Elucidation of the solution structure and water-exchange mechanism of paramagnetic [FeII(edta)(H2O)]2-

The lability and structural dynamics of [Fe-II(edta)(H2O)](2-) (edta = ethylenediaminetetraacetate) in aqueous solution strongly depend on solvent interactions. To study the solution structure and water-exchange mechanism, H-1, C-13, and O-17 NMR techniques were applied. The water-exchange reaction was studied through the paramagnetic effect of the complex on the relaxation rate of the O-17 nucleus of the bulk water. In addition to variable-temperature experiments, high-pressure NMR techniques were applied to elucidate the intimate nature of the water-exchange mechanism. The water molecule in the seventh coordination site of the edta complex is strongly labilized, as shown by the water-exchange rate constant of (2.7 +/- 0.1) x 10(6) s(-1) at 298.2 K and ambient pressure. The activation parameters Delta H-not equal, Delta S-not equal, and Delta V-not equal were found to be 43.2 +/- 0.5 kJ mol(-1), +23 +/- 2 J K-1 mol(-1), and +8.6 +/- 0.4 cm(3) mol(-1), respectively, in line with a dissociatively activated interchange (I-d) mechanism. The scalar coupling constant (A/h) for the Fe-II-O interaction was found to be 10.4 MHz, slightly larger than the value A/h = 9.4 MHz for this interaction in the hexa-aqua Fe-II complex. The solution structure and dynamics of [Fe-II(edta)(H2O)](2-) were clarified by H-1 and C-13 NMR experiments. The complex undergoes a Delta,Lambda-isomerization process with interconversion of in-plane (IP) and out-of-plane (OP) positions. Acetate scrambling was also found in an NMR study of the corresponding NO complex, [Fe-III(edta)(NO-)](2-).