methane, nitrogen and hydrogen. Somewhat surprisingly,
compound 1 displayed negligible sorption of each of these
gases, displaying a maximum uptake at 2500 kPa of less than
and the Science and Industry Endowment Fund (M.R.H., B.F.A.)
for financial support.
3
ꢁ1
1
5 cm (STP) g
for H2 across the temperature range
Notes and references
7
7–273 K (Fig. S8, ESIw), and similarly minimal uptake of
1
2
3
N. Armaroli and V. Balzani, Chem.–Asian. J., 2011, 6, 768–784;
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2
4
tively). In an attempt to quantify this apparent selectivity, a
high-resolution sorption isotherm was measured for CO at
73 K in the range 0.1–121 kPa (Fig. S10, ESIw), however,
analysis of the sorption by the Ideal Adsorbed Solution Theory
2
2
2
009, 2, 148–173.
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1
4
(
IAST) method was hampered by the extremely low uptake of
methane and nitrogen under the same conditions, and as such,
no quantification for selectivity under these industrially relevant
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CO isotherm displayed a prominent step at ca. 66 kPa, further
2
4 Metal–Organic Frameworks: Applications from Catalysis to
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indicative of a cooperative sorption process.
Several factors have been identified to account for sorption
discrimination between these gases in PCPs, including the
limiting dimensions of the pores better matching the smaller
2
kinetic diameter of the CO molecules relative to the other gases,
and differences in the gases electronic properties i.e. quadrupolar
moments and polarizabilities, which lead to differing interactions
with the host. However, we would not anticipate that the larger
channels in 1 would permit such significant size discrimination
7, 90–95; N. R. Kelly, S. Goetz, S. R. Batten and P. E. Kruger,
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5 J. R. Li, J. Sculley and H. C. Zhou, Chem. Rev., 2012, 112, 869–932;
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˚
kinetic diameter of CH = 3.8 A, cf. pore window 4.7 A), so the
˚
(
4
differentiation between CO and CH , N and H by 1 may be
2
2
4
2
1
1586–11596; E. D. Bloch, W. L. Queen, R. Krishna, J. M.
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enthalpic in origin; the low enthalpy of sorption for CO suggests a
2
plausible mechanism for the sorption selectivity may exist simply
in the unusually weakly interacting nature of the pore walls.
However, it is evident from the hystereses observed for the
1
2
R. Matsuda, J. Chen, M. Takata, Y. Kubota and S. Kitagawa, J. Am.
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2
sorption profiles of CO in 1, as well as the clearly defined step
in the high-resolution sorption data, that some dynamic
behaviour within the framework is induced by the presence
6 L. Huang, H. Wang, J. Chen, Z. Wang, J. Sun, D. Zhao and
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2
of the CO sorbate. Dynamic behaviour in PCP materials may
1
5
conceivably take several forms including reversible frame-
work collapse; pore-size/shape change; ‘gate-opening’ or other
structural rearrangement to allow (better) access to the pores.
The verifiably robust nature of 1 would discount framework
collapse and significant structural rearrangement, and would
tend to suggest a more subtle rearrangement or breathing process
is operative in 1, possibly involving the hydrogen bonding
interactions between the interpenetrated frameworks.
7 V. Colombo, S. Galli, H. J. Choi, G. D. Han, A. Maspero,
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5
1
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8
9
In conclusion, we have prepared a novel porous coordina-
tion polymer 1 from 1H-indazole-5-carboxylic acid, which, in
addition to representing the first structurally characterised
PCP containing an indazole moiety, is capable of adsorbing
10 The accessible surface area and pore volume of 1 was calculated
˚
using a probe molecule of 3.681 A diameter and based on helium
adsorption. The Lennard-Jones parameters for the framework
atoms were taken from the Dreiding force field.
1
2
1 The percentage uptake is based upon (m(CO )/[m(host network)].
2
4% CO by weight, and displays substantial hysteresis beha-
2
12 D. M. D’Alessandro, B. Smit and J. R. Long, Angew. Chem., Int.
Ed., 2010, 49, 6058–6082.
viour. The unusual sorption properties of 1 are probably best
explained by a cooperative sorption mechanism. The precise
nature of this cooperativity, as well as the utility of related
indazole-based ligands in similar systems, are currently under
investigation in our group and will be reported in due course.
The authors thank the University of Canterbury (Scholarship to
C.S.H.); the Royal Society of New Zealand Marsden Fund (P.E.K)
1
3 K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch,
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1
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1
1560 Chem. Commun., 2012, 48, 11558–11560
This journal is c The Royal Society of Chemistry 2012