Understanding the Interactions of Anions with Lewis Acids and π-Acids to Develop Efficient Ion-Pair Receptors

Anions play vital roles in many industrial, environmental, chemical, and biological processes. Due to their adverse effects on our health and ecosystem, precise regulation of toxic anions and ion-pairs are of prime importance. A majority of anion receptors rely on subtle supramolecular forces like electrostatic interactions and hydrogen bonding to bind anions, and the main source of selectivity comes from the shape and size complementarity of the receptors and guest anions. In aprotic solvents, interactions of anions with π-acidic receptors such as naphthalenediimides (NDIs) and perylenediimides (PDIs), and Lewis acids, such as silver ion, depend on their electron donating ability, which runs parallel to their Lewis basicity. While non-basic larger charge-diffuse anions engage in anion–π and CH···anion interactions in aprotic solvents, stronger Lewis basic and less solvated hard anions participate in electron transfer (ET) and weaker Lewis basic anions establish charge transfer (CT) interactions. The anion-induced electron transfer from strong Lewis basic anions produces colorful radical anions and dianions of π-acids leading to colorimetric anion sensing. Anion-induced electron transfer (AIET) is not only limited to π-acids. Reduction of silver(I) to produce silver(0) mirrors and luminescent silver(0) nanoparticles show that AIET can happen from fluoride to a Lewis acid, too. On the other hand, NDI-imine-based heteroditopic ion-pair receptors are developed to capture both cations and charge diffuse anions, cooperatively. These synthetic receptors exploited anion–π and CH···anion interactions to bind anions cooperatively in the presence of metal ions such as Zn(II) and Cd(II)