A Reaction-Induced Localization of Spin Density Enables Thermal C-H Bond Activation of Methane by Pristine FeC4.

Caiyun Geng,Jilai Li,Thomas Weiske,Helmut Schwarz

doi:10.1002/chem.201902572

Caiyun Geng, Jilai Li + Show 2 more

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https://doi.org/10.1002/chem.201902572

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Journal: Chemistry	Publication Date: Aug 13, 2019
Citations: 21	License type: CC BY 4.0

Affiliation: Technische Universität Berlin, Jilin University

Abstract

The reactivity of the cationic metal‐carbon cluster FeC4 + towards methane has been studied experimentally using Fourier‐transform ion cyclotron resonance mass spectrometry and computationally by high‐level quantum chemical calculations. At room temperature, FeC4H+ is formed as the main ionic product, and the experimental findings are substantiated by labeling experiments. According to extensive quantum chemical calculations, the C−H bond activation step proceeds through a radical‐based hydrogen‐atom transfer (HAT) mechanism. This finding is quite unexpected because the initial spin density at the terminal carbon atom of FeC4 +, which serves as the hydrogen acceptor site, is low. However, in the course of forming an encounter complex, an electron from the doubly occupied sp‐orbital of the terminal carbon atom of FeC4 + migrates to the singly occupied π*‐orbital; the latter is delocalized over the entire carbon chain. Thus, a highly localized spin density is generated in situ at the terminal carbon atom. Consequently, homolytic C−H bond activation occurs without the obligation to pay a considerable energy penalty that is usually required for HAT involving closed‐shell acceptor sites. The mechanistic insights provided by this combined experimental/computational study extend the understanding of methane activation by transition‐metal carbides and add a new facet to the dizzying mechanistic landscape of hydrogen‐atom transfer.

Highlights

To locate the most stable structure of FeC4+, a Fortran-based genetic algorithm[6] to generate initial guess structures of FeC4+, followed by density functional theory (DFT) calculations, were conducted; these results point to A01 as the most stable species (Figure S1), in agreement with a previous study.[7]
The minimum structures reported in this paper show only positive eigenvalues of the Hessian matrix, whereas the transition states (TSs) have only one negative eigenvalue
Intrinsic reaction coordinate (IRC)[14,15,16,17] calculations were performed to confirm that the transition states correlate between designated intermediates

Summary

Introduction

2. Computational Details The calculations of the electronic structures were performed with Gaussian 16 and ORCA.[4,5] To locate the most stable structure of FeC4+, a Fortran-based genetic algorithm[6] to generate initial guess structures of FeC4+, followed by density functional theory (DFT) calculations, were conducted; these results point to A01 as the most stable species (Figure S1), in agreement with a previous study.[7] The most stable structure of FeC4+ corresponds to a linear arrangement of these five atoms with the iron atom located at one end of the carbon chain. To elucidate whether the ground state of A01 corresponds to a sextet or a quartet state, quite elaborate multireference (MR) calculations were conducted.

Results

Conclusion