Functional group effects on metal-organic framework topology

Article abstract of DOI:10.1039/c2cc34938h

Functionalization of the ligand 1,3,5-tris(4-carboxyphenyl)benzene (H ₃BTB) has been realized with methoxy (H₃BTB-[OMe] ₃) and hydroxy (H₃BTB-[OH]₃) groups. Combining H₃BTB-[OMe]₃

Full text of DOI:10.1039/c2cc34938h

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Functional group effects on metal–organic framework topologywz
Phuong V. Dau, Kristine K. Tanabe and Seth M. Cohen*
Received 11th July 2012, Accepted 7th August 2012
DOI: 10.1039/c2cc34938h
Functionalization of the ligand 1,3,5-tris(4-carboxyphenyl)benzene
(H3BTB) has been realized with methoxy (H3BTB–[OMe]3) and
hydroxy (H3BTB–[OH]3) groups. Combining H3BTB–[OMe]3 and
Zn(II) results in the formation of the first isostructural, functionalized
analogue of MOF-177 (MOF-177–OMe), while the combination of
H3BTB–[OH]3 and Zn(II) generates a rare, interpenetrated pcu-e
framework.
H3BTB together with various dicarboxylate ligands and Zn(II)
has produced a series of high surface area UMCM frameworks
(UMCM-1, UMCM-2 with BET surface areas >4000 m2 gꢀ1).20,21
Thus, the functionalization of H3BTB is an attractive target for
modifying the properties of these highly porous MOFs.
Only a few studies of modified H3BTB ligands have been
reported. In most cases, these studies extended the length of
the H3BTB ligand or functionalized the central benzene ring of
H3BTB.22–24 Only one example of a functionalized H3BTB
ligand25 with chiral oxazolidinone groups on the three benzoic
acid aryl rings has been described, which formed a family
of chiral MOFs with Zn(II). However, these chiral H3BTB
analogues did not produce a framework that was isostructural
with MOF-177. The limited number of reports on functionalized
H3BTB ligands is likely due, in part, to the synthetic challenges of
making derivatized analogues. Herein, we report the synthesis of
functionalized H3BTB ligands with methoxy (H3BTB–[OMe]3)
and hydroxy (H3BTB–[OH]3) (Fig. 1) groups. These ligands
were prepared via Pd-mediated cross-coupling reactions. The
combination of H3BTB–[OMe]3 and Zn(II) allows for the
formation of the first functionalized MOF-177 analogue,
MOF-177–OMe. In contrast, the combination of H3BTB–[OH]3
and Zn(II) generates the first functionalized interpenetrated pcu-e
(primitive cubic unit expansion) framework,26,27 wherein the
hydroxy groups remain free (not coordinated to the SBUs).
In the earlier report of Kaskel and co-workers,25 the function-
alization of H3BTB was obtained via the trimerization of an
acetophenone derivative. In our study, the functionalization of
H3BTB is achieved by the Pd cross-coupling reaction between
1,3,5-tris(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzene and
methyl 4-iodo-2-methoxybenzoate (Scheme S1, ESIz), the product
of which can be used to obtain both H3BTB–[OMe]3 and
H3BTB–[OH]3. In an attempt to make isostructural functional
MOF-177, H3BTB–[OMe]3 was combined with Zn(II) in
N,N-dimethylformamide (DMF) under solvothermal conditions,
which produced the desired MOF-177–OMe as clear colorless
Metal–organic frameworks (MOFs) continue to be one of the
most highly investigated porous materials for a wide range of
applications such as gas storage,1,2 catalysis,3 and chemical
sensors.4 Such applications may benefit from MOF materials
due to the ability to alter their chemical composition, function-
ality, and molecular dimensions systematically without changing
their underlying topology. The ability to control MOF topology
was termed reticular synthesis by Yaghi and co-workers.5 For
example, Yaghi and co-workers pioneered the synthesis of
cubic IRMOFs (IRMOFs = isoreticular MOFs) with different
functional groups (e.g., bromo, amino, naphthyl), and with
increased dimensions (e.g., biphenyl).6 Notably, Yaghi and
co-workers have also showed the ability to incorporate multiple
functional groups into one single IRMOF crystal.7 One crucial
factor of reticular synthesis relies on the fact that the functional
groups do not alter the topology of the desired MOF, which
occurs most often when the functional groups can interact with
metal ions in a way that would interfere with MOF synthesis.
Reticular synthesis has been successfully applied to many MOF
systems, including pillared paddlewheel MOFs,8,9 UMCM
(UMCM = University of Michigan Crystalline Materials),10
UiO (UiO = University of Olso),11 and others.12
In recent years, multitopic ligands have been of interest due
to their potential to generate MOFs that have high surface
areas for applications in gas storage.13–15 Among these multitopic
ligands, 1,3,5-tris(4-carboxyphenyl)benzene (H3BTB) has been
studied extensively by Matzger16 and others to generate MOFs
with high porosity.17,18 For example, the combination of H3BTB
and Zn(II) generated the high surface area MOF-177 (BET surface
area B4500 m2 gꢀ1).19 Additionally, the copolymerization of
Department of Chemistry and Biochemistry, University of California,
San Diego, La Jolla, CA, USA 92093. E-mail: scohen@ucsd.edu
w This article is part of the ChemComm ‘Metal–organic frameworks’
web themed issue.
z Electronic supplementary information (ESI) available: Detailed
synthesis and other charaterization of materials. CCDC 890248. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2cc34938h
Fig. 1 Molecular structures of H3BTB–[OMe]3 (left) and
H3BTB–[OH]3 (right).
c
9370 Chem. Commun., 2012, 48, 9370–9372
This journal is The Royal Society of Chemistry 2012

Fig. 3 Structure of the SBU of Zn(II)–MOF–BTB[OH]3 (left), view of
the octahedral cage (middle, SBUs are displayed in polyhedron style),
and space-filling model of the octahedral cage with the removal of two
BTB–[OH]3 ligands, showing the void space of the cage (right).
Hydrogen atoms and some disordered hydroxyl groups are omitted
for clarity. Color scheme: Carbon (grey), nitrogen (blue), oxygen (red),
and zinc (green).
Fig. 2 PXRD patterns of MOF-177 (simulated, black), MOF-
177–OMe (red), Zn(II)–MOF–BTB[OH]3 (simulated, blue), and
Zn(II)–MOF–BTB[OH]3 (magenta).
in the framework, but were not located in the XRD analysis.
The underlying topology of the framework is an (8,3)-coordinated
net with the 8-fold vertex centered on the SBU and the 3-fold
vertex centered on the BTB–[OH]3 ligands.
hexagonal crystals (Fig. S1, ESIz). H3BTB–[OH]3 combined
with Zn(II) under the same reaction conditions did not yield
any crystals. Interestingly, the combination of H3BTB–[OH]3
and Zn(II) in DMF and H2O (2 : 0.1 DMF : H2O ratio) under
solvothermal condition yields clear colorless crystals suitable
for single-crystal X-ray diffraction analysis (XRD). This new
phase, designated Zn(II)–MOF–BTB[OH]3 was not isostructural
to MOF-177.
Examination of Zn(II)–MOF–BTB[OH]3 shows that the
structure is comprised of large pseudo-octahedral cages with
the BTB–[OH]3 ligands acting as the faces and the SBUs acting
as the vertices of the cages (Fig. 3). These cages are connected
together via the vertices to form a cubic framework that is
classified as
a
pcu-e net (Fig. 4).26 Interestingly,
The powder X-ray diffraction (PXRD) analysis of MOF-
177–OMe verifies the isostructural relationship when
compared to the parent MOF-177 material (Fig. 2). Although
large single crystals of MOF-177–OMe were produced
(Fig. S1, ESIz), the crystals only diffracted to low resolution
(B1.7 A), limiting the data analysis to determination of the
cell parameters. Nevertheless, the unit cell parameters of
MOF-177–OMe are very similar to MOF-177,19 and the space
group assignment is identical (Table S1, ESIz). Therefore, the
topology of MOF-177–OMe, being the same as MOF-177, is a
(6,3)-coordinated net with the 6-fold vertex centered on the
Zn4O SBUs and the 3-fold vertex centered on the BTB–[OMe]3
ligands. A high Brunauer–Emmett–Teller (BET) surface area of
B3900 m2 gꢀ1 was determined for MOF-177–OMe, which also
supports the formation of the isostructural material.
Zn(II)–MOF–BTB[OH]3 is comprised of two frameworks,
which are interpenetrated and interact with each other via
p–p stacking between the BTB–[OH]3 ligands (Fig. 5). Only two
MOFs have been reported to have the similar interpenetrated,
pcu-e net topology found with Zn(II)–MOF–BTB[OH]3. Two
early reports by Robson show that combining Zn(II) or Cu(I)
together with the monodentate, tritopic 2,4,6-tri(4-pyridyl)-
1,3,5-triazine) ligands produces interpenetrated, pcu-e nets.28,29
The topology of Zn(II)–MOF–BTB[OH]3 provides an example
of the first, rare functionalized interpenetrated pcu-e frame-
work, as the hydroxy groups on the ligand are not coordinated
to the SBUs, but remain free in the pores of the MOF (Fig. 4).
The pseudo-octahedral cages are large enough to accommodate
guest molecules (Fig. S3, ESIz, B2700 A3); however, guests
within the cage were disordered in the XRD analysis and could
PXRD analysis of Zn(II)–MOF–BTB[OH]3 reveals the frame-
work possesses an entirely different structure when compared to
MOF-177 (Fig. 2). Single-crystal XRD shows the structure of
Zn(II)–MOF–BTB[OH]3 has a zinc oxide SBU with an overall
molecular formula of [Zn2(OH)](BTB–[OH]3)2(DMF) (Fig. 3).
The SBUs of this framework consist of four Zn(II) ions linked
together by hydroxy bridges with the formula Zn4(OH)2. While
the two terminal Zn(II) ions of the SBUs are 6-coordinate, bound
by the BTB–[OH]3 ligands, hydroxy bridges, and DMF molecules,
the other two Zn(II) ions are 5-coordinate with donation from the
hydroxy bridges and BTB–[OH]3 ligands. The SBUs of the
Zn(II)–MOF–BTB[OH]3 are coordinated by eight different
BTB–[OH]3 ligands, where six of them act as bridging, bidentate
ligands, and the other two act as monodentate donors (Fig. 3).
The non-coordinated oxygen atoms of the two monodentate
carboxylate donors from BTB–[OH]3 interact with the hydroxy
bridges via hydrogen bonding forming a six-membered ring
(Fig. 3, O–H–O B 1.8 A). Charge balance indicates that the
SBU is overall a dianion, thus, counter cations must be present
Fig. 4 Structure of one pcu-e net of Zn(II)–MOF–BTB[OH]3,
consisting of pseudo-octahedral cages. The framework is highlighted
in orange, free hydroxy groups are displayed as red balls, and
Zn atoms are displayed as green balls.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9370–9372 9371

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This journal is The Royal Society of Chemistry 2012