Ligand-Directed Self-Assembly of Binuclear/Hexanuclear Lanthanide Complexes for Functional Catalysis

Abstract

N-heterocyclic carboxylic acid ligands are an important class of ligands in coordination chemistry, which combine the advantages of N and O atoms coordinating with metal centers. In this study, two N-heterocyclic carboxylic acid ligands were synthesized and six complexes were successfully constructed, namely, [La(L1)(NO3)2(H2O)2]2(H2O)2 (1), [Nd(L1)(NO3)2(H2O)2]2(H2O)2 (2), [Sm(L1)(NO3)2(H2O)2]2(H2O)2 (3), [Dy(L1)(NO3)2(H2O)2]2(H2O)2 (4), [Ho6(μ6-O)(μ3-OH)8(L2)4(H2O)16] (5) and [Er6(μ6-O)(μ3-OH)8(L2)4(H2O)16] (6). Among them, HL1 is 2-(1H-imidazole-5-yl)-1H-benzimidazole-5-carboxylic acid, and HL2 is 2-(1H-imidazole-2-yl)-1H-benzimidazole-5-carboxylic acid. Structural analysis shows that complexes 1-4 are zero-dimensional binuclear structures, and complexes 5 and 6 form polyhedral erbium clusters through Er-O-Er bridge bonds. Catalytic performance studies demonstrated that complexes 1-6 showed moderate high yields for cyanosilylation reaction and CO2 cycloaddition reaction under mild conditions. Taking complexes 4 and 6 as examples, the influencing factors, substrate universality, catalyst recyclability and possible reactions mechanism in the catalytic reaction were investigated in detail. Through experimental investigation, the synthesized complexes are expected to become a highly promising heterogeneous catalyst for CO2 chemical fixation and cyanosilylation reactions.