Self-assembly of functional metal-seamed pyrogallol[4]arene nanocapsules
Authors:   , Hu Xiangquan 
Published: February 2023

[EMBARGOED UNTIL 8/1/2023] C-alkylpyrogallol[4]arenes (PgCns, where n is the represents pendant alkyl chain lengths), bowl-shaped macrocycles, upon deprotonation, can self-assemble with metal ions into dimeric (M8L2) and hexameric (M24L6) metal-organic nanocapsule (MONCs). Herein we exploited this feature and tried to construct new functional MONCs. A Co2+-seamed hexameric C-butylpyrogallol[3]resorcinol[1]arene nanocapsule has been constructed and characterized using biomimetic self-assembly as the synthetic methodology. Akin to the biological behavior of zinc-finger proteins' release, uptake, and electrophilic substitution of Zn2+ ions, the assembly of this novel MONC has been accomplished by employing three sequential processes: assembly of the framework, metal ion insertion, and metal exchange, resulting in the formation of a Co2+18L6 MONC. The synthesis of MONC-based hierarchical superstructures (HSSs) has been accomplished by employing PgC3OH, Mn2+ ions, and proline using a solvothermal method. Moreover, upon increase of the water content in the reaction mixture, the resulting assemblies transform from 2D to 3D HSSs. The MONC subunits within two distinct coordinate polymers were both assembled from six PgC3OH molecules and 24 Mn2+ ions, while their exterior surface properties show remarkable differences. Within the 2D system, each capsule is connected to four adjacent capsules via double Mn-hydroxyl coordination, while in the 3D analog, each capsular subunit is linked to eight adjacent MONCs via single Mn-hydroxyl coordination. In addition, endohedral functionalization within 2D one has been achieved via chaperone guest encapsulation. The control of surface properties of the capsule subunits has been shown as an effective strategy for fine-tuning of magnetic properties. Both HSSs show excellent catalytic activity and stability toward water oxidation with a relatively low overpotential of only +368 mV. Moreover, a MONC constructed from six PgC5 units, which are stitched together by 16 Fe3+ and 16 Co2+ ions has been constructed. This supramolecular structure is the first instance of a spheroidal heterometallic nanocage assembled through a one-step metal-ligand coordination approach. This assembly also demonstrates an important proof of concept through the formation of multiple heterometallic metal-metal interactions within the MONC framework. Photophysical and electrochemical studies of self-assembled MONC films indicate their potential as semiconductors. These materials display photoelectric conversion properties. Further, a Cu2+-seamed hexameric MONC has been constructed through an in situ redox reaction facilitated self-assembly of cuprous bromide and PgC5. Solid-state spectroscopic and electrochemical studies revealed its strong visible light absorption and semiconducting properties. This MONC exhibited both outstanding photostability and efficient heterogeneous photocatalytic activity toward visible light-driven aerobic oxidation of organoboron compounds in aqueous solution. Finally, a mixed-valence vanadium-seamed MONC has been synthesized and structurally characterized. This MONC is assembled from six PgC5 units, 18 V3+ ions, and six VO2+ ions. It exhibits ferromagnetic and photoelectric conversion behaviors.