Nickel boryl complexes and nickel-catalyzed alkyne borylation.

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))].