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)WMe and (≡SiO)TaMe , while (≡SiO)TaMe Cp* is inactive. The contrasting reactivity is paralleled by differences in the C NMR signature; the former display significantly more deshielded isotropic chemical shifts (δ ) and almost axially symmetric chemical shift tensors, similar to what is observed in their molecular precursors TaMe and WMe . 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.