Clathrochelate
In chemistry, clathrochelates are ligands that encapsulate a metal ion. Chelating ligands bind to metals more strongly than related monodentate ligands, and macrocyclic ligands bind more strongly than typical chelating ligands. It follows, that bi- or polymacrocyclic ligands would bind to metals particularly strongly. Clathrochelates are usually derived from bimacrocyclic ligands.
The first examples were derived from the tris(dioximate)s of cobalt(II) and iron(II). The synthesis entails replacement of the hydrogen-bonded proton center with BF2+ or BOR2+ group:[1]
- [Fe(HON=CMeCMe=NOH)(ON=CMeCMe=NO)2]2- + 2 BF3 → Fe(ON=CMeCMe=NO)3(BF)2 + 2 HF2-
Also well known is the clathrochelate derived from tris(ethylenediamine)cobalt(III), which are called sepulchrates:[2]
- [Co(H2NCH2CH2NH2)3]3+ + 6 CH2O + 2 NH3 → [Co[N(CH2HNCH2CH2NHCH2)3N]3+ + 6 H2O
The insertion and removal of metals from the binding pocket of clathrochelates can be very slow. For this reason, many clathrochelates are prepared by the reactions of pre-coordinated ligands. These reactions often do not directly break any metal-ligand bonds, but occur in the second coordination sphere. The slowness of the metal ion exchange allows for certain experiments that would otherwise be difficult or impossible. For example, it is possible to optically resolve cobalt(II) complexes, whereas in the absence of the special geometry imposed by the clathrochelate the lifetime of Co(II)-amine complexes is typically very short. Furthermore, this family of complexes enables studies on self-exchange redox reactions between couples (e.g. Co(II)/Co(III)) that would be impossible with simpler ligand systems.
See also
References
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<references />, or <references group="..." />Further reading
- Krämer, Roland; Voloshin, Yan Z.; Kostromina, N. A. (2002). Clathrochelates: synthesis, structure and properties. Amsterdam: Elsevier. ISBN 0-444-51223-3.
