Linear hexadentate ligands as iron chelators†

Abstract

3LIHOPO2TAM, 4LIHOPO2TAM, 5LIHOPO2TAM, 6LIHOPO2TAM and 5LIOHOPO2 TAM represent a new class of linear, hexadentate hydroxypyridinone-containing ligands explored as iron(III) sequestering agents. The ligands incorporate the bifunctional 2,3-dihydroxyterephthalamide (TAM) unit as an integral part of the backbone linking two terminal 3,2-hydroxypyridinone (HOPO) units. The ferric complexes of 5LIHOPO2TAM, 6LIHOPO2TAM and 5LIOHOPO2TAM have been prepared and structurally characterized by X-ray diffraction. Fe[5LIHOPO2TAM] crystallizes in the triclinic space group P with cell parameters a = 12.4679(16), b = 12.7498(16), c = 15.1475(19) Å, α = 78.163(2), β = 69.841(2), γ = 89.671(2)°, Z = 2. Fe[6LIHOPO2TAM] crystallizes in the monoclinic space group I2/a with cell parameters a = 28.1875(29), b = 15.6457(16), c = 22.6837(23) Å, β = 90.080(2)°, Z = 8. Fe[5LIOHOPO2TAM] crystallizes in the monoclinic space group P21/c with cell parameters a = 20.6200(19), b = 9.2365(9), c = 23.8669(22) Å, β = 113.406(1)°, Z = 4. These ligands form mononuclear iron complexes with a ligand chirality independent of the metal center chirality. Fe[5LIOHOPO2TAM] and Fe[5LIHOPO2TAM] have different ligand chiralities for the same chirality metal center, and this difference is attributed to the effect of intramolecular hydrogen bonding between the ether oxygen and amide protons in 5LIOHOPO2TAM. The aqueous coordination chemistry of 5LIOHOPO2TAM with ferric ion has been examined using spectroscopic methods, giving a log formation constant of 32.1(1) (β110) and a pM of 30.4 for ferric 5LIOHOPO2TAM. The ferric complex stability is comparable to that of an analogous, previously described tripodal mixed terephthalamide/hydroxypyridinone ligand. The thermodynamic data validate the linear design strategy for this new class of hydroxypyridinone-containing ligands.