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Mo/W Imido and W Oxo Alkylidene N-Heterocyclic Carbene Complexes for Olefin Metathese

The synthesis of the first molybdenum imido alkylidene N-heterocyclic carbene (NHC) bistriflate catalysts in 2014 and the following mechanistic investigations resulted in the fast development of a large catalyst library. Variations in the metal or in the ligand sphere, such as the imido ligand, the alkoxide ligand and the NHC moiety paved the way for numerous applications. Remarkably, due to the excellent s-donor properties of the NHCs, the first highly active cationic group 6 metal alkylidene NHC were prepared that allowed to run various olefin metathesis reactions with turnover numbers >500,000. These olefin metathesis reactions entail ring opening metathesis polymerization (ROMP), cross-metathesis (CM) and ring closing metathesis (RCM), therefore making the catalysts interesting targets even beyond polymer chemistry. Their immobilization on silica, their application in biphasic metathesis and their high functional group tolerance further highlight the versatile properties of this new class of olefin metathesis catalysts and led to cooperations, further widening the scope of the catalyst system.
Figure 1: Exemplary view of molybdenum and tungsten imido/ oxo alkylidene NHC complexes.

Mechanistic studies
The unprecedented catalytic activity of 16-electron molybdenum and tungsten complexes in olefin metathesis reactions raised the question about mechanistic details, since the usual Schrock type olefin metathesis catalysts display a 14-electron architecture. In situ 19F NMR measurements revealed the reason for the unexpected catalytic properties. The synergistic effect of the excellent donor ligand NHC and the excellent leaving group character of the triflates results in the formation of the catalytically active cationic species.
Figure 2: Possible reaction pathways for the formation of the catalytically active cationic species.

Molybdenum imido alkylidene NHC bistriflate complexes display a characteristic coalescence temperature, Tc, at which the catalysts have a square pyramidal geometry and the trans-effect of the NHC ligand on the triflate ligand becomes fully operative, enabling the fast formation of the cationic active species. Variations of the carbene ligand, covering carbenes with large Tolman electronic parameters (TEP) and therefore less pronounced donor properties, such as 1,2,4- triazol-4-ylidenes, and carbenes with small Tolman electronic parameters, e.g., mesoionic carbenes, allowed for the fine tuning of Tc. This opened the way to tailor-made catalysts with special operating windows.

Cationic olefin metathesis catalysts
Molybdenum imdo, tubgsten imido and tungsten oxo alkylidene NHC complexes can be transferred into cationic complexes by the replacement of one anionic X ligand (e.g., triflate, chloride, alkoxide) by a weakly coordinating anion like BF4 or B(ArF)4 (tetrakis-(3,5-trifluoromethylphenyl)borate). The stable tetracoordinated cationic species exhibit high activity in olefin metathesis reactions, which sets them apart from the few examples of cationic alkylidene complexes published by the Schrock group. The influence of the weakly coordinating anion on stability and activity of the cationic systems is currently investigated in our laboratories.
Figure 3: Cationic metal imido alkylidene NHC complexes (metal = Mo, W).

Stereoselectivity in olefin metathesis reactions can be addressed by forcing the intermediate metallacyclobutane into a preferred configuration. This can most possibly be realized by the employment of sterically demanding alkoxides, chiral alkoxides, chiral carbenes and bidentate (chiral) ligands. Several complexes with such ligands have already been synthesized in our laboratories. Furthermore, imido alkylidene NHC complexes of group 6 metals exist in two configurations, the syn- configuration with the substituent R at the metal carbon double bond pointing in the direction of the imido ligand and the corresponding anti- configuration. Syn- and anti- isomers can be interconverted by simple irradiation with UV- light. Studies on the rate of syn/anti-interconversion in group 6 imido alkylidene NHC complexes and the influence on the selectivity in olefin metathesis are ongoing.
Figure 4: Syn/anti- interconversion in molybdenum imido alkylidene NHC complexes. Schematic view of syn- and anti-signals in proton NMR spectroscopy.

Molybdenum imido alkylidene NHC bistriflate catalysts have been immobilized via coordination of a silica-bound NHC to standard bistriflate complexes. Employment of those immobilized analogues in RCM and CM lead to the isolation of metal-free olefin metathesis products. Furthermore, tungsten oxo alkylidene complexes and molybdenum imido alkylidene mono triflate mono alkoxide complexes have been immobilized on silica by replacement of the X ligand with a Si-O bond in cooperation with the Copéret group at the ETH Zurich. Especially the immobilized tungsten based catalyst 2@SiO2 exhibits unprecedented activity in self metathesis reactions with turnover numbers >1,200,000. In depth solid-state NMR investigations allowed for anylzing the geometry of the intermediary metal cyclobutanes.
Figure 5: Immobilized NHC alkylidene complexes.

From alkylidenes to alkylidynes
Recently, our group transferred the concept of stabilizing high oxidation state group 6 metal complexes with NHCs from alkylidene to alkylidyne metathesis catalysts. Several molybdenum and tungsten alkylidyne carbyne complexes have successfully been synthesized and proved to be active in alkyne metathesis. Mechanistic investigations and the employment of more sophisticated carbene ligands, like e.g. donor- functionalized carbenes, to enable the synthesis of cationic alkylidyne complexes, are under way.
Figure 6: Molybdenum NHC alkylidyne complexes tested in a model reaction.

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