Selective Solvent-Assisted Linker Exchange (SALE) in a Series of Zeolitic Imidazolate Frameworks

Article abstract of DOI:10.1021/acs.inorgchem.5b01231

Solvent-assisted linker exchange (SALE) has recently emerged as an attractive strategy for the synthesis of metal-organic frameworks (MOFs) that are unobtainable via traditional synthetic pathways. Herein we present the first example of selective SALE in which only the benzimiadazolate-containing linkers in a series of mixed-linker zeolitic imidazolate frameworks (ZIF-69, -78, and -76) are replaced. The resultant materials (SALEM-10, SALEM-10b, and SALEM-11, respectively) are isostructural to the parent framework and in each case contain trifluoromethyl moieties. We therefore evaluated each of these materials for their hydrophobicity in condensed and gas phases. We expect that selective SALE will significantly facilitate the design of improved, and potentially complex, MOF materials with new and unusual properties.

Full text of DOI:10.1021/acs.inorgchem.5b01231

Communication
pubs.acs.org/IC
Selective Solvent-Assisted Linker Exchange (SALE) in a Series of
Zeolitic Imidazolate Frameworks
Marianne B. Lalonde,† Joseph E. Mondloch,‡ Pravas Deria,† Amy A. Sarjeant,† Salih S. Al-Juaid,§
Osman I. Osman,§ Omar K. Farha,*,†,§ and Joseph T. Hupp*,†
†Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208 United States
‡Department of Chemistry, University of WisconsinStevens Point, 2001 Fourth Avenue, Stevens Point, Wisconsin 54481, United
States
§Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia
S
* Supporting Information
closing metathesis,17 enhance proton conductivity18 as well as
photochemical H2 production,19 and incorporate free carboxylic
ABSTRACT: Solvent-assisted linker exchange (SALE)
has recently emerged as an attractive strategy for the
synthesis of metal−organic frameworks (MOFs) that are
acid groups by functionalizing defect sites.20
While SALE has proven to be a powerful tool for imparting
unobtainable via traditional synthetic pathways. Herein we
functionality within MOFs, it has been limited to the exchange of
present the first example of selective SALE in which only
MOFs that contain organic linkers with only one type of
the benzimiadazolate-containing linkers in a series of
coordination bond to the metal-based nodes. (One notable
mixed-linker zeolitic imidazolate frameworks (ZIF-69, -78,
exception is pillar-paddlewheel-type MOFs, which have two
and -76) are replaced. The resultant materials (SALEM-10,
different functional groups attaching the organic linkers to the
metal nodes. However, SALE has been limited to exchange at the
dipyridyl struts and does not occur at the carboxylate-terminated
linkers.9) A new and highly attractive concept would be selective
SALE, in which only one linker is replaced in a MOF containing
at least two different linkers but with the same coordination
chemistry (see Figure 1, for example).
SALEM-10b, and SALEM-11, respectively) are isostruc-
tural to the parent framework and in each case contain
trifluoromethyl moieties. We therefore evaluated each of
these materials for their hydrophobicity in condensed and
gas phases. We expect that selective SALE will significantly
facilitate the design of improved, and potentially complex,
MOF materials with new and unusual properties.
etal−organic frameworks (MOFs) are porous crystalline
Mmaterials composed of organic linkers and metal-based
nodes.1−3 By using a building-block-like approach with
appropriate metals and organic linkers, MOFs can be tailored
for a wide variety of potential applications.3 MOFs are typically
synthesized under solvothermal conditions (i.e., de novo
conditions), and this approach has led to over 20000 variants.3
However, the de novo synthesis of MOFs can be challenging; it
can lead to, for example, an undesired number of topologies,
catenated networks, and a lack of desired functionality within the
framework.4 High-throughput screening along with postsyn-
thesis modification have been utilized to overcome some of these
difficulties,5−7 yet more general synthetic strategies are still
needed.
Recently, solvent-assisted linker exchange (SALE) has
emerged as an attractive alternative to the de novo synthesis of
MOFs.4,8 SALE involves the placement of template MOF
crystals in an excess solution of a secondary linker. Following a
SALE reaction, the daughter framework retains the original MOF
topology, but linkers from the reaction solution will have
replaced the original MOF linkers. Along with the incorporation
of the desired linkers in MOFs that are inaccessible de novo,9−13
SALE can also impart new properties onto the framework. For
example, SALE has been used to introduce catalytically active
moieties,14,15 gain control over catenation,16 demonstrate ring-
Figure 1. (a) Illustration of the MOF ZIF-69. SALE occurs selectively at
the benzimidazolate linkers that line the pores indicated with yellow
spheres but not the 2-nitroimidazolate linkers highlighted with the
purple spheres (b and c).
Received: May 31, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b01231
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Selective SALE has the potential to (i) greatly broaden the
variety of accessible frameworks accessible via SALE, by giving
additional control over the incorporation of linkers into a specific
topology, (ii) create complex pore environments (e.g.,
containing linkers with multiple functionalization) that could
provide opportunities for uncovering new and unusual proper-
ties, and (iii) allow functional groups that are incompatible under
de novo synthetic conditions to be found in close proximity to
each other. To date, selective SALE has not been demonstrated
in a MOF with two linkers that have identical coordination
chemistry at the metal nodes.
NMR spectrum of SALEM-10 (Figure 2). The ratio between the
total amount of benzimidazole (i.e., the remaining parent cbim
and fbim) and imidazole remains constant. The selective nature
of the SALE reaction was corroborated by single-crystal X-ray
diffraction (Figure 1a), which reveals that fbim selectively
replaced the cbim linker. [See the Supporting Information (SI)
for additional details.] Finally, the bulk phase purity and
permanent porosity of SALEM-10 were confirmed by PXRD
(Figure S3 in the SI) and N2 adsorption measurements (Figure
S6 in the SI).
To further probe the selectivity of the SALE process, we
pursued analogous experiments with two other ZIFs: ZIF-78,21
a
Herein we demonstrate the first examples of selective SALE by
using a series of zeolitic imidazolate frameworks (ZIFs),5 namely,
ZIF-69, ZIF-78, and ZIF-76. Each of these ZIFs contains two
types of linkers, but in each case, both linkers are connected to
the metal node via imidazolate-terminated coordination bonds,
and thus they are attractive candidates for selective SALE. The
resultant materials are termed solvent-assisted linker exchange
material-10 (SALEM-10), SALEM-10b, and SALEM-11, re-
spectively. In our prototype system (the conversion of ZIF-69 to
gme topology ZIF analogous to ZIF-69 but containing 5-
nitrobenzimidazolate (nbim) linkers instead of cbim linkers, and
ZIF-76, an lta topology ZIF composed of imidazole and cbim.
The 1H NMR results for the SALE reactions are summarized in
Table 1. In addition to replacing 95% of cbim in ZIF-69, 5-
Table 1. MOFs, Their Linkers, pKa Values, and Percentage of
Linker Exchange
1
SALEM-10), both materials have been characterized by H
a
b
MOF
ZIF-69
linkers
pKa values
% linker exchange
NMR, powder X-ray diffraction (PXRD), N2 gas adsorption,
single-crystal X-ray diffraction, condensed-phase contact-angle
measurements, and water vapor adsorption measurements.
To start, selective SALE was carried out with ZIF-69, which
exhibits gme topology and is composed of a 1:1 ratio of 2-
nitroimidazolate (nim) and 5-chlorobenzimidazolate (cbim)
linkers coordinated to tetrahedral zinc nodes (Figure 1a). ZIF-69
and other gme ZIFs are attractive candidates for selective SALE
because nim has shown resistance to SALE in single-linker ZIFs,
such as CdIF-4.10 Hence, we hypothesized that nim in ZIF-69
would remain in place, while cbim would be susceptible to
replacement via SALE. When selecting a candidate linker for
selective SALE, we opted for 5-(trifluoromethyl)benzimidazole.
The goal of incorporating fluorinated functional groups was to
demonstrate selective SALE and to increase the hydrophobicity
of the framework.
nim and cbim
nim and fbim
nim and nbim
nim and fbim
im and cbim
−1.73 and 5.74
−1.73 and 5.74
−1.73 and 5.03
−1.73 and 5.74
6.97 and 5.74
SALEM-10
ZIF-78
95
90
90
SALEM-10b
ZIF-76
SALEM-11
im and fbim
6.97 and 5.74
a
b
1
For the protonated N3. Determined by H NMR.
(trifluoromethyl)benzimidazole replaced 90% of nbim in ZIF-78
and 90% of cbim in ZIF-76. The resulting daughter ZIFs are
named SALEM-10b and SALEM-11, respectively. PXRD and N2
adsorption analyses confirmed the bulk phase topology and
permanent porosity of SALEM-10b and SALEM-11. (See the SI
for additional details.) Notably, attempts to synthesize SALEM-
10 and -11 de novo were unsuccessful; see the SI.
Insight into selective SALE can be gained by examining the
pKa values for the nitrogen atoms of imidazole and imidazolium
versions of the molecules deployed as linkers. Table 1 lists pKa
values for N3 in each linker precursor, and an identical trend can
be observed for the N1, the details of which are given in the SI. As
we previously noted,22 the linker basicity (as reflected in the pKa
values of the corresponding acids) can be used as a proxy for the
relative Zn−L bond strength, and it is clear that thermodynamic
factors are playing a large role in governing the selective nature of
the SALE reaction. For example, 5-(trifluoromethyl)-
benzimidazole (5.74) selectively replaces cbim (5.74). However,
Zn−L bond strengths alone cannot explain all of the observed
data; on the basis of pKa values, one would expect 5-
(trifluoromethyl)benzimidazole (5.74) to also replace nim
(−1.73) in ZIF-69, yet this is not observed experimentally. At
this point in time, we cannot unequivocally determine whether
this is due to steric and/or kinetic effects.
SALE was performed by submerging 30 mg of ZIF-69 in 10
mL of a 2 M solution of 5-(trifluoromethyl)benzimidazole in n-
butanol at 120 °C for 3 days. 1H NMR indicated that 95% of cbim
was replaced by 5-(trifluoromethyl)benzimidazolate (fbim;
Figure 2). If SALE occurs selectively at the cbim linker, the
ratio between the total benzimidazolate and nim linkers should
1
remain unchanged. This was confirmed by examining the H
Fluoroalkane-containing linkers within MOFs have received
attention because they are often hydrophobic. For example, Yang
et al. presented a series of fluorous MOFs with notable
hydrophobicity for the adsorption of aromatic and aliphatic
components of oil.23 Comparing the results of water contact-
angle measurements of the ZIFs and the fluorinated SALEM
materials reveals significantly larger contact angles for the
SALEMs (Figure S9 in the SI). Recognizing that contact-angle
measurements report exclusively on interactions of the exterior
Figure 2. Reaction progress followed by 1H NMR for conversion of ZIF-
69 into SALEM-10.
B
DOI: 10.1021/acs.inorgchem.5b01231
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
surfaces of materials with liquid-phase water, we also measured
the water vapor uptake of ZIF-69 and SALEM-10. In contrast to
the contact-angle measurements, vapor sorption measurements
showed no enhancement of the hydrophobicity for SALEM-10
relative to ZIF-69 (Figure 3). The contrast between these two
Grant 23-130-36-HiCi. The authors, therefore, acknowledge
with thanks DSR technical and financial support.
REFERENCES
■
(1) Ferey, G. Chem. Soc. Rev. 2008, 37, 191−214.
(2) Horike, S.; Shimomura, S.; Kitagawa, S. Nat. Chem. 2009, 1, 695−
704.
(3) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. Science
2013, 341, 1230444.
(4) Karagiaridi, O.; Bury, W.; Mondloch, J. E.; Hupp, J. T.; Farha, O. K.
Angew. Chem., Int. Ed. 2014, 53, 4530−4540.
(5) Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.;
O’Keeffe, M.; Yaghi, O. Science 2008, 319, 939−943.
(6) Sumida, K.; Horike, S.; Kaye, S. S.; Herm, Z. R.; Queen, W. L.;
Brown, C. M.; Grandjean, F.; Long, G. J.; Dailly, A.; Long, J. R. Chem. Sci.
2010, 1, 184−191.
(7) Cohen, S. M. Chem. Rev. 2012, 112, 970−1000.
(8) Deria, P.; Mondloch, J. E.; Karagiaridi, O.; Bury, W.; Hupp, J. T.;
Farha, O. K. Chem. Soc. Rev. 2014, 43, 5896−5912.
(9) Burnett, B. J.; Barron, P. M.; Hu, C.; Choe, W. J. Am. Chem. Soc.
2011, 133, 9984−9987.
(10) Karagiaridi, O.; Bury, W.; Sarjeant, A. A.; Stern, C. L.; Farha, O.
K.; Hupp, J. T. Chem. Sci. 2012, 3, 3256−3260.
(11) Kim, M.; Cahill, J. F.; Fei, H.; Prather, K. A.; Cohen, S. M. J. Am.
Chem. Soc. 2012, 134, 18082−18088.
(12) Kim, M.; Cahill, J. F.; Su, Y.; Prather, K. A.; Cohen, S. M. Chem.
Sci. 2012, 3, 126−130.
Figure 3. Water vapor adsorption isotherms for ZIF-69 (blue) and
SALEM-10 (red).
(13) Lee, C.-Y.; Farha, O. K.; Hong, B. J.; Sarjeant, A. A.; Nguyen, S. T.;
Hupp, J. T. J. Am. Chem. Soc. 2011, 133, 15858−15861.
(14) Karagiaridi, O.; Lalonde, M. B.; Bury, W.; Sarjeant, A. A.; Farha,
O. K.; Hupp, J. T. J. Am. Chem. Soc. 2012, 134, 18790−18796.
(15) Takaishi, S.; DeMarco, E. J.; Pellin, M. J.; Farha, O. K.; Hupp, J. T.
Chem. Sci. 2013, 4, 1509−1513.
(16) Bury, W.; Fairen-Jimenez, D.; Lalonde, M. B.; Snurr, R. Q.; Farha,
O. K.; Hupp, J. T. Chem. Mater. 2013, 25, 739−744.
(17) VermeUlen, N. A.; Karagiaridi, O.; Sarjeant, A. A.; Stern, C. L.;
Hupp, J. T.; Farha, O. K.; Stoddart, J. F. J. Am. Chem. Soc. 2013, 135,
14916−14919.
(18) Kim, S.; Dawson, K. W.; Gelfand, B. S.; Taylor, J. M.; Shimizu, G.
K. H. J. Am. Chem. Soc. 2013, 135, 963−966.
(19) Pullen, S.; Fei, H.; Orthaber, A.; Cohen, S. M.; Ott, S. J. Am. Chem.
Soc. 2013, 135, 16997−17003.
(20) Karagiaridi, O.; Vermeulen, N. A.; Klet, R. C.; Wang, T. C.;
Moghadam, P. Z.; Al-Juaid, S. S.; Stoddart, J. F.; Hupp, J. T.; Farha, O. K.
Inorg. Chem. 2015, 54, 1785−1790.
(21) Banerjee, R.; Furukawa, H.; Britt, D.; Knobler, C.; O’Keeffe, M.;
Yaghi, O. J. Am. Chem. Soc. 2009, 131, 3875−3877.
(22) Karagiaridi, O.; Bury, W.; Tylianakis, E.; Sarjeant, A. A.; Hupp, J.
T.; Farha, O. K. Chem. Mater. 2013, 25, 3499−3503.
(23) Yang, C.; Kaipa, U.; Mather, Q. Z.; Wang, X.; Nesterov, V.;
Venero, A. F.; Omary, M. A. J. Am. Chem. Soc. 2011, 133, 18094−18097.
results can likely be attributed to the cohesive forces between
water molecules, resulting in higher surface tension in the
contact-angle measurements. This discrepancy in hydrophobic-
ity measurements will influence how hydrophobicity is quantified
on the surface and in the pores of future materials.
In conclusion, we have presented the first examples of selective
SALE within MOFs. The structure and permanent porosity of
the examined ZIFs are retained in SALEM-10, -10b, and -11, as
demonstrated by PXRD and N2 adsorption. The selective nature
1
of the SALE reaction was confirmed by H NMR and in our
prototypical example (SALEM-10) by single-crystal X-ray
analysis. While the selective nature of the SALE process is
largely governed by the pKa values of the linkers, steric and/or
kinetic factors must also play a key role. We look forward to
exploring new materials with potentially unique properties
prepared by selective SALE.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic data in CIF format, experimental details,
NMR, PXRD, TGA, BET surface area, single-crystal X-ray
parameters, and contact-angle measurements. The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/acs.inorgchem.5b01231.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: o-farha@northwestern.edu.
*E-mail: j-hupp@northwestern.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.T.H. gratefully acknowledges funding from the Army Research
Office (Project W911NF-13-1-0229). Contributions by O.K.F.,
O.I.O., and S.S.A. to this project were funded by the Deanship of
Scientific Research (DSR), King Abdulaziz University, under
C
DOI: 10.1021/acs.inorgchem.5b01231
Inorg. Chem. XXXX, XXX, XXX−XXX