<i>In Situ</i> FT-IR Spectroelectrochemistry\nReveals Mechanistic Insights into Nitric Oxide Release from Ruthenium(II)\nNitrosyl Complexes

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