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.