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

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 (TPPBrx)Co(x = 0-8) and (TPPBr4NO2)Co(NO). 2CH(3)CH(2)OH was structurally characterized. The porphyrin ring possesses a saddle-shaped conformation as a result of a strong steric interaction which exists between the mesophenyl and beta-pyrrole groups on the macrocycle. The bound nitrosyl group has a bent conformation with a 125 degrees angle and is disordered at two equivalent positions. The nitro substituted beta-pyrrole group shows a tilted conformation and has a dihedral angle of 46 degrees with respect to the mean plane of the porphyrin macrocycle in order to minimize interactions with the closest phenyl group.