The stereospecific ring-opening metathesis polymerization (ROMP) of endo,exo-2,3-dimethoxymethylnorborn-5-ene (DMMNBE) was accomplished using cationic tungsten imido alkylidene N-heterocyclic carbene (NHC) complexes as initiators. These even outperform a cationic molybdenum imido alkylidene...

A series of molybdenum alkylidyne silyloxy N-heterocyclic carbene (NHC) complexes of the general formula [Mo(�C-(R))(OSiPh 3 ) 3 (NHC)] (R = tBu, 4-methoxyphenyl, 2,4,6-trimethylphenyl; NHC = 1,3-diisopropylimidazol-2-ylidene, 1,3-dicyclohexylimidazol-2-ylidene, 1,3-dicyclohexyl-4,5-dihydroimidazol-2ylidene, 1,3-dimethylimidazol-2-ylidene, 1,3-dimethyl-4,5-dichloroimidazol-2-ylidene) was synthesized. Single crystal X-ray analyses revealed that with increasing steric demand of the alkylidyne group, enhanced air-stability of the complexes in the solid-state is achieved with the most stable complex (R = 2,4,6trimethylphenyl, NHC = 1,3-diisopropylimidazol-2-ylidene) being stable in air for 24 h without showing signs of decomposition in 1 H NMR. In contrast to previously reported air-stable molybdenum-based complexes, the novel catalysts proved to be highly active in alkyne metathesis, allowing for turnover numbers (TONs) of up to 6000 without further activation, and tolerant towards several functional groups such as tosyl, ether, ester, thioether and nitro moieties. Their air stability allows for facile handling of the catalysts in air and even after exposure to ambient atmosphere for one week, the most stable representative still displayed high productivity in alkyne metathesis.

A system for the visible-light-controlled ring-opening metathesis polymerization (ROMP) of endo,endo-dicyclopentadiene (DCPD) was developed by combining thermally latent group 6 olefin metathesis initiators, whose vacant binding site is occupied by an N-heterocyclic carbene (NHC), with a photoredox catalyst. Similar ruthenium-based systems have previously been utilized in numerous photolithographic applications. Here, we were able to adapt the approach to group 6 metathesis catalysts for the first time. Extensive screening of 31 different olefin metathesis initiators and five different photoredox catalysts revealed that both tungsten oxo and tungsten imido alkylidene NHC complexes together with 2,4,6-triphenylpyrylium tetrafluoroborate (TPPT) as a photocatalyst are most suitable for the desired application. We propose that, upon irradiation of a mixture of the corresponding olefin metathesis catalyst and TPPT with visible light (440 nm), the excited pyrylium compound oxidizes the NHC ligand and a metathesis-active Schrock-type catalyst is formed. This system can be applied to two-dimensional (2D) photolithography in the ROMP of DCPD. Photomasks were employed to produce cross-linked polymers with great spatial resolution within minutes by applying visible light as the sole stimulus giving access to polymers those compared to many other systems do not require thermal or UV post-curing. Notably, mixtures of the two catalysts and DCPD were latent for several days, and catalyst loadings as low as 100 ppm were sufficient for the desired applications proving the robustness and high activity of the developed system.

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