Twelve-connected porous metal-organic frameworks with high H2 adsorption

Article abstract of DOI:10.1039/b614254k

The twelve-connected metal-organic frameworks {[Ni₃(OH)(L) ₃]·n(solv)}_∞ 1 and {[Fe₃(O)(L) ₃]·n(solv)}_∞ 2 [LH₂ = pyridine-3,5-bis(phenyl-4-carboxylic acid)] have been prepared and

Full text of DOI:10.1039/b614254k

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Twelve-connected porous metal–organic frameworks with high H2
adsorption{
Junhua Jia,a Xiang Lin,a Claire Wilson,a Alexander J. Blake,a Neil R. Champness,*a Peter Hubberstey,*a
Gavin Walker,b Edmund J. Cussena and Martin Schro¨der*a
Received (in Cambridge, UK) 2nd October 2006, Accepted 2nd November 2006
First published as an Advance Article on the web 7th December 2006
DOI: 10.1039/b614254k
The twelve-connected metal–organic frameworks {[Ni3(OH)
(L)3]?n(solv)}‘ 1 and {[Fe3(O)(L)3]?n(solv)}‘ 2 [LH2 = pyridine-
3,5-bis(phenyl-4-carboxylic acid)] have been prepared and
characterised: these materials can be desolvated to form porous
materials that show adsorption of H2 up to 4.15 wt% at 77 K.
Metal–organic co-ordination framework materials have attracted
much attention in recent years due to their inherent design flexi-
bility,1 and their potential to form stable, highly-porous scaf-
Fig. 1 View of [Ni3(OH)(O2CR)6] cluster along c axis (left) and b axis
folds,2,3 which can be used for gas storage4 and for the adsorption
(right).
of volatile organic compounds.5 In particular, frameworks
constructed from metal–carboxylate interactions can exhibit high
˚
hydroxo ligand, Ni–O 1.9824(4) A, and each pair of Ni centres is
thermal stability and permanent porosity upon desolvation.2–5
Trinuclear clusters of the type [M3(m3-O)(O2CR)6(X)3]n2 (M = Cr,
Fe) have been reported as building blocks for metal–organic
framework construction;6 however, in most of these reported
cases, the metal ions bind to a terminal solvent molecule (X),
typically water or pyridine. We reasoned that angular pyridyldi-
carboxylate ligands7 would template the formation of trinuclear
clusters in a divergent synthesis in which the ligand building block
would act not only as a bridging carboxylate, but would also
involve the terminal pyridyl N-donor to replace X in the trinuclear
fragment [M3(m3-O)(O2CR)6(X)3]n2 and afford extended and
highly-connected frameworks. We are especially interested in
developing frameworks of high connectivity8 that might show
enhanced stability and stable porosity for reversible gas adsorp-
tion. We report herein the use of the angular ligand pyridine-3,5-
bis(phenyl-4-carboxylic acid) (LH2) in the construction of two
isomorphous 12-connected 3D porous frameworks, which can be
desolvated to afford porous materials that can adsorb H2.
bridged by two carboxylate groups from separate ligands above
and below the Ni3(OH) plane (Fig. 1). When the pyridine nitrogen
is included, the local stereochemistry at each Ni centre can be
viewed as octahedral. Each trinuclear cluster [Ni3(OH)(O2CR)6]
acts as a node and is linked to six nearby cluster nodes (green
spheres in Fig. 2(b)) to form an a-Po lattice (Fig. 2(c)), with an
˚
internode separation of 13.4017(13)–13.402(2) A. This array
˚
A diameter as calculated by
affords pores of some 10
PLATON9 and these are filled with solvent molecules. In addition,
the L22 bridges extend to six further cluster nodes (red spheres
˚
in Fig. 2(b)) at a distance of 18.848(2) A to give an overall
12-connected framework of 31844256 topology (Fig. 2(b)). The
12-membered polyhedron defined by the six close and six further
distant nodes is a highly distorted cube-octahedron in which the
two triangles above and below the central hexagon are displaced in
opposite directions. Alternatively, this polygon can also be viewed
as an octahedron capped by two triangular units (see ESI{).
Twelve-connected metal–organic frameworks are very rare and the
two reported examples are based on a more regular cube-
octahedral structural matrix.10 The topology of 1 can be described
in an alternative manner in which the cluster node
[Ni3(OH)(O2CR)6] links to ligand nodes. In this analysis, the
cluster node forms a highly novel 9-connected tricapped trigonal
prismatic polyhedron of 4662189 topology (see ESI{), with the
3-connected ligand nodes having 426 connectivity.
Reaction of Ni(NO3)2?6H2O or Fe(NO3)3?9H2O with LH2 in
dmf afforded the two compounds {[Ni3(OH)(L)3]?n(solv)}‘ 1 and
{[Fe3(O)(L)3]n(solv)}‘ 2, respectively. The structure of 1{ is based
around the trinuclear cluster node [Ni3(OH)(O2CR)6] (Fig. 1)
extended by the branched organic spacer L22 via carboxylate
bridging and, as predicted, N-binding to each Ni centre (Fig. 2(a)).
The trinuclear cluster is held together by a central bridging
aSchool of Chemistry, University of Nottingham, University Park,
Nottingham, UK NG7 2RD
Compound 2 was obtained by the same procedure as above but
substituting Fe(NO3)3?9H2O for Ni(NO3)2?6H2O. The framework
structure of 2{ is isomorphous to that of 1 except that the
trinuclear cluster node in 2 comprises a [Fe3(O)(O2CR)6] moiety
rather than [Ni3(OH)(O2CR)6] as in 1. Thus, in both 1 and 2 the
central oxygen lies on a site of crystallographic 32 (D3) symmetry
with the metal centres occupying two-fold sites. There are a
considerable number of analogous mixed-valence Fe3O cluster
complexes reported in the literature,11 but fewer related Ni-based
bSchool of Mechanical Materials & Manufacturing Engineering,
University of Nottingham, University Park, Nottingham, UK NG7 2RD.
E-mail: m.schroder@nottingham.ac.uk; Fax: +44 (0)115 9513563;
Tel: +44 (0)115 9513490
{ Electronic supplementary information (ESI) available: Experimental
details and characterization of ligands and complexes, single-crystal X-ray
structure analyses, PXRD and TGA data, details of bond valence sum
(BVS) analyses, N2 isotherms for 1 and 2, and magnetochemical analysis
on 1. See DOI: 10.1039/b614254k
840 | Chem. Commun., 2007, 840–842
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Fig. 2 (a) Schematic representation of the tri-branched ligand L22; (b) 12-connectivity about the [Ni3(OH)(O2CR)6] cluster node (shown as green and red
spheres) and (c) pores in 1 shown as yellow spheres.
systems.12 Significantly, a literature search using CCDC ConQuest
version 1.8 confirms that the majority of Ni3 cores are based
around m3-OH bridging (119 hits) as in 1 with only 2 hits13 for
complexes showing a planar Ni3O unit. No hits were observed for
m3-O bridging to Ni3 cores. Thus, the assignments of m3-OH and
m3-O moieties in 1 and 2, respectively, are based upon literature
precedents and upon bond valence sum analyses (see ESI{).14
Although the latter approach is not definitive, these structural
assignments do not affect the subsequent discussion of the porosity
of the desolvated materials. The assignment for 1 as incorporating
a m3-OH moiety is further supported by magnetochemical
The N2 adsorption of desolvated 1 and 2 at 77 K each shows a
reversible type-I isotherm characteristic of microporous material.
The BET surface areas for 1 and 2 were calculated to be 1553 and
1200 m2 g21, respectively. Sorption data for desolvated 1 and 2 are
listed in Table 1. D2 and H2 sorption were measured to 1 bar for
both samples and the molar ratio of adsorbed D2 or H2 (Fig. 3) is
within the anticipated range of 1.0–1.1.15 This consistency between
H2 and D2 sorption results for both compounds confirms the
accuracy of the recorded data for H2 adsorption and the absence
of any significant uptake of impurities. In addition, all H2
adsorption results were corrected for buoyancy effects. At 1 bar,
the wt% adsorption of H2 is 1.99% for 1 and 1.60% for 2 at 77 K.
At 20 bar (Fig. 4) this reaches a maximum at 4.15 and 3.05% for 1
and 2, respectively, corresponding to 23.7 and 17.3 H2 molecules
per formula unit for 1 and 2, respectively.
measurements
confirming
the
formation
of
formal
Ni(II)Ni(II)Ni(III) mixed-valence centres (see ESI{).
Thermal gravimetric analyses show one main weight loss of 38%
for 1 and 31% for 2 between 20 and 250 uC. A plateau is reached
above 250 uC before the structure decomposes at 350 uC. To
determine the stability of the framework after removal of solvent,
both in situ variable-temperature powder X-ray diffraction
(PXRD) measurements under vacuum and single-crystal X-ray
data of 1 following in situ heat treatment of the crystal were carried
out. In situ PXRD measurements were taken every 10 uC from 30
to 120 uC with the sample heated at 2 uC min21. The temperature
was held for 1 h at each setting point before measuring the X-ray
diffraction pattern. All experimental PXRD data match the
calculated X-ray pattern derived from the single crystal structures
very well, and thus confirm high thermal stability of the
framework under vacuum. Even after 20 h at 100 uC under
vacuum, the PXRD pattern for the frameworks 1 and 2 as defined
by the single-crystal structure analyses can be clearly identified.
Moreover, single-crystal X-ray diffraction data for 1 also confirm
the stability of the framework structure on heating and desolva-
tion. Thus, a single crystal of 1 was taken directly from the
crystallisation solvent and mounted in a dry N2 flow at 2123 uC
and a data set collected for the fresh sample. The crystal was then
heated in situ to 50 uC at a rate of 2 uC/min, and held at this
temperature for 1 h. The crystal was then cooled to 2123 uC and a
full data set collected. The procedure was repeated with heating to
100, 150, 200, and 227 uC with the crystal held in a dry N2 gas flow
at all times. The framework, as described above, is clearly
conserved at all points in this treatment, reinforcing the PXRD
results and confirming the thermal stability of this framework.
Only very small crystals were obtained for compound 2 and a
single-crystal X-ray diffraction study has been carried out on the
fresh sample only; however, in situ PXRD results confirm high
thermal stability for this framework as well.
In summary, through the use of a designed angular ligand with
two different donors in a 2 : 1 carboxylate : amine ratio, two
isomorphous 12-connected metal–organic framework materials
have been successfully synthesized and characterised. These
Table 1 Sorption data for compound 1 and 2 at 77 K
H2 (D2)
wt%
uptake
No. of H2
molecules per
H2 wt%
a
N2/
Asurf
/
formula unitb at adsorption
Material mg g21 m2 g21 at 1 bar
1 bar (20 bar)
at 20 bar
1
2
a
547
425
1553
1200
1.99 (4.47) 11.4 (23.7)
1.60 (3.46) 9.1 (17.3)
4.15
3.05
BET surface area calculated assuming a monolayer coverage of
.
Formula unit is [Ni3(OH)(L)3] for 1 and [Fe3(O)(L)3] for 2.
close-packed N2 with a cross-sectional area of 16.2 A molecule21
2
˚
b
Fig. 3 H2 (circles) and D2 (squares) isotherms for 1 (red) and 2 (black) at
77 K.
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We thank EPSRC (UKSHEC) for support, the CVCP and
University of Nottingham for funding (to JJ). We are grateful to
CCLRC for access to Station 9.8 on the Daresbury SRS and to Dr
J. E. Warren and Dr T. J. Prior for experimental advice. MS
acknowledges receipt of a Royal Society Wolfson Merit Award
and of a Leverhulme Trust Senior Research Fellowship. We thank
Dr Jonathan McMaster for helpful discussions regarding bond
valence sum analysis.
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842 | Chem. Commun., 2007, 840–842
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