Effects of N-methylation on the rates and equilibrium constants for protonation of meso-tetraphenylporphyrin in 98%(v/v) dimethyl sulphoxide–water
Abstract
Equilibrium constants for the first and second protonations of N-methyl-meso-tetraphenylporphyrin have been measured in 90% and 98%(v/v) Me2SO–H2O. The interconversion of the mono- and di-protonated forms of N-methyl-meso-tetrakis-(p-methoxyphenyl)porphyrin occurs slowly and the rate coefficient for thermodynamically favourable protonation of the monoprotonated form by hydronium ion has a value of 4.3 × 105 I mol–1 s–1 in 98%(v/v) Me2SO–H2O. The results are compared with kinetic and equilibrium results for meso-tetraphenyl- and meso-tetrakis-(p-methoxyphenyl)-porphyrins for which only the overall equilibrium between the porphyrin and diprotonated species could be studied since the monoprotonated species were undetectable in 90% and 98%(v/v) Me2SO–H2O. Possible reasons for the differences in behaviour are discussed.