Hydroxypyronate, thiohydroxypyronate and hydroxypyridinonate derivatives as potential Pb2+ sequestering agents

Solution equilibrium study on Pb(II) complexes of 3-hydroxy-2-methyl-4-pyrone (maltol, maltH) and its two derivatives, 3-hydroxy-2-methyl-4H-pyran-4-thione (thiomaltol, thiomalH) and 3-hydroxy-1,2-dimethyl-4-pyridinone (dhpH) have been performed by using pH-potentiometry, 1H NMR, ESI MS methods. Out of the studied ligands, the (S,O)-chelating thiomalt was found to form the most stable mono- and bis-chelated type species, [PbL]+ and [PbL2], but limited water solubility hindered the examination on this system above pH 7. The Pb(II)-binding capabilities of malt and especially dhp are still very good and the higher extent of electron delocalization in dhp compared to malt results not only in the increased stability of the 5-membered (O,O) dhp chelate, but also the possibility of some involvement of the 6s2 lone electron pair of Pb(II) in the bonding. Furthermore, dhp, compared to malt, is not just a more effective Pb(II) chelator, but also shows better selectivity toward Pb(II) against Zn(II).

Results on the title systems indicate the thiohydroxypyronate to form the most stable, but very water unsoluble Pb(II) complexes. In solution, under physiological conditions, the hydroxypyridinonate, dhp, seems to be the best Pb(II) chelator and exhibits the highest selectivity against Zn(II) what was interpreted by the most pronounced delocalization along the chelating O donors in this ligand.Figure optionsDownload as PowerPoint slide