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

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 pi*-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.