What is a Macrocycle

Macrocyclic ligands have played an important part in the development of modern co-ordination chemistry. But what exactly is a macrocycle As far as a co-ordination chemist is concerned, the definition of Melson is probably the most useful. Melson stated that a macrocycle is a cyclic molecule with three or more potential donor atoms in a ring of at least nine atoms. Thus, ethylene oxide (6.1), 1,4-dithiane (6.2), cyclotetradecane (6.3) and cyclooctatetraene (6.4) are not commonly thought of as macrocycles (Fig. 6-1), whereas molecules such as cyclam (6.5), phthalocyanine (6.6), 1,4,7-trithiacyclononane (6.7) and dibenzo-18-crown-6 (6.8) fit the definition (Fig. 6-2). 

This definition is framed to emphasise the metal-binding properties of the ligands and includes most cyclic polydentate ligands that can incorporate a metal ion into the middle of a bonding cavity. It is also worth noting that many macrocyclic ligands have trivial names. This reflects the unfriendly nature of the systematic names for example, the systematic names of 6.5 and 6.8 are 1,4,8,11-tetraazatetracyclodecane and 2,5,8,15,18,2l-hexaoxatricyclo[20.4.0.09 14]hexacosa-l(22),8,11,13,23,24-hexaene, respectively. 

Metals and Ligand Reactivity, New Edition. Edwin C. Constable Copyright 1996 VCH Verlagsgesellschaft mbH, Weinheim ISBN 3-527-29278-0 

In the past thirty years, the study of the co-ordination chemistry of macrocyclic ligands has expanded from the investigation of a chemical curiosity to a major area of research. The origins of this flurry of activity are numerous, but the development of synthetic methodology for the preparation of macrocyclic molecules has played a decisive role. In this chapter and the one following, we will consider one of the primary synthetic methods that is used for the synthesis of macrocyclic ligands and their co-ordination compounds. In doing so, we will see many of the effects that we have discussed in the earlier chapters in operation. 

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