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

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. C-s-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 degrees 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-N-pyrrole [1.878(5)-1.950(4) angstrom] and longer Zn-N-pyridine [2.108(5)-2.509(3) angstrom], Zn-N-pendant pyridine [2.802(2)-3.039(3) angstrom] 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 H-1 NMR spectroscopy.