Bimetallic Fluorine-Substituted Anilido−Aldimine Zinc Complexes for CO2/(Cyclohexene Oxide) Copolymerization

Abstract

Regioselective nucleophilic aromatic substitution of an o-fluorine occurs to afford fluorine-substituted o-phenylene-bridged bis(anilido-aldimine) compounds o-C6H4[(C6H2R2)N=CH-C6F4-(H)N(C6H3R'2)]2 when Li(H)N-C6H3R'2 (R' = iPr, Et, Me) is reacted with o-C6H4[(C6H2R2)N=CH-C6F5]2 (R = iPr, Et, Me) in a nonpolar solvent such as diethyl ether or toluene. Successive additions of Me2Zn and SO2 gas to the bis(anilido-aldimine) compounds afford quantitatively dinuclear mu-methylsulfinato zinc complexes o-C6H4[[(C6H2R2)N=CH-C6F4-N(C6H3R'2)-kappa2N,N]Zn(mu-OS(O)Me)]2 (R = iPr, R' = iPr, 3a; R = iPr, R' = Me, 3c; R = Et, R' = (i)Pr, 3d; R = Et, R' = Et, 3e; R = Et, R' = Me, 3f; R = Me, R' = iPr, 3g; R = Me, R' = Et, 3h; R = Me, R' = Me, 3i). The molecular structure of 3c was confirmed by X-ray crystallography. Fluorine-substituted complexes 3a-i show significantly higher TOF (turnover frequencies) than the unfluorinated analogues for CO2/(cyclohexene oxide) copolymerization. The TOF is highly sensitive to the substituents R and R', and the highest TOF (2480 h(-1)) is obtained with 3g (R = Me, R' = iPr). Complex 3g is less sensitive to the residual protic impurities present in the monomers and shows activity at such a low catalyst concentration as [Zn]:[cyclohexene oxide] = 1:50,000, at which the unfluorinated analogue is completely inactive. By realizing the activity at such an extremely low [Zn]:[cyclohexene oxide] ratio, we achieve a high TON (turnover number) up to 10,100. High-molecular-weight polymers (M(n), 100,000-200,000) are obtained with a rather broad molecular-weight distribution (M(w)/M(n), 1.3-2.5). The obtained polymers are not perfectly alternating, and variable carbonate linkages (65-85%) are observed depending on the N-aryl ortho substituents R and R' and the polymerization conditions.