Tetrameric Cyclic Double Helicates as a Scaffold for a Molecular Solomon Link

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.

Triple Self-Sorting in Constitutional Dynamic Networks: Parallel Generation of Imine-Based CuI , FeII , and ZnII Complexes

Three imine-based metal complexes, having no overlap in terms of their compositions, have been simultaneously generated from the self-sorting of a constitutional dynamic library (CDL) containing three amines, three aldehydes, and three metal salts. The hierarchical ordering of the stability of the three metal complexes assembled and the leveraging of the antagonistic and agonistic relationships existing between the constituents within the constitutional dynamic network corresponding to the CDL were pivotal in achieving the sorting. Examination of the process by NMR spectroscopy showed that the self-sorting of the FeII and ZnII complexes depended on an interplay between the thermodynamic driving forces and a kinetic trap involved in their assembly. These results also exemplify the concept of "simplexity"-the fact that the output of a self-assembling system may be simplified by increasing its initial compositional complexity-as the two complexes could self-sort only in the presence of the third pair of organic components, those of the CuI complex.

A Six-Crossing Doubly Interlocked [2]Catenane with Twisted Rings, and a Molecular Granny Knot

A molecular 6 2 3 link (a six crossing, doubly interlocked, [2]catenane with twisted rings) and a 31 #31 granny knot (a composite knot made up of two trefoil tangles of the same handedness) were constructed by ring-closing olefin metathesis of an iron(II)-coordinated 2×2 interwoven grid. The connections were directed by pendant phenyl groups to be between proximal ligand ends on the same faces of the grid. The 6 2 3 link was separated from the topoisomeric granny knot by recycling size-exclusion chromatography. The identity of each topoisomer was determined by tandem mass spectrometry and the structure of the 6 2 3 link confirmed by X-ray crystallography, which revealed two 82-membered macrocycles, each in figure-of-eight conformations, linked through both pairs of loops.

Molecular Knots

The first synthetic molecular trefoil knot was prepared in the late 1980s. However, it is only in the last few years that more complex small-molecule knot topologies have been realized through chemical synthesis. The steric restrictions imposed on molecular strands by knotting can impart significant physical and chemical properties, including chirality, strong and selective ion binding, and catalytic activity. As the number and complexity of accessible molecular knot topologies increases, it will become increasingly useful for chemists to adopt the knot terminology employed by other disciplines. Here we give an overview of synthetic strategies towards molecular knots and outline the principles of knot, braid, and tangle theory appropriate to chemistry and molecular structure.

Tight Embrace in a Molecular Knot with Eight Crossings

When it takes eight to tango: Since the synthesis of the simplest of all knots, the trefoil knot, in 1989, the formation of more complex knots has remained extremely challenging. The synthesis of the most complex molecular knot made to date, the 8₁₉ , by Leigh and co-workers thus represents a major step towards building topologically complex architectures.

A Solomon link through an interwoven molecular grid

A molecular Solomon link was synthesized through the assembly of an interwoven molecular grid consisting of four bis(benzimidazolepyridyl)benzthiazolo[5,4-d]thiazole ligands and four zinc(II), iron(II), or cobalt(II) cations, followed by ring-closing olefin metathesis. NMR spectroscopy, mass spectrometry, and X-ray crystallography confirmed the doubly interlocked topology, and subsequent demetalation afforded the wholly organic Solomon link. The synthesis, in which each metal ion defines the crossing point of two ligand strands, suggests that interwoven molecular grids should be useful scaffolds for the rational construction of other topologically complex structures.

Catenanes: fifty years of molecular links

Half a century after Schill and Lüttringhaus carried out the first directed synthesis of a [2]catenane, a plethora of strategies now exist for the construction of molecular Hopf links (singly interlocked rings), the simplest type of catenane. The precision and effectiveness with which suitable templates and/or noncovalent interactions can arrange building blocks has also enabled the synthesis of intricate and often beautiful higher order interlocked systems, including Solomon links, Borromean rings, and a Star of David catenane. This Review outlines the diverse strategies that exist for synthesizing catenanes in the 21st century and examines their emerging applications and the challenges that still exist for the synthesis of more complex topologies.

Guest-induced transformation of a porphyrin-edged Fe(II)4L6 capsule into a Cu(I)Fe(II)2L4 fullerene receptor

The combination of a bent diamino(nickel(II) porphyrin) with 2-formylpyridine and Fe(II) yielded an Fe(II) 4 L6 cage. Upon treatment with the fullerenes C60 or C70 , this cage was found to transform into a new host-guest complex incorporating three Fe(II) centers and four porphyrin ligands, in an arrangement that is hypothesized to maximize π interactions between the porphyrin units of the host and the fullerene guest bound within its central cavity. The new complex shows coordinative unsaturation at one of the Fe(II) centers as the result of the incommensurate metal-to-ligand ratio, which enabled the preparation of a heterometallic cone-shaped Cu(I) Fe(II) 2 L4 adduct of C60 or C70 .

A simple and highly effective ligand system for the copper(I)-mediated assembly of rotaxanes

A [2]rotaxane was produced through the assembly of a picolinaldehyde, an amine, and a bipyridine macrocycle around a Cu(I) template by imine bond formation in close-to-quantitative yield. An analogous [3]rotaxane is obtained in excellent yield by replacing the amine with a diamine, thus showing the suitability of the system for the construction of higher order interlocked structures. The rotaxanes are formed within a few minutes simply through mixing the components in solution at room temperature and they can be isolated through removal of the solvent or precipitation.

The self-sorting behavior of circular helicates and molecular knots and links

We report on multicomponent self-sorting to form open circular helicates of different sizes from a primary monoamine, Fe(II) ions, and dialdehyde ligand strands that differ in length and structure by only two oxygen atoms. The corresponding closed circular helicates that are formed from a diamine--a molecular Solomon link and a pentafoil knot--also self-sort, but up to two of the Solomon-link-forming ligand strands can be accommodated within the pentafoil knot structure and are either incorporated or omitted depending on the stage that the components are mixed.