Template-Directed Assembly of Dinuclear Triple-Stranded Helicates
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Helicates are oligonuclear coordination compounds in which linear organic oligodonor ligands wrap around two or more metal centers. The main focus in helicate chemistry is on double-stranded or triple-stranded helicates, but quadruple-stranded or circular helicates are also well known.
In dinuclear helicates, both metal complex units possess the same configuration, and a left-handed as well as a right-handed helix can be present. Usually the helicate is formed as a racemic mixture but some examples of enantiomerically pure helicates are known. If the two metal complex units of a dinuclear complex are oppositely configurated, an achiral meso-type coordination compound is obtained—the meso-helicate (“side-by-side complex,” “meso-cate”).
Since the introduction of the term “helicate” by Lehn et al. in 1987, the chemistry of helicates has become one of the most intensely studied topics of metallosupramolecular chemistry. This is because of the simplicity of the helicates, which allows the study of fundamental aspects of metal-directed self-assembly such as mechanisms, stereochemistry, or regiochemistry. In this chapter, we will focus on an important mechanistic feature of helicate chemistry—the influence of templates on the formation of triple-stranded dinuclear helicates. This can be observed for several helicates, although templation is not a general requirement for the formation of the complexes. Many helicates that are formed without a template present were observed. Here, very often, other stabilizing interactions (e.g., steric constrains) are at work, which support the formation of the helicate.