Oxygen-induced decomposition in Ru-catalyzed olefin metathesis – experimental and theoretical exploration of the reaction pathway

In mechanistic studies of Ru-catalyzed olefin metathesis using UV/Vis spectroscopy, the origin of multiple bands has been elusive. This study uncovers the origin of those bands and assigns them to previously unknown Ru species that arise during the oxygen-induced decomposition of the active catalyst. Pathways for such decomposition have been proposed, but spectroscopic evidence of the respective Ru species has not yet been reported. Therefore, in a multi-spectroscopic and theoretical approach to mechanistic investigation, using operando nuclear magnetic resonance (NMR), operando UV/Vis, operando combined NMR-UV/Vis, high-resolution mass spectrometry (HR-MS) and time-dependent density functional theory (TD-DFT), we connect two open strands in metathesis literature: i) explaining the origin of the previously unassigned UV/Vis bands and ii) experimentally substantiating and refining the proposed pathway for oxygen-induced decomposition. With that, we propose the occurrence of the 505 and 640 nm UV/Vis bands as a convenient spectroscopic indicator for oxygen-induced decomposition occurring in Ru-catalyzed olefin metathesis.