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COMMUNICATION
Cite this: CrystEngComm, 2013, 15,
9368
Received 7th July 2013,
Accepted 30th August 2013
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Alternative synthetic methodology for amide
formation in the post-synthetic modification of TiMIL125-NH2†
Adam P. Smalley,a David G. Reid,a Jin Chong Tanb and Gareth O. Lloyd‡*a
DOI: 10.1039/c3ce41332b
www.rsc.org/crystengcomm
There are relatively few metal–organic framework materials that
are robust enough to survive post-synthetic modification of their
structures. We present test modifications of Ti-MIL125-NH2 leading towards tuning of the porous and catalytic properties of the
material. We also present the first use of a mild amide synthesis
method for post-synthetic modification.
Coordination polymers are a class of inorganic–organic
hybrid materials, both crystalline and non-periodic, that can
be modulated to give rise to a large variety of properties.1
One of the key properties of the crystalline materials that has
been extensively studied and developed over the last two
decades is that of porosity.2,3 The porosity has led to several
functions for the materials being envisioned, some of which
have been gas separation and storage,4,5 catalysis,6,7 drug
delivery8 and sensing.9,10 One of the major successes of these
materials has been the ability to tune the pore functionality
through the modulation of the organic linkers.11
Modification after synthesis of the materials has become an
increasingly popular means to functionalise the pore
surfaces, and this method has become known as postsynthetic modification (PSM).12 The most common reactive
group to have been studied to date has been the amino
group.13–16 This has partially been due to the success of
isoreticular synthesis of porous coordination polymers (PCPs)
using terephthalic acid derivatives.17 Aminoterephthalic
acid has been successfully incorporated into a number of
PCPs and a large number of these materials have had
their properties successfully altered using PSM.18 A recently
reported amino-terephthalic acid containing material
Ti8O8(OH)4(aminoterephthalate)6, MIL125-NH2,19 has not to
date been studied for its potential to undergo PSM. Thus, the
aim of this work is to present PSM studies of MIL125-NH2 in
which we show an unreported method to synthesise amide
groups and functionalise the materials to introduce potentially useful groups (Scheme 1).
MIL125-NH2 was synthesised using a scaled up version
of the literature method utilising 2-aminoterephthalic acid
and titanium isopropoxide in a DMF–MeOH solvent mixture
(see ESI† for details).19 We performed Rietveld refinement20
utilising an amino modified structural model based on the
known MIL125 structure.19 This confirmed the structure as
represented in Fig. 1. The material consists of inorganic clusters of eight Ti metal centres connected together with eight
oxides, four hydroxides and six aminoterephthalates. The
aminoterephthalate organic linkers join the inorganic clusters into a quasi-cubic tetragonal structure that can be
described as an enlarged version of a centered cubic structure. There are two types of cavities that can be best
described as tetrahedral and octahedral in shape, with
a
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge,
CB2 1EW, United Kingdom. E-mail: [email protected]; Fax: +44 (0) 131 451
3180; Tel: +44 (0) 131 451 4167
b
Department of Engineering Science, University of Oxford, Parks Road, Oxford,
OX1 3PJ, United Kingdom
† Electronic supplementary information (ESI) available: CIF (CCDC 948966),
crystallographic details for powder diffraction and Rietveld, and analytical data
(FT-IR, porosity, TGA and solid state NMR) of synthesised materials. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3ce41332b
‡ Current address: Institute of Chemical Sciences, School of Engineering and
Physical Sciences, William Perkin Building, Heriot-Watt University, Edinburgh,
EH14 4AS, United Kingdom.
9368 | CrystEngComm, 2013, 15, 9368–9371
Scheme 1 Reaction scheme for the activation of carboxylic acids with ethyl
chloroformate to react with the PCP in a two step, one pot reaction.
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Fig. 1
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MIL125-NH2 structure consisting of octahedral and tetrahedral cavities.
triangular faces of the polyhedra acting as windows
connecting the cavities traced out by the polyhedra.
To investigate if this material could be modified using
amide synthesis we utilised three known amide synthetic
methodologies; acid chlorides, symmetric anhydrides and the
use of ethyl chloroformate to activate carboxylic acids by the
generation of asymmetric anhydrides (Scheme 1). Acid chloride derivatives react with amines to give amides and HCl.
The HCl acid by-product is detrimental to the majority of
PCPs and this is also found in the case of MIL125-NH2, even
in the presence of base. Thus acid chlorides were found to be
inappropriate for modifying the PCP, similar to how phosgene as a PSM reagent is limited to acid stable PCPs.21 The
reaction with symmetric anhydrides is a very common
method used to modify amino containing PCPs. The
by-product in this procedure is the carboxylic acid derivative
of the anhydride. Although this often does not interact with
the PCPs, it has been shown that carboxylic acids can modify
reactive surfaces on PCPs.22 We reacted MIL125-NH2 with
acetic anhydride and found a conversion of 45(±5)%. The
digested material (digested by sonication in DCl–d6-DMSO)
was used to determine the conversion percentage using 1H
solution NMR by comparison of the integrated areas of the
aromatic resonances of the two compounds of interest
(acetylated and amino terephthalic acids). The solid material was analysed using solid-state 13C NMR, FT-IR and
PXRD to confirm conversion had indeed occurred on the
stable crystalline framework. We state the conversion as a
lower limit as there may be some cleavage of the amide
bonds under these acidic conditions.
The analysis by solid state 13C NMR of the modified PCP
gave the expected sp3 and sp2 peaks due to the incorporated
CH3 and CO. The carbon next to the nitrogen is also
shifted upfield by this acetylation which indicates a successful reaction rather than uptake of acetic anhydride into the
pores (Fig. 2). Additional evidence for this reaction was
obtained by FT-IR where the anticipated amide carbonyl
stretch at 1686 cm−1 was seen with no evidence of the symmetric and antisymmetric anhydride stretches. PXRD was
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13
Fig. 2 Solid state
C NMR of the acetic anhydride modified PCP (red)
compared with the unmodified Ti-MIL125-NH2 (black). The new peaks at 26 ppm
and 172 ppm are due to the new methyl and CO respectively while the aromatic
carbon attached to nitrogen (*) has shifted from 152 ppm to 142 ppm as a result of
amide formation.
used to confirm the retention of the crystal structure which
was effectively unchanged by modification.
To synthesise asymmetric anhydrides we utilised the reaction of ethyl chloroformate with carboxylic acids. This reaction also generates HCl, but this is not a problem here as the
synthesis of the asymmetric anhydride is performed before
addition of the PCP. Triethylamine, or any other organic
base, captures the HCl. It is possible to separate the asymmetric anhydride from the initial reactants if necessary, however, in our case the PCP was subsequently added only once
the carboxylic acid had been activated in situ (i.e. a one-pot
synthetic methodology). Scheme 1 shows the one-pot reaction
procedure and the by-products, which are carbon dioxide and
ethanol. The by-products have mild reactivity and should not
detrimentally modify the majority of PCPs known. These
by-products are also highly volatile and dissolve in most
organic solvents, meaning they can be removed from the
materials utilising standard methodology.
The product formed from the acetic anhydride modification and the acetic acid activated asymmetric anhydride was
shown to be the same by PXRD (Fig. 3), solid-state 13C NMR
and FT-IR and also gave a similar conversion percentage of
40(±5)% (see ESI† for details). Indeed, characterisation of the
porosity of the materials indicates there is little difference
between the two modification techniques, with BET surface
areas obtained for our synthesised materials being 775 m2 g−1,
596 m2 g−1 and 642 m2 g−1 for the unmodified, acetic anhydride modified and the asymmetric anhydride modified
MIL125-NH2, respectively. The pore size and distribution
from the sorption data is also very similar for the two PSM
methods (see ESI† for details). Therefore this acetylation is
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Fig. 3 PXRD pattern of the asymmetric anhydride modified PCP (red, top)
compared with the unmodified Ti-MIL125-NH2 (blue, bottom) indicating no
significant change to the crystallinity, phase purity and structural integrity of
the material.
competitive with the more established method in terms of
conversion, but it is milder and more diverse chemically. It
can potentially be more widely used to add a large variety of
functionalities to PCPs by utilising PSM with greater functional group tolerance.
An important aspect of PSM is that it can provide a synthetic tool to access structures that cannot be obtained via
traditional methodology, such as solvothermal synthesis. To
test this we synthesised amido-derived terephthalic acid by
reacting 2-amino-terephthalic acid with acetic anhydride in
dichloromethane and in the presence of triethylamine. The
amido-derived terephthalic acid was then utilised in the standard solvothermal synthesis of MIL125. This resulted in
black non-crystalline precipitates, probably titanium oxides,
indicating that PSM of MIL125-NH2 may be the only reaction
pathway to the amide functionalised MIL material.
To investigate the general applicability of the ethyl
chloroformate amide synthetic methodology we further
tested modifications on a different PCP, UiO-66-NH2, and
tested MIL125-NH2 with different carboxylic acids. The PSM
of UiO-66-NH2 has been extensively studied and therefore
makes a good test material for the general use of ethyl
chloroformate.11–13 FT-IR and the digested sample show the
modification was successful. Unfortunately, a percentage
conversion could not be ascertained from the NMR of the
digested samples due to overlap of peaks.
The reactivity of benzoic acid and BOC-L-proline utilising
ethyl chloroformate with MIL125-NH2 was successful, as
determined by FT-IR. Conversion levels were very low, probably due to the bulky nature of the acids compared to the pore
windows of the material.
The bulkiness of the reactive groups was further tested by
utilising ethyl isocyanate. Urea formation through the reaction between an amine and isocyanate is an atom efficient
click process that can add reactivity to a large number of
materials.23 The MIL125-NH2 material was successfully modified using ethyl isocyanate as shown by FT-IR and solid-state
13
C NMR (Fig. 4).
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Fig. 4 Solid state C NMR of the ethyl isocyanate modified PCP (red, top) compared
with the unmodified Ti-MIL125-NH2 (black, bottom). The new peaks at 11 ppm, 35 ppm
and 173 ppm are due to the new methyl, methylene and CO groups, respectively. The
peak at 52 ppm is potentially due to residual DMF–MeOH in the PCP pores.
The catalytic properties of titanium containing materials are
well known. Related to that, it has been recently shown that
MIL125-NH2 has photo-catalytic properties.19c We thus aimed
to test the effect of the PSM of the MIL125-NH2 material on its
catalytic potential. This was done using the methodology
shown by Li et al. where the MIL125 materials were exposed to
visible light under a N2 atmosphere in acetonitrile and
triethanolamine.19c The pristine MIL125-NH2 material changes
colour under these conditions from yellow to green.19c Exposing the two PSM MIL125 materials to these conditions result in
no change in colour (Fig. 5). This indicates that the PSM procedure significantly reduces or switches off the photo-catalytic
properties even when only 40–50% of the amine groups are
modified. Further computational and experimental work is
required to understand this change.
In conclusion we have shown the mild modification of
amino containing PCPs using carboxylic acids to generate
amide coupling by utilising the formation of an asymmetric
anhydride through the reaction of the carboxylic acid with
ethyl chloroformate. In particular, we have successfully
Fig. 5 Experiments to determine the catalytic potential of the materials. a) Pristine
MIL125-NH2 shows the photo-catalytic potential by changing colour. b) Acetic anhydride modified MIL125-NH2 and c) asymmetric anhydride modified MIL125-NH2
show no clear catalytic potential.
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modified MIL125-NH2 which resulted in reduced porosity
and altered photo-catalytic properties of the material. As only
ethanol and CO2 are produced during the synthesis of the
amide from the asymmetric anhydride, this technique should
be a valuable tool to couple a multitude of carboxylic acids to
PCPs to introduce a more varied quantity of functionalities.
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Acknowledgements
G. O. L. thanks the Herchel Smith Fellowship Fund
(Cambridge) and Heriot-Watt University. J. C. T. thanks the
European Research Council. Prof. Anthony Cheetham and
Mihails Arhangelskis (both of Cambridge) are thanked for
access to porosity equipment and Rietveld assistance. D.G.R.
would like to acknowledge the BBSRC for support.
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