Beyond metal oxides, various natural minerals and artificial solid-state materials, such as silicates, halides, borates, and perovskites, have been extensively studied for their applications in catalysis, energy storage, and optoelectronic devices. In these material systems, electrostatic interactions profoundly influence synthetic methods and ultimately determine their functional performance. However, the controlled assembly of multiple cationic and anionic clusters into well-defined architectures remains one of the major challenges in chemistry.

In a study published in Angew. Chem. Int. Ed., the research group led by Prof. ZHANG Jian from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences,reported the successful synthesis of a series of charged boron-oxo cluster-based structures, named BOC‑n (n = 1–8), via an in situ tandem strategy.

In this study, the researchers combined designed cationic metal boron-oxo clusters with various anionic counterions. Typically, the cationic metal boron-oxo cluster can be regarded as a porphyrin-like boron-oxo macrocycle assembled from four 8-quinolineboronic acid (8-QBA) dimers, chelating one or two Cu(II) ions at its center. The quinoline units on the periphery of the boron-oxo macrocycle exhibit a precise spatial arrangement, endowing the cationic clusters with directionality and thus facilitating the formation of three-dimensional porous architectures through noncovalent interactions (including π-π stacking and C-H⋅⋅⋅π hydrogen bonding). The Cu(II) ion at each center exposes an axial vacant site that can accept a series of organic ligands through coordination interactions, thereby enabling fine-tuning and stabilizing the entire arrangement.

Therefore, the spatially well-defined assembly of cationic metal boron-oxo clusters affords dynamic and flexible cavities with counterion-adaptive feature, that can encapsulate different types anions, such as 0-D anions of [CF₃SO₃]^-, [CuCl₂]^-, [B₅O₆(OH)₄]^-, and 1-D chain of [Cu(SCN)₂]_n^n-.

Furthermore, these materials can reversibly respond to guest molecules through electrostatic interaction to facilitate anion exchange. Moreover, the modulation of axial ligand and guest anions allows for the regulation electrostatic interactions, charge density, and π-π stacking within the structure, thereby influencing the optical third-order nonlinear properties of these materials. All these materials show a typical reverse saturated absorption (RSA) response (optical limiting performance), among which BOC-6 shows the lowest normalized transmittance (T_min) of 0.228.

This study not only provides an efficient methodology to synthesisze organic-inorganic boron-oxo clusters but also opens a new way to build well-defined architectures with multiple cationic and anionic clusters.

Schematic illustration of the covalent and coordinate assembly of BOCs with their cation/anion engineering strategies.(Image by Prof. ZHANG’s group)

Contact:

Prof. ZHANG Haixia

Fujian Institute of Research on the Structure of Matter

Chinese Academy of Sciences

E-mail: zhanghaixia@fjirsm.ac.cn