Tailored molybdenum imido alkylidene N‐heterocyclic carbene complexes as latent catalysts for the polymerization of dicyclopentadiene

Abstract

Three novel molybdenum imido alkylidene N‐heterocyclic carbene (NHC) pre‐catalysts, that is, Mo(N‐t‐Bu)(1‐(2,6‐diisopropylphenyl)‐3‐isopropyl‐4‐phenyl‐1H‐1,2,3‐triazol‐5‐ylidene)(CHCMe2Ph)(OTf)2 (I1, OTf = CF3SO3), Mo(N‐t‐Bu)(1‐(2,6‐diisopropylphenyl)‐3‐isopropyl‐4‐phenyl‐1H‐1,2,3‐triazol‐5‐ylidene)(CHCMe2Ph)(OTf)(t‐BuO) (I2) and Mo(N‐2,6‐Me2‐C6H3)(1,3,4‐triphenyl‐4,5‐dihydro‐1H‐1,2,4‐triazol‐5‐ylidene)(CHCMe2Ph)(OTf)2 (I3) are presented. Compared to complexes based on imidazol‐2‐ylidenes or imidazolin‐2‐ylidenes, (1‐(2,6‐diisopropylphenyl)‐3‐isopropyl‐4‐phenyl‐1H‐1,2,3‐triazol‐5‐ylidene) used in precatalysts I1 and I2 exerts a comparably strong trans effect to the triflate groups trans to the NHC, while (1,3,4‐triphenyl‐4,5‐dihydro‐1H‐1,2,4‐triazol‐5‐ylidene) used in I3 has a weaker trans effect on the triflate. In combination with a suitable second anionic ligand at molybdenum, that is, OTf, t‐BuO, compounds I1–I3 require higher temperatures to become active and can thus be used as truly room temperature latent pre‐catalysts, even for a highly reactive monomer such as dicyclopentadiene (DCPD). When used as latent precatalysts, I1–I3 offer access to poly‐DCPD with different degrees of cross‐linking and glass‐transition temperatures (Tg). © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3028–3033

Understanding Synthetic Peculiarities of Cationic Molybdenum(VI) Imido Alkylidene N‐Heterocyclic Carbene Complexes

“…The geometry at the metal is an intermediate between SP and TBP ( τ 5 = 0.495). Contrary to previously reported NHC bistriflate alkylidenes,, [19a], , this structure seems to be highly affected by the greater N‐C NHC ‐N angle of the 6‐membered carbene ligand, which results in increased steric bulk. The metal–NHC bond is considerably longer (235 pm vs. ≈ 220 pm in previous NHC alkylidene complexes).…”

“…The preference for the protonation of alkoxides/aryloxides might evoke as the key feature in rendering such catalysts tolerant towards protic functional groups. Notably, there are many examples of pentacoordinated molybdenum imido alkylidene NHC complexes containing two different anionic ligands that are not prone to ligand scrambling , …”

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

“…Also, 1,1′‐bipyridine and phenanthroline can only be removed from the catalyst by the addition of an additional Lewis acid, for example, ZnCl 2 . So far, pentacoordinated 16‐valence electron (VE) molybdenum imido alkylidene NHC bistriflate complexes with 1,2,3‐ and 1,3,4‐triazol‐5‐ylidenes have been shown to be potent candidates for the latent polymerization of dicyclopentadiene . In search for alternatives, we extended the base of thermally latent pentacoordinated molybdenum imido bistriflate alkylidene NHC complexes but also aimed at the synthesis and use of the first hexacoordinated, 18‐VE molybdenum imido bistriflate alkylidene NHC complexes containing an additional chelating ligand.…”

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

“…[15d] high TOFst hat have been found for these catalysts. [28] Based on current knowledge,t he active species in pentacoordinated complexes is believed to be at etracoordinated, cationic one as demonstrated by 19 FNMR studies on the activation of pentacoordinated pre-catalysts. This requires the dissociation of one anionic ligand in pentacoordinated species.N onetheless, in such activated pentacoordinated precursors, the anion stays in close contact to the cationic forming an ion pair.T he extento f ion pairing obviously depends on solvent polarity.I nv iew of the poor leaving group character of (fluorinated) alkoxides, it is not surprising that monoalkoxide monotriflate pre-catalysts display generally ah igherp roductivity than bis-alkoxide precursors.I nc ationic complexes, fluorinated alkoxides and phenoxides increase catalytic activity, while electron-rich alkoxides such as tert-butoxide can stabilizet he cationic metal center very efficiently,w hich can result in an almost complete shutdown of catalytic activity.…”

“…Below T c , the pre‐catalysts are inactive even in the presence of highly reactive substrates and monomers, for example, dicyclopentadiene (DCPD), a pure hydrocarbon‐based crosslinked thermoset polymer with high notch‐impact strength and high resistance to chemicals. In due consequence, they can be used as thermally fully latent pre‐catalysts for the ring‐opening metathesis polymerization (ROMP) of DCPD, thereby allowing true “single‐component” mixtures of both the pre‐catalyst and the monomer that can be stored for prolonged time prior to thermal curing. 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).…”