Latent and Air‐Stable Pre‐Catalysts for the Polymerization of Dicyclopentadiene: From Penta‐ to Hexacoordination in Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Complexes

Abstract

The pentacoordinated, 16-valence electron (VE) Mo imido alkylidene N-heterocyclic carbene (NHC) complexes I1-I5 and the hexacoordinated 18-VE Mo imido alkylidene NHC complexes 1-4, 8, 10 and 12 containing a chelating ligand have been prepared and used as thermally latent catalysts in the ring-opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD). Both 10 and 12 are the first Mo imido alkylidene complexes with a chelating alkylidene featuring a carboxylate group. Complexes I1-I3 and I5 as well as 1-4 proved to be fully thermally latent in the presence of DCPD. With the changes in both the electronic and steric situation at the imido ligand provided by these pre-catalysts, different temperatures of the onset of polymerization (T =65-140 °C) and for the exothermic maximum of the curing curve (T =98-183 °C) of DCPD were achieved. Also, the degree of crosslinking was successfully varied as indicated by swelling experiments in toluene, which revealed degrees of swelling between 0 and 50 %. While the introduction of a chelating alkylidene increases T , the introduction of more electron-donating anionic ligands (tert-butoxide, phenoxide) resulted in a drastic reduction in T , underlining the high flexibility of these systems. The hexacoordinated high-oxidation state molybdenum imido alkylidene NHC complexes 2, 3 and 4 were stable under air for at least twelve hours in the solid state.

Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalysts

“…Catalysts containing the more basic alkyl‐substituted imido ligands ( 8 , 11 , 17 a ) displayed decreased reactivity compared to complexes with aromatic imido ligands. This was also observed earlier . To demonstrate that this is a reactivity issue, we mixed catalyst 11 with 10 equiv 10‐undecenoic acid and monitored the reaction by 1 H NMR (Figure S122).…”

“…Until now, monocarboxylate species of Schrock‐type catalysts have not been reported. However, the monotriflate‐monocarboxylate species Mo( N ‐ t Bu)(CHCMe 2 Ph)(IMes)(O 2 CC 6 F 5 )(OTf) 10 can readily be prepared from 9 a (Scheme ). Subsequent transformation into the cationic monocarboxylate species 11 is facile and high‐yielding employing NaB(Ar F ) 4 .…”

Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalysts

“…Recently, we have shown that tailored six‐coordinate neutral Mo‐imido alkylidene NHC catalysts can serve as latent pre‐catalysts in the polymerization of dicyclopentadiene (DCPD) and are stable to air for at least 12 h . Here, we tested the air stability of our cationic complexes for a minimum of 12 hours.…”

“… Synthesis of cationic monocarboxylate species 11 – 15 from bistriflates 9 a – d via mixed monotriflate/monocarboxylate complex 10 or in situ generation of cationic monotriflate species and subsequent salt metathesis. Synthesis of cationic monocarboxylate complexes 17 a , b from dichloride NHC alkylidenes 16 a , b .…”

Molybdenum Imido, Tungsten Imido and Tungsten Oxo Alkylidene N‐Heterocyclic Carbene Olefin Metathesis Catalysts

“…Such setup avoids any fast pre‐mixing of the components and thus simplifies the production process of poly‐DCPD, which is usually accomplished via reaction injection molding (RIM) or, in the case of fiber‐reinforced structures, via vacuum‐assisted resin‐transfer molding (RTM). Very recently, this concept has been extended to hexacoordinated 18‐VE complexes containing chelating NHCs and alkylidenes, respectively (Figure ) …”