Coordination polymer

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. 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 such as organic and inorganic chemistry, biochemistry, materials science, electrochemistry, and pharmacology, having many potential applications. This interdisciplinary nature has led to extensive study in the past few decades. Coordination polymers can be classified in a number of different 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. The work of Alfred Werner and his contemporaries laid the groundwork for the study of coordination polymers. Terms ubiquitous in the field, such as coordination number, were coined by Werner. Many time honored materials are now recognized as coordination polymers. These include the cyanide complexes Prussian blue and Hofmann clathrates. 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 synthesis methods utilized to produce coordination polymers are generally the same methods used to grow any crystal. These generally include solvent layering (slow diffusion), slow evaporation, and slow cooling. (Because the main method of characterization of coordination polymers is X-ray crystallography, growing a crystal of sufficient size and quality is important.) Forces that determine metal-ligand complexes include van der Waals forces, pi-pi interactions, hydrogen bonding, and stabilization of pi bonds by polarized bonds in addition to the coordination bond formed between the metal and the ligand. These intermolecular forces tend to be weak, with a long equilibrium distance (bond length) compared to covalent bonds. The pi-pi interactions between benzene rings, for example, have energy roughly 5–10 kJ/mol and optimum spacing 3.4–3.8 Ångstroms between parallel faces of the rings. The crystal structure and dimensionality of the coordination polymer is determined by the functionality of the linker and the coordination geometry of the metal center. Dimensionality is generally driven by the metal center which can have the ability to bond to as many as 16 functional sites on linkers; however this is not always the case as dimensionality can be driven by the linker when the linker bonds to more metal centres than the metal centre does linkers. The highest known coordination number of a coordination polymer is 14, though 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.