Activation of alkanes by ruthenium, rhodium, and palladium ions in the gas phase: striking differences in reactivity of first- and second-row metal ions

Abstract

The reactions of Ru⁺, Rh⁺, and Pd⁺ with alkanes are studied in the gas phase by using an ion beam apparatus. The reactivity of the second row group 8-10 metal ions is shown to be dramatically different than that of their first-row congeners. Studies with deuterium labeled alkanes reveal that Ru⁺, Rh⁺+, and Pd⁺ all dehydrogenate alkanes by a 1,2-mechanism, in contrast to the 1,4-mechanism of Co⁺ and Ni⁺ and the combination of 1,2- and 1,4-processes for Fe⁺. In most respects, Ru⁺ and Rh⁺ exhibit similar reactivity quite distinct from that observed for Pd⁺. The reactions of Ru⁺ and Rh⁺ are dominated by the loss of one or more molecules of hydrogen, via mechanisms characterized by C-H bond insertions and β-H transfers. In contrast to the reactions of their first-row congeners, neither β-methyl transfers nor C-C bond insertions occur competitively at Ru⁺ and Rh⁺ centers. Furthermore, evidence is presented which indicates that the barriers for reductive elimination of H₂ and HR from Rh-(olefin)⁺ complexes are much smaller than the corresponding barriers for the first row group 8-10 metal ions. These low barriers may result in the formation of internally excited products able to undergo a second exothermic elimination reaction. The differences in reactivity of the first and second row group 8 and 9 metal ions are proposed to be due to differences in the sizes and shapes of the orbitals used for bonding. Although the reactivity of Pd⁺ appears in some ways to be quite similar to that of Ni⁺, the mechanism by which alkanes are activated by Pd⁺ may be quite different than for any of the first-row metal ions. It is proposed that the uniquely high Lewis acidity of Pd⁺ results in hydride abstraction as a first step in the mechanism for C-H bond activation, leaving the hydrocarbon fragment with an appreciable amount of carbonium ion character in the reaction intermediate. This mechanism is supported by the fact that Pd⁺ dehydrogenates n-butane by a 1,2-elimination across the central C-C bond exclusively. Palladium is the only metal ion studied to date which undergoes this selective elimination.