Anion Recognition by a Pincer-Type Host Constructed from Two Polyamide Macrocyclic Frameworks Jointed by a Photo-Addressable Azobenzene Switch.

Patryk Niedbała,Wiktor Ignacak,Mart Mehmet,Kajetan Dąbrowa,Magdalena Ceborska,Janusz Jurczak

doi:10.3390/ma15020692

Patryk Niedbała, Wiktor Ignacak + Show 4 more

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https://doi.org/10.3390/ma15020692

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Abstract

A sterically crowded light-responsive host 1 was synthetized with a 93% yield by applying a post-functionalization protocol utilizing the double amidation of 4,4′-azodibenzoyl dichloride with a readily available 26-membered macrocyclic amine. X-ray structures of two hydrates of trans-1 demonstrate a very different alignment of the azobenzene linkage, which is involved in T-shape or parallel-displaced π⋯π stacking interactions with the pyridine-2,6-dicarboxamide moieties from the macrocyclic backbone. Despite the rigidity of the macrocyclic framework, which generates a large steric hindrance around the azobenzene chromophore, the host 1 retains the ability to undergo a reversible cis⟷trans isomerization upon irradiation with UVA (368 nm) and blue (410 nm) light. Moreover, thermal cis→trans back-isomerization (ΔG0 = 106.5 kJ∙mol−1, t½ = 141 h) is markedly slowed down as compared to the non-macrocyclic analog. 1H NMR titration experiments in DMSO-d6/0.5% water solution reveal that trans-1 exhibits a strong preference for dihydrogenphosphate (H2PO4−) over other anions (Cl−, MeCO2−, and PhCO2−), whereas the photogenerated metastable cis-1 shows lower affinity for the H2PO4− anion.

Highlights

IntroductionThe recognition and transport of anionic species play a central role in many fundamental chemical, biological, or environmental processes [1,2]
Academic Editor: Barbara PawelecThe recognition and transport of anionic species play a central role in many fundamental chemical, biological, or environmental processes [1,2]
The conformation of the anionic complex is further stabilized by numerous intramolecular hydrogen bonding interactions that hinder the binding of the second guest. These results indicate that intramolecular hydrogen bonding and increased crowding cause the host 1 to be unable to adopt an optimal conformation to bind the H2PO4−, resulting10inof the decreased stability of the complex

Summary

Introduction

The recognition and transport of anionic species play a central role in many fundamental chemical, biological, or environmental processes [1,2]. Anion recognition is highly selective, and it is primarily accomplished in a tailored binding cavity by multiple directional hydrogen bonds derived from amide and hydroxyl groups of amino acid residues [3,4,5,6]. Molecular recognition can be aided by additional and less-directional ionic, ion-π, hydrophobic, and stacking interactions [7,8]. Together, all of these interactions allow for the complete separation of the often initially highly solvated guest molecule from the external environment [9,10]. A strong and selective binding of anions, such as chloride [11], carboxylates [12,13,14,15], and phosphates [16,17], might be realized by the selection of a proper molecular platform and decorating it with neutral (thio)amide [18,19,20],

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