Synthesis and electrochemistry of cobalt β-halogenated meso-tetraphenylporphyrins containing a nitrosyl axial ligand. Crystal structure of (TPPBr4NO2)Co(NO)

Abstract

Three new cobalt porphyrins, two of which are coordinated by a nitrosyl group have been synthesized and characterized by spectral and electrochemical methods. The investigated compounds are represented as (TPPBr₄)Co(NO), (TPPBr₄NO₂)Co and (TPPBr₄NO₂)Co(NO) where TPPBr₄ and TPPBr₄NO₂ are the dianions of 7,8,17,18-tetrabromo-5,10,15,20-tetraphenylporphyrin and 2-nitro-7,8,17,18-tetrabromo-5,10,15,20-tetraphenylporphyrin, respectively. Up to three oxidations and three reductions are observed for each compound in CH₂Cl₂ or pyridine containing 0.1 M TBAP. The first one-electron oxidation leads in all cases to formation of a Co(III) species while the first one-electron reduction leads invariably to a Co(I) porphyrin. The nitrosyl group remains coordinated to the Co(I) ion after electrogeneration of [(TPPBr₄NO₂)Co(NO)]^– and [(TPPBr₄NO₂)Co(NO)]^2– on the thin-layer spectroelectrochemical timescale and this enables the first UV–vis characterization of reduced cobalt porphyrins containing a bound NO axial ligand. The oxidation and reduction potentials of (TPPBr₄)Co(NO), (TPPBr₄NO₂)Co and (TPPBr₄NO₂)Co(NO) are analyzed in terms of related redox reactions involving (TPP)Co(NO) and (TPPBr_x)Co (x = 0–8) and (TPPBr₄NO₂)Co(NO)·2CH₃CH₂OH was structurally characterized. The porphyrin ring possesses a saddle-shaped conformation as a result of a strong steric interaction which exists between the meso-phenyl and β-pyrrole groups on the macrocycle. The bound nitrosyl group has a bent conformation with a 125° angle and is disordered at two equivalent positions. The nitro substituted β-pyrrole group shows a tilted conformation and has a dihedral angle of 46° with respect to the mean plane of the porphyrin macrocycle in order to minimize interactions with the closest phenyl group.