Dynamics of the 9,10-dihydroanthracene type inversion of the six-membered palladocycle in chloro(ligand)[2-(2′-pyridylmethyl)phenyl]palladium(II) complexes

Abstract

The dynamics of inversion of the six-membered palladocycle in chloro(ligand)[2-(2′-pyridylmethyl)phenyl]palladium(II) complexes, where ligand = substituted pyridine, PPh₃, or PMePh₂, has been studied by ¹H n.m.r. spectroscopy in CDCl₃ in the temperature range 213–323 K. The pseudo-firstorder rate constants for the inversion are independent of the total concentration of the complex as well as of the concentration of added free pyridine. Although the inversion proceeds without dissociation of N-bound pyridines, the free energies of activation, ΔG^#, are virtually insensitive to electronic effects brought about by pyridine substituents. The proposed inversion mechanism involves the formation of a planar intermediate or transition state without any bond breaking. The role of steric factors was demonstrated by the measuring rates of isomerisation of the phosphine complexes. The results obtained are discussed in connection with the dynamic behaviour of 9,10-dihydroanthracenes.