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

Abstract

C−H σ-bond activation of arene (represented here by benzene) by the Ni 0 propene complex Ni 0 (IMes)-(C 3 H 6 ) (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)(C 3 H 7 )(Ph) (C 3 H 7 = 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-toligand 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 (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.

Computational Approach to Molecular Catalysis by 3d Transition Metals: Challenges and Opportunities

“…The activation of Ar–H bond by Ni, Pd, and Pt NHC-complexes was calculated at the M06/6-311G(d)&SDD//M06/6-31G(d)&LANL2DZ level of theory with bulk solvent effects modeled with C-PCM method. 631 The authors showed that while Pd and Pt easily form the [(NHC)Ar-M-H(alkyl)] hydride complex, Ar–H activation on the Ni-NHC complexes proceeds through the direct aryl-to-alkyl transfer, with propene being the proton acceptor and the coupling partner, which is a consequence of the Ni–H bond weakness and the lower aryl-to-alkyl distance in the Ni species. It should be noted that in C–H activation by 3d transition metals, the so-called two-state reactivity phenomenon is common which makes the computational modeling of these rather challenging ( vide infra ).…”

Efficient alkene hydrosilation with bis(8-quinolyl)phosphine (NPN) nickel catalysts. The dominant role of silyl-over hydrido-nickel catalytic intermediates

“…Finally, the lack of an oxidizing Ni/H interaction at the transition state is not solely due to the expected difficulty in oxidizing a Ni(II) cationic centre, as a similar absence of redox character has been suggested for s-bond exchanges involving Ni(0) centres, 79,80 but this does not extend to heavier metals of Group 10. 81 The two catalytic cycles of Scheme 3 are quite similar and are based on reactions of analogous hydride (cycle A) or silyl (cycle B) complexes. In A, the silyl group migrates to an alkyl ligand, while in B, hydride migrates to a silyl-substituted alkyl ligand.…”

Nickel-Catalyzed C−H Functionalization Using A Non-directed Strategy

“…The LLHT mechanism is now commonly proposed in many catalytic reactions involving the addition of a C-H bond to C-C multiple bond. 67,[136][137][138] In 2017, Sakaki performed an in-depth DFT study of the C-H activation of benzene by Ni(0)-, Pd(0)-and Pt(0)-propene complexes. 136 In the case of the Ni(0) complex, C-H bond activation occurs via a LLHT mechanism without generating a Ni(II) hydride phenyl intermediate because of the compact size of the nickel metal which decreases the distance between the benzene and alkene in the transition state and the intermediate.…”