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 sigma-bond activation of arene (represented here by benzene) by the Ni-0 propene complex Ni-0 (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 Ni-0 complex, the C-H activation occurs through a ligand-to-ligand H transfer mechanism to yield Ni-II(IMes)(C3H7)(Ph) (C3H7 = propyl; Ph = phenyl). In Pd-0 and Pt-0 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 Pd-0 and Pt-0, in which CT occurs only from the metal to the benzene. Preference for ligand-to-ligand H transfer over concerted oxidative addition in the Ni-0 complex is shown to be due to the smaller atomic radius of Ni in comparison to those of Pd and Pt and the smaller Ni-II-H bond energy relative to the Pd-II-H and Pt-II-H energies. Interestingly, the bulky ligand accelerates the ligand-to-ligand H transfer in the Ni-0 complex by decreasing the distance between the coordinated benzene and alkene substrates. Thus, the Gibbs activation energy (Delta G(circle double dagger)) decreases in the case of cyclic-alkylaminocarbene with bulky substituents (CACC-K3), while the Delta G(circle double dagger) 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.