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 (H-2(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 degreesC and 0.16 M NaCl: pK(a1) = 3.67+/-0.07, pK(a2) = 5.82+/-0.07, pK(a3) = 7.96+/-0.04, pK(a4) = 8.77+/-0.03 for [H-4(en(ama)(2))](2+), pK(a) = 8.04+/-0.02 for Hama and pK(a) = 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. H-1 NMR was used to assign the protonation constant values to the different protons in H-2(en(ama)(2)). A pM versus pH plot confirmed that H-2(en(ama)(2)) has much stronger complexation than its bidentate analogues, in the same pH range. (C) 2002 Elsevier Science B.V. All rights reserved.