Kaixiu
Li
a,
Zhengguang
Li
a,
Jie
Yuan
c,
Mingzhao
Chen
b,
He
Zhao
a,
Zhiyuan
Jiang
a,
Jun
Wang
b,
Zhilong
Jiang
b,
Yiming
Li
a,
Yi-Tsu
Chan
d,
Pingshan
Wang
*ab and
Die
Liu
*a
aDepartment of Organic and Polymer Chemistry, Hunan Key Laboratory of Micro & Nano Materials Interface Science, College of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan-410083, China
bDepartment Institute of Environmental Research at Greater Bay Area, Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Guangzhou Key Laboratory for Clean Energy and Materials, Guangzhou University, Guangzhou-510006, China
cSchool of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, China
dDepartment of Chemistry, National Taiwan University, Taipei 10617, Taiwan
First published on 8th May 2024
The construction and application of metal–organic cages with accessible internal cavities have witnessed rapid development, however, the precise synthesis of complex metal–organic capsules with multiple cavities and achievement of multi-guest encapsulation, and further in-depth comprehension of host–multi-guest recognition remain a great challenge. Just like building LEGO blocks, herein, we have constructed a series of high-order layered metal–organic architectures of generation n (n = 1/2/3/4 is also the number of cavities) by multi-component coordination-driven self-assembly using porphyrin-containing tetrapodal ligands (like plates), multiple parallel-podal ligands (like clamps) and metal ions (like nodes). Importantly, these high-order assembled structures possessed different numbers of rigid and separate cavities formed by overlapped porphyrin planes with specific gaps. The host–guest experiments and convincing characterization proved that these capsules G2–G4 could serve as host structures to achieve multi-guest recognition and unprecedentedly encapsulate up to four C60 molecules. More interestingly, these capsules revealed negative cooperation behavior in the process of multi-guest recognition, which provides a new platform to further study complicated host–multi-guest interaction in the field of supramolecular chemistry.
To achieve structural complexity and further diverse functionalities, the coordination-driven supramolecular cages may be the most suitable candidate due to the significant cavity, which can be exploited in diverse fields, such as biomedicine,28 separation,29 catalysis,30,31 and so on.32 However, most of the efforts have been focused on the metal–organic polyhedron featuring one single cavity.33,34 Therefore, the construction methods of metal–organic capsules with multiple cavities, namely multiple separate cavities in a single entity, are highly desirable.35,36 Three strategies are available to create multiple cavities in metallo-supramolecular architectures: interpenetrating architectures to increase the number of cavities,37–39 vertical extension of single cavity systems40–45 and expanding the structure on a two-dimensional plane.46,47 For example, Crowley et al. reported the first example of a [Pd4(L)4]8+ cage with three cavities using the long backboned multi-pyridine ligands.43 Soon after, Clever's group synthesized a peanut-shaped cage by assembling a tris-monodentate ligand with PdII cations, followed by quantitative catenation to give a five-cavity-containing compound.37 However, the previously reported multi-cavity supramolecular capsules can only recognize multiple anions or small molecules due to a small cavity volume.43–45 By comparison, those capsules being able to encapsulate large guests have rarely been documented due to the difficulties in synthesis and the assembly strategy.42 Moreover, the reports of cooperative behaviors of the homotropic binding or heterotropic binding guests in multi-cavity capsules remain elusive in spite of the fact that such behavior has been intensively investigated in a single cavity.36 Therefore, constructing capsules with multiple large cavities and further researching the mutual influence between the host and binding large guests is meaningful, yet especially challenging.
Encouraged by previous excellent and appealing studies on multi-cavity cages, herein, a series of layered metal–organic capsules with large cavities varying from one to four pockets have been synthesized via the multicomponent self-assembly of terpyridine ligands and Cd2+. Furthermore, the large guest fullerene (C60) can be homogeneously bound by these capsules. Particularly noteworthy are the discrete single-molecular capsules that can wrap up to four C60 molecules (Fig. 1). More interestingly, based on the results of experimental and theoretical calculations, the negative cooperation effect between binding C60 has been also revealed.
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Fig. 1 Cartoon representation of a series of multi-cavity cages and their C60 complexes (the darker colored ball represents a more prioritized binding). |
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Scheme 1 Chemical structure of ligands L1 and L5, and schematic illustration of the self-assembly and host–guest recognition to C60 of G1. |
The assembly of G1 was carried out by mixing L1, L5 and Cd(NO3)2·4H2O at a precise stoichiometric ratio of 2:
1
:
4 in MeOH/CHCl3 (1
:
1) and heating at 70 °C for 8 h (Scheme 1). After cooling to room temperature, an excessive CH3OH solution of NH4PF6 was added to exchange the anion NO3− to PF6−, giving a deep purple precipitate. Based on the comparison of 1H NMR spectra of L1, L5 and supramolecular G1, the tpy-H3′ of ligand L1 shifted to the low-field (δ = 9.09 ppm, Δδ = 0.43 ppm) due to the electron-withdrawing effect of the metal center after coordination with Cd2+. In contrast, the tpy-H3′ of ligand L5 shifted to high-field (δ = 8.53 ppm, Δδ = 0.37 ppm) in G1 because of the strong shielding effect of 2,6-dimethoxyphenyl groups. These results indicated that L1 and L5 had formed heteronuclear coordination assembly (Fig. 2A). Moreover, two characteristic signals of tpy-H3′ displayed an integral ratio of 1
:
1, and the triple peak at 4.63 ppm combining with two singlet peaks at 4.17 and 2.77 ppm in the non-aromatic region, respectively attributed to the methylene and methoxyl groups, showed an integral ratio of 2
:
3
:
12, which was completely consistent with the desired structure (Fig. S62 and S63†). All proton signals were fully assigned by 2D COSY and 2D NOESY, verifying the successful formation of G1 (Fig. S64 and S65†). Through ESI-MS, a series of signal peaks from +11 to +5 corresponding to moieties of continuously losing various numbers of PF6− were observed. And the experimental charge-to-mass ratio (m/z) values were consistent with the calculated ones (Fig. 2B and S80†). Further, TWIM-MS presented a group of signals ranging from +11 to +6, with a narrow drift time and no signal of other isomers, indicating that a single and discrete species G1 was formed (Fig. S79†).
By using a similar method to G1, multi-cavity supramolecular capsules G2–G4 were obtained by heating the mixture of ligands L2/L3/L4, L5 and Cd(NO3)2·4H2O at accurate stoichiometric ratios (Fig. 3). The structural evidence of supramolecular G2–G4 was first collected by NMR experiments. Despite the large size and complicated composition of assembled architectures, sharp and distinct 1H NMR patterns were still obtained. From the comparison of G2–G4 with corresponding ligands (L2–L5), the signals assigned to the tpy-H3′ situated on ligands L2/L3/L4 showed low-field upon complexation, whereas the tpy-H3′ and the tpy-H6 located at ligand L5 obviously shifted to high-field due to the shielding effect of 2,6-dimethoxyphenyl, respectively, supporting the formation of complexes (Fig. 4A, S82 and S85†). Further exemplified with G3, as shown in Fig. 4A, structural information was collected: (i) four characteristic single peaks at 9.11, 9.03, 8.42 and 8.35 ppm attributed to tpy-H3′ with an integral ratio of 1:
1
:
1
:
1 were observed; (ii) three triple peaks at 4.70 ppm, 4.52 ppm, and 4.34 ppm showed an integral ratio of 1
:
2
:
1 attributed to –OCH2–, and two single peaks at 2.68 and 2.51 ppm with an integral ratio of 1
:
1 attributed to –OCH3; (iii) two single peaks at −3.03 and −3.32 ppm with an integral ratio of 1
:
1 attributed to pyrrole proton HP (Fig. S70 and S71†). The integral ratio of these characteristic peaks was completely consistent with the desired structure. Similarly, the detailed structural analysis of G2 and G4 was also presented in the ESI.† The attribution of all protons through the 1H NMR, 2D COSY and NOESY proved the successful assembly of the supramolecular cages G2–G4 (Fig. S66–S77†).
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Fig. 3 Chemical structure of ligands L2–L4 and schematic illustration of multicomponent self-assembly of multideck complex metal–organic capsules G2–G4. |
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Fig. 4 (A) 1H NMR spectra (500 MHz, 298 K) of G3 in CD3CN and L3 and L5 in CDCl3; (B) ESI-MS spectra and (C) 2D ESI-TWIM-MS plots of G3. |
ESI-MS and TWIM-MS experiments were also performed to confirm the composition of G2–G4. A series of peaks with continuous mass charge ratio (m/z) from +16 to +7, +22 to +10 and +22 to +12 respectively correspond to charged moieties of G2 {[Cd12L24L53(PF6−)24]-nPF6−}n+ (n = 7–16) (Fig. S83A†), G3 {[Cd16L34L54(PF6−)32]-nPF6−}n+ (n = 10–22) (Fig. 4B) and G4 {[Cd20L44L55(PF6−)40]-nPF6−}n+ (n = 12–22) (Fig. S86A†). Further investigation revealed that the experimental m/z values matched well with theoretical values of G2 (with a molecular weight of 21731.48 Da), G3 (with a molecular weight of 29
138.14 Da) and G4 (with a molecular weight of 36
544.80 Da), respectively. In addition, TWIM-MS spectra showed a series of bands with a narrowly distributed drifting time at each charge state from +16 to +7 (Fig. S83B†), +18 to +9 (Fig. 4C) and +21 to +16 (Fig. S86B†) for G2–G4, respectively. And no signals of other unexpected isomers were found, verifying accurate assembly of single and discrete species.
In terms of host capsule G2, the host–guest recognition complex system (C60)n@G2 consisted of three species, namely G2, C60@G2 and (C60)2@G2, which was proved by NMR and ESI-MS. The 1H NMR spectrum showed that the HP signals can be fitted to three sets, respectively corresponding to G2, C60@G2 and (C60)2@G2, and the integration ratio of G2, C60@G2 and (C60)2@G2 was determined to be 5:
9
:
4, manifesting that C60@G2 was the dominant product (Fig. 6C and S99†). Meanwhile, compared with G2, the 13C NMR of (C60)n@G2 displayed a new broad single peak around 140.3 ppm composed of two slightly different signals belonging to C60, indicating that C60 was successfully wrapped into the cavity because C60 was originally nearly insoluble in acetonitrile (Fig. S100†). Moreover, as shown in Fig. 6A, the ESI-MS spectra in both CH3CN and DMF clearly showed three sets of consecutive peaks assigned to G2, C60@G2 and (C60)2@G2, respectively. And the content distribution of component G2, C60@G2 and (C60)2@G2 was further supported by the ESI-MS spectra (Fig. 6A and S102–S104†), which matched with the above-mentioned 1H NMR analysis. To shed light on the recognition process, by using similar methods to C60@G1, the binding constants of C60@G2 and (C60)2@G2 were determined by UV titration as follows: K1 = (3.06 ± 0.5) × 104 M−1 and K2 = (1.99 ± 0.5) × 104 M−1 (Fig. S124 and S125†). Meanwhile, the kinetic process of G2 wrapping C60 was monitored through time-dependent 1H NMR experiments at 353 K. These 1H NMR spectra showed that it took 18 h to achieve dynamic equilibrium of the (C60)n@G2 system (Fig. S101†), whereas just 13 h were required for C60@G1. Combining thermodynamics with kinetic data analysis, especially the cooperativity parameter (α = 0.65 < 1) in the process of G2 host–guest recognition (Fig. S125†),60 we rationally proposed that the encapsulation of two guest molecules is a negative cooperation process,61 namely one C60 molecule entering the host cavity relatively increases the difficulty of accommodating the other one. To further prove this hypothesis, the total energy of states G2, C60, C60@G2 and (C60)2@G2 were also calculated using the semiempirical quantum mechanical GFN1-xTB method, and the corresponding binding energy (ΔE) for sequentially wrapping two C60 was calculated to be −37.27 kcal mol−1 (G2 + C60 → C60@G2) and −15.56 kcal mol−1 (C60@G2 + C60 → (C60)2@G2) (Fig. 7A and S128†). Compared with the binding energy of C60@G1 (−23.47 kcal mol−1) (Fig. S127†), G2 revealed stronger binding affinities to the first C60, whereas the binding energy of further encircling the second one to form (C60)2@G2 remained relatively lower, supporting that negative cooperation encapsulation behavior of two guest molecules.
Moving to (C60)n@G3 and (C60)n@G4, in spite of overly complex resonance signals which cannot be specifically assigned due to the possible presence of multiple adducts for (C60)n@G3 and (C60)n@G4, respectively (Fig. S126†), the fact that G3/G4 packaged multiple C60 molecules can be supported by the multiple shifts of HP in 1H NMR spectra in comparison to empty capsules G3/G4 (Fig. 6E, G, S106 and S112†). The asymmetric broad peaks belonging to C60 in their 13C NMR spectra further supported the encapsulation of multiple C60 by G3 and G4 (Fig. S107 and S113†). Convincingly, the ESI-MS spectra clearly revealed the composition of multiple components for (C60)n@G3 and (C60)n@G4, in which (C60)n@G3 mixtures contained the main component (C60)2@G3 in addition to a small portion of C60@G3 and (C60)3@G3 (Fig. 6D, S109 and S110†), and (C60)n@G4 mixtures consisted of consecutive three sets of signal peaks belonging to (C60)2@G4, (C60)3@G4, and (C60)4@G4, respectively, with (C60)3@G4 being the main component (Fig. 6F, S115 and S116†). Monitoring of the dynamic process of G3/G4 wrapping C60 at 353 K showed that G3/G4 wrapping C60 to reach dynamic equilibrium needed nearly 20 hours (Fig. S108 and S114†), manifesting the slower multiple binding process. Finally, the total energy of various possible products for (C60)n@G3 and (C60)n@G4 was also calculated (Fig. S129 and S130†). Based on the tendency of each step to generate the corresponding host–guest complexes with the lowest energy, the proposed processes of G3 and G4 wrapping multiple C60 and these corresponding binding energies (ΔE) were given (Fig. 7B and S131†). In the process of sequentially encapsulating C60, the binding energy revealed a downward trend and was the smallest for the last one which entered the cavities to form (C60)3@G3 and (C60)4@G4, which was calculated to be ΔE3 = −24.41 kcal mol−1 and ΔE4 = −14.24 kcal mol−1, respectively (Fig. 7B and S131†). Such results were in accordance with the wrapping behavior of G2, verifying the negative cooperation performance in the process of multiple host recognition to guest C60 molecules for multideck capsules G2–G4. The negative cooperation behavior can be attributed to conformational adaptation, in which the entry of large-size guest molecules affects the molecular configuration, resulting in the volume decrease of remaining cavities, thereby increasing the energy of the entire system after subsequent guest molecule binding.42,62 Similar collaborative behavior for the multi-guest recognition process in a single cavity was understandable because the electronic effects and spatial complementarity between multiple guests in a single-cavity host were strong.51,63–65 However, the synergistic effect of capsules with multiple separate cavities is rarely reported, only observed in a few capsules with flexible cavities and strong conformational adaptation,66,67 For herein reported capsules with rigid cavities, the negative cooperation behavior was possibly attributed to the large size of guest molecules and strong host–guest interactions.
In addition, the photosensitivity of capsule complexes with C60 ((C60)n@Gn) to generate singlet oxygen has been studied. The time-dependent UV-vis spectra under 405 nm irradiation were recorded and showed the gradually decreasing absorption intensity of DPHA, which can be ascribed to the formation of endoperoxide via singlet oxygen-mediated oxidation.68 Such a result indicates that capsules loaded with C60 possess photosensitive properties (Fig. S133 and S134†). As the number of cavities increases, the ability of capsules to generate 1O2 increases, principally because of improved photosensitive performance via enhanced light capture as the number of porphyrin rings increases (Fig. S135†). These results provide a method for designing and synthesizing supramolecular structures with enhanced photosensitivity in the future.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4sc01204f |
This journal is © The Royal Society of Chemistry 2024 |