Well-Defined Silica-Supported Tungsten(IV)–Oxo Complex: Olefin Metathesis Activity, Initiation, and Role of Brønsted Acid Sites

Abstract

Despite the importance of the heterogeneous tungsten-oxo based olefin metathesis catalyst (WO 3 /SiO 2) in industry, understanding of its initiation mechanism is still very limited. It has been proposed that reduced W(IV)-oxo surface species act as precatalysts. In order to understand the reactivity and initiation mechanism of surface W(IV)-oxo species, we synthesized a well-defined silica-supported W(IV)-oxo species (≡SiO)WO(OtBuF 6)(py) 3 (OtBuF 6 = OC(CH 3)(CF 3) 2 ; py = pyridine) (F6@SiO 2-700) via Surface Organometallic Chemistry (SOMC). F6@SiO 2-700 was shown to be highly active in olefin metathesis upon removal of pyridine ligands through the addition of tris(pentafluorophenyl)borane (B(C 6 F 5) 3) or thermal treatment under high vacuum. The metathesis activity towards olefins with and without allylic C−H groups, namely β-methylstyrene and styrene, respectively, was investigated. In the case of styrene, we demonstrate the role of surface OH groups in initiating metathesis activity. In particular, the presence of strong Brønsted acidic surface OH sites, revealed by 15 N-labeled pyridine, likely arises from the presence of adjacent W sites in the catalyst and assist styrene metathesis, an olefin having no allylic C-H bond. In contrast, initiation of olefins containing allylic C−H group (e.g. β-methylstyrene) is independent of surface OH group concentration and likely involves allylic C−H activation mechanism like the molecular W(IV)-oxo species. This study indicates that initiation mechanisms depend on the olefinic substrates and reveals the synergistic effect of Brønsted acidic surface sites and reduced W(IV) sites in the initiation of olefin metathesis.

Olefin metathesis: what have we learned about homogeneous and heterogeneous catalysts from surface organometallic chemistry?

“…6a) was characterized by spectroscopic techniques (IR, NMR, and EXAFS) and shown to initiate metathesis in the presence of Lewis acids like B(C 6 F 5 ) 3 for all types of olens, including those without allylic C-H bonds, suggesting the opening of alternative and/or additional initiation mechanisms in the supported system. 164 Investigation of this system on silica supports prepared at different dehydroxylation temperatures shows that the rate of metathesis for olens having no allylic C-H bond increases with increasing amounts of residual OH groups, while olens having allylic C-H bonds do not show such behaviour (Fig. 6b), suggesting that surface protons can also be involved in the initiation, similar to what was suggested earlier for supported Mo metathesis catalysts.…”

“…In order to further understand the formation of alkylidenes from the putative M(IV) oxo species, tailored W(IV) oxo complexes, [WO(OR) 2 (py) 3 ] (R ¼ Si(OtBu) 3 , tBu F 6 , and tBu F 9 ), and the corresponding well-dened supported [(^SiO) WO(OtBu F 6 )(py) 3 ] were synthesized as molecular and supported model systems, respectively. 163,164 With the molecular system, it was shown that addition of tris(pentauorophenyl)borane (B(C 6 F 5 ) 3 ) or alternative Lewis acids was essential to switch on metathesis activity for olens containing allylic C-H bonds, though interestingly no metathesis activity was observed for olens without allylic C-H bonds.…”

“…One possibility is the formation of surface propoxy species 190 that can be used to reduce the metal sites, hence opening reaction pathway possibly related to what is proposed on Mo and W-based catalysts. 150,164 However, further spectroscopic and computational studies are clearly required to solve this longstanding problem and to allow the further development and implementation of Re-based metathesis catalysts.…”

A Formulation Protocol with Pyridine to Enable DNP-SENS on Reactive Surface Sites: Case Study with Olefin Polymerization and Metathesis Catalysts

“…These Brønsted acid sites are most likely attributed to OH-group in close proximity to cationic Mo-sites as recently discussed for the related silica-supported W-oxo system. [39] Regarding the 13 C DNP-SENS spectrum of Mo=CHR + @SiO2-700 with pre-adsorbed Py (Fig. 4d), besides the signals of Py (147, 136 and 126 ppm) and TCE (75 ppm), these at 20, 130, 142 and 324 ppm are consistent with the presence of the cationic Mo alkylidenes, in view of their similar chemical shift with the molecular compound.…”

Lithium Ion Battery Materials as Tunable, Redox Non-Innocent Catalyst Supports

“…4,5 These support materials, most commonly silica (SiO 2 ) and alumina (Al 2 O 3 ), play a crucial role in stabilizing and site-isolating reactive intermediates on the surface. [6][7][8][9][10][11] While this strategy has been leveraged to great effect, the chemical properties of the support are seldom employed to modulate catalyst reactivity, despite the fact that the surface occupies the inner-coordination-sphere for single-site catalysts, and therefore plays an intimate role in determining the electronic properties of the catalyst. The treatment of supports as chemically dynamic ligands, as opposed to highsurface-area dispersants would allow for the application of lessons learned from the homogeneous catalysis literature, unlocking unique reactivity.…”