The Incorporation of Zinc Ion into a Synthetic Water-Soluble Porphyrin

The synthesis of a water-soluble porphyrin, C₃₈H₅₄N₁₂O₂. 8H₂O, is described. Kinetics of the zinc incorporation of the porphyrin are reported for aqueous solutions. The rate law at fixed pH (6.23) and 30˚ is d[ZnP]/dt = (14.2 M^-1 min^-1)[Zn²⁺][H₂P]. The reaction is reversible, and the first-order rate constant for zinc replacement is 1.7 × 10^-4 min^-1. The forward reaction is shown to be catalyzed by pyridine, 2^-, 3^-, and 4-methylpyridines, imidazole, and methylimidazole. Of these, imidazole shows the greatest catalysis, and it also catalyzes the reverse reaction. There is no catalysis by the sterically hindered nitrogen base collidine which serves as buffer for the rate experiments. The activation energy measured for the uncatalyzed zinc incorporation is 11.7 kcal/mole, and 12.3 kcal/mole for the pyridine-catalyzed path indicating a preequilibrium having favorable entropy. Formation and absorption spectra of metalloporphyrins of Ag(II), Sn(IV), Cu(II), In(III), and Mn(II) are also described.