Competitive/co-operative interactions in acid base sandwich: role of cation vs. substituents

The cation-π interaction can be envisaged as a lewis acid base interaction, and it is in line with Pearson's acid base concept. The critical examination of interactions between the π-acids (alkali metal cations - Li⁺, Na⁺ and alkaline earth metal cations Mg²⁺, Ca²⁺) on one face and tripodal Cr(CO)₃ moiety on the other π face of substituted arenes demonstrates the role of cation and substitutents in manipulating the interactions between them. The interaction of the two π acids on both faces of arene is not expectedly additive, rather it shows either depreciation of interaction energy revealing the competition of acids toward the base or enhancement of interaction energy denoting a cooperative effect. Among the metal cations under study, Mg²⁺ shows a cooperative gesture. Although the substituents play a meek role, they unfailingly exert their electronic effects and are amply documented by excellent correlation of various parameters with the Hammett constant σ_m. The elusive switching of λ_max from the UV to IR region on binding Mg²⁺ with substituted arene-Cr(CO)₃ complex is a characteristic clue that TDDFT can help design the ionic sensors for Mg²⁺ cations.