Oxidation State Dependent Coordination Modes: Accessing an Amidate-Supported Nickel(I) δ-bis(C−H) Agostic Complex

Computational Study of the Ni-Catalyzed C-H Oxidative Cycloaddition of Aromatic Amides with Alkynes

The mechanism of Ni-catalyzed ortho C(sp²)-H oxidative cycloaddition of aromatic amides with internal alkynes containing 2-pyridinylmethylamine directing group was investigated using density functional theory (DFT) calculations. The C-H cleavage step proceeds via σ-complex-assisted metathesis (σ-CAM) with an alkenyl-Ni(II) complex. This is in contrast to the more common carboxylate/carbonate-assisted concerted metalation-deprotonation mechanism in related Ni-catalyzed C-H bond functionalization reactions with N,N-bidentate directing groups. In this reaction, the alkyne not only serves as the coupling partner, but also facilitates the σ-CAM C-H metalation both kinetically and thermodynamically. The subsequent functionalization of the five-membered nickelacycle proceeds via alkyne insertion into the Ni-C bond to form a seven-membered nickelacycle. This process proceeds with high levels of regioselectivity to form a C-C bond with sterically more encumbered alkyne terminus. This unusual regioselectivity is due to steric repulsions with the directing group that is coplanar with the alkyne in the migratory insertion transition state. The C-N bond reductive elimination to form the isoquinolone cycloadduct is promoted by PPh₃ complexation to the Ni center and the use of flexible 2-pyridinylmethylamine directing group. The origin of the cis-trans isomerism of alkene byproduct was also explained by computations.

The Importance of Ligand-Induced Backdonation in the Stabilization of Square Planar d10 Nickel π-Complexes

The electronic nature of Ni π-complexes is underexplored even though these complexes have been widely postulated as intermediates in organometallic chemistry. Herein, the geometric and electronic structure of a series of nickel π-complexes, Ni(dtbpe)(X) (dtbpe=1,2-bis(di-tert-butyl)phosphinoethane; X=alkene or carbonyl containing π-ligands), is probed using a combination of ³¹ P NMR, Ni K-edge XAS, Ni K_β XES, and DFT calculations. These complexes are best described as square planar d¹⁰ complexes with π-backbonding acting as the dominant contributor to M-L bonding to the π-ligand. The degree of backbonding correlates with ² J_PP from NMR and the energy of the Ni 1s→4p_z pre-edge in the Ni K-edge XAS data, and is determined by the energy of the π*_ip ligand acceptor orbital. Thus, unactivated olefinic ligands tend to be poor π-acids whereas ketones, aldehydes, and esters allow for greater backbonding. However, backbonding is still significant even in cases in which metal contributions are minor. In such cases, backbonding is dominated by charge donation from the diphosphine, which allows for strong backdonation, although the metal centre retains a formal d¹⁰ electronic configuration. This ligand-induced backbonding can be formally described as a 3-centre-4-electron (3c-4e) interaction, in which the nickel centre mediates charge transfer from the phosphine σ-donors to the π*_ip ligand acceptor orbital. The implications of this bonding motif are described with respect to both structure and reactivity.