An induced-fit process through mechanical pivoting of aromatic walls in host-guest chemistry of calix[6]arene aza-cryptands.

The per-ipso-nitration of a TMPA-capped calix[6]arene has been achieved. The substitution of the six bulky tBu substituents for nitro groups has a strong impact on the behavior of the ligand during guest recognition. The complexation of the aza cap (by H(+) or Cu(+)) associated with the encapsulation of a guest triggers an induced-fit process leading to the loss of the cone conformation of the host in favor of alternate conformations. Such a "pivoting" response of one or two walls of the calixarene core induces a large mechanical motion of the corresponding aromatic units. This stands in strong contrast with the "breathing" phenomena previously identified with other calix[6]arene-based complexes that expand or shrink the size of their cone as a function of the guest. Because of the covalently attached rigid TMPA cap, three arene units of this new calixarene host have a restricted mobility, which forces it to respond in a different manner to a supramolecular stress.