Lithium complexes and the kinetics of interactions of zinc ions with tetra(N-methyl-4–pyridyl)porphyrins in basic solution

The acidity of the free base (H2–P(X)) forms of the tetra- (N-methyl-4–(3 or 2)pyridyl)porphyrins were studied in basic solutions at 25 C, I=0.50. Equilibrium constants for both the H2– P(X)=P(X)2–+2H+ and the Li++ P(X)2–=Li- P(X)– reactions are reported. Log (KS) values for the Mn++P(4)2–=M- P(4)(n–2) reactions are 2.6 for Li+, 17.6 for Cd2+, 17.8 for Pb2+, 19.6 for (OH)2Hg, 25.9 for Zn2+ and 19.6 for the formation of Hg2–P(4)2+. Zn(OH)3– shows similar kinetic reactivity with both H2–P(4) and P(4)2– to form Zn- P(4) and HO-Zn- P(4)–,whileZn(OH)42– is unreactive with either species. For Zn2+, Zn(OH)+ and Zn(OH)3– with H2–P(4) the relative kinetic order for this tetrapositive macrocycle was ca. 1:300:20,000, while the trend Zn(OH)+>Zn2+>Zn(OH)3– is the usual pattern for peripherally negatively charged porphyrins.