Browsing by Author "Schowner, Roman"

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    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.
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    Predicting catalytic activity from 13CCH alkylidene chemical shift in cationic tungsten oxo alkylidene N‐heterocyclic carbene complexes
    (2021) Musso, Janis V.; Schowner, Roman; Falivene, Laura; Frey, Wolfgang; Cavallo, Luigi; Buchmeiser, Michael R.
    A series of cationic tungsten oxo alkylidene N‐heterocyclic carbene (NHC) complexes was synthesized and structurally characterized by single crystal X‐ray diffraction. The 13C NMR chemical shifts of the alkylidene C atoms of these complexes were correlated with the diamagnetic, paramagnetic and spin‐orbit chemical shifts calculated by DFT. A good correlation (R2=0.90) between the DFT isotropic chemical shifts and the experimental chemical shift as well as a strong correlation between the DFT isotropic chemical shifts and the TOF1min for the RCM of 1,7‐octadiene was found. Further, a comparison of the catalyst geometries allowed for assigning tetracoordinate pseudotetrahedral catalysts to the most deshielded alkylidenes and to the highest TOF1min, pentacoordinate square‐planar catalysts to the intermediate deshielded alkylidenes and intermediate TOF1min, and hexacoordinate and octahedral catalyst to the most shielded alkylidene and lowest TOF1min. Analysis of the magnetic shielding tensors allowed for ascribing variations in the chemical shifts to electronic transitions between occupied molecular orbitals corresponding to the alkylidene‐C and alkylidene‐H σ‐bonds and the empty molecular orbital corresponding to the W‐alkylidene σ*‐bond.
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    Synthesis, structure and reactivity of ionic olefin metathesis catalysts of group VI
    (2020) Schowner, Roman; Buchmeiser, Michael R. (Prof. Dr.)
    Die vorliegende Arbeit befasst sich mit dem Design und der Entwicklung neuer Katalysatoren und Katalysatorsysteme für die Olefin Metathese. Der erste Teil der Dissertation behandelt die Erarbeitung eines zweiphasigen, flüssig-flüssig Systems für die Anwendung mit vergleichsweise empfindlichen Molybdän- und Wolfram Alkylidenkomplexen. Dadurch konnten Verunreinigungen der Reaktionsprodukte durch Katalysatorrückstände minimiert werden. Der zweite, wesentliche Aspekt dieser Arbeit ist die Erweiterung der Katalysatorbibliothek der in diesem Arbeitskreis eingeführten Molybdän- und Wolfram Alkyliden NHC Katalysatoren. Durch gezielte Variation der Liganden konnten so eine große Anzahl an Metallkomplexen dargestellt und charakterisiert werden. Diese Verbindungen sollten dazu beitragen, die Anwendungsgebiete der neuen Katalysatorklasse zu erweitern. Daher wurden sie auf ihre Reaktivität gegenüber Olefinen, mit und ohne funktionelle Gruppen, geprüft. Bei diesen Untersuchungen lag auch auf Nebenreaktionen und Zersetzungsprodukten ein Augenmerk.
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    Synthetic and structural peculiarities of neutral and cationic molybdenum imido and tungsten oxo alkylidene complexes bearing weakly coordinating N‐heterocyclic carbenes
    (2024) Buchmeiser, Michael R.; Wang, Dongren; Schowner, Roman; Stöhr, Laura; Ziegler, Felix; Sen, Suman; Frey, Wolfgang
    The syntheses of the neutral molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes of the general formula [Mo(NAr)(CHCMe2Ph)(NHC)XY] (Ar=2-tBu-C6H4, 2-CF3-C6H4, 2,6-Me2-C6H3, 2,6-Cl2-C6H3, adamantyl; X, Y=OTf, OC(CF3)3, OCH(CF3)2, OC6F5, SC6F5, 2,5-bis(pentafluorophenyl)phen-1-yl) bearing electron-withdrawing NHCs (1,3-dimethyl-4,5-dichloroimidazol-2-ylidene (IMeCl2), 1,3,4-triphenyl-1,2,4-triazol-5-ylidene (TPT)) are reported. Complementary, the corresponding cationic molybdenum imido alkylidene NHC complexes of the general formula [Mo(NAr)(CHCMe2R)(NHC)X+][B(ArF)4−/Al(OC(CF3)3)4−] (R=Me, Ph; B(ArF)4-=tetrakis (3,5-bis(trifluoromethyl)phenyl)borate) have been prepared. Aiming at tungsten oxo complexes, reaction of [W(O)Cl2(CHCMe2Ph)(PMe2Ph)2] with [1,3-dimethyl-4,5-dichloroimidazol-2-ylidene⋅AgI] (IMeCl2⋅AgI) followed by the addition of lithium terphenoxide yields [W(O)(CHCMe2Ph)(IMeCl2)(DPPO)2]. For comparison, [W(O)Cl(CHCMe2Ph)(IMes)(OSi(OtBu)3)] was prepared via reaction of [W(O)Cl2(CHCMe2Ph)(PMe2Ph)(IMes)] with KOSi(OtBu)3. [W(O)(CHCMe2Ph)(IMeCl2)(DPPO)(Et2O)+][B(ArF)4−] (DPPO=2,6-diphenylphenoxide) became accessible via reaction of [W(O)(DPPO)2(CHCMe2Ph)(IMeCl2)] with anilinium B(ArF)4-. The structural peculiarities of selected complexes are reported. Benchmark ring-closing metathesis and homometathesis reactions revealed that the neutral complexes bearing weakly coordinating NHCs such as IMeCl2 and TPT possessed only moderate activity, which could, however, be improved by preparing the corresponding cationic metal alkylidene complexes.