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CrystEngComm Published on 02 September 2013. Downloaded by University of Oxford on 28/04/2014 14:24:42. COMMUNICATION Cite this: CrystEngComm, 2013, 15, 9368 Received 7th July 2013, Accepted 30th August 2013 View Article Online View Journal | View Issue 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. This journal is © The Royal Society of Chemistry 2013 View Article Online Published on 02 September 2013. Downloaded by University of Oxford on 28/04/2014 14:24:42. CrystEngComm Fig. 1 Communication 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 CO. 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 This journal is © The Royal Society of Chemistry 2013 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 CO 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 CrystEngComm, 2013, 15, 9368–9371 | 9369 View Article Online Published on 02 September 2013. Downloaded by University of Oxford on 28/04/2014 14:24:42. Communication CrystEngComm 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). 9370 | CrystEngComm, 2013, 15, 9368–9371 13 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 CO 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. This journal is © The Royal Society of Chemistry 2013 View Article Online CrystEngComm 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. Published on 02 September 2013. Downloaded by University of Oxford on 28/04/2014 14:24:42. 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. Notes and references 1 S. Kitagawa, R. Kitaura and S. Noro, Angew. Chem., Int. Ed., 2004, 43, 2334–2375. 2 S. Bureekaew, S. Shimomura and S. Kitagawa, Sci. Technol. Adv. Mater., 2008, 9, 014108. 3 Z. Wang, K. K. Tanabe and S. M. Cohen, Chem.–Eur. J., 2010, 16, 212–217. 4 H. Wu, R. S. Reali, D. A. Smith, M. C. 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