IR FLASH KINETIC SPECTROSCOPY OF C-H BOND ACTIVATION OF CYCLOHEXANE-D(0) AND CYCLOHEXANE-D(12) BY CP(ASTERISK)RH(CO)(2) IN LIQUID RARE-GASES - KINETICS, THERMODYNAMICS, AND AN UNUSUAL ISOTOPE EFFECT

Flash kinetic spectroscopy with infrared detection is used to probe C-H activation of cyclohexane-d(0) and -d(12) by intermediates generated upon ultraviolet irradiation of Cp*Rh(CO)(2) (Cp* = C-5(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 (k(2)(H)/k(2)(D) approximate to 10) on the C-H activation step k(2), but, most unusually, a large inverse isotope effect on the preequilibrium constant (K-eq(H)/K-eq(D) approximate to 0.1), implying that C6D12 binds more strongly to the rhodium center than does C6H12 From the temperature dependence of k(2), we derive Eyring activation energies of 4.2 +/- 0.5 kcal/mol for k(2)(H) and 5.3 +/- 0.5 kcal/mol for k(2)(D).