Zinc Complexes with Tridentate Pyridyl‐Pyrrole Ligands and their Use as Catalysts in CO2 Fixation into Cyclic Carbonates

Abstract

Pyridyl‐pyrrole ligands usually mimic the polypyridine ligand, and their metal complexes are interesting due to their rich coordination chemistry and their use in molecular devices, biological therapeutics, and catalytic transformations. Cs‐symmetric HL3 ligand Br‐substituted on one of the pyridyl rings was designed and synthesized by Paal–Knorr pyrrole condensation. Ligand HL4 was synthesized by Suzuki‐coupling reaction between HL3 and 2‐thiopheneboronic acid in the presence of K2CO3 and Pd(PPh3)4 in degassed THF/H2O solution. Zinc complexes 1–5 with HL1 to HL5 were prepared by treatment of the corresponding pyridyl‐pyrrole ligands with diethylzinc in toluene at 0 °C in high yield. Ligand HL3 and all zinc complexes are characterized by X‐ray diffraction analysis. The structure of free HL3 is planar. The geometries at Zn in 1, 2, and 5 are compressed octahedra, while in 3 and 4 they are perhaps best described as being between cis‐divacant octahedral and distorted tetrahedral. In these zinc complexes, shorter Zn–Npyrrole [1.878(5)‐1.950(4) Å] and longer Zn–Npyridine [2.108(5)‐2.509(3) Å], Zn‐Npendant pyridine [2.802(2)‐3.039(3) Å] bond lengths are observed. Complexes 2 and 3 were used as catalysts to promote the coupling reaction of CO2 and various epoxides under solvent‐free conditions, and the catalytic mechanism was investigated by 1H NMR spectroscopy.