Angewandte Communications Chemie International Edition: DOI: 10.1002/anie.201901668 Microporous Materials German Edition: DOI: 10.1002/ange.201901668 A Coordinative Solubilizer Method to Fabricate Soft Porous Materials from Insoluble Metal–Organic Polyhedra** Arnau Carn-Snchez,* Gavin A. Craig, Patrick Larpent, Vincent Guillerm, Kenji Urayama, Daniel Maspoch, and Shuhei Furukawa* Abstract: Porous molecular cages have a characteristic proc- this can make it difficult to engineer the size of the pores. One essability arising from their solubility, which allows their possible approach to control pore size is to use molecular incorporation into porous materials. Attaining solubility often cages such as porous organic cages or metal–organic poly- requires covalently bound functional groups that are unneces- hedra (MOPs) as pre-synthesized building blocks that will sary for porosity and which ultimately occupy free volume in retain their known porous structure even in the amorphous the materials, decreasing their surface areas. Here, a method is state.[2] The processability of molecular cages depends on described that takes advantage of the coordination bonds in being able to manipulate them in the solution phase. To metal–organic polyhedra (MOPs) to render insoluble MOPs ensure solubility, and avoid uncontrolled aggregation into soluble by reversibly attaching an alkyl-functionalized ligand. insoluble phases, the cages are often covalently functionalized We then use the newly soluble MOPs as monomers for with bulky side groups.[3] The cost of this approach is that supramolecular polymerization reactions, obtaining perma- these covalently bound moieties can be detrimental to the gas nently porous, amorphous polymers with the shape of colloids uptake of the materials.[4] and gels, which display increased gas uptake in comparison Herein, we show how unfunctionalized, insoluble MOPs with materials made with covalently functionalized MOPs. can be incorporated into porous, amorphous polymers (Figure 1). We build on our recent study of polymeric Amorphous, porous materials are attractive for materials processing because their principal functionality, the pore, does not depend on order in the network.[1] At the same time, [*] Dr. A. Carn-Snchez, Dr. G. A. Craig, Dr. P. Larpent, Prof. S. Furukawa Institute for Integrated Cell-Material Sciences (WPI-iCeMS) Kyoto University Yoshida, Sakyo-ku, Kyoto 606-8501 (Japan) E-mail: Shuhei.furukawa@icems.kyoto-u.ac.jp Dr. A. Carn-Snchez, Dr. V. Guillerm, Prof. D. Maspoch Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC The Barcelona Institute of Science and Technology Campus UAB, Bellaterra, 08193 Barcelona (Spain) E-mail: arnau.carne@icn2.cat Prof. K. Urayama Department of Macromolecular Science and Engineering Figure 1. The strategy used in this paper to solubilize MOPs. Assem- Kyoto Institute of Technology blies of insoluble MOPs are made soluble by the attachment of Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan) a monodentate, coordinative solubilizer. These monodentate ligands Prof. D. Maspoch can then be replaced with bidentate linkers, driving the assembly of ICREA the MOPs into porous polymers. Pg. Llus Companys 23, 08010 Barcelona (Spain) Prof. S. Furukawa Department of Synthetic Chemistry and Biological Chemistry, MOPs (polyMOPs) with permanent microporosity synthe- Graduate School of Engineering, Kyoto University sized through coordination-driven supramolecular polymeri- Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan) zation,[5] in which the soluble MOP [Rh2(C12-bdc)2]12 [**] A previous version of this manuscript has been deposited on (C12RhMOP; C12-bdc2 = 5-dodecoxybenzene-1,3-dicarboxy- a preprint server (https://doi.org/10.26434/chemrxiv.7460618.v1). late)[6] was used as an intrinsically porous monomer. There, Supporting information and the ORCID identification number(s) for the dodecoxy chains that made the MOPs soluble occupied the author(s) of this article can be found under: free volume in the final assembly, hindering gas diffusion, and https://doi.org/10.1002/anie.201901668. lowering the capacity of the polyMOPs. In this study, we  2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. attach a molecule at the periphery of unfunctionalized, KGaA. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, insoluble MOPs through labile coordination bonds: a coor- distribution and reproduction in any medium, provided the original dinative solubilizer. In the subsequent polymerization reac- work is properly cited and is not used for commercial purposes. tion, this molecule detaches and is replaced by a bis-mono- Angew. Chem. Int. Ed. 2019, 58, 6347 –6350  2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 6347 Angewandte Communications Chemie dentate linker. The obtained polymers show superior gas uptake compared to the covalently functionalized polymers. First, we chose the cuboctahedral MOP [Rh2(bdc)2]12 (HRhMOP; bdc2 = benzene-1,3-dicarboxylate; Supporting Information, Figure S1) because the complex is microporous, presenting one of the highest BET surface areas[7] reported for MOPs (SBET = 915 m2 g1).[8] However, HRhMOP is insoluble in organic solvents, impeding its use as a porous monomer. Thus, we exploited the reactivity of the axial site of the dirhodium paddlewheel to coordinate the alkyl-imidazole ligand 1-dodecyl-1H-imidazole (diz) as a coordinative solu- bilizer to the external axial site.[9] First, crystals of HRhMOP were dispersed in dichloromethane (DCM), before addition of 15 mol equiv (with respect to HRhMOP) of diz dissolved in DCM. Immediately, the green suspension became a purple solution, suggesting the coordination of diz to HRhMOP (Figure 2). The solvent was removed in vacuo, and the solid Figure 3. a) In black, the size evolution of the colloids observed during Figure 2. Representation of the molecular structure of HRhMOP, with the titration of [HRhMOP(diz)12] with bix, as determined by DLS hydrogen atoms and coordinated solvent molecules omitted for clarity. measurements (0.23 mm in DMF). Error bars correspond to the Photographs of a suspension of HRhMOP in DCM, and the solution standard deviation of three repetitions. In gray, the decrease in the free of [HRhMOP(diz)12] obtained following addition of the coordinative HRhMOP molecules in solution as the particles of CPP grow, solubilizer diz. determined from the supernatant of the suspensions. b) Representa- tive dark field STEM image of the colloids of CPP obtained after the residue washed to remove excess diz. The obtained solid was titration; scale bar: 200 nm. c) In red, evolution of the cross-linking soluble in common organic solvents such as tetrahydrofuran, degree (CLD) during particle growth. In blue, the degree of substitu- tion of diz by bix. dimethylformamide (DMF), DCM, and chloroform, allowing its characterization with spectroscopic techniques. Dynamic light scattering (DLS) analysis showed the presence of of coordination polymer particles (CPP) with an average size isolated molecules of an average size of 3.3  0.5 nm. UV/ of 40  7 nm (Figure 3 b). The infrared (IR) spectrum of CPP Vis spectroscopy revealed that the maximum (lmax) of the confirmed the presence of HRhMOP units and bix linkers, band associated to the dirhodium paddlewheel was centered and also showed a decrease but not a total disappearance of at 560 nm, corresponding to the coordination of one diz ligand the bands arising from diz (Supporting Information, Fig- per paddlewheel (Supporting Information, Figure S2). NMR ure S5); that is, diz was only partially replaced by bix during spectroscopy showed the coordination of diz to HRhMOP in polymerization. This was further evidenced by NMR analysis solution and was used to determine the molecular formula of of the acid-digested particles, which displayed peaks from the functionalized HRhMOP as [Rh2(bdc)2(diz)]12 both bix and diz. The relative integrations for the linkers ([HRhMOP(diz)12]; Supporting Information, Figure S3). indicated the composition of CPP to be [HRhMOP(bix)8- We tested the reactivity of this soluble, isolated MOP (diz)2.7] (Supporting Information, Figure S6). Because there towards the imidazole-based ditopic ligand bix (1,4-bis(imi- are 12 paddlewheels in HRhMOP and each paddlewheel only dazol-1-ylmethyl)benzene). A 0.23 mm DMF solution of coordinates one N-donor ligand on the exohedral site, it [HRhMOP(diz)12] was titrated with bix until 12 mol equiv follows that not all of the bix molecules in CPP act as had been added. While UV/Vis spectroscopy showed no bidentate ligands, as this would limit the maximum number of change in lmax (Supporting Information, Figure S4), DLS bix ligands to 6. We define the cross-linking degree (CLD) as measurements revealed a self-assembly process: upon addi- the fraction of bix molecules acting as bidentate ligands tion of 12 mol equiv of bix, the particle size grew from 3.3  [Eq. (1)]. Therefore, the CLD of CPP is 16 %. 0.5 nm to 44.6  1.6 nm (Figure 3 a). The constant value of lmax throughout self-assembly suggests that any ligand bidentate bix 12dizbix CLD ¼ ¼  100 ð1Þ exchange reaction occurs concertedly, with the number of total bix bix axial sites occupied by imidazole ligands remaining constant at 12 during the process. The colloidal suspension was These characterization data are consistent with a self- centrifuged to isolate an amorphous purple solid consisting assembly mechanism entailing ligand exchange between diz 6348 www.angewandte.org  2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2019, 58, 6347 –6350 Angewandte Communications Chemie and bix. However, the growth profile observed by DLS measurements suggests a complex pathway, whereby a thresh- old quantity of bix is required to begin particle growth. Above 5 mol equiv of bix, particle growth stops, as beyond this point most of the free HRhMOP molecules in solution have been incorporated into the polymers (Figure 3 a). Although con- tinued addition of bix beyond 5 mol equiv leads to substitu- tion of diz, the bix molecules act as monodentate ligands, decreasing the CLD (Figure 3 c). This mechanism was found to be independent of the concentration at which polymeri- zation was performed; higher concentrations simply led to larger colloids (average size 247  70 nm; Supporting Infor- mation, Figure S7). More details of the mechanism underlying the self-assembly are provided in the Supporting Information. As found previously for C12RhMOP, the self-assembly mechanism could be controlled by changing how bix was added.[5] Thus, 12 mol equiv of bix were added in one portion to a 0.93 mm DMF solution of [HRhMOP(diz)12]. In the case of C12RhMOP, this yielded a kinetically trapped, isolated molecule with 12 molecules of bix coordinated to the MOP in a monodentate fashion [C12RhMOP(bix)12]. Here the situa- tion is different because the axial sites to be occupied by bix are already filled by diz. The kinetically trapped molecule formed from concerted addition of bix to the solution of [HRhMOP(diz)12] was obtained by precipitation with diethyl ether, with analysis revealing the composition of these molecules to be [HRhMOP(bix)9.6(diz)2.4] (Supporting Infor- Figure 4. a) Photograph of the supramolecular colloidal gel SCG. mation, Figures S8 and S9). Heating the DMF solution of b) Storage and loss moduli of SCG as a function of angular frequency. c) Photograph of the supramolecular aerogel SAG obtained after these molecules at 80 8C led to the formation of a transparent drying SCG with supercritical CO2. d) Representative FESEM image of gel within a few minutes (SCG: supramolecular colloidal gel), SAG; scale bar: 200 nm. with gelation also occurring at room temperature, albeit at a slower rate (Figure 4). Gel-like behavior was confirmed by rheological measurements, which determined a frequency- independent storage modulus E’ of about 4 kPa, which is one order of magnitude higher than the loss modulus. The composition of SCG was found to be [HRhMOP(bix)9.5- (diz)1.3], corresponding to a CLD of 12 % (Supporting Information, Figure S10). The decrease in diz content upon assembly of the gel while the bix content is retained demonstrates that the self-assembly process is driven by the reactivity of the monodentate bix molecules replacing diz to crosslink the MOPs. Through solvent exchange with acetone followed by supercritical CO2 drying, the gel was converted to a supramolecular aerogel (SAG) consisting of fused nano- particles with an average size of 29.8 nm (Figure 4; Support- Figure 5. a) Ar adsorption isotherm at 87 K for HRhMOP (black), CPP ing Information, Figure S11). (gray), and SAG (blue). b) Comparison of the N2 uptake at 77 K for the The aim of these assembly strategies was to demonstrate supramolecular aerogels containing HRhMOP and the previously that an otherwise insoluble MOP could be incorporated into reported C12RhMOP. Closed and open symbols correspond to the amorphous, supramolecular polymers with permanent micro- adsorption and desorption branches, respectively. porosity. For these types of polymers, it can be difficult to distinguish between the contributions of intrinsic porosity and SAG all show sharp uptake of Ar at low pressures, (arising from the cavity of the MOP) and extrinsic porosity characteristic of microporous materials (Figure 5; for volu- (arising from the free volume between MOPs or particles). To metric data see the Supporting Information, Figure S12). The determine the impact of the intrinsic microporosity of pore size distribution (PSD) demonstrates that all of the HRhMOP on the overall porosity of CPP and SAG, we samples have pores in the microporous range, in good performed Ar adsorption measurements at 87 K. The low agreement with the dimensions of the cuboctahedral cage dipole moment of Ar prevents any chemical interaction (Supporting Information, Figure S13). The shaping process between the cavity walls and the gas, providing higher quality and polymer morphology is important: while for CPP 54 % of physisorption measurements.[10] At 87 K, HRhMOP, CPP, the porosity arises from the micropores, SAG presents a much Angew. Chem. Int. Ed. 2019, 58, 6347 –6350  2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org 6349 Angewandte Communications Chemie wider PSD, where the microporosity accounts for 21 % of its X. Jiang, F. Su, A. Trewin, C. D. Wood, N. L. Campbell, H. Niu, porosity. Furthermore, SAG displays interstitial voids C. Dickinson, A. Y. Ganin, M. J. Rosseinsky, Y. Z. Khimyak, between the spheres, as shown by increased gas uptake in A. I. Cooper, Angew. Chem. Int. Ed. 2007, 46, 8574; Angew. Chem. 2007, 119, 8728; c) Y. Zhao, S.-Y. Lee, N. Becknell, O. M. the higher-pressure region. Comparison of the N2 sorption Yaghi, C. A. Angell, J. Am. Chem. 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Commun. 2016, 52, 8850; i) G. Lal, M. Acknowledgements Derakhshandeh, F. Akhtar, D. M. Spasyuk, J.-B. Lin, M. Trifkovic, G. K. H. Shimizu, J. Am. Chem. Soc. 2019, 141, 1045. A.C.-S., G.A.C., and P.L. thank the Japan Society for the [4] a) M. W. Schneider, I. M. Oppel, H. Ott, L. G. Lechner, H.-J. S. Promotion of Science (JSPS) for Post-Doctoral Fellowships. Hauswald, R. Stoll, M. Mastalerz, Chem. Eur. J. 2012, 18, 836; This work was supported by JSPS KAKENHI grant number b) E. V. Perez, K. J. Balkus, J. P. Ferraris, I. H. Musselman, J. 18H01995 (Kiban B) and 17H05367 (Coordination Asymme- Membr. Sci. 2014, 463, 82; c) T.-H. Chen, L. Wang, J. V. try). This work was also supported by the Spanish MINECO Trueblood, V. H. Grassian, S. M. Cohen, J. Am. Chem. Soc. 2016, 138, 9646; d) P. S. Reiss, M. A. Little, V. Santolini, S. Y. (projects PN MAT2015-65354-C2-1-R), the Catalan AGAUR Chong, T. Hasell, K. E. Jelfs, M. E. Briggs, A. I. Cooper, Chem. (project 2014 SGR 80), and the ERC under the EU-FP7 Eur. J. 2016, 22, 16547. (ERC-Co 615954). It was also funded by the CERCA [5] A. Carn-Snchez, G. A. Craig, P. Larpent, T. Hirose, M. Program/Generalitat de Catalunya. iCeMS is supported by Higuchi, S. Kitagawa, K. Matsuda, K. Urayama, S. Furukawa, the World Premier International Research Initiative (WPI), Nat. Commun. 2018, 9, 2506. MEXT, Japan. ICN2 is supported by the Severo Ochoa [6] R. Kawano, N. Horike, Y. Hijikata, M. Kondo, A. Carn- program from the Spanish MINECO (Grant No. SEV-2017- Snchez, P. Larpent, S. Ikemura, T. Osaki, K. Kamiya, S. Kitagawa, S. Takeuchi, S. Furukawa, Chem 2017, 2, 393. 0706). A.C.S. also thanks the Spanish MINECO for a Juan de [7] C. A. Rowland, G. R. Lorzing, E. J. Gosselin, B. A. Trump, la Cierva fellowship (IJCI-2016-29802 and FJCI-2017-31598). G. P. A. Yap, C. M. Brown, E. D. Bloch, J. Am. Chem. Soc. 2018, 140, 11153. [8] S. Furukawa, N. Horike, M. Kondo, Y. Hijikata, A. Carn- Conflict of interest Snchez, P. Larpent, N. Louvain, S. Diring, H. Sato, R. Matsuda, R. Kawano, S. Kitagawa, Inorg. Chem. 2016, 55, 10843. [9] A. Carn-Snchez, J. Albalad, T. Grancha, I. Imaz, J. Juanhuix, P. The authors declare no conflict of interest. Larpent, S. Furukawa, D. Maspoch, J. Am. Chem. Soc. 2019, 141, 4094. Keywords: coordination cages · metal–organic polyhedra · [10] K. A. Cychosz, R. Guillet-Nicolas, J. Garca-Martnez, M. microporous materials · polymers · supramolecular chemistry Thommes, Chem. Soc. Rev. 2017, 46, 389. How to cite: Angew. Chem. Int. Ed. 2019, 58, 6347 – 6350 Angew. Chem. 2019, 131, 6413 – 6416 Manuscript received: February 7, 2019 [1] a) P. M. Budd, B. S. Ghanem, S. Makhseed, N. B. McKeown, Accepted manuscript online: March 7, 2019 K. J. Msayib, C. E. Tattershall, Chem. Commun. 2004, 230; b) J.- Version of record online: April 2, 2019 6350 www.angewandte.org  2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2019, 58, 6347 –6350