Cobalt complexes optical rotation

Approximately 25 g of finely ground A-( — )D-[Co(N02)2(S-aigH)2]Cl is sieved with a 140-mesh screen, and the fines are discarded. The 20 g of complex which does not pass through the screen are slurried in toluene and packed into a chromatography column 1 cm X 50 cm. A 0.2-g sample of tris(2,4-pentane-dionato)cobalt(III)9 dissolved in 5 mL of toluene is applied to the top of the column. The column is eluted with toluene and 35 mL of eluate is collected prior to the emergence of the sample band. Sixteen fractions of 0.8 mL each (15 drops) are collected and each is diluted to 3.0 mL. From the absorbance of the solutions at 590 nm (e = 160 L mole- em1) and the optical rotations at 546 nm, molar rotations are obtained for the earliest and latest fractions collected. The middle fractions are almost inactive. There is a large error at the last two concentrations. 

For some complexes, the isomerization is very slow (as for the inert cobalt(III) complex [Co(en)3]3+), whereas for others the process is sufficiently fast (as for thenickel(II) analogue CNi(en)3]3+) that the complex cannot be easily resolved into its optical forms through conventional crystallization methods. The isomerization reaction can be readily followed by observing the loss of optical rotation at a selected wavelength over time this is usually a simple first order exponential decay process. 

Cobalt, rhodium and platinum complexes modified with numerous chiral phosphanes have been used in asymmetric hydroformylation of styrene. The results are compiled in Table 4. Iso-product selectivities of >95% and stereoselectivities of >90% ce are reported, in many cases, however, only with low conversion rates and yields. Early results based on optical rotation measurements had to be reevaluated due to wrongly adopted rotation values for hydra tropaldehyde4-. ... 

The use of an analogous chiral bis-bidentatesyn- encapsulated tetrafluoroborate anion [96]. Their tone 328 allowed diastereoselectively obtain IVLiLg only diastereomers showing more than 30-fold pseudotetrahedral 1 1 cage complexes of coordina- inaease in the optical rotation per mol have a T tion capsules 633 and 634 (Schane 4.99) with ver- synunelry. The conformational changes at this dia-lex cobalt and zincfll) cations, respectively, and stereoselective formation of a cage complex are... 

The following table shows the rotation of the chromic salts described and of the corresponding optically active dichloro-cobalt salts,2 showing that the optical activity depends on the central atom as well as the surrounding groups in the complex.3... 

I would suggest that the formation of metal chelate complexes, with a four or six-coordinate metal partly bound to an optically active protein and partly bound to a substrate molecule can explain this stereospecificity. The optically active coordination compounds of metals, such as cobalt, have extraordinarily high molecular rotation, and so the difference in chelation powers of the d and I forms of a substrate may be very great. As Dr. Chaberek has pointed out (Lecture 33), this chelation may involve both metals of constant valency, e.g.. Mg, Zn, and those of variable valency. Metallic ions of both types are proven essential trace metals in biological systems. 

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