In Situ FT-IR Spectroelectrochemistry Reveals Mechanistic Insights into Nitric Oxide Release from Ruthenium(II) Nitrosyl Complexes

Ruthenium­(II) tetraamine nitrosyl complexes with N-heterocyclic ligands are known for their potential as nitric oxide (NO•) donors, capable of releasing NO• through either direct photodissociation or one-electron reduction of the Ru­(II)­NO+ center. This study delivers a novel insight into the one-electron reduction mechanism for the model complex trans-[RuII(NO)­(NH3)4(py)]3+ (RuNOpy, py = pyridine) in phosphate buffer solution (pH 7.4). In situ FT-IR spectroelectrochemistry reveals that the pyridine ligand is readily released upon one-electron reduction of the nitrosyl complex, a finding supported by nuclear magnetic resonance spectroscopy (1H NMR) and electrochemistry coupled to mass spectrometry (EC-MS), which detect free pyridine in solution. However, direct evidence of NO• release from RuNOpy as the primary step following reduction was not observed. Interestingly, FT-IR results indicate that the isomers of the nitrosyl complex, cis-[Ru­(NO)­(NH3)4(OH)]+ and trans-[Ru­(NO)­(NH3)4(OH)]+, are formed following reduction and pyridine labilization, initiating an outer-sphere electron transfer process that triggers a chain electron transfer reaction. Finally, nitric oxide is liberated as an end product, arising from the reduction of the hydroxyl isomer complexes cis-[Ru­(NO)­(NH3)4(OH)]2+ and trans-[Ru­(NO)­(NH3)4(OH)]2+. This study provides new insights into the reduction mechanism and transformation pathways of ruthenium nitrosyl complexes, contributing to our understanding of their potential as NO• donors.