Borane-Mediated Polyhedral Expansion to Access Metal-Free Neutral and Cationic Derivatives of closo-Heptaboranes

Boranes with closed polyhedral structures feature peculiar bonding and structural characteristics, rendering them widely applicable in diverse research areas ranging from basic functionalization reactions to applications such as medicine, nanomaterials, molecular electronics, and neutron capture therapy. Among the closed borane family, the neutral and cationic heptaborane B₇ clusters have been missing in contemporary boron cluster chemistry to date. Herein, we report a polyhedral expansion protocol to construct a neutral derivative of closo-heptaborane (B₇) from closo-hexaborane (B₆) mediated by borane. Conversion of the neutral derivative of closo-heptaborane to a cationic derivative is also demonstrated. X-ray crystallographic and spectroscopic analyses with the aid of quantum chemical calculations reveal that both neutral and cationic derivatives of closo-heptaborane exhibit a pentagonal-bipyramidal geometry and involve the delocalized σ skeletal electrons, leading to three-dimensional aromaticity. Moreover, the B₇ core of the former undergoes a complexation reaction with silver tetrafluoroborate, representing the first experimental demonstration of the nucleophilic nature of the closo-heptaborane.