Photochemical activation of a metal-organic framework to reveal functionality

Article abstract of DOI:10.1002/anie.201004736

Seeing the light: Two highly porous metal-organic frameworks (MOFs) were transformed using UV light to produce MOFs with hydroxy and catechol groups through an unusual postsynthetic deprotection reaction (see scheme). Copyright

Full text of DOI:10.1002/anie.201004736

Communications
DOI: 10.1002/anie.201004736
Light-Activated MOFs
Photochemical Activation of a Metal–Organic Framework to Reveal
Functionality**
Kristine K. Tanabe, Corinne A. Allen, and Seth M. Cohen*
In the field of metal–organic frameworks (MOFs), the
concept of postsynthetic modification (PSM) has garnered
increasing interest as a useful synthetic approach.[1] PSM has
been found to be a powerful tool for introducing new
functionality and hence new physical and chemical properties
into these porous materials. In general, PSM has largely been
described in the context of treating MOFs with reagents that
can react with an available chemical “handle” on the MOF
framework.[2–8] In contrast, the removal of chemical groups on
and within a MOF to reveal new chemical functionality is far
less well studied.[9,10] Here we describe how light can be used
to remove protecting groups from the organic components of
a MOF lattice in a single-crystal-to-single-crystal (SCSC)
fashion. In this manner the MOF lattice is kept pristine, while
unmasking new chemical functionality throughout the pores
of the MOF. Our findings suggest that this kind of postsyn-
thetic “deprotection” of MOFs may be an excellent strategy
for producing porous materials containing complex function-
ality.
A small number of studies have described the deprotec-
tion of protecting groups within a MOF lattice using either
chemical reagents or thermal treatment.[9,10] Recently, Telfer
et al. performed a controlled postsynthetic deprotection by
using a thermally labile protecting group (e.g., Boc = tert-
butoxycarbonyl) to yield a free amino group within a MOF.[11]
The MOF, which was prepared from Zn2+ and 2-(tert-
butoxycarbonylamino)biphenyl-4,4’-dicarboxylic acid, was
heated to > 1508C to remove the Boc protecting group.
1H NMR analysis of the digested sample and thermal
gravimetric analysis (TGA) confirmed the successful depro-
tection. The MOF was found to remain intact by single-crystal
X-ray diffraction, but attempts to confirm the deprotection by
gas sorption were not achieved due to pore collapse upon
solvent evacuation.
To the best of our knowledge, there are very few studies
that have reported on the light-driven chemical modification
or alteration of a MOF.[12–16] Light driven processes should be
an efficient and relatively gentle method by which to initiate
chemical reactions within a MOF lattice. Nitrobenzyl groups
have been well established in the literature as photocleavable
protecting groups for alcohols and amines.[17] A variety of
nitrobenzylethers can be cleaved by irradiation with light
ranging from ultraviolet to visible range. Herein, we demon-
strate that photolabile protecting groups can be liberated
within MOFs to yield materials with free hydroxy groups
(Scheme 1). This is significant, as reports of MOFs with free,
uncoordinated hydroxy groups are extremely rare.[18,19]
2-Hydroxy-1,4-benzenedicarboxylic acid (HO-BDC)[18]
and 2,3-dihydroxy-1,4-benzenedicarboxylic acid (CAT-BDC,
CAT= catechol)[20] were combined with o-nitrobenzyl bro-
mide to generate protected dicarboxylate building blocks
suitable for MOF construction (see Supporting Information).
The resulting ligands, 2-((2-nitrobenzyl)oxy)terephthalic acid
(NO2BnO-BDC) and 2,3-bis((2-nitrobenzyl)oxy)terephthalic
acid ((NO2BnO)2-BDC), were combined separately with
4,4’,4’’-benzene-1,3,5-triyl-tribenzoate (BTB)[21] and Zn-
(NO3)2·6H2O to obtain UMCM-1-OBnNO2 and UMCM-1-
(OBnNO2)2 (UMCM = University of Michigan Crystalline
Material, Scheme 1) as colorless needles (Supporting Infor-
mation, Figures S1, S2). Both MOFs were found to be
structural analogues of UMCM-1, a highly porous MOF
that contains BDC and BTB.[22] The structure and composi-
tion of UMCM-1-OBnNO2 and UMCM-1-(OBnNO2)2 were
conclusively established by several methods. Digestion of the
1
MOFs in dilute acid followed by H NMR analysis showed
that the materials contained both the BTB and the appro-
priate nitrobenzyl-protected BDC ligand (Figure 1). Powder
X-ray diffraction (PXRD) of the MOFs gave a similar pattern
as observed for UMCM-1 (Figures S3 and S4). Finally, gas
sorption experiments with N2 at 77 K indicated that UMCM-
1-OBnNO2 and UMCM-1-(OBnNO2)2 were highly porous
and showed a characteristic step in the isotherm (Figure 2).
The Brunauer–Emmett–Teller (BET) surface areas of
UMCM-1-OBnNO2 and UMCM-1-(OBnNO2)2 were found
to be 3219 Æ 150 m2 gÀ1 and 2661 Æ 172 m2 gÀ1, respectively.
Single-crystal X-ray diffraction of UMCM-1-OBnNO2
and UMCM-1-(OBnNO2)2 unambiguously showed that both
MOFs had the same topology as UMCM-1 (Tables S1–S4). A
disordered oxygen atom was located and assigned on the
BDC ligand of UMCM-1-OBnNO2; however, the nitrobenzyl
substituent could not be located in the electron density map
due to positional disorder. In contrast, both protecting groups
[*] K. K. Tanabe, C. A. Allen, Prof. S. M. Cohen
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive, La Jolla, CA 92093 (USA)
Fax: (+1)858-822-5598
E-mail: scohen@ucsd.edu
[**] C.A.A. and K.K.T. were equal contributors to this work. We thank Dr.
M. Rouffet for synthesis of CAT-BDC, D. Martin for assistance with
X-ray crystallography, and Dr. Y. Su for performing mass spec-
trometry experiments. We thank Prof. R. Snurr (Northwestern
University) and Dr. Y.-S. Bae for BET calculations. This work was
supported by UCSD, the NSF (new MOF synthesis CHE-0952370;
instrumentation grants CHE-9709183, CHE-0116662 and CHE-
0741968), and the DOE (gas sorption studies, PXRD, BES Grant No.
DE- FG02-08ER46519).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004736.
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Figure 1. 1H NMR spectra of digested UMCM-1-OBnNO2, UMCM-1-
OH, UMCM-1-(OBnNO2)2, and UMCM-1-CAT (DCl/[D6]DMSO).
Scheme 1. Synthesis and postsynthetic photochemical deprotection of
UMCM-1-OBnNO2 (left) and UMCM-1-(OBnNO2)2 (right).
were located and assigned for the structure of UMCM-1-
(OBnNO2)2 (Figure 3, Figures S7, S8). Electron density for
the NO2 groups was located, but could not be effectively
refined due to severe disorder.
UMCM-1-OBnNO2 and UMCM-1-(OBnNO2)2 were irra-
diated with 365 nm light for 24–48 h; both MOFs underwent a
distinct color change from colorless to orange, which was
indicative of the photochemical reaction (Figures S1, S2), and
release of 2-nitrosobenzaldehyde (which was confirmed by
electronic spectroscopy and ESI-MS, data not shown).[17,23,24]
Examination by 1H NMR spectroscopy upon digestion of the
MOFs in dilute acid showed that the protecting groups were
cleaved (Figure 1). UMCM-1-OBnNO2 readily underwent
quantitative deprotection to UMCM-1-OH within 24 h.
UMCM-1-(OBnNO2)2 was found to achieve ca. 75% depro-
tection to UMCM-1-CATunder the best conditions identified
to date. As additional evidence for the deprotection process,
Figure 2. N2 sorption and desorption isotherms for UMCM-1-
OBnNO2/UMCM-1-OH (top) and UMCM-1-(OBnNO2)2/UMCM-1-CAT
(bottom) at 77 K.
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To show that the newly revealed functional groups are
accessible, both UMCM-1-(OBnNO2)2 and UMCM-1-CAT
were treated with [Fe(acac)3] in EtOAc. After 24 h, UMCM-
1-(OBnNO2)2 was a light orange color while UMCM-1-CAT
was a deep red-purple color (Figure S21). These color changes
were confirmed by diffuse reflectance solid-state electronic
spectroscopy
of
UMCM-1-(OBnNO2)2,
UMCM-1-
(OBnNO2)2 treated with [Fe(acac)3], UMCM-1-CAT, and
UMCM-1-FeCAT, which showed clear differences in absorb-
ance between non-metalated and metalated samples (Fig-
ure S22). As expected, UMCM-1-(OBnNO2)2 did not show
any significant absorbance above 400 nm. Both UMCM-1-
CAT and UMCM-1-(OBnNO2)2 treated with [Fe(acac)3]
showed a transition at 500 nm. UMCM-1-CAT also had an
additional shoulder around 400 nm, which was indicative of
Figure 3. X-ray crystal structure of UMCM-1-(OBnNO2)2 (left) and after
photochemical deprotection to produce UMCM-1-CAT (right).
UMCM-1-CAT was found to fluoresce blue (lex = 365 nm),
which is characteristic of CAT-BDC ligand, while UMCM-1-
(OBnNO2)2 showed no fluorescence emission. TGA analysis
also revealed differences between the protected and depro-
tected UMCM MOFs (Figures S9, S10). It is important to
note that neither UMCM-1-OH nor UMCM-1-CAT could be
prepared by direct solvothermal synthesis from HO-BDC and
CAT-BDC (see Supporting Information), thus demonstrating
the usefulness of the postsynthetic approach.
Unambiguous evidence for removal of the nitrobenzyl
groups in a SCSC fashion was provided by single-crystal X-ray
diffraction. The X-ray structure of UMCM-1-OH showed that
the framework remained intact after irradiation (Figures S11,
S12) and electron density for the hydroxy group on the BDC
ligand was resolved. In contrast, deprotection of UMCM-1-
(OBnNO2)2 to UMCM-1-CAT was readily evident by the X-
ray structure with the disappearance of the nitrobenzyl
protecting groups from the electron density map and appear-
ance of the expected catechol group (Figure 3).
the catechol substituent. UMCM-1-FeCAT exhibited
a
smaller shoulder at 400 nm, but displayed a strong absorbance
between 500 and 600 nm consistent with known Fe3+–catechol
complexes.[23,24]
In summary, hydroxy and catechol moieties were success-
fully incorporated into a MOF by using postsynthetic photo-
chemical deprotection. UMCM-1-OH and UMCM-1-CAT
maintained their structural and thermal stabilities under the
photochemical conditions. Moreover, UMCM-1-OH and
UMCM-1-CAT exhibited significant increases in porosity,
which is consistent with the liberation of the bulky nitrobenzyl
substituents. Preliminary metalation tests with UMCM-1-
CAT confirmed the presence of the catechol unit, as
evidenced by binding of Fe3+ to the MOF. UMCM-1-OH
and UMCM-1-CAT demonstrate the promise of postsynthetic
deprotection to obtain MOFs with unprecedented function-
ality, which may lead to new applications of these materials.
Postsynthetic deprotection of the MOFs was clearly
manifest in the gas sorption behavior. N2 isotherms (77 K)
of UMCM-1-OH and UMCM-1-CAT showed significant
increases in BET surface area (Figure 2). The BET surface
areas were determined to be 3705 Æ 177 m2 gÀ1 and 3541 Æ
38 m2 gÀ1 for UMCM-1-OH and UMCM-1-CAT, respectively.
The difference between the protected and deprotected MOFs
is ca. 500 m2 gÀ1 and ca. 900 m2 gÀ1, which is reasonable for the
removal of either one (UMCM-1-OH) or two (UMCM-1-
CAT) protecting groups from each structure. Although it is
not intuitively obvious whether the surface area should
increase or decrease upon removal of the protecting groups,
calculated surface areas for UMCM-1-(OBnNO2)2 and
UMCM-1-CAT confirm that the there should be an increase
upon removal of the protecting groups (see Supporting
Information). Furthermore, the changes in BET surface
area are consistent with our observations in related systems,
where upon introducing substituents by postsynthetic modi-
fication we see a decrease in BET surface area.[4] The
complete gas sorption isotherms show a substantial increase
in capacity at saturation pressures, also consistent with
removal of the nitrobenzyl protecting groups. Finally, in
both systems, the average pore size distribution changes from
ca. 7.5 to 7.9 ꢀ upon removal of the protecting groups
(Horvath–Kawazoe model, data not shown). Overall, the gas
sorption experiments show that the lattice is intact, the
material is highly porous, and that the deprotection reaction
results in the expected increase in surface area.
Received: July 30, 2010
Revised: September 13, 2010
Published online: November 9, 2010
Keywords: catechols · metal–organic frameworks ·
.
photochemistry · postsynthetic modifications · zinc
[1] Z. Wang, S. M. Cohen, Chem. Soc. Rev. 2009, 38, 1315.
[2] C. Volkringer, S. M. Cohen, Angew. Chem. 2010, 122, 4748;
Angew. Chem. Int. Ed. 2010, 49, 4644.
[3] Z. Wang, S. M. Cohen, J. Am. Chem. Soc. 2007, 129, 12368.
[4] K. K. Tanabe, Z. Wang, S. M. Cohen, J. Am. Chem. Soc. 2008,
130, 8508.
[5] W. Morris, C. J. Doonan, H. Furukawa, R. Banerjee, O. M.
Yaghi, J. Am. Chem. Soc. 2008, 130, 12626.
[6] T. Gadzikwa, O. K. Farha, K. L. Mulfort, J. T. Hupp, S. T.
Nguyen, Chem. Commun. 2009, 3720.
[7] T. Kawamichi, T. Haneda, M. Kawano, M. Fujita, Nature 2009,
461, 633.
[8] S. C. Jones, C. A. Bauer, J. Am. Chem. Soc. 2009, 131, 12516.
[9] T. Gadzikwa, O. K. Farha, C. D. Malliakas, M. G. Kanatzidis,
J. T. Hupp, S. T. Nguyen, J. Am. Chem. Soc. 2009, 131, 13613.
[10] T. Yamada, H. Kitagawa, J. Am. Chem. Soc. 2009, 131, 6312.
[11] R. K. Deshpande, J. L. Minnaar, S. G. Telfer, Angew. Chem.
2010, 122, 4702; Angew. Chem. Int. Ed. 2010, 49, 4598.
[12] S. S. Kaye, J. R. Long, J. Am. Chem. Soc. 2008, 130, 806.
[13] A. J. Blake, N. R. Champness, T. L. Easun, D. R. Allan, H.
Nowell, M. W. George, J. Jia, X.-Z. Sun, Nat. Chem. 2010, 2, 688.
9732
www.angewandte.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 9730 –9733

Angewandte
Chemie
[14] K. Ohara, Y. Inokuma, M. Fujita, Angew. Chem. 2010, 122, 5639;
Angew. Chem. Int. Ed. 2010, 49, 5507.
[15] H. Sato, R. Matsuda, K. Sugimoto, M. Takata, S. Kitagawa, Nat.
Mater. 2010, 9, 661.
[16] Q. K. Liu, J. P. Ma, Y. B. Dong, J. Am. Chem. Soc. 2010, 132,
7005.
[20] K. Raymond, J. Xu in United States Patent and Trademark Office
(Ed.: U.S.P.T. Office), The Regents of the University of
California (Oakland, CA), Pat. No. 5,892,029, US, 1999.
[21] S. B. Choi, M. J. Seo, M. Cho, Y. Kim, M. K. Jin, D.-Y. Jung, J.-S.
Choi, W.-S. Ahn, J. L. C. Rowsell, J. Kim, Cryst. Growth Des.
2007, 7, 2290.
[17] G. Mayer, A. Heckel, Angew. Chem. 2006, 118, 5020; Angew.
Chem. Int. Ed. 2006, 45, 4900.
[22] K. Koh, A. G. Wong-Foy, A. J. Matzger, Angew. Chem. 2008,
120, 689; Angew. Chem. Int. Ed. 2008, 47, 677.
[18] D. Himsl, D. Wallacher, M. Hartmann, Angew. Chem. 2009, 121,
4710; Angew. Chem. Int. Ed. 2009, 48, 4639.
[23] T. B. Karpishin, T. D. P. Stack, K. N. Raymond, J. Am. Chem.
Soc. 1993, 115, 6115.
[19] T. Devic, P. Horcajada, C. Serre, F. Salles, G. Maurin, B. Moulin,
D. Heurtaux, G. Clet, A. Vimont, J.-M. Grenꢁche, B. Le Ouay, F.
Moreau, E. Magnier, Y. Filinchuk, J. Marrot, J.-C. Lavalley, M.
Daturi, G. Fꢂrey, J. Am. Chem. Soc. 2010, 132, 1127.
[24] T. B. Karpishin, T. D. P. Stack, K. N. Raymond, J. Am. Chem.
Soc. 1993, 115, 182.
Angew. Chem. Int. Ed. 2010, 49, 9730 –9733
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
9733