Al42−; the anion–π interactions and aromaticity in the presence of counter ions

Abstract

The influence of presence of counter ions and π-complexation with benzene on the bonding and magnetic properties of Al₄²⁻, the most studied all-metal cluster, is studied here. It is shown that complexation by either counter ions or benzene decreases the delocalization index between Al atoms and the magnitude of bond magnetizability, that is a Quantum Theory of Atoms in Molecules, QTAIM, -based magnetic index of aromaticity. Benzene forms two types of π-complexes with the Al₄ framework; CH–π (T-shaped) complexes and parallel π–π stacking (PPS) complexes. It is shown that variation in the π-charge of the Al₄ framework affects the relative stability of the T-shaped/PPS complexes. Free Al₄²⁻ forms a stable T-shaped anion–π complex with benzene but in the presence of cations, formation of PPS complexes is more favourable, energetically. It is suggested that this property could be used for designing molecular switches and tuneable anion sensors.