Ru-Based Olefin Metathesis Catalysts Bearing pH-Responsive N-Heterocyclic Carbene (NHC) Ligands: Activity Control via Degree of Protonation

Olefin metathesis catalysts (H(2)ITap)(PCy3)Cl2Ru=CHPh (4) and (H(2)ITap)Cl2Ru=CH-(C6H4-O-iPr) (5) [H(2)ITap = 1,3-bis(2',6'-dimethyl-4'-dimethylaminophenyl)-4,5-dihydroimidazol-2-ylidenel were used for the ring-opening metathesis polymerization (ROMP) of exo-7-oxanorbornene derivative 7 in the presence of various amounts of acid. Upon gradual protonation of the NMe2 groups of the H(2)Tap ligand, the metathesis activity of both catalysts were gradually reduced due to electronic changes of the N-heterocyclic carbene (NHC) ligand donor capability. The investigation of the ROMP polymer 8, DFT calculations and measurements of the initiation kinetics prove that the reduced activity is solely due to reduced rates of propagation. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)