Structural macrocyclic supramolecular chemistry

We are delighted to assemble this issue of CrystEngCommun on the field of macrocyclic supramolecular chemistry. Since its birth, supramolecular chemistry has been heavily focused on macrocyclic systems. The heavy focus is owing to the pioneering work of Cram, Pedersen and Lehn who in the late 1960's developed macrocycles able to selectively bind ions and/or molecules. In 1987, their work on spherands, crown ethers and cryptands was awarded a Nobel Prize under the title “for their development and use of molecules with structure-specific interactions of high selectivity”. One year later, Lehn formulated a new branch of chemistry, termed supramolecular chemistry¹ as “chemistry beyond the molecule”. In crystal terms, supramolecular chemistry can be, and is often, deemed synonymous to what is a cornerstone of supramolecular chemistry, namely, host–guest chemistry wherein there are continuous and vigorous efforts by researchers worldwide to understand and exploit the binding properties of macrocycles to form solids with useful properties and functions (e.g. recognition, sensing, separations and storage).

Given that single-crystal X-ray crystallographic studies can now be accomplished on timescales of hours to minutes, our understandings of host–guest chemistry and supramolecular complexes are, indeed, advancing at an ever increasing rate. In is now clear, for example, that macrocycles are to be considered well-defined building blocks, or essentially, tools to construct or crystal engineer² organic and metal–organic solids with complex topologies and more downstream functions. Here, we present a collection of contributions that shed light on the status of macrocyclic supramolecular chemistry and, we expect, will provide a foundation for new creative endeavors in the exciting area.

In the themed issue, accounts on contemporary research that involve cyclotriveratrylenes, calixarenes, resorcinarenes, cyclens, and cucurbiturils, as well as imidazole and pyridine-based cyclophanes, are presented.

Thus, lanthanide coordination polymers with pyridyl-N-oxide or carboxylate functionalized CTV ligands are reported by Hardie et al. There are also six contributions based on calixarenes. In particular, Gruber and Weber et al. report on dinitro-substituted calix[4]arenes that form a honeycomb-like architecture with hydrophobic channels. Molecular tectonics based on calixarenes that generate grid and porous diamondoid coordination networks are also described by Ferlay and Hosseini et al. Gorbatchuk et al. report an anti-sieve effect in guest inclusion by thiacalix[4]arene with enhanced thermal stability, while a ladder-type coordination polymer involving two different macrocyclic building units, viz. calix[4]arene tetracarboxylate and a hexaazamacrocyclic nickel(II) complex, is reported by Lampeka et al. Oliver et al. also demonstrate how a C-shaped p-sulfonatocalix[4]arene acts a building block for mutual-inclusion and bilayer insertion of dipicolinate. Bombicz et al. also highlight calixarene-based synthon engineering for the fine tuning of crystal lattices.

Resorcinares are represented in two studies. Firstly, the use of a macromolecular structure solution program PHASER is demonstrated by Szumna, Jaskolski, and Rissanen in a study of a dimeric resorcinare capsule. Secondly, complexation and intermolecular interactions involving deprotonated resorcinarenes is reported by Rissanen.

Various heterocyclic building blocks can be used in the construction of macrocyclic host molecules. Thus, two articles by Beer et al. describe a ferrocene imidazolium-based macrocycle as an electrochemical chemosensor for halide anions and a very topical³ study of an observation of strong halogen bonds to anions by bis-haloimidazolium macrocycles. In related work, a cyclen derivative is shown to self-assemble to discrete and polymeric metallosupramolecular architectures by Batten and Turner et al.

Cucurbiturils are an ongoing and hot topic in supramolecular macrocyclic chemistry owing to the formation of very stable inclusion complexes with spherical hydrocarbons. From a crystallographic perspective, cucurbiturils still remain relatively unexplored. In this context, Danylyuk et al. report on an unexpected crystallization of the metastable tubular coordination polymer of cucurbit[6]uril with magnesium ions. As a complement to the variety of the single-crystal X-ray studies of macrocycles presented in the themed issue, Ratcliffe et al. report the use of high-field solid-state ¹³C NMR spectroscopy as a very promising tool to study crystalline cucurbituril materials.

We hope you enjoy these timely and important contributions to the themed issue of the journal.

References

1. J.-M. Lehn, Angew. Chem., Int. Ed. Engl., 1988, 27, 89–112.
2. G. R. Desiraju, J. Am. Chem. Soc., 2013, 135, 9952–9967.
3. G. R. Desiraju, P. S. Ho, L. Kloo, A. C. Legon, R. Marquardt, P. Metrangolo, P. A. Politzer, G. Resnati and K. Rissanen, Pure Appl. Chem., 2013, 85, 1711–1713.

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