Spectroelectrochemical investigation of a dinuclear cobalt porphyrin complex with ionic groups in DMSO

Abstract

The dinuclear cobalt porphyrin of (5,10,15,20-tetrakis(N-methyl-4-pyridinio)porphyrinato)cobalt(II) (CoTMPyP) with (5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinato)cobaltate(II) (CoTPPS) has allowed spectroelectrochemical analysis of electron transfer processes in DMSO. In the II–II valence state it undergoes stepwise two-electron transfer accompanied by a dissociation equilibrium (K_d = 1.3 × 10⁻⁶ M) with redox potentials at −0.37 and −0.17 V vs. Fc–Fc⁺. The kinetics of the dissociation allows a snapshot of the species present without perturbing the equilibrium distribution. The dissociation in the II–II valence state is promoted by oxidation to the higher valence states due to the increasing charge on the complex and solvation of DMSO at the cobalt. The dissociation constants of the dinuclear porphyrin in the II–III and III–III valence states were determined to be 2.5 × 10⁻⁶ and 5.0 × 10⁻⁶ M, respectively, using Job plots. EXAFS analysis reveals that the Co ⋯ Co distance in the dimeric porphyrins is 3.37 Å which is not drastically influenced by the valence state in the solid state. The dinuclear cobalt porphyrin complex acts as an excellent catalyst for reduction of oxygen in which the number of electrons transferred was determined to be n = 3.9 based on Koutecky–Levich plots.