A Modular Approach to Achiral and Chiral Nickel(II), Palladium(II), and Platinum(II) PCP Pincer Complexes Based on Diaminobenzenes

The synthesis and characterization of a series of nickel, palladium, and platinum complexes containing new achiral and chiral PCP pincer ligands based on 1,3-diaminobenzene, 5-trifluoromethyl-1,3-diaminobenzene, and 3,5-diamino-4-chloroisobutylbenzoate are reported. The new PCP ligands are prepared conveniently in high yield by treatment of the respective diaminobenzene with 2 equiv of a variety of achiral and chiral R 2 PCl compounds in the presence of base. PCP complexes of Ni(II), Ni(PCP)Cl, were synthesized by the reaction of NiCl 2 ·6H 2 O with 1 equiv of a PCP ligand. In similar fashion, treatment of M(COD)X 2 (M = Pd, Pt; X = Cl, Br) with 1 equiv of a PCP ligand yields the square-planar complexes M(PCP)X. Palladium PCP complexes featuring a coordinated TFA ligand (TFA = CF 3 COO - ) are obtained by the reaction of Pd(TFA) 2 with 1 equiv of a PCP ligand. Alternatively, palladium PCP complexes can also be generated via an oxidative addition route. Addition of 2 equiv of PCP ligands based on 3,5-diamino-4-chloroisobutylbenzoate to Pd 2 (dba) 3 affords the respective Pd(PCP)Cl pincer complexes in high yields. X-ray structures of representative Ni, Pd, and Pt PCP complexes have been determined. Finally, the use of the palladium complexes as catalysts for the Suzuki−Miyaura coupling of some aryl bromides and phenyl boronic acid has been examined.