Dynamics of C-C coupling reactions of activated carbon centers by crossed beam imaging

• Applicant: Dr. Jennifer Meyer
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 500279291

Carbon-carbon coupling reactions are an integral part of synthetic protocols and often make use of organometallic catalysts with intermediate reactive carbon centers being formed through interaction with a metal center. This project focuses on two types of reactive carbon centres (1) alkylidens M-CH3 and (2) carbenes M=CH2 with the first important in Ziegler-Natta type chemistry and the second in olefin metathesis. Despite the very different nature of the metal carbon bond, it is accepted that for both intermediates the coupling reaction passes through a four-membered cyclic transition state. The currently used transition metals are foremost from the 4d and 5d series. These elements are rare and expensive, thus research tries to replace them by more abundant 3d transition metals, for example iron or cobalt which will be our transition metals of choice. The outcome of transition metal chemistry is hard to predict due to the number of close lying electronic states of the metal which can for example lead to state-selective reactivity, non-statistical product formation or efficient crossing of spin-surfaces. The aim of this project is to investigate the underlying atomistic dynamics of the carbon bond forming reactions in gas phase using M-CH3+ and M=CH2+ model systems in reactions with small olefines, namely ethene and propene. In-situ ion formation in an radio frequency multipole ion trap allows us to omit any stabilizing ligands needed in condensed phase. We will use crossed beam 3D velocity map imaging to investigate the atomic-level dynamics to record product ion velocity distributions, i.e. differential cross sections. From the experimental data, we can extract information on the atomistic mechanisms, that means how atoms rearrange during the reactive encounter and if a reaction is direct or indirect. Further, we learn about the influence of barriers along the reaction coordinate or if reaction follows may be non-statistical. Recording the full velocity vector allows us to investigate competing product channels in the same experiment and derive branching ratios. Collision energy dependent experiments will give additional insight into these competitions, importance of barriers and stability of encounter complexes. We will study the benchmark reaction of methane coupling by Ta+ for which the first step is the Ta=CH2+ formation followed by a reaction with a second methane molecule. We will concentrate on the role of the four-membered transition state and if direct reaction dynamics are possible despite the complex transition state structure and if a common signature emerges. Depending on the chosen transition metal, barriers along the reaction coordinate might be submerged or not. Their role will be studied in reactions with M-CH3+. Bond insertion reactions are a competing with metathesis reactions, therefore we will carefully study this competition as function of collision energy.

• DFG Programme: Research Grants
• International Connection: Austria, USA
• Major Instrumentation: Radio-Frequency Multipole Ion Trap
• Instrumentation Group: 1790 Spektrometer (Massen-, NMR-, außer 170-178)
• Cooperation Partners: Shaun Gerald Ard, Ph.D.; Professor Milan Oncak, Ph.D.; Albert A. Viggiano, Ph.D.; Professor Dr. Roland Wester