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.

Stereochemistry of molecular figures-of-eight

A trans isomer of a figure-of-eight (Fo8) compound was prepared from an electron-withdrawing cyclobis(paraquat-p-phenylene) derivative carrying trans-disposed azide functions between its two phenylene rings. Copper(I)-catalyzed azide-alkyne cycloadditions with a bispropargyl derivative of a polyether chain, interrupted in its midriff by an electron-donating 1,5-dioxynaphthalene unit acting as the template to organize the reactants prior to the onset of two click reactions, afforded the Fo8 compound with C(i) symmetry. Exactly the same chemistry is performed on the cis-bisazide of the tetracationic cyclophane to give a Fo8 compound with C(2) symmetry. Both of these Fo8 compounds exist as major and very minor conformational isomers in solution. The major conformation in the trans series, which has been characterized by X-ray crystallography, adopts a geometry which maximizes its C-H···O interactions, while maintaining its π···π stacking and C-H···π interactions. Ab initio calculations at the M06L level support the conformational assignments to the major and minor isomers in the trans series. Dynamic (1)H NMR spectroscopy, supported by 2D (1)H NMR experiments, indicates that the major and minor isomers in both the cis and trans series equilibrate in solution on the (1)H NMR timescale rapidly above and slowly below room temperature.

Putting anion-π interactions into perspective

Supramolecular chemistry is a field of scientific exploration that probes the relationship between molecular structure and function. It is the chemistry of the noncovalent bond, which forms the basis of highly specific recognition, transport, and regulation events that actuate biological processes. The classic design principles of supramolecular chemistry include strong, directional interactions like hydrogen bonding, halogen bonding, and cation-π complexation, as well as less directional forces like ion pairing, π-π, solvophobic, and van der Waals potentials. In recent years, the anion-π interaction (an attractive force between an electron-deficient aromatic π system and an anion) has been recognized as a hitherto unexplored noncovalent bond, the nature of which has been interpreted through both experimental and theoretical investigations. The design of selective anion receptors and channels based on this interaction represent important advances in the field of supramolecular chemistry. The objectives of this Review are 1) to discuss current thinking on the nature of this interaction, 2) to survey key experimental work in which anion-π bonding is demonstrated, and 3) to provide insights into the directional nature of anion-π contact in X-ray crystal structures.

Synthetic molecular motors and mechanical machines

The widespread use of controlled molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems, which by and large rely upon electronic and chemical effects to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movements of smaller parts to perform specific tasks. This is a scientific area of great contemporary interest and extraordinary recent growth, yet the notion of molecular-level machines dates back to a time when the ideas surrounding the statistical nature of matter and the laws of thermodynamics were first being formulated. Here we outline the exciting successes in taming molecular-level movement thus far, the underlying principles that all experimental designs must follow, and the early progress made towards utilizing synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.

Toward Electrochemically Controllable Tristable Three-Station [2]Catenanes

Encouraged by the prospect of producing an electrochemical, color-switchable red-green-blue (RGB) dye compound, we have designed, synthesized, and characterized two three-station [2]catenanes. Both are composed of macrocyclic polyethers containing three pi-electron-rich stations, which act as recognition sites for a pi-electron-deficient tetracationic cyclophane. The molecular structures of the two three-station [2]catenanes were characterized fully by mass spectrometry and 1H NMR spectroscopy. To anticipate the relative occupancies of the three stations in each [2]catenane by the cyclophane, model compounds with the same constitutions in the vicinity of the stations were synthesized. The relative ground-state populations of the three stations occupied in both [2]catenanes were estimated from the thermodynamic parameters for 1:1 complexes between all these model compounds and the cyclophane, obtained from isothermal titration calorimetry (ITC). The electrochemical and electromechanical properties of the three-station [2]catenanes were analyzed by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and spectroelectrochemistry (SEC). The first three-station [2]catenane was found to behave like a bistable system, whereas the second can be described as a quasi-tristable system.

Synthesis and Self-Assembly of a Rigid Exotopic Bisphenanthroline Macrocycle: Surface Patterning and a Supramolecular Nanobasket

The synthesis and characterisation of a rigid nanoscale macrocycle with two exotopic phenanthroline binding sites is reported. Scanning tunnelling microscopy (STM) at the solid-liquid interface reveals the formation of highly ordered monolayers of macrocycles with dimensions that are in good agreement with the calculated structure. Using the HETPHEN concept several bisheteroleptic coordination complexes with other phenanthrolines and a nanoscale basket assembly were prepared in presence of copper(I) ions. NMR spectroscopy, mass spectrometric data and elemental analysis point to three distinct isomers of the basket assembly in solution. A silver basket was prepared and readily converted to its copper analogue. Electrospray ionisation (ESI)-MS and spectrophotometric investigations provided additional mechanistic insight into the assembly process. Hence, the exotopic bisphenanthroline macrocycle in combination with HETPHEN concept proves to be very effective in controlling the compositional aspects of multicomponent self-assembly.

Self-assembly of dialkyltin(IV) moieties and aromatic dicarboxylates to complexes with a polymeric or a discrete trinuclear macrocyclic structure in the solid state and a mixture of fast interchanging cyclooligomeric structures in solution

It is well-known that the structures of trialkyltin(IV) carboxylates can be either monomeric, polymeric, or cyclooligomeric in the solid state. In contrast, all dialkyltin(IV) dicarboxylates characterized so far in the solid state have monomeric or polymeric structures, however, for some cases it has been proposed that their solution-state structure is cyclooligomeric. In order to generate more information on this subject, dimethyl- and di-n-butyltin(IV) complexes with phthalic and isophthalic acid have been prepared and analyzed both in solution and in the solid state. The solid-state structures of the two dialkyltin(IV) phthalates examined herein contain polymeric molecular chains, however, with supramolecular Sn.O' interactions, which result in the generation of cyclooligomeric units. This provides evidence for the presence of discrete cyclooligomeric structures in solution, which are involved in fast dynamic exchange equilibria as evidenced by (1)H, (13)C, and (119)Sn NMR spectroscopy. In the case of the two dialkyltin(IV) isophthalate complexes studied herein (R = Me, n-Bu), only the di-n-butyltin derivative is soluble and NMR spectroscopy as well as FAB(+) spectrometry indicates the formation of cyclic dinuclear, trinuclear, and/or tetranuclear species in solution, which may be involved also in fast dynamic exchange equilibria. In the solid state, however, discrete cyclotrinuclear units can be identified, in which the 24-membered macrocyclic cavity is almost completely planar, having six oxygen atoms directed into its interior and six Sn-n-butyl groups approximately perpendicular to the molecular plane. The diameter of the cyclic cavity can be described by the transannular O.O distances that vary from 7.68 to 7.84 A, being large enough for the introduction of linear alkyl groups. This can be demonstrated by the supramolecular structure of this compound, which contains a new type of bis[2]pseudorotaxane formed between two molecules through mutual threading via two of the Sn-butyl groups. Such a supramolecular entity has been unknown so far, since the usual composition of bis[2]pseudorotaxanes is the trimolecular combination of a macrocyclic ring system with two threads.