Mixed-Donor, α-Hydroxy Acid-Containing Chelates for Binding and Light-Triggered Release of Iron

A series of five new α-hydroxy acid-containing chelates inspired by photoactive marine siderophores, along with their Fe(III) complexes, have been synthesized and characterized. These chelates, designated X-Sal-AHA, each contributes a bidentate salicylidene moiety (X-Sal, X = 5-NO2, 3,5-diCl, H, 3,5-di-tert-butyl, or 3-OCH3 on the phenolate ring) and a bidentate α-hydroxy acid moiety (AHA). The X-ray crystal structure of Na[Fe3(3,5-diCl-Sal-AHA)3(μ3-OCH3)] shows an Fe(III) trimer with the triply deprotonated, trianionic ligands each spanning two Fe(III)’s that are bridged by the hydroxyl group of the ligand. Additionally, a μ3-methoxy anion caps the Fe(III)3 face. Electrospray ionization mass spectra demonstrate that this structure is representative of the Fe(III) complexes of all five derivatives in methanol solution, with the exception of the X = 3,5-di-t-Bu derivative having a μ3-OH bridge rather than a methoxy bridge. Stability constants determined from reduction potentials range from 1034 for the 5-NO2 derivative to >1040 for the 3,5-di-tBu derivative. All five complexes are photoactive when irradiated by sunlight, with the relative rate of photolysis as monitored by Fe(II) transfer correlating with the Hammett σ+ parameter for the phenolate ring substituents.