Thiocyanate‐assisted demetallation of α,β,γ,δ‐tetra(p‐sulfophenyl)porphinatoindium(III) in aqueous media

Abstract

AbstractThe rate law for the demetallation of the title indium(III)‐porphin complex in aqueous acidic thiocyanate media at 3.00M ionic strength was found to be of the form where [H4P2−] is the concentration of the diacid product formed, [InP]t is the total concentration of all forms of indium(III)‐porphin complex present, and a and b are constants. The constant a is a pseudo‐third‐order rate constant with the value (0.057 ± 0.005)M−2 s−1 and b has the value 0.704M−2 at 50.5°C. If the mechanism for demetallation involves ringpuckering with the attachment of two H+ ions, then 1/b can be identified with the product K1K2 for the stepwise dissociation of two protons from two ring pyrrolic nitrogen atoms of H2InP−. In the sulfonated tetraphenylporphin used for these studies the ring pyrrolic nitrogen atoms seem to be the most probable sites for protonation. If this identification is correct, the value of 1.42 ± 0.13 found for the product K1K2 shows the enormous effect that the presence of the In3+ center has on the ionization constants of these two protons. That the kinetic studies show saturation effects with respect to proton addition to InP3− may result from the fact that In3+ sits about 0.6 Å above the porphin ring.