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.