Binuclear Enamino-Oxazolinate Rare-Earth Metal Complexes: Synthesis and Their Catalytic Performance in Isoprene Polymerization

We have synthesized a series of binuclear rare-earth metal complexes bearing the newly designed enamino-oxazolinate ligands that feature bridging para-phenyl, meta-phenyl, 1,5-naphthalenyl, and 1,5-naphthalenyl moieties. NMR and X-ray diffraction analyses confirmed the binuclear structures of the obtained complexes with two enamino-oxazolinate-metal units located at a trans position against the bridged aryl plane. After activation by [Ph3C][B(C6F5)(4)], all the rare-earth metal complexes served as efficient catalysts for isoprene polymerization, producing polymers with high cis-1,4 regularity (up to 96.1%) and high molecular weight. The steric and electronic effects exerted on the active metal centers, as well as the radius of metal centers, were the major contributing factors for determining both the catalytic activity and cis-1,4-selectivity of the binuclear catalytic systems. Compared to its mononuclear analogue, the binuclear yttrium catalytic system with a para-phenyl bridge exhibited a higher thermostability and catalytic efficiency during polymerization, revealing a special binuclear effect in this binuclear catalytic system.