Na<sub>5</sub>[MB<sub>24</sub>O<sub>34</sub>(OH)<sub>12</sub>]·<i>n</i>H<sub>2</sub>O (M = Cr<sup>3+</sup>, Al<sup>3+</sup>): Unprecedented Spherelike Polyborate Clusters from Boric Acid Flux Synthesis
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Two novel isotypic polyborate cluster compounds Na5[MB24O34(OH)12]·nH2O (M = Cr3+, Al3+) have been synthesized in a boric acid flux closed system at 220 °C by using gel as precursors. The structure of the Cr compound was determined by low-temperature single-crystal X-ray diffraction, and the Al compound was confirmed to be isostructural by powder X-ray diffraction and similar FT-IR spectra and thermal behavior. Na5[CrB24O34(OH)12]·nH2O crystallizes in the monoclinic space group P21/c with the cell parameters a = 17.4549(3) Å, b = 11.1976(2) Å, c = 21.2403(3) Å, β = 94.644(2)°, V = 4137.85(12) Å3, and Z = 4. The predominant feature of the structure are the spherelike polyborate clusters [MB24O34(OH)12]5−. The M3+ cation is located at the center of the cluster, around which four hexaborate units ([B6O10(OH)3]5−) are tetrahedrally assembled in almost a perfect Td symmetry. To the best of our knowledge, [MB24O34(OH)12]5− is the first example of a polyborate cluster that contains 24 boron atoms. Upon heating, the compounds show two steps of weight loss, including the loss of the disengaged water and dehydration of the hydroxy groups. The first step is reversible, retaining the structure framework. The second weight loss is continuous and irreversible.
