Metal Alkyls with Alkylidynic Metal-Carbon Bond Character: Key Electronic Structures in Alkane Metathesis Precatalysts

The homologation of alkanes via alkane metathesis is catalyzed at low temperatures (150 °C) by the silica-supported species (≡SiO)WMe₅ and (≡SiO)TaMe₄, while (≡SiO)TaMe₃Cp* is inactive. The contrasting reactivity is paralleled by differences in the ¹³C NMR signature; the former display significantly more deshielded isotropic chemical shifts (δ_iso) and almost axially symmetric chemical shift tensors, similar to what is observed in their molecular precursors TaMe₅ and WMe₆. Analysis of the chemical shift tensors reveals the presence of a triple-bond character in their metal-carbon (formally single) bond. This electronic structure is reflected in their propensity to generate alkylidynes and to participate in alkane metathesis, further supporting the role of alkylidynes as key reaction intermediates. This study establishes chemical shift as a descriptor to identify potential alkane metathesis catalysts.