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Recap
So in the last article, we learnt about three types of addition compounds, which are also a part of host-guest chemistry; the host and the guest, the reactants, ‘combine’ to form a single product. As we have mentioned previously, the definition of addition reactions (and therefore, addition compounds) itself can be highly sketchy, and many conventional sources disagree on how to define an addition reaction. It can be argued that addition reactions should retain the definition as given by the IUPAC Gold Book, while addition compounds are separate from products, or adducts, formed by addition reactions. Questionable also is the fact that some of the compounds we will cover next, such as inclusion compounds, are typically considered addition compounds, but it appears that there is no real ‘reaction’ taking place, and technically the structures of the host and guest are retained, so it could be said that there is no chemical transformation. Nevertheless, it is always better to be more general than specific, so here we are, covering addition compounds.
Now, for the recap. The three types of addition compounds covered were crown ether complexes, cryptands and calixarenes. Crown ether complexes and cryptands are similar in that both of them involve so-called ‘true’ bonds, where a metal ion forms coordinate bonds with the oxygen atoms present in crown ether complexes and cryptands. However, in calixarenes, there is no coordination or bonding between the host and the guest, and the only factors that allow the host to retain the guest are van der Waals forces (which are not considered bonds) as well as steric factors; the shape of the calixarene is that of a chalice, making it more difficult for the guest to ‘escape’ the host. Of course, the steric factors are present as well in the cryptands, but in these complexes, the dative covalent or coordinate bond is generally a factor that takes greater precedence toward retaining the metal ion rather than the steric factors. As for crown ethers, there is generally no steric factor preventing the metal ion from ‘escaping’.
Inclusion Compounds
The first type of addition compounds that will be discussed in this article is a compound that has already been briefly mentioned, inclusion compounds. Inclusion compounds are quite related to calixarenes in that both of them do not feature any type of bonding or interaction apart from pure van der Waals interactions. Surprisingly, a simple example of a inclusion compound is that of the organic molecule urea, which, when a guest is present, forms several long hexagonal lattices, becoming similar in shape to a honeycomb. The diameter of this hexagonal lattice is very small; one of the figures I have seen suggests five angstroms. To give an idea of how small this is, the C-C single bond length is 1.5 angstroms; as such, only smaller atoms or very small molecules can fit within the hexagonal cavity. The hexagonal lattice can be of very long length, however, and this allows long, straight-chain molecules to fit within the cavity as well. As such, a possible candidate for the guest would be octane. While there are indeed interactions between the host and guest in such cases, the inclusion compound does not appear to affect the physical properties of urea.
Hosts of inclusion compounds usually cannot ‘retain’ their guest for a long period of time, because it is easy to overcome the weak van der Waals interactions between the two molecules. However, there is a special type of inclusion compound, known as a carciplex (Fig. 1). The carciplex is composed of carcerands, which are in turn synthesized from two calixarenes, which have already been shown and explained in the previous article. As we can observe from the diagram, the carciplex looks and acts similarly to a cage, completely entrapping the guest molecule within, making the inclusion compound highly stable. This utility has been exploited, and highly unstable molecules have been isolated within a carciplex with great success. One example of this is of cyclobutadiene, which has been trapped inside a carciplex. Cyclobutadiene is typically highly unstable due to its antiaromaticity (which is a topic for another day!), but it has been successfully isolated in a carciplex due to its ‘stabilizing’ properties.
Fig. 1: Structure of a carciplex.
Cyclodextrins
Cyclodextrins (Fig. 2) are host compounds which possess a frustum-like structure (a frustum is basically the shape remaining after a smaller cone is cut out from the top of a larger cone). This means that it possesses two holes and one of them is larger than the other. If a molecule is able to pass through the larger hole, but not the smaller hole, it could possibly be trapped inside the cyclodextrin. Interestingly for cyclodextrins, they can act as both a cage compound and a channel compound. If several cyclodextrin molecules stack themselves on top of each other, a long, tunnel-like structure is created, in which long molecules can be accomodated. An example of such a compound is valeric acid, which is quite a long molecule, featuring a five-carbon backbone. Note that in such compounds, there should be an equimolar ratio of hosts and guests, meaning that only one guest can be trapped within the cyclodextrin.
Fig. 2: Structure of a cyclodextrin.
Catenanes and Rotaxanes
We have already finished discussing the so-called ‘main’ addition compounds, which are electron donor-acceptor complexes (in Part 1), crown ether complexes, cryptands and calixarenes (in Part 2), as well as carciplexes and cyclodextrins. Now, we will explain catenanes and rotaxanes, which are not as commonly encountered in chemistry. Catenanes refer to a molecule which is constituted of two linked rings of separate molecules (Fig. 3). While the constituent molecules in catenanes lack any form of intermolecular bonding, they are ‘forced’ to remain together, since only the breaking of individual bonds, which requires a relatively higher amount of energy, can be done to separate the indicidual molecules. The nomenclature of catenanes is very simply; it is [n]-catenane, where n refers to the number of linked rings in the molecule. One of the interesting features of catenanes is the ability to ‘rotate’ one of the rings so that two structurally distinct catenanes, known as topological isomers or topoisomers, can be produced (this should not be confused with stereoisomers).
Fig. 3: Structure of [2]-catenanes.
The last type of molecules we will discuss are rotaxanes, which possess a notably ‘interesting’ structure, as it will seem to many undergraduates. The simpler rotaxanes involve two molecules, a molecular chain and a molecular ring, with the chain threaded through the ring. To prevent separation of the two molecules (due to a lack of attractive forces), the molecular chain is ‘capped’ at its two ends with very large, sterically hindered molecules, such as porphyrin rings. As for the nomenclature of rotaxanes, it is very similar to the nomenclature for catenanes, with the general name [n]-rotaxane, and n referring to the number of components of the rotaxane.
Worth discussing are the synthetic methods of creating rotaxanes (Fig. 4), of which there are surprisingly quite a few. The first method is the ‘capping method’, with a ringed molecule and a straight-chain molecule as the reactants. Hydrophobic interactions cause the straight-chain molecule to be forced into the ringed molecule. Next, by addition reactions, the straight-chain molecule is capped forming the rotaxane. In the next method, the ‘clipping method’, there is a partial macrocycle (i.e. a partial ringed molecule) as well as the capped straight-chain molecule. Weak intermolecular interactions cause the straight-chain molecule to enter the partial macrocycle; a ring-closing reaction then occurs to form the rotaxane. Lastly, there is the ‘slipping method’. The reactants are a ringed molecule and a capped straight-chain molecule. While thermodynamically stable, they can be kinetically unstable at higher temperatures, and reaction occurs with the ringed molecule ‘slipping’ through the straight-chain molecule (hence the name of the reaction). Cooling causes kinetic and thermodynamic stability.
Fig. 4: Synthesis methods for rotaxanes.
The ‘active template’ method as seen in Fig. 4 is still in development and will not be discussed.
Conclusion
In conclusion, there are nine types of addition compounds, EDA complexes, organic acceptor complexes, crown ether complexes, cryptands, calixarenes, cyclodextrins, inclusion compounds, catenanes and rotaxanes. These compounds all are generally formed by the combination of two or more reactants into a single product, and are thus classified under addition compounds.
For Part 2 of this article, it is here.