Activation of the C-H Bonds in Neopentane and Neopentane-d₁₂ by (η⁵-C₅(CH₃)₅)Rh(CO)₂: Spectroscopic and Temporal Resolution of Rhodium-Krypton and Rhodium-Alkane Complex Intermediates

The photoinitiated reaction of Cp^*Rh(CO)₂ with neopentane and neopentane-d₁₂ in liquid krypton has been studied with low-temperature IR flash kinetic spectroscopy. Photolysis of Cp^*Rh(CO)₂ generates a single transient absorption at 1946 cm^-1 which is assigned to the Cp^*Rh(CO)(Kr) complex. This complex reacts with (CH₃)₄C to form the C-H activated neopentyl hydride product observed at 2008 cm^-1. Confirming earlier flash kinetic results with cyclohexane, the results are consistent with a pre-equilibrium mechanism in which an initially formed transient krypton complex Cp^*Rh(CO)(Kr) is in rapid equilibrium with a transient (uninserted) alkane complex Cp^*Rh(CO)((CH₃)₄C) which then proceeds to form the neopentyl hydride in a unimolecular step. Under most conditions our mechanism requires that both Cp^*Rh(CO)(Kr) and Cp^*Rh(CO)(alkane) exhibit unresolved carbonyl stretching absorptions at 1946 cm^-1. However, use of (CD₃)₄C as the alkane substrate allows us, for the first time, to spectroscopically and temporally resolve both the rhodium-krypton and rhodium-alkane complex intermediates by 1 cm^-1, lending further support for the pre-equilibrium mechanism proposed to be operative in these systems. As was observed with cyclohexane and cyclohexane-d₂, a normal isotope effect is observed for the unimolecular C-H(D) insertion step but an unusual equilibrium isotope effect (EIE) is measured for the pre-equilibrium step of the reaction: rhodium is bound an order of magnitude more strongly to (CD₃)₄C than to (CH₃)₄C.