Influence of Pyridine-Imidazoline Ligands on the Reactivity of Palladium-Methyl Complexes with Carbon Monoxide

Two series of monocationic Pd-methyl complexes [Pd(Me)(NCMe)(N−N‘)][X] (X = PF₆^-, BAr‘₄^-) were synthesized, where N−N‘ are pyridine-imidazoline ligands of C₁ symmetry, electronically modified with various R substituents at the aminic N atom of the imidazoline ring. These substituents make it possible to vary the electronic properties of the nitrogen-donor atoms. The crystal structures of two neutral palladium precursors [Pd(Me)_2-nCl_n(N-N‘)] (n = 1, 2) with different R substituents show different Pd−N coordination distances and geometrical distortions in the imidazoline ring. The characterization in solution of the neutral derivatives evidences the presence of the complex with the Pd-Me group cis to the imidazoline ring (cis isomer). For the cationic complexes, the number and the kind of stereoisomers present in solution depend on the nature of both the ligand and the anion. One isomer is always observed when the anion is BAr‘₄^-:  the cis when electron-donating substituents are present on the imidazoline ring and the trans when electron-withdrawing groups are present. Both cis and trans isomers were found for the latter kind of ligands, when PF₆^- was the anion. The reactivity of the cationic complexes with carbon monoxide was studied in solution by multinuclear NMR spectroscopy, and it was shown that the Pd-acyl-carbonyl species was formed as the final product. For the complexes with electron-donating substituents, the intermediates of the insertion reaction were detected, namely, Pd-methyl-carbonyl and Pd-acyl-acetonitrile species. Moreover, only one stereoisomer was seen to form:  the one with the Pd-acyl group trans to the imidazoline ring. For the other ligands, the final product is a mixture of cis and trans stereoisomers for both anions, and their distribution depends on the type of ligand. The cationic complexes with BAr‘₄^- behave as catalysts in the CO/4-tert-butylstyrene copolymerization and yield polyketones whose stereoregularity depends on the nature of the ligand. The stereocontrol in the copolymerization process is tentatively explained on the basis of the results of this mechanistic investigation.