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‐MeOC 6 H 4 NC( t Bu)NH(2,6‐R 2 C 6 H 3 ) (R = Me, i Pr), were synthesized and successfully employed as tridentate ligands for the preparation of the dialkyl rare earth complexes [2‐MeOC 6 H 4 NC( t Bu)N(2,6‐R 2 C 6 H 3 )]Ln(CH 2 SiMe 3 ) 2 (L) n {Ln = Y, Lu; R = Me, i Pr; 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‐MeOC 6 H 4 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‐MeOC 6 H 4 NC( t Bu)N(2,6‐Me 2 C 6 H 3 )]Lu(CH 2 SiMe 3 ) 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‐MeOC 6 H 4 NC( t Bu)N(2,6‐ i Pr 2 C 6 H 3 )]Y(CH 2 SiMe 3 ) 2 (thf) with 2,6‐diisopropylaniline in hexane at –70 °C, regardless of the ratio of the reagent, afforded the six‐coordinate diamido compound [2‐MeOC 6 H 4 NC( t Bu)N(2,6‐ i Pr 2 C 6 H 3 )]Y(NHC 6 H 3 ‐2,6‐ i Pr 2 ) 2 (thf), for which intramolecular coordination of the oxygen atom of the side chain was detected by an X‐ray study. Complex [2‐MeOC 6 H 4 NC( t Bu)N(2,6‐ i Pr 2 C 6 H 3 )]Y(CH 2 SiMe 3 ) 2 (thf) was evaluated as a precatalyst for isoprene polymerization. The ternary system [2‐MeOC 6 H 4 NC( t Bu)N(2,6‐ i Pr 2 C 6 H 3 )]Y(CH 2 SiMe 3 ) 2 (thf)/[Ph 3 C][B(C 6 F 5 ) 4 ]/Al i Bu 3 provided 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 M n of 13.0 × 10 4 and moderate polydispersity (2.12).