Spectral and Structural Characterization of Two Ferric Coordination Modes of a Simple Bis(catecholamide) Ligand: Metal-Assisted Self-Assembly in a Siderophore Analog

The structurally diverse group of bis(catecholamide) bacterial siderophores is intriguing because these molecules lack a sufficient number of ligating atoms to fully complex one iron atom as a 1:1 complex. Any complex containing fully-complexed iron ligated exclusively by these siderophores inherently requires multiple ligands to bind multiple metals, demonstrating a biological example of metal-assisted self-assembly. Two new ferric coordination modes of a bis(catecholamide) ligand are structurally and spectroscopically characterized as a 2:3 dinuclear triple helix and a 2:2 bis(μ-hydroxo) dimer. The dimer represents an intermediate in the assembly pathway of the dinuclear triple helix, as it is readily converted to the triple helix upon addition of a stoichiometric amount of ligand. The potential relevance of these complexes to biological siderophores is discussed.