Cis/Trans Isomeric Valence Tautomeric Compounds with Guest-Driven Conformational Adaptation in Pillar[5]arene Structures

The pursuit of ancillary ligands is crucial for constructing new valence tautomeric (VT) complexes based on the dioxolene-cobalt(II/III) moiety. In this study, we adopted pillar[5]arene derivatives with guest-adaptable conformationsspecifically, phenylpyridine-containing pillar[5]arene (pyphp[5]) and alkynylpyridine-containing pillar[5]arene (pyetp[5])as ancillary and bridging ligands to synthesize a series of cobalt-based VT complexes: [Co^III(Sq^•–)(Cat^2–)(pyphp[5])]·2CH₂Cl₂·2CH₃OH (<b>1a·<i>S</i></b>), [Co^III(Sq^•–)(Cat^2–)(pyphp[5])]·4Cl₂CH₂CH₂Cl₂·2CH₃OH (<b>1b·<i>S</i></b>), [Co^III(Sq^•–)(Cat^2–)(pyetp[5])]·2CH₂Cl₂·3CH₃OH (<b>2a·<i>S</i></b>), and [Co^III(Sq^•–)(Cat^2–)(pyetp[5])]·3Cl₂CH₂CH₂Cl₂·2CH₃OH (<b>2b·<i>S</i></b>) (Sq^•– = 3,5-di-<i>tert</i>-butylsemiquinonate, Cat^2– = 3,5-di-<i>tert</i>-butyl-catecholate). Single-crystal X-ray diffraction analyses revealed that all complexes possess one-dimensional structures. Complexes <b>1a·<i>S</i></b> and <b>1b·<i>S</i></b> exhibit the common trans configuration (<i>trans</i>(N)–<i>trans</i>(^<i>t</i>Bu)) of the cobalt-dioxolene-pyridine species, whereas <b>2a·<i>S</i></b> and <b>2b·<i>S</i></b> adopt a rare <i>C</i>_2h-symmetric cis one (<i>trans</i>(N)–<i>cis</i>(^<i>t</i>Bu)). Variations in guest molecule size and host–guest interactions within the pillar[5]arene cavity led to distinct pore conformations and stacking patterns. This work represents the first example of both the trans and <i>C</i>_2h-symmetric cis isomers of the cobalt-dioxolene-pyridine moiety being obtained through synthesis. Our findings highlight the potential to synthesize VT isomers and the critical role of host–guest interactions in modulating VT behavior, offering valuable insights into the design of multifunctional materials with controllable magnetic properties.