Transition-Metal-Carbene Chemistry. Structure, Thermodynamics, and Reactivity of RhCH2+ in the Gas Phase

Article abstract of DOI:10.1021/ja00307a007

The first studies invoving a second-row transition-metal methylidene ion, RhCH2⁺, in the gas phase using Fourier transform mass spectrometfy (FTSM) are described.Product distribution for the reactions of RhCH2⁺ with hydrogen, methane, ethane, ethene, propene, and cyclopropane are reported.The methylene bond strength, D⁰(Rh⁺-CH2), was bracketed between 92 and 95 kcal/mol from which D⁰(Rh⁺-CH2) = 94 +/- 5 kcal/mol is assigned.Structural studies clearly indicate formation of a methylidene-rhodium complex instead of a hydrido-metyhylidene species.Collisional activation of RhCH2⁺ yields both RhC⁺ and Rh⁺, suggesting that α-hydride abstraction in RhCH2⁺ can occor.RhCH2⁺ reacts readily with both hydrogen and methane and represents the first example of methane activation by a cationic mononuclear transition-metal complex in the gas phase.The activation energy for methane elimination from Rh(H)(CH3)⁺ appears to be less than 5 kcal/mol. α-Hydride abstraction from Rh(CH3)⁺ resulting in methane elimination is found to be facile.Reactions of Rh⁺ with cyclopropane and RhCH2⁺ with ethene suggest that β-hydride elimination from a rhodacyclobutane intermediate occurs readily.Therefore, the rhodacyclobutane⁺ ring is belived to have significant puckering.Finally, metathesis products are observed for reactions of RhCD2⁺ with both ethene and propene.The results of this work are compared to earlier studies on first-row MCH2⁺ ions.