Electrochemistry of Co–Ru hetero-dinuclear porphyrin complex in a Nafion matrix

Abstract

The metallo porphyrins possessing four ionic substituents of (meso-tetrakis(N-methyl-4-pyridiniumyl)porphyrinato)ruthenium(II) (RuTMPyP) and (meso-tetrakis(4-sulfonatophenyl)porphyrinato)cobalt(II) (CoTPPS) spontaneously associate. The equilibrium constant of dissociation (K_d) of the Co–Ru porphyrin was estimated to be 5 × 10⁻⁷ mol dm⁻³ by Job's titration. A single redox wave based on a 2-electron transfer was observed at 0.19 V s. SCE using cyclic voltammetry in a Nafion matrix. Potential-step chronocoulometry (PSCC) and potential-step chronoamperometry (PSCA) determined the kinetics of the electron transfer (k₀ = 1.47 cm s⁻¹) from the electrode to the complex in the Nafion film, and the apparent diffusion coefficient of electrons (D_app = 8 × 10⁻⁸ cm² s⁻¹) in the Nafion film. Dimerization of the Co and Ru porphyrins results in an accelerated 2-electron transfer because the electrons are transferred ia overlapping π orbitals on the porphyrin ring. The film containing the dinuclear porphyrins shows sufficient catalytic activity toward dioxygen with a 4-electron reduction, based on successive electron transfer from the electrode to dioxygen.