Coordination polymer

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Figure 1. An illustration of 1- 2- and 3-dimensionality. DimensionalityandCoordination.png
Figure 1. An illustration of 1- 2- and 3-dimensionality.

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. [1] [2]

Contents

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. [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 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). [4] 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. [5]

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. [3]

Figure 2. Shows planar geometries with 3 coordination and 6 coordination. Planer3and6Coordination.png
Figure 2. Shows planar geometries with 3 coordination and 6 coordination.

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

Figure 3. Three coordination polymers of different dimensionality. All three were made using the same ligand (4,5-dihydroxybenzene-1,3-disulfonate (L)), but different metal cations. All of the metals come from Group 2 on the periodic table (alkaline earth metals) and in this case, dimensionality increases with cation size and polarizability. A. [Ca(L)(H2O)4]*H2O B. [Sr(L)(H2O)4]*H2O C.[Ba(L)(H2O)]*H2O In each case, the metal is represented in green. Coordination figure.jpg
Figure 3. Three coordination polymers of different dimensionality. All three were made using the same ligand (4,5-dihydroxybenzene-1,3-disulfonate (L)), but different metal cations. All of the metals come from Group 2 on the periodic table (alkaline earth metals) and in this case, dimensionality increases with cation size and polarizability. A. [Ca(L)(H2O)4]•H2O B. [Sr(L)(H2O)4]•H2O C.[Ba(L)(H2O)]•H2O In each case, the metal is represented in green.

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. [8]

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).

Ligands

Coordination polymers require ligands with the ability to form multiple coordination bonds, i.e. act as bridges between metal centers. Many bridging 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 a ligand.

Very elaborate ligands have been investigated. [9] and phosphorus, [10] [11] have been observed.

Structural orientation

1,2-Bis(4-pyridyl)ethane is a flexible ligand, which can exist in either gauche or anti conformations. Anti-gauche.svg
1,2-Bis(4-pyridyl)ethane is a flexible ligand, which can exist in either gauche or anti conformations.

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 different conformations 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.

Ligand length

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]

Other factors

Counterion

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. The counterion 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]

Crystallization environment

Additionally, variations in the crystallization environment can also change the structure. Changes in pH, [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.

Guest molecules

The addition and removal of guest molecules can have a large effect on the resulting structure of a coordination polymer. A few examples are (top) change of a linear 1D chain to a zigzag pattern, (middle) staggered 2D sheets to stacked, and (bottom) 3D cubes become more widely spaced. Conformational change.jpg
The addition and removal of guest molecules can have a large effect on the resulting structure of a coordination polymer. A few examples are (top) change of a linear 1D chain to a zigzag pattern, (middle) staggered 2D sheets to stacked, and (bottom) 3D cubes become more widely spaced.

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 as oxygen, 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.

Applications

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]

2-aminophenol coord.png

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]

Molecular storage

Although not yet practical, porous coordination polymers have potential as molecular sieves in parallel with porous carbon and zeolites. [5] The size and shapes of the pore can be controlled by the linker size and the connecting ligands' length and functional groups. To modify the pore size in order to achieve effective adsorption, nonvolatile guests are intercalated in the porous coordination polymer space to decrease the pore size. Active surface guests can also be used contribute to adsorption. 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] The Metal-organic framework page has a detailed section dealing with H2 gas storage.

Luminescence

Luminescent coordination polymers typically feature organic chromophoric ligands, which absorb light and then pass the excitation energy to the metal ion. Coordination polymers are potentially the most versatile luminescent species due to their emission properties being coupled with guest exchange. Luminescent supramolecular architectures have recently attracted much interest because of their potential applications in optoelectronic devices or as fluorescent sensors and probes. Coordination polymers are often more stable (thermo- and solvent-resistant) than purely organic species. For ligands that fluoresce without the presence of the metal linker (not due to LMCT), the intense photoluminescence emission of these materials tend to be magnitudes of order higher than that of the free ligand alone. These materials can be used for designing potential 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]

Electrical conductivity

Structure of coordination polymers that exhibit conductivity, where M = Fe, Ru, OS; L = octaethylporphyrinato or pthalocyaninato; N belongs to pyrazine or bipyridine. Coord Poly Conductivity.png
Structure of coordination polymers that exhibit conductivity, where M = Fe, Ru, OS; L = octaethylporphyrinato or pthalocyaninato; N belongs to pyrazine or bipyridine.

Coordination polymers can have short inorganic and conjugated organic bridges in their structures, which provide pathways for electrical conduction. example of such coordination polymers are conductive 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 metal d-orbital and the pi* level of the bridging ligand. In some cases coordination polymers can have semiconductor 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]

Magnetism

Coordination polymers exhibit many kinds of magnetism. Antiferromagnetism, ferrimagnetism, and ferromagnetism 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]

Sensor capability

Coordination polymers can also show color changes upon the change of solvent 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 with tetrahydrofuran, and blue upon the addition of diethyl 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]

Related Research Articles

<span class="mw-page-title-main">Coordination complex</span> Molecule or ion containing ligands datively bonded to a central metallic atom

A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the coordination centre, and a surrounding array of bound molecules or ions, that are in turn known as ligands or complexing agents. Many metal-containing compounds, especially those that include transition metals, are coordination complexes.

<span class="mw-page-title-main">Metallocene</span>

A metallocene is a compound typically consisting of two cyclopentadienyl anions (C
5H−
5, abbreviated Cp) bound to a metal center (M) in the oxidation state II, with the resulting general formula (C5H5)2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride or vanadocene dichloride. Certain metallocenes and their derivatives exhibit catalytic properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to [Cp2ZrCH3]+ catalyze olefin polymerization.

<span class="mw-page-title-main">Host–guest chemistry</span> Supramolecular structures held together other than by covalent bonds

In supramolecular chemistry, host–guest chemistry describes complexes that are composed of two or more molecules or ions that are held together in unique structural relationships by forces other than those of full covalent bonds. Host–guest chemistry encompasses the idea of molecular recognition and interactions through non-covalent bonding. Non-covalent bonding is critical in maintaining the 3D structure of large molecules, such as proteins and is involved in many biological processes in which large molecules bind specifically but transiently to one another.

<span class="mw-page-title-main">Supramolecular assembly</span> Complex of molecules non-covalently bound together

In chemistry, a supramolecular assembly is a complex of molecules held together by noncovalent bonds. While a supramolecular assembly can be simply composed of two molecules, or a defined number of stoichiometrically interacting molecules within a quaternary complex, it is more often used to denote larger complexes composed of indefinite numbers of molecules that form sphere-, rod-, or sheet-like species. Colloids, liquid crystals, biomolecular condensates, micelles, liposomes and biological membranes are examples of supramolecular assemblies, and their realm of study is known as supramolecular chemistry. The dimensions of supramolecular assemblies can range from nanometers to micrometers. Thus they allow access to nanoscale objects using a bottom-up approach in far fewer steps than a single molecule of similar dimensions.

<span class="mw-page-title-main">Cryptand</span> Cyclic, multidentate ligands adept at encapsulating cations

In chemistry, cryptands are a family of synthetic, bicyclic and polycyclic, multidentate ligands for a variety of cations. The Nobel Prize for Chemistry in 1987 was given to Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen for their efforts in discovering and determining uses of cryptands and crown ethers, thus launching the now flourishing field of supramolecular chemistry. The term cryptand implies that this ligand binds substrates in a crypt, interring the guest as in a burial. These molecules are three-dimensional analogues of crown ethers but are more selective and strong as complexes for the guest ions. The resulting complexes are lipophilic.

<span class="mw-page-title-main">Octahedral molecular geometry</span> Molecular geometry

In chemistry, octahedral molecular geometry, also called square bipyramidal, describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The octahedron has eight faces, hence the prefix octa. The octahedron is one of the Platonic solids, although octahedral molecules typically have an atom in their centre and no bonds between the ligand atoms. A perfect octahedron belongs to the point group Oh. Examples of octahedral compounds are sulfur hexafluoride SF6 and molybdenum hexacarbonyl Mo(CO)6. The term "octahedral" is used somewhat loosely by chemists, focusing on the geometry of the bonds to the central atom and not considering differences among the ligands themselves. For example, [Co(NH3)6]3+, which is not octahedral in the mathematical sense due to the orientation of the N−H bonds, is referred to as octahedral.

The coordination geometry of an atom is the geometrical pattern defined by the atoms around the central atom. The term is commonly applied in the field of inorganic chemistry, where diverse structures are observed. The coodination geometry depends on the number, not the type, of ligands bonded to the metal centre as well as their locations. The number of atoms bonded is the coordination number. The geometrical pattern can be described as a polyhedron where the vertices of the polyhedron are the centres of the coordinating atoms in the ligands.

<span class="mw-page-title-main">Uranyl</span> Oxycation of uranium

The uranyl ion is an oxycation of uranium in the oxidation state +6, with the chemical formula UO2+
2. It has a linear structure with short U–O bonds, indicative of the presence of multiple bonds between uranium and oxygen. Four or more ligands may be bound to the uranyl ion in an equatorial plane around the uranium atom. The uranyl ion forms many complexes, particularly with ligands that have oxygen donor atoms. Complexes of the uranyl ion are important in the extraction of uranium from its ores and in nuclear fuel reprocessing.

<span class="mw-page-title-main">Coordination sphere</span> All ligands directly bound to the central metal atom of a chemical complex

In coordination chemistry, the first coordination sphere refers to the array of molecules and ions directly attached to the central metal atom. The second coordination sphere consists of molecules and ions that attached in various ways to the first coordination sphere.

<span class="mw-page-title-main">Metal–organic framework</span> Class of chemical substance

Metal–organic frameworks (MOFs) are a class of compounds consisting of metal clusters coordinated to organic ligands to form one-, two-, or three-dimensional structures. The organic ligands included are sometimes referred to as "struts" or "linkers", one example being 1,4-benzenedicarboxylic acid (BDC).

In chemistry, a Zintl phase is a product of a reaction between a group 1 or group 2 and main group metal or metalloid. It is characterized by intermediate metallic/ionic bonding. Zintl phases are a subgroup of brittle, high-melting intermetallic compounds that are diamagnetic or exhibit temperature-independent paramagnetism and are poor conductors or semiconductors.

Zinc compounds are chemical compounds containing the element zinc which is a member of the group 12 of the periodic table. The oxidation state of zinc in most compounds is the group oxidation state of +2. Zinc may be classified as a post-transition main group element with zinc(II). Zinc compounds are noteworthy for their nondescript appearance and behavior: they are generally colorless, do not readily engage in redox reactions, and generally adopt symmetrical structures.

Covalent organic frameworks (COFs) are a class of materials that form two- or three-dimensional structures through reactions between organic precursors resulting in strong, covalent bonds to afford porous, stable, and crystalline materials. COFs emerged as a field from the overarching domain of organic materials as researchers optimized both synthetic control and precursor selection. These improvements to coordination chemistry enabled non-porous and amorphous organic materials such as organic polymers to advance into the construction of porous, crystalline materials with rigid structures that granted exceptional material stability in a wide range of solvents and conditions. Through the development of reticular chemistry, precise synthetic control was achieved and resulted in ordered, nano-porous structures with highly preferential structural orientation and properties which could be synergistically enhanced and amplified. With judicious selection of COF secondary building units (SBUs), or precursors, the final structure could be predetermined, and modified with exceptional control enabling fine-tuning of emergent properties. This level of control facilitates the COF material to be designed, synthesized, and utilized in various applications, many times with metrics on scale or surpassing that of the current state-of-the-art approaches.

<span class="mw-page-title-main">Metal halides</span>

Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. A few metal halides are discrete molecules, such as uranium hexafluoride, but most adopt polymeric structures, such as palladium chloride.

<span class="mw-page-title-main">3-Pyridylnicotinamide</span> Chemical compound

The organic compound 3-pyridylnicotinamide (3-pna), also known as N-(pyridin-3-yl)nicotinamide, is a kinked dipodal dipyridine that is synthesized through the reaction of nicotinoyl chloride and 3-aminopyridine. The nitrogen atoms on its pyridine rings, like those of its isomer 4-pyridylnicotinamide, can donate their electron lone pairs to metal cations, allowing it to bridge metal centers and act as a bidentate ligand in coordination polymers. It can be used to synthesize polymers that have potentially useful gas adsorption properties.

<span class="mw-page-title-main">Two-dimensional polymer</span>

A two-dimensional polymer (2DP) is a sheet-like monomolecular macromolecule consisting of laterally connected repeat units with end groups along all edges. This recent definition of 2DP is based on Hermann Staudinger's polymer concept from the 1920s. According to this, covalent long chain molecules ("Makromoleküle") do exist and are composed of a sequence of linearly connected repeat units and end groups at both termini.

Metallogels are one-dimensional nanostructured materials, which constitute a growing class in the Supramolecular chemistry field. Non-covalent interactions, such as hydrophobic interactions, π-π interactions, and hydrogen bonding, are among the responsible forces for the formation of those gels from small molecules. However, the main driving force for the formation of a metallogel is the metal-ligand coordination. Once the structure has been established, it resists gravitational force when inverted.

Some metal-organic frameworks (MOF) display large structural changes as a response to external stimuli, and such modifications of their structure can, in turn, lead to drastic changes in their physical and chemical properties. Such stimuli-responsive MOFs are generally referred to as a flexible metal-organic frameworks. They can also be called dynamic metal-organic framework, stimuli-responsive MOFs, multi-functional MOFs, or soft porous crystals.

Carboxylate–based metal–organic frameworks are metal–organic frameworks that are based on organic molecules comprising carboxylate functional groups.

<span class="mw-page-title-main">Hydrogen-bonded organic framework</span>

Hydrogen-bonded organic frameworks (HOFs) are a class of two- or three-dimensional materials formed by hydrogen bonds among molecular monomer units to afford porosity and structural flexibility. There are diverse hydrogen bonding pair choices that could be used in HOFs construction, including identical or nonidentical hydrogen bonding donors and acceptors. For organic groups acting as hydrogen bonding units, species like carboxylic acid, amide, 2,4-diaminotriazine, and imidazole, etc., are commonly used for the formation of hydrogen bonding interaction. Compared with other organic frameworks, like COF and MOF, the binding force of HOFs is relatively weaker, and the activation of HOFs is more difficult than other frameworks, while the reversibility of hydrogen bonds guarantees a high crystallinity of the materials. Though the stability and pore size expansion of HOFs has potential problems, HOFs still show strong potential for applications in different areas.

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