ALUMINUM(III) COORDINATION TO HYDROXY CARBOXYLATES - THE INFLUENCE OFHYDROXY SUBSTITUENTS ENABLING TRIDENTATE BINDING

Potentiometric evalution of the interaction between aluminium(III) and the hydroxy carboxylates 3-deoxy-D-ribo-hexonate (L(1)H) and 2,5-dihy droxypentanoate (L(2)H) provided evidence for the formation of 1:1 com plexes only, though with the ligand being present in singly, doubly an d quadruply deprotonated forms in the hexonate complexes, and in singl y, doubly and triply deprotonated forms in those of the pentanoate. At 298 K in aqueous media, I = 0.1 (NaCl), equilibrium quotient values o btained through use of the programs PKAS and BEST were: -log([(L(1))(- )][H+]/[L(1)H]) (or pK(a)(L(1)H)) = 3.58 +/- 0.03 log([(AlL(1))(2+)]/[ Al3+][(L(1))(-)]) = 1.97 +/- 0.03 log([(AlH(-1)L(1))(+)][H+]/[(AlL(1)) (2+)]) = -2.83 +/- 0.05 log([(AlH(-3)L(1))(-)][H+](2)/[(AlH-(1)L(1))()]) = -9.17 +/- 0.08 -log([(L(2))(-)][H+]/[L(2)H]) (or pK(a)(L(2)H)) = 3.92 +/- 0.03 log([(AlL(2))(2+)]/[AI(3+)][(L(2))(-)]) = 2.04 +/- 0.03 log([(AlH(-1)L(2))(+)][H+]/[AlL(2))(2+)]) = -3.14 +/- 0.06 log([AlH(- 2)L(2)][H+]/[(AlH(-1)L(2))(+)]) = -2.25 +/- 0.09. The very close simil arity of the results for 3-deoxy-D-ribo-hexonate (3-deoxygluconate) an d those known for D(+)-gluconate has led to the suggestion that the co mplexes of the quadruply deprotonated ligands may involve trialkoxo co ordination through the C1, C5 and C6 substituents.