Prediction of endohedral borafullerenes X@BC (X = CH, BH, HO, and NH) with a BC shell isovalent with C.

Context: Inspired by the newly synthesized endohedral fullerene T CH@C (1) and based on extensive density functional theory calculations, we predict herein a series of endohedral borafullerenes C CH@BC (4), T BH@BC (5), C HO@BC (6), C NH@BC (7), and T C@BC (8) which possess a BC (3) shell isovalent with C, with the neutral D C@BC (9) obtained from C@BC (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@BC) in HO@BC (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 C CH@BC (4), T BH@BC (5), C HO@BC (6), C NH@BC (7), T C@BC (8), and D C@BC (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.