Cyclobis(paraquat-p-phenylene)

Cyclobis(paraquat-p-phenylene)
Identifiers
117271-76-8 (Tetracation)
117271-78-0 (Tetrachloride)
Properties
C36H32N4
Molar mass 520.663 g·mol−1
Appearance weißer Feststoff[1]
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Cyclobis(paraquat-p-phenylene) (formally a derivative of paraquat) belongs to the class of cyclophanes, since it consists of aromatic units bridged by aliphatic segments. It is able to incorporate a smaller molecule (a "guest") in it. It plays an important role in host-guest chemistry and is perhaps the best known cyclophane of supramolecular chemistry.[2]

It is also called Stoddart's blue box because its inventor, J. Fraser Stoddart, drew electron-poor areas in molecules in blue color.[3]

Synthesis

For synthesis of cyclobis(paraquat-p-phenylene) at first 4,4'-bipyridine is reacted with 1,4-bis(bromomethyl)benzene to 1,1′-[1,4-phenylenebis-(methylene)]bis(4,4′-bipyridine), which is reacted in a template synthesis again with 4,4′-bipyridine to the final product. As template serves 1,5-bis[2-(2-methoxyethoxy)ethoxy]naphthalene.[1]

Host guest chemistry

Cyclobis(paraquat-p-phenylene) ist able to incorporate a guest in itself, a host-guest complex is formed then. The interactions required for complex formation are donor-acceptor interactions, their strength is highly dependent on the ability of the donor to provide π-electron density - the complexation is the stronger the stronger the π-electron donor is. Also an enlargement of the π-system enhances the binding. However, the kinetics of complex formation and dissociation depends on the bulkiness of the guest.[4]

A molecule which is able to form stable complexes with cyclobis(paraquat-p-phenylene) is tetrathiafulvalene (TTF). Numerous derivatives are based on the chelating ability of tetrathiafulvalene. The modifications include mechanically entrapped compounds such as catenanes and rotaxanes, molecular switches and larger supramolecular structures.[4]

The charge-transfer interactions, present in complexes of cyclobis(paraquat-p-phenylene), can be compared as structural motif with the more commonly used hydrogen bonds, especially in terms of directionality and complementarity (lock-and-key model). Charge-transfer complexes are easierer zu detect by spectroscopic methodes and have a greater tolerance to various solvents, but also generally a lower association constant. Due to the lower association constant much less charge transfer complexes are known. Other non-covalent bonds (e. g. solvophobic forces, metal-ligand interaction) can be used to increase the association constant; numerous structures based on this strategy are known in literature.[5]

It was shown that the choice of the counterion of cyclobis(paraquat-p-phenylene) takes a big influence on the binding constant of the corresponding host-guest complex.[6] It is mostly used as hexafluorophosphate salt.

Utilization

To create catenanes, the cyclobis(paraquat-p-phenylene) can be used as a template to "thread" a crown ether with a π-donor component. Subsequently, its still open ends are linked with each other to obtain two closed rings.[7] A bistable catenane (a ring with two π-donor components) is already a simple example of a molecular switch. In the present example, a cyclic ether has been selected with a TTF and a DNP moiety. While the cyclobis(paraquat-p-phenylene) surroundings the TTF unit in the rest position, the DNP unit is stable when the TTF is (reversible) is oxidized. The ring rotates in this case due to the coulomb repulsion around itself until the cyclobis(paraquat-p-phenylene) encloses the DNP unit. A reverse movement occurs when the TTF unit is reduced again. This first example that proved the general feasibility, many more have followed.[8]

Derivates

Numerous derivatives of cyclobis(paraquat-p-phenylene) have been developed, including an enlarged version of the molecule, in the literature referred ExnBox4+, where n is the number of p-phenylene rings (n = 0-3).[2] These variants with larger apertures are capable to included larger, different sized molecules. Based on the charge-transfer complexation of CBPQT4 + many supramolecular structures have been created, including fibrillar gels, micelles, vesicles, nanotubes, foldamers and liquid crystalline phases. In analogy to biological systems, which are assembled by hydrogen bonds to form supramolecular structures, the charge-transfer complexation is here an alternative.[5]

References

  1. ↑ 1.0 1.1 Masumi Asakawa, Wim Dehaen, Gerrit L’abbé, Stephan Menzer, Jan Nouwen, Françisco M. Raymo, J. Fraser Stoddart, David J. Williams (01 1996), "Improved Template-Directed Synthesis of Cyclobis(paraquat-phenylene)" (in German), The Journal of Organic Chemistry 61 (26): pp. 9591–9595, doi:10.1021/jo961488i, ISSN 0022-3263
  2. ↑ 2.0 2.1 Jonathan C. Barnes, Michal Juríček, Nicolaas A. Vermeulen, Edward J. Dale, J. Fraser Stoddart (2013), "Synthesis of ExnBox Cyclophanes" (in German), The Journal of Organic Chemistry 78 (23): pp. 11962–11969, doi:10.1021/jo401993n
  3. ↑ Atwood, Jerry L.; Steed, Jonathan W. (2013). Supramolecular chemistry. Hoboken, N.J.: Wiley. ISBN 978-1-118-68150-3.
  4. ↑ 4.0 4.1 Nielsen, Mogens Brøndsted; Jeppesen, Jan Oskar; Lau, Jesper; Lomholt, Christian; Damgaard, Dorthe; Jacobsen, Jens Peter; Becher, Jan; Stoddart, J. Fraser. "Binding Studies between Tetrathiafulvalene Derivatives and Cyclobis(paraquat-phenylene)". The Journal of Organic Chemistry 66 (10): 3559–3563. doi:10.1021/jo010173m.
  5. ↑ 5.0 5.1 Das, Anindita; Ghosh, Suhrit (2014-02-17). "Supramolecular Assemblies by Charge-Transfer Interactions between Donor and Acceptor Chromophores". Angewandte Chemie International Edition 53 (8): 2038–2054. doi:10.1002/anie.201307756.
  6. ↑ Andersen, Sissel S.; Jensen, Morten; Sørensen, Anne; Miyazaki, Eigo; Takimiya, Kazuo; Laursen, Bo W.; Flood, Amar H.; Jeppesen, Jan O. "Anion effects on the cyclobis(paraquat-p-phenylene) host". Chemical Communications 48 (42): 5157. doi:10.1039/c2cc31225e.
  7. ↑ Miljanić, Ognjen Š.; Dichtel, William R.; Mortezaei, Shahab; Stoddart, J. Fraser. "Cyclobis(paraquat-phenylene)-Based [2]Catenanes Prepared by Kinetically Controlled Reactions Involving Alkynes". Organic Letters 8 (21): 4835–4838. doi:10.1021/ol061864d.
  8. ↑ Fahrenbach, Albert C.; Warren, Scott C.; Incorvati, Jared T.; Avestro, Alyssa-Jennifer; Barnes, Jonathan C.; Stoddart, J. Fraser; Grzybowski, Bartosz A. (2013-01-18). "Organic Switches for Surfaces and Devices". Advanced Materials 25 (3): 331–348. doi:10.1002/adma.201201912.