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 [(Na1/4)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 [(Na1/4)4(H2O)4]+ in a 1:1 ratio. In this assembly,
each [(Na1/4)4(H2O)4]+ is connected to four ε-K–Al13 and
each ε-K–Al13 is also connected to four [(Na1/4)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.

该研究确定了由Na-(H2O)4+-连接的无限ε-凯金-铝13物种（Na-ε-K-Al13）的绝对结构，这些物种互为倒转结构和镜像结构。通过向NaOH水解的AlCl3溶液中加入A2SO4（A代表Li、Na、K、Rb或Cs）溶液，获得的单晶进行了X射线结构分析。36个单晶的统计分析结果显示，所有晶体具有几乎相同的晶胞参数，属于同一F4̅3m空间群，并具有相同的结构式[Na-(H2O)4AlO4Al12(OH)24(H2O)12]-(SO4)4·10H2O。然而，晶体具有两种相反的绝对结构（标记为A和B），其结晶比为1:1。从Li+到Cs+，随着母液中阳离子体积的增加，Na-(H2O)4+水合离子的四个H2O分子的无序度持续降低；当阳离子为Cs+时，它们变得有序。绝对结构A和B是首次发现的通过统计占据的[(Na1/4)4(H2O)4]+水合离子连接的两个无限铝多价阳离子。无限Na-ε-K-Al13物种的三维结构可以视为有限ε-K-Al13物种通过[(Na1/4)4(H2O)4]+以1:1的比例连接的组装。在这个组装中，每个[(Na1/4)4(H2O)4]+连接到四个ε-K-Al13，每个ε-K-Al13也以四面体取向连接到四个[(Na1/4)4(H2O)4]+，形成一个连续的刚性框架结构，其中绝对结构A和B之间存在反空间取向。这一发现阐明，在Na-[MO4Al12(OH)24(H2O)12]-(XO4)4·nH2O（M=Al、Ga；X=S、Se；n=10-20）中的ε-K-Al13（或ε-K-GaAl12）物种以离散组的形式存在，并加深了对强制水解铝盐溶液中多铝物种形成和演化过程的理解。对Na+统计占据四个位置的原因进行了考察，并提出了无限Na-ε-K-Al13物种的形成和演化机制。