Alkane Complexes and C-H Activation Reactions

3 Mechanistic Studies using High Pressure IR Spectroscopy 1145 

This mechanism clearly implicated alkane complexes as precursors to C-H activation but the IR absorptions of [Cp Rh(CO)Kr] and [Cp Rh(CO)(C6Hi2)] were not resolved and were presumed to be coincident. The temperature dependent data gave values of AH = 18 (or 22) kj mol for the unimolecular C-H (or C-D) activation step representing a normal kinetic isotope effect, kn/fco 10- However, an inverse equilibrium isotope effect (K /Kq 0.1) was found for the slightly exothermic pre-equilibrium displacement of Kr by CoHn/C Dn implying that C6Dj2 binds more strongly to the rhodium center than does C Hn- 

A subsequent study using neopentane as the alkane substrate gave evidence in support of the same mechanism, and also allowed resolution of near-coincident y(CO) absorptions due to [Cp Rh(CO)Kr] (1946 cm ) and [Cp Rh(CO)(di2-neopen-tane)] (1947 cm ) [18]. Further studies were able to quantify the reactivity of [Cp Rh(CO)Kr] towards a range of alkanes [20]. It was found that binding of the alkane to Rh becomes more favorable, thermodynamically, as the alkane size is increased, but that the rate of the C-H oxidative addition step shows less variation with linear alkane chain length. No reaction with methane was observed, which was explained by the ineffective binding of methane (relative to excess Kr) to Rh. 

Foster, D. J. Cole-Hamilton, Inorg. Chem. Commun. 2000, 3, 617-619. 

Mokheseng, S. Otto, P. Steyn-berg, Dalton Trans. 2003, 2036-2042 C. L. Dwyer, H. Assumption, J. Coet-zee, C. Crause, L. Damoense, M. Kirk, Coord. Chem. Rev. 2004, 248, 653-669 L. Damoense, M. Datt, M. Green, C. Steenkamp, Coord. Chem. Rev. 2004, 248, 2393-2407. 

---