A Cobalt Supramolecular Triple-Stranded Helicate-based Discrete Molecular Cage

We report a strategy to achieve a discrete cage molecule featuring a high level of structural hierarchy through a multiple-assembly process. A cobalt (Co) supramolecular triple-stranded helicate (Co-TSH)-based discrete molecular cage (1) is successfully synthesized and fully characterized. The solid-state structure of 1 shows that it is composed of six triple-stranded helicates interconnected by four linking cobalt species. This is an unusual example of a highly symmetric cage architecture resulting from the coordination-driven assembly of metallosupramolecular modules. The molecular cage 1 shows much higher CO2 uptake properties and selectivity compared with the separate supramolecular modules (Co-TSH, complex 2) and other molecular platforms.

A more advanced assembly mode can be used to produce discrete molecular platforms with a structural hierarchy that has previously been unattained in the field of metallosupramolecular chemistry. This assembly mode, corresponding to "tertiary" assembly, as shown in Fig. 1, is concerned with the use of predefined, well-organized, and secondarily-assembled metallosupramolecular modules, permitting access to systematic molecular architectures. In other words, such molecules can be built from three distinct levels of structural hierarchy. However, it should be emphasized that tertiary assemblies are discrete rather than infinite, highlighting their difference from polymeric frameworks [18][19][20][21][22][23]37 . Clear-cut innovations of tertiary assembly over the corresponding primary and secondary assemblies can be addressed as follows: first, structurally well-organized, highly-ordered metallosupramolecular modules acting as the basic platforms can provide diverse assembly fashions. This enables the construction of hierarchical discrete platforms possessing target-oriented properties that can rarely be obtained in individual units. Second, the resultant molecules would have the maximum expression of unique functional sites in specific geometrical arrangements through rational supramolecular module design and appropriate self-assembly considerations.
We report a discrete molecular platform with a high level of structural hierarchy through the coordination-driven assembly of supramolecular modules. A cobalt supramolecular triple-stranded helicate (Co-TSH)-based discrete high-order molecular cage is successfully synthesized. PDA ligand is chosen due to its considerable importance in the formation of tetranuclear cobalt cluster as a primary assembly (PDA = 2,6-pyridi nedicarboxylate) 35 . Two tetranuclear clusters with tbu-PTA generate a Co-TSH as a conceptual secondary assembly (tbu-PTA = 5-tert-butyl isophthalate). The six well-organized Co-TSHs generated in-situ are supramolecular modules; their assembly leads to the formation of a discrete molecular cage. Moreover, the synthesized molecular  temperatures 4 K ≤ T ≤ 300 K with a 500 Oe applied fields ( Supplementary Fig. S6A). The magnetic behavior of 1 is described by Curie-Weiss law [38][39][40][41] ; and the corresponding fitting (1/χ vs T) ( Supplementary Fig. S6B) yields a value of θ = − 9.92 K from the intercept, which suggests the antiferromagnetic interaction between cobalt ions 40,42,43 , and C = 0.0103 emu K g −1 from the slope (1/C). The measurement χ M T at 300 K is ca. 2.86 (emu K mol −1 ) ( Supplementary Fig. S6C). This value, while higher than the estimated spin-only value of 1.88 (emu K mol −1 ) for S = 3/2, still falls within an acceptable range when compared to other experimentally observed high-spin octahedral Co (II) ions with an orbital angular momentum contribution [42][43][44][45][46] . Upon cooling, χ M T continuously decreases to a value of 1.73 (emu K mol −1 ) at 6.4 K. The chemical states of the Co species in 1 are also investigated by X-ray photoelectron spectroscopy (XPS) (Supplementary Fig. S7). An intense and characteristic satellite at ca. 786 eV (no other satellites appearing in the area of over 790 eV from the Co 2p 1/2 and Co2p 3/2 spectra) indicates that all cobalts show 2+ states 47 . This result is further confirmed by calculating the bond valence sums using the observed bond distances in the crystal structure data (Supplementary Table S3) 48,49 . The phase purity of the as-synthesized 1 is confirmed using powder X-ray diffraction (PXRD) (Supplementary Fig. S8). The data show considerable similarities between the experimental and simulated PXRD patterns. The crystalline stability is proved to be retained below 120 °C by variable temperature PXRD experiments (Supplementary Figs S9 and S10). The thermogravimetric analysis (TGA) ( Supplementary Fig. S11) of 1 shows minor weight loss below 350 °C, which is attributed to the removal of coordinated DMF and H 2 O molecules. Above 350 °C, the complete decomposition of 1 occurred.
It is interesting to note that the molecular cage 1 is considered a result of the assembly of Co-TSHs, as shown in Fig. 1. Although similar concepts of higher-order discrete platforms based on supramolecular assemblies have been previously demonstrated 50,51 , they are mainly based on noncoordinative interactions (e.g. van der Waals, electrostatic, π -π interaction, or hydrogen bonding). Notably, all the assemblies that form 1 are coordination-driven. It is well established that coordination-driven motifs not only affect the stability, but also often provide unique design features in the assemblies because of the higher directionality offered by metal-ligand coordinative bonding 36 . To the best of our knowledge, and based on our conceptual assembly model (Fig. 1), the molecular cage 1 is the first example of a discrete cage architecture exhibiting an unprecedentedly higher order of hierarchy resulting from the metal-directed tertiary assembly of preassembled secondary metallosupramolecules.
The discrete cobalt triple-stranded helicate (Co-TSHs) is successfully prepared by separate experiments. Treatment of 2 equiv. of Co(NO 3 ) 2 ·6H 2 O, 1 equiv. of H 2 PDA, and 1 equiv. of H 2 tbu-PTA in DMF at 120 °C for 36 h affords {Co 8 (PDA) 6 (tbu-PTA) 3 (DMF) 6 } (2). Complex 2 is isolated as purple rhombic crystals. The solid-state structure of 2 is determined by SXRD, and solved and refined as the space group of P2 1 /n ( Supplementary Fig. S13 and Supplementary Table S2). The solid-state structure of 2 shows that two distinct conformations, left-and right-handed, exist simultaneously in a single unit cell. Apart from the orientation difference of interconnecting tbu-PTA ligands, both conformations are geometrically similar to each other ( Supplementary Fig. S13). It should be noted that the left-handed conformation is topologically similar to that reported previously 35 , whereas the right-handed conformation shows a structure similar to that of a basic supramolecular module of 1. Variable-temperature (4-300 K) magnetic measurement of complex 2 is conducted under an applied field of 500 Oe (Supplementary Fig. S14A). Curie-Weiss fitting (1/χ versus T) yields a value of θ = − 23.85 K and C = 0.0086 emu K g −1 (Supplementary Fig. S14B) and the value χ M T at 300 K is found to be 2.59 (emu K mol −1 ) which could be assigned to high-spin octahedral Co(II) ions ( Supplementary Fig. S14C) [42][43][44][45][46] . The existence of Co(II) in complex 2 is also confirmed by XPS (Supplementary Fig. S15) and calculation of the bond valence sums using the observed bond distances in the crystal structure data (Supplementary Table S4). The phase purity of 2 is confirmed by a good match between the experimental and simulated PXRD patterns ( Supplementary Fig. S16). The TGA (Supplementary Fig. S17) also indicates that the removal of coordinated DMF and H 2 O in 2 occurred below 350 °C.
The transformation of 2 to 1 is confirmed by XRD experiments. In the treatment of 2 with excess Co(NO 3 ) 2 ·6H 2 O in DMF at 50 °C, slow generation of rectangular crystals is observed (2 generally shows rhombic crystals, Supplementary Fig. S19). The SXRD and PXRD analyses confirm that the newly-generated crystals in the reaction mixture are 1 (Supplementary Fig. S20). The assembly processes to give the discrete molecular cage can be achieved by both direct and step-wise approach.
Efficient CO 2 capture and separation play a vital role in both environmental and industrial applications. In particular, a high selectivity of CO 2 over other components of gas mixtures is essential [52][53][54] . The CO 2 capture performances and selectivity over other gases are tested (e.g. N 2 and CH 4 ) at room temperature and ambient pressure. Prior to the gas adsorption experiments, the solvent molecules of 1 are removed by successive acetone solvent exchanges and heating at 60 °C under a vacuum for 24 h. After the activation step, the crystallinity of 1 is still maintained, as confirmed by the PXRD analysis ( Supplementary Fig. S8). The gas adsorption measurements at 196 K (for CO 2 and CH 4 ) and 77 K (for N 2 ) (Fig. 4A) show significant uptakes of CO 2 (ca. 132 cm 3 g −1 ) for 1, almost excluding N 2 and CH 4 (ca. 6.1 and 11.9 cm 3 g −1 , respectively). Moreover, as shown in Fig. 4B and C, 1 has a high CO 2 uptake (83 and 61 cm 3 g −1 ), but slight CH 4 (8.3 and 5.1 cm 3 g −1 ) and N 2 (3.7 and 2.0 cm 3 g −1 ) adsorption capacities at 273 and 298 K, respectively (at 1 atm). High selective CO 2 adsorption over other gases can be attributed to Co sites, particularly Co5, resulting from the desolvation step, which could induce better interactions with a higher quadrupole moment and polarizability of CO 2 compared with CH 4 and N 2 52,55-57 . The CO 2 isosteric heat of adsorption (Q st ) of 1 (Fig. 4D), calculated by fitting the 273 and 298 K isotherms to the virial-type expression 41 , is found to be ca. 24.1 kJ mol −1 at a low loading. This Q st value falls within the range of most materials with high CO 2 adsorption capacity caused by exposed metal sites 52 . A variety of materials with high affinity towards CO 2 caused by favored interactions with exposed metal sites have also been reported 52,58,59 . Given that CO 2 molecule has a lower kinetic diameter than those of CH 4 and N 2 52,55-57 , the confined cage structure of 1 could preferentially entrap CO 2 , thus improving the CO 2 selectivity over others. The CO 2 adsorptions of 2 exhibit much lower uptakes of 17.7, 9.7, and 7.1 cm 3 g −1 at 196, 273, and 298 K at 1 atm, respectively ( Fig. 4A-C). This remarkable difference in adsorption suggests that confined cavities of 1 significantly enhance the CO 2 adsorption performance of 1. Complex 2 also shows little adsorption towards CH 4 and N 2 ( Supplementary Figs S21-S23). To the best of our knowledge, the CO 2 uptake capacity of 1 at 298 K and 1 atm is among the highest values for discrete molecular platforms constructed from metal ions/clusters with organic ligands (Supplementary Table S5) 9-11,60-68 .

Conclusions
In conclusion, a facile strategy is developed to achieve a discrete molecular platform through the assembly of well-organized supramolecules. A novel cobalt supramolecular triple-stranded helicate (Co-TSH)-based molecular platform, (1), is successfully synthesized. Complex 1 is a structurally well-defined, highly systematic, and discrete cage architecture resulting from the coordination-driven assembly of in-situ-generated supramolecular modules. The right-and left-handed Co-TSHs, (2), structurally analogous to the Co-TSHs of 1, are also successfully isolated from a separate reaction. The molecular cage 1 shows much higher CO 2 capture capacity and selectivity compared with 2 and other single molecules, including cage complexes. The CO 2 uptake capacity of 1 at 298 K and 1 atm is among the highest values for discrete molecular platforms constructed from metal ions/ clusters. and DMF (6 mL) are mixed in a 20 mL vial at room temperature. The vial is sealed tightly and heated to 50 °C, and then the reaction mixture is maintained for 3 days, and cooled down to room temperature. The generation of purple rectangular crystals begins to be observable after 4 hours, and more formed as the reaction proceeded. Purple rectangular crystals are collected and analyzed through the single crystal X-ray diffraction (SXRD) and powder X-ray diffraction (PXRD) methods.

Synthesis of {[Co
Single crystal X-ray diffraction. The diffraction data were collected at 100 K on a ADSC Quantum 210 Gas adsorption measurements. Gas adsorption isotherms were obtained using BELSORP-mini II (BEL Japan, Inc.). The gases used throughout adsorption experiments were highly pure (99.999%). Prior to the adsorption experiments, all the samples were activated as follows: First, the as-synthesized sample was thoroughly rinsed with DMF (3 × 10 mL) and immersed in 10 mL acetone for 24 h for solvent exchange; the acetone was decanted and replenished with fresh solvent. This procedure was repeated three times. Finally, the sample was dried under vacuum at 60 °C for 24 h prior to the gas sorption measurements.