Macrocycle with Equatorial Coordination Sites Provides New Opportunity for Structure-Diverse Metallacages

Reported here is the synthesis of a macrocycle with equatorial coordination sites for the construction of self-assembled metallacages. The macrocycle is prepared via a post-modification on the equator of biphen[n]arene. Utilizing this macrocycle as a ligand, three prismatic cages and one octahedral cage were synthesized by regulating the geometric structures and coordination number of metal acceptors. The multi-cavity configuration of prismatic cage was revealed by single-crystal structure. We prove that a macrocycle with equatorial coordination sites can be an excellent building block for synthesizing structure-diverse metallacages. Our results provide a typical example and a general method for the design and synthesis of metallacages.


Introduction
Molecular cages are widely utilized in catalysis, drug delivery, smart materials, chemical sensors, etc., due to their diverse building blocks, various geometry, and rich inner cavities [1][2][3][4][5][6]. Rebek and coworkers employed a cylindrical capsule as a nanoscale container for the 1,3-dipolar cycloaddition reaction between phenylazide and phenylacetylene. The cylindrical cavity of the capsule constrains the guests to be arranged in an edge-to-edge manner, resulting in the exclusive formation of the 1,4-triazole product after several days. Tiefenbacher et al. reported the application of an organic molecular cage as a catalyst for the selective hydrolysis of acetal derivatives in organic solvents. Mukherjee et al. demonstrated the utility of a trifacial molecular barrel-type molecular cage as a homogeneous catalyst in the efficient synthesis of xanthenes and their derivatives in aqueous media. Xu and colleagues employed an organic cage as a template to facilitate the synthesis of metal nanoparticles. In 2014, Zhang and coworkers presented a novel approach for the synthesis of gold nanoparticles (AuNPs) with controlled sizes based on a specific covalent organic cage. Therrien et al. demonstrated the formation of triangular prismatic hostguest compounds by self-assembling 2,4,6-tris(pyridin-4-yl)-1,3,5-triazine triangular panels with p-cymene ruthenium building blocks and 2,5-dioxydo-1,4-benzoquinonato bridges in the presence of a pyrenyl derivative (pyrene-R) that was functionalized. Nitschke et al. reported a fascinating collection of aromatic-paneled Fe 4 L 6 cages created through iron(II)templated subcomponent self-assembly of 2-formylpyridine and C 2 -symmetric diamine building blocks. The researchers discovered that both the size and the arrangement of the aromatic panels played a critical role in achieving successful encapsulation of large hydrophobic guests, such as fullerenes, polycyclic aromatic hydrocarbons, and steroids. They found that even minor differences in the structure of the subcomponents had obvious effects on the binding abilities of the resulting hosts.
Coordination-driven self-assembly is a simple and efficient method for constructing molecular cages [7][8][9][10][11][12][13]. A series of discrete or consecutive self-assembled metallacages with These cages exhibited hexagonal bipyramidal architectures, resulting from coordination between the uranyl cation and three carboxylate groups located in the equatorial plane. Sue et al. reported Ag n L 2 metal-organic pillars assembled from pillar [5]arene, which paved the way for the construction of deep-cavity metallocavitands and nanochannels with unique molecular recognition and transportation properties [29][30][31] (Figure 1). We envision that macrocycles bearing equatorial coordination sites would provide a new opportunity for structure-diverse metallacages. However, the introduction of equatorial coordination sites into traditional macrocycles is usually difficult. Firstly, the modification on the skeleton is prohibitive for most macrocyclic compounds; secondly, the stereochemical structures of most macrocycles make it impossible to produce a derivate with equatorial coordination sites [32][33][34][35][36][37]. Therefore, the design and synthesis of macrocycle ligands with equatorial coordination sites remain a formidable challenge.
Molecules 2023, 28, x FOR PEER REVIEW 3 of 13 complexity of calix [4]resorcinarene-containing metallo-supramolecules. Martinez-Belmonte et al. prepared a novel class of self-assembled cages by conical-shaped carboxylic acid derivatives of calix [4]arene and calix [5]arene ligands and the metallic counterpart of uranyl cation. These cages exhibited hexagonal bipyramidal architectures, resulting from coordination between the uranyl cation and three carboxylate groups located in the equatorial plane. Sue et al. reported AgnL2 metal-organic pillars assembled from pillar [5]arene, which paved the way for the construction of deep-cavity metallocavitands and nanochannels with unique molecular recognition and transportation properties [29][30][31] (Figure 1). We envision that macrocycles bearing equatorial coordination sites would provide a new opportunity for structure-diverse metallacages. However, the introduction of equatorial coordination sites into traditional macrocycles is usually difficult. Firstly, the modification on the skeleton is prohibitive for most macrocyclic compounds; secondly, the stereochemical structures of most macrocycles make it impossible to produce a derivate with equatorial coordination sites [32][33][34][35][36][37]. Therefore, the design and synthesis of macrocycle ligands with equatorial coordination sites remain a formidable challenge. Herein, we report the synthesis of a macrocyclic compound containing pyridine groups at the equatorial plane for the construction of self-assembled metallacages. As far as we know, this is the first report of synthesizing metallacages from macrocyclic ligands bearing equatorial coordination sites.

Results and Discussion
In our previous work, we developed a modular synthetic strategy for functional macrocycles, which realizes the customization of size, functional backbones, and endo-binding sites of macrocycles [38][39][40]. This strategy endows macrocycles with potential applications in gas chromatography, pollutant capture, and physical adsorption separation [41,42]. Benefiting from the modular synthesis and plentiful molecular supplies, we readily synthesized the macrocyclic compound possessing pyridine groups on the outer side of the cavity. As illustrated in Scheme 1, the synthesis process of macrocyclic ligand TP3 can be divided into three steps: firstly, the functional module tribromobenzene is coupled with the 2,5-dimethoxyphenyl reaction module via a Pd-catalyzed Suzuki-Miyaura cross-coupling reaction to obtain the monomer; secondly, the monomer is condensed with polyformaldehyde under the catalysis of Lewis acid boron trifluoride diethyl etherate to Herein, we report the synthesis of a macrocyclic compound containing pyridine groups at the equatorial plane for the construction of self-assembled metallacages. As far as we know, this is the first report of synthesizing metallacages from macrocyclic ligands bearing equatorial coordination sites.

Results and Discussion
In our previous work, we developed a modular synthetic strategy for functional macrocycles, which realizes the customization of size, functional backbones, and endobinding sites of macrocycles [38][39][40]. This strategy endows macrocycles with potential applications in gas chromatography, pollutant capture, and physical adsorption separation [41,42]. Benefiting from the modular synthesis and plentiful molecular supplies, we readily synthesized the macrocyclic compound possessing pyridine groups on the outer side of the cavity. As illustrated in Scheme 1, the synthesis process of macrocyclic ligand TP3 can be divided into three steps: firstly, the functional module tribromobenzene is coupled with the 2,5-dimethoxyphenyl reaction module via a Pd-catalyzed Suzuki-Miyaura cross-coupling reaction to obtain the monomer; secondly, the monomer is condensed with polyformaldehyde under the catalysis of Lewis acid boron trifluoride diethyl etherate to obtain TP3-Br; thirdly, the TP3-Br is coupled with pyridine boric acid to obtain the macrocycle TP3. Furthermore, the structure of TP3 was unambiguously confirmed by the single-crystal structure (Scheme 1). TP3 has a regular hexagonal cavity with a diameter of 11.5 Å (C-C distance). The pyridine groups are located at the equator of the macrocycle molecules with a distance of~21.0 Å (N-N distance). It is worth noting that the stacking structure of TP3 is not parallel ( Figure S33). The N atom of pyridine group forms an intermolecular hydrogen bond with the H atom on the OMe group of adjacent macrocycles (C-H···N 2.9 Å and 3.7 Å).
Molecules 2023, 28, x FOR PEER REVIEW 4 of 13 obtain TP3-Br; thirdly, the TP3-Br is coupled with pyridine boric acid to obtain the macrocycle TP3. Furthermore, the structure of TP3 was unambiguously confirmed by the single-crystal structure (Scheme 1). TP3 has a regular hexagonal cavity with a diameter of ~11.5 Å (C-C distance). The pyridine groups are located at the equator of the macrocycle molecules with a distance of ~21.0 Å (N-N distance). It is worth noting that the stacking structure of TP3 is not parallel ( Figure S33). The N atom of pyridine group forms an intermolecular hydrogen bond with the H atom on the OMe group of adjacent macrocycles (C-H•••N 2.9 Å and 3.7 Å). Scheme 1. The synthesis and crystal structure of macrocyclic ligand (TP3) with equatorial coordination sites (top). TP3 has a regular hexagonal cavity with a diameter of ~11.5 Å (C-C distance) and pyridine distance of ~21.0 Å (N-N distance). Cartoon representations of the assembly of [2 + 3] prismatic cages [4 + 6] and octahedron cage by macrocyclic ligand TP3 and metal acceptors Ru(II) and Pt(II) (bottom).

Scheme 1.
The synthesis and crystal structure of macrocyclic ligand (TP3) with equatorial coordination sites (top). TP3 has a regular hexagonal cavity with a diameter of~11.5 Å (C-C distance) and pyridine distance of~21.0 Å (N-N distance). Cartoon representations of the assembly of [2 + 3] prismatic cages [4 + 6] and octahedron cage by macrocyclic ligand TP3 and metal acceptors Ru(II) and Pt(II) (bottom).
The self-assembly behavior between the acceptors A1-A4 and the ligand TP3 was carefully studied using NMR spectroscopy in CD 3 CN or CD 3 OD. The 1 H NMR spectra of the resulting metallacages M1-M4 revealed that they possess highly symmetric and discrete structures. Upon the formation of the metallacages, the partial proton signals of the ligand TP3 displayed significant shifts, which can be attributed to the loss of electron density upon ligand-to-metal coordination (Figures 2a and 3a). In addition, diffusion-ordered NMR spectroscopy (DOSY) analysis was conducted, which further confirmed the formation of the metallacages in solution (Figures 2a, 3a, S13 and S23). Specifically, in the DOSY spectra, M1-M4 displayed a single band of signals with a diffusion coefficient (D) of 2.8 × 10 −6 cm 2 /s for M1 in CD 3 CN, 3.5 × 10 −6 cm 2 /s for M2 in CD 3 CN, 2.1 × 10 −6 cm 2 /s for M3 in CD 3 CN, and 2.6 × 10 −6 cm 2 /s for M4 in CD 3 OD at 298 K, indicating the presence of a single species of metallacages. These results provide strong evidence for the successful formation of the highly symmetric and discrete metallacages through self-assembly between the acceptors and the ligand TP3.   Further confirmation of the formation of the prismatic metallacages M1-M3 and octahedral cage M4 was obtained through ESI-MS studies. The ESI-MS spectrum of M1-M3 displayed peaks corresponding to the assigned [2 + 3] assembly, including peaks with continuous charge states ranging from 3 + to 6 + , resulting from the successive loss of the counteranion OTf − (as shown in Figures 2b and S29-S31). Similarly, the ESI-MS results for M4 revealed peaks for the assigned [4 + 6] assembly (Figures 3b and S32). All peaks in the mass spectra were isotopically resolved and agreed well with their calculated theoretical distributions. These results provide further evidence of the successful formation of the desired metallacages.  Further confirmation of the formation of the prismatic metallacages M1-M3 and octahedral cage M4 was obtained through ESI-MS studies. The ESI-MS spectrum of M1-M3 displayed peaks corresponding to the assigned [2 + 3] assembly, including peaks with continuous charge states ranging from 3 + to 6 + , resulting from the successive loss of the counteranion OTf − (as shown in Figures 2b and S29-S31). Similarly, the ESI-MS results for M4 revealed peaks for the assigned [4 + 6] assembly (Figures 3b and S32). All peaks in the mass spectra were isotopically resolved and agreed well with their calculated theoretical distributions. These results provide further evidence of the successful formation of the desired metallacages. Single-crystal X-ray diffraction analysis further elucidated the structure of prismatic cage M2. The X-ray-quality single crystal of the self-assembly metallacage M2 was obtained as atrovirens cube crystal by vapor diffusion of i-propyl ether into acetonitrile. As illustrated  Figure 2c, the solid-state structure of M2 revealed that two pyridine-based macrocycle TP3 were connected by three ruthenium(II) acceptors. Two macrocyclic molecules form the bottoms of the triangular prism. Interestingly, they are not parallel to each other, which may contribute to an increase in the C-H···π interactions (2.6, 2.8, and 3.0 Å). Some methoxy groups on the macrocyclic molecule inserted into the cavity of another macrocycle via multiple C-H···π interactions. The hydrophobic cavities of the upper and lower were arranged together to form a channel. The Ru(II) acceptor forms the three sides of the triangular prism. They are also not completely vertical. The side length of the bottom surface of the triangular prism is~25.6 Å (Ru-Ru distance), and the height of the triangular prism is~8.3 Å (Ru-Ru distance). For the packing structure, adjacent metallacage units are packed in a parallel stacking with a separation of~5.6 Å ( Figure S34); no C-H···π hydrogen bonds can be found.
Despite multiple attempts, it was not possible to obtain a single crystal of M4. Therefore, geometry optimizations were performed using density functional theory (DFT) calculations with the B3LYP/3-21G method. The results of these calculations revealed that M4 has an explicit octahedral conformation. Specifically, the octahedral cage is formed by four macrocyclic ligands (TP3), which form the four faces of the cage, and six Pt(II) acceptors, which form the verticals. The side length of this octahedral cage is 25.7 Å, which is the distance between the Pt atoms. The simulated structure provides a detailed representation of M4's geometry, which is important for understanding its properties and potential applications. Despite the inability to obtain a single crystal of M4, the DFT calculations provide valuable insights into its structural characteristics.

Materials and Methods
All the chemicals used in this study were purchased from commercial sources and were not subjected to any further purification. The nuclear magnetic resonance (NMR) spectra, including 1 H, 31 P (only for M4), 13 C, COSY, DOSY, and NOESY, were recorded on a Bruker Avance 400/600 MHz spectrometer. The chemical shifts in the NMR spectra are reported in parts per million (ppm) relative to the proton resonance resulting from incomplete deuteration of the NMR solvents, which were CD 3 OD (3.33 ppm for 1 H and 49.0 ppm for 13 C), CD 3 CN (1.94 ppm for 1 H and 118.3 ppm for 13 C), and CDCl 3 (7.26 ppm for 1 H and 77.2 ppm for 13 C). High-resolution electrospray ionization (HR-ESI) mass spectral analyses were performed using the Thermo Fisher Q Exactive™ HF/UltiMate™ 3000 RSLCnano. Matrix Assisted Laser Desorption Ionization (MALDI) mass spectra were performed on Bruker Daltonics UltrafleXtreme time of flight (TOF) equipment. Single crystals suitable for X-ray crystallographic analysis were selected, and their X-ray diffraction intensity data were collected on a rotating anode diffractometer equipped with a hybrid photon counting detector. Graphite-monochromated CuKα radiation with a wavelength of 1.54184 Å was used at a temperature of 200 K. To simulate the geometry optimizations of the metallacage M4, the Gaussian 09 program was used with B3LYP/3-21G computations. The obtained results provide valuable insights into the structure and properties of the metallacage M4, which are important for understanding its potential applications. The use of advanced techniques, such as HR-ESI mass spectrometry and X-ray crystallographic analysis, ensures the accuracy and reliability of the results, while the NMR spectra provide additional information about the chemical environment of the metallacages.

Conclusions
In summary, we successfully synthesized a macrocycle TP3 with pyridine units on the equator via a post-modification of biphenarene. Benefiting from the equatorial coordination sites, the macrocycle could be an excellent ligand for the construction of four novel prismatic/octahedral cages M1-M4 via coordination-driven self-assembly with Ru(II) and Pt(II) building acceptors. All these metallacages were fully characterized by NMR and ESI-MS spectroscopic studies. Moreover, the multi-cavity configuration of the prismatic cage was proven by X-ray crystallographic analysis. This work enriches the toolbox of macrocycle ligands and provides a general method for the design and synthesis of structure-diverse metallacages. More macrocycles with equatorial coordination sites can be synthesized, various macrocycle-based metallacages can be assembled, and further functions and applications can be explored.