Chemistry and Structure
of a Host–Guest Relationship:
The Power of NMR and X‑ray Diffraction in Tandem
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Abstract
An amine/amide mixed covalent organic tetrahedral cage <b>1</b> (<i><b>H</b></i><sub><b>12</b></sub>) was
synthesized and characterized. The <i><b>H</b></i><sub><b>12</b></sub> cage contains 12 amide NH groups plus
four tertiary amine N groups, the latter of which are positioned in
a pseudo-tetrahedral array. Crystallographic findings indicate that
the tetrahedral host can adopt either a pseudo-<i>C</i><sub>3</sub> symmetric “compressed tetrahedron” structure,
or one in which there are two sets of three stacked pyridine units
related by a pseudo-S<sub>4</sub> axis. The latter conformation is
ideal for encapsulating small pentameric clusters, either a water
molecule or a fluoride ion surrounded by a tetrahedral array of water
molecules, i.e., H<sub>2</sub>O·4H<sub>2</sub>O or F<sup>–</sup>·4H<sub>2</sub>O, as observed crystallographically. In solution,
however, <sup>19</sup>F NMR spectroscopy indicates that <i><b>H</b></i><sub><b>12</b></sub> encapsulates fluoride
ion through direct amide hydrogen bonding. By collectively combining
one-dimensional <sup>1</sup>H, <sup>13</sup>C, and <sup>19</sup>F
with two-dimensional <sup>1</sup>H–<sup>1</sup>H COSY, <sup>1</sup>H–<sup>13</sup>C HSQC, and <sup>1</sup>H–<sup>19</sup>F HETCOR NMR techniques, the solution binding mode of fluoride
can be ascertained as consisting of four sets of independent structural
subunits with <i>C</i><sub>3</sub> symmetry. A complex deuterium
exchange process for the fluoride complex can also be unraveled by
multiple NMR techniques