Combining relaxometric, potentiometric and spectrophotometric data for the characterization of Fe(III)-based complexes in aqueous solution

NMR relaxometry is a very powerful technique for investigating the solution structural and dynamic properties of paramagnetic complexes. Due to their peculiar magnetic characteristics and their great success as diagnostic probes for applications in MRI, many studies have been carried out on Gd(III)- and Mn(II)-based systems. Surprisingly, very few investigations have been reported on Fe(III) complexes despite their considerable importance in various fields of chemistry. Fe(III), with five unpaired electrons in the d orbitals, a 6S configuration and a high magnetic moment, is very well suited to be studied with this technique. In particular, in this work we focused on well-defined type of catecholate complexes of iron(III), Fe-Tiron (Tiron® = disodium 4,5-dihydroxy-1,3-benzenedisulfonate). The formation constants should allow the individual study of these complexes by an appropriate choice of starting pH and Tiron concentration. We report for the first time the complete characterization of the various species through a combined 1H and 17O NMR relaxometric, spectrophotometric and potentiometric study. In particular we investigated [Fe(Tiron)(H2O)4]- (pH = 2), [Fe(Tiron)2(H2O)2]5- (pH = 4) and [Fe(Tiron)3]9- (pH = 8). Through a simultaneous fit of 1H Nuclear Magnetic Resonance Dispersion profiles (from 0.01 up to 500 MHz), 17O transverse relaxation rates (R2) and shifts () (measured at 11.7 T as a function of temperature) we were able to obtain accurate value of the structural and dynamic parameters that adequately describe the behaviour of these paramagnetic complexes in aqueous solution. UV-Vis spectrophotometry and potentiometric titrations were conducted in order to thoroughly evaluate the kinetic inertness and thermodynamic stability of the system.