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.