(P,N,P)Pd- versus (P,N,P)Ni-Catalyzed Suzuki-Miyaura Cross-Coupling Reactions under Green Conditions†

Palladium dichloride (6-10) and nickel dichloride (11-15) complexes of a series of N-substituted/functionalized bis(diphenylphosphino)amine-type ligands (1-5) were synthesized and characterized, using multinuclear NMR and FT-IR spectroscopic techniques, mass spectrometry and elemental analysis. The solid-state structures of two palladium(II) (6 and 8⋅2CHCl₃) and two nickel(II) (12 and 15) complexes could be confirmed by single-crystal X-ray diffraction studies. All the complexes were evaluated as catalysts in the Suzuki-Miyaura cross-coupling reaction. Applying what appeared, from the literature, as the optimized reaction conditions for such complexes, i. e. Cs₂CO₃ as base and 1,4-dioxane as solvent, the palladium catalysts were found very efficient for the coupling of phenylboronic acid and aryl bromides functionalized with weak and moderate electron donating groups (EDGs) and weak and moderate electron withdrawing groups (EWGs). In contrast, reduced activity was observed for aryl bromides functionalized with strong EDGs and EWGs and no activity for aryl chlorides. Under the same reaction conditions, the nickel catalysts were completely inactive. Applying different and “greener” reaction conditions, i. e. K₃PO₄ as base and tert-amyl alcohol as solvent, the nickel catalysts efficiently converted various aryl bromides and chlorides with a pool of aryl boronic acids, therefore clearly surpassing their palladium analogues at lower cost, economically and environmentally. Surprisingly, these reaction conditions could also be successfully applied to the palladium complexes, improving their performances (activity towards aryl chlorides) and offering a “greener” alternative to the common conditions.