Synthesis and solution studies of the complexes of pyrone analogue ligands with vanadium(IV) and vanadium(V)

A new potentially tetradentate chelator N,N′-bis(3-hydroxy-6-methyl-2-methylene-4-pyrone)ethylenediamine (H2(en(ama)2)), has been synthesized and its protonation constants, as well as those of two other 3-hydroxy-4-pyrone ligands (allomaltol (Hama) and methylmaltol (Hmma)) determined potentiometrically at 25 °C and 0.16 M NaCl: pKa1=3.67±0.07, pKa2=5.82±0.07, pKa3=7.96±0.04, pKa4=8.77±0.03 for [H4(en(ama)2)]2+, pKa=8.04±0.02 for Hama and pKa=8.82±0.02 for Hmma. Potentiometric pH titrations were also used to measure the stability constants of these ligands with V(IV) and V(V) and to study the structures of the complexes of (en(ama)2)2− in aqueous solution. 1H NMR was used to assign the protonation constant values to the different protons in H2(en(ama)2). A pM versus pH plot confirmed that H2(en(ama)2) has much stronger complexation than its bidentate analogues, in the same pH range.