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. Coordinate bonds are useful in such supramolecular self-assembly because of their versatile geometries. However, there is controversy over calling coordinate bonds noncovalent, as they are typically strong bonds and have covalent character. The combination of a coordination cage and a guest is a type of inclusion compound. Coordination complexes can be used as 'nano-laboratories' for synthesis, and to isolate interesting intermediates. The inclusion complexes of a guest inside a coordination cage show intriguing chemistry as well; often, the properties of the cage will change depending on the guest. Coordination complexes are molecular moieties, so they are distinct from clathrates and metal-organic frameworks. 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. Coordinate bonds are useful in such supramolecular self-assembly because of their versatile geometries. However, there is controversy over calling coordinate bonds noncovalent, as they are typically strong bonds and have covalent character. The combination of a coordination cage and a guest is a type of inclusion compound. Coordination complexes can be used as 'nano-laboratories' for synthesis, and to isolate interesting intermediates. The inclusion complexes of a guest inside a coordination cage show intriguing chemistry as well; often, the properties of the cage will change depending on the guest. Coordination complexes are molecular moieties, so they are distinct from clathrates and metal-organic frameworks. Chemists have long been interested in mimicking chemical processes in nature. Coordination cages quickly became a hot topic as they can be made by self-assembly, a tool of chemistry in nature. The conceptualization of a closed-surface molecule capable of incorporating a guest was described by Donald Cram in 1985. Early cages were synthesized from bottom-up. Makoto Fujita introduced self-assembling cages, which are less tedious to prepare. These cages arise from the condensation of square planar complexes using polypodal ligands. There are five main methodologies to create coordination cages. In directional bonding, also called edge-directed self-assembly, polyhedra are designed using a stoichiometric ratio of ligand to metal precursor. The symmetry interaction method involves combining naked metal ions with multibranched chelating ligands. This results in highly symmetric cages. The molecular paneling method, also called the face-directed method, was the method developed by Fujita. Here, rigid ligands act as 'panels' and coordination complexes join them together to create the shape. In the figure above, the yellow triangles represent panel ligands, and the blue dots are metal complexes. The ligands of the complex itself helps enforce the final geometry. In the weak link method, a hemilabile ligand is used: a weak metal-heteroatom bond is the 'weak link.' 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. The metals used in the assembly must be available to perform further in the final structure, without compromising the cage structure. The initial structure is referred to as 'condensed.' In the condensed structure, the weak M-X bond can be selectively replaced by introducing an ancillary ligand with a higher binding affinity, leading to an open cage structure. In the figure to the right, the M is the metal, the orange ellipses are ligands, and the A is the ancillary ligand. For the dimetallic building block method, two pieces are needed: the metal dimer and its nonlinking ligands, and linking ligands. The nonlinking ligands need to be relatively nonlabile, and not too bulky; amidinates, for instance, work well. The linking ligands are either equatorial or axial: equatorial ligands are small polycarboxylato anions, and axial linkers are usually rigid aromatic structures. Axial and equatorial ligands may be used separately or in combination, depending on the desired cage structure.