Synthesis of Imidazolium Salts Linked to a t-Butylcalix[4]arene Framework and the Isolation of Interesting By-Products

TL;DR

Scientists synthesized new calix[4]arene compounds with imidazolium groups and discovered unexpected by-products, which could be useful for creating anion-complexing materials and nickel complexes.

Contribution

The novel synthesis of imidazolium-substituted calix[4]arenes and the discovery of unexpected bis-calix[4]arene by-products with unique bridging structures.

Findings

Mono- and bis-imidazolium salts were successfully synthesized and linked to a t-butylcalix[4]arene framework.

Unexpected bis-calix[4]arene by-products were isolated, featuring unique bridging organic linkers.

The nickel complex of an N-heterocyclic carbene was formed from a mono-imidazolium-substituted calix[4]arene.

Abstract

A series of functionalized calix[4]arenes were prepared that contain mono- and bis-(alkoxy)imidazolium groups that are linked to the lower rim of a t-butylcalix[4]arene framework. These molecules have potential as anion-complexation reagents and as precursors to N-heterocyclic carbene complexes that are attached to a calixarene framework. They were prepared by the preliminary reaction α,ω-dibromoalkanes with the parent t-butylcalix[4]arene to give bis-ω-bromoalkoxy groups that are connected to the calix[4]arene framework in the 25- and 27-positions. The reaction of the bis-substituted calixarenes with TiCl4 led to the removal of one bromoalkoxy group to give mono-substituted derivatives. Both the mono- and bis-functionalized calixarenes were reacted with N-substituted imidazoles to give a series of mono- or bis-imidazolium salts with the imidazolium group tethered to the calix[4]arene via O–(CH2)n– linkages (n = 2, 4, or 6). Unexpected bis-calix[4]arene products, in which the calixarenes are linked together via bridging organic groups, were obtained in some of these reactions. One bridge consists of two calixarenes linked together via two –C2H4– groups while the other had a –O–C4H8–imidazolium-C4H8–O– linker tethering the two calix[4]arenes together. Both these species were characterized by single crystal X-ray diffraction studies. The structures both had significant disorder but, nevertheless, the data do establish their structures. That the imidazolium-substituted calix[4]arene cations are precursors to N-heterocyclic carbene complexes of nickel was demonstrated by the reaction of a mono-imidazolium-substituted calix[4]arene with nickelocene to give the fully characterized N-heterocyclic carbene nickel complex linked to the calix[4]arene group.

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