7 with no apparent stereospecificity

In document Stereospecific effects of asymmetric ligands in cobalt (III) complexes (Page 130-135)

DL-|Co(tpn)2ox| D-Jco en ox2] F iltra te

7 with no apparent stereospecificity

9

(5) An asymmetric synthesis was claimed for the hydrolysis of the reaction product between dichlorotriethylenetetraminecobalt(ill) ion and d-antimonyltartrate ion.

The various tests to which these systems and the conclusions previously drawn from them, have been submitted, are described in the following sections,

*

7.2, Carbonatobis(l-propylenediamine)cobalt(lll) Ion. 7.21. Historical.

This complex cation has been extensively studied b y Bailar and 5 45 46

his coworkers * 9 . They prepared isomers, designated Dll and 45

Lll, by different preparative reactions and showed that these 46 isomers can be interconverted by heating in aqueous solution •

46

The isomers were obtained analytically pure , and their identity was based on the rotatory dispersion curves shown in Fig. 7.21.

mjj

X

These isomers were used in further studies by Bailar and his

coworkers^* in an attempt to obtain a stereospecific

replacement of the carbonato group. 7.22. Results and Conclusions,

The preparation of carbonatobis(l-propylenediamine)cobalt(lIl) chloride was similar to that for the corresponding oxalato complex described in Chapter Six. The product of the lead(iv) oxide oxidation of cobalt(ll) chloride in the presence of 1-propylene- diamine and hydrochloric acid was treated with lithium carbonate, and the product was precipitated as the chloride in 65^ yield. This chloride was transposed to the less soluble iodide which was dissolved in a water-acetone mixture. The solution was allowed to stand at 30° for several days, during which time a crystalline deposit formed and was collected. This product was shown by

analyses to be the anhydrous iodide. Its rotatory dispersion curve is given in Pig. 7.22 and compared with that for the L[Co en^CO^]* ion. Such a comparison indicates that this is the L-isomer and that

probably optical purity has been attained.

5

It has already been demonstrated that an equilibrium exists between the Dll and Lll isomers of the carbonato complex. The present work substantiates these findings since it seems that as the less soluble L isomer is deposited fron the aqueous acetone

* (previous page)

These molecular rotations are for the carbonate salt and, since the complex cation is univalent, should be about double those for, say, the iodide.

400 mjj

500

P ig . 7 .2 2 . R o ta to ry D is p e r s io n C u rv e s, ( l ) L f C o ^ - p n ^ C O ^ ] ! ; (2 ) L[Co en2C03 ] l .

s o l u t i o n th e e q u ilib r iu m s h i f t s to g iv e more o f th e L iso m er u n t i l alm o st th e e n t i r e r e a c t i o n p ro d u c t i s o b ta in e d as th e one iso m e r.

T h is stu d y does n o t d e m o n stra te t h a t th e L l l isom er h a s th e lo w e st f r e e e n erg y , b u t sim ply t h a t i t h a s th e lo w e st s o l u b i l i t y

i n th e s o lv e n t u s e d . Com parison o f r o t a t o r y d i s p e r s i o n c u rv e s how ever, 5 45 46 show t h a t th e “ iso m e rs” d e s c rib e d by B a i l a r and h i s cow orkers 9 9

are in fact mixtures containing different proportions of the Dll 6 50 51 and Lll isomers* The stereospecific substitution reactions * * carried out on such products could not be more than partially successful*

7*5* Cis and trans dichlorobis(l-propylenediamine)cobalt(lIl)chloride, 31

These complexes were first prepared by Werner and Frolich

g

Their method was somewhat modified by Bailar and his coworkers and 27

the preparation was repeated by Mathieu

The present preparation was incidental to the preparation of the above carbonato complex* After the carbonato complex had been

filtered from the reaction mixture in 6 5 ^ yield, the filtrate was evaporated to a small volume, in the presence of hydrochloric acid, to give an almost quantitative yield of the green trans dichloro compound* It was obtained free from hydrochloric acid of

crystallisation by warming in methanol solution with solid lithium carbonate *

31

The cis complex was made from the trans by Werner's method of

repeatedly evaporating a neutral aqueous solution of the trans compound at 60°• It was a purple glassy solid which was very soluble in water* All attempts at resolution failed to give insoluble salts with this complex so no evidence was obtained for the existence of more than one optical isomer*

The cis dichloro complex is a key compound in the confoimational

analyses treatment of ethylenediamine complexes* The chlorine

reveal that their environment is the same in the various conformational isomers, so that interactions involving these chlorines make no contribution to the free-energy differences between isomers# The 3ame argument applies to the donor nitrogen atoms of the ethylenediamine groups, hence only the H-H, C-C and C-H

interactions need be considered. This compound is, therefore,

relatively simple and lends itself to conformational analyses. It is unfortunate that attempts to resolve it and to ascertain the point of equilibrium between its isomers, were unsuccessful, 7,4, Dinitrobis(l-propylenediamine)cobalt(lIl) ions,

7.41. Historical,

0

The preparation by Hurlimann of this complex and his inability 23

to resolve it was taken by Jaeger to mean that this was a case of 27

complete stereospecificity, Mathieu prepared the corresponding

33

[ ( ^ ( d - p ^ ^ N O g ) ^ ] ion first reported by Watts , and obtained an identical compound, Mathieu recorded the rotatory dispersion curve for the isomer he isolated (Fig, 7,41) and this is the same as that for one of the isomers isolated in the present study,

7.42. Results and Discussion,

82

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