Alkylidene and alkylidyne surface complexes: Precursors and intermediates in alkane conversion processes on supported single-site catalysts

Well-defined surface alkylidenes and alkylidynes can be compared to their molecular counterparts in structural features and formation pathways, but their reactivity towards alkanes is strikingly different. They catalyse the metathesis of alkanes and cross-metathesis of alkanes, whereas no single-component molecular system is known to do so. Herein, we study such reactivity in terms of structure–activity relationships, to further propose a comparison to the reactivity of well-defined group 4–6 supported hydrides, focused on the alkane metathesis and alkane hydrogenolysis processes. There, the formation of intermediates containing alkylidenes and alkylidynes nicely pinpoints their pre-eminence in the catalytic conversion of alkanes, always in agreement with the elementary steps of molecular organometallic chemistry. Finally, a number of relevant reactions where the formation of alkylidene/alkylidyne intermediates has been presumed are also presented.

Alkylidenes and alkylidynes prepared on surfaces (SiO2, Al2O3) exhibit unique catalytic properties for the conversion of alkanes, absent in molecular entities. Structure–activity relationships and mechanistic studies point to the formation of low-coordinated and reactive—yet stable—alkyl–alkylidene intermediates. Surface hydrides, where alkylidene and alkylidyne intermediates have also been observed are also discussed and compared.Figure optionsDownload as PowerPoint slideHighlights
► Well-defined surface alkylidenes/alkylidynes are active in alkane metathesis, their molecular counterparts are not.
► Alkyl–alkylidene intermediates are key to trigger such reactivity, site isolation is crucial.
► Alkylidenes and alkylidynes are also key intermediates in the reactivity of surface hydrides.
► The elementary steps observed are consistent with molecular organometallic chemistry.