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 (1a·S), [Co^III(Sq^•-)(Cat^2-)(pyphp[5])]·4Cl₂CH₂CH₂Cl₂·2CH₃OH (1b·S), [Co^III(Sq^•-)(Cat^2-)(pyetp[5])]·2CH₂Cl₂·3CH₃OH (2a·S), and [Co^III(Sq^•-)(Cat^2-)(pyetp[5])]·3Cl₂CH₂CH₂Cl₂·2CH₃OH (2b·S) (Sq^•- = 3,5-di-tert-butylsemiquinonate, Cat^2- = 3,5-di-tert-butyl-catecholate). Single-crystal X-ray diffraction analyses revealed that all complexes possess one-dimensional structures. Complexes 1a·S and 1b·S exhibit the common trans configuration (trans(N)-trans(^tBu)) of the cobalt-dioxolene-pyridine species, whereas 2a·S and 2b·S adopt a rare C_2h-symmetric cis one (trans(N)-cis(^tBu)). 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 C_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.