Electrochemical and Spectroelectrochemical Investigations of “Figure-Eight” Octapyrrolic Macrocycles, Free Bases, and Their Mononuclear, Homo- and Heterodinuclear Complexes with Divalent Transition Metals

Abstract

This work reports on electrochemical investigations of “figure-eight”-shaped cyclooctapyrrolic macrocycles (octaphyrins) 1−3 and of some of their mono- and dinuclear transition metal complexes. The octaphyrins 1−3 differ mainly in the size of the main conjugation pathway, which involves either 4n π-electrons for 1 and 3, or (4n + 2) π-electrons for 2. All species studied undergo six redox steps, namely two reductions and four oxidations. The two reductions and the first three oxidations are reversible one-electron transfers. The fourth oxidation is either reversible, or irreversible due to a subsequent chemical reaction. Depending on the complex studied, the third and fourth oxidations occur either at similar potentials, so that a global two-electron step is observed, or at distinct potentials, leading to two well-separated signals. Spectroelectrochemical investigations demonstrate that all redox processes are ligand centred. The reported results clearly indicate that the redox potential changes in these new octapyrrolic macrocycles depend much more on structural contributions, due to their figure-eight topography, than on characteristics such as ionic radius or electronegativity of the coordinated metal. The HOMO-LUMO gap in the series of octaphyrins studied is rather small (about 1 eV), and is not only related to the macrocyclic ring size, but also depends on whether the main conjugation pathway involves 4n or (4n + 2) π-electrons. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)