Chemoselective homogeneous hydrogenation of phenylacetylene using thiosemicarbazone and thiobenzoylhydrazone palladium(II) complexes as catalysts

A series of monometallic palladium(II) complexes with thiosemicarbazones and thiobenzoylhydrazones has been synthesized and characterised by spectroscopic methods. The crystal structures of chloro(phenyl 2-pyridyl ketone thiosemicarbazonato)palladium(II) 1a and chloro(phenyl 2-pyridyl ketone thiobenzoylhydrazonato)palladium(II) 2 were also determined. The catalytic activity of the complexes in the homogeneous hydrogenation of phenylacetylene was tested with particular regard to the chemoselectivity from a triple to a double bond. Using chloro-complexes a high chemoselectivity was always observed. Results of a kinetic study of the hydrogenation of phenylacetylene in the presence of chloro(methyl 2-pyridyl ketone thiosemicarbazonato)palladium(II) 3 as catalyst provided suggestions for the elucidation of the catalytic cycle of the reaction.