Density functional theory studies on a designed photoactive {FeNO}6 nitrosyl and the corresponding photoinactive {FeNO}7 species: Insight into the origin of NO photolability

Density functional theory (DFT) and time-dependent DFT (TDDFT) studies on a photoactive {FeNO}6 nitrosyl [(PaPy3)Fe(NO)](ClO4)2 (1) and the corresponding light-insensitive {FeNO}7 species [(PaPy3)Fe(NO)](ClO4) (2) have been carried out to determine the origin of NO photolability of 1. The iron center in these two nitrosyls formally exists in 2+ oxidation state and the difference in π-accepting ability of NO+ in 1 versus NO in 2 greatly affects the extent of NO photolability of these two nitrosyls. Low energy transitions from the carboxamido/π(FeNO) to the FeNO antibonding molecular orbitals lead to release of NO from 1 upon exposure to visible light. The decreased π-accepting ability of the NO moiety in 2 does not favor such transitions; instead transitions from orbitals centered at the FeNO unit to the πpy orbitals of the ligand frame become more favorable and the photolability of NO is lost in 2.