LASER PHOTOLYSIS OF HEMOCHROMES - KINETICS OF NITROGENOUS BASES BINDING TO 4-COORDINATED AND 5-COORDINATED IRON(II) TETRAPHENYLPORPHINE

Six-coordinated complexes of iron(Il) porphyrins with nitrogenous bases as axial ligands (hemochromes) can be photodissociated by a laser pulse. The photodissociation of hemochromes has been used in order to investigate the reactivity of a number of nitrogenous bases toward iron(II) tetraphenylporphin and two derived compounds in which one base (imidazole or pyridine) is covalently linked to the porphyrin macrocycle (chelated models). The recombination of photodissociated hemochromes exhibits a variety of kinetic patterns, which are attributed to the spontaneous dissociation of the unstable five-coordinated species formed upon photolysis. The rate constants for axial ligand binding and dissociation are found to decrease upon increasing the pKa of the external ligand, in contrast to the opposite pKa dependence of the equilibrium constants. Examination of the potential energy terms governing the reaction rates suggests that electrostatic repulsion between ligand and five-coordinated porphyrin dipole moments might account for the variation of the association rate constant. © 1979, American Chemical Society. All rights reserved.