1H and 17O NMR relaxation studies of the Fe(III)-Tiron system

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, starting from bioinorganic 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 for independent 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 study. In particular we investigated [Fe(Tiron)(H2O)4]- (pH=2.3), [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 (Dw) (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.