The stability of porphyrin and metalloporphyrin molecular complexes in solution

Stability constants for the molecular complexes formed by protoporphyrin IX dimethyl ester and its metal chelates with Co2+, Ni2+, Cu2+, Zn2+, Mg2+ and Fe3+ with nitroarene acceptors are determined in non-aqueous solvents. The stability of the complexes is found to be largely insensitive to variations in the central metal ions provided they are in the same oxidation state. Solvent-solute interactions are seen as being important in the formation of such complexes giving rise to large variations in their apparent stability. Some of the other factors that affect the stability of the molecular complexes are also evaluated. No evidence has been found for the appearance of any intermolecular charge-transfer absorption bands. Infrared studies, however, have provided some evidence for a substantial loss of electron density from the porphyrin ring upon interactions with the acceptors.