Di-Zinc-Aryl Complexes: CO2 Insertions and Applications in Polymerisation Catalysis

Abstract

Two new di-zinc-aryl complexes, [LZn₂ Ph₂ ] and [LZn₂ (C₆ F₅ )₂ ], coordinated by a diphenol tetraamine macrocyclic ligand have been prepared and fully characterised, including by single-crystal X-ray diffraction experiments. The complexes' reactivities with monomers including carbon dioxide, cyclohexene oxide, phthalic anhydride, isopropanol and phenol were investigated using both experimental studies and density functional theory calculations. In particular, [LZn₂ Ph₂ ] readily inserts carbon dioxide to form a carboxylate, at 1 bar pressure, whereas [LZn₂ (C₆ F₅ )₂ ] does not react. Under these conditions [LZn₂ Ph₂ ] shows moderate activity in the ring-opening copolymerisation of cyclohexene oxide/carbon dioxide (TOF=20 h^-1 ), cyclohexene oxide/phthalic anhydride (TOF=33 h^-1 ) and the ring-opening polymerisations of rac-lactide (TOF=99 h^-1 ) and ϵ-caprolactone (TOF=5280 h^-1 ).

Keywords: CO2 insertion; organozinc catalysts; reactivity studies; ring-opening copolymerisation; ring-opening polymerisation; zinc.

Scheme 1

Reactivity overview of the plausible reactions of di‐zinc–aryl complexes with monomers CO₂, CHO and PA, in the presence of chain‐transfer agent 1,2‐cyclohexenediol.

Scheme 2

Reactivity overview showing the synthesis of zinc complexes 1–5. Reaction conditions: i) −40 °C to 25 °C, THF solvent, 18 h; 1, ZnPh₂ (2 equiv), 81 % crystalline yield; 2, Zn(C₆F₅)₂ (2 equiv), 52 % crystalline yield; ii) [Zn(OCO‐Ph)₂] (2 equiv), −40 °C to 25 °C, THF, 18 h, 72 % yield; iii) Starting from 1, CO₂ (1 bar), 2 h at 25 °C or 5 minutes at 80 °C; iv) Starting from 1 (1 equiv); 4, isopropanol (2 equiv), 60 °C, THF, 18 h; 5, phenol (2 equiv), 25 °C, THF, 18 h, 32 % crystalline yield.

Figure 1

Molecular structures of a) [LZn₂Ph₂] and b) [LZn₂(C₆F₅)₂]. Hydrogen atoms and benzene molecules are omitted for clarity.

Figure 2

Molecular structure of 3. Hydrogen atoms and a benzene solvent molecule are omitted for clarity.

Figure 3

Potential energy surface for the first single CO₂ insertion into the zinc aryl bond of 1 (black) and 2 (blue); DFT protocol: ωb97xd/6‐31G(d)/cpcm=CH₂Cl₂/Temp=353 K. The ancillary ligand structure is omitted for clarity. Interactive version of the figure available at doi.org/10.14469/hpc/2222.

Figure 4

Molecular structure of 5. Hydrogen atoms and one CH₂Cl₂ molecule are omitted for clarity.

Figure 5

Potential energy surface for the first and second protonolysis of the zinc–aryl bond of 1, with isopropanol; DFT protocol: ωb97xd/6‐31G(d)/cpcm=CH₂Cl₂/Temp=353.15 K (data available at doi.org/10.14469/hpc/2144). The ancillary ligand structure is omitted for clarity.