Oxidative C–C/C–X coupling in organometallic nickel complexes: insights from DFT

Abstract

Nickel-based catalysts are employed in organometallic transformations such as C–C/C–heteroatom cross-coupling reactions. The work presented here describes the reactivity of high valent nickel complexes using DFT computations performed at the B3LYP/BS1 level. The calculated bond parameters shed light on how the ligand flexibility changes the reactivity of the nickel centre in various oxidation states of nickel. The reactivity of Ni^III-center (2⁺) and Ni^IV-center (3²⁺) complexes reveals that direct reductive elimination leading to the C–C coupled product of 1,1-dimethylbenzocyclobutene (P1) is more feasible than the path via five-coordinate intermediates (2⁺-Int1 and 3²⁺-Int1) as the latter path involves ligand-conformation-change transition states that demand high energy. However, the five-coordinate 3²⁺-Int1 intermediate reacts with MeCN in the sixth position of Ni^IV-center to generate the 3²⁺-Int2 intermediate that readily undergoes barrierless reductive elimination leading to the product (P1) and this also explains the high yield under blue LED illumination that is observed experimentally. Solvent participation and ligand flexibility are essential for reductive elimination pathways. The Kohn–Sham energy levels of d-orbitals are progressively stabilized when going from the Ni^II to Ni^III to Ni^IV oxidation states. The HOMO–LUMO gaps (ΔE_H–L) show that the overall reactivity decreases in the order 3²⁺ > 2⁺ > 1. Finally, the reaction of the Ni^IV-center 3²⁺ complex in the presence of the Cl⁻ nucleophile is investigated. This shows that the reductive elimination of C–C/C–heteroatom coupling can occur through three pathways with almost similar activation barriers (∼18.0 kcal mol⁻¹). This agrees well with the experimental observation that the three products are formed in the reaction almost in the same percentages. QTAIM pictures reveal that the transition-state structures of 3²⁺-TS6, 3²⁺-TS7 and 3²⁺-TS8 relating to reductive elimination from the Ni^IV-center complex involves a three-membered cyclic transition state.