Industrial Roles for Ligands and Coordination Complexes

Coordination chemistry offers many examples of applications in industry beyond those addressed above. Recent developments in chemistry promise greater applications in the future. Here a limited number of examples from two fields are presented to give a sense of opportunities for coordination complexes in commercial roles. Complexes as Catalysts and Asymmetric Catalysts ,The chemical industry as we currently know it would be markedly different without transition metal catalysts, as these play roles in a wide range of processes. The key task of a catalyst is to accelerate a reaction by effectively lowering the activation barrier for the reaction. Apart from acceleration, a catalyst may also be able to induce optical activity in an organic product if it includes a chiral ligand. The success of an asymmetric catalyst is defined by the enantiomeric excess, which is the difference in percentage yields of the major and minor enantiomers of the product. If 90% of one optical isomer forms and 10% of the other, the enantiomer excess is 80%; obviously the closer that this value is to 100% (which means stereospecificity is achieved) the better. Asymmetric synthesis in industry depends fully upon transition metals as the active site of the catalysis.

One of the best-known coordination complexes that acts as a catalyst of chiral epoxidations is the Jacobsen catalyst. This is a manganese (III) complex of a sterically demanding chiral ligand (Figure 9.7) which is used in conjunction with the oxidant hypochlorite (CIO). The oxidant initially oxidizes the complex to form an oxomanganese(V) com- pound, which then is able to deliver the oxygen to an alkene to form an epoxide, returning to the Mn(III) state. The epoxidation occurs highly steroselectively with enantiomer excesses of usually greater than 95% achieved routinely. It is believed that the substrate is bound to the metal ion in the activated state, with oxygen transfer occurring in the chiral environment of the tetradentate N₂O₂ ligand leading to chirality in the epoxide product. Since the Mn (III) complex is regenerated, it is able to act as a catalyst undergoing many 'turnovers' before degradation through ligand dissociation reactions.

The above example is of a homogeneous catalyst which is one that is dissolved in solution for the reaction. One of the best understood and long-established coordination complexes

Figure 9.7

Chiral epoxidation with Jacobsen’s catalyst. Chiral centres in the ligand and product are marked (*).

acting as a catalyst is the simple square planar Rh(I) complex [RhCl{P(C6H5)3}3]. Wilkinson's catalyst, used for the hydrogenation of terminal alkenes (see Figure 6.1). It involves an intermediate where both hydrogen and the alkene are coordinated, allowing an intramolecular reaction between these to form an alkane product-effectively a reaction of coordinated ligands, but one where the product can depart, so that the original complex is reformed, allowing the process to occur again, a key requirement for catalysis.

There are a number of others in common industrial use, such as those used for polymerization of alkenes. One example of an organometallic homogeneous catalyst is the Zr(IV) complex [Zr(CH3)(n⁵-Cp)2X], which operates by binding alkene monomers to the metal prior to addition to a growing carbon chain. A similar coordination of substrate is involved in the use of [Co(CO)4H] as catalyst in the hydroformylation of alkenes to aldehydes. Likewise, the [Rh(CO)2I2]- ion formed in situ, catalyses the carbonylation of methanol to acetic acid.

Another catalyst type is the heterogeneous catalyst, which remains as a solid and pro- motes chemistry at the surface. To function well they require high surface areas per unit mass. Metal oxides and hydroxides are common examples. A vanadium(V) oxide is employed in the formation of ammonia from nitrogen and hydrogen under elevated temperature and pressure, for example. Polyoxometallate metal clusters, which are oxo-ligand coordination complexes employing dominantly O2 and HO as ligands, have some catalyticroles.

The two types of catalysts merge with the development of what are called 'tethered' catalysts where a homogeneous catalyst is amended so that it is able to be attached covalently to an inert surface, such as silica. This is also called a supported catalyst. Having the active component available as part of a solid can assist processes where carrying the catalyst forward in solution to another stage of the process may lead to contamination or catalyst destruction. Further, surface attachment also can alter catalytic activity favourably in certain cases.

Nanotechnology is one of the new frontiers of chemistry - the development of new mate- rials with well-defined structure within the size range of from 1 to~100 nm. In particular, the properties of the nanomaterial should differ from those of both conventional molecular compounds and bulk solids, and it this difference that is the key to their attraction and potential applications. This definition can include metal complexes, particularly larger polymetallic systems. Of course, many biomolecules fall within the definition of nanomaterials in terms of size, but the interest in the field at present lies in the development of synthetic more than natural materials.

Fabrication of nanomaterials divides into two approaches - 'top down' which relies on starting with bulk materials and processing them to yield nanoscale materials, and 'bottom up' which employs atomic and molecular species aggregated to form larger nanoscale materials. It is the latter of these two approaches that may involve coordination chemistry. A simple example may suffice to display this in application. It is possible to produce (cation)⁺[AuCl₄]- as a finely-divided particulate by choosing an appropriate cation that limits complex solubility in the chosen solvent. In the presence of a thiol (RSH), substitution of chloride ligands on the gold (III) by thiolate (RS-) ligands can occur. When the resultant gold(III) thiolate is treated with a suitable reducing agent, the Au(III) is reduced to form nanoscale metallic gold particles that remain coated with thiol, of type {Au_x(RSH)y}. This

Figure 9.8 Assembly of rigid precursor units into a larger nano-assembly is directed by the shape of components.

represents a new form of gold particles with special properties, whose size and surface character can be controlled through manipulation of reaction conditions and reagents. Building polymetallic clusters into new nanomaterials through combination of particularly shaped ligands and monomer complex components is an area of growing development. This is an extension of concepts we have already developed in Chapter 6 (see Figure 6.4). This is really molecular Lego– there is only a limited number of ways ligand and complex precursors of particular and usually rigid shapes can combine as larger units, just like Lego blocks can only fit together in certain ways. An example of the concept is illustrated in Figure 9.8. Loss of the two cis X-groups from the complex allows coordination of terminal pyridine groups from the Y-shaped ligands in their place. However as a result of the rigid ligand shape, any ligand can only attach as a monodentate to a single metal and so a network of attachments is built up to satisfy the demands of the ligand for tridentate coordination and the metal for filling two adjacent coordination sites. A large number of complex, shape-directed nanosized clusters have been developed using this type of approach.

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مواضيع ذات صلة

Molecular Symmetry: The Point Group

Organometallic Compounds

Bridging Ligands

Organic Molecules as Ligands

Multipliers

Polydenticity

Crystal Ball Gazing

Taking Stock

Complex Futures

Better Ways to Synthesize Fine Organic Chemicals

Metal Extraction

Fluoroionophores