Nickel boryl complexes and nickel-catalyzed alkyne borylation.

The first nickel bis-boryl complexes -[Ni( PrIm)(Bcat)], -[Ni( PrIm)(Bpin)] and -[Ni( PrIm)(Beg)] are reported, which were prepared the reaction of a source of [Ni( PrIm)] with the diboron(4) compounds Bcat, Bpin and Beg ( PrIm = 1,3-di-iso-propyl-4,5-dimethylimidazolin-2-ylidene; Bcat = bis(catecholato)diboron; Bpin = bis(pinacolato)diboron; Beg = bis(ethylene glycolato)diboron). X-ray diffraction and DFT calculations strongly suggest that a delocalized, multicenter bonding scheme dictates the bonding situation of the NiB moiety in these square planar complexes, reminiscent of the bonding situation of "non-classical" H complexes. [Ni( PrIm)] also efficiently catalyzes the diboration of alkynes using Bcat 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( PrIm)] and subsequent borylation at the coordinated and, thus, activated alkyne to yield complexes of the type [Ni(NHC)(η--(Bcat)(R)C[double bond, length as m-dash]C(R)(Bcat))], exemplified by the isolation and structural characterization of [Ni( PrIm)(η--(Bcat)(Me)C[double bond, length as m-dash]C(Me)(Bcat))] and [Ni( PrIm)(η--(Bcat)(HC)C[double bond, length as m-dash]C(CH)(Bcat))].