REDOX PROPERTIES OF PORPHYCENES AND METALLOPORPHYCENES - A COMPARISON WITH PORPHYRINS

An electrochemical analysis of a series of porphycenes and metalloporphycenes in tetrahydrofuran, N,N-dimethylformamide, and dichloromethane is reported. This analysis combined the use of polarographic, steady-state voltammetric, cyclic voltammetric, and spectroelectrochemical methods. The free-base porphycenes studied were porphycene (H2PC), 2,3,6,7,12,13,16,17-octaethylporphycene (H2OEPc), 2,7,12,17-tetra-n-propylporphycene (H2TPrPc), and its tetrabromo derivative 3,6,13,16-tetrabromo-2,7,12,17-tetra-n-propylporphycene (H2TBrTPrPc), respectively. Each free-base and nickel porphycene was found to undergo two reversible one-electron reductions as well as two oxidations all centered on the tetrapyrrolic ring system. CoTPrPc was reduced in two one-electron steps and oxidized in three one-electron redox processes. The reductions were assigned as occurring on the tetrapyrrolic ligand while the first oxidation step was ascribed to a Co(II) to Co(III) redox process. (Fe(III)TPrPc)Cl was reduced in three steps, and it was oxidized in two one-electron steps. The first of these reduction processes was clearly metal centered [Fe(III) --> Fe(II)]. All four other electron transfers were ligand centered. The corresponding iron mu-oxo dimer (Fe(III)TPrPC)2O underwent four one-electron oxidations as well as four one-electron reductions. The studies reported herein indicate that porphycenes, structural isomers of porphyrins, behave electrochemically much like bacteriochlorins: only two reduction steps are observed in the available potential range, and as compared to those of the porphyrins, smaller potential differences are seen between the first oxidation and the first reduction waves.