Mathieu Chabanas

β-H Transfer from the Metallacyclobutane: A Key Step in the Deactivation and Byproduct Formation for the Well-Defined Silica-Supported Rhenium Alkylidene Alkene Metathesis Catalyst

The surface complex [([triple bond]SiO)Re([triple bond]CtBu)(=CHtBu)(CH2tBu)] (1) is a highly efficient propene metathesis catalyst with high initial activities and a good productivity. However, it undergoes a fast deactivation process with time on stream, which is first order in active sites and ethene. Noteworthy, 1-butene and pentenes, unexpected products in the metathesis of propene, are formed as primary products, in large amount relative to Re (>>1 equiv/Re), showing that their formation is not associated with the formation of inactive species. DFT calculations on molecular model systems show that byproduct formation and deactivation start by a beta-H transfer trans to the weak sigma-donor ligand (siloxy) at the metallacyclobutane intermediate having a square-based pyramid geometry. This key step has an energy barrier slightly higher than that calculated for olefin metathesis. After beta-H transfer, the most accessible pathway is the insertion of ethene in the Re-H bond. The resulting pentacoordinated trisperhydrocarbyl complex rearranges via either (1) alpha-H abstraction yielding the unexpected 1-butene byproduct and the regeneration of the catalyst or (2) beta-H abstraction leading to degrafting. These deactivation and byproduct formation pathways are in full agreement with the experimental data.

Molecular Insight Into Surface Organometallic Chemistry Through the Combined Use of 2D HETCOR Solid‐State NMR Spectroscopy and Silsesquioxane Analogues

Reference EPFL-ARTICLE-204460doi:10.1002/1521-3773(20011203)40:23 3.0.CO;2-XView record in Web of Science Record created on 2015-01-08, modified on 2017-12-03

Catalytic properties of silica-supported tantalum and tungsten hydrides in the cleavage and formation of C-C bonds of alkanes

Heterogeneous catalysts are widely used in industrial applications; they are often easily prepared at a low cost and can be conveniently separated from the reaction medium. However it is always difficult to define and control the active site and then to determine the various elementary steps of a catalytic reaction. On the contrary, homogeneous catalysis has been based on the rules of molecular organometallic chemistry, which affords a better understanding of what are the active species and the elementary steps of a process. Surface Organometallic Chemistry (SOMC) is aimed at the preparation of a new type of heterogeneous catalysts for which the concepts and the rules of molecular organometallic chemistry could be transposed. The reaction of molecular complexes with functional groups of a surface affords the formation of highly reactive well-defined supported organometallic species, usually unprecedented in solution.