Abstract
A series of neutral molybdenum imido alkylidene N-heterocyclic carbene (NHC) bistriflate and monotriflate monoalkoxide complexes as well as cationic molybdenum imido alkylidene triflate complexes have been subjected to NMR spectroscopic, X-ray crystallographic, and reaction kinetic measurements in order to gain a comprehensive understanding about the underlying mechanism in olefin metathesis of this new type of catalysts. On the basis of experimental evidence and on DFT calculations (BP86/def2-TZVP/D3/cosmo) for the entire mechanism, olefinic substrates coordinate trans to the NHC of neutral 16-electron complexes via an associative mechanism, followed by dissociation of an anionic ligand (e.g., triflate) and formation of an intermediary molybdacyclobutane trans to the NHC. Formation of a cationic complex is crucial in order to become olefin metathesis active. Variations in the NHC, the imido, the alkoxide, and the noncoordinating anion revealed their influence on reactivity. The reaction of neutral 16-electron complexes with 2-methoxystyrene is faster for catalysts bearing one triflate and one fluorinated alkoxide than for catalysts bearing two triflate ligands. This is also reflected by the Gibbs free energy values for the transition states, ΔG‡303, which are significantly lower for catalysts bearing only one triflate than for the corresponding bistriflate complexes. Reaction of a solvent-stabilized cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) monotriflate complex with 2-methoxystyrene proceeded via an associative mechanism too. Reaction rates of both solvent-free and solvent-stabilized cationic Mo imido alkylidene NHC catalysts with 2-methoxystyrene are controlled by the cross-metathesis step but not by adduct formation.
Highlights
Activation of the catalysts through the release of one triflate in the square pyramidal (SP) configuration is in full accordance with the observed reactivity of both neutral and cationic Mo−imido alkylidene N-heterocyclic carbene (NHC) complexes and with 19F NMR.[40]
An olefin metathesis reaction starting from neutral, pentacoordinated 16-electron Mo/W imido or oxo alkylidene NHC complexes can occur in a dissociative fashion, with one triflate leaving the complex, thereby generating a cationic 14-electron species, followed by coordination of substrate
The reaction can proceed in an associative fashion where substrate coordination is followed by the dissociation of triflate (Scheme 1)
Summary
Olefin metathesis with well-defined metal alkylidenes has long been dominated by Schrock- and Grubbs-type catalysts,[1−7] mostly because of their most favorable properties in terms of regio-, chemo-, and stereoselectivity[8−22] and often specificity in many olefin metathesis reactions including those related to polymer chemistry.[23−25] Aiming at ionic olefin metathesis catalysts for use in biphasic reactions, we recently reported on a new class of neutral and cationic molybdenum imido, tungsten imido, and tungsten oxo alkylidene N-heterocyclic carbene (NHC) complexes as a new family of group 6 olefin metathesis catalysts and successfully carried out substantial variations in both the imido and the NHC ligand.[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), 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), crossmetathesis (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, Tc, for the two triflates and that there is a correlation between Tc 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. Considering the complete reaction pathway based on the predicted energies as depicted in Scheme 3 and Figures 4 and 5, the predominant mechanism with the lowest relative energies of the transition states comprises the direct formation of the neutral adduct 21, where the substrate is coordinated to the neutral catalyst. In the case of 2,2-dimethylpent-4-ene, this destabilizing steric effect is less pronounced because the t-Bu group points away from the N-aryl group
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