Calix[4]arene based molecular container for water

Calix[n]arenes are polyphenolic platforms used for the design of molecular receptors in host-guest chemistry. We have previously described the use of bulky tert-butylaminocarbonyl (i.e. Bac) and tert-butoxycarbonyl (i.e. Boc) groups for selective functionalization of calixarenes and also for the development of calix[6]arene based molecular boxes. Recently, we have synthesized a calix[4]arene derivative bearing four bulky Bac groups (i.e. p-tBu-calix[4]arene-tetra-Bac), and it was found that this compound binds water selectively and rather strongly. X-ray diffraction and solution-state NMR spectroscopy revealed that the receptor exhibits a 1,3-alternate conformation with two buried binding sites, leading to full encapsulation of up to two water molecules. The selectivity toward water is explained by the combination of suitably oriented H-bond donors and a tiny inner cavity (69 Å3 = 0.07 yL). In addition, both the first and second association constants Ka1 and Ka2 could be determined by NMR spectroscopy and revealed a weak positive cooperativity. Considering its shape and its ability to encapsulate two water molecules, p-tBu-calix[4]arene-tetra-Bac can be seen as the smallest water bottle known so far.0