Cobalt, tris racemization

Cobalt, tris(l,2-ethanediamine)-conformation, 1,25,197 polarography, 1,481 racemization, 1, 466 solid state, 1,466,467 reactions, 1, 27 redox potential, 1, 514 structure, 1, 67... 

Cobalt, tris(oxalato)-racemization solid state, 1, 467 structure, 1, 68... 

Cobaltate, tris(l, 2-ethanediamine)-racemization solid state, 466 Cobaltate, tris(oxalato)-racemization solid state, 467 Cobaltates sepulchrates, 22 Cobalt complexes geometric isomerism, 11 hexaammine... 

The question might be asked Are there similar mechanisms for changing the configuration of molecules without breaking bonds in molecules with coordination numbers other than 3 and 5 The answer is yes. One of the most important series of inorganic compounds consists of six-coordinate chelate compounds exemplified by the tris(ethylenediamine)cobalt(lIl) ion. Because of the presence of the three chelate rings, the ion is chiral and racemization can take place by a mechanism that is closely related to atomic inversion or Berry pseudorotaiion (the mechanism for six-coordination is termed the Bailar twist see Chapter 13k... 

The resolution of racemic amino acid mixtures via coordination to a metal ion has been a popular field of study. [Cu(L-aa)2] complexes can be used to resolve DL-Asp, dl-G1u and DL-His.58,59 (—)-[Co(EDTA)] has been used to resolve DL-His having first resolved the racemic [Co(EDTA)] ion using the L-histidinium cation.60 Schiff base complexes of both Co111 and Ni11 have also been used to resolve amino acids.61,62 A more esoteric finding is that the bacterium Enterobacter cloacae prefers to metabolize the A-( —) isomer of/ac-[Co(GlyO)3] rather than the A-(+) form,63 an observation reminiscent of that made by Bailar using tris(ethylenediamine)cobalt(III) salts. 

Levo iodide. The Zeyo-tris(ethylenediamine) cobalt (III) chloride d-tartrate remaining in the solution above is treated with concentrated ammonia solution (0.5 ml.) and the mixture is heated to about 80°. Solid sodium iodide (35 g.) is stirred in and the whole cooled in ice. The impure levo iodide is filtered off and washed with ice-cold 30% sodium iodide (25 ml.) and then with alcohol and air-dried, resulting in a yield of 27 g. Purification is effected by stirring the whole of the crude material into 65 ml. of water heated to 50°. The racemate remains undissolved and is filtered off. Sodium iodide (10 g.) is added to the warm filtrate (50°) and crystallization allowed to take place. After cooling in ice the solid is filtered, washed with ethanol and then acetone, and air-dried. The yield is 18 g. (26%), and [a]D = -90°. Anal. Calcd. for [Co(en)3]I3-H20 C, 11.26 H, 4.11. Found C, 11.43 H, 4.01. 

Both dextro- and levo-tris (ethylenediamine) cobalt (III) iodide monohydrate crystallize in deep orange needles or rhombs. The racemic phase which separates as the 1-hydrate is much less soluble in water. Racemization ensues... 

The first report regarding quenching of DNA-bound metal complexes was of enantiomeric ruthenium(II)tris-diammine complexes by racemic cobalt(III)tris-... 

The tris(oxalato)chroinate(III) ion, [Cr(Cj04)3P , possesses the double historical distinction of being both the first resolved complex anion and the first resolved complex that did not contain nitrogen. Since it racemizes rapidly in aqueous solution, more so than the corresponding cobalt(III) ion, virtually all of this labile complex can be separated as a single en-... 

Five grams (0.0127 mol) of (-t-)-bis(l,2-ethanediamine)(oxalato)-cobalt(III) iodide ([a] = -1-720°) is converted to the acetate by shaking for 10 min with 2.11 g (0.0126 mol) of solver acetate suspended in 30 mL of hot (50°) water contained in a 100-mL flask. The precipitated silver iodide is removed by filtration and washed with 10 mL of hot water. The combined red filtrate and washings are added with stirring to a solution of 2.00 g (0.00409 mol) of racemic potassium tris(oxalato)chromate(III)... 

In aqueous solution potassium tris(oxalato)chromate(III) racemizes faster than the analogous cobalt compound but more slowly than the corresponding iron compound. Many studies of the racemization rate have been made under various conditions. - The rate is dependent on the nature of the solvent it is lowered by addition of organic solvents to aqueous solutions. - Both enantiomers have been the subject of numerous optical rotatory dispersion studies.No exchange has been shown to occur between oxalate ion containing radioactive carbon and the tris(oxalato)-chromate(III) ion. ... 

It must be concluded, therefore, that the ligands do not become completely detached from the metal ion in isomerization reactions. Comparable results have been observed in the isomerization of potassium diaquodioxalatochromium(III) and the racemization of optically active potassium tris(oxalato)chromium(III) when no exchange with free ligand in solution occurs. Thus, although it is not practicable to take advantage of the desirable properties of individual isomers of 2 1 chromium and cobalt complexes of tridentate azo compounds because of the facility with which such compounds isomerize in solution, the technically important unsymmetrical 2 1 complexes are capable of practical application because they show little or no tendency to disproportionate in solution. 

The optical isomers of cis-Cr(benz)3 are quite stable in dry chloroform in the dark, yet isomerize rapidly in methanolic solution. The trans-isomer is moderately stable in alcohols, with a half-life for racemization of a few days. Due to the instability with respect to decomposition of the neutral tris(benzohydrox-amato)cobalt(III) complex, only the CD spectrum of the anionic species has been recorded (Figure 7). It resembles that of the cis-Cr(hydroxamate)3 complexes. 

It was mentioned above that tris(chelate) complexes of the type [Co(en)jp+ lack an improper axis of rotation. As a result, such complexes can exist in either of two enantiomeric forms (or a racemic mixture of the two). Figure 12.20 illustrates the complex ions [Co(en)3]3+ and [Cr(ox)3]3-, each of which is chiral with D3 symmetry. It is not necessary to have three chelate rings present. The cation dichloro-bis(ethylenediamine)cobalt(lIl) exists as two geometric isomers, cis and trans. The trans isomer has approximate Dlh symmetry (Fig. 12.21b). Because it has three internal mirror planes, it is achiral.40 The cis isomer has C2 symmetry and is chiral (Fig. I2.2la). Since the two chloride ions replace two nitrogen atoms from an eth-... 

Bridged hydroxy complexes (208) of the bisoxalate type have been known for many years. Both a trihydrate and an anhydrous form are known. The trihydrate is isomorphous with the corresponding cobalt(III) complex (Durrant s salt) and the hydrous complex is slowly converted to the hydrate on exposure to moisture. Only a preliminary crystallographic characterization has appeared " it crystallizes as a meso rather than racemic complex. IR and magnetic studies are in accord with the hydroxy-bridged structure. Despite the crystallographic meso structure, the resolution of the complexes has been reported either photochemically or with cis- — )bis(ethylenediamine)dinitrocobalt(III) and leads to optically active complexes with spectra similar to those of the corresponding tris oxalate. 

Tris(propylenediamine)cobalt(III) salts were first synthesized from racemic pro-pylenediamine Later, the synthesis with the optically active base showed that the action of S(+)-pn on [CoCl2(R-pn ] ve a mixture of —)[Co(R-pn)j] and (-t-)[Co(S-pn)3] rather than [Co(R-pn>2(S-pn)p. After all the salts prov to be (+)[Co(S-pn)3]Br3 from the S-base and (—)[Co(R-pn)3]Br3 from... 

Resolution of a complex even to a very small extent could demonstrate tiiat the original complex was a racemic mixture. Thus, complexes like [Co(dimethylglyoximo)2(NH3)Cl] were partly resolved on quartz powder. The spatial configuration of this complex was unknown. From file theoretical point of view it was expected that only the c/5-form of the complex should exhibit optical activity. In agreement with this expectation, resolution of the tris(dimethylglyoximo)cobaltate(lll) complex on quartz exhibited a poor but definite optical activity revealing the configuration of the complex to be cis rather than trans... 

Three X-iay analyses of racemic tris(ethy]enediamine)cobalt(iii) salts have been described. In [Co(en)3]a(HP04)3,9H20 and [Co(en)3]j[CdCle]Clg,-... 

The hexol cation in crystals of racemic tris[di-/i-hydroxobis(ethylene-diamine)cobalt]cobalt dithionate octahydrate (35) has the structure pre-... 

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