17 O MAS NMR studies of oxo-based olefin metathesis catalysts: a critical assessment of signal enhancement methods

International audienceThe DFS enhancement method as applied to 17O MAS NMR was critically assessed, first on NaPO3, a simple binary glass system, and in a second step, on a series of catalysis-related organometallic molecules and materials. The robustness of DFS was investigated for the wide range of anisotropic parameters (quadrupolar coupling and chemical shift anisotropy) encountered in these samples. Emphasis has been put on the variation of signal enhancements with respect to the DFS final sweep frequency, pulse amplitude and pulse duration, while line shape distortion issues were also addressed. Finally, the robustness of DFS enhancement of the 17O MAS NMR signal is shown through its successful application to silica-supported olefin metathesis catalysts

On the use of 17 O NMR for understanding molecular and silica-grafted tungsten oxo siloxide complexes

International audience17 O-labelled tungsten siloxide complexes and grafted materials were prepared and characterized by 17 O MAS NMR, with input from DFT calculations. Guidelines linking 17 O NMR parameters and the tungsten oxo species coordination sphere are proposed

Modification of silica-supported tungsten neosilyl oxo precatalysts: impact of substituted phenol on activity and stability in olefin metathesis

International audienceA novel strategy to access a large variety of tungsten oxo pre-catalysts for olefin metathesis involving post-treatment of [(SiO)W(O)(CH2SiMe3)3], 1b, with the desired spectator ligands is presented. This method allowed establishing structure–reactivity relationships in propylene metathesis as a function of the spectator ligands of tungsten oxo species. Thus, the novel pre-catalysts [(SiO)W(O)R2X], 2a–c (X = 2,6-dimethylphenoxide, 2-trifluoromethylphenoxide, pentafluorophenoxide), are obtained by modification of well-defined monopodal species 1b by substituted phenols along with TMS release. These were demonstrated by mass balance analysis, elemental analysis, IR, advanced solid-state NMR (1D and 2D 1H, 13C and 19F) and EXAFS. The modified materials were proved to lead to stable and efficient supported tungsten oxo catalysts for propene metathesis under dynamic conditions at 80 °

Accessing Realistic Models for the WO 3 –SiO 2 Industrial Catalyst through the Design of Organometallic Precursors

International audienceIn order to access realistic models to the industrial olefin metathesis catalyst WO3/SiO2, which is the bigrafted tungsten oxo alkylidene species [(≡SiO)2WO(═CHR)], we targeted the parent bis-alkyl oxo derivative [(≡SiO)2WOR2] prone to carbene formation. Thus, grafting of [WO(CH2EMe3)3Cl] (E = C, 1-Np; E = Si, 1-Ns) onto silica dehydroxylated at 200 °C was performed. While 1-Np affords the monopodal species [(≡SiO)WO(CH2CMe3)3] (2-Np), the neosilyl derivative 1-Ns reacts to yield the well-defined bipodal species [(≡SiO)2WO(CH2SiMe3)2] (2-Ns), via consecutive HCl and SiMe4 release. This was demonstrated by mass balance analysis, elemental analysis, IR, advanced solid-state NMR (1D and 2D 1H, 13C, 29Si, and 17O), and EXAFS. Furthermore, DFT calculations allowed understanding and rationalizing the experimental results regarding grafting selectivity. The material 2-Ns proved to lead to the most stable and efficient supported tungsten oxo catalyst for propene metathesis under dynamic conditions at 80 °C

Silica-Supported Tungsten Neosilyl Oxo Precatalysts: Impact of the Podality on Activity and Stability in Olefin Metathesis

International audienceIn order to establish structure–reactivity relationships in propylene metathesis as a function of the podality of tungsten oxo species bearing neosilyl ligands, we targeted the parent tris alkyl [(≡SiO)WOR3] and bis alkyl oxo [(≡SiO)2WOR2] derivatives prone to carbene formation. Thus, [WO(CH2SiMe3)3Cl] (1) was grafted onto silica dehydroxylated at 700 °C (SiO2–700), proceeding via W–Cl cleavage to yield well-defined monopodal species [(≡SiO)WO(CH2SiMe3)3] (2a) along with HCl release. On the other hand, the corresponding bipodal species [(≡SiO)2WO(CH2SiMe3)2] (2b) was obtained on SiO2–200 by release of both HCl and TMS. The formation of these species were demonstrated by mass balance analysis, elemental analysis, IR, advanced solid-state NMR (1D and 2D 1H, 13C, 29Si, and 17O), and EXAFS. Furthermore, DFT calculations allowed understanding and rationalizing the experimental results regarding grafting selectivity. Materials 2a and 2b proved to lead to stable and efficient supported tungsten oxo catalysts for propene metathesis under dynamic conditions at 80 °C. The symmetric bipodal precatalyst (expressed as [W(═E)(═CHR)(X)(Y)] (X = Y, E = spectator ligand)) showed somewhat higher activity than the asymmetric (X ≠ Y) counterparts

Turning unreactive copper acetylides into remarkably powerful and mild alkyne transfert reagents by oxidative umpolung

International audienceThis is not breaking news: copper acetylides, readily available polymeric rock-stable solids, have been known for more than a century to be unreactive species and piteous nucleophiles. This lack of reactivity actually makes them ideal alkyne transfer reagents that can be easily activated under mild oxidizing conditions. When treated with molecular oxygen in the presence of simple chelating nitrogen ligands such as TMEDA, phenanthroline or imidazole derivatives, they are smoothly oxidized to highly electrophilic species that formally behave like acetylenic carbocations and can therefore be used for the mild and practical alkynylation of a wide range of nitrogen, phosphorus and carbon nucleophiles

Accessing Realistic Models for the WO₃–SiO₂ Industrial Catalyst through the Design of Organometallic Precursors

In order to access realistic models to the industrial olefin metathesis catalyst WO₃/SiO₂, which is the bigrafted tungsten oxo alkylidene species [(SiO)₂WO­(CHR)], we targeted the parent bis-alkyl oxo derivative [(SiO)₂WOR₂] prone to carbene formation. Thus, grafting of [WO­(CH₂EMe₃)₃Cl] (E = C, <b>1-Np</b>; E = Si, <b>1-Ns</b>) onto silica dehydroxylated at 200 °C was performed. While <b>1-Np</b> affords the monopodal species [(SiO)­WO­(CH₂CMe₃)₃] (<b>2-Np</b>), the neosilyl derivative <b>1-Ns</b> reacts to yield the well-defined bipodal species [(SiO)₂WO­(CH₂SiMe₃)₂] (<b>2-Ns</b>), via consecutive HCl and SiMe₄ release. This was demonstrated by mass balance analysis, elemental analysis, IR, advanced solid-state NMR (1D and 2D ¹H, ¹³C, ²⁹Si, and ¹⁷O), and EXAFS. Furthermore, DFT calculations allowed understanding and rationalizing the experimental results regarding grafting selectivity. The material <b>2-Ns</b> proved to lead to the most stable and efficient supported tungsten oxo catalyst for propene metathesis under dynamic conditions at 80 °C