Tuning the Reversible Binding of NO to Iron(II) Aminocarboxylate and Related Complexes in Aqueous Solution

Chelate complexes of FeII were investigated with respect to their reactivity against nitric oxide and dioxygen. Through a systematic variation of the structure of the polyaminocarboxylate EDTA, a series of 38 potential chelate ligands were selected for FeII. The nitrosyl complexes were prepared from the FeII chelates with NO gas and examined spectroscopically by UV/Vis and ATR-IR techniques, and themodynamically by determining the overall binding constants for NO. In addition, the reversibility of NO binding to these FeII chelates and the rate of the competing oxidation by dioxygen were studied qualitatively. Whereas the studied complexes all form more or less stable nitrosyl complexes with a characteristic band pattern in the UV/Vis spectra and only slightly diverging frequencies for the NO stretching vibration in the IR spectra, they differ considerably in the reversibility of NO binding, the overall NO binding constants and the sensitivity towards dioxygen. It was found that an increasing number of donor groups on the chelate ligand causes a stronger coordination to FeII, and increases the tendency of the FeII chelates to transfer electron density from iron to substrates like dioxygen or nitric oxide. This results in an accelerated oxidation of FeII to FeIII by dioxygen and a more pronounced tendency of the corresponding nitrosyl complexes to slowly decompose to FeIII and N2O. In addition, the overall binding constant for NO (KNO), which spans a range from 1·103 to 2·107 M−1, increases in the same direction as a result of the inductive effect of the chelate ligand.