Steric modulation of nitric oxide photodissociation kinetics in iron–nitrosyl complexes

Abstract

Nitric oxide (NO) photorelease from transition metal–nitrosyl complexes enables spatiotemporal control of NO delivery for biological and therapeutic applications. Here, we investigated how steric modification of a pyridinophane ligand influences NO photorelease behavior in nonheme iron–nitrosyl complexes. Replacement of tert ‐butyl substituents with cyclohexyl groups preserved the tetradentate coordination architecture while altering the steric environment around the iron center. Photochemical studies revealed that [Fe(CHDAP)(NO)(H ₂ O)] ²⁺ underwent slower NO photorelease than [Fe(TBDAP)(NO)(H ₂ O)] ²⁺ under identical irradiation conditions. Structural comparisons based on density functional theory indicated subtle differences between the two complexes, suggesting that modest ligand‐derived perturbations can influence Fe–NO bonding and photodissociation behavior. Electrochemical analysis also showed ligand‐dependent changes in redox properties. Taken together, these results show that steric modification within a conserved macrocyclic scaffold can produce a change in NO photoreactivity without altering the ligand framework.