Tungsten(VI) Carbyne/Bis(carbene) Tautomerization Enabled by N-Donor SBA15 Surface Ligands: A Solid-State NMR and DFT Study

Designing supported well‐defined bis(carbene) complexes remains a key challenge in heterogeneous catalysis. The reaction of W(≡CtBu)(CH2tBu)3 with amine‐modified mesoporous SBA15 silica, which has vicinal silanol/silylamine pairs [(≡SiOH)(≡SiNH2)], leads to [(≡SiNH2−)(≡SiO−)W(≡CHtBu)(CH2tBu)2] and [(≡SiNH2−)(≡SiO−)W(=CHtBu)2(CH2tBu). Variable temperature, 1H–1H 2D double‐quantum, 1H–13C HETCOR, and HETCOR with spin diffusion solid‐state NMR spectroscopy demonstrate tautomerization between the alkyl alkylidyne and the bis(alkylidene) on the SBA15 surface. Such equilibrium is possible through the coordination of W to the surface [(≡Si−OH)(≡Si−NH2)] groups, which act as a [N,O] pincer ligand. DFT calculations provide a rationalization for the surface–complex tautomerization and support the experimental results. This direct observation of such a process shows the strong similarity between molecular mechanisms in homogeneous and heterogeneous catalysis. In propane metathesis (at 150 °C), the tungsten bis(carbene) tautomer is favorable, with a turnover number (TON) of 262. It is the highest TON among all the tungsten alkyl‐supported catalysts. O brave HETCOR! Look how it binds! Solid‐state NMR was used to demonstrate the tautomerization of the alkyl alkylidyne and bis(alkylidene) forms of a tungsten complex on a SBA15 silica surface. Such equilibrium is possible through the coordination of W to the surface‐adjacent [(≡Si−OH) (≡Si−NH2)] groups, which behave as a [N,O] pincer ligand. {W}=W(≡CtBu)(CH2tBu)3.