Characterization of the Fe(III)-Tiron System in Solution through an Integrated Approach Combining NMR Relaxometric, Thermodynamic, Kinetic, and Computational Data
Alessandro Nucera, Fabio Carniato, Zsolt Baranyai, Carlos Platas‐Iglesias, Mauro Botta
Abstract
High Resolution Image Download MS PowerPoint Slide The Fe(III)-Tiron system (Tiron = 4,5-dihydroxy-1,3-benzenedisulfonate) was investigated using a combination of 1 H and 17 O NMR relaxometric studies at variable field and temperature and theoretical calculations at the DFT and NEVPT2 levels. These studies require a detailed knowledge of the speciation in aqueous solution at different pH values. This was achieved using potentiometric and spectrophotometric titrations, which afforded the thermodynamic equilibrium constants characterizing the Fe(III)-Tiron system. A careful control of the pH of the solution and the metal-to-ligand stoichiometric ratio allowed the relaxometric characterization of [Fe(Tiron) 3 ] 9–, [Fe(Tiron) 2 (H 2 O) 2 ] 5–, and [Fe(Tiron)(H 2 O) 4 ] − complexes. The 1 H nuclear magnetic relaxation dispersion (NMRD) profiles of [Fe(Tiron) 3 ] 9– and [Fe(Tiron) 2 (H 2 O) 2 ] 5– complexes evidence a significant second-sphere contribution to relaxivity. A complementary 17 O NMR study provided access to the exchange rates of the coordinated water molecules in [Fe(Tiron) 2 (H 2 O) 2 ] 5– and [Fe(Tiron)(H 2 O) 4 ] − complexes. Analyses of the NMRD profiles and NEVPT2 calculations indicate that electronic relaxation is significantly affected by the geometry of the Fe 3+ coordination environment. Dissociation kinetic studies indicated that the [Fe(Tiron) 3 ] 9– complex is relatively inert due to the slow release of one of the Tiron ligands, while the [Fe(Tiron) 2 (H 2 O) 2 ] 5– complex is considerably more labile.