Abstract
The synthesis of a novel class of exocyclic bis- and tris-3,2-hydroxypyridinone (HOPO) chelators built on N(2) and N(3) aza-macrocyclic scaffolds and the thermodynamic solution characterization of their complexes with Fe(III) are described. The chelators for this study were prepared by reaction of either piperazine or N,N',N''-1,4,7-triazacyclononane with a novel electrophilic HOPO iminium salt in good yields. Subsequent removal of the benzyl protecting groups using HBr/acetic acid gave bis-HOPO chelators N(2)(etLH)(2) and N(2)(prLH)(2), and tris-HOPO chelator N(3)(etLH)(3) in excellent yields. Solution thermodynamic characterization of their complexes with Fe(III) was accomplished using spectrophotometric, potentiometric, and electrospray ionization-mass spectrometry (ESI-MS) methods. The pK(a)'s of N(2)(etLH)(2), N(2)(prLH)(2), and N(3)(etLH)(3), were determined spectrophotometrically and potentiometrically. The Fe(III) complex stability constants for the tetradentate N(2)(etLH)(2) and N(2)(prLH)(2), and hexadentate N(3)(etLH)(3), were measured by spectrophotometric and potentiometric titration, and by competition with ethylenediaminetetraacetic acid (EDTA). N(3)(etLH)(3) forms a 1:1 complex with Fe(III) with log β(110) = 27.34 ± 0.04. N(2)(prLH)(2) forms a 3:2 L:Fe complex with Fe(III) where log β(230) = 60.46 ± 0.04 and log β(110) = 20.39 ± 0.02. While N(2)(etLH)(2) also forms a 3:2 L:Fe complex with Fe(III), solubility problems precluded determining log β(230); log β(110) was found to be 20.45 ± 0.04. The pFe values of 26.5 for N(3)(etLH)(3) and 24.78 for N(2)(prLH)(2) are comparable to other siderophore molecules used in the treatment of iron overload, suggesting that these hydroxypyridinone ligands may be useful in the development of new chelation therapy agents.