Cationic Phenoxyimine Complexes of Yttrium: Synthesis, Characterization, and Living Polymerization of Isoprene

The phenoxyimine cationic complex [(LY)-Y-1(CH2SiMe2Ph)-(THF)(3)][BArF] (L-1 = (3-Bu-t)-(O)-C6H3-CH=N-(2,6-iPr-C6H3)) was prepared starting from the homoleptic [Y(CH2SiMe2Ph)(3)(THF)(2)] yttrium complex and the phenoxyimine ligand HL1 and the subsequent cationization by the anilinium borate salt. The resulting complex was characterized by different techniques such as elemental analysis, NMR (H-1, C-13, and Y-89), and more specifically by the H-1-coupled Y-89 INEPT. The reactivity of the cationic complex toward isoprene polymerization was evaluated in the presence of trialkylaluminum. This catalyst enables the living cis-1,4 polymerization of isoprene selectively. The influence and the role played by alkylaluminum are discussed based on the screening of a set of aluminum reagents. By means of a computational mechanistic investigation performed at the DFT level, a cationic complex that accounts for the cis/trans/3,4 selectivities experimentally observed is identified. Additionally, the in silico speciation of complexes resulting from the precatalytic mixture revealed the formation of stable Y/Al heterobimetallic complexes. Finally, for comparison purposes, the cationic amidinate yttrium complex [(LY)-Y-2(CH2SiMe2Ph)(THF)(3)][B(C6F5)(4)] (L-2 = PhC(N-2,6-(Pr2C6H3)-Pr-i)(2)) was synthesized and evaluated toward isoprene polymerization under similar conditions. This complex turned out to be active and selective toward the formation of 3,4-units.