Author: Kisholoy Bhattacharya

Coordination chemistry is a branch of chemistry that deals with compounds formed by formation of coordinate covalent bond between a metal and a neutral or negatively charged molecule called ligand. Although this is a part of inorganic chemistry but coordination chemistry plays very crucial role in biological systems. Biological system is a diverse system. Varieties of metal ions are found in the biological systems and so it is expected that coordination compounds will be found in the system. Roles of few such coordination compounds are listed below.

1. Metalloproteins: Metals are commonly found as natural constituent of proteins. Nature has learned to use the special properties of metal ions to perform a wide variety of specific functions associated with life processes. Examples of such metalloproteins and their roles in biological processes are as follows.

1.1 Hemoglobin and myoglobin: Myoglobin is monomeric complex of Iron-porphyrin whereas hemoglobin is tetrameric complex of same unit. Myoglobin acts as oxygen storage protein whereas hemoglobin can store and transport proteins. This protein contains Iron atom that has coordination number six that is like a man with six hands. In the metalloprotein only five atoms coordinate and the sixth position remains vacant that means five hands are blocked but the sixth hand is empty and so this hand can hold a molecule. That is a ligand from outside can coordinate at the sixth coordination site. Oxygen molecule can reversibly bind at the sixth position. Since the binding with oxygen is reversible that is two way process so it can release the oxygen molecule whenever necessary. Thus hemoglobin transports oxygen in blood. It takes oxygen from air in the lungs and deliver it to myoglobin which are found in tissues where myoglobin stores the oxygen and releases during respiration.

1.2 Electron transfer proteins: This kind of metalloproteins act as electron carriers during redox reactions that is reactions occurring with transfer of electrons in biological system. Such biological current carriers usually pass electrons to or from enzymes that require redox chemistry in order to perform a specific function. Two such metalloproteins widely encountered in biological systems are cytochromes and iron-sulfur clusters. In iron-sulfur clusters the iron atom is coordinated by cysteine sulfur as well as inorganic sulfide sulfur whereas cytochrome is a complex of iron-porphyrin system that has resemblance with the structure of hemoglobin used for oxygen transport. Both of these metalloproteins are situated in the mammalian respiratory chain and plays crucial role in the transportation of electrons from one substance to another in the respiratory chain during respiration process.

2. Metalloenzymes: Metalloenzymes are a subclass of metalloproteins that perform specific catalytic functions. The metal centre presents in such kind of molecules, catalyze the required chemical transformation. Different kinds of metalloenzymes are found in the biological system. They are classified according to their functions as follows:

2.1 Hydrolytic enzymes: These type of proteins catalyze addition or removal of water in a substrate molecule. Notable example includes carbonic anhydrase which promote hydrolysis of CO2; peptidases and esterases which catalytically hydrolyze carbonyl compounds; and phosphatases which catalytically cleavage the phosphate esters. In most of the cases the metalloenzyme contains Zn²⁺ ion. Presence of Zn²⁺ ion helps in lowering the pK_a of coordinated oxygen containing ligands, such as the above said substrates, and also can avoid the potential for undesired electron transfer chemistry, since divalent zinc does not have any readily accessible redox states. Other metal ions found in hydrolytic enzymes,  also known as hydrolases are Mn²⁺, Ni²⁺, Ca²⁺, and Mg²⁺. All these metal ions have the ability to avoid redox activity and only alters the pKa value of the oxygen containing ligands so that addition or removal of water molecule in a substrate can be catalyzed.

2.2 Two electron redox enzymes: Many metalloenzyme catalyze reactions that involve either oxidation or reduction of substrate. These reactions are generally two electron redox process. Most common reactions of this type is addition of oxygen atom to the substrate. For example oxidation of hydrocarbons to alcohols catalyzed by cytochrome P-450 enzymes containing iron-porphyrin moiety as the active centre, ortho-hydroxylation of phenolic substrates catalyzed by tyrosinase, which contains a dsinuclear copper in the active site etc. Addition of oxygen is two electrons oxidation process and in biological system these processes are catalyzed by such metalloenzymes due to the presence of redox active metal centres in the active site of the enzyme. Some metalloenzymes remove oxygen atoms from a substrate. This process is accompanied by two electrons reduction and are catalyzed by metalloenzymes, For example nitrate reductase which catalyzes the reduction of nitrate to nitrite and the redox active metal in the enzyme that catalyzes the process is molybdenum.

2.3 Multielectron pair redox enzymes: Metalloenzymes also take part in multi electron pair redox transformations. One of most important such reaction occurring in the respiratory chain is the conversion of oxygen molecule into water that involve four electrons transfer process. Cytochrome c oxidase, a highly complex enzyme containing two copper and two iron-porphyrin centres, catalyzes the reduction of oxygen molecule into water. Another iron containing enzyme catechol dioxygenase cleaves the aromatic ring of catechol and this is also a multi electron transfer process.

3. Metal complexes as medicines: Many metal complexes can also be used as drugs in the treatment of chronic diseases. Cisplatin is a complex of platinum that is used as anticancer drug; Auranofin is an oral rheumatoid arthiritis drug which is a complex of gold: Cardiolyte is a technicium containing complex used as heart imaging agent. Calcium-edta (edta = ethylenediaminetetraacetic acid) complex is injected in our body to remove lead poisoning. Moreover metal ions like Hg²⁺ have been commonly used for the treatment of syphilis, Mg²⁺ for intestinal disorders, and Fe²⁺ for anaemia, over the centuries.

In conclusion we can say that coordination chemistry and coordination complexes or metal complexes play a very crucial role in different biological processes and biochemical field. In this way metal complexes take a part in sustaining the life in the earth.

Shortage of energy and environmental pollution is a growing problem nowadays. To get rid of pollution, attention has been given in the development of environmentally friendly and clean fuels such as H₂, CH₄ etc. So it is always a great challenge for society as well as scientists to store these fuels and use them efficiently with low cost and safety. Environment is getting polluted due to unreasonable use of fossil fuels and liberation of harmful and toxic gases (such as NO_x, SO_x, CO, H₂S, NH₃ etc.) from various industries into the environment. In this continuous developing age this has become an worldwide problem. Moreover these harmful gases are major threat to human health also. So effective removal of these gases are an urgent need to protect the environment as well as prevent the health problems.

Porous materials are widely used in the field of energy and environment as they contain adequate pore structures. Metal organic frameworks (MOFs) are these kind of porous materials. These are actually a kind of periodic porous materials and these are formed through self assembly of metal ions with organic ligands. These compounds posses the properties of both organic as well as inorganic materials. MOFs have special properties like large surface area, tunable pore sizes and high porosity and by virtue of these properties these materials can be used in gas storage and separation. All these qualities have led scientists to utilize MOFs in the field of energy and environmental application like energy storage, capture and separation of pollutants, sensing of harmful gases etc.

Energy storage: Microporous MOF materials with high surface area have the advantage in the storage of environment friendly fuels such as H₂ and CH₄. In these kind of compounds the metal centre remains coordinatively unsaturated i.e. one or more coordination site remains vacant and as a result the metal can bind hydrogen very easily. So they can be used in hydrogen storage. Hydrogen storage capacity can be enhanced by synthesizing MOF composites. MOF composites are synthesized by combination of MOFs with other materials such as metal nanoparticle, metal oxides, polyoxometallates etc. Such compounds optimizes the large void space that help in storage of hydrogen. These kinds of materials are also potential adsorbent for methane and depending on the pore size they can be utilized for storage of methane gas which is also an environment feiendly fuel.

Capture and separation of pollutants: All of us know that CO₂ is a green house gas causing global warming. Moreover presence of CO₂ gas with other natural gases prevent full combustion and leads to potential security problem. MOFs having high porosity, specific pore size and tunable chemical affinity have been extensively developed by chemists and these have been utilized in capture and separation of CO₂. MOF composites enhance the CO₂ uptake ability due to creation of new pores at the interface of two materials. Tunable pore size and modifiable functional groups help MOFs to show selectivity for a particular gas over others. That is if a MOF contains large pore size then it do not accommodate small molecule as in that case interaction becomes poor. Similarly if the pore size is small then it cannot accommodate large molecules as in that case the guest molecule do not get sufficient space to be seated in the cavity. Change in polarity of functional groups also changes the affinity of the framework for a particular molecule over others. Combination of all these properties of MOFs helped scientists to utilize these compounds in the field of separation of pollutant gas molecule from other gases in the environment.

Adsorption of harmful and toxic gases: Since MOFs contain large surface area so they can be utilized for adsorption of gases and thus many harmful gases like H₂S, NH₃, CO, NO, etc. can be removed from environment. Hydrogen sulfide (H₂S) is a toxic chemical gas released from petroleum and natural gas industries. Humans are extremely sensitive to H₂S due to its rotten egg smell. However removal of this kind of gas from environment is necessary to avoid pollution. Adsorption by MOFs is a highly effective technology for the removal of H₂S gas from environment. Due to presence of sulphur atom this gas can bind with many metal centres especially with soft metal ions and this property has been utilized in the separation of H₂S from the environment.

Ammonia (NH₃) is a harmful and irritating gas. It gives rise to accidents from spill or explosions at chemical industries. So it is very necessary to remove NH₃ from the environment. NH3 can interact with -OH and -NH₂ group present in the organic ligand part of the framework through hydrogen bonding. Moreover if the MOF contain coordinatively unsaturated metal centre then NH₃ can bind it by coordination and so use of MOFs containing such properties has been found highly effective in the removal of NH₃ from the environment.

Carbon monoxide is a colourless, odourless gas that can bind with the iron (Fe) atom present in the hemoglobin resulting in decrease of oxygen carrying capacity of blood. Inhalation of a large amount of CO can cause poisoning and even life threatening to human being. So removal of this gas is very much necessary from the environment. Utilization of copper (Cu) containing MOF has been found effective in the removal of CO from environment due to electrostatic interaction of CO with the framework. Such kind of MOFs have also been found effective in the removal of nitric oxide gas (NO) which is also harmful for our respiratory system and hence should be removed from environment.

Chemical sensor: Identification of harmful and toxic gases from environment by sensing is the preliminary step to protect human’s health. It requires initially adsorption of the gas molecules on the surface producing a signal. Among the various sensing materials available, MOFs are very promising materials to build up such sensors due to their various properties. Due to high porosity they can act as good host for reversible adsorption and desorption of guest molecules and it ensures the repeatability of detection. The tunable pore size and large surface area help them in enhancing the selectivity and sensitivity. As a combination of all these factors we can utilize MOFs as sensors for detection of harmful and toxic gases in the environment and then separation of such gases by using suitable MOFs.

In conclusion, we can say that MOFs are a kind of porous materials that can be utilized in the application field of energy storage, separation of pollutant gases from atmosphere, sensing, adsorption and removal of harmful and toxic gases from the environment. That is they are very much useful materials for environment.