Single-Site β-Diiminate Zinc Catalysts for the Alternating Copolymerization of CO₂ and Epoxides:  Catalyst Synthesis and Unprecedented Polymerization Activity

Abstract

Synthetic routes to zinc beta-diiminate complexes are reported. The synthesis of 11 beta-diimine [(BDI)-H] ligands, with varying N-aryl substituents and bridging structures, is described. These ligands are converted to (BDI)ZnX complexes (X = OAc, Et, N(SiMe3)2, Br, Cl, OH, OMe, O(i)Pr). X-ray structural data revealed that all zinc complexes examined exist as micro-X-bridged dimers in the solid state, with the exception of the zinc ethyl and amido complexes which were monomeric. Complexes of the form (BDI)ZnOR (R = alkyl, acyl) and (BDI)ZnN(SiMe3)2 are highly active catalysts for the alternating copolymerization of epoxides and CO2. Copolymerizations of cyclohexene oxide (CHO) and CO2 with (BDI-1)ZnX [(BDI-1) = 2-((2,6-diisopropylphenyl)amido)-4-((2,6-diisopropylphenyl)imino)-2-pentene)] and (BDI-2)ZnX [(BDI-2) = 2-((2,6-diethylphenyl)amido)-4-((2,6-diethylphenyl)imino)-2-pentene)], where X = OAc, Et, N(SiMe3)2, Br, Cl, OH, OMe, O(i)Pr, were attempted at 50 degrees C and 100 psi CO2. Complexes with X = OAc, N(SiMe3)2, OMe, O(i)Pr all produced polycarbonate by the alternated insertion of CHO and CO2 with similar catalytic activities, comparable molecular weights, and narrow molecular weight distributions (MWD approximately 1.1), indicating the copolymerizations are living. Furthermore, ligand effects were shown to dramatically influence the polymerization activity as minor steric changes accelerated or terminated the polymerization activity.

Solid-state and Solution Studies on a β-Diketiminate Zinc Hydride Complex

“…Complex 1a shows a crystallographic inversion center whereas 1b displays a crystallographically imposed C 2 axis. The C 3 N 2 Zn rings in both 1a and 1b are almost planar with the Zn atoms slightly out of the plane, as was previously observed in [Mes nacnacZn(µ-H)] 2 [5], DippnacnacZnH [7], DippnacnacZnN(SiMe 3 ) 2 [13], and DippnacnacZn(µ-H) 2 BH 2 [14]. In 1a, the almost planar C(CN) 2 Zn six-membered rings adopt a co-planar orientation, whereas in 1b the rings are twisted by 42.6 • as was observed for [Mes nacnacZn(µ-H)] 2 [5].…”

Copolymerization of carbon dioxide and epoxide

“…Coates and his coworkers [23][24][25][26][27] have reported that zinc bis(␤-diiminates) (4) are more active than Darensbourg's zinc bis(phenoxides) (1) for the alternating copolymerization of carbon dioxide and cyclohexene oxide. The zinc bis(␤-diiminates) (4) were dimeric in the solid state, as shown by X-ray crystallographic analysis, 23 whereas the 1 H NMR data showed that 4 was monomeric in solution.…”

Structural Organozinc Chemistry

“…A series of compounds with various organic groups bound to zinc, i.e. 181 266 , 182 267 , 183 266 , 184 266 and 185 268 ( Figure 90), have been prepared and were structurally characterized in the solid state. All compounds have comparable structural features.…”