Chiral metal complexes absolute configuration

By far the most reliable method to determine the absolute configuration of chiral inorganic and organic compounds is by single-crystal X-ray crystallographic analysis.4 By 1974 Saito listed 59 chiral metal complexes whose absolute stmctures were determined by X-ray diffraction.181 Advances in the speed of computer processing and other aspects of X-ray instrumentation have resulted in the ability to determine absolute structures from much smaller crystals. As a consequence, the number of crystal structure determinations of inorganic compounds has increased enormously in recent years. 

The problem of distinguishing Tweedledum from Tweedledee is not peculiar to Alice(1). Determination of absolute configurations of chiral metal complexes, apart from being of basic chemical significance, has proved an intriguing challenge to a range of techniques. Circular dichroism in particular, because of the ease of measurement, has enjoyed considerable popularity as a stereochemical probe, albeit with incommensurate success. 

The structure of the major diastereomer 4.18a in sample 1 was determined by X-ray diffraction analysis. Complex 4.18a crystallizes in the chiral space group P2i, a view of the complex is shown in Figure 4.10. The absolute configuration of the molecules in the structure was confirmed by refining the Flack s x parameter and was equal to —0.02(1), attesting to the enantiopure character of the crystal.This complex possesses planar chirality and the absolute configuration of the metal centres in the cationic species... 

There are more examples of a second type in which the chirality of the metal center is the result of the coordination of polydentate ligands. The easiest case is that of octahedral complexes with at least two achiral bidentate ligands coordinated to the metal ion. The prototype complex with chirality exclusively at the metal site is the octahedral tris-diimine ruthenium complex [Ru(diimine)3 with diimine = bipyridine or phenanthroline. As shown in Fig. 2 such a complex can exist in two enantiomeric forms named A and A [6,7]. The bidentate ligands are achiral and the stereoisomery results from the hehcal chirality of the coordination and the propeller shape of the complex. The absolute configuration is related to the handness of the hehx formed by the hgands when rotated... 

The Pfeiffer effect (the shift in a chiral equilibrium on the addition of an optical isomer of a different compound) of racemic [Cr(ox)3]3- has been examined using for the first time optically stable metal complexes cis-[MXY(AA)2]"+ (where M = Cr3+ or Co, AA = en or tmd and X and Y = anionic monodentate ligand). It was found that the chiral equilibrium of [Cr(ox)3]3-was always displaced in favour of its A enantiomer in the presence of A enantiomers of the cis complexes, and it is proposed that the absolute configurations of cis complexes could be inferred from the equilibrium shift induced in [Cr(ox)3], 438 Laser irradiation of an aqueous solution of racemic [Cr(ox)2(phen)] or [Cr(ox)(phen)2]+ in the presence of ( + )- or (- )-cinchonine hydrochloride rapidly shifts the chiral equilibrium in a direction opposite to that induced by the usual Pfeiffer effect in the dark.439... 

The availability of commercial instrumentation in the 1960s led to numerous investigations aimed at the determination of chiral absolute structures through the measurement of ECD as well as associated advances in the chiroptical (chiral optical) theory for metal complexes. The relationships between chiral structure and ECD for inert cobalt(III) and chromium(III) complexes with d6 and d3 configurations have... 

Von Zelewsky has published many examples of the stereoselective synthesis of metal complexes using what he refers to as chiragen ligands. These are enantiopure natural compounds synthesized from the natural product (—)-a-pinene, which is combined with species such as bipyridine units to provide impressive control of metal-centered chirality.159-161 In this section, we will focus on the determination of absolute configurations in terms of stereospecific formation from different points of view in connection with absolute conformations in the ligands. 

The examples of metal-chiral structures are mainly cationic mixed-ligand cobalt(III) complexes of ethylenediamine and its monodimethylphosphine analogue [Co(H2NCH2CH2NH2)3(H2NCH2CH2PMe3)3 J3+. Their synthesis, separation to enantiomers, and establishment of absolute configuration have been carried out for these compounds [276]. The binuclear cobalt(III) complexes 924 possess similar optical properties [277] ... 

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