Two Nanometer-Sized High-Nuclearity Homometallic Bromide Clusters (M₂₆Br₃₈)^12– (M = Cu, Ag): Syntheses, Crystal Structures, and Efficient Adsorption Properties

The spontaneous formation of discrete spherical nanosized molecules is ubiquitous in nature; however, the actual structural imitation of such high-symmetry polyhedra from the edge sharing of regular polygons has still proved to be elusive. Herein, two high-nuclearity metal clusters, namely (TTB)₄·M₂₆Br₃₈ (M = Cu (1), Ag (2), TTB·Br₃ = 1, 3, 5-tris­(triethylammoniomethyl)­benzene tribromide), have been rationally and solvothermally synthesized and structurally characterized. Single-crystal X-ray analysis confirmed that 1 has I4̅3m symmetry with a (Cu₂₅Br₃₄)^12– anion shell enwrapping a CuBr₄ tetrahedron and 2 has I4̅3m symmetry with a [Ag₂₆Br₃₄]^12– anion shell enwrapping a Br₄ pyramid. The diffuse-reflectance UV–vis measurements showed that both compounds possess proper semiconductor behaviors with tunable band gaps of 1.87 eV for 1 and 1.90 eV for 2. Interestingly, all the samples feature distinct adsorption speed and compound 1 shows good adsorption activity for methyl orange (MO) under the same reaction conditions. The effects of pH, temperature, and cyclicity on dye adsorption, together with the thermal stabilities and luminescence properties of the compounds, were also studied. From the cyclic adsorption of compound 1 to the anionic dye MO, it was found that the adsorption of MO was good over three cycles, and the third adsorption rate was still 93.90%.