Spectroscopic characterization of coaxially and π—π stacked binuclear copper(II) complexes of ω,ω′-bis(pyridine-2-carboxamido)alkanes in acidic solution

Abstract

Using electron spin resonance (ESR) and visible spectroscopies, coaxially stacked binuclear copper(II) complexes of ω,ω′-bis(pyridine-2-carboxamido)alkanes in acidic aqueous solution (pH 3.8–5.0), H 2peda (alkane = ethane), H 2ppda (alkane = propane), and H 2pbda (alkane = butane), were characterized. trans-1,2-Bis(pyridine-2-carboxamido)cyclohexane or trans-H 2pcyhda (alkane = trans-1,2-cyclohexane) did not provide coaxially stacked binuclear complex, because of the lower conformational freedom at the HNCH-CHNH bond. The coaxially stacked structure causes π—π stacking between the pyridyl rings. The conformation of the propanediyl chain is gauche—gauche, while the ethanediyl and butanediyl chains contain an eclipsed conformation. The driving force for the formation of these binuclear complexes in acidic solution is intramolecular π—π stacking interaction between the pyridyl rings in concert with hydrophobic interaction. Although H 2ppda gave crystals of a binuclear complex with many lattice water molecules, H 2peda and H 2pbda ligands provided crystals of copper(II) polymer complexes from the respective acidic solution. The trans-H 2pcyhda ligand forms a copper(II) binuclear complex with open-chain structure in acidic solution. Crystal structures of the polymer complexes are also presented.