Aqueous electrochemistry of mono- and bi-nuclear copper(II) complexes with polyaza[n]paracyclophane ligands

Abstract

The aqueous electrochemistry of the Cu²⁺ complexes of the azaparacyclophanes 2,6,9,13-tetraaza[14]paracyclophane (L¹), 2,5,8,11-tetraaza[12]paracyclophane (L²) and 2,5,8,11,14-pentaaza[15]paracyclophane (L³) has been studied. Copper(II) forms stable mononuclear [CuH_rL]^(2+r)+ species with all three ligands (r=–1 to 1, L¹ and L²; –1 to 2, L³) while L² and L³ also form binuclear [Cu₂H_rL]^(4+r)+ species (r=–2, L²; 0 to –2, L³) in a pH-dependent stepwise manner. Frozen-solution ESR measurements show that all the complexes prefer a square-planar co-ordination. In neutral and basic media reduction at mercury and glassy carbon electrodes occurs in two quasi-reversible steps, indicating the formation of stable Cu⁺ species. Stabilization of the Cu⁺ complexes with respect to disproportionation in the order L³ > L¹ > L² has been found.