Stereospecific effects of asymmetric ligands in cobalt (III) complexes

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LIGANDS IN C O B A Iff(lIl) COMPLEXES".

A T h e s is

s u b m i t t e d f o r t h e D e g r e e o f D o c to r o f P h i l o s o p h y

i n t h e

A u s t r a l i a n N a t i o n a l U n i v e r s i t y

Dy

T . E . M a c D e m o tt B * S c , ( S y d . )

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The work d e s c r ib e d i n t h i s t h e s i s h a s b een c a r r i e d o u t by th e c a n d id a te h im s e lf , u n d e r th e s u p e r v is io n of P r o f e s s o r F .P . Dwyer and i n p a r t i n c o l l a b o r a t i o n w ith D r. A*M. S a rg e s o n .

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PREFACE

One o f th e most co n v in cin g ap p ro ach es to s te re o c h e m ic a l problem s i s to s e t up a sy stem , p r e d i c t th e isom erism , and th e n i s o l a t e and i d e n t i f y th e p r e d ic te d isom ers# When, how ever, such a system f a i l s to e x h i b i t th e p r e d ic te d isom erism th e i n v e s t i g a t i o n changes ground somewhat and becomes an a tte m p t to j u s t i f y th e p re se n c e o f some and ab sen ce o f o t h e r isom ers w ith in th e g e n e r a l framework of w e l l - e s t a b l i s h e d th eo ry #

Such a problem h a s lo n g b een s tu d ie d i n th e m e ta l com plexes o f o p t i c a l l y a c t i v e l i g a n d s . I n many o f th e s e compounds o n ly a few of th e la r g e number o f p o s s ib le iso m e rs o ccu r i n d e te c ta b le am ounts, and i t h a s been th e aim o f t h i s t h e s i s to a s s e s s what h as a lr e a d y b een done, and to d e s c r ib e th e stu d y of v a rio u s o th e r system s c a l c u l a t e d t o throw a d d i t i o n a l l i g h t on t h i s su b je c t#

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with the material presented in Chapters 4,5,6 and 8 below«

T.E. MacDennott.

Biological Inorganic Chemistry Section, John Curtin School of Medical Research, Institute of Advanced Studies,

A.N.U.,

Canberra, Australia.

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ACKNOWLEDGEMENTS

I acknowledge the supervision of this work by Professor

F.P. Dwyer and thank him for his continual help and encouragement * M y thanks go also to the other members of the Biological Inorganic

Chemistry Section for the strictly honest, scientific atmosphere

in which I worked. I must single out Dr. A.M. Sargeson for

thanks since, besides general encouragement and advice, he entered with me into certain experimental parts of this work (Chapters 4 and 5).

The microanalyses recorded in this thesis were carried out by the Microanalytical Service of the Department of Medical Chemistry, A.N.U., and the reproducibility of their results evidences the high standard of their work.

The efficient typing of Miss Eve Freeman and the painstaking proof-reading by Mr. K.R. Turnbull and Dr. B.N. Preston have made the writing of this thesis so much the easier and I am very

grateful.

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TABLE OF CONTENTS

Section Page No*

1.1 1.2 2.1 2.2 2.3 2.4 2.5 2.6 3.1 Preface iii

Acknowledgement s V

Abbreviations X

C H A P T E R O N E

Introduction 1

Nomenclature 5

C H A P T E R T W O

"SOURCES OP OPTICAL ACTIVITY IN OCTAHEDRAL

METAL COMPLEXES". 8

Molecular Dissymmetry 8

Asymmetry resulting from Chelation 10

Asymmetry in Donor Atoms due to Complex

Formation 13

Complexes containing Asymmetric Ligands 17

The Development of the Theory of Stereospecific

Influences of Asymmetric Ligands in Metal 21

Complexes

Conformational Analyses of Stereospecific Complexes

C H A P T E R T H R E E

•»HISTORICAL SURVEY OP METAL COMPLEXES CONTAINING OPTICALLY ACTIVE LIGANDS". Werner and his Associates 1899-1920

24

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Section 3.2 3.3 3.4 3.5 3.6 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10

Jaeger and his Associates 1922-1937 Mathieu 1944

Bailar and his Associates 1934-1959 Dwyer and his Associates 1958-1962 Observations

C H A P T E R F O U R “OPTICAL ROTATORY DISIERSION“ . Optical Rotation

Rotatory Dispersion

Comparison of Rotatory Dispersion Curves The Assignment of Absolute Configuration Absolute Configuration of [Co 1-PDTA]" Isomerisation with Complete Inversion of

Configuration

Series of Reactions with Complete Retention of Configuration

Further Comparisons of Rotatory Dispersion Curves

Conclusion Experimental

C H A P T E R F I V E “ISOMERISM OF COBALT(ill) COMPLEXES

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S e c t i o n P a g e N o 5 . 2 R e s u l t s a n d D i s c u s s i o n 78 5 . 3 T h e o r e t i c a l C o n s i d e r a t i o n s 8 5 5 . 4 E x p e r i m e n t a l

C H A P T E R S I X

•’T H E I S O M E R I S M O F T H E 0 X A L A T 0 1 P R O P Y L E N E

-87

D I A M I N E C O B A L T ( i l l ) S E R I E S ” .

98

6 . 1 I n t r o d u c t i o n 9 8

6 . 2 P r e p a r a t i o n a n d R e s o l u t i o n o f

9 8 N a [ C o 1 - p n 0X g ] 0 . 5 H g O

C

O

t

o E q u i l i b r a t i o n of D a n d L - [ C o 1 - p n o x ^ ] “ 99

6 . 4 P r e p a r a t i o n a n d R e s o l u t i o n of [Co ( l - p n ) g O x j l 1 0 2 6 . 5 E q u i l i b r a t i o n o f D a n d L - [ C o l - p n ^ o x ] * 1 0 5 6 .6 D i s c u s s i o n of R e s u l t s 1 0 4 6 . 7 E x p e r i m e n t a l

C H A P T E R S E V E N

1 1 0

'•THE S T U D Y O F S O M E P R E V I O U S L Y R E P O R T E D W O R K ” . 1 2 0

7.1 I n t r o d u c t i o n 1 2 0

7 . 2 C a r b o n a t o b i s ( 1 - p r o p y l e n e d i a m i n e )c o b a l t ( l l l ) I o n

1 2 1

* 7 , 2 1 H i s t o r i c a l 1 2 1

7 . 2 2 R e s u l t s a n d C o n c l u s i o n s 1 2 2 7 . 3 c i s a n d t r a n s d i c h l o r o b i s ( l p r o p y l e n e d i a m i n e )

-1 2 4 c o b a l t ( l l l ) c h l o r i d e

7 . 4 D i n i t r o b 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 ) I o n s 1 2 5

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7*42 R e s u lt s and D is c u s s io n 125 7 .5 The D i n i t r o ( e t h y 1e n e d ia m in e ) p ro p y le n e -

d i a m i n e c o b a l t ( l l l ) System 127

7 .5 1 H i s t o r i c a l 127

7 .5 2 R e s u l t s and D is c u s s i o n 127

7 .6 B i s ( d - a n t i m o n y l t a r t r a t o ) t r i e t h y l e n e t e t r a m i n e -

c o b a l t ( l l l ) I o n 129

7 .7 E x p e rim e n ta l 130

C H A P T E R E I G H T

"ISOMERISM OF 4-METHYL-l, 8-B IS (

SALICYLIDENE-IMINO) 3 , 6 -DITHIAOCTANECOBALT( I I I )"* 136

8 .1 I n t r o d u c t i o n 136

8 .2 R e s u lt s and D is c u s s io n 138

8 .3 T h e o r e t i c a l C o n s id e r a t io n s 143

8 .4 E x p e rim e n ta l 149

In d e x o f F ig u r e s 167

In d e x o f E x p e rim e n ta l D e t a i l 171

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Ethylenediamine en Propylenediamine pn Cyclopentane-1,2-diamine cpn Cyclohexane-1,2-diamine chxn Stilbenediamine stien iso-Butylenediamine iso-bn

2,3-Butyl enediamine d,l-bn

Trie thylenetetramine trien

Oxalate ox

Tartrate tart

Ant imonyltartrate SbOtart

Ethylenediaminetetraacetic acid EDTA

Propylenediaminetetraacetic acid PDTA

l,8-Bis(salicylideneimino)3t6-dithiaoctane * * ** F.EE 1,10-Bis(salicylideneimino)4,7-dithiadecane * * TET 4-Methyl-l,8-bis(salicylideneimino)3,6-dithia-octane * * SEPE

4-Methyl-3,6 -dithiaoc t ane -1,8-diaminebis -(acetylacetone)

* * AEPE

Free Energy Difference A F °

Molecular Equivalents mols.

Other abbreviations are explained where they are first used.

*

The abbreviations given in each case are the ones used for the complexed organic compound.

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CHAPTER ONE

1»1« Introduction*

In the study of the effeots of asymmetric groups in metal complexes, by far the most commonly used ligand has been

propylene-diamine« This base is a bidentate ligand and its metal complexes

show a d o s e chemical resemblance to those of ethylenediamine, so that any complex of ethylenediamine can be paralleled using propy-lenediamine •

1 2

X-ray and infra-red studies show that the metal ligand ring in both ethylenediamine and propylene diamine complexes assumes a

non-planar configuration« The atoms are distributed in a staggered

conformation so that the angles between bonds deviate as little as

possible from the tetrahedral and octahedral angles« This

puckering, shown in Fig* 1*11 for the ethylenediamine case, gives

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rise to isomerism such as that found in the cyclopentane system*

The h y d r o g e n atoms attached to the methylene carbons can be either

"axial” or "equatorial"* U s i n g conformational analyses Bailar

and C o r e y were able to predict w h i c h isomers should be energetically

preferred to others in a system such as t r is (e thy lenedi amine) cob a l t

-(ill)* T h e y also applied this treatment to the corresponding

propylenediamine complex and predicted the predominance of some

isomers*

These predictions of "Stereospecifioity" have b e e n tested b y

the study of the series, [Co en„] , [Co en^l-pn] , [Co en(l-pn)2 J

and [Co(l~pn)3 J All the members of this series are chemically

similar compounds and differences m u s t b e due to the introduction

of asymmetry into the ligands* H o w e v e r the preparation of such

4

complexes is b y no m e a n s simple* D w yer and Sargeson reported that

the reaction of cis dichlorobis(ethylenedianine)oobalt(lIl) ion wit h

1 -propylenediamine yi e l d e d a mixture of products containing some

7, *T ,

[Co en^] and some [Co(l-pn)^] ions* In the present study a

quantitative separation of this reaction mixture, a nd also the

product of the direct oxidation of a mixture of all the components,

has b e e n achieved using paper chromatography* All the complexes

in the series are formed in each case, and the relative amounts of

the various complexes correspond to a statistical distribution of

the ligands among them* Paper chromatography was also u s e d to

separate the optical isomers of those complexes containing 1-propy­

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calculated from the relative amounts of their D and L isomers* The results were of the same order of magnitude as the predictions of Bailar and Corey*

The complexes [Co(l-pn)gOx]* and [Co 1-pn ox,,] are members of a series which shoulddiow a gradation of effect due to the varying

numbers of 1-propylenediamine molecules present* These complex

ions were prepared and resolved and the stereospecific effect of the optically active propylenediamine groups was found to be greatly altered by the presence of the oxalato groups* An attempt has been made to relate this finding to steric interactions in the various isomers.

The stereospecific effects observed in these studies are not

consistent with some previously reported work* For this reason

the following complexes have been re-investigated#

Complex ion Reference

[Co(l-pn)gC 0 3 ]+ 5

[Co(l-pn)2C X 2 ]+ 6

[Co 1-pn en(N02 )2 ]+ 7

[Co(l-pn)2(N02 )2 ]+ 8

[Co trien (Sb0tart)2 ]+ 9

The carbonato complex was found to be very similar to the corres­

ponding oxalato compound* Though the cis dichloro complex proved

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number of species by paper chromatography. They each appear to be a mixture of compounds as well as a mixture of isomers* The tri­ ethylene te tram ine complex was found to be a 2*1 electrolyte and so

cannot havethe proposed structure* No detailed study of these compounds has been undertaken, but the results indicate that the previous work was incomplete and so does not effectively contradict the results obtained in the present study.

The stereospecific effect of introducing an asymmetric centre into a multidentate ligand was first studied by Dwyer and G a r v a n ^ *

They found that each optical isomer of propylene diamine tetraacetic acid gave only one optical form of the cobalt(lll) complex; further­ more the resolution of the complex prepared from the racemic acid

gave only the same two products. The system then is almost

completely stereospecific. In an attempt to find another completely

stereospecific system, the cobalt(lll) complex of 4-methyl-l,8-bis-(salicylideneimino)3,6-dithiaoctane (Fig. 1.12) was prepared using

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the racemic and optically-active forms of the base* The system was found to be only partially stereospecific and inspection of models shows that either optical isomer of the base can fit round both D and L forms of the complex to give reasonably strain-free molecules.

In these investigations strong evidence for the separation of optical isomers and for their absolute configuration was obtained by plotting their rotatory dispersion curves. These curves - the plot of molecular rotation against wave-length - cannot as yet be firmly based on theory, and an attempt has been made to demonstrate the worth of the empirical use of this rotatory dispersion evidence. Attention has been drawn to the deviations one might expect in going from one isomer of a complex to another, or from one compound to a chemically similar one. The introduction of an asymmetrio centre into a previously inactive ligand does not greatly alter the shape of the rotatory dispersion curves of the optical isomers of the complexes it forms.

1.2. Nomenclature.

(i) Names of Complexes.

The nomenclature used "throughout this thesis follows the 1957 Report of the Commission on the Nomenclature of Inorganic Chemistry, appointed by the International Union of Pure and Applied Chemistry”^ •

There is one major exception and that is in formulating complexes containing both neutral and anionic ligands. In this work the neutral ligand will be written first and the anion second,

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so that c is -dichlorobis (ethylenediamine )cobalt(III) chloride will he written [Co en^ClglCl, and sodium bis(oxalato)ethylenediamine-cobaltate(lll) will be written Na [Co en o xg]. This seems to be

more in acoord with common usage. The names, and their abbreviations,

of many of the organic compounds quoted in this thesis are listed in a table immediately after the Table of Contents.

(ii) Notation of Optical Isomers.

The optical antipodes of metal complexes will be referred to as D or L for dextro- and levo-rotatory isomers respectively. Where there is any doubt as to the reference wave-length this will be added as a subscript, e.g. Egggg» The optical antipodes of organic compounds used will be called d or 1 for dextro- or levo-rotatory

compounds. If the absolute configuration of a metal complex is

discussed it will b e designated D or L for absolute-D and absolute-L configurations respectively. X) refers to the absolute configuration

of dextrorotatory-tris(ethylenediamine)cobalt(lIl)chloride (Fig. 1.21).

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The absolute configuration of organic ligands will be given as e*g* D(+) or D(-) for the isomer having absolute-D configuration* The (+) and (-) refer to the sign of the rotation of the iscmer at the sodium-D line*

Example t Dextro-tris(levo-propylenecLiamine)-cobalt(lIl) ion

has a positive rotation in the sodium-D line* Its absolute

configuration is D* Levo-propylenediamine has the absolute-D

configuration* In a general discussion this complex would be

written D[Co(l-pn)3 ] and called the Dill isomer. If its absolute

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CHAPTER TWO

SOURCES OF OPTICAL ACTIVITY IN OCTAHEDRAL METAL COMPLEXES

2.1« Molecular Dissymmetry. 13

van't Hoff and Le Bel in their original postulates

ascribed the essential source of optioal activity to molecular dissymmetry rather than to the presence of an asymmetric centre in the molecule. A molecule is dissymmetric if it cannot he

exactly superimposed on its mirror-image. This phenomenon is

illustrated b y numerous octahedral metal complexes*

If X is a unidentate ligand and AA a symmetrical bidentate group, then successive replacements of pairs of the ligand X from the octahedral complex [MXg] with the bidentate ligand AA, gives a series of compounds, the isomers of which are shown in Fig. 2.11* The complex [M AA X 4 ] cannot exhibit optical isomerism; it has two planes and one 2-fold axis of symmetry. ^(AAjgXg] can have the X groups either cis or trans to each other. The trans

isomer exhibits no optical isomerism. It has two planes of

symmetry and one 2-fold axis of symmetry. The cis isomer has one 2-fold axis of symmetry yet has non-superimposable mirror-image forms and so is capable of resolution. The complex [M(AA)^] has

three 2-fold and one 3-fold axes of symmetry. It can also exhibit optical isomerism.

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X

X

x

[M AA X j

D Isomer

X

trans [M(AA)2X2]

L Isomer

D Isomer

L Isomer

[H(AA)3]

F ig. 2.11« General Isomerism of Octahedral Complexes of

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complete picture of the stereoisomerism of its octahedral metal complexes* When the hident&te ligand is not rigid and is not constrained to lie in a plane, then its flexibility may lead to

puckering of the metal ligand ring. ©iis would introduce further

asymmetry and dissymmetry into the molecule. These secondary

sources of optical activity are discussed in the following sections# 2.2. Asymmetry resulting from Chelation.

There are a large number of bidentate ligands which, on being introduced into a metal coordination sphere are not constrained by

resonance to assume a rigid planar conformation. An example of

this type of ligand is ethylenediamine. If we consider the

hypothetical case of [M en X^] where X is a simple unident ate group, 14

bond lengths and steric requirements of atoms in the metal-ethylene diamine ring show that this ring is not planar and that

the isomerism depicted in Fig. 2.21 should occur. This non­

planarity has recently been demonstrated by X-ray analyses, e#g.

X

X

X

X

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in the case of trans dichlorobis(ethylenediamine)cobalt(lIl) chloride1 5 .

These isomers are non-superimposable and so are theoretically,

optical isomers. However they can be inter-converted by a simple

internal rotation, the energy barrier of which would be only of the order of that for the rotation of a substituted ethane molecule, i.e. about 5 Kcal/g mole. This value is far too low to allow the

separation of such isomers. The predicted optical isomerism of

this complex is due to the asymmetry of the five-membered metal-ethylene diamine ring. The asymmetric chelate rings are called d or 1 according to the arbitrary designation given in Fig. 2.21.

The introduction of a second molecule of ethylenediamine into this system to give compounds [M(en)gXg] further complicates the picture. As has be e n pointed out there is the possibility of

cis-trans isomerism in this system* The trans isomer can theoretically exist in the three epimerio forms shown in Fig. 2*22. Again the optioal isomerism is due entirely to the asymmetry of the metal-ligand rings*

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The els isomer can also have the same combinations of

asymmetric metal-ligand rings* It also possesses total

molecular dissymmetry so that six forms become possible (Fig* 2*23)•

0 Isomers

X

Nte^-r^-NH

OH

2

CH

5

NH-NH-

-X.

nh

J

'2

ch

2

\

nh

---- ---- /

NH

2

CH

2

N H f ^ - r — 1

NN

2

21

"C H

'tlH-L Isomers

X

Fig* 2*23* Isomers of _cis[M en^Xg]*

All the dd and 11 isomers have a single 2-fold axis of symmetry while none of the dl forms possess any true symmetry elements*

The replacement of the final two uni dentate groups with another ethylene diamine molecule gives the tris complex [M(en)g]* This is a system in which there are three asymmetric

metal-ligand rings in a complex which is dis symmetric* Each

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either D or L so that there are eight possible isomers, which may­ be designated Dddd, Dddl, Ddll, Dill and Lddd, Lddl, Ldll, Llll. The isomers Dddd, Llll, Dill and Lddd each have one three-fold and

three two-fold axes of symmetry* The isomers Dddl, Ddll, Lddl, Ldll each have a single two-fold axis of symmetry#

In all these complexes of ethylene diamine the isomerism due to the asymmetry of the metal-ethylenediamine ring cannot be demonstrated. As has been pointed out, the energy barrier to the internal rotation of any single metal-ethylenediamine ring is of the order of 5 Kcal/g mole and about five times this value is required before separation of isomers becomes possible.

This section has dealt solely with flexible bidentate ligands# Other flexible multidentate ligands can be treated in the same way although the stereochemistry is in general more complicated and it becomes increasingly difficult to generalise. Some specific examples of this sort are given in -the next section.

2.3. Asymmetry in Donor Atoms due to Complex-Formation. X6

Mann showed that bis(ß-aminoethyl)sulphide acted as a

bidentate group with Pt(ll). The complex formed, [Pt(ll)baseCl2]°,

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forms of this ring would be readily interconvertible and the isolation of two optical isomers doubtless is dependent on the asymmetry about the sulphur atom.

(CH2)

t

NH

\

•NH

p\ l 9 H r C H

(CH2)-NH3

.N H J

2/ C

i

S

CL

Fig. 2.31. Asymmetric Donor Sulphur Atom.

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Fig. 2.32. [Co EEE]*.

non-aromatic type metal-ligand cycles which are formed. The

complex as a whole is dissymmetric. These six sources of optical activity are "by no means independently variable and it is clear from models that only certain combinations can occur. When the configuration of one sulphur atom is fixed the other

sulphur atom can assume only one configuration. The Co-S-C-C-8 ring will probably assume a preferred conformation and so pre­ dispose the other four metal-ligand rings to take certain

unstrained conformations. This is fully discussed in Chapter

Eight.

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and it can be seen from models that both nitrogen atoms eure asymmetric; all rings of which the metal is a member are also asymmetric, and the entire metal complex is dissymmetric*

Fig, 2,33. [Co EDTA]“ • 18

X-ray analyses substantiate these postulates in general,

though two of the Co-N-C-C-0 rings are essentially planar. The asymmetry of the other Co-N-C-C-0 rings is easily inverted b y an interned rotation, without affecting the rest of the molecule,

so will not be important. The fixing of any other asymmetric centre -e,g, one of the nitrogen atoms - automatically fixes the

configuration of the rest of the complex. The Co-N-C-C-N ring

is forced to take up a definite configuration as is also the other

nitrogen atom and consequently the complex as a whole. The

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2*4* Complexes containing Asymmetric? Ligands.

The stereochemistry of octahedral meted complexes containing bident&te ligands is further complicated if the ligands themselves

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are asymmetric* Such ligands as trans-l,2-cyclopentanediamine ,

20 21 22

trans-l, 2-cyclohexanediamine , alanine and propylene dims ine are in this category*

If the optioal isomers of the ligand are referred to as d or 1, and the confoxmations of the metal-ligand ring as (+) or (-) then the simplest compound [M AA X^] (where AA has a symmetrical structure) now has the isomers [M (d(+)AA)X4 ], [M(d(-)AA)X4 ], [m(1(+)AA)X4 ] and

[M(1(-)AA)X4 ]. In a similar way this additional isomerism can be superimposed on all the isomers discussed in Section 2*2*

Trans [MCAAjgXg] can have the following isomers*

d(+> d(+)

d(+> d(-)

d(-) d(-)

d(+ ) K + ) d(+) i(-) d(-) K + ) d(-) i(-)

K+) l(+)

1(+) l(-)

l(-) x(-)

Cis [M(AA)gXg] can also exhibit total molecular dissymmetry so that the above ten isomers will occur for both the D and the L forms of the complex, giving a theoretical total of twenty optical isomers* The [m(AA)3 ] complex of this series can theoretically exist in forty isomeric forms, comprising all the possible combinations of the d and 1 forms of the ligand with the (+) and (-) conformations of the metal-ligand ring, in both the D and L forms of the complex*

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-say d AA. [M(d AA)X4 ] can now have only two isomers s d(+) and d(-). Trans [M(d AA)gXg] may exist in three forms d(+) d(+), d(+) d(-) and d(-) d(-); while the cis isomer is reduced to six possible forms, D and L d(+) d(+), D and L d(+) d(-) and D and L d(-) d(-). The [M(d AA)^] system is greatly simplified, there now being only eight possible isomeres

D d(+)d(+)d(+), D d(+)d(+)d(-), D d(+)d(-)d(-), D d(-)d(-)d(-)

i d(+)a(+)d(+),

l

d(+)d(^)d(-),

l

a(+)a(-)d(-),

l

d(-)d(-)d(-)

An asymmetric ligand of especial interest is propylenediamine• Besides being asymmetric this ligand is also unsymmetrioal, and so

can theoretically give rise to geometric isomerism in many of its metal complexes.

The complex [M pn X^] can show only the same isomerism as [M AA X^] discussed above.

Trans [M p n g X g ] has twenty possible isomeric forms since the methyl groups in the two propylenediamine molecules can be either cis or tran8 to each other*

Cis [M pn2 Xg] should be capable of existing in sixty-eight isomeric forms; while the complex [M pn^] will have one hundred and twenty-eight theoretical isomers.

Again the number of theoretically possible isomers is greatly reduced in practice by the use of only one optical ieomer of the base (in this case 1-propylenediamine).

There are two isomeric forms possible for [M(l pn)X4 ] - the 1(4) said 1 (-) isomers. The trans compound of [M(l pn)gX2 ] can

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_Cis[M(lpn)2Xp] can have the three geometric isomeric forms shown in Fig. 2.41. In each of these the ligands can be (+)(+), (+)(-) or (-)(-), so that there are nine such forms of both the D and L isomers of the complex.

Fig. 2.41. Geometric Isomers of D cis [M(lpn)gXg].

[M(lpn)3 ] has a total of twenty possible isomeric fonns, in ten of which the metaJ has the I) configuration while in the other ten it has the _L configuration. The D isomers are shown in Fig. 2.42.

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This complexity of isomerism is not found in practice. In a recent exhaustive study of the [Co(l-pn)g] system most of the material used in the preparation was accounted for as two isomers

designated Llll and Dill. No geometric isomerism was observed, and the Llll isomer was thirteen times as abundant as the D isomer (see Chapter 5 below). This absence of some isomers and predominance of others at equilibrium is called "Stereospecifioity". It is brought about by the "Stereospecific Influence" of the asymmetric

ligands present; and almost all octahedral metal complexes containing asymmetric ligands exhibit the phenomenon.

The theories put forward to account for this stereospecificity are discussed in the following sections.

2.5. The Development of the Theory of Stereospecific Influences of Asymmetric Ligands in Metal Complexes.

The first reference to any principle underlying stereospecificity 23

in metal complexes was made by Jaeger • He had observed the effect in his work on the tris(cyclopentanediamine) rhodium(lll) complex and concluded that* "The levo-rotatoiy configuration of the complex is incompatible with the presence of three levo

molecules of the base; and three dextro-rotatory molecules of the base are incompatible with the dextro-rotatory configuration of the complex".

0

(31)

if the molecules of the optically active base be introduced into the complex ions; so that only one of its two possible enantio-morphic configurations is compatible with the presence of these active molecules’*.

Jaeger suggested that this incompatibility depended on symmetry

considerations. He pointed out that in the cases he had cited the

isomer exhibiting the highest degree of symmetry was formed to the

exclusion of other isomers. This argument from symmetry constituted

the first ’Theory of Stereospecificity in Metal Complexes'.

*

From a study of the dispersion ratios of a number of

24

”stereospecificM metal complexes O ’Brien and Toole concluded that

complexes of optically active ligands differ from ordinary optically active complexes in that* "1. The source of optical activity in metal complexes with optically active diamines is different than for the resolved metal complexes with inactive diamines, and

2 . .... because of the constant dispersion ratio for the optically active amines and for the metal complexes with these amines, it seems fair to presume that the optical activity of the complex arises only from the ligands and not from any induced asymmetry around the centred atom".

The logical conclusion from these ideas is that the metal complex of an optically active ligand is a unique species and has no other possible optical isomers. This contradicts basic theory

* 1

(32)

and was probably not meant by the authors. More than likely 25

Bail&r was correct when he took their conclusions to mean that they thought that all possible isomers occurred to the same

26

extent. Garvan’s agrument that O'Brien and Toole were inconsistent

in the choice of isomers of the compounds they compared - comparing the resolved complex of a racemic base with the unresolved complex of a resolved base - indicated the weakness of their technique.

His separation of the [Co(l-pn)g] ion into D and L isomers and

the demonstration that these isomers occurred in unequal amounts at

equilibrium completely disproves their conclusions. This

refutation is supported by work described in this thesis (see Chapters 5 and 6 below).

27

Independently of this treatment by O'Brien and Toole, Mathieu proposed his "Vicinal Effect". This concept was introduced to account for the appearance of the Cotton effect in some square-planar complexes of optically-active propylenediamine (see Section 4.3. below). In its general development this treatment would seem to endorse the ideas put forward by O'Brien and Toole. However Mathieu went further and postulated a physical basis for the effect

pi b y suggesting that one of the two puckered forms of the [Cu d-pn^ ]

i o n ^ (Fig. 2.51) predominated in the reaction mixture.

Mathieu gave no reason why one form should predominate; but

(33)

ch

3

24

-Pig. 2.51. Puckered Poims of [Cu(d-pn)g] Ions.

ideas more fully developed b y Bailor and Corey (see Section 2.6 below).

Mathieu also suggested that steric interactions could explain the stereospecificity of metal complexes containing optically active ligands. He used models to estimate the distances between methyl groups in the various possible isomers of [Co^-pn)^]^]*, and then proceeded to calculate the repulsive interactions between them. His results did not account for the specifioity observed since his calculations did not involve the sterically interacting

hydrogen atoms (see Section 2.6). However his prediction that*

"It is the interaction of the two molecules of propylene diamine that determines their mutual orientation in the complex considered", has been justified by more recent work •

2.6. Conformational Analyses of Stereospecific Complexes.

The original suggestion that the metal -ethylenediamine ring 14

system was not planar was made by Theilacker • He studied the

2*

(34)

Isomer (i) has a non-superimposable mirror-image form, giving a total of five possible isomers.

& H

H H

h

>-

n

.

'H ^

PtCf^rcH

,N\

H H

(i) U

(ii)dl

/ C H 2

CH^-N

i

H /

ü

C\

H2

ch

2

hX

r \ ,

N CH2 ;N

'CH,

H

/CH2

CH2N/

x

H

\

(

i

i

i

)

m m

O i Pig. 2.61. Isomers of [Pt en2 ] .

rz

Bailar and Corey point out that in such a system (they studied the corresponding cobalt(lll) complexes (as in Pig. 2.22)) the most staggered configuration i.e. the one involving the least energetic non-bonded atom repulsions, will be the most stable. Isomer (i) of

Pig. 2.61 will therefore be the preferred configuration of the 2+

[Pt en^] ion since the other isomers all show some eclipsing of groups, when viewed either down tiie line joining two adjacent nitrogen atoms of different ethylenediamines (ii Pig. 2,61), or along the

(35)

The eclipsed conformation of ethylenediamine, illustrated by iii and iv of Fig. 2.61 is independent of the neighbouring ligands« It is obviously a high-energy conformation and ligands will always tend to assume a staggered conformation« There is, therefore, no point in considering this conformation in other structures«

When this conformational analysis is extended to tris

(ethylenediamine) metal complexes it becomes somewhat more difficult to see which isomer involves the least crowding« If one takes a scale molecular model of such a complex (see Figs« 2«62 and 2«63), and holds it in such a way that the short axis is vertical, then it is clear that, if the ethylenediamine groups have the same

conformation, this axis is a three-fold axis of symmetry« The

carbon-carbon bonds within the three ethylenediamines will all be either oblique to this three-fold axis, or parallel to it« In this thesis the oblique form will be referred to as the o-form and the

*

parallel conformation as the p-form $ so that the two isomers

* 3

Bailar and Corey refer to ligand conformations as k and k*, indicating their absolute configuration« This is equivalent to the d and 1 notation used in Section 2«2 or the (+) and (-)

isomerism of Section 2«4« Such notation is confusing in the

(36)

described will be the ooo and ppp isomers respectively. The mixed oop and opp conformations are also possible but will have free

energies intermediate between the other two. It is merely a question of deciding which of the two extreme forms is the more stable.

Bailar and Corey did this b y estimating the repulsive energies due to the crowding together of non-bonded atoms in these isomers. Since the force laws involved indicate a rapid fall off of repulsive interactions with distance, the effect of atoms relatively close to each other will be much greater than for those further apart.

Taking this into account these workers showed that only a few atoms are actually involved in the determination of the relative

stabilities of ttie compounds in question. Fig. 2.62 shows the

(37)

ooo and ppp forms of a tris(ethylenediamine) complex of an octahedral metal ion.

The hydrogen atoms involved in this crowding are distributed in three identical groups about the molecule. In the ooo form four atoms are involved in each group, while in the ppp isomer only three atoms clash at each site. Fig* 2*63 shows photographs

*

of scale molecular models of the two isomers. Although the whole molecule cannot be seen, an idea of the total picture can be gained by realising that this grouping is repeated exactly at three symmetrically disposed sites about the molecule.

Using the best known values for bond-lengths and angles,

Bailar and Corey wo iked out the distances between these interacting

atoms. They then estimated the repulsive interactions using the

28

empirical force laws described by Mason and Kreevoy ; but

because these cal dilations involved numerous approximations, the results were regarded as being only semi-quantitative. The free energy difference between the ooo and ppp isomers thus calculated amounted to some 1.8 K cal/g. mole in favour of the latter.

The energy barrier to the inversion of a metal-ethylenediamine ring might be expected to be about the same as for the inversion

(38)
(39)

of a cyclohexane ring, or the rotation of a substituted ethane molecule about the carbon-carbon bond. This is of the order of

5 K cal/g. mole and is clearly too small to allow for the separation of the ooo and ppp forms of either optical isomer.

In propylene diamine complexes the metal-propylenediamine ring will exhibit the same puckering and models show that the methyl

groups can be either "axial" or "equatorial". (Fig. 2.64), As in substituted cyclohexane the methyl group in the "equatorial"

position is much less crowded than in the "axial" position. Bailar and Corey give the free energy difference between the two forms as "more than 2 K cals/g. mole" so that in all propylenediamine

complexes the methyl group will be almost entirely "equatorial".

Fig. 2.64. "Axial" and "equatorial" methyl groups in the

Metal-* These groups are pseudo-axial and pseudo-equatorial since the ring differs drastically from the cyclohexane type. The italicising of the words axial and equatorial in the text is meant to indicate this difference.

(40)

It can be seen from modele that when the methyl groups are "equatorial” the only significant non-bonded atom repulsions in a tris( propylene diamine) complex are identical with those for the

corresponding tris(ethylenediamine) species. Therefore the

conformational analysis developed above for the latter complex

can be applied precisely to the former. The free-energy difference between the ooo and ppp conformations should also be of the order of 1.8 K cal/g. mole.

If one optical isomer of propylenediamine (say levo) is used in complex formation then, in order to keep the methyl groups "equatorial” , the Llll isomer will assume a ppp conformation while the Bill will be constrained to adopt the ooo form. The

resolution of such a complex and the establishment of the

equilibrium between its isomers should give a quantitative check on Bailar and Corey*s predicted free energy difference. When this was done the free energy difference was found to be 1.5 K cal/g. mole, (see Chapter 5 below).

This confirmation of theory b y experiment, encourages the hope

3

that the conformational analytical approach of Bailar and Corey may be developed to give a complete and quantitative explanation

(41)

CHAPTER THREE

HISTORICAL SURVEY OF METAL COMPLEXES CONTAINING OPTICALLY ACTIVE LIGANDS*

5*1. Werner and his Associates 1899-1920.

This topic was first studied in Werner's laboratories* In 29

1899 the purple tris(propylenediamine)nickel(ll) complex was

prepared from racemic propylenedi amine and isolated as the sulphate* [Ni pnClg]0 » [Pt pnClg]0 , [Pt pn(HH3 )2 ]Cl2 and [Pt png Clg] were also prepared, using racemic propylenediamine • No comment was made on the optical isomerism of these complexes and no attempt was made

30

to separate any isomers* Pfeiffer and Gassman in 1906 prepared

the analogous tris-propylenediamine cobalt(lll) cation but did not 31

separate any isomers. In 1907 Werner and Frolich prepared

cis-and trans- [CoCl-pn^Clg]* ions cis-and, though they did not isolate any isomers, they pointed out that the [Co pn^Cl^]* ion can exist in twenty isomeric forms.

32

In 1909 Tschugaeff and Sokoloff prepared the

bis(1-propylene-diamine )platinum(ll) cation and pointed out that the molecular

o 2+

rotation, [m ]q « +192 , was just double that of the [Pt(l-pn)en]

ion for which [m ]^ = +96° * These workers suggested that 1-propylene-diamine made a constant contribution of +96° to the molecular

(42)

"band. Tsohugaeff and Sokoloff also prepared the

tris(l-propylene-diamine)cobalt(lll) cation. The rotation of the reaction product

was given as [a]^ * +16 5°, though the sign of this rotation is

22 o

probably wrong, as later workers obtained a value of -180° in a careful investigation.

33

Following Watt*s description in 1912 of some flavo (cis)

complexes of the type [ C o ^ - p n ) , ^ ] 1' Hurlimann investigated the systems [Co(l-pn)2(N02 )2 ]4' and [Co(d-pn)2(N02 )2 ]+ . He was unable to separate any isomers of these compounds and J a e g e r ^ suggested that this was a “partially asymmetric synthesis” . Hurlimann also failed to separate any trans isomers or any geometric isomers from

the reaction mixture. He resolved the mixed complex

[Co(d-pn)(l-pn)(N02 )2 ]+ with d-bromcamphor-n-sulphonio acid into its I) and L forms.

7 85

In 1918 Werner and Smirnoff repeated the work of Heiberg on [Co en(d,l-pn)(N02 )2 ]+ . This complex cation can exist in two trans isomers and eight cis isomers, of which four are optical isomers and the other four are geometric variations of these. They claimed to have isolated all ten isomers of this complex.

8 7

The identity of the compounds studied by Hurlimann and Werner is uncertain and experimental evidence is presented in Chapter 7 below which strongly suggests that these compounds were in fact mixtures of several complexes.

Although the tris(d,l-propylenediamine)cobalt(lIl) cation was 30

(43)

35

until 1919 when Lifsehitz and Rosenbohm attempted unsuccessfully

to resolve it using d-tartaric acid and d-a-brom-camphor-TT-sulphonic 36

acid. In the following year Smirnoff carried out a similar

investigation into the tris(d,l-propylenediamine)platinum(lV) ion. The bromides prepared from the 1- and d-forms of the base had

[a]ß * - 140° respectively and these values were unchanged on

transformation to the 1- and d-tartrates respectively, recrystallisation of the diastereoisomers and regeneration of the bromides. The

bromides prepared from DL-[Pt(d,l-pn)^]Cl^ by resolution through the 1- and d-tartrates had [a]^ * - 137.4° respectively. Hie isomers were designated Dill and Lddd and the base recovered from

the former had [a]^ * - 26.71 . Recent work 9 shows that

Smirnoff* s results are by no means reproducible and that some isomers which he concluded did not exist can be isolated from the reaction mixture.

By analogy with his researches into the platinum complex cited above, Smirnoff, in the same paper, explains a parallel though much less complete investigation into the corresponding cobalt(lll) complex. Smirnoff repeated the preparations of Tschugaeff and

32

Sokoloff preparing the *tris* complex with both d- and

1-propy-lenediamine. These cations were isolated as the bromides and reciystallised from hot water, thus removing the very soluble Dill

22

and Lddd isomers subsequently found by Dwyer, Garvan and Shulman

The compounds isolated were designated as the Dddd and Llll isomers

(44)

investigation - [m J^ * - 223° respectively - do not agree with those of Dwyer and his coworkers , viz. [m ]^ = - 162 ♦ The rotatory dispersion curves which Smirnoff prepared for each isomer almost exactly parallel the corresponding curves for tris(ethylene-diamine)cobalt(lll) ions hut do not correspond with curves reported in this work (see Section 5.2).

A remarkable instance of a dissymmetric metal complex containing an optically active centre was reported by Meisenheimer and his

37

coworkers in 1924. They reacted

dichlorobis(ethylenediamine)-cobalt(lll) chloride with glycine and N-methylglycine respectively in alkaline solution to give glycinatobis(ethylenediamine) cobalt (ill) and K-methylglycinatobis(ethylenediamine)cobalt(lIl) ions (Fig. 3.11).

N

(2)

Fig. 3.11. (l) Glycinatobis(ethylenediamine)cobalt(lIl) and

(2) N-methylglycinatobis(ethylenediamine)cobalt(lIl) Ions.

Both complexes were resolved with d-a-bromcamphor- ft -sulphonic acid* In the second case the more soluble diastereoisomer had [ M ^ * +2020°

(giving a free complex with [M]^ - 4-1470°)• This diastereoisomer

(45)

which reverted to +2020° in 24 hours* A more soluble fraction of this compound had an initial rotation somewhat lower than +2020°, increasing to this value on standing* These workers claimed to have thus demonstrated the asymmetry of the nitrogen atom on complex formation. They pointed out that although the optical isomers of

the metal complex were stable, the optically active nitrogen was 38

unstable and racemised on standing* However Basolo could not

4

repeat this work, and recent investigations into the reaction of the cisdichlorobis(ethylenediamine)cobalt(lIl) ion must make the preparative methods seem doubtful*

5*2* Jaeger and his Associates* 1922-1937*

39

Lifsohitz in 1922 claimed to have carried out a partial

asymmetric synthesis when he prepared the internal salts of d-nitro-camphor* The high rotations of complexes of cobalt(ll), nickel(ll) and copper(ll) were taken to mean that the metals had been rendered

asymmetric* The abnormal rotatory dispersion curves of the

cobalt(ll) derivative of l*4iydroxymethylenecamphor and also of the

tris(d-hydroxymethylenecamphor)cobalt(lIl) and chromium (ill)

complexes were taken as a confirmation of the partial asymmetric synthesis.

(46)

curves indicated that they were enantiomeric with respect to the

metal. These isomers were called Dddd and Lddd respectively, and

were present in equal amounts.

5 27

More recent work by Bailar and Mathieu also indicated that

the stereospecificity involved was partial and not absolute; but results which pointed to this conclusion were taken, together with

41

the work of Lifschitz, to be anomalous • It has only been quite

22

a recent development to accept this partial stereospecificity as

the rule rather than the exception.

It may be of interest to note here that molecular models show that the tris(d-alaninato)cobalt(lIl) ion has few of the non-bonded

3

atom interactions which Bailar and Corey suggest are the chief cause of specificity. This may account for the failure to observe the effect in this case. No quantitative experiments were u nder­ taken by Lifschitz and a small effect might easily have escaped his notice.

19

In 1928 Jaeger and Blumendal carried out a series of researches

designed to test the stereospecific effects reported by previous

workers. They used racemic and optically active

(47)

The iso n erB were l a b e l l e d Lddd and D i l l and 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 s u b s t a n t i a t e d t h i s c o n c lu s io n . The racem ic p e r c h lo r a te c o u ld be s e p a r a te d i n t o i t s a n tip o d e s by h a n d -p ic k in g th e c r y s t a l s a f t e r c r y s t a l l i s a t i o n a t room te m p e ra tu re .

The c o rre sp o n d in g c o b a l t ( i l l ) com plexes were p re p a re d and shown by J a e g e r and B lum endal to be analogous to th e rh o d iu m (m ) compounds. I n t h i s c a s e to o th e c r y s t a l l i n e p e r c h lo r a te o f th e racem ic com plex was s e p a r a te d by h a n d - p ic k in g . J a e g e r commented t h a t th e s e s a l t s w ere p in k and t h a t t h i s was Mrem a rk a b le M.

I t i s w o rth n o tin g h e r e t h a t i f m o le c u la r m odels a re made o f c y clo p e n ta n ed iam in e i n i t s v a r io u s is o m e ric form s i t would a p p e a r to b e a po o r b id e n t a te c h e l a t e . The l a r g e s t p o s s ib le an g le b etw een th e two n itr o g e n - m e ta l bonds i s ab o u t 40° ( F i g . 3 .2 1 ) .

40-50

F i g . 3 .2 1 . C h e la te r i n g i n C y clo p en tan ed iam in e Complexes

The s t r u c t u r e would a p p ear t o be v e ry u n s ta b le and such com plexes a r e u n l i k e l y t o be foimed* The r e p e t i t i o n o f t h i s work which i s

42

(48)

J a e g e r a l s o prepared'*'9 th e b is ( e th y le n e d i a m i n e ) l - c y c l o - p e n t a n e d ia m in e c o h a lt( lll) io n by r e a c t i n g le v o cy c lo p e n ta n e d ia m in e w ith c i s - d i o h l o r o b 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 ) c h l o r i d e . T h is compound was r e s o lv e d th ro u g h i t s brom ide d - t a r t r ä t e . The r o t a t o r y d i s p e r s i o n c u rv e s were ta k e n t o co n firm th e r e s o l u t i o n , and th e v a s t d if f e r e n c e o f th e s e c u rv e s from Hie c o rre s p o n d in g

rz M

c u rv e s f o r [C o (l- c p n ) g ] was ta k e n t o s u p p o rt th e claim ed i d e n t i t y o f th e compounds s tu d ie d . T h is e v id e n c e from 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 .

The b is ( e th y le n e d ia m in e ) c y c l o p e n ta n e d ia m i n e c o b a lt( lI l ) c a t i o n was a ls o p re p a re d u s in g ra c em ic c y c lo p e n ta n e d ia m in e . R e s o lu tio n w ith d - t a r t a r i c a c id y ie ld e d o n ly two is o m e rs, one o f w hich resem b led

rz .

D [C o (e n )g l-c p n ] • The o t h e r was n o t l i k e e i t h e r iso m er r e p o r te d above and, a lth o u g h i t was n o t o b ta in e d p u r e , was ta k e n to be th e

3+

e n a n tio m e ric L f C o ^ n Jg d -c p n ] i o n . J a e g e r co n clu d ed t h a t when o n ly one o p t i c a l l y a c t i v e lig a n d was p r e s e n t i n a m e ta l complex th e s t e r e o s p e c i f i c e f f e c t was g r e a t l y re d u c ed compared w ith th e a b s o lu te s t e r e o s p e c i f i c i t y o f the t r i s com plex. He d id n o t e x p la in why th e racem ic b a s e sh o u ld d is p la y g r e a t e r s t e r e o s p e c i f i c i t y th a n th e two o p t i c a l iso m ers s e p a r a t e l y .

F i n a l l y J a e g e r ^ 9 p re p a re d th e c i s - d ic h lo r o b is ( d - o y c lo p e n ta n e - d i a m i n e ) c o b a l t ( l l l ) io n and r e a c t e d t h i s w ith 1-c y c lo p e n ta n e d ia m in e . The p r o d u c ts from t h i s r e a c t i o n w ere L [C o (d -c p n )3 ] and

r z .

(49)

gave [C o(en) ] and D [C o(d-cpn) ] as th e o n ly p r o d u c ts .

ö U

43 An im p o r ta n t p a p e r was p u b lis h e d by J a e g e r and D ip p le i n 1931, i n w hich th e y d e s c rib e d a v a r i e t y o f com plexes o f th e o p t i c a l l y - a c t i v e b i d e n t a t e 2 ,4 -d ia m in o p e n ta n e . They f i r s t s e p a r a te d th e b a s e i n t o i t s meso and racem ic form s, a lth o u g h th e y d id n o t r e s o lv e th e racem ic m ix tu r e . T r is - ( 2 ,4 - d ia m in o - p e n ta n e ) if c o d iu m ( lll) com plexes w ere p r e p a re d and r e s o lv e d , and

r o t a t o r y d i s p e r s i o n cu rv es w ere o b ta in e d f o r th e r e s o l u t i o n p r o d u c ts . These c u rv e s i n d i c a t e d t h a t th e p r o d u c ts had o p p o s ite c o n f i g u r a t i o n s . The b a s e r e c o v e re d from th e d e x tro iso m er o f th e complex was found t o b e d e x tr o ( [ a j ^ « + 1 4 .5 ° compared w ith +17° f o r th e o p t i c a l l y p u re b a s e ) . Only 40‘fo o f s t a r t i n g m a t e r i a l s were i s o l a t e d from th e r e a c t i o n m ix tu re i n t h i s work and th e c o n c lu s io n th a t* "T his i s a c a s e o f asym m etric s y n th e s is i n th e fo r m a tio n o f th e io n l i k e t h a t a lr e a d y o b serv ed i n s i m i l a r cases*' - i s h a r d ly j u s t i f i a b l e . O th er com plexes d e s c r ib e d by th e s e w o rk ers in c lu d e d [Co b^ C lg ]* ,

[Co b 2 (N02 ) 2 ]+ , [Co b 2 C03 ]+ , [Co b g o x ]+ , [ I r b 2 (K 0g)2 ]+ and

O , ~

[Cu b g ] (w here b i s racem ic o r meso 2 ,4 -p e n ta n e d ia m in e ). 44

(50)

In another investigation trans-dichlorobis(d-cyclohexane-diamine) cohalt (ill) chloride was reacted with 1-cyclohexanediamine and also with ethylene diamine« The first reaction gave only the Dill and Lddd isomers, while the second gave the ion

[Co (d-chxn^en]^*, The complex cation [Co engd-chxn]^* was also

prepared from [Co en^Cl^]* but was found to give only [Co and L[Co (d-chxn)3 J when efforts were made to resolve it«

All the results obtained paralleled the previous work on

trans-l,2-cyclopentanediamine« Attempts to make mixed

cyclo-pentanediamine-cyclohexanediamine cobalt(lll) complexes resulted in mixtures of isomers of the 'tris' species of each ligand* 3«3» Mathieu. 194 4 «

27

A thorough study of the subject was made by Mathieu in 1944* Much of the early work was repeated using ^dextro-propylenediamine

instead of the levo-isomer which previous workers used. Mathieu prepared the [Co ( d - p n ^ N O g ^ ] * ion and obtained the same compound

Q

as that described by Hurlimsnn and resolved this into D- and L-isomer8. The preparation of [Co en(d-pn)(N02 )2 ]+ cation paralleled

7

Werner and Smirnoff's preparation except that the geometric isomers were isolated first, and then one of these was resolved into its optical isomers. Mathieu queries the rotatory dispersion curves given by Werner and Smirnoff since they differed markedly from his own« He also points out that the straight line optical rotatory dispersion obtained for the compound labelled

D,L[Co en l-pn(N02 )2 ]+ is unacceptable since when one adds the

(51)

ions a non-linear plot is obtained.

Mathieu prepared cis- and trans-dichlorobis(d-propylenediamine)-31

cobalt (ill) ions by the method of Werner and Frolich and then attempted to complete the series of dichloro complexes by reacting the dinitroethylenediamine-d-propylenediaminecobalt(lll) chloride

with hydrochloric acid. Two products were formed and, since one

was water-soluble and the other alcohol-soluble, Mathieu suggested they were [Co(en)2Cl2 ]Cl and [Co(d-pn)2C l 2 ]Cl respectively*

Disproportionation was also reported in the reaction of

_trans-[Co(d-pn)2C l 2 ]+ ion with a hot solution of ethylene diamine. Fractionation of the product showed inconsistencies which Mathieu explained as due to the formation of the [Co(d-pn)„J and [Co(en)„]

ions. However no mention of disproportionation was made in his

preparation of [Co(d-pn)en2 ] ion from the oi3-dichlorobis( ethylene

-diamine)cobalt(ill) cation. This mixed complex Mathieu claimed to

have partly resolved through its bromide-d-tartrate salt. Mathieu prepared the square-planar complex cations

[Ni(ll)(d-pn)2 ]2+ and [Cu(ll)(d-pn)2 ]2+ in solution following 29

Werner . He prepared the rotatory dispersion curves for these ions

and pointed out that they each exhibit the Cotton effect. These

phenomena were explained by a stereochemical, model which fore­ shadowed the semiquantitative conformational analyses of Bailar and

3

Corey . Just as important was Mathieu*s explanation of the stereo­ specificity of the [Co(d-pn)2C l 2 ]+ ion in terms of non-bonded atom

(52)

5»4. Bailar and his Associates, 1934-1959, 45

In 1934 Stiegman reported the existence of two isomers of carbonatobis(l-propylenediamine)cobalt(lIl) cation, which he

concluded were the dextro- and levo-forms. He was unable to isolate these optical isomers and it was not until 1939 that Bailar and

46

McReynolds reported the preparation of each isomer by separate

stereospecific reactions, and their isolation as carbonate salts. These salts were not analytically pure and in 1952 Bailar and

5

Martinette repeated the work and isolated both isomers as chlorides

which were identifiable by elemental analyses* Both these last

investigations used rotatory dispersion curves to prove that a separation had taken place. The significance of such curves is

discussed in Chapter 4 below. Some of the work has been repeated

in the present study and is reported in Section 7.2.

The possibility of using a stereospecific complex as a resolving agent by selective reactivity was first put forward by Bailar,

g

Stiegman, Balthis and Huffman in 1939« They reacted, in turn,

racemic propylenediamine, leucine and alanine with cis-dichlorobis-(l-propylenediamine)cob alt(ill) salts in the belief that this

complex was completely stereospecific in its preparation. No

resolution was achieved nor were the reactions of racemic propylene­ diamine with the complex cations [Co(l-pn)2Cl2 ]+ , [Co(l-pn)2C03 ]+ and [Co(l-pn)2 (N02 )2 ]* any more successful.

47

Hamilton in 1947 succeeded in using a stereospecific complex

(53)

acid* The resulting uncoordinated acid had a small levo-rotation showing that one isomer of tartaric acid was replaced in the

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

Figure

Fig. 1.12.
Fig. 1.12. p.14
Fig. 1.21. D[Co ei^]
Fig. 1.21. D[Co ei^] p.16
Fig. 2.11«
Fig. 2.11« p.19
Fig. 2.22. Isomers of trans[M en^Xg]
Fig. 2.22. Isomers of trans[M en^Xg] p.21
Fig. 2.31. Asymmetric Donor Sulphur Atom.
Fig. 2.31. Asymmetric Donor Sulphur Atom. p.24
Fig. 2.32.
Fig. 2.32. p.25
Fig. 2.41. Geometric Isomers of D cis [M(lpn)gXg].
Fig. 2.41. Geometric Isomers of D cis [M(lpn)gXg]. p.29
Fig. 2.62 shows the
Fig. 2.62 shows the p.36
Fig. 2.64.
Fig. 2.64. p.39
Fig. 3.11.
Fig. 3.11. p.44
Fig. 4.21. Typical Rotatory Dispersion Curve within an
Fig. 4.21. Typical Rotatory Dispersion Curve within an p.65
Fig. 4.61.
Fig. 4.61. p.73
Fig. 4 .7 1.
Fig. 4 .7 1. p.77
Fig. 5.21.
Fig. 5.21. p.88
Fig. 6.21. Rotatory Dispersion Curves: (l) L[Co 1-pn ox^]”;
Fig. 6.21. Rotatory Dispersion Curves: (l) L[Co 1-pn ox^]”; p.109
Fig. 6.61 shows photographs of models of the D-
Fig. 6.61 shows photographs of models of the D- p.115
 Pure Isomerised D isomerIsomerised L isomer L isomer present L isomer present isomerD Pure L isomer Run 1 23fo Run 1234faTable I
Pure Isomerised D isomerIsomerised L isomer L isomer present L isomer present isomerD Pure L isomer Run 1 23fo Run 1234faTable I p.123
Fig. 7.21. Rotatory Dispersion Curves46* (l) LfCo^-pn^CO^gCO^;
Fig. 7.21. Rotatory Dispersion Curves46* (l) LfCo^-pn^CO^gCO^; p.131
Fig. 8.11 shows four possible isomers of a complex
Fig. 8.11 shows four possible isomers of a complex p.146
Fig. 8.12,
Fig. 8.12, p.148
Fig. 8.21, Preparation of DL-[Co dl-SEPE]+ .
Fig. 8.21, Preparation of DL-[Co dl-SEPE]+ . p.149
Fig. 8.22. Rotatory Dispersion Curves:
Fig. 8.22. Rotatory Dispersion Curves: p.150
Fig. 8*41*. Fractional C rystallisation  of D and L Isomers of [Co dl-SEPEf
Fig. 8*41*. Fractional C rystallisation of D and L Isomers of [Co dl-SEPEf p.171
Fig. 1.12
Fig. 1.12 p.177
Fig. 2.51
Fig. 2.51 p.177
Fig* 4*21
Fig* 4*21 p.178
Fig. 6*6l(i)
Fig. 6*6l(i) p.179

References

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