Browsing by Author "Benedikter, Mathis J."

Filter results by typing the first few letters
Now showing 1 - 5 of 5
  • Thumbnail Image
    ItemOpen Access
    Cationic group VI metal imido alkylidene N‐heterocyclic carbene nitrile complexes : bench‐stable, functional‐group‐tolerant olefin metathesis catalysts
    (2020) Benedikter, Mathis J.; Musso, Janis V.; Frey, Wolfgang; Schowner, Roman; Buchmeiser, Michael R.
    Despite their excellent selectivities and activities, Mo‐and W‐based catalysts for olefin metathesis have not gained the same widespread use as Ru‐based systems, mainly due to their inherent air sensitivity. Herein, we describe the synthesis of air‐stable cationic‐at‐metal molybdenum and tungsten imido alkylidene NHC nitrile complexes. They catalyze olefin metathesis reactions of substrates containing functional groups such as (thio‐) esters, (thio‐) ethers and alcohols without the need for prior activation, for example, by a Lewis acid. The presence of a nitrile ligand was found to be essential for their stability towards air, while no decrease in activity and productivity could be observed upon coordination of a nitrile. Variations of the imido and anionic ligand revealed that alkoxide complexes with electron‐withdrawing imido ligands offer the highest reactivities and excellent stability compared to analogous triflate and halide complexes.
  • Thumbnail Image
    ItemOpen Access
    Dual catalysis with an N‐heterocyclic carbene and a Lewis acid : thermally latent precatalyst for the polymerization of ε‐caprolactam
    (2020) Altmann, Hagen J.; Steinmann, Mark; Elser, Iris; Benedikter, Mathis J.; Naumann, Stefan; Buchmeiser, Michael R.
    So far, the earlier reported strong correlation between basicity of an N‐heterocyclic carbene (NHC) and its reactivity in poly(ε‐caprolactam) (PA6) synthesis resulted in a substantial limitation of applicable carbenes. Here, to overcome this issue, 1,3‐dimethylimidazolium‐2‐carboxylate, an easily accessible, air and moisture‐stable NHC, was applied as thermally latent initiator. In order to compensate for its low basicity, reactivity was enhanced by the addition of both a Lewis acid and an activator to ease the initial polymerization step. The resulting mixtures of ε‐caprolactam, the CO2‐protected NHC, a Lewis acid and N‐acylazepan‐2‐one constitute homogeneous, thermally fully latent “single‐component” blends for the anionic polymerization‐based synthesis of PA6. They can be stored both in the liquid and solid state for days and months, respectively, without any loss in activity. The role of the Lewis acid as well as technical implications of the prolonged pot‐times are discussed.
  • Thumbnail Image
    ItemOpen Access
    Neutral and cationic molybdenum imido alkylidene cyclic alkyl amino carbene (CAAC) complexes for olefin metathesis
    (2023) Kundu, Koushani; Musso, Janis V.; Benedikter, Mathis J.; Frey, Wolfgang; Gugeler, Katrin; Kästner, Johannes; Buchmeiser, Michael R.
    The first neutral and cationic Mo imido alkylidene cyclic alkyl amino carbene (CAAC) complexes of the general formulae [Mo(N-Ar)(CHCMe2Ph)(X)2(CAAC)] and [Mo(N−Ar)(CHCMe2Ph)(X)(CAAC)][B(ArF)4] (X=Br, Cl, OTf, OC6F5; CAAC=1-(2,6-iPr2-C6H3)-3,3,5,5-tetramethyltetrahydropyrrol-2-ylidene) have been synthesized from molybdenum imido bishalide alkylidene DME precursors. Different combinations of the imido and “X” ligands have been employed to understand synthetic peculiarities. Selected complexes have been characterized by single-crystal X-ray analysis. Due to the pronounced σ-donor/π-acceptor characteristics of CAACs, the corresponding neutral and cationic molybdenum imido alkylidene CAAC complexes do not require the presence of stabilizing donor ligands such as nitriles. Calculations on the PBE0-D3BJ/def2-TZVP level for PBE0-D3BJ/def2-SVP optimized geometries revealed partial charges at molybdenum similar to the corresponding molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes with a slightly higher polarization of the molybdenum alkylidene bond in the CAAC complexes. All cationic complexes have been tested in olefin metathesis reactions and showed improved activity compared to the analogous NHC complexes for hydrocarbon-based substrates, allowing for turnover numbers (TONs) up to 9500 even at room temperature. Some Mo imido alkylidene CAAC complexes are tolerant towards functional groups like thioethers and sulfonamides.
  • Thumbnail Image
    ItemOpen Access
    Reversible N‐heterocyclic carbene‐induced α‐H abstraction in Tungsten(VI) imido dialkyl dialkoxide complexes
    (2020) Musso, Janis V.; Benedikter, Mathis J.; Wang, Dongren; Frey, Wolfgang; Altmann, Hagen J.; Buchmeiser, Michael R.
    The first reversible N‐heterocyclic carbene (NHC) induced α‐H abstraction in tungsten(VI) imido‐dialkyl dialkoxide complexes is reported. Treatment of W(NAr)(CH2Ph)2(OtBu)2 (Ar=2,6‐dichlorophenyl, 2,6‐dimethylphenyl, 2,6‐diisopropylphenyl) with different NHCs leads to the formation of complexes of the type W(NAr)(CHPh)(NHC)(CH2Ph)(OtBu) in excellent isolated yields of up to 96 %. The highly unusual release of the tert‐butoxide ligand as tBuOH in the course of the reaction was observed. The formed alkylidene complexes and tBuOH are in an equilibrium with the NHC and the dialkyl complexes. Reaction kinetics were monitored by 1H NMR spectroscopy. A correlation between the steric and electronic properties of the NHC and the reaction rates was observed. Kinetics of a deuterium‐labeled complex in comparison to its non‐deuterated counterpart revealed the presence of a strong primary kinetic isotope effect (KIE) of 4.2, indicating that α‐H abstraction is the rate‐determining step (RDS) of the reaction.
  • Thumbnail Image
    ItemOpen Access
    Silica‐supported cationic tungsten imido alkylidene stabilized by an N‐heterocyclic carbene ligand boosts activity and selectivity in the metathesis of α‐olefins
    (2020) De Jesus Silva, Jordan; Mance, Deni; Pucino, Margherita; Benedikter, Mathis J.; Elser, Iris; Buchmeiser, Michael R.; Copéret, Christophe
    A well‐defined silica‐supported cationic W imido alkylidene was prepared through surface organometallic chemistry. This catalyst shows preferential activity towards α‐ over internal olefins, which is atypical for W‐based catalysts, but consistent with the strong σ‐donating ability of the NHC ancillary ligand. Complementing the studies on tungsten‐based d0 metathesis catalysts, the silica‐supported cationic W imido alkylidene displays the highest activity among W imido catalysts for α‐olefins and shows improved selectivity for this class of olefins compared to Mo‐based catalysts.