An Unprecedented Trimeric Palladium(0) Complex with Bridging Nitrogen Ligands

While studying the deactivation of the palladium catalyst in the CO/styrene copolymerization reaction, we got kinetic evidence for the formation of Pd(Ar-BIAN)(styrene) complexes. We thus attempted preparing such complexes by reacting a Pd(II) precursor in the presence of the olefin and a reduced Ar-BIAN compound of a type recently developed by us. Although the desired complex is apparently intermediately formed, it decomposed to a complex whose identity was independent of the olefin employed. Characterization of this complex by X-ray diffraction showed it to be an unprecedented Pd(0) trimer in which each palladium atom is normally coordinated to an Ar-BIAN molecule (eta-1 to both N atoms) and unusually eta-2 coordinated to the C=N double bond of another Ar-BIAN molecule, which is in turn normally coordinated to a second Pd atom and so on until the cycle is closed as a trimer.