Tungsten Oxo and Tungsten Imido Alkylidene N-Heterocyclic Carbene Complexes for the Visible-Light-Induced Ring-Opening Metathesis Polymerization of Dicyclopentadiene

Abstract

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.

PhotoROMP: The Future Is Bright

“…In addition to the benefits of visible light over UV-initiation from the perspective of energy input and user safety, this system exhibited excellent spatial control and microscale resolution. The Buchmeiser group demonstrated a similar system employing NHC-coordinated W-vinylidene species (e.g., W-1 ) in 2D photolithography . Pushing toward even lower-energy radiation sourcesa near-IR-initiated ROMP system using an osmium photooxidant was also investigated .…”

“…The Buchmeiser group demonstrated a similar system employing NHC-coordinated W-vinylidene species (e.g., W-1 ) in 2D photolithography. 53 Pushing toward even lower-energy radiation sources—a near-IR-initiated ROMP system using an osmium photooxidant was also investigated. 54 Though this system required acetone as a cosolvent and therefore exhibited limited feasibility for applications in photolithography, the use of NIR radiation as the activating stimulus enabled through-barrier initiation, a feature impossible with visible light methods.…”

Olefin Metathesis and Stereoselective Ring-Opening Metathesis Polymerization with Neutral and Cationic Molybdenum(VI) Imido and Tungsten(VI) Oxo Alkylidene Complexes Containing N-Chelating N-Heterocyclic Carbenes

“…The area of stereoselective ring-opening metathesis polymerization − (ROMP) by well-defined group 5, 6 and group 8 metal alkylidenes has blossomed into a mature field over the last two decades, given the large number of publications and that all possible stereoregular structures can now be accessed, though with different ease and generality. In this regard, neutral and cationic molybdenum imido, tungsten imido, and tungsten oxo alkylidene N-heterocyclic carbene (NHC) complexes have evolved as highly active and stereoselective catalysts. ,,,− Recently, we reported on molybdenum imido alkylidene complexes containing N-, O-, and C-chelating NHCs. , In view of their unique reactivity but also stability, we extended our research to cationic molybdenum and tungsten­(VI) imido and oxo alkylidene complexes containing N-chelating NHCs. Notably, to date, no neutral or cationic tungsten­(VI) oxo alkylidene complexes containing N-chelating N-heterocyclic carbene have been reported.…”