Abstract:
Design and development of cation receptors
represent a fascinating area of research, particularly in dealing
with chemical and biological applications that require finetuning
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 (EM
+) can be
achieved for a single phenyl ring compared to benzene. The EM
+ (M+ = NH4
+, NMe4
+) of 4···M+, wherein M+ interacts with only
one phenyl ring, is significantly higher than EM
+ 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.
