Bis(alkyl) rare-earth complexes coordinated by bulky tridentate amidinate ligands bearing pendant Ph2PO and Ph2PNR groups. Synthesis, structures and catalytic activity in stereospecific isoprene polymerization

Abstract

A series of new tridentate amidines 2-[Ph₂PX]C₆H₄NHC(tBu)N(2,6-R₂C₆H₃) (X = O, R = iPr (1); X = S, R = Me (2); X = NPh, R = Me (3); X = N(2,6-Me₂C₆H₃), R = Me (4)) bearing various types of donor Ph₂PX groups in a pendant chain was synthesized. Bis(alkyl) complexes {2-[Ph₂PX]C₆H₄NC(tBu)N(2,6-R₂C₆H₃)}Ln(CH₂SiMe₃)₂ (Ln = Y, X = O, R = iPr (5); Ln = Er, X = O, R = iPr (6); Ln = Lu, X = O, R = iPr (7); Ln = Y, X = NPh, R = Me (8); Ln = Lu, X = NPh, R = Me (9); Ln = Lu, X = N(2,6-Me₂C₆H₃), R = Me (10)) were prepared using alkane elimination reactions of 1, 3 and 4 with Ln(CH₂SiMe₃)₃(THF)₂ (Ln = Y, Er, Lu) in toluene and were isolated in 45 (5), 62 (6), 56 (7), 65 (8), 60 (9), and 60 (10) % yields respectively. The X-ray diffraction studies showed that complexes 6–8 are solvent free and feature intramolecular coordination of the PX (X = O, NPh) group to the lanthanide ions. The ternary systems 5–10/borate/AlR₃ (borate = [PhNHMe₂][B(C₆F₅)₄], [Ph₃C][B(C₆F₅)₄], AlR₃ = AliBu₃, AliBu₂H; molar ratio = 1/1/10 or 1/1/1, toluene) proved to be active in isoprene polymerization and enable complete conversion of 1000–10 000 equivalents of the monomer into a polymer at 25 °C within 0.5–24 h affording polyisoprenes with polydispersities M_w/M_n = 1.22–3.18. A comparative study of the catalytic performance of the bis(alkyl) complexes coordinated by tridentate amidinate ligands containing different pendant donor groups demonstrated that replacement of the Ph₂PO group by Ph₂PNPh leads to a switch of stereoselectivity in isoprene polymerization from cis-1,4 (up to 98.5%) to trans-1,4 (up to 84.8%). And conversely introduction of methyl substituents in the 2,6 positions of the phenyl group of the Ph₂PNPh fragment allows us to restore the 1,4-cis stereoselectivity of the catalytic systems based on bis(alkyl) complex 10 (up to 86.4%).