Reactions of Transition-Metal Carbyne Cations with Ethylene in the Gas Phase

The reactions of iridium- and osmium-carbyne hydride cations [HIrCH]⁺ and [HOsCH]⁺ with ethylene have been studied using mass spectrometry with isotopic-labeling in the gas phase. The carbyne reactivity is compared with that of the rhodium, cobalt, and iron analogues [TMCH₂]⁺ (TM = Fe, Co, and Rh), which were determined to have the carbene structures. Besides the cycloaddition/dehydrogenation reaction in forming the [TMC₃H₄]⁺ + H₂ (TM = Ir and Os) products, a second reaction pathway producing the [TMC₂H₂]⁺ ion and CH₄ via triple hydrogen atom transfer reactions to the carbyne carbon is observed to be the major channel. The latter channel is not observed in the rhodium, cobalt, and iron carbene cation reactions. Quantum-chemical calculations indicate that the distinct reactivity is not due to different initial structures of the reactants. Both reaction channels are predicted to be thermodynamically exothermic and kinetically facile for the carbyne cations, and the reactions proceed with the initial formation of a carbene intermediate via hydride-carbyne coupling. The latter channel is also exothermic but kinetically unfavorable for the rhodium, cobalt, and iron carbene cations.