices has attracted researchers’ attention. One
of the most remarkable features of MOF based magnetic
materials is that the magnetic properties could be altered by
4
small and subtle variations in the level of molecular chemistry.
As a part of this active research on MOF based molecular
magnets, single-chain magnets (SCMs) or double chain magnets
(
DCMs) have become important for their potential applications
5
in high-density information storage devices. However, the
successful construction of SCMs is still a great challenge,
because it is difficult to experimentally achieve various
constraints that will essentially create pre-designed 1D chains.
There are several reports of SCMs with Co(II), Ni(II), Mn(III),
6
Fe(II) etc but reports of SCMs with Cu(II) are rare. Moreover,
there are only a handful of reports where two of these
magnetic single chains are connected to each other by coordi-
7
nate or covalent bonds to form an isolated double chain.
Here, for the first time, we report the design and in situ
synthesis of a porous 1D Double-Chain Magnetic (DCM)
Framework, Cu
2
(Phen-COOH)
2
(BTC)ꢀ3H
2
O (Cu-DCM).
a
Physical/Materials Chemistry Division, National Chemical
Laboratory, Dr Homi Bhabha Road, Pune 411008, India.
E-mail: p.poddar@ncl.res.in, r.banerjee@ncl.res.in;
Tel: +912025902535
Department of Chemistry, Pondicherry University, R. V. Nagar,
Kalapet, Pondicherry-605014, India
Fig. 1 (a) Schematic representation of in situ Cu-DCM synthesis. (b)
Complexation of 2-carboxy phenanthroline with Cu(II) and intra-chain
and inter-chain connectivity of BTC. (c) One isolated fragment of Cu-
DCM showing the intra-chain and inter-chain Cu–Cu distance. (d)
Interdigitation of four such fragments via phenanthroline rings creates
one dimensional channel, (e) space-filled model showing the arrange-
ment of the pores in bc-plane [colour code; black-C, green-Cu, red-O,
blue-N, yellow-H].
b
w Electronic supplementary information (ESI) available: Experimental
procedures, crystallographic data (CIF) and additional supporting
data. CCDC 818302. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1cc13418c
z C.D. and R.D. contributed equally to the work.
1
1008 Chem. Commun., 2011, 47, 11008–11010
This journal is c The Royal Society of Chemistry 2011

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decarboxylated possibly due to the high reaction temperature
B170 1C) and high pressure generated during the reaction.
(
In situ oxidation of neocuproine and subsequent decarboxylation
yields 2-carboxy phenanthroline, which upon coordination to
Cu(II) creates a 1D ‘ꢀ ꢀ ꢀ–m -OCO–Cu(II)–m -OCO–ꢀ ꢀ ꢀ’ type
2
2
chain. Two of these chains are connected via the coordination
of neighboring Cu(II) atoms to form an isolated double chain.
In the crystal structure of Cu-DCM (space group, Pnmay), the
Cu(II) atom is equatorially connected to two nitrogen atoms and
one oxygen atom of 2-carboxy phenanthroline and one oxygen
atom from BTC-carboxylate. The other oxygen atom of the same
BTC-carboxylate is axially coordinated to another Cu(II) atom
(Fig. 1b). This unit repeats along the crystallographic a-axis to
form a 1D zig-zag ‘ꢀꢀꢀ–m -OCO–Cu(II)–m -OCO–ꢀꢀꢀ’ chain with
2
2
˚
‘
Cu–Cu’ distance of 3.99 A and ‘Cu–Cu–Cu’ angle of 128.41. The
Fig. 2 (a) H
2
adsorption isotherm for Cu-DCM at 77 K below 1.0
two BTC-carboxylates eventually connect two such chains to
2
bar. Inset showing N adsorption isotherm for Cu-DCM at 77 K.
form a 1D ladder like an isolated double chain structure with
˚
Filled and open circles represent adsorption and desorption data,
respectively. (b) The temperature dependence of the magnetic suscept-
ibility w (left axis), and wT (right axis) for Cu-DCM; inset image shows
the finite length of an isolated Cu-DCM (c) The temperature depen-
dence of the magnetic susceptibility w for Cu-DCM at different applied
fields. (d) The temperature dependence of the magnetic susceptibility w
‘
Cu–Cu’ inter-chain distance of 8.39 A (Fig. 1c). Inter-digitations
of four such isolated ladders in the bc-plane via p–p interactions
˚
between the closest aromatic rings (B3.53 A) of phenanthroline
create a one-dimensional channel through the crystallographic
a-axis (Fig. 1d). These channels are filled with water molecules,
which form a hydrogen bonded tetramer with an ‘OꢀꢀꢀO’
(
thick: ZFC & thin: FC) for Cu-DCM (black), Cu-DCM-Eva (blue);
inset temperature dependence of real part ac susceptibility for Cu-
˚
distance of 2.74 A. All major peaks of the experimental powder
DCM (Fig. S22–S27, ESIw).
X-ray diffraction (PXRD) of Cu-DCM match well with simulated
PXRD, indicating their crystalline phase purity (Fig. S2, ESIw).
Thermal gravimetric analysis (TGA) was performed on the
as-synthesized compound in order to understand the behaviour
of the solvent with respect to increase in temperature (Fig. S7, ESIw).
The initial weight loss (B6.38 wt%) at 90–100 1C is due to the
loss of all the three water molecules inside the 1D channels and
the subsequent collapse of the hydrogen bonded structure, as
evident from the different PXRD patterns of the evacuated
sample (Fig. S8, ESIw) followed by a plateau (100–250 1C).
The sharp weight loss at B270 1C indicates the decomposition
point of the compound.
value, below which it falls rapidly until 8 K and then again
rises until temperature of 3 K. The rapid increase of wT (ZFC)
below 14 K shows the signature of the onset of ferromagnetic
exchange interactions between the closest Cu(II) atoms within
8
˚
a chain, [Cu(II)–Cu(II) distance 3.99 A]. The peak at 12 K
indicates the occurrence of an antiferromagnetic transition due
to the inter-chain magnetic interaction. The rise below 8 K indicates
the second order phase transition to a totally magnetically
ordered state. The reason for this ferromagnetic behaviour
is due to the parallel and stacked orientation of two ‘ꢀ ꢀ ꢀ–m
2
-
OCO–Cu(II)–m
2
-OCO–ꢀ ꢀ ꢀ’chains in Cu-DCM [with a Cu–Cu
˚
The N adsorption isotherm exhibits type-I sorption behaviour
2
distance of 8.39 A]. To investigate the nature of the 3D
magnetic ordering in Cu-DCM and to probe the field depen-
dence of the magnetic susceptibility, we measured ZFC and
FC w at different applied fields (Fig. 2c). Up to an applied field
of 1000 Oe, the ZFC-FC curves show strong bifurcation, and
show the onset of a ferromagnetic transition at around 14 K
and an inflection point at around 8 K. This bifurcation
disappears at a field of 10 kOe as a sufficiently higher field
can overcome both intra and inter-chain magnetic interactions.
At this field the Zeeman energy will be higher than the uniaxial
anisotropy energy, therefore an increasing number of spins
will orient in the field direction with decreasing temperature.
There is no report in the literature where a ZFC curve shows
an upright behaviour below 8 K after showing a peak such as
we have observed at 12 K due to long range antiferromagnetic
ordering. There are only a handful of examples where the
SCM type of interaction (intra-chain ordering) is followed by
2
ꢁ1
with Langmuir and BET surface areas of 31.904 m g and
2
ꢁ1
1
6.326 m g respectively. At 760 Torr and 77 K, Cu-DCM
adsorbs 0.42 wt% H (Fig. 2a). The framework of Cu-DCM
exhibits a typical type-I profile [Fig. S11, ESIw] for CO
2
˚
2
(3.4 A)
at 298 K
molecules
ꢁ1
adsorption. Cu-DCM adsorbs 0.95 mmol g of CO
and 1 bar pressure. Hysteretic sorption of CO
2
2
suggests strong interactions with the pore surfaces. The low
gas adsorption values for Cu-DCM could be attributed to the
soft evacuation of the sample at room temperature (B30 1C)
to prevent structural collapse.
We anticipated that down to a certain temperature, Cu-
DCM should exhibit 1D magnetic character (similar to SCMs)
since the intra-chain magnetic coupling of Cu(II) ions via the
˚
carboxylate linkage is stronger due to a shorter distance (3.99 A)
˚
than the inter-chain coupling via the BTC ligand (8.39 A). The
temperature dependence of magnetic susceptibility (w) and wT
for Cu-DCM is shown in Fig. 2b. The wT value for two Cu(II)
the modified-SCM type of behaviour when inter-chain magnetic
9,10
ꢁ1
ꢁ1
of Cu-DCM at 300 K is 1.08 emu K mol Oe (Fig. S12, ESIw).
Upon cooling, the wT (ZFC) value remains almost constant
down to B120 K, followed by a gradual decrease until B40 K.
Below 40 K, the wT (ZFC) value falls at an increased rate
down to 15 K, then again increases with a peak at the 12 K
coupling decreases uniaxial anisotropy.
The ac susceptibility
measurements also showed these peaks but without any
frequency shift, thereby ruling out spin-type behaviour (inset
Fig. 2d). Probably, this is the first example of a Cu(II) based
double chain magnetic framework where such types of
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11008–11010 11009

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competing magnetic interactions and their temperature and
field dependencies are observed. So far only two Cu(II) based
MOFs are reported where antiferromagnetic and ferromagnetic
Notes and references
y Cu-DCM: C35
a = 7.1889(10) A; b = 24.444(3) A; c = 18.818(2) A; V = 3306.7(7) A ,
H23 Cu
2
N
4
O
15, M
r
= 866.67, orthorhombic, Pnma,
˚
3
˚
˚
˚
1
1
ꢁ3
ꢁ1
interactions have been observed. One of them being [Cu
F-pymo) (H O)1.25]n where both antiferromagnetic (a broad
c
Z = 4, D = 1.729 g cm , m = 0.94 mm , 8914 total reflections,
3947 unique, T = 293(2) K, final R indices (I 4 2s(I): R
1
= 0.0554,
(
2
2
wR = 0.1026, GOF = 0.843, CCDC 818302.
2
hump) and weak ferromagnetic (spin canting) interactions
1
1a
were observed at 60 K and 17 K respectively.
another Cu(II) based MOF [Cu (PTMTC) (py)
O)], long range antiferromagnetic ordering was observed
Whereas in
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3
2
6
(CH CH OH)
3
2
2
(H
2
11b
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and the effect of the perturbation exerted by the guest (water)
molecules in Cu-DCM, we compared the w-T and M-H
measurements on as synthesized, evacuated and re-solvated
samples (Fig. 2d). As mentioned previously the PXRD study
showed that the evacuated and re-solvated Cu-DCM crystal
structure is different from the as-synthesized Cu-DCM. How-
ever, the magnetic study of as-synthesized, evacuated and
re-solvated (Fig. S20 & 21, ESIw) Cu-DCM shows that the
magnetic behaviour of these three phases remains largely
unchanged with little difference at low temperatures. This
indicates that the removal of the guest molecule does not
affect the overall intra and inter-chain magnetic coupling.
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absence of any magnetic interaction (dipolar or exchange)
outside these isolated double chain magnets in 3D and the
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individual Cu-DCM behaviour. Additionally, the major peaks
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ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
at 722 cm , 846 cm , 916 cm , 1365 cm , 1435 cm , 1566 cm ,
ꢁ1
and 1622 cm in the IR spectra were similar for as-synthe-
sized, evacuated and re-solvated samples (Fig. S28, ESIw),
indicating the retention of the basic skeleton (double chain)
even though the H-bonded structure collapses after evacuation
of the guest water molecules. The SEM and TEM images
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morphological change after evacuation (Section S8, ESIw).
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4
2
1
2
ing its application.
In conclusion we could state that we have successfully
designed and synthesized one DCM framework. The synthesis
was pre-designed and performed in situ which decreased the
number of steps. Cu-DCM is the first example of a Cu(II)
based DCM framework where we have demonstrated a
modified-SCM type of behaviour with rich interplay between
intra and inter-chain Cu(II) spins at low temperatures, resulting
in competing ferromagnetic and antiferromagnetic inter-
actions. Additionally, it is remarkable that the inherent magnetic
behaviour of Cu-DCM remains intact even after the frame-
work collapses, indicating the absence of magnetic interactions
outside these isolated double chains. This robust magnetic
nature of Cu-DCM type materials may be useful in magnetic
device related applications.
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1010 Chem. Commun., 2011, 47, 11008–11010
This journal is c The Royal Society of Chemistry 2011