Unraveling the Structural Evolution of Aluminum Polyoxocations in Solution

The primary objective within the realm of aluminum solution chemistry is to elucidate the structural changes in aluminum polyoxocations under the influence of altered solution conditions. Notably, previous reports are primarily focused on specific types, such as aluminum monomers, species from the Keggin series, and the planar Flat-Al₁₃¹⁵⁺ (F-Al₁₃) cluster. As a result, there is a lack of comprehensive understanding of the remaining aluminum polyoxocations and their respective transformation pathways. In response to this lack, we adopt a combined experimental and theoretical approach to explore the spectral properties of aluminum polyoxocations. Specifically, we analyze infrared spectra, Raman spectra, and aluminum-27 nuclear magnetic resonance (²⁷Al NMR) spectra. Notably, the changes in the spectral features originate from varying solution basicity levels. Through our findings, we can categorize the Al-O clusters into three primary groups: Al(H₂O)₆³⁺ (Al₁), ε-Keggin-[AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ (ε-Al₁₃), and 6-coordinated aluminum species. Notably, the Raman spectra exhibit prominent peak shifts at 559 and 595 cm^-1, indicating the existence of Al₃(1) intermediates during the transition from the Al monomer to the ε-Al₁₃ cluster. Overall, this paper presents a comprehensive summary of the possible mechanisms that govern the formation of ε-Al₁₃ from Al₃(1), offering a clearer picture of the aluminum polyoxocation landscape and its dynamics under various solution conditions.