| Summary: | Hexahexyloxycalix[6]arene <b>2b</b> leads to the <i>endo</i>-cavity complexation of linear and branched alkylammonium guests showing a conformational adaptive behavior in CDCl<sub>3</sub> solution. Linear <i>n</i>-pentylammonium guest <b>6a<sup>+</sup></b> induces the cone conformation of <b>2b</b> at the expense of the 1,2,3-alternate, which is the most abundant conformer of <b>2b</b> in the absence of a guest. In a different way, branched alkylammonium guests, such as <i>tert</i>-butylammonium <b>6b<sup>+</sup></b> and isopropylammonium <b>6c<sup>+</sup></b>, select the 1,2,3-alternate as the favored <b>2b</b> conformation (<b>6b<sup>+</sup></b>/<b>6c<sup>+</sup></b>⊂<b>2b</b><b><i><sup>1,2,3-alt</sup></i></b>), but other complexes in which <b>2b</b> adopts different conformations, namely, <b>6b<sup>+</sup></b>/<b>6c<sup>+</sup></b>⊂<b>2b</b><b><i><sup>cone</sup></i></b>, <b>6b<sup>+</sup></b>/<b>6c<sup>+</sup></b>⊂<b>2b</b><b><i><sup>paco</sup></i></b>, and <b>6b<sup>+</sup></b>/<b>6c<sup>+</sup></b>⊂<b>2b</b><b><i><sup>1,2-alt</sup></i></b>, have also been revealed. Binding constant values determined via NMR experiments indicated that the 1,2,3-alternate was the best-fitting <b>2b</b> conformation for the complexation of branched alkylammonium guests, followed by cone > paco > 1,2-alt. Our NCI and NBO calculations suggest that the H-bonding interactions (<sup>+</sup>N–H···O) between the ammonium group of the guest and the oxygen atoms of calixarene <b>2b</b> are the main determinants of the stability order of the four complexes. These interactions are weakened by increasing the guest steric encumbrance, thus leading to a lower binding affinity. Two stabilizing H-bonds are possible with the 1,2,3-alt- and cone-<b>2b</b> conformations, whereas only one H-bond is possible with the other paco- and 1,2-alt-<b>2b</b> stereoisomers.
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