Hexahexyloxycalix[6]arene, a Conformationally Adaptive Host for the Complexation of Linear and Branched Alkylammonium Guests.

Hexahexyloxycalix[6]arene leads to the -cavity complexation of linear and branched alkylammonium guests showing a conformational adaptive behavior in CDCl solution. Linear -pentylammonium guest induces the cone conformation of at the expense of the 1,2,3-alternate, which is the most abundant conformer of in the absence of a guest. In a different way, branched alkylammonium guests, such as -butylammonium and isopropylammonium , select the 1,2,3-alternate as the favored conformation (/⊂), but other complexes in which adopts different conformations, namely, /⊂, /⊂, and /⊂, have also been revealed. Binding constant values determined via NMR experiments indicated that the 1,2,3-alternate was the best-fitting 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 (N-H···O) between the ammonium group of the guest and the oxygen atoms of calixarene 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- conformations, whereas only one H-bond is possible with the other paco- and 1,2-alt- stereoisomers.