Aromatic C–H σ‑Bond Activation by Ni0, Pd0, and Pt0 Alkene Complexes: Concerted Oxidative Addition to Metal vs Ligand-to-Ligand H Transfer Mechanism

C–H σ-bond activation of arene (represented here by benzene) by the Ni0 propene complex Ni0(IMes)­(C3H6) (IMes = 1,3-dimesitylimidazol-2-ylidene), which is an important elementary step in Ni-catalyzed hydroarylation of unactivated alkene with arene, was investigated by DFT calculations. In the Ni0 complex, the C–H activation occurs through a ligand-to-ligand H transfer mechanism to yield NiII(IMes)­(C3H7)­(Ph) (C3H7 = propyl; Ph = phenyl). In Pd0 and Pt0 analogues, the activation occurs through concerted oxidative addition of the C–H bond to the metal. Analysis of the electron redistribution during the C–H activation highlights the difference between the two mechanisms. In the ligand-to-ligand H transfer, charge transfer (CT) occurs from the metal to the benzene. However, the atomic population of the transferring H remains almost constant, suggesting that different CT simultaneously occurs from the transferring H to the LUMO of propene. The electron redistribution contrasts significantly with that found for Pd0 and Pt0, in which CT occurs only from the metal to the benzene. Preference for ligand-to-ligand H transfer over concerted oxidative addition in the Ni0 complex is shown to be due to the smaller atomic radius of Ni in comparison to those of Pd and Pt and the smaller NiII–H bond energy relative to the PdII–H and PtII–H energies. Interestingly, the bulky ligand accelerates the ligand-to-ligand H transfer in the Ni0 complex by decreasing the distance between the coordinated benzene and alkene substrates. Thus, the Gibbs activation energy (ΔG°⧧) decreases in the case of cyclic-alkylaminocarbene with bulky substituents (CACC-K3), while the ΔG°⧧ values are similar in X-Phos, IMes, and nonsubstituted cyclic alkylaminocarbene (CAAC-K0). An electron-withdrawing substituent on the arene accelerates the C–H activation by favoring the metal to arene CT.