Nickel boryl complexes and nickel-catalyzed alkyne borylation.

The first nickel bis-boryl complexes cis -[Ni( i Pr2 ImMe )2 (Bcat)2 ], cis -[Ni( i Pr2 ImMe )2 (Bpin)2 ] and cis -[Ni( i Pr2 ImMe )2 (Beg)2 ] are reported, which were prepared via the reaction of a source of [Ni( i Pr2 ImMe )2 ] with the diboron(4) compounds B2 cat2 , B2 pin2 and B2 eg2 ( i Pr2 ImMe = 1,3-di-iso-propyl-4,5-dimethylimidazolin-2-ylidene; B2 cat2 = bis(catecholato)diboron; B2 pin2 = bis(pinacolato)diboron; B2 eg2 = 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( i Pr2 ImMe )2 ] also efficiently catalyzes the diboration of alkynes using B2 cat2 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 Pr2 ImMe )2 ] and subsequent borylation at the coordinated and, thus, activated alkyne to yield complexes of the type [Ni(NHC)2 (η2 - cis -(Bcat)(R)C[double bond, length as m-dash]C(R)(Bcat))], exemplified by the isolation and structural characterization of [Ni( i Pr2 ImMe )2 (η2 - cis -(Bcat)(Me)C[double bond, length as m-dash]C(Me)(Bcat))] and [Ni( i Pr2 ImMe )2 (η2 - cis -(Bcat)(H7 C3 )C[double bond, length as m-dash]C(C3 H7 )(Bcat))].