From inverse sandwich Ta₂B₇⁺ and Ta₂B₈ to spherical trihedral Ta₃B₁₂⁻: prediction of the smallest metallo-borospherene

Abstract

Transition-metal-doped boron nanoclusters exhibit interesting structures and bonding. Inspired by the experimentally discovered inverse sandwich D_6h Ta₂B₆ and spherical trihedral D_3h La₃B₁₈⁻ and based on extensive first-principles theory calculations, we predict herein the structural transition from perfect di-metal-doped inverse sandwich D_7h Ta₂B₇⁺ (1) and D_8h Ta₂B₈ (2) to tri-metal-doped spherical trihedral D_3h Ta₃B₁₂⁻ (3). As the smallest metallo-borospherene reported to date, Ta₃B₁₂⁻ (3) contains three octa-coordinate Ta atoms as integral parts of the cage surface coordinated in three equivalent η⁸-B₈ rings which share two eclipsed equilateral B₃ triangles on the top and bottom interconnected by three B₂ units on the waist. Detailed orbital and bonding analyses indicate that both Ta₂B₇⁺ (1) and Ta₂B₈ (2) possess σ + π dual aromaticity, while Ta₃B₁₂⁻ (3) is σ + π + δ triply aromatic in nature. The IR, Raman, and UV-vis or photoelectron spectra of the concerned species are computationally simulated to facilitate their future spectroscopic characterizations.

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1. Introduction

Boron exhibits strong propensity to form multicentre-two-electron bonds (mc-2e bonds) in both polyhedral molecules and bulk allotropes due to its prototypical electron-deficiency.^1–4 Joint photoelectron (PE) spectroscopy experimental and first-principles theory investigations in the past two decades have unveiled a rich landscape for B_n^−/0 boron nanoclusters from planar or quasi-planar structures (n = 3–38, 41–42) to cage-like borospherenes (n = 39, 40).^3–7 Cage-like D_2d B₄₀^−/0 are the first borospherenes discovered in 2014,⁵ while C₃/C₂ B₃₉⁻ are the first axially chiral borospherenes confirmed in experiments as the global minima (GM) of the B_n⁻ monoanions.⁶ These discoveries mark the onset of borospherene chemistry parallel to fullerene chemistry. The borospherene family has been systematically expanded by our group at first-principles theory levels to the cage-like B_n^q series in different charge states (n = 36–42, q = n–40) which all possess twelve delocalized π bonds on the surface and follow the universal σ + π double delocalization bonding pattern.^5–10 Sea-shell-like C₂ B₂₈^−/0 and C_s B₂₉⁻ were later observed as minor isomers of the monoanions in PES experiments^11,12 and sea-shell-like C_s B₂₉⁺, C₂ B₃₁⁺, C₂ B₃₂, C₂ B₃₄, C₂ B₃₅⁺, and cage-like C_s B₃₉⁺ have also been predicted recently in theory.^13–16

Transition-metal (TM)-doping induces dramatic structural changes and leads to earlier planar → tubular → spherical structural transitions in boron clusters due to effective TM–B coordination interactions, as evidenced by the experimentally observed pyramidal C_8v Ta©B₈⁻,¹⁷ C_s Ta©B₉⁻,¹⁸ and perfect planar D_10h Ta©B₁₀⁻ which has the highest coordination number of CN = 10 in planar species¹⁹ and the theoretically predicted half-sandwich TaB₁₂⁻.²⁰ A family of transition-metal-centred double-ring tubular boron complexes in staggered motifs were recently observed in experiments, including D_8d Co@B₁₆⁻,²¹ D_9d Rh@B₁₈⁻,²² and C_s Ta@B₂₀⁻,²³ with C_s Ta@B₂₀⁻ (B₂–[Ta@B₁₈]⁻) behaving like a tubular molecular rotor at 900 K facilitated by prototypical multicentre fluxional bonds (FBs) which form and break constantly.²⁴ A series of perfect di-metal-doped inverse sandwich complexes have also been observed in PE experiments, including D_6h Ta₂B₆^−/0,²⁵ D_8h La₂B₈, and D_8h Pr₂B₈.²⁶ Our group recently reported the smallest core-shell-like tubular molecular rotor C_2h La₂[B₂@B₁₈] which contains a B₂ core rotating inside a B₁₈ tube almost freely.²⁷ The first experimentally observed tri-metal-doped inverse triple-decker C_2v La₃B₁₄⁻ exhibits a tilted La–B₈–La–B₈–La triple-decker structure with two conjoined η⁸-B₈ rings sharing a B–B unit on one edge.²⁸ With increasing cluster size, the first perfect spherical trihedral metallo-borospherenes D_3h Ln₃B₁₈⁻ (Ln = La, Tb) were very recently discovered in a joint PE experimental and first-principles theory investigation.²⁹ These metallo-borospherenes represent a new class of unusual geometry with tunable magnetic or catalytic properties, with three deca-coordinate Ln centres as integral parts of the cage surface coordinated in three equivalent η¹⁰-B₁₀ decagons which share two eclipsed triangular B₆ units interconnected by three B₂ units. Metallo-borospherenes were previously proposed in theory for MB₄₀ (M = Be, Mg) and Ni_nB₄₀ (n = 1–4) which contain hepta-coordinate metal centres on the cage surface of B₄₀, with the corresponding metallo-borophenes with hepta-coordinate metal centres as precursors.^30,31 However, it remains unknown at current stage what is the possible smallest size of metallo-borospherenes and if such coordination-stabilized metallo-borospherenes can be characterized in experiments.

Based on extensive first-principles theory calculations, we predict in this work the possibility of the perfect di-TM-doped inverse sandwich D_7h Ta₂B₇⁺ (1) and D_8h Ta₂B₈ (2) and, more importantly, the smallest tri-TM-doped cage-like metallo-borospherene D_3h Ta₃B₁₂⁻ (3) which contains three octa-coordinate Ta centres as integral parts of the cage surface coordinated in three equivalent η⁸-B₈ octagons that share two equilateral B₃ triangles interconnected by three B₂ units. Detailed analyses indicate that both Ta₂B₇⁺ (1) and Ta₂B₈ (2) are σ + π dually aromatic in nature, while Ta₃B₁₂⁻ (3) is the first transition-metal-doped boron complex reported to date with σ + π + δ triple aromaticity.

2. Theoretical procedure

Extensive global minimum (GM) searches were performed on Ta₂B₇⁺, Ta₂B₈, and Ta₃B₁₂⁻ using the Tsinghua Global Minimum (TGMin) package^32,33 based on a constraint Basin-Hopping algorithm,³⁴ with more than 2000 singlet or triplet stationary points explored for each cluster at PBE/DZVP level.³⁵ Low-lying isomers were then fully optimized at the PBE0 (36) level with the 6-311+G* basis set³⁷ for B and Stuttgart relativistic small-core pseudopotential^38,39 for Ta using the Gaussian 16 program suite,⁴⁰ with vibrational frequencies checked to make sure that all isomers reported are true minima. The 15 lowest-lying isomers were subsequently fully re-optimized at both PBE0 and BP86 (ref. 41 and 42) levels with the basis sets of aug-cc-pVTZ for B^43,44 and Stuttgart+2f1g for Ta. Relative energies for the four lowest-lying isomers were further refined using the more accurate coupled cluster method with triple excitations CCSD(T)^45–47 implemented in Gaussian 16 with the same basis sets. Natural bonding orbital (NBO) analyses were performed using the NBO 6.0 program⁴⁸ and detailed bonding analyses carried out utilizing the adaptive natural density partitioning (AdNDP) approach.^49,50 Molecular dynamics (MD) simulations were performed on Ta₂B₇⁺ (1), Ta₂B₈ (2), and Ta₃B₁₂⁻ (3) for 30 ps using the CP2K software suite at different temperatures.⁵¹ The anisotropy of the current-induced density (ACID) analysis⁵² was realized using the ACID code, with the ring-current maps generated using POV-Ray 3.7.⁵³ The iso-chemical shielding surfaces (ICSSs)^54,55 were generated with the Multiwfn 3.7 code.⁵⁶ The UV-vis and PES spectra were simulated using the time-dependent TD-DFT-PBE0 approach.⁵⁷

3. Results and discussions

3.1 Structures and stabilities

We start from Ta₂B₇⁺, the smallest di-metal-doped Ta–B complex concerned in this work. Extensive GM searches indicate that inverse sandwich D_7h Ta₂B₇⁺ (1, ¹A₁) is the deep-lying GM of the monocation which lies 1.58 eV lower than the second lowest-lying isomer C_2v Ta₂B₇⁺ (¹A₁) at CCSD(T) level (Fig. S1†), with the large HOMO–LUMO gap of ΔE_gap = 3.77 eV at PBE0. The monocation contains a perfect B₇ ring sandwiched between two Ta atoms along the C₇ molecular axis at the two ends (similar to the experimentally observed inverse sandwich D_6h Ta₂B₆ which has a perfect B₆ ring sandwiched between two Ta atoms²⁴), with the B–B bond length of r_B–B = 1.54 Å, Ta–B distance of r_Ta–B = 2.28 Å, and Ta–Ta distance of r_Ta–Ta = 2.86 Å (which is obviously larger than the proposed Ta–Ta single bond length of 2.46 Å⁵⁸). With one more B atom added in, the perfect inverse sandwich D_8h Ta₂B₈ (2, ¹A_1g) is achieved which is the deep-lying GM of the neutral lying 0.62 eV lower in energy than the second lowest-lying isomer C_s Ta₂B₈ (¹A′) at CCSD(T) (Fig. S2†). It encompasses a perfect B₈ ring sandwiched between two Ta atoms along the C₈ molecular axis at the two ends, with the HOMO–LUMO gap of ΔE_gap = 2.15 eV at PBE0. With a B₈ octagon as the ligand, the Ta–Ta distance in Ta₂B₈ (2) is effectively shortened to r_Ta–Ta = 2.52 Å which is close to the proposed average Ta–Ta single bond length.⁵⁸ The highly stable inverse sandwich Ta₂B₇⁺ (1) and Ta₂B₈ (2) may serve as building blocks to form tri-metal-doped Ta–B complexes, similar to the situation in the experimentally observed triple-decker C_2v La₃B₁₄⁻ (28) and spherical trihedral D_3h La₃B₁₈⁻.²⁹

Following the direction, we obtained the lowest-lying C_2v Ta₃B₁₀⁺ (¹A₁) for Ta₃B₁₀⁺, C_s Ta₃B₁₁⁻ (¹A′) for Ta₃B₁₁⁻, and C_s Ta₃B₁₁⁻ (²A′) for Ta₃B₁₁⁻ which all possess incomplete spherical-trihedral structures with one edge broken (Fig. S3†). The smallest perfect spherical trihedron was achieved at D_3h Ta₃B₁₂⁻ (3, ) which, as the GM of the monoanion, lies 0.24 eV lower than the second lowest-lying C_2v Ta₃B₁₂⁻ (¹A₁) and 1.06 eV lower than the third lowest-lying C_s Ta₃B₁₂⁻ (¹A′) (which both belong to metallo-borospherenes) at the most reliable CCSD(T) level (Fig. S4†). As shown in Fig. 1, Ta₃B₁₂⁻ (3) contains three equivalent octa-coordinate Ta centres as integral parts of the cage surface coordinated in three equivalent η⁸-B₈ rings which share two eclipsed equilateral B₃ triangles at the top and bottom interconnected by three B₂ units on the waist. It has the calculated B–B bond lengths of r_B–B = 1.58–1.74 Å, Ta–B coordination distances of r_Ta–B = 2.26–2.27 Å, and elongated Ta–Ta distances of r_Ta–Ta = 3.04 Å, respectively, with the large HOMO(e′′)–LOMO(e′) gap of ΔE_gap = 2.50 eV at PBE0 which is comparable with the corresponding value of 2.70 eV calculated for the experimentally observed D_3h La₃B₁₈⁻ (29) at the same theoretical level. Ta₃B₁₂⁻ (3) can be constructed from Ta₂B₈ (2) by adding one B₄ edge ([double bond splayed left]B–B–B–B[double bond splayed right]) perpendicular to the B₈ ring in the front and one Ta atom coordinated to the newly formed B₈ ring, forming three interconnected Ta₂B₈ inverse sandwiches around the C₃ main molecular axis (Fig. 1). In comparison with D_3h Ln₃B₁₈⁻ (Ln = La, Tb) which possess a perfect cage-like D_3h B₁₈ ligand with three equivalent η¹⁰-B₁₀ decagons sharing two eclipsed B₆ triangles at the top and bottom interconnected by three B₂ units on the waist,²⁹ Ta₃B₁₂⁻ (3) has a perfect cage-like D_3h B₁₂ ligand with three equivalent η⁸-B₈ octagons sharing two eclipsed B₃ triangles at the top and bottom interconnected by three B₂ units on the waist. Ta₃B₁₂⁻ (3) has the same symmetry (D_3h) as Ln₃B₁₈⁻, but smaller in size with six less B atoms in the ligand. Such a structural change can be understood based on the fact that Ta ([Xe]5d³6s²) has a smaller atomic radius (1.46 Å) than that (1.83 Å) of La ([Xe]5d¹6s²),⁵⁹ but possesses two more 5d valence electrons than the latter. The Ta centres appear to have the right atomic radii and electronic configurations to match the D_3h B₁₂ ligand both electronically and geometrically. Ta₃B₁₂⁻ (3) is therefore the smallest metallo-borospherene reported to date. Perfect spherical trihedral D_3h Nb₃B₁₂⁻ and D_3h V₃B₁₂⁻ also appear to be true minima of the systems at both PBE0 and BP86 levels (Fig. S5†), with the HOMO–LUMO gaps of ΔE_gap = 2.59 and 2.67 eV at PBE0, respectively.

Fig. 1 Optimized structures of D_7h Ta₂B₇⁺ (1), D_8h Ta₂B₈ (2), and D_3h Ta₃B₁₂⁻ (3) at PBE0//B/aug-cc-pVTZ/Ta/Stuttgart+2f1g level, with bond lengths indicated in Å.

Extensive MD simulations (Fig. S6†) indicate that Ta₂B₇⁺ (1), Ta₂B₈ (2), and Ta₃B₁₂⁻ (3) are dynamically stable at 1200 K, 700 K, and 1200 K, with the small calculated average root-mean-square-deviations of RMSD = 0.11, 0.10, and 0.11 Å and maximum bond length deviations of MAXD = 0.27, 0.24, and 0.29 Å, respectively. No high-lying isomers were observed during the MD simulation. Ta–B complexes with larger HOMO–LUMO energy gaps appear to be dynamically more stable than the ones with narrower energy gaps.

3.2 Natural orbital and bonding analyses

The high stabilities of these high-symmetry complexes originate from their unique electronic structures and bonding patterns. Detailed natural bonding orbital (NBO) analyses show that the Ta centres in Ta₂B₇⁺ (1), Ta₂B₈ (2), and Ta₃B₁₂⁻ (3) possess the electronic configurations of Ta [Xe]6s^0.215d^3.46, Ta [Xe]6s^0.225d^3.64, and Ta [Xe]6s^0.245d^3.72, natural atomic charges of q_Ta = +1.34|e|, +1.12|e| and +1.01|e|, and total Wiberg bond indexes of WBI_Ta = 4.85, 4.80, and 5.07, respectively, indicating that each Ta centre in these complexes donates its 6s² electrons almost completely to the B_n ligand (n = 7, 8, 12), while, in return, accepts roughly one electron in its partially filled 5d orbitals from the B_n ligands via p → d back-donations. The formations of effective Ta–B coordination interactions in 1–3 are strongly supported by the calculated Ta–B bond orders of WBI_Ta–B = 0.59, 0.40, and 0.50–0.53 (Ta–B interactions within TaB₇ or TaB₈ pyramids) which appear to be systematically higher than the typical Cr–C coordination bond orders of WBI_Cr–C = 0.34 in D_6h Cr(C₆H₆)₂. As mentioned above, a Ta centre in Ta₃B₁₂⁻ (3) has the total bond order of WBI_Ta = 5.07 which is also obviously higher than the corresponding value of WBI_Cr = 4.12 calculated for Cr in D_6h Cr(C₆H₆)₂.

Detailed AdNDP bonding analyses unveil both the localized and delocalized bonds of the concerned systems. As shown in Fig. 2a, Ta₂B₈ (2) possesses 8 2c–2e B–B σ bonds on the periphery of the η⁸-B₈ ligand with the occupation numbers of ON = 1.82|e| and 1 2c–2e Ta–Ta σ bond between the two Ta centres with ON = 2.00|e| in the first row. The remaining 16 valence electrons form 8 totally delocalized 10c–2e coordination bonds between the η⁸-B₈ ligand and two Ta coordination centres with ON = 2.00|e|, including 3 10c–2e σ bonds in the second row, 3 10c–2e π bonds in the third row, and 2 10c–2e (p–d) δ bonding interactions in the fourth row. Both the delocalized 6e σ-system and delocalized 6e π-system match the 4n + 2 aromatic rule (n = 1) and together they render σ + π dual aromaticity to Ta₂B₈ (2). Similarly, both the experimentally observed D_6h Ta₂B₆ (ref. 25) and D_7h Ta₂B₇⁺ (1) are σ + π doubly aromatic in nature, as clearly indicated in their canonical molecular orbitals (CMOs) depicted in Fig. S7.†

Fig. 2 AdNDP bonding patterns of (a) D_8h Ta₂B₈ (2) and (b) D_3h Ta₃B₁₂⁻ (3), with the occupation numbers (ON) indicated.

The AdNDP bonding pattern of spherical trihedral Ta₃B₁₂⁻ (3) (Fig. 2b) exhibits certain similarity with that of the inverse sandwich Ta₂B₈ (2), but it is more complex and intriguing. Ta₃B₁₂⁻ (3) possesses 9 localized 2c–2e σ B–B bonds in the three B₂ units and between the B₂ units and the six apexes of the two B₃ triangles and 2 equivalent 3c–2e σ bonds on the two B₃ triangles at the top and bottom in the first row and 6 equivalent 3c–2e B₂–Ta σ bonds between the three Ta atoms and the two B₃ triangles at the top and bottom in the second row. The remaining 18 valence electrons form 9 delocalized 9c–2e bonds evenly distributed on three equivalent C_2v TaB₈ octagonal pyramids (which are similar to the experimentally observed C_8v TaB₈⁻ octagonal pyramid¹⁷) around the C₃ main molecular axis, including 3 equivalent 9c–2e B₈(σ)–Ta(d_π) bonds in the third row, 3 equivalent 9c–2e B₈(π)–Ta(d_σ) bonds in the fourth row, and 3 equivalent B₈(π)–Ta(d_δ) bonds in the fifth row. Each Ta&B₈ octagonal pyramid thus possesses 1 9c–2e B₈(σ)–Ta(d_π) bond, 1 9c–2e B₈(π)–Ta(d_σ) bond, and 1 B₈(π)–Ta(d_δ) bond. Overall, the Ta₃&B₁₂⁻ (3) spherical trihedron has 3 equivalent (d–p)σ bonds, 3 equivalent (d–p)π bonds, and 3 equivalent (d–p)δ bonds evenly distributed on three equivalent Ta&B₈ octagonal pyramids around the C₃ molecular axis, forming one delocalized 6e σ-system, one delocalized 6e π-system, and one delocalized 6e δ-system on the cage surface each matching the 4n + 2 rule (n = 1) independently. Such a delocalized bonding pattern renders σ + π + δ triple aromaticity to the smallest metallo-borospherene of Ta₃B₁₂⁻ (3) which is a highly stable D_3h spherical trihedron with 18 delocalized electrons evenly distributed on the cage surface.⁶⁰

The aromatic nature of Ta₂B₇⁺ (1), Ta₂B₈ (2), and Ta₃B₁₂⁻ (3) is further evidenced by their calculated nucleus-independent chemical shift (NICS) values of NICS = −110.5, −101.9, and −86.6 ppm at the geometrical centres, respectively. Based on the calculated NICS-ZZ components, Fig. 3 plots their iso-chemical-shielding surfaces^54,55 with Z-axis parallel to the designated C₇, C₈, or C₂ molecular axes to illuminate the chemical shielding around the TaB₇ or TaB₈ pyramids in these complexes. Obviously, the space inside the spherical trihedron surrounded by the delocalized Ta–B_n coordination bonds in horizontal direction or within about 1.0 Å above the Ta centres in vertical direction belong to chemical shielding regions (highlighted in yellow) with negative NICS-ZZ values, indicating that the aromatic contribution mainly comes from Ta(5d)–B(2p) coordination interactions between the Ta centres and B_n ligands around them (n = 7 or 8), while the chemical de-shielding areas (highlighted in green) with positive NICS values are located outside the B_n ring in horizontal direction. The ICSSs of these complexes appear to be analogous to that of the prototypical aromatic benzene.^54–56

Fig. 3 Calculated iso-chemical shielding surfaces (ICSSs) of (a) Ta₂B₇⁺ (1), (b) Ta₂B₈ (2), and (c) Ta₃B₁₂⁻ (3), with the corresponding NICS-ZZ components indicated. The C₇ axis of Ta₂B₇⁺ (1), C₈ axis of Ta₂B₈ (2), and C₂ axis of Ta₃B₁₂⁻ (3) are designated as z axis in vertical direction to compare the shielding effects around the TaB_n pyramids (n = 7, 8). Yellow regions stand for chemical shielding areas, while green areas represent chemical de-shielding areas.

Fig. S8† depicts the corresponding ring current maps of Ta₂B₇⁺ (1), Ta₂B₈ (2), and Ta₃B₁₂⁻ (3). Consistent magnetic responses occur in 1, 2, and 3 in an external magnetic field in the vertical direction parallel to the assigned molecular axes, irrespective of the electron type, similar to the situations in both benzene^52,53 and double-ring tubular B₂₀ (61) which are known to possess typical planar or tubular aromaticity. Such diatropic ring currents well support the aromatic nature of these Ta-doped boron complexes.

3.3 Simulated IR, Raman, and PE spectra of Ta₃B₁₂⁻ (3)

We computationally simulate the IR, Raman, and UV-vis or PES spectra of Ta₂B₇⁺ (1), Ta₂B₈ (2), and Ta₃B₁₂⁻ (3) at PBE0 level to facilitate their future spectral characterizations. PE measurements and infrared photodissociation (IR-PD) spectra in combination with first-principles theory calculations have proven to be powerful approaches in characterizing novel clusters in gas phases.^{3–7,17–28,62} As shown in Fig. 4, the high-symmetry Ta₃B₁₂⁻ (3) exhibits relatively simple IR and Raman spectral patterns, with the major IR active peaks at 507 (e′), 559 (e′), 668 , and 881 (e′) cm⁻¹ (Fig. 4a) and main Raman spectral features at 209 , 881 (e′), 998 (e′′), and 1201 cm⁻¹ (Fig. 4b), respectively. The first Raman active symmetrical vibrational mode at 209 cm⁻¹ corresponds to typical “radial breathing mode” (RBM) of the metallo-borospherene which may be used to characterize the hollow structures of single-walled boron nanoclusters in experiments.⁶³

Fig. 4 Simulated (a) IR, (b) Raman, and (c) PES spectra of D_3h Ta₃B₁₂⁻ (3) at PBE0//B/aug-cc-pVTZ/Ta/Stuttgart+2f1g level.

The calculated PE spectrum of Ta₃B₁₂⁻ (3) in Fig. 4c exhibits major spectral features at 2.95 (²E′′), 3.17 , 3.66 , 3.98 (²E′), 4.17 , 5.32 (²E′), 5.68 (²E′), and 5.98 , respectively, corresponding to vertical electronic transitions from the ground state of the anion to the excited states of the neutral at the ground-state geometry of the anion. The open-shell neutral Ta₃B₁₂ has a slightly distorted C_2v Ta₃B₁₂ (²A₂) ground-state structure due to Jahn–Teller effect, with the calculated electron affinity of EA = 2.87 eV. The simulated IR, Raman, and UV-vis spectra of Ta₂B₇⁺ (1) and Ta₂B₈ (2) are depicted in Fig. S9.†

4. Conclusions

Extensive first-principles theory calculations performed in this work indicate a structural transition from perfect inverse sandwich D_7h Ta₂B₇⁺ (1) and D_8h Ta₂B₈ (2) to the smallest metallo-borospherene D_3h Ta₃B₁₂⁻ (3). As the first transition-metal-doped boron complex reported to date with σ + π + δ triple aromaticity, Ta₃B₁₂⁻ (3) possesses three octa-coordinate Ta centres as integral parts of the cage surface coordinated in three equivalent η⁸-B₈ octagons. The results obtained in this work suggest that a large family of coordination-stabilized multi-TM-doped TM_mB_n metallo-borospherenes with tunable magnetic and electronic properties may exist in experiments in which the transition metal dopants and B_n ligands match both geometrically and electronically.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (21720102006 and 21973057 to S.-D. Li).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra05570k

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