Dialkyl Rare Earth Complexes Supported by Potentially Tridentate Amidinate Ligands: Synthesis, Structures, and Catalytic Activity in Isoprene Polymerization

Two new amidines that contain a pendant Lewis base in the side arm, 2-MeOC6H4NC(tBu)NH(2,6-R2C6H3) (R = Me, iPr), were synthesized and successfully employed as tridentate ligands for the preparation of the dialkyl rare earth complexes [2-MeOC6H4NC(tBu)N(2,6-R2C6H3)]Ln(CH2SiMe3)2(L)n {Ln = Y, Lu; R = Me, iPr; L = thf (n = 2), dme (n = 1)}. These ligands provided enhanced stability for the complexes. The X-ray structure determinations revealed that intramolecular coordination of the 2-MeOC6H4 group is realized if a thf molecule is coordinated to the metal center (Ln = Y, Lu), which results in the coordination number of six. The treatment of complex [2-MeOC6H4NC(tBu)N(2,6-Me2C6H3)]Lu(CH2SiMe3)2(thf) with dme afforded a six-coordinate dme adduct in which the methoxy group of the amidinate ligand has drifted out from the metal coordination sphere. The reaction of [2-MeOC6H4NC(tBu)N(2,6-iPr2C6H3)]Y(CH2SiMe3)2(thf) with 2,6-diisopropylaniline in hexane at 70 degrees C, regardless of the ratio of the reagent, afforded the six-coordinate diamido compound [2-MeOC6H4NC(tBu)N(2,6-iPr2C6H3)]Y(NHC6H3-2,6-iPr2)2(thf), for which intramolecular coordination of the oxygen atom of the side chain was detected by an X-ray study. Complex [2-MeOC6H4NC(tBu)N(2,6-iPr2C6H3)]Y(CH2SiMe3)2(thf) was evaluated as a precatalyst for isoprene polymerization. The ternary system [2-MeOC6H4NC(tBu)N(2,6-iPr2C6H3)]Y(CH2SiMe3)2(thf)/[Ph3C][B(C6F5)4]/AliBu3provided isoprene polymerization with moderate activity but without control of the regioselectivity (3,4-regularity was slightly predominant at 52?%). Nevertheless, high 1,4-trans-selectivity was found (96%) for the 1,4-polyisoprenes. The obtained polyisoprene has a Mn of 13.0 x 10(4) and moderate polydispersity (2.12).