The term Organometallic arises from the covalent bond existing between a carbon atom and a metal atom and as the name suggests, it is a compound that is partly organic and partly metallic. There are, however, compounds containing metal to hydrogen bonds and also other compounds containing non-metallic atoms bonded to carbon which are classed under Organometallic compounds. The earliest synthesis of organometallic compounds originated form an English Chemist, Sir Edward Franklin, in 1845 who worked on the interaction between zinc (Zn) and an alkyl halide. It is believed he was attempting to synthesize alkyl radicals. The key to enable one to class a compound as an organometallic compound however, is for the compound to have at least one carbon to metal bond (which is referred to as a sigma bond) or a carbon to a metal complex bond (which is known as a pi bond). Hunt, I. Department of Chemistry, University of Calgary.
To gain understanding of the general reactivity of organometallic compounds, it would be prudent to view them as ionic compounds based on the values of their electrostatic potentials which are similar to that of ionic compounds. These organometallic compounds react as electron rich compounds and have anionic carbon atoms which are often referred to as carbanions. Carbanions function as either bases or nucleophiles and their reactivity is determined by a number of factors including inductive effect, hydilisation, and the extent of conjugation of the anion. There are several synthetic ways of forming a metal to carbon bond and some of these common but important processes are as outlined below.
- The direct reaction of the metal – this involves the direct reaction of an alkyl halide or an aryl halide with mainly lithium (Li), sodium (Na), potassium (K), magnesium (Mg), calcium (Ca) and zinc (Zn) resulting in what is known as the Grignard reagent.
- The use of alkylating agents – this relates to the use of other organometallic compounds such as aluminium alkyls as reactants.
- Insertion reactions – as the term suggests, the chemical compound interposes itself into an existing bond of typically the second chemical compound.
- Oxidative addition reaction – here, a metal complex with vacant coordination sites and a relatively low oxidation state is oxidized by the insertion of the metal into a covalent bond. Cotton et al. (1987)
The various methods of forming an organometallic compound are crucial as this has a great bearing on it uses. It is essential to understand the basics of these organometallic reactions and synthesis and also to know the appropriate conditions for particular products as they have a wide range of uses across individual industries.
A good and popular example of an organometallic reaction is the formation of an organomagnesium halide (RMgX) commonly known as a Grignard reagent named after the French Chemist Victor Grignard who acquired a Nobel Prize for his work in 1912 surrounding organometallic chemistry. They are generally reagents that find application in the synthesis of general bases, heterogeneous catalyst supports, intermediates, polymers and pharmaceuticals. A Grignard reagent is formed as a result of the reaction between magnesium metal and an organohalogen compound in the presence of ether as solvent.
The most appropriate reagent/solvent to use for a Grignard reaction is diethey ether as it does not react with the final product but rather donates unshared electrons to help dissolve the organometallic compound (Fessenden et al., 1990).
In general, organometallic compounds are known to be highly reactive species particularly with water and air due to their ionic nature and are therefore stored or kept in organic solvents. Some common properties of organometallic compounds are relatively low melting points, insolubility in water, solubility in ether and related solvents, toxicity, its ability to oxidise and high reactivity. As with all other compounds, these properties are what determine how and where these compounds can be useful or cause harm. In synthesis, they act as nucleophiles that can bond with relatively positive carbon atoms in compounds such as alkyl halides, aldehydes, and ketones. A typical example of these syntheses is as shown above in the production of a Grignard reagent (Crabtree, 2005).
The importance of organometallic compounds in our day to day lives can almost be associated with basic necessities. There are several uses of organometallic compounds across a wide range of industries including agriculture, medical/pharmaceutical, petroleum, polymer etc. In the agricultural industry for instance there are a number of popular herbicides, insecticide and fungicides used which are organometallic compounds. Take the paraquat, the trade name for N,N’-dimethyl-4,4′-bipyridinium dichloride, one of the most commonly used herbicides available today (Dikshith et al., 2003).
Unlike other herbicides, the organometallic characteristics of paraquat allow it to shunt the growth of weeds and grasses by rapid and non-selective disintegration via translocation.
Its effect however is less on deep rooted plants such as fruits and herbs. Due to this, the use of paraquat is well known for the control of weeds in fruits orchards, and a wide range of crop plantations such as rubber, vines, coffee, tea and palm oil. They are also known to be used in the cultivation of asparagus, sugar beet, onions and leeks. Aside from the herbicidal properties of paraquat, it has been known to be useful as a defoliant for certain crops such as cotton and hops with the aim to aid harvesting.
A large number of insecticides are organometallic compounds. Common examples of an organometallic insecticide are the Organophosphorous compounds. They are simply compounds made up of an organic molecule to which a Phosphorous atom is bonded. Chlorpyrifos and Chlorpyrifos-methyl are typical examples of Organophosphorous insecticides. Most organometallic insecticides work effectively by inhibiting an enzyme vital to the nervous systems of humans and animals. This enzyme is known as acetylcholinesterase (Ach-ase). Ach-ase breaks down the neurotransmitter acetylcholine at the nerve junctions or synapse (the space between two nerve cells) with the view to allow nerve impulses to be transmitted across the synaptic gap. Organometallic insecticides work to inhibit the stimulant effect of Ach-ase resulting in major disruptions in the nervous system. In humans, the general side effects/symptoms of Chlorpyrifos include increased sweating, dizziness, headaches, muscle twitching and nausea. These symptoms are common to all organophosphate insecticides with a tendency of symptom delay from one to four weeks resulting in numbness, tingling, weakness and cramping in the lower limbs and eventually paralysis. Some other popular insecticides include Dieldrin, Toxaphene, DDT, Mirex, Hexachlorobenzene (fungicide for seed treatment) and chlordane. PAN Pesticides (2009), MedicineNet.com (2009).
In the polymer industry, organometallic compounds play an important role and are widely used as intermediates and initiators in polymerisation. There are numerous amounts of beneficial complexes which are formed between polymers and organometallic compounds. These complexes formed are very stable which in turn allow long shelf life and durability. For instance, Butyllithium is currently the most prominently used organolithium reagent in the production of elastomers. Butyllithium works as strong base to deprotonate carbon acids during synthesis. Other important organometallic compounds are those consisting of the semimetals, boron, and silicon which are also used in synthesis; while organosilicones are polymerized to manufacture plastics and elastomers. Another important and organometallic compound is the Ziegler-Natta catalyst. This is used as a reagent in the production of polymers of 1-alkaeynes. The chemical structure of Ziegler-Natta catalysts are typically based on titanium or vanadium compounds and organometallic aluminium compounds. Ziegler-Natta catalyst are used worldwide to produce a wide range of polymers which include: High density polyethylene (HDPE), Linear low density polyethylene (LLDPE), Ultra-high molecular weight polyethylene (UHMWPE), Polypropylene (PP)–homopolymer, random copolymer and high impact copolymers, Thermoplastic polyolefins (TPO’s), Ethylene propylene diene monomer polymers (EPDM) and Polybutene (PB) (Craig, 2003).
For biomedical applications and uses, caution need be adhered to in order to prevent any detrimental effects of human physiology. There is continuous research being carried out on the area of metal ions bonded to biological molecules. The use of organometallic compounds in medicine today is classed as highly innovative owing to their unusually impressive reactivities. The first significance of organomeallic drugs were from France in the mid seventeenth century where the name white arsenic or inheritance powder arose. Further on in nineteenth century Europe, two alchemists, Bunsen and Frankland managed to synthesize alkyl mercury compounds. Further work was carried out between 1850 and 1915 on arsenic complexes and came to realization that these complexes are less toxic as previously anticipated and had more manipulation flexibility than their organic counterparts.
Cancer has been a major adult killer since the 1970’s. In recent years organometallic compounds have played an important role in the development of anti cancer and chemotherapy drugs. One common version of this drug is Cisplatin or cis-diamminedichloroplatinum (II) which is a platinum based organometallic drug. The main target of action for the cis-diamminedichloroplatinum(II) is the DNA of cancerous cells. Cisplatin focuses on interfering with cells replication/division or mitosis of cancerous cells. By forming covalent bonds with the DNA of cancerous cells, it is believed that they unleash toxic properties resulting in bifunctional -DNA adduct information. The manner in which proteins are affected by the processing of cisplatin-DNA adducts can help us determine whether a living cell repairs the damage or activate an irreversible death program. The different responses from protein expression during this bonding can explain why certain cancerous cells such as ovarian, testicular and tumors are successfully treated by cisplatain drugs.
In the human body, there are a variety of metal containing molecules which include iron seen as hemoglobin, molybdenum as xanthine oxidase, vanadium as hemovanadin, zinc as carbonic anhydrase and copper hepatocuprein. Below are examples of organometallic based drugs which are currently on the market.
In enzyme catalysed reactions, organometallic compounds cans help us gain insight in the workings of living organisms. As detailed above, there are naturally occurring organometallic compounds in living tissues however some organometallic compounds can be equally potent to living tissues. Examples of these are lead, mercury and Arsenic. These examples were known in the Roman and medieval eras. In biomedical reactions which primarily involve enzymes, the presence of organometals is extremely common. For enzymic reactions in living tissues, enzymes play an essential role via mediation. Because enzymes are proteins, they are formed form α-chiral amino acids which make them chiral molecules. This nature gives them ability to hold a substrate in the appropriate orientation such that only one stereoisometic product results. The involvement of organomentals in these reactions comes from the need of what is known as a cofactor. The use of having the cofactor is to activate the enzyme by altering its shape to participate in the chemical reaction.