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Organometallic Compounds: The Fundamental Aspects

Geetha B. Markandeya and Srinivasa Budagumpi

Jain University, Centre for Nano and Material Sciences, Jain Global Campus, 45 km, NH - 209, Jakkasandra Post, Kanakapura Taluk, Ramanagaram 562112, Bangalore, Karnataka, India

1.1 Introduction

1.1.1 Organometallic Chemistry

The branch of chemistry deals with the study of molecules having a metal-carbon bond, in which a compound is said to be an organometallic compound when the metal-carbon bond in a molecule should be completely or partially covalent. Depending upon the elements in the periodic table, organometallic chemistry is mainly classified into main groups metal, transition metal, lanthanide, and actinide-based organometallics.

1.1.2 Organometallic Compounds

The organic compounds contain at least one metal-carbon bond in which metal is directly attached to the carbon atom in which there should be a bonding interaction (covalent, ionic, localized/delocalized) between the metal and the carbon atom are defined as organometallic compounds. The metals may be alkaline metals, alkaline earth metals, and metalloids (boron, silicon, arsenic, germanium, tellurium, and selenium). The bond present between the metal atom and the carbon is likely covalent in nature.

Examples: Organocadmium compounds, organoboron compounds, organozinc compounds, organomagnesium compounds, organolithium compounds, organolead compounds, and organotin compounds.

1.1.3 Structure of Organometallic Compound

The nature of the metal-carbon bond varies from ionic to covalent. The organometallic compounds have some effect on the nature of metal-carbon and these compounds have both organic and metal portions in which the metallic portion has greater importance. The electropositive nature of metal will be the ionic nature of the metal-carbon bond.

In organometallic compounds, the carbon is bonded to an electropositive atom, which has a negative charge, whereas the metal has a slightly positive charge; hence, the organometallic compounds having the organic part behave as nucleophilic or basic as shown in Figure 1.1.

Figure 1.1 General representation of organometallic compound.

1.2 Milestones in Organometallic Compounds

In 1760, the first organometallic compound of the main group (Eq. (1.1)), cadet fuming liquid was discovered. In the Paris military pharmacy, a cadet discovered a fuming liquid while working on ink while preparing cobalt salt from cobalt minerals consisting of arsenic, which is called cacodyl(malodorous)oxide [1, 2].

1.2.1 Equation (1.1): Synthesis of First Organometallic Compound

In 1827, W. C. Zeise, a Danish pharmacist, discovered the first organometallic compound with a transition metal called Zeise's, K[PtCl3C2H4]H2O. When K2PtCl4 was refluxed in ethanol, it resulted in the formation of Zeise's salt (Eq. (1.2)). It was characterized as the first organometallic olefin complex. Herein, they have used platinum because the Nobel metal complexes are stable toward air and moisture [3, 4].

1.2.2 Equation (1.2): Preparation of Zeise's Salt

In 1849, Edward Frankland prepared diethylzinc (a pyrophoric liquid), while trying to prepare an ethyl radical, he ended up with ethylzinc iodide (solid) and diethylzinc (liquid) [5, 6]. Whereas in 1852, he used sodium amalgam and methyl halide to prepare dimethyl mercury. Furthermore, many people have used R2Hg and R2Zn following an alkyl transfer reaction to prepare main group elements of organometallic compounds.

In the same year 1852, Schweizer and Lowig used an alloy of Na/Pb to prepare tetraethyllead (Eqs. (1.3) and (1.4)). In 1863, organochlorosilane was prepared by Friedel and Craft using alkylzinc as a reagent (Eq. (1.5)).

1.2.3 Equations (1.3)-(1.5): Preparation of Organochlorosilane Compound

In 1868, [PtCl2(CO)]2 the first metal carbonyl compound (Eq. (1.6)) was prepared by Schutzenberger [7].

1.2.4 Equation (1.6): Synthesis of First Metal Carbonyl Compound

In 1890, Ludwig Mond prepared the first binary metal carbonyl Ni(CO)4 (Eq. (1.7)), which is used for the refining of nickel [8-10].

1.2.5 Equation (1.7): Synthesis of First Binary Metal Carbonyl Complex

In 1899, the Philippe Barbier, Grignard's teacher introduced Barbier reaction (Eq. (1.8)). It was a one pot reaction carried out in the presence of water. Compared to Grignard reaction this is less versatile.

1.2.6 Equation (1.8): Barbier Reaction

Whereas in 1900, in RMgX, Zn was replaced by Mg in Barbier by Grignard and called as Grignard reagent (Eq. (1.9)). This reagent is more versatile than the Barbier reaction and has more applications compared to organozinc reagents [11].

1.2.7 Equation (1.9): Synthesis of Organic Compound Using a Grignard Reagent

In 1912, P. Sabatier and V. Grignard were awarded the Nobel Prize for Grignard reagent, and they followed Sabatier's method for the hydrogenation by using metal powders.

In 1917, the first alkyllithium derivatives (Eqs. (1.10) and (1.11)) were prepared by Wilhelm Schlenk, and he followed the transalkylation of organomercury compounds. Whereas the synthetic strategies of lithium derivatives replace the Grignard reagent as the primary anionic intermediate (1.10).

1.2.8 Equations (1.10) and (1.11): Synthesis of Alkyllithium Compound

In 1921, Thomas Midgley introduced tetraethyllead as an additive in gasoline while working on GE motors [12].

In 1930, K. Ziegler prepared organolithium compounds (Eqs. (1.12) and (1.13)) by a simple synthetic procedure and, furthermore, fine-tuned by Gilman. Later on, this compound was widely used.

1.2.9 Equations (1.12) and (1.13): Synthesis of Organolithium Compound

In 1938, hydroformylation (Eq. (1.14)) was discovered by Otto Roelen, for the first time in homogeneous catalysis an organometallic compound has been used [13, 14].

1.2.10 Equation (1.14): Hydroformylation Reaction

In 1943, direct synthesis of organochlorosilane (Eq. (1.15)) was discovered by E G Rochow, further it initiated for large scale production of silicones [15].

1.2.11 Equation (1.15): Synthesis of Organochlorosilane Compound

In 1948, acetylene trimerization (Eq. (1.16)), which was catalyzed by nickel was discovered by W. Reppe [16].

1.2.12 Equation (1.16): Trimerization of Acetylene

In 1951, the sigma bonded structure of ferrocene (Eq. (1.17)) was suggested and independently prepared by two groups. Fischer, Woodward, and Wilkinson proposed the sandwich structure of ferrocene [17].

1.2.13 Equation (1.17): Synthesis of Ferrocene

In 1955, W. Hafner and E. O. Fischer followed rational synthesis to prepare bis(benzene)chromium (Figure 1.2), even though in 1919 same reaction was carried by F. Hein with CrCl3 and PhMgBr [18, 19].

Figure 1.2 Bis(benzene)chromium compound.

In 1955, G Natta and K Ziegler used a mixed metal catalyst to develop olefin polymerization at lower pressure.

In 1959, the stabilization of cyclobutadiene was done by complexation in [(C4Me4)NiCl2]2.

In 1961, Vaska's complex was discovered, which binds reversibly to O2 trance IrCl(CO)(PPh3)2 [20].

In 1963, Ziegler and Natta both were awarded Nobel prize for Ziegler Natta catalyst.

In 1964, the first carbene complex with metal W (tungsten) (Figure 1.3) by E. O. Fischer [21].

Figure 1.3 First carbene complex with tungsten.

In 1965, Coffey and Wilkinson, for hydrogenation of alkenes, they have used (PPh3)3RhCl as a homogeneous catalyst [22].

In 1968, asymmetric catalysis (Eq. (1.18)) was discovered by William S. Knowles, the achiral substrates can be converted into chiral products with the help of complexes consisting of ligands that are chiral with high enantiomers [23].

1.2.14 Equation (1.18): Asymmetric Catalysis Reaction

In 1972, T Mizorki and R. F. Heck discovered the...