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

Abstract

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.

Solid-State NMR Spectra of Protons and Quadrupolar Nuclei at 28.2 T: Resolving Signatures of Surface Sites with Fast Magic Angle Spinning

“…The compound exhibits a 27 Al isotropic shift of 44 ppm and a quadrupolar coupling constant of 29.6 MHz, in very good agreement with values predicted from first-principles calculations (Table S4.1). The 27 Al chemical shift of this compound is significantly shielded compared to what is expected for tricoordinate Al in aluminosilicates (δ iso = 87 ppm and C Q = 35 MHz), due to the differences of aryloxide vs (surface) siloxide ligands and associated σ/π effects, but shows similar quadrupolar coupling constants as expected from their similar trigonal planar geometry . At lower magnetic field strengths or without adequate MAS rates, static wide-line excitation and detection methods are necessary to extract the 27 Al parameters of such Al sites, which would limit spectral resolution.…”

NMR Signatures and Electronic Structures of Ti-sites in Titanosilicalite-1 from solid-state 47/49Ti NMR Spectroscopy

“…3c S9), in line with recent NMR studies. 44,45 In conclusion, we obtained the 47/49 Ti ssNMR signatures of TS-1, despite the low Ti loading (< 2 wt%).…”

Surface immobilization of nitrogen-coordinated iron atoms: a facile and efficient strategy toward MNC sites with superior catalytic activities

“…[10][11][12][13] The resulting atomically dispersed metal sites make it easy to understand the catalysis reaction at the molecular level, but this strategy is usually only effective for the fabrication of early transition metal sites on the substrate surface. [14][15][16][17][18] Later, metal-organic frameworks (MOFs) were developed as uniform heterogeneous catalysts. 10,19 MOFs are crystalline porous materials composed of highly ordered metal nodes and organic ligands, 20,21 and the atomically dispersed metal sites can be generated via the incorporation of metal sites into the metal nodes, organic ligands, and well-defined pores.…”

“…10–13 The resulting atomically dispersed metal sites make it easy to understand the catalysis reaction at the molecular level, but this strategy is usually only effective for the fabrication of early transition metal sites on the substrate surface. 14–18…”