Janus-Like squaramide-based hosts: dual mode of binding and conformational transitions driven by ion-pair recognition

Abstract

[eng] New tripodal squaramide based</p><p>hosts 6 and 7, have been</p><p>synthesized and structurally</p><p>characterized by spectroscopic methods.</p><p>In 2.5% [D6]DMSO in CDCl3 (v/v),</p><p>compound 6 was insoluble, whereas 7</p><p>formed dimeric assemblies (log Kdim =</p><p>3.68(8)) as demonstrated by 1H NMR</p><p>and UV dilution experiments. AFM and</p><p>SEM analyses performed by</p><p>evaporation of 7 revealed the formation</p><p>of an entangled network of bundled</p><p>fibres, indicating a preferential</p><p>mechanism of aggregation. These C3-</p><p>symmetric tripodal hosts exhibited two</p><p>different and mutually exclusive modes</p><p>of binding each one easily accessible by</p><p>simultaneous re-orientation of</p><p>squaramide groups. First, a convergent</p><p>disposition of the NH squaramide</p><p>protons allowed the formation of an</p><p>array of N–H···X– hydrogen bonds</p><p>with anions. Thus, addition of the</p><p>tetraalkylammonium salts of trimesoate</p><p>anion to 6 or 7 resulted in the formation</p><p>of 1:1 ([D6]DMSO) and of 1:1 and 2:1</p><p>(2.5%[D6]DMSO/CDCl3) host-guest</p><p>complexes with 7. In a second mode,</p><p>re-orientation of carbonyl squaramide</p><p>groups allowed multiple C=O···H</p><p>interactions with ammonium cations.</p><p>However, titration of 7 with different</p><p>tetraalkylammonium iodides (NR4)I</p><p>persistently showed the formation of</p><p>1:1 complexes, as well as 1:2 and 1:3</p><p>complexes. The corresponding</p><p>stoichiometries and binding affinities of</p><p>the complexes were evaluated by</p><p>multiregression analysis.</p><p>The formation of high order complexes,</p><p>which is supported by ROESY,</p><p>NOESY, and mass spectrometry</p><p>experiments, was assigned to the</p><p>insertion of NR4I ion-pairs between the</p><p>carbonyl and NH protons of</p><p>squaramide groups located in adjacent</p><p>arms of 7. The emerging picture</p><p>reflects the induction of significant</p><p>conformational changes in the hosts</p><p>concerning mainly the relative</p><p>orientation of the squaramide groups to</p><p>adapt their geometries to incoming ionpair</p><p>complementary substrates. The</p><p>results presented herein identify and</p><p>fully describe two different modes of</p><p>ion-pair recognition aimed at directing</p><p>conformational transitions on the host,</p><p>therefore establishing a base for</p><p>controlling more elaborate movements</p><p>of molecular devices through ion-pair</p><p>recognition.</p>