The quest for modular nanocages: Tbo -MOF as an archetype for mutual substitution, functionalization, and expansion of quadrangular pillar building blocks

Article abstract of DOI:10.1021/ja205658j

A new blueprint network for the design and synthesis of porous, functional 3D metal-organic frameworks (MOFs) has been identified, namely, the tbo net. Accordingly, tbo-MOFs based on this unique (3,4)-connected net can be exclusively constructed utilizing a combination of well-known and readily targeted [M(R-BDC)]_n MOF layers [i.e., supermolecular building layers (SBLs)] based on the edge-transitive 4,4 square lattice (sql) (i.e., 2D four-building units) and a novel pillaring strategy based on four proximal isophthalate ligands from neighboring SBL membered rings (i.e., two pairs from each layer) covalently cross-linked through an organic quadrangular core (e.g., tetrasubstituted benzene). Our strategy permits the rational design and synthesis of isoreticular structures, functionalized and/or expanded, that possess extra-large nanocapsule-like cages, high porosity, and potential for gas separation and storage, among others. Thus, tbo-MOF serves as an archetypal tunable, isoreticular MOF platform for targeting desired applications.

Full text of DOI:10.1021/ja205658j

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The Quest for Modular Nanocages: tbo-MOF as an Archetype for
Mutual Substitution, Functionalization, and Expansion of
Quadrangular Pillar Building Blocks
Jarrod F. Eubank,† Hasnaa Mouttaki,† Amy J. Cairns,‡ Youssef Belmabkhout,‡ Lukasz Wojtas,† Ryan Luebke,‡
Mohamed Alkordi,‡ and Mohamed Eddaoudi*,†,‡
†Department of Chemistry, University of South Florida, 4202 East Fowler Avenue (CHE 205), Tampa, Florida 33620, United States
‡KAUST Advanced Membranes and Porous Materials Center, 4700 King Abdullah University of Science and Technology,
Thuwal 23955-6900, Kingdom of Saudi Arabia
S Supporting Information
b
3-connected nodes and squares for 4-connected nodes).6 The
ABSTRACT: A new blueprint network for the design and
synthesis of porous, functional 3D metalꢀorganic frame-
works (MOFs) has been identified, namely, the tbo net.
Accordingly, tbo-MOFs based on this unique (3,4)-con-
nected net can be exclusively constructed utilizing a combi-
nation of well-known and readily targeted [M(R-BDC)]n
MOF layers [i.e., supermolecular building layers (SBLs)] based
on the edge-transitive 4,4 square lattice (sql) (i.e., 2D four-
building units) and a novel pillaring strategy based on four
proximal isophthalate ligands from neighboring SBL mem-
bered rings (i.e., two pairs from each layer) covalently cross-
linked through an organic quadrangular core (e.g., tetrasub-
stituted benzene). Our strategy permits the rational design and
synthesis of isoreticular structures, functionalized and/or ex-
panded, that possess extra-large nanocapsule-like cages, high
porosity, and potential for gas separation and storage, among
others. Thus, tbo-MOF serves as an archetypal tunable,
isoreticular MOF platform for targeting desired applications.
generated (obtained) geometrical information is then employed
to determine, design, and target prospective molecular building
block (MBB) counterparts.7 That is, such building units, as
geometric entities, dictate the points of extension of the desired
MBBs. The preferred MBBs can then be targeted by a combina-
tion of organic ligand synthesis and controlled metalꢀligand-
directed coordination.8 The latter is the determining and critical
factor in the successful practice of isoreticular chemistry, since
access to precise reaction conditions is necessary for consistent
generation of the inorganic MBB in situ.2
In our continuing quest to develop new strategies necessary
for the rational assembly of MOF platforms, particularly those
containing tunable cages (i.e., confined spaces) and readily amen-
able to isoreticulation, we re-examined augmented three-dimen-
sional (3D) edge-transitive nets.3 The deliberate intent was to
identify hierarchal building units2,9 (i.e., periodic 0D, 1D, or 2D
building units10) containing elaborate, inbuilt structural informa-
tion that is preferably distinctive to the given deconstructed parent
augmented net. It should also be mentioned that interconnected
cages are of interest because of their great importance for many
applications, as has been observed in application-rich zeolites.10
We successfully implemented this strategy when we introduced
the supermolecular building block (SBB) approach,2 wherein
sophisticated hierarchical construction information is embedded
into the building unit(s), to deliberately assemble in situ a metalꢀ
organic polyhedron (MOP) with 24 additional, peripheral func-
tional moieties (i.e., 24 points of extension) that, when combined
with triangular building units, can exclusively assemble into a
singular (3,24)-connected net, rht,3 having extra-large cages.
During our quest, we recognized another distinctive net, tbo-
a,3 that exhibits explicit features of a potential blueprint net for
logical design and directed assembly of MOF platforms. Un-
iquely, the tbo-a net is composed of 2D periodic arrays of
squares, each consistent with an augmented 4,4 square grid or
lattice (sql-a),3 that are cross-linked through 4-connected
(quadrangular) pillars (Figure 1b) and encompass inherent poly-
hedralcavities. The sql-a net, as one of only two edge-transitive 2D
nets based on the assembly of squares,3 is more easily attained in
crystal chemistry. In fact, MOFs having sql topology based on
etalꢀorganic frameworks (MOFs) are a burgeoning class
M
of functional solid-state materials that are already being
explored for use in multiple areas ranging from catalysis to
surface chemistry, hydrogen storage, and carbon capture.1 These
promising materials are attractive to scientists in academia and
industry alike because of their unique and readily functionalizable
structures. In other words, their design and construction can be
directed toward a targeted use: certain MOFs can be designed as
platforms with controlled pore size, shape, and functionality for
specific applications. To achieve framework design, however, a
high degree of predictability must be integrated prior to synthe-
sis, and it is still an ongoing challenge to absolutely predict the
network topology of the constructed MOF.2 To address this, our
group has devoted its efforts to pinpointing particular blueprint
frameworks that underpin our design strategies (preferably based
on singular, exclusive nets for a combination of specific building
blocks3) and are amenable to isoreticular chemistry.2,4
Edge-transitive nets are an obvious choice because they are
typically the most appropriate targets in crystal chemistry and are
suitable for the practice of reticular chemistry.5 A given augmen-
ted net can be deconstructed into its basic building units, which
are equivalent to vertex figures of the parent net (e.g., triangles for
Received: June 17, 2011
Published: August 05, 2011
r
2011 American Chemical Society
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Figure 1. (a) Substitution of the vertex figures in the augmented tbo
network, tbo-a, with analogous chemical entities results in a tbo-MOF.
(b) tbo-MOFs can be targeted from sql SBLs (green) linked by
4-connected core pillars (blue), which can be functionalized. (c)
Extension of the ligand between the core (blue) and the BDC results
in a greater distance between the SBLs and increased confined space.
Figure 2. (a) (left) L1 ligand with tetraisophthalate groups connected
via alkoxy links to a 4-connected benzene core. (right) Representative
section of the crystal structure of 1 showing the polyhedral cages. (b)
Three types of polyhedral cage in 1: truncated tetrahedron (red),
truncated cube (yellow), and truncated cuboctahedron (green).
square paddlewheel dimer MBBs [M2(O2CR)4(A)2; M = metal,
A = axial ligand] bridged by ditopic organic ligands [e.g., benze-
nedicarboxylates (BDCs) such as terephthalate or isophthalate]
are well-known and readily constructed.8,11 Thus, surface-decorated
sql-MOFs (e.g., [M(R-BDC)]n) could be employed as super-
molecular building layers (SBLs)11b amenable to pillaring12 via
cross-linking through 4-connected organic building blocks to con-
struct the desired tbo-MOF platform. The ability to generate [M(R-
BDC)]n SBLs consistently in situ and space them using organic
quadrangular pillars would permit the relatively small four-mem-
bered-ring (4MR) windows of the SBLs to be preserved, prohibiting
self-interpenetration, while functionalizing and/or unilaterally en-
larging the intrinsic confined space (e.g., distinct cages) delimited by
the quadrangular pillars. This would permit access to made-to-order
MOFs and exploitation of their resultant functionalized nanocages.
To implement our design strategy, we designed and synthe-
sized the octacarboxylate ligand 5,50,500,5000-[1,2,4,5-phenyltetra-
methoxy]tetraisophthalate (L1; Figure 2a) wherein an ether
linkage was chosen to allow flexibility of the pendant BDC arms
positioned in a squarelikegeometry and retain their ability to freely
form the intended SBL (sql-MOF). As expected, reactions of
quadrangular-core tetra-BDC ligand L1 with Cu permitted the
construction of the desired tbo-a framework, in which the sql-
MOF SBLs are bridged by the organic quadrangles. Solvothermal
corresponds to the expected tbo,3 that is, 1 can be considered as a
tbo-MOF (analogous to one prototypical MOF, HKUST-113).
Alternatively, 1 can be interpreted as a MOF consisting of
polyhedral cages2,14 (Figure 2a), specifically face-sharing trun-
cated cuboctahedra3 [24 functionalized isophthalate ligands
connected by eight Cu2(O2CR)4 centers and four L1 benzene
cores]. Each inorganic paddlewheel MBB is dinuclear and
consists of two copper ions with the expected CuO5 square-
pyramidal geometry, with each Cu coordinated to four oxygen
atoms of four carboxylates and one axial water molecule. Each
carboxylate moiety of the octuply deprotonated L1 ligand
coordinates in a bis-monodentate fashion to two Cu atoms,
forming the Cu2(O2CR)4 MBBs. The packing of these face-
shared (through 4MRs) truncated cuboctahedral cages generates
interpolyhedral cavities delineating two other cage types, trun-
cated cubes and truncated tetrahedra (Figure 2b).
Thus, the overall neutral framework of 1 consists of three types
of open cages (Figure 2b). The largest, truncated cuboctahedra
having diameters of 17.787 and 18.349 Å [14.387 Å sphere
including van der Waals (vdw) radii], are surrounded by six
truncated cubes (Figure S2b) and eight truncated tetrahedra.
The truncated cubes have diameters of 15.424 Å (height) and
16.102 Å (width) (12.024 Å vdw sphere) and are surrounded by
six truncated cuboctahedra and eight truncated tetrahedra. The
truncated tetrahedra have diameters of 12.521 Å (height) and
10.220 Å (width) (6.820 Å vdw sphere) and are surrounded by
four truncated tetrahedra and four truncated cubes.
reaction between the rectangle-likeH8L1 and Cu(NO3)2 2.5H2O
3
in an N,N-dimethylformamide (DMF)/water solution yielded a
homogeneous crystalline material whose purity was confirmed by
similarities between the experimental and calculated powder X-ray
diffraction (PXRD) patterns (Figure S3 in the Supporting In-
formation). The as-synthesized compound was characterized by
To confirm the efficacy of our design strategy, we function-
alized ligand L1 with two extra pendant isophthalate arms
(Figure 3a), which in principle could also potentially coordinate
to metal cations and direct the formation of a different network
(but only if the structure deviates from the targeted sql SBLs). To
our satisfaction, the reaction of 5,50,500,5000,50000,500000-[1,2,3,4,5,6-
phenylhexamethoxy]hexaisophthalic acid (H12L2) with copper
under reaction conditions similar to those for 1 yielded a MOF
analogous to 1 wherein the two added pendant arms freely point
into the generated cavities with no metal coordination.
single-crystal XRD as [Cu4L1(H2O)4 (solvent)]n (1).
3
The resulting MOF can be viewed as a pillared sql-MOF based
on L1 cores substituting the interlayer quadrangles and essentially
pillaring 2D Cu-(5-R-isophthalate) sql SBLs. Thus, the quadran-
gular-pillared sql-MOF can be viewed as a 3D MOF wherein each
L1 core serves as a 4-connected node, each 5-R-isophthalate
moiety as a 3-connected node, and each Cu2(O2CR)4 paddle-
wheel cluster asanother4-connected node. Topological analysisof
the resultant (3,4)-connected net reveals that the topology of 1
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Solvothermal reaction between rectangle-like H12L2 and Cu-
(NO3)2 2.5H2O in a DMF/water solution yielded a homoge-
3
neous crystalline material whose purity was confirmed by
similarities between the experimental and calculated PXRD
patterns (Figure S4). The as-synthesized compound was char-
acterized by single-crystal XRD as [Cu4(H4L2)(H2O)x-
(DMF)4ꢀx (solvent)]n (2). As expected, the reaction gave the
3
desired tbo-MOF, in which the necessary tetraisophthalate
moieties are coordinated to Cu to form paddlewheels while the
two extra isophthalates remain unbound and point into the large
polyhedral cavities (Figure 3b).
Additionally, the uniqueness of our pillaring approach to tbo-
MOFs should permit the construction of isoreticular MOFs with
the same tbo topology via the expansion of the organic pillars,
specifically by extension of the links between the quadrangular
benzene core and the isophthalate moieties shown in Figure 1a,
resulting in SBLs that are pillared and separated at a greater
distance by the expanded quadrangles. To support this concept,
we synthesized 5,50,500,5000-[1,2,4,5-benzenetetrakis(4-methyle-
neoxyphenylazo)]tetraisophthalic acid (H8L3), wherein the iso-
phthalate moieties and benzene core of L1 were kept intact but
the length between them was extended by using 4-hydroxyphe-
nyl-1-diazene moieties as the links.
Figure 3. (a) Ligand L2, a functionalized version of L1. (b) Represen-
tation of the largest cage in 2, in which the open space is reduced
because of the pendant groups. (c) Representation of a nanocapsule cage
in 3. The vdw sphere in (b) and cylinder in (c) are shown in gold.
As projected, solvothermal reaction between H8L3 and cop-
per in a DMF/water solution afforded green crystals that were
characterized by single-crystal XRD studies as [Cu4(L3)(H2O)3-
of CO2, it was anticipated that the free carboxylic acids exposed in
the pores would enhance the energetics of CO2 sorption; CO2 is
recognized to act as a Lewis base, leading to relatively strong
specific interactions with carboxylic acids.18 Indeed, the Qst for
CO2 was found to be higher for 2 than for 1 over the entire
studied range, with a more pronounced difference at low loading
(35.2 vs 29.7 kJ molꢀ1) (Figure 4a inset). It should be mentioned
that at 258 K, the uptake was more pronounced for 2 than for 1 at
reduced pressures; this trend was reversed once the uptake
reached 8.5 mmol/mmol (at 1 bar, the maximum uptake was
10.1 for 1 vs 9.7 mmol/mmol for 2) (Figure 4a).
For further confirmation and elucidation of the impact of the
free carboxylic acids on the CO2 sorption energetics in the case of
compound 2, we elected to perform comparative sorption studies
at room temperature and elevated pressures using a sorbate
probe molecule without a quadrupole, namely, CH4. In fact, a
more pronounced difference between CO2 and CH4 uptake for 2
versus 1 was observed in the lower-pressure range (i.e., 0ꢀ5 bar)
of the sorption isotherms (Figure 4b), indicative of enhanced
CO2 interactions with the framework, as also evidenced by the
steepness of the CO2 sorption isotherm at low loading for 2. This
result is in complete agreement with the relatively high Qst
observed at low loading for compound 2 versus 1. It is worth
mentioning that the CO2 sorption isotherms at 298 K for 1 and 2
intersect at ∼6 bar and at an uptake (∼8.5 mmol/mmol)
comparable to that observed in the sorption isotherms at lower
temperature (258 K) (Figure 4a). Also, a noticeable difference in
the CH4 sorption isotherms was primarily observed only at
higher pressures, which is likely correlated with the difference
between the pore size distributions of compounds 1 and 2, since
CH4-specific interactions with the free carboxylic acid groups are
relatively less dominant than for CO2. It is apparent that
functionalizing the pores with moieties that can favorably interact
with CO2 should result in made-to-order MOFs with desired
affinities and capacities for CO2 capture and storage.
(DMF) (solvent)]n (3), a 3D tbo-MOF isoreticular to 1 and 2.
3
The extra-large truncated cuboctahedral cages in 3 are more like
nanocapsules (Figure 3c) and possess diameters in the mesopor-
ous range (up to 29.445 Å ꢁ 18.864 Å and 26.045 Å ꢁ 15.464 Å
including vdw radii). The truncated cubes have diameters of
28.682 Å (height) and 15.612 Å (width) (25.282 ꢁ 12.212 Å
vdw), and the truncated tetrahedra have diameters of 26.499 Å
(height) and 9.081 Å (width) (23.099 ꢁ 5.681 Å vdw).
The total solvent-accessible volumes of 1, 2, and 3 were
estimated to be ∼72%, ∼56%, and ∼76%, respectively, by
summing voxels more than 1.2 Å away from the framework
using PLATON software.15 Sorption studies performed on each
compound confirmed their permanent porosity. Argon sorption
experiments on compounds 1 and 2 gave fully reversible type-I
isotherms (Figure S9) characteristic of microporous materials.
The Langmuir apparent surface area of 1 was estimated to be
2896 m2/g, which is higher than that of prototypical HKUST-1
(692ꢀ1944 m2/g).13,16 The calculated total free volume was
estimated to be 0.97 cm3/g. As anticipated, the effect of the
ancillary pendant groups in compound 2 is reflected by the
reduced surface area and free pore volume, which were
estimated as 1490 m2/g and 0.47 cm3/g, respectively. Thus,
isoreticular compounds 1 and 2 are attractive sorbents for
evaluating the impact of pore size, shape, and functionality on
the sorption energetics and uptake of industrially and envir-
onmentally relevant gases such as CH4, H2, and CO2.
Accordingly, H2, CO2, and CH4 sorption experiments were
carried out at various temperatures and pressures. For H2, the
isosteric heat of adsorption (Qst) was found to be higher for 2
than for 1 over the entire studied range (e.g., 7.6 vs 6.8 kJ molꢀ1
at low loading); nevertheless, the maximum uptake at 77 K and 1
bar was higher for 1 than for 2 (2.37 vs 1.80 wt %) (Figure S10).
The observed improvement in the H2 sorption energetics in the
case of 2 is likely due to the combined size and surface effects of
the exposed free carboxylic acids, which reduce the confined
space and promote a localized higher charge density.17 In the case
We have successfully adapted a modular pillaring strategy to
the design and synthesis of isoreticular tbo-MOF platforms,
including functionalization and expansion of cavities. The novel
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’ ACKNOWLEDGMENT
This work is dedicated to the memory of I. Dale Shellhammer
(1924ꢀ2010). The authors gratefully acknowledge funds from
NSF (DMR 0548117) and KAUST as well as beamline 15ID-C
of ChemMatCARS Sector 15 at APS, ANL (DOE DE-AC02-
06CH11357, NSF CHE-0822838).
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tbo-MOFs (i.e., quadrangular-pillared sql-MOFs) maintain ther-
mal stability up to nearly 300 °C (Figures S7 and S8) and exhibit
permanent porosities higher than those observed for the analo-
gous HKUST-1 material. The addition of pendant functional
groups, in this case carboxylic acids, enhances the affinity for
guest molecules, particularly CO2. Further sorption studies are
underway to evaluate other potential guest molecules. Future
work will include encapsulation of porphyrins and analogous
molecules as well as corresponding catalytic studies. Also, the
facile nature of MOF synthesis combined with our platform
technique will allow us to integrate a variety of metals (e.g., Mo-,
Fe-, and Cr-HKUST-1 analogues are already known,19 and
numerous metals are known to form the paddlewheel MBB11a).
In addition, this approach based on pillaring of SBLs as the main
periodic building units will permit the generation of MOFs with
larger surface areas that can be readily functionalized prior the
assembly process. Work is in progress to introduce various
additional functionalities (e.g., various amines to evaluate their
impact on CO2 uptake) for targeted applications such as gas
separation and development of thin-film-based MOFs.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, additional
b
(19) Mo3(BTC)2: Kramer, M.; Schwarz, U.; Kaskel, S. J. Mater.
Chem. 2006, 16, 2245. Fe3(BTC)2: Xie, L.; Liu, S.; Gao, C.; Cao, R.;
Cao, J.; Sun, C.; Su, Z. Inorg. Chem. 2007, 46, 7782. Cr3(BTC)2: Murray,
L. J.; Dinca, M.; Yano, J.; Chavan, S.; Bordiga, S.; Brown, C. M.; Long,
J. R. J. Am. Chem. Soc. 2010, 132, 7856.
results, and crystallographic data (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
mohamed.eddaoudi@kaust.edu.sa
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