Cage-type structures cyclodextrin complexes

Metalated container molecules can be viewed as a class of compounds that have one or more active metal coordination sites anchored within or next to a molecular cavity (Fig. 2). A range of host systems is capable of forming such structures. The majority of these compounds represent macrocyclic molecules and steri-cally demanding tripod ligands, as for instance calixarenes (42), cyclodextrins (43,44), and trispyrazolylborates (45-48), respectively. In the following, selected types of metalated container molecules and their properties are briefly discussed and where appropriate the foundation papers from relevant earlier work are included. Porphyrin-based hosts and coordination cages with encapsulated metal complexes have been reviewed previously (49-53) and, therefore, only the most recent examples will be described. Thereafter, our work in this field is reported. 

Cyclodextrins are well known [1] to form a number of crystalline adducts or inclusion complexes with a variety of substances. In the crystalline lattice, cyclodextrins form two types of packing One is a cage type where the cavity of one cyclodextrin molecule is closed on both sides by ad jacent molecules. The other is a channel-type, in which the cyclodextrin rings are packed on top of each other to produce cylinders with infinite centralcavity. Formation of one or the other type crystal structure depends on the size and molecular character of the guest molecule. 

Fig. 1 a-c Schematic representation of a channel type b cage herringbone type c cage brick type, crystal structures formed by crystalline cyclodextrin inclusion complexes. (Adopted from [18] with permission)... 

When complexes are prepared in the presence of iodide, four different complex types can be obtained, depending on the conditions. Here we will only discuss the blackish-brown complex (a-cyclodextrin)2-Cdo.5-l5-27H2O. On the Cf, axis of the molecule are found four of the five iodine atoms of Is", the central fifth one being disordered. The disordered iodine atom is located between the two a-cyclodextrin molecules which face each at their wide bases, 11.18. Structures of 11.18 are packed one on top of the other in a way as to create a continuous channel filled with iodine atoms. This is a channel-type complex (Noltemeyer and Saenger 1980). With guest molecules other than iodine, either one or the other of the two types of crystalline structures discussed here, i.e. cage or channel, can be observed. 

When a large number of crystalline a-cyclodextrin inclusion complexes was investigated according to space group symmetries and cell dimensions it was found that essentially two types of crystal structures do exist, the cage and the channel structures [11],... 

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