Classifying and Understanding the Reactivities of Mo-Based Alkyne Metathesis Catalysts from ⁹⁵Mo NMR Chemical Shift...

The most active alkyne metathesis catalysts rely on well-defined Mo alkylidynes, X₃Mo≡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 ⁹⁵Mo chemical shift tensors encode detailed information on the electronic structure of these catalysts. Analysis by first-principles calculations of ⁹⁵Mo chemical shift tensors extracted from solid-state ⁹⁵Mo 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 ⁹⁵Mo chemical shifts to reactivity. In particular, the ⁹⁵Mo 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.