Photolysis of the [Fe(CN)5(NO)]2– ion in water and poly(vinyl alcohol) films: evidence for cyano radical, cyanide ion and nitric oxide loss and redox pathways

Abstract

Ultraviolet–visible and IR spectroscopy and mass spectrometry have been used to investigate photolysis of the [Fe(CN)₅(NO)]^2– ion upon irradiation with UV/VIS light in aqueous solutions and in poly(vinyl alcohol) films at 12 and 298 K. Changes in the ν(CN) and ν(NO) bands in the IR and in the d–d and charge-transfer bands in the UV/VIS region were used to monitor the appearance and disappearance of complex ions as a function of photolysis time. Mass spectrometric analysis of the gaseous products released during the irradiation of aqueous solutions revealed NO, HCN and (CN)₂. The combined results showed that the [Fe(CN)₅(NO)]^2– ion undergoes photoaquation and photoreduction, producing aquacyanoferrate-(III) and -(II) species. The origin of the iron(II) species was shown to be mainly due to the photoreduction of the iron(III) species produced after primary loss of the nitrosyl ligand as molecular NO and not as NO⁺. Subsequent thermal reactions between the iron-(II) and -(III) species led to the formation of mixed-valence compounds, e.g. Prussian blue. A scheme for the photochemical and thermal reactions with CN˙, CN^– and NO loss pathways is proposed. The possible implications of the results for the use of [Fe(CN)₅(NO)]^2– as a vasodilator are discussed.