Controlling the Selectivity for C-H and C-CN Bond Activation at Rhodium: A DFT Examination of Ligand Effects

Density functional theory has been used to evaluate the energetics of C H and C-CN bond activation Of of acetonitrile by two rhodium fragments, [(C5Me5)Rh(CNMe)] and [TpRh(CNMe)], each containing a pi-acceptor ligand (isocyanide) (Tp = tris(pyrazolyl)borate). These new results are evaluated against previous calculations on similar rhodium systems, [(C5Me5)Rh(PMe3)] and [TpRh(PMe3)], which contain a sigma-donating ligand (phosphine). Our new DFT results show the barrier to C-H bond activation to be lower than the barrier to C-CN activation by 6.7 kcal mol(-1) (for [(C5Me5)Rh(CNMe)]) and 9.3 kcal mol(-1) (for [TpRh(CNMe)]). The addition of a PCM solvation correction did not change the relative energies of these two barriers for either complex, which is in contrast to previous results observed for the two analogous rhodium phosphine systems. These new calculations along with experimental results demonstrate the importance of an electron-rich metal center for the stabilization of the transition state for C-CN cleavage and ultimately on the metal's ability to cleave the C-CN bond.