Coordination compounds optical isomers

This chapter has reported the only extensive and coordinated investigation of the effects of chirality on the properties of monolayer films spread at the air-water interface. Twenty compounds of varied headgroup and chain length have been examined carrying one and two chiral centers. In every case, all of the optical isomers—enantiomers and diastereomers—were made and their properties measured both as pure compounds and as mixed monolayers in order to compare phase changes in the films with mixed melting points of the crystals. 

As noted previously, the classification of stereoisomers preferred by contemporary organic chemists is the enantiomer-diastereomer dichotomy17 and this may be quite conveniently applied to coordination compounds. Thus, complexes (9a) and (9b) are enantiomers, but (9a) and (9c), and (9b) and (9c), are diastereomers. Older terminology might have led to the description of A and B as optical antipodes and to (A+B) and C as geometrical isomers. 

A tetrahedral platinum atom was not possible in these molecules, since the isomers were not optically active. The direct linking to the metal of the non-metallic groups added in the formation of these supposed molecular compounds was known as coordination and the resulting molecules were coordination compounds. Molecules that we consider coordination compounds today include hemoglobin and chlorophyll, which are vital to animal and plant life. 

The planar square molecular structure turns out to be common among coordination compounds. The other common structure is octahedral, where the metal is at the center of an eight-sided geometric solid with six vertices, formed by joining two square pyramids at their base. For example, a platinum atom can be linked to four chlorine atoms and two molecules of ammonia in this fashion. If the four chlorine atoms are at the corners of the square, and the ammonia molecules at the apexes, then they form one optical isomer. If one of the ammonia molecules and three of the chlorine atoms form the square, and one chlorine atom and one ammonia molecule occupy the apexes, then we have another optical isomer. The two forms are enantiomers of one another ... 

Metal-catalyzed cyclopropanation of an alkene by a diazo compound, reaction 7.33, is another reaction where new C-C bonds are formed. This reaction finds use in the industrial manufacture of synthetic pyrethroids. The precatalysts for carbene addition reactions are coordination complexes of copper or rhodium. It should be noted that reaction 7.33 gives a mixture of isomers (syn plus anti) of the cyclopropane derivative. However, with some chiral catalysts, only one optical isomer with good enantioselectivity is obtained (see Section 9.5). 

Many coordination compounds are chiral and thus exhibit optical activity if they can be resolved into the two isomers. One of these is [Ru(NH2CH2CH2NH2)3], with... 

The proposals of Werner (49) about the structure of coordination compounds were amply justified by the chemical reactions exhibited by them, by the identification of geometrical isomers, and by the resolution of certain compounds into their optical antipodes. Modern methods for the determination of structures and for studying the nature of charged or neutral species in solution have demonstrated the fundamental soundness of Werner s views. 

The directional nature of secondary bonds on a metal ion can act as a template to hold specific configurations (see the example from evolution in Figure 3.4) and to produce isomers which may be optically active. Such coordination compounds have given our evolution a great boost and added a whole new dimension to life s biochemistry. 

When a species cannot be superimposed on its mirror image the two forms are known as enantiomers or optical isomers. Most examples with coordination compounds have chelating (e.g. bidentate) ligands (see Topic E3 ). Structures 10 and 11 show respectively the delta and lambda isomers of a tris(chelate) complex, with the bidentate ligands each denoted by a simple bond framework. As discussed in Topic C3. optical isomerism is possible only when a species has no improper symmetry elements (reflections or inversion). Structures 10 and 11 have the point group D3, with only C3 and C2 rotation axes. 

At an early stage of the development, the structural information was rather fragmentary. Nowadays, however, the accumulation of structural data for isomers has enabled us to understand structural principles and the optical properties of chelate complexes in considerable detail. In this connection, column chromatography on SP Sephadex has played an important role in the separation of isomers of coordination compounds (2). In view of the large number of structures, a few basic series of structures will be taken up and discussed. 

Stereoisomers are compounds that are made up of the same types and numbers of atoms bonded together in the same sequence but with different spatial arrangements. There are two types of stereoisomers geometric isomers and optical isomers. Coordination compounds may exhibit one or both types of isomerism. Note, however, that many coordination compounds do not have stereoisomers. 

Stereoisomers Different Spatial Arrangements of Atoms In the case of stereoisomers, the atoms have the same connections but different spatial arrangements. The two types we discussed for organic compounds, called geometric and optical isomers, occur with coordination compounds as well ... 

Transition metal complexes can interact with the DNA biomolecule either covalently, as with c/i-platin, or noncovalently, when coordinatively saturated octahedral [Ru (dimine)3] " complexes or related are employed. The latter exists in two enantiomeric forms designated as the A and A optical isomers (Figure 4.15). In solution at room temperature they are configurationally stable and kinetically inert to ligand substitution. Due to their geometry these compounds are ideally suited for DNA binding studies. There are three kinds of noncovalent interactions ... 

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