Acyclic Oligopyrrolic Anion Receptors

Abstract

Recent progress in the guest-binding and supramolecular chemistry of anion-responsive acyclic oligopyrroles is summarized here in this chapter. The hydrogen-bonding properties of the pyrrole NH sites determine anion binding in acylic oligopyrroles such as guanidinocarbonyl and amidopyrroles, dipyrrins and their analogs, dipyrrolylquinoxalines, and boron complexes of dipyrrolyldiketones. Linear oligopyrroles can be incorporated as subunits in various macromolecules and complexes by means of covalent and noncovalent interactions; in fact, boron complexes of dipyrrolyldiketones form assembled structures and, with the appropriate substituents, soft materials such as anion-responsive supramolecular gels.

Pyrrole N–H anion complexes

“…The amido- and bisamidopyrroles prepared by Gale and coworkers have shown selective oxoanion complexation and have even been employed as transmembrane anion carriers, making anion transport a fast growing subfield of supramolecular chemistry. Since these first reports descriptions of other monopyrrolic anion receptors followed [20,28,29]. Taken in concert, this work provided the antecedents for the preparation of more complex and efficient anion receptors.…”

“…The synthesis and anion binding properties of a number of these systems were reviewed in a book chapter by Maeda [29]. Aryl-bridged bispyrroles have also been used extensively as building blocks in the construction of macrocycles [11] and for preparing low molecular weight compounds with membrane permeability [58].…”

Synthesis, Crystal Structures, and Supramolecular Assemblies of Pyrrole-Based Anion Receptors Bearing Modified Pyrrole β-Substituents

“…On the other hand, pyrrole β-substituents such as those bearing methyl ( 2a ), ethyl ( 3a ), and fluorine ( 4a ) moieties are also essential factors for controlling the anion-binding and assembling properties of the anion receptors. In contrast to the cases of 1a , 2a , and 4a , the β-ethyl groups in 3a interfere with the construction of efficient stacking structures, as suggested by the solid-state assembly. ,, However, π-extension at pyrrole α-positions provides solid-state stacking dimers, such as those found in α-phenyl 3b and related derivatives. , …”

“…However, further detailed investigations of such soft materials comprising charged species require the preparation and examination of suitable π-conjugated molecules that efficiently bind anions. As potential candidates, boron complexes of 1,3-dipyrrolyl-1,3-propanediones (e.g., 1a , b , Figure a) , form anion-responsive supramolecular gels via the introduction of aliphatic chains to α-aryl moieties such as 1c . On the other hand, pyrrole β-substituents such as those bearing methyl ( 2a ), ethyl ( 3a ), and fluorine ( 4a ) moieties are also essential factors for controlling the anion-binding and assembling properties of the anion receptors.…”

“…The fluorine moiety does not significantly affect the geometries of molecules because of its small size but can perturb the states of π-conjugated systems, resulting in the stabilization of molecules as well as the modulation of electronic properties and assembled structures. In this paper, on the basis of the initial findings by our group, , we show the modifications of the α-positions of the β-substituted receptors to form functional assembled structures, especially, anion-responsive supramolecular gels. In particular, introduction of fluorine moieties at the pyrrole β-positions was found to enable the formation of supramolecular gel at rt even in the presence of Cl – as a tetraalkylammonium salt, suggesting that more stable and useful soft materials consisting of charged species would be prepared using β-fluorinated receptors if anions as salts of appropriate countercation are incorporated.…”