Acoordination polymer is aninorganic ororganometallicpolymer structure containing metal cation centers linked byligands. More formally a coordination polymer is a coordination compound with repeating coordination entities extending in 1, 2, or 3 dimensions.[1][2]
It can also be described as a polymer whose repeat units arecoordination 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 themetal-organic frameworks, or MOFs, that are coordination networks with organic ligands containing potential voids.[1]
Coordination polymers are relevant to many fields, having many potential applications.[3]
Coordination polymers can be classified in a number of ways according to their structure and composition. One important classification is referred to asdimensionality. 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).[4] This is depicted in Figure 1.
The work ofAlfred 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 complexesPrussian blue andHofmann clathrates.[5]
Coordination polymers are often prepared byself-assembly, involvingcrystallization of a metalsalt with a ligand. The mechanisms ofcrystal engineering andmolecular self-assembly are relevant.[3]
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.[6] 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, 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 thecoordination number, which, along with the angles they are held at, determines the dimensionality of the structure. Thecoordination number andcoordination geometry of a metal center is determined by the nonuniform distribution ofelectron density around it, and in general the coordination number increases with cation size. Several models, most notablyhybridization model andmolecular 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 onelectron density distribution.[8]
Transition metals are commonly used as nodes. Partially filled dorbitals, either in theatom orion, canhybridize differently depending on environment. Thiselectronic structure causes some of them to exhibit multiplecoordination geometries, particularly copper and gold ions which as neutral atoms have full d-orbitals in their outer shells.
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 andalkaline 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 thecounterion 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 (fromcalcium tostrontium tobarium).
Coordination polymers require ligands with the ability to form multiple coordination bonds, i.e. act as bridges between metal centers. Manybridging ligands are known. They range from polyfunctional heterocycles, such as pyrazine, to simple halides. Almost any type of atom with a lone pair of electrons can serve as aligand.
Very elaborate ligands have been investigated.[9] andphosphorus,[10][11] have been observed.
Ligands can be flexible or rigid. A rigid ligand is one that has no freedom to rotate around bonds or reorient within a structure. Flexible ligands can bend, rotate around bonds, and reorient themselves. These differentconformations create more variety in the structure. There are examples of coordination polymers that include two configurations of the same ligand within one structure,[12] as well as two separate structures where the only difference between them is ligand orientation.
A length of the ligand can be an important factor in determining possibility for formation of a polymeric structure versus non-polymeric (mono- or oligomeric) structures.[13]
Besides metal and ligand choice, there are many other factors that affect the structure of the coordination polymer. For example, most metal centers are positively charged ions which exist as salts. Thecounterion in the salt can affect the overall structure. For example, when silver salts such as AgNO3, AgBF4, AgClO4, AgPF6, AgAsF6 and AgSbF6 are all crystallized with the same ligand, the structures vary in terms of the coordination environment of the metal, as well as the dimensionality of the entire coordination polymer.[14]
Additionally, variations in the crystallization environment can also change the structure. Changes inpH,[15] exposure to light, or changes in temperature[16] can all change the resulting structure. Influences on the structure based on changes in crystallization environment are determined on a case by case basis.
The structure of coordination polymers often incorporates empty space in the form of pores or channels. This empty space is thermodynamically unfavorable. In order to stabilize the structure and prevent collapse, the pores or channels are often occupied by guest molecules. Guest molecules do not form bonds with the surrounding lattice, but sometimes interact via intermolecular forces, such as hydrogen bonding or pi stacking. Most often, the guest molecule will be the solvent that the coordination polymer was crystallized in, but can really be anything (other salts present, atmospheric gases such asoxygen,nitrogen,carbon dioxide, etc.) The presence of the guest molecule can sometimes influence the structure by supporting a pore or channel, where otherwise none would exist.
Coordination polymers are found in some commercialized as dyes.. Metal complex dyes using copper or chromium are commonly used for producing dull colors.Tridentate ligand dyes are useful because they are more stable than their bi- or mono-dentate counterparts.[17][18]
Some early commercialized coordination polymers are the Hofmann compounds, which have the formula Ni(CN)4Ni(NH3)2. These materials crystallize with small aromatic guests (benzene, certain xylenes), and this selectivity has been exploited commercially for the separation of these hydrocarbons.[19]
Although not yet practical, porous coordination polymers have potential asmolecular sieves in parallel with porous carbon andzeolites.[5] The size and shapes of the pore can be controlled by the linker size and the connecting ligands' length andfunctional groups. To modify the pore size in order to achieve effective adsorption, nonvolatileguests areintercalated in the porous coordination polymer space to decrease the pore size. Active surface guests can also be used contribute toadsorption. For example, the large-pore MOF-177, 11.8 Å in diameter, can be doped by C60 molecules (6.83 Å in diameter) or polymers with a highly conjugated system in order to increase the surface area for H2 adsorption.
Flexible porous coordination polymers are potentially attractive for molecular storage, since their pore sizes can be altered by physical changes. An example of this might be seen in a polymer that contains gas molecules in its normal state, but upon compression the polymer collapses and releases the stored molecules. Depending on the structure of the polymer, it is possible that the structure be flexible enough that collapsing the pores is reversible and the polymer can be reused to uptake the gas molecules again.[20] TheMetal-organic framework page has a detailed section dealing with H2 gas storage.
Luminescent coordination polymers typically feature organic chromophoric ligands, which absorb light and then pass the excitation energy to the metal ion. For ligands that fluoresce without the presence of the metal linker (not due to LMCT), the intensephotoluminescence emission of these materials tend to be magnitudes of order higher than that of the free ligand alone. These materials are candidates for light emitting diode (LED) devices. The dramatic increase in fluorescence is caused by the increase in rigidity and asymmetry of the ligand when coordinated to the metal center.[21][22]
Coordination polymers can have short inorganic and conjugated organic bridges in their structures, which provide pathways forelectrical conduction. example of such coordination polymers areconductive metal organic frameworks. Some one-dimensional coordination polymers built as shown in the figure exhibit conductivities in a range of 1x10−6 to 2x10−1 S/cm. The conductivity is due to the interaction between the metald-orbital and the pi* level of the bridging ligand. In some cases coordination polymers can havesemiconductor behavior. Three-dimensional structures consisting of sheets of silver-containing polymers demonstrate semi-conductivity when the metal centers are aligned, and conduction decreases as the silver atoms go from parallel to perpendicular.[21]
Coordination polymers exhibit many kinds ofmagnetism.Antiferromagnetism,ferrimagnetism, andferromagnetism are cooperative phenomena of the magnetic spins within a solid arising from coupling between the spins of the paramagnetic centers. In order to allow efficient magnetic, metal ions should be bridged by small ligands allowing for short metal-metal contacts (such as oxo, cyano, and azido bridges).[21]
Coordination polymers can also show color changes upon the change ofsolvent molecules incorporated into the structure. An example of this would be the two Co coordination polymers of the [Re6S8(CN)6]4− cluster that contains water ligands that coordinate to the cobalt atoms. This originally orange solution turns either purple or green with the replacement of water withtetrahydrofuran, and blue upon the addition ofdiethyl ether. The polymer can thus act as a solvent sensor that physically changes color in the presence of certain solvents. The color changes are attributed to the incoming solvent displacing the water ligands on the cobalt atoms, resulting in a change of their geometry from octahedral to tetrahedral.[21]