Binding of Inorganic Oxoanions to Macrocyclic Ligands: Effect of the Degree of Protonation on Supramolecular Assemblies Formed by Phosphate and [18]aneN6

Five macrocycle-oxoanion adducts have been isolated from aqueous solutions containing 1,4,7,10,13,16-hexaazacyclooctadecane ([18]aneN6, L) and phosphoric acid whose pH had been adjusted to selected values in
the 1−8 range. Four products, (H6L)(H2PO4)6·2H3PO4 (1), (H6L)(H2PO4)6 (2), (H4L)(H2PO4)4·2H2O (4), and
(H4L)(HPO4)2·7H2O (5) crystallized from aqueous solutions at pH 1, 3, 6, and 8, respectively, while (H4L)(H2PO4)4
(3) crystallized on diffusion of EtOH into an aqueous reaction mixture at pH 6. Single-crystal X-ray structure
determinations enabled an examination of supramolecular interactions between protonated forms of [18]aneN6,
phosphoric acid and its conjugate bases, and water of solvation. The macrocycle adopts a variety of conformations
in order to accommodate the supramolecular constructs formed by the oxoanions and solvent molecules as the
relative proportions of interacting species are altered. At pH 1 and 3, the fully protonated macrocycle, [LH6]6+, is
found with six H2PO4- anions. At pH 6 and 8, the tetraprotonated macrocycle, [LH4]4+, crystallizes with four H2PO4-
and two HPO42-, respectively. Variations in the solute of crystallization are evident, with phosphoric acid being
present at the lowest pH and water at pH 6 and 8. In 5, the seven unique water molecules form a string-of-pearls
motif within which a new heptameric isomer, consisting of a water pentamer that uses a single water to interact
with the other two unique water molecules, is found. Structures 1, 2, 4, and 5 exhibit η-3 H-bonding of ammonium
protons to a single oxygen of the guest phosphates located above and below the macrocyclic ring. In 3, two
phosphate oxygens of the cavity anion interact with the macrocycle, one of which participates in η-2 H-bonding
with ammonium groups.

五种大环-氧负离子加合物已从含有1,4,7,10,13,16-六氮杂环十八烷([18]aneN6, L)和磷酸的溶液中分离出来，其中溶液的pH值已被调节至1至8的选定范围内。在pH值为1、3、6和8时，四种产物分别结晶，即(H6L)(H2PO4)6·2H3PO4 (1)、(H6L)(H2PO4)6 (2)、(H4L)(H2PO4)4·2H2O (4)和(H4L)(HPO4)2·7H2O (5)，而(H4L)(H2PO4)4 (3)则在pH 6时通过乙醇扩散进入水相反应混合物中结晶。单晶X射线衍射分析使得对[18]aneN6的质子化形式、磷酸及其共轭碱以及溶剂水的超分子相互作用进行了研究。大环分子根据相互作用的物种相对比例的变化，采取多种构象以适应由氧负离子和溶剂分子形成的超分子结构。在pH 1和3时，完全质子化的六质子化大环[LH6]6+与六个H2PO4-阴离子结合。在pH 6和8时，四质子化的大环[LH4]4+分别与四个H2PO4-和两个HPO42-结合。结晶溶剂的变化显而易见，磷酸在最低pH值时存在，而水则在pH 6和8时存在。在5中，七个独特的水分子形成一串珍珠状结构，其中发现了一种新的七元异构体，由一个五元水分子和一个与另外两个独特水分子相互作用的单水分子组成。结构1、2、4和5表现出铵离子质子与位于大环上下的客磷酸单氧的η-3氢键结合。在3中，腔阴离子的两个磷酸氧与大环相互作用，其中一个参与与铵基的η-2氢键结合。