Are the Physical Properties of Xe@Cryptophane Complexes Easily Predictable? The Case of <i>syn</i>- and <i>anti</i>-Tris-aza-Cryptophanes

Abstract

We report the synthesis and optical resolution of <i>C</i>₃-symmetrical tris-aza-cryptophanes <i>anti</i>-<b>3</b> and <i>syn</i>-<b>4</b>, as well as the study of their interaction with xenon <i>via</i> hyperpolarized ¹²⁹Xe NMR. These molecular cages are close structural analogues of the two well-known cryptophane-A (<b>1</b>; chiral) and cryptophane-B (<b>2</b>; achiral) diastereomers since these new compounds differ only by the presence of three nitrogen atoms grafted onto the same cyclotribenzylene unit. The assignment of their relative (syn vs anti) and absolute configurations was made possible, thanks to the combined use of quantum calculations at the density functional theory level and vibrational circular dichroism spectroscopy. More importantly, our results show that despite the large structural similarities with cryptophane-A (<b>1</b>) and -B (<b>2</b>), these two new compounds show a very different behavior in the presence of xenon in organic solutions. These results demonstrate that prediction of the physical properties of the xenon@cryptophane complexes, only based on structural parameters, remains extremely difficult.