Differentiation of cation-π bonding from cation-π intermolecular interactions:: A quantum chemistry study using density-functional theory and Morokuma decomposition methods

A strong interaction called cation-pi bonding, which we named because it occurs between aromatics and divalent metal cations, has been successfully differentiated from the normal cation-pi intermolecular interactions. Our findings were based on the B3LYP/6-311++G(d,p) calculations and Morokuma decomposition analyses on the complexes formed by substituted benzenes with alkaline metal and alkaline earth metal ions. In comparison with the common cation-pi intermolecular interaction, the cation-pi bond in the complexes of either Be2+ or Mg2+ with the aromatics has its own characteristics: (a) short bond lengths, (b) very strong binding strength, (c) significant nonelectrostatic interaction that constitutes more than 50% of the total binding strength, (d) obvious cation-pi orbital interaction, and (e) special orbital interaction pattern that only the pi orbitals of the aromatics interact with the s, p(x), and p(y) atomic orbitals of metal cations for forming bonding MOs. While the electrostatic interaction is significantly affected by the nature of the substituents attached to the benzene, the nonelectrostatic interaction and orbital interaction are not. Furthermore, the total binding strength and electrostatic interaction are well correlated with the Hammett electronic parameters. This structural and thermochemical information is highly useful in identifying cation-pi bonds. Moreover, they are equally helpful for modifying current force fields in reproducing this unusual chemical bond that is commonly encountered in both chemical and biological systems.