Discrete metal-organic macrocycles and cages have been rapidly developed in the past few decades by using the efficient coordination-driven self-assembly strategy. These metal-ligand edifices are mostly charged and their functions, such as host-guest encapsulation and catalysis, are highly sensitive to their charges. However, it remains a challenge to prepare coordination hosts that can regulate their charges orthogonally without affecting the structural integrity of metallosupramolecular assemblies.
In a study published in Journal of the American Chemical Society, the research group led by Prof. SUN Qingfu from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences reported the condition-dependent self-assembly of charge-reversible lanthanide-organic tetra-capped octahedral cages for acid/base-triggered guest uptake/release.
Taking advantage of the distinct pKa of the two acidic protons and the keto-enol tautomerization on 2-[(8-hydroxyquinolinyl)methylene]-benzo-hydrazide, the researchers synthesized a tritopic tetradentate ligand (H6L) featuring multiple deprotonation states and propeller conformations and demonstrated its lanthanide-directed coordination self-assembly with lanthanide ions (LnIII: LuIII, YbIII, EuIII) under different conditions.
Nuclear Magnetic Resonance (NMR), electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) and X-ray crystallography confirmed the formation of tetra-capped octahedra, including positive [Ln6(H3L)4]6+ and negative [Ln6L4]6-. While direct self-assembly under basic conditions produced a mixture of various ΔxΛ6-x-[Ln6L4]6- (x = 0-6) stereoisomers, racemic Δ6- and Λ6-[Ln6L4]6- could be exclusively obtained from the first self-assembly of Δ6- and Λ6-[Ln6(H3L)4]6+ under neutral conditions followed by post-assembly deprotonation.
In addition, the coupling between the metal-centered Δ/Λ chirality and the ligand conformations, shown in the X-ray single crystal structures of the C3-symmetric Δ3Λ3-Ln6L4 and the T-symmetric Δ6/Λ6-Ln6L4 complexes, resulted in rich isomerism on the tetra-capped octahedral cages.
Further host-guest studies indicated that positively charged rac-Δ6/Λ6-[Ln6(H3L)4]6+ can bind anionic sulfonates guests in fast-exchange mode, while the negatively charged rac-Δ6/Λ6-[Ln6L4]6- can strongly encapsulate ammonium guests. By virtue of the charge reversibility on the cage, the researchers well regulated the uptake/release of these ionic guests by acid/base. This is the first example of lanthanide-organic cage capable of responsive guest encapsulation.
Moreover, photophysical studies revealed visible-light sensitized and guest-encapsulation-enhanced NIR emissions on the rac-Δ6/Λ6-Yb6L4 cage.
This study not only enriches the library of functional lanthanide-organic cages, but also provides a promising host candidate with charge reversibility for the development of smart supramolecular materials.
Schematic illustration of acid/base-triggered guest uptake/release taking advantage of the charge reversibility of the cage (Image by Prof. SUN’s group)
Contact:
Prof. SUN Qingfu
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences
Email: qfsun@fjirsm.ac.cn
