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 Unusual Isotope Effect

Abstract

Flash kinetic spectroscopy with infrared detection is used to prove C-H activation of cyclohexane-d[sub 0] and -d[sub 12] by intermediates generated upon ultraviolet irradiation of Cp*Rh(CO)[sub 2] (Cp* = C[sub 5](CH[sub 3])[sub 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[sup [minus]1] in Kr and at 1941.5 cm[sup [minus]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[sup [minus]1] in Kr and 1998 cm[sup [minus]1] in Xe) grows in at the same observed rate as the disappearance of the 1946.5-cm[sup [minus]1] band. We attribute this second band to the alkyl hydride product, Cp*Rh(CO)(H)(C[sub 6]H[sub 11]). 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. 53 refs., 13 figs., 1 tab.