Thermal Decomposition Modes for Four-Coordinate Ruthenium Phosphonium Alkylidene Olefin Metathesis Catalysts

The four‐coordinate ruthenium phosphonium alkylidenes 1‐Cy and 1‐iPr, differing in the substituent on the phosphorus center, were observed to decompose thermally in the presence of 1,1‐dichloroethylene to produce [H3CPR3][Cl]. The major ruthenium‐containing product was a trichloro‐bridged ruthenium dimer that incorporates the elements of the 1,1‐dichloroethylene as a dichlorocarbene ligand and a styrenic vinyl group on the supporting NHC ligand. Spectroscopic, kinetic, and deuterium‐labeling experiments probed the mechanism of this process, which involves a rate‐limiting C–H activation of an NHC mesityl ortho methyl group. These studies provide insight into intrinsic decomposition processes of active Grubbs type olefin metathesis catalysts, pointing the way to new catalyst design directions. Death of a catalyst: Rate‐limiting CH activation at an ortho methyl group of the NHC mesityl group leads to catalyst deactivation (see scheme). Chemical, spectroscopic, kinetic, and deuterium‐labeling experiments were deployed to probe the mechanism of this process and point the way to design principles for next generation catalysts.