Catecholate/salicylate heteropodands: demonstration of a catecholate to salicylate coordination change

While iron release from enterobactin-mediated iron transport occurs primarily via an esterase that destroys the siderophore, other catechol siderophores that are not susceptible to hydrolysis act as bacterial growth factors. Elucidating the structures of protonated ferric enterobactin may reveal the pathway by which synthetic analogues fulfill bacterial iron requirements. In order to more completely model this potential delivery pathway for ferric iron, as well as to understand the pH dependent structural dynamics of ferric enterobactin, two ligands, (2-hydroxybenzoyl-2-aminoethyl)-bis(2,3-dihydroxybenzoyl-2-aminoethyl)amine (TRENCAMSAM) and (2-hydroxy-3-methoxybenzoyl-2-aminoethyl)-bis(2,3-dihydroxybenzoyl-2- aminoethyl)amine (TRENCAM(3M)SAM), have been synthesized as models for monoprotonated enterobactin. The coordination chemistry of these ligands with Fe3+ and Al3+ has been investigated. Fe[TRENCAMSAM]2- crystallizes in the triclinic space group P1: Z = 1, a = 11.3307(6) A, b = 12.5479(7) A, c = 15.5153(8) A, alpha = 94.513(1) degree, beta = 105.867(1) degree, gamma = 94.332(1) degree. The structure is a two-metal two-ligand dimer supported by mu-oxo bridges from two catecholate moieties. Al[TRENCAMSAM]2- crystallizes in the triclinic space group P1: Z = 2, a = 9.1404(2) A, b = 13.3570(1) A, c = 15.5950(1) A, alpha = 95.711(1) degree, beta = 104.760(1) degree, gamma = 92.603(1) degree. The complex is a monomer with a five-coordinate, square-pyramidal aluminum cation. Al[TRENCAM(3M)SAM]2- crystallizes in the monoclinic space group C2/m: Z = 8, a = 34.244(2) A, b = 11.6206(6) A, c = 21.9890(12) A, beta = 101.478(1) degree. The complex is also a monomer, but with a highly distorted five-coordinate, square-pyramidal aluminum cation coordination sphere. At high pH these complexes do not display a salicylate mode of binding; however, at low pH Al[TRENCAMSAM]2- converts to protonated Al[H3TRENCAMSAM]+, which is a six-coordinate, tris-salicylate complex. Al[H3TRENCAMSAM]+ crystallizes in the triclinic space group P1: Z = 2, a = 11.5475(4) A, b = 12.1681(4) A, c = 12.5094(4) A, alpha = 109.142(1) degree, beta = 104.327(1) degree, gamma = 103.636(1) degree. This is the first catecholamide enterobactin analogue that has been structurally characterized in both a catecholate and salicylate mode of coordination.