High-symmetry cage-like molecule N₂₀(C₂B₂)₃₀: computational insight into its bonding and reactivity

Abstract

On the basis of density functional theory (DFT) calculations and AIMD simulations, a novel I_h-symmetry cage-like molecule N₂₀(C₂B₂)₃₀ is constructed and characterized computationally. It is found that N₂₀(C₂B₂)₃₀ is structurally similar to fullerene C₂₀, but it has high thermodynamic and kinetic stability. The designed N₂₀(C₂B₂)₃₀ exhibits strong chemical reactivities, including reactivities for the Diels–Alder reaction with butadiene (C₄H₆) and cyclopentadiene (C₅H₆), as well as for the [3+2] addition reaction with diazomethane (N₂CH₂). In addition, the presence of the boron site and the inverted C=C bond with the charge-shift (CS) bonding in N₂₀(C₂B₂)₃₀ make it quite active not only for cycloaddition reactions but also for capture of small molecules (e.g. H₂, CO, NO, and NO₂). Once N₂₀(C₂B₂)₃₀ complexes with a transition metal (TM) ion, the resultant complexes (TM)N₂₀(C₂B₂)₃₀⁺ (TM = Cu, Ag, and Au) can bind inactive CO₂ and N₂O at the TM site. Furthermore, AuN₂₀(C₂B₂)₃₀⁺ is able to effectively separate CO₂ and N₂O. Owing to its unique porous structure and reactivity as well as high stability, N₂₀(C₂B₂)₃₀ may further enrich the diversity of a highly symmetrical molecular family.

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1. Introduction

Highly symmetrical molecules with I_h symmetry can be traced back to closo-[B₁₂H₁₂]²⁻, as reported both experimentally and theoretically.^1–3 Owing to its perfect structure and stable physicochemical properties in the closed borane family, this closo-[B₁₂H₁₂]²⁻ has begun to play important roles in medicine, energetic materials, new energy, environmental management and optoelectronic materials.⁴ In 1985, another kind of soccer-like cage C₆₀ with I_h symmetry was reported by Kroto et al.⁵ Henceforth, there has been long standing interest in using C₆₀ in experimental and theoretical domains due to its perfect symmetry and applications in electrochemistry, organic chemistry, nanoscience, nanotechnology, and so on.^6–9 Previous studies have shown that C₆₀ can act as a dienophile to undergo a cycloaddition reaction because of its C=C double bond character. Moreover, the Diels–Alder reactions of C₆₀ with the derivatives of butadiene and cyclopentadiene, as well as the [3+2] cycloadditions of C₆₀ with N₂CR₂ and ONCR are favorable to occur as usual across a 6,6-bond of C₆₀.^10–13 Recently, the aggregation of Ti atoms on the surface of C₆₀ enables it as a potential hydrogen storage material.¹⁴ A C₆₀-modified gold electrode is employed for the determination of dopamine in the excess of ascorbic acid using square-wave voltammetry.¹⁵ C₆₀-buffered Cu/SiO₂ can decrease the energy barriers to effectively synthesize ethylene glycol (EG) under ambient pressure conditions.¹⁶ Thus, C₆₀ has been broadly applied in designing functional materials.

Besides closo-[B₁₂H₁₂]²⁻ and C₆₀, some molecules and clusters with I_h symmetry are also reported experimentally or theoretically, such as B₈₀,¹⁷ C₆₀H₆₀, C₆₀F₆₀, Si₂₀H₂₀, Si₆₀H₆₀, and so on.^18,19 The edges of high symmetry molecules and clusters mentioned above are normal B–B, C–C, and Si–Si bonds. Inspired by the synthesis methods of covalent organic frameworks (COFs),²⁰ we wonder whether high-symmetry cages with linkers on the edges can be obtained by the [C₃+C₂] topology. Herein, a type of high-symmetry cage N₂₀(C₂B₂)₃₀, mimicking fullerene C₂₀ composed of C₂ and C₃ building blocks, is constructed using C₂B₂ linkers and N nodes, as illustrated in Fig. 1, in which C₂B₂ is taken from the molecule C₂B₂H₂ reported experimentally in 2017.²¹

Fig. 1 The optimized structure of N₂₀(C₂B₂)₃₀, as well as selected bond lengths, bond angles, and calculated Wiberg bond indices (WBIs, in bracket) are presented. Color scheme: B atom (in pink), C atom (in green), and N atom (in blue).

2. Computational methods

The geometry optimization of N₂₀(C₂B₂)₃₀ and small molecules (H₂, CO, NO, NO₂, CO₂, N₂O, C₄H₆, C₅H₆, and N₂CH₂), as well as the corresponding molecular complexes is performed by using the DFT/M06-2X method,²² in combination with the SDD basis set²³ for Cu, Ag, and Au atoms and the 6-31G(d,p) basis set²⁴ for the non-metallic atoms. Vibrational frequencies of the studied molecules are calculated at the same level of theory to confirm the true minima on the potential energy surfaces (PESs). The adsorption energies of small gas molecules are estimated by the difference in the total energies of their respective constituents, including the zero-point energy (ZPE) correction and the basis set superposition error (BSSE) correction.²⁵ The Wiberg bond indices (WBIs) are derived via the natural bond orbital (NBO) analysis.²⁶ The electron density (ρ) and Laplacian values (∇²ρ) of certain chemical bonds are calculated using the Multiwfn 3.8 program.²⁷ Key molecular orbital analysis and the pore diameter of N₂₀(C₂B₂)₃₀ can be exported and visualized by using the Multiwfn 3.8 program²⁷ and VMD 1.9.4 software,²⁸ respectively. Nucleus independent chemical shifts (NICS) at the centers of the three-membered rings of CCB in N₂₀(C₂B₂)₃₀ are calculated using the gauge-independent atomic orbital (GIAO) method.²⁹ All the above calculations are performed with the Gaussian16 program.³⁰

Ab initio molecular dynamics (AIMD) simulations with N₂₀(C₂B₂)₃₀ are carried out with the Vienna ab initio simulation package (VASP) program,³¹ where the core and valence electrons are represented by using the projector augmented wave (PAW)³² method and plane-wave basis functions with a kinetic energy cut-off of 520 eV. The generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE)³³ exchange–correlation functional is used in the AIMD calculations. The finite temperature simulations of the dynamic properties are performed at temperatures 298 and 600 K using the exact Hellmann–Feynman forces and applying the statistics of the canonical ensemble to the motion of atomic nuclei by means of a Nosé thermostat.³⁴ Newton's equations of motion are integrated using the Verlet algorithm³⁵ with a time step of 1 fs, and the total duration is 10 ps. N₂₀(C₂B₂)₃₀ is confined in a cubic box of 25 × 25 × 25 Å, and a vacuum distance of more than 10 Å is set to keep the interactions between the molecules in the adjacent boxes negligible. The Brillouin-zone sampling is restricted to the Γ-point. The geometry optimization is performed via a conjugate gradient algorithm until the residual forces acting on atoms are less than 0.1 eV Å⁻¹.

The thermodynamic stability of a molecule can generally be measured by its cohesive energy. Generally, the greater the cohesive energy, the better the stability. The cohesive energy of N₂₀(C₂B₂)₃₀ is computed as follows

where E_c is the calculated cohesive energy, E_(tot), E_(B), E_(C), and E_(N) are the total energies of N₂₀(C₂B₂)₃₀, B, C, and N atoms, respectively, x, y, and z are the numbers of atoms B, C, and N, respectively, and n is the total number of all atoms in N₂₀(C₂B₂)₃₀.

3. Results and discussion

3.1. Structure and stability of N₂₀(C₂B₂)₃₀

The optimized structure of the novel high-symmetry cage-like molecule N₂₀(C₂B₂)₃₀, as well as its key lengths and WBIs are shown in Fig. 1. Notably, this molecule is structurally similar to fullerene C₂₀, i.e. this molecule contains twelve regular pentagons (see Fig. S1 in the ESI†), in which twenty vertexes are nitrogen atoms and thirty edges are rhombus C₂B₂ units. In contrast to fullerene C₂₀,^36–38 the optimized N₂₀(C₂B₂)₃₀ with I_h symmetry has the minimum on the PES without any imaginary frequency and Jahn–teller distortion. In addition, the lengths of B–N, B–C, and inverted C=C bonds in N₂₀(C₂B₂)₃₀ are 1.43, 1.47, and 1.49 Å, respectively. The C=C bonds in N₂₀(C₂B₂)₃₀ are inverted compared with the C=C bond in ethylene, thus, we describe the C=C bond to be the inverted C=C bond. The corresponding WBIs of the B–N, B–C, and inverted C=C bonds are 0.88, 1.07, and 1.38, respectively. Thus, the inverted C=C bond has double bond character which is consistent with B-heterocyclic carbene (BHC), double B-heterocyclic carbene (DBHC), and Si, B-heterocyclic carbene (SiBHC),^39–42 while the B–N and B–C are single bonds. Moreover, the key occupied molecular orbitals (MOs, HOMO−4 to HOMO) involving C=C bonds of N₂₀(C₂B₂)₃₀ are evaluated in order to gain insights into its electronic properties, as shown in Fig. S2 (see the ESI†). It is found that these MOs are formed by the sp² hybrid orbital of carbon atoms in the C=C bonds, which is well consistent with the reported BHCs, DBHCs, and SiBHCs. Thus, the inverted C=C in N₂₀(C₂B₂)₃₀ contains one π bond and one charge-shift (CS) bond.^39–44

In order to compare the size of N₂₀(C₂B₂)₃₀ and C₆₀, we optimize the structure of C₆₀ using the M06-2X/6-31G(d,p) approach. Additionally, the pore diameters of the cavity of N₂₀(C₂B₂)₃₀ and C₆₀ are calculated using the Multiwfn 3.8 program and are shown in Fig. S3 (see the ESI†). The computational results show that the pore diameter of N₂₀(C₂B₂)₃₀ is 11.29 Å, which is approximately three times as large as that of C₆₀ (3.68 Å). Calculated cohesive energy of N₂₀(C₂B₂)₃₀ is −6.00 eV per atom, indicating that it has relatively high thermodynamic stability. To further confirm the chemical stability of N₂₀(C₂B₂)₃₀, AIMD simulations of N₂₀(C₂B₂)₃₀ are performed at T = 298 and 600 K and the specific details are shown in Fig. 2. The structure of N₂₀(C₂B₂)₃₀ has no significant changes at 298 and 600 K for 10 ps. According to the radial distribution of interatomic distances, it is suggested that during a 10 ps of AIMD simulation at both 298 and 600 K, the stability of the backbone of N₂₀(C₂B₂)₃₀ still remains unchanged. Additionally, all of the B–N, B–C, and C–C bonds of N₂₀(C₂B₂)₃₀ are not broken at T = 298 and 600 K during AIMD simulations for 10 ps. Thus, N₂₀(C₂B₂)₃₀ has high kinetic stability.

Fig. 2 Radial distribution of interatomic distances at 298 and 600 K. Color scheme: B atom (in pink), C atom (in green), and N atom (in blue).

A previous study shows that C₂B₂H₂ has the aromatic character according to the NICS(0) and NICS(1) values at the ring center of the three-membered CCB moiety in C₂B₂H₂.²¹ The NICS(0) and NICS(1) values (inside and outside of the cage) of N₂₀(C₂B₂)₃₀ are calculated and shown in Fig. S4 (see the ESI†). It is shown that the NICS(0) value of N₂₀(C₂B₂)₃₀ is −17.49, and the NICS(1) values of inside and outside of the cage of N₂₀(C₂B₂)₃₀ are −21.32 and −15.30, respectively. Note that the NICS(1) value inside the cage is close to that of C₂B₂H₂ molecules,²¹ indicating that N₂₀(C₂B₂)₃₀ also possesses aromatic character.

3.2. Cycloaddition reaction of N₂₀(C₂B₂)₃₀

BHCs, DBHCs, and SiBHCs can behave as dienophiles to undergo Diels–Alder reactions with dienes due to their C=C bond character.^39,40,42 Herein, in order to investigate the reactivity of the C=C bond, the mechanism for the Diels–Alder reactions of N₂₀(C₂B₂)₃₀ with butadiene and cyclopentadiene is investigated. The predicted relative free energy profiles of the two Diels–Alder reactions, along with the optimized structures of transition states and products for analyzing the formation of chemical bonds, are shown in Fig. 3. Note that Diels–Alder reaction of N₂₀(C₂B₂)₃₀ with butadiene is exothermic and yields the product (PC1) via the transition state (TS1) with a free energy barrier of 20.9 kcal mol⁻¹ and an energy release of 48.0 kcal mol⁻¹, which is similar to the addition reaction of BHCs with butadiene.⁴⁰ In the Diels–Alder reaction of N₂₀(C₂B₂)₃₀ with cyclopentadiene, a loose intermolecular complex (RC) is formed at first, and then the reaction undergoes the transition state (TS2) to generate the product (PC2), which is consistent with the Diels–Alder reaction of fullerene C₆₀ with cyclopentadiene.⁴⁵ The free energy barrier for the Diels–Alder reaction of N₂₀(C₂B₂)₃₀ with cyclopentadiene is 20.6 kcal mol⁻¹, and the Gibbs free energies of the reaction ΔG is −15.5 kcal mol⁻¹. Clearly, the Diels–Alder reactions of N₂₀(C₂B₂)₃₀ with butadiene and cyclopentadiene are feasible, both thermodynamically and kinetically.

Fig. 3 The predicted relative free energy profiles (in kcal mol⁻¹) for the Diels–Alder reactions of N₂₀(C₂B₂)₃₀ with butadiene and cyclopentadiene are shown. Optimized structures, key bond lengths (Å, in plain), and WBIs (in italic). Color scheme: H atom (in white), B atom (in pink), C atom (in green), and N atom (in blue).

The [3+2] cycloaddition reaction of C₆₀ as well as its derivatives towards N₂CH₂ undergo either thermal or photochemical loss of dinitrogen to give C₆₀CH₂.¹³ Here, the mechanism for the [3+2] cycloaddition reaction of N₂₀(C₂B₂)₃₀ with N₂CH₂, including loss of dinitrogen, is illustrated in Fig. 4. It is revealed that the [3+2] cycloaddition proceeds to an intermediate (IM) through the transition state (TS3) with an energy barrier of 14.6 kcal mol⁻¹ and an energy release of 25.8 kcal mol⁻¹. Thus, the [3+2] cycloaddition is feasible both thermodynamically and dynamically. Followed by the release of N₂, i.e., N₂₀(C₂B₂)₃₀N₂CH₂ (IM) → N₂₀(C₂B₂)₃₀CH₂ (PC3) + N₂, the final product N₂₀(C₂B₂)₃₀CH₂ (PC3) of the [3+2] cycloaddition reaction, containing a 3, 4-boratricyclo[1.1.1]pentane, is formed, and the overall Gibbs free energies of the reaction ΔG is −47.6 kcal mol⁻¹ relative to the initial reactants, showing the [3+2] cycloaddition is quite feasible, both thermodynamically and kinetically.

Fig. 4 The predicted relative free energy profile (in kcal mol⁻¹) for the [3+2] cycloaddition of N₂₀(C₂B₂)₃₀ with diazomethane is shown. Optimized structures, key bond lengths (Å, in plain), and WBIs (in italic). Color scheme: H atom (in white), B atom (in pink), C atom (in green), and N atom (in blue).

The lengths (WBIs) of the C1–C2 bond in PC1, PC2, IM, and PC3 are 1.73 (0.66), 1.69 (0.74), 1.68 (0.72), and 1.73 Å (0.39), respectively. Thus, the C1–C2 bond is a weak single bond, as found in the products of the Diels–Alder reaction of BHCs.⁴⁰

We have further investigated the reaction mechanism of the successive addition of three butadiene molecules to N₂₀(C₂B₂)₃₀. The results show that the free energy barriers at different stages of addition remain essentially unchanged, and the reaction exhibits sustained exergonicity, although the trend of exergonicity slows down. Therefore, it can be inferred that N₂₀(C₂B₂)₃₀ may undergo successive addition with butadiene molecules until saturation (see Fig. S5 in the ESI†). In addition, we have compared the addition reaction activities of the C=C bonds in the exohedral and endohedral additions of the cluster with butadiene. The results indicate that both have comparable addition energy barriers. However, it should be noted that due to the spatial effects within the cluster, the stability of the endohedral addition product is slightly lower than that of the corresponding exohedral product (see Fig. S6 in the ESI†).

3.3. Binding interactions of N₂₀(C₂B₂)₃₀ and its ionic complexes with small molecules

Due to the CS bonding character of the inverted C=C bond in N₂₀(C₂B₂)₃₀, the carbon atoms can serve as the active sites to bind small molecules. Accordingly, the binding behaviors of N₂₀(C₂B₂)₃₀ toward small gas molecules, such as H₂, CO, NO, and NO₂, are considered here. Optimized structures of complexes N₂₀(C₂B₂)₃₀(H₂)₂, N₂₀(C₂B₂)₃₀(CO)₂, N₂₀(C₂B₂)₃₀(NO)₂, and N₂₀(C₂B₂)₃₀(NO₂)₂ are illustrated in Fig. 5, and the vibrational analysis reveals that they are all stable without any imaginary frequency. In addition, the evaluated binding energies of N₂₀(C₂B₂)₃₀ with H₂, CO, NO, and NO₂ are −3.96, −3.16, −1.72, and −2.57 eV, respectively, indicating that N₂₀(C₂B₂)₃₀ can effectively capture H₂, CO, NO, and NO₂. Moreover, since the binding energies are significantly different from each other, N₂₀(C₂B₂)₃₀ might be used to separate H₂, CO, NO, and NO₂.

Fig. 5 The complexes of N₂₀(C₂B₂)₃₀ with small gas molecules H₂, CO, NO, and NO₂ are shown. Selected bond lengths are given in Å. Color scheme: H atom (in white), B atom (in pink), C atom (in green), N atom (in blue), and O atom (in red).

Every carbon atom of the C₂B₂ unit in N₂₀(C₂B₂)₃₀(H₂)₂ and N₂₀(C₂B₂)₃₀(CO)₂ forms two C–H bonds with H₂ and a C=CO bond with CO, respectively, resulting in the cleavage of the inverted C=C bond of N₂₀(C₂B₂)₃₀. Interestingly, C=N (left) and C–N (right) in N₂₀(C₂B₂)₃₀(NO)₂ are obviously different with the bond lengths of 1.20 and 1.42 Å, respectively. In order to better understand the bonding nature, the natural bond orbital analysis of N₂₀(C₂B₂)₃₀(NO)₂ is performed. As seen from Fig. S7 (see the ESI†), there is a significant electron transfer from the N₁ atom to the C₂ atom, resulting in the C₂ atom with a net negative charge of −0.64. The 2p orbitals of the left O, N, and C atoms form a linear structure with Π₂² and Π₃⁴ bonds when the N atom loses an electron. In contrast, the 2p orbitals of the right O, N and C atoms form a V-like structure with the Π₃⁴ bond when the C atom accepts an electron. The lengths of two N=O bonds in N₂₀(C₂B₂)₃₀(NO)₂ are around 1.20 Å. Two carbon atoms in the C₂B₂ unit in N₂₀(C₂B₂)₃₀(NO₂)₂ form two C–N bonds and an inverted C–C bond, and the latter bears a weak π bond character with a distance of 1.71 Å. Thus, N₂₀(C₂B₂)₃₀ accommodates two NO₂ through the covalent form of the CS bond of the inverted C⋯C.

In particular, due to the empty 2p_z orbital of the boron atom in N₂₀(C₂B₂)₃₀, boron atoms may bind CO through a dative bond interaction. The optimized structure of the binding complex N₂₀(C₂B₂)₃₀(CO) is depicted in Fig. 6, and it is a minimum on the PES predicted by the frequency analysis. The calculated adsorption energy of N₂₀(C₂B₂)₃₀ + CO → N₂₀(C₂B₂)₃₀CO is −0.02 eV, suggesting that the boron atom in N₂₀(C₂B₂)₃₀ is also an active site for CO adsorption.

Fig. 6 The adsorption of CO at the B site of N₂₀(C₂B₂)₃₀ is shown, where key bond lengths are given in Å. Color scheme: B atom (in pink), C atom (in green), N atom (in blue), and O atom (in red).

Although it can bind with the aforementioned small molecules, N₂₀(C₂B₂)₃₀ cannot effectively bind with CO₂ and N₂O. Note that a zeolite recently reported can be used to separate CO₂ and N₂O through the Ag⁺ site.⁴⁶ The carbon atoms of the inverted C=C bond in N₂₀(C₂B₂)₃₀ can coordinate to the transition metal (TM) to form a complex (TM)N₂₀(C₂B₂)₃₀.³⁹ Interestingly, it is found that the newly generated (TM)N₂₀(C₂B₂)₃₀⁺ (TM = Cu, Ag, and Au) could capture CO₂ and N₂O. The optimized structures of (TM)N₂₀(C₂B₂)₃₀⁺, (CO₂)(TM)N₂₀(C₂B₂)₃₀⁺, and (N₂O)(TM)N₂₀(C₂B₂)₃₀⁺, and the corresponding geometric parameters are illustrated in Fig. 7. Note that TM in (TM)N₂₀(C₂B₂)₃₀⁺ is coordinated with the terminal C atoms of three inverted C=C units. Specifically, Cu⁺ in CuN₂₀(C₂B₂)₃₀⁺ binds with CO₂ and N₂O through the interaction of their O and N atoms with Cu, where the newly formed Cu–O or Cu–N bond replaces one Cu–C bond connected to the inverted C=C unit. Differing from CuN₂₀(C₂B₂)₃₀⁺, the formation of the TM–O/N bond in (TM)N₂₀(C₂B₂)₃₀⁺ in (TM = Ag and Au) may weaken the Cu–C (inverted C=C) bond to some extent. Calculated binding energies for (TM)N₂₀(C₂B₂)₃₀⁺ + CO₂ → (CO₂)(TM)N₂₀(C₂B₂)₃₀⁺ (TM = Cu, Ag, and Au) are −0.65, −0.38, and −0.09 eV, respectively. Similarly, the binding energies for (TM)N₂₀(C₂B₂)₃₀⁺ + N₂O → (N₂O)(TM)N₂₀(C₂B₂)₃₀⁺ (TM = Cu, Ag, and Au) are −0.68, −0.34, and −0.21 eV, respectively. Here the predicted binding energies of CO₂ and N₂O bound at the Ag⁺ site of AgN₂₀(C₂B₂)₃₀⁺ are well consistent with those of these small molecules bound at the Ag⁺ site of the reported zeolite.⁴⁶ Accordingly, the introduction of transition metal ions can remarkably enhance the capability of N₂₀(C₂B₂)₃₀ toward the binding of inactive molecules, such as CO₂ and N₂O. Furthermore, the adsorption energy difference of 0.12 eV between AuN₂₀(C₂B₂)₃₀⁺ with CO₂ and N₂O indicates that these two gas molecules can be effectively separated. In order to better understand the interaction between TM (TM = Cu, Ag, and Au) and CO₂, as well as TM and NO₂, the electron density and Laplacian values of the critical points (CPs) of TM–O and TM–N bonds in (CO₂)(TM)N₂₀(C₂B₂)₃₀⁺ and (N₂O)(TM)N₂₀(C₂B₂)₃₀⁺ are calculated and listed in Table S1 (see the ESI†). The electron density and Laplacian values of the CPs of TM–O and TM–N bonds are in the range of 0.03–0.07 and 0.16–0.37, respectively, indicating that the TM–O and TM–N bonds are ionic bonds.⁴³

Fig. 7 The optimized complexes of AgN₂₀(C₂B₂)₃₀⁺, CuN₂₀(C₂B₂)₃₀⁺, and AuN₂₀(C₂B₂)₃₀⁺ with CO₂ and N₂O, where key bond lengths are given in Å. Color scheme: B atom (in pink), C atom (in green), N atom (in blue), O atom (in red), Ag atom (in silver), Cu atom (in copper), and Au atom (in gold).

4. Conclusions

A high-symmetry cage-like molecule N₂₀(C₂B₂)₃₀, resembling the fullerene C₂₀ structurally, has been constructed by taking C₂B₂ and N motifs as linkers and nodes, respectively, and its stability and reactivity for cycloaddition and adsorption reactions are further demonstrated using density functional theory (DFT) calculations and AIMD simulations. The calculated results show that three cycloaddition reactions of N₂₀(C₂B₂)₃₀ with butadiene, cyclopentadiene, and diazomethane are all exothermic, and the corresponding free energy barriers are 20.9, 20.6, and 14.6 kcal mol⁻¹, respectively, showing that these reactions are feasible, both thermodynamically and kinetically. For the adsorption reactions, the carbon atoms of the inverted C=C bond in N₂₀(C₂B₂)₃₀ can effectively bind with H₂, CO, NO, and NO₂, whereas the boron atom site only accommodates CO. After the modification with transition metal (TM) ions (Cu⁺, Ag⁺, and Au⁺), CO₂ and N₂O can bind at the TM site of the obtained complexes (TM)N₂₀(C₂B₂)₃₀⁺. Among these transition metal complexes, only AuN₂₀(C₂B₂)₃₀⁺ shows obviously different adsorption energies for CO₂ and N₂O, and may be applied to effectively separate these two gas molecules. The present findings for novel structure and reactivity of N₂₀(C₂B₂)₃₀ may endow it with potential for applications in materials chemistry.

Author contributions

All authors contributed to the writing of the manuscript and approved the final version of the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Science Foundation of China (22033006 and 92372105).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cp04653f

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