Preliminary note
Electrocatalytic reduction of molecular oxygen using non-planar cobalt tetrakis-(4-sulfonatophenyl)-β-octabromoporphyrin

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Abstract

Synthesis of a new water soluble porphyrin, cobalt(II) tetrakis-(4-sulfonatophenyl)-β-octabromoporphyrin (Br8TPPS)Co and catalytic reduction of molecular oxygen to hydrogen peroxide using this porphyrin is described. The investigated porphyrin is highly non-planar and water soluble due to the presence of eight bromides on the β-pyrrole positions and four negative charges of the sulfonatophenyl groups on the meso positions. The first reversible reduction of (Br8TPPS)CoII is diffusion controlled one-electron transfer, located at E12 = − 0.59V vs. Ag|AgCl in a pH 8.9 solution and leads to the formation of a stable [(Br8TPPS)CoI]. The synthesized cobalt(II) porphyrin binds dioxygen and catalytically reduces it to peroxide with an over voltage for dioxygen reduction of nearly 250 mV. Cyclic and rotating ring-disc voltammetric techniques are used to examine the mechanistic aspects of the catalytic process.

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      These observed anodic shifts following β-bromination could be ascribed to effects of the bulky and electron-withdrawing bromine substituents in the β-pyrrole positions of the porphyrin ring. In fact, several electrochemical studies have reported that the type and number of β-halogen atoms in a porphyrin ligand as well as the nature of the metal center crucially affect the redox behavior of these systems [21–24,26,46,47,51,53,55,57,58,111,126–131]. The hydroxylation of cyclohexane by PhIO or PhI(OAc)2 was carried out at room temperature, using Mn(III)T3PyPCl or Mn(II)Br8T3PyP as catalysts.

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