Synthesis of an Aluminum Hydroxide Octamer through a Simple Dissolution Method

Multimeric oxo-hydroxo Al clusters function as models for common mineral structures and reactions. Cluster research, however, is often slowed by a lack of methods to prepare clusters in pure form and in large amounts. Herein, we report a facile synthesis of the little known cluster Al₈ (OH)₁₄ (H₂ O)₁₈ (SO₄ )₅ (Al₈ ) through a simple dissolution method. We confirm its structure by single-crystal X-ray diffraction and show by ²⁷ Al NMR spectroscopy, electrospray-ionization mass spectrometry, and small- and wide-angle X-ray scattering that it also exists in solution. We speculate that Al₈ may form in natural water systems through the dissolution of aluminum-containing minerals in acidic sulfate solutions, such as those that could result from acid rain or mine drainage. Additionally, the dissolution method produces a discrete Al cluster on a scale suitable for studies and applications in materials science.