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 Mw/Mn = 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%).