A coordination polymer is an inorganic or organometallic polymer structure containing metal cation centers linked by ligands. More formally a coordination polymer is a coordination compound with repeating coordination entities extending in 1, 2, or 3 dimensions.
It can also be described as a polymer whose repeat units are coordination complexes. Coordination polymers contain the subclass coordination networks that are coordination compounds extending, through repeating coordination entities, in 1 dimension, but with cross-links between two or more individual chains, loops, or spiro-links, or a coordination compound extending through repeating coordination entities in 2 or 3 dimensions. A subclass of these are the metal-organic frameworks, or MOFs, that are coordination networks with organic ligands containing potential voids.
Coordination polymers are relevant to many fields, having many potential applications.
Coordination polymers can be classified in a number of ways according to their structure and composition. One important classification is referred to as dimensionality. A structure can be determined to be one-, two- or three-dimensional, depending on the number of directions in space the array extends in. A one-dimensional structure extends in a straight line (along the x axis); a two-dimensional structure extends in a plane (two directions, x and y axes); and a three-dimensional structure extends in all three directions (x, y, and z axes). This is depicted in Figure 1.
History
The work of Alfred Werner and his contemporaries laid the groundwork for the study of coordination polymers. Many time-honored materials are now recognized as coordination polymers. These include the cyanide complexes Prussian blue and Hofmann clathrates.
Synthesis and propagation
Coordination polymers are often prepared by self-assembly, involving crystallization of a metal salt with a ligand. The mechanisms of crystal engineering and molecular self-assembly are relevant.
The structure and dimensionality of the coordination polymer are determined by the linkers and the coordination geometry of the metal center. Coordination numbers are most often between 2 and 10. Examples of various coordination numbers are shown in planar geometry in Figure 2. In Figure 1 the 1D structure is 2-coordinated, the planar is 4-coordinated, and the 3D is 6-coordinated.
Metal centers
Metal centers, often called nodes or hubs, bond to a specific number of linkers at well defined angles. The number of linkers bound to a node is known as the coordination number, which, along with the angles they are held at, determines the dimensionality of the structure. The coordination number and coordination geometry of a metal center is determined by the nonuniform distribution of electron density around it, and in general the coordination number increases with cation size. Several models, most notably hybridization model and molecular orbital theory, use the Schrödinger equation to predict and explain coordination geometry, however this is difficult in part because of the complex effect of environment on electron density distribution.
Transition metals
Transition metals are commonly used as nodes. Partially filled d orbitals, either in the atom or ion, can hybridize differently depending on environment. This electronic structure causes some of them to exhibit multiple coordination geometries, particularly copper and gold ions which as neutral atoms have full d-orbitals in their outer shells.
Lanthanides
Lanthanides are large atoms with coordination numbers varying from 7 to 14. Their coordination environment can be difficult to predict, making them challenging to use as nodes. They offer the possibility of incorporating luminescent components.
Alkali metals and alkaline earth metals
Alkali metals and alkaline earth metals exist as stable cations. Alkali metals readily form cations with stable valence shells, giving them different coordination behavior than lanthanides and transition metals. They are strongly affected by the counterion from the salt used in synthesis, which is difficult to avoid. The coordination polymers shown in Figure 3 are all group two metals. In this case, the dimensionality of these structures increases as the radius of the metal increases down the group (from calcium to strontium to barium).