Hydrogen Bonding in Catechoylamides

The siderophore enlerobactin has an extraordinary affinity for ferric ion. Hydrogen bonding has been proposed to play a significant role in the extra binding affinity enterobactin shows relative to synthetic analogues. These analogues contain 2,3-dihydroxybenzoyl subunits or, as in the present study, 2,3-dihydroxyterephthalamide ligand groups. The bisethyl amide of 2,3-dihydroxybenzoic acid (1) crystallizes in space group P21/c with a = 7.387(2), b = 25.068(4), c = 7.198(2) Å, β = 100.42(2)[ddot] with z = 4. The structure of 1 shows that the conformation of the acid form of these ligands has the amide carbonyl hydrogen bonded to the catechol proton. The cyclic trimer diamide of ethylenediamine with 2,3-dimethoxyterephthalic acid (2) crystallizes in Pbca with a = 25.891(2), b = 17.361(3), c = 33.665(2) Å, z = 16. The two independent molecules of 2 have slightly different conformations largely due to the effects of hydrogen bonding from the amide proton to the catechol oxygen. This is the hydrogen bond structure found in the metal complexes of these ligands and represents a 180[ddot] rotation around the amide C-N bond from the free ligand to the metal complex forms of these compounds—a dominant feature of the metal complexation process.