Tuning the reversible binding of NO to iron(II) aminocarboxylate and related complexes in aqueous solution

Chelate complexes of Fe-II 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 Fe-II. The nitrosyl complexes were prepared from the Fe-II 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 Fe-II 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 Fe-II, and increases the tendency of the Fe-II chelates to transfer electron density from iron to substrates like dioxygen or nitric oxide. This results in an accelerated oxidation of Fe-II to Fe-III by dioxygen and a more pronounced tendency of the corresponding nitrosyl complexes to slowly decompose to Fe-III and N2O. In addition, the overall binding constant for NO (K-NO), which spans a range from 1.10(3) to 2.10(7) M-1, increases in the same direction as a result of the inductive effect of the chelate ligand.