Coordination cage
Coordination cages are three-dimensional ordered structures in solution that act as hosts in host–guest chemistry.
They are self-assembled in solution from organometallic precursors, and often rely solely on noncovalent interactions rather than covalent bonds.
Coordination cages quickly became a hot topic as they can be made by self-assembly, a tool of chemistry in nature.
[7] In directional bonding, also called edge-directed self-assembly, polyhedra are designed using a stoichiometric ratio of ligand to metal precursor.
[4] The symmetry interaction method involves combining naked metal ions with multibranched chelating ligands.
[4][8] In the figure at left, the yellow triangles represent panel ligands, and the blue dots are metal complexes.
The formation of the complexes is driven by favorable π-π interactions between the spacers and the ligands, as well as the chelation of the metal.
Axial and equatorial ligands may be used separately or in combination, depending on the desired cage structure.
Four of the triangular faces of this shape are occupied by Lmes, which acts as a triply bridging ligand.
[3] In order for a cavitand cage to efficiently self-assemble, the following requirements must be met: The cavitand scaffold must be rigid, the incoming metal complex must impose cis geometry, and there must be enough preorganization in the structure such that the entropic barrier to create the cage can be overcome.
[3] The complexes used to assemble cavitand cages are square planar with one η2 ligand; this helps enforce the final geometry.
Self-assembly also requires a ligand exchange; weakly bound ions such as BF4- and PF6- promote assembly because they leave the complex so it can bind with the nitriles on the rest of the structure.
[13] In the cage above, the outer shell is a cuboctohedron; its structure comes from two adjacent benzoate moieties from the m-BTEB ligand.
In some instance, planar aromatic molecules stack inside of metalloprisms, as can be observed by UV-visible spectroscopy.
