Activation of C–H Bonds in Pt^+ + x CH_4 Reactions, where x = 1–4: Identification of the Platinum Dimethyl Cation

Activation of C–H bonds in the sequential reactions of Pt^+ + x(CH_4/CD_4), where x = 1–4, have been investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and theoretical calculations. Pt^+ cations are formed by laser ablation and exposed to controlled amounts of CH_4/CD_4 leading to [Pt,xC,(4x-2)H/D]^+ dehydrogenation products. Irradiation of these products in the 400–2100 cm^(–1) range leads to CH_4/CD_4 loss from the x = 3 and 4 products, whereas PtCH_2^+/PtCD_2^+ products do not decompose at all, and x = 2 products dissociate only when formed from a higher order product. The structures of these complexes were explored theoretically at several levels of theory with three different basis sets. Comparison of the experimental and theoretical results indicate that the species formed have a Pt(CH_3)_2^+(CH_4)_(x-2)/Pt(CD_3)_2^+(CD_4)_(x-2) binding motif for x = 2–4. Thus, reaction of Pt^+ with methane occurs by C–H bond activation to form PtCH_2^+, which reacts with an additional methane molecule by C–H bond activation to form the platinum dimethyl cation. This proposed reaction mechanism is consistent with theoretical explorations of the potential energy surface for reactions of Pt^+ with one and two methane molecules.