Activation of methane by gold cations: Guided ion beam and theoretical studies

Abstract

The potential energy surface for activation of methane by the third-row transition metal cation, Au+, is studied experimentally by examining the kinetic energy dependence of this reaction using guided ion beam tandem mass spectrometry. A flow tube ion source produces Au+ primarily in its 1S0 (5d10) electronic ground state level but with some 3D (and perhaps higher lying) excited states that can be completely removed by a suitable quenching gas (N2O). Au+ (1S0) reacts with methane by endothermic dehydrogenation to form AuCH2+ as well as C-H bond cleavage to yield AuH+ and AuCH3+. The kinetic energy dependences of the cross sections for these endothermic reactions are analyzed to give 0 K bond dissociation energies (in eV) of D0(Au+ - CH2) = 3.70 +/- 0.07 and D0(Au+ -CH3) = 2.17 +/- 0.24. Ab initio calculations at the B3LYPHW + /6-311++G(3df,3p) level performed here show good agreement with the experimental bond energies and previous theoretical values available. Theory also provides the electronic structures of the product species as well as intermediates and transition states along the reactive potential energy surface. Surprisingly, the dehydrogenation reaction does not appear to involve an oxidative addition mechanism. We also compare this third-row transition metal system with the first-row and second-row congeners, Cu+ and Ag+. Differences in thermochemistry can be explained by the lanthanide contraction and relativistic effects that alter the relative size of the valence s and d orbitals.

Bonding in Cationic MCH2+ (M=K–La, Hf–Rn): A Theoretical Study on Periodic Trends

“…Understanding of both the bonding and the reactivity of naked MCH 2 + sheds light on mechanistic aspects of catalytic processes in which these simple complexes are involved. One of the most relevant examples concerns the stoichiometric activation of methane by bare transition-metal cations M + , [1] as show in Equation (1), which represents a combination of Equations (2) and (3).…”

“…Given the fact that DE 2 is constant, the reaction energy (DE 1 ) for various M + is thus solely determined by the bond dissociation energy (BDE) of the resulting M À CH 2 bond in MCH 2 + . To achieve thermal activation of methane, the BDE of the MÀCH 2 bond in MCH 2 + has to exceed DE 2 .…”

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

“…Ir (16), and Au (17). Several metal dications were also found to activate methane in analogy to Reaction 1 (18).…”

Chemie mit Methan: Studieren geht über Probieren!

“…[24] Während Methan gegenüber atomarem Au + in dessen Grundzustand ( 1 S 0 ) vçllig inert ist, [25] vermitteln isolierte Au 2 + -Cluster bei relativ hohem Substratdruck die C-C-Kupplung von Methan unter Bildung von Ethen [Gleichung (6)]. …”