Control over catenation in metal-organic frameworks via rational design of the organic building block

Article abstract of DOI:10.1021/ja909519e

(Chemical Equation Presented) Metal-organic frameworks (MOFs), a hybrid class of materials comprising inorganic nodes and organic struts, have potential application in many areas due to their high surface areas and uniform pores and channels. One of the key challenges to be overcome in MOF synthesis is the strong propensity for catenation (growth of multiple independent networks within a given crystal), as catenation reduces cavity sizes and diminishes porosity. Here we demonstrate that rational design of organic building blocks, which act as strut-impervious scaffolds, can be exploited to generate highly desired noncatenated materials in a controlled fashion.

Full text of DOI:10.1021/ja909519e

Published on Web 12/29/2009
Control over Catenation in Metal-Organic Frameworks via Rational Design of
the Organic Building Block
Omar K. Farha,* Christos D. Malliakas, Mercouri G. Kanatzidis, and Joseph T. Hupp*
Department of Chemistry and International Institute for Nanotechnology, Northwestern UniVersity,
2145 Sheridan Road, EVanston, Illinois 60208
Received November 9, 2009; E-mail: j-hupp@northwestern.edu; o-farha@northwestern.edu
Metal-Organic Frameworks (MOFs) are a novel class of
hybrid materials made from metals and organic compounds with
numerous potential applications.1 MOFs are one of the most
significant breakthroughs in solid-state science and have garnered
significant attention from a number of researchers in chemistry,
physics, materials science, and biology. These fascinating
materials are characterized by large internal surface areas,
ultralow densities, and uniformly structured pores and channels.
Among the many applications that may capitalize on these
extraordinary properties are gas and chemical storage,2 chemical
separations,3 sensing,4 selective catalysis,5 ion exchange,6 and
drug delivery.7
as a general method to completely avoid interpenetration. For
example, IRMOF-0 (acetylenedicarboxylic acid is used as ligand)
could only be made in its 2-fold catenated version.14 Ma and
co-workers utilized an alternative strategy, based on templating
agents (oxalic acid), to produce noninterpenetrated materials.15
To date, only a single example has been reported, and no
apparent generality demonstrated for this method.
Most of the applications mentioned above require pure-phase
materials that retain their full porosities after guest removal. It
is also imperative that pore cavities in MOFs have the largest
void volume possible. Indeed, we have recently proposed a
solution to circumvent a typical MOF collapse scenario upon
thermal activation by introducing a supercritical treatment
method.8 We have also developed a method that allows for the
separation of different MOF phases based on density differ-
ences.9 Although density-based separation is a powerful tech-
nique, it does not address the principal problem that results in
the formation of multiphased products: catenation (i.e., inter-
penetration or interweaving of two or more identical and
independent frameworks). Obtaining a single network (noncat-
enated) MOF product, which is a direct route to the highest
porosity, has proven to be problematic in the majority of cases,
especially when the strut (ligand) used to construct the frame-
work is large. Catenation imposes a severe limitation on efforts
to expand surface area and/or decrease density by employing
lengthier ligands for the framework structures. High degrees of
catenation overall tend to diminish the porosity. While 2- and
3-fold catenation are routinely encountered in MOF structures,10
examples of significantly higher degrees of catenation are also
known.11 Noncatenated systems should have larger channels and
pores than their catenated counterparts, a feature of value in
certain catalysis applications. (On the other hand, catenation can
be useful for precise pore sizes, essential for separations.)
Figure 1. Cartoon representation of the 2D sheet formation by the tetraacid
ligand (red) pillared by a dipyridal strut (blue). The green corners are the
zinc paddlewheel nodes.
More recently, Shekhah and co-workers have shown that they
can suppress catenation by using “liquid-phase epitaxy” on an
organic template16 and then employing a step-by-step growth
method.17 This elegant work demonstrated that the paddlewheel
compound, MOF-508, could be synthesized as a noncatenated
material in contrast to the solvothermal method, which always
generates two networks.18 Unfortunately, liquid-phase epitaxy
is only applicable for small-scale MOF fabrication, limiting the
use of this method to surface-related applications only. Yet there
are numerous cases where bulk quantities of these materials are
required. Currently, a versatile method that can produce large
quantities of both the catenated and noncatenated versions under
the same condition is nonexistent.
Scheme 1. Representation of Structures of Struts and Starting
Materials
In their ground breaking work on the IRMOF (isoreticular
metal-organic framework) homologous series,12 Yaghi and co-
workers were the first to show the systematic assembly of MOFs
without changing the topology. In order to produce a reasonable
quantity of a noncatenated version of IRMOFs, however, very
high dilutions are required. Still, the well-known IRMOF-1
(MOF-5) can be contaminated with its doubly catenated coun-
terpart,13 thus complicating the study of its properties such as
surface area and gas sorption. Thus, dilution did not prove itself
Herein we demonstrate an approach to suppress catenation
through careful ligand design that controls network growth. We
have previously reported a tetracarboxylic acid ligand (4,4′,4′′,4′′′-
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C O M M U N I C A T I O N S
benzene-1,2,4,5-tetrayl-tetrabenzoic acid, L5; Scheme 1).19 We
envisioned that the deprotonated L5 would favor the formation
of comparatively large cavities that resist the formation of
catenated MOFs. Also, we rationalized the use of a tetratopic
building block to produce robust frameworks, which is desirable
for MOF applications. Indeed, the solvothermal reaction of L5
and Zn(NO3)2 · 6H2O in DMF (Dimethylformamide) at 80 °C for
24 h afforded in high yield a noncatenated MOF.19 Then we
examined the use of L5 to form pillared paddlewheel MOFs,
which we have previously described based upon mixed-ligand
Zn(II) coordination to linear dicarboxylates and dipyridyls.18a
We reasoned that ligand L5 would form a 2D sheet within the
xy-plane, which could be pillared by a dipyridyl ligand (see
Figure 1). The 2D sheets can function as scaffolds for dipyridyl
ligands. This allows for diverse functionalities to be incorporated
into new functional MOFs. MOF materials that were made in
this fashion produced several examples of noncatenated5a,20 as
well as catenated structures.21 We noticed that in the instances
where noncatenated MOFs formed, we used either a sterically
demanding (porphyrin-based or trimethylsilane-protected struts)
or a hydrogen-bonding capable (diol-containing) dipyridyl ligand.
On the other hand, the linear dipyridyl ligand N,N′-di-(4-pyridyl)-
1,4,5,8-naphthalenetetracarboxydiimide produced a pillared pad-
dlewheel structure that is catenated where the dipyridal strut
resides directly in the middle of the diamond-shaped cavities
formed by two of the L5 ligands (see Scheme 2). From this
structure, we concluded that the sterics of the dipyridyl moiety
plays a significant role in the control of catenation.
crystal data reveals a framework formula of Zn2(Lbp)(Lta) with
either noncatenated or 2-fold catenated networks (Figure 2). Lta
bridges the Zn(II) dimers and forms flat two-dimensional sheets
that can be pillared by Lpb. Thermogravimetric analysis (TGA)
of 1-8 revealed significant mass loss between 115 and 175 °C
depending on the material. This is assigned to DMF molecules,
and no further mass loss was observed until 300 or 400 °C,
conditional on the material. Materials 2, 4, 6, and 8 show greater
solvent loss than 1, 3, 5, and 7, which is expected going from
a 2-fold catenated material to noncatenated material (see Figure
3a and Figures SI9-11). The purities of the samples were
determined by powder X-ray diffraction (PXRD) (see Figures
SI1-8).
Scheme 2. Crystallographically derived space-filled ab-plane
looking down the c-channel using L5 (left) and L6 (right) (yellow )
Zn, red ) O, green ) Br, blue ) N, gray ) C, white ) H).
We therefore redesigned ligand L5, expecting to suppress
catenation by introducing additional steric blockage in the xy-
plane. We reasoned that ligand L6, bearing two large bromine
atoms instead of hydrogen atoms (as in L5), would suppress
the formation of interpenetrated structures. To verify our
hypothesis, we constructed paddlewheel MOFs using L6 with
the same dipyridyl moieties used to create paddlewheel structures
with L5. Indeed, the approach using L6 produced only noncat-
enated materials, where under the identical conditions L5
produced 2-fold catenated structures. Overall, a rational com-
parison between a total of eight materials, four with ligand L5
(2-fold catenated; 1, 3, 5 and 7) and four with ligand L6
(noncatenated; 2, 4, 6 and 8), supports this hypothesis. The only
variable in the synthesis is the identity of the tetra-acid (L5 or
L6). Thus a change from aryl-H moieties to aryl-Br furnished
on the tetraacid ligand is sufficient to suppress an undesired
catenation by contracting the diamond shaped opening (Scheme
2). The general method to assemble these materials via static
heating of Lbp (L1, L2, L3, or L4), Lta (L5 or L6), and
Zn(NO3)2 · 6H2O in DMF and one drop of concentrated HCl at
80 °C for 24 h resulted in single crystals suitable for X-ray
diffraction (bp ) bipyridal; ta ) tetraacid). Analysis of the single
Figure 2. Single crystal X-ray structures: 2-fold catenated structure with
building block L5 (left) and noncatenated structures with building block
L6 (right).
Unlike most pillared paddlewheel MOF structures, those of
1, 3, 5, and 7 are interpenetrated as opposed to interwoVen, such
that the networks are maximally displaced from one another.1
All have Lbp pillars that reside directly in the center of the
diamond-shaped cavities formed by two of the Lta ligands.
Compound 7 has a unique structure in that it is interpenetrated
instead of interwoven with respect to the x,y-plane as well as
the z-direction. The other three (1, 3, and 5) are only interpen-
etrated in the z-direction and interwoven in the x,y-plane.
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C O M M U N I C A T I O N S
Acknowledgment. J.T.H. gratefully acknowledges the DOE
(Grant No. DE-FG02-08ER15967) and the Northwestern Uni-
versity NSEC. M.G.K. acknowledges the ChemMatCARS Sector
15 supported by NSF/ DOE (Grant No. CHE-0535644) and the
APS at ANL (DOE Contract No. DE-AC02-06CH11357).
Supporting Information Available: Experimental procedures, X-ray
crystallographic files in CIF format, PXRD, TGA, and sorption isotherm
data. This material is available free of charge via the Internet at http://
pubs.acs.org.
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