Photochemical NO-removal and NO x-release in the presence of Fe–EDTA complexes. DFT calculations of electronic structure and spectroscopy of the [Fe(edta)(NO)] 2− complex

Abstract

The calculations for iron complexes with EDTA and NO, [Fe II(edta)(NO)] 2− and [Fe III(edta)(NO)] −, were performed with the DFT/B3LYP method. The lowest energy structure for the Fe(II) complex is six-coordinated with one carboxylate group unbound. The Fe–NO bond is here short and strengthen additionally by the charge shift to form Fe IIINO. The low stability of the [Fe III(edta)(NO)] − complex can be interpreted in terms of the weak Fe–NO bond calculated for this species. Photoreduction of the [Fe III(edta)(H 2O)] − or [Fe III(edta)(OH)] 2− complex by EDTA in the NO atmosphere was studied and is suggested as convenient method of the [Fe(edta)(NO)] 2− synthesis. The results of the [Fe(edta)(NO)] 2− irradiation connotes also a possibility of alternative release NO or N 2O in acidic or alkaline solution, respectively. The calculated electronic spectrum of [Fe(edta)(NO)] 2− showed that low and medium energy bands are mainly of π NO ∗ / d → d / π NO ∗ , while transitions from free electron pairs of nitrogen and oxygen atoms to metal d and nitrosyl π NO ∗ orbitals are responsible for the high energy band. The former favors homolytic dissociation of the neutral NO molecule, whereas the latter heterolytic Fe–NO bond cleavage. In both cases excitation leads to weakening of the Fe–NO bond, enabling the pH effect on dissociation of neutral NO molecule or NO anion. This outcome allows to control the NO x delivery by two stimuli: light and solution pH.