The metalation kinetics of water-soluble porphyrins

The kinetics of Mg2+ incorporation into tetra(N-methyl-X-pyridyl)porphyrin where X = 2, 3, or 4 and beta-octabromo-tetra(N-methyl-4-pyridyl)porphyrin [BrP(4)] were investigated spectrophotometrically from pH 6.5 to 8.9 at an ionic strength of 2.6. The rate law was first order in porphyrin and Mg 2+ concentrations. The free base (H₂-P), monoanion (H-P −), and the dianion (P2−) reacted in the order P2− > H-P− > H₂-P for all of the compounds. In solutions containing the mono- and di-anions of BrP(4) Mg2+ reacted with only the dianion.; The kinetics of Cu2+, Zn2+, Co2+ , and Ni2+ incorporation into BrP(4) was studied below pH 6. The H₃-P+ was un-reactive and the H-P − reacts 500–1000 times faster than does H₂-P. The reactivity paralleled the water exchange rate constants of the entering ions, Cu2+ > Zn2+ > Co2+ > Ni 2+, suggesting a dissociative-interchange mechanism. The brominated porphyrin reacted with these cations much more rapidly than did the parent non-brominated derivative.; The kinetics of HO-Zn-BrP(4) formation in base from the dianion BrP(4) 2+ in base indicated that Zn(OH)₂ and Zn(OH)₃ − were reactive, while Zn(OH)₄2− was unreactive. Lithium forms a mononuclear complex with the dianion. With both forms present, the Li-BrP(4)3+ was unreactive, and only the dianion reacted with the zinc species to form the zinc porphyrin.; Numerous studies have been done on the reactivity of metalloporphyrins with small gaseous molecules. These studies lead to the possibility of using metal loporphyrins as sensors for trace amounts of gases. The octa-brominated compound has the ability to stabilize metals in lower oxidation states and can be possibly used as a sensor for gases.