Classifying and Understanding the Reactivities of Mo-Based Alkyne Metathesis Catalysts from Mo NMR Chemical Shift Descriptors.

The most active alkyne metathesis catalysts rely on well-defined Mo alkylidynes, XMo≡CR (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 context of alkyne metathesis that Mo chemical shift tensors encode detailed information on the electronic structure of these catalysts. Analysis by first-principles calculations of Mo chemical shift tensors extracted from solid-state Mo NMR spectra shows 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 Mo chemical shifts to reactivity. In particular, the Mo chemical shifts are driven by ligand electronegativity (σ-donation) and electron delocalization through Mo-O π interactions, thus explaining the reactivity patterns of the silanolate canopy catalysts. These results further motivate exploration of transition metal NMR signatures and their relationships to electronic structure and reactivity.