Nickel boryl complexes and nickel-catalyzed alkyne borylation

Abstract

The first nickel bis-boryl complexes cis-[Ni(ⁱPr₂Im^Me)₂(Bcat)₂], cis-[Ni(ⁱPr₂Im^Me)₂(Bpin)₂] and cis-[Ni(ⁱPr₂Im^Me)₂(Beg)₂] are reported, which were prepared via the reaction of a source of [Ni(ⁱPr₂Im^Me)₂] with the diboron(4) compounds B₂cat₂, B₂pin₂ and B₂eg₂ (ⁱPr₂Im^Me = 1,3-di-iso-propyl-4,5-dimethylimidazolin-2-ylidene; B₂cat₂ = bis(catecholato)diboron; B₂pin₂ = bis(pinacolato)diboron; B₂eg₂ = bis(ethylene glycolato)diboron). X-ray diffraction and DFT calculations strongly suggest that a delocalized, multicenter bonding scheme dictates the bonding situation of the NiB₂ moiety in these square planar complexes, reminiscent of the bonding situation of “non-classical” H₂ complexes. [Ni(ⁱPr₂Im^Me)₂] also efficiently catalyzes the diboration of alkynes using B₂cat₂ as the boron source under mild conditions. In contrast to the known platinum-catalyzed diboration, the nickel system follows a different mechanistic pathway, which not only provides the 1,2-borylation product in excellent yields, but also provides an efficient approach to other products such as C–C coupled borylation products or rare tetra-borylated compounds. The mechanism of the nickel-catalyzed alkyne borylation was examined by means of stoichiometric reactions and DFT calculations. Oxidative addition of the diboron reagent to nickel is not dominant; the first steps of the catalytic cycle are coordination of the alkyne to [Ni(ⁱPr₂Im^Me)₂] and subsequent borylation at the coordinated and, thus, activated alkyne to yield complexes of the type [Ni(NHC)₂(η²-cis-(Bcat)(R)CC(R)(Bcat))], exemplified by the isolation and structural characterization of [Ni(ⁱPr₂Im^Me)₂(η²-cis-(Bcat)(Me)CC(Me)(Bcat))] and [Ni(ⁱPr₂Im^Me)₂(η²-cis-(Bcat)(H₇C₃)CC(C₃H₇)(Bcat))].