Ground-State Proton Transfer Tautomer of Al(III)−Salicylate Complexes in Ethanol Solution

Abstract

The tautomerization of salicylate anion in the presence of Al(III) in ethanol was studied by UV−visible absorption spectroscopy and fluorescence spectroscopy, anisotropy, and lifetime measurements from 100 to 298 K. Complexation with Al(III) causes an equilibrium shift from the normal form of the salicylate anion toward the tautomer form, demonstrating that the presence of a highly charged cation, Al(III), stabilizes the tautomer form of salicylate. Spectra and fluorescence lifetimes of salicylate and other salicyl derivatives in the presence of Al(III) reveal three types of Al(III)−salicylate complexes. In type I complexes, salicylate binds to Al(III) through the carboxylate group, preserving the intramolecular hydrogen bond between the carbonyl oxygen and the phenol group, as indicated by the largely Stokes-shifted fluorescence emission following an excited state proton transfer process. In type II complexes, salicylate binds to Al(III) through the carboxylate group, but the phenol proton is oriented aw...