C-H Bond Activation and C-C Coupling of Methane on a Single Cationic Platinum Center: A Spectroscopic and Theoretical...

We spectroscopically investigated the activation products resulting from reacting one and multiple methane molecules with Pt⁺ ions. Pt⁺ ions were formed by laser ablation of a metal target and were cooled to the electronic ground state in a supersonic expansion. The ions were then transferred to a room temperature ion trap, where they were reacted with methane at various partial pressures in an argon buffer gas. Product masses observed were [Pt,C,2H]⁺, [Pt,2C,4H]⁺, [Pt,4C,8H]⁺, and [Pt,2C,O,6H]⁺, which were mass-isolated and characterized using infrared multiple-photon dissociation (IRMPD) spectroscopy employing the free electron laser for intra-cavity experiments (FELICE). The spectra for [Pt,2C,4H]⁺ and [Pt,4C,8H]⁺ have several well-defined bands and, when compared to density functional theory-calculated spectra for several possible product structures, lead to unambiguous assignments to species with ethene ligands, proving Pt⁺-mediated C-C coupling involving up to four methane molecules. These findings contrast with earlier experiments where Pt⁺ ions were reacted in a flow-tube type reaction channel at significantly higher pressures of helium buffer gas, resulting in the formation of a Pt(CH₃)₂⁺ product. Our DFT calculations show a reaction barrier of +0.16 eV relative to the PtCH₂⁺ + CH₄ reactants that are required for C-C coupling. The different outcomes in the two experiments suggest that the higher pressure in the earlier work could kinetically trap the dimethyl product, whereas the lower pressure and longer residence times in the ion trap permit the reaction to proceed, resulting in ethene formation and dihydrogen elimination.