Comparison of Rotatory Dispersion Curves.

In document Stereospecific effects of asymmetric ligands in cobalt (III) complexes (Page 65-81)

The optical isomers of the metal complexes studied in this work have all optically active absorption bands in the visible due to d-d transitions within the metal* The first optically

L

a c t i v e a b s o r p tio n bands ( i . e . th e bands o f lo n g e s t w av elen g th ) a re r e l a t i v e l y i s o l a t e d and th e r o t a t o r y d is p e r s io n c u rv e s

w ith in th e s e bands a re more o r l e s s i d e a l . FOr d i f f e r e n t com plexes w hich can e x i s t i n two m irro r-im a g e form s o n ly , and which have o p t i c a l l y a c t i v e a b s o r p tio n s i n th e same r e g io n , a s i m i l a r i t y o f r o t a t o r y d i s p e r s i o n c u r v e s , w ith in th e s e a b s o r p tio n bands f o r iso m e rs h a v in g th e same a b s o lu t e c o n f ig u r a tio n , m ight be e x p e c te d . Thus i f one ta k e s th e s e r i e s [Co en^ NH^X]11* (w here X i s NOg, NHg, HgO, C l- ) th e s h i f t o f th e p o in ts o f z e ro r o t a t i o n i n th e r o t a t o r y d is p e r s io n c u rv e s fo llo w s th e spectrochemicaJL fi *7 s e r i e s • A p a rt from t h i s s h i f t , th e c u rv e s (shown i n F ig . 4*31) a r e v e ry s i m i l a r and th e iso m e rs th e y r e p r e s e n t a r e a l l ta k e n t o have th e same a b s o lu te c o n f i g u r a t i o n . S i m i l a r l y th e r o t a t o r y d i s p e r s i o n c u rv e s f o r one s e t o f iso m ers o f th e e th y le n e d ia m in e o x a la to c o b a l t ( i l l ) com plexes show a s h i f t tow ards lo n g e r w av elen g th s a s e th y le n e d iam in es a r e re p la c e d by o x a la te io n s i n th e com plex. A ll th e c u rv e s shown i n F ig . 4 .3 2 a r e ta k e n to r e p r e s e n t iso m ers h a v in g th e same a b s o lu te c o n f i g u r a t i o n .

Such assig n m en t o f r e l a t i v e c o n f i g u r a t i o n by com parison o f 69 r o t a t o r y d is p e r s i o n c u rv e s i s by no means new. M athieu

rev iew ed t h i s phenomenon and t r i e d to j u s t i f y i t by c o r r e l a t i n g i t w ith o t h e r m ethods such as th e s o l u b i l i t i e s o f th e d i a s t e r e o - iso m ers form ed d u rin g r e s o l u t i o n . I t i s in te n d e d h e r e to p u t fo rw a rd a d d i t i o n a l ev id en ce showing th e r e a s o n a b le n e s s o f such a ssig n m e n ts

Pig. 4.31. Rotatory Dispersion Curves^0 , (l) [Co engNH^Cl]^; (2) [Co e n g N H ^ o ] 15*} (3) [Co e n ^ N H ^ ] 3*;

Scale

400nryj

500

P ig . 4 .3 2 . R o ta to ry D is p e r s io n C u rv es, ( l ) D[Co en ^ ] (2 ) [Co engox]*? (3 ) [Co en ox2 ]~ ; (4 ) [Co ox3 ]3~#

4 . 4 . The Assignment o f A b so lu te C o n fig u ra tio n .

The r e c e n t d e te r m in a tio n o f th e a b s o lu te c o n f ig u r a tio n o f th e d e x t r o - _ 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 n by X -ray

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te c h n iq u e s h a s g iv en an a b s o lu te r e f e r e n c e compound. P r o v id in g th e co m p ariso n s a r e v a l i d , i t i s now p o s s ib le to a s s ig n a b s o lu te c o n f ig u r a tio n s to th e iso m e rs o f th e c o b a l t ( l l l ) com plexes b e in g i n v e s t i g a t e d . The r o t a t o r y d i s p e r s i o n curve o f I)[Co e n ^ J l^ i s g iv e n in b o th F ig . 4 .3 1 and 4 .3 2 and a l l th e iso m e rs re p r e s e n te d i n each f i g u r e a r e a s s ig n e d th e a b s o lu te 1) c o n f ig u r a tio n o f

[Co e r u j • T h is c o n f ig u r a tio n i s shown i n F i g . 4 .4 1 .

F i g . 4 .4 1 . D[Co e n ^ 3* .

4 . 5 . A b so lu te C o n f ig u ra tio n o f [Co 1-FDTAl .

X -ray a n a l y s i s h a s a ls o r e c e n t l y g iv en th e a b s o lu te 70

c o n f i g u r a t i o n o f l e v o -p ro p y len e d ia m in e . Levo- p r o p y le n e - d i a m i n e t e t r a a c e t i c a c id can be p re p a re d d i r e c t l y from th e

le v o -p ro p y le n e d i a m i n e ^ so i t s a b s o lu te c o n f ig u r a tio n i s a lso known ( F i g . 4 * 5 1 ).

CH2NR2

NR2- — C— H

CH5

F ig . 4 .5 1 . A b so lu te C o n f ig u r a tio n o f D (-)p n (R*H) and D(-)PDTA (R - -CHgCOCE).

The c o b a l t ( l l l ) com plex o f racem ic p ro p y le n e d ia m in e te tr a - a c e t i c a c id can be r e s o lv e d i n t o two isomers'*’0 . These same two iso m ers c an be p re p a re d s e p a r a t e l y , one from each o p t i c a l isom er o f PDTA. 1-PDTA can f i t o n ly one c o n f ig u r a tio n o f th e o c ta h e d r a l m e ta l w ith o u t undue h in d r a n c e . I n s p e c tio n o f th e model o f 1-PDTA on th e o p p o s ite c o n f i g u r a t i o n o f th e m e ta l

r e v e a ls an i n t e r f e r e n c e o f th e m eth y l group w ith th e c a rb o n y l group o f th e n e a r e s t c o o rd in a te d c a rb o x y l which w i l l make t h i s isom er o f h ig h e r f r e e energy th a n th e o t h e r . A d is c u s s io n o f

this phenomenon is given in Chapter Eight below. The two possible isomers are shown in Pig. 4 «52.

Pig. 4*52. (l) Unhindered 011(1 (2) Hindered

Isomers of [Co 1-FDTA]” *

It is presumed that isomer (l) in Pig. 4*52 is the isomer formed in the stereospecific reaction described above and it seems likely that this isomer has the same absolute configuration as the

I f th e r o t a t o r y d i s p e r s i o n c u rv e s o f th e s e two compounds, and a ls o t h a t f o r L (+ )5461[Co EDTA]” io n a r e compared ( F ig . 4 .5 4 ) i t i s c l e a r t h a t w ith in th e f i r s t a b s o r p tio n band th e y a re a lm o st i d e n t i c a l . T h is accords w ith th e above p r e d i c t i o n s .

400mjj

500

F ig . 4 .5 4 . R o ta to ry D is p e r s io n C urves of ( l ) [C° en 0Xg]~;

(2 ) l ( + ) ^ n [Co EDTA]“ ) (3 ) ! ; ( + ) _ , [Co 1-H>TA]". 4 . 6 . I s o m e r is a ti o n w ith Com plete I n v e r s io n o f C o n f ig u r a tio n .

An i n t e r e s t i n g c a s e o f i s o m e r is a tio n r e p o r te d by Dwyer, 71

of the [Co(III)TET]+ ion (Fig. 4.61).

Fig. 4.61. [CoTET ]+ .

This complex cation normally exists in a green isomeric form which can be resolved into D and L isomers. If one of these optical isomers is dissolved in methanol and recovered by

evaporation a brown compound is obtained. The rotatory

dispersion curves for the green D[Co TET ] h ion and its brown isomerisation product are compared in Fig. 4*62.

These curves indicate isomerisation with complete inversion

of configuration. This conclusion is supported by stereo­

chemical considerationss

Inspection of a model of D[Co TET]+ reveals that the only isomerisation process which seems possible without racemisation is the interchange of the two terminal oxygen atoms* This is demonstrated in Fig. 4.63.

J +

[M] 0

100QQ

20000

300QQ

40000

50000

400m jj

500

* 6 0 0

700

Pig* 4*62* R o ta to ry D is p e r s io n Curves* ( l ) D[CoTET]+ ( g r e e n ) , (2 ) L[CoTET]* (b ro w n ).

■»(brown) L[Co TET] Pig* 4*63. (green) D[Co TET]

If the S - N links are omitted in drawing these models the isomerisation appears as an inversion (Pig. 4.64)*

Pig. 4*64* green D --- -— >brown L[Co TET]"1*.

This sort of rationale is in agreement with the rotatory dispersion evidence•

4.7» Series of Reactions with Complete Retention of Configuration. In the reaction sequence*

D[Co erigC lr,]---L -^ D [Co erigCl HgO]2* HOC 3 \ D[Co en2 C03 ]* D[Co e n 2 (N02 ) 2 ] < --- i - D [ C o e n 2 (H2O j2 ] 3+ 72 t h e r e i s no d i s c e m a b l e r a c e m is a tio n • R o ta to ry d is p e r s i o n c u rv e s o f th e compounds fo n n ed a r e i d e n t i c a l w ith th o s e o f t h e i r o p t i c a l l y p u re isom ers* Com parison o f th e s e c u rv e s (Fig* 4*71) shows t h a t a l l compounds i n th e sequence have th e same a b s o lu te c o n f ig u ra tio n * R o ta to ry d i s p e r s i o n ev id en ce th e n r e q u i r e s t h a t a l l th e r e a c t io n s go w ith com plete r e t e n t i o n o f c o n f ig u r a tio n *

Oxygen exchange s t u d i e s on th e s e system s show t h a t th e m e ta l- oxygen bond i s n o t b ro k e n d u rin g such a r e a c t i o n 73* ?^»75,76^

Complete i n v e r s i o n w ith e v e iy a c t o f s u b s t i t u t i o n i s v e ry u n lik e ly in d e e d , so i n th e p r e s e n t system th e fo rm a tio n o f o p t i c a l l y p ure compounds in each r e a c t i o n s t e p , a lm o st c e r t a i n l y stem s from com plete r e t e n t i o n o f c o n f ig u r a tio n * T h is i s i n a c c o rd w ith th e c o n c lu s io n s drawn from th e r o t a t o r y d i s p e r s i o n c u rv e s d e s c rib e d ab o v e.

From th e same ty p e o f exchange s t u d i e s i t seems c l e a r t h a t i n th e r e a c t i o n s

D [Co engNH^Cl ]

500 X ^ 600

F ig . 4 .7 1 . R o ta to ry D is p e r s io n C urvesa ( l ) D[Co en g C l^]* ; ( 2 ) D[Co engCl HgO]2+; (3 ) D[Co e n g C O g jV

( 4 ) D[Co en2 (H20

) 2 f + ;

(5 ) D[Co en 2 (N02 ) 2 ]+.

The r o t a t o r y d is p e r s io n c u rv e s f o r th e two compounds co n cern ed a re

72

They are all shown in Fig* 4.21* The suggestion that all three isomers have the same absolute configuration seems very reasonable. 4.6. Further Comparisons of Rotatory Dispersion Curves*

In the remaining chapters of this thesis there are reported several series of chemically similar compounds which have been

separated into optical isomers. The absolute configuration of these isomers was assigned by comparison of their rotatory dispersion

34-

curves with that of the D[Co en^] ion. The probability of this comparison being oorrect has already been established in this Chapter and in these compounds it is often strikingly illustrated. They also serve to demonstrate the effect of asymmetry in the ligands on the rotatory dispersion curves.

rz, rr ,

One such set of complex ions is [Co en^] , [Co en^ 1-pn] , [Co en(l-pn)2 ] and [Co(l-pn)3 ] The preparation of the optical isomers of these compounds is described in Chapter Five* A

In document Stereospecific effects of asymmetric ligands in cobalt (III) complexes (Page 65-81)