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

Abstract

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.

Biological Reduction of Nitric Oxide in Aqueous Fe(II)EDTA Solutions

“…A chemical NO reduction to N 2 occurred as well (Table 1) but at a much lower rate, ±0.1 mmol L −1 d −1 (Figure 1). Chemical NO reduction in aqueous Fe(II)EDTA solutions at lower temperature (23 °C) and lower Fe(II)EDTA concentration (1 mM) was reported before by Van Eldik et al (19). They observed a slow and irreversible redox process accompanied by the dissociation of the nitrosyl complex to Fe(III)EDTA and N 2 O (19).…”

“…In that reaction, oxidation of iron takes place according to reaction 5. Van Eldik et al (19) proposed that the same reaction occurs during the chemical NO reduction in aqueous Fe(II)EDTA‐NO solutions: …”

[Fe(H2O)5(NO)]2+, das Chromophor des “braunen Rings”

“…Dementsprechend nutzte die van‐Eldik‐Gruppe bei ihren kinetischen Messungen in wässriger Lösung den Umstand, dass 1 schlagartig durch sichtbares Licht praktisch jeder Wellenlänge photolysiert wird. In ihrer Übersicht über mehr als 100 Quartett‐{FeNO} 7 ‐Komplexe zeigte die Stammspezies 1 die geringste Stabilität von allen . Es verwundert daher nicht, dass die von Manchot für seine großen, schokoladebraunen Kristalle (Ref.…”

Reaction Systems for Bubbly Flows

“…Van Eldik et al investigated kinetic, spectroscopic and thermodynamic properties of aqueous {FeNO} 7 ( S = 3/2) complexes with various aminocarboxylate chelate ligands (e.g. edta, edda, hedtra, ida, oda) (Figure ) …”