Al₂B₇: an extension of the inverse sandwich B₉ cluster featuring Lewis acid sites and planar aromaticity

In this work, we employ density functional theory (DFT) to explore the structure of boron clusters doped with two aluminum atoms (Al₂B₇). The results show that the most stable structure is an inverted sandwich bipyramidal configuration formed by a B₇ ring coordinated with two Al atoms, as an inverse sandwich motif similar to B₉, while the higher energy isomers correspond to peripheral Al₂-doped B₇ structures. In Al₂B₇, the formation of two separate Lewis acid sites serves for further exploration of small and light clusters with catalytic activity. The bonding features reveal the decrease of the bond order between the capped Al₂ motif, in contrast to the recently characterized B₉ cluster. Interestingly, the aromatic behavior of such clusters is of planar aromatic character despite their three-dimensional structures, which adds to the scarcity of examples in the literature. Our observation further encourages the quest for small and light clusters featuring localized Lewis sites, which are introduced by a certain number of heteroatoms and serve as building blocks of larger and extended architectures.