The most active alkyne metathesis catalysts rely on well-defined Mo alkylidynes, X3MoCR (X = OR), in particular the recently developed canopy catalyst family bearing silanolate ligand sets. Recent efforts to understand catalyst reactivity patterns have shown that NMR chemical shifts are powerful descriptors, though previous studies have mostly focused on ligand-based NMR descriptors. Here, we show in the con-text of alkyne metathesis that 95Mo chemical shift tensors encode detailed information on the electronic structure of potent catalysts. Analysis by first principles calculations of 95Mo chemical shift tensors ex-tracted from solid-state 95Mo NMR spectra show a direct link of chemical shift values with the energies of the HOMO and LUMO, two molecular orbitals involved in the key [2+2]-cycloaddition step, thus linking 95Mo chemical shifts to reactivity. In particular, the 95Mo chemical shifts are driven by ligand electronega-tivity (σ-donation) and electron delocalization through Mo-O π-interactions, thus explaining the unique reactivity of the silanolate canopy catalysts. These results further motivate exploration of transition-metal NMR signatures and their relations to electronic structure and reactivity.
Experimental section, solution NMR characterization of all compounds, additional solid-state NMR and DFT results.