Rigid Acridane-Based Pincer Supported Rare-Earth Complexes for cis-1,4-Polymerization of 1,3-Conjugated Dienes

A convenient synthetic route has been developed for preparing the novel rigid 4,5-(PR₂)2-2,7,9,9-tetramethylacridane-based pincer ligands (^acri-RPNP; R = Pr and Ph), and the first rare-earth (Ln = Y, Lu) alkyl complexes bearing the ^acri-RPNP ligands were synthesized by a salt metathesis reaction (for the isopropyl-substituent ^acri-PNP complexes, 1-Ln) or direct alkylation (for the phenyl-substituent ^acri-PhPNP complexes, 2-Ln). For both 1-Ln and 2-Ln, the NMR spectroscopy and X-ray diffraction study confirmed the successful coordination of the ^acri-RPNP ligand to the central metal ion in a tridentate manner via the two phosphine and the nitrogen donors. In contrast to 1-Ln that are solvent-free complexes, the metal centers in 2-Ln are each coordinated with one tetrahydrofuran molecule. Upon activation by [Ph₃C][B(C₆F₅)₄], 1-Y and 2-Lu could catalyze the living polymerization of isoprene and β-myrcene with high catalytic activity and high cis-1,4-selectivity (up to 92.3% for isoprene and 98.5% for β-myrcene). Moreover, the 1-Y/[Ph₃C][B(C₆F₅)₄] catalytic system also could promote the polymerization of butadiene and its copolymerization with isoprene to produce copolymers with high cis-1,4-selectivity and narrow polydispersity.