ChemComm This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online COMMUNICATION View Journal | View Issue Stepwise assembly of heterometallic, heteroleptic ‘‘triblock Janus-type’’ metal–organic polyhedra† Open Access Article. Published on 13 February 2023. Downloaded on 1/12/2024 11:49:37 AM. Cite this: Chem. Commun., 2023, 59, 3423 Cornelia von Baeckmann,ab Sara Ruiz-Relaño,ab Inhar Imaz,ab Marcel Handke,ab Received 14th December 2022, Judith Juanhuix, c Felipe Gándara, d Arnau Carné-Sanchez *ab and Accepted 13th February 2023 Daniel Maspoch *abe DOI: 10.1039/d2cc06815j rsc.li/chemcomm Increasing the chemical complexity of metal–organic cages (MOCs) Specifically, heteroleptic cages have been assembled through the or polyhedra (MOPs) demands control over the simultaneous orga- use of shape complementary linkers,13,14 asymmetric linkers,15–17 nization of diverse organic linkers and metal ions into discrete and coordination sphere engineering.18 Interestingly, heterometallic caged structures. Herein, we show that a pre-assembled complex cages19 are even rarer than heteroleptic ones. The most common of the archetypical cuboctahedral MOP can be used as a template route to them entails the use of heterotopic ligands, which can bind to replicate such caged structure, one having a ‘‘triblock Janus- two different metal ions according to their different coordination type’’ configuration that is both heterometallic and heteroleptic. preferences.20–22 Alternatively, pre-synthesized hetero-bimetallic clusters can be employed.23 Discrete metal–organic assemblies featuring empty cavities, gen- Herein, we report a new multicomponent assembly strategy that erally known as coordination-/metal–organic cages (MOCs)1,2 or uses a pre-assembled metal–organic fragment of a known caged metal–organic polyhedra (MOPs),3 are receiving great attention structure as a building unit. We envisioned that such a pre- due to their rich structural versatility and potential applications in assembled fragment could template the formation of the parent areas such as molecular separation, catalysis, gas storage and cage when combined with complementary organic linkers and/or biomedical applications.4–7 To date, most of these assemblies have metal ions. We reasoned that, if these complementary organic been constructed from one type of organic linker and one type of linkers and metal ions were different to those comprising the metal ion. This is mainly due to the inherent difficulty of control- metal–organic fragment, then our templated synthesis would enable ling the assembly of multiple ions and linkers into such discrete assembly of heteroleptic and bimetallic versions of the parent caged caged structures.8 However, there is an increasing interest in structure. To validate our hypothesis, we applied our strategy to the developing more complex, multicomponent MOCs that may pre- archetypical cuboctahedral Cu24bdc2424 or Rh24bdc2425 (where bdc sent emerging properties arising from the intra-MOC interactions stands for 1,3-benzenedicarboxylic acid) MOP, aiming to assemble between different adjacent building blocks, such as increased heteroleptic and/or bimetallic cuboctahedral caged structures hav- chemical stability, tandem catalysis, optical or magnetic ing a ‘‘triblock Janus-type’’ configuration. properties.9,10 In this context, multicomponent assembly strategies We began with a retrosynthetic analysis to identify a repe- for the construction of cages with two or more metal ions (known titive, non-interconnected, metal–organic fragment within the as multimetallic cages), or with two or more organic linkers cuboctahedral MOP. We found the Rh2(bdc)4 fragment in the (known as heteroleptic cages), have recently been proposed.11,12 central part (Fig. 1, in magenta) of the cage. It is a Rh(II) paddlewheel complex comprising four bdc linkers, each show- a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, and The ing a free carboxylic acid group. We reasoned that four of these Barcelona Institute of Science and Technology, Campus UAB, Bellaterra, Barcelona pre-synthesized Rh2(bdc)4 complexes should template the for- 08193, Spain. E-mail: arnau.carne@icn2.cat, daniel.maspoch@icn2.cat b mation of the cuboctahedral MOP when connected via 16 Departament de Quı́mica, Facultat de Ciències Universitat Autònoma de Barcelona, Bellaterra 08193, Spain additional metal ions and 8 bdc linkers (Fig. 1; in cyan). This c Alba Synchrotron Light Facility, Cerdanyola del Vallès, Barcelona 08290, Spain assembly involves three different components that ultimately d Materials Science Institute of Madrid (ICMM), Consejo Superior de Investigaciones provide access to cuboctahedral MOP whose central portion Cientı́ficas (CSIC), Calle Sor Juana Inés de la Cruz, 3, Madrid 28049, Spain differs in composition from their two lateral portions, analo- e ICREA, Pg. Lluı́s Companys 23, Barcelona 08010, Spain gously to the well-known, ‘‘triblock Janus-type’’ particles.26 † Electronic supplementary information (ESI) available: Experimental procedures and characterization data. CCDC 2190388, 2190389, 2184149, and 2190390. For Accordingly, the cuboctahedral structure of the resulting MOPs ESI and crystallographic data in CIF or other electronic format see DOI: https:// will be divided in three distinct parts that exhibit two different doi.org/10.1039/d2cc06815j chemistries. This journal is © The Royal Society of Chemistry 2023 Chem. Commun., 2023, 59, 3423–3426 | 3423 View Article Online Communication ChemComm This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 13 February 2023. Downloaded on 1/12/2024 11:49:37 AM. Fig. 1 Schematic illustration of the multicomponent assembly strategy used to synthesize heteroleptic and heterometallic, ‘‘triblock Janus-type’’, MOPs. Having identified the Rh2(bdc)4 fragment, we next pro- ceeded with its synthesis. As this complex contains free car- boxylic acid groups, we used a protection/deprotection strategy for one of the two carboxylic groups in the bdc linker.27 In the first step, the carboxylic acid group was protected using 2-(tri- methylsilyl)ethanol (see synthetic details in the ESI,† and Fig. S5 and S6). This step was essential, as it prevented reaction of the two carboxylic acid groups of each bdc with Rh(II), thus promoting formation of the protected Rh2(bdc)4 and conse- quently, precluding generation of homometallic MOPs or extended structures. The protected Rh2(bdc)4 complex was synthesized by reacting the half-protected bdc linker with Rh(II) acetate in chlorobenzene at 150 1C overnight. The result- ing complex was purified through column chromatography, and its formation was confirmed by nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS) (Fig. S7 and S8, ESI†). Finally, the four carboxylic acid groups of the complex were deprotected by treatment with tetra-n- butylammonium fluoride at room temperature overnight. The formation of Rh2(bdc)4 with the four deprotected carboxylic acid groups was confirmed by single-crystal X-ray diffraction Fig. 2 (a and b) Crystal structures of (a) deprotected Rh2(bdc)4 complex (SCXRD, Fig. 2a), 1H NMR and MS (Fig. S9–S11, ESI†). and (b) HET-MOP-1. (c) 1H NMR spectrum (300 MHz, DMSO-d6) indicating With the complex in hand, we first used it as a building disassembly of the structure into the Rh2(bdc)4 complex (purple) and the block to assemble a three-component, bimetallic cuboctahedral free bdc linker (red). MOP. Based on the expected stoichiometry between the three components in the final caged structure, Rh2(bdc)4 was reacted with 4 mol. eq. of Cu(NO3)2 and 2 mol. eq. of bdc in dimethy- but a discrete molecule. Exposure of the DMF solution in the lacetamide (DMA) at room temperature for 1 week to obtain a presence of 4-tert-butylpyridine to ether vapors let to formation green solid, herein named HET-MOP-1 (where HET stands for of crystals suitable for SCXRD, which revealed formation of the heteroleptic and/or heterometallic). The solid was washed with targeted, bimetallic, cuboctahedral HET-MOP-1, of formula DMA and diethyl ether to remove unreacted reagents and left to [Rh8Cu16(bdc)24] (Fig. 2b). However, in this structure, the posi- dry in open air (yield: 13%). To confirm the MOP structure, we tion of each metal could not be elucidated, due to the high performed N2 and CO2 adsorption of the obtained solid symmetry of the cage, whereby Rh(II) and Cu(II) can occupy the (Fig. S12, ESI†), observing a Brunauer–Emmett–Teller (BET) same crystallographic positions (Fig. S13, ESI†), and where all surface area of 411 m2 g 1 and a maximum uptake at 1 bar paddlewheels are topologically equivalent, corresponding to and 298 K of 1.03 mmol CO2 g 1. Importantly, the solid could the only kind of vertex present in the cuboctahedron.28,29 be redissolved in dimethylformamide (DMF), which suggested Considering this, a satisfactory refinement was obtained when that the product of the reaction is not an extended framework the occupancy of each metal site was fixed to 1/3 for Rh(II) and 2/3 3424 | Chem. Commun., 2023, 59, 3423–3426 This journal is © The Royal Society of Chemistry 2023 View Article Online ChemComm Communication for Cu(II), as expected from the molecular formula of HET-MOP-1. disassembly of the MOP into the deprotected Rh2(bdc)4 Note that the powder-XRD pattern matched the calculated pattern, complex and bdc linkers at a molar ratio of 1 : 2, as expected confirming the purity of the sample (Fig. S14, ESI†). from the molecular formula of HET-MOP-1 (Fig. 2c and Fig. S20 Next, we further confirmed the bimetallic character of and S21, ESI†). This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. the cuboctahedral HET-MOP-1 by matrix-assisted laser desor- We then envisioned incorporating a functionalized bdc ption/ionization-time-of-flight (MALDI-TOF) spectrometry, linker into the self-assembly process to assemble cuboctahedral from which the solubilized HET-MOP-1 showed a peak at MOPs that would be both bimetallic and heteroleptic. With the m/z = 5779.8, which is in very good agreement with the expected use of functionalized bdc linkers, we expected to introduce molecular mass ([M + H+] = 5779.4) (Fig. S15, ESI†). Also, high such functional groups into both side portions of the ‘‘triblock resolution SEM/EDX performed on several single crystals con- Janus-type’’ MOPs. We imagined replacing the complementary Open Access Article. Published on 13 February 2023. Downloaded on 1/12/2024 11:49:37 AM. firmed the presence of both Rh and Cu in the expected bdc with functionalized versions of it, at its 5-position (Fig. 3a). proportion of 1 : 2 (Rh/Cu) (Fig. S16, ESI†). Furthermore, EDX Thus, we decided to run a co-assembly reaction of Cu(II), mapping performed on several single crystals of HET-MOP-1 Rh2(bdc)4 and either 5-hydroxyl-isophthalic acid (OH-bdc) or revealed the homogeneous distribution of both metal ions 5-sulfoisophthalic acid sodium salt (NaSO3-bdc). In a first through the crystal (Fig. S17, ESI†). In addition, UV-Vis analysis assembly, OH-bdc, Cu(NO3)2 and Rh2(bdc)4 were reacted in of the DMF solution of HET-MOP-1 revealed the presence of two DMA at room temperature for 24 hours, after which a green broad bands centered at 588 nm and 746 nm, which were precipitate (hereafter named HET-MOP-2) was obtained, which ascribed to the lmax of both Rh(II)- and Cu(II)-based paddle- was isolated and washed with DMA and ether (yield: 14%). The wheels, respectively (Fig. S18, ESI†).30,31 Finally, to further green solid was solubilized in DMF and exposed to ether demonstrate the integrity of Rh2(bdc)4 throughout the self- vapours to obtain crystals suitable for SCXRD. Next, a second assembly process, HET-MOP-1 was disassembled into its initial assembly reaction was performed under similar conditions components by exposing it to mild acidic conditions at room with respect to the stoichiometry of the different building temperature. The titration of a DMF solution of HET-MOP-1 blocks (85 1C for 48 hours), except that NaSO3-bdc was used with HCl was monitored through UV-Vis (Fig. S19, ESI†). instead of OH-bdc. This led to a similar product, named HET- Among the two types of paddlewheels forming HET-MOP-1, MOP-3 (yield: 25%). Here, single crystals suitable for SCXRD titration revealed that the Cu(II) one could be fully disas- analysis were directly harvested from the solvothermal reaction. sembled, whereas the Rh(II) paddlewheel could not be, as it SCXRD confirmed formation of two cuboctahedral cages was maintained even upon addition of 12 mol. eq. of acid. built up from two types of linkers (bdc and either OH-bdc or This disassembly experiment further confirms the absence of NaSO3-bdc) and two types of metal ions [Cu(II) and Rh(II)] metal exchange between the Rh2(bdc)4 and Cu(II) ions (Fig. 3b and e). Analysis of the crystal structures of HET-MOP- during the synthesis of the HET-MOP-1, which is ascribed to 2 and HET-MOP-3, which followed the same principles as for inertness of the equatorial position of Rh(II)-carboxylate pad- HET-MOP-1, confirmed occupancy values at each metal site of dlewheel structures.10 The 1H-NMR spectrum of the acid- 1/3 for Rh(II) and of 2/3 for Cu(II). Similarly, based on these treated HET-MOP-1 (in DMSO-d6) further confirmed selective symmetric effects, the electron density of all functional groups Fig. 3 (a) Schematic illustration of the formation of HET-MOP-2 and HET-MOP-3. (b) Crystal structure of HET-MOP-2. (c) MALDI-TOF spectrum of HET-MOP-2, with the peak at m/z = 5996.1 highlighted. (d) 1H NMR spectrum (300 MHz, DMSO-d6) confirming the disassembly of HET-MOP-2 into its constituent Rh2(bdc)4 complex (purple) and OH-bdc linker (green). (e) Crystal structure of HET-MOP-3. (f) MALDI-TOF spectrum of HET-MOP-3, with the peak at m/z = 6409.2 highlighted. (g) 1H NMR spectrum (300 MHz, DMSO-d6) confirming the disassembly of HET-MOP-3 into its constituent Rh2(bdc)4 complex (purple) and NaSO3-bdc linker (green). This journal is © The Royal Society of Chemistry 2023 Chem. Commun., 2023, 59, 3423–3426 | 3425 View Article Online Communication ChemComm of the linkers are a sum of hydrogen (for bdc) and OH (for OH- Conflicts of interest bdc) or NaSO3 (for NaSO3-bdc). Here, the molecular formula suggested a ratio of 1/3 R-bdc (R = OH, NaSO3) to 2/3 bdc, which There are no conflicts to declare. were confirmed during the refinement process. Powder-XRD This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. patterns further confirmed the purity of the samples (Fig. S22 and S28, ESI†). We then sought further evidence of formation of Notes and references the above heterometallic and heteroleptic cages, through 1 S. Zarra, D. M. Wood, D. A. Roberts and J. R. Nitschke, Chem. Soc. MALDI-TOF (Fig. 3c and f), which revealed a peak at m/z = Rev., 2015, 44, 419–432. 2 M. Yoshizawa, J. K. Klostermann and M. Fujita, Angew. Chem., Int. 5996.6 (for HET-MOP-2) and m/z = 6409.2 (for HET-MOP-3), which Ed., 2009, 48, 3418–3438. are in agreement with the corresponding postulated formulas: 3 D. J. Tranchemontagne, Z. Ni, M. O’Keeffe and O. M. Yaghi, Angew. Open Access Article. Published on 13 February 2023. Downloaded on 1/12/2024 11:49:37 AM. [Rh8Cu16(bdc)16(OH-bdc)8] ([M + H3O+ + DMF] = 5996.3); and Chem., Int. Ed., 2008, 47, 5136–5147. 4 D. Zhang, T. K. Ronson, Y.-Q. Zou and J. R. Nitschke, Nat. Rev. [Rh8Cu16(bdc)16(NaSO3-bdc)8] ([M + H+] = 6409.6). Consistently, mild Chem., 2021, 5, 168–182. acidic disassembly of both HET-MOP-2 and HET-MOP-3 at room 5 A. J. Gosselin, C. A. Rowland and E. D. Bloch, Chem. Rev., 2020, 120, temperature revealed the expected molar ratio of 1 : 2 (Rh2(bdc)4 8987–9014. 6 S. K. Samanta and L. Isaacs, Coord. Chem. Rev., 2020, 410, 213181. complex/R-bdc) (Fig. 3d and g) (for UV/Vis analysis see Fig. S23 and 7 B. S. Pilgrim and N. R. Champness, ChemPlusChem, 2020, 85, S29, ESI†). Identification of Rh2(bdc)4 in this disassembly process 1842–1856. clearly evidenced preservation of this complex throughout the self- 8 T. Tateishi, M. Yoshimura, S. Tokuda, F. Matsuda, D. Fujita and S. Furukawa, Coord. Chem. Rev., 2022, 467, 214612. assembly process, thus excluding any possible ligand exchange. 9 D. Nam, J. Kim, E. Hwang, J. Nam, H. Jeong, T.-H. Kwon and Acidic disassembly at high temperature (T = 100 1C) enabled full W. Choe, Matter, 2021, 4, 2460–2473. disassembly of both HET-MOP-2 and HET-MOP-3 into their corres- 10 A. Khobotov-Bakishev, C. von Baeckmann, B. Ortı́n-Rubio, L. Hernández-López, A. Cortés-Martı́nez, J. Martı́nez-Esaı́n, ponding free linkers (Fig. S24 and S30, ESI†). 1H-NMR spectra of the F. Gándara, J. Juanhuix, A. E. Platero-Prats, J. Faraudo, A. Carné- resulting solutions revealed molar ratios of 1 : 0.5 for HET-MOP-2 Sánchez and D. Maspoch, J. Am. Chem. Soc., 2022, 144, 15745–15753. (bdc/OH-bdc) and HET-MOP-3(bdc/NaSO3-bdc), which are in good 11 H. Li, Z.-J. Yao, D. Liu and G.-X. Jin, Coord. Chem. Rev., 2015, 293, agreement with the expected values (1 : 0.5). High-resolution SEM/ 139–157. 12 S. Pullen and G. H. Clever, Acc. Chem. Res., 2018, 51, 3052–3064. EDX analysis and EDX mapping of single crystals confirmed the 13 K. Wu, D. Zhang, C. Drechsler, J. J. Holstein and G. H. Clever, Angew. expected Cu : Rh ratio (2/3 Cu and 1/3 Rh) and the homogeneous Chem., Int. Ed., 2021, 60, 6403–6407. distribution of both metals within the same crystal for both HET- 14 S. Saha, I. Regeni and G. H. Clever, Coord. Chem. Rev., 2018, 374, 1–14. MOP-2 and HET-MOP-3 (Fig. S25, S26, S31 and S32, ESI†). Finally, 15 S. Pullen, J. Tessarolo and G. H. Clever, Chem. Sci., 2021, 12, we found that HET-MOP-2 is porous to N2 (BET surface 7269–7293. area = 359 m2 g 1) and CO2 (uptake at 1 bar and 298 K = 16 S. C. Bete and M. Otte, Angew. Chem., Int. Ed., 2021, 60, 18582–18586. 1.2 mmol CO2 g 1), whereas HET-MOP-3 is only porous to CO2 (uptake 17 W. M. Bloch and G. H. Clever, Chem. Commun., 2017, 53, 8506–8516. at 1 bar and 298 K = 1.2 mmol CO2 g 1; Fig. S27 and S33, ESI†). 18 B. Chen, J. J. Holstein, A. Platzek, L. Schneider, K. Wu and In conclusion, we have developed a stepwise synthetic method G. H. Clever, Chem. Sci., 2022, 13, 1829–1834. 19 L. S. Lisboa, D. Preston, C. J. McAdam, L. J. Wright, C. G. Hartinger that employs pre-assembled complexes of a known cage to replicate and J. D. Crowley, Angew. Chem., Int. Ed., 2022, 61, e202201700. it with a multicomponent character. The value of our method stems 20 Y.-Y. Zhang, W.-X. Gao, L. Lin and G.-X. Jin, Coord. Chem. Rev., 2017, from the fact that the pre-assembled Rh2(bdc)4 complex determines 344, 323–344. 21 J.-F. Ayme, J. E. Beves, C. J. Campbell and D. A. Leigh, Angew. Chem., the position of both the additional inorganic and organic building Int. Ed., 2014, 53, 7823–7827. blocks in the final cuboctahedral structure. This enables conver- 22 J. P. Carpenter, T. K. Ronson, F. J. Rizzuto, T. Héliot, P. Grice and gence of four different molecules (two types of metal ions + two J. R. Nitschke, J. Am. Chem. Soc., 2022, 144, 8467–8473. types of organic linkers) into a single, well-defined caged structure. 23 J. M. Teo, C. J. Coghlan, J. D. Evans, E. Tsivion, M. Head-Gordon, C. J. Sumby and C. J. Doonan, Chem. Commun., 2016, 52, 276–279. We are confident that this degree of structural control in discrete 24 M. Eddaoudi, J. Kim, J. B. Wachter, H. K. Chae, M. O’Keeffe and metal–organic assemblies should pave the way to achieving greater O. M. Yaghi, J. Am. Chem. Soc., 2001, 123, 4368–4369. chemical complexity in cages, expanding the catalog of synthetically 25 S. Furukawa, N. Horike, M. Kondo, Y. Hijikata, A. Carné-Sánchez, P. Larpent, N. Louvain, S. Diring, H. Sato, R. Matsuda, R. Kawano accessible cages, and optimizing cages for functional applications and S. Kitagawa, Inorg. Chem., 2016, 55, 10843–10846. such as catalysis and gas storage. 26 A. H. Gröschel, A. Walther, T. I. Löbling, J. Schmelz, A. Hanisch, This work was supported by the Spanish MINECO (project H. Schmalz and A. H. E. Müller, J. Am. Chem. Soc., 2012, 134, 13850–13860. RTI2018-095622-B-I00), the Catalan AGAUR (project 2021 SGR 27 S. Matasunaga, K. Hasada, K. Sugiura, N. Kitamura, Y. Kudo, 00458), the CERCA Program/Generalitat de Catalunya, and the N. Endo and W. Mori, Bull. Chem. Soc. Jpn., 2012, 85, 433–438. MCIN/AEI/10.13039/501100011033. ICN2 is supported by the 28 D. J. Tranchemontagne, Z. Ni, M. O’Keeffe and O. M. Yaghi, Angew. Chem., Int. Ed., 2008, 47, 5136–5147. Severo Ochoa program from the Spanish MINECO (grant SEV- 29 M. O’Keeffe, M. A. Peskov, S. J. Ramsden and O. M. Yaghi, Acc. Chem. 2017-0706). The project that gave rise to these results received Res., 2008, 41, 1782–1789. the support of a fellowship (LCF/BQ/PR20/11770011) from ‘‘la 30 P. Mosae Selvakumar, S. Nadella, J. Sahoo, E. Suresh and Caixa’’ Foundation (ID 100010434). C.v.B. thanks the Austrian P. S. Subramanian, J. Coord. Chem., 2013, 66, 287–299. 31 A. Carné-Sanchez, J. Albalad, T. Grancha, I. Imaz, J. Juanhuix, Science Fund (FWF), Erwin Schrödinger fellowship for support- P. Larpent, S. Furukawa and D. Maspoch, J. Am. Chem. Soc., 2019, ing the project J 4637. 141, 4094–4102. 3426 | Chem. Commun., 2023, 59, 3423–3426 This journal is © The Royal Society of Chemistry 2023