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Mechanism of the Stepwise Dissociation of FeIII Complexes with Tripodal Ligands as Siderophore Models

The acid hydrolysis and formation kinetics of 1:1 ferric complexes with several tripodal hexadentate ligands were investigated over the [H+] range of 0.02−1.0 M at 25 °C and at an ionic strength of 2.0 M (NaClO4/HClO4). These ligands are based on a tris(2-aminoethyl)amine (TREN) spacer connected through a tris(amide) moiety to identical pendant bidentate chelating group arms such as catecholate (TRENCAMS, L3), 8-hydroxyquinolinate (O-TRENSOX, L0) or pyridinophenolate (TRENPYPOLS, L5). Mixed ligands with chelating arms such as catecholate and 8-hydroxyquinolinate (TRENSOX2CAMS, L1 and TRENSOXCAMS2, L2) were also investigated. An analogue of L0 (C-SOX, L4) with a C-pivot scaffold instead of TREN was studied for comparison. The acid hydrolysis reaction was found to proceed in four kinetically distinguishable stages. A common mechanism in the series was followed when the ligand structure was varied. The first stage is very fast and can be attributed to the dissociation of one arm of the tripodal ligand. The second and third stages involve protonation of the complex which induces a change of coordination giving the corresponding tetracoordinated bis(salicylate) complex (coordination with carbonyl and o-hydroxy oxygen atoms). The final dissociation of the ligand proceeds in the fourth stage by one or two kinetically detectable steps. For the last step an important structural effect may be observed from 1.2 × 10−5 to 2.2 × 10−2 s−1 for the proton-independent step and from 1.2 × 10−6 to 0.18 M−1s−1 for the proton-dependent step. Differences in the rates and mechanism between the different tripodal ligands are discussed in terms of the global electrostatic charge of the ligand and the donor ability of the chelating subunit. The C−N bond rotation of the amide moiety and the importance, depending on the charge, of the H-bond network in the partially uncoordinated complexes have also been considered. The role of electrostatics in complex formation is discussed in relation to the Eigen-Wilkins mechanism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)