Absolute Structures of a Mirror Pair of Infinite Na(H₂O)₄⁺‑Connected ε‑Keggin–Al₁₃ Species

The absolute structures of a pair of infinite Na­(H₂O)₄⁺-connected ε-Keggin–Al₁₃ species (Na−ε-K–Al₁₃) that were inversion structures and mirror images of each other were determined. Single crystals obtained by adding A₂SO₄ (A = Li, Na, K, Rb, or Cs) solution to NaOH-hydrolyzed AlCl₃ solution were subjected to X-ray structure analyses. The statistical results for 36 single crystals showed that all the crystals had almost the same unit cell parameter, belonged to the same F4̅3m space group, and possessed the same structural formula [Na­(H₂O)₄AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]­(SO₄)₄·10H₂O. However, the crystals had two inverse absolute structures (denoted A and B), which had a crystallization ratio of 1:1. From Li⁺ to Cs⁺, with increasing volume of the cation coexisting in the mother solution, the degree of disorder of the four H₂O molecules in the Na­(H₂O)₄⁺ hydrated ion continuously decreased; they became ordered when the cation was Cs⁺. Absolute structures A and B are the first two infinite aluminum polycations connected by statistically occupied [(Na_1/4)₄(H₂O)₄]⁺ hydrated ions. The three-dimensional structure of the infinite Na−ε-K–Al₁₃ species can be regarded as the assembly of finite ε-K–Al₁₃ species linked by [(Na_1/4)₄(H₂O)₄]⁺ in a 1:1 ratio. In this assembly, each [(Na_1/4)₄(H₂O)₄]⁺ is connected to four ε-K–Al₁₃ and each ε-K–Al₁₃ is also connected to four [(Na_1/4)₄(H₂O)₄]⁺ in tetrahedral orientations to form a continuous rigid framework structure, which has an inverse spatial orientation between absolute structure A and B. This discovery clarifies that the ε-K–Al₁₃ (or ε-K–GaAl₁₂) species in Na­[MO₄Al₁₂(OH)₂₄(H₂O)₁₂]­(XO₄)₄·nH₂O (M = Al, Ga; X = S, Se; n = 10–20) exists as discrete groups and deepens understanding of the formation and evolution process of polyaluminum species in forcibly hydrolyzed aluminum salt solution. The reason why Na⁺ statistically occupies the four sites was examined, and a formation and evolution mechanism of the infinite Na−ε-K–Al₁₃ species was proposed.