Nickel(0) Complexes of N-Alkyl-Substituted N-Heterocyclic Carbenes and Their Use in the Catalytic Carbon−Carbon Bond Activation of Biphenylene

The reaction of [Ni(COD)2] with stable N-heterocyclic carbenes R2Im (R2Im = 1,3-di(R)imidazole-2-ylidene; R2 = Me2, nPr2, MeiPr, iPr2) affords homoleptic [Ni(Me2Im)3] (1) or dinuclear, COD-bridged
complexes of the type [Ni2(R2Im)4(COD)] (R2 = nPr2, 2; MeiPr, 3; iPr2, 4). Compounds 1−4 are suitable
precursors for the synthesis of [Ni(R2Im)2]-containing complexes in solution, exemplified by the reaction
with CO under atmospheric pressure, with equimolar amounts of diphenyl acetylene or with biphenylene
to give carbonyl complexes [Ni(R2Im)2(CO)2] (R2 = Me2, 5; nPr2, 6; MeiPr, 7; iPr2, 8), diphenyl acetylene
complexes [Ni(R2Im)2(η2-C2Ph2)] (R2 = Me2, 9; nPr2, 10; MeiPr, 11; iPr2, 12), and biphenylene complexes
[Ni(R2Im)2(2,2‘-biphenyl)] (R2 = Me2, 13; nPr2, 14; MeiPr, 15; iPr2, 16). Furthermore, the reaction of 4
with 3-hexyne or 2-butyne afforded [Ni(iPr2Im)2(η2-C2R2)] (R = Me, 18; Et, 19) in good yields. The
compounds 1, 11, 13, 17, and 18 have been structurally characterized. Complexes 13−16 are the products
of a stoichiometric carbon−carbon activation of biphenylene, and compounds 1−4 (as well as 9−12)
are efficient catalysts for the insertion of diphenyl acetylene into the C−C bond of biphenylene, a process
in which the C−C activation of biphenylene is incorporated into a catalytic cycle. The reaction rate of
the formation of 9,10-di(phenyl)phenanthrene depends on the nature of the carbene ligand of the catalyst;
the highest was observed for [Ni2(iPr2Im)4(COD)] (4).