New Transformations of Unsaturated Hydrocarbons with Polynuclear Complexes of Pd (I)

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Increasing attention has been paid to rapid progress in the field of organic synthesis by means of palladium complex based catalyst. In most of the catalysis, key intermediates are believed to have the oxidation state of Pd (0) and Pd(II). However, there is an urgent need to revolutionize this methodology. The aim of the present project is to lay a molecular level basis for the synthetic method of the next generation by studying preparation, structure and reactions of polynuclear Pd (I) complexes containing bridging ligands such as allyl, propargyl, vinyl, alkynyl, and carbene. First of all, the Pd (I)-Pd (I) complexes containing 4 and 6 acetonitrile ligands, which can readily be replaced by weak ligands such as unsaturated hydrocarbons, have been prepared. X-ray structure determination established coordination of two PPh_3 ligands to the Pd (I)-Pd (I) unit having four acetonitrile ligands. This complex reacted with two equiv. phenylacetylene to give the Pd-Pd complex containing the bridging butenediylidene ligand made by head-to-tail dimerization of the acetylene. The Pd (I)-Pd (I) complex having six acetonitrile ligands reacted with 1, 8-diphenyloctatetraene in the presence of Pd (0) complex to give novel sandwich complex made by two tetraen ligands coordinating to the linear Pd_4 chain. Next the Pd (I)-Pd (I) complexes containing bridging diene and allene ligands were synthesized, and their structures determined by NMR and X-ray analysis as well as by MO calculation. These studies established that considerable electron density flows from the Pd-Pd σ orbital to the antibonding MO of the organic ligand through back bonding interaction.