Zn–N3 complexes for the cycloaddition of epoxides with CO2 at atmospheric pressure

Abstract

Tridentate NNN donor ligands (N3) L1–L7, synthesized by the reaction of 1-(6-bromopyridin-2-yl)ethan-1-one and 2-aminobenzaldehyde, reacted with different N-heterocycles, respectively, and then with ZnX₂ in a molar ratio of 1.2 : 1 in THF at 45 °C to obtain eight Zn–N3 complexes 1a–1h. All ligands and Zn complexes are characterized by ¹H NMR, ¹³C NMR, HRMS, FT-IR spectroscopy, elemental analysis, and single crystal X-ray analysis of 1a and 1d. The tridentate NNN donor ligands coordinate with ZnX₂, and formed the five-coordinated Zn–N3 complexes. The Zn–N3 complexes were used for the cycloaddition of epoxides and CO₂ with tetrabutylammonium iodide (TBAI) under 1 bar CO₂ and 30 °C conditions. This catalytic system exhibited good performance with terminal and internal epoxides, forming desired cyclic carbonates with isolated yields of 53–94%. The mechanism was proposed based on FT-IR, UV-vis and HRMS spectroscopy, thereby confirming that [ZnLCl₂I]⁻Bu₄N⁺ was the active intermediate.