Polynuclear alkoxy–zinc complexes of bowl-shaped macrocycles and their use in the copolymerisation of cyclohexene oxide and CO2

Abstract

The reactions between alcohols and the tetranuclear ethyl–Zn complexes of an ortho-phenylene-bridged polypyrrole macrocycle, Zn₄Et₄(L¹) 1 and the related anthracenyl-bridged macrocyclic complex, Zn₄Et₄(THF)₄(L²) 2 have been studied. With long-chain alcohols such as n-hexanol, the clean formation of the tetranuclear hexoxide complex Zn₄(OC₆H₁₃)₄(L¹) 3 occurs. In contrast, the use of shorter-chain alcohols such as i-propanol results in the trinuclear complex Zn₃(μ²-OⁱPr)₂(μ³-OⁱPr)(HL¹) 4 that arises from demetalation; this complex was characterised by X-ray crystallography. The clean formation of these polynuclear zinc clusters allowed a study of their use as catalysts in the ring-opening copolymerisation (ROCOP) reaction between cyclohexene oxide and CO₂. In situ reactions involving the pre-catalyst 1 and n-hexanol formed the desired polymer with the best selectivity for polycarbonate (90%) at 30 atm CO₂, whilst the activity and performance of pre-catalyst 2 was poor in comparison.