Dipartimento di Chimica e Biologia "A. Zambelli", Università di Salerno, Via Giovanni Paolo II 132, I-84084 Salerno, Italy.
Molecules. 2023 Jun 13;28(12):4749. doi: 10.3390/molecules28124749.
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.
六氧杂环十六烷杯[6]芳烃导致线性和支链烷基铵客体与空腔配位，在 CDCl 溶液中表现出构象自适应行为。线性戊基铵客体以牺牲 1,2,3-交替构象为代价诱导杯[6]芳烃的锥形构象，而在没有客体的情况下，1,2,3-交替构象是杯[6]芳烃最丰富的构象。以不同的方式，支链烷基铵客体，如正丁基铵和异丙基铵，选择 1,2,3-交替构象作为有利的 构象 (/⊂)，但也揭示了其他采用不同构象的配合物，即 /⊂、/⊂和 /⊂。通过 NMR 实验确定的结合常数值表明，1,2,3-交替构象是支链烷基铵客体配位的最佳拟合杯[6]芳烃构象，其次是锥形构象>paco 构象>1,2-alt 构象。我们的 NCI 和 NBO 计算表明，客体铵基团与杯芳烃之间的氢键相互作用 (N-H···O) 是四个配合物稳定顺序的主要决定因素。这些相互作用被客体的空间位阻增加所削弱，从而导致结合亲和力降低。1,2,3-alt 和锥形构象可能存在两个稳定氢键，而其他 paco 和 1,2-alt 立体异构体仅可能存在一个氢键。
