Molecular Recognition of Trigonal Oxyanions Using a Ditopic Salt Receptor: Evidence for Anisotropic Shielding Surface around Nitrate Anion

A ditopic, macrobicyclic receptor with adjacent anion and cation binding sites is able to extract
a range of monovalent salts into chloroform solution. The structures of the receptor complexed with KAcO,
LiNO3, NaNO3, KNO3, and NaNO2 are characterized in solution by NMR spectroscopy and in the solid
state by X-ray crystallography. The sodium and potassium salts are bound to the receptor as contact ion-pairs, with the metal cation located in the receptor's crown ether ring and the trigonal oxyanion hydrogen
bonded to the receptor NH residues. The solid-state structure of the LiNO3 complex has a bridging water
molecule between the cation and anion. In all solid-state structures, the trigonal oxyanion is not located
symmetrically inside the receptor cavity. It appears that anion orientation is controlled by a complex interplay
of steric factors, coordination bonding to the metal cation, and hydrogen bonding with the receptor NH
residues. An important feature with this latter effect is the fact that hydrogen bonds directed toward the
oxygen lone pairs on a trigonal oxyanion are stronger than hydrogen bonds to the π-electrons. In solution,
the 1H NMR spectra of the nitrate and nitrite salt complexes are noteworthy because several receptor
signals, including the NH protons, undergo unusual upfield movements in chemical shift upon complexation.
This is a reflection of the diamagnetic anisotropy of these trigonal oxyanions. The magnetic shielding surface
for the NO3- anion is calculated using density functional theory and shown to have a shielding region
directly above the central nitrogen.

一种带有相邻阴、阳离子结合位点的双位点大环双环受体（ditopic macrobicyclic receptor），可将多种一价盐萃取至氯仿溶液中。该受体与乙酸钾（KAcO）、硝酸锂（LiNO₃）、硝酸钠（NaNO₃）、硝酸钾（KNO₃）及亚硝酸钠（NaNO₂）形成的复合物，已通过溶液态核磁共振波谱法（NMR spectroscopy）与固态X射线晶体学（X-ray crystallography）完成结构表征。钠盐与钾盐以接触离子对（contact ion-pairs）的形式与受体结合：金属阳离子位于受体的冠醚环（crown ether ring）内，三角平面含氧阴离子（trigonal oxyanion）则通过氢键与受体的NH残基（NH residues）相互作用。硝酸锂复合物的固态结构中，阳离子与阴离子之间存在一个桥连水分子（bridging water molecule）。在所有固态复合物结构中，三角平面含氧阴离子均未对称分布于受体空腔内部。研究表明，阴离子的取向受到空间位阻因素（steric factors）、与金属阳离子的配位键（coordination bonding）作用以及与受体NH残基的氢键作用三者的复杂协同调控。该氢键作用的一项重要特征为：指向三角平面含氧阴离子氧原子孤对电子的氢键，其强度要高于针对其π电子的氢键。在溶液中，硝酸根与亚硝酸根盐复合物的氢谱（¹H NMR spectra）具有显著特征：包括NH质子在内的多个受体信号峰，在形成复合物后会发生反常的高场化学位移。这一现象源于这些三角平面含氧阴离子的抗磁各向异性（diamagnetic anisotropy）。研究人员通过密度泛函理论（density functional theory）计算了硝酸根（NO₃⁻）阴离子的磁屏蔽表面（magnetic shielding surface），结果显示其中心氮原子上方存在一个屏蔽区域。