Reactive intermediates for the CH bond activation of methane synergistic experimental and computational studies – RICH

The long term goal of RICH is to gain insight into the design of new catalysts of the functionalisation methane. RICH aims at (i) the synthesis of cyclopropyl ETM complexes capable of generating reactive intermediates and their implication in the CH bond activation of methane (partner P1, Michel Etienne LCC Toulouse); (ii) studying mechanistic issues of the global CH bond activation event thanks to kinetic and DFT computational stuides ((P1, P2 Karinne Miqueu UPPA Pau, P3 Véronique Pimienta IMRCP Toulouse), and (iii) direct detection in the gas phase of the reactive intermediates by Photoelectron Spectroscopy with UV radiation (SPE-UV) (P2).

Our first objective is to synthesise organometallic precursors of general formula [LnMR(c-C3H5)], (Ln = monoanionic multidentate ligand ; R = alkyl) that will generate reactive intermediates [LnM(eta2-c-C3H4)] capable of activating a CH bond of methane. Metal of interest are from the groups 3-6. The niobium complexes [TpMe2NbR(c-C3H5)(MeCCMe)] indeed lead to methane activation via the reactive intermediate [TpMe2Nb(eta2-c-C3H4)(MeCCMe)] to give[TpMe2NbCH3(c-C3H5)(MeCCMe)]. Using isotopomers (CD4, 13CH4) and with CH4 pressures up to 40 bars in solution, we have studeid the stereochemistry and, by dynamic NMR, the kinetics of the reaction. Due to intrinsic physical limitations we have not been able to gain fully quantitative data on the kinetics of the elementary steps. However, those remarkable results have been published in J. Am. Chem. Soc.. In order to better understand the kinetics and mechanism of these reactions, we have synthesised tungsten complexes of the type [Cp*W(NO)R(c-C3H5)]. The synthesis is not straightforward as isomerisation to an eat3-allyl complex [Cp*W(NO)R(eta3-C3H5)] is observed. Together with P2 and P3, we will study the mechanism of this reaction as it can guide us to design and synthesise new precursors for methane activation. In parallel, P1 has synthesised several families of cyclopropyl complexes whose reactivity in the gas phase by SPE-UV will be studied by P2. It is one of the objectives of RICH to directly characterise putative reactive intermediates such as [LnM(eta2-c-C3H4)]. Coupled to flash pyrolysis of the diphenyl precursors [Cp2MPh2] (M = Ti, Zr, Hf), SPE-UV has allowed the direct characterisation of the reactive benzyne transients [Cp2M(eta2-c-C6H4)]. These promising results pave the way for the study and design of new precursors for methane activation.

Following results just presented, we will focus on the synthesis of tungsten complexes that will be more electron rich and on the synthesis of other group 4 metal complexes known to activate hydrocarbon CH bonds and modify the ligands to make them react with methane. Thanks to mechanistic studies using kinetics (in solution with P3), SPE-UV (in the gas phase with P2, thanks to an improved set up being buit up) and computational modeling (DFT, P1 and P2), we hope to find new examples of reactive intermediates. Comparison of their reactivity with methane will hopefully suggest the design of efficient catalysts for methane functionalisation.

Article:
“CH Bond Activation of Methane by a Transient eta2-Cyclopropene / Metallabicyclobutane Complex of Niobium” C. Li, C. Dinoi, Y. Coppel, M. Etienne, J. Am. Chem. Soc. 2015, 117, 12450-12453.

This project revolves around (i) the synthesis of early transition metal (ETM) cyclopropyl complexes able to generate reactive eta2-cyclopropene intermediates and the study of their involvement in the CH bond cleavage of CH4 by a 1,3-CH addition mechanism; (ii) the mechanistic investigation of the whole CH activation process by kinetic studies in solution and DFT modeling; (ii) the direct detection by UV- Photoelectron Spectroscopy (UV-PES) of eta2-cyclopropene intermediates generated in situ from thermolysis of different precursors.
Methane is the main component of natural and bio-gas. Current transformations – syngas production and oxidation to provide heat – not only are not selective, but also produce large amounts of CO2, source of serious environmental problems. Developing clean ways of selectively activating methane to make high value-added chemicals is therefore an up-to-date, though long term, challenge. RICH is a fundamental research project aimed at identifying reactive intermediates involved in the activation of the inert CH bonds of methane by ETM complexes. Usually, the CH bond activation with ETM compounds involves two main mechanisms: sigma bond metathesis or alpha-H abstraction/1,2-CH bond addition. We recently discovered that ß-H abstraction from a methyl cyclopropyl Nb complex generates a transient eta2-cyclopropene intermediate, that is capable of activating aromatic and aliphatic C-H bonds by a 1,3-CH bond addition across a Nb(CC) bond. RICH will allow us to investigate the fundamentals of this new mechanism towards the development of more selective catalytic reactions involving CH4.
Our first objective is to prepare cyclopropyl organometallic precursors of the general formula LnMR(c-C3H5), (Ln = monoanionic multidentate ligand; R = alkyl group), that will generate LnM(eta2-c-C3H4) intermediates able to activate the CH bond of CH4. The metals will be those of groups 3 to 6. Various ligands Ln (Cp, Tp, etc.) forcing cis-arrangement needed for RH elimination will provide scaffolds whose steric and electronic properties can be tuned conveniently. The CH bond activation of CH4 by a 1,3-CH bond addition pathway across the LnM(eta2-c-C3H4) intermediates will be further addressed.
The second objective will be to achieve a full understanding of the whole beta-H abstraction/1,3-CH bond addition mechanism by kinetic studies (1H, 2H, 13C NMR), including numerical simulations of concentration profiles and trapping experiments. Particular focus will be given to the formation and reactivity of the eta2-cyclopropene intermediate towards methane. For this purpose, the use of labelled CD4 and 13CH4 will be especially usefull. Reaction pathways, including the structure of the intermediates and transition states, will be unveiled by DFT calculations.
The third objective will be the direct detection of the LnM(eta2-c-C3H4) intermediates by UV-PES. UV-PES allows the determination of ionization potentials of the molecules, real electronic “fingerprints”, which can be directly correlated with the molecular orbitals energy levels. Coupled with flash vacuum thermolysis, UV-PES will allow to monitor on-line the thermal degradation of the cyclopropyl complexes ETM. The transient LnM(eta2-c-C3H4) species will be characterized in the gas phase. If necessary, in collaboration with the OMICRON Company, original devices may be designed to perform specific PES experiments and improve analysis and detection conditions.
Owing to the ambitious nature of this proposal, the work program is distributed over a period of 48 months. In addition to the permanent members involved in the proposal, two postdoctoral researchers will be recruited: one will work with PI 1 in Toulouse and another one with PI 2 in Pau.