The selective recognition of substrates or cofactors is a key feature of biological processes. It involves coordination bonds, hydrogen bonding, charge/charge and charge/dipole interactions. In this Perspective, we describe how the calix[6]arene core can be functionalized and shaped to act as a biomimetic molecular receptor. The strategy relies on the selective introduction of three amino arms on alternate phenolic positions. Upon metal ion binding or self-assembly via multiple ion-pairing and H-bonding, these amino arms are projected towards each other, thus closing the calixarene small rim. The resulting cone-shaped receptors act as molecular funnels displaying high affinities for a variety of neutral guests. Their hosting properties can be finely tuned by changing the small or the large rim or by allosteric effects. Induced-fit processes are also often observed as the cavity can expand for large guests or shrink for small ones. Hence, the different families of calix[6]arene-based receptors presented here highlight the importance of having a flexible and polarized hydrophobic structure to accommodate the guest.
This critical review describes recent efforts in the field of chromophoric scaffolding. The advances in this research area, with an emphasis on rigid scaffolds, for example, synthetic polymers, carbon nanotubes (CNTs), nucleic acids, and viruses, are presented (166 references).
The straightforward syntheses of C3v symmetrical calix[6]trisureas and -thiourea have been achieved. NMR studies have shown that these flexible compounds possess a major cone conformation. While these neutral hosts can strongly bind anions such as AcO(-) or HSO4(-) through induced fit processes, they can also behave as unique heteroditopic receptors for organic ion pairs with a remarkable positive cooperativity in the complexation process, the anion acting as an allosteric effector.
Metal ion migration in a bis-strapped porphyrin ligand with overhanging carboxylate groups has been investigated in solution. Two types of homobimetallic complexes are generated with Pb(II) and Bi(III) cations, which stand on both sides of the macrocycle: (i) a dissymmetric complex with one cation bound to the porphyrin N core and the other cation hung over the N core through bonding with a carboxylate of a strap; (ii) a C(2)-symmetric complex with both cations coordinated to the N core and to the carboxylate groups of the straps. Variable-temperature NMR studies and 2D rotational Overhauser effect spectroscopy NMR experiments have shown that in the former dissymmetric complexes, the two cations undergo a coupled intramolecular migration resulting in exchange of their coordination modes. Such complexes constitute active states of Newton's cradle-like devices (NCDs), with the ion migration rate depending on the lability of the metal-ligand interactions [Pb(II) faster than Bi(III) NCDs]. On the other hand, the C(2)-symmetric complexes constitute either an inactive state [with Pb(II)] or a resting state [with Bi(III)] of an NCD, since they correspond respectively to a precursor or an intermediate in the motion of the cations. The NCDs are under both allosteric and acid-base control: (i) with Pb(II), the addition of an allosteric effector such as an acetate anion to the medium allows the conversion of the symmetric form to the dissymmetric one, thus triggering the Newton's cradle-like motion of the cations; (ii) with Bi(III), a lifted state was converted to a resting one by the addition of protons and then restored by the addition of a base. As an extension to nondegenerate systems, a heterobimetallic Bi(III)-Pb(II) complex was selectively obtained, and it constitutes a frozen lifted state of a dissymmetric NCD. All of these homo- and hetero-NCDs could be successively formed by selective metal ion exchange. These unique findings open the way to novel tristable devices.
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