Endohedral C₃Ca@B₃₉⁺and C₂Ca@B₃₉⁺: axially chiral metalloborospherenes based on B₃₉⁻

Abstract

Using the newly discovered borospherenes C3 B39(-) and C2 B39(-) as molecular devices and based on extensive global-minimum searches and first-principles calculations, we present herein the possibility of the first axially chiral metalloborospherenes C3 Ca@B39(+) (, (1)A) and C2 Ca@B39(+) (, (1)A), which are the global minimum and the second lowest-lying isomer of CaB39(+), respectively. These metalloborospherene species turn out to be charge-transfer complexes Ca(2+)@B39(-) in nature, with the Ca centre on the C3 or C2 molecular axis donating one electron to the B39 cage which behaves like a superhalogen. Molecular orbital analyses indicate that C3/C2 Ca(2+)@B39(-) possess the universal bonding pattern of σ plus π double delocalization, similar to their C3/C2 B39(-) parents. Molecular dynamics simulations show that both C3 Ca@B39(+) () and C2 Ca@B39(+) () are dynamically stable at 200 K, with the former starting to fluctuate structurally at 300 K and the latter at 400 K, again similar to C3/C2 B39(-). The infrared and Raman spectra of C3/C2 Ca@B39(+) (/) are simulated and compared with those of C3/C2 B39(-) to facilitate their forthcoming experimental characterization.

From Quasi-Planar B56 to Penta-Ring Tubular Ca©B56: Prediction of Metal-Stabilized Ca©B56 as the Embryo of Metal-Doped Boron α-Nanotubes

“…S1(a), the previously predicted quasi-planar C 2 v B 56 ( A-1 ) with two equivalent hexagonal holes27 appears to be 1.05 eV and 1.06 eV more stable than the quasi-planar C 1 B 56 ( A-2 ) with three hexagonal holes around the center and the PR tubular C 2 h α-B 56 ( A-3 ) with four equivalent hexagonal holes in the middle at PBE0 level, respectively. Other low-lying isomers lie at least 1.19 eV above B 56 ( A-1 ), with the much concerned cage-like borospherene C 2 B 56 ( A - 7 ) composed of eighteen interwoven BDCs141519202123 and the irregular cage-like C 1 B 56 ( A-9 ) obtained using first-principles simulated annealing34 being 1.78 eV and 2.64 eV less stable, respectively. With two extra electrons, a quasi-planar C 2 v B 56 2− similar to B 56 ( A-1 ) appears to be 1.07 eV more stable than the PR tubular D 4 h α-B 56 2− similar to α-B 56 ( A-3 ) at PBE0.…”

“…Ca©B 56 ( B-1 ) contains an almost perfect PR tubular C 4 v α-B 56 ligand similar to α-B 56 ( A-3 ) which can be constructed by rolling up the most stable boron α-sheet3233. From another perspective of view, Ca©B 56 ( B-1 ) can be viewed as a PR tube composed of interwoven BDCs with four hexagonal holes evenly distributed in the middle, in a structural pattern similar to that of the B n q borospherene family (q = n-40, n = 36–42)141519202123. C 4 v Ca©B 56 ( B-1 ) turns out to lie 1.18 eV and 1.71 eV lower than the Ca-centered PR tubular D 4 h Ca©B 56 ( B-2 ) based on α-B 56 ( A-3 ) and the face-capped quasi-planar C s CaB 56 ( B-3 ) based on C 2 v B 56 ( A-1 ), respectively.…”

“…The high stability of the 3D aromatic C 4 v Ca©B 56 ( B-1 ) originates from its unique bonding pattern which possesses a perfect delocalized π system over a σ-skeleton on the tube surface. Metal dopants encapsulated in cage-like borospherenes to form metalloborospherenes171920212223, inserted in planar borophenes to form metalloborophenes9, or wrapped up in boron nanotubes to form metal-doped boron nanotubes may effectively enhance the chemical stabilities and tune the transport properties of the boron nanostructures. Metal-stabilized boron nanostructures are expected to be complementary with the corresponding carbon nanomaterials in applications and warrant further theoretical and experimental investigations.…”

“…Cage-like borospherene structures composed of interwoven BDCs were also built for B 56 according to the structural pattern of borospherenes141519202123. In particular, a PR tubular α-B 56 ( A-2 ) with four hexagonal holes evenly distributed in the middle was constructed by rolling up the most stable boron α-sheet3233.…”

“…Two cage-like cations C 1 B 41 + and C 2 B 42 2+ predicted at density functional theory (DFT) have also been presented to the B n q borospherene family (q = n-40) which are all composed of twelve interwoven boron double chains (BDCs) with six hexagonal or heptagonal faces18. Encapsulations of alkaline earth or transition metals have proven to be an effective approach to stabilize borospherene anions like T h B 36 4− in Li 2 &[Ca@B 36 ]19, C s B 37 3− in Ca@B 37 −  20, C s B 38 2− in Ca@B 38  21 and M@B 38 (M = Sc, Y, Ti)2122, and C 3 / C 2 B 39 − in Ca@B 39 +  23. The discovery of the first double-ring (DR) tubular B 20 and the experimental confirmation of the double-ring (DR) tubular B n + monocations (n = 16–25)24, on the other hand, unveils another important domain in the structural evolution of boron clusters.…”