Living 3,4-Polymerization of Isoprene by Cationic Rare Earth Metal Alkyl Complexes Bearing Iminoamido Ligands

Abstract

in 65-85% isolated yields. X-ray analyses show these complexes have decreasing steric hindrance in the coordination spheres of the metal centers in the order 1 > 2 > 3 > 4 > 5 > 6. A mechanism involving intramolecular alkyl and hydrogen migration is supported on the basis of DFT calculations to account for ligand alkylation. Activated by [Ph 3 C]-[B(C 6 F 5 ) 4 ], all of these iminoamido rare earth metal dialkyl complexes are active for living polymerization of isoprene, with activity and selectivity being significantly dependent on the steric hindrance around the metal center to yield homopolyisoprene materials with different microstructures and compositions. The sterically crowded complexes 1-3 give a mixture of 3,4-and trans-1,4-polyisoprenes (3,4-selectivities: 48-82%, trans-1,4-selectivities: 50-17%), whereas the less sterically demanding complexes 4-6 show high 3,4-selectivities (3,4-selectivities: 90-100%). In the presence of 2 equiv of Al i Bu 3 , the complexes 1-6/activator systems exhibit higher activities and 3,4-selectivities in the living polymerization of isoprene. A similar structure-reactivity relationship in polymerization catalysis can be also observed in these ternary systems. A possible mechanism of the isoprene polymerization processes is proposed on the basis of the DFT calculations.

Yttrium-Alkyl Complexes Supported by a Ferrocene-Based Phosphinimine Ligand

“…17,[35][36][43][44][45][46][47][48][49][50] On the other hand, reports of structurally characterized alkyl or aryl rare earth complexes bearing an imine functionality in the backbone are rare, [51][52] the majority being represented by phosphinimine ligands. [53][54][55][56][57][58][59][60] In our lab, the synthesis of an electrophilic metal alkyl complex supported by a Schiff base ferrocene ligand would continually fail. Although a few examples of alkyl yttrium complexes supported by a ligand containing an imine group exist, 53,61 in general, attempts to isolate such complexes resulted in the migration of the alkyl ligand to the imine group.…”

“…Although a few examples of alkyl yttrium complexes supported by a ligand containing an imine group exist, 53,61 in general, attempts to isolate such complexes resulted in the migration of the alkyl ligand to the imine group. 57,[62][63] We and others [64][65] have reasoned that replacing the imine with a phosphinimine backbone would increase the robustness of the resulting yttrium alkyl complexes. Herein, we report the synthesis, characterization, and reactivity of two yttrium alkyls supported by a ferrocene-based diphosphinimine ligand.…”

Novel cationic rare earth metal alkyl catalysts for precise olefin polymerization

“…The one‐pot acid–base reactions between the rare earth metal trialkyl complexes Ln(CH 2 SiMe 3 ) 3 (THF) 2 (Ln = Sc, Lu, and Y) and 1 equiv. of substituted α‐diimine ligands 2,6‐R 2 C 6 H 3 N = CH – CH = NC 6 H 3 R 2 ‐2,6 (R = i ‐Pr, Me) in toluene at room temperature gave straightforwardly the monoanionic iminoamido rare earth metal dialkyl complexes [2,6‐R 2 C 6 H 3 NCH 2 C(CH 2 SiMe 3 )=NC 6 H 3 R 2 ‐2,6]Ln(CH 2 Si Me 3 ) 2 (THF) 5a–5f ( 5a : Ln = Sc, R = i‐ Pr; 5b : Ln = Lu, R = i‐ Pr; 5c : Ln = Y, R = i‐ Pr; 5d : Ln = Sc, R = Me; 5e : Ln = Lu, R = Me; 5f : Ln = Y, R = Me) in high yields . All complexes 5a–5f are soluble in common organic solvents such as THF, toluene, and hexane [Fig.…”

“…In 2011, we reported that the cationic rare earth metal alkyl species generated by the monoanionic iminoamido dialkyl complexes 5a–5f and [Ph 3 C][B(C 6 F 5 ) 4 ] and Al i ‐Bu 3 could promote the living 3,4‐polymerization of isoprene to afford the mainly 3,4‐poly(IP) (3,4‐selectivity: 48–99%) with narrow molecular weight distribution [Fig. (b)] . The structure–reactivity relationship was observed in the isoprene polymerization catalyzed by these cationic iminoamido rare earth metal alkyl catalysts.…”

Precisely Controlled Polymerization of Styrene and Conjugated Dienes by Group 3 Single‐Site Catalysts

“…Li et al. reported the one‐pot synthesis of a new class of monoanionic dialkyl rare‐earth metal complexes 122 and 123 bearing iminoamido ligands . Activated by [Ph 3 C][B(C 6 F 5 ) 4 ], all of these complexes showed high catalytic activities for living polymerization of IP with narrow molecular weight distributions.…”