Carcerand

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A carcerand in host-guest chemistry is a container molecule consisting of two hollow molecular spheres, called hemicarcerands, connected to each other rim to rim via a spacer [1]. When the internal cavity is large enough, a molecule can fit in and the resulting complex is called a carceplex. This type of molecule was first described and so named by Donald J. Cram in 1985 as a contraction of the words ligand and carcer, Latin for prison [2]. He also called the interior of the container compound the inner phase. Potential applications for this type of compound are slow-release drugs or pesticides.

Carcerands

The first generation carcerands are based on calixarene hemicarcerands with 4 alkyl substituents on the upper rim and 4 reactive substituents on the lower rim. The coupling of both hemicarcerands takes place through a spacer group. In the original 1985 publication two different hemicarcerands react, one with chloromethyl reactive groups and one with thiomethyl reactive groups in a nucleophilic displacement and the resulting the spacer group is a dimethylsulfide (CH2SCH2). In this experiment the quests were the molecules already present in the reaction medium such as argon anddimethylformamide.

In another configuration the 4 lower rim functional groups are aldehydes which condense with O-Phenylenediamine to the corresponding di-imines. The 4 spacer groups connecting the two spheres are now much longer and consequently the internal cavity is much larger. Compounds trapped in the cavity are said to be held there by constrictive binding [3]. They can be introduced by simply heating in neat solvent like hexachlorobutadiene (a fungicide). The half-life of the reverse process is 3.2 hours at 25 °C in CDCl3 by NMR analysis. Ferrocene can be introduced by heating with the hemicarcerand in a large bulky solvent such as tripiperidylphosphine oxide. The half-life for ferrocene liberation is 19.6 hours at 112 °C.

Octahedral Nanocontainer Molecule ref Liu 2006. Each sphere represents a hemicarcerand, the blue lines represent flexible aliphatic spacers

The internal cavity of a carcerand can be as large as 1700 Å3 (1.7 nm3) when six hemicarcerands form a single octahedral compound. This is accomplished by dynamic covalent chemistry in a one-pot condensation of 6 equivalents of a tetraformyl calixarene and 12 equivalents of ethylene diamine with trifluoroacetic acid catalyst in chloroform at room temperature [4] followed by reduction of the imine bonds with sodium borohydride.

[edit] References

  1. ↑  The Inner Phase of Molecular Container Compounds as a Novel Reaction Environment Ralf Warmuth Journal of Inclusion Phenomena and Macrocyclic Chemistry 37: 1–38, 2000.
  2. ↑  Shell closure of two cavitands forms carcerand complexes with components of the medium as permanent guests Donald J. Cram, Stefan Karbach, Young Hwan Kim, Lubomir Baczynskyj, Gregory W. KallemeynJ. Am. Chem. Soc.; 1985; 107(8); 2575-2576. Abstract
  3. ↑  Constrictive binding of large guests by a hemicarcerand containing four portals Mimi L. C. Quan, Donald J. Cram J. Am. Chem. Soc.; 1991; 113(7); 2754-2755. Abstract
  4. ↑  One-Pot, 18-Component Synthesis of an Octahedral Nanocontainer Molecule Xuejun Liu, Yong Liu, Gina Li, Ralf Warmuth, Angewandte Chemie International Edition Volume 45, Issue 6 , Pages 901 - 904 2006 Abstract
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