Well‐Defined Ni(0) and Ni(II) Complexes of Bicyclic (Alkyl)(Amino)Carbene (^MeBICAAC): Catalytic Activity and Mechanistic Insights in Negishi Cross‐Coupling Reaction

Abstract

Negishi cross‐coupling reaction of organozinc compounds as nucleophiles with aryl halides has drawn immense focus for C−C bond formation reactions. In comparison to the well‐established library of Pd complexes, the C−C cross‐coupling of this particular approach is largely primitive with nickel‐complexes. Herein, we describe the syntheses of Ni(II) complexes, [(MeBICAAC)2NiX2] (X=Cl (1), Br (2), and I (3)) by employing the bicyclic (alkyl)(amino)carbene (MeBICAAC) ligand. The reduction of complexes 1–3 using KC8 afforded the two coordinate low valent, Ni(0) complex, [(MeBICAAC)2Ni(0)] (4). Complexes 1–4 have been characterized by spectroscopic techniques and their solid‐state structures were also confirmed by X‐ray crystallography. Furthermore, complexes 1–4 have been applied in a direct and convenient method to catalyze the Negishi cross‐coupling reaction of various aryl halides with 2,6‐difluorophenylzinc bromide or phenylzinc bromide as the coupling partner in the presence of 3 mol % catalyst. Comparatively, among all‐pristine complexes, 1 exhibit high catalytic potential to afford value‐added C−C coupled products without the use of any additive. The UV‐vis studies and HRMS measurements of controlled stochiometric reactions vindicate the involvement of Ni(I)−NI(III) cycle featured with a penta‐coordinated Ni(III)‐aryl species as the key intermediate for 1 whereas Ni(0)/Ni(II) species are potentially involved in the catalytic cycle of 4.

An isolable, chelating bis[cyclic (alkyl)(amino)carbene] stabilizes a strongly bent, dicoordinate Ni(0) complex

“…Ni–C 1.91(2) Å and Ni–Br 2.32(2) Å, 50 structures) 42 and trans -(Me 2 CAAC) 2 NiX 2 (X = Cl, Br, I; av. Ni–C 1.935(5) Å and Ni–Br 2.300(8) Å, 4 structures) 43 , 44 . As 3 induces a relatively weak-field splitting in the pseudotetrahedral coordination geometry it imposes 45 , it allows for an interesting high-spin configuration in the case of 4 , in agreement with theoretical calculations.…”

“…Crystallographic characterization revealed that the isocrystalline complexes 4-7 feature a pseudotetrahedral coordination environment at the metal (τ 4 ' = 0.96-1.01) with a chelating ligand 3 (Fig. 4) 41 43,44 . As 3 induces a relatively weak-field splitting in the pseudotetrahedral coordination geometry it imposes 45 , it allows for an interesting high-spin configuration in the case of 4, in agreement with theoretical calculations.…”