Rare-Earth-Metal Complexes Bearing Phosphazene Ancillary Ligands: Structures and Catalysis toward Highly Trans-1,4-Selective (Co)Polymerizations of Conjugated Dienes

The bis-arylated phosphazene compounds [HN­(PPh2NAr)2] (Ar = phenyl (HL1), 2,6-dimethylphenyl (HL2), 2,6-diisopropylphenyl (HL3)) and the imidodiphosphinate compound HN­(PPh2O)2 (HL4) have been prepared via the Staudinger reaction. Treatment of the neutral compounds HL1, HL2, and HL3 with Ln­(CH2SiMe3)3(THF)2 (Ln = Sc, Y, Lu) generated the solvent-free bis­(alkyl) complexes L1Ln­(CH2SiMe3)2 (Ln = Sc (1a), Y (1b), Lu (1c)), L2Sc­(CH2SiMe3)2 (2a), L3Y­(CH2SiMe3)2 (3b), and L3Lu­(CH2SiMe3)2 (3c), respectively. The reaction between HL4 and Y­(CH2SiMe3)3(THF)2 gave the rare zwitterionic complex 4b. Lithiation of the ligand HL1 by nBuLi followed by a metathesis reaction with Nd­(BH4)3(THF)3 afforded the corresponding complex L1Nd­(BH4)2(THF)2 (5). Complexes 1 upon incorporation of [Ph3C]­[B­(C6F5)4] and AliBu3 led to ternary systems that initiated isoprene polymerization with high activities, among which complex 1a was the first example of a scandium catalytic precursor providing trans-1,4-selectivity (90.0%), while the lutetium analogue 1c had medium trans-1,4-selectivity (54.3%) and the yttrium complex 1b exhibited high cis-1,4-selectivity (76.3%). The ternary system based on the zwitterion 4b displayed the highest activity for the isoprene polymerization among these complexes and gave cis-1,4-regularity-enriched polyisoprene (70.6%). Highly stereospecific homopolymerizations of isoprene (trans-1,4-content: 97.0%) and butadiene (trans-1,4-content: 94.0%) were achieved by using the borohydrido complex 5 upon the activation of dibutylmagnesium. The copolymerization of isoprene and butadiene with 1a/[Ph3C]­[B­(C6F5)4/AliBu3] gave randomly arranged trans-1,4-regulated polybutadiene and polyisoprene sequences. The kinetics study displayed competitive polymerization rates of rBD = 2.89 and rIP = 0.41. The thermal behaviors of the (co)­polymers were investigated.