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

The phenoxyimine cationic complex [L1Y(CH2SiMe2Ph)(THF)3][BArF] (L1 = (3-tBu)-(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 (1H, 13C, and 89Y), and more specifically by the 1H-coupled 89Y 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 [L2Y(CH2SiMe2Ph)(THF)3][B(C6F5)4] (L2 = PhC(N-2,6-iPr2C6H3)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.