Efficient single-component nickel catalysts with tetradentate aminopyridine ligands for cycloaddition reactions of CO2 and epoxides under mild conditions†

A series of single-component nickel catalysts (L1-NiBr₂/L2-NiBr₂/L3-NiBr₂) with tetradentate aminopyridine ligands are presented, which exhibit excellent capabilities and selectivity for the synthesis of cyclic carbonates from epoxides and carbon dioxide. A green crystal of L1-NiBr₂ was obtained in CH₃CN, and the ligand adopted a cis-α conformation in the complex. The conversion of styrene oxide could reach 98%, providing 100% selectivity at 90 °C, 1 MPa CO₂ pressure and 5 mol% of L1-NiBr₂ under solvent-free conditions, while the yield and selectivity values were still as high as 92% and 99%, respectively, under 1 atm CO₂ and 0.5 mol% of the catalyst at the same temperature. The catalysts also exerted efficient catalytic coupling reactions of terminal epoxides (90–98% yields of cyclic carbonates), except those bearing a long aliphatic chain under mild conditions. The catalyst exhibited excellent recyclability and stability, which were further proved through ICP to test the catalyst leaching, TGA and IR spectra. Moreover, the catalytic cycloaddition reaction mechanism was investigated using density functional theory (DFT) calculations.