It is reasonable to suppose that, due to the presence of the acid sulfonic group, H2SPP in water is much more solvated with respect to H2PP in propanol: the stronger solvent interaction should make H2SPP less reactive than H2PP toward Zr(IV). Moreover, one should also consider that the −SO3H group in the meta-position on the aryl ring in H2SPP could reduce the electron density of the −PO3H2 group and, consequently, its reactivity toward Zr(IV).