Evaluation of heterogeneous metal-organic framework organocatalysts prepared by postsynthetic modification

Article abstract of DOI:10.1021/ic1011549

A metal-organic framework (MOF) containing 2-amino-1,4-benzenedicarboxylate (NH₂-BDC) as a building block is shown to undergo chemical modification with a set of cyclic anhydrides. The modification of the aluminum-based MOF known as MIL-53(Al)-NH₂ (MIL = Material Institut Lavoisier) by these reagents is demonstrated by using a variety of methods, including NMR and electrospray ionization mass spectrometry (ESI-MS), and the structural integrity of the modified MOFs has been confirmed by thermal gravimetric analysis (TGA) and powder X-ray diffraction (PXRD). Reaction with these cyclic anhydrides produces MOFs that display carboxylic acid functional groups within their pores. Furthermore, it is shown that maleic acid functionalized MIL-53(Al)-AMMal can act as a Bronsted acid catalyst and facilitate the methanolysis of several small epoxides. Experiments show that MIL-53(Al)-AMMal acts in a heterogeneous manner and is recyclable with consistent activity over at least three catalytic cycles. The findings presented here demonstrate several important features of covalent postsynthetic modification (PSM) on MOFs, including (1) facile introduction of catalytic functionality using simple organic reagents (e.g., anhydrides); (2) the ability to utilize and recycle organocatalytic MOFs; (3) control of catalytic activity through choice of functional group. The findings clearly illustrate that covalent postsynthetic modification represents a powerful means to access new MOF compounds that serve as organocatalytic materials.

Full text of DOI:10.1021/ic1011549

NIH Public Access
Author Manuscript
Inorg Chem. Author manuscript; available in PMC 2011 September 6.
Published in final edited form as:
Inorg Chem. 2010 September 6; 49(17): 8086–8091. doi:10.1021/ic1011549.
Evaluation of Heterogeneous Metal-Organic Framework
Organocatalysts Prepared by Postsynthetic Modification
*
Sergio J. Garibay, Zhenqiang Wang, and Seth M. Cohen
Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman
Drive, La Jolla, California 92093-0358
Abstract
A metal-organic framework (MOF) containing 2-amino-1,4-benzenedicarboxylate (NH -BDC) as
2
a building block is shown to undergo chemical modification with a set of cyclic anhydrides. The
modification of the aluminum-based MOF known as MIL-53(Al)-NH (MIL = Matérial Institut
2
Lavoisier) by these reagents is demonstrated by using a variety of methods, including NMR and
ESI-MS, and the structural integrity of the modified MOFs has been confirmed by TGA, PXRD,
and gas sorption analysis. Reaction with these cyclic anhydrides produces MOFs that display
carboxylic acid functional groups within their pores. Furthermore, it is shown that maleic acid
functionalized MIL-53(Al)-AMMal can act as a Brønsted acid catalyst and facilitate the
methanolysis of several small epoxides. Experiments show that MIL-53(Al)-AMMal acts in a
heterogeneous manner and is recyclable with consistent activity over at least three catalytic cycles.
The findings presented here demonstrate several important features of covalent postsynthetic
modification (PSM) on MOFs, including: 1) facile introduction of catalytic functionality using
simple organic reagents (e.g. anhydrides); 2) the ability to utilize and recycle organocatalytic
MOFs; 3) control of catalytic activity through choice of functional group. The findings clearly
illustrate that covalent postsynthetic modification represents a powerful means to access new MOF
compounds that serve as organocatalytic materials.
Introduction
Metal-organic frameworks (MOFs) are crystalline materials composed of metal ions or
1-3
metal cluster nodes connected by organic ligands. The ability to design MOFs with
4,5
varying pore volumes and surface areas has made these materials attractive for
6-12
applications in gas storage.
However, MOFs have shown promise in a diverse set of
13,14
15-17
18
technologies including separation,
drug delivery,
and catalysis. Throughout the
last several years, several groups have shown that MOFs can act as heterogeneous catalysts
19 20
21,22
in a variety of reactions such as alkene oxidation,
aldol condensations,
Compared to homogeneous reactions,
23-25
26,27
hydrogenation,
and epoxide ring opening.
the use of heterogeneous catalysts simplifies the reaction work-up by requiring only
filtration to separate the catalyst from the product for reuse.
The alcoholysis of epoxides is a well-studied reaction that is promoted by a variety of
homogeneous and heterogeneous acid catalysts. Historically, the methanolysis of epoxides is
28
achieved by using sulfuric acid. Heterogeneous catalysts based on polymers or silica-
29,30
based materials have also been widely studied for the methanolysis of epoxides.
With
respect to MOFs, Baiker and co-workers utilized a bipyridine Cu(II) based MOF Cu(bpy)
* To whom correspondence should be addressed. scohen@ucsd.edu. Telephone: (858) 822-5596. Fax: (858) 822-5598.
Supporting Information Available. Figures S1-S22. This material is available free of charge via the Internet at http://pubs.acs.org.

Garibay et al.
Page 2
(H O) (BF ) (bpy) for the methanolysis of epoxides demonstrating high activity and
2
2
4 2
31
selectivity. However, further investigation of this system revealed that the excellent
catalytic properties were due to the structural rearrangement of the MOF and subsequent
32
release of multicopper clusters upon exposure to methanol. Rosseinsky and co-workers
27
cleverly developed catalytically active chiral aspartate MOFs. With the careful addition of
HCl the MOFs could be protonated at a carboxylate group bound to the Cu(II) or Ni(II)
metal centers, thereby introducing Brønsted acid sites. While the Brønsted acid
functionalized materials showed modest activity and enantioselectivity in the methanolysis
of small epoxides, the recyclability and robustness of these catalytic materials was not
described.
Our group and others have recently demonstrated the practical use of postsynthetic
modification (PSM) to produce functionalized systems by targeting the organic linking
33
group or secondary building unit of pre-fabricated MOFs. We have become increasingly
interested in the use of PSM to develop MOFs as catalytic materials. Several recent studies
from our laboratory have focused on the generation of metal ion sites within a MOF to
34,35
generate Lewis acid catalytic centers.
A recent study utilized coordinative postsynthetic
modification with pyridyl L-proline onto the open metal coordination sites of MIL-101
22
(MIL = Material Institut Lavoisier). Through this approach MIL-101 was transformed into
a catalytically active homochiral material that was competent for asymmetric aldol reactions
between aldehydes and ketones. While the material displayed high catalytic activities and
enantioselectivity, its catalytic properties diminished over time on account of leaching of the
pyridyl groups after repeated use. The experiments here focus on the heterogeneous
acylation of the pendant amino group of MOFs employing 2-amino-1,4-
benzenedicarboxylate (NH -BDC) as an organic building block. Covalent PSM of these
2
systems provides an opportunity to develop MOFs with stable and reusable organocatalytic
functionality. Herein, we demonstrate the utilization of carboxylic acid functionalized MOFs
as a solid state Brønsted acid catalyst for the ring opening of small epoxides by methanol.
The findings described here demonstrate that these modified MOFs act as a heterogeneous
catalysts that can be reused after regeneration without loss of activity.
Experimental Methods
General
Starting materials and solvents were purchased and used without further purification from
commercial suppliers (Sigma-Aldrich, Alfa Aesar, EMD, TCI, Cambridge Isotope
36
37
Laboratories, Inc., and others). MIL-53(Al)-NH and MIL-53(Al)-AM1 were
2
synthesized and activated as described previously.
Synthesis
Synthesis of 4-((2,5-bis(methoxycarbonyl)phenyl)amino)-4-oxobut-2-enoic
acid (DMT-AMMal)—Maleic anhydride (17.4 g, 177 mmol) was added to a solution of
dimethyl amino terephthalate (1.06 g, 5.1 mmol) in ∼125 mL of CHCl . The solution was
3
stirred for 24 h at room temperature. The solvent was removed by rotary evaporation. The
brown solid was washed with ∼100 mL of ethyl acetate, filtered, and washed with ∼100 mL
1
6
of CHCl . The product was dried at ∼85 °C. Yield = 27%. H NMR (500 MHz, d -DMSO):
3
δ 10.75 (s, 1 H), 8.7 (s, 1H), 7.96-7.6 (d, 2H), 6.5-6.34 (d, 2H), 3.8 (s, 6H). ESI-MS m/z
-
306.07 [M-H] .
Synthesis of 4-((2,5-bis(methoxycarbonyl)phenyl)amino)-4-oxobutanoic acid
(DMT-AMSuc)—Succinic anhydride (17.9 g, 177 mmol) was added to a solution of
dimethyl amino terephthalate (1.06 g, 5.1 mmol) in ∼125 mL of CHCl . The solution was
3
Inorg Chem. Author manuscript; available in PMC 2011 September 6.

Garibay et al.
Page 3
stirred for 24 h at room temperature. The solvent was removed by rotary evaporation. The
brown solid was washed with ∼100 mL of ethyl acetate filtered and washed with ∼100 mL
1
6
of CHCl . The product was dried at ∼85 °C. Yield = 67%. H NMR (500 MHz, d -DMSO):
3
δ 12.19 (s, 1H), 10.54 (s, 1 H), 8.7 (s, 1H), 7.96-7.6 (d, 2H), 3.8 (s, 6H), 2.6-2.5 (d, 4H).
-
ESI-MS m/z 308.00 [M-H] .
MIL-53(Al)-AMMal—In a typical reaction, residual DMF was removed from
microcrystalline MIL-53(Al)-NH (∼60 mg) by heating at 150 °C for 5 h. The activated
2
MIL-53(Al)-NH (∼50 mg 0.2 mmol-NH ) was treated with a 5 mL solution of CH CN
2
2
3
containing the anhydride (12 mmol) and heated at 80 °C for 24 h. After the reaction was
complete the sample was rinsed with CH CN (3×6 mL), centrifuged, and dried at 80 °C for
3
up to 2 h resulting in 43±4% (-AMMal), 39±4% (-AMSuc), and 34±4% (-AMCrot)
conversion.
Characterization of MILs
1
H NMR Digestion and Analysis—Approximately 10 mg of microcrystalline MIL was
digested by sonication in one of two digestion cocktails: a) 400 μL D O and 200 μL of
2
NaOH 40 wt. % of D O; or b) 570 μL of d -DMSO and 30 μL of HF. After complete
2
6
1
1
dissolution of the crystals, the solution was used to collect a H NMR spectrum. H NMR
spectra were recorded on a JEOL ECA spectrometer (500 MHz).
ESI-MS Analysis—Electrospray ionization mass spectrometry (ESI-MS) was performed
using a ThermoFinnigan LCQ-DECA mass spectrometer and the data were analyzed using
the Xcalibur software suite in negative ion mode. MIL samples were digested by sonicating
the materials in a mixture of 10 μL of HF and 1.0 mL of CH CN.
3
Thermal Gravimetric Analysis—Approximately 10-20 mg of modified MIL samples
were dried at 150 °C for 5 h and used for TGA measurements. Samples were analyzed under
a stream of dinitrogen using a TA Instrument Q600 SDT running from room temperature to
800 °C with a scan rate of 5 °C/min.
PXRD Analysis—PXRD data were collected at ambient temperature on a Bruker Advance
D8 diffractometer at 40 kV, 40 mA for Kα (λ = 1.5418 Å) with a scan speed of 3 sec/step, a
step size of 0.02° in 2θ, and a 2θ range of 5-45°. Approximately 15 mg of microcrystalline
MIL samples were dried at 150 °C for at least 2 h before PXRD analysis. The experimental
backgrounds were corrected using the Jade 5.0 software package.
FT-IR Analysis—Approximately 5-10 mg of modified MIL was dried at 150 °C for at
least 2 h before FT-IR analysis. FT-IR spectra were collected using a Bruker ALPHA-P FT-
IR spectrometer with a diamond ATR.
Catalysis
Catalysis Experiments—In a typical reaction, the MIL microcrystalline solids (ca. 0.20
mmol equiv of BDC ligand) were pre-dried for ∼5 h were and added to a CD OD solution
3
(2 mL) containing the epoxide substrate (0.40 mmol). The reaction was allowed to stand for
2 d at 25 °C. At the end of the reaction, ca. 0.8 mL aliquot was taken from the mixture and
1
used for H NMR measurements. Note that the chemical formula for each MOF that was
examined for catalysis are as follows: MIL-53(Al)-NH = Al(OH)(C H NO ), MIL-53(Al)-
2
8
5
4
AM1 = Al(OH)(C H NO ), MIL-53(Al)-AMCrot = Al(OH)
10
7
5
(C H NO ) (C H NO ) , MIL-53(Al)-AMSuc = Al(OH)
8
5
4 0.66 12
9
5 0.34
(C H NO ) (C H NO ) , and MIL-53(Al)-AMMal = Al(OH)
8
5
4 0.6 12
9
7 0.4
(C H NO ) (C H NO ) .
7 0.43
8
5
4 0.57 12
7
Inorg Chem. Author manuscript; available in PMC 2011 September 6.

Garibay et al.
Page 4
Catalyst Recycling—After the completion of a reaction with epoxide, the CD OD
3
solution was decanted and the MIL solids were washed with CHCl (3×6 mL, isolated by
3
centrifugation between washes) before soaking in 10 mL of pure CHCl for three days, with
3
fresh CHCl added every 24 h. After three days of soaking the solids were stored in the last
3
CHCl solution until needed. Samples treated this way were showed a large reduction in
3
activity, which required regeneration as described below.
Catalyst Regeneration—The CHCl storage solution was decanted away from the
3
inactive MOF catalysts (see Catalyst Recycling above) and the solids was dried at 55 °C for
up to 2 h. To the dried MOFs, ∼13 mL of a 0.1 M HCl solution was added and allowed to
stand for ∼18 h. The solids were centrifuged and washed with H O (3×6 mL) and dried at
2
100 °C for at least 2 h prior to use.
Results and Discussion
Modification of MIL-53-NH with Anhydrides
2
In order to develop a heterogeneous MOF catalyst, a chemically robust framework was
selected for compatibility with the protic solvents of the ring opening reaction conditions.
The MIL-53(Al) framework is chemically stable to many protic solvents and acidic
38
environments, unlike zinc(II)-carboxylate MOFs (IRMOF-3 derivatives), which degrade
under these reaction conditions described below (data not shown). Recently MIL-53(Al)-
NH , a MIL-53(Al) analogue incorporating NH -BDC, was synthesized and could be
2
2
36
postsynthetically modified with formic acid. PSM of MIL-53(Al)-NH was attempted with
2
a variety of anhydrides (Scheme 1). The use of maleic and succinic anhydride generated
carboxylic acid functionalized MIL-53(Al)-AMMal and MIL-53(Al)-AMSuc with modest
conversions of ∼43% and ∼40%, respectively (Figure 1). The percent conversions were
1
37,39
determined by H NMR analysis of digested materials, as previously described.
Specifically, conversion was determined by comparing the relative integration areas of the
singlet aromatic resonance (corresponding to the C3-position) of the modified to the
unmodified NH -BDC ligands (Figure 1). Fourier transform infrared spectroscopy (FTIR)
2
-1
on MIL-53(Al)-AMMal and MIL-53-(Al)-AMSuc showed new stretches around 1720 cm
characteristic of carbonyl functionalities. In addition, the N-H stretches at 3500 to 3387
-1
cm , associated with the amine of the NH -BDC ligand, were notably diminished (Figure
2
S1-S2). MIL-53(Al)-NH treated under identical reaction conditions with crotonic and acetic
2
anhydride resulted in MIL-53(Al)-AMCrot and MIL-53(Al)-AM1 modified with ∼34% and
37
∼95% conversion. As expected, the N-H stretches associated with the NH -BDC ligand in
2
-1
the FTIR spectrum of MIL-53(Al)-AM1 disappeared, while a new vibration at 3361 cm
emerged, corroborating the nearly quantitative conversion to an amide (Figure S1). In
addition, electrospray ionization mass spectroscopy (ESI-MS) of modified materials
(digested in an HF/CH CN solution) confirmed PSM of the NH -BDC ligands (Figures S3-
3
2
S6). The structural and thermal properties of the modified MILs were found to closely
resemble that of MIL-53(Al)-NH as evidenced by powder X-ray diffraction (PXRD)
2
(Figure S7) and thermogravimetric analysis (TGA) (Figure S8).
Catalytic Properties of MIL-53-(Al)-AMMal and MIL-53(Al)-AMSuc
Having demonstrated that MIL-53(Al)-NH could be modified with cyclic anhydrides
2
without degradation of the framework, the ability of these solid-state Brønsted acid materials
in the methanolysis of cis-2,3-epoxybutane was explored. In order to confirm the origin of
catalysis (i.e. Brønsted acid catalysis), control experiments with the structural surrogates
MIL-53(Al)-NH , MIL-53(Al)-AM1, and MIL-53(Al)-AMCrot were performed in parallel
2
(Scheme 2). The MOF solids (∼0.08 mmol of carboxylate group) were pre-dried for ∼5 h
and were then added to a CD OD solution containing the epoxide (0.40 mmol). After two
3
Inorg Chem. Author manuscript; available in PMC 2011 September 6.

Garibay et al.
Page 5
1
days under ambient conditions, H NMR revealed that the epoxide essentially underwent
complete (>95%) turnover with MIL-53(Al)-AMMal, but there was no conversion with
MIL-53(Al)-AMSuc or any of the control materials (Figure 2). These results strongly
suggested that the carboxylate groups in MIL-53(Al)-AMMal were essential to catalyze the
methanolysis of cis-2,3-epoxybutane.
The MIL-53-(Al)-AMMal system maintained its structural integrity after completion of the
methanolysis reaction as evidenced by PXRD (Figure S9). To confirm that MIL-53(Al)-
AMMal was acting in a heterogeneous manner, aliquots of the reaction mixture were taken
1
after undergoing partial conversion and monitored by H NMR over the course of several
days for up to one week. The methanolysis of cis-2,3-epoxybutane is indeed a heterogeneous
reaction, as the signals associated with product formation did not change after removal of
the MIL catalysis (Figure S10).
The observation that MIL-53(Al)-AMSuc did not show good catalytic activity when
compared with MIL-53(Al)-AMMal was particularly intriguing. While MIL-53(Al)-AMSuc
contains approximately the same number of free carboxylate groups, it does not show
catalytic activity above that observed with the control materials studied (Scheme 2, Figure
2). The disparity in catalytic activity likely originates from the difference in the acidity of
the carboxylic acids. Unlike the succinic acid functionality, the maleic acid contains a
conjugated C=C double bond, which can stabilize the conjugate base resulting in higher
acidity. By comparison, propionic acid has a pK of 4.87, while its conjugated analogue,
a
acrylic acid, has a pK of 4.25, more than a half a log unit lower. Consistent with the MOF
a
findings, homogenous experiments for the methanolysis of cis-2,3,-epoxybutane using
maleic and succinic modified dimethyl amino terephthlate ligands (DMT-AMMal, and
DMT-AMSuc) gave similar results (Figure S11), with the DMT-AMMal ligand promoting
the reaction much more effectively than DMT-AMSuc (21% compared to 2%, respectively).
In order to establish the recyclability of MIL-53(Al)-AMMal, the reaction mixtures were
removed and extensively washed with CD OD. The solid catalyst then underwent a second
3
catalytic reaction employing the same reaction conditions. Unfortunately, the recycled
catalyst showed a significant reduction in activity (95% compared to 10%) (Figure S12).
Given that the MOF is exposed to a large excess of methanol during catalysis, it was
suspected that esterification of the carboxylic acid moieties could be responsible for the
reduction in activity of subsequent catalytic reactions. Attempts to spectroscopically
1
(FTIR, H NMR) confirm esterification of MIL-53(Al)-AMMal after catalysis were
unsuccessful. However, prolonged exposure of MIL-53(Al)-AMMal to methanol prior to the
first catalytic cycle significantly hampered its ability to perform the methanolysis of cis-2,3,-
epoxybutane. For example, freshly synthesized MIL-53-AMMal samples were treated with
CD OD (2 mL) and allowed to stand at ambient conditions for 2, 4, and 7 days. These pre-
3
treated samples were utilized as catalysts in the methanolysis of cis-2,3,-epoxybutane,
1
but H NMR revealed a decrease in product formation as a consequence of prolonged
1
exposure to CD OD (Figure S13). Based on H NMR integration, conversion from these
3
pretreated MIL-53-AMMal samples for 2, 4, and 7 days were 43, 33, and 2%, respectively.
These findings support the hypothesis that esterification of the carboxylate moieties is
responsible for the reduced catalytic properties of MIL-53-AMMal after the first round of
catalysis.
We reasoned that if the carboxylate groups of MIL-53(Al)-AMMal were undergoing
esterification then hydrolysis should re-establish the activity of the solid state Brønsted acid
catalyst. To test this hypothesis, inactive MIL-53(Al)-AMMal that had been through one
catalytic cycle was soaked 0.1 M HCl (13 mL) overnight, followed by washing with H O
2
(3×6 mL) and drying at 100 °C. Treatment with HCl successfully restored the catalytic
Inorg Chem. Author manuscript; available in PMC 2011 September 6.

Garibay et al.
Page 6
activity of MIL-53(Al)-AMMal (Figure S13). Due to the robust nature of the MIL-53(Al)
system, treatment with HCl does not degrade the structural integrity of MIL-53(Al)-NH or
2
MIL-53(Al)-AMMal (Figure S14). The catalytic properties of inactive MIL-53(Al)-AMMal
(turnover <2%), pretreated with CD OD for a week, showed complete restoration of activity
3
upon treatment with HCl. We confirmed that the renewed catalytic activity is not the result
of residual HCl, as a control reaction involving MIL-53(Al)-NH , MIL-53(Al)-AM1, and
2
MIL-53(Al)-AMSuc treated identically with HCl led to insignificant catalytic activity (<2%,
Figure S15). These findings suggest that the observed reduction in reactivity of MIL-53(Al)-
AMMal is likely due to ester formation, which can be reversed upon hydrolysis with acid.
With acidic treatment, MIL-53(Al)-AMMal provides a robust and recyclable solid-state
catalyst. In this study, MIL-53(Al)-AMMal was found to undergo at least three catalytic
cycles with consistent activity (Figure 3).
Attempts were made to determine the minimum catalyst loading for these reactions.
MIL-53(Al)-AMMal samples containing 0.08 mmol of the catalytic group (maleic acid
functionality) were introduced to solutions containing 8, 0.8, and 0.4 mmol of cis-2,3-
1
epoxybutane, which corresponds to 1, 10, and 20% catalyst loadings. H NMR of these
reaction mixtures after two days under ambient conditions showed that the methanolysis
reactions with 1, 10, and 20% catalyst loading led to 10%, 90%, and ∼95% turnover,
respectively (Figure S16). Therefore, under the present reaction conditions, at least 10%
loading was required to obtain high conversions within two days.
Given the modest success in facilitating the methanolyis of cis-2,3,-epoxybutane, the
catalytic scope of MIL-53(Al)-AMMal in the transformation of different epoxides was
investigated. Under identical conditions, MIL-53(Al)-AMMal initiates the methanolysis of
several epoxides (Table 1). MIL-53(Al)-AMMal promotes the methanolysis of cyclopentene
oxide, cyclohexene oxide, styrene oxide, and trans-stilbene oxide, while the asymmetric 1,2-
epoxyhexane is not activated by the catalyst (Figures S17-S21). It is unclear why the latter
substrate is not efficiently turned over by the MIL-53(Al)-AMMal catalyst. Size exclusion
from the MIL pores does not explain this result as the sterically more crowded cyclohexene
oxide, styrene oxide, and trans-stilbene oxide are converted with MIL-53(Al)-AMMal as a
catalyst. The only apparent feature of 1,2-epoxyhexane is its asymmetric structure, although
it is unclear why this feature would inhibit catalytic turnover, as it does not appear to inhibit
the reaction with styrene oxide. Nonetheless, consistent with this finding, (R)-(+)-propylene
oxide was a significantly poorer substrate for MIL-53(Al)-AMMal than cis-2,3,-
epoxybutane, giving a yield of only 59% (Figure S22).
Conclusions
In summary, the findings presented in this study demonstrate that covalent postsynthetic
modification is a viable method for synthesizing MIL-based catalysts. In contrast to our
earlier studies, which used Lewis acid metal sites for catalysis, the studies presented here
incorporate Brønsted acid, organocatalytic groups into a MOF. MIL-53(Al)-NH was treated
2
with maleic anhydride producing a carboxylic acid functionalized material. The carboxylic
acid groups residing in the framework were utilized as Brønsted acid catalytic sites for the
methanolysis of epoxides. While MIL-53(Al)-AMMal facilitated the ring opening of several
epoxides, another carboxylate functionalized MIL possessing a weaker acidic group,
MIL-53(Al)-AMSuc, did not perform the same catalysis. The methanolysis reaction was
found to be genuinely heterogeneous, but required regeneration with exogenous acid for
recovery as a reusable catalyst. Presently, our data do not provide clear evidence whether
catalysis is occurring in the interior or only on the surface of the MOFs. In addition, not all
epoxide substrates were effectively turned over by MIL-53(Al)-AMMal, and the origins of
this apparent selectivity require further investigation. Ongoing efforts will focus on
Inorg Chem. Author manuscript; available in PMC 2011 September 6.

Garibay et al.
Page 7
developing catalytic materials capable of asymmetric ring opening reactions as well as other
organocatalytic transformations.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgments
We thank Dr. Y. Su (U.C.S.D.) for performing the mass spectrometry experiments. This work was supported by
U.C.S.D. the National Science Foundation (CHE-0546531, new MOF synthesis; CHE-9709183, CHE-0116662 and
CHE-0741968 instrumentation grants), and the Department of Energy (DE-FG02-08ER46519, MOF modification
for gas sorption, PXRD instrumentation). S.J.G. was supported by a supplement to NCI grant 3R01
CA095298-07S1.
References
1. Yaghi OM, O'Keeffe M, Ockwig NW, Chae HK, Eddaoudi M, Kim J. Nature. 2003; 423:705–714.
[PubMed: 12802325]
2. Kitagawa S, Kitaura R, Noro Si. Angew Chem Int Ed. 2004; 43:2334–2375.
3. Ferey G. Chem Soc Rev. 2008; 37:191–214. [PubMed: 18197340]
4. Eddaoudi M, Moler DB, Li H, Chen B, Reineke TM, O'Keeffe M, Yaghi OM. Acc Chem Res. 2001;
34:319–330. [PubMed: 11308306]
5. Ockwig NW, Delgado-Friedrichs O, O'Keeffe M, Yaghi OM. Acc Chem Res. 2005; 38:176–182.
[PubMed: 15766236]
6. Furukawa H, Yaghi OM. J Am Chem Soc. 2009; 131:8875–8883. [PubMed: 19496589]
7. Rowsell JLC, Yaghi OM. Angew Chem Int Ed. 2005; 44:4670–4679.
8. Llewellyn PL, Bourrelly S, Serre C, Vimont A, Daturi M, Hamon L, De Weireld G, Chang JS, Hong
DY, Kyu Hwang Y, Hwa Jhung S, Ferey G. Langmuir. 2008; 24:7245–50. [PubMed: 18355101]
9. Latroche M, Surble S, Serre C, Mellot-Draznieks C, Llewellyn PL, Lee JH, Chang JS, Jhung SH,
Ferey G. Angew Chem Int Ed. 2006; 45:8227–8231.
10. Dinca M, Dailly A, Liu Y, Brown CM, Neumann DA, Long JR. J Am Chem Soc. 2006;
128:16876–16883. [PubMed: 17177438]
11. Ma S, Zhou HC. Chem Commun. 46:44–53.
12. Britt D, Tranchemontagne D, Yaghi OM. Proc Natl Acad Sci U S A. 2008; 105:11623–7.
[PubMed: 18711128]
13. Ahmad R, Wong-Foy AG, Matzger AJ. Langmuir. 2009; 25:11977–11979. [PubMed: 19754060]
14. Nuzhdin AL, Dybtsev DN, Bryliakov KP, Talsi EP, Fedin VP. J Am Chem Soc. 2007; 129:12958–
12959. [PubMed: 17924635]
15. Horcajada P, Serre C, Maurin G, Ramsahye NA, Balas F, Vallet-Regi M, Sebban M, Taulelle F,
Ferey G. J Am Chem Soc. 2008; 130:6774–6780. [PubMed: 18454528]
16. Horcajada P, Serre C, Vallet-Regi M, Sebban M, Taulelle F, Ferey G. Angew Chem Int Ed. 2006;
45:5974–5978.
17. Taylor-Pashow KM, Della Rocca J, Xie Z, Tran S, Lin W. J Am Chem Soc. 2009; 131:14261–
14263. [PubMed: 19807179]
18. Lee J, Farha OK, Roberts J, Scheidt KA, Nguyen ST, Hupp JT. Chem Soc Rev. 2009; 38:1450–
1459. [PubMed: 19384447]
19. Tonigold M, Lu Y, Bredenkotter B, Rieger B, Bahnmuller S, Hitzbleck J, Langstein G, Volkmer
D. Angew Chem Int Ed. 2009; 48:7546–7550.
20. Maksimchuk NV, Kovalenko KA, Arzumanov SS, Chesalov YA, Melgunov MS, Stepanov AG,
Fedin VP, Kholdeeva OA. Inorg Chem. 2010; 49:2920–2930. [PubMed: 20166742]
21. Dhakshinamoorthy A, Alvaro M, Garcia M. Adv Synth Catal. 2010; 352:711–717.
Inorg Chem. Author manuscript; available in PMC 2011 September 6.

Garibay et al.
Page 8
22. Banerjee M, Das S, Yoon M, Choi HJ, Hyun MH, Park SM, Seo G, Kim K. J Am Chem Soc.
2009; 131:7524–7525. [PubMed: 19438178]
23. Hu A, Ngo HL, Lin W. J Am Chem Soc. 2003; 125:11490–1. [PubMed: 13129339]
24. Xamena FXLI, Abad A, Corma A, Garcia H. J Catal. 2007; 250:294–298.
25. Prats AEP, de la Pena-O'Shea VA, Iglesias M, Snejko N, Monge A, Gutierrez-Puebla E.
Chemcatchem. 2010; 2:147–149.
26. Tanaka K, Oda S, Shiro M. Chem Commun. 2008:820–822.
27. Ingleson MJ, Barrio JP, Bacsa J, Dickinson C, Park H, Rosseinsky MJ. Chem Commun.
2008:1287–1289.
28. Reeve W, Christoffel I. J Am Chem Soc. 1950; 72:1480–1483.
29. Olah GA, Fung AP, Meidar D. Synthesis. 1981:280–282.
30. Salehi P, Dabiri M, Zolfigol MA, Fard MAB. Phosphorous, Sulfur, and Silicon. 2004; 179:1113–
1121.
31. Jiang D, Mallat T, Krumeich F, Baiker A. J Catal. 2008; 257:390–395.
32. Jiang D, Urakawa A, Yulikov M, Mallat T, Jeschke G, Baiker A. Chem Eur J. 2009; 15:12255–
12262.
33. Wang Z, Cohen SM. Chem Soc Rev. 2009; 38:1315–1329. [PubMed: 19384440]
34. Tanabe KK, Cohen SM. Angew Chem Int Ed. 2009; 48:7424–7427.
35. Tanabe KK, Cohen SM. Inorg Chem. 2010; 49:6766–6774. [PubMed: 20565054]
36. Ahnfeldt T, Gunzelmann D, Loiseau T, Hirsemann D, Senker J, Ferey G, Stock N. Inorg Chem.
2009; 48:3057–3064. [PubMed: 19245258]
37. Nguyen JG, Cohen SM. J Am Chem Soc. 2010; 132:4560–4561. [PubMed: 20232871]
38. Garibay SJ, Wang Z, Tanabe KK, Cohen SM. Inorg Chem. 2009; 48:7341–7349. [PubMed:
19580256]
39. Volkringer C, Cohen SM. Angew Chem Intl Ed. 2010; 49:4644–4648.
Inorg Chem. Author manuscript; available in PMC 2011 September 6.

Garibay et al.
Page 9
Figure 1.
1
H NMR spectra of modified MIL-53(Al)-NH samples digested in HF/d -DMSO. Red
2
6
squares and black circles represent signals of aryl protons in modified and unmodified NH -
2
BDC, respectively. All other peaks associated with modifications or solvent are labeled
explicitly in red.
Inorg Chem. Author manuscript; available in PMC 2011 September 6.

Garibay et al.
Page 10
Figure 2.
1
H NMR of catalytic reaction aliquots employing MIL-53(Al)-AMMal (green),
MIL-53(Al)-AM1 (blue), MIL-53(Al)-AMCrot (red), and MIL-53(Al)-NH (black).
2
Inorg Chem. Author manuscript; available in PMC 2011 September 6.

Garibay et al.
Page 11
Figure 3.
1
H NMR spectra of three consecutive reactions of the methanolysis of cis-2,3,-epoxybutane
utilizing MIL-53(Al)-AMMal showing essentially quantitative conversion (black), ∼97%
conversion (red), and 90% conversion (blue). The MIL was regenerated with HCl between
each reaction cycle.
Inorg Chem. Author manuscript; available in PMC 2011 September 6.

Garibay et al.
Page 12
Scheme 1.
Synthesis (top left) and structure (top right) of MIL-53(Al)-NH . Postsynthetic modification
2
reactions performed on MIL-53-(Al)-NH relevant to this study (bottom).
2
Inorg Chem. Author manuscript; available in PMC 2011 September 6.

Garibay et al.
Page 13
Scheme 2.
Attempted methanolysis of cis-2,3-epoxybutane with carboxylate functionalized
MIL-53(Al)-AMMal and MIL-53(Al)-AMSuc. Letters a∼f represent the protons associated
with starting material and the ring opening product. Also listed are structural analogues
MIL-53(Al)-NH , MIL-53(Al)-AM1, MIL-53(Al)-AMCrot that were used for control
2
reactions.
Inorg Chem. Author manuscript; available in PMC 2011 September 6.

Garibay et al.
Page 14
Table 1
Methanolysis of epoxides with MIL catalysts. MIL catalysts (ca. 0.08 mmol based on free carboxylate groups)
were added to a CD OD solution (2 mL) containing the epoxide substrate (ca. 0.40 mmol) and the reaction
3
was allowed to stand for 2 days at 25 °C. The conversions are based on at least three independent trials.
Reagent
Product
Catalyst
Conversion [%]
cis-2,3,-epoxybutane
MIL-53(Al)-AMMal
MIL-53(Al)-AMSuc
95 ± 2
<2
MIL-53(Al)-AMCrot <2
MIL-53(Al)-AM1
<2
MIL-53(Al)-NH2
<2
Cyclopentene oxide
MIL-53(Al)-AMMal
44 ± 4
Cyclohexene oxide
MIL-53(Al)-AMMal
100 ± 2
trans-stilbene oxide
MIL-53(Al)-AMMal
MIL-53(Al)-AMMal
37 ± 4
Styrene oxide
100 ± 2
1,2-epoxyhexane
No reaction
MIL-53(Al)-AMMal
No reaction
Inorg Chem. Author manuscript; available in PMC 2011 September 6.

Garibay et al.
Page 15
Reagent
Product
Catalyst
Conversion [%]
(R)-(+)-propylene oxide
MIL-53(Al)-AMMal
59± 4
Inorg Chem. Author manuscript; available in PMC 2011 September 6.