Halogen Bonds Formed by Halogen’s <i>p</i>/π-Hole in Molecules Help Shape Crystalline Materials

This overview shines a spotlight on the often-neglected realm of p-/π-hole halogen bonds, a topic that has garnered relatively scant attention despite being pivotal in molecular chemistry over the past two decades. Historically, research on halogen bonding has predominantly centered on the more familiar σ-hole interactions within molecules, complexes, and crystals, as well as efforts to define noncovalent interactions involving main group elements of the periodic table. The academic landscape is replete with original studies, comprehensive reviews, and detailed analyses of σ-hole halogen bonds, including both experimental and computational investigations. These studies frequently highlight instances where ordinary halogen derivatives serve as σ-hole bond donors. Yet, there remains a notable absence of rigorous exploration into the fundamental principles governing the physical chemistry and chemical physics of electrophilic p-/π-holes within halogen-containing molecules, particularly those featuring hypervalent halogens. A p-/π-hole (or p-/π-belt) halogen bond arises when a halogen atom, possessing a p-/π-hole (or p-/π-belt), engages inductively with a nucleophile from a neighboring molecule, which may be identical or distinct. This overview aims to address this gap by delving into the conceptual evolution of p-/π-hole (or p-/π-belt) across various representative molecules, with or without halogen derivatives, thereby advancing our understanding of this crucial yet underexplored facet of molecular interactions.