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

Abstract

The photoinitiated reaction of Cp*Rh(CO)[sub 2] with neopentane and neopentane-d[sub 12] in liquid krypton has been studied with low temperature IR flash kinetic spectroscopy. Photolysis of Cp*Rh(CO)[sub 2] generates a single transient absorption at 1946 cm[sup [minus]1] which is assigned to the Cp*Rh(CO)(Kr) complex. This complex reacts with (CH[sub 3])[sub 4]C to form the C-H activated neopentyl hydride product observed at 2008 cm[sup [minus]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)(CH[sub 3][sub 4]C) 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[sup [minus]1]. 44 refs., 7 figs., 1 tab.