‘Pincer-tweezer’ tetraimidazolium salts as hosts for halides

Abstract

In this work we describe the preparation of two tetracationic molecular tweezers that were used as hosts for the recognition of chloride, bromide and iodide. These two hosts contain a rigid bis-alkynyl spacer that connects two arms built with two pincer bis-imidazolium units. The nature of the spacer confers distinct structural features to these two molecular ‘pincer-tweezers’. The tweezer with the anthracenyl linker has a quasi-parallel disposition of the two arms of the tweezer, while in the carbazolyl-linked tweezer the two arms are significantly more separated. The study of the binding affinities of these two hosts with the three target halides showed substantial differences depending on the tetra-imidazolium tweezer used. In particular, the carbazolyl-based tweezer follows a 1:2 host:guest binding model, while the binding of the anthracenyl-linked tweezer is best explained using a 1:1 model. The different behavior is ascribed to the subtle different structural features of the two tetra-azolium receptors. The comparison of the affinities of the two tetracationic hosts with the related bis-imidazolium mono-pincer host indicates significantly enhanced association constants shown by the pincer-tweezer hosts, which are ascribed to a combination of the larger electrostatic attraction provided by the use of the tetracations together with the converging orientation of the two pincer bis-imidazoliums in the structure of the tweezers. These new tweezer-shaped tetra-imidazolium receptors are significantly different to other traditional tetrazoliums used as hosts for anions, which are mostly based on cyclic and more flexible structures. The pincer-tweezer hosts are acyclic and nevertheless more rigid than most of the poly-azolium-based receptors used until now.