Exploring Anion-π Interactions and Their Applications in Supramolecular Chemistry

CONSPECTUS: Noncovalent bond interactions provide primary driving forces for supramolecular processes ranging from molecular recognition to self-assembly of sophisticated abiotic and biological machineries. While hydrogen bonding and pi-pi interactions are arguably textbook concepts playing indispensable parts in various scientific disciplines, noncovalent anion-pi interactions have been emerging as attractive forces between pi systems and negatively charged species for just about two decades. At the beginning of this century, three research groups reported independently their computational studies on the interactions between anions and aromatic compounds, proposing attractive anion-pi interactions. Since pi systems such as aromatic rings are traditionally noted as electron rich entities, anions and pi systems would be repulsive to each other if there are any interactions. In stark contrast to the acknowledged cation-pi interactions, the seemingly counterintuitive noncovalent anion-pi bindings invoked great interest in the following years. Although a plethora of calculations had been published, the lack of experimental evidence cast doubt on the existence of anion-pi interactions between anions and charge-neutral aromatic systems. During the same time when anion-pi interactions were coined, we were studying the chemistry of novel macrocyclic compounds, namely, heteracalixaromatics, and their applications in supramolecular chemistry. It has been shown that heteracalixaromatics are powerful and versatile macrocyclic hosts to bind various guest species forming interesting assembled structures and organometallic complexes. Being a member of heteracalixaromatics, tetraoxacalix[2]arene[2]triaizne adopts a 1,3-alternate conformational structure yielding a V-shaped cavity or cleft formed by two electron-deficient triazine rings. Advantageously, the macrocycle is able to self-tune the cavity sizes by altering the degrees of conjugation between the bridging oxygen atoms with their bonded aromatic rings in response to the guest species in present, rendering it an ideal tool to explore anion-pi interactions. We initiated our study on anion-pi interactions using tetraoxacalix[2]arene[2]triazine as a molecular tool with the primary aim to clarify experimentally the uncertainty of whether exclusive anion-pi interactions exist between anions and charge-neutral aromatic rings. We provided for the first time concrete evidence substantiating the formation of typical anion-pi interaction between the anions and 1,3,5-triazine ring and demonstrated subsequently the generality and binding motifs of anion-pi interactions. We have then extended our study to anion-pi interaction-directed or -driven anion recognition and selective sensing, transmembrane anion transport, molecular self-assembly, and stimuli-responsive aggregation systems. A number of new generation macrocycles and cages constructed from electron-deficient tetrazine and benzenetriimide segments have also been developed in the meantime, advancing the study of anion-pi interactions. This Account summarizes our endeavors to explore nascent anion-pi interactions and their applications in supramolecular chemistry. We hope this Account will inspire scientists from various disciplines to explore all aspects of the nascent yet fruitful research area of anion-pi interactions.