Prediction of endohedral borafullerenes X@B32C36 (X = CH4, BH4−, H2O, and NH3) with a B32C36 shell isovalent with C60

Context

Inspired by the newly synthesized endohedral fullerene T CH₄@C₆₀ (1) and based on extensive density functional theory calculations, we predict herein a series of endohedral borafullerenes C₃ CH₄@B₃₂C₃₆ (4), T BH₄@B₃₂C₃₆^– (5), C₁ H₂O@B₃₂C₃₆ (6), C₃ NH₃@B₃₂C₃₆ (7), and T C₈@B₃₂C₃₆^2– (8) which possess a B₃₂C₃₆ (3) shell isovalent with C₆₀, with the neutral D₂ C₈@B₂₄C₄₄ (9) obtained from C₈@B₃₂C₃₆^2– (8) by symmetric C─B substitutions. Detailed adaptive natural density partitioning (AdNDP) bonding analyses and iso-chemical shielding surfaces (ICSSs) calculations indicate that these core–shell species are spherically aromatic in nature, rendering high stability to the systems. More interestingly, based on the calculated effective donor–acceptor interaction between LP(O) → LV(B@B₃C₃) in H₂O@B₃₂C₃₆ (6), we propose the concept of boron bond (BB) in chemistry which is defined as the in-phase orbital overlap between an electronegative atom A as lone-pair (LP) donor and an electron-deficient boron atom with a lone vacant (LV) orbital as LP acceptor. A boron bond appears to possess about 20 ~ 30% of the bond dissociation energy of a typical A-B covalent bond.

Methods

Extensive density functional theory investigations at the hybrid M06-2X-D3 and PBE0-D3 levels with the basis set 6-311 + G(d) were employed to fully optimize the structures of endohedral C₃ CH₄@B₃₂C₃₆ (4), T BH₄@B₃₂C₃₆^– (5), C₁ H₂O@B₃₂C₃₆ (6), C₃ NH₃@B₃₂C₃₆ (7), T C₈@B₃₂C₃₆^2– (8), and D₂ C₈@B₂₄C₄₄ (9), with natural bonding orbital (NBO) and adaptive natural density partitioning (AdNDP) analyses performed to analyze the bonding patterns of the concerned species and the non-covalent interactions reduced density gradient (NCI-RDG) approach utilized to identify the types of the intramolecular non-covalent bonding interactions.

Graphical abstract