Cyclodextrins as supramolecular hosts for organometallic complexes

Cyclodextrins are oligosaccharides composed of six or more D-glucopyranose residues attached by α-1,4-linkages in a cyclic array. The most current cyclodextrins contain six, seven, or eight glucose residues and are named α-cyclodextrin (α-CD), β-cyclodextrin (β-Cd), and γ-cyclodextrin (γ-CD). CDs have the ability to include a part of an organometallic complex in their internal hydrophobic cavities. When this happens, four types of adducts can be observed: the CD behaves as a second-sphere ligand; the CD acts simultaneously as a first- and second-sphere ligand; the CD acts simultaneously as a first-sphere ligand; and the CD can be considered as a transient second-sphere ligand. The characteristics and the properties of these four adducts are reviewed. Focus is on studies in which at least one direct metal-carbon bond is formed in organometallic supramolecular systems that have been well-identified or are strongly suspected of existing.