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 R(2)Im (R(2)Im = 1,3-di(R) imidazole-2-ylidene; R-2 = Me-2, Pr-n(2), (MePr)-Pr-i, Pr-i(2)) affords homoleptic [Ni(Me(2)Im)(3)] (1) or dinuclear, COD-bridged complexes of the type [Ni-2(R(2)Im)(4)(COD)](R-2 = Pr-n(2), 2; (MePr)-Pr-i, 3; Pr-i(2), 4). Compounds 1- 4 are suitable precursors for the synthesis of [Ni(R(2)Im)(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(R(2)Im)(2)(CO)(2)](R-2 = Me-2, 5; Pr-n(2), 6; (MePr)-Pr-i, 7; Pr-i(2), 8), diphenyl acetylene complexes [Ni(R(2)Im)(2)(eta(2)-C2Ph2)] (R-2 = Me-2, 9; Pr-n(2), 10; (MePr)-Pr-i, 11; Pr-i(2), 12), and biphenylene complexes [Ni(R(2)Im) 2(2,2'- biphenyl)] (R-2 = Me-2, 13; Pr-n(2), 14; (MePr)-Pr-i, 15; Pr-i(2), 16). Furthermore, the reaction of 4 with 3-hexyne or 2-butyne afforded [Ni((i)Pr(2)Im) 2(eta(2)-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 [Ni-2((i)Pr(2)Im)(4)(COD)] (4).