Metallo-Supramolecular Hexagonal Wreath with Four Switchable States Based on a pH-Responsive Tridentate Ligand

Abstract

In biological systems, many biomacromolecules (e.g., heme proteins) are capable of switching their states reversibly in response to external stimuli, endowing these natural architectures with a high level of diversity and functionality. Although tremendous efforts have been made to advance the complexity of artificial supramolecules, it remains a challenge to construct metallo-supramolecular systems that can carry out reversible interconversion among multiple states. Here, a pHresponsive tridentate ligand, 2,6-di(1H-imidazole-2-yl)pyridine (H 2 DAP), is incorporated into the multitopic building block for precise construction of giant metallo-supramolecular hexagonal wreaths with three metal ions, i.e., Fe(II), Co(II), and Ni(II), through coordination-driven self-assembly. In particular, a Co-linked wreath enables in situ reversible interconversion among four states in response to pH and oxidant/reductant with highly efficient conversion without losing structural integrity. During the state interconversion cycles, the physical properties of the assembled constructs are finely tuned, including the charge states of the backbone, valency of metal ions, and paramagnetic/diamagnetic features of complexes. Such discrete wreath structures with a charge-switchable backbone further facilitate layer-by-layer assembly of metallo-supramolecules on the substrate.

Constructing Ultrastable Metallo-Cages via In Situ Deprotonation/Oxidation of Dynamic Supramolecular Assemblies

“…Considering the acid/base responsibility of the H 2 DAP moiety and the high reducibility of < H 2 DAP -Co­(II)- H 2 DAP > under alkaline conditions, the consequent metallo-supramolecules can undergo interconversion among multiple states with various charges and magnetic properties . Moreover, we reason that the in situ deprotonation/oxidation of the system can lock geometries of assembled metallo-cages, thereby converting their dynamic structures into static ones with high fidelity, attributing to the kinetic inertness of < DAP -Co­(III)- DAP > coordination, as well as rapid and complete deprotonation/oxidation processes.…”

“…Analysis of the single-crystal data for the model complexes [Co II ( H 2 DPA ) 2 ] and [Co III ( DPA ) 2 ] (CCDC numbers 22218720 and 22218728, respectively) revealed a significant shortening in the N–Co bond lengths, with values shrinking from 2.07 Å for Py-N–Co(II) to 1.88 Å for Py-N–Co(III) (Figure S156), indicating that the coordination bonds are strengthened after the in situ transformation. Density functional theory (DFT) calculations were conducted by using the crystal data to assess the binding strength of these two types of connectivity.…”

Near-infrared metal agents assisting precision medicine: from strategic design to bioimaging and therapeutic applications

“…Although many terpyridinebased MOCs composed of other types of metal ions (e.g., Zn, Fe, Cd, and Co) have recently been reported, most of the terpyridine-based MOCs are located in visible light region, which limits their further biomedical applications. [331][332][333] Therefore, it is critical to enrich the molecular library of NIR MOCs and further screen MOCs with excellent performance. In particular, the successful development of NIR-IIa/IIb absorption/emission (NIR-IIa, 1300-1400 nm; NIR-IIb, 1500-1700 nm) MOCs would represent a qualitative leap in precision diagnosis and treatment.…”

Responsive materials based on coordination bonds

“…The material reported by Xia et al only responds to reducing agents, while Su and Wei et al [230] synthesized a material that can respond to both oxidizing and reducing agents. In recent years, coordination-driven self-assembly has attracted attention due to its ability to increase the complexity of the metallosupramolecular system to obtain properties comparable to biological systems.…”