Reticular Chemistry and the Discovery of a New Family of Rare Earth (4, 8)-Connected Metal-Organic Frameworks with csq Topology Based on RE₄(μ₃‑O)₂(COO)₈ Clusters

In recent years, the design and discovery of new metal-organic framework (MOF) platforms with distinct structural features and tunable chemical composition has remarkably enhanced by applying reticular chemistry rules and the molecular building block (MBB) approach. We targeted the synthesis of new rare earth (RE)-MOF platforms based on a rectangular-shaped 4-c linker, acting as a rigid organic MBB. Accordingly, we designed and synthesized the organic ligand 1,2,4,5-tetrakis­(4-carboxyphenyl)-3,6-dimethyl-benzene (H₄L), in which the two methyl groups attached to the central phenyl ring lock the four peripheral carboxyphenyl groups to an orthogonal/vertical position. We report here a new family of RE-MOFs featuring the novel inorganic building unit, RE₄(μ₃-O)₂ (RE: Y³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, and Yb³⁺), with planar D_2h symmetry. The rigid 4-c linker, H₄L, directs the in situ assembly of the unique 8-c RE₄(μ₃-O)₂(COO)₈ cluster, resulting in the formation of the first (4, 8)-c RE-MOFs with csq topology, RE-csq-MOF-1. The structures of the yttrium (Y-csq-MOF-1) and holmium (Ho-csq-MOF-1) analogues were determined by single-crystal X-ray diffraction analysis. Y-csq-MOF-1 was successfully activated and tested for Xe/Kr separation. The results show that Y-csq-MOF-1 has high isosteric heat of adsorption for Xe (33.8 kJ mol^–1), with high Xe/Kr selectivity (IAST 12.1, Henry 12.9) and good Xe uptake (1.94 mmol g^–1 at 298 K and 1 bar), placing this MOF among the top-performing adsorbents for Xe/Kr separation.