Induced Aggregation and Synergistic Coordination Strategy in Cluster Organic Architectures

Developments in strategies for the synthesis of oxo-hydroxo lanthanide (Ln) clusters during the past few decades have resulted in abundant relevant crystalline materials that exhibit attractive structures and interesting properties. The combination of these oxo-hydroxo Ln clusters and organic ligands has led to the formation of new extended arrays of Ln cluster organic frameworks (LnCOFs). In contrast to metal-organic frameworks, the incorporation of particular characteristics of clusters provides the opportunity to develop performances not available in single-metal compounds. Even with steady advances in oxo-hydroxo Ln clusters, progress in LnCOFs is less developed. To obtain LnCOFs, one premise is to induce the oxophilic Ln ions undergoing aggregation. Meanwhile, the organic ligands should have extra coordination sites for further expansion. Multidentate organic ligands like pyrazinecarboxylic acid and pyridinecarboxylic acid containing O and N donors will meet these two requirements. Their carboxyl groups will induce the aggregation of Ln ions, while the N donors can serve as potential extension sites. To make more open frameworks or if the oxo-hydroxo Ln clusters fail to be congregated or connected, then a second ligand is necessary. The introduction of the suitable second ligand may occupy a partial coordination sphere of Ln ions and ultimately benefit the connection process. In this Account, we introduce the origin and evolution of the induced aggregation and synergistic coordination strategy. According to the attributes of the organic ligands in the documented LnCOFs, we classify them into linear and nonlinear groups in the second and third parts. From the aspect of ligand-induced aggregation, isonicotinic acid (HIN) and lengthened 4-(4-pyridyl)benzoic acid (HPBA) ligands as well as their nonlinear analogues are settled as typical models. From the aspect of synergistic coordination, chelating ligands like 1,2-benzenedicarboxylic acid (1,2-H₂BDC) and acetic acid (HOAc) play significant roles. Moreover, three types of synergistic coordination are discussed in detail: synergistic coordination between two types of organic ligands, synergistic coordination between organic and inorganic ligands, and simultaneous synergistic coordination of aforementioned two types. From the aspect of LnCOF products, in addition to traditional pure LnCOFs, new types of heterometallic frameworks containing two types of cluster building units have been developed. Although this Account focuses on the nuclearity and coordination aspects of LnCOFs, we anticipate that it will stimulate more efforts in the further study of their properties beyond the exploratory synthesis. More importantly, synergistic coordination may be applied to other systems and inspire crystal design and targeted assembly of new functional materials.