Photochemical reactivity of the iron(III) complex of a mixed-donor, α-hydroxy acid-containing chelate and its biological relevance to photoactive marine siderophores.

The trimeric clusters [Fe(III)3(X-Sal-AHA)3(μ3-OCH3)]−, where X-Sal-AHA is a tetradentate chelate incorporating an α-hydroxy acid moiety (AHA) and a salicylidene moiety (X-Sal with X being 5-NO2, 3,5-diCl, all-H, 3-OCH3, or 3,5-di-t-Bu substituents on the phenolate ring), undergo a photochemical reaction resulting in reduction of two Fe(III) to Fe(II) for each AHA group that is oxidatively cleaved. However, photolysis of structurally analogous mixed Fe/Ga clusters demonstrate that a similar photolysis reaction will occur with only a single Fe(III) in the cluster. Quantum yields of iron reduction for the series of [Fe(III)3(X-Sal-AHA)3(μ3-OCH3)]− complexes measured by monitoring Fe(II) production are twice those for ligand oxidation, measured by loss of the CD signal for the complex due to cleavage of the chiral AHA group. These moderate quantum yields, around 1–2% in the UVA and UVB range, are higher for complexes with electron-withdrawing X groups than for electron-donating X groups. The observed final p...