Dual Pathways and Transient Intermediates Reveal Hydrolysis-Driven Assembly of Aluminum Keggin Clusters

Despite the critical role of aluminum hydrolysis and condensation in the synthesis of aluminum-based compounds and their chemical behavior, experimental evidence regarding the formation and transformation pathways of species in solution remains scarce. This study systematically elucidates the hydrolysis-driven structural evolution of aluminum polyoxocations through integrated liquid- and solid-state ²⁷Al nuclear magnetic resonance (NMR), wide-angle X-ray scattering (WAXS), and spectroscopic analyses. Our study elucidates the dual competitive pathways governing the formation of <i>ε</i>-Al₁₃ species within the Al³⁺ hydrolysis system, that is, oligomeric species undergo stepwise dehydration-oligomerization with deprotonated monomers, culminating in <i>ε</i>-Al₁₃, and <i>F</i>-Al₁₃ undergoes partial dissociation followed by reorganization to generate <i>ε</i>-Al₁₃. Furthermore, we demonstrate that five-coordinate aluminum serves as a pivotal intermediate in the transition from a six-coordinate to a four-coordinate unit. This mechanistic insight, supported by multiscale structural characterization, redefines the understanding of the structural evolution of aluminum polyoxocations in aqueous systems.