Exclusive C-C activation in the rhodium(I) PCN pincer complex. A computational study

The mechanism of competitive intramolecular C-C and C-H bond activation in PCN pincer complexes of rhodium(I) has been studied computationally. Experi mentally this system exhibits exclusive cleavage of an aryl-methyl C-C bond ; no products corresponding to C-K bond cleavage in the methyl group have b een observed. The calculated relative energies indicate that the C-C activa tion product is the most stable one (DeltaE < -50 kJ<bullet>mol(-1) relativ e to the C-H activation product) and that its formation is irreversible. C- K activation is not kinetically forbidden, but it is fast and reversible. B ecause of the weak Rh-N bond several intermediates on the C-H activation pa ths are accessible; therefore low concentrations of these species are a pla usible reason for the experimental result. Further, this study reveals that the unique preference of C-C bond activation in the PCN system results fro m insufficient stabilization of the C-H activation product due to strain. T o model the complete ligand including all bulky substituents used in the ex periment, the ONIOM method has been employed, using the B3LYP/lanl2dz level for the inner layer and the HF/lanl1mb level to model substituent effects.