Cobalt complexes absolute configurations

The circular dichroism (CD) spectra of optically active di-, tri-, and tetranuclear complexes of chromium(III) and cobalt(III) have been reported and used to establish the complexes absolute configurations (55 59, 111, 115, 116, 152-157). The changes in circular dichroism resulting from ion pairing have been studied for the tetranuclear hexol Co j(OH)2Co(NH,)4J, h+ and have been shown to be attributable to the vicinal effect of the chiral oxygen centers produced stereospecifically by the ion-pair formation (56). For a series of trinuclear cobalt (III) amine complexes, cis-Co(CN)2[(OH)2Co(N4)2 J3 +, it was shown that the main CD contributions due to the two chiral Co(OH)4(CN)2 and Co(N4)(OH)2 centers are additive (155). In the case of the related tetranuclear complex Co((OH)2Co(en)2J,< + this postulate of additivity of CD spectra proved unsatisfactory (57). 

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] ... 

The application of 59Co n.m.r. spectroscopy to the study of cobalt(m) complexes is beginning to expand.285,286 An n.m.r. study of the stereoisomers of propane-1,2-diamine (pn), en, acac, CF3acac, etc., showed that the chemical shifts do not appear to be related to the absolute configuration, but that larger linewidths are observed with lower symmetry complexes 286 The chemical shifts are larger for ob-lel than for mer-fac isomerism in Co(pn)3 +. 285... 

For many years no attempt was made to determine the absolute stereochemistry of transition metal complexes, although Kuhn (8) calculated the absolute configuration of (—)n-[Co(C204)3]3- from its anomalous optical rotatory dispersion, using classical coupled oscillator theory. He later (9) extended his theory to other tris-chelated cobalt complexes such as (+)n-[Co(en)3]3+. However, in 1955 Saito (10) showed by anomalous X-ray dispersion that Kuhn s suggested configuration for (+)n-[Co(en)3]3+ was incorrect. 

The X-ray method was first applied (52) to a chelated inorganic molecule4) in the case of the most accessible product of the reaction of aqueous L(+)alanine with cobalt(III) hydroxide, the violet crystalline, a(+)-[Co(L-ala)s]. The absolute configuration of L(+)alanine is known and could be projected only on to the configuration shown. The configuration of the whole complex (IV) was thus established as D. 

The twist boat form is chiral and there are two enantiomeric conformations. The chirality can be defined in the same way as a cobalt-ethylenediamine ring and designated as S and X. The third form, a boat form cannot be accommodated to form a tris(bidentate) complex and, in fact, such a structure has not yet been reported. Niketii and Woldbye showed that there exist 16 possible conformers of the [M(tn),] system for each of the absolute configurations A and A, in which they adopt the three stable configurations chair andS- and X-skew boat forms (32). 

Using all of the recorded CD data for tris-diamine cobalt-(III) complexes of known absolute configuration, an empirical rule relating the absolute configuration to CD spectra of tris-diamine cobalt(III) complexes in the charge-transfer region was estab-... 

While the absolute configuration of transition metal catecholates has not been determined by x-ray diffraction, the assignments are based on several lines of reasoning. In particular, for both the rhodium(III) and chromium(III) complexes, that isomer precipitated by A-tris(ethylenediamine)cobalt(III) is assigned the A configuration. 

Werner resolved the tris oxalate using strychninium ion in 1912. Other effective resolving agents for the tris complex include (+ )-tris(l,10-phenanthroline)nickel(II), ( + )- or (- )-tris(ethylenediamine)cobalt(III) and (- )-tris(ethylenediamine)rhodium(III). The (- )-[Co(ox)3] ion has been studied by single-crystal methods and has the d configuration. It has been related to (-i-)-[Cr(ox)3] by X-ray powder photography, so its absolute configuration is also established as d by the more exact form of the rule of least-soluble diastereoisomers. ... 

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