Synthesis, structural preference and catalytic activity of neutral and cationic methylpalladium(II) complexes containing N-arylpyridine-2-carbaldimine chelating ligands

The syntheses and structures of neutral complexes [PdCl(Py-2-CH=NAr)(Me)] (Ar = 4-MeC6H4, 4-MeOC6H4, 4-CF3C6H4) and cationic complexes [Pd(Py-2-CH=NAr)( Me)(MeCN)]SbF6 (Ar = 4-MeC6H4, 4-MeOC6H4, 4-CF3C6H4) are described. The preference for the trans-isomers in the cationic complexes and for the cis-isomers in the neutral complexes is discussed on the basis of electronic arguments and supported by DFT calculations. The observed preference seems to follow the maximum hardness principle (MHP) introduced by Pearson. On the basis of the application of this principle to square planar complexes of palladium(II) and platinum( II) we propose the trans choice, which means that the hardest ligand arranges trans to the softest one. The synthesis and crystal structure of the related neutral complex trans-[Pd(CF3COO)(Py-(2)-CH=NC6H4-4-OMe)(Me)] is also described and allows to rule out the charge of the complex as the cause of isomeric preference. We also report our preliminary studies dealing with the catalytic activity of the cationic complexes in alkene oligomerization and copolymerization with CO.