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

Abstract

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(CH 3 ) 2 + product. Our DFT calculations show a reaction barrier of +0.16 eV relative to the PtCH 2 + + CH 4 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.

Quantitative Aspects of Gas-Phase Metal Ion Chemistry: Conservation of Spin, Participation of f Orbitals, and C–H Activation and C–C Coupling

“…Note that the single dehydrogenation observed is in contrast to the observations of Irikura and Beauchamp, but this difference could be attributed to the high-pressure conditions of the molecular beam versus the low-pressure conditions in the ICR experiments. This conclusion was confirmed by later repeating the experiment using a different apparatus operating under low-pressure conditions . Now, both PtC 2 H 4 + and PtC 4 H 8 + (but no PtC 3 H 6 + ) were observed and shown to have Pt + (ethene) and Pt + (ethene) 2 structures.…”

“…This conclusion was confirmed by later repeating the experiment using a different apparatus operating under low-pressure conditions. 112 ) suggests that this species reacts rapidly with a fourth methane molecule under our experimental conditions.…”

IR spectroscopic characterization of 3d transition metal carbene cations, FeCH2+ and CoCH2+: periodic trends and a challenge for DFT approaches

“…Previously, we found that relatively standard density functional theory (DFT) methods proved sufficient to accurately predict the IR fingerprints, both in terms of frequencies and IR intensities, necessary for assigning the experimental IR spectra of [M,C,2H] + species with M 5d elements. 8–11,31 For the 4d elements M = Ru and Rh, the same conclusion holds, even though the assignment for the [Rh,C,2H] + species was complicated by the multi-reference character of RhCH 2 + . 20 In contrast, here, we find that these comparisons are not so straightforward for 3d elements.…”

NO Bond Cleavage on Gas-Phase Irn+ Clusters Investigated by Infrared Multiple Photon Dissociation Spectroscopy

“…This thermalization was recently illustrated by comparing the product formation upon activation of two methane molecules by Pt + , which in an ion trap leads to formation of ethene Pt + C 2 H 4 , whereas in a similar source to the one used here, the dehydrogenation was throttled, and a dimethyl product Pt + (CH 3 ) 2 is formed instead. 27,28 The reaction gas cell was cooled to a temperature of ∼200 K by liquid N 2 to facilitate the attachment of Ar atoms to the clusters, forming Ir n NO + -Ar p (p = 1−3). Internal energies of the clusters in good thermal equilibrium with the coolant gas were calculated to be 0.1−0.2 eV, or just below the binding energies of the Ar atom to the cluster (0.1−0.3 eV).…”