ISCMERISM OF COBALT(lIl) COMPLEXES CONTAINING ETHYLENE - DIAMINE AM) 1-PROPYLENEDI AMINE.

5.1* Introduction.

The replacement of the ethylenediamine groups in the tris- (ethylenediamine)cohalt(lll) ion, one at a time, with 1-propylene- diamine results in a series of complex cations - [Co en^]3*,

[Co engl-pn]3*, [Co e n ^ - p n ) ^ ] 3*, [Co(l-pn)^]3* - whose chemical properties might be expected to be very similar, while the energy differences between their various confonnational isomers as has been pointed out in Section 2*8, will be quite different. This

system was studied in the hope of establishing quantitatively the

stereospecific influence of the optically active 1-propylenediamine molecules present.

A similar system has already been studied3 '**38 in the series [Pt en^]4*, [Pt en2l-pn]4 *, [Pt en(l-pn)2 ]4+ and [Pt(l-pn)3 ]4 + . All of these complexes were isolated and each was resolved into

two optical isomers. As the authors point out, the isomers obtained did not appear to racemise and the preparations were not carried out under equilibrium conditions. Because of this no quantitative conclusions could be drawn about the predominance of some isomers and the absence, or seeming absence, of others in the reaction mixture. Geometric isomerism was not observed in this system

The t r i s ( l - p r o p y l e n e d i a m i n e ) c o b a l t ( l I l ) system h a s been s tu d ie d

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i n g r e a t d e t a i l , Of th e tw enty t h e o r e t i c a l l y p o s s ib le iso m ers ( S e c tio n 2#9) o n ly two have been i s o l a t e d . These were d e s ig n a te d D i l l and L l l l , and a g a in g e o m e tric a l isom erism was n o t observed*

These two iso m e rs were s e p a r a t e l y ra c e m ise d on c h a r c o a l and th e p o in t o f e q u ilib r iu m a r r iv e d a t by e ach i n d i c a t e d a f r e e en erg y d i f f e r e n c e betw een them o f - 1 .0 2 K c a l / g . mole i n fa v o u r o f the L -iso m e r.

The r e i n v e s t i g a t i o n o f t h i s system h a s b e e n c a r r i e d o u t as p a r t o f th e work d e s c r ib e d i n t h i s C h a p te r. The same two iso m ers were found and were c h a r a c t e r i s e d by t h e i r r o t a t o r y d is p e r s io n cu rv es ( F ig . 5 .2 2 ) . The f r e e en erg y d if f e r e n c e h a s proved to be somewhat h ig h e r ( - 1 . 5 K c a l / g . m ole) th a n th e - 1 .0 2 K c a l / g . mole o f Dwyer,

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G arvan and Shulman , T his d is c re p a n c y i s e x p la in e d by two

f a c t o r s a 1 . th e p a r t i a l a b s o r p tio n o f some m a t e r ia l on th e c h a rc o a l in th e p ro c e s s o f r a c e m is a tio n ^ 0, and 2 . th e f a i l u r e o f

f r a c t i o n a l c r y s t a l l i s a t i o n te c h n iq u e s to s e p a r a te th e D -iso m er i n an o p t i c a l l y p u re s t a t e .

3+ 3+

The m ixed com plexes [Co e n ^ l- p n ] and [Co e n ^ - p n ) ^ ] have 80

b o th been r e p o r t e d and th e p r e p a r a t i o n s have been re p e a te d by 27

M athieu • There seems to be l i t t l e p o in t i n d is c u s s in g h e r e , e i t h e r th e e a r l y work, o r M a th ie u ’ s r e p e t i t i o n o f i t s in c e th e r e a c t i o n s u se d were of th e type j

[Co engC lg]* + 1 - p n --- ►[Co en^ 1 -p n ]34" + 201“ . 4

3+ 34*

mixture of ions containing some [Co en^] and some [Co(l-pn)3 ] ions. The present work substantiates this finding - and indeed gives it a quantitative basis (Fig. 5.21 (l)). ,,Isomers,, from the resolution of such reaction products cannot be quoted with any confidence.

5.2. Results and Discussion.

Aerial oxidation in the presence of charcoal, of solutions

containing cobalt(il) chloride, hydrochloric acid, 1-propylenediamine and ethylenediamine in the exact proportions represented by the

two equations*

Co Clg * HC1 4- 2(l-pn) + e n --- ►Products and Co Clg + HC1 + 1-pn + 2en --- > Products, gave two products which were studied separately.

Cellulose partition chromatography, using wet-butanol/hydro­ chloric acid as the eluent, separated each product into four bands*

(1) Ii[Co(l-pn)g]Clj (2) L[Co(l-pn)2en]013

(3)

L[Co 1-pn en

]ci„

(4) A mixture.

The fourth band in each case, (i.e. the band least developed), was separated by paper chromatography, using wet-butanol/perchloric

acid, into four separate compounds which were shown to be* (1) D[Co(l-pn)3 ](C104 )3

(2) D[Co(l-pn)2en](C104)3 (3) D[Co 1-pn en2 ](C104)3 (4) DL[Co en3 ](C104 )3

The i d e n t i f i c a t i o n o f th e v a rio u s bands i n th e s e ch ro m ato g rap h ic s e p a r a tio n s was made e a s i e r by th e f a c t t h a t b o th iso m ers o f t r i s - ( 1 -p ro p y len e d iam in e) cob a l t ( i l l ) io n were known. The u se o f th e s e as m arker compounds i n th e c h ro m ato g rap h ic s e p a r a tio n s showed t h a t th e f i r s t band o f the f i r s t chromatogram c o n s is te d o f th e

L [C o (l-p n )s ]3* c a t i o n (P ig* 5 .2 1 ) . When th e l a s t band o f t h i s

H

t «

M

« »

• I

»

I

• « •

t

CD CCoen^C^lCl + lfP " C4) L-CCo l-pn er^]' ßK C oer^ l-pn] 3+ Equil.Mixture.

(3)L-[CoUpn3l 3+

(71 [Col-pn2en l3+ Equil. Mixt.

C5) L-CCo l-pr^en]3* (6) [Co en 3D3+

F ig . 5 .2 1 . P h o to g rap h o f Chromatogram i n U ltr a - V i o l e t L i g h t, s e p a r a tio n was re-chrom atogram m ed, m arker compounds showed t h a t th e f i r s t band c o n s is te d o f th e U [C o (l-p n )3 J io n , w h ile th e

l a s t band c o n ta in e d racem ic t r i s ( e t h y l e n e d i a m i n e ) c o b a l t ( l I l ) s a l t s . The second and t h i r d bands on each chrom atogram were re a s o n a b ly

(7 i * 2 , i n th e f i r s t ? and D [C o (l-p n )2e n ] and D[Co 1-pn en g] r e s p e c t i v e l y i n th e second.chrom atogram *

The ch em ical c o n s t i t u t i o n o f th e s e fo u r hands was d e term in e d by e le m e n ta l a n a ly s e s , and a ls o i n th e c a se of th e le v o is o m e rs, by th e e s ti m a ti o n o f th e r a t i o o f e th y le n e d ia m in e to 1 - p r o p y le n e - diam in e p r e s e n t i n th e complex* Such a n a ly s e s a g re e d w ith th e p r e d i c t e d fo rm u la e . The d e te r m in a tio n o f th e r a t i o o f e n /1 -p n was c a r r i e d o u t by th e d e s t r u c t i v e e x t r a c t i o n te c h n iq u e d e s c r ib e d

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by Dwyer, G arvan and Shulman x The complex was decomposed by h e a ti n g w ith c a u s t i c soda and th e d ia m in e s p r e s e n t were e x t r a c t e d

i n t o benzene* The c o n c e n tr a tio n o f t o t a l diam ine was o b ta in e d by a c id t i t r a t i o n , and th e o p t i c a l r o t a t i o n was a ls o m easu red . By

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u s in g th e b e s t v a lu e f o r th e r o t a t i o n o f 1-p ro p y len e d ia m in e i n d ry benzene ( [ a ] ^ » - 3 4 .4 ° ) , th e r e l a t i v e amounts o f e th y le n e d ia m in e and 1-p ro p y len e d ia m in e were e a s i l y c a l c u l a t e d s

F i r s t Chromatograms

r a t i o e n slp n S u g g ested Form ula

2nd band 1 x 2 L [C o (l-p n ) ^ e n ]3* 3 rd band 2 3 1 L[Co 1 -p n e n ^ ]3*

The a b s o lu te c o n f ig u r a tio n o f th e s e complex io n s was d e term in e d by a co m p ariso n o f t h e i r r o t a t o r y d is p e r s i o n c u rv e s w ith th e c u rv e s f o r tiie p u re o p t i c a l iso m ers o f t r i s ( e t h y l e n e d i a m i n e ) c o b a l t ( l I l ) io d id e * Such a com parison f o r th e t r i s ( l - p r o p y l e n e d i a m i n e ) -

cobalt(lll) salts is shown in Fig. 5.22. That for the ethylene- diaminebis(l-propylenediamine)cobalt(lIl) system is given in Fig. 5.23; while the bis(ethylenediamine)i-propylenediaminecobalt(lIl) series is shown in Fig. 5.24.

400mp

500

3+

In document Stereospecific effects of asymmetric ligands in cobalt (III) complexes (Page 85-90)