Structural and dynamic properties of propane coordinated to TpRh(CNR) from a confrontation between theory and experiment

Density functional calculations with the B3PW91 functional have been carried out on the TpRh(CNMe) species [Tp = HB(pyrazolyl)(3)] as a model for Tp'Rh(CNCH2CMe3) [Tp' = HB(3,5-dimethylpyrazolyl)(3)] in interaction with propane. Two sigma, complexes have been found as minima coordinated through either a methyl or a methylene C-H bond, the former being more stable. The approach of the alkane to TpRh(CNMe) has been studied. Although no transition state could be located, study of this path reveals the key importance of the partial decoordination of one pyrazole ring. The full coordination of the alkane can only be achieved when the metal is essentially in a square pyramid coordination with one of the three pyrazole groups only weakly interacting with Rh. The main reaction of the methyl or complex is oxidative addition, leading to the n-propyl hydride complex. In contrast, two reactions are found for the methylene sigma complex: (i) oxidative addition to give the isopropyl complex and (h) exchange between the secondary and primary C-H bonds to convert the methylene complex of propane into a methyl complex of propane. This latter reaction has a much lower barrier than the oxidative addition at the methylene C-H bond. The results account well for most of the experimental results obtained from kinetic studies. Steric factors are found to control the energy barriers between these various processes, disfavoring any process that brings the central carbon into close proximity to Rh.