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

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 Ag 2 O 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 C carbene –Ag–C carbene 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 Ag 2 O 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, 1 H NMR and 13 C 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.