Self-assembled one dimensional functionalized metal-organic nanotubes (MONTs) for proton conduction

Article abstract of DOI:10.1039/c2cc31527k

Two self-assembled isostructural functionalized metal-organic nanotubes have been synthesized using 5-triazole isophthalic acid (5-TIA) with In(iii) and Cd(ii). In- and Cd-5TIA possess one-dimensional (1D) nanotubular architecture and show proton conducti

Full text of DOI:10.1039/c2cc31527k

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COMMUNICATION
Self-assembled one dimensional functionalized metal–organic nanotubes
(MONTs) for proton conductionw
Tamas Panda,z Tanay Kunduz and Rahul Banerjee*
Received 29th February 2012, Accepted 12th April 2012
DOI: 10.1039/c2cc31527k
Two self-assembled isostructural functionalized metal–organic
nanotubes have been synthesized using 5-triazole isophthalic acid
(5-TIA) with In(III) and Cd(II). In- and Cd-5TIA possess one-
dimensional (1D) nanotubular architecture and show proton
conductivity along regular 1D channels, measured as 5.35 ꢀ 10ꢁ5
and 3.61 ꢀ 10ꢁ3 S cmꢁ1 respectively.
based MONT (In-IA)7 to systematically study the proton
conduction pathways in these MONTs (Fig. 1). To the best of
our knowledge, In-5TIA and Cd-5TIA are the first examples
of self-assembled functionalized metal–organic nanotubes
(MONTs) which show properties like proton conductivity.
Isophthalate building blocks are well-known for the synthesis
of Metal–Organic Polyhedras (MOPs).8 Self-assembled MONT
architecture has already been reported7 with isophthalic acid as
the organic building unit and In(III) as a metal center with a
small inner dimension (4.8 A) (Fig. 1a). One would assume that
the angle between two carboxylate groups of the isophthalate
scaffold (1151) favors the formation of the nanotubular archi-
tecture. This motivated us to choose 5-TIA as the organic
ligand, which possesses the specific geometry as well as functionali-
zation of the outer core to achieve these large functionalized
nanotubular architectures (Fig. 1b). 5-TIA offers a triazole moiety
on the five positions of the isophthalate scaffold, which could hold
these nanotubes in close proximity via hydrogen bonding and
Metal–Organic Frameworks (MOFs) are a new class of
materials that offer the advantage of precise control over
architectural as well as framework topologies with specific
applications such as gas storage, catalysis and drug delivery.1
Although there have been several reports of MOFs with diverse
architectures, still discrete metal–organic nanotubular structures are
extremely rare.2 Only a handful of literature reports are available
where self-assembled Metal–Organic Nanotubes (MONTs) have
been constructed using capping agents (like ethylene diamine,
phenanthroline) or secondary linkers (like 4,40 bipyridine, iodine).3
However most of these reports reveal only the structural details of
the MONTs without showcasing any fundamental application.
Moreover, most of the MONTs reported in the literature are
interconnected by metal ions4 leaving only one or two examples
of self-assembled hydrogen bonded MONTs.5
Herein, we report the design and synthesis of two isostructural
self-assembled functionalized MONTs, In-5TIA [In(C10O4N3H5)2-
(C2H8N)(H2O)] and Cd-5TIA [Cd(C10O4N3H5)2(C2H8N)2(H2O)],
using In(III) and Cd(II) ions as metal centers with 5-triazole
isophthalic acid (5-TIA) as an organic building block. Both
In-5TIA and Cd-5TIA consist of one dimensional single walled
self-assembled nanotubes with 7.85 and 8.23 A inner dimensions
respectively. These MONTs are held together by weak C–Hꢂ ꢂ ꢂO
hydrogen bonding6 to form the self-assembled architecture. It is
also noteworthy that Cd-5TIA and In-5TIA MONTs show high
proton conductivity of 3.61 ꢀ 10ꢁ3 S cmꢁ1 and 5.35 ꢀ 10ꢁ5 S cmꢁ1
at ambient temperature (301 K) and 98% relative humidity
(RH). We have also collected the proton conductivity data of
the literature reported self-assembled indium isophthalic acid
Physical/Materials Chemistry Division, National Chemical
Laboratory, Dr Homi Bhabha Road, Pune 411008, India.
E-mail: r.banerjee@ncl.res.in; Tel: +91 2025902535
w Electronic supplementary information (ESI) available: Experimental
procedures and additional supporting data. CCDC 845047 and
845048. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2cc31527k
Fig. 1 Structures of In-IA, In-5TIA and Cd-5TIA MONTs: (a) schematic
representation of IA ligand forming a three centered MONT with In(III)
and its proton conductivity at 301 K and 98% humidity. (b) Schematic view
of functionalized 5-TIA ligand forming a four centered MONT with In(III)
and its proton conductivity at 301 K and 98% humidity. (c) Schematic view
of functionalized 5-TIA ligand forming a four centered MONT with Cd(II)
and its proton conductivity at 301 K and 98% humidity.
z TP and TK contributed equally to the work.
c
5464 Chem. Commun., 2012, 48, 5464–5466
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Thermal gravimetric analysis (TGA) revealed that both
In-5TIA and Cd-5TIA retain their stability at temperature
as high as 150 1C (see Fig. S13–S15 in ESIw), which was further
confirmed by in situ Variable Temperature Single Crystal
X-Ray Data (see Section S7 in ESIw). In order to confirm
the phase purity of the bulk materials, powder X-ray diffrac-
tion (PXRD) experiments were carried out on In-5TIA and
Cd-5TIA. All major peaks of experimental PXRDs of In-5TIA
and Cd-5TIA match well with those of simulated PXRDs,
indicating their reasonable crystalline phase purity (Fig. S4
and S7, ESIw). Variable Temperature Powder X-Ray Diffrac-
tion (VTPXRD) performed on as-synthesized In-5TIA and
Cd-5TIA (from 25 1C to 150 1C) further confirms their stability
and crystallinity at elevated temperatures (see Fig. S16–S17
in ESIw).
Ion conduction can occur intrinsically through the solid10
material by some carrier-mediated pathway (e.g., water/
hydronium for H+ ions) in pores of the material. The proton
conductivity values were measured for In-5TIA and Cd-5TIA
as 5.35 ꢀ 10ꢁ5 S cmꢁ1 and 3.61 ꢀ 10ꢁ3 S cmꢁ1, respectively, at
ambient temperature (301 K) with 98% RH (Fig. 1b and c).
The conductivities were determined from the semicircle in the
Nyquist plots, as shown in Fig. 1. These values are highly
humidity-dependent and dropped from 5.35 ꢀ 10ꢁ5 S cmꢁ1
and 3.61 ꢀ 10ꢁ3 S cmꢁ1 at 98% RH to 1.25 ꢀ 10ꢁ5 S cmꢁ1 and
3.14 ꢀ 10ꢁ5 S cmꢁ1 at 20% RH, respectively, for In-5TIA and
Cd-5TIA at 301 K (Fig. S33 and S35, ESIw), which indicates
gradual decrease in proton conductivity upon decreasing
humidification and establishes the role of water in proton
conduction. At 298 K and 1 bar pressure, the water vapor
uptake of In-5TIA and Cd-5TIA is 15 wt% and 14 wt%
respectively. Interestingly, proton conductivity of Cd-5TIA is
far more humidity sensitive than that of In-5TIA, evident from
a sharp drop in proton conductivity upon lesser humidification
(Fig. S33–S36, ESIw). This humidity sensitive proton con-
ductivity behavior of Cd-5TIA is similar to that of
(NH4)2(adp)[Zn2(ox)3]ꢂ3H2O (8 ꢀ 10ꢁ3 S cmꢁ1 at 98% RH
while 6 ꢀ 10ꢁ6 S cmꢁ1 at 298 K 70% RH).11e Ramping the
sample results in dislocation of the adsorbed water molecules,
leading to lower proton conductivity at elevated temperature
(Fig. 2c and d). In-5TIA outperforms MIL-53 based MOFs
Fig. 2 (a) Simplified view of an eight-connected indium center, yellow
sticks indicate each m1-CO2ꢁ carboxyl group connected with an octahedral
In(III) node and self-assembled MONTs are arranged in a three-
dimensional manner. (b) Packing view of simplified self-assembled
MONTs. (c) Proton conductivity plots of In-5TIA at higher temperature
showing decrease in proton conductivity. (d) Proton conductivity plots of
Cd-5TIA at higher temperature showing decrease in proton conductivity.
create functionalization on the outer wall of these nanotubes.
The asymmetric unit of In-5TIA (space group P4/n; isostructural
to Cd-5TIA) consists of one octahedral In(III) ion and two 5-TIA
molecules. Each In(III) SBU is coordinated to eight oxygens from
ꢁ
four m1-CO2 functionalities and each carboxylate group of
5-TIA chelates to one In(III) site (Fig. 2a). Each 5-TIA ligand
is coordinated to two different In(III) ions with an In–5TIA–In
angle of 1261 (for Cd-5TIA, the Cd–5TIA–Cd angle is 124.71).
This M(CO2)4 type of SBU could be found in some In(III) based
MOFs9 but it is very rare among Cd(II) based MOFs. In the
crystal structure of In-5TIA each In(III) SBU extends through
a and b axes where four In(III) ions are linked by four 5TIA via
m1-CO2ꢁ carboxyl groups to generate an [In4(5TIA)4] square grid
(Fig. 2a). These [In4(5TIA)4] squares are infinitely connected to
ꢁ
m1-CO2 functionalities of 5TIA ligands along the c axis to
produce a 1D nanochannel containing [In8(5TIA)12] boxes
(Fig. 2a) where all the triazole moieties of 5TIA molecules point
to the periphery of the MONT (Fig. 2b). The interior and
exterior wall cross-sectional diameter of these nanotubes of
In-5TIA is 7.85 and 26.6 A whereas for Cd-5TIA, it is 8.23
and 27.4 A respectively. These independent MONTs are further
self-assembled via C–Hꢂ ꢂ ꢂO hydrogen bonding [D = 3.645(7) A,
d = 2.811(5) A, y = 150.0(3)1] to produce an interesting 3D
supramolecular aggregate along the bc plane where the triazole
functionality of one MONT is interdigitated with another
MONT and vice versa (Fig. 2b). The whole framework of
In-5TIA and Cd-5TIA bears negative charge due to the coordi-
nation of each metal [In(III) or Cd(II)] atom to the four carboxylate
ligands. However, after careful investigation of these two
isostructural MOFs, we propose that the single negative charge
of In-5TIA SBU was balanced by one dimethyl ammonium
cation. Whereas in Cd-5TIA, two negative charges at SBU were
neutralized by two dimethyl ammonium cations. These cations
were heavily disordered, which was confirmed by the elemental
analysis of In-5TIA and Cd-5TIA (see Section S2 in ESIw).
(10ꢁ6–10ꢁ7 S cmꢁ1 11a and PCMOF-3 (3.5 ꢀ 10ꢁ5 S cmꢁ1),11b
)
whereas Cd-5TIA outperforms ferrous oxalate dehydrate
(1.3 ꢀ 10ꢁ3 S cm
)
although it possesses a lower proton conductivity value than
ꢁ1 11c and cucurbituril (1.3 ꢀ 10ꢁ3 S cmꢁ1),11d
(NH4)2(adp)[Zn2(ox)3]ꢂ3H2O (8 ꢀ 10ꢁ3 S cmꢁ1 11e
)
at 298 K
(Table S5, ESIw). Contrary to other proton conducting
MOFs,11h In-5TIA and Cd-5TIA show proton conductivity in
a wide range of temperatures, from 277 K (2.8 ꢀ 10ꢁ5 S cmꢁ1
for In-5TIA and 3 ꢀ 10ꢁ5 S cmꢁ1 for Cd-5TIA) to 368 K
(2.7 ꢀ 10ꢁ5 S cmꢁ1 for In-5TIA and 1.2 ꢀ 10ꢁ5 S cmꢁ1 for
Cd-5TIA) [Tables S3–S4 and Fig. S27–S32, ESIw],12 proving
their potential in various applications. For In-5TIA, the proton
conductivity value increases with increase in temperature from
277 K to 312 K, and then decreases steadily up to 368 K
whereas for Cd-5TIA proton conductivity increases with
increase in temperature from 277 K to 301 K and then
decreases up to 368 K (Fig. 2 and Fig. S27–S32, ESIw).
However, we acknowledge that both In-5TIA and Cd-5TIA
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provide a unique pathway for proton conduction under humid
conditions, showing noticeably high proton conductivity
values across a wide range of temperatures and 98% relative
humidity. Moreover, proton conductivity shown by Cd-5TIA
(3.61 ꢀ 10ꢁ3 S cmꢁ1 at 301 K) is at a high position among the
proton conducting MOFs reported till date. We have also
studied and compared the proton conductivity of the literature
reported MONT architecture (In-IA). We expect that this
finding will lead to many new applications previously unrealized
in nanotube based porous materials.
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Fig. 3 Schematic representation of the Grotthuss proton hopping
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The arrows indicate the possible movement of the H+
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show proton conductivity till 368 K. To further compare the
proton conducting efficiency of In-5TIA and Cd-5TIA, we
have collected the proton conductivity data of the literature
reported MONT architecture (In-IA) (Fig. 1a), which reveal
that In-IA and In-5TIA have comparable proton conductivity
(5.35 ꢀ 10ꢁ5 S cmꢁ1 for In-5TIA and 2.20 ꢀ 10ꢁ4 S cmꢁ1 for
In-IA, respectively, at 301 K and 98% RH) as both materials
contain one dimethyl ammonium cation per SBU within the
framework. Accordingly, Cd-5TIA shows higher proton
conduction (3.61 ꢀ 10ꢁ3 S cmꢁ1 at 301 K and 98% RH) than
In-5TIA and In-IA as Cd-5TIA possesses two dimethyl
ammonium cations per SBU inside the framework (Fig. 3,
Fig. S43 and S44 and page S51, ESIw). In-5TIA and Cd-5TIA
show considerably lower activation energy values than In-IA
(0.137 eV for In-5TIA and 0.163 eV for Cd-5TIA whereas
0.47 eV for In-IA). Thus, proton conductivity in In-5TIA and
Cd-5TIA follows mainly the Grotthuss proton hopping
mechanism13 whereas proton conductivity of In-IA follows chiefly
a vehicular mechanism, which hints at the role of the triazole
moiety in providing proton conducting pathways in In-5TIA and
Cd-5TIA and hence the advantages of functionalization of the
framework. The activation energy value of In-5TIA (0.137 eV) is
comparable to that of nafion based membrane electrolytes
(0.22 eV),14 and is the lowest activation value reported till date for
MOF based proton conducting materials (Table S5, ESIw).
Cd-5TIA also possesses a low activation energy value of 0.163 eV.
In conclusion, we could design and synthesize two isostructural
single-walled functionalized metal–organic nanotubes (MONTs)
by using 5-TIA as a single organic building block and In(III) or
Cd(II) as a metal node. These large MONTs are held together
by hydrogen bonding interactions, leading to unique supra-
molecular nanotubular arrays. In this work, 5-TIA serves a
dual purpose, constructing nanotubular architecture using a
single organic precursor linked with an In(III) or Cd(II) node,
as well as functionalizing the pore wall with triazole moieties.
The triazole decorated pores along with dimethyl ammonium
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12 At 95 1C proton conductivity values of In-5TIA (2.7 ꢀ 10ꢁ5 S cmꢁ1
)
and Cd-5TIA (1.15 ꢀ 10ꢁ5 S cmꢁ1) are comparable to those of
PCP*Im (2.2 ꢀ 10ꢁ5 S cmꢁ1 at 120 1C)[11g] and b-PCMOF2*tz
(5 ꢀ 10ꢁ4 S cmꢁ1 at 150 1C).[11f]
.
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c
5466 Chem. Commun., 2012, 48, 5464–5466
This journal is The Royal Society of Chemistry 2012