σ-Bond Metathesis Reactions of Sc(CH_3)_2^+ with Secondary C−H Bonds:  Reactivity with Cyclohexane and Cyclopentane

Fourier transform ion cyclotron resonance mass spectrometry has been used to examine the σ-bond metathesis reactions of Sc(CH_3)_2^+ with the secondary C−H bonds of cyclohexane and cyclopentane. Methane elimination, accompanied by further dehydrogenation, is the initial and dominant reaction observed. These processes are facile at room temperature and involve little or no activation energy. Measured total bimolecular rate constants for the reaction of Sc(CH_3)_2^+ with cyclohexane and cyclopentane are 6.2 × 10^(-10) and 5.1 × 10^(-10) cm^3 s^(-1) molecule^(-1), respectively. The total bimolecular rate constant for the reaction of Sc(CD_3)_2^+ with cyclohexane is measured to be 6.6 × 10^(-10) cm^3 s^(-1) molecule^(-1). In accordance with earlier theoretical predictions these metathesis reactions appear to proceed via an allowed four-center mechanism similar to that of a 2_σ + 2_σ cycloaddition. The observed gas phase reactivity is also compared to the liquid phase reactivity of similar complexes and the implications are discussed.