Coordination polymers
Encyclopedia
A coordination polymer is an inorganic or organometallic polymer
structure containing metal cation centers linked by ligands, extending in an array. It can also be described as a polymer whose repeat units are coordination complexes. Similar supramolecular architectures are also called Metal-organic framework
s (MOFs), and coordination
networks, with some inconsistency in the distinctions between the terms. (An IUPAC project was initiated in 2009 to address the terminology issues in this area and will deliver its final report in 2011.) Coordination polymers span scientific fields such as organic
and inorganic chemistry
, biology
, 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 due to particular aspects of 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.
, sometimes called the father of coordination chemistry, won the Nobel Prize
for Chemistry in 1913 "for his work on the linkage of atoms in molecules, by which he has thrown new light on earlier investigations and opened up new fields of research especially in inorganic chemistry." This work laid a lot of the groundwork for the future study of coordination polymers. Terms ubiquitous in the field, such as coordination number
, were coined by Werner. He also determined that even neutral, stable molecules in their own right (such as ammonia
or water) can behave like ligands, and enter into coordination bonds with metals.
In the past few decades, research in this field has boomed, partially because of the potential applications for metal-organic frameworks (MOF's) in areas such as hydrogen storage.
, involving crystallization
of a metal salt
with a ligand. This can be achieved by known crystallization and crystal engineering
techniques. The mechanisms of Molecular self-assembly
processes on a molecular level are currently being investigated, however it is accepted that coordination generally emerges due to the interaction of lone pairs of electrons on the ligand with the regions of low electron density on the cationic metal ion. Crystals of coordination polymers are nucleated when an aggregate of coordinated ligand/metal complexes reaches a size where lowered energy of the aggregate as a whole is more significant than the added surface energy. This means that the crystal will grow more easily in terms of energy as opposed to de-aggregation. This phenomenon is known as nucleation
. The nucleus
is a localized, thermodynamic site for crystal growth
to occur. More molecules (metals and ligands) can self-assemble onto the growing crystal in a periodic array.
For the most part, it is not possible right now to draw any direct correlations between particular structural aspects and applications.
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.)
s, pi-pi interactions, hydrogen bonding, and stabilization of pi bond
s by polarized bonds in addition to the coordination bond formed between the metal and the ligand. These intermolecular force
s 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 Ångstrom
s between parallel faces of the rings.
Metal Centers, also 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.
, 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.
(attaching to multiple linkers) have characteristic luminescent wavelengths.
to strontium
to barium
).
(atom or group of atoms) will formally donate a lone pair
of electron
s to a metal cation and form a coordination complex via a Lewis acid/ base relationship (lewis acids and bases). Coordination polymers are formed when a ligand has the ability to form multiple coordination bonds and act as a bridge between multiple metal centers. Ligands that can form one coordination bond are referred to as monodentate, but those which form multiple coordination bonds, which could lead to coordination polymers are called polydentate. Polydentate ligands are particularly important because it is through ligands that connect multiple metal centers together that an infinite array is formed. Polydentate ligands can also form multiple bonds to the same metal (which is called chelation
). Monodentate ligands are also referred to as terminal because they do not offer a place for the network to continue. Often, coordination polymers will consist of a combination of poly- and monodentate, bridging, chelating, and terminal ligands.
. Ligands that are commonly found in coordination polymers include polypyridines, phenanthrolines, hydroxyquinolines and polycarboxylates. Oxygen and nitrogen atoms are commonly encountered as binding sites, but other atoms, such as sulfur
and phosphorus
, have been observed. Ligands and metal cations tend to follow hard soft acid base theory (HSAB) trends. This means that larger, more polarizable soft metals will coordinate more readily with larger more polarizable soft ligands, and small, non-polarizable, hard metals coordinate to small, non-polarizable, hard ligands.
, exposure to light, or changes in temperature can all change the resulting structure. Influences on the structure based on changes in crystallization environment are determined on a case by case basis.
, 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. Figure 5 shows a schematic of a few different ways a structure can change upon the addition or evacuation of a guest molecule.
s. To modify the pore size in order to achieve effective adsorption, nonvolatile guests (host-guest chemistry
) 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 also convenient for molecular storage, since their pore sizes can be easily altered by physical changes. And 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 crystal 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. The Metal-organic framework
page has a detailed section dealing with H2 gas storage.
arises from electronic transitions from an excited state
, caused by photoexcitation, to the ground state
, resulting in the emission of light
. Most luminescent compounds require organic chromophoric ligands, which absorb light and then pass the excitation energy to the metal ion, which is referred to as a ligand-to-metal charge-transfer
process (LMCT). 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 or 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.
. Antiferromagnetism
, ferrimagnetism
, and ferromagnetism
are cooperative phenomena of the magnetic spins within a solid. They require an interaction or coupling between the spins of the paramagnetic centers. Since intramolecular interactions (such as bonding) are much more efficient than intermolecular interactions (i.e. through space), three-dimensional coordination polymers are of interest because they connect their magnetic centers through direct coordinative links, and have the possibility for displaying high critical temperatures. 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).
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.
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
structure containing metal cation centers linked by ligands, extending in an array. It can also be described as a polymer whose repeat units are coordination complexes. Similar supramolecular architectures are also called Metal-organic framework
Metal-organic framework
Metal-Organic Frameworks are crystalline compounds consisting of metal ions or clusters coordinated to often rigid organic molecules to form one-, two-, or three-dimensional structures that can be porous. In some cases, the pores are stable to elimination of the guest molecules and can be used for...
s (MOFs), and coordination
Coordination
Coordination is the act of coordinating, making different people or things work together for a goal or effect to fulfill desired goals in an organization.Coordination is a managerial function in which different activities of the business are properly adjusted and interlinked.Coordination may also...
networks, with some inconsistency in the distinctions between the terms. (An IUPAC project was initiated in 2009 to address the terminology issues in this area and will deliver its final report in 2011.) Coordination polymers span scientific fields such as organic
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
and inorganic chemistry
Inorganic chemistry
Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. This field covers all chemical compounds except the myriad organic compounds , which are the subjects of organic chemistry...
, biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
, materials science
Materials science
Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates...
, electrochemistry
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
, 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 due to particular aspects of 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
Even though coordination polymers have experienced a research boom in the past decade or so, research in this field has been around for a long time. Alfred WernerAlfred Werner
Alfred Werner was a Swiss chemist who was a student at ETH Zurich and a professor at the University of Zurich. He won the Nobel Prize in Chemistry in 1913 for proposing the octahedral configuration of transition metal complexes. Werner developed the basis for modern coordination chemistry...
, sometimes called the father of coordination chemistry, won the Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...
for Chemistry in 1913 "for his work on the linkage of atoms in molecules, by which he has thrown new light on earlier investigations and opened up new fields of research especially in inorganic chemistry." This work laid a lot of the groundwork for the future study of coordination polymers. Terms ubiquitous in the field, such as coordination number
Coordination number
In chemistry and crystallography, the coordination number of a central atom in a molecule or crystal is the number of its nearest neighbours. This number is determined somewhat differently for molecules and for crystals....
, were coined by Werner. He also determined that even neutral, stable molecules in their own right (such as ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
or water) can behave like ligands, and enter into coordination bonds with metals.
In the past few decades, research in this field has boomed, partially because of the potential applications for metal-organic frameworks (MOF's) in areas such as hydrogen storage.
Synthesis and Propagation
Much of the mechanism for growth coordination polymers is based on self-assemblySelf-assembly
Self-assembly is a term used to describe processes in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction...
, involving crystallization
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
of a metal salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
with a ligand. This can be achieved by known crystallization and crystal engineering
Crystal engineering
Crystal engineering is the design and synthesis of molecular solid-state structures with desired properties, based on an understanding and exploitation of intermolecular interactions. The two main strategies currently in use for crystal engineering are based on hydrogen bonding and coordination...
techniques. The mechanisms of Molecular self-assembly
Molecular self-assembly
Molecular self-assembly is the process by which molecules adopt a defined arrangement without guidance or management from an outside source. There are two types of self-assembly, intramolecular self-assembly and intermolecular self-assembly...
processes on a molecular level are currently being investigated, however it is accepted that coordination generally emerges due to the interaction of lone pairs of electrons on the ligand with the regions of low electron density on the cationic metal ion. Crystals of coordination polymers are nucleated when an aggregate of coordinated ligand/metal complexes reaches a size where lowered energy of the aggregate as a whole is more significant than the added surface energy. This means that the crystal will grow more easily in terms of energy as opposed to de-aggregation. This phenomenon is known as nucleation
Nucleation
Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...
. The nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
is a localized, thermodynamic site for crystal growth
Crystal growth
A crystal is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. Crystal growth is a major stage of a crystallization process, and consists in the addition of new atoms, ions, or polymer strings into...
to occur. More molecules (metals and ligands) can self-assemble onto the growing crystal in a periodic array.
For the most part, it is not possible right now to draw any direct correlations between particular structural aspects and applications.
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
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
, growing a crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
of sufficient size and quality is important.)
Intermolecular Forces and Bonding
The forces that led to spontaneous arrangement of molecules (metal-ligand complexes in this case) are thought to be van der Waals forceVan der Waals force
In physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...
s, pi-pi interactions, hydrogen bonding, and stabilization of pi bond
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
s by polarized bonds in addition to the coordination bond formed between the metal and the ligand. These intermolecular force
Intermolecular force
Intermolecular forces are forces of attraction or repulsion which act between neighboring particles: atoms, molecules or ions. They are weak compared to the intramolecular forces, the forces which keep a molecule together...
s tend to be weak, with a long equilibrium distance (bond length) compared to covalent bonds. The pi-pi interactions between benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
rings, for example, have energy roughly 5-10 kJ/mol and optimum spacing 3.4-3.8 Ångstrom
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
s between parallel faces of the rings.
Coordination
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 16 functional sites on linkers. This number of possible bonds is the coordination number. Coordination numbers are more often between 2 and 10, and the bonding sites are separated by distinct angles. Coordination numbers are shown in planer geometry in Figure 2. In Figure 1 the 1D structure is 2-coordinated, the planer is 4-coordinated, and the 3D is 6-coordinated. Figure 3 shows structures hinting at the complexity that is possible by changing coordination geometry.Metal Centers
Metal Centers, also 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
Coordination number
In chemistry and crystallography, the coordination number of a central atom in a molecule or crystal is the number of its nearest neighbours. This number is determined somewhat differently for molecules and for crystals....
, which, along with the angles they are held at, determines the dimensionality of the structure. The coordination number
Coordination number
In chemistry and crystallography, the coordination number of a central atom in a molecule or crystal is the number of its nearest neighbours. This number is determined somewhat differently for molecules and for crystals....
and coordination geometry
Coordination geometry
The term coordination geometry is used in a number of related fields of chemistry and solid state chemistry/physics.-Molecules:The coordination geometry of an atom is the geometrical pattern formed by atoms around the central atom....
of a metal center is determined by the nonuniform distribution of electron density
Electron density
Electron density is the measure of the probability of an electron being present at a specific location.In molecules, regions of electron density are usually found around the atom, and its bonds...
around it, and in general the coordination number increases with cation size. Several models, most notably hybridization
Orbital hybridisation
In chemistry, hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. Hybridised orbitals are very useful in the explanation of the shape of molecular orbitals for molecules. It is an integral part...
model and molecular orbital theory
Molecular orbital theory
In chemistry, molecular orbital theory is a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule...
, 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
Electron density
Electron density is the measure of the probability of an electron being present at a specific location.In molecules, regions of electron density are usually found around the atom, and its bonds...
distribution.
Transition Metals
Transition metals are commonly used as nodes. Partially filled d orbitalsAtomic orbital
An atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus...
, either in the atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
or ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
, can hybridize
Orbital hybridisation
In chemistry, hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. Hybridised orbitals are very useful in the explanation of the shape of molecular orbitals for molecules. It is an integral part...
differently depending on environment. This electronic structure causes some of them to exhibit multiple coordination geometries
Coordination geometry
The term coordination geometry is used in a number of related fields of chemistry and solid state chemistry/physics.-Molecules:The coordination geometry of an atom is the geometrical pattern formed by atoms around the central atom....
, 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 10 due to filling of f orbitals. They are strongly affected by their environment and difficult to predict, making them challenging to use as nodes. This challenge may be worth it however, because the bonds formed by multiplexingMultiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
(attaching to multiple linkers) have characteristic luminescent wavelengths.
Alkali/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 effected by the counter ion 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 the of these structures increases as the radius of the metal increases down the group (from calciumCalcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
to strontium
Strontium
Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
to barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
).
Ligands
In most coordination polymers, a ligandLigand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
(atom or group of atoms) will formally donate a lone pair
Lone pair
In chemistry, a lone pair is a valence electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons...
of electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s to a metal cation and form a coordination complex via a Lewis acid/ base relationship (lewis acids and bases). Coordination polymers are formed when a ligand has the ability to form multiple coordination bonds and act as a bridge between multiple metal centers. Ligands that can form one coordination bond are referred to as monodentate, but those which form multiple coordination bonds, which could lead to coordination polymers are called polydentate. Polydentate ligands are particularly important because it is through ligands that connect multiple metal centers together that an infinite array is formed. Polydentate ligands can also form multiple bonds to the same metal (which is called chelation
Chelation
Chelation is the formation or presence of two or more separate coordinate bonds between apolydentate ligand and a single central atom....
). Monodentate ligands are also referred to as terminal because they do not offer a place for the network to continue. Often, coordination polymers will consist of a combination of poly- and monodentate, bridging, chelating, and terminal ligands.
Chemical Composition
Almost any type of atom with a lone pair of electrons can be incorporated into a ligandLigand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
. Ligands that are commonly found in coordination polymers include polypyridines, phenanthrolines, hydroxyquinolines and polycarboxylates. Oxygen and nitrogen atoms are commonly encountered as binding sites, but other atoms, such as sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
and phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
, have been observed. Ligands and metal cations tend to follow hard soft acid base theory (HSAB) trends. This means that larger, more polarizable soft metals will coordinate more readily with larger more polarizable soft ligands, and small, non-polarizable, hard metals coordinate to small, non-polarizable, hard ligands.
Structural Orientation
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. This creates more variety in the structure. There are examples of coordination polymers that include two configurations of the same ligand within one structure, as well as two separate structures where the only difference between them is ligand orientation. An example of a flexible ligand is shown in Figure 4.Other Factors
Counter Ion
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 counter ion in the salt can effect the overall structure. For example, silver salts such as AgNO3, AgBF4, AgClO4, AgPF6, AgAsF6 and AgSbF 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.Crystallization Environment
Additionally, variations in the crystallization environment can also change the structure. Changes in pHPH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
, exposure to light, or changes in temperature 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 structure of coordination polymers oftentimes 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 oxygenOxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, etc.) The presence of the guest molecule can sometimes influence the structure by supporting a pore or channel, where otherwise none would exist. Figure 5 shows a schematic of a few different ways a structure can change upon the addition or evacuation of a guest molecule.
Molecular Storage
Thanks to coordination polymerization in three dimensions, pores within the structure can be made. The size and shapes of the pore can be controlled by the linker size and the connecting ligands’ length and functional groupFunctional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...
s. To modify the pore size in order to achieve effective adsorption, nonvolatile guests (host-guest chemistry
Host-guest chemistry
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...
) are intercalated in the porous coordination polymer space to decrease the pore size. Active surface guests can also be used contribute to adsorption
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
. 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 also convenient for molecular storage, since their pore sizes can be easily altered by physical changes. And 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 crystal 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. The Metal-organic framework
Metal-organic framework
Metal-Organic Frameworks are crystalline compounds consisting of metal ions or clusters coordinated to often rigid organic molecules to form one-, two-, or three-dimensional structures that can be porous. In some cases, the pores are stable to elimination of the guest molecules and can be used for...
page has a detailed section dealing with H2 gas storage.
Luminescence
LuminescenceLuminescence
Luminescence is emission of light by a substance not resulting from heat; it is thus a form of cold body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal. This distinguishes luminescence from incandescence, which is light emitted by a...
arises from electronic transitions from an excited state
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
, caused by photoexcitation, to the ground state
Ground state
The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state...
, resulting in the emission of light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
. Most luminescent compounds require organic chromophoric ligands, which absorb light and then pass the excitation energy to the metal ion, which is referred to as a ligand-to-metal charge-transfer
Charge-transfer
Charge-transfer may refer to:* Intervalence charge transfer* Charge-transfer complex* Charge-exchange ionization, a form of gas phase ionization...
process (LMCT). 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 or the free ligand alone. These materials can be used for designing potential candidates for light emitting diode (LED
LEd
LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....
) devices. The dramatic increase in fluorescence is caused by the increase in rigidity and asymmetry of the ligand when coordinated to the metal center.
Electrical Conductivity
Coordination polymers can have short inorganic and conjugated organic bridges in their structures, which provide pathways for electrical conduction. Some one-dimensional coordination polymers built as shown in Fig. X 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 semi-conductor 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.Magnetism
Interest in coordination polymers is continued by the application of magnetismMagnetism
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
. Antiferromagnetism
Antiferromagnetism
In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usuallyrelated to the spins of electrons, align in a regular pattern with neighboring spins pointing in opposite directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism...
, ferrimagnetism
Ferrimagnetism
In physics, a ferrimagnetic material is one in which the magnetic moments of the atoms on different sublattices are opposed, as in antiferromagnetism; however, in ferrimagnetic materials, the opposing moments are unequal and a spontaneous magnetization remains...
, and ferromagnetism
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
are cooperative phenomena of the magnetic spins within a solid. They require an interaction or coupling between the spins of the paramagnetic centers. Since intramolecular interactions (such as bonding) are much more efficient than intermolecular interactions (i.e. through space), three-dimensional coordination polymers are of interest because they connect their magnetic centers through direct coordinative links, and have the possibility for displaying high critical temperatures. 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).
Sensor Capability
Coordination polymers can also show color changes upon the change of solventSolvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
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
Tetrahydrofuran
Tetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...
, 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.