Synthesis, characterization and X-ray structural determination of palladium(0)-olefin complexes containing pyridin-thioethers as ancillary ligands. Equilibria and rates of olefin and ligand exchange

The synthesis of Pd(0)-olefin complexes with pyridin-thioether ligands R'N-SR is reported. X-ray structure determinations of selected species are described. The dynamic behavior was studied by variable-temperature H-1-NMR spectrometry. Equilibrium constants for olefin and chelate ligand exchange were determined by UV-vis spectrophotometry in chloroform at 25 degrees C. The following metal-olefin stability order was observed: tetramethylethylenetetracarboxylate (tmetc) approximate to naphthoquinone (nq) < fumaronitrile (fn) approximate to maleic anhydride (ma) << tetracyanoethylene (tcne). The ligand exchange equilibrium constants indicate that alpha-diimines and pyridin-thioethers affect the stability of the metal-bidentate ligand arrangement to a similar extent, as found in similar Pd(II) complexes. When the entering olefin is tmetc, the approach to equilibrium is slow so that both second-order rate constants k(2) and k(-2) could be determined along with their activation parameters for the reversible reaction of [Pd(eta(2)-nq)(HN-(SPr)-Pr-i)] with tmetc. The results indicate an associative mechanism to be operative in these olefin exchange processes. (C) 2000 Elsevier Science S.A. All rights reserved.