The Platinum Hydrido-Methyl Complex:  A Frozen Reaction Intermediate?

Abstract

Reactions of platinum-argon complexes Pt + Ar m , m ) 1-6, with methane (CH 4 ) and methane-d 4 (CD 4 ) were investigated by means of FT-ICR mass spectrometry and DFT calculations. Ligand exchange reactions are observed for Pt + Ar m , m ) 2-6, in which up to four argon ligands are replaced by methane. In contrast the bare platinum ion and platinum solvated with one argon ligand lead to the formation of a platinum-carbene complex. Gibbs free enthalpies from ligand exchange reactions of Pt + CH 4 with CD 4 and H 2 O provide evidence for the inserted hydrido-methyl complex HPt + CH 3 which is corroborated by high-level DFT calculations. No isotopic scrambling is observed for the reaction of Pt + CH 4 with CD 4 (and the reverse reaction). This is attributed to the inability of the platinum cation to form more than three covalent bonds.

Gas-phase activation of methane by ligated transition-metal cations

“…With the increased knowledge gathered in recent years, both experimentally and theoretically, some mechanistic guidelines for the interpretation of the primary, intramolecular KIEs associated with the C-H(D) bond activation of methane can be provided, however. For the platinum species listed in the left side of Table 1, the KIEs are generally quite low, which is consistent with the view that for platinum not the activation step itself, but the release of the products is rate-determining (5,27,79). In contrast, a sizable KIE occurs in the case of the nickel(II) fluoride cation NiF ϩ , which is consistent with the view that this reaction proceeds as a -bond metathesis in which C-H(D) bond cleavage and H(D)-F bond formation contribute to the rate-determining step (40).…”

“…In this respect, Tjelta and Armentrout (25) reported impressive examples for the ability of tuning selectivities in C-H-versus C-Cbond activation of small alkanes by Fe(L) ϩ cations with L ϭ H 2 O and CO; the former ligand induces a preferential attack of C-H bonds, and the latter also promotes the cleavage of C-C-bonds in the substrate. Similarly, PtAr ϩ is able to dehydrogenate methane, but here, the ligand damps the reactivity in that the rate constant for bare Pt ϩ is Ϸ7 times larger; higher PtAr n ϩ clusters (n ϭ 2-6) do not activate methane any more (27). These simple examples thereby illustrates the general consideration that ligated M(L) ϩ species are much less reactive than the corresponding bare metal cations and often even act as sinks of catalytic cycles.…”

Ion–molecule reactions of CoAr6+ with nitrogen oxides N2O, NO, and NO2: measuring absolute pressure by shock-freezing of the collision complex

“…This is in line with our earlier observation that dehydrogenation of hydrocarbons is reduced if metal clusters are decorated with argon atoms [59][60][61]. Another interesting and valuable result is that collisions lead to loss of at most one argon atom, which shows that the collision energy in our experiment is small: It takes only 10 kJ/mol to remove an Ar from 6 CoAr + .…”

Activated Methane on Small Cationic Platinum Clusters

“…[8][9][10] Similar changes in reactivity with argon coverage, from rapid dehydrogenation to sticking of CH 4 , have been observed for Pt + atomic ions. [18] The Ar coverage dependence of the reactions of Rh n Ar m + with methane has been investigated in detail, [19] showing the formation of Rh n [C,4H] + and Rh n -[C,2H] + complexes, while most bare Rh n + clusters (except Rh 2 + ) are essentially unreactive towards methane. Mass spectrometry alone cannot, however, provide detailed information about the structures of the complexes.…”