Hosts cucurbiturils

Fig. 9. Cucurbituril inclusion chemistry (a) tridimensional stmcture of cucurbit[6]uril (b) conjectured cross-sectional representation of a host—guest...

This entry to supramolecular chemistry allows one, at the same time, to develop a host-guest chemistry because the space inside the cucurbituril barrel is sufficient to confine small "guesf molecules. This is illustrated with the crystallization of the supramolecular adduct [W3S4(H20)8Cl](pyH ccuc) Cl4a 5.5 H2O with a pyridinium cation inside the cucurbituril cavity [48]. The introduction of guest molecules may vary the net charge on the assembly and consequently the whole packing in the solid state. 

The network structures to be discussed will all involved hydrogen bonding as the supramolecular synthon. It should be noted however that other interactions such as coordinate bonds and host-guest interactions may also organise host molecules into network structures. Coordination polymers constructed from molecular hosts may involve functionalised calixarenes [8-11], cyclotriveratrylene [12], or cucurbituril [13]. Calixarenes have also been used to build up network structures via host-guest interactions [14,15]. It is also notable that volatile species may be trapped within the solid state lattice of calix[4] arene with a structure entirely composed of van der Waals interactions [16]. 

Buschmann HJ, Schollmeyer E (1997) Cucurbituril and P-cyclodextrin as hosts for the complexation of organic dyes. J Incl Phenom Macrocycl Chem 29 167-174... 

Gadde S, Batchelor EK, Weiss JP, Ling Y, Kaifer AE (2008) Control of H- and J-aggregate formation via host-guest complexation using cucurbituril hosts. J Am Chem Soc 130 17114-17119... 

Gadde S, Batchelor EK, Kaifer AE (2009) Controlling the formation of cyanine dye H- and J-aggregates with cucurbituril hosts in the presence of anionic polyelectrolytes. Chem Eur J 15 6025-6031... 

On the contrary, cucurbiturils preferentially coniine charged, but not neutral, species. Cucurbiturils also possess cylindrical cavities suitable for the inclusion of organic molecules. Cucurbiturils are a family of macrocyclic host molecules consisting of methylene-bridged glucouril units (see Scheme 2.44). 

Glycoluril 1 is a rigid, concave molecule, which can be easily functionalized via its four ureido nitrogen atoms. The two carbonyl groups in 1 are potential hydrogen bond acceptor sites. These features make 1 an excellent building block for synthetic host molecules. Glycoluril is the monomeric unit of cucurbituril,... 

Cavilaics include crown rnacroring inclusion compounds (coronates), cryptates, podates. cytlophane host inclusion compnunds. calixarene inclusion compounds, cyclodextrin and amylose inclusion compounds, cucurbituril inclusion compounds, molecular deft inclusion compounds, and anionic guest inclusion compounds. 

Cucurbit [6] uril and increasingly its higher homologues have been used extensively in a variety of host-guest chemistry and nanostructure assemblies. Of particular appeal are rotaxanes and molecular necklaces, mechanically interlocked assemblies of molecules based on CB [6]. Efficient synthesis of ID, 2D and 3D polyrotaxanes and molecular necklaces (cucurbituril beads linked by a macrocyclic molecule string ) has been achieved by a combination of self-assembly and coordination chemistry. We discuss rotaxanes and molecular necklaces in Section 10.7, and cucurbil-based systems are summarised in a recent review.23... 

Looking more at host-guest type systems, the cucurbiturils are often very efficient catalysts in cases where they simultaneously bind two guests, thus increasing effective concentration and changing a bimolecular reaction into a guas/ -unimolecular one. To take one recent example, cucurbit[6]uril (Section 6.2.4) proves to be a very efficient catalyst for the click 1,3-dipolar cycloaddition reaction of an azide with an alkyne, particularly where the substrates are suitable guests for the cavity, as in Scheme 12.20. Cucurbituril binds very effectively to ammonium ions and addition of a small amount... 

A large range of rotaxanes are based on the inclusion complexes of a pyridinium guest in a flexible host containing electron-rich Jt-systerns. In the case of the cucurbituril derivatives, the complex 76 can be capped to give the rotaxane 94 <2004OL2665>. 

Based on these characteristics, cage 2 shows unique properties not demonstrated in previously known organic hosts such as crown ethers [18], cyclodextrins [19], calixarenes [20,21], calixresorcarenes [21], cucurbituril [22], and so on. The details of these properties are discussed in the following sections. 

Fig. 1. Schematic representation of some host systems used for host-guest assemblies from left calixarene, cucurbituril, and p-cyclodextrine.

A cucurbituril macropolycycle composed of six units, and a side-on view of the cucurbituril acting as a host to an organic guest cation. 

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