Carrier-facilitated bulk liquid membrane transport of iron(iii) hydroxamate complexes utilizing a labile recognition agent and amine recognition in the second coordination sphere

Abstract

Carrier-facilitated bulk liquid membrane transport from an aqueous source phase through a chloroform membrane phase to an aqueous receiving phase was studied for various Fe(III) hydroxamate complexes (siderophore mimics) using second coordination sphere recognition. Iron transport systems were designed using a strategy whereby a tetradentate siderophore mimic sequesters iron(III), leaving two labile aquated coordination sites for ternary complex formation. This aquated complex reacts with a bi-functional host/guest molecule capable of acting as a host for the iron complex via ternary complex formation, while simultaneously serving as a guest for a membrane-bound host (carrier). The bi-functional molecules utilized contain a hydroxamate host for Fe(III) binding and a protonated primary amine that can be “recognized” by a liquid membrane-bound hydrophobic ionophore, which carries the hydrophilic Fe(III)-complex across the hydrophobic membrane to an aqueous receiving phase. Four protonated amine hydroxamic acids were investigated as bi-functional host/guest molecules: β-alanine hydroxamic acid (H₂L^ala)⁺, L-glutamic acid γ-monohydroxamic acid (H₃L^glu)⁺, glycine hydroxamic acid (H₂L^gly)⁺, and L-lysine hydroxamic acid (H₃L^lys)²⁺. These four bidentate ligands were each coordinated to Fe(III) along with the tetradentate N,N′-dihydroxy-N,N′-dimethyldecanediamide (H₂L⁸) to form ternary complexes [Fe(L⁸)(H_xL^y)^z; x = 1 or 2; y = ala, glu, gly, or lys; z = 0, +1, or +2] that were transported through a chloroform bulk liquid membrane by the lipophilic host carrier cis-dicyclohexano-18-crown-6 (DC18C6). No carrier-dependent flux was observed for Fe(L⁸)(HL^glu), probably due to intramolecular H-bonding. Flux values for the transport of Fe(L⁸)(H_xL^y)^z (x = 1 or 2; y = ala, gly, or lys; z = +1 or +2) facilitated by the membrane carrier (DC18C6) were highest when y = gly and lowest when y = ala. Equilibrium constants pertaining to two-phase distribution or ion pairing, second coordination sphere host–guest formation, and overall extraction were determined and used to rationalize variations in flux values.