Application of Nickel Complexes Modified by Tridentate O,N,O′‐Ligands as Precatalysts in Nickel‐Catalyzed C(sp2)–C(sp3) Bond Formations

Abstract

Abstract1‐Acetyl‐ [1a; 3,5‐CF3, 1‐C(=O)CH3] and 1‐benzoyl‐5‐hydroxypyrazolines [1b; 3,5‐CF3, 1‐C(=O)C6H5] 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.