Cationic versus Neutral RuIIN-Heterocyclic Carbene Complexes as Latent Precatalysts for the UV-Induced Ring-Opening Metathesis Polymerization

摘要

A series of cationic and neutral RuII complexes of the general formula [Ru(L)(X) (tBuCN)₄]+X− and [Ru(L)(X)₂(tBuCN)₃)], that is, [Ru(CF₃SO₃){NCC(CH₃)₃}₄(IMesH₂)]+[CF₃SO₃]− (1), [Ru(CF₃SO₃){NCC(CH₃)₃}₄(IMes)]+[CF₃SO₃]− (2), [RuCl{NCC(CH₃)₃}₄(IMes)]+Cl− (3), [RuCl{NCC(CH₃)₃}₄(IMesH₂)+Cl−]/[RuCl₂{NCC(CH₃)₃}₃(IMesH₂)] (4), and [Ru(NCO)₂{NCC(CH₃)₃}₃(IMesH₂)] (5) (IMes=1,3-dimesitylimidazol-2-ylidene, IMesH₂=1,3-dimesityl-imidazolin-2-ylidene) have been synthesized and used as UV-triggered precatalysts for the ring-opening metathesis polymerization (ROMP) of different norborn-2-ene- and cis-cyclooctene-based monomers. The absorption maxima of complexes 1–5 were in the range of 245–255 nm and thus perfectly fit the emission band of the 254 nm UV source that was used for activation. Only the cationic RuII-complexes based on ligands capable of forming μ2-complexes such as 1 and 2 were found to be truly photolatent in ROMP. In contrast, complexes 3–5 could be activated by UV light; however, they also showed a low but significant ROMP activity in the absence of UV light. As evidenced by 1H and 13C NMR spectroscopy, the structure of the polymers obtained with either 1 or 2 are similar to those found in the corresponding polymers prepared by the action of [Ru(CF₃SO₃)₂(IMesH₂)(CH-2-(2-PrO)-C₆H₄)], which strongly suggest the formation of Ru-based Grubbs-type initiators in the course of the UV-based activation process. Precatalysts that have the IMesH₂ ligand showed significantly enhanced reactivity as compared with those based on the IMes ligand, which is in accordance with reports on the superior reactivity of IMesH₂-based Grubbs-type catalysts compared with IMes-based systems.