Catalytic C−C Bond Activation in Biphenylene and Cyclotrimerization of Alkynes: Increased Reactivity of P,N- versus P,P-Substituted Nickel Complexes

Abstract

Reaction of Ni(COD)2 with the bidentate hybrid ligand (iPr)2CH2CH2NMe2 (PN) and PhC⋮CR (R = Ph, tBu, CF3, C⋮CPh) afforded the donor-stabilized mononuclear Ni(0) complexes (PN)Ni(η2-PhC⋮CR) (R = Ph (1), tBu (4), CF3 (5)) and the dinuclear compound (PN)Ni(η2;η2-PhC⋮C−C⋮CPh)Ni(PN) (3), respectively. In the presence of excess alkyne, compounds 1, 3, and 4 turned out to be active species for the catalytic carbon−carbon bond activation in biphenylene and formation of the corresponding 9,10-disubstituted phenanthrenes. However, cyclotrimerization to benzene derivatives instead of C−C bond cleavage occurred with complex 5 in the presence of excess CF3C⋮CPh and biphenylene. In contrast, the corresponding bis-phosphino-substituted species (dippe)Ni(η2-PhC⋮CR) (R = Ph (2), CF3 (9)) showed substantially reduced catalytic activity toward phenanthrene formation or cyclotrimerization reactions.