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Introduction
It is always apt to start with a basic summary of Part 1 of this article. In Part 1, we learnt about the definition of addition compounds, as well as one type of addition compound, the EDA (electron donor-acceptor) complex. Firstly, the definition of addition compounds is simply a compound formed from an addition reaction; and the definition of addition reactions is two or more chemical species reacting to form a single product. Specifically, the addition reaction must result in the break of a pi bond (reduction in bond multiplicity, since a pi bond corresponds to a multiple bond) and two new bonds must be formed. Regarding the previous article on EDA complexes as addition compounds, it may not seem accurate when we look at the definition as given by the IUPAC Gold Book. We can, however, broaden the scope of the definition of addition reactions (although this is very debatable) to simply describe two chemical species reacting to form a single product, not necessarily with a drop in bond multiplicity or formation of new bonds.
The reason for this broadening of definition is that many so-called ‘non-conventional’ species are increasingly considered addition compounds; particularly, inclusion compounds involve nothing but instantaneous dipole interactions (van der Waals forces) to hold the two molecules together, or that the host species ‘traps’ the guest species inside. Cryptands, crown ether complexes, inclusion compounds and cyclodextrins are grouped together under ‘host-guest chemistry’, which is in turn sometimes considered under addition compounds, since it is indeed true that the host and guest combine to form only a single product. The first two of these compounds, cryptands and crown ether complexes, will be considered in this article. For other interesting types of compounds, catenanes and rotaxanes, their position in chemistry is also highly questionable, as they can involve two reactants combining two form a single product; however, this does not always mean that a single reaction is undergone to combine these two reactants. It is, in fact, sometimes unclear whether reactants have undergone a single reaction or more, as we will see with rotaxanes.
Crown Ethers
Crown ethers are very interesting cyclical compounds. Before we begin to discuss the role of crown ethers in addition compounds, it is first important to understand what crown ethers are in the first place. Crown ethers involve a cyclic compound where there is an alternating structure of ethers, and ethers involve oxygen atoms bonded to two alkyl groups. Below in Fig. 1 we show the structure of a crown ether. As for the nomenclature of crown ethers, the general name is a-crown-b, where a is the number of atoms in the ether (including both oxygen and carbon atoms), while b is the number of oxygen atoms. As such, the crown ether in Fig. 1 is known as 12-crown-4. In terms of molecular geometry, it should be noted that there is a cavity inside the crown ether that is able to ‘capture’ molecules inside of it. Furthermore, the oxygen atoms surrounding the cavity are electronegative and draw electron densities toward themselves, making the crown ether ring cavity possess a partial negative charge.
Fig. 1: Structure of 12-crown-4.
The partial negative charge is an important part of the host-guest chemistry concept, as positively-charged (and electropositive) metal ions are attracted to the space within the cavity. Furthermore, the oxygens of the crown ether possess unpaired pairs of electrons, which can be used to coordinate to the metal ion. This is, in fact, what occurs for crown ether complexes; the metal ion fits within the cavity, and is coordinated to by the oxygen atoms, creating a relatively stable crown ether complex (it is also a coordination compound, but terming it a crown ether complex is generally more specific). Of course, that is not to say that metal ions larger than the size of the cavity cannot form a crown ether complex with the crown ether; this is because the metal ion may be suspended above the cavity, where the distance is still close enough for coordination. However, we should be aware that the coordinate bond gets weaker the further the metal ion from the crown ether; as such, the larger the metal ion, the weaker and less stable the crown ether complex. There is some utility of crown ether complexes as they are able to approximate the relative sizes of metal ions, allowing for separation and identification of different metal ions.
Additionally, there is a special type of compound known as a Lariat ether. In Lariat ethers, there is a typical crown ether with a side chain, and this side chain is typically electron-donating. In such cases, of course, one of the oxygen atoms must be replaced with another atom, such as a nitrogen atom, so that it can participate as a member of the crown ether but be able to bond to an additional side chain (requiring three bonds). Lariat ethers are special because the presence of the additional side chain allows for more coordination to the metal ion in the cavity (since the side chain is typically a donor), making the crown ether complex more stable.
Cryptands and Calixarenes
In this next part, we will be explaining cryptands but also calixarenes, which are somewhat similar complexes. Cryptands are slightly related to crown ethers, and they look just like three-dimensional crown ethers. In the case of the cryptand shown below (Fig. 2), it is known as a bicyclic cryptand, because two structurally distinct rings can be identified in this cryptand. Generally, a cryptand has the same function as a crown ether, because in both cases it involves a metal ion inside the cavity coordinating to each of the electronegative donor atoms within. As for the nomenclature of the crown ether below, it is named [2.2.2] cryptand, because in each of the chains between the nitrogens, there are two oxygen atoms available for coordination. Cryptands, especially cryptands with larger numbers of cycles, are interesting because it is sometimes possible for them to completely encapsulate the metal ion, making the metal ion extremely stable. I believe the function of this is that it is sometimes used to encase unstable metal ions; the steric hindrance posed by the linkages between the nitrogen atoms prevents reactants from approaching the metal ion and reacting with it.
Fig. 2: Structure of a cryptand.
Other than cryptands, there are also compounds known as calixarenes (Fig. 3). For this compound the derivation of the name might aid in the memory of the compound. Calixarene is derived from the words ‘calix’ and ‘arene’, where ‘calix’ means ‘chalice’; this is indeed so, as the calixarene has a chalice-like structure. The reason why there is no planarity possible for calixarenes is because of the repulsion between electronegative oxygen atoms, which draw clouds of electron density towards themselves, resulting in the repulsion. Yet, complete repulsion is not possible as there is, at the same time, attraction between the -OH groups because of intramolecular hydrogen bonds. The push and pull effects then shape the calixarene into a chalice-like structure. As for the nomenclature of such compounds, the general name is calix[n]arene, with n being the number of repeating arene subunits in the molecule, making the naming of calixarenes fairly basic. For instance, the calixarene in Fig. 3 is known as calix[6]arene.
Fig. 3: Structure of calix[6]arene.
Conclusion
In this article, we have learnt about two extra types of addition compounds of host-guest chemistry, which are crown ether complexes and cryptands (as well as calixarenes, which are related to cryptands). There are other types of addition compounds, of course, that have been mentioned but not yet discussed, such as inclusion compounds and cyclodextrins. Hopefully, the discussion of such compounds can be concluded in the next article.
Part 1 of this article is here.
Part 3 of this article is here.