Coordination Chemistry of Iron Porphycenes in the Presence of CO, CO₂, and N-Methylimidazole:  Electrochemical, ESR, and UV−Vis Study

The electrochemical behavior of iron porphycenes in benzonitrile, tetrahydrofuran, N, N -dimethylformamide, and dichloromethane in the presence of Lewis bases is reported. This analysis combined the use of polarographic, steady-state voltammetric, cyclic voltammetric, and spectroelectrochemical methods. Iron(III) 2,7,12,17-tetra- n -propylporphycene ([Fe III TPrPn]Cl) is reduced in three one-electron steps and is oxidized in two one-electron steps: All the processes are reversible. The corresponding iron μ-oxo dimer [Fe III TPrPn] 2 O undergoes four one-electron oxidations as well as four one-electron reductions. [Fe III TPrPn]Cl forms a stable [Fe III TPrPn](N-MeIm) 2 + complex in the presence of N-MeIm. This complex is reduced stepwise to [Fe II TPrPn]N-MeIm, the iron(II) radical anion [Fe II TPrPn • ] - and the iron(II) dianion [Fe II TPrPn] 2 - as confirmed by UV−vis and ESR spectroscopy of the electrogenerated species. In the presence of CO, the reduction of iron(III) generates [Fe II (CO)TPrPn]. In contrast to porphyrins, one- and two-electron reductions of the latter compound are shifted to more negative potentials in DMF and THF and these electron transfers remain reversible. UV−vis spectroelectrochemical experiments at an optically transparent thin layer electrode (OTTLE) demonstrate that the generated species are respectively [Fe II (CO)TPrPn • ] - and [Fe II (CO)TPrPn] 2 - . This redox behavior is distinctively different from that of Fe−CO porphyrins. In the presence of CO 2, electrocatalytic reduction of CO 2 is observed, as for porphyrins.