Unusual preservation of polyhedral molecular building units in a metal-organic framework with evident desymmetrization in ligand design

Article abstract of DOI:10.1039/c3cc48089e

A new porous metal-organic framework (MOF) with three types of preserved polyhedral molecular building units, all of which have been observed in a series of isostructural MOFs constructed from highly symmetrical hexacarboxylic ligands, was formed even from a pre-designed related tetracarboxylic ligand with drastically reduced symmetry. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cc48089e

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Unusual preservation of polyhedral molecular
building units in a metal–organic framework with
evident desymmetrization in ligand design†
Cite this: Chem. Commun., 2014,
50, 563
Received 22nd October 2013,
Accepted 5th November 2013
Yabo Xie,a Hui Yang,a Zhiyong U. Wang,b Yangyang Liu,c Hong-Cai Zhouc and
Jian-Rong Li*a
DOI: 10.1039/c3cc48089e
www.rsc.org/chemcomm
A new porous metal–organic framework (MOF) with three types of with preserved original polyhedral molecular building units but
preserved polyhedral molecular building units, all of which have assembled from desymmetrized ligands has not been reported to
been observed in a series of isostructural MOFs constructed date. Here we wish to report such a case which demonstrates the
from highly symmetrical hexacarboxylic ligands, was formed even complicated aspect of the desymmetrization in ligand design.
from a pre-designed related tetracarboxylic ligand with drastically
reduced symmetry.
In our and other previous studies, a series of (3,24)-connected
isoreticular MOFs (including PCN-61, -66, -68, -69, NOTT-113, -114,
-115, PMOF-2, NU-100, etc.) with three types of metal–organic
Metal–organic frameworks (MOFs) have attracted lots of interest polyhedra (MOP) acting as building units were constructed
because of their diverse compositions and framework aesthetic from similar dendritic hexacarboxylate ligands and paddle-wheel
appeal, as well as their unique properties and various potential Cu2 nodes.8 These MOFs exhibited not only graceful architecture but
applications.1 The architectural design of MOFs is mainly depen- also excellent gas adsorption properties. To construct this series of
dent on the assembly of metal centers and organic ligands.2 Of MOFs, it was found that the hexacarboxylate ligands should have a
particular importance are the design and use of organic ligands, C3 symmetry and the three isophthalate moieties must be coplanar.
which add flexibility and diversity to the chemical structures and In this work, we explored the construction of MOFs from a
functions of these materials.3–6 Most of the carboxylate ligands novel desymmetrized ligand 5,50-(9-oxo-9,10-dihydroacridine-2,7-diyl)-
used in MOF construction have been designed with a high diisophthalic acid (H4OADDI), which was derived from the reported
symmetry for both aesthetic reasons and practical convenience hexacarboxylate ligand 40,400,40 00-nitrilotribiphenyl-3,5-dicarboxylic
in predicting the crystalline MOF structures. In comparison, acid (H6NBPD)8f (Scheme 1). We employed a shearing strategy to
ligands with lowered symmetry are rarely explored largely due to desymmetrize H6NBPD into 5,50-(acridine-2,7-diyl)diisophthalic acid
the uncertainty in MOF structure prediction. However, this could (H4ADDI), which got oxidized in situ during the synthesis of a new
be considered as an advantage instead of a disadvantage because MOF [Cu6(OADDI)3(S)6]ꢀnS (1ꢀnS, S represents non-assignable solvent
novel structures might be generated from desymmetrized ligands. molecules). Interestingly, 1ꢀnS preserved three types of MOP building
Recently, a new strategy called linker-directed vertex desymme- units similar to those observed in NOTT-115,8f the corresponding
trization was proposed by Matzger and co-workers to construct MOF constructed from the high symmetry ligand H6NBPD.
MOFs by using reduced symmetry ligands, which were achieved
1ꢀnS was solvothermally synthesized by the reaction of
through removing or adding chemical entities from/into high Cu(NO)2ꢀ2.5H2O with H4ADDI.‡ Its crystal structure was determined
symmetry ligands.7 The resulting MOFs presented diverse struc-
tures distinct from those constructed from the corresponding
high symmetry ligands. To the best of our knowledge, a MOF
a Department of Chemistry and Chemical Engineering, College of Environmental
and Energy Engineering, Beijing University of Technology, Beijing 100124,
P. R. China. E-mail: jrli@bjut.edu.cn
b Department of Chemistry and Physics, Troy University, Troy, AL 36082, USA
c Department of Chemistry, Texas A&M University, College Station, TX 77843, USA
† Electronic supplementary information (ESI) available: More experimental
details of synthesis and characterization, as well as additional structural figures.
CCDC 965271 and 965272. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3cc48089e
Scheme 1 The design and in situ generation of the desymmetrized
H4OADDI ligand.
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by single-crystal X-ray diffraction. The phase purity of the product faces with four T-Oh polyhedra and its four trigonal faces with four
was confirmed by powder X-ray diffraction (PXRD), where the C-Oh polyhedra. Finally, these three types of polyhedra share their
experimental pattern matches well with the simulated one (Fig. S3, faces and extend to form a three-dimensional (3D) framework with
ESI†). Thermogravimetric analysis (TGA) of the as-synthesized 1ꢀnS pores (Fig. 1c). The pore volume ratio in 1 is about 70% without
indicated a weight loss of 50% before about 250 1C, above which guest solvent molecules, as calculated using the PLATON program.
the compound starts to decompose (Fig. S5, ESI†).
Topologically, the framework of 1 can be simplified as a
As reported previously,8 all (3,24)-connected MOFs crystallized in novel (3,4)-connected ternary net (i.e., trinodal) when consider-
%
a highly symmetric cubic space group of Fm3m as represented by ing each Cu2 entity as a planar 4-connected node and each
NOTT-115.8f The structure of these MOFs can be described as the tetracarboxylate OADDI4ꢁ ligand as two 3-connected nodes
packing of three types of polyhedra: cuboctahedra (C-Oh), truncated (Fig. S1, ESI†). For NOTT-115, each hexacarboxylate ligand is
tetrahedra (T-Td), and truncated octahedra (T-Oh). In comparison, considered as four 3-connected nodes to give also a (3,4)-connected
single-crystal X-ray diffraction analysis revealed that 1ꢀnS crystallized net, but being different from that of 1 (Fig. S2, ESI†).
in the centrosymmetric tetragonal space group of P4/mnc. Similar to
The permanent porosity of 1 was demonstrated by N2 gas
NOTT-115, 1 also possessed dimeric paddle-wheel [Cu2(COO)4] adsorption at 77 K. Before the gas adsorption experiment, a freshly
nodes. All the carboxylate groups in OADDI4ꢁ took part in the prepared sample was activated by supercritical carbon dioxide (SCD).
coordination in the same mode. It is quite interesting that in 1 three The N2 adsorption isotherm showed the characteristics of a micro-
kinds of polyhedra are formed (Fig. 1a): twelve Cu2 nodes are porous material (Fig. 2). The maximum N2 uptake at 1 bar for 1 is
connected together by twenty isophthalate moieties of different 568 cm3 ꢁ1. The Brunauer–Emmett–Teller (BET) and Langmuir
g
OADDI4ꢁ ligands to give a C-Oh polyhedron; twelve Cu2 entities surface areas of 1 are evaluated to be 1475 and 1736 m2 gꢁ1
,
are connected together by ten OADDI4ꢁ ligands to form a T-Td respectively. The pore size distribution evaluation (by PSD theory)
polyhedron; and twenty-four Cu2 entities are assembled together by demonstrated three kinds of pores in the material with the pore size
sixteen OADDI4ꢁ ligands to form a T-Oh polyhedron. All these of about 9.1, 11.8, and 14.8 Å, respectively, corresponding to the T-Td,
polyhedra are similar to those observed in NOTT-115, but distorted T-Oh, and C-Oh polyhedral cages in the structure of 1 (Fig. 1).
for T-Td and T-Oh ones because the carboxylate groups are not
Another interesting aspect of this work is the in situ oxidation of
coplanar in OADDI4ꢁ. Furthermore, as shown in Fig. 1b, the H4ADDI to H4OADDI during the formation of 1. This has been
truncated triangular face (distorted hexagon) of the T-Td or the confirmed by single-crystal X-ray diffraction of 1 and NMR analysis
T-Oh polyhedron is constructed from three ligands with each linked of the recovered ligand after acidic decomposition of 1 (Fig. S7, ESI†).
to four shared Cu2 nodes, even if fully occupied by only one. This is This in situ reaction was also observed in the process of using a relevant
different from that in NOTT-115, where only one ligand links six Cu2 dicarboxylic acid ligand to construct a MOP as discussed below.
nodes in each hexagonal face.8f The capability of forming these
Further removal of two carboxylate groups from H4ADDI gave
hexagonal faces in both MOFs should be key for the similar the ligand 3,30-(acridine-2,7-diyl)dibenzoic acid (H2ADDB, Fig. 3a),
polyhedral structural features of 1 and NOTT-115. In the polyhedral from which a new MOP, [Cu4(OADDB)4(DMA)4]ꢀnS (2ꢀnS, OADDB =
packing of 1, similar to that in NOTT-115, eight trigonal faces of each 3,30-(9-oxo-9,10-dihydroacridine-2,7-diyl)dibenzoate), with the same
C-Oh polyhedron are shared by eight T-Td polyhedra, and the Cu2 nodes in 1 was constructed. 2ꢀnS was solvothermally synthe-
remaining six square faces are shared with six T-Oh polyhedra. sized by the reaction of Cu(NO)2ꢀ2.5H2O with H2ADDB.‡ Single-
Simultaneously, each T-Td polyhedron shares its four hexagonal crystal X-ray diffraction revealed that 2ꢀnS crystallized in the triclinic
%
space group P1 and possesses a molecular cage structure
(Fig. 3b). In the structure two Cu2 entities are connected by four
Fig. 1 The structural presentation of 1: (a) the T-Td, T-Oh, and C-Oh
polyhedra; (b) the hexagonal face shared by T-Td and T-Oh polyhedra;
(c) the face-shared polyhedral packing.
Fig. 2 N2 adsorption and desorption isotherms at 77 K and pore size
distribution (inset) of 1.
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This work was supported by NSFC (No. 21271015) and Beijing
Municipal NSF (No. 2132013). Z. U. Wang is grateful for financial
support from Troy University (2013 Summer Research Grant).
Notes and references
‡ Synthesis of 1ꢀnS. A solution of Cu(NO)2ꢀ2.5H2O (116 mg, 0.5 mmol)
and H4ADDI (101 mg, 0.2 mmol) in the mixed solvent of 8 mL of
dimethyl sulfoxide (DMSO) and 1 mL of methanol was sealed in a 20 mL
glass vial and heated at 100 1C for 6 days. Dark-green block crystals were
collected, washed with DMSO and acetone, and then dried in air (yield
43%). The PXRD pattern of as-synthesized 1ꢀnS is shown in Fig. S3
(ESI†). For TGA and FT-IR, see Fig. S5 and S9 (ESI), respectively.
Synthesis of 2ꢀnS. A mixture of Cu(NO)2ꢀ2.5H2O (46 mg, 0.2 mmol),
H2ADDB (83 mg, 0.2 mmol), and 3 drops of HBF4 (48% aqueous solution)
in 3 mL of N,N-dimethylacetamide (DMA) was sealed in a 20 mL glass vial
Fig. 3 (a) The design and in situ generation of H2OADDB; (b) the structure
of 2 (yellow sphere shows the internal cavity); (c) lantern shaped configu- and heated at 85 1C for 3 days. The resulting dark-yellow solution was
allowed to stand at room temperature for 6 days to obtain dark-green
block crystals, which were washed with DMA and then dried in air (yield
52%). The PXRD pattern of as-synthesized 2ꢀnS is shown in Fig. S4 (ESI).
For TGA and FT-IR, see Fig. S6 and S10 (ESI), respectively.
ration of 2.
OADDB2ꢁ ligands to form a big paddle-wheel with the distance
between two Cu2 nodes being about 5.2 Å. The overall molecular
cage can also be considered to have a lantern shaped configu-
ration as presented in Fig. 3c. The H2ADDB ligand was in situ
oxidized to H2OADDB during the formation of 2, similar to
what happened during the synthesis of 1. The oxidation of an
acridine to an acridin-9(10H)-one compound is a well-documented
organic transformation.9
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molecular cage of MOP 2 is not a structural unit of MOF 1.
It should be pointed out that based on a similar system of
tetracarboxylate and dicarboxylate ligands we have recently
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structure with three kinds of preserved polyhedral molecular cage
building units as observed in the corresponding MOF (NOTT-115)
constructed from the highly symmetrical hexacarboxylic ligand.
The MOP has a lantern shaped structure, but it was not incorpo-
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reported by us. These results demonstrate both the power of the
ligand desymmetrization strategy in generating novel MOFs
and the challenge in the precise design and control of MOFs, in
which even a slight perturbation of ligand structures may lead
to drastically distinct MOFs.
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