Resonance Enhancement via Imidazole Substitution Predicts New Cation Receptors

Design and development of cation receptors represent a fascinating area of research, particularly in dealing with chemical and biological applications that require fine-tuning of cation−π interactions. The electronic nature of a substituent is largely responsible for tuning the strength of cation−π interaction, and recent studies have shown that substituent resonance effect contributes significantly to such interactions. Using substituent resonance effect as a key electronic factor, we have proposed new cation−π receptors (1···M⁺−4···M⁺; M⁺ = Li⁺, Na⁺, K⁺, NH₄⁺, and NMe₄⁺). B3LYP/6-311+G­(d,p) density functional theory (DFT) calculations show that by using a strategy of resonance donation from six nitrogen atoms via three substituted imidazole subunits, more than 4-fold increase in cation−π interaction energy (E_M⁺) can be achieved for a single phenyl ring compared to benzene. The E_M⁺ (M⁺ = NH₄⁺, NMe₄⁺) of 4···M⁺, wherein M⁺ interacts with only one phenyl ring, is significantly higher than E_M⁺ of a known cation host with several aromatic rings (abstract figure). Our hypothesis on resonance enhancement of cation−π interaction is verified using several π-systems (5–10) containing a lone pair bearing six nitrogens and observed that a nitrogen lone pair attached to a double bond is more effective for donation than the lone pair that is directly attached to the benzenoid ring. Further, a convenient strategy to design electron rich π-systems is provided on the basis of topographical analysis of molecular electrostatic potential.