¹H NMR direct observation of enantiomeric exchange in palladium(II) and platinum(II) complexes containing N,N' bidentate aryl-pyridin-2-ylmethyl-amine ligands

The complexes [MCl2(2-N~N')] (N~N' = 2-C5H4N-CH2-NHAr; Ar = 4-MeC6H4, a; 2,6-Me2C6H3, b; 4-MeOC6H4, c; 4-CF3C6H4, d; M = Pd, 1a-d; Pt, 2a-d) have been prepared and fully stereochemically characterized both in the solid state and in solution. Their behavior in DMSO-d6 solution is dependent on the substituents of the aryl group and on the metal. Complexes of palladium with substituents at the para position (1a, 1c, 1d) display a dynamic 1H NMR pattern when the solutions are heated. An enantiomeric exchange S/R is suggested to explain such behavior. On the basis of the calculated negative S values, an associative mechanism involving the solvent is proposed. Under the same conditions, analogous complexes of platinum (2a, 2c, 2d) proved to be unstable, and release of the N~N' ligand was observed. Complexes 1b and 2b show temperature-variable 1H NMR spectra without any evidence accounting for enantiomeric exchange or decoordination. DFT calculations on models of 1a and 1b show that diastereomeric exchange S/S is a process where the complex with the higher sterical hindrance, 1b, has a lower energy barrier.