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What is Coordination Chemistry?
What are Rare Earths?
What are Ionic Liquids?

What is Coordination Chemistry?

In the past, chemistry as a scientific discipline was divided into different subfields: analytical chemistry, organic chemistry, inorganic chemistry, physical chemistry and biochemistry. Because of the progress in scientific research, chemistry is becoming very interdisciplinary, so that is more and more difficult to adhere to the classical subdivisions of the chemical sciences.

It is not easy to explain to an outsider what Coordination Chemistry is. Coordination Chemistry is the chemistry of coordination compounds or metal complexes. A coordination compound consists of a central metal ion surrounded by a certain number of ligands. These ligands can be simple inorganic anions (F-, Cl-, Br-, I- , NO3-, SO42- ,...) or molecules (H2O, NH3 , ...), but also large organic molecules can act as ligands. The ligands are bound via so-called dative or coordinative bonds to the central metal ion. The ligands don't really need the metal to complete their valence shell, but the interaction with the metal ion will result nevertheless in a coordination compound, which is more stable than the metal ion and the ligands separately. Because the central metal ion is very often a d-block element (transition metals) or an f-block element (lanthanides and actinides), coordination chemistry is for many scientist a synonym for the chemistry of the transition metals, lanthanides and actinides. However, s-block and p-block elements can also form coordination compounds. The difference between a coordination compound and a organometallic compound is that in an organometallic compound a direct metal-carbon (M-C) bond is present, whereas in a coordination compound there is always a heteroatom (O,N, S, P, ..) located between the metal and the carbon atom or there is no carbon atom present at all. Carbonyl compounds (with CO as the ligand) and cyano compounds (with CN- as the ligand) are borderline cases of compounds with direct metal-carbon bonds. In general, carbonyl complexes are considered as organometallic compounds and cyano complexes as coordination compounds. Due to the presence of a metal ion with unpaired electrons, coordination compounds can have interesting spectroscopic and magnetic properties. Often, coordination compounds are intensively colored.

Although coordination compounds such as the tetramminecopper(II) complex, [Cu(NH3)4]2+ , and 'Berlin blue', Fe4 [Fe(CN)6]3 , have been known for centuries, Coordination Chemistry is a branch of chemistry since 1893. The foundations of Coordination Chemistry were laid by the Swiss Alfred Werner (1866-1919), who was awarded the Nobel Prize of Chemistry in 1913 for his pioneering work on metal complexes. In honor of Werner, simple coordination compounds are often called Werner-complexes. It was not before the end of World War II that Coordination Chemistry became a popular research field, thanks to the impulse programmes for the development of nuclear energy and to the development of theoretical models that allowed to explain the chemical bonding in metal complexes (for instance the Ligand Field Theory). Presently, Coordination Chemistry is an important branch of chemistry. This is reflected by the fact that international conferences on Coordination Chemistry are often attended by more than 1000 scientists and by the fact that a major part of the papers published in international journals on inorganic or general chemistry are devoted to coordination compounds. In the subfield of Supramolecular Coordination Chemistry, metal complexes that form larger entities or new types of aggregation states by non-covalent interactions (e.g. by hydrogen bonding or by dipole-dipole interactions) are being studied. An example of supramolecular coordination compounds are the liquid-crystalline metal complexes (metallomesogens).

Coordination compounds are a very important class of chemicals, because some of them play an essential role in the biochemical processes of living beings. For instance, chlorophyll (the green plant pigment), hemoglobin (the red blood pigment) and vitamin B12 are coordination compounds. Many enzymes contain a metal ion, and as such they can be considered as coordination compounds. By studying simple coordination compounds, one can gain insight in the mechanism of complex biochemical processes based on the use of a metal ion inside the cell. Many dyes and pigment, for instance the blue color of writing ink, are metal complexes. Coordination compounds are of importance for medical diagnosis and therapy: contrast agents for magnetic resonance imaging (MRI), the active compounds in chemotherapy and in photodynamic therapy for the treatment of cancer contain a metal ion as an essential component. Metal complexes are being studied as potential new drugs (metallopharmaceutics). A number of catalysts used in the chemical industry make use of coordination compounds.

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What are Rare Earths?

The name ' rare earths ' is given to the series of 17 elements consisting of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu). The term ' lanthanides ' used for the series of the rare earths, with the exception of yttrium and scandium. The symbol Ln (which is also enclosed in the logo of our lab) stands for any of the lanthanides. The elements of the lanthanides differ from one another by the number of electrons in the 4f-orbitals. The name 'rare earths' is an unfortunate choice. Although the rare-earth elements have exotic names, most of them are not rare at all. The natural abundance of cerium is comparable with that of copper. Even the rarest elements among the rare earth (thulium and lutetium) are more abundant than mercury, silver or gold. The reason of the misconception is a historical one, because in contradiction to many other metals, the rare earths are in nature not often concentrated in ore bodies, but they are dispersed in silicate rocks. This gives the impression that they are rare.

The lanthanides (or the rare earths in general) find many applications in our modern society, although only very few people are aware of these applications. Indeed, the rare earths play a role in many crucial components of electronic devices, but as such they are not visible for the users. But without strong permanent magnets based on elements of the lanthanide series (mainly neodymium) it would have been impossible to design small mobile telephones. These magnets find also an application in loud speakers, electric motors, ... Lanthanides can provide light. The white phosphor layer of luminescent lamps (fluorescent discharge lamps) contain europium (Eu3+ for the red color, Eu2+ for the blue color) and terbium (Tb3+ for the green color), in combination with yttrium, cerium and gadolinium. By combination of the blue, the green and the red phosphor in the appropriate proportions, a white emission color is obtained. The red color of a CRT display (television or computer screens) is due to an europium compound. Many lasers contain lanthanide ions as the active component. The best known laser based on lanthanide ions is without doubt the Nd-YAG laser, in which a trivalent neodymium ion is embedded in an yttrium aluminum garnet host crystal (Y3Al5O12 ) and which emits infrared light at a wavelength of 1064 nm. Besides a number of other important applications such as catalysts for the synthesis of synthetic rubber or as active component in the three-way catalysts for cars, we want to point on the importance of gadolinium as contrast agent for magnetic resonance imaging (MRI). The rare earths thank their applicability to their unique electronic and magnetic properties. More information about these elements on the Internet can be accessed via these links .

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What are Ionic Liquids?

Just as ionic inorganic compounds (for instance sodium chloride), ionic liquids consist entirely of positive and negatively charged ions. While the melting point of ionic inorganic compounds is typically several hundreds degrees centigrade, the melting point of ionic liquids is by definition below 100 °C. Several types of ionic liquids are liquid at room temperature (room temperature ionic liquids or RTILs).

The cation of an ionic liquid is often a large organic cation, such as an imidazolium, a pyridinium or a quaternary ammonium ion. As anions, Cl-, Br- , BF4-, PF6- or CF3SO3- are popular choices. An example of a well-known ionic liquid is 1-ethyl-3-methylimidazolium hexafluorophosphate, which is abbreviated to [C2mim][PF6]. Because the properties of ionic liquids (miscibility with water and other solvents, dissolving ability, polarity, viscosity, density) can be tuned by an appropriate choice of the anion and the cation, ionic liquids are often considered as designer solvents. Presently, there exists worldwide an intense research activity in the field of ionic liquids, because these solvents have several interesting properties. The vapour pressure of an ionic liquid is extremely low, so that ionic liquids are non-volatile and do not evaporate. Ionic liquids can be used as environmentally friendly substitutes for volatile organic compounds (VOCs).
After reaction, the reaction products can be separated from the ionic liquid by distillation. Ionic liquids are fluid over a broad temperature range, from the melting point to the onset of thermal decomposition. Because ionic liquids are non-flammable and non-explosive, they are much safer to work with in the lab than the conventional organic solvents. Due to their ionic nature, ionic liquids conduct electricity. Ionic liquids have a high electrochemical stability, which means that they are very resistant to oxidation and reduction. By dissolving metal salts in ionic liquids, reactive metals can be deposited and purified by electrolysis. Ionic liquids are polar solvents, and their polarity is comparable with the polarity of the lower alcohols. In contrast to other polar organic solvents, ionic liquids are non-coordinating and non-solvating.
Ionic liquids are good solvents for many organic and inorganic compounds, but at the same time they are not very corrosive. For instance, ionic liquids are one of the very few solvents in which cellulose is soluble. By choosing ionic liquids as the solvent for organic reactions, it is possible to obtain a higher reaction rate, higher yields and a better selectivity than when conventional organic solvents are used. For a long time, ionic liquids were an academic curiosity. Research was mainly focused on haloaluminate ionic liquids as ion-conductive electrolytes for batteries and as a medium for electrodeposition of reactive metals.

Typical first-generation ionic liquids are mixtures of aluminium chloride and N-butylpyridinium chloride in different molar ratios. Although these first-generation ionic liquids performed well, they were cumbersome to handle due to their moisture- and oxygen-sensitivity. Therefore, one has to work with these solvents in a glove-box. A major breakthrough was the development of the second-generation of ionic liquids like the 1-alkyl-3-methylimidazolium hexafluorophosphates, which are moisture and air-stable. Because these ionic liquids can be handled on the lab bench like an conventional organic solvent, and because they have at the same time the advantageous properties of molten salts like a low vapour pressure and remarkable solubilising properties, this second generation of ionic liquids attracted the attention of the wide scientific community. Researchers began to use the ionic liquids for applications other than battery electrolytes and electrolyte baths for electrodeposition of metals, for instance as solvents for organic synthesis and especially as solvents for catalytic reactions. Of special interest is the fact that enzymes retain their activity in ionic liquids, so that these solvents can be used in biocatalysis and in biochemical technology.

More information on ionic liquids as well as many interesting links can be found on the Ionic Liquids @ K.U.Leuven website.

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URL: http://www.chem.kuleuven.be/research/coord/coc_eng/generalinfo/introduction/coc.htm