48 complex hy the other A similar attempt was made by Johnson in

In document Stereospecific effects of asymmetric ligands in cobalt (III) complexes (Page 53-63)

K+) l(+) 1(+) l(-) l(-) x(-)

48 complex hy the other A similar attempt was made by Johnson in

1948 when he reacted a mixture which he claimed consisted of

[Co(d-.pn)gd-taxt]C1 and [Co(l-pn)gd-tart]C1 with racemic propylene- diamine in the solution. The resolution was never more than fifteen percent effective.

The first example of an asymmetric synthesis in inorganic 49

chemistry was that reported by Bailar, Jonassen and Huffman in 1948. They reacted d-tartaric acid with racemic carbonatobis(ethylene-

diamine)cobalt(ill) salt to obtain a mixture of dextro- and levo- d-tartratobis(ethylenediamine)cobalt(lIl) ions. When this mixture was reacted with ethylenediamine the chief product was the dextro- tris(ethylenediamine)cobalt(lIl) ion. That the greater reactivity of the dextro form of the d-tartrato complex was responsible for

the predominance of one isomer in the reaction product, was

substantiated by the fact that calcium ions precipitated only 50ft of the tartrate present. Stepwise precipitation with hydrochloric acid left filtrates which were increasingly levo-rotatoryt

demonstrating further selective reactivity.

Partial resolution of racemic acids was obtained in 1952 by 50

Bailar and Gott They reacted racemic tartaric, chloropropionic and lactic acids with the carbonatobis(l-propylenediamine)cobalt(ill) ion and achieved an resolution for the first acid and 30^ for the other two. The reaction mixture was evaporated to dryness in

each case and extracted with ether to isolate the unreacted acid* 51 The opposite approach was made by Bailar, Jonas3en and Gott who reacted ci3-dichlorobis(l-propylenediamine)oobalt(lIl) chloride

with silver carbonate and then with racemic tartaric acid. The

product was presumed to be a mixture of [Co(l-pn)gd-tart ]+ and [Co(l-pn)gl-tart]+ ions, and reaction with 1-propylenediamine was predicted to displace one isomer of tartaric acid before the other. When the reaction was carried out at 70°C a partial resolution of

the tartaric acid was observed.

Another asymmetric synthesis was claimed by Bailar and DasSarma 9

in 1955 in their reaction of d-antimonyltartrate ion with the cis-

dichloro(triethylenetetramine)cobalt(lIl) cation. These workers

claimed that the antimonyltartrate ion replaced the two chloro groups attached to the metal, in a stereospecific reaction, so that when the product was reacted with barium hydroxide an optically active

dihydroxy complex was formed. This was readily transformed into

the carbonato or dichloro compounds which were also partly active* The basis of this work rests on the elemental analyses of the product written as [Cotrien(d-Sb0tart)2]d-Sb0tart.2CHgGH. Partial repetition of this work (Chapter 7 below) shows that this product is in fact a 2*1 electrolyte*

52

Following Dwyer, Gyarfas and Mellor , who, in 1955, showed that the reactions

D[CoEDTA]~ + 3 e n --- > D [ C o e n 3 ]3+ + EDTA4’

53

K irs c h n e r and Wei i n 1957 c a r r i e d o u t th e r e a c t io n !

DfCoEDTA]” + 6 d , l ~ p n --- ► D [C o (d -p n )3 ]Sjf + 3 1-pn & EDTA 4” . They showed t h a t th e p ro p y le n e diam ine was r e s o lv e d to ab o u t 17J&. T h is was e x p la in e d by assum ing th e same r e t e n t i o n o f c o n f ig u r a tio n as t h a t o b s e rv e d by Dwyer and h i s cow orkers f o r th e e th y le n e d ia m in e c a s e , and th e n p o s t u l a t i n g t h a t one iso m er o f p ro p y len ed iam in e p r e f e r r e d t h a t c o n f ig u r a tio n o f th e m e ta l and so r e a c te d more r e a d ily *

S t i l b enediam ine h as d - , 1 - and meso- f orms b u t i s n o t unsy m m etrical and so i t s m e ta l com plexes can n o t e x h i b i t g eo m etric iso m erism .

54

F o r t h i s r e a s o n F erro n e i n 1957 i n v e s t i g a t e d i t s c o b a l t ( l l l ) co m p lex es. He p re p a re d th e _ c is - d ic h lo r ob i s ( l - s t i l b e n e diam ine ) - c o b a l t ( l l l ) c a t i o n and r e a c t e d i t s e v e r a l l y w ith racem ic t a r t a r i c a c i d , p ro p y len ed iam in e and c y c lo h e x a n e d ia m in e . T h ere was no

a p p a re n t r e s o l u t i o n o f th e a t t a c k i n g b a s e s . F e rro n e a ls o c a r r i e d o u t th e f o llo w in g s e r i e s o f r e a c t i o n s !

[C o^-stienJgC lg]*

+ 2 d , l - s t i e n . . . ( l ) »* + 1 - s t i e n . . . (2) ” + 2 l - s t i e n . . . ” + m - s tie n . . . (4 ) " + d - s t i e n • • •

The diam ine re c o v e re d from r e a c t i o n ( l ) was 6 5 ^ l e v o , w h ile t h a t from r e a c t i o n s (3 ) and ( 4 ) was i n a c t i v e . The p ro d u c t o f r e a c t i o n (3 ) was i n a c t i v e . The p ro d u c ts o f r e a c t i o n s ( l ) , (2 ) and (4 ) were each s p l i t i n t o two f r a c t i o n s , one o f which had a p o s i t i v e r o t a t i o n

and th e o t h e r a n e g a tiv e one i n each case*

R e a c tio n (5 ) y ie ld e d two p ro d u c ts h a v in g [a ]^ * - 6 5 .5 ° and 4-208° r e s p e c t i v e l y . P er rone c a l l e d th e s e th e Lddd and Dddd

is o m e rs . U nder p ro lo n g e d h e a ti n g th e Dddd iso m er ( [ a » 4222°) was tra n s fo rm e d i n t o th e Lddd form ( [gl * -37° )•

The p ro d u c t o f r e a c t i o n (4 ) was shown t o c o n ta in tw o - th i r d s a c t i v e b a s e . S in ce th e t r i s ( m e s o - s tilb e n e d ia m in e ) complex i s s t e r i c a l l y h in d e re d from fo rm in g , th e n t h i s p ro d u c t m ust have b een th e b is ( d - s tilb e n e d ia m in e ) m - s tilb e n e d ia m in e c o b a lt ( i l l ) c a t i o n .

The f r a c t i o n a t i o n o f th e p ro d u c t o f r e a c t i o n (2 ) was assum ed to y i e l d th e d e x t r o - and le v o - iso m ers o f th e m ixed b i s ( d - s t i l b e n e - d i a m i n e ) ( l - s t i l b e n e d i a m i n e ) c o b a l t ( l I l ) com plex.

F e r r o n e ’ s work p o in ts to th e e x is te n c e o f tw elve o f th e

e ig h te e n p o s s ib le iso m ers o f th e [ C o ^ t i e n ) ^ ] c a t i o n , h i s p ro o f o f th e i s o l a t i o n o f m ost o f th e s e iso m e rs r e s t i n g on r o t a t o r y d i s p e r s i o n c u r v e s . There i s c e r t a i n l y ev id en ce t h a t two p r o d u c ts a re o b ta in e d from each o f r e a c t i o n s ( l ) , ( 2 ) , (4 ) and (5 ) ab o v e, b u t th e o p t i c a l p u r i t y o f such ieom ers i s d o u b tfu l*

An i n t e r e s t i n g r e s e a r c h was t h a t r e p o r te d by B a i l a r , Cooley and 55

L iu i n 1959. D in itr o e th y le n e d ia m in e - d ,l- b u ty le n e d ia m i n e c o b a l t- ( i l l ) can e x i s t i n th e o p t i c a l iso m ers d e s ig n a te d Dd, Ld, D1 and L I ; w h ile d in itr o e th y l e n e d ia m in e - i s o -b u ty le n e d ia m in e can e x h i b i t

g eo m etric isom erism due to th e u n sy m m etrical n a tu r e o f i s o - b u ty le n e - d ia m in e . I t s iso m e rs a re Da, La, Dß and Lß, where a and ß

7

r e p r e s e n t g e o m etric is o m e rs . W e r n e r^ p r e p a r a t io n f o r th e [Co en ( l - p n ^ N O g ^ j * io n was fo llo w e d c l o s e l y i n each c a s e and

th e f o u r iso m e rs o f each complex w ere i s o l a t e d .

W e rn e r's work h a s now been r e p e a te d (C h ap ter 7 below ) and th e r e s u l t s c a s t some doubt on th e p u r i t y o f h i s p r o d u c ts . The d if f e r e n c e i n c r y s t a l form o f th e g eo m etric iso m ers i s o l a t e d by B a i l a r and h i s cow orkers h a s no p r e c e d e n t o th e r th a n W e rn e r's work

and i f t h a t i s in o o r r e c t th e n t h i s s e p a r a tio n may a ls o be i n d o u b t. The d e m o n s tra tio n o f th e s e p a r a tio n o f th e o p t i c a l iso m ers o f the a c tiv e -b u ty le n e d ia m in e complex on th e grounds o f t h e i r r o t a t o r y d i s p e r s i o n c u rv e s i s n o t c o n c lu s iv e ( s e e S e c tio n 4 . 8 ) .

2

F i n a l l y in 1959 B a i l a r p u b lis h e d , i n c o n ju n c tio n w ith Corey a sem i-m a th em a tic a l model c a p a b le o f e x p la i n in g th e s t e r e o s p e c i f i c e f f e c t s o f lig a n d s l i k e p ro p y le n e d iam in e and c y clo p e n ta n ed iam in e i n t h e i r m e ta l com plexes. The m ethods and c o n c lu s io n s of B a i l a r and Corey a r e d is c u s s e d f u l l y i n S e c tio n 2.6 above.

5 .5 . Dwyer and h i s A s s o c ia te s , 1958-1962»

T r i s ( d i h y d r o x y b i s ( e t h y l e n e d i a m i n e ) c c b a l t ( i l l ) ) c o b a l t ( l I l ) h e x a n i t r a t e - [ C o ( l l l ) ( c o ( l l l ) en 2(0H )2j 3 ] (N03) 6 - h a s th r e e d issy m m etric b id e n ta te lig a n d s s u rro u n d in g th e c e n t r a l m e ta l atom i n such a way t h a t i t i s a ls o d is sy m m e tric . Goodwin, G y a rfa s and

56

M e llo r i n 1958 s e p a r a te d f o u r of th e e ig h t p o s s ib le is o m e rs . TJsing d - and th e n 1- a n t i m o n y l t a r t a r i c a c id as a r e s o lv in g a g e n t,

th e y i s o l a t e d iso m ers w hich th e y l a b e l l e d D^, L^, D jj» k jj» and- which seemed to be i n f a c t two e n a n tio m e ric p a i r s . No

v e r i f i c a t i o n o f t h i s r e l a t i o n s h i p was o b ta in e d s in c e th e o p t i c a l iso m ers had h a l f l i v e s o f th r e e h o u r s , so t h a t f r a c t i o n a l

r e s u l t s to be o b ta in e d * F u rth erm o re p a r t i a l d e c o m p o sitio n o f t r i s ( d i h y d r o x y b i s ( e t h y l e n e d i s m i n e ) c o b a l t ( l I l ) ) c o b a l t ( l I l ) n i t r a t e to g iv e th e b id e n t a te group [ C o ^ I l J e n ^ O H ) ^ ] * b ro u g h t about ra c e m is a tio n and th e r o t a t i o n o f th e lig a n d c o u ld n o t be a s c e r ta in e d *

22

I n 1959 Dwyer, G arvan and Shulman r e - i n v e s t i g a t e d th e isom erism o f th e t r i s ( 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 ) c a tio n * They p re p a re d th e t r i s com plexes from e a c h o f th e p u re o p t i c a l iso m ers o f th e base* Each o f th e s e compounds was r e s o lv e d th ro u g h i t s c h lo r id e d - t a r t r a t e s a l t u s in g e th y le n e g ly c o l a s s o lv e n t* Only two iso m ers were o b ta in e d i n each c a s e even a f t e r a th o ro u g h f r a c t i o n a t i o n o f th e p ro d u c ts o f th e r e s o lu tio n s * These iso m ers

In document Stereospecific effects of asymmetric ligands in cobalt (III) complexes (Page 53-63)