Porous crystal derived from a tricarboxylate linker with two distinct binding motifs

Article abstract of DOI:10.1021/ja0753952

The use of a reduced symmetry organic linker for the preparation of porous coordination polymers is demonstrated. The solvothermal reaction of the unsymmetrically substituted biaryl compound biphenyl-3,4′,5-tricarboxylic acid with Cu(II) ions produces a [3,4,6]-connected coordination polymer exhibiting very high porosity and surface area (S_Langmuir = 3100 m²/g). A striking feature of the structure is its incorporation, in a single material, of both the ubiquitous dinuclear Cu(II) paddlewheel motif and the rarely observed trinuclear Cu(II) cluster. Saturation H₂ uptake, measured at 77 K, shows an excess gravimetric uptake of 5.7 wt % at 45 bar with a steep rise at low pressures. Copyright

Full text of DOI:10.1021/ja0753952

Published on Web 12/04/2007
Porous Crystal Derived from a Tricarboxylate Linker with Two Distinct
Binding Motifs
Antek G. Wong-Foy, Olivier Lebel, and Adam J. Matzger*
Department of Chemistry, Macromolecular Science and Engineering Program, UniVersity of Michigan,
930 North UniVersity AVenue, Ann Arbor, Michigan 48109-1055
Received July 19, 2007; Revised Manuscript Received November 10, 2007; E-mail: matzger@umich.edu
Although remarkable advances in the understanding of structure-
property relationships of porous coordination polymers have taken
place over the past decade,1 there remains a pressing need to design
higher performance materials. However, it is just a matter of time
before commercial applications in gas storage, catalysis, and
separations are realized because the combination of high surface
area, tunable functionality, and defined pore structure are unrivaled
by other classes of materials.2 The fevered pitch of research in this
area has centered around high-symmetry structures mainly for
aesthetic reasons, although the resultant simplification in predicting
connectivity is much heralded.3 The first materials that showcased
the potential of metal-carboxylates as network building nodes,
MOF-54 and HKUST-1,5 are derived from highly symmetrical
organic linkers (1 and 2, respectively) bound to high-symmetry
metal clusters. MOF-177,6 the material exhibiting the highest
gravimetric levels of physisorbed hydrogen reported to date,7 derives
from a carboxylate linker (3) that has idealized D3h symmetry. On
DMF/dioxane/H2O 4:1:1 at 100 °C for 4 h in 78% yield. Multiple
solvent exchanges followed by evacuation at 100 °C for 4 h led to
a change in color of the material to deep purple. The evacuated
material was formulated as Cu3(C15H7O6)2 by elemental analysis,
and this assignment is corroborated by X-ray crystallography (vide
infra). No significant change in the PXRD pattern was observed
after solvent removal (see Supporting Information).
The structure of UMCM-150 was solved by single-crystal X-ray
diffraction in the space group P63/mmc.11 Views of the crystal
lattice along the a and c axes are provided in Figure 1a and 1b.
The carboxylates on the 3- and 5-positions (isophthalate ring) of
the linker each form Cu paddlewheel clusters with three neighboring
molecules and this is a ubiquitous motif in network solids derived
form Cu carboxylate clusters. Notably, the carboxylate on the 4′-
position (carboxyphenyl ring) assembles into a Cu3(O2CR)6 cluster
(Figure 1c), which is unknown in coordination polymers despite
the hundreds of examples reported.12 UMCM-150 derives from a
[3,4,6]-connected net consisting of three types of cagelike structures.
One type of cage possesses a hexagonal bipyramidal shape with
an aperture (accounting for the van der Waals radii) of ap-
proximately 5.2 Å × 5.8 Å (Figure 1d). The walls of the cages are
formed by the faces of six linker molecules. Trinuclear Cu clusters
are located in the apical positions, whereas Cu paddlewheels occupy
the equatorial positions. The second type of cage possesses a
trigonal bipyramidal shape with a larger aperture of 10.2 Å × 13.7
Å. In the apical positions, small cylindrical pores about 4.2 Å in
diameter resulting from the assembly of three linker molecules by
three paddlewheels are observed; Cu3 clusters are located in the
equatorial positions and linker molecules occupy the faces (Figure
1e). The third type of cage possesses all three different apertures
observed in the other two types of cages (Figure 1f). Each apical
position of the cage is formed from three paddlewheels and three
linker molecules pointing outward, while three Cu3 clusters are
located in the equatorial positions. The edges of twelve linker
molecules form the cage walls. Notably, for the first two types of
cages only aromatic groups face toward the inside, whereas the
axial coordination sites of the Cu paddelwheels are directed inward
in the third type of cage.
the other hand, the use of organic linkers in which the coordinating
functionalities are not symmetry equivalent (termed “unsymmetri-
cally substituted”) may allow the formation of coordination
polymers with geometries and properties previously unseen. Few
examples of such linkers have been reported in the literature,8,9
and unsymmetrical linkers with identical coordinating functionality
have not yet been reported for carboxylates, the most important
class of linkers.10 Although the increased topological complexity
introduced by less symmetrical linkers may frustrate attempts at
prediction, there is an opportunity to drive new modes of network
assembly required to satisfy the constraints imposed by unique
linker geometries. These reasons make it appealing to explore the
use of unsymmetrically substituted linkers to generate novel porous
coordination polymers.
Herein we report the use of biphenyl-3,4′,5-tricarboxylate (4)
with Cu(II) to generate a highly porous crystalline material
containing both the common binuclear Cu(II) paddlewheel cluster,
[Cu2(O2CR)4], and the exotic trinuclear Cu(II) cluster, [Cu3(O2-
CR)6]. This material, termed UMCM-150 (University of Michigan
crystalline material), shows relatively high excess gravimetric and
volumetric H2 uptake.
The N2 adsorption isotherm of UMCM-150 (see Supporting
Information) reveals type I behavior with no hysteresis upon
desorption. From these data, a total pore volume of 1.0 cm3/g was
determined, and the apparent surface area was calculated using the
Langmuir method to be 3100 m2/g (2300 m2/g BET), thus
confirming the permanent porosity of UMCM-150. Ar adsorption
(see Supporting Information) yields the following pore distribution
diameters: 6.5-8 Å, 10.5-12 Å, and 12.5-13 Å. These pore sizes
are consistent with the three types of cages observed in the crystal
structure.
Aquamarine hexagonal plates of UMCM-150 were synthesized
by the solvothermal reaction of H3-4 with Cu(NO3)2‚2.5H2O in
The H2 sorption isotherm recorded at 77 K shows type I behavior
with no hysteresis and no noticeable change in properties upon
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J. AM. CHEM. SOC. 2007, 129, 15740-15741
10.1021/ja0753952 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
region, suffered from poor capacity.7 Gratifyingly, high-pressure
studies on UMCM-150 revealed 5.7 wt % at 45 bar (Figure 2).
From the crystallographic density of the fully evacuated material
(0.636 g/cm3), an upper limit on the excess volumetric uptake could
be estimated as 36 g/L. These values are among the highest reported
for a copper-based coordination polymer.
In conclusion, we have demonstrated that a linker with carboxy-
late groups distributed in an unsymmetrical fashion leads to a
coordination polymer possessing rare trinuclear Cu(II) clusters.
Lifting the constraint of symmetry equivalent functional groups
offers entry into a variety of new linker types and represents a
starting point toward a new generation of high performance
coordination polymers.
Acknowledgment. The authors would like to thank Dr. William
W. Porter III for assistance with X-ray data collection and funding
provided by the U.S. Department of Energy (Grant No. DE-FC26-
05NT42447).
Supporting Information Available: Synthetic procedures, crystal-
lographic data, powder X-ray diffraction data, thermogravimetric
analysis, N2 and Ar sorption isotherms, pore size distribution and heat
of adsorption data. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Figure 1. (a) View along the a-axis of a 2 × 2 × 1 array of unit cells of
the crystal structure of UMCM-150. C atoms (dark gray), H atoms (white),
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(b) View along the c-axis. (c) Detail of a Cu3(O2CR)6 cluster. (d) Hexagonal
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