Methyl-l,8-bis(salicylideneimino)3,6-dithiaoctanecobalt(lIl)ion was prepared by the reaction sequence shown in Pig 8.21 The diamine

In document Stereospecific effects of asymmetric ligands in cobalt (III) complexes (Page 148-165)

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

dl 4- Methyl-l,8-bis(salicylideneimino)3,6-dithiaoctanecobalt(lIl)ion was prepared by the reaction sequence shown in Pig 8.21 The diamine

CHj— C H — SH CH— SH C H j-C H — S— CH^C H —

0^_>N-cHi-CH,-er

+ NH— NH2.H20 CH3— CH— S— CH2-C H 2— NH CH— S— CH— CHj—NH CHj-CH— S ^CH C H j-C H — S— CH— C H r -N ^ \ .CH

,)0

CH— S HO' un CH — CH* 'CH Fig. 8.21, Preparation of DL-[Co dl-SEPE]+ .

was purified by distillation and reacted with salicylaldehyde to give an oily bis-Schiff base, (d,l-SEPE), which was used, without further purification, to prepare the cobalt(lll) complex. The preparations paralleled those for similar compounds described by

71

Dwyer, Lions and their coworkers .

The [Co d,l-SEPE]* cation was split into two isomers by a long and thorough fractional crystallisation sequence (Fig. 8.41).

d-Antimonyltartrate, 1 -antimony It art rate and iodide were employed as "the anions in various stages of the fractionation and the two

optically pure isomers, =*_t7800°> were isolated as iodides.

The rotatory dispersion curves for these two isomers are shown in Fig. 8.22, and comparison with that for green D[Co EEE]+ ion

Fig. 8.22. Rotatory Dispersion Curves: (l) D[Co 1-SEPE]*; (2) D[Co EEE]+ ; (3) L[Co d-SEPE]*.

the metal. The presence of geometric isomers or other optical isomers could not be inferred from any stage of the fractionation.

A similar bis-Schiff base was prepared using acetylacetone instead of salicylaldehyde and its oobalt(lll) complex was isolated, but it could not be resolved and so was of no further use in the present study»

The free diamine - dl 4-methyl-l,8-diamino-3,6-dithiaoctane - was resolved "through its d- and 1-dibenzoyltartrate salts and the condensation of these optical isomers, [ßJgggg = ± 50°» with salicyl­ aldehyde gave the optical isomers of the sexadentate ligand,

[«]5893 - + S 2 C • These Schiff bases «ere again obtained as yellow

oils and used to prepare cobalt(lll) complexes. The two complexes so obtained were isolated almost quantitatively as iodides which had rotations 134.51 = +5100° and -5900° for the complexes from the

levo and dextro-base respectively. The rotatory dispersion curve

of these isomers were similar to those shown in Pig. 8.22 though all rotations were about 30$£ lower.

These two products were transformed to chlorides and about 80^ of each was precipitated - the first with 1-antimonyItartrate ion, and the other as the d-antimonyltarträte salt. Treatment of the filtrates with sodium iodide gave small amounts of the residual

fractions in each case. In the first case this small second fraction

has a negative rotation = ”2500°» while in the second it was

The diamine gave a crystalline 5-bromosalicylidene derivative and also gave crystalline hydrochlorides, hydrogen tartrates and hydrogen dibenzoyltartrates, so was readily characterised.

zero though th e f i n a l f i l t r a t e from which i t p r e c i p i t a t e d was

d e x tr o r o ta to r y * The sm all le v o f r a c t i o n , o b ta in e d from th e complex o f th e levo isom er o f th e b a s e , gave a r o t a t o r y d i s p e r s i o n curve w ith th e same sh ap e, b u t o n e - t h i r d th e m agnitude o f L[Co d-SEPE]^ shown i n F i g . 8 .2 2 .

The i n a b i l i t y to re c o v e r th e lig a n d o r th e f r e e diam ine from th e v a rio u s isom ers o f th e c o b a l t complex d e s c r ib e d above h as

*

p re v e n te d a com plete p ro o f o f th e isom erism from b e in g o b ta in e d . The most l i k e l y e x p la n a tio n o f th e e x p e rim e n ta l r e s u l t s i s t h a t a t

l e a s t fo u r iso m ers o f t h i s c o b a l t ( l l l ) complex a re form ed u s in g th e racem ic l i g a n d . These same f o u r can be p re p a re d as two p a i r s by u s in g th e o p t i c a l fo im s of th e b a se and can be l a b e l l e d D1 and LI

i n one c ase and Ld and Dd i n th e o t h e r .

I f i t i s assumed t h a t th e sm a ll f r a c t i o n s - Ld and LI - which were n o t o b ta in e d p u re i n e i t h e r c a s e , have e q u al and o p p o s ite r o t a t i o n s t o th e predom inant f r a c t i o n s - i . e . [ a ] 54^ a ± 78^ ° - th e n th e f r e e - e n e r g y d if f e r e n c e betw een each p a i r o f iso m ers can be c a l c u l a t e d . So f o r th e system c o n ta in in g L I and LI iso m ers th e a p p a re n t c o n s t i t u t i o n i s 8 3 LI and Yl°fo LI g iv in g a f r e e - e n e r g y d if f e r e n c e o f 0.93K c a l / g m o le. I n th e o th e r c a se th e Ld isom er c o n s t i t u t e s some 12^ and th e Ld iso m e r 88j£ o f t h e r e a c t i o n m ix tu re

The r e s u l t s c o u ld be e x p la in e d a s th e in c o m p le te r e s o l u t i o n o f th e d ia m in e , though th e r e c r y s t a l l i s a t i o n and f r a c t i o n a t i o n

o f th e d ia s te r e o is o m e r s to c o n s ta n t r o t a t i o n makes t h i s r a t h e r im probable

g iv in g a f r e e - e n e r g y d if f e r e n c e o f ab o u t 1 .1 7 K c a l / g m ole. A ttem p ts to is o m e ris e th e o p t i c a l l y p u re iso m ers were

u n s u c c e s s f u l so no c o n f ir m a tio n o f th e above v a lu e s c o u ld be o b tain ed * 8*3» T h e o r e tic a l C o n s id e r a tio n s *

I f a m odel o f one iso m e r o f SEPE— say th e L -iso m er r e p r e s e n te d

by CHg - SR

H--- C--- SR

iH

3

i s made, u s in g Leybold m o d els, i t can b e f i t t e d around an atom ic model o f an o c ta h e d r a l m e ta l t o g iv e te n is o m e rs . The m etal can have a D o r L c o n f i g u r a t i o n ; th e m eth y l group can be " e q u a t o r i a l " o r " a x i a l ” (e o r a ) , and t h e m e ta l- lig a n d r i n g s c o n ta in in g benzene groups can assume two c o n fo rm a tio n s i n one o f which th e benzene r i n g l i e s a g a in s t a s u lp h u r and i n th e o t h e r a g a in s t an oxygen atom ,

( F ig . 8 .3 1 ) . S in c e th e m eth y l group makes th e lig a n d unsym m etrical as w e ll as a sy m m e tric a l, th e movements o f th e two benzene r in g s a re n o t e q u iv a l e n t. I f th e benzene r i n g n e a r e s t th e asym m etric carb o n atom i s l a b e l l e d bz^ and th e o liie r b z ^ , and th e p ro x im ity o f a s u lp h u r o r oxygen atom to su ch a benzene r i n g i n d ic a te d by , f o r exam ple, bz^S o r bz^O, th e n th e iso m e rs o f th e m e ta l complex o f th e L-form o f th e lig a n d w hich c a n be made u s in g 3 c a le m odels, a r e s

(1 ) D e bz^S bZgS (2 ) D a bz-jS bZgS ( 3 ) D a b z xO bZgS

F i g . 8 . 3 1 . A l t e r n a t i v e C o n f o r m a tio n s o f a b c d e f r i n g - bz..S and * z i ° - ( 4 ) 3) a bz^S bZgO ( 5 ) D e bz^S bz^O ( 6 ) L e bz^S bz^S ( 7 ) L a bz^S bz^S ( 8 ) L a bz^S bz^O ( 9 ) L e bz^C bz^S ( lO ) L e bz^S bZgO

Most of these isomers are obviously crowded and strained and so probably do not occur in appreciable amounts in an equilibrium mixture. Isomers (l), (6) and (1C) appear to be more or less

strain-free and it would require a more detailed study to determine their relative free-energies. Models of these three isomers are shown in Fig. 8*32 (a), (b) and (c) respectively. An attempt has been made in these photographs to display prominently those

sections of the molecule in which the most serious non-bonded atom interactions would seem to occur.

The above considerations do not indicate complete stereo­

specificity in this case, since two of the strain-free isomers have the L-configuration while the other has the D. The experimental results do not clash with this observation and could well be explained by the conformational analyses of these isomers.

If indeed these three isomers do occur in appreciable amounts at equilibrium, the free-energy differences between them cannot be

calculated until each isomer has been isolated. Also the free-

energies differences reported in Section 8.2 will have little significance

8.4, Experimental.

dl 4^Methyl-lt8-bis(phthalimido)5,6-dithiaoctane,

afc

dl-Propylenedithiol (57g) in absolute ethanol (250ml.) was added to a solution of sodium (24.3g) in dry absolute ethanol (600ml.). ß-bromoethylphthalimide (270g), prepared by the method of Salzberg and Supniewski , in ethanol (800ml.) was added and the mixture

refluxed for 3.5hr. Solid sodium bromide was filtered off and washed with ethanol. The solvent was distilled from the filtrate and washings until the volume reached 500ml. This solution was poured into water

(1500ml.) and the product separated as an oil. The mother liquor was decanted and again diluted to give further oily product from which the supernatant liquid was again decanted. This additional product was dissolved in acetone and added to the initial product. All the solvents were removed in a stream of air at 80°. The crude yield was 240g (~100$&). It could be used for the next stage without further purification but a small sample was induced to crystallise by rubbing with acetone and then recrystallised several times from hot ethanol, from which it separated as white needles, m.p. 78°C.

Analyses for C23H 22N 20 4S 2

C H N

Calculated? 60.80 4.85 6.17

Founds 60.37 5.14 5.72

*

d l 4 -M e th y l» lt 8~diam ino»3t 6 - d i t h i a o c t a n e .

The o i l y p ro d u c t from th e p r e v io u s r e a c t i o n (240g) was suspended i n e th a n o l (12 0 0 m l.) and "boiled u n d e r r e f lu x f o r a few m in u te s . H ydrazine h y d r a te ( 1 1 7 .5 m l., 80^) i n w a te r (8 0 m l.) was added and th e

r e f lu x in g c o n tin u e d f o r a f u r t h e r 1 .2 5 h r . The r e a c t i o n m ix tu re was s t i r r e d from time to tim e to a c c e l e r a t e th e d i s s o l u t i o n o f th e s t a r t i n g m a t e r i a l . Large amounts o f p h th a lh y d ra z id e s e p a r a te d . E xcess h y d ro ­ c h lo r ic a c id (2 5 0 m l., 11N) was added c a u ti o u s ly down th e c o n d en ser and th e r e f l u x i n g c o n tin u e d f o r a f u r t h e r 30m ins. P h th a lh y d ra z id e was f i l t e r e d o f f and washed w ith a l i t t l e w a te r . S o lv e n t was d i s t i l l e d from the f i l t r a t e u n t i l no more came o v e r, th e a d d itio n a l h y d ra z id e which s e p a r a te d was f i l t e r e d and more s o lv e n t removed by d i s t i l l a t i o n u n t i l th e volume re a c h e d 300m l. The amount o f p h th a lh y d ra z id e removed was 150g ( ~ 9 0 /o o f th e e x p ec te d amount).

Sodium h y d ro x id e p e l l e t s (300g) were added c a u ti o u s ly to th e f i l t r a t e and cau sed sodium c h lo r id e to p r e c i p i t a t e and th e diam ine to s e p a ra te a s a d ark o i l y u p p e r l a y e r . The p re se n c e o f s o l i d sodium c h lo r id e p re v e n te d th e u se o f a s e p a r a tin g fu n n e l i n th e e x t r a c t i o n , and th e p ro d u c t was s e p a r a te d by f i r s t g e n tly s w i r lin g th e c o n te n ts of th e f l a s k w ith e t h e r ( 1000m l.) i n 250ml. l o t s and d e c a n tin g th e e t h e r l a y e r , and th e n s h a k in g v ig o r o u s ly w ith e t h e r (20 0 0 m l., 300ml. l o t s ) and a g a in d e c a n tin g th e e t h e r l a y e r s . The e t h e r e a l e x t r a c t

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