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

Abstract

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 ([FeIIITPrPn]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 [FeIIITPrPn]2O undergoes four one-electron oxidations as well as four one-electron reductions. [FeIIITPrPn]Cl forms a stable [FeIIITPrPn](N-MeIm)2+ complex in the presence of N-MeIm. This complex is reduced stepwise to [FeIITPrPn]N-MeIm, the iron(II) radical anion [FeIITPrPn•]- and the iron(II) dianion [FeIITPrPn]2- as confirmed by UV−vis and ESR spectroscopy of the electrogenerated species. In the presence of CO, the reduction of iron(III) generates [FeII(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 [FeII(CO)TPrPn•]- and [FeII(CO)TPrPn]2-. This redox behavior is distinctively different from that of Fe−CO porphyrins. In the presence of CO2, electrocatalytic reduction of CO2 is observed, as for porphyrins.