Origin of Reactivity Trends of an Elusive Metathesis Intermediate from NMR Chemical Shift Analysis of Surrogate Analogs

Olefin metathesis has been established as an efficient tool to build carbon-carbon bonds, and its widespread applications in organic synthesis have been made possible by the development of efficient homogeneous catalysts – Grubbs and Schrock-type – that operate through the same intermediates and Chauvin mechanism. With d0 Schrock-type catalysts, the first elementary step, olefin-coordination, is often rate determining, but it been rarely explored due to the lack of accessible relevant molecular analogs. Here, we develop a surrogate of this key olefin-coordination intermediate, namely a cationic d0 tungsten oxo-methylidene complex bearing two N-heterocyclic carbene ligands, [WO(CH2)Cl(IMes)2](OTf) (1) (IMes = 1,3-dimesitylimidazole-2-ylidene; OTf – triflate counter-anion) resulting in a trigonal bipyramidal (TBP) geometry, along with its neutral octahedral analog [WO(CH2)Cl2(IMes)2] (2), and an isostructural oxo-methylidyne derivative [WO(CH)Cl(IMes)2] (3). These compounds were fully characterized by state-of-the-art methods, including single-crystal X-ray diffraction (scXRD) and multinuclear (1H, 13C, 183W) solution NMR spectroscopy. The analysis of their solid-state 13C and 183W MAS NMR signatures, along with computed 17O NMR parameters, helps to correlate their electronic structures with NMR patterns and evidences the importance of the competition between the three equatorial ligands in the TBP complexes. Further analysis validates the suitability of complex 1 as a surrogate molecule to interrogate the electronic structure of the key olefin coordination intermediate of the Chauvin cycle. Anchored on experimentally obtained NMR parameters for 1, computational analysis of a series of olefin coordination intermediates highlights the interplay between σ- and π-donating ligands in modulating the stability of TBP olefin-coordination intermediates, paralleling their reactivity. In this work, NMR spectroscopy descriptors reveal the origin for the advantage of the dissymmetry in σ-donating abilities of ancillary ligands in Schrock-type catalysts: weak σ-donors avoid the orbital-competition with the oxo ligand upon formation of a TBP olefin-coordination intermediate, while stronger σ-donors compromise M≡O triple bonding and thus render olefin coordination step energy expensive.