Electrochemical properties of 'figure-eight'-cyclooctapyrroles (octaphyrins) and their metal complexes

In this paper the first electrochemical results obtained on 'figure-eight'-cyclooctapyrrolic macrocycles (octaphyrins) 1 to 3, and on some of their mono- and bi-nuclear transition metal complexes, i.e. compounds 5-8 and 9-16, respectively, are reported. All these species undergo five distinct redox steps. The two reductions and the first two oxidations are reversible one-electron transfers. The third oxidation is a global two electron reaction except in the Co(IT) complexes. In the latter, this step corresponds to the Co(II) --> Co(III) oxidation. The free base octaphyrins display electrochemical gaps (E-ox1 - E-red1) from 1.43 to 0.85 V, lower than any previously observed gap in porphyrins and porphycenes, and-at further variance with these tetrapyrrolic molecules - this gap changes with the coordinated metal. Another special feature of binuclear metallo-octaphyrins is an intense and sharp absorption band exhibited at 700 nm and beyond in their two electron reduced form. (C) 1999 Elsevier Science S.A. All rights reserved.