Ligand exchange reactions in cyclopalladated complexes of benzylideneanilines. Crystal structure of [Pd(C6H4CHNC6H5)Br(PPh3)2]

Abstract

The ligand exchange reactions between [{[graphics omitted]C₆H₅)(O₂CMe)}₂] and the free imines p-RC₆H₄CHNC₆H₅(R = H, Cl, NMe₂, or NO₂)(N[graphics omitted]CH) in acetic acid have been studied. The substitution process is favoured when the incoming ligand contains electron-withdrawing substituents. From the results obtained a new mechanism involving a bis-cyclometallated complex ([graphics omitted]P[graphics omitted])] as intermediate is proposed. The dimeric cyclopalladated compounds are obtained by metallation of the imines by Pd(O₂CMe)₂; by reaction with phosphines PR′₃ and LiBr, [[graphics omitted]C₆H₅)Br(PR′₃)] and [Pd(p-RC₆H₃CHNC₆H₅)Br(PR₃′)₂] are formed (R′= Ph or Et). All the compounds have been fully characterized and their ¹H and ¹³C n.m.r. spectroscopic data recorded. The molecular structure of [Pd(C₆H₄CHNC₆H₅)Br(PPh₃)₂] has been determined by a single-crystal X-ray analysis. The crystals are monoclinic, space group P2₁/a, a= 17.427(3), b= 24.790(4), c= 10.424(2)Å, β= 98.91(2)°, and Z= 4; R= 0.053 for 6 579 reflections. The palladium atoms are five-co-ordinated, the Pd–N distance being 2.710(6)Å.