Abstract
The first nickel bis-boryl complexes cis-[Ni( (i) Pr(2)Im(Me))(2)(Bcat)(2)], cis-[Ni( (i) Pr(2)Im(Me))(2)(Bpin)(2)] and cis-[Ni( (i) Pr(2)Im(Me))(2)(Beg)(2)] are reported, which were prepared via the reaction of a source of [Ni( (i) Pr(2)Im(Me))(2)] with the diboron(4) compounds B(2)cat(2), B(2)pin(2) and B(2)eg(2) ( (i) Pr(2)Im(Me) = 1,3-di-iso-propyl-4,5-dimethylimidazolin-2-ylidene; B(2)cat(2) = bis(catecholato)diboron; B(2)pin(2) = bis(pinacolato)diboron; B(2)eg(2) = 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(2) moiety in these square planar complexes, reminiscent of the bonding situation of "non-classical" H(2) complexes. [Ni( (i) Pr(2)Im(Me))(2)] also efficiently catalyzes the diboration of alkynes using B(2)cat(2) 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( (i) Pr(2)Im(Me))(2)] and subsequent borylation at the coordinated and, thus, activated alkyne to yield complexes of the type [Ni(NHC)(2)(η(2)-cis-(Bcat)(R)C[double bond, length as m-dash]C(R)(Bcat))], exemplified by the isolation and structural characterization of [Ni( (i) Pr(2)Im(Me))(2)(η(2)-cis-(Bcat)(Me)C[double bond, length as m-dash]C(Me)(Bcat))] and [Ni( (i) Pr(2)Im(Me))(2)(η(2)-cis-(Bcat)(H(7)C(3))C[double bond, length as m-dash]C(C(3)H(7))(Bcat))].