Aluminum(III) Coordination to Hydroxy Carboxylates: the Influence of Hydroxy Substituents Enabling Tridentate Binding

Abstract

Potentiometric evalution of the interaction between aluminium(III) and the hydroxy carboxylates 3-deoxy-D-ribo-hexonate (L¹H) and 2,5-dihydroxypentanoate (L²H) provided evidence for the formation of 1:1 complexes only, though with the ligand being present in singly, doubly and quadruply deprotonated forms in the hexonate complexes, and in singly, doubly and triply deprotonated forms in those of the pentanoate. At 298 K in aqueous media, I = 0.1 ( NaCl ), equilibrium quotient values obtained through use of the programs PKAS and BEST were:

-log([(L¹)^-][H⁺]/[L¹H])(or pK_a(L¹H))= 3.58±0.03

log([(AlL¹)²⁺]/[Al³⁺][(L¹)^-])=1.97±0.03

log([(AlH-₁L¹)⁺][H⁺]/[(AlL¹)²⁺])= -2.83±0.05

log([(AlH-₃L¹)^-][H⁺]²/[(AlH_-1L¹)⁺])= -9.17±0.08

-log([(L²)^-][H⁺]/[L²H])(or pKa(L²H))= 3.92±0.03

log([(AlL²)²⁺]/[Al³⁺][(L²)^-])= 2.04±0.03

log([(AlH-₁L²)⁺][H⁺]/[AlL²)²⁺])= -3.14±0.06

log([AlH-₂L²][H⁺]/[(AlH_-1L²)⁺])= -2.25±0.09

The very close similarity of the results for 3-deoxy-D-ribo-hexonate (3-deoxygluconate) and those known for D(+)-gluconate has led to the suggestion that the complexes of the quadruply deprotonated ligands may involve trialkoxo coordination through the C1, C5 and C6 substituents.