IR flash kinetic spectroscopy of C-H bond activation of cyclohexane-d0 and -d12 by Cp*Rh(CO)2 in liquid rare gases: Kinetics, thermodynamics, and an unusual isotope effect

Article abstract of DOI:10.1021/ja00095a047

Flash kinetic spectroscopy with infrared detection is used to probe C-H activation of cyclohexane-d₀ and -d₁₂ by intermediates generated upon ultraviolet irradiation of Cp*Rh(CO)₂ (Cp* = C₅(CH₃)₅) in liquid rare gas (Rg = Kr or Xe) solution at low temperature (163-193 K). Upon UV photolysis, a new C-O stretching band (at 1946.5 cm^-1 in Kr and at 1941.5 cm^-1 in Xe) appears promptly, which we attribute to Cp*Rh(CO)(Rg). In the presence of hydrocarbon a second C-O stretching band (2002.5 cm^-1 in Kr and 1998 cm^-1 in Xe) grows in at the same observed rate as the disappearance of the 1946.5-cm^-1 band. We attribute this second band to the alkyl hydride product, Cp*Rh(CO)(H)(C₆H₁₁). The concentration dependence of the observed reaction rate in Kr solution shows behavior consistent with a preequilibrium mechanism in which the initial Cp*Rh(CO)(Rg) complex equilibrates with a weakly bound Cp*Rh(CO)(alkane) complex (which has an IR carbonyl frequency unresolvable from that of the rare gas complex) followed by C-H activation of the latter. We observe a large normal kinetic isotope effect (k₂^H/k₂^D ≈ 10) on the C-H activation step k₂, but, most unusually, a large inverse isotope effect on the preequilibrium constant (K_eq^H/K_eq^D ≈ 0.1), implying that C₆D₁₂ binds more strongly to the rhodium center than does C₆H₁₂. From the temperature dependence of k₂, we derive Eyring activation energies of 4.2 ± 0.5 kcal/mol for k₂^H and 5.3 ± 0.5 kcal/mol for k₂^D.