The Story of the Little Blue Box: A Tribute to Siegfried Hünig

The tetracationic cyclophane, cyclobis(paraquat-p-phenylene), also known as the little blue box, constitutes a modular receptor that has facilitated the discovery of many host-guest complexes and mechanically interlocked molecules during the past 35 years. Its versatility in binding small π-donors in its tetracationic state, as well as forming trisradical tricationic complexes with viologen radical cations in its doubly reduced bisradical dicationic state, renders it valuable for the construction of various stimuli-responsive materials. Since the first reports in 1988, the little blue box has been featured in over 500 publications in the literature. All this research activity would not have been possible without the seminal contributions carried out by Siegfried Hünig, who not only pioneered the syntheses of viologen-containing cyclophanes, but also revealed their rich redox chemistry in addition to their ability to undergo intramolecular π-dimerization. This Review describes how his pioneering research led to the design and synthesis of the little blue box, and how this redox-active host evolved into the key component of molecular shuttles, switches, and machines.

Through-the-annulus threading of the larger calix[8]arene macrocycle

A complete study of the through-the-annulus threading of the larger calix[8]arene macrocycle with di-n-alkylammonium cations has been performed in the presence of the "superweak" TFPB counterion. Thus, it was found that such threading occurs only upon partial preorganization of the calix[8]arene macroring by intramolecular bridging. In particular, 1,5-bridged calix[8]arenes with a meta- or para-xylylene bridge (2 and 3) gave pseudo[2]rotaxanes in which one dialkylammonium axle (4a-4e(+)) was threaded into one of the two subcavities of the calix[8]-wheel. Conformational studies by using chemical shift surface maps and DFT calculations evidenced a 3/4-cone geometry for these subcavities. Higher pseudorotaxane K(ass) values were obtained for calix[8]-wheels 2 and 3, with respect to calix[6]-host 1a, due to the cooperative effect of their two subcavities. Dynamic NMR studies on calix[8]-pseudorotaxanes evidenced a direct correlation between K(ass) (and ΔG(ass)) values and energy barriers for calix inversion due to the effectiveness of thread templation. In accordance with DFT calculations, an endo-alkyl preference, over the endo-benzyl one, was observed by threading calix[8]-wheel 3 with the directional n-butylbenzylammonium axle 4d(+).

A pH-sensitive lasso-based rotaxane molecular switch

The synthesis of a pH-sensitive two-station [1]rotaxane molecular switch by self-entanglement of a non-interlocked hermaphrodite molecule, containing an anilinium and triazole moieties, is reported. The anilinium was chosen as the best template for the macrocycle benzometaphenylene[25]crown-8 (BMP25C8) and allowed the self-entanglement of the molecule. The equilibrium between the hermaphrodite molecule and the pseudo[1]rotaxane was studied by (1)H NMR spectroscopy: the best conditions of self-entanglement were found in the less polar solvent CD(2)Cl(2) and at high dilution. The triazole moiety was then benzylated to afford a benzyltriazolium moiety, which then played a dual role. On one hand, it acts as a bulky gate to trap the BMP25C8, thus to avoid any self-disentanglement of the molecular architecture. On another hand, it acts as a second molecular station for the macrocycle. At acidic pH, the BMP25C8 resides around the best anilinium molecular station, displaying the lasso [1]rotaxane in a loosened conformation. The deprotonation of the anilinium molecular station triggers the shuttling of the BMP25C8 around the triazolium moiety, therefore tightening the lasso.

Catenation of calixarene annulus

Through-the-annulus-catenated calixarenes have been obtained, for the first time, by exploiting the "superweak anion" approach that allows the threading of the calix cavity with functionalized dialkylammonium axles. In addition, the first example of a stereoprogrammed synthesis of a catenane orientational isomer (an oriented calix[2]catenane) has been obtained, after macrocyclization, by using a directional alkylbenzylammonium axle.