Structure-property insights into molybdenum imido alkylidene N-heterocyclic carbene complexes for olefin metathesis

Abstract

In the past six years, our group has developed neutral, 16-valence electron (VE) molybdenum imido alkylidene N-heterocyclic carbene (NHC) bistriflates as well as the corresponding cationic 14-VE molybdenum imido alkylidene NHC monotriflate and monoalkoxide complexes, which display high activity, productivity and functional group tolerance in various olefin metathesis reactions. This thesis elaborates and extends the existing catalyst library, thereby enabling stereoselective olefin metathesis reactions, specifically ring-opening cross-metathesis (ROCM). 2,3-Disubstituted norbornene derivatives and a variety of terminal olefins as cross-partners were evaluated, employing tailored cationic molybdenum imido alkylidene monoalkoxide and monopyrrolide catalysts. The choice of substituents at the imido ligand allows for tuning of its steric demand, typically expressed as the buried volume %Vbur. By variation of the buried volume of the imido ligand and the anionic alkoxide or pyrrolide ligands, high E-selectivity of up to 99% as well as high Z-selectivity of up to 96% could be achieved. “Sterically undemanding” allyl ethers used as a cross-partners experienced O-chelation to the cationic metal center, which led to high E-selectivity. The reaction progress of representative high E- and high Z-selective catalysts for all cross-partners was monitored via GC-MS to check for any post-metathesis isomerization. With only one single exception, no post-metathesis isomerization was observed, even though the catalyst remained active after the metathesis reaction. The catalysts delivered high stereoselectivity under thermodynamic but not under kinetic control. Next, thermally switchable hexacoordinated molybdenum initiators bearing a chelating alkylidene were investigated as pre-catalysts for the ring-opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD). The ultimate aim of the current work was to achieve latent pre-catalysts with sufficiently low Tonset (temperature at which exotherm starts) along with quantitative and fast initiation once activated. In the present work, several new molybdenum(VI) imido alkylidene NHC complexes bearing O- as well as N-chelating alkylidene moieties, mostly derived from commercially available, inexpensive olefins viz. N-vinyl-2-pyrrolidone and 2-vinylpyridine, were synthesized and compared with respect to their ROMP behavior in the polymerization of DCPD by differential scanning calorimetry. Olefin metathesis pre-catalysts derived from 2-vinylpyridine provided 4-membered chelating alkylidene motifs and were structurally characterized by single-crystal X-ray analysis. The labile nature of the four-membered chelates provides high activity together with adequate latency caused by the hexacoordination of the metal center. These hexacoordinated complexes were subjected to air stability studies in the solid state. The necessary fine-tuning of Tonset was accomplished by the use of tailored pre-catalysts. In addition to bistriflate complexes, molybdenum imido alkylidene NHC monoalkoxide monotriflate complexes also delivered high activity without sacrificing latency. Furthermore, a rare example of a molybdenum η2-olefin NHC complex obtained through the cross-metathesis of 2-vinylpyridine with Mo(N-2,6-Me2-C6H3)(CHCMe2Ph)(IMes)(OTf)(O-2-Cl-C6H4) is presented. Finally, complexes bearing electron-poor imido ligands were found to be more productive than their analogs bearing electron-rich imido ligands, while being tolerant towards hydroxyl groups and air. In this regard, the novel neutral or cationic molybdenum 2,6-difluorophenylimido alkylidene NHC complexes Mo-19 - Mo-24 were prepared.