Mixed hydroxypyridinonate ligands as iron chelators.

New ligands based on hydroxypyridinonate (HOPO) and other bidentate ligands are explored as iron(III) sequestering agents. These are based on the N,N',N"-tris[(3-hydroxy-1-methyl- 2-oxo-1,2-didehydropyrid-4-yl)-carboxamidoethyl]amine (TREN-Me-3,2-HOPO) platform in which one Me-3,2-HOPO ligand group is substituted with either a 2-hydroxyisophthalamide (TREN-Me-3,2-HOPOIAM) or a 2,3-dihydroxyterephthalamide (TREN-Me-3,2-HOPOTAM) moiety. The ferric complexes have been prepared and structurally characterized by X-ray diffraction: Fe[TREN-Me-3,2-HOPOIAM] crystallizes in the monoclinic space group C2/c with cell parameters a = 18.1186(3) A, b = 17.5926(2) A, c = 25.0476(2) A, beta = 98.142(1) degrees, Z = 8. Fe[TREN-Me-3,2-HOPOTAM]- crystallizes in the monoclinic space group C2/c with cell parameters a = 31.7556(12) A, b = 14.0087(6) A, c = 22.1557(9) A, beta = 127.919(1) degrees, Z = 8. The aqueous coordination chemistry of these ligands with both the ferric and ferrous redox states of iron has been examined using spectroscopic and electrochemical methods, giving log formation constants of 26.89(3) (beta 110), 31.16(6) (beta 111) for the ferric TREN-Me-3,2-HOPOIAM complexes and 33.89(2) (beta 110), 38.45(2) (beta 111) for the ferric TREN-Me-3,2-HOPOTAM complexes. For the reduced (ferrous) complexes values of 10.03(9) (beta 110) and 13.7(2) (beta 110) were observed for the Fe[TREN-Me-3,2-HOPOIAM]- and Fe[TREN-Me-3,2-HOPOTAM]2- complexes, respectively. These data provide a complete description of metal-ligand speciation as a function of pH and of redox activity. The ligands described in this work are part of a new class of heteropodate ligands which exploit the various chelating properties of several binding units within a single tripodal ligand and allow for systematic variation of the properties for medical or other applications.