Latent Ruthenium Complexes Supported by Two <i>N</i>‐Heterocyclic Carbene Ligands: Synthesis and Catalytic Activity with Distinctive Activation

Novel bis( N ‐heterocyclic carbene [NHC]) ruthenium complexes of the form (NHC 1 )(NHC 2 )Ru(=CRR')Cl 2 (where NHC represents N ‐heterocyclic carbene ligands and CRR’ denotes Ph‐indenylidene or benzylidene moieties) are prepared from second‐ and third‐generation Grubbs catalysts. These complexes exhibit thermal latency in ring‐closing metathesis (RCM), cross‐metathesis (CM), and several ring‐opening metathesis polymerization (ROMP) reactions. Upon UV‐C (255 nm) irradiation, two complexes that contained benzylidene ligands prove to be highly effective for the ROMP of cyclooctadiene. The most active species incorporate IPr or SIPr as at least one NHC ligand, highlighting how bulky isopropyl substituents enhance NHC lability. Beyond their utility in UV‐C‐initiated ROMP, these complexes show promising activity for high‐temperature CM and RCM of alkenyl‐nitrile substrates.