Catalytic C-C bond activation in biphenylene and cyclotrimerization of alkynes:: Increased reactivity of P,N- versus P,P-substituted nickel complexes

Reaction of Ni(COD)(2) with the bidentate hybrid ligand (Pr-i)(2)CH2CH2NMe2 (PN) and PhCdropCR (R = Ph, Bu-t, CF3, C=CPh) afforded the donor-stabilized mononuclear Ni(0) complexes (PN)Ni(eta(2)-PhCdropCR) (R = Ph (1), Bu-t (4), CF3 (5)) and the dinuclear compound (PN)Ni(eta(2);eta(2)-PhCdropC-CdropCPh)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(eta(2)-PhCetadropCR) (R = Ph (2), CF3 (9)) showed substantially reduced catalytic activity toward phenanthrene formation or cyclotrimerization reactions.