This is the accepted version of the article: Luis-Barrerra J., Cano R., Imani-Shakibaei G., Heras-Domingo J., Pérez-Carvajal J., Imaz I., Maspoch D., Solans-Monfort X., Alemán J., Mas-Ballesté R.. Switching acidic and basic catalysis through supramolecular functionalization in a porous 3D covalent imine-based material. Catalysis Science and Technology, (2019). 9. : 6007 - . 10.1039/c9cy01527b. Available at: https://dx.doi.org/10.1039/c9cy01527b Switching acidic and basic catalysis through supramolecular functionalization in a porous 3D covalent imine-based material Javier Luis-Barrerra, a Rafael Cano, b Ghazaleh Imani-Shakibaei, a Javier Heras-Domingo, c Javier Pérez-Carvajal, d Inhar Imaz, d Daniel Maspoch, d,e Xavier Solans-Monfort, * c José Alemán* a,f and Rubén Mas-Ballesté * b,f Non-covalent inclusion of small acid and base molecules (CH 3COOH and NEt3) in an imine structure based on micrometre COF-300 crystals and amorphous nanoparticles results on the facile modulation of its acid-base properties. Such strategy results in the triggering of acidic/basic catalytic activity of the otherwise inactive materials towards ring-opening epoxide and Knoevenagel condensation reactions. For both reactions, amorphous nanoparticles are better catalysts than micrometre crystals as they exhibit a higher external surface area. The found activities and stability of this supramolecular functionalization are modulated by confinement effects, which are rationalized with the help of DFT theoretical calculations. All results obtained suggests that reaction catalysed by these functionalized materials occur confined at pores closer to the material surface, which makes possible size-discrimination phenomena, and explains the major activity of the nanoparticulated material. zeolites,6 mesoporous silica7 and metal-organic frameworks Introduction (MOFs).8 In 2005, Yaghi and co-workers described a new concept on The development of molecules and materials with catalytic the reticular chemistry field: the covalent-organic frameworks cavities, inspired on biological systems, such as enzymes, has (COFs).9 Their use as heterogeneous catalysts offers advantages been a main topic in modern chemistry. For instance, different compared to MOFs such as increased activity, durability and molecules known as molecular flasks1 are able to catalyse a recyclability. COFs are based on the covalent linkage of organic variety of reactions in confined spaces. The outstanding subunits into two- or three-dimensional porous crystalline performance of such molecular systems and other porous structures. Therefore, they show predictable composition, materials lays not only on the design of active catalytic centres, topology and porosity. As a consequence, COFs have been but also on the easy tuning of environment in a confined space, explored for applications related to gas storage and resulting on different regio-,2 chemo-3 and stereo-selectivities4 separation,10 ultrasensitive sensing,11 electronic applications,12 in different reactions. In addition, the proximity between the and clean energy technologies.13 However, only few recent different reactants in the confined space can result in an studies initiated the exploration of their catalytic applications. 14 enhancement of the catalytic activity.5 Furthermore, confined In 2014, Yan et al. described the intrinsic basic property of a catalysis offers the possibility to perform size-selective three-dimensional (3D) imine-based COF useful for the catalytic reactions, especially important when working with mixtures of Knoevenagel reaction.15 After this achievement, other groups compounds with similar chemical and physical properties. have employed two main covalent strategies in order to tune During the last years, confinement effects have been the catalytic activity of different COFs. The first one consisted in particularly studied in different porous materials such as the post-functionalization of the material, attaching organic pending arms (top-left, Scheme 1)16 or metal nano-particles,14 a. Department of Organic Chemistry (module 01), Facultad de Ciencias, Universidad which are both s irreversible functionalization processes. A Autónoma de Madrid, 28049 Madrid, Spain. E-mail: jose.aleman@uam.es; second strategy relayed in the use of catalytic backbones, which www.uam.es/jose.aleman often requires complex synthetic modifications of building b. Department of Inorganic Chemistry (module 07), Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain. E-mail: blocks (top-right, Scheme ruben.mas@uam.es c. Department of Chemistry, Facultat de Ciències, Universitat Autònoma de Barcelona, 08191 – Bellaterra, Spain. E-mail: xavier.solans@uab.cat d. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, 08191 – Bellaterra, Spain. E-mail: daniel.maspoch@icn2.cat e. ICREA, Pg. Lluís Companys 23, Barcelona, 08010, Spain. f. Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de Madrid, 28049 Madrid, Spain. † Footnotes relating to the title and/or authors should appear here. Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x Figure 1. Schematic representation of the preparation of a-3, c-3 and related Scheme 1 Previous strategies for the catalytic functionalization materials and the supramolecular materials. supramolecular used in this work. 1).17 Although these are interesting approaches, they require presence of acetic acid allowed the formation of the amorphous predetermined specific architectures or, in some cases, imine-based polymer nanoparticles a-3, which contains complicated post-functionalization processes. CH3COOH molecules inside its structure. 23 Then, in a second As an alternative to these strategies shown above, step, these nanoparticles were subjected to a dynamic covalent supramolecular functionalization appears as an easy way to chemistry to crystallize them under similar conditions to those modulate and direct several properties of COFs. In particular, reported by Dichtel et al.22 hydrogen bonding interactions between COFs and small, SEM images of the solid collected in the first step revealed molecules have been recently demonstrated and applied to the formation of amorphous-nanoparticles with diameters of drug delivery and molecular recognition systems. 18 In addition nearly 100 nm (Figure 2; see S. I., Figures S1-3 for powder X-ray several porous materials with pre-designed structures that diffraction (PXRD) data). Solid-state MAS-13C-NMR and FT-IR could make possible proton conduction mechanism have been spectra confirmed the chemical identity of the material (see S. applied to proton-exchange in fuel membranes.19 However, to I., Figures S6 and S9-12, respectively). The FT-IR spectrum of the best of our knowledge, such supramolecular COF-300 showed the C═N stretching modes characteristic for functionalization strategy, to obtain catalytic COF-molecule imines at 1620 and 1202 cm−1. The solid-state MAS-13C-NMR confined systems, have never been applied to catalyse chemical spectrum was also in agreement with the formation of the imine organic transformations. Herein we show that this latter strategy can be applied to bonds. Thermogravimetric analysis (TGA) data revealed a 2.6 % functionalize COF-related materials for catalytic applications of volatile molecules lost in the range of 50-400 ºC, which was (bottom, Scheme 1). These materials can be reversibly consistent with the presence of acetic acid even after numerous functionalized through supramolecular interactions with two washing procedures with distinct solvents (see S. I., Figure S14). simple model molecules: a carboxylic acid (acetic acid) that has The presence of acetic acid was further confirmed by mass been used to catalyse ring-opening reactions of epoxides; and a spectrometry coupled to TGA analysis, which showed peaks at Brønsted base (triethylamine) that has been used to catalyse m/z = 15 and 43 that can be assigned to [CH3]+ and [CH3CO]+, Knoevenagel condensation. respectively. Further evidence of the presence of acetic acid molecules inside a-3 was acquired by solid-state MAS-13C-NMR. To this end, a-3 was synthesized in the presence of 13C Results and discussion isotopically labelled acetic acid. Solid-state MAS-13C-NMR of this Preparation and characterization of a-3, c-3 and derivatized newly synthesized a-3 gave a clear evidence of the presence of materials. 13C-enriched acetic acid molecules by means of the CH signal at 3 We have demonstrated that these porous materials can be 20 ppm (see S.I., Figure S8). Altogether, these observations reversibly functionalized (Figure 1) through supramolecular confirmed the formation of CH3COOH@a-3.24 interactions. We selected the 3D COF-30020 made of The evacuation of all CH3COOH molecules in CH3COOH@a- terepthalaldehyde (1) and tetra-(4-anilyl)-methane (2) as it is 3 was achieved by heating it at 80 ºC during 12 hours under stable up to 490 °C, in water and common organic solvents. vacuum (1 mmHg). The resulting material showed the same COF-300 was synthesized by a well-known two-step morphology than its precursor (see S. I., Figure S5). First process:21,22 first, amorphous imine polymer nanoparticles evidence of the evacuation of all acetic acid molecules was (hereafter called amorphous-3 or a-3) were formed; and obtained by TGA analysis, from which no significant weight loss subsequently, these a-3 nanoparticles were further reacted to in the range of 50-400 ºC was observed (see S. I., Figure S15). form micrometre COF-300 crystals (hereafter called crystalline- We also demonstrated the removal of acetic acid 3 or c-3). In a first step, a room temperature synthesis in the confirming the evacuation of guest molecules from its framework (see S. I., Figure S18). This observation was further corroborated by N2 sorption measurements at 77 K. As shown in Figure 3c, evacuated c-3 showed to be porous after mild activation conditions (2 hours of primary vacuum at room temperature). The properties of this material matched with the previously reported data:25 the adsorption isotherm presented a S-shape (Type V) centred at 0.1 P/P0 and with a plateau at 400 cm3 g-1. Figure 2 Representative SEM images of CH3COOH@a-3. molecules by performing solid-state MAS-13C-NMR of a sample of 13C enriched CH3COOH@a-3 exposed to identical activation conditions (see S. I., Figure S8). In our quest of switching from acidic to basic properties of the material, a sample of CH3COOH@a-3 was treated with pure triethylamine (1 mL per 10 mg of CH3COOH@a-3) overnight at 80 ºC. Again, SEM images confirmed the retention of the size and morphology of the nanoparticles after both amine treatments (see S. I., Figure S5). TGA analysis of CH3COOH···NEt3@a-3 showed a larger weight loss from 50 ºC to 400 ºC in comparison to CH3COOH@a-3 (4.6 % versus 2.6 %). Here, at this temperature range, mass spectrometry coupled to TGA revealed peaks at m/z = 43 and 51 in the ionization cell, which were assigned to [CH3CO]+ and [HNEt3]2+, respectively, thereby confirming the presence of both acetic acid and triethylamine molecules (see S. I., Figure S17). The supramolecular interaction is strong enough to prevent leaching after 14 hours of stirring the material in toluene. Such stability is suggested by the fact that after such treatment no evidences of acetic acid nor NEt3 were detected NMR measurements. In order to determine the reversibility of the functionalization of this material, we performed vacuum in a sample of CH3COOH···NEt3@a-3, recovering the evacuated a-3, as revealed by TGA. It is worth mentioning that direct reaction of evacuated a-3 with pure Et3N results on a functionalized material in which the interactions between the imine structure and the guest molecules are very weak. Therefore, such functionalized material resulted in massive leaching of Et 3N molecules under catalytic conditions (vide infra). Consequently, such strategy was not useful to achieve confined catalysis. In a subsequent synthetic step, amorphous a-3 nanoparticles were converted to micrometre crystalline c-3 particles by heating them at 120 ºC for 7 days in a mixture of dioxane, mesitylene and acetic acid. After exposing the treated solid under vacuum conditions, evacuated c-3 was obtained in the form of microcrystals with dimensions of nearly 1 m in diameter (Figure 3a). PXRD showed the characteristic pattern of Figure 3 Characterization of c-3 and derivatized materials. a) Representative SEM COF-300 (Figure 3b).20 Moreover, c-3 did not show significant image of c-3; b) PXRD pattern of c-3; c) Nitrogen adsorption analysis of c-3 (black), crystalline CH3COOH@c-3 (red) and crystalline CH3COOH···NEt3@c-3 (blue). spectroscopic changes in both FT-IR and solid-state 13C MAS NMR spectra in comparison to those seen for a-3 (see S. I. Figures S13 and S7, respectively). TGA analysis for the c-3 did not display any significant weight loss in the range of 50-400 ºC, Then, evacuated c-3 was functionalized with CH3COOH or CH3COOH···NEt3 following the same methodology described for the amorphous materials (see S. I., section 1). It should be highlighted that N2 sorption measurements in both CH3COOH@c-3 and CH3COOH···NEt3@c-3 showed a dramatic loss of porosity, with total uptakes decreasing below 50 cm 3 g-1 (49 cm3 g-1 for CH3COOH@c-3, and 23 cm3 g-1 for CH3COOH- NEt3@c-3). Moreover, addition of CH3COOH or CH3COOH··· NEt3 adduct resulted on the change of the isotherms observed to Type III. These changes in the isotherm trend and uptake were indicative of the blocking of the pores by the post-synthetic inclusion of CH3COOH and NEt3 molecules. Catalysis of the materials. Comparison of the catalytic activity of amorphous a-3 and crystalline c-3 based materials. We started evaluating the catalytic activity of crystalline evacuated c-3 and amorphous evacuated a-3 and the corresponding functionalized materials in two model reactions: the ring-opening epoxide reaction of 4a with epoxide 5 Scheme 2 Catalytic activity of the prepared materials for a) ring-opening epoxide obtaining product 6a under acidic conditions and the reaction and b) Knoevenagel reaction. Knoevenagel reaction between malononitrile 7 and aldehyde 8a, obtaining product 9a under basic conditions.26 The results obtained are summarized in Scheme 2. The evacuated materials a-3 and c-3 showed a negligible catalytic activity for the two processes studied, being observed amounts of product similar to those obtained in control experiments without catalyst. On the contrary, crystalline acidic CH3COOH@c-3 and basic CH3COOH···NEt3@c-3 materials showed a moderate catalytic performance for both ring opening epoxide and Knoevenagel reactions, respectively. We attributed this moderate activity to the fact that inclusion of these molecules dramatically reduces the porosity of c-3 (as confirmed by sorption N2 measurements, Scheme 3 Scope of epoxide ring opening reactions explored in this work catalysed vide supra), thereby limiting the access of reagents to the by CH3COOH@a-3. internal parts of material. According to this hypothesis, we decided to study the catalytic activity of both amorphous CH3COOH@a-3 and CH3COOH···NEt3@a-3, because they show Acid catalysis: ring-opening epoxide reaction using CH3COOH@a-3. a much higher external surface area due to their nanosized To generalize the use of CH3COOH@a-3 to catalyse opening character. The higher dimension of the particle was 145±16 nm epoxide reactions, we performed different reactions with for amorphous materials and 1100±400 nm for crystalline COFs secondary amines (4b and 4c) following a solvent free (see ESI). Indeed, amorphous materials, even though they show procedure. Under these reaction conditions, CH3COOH@a-3 lower loadings of active molecules (see S. I. section 6), showed catalysed the formation of product 6b with a 55 % yield (Scheme better catalytic performances for both acidic and basic catalytic 3). Moreover, it could also catalysed the formation of 6c in low systems. yields from Et2NH (4c). Accordingly, while we observed a 20% of We also studied the stability of such materials in the conversion that corresponds to the background reaction in the presence of the reactants used in both reactions (see section case of 4a and 4b, the uncatalyzed reaction did not generate a 12.3 in S.I. for more details). Overall, imine structures are measurable amount of product when 4c was used as reagent. sensitive to the nucleophilic attack that result in erosion of the (see. S. I., section 7). organic framework as a consequence of the reversible nature of the imine bond. Such decomposition reactivity competes with Basic Catalysis: Knoevenagel Reaction using CH3COOH···NEt3@a-3. the catalysed reaction. Consequently, imine structure reactivity We next expanded the scope of Knoevenagel reaction using limits the output of the catalytic reaction and the recyclability different aldehydes 8b-g (Scheme 4). We found an excellent of the corresponding material. catalytic outcome, which was maintained with electron donating groups, such as methyl or methoxy groups (9b-c), alkyl groups (9d), and more sterically demanding aldehydes (9e and 9f). However, further increase of the steric hindrance Scheme 4 Knoevenagel reactions explored in this work between malononitrile and aldehydes catalysed by CH3COOH···NEt3@a-3. [a] All the reactions were performed using 0.11 mmol of acetonitrile 7, 0.1 mmol of aldehyde 8 and 5 mg of CH3COOH···NEt3@a-3 in 0.6 mL of toluene at room temperature. [b] mol% obtained from TGA analysis (4.6 % of total mass). [c] A reaction was performed under homogeneous conditions (1.4 mol% of adduct CH3COOH···NEt3) in absence of the material, obtaining product 9a in 77% yield after 14h. [d] Full conversion to the product was obtained when free NEt 3 was used instead of the porous material. in reagent 8g resulted in null conversion to the Knoevenagel product 9g. This result is very interesting considering that product 9g could be obtained in homogenous catalysis using NEt3. Therefore, the null reactivity observed using CH3COOH···NEt3@a-3 can be understood by a size discrimination effect that seems to point to a catalysis taking place inside the pores of the material. Moreover, the different catalytic activity showed by c-3 and a-3 materials in addition with N2 adsorption experiments of the pre- and post- functionalized materials, led us to the hypothesis that catalysis Figure 4 Optimized imine structures where CH 3COOH, NEt3 and CH3COOH···NEt3 using these materials, although confined, should be restricted adducts are included (distances in Å), and interaction energies (kJ mol -1) between imine structure and the incorporated molecules. to pores close to material surface. This hypothesis is also corroborated by DFT calculations (see below). We also discarded the possibility of significant leaching of molecular interaction between the imine structure of a-3/c-3 and catalytic species (ie. CH3COOH and NEt3) from a-3. (see S. I., CH3COOH, NEt3 and CH3COOH···NEt3 adduct (see S.I., section section 10), except for the case NEt3@a-3 and NEt3@c-3 12). Results are presented in Figure 4, showing the optimized materials. The weakness of the interaction between the amine geometries, selected geometrical parameters and the molecule (Et3N) and the porous material, either crystalline or adsorption energies of these imine structures functionalized amorphous, resulted in complete leaching of catalytic with CH3COOH, NEt3 and CH3COOH···NEt3 adducts. The molecules. interaction between acetic acid and the imine structure is stablished through hydrogen bonding between the acidic DFT analysis. proton of the acetic acid and the imine group of the structure (Figure 4, top). The OH···N distance is 1.66 Å, which is indicative DFT calculations for the imine structure-adsorbed molecules of a strong hydrogen bond. The associated adsorption energy is interactions. -79.4 kJ mol-1. With the aim of further characterizing these materials and On the other hand, the imine of the structure and the supporting the experimental data, DFT calculations with adsorbed amine (NEt3) are both basic and no hydrogen bonding periodic boundary conditions were used to evaluate the is possible between them. In this context, the adsorption takes place only through Van der Waals interactions and the preferred structure presents the NEt3 close to the walls of the imine material. The shortest distances between atoms of NEt 3 and those of the imine structure are smaller than 4 Å (Figure 4, middle). The resulting adsorption energy is -43.6 kJ mol-1. Finally, we studied the interaction of the CH 3COOH···NEt3 mixture with the imine structure, considering a 1 to 1 relationship between CH3COOH and NEt3 (Figure 4, middle). Within these initial structures, two possibilities almost equally raised as the most favorable situations (see Figure 4 and S. I. Figure S21). The first one can be described as an CH 3COOH molecule interacting with the imine structure through hydrogen bonding, while the remaining NEt3 molecule interacts with the imine structure and CH3COOH through van der Waals Figure 5. Energy profile in kJ mol-1 (a) Knoevenagel reaction with 8a and (b) 8g. interactions. The second one implies the formation of a neutral Grey curves correspond to the M06/6-311+G(d,p) values associated with reaction in solution, while values in black correspond to the best estimated values for the CH3COOH···NEt3 aggregate, interacting with the imine structure reactions inside a cavity defined by the imine structure. R stands for separated through van der Waals interactions. Therefore, the mixture reactants, PC for the reacting precomplex, TS for transition state and P for the products of the elementary step considered. CH3COOH + NEt3 in the imine structure shows a combination of intermolecular and molecule-imine structure interactions that enhance the adsorption of the base molecule. orientation. Moreover, although several arrangements of Analysis of Influence of Confinement of the Active Site by DFT the transition state of 8a without steric hindrance from the Calculations. cavity could be envisaged, we could only find two possibilities to settle the transition state associated to 8g, but with severe We next explored the Knoevenagel reaction considering two geometrical constrictions (S. I. Figures S23-29). Therefore, different aldehydes: benzaldehyde (8a) and anthracene-9- diffusion of the reactants and products along the material carbaldehyde (8g) (see S. I. section 12.2 for confinement DFT seems to be much more favourable for 8a/9a than for 8g/9g. calculations for the ring-opening epoxide reaction). The Overall, although the best estimated energies of the transition reaction of 8a inside the cavity defined by the imine structure states inside the cavity are similar to those in solution, led to the formation of 9a with a 71% yield, but 8g did not react computations suggest that the system involving 8g has much inside this cavity. Therefore, these two reactions are ideal less flexibility inside the cavity and it would require a very examples to study the size selectivity. specific orientation, which suggest that this would impose a Considering the homogenous system, molecular high entropic penalization. This is especially relevant calculations in gas phase and solution showed that, regardless considering that the model used does not include neither the the reacting aldehyde, the nucleophilic attack to the carbonyl acetic acid needed to trap the triethylamine nor the solvent. carbon directly leads to the formation of the alcohol product, Therefore, the diffusion of the resulting product 9g would be resulting from the addition of the deprotonated malononitrile challenging if ever formed. All this data together suggest that (see S. I.). In solution, the global process is energetically highly while 8a and 8g should react similarly in solution, the reaction favourable (Figure 5, ΔE = -82.2 and -86.4 kJ mol-1 for 8a and 8g of 8g becomes more difficult inside the cavity. This reinforces respectively). The energy barrier with respect to the pre- the fact that the reaction takes place inside the material and complex are very low in these two cases (less than 26 kJ mol -1). that size selectivity would appear when the reactant is wider It is worth mentioning that the main difficulty to perform the than 9 – 10 Å, such is the case of the three fused aromatic rings. reaction is to approach the three reactants (base, malononitrile and aldehyde). However, after thermal corrections, the global activation Gibbs energies are still consistent with a feasible Conclusions process (ΔG‡ = 96.8 and 87.6 KJ mol -1 for the 8a and 8g respectively, see Figure 5 and S. I., Figures S23-29). This is In this work we successfully carried out the supramolecular consistent with a relatively fast reaction, taking place at room functionalization of crystalline COF-300 and related amorphous temperature. imine polymer nanoparticles with CH3COOH molecules and the Several key features appeared when analysing the reactivity adduct CH3COOH···NEt3. Gas adsorption experiments on of 8a and 8g inside the cavity defined by the imine structure. In crystalline samples corroborated that functionalization by particular, aldehyde 8a is sufficiently small to be inclusion of CH3COOH molecules or CH3COOH···NEt3 adducts accommodated inside the cavity either with the phenyl ring occurs inside the pores of material. Considering the accessibility placed along or perpendicular to the channels of the material. of active centres, we increased the acid/base catalytic activity This is due to the fact that the width of the phenyl ring is less of these materials by using amorphous imine polymer than half of the cavity section. In contrast, aldehyde 8g is nanoparticles. Despite that these nanoparticles are amorphous significantly wider than 8a. Therefore, this imposes that the and non-porous, its functionalization with CH3COOH molecules anthracene aldehyde 8g can only diffuse in the less hindered and CH3COOH···NEt3 adducts allowed to use them as efficient catalysts for acidic ring opening epoxide and basic Knoevenagel reactions, respectively. Moreover, this catalysis exhibited size selectivity in Knoevenagel transformations, suggesting that Xu, A. Nagai and D. Jiang, J. Am. Chem. Soc., 2013, 135, 17310– reactions must take place into cavities of the imine structure 17313. 12 a) M. Dogru, M. Handloser, F. 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Soc., 2018, 140, 6763-6766. 26 All the catalytic experiments were performed during a two weeks period from the synthesis of material, because we observed that storage of the materials for longer periods results in the decrease of catalytic activity, probably due to loss CH3COOH and/or NEt3 volatile molecules. GRAPHICAL ABSTRACT and TEXT (20 words) Non-covalent inclusion of small acid and base molecules in an imine structure based on COF-300 nanoparticles is presented.