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 gro up have been synthesized and characterized by spectral and electrochemical methods. The investigated compounds are represented as (TPPBr4)Co(NO), (TPP Br4NO2)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-7,8,1 7,18-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 re duction leads invariably to a Co(I) porphyrin. The nitrosyl group remains c oordinated to the Co(I) ion after electrogeneration of [(TPPBr4NO2)Co(NO)]( -) and [(TPPBr4NO2)Co(NO)](2-) on the thin-layer spectroelectrochemical tim escale 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 anal yzed 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 character ized. The porphyrin ring possesses a saddle-shaped conformation as a result of a strong steric interaction which exists between the mesophenyl and bet a-pyrrole groups on the macrocycle. The bound nitrosyl group has a bent con formation with a 125 degrees angle and is disordered at two equivalent posi tions. 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 t he porphyrin macrocycle in order to minimize interactions with the closest phenyl group.