Promoting Terminal Olefin Metathesis with a Supported Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalyst

Abstract

Silica-supported cationic Mo-imido alkylidene N-heterocyclic carbene catalysts, prepared by surface organometallic chemistry, display contrasting olefin metathesis activity for terminal and internal olefins. The high metathesis activity towards terminal alkenes is attributed to the strong σ-donating property of the NHC ancillary ligand, which disfavors the formation of the parent square-planar metallacyclobutane, an off-cycle reaction intermediate resulting from the reaction with ethylene, one of the metathesis products. This tailored ligand environment also allowed the first trigonal bipyramidal (TBP) metallacyclobutane reaction intermediate for supported Mo metathesis catalysts to be identified.

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

“…Coordination always occurs trans to the NHC. This is supported by the single crystal X‐ray structures reported here and several other already reported crystal structures of similar compounds, recently published mechanistic studies from our group and mechanistic studies for MAP (monoalkoxide pyrrolide) type Schrock‐catalysts, which indicate that coordination occurs preferably trans to the strongest σ‐donor ligand . Hence, the NHC dictates the coordination of molecules to the metal center.…”

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

“…Other than the longer Mo–NHC distance, the overall structure is highly similar to related cationic molybdenum imido alkylidene NHC complexes, in bond angles and bond lengths. [15a]…”

“…Also worth to be mentioned, in olefin cross metathesis reactions of non‐functionalized olefins, the cationic Mo‐imido alkylidene NHC complexes allow for turnover numbers (TONs) up to several hundred thousands and are active up to 140 °C . Very recently, silica‐supported versions of Mo imido alkylidene NHC complexes, which displayed high reactivity vs. terminal olefins, have been reported . Here we provide a more comprehensive insight into this new class of metal alkylidenes, both in terms of their synthesis and structure.…”

Mechanism of Olefin Metathesis with Neutral and Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes

“…26−35 While molybdenum imido alkylidene NHC bistriflate complexes display substantial activity and functional group tolerance both in the cyclopolymerization of α,ω-diynes and in ring-opening metathesis polymerization (ROMP), particularly cationic molybdenum imido, tungsten imido, and tungsten oxo alkylidene NHC complexes and their silica-supported versions show high activity and productivity in ring-closing metathesis (RCM), cross-metathesis (CM), and homometathesis (HM), reaching turnover numbers > 1 200 000. 27,36,37 We found that molybdenum imido alkylidene NHC bistriflate complexes possess a coalescence temperature, T c , for the two triflates and that there is a correlation between T c and both productivity and activity at a given temperature. 27,33 Further important findings were that neutral complexes containing at least one triflate are activated by the release of triflate.…”