The effect of end-group substitutions and use of a mixed solvent system on β-diketones and their iron complexes

The protonation constants and the Fe(LT) and Fe(III) binding constants have been determined potentiometrically in a solvent consisting of 75% ethanol and 25% water (vol./vol.) for the anions derived from four beta-diketones: acetylacetone, benzoylacetone, 1,1,1-trifluoro-2, 4-pentanedione, and 1-phenyl-4,4,4-trifluoro-1,3-butanedione. Replacement of a methyl group by a phenyl group results in a small increase in basicity and small increases of the stability constants of the Fe(II) and the Fe(LII) chelates. Replacement of a methyl group by trifluoromethyl results in considerable reduction in the basicity of the ligand and considerable decreases in the stabilities of the Fe(II) and Fe(III) chelates. The metal ions are found to form successively 1:1, 1:2 and 1:3 metal-ligand chelates as the pH is increased. Protonation constants of the ligands and the stability constants of the 1:1 Fe(III) chelates are also determined spectrophotometrically. The use of the mixed alcohol-water solvent, required to increase the solubility of the chelates formed, results in increases in the protonation constants and the stability constants of the metal chelates, compared with the values of these constants obtained in water. (C) 1998 Elsevier Science S.A. All rights reserved.