Synthesis of multi-imidazolium salt ligands containing calixarene fragments and their N-heterocyclic carbene Ag(I) macrocyclic complexes

Abstract

The unsymmetric bis-imidazolium salts ligands 4a and b were prepared by stepwise reactions of 2,6-bis(bromomethylene)-4-(t-butyl)anisole (1) with two different N-substituted imidazoles. Reaction of the bis-imidazolium ligands with Ag2O in acetonitrile gave the dinuclear bis(N-heterocyclic carbene) complexes 5a and b. The molecular structure of complex 5a was determined by single-crystal X-ray diffraction analysis. In complex 5a and b, the two Ccarbene–Ag–Ccarbene axes are in a crossed conformation, two O atoms from phenoxyl groups weakly coordinate to different Ag ions and the two p-t-butylphenoxyl groups adopted a staggered conformation. To regulate the molecular shape of the N-heterocyclic carbene Ag(I) macrocyclic complexes, two tetradentate imidazolium ligands (9a and b) were prepared. The bimetallic NHC–Ag(I) complexes 10a and b were obtained by the reaction of 9a and b and Ag2O in acetonitrile. Constrained by the chain (the dioxoethylene or dioxomethylenebenzene chain in 10a or 10b, respectively) at the O-position of the phenoxyl group, the two p-methylphenoxyl groups in complexes 10a and b might be predicted to have a cone conformation. The ligands and the corresponding NHC metal complexes have been characterized by elemental analysis, 1H NMR and 13C NMR, HRMS spectra, as well as TGA and DTA spectra being determined for complexes 10a and b. The TGA curves show that these NHC–Ag complexes have good thermal stability.