The Association Equilibrium Constants and Magnetic Susceptibilities of Hexacoordinate and Pentacoordinate (Nitro)(tetraphenylporphyrinato)cobalt(III) Derivatives

Spectrophotometric titrations have been used to study the equilibrium of association of pyridine ligands with (nitro)cobalt tetraphenylporphyrin in CH2Cl2 solution, and thus to confirm the existence of the reactive pentacoordinate (nitro)cobalt tetraphenyl porphyrin in solution. The PC version of the SPECDEC program was used for data analysis. The value of the equilibrium constant at 25 °C for association of the pyridine was 1.08 x 108 with x2 = 1.29 x 104. The value for the 3,5-dichloropyridine ligand was 6.94 x 106 with x2= 7.22 x 10-4. The formation of the pentacoordinate nitro complex was also achieved by addition of lithium perchlorate and confirmed by its UV-VIS spectrum. In using NMR method to characterize the novel pentacoordinate nitro complex, line broadening was observed. The Evans method was used to assess any paramagnetism in the complex. Lack of paramagnetism suggests that an exchange mechanism may be responsible for the line broadening