A theoretical study of tautomerism in hexa-aza macrocycles containing 2,2′-bipyridine and 1,10-phenanthroline and their ability to form lithium complexes

Abstract

Calculations using ab initio restricted Hartree–Fock molecular orbital (RHF MO) theory and the density functional theory (DFT) are reported for several structural isomers of hexa-aza macrocycles 1– 3 containing 2,2′-bipyridine and 1,10-phenanthroline, as well as their lithium complexes. Based on the computational results, we describe the geometries, imine/amine tautomerism, and lithium complexation of these hexa-aza macrocycles. Metal-free macrocycles have the most stable imine form with planar structures, whereas their lithium complexes tend to be in the amine form. We discuss protonation/deprotonation processes with proton transfer in a cavity of macrocycles as a means of interpreting imine/amine tautomerism. Deprotonation and the subsequent geometric changes in tautomeric isomers lead to structural changes, i.e. macrocycles 2 and 3 having 2,2′-bipyridines become non-planar; this theoretical result is consistent with observed experimental behaviors. The present results indicate that unlike macrocycle 1, macrocycles 2 and 3 with 2,2′-bipyridine have the capacity for amine/imine tautomerism, as well as the ability to form complexes with lithium.