The nature of the M–NO bond in [M(Imidazole)(PPIX)(L)]q complexes (M = Fe2+, Ru2+; L = NO+, NO[rad] and NO−; PPIX = Protoporphyrin IX)

Abstract

Aiming at understanding the effect of the replacement of iron by ruthenium in the Protoporphyrin IX (PPIX) system of the heme group, in this work density functional theory calculations were performed to investigate the structures, nature of the M–NO bond and electronic spectra of the complexes [M(Im)(PPIX)(L)]q, where M=Fe2+ and Ru2+, Im=Imidazole ring, PPIX=Protoporphyrin IX of the heme group and L=NO+, NO and NO−. Energetic and structural results obtained at the B3LYP/SBKJC/6-31G(d) level of theory indicates that the NO coordinates preferentially through the nitrogen atom and its oxidized form NO+ produces more stable complexes. Charge and energy decomposition analysis revealed that, independent of the coordination mode, the interaction of NO species with the ruthenium fragment is stronger than with iron. TD-DFT calculations predict that the transition that can lead to photodissociation of the Ru–NO bond occurs at 419nm and if the NO coordinates to ruthenium in its oxidized form this transition is blue-shifted to 384nm. The results obtained in this work also gives support to conclude that ruthenium complexes with the protoporphyrin IX (PPIX) system can strongly bind NO, which can be used to capture NO species, and releasing it upon irradiation with visible light.