ORGANOMETALLIC CHEMISTRY Metal-metal multiple bonding is an important feature of the chemistry of many transition elements and is very important to the field of metal cluster chemistry. A typical property of metals is that rather than forming straight chains or rings, metals tend to agglomerate so as to form maximum number of bonds with minimum number of adjacent metal atoms, giving rise to either metal-metal multiple bonds or metal cluster compounds. The existence of a quadruple bond in inorganic systems was first recognized in 1964 when the compound [Re2Cl8] 2– was isolated. Inorganic clusters may contain quadruple bonds due to the formation of an extra bond called δ bond which is formed by the overlapping of the dx2-y 2 or dxy orbitals. [Re2Cl8]2- is characterised by shorter M-M bond distances Re-Re 0.224nm and eclipsed configuration of Cl atoms linked to Rhenium. Since Re-Re bond distance is shorter, the Cl atoms are very close at a distance less than the sum of their van der Waal’s radii. So they are expected to be in staggered configuration. [Re2Cl8]2- contains two ~square planar ReCl4 units Re [Xe] 4f14 5d5 6s2 Re3+ [Xe] 4f14 5d4 6s0 2In [Re2Cl8] the Z axis is taken as the line joining the two Re atoms. Each Re atom is linked to four Cl atoms through dsp2 hybridised orbitals involving dx2-y2 orbital. dz2 forms Re-Re σ bond. dxz and dyz form two π bonds. The dxy orbitals of both Re atoms overlap confacially to form δ bond. Thus there is quadruple bonding between Re-Re atoms in [Re2Cl8]2-. This δ bond can be formed only if Cl atoms are eclipsed, thereby forcing Cl atoms to take eclipsed configuration. The degree of overlapping and hence the bond strength is in the order σ > π > δ. Other compounds with δ bonding- Catalysis A catalyst is a substance that increases the rate of a reaction but it is not itself consumed. A catalyzed reaction is faster (or in some cases more specific) than an uncatalyzed version of the same reaction because the catalyst provide a different reaction pathway with a lower activation energy. Catalysis plays a vital role in the production of fuels, commodity chemicals, fine chemicals and pharmaceuticals as well as providing the means for experimental safeguards all over the world. More than 60% of all chemical products and 90% of all chemical processes are based on catalysis. A whole new technology appeared based on organometallic catalysis in olefin polymerization. [ Nobel prizes for chemistry have been awarded to Ziegler and Natta (1963), Fischer and Willkinson (1973) for their discoveries in organometallic chemistry and homogeneous catalysis • More recently, in 2005, Chauvin, Schrock, and Grubbs were awarded Nobel Prize for developing organometallic catalysts for olefin metathesis. ] Desired Properties of Catalysts (a) Selectivity • A selectivity catalyst yields a high proportion of the desired product with minimum amounts of side product. In industry there is considerable economic incentive to develop selective catalysts. (b) Lifetime • A small amount of catalyst must survive through a large number of cycles of it is to be economically viable • A catalyst may be destroyed by side reactions to the main catalytic cycle by the presence of small amounts of impurities in the starting material. • For example, many alkene polymerization catalysts are destroyed by O2, so these polymerizations are carried out in absence of air. Catalysis can be of two types: Heterogeneous and Homogeneous Homogeneous Catalysis: They are present in the same phase as the reagents Heterogeneous Catalysts: They are present in a different phase from that of the reactants Steps involved in catalysis In oxidative addition, a metal M inserts into a covalent bond of a compound XY. The XY bond is broken and two new bonds form: MX and MY. The metal .loses two valence electrons and gains two new ligands, X and Y. Oxidative addition is a key step in many catalytic cycles. Often, it is the slow (i.e. rate-determining) step, because a covalent bond (usually in the substrate) is broken. This creates a metastable species that easily reacts further in the cycle. Reductive elimination involves the elimination or expulsion of a molecule from a transition metal complex. In the process of this elimination, the metal centre is reduced by two electrons. An insertion or migration step involves the introduction of one unsaturated ligand into another metal–ligand bond on the same complex. Insertion and reductive elimination are common bond-forming steps, just as oxidative addition is a common bond-breaking step. Wilkinson’s catalyst - Hydrogenation of alkenes is an important industrial reaction that requires high temperature and pressure. Wilkinson’s catalyst can bring about hydrogenation of a wide variety of alkenes at pressure of hydrogen close to 1 atm or less • Wilkinson’s catalyst is highly sensitive to the nature of phosphine ligand and alkene substrate. Ph3P Rh Ph3P PPh3 H + H2 H Cl Ph3P Rh PPh3 Cl PPh3 - PPh3 R Rh Ph3P PPh3 - PPh3 Cl + H2 R H H Rh Ph3P H PPh3 H Rh Cl Cl Ph3P R H H Rh PPh3 PPh3 Cl Ph3P R Disadvantages 1. Rhodium is costly and the conversion of rhodium to the catalyst is also a costly process. 2. Wilkinson’s catalyst is a soluble catalyst, separation of the catalyst from the product is difficult. Hydroformylation reaction is the addition of H2 and CO to an alkene to form an aldehyde. Hydroformylation increases the carbon chain by one carbon and introduces O atom in the chain. Hydroformylation was discovered by Otto Roelen in 1938 during an investigation of the origin of oxygenated products occurring in cobalt catalyzed Fischer-Tropsch reactions. Roelen's observation that ethylene, H2 and CO were converted into propanal, and at higher pressures, diethyl ketone, marked the beginning of hydroformylation. Roelen's original research into hydroformylation involved the use of cobalt salts that, under H2/CO pressure, produced HCo(CO)4 as the active catalyst. In 1960 and 1961 Heck and Breslow proposed what is now accepted as the general mechanism for hydroformylation Roelen’s catalyst - Co2CO8 Mechanism 1) In the first step Co2CO8 combines with H2 at high pressure and forms 18 electron tetracarbonyl hydrido cobalt complex. Co2CO8 + H2 → 2 HCoCO4 HCoCO4 → HCoCO3 + CO 2) HCoCO3 gets co-ordinated to the alkene and forms 18 electron complex which undergoes insertion reaction with the hydrido ligand. CH2 HCoCO3 + CH2 CHR→ H (CO)3Co → (CO)3CoCH2 CH2R CHR 3) At high pressure the alkyl complex undergoes migratory insertion and forms an acyl complex. O C (CO)3CoCH2 CH2R +CO→(CO)4CoCH2 CH2R→ (CO)3CoCH2 CH2R 4) In the next step the reaction with H2 finally releases the aldehyde and regenerates the catalyst. O C (CO)3CoCH2 CH2R→ HCoCO3 + R CH2 CH2 CHO HCoCO3 + CO→ HCoCO4 In 1965 Osborn, Young and Wilkinson reported that Rh(I)-PPh3 complexes were active and highly regioselective hydroformylation catalysts for 1-alkenes, even at ambient conditions, it was Wilkinson's work that really ignited serious interest in rhodium phosphine hydroformylation catalysts. The initial catalyst system was derived from Wilkinson's catalyst, RhCl(PPh3)3, but it was rapidly discovered that halides were inhibitors for hydroformylation. Polymerisation of Alkenes: The Ziegler-Natta catalyst, is a powerful tool to polymerize αolefins with high linearity and stereoselectivity. A typical Ziegler-Natta catalyst system usually contains two parts: a transition metal (Group IV metals, like Ti, Zr, Hf) compound and an organoaluminum compound (co-catalyst). The common examples of Ziegler-Natta catalyst systems include TiCl4 + Et3Al and TiCl3 + AlEt2Cl. In 1953, German chemist Karl Ziegler discovered a catalytic system able to polymerize ethylene into linear, high molecular weight polyethylene which conventional free radical polymerisation techniques could not make. The system contained a transition metal halide with a main group element alkyl compound. This was the first synthesis of high density polyethylene (HDPE), Following the catalytic design, Italian chemist Giulio Natta found that polymerisation of αolefins resulted in stereoregular structures, either syndiotactic or isotactic, depending on the catalyst used. Using a mixture of triethylaluminum and titanium tetrachloride, Natta reported the first synthesis of linear, isotactic and syndiotactic polypropylene, which result in crystalline polymers opposed to the atactic form which is amorphous. Because of these important discoveries, Karl Ziegler and Giulio Natta shared the Nobel Prize in Chemistry in 1963. The activation of ZN catalyst system by coordination of AlEt 3 to Ti atom.
