Application of nickel complexes modified by tridentate O,N,O'-ligands as precatalysts in nickel-catalyzed C(sp²)-C(sp³) bond formations

1-Acetyl- [1a; 3,5-CF₃, 1-C(=O)CH₃] and 1-benzoyl-5-hydroxypyrazolines [1b; 3,5-CF₃, 1-C(=O)C ₆H₅] have been synthesized and treated with Ni(OAc) ₂·4H₂O in the presence of an excess of base [NH₃ or 4-(dimethylamino)pyridine (DMAP)] to form the nickel complexes 4a-c. These complexes have been characterized by various techniques, which indicate a tridentate coordination mode of the ligands. X-ray crystallography determined an O,N,O'-coordination of the ligands, in which the ligand is planar, the oxygen donors are trans to each other, and the nitrogen donor is in a cis position. The other coordination sites on the nickel centre are occupied by the added base molecules (NH₃ or DMAP). The number of NH₃ or DMAP ligands depends on the nature of the base; in the case of ammonia, one molecule is coordinated to the nickel centre to form a diamagnetic square-planar complex, whereas with DMAP, an octahedral paramagnetic complex with three additional DMAP ligands was observed. Initial catalytic experiments have been performed by applying the complexes in the nickel-catalyzed C(sp²)-C(sp³) cross-coupling of aryl halides with benzylzinc bromides or dialkylzinc reagents; excellent yields and selectivities have been achieved.