Electrooxidation of Cobalt(II) β-Brominated-Pyrrole Tetraphenylporphyrins in CH₂Cl₂ under an N₂ or a CO Atmosphere

Abstract

The electrochemical behavior of (TPPBrx)Co (TPPBrx = the dianion of β-brominated-pyrrole tetraphenylporphyrin and x = 0−8) is reported under an N2 or a CO atmosphere in dichloromethane containing tetra-n-butylammonium hexafluorophosphate as supporting electrolyte. Each investigated compound undergoes three reversible one-electron oxidations within the potential window of the solvent, and this results in the ultimate formation of a cobalt(III) dication as the final three-electron-oxidation product under both an N2 and a CO atmosphere. The initial one-electron abstraction from (TPPBrx)Co involves the central metal ion for derivatives with x ≤ 5 and the conjugated porphyrin π ring system for derivatives with six, seven, or eight Br groups. Thin-layer infrared spectroelectrochemical measurements were performed during the first oxidation of (TPPBrx)Co in dichloromethane and indicate that in situ generated [(TPPBrx)Co]+ forms mono- and bis(carbon monoxide) adducts for complexes with zero to five Br groups but that no CO molecule binds to the singly oxidized derivatives with six, seven, or eight Br groups, all of which exist as Co(II) π cation radicals.