Stability Constants of Some Bivalent Metal Chelates of 3-Aryl-2,4-pentanedione in 75% v/v 1,4-Dioxane-Water Medium

A new ligand, 3-(1-naphthyl)-2,4-pentanedione, was prepared. Concentration proton-dissociation constants of 3-aryl-2,4-pentanedione (aryl group; phenyl, 2,4-dinitrophenyl, and 1-naphthyl) and concentration stability constants of some bivalent metal chelates of 3-aryl-2,4-pentanedione were determined in a 75% v/v 1,4-dioxane–water medium containing 0.05 mol dm−3 sodium perchlorate by a pH-titrimetric method. The dissociation constant (pKa) of 3-(2,4-dinitrophenyl)-2,4-pentanedione, 2,4-pentanedione, 3-phenyl-2,4-pentanedione, and 3-(1-naphthyl)-2,4-pentanedione were found to be 8.87, 10.93, 11.50, and 11.71, respectively. The order of acidity in 2,4-pentanedione was explained by the field effect of the aryl group. The stability constants of the bivalent metal chelates of these ligands were in accord with the Irving-Williams series. The stepwise stability constant of the chelate increased with an increasing pKa value of 3-aryl-2,4-pentanedione. This result cannot be explained by the steric and solvation effect of an aryl substituent.