Unravelling the Metastable Nature of the Single Site Tungsten Hydride Metathesis Catalyst Supported on γ-Alumina from First Principles

Abstract

We explore the structure and formation mechanism of the surface tungsten hydride complex supported on alumina, which is a catalyst for alkane and alkene metathesis and crossmetathesis. We show that the kinetics for the formation reaction of the hydride from the grafted alkylidyne-alkyl W complex strongly favors the creation of a metastable W VI trihydride, which is less coordinated by the alumina surface than the most stable isomer. The reaction network for the hydride formation involves hydrogenation of the WºC or W=C bond, hydrogenolysis of W-C bonds, and a second grafting step. The creation of metastable surface complexes is proposed be a key path for the design of active and selective catalysts.

High‐Field NMR, Reactivity, and DFT Modeling Reveal the γ‐Al2O3Surface Hydroxyl Network**

“…In particular, the contribution of site C decreases. Concomitantly, the 27 Al signal in the 1 H- 27 Al D-HMQC-NMR spectrum for sites bearing terminal hydroxyl groups (Figure 4a, see also Figure S6 for the full spectra) broadens with increasing dehydroxylation temperature, consistent with the variation in relative abundance of the A, C and F sites and/ or with increased quadrupolar coupling due to more severe structural distortion. A or F sites may contain either Al T or Al P centers, although the poor resolution in the 1 H…”

“…Chemie dimension precludes a more precise assignment at this time. The low resolution in the HMQC experiment is due to the similarity in 1 H-NMR chemical shifts and large 27 Al quadrupolar coupling constants which cause signal overlap. At 18.8 T, the interpretation of these spectra has reached its current technical limit, although we know that the most abundant type of terminal hydroxyl group is associated with Al T , and the next most abundant with Al P .…”

“…Among the doubly-bridging hydroxyl groups (μ 2 -OH, 1 H CS 1-3 ppm), sites D and E are mostly correlated to 27 Al sites with CS values below 40 ppm, i.e., Al p and Al H sites.…”

High‐Field NMR, Reactivity, and DFT Modeling Reveal the γ‐Al2O3Surface Hydroxyl Network**

“…High-resolution transmission electron microscopy has also been harnessed to study the surface of this material, although the relationship to results from other techniques is not straightforward. [5] In principle, solid-state NMR spectroscopy can provide much more detailed information about the nature of the surface hydroxyls and their relationships, due to the resolution of both 1 H and 27 Al NMR 1D spectra at high magnetic fields, as well as the power of advanced NMR methods, namely, homo-and heteronuclear correlations. [6] Together, they offer the possibility of advancing our current state-of-the-art understanding of the alumina surface.…”

High-field NMR, reactivity, and DFT modeling reveal the gamma-Al2O3 surface hydroxyl network.

“…On the other hand, recent theoretical studies on the nature of the active sites formed by hydrogenolysis of [(Al-O)[W(CtBu)(CH 2 tBu) 2 ] suggest that the real structure is [(Al-O) n [WO(H) 4-n ], in which an oxygen extracted from the alumina support serves to generate the W=O moiety. [23] Calculations show that the participation of neighboring (bridging) hydroxyl groups is essential for this transformation. It may well be that a distinct active structure is formed in the absence of neighbouring OH groups, with superior catalytic performances.…”