ACTIVATION OF THE C-H BONDS IN NEOPENTANE AND NEOPENTANE-D(12) BY (ETA(5)-C-5(CH3)(5))RH(CO)(2) - SPECTROSCOPIC AND TEMPORAL RESOLUTION OF RHODIUM-KRYPTON AND RHODIUM-ALKANE COMPLEX INTERMEDIATES
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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. 1994, American Chemical Society. All rights reserved.