Investigations of the Boron Buckyball B₈₀: Bonding Analysis and Chemical Reactivity

Abstract

The boron fullerene B₈₀ is a spherical network of 80 boron atoms, which has a shape similar to the celebrated C₆₀. The 80 Bs span two orbits: while the first contains 60 atoms localised on the vertices of a truncated icosahedron like C₆₀, the second includes 20 extra B atoms capping the hexagons of the frame. Quantum chemical calculations showed that B₈₀ is unusually stable and has interesting physical and chemical properties. Its geometry is slightly distorted from I _h to T _h symmetry. However, the boron buckyball is only observed in silico, so far the synthesis of this molecule is only a remote possibility. Using DFT at the B3LYP/SVP level, we have analyzed the chemical bonding in B₈₀, the possibility of methyne substitution and the stability of endohedral boron buckyball complexes. A symmetry analysis revealed a perfect match between the occupied molecular orbitals in B₈₀ and C₆₀. The cap atoms transfer their electrons to the truncated icosahedral frame, and they contribute essentially to the formation of σ bonds. The frontier MOs have π character and are localised on the B₆₀ truncated icosahedral frame. The boron cap atoms can be replaced by other chemical groups, such as methyne (CH), which are also able to introduce three electrons in the cage. Symmetrical substitutions of the boron cap atoms by methyne groups in T and T _h symmetries revealed two stable endo methyne boron buckyballs, endo-${\mathrm{B}}_{80-\mathrm{x}}\mathrm{{(CH)}_{x}}$, with x = 4, 8. The stability of these compounds seems to be due to the formation of six boron 4-centre bonding motifs in between the substituted hexagons. These localized bonding motifs are at the basis of the observed symmetry lowering, via a pseudo-Jahn-Teller effect. The methyne hydrogen atoms in the two endohedral fullerenes can be replaced by other atoms, which can lead to cubane or tetrahedral endohedral boron fullerenes. Theoretical study on encapsulated small bases molecules, tetrahedral and cubane like clusters of Group V atoms, showed that the boron buckyball is a hard acid and prefers hard bases like NH₃ or N₂H₄, to form stables off-centred complexes with B₈₀. Tetrahedral and cubane like clusters of this family are usually metastable in the encapsulated state, due to steric strain. The most favorable clusters are mixed tetrahedral and cubane clusters formed by nitrogen and phosphorus atoms such as $\mathrm{{P}_{2}{N}_{2}@{B}_{80},\ {P}_{3}N@{B}_{80}}$ and P₄N₄@B₈₀. The boron cap atoms act as electrophilic centres, which react with nucleophilic sites rich in electrons.