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.