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An Organic Receptor Isolated in an Unusual Intermediate Conformation: Computation, Crystallography, and Hirshfeld Surface Analysis
1,1″-1,4-Phenylene-bisÂ(methylene)Âbis-4,4′-bipyridinium
cation [C<sub>28</sub>H<sub>24</sub>N<sub>4</sub>]<sup>2+</sup> (<b>c</b>), an organic receptor that generally crystallizes in its
anti conformation, has recently been shown to be isolated in its syn
conformation in an ion-paired compound [C<sub>28</sub>H<sub>24</sub>N<sub>4</sub>]Â[ZnÂ(dmit)<sub>2</sub>]·2DMF (dmit<sup>2–</sup> = 1,3-dithiole-2-thione-4,5-dithiolate; DMF = dimethylformamide).
In this article, we demonstrated that the same receptor [C<sub>28</sub>H<sub>24</sub>N<sub>4</sub>]<sup>2+</sup> (<b>c</b>) can also
be stabilized in an unusual intermediate conformation (neither syn
nor anti) with PF<sub>6</sub><sup>–</sup> anion in compound
[C<sub>28</sub>H<sub>24</sub>N<sub>4</sub>]Â(PF<sub>6</sub>)<sub>2</sub>·(1,4-dioxane) (<b>1</b>·(1,4-dioxane)). The energetically
favored anti conformation has been described in its nitrate salt [C<sub>28</sub>H<sub>24</sub>N<sub>4</sub>]Â(NO<sub>3</sub>)<sub>2</sub>·2H<sub>2</sub>O (<b>2</b>·2H<sub>2</sub>O). Compounds <b>1</b>·(1,4-dioxane) and <b>2</b>·2H<sub>2</sub>O, crystallizing
in triclinic and monoclinic systems with space groups <i>P</i>1Ì… and <i>P</i>2<sub>1</sub>/<i>n</i>,
respectively, were additionally characterized by Hirshfeld surface
analysis. The density functional theory calculations are performed
to understand the internal mechanism of the stability of various conformers
of cationic receptor <b>c</b>, compound <b>1</b>, and
compound <b>2</b>. In conjunction with the electronic stability
of the conformers, the natural bond orbital analysis and conformational
equilibrium constants at different temperatures are also calculated
to find out the sources of the different stability of the various
conformers of experimentally synthesized compounds