Kinetico–mechanistic studies of C–H bond activation on new Pd complexes containing N,N′-chelating ligands

Abstract

The hybrid imine/amine palladium(II) coordination complexes [PdX₂(κ²-N_imino,N_amino)] (X = Cl, AcO; κ²-N_imino,N_amino = 4ClC₆H₄CHNCH₂(CH₂)_nN(CH₃)₂, n = 1, 2) have been prepared in different isomeric forms which include E/Z arrangement around the CN bond of the hybrid ligand and {Pd(κ²-N_imino,N_amino)} ring conformation. The crystal structures of four of them, E-1AcO, Z-1AcO, E-2AcO and E-2Cl, have been determined and the solution behaviour in acetic acid, the common cyclometallating solvent, for all these systems studied. The complexes in acetic acid solution are shown to maintain the structure determined by X-ray crystallography, as they do in deuterated chloroform. Nevertheless, a partial opening equilibrium of the {Pd(κ²-N_imino,N_amino)} ring is observed by NMR experiments. When the complexes are held in solution for longer periods the corresponding cyclometallated derivatives, 1AcO-CM, 2AcO-CM, 1Cl-CM and 2Cl-CM, containing the {Pd(κ²-C,N_imino)} palladacycle are obtained, as characterized by ¹H NMR spectroscopy. In these compounds the total opening of the N_amino moiety of the ligand has occurred. The C–H bond activation process has been studied kinetico–mechanistically at different temperatures, pressures and acid concentrations; the results agree with the need of an opening of the chelate ring in [PdX₂(κ²-N_imino,N_amino)] prior to the proper cyclometallation reaction. The values of the enthalpies of activation are higher than those observed for known N-monodentated cyclometallating ligands, as should correspond to the contribution of a ligand dechelation pre-equilibrium. The entropies and volumes of activation are also indicative of this predissociation that include an important amount of contractive ordering. The presence of small amounts of triflic acid in the reaction medium accelerates the reaction to the value observed for N_imino-monodentate systems, indicating that the full opening of the chelate ring has taken place. For the badly oriented isomeric forms of the ligand in the chelated complex (Z), the cyclometallation process is even more slow and corresponds directly to the reorganization of the ligand to its cyclopalladation-active (E) conformation.