Cation–π interaction in potassium–polyene complexes and the fate of potassium ion: A theoretical study

Ab initio studies have been done at B3LYP/6-31G* level of theory to determine the structural changes on the substitution of potassium to odd-numbered all-trans conjugated polyenes ranging from C5 to C13. The results show that potassium is always positioned in the form of K+ above an odd carbon other than the terminal carbons and that the stablest structural isomer is the one with K+ lying above the central odd carbon. If a central odd carbon does not exist (as in C11-system), then K+ will be positioned above the odd carbon nearest to the central carbon to achieve maximum stability. The difference in the binding energies of the isomers is generally small and it becomes insignificantly small as the carbon chain length increases so that under suitable conditions K+ ion may be made to move between the ends of the polyene. The metal–polyene complexes are seen to have a considerably reduced HOMO–LUMO gap. Further, the interaction of potassium with the polyene not only caused a total rearrangement of the bond lengths, bond angles and dihedral angles, but also induced a bend (warping) to the polyenic fragment that pockets K+. The structural changes and stability of the metal–polyene complexes are explained in terms of electrostatic interaction and cation–π cloud interaction.