Stepwise Synthesis of Tetra-imidazolium Macrocycles and Their N-Heterocyclic Carbene Metal Complexes

Abstract

A modular stepwise synthetic method has been developed for the preperation of tetra -imidazolium macrocycles. Initially a series of three bis (imidazolylmethyl)benzene precursors were alkylated with 1,2-dibromoethane to produce the corresponding bis -bromoethylimidazolium bromide salts. In the second step the bis -bromoethylimidazolium bromide salts were reacted with selected bis (imidazolylmethyl)benzene molecules to produce a series of two symmetrical and three asymmetrical tetra -imidazolium macrocycles. These tetra -imidazolium salts act receptors for anions and 1 H-NMR titration studies were used to determine the association constants between two of the macrocycles and the halide anions chloride, bromide and iodide. The tetra -imidazolium salts are precursors for N -heterocyclic carbene (NHC) ligands and the corresponding silver(I), gold(I), and palladium(II) NHC complexes have been prepared. Varied structures were obtained, which depend on the chosen macrocyclic ligand and metal ion and in the case of the coinage metals Ag(I) and Au(I), mono, di, and hexanuclear complexes were formed.

Step-Growth Cyclo-Oligomerization for the Preparation of Functionalized Pillar[6]arenes with Alternating Methylene Bridge Substitutions

“…Step-growth cyclo-oligomerization is a stepwise synthetic methodology, often used to prepare complex macrocyclic host molecules. − Unlike a one-pot reaction, which often generates a mixture of macrocyclic homologues, step-growth cyclo-oligomerization is able to yield a major product for every step. Therefore, the size of the macrocycle and the position of the substitution could be precisely controlled without laborious separation.…”

Bimetallic Platinum Group Complexes of a Macrocyclic Pyrazolate/NHC Hybrid Ligand

“…While this approach is synthetically elegant, only a handful of compounds have been prepared by metal template synthesis as it is not simply transferrable to different metals and ligand geometries . Instead, the majority of macrocyclic NHC complexes were prepared by deprotonation of macrocyclic imidazolium precursors. − Two strategies toward the synthesis of macrocyclic imidazolium salts have been developed: In a “direct” approach, equimolar amounts of an α,ω-diimidazole linker and a α,ω-dihaloalkane are combined under high dilution conditions. , The problem of low yields associated with this strategy is circumvented in a “sequential” approach, in which the desired cyclophane is constructed stepwise, including the isolation of intermediate species . Apart from the deprotonation of imidazolium salts, another extensively used route toward structurally diverse macrocyclic NHC complexes is the transmetalation of the respective Ag I –NHC congeners. − Here, the latter is treated with a metal precursor (often a chloride salt) to form the desired metal–NHC complex, driven by the precipitation of insoluble Ag I salts in polar aprotic media.…”