Latent Thermo-Switchable Olefin Metathesis Initiators Bearing a Pyridyl-Functionalized Chelating Carbene: Influence of the Leaving Group’s Rigidity on the Catalyst’s Performance

The synthesis and characterization of two ruthenium complexes bearing chelating carbene ligands is described. Carbene precursors, 2-(2-vinylphenyl)pyridine and 10-vinylbenzo[h]quinoline, are applied to prepare (SPY-5-31)-dichloro-(κ2(C,N)-N-2-(2-vinylbenzylidene)pyridine(1,3-bis(2,4,6-trimethylphenyl)4,5-dihydroimidazol-2-ylidene)ruthenium (VIII) and (SPY-5-31)-dichloro-(κ2(C,N)-2-(benzo[h]quinolin-10-yl)methylidene)(1,3-bis(2,4,6-trimethylphenyl)4,5-dihydroimidazol-2-ylidene)ruthenium (IX). Both catalysts/initiators are used to perform ring-closing metathesis (RCM) and ring-opening metathesis polymerizations (ROMP). RCM experiments reveal significant thermal stability of the catalysts under forcing reaction conditions such as boiling toluene for 48 h. Even challenging substrates such as diethylallyl(2-methylallyl)malonate are completely transformed with low catalyst loadings (0.1 mol % at 110 °C). The high thermal stability, i.e., latency, might be explained by a slow generation of the catalytically active methylidene species. This feature leads to high molecular weight polymers and a thermal switchability in ROMP. Initiation of polymerizations of several norbornene derivatives occurs at about 48 ± 5 °C in the case of initiator VIII and at 110 ± 9 °C in the case of IX. A substantial increase of the switching temperature in the following cases could be supported with higher rigidity of the chelating carbene moiety in IX when compared to VIII.