Deciphering Pyramidanes: A Quantum Chemical Topology Approach

Abstract

C[C4H4], the simplest compound of the [4]‐pyramidane family, has so far eluded experimental characterization, although several of its analogs E[C4(SiMe3)4], in which the E apex atom is a tetrel group element, have been successfully prepared. The non‐classical bonding mode of E, similar to that found in propellanes, has prompted a considerable number of theoretical studies to unravel the nature of the apex‐base interaction. Here, we contribute to this knowledge by analyzing the electron localization function (ELF) and classical QTAIM descriptors; as well the statistical distribution of electrons in atomic regions by means of the so‐called electron distribution functions (EDFs), calculation of multicenter indices (MCI) as aromaticity descriptors and by performing orbital invariant energy decompositions with the interacting quantum atoms (IQA) approach on a series of E[C4(SiMe3)4] compounds. We find that the bonding evolves from covalent to electrostatic as E changes from C to Pb, with an anomaly when E=Si, which is shown to be the most charged moiety, compatible with an aromatic [C4(SiMe3)4]2‐ scaffold in the pyramidane base.