Functionalized defects through solvent-assisted linker exchange: Synthesis, characterization, and partial postsynthesis elaboration of a metal-organic framework containing free carboxylic acid moieties

Article abstract of DOI:10.1021/ic502697y

Intentional incorporation of defect sites functionalized with free carboxylic acid groups was achieved in a paddlewheel-based metal-organic framework (MOF) of rht topology, NU-125. Solvent-assisted linker exchange (SALE) performed on a mixed-linker deriva

Full text of DOI:10.1021/ic502697y

Article
pubs.acs.org/IC
Functionalized Defects through Solvent-Assisted Linker Exchange:
Synthesis, Characterization, and Partial Postsynthesis Elaboration of
a Metal−Organic Framework Containing Free Carboxylic Acid
Moieties
Olga Karagiaridi,† Nicolaas A. Vermeulen,† Rachel C. Klet,† Timothy C. Wang,† Peyman Z. Moghadam,‡
Salih S. Al-Juaid,§ J. Fraser. Stoddart,*,† Joseph T. Hupp,*,† and Omar K. Farha*,†,§
†Department of Chemistry and International Institute for Nanotechnology and ‡Department of Chemical and Biological Engineering,
Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
§Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia
S
* Supporting Information
ABSTRACT: Intentional incorporation of defect sites func-
tionalized with free carboxylic acid groups was achieved in a
paddlewheel-based metal−organic framework (MOF) of rht
topology, NU-125. Solvent-assisted linker exchange (SALE)
performed on a mixed-linker derivative of NU-125 containing
isophthalate (IPA) linkers (NU-125-IPA) led to the selective
replacement of the IPA linkers in the framework with a conjugate base of trimesic acid (H3BTC). Only two of the three
carboxylic acid moieties offered by H3BTC coordinate to the Cu2 centers in the MOF, yielding a rare example of a MOF
decorated with free −COOH groups. The presence of the −COOH groups was confirmed by diffuse reflectance infrared
Fourier-transformed spectroscopy (DRIFTS); moreover, these groups were found to be available for postsynthesis elaboration
(selective monoester formation). This work constitutes an example of the use of SALE to obtain otherwise challenging-to-
synthesize MOFs. The resulting MOF, in turn, can serve as a platform for accomplishing selective organic transformations, in this
case, exclusive monoesterification of trimesic acid.
bonds with the MOF metal centers and are therefore
commonly employed as MOF linkers. To prevent unwanted
coordination of carboxylates to metal nodes, several strategies
have been employed, including reliance on the steric demands
of the framework,20,24 variation of synthetic conditions (choice
of solvent,21 synthesis in the absence of bases,22 or use of acidic
conditions23), and postsynthesis deprotection.25 Nevertheless,
the overall number of reports outlining the successful,
intentional incorporation of free carboxylic acid groups into
MOFs remains small. Since free carboxylic acid-containing
MOFs are anticipated to have great utility in a range of
applications (including organocatalysis, enhancement of proton
conductivity, and capture of vapor-phase ammonia26), the array
of methods available for their synthesis merits expansion and
diversification.
Recently, solvent-assisted linker exchange (SALE) has been
shown to be remarkably versatile and effective for the synthesis
of MOFs that are difficult to access de novo (i.e., through a
one-pot solvothermal reaction).27−42 Briefly, SALE involves
replacing the linkers in a parent metal−organic framework with
linkers of choice (e.g., functionalized linkers,29 linkers that can
serve as a catalyst precursor,30 longer linkers36) via a
INTRODUCTION
■
Hybrid materials that combine the rigidity of inorganic
compounds with the flexibility and the tunability of organic
matter are attractive candidates for several industrially relevant
tasks, including new tasks pertaining to alternative energy and
sustainability. It is hardly surprising, therefore, that the
emerging class of porous hybrid materials known as metal−
organic frameworks (MOFs)1 has generated an impressive
amount of research over the past two decades. MOFs have
been studied as promising candidates for, among other things,
gas storage,2−4 separation,5−7 catalysis,8−10 sensing,11 toxic gas
removal,12,13 and light harvesting.14−16
Despite the fact that by definition MOFs are highly modular
hybrid materials (composed of metal-based nodes connected by
organic linkers, with a large variety of structural building blocks
available for the synthesis of “tailor-made” products), taking
advantage of their signature tailorability is not always
straightforward.17−19 In particular, incorporation of reactive
organic functionalities into MOF cages has proven to be a
formidable task, as these moieties tend to react with the MOF
metal precursors under the solvothermal conditions typically
employed to synthesize MOFs rather than remain free to be
harnessed for further application. This complication is
particularly relevant for the incorporation of free carboxylic
acid groups. Carboxylates form relatively strong coordination
Received: November 10, 2014
© XXXX American Chemical Society
A
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Scheme 1. Idealized Representation of the Synthesis of NU-125-HBTC by SALE of IPA for HBTC in NU-125-IPA
heterogeneous reaction in a carefully selected solvent.27,28 We
hypothesized that SALE would be an effective method for
obtaining MOFs featuring free (i.e., linker-pendant, rather than
node-coordinated) carboxylic acid moieties. Relevant to this
goal, we43 and others44 have reported on intentional
introduction of structural defects (e.g., missing phenyl
carboxylate fragments) into copper-paddlewheel-based MOFs.
Barin et al. showed that it was possible to incorporate defects
into the framework of NU-125 (Cu3L),45 a highly porous
paddlewheel-based MOF of rht topology that features as a
linker the fully deprotonated conjugate base of 5,5′,5″-(4,4′,4″-
(benzene-1,3,5-triyl)tris(1H-1,2,3-triazole-4,1-diyl))-
triisophthalic acid (H6L).43 Synthesis of NU-125 in the
presence of diacid linker precursors, such as isopthalic acid
(H2IPA), led to framework incorporation of IPA and
associated node vacancies, in lieu of L. The resulting structural
defects (voids) significantly alter the material’s textural
properties as shown by augmentation of the Brunauer−
Emmett−Teller (BET) area and the pore volume. Herein, we
apply SALE to a sample of NU-125 featuring IPA defects to
selectively replace the diacid molecules with a monoprotonated
conjugate base of benzene-1,3,5-tricarboxylic acid (H3BTC,
Scheme 1). The rht topology of NU-125 in combination with
the steric demands of the framework allows the H3BTC
molecules to coordinate to the Cu2 centers using only two of
the three carboxylic acid groups, resulting in the functionaliza-
tion of the defect voids with free carboxylic acid moieties (one
per added BTC unit) as shown in Scheme 1. The porosity and
crystallinity of the framework is preserved, and the presence of
free carboxylic acid moieties was confirmed by diffuse
reflectance infrared Fourier-transformed spectroscopy
(DRIFTS). Additionally, we found that the free carboxylic
acid groups can be elaborated, without damage to surrounding
moieties. Thus, reaction of NU-125 containing HBTC defects
with trimethylsilyldiazomethane (TMS−CHN2) culminates in
selective methylation of only the free carboxylic acid group of
the HBTC linkers, resulting in the selective synthesis of the
monomethyl ester of BTC (within the MOF) or H3BTC (once
released from the MOF).
pattern of NU-125; Figure S13, Supporting Information). The
1H NMR spectrum of the digested BTC-functionalized product
indicated the presence of BTC in the powder with the L:BTC
ratio of 1:2 (Figure S3, Supporting Information). However,
scanning electron microscopy (SEM) images of the powder
revealed the absence of the characteristic octahedral crystals
associated with the rht topology of NU-125 (Figure 1). Instead,
an agglomerated and likely polycrystalline phase was observed
on top of a large amount of possibly amorphous plates (Figure
1a). Furthermore, upon activating the material by solvent
exchange to ethanol (EtOH), supercritical CO2 drying46,47 and
heating under vacuum to remove solvent molecules coordi-
nated to the Cu2 centers, and collecting its N2 isotherm, it was
discovered that the material did not possess the exceptional
porosity of NU-125 (∼3000 m2/g), as its BET area was only
470 m2/g (Figure 1c). The PXRD, SEM, and N2 sorption data
together suggest that the de novo synthesized material is not
isostructural with NU-125 and that defect incorporation takes
place in a different manner from that observed in the published
IPA-functionalized material (NU-125-IPA).
To achieve the desired HBTC incorporation into NU-125,
we turned to SALE. We synthesized the NU-125 analogue
featuring IPA defects using a previously optimized H6L:H2IPA
feed ratio of 1:3. The resulting product, hereafter referred to as
NU-125-IPA, possesses a L:IPA ratio of 1:1, as indicated by 1H
NMR spectroscopy, and is isostructural with NU-125 according
to its PXRD pattern (Figures S1 and S13, Supporting
Information). After being soaked in N,N-dimethylformamide
(DMF), NU-125-IPA was subjected to SALE with 3 equiv of
1
H3BTC at 80 °C overnight. The H NMR spectrum of the
product NU-125-HBTC showed complete replacement of the
IPA linkers with BTC, while the L:defect ratio remained 1:1
(Figure S4, Supporting Information). No leaching of L linkers
into the solution took place during the SALE reaction, as
1
evidenced by H NMR spectroscopy of the evaporated (and
redissolved) reaction solution, even when increased amounts of
IPA were used or when the reaction was allowed to run for
more than 48 h (Figure S6, Supporting Information). The
PXRD pattern of NU-125-HBTC confirms isostructurality with
NU-125, matching well with the simulated pattern of NU-125,
in terms of both peak positions and intensities (Figure S13,
Supporting Information).
Remarkably, in contrast to the de novo synthesized product,
the SEM images of the SALE-obtained NU-125-HBTC clearly
showed the presence of a single phase composed of well-
formed octahedral crystals varying in diameter from 2 to 5 μm
(Figure 1b). After the material was activated, N2 isotherm
measurements revealed a BET area of 3030 m2/g, providing
further evidence that the NU-125-HBTC material obtained
through SALE is structurally similar to NU-125-IPA (Figure
1c). Taking into account the L:BTC ratio of 1:1 as shown by
1H NMR measurements along with the fact that each hexa-
RESULTS AND DISCUSSION
■
Synthesis of NU-125-HBTC. Initially, incorporation of
HBTC defects into NU-125 was attempted in a similar fashion
to the de novo method that was reported by Barin et al.
Following the procedure established by the authors, NU-125
was synthesized at 80 °C in the presence of H3BTC, utilizing
the reported optimal H6L:H3BTC feed ratio of 1:3. A blue
crystalline powder was obtained, and its powder X-ray
diffraction (PXRD) pattern resembles that of NU-125 in
terms of peak positions but differs from it in terms of peak
intensities (the peak at 2θ = 5.35 exhibits a much elevated
intensity compared to the corresponding peak in the simulated
B
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the digested MOF are collected in a medium that contains
D2SO4, labile protons, such as those of the free −COOH
groups, cannot be detected. In other words, 1H NMR spectra of
the digested crystals can only confirm incorporation of BTC
linkers, without providing any information regarding their
mode of coordination to the Cu2 centers.
To elicit the needed information we resorted to diffuse
reflectance infrared Fourier-transformed spectroscopy
(DRIFTS). Notably, DRIFTS is nondestructive, so it can be
directly applied to the intact MOF. DRIFTS allows differ-
entiation between coordinated and noncoordinated carboxylate
and carboxylic acid moieties, since the vibration associated with
the COO−H bond stretch of a free carboxylic acid group
exhibits a characteristic band position. Furthermore, DRIFTS
can be performed in a dry and air-free atmosphere by using a
Praying Mantis cell, thereby ensuring that the integrity of the
activated framework is preserved and that no solvent (e.g.,
H2O) molecules are coordinated to the free Cu2 centers.
DRIFTS spectra of both NU-125-IPA and NU-125-HBTC
were collected and compared. Two important differences were
observed (Figure 2). The spectrum of NU-125-HBTC exhibits
Figure 2. Full DRIFTS spectra of NU-125-IPA and NU-125-HBTC
(black and red, respectively) and highlights of the spectral regions for
the non-hydrogen bonding carboxylic acid O−H stretch (2700−3700
cm−1) and the carbonyl stretch of free carboxylic acids (1500−2000
cm−1).
Figure 1. (a) SEM images of the products of the de novo attempt to
synthesize NU-125-HBTC and (b) the product of SALE of HBTC
into NU-125-IPA. (c) N2 isotherms of the products resulting from the
de novo and the SALE attempts to synthesize NU-125-HBTC.
bands at 1750 and 3600 cm−1 that are absent from the
spectrum of NU-125-IPA. As noted in previous work, the band
at 1750 cm−1 corresponds to the carbonyl stretch of a free
−COOH group.25 The very sharp band at 3600 cm−1 elicits a
more intriguing interpretation. From tabulated data of infrared
absorption frequencies one might naively expect to observe for
the O−H stretch of the −COOH group a broad band at 2500−
3300 cm−1. These data, however, correspond to nonporous
samples, in which neighboring carboxylic acid molecules are
able to engage in hydrogen bonding (which leads to band
broadening). In contrast, the IR spectrum of vaporized benzoic
anionic L linker is replaced by three HBTC linkers to achieve
the expected defect configuration, the proposed formula for
NU-125-HBTC is Cu3L0.75(HBTC)0.75
.
Detection of Free Carboxylic Acid Groups in NU-125-
HBTC through DRIFTS Experiments. In search of
corroborative (or not) evidence for the proposed SALE-based
defect incorporation model (leading to the presence of one free
carboxylic acid group per HBTC linker) we initially considered
1H NMR spectroscopy. Unfortunately, since solution spectra of
C
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acid (with absence of hydrogen bonding) features a sharp band
at 3600 cm−1, similar in shape to that observed in the spectrum
of NU-125-HBTC.48 The observed similarity is consistent with
the proposed model of defect incorporation into the NU-125
framework, which suggests site isolation of the HBTC
molecules and precludes hydrogen bonding. The DRIFTS
spectrum, therefore, provides strong evidence for the presence
of free carboxylic acid moieties inside the framework of NU-
125-HBTC.
Postsynthesis Methylation of NU-125-HBTC. In order
to investigate the availability of the free carboxylic acid moieties
for postsynthesis chemical elaboration inside the MOF
framework, we attempted an esterification reaction. We
reasoned that the results of such an experiment could provide
additional evidence for the presence (or not) of the proposed
free −COOH groups. Additionally, such results could provide
an attractive route for selective monomethylation of H3BTC,
since the Cu2 centers of the paddlewheel structural building
units in NU-125 essentially act as protecting groups for two of
the three −COOH functionalities of the HBTC molecule.
Methylation of HBTC to form the methyl ester Me-BTC
was achieved by subjecting activated NU-125-HBTC to 2 equiv
of TMS-CHN2 in a toluene−methanol solution (2:1 toluene/
1
MeOH v/v) at 0 °C (Figure 3). A H NMR spectrum in a d6-
dimethyl sulfoxide-D2SO4 solution of crystals that had been
subjected to the reaction for 24 h revealed a new peak at 3.85
ppm, indicating the presence of a methyl ester group (Figure
S7, Supporting Information). The relative integration of the
peak did not change upon extensive solvent exchange of the
crystals to EtOH and activation by supercritical CO2 drying,
thus demonstrating that the monomethylated Me-BTC units
remain part of the framework (Figure S8, Supporting
Information). The extent of the methylation, as determined
by 1H NMR, was 30−50%. Attempts to achieve more extensive
methylation by subjecting the framework to larger amounts of
TMS−CHN2 resulted in partial degradation of the Cu2 sites
(observed by the evolution of black particlesmost likely, Cu
nanoparticlesupon addition of excessive methylating agent)
and subsequent multiple methylation of released BTC, as
Figure 3. (a) Selective methylation of the HBTC unit inside the NU-
125-HBTC framework and its subsequent extraction by SALE with
H2IPA. (b) HPLC analysis data. Elution profiles of the trimethyl ester
of H3BTC, the dimethyl ester of H3BTC, and the monomethyl ester
of H3BTC as well as H3BTC and H2IPA (black). Elution profiles of
the diacid linker extracted from NU-125-IPA (blue), the diacid linker
extracted from NU-125-HBTC (green), and diacid linkers extracted
from NU-125-Me-BTC (red). The peak marked with an asterisk (*)
corresponds to excess H2IPA from the SALE solution.
1
evidenced by H NMR data.
The partially methylated NU-125-Me-BTC product retained
its crystallinity, yielding a PXRD pattern that matched that of
its parent to an excellent degree (Figure S14, Supporting
Information). The material was activated for N2 sorption
studies by supercritical CO2 drying without further thermal
activation, since monomethylated H3BTC was found to be
thermally sensitive; this activation method leaves residual
coordinated EtOH molecules. The material is still highly
porous, as illustrated by its large BET area (2050 m2/g; see
Figure S20, Supporting Information; for comparison, the BET
area of an NU-125-IPA sample that had been activated by a
similarly mild protocol was 2400 m2/g). Furthermore,
thermogravimetric analysis (TGA) data suggest that the three
materials (NU-125-IPA, NU-125-HBTC, and NU-125-Me-
BTC) have similar thermal stabilities (with thermal decom-
position occurring around 300 °C).
In order to quantify the extent of HBTC methylation as well
as to confirm the selectivity of the reaction, we utilized high-
performance liquid chromatography (HPLC). Analysis by
HPLC requires that the diacid linkers be removed from the
MOF framework and dissolved in a suitable solution medium.
Conveniently, SALE provides a facile route for the realization of
this goal, as the HBTC and Me-BTC linkers inside NU-125-
Me-BTC can be replaced by IPA due to the small difference in
the pKa of these diacids (Figure 3). Prior to SALE, the TMS-
CHN2-treated MOF was extensively solvent exchanged with
EtOH to ensure complete removal of the methylating agent
and then activated via supercritical CO2 drying. The material
was then subjected to SALE with IPA in DMF. After 24 h, an
aliquot of the SALE solution was removed from the reaction
vessel, the solvent was removed in vacuo, and the solid residue
was dissolved in an appropriate HPLC solvent and analyzed by
HPLC (see Supporting Information for details). Figure 3 shows
the HPLC elution profile of the sample as well as elution
profiles of relevant molecular standards (H2IPA, H3BTC, and
mono-, di-, and trimethyl esters of H3BTC) and samples of
NU-125-IPA and NU-125-HBTC (see Supporting Informa-
tion for sample preparation details). From the elution profiles,
it is evident that the only species present in the trace
corresponding to NU-125-Me-BTC are H3BTC, H2BTC-Me,
and H2IPA (present in excess in the SALE solution); no di- or
trimethyl ester products are observed, illustrating the selectivity
of the reaction. Mass spectra collected on the sample further
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support these assignments. Integration of the HPLC peaks
shows that the extent of methylation is 40%.49
The authors would like to thank both KACST and NU for their
continued support of this research.
Notably, treatment of NU-125-IPA with the methylating
agent under conditions identical to those used for NU-125-
HBTC (2 equiv of TMS-CHN2 at 0 °C in 2:1 toluene/MeOH
solution for 24 h) resulted in no detectable methylation, as
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CONCLUSIONS
■
Desired free carboxylic acid moieties can be installed in a
congener of NU-125 featuring a high density of ditopic IPA
units by exchanging these units for HBTC. The resulting
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ASSOCIATED CONTENT
■
S
* Supporting Information
1
Experimental details, H NMR, PXRD, N2 sorption, and TGA
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: stoddart@northwestern.edu.
*E-mail: j-hupp@northwestern.edu.
*E-mail: o-farha@northwestern.edu.
■
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ACKNOWLEDGMENTS
■
Work in the Hupp/Farha lab was supported by the U.S.
Department of Energy, Office of Basic Energy Sciences,
Separations and Analysis program, award no. DE-FG02-
12ER16362. P.Z.M. thanks the Army Research Office (grant
W911NF-12-1-0130) for financial support. This work made use
of Northwestern’s CleanCat Core facility. The CleanCat Core
facility acknowledges funding from the Department of Energy
(DE-FG-02-03ER15457) used for the purchase of the Thermo
Nicolet/Harrick DRIFTS system. This research is part (Project
34-944) of the Joint Center of Excellence in Integrated Nano-
Systems (JCIN) at King Abdulaziz City for Science and
Technology (KACST) and Northwestern University (NU).
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