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

Abstract

Designing supported well-defined bis(carbene) complexes remains akey challenge in heterogeneous catalysis. The reaction of W(CtBu)(CH 2 tBu) 3 with amine-modified mesoporous SBA15 silica, which has vicinal silanol/silylamine pairs [( SiOH)( SiNH 2 )],l eads to [( SiNH 2 À )( SiO À )W( CHtBu)(CH 2 tBu) 2 ]a nd [( SiNH 2 À )( SiO À )W( = CHtBu) 2 -(CH 2 tBu). Variable temperature, 1 H-1 H2 Dd ouble-quantum, 1 H-13 CH ETCOR, and HETCOR with spin diffusion solidstate NMR spectroscopydemonstrate tautomerization between the alkyla lkylidyne and the bis(alkylidene) on the SBA15 surface.Suchequilibrium is possible through the coordination of Wt ot he surface [( Si À OH)( Si À NH 2 )] groups,w hich act as a[ N,O] pincer ligand. DFT calculations provide ar ationalization for the surface-complex tautomerization and support the experimental results.T his direct observation of such ap rocess shows the strong similarity between molecular mechanisms in homogeneous and heterogeneous catalysis.I n propane metathesis (at 150 8 8C), the tungsten bis(carbene) tautomer is favorable,w ith at urnover number (TON) of 262. It is the highest TONa mong all the tungsten alkyl-supported catalysts.

Tungsten Catalyst Incorporating a Well‐Defined Tetracoordinated Aluminum Surface Ligand for Selective Metathesis of Propane, [(≡Si−O−Si≡)(≡Si−O−)2Al−O−W(≡CtBu) (H)2]

“…The W low loading in 4 (1.43 wt.%, Table S3, SI) does not allow clear observation of the quaternary carbon of W‐carbyne. This result was expected, since earlier work demonstrated that to measure the resonance of the carbyne in a species immobilized on oxide support it is necessary to have a high W loading (8 wt.%) and/or isotopically labeled W‐containing species …”

“…The 13 C SS NMR signals characteristic of the neopentyl ligands were observed at 34 and 102 ppm and assigned to CH 2 and CH 3 , respectively. Nonetheless, we observed a weak signal at 32 ppm that we assigned to the CH 3 group of the neopentylidyne fragment (Figure S13, SI) …”

A Silica-Supported Monoalkylated Tungsten Dioxo Complex Catalyst for Olefin Metathesis

“…Importantly, no clear indication of the formation of a Walkylidene complex, expected to display a signal at ∼10−11 ppm in the 1 NMR spectrum and in the 220−300 ppm region in the 13 C NMR spectrum, 16,84 was observed. Whereas tungsten forms stable metallacyclobutane intermediates, 74 tungsten alkylidene intermediates generated from supported tungsten alkyl complexes are generally difficult to observe, as a consequence of their high reactivity and lability.…”

Density Functional Theory Study of the Reaction between d0 Tungsten Alkylidyne Complexes and H2O: Addition versus Hydrolysis

“…32 Previous computational studies of early-transition-metal alkylidyne complexes mainly focused on metathesis 28,45,46 and t a u t o m e r i z a t i o n b e t w e e n a l k y l i d y n e a n d b i s -(alkylidene). 31,[33][34][35][36]47,48 The grafting reaction of C-1 onto the γ-alumina surface has been studied by density functional theory (DFT) calculations. 17 The calculations showed that the grafting of C-1 occurs via direct electrophilic cleavage of the W−C bond by Al s OH to form the monografted surface complex [(SiO)W(C t Bu)(CH 2 t Bu) 3 ].…”