Synthesis of tris(β-diketones) and study of their complexation with some transition metals

New chelating ligands consisting of three beta-diketone fragments, viz., 1,3,5-tris[(acetylaceton-3-yl)methyl]benzene, 1,3,5-tris[(benzoylaceton-3-yl)methyl]benzene, and 1,3,5-tris[(dibenzoylmethan-1-yl)methyl]benzene, linked to each other through the mesitylene spacer were synthesized by the reaction of 1,3,5-tris(bromomethyl)benzene with the corresponding beta-diketone sodium salt. The acidity of these compounds and their complexation properties were studied by pH-potentiometry in aqueous-ethanol solutions. Tris(beta-diketones) form mononuclear complexes with lanthanide ions, whose stability increases in the series La3+ < Gd3+ < Lu3+. 1,3,5-Tris[(acetylaceton-3-yl)methyl]benzene forms both mononuclear and polynuclear complexes with the Ni2+ and Cu2+ ions. The stability constants and selectivity of complex formation increase substantially with an increase in the degree of deprotonation of the ligands, thus indicating that all deprotonated chelate groups are involved in coordination with the metal ion. The replacement of the methyl groups by the phenyl substituents in the beta-diketone fragments of the molecules affects substantially the composition and stability of the complexes formed.