Triazine‐based multicomponent metallacages with tunable structures for SO2 selective capture and conversion

The design of novel materials for sulfur dioxide (SO2) capture and conversion with considerable efficiency under mild conditions is of great significance for human health and environmental protection yet highly challenging. Herein, we report a series of triazine‐based multicomponent metallacages via coordination‐driven self‐assembly of 2,4,6‐tri(4‐pyridyl)‐1,3,5‐triazine, cis‐Pt(PEt3)2(OTf)2 and different tetracarboxylic ligands. As the increase of the length of the tetracarboxylates, the structures of the metallacages change from pyramids to extended octahedrons. Owing to their N‐rich structure, these metallacages are further used for selective SO2 capture, showing good adsorption capacity and remarkable SO2/CO2 selectivity in ambient conditions, suggesting their potential applications toward real flue gas desulfurization. The metallacages are further employed for the conversion of SO2 into value‐added compounds, showing exceptional efficiency even dilute SO2 is used as the reactant. This study represents a type of structure‐tunable triazine‐based metallacages for SO2 capture and conversion, which will pave the way on the applications of metal‐organic complexes for gas adsorption.

SO 2 adsorption capacity, benefiting from their uniform pore size, tailorable pore environment and large surface area. [16,17]owever, in most MOF-based systems, [18,19] the adsorption is attributed to the coordination of metal centers with sulfur atoms, which may compete with the inherent coordination bonds in MOFs, resulting in structural degradation upon desorption.Moreover, such chemisorption process is often irreversible, offering difficulties for the efficient conversion of SO 2 . [20][23][24][25] However, it is still challenging to design adsorbent materials for reversible SO 2 adsorption and conversion.
Metallacages, [26][27][28][29][30][31] as a type of discrete supramolecular coordination complexes, have been widely used as supramolecular hosts capable of guest encapsulation and applied in chemical and biological sensing, [32,33] drug delivery [34] and confined catalysis [35,36] .Compared with MOFs, the development of metallacages for gas capture is far less behind, owing to their low BET surface area and porosity in the solid state. [37]Recent study indicates that some organic cages exhibit good SO 2 adsorption capacity based on the interactions between SO 2 and N-containing ligands. [38,39]Moreover, the reversibility of such adsorption can be manipulated by the basicity of the N species in the adsorbent. [40,41]However, compared with MOFs and covalent cages, the study of metallacages for SO 2 adsorption has been rarely reported. [42,43]Moreover, these examples only used pure SO 2 for the adsorption process; the selective adsorption of SO 2 over other gases, especially CO 2 and N 2 in flue gas by metallacages has never been reported.In addition, the use of metallacages for the conversion of SO 2 into value-added compounds has never been realized yet.
[46] The most famous and representative triazine-based metallacages are Fujita's Pd 6 L 4 octahedron [44] (Scheme 1) self-assembled by cisblocked Pd(II) ligands and 2,4,6-tri(4-pyridyl)−1,3,5-triazine (1), which have been widely applied as molecular flasks for confined catalysis. [47]Herein, by using cis-Pt(PEt 3 ) 2 OTf 2 (2) instead of cis-blocked Pd(II) ligands as the metal nodes in the self-assembly process, an analogous Pt 6 L 4 octahedron (3) is first prepared.Interestingly, the self-assembly of 1, 2 with different tetracarboxylic ligands (4a-e) offers a series of structurally different multicomponent metallacages (5ae), in which two different coordination modes are observed in a single metallacage.The geometric structures of these multicomponent metallacages depend on the chemical structures of the tetracarboxylic ligands, ranging from pyramids to extended octahedrons.Our study shows that the metallacages display SO 2 adsorption capacity as high as 4.66 mmol/g, with SO 2 /CO 2 selectivity of 538 under ambient conditions (i.e., 10:90 mixture at 298 K and 1 bar).Moreover, such metallacages are successfully used as containers for the cycloaddition reaction of 2,3-dimethylbutadiene and SO 2 , showing good performance for SO 2 conversion.This study not only provides a series of triazine-based multicomponent metallacages with tunable geometries but also explores their applications for SO 2 capture and conversion, which will enrich the study on metallacages toward the adsorption and utilization of pollutive gases.

Preparation, characterization, and structural analysis of metallacages
Metallacage 3 was prepared by the self-assembly of 2,4,6-tri(4-pyridyl)−1,3,5-triazine (TPT, 1) and cis-Pt(PEt 3 ) 2 (OTf) 2 2 in 2:3 molar ratio in acetone.Single crystals of 3 suitable for X-ray diffraction analysis were obtained by vapor diffusion of isopropyl ether into acetonitrile.Metallacage 3 showed an octahedral structure, with the distance between two adjacent platinum atoms to be 1.31 nm (Figure 1A), which is similar with Fujita's Pd 6 L 4 metallacage. [44]Multicomponent metallacages 5a-e were prepared by the self-assembly of 1, 2 and corresponding tetracarboxylic ligands 4a-e in 2:5:1 molar ratio (See supporting information for synthetic details).Owing to the complexity of the coordination modes of these metallacages, their structures were not revealed until their solid-state structures (Figure 1B-E) were determined by X-ray diffraction analysis.Interestingly, two different coordination modes of the platinum (II) nodes are found in a single metallacage, which is different from previously reported results. [48,49]he first one is the homoleptic coordination of platinum (II) nodes with two identical pyridyl ligands, the other one is the heteroleptic coordination of platinum (II) nodes with one carboxylic and one pyridyl ligand, which is induced by charge separation. [50]The integration of these two coordination modes increases the complexity of structures of the as-formed metallacages, leading to three different types of geometries for metallacages 5a-e.Metallacage 5a displayed a pyramid-shaped structure (Figure 1B), while extended octahedrons were found for metallacages 5c, 5d and 5e (Figure 1C-E).It is worth mentioning that the orientation of the tetracarboxylic ligands in 5c/5d and 5e is different, offering two different types of extended octahedron (Type I for 5c and 5d, Type II for 5e) with different cavity sizes.Based on the calculation from the VOIDOO program, the cavity spaces of metallacages 3, 5a, 5c, 5d and 5e are 507, 157, 1177, 1337 and 3565 Å 3 , respectively.These results indicated that the chemical structures of tetracarboxylic ligands played an important role in the formation of the final coordination constructs, offering a convenient strategy to finely tune the shapes and sizes of the metallacages.
In order to reveal the assembly mechanism, detailed structure analyses on metallacages 5a-e were performed (Figure 2).The pyramid-shaped structure of 5a can be regarded as that the tetracarboxylic ligand 4a inserts into the central plane of the octahedral metallacage 3 and breaks it into two half metallacages, owing to the stronger coordination of N-Pt-O bonds compared with N-Pt-N bonds.However, one N-Pt-N coordination bond is remained because such combination of coordination affords the smallest discrete structure, which is generally most energetically favorable in coordinated structures (Figure 2A). [51]For tetracarboxylic ligands 4c and 4d, their length (1.44 nm) does not match with the suitable coordination in the pyramid-shaped structure, so they have to shift to another direction to connect two tri(4-pyridyl)−1,3,5-triazine units, offering metallacages 5c and 5d with extended octahedron-type structures (Type I, Figure 2B).Different from 5c and 5d, 5e displays another extended octahedron (Type II) structure, which can be explained by their different molecular packing modes.In the solid-state structure of 5c, different cages are connected via the van der Waals forces between the peripheral PEt 3 units, offering nearly parallel side-to-side packing along the tetracarboxylic ligands (Figure 2C).However, in the solid-state structure of 5d, the center naphthyl units in the tetracarboxylic ligand 4d is tilted, with the dihedral angle between naphthyl and isophthalic groups of 62 • , which weakens the van der Waals forces between the peripheral PEt 3 F I G U R E 1 Crystal structures of (A) 3, (B) 5a, (C) 5c, (D) 5d, and (E) 5e and their related VOIDOO-calculated cavity spaces (blue clouds).Counterions, solvent molecules and triethylphosphine units were omitted for clarity.A 1.6 Å probe was used for VOIDOO calculation.units owing to steric hindrance, so additional C-H⋅⋅⋅O hydrogen bonding and C-H⋅⋅⋅π interactions are needed for the molecular packing.As a result, the units of 5d are staggered, offering a shoulder-to-head packing (Figure 2D).When the central units in the tetracarboxylic ligand are changed into anthracene groups (4e), the increased dihedral angle (81 • ) between anthracene and isophthalic groups further weakens the van der Waals forces between the peripheral PEt 3 units, so the interactions between the 5e units are not sufficient to afford reasonable packing for the metallacages.Therefore, the orientation of the tetracarboxylic ligand changes, offering Type II extended octahedron configuration for 5e (Figure 2E).In such structures, abundant C-H⋅⋅⋅O hydrogen bonds form between the protons of PEt 3 and the oxygen atoms of carboxylic groups, which help to stabilize the head-to-head packing of 5e in the solid state.
Encouraged by the X-ray crystal structure results, the formation of metallacages 3 and 5a-e was further studied by 31 P{ 1 H} and 1 H nuclear magnetic resonance (NMR) spectroscopy and electrospray ionization time-of-flight mass spectroscopy (ESI-TOF-MS).The 31 P{ 1 H} NMR spectrum of 3 exhibited one singlet peaks (P 3 ) at −1.02 ppm with concomitant 195 Pt satellites, corresponding to its highly symmetric octahedron structure (Figure 3A).However, for multicomponent metallacages 5a-5e, the 31 P{ 1 H} NMR spectra of such metallacages generally exhibited two doublets of equal intensity with concomitant 195 Pt satellites, as well as an extra singlet peak (Figure 3B-F), which is different from previously reported multicomponent metallacages. [48,49]ch results are consistent with the chemical environments of the phosphorus atoms in their crystal structures: the two doublet peaks (P 1 and P 2 ) corresponded to the heteroleptic coordination of Pt(II) atoms with carboxylic and pyridyl ligands, while the singlet peak (P 3 ) was derived from the coordination of Pt(II) atoms with two nitrogen atoms.For the two doublet peaks, the one at ca. 5.5 ppm corresponded to the phosphorus (P 1 ) opposite to the carboxylic groups, and the one at ca. 0.5 ppm was derived from the phosphorus nuclei (P 2 ) trans to the pyridyl groups. [52]Interestingly, the chemical shifts of the phosphorus nuclei were closely related to their structures.For pyramid-shaped metallacages 5a and 5b, the chemical shift of P 2 resided more downfield than that of P 3 .For metallacage 5c, the chemical shifts of P 2 and P 3 were very close, with P 2 slightly more downfield than P 3 .For metallacage 5e, P 3 located more downfield than P 2 .Interestingly, for 5d, the 31 P{ 1 H} NMR spectrum was similar to the sum of the individual spectra of 5c and 5e, suggesting that both extended octahedron-shaped structures (Type I and Type II) existed in solution for 5d, even only Type I structure was crystallized in the solid state.The 1 H NMR spectroscopy results provided further evidence for their coordinated structures.In the 1 H NMR spectra of 3 (Figure 3G in multicomponent metallacages were observed, proving the coexistence of two coordination modes again (Figures 3H-I).In addition, 1 H NMR spectrum of 5d could also be identified as the sum of the two metallacages with different configurations, meaning that 5d is a mixed assembly in solution.Both 31 P{ 1 H} NMR and 1 H NMR spectra observation indicated that as the increase of the central aromatic groups from benzene to naphthalene and then to anthracene, the structure of the metallacages gradually changed from Type I extended octahedron to Type II extended octahedron, which were consistent with the above-mentioned structural changes of the metallacages.Diffusion-ordered 1 H NMR spectroscopy from its 1 H NMR spectrum (Figure 3K), the size of these two configurations was very close, leading to only one diffusion coefficient for 5d (Figure S15).ESI-TOF-MS provided evidence of the coordination stoichiometry of 3 and 5a-5e (Figure S21).Prominent sets of peaks with different charge states were observed for all the metallacages, which agreed well with their experimental structures.For example, peaks at m/z = 1725.8485,1865.7576,1863.1941,2569.2556,1501.9647, and 1939.6998 were found, corresponding to [3 5+ , and [5e − 4OTf] 4+ , respectively.
The thermal stability of metallacages 3 and 5a-5e was studied by thermal gravimetric analysis (TGA) (Figure S22a).Only ca. 5% weight loss was found for metallacages 3 and 5a-5e up to 250 • C under N 2 atmosphere, which was probably due to the loss of solvent molecules in their structures.This result suggested that these metallacages were stable below 250 • C, which is important for their adsorption applications.Therefore, metallacages 3 and 5a-5e were activated at 90 • C under vacuum for powder X-ray diffraction (PXRD) study (Figure S23).For metallacage 3, PXRD pattern indicated that it maintained certain crystallinity, which was probably because its high symmetric octahedral structures offered dense molecular packing in the solid state.Metallacages 5a-5e were amorphous as indicated from the diffraction spectra, suggesting that they lost their crystallinities upon desolvation, which is a general phenomenon for coordinated metallacages. [37]Such crystalline differences do not have an adverse impact on their adsorption properties, because the metallacages mainly use their intrinsic porosity derived from the cavities for the capture of gas molecules. [54] 2 adsorption-desorption experiment (at 77 K) analyses were conducted in order to study the Brunauer-Emmett-Teller (BET) surface areas in 3 and 5a-5e.However, no good results were obtained due to the very low uptake of N 2 .The most likely reason is that the affinity between metallacage and N 2 is too weak.Therefore, the CO 2 adsorption isotherm of metallacages 3 and 5a-5e were examined at 195 K to calculate BET surface areas.As shown in Figure S22b, the calculated BET surface areas of 3 and 5a-5e were 204, 57, 78, 131, 88, and 76 m 2 /g, respectively, which falls in the range of common metallacages. [37,55]2 SO 2 capture from flue gas SO 2 , as a harmful acidic gas, prefers to be adsorbed on Ncontaining functional groups due to the basicity of the N species.[38,39] Considering the abundant N sites in triazinebased metallacages 3 and 5a-5e, as well as their diverse intrinsic cavities, the desulfurization performance of these metallacages were investigated.As shown in Figure 4A-F, metallacages 3 and 5a-5e showed remarkable adsorption capacities for SO 2 , much larger than those of CO 2 and N 2 .
The SO 2 adsorption capacities of 3, 5a-5e were 4.26, 3.76, 3.48, 3.21, 3.61, and 4.66 mmol⋅g −1 , respectively, at 298 K and 1.0 bar (Figure 4A-F), which are among the high values for porous cage-based materials (Table S6). [21,22]As indicated from the PXRD results, the good SO 2 adsorption performance of metallacage 3 was because the good molecular packing in their structures would also provide extra adsorption sites between the molecules, while the adsorption inside the metallacage was also well retained.The SO 2 adsorption capacity of all the multicomponent metallacages was associated with their different cavity volumes induced by different geometries.As the geometry of multicomponent metallacages changed from pyramid to extended octahedron I and then to extended octahedron II, gradual size increase of their internal cavities was observed.The bigger cavity volume suggested that metallacage had more space to accommodate more SO 2 molecules; hence 5e with extended octahedron II geometry displayed the best adsorption capacity.All the metallacages showed a moderate degree of hysteresis due to the swelling effects of such flexible metal-organic complexes, which is also observed for some soft materials. [56]The selectivity for SO 2 /CO 2 adsorption of all the metallacages was calculated using ideal adsorbed solution theory (IAST).As shown in Figure 4G, when the molar fraction of SO 2 /CO 2 changed from 0.1 to 0.9 (v/v), the SO 2 /CO 2 selectivity of metallacages 3 and 5a-5e decreased gradually.For metallacage 5e, the SO 2 /CO 2 selectivity reached 538 when the SO 2 /CO 2 molar fraction is 0.1 (v/v), displaying an excellent selectivity under low SO 2 concentration.This value is among the highest numbers in SO 2 adsorbent porous materials, [23,57] which is attributed to the very high N atom densities in such structures (Figure 4H).Therefore, we further examined the SO 2 capture ability of metallacage 5e from flue gas, in which the main competitive components were CO 2 and N 2 .Con-sidering that the SO 2 concentration in actual flue gas was very low, the molecular fraction of N 2 was kept at 89.50% and the fraction of SO 2 was changed from 0.05% to 0.30% in the ternary mixtures (SO 2 /CO 2 /N 2 ) to study the SO 2 /CO 2 adsorption selectivity of 5e (Figure 4I).Even at 0.05% molar fraction of SO 2 , metallacage 5e still exhibited a well-pleasing SO 2 /CO 2 adsorption selectivity of 86, suggesting that it can be applied for real flue gas desulfurization. [58]Owing to the acidic and corrosive nature of SO 2 , the recyclability and stability of metallacages 5e upon SO 2 adsorption were also evaluated.When 5e was only activated under vacuum at room temperature, we observed a slight decrease in SO 2 uptake (about 6%) after three cycles (Figure S25a).While when metallacage 5e was reactivated upon heating at 90 • C under vacuum for 120 min, the results showed that 5e did not lose any of its adsorption capacity after 5 adsorption-desorption recycles at 298 K and 1.0 bar (Figure S25b), proving its good recyclability.Moreover, 31 P{ 1 H} and 1 H NMR spectra of metallacage after 5 adsorption-desorption recycles (Figure S26) indicated that all the metallacages remained stable after the exposure to SO 2 , suggesting that this is a physical adsorption of SO 2 for these metallacages.The evaluation of F I G U R E 5 Density functional theory calculations of (A) N 2 , (B) CO 2 , (C) SO 2 adsorption by metallacage 5e based on its crystal structure.About 8 N 2 molecules, 47 CO 2 molecules and 78 SO 2 molecules are calculated to be adsorbed per one metallacage.Optimized geometries of the complexes of (D) N 2 @5e, (E) CO 2 @5e and (F) SO 2 @5e and corresponding intermolecular interactions (CF 3 SO 3 − , pink).
adsorption kinetics parameters as an important index was also carried out.Time-dependent adsorption performance of SO 2 and CO 2 in 5e was shown in Figure S27.The adsorption rate of SO 2 reached the highest values in 20 sec, while the adsorption rate of CO 2 reached the highest values in 30 sec and then gradually decreased.Obviously, 5e showed a much higher adsorption rate of SO 2 (0.911 mmol⋅g −1 ⋅sec −1 ) than that of CO 2 (0.049 mmol⋅g −1 ⋅sec −1 ).Furthermore, the adsorption rate of SO 2 was much faster than that of CO 2 during the whole test period, indicating that SO 2 was preferentially adsorbed on 5e, which could lead to high SO 2 /CO 2 separation performance.Motivated by the excellent SO 2 /CO 2 adsorption selectivity of 5e shown by single component adsorption, we further investigated its SO 2 /CO 2 separation performance by dynamic breakthrough test.The breakthrough test experiments of ternary gas mixture SO 2 /CO 2 /N 2 (0.20/10.00/89.80,v/v/v) was also investigated, and the results were shown in Figure S28.As expected, SO 2 could be effectively adsorbed in the bed until about 5 min⋅g −1 , while N 2 and CO 2 broke through at the very start.We could conclude from the above results that metallacages showed good recyclability, stability, and adsorption selectivity for SO 2 , which is of vital importance for their applications in SO 2 adsorption and conversion.
Since metallacage 5e showed better SO 2 adsorption capacity and selectivity compared with other metallacages, theoretical calculation was performed on 5e to reveal structure-properties relationship in the adsorption process.Molecular simulation indicated that 8 N 2 molecules could be adsorbed inside 5e (Figure 5A) and 47 CO 2 molecules could be adsorbed both inside and in-between metallacage 5e (Figure 5B), which were far less than the theoretical adsorption capacity of SO 2 molecules (78 SO 2 molecules/unit cell) (Figure 5C).Such impressive selectivity toward SO 2 is probably owing to the better interactions between SO 2 and the metallacage compared with CO 2 and N 2 .However, based on the experimental results, 39 SO 2 molecules were calculated to be adsorbed by one cage at 1 bar.The decrease in the adsorption capacity is because the crystallinity of metallacage 5e was lost after activation, suggesting that the SO 2 molecules were mainly adsorbed in the cavity of 5e in the practical adsorption experiments.Figure 5D-F showed the optimized structures of N 2 , CO 2 and SO 2 adsorption in 5e and revealed that 5e exhibited richer non-covalent interactions with SO 2 molecules for better SO 2 adsorption performance.Figure 5D showed that N 2 molecules were primarily adsorbed through the weak N δ− ⋅⋅⋅H δ+ dipole-dipole interaction with (tetracarboxylates)H⋅⋅⋅N distances of 3.68 Å.While for CO 2 , the primary adsorption forces were from enhanced (C)H δ+ ⋅⋅⋅O δ− (S) interactions.The optimized H⋅⋅⋅O distances of 2.68 Å between methyl groups and CO 2 (Figure 5E).However, this force was still weak compared to the abundant interactions between SO 2 and the metallacage (Figure 5F). the Monte Carlo simulation, the calculated binding energy (ΔE) of SO 2 @5e (−40.52 kJ⋅mol −1 ) was higher than that of CO 2 @5e (−32.14 kJ⋅mol −1 ) and N 2 @5e (−17.81 kJ⋅mol −1 ), indicating that 5e exhibited better affinity for SO 2 .This is probably because the stronger acidity of SO 2 (compared with CO 2 and N 2 ) would offer better interactions toward basic N-containing functional groups, thus increasing the adsorption selectively of SO 2 over CO 2 and N 2 for the metallacages.

SO 2 conversion
In order to explore the applications of such triazine-based multicomponent metallacages, the use of 5e as a molecular container for the conversion of SO 2 into value-added compounds was further explored.The cycloaddition reaction of 2,3-dimethylbutadiene and SO 2 , which generally requires SO 2 -based complexes (i.e., DABSO) as the gas source, was chosen as the model reaction to investigate the conversion. [59]ince metallacage 5e can selectively capture trace amount of SO 2 , here only 1% SO 2 (SO 2 /N 2 = 1:99, v/v) was used in the chemical reaction.No significant chemical shift changes were observed in the 1 H NMR spectra before and after the addition of 1% SO 2 (Figure S30), suggesting that metallacage 5e was stable under 1% SO 2 , which is of vital importance for its use as adsorbent for chemical reactions.The reactions were performed under different conditions and monitored by 1 H NMR spectroscopy (Figure S31-S38).It can be concluded from the experimental results that the conversion of the reaction increased as the temperature increased from 25 • C to 90 • C (Table 1).However, some byproducts would form at elevated temperature (90 • C), which decreased the yield of the formed 2,3-dimethyl sulfolene.The reaction at 70 • C for 24 h was found to be the best condition, offering the product with 99% selectivity and 47% yield.Considering the extremely low concentration of SO 2 gas used for the reaction, such results are very satisfying.In this reaction, the metallacage serves as a molecular container which captures the purged SO 2 gas effectively for the chemical reaction in solution.Compared with the small amount of SO a Reaction conditions: 2,3-dimethyl-butadiene (10 μmol), hydroquinone (10 μmol), 5e (10 mg), 1% SO 2 (SO 2 /N 2 = 1:99, v/v) in balloon, CD 3 CN.b The yields were determined by 1 H NMR using 1,3,5-trimethylbenzene as the internal standard.The values of conversion, selectivity and yield were the average of the three repeated experiments.
so it can accelerate the reaction and reduce the intermolecular collision in the free state, which can effectively improve the activity and selectivity of the reaction.This study explores the applications of triazine-based metallacage for SO 2 conversion, which is promising for the treatment of trace amount of SO 2 in real flue gas.

CONCLUSION
In summary, a series of triazine-based metallacages with different geometrical structures were prepared via multicomponent coordination-driven self-assembly.The length and central groups of the tetracarboxylic ligands were responsible for the assembled structures, offering metallacages with shapes ranging from pyramid to extended octahedron.Owing to the abundant N atoms in triazine units and porous structures of the metallacages, they were further employed for SO 2 adsorption, showing good adsorption capacity and remarkable SO 2 /CO 2 selectivity.Moreover, their applications as molecular containers for the chemical conversion of SO 2 under low concentrations were also developed with reasonable conversions and yields.This study not only offers a noncovalent strategy to modify and functionalize the existing sophisticated metallacages via multicomponent self-assembly but also explores their applications as the adsorbents for flue gas desulfurization, which will promote the further development of metallacages for the treatment of harmful gases.
[CCDC deposition numbers 2251300 (for 3), 2251301 (for 5a), 2251302 (for 5c), 2251303 (for 5d), and 2251304 (for 5e) contain the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interests.

D ATA AVA I L A B I L I T Y S TAT E M E N T
All data are reported in this article and available upon request.

R E F E R E N C E S
), clear downfield chemical shift changes were found for αpyridyl protons H a* (9.22 ppm) and β-pyridyl protons H b* (8.81 ppm) compared with their TPT precursor (Figures S1-2), indicating the formation of highly symmetric structure.However, except for H a* and H b* , another set of signals of α-pyridyl protons H a and β-pyridyl protons H b of TPT F I G U R E 2 Representative size-dependent self-assembly of metallacages 5a (A) and 5c (B).Molecular packing of metallacages 5c (C), 5d (D) and 5e (E) in the solid state.
Multiple (C)H δ+ ⋅⋅⋅O δ− (S) interactions between pyridine, -PEt 3 and SO 2 also contributed to the structure stabilization with H⋅⋅⋅O distances of 2.65-3.53Å.Furthermore, electrostatic interactions with the CF 3 SO 3 − group acted as extra binding forces with a S⋅⋅⋅O distance of 3.44 Å.The lone pairs of electrons from N of triazine groups facilitated the formation of a charge-transfer complex (N→SO 2 ) with N-S distance of 2.77 Å. Besides, based on S C H E M E 1 Cartoon representations for the self-assembly of metallacages 3 and 5a-e.
ac.uk/data_request/cif.] A C K N O W L E D G M E N T S RZ thanks the China Postdoctoral Science Foundation (grant number: 2021M702588) and Shaanxi Provincial Natural Science Foundation of China (grant number: 2023-JC-QN-0105) for financial support.MZ thanks the National Natural Science Foundation of China (grant numbers: 22171219 and 22222112), Innovation Talent Promotion Plan of Shaanxi Province for Science and Technology Innovation Team (grant number: 2023-CX-TD-51) and the Fundamental Research Funds for the Central Universities for financial support.

F
U N D I N G I N F O R M AT I O N The China Postdoctoral Science Foundation, Grant Number: 2021M702588; Shaanxi Provincial Natural Science Foundation of China, Grant Number: 2023-JC-QN-0105; The National Natural Science Foundation of China, Grant Numbers: 22171219 and 22222112; Innovation Talent Promotion Plan of Shaanxi Province for Science and Technology Innovation Team, Grant Number: 2023-CX-TD-51; Fundamental Research Funds, The Central Universities Cycloaddition of SO 2 and 2,3-dimethyl-butadiene in the presence of metallacage 5e a .
2 simply dispersed in solution, SO 2 enriched in metallacage has a larger local concentration, TA B L E 1