The Direct Electrochemical Synthesis of Selected Group-Iib Organometallic Compounds

University of Windsor Scholarship at UWindsor

Electronic Theses and Dissertations Theses, Dissertations, and Major Papers

1-1-1979

THE DIRECT ELECTROCHEMICAL SYNTHESIS OF SELECTED GROUP-IIB ORGANOMETALLIC COMPOUNDS.

AKHTAR OSMAN University of Windsor

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Recommended Citation OSMAN, AKHTAR, "THE DIRECT ELECTROCHEMICAL SYNTHESIS OF SELECTED GROUP-IIB ORGANOMETALLIC COMPOUNDS." (1979). Electronic Theses and Dissertations. 7187. https://scholar.uwindsor.ca/etd/7187

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NAME OF AUTHOR/NOM O f L’AUTEUR. A khtar Osman

TITLE OF THEStS/7777?£ O f LA THESE- The direct electrochemical synthesis of selected

grouo IIB organonetallic compounds.

UNIVERSITY/LW/ VERS/ TE. U niversity of ~Windsor. Windsor ..Ontario

DEGREE FOR WHICH THESIS WAS PRESENTED/. GRADE POUR LEQUEL CETTE THESE FUT PRESENTEE _ Ph.D.

YEAR THIS DEGREE CONFERRED /A NNiE D'OBTENTlON DE C£ GRADE Mav 19 SO

NAME OF SUPEKVISOR//VOM DU D1RECTEUR DE THESE _ U r. D. G. Tuck

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Ottawa, Canada K 1 A 0 N 4

NL-339 {Rev. S/80)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. THE DIRECT ELECTROCHEMICAL SYNTHESIS OF SELEe?ED GROUP' IIB v ORGANObETALLIC COMPOUNDS)^£TALL]

. by . ■ C Akhtar Osman

B.Sc., University of Karachi, 1970 M.Sc., University of Karachi, ,1972 M.Sc. , Middle East Technical University, Ankara, 1975'

A D issertation

Submitted to the Faculty of Graduate Studies ' through the Department of Chemistry in Partial Fulfillm ent of the Requirements fo r’the Degree of. Do'ctor of Philosophy

a t The "University of Windsor W indsorOntario, Canada 1979

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. This dissertation has been examined and approved by -

Dr. D.S. Tuck ------1 /

Dr. J.E. Drake < U !.

Dr. B.R. McGarvey

Dr. J.W. McConkey

Dt^vJ. Oliver

Wayne/ State University 4 it, Michigan, U.S.A. (External Examiner)

X r \ / ______Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENT

The investigations described in this dissertation were

carried out in the Department of Chemistry of the University

of Windsor, Ontario, Canada between 1975 and 1979 under the supervision of Dr. D. G. Tuck. To him I would like to express my utmost gratitude, for his tremendous help, guidance and encouragement_^£hroughout the research. His

drive and boun^l^ss enthusiasm w ill always remain a source

of inspiration for me.

I would also like to acknowledge the warmth and the

invaluable mutual support of my colleagues, in particular

Dr. R. Steevensz, Dr. F. Said, Dr. J. Habeeb, L. Victoriano.

and S. Zhandire. Their helpful discussions and stimulating companionship made these years a most rewarding and

delightful experience. To my parents and elder brother, I owe special gratitude,

for even though they were far away, their concern and moral support have always provided the meaning and purpose to my

endeavours.

I also thank the technical staff and the secretaries

of the Department of Chemistry for their valuable assistance

and cooperation. At the same tim e, my thanks are also due

to Mrs. Joyce Popovich of the Faculty of Law for her patience

in typing this manuscript, the quality of~which~''speaks for i t s e l f .

i i i

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Finally, I shall take this opportunity to acknowledge the invaluable financial assistance provided by the National

Research Council of Canada and the University of Windsor, ■

without which it would have been difficult indeed to coup let e

t h i s w o rk .

Windsor, Ontario September, 1979

i v

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. . ABSTRACT

A review of the electrochemical synthesis in non-aqueous

•media involving main group metals and the transition metals

as the^acrificial electrode (cathode or anode) 'is presented.

Electrochemical insertion reactions were successfully

investigated and a range of neutral and anionic organo-

m etallic compounds synthesised 'by sacrificial dissolution

‘of anode metal in non-aqueous media. The products have been identified using physicochemical means. The'possible reaction

mechanism of these direct electrochemical reactions by measuring the current efficiency of .the system, and the

advantages of electrochemical preparative methods, are also

d is c u s s e d .

X. Synthesis of neutral organocadmiurn halides . .

The electrochemical oxidation of cadmium metal in the

presence of alkyl or aryl halides leads to the formation of

unstable RCdX (where R = Me, Et, n-Bu, CF^ > C^F^'and X - Br, Cl, I). These compounds are•stabilized by either

mono or bidentate neutral ligands; these adducts are formed ■

t in good yield.

II. Synthesis of anionic organocadmlum dihalxdes

Direct electrochemical synthesis gave rise to a series

of previously unknown salts or anions, such as RCcD^ (where R = Me, Et, n-Bu, t-Bu, CF^, CgF^; X = Cl, Br, I ) , by the

oxidation of cadmium (anode) in a chemical cell in which the

v *

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. solution phase was a methanol/benzene mixture, containing the appropriate tetra-n-propylammonium. halide and alkyl *

halide. Gram quantities of product were obtained at or near * room temperature.- In general the .compounds hydrolysed and_

decomposed’on exposure' to. air or m oisture.

I I I . ' Synthesis of LCdBr, LpCd, LCdMe and LCdPh (L = o-bromo-N,N-dimethylbenzylamine)

Oxidative insertion of cadmium metal in between carbon- bromine bond of o-bromo-N ,N-dime thy lbenzylamine was carried

out by the electrolysis of o-bromo-N ,N-dimethylbenzylamine

in an acetonitrile/benzene mixture and a few mg of Et^NClO^. The sacrificial anode was cadmium, the cathode was tungsten,

and all reactions were carried out in a dry nitrogen at

mosphere. The insertion product LCdBr.so obtained was

successfully utilized to prepare LCdMe and LCdPh quanti- * tatively by reaction with MeLi and PhLi. respectively. .Four coordinate bis 2-[Cdime thy lamino)’methyl] phenyl

' cadmium has~heen prepared by the reaction, of 2:1 molar ratio

o f 2[Cdimethylamino)methyl phenyl]lithium with anhydrous

CdCl2 in diethyl ether. All the compounds were isolated and charadt^t^zed using I.P., NMR, and mass spectrophoto

m e try . —N \ IV. Synthesis of neutral and anionic organozinc halides

The electrochemical oxidation of zinc in the presence of an organic solution containing alkyl or aryl halide RX

an d 2 , 2 ’-bipyridine yields in gram quantity a series of

• / Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2,2'-bipyridine adducts of the organozinc halides RZuX,

w here R = Me; E t, CF3 , C 2E3> CgH5 , , C6H5CH2 ; X = C l, B r, I.

Similar electrochemical oxidation of zinc metal in the

presence of a solution mixture containing tetra-n-propylam- moniurn halide and RX leads to the direct synthesis of tetra-

n-propylammonium salts of RZnX^ anions. .

V. Synthesis of some Ph3SnMCl adducts. (where M = Zn, Cd, Hg)

Metal-metal bonded products were synthesised electro-

chemically and isolated in good yield as the 2 ,2 ’ - b i p y r id i n e

or N,N,N' ,-N* -tetram ethylethylenediam ine adducts by the electrochemical oxidation- of zinc, cadmium or mercury in a

nonaqueous solution of triphenyltin chloride containing a few mg of Et,NC10, as a supporting electrolyte. The proton

n.m.r. ^ and the far * infrared spectra C50Q-50cm -1 ) of these

compounds are also reported.

VI. Synthesis of dialkylamides of zinc, cadmium and mercury The electrochemical oxidation of zinc, cadmium or mercury

in a non-aqueous solution ^containing secondary ’ amine (eg- »

■isopropylamine , 2 , 2,6 , 6-tetram ethylpiperidine and 1 , 1 , 1 ,3,3,3- XI hexamethyldisilazane) yields metal-nitrogen bonded M (dia'l- kylamide) species. Except for bis(disilylam ino)zinc and

bis(disilylamino)cadmium, the 2 , 2 '-bipyridine adducts of the

di alkyl amide of were also obtained. The results are

discussed, and compared with other preparative routes to II metal di alkyl ami de s . . v i i

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

^ Page Acknowledgement ...... ' ...... i i i A b s tr a c t ...... v

List of Tables ...... ■...... v i i i List of Abbreviations ...... x i

CHAPTER I Electrochemical Synthesis of Organo- m etallic Compounds in Nonaqueous Solvents 1 1.1 Introduction ...... 1

1.2 Advantages in Direct Electrochemical R e a c tio n s ...... ‘...... 3 r.3 Literature Survey of Preparative Electro chemistry of Organometallic Compounds of Main Group Elements ...... 4

(a) Group II and III as a Sacrificial Anode 4

(b) Group II and III as a Sacrificial .C ath o d e ...... '...... 8

1.4 The Electrochemical Synthesis of Transition M etal Compounds ...... 17

1.5 Objective of Work ...... 26

CHAPTER II Experimentation ...... 27

2.1 Introduction ' 27

2.2 Electrochemical Cell ...... 27

C e ll "A” ...... • 27

C e ll "B" ...... : ...... 28 2.3 Power Supply ...... 28

2.4 Purification and Drying of Solvents and R eag en ts ...... 28

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. \

2.5 Physical Measurements ...... ^...... 31

Conductivity Measurement ...... 32 2.6 Analytical Techniques ...... 32

Metal Analysis ...... 32

Halogen Analysis ...... 32

Gas Analysis ...... 33 2 .7 M easurem ent' o f C u rre n t E f f i c ie n c y ...... 33

CHAPTER III The Direct Electrochemical Synthesis of * Neutral Organocadmium Halides ...... 36

3.1 Introduction ...... 36 3.2 Synthesis of Organocadmium Compounds . r: 37

Literature Survey ...... 37 3.3 Organocadmium Compounds as Alkylating R eag e n ts ...... 41

% 3.4 Co-ordination Complex with Ligands not Containing Acidic Hydrogen ...... 42 3.5 Some Chemical Properties of Organocadmium Compounds ...... 43

3 . 6- Application in Organic Synthesis ...... 43 3.7 Electrochemical Preparation of Neutral Organocadmium Halides ...... 45

G e n e ra l ...... 46 3.8 Isolation of Product ...... -...... 47

3.9 Preparative Chemistry ...... 47

(a) Electrochemical Preparation of MeCdl ... 47 if (b) Electrochemical Preparation of E'tCdl ... 48

(c) Electrochemical Preparation of DMSO Adduct of MeCdl ...... ; 48 (d) Electrochemical Preparation of 2,2’- bipyridine Adduct of MeCdl ...... 49

< •

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ce) Electrochemical Preparation of. 2,2*- bipyridine Adduct of EtCdBr ...... -... 49

Cf) Electrochemical Preparation of 2 , 2 *- bipyridine Adduct of h-BuCdBr ...... 50

Cg) Electrochemical Preparation of 2,2*- bipyridine Adduct of CgF^CdBr ...... 50

Ch) Electrochemical Preparation of 2,2’- bipyridine Adduct.of CF^Cdl ...... 51

Ci) Electrochemical Preparation of 1,4-Dioxane Adduct of EtCdBr ...... 51

Cj) . Electrochem ical'Preparation of 1,4-Dioxane - Adduct of EtCdl ...... ;..... 52 (k) Electrochemical .Preparation of 1,4-Dioxane t Adduct of PhCdBr ...... •...... 52

Cl) Electrochemical Preparation of 1,10-Ph.enan- throline Adduct.of EtCdBr ...... 52 3.10 Results and “Discussion ...... -.. 58

V ibrational Spectroscopy-n...... s?-...... 60 Reaction Mechanism 60

3.11 Conclusion 62

CHAPTER IV the D irect Electrochemical* Synthesis •Qf , ■ . Anionic OrganodihahocadmateCH) Comp_lexes ■ 63

4.1.“ Introduction \ ...... 63 , ■ 4.2 Experimental ...... 64 * General •...... 64 . ' 4.3 Isolation of Product ...... ; ...... '. 65 * * 4.4 Preparative Chemistry ...... e. ? ...... 65 © Ca) Electrochemical Preparation of iMeCdBr^]- Anion ...... 65 - . « ’ Cb) Electrochemical Preparation of [E tC dB r2I A nion ...... 65. Cc) Electrochemical Preparation of [t-BuCdB^J Anion ...... *...... _...... 66

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (d) Electrochemical Preparation of [MeCd^J- A nion ...... f*...... 66 (e) Electrochemical Preparation of lEtCd^J Anion ...... 66 '(f) Electrochemical Preparation o£- [n^-BuCd^] Anion ...... 67 (g) Electrochemical Preparation of [CF3CdI2]" Anion ...... : ...... 67 (h) Electrochemical Preparation of [P h C d C ^ l* A n i o n ...... • ...... V . .. 68 (i) Electrochemical Preparation of [PhCdBr 2] Anion .j, ...... 68 (j) Electrochemical Preparation of [CgF^CdBr]- A nion ...... 68

4.5 Results and Discussion ...... 71

Molar Conductivity ...... 73

Gas Analysis ...... 74 Reaction Mechanism ...... 74

CHAPTER V Electrochem ical and Chemical. Synthesis of Arylcadmium Compounds Containing (N^N-dimethylaminoJmethyl Group' at : the Aryl Nucleus ...... 76 5 ♦ 1 I n t r o d u c t io n ...... 76

5 . 2 E x p e rim e n ta l ...... 79

5.3 Synthesis of o-bromo-(N ,N- dime thy lbenzyl- am ine) ...... 80 5.4 Lithiation of N,N-dimethyIbenzylamine 80

5.5 Analytical Techniques ...... 81

5 .6 Preparative Chemistry ...... 81 (a) Electrochemical Preparation of LCdBr .. 81

(b) Preparation of LCdMe and LCdPh ...... 82

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5.7 Results and Discussion • 90

NMR S tu d ie s .*■...... 1...... 93 r . . V ibrational Analyst* of LCdBr, LCdPh, LCdHe, and - I ^ C d ...... 93

Mass Spectral Studies on\ LCdPh ...... 93 5.8 General Conclusion — i. .Jt...... ' 94

CHAPTER VI The Direct Electrochemical Synthesis of Neutral and Anionic Organozinc Halides . 95'

6 .1 Introduction ...... '95.

6 .'2 Synthesis of Organozinc Compounds ...... 96

Literature Survey ...... '...... ,...... 96

Preparation of Organozinc Compounds ...... 97

6.3 Co-ordination Complexes with Ligands not Containing Acidic Hydrogen ...... 99

6.4 Some Chemical Properties of Organozinc Compounds ...... *. 100

V6.5 . Application in Organic Synthesis . \ ...... 101.

(a) Reformatsky Reaction ...... 101. (b) Simmons^^ith. Seactibn ...... 103.

6.6 Electrochemical Preparation of Neutral and ionic Organozinc Halides ...... , ...... ' 104

• i 6 .7 Experimental ...... 105

_ G e n e ra l ...... 105- ~ V f' Isolation of Product ...... 105

Analysis . 106

6.8 Preparative Chemistry of Neutral Organozinc * H a lid e s ...... 106

(a) Electrochemical Preparation of MeZnI.bipy 106

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. \ , •

.(b) Electrochemical Preparation of-EtZnI.hipy 1Q£

/^Cc) Electrochemical Preparation of EtZnBr.bipy 107

(d) Electrochemical Preparation of CF^Znl.bipy ...... 107

(e) Electrochemical Preparation of HC2=CH-Za-Br.bipy ...‘if ...... 108

(f) Electrochemical Preparation of PhZnCl .bipy 108

(g) Electrochemical Preparation of PhZnX.bipy -(where X = Br, I) ...... '. 108

(h) Electrochemical Preparation of CeFcZnBr. b ip y ...... 109

(i) Electrochemical Preparation of PhCH 2ZnX. ■ b ip y (■where X = C l, ^Br) ...... 109

(j) Electrochemical Preparation of PhCH^Znl. b ip y ...... 109

6 .9 Preparative Chemistry of Anionic Organozinc H a lid e s ...... 110 (a) Electrochemical Preparation of [MeZnl^] A n i o n ...... 110

(b) Electrochemical Preparation of [EtZnI2] A nion ...... '...... 110 (c) Electrochemical Preparation of [CF-Znl,] A nion ...... I l l (d) Electrochemical Preparation of [EtZnBr~] A n i o n ...... ‘...... I l l (e) Electrochemical Preparation of [PhZnXr,] Anion . 112

(f) Electrochemical Preparation of [PhZnCl«] A n i o n ...... : ...... 112

6 .10" Results and Discussion ...... 121' Molar Conductivity i . .. 122

Gas Analysis ...... : ...... 122

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ' yibra^on -'Si^tn^scp"^-': . r.

^H NMR Spectroscopy ...... i ...... Reaction Mechanism ......

6.11 Conclusion ...... -... CHAPTER VII Electrochemical Synthesis of Some Ph^SnMCl Adducts (M = Zn, Cd, Hg)

7.1 .Introductipn ...... '......

' 7.2 Experimental ...... 7. 3 Isolation of Pro‘duct ...... f. . . . .

7.4 Analysis ......

7.5 . Preparative Chemistry ...... (a) Electrochemical Preparation of Ph 3SnZnClNTMED ...... • (b) Electrochemical Preparation of Ph 3SnCdCl.TMED ......

Cc) E le c tr o c h e m ic a l P r e n a iia tio n o f Ph 3SnCdCl.bfpy * ...... (d) Electrochemical Preparation oj Ph *3 SnHg C1. TME D ...... 7.6 Results and Discussion ......

Vibrational Analysis ...... NMR Spectroscopy ......

> Reaction Mechanism ......

7.7 Conclusion ...... CHAPTER-VIII The Direct Electrochemical Synthesis 8 .1 of Group IIB Metal-Dialkylamides ....

8 .2 The Preparation of Metal-Dialkylamides ..

8 .3 Present Work ...... 8.4 Literature Survey ......

—U Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ca) Reaction of H-H with Secondary Amine ... 140 Cb) Reaction of Metal-AlkyIs with Secondary Amine ...... 141 Cc) Reaction of Metal Chloride with an ' Alkali Metal (Li or Na) Dialkylamide . .. 141

Cd) By. Direct Reaction of Metal with Secondary Amine ...... - 144

8 .5 Experimental ...... 144 Solution Composition ...... 145

Isolation of Product ...... 145

8.6 Preparative Chemistry ...... ^""146 (a) Electrochemical Preparation of 2,2’- < bipyridine Adduct of BisClsopropylamino)- z i n c ...... 146 Cb) Electrochemical Preparation of 2,2'- bipyridine Adduct of BisClsopropylamino)- ca^gri. u r n ...... 146

Cc) Electrochemical Preparation of 2,2'- bipyridine Adduct of BisC2,2,6,6- Tetramethylpiperidino) zinc ...... 146

Cd) Electrochemical Preparation of 2,2'- bipyridine Adduct .-of Bis C2 ,2,6,6 - TetramethyIpiperiiiino) cadmium ...... 147

(_e) Electrochemical Preparation of 2,2’- bipyridine Adduct of BisC2,2,6,6- Tetrame thy Ip iperidino) mercury ...... '•147

(f) Electrotsijemical Preparation of BisChexa- me thy 1 disily lamino) zinc ■ 148 Cg) Electrochemical Preparation of BisChexa- m e th y ld is ily la m in o ) c a d m iu m ...... 148

8 .7 Results and Discussion ...... 154

Vibrational Spectroscopy ...... 155 NMR Spectroscopy ...... 156

8 . 8 Conclusion ’ ...... 156

k with permission of the copyright owner. Further reproduction prohibited without permission. L ist of Organometallic Compounds Prepared

References ......

Vita Auctoris ...... ' ......

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES ' i ' TABLE TITLE PAGE > — \ ------1.1 Electrolysis of Gipignard Reagents ...... 4

1.2 Electrolysis o^Alumini'um and .Boron Com plexes ...... 6 1.3 Formation of Metal Alkyls from Group III Sacrificial Cathode ...... 12

1.4 • Formation of Metal Alkyls from Group IV Sacrificial Cathode ...... 16

3.1 Reaction Condition for Direct Electro chemical, Synthesis of Organocadmium ^ H a lid e s ...... 53 3.2 Analytical Results for Organocadmium Halides and their Adducts .. . . 55

. 3.3 Gas Analysis for Some Organocadmium Halides and their Adducts •.... 56

3.4 (Cd-C) Modes in the Infrared Spectra op RCdX A ddition Compounds ...... 56 3.5 ' Measurement of Current Efficiency ...... 57

4.1 Reaction Conditions for Direct Electro- Chemical Synthesis of (C.,H7) , N[RCdXj3 S a l t s ...... - 69 4.2 Analytical Results for Salts of Organodihalocadmate CII) Anions ...... 70 4.3- Conductivity Measurement ...... 70

4.4 Gas Analysis for Some Organodihalo- • cadmateCH) Complex ...... ' . i ...... 71

5.1 Analytical Results of LCdBr, LCdMe, * LCdPh, and I^Cd Compounds ...... 85

•« 5..2 Infrared Spectra of LCdBr, LCdMe, LCdPh and L7Cd Compounds ...... 86 5.3 * H NMR Spectra of N.N-Dimethyl-benzylamine and L 2C d ...... 87

v i i i t Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 .4 Mass S pectrum o f LCdCgH^ 88

6.1 Reaction Condition for the Direct Electrochemical Synthesis of Neutral Organozinc Halides : ...... 113 6.2 ' Reaction Condition for the Direct Electrochemical Synthesis of OrganodihologenozincateCH) Anion ..... 115 i 6.3 Analytical Results for 2,2'-bipyridine Adduct of Organozinc Halides ...... 116 % 6.4 Analytical Results fdt Tetra-n-propylammonium Salts of Organodihalogenozincate CII) Anion ...... 117 6.5 Conductivity Measurement for Some » Anionic Organozinc Halides ...... 118 6.6’ Analysis of Organozinc Halides by Acid 'Decomposition ...... 118 i. 6.7 Infrared Spectra of 2 ,2*-bipyridine Adduct of RZnX ...... 119

6.8 Measurement of Current Efficiency for Some Neutral and Anionic Organozinc H a lid e ...... 120

7.1 Experimental Condition for the Electro chemical Synthesis of Ph^SnMCl.L Compounds (where M = Zn, Cd, Hg) .T ...... 132

7.2 Analytical Results-for Ph-SnMCl.L Compounds ...... 133 7.3 ' * Far Infrared Spectra of Ph,SnCMl.TMED Compounds ...... 133

7.4 ^H MNR Spectra ...... 134 t 8.1 ’ Reaction Condition for Direct Electro chemical Synthesis of M etal^ Amides .. 149

8.2 Metal Analysis of Metal^ Amides ...... 151

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8.3 Infrared Spectra for some 2,2’-bipyridine Adduct of"Metal 11 Alkylamide ......

8.4 Infrared Spectra of Bis(hexamethyldisi- l y lam in o ) •where M = Zn, C d ...... 8.5 NMR Chemical Shifts of Mr i £CNCSiMe3) 2] 2 '......

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF ABBREVIATIONS

a c a c acetylacetone

a p p ro x approximate(ly) ' a s yin asymmetric

b .p boiling point

b ip y 2 , 2 ' -bipyridine n -B u n - b u t y l t-B u tertiary butyl

CDT 1,5,9-cyclodecatriene

COD 1 , 5-cyclooctadiene

COT cycloocatatetraene

Cp cyclopentadienyl

Cm' 1 Wave number

d ip h o s 1 , 2-bisCiipbenylphosphino)ethane dime thy If ormami de V DMSO dime thy Is u lf oxi de

e.g. for example

en ethylenediamine . v Et e th y l

F Faraday constant

glym e 1,2 dimethoxyethane HMPA hexamethylphosphorictrianh.de

/~ I c u r r e n t I.R I n f r a r e d mA milliAmpere

Me m eth y l

x r

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. _3 m.m * ail lime ter CIO meters)

m.'p. melting point

M ; Molar Conductivity

m/e mass to charge ratio NMR proton nuclear magnetic resonance

NTP normal temperature and pressure

QAc acetate Fh p h e n y l n-Pr n-propyl

i-P'r isopropyl Pent n-pentyl

Pet petroleum ppm parts per million

ph en 1 , 10-phenanthroline

S. organic moiety 70RA , percentage relative abundance *■ STP s ta n d a r d te m p e ra tu re an d p r e s s u r e

t tim e THE tetrahydrofuran

TMS • tetramethylsilane TMED NjN.N'’ ,N‘-tetramethyle^fxyldnediamine

TBAP tetra-n-butylammonium perchlorate

V v o l t s w

x i i

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. t CHAPTER I

ELECTROCHEMICAL SYNTHESIS OP ORGANOMETALLIC COMPOUNDS' IN NON AQUEOUS SOLVENTS' " •

1.1 Introduction

Electrochemical techniques have been extensively used

for many years for both the synthesis and analysis of •

aqueous inorganic and organic systems. Experiments in ✓ aqueous media impose serious restrictions which are often ' 4 impossible to overcome, especially with organometallic

compounds. Although in the last ten years significant contributions.have been made in electrochemical synthesis

in non-aqueous solvents in inorganic and organometallic

chemistry, many systems have yet to be investigated.

The electrochemical technique represents the simplest

and most direct method of carrying out oxidation or

reduction reactions, since the removal or addition of an electron can be achieved without any of the complications

involved in the use of redox reagents. An electrochemical system consists essentially of a

voltage source, electrolyte solution, a cathode and an

anode either of which may be inert or active. The latter

case involves a sacrificial electrode, while in the former

case the electrodes act as the source of, or sink for, electrons in the reduction or oxidation of solute species

in the non-aqueous solution phase.

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A number of solvents and supporting electrolytes have been used in non-aqueous studies, and each combination has

its own characteristics. The choice of a particular combin

ation of solvent and electrolyte is primarily empirical, and can be made only after consideration of such factors as

chemical inertness towards starting and end products, greater resistance to reduction or oxidation than.the

starting reagents, ease of separation of• products after

electrolysis, and the electrical conductivity of the solu tion. The properties of the desired product must also be ■v • considered. Thus, for example protic solvents or H-acidic '

electrolytes cannot be used for the synthesis of organo- m etallic compounds. The solvents most often'used have been

acetonitrile, methanol, dimethylformamide, propylene carbon

ate,- dimethyIsulfoxide, tetrahydrofuran, hexamethyl-

phosphoramide, pyridine, dichloromethane and 1 , 2-dimethoxy-

ethane (glyme) . The supporting electrolytes which offer the widest

potential range are the tetraalkylammonium perthlorate

and halides. Lithium salts may also be used as supporting ,

electrolyte for reduction unless mercury electrodes are

used when amalgum formation is a problem.

In the synthesis of inorganic and organo-metallic

compounds relatively few synthetic^methods use the metal as

the starting m aterial. ' " Grignard reagent, and Frankland's synthesis of organo-

zinc halides are salutory examples of the ability of metals

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to cleave strong chemical bonds. In recent years, the use

of metals has increased the development of vapour phase

reactions,' in which vaporization of metal at elevated

temperature is followed by low temperature reaction in * condensed phase, or by high temperature reaction in vapour

phase. In general sophisticated apparatus including a high vacuum system is required in such vapour phase-reactions

. \ 1.2 Advantages in direct electrochemical reactions

- In electrochemical reactions, one normally works at

or near room temperature. - The compounds are produced by the oxidation or reduc

tion of metal electrode, using relatively unsophisti

cated apparatus. - The method involved is a direct one in which the

product(s) precipitate down during the electrolysis

in many cases. - Metals are used as starting m aterials, and axe

generally stable, easily stored.and available in high c. p u rx ty -. - The chemical yield is generally high in terms of

metal consumed during electrolysis. - It has been observed that it is possible to- p e rfo rm

• electrochemically a reaction which is not feasible

by direct reaction of components because of kinetic restraints resulting from a high activation energy.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1.3 Literature Survey on Preparative Electrochemistry of Organometallic Compounds of Main Group Elements

(a) Group II and III as a sacrificial anode

In 1930 French and Drane^ had shown that zinc and cadmium could .be used as sacrificial anodes in the

electrolysis of isopentylmagnesium in the ether solution.

The electrolysis products were not isolated, but were

assumed to be alkyl of the anode m aterial dissolved.

Grignard reagents were then found to be generally electro- active at both sacrificial and inert electrodes in non- 2 aqueous solvents. Later, it was suggested by Evans and Lee

that dissolution of an aluminum anod» in e thy lmagnes ium

bromide yields triethylaluminum. Some related examples of

organometallic compounds obtained from the electrolysis of

Grignard reagents are given in table tl-1) • Table 1.1 Electrolysis of Grignard Reagents #

Anode Electrolyte Solvent Product Reference

Z in c c 6H i3MgBr H e x y le th e r CC6H i3) 2Zn 3 of diethylene g ly c o l

B oron EtMgCl Diethyl ether B (E t) 3 3

A lum inium EtM gl . A l(E t) 3 2

The anodic oxidation of metal complexes containing carbonions is a widely used method for the preparation.of organo- 4 5 ^ m etallic compounds. ' Solutions of alkyl compounds of the

alkali metals in dialkylzinc or trialkylaluminium, which

are relatively good conductors, produce alkyl radicals .on

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anodic oxidation. These "radicals dissolve the metal anode with the form of its alkyl metal compound. With metals

which do not form stable alkyl compopr£ds, disproportionation or dimerization products of the alkyl radicals are formed.^

e g . ,

-> M + CC2H5J - e

y ? -c 2n5

C4H10 o.

+ c 2h 6 + c2h 4 .

M1 - Na, L i, K, 1/3A1~ % Mg.

M2 = % Zn, % Cd, % Hg, 1/3 Al, 1/3 Ga,**l/3 In. ~

Ziegler has reported that^ when a complex of the type NaAlEt,

has electrolysed on an aluminium anode, AlEt^ was produced on the anode and sodium deposited on the cathode. The organo

boron complexes of the type M.Br,, on the other hand, are

of interest in that they are ^^fe compounds with which to" work, and a,number of alkyl metal compounds can be prepared g by electrolysis in ether. Some examples - of such syntheses are given in Table 1.2.

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Table 1.2 Electrolysis of Aluminium and Boron Complexes

Anode Electrolyte Product R e fe re n c e

Z in c N a F .2 A l£ t3 Z nE t 2

Magnesium .N aA lEt^ MgEt2 9

Cadmium NaAlEt4 + K CdEt2 10

M drcury NaF. 2 A lE t3 H gEt2 11 .

A lum inium N aA lEt4 A lE t3 9

In d iu m N aF .2 A lE t3 I n E t j 2

M ercury NaBEt4 HgE£2 8

V The electrolysis of 3-iodo propionitrile in 0.5MH 2S04 o r

aqueous sodium sulphate with mercury as an anode results 12 in the formation of the corresponding organometallic compound.

E lectro chem ically th is b is ( di chi or o aluminium) me thane

has been synthesised by electrolysing a dichloromethane

•solution containing aluminium triiodide or trichloride

and Et^NCl as a supporting electrolyte, the aluminCum 13 14 • • • metal acting as a sacrificial anode'. ’ The. mechanism

proposed involves the following sequence.

C athode

CH2C12 + e " ------> C l" + C1CH2

CH2C12 + 2e~ > 2C l” + CH2 : \ The methylene radical so generated might also undergo -the

♦ ’ following reaction:

2CH2 : A ------> CH2= CH2

CH2 , : CK2 -r CH2-CH2 > ch 2 _ ------CK2

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:CH2 + CH2C12 ------>--Cl-CU 2-CH2- C l. Anode

Cl + A l ------> [A1 - Cl] - -e~

[A1C1] + % CH2C12 ------% C12A1-CH2-A1C12 .

Chadwick and K insella have reported a very simple prepara

tion of methyl aluminium diiodide by electrolyzing a solution of aluminium triiodide in idodomethane using an

aluminium electrode.^ Tuck et a l.^ have determined recently an electrochemical route for the synthesis of

neutral and anionic organoindium compounds. Neutral

compounds of the type R^nX or RXn^ are obtained as adducts

w ith 2 , 2 ’ -bipyridine by . the electrolysis of alkyl or aryl h a li d e s i n CH^CN/CeH.- m ix tu re ^ i t h a s m a ll am ount o f J O O Et^NBr as a supporting electrolyte.

I?1^ C 5H5/CH3CN + bipy^ RInX2‘ bipy where R '= C 2H3 , > ^ 6^5 ’ ^ 6^5 ^ 2 ' X = C l, B r, I . I n ^ ^ CR31______^ (CH3) 2I n I bip-y

The anionic complexes of the type [R^N][RInX 3J , [R^N ]-

[R2InX2] were synthesised under essentially the same

conditions as used for tl^~neutral compounds, except that

in this case excess R^NX was added to the electrolysis

solution; indium metal again acted as a sacrificial anode. Inco rc + (C2H5)4NX ^ C(C2H5)J][RInX3] • c 6h 6 /c h 3cn

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R = 'CH3 , C2H5> C6H5 CH2 .

X = C l, B r, I . * CH X + I n c o 3 Cc 4H9) 4Si: ^ [ ( c 4H9) 4N][(CH 3) 2I n I 2 ] C6H5/CH3CN 'A

(b) Group II and III as a Sacrificial Cathode

Electrochemical synthesis of organometallic compounds

can often be carried out at the cathode, starting from a

metal and organic substance in non-aqueous solvents. Synthesis of 1-methyl-2 ,1 '-diphenylcyclopropane has been

' achieved by electrolyzing 1-iodo-l-methy 1- 2 ,2 ' -diphenylcyclo

propane in CH 3CN, using Et4NBr as a supporting electrolyte

and Hg as a cathode. The intermediate was identified as

dialkyl mercury, not the alkyl mercury halide.^

The mechanism involved in this reaction is given in

the following successive steps. * R -I + e " ------— ' [R * -I]~ [R*-I]_ ------■ * R* + 1 “ 'vV.-

R' + e ” ------;------> R~ Q

R- + Hg£ ------:------> .

R ‘ +

EHSnX+ * 2151^ » H g ^ +• RHgS.

R" + Et4NBr ------* EH + CH 2 = CH2 + Et3N + Br" . •

The ^eieqtro lysis of benzyl bromide or substituted benzyl bromides at' a mercury cathode in the presence of 1M LiBr '

in methanol results in the formation of mixtures of the

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diorganoniercury compounds and dimeric hydrocarbon in

varying quantities. The proportion of the products are

dependent on the substitutent R, the electrolyte and the 18 cathode potential.

R Hg-

R y i e l d %

4 -tB u 50 H 41 % 4-CH3 64

3 , 4 - d i o l 39

19 Dessy et al. have reported the synthesis of some organo-

mercury compounds by the electrochemical reduction of other

organometallic compounds .(triphenyllead chloride,

diphenyllead dichloride, and diphenyllead diacetate) at

mercury which is acting as a sacrificial cathode.

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2(C6H5) 3PbCl + 4 Hg(_) -» 3(C6H5) 2Hg + 2 Pb + HgCl2 .

( ) 2?b Cl 2 + 3 Hg ^ -> (C6H5) 2Hg + Pb + HgCl2 -

■ (C6H5) 2Pb(OAc ) 2 + 3 H g, (CgH5) 2Rg + Pb + Pb(OAc)2-

« 3 Sim ilarly the reduction of alky 1-mercury bromide to dialkylmercury using mercury as a cathode has been

reported. ^ ’* Moreover a complete«scheme of cathodic reduction of 20 21 organometallic compounds was derived by Dessy ’ and

the fate of the species produced in the electrochemical reduction of ^various organometallic compounds has been \ 2 2 ,2 3 reviewed by Lehmkuhl. [R-M* -X] “ s t a b l e

R-M-X + e~ -[R-M-X]" r

2 -> [R-M-X-X-M-R]

solvent cleavage eg., T.H.F. ------R-M • CH, -X \ / ' 0-CH2

+A ^Cr - m- a ]' +5 (solvent^) R-M- H V

d-finerizatioi^ R-M-M-R

disproportionation^ R 2M + M

fag ( c a th o d e ) v + 2M alkyl exchange

x I

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Hg (cathode) ^ (R-M) ^Hg mercuratxon

Homo ly s i s ______^ (R‘ ) + M

where R-M is a neutral or ionic organometallic group X = an organic or inorganic rest or a complex forming

lig a n d .

A = an electron affinity hydrocarbon. The electrochemical reduction of ketones and some

aldehydes in acid solution at a sacrificial mercury cathode give rise eventually to organometitLlic-mercury

compounds which separate as heavy oils from the aqueous

solution. The overall stoichiometry is

2 R ^ O + Hg + 6H+ ------> ( R ^ ) ^ Hg + 2H20 . and the mechanism involved in the cathodic reduction of

organic\pompound is

C '-OHC=0 + H + e C'-OHC=0

R2-

w here M =■ % Hg

R1 r e f 24 ■ < R1 = CH r e f 25 3 ■ R C2H5 I —1 s2 = r e f 24 - -5 . R1 r e f 25 ■ i _2 R„1 = R r e f 27 „1 _2 R , R >- -Cch2V r e f 24 „1 7 R ’ ip tt / \ CH R, 3 7 3 r e f 28 H .

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3 H (-) HC CH - H2C ) - ( Hg , - 2 1 - 2 iH 1t 2 2 30 CH2) CH 3 2 2 ) C CH - C - 2 2 .I 31 T.1I I 32 1I T 31 ... 3 ■'Apr -1 3 -

The use of Grignard reagents in the syntheses of tetraalkyllead has been a subject of considerable interest

in the last ten years because of' the 'economic importance

of this compound due to its use as a fuel anti-knocking

addititive. A direct result of this work was the commercial

electrolytic preparation of tetraalkyllead compounds invol

ving the' 'use of sacrificial lead anode and an inert cathode , f .in ether solution. The NaLco Chemical Company and other 33 ^ investigators - have investigated such variables as cell

design, the concentration ra^io of Grignard reagent and excess alkyl halide, temperature, pressure and electrolyte

flow-rate. With optimum conditions, the yield of the alkyl

, lead compound was 96%. Solvents generally used are mixtures

of ether such as THF, diethylene glycol and diethyl ether;

the stainless steel wall of the cell acts as a cathode and

lead as a sacrificial anode. v The mechanism involved in the preparation of tetra-

KMgCl - — > R" + KgCl+ ( 1)

le a d anode ( 2) 4R + Pb * R4Pb + 4e

steel cathode

4MgCl+ + 4e * 2 Mg + 2 MgCl 2 (3)

o v e r a l l 4R“ + Pb + 4 MgX+ * R4Pb + 2 Mg + 2 Mg^ (4)

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(where X = Cl, Br, I and R = alkyl or aryl radical). Braithwaite et al^6,47,48 ^ave found .that tetraalkyllead

compounds containing different alkyl groups in the same

molecule have been prepared by the electrolysis of Grignard

reagent in the presence of an excess alkyl (aryl) halide, wh'ose alkyl (aryl) group 'differed from that of the original

Grignard reagent. For exam pleEt 2pbMe 2 was obtained by

electrolyzing C 2H^Br and MeMgBr (Grignard reagent) in THF

and diethylenel glycol; dibutyl ether, using lead as a

v sacrificial anode.

The organic halide "RX" which is the source of the

carbonion in these syntheses could be successfully

replaced by such species as trialky lb oron and -tri alkyl-

aluminium compounds.^

Mixtures of trim ethylvinyllead, dime thy Iviny Head

and tetramethyllead as well as m ethyltrivinyllead have

been synthesized by the electrolysis of vinylmagnesium chloride,- trimethylboron, and trimethylaluminium in THF

-ethylene glycolmonoether. Comparable results were obtained with dime thy Imethoxyaluminium, triphenylaluminium and

triphenoxyaluminium employing a steel cathode and lead

an o d e. Tetraalkyllead compounds have also been prepared by

the electrolysis of metal alkyls such as diethylmagnesium,-.

tetraethyltin and triethylaluminium in •ether at a lead 50 anode and a platinium cathode. Hydrogen and cC olefins

were continuously passed into the cathode chamber.

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Ziegler in 1956^ using the complex NaF.ZAlEt^ as electrolyte, prepared PbEt^ electrochemically at a lead

anode. Sim ilarly an organoboron complex of the type 8 NaBMe^ on electrolysis produced PbMe^ at a lead anode. 52 Dibutyltin dibromide has been prepared by electrolyzing a system containing butyl bromide in butyl acetata^and

employing tin as a sacrificial anode. If the solvent

was- changed to methanol, butyl alcohol, ethyl acetate or

i-butyl acetate anci a mixture thereof, the only tin compound isolated was tributyltin bromide. 53 Tuck and Habeeb have reported recently a direct >

electrochemical route for the synthesis of organotin .

compounds of the type R 2SnX 2 » R^Sn, RgS^, R2$nX22L and

R2SnX 2L where R = Me, Et.nBu or Ph •

X = I, Br, or Cl L .= CILjCN, DMSO, 2 , 2 ’ - b i p y r i d i n e ,

1 , 10-phenan thro line . ♦ The organotin dihalides were prepared in high yield by the direct electrochemical reaction'of metallic tin as sacri

ficial- anode with alkyl or aryl halides in organic media

consisting of MeOH/CgHg and small amount of Et^NClO^.

The adducts were prepared by the addition of appropriate

ligand to the electrolysis solution. In the' electro

chemical synthesis of the following mechanism

was proposed.

■* ■* /

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s ' • .

Cathode— RX + e” -i =: ------> ' R* + X". ’ ' ✓ •• Anode X~ ------> X* + e - .

X’ + Sn ------* SnX . "*=• SnX + RX ------* RSnX+ X‘ XX ■ * Organotin species are unstable, and w ill react by

• ’ RSnX + RX ------» R2SnX 2 .

/ thus explaining the unique formation of the diorganotin - •

dihalides in this synthetic method. c

Group XV As a S acrificial Cathode » The formation of group XV metal alkyls by the irreversible electrochemical reduction of an o'granic halide

. "RX" at sacrificial cathode are given in Table 1.4. o

Table 1.4 Formation of Metal Alkyles from Group XV S acrificial ^Cathode

S u b s tr a te Cathode, supporting Product Reference electrolyte, solvent

E tB r Pb, Et,NBr P b (E t ) 4 • 5 4,55 propylene carbonate

E tB r Pb, various salts P b (E t ) 4 56 propylene carbonate

E tB r ’^'Pb, 'Et,NBr Pb ( E t ) 4 '5 4

various solvents » <3 E tB r Pb, LiBr, various Pb (E t) 4 - 57 s o lv e n ts

M eCl.Br.X P b , E t,N B r .*1*4 57 E t C l .I ch 3 cn *

E tB r Sn, Et^NBr, CH^CN Sn (E t) 4 57

RX Sn, various, CH-OH or SnR 4 . 5 8 ,5 9 ,6 0

<^3 cn ' *

u v V -

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1.4 The Electrochemical Synthesis of Transition Metal COnipoun5s~ Organometallic compounds of the transition metals are

of great industrial importance as active catalysts for the

•conversion of unsaturated organic compounds by hydrogenation

and in isomerization, dimerization, oligomerization, polymeri- 23 zation or hydroformylation processes. Some of these com

pounds also have the ability to complex hydrogen, nitrogen or oxygen and so bring these to an active form, which shows

a special significance in the chemical conversion of

n i tr o g e n .

Organ o tran sit ion metal compounds in-^rhich the alkyl

or aryl group is bonded to the transition metal by a sigma

bond are often unstable. The alkyl or aryl compounds often exist in equilibrium with another class of organotransition ;

metal compounds in which the C-C m ultiple bonds are joined

. to the metal through the "K electron system.

M-CH^-CH^-R " N H-M---' II R

o r

m -ch 2-ch=ch-r - — n M ^C-H

M= one or more equivalent transition metals . Manahan^

was the first chemist to apply electrochemical techniques

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II ■ i n t h e jsyn thesis of tietal-olefin complexes. Copper perchlorate was- electrolysed between copper electrodes

'"in the presence of 1,5-cyclo-octadiene, and bis-C.1,5

cyclo-octadieiie)-copper^ perchlorate was obtained. Cu+ CuCC10^)2 Cu-/olefin^ bis-Cl >5 cyclo-octadiene) copper^ perchlorate. : Modifications of this basic method hSve been used exten

sively for the synthesis of other transition metal—cyclo- .

olefins complexes. The general principle has been the cathodic reduction of mixtures of readily accessible 62 63 64 transition'm etal compounds with suitable olefins. ’ ’

Depending on the magnitudes of the electron ^affinities

of the metal compound and the ligand, either the metal

■ compound or the ligand may be electro chemically reduced.

Thus, the reaction may be obtained by combination of

one or the other pairs of part reaction, i.e ., either-

M+ + e ------^(o) + ligand ------^ . . . 1-ig] 65 W. Leuchte has reported, electrochemically it is possible with nickel anode and cyclo-octatetraene (COT)- in dime thy 1-

formamide (DMT). The weight loss of the anode corresponds

to 90% of the current and the desired' complex was identified

•- as Ni (COT) .

Ni + The dissolution of the nickel anode is less complete when,

the electrolysis is carried out in pyridine--or tetrahydro-

furan. If the organometallic complexes formed by cathodic

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reduction are more readily oxidizable than the metal itself, the reaction may be reversed by anodic oxidation of the

product. This can be prevented by using a diaphragm to

prevent the product 'formed at the ^cathode from reaching the

anode, or by using a more electropositive metal for the ■ anode which is then more readily oxidizable than the product.

3 Ni (acac) 2 + 6 CED + 2 AlCanode) ■ > 3 NiCCOD ) 2

+ Al(.acac), ,

3 Ni (acac) 2 + 3 CDT + AlCanode) — ------^ 3 NiCCDT) + A lC acac>2 and

F eC acac) 2 + 2 COT + AlCanode) ------>FeCCOT) 2

v ' + A1 Cacac) ^

E arlier attempts to prepare "ft-cyclo-octenylcobaltcyclo-

octadiene by electrochemical techniques were at first

unsuccessful becaush the compound formecPby cathodic

reduction was readily oxidized at .an aluminium' anode in the .. presence of 1,5 cyclo-octadiene. ■ This problem has easily

been overcome with ‘the- use of^'proton donating solvents,

eg. , methanol and ethanol, which stabilized the cobalt

anion by, abstraction of hydrogen from the reaction medium to form ~K -allyl cobalt derivatives. '

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Co(acac),2e~ cathode. ['Co °] ads + 2(acac)

[C o °]ad s COD +e Co Co

COD

Co-H is omerrzatxon

+ COD isomerization <

+H.+ Co

In the absence of alcohol and with separation of anode

and cathode by\ a diaphragm, cobalt complexes can also

be prepared. The necessary hydrogen is abstracted from

the reaction medium. Cyclooctatrienyl-cobalt-cyclooctatetraene can 23 be synthesised analogously.

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CoCacac ) 2 + 2 COT + 2e" ^ ( ^ c)------> l^ j)“Co.COT ' Ferrocene has been synthesised electro chemically by the

electrolysis of cyclo-pentadieny1-thallium in DMF at an iroh anode.^ {

Fe^ > O reO Sim ilarly, cyclopentadienyl compounds of alkali metals can be electrolyzed to produce the cyclopentadienyl- 66 transition metal complex.

A procedure for the preparation of transition metal- carbonyls has been described by Ercoli et a l.^

This involves electrolysis of a metal acetylacetone in a ^ pyridine , • tetra-n-butyl-ammonium bromide solution at an

aluminium anode and a graphite or steel cathode in a

carbon monoxide atmosphere.

The following compounds were obtained.

CrCCO£, *fa2 CCO)10, FeCCO)5 , NiCC0)4. Substituted cyclopentadienylmaganesetricarbonyl compounds

have been prepared by the electrolysis of a solution of a

Mn^ salt, a cyclopentadiene hydrocarbon and a transition

metal carbonyl in such solvents as DMF, diethylene glycol,

dibutyl ether, or hexane thy lphosphor amide under a pressure

of carbon monoxide. ^

Thus by applying a potential of 25-30 V across

manganese electrodes, methylcyclopentadieny 1-manganese

tricarbonyl and fluoreny 1-manganese tr i c'arbonyl were synthesised.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Electrolysis of the sodium-alkyl Caryl) cyclo-

pentadienide and excess alkyl Caryl) cyclopentadiehe in THF or pyridine/diethylene glycol, dimethyl ether at a

manganese anode and copper cathode under nitrogen atmosphere yields the manganese-alkyl- Caryl)-cyclopentadienides^

in 257. yield. Manganese methylcyclopentadienide and-

cyclopentadienyl-manganesetricarbonyl were prepared in the

same way. Tuck and Habeeb^ have reported the direct electro chemical synthesis of organometallic nickel and palladium

complexes of the type

RNiX.2L where R= CgF^, CH3> C 6H5CH2 ,

L = Et^P, diphos or pyridine. ----

The electrochemical oxidation of nickel or palladium anode

in the presence of certain organic halides and' ethanol

Cl:1 ratio) containing a few mg of Et^Br leads to the

formation of RNiX, which can be stabliized with some neutral ligand like Et^P or pyridine, sim ilar procedure

leads to the corresponding palladium compound.

Tuck et al. have also reported that the electro

chemical anodic oxidation of titanium , zirconium’ or 72 hafnium leads to the synthesis of the complexes

.2CH3CN (R = C6H5CH2 , X = C l,B r)

by carrying out the electrolysis of a solution containing

CR3CN and aryl or alkyl halide. _ -The 2 ,2 '-bipyridine

adduct are readily obtained by the addition of 2 , 2 *- b i p y r i

dine to the electrolysis solution.

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Dessy et al. have investigated the polarographic 73 behaviour of .large numbers of metal carbonyl systems and

found that stable, radical anions or dianions could be empe-

ted from bridged- bim etallic species having a metal-metal .

bond, while the radical cation may be formed^ from those not

having such, metal-metal bonding. The electrochemical

scission of metal-metal bonds may proceed by two different

pathways. ^ __

2e~ M" : + M* : '

l e H : + M* . Homodimetallic compounds accept two electrons per molecule

and form two uninegative anions, while heterodim etallie

compound may do the same, or may accept only one electron

per molecule and form a radical and an anion.

' In systems involving compounds of the type

(CO)5M-M(CO) 5 w here M = Re, Mn.

ICpMCCO) w here M : F e , Mo o r W.

CpMCCO^l.M = Fe, W, or Mo and Cp = cyclopentadienyl,

compounds of the type IMCCCO^^S £CpM(CO) 2^2^ w ere characterized.. A mercury cathode was used in these reactions.

Electrolytically generated anions w ill react with

organometallic halides in accordance with the.nucleaphilicity

of the anion and the respective strength >of the bonds which

may be formed. The following are possible general reactions.

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M-X + M

■> ' M-M' -X+L

■> M-M C^edis tribution)

■> , M-H Cabs traction)

Thus, [C C^H^FeCCO^J” ion forms the di-iron compound

w ith C C-^H^) Fe(CO) 2I , but the EMnCCO^J ion does not

react with CCO)3MhCl to form Mn 2 CCO)^g b e c a u s e o f i t s low

nucleophilicity.

The addition of triphenyltin chloride to the [MnCCO)

ion yielded a mixture of dimanganese, ditin, and manganese- ■•r' 73 tin compounds in approximately equal proportions.

Redistribution reactions may also be used for synthetic f purposes. An electrolytically generated anion w ill react with a polym etallic compound to form a new polym etallic

compound and the least nucleaphilic or most stable anion.

(c h 3ock 2) 2 V M:~ + M-M ------> M-M + Mr" TBAP This reaction has been used to prepare CCgH^) ^SnFeCCO) 2~

C C5H5) from ICCgH-^Sn]- , and E(*-C 5H5)FeCCO)232 ,

ECC5H5)Fe.CCO) 2] 2 and E>biCCO)5]" from EC*-C 5H5)FeCCO)2ET

and Mn 2 CC0) .

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74 Tuck and Zhandire, recently have been able to prepare, polym etallic compounds by the electrochemical

procedure. The salient features of these reactions are

illustrated below.

M i- CCO), n ^ ------> MDfaCCO),-32.bipy . • L J CgHg + CH3OH * b ip y 3 c ^

' M = Zn, Cd.

Co- Cco)- C d ,.. —± ------2------:------CdlCoCCO) . ^ 1 ■ CH-OH + C,H, + Et/NCIO, 3 6 o < 4 4

Co 2 Cco) o y t M,,. — ------2------* MlCoCCO) J o L 11. w CH3OH. + CgHg + E t 4NC104 *

M = Zn, Cd. - = 2,2' bipyridine, TMED.

Co-CCO)- I n , , . 2------^ In£CoCCO) bipy CgHg + CH3OH + Et4^Cl04 J

+ 2 , 2 ’- b ip y

Mn-o CC°) -i n I n , ------±2------:------=------> Bipy In DfoCCO)-]-. CgHg + CE3OH + E t4NCl04 3 J

4* 2 ,2 '-bipyridine

Organometallic catalysts have been generated electro-

chemically in situ for the polymerization of butadiene

to polybutadiene,^ie .' ethylene to polyethylene, and alkynes 76 to polyalkynes.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1.5 Objective of Work In principle, the electrochemical technique is the

easiest and most direct method of carrying out oxidation or reduction reactions, for the reason that the removal or

addition of electron can be achieved without the attendant

complications otherwise involved in the addition of a

redox reagent. Despite this inherent sim plicity and the amount of

information available from polapographic studies and other

physical measurements, .chemists have not availed themselves

of the advantages of electrochemical methods to the extent

which might have been expected. * Some workers have concentrated particularly on the.

determination of fundamental parameters like E°, the number

of electrons involved in oxidation *or reduction, and current/voltage relationships. The present investigation

does not depend upon these fundamental variables, but

rather sets .out to carry oxidative addition and insertion

reactions by -the general application of the anodic oxidation

of the metals zinc, cadmium or mercury in the presence of

alkyl (aryl) halides in npn-aqueous solution, -which leads

to the formation of species of the type RMX.

It is conceivable that this method may be of general

application for a wide variety of main group and transition

metals. Reaction mechanisms involved in the electrochemical synthesis have been proposed from the measurement of current

efficiency. &

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2.1- Intro duct ion Due to. the extreme reactivity of organometallic

compounds and their adducts towards moisture and air, ■

extreme preacua£ions were taken to synthesis and isolate the compounds described later, under dry nitrogen, or in

a nitrogen filled dry-=£ox, (.type HE 43-2 DRI-LAB, supplied by Vacuum Atmosphere Company, Hawthorne, C alifornia).

V- 2.2 Electrochemical Cell Direct electrochemical syntheses have been carried out

in two differently.designed cells.

.C ell ”A” (Fig. 2.1). A tall-form 200cm^ pvrex beaker 3 containing approximately 100cm of solution phase, served

as the reaction vessel. The cathode was a platinum wire

approximately 1mm diameter. It was sometimes essential

to spiral the lower end of wire to increase the available

electrode surface area. The form of the anode depended

upon the properties of the metal concerned. In most cases

the metal (2-4g) was hammered flat and suspended on a platinum wire. Both electrodes were supported by a rubber

bung fitted tightly into the neck of the vessel. This rubber,

bung was also provided with glass inlet and outlet- tubes r> which served to maintain a chemically inert atmosphere of nitrogen in the electrochemical cell. The outlet in turn

was connected to a non-return glass t>nbbler.

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Cell "B" 0?ig* 2.2) was a two arm cell ( H - f o r m ) , eac h arm being approximately 15cm long and 2cm in diameter with

the arms joined by a small tube (. 5cm long and 1 . 2cm in diameter). One arm served for degassing the solution in

vacuo by a series of evacuation/freezing cycles to remove

dissolved oxygen. The degassed solution was flushed with

dry nitrogen and then decanted into the second arm which

actually served as the electrochemical cell, in which the

anode was suspended on a platinum wire entering through a teflon plug; a tungsten wire ring fused into the side of

the vessel below the anode served as the cathode.

2.3 Power Supply .

The power supply used was a Coutant LQ 50/50, eatable

of supplying' 'maximum 50 V D.C. and 500 mA.

2.4 Purification and Drying' of Solvents and Reagents

Due to the extreme reactivity of organometallic compounds towards water all'solvents were rigorously

purified, handled and stored under nitrogen.^

A cetonitrile

Acetonitrile "A.C.S." grade was dried by first treating 3 acetonitrile (500cm.) with phosphorus pentoxide then

refluxed with finely divided calcium hydride C3-4g) for

5h and finally distilled at boiling point (81.6°C) under

nitrogen. A cetonitrile was stored over Linde 4A molecular

s i e v e s . .

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A ceto n e 3 Analytical grade acetone (500cm ) was stirred with

anhydrous MgSO^ (5g). artd'then distilled at the boiling

point (56.2°C). The dist^Lled acetone was stored over

Linde molecular^sieves.

B enzene f \ 3 Analytical grade benzene (500cm ) was dried by

refluxing it with sodium naphthaquinone (ca 5g) for a few hours and then distilled under nitrogen atmosphere at the

boiling point (80.1°C). D istilled benzene was stored over

strips of sodium.

Diethyl ether 3 Anhydrous diethyl ether (500cm ) was stirred for 12

hours with CaC^ ( 5g) , filtered, re fluxed over sodium wire

for 3h and distilled at 34.6°C. It was stored over

calcium hydride in-a dark and cool place.

1.4-dioxane i I ■ — - — 1.4-dioxane was purified by refluxing over sodium,

until the surface of the metal was not further oxidized. The solvent was then distilled at boiling point (101.3°)

and stored over calcium hydride.

Dimethyl sulphoxide

It was dried over Linde type 4A molecular sieve.

( ____ Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Hexane D istilled from sodium under nitrogen at its boiling

point (63°C} * «

M eth an o l ■ * Anhydrous methanol was obtained from absolute methanol 'VV by ,refluxing it with calcium hydride for 2h. The solvent

was then distilled off at boiling point (d4.5°C) in a dry

nitrogen- atmosphere .and collected over Linde type 4A

molecular sieves.

n - P e n ta n e Dried over anhydrous, magnesium sulphate or sodium * * . *• ' sulphate and then P 205 ^'distilled at the boiling point

C36.I°C) and stored over sodium strips.

Petroleum Ether .

Petroleum ether was stirred with CaCl 2 o r Na 2S0^ f o r

12 hours , filtered and further dried over sodium wire.

> Tetrahydrofuxan 3 H Tetrahydrofuran C500cm 1 was refluxed with LiAlH^ C.3g)

for 3 hours and then distilled under nitrogen at its boiling'

point C65.4°C). The distilled solvent was stored above

calcium hydride.

■ , N, N * , N1 -T e tra m e th y le th y le n e di am ine N’N,Nf ,N'-Tetramethylethylenediamine w as'ref luxe d with A ,XOH pellets and distilled under nrtrogen at its boiling

po’in t 122°C.

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AlkyX and Aryl Halides

The alkyl and aryl halides used for the preparative work were of A.C.S. grade and the products of Fischer,

Aldrich, Baker or P.C.R. They were dried over Linde 4A

molecular sieves, except CH^Br and-CF^I which-because of

their very low boiling point were used without further purification.

Di-isopropylaraine

This was used without any purification and was the

product of "Aldrich". '

2,2,6,6-Tetramethylpiperidine

It was the product of "Aldrich" and used without any purification. 4

1,1,1,3,3,3-Hexaraethyldisilazane

It was supplied- by "Aldrich" and was used without any further purification.

2.5 Physical Measurements

The infrared spectra between 50-4000cm-^ were recorded

w ith Efeckman I.R -1 2 a n d /o r P e rk in -E lm e r 180 in s tr u m e n ts ,

using ’ Nujol' mull between KBr or Csl pellets as appropriate.

The Raman Spectra were recorded on a Beckman 700 laser

Raman (Argon ion 488.0 nm excitation) instrument, and the

N.M.R. Spectra with Varian EM-360 instrument operating at 60 MHZ.'

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The mass spectra were recorded w ith Varian MAT CH-5DF

mass spectrometer, equipped with a combined F.d/F.i/C.i source. All spectra, were recorded'thourgh an..Incos model

2000 computer interfaced w ith the ,mass _ spectrometer.

Conductivity Measurement Conductivity measurements were performed with 10

solutions in acetonitrile. A Phillips conductivity bridge •

PR 9501 and a cell 3402 Callow Spring Instrument Co.' Inc.)

having a cell constant 0.106 were used to measure the

conductivity.

2.6 Analytical Techniques

Metal analysis were determined by atomic absorption

spectroscopy, using an IL-250 atomic absorption emission spectrophotometer. • • -

The apparatus was calibrated with metal standard 3 ' " solutions ranging in .concentration from 1 to IQ mg/cm .

Halogen Analysis 78 ■ The Volhard method was used for halogen analyses.

A sample of approximately 20-35 mg was dissolved in 50-80

cm 3 distilled water to which 5cm 3 concentrated n itric aci-d

was added, followed by.-a known excess of standard silver

nitrate, 5cm 3 nitrobenzene and 2cm 3 ferric ■ alum indicator

solution. The excess AgNO^ was then titrated, with vigorous agitation against standard potasium thiocyanate, until the

red brown colour of FeSCN^+ was oermanent for, 1 minute. •

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Gas Analysis Gas analyses were carried out by decomposing the known

quantity of substance with acetic acid and measuring the

amount of gas evolved, corrected to STP.

2.7 Measurement of Current Efficiency

. The current efficiency is 'defined as the number of

moles of metal dissolved per Faraday of electricity. The

measurement was carried out at a controlled constant current., under exactly the same conditions as in the actual- experi

ment in which the products were isolated and characterized.

The electrode which undergoes dissolution is weighed before and after-electrolysis, and the number of moles dissolved

so calculated. The amount of .electricity passed through the cell in

Faradays is also calculated. Number o f F a ra d a y s = I (Amp) x t(S e c )

I = the controlled constant current.

-t = the time of electrolysis

and Ixt - the number of coulombs.

F = Fars^ays’ Constant = 96500 Coulombs. Then E— = number of m oles-dissolved • m r m h p .-r o f FaradayR . ~

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^ t0 b u b b le r

Rubber tubing

Platinum wire Glass tubing

Air tight rubber . ' stopper

Platinum cathode

Anode S o lu tio n

ELECTROCHEMICAL CELL A

FIGURE 2 .1

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T e flo n ta p

Platinum wire

Anode

1 / Tungsten wire ''c a th o d e

ELE CTROCHEMICAL CELL B

FIGURE 2 .2

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CHAPTER I I I

THE DIRECT ELECTED CHEMICAL SYNTHESIS OF • — NEUTRAL ORGANO CADMIUM HALIDES

3.1 Introduction *

The second member of group ITb of the periodic table

in cadmium (Z * 48, atomic weight 112.40), discovered by

Stromeyer in 1817 in zinc im purities. It is a blueish-white

soft metal. The organic compounds of group II sub-group

"b" elements show a most striking gradation of properties

and chemical..;reactivity, running parallel to the electrone- 79 gativity of the elements.

Metals such as. zinc, cadmium and mercury which are more

electronegative than magnesium form organic compounds which are comparatively non reactive and which undergo fewer

useful chemical reactions than the magnesium analogues.

The organic derivatives of zinc, cadmium and mercury are covalent compounds of normal structure. These elements

do not have sufficient tendency to increase their coordina

tion number above the group valency of-two to give rise to

many compounds with electron-deficient structures. The

decrease of reactivity with increasing electronegativity in the sequence, zinc, cadmium, mercury is illustrated by

the behaviour of the lower alkyls towards water. Organo-

cadmium compounds are less thermally stable than the organic compounds of zinc, and less work has been published

on these compounds than for the other members of Group lib. •

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The main interest in organocadmium compounds has centred on their application in organic synthesis.

3. 2 Synthesis of Or gano cadmium Compounds Literature Survey: 80 Chenault and Talibonet showed that cadmium w ill

react with alkyl iodides (.* RI) (but not chlorides' or

bromides) in hexamethylphosphoramide (= HMPA) , and isolated

b o th C dl 2 . 2HMPA and CdJ^-^HMPA from the reaction m ixture.

An improved method for the isolatiC^Sof dialkylcadmium involves a reaction between alkyl magnesium bromide and Cl cadmium bromide in the presence of Et 20 as a solvent. •Et«0 2RMgBr + CdBr2 ------> + 2 MgBr2

■ 2MgBr2_ 2HMPA t ^ I ' .

where (R = Me, Et, Pr and Bu). The addition of HMPA to the

diethyl ether solution of the "in situ" alkylcadmium . ' reagent causes precipitation of an insoluble magnesium

halide complex which can be isolated by fractional d istil lation of the dialkylcadmium compound. 82 Gaudemar has obtained an organocadmium analog of the well known Reformatsky reagent from the reaction of

tert-butyl oc-bromoacetate with m etallic cadmium in DMSO.

The IR data for the product suggested that it has a C-metallated structure.

0 DMSO 0 DMSO 1 N Br-CHo-C-0-t~Bu-+ Cd : * BrCdCH~-C-0-t-8n 2 , 2 DMSO

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83 Ashby- and Saunders published a detailed procedure for the

• preparation of di-n-butylcadmium which was obtained as a

colourless light sensitive liquid. E t« 0 2-n-BuMgCl + CdCl 2 — > n-Bu2Cd + MgCl 2 . ^ -7 8 °

They have also investigated in detail the nature" of the

compound obtained by reacting Grignard reagent with

Cd^ CX = Cl, Br, I) in 1/1 and 2/1 molar ratio in diethyl 83 ether. The following equation shows the equilibra. RMgX + CcD^ > RCdX > MgX£ t ■ RCdX. M g ^

2RMgX + C d X ^ > RjCd + R2Cd. + M g ^ . R ^ d + C d ^ 2RCdX.

RCdX. I^lg^ has been isolated from 1/1 reaction mixture.

Infrared results support the conclusion that both CdX 2 and

HgX2 coordinate to n-B^Cd in either Cl) or single CH) halogen bridge-arrangements.

Cd ----- X M X

II. Divinylcadmium has been synthesised by reacting divinyl-

mercury and dimethylcadmium.

HgCCH=CH2) 2 + CdCCH3) 2 ------> CH3HgCH=CH2 + CH3CdCH=CH2 ^

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2CR3CdCR«CH2- ? = * CdCCH3>2 + CdCCH=CH2l (2)

CH3CdCH-CH2' + HgCCH=CH^)2 ^ = ? CH3HgCH=CH2 + cdCCH»CH2) 2 O l Step 2 is the predominant path for the formation of

d iv in y lc a d m iu m . CdCCH=CH 2 l 2 is a readily sublimahle

crystalline solid, whereas ZnCCH=CH 2) 2 and HgCCE=CH 2)_2

are relatively involatile liquids.

Several procedures for the preapration of perhalo-aryl-

cadmium derivates have been reported. In one of these the

thermal decarboxylation of cadmium pentachlorobenzoate gives an excellent synthesis of bisCpentachlorophenyl)

cadmium. ^

CdCC2C l5C02) 340°/0 .01 mm ^ Cd(.C6C l5) 2 + 2 C02 -

Bi^Xpentafluorophenyl) cadmium has also been obtained by heating bis (pentafluorophenyl) thallium with m etallic 86 cadmium in the absence of solvent.

CCgF5)2TiBr + Cd 100°/5 days ) CC6F5‘)2Cd + TlBr. 87 Dyatkin et al. have reported the_presence of methyl

trifluoromethylcadmium and bistrifluoromethylcadmium in

solution, but these compounds were not isolated. Dyatkin

also carried out some studies on alkyl, group exchange

between dime thy Icadmium and .bis (trifluorom ethyl)'m ercury

and the results were discussed in terms of following

equilibria.

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CCF3) 2Hg + CCH3) 2Cd ■ > CH3HgCF3 + CH3CdCF3 C4)

CCF3) 2Hg + CH3CdCF3 — ► C^3HgCF3 + CCF3) 2Cd C5)

(CH3>2Cd + CH3HgCF3 — ----- ► CCH3) 2Hg + CF3CdCF3 C6)

CCH3) 2Cd + CCF3) 2Cd :----- ► 2 CH,CdCF,, 3 3 C7) Diethylcadmium differs from diethylzinc in reactivity with

CCl^. Diethylzinc with CC1 4 gives zinc chloride', whereas diethylcadmium does not. However, if the reaction is

carried out in the presence of cyclohexene, then EtCdl is

form ed. Cadmium metal reacts with pentafluoroiodobenzene

in a variety of different coordinating solvents such as

THF, and DMSO to give almost quantitative yields of 88 pentafluorophenylcadmium iodide.

Cd + C6F5I ------: CgF5C dI. A diGrignard reagent in 1/1 ratio with cadmium halide forms

a compound which subsequently was found to-be a monomeric •89 cadmium heterocycle .which has been isolated by d istillation.

CdBr2 + BrMgCCH2 ) 4MgBr ------

The half reaction of diGrignard reagent with cadmium halide

forms an organocadmium compound of the type XCdCCI^^CdX

the reactivity of which differs, from.that of the monomeric 90 cadmium heterocycle formed in 1 /1 r e a c t i o n .

2 CdBr2 + BrMgCCH2) 4MgBr ------> BrCd(CH2) 4CdBr +. 2MgBr2. • 91 In a recent communication, -a new method has been proposed for the formation of I^Cd. Active cadmium and zinc slurries were prepared by the cocondensation of the

metal vapours with excess solvent at 73°K followed by

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warming to room temperature. The metals were then allowed

to react with alkyl bromide and iodide in several solvents;

both polar and nonpolar.

Cd vapours + hexane 77^C Cd- hexane slurry . warm % I^'Cd + Cdl2 RI r e f l u x

An' improved method for the synthesis of dicyclopentadienyl- 92 cadmium has been reported by J, Lorberth. Acidolysis of bisCtrimethyIsilylamino)cadmium with cyclopentadiene in

E t 20 at room temperature gives dicyclopentadienylcadmium

in high yield and purity.

Cd£NCSiMe3) 2] 2 + 2C5H6 C^H^Cd + 2 HNCSiMe^.

3. 3 Organocadmium as an Alkylating Reagent

Birch and Pearson 93 have reported a method for the

application of a■ ■‘tricarbotiylcyclohexadienyliron cationic

complex with organocadmium reagent in dry THF cooled to

0°C under nitrogen. Alkylation occurs on the opposite side

of the ring to the Fe(£0 ).3 group. The reaction is found to

be regioselective in the two substituted salts (1, R* = Me

or MeO) showing greater than 90% addition to C-5 to give „ C2, R' = Me o r MeO).

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CCO)3Fe

R' R" H Y ie ld %

H ch 2= chch, Cd 82 H He2CH Cd • 52

H ■ MeCH=CH Cd 40 H' Ph Cd 83 • E PhCH„ Cd 72

,94 Birch and Pearson further reported^-’’ the 'alkylation of

tricarbonyldienyl iron salt with organocadmium reagents.

The tricarbonyl (2-methylcyclohexadienyl) iron cation reacts

regioselectively, mainly at the unhindered terminus and

stereospecifically on the face opposite to the Fe(C0 ) 3 group, and the corresponding alkylated tr i carbonyl eye lohexa-

diene iron was obtained.

R* R"„Cd CCO)3Fe

CCO) 3Fe : + X Orgahocadmium

X = BF^_ o r PFg . _

3.4 Co-ordination Complex'with Ligands not Containing Acidic Hydrogen In contrast to dimethyIzinc which forms a stable

2 ,2T -bipyridine complex Me2Zn b ip ^ dime thy lcadmium forms

an unstable complex Me2Cd bipy, also coloured yellow, from

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X which dime thy lcadmium can he pumped at room temperature 95 leaving a residue of 2 , 2 ’-bipyridine. Mixture of dime thy lcadmium and - dioxane in 1:1 molar

proportion are very largely dissociated into their compo

nents in benzene solution, the corresponding Me 2Zn-dioxane mixture being only very slightly dissociated under the same

ccndictions. Dimethylcadmium, however, forms a series of 96 complexes wi£h chelating diamine (TMED) and diethers.

3. 5 Some Chemical Properties of Organocadmium Compounds

1^2Cd reacts with compounds having reactive S-H, C-H,

0-H, P-H, Si-H, Ge-H etc., bonds to give the following '

compounds, and eliminating RH. ‘ - . Et,SiH R’OH ref. 99 (Et 3S i ) 2Cd * *---- — » RCdOR' r e f . 97 ' .PhC-C-H £Pilc=C)2Cd xef. 97a Et^G e-H tBuSH ref. 100 CEc3 G e)2Cd ^------’ CdB^------> RCdS-Bu* ref. 97

X Me,SiOH r ' 2ph re f. 101 MeCdOSiMe-3 RCdPR’o2 ref. 98

3 .6 Applications in Organic Synthesis

The most important reaction of organocadmium compounds •

in organic synthesis is with acid chlorides to formed ketones.

Asymmetric ketones have been synthesised by reacting dicyclo- 102 hexylcadmium w ith aceyl or benzyl ..halide. E t -0 CdCCgH11) 2 + 2RC0C1 — 2CgH11COR + CdCl2 .

Cwhere R = Me, Ph) Phosgene with diphenylcadmium in 2/1 molar ratio gave benzophenone rather than benzoyl chloride, indicating that

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. benzoyl chloride reacts more rapidly than phosgene. 1 0 3

0 0 ' II II C6R5 "C“ C1 + CPh) £ c 3. ------> G5H5C P_h>f. PhC dCl.

<* This preparative method is more convenient than the direct *• use of Grignard reagent, since Grignard reagents combine. < not only with acid halides but also with the ketones p ro d u c e d .

p» B-hydroxyesters have been obtained using an organo- - cadmium analog of the Ref ormatsky reagent. but organozinc

•appears to be superior for such reactions.

• 0 0 OH 0 !( t- I1 m e n i 11 <- BrCdCH2C-0-Bu + Pr-C-H -g^ - > Pr-CH-CH^-C-O-Eu .

Jones and Castanzo have studied the mechanism of displace

ment of the halogen from a saturated carbon atom by

phenylcadmium chloride. It has also been shown' that the displacement reaction proceeds with raceminization. The

free radical intermediate was detected by its E.S.R.; signal.

The mechanism was also confirmed by the formation of (.-)

methylhydrotropate from (K) - (+) bromopropionate when the .starting ester was optically stable.

0 0. .11 ' ‘ 607 II ■G0-C+) CH3-oi-CrvocH3 + Phcdci c+) ch 3 - ch - c - och 3 . . B r 2 X . Ph

Emptoz and H uet^^ have reported thqt when dime thy lcadmium,

diphenylcadmium or di-p-polylcadmium were reacted with

benzyl halite in benzene or ether, the substituted;.products

PhCH2R were formed;, in some cases small amounts of the

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coupling products PhC^G^Ph were also found. When the

reactions were carried out in benzene solution, the alkyla-

tioh-:pf~the benzene was observed and the reaction rate -

.'increased considerably. The presences of MgBr 2 had an over

all accelerating effect, but also increased the amount of

the; coupling product. Diphenyl cadmium was found to

react with benzene sulphonylchloride in benzene or ether

Solution to yield diphenylsulphones and biphenylbenzenesul-

phonic acid along with small amounts of chlorobenzene Sim ilarly, diphenylcadmium reacts with p- toluene

sulphonylchloride to give p- tolylphenylsulphones, biphenyl

. and toluene sulphonic acid.

Cc 6H5) 2 cd + 2 KS02C1 > 2 RS02C6H5 + CdCl2

CCgH^Cd + RS02C1 ------=—> C6H5C6R5 + RS02CdCl

+ CRS02)2Cd + CdCl2

3.7 Electrochemical Preparation of Neutral Organocadmium Halides • "

Organocadmium halides 'cannot be prepared ‘by direct

catalytic or photochemical reaction of the metal and

organic halide. 79 The work reported in this part of the

thesis deals with the direct electrochemical preparation of organocadmium halides, and the chemical characterization

and vibrational spectra, of the complexes .

Footnote: The electrochemical preparation of neutral ” organocadmium halides has been carried out in colloboration with Dr. J. J. Habeeb.

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G e n e ra l

In general the electrochemical syntheses were carried

'cut in two different cells 0 ?ig- C2 . l l and ( 2 . 2), depending upon the alkyl or aryl halides used.

Cadmium metal (.Alfa Inorganics) was falttened out in a

form of a sheet . C .6 cm to 2.5 cm .by 0.15 mm) of 3'.5 to 0.8

cm by 0 .12 mm) again depending upon the cell used, and

supported on platinum wire. The cathode was a platinum

wire CIO cm long and 1 diameter) or semicircular form of tungsten wire fused into the pyrex glass -vessel'.

The solution phase consisted of an alkyl or aryl halide, in some organic solvent with or without ligands.

In a ll experiments Et^NClO^ or Et^NBr (10-20mg) were

used to enhance the conductivity of the solution. Solution

composition and analytical data in each and every experimen

tal setup is given in Tables 3,1 and 3.2.

The applied voltage was 20 - 50 V, as dictated by the solution conditions, given that a current of 10-50 mA

produced a reasonable rate of reaction at room, temperature.

In some cases external colling was necessary, especially

when very volatile alkyl halides were used in the electro chemical reactions.

It was also observed that the surface of the cadmium

metal electrodes tended to distintegrate physically during

long electrolyses, and especially at high (20 V) voltages. Perceptible particles of cadmium metal contaminate the

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product in these circumstances andf?are difficult to remove, since the adduct of RCdX are insoluble in most common organic solvents. This particular problem was overcome by

restricting the operation to low voltages and short periods

of '^len'trblysis . .'When more-extreme conductions were'

necessary, it was. found possible to effect' a separation of

cadmium particles from the product by collecting the former' in a small vial placed immediately below the cadmium anode. The cadmium particles fall into this vial, while most of

the lighter insertion products fall outside it. <

3.8 Isolation of Product

Insertion products of the electrochemical reaction

were precipitated by subsequent slow addition of diethyl

ether, pet ether or other organic solvents and/or by reducing the total volume of the solution. A ll' chemical manipulations

such as filtration, were carried out under nitrogen as all the compounds prepared are extremely air sensitive. Volume

reduction and sample drying were done in vacuo.

3 .9 Preparative Chemistry Ca) - Electrochemical preparation of methyl cadmium io d id e

Electrochemical insertion of anodic cadmium metal

takes place readily in methyl iodide, when a mixture • 3 3 consisting of 20 mg of Et^NClO^, 30cm -methanol, 70cm of

methyl iodide is electrolyzed for two ho'urs , using a

current of-60 mA and an in itial voltage of 50 volts.

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At the end of the time period the solution was filtered

and a white and air sensitive product isolated. Analysis

showed 44.4% cadmium, and 49.7% halogen (see Table 3.2).

The total yield of the product was 0.71g (74% based on

cadmium dissolved)

(b) - Electrochemical preparation of ethylcadmium io d id e Using an in itial voltage of 45 volts and a current of 3 3 50 mA, a solution of 60cm ethyl iodide in 20cm of methanol and a few mg of Et^NClO^ was electrolyzed for 2 hours. The *

colourless or off-white products precipitated in the cell.

The mixture was filtered and dried, yielding 0.72g of

product (.81% yield on the' basis of cadmium dissolved) .

Approximately 0.37g of cadmium dissolved from the anode. .

The white product so obtained.was found to contain

41.‘8% cadmium and"4711% iodide, and shows insolubility

in most, of"the organic solvents. •v/41 (c) - Electroch^Tm' cal preparation of DMSO adduct of MeCdl : Electrochemical oxidation of cadmium metal in a solution 3 3 containing 25cm of methyl iodide, 25cm of DMSO and 15 mg

of Et^NClO^ was carried out for 12 hours using an initial

voltage of 25 volts and a current of 100 mA. The reaction

mixture was then filtered to isolate the product, which was

subsequently dried in vacuo. The product obtained was found to contain 27.1% cadmium.

and 31.1% iodine. This particular compound proved to be very

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unstable; two days after the original preparation, the

iodine content of the solid'was 62.0% which is close to

that for unsolvated Cd^ (.calculated 69.4%).

Cd) - Electrochemical preparation of 2,2 ’ - b i p y r id i n e adduct of MeCdl 3 ■ 3 A mixture consisting of 0.7g 2,2’-bipyridine, 5cm 3 of Mel, 40cm. acetone and 15mg Et^NClO^ was electrolyzed

for 2.5 hours using ah in itial voltage 35 volts and a current

of 100. mA. As the electrolysis progressed, the product was

deposited at the bottom of the cell; this m aterial was •

^collected, washed with benzene, to remove excess 2 ,2 ’-bipyridine,

and dried in vacuo. The product was analysed for cadmium and

iodine. The total yield of 0.9g was obtained (.95%, on the

basis of cadmium dissolved). Experimental details and

analytical data are listed in Tables 3.1 and 3.2 respectively.

Ce) - Electro che-tTn cal preparation of 2,2’-bipyridine adduct of EtCdBr 3 A solution of 0.7g of 2,2*-bipyridine, 15cm EtBr, 3 30cm acetone and 15mg of Et^NClO^ was electrolysed for

2 hours at 40 volts and 80mA current. After this period

the reaction mixture was treated in the same way as

described above. The product (0.84g) was dried in vacuo

and analysed for cadmium and iodine. The product was

found to be insoluble in most of the organic solvents. The yield of the product and the amount of metal dis

solved during the electrolysis are given in Table 3.1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (f) - Electrochemical preparation of 2,2'-bipyridine adduct of nBuCdBr

A mixture consisting of 15mg Et^NClO^, 0.5g 2,2’- 3 3 bipyridine, ,30cm n-butyl chloride and 15cm MeOH was

electrolyzed for 5 hours, using a voltage of 10 volts and maintaining a current of 20mA. The precipitated pi^jduct

in the cell was collected, washed with benzene to remove

any unreacted bipyridine and then dried in'vacuo . The s * product was analysed for cadmium and iodine contents

(Table 3.2).

(g) - Electrochemical preparation of 2,2’-bipyridine dociuct o f CgF^CdBr "

Electrochemical preparation of CgF^CdBr bipy was

carried out in' 2 armed cell B (Fig- 2 . 2) .

The mixture of lOmg of Et^NClO^, 2cm"^ of CgF^Br, 0.35g 3 2 , 2 '-bipyridine and 4cm of acetone was poured into one

of the arm of H-Cell, and after a series of evacuation/ freezing cycles to remove dissolved oxygen, the solution ■ mixture was decanted into the second arm which actually

served as the electrochemical cell. Details of the electri

cal condictions are given in Table 3.1. After six hours

of electrolysis the product was filtered and washed with

benzene to remove unreacted 2 , 2 ’-bipyridine and dried in v a c u o .

The product was analysed for cadmium and iodine (Table 3.2). The yield of the product and the amount of metal

dissolved during the electrolysis are given in Table 3.1.

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Ch.) - Electrochemical -preparation of 2 ,2 '-bipyridine adduct bf~CF3Cdl ' The preparation of CF^Cdl bipy was carried out in 2 armed cell. The solution consisted of 0.8g of 2,2'-bipyridine, 3 3 3cm benzene, 8 cm acetonitrile, arid few mg of Et^NClO^, 3 into which was distilled 2 cm o f CT^L.

After a series of evaucaticn/freezihg cycles, the rest

of _the procedure was the same as "that described above for -

the other adducts.. The mixture was then electrolyzed for 1.5 hours at

8 volts' and a current o £ 10mA; a s th e e l e c t r o l y s i s p ro c e e d e d , a light yellow*solid precipitated in the cell. On filtering the cell contents and.treating the filtrate with petroleum

ether more light yellow product precipitated out, and was

collected, washed with benzene and dried in vacuo.

The* yield of the product was 0.22g (66.7% on the basis of cadmium dissolved). The isolated product was analysed for cadmium and iodine and results are .given in Table 3.2.

(i) - Electrochemical preparation of 1,4-Dioxane adduct of EtCdBr : 3 ’ 3 A mixture consisting of 15cm EtBr, 10cm acetone, 3 " 3 5cm o f OLjOH. 15cm 1,4-dioxane and 15mg Et^NC10^_ was electrolyzed using a current of 40mA (14 volts) for 10'

hours. After this period of time, the reaction mixture

was filtered to isolate the product, which was washed with pet ether and dried in vacuo. ^The total yield of the product

was 1.21g (.96% based on cadmium dissolved). Details of

solution composition, electrical condiction etc. are given

in Table 3.1 and the analytical data in Table 3.2.

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(j) - Electrochemical preparation of 1,4-Dioxane adduct of EtCdif ’ 3 A .mixture consisting of 15mg Et^NClO^, 15cm EtI, 3 3 10cm acetone, 15cm dioxane was electrolysed for 8 h o u rs ,

using a current of 40mA and in itial voltage of 15 volts.

After 'the electrolysis the product was isolated in the

manner described above. The total yield of the product was 0.87% (78% based

on cadmium dissolved) .

(k} - Electrochemical preparation of 1,4-Dioxane adduct of PhCdBr 3 3 A solution consisting of 30cm PhBr, 10cm acetone, 3 30cm dioxane and a few mg of Et^NC10^_ was electrolysed

f o r 6 hours ■using a voltage of 18 volts and maintaining a

•current of 40mA, again the isolation method was the same

as for other 1,4 dioxane adducts. Details of the amount .

of substance obtained, and %-yield are given in Table 3.1

and the analytical data in Table 3.2.

Cl) - Electrochemical preparation of 1 ,10-phenathro- ' line adduct of EtCdBr The solution composition,- electrical conditions etc.

are given in Table 3.1. After a period of 2 hours, Q.27g

of cadmium had dissolved. The product was collected and

dried in vacuo; analysis showed 28.2% cadmium and 20.3%

io d in e . Further characterization of the product was not

possible because of insolubility in most organic solvents.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 3 - 8 (mA) I n i t iC a u rre l n t 80. 60 90 io 20 40 40 100 100 \ 15 4525 50 14 . 40 10 50 17 18 8.0 3 3 3 3 g , 8 . experiment performed in small cell 15 cm 15 cm V/t., or volume, Voltage 0 0 0 . 7g0 . 75g 40 40 of ligand . (V) 0 . 7g 35 0 . 5g 0 • • 25 cm TABLE TABLE 3 .1

T -o

. . 3b

d O ' 30 cm

h 0 -1 8 0 ,5 d 0 . 35g (cm3) a 30d 20 30 10 15d 30 10 40 5 2 2 7° 15 15 15 15 30 60 25 Vol. of organic Vol. of acetone 30 halide (cm ) CH3OH CH3OH to reduce voltage V 3 c a- of acetone f ' _ , , Acetonitrile/benezene mixture used instead ^ot recorded a15mg of Et^Ncio^ added in each experiment ^Methanol used instead of acetone ° p lu s 5 cm i • HgCdCl bipy 4 H3CdBr phen H3CdBr bipy ll5CdI F5CdBr bipy H,jCdI diox H3CdBr diox 5 2 2 2 2 2 b 6 REACTION REACTION CONDITIONS FOR HALIDES DIRECT SYNTHESIS ELECTROCHEMICAL OF ORGANOCADMIUM Compound CH^Cdl bipy CH3CdI 2 dmso C CH3CdI C C Cz-HcCdBr d io x C C CF3CdI b ip y f C n-C

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■

01 ✓- mt r-t m VO N vO CO < t \£5 >* >- Is * CO O '! cn O' c\ r-* SQ- SO

4-> O 3 &c o u • a cu a u <4-1 a} o > m o t-l CM 01 S OOOQJ O O f H O W t—4 O O

T3 CD 3 C TO t 4 o o 00 o m cm r*>» vo vo» p** vo m CO P '. r—i CU X g m m C3 T-l X • cm o E-* CM CM CM 'i—I CMCMi—IC O lOOCM i—I

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE 3 .2 ANALYTICAL RESULTS FOR ORGANOCADMIUM HALIDES, AND THEIR ADDUCTS. . . Compound Analysis (Found (C a lc d v) (7*) )

♦ Cd H alogen

MeCdl . 4 4 .4 4 9 .7 (4 4 .1 ) (4 9 .9 )

E tC d l 4 1 .8 4 7 .1 (4 1 .9 ) (4 7 .3 )

MeCdl. bipy 2 7 .3 3 0 .9 ^ (27.3) • (30.9)

MeCdl* 2dmso 2 7 .1 3 1 .1 (2 7 .3 ) (3 0 .9 )

EtCdBr. bipy 2 9 .4 21.2 * (29.8) C21.3) . EtCdBr. Phen • 28.2 2 0 .3 (2 8 .0 ) (1 9 .9 ) EtCdBr* diox 36.6 2 5 .8 (3 6 .4 ) (2 5 .9 )

E tC d l.d io x 3 2.0* 3 5 .6 (3 1 .5 ) (3 5 .7 )

nBuCdCl- bipy 31.4 9.7 (3 1 .0 ) ■ (9 .8 ) '

PhCdBr. diox 32.2 22.1 (3 1 .5 ) (2 2 .4 )

CcF-CdBr.bipy 21.6 1 5 .4 0 D ■ ( 21-. 8 ) g (1 5 .5 ) CF~CdI.bioy 24.4- 2 7.4 5 (2 4 .2 ) (2 7 .3 4 ) •

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- . . TABLE 3 .3

GAS ANALYSIS FOR SOME ORGANOCADMIUM HALIDES, AND THEIR ' ADDUCTS.

Compound W eight o f Gas measured at NTP Compound 3 ST i n (g) i n 'em fo u n d c lc d .

MeCdl 0 .4 2 8 • 37. 5 3 7 .7

MeCdl# b ipy 0 .2 1 7 ' 1 1 .4 11.8

EtCdBr#bipy 0 .1 7 3 10.2 10.2

TABLE 3 .4

^C C d-C ) MODES IN THE INFRARED SPECTRA OF R-CdX ADDITION COMPOUNDS

Compound Ca) CCd.-C) (Cm- 1 )

MeCdlib'ipy ■ 445

EtCdBr# bipy 478 ' EtCdBr# phen 479 EtCdBr# diox 474 EtCdl# diox 471 n -BuCdC 1# b ip y 472

(a) The presence of the appropriate neutral, ligand Was demonstrated in each case by established infrared spectral criteria.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE 3 .5 MEASUREMENT OF CURRENT EFFICIENCY

System E^Cmol (Cd) F ~ ^ ^ • • ^

/EtBr + bipy/Pt^ 1.1

Cdc+)/MeI + bipy/Pt^^ 2.4

Cd^ CgF^Br '+ bipy/Pt^ - 2.0

'<*1 ••

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.10 Results and Discussion The most important feature of this method of synthesis

is tha£ itvnf fers a rapid convenient • and simple route for the direct electrochemical synthesis of organometallic

complexes, especially since the oxi-da'tion of metal occurs

at room temperature and good crystalline samples are obtained.

The range of experimental systems investigated and the

variety.of products obtained suggest that direct electro-

cheqiical synthesis is definitely superior to other ccnven- *■ “■*- ■ - . tional synthetic m ethods^-^ Zinc and mercury undergo, direct catalytic or photo- ..

chemical reaction with alkyl halide to give the compound of the type KMX, but the analogous alkylcadmium halides

cannot be prepared in this way and the only possible route

involves an exchange reaction between cadmium dihalide 79 8 8 and appropriate dialkylcadmium.- * In this chapter,-characterization of the products

was established by elemental analysis (Table 3.2) gas

analysis (Table 3 , 3 ) and infrared spectroscopy (Table 3.4).

Electrochemical oxidation of .cadmium under the various

conditions described in Table 3.1 gave rise to a series;of

compounds and its adducts. The infrared spectra of the

2 , 2 * -bipyridine and 1 , 1 0 -phenanthro.line. adducts confirm the bidentate coordination of these ligands and thus the

w - - compounds are formulated as, the. four-coordinate RCdX.bipy etc. species.-

' *

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The-.compound MeCdI.2dmso proved to be very unstable,

sim ilar loss of dmso from adducts of main group compounds was reported previously.

The CFgBr and CgF^Br systems are interesting because

of their relevance to "the postulated Schlenk equilibra in

solution. The isolation of CF^CdX and CgF^CdX adducts

suggests that the method could be used in circumstances in. which the preparation of perfluoroalkyl-Grignard reagent

is inconvenient. When the electrolysis was carried out

with a solution phase consisting of only CgF^Brand

acetonitrile, cadmium dissolved and the m aterial isolated

contained acetonitrile (identified by infrared absorption

at 2284 cm-^). The' absence of the CgF^- vibration in the ether-washed solid, and the analytical results

(found: Cd 25.3; Br 36.17o) are in agreement with the

formation of CdBr2. (Calcd: Cd 25.5; Br 36.3%) which suggests that in the presence of acetonitrile, the initial product ib CgF^CdBr. 2CH.gCN, which is soluble in excess

acetonitrile, and dissociates via the equilibrium below,

which is. followed by precipitation of the cadmium, dibromide

a d d u c t.

2C6 F5 CdBr.L2 ; - — * CCgF^Cdl^ + CdBr 2 -L2 . 81 This mechanism was confirmed by Chenault and Talibonet for the reaction of cadmium with alkyl iodides in HMPA and 88 also by Evan and Phillips . in NMR studies of the CgF^I/Cd

sy ste m . ^

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V ibrational Spectrqpcopy The Raman spectra of these compounds could not be

recorded because of the decomposition of these compounds

in the .laser beam. The infrared spectra of the addition compounds in the ^ (Cd-C) region'are shown in Table 3.4.

The results show some dependence on the mass of the

alkyl group, and equally shows, that ^ CCd-Et) is essentially unaffected by the change- in the bidentate ligand or halogen. 108 Cavanagh and Evan have studied the infrared and jjipman spectra of solutions prepared by mixing an equimolar

quantity of CMe^Cd and (X = Cl, Br, I) in THE or DME CX = I only). In these systems they also identified • vibrations between 475cm~^ and 482cm~^ assigned to

CCd-Me) t •

• These reported values are slightly higher than 445cm-^

for MeCdl.bipy, which may be accounted for as the

difference between the crystalline and solution phase. A. J . Downs and G. E. Coatesidentified vibrations

between 260-270cm” due to ^ (Jlg-CE^) in compounds of the

type CE^HgX/where X = Cl, Br, I. or CE^) . Inspection of

the vibration spectra of CE^Cdl.bipy shows that frequency at 275cm- may be due to the CCE^-Cd) stretching vibration. « Reaction mechanism

It has beest-suggested that a knowledge of the number of 'jT a h electrons involved in. the electrochemical process can give > •* * some ^nsight int.o the mechanism of the electrochemical

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reactions. In the present work, as in the related studies 53 71 with tin and nickel , the electrochemical reaction has

a stoichiometry which corresponds to the oxidative

insertion reaction. nRX -+ M ------> R MX • n n where M = Cd or Ni, n = 1; M * Sn, n = 2

The current efficiencies .in the experiments reported in

Table 3.5 were between l.i - 2.4 mol F-^ based on weight

of cadmium dissolved. These values are sim ilar to those 53 reported for tin dissolving in Mel or EtI. .All this

information leads to a mechanism in which reduction of 0 RX at the cathode is followed by the anodic processes described below.

Cathode ' ’

^ RX + e~ ------> R' + R~ C8 a)

The formation of R 2 tiay proceed either by coupline

2R’ ------* R ^ . ( 8b) <^r by abstraction-^

R‘ + RX > R^ + X* ( 8 c)

Anode f t - X" ------> X* + e ^ (9a)

X* + Cd >CdX. (9b)

CdX + RX >RCdX + X* (9 c)

X' + Cd ------> CdX (9d)

The relatively high current efficiency was explained in terms of the abovd scheme with (9c) and (9d) constituting a chain

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process and requiring that E^. be greater than 1 b e c a u se

m ore th a n 1 mole of the cadmium anode is consumed per

F a ra d a y .

3.11 Conclusion The conclusion one can draw from this chapter is that

the electrochemical reaction involving cadmium as the

sacrificial anode offer.a clean and rapid synthetic route

to a variety of organocadmium halides at ambient tempera ture, with high product yield. These’ compounds may serve

as a convenient substitute for Grignard reagents in

certain situations.

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CHAPTER IV

THE DIRECT EIECTRDCHEMICAL SYNTHESIS OF ANlCNIC ORGANODIHALOCADMATE(II) COMPLEXES

4.1 Introduction

Tuck and Contreras^^’^ ^ have reported the formation

of anionic complexes of cadmium of the type ICdX^J and

[Cd^Y'.]*, where X and Y are_chlorine, bromine or iodine,

all as the -tetrapropylammonium salt. Raman spectra have

been recorded, and serve to establish that these anions have C2v symmetry.in the solid state and are present as discrete three-coordinate cadmium units in the crystalline

l a t t i c e . - As discussed in Chapter III the electrochemical oxi

dation of cadmium metal in the presence of an alkyl or aryl

halide provide the easiest route for the synthesis of

organocadmium halides, 'which can be stabilized by neutral

organo mono CL] or bidentate jligands. This chapter is devoted to a second group of derivatives

in which the parent RCdX compounds are stabilized by

further coordination- to give the anionic complex fRCd^]

(where R - Me, Et, CF^> hBu, Ph, CgH^ and X = Cl, Br, I)

as their salts with tetrapropylammonium cation. These * appear to be the first examples of the preparation of

organocadmium anions of this type. The d i r e c t electrochemical technique has been successfully utilized,

so that the electrochemical oxidation of anodic cajjziium

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metal in a Benzene/methanol mixture containing (CgHy^NX

and RX leads to the formation of the anionic organohihalio-

cadmateCH) complexes. CH., OH /a. H. n=Pr4NX + RX + C d^ lT|- fatio n-Pr^RCdX^'.

4.2 Experimental .

G e n e ra l

The general experimental arrangements are shown in

f i g u r e 2 .1 and figure 2 . 2 , and these two electrochemical

cells were exclusively used for the preparation of anionic

organodihalocademateCH) complexes. The detailed description of the cells has been already given in Chapter II.

Cadmium M4N (Alfa inorganic) was used in the form of

a rod, 8.5 cm long, 0.25 cm in diameter, or beaten into a

sheet (Ca 2 x 2 cm) and a platinum wire, 10cm long, lnm dia

meter, was used as.the cathode. Solvents and alkyl or aryl halides were as described

in Chapter II. Tetra-n-propylammonium halides (Eastman organic chemicals) were used without any further purifi

cation. All preparation, isolation and handling of products

were carried out under dry nitrogen.

The applied voltage was 10-50 volts, as dictated by

the solution condition, given that a current of 20-30mA.

produced a reasonable rate of reaction without overheating the solution. *

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.3 Isolation of product

During the electrolysis, colourless crystals deposited in the cell and in' each case these were collected, washed

with acetonitrile and dried in vacuo. As described in the

previous chapter on neutral organocadmium compounds, care

was taken to avoid the disintegration of the surface of the

cadmium anode, which caused contamination of the product with particles of cadmium metal.

Solution composition and analytical data for each and every experimental setups are given in Tables 4.1, 4.2 and 4 .3 .

4.4 Preparative Chemistry

(a) Electrochemical preparation of [MeCdB^J anion 3 3 A mixture consisting of 2.7g nPr^NBr, 2cm MeBr, 8 cm 3 MeOH and. 20cm of CgH.g was eldctrolyzed using the electro-

c-emical cell A CFig. 2.1). The electrochemical cell was cooled between 5-10°C to prevent excessive loss of MeBr from the solution. After 5 hours of electrolysis with an

in itial current of 25mA and 40 volts, the product deposited

at the bottom of the cell, and was collected, washed with

acetonitrile to remove unreacted nPr^NBr, and dried in vacuo

The product was analysed for cadmium and iodine.

’■ Cb) Electrochemical preparation of [EtCdBr^iP anion

Electrochemical' oxidation of cadmium metal in.a solu- 3 tion mixture consisting of 2.7g NPr^NBr,. 3.5cm of EtBr,

' ' i

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10cm CH^OE and 10 cm gHg was , carried out for 6 h o u r s , using an in itial voltage of 40V and current of 25mA. As

the electrolysis proceeded, colourless crystals deposited

'in the cell; these were collected, washed with acetonitrile

and subsequently dried in vacuo. Details of experimental condition and analytical results are given in Table 4,1

and 4 .2 .

Cc) Electrochemi cal preparation of [t-BuCdBrpJ 3 A solution phase consisting of 2.7g nPr^NBr, 3.5cm 3-^3 t-C ^E g B r, 8 cm CSL^OH and 20cm CgHg was electrolysed-for 5 hours, using a current-of 25mA and 40 volts. After the

electrolysis the reaction mixture was filtered to isolate the product, which was dried in vacuo. Details of solution

composition, electrical condition etc. are given in Table

4.1 and analytical data in Table 4.2 and 4.3 respectively.

Cd)' Electrochemical preparation of [MeCdl,,] anion 3 The solution consisted of. 1.5g of nPr^NI, 2.5cm of 3 3 Mel, 10cm MeOE and 30cm , with an initial voltage of

40V and a c u r r e n t o f 25mA. A f t e r th e p e r i o d o f 6 h o u r s ,

colourless crystals deposited in the cell; these were

collected by filtering the reaction mixture under dry

nitrogen. This product typically represented a 30-40%

yield of the salt, based on dissolved cadmium CTalbe 4.1).

(e) Electrochemical preparation of [EtCdlpJ anion 3 3 A mixture consisting of 1.5g nPr^NI, 2.5cm EtI, 10cm

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 Me OH and 30 cm CgRg was eldctrolysed for 5 hours using a

current of 25mA and 35 volts. The isolation technique'

was exactly the same as in Cd) above. The product was

then filtered under nitrogen Cglove box) , washed with

acetonitrile and dried in vacuo.

Cf) Electrochemical preparation of [n-C^HgCdlp]" anion 3 2.5g nPr^NI was dissolved in a mixture of 8 cm Me OH, 3 3 20cm CgHg and 3.0cm n-C^Hgl. The solution mixture was

electrolyzed using cadmium, as an anode. After 4 hours of electrolysis, the white product was collected by filtering

the reaction mixture and dried'in vacuo. Details of solu

tion composition, electrical condition are given in Table

4 .1 . v *

Cg) Electrochemical preparation of

Electrochemical preparation of [CF^Cd^] was carried

out in CH-form) Cell B (Jig. 2.2). The preparative method followed the method described in. Chapter II of this disser- . 3 tation. CF^I Cl-5cm ) was distilled into one of the arms 3 of the cell containing a mixture of 1.2g of Pr^NI, 3cm 3 CH^OH and 8 cm CgHg. After degassing the solution in vacuo by a series of evacuation/freezing cycles, the degassed

■ solution was/-flushed with dry ^ and then decanted into

the second arm which served as the electrochemical cell. \ After the electrolysis the reaction mixture was filtered to isolate the product, which was washed with MeCN and

dried in vacuo. Details of solution composition, electri-

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cal condition are given in Table 4.1.

Ch) Electrochemical preparation of [PhCdCl^J anion

The electrochemical preparation of the [PhCdC^'] anion

was carried out in Cell A CFig. 1.1). Electrochemical

oxidation of cadmium metal in the solution mixture con

sisting of l,5g nPr^NCl, IQcm^ MeOH, 30cm^ and 3cm^

PhCl was carried out for 6 hours, using an in itial voltage

t of 50V and current of 25mA. As the electrolysis proceeded, off-white' crystals deposited in the cell^these were collec- ( ted, washed with MeCN and dired in vacuo. The product was / analysed for cadmium and iodine

Ci) Electrochemical preparation of £PhCdBr 2] a n io n 3 A solution mixture consisting of 1.3g nPr^NBr, 10cm ■3 3 Me OH, 30cm CgHg and 3.0cm PhBr was electro lyzted for 6

hours using a current of 25mA and 30 volts. After the

electrolysis the product was isolated and analysed for

cadmium and halogen.

_ jCj) -Electrochemical preparation of [CgF^CdBr^] anion

- ■ • 3 A solution mixture consisting of 2.7g nPr^NBr, 8 cm 3 3 MeOH, 20cm and 2cm CgF^Br was electrolyzed for 5 _

hours. After the electrolysis the reaction mixture was

-filtered to isolate the product, which was washed with MeCN and dried in vacuo. Details of solution composition,

electrical condition etc. are given in Table 4.1 and the

analytical data in Table 4.2.

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T 3 O fa 'O O 0) > 4-> fa fa o .co 60 CO- oootNiftiocofloiflinfa fa 03 y-s.

CM CO fa CO g fa w* fa z fa o Mf o u ■U a o 5 co fa fa fa o fa —I fa u. CO f a 5-1 • w C 3 mmmmommmmmo fa O' NNNNClNCMNNNcn w CO CQ 0560 < «3 fa fa fa3 o > omoommooooo z > 'fa •jN CM CM CO CM CO CO CO CO CO f a CM fa a z 1 o I CM u 1 CM 5-1 | I I I CM I f a O CM CM I CM z CM I U CM'O -o fa 54 CM| 5-1 o ^ cm ca m o o o ca fa cMca fa ca fa "O "O cr> 0^*0 -o *o i—i ■o fa C ’O'Ouoxaauoo'uo o o O O to in -j

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TABLE 4 .2

ANALYTICAL RESULTS FOR SALTS OF ORGANODIHALOCADMAXE ( I I ) ANIONS •

Complex Found (X) C a lc u la te d (7.) anxon 3 . H alogen Cd H alo g en Cd

CH3CdBr2 33.5 2 3 .6 3 3 .8 2 3 .7 CH3C dI2" 4 5 .0 • 1 9 .8 ' 4 4 .8 '1 9 .8 C2H5CdBr2~ 3 2 .7 2 3 .2 3 2 .8 • 2 3 .3 C2H5C dI2- 4 3 .7 1 9 .2 4 3 .6 1 9 .3 n -C 4H9C dI2" 4 1 .5 4 1 .7 ------t- C 4HgCdBr2" 3 1 .0 3 1 .0 C6H5CdCl2" 1 5 .3 2 5 .2 1 5 .9 2 5 .2 CgH5CdBr2" 29.2 21.4 29.0 21.0 C6H5c d I 2- 4 0 .9 • 1 8 .1 4 1 .1 1 7 .9 CF3C dI^“ 2 5 .8 1 7 .7 2 5 .6 1 8 .0 CgF3CdBr2~ 42.6 17.6 41.8 1 8.5

As the tetra-n-proplyammonium salt in each case. The presence of tetra-n-proplyammonium cation was established by infrared-spectroscopy.

TABLE 4 .3

CONDUCTIVITY MEASUREMENTS

Corapound ■ -Conductivity

Ohmnu -1 cm 2 m ol „ , - l

(C3H7 ) 4NIMeCdBr2] ' 147 CC3H7 ) 4N[MeCdI2] 138 (C3H7 ) 4N[Et.CdBr2] 136 (C3H7 ) 4N[EtCdI2] 140

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TABLE 4 .4

GAS ANALYSIS FOR SOME ORGANODIHALQGADMATE(II) COMPLEXES '

A nion Wt of Compound Gas - measured a t NTP 3 in (g) r n cm • Found _ Calculated

MeCdI2” 0.201 8 .0 7 .9

E tC dB r2~ ' 0 .1 8 3 8 .4 8 .4

E tC d I2 0 .1 8 5 7 .0 ■. 1 7 .1 * t

c ^ 4.5 Results and discussion Electrochemically a series of tetra~n-propylammonium

salts of organodlhalocadmate (II) anions have been pre pared. These form a new group of organocadmium halide

been prepared, although Goggin, Goodfellow and H urst^^

have identified RHgX^ species (R = Me, Et,n-C 2Hy, n-C^K^ ;

X = Cl,Br, I, SCN) spectroscopically in various organic T17 1 solvents. Roder and Dehnicke7 have presented H NMR

evidence for the occurence of the following exchange

equilibria in the mixture obtained by dissolving

tetramethy 1 ammonium cyanate and thiocyanate in dimethyl-

cadmium (X = OCN, SGN) ,

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[CCH* 3) 2Cd*X]“ + ICCH 3) 2c

but they failed to isolate this particular anionic species.

The direct electrochemical synthesis which.leads to the formation of RCd^ salts with- each of the elements ::f ’

chlorine, bromine and iodine and with a variety of organic groups and the synthesis appears to be a. general one for

these~lkiions. Not-only are cadmlum-alkyl and aryl bonds

formed/in this way, but the perfluoro derivatives are also

accessible; .the anion [CF 3CdI2 ] appears to be the first

example of a crystalline compound containing.Cd-CF 3 b o n d . The tetra-n-propylammonium salts of the RCd^ anions

are m oisture-sensitive m aterials, and insoluble in the common organic solvents, which unfortunately prevented any

studies of their nmr spectra. The''presence of the tetra-n-

propylammonixm cation was established by infrared spectro- .

scopy for each compound prepared. ' The stoichiometry suggests a structural sim ilarity to

the mononuclear CdX^Y anion (X ^ Y = Cl,Br,I) reported 112 recently and shwon to involve three coordinate cadmium,

but a detailed structural investigation, probably at the

level of x-ray crystallography, w ill be required to inves

tigate this analogy. Indeed, the solubilities of the respective compounds suggest some structural differences,

since the salts of the perhalogeno species CdX3~ and Cd^Y-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. , are soluble in acetone/acetonitrile or m ethanol/acetonitrile r mixtures, whereas such solvents. could not he used for

recrystallizing the RCd^ salts. The general insolutbility in organic solvents contrasts

with the' appreciable solubility in the electrolyte solution,

# which may be due to the fomration of [RCcD^S} anion in the latter pr to association with halide ion to give

2— [RCdX^] ^ . Both these proposed species involved four coordinate cadmium, and a sim ilar increase in coordination number in the solid state by halide bridging, reported for

many cadmium complexes, may be responsible for the insolu-

'b ility of the crystalline material.

Molar Conductivity Despite the low solubility of the salts, it was possible

to dissolve sufficient material in acetonitrile to allow

determination of the molar conductivities of mM solutions. The results were given in Table 4.3. The normal range of

values for 1:1 electrolytes in acetonitrile is 120-160 —2 -1 118 ohm cm mol which agrees with the formulation of

the compounds in question as 1:1 electrolytes.in solution.

This conclusion is not however, unambiguous (_cf- - ref. 118) , and in- any case such results do nod*.establish the solid

state structure. The main conclusion must be that the

compounds are indeed salts' of organohalocadmate anions.

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Gas Analysis Results given in Table 4.4 shows that for each mole

of acetic acid, 1 mole of alkane gas was-evolved, which

is consistent with the following equation.

AcOH + iRCdX^'------> [AcOCdX^” + RE

w here AcOE = CE^COOH R = Me, Et,

- X - I, Br

Reaction Mechanism The reaction mechanism involved in the formation of

ERCdX^ has not been investigated, since it seems clear

that the formation of RCdX in the presence of excess ^

Pr^NX w ill readily give rise to Pr^N[RCdX 2.]. 113 The electrochemical oxidation of cadmium to give

■ RCdX has been discussed in the Chapter III, at which time it was reported that the electrochemical yield corresponds

to the reaction sequence shown in equations (4.1 - 4.io with

C athode

■ RX + e “ ------^ R‘ + X~ ...... 4 .1 Anode X" + Cd ------> CdX + e~...... 4.2

CdX + RX — ------* RCdX + X* / . . . . 4 .3

X' + Cd ------^ CdX. ' ...... 4 .4

Equations 4.3 and 4.4 providing a sequence of reactions to

explain the current efficiencies (r^2 mol cadmium dissolved/Faraday (cf. ref. 53} The stabilization of

RCdX by R4 NX

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. RCdX + R’ 4NX ------+ R*4N £RCdX23 then yields the observed product.

The electrochemical method presents a convenient, ' ' * simple and efficient method of preparing new organocadmium compounds in gram quantities.

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CHAPTER V

ELECTROCHEMICAL AND CHEMICAL SYNTHESIS OP ARYLCADMTUM COMPOUNDS CONTAINING 2 - CNYN-DIMETHYLAMIN0)M£IHYL GROUP :------T ------AT th E ar YL NttCLEtS------

5.1 Intro duction'

Several workers have prepared complexes containing alkyl

or aryl ligands which chelate to the metal via a substituent donor. In the present works (N, N- dime thy 1 amino) me thy lpheny 1

was selected since this ligand shows important coordinating

properties. Its stability can be attributed to the fact •

that the ligand is sterically bulky,, stereo chemically rigid,

and chelates' strongly. 119 V J_. G. Noltes et al. has reported the synthesis o'f 21 (N,N-dimeth.ylamino);meth.ylJphenylcopper as w ell as its 3

or 5-substituted methyl, methoxy and chloro-derivatives

by methathesis of the corresponding organolithium compounds with cuprous bromide, using diethyl ether as a solvent.

RT R’

A / ® 2 ' E t~ 0 NMe0 + CuBr ------* NMe^ 2 -20 :25 L i

R + L iB r

w here R R* = H R Me, Rf .= H

R MeO, R‘ - H

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l / - R -.C l, R* - H R - H, R’ *-Cl

The analogous 4 Qn, N- dime thy lamino) me th y l - sub s t i tu ted % copper compound was also synthesised, and found to be

insoluble in hydrocarbon and ether solvents.

» ■ OMe iMe

+ CuBr + L iB r

L i cr2- NMe, CH.

/ NMe2 The reaction of 2- (N, N- dime thy lanrino) phenyl lithium w ith cuprous bromide yields ether and hydrocarbon insoluble 120 2- CN, N-dimethylamino) phenylcopper.

+ CuBr + L iB r

L i

The reverse addition of both reagents Ci-e-> addition of

2- CN,N-dimethylamino)phenyllithium to a suspension of

cuprous bromide in ether) gives rise to the following pro d u c ts .

NMe., NMe.

F a s t -20 C + 3CuBr ------> * CuBr -j- LiBr. -2 L iB r; 2. h r s. 2

Slow; 25 NMe.

-LiBr; 22 hrs.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Lexis ink, Koten and Noltes have prepared 2-C.N, N-dimethylamino) 121 methyl ]pheny Is ilver by the reaction of 2- CN.N-dimethyl-'

amino) me thy lpheny 1 li thium with silver, bromide in diethyl-

ether, which affords an almost, colourless benzene soluble

compound. -

2NMe2 . ,CH2-NMe2 E t 20 AgBr g '+ L iB r - 6 0 ° , 0° 16 h r s

Bis 2 -[CN.N-dime thylamLno)me thy Ijphenylgold lithium was

syrffchesised by the reaction of 2 CN,N-dimethylamino)pheny 1-

— 122 lithium with gold bromide triphenylphospine adduct.

Cl) -PPh3; LiBr

NMe2 + BrAu.PPh^ E t 20 ; 25 £ .

L i

CH.

Noltes et al. have further reported a novel reaction of

Z M e ^ -C t^ , 2Me2N- and di and trimethoxy- substituted *

phenylcopper compounds with alkyl or aryl-tin bromide to

give exclusively the mixed Csubstituted phenyl) dime thy 1't in

b ro m id e s .

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Cu + R^SnBr 2

X ' X w here . .

R » Me, X =' Z - M e ^ - C ^ .

R - Me, X = 2-Me2N- '

R * Me, X » 4-Me0N.

R = Me, X - 2-MeO. V,R = Me, X = 2,6-(MeO)2 .

R - Me, X =» 2,4,6-(MeO)3.

R = P h, X = 2-Me 2NCH2 . T Work has also been reported on the synthetic •utility of'

N, N- dime thy lbenzylamine and its complex formation with 10/ some transition metals like chromium, .platinum and 125 126 palladium, titanium and vanadium.

The work reported in this chapter deals with the electrochemical synthesis, chemical synthesis and charac

terization of LCdBr, LCdMe, LCdPh and L2Cd compounds.

where.L is N,N-dime thy lbenzylamine *> 5.2 Experimental

All reactions were carried out in an atmosphere of dry

oxygen-free nitrogen. Solvents were carefully purified and

distilled under nitrogen before use.

In the electrochem ical prepa^aniQT^gf 2-£ CN,N-dimethyl-.

amino)methyl]phenylcadmium(II) bromide, o-bromo CN,N-dime thy 1-

benzylamine) was used as a starting reagent, which undergoes

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'oxidative insertion with cadmium, sim ilar to that of aryl

halide in the electrochemical cell. * Dr. Richard Steevenz is thanked for helping in the

preparation of starting m aterials, and for helpful discussions.

5.3 Synthesis of o-bromo .CN,N-dime thy lbenzylamine)

To a preheated (.50°) 40.0% aqueous C32g/55cm^ H«0) 127 1 70.0 mmol dime thy lamine 'solution was slowly added 20.0mmol C48.0g) dibromotoluene. The temperature was

increased to 70°C after 2 hours, the second portion of

40% aqueous 70.0 mmol dime thy lamine added, and the mixture

heated for a further 3.5 hours. The light greenish-yellow

organic layer was separated and the aqueous layer washed

several times with benzene. The combined benzene and

organic phase was dried over anhydrous sodium sulphate,

concentrated and finally distilled (b.p 60°G at Q.Imm) to

give a clear colourless liquid with an overall yield .of

3 4 .Og C78.0%). ’X

5.4 L ithiation of N,N- dime thy lbenzylamine

. In the chemical preparation of bis^2-[ CN, N-dime thy 1- amino) me thy l^phenyl^ cadmium, o-lithio-N ,N- dim ethylbenzyl-

amine- was used as one of the starting reagents.

N, N-dime thy laminome thy 1 substituted aryllithium

m aterial has been prepared via Li-H exchange reaction of 128 129 the corresponding arene with n-butyllithium . y 'I t was observed that the reaction of N, N- dime thy 1-

benzy lamine with n-butyllithium ( 1 : 1) r e s u l t s in a lm o st

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. quantitative lithiatro^ ( 90- 100%) at the ortho position ■

of dime thy lbenzy lam ine.

N, N-dimethylbenzylamine CIO mmol) was added at room

temperature to a. solution of 10 mmol of butyl lithium 3 (2M solution in hexane) in ether/hexane C50cm ). The

reaction mixture was stirred at room temperature until 79 Gilman test II for the presence of butyllithium was

n e g a tiv e .

5.5 Analytical techniques

Analysis for metal was by atomic"absorption spectro

photometry and for bromine by Vo Ihard titratio n as described

in Chapter III of this dissertation. • Proton NMR spectra were run in the solvent CDCl^', using TMS as internal standard (see Table 5.3). The infrared

spectra were recorded, •using Cesium iodide pellets (Table

5.2). The mass spectra of LCdPh. compound was recorded by

th e E.I. technique (Table 5.4).

5 .6 Preparative chemistry

Ca) Electrochemical preparation of LCdBr i.e. , 2-£(N, N-dimethylamino (methyl]phenylcadmium(II)

b ro m id e .

The reaction was. carried out in a vessel of twp arm

Cell B (Fig- 2.1). The solution, consisting of 2g of i o-bromo(N,N-dimethy.l)benzylamine, acetonitrile/benzene

CCa 9cm ; 2:1 V/V mixture) containing approximately lOmg **• of Et^NClO^, was poured into one of the arms of the H-Cell,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and after a series of evacuation/freezing cycles to remove

dissolved oxygen, the solution mixture was decanted into

the second arm where a piece of cadmium metal (anode) was - suspended on a thin platinum wife, which entered the

vessel through a "Teflon** plug. The cathode was a tungsten

wire fused into the side of electrochemical cell. The

cadmium was electrochem ically oxidized for 5 to 6 h o u rs

with an in itial current of 30mA. and 40 volts. The white

m aterial which formed in the cell was collected, dried in

'vacuo, and analysed for cadmium and bromine (Table 5.1) . In two separate experiments, 98mg and 75mg of cadmium was

lost from the anode and 186mg and 154mg of product obtained

representing yields of 65-70%. The product was shown to be LCdBr and the process

represents stoichiom etrically the electrochemical insertion

of cadmium into a cargon-bromine bond, a reaction previously

reported for a variety of alkyl apd aryl halides. T^ie

product is insoluble in acetonitrile, chloroform, benzene

and acetone, but shews, solubility- in warm pyridine.

(b) Preparation of- LCdMe and LCdPh

(I) 110 mg of LCdBr (approximately 0 .34 mmol) and an

^ 3 equimolar quantity of MeLi were refluxed in 5cm of dry

benzene diethyl ether (1:1 V/V mixture) under nitrogen for 3-4 hours with 'continuous mechanical stirring. The solu

tion was then filtered, which removed LiBr and the product

washed several times with diethyl ether. The product is-

: ------— ------:------/ ^ r ■ ------Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. -8 3 -

LCdMe (76mg, 85% yield) *diich is insoluble in acetone, acetonitrile or benzene at room temperature, but shows

solubility 'in pyridine' and is slightly soluble in hot

b e n z e n e .

Cii). • Sim ilar reaction with PhLi(Ca 0.35 mmol, 107mg 0.33 mmol LCdBr) yielded LCdPh (95mg, 89% yield) which has

1.e., MeCdL-and PhCdL were. analysed for cadmium (Table 5.1). 1 (c) Preparation -of 'Bis[2-(N^-dlmethylaminomethyl)phenylj- cadmium - i "

A sample^o^O .9g C5.0 mmol) solid anhydrous CdClp^was

slowly added \(in about 1.5 hours) at room tanperature to a

freshly prepared suspension of 2- [(dimethylaminomethy 1) p h e n y l] 3 lith iu m (10 mmol) in 60cm of ether. The mixture was then left stirring for an additional

3 hours at which time the reaction mixture showed a very light grey suspension. The insoluble m aterial was filtered

off and the organic solution concentrated to a yellow oil. 3 The viscous o il was redissolved in 10cm benzene and the

murky solution refiltered and concentrated again to a thick yellow liquid. The resulting oil was treated dropwise with

5-10cm petroleum ether (b.p 30-60°) and immediately cooled

by rapid evaporation .of a small portion of solvent in vacuo.' This afforded white crystalline particles which were ^

filtered off and washed with cold petroleum ether solvent

and then dried in vacuo. An average yield of 0.3g (16.0%)

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» of the organocadmium compound was obtained.

(ii) Comments

.In the above reaction solid CdCl z.0 was added to the organolithium suspension affording the desired I^Cd and

LiCl. The total removal of LiCl was effectively accom

plished by the addition of benzene and subsequent filtrationl

The reverse addition produces a less selective reaction, as indicated by isolation of many unidentified by-products

■which caused difficulty in the isolation and purification

of the L^Cd. In the primary studies, CdB^ was used rather

■than the corresponding CdC^. The final I^Cd product was ' obtained, but was contaminated with trace amounts of LiBr.

It appeared that LiBr is slightly soluble in most organic

solvents especially in benzene. During the course of the reaction, pieces of suspended grey m aterial were observed

in solution (presumably cadmium metal) . This; was confirmed by analysis, when it was noted that the insoluble by-products contained 37.0% cadmium. The white crystalline m aterial

L2Cd, readily decomposed in. air as observed in NMR and

infrared, spectroscopy.

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A nalytical results for LCdBr, LCdMe, LCdPh & L^Cd Compounds

i Compounds Analysis (Found (Calcd) (%)

ftadmi tttti H alo g en

LCdBr 3 4 .7 2 4 .6 * (3 4 .4 ) (2 4 .5 )

LCdMe 4 1 .9 _ _ _ _ (4 2 -9)

LCdPh 3 4 .9 (3 4 .7 5 ) L«Cd 29.0 z ■ (2 9 .5 )

where L = N, N' - dime thy lb' enzy-lamine

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Infrared spectra of LCdBr, LCdMe ( LCdPh, & I^Cd Compounds Cln cm )

Compound IR Vibration Cin cm~^)

LCdBr 455 On)'/ 625 (m) , 675 Cm), -800-820Cst) , .

1025 Cst) , 1100Cst), 14600n.br)

2830(w), 2950Cm).

LCdMe 455 (w), 520-540(m.br) , 625 fct) ,

6 8 0 Cm), 8 0 0 -8 2 5 C s t) ,

1100-Cst)", 1410 Cm), 1460 Cm), .

2820 Cv), 2950 Cm) .

LCdPh 410 Cw)., 460 Cm) , 680 Cm) , 800-830 Cst) ", lQ20Cst), 1100 Cst) ,

1450Cm), 1600Cw), 283aCw) ,

2990 Cm).

L2Cd . - ' 4 1 0 Cm), 670(m ), 8 2 0 -8 3 0 C st) , - 1020 C st), 1 1 0 0 C st), 1450 Cm), * 2835Cw), 2835Cw), 2980Cm). *

where m = medium, st - strong, w = weak, m.br - medium broad

■a

•

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TABLE 5J3

^H.NMR Spectra in (ppm)

Compound CCH3>2N -CH2N, -CH2-Ph Aromatic

, ■ " > N, N-dime thy lbenzy lamine 2.22 (s ) 3 .3 9 Cs) ' 7 .2 7 (m)

< Bis 2-(N,N-dimethy1- 1 .2 (s ) 3 .4 5 (s ) 7 . 0- 8.0 (m) aminomethyl) pheny 1- cadmium - *"

Run. in CdCl^ solvent, reported in ppm (s). downfield from internal TMS, s = singlet, m = multiplet

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Mass Spectrum of 2- [ (N. N-dime thy lflTTn-no)ine thy Ijpheny lcadmiuni(II) - - ■ pfrgpyj^ ' “

Electron impact excitation ion source * 190°

lis t t&reshold «* 0.10% relative abundance

. m /e / ^ intensity provisional assignment Cmonopositive species) \ ^ 42.06 892 c 2H4 N

4 3 .0 6 1162 C ^ N ■

5 8 .0 9 4416 - W

76.09 90 ■ , G6E4 77.09 230

9 0 .1 2 . 551 ■ C7H7 • 4 9.1.12 • 690 A

109.15 221 -

110.18 . 86 ' 1 1 1 .1 8 313

1 1 2 .1 8 •65 Cd 113.21 83 v

. 2^132.18 c 9h 12n

- 133.15 79 • > 134.18 283 CgH13N

** .

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m /e i n t e n s i t y provisional positive s p e c ie s

- 189 • - . 16

190 14 C6Hs Cd . • ■ 1 9 1 .3 1 • 68 v .

212.15 578

213.15 1216 ' C?H4NCd 214.15 622

215.15 1220 ' •

247 34 11 CgH1QNCd 2 4 8 .4 3 ' 8 2 4 9 .4 3 6 • * 0 265.46 io- Cl 2HlQ Cd 2 67 .46 9 •

3 2 2 .4 9 25 . C15H17NCd 3 23 .59 20 Cmol^.cular ion)

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5.7 . Results *and Discussion — • > -- The electrochemical, reduction of o-bromo N,N-dimethyl-

benzyl amine » sim ilar to that of alkyl or aryl halide^^’^^ in the presence of anodic cadmium metal gives a crystalline

product whose analysis CTable 5.1) showed that the product

N,N-dime thy lbenzy lam ine-Cd^ bromide has been formed.

The collection of spectral data and other date which could

have given information about the structure of-this complex was hampered by the insolubility in hydrocarbon and some . ♦ polar, solvent like acetonitrile, chloroform and acetone. The insolubility o f phenyl cadmium bromide has .been used

by several authors as an argument for polymeric structure.

This m aterial which analysed as LCdBr might have a four

coordinate dimeric structure as shown below.

Phenyl or methyl groups bonded to lithium readily exchange

$with metal-hailide bonds; Noltes et a l.^ ^ have prepared phenyl

copper from the reaction of PhLi with cuprous bromide and

have also reported that the^eaction of PhLi with

gives diphenyl dime thy It in as the major product 88 %. T h is

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method was successfully applied to prepare LCdPh and LCdMe

in quantitative yield by the reaction of LCdBr and PhLi or

MeLi respectively. - The structural information on these two compounds is

unclear at this moment since no conclusive molecular weight data could be obtained. It is noteworthy that LCdMe-and-

' ■ # . * ■ LCdPh compounds dissolve in strong coordinating solvents

such as pyridine^- which suggests that the polymeric struc £

ture breaks down as a result of coordination of the

pyridine to cadmium, i.e ., the aryl or alkyl group change * * * , * * from an unsaturated coordinate bonding'into a saturated

coordination.

•NMe

Noltes et al. have proposed a coordinated polymeric structure

120 * f o r 2 (dimethylamino)phenylcopper, ther-J&- proposal based

on the data .for insoluble and. presumably polymeric dime thy 1-

amino substituted phenylcopper. ■•jit .

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Recently, the x-ray structural determination of the aryl- 130 1 31 •. - ^ copper compound ’ shows’ that each pheynl group bridges"' ;..-'

two copper atoms by two-electron three-centre bonds.

The reaction of 2-[(dimethylamino)methyl]phenyllithiuni

with anhydrous CdC^ in diethylether at room temperature resulted in the formation of an chloroform'soluble white

pro duct b is ^2- £(N, N-dimethylamino) me thy lj phenyl^ cadmium,

which was isllated in quantitative yields Elemental analysis

(Table 5.1) of the solid isolated from a series of prepara

tions confirmed the composition CdC^gH^^N^- Four * coordinate

I^Cd is less stable than the corresponding 132 133 122 organosilver, copper and gold compounds..

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N.M.R. Studies The ^ NMR spectrum of bis ^2— (N,N-dimkthylamino-

methyl) phenylj cadmium, taken at ambient temperature in'

■solvent CDClo using TMS internal standard, shows die . 3 .. ♦. - • • •«*«& expected resonance pattern (Table 5 .-3). The spectrum of

pure N, N- dime thy lbenzy lamine consisted of three sharp

singlets at S 2.22 (NMe2) , S 3.39 (CH 2-N) and at S 7.27 due

to aromatic protons. Comlexation with cadmium results in

a substantial upfield'shift for (NMe2) proton, at S 1.2, a"

downfiled- shift -for (-CE^N) at S 3.45, and a m ultiplet ,

between S 7.0-8.0 du^to aromatic protons. . The chemical

shift is in a sim ilar range to that of the analogous organo-

silver, copper and gold compounds.

V ibrational Analysis for LCdBr, LCdPh, LCdMe and L^Cd

Th/I IR spectra of these compounds are almost

identical, which revealed that exclusively 2-m etallated " 128" benzylamine was present, ( 1 , 2 substitution pattern of

the axmoatic ring in the 830-675 cm ^ region). The characteristic absorption between 820-840 cm ^ is" ' 129 diagnostic, for Aryl-CH. 2~NMe2 group, and the band at

2830 cm ^ (C-H symm stretch)-is typical fo.r the CH., group

in aliphatic NMe2. ~

Mass spectral studies on LCdPh

The El mass spectrum of LCdPh was recorded at 190°C

■probe temperature (Table 5.4). The main feature of this

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spectrum is the weak molecular ion peak M at 323 m/e for NCd,and a base peak due to the substituted nitrogen .

donor ligand (C^H^N) 'at m/e 58. (relative intensity of 100%)-

other common fragments of high relative : abundance like.

CgH^, C5H5 > G7H7 > C9H-£2N>*C 9H;l3N 311(1 CyH^NCd, suggests and 'supports the formulation of LCdPh:

5 .8 Conclusions -.Although the electrochem ical (LCdBr) and conventional

chemical (LCdMe, LCdPh, L^Cd) methods of synthesis are

proved to be very successful, little is known about the ' structure of these compounds.

Further investigation by x-ray diffraction, together 1 13 " with H and' C spectroscopy w ill provide sufficient data

to draw-conclusions on the configuration of the substituted-

aryl group(s) around the cadmium metal.

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CHAPTER VI

THE DIRECT ELECTROCHEMICAL SYNTHESIS OF NStrtRAL 'And a n io n ic organozinc h a l id £ 5 ~

6,1. Introduetion History: the element zinc,, the..second member of group

lib of the periodic table (At Wt 65.38, 5 * 30) is a

bluish-white lustrous metal, b rittle at ordinary temperature,

but malleable at 100° to 150°C and a fairly good conductor

of electricity. M etallic zinc was produced in the 13th century A.D. in .

India by reducing calamine .with organic substances, an<£.the

metal was later rediscovered in Europe by Marggrof in 1746,

who* showed that the metal could be obtained by reducing 134 ' calamine with charcoal. The principal ores of zinc are ' sphaliriteor blende Csuplhide) , Smithsonite (carbonate) ,

calamine (silicate), and . franklinite '*£zinc, manganese, iron

o x id e ) . Zinc can be obtained by roasting- its ores to form the

oxide or by reducing the oxide with coal or carbon, with

subsequent. distillation of metal. "T *\f In the electrolytic process, the roasted 'concentrate

of crude zinc oxide is dissolved in dilute sulphuric acid,

and the solution then electrolysed in banks of cells with

cathodes of aluminium and anodes of lead alloyed with a

little silver. Oxygen is liberated at the latter and the

.zinc of at least 99.95% purity deposits on the cathode.

Further purification can be effected by re-electrolysis and

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the metal is commercially available.in at least 99.999%

p u r i t y . ’ Like the other members of group IIB, zinc forms many V compounds ' in the +2 oxidation state. This chapter w ill

deal mainly with the organometallic chemistry of zinc in its oxidation state of +2. As noted in Chapter III, the

organic compounds of group IIB elements, shows a most striking

gradation of properties and chemical reactivity, running

parallel to the electronegativity of the elements.

The organic derivatives of zinc, cadmium and mercury

■are covalent compounds of either linear, planar or tetrahed-

■ ral structures. '79 These elements do not have sufficient

tendency to increase their covalency above the group valency

-'..of two, to give rise to many compounds with electron deficient, structures.- The decrease of reactivity with

increasing electronegativity in the sequence zinc>cadmium^

mercury is illustrated by the behaviour of the lower alkyls

towards water; the dialkyls ojf zinc are hydrolysed with explosive violence, those of cadmium slowly and those of

mercury, not at all. 4

6 .2 Synthesis of organozinc compounds

Literature survey ' Organozinc compounds are historically important, since

they were the first organometallic compounds . to be prepared 79 by Frankland in 1849. Moreover, ethylzinc iodide and

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diethylzinc were among the earliest organometallies'to be

studied, (Zeise’s salt was described in 1827 and Cacodyl v - \ • • in 1842) and were certainly the first to achieve importance

as chemical reagents. Due to their low reactivity towards

/•certain organic functional groups, organozinc compounds have

' unique synthetic potentialities.

a. Preparation of organozinc- compounds 135 The dialkyl‘of zinc may be prepared by the thermal

disproportionation. of alkylzinc chloride (this was Frankland's ' K ■ original method) . The alkylzinc chlorides themselves are

obtained by direct reaction between alkyl iodide and zinc

or zinc/copper couple. Zn/Cu + RBr/RI ------RZnX 110a t > ZnB^ + ZnX2>

(R - Me, Et, n-Pr, n-Bu) Improvements on the original method liave mainly been Carried

out by the use of improved Zn/Cu.couple (zi^c dust and dry II copper citrate), and the use of high boiling ethers as , ■ 136 s o l v e n ts . 137 ^ —^-Abraham and Rolff have reported that neither ethyl

bromide nor ethyl chloride react with Zn/Cu and hence studies

in these cases are limited to the action of zinc bromide or

zinc chloride on diethylzinc.

Some workers 138 ’ 139• have reported that methyl bromide

reacts with zinc dust in specific solvents like N;N-dimethy 1- formamide, tetrahydrofuran, dimethylsulphoxide or glyme to

form methylzinc bromide.

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Zn(dust) + MeBr ------MeZnBr.

The alkyls may also be'prepared, and the diary Is are most conveniently obtained by the reaction of zinc metal with an 140 oxganomercury compound in boiling xylene solution.

I^Hg + Zn —:----- ^ 2 n + Hg.

CR Me, i-B u , pCgH^F) . or by the reaction of anhydrous zinc chloride Car b ro m id e) 141 'w ith an organolithium' reagent in ether.

Zn^ + 2RLi ------> R2Zn + 2LiX

R - P h , C 6P5 . Solutions of dialky Is, diaryls or organozinc halides may be

obtained by the addition of anhydrous zinc chloride in 142 ether to a Grignard reagent.

ZnCl 2 + RMgX e- - e-^RZnCl + MgXCl.

CR = E t, Vinyl, t-bu) . Zincalkyls were prepared on large scale by the reaction

between trimethylaluminum.and anhydrous zinc chloride, with

a mineral o il .free from oxygen or sulphur compounds being 143 used to moderate the reaction. r 3Me3A l + Z nC l 2 ------> 2Me2A lC l + Me2Zn. 144 Heptafluoropropylzinc iodide has been prepared by the

reaction between zinc dust and. heptafluoropropyl iodide in

ether,*'tetrahydropyran, dioxane*, or 1 , 2-dimethoxyethane.

C3F7I + Zn ------* C 3F7ZnI + C 3F6 + C6F14 + C3F?H.

J./ Lorberth has reported the formation of biscyclopentadienyl

zinc by reaction of bisC l,l»l, 3 , 3 , 3 ,-hexamethyldisilazane) 145 with cyclopentadiene in ether at room temperature.

with permission of the copyright owner. Further reproduction prohibited without permission. -9 9 -

ZnlNCSi-Me 3) 2J 2 + 2CpH r a g ^ 3 . Cp2Zn + 2HHCS1Mb 3) 2 * Murdoch, and KlaBun.de have prepared a highly reactive zinc slurry by cocondensation of the metal vapour- and excess o ' ' 146 solvent at 77 si, followed by "warming to ambient temperature.

This slurry was reacted with alkyl halides in all types of both polar and nonpolar solvents, and the appropriate di-

alkylzinc compound was Isolated. Diglyme and dioxane appeared

to be the best solvents, but toluene and hexane were also u s e d .’

Crystalline dipentadienylzinc-tetrahydrofuranate has_

been prep are d^^ by the reaction of pentadienylpotas-sium with

zinc chloride in THF at low temperature.

- 20 ° 1 ’ 2KCcH, + ZnCl 0 —---- > ZnCC.HO-THF + 2KC1. 5 7 THF 5 7 2 Dipen tadienylzinc-tetrahydrofuranate is thermally unstable

and decomposes at 10°C into m etallic zin:c and a mixture of

o l e f i n s . _ 148 5is.CdiphenylmethylCzinc.has been prepared by reac

ting diphenylme thy Is odium as its dioxanate with zinc chloride

in THF. The complex so formed is thermally unstable and

decomposes within a few hours at room temperature into

m etallic zinc and tetraphenylethane.

Fh 2CHNa.C4Hg02 + Z nC l 2 . 7117 > ;lF h 2CE]22n.THF + NaCl.

6.3 Co-ordination complexes with ligands not. containing acidic hydrogen - ■* The' dialkylzinc compounds form relatively unstable

complexes with simple ethers; for example the dimethyl ether .

• - com plex Me’ 2ZnOMe 2 has a boiling point ^47°) sim ilar to that -

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of dime thy Iz in c (44°) . Cyclic ether's and chelating diethers

form rather more stable complexes (cf. ref. 79) i The

bistetrahydrofuran complex was prepared conveniently by

boiling zinc turnings (.containing 10% copper) with methyl

iodide in tetrahydrofuran. Unlike dime thy lzinc it: does

not catch fire in the air - and. has- a boiling point, of 83°. ^

Additioii of TMED to a freshly prepared mixture of ZnEt 2

/ ( ' and Za^ in^THF gave the adduct Zn^TMED, but addition to the same solution after a week at room temperature gave 137 four coordinate EtZnI.TMED. This result is also in agree ment with those of other w o rk e rs;,150,151 ^ ^^enticai

m aterial was also prepared using an ethereal solution of

EtZnI and TMED. ' (Me^SiCE^) qZtx., f produced either by a Grignard

reaction or from MegSiCI^X and Zn-Cu, forms adducts with 152 TMED, quin, py, bipy and phen.

The most interesting four-coordinate complexes formed

by organozinc compounds are those containing 2 , 2 ’-bipyridine

as a bidentate ligand. Many of these complexes have bright

colours (cf. ref. 79) which are ascribed to charge transfer

transitions.

6.4 Some chemical properties of organozinc compounds

Organozinc compounds are very sensitive to moisture,

the more volatile of them being spontaneously inflammable,

while others fume strongly in a ir..

Compounds with reactive C-H, Sn-H, 0-H, S-H, N-H, P-H

etc. bonds react readily with Znl^ according to the scheme

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below, giving RR.

PhC = CH -*• PhC=CZnR

Phg'SnH ^ (Ph^Sn) 22xi

/ HAcac RZnCacac)

K1QH.. ^ RZnOR'.

R'SK ^ R Z nSR '. r : 2nh —» RZnNR'2 ; Zn(NR '2) 2

r «2ph ------> RZnPR'2; Zn(PR*2)2 -

6 .5 Application in Organic Synthesis

• The chemical reactions -of organozinc compounds are

generally sim ilar to those:'of magnesium alkyls- and Grignard

reagents, the essential difference being in the ■ considerably

lower reactivity of the former. The relative low rate of reaction with cyanides, isocyanates, esters and ketones allow these groups to remain unaffected in 'reaction involving

some others and more, active functional group such as -C0C1. Combine acid chlorides rapidly with organozinc compounds,

a reaction which has been applied to the synthesis of ketones. 79

0 I! R0C1 + R l^?n Cor R’ZnCl) - RCR’ .

a. The Reformatsky reaction

The Reformatsky reagents which continues to find numerous applications in synthetic organic ehcmistry, is formed from

OC haloester and zinc. This intermediate has been, used in

the Reformatsky synthesis by reaction with aldehyde or a

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k e to n e . 153 &

ClCH^COOEt -KZn -me'W 1.al> ClZnCH^COEt

0 “OH 0 II I . - II. Cl) RCMe R-C-CH„-C-OEt. C2) H20 CH.

Sim ilarly, ^-hydroxyester can be prepared by means of the

Reformatsky reaction.

0 0 - OH . 0 II 1 ' II RCH + BrCH2COEt + Zn RCH-O^-COEt.' Upon dehydration Coften just by heating) ^-hydroxyesters ■r revert to the <£,^ unsaturated acids.

OH R I I 0 R-CH-C-C-OEt h e a t R-CH—CH-C-OEt T C-C-OI -VL^S~ I II*I K II H O O

A modification of the Reformatsky reaction, involving the

use of .*zinc-copper instead of zinc powder alone, was used to prepare kavain and dihydrokavain in much higher yields 154 than reported earlier OMe OMe RCHO B r CH2 C=CH COOE t Z n/C u, CgHg

K av in ; R = PhCH=CH-

Dihydrokavin; R = PhCH 2-CR2-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. » * _ A series of twelve OC-ketonosulphides was prepared by reacting RCOCR,R"ZnBr w ith R"*SC1.

RCOCR* R”B r + R "’ SCI — : — ------> RCOCR’ R"SR'

b . StTnrnons-Smith reaction / 156 Io dome thy lziac iodide is obtained in ether solution,

by the reaction between methylene iodide and zinc/copper, and

relatively recently it was found that when the reaction was

carried out in the presence of olefins, cyclopropanes are 157 ' " obtained in good yield. This reaction has been named'the

Simmons-Smith reaction. .-•V V c" H. CH2I 2 + Zn/Cu £ther. ICH2ZnI > 2 + Cu

, A mixture of cyclopropylether and allylic ether was formed

when 1-methoxy-and 1-ethoxycyclohexene were reacted with 158 zinc-copper couple and methylene iodide.

+ Zn/Cu *f C H ^ ------>

CR “ OMe, OEt) .

c. As a catalyst in polymerization

As increasingly important industrial application of

organozinc compounds is in the production of polymerization 159 catalysts. Thus Et2Zn complexes with various Lewis bases catalyse the formation of polyalkaneoxide from epoxides,

and the products from the reaction of ZnEt 2 with TlCl^

j ■

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. promote the polymerization of vinylmonomers, propene, butadiene, and isoprene. Anionic complexes such as CaZnEt^ are useful

in the'preparation of polyacrylate, polystyrene and poly-

acrylionitrile.

6 .6 Electrochemical preparation of neutral and anionic organozine halides The reaction of m etallic zinc or zinc/copper with alkyl

iodide to yield alkylzinc iodide is one of the simplest'

classical routes to an organometallic compound. r~ According to'the literature neither ethyl bromide, ethyl chloride nor any phenyl halides reacts with Zn/Cu couple 137 to form the corresponding organozinc halide insertion product,

and the usual method involves the reaction of anhydrous

zinc halide with the appropriate Grignard reagent.

Following the work on the direct synthesis of neuftral and anionic organocadmium halides described in Chapters

III and IV of this dissertation, this par.t of the dissertation deals with the electrochemical oxidation of zinc metal

in the presence of alkyl or aryl halides to give the corres p o n d in g RZnX compounds, which are conveniently prepared and

stabilised as the addition compounds with 2 , 2 ’-bipyridine

Cbipy) . • A lternatively, oxidation in the presence of EX and

R^NX gives the salts R'^NtRZn^.], which are the first •-

examples of dihalogeno-organozincate(II) anions.

-S’ ^

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Experimental '

6.7 General

The electrochemical synthesis was carried out in a

two arm ed.cell CFig. 2 . 2) and the detailed description of

the cell has been already given in Chapter II. Zinc metal (m4N5) (Alfa inorganic) was used in the form of discs,

approximately 1 x 1 x 0 . 2cm, obtained by hammering pieces of zinc rod. The'purification of solvents, alkyl and aryl-

halides were as described in Chapter II. Tetra-n-propyl- > ammonium halides (Eastman organic chemcials) were, used V; * without any further purification, and thev' spectroscopic

investigation of product m aterials followed techniques described in the previous chapters of this dissertation.

All experiments were carried out in an atmosphere of dry

nitrogen. ’ The applied voltage was 10-50*volts, as dictated

By the solution condLction, given that a current of 20-30mA produced a reasonable rate of reaction at room temperature.

The composition of solution phase, electrochemical conditions,

time of electrolysis and the yields are given in Table 6.1

for neutral RZnX.bipy compounds, and in Table 6.2 for .the

tetrapropylammonium salts of [RZn^] anions.

Isolation of product In the case of 2,2'-bipyridine adducts of RZnX, the

solution resulting from the electrochemical oxidation was

filtered, and petroleum ether added dropwise to the filtrate.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The yellow precipitate which formed was washed several times

with benzene to remove excess bipy and then dried in vacuo. In the anionic complexes, addition of diethyl .ether to

the filtrate precipitated both the product and excess

tetrapropylammonium bromide or. iodide, but the latter was ♦ removed by successive washing with dry chloroform. In the

cases of chlorides, only the product was precipitated by

diethyl ether, 'and this, solid was washed.with-petroleum ether and dried.

A n a ly s is

Analysis for zinc, was by atomic absorption spectro

photometry, and for halogen by Volhard titration;, the

results are given .in Tables 6.3 and 6.4.

« ■ ' 6.8 Preparative chemistry of neutral organozinc halides

(a) Electrochemical preparation ’of HeZnI .bipy - •

Electrochemical insertion.of anodic zinc takes place

• ■ • 3 readily in methyl iodide, when a mixture consisting of 2.5cm ■7 3 M el, 0 . 8 gm 2, 2* -bipyridine, .'15cm acetonitrile mixture ' (2:1 ratio) and 5 mg of Et^NClO^ was electrolyzed for 6 h o u rs * On filtering the cell contents treating the filtrate with

petroleum either, a yellow product precipitated out and was

collected,, washed.-with benzene, to remove excess 2 , 2 ’b i p y r i - dine and dried in vacuo.

(b) Electrochemical preparation of EtZnI.bipy

Electrochemical oxidation of zinc metal occurred in a 3 3 solution mixture consisting of 2.5cm ethyl iodide, 15cm_J

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of acetonitrile/benzene, 0 . 8 gm 2 , 2 'bipyridine and 8mg o f

Et^NClO^. A fter 6 hours of electrolysis, using.a current

of 25mA. and 40 volts, the product Cpale yellow in colour)

was isolated, and analysed for metal and halogen.

. # ' • Cc) Electrochemical preparation of EtZnBr.hipy\

— : -■ . •' ‘ 3~ ‘ • ^ 3 A solution mixture consisting of 2.5cm- EtBr. 15cm

acetonitrile/benzene in 2:1 V / ^ 0 . 8 gm o f 2 , 2 ’-bipyridine

an d 8mg of Et^NClO^ was electrolyzed for & hours using _ a current of 25mA and 50 vgj^fcs . ' After the electrolysis

the pale yellow product was isolated, dried and analyzed^

for zinc and haiogen.

'(d) - Electrochem ical preparation' of. CF^Znl.bipy

Electrochemical preparation of CF^Znl.h'ipy was carried

out in two armed cell (Fig. 2.2). The preparative method 3 ' followed the method described in Chap^r II. CF^I. CZ'.5cm-)• was distilled into one'of the arms of the. cell containing a 3 mixture of 0.8gm 2 , V - b i p y r i d i n e , 15 cm o f b e n z e n e /a c e to n i-

£3)le 0-:2 v/v) and 9mg of Et^NClO^. After degassing the

solution .in vacuo by a series of evacuation/freezing cycles,

the degassed solution was flushed with dry nitrogen and

then decanted into second arm of the cell which s/rved as the electrochemical cell. After the electrolysis the

reaction mixture was filtered and petroleum ether added

slowly to it; the brown precipitate which formed was

washed several times with benzene to remove excess non

coordinated 2 , 2 *-bipyridine J^id then dried in vacuo.

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- ( Details of solution! compositioi^*- electrical conditions are

given in Table'6.1. ’ ’ X

(e) Electrochemical preparation of HC^CH-Zn-Br.b-ipy

A mixture consisting of 15 mg Et^NClO^, 0.8gm 2,2*- 3 3 ■ bipyridine, 2.5cm vinyl bromide and 15cm of acetonitrile/

benzene was electro lyzed for 3 hours,' using a voltage, of.

^30 volts and maintaining a current of'30mA.. The brownish:

yellow product was precipitated after the addition of pet

ether, washed with benzene and dried in vacuo.

Cf) Electrochemical preparation of PhZnCl.bipy.. 3 A mixture consisting of 15cm of^benzene/acetonitrile * 3 in 1:2 ratio, 0.8g 2,2-bipyridine, 3.0cm of chlorobenzene and lOmg of Et^NClO^ was electrolyzed using a current of

^ 25mA f o r 6 hours. After the electrolysis the reaction-

mixture was filtered and petroleum ether added dropwise.

The greenish yellow precipitate which formed was washed ■ .9 several times with benzene to remove free 2 , 2 r-b^pyridine

andrn^'ied in vacuo.

Sg) Electrochemical preparation of PhZnX.bipy (where X - Br, I) "

Electrochemical oxidation of anodic zinc in the solu

tion mixture consisting of bromobenzene-or iodobenzene 3 3 C3.0cm ) 0 . 8 g of 2,2'-bipyridine, 15cm of acetonitrile/

benzene in a ratio of 2:1 and 15mg of Et^NClO^ was carried

o u t f o r 6 hours. After.-_.the electrolysis the reaction

mixture was filtered and treated with petroleum ether.

- - Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■ - -W 9- < . •. V.

?' ' • S’) In both cases the yellow product was wahsed several times

with benzene to remove excess 2 , 2 ’-bipyridine and then dried in vacuo. "Details of solution composition, electrical

condition etc. are given in Table 6.1 and analytical

in.Table 6.3. '

# . ~Ch) Electrochemical preparation of CgF^ZnBr.bipy 3 . .A solution mixture consisting of 3.0cm of pehtafluoro-

•■'3 bromobenzene, 15cm benzene/acetonitrile in 1:1 ratio by volume, 0.8 g 2,2’-bipyridine and lOmg of Et^NClO^ was

electrolyzed under conditions stated in Table 6.1. The

isolation and subsequent treatment of product were as for

FhZnI.bipy.

Ci)' Electrochemical preparation of PfcQL,ZnX.bipy " Cwhe^e X = Cl, Br)

Zinc metal Can ode) was electro lyzed in a -solution 5* 3 3 consisting, of 2 .bcm benzyl bromide or benzyl chloride, 16cm

acetonitrile/benzene CV :1 v / v l , 0 . 8 g 2 , 2 ’-bipyridine and

15mg Et^NClO^ under the stated conditions CTable 6.1). After the electrolysis the reaction mixture was treated as described above. The products were dried in vacuo and analyzed

for zinc, and halogen CTable 6.3) . The identified products

PhCH^ZnBr.bipy and PhCI^ZnCl.bipy are pale yellow and yellow

in colour, and^insoluble in-most organic solvents.

■‘.X : . Cj).Electrochemical preparation of PhCHpZnl.bipy

A solution of Q/. 8 g 2,2’ -bipyridine,. l.Og of benzyl 3 iodide, 15mg Et^NC10^_ was prepared in 13cm of acetonirrile/

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. -no-

benzene and electrolysed for 6 hours at an initial current of < * 25mA C40 volts). After this period the reaction mixture

was filtered and then treated with petroleum, ether. The orange-yellow precipitate which formed was wahsed several

times with benzene to remove excess free 2 , 2 '-bipyridine

and then dried in vacuo.

6 .9 Preparative chemistry of anionic organozinc halides

(a) Electrochpnn cal preparation of [MeZnl^J" anion

Electrochemical oxidation of anodic zinc in the solu tion mixture consisting of 1.5g of tetra-n-proply iodide, 3 / 3 3 1.5cm methyl iodide, 10cm acetonitrile- and 3cm benzene

was carried out for 3 hours,, using an in itial current of

25mA (20* volts) . In some experiments the solid was depo

sited at the anode and adhered to the surface, thus_causing

the current to drop gradually .to zero when no more metal

was dissolved. This could be overcome by repeated cleaning of the surface of anode or by scratching of the deposited

p r o d u c t.

After the electrolysis the reaction-mixture was filtered

and treated .with diethyl ether cSropwise. This precipitated

both the product-and unreacted tetrapropylammonium iodide;

the latter was removed by successive washing with dry

chloroform and the product dried in vacuo. The pale yellow

^material was analysed for zinc and iodine CTable 6.4).

(b) Electrochemical preparation of [EtZn^J ~ anion

A mixture consisting of 1.5g tetra-n-proply ammonium 3 ' 3 3 iodide, 12cm EtI, 10cm- acetonitrile and 3cm benzene was

-—:— ’ ~-- & Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. - 111-

electro lyzed -using electrochemical cell B (Jig. 2,2). After four hours of electrolysis with an in itial current of "25mA

and 20 volts, the light brown solution resulting from

electrolysis was, filtered and treated with diethyl ether.

The solid which .precipitated was washed thoroughly with ^^chloroform to dissolve excess tetra-n-propl iodide. The.

• pale yellow solid, which is ins’oluble in chloroform was

characterized as [nPr^lQCEtZn^J , slightly soluble in ;

acetonitrile but insoluble in hydrocarbons .

Cc) Electrochp-nvi cal preparation of CF^Znlp anion

CF^I (2.5 cm ) was distilled into one of the arm of

the K-type of cell containing a mixture of 1.5g tetra-n- 3 3 propylammonium iodide, :10cm acetonitrile and 5cm benzene.'

After degassing the solution it was flushed with dry n it

rogen and transfered into the second arm of the cell which

served as the electrochemical cell. Details of the electf- rical conditions are given in Table' 6.2. The isolation of

the product is essentially identical to that described for

iMeZn^j” and fEtZn^]- anionic complexes.

(d) Electrochemical preparation of' EtZnB^ ■ anion y Electrochemical oxidation of zinc metal in a solution mixture stated in .Table 6.2 was carried out for 4 hours,

using in itial current of 25mA (.30 volts) . The yellowish

brown solution resulting from the electrolysis- was filtered and treated with diethyl ether which precipitated both the

produfct and unreacted tetra-n-propylammonium bromide. The

■** ^ • r xtfs

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. latter was removed by successive washing with dry chloroform and dried in vacuo. The product was analysed for zinc and

bromine CTable 6.4). The 7. yield of the product on the

basis of metal dissolved during electrolysis are given in

Table. 6.4.

* a Ce) Electrochemical preparation of PhZnX^ anions

Electrochemically zinc metal was oxidized in a solution mixture R etails of solution composition are given in Table

6 .2 for the period of five to six hours at an-initial

current of 30mA. After the eldctrolysis the reaction

mixture was filtered and treated with diethyl ether. The .

isolation of the product was identical to that described

above for [EtZn^J and tEtZnB^J anions. # '

Cf) Electrochemical preparation of PhZnClanion

A solution consisting of 1.2g of tetra-n-propylammonium 3 ' 3 c h lo r i d e , 2cm chlorobenzene, 10cm of acetonitrile and 3 5cm benzene, was eldctrolyzed for 5 hours at room tempera

ture. After the electrolysis, the electrochemical solution

was filtered to remove zinc particles, and diethyl ether

added dropwise. The product above precipitated, and was

washed with petroleum etl^er and dried in vacuo.

• -V

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced

TABLE 6 . .03 '4-1 “0) «vt r-4 . C J H 0) *H •H •H i-i 0-1 1-1 •o t - i X X •o r-M 1-t 1-1 i v X u ■U J> 4J 4J ■u •u # . # •u u u 03 x o J X E y 3 o 05 s' > c 0) 1-1 0) c 3 O • o o 0) 05 o CO u U ' OS c <55 03 y y Vi y os y y o 3 y 60 O N . c o c o y y y ■ , qq • •» Ss o • ,i-t t—t 'w' w E + «w I / O N i— I—t v a i r > i-t i-t i—1 1-' i-t i-i M-t C > w M ■w > ? u u •U 'X O o o o » «a a S 3 3 y ” * ♦ * t CO , i' • ' ’i w 2 3 * y X I—i •H "O i-t "0 ■U CM X W t s vt • > 5-t s y oS 60 a 3 o y y ‘ . ‘‘ o y 5-1 CO a -' 80v 60 0 x . ■ c o s to ] x o v VO' S x O . V—' V-/ s • ' X 3 H u y Cu 3 £ o ; c [ y

' - t—t CM M m CM - « MCM CM o -r* tn X to o i—i na CM m ao tn o 3 n T COX - . ■tn1 t— m CM CM O M X X tn O n•c • tn oo r-. Vi e CM tn - • t , tn CM CM CM * x tn i-M tn X O CM X m 113- 3 1 -1 — i-t *—t c cm m • • V r=* t - i . cm tn m CM t - r 1^ i—i* o X * O m r-t o m 3 n X X on - - - - CM MCO CM r-t X . x CO CO tn m tn o o vo i—t O i-M CO c Vi U cu Cu. QJ > I X X x Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced

TABLE 6 .1 continued n-i 3 a S u O’Oz—v ■u fiO o a -cscsrr)

(d) based upon metal dissolved -1 1 5 - 79 91 67 73 69 67 % 60 ......

* ? 86 101 125 115 *

1 \ \ 65 / 77 ><( 0 \. . 3 3 • fe j 6 2 (h) ------Q:.:: Time of Zn Electrolysis dissolved yield . . — 25. 30 25 30 15 30 (mA) I n i t i a l 30 25 20 20 30 20 A0 (v) . . I n i t i a l voltage current (II) anions . H 1 .5 2 .0 1.2 T. 5 1 .3 A0 J / 3 3 . 1 .5 5 5 5 5 1 .5 . 5 * 10 10 10 10 10 10 10 V ol(cm 3) (C-H-A.NX

... 1.5 2.0 2.0 ZnBr2~ 2.5 Z n I2“ 1 .5 Z n I2“ . . • . TABLE 6 .2 Reaction conditions for the direct electrochem ical'synthesis of organodihalogenozincate 5 5 5 Z n I2" 3 H H H 2 2 6 CH3ZnI2 CH3ZnI2 C C6H5ZnCl2" C6H5ZnCl2" 2.0 C A nion (a) RX CH3CN CgHg (g). C6H5ZnBr2~ C6H5ZnBr2~ 2.0 CF C ' * i i (aV i i j j v 'As the tetra-n-propylammonium salt i- j ■ ' I j j • i i i

“5 o Q. with permission of the copyright owner. Further reproduction prohibited without permission. -lie- <

TABLE 6 .3 '

Analytical re'sults for 2,2*-bipyridine adducts of organozinc halides

A n a ly s is (F o u n d (c a lc d ) ) • (/£) fa ’s •V? Compound 1 C o lo u r Z in c H alogen

C Hj ,Z nI.L y e llo w 18.1 (18.0) 35.1 (35.0) C2H^ZnBr?L pale yellow 20 ;0 (19.8)' 24.6 (2 4 .2 )

C2H52n I .L pale yellow i t . 5 (1 7 .3 ) 3 3 .8 (3 3 .7 ) / ■V A r C2H22n B r. L brown-yellow 19.8 (19.9) 2 4 .4 ' (2 4 .3 )

C6H52n C l.L green-yellow 1 9 .7 . (1 9 .6 ) 10.6 ( I P . 6)

CgH^ZnBr.L y e llo w 1 7 .2 (1 7 .3 ) 2 1 .3 ( 2 1 . 1)

C6H5Z n I.L v y e llo w 1 5 .5 (15 .4)' 29.9 (2 9 .8 ) ''s-» CgF^ZnBr.L. • off-white 14.2 (1 4 .0 ) 1 7 .2 ■(1 7 .1 ) (1 8 .8 ) 2 3 .2 (2 3 .0 ) C6H5GH2ZnC1*L y e llo w 1 8 .7 CgH^ C I^ZnB r. L pale yellow 1 6 .6 (1 6 .7 ) 2 0 .3 (2 0 .4 )

C6H5CH2Z n I.L orange-yellow 14.8 (14.9) 2 9 .0 (2 8 .9 )

CFgZnI.L brown * 15.7 (1 5 .7 ) 3 0 .4 (3 0 .4 )

^ L' =2,2’-bipyridine

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. TABLE 6 .4 gg

Analytical results for tetra-n-propylammonium salts of organodihalogenozincateCU) ancons

Analysis(Found(calcd) )* (%)

A nion C olour Zinc Halogen

CH3Z n I2~ pale yellow 12.6 (1 2 .5 ) 4 8 .9 (4 8 .8 )

C2R3ZnBr2~ pale orange 14.7 (1 4 .8 ) 3 6 .3 (3 6 .3 )

C2H5Z n I2" pale yellow 12.3 ( 1 2 . 2) 4 7 .4 (4 7 .5 )

C6H5ZnCl2". off-white 16.4 (1 6 .3 ) 1 7 .7 (1 7 .7 )

CgH^ZnBr 2 ye llow- or ange 13.4 (13.4) '3 2 .7 (3 2 .7 )

pale yellow 11.2 43.7- (4 3 .6 ) C6H5Z n I2_ ( 1 1 . 2) CF3Z n I2" y e llo w 1 1 .5 (1 1 .4 ) 4 4 .4 (4 4 .2 )

.-v

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Compound Conductivity

• -1 2 :-l ohm on m ol

140 / [C3H7 ] 4N[MeZaI2]-* •

[C3H7 ] 4N [EtZnB r2] 3.34

138 [C3H7 ] 4N [E tZ n I 2' l ' . -

TABLE 6.6

Analysis of organozinc halides by acid decomposition

W t. cp d . Vol. of gas (cm3, STP) Compound (®g) Found / C a lc d .

CH3ZnI.bipy 87.3 5 .8 (5 .9 )

C^^ZnBr .bipy 9 5 .0 7 .0 ( 6 - 8 )

C^H^Znl .bipy 9 .1 2 5 .9 (5 .7 ) ' '

[CH3ZnI2]_ (a) - 8 0 .0 3 .5 (3 .7 )

[C2H3ZnBr2] (a) 75.3 * -4.0 (4.1)

[C2H5Z n I2 ] _ (a) 121.0 5 .3 (5 .5 )

/ i v ' As the tetra-n-propylammonium. £alt

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TABLE 6 .7

I.R spectra of 2,2'-bipyridine adducts of RZnX

b i p y ^ MeZnI.bipyk Et2nl.bipyb EtZnBr .bipy *3

4 0 4 ^ s 410 m 410 s • 410 s .429 w 510 s 515 s ' ' 545 mb, . 618 s 650 m 630 sb 635 m . 640 sh 650 w 652 s 675 w 665 w 670 m 740 m 735 s 735 s 740 s 752 s 770 s, ■ 760 s , 765 s 892 m 900 w 890 v 900 m 992 .s ' 970~m 965 m ___980 m 1039 m 1020 m 1025 s 1020 m 1062 m ; 1035 m 1035 m 1040 v 1083 m 1080 w 1090 m ' 1075 m 1139 m 1140 m 1145 s 1140 m 1249 m 1233 s 1240 m 1230 m 1312 s 1300 s 1310 s 1305 s 1418 s 136Q s 1370 s 1365 .mb * •1552 s 1445 s 1430 s 1450 m 1575 s - 1475 s 1465 s 1470 s 1570 m 1580 in— 1590 m 1615 s. 1610 1610 s a- - r e f . b = Frequencies in cm , • sample run -as Nujol- mull- between KBr plates.

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• TABLE 6.8 Measurement of current efficiency for the preparation' of some neutral and anionic organozinc halides

S ystem .t 5tadlCZn)F"i

Zn(+ ) /M eI 0 .9 9

Zn^^ /EtI /Pt 1 .0 8

Zn^/Mel + b.ipy/Pt^ .0.89

Zn^/Etl + bipy/Pt^ 0 .7 6

2n^/M eI + nPr^NI/Pt^ .. 0 .8 3

Z n^/E tl + nPr^NI/Pt^ 0 .7 9

Experiments run for 2-3 hrs. at a constant current of 20-30mA.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6.10 Results and Discussion

The electrochemical oxidation of zinc, sim ilar;to that 27 28 29 of cadmium ' ’ in non-aqueous media offers, a simple and

a convenient route to the synthesis of a series of neutral

and anionic organozihc halides. . / • * » The zinc metal undergoes- direct catalytic reaction

with- alleylhalides to give the compounds of the type RZnX,

but the analogous aryl zinc halides cannot be prepared by this conventional chemical way, and the easiest available

route involves an electrochemical oxidation of zinc in the presnece of .aryl halides-. The adducts all involved the

bidentate'donor. 2 , 2 ?-bipyridine, but there is no doubt on

the basis of previous studies-.that compounds with other neutral ligands could readily be prepared by this route.

The 2 r2 * -bipyridine adducts or organozinc halides are

' ft> crystalline coloured (yellow-white) compounds, stable ••irv'£,> dry nitrogen, insoluble in acetone, benzene, diethyl ether

or petroleum ether and slightly soluble in acetonitrile.„

All show solubility in a mixture of acetonitrile "and the

parent organohalide. The method of direct electrochemical synthesis also

leads' to the formation of RZn^- salts with each of the

elements chlorine, bromine, and iodine and with R = alkyl

or aryl groups, and here again there seems every reason to » • .* /* assume that the parent RZnX compounds are stabilized by

further coordination are the anionic complexes RZnX^ (as.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the salt with the tetra-n-propylammbnium cation) . The

salts have sim ilar stabilities and solubilities to the neutral adducts discussed above. In both casesi, the low

solubilities may reflect same degree of polymerization in the solid state Ccf. ref. 134).

Molar conductivity >*>

The formulation of these compounds as salts, was con- • ’ T h . v firmed by.measuring the molar conductivities for nM solution

in acetonitrile. The results were given in Table 6.5, the 2-1 values were in the range of 135 + 5 ohms cm mol which 2 -1 is within the lim its of 120-160 ohms- cm mol , regarded as

. - 118 being typical for 1:1 electrolytes in this solvent. The

conductivity and analytical data (Table 6.4) establishes the ionic structure with certainty.

Gas analysis

In neutral and anionic organozinc halide compounds where R = Me or Et group, decomposition by the addition of

dilute and release methane or ethane gas which was collected •*» and whose volume was measured (.Table 6. 6) shows that for each mole of dilute acid, one mole of alkane was evolved.

Vibrational spectroscopy

The information which can be obtained in the potassium

bromide region of the infrared spectrum is limited. The

infrared spectra of the 2, 21-bipyridine or the tetra-n-

propylammonium cation as appropriate was clearly demonstrated

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in every case by established infrared spectroscopic methods. -1 The (Zn-Me) band at 510 cm was readily identified _v w in (MeZnI.bipy), as was (Zn-Et) at 515 cm (EtZnI .bipy^,

or 545 cm*. -'In- (EtZnBr.bipy) . This region of the spectrum

has been assigned in a variety of papers to the' carbon-zinc

stretching frequency. ^

The p re s e n c e o£ CF^ ° r £ 5^5 was equally s&bwn by ^(.C-F) with m ultiple strong bands appearing over the (1040-1150)cm broad region of carbon-fluorine absorption. No attempts were made to identify the "Q(Zn-X) or ^(Zn-N)" modes of

v i b r a t i o n .

Proton N.M.R. spectroscopy

•'The low solubility of the neutral 2,2’-bipyridine * adducts of organozinc halides in organic solvents prevented

-studies of the n .m.r . spectra of these compounds. In

deuterated acetonitrile, the compound Pr,N£PhZnI7j shows

the expected resonance due 'to cation plus a mult ip let at

S 7.2 assigned to the phenyl protons of the anion. Despite

their low solubility, the corresponding chloro and bromo

compounds show signals for the phenylic protons at $ 7 . 2 ,

and £ 7..3. For Pr^NCMeZn^J / the trip let at £ 0.9 of the -CH^ protons of the cation shows some distortion and extra inten sity on integration compared to the spectrum of tetra-n-

propylammonium iodide, from which one can deduce that CH^

group present in the anion MeZn^ has a resonance in this region.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reaction mechanism In a previous Chapter, it was pointed out that measure

ment of th e. current efficiency expressed as mole metal

dissolved per Faraday of electricity gives a good indication of the possible reaction mechanism. The current efficiencies

measured for a number of Zn,,%/RX systems at constant current

(20-30mA) for 2-3 Hours are reported in Table 6. 8 . T hese » f . •* values are remarkably constant, and significantly lower

than those found for the. corresponding cadmium system. In this latter case, as with tin the high current efficiency,

was e x p la in e d i n ••■terms o f th e fo llo w in g schem e.

C athode

R X f e " ------:------» R* + X' Cl) Anode X_ ■+ M ------» HX * e~ C2) MX + RX ------> RMX' + X* : C3) * X* + M ------> MX C4) Reactions C3) and C4) constitute a chain process whose length is independent of the electrochemical anode' reaction

C2) : The current efficiency for the simplest system Zn^/M el

or EtI in the absence of ligand C2, 2’-bipyridine) was found

to be unity, within experimental error '7 which implies that

reaction GO is of little importance.

Any halogen formed in eq C3) w ill then most likely go

to X2 (e g - I 2) I 11 keeping with this^, a purple colour . was observed near the anode in unstirred solutions. The other system gives Ep values below unity in the presence,

o f 2 , 2 * -bipyridine or tetrapropylammonium iodide which may

be due to the formation of insoluble reaction products' on

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the metal surface. . One possible rationalization for the. difference between zinc on the one hand and cadmium and

tin on the' other, may- lie in the energetics of the reaction.

(4); zinc has a higher ionization enthalpy than cadmium

(.905 KJ mol”^ for zinc, 867 KJ mol~^ for cadmium) and this

may be sufficient to prevent eq (4) frc& occurring.

6 .11 Conclusion.

The electrochemicil .oxidation of zinc as the sacrifi cial anode offers a simple, convenient and rapid synthetic

route to -a series of neutral and anionic organozinc halides at room tem perature. _ Although spectroscopic and analytical

results confirm the nature of, reaction products, a detailed structural investigation, probably -at the level

of x-ray crystallography, w ill be required to draw a final

conclusion about the structures.

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L ■ • CHAPTER V II

ELECTROCHEMICAL SYNTHESIS OF SOME PhjSnM Cl ADDUCTS

. ' iJ - 7.1 Introduction

A considerable number of compounds containingtin bonded 1 1 to a transition metal or to a main group metal * o f

group V CSb, Bi)’ or group IV CGe, Sn, Pb)^^ have been '\\

reported. ' The chemistry of metfal-metal bonded compounds

of main group metals, and especially of group IIB and IVA. elements, h*s been the subject of a number of studies in recent years. '

One preparative route involves the reaction of ZnCl 2 16S with Ph^SnK in liquid ammonia followed by extraction .

with tetrahydrofuran which yields unsolvated CPh^Sn) . • * % The preparation of such compounds, and hence their

adducts has been also achieved by electrophilic attack of a triorganotin hydride on dialkyltinc or cadmium or their

adducts with some bidentate neutral or donor ligand, where

upon' the formation of G^Sn) 2CdL. i s acco m p an ied by th e

release of alkane.^^

SPh^SnH + ^KLL ------;------> CPh3S n )2M.L + 2RH

R = Et, M = Zn; R = Me, M = Cd

CL = THE, DME, TMED, o r B ipy)

The mechanism of sucb reactions has been discussed, as has the importance of the, corresponding halide complexes

eg. , R^SnZnX.L which can be prepared by this reaction.

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The role of a strong coordinating ligand CL) in preveri- • * X68 t i n g 1,2 shifts of phenyl groups has been emphasized. Eabom, Thomson and Walton have prepared bisCtriphenylstannyl)-

mercury^^ by the reaction, of triphenylstann'e are with

_di£bis-Ctrim ehylsilyl) amidojmercury in 2:1 mole ratio at

room temperature.' Hexamethyldisilazane was 'quantitatively evolved, leaving pure bis CtriphenyIstannyl)mercury as a

7 ■ • bright yellow solid. '

2Ph3SnH + HgrNCSiMe 3} 23 2 > HgCSnPh3) 2 + 2CMe3S i ) 2NH. This chapter deals with the electrochemical oxidation

of group IIB metals CM = Zn, Cd or Hg) in a non-aqueous J

solution of Ph3SnCl to yield the metal-metal bonded insertion

products Ph 3SnHCl, which'were isolated as an adduct of a

Bidentate ligand present in the solvent phase. 114 The direct electrochem ical synthesis of RCdX-, RCdX-.L

and RZaX.L^^ compounds has been discussed in Chapter III

and VI. The overall reaction

RX + M ------>RMX. which describes the stoichiometry of these reactions is

apparently not restricted to organic halides, but also

applies to those in'which R = Ph 3S n.

7.2 . Experimental .

In general the electrochemical preparation of these

metal-metal bonded compounds were carried out in an H-form

Cell B (Fig. 2.2, p. 35). The detailed description of the

cell has been already given in Chapter II.

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+ * ' Zinc CM5N) and cadmium (M4N) CAlfa Inorganics}-~were used as anodic plates, approximately 2 x 0.5 x 0.2 cm,

made by mechanically flattening metal rods of the appropriate

size. Mercury was triple distilled (Engelhard Industries

of Canada L td.); a pool of mercury in the bottom of the cell

was the. anode. Triphenyltin chloride and 2,2*-bipyridine were used as supplied. ' N,N,N’ -tetramethylethylene'diamine

aceonitrile and benzene were dried as described in Chapter II.

The solution phase consisted 'of triphenyltin chloride , in same organic solvent together with the neutral donor

_ ligand. In a ll experiments Et^NClo^ C1Q-20 mg) were used to

enhance the. conductivity; applied voltages and analytical

data in each and every experiment are given Table 7.1 and 7.2.

The exclusion of moisture and air from the reaction media is a most important experimental precaution, and all

reactions and operations were therefore performed under nitrogen. -

7.3 Isolation of product

The product was isolated by adding petroleum ether or-

diethyl ether dropwise to the filtrate from the electro

chemical experiment. The m aterial was washed several times * with petroleum ether and dried .in vacuo.

7.4 ' Analysis

Analysis for metals was by atomic absorption spectro photometry and for chlorine by Volhard titration as described in Chapter II of this dissertation. The infrared spectra

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were recorded, using cesium iodide pellets, which confirmed the presence of the phenyl group and the neutral ligand-.

Farinfrared spectra (500-50cm~^) were recorded with samples

as Nujol mulls between polyethylene plates (Table 7.3).

Proton-NMR spectra for each of the N,N,N' ,N’-te.tra- * * Ns , me thy le thy lene diamin e adducts, and free bidentate ligand

- (N,N,N*-,N*-tetr amethyle thy lene diamine) were run in CDCl^ at 60MHZ, using TMS as an- internal standard (Table 7.4).

. 7.5 Preparative chemistry (a) Electrochpffl-i cal preparation of Ph^ShZriCl.TMED

The preparation of Ph^SnCdCl.TMED was carried out

in two armed cell B (Fig. 2.2). The solution consisting of . 3 3 0.8g Ph^SnCl, 6cm a c e o n i t r i l e , 8 cm benzene, l.lg TMED

and 15mg of Et^CNlO^ was’poured into one of the arm of the

E-cell-, and after a series-of evacuation/freezing cycles to

remove dissolved oxygen, the'solution mixture was decanted into the second arm which served as the electrochemical cell.

Details of the electrical conditions are given in Table 7.1. After 3 hours of electrolysis-, the electrochemical solution

was filtered to remove any zinc particles.’

A white insertion product of the electrochemical reac tion was precipitated from the filtrate by subsequent slow

addition of diethyl ether.‘or pet ether. The material was to’ washed several times with petroleum ether /diethyl ether and

; dried in vacuo. The product was analysed for zinc, tin and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. chlorine CTable 72.)-/ The yield of the product and the metal dissolved during electrolysis- are given in Table 7.1.‘

(b) Electrochemical preparation of PhgSnCdCl. TMED

-• The preparation of PhgSnCdClTMED was also carried out

in two arm Cell B (Fig. 2.2). The mixture of 15mg Et^NClO^, . 0.5g Ph^SnCl, l.Og TMED, 2.5cm acetonitrile and 5cm

benzene was electrolyzed after a series of evacuation/ freezing'cycles. The;‘procedure for the isolation of product was the same as that described above for the Sn-Zn compound.

Details of the electrical condiction and analytical'results ^ are given-in Tables' 7.1.and 7.2.

Electrochemical preparation of' Ph^SnCdCl.bipy was ^

performed in Cell B (Fig. 2.2). The mixture of 0.07g

Fh^SnCl, 0.3g 2,2'-bipyridine, 2.5cm^ acetonitrile, 5cm^

benzene and 15mg of Et^NClO^ was added into one of the arm

of the Cell and^stfter degassing transferred into the other

arm. After 1.5 hours of electrolysis at 8 volts, 18mA, the- - white m aterials which formed in the cell was collected;

further quantities of product were obtained by adding pet

roleum ether dropwise to the filtr^teT

The m aterial was washed with diethyl ether to remove

excess 2,2'-bipyridine and then dried in vacuo. The product

was analyzed for cadmium, tin and chlorine (Table 7.2). • The yield of the product and the metal dissolved .during

the electrolysis are given in Table 7.1.

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(d) ' Electrochem ical preparation off- Ph^SnHgCl.TMED

Electrochem ical preparation of PhiphHgCl.TMED was carried out in the Cell S. (Fig. 2.2); a pool of mercury in

the bottom of the cell was the anode, with the semicircular

tungsten wire fused into the side of the arm acted as the c a th o d e . ' 3 The mixture of 0.8g Ph^SnCl, l.Sg TMED, 6cm acetoni- 3 ' t r i l e , 8 cm benzene and few mg of Et^NClO^was degassed,

flushed with dry nitrogen*, and then decanted into the other arm. After four hours of electrolysis with an initial

current of 25mA and 30 volts, the electrochemical solution

was removed and then filtered; a white product was pre cipitated from the filtrate by slow addition of diethyl 4' ****&. ether/pet ether. The m aterial fl^s washed s.everal times < ' with pet ether/diethyl ether,- dried in vacuo and analyzed

(Table 7.2) . It is much less stable than the Sn-Zn or ■ Sn-Cd bonded compounds, and shows decomposition on long exposure to light.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I N> N> . CO i i . , . ? ? * 30 43 , 7a 100 - Y ie ld ? 0 .6 5 0 $ 3 0.2 > — > .... (g) Mass'ofMetal Mass of Product Dissolved'(g) 0 .3 5 0 .0 8 4 * * I ’ 3 .0 4 .5 4 .0 0 .0 7 8 18 1 .5 0 .0 8 0 30 25 ? ? * 8 (V) (mA) TABLE TABLE 7 .1 30 25 Initial Initial Time of 30 Voltage Current Electro- .

(g) o f L 1.0 1.1 Maas 1 .5 0.8 0 .5 0 .2 7 0 .3 0.8 (£> Maas o f Ph-jSnCl i 6 H 6 5 2 o CH3CN CH3CN C 2 V ol(cm 3) a 6 8 2 .5 6 8 Experimental Conditions for the Electrochemical Synthesis of PhgSnMCl.L Compounds .. .. . • ' SnCdCl.TMED SnCdCl.blpy SnHgCl.bipy 3 3 3 3 All solutions contained'Ca. 15 mg Et^NClO^. Based on mass of metal dissolved; n n K P ro d u c t Ph0SnZnCl TMED Ph Ph P h

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TABIZ 7.2. * £3 ... Analytical. Results for^h^SnMCl .T.'Gompoundsa

Compound SnC7.) A CIO*) (Zn.Cd.HgXZ)

Ph 3Sn2nCl.TMED .0) ■- . 11 „5.(1-1.5), 6 .3 ( 6 .3 )

Ph-SnCdCLJ.TMED 19.3(19.3) 18.3(18.3) 5 . 8 ( 5 . 8 ) < Ph 3SnCdC1 .b ipy 18.4(18.2) 17.2(17.1) 5 . 6 ( 5 . 4 # * • Ph^nH^l.TMED 17.1(16.9) — 5 .2 ( 5 .0 ) s . Calculated values given in parentheses.

' TABLE 7 .3

Far Infrared Spectra of Ph 3SnMCl.TMED Compounds

(x/-• n cm -lx )

. M ^(SnPh3) S(M-C1) ^(M-N2) ^(Sn-M )

Zn 452 sh 335 250,310 200 446 s 435 sh

Cd 460 sh 260 190,205 ^ 5 0 " . 450 s • Hg 45 3 ms 272 190,210 . 155 445 sh

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NMR Spectra

Compound Me2N N-CH2-N P h e n y lic

)

V TMED 1 .0 8 (s ) 1 .1 5 (S) Ph^SnZnCl .TMED 2.55 (s) 2.68 (s ) / 7 .3 5 (m) Ph^SnCdCl.TMED 2 .4 0 (s ) 2 .5 3 (s ) J 7 .2 (m) Ph^SnHgCl.TMED 2.80 3 .1 (s) 7 .8 (m)

Run in CDCl^ solvent; Reported in ppm (S) downfield from internal TMS s = Singlet, m = m ultiplet.

7.6 Results and Discussion‘

The electrochemical oxidation of 2inc, cadmium or

mercury in the presence of Ph^SnCl provides a simple con-

venient preparative route to the.synthesis of the metal-metal

.bonded products Ph^SnMCl where M = (Zn,Cd,Hg) .' The percen tage yields (Table 7.1) are adequate, except that in the

case of zinc the yield approaches almost theoretical, and in

general are sim ilar to, but lower than those reported by 167 Destombe, Vanderkerk and Noltes. When ethylzinc chloride

reacts with triphenyltin hydride, selective hydrostannolysis

of the zinc-carbon bond occurs with formation of the corres

ponding (triphenyltin) zinc chloride complexes .

P h3SnH + E tZ n C l.L ------> Ph^nZnCl.L + EtH.

,(L - DME, TMED) ' The scale of the electrochemical experiments could be easily

increased i£ large quantities of product are desired.

s '

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The chemistry of the Sn-Zn bonded species has been -discussed by the previous authors,^7,168 gome Spectro-.

scopic information isT also available on these and the related •cadmium compound. ”

The present^studies add briefly to these results. The *•* 2,2'-bipyridine adduct of Ph^SnCdCl is much less stable a t-' room temperature even, under nitrogen than is the TMED analogu^

The decomposition product has cadmium 30.9%, chlorine .20.1%, close-to the values for CdC^bipy (Cadmium 32.9%; chlorine

20.9%). It seems - likely that the difference in stability % between the bipy and-TMED adducts lies in different bite of

these two ligands,reflecting the extra flexibility of the la tr e ^ r .

Vibrational analysis-

The far-infrared spectra of the three TMED adducts have

been recorded in the region 500-50 cm The tentative •

vibration assignments are given in Table 7.3. The (SnPh^)

■ region is almost invariant in the three spectra, showing only

slight dependence on M. The *?(M-C1) vibration are assigned by comparison with- other halides of the group IIB elements,

and theire are two other metal dependent bands assigned to

• the complex stretching modes of the M i moiety. The N remaining low frequency band is then identified as the Sn-M - stretching vibration; the frequencies assigned to (Sn-Zn) and • 166 ■^(Sn-Cd) are close to those reported formas in S^Zn and S^Cd compounds. ‘It is also worth noting that the -^(Sn-Cd)

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frequencies in both type of compounds are close to the 173 value for the stretchinget< mode of the isolectronic

2- In-In bond in. I^C lg

P ro to n NMR

. The ^HNMR sp ectra o f three Fh^SnMCl .TMED- compounds,

and of TMED itse lf in CDCl^ are shown in Table 7.4. The

free ligand spectrum 'consists of two sharp singlets at

S * 1 .0 8 (NMe 2) 311 <3- 1-15 ppm'from TMS. The three adducts have m ultiplet resonance absorptions due to phenylic protons of the Ph^Sn group centered at approximately 7.35 CZn) , 7.2 (Cd) and 7.8 CHg) ppm.

The TMED resonance shows,substantial downfield shifts on complexation, w ith CCH0) resonances at S *"2.68 (Zn) , 2.53* V - z -.*v ■ . ^ CCd) and 3.10 (Hg), and 2.80 (Hg) . These absorptions are all sharp singlets, except that

th e CH 2 resonance of the mercury shows a reproduceable absorption approximately 0.05 ppm downfield of -the main resonance. .

There is no obvious relationship between the complexa-

-\tion shifts of. TMED and the properties of the group IIB

metals involved, since for each resonance the order is

Cd<(Zn^Hg. It seems probable that in addition to factors

such as charge density, the size of the metal may be involved,

since thi^ w ill affect the bond angles ( a n d hence resonance ' frequencies) in the N-C^-CH^-N chain of the ligand. Further work on this point would'be of interest.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reaction mechanism /. As in other electrochemical work, the efficiency of the

reaction can be measured in electrical, as well as in

chemical terms. Following normal practice, the loss of '

weight of both zinc and cadmium electrodes was measured at

constant current (20mA) over a period of 2 hours; the current

efficiencies for each system was found- to be 0.61 + 0.02

mole per Faraday. The electrochemical reduction of Ph^SnCl

at a dropping mercury electrode has been discussed by Dessy . *73 174 175 and co-workers ’ ~ who sugges ted that the process is •

e i t h e r .

Ph3'SnCl ------:— » Ph3Sn* + Cl" - (1)

2Eh 3Sn* ^ P h 3SnSnPh 3 , (2) / o r

Ph3SnC l ----- —------> Ph 3Sn" +-C1* (3)

Ph 3Sn" + Ph3SnCl -----:------» Ph3SnSnPh 3 + C l"______(4) Reaction (1) or (3) + (4), both of which are possible in

electrochemical conditions, give rise to chloride ions, whose

migration to the anode would be followed, according to

previous postulates, by the reactions

C l" ------>C1‘ + e" (5)

Cl' +M ------»MC1 (6)

and the reaction of M^Cl on the Ph 3SnCl- substrate would give the primary product,

Ph3SnCl + H^Cl ------* Ph3SnMCl + Cl _____ (7)

w here M = Zn,Cd,H g ^ . -

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with a chain reaction sequence of (. 6) + (7 ) then expected.

Such sequences have been identified by resulting

current efficiencies, which, are greater than unity and sometimes considerably so. 53 _ In the present case, the

current efficiency obviously does not demonstrate such

reactions, despite the earlier evidence^"* as to the *v cathode reactions. It also suggests that chlorine atoms generated in (5)

may react with Ph^SnCl to give Ph^SnC^ or Ph^Sn" + C ^, and so remove chlorine from the system more efficiently

than by attack on the anode. The fact that the current

efficiencies are identical for two different metals points to some metal independent process being responsible for

the lowering of current efficiency below that observed in'

otherwiise formally sim ilar systems.

7.7 Cone-1 us ion

The direct electrochemical synthesis, of m etal-m etal„

bonded compounds described in this chapter is one.of t&e

easiest, simplest and most convenient, and can be performed

at room-temperature. This suggests that it has many advan

tages over the other ^commended conventional methods, in- which one has to use apparatus such as high vacuum line,

and Schlenk apparatus to handle air sensitive starting ^ m aterials like dialkylzinc, dialkyl cadmium or dialky Imercury

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CHAPTER V I I I

8 T '.THE DISECT ELECTROCHEMICAL SYNTHESIS OF GROUP IIB — — :------;------WfMHSi^YtAKt&Sg ------:------r——

The m etal, dlalkylamid.es MCNB^x are of special interest

in that they.contain covalent metal-nitrogen bonds and so ^ 9 3 occupy a position between metal alkoxides and metal alkyls.

Due to the reactivity of metal-dialkylamido bonds, the compounds have considerable versatility as synthetic r e a g e n t s .

For covalently bonded dialkylamido groups, there are

three separate possible structures •

HIT R, N M \ \ + A- . / N = M R r X M 2 <

x .• - I m

' - . I P ' s1 ■

Structure (I) involves a m etal-nitrogen sigma bond with a

pyramidal nitrogen containing a .basic lone pair of electrons.

If steric factors allow, this lone pair may donate to

another metal, giving rise to a dialky lami do bridge . .(II) .

If bridging is precluded by steric factors, the' nitrogen lone pair may engage in bonding giving the trigonal planar 9

nitrogen as shown in structure (III) .

cv-. . — . Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8.2 The Preparation of Metal DialkfrhAmides

, There are three main preparative procedures for syn-' X76 thesizihg metal dialkylamides. % (i) By treating the metal hydride with a secondary

amine. -' 'V

m x + :KR2m ------* M(NR2^x + = 2 - (ii) By the reaction between the metal, alky I van d .

secondary amine. MS^ + XB^NH ------■----> + XBH.

chloride) with an alkali metal (Li or Na)dialkylamide *

MC1X + xLiNI^ ------> H(NR2) x + X L iC l.

8.3- Present work This chapter^gals with the electrochemical oxidation

b’jc group I IB metals (Zn, Cd or Hg) in the presence of

secondary amine to produced metal dialkylam ides, which were

isolated as the adduct of a bidentate ligand preseht in the

solvent phase. CHoCN/Et/NC10. f-n u v r R,NH + M -2 ------4 2, 25??bipyridine

R^N = ( N (Me2CH)2N, (He3S i ) 2N.

8.4 Literature Survey (a) Reaction of K-H with secondary amine

Derivatives of aluminum hydride were used by Ruff^"^ to

permission of the copyright owner. Further reproduction prohibited without permission. prepare aliminum■ dialkylamides

LiAlH 4 + 4Me'2NH ------>LiAl(NMe2) 4 + 4H2 *

3LiAlCNMe2) 4 + A1C13 — :------> 2A l2 (NMe2)g + 3L1C1.

AIH3 (NMe3) ' + 3 iP r 2NH — A l O ^ P r ^ + Me3N + 3Hr

- ' - * (b) Reaction of meta-lalkyls with secondary amines

The reaction .between diethylzinc and diethlyamine was 178 .• - . reported by Prankland in 1867, but the chemistry of zinc- *• nitrogen derivatives received no further. attention until.

1965, when w ell defined organozinc amides R2nNR *2 (R and R 1 alkyl or aryl group) .were prepared by controlled acidolysis 179 of diorganizincs with secondary amines. Coates and i g n Ridley have reported the formation of bis Cdiniethylamino)-

z in c [(M e 2N>2Zn]x on reaction of dimethylzinc with dimethyl- 181 amine. Coates and Glockling have also reported the formation of bisCdimethylamino)beryllium by the reaction of'

di me thy lb.ery Ilium w ith secondary amines...

BeMe2+ ( ^ Be(NMe2) 2 + MeR.

Mb'.

On the basis of molecular weight results, they predicted •_

the structure to be the cyclic, trimers Be 3 (NMe2) g .

Cc) - Reaction of Metal Chloride with an-alkali metal (Li or Ha) dialkylamxde ' T- By the reaction of sodium bis (trimethylsilyl).amide * 182 with ZnCl2, Cdl 2 and HgBr 2 respectively, the monomeric,

volatile" silylam ides M[NCSiMe 3) 2] 2 (M\= Zn, Cd, Hg) are

permission of the copyright owner. Further reproduction prohibited without permission. formed. Infrared and Raman results yielded force constants which show that the M-N bond is a covalent single bond.

K.^J. Fischer^^ has reported that Co[N(SiMe 3) 2J 2 ,

and Zn[NCSiMe 3>232' can PrePare<* by .the reaction of the 'anhydrous metal halides CoBr 2 and ZnCl 2 with hexamethyl- disilylaminolithium. They also"investigated the reactions

of pyridine and substituted pyridine with Co[NCSiMe3)2.]2,and

the zinc analogue.

'Sim ilarly bis[diCtrimethylsilylCaminojberyIlium had

been prepared^®^ as a monomer in'organic solvents; this

species forms no adducts even with strong complexing agents. Lappert and Harris have reported the synthesis of monomeric, 185 volatile bivalent amides of group IVB elements. The

reaction of MC12(M = Sn or Pb) or GeCl 2 • dio x ah e w ith .

LiN (SiM e3) 2OEt2 of LiN(SiMe3) CCMe^OEt^, in Et20 at 0° gives

yellow crystalline MCNR '2) 2 = Sn or Pb; R* = Me 3S i) ,

2 2 * • or orange to red liquids CMCNB.’R ) 2 CR 53 Me3C) in high • yield at ambient tempera^inre! Similarly Lappert et al. have

reported the preparation of the trivalent silicon, germanium

an d t i n species^^^’ M[NCSiMe 3) 2]3 CM = Ge or Sn) and the

isoelectronic M* [CHCSiMe 3) 233 ^ = ^ o r -Sn). T hese compounds were obtained by photochemical disproportionation

using metal CH) halide precursors

2LiNCSiMe, ) 9 ' r ' MC12 ------H|NCSiMe.2) h ^ [CMe3S i ) 2N]3M- where M = Ge, Sn.

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The compounds [(Me^Si) 2^ 2^ ! CM =■ P or'As) were prepared by reactions between the- element trichlorides and (Me^Si^NLi- IS 8 xn e t h e r .

The first transition metail’ derivative synthesised was

TiENCCgH^^l^, which was-prepared' frop TiCl^ and potassium- 189 dipheny lamide. . The only uranium derivative prepared- is

U(NEt^)^, which was obtained from UCl^' and lithium diethyla-

m ide.^^ In recent yeras tetradialkylamides .of titanium, TQ? ' 19? 195 194. ■ 195 vanadium, chromium, ’ zirconium, niobium, v 196 197 195 hafnium, thorium, pentadi alkyl amides of niobium 198 ' " 199 and tantalum, and the hexadime. thy lami de of tungsten v ' *» have all obtained by means of metal chloride/lithium- ^

dialky lamide reaction. Burger and Wannagot^^ have

prepared a number of interesting bisCtrimethylsilyl)amido

derivatives M[N(SiMeg) 2lx using, the reaction of the sodium

derivative NaNCSiM^^ with metal halides. Thus, the tris derivatives of chromium^^^^ and iro n ^^^^;, the bis

derivatives of manganese^^^^^ and cobalt^^^^ , end n ickel^^^^ were isolated by general method:

- ■ MXx + xNaN(-SiMe 3) 2 ------:---- > MCNCSiMep^]^. + NaCl

The new compounds M[N(SiMe 3) 2^ M= La, Er, Sm,.Eu Gd, Lu) .

have been isolated by the reaction of LiNCSiMe^) 2 ^w ith lanthanides^^chloride in THF, n-pentane or- in ether

202 . s o l u t i o n .

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(d) By direct reaction of metal with secondary a-mine In addition to tha^ three main preparative methods

mentioned above, a novel method has recently been reported 203 by Ashby and Kovar for the synthesis of aluminum ^ tris diethylamide and diethylaminoalanes. » ’ A1 + |h 2 + nEt2NH ----- > H ^^AlCNEt^.+nH^

n.** 1,2,3 • The reaction was carried out at moderate temperature and

pressure in benzene solution. A 91% yield of AlCNEt^^ was obtained using di ethyl amine as a solvent at 15.0° under 3000

psig of hydrogen for 4 hours. This direct synthesis of aluminium diethylamide from the metal may well have ■ commercial application.

8 .5 Experimental

In general" the electrochemical synthesis of some group IIB metal dialkylamides were carried out in the H-form cell

'B' ‘(Fig* 2.2). The detailed description of the cell has been - already given in Chapter II of this dissertation.

-.Zinc (m5N) and cadmium (jn4N) (A lfa I n o r g a n ic s ) w ere

used as anodic plates after mechanically flattening the

metal to give sheets of approximate size 2 x 0.5 x 0.2cm.

Mercury was triple distilled (Engelhard Industries of Canada Ltd.); a pool of mercury in the bottom of the cell was the_ ano de.

Isopropylamine, 2,2,6,6 ,-tetram ethylpiperidine and

1,1,1,3,3,3,-hexam ethyldisilazane were used as supplied..

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. N,N,N' ,N' -tetramethylethylenediamine and aceonitrile were

dried as described in Chapter II.

Solution Composition 3 The solution phase consisted of approximately; 3-4cm

secondary gmine in acetonitrile together with -same neutral

bidentate donor ligand.. In each set of electrochemical

experiments Et^NClO^ ( 10“ 20mgl was used to enhance the conductivity of the solution. The solution composition and

electrical conditions are given in Table 8.1. The electro chemical solution was degassed to exclude . moisture and

air from the reaction media. All subsequent experimental

operations were .performed under nitrogen in a dry box.

Isolation of product ' , ' After the electrolysis, the electrochemical solution

was filtered through a coarse sintered crucible to remove any anoidic metal particles formed by disintegration during

the electrolysis. I * In case of 2,2',-b'ipyridine adducts of group IIB dial

kylamides , the solution resulting from the electrochemical

' oxidation was treated dropwise with diethyl ether. The

coloured precipitate which formed was washed several times

with benzene to remove excess 2 , 2 *-bipyridine and then dried

i n v a c u o . Bis(hexamethyldisilylamino)metal^ was isolated as a

white product by removing the solvent and excess ligand

from the electrochemical solution in vacuo.

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8.6 Preparative Chemistry ■ '. (a.) SIectroch emica 1 prep aration o£ 2,2l-bipyridine adduct of bis(isopropylgmTr>o) zinc " -

2 , 2 ’-bipyridine and a few mg of Et^NClO^ was carried out

for 5 hours using an in itial voltage- of 40 volts and a current of 30mA. The reaction mixture was- then filtered;

a greenish yellow product was precipitated from the filtrate by subsequent slow addition of diethyl ether. The m aterial

was washed several times with pet ether/diethyl ether,

dried in vacuo and analysed (Table 8 . 2).

Cb) Electrochemical preparation of 2,2*-bipyridine adduct of~ bis(isopropylam ino) carfcni urn

Electrochemical oxidation of cadmium^ anode in a solution mixture consisting of 5cm isopropylamine,3 10cm 3

acetonitrile,- 0.85g 2,2'-bipyridine and a few mg of Et^NClO^,

was carried out under condition stated in Table 8.1. The

isolation and subsequent treatment of product were , as for

a n a lo g o u s bis(isopropylam ino) z i n c . b i p y .

When z in c m e ta l (a n o d e ), was electrolyzed in a solution 3 3 consisting of 3 . 5cm 2 , 2 , 6 , 6-tetram ethylpiperidine, 8 cm

acetonitrile, 0 . 8 g o f ’ 2 , 2 ’-bipyridine and a few mg of Et^_NC 10^ , a violet colouration formed at the cathode. This coloured

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species migrated to the anode, where the colour-was imme diately dissipated. After the electrolysis, the ■'reaction

mixture was filtered and treated dropwise with diethyl ether. The greenish yellow precipitate which formed was washed

several times with diethyl ether and .dried in vacuo.

(d) Electrochemical preparation of 2.2 * -bipyridine adduct of bxs( 2 , 2 , 6 , 6-~tetram ethylpiperidino) cadmium. 3 A solution mixture consisting of 3.5cm 2,2,6,6-tetra- 3 m ethylpiperidine, 8 cm • of acetonitrile containing 0 . 8 g

2,2*-bipyridine and 15mg of Et^NBr was electrolysed for

6 hours,- using a current of 25mA and 40 volts. After the

electrolysis a greenish yellow product was isolated, dried and analysed for cadmium. ■

(e) Electro chemical prep R ation of 2,2' -bipyridrne adduct of bis( 2 , 2 , 6 , 6 -1 e tram ethylpiperldino)m ercury

Eelctrochemical preparation was done in an H-shaped

Cell B (Fig. 2.2) ; a pool of mercury in the bottom of the cell was the anode, with a tungsten wire fused into the

side arm acting as the cathode, fhe solution mixture as

described in Table 8.1 was degassed, flushed with dry nitrogen,

and then decanted into the other arm. After 5 hours of

electrolysis, the electrochem ical'solution was pipetted out

and then treated with diethyl ether slowly to isolate the

product, which was dried in vacuo and analysed (Table 8.2) .

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(f) Electrochemical -preparation of bisCheyfl-methyldisil- • ylamino)zinc ' “

Electrochemical oxidation of zinc anode in a. solution 3 - 3 consisting of 4cm 1,1.1,3,3, 3-hexamethyldisilazane, 8 cm 3 of acetonitrile, 2cm N,N,Nf,N^’-tetramethylethylenediamine

and a few mg of Et^NClO^ was carried out for 5 hours, using

an in itia l voltage 40V and a current of 25mA at low tempera ture 10°C. Theu-.reaction mixture was then filtered to remove

the disintegrated zinc particles. The solvent and unreacted TMED was removed in vacuo leaving a white solid, which is'

highly unstable and melted into light yellow liquid even at

room tem perature.

Cg) Electrochem ical'preparation of bisQiexamethyldisil- ylamino) cadmium :

Cadmium was oxidized electrochem ically in a soTutidk 3 3 containing 40cm 1,1, 1 ,3,3,3-hexamethyldisilazane, 8.0cm. 3 acetonitrile, 2cm TMED and a few mg of Et^NClO^, keeping the solution temperature at 10°C for 5. hours. After the

electrolysis, a white unstable product is obtained on removing the solvent and unreacted TMED in"vacuo. This

solid shows solubility in a wide variety of organic solvents-,

It is highly unstable and decomposed immediately in pro- tonated solvents.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 5 (hre) Time 30 30 25 25 25 25 ’25 (mA) (mA) / c u r r e n t II 40 40 30 30 40 40 40 (V) <&' \ b b

n 8 8 .0 8 .0 10 cm ) / 3 ( TABLE TABLE 8 .1 /■• /■• 3S (cm ) 3 .5 '3.5 4 .0 4 .0 4 .0 Vol. of Sec amine Vol. of acetonitrile Voltage Initial Cd.bipy 5.0 Zn.bipy 5.0 2 2 N j N] n | 2Zn N J^nbip/ N a 2 2 2 CH) CH) 2 REACTION REACTION CONDITIONS FOR DIRECT SYNTHESIS ELECTROCHEMICAL OF METAL AMIDES 2 ( < 3 ^ j(Me |(Me J(Me^ S i)2Njcd J(Me^ [(Me S l ) c ~ n o t re c o r d e d . a » l5mg of Et^NClO^ added in each experiment, b » 4cnT* of was TMED used in each experim ent. Compound

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. without prohibited reproduction Further owner. copyright the of permission with Reproduced

Table 8.1 c o n tin u e d 3 T jj <0 ca >

TABLE. 8 .2

Metal Analysis of Metal II Amides

Compound A n a ly s is (fo u n d (c a lc d ) (%) )

[ (Me2CH)2N ]Z n -b ip y 1 5 .0 1 (1 5 .4 9 )

[(M e2CH)2N ]C d .b ip * 2 3 .4 6 (2 3 .9 6 )

Zn b ip y ' 18.25 (1 8 .0 6 ) J f

2 7 .6 3 Cd b ip y (2 7 .4 9 )

4 1 .2 . S g b ip y - (4 0 .3 5 )

[ ( M e ^ D ^ Z n 1 7 .1 2 (1 6 .9 2 )

[(Me^Si^N^Cd 2 6 .3 4 (25 .94)

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TABLE 8 .3

Infrared data for some 2,2'-bipyridine adduct of alkylam ide

Compound A b s o rp tio n i n cm ”1

[ (Me 2CH) 2^ ] 2Zn • b ip y 410(w) , 480(st) (*^asZnN«) , 620 (m), 650 (m), 720 (m), 760(m) , 1010(m) , 1090( s t ) , . - 1150(m) , 1 5 9 5 ( s t ) * -^(C -C )

[(M e2CH) ] 2C d .b ip y 410 (m) , 455 (w) C?asCdN9) , ~ 620 (nO , 730 (m), 770(mf, lO lO (m ), 1 050 (m) , 1 5 9 0 (s t) ,

390(w), 410 (w), 485 (st) ( r ^ 2\ 5as(Z nN 9) , 670(m) , 720(m) , I / N j Zn.bipy 765 (m) , 1090(m) , 1580(a)^ (C=C)

^ 2 ?

410 (w) . 4 6 5 ( s t .b r o a d ) 5 a s CdN9) , 620 (m ), 665 (ni) ; N 1 Cd.bipy 7 3 0 ( s t ) , 7 6 0 (m ), 1040(m) , ' -1085 (m) , 1150(m) , 1580 (m*

lte2\' 390 (w) , 4 1 0 ( s h ) , 4 5 5 ( s t) (oJasHgN9) , 620 (m) , 660 (m) , N J Hg.bxpy 750(st)7 790(st) , 1090(st), C 1590(isWC=C) >5*2' 2

m = medium, s = strong, v .s.'=■ very strong, w = weak

. • ■ ______Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE 8 .4

Infrared spectra of bis(hexa 3neth.yldisilylamino)M ^ recorded '.in- Csl

C(Me3S i) 2N ]2Zn C(Me3S i ) 2N]2 Cd A ssignm ent

260 (w) S a s(S iC 3) 2 7 5 § '

290 ( id ) -29 5 0 ?) ? (S iC 3) 3 70.(b ro a d ) 330 (b ro a d ) . 440 (m) 410 (m) ^as(M N 2)

530 (broad) —

615 (m) - 615 (m) *^(SiC3)

6 7 5 ( s t ) 670 (st) ^CSiC3) ’ 750 (ml.. 7 4 5 ( s t )

835 (m) 8 3 0 ( s t )

. 945 (v.st) 930(v.sr) ^as(NSi2)

1257 (m ) 1240(st) s s ( ch 3) - 1265 (m) 1 2 5 5 (s t) 1440 (w) 1445 (cO • Sas(CH 3)

2900(m) 2910(m) ^as(CH3)

■m = medium, s.t = strong, v .s t- very" strong, w = weak

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE 8 ,5

XH NMR S p e c tr a o f MI I I.CNCSLMe3) £ 2

Compound SiCH3

ZnlN (SiM e 3) 2J 2 0.00

Cd£N(SiMe3) 2] 2 -Q .01

8.7 Results and Discussion -

The techniques which are discussed in the previous

chapters on the direct electrochemical synthesis of organometal halides were successfully and conveniently utilized

in the electrochemical synthesis of some metal-nitrogen

covalently bonded products. The present studies show that

it is also possible to synthesize these QOmpounds electro- 176 182 •chemically as well as by the accepted conventional methods

on a reasonable s-cale and in a short period of time, (Table- 8,1) .

The 2,2'-bipyridine adducts of bis(isopropylamino)zinc

or cadmium, sim ilar to 2 , 2 '-bipyridine adduct of bis( 2 , 2 , 6 , 6-

tetram ethylpiperidino)H ^ (where M = Zn,Cd.,Hg) shows insolu- . bility in most of the organic solvents. This lack of

solubility precluded proton n.m.r. or molecular weight

measurements and the compounds are assumed to be polymeric. It is remarkable that they are also insoluble in donor

solvents such as dimethylformamide, pyridine or a solution

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of 2,2'-bipyridine in benzene.. The-donor-character of the dime thy lamino group when bonded to zinc is so enhanced that

* even 2,2*-bipyridine causes no displacement. An attempted preparation of (M ^^Zn.-bipy from the dime thy lz in c complex

Me22n.bipy and dimethylamine at 80° yielded, only black

insoluble involatile material- in addition to methane.

- Bis.Qhexame thy Id isily lamino) zinc and bis (hexamethyldisilylamino) cadmium.were also successfully prepared

electrochem ically. -Bpth compounds, are soluble in common organic solventslike chloroform,, pentane, hexane etc. and. •

they react with compounds containing an active hydrogen

eg., water and alcohols. ; Because of the sensitivity of the

compounds good analytical data were hard to obtain. The

metal analysis indicated the presence of-two hexamethyl- disilylam ino group per atom (Table 8.2) . ...

The compounds form-no ■adducts'even w ith strong c du plexing agents like 2,2*-bipyridine or N,N,N',N’-tetram ethyl-

ethylenediamine. Models of M^CUCSiMe^^^ indicated that the steric hindrance provided by the amine group would be

sufficient to prevent access by the bidentate ligand.

Vibrational spectroscopy

The infrared spectra of-the five 2,2*-bipyridine adducts

of m etal^ alkyls ’have been recorded between 200cm'* ^ to

4000cm \ .generally as Nujol mulls or Csl pellet. The

absorptions are listed in Table 8.3. The*^(M-N) vibrations

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are assigned by. comparison with the results of previous,

workers on group IIB metal alkylam ides.^^’^ ^ ’^ ^ ’^ '’ The infrared spectra of bis Chexamethyldisilylamino) zinc^-

and the corresponding analog of. cadmium are "shown in'Table

8.4. The vibrations in the region -1260cm ^ to-610cm~^ are

indicative of the presence of NCSiMe^^ groups, and the intense bands at 440cm ^ and *410 cm” ^ correspond to CZn^)

and CCdN2) stretching vibrations. In all compounds,' the infrared spectra show the absence of absorptions due to

"3 (N-H) around 3300cm

NMR . The NMR chemical shifts for bisChexamethylsilyl-

amidp)'zinc and cadmium are given in Table 8.5. The spectra

show the singlets at S 0 . 0- in CDCl^ for the zinc complex

and. a t S -0 .1 for cadmium, indicating the presence of -

S'iQfe^ protons. •

8.8 Conclusion

The synthetic technique discussed in this chapter is

a very easy and convenient route to metal alkylamide. The

method has many advantages over the other recommended .conventional methods, in which sophisticated systems are

used-to handle-air sensitive starting m aterials.

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The folld&ing is a lis t of the organometallic .compound^ •prepared by the author during the period covered by. this

thesis. - .

I. Neutral organocadmium.halides ‘ ;•

CD MeCdl . a- C2) E tC d l a C3) ,MeCdI.2DMS0 C4) •MeCdl.bipy a C5) E tC d l b ip y a C6) EtCdI.1,4 dioxane a C7) CF^Cdl.bipy • C8 ) EtCdBr.bipy • a C9) EtCdBr* 1,4-dioxane a CIO) EtCdBr.phen a C ll) n-BuCdBr. b ipy a C12) . PhCdBr* 1 ,4 -dioxane a C13) Cg.F^ CdBr. b ip y a

II. Neutral organozinc halides

■ Cl 4) JfeZnl.bipy a C15) E tZ n I .b ip y a ■ C16) CF^Zal.bipy .0-7) EtZnBr.bipy a ■ C18) ^CKIH-ZnBr. b ipy a CIS) P h Z n I.b ip y “' a C20) . PhZnBr.b^fpy a C21) PhZnCl.bipy a h 2) PhQ^Znl-. b ipy $ a , / C23) P h C ^ Z n B r. b ip y a C24) PhCI^ZnCl.bipy a ■ 7* * ' • I I I . Anionic ' organocadmium. halides

C25) DfeCdI2J~ b C26) [E tC d I2] " b C27) [CF3C dI2 ] " * b C2S) [^-B uC dI23- b C29) £MeCdBr23” * ~. b cm £EtCdBr2 ] b- C31) [t-B uC dB r2J~ b

' \ v ' * ’ Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. -1 5 8 -

(.32) IP hC dI2J ‘

(33) lPhC dB r 2J' b (34) lPhCdCl2]“ b (35) £C6F5Cc3Br2J b

IV. Anionic Organozinc halides

(36) [MeZnI2]- b (37) £EtZnI2J" b (38) rCF 3Z n I2] _ ‘ b (39) [EtZnBr2]~ b (40) [PhZnI2]_ b (41) £PhZnBr2]_ b (42) . £PhZnCl 23 ‘ b

Aryl cadmium compounds containing rK -me thy lamino .group as a built-in ligand , '

(43) 2-£ (N, N' -dime thy lamino.) me thy 1] phenyl cadmium(II) - b ro m id e. b (44) ' 2-£ (N,N* - dim^r.hyl arm'n^-mpf-hyl jphpnyl r^ dmi tttti(TT) - • m e th y l b (45) 2- £ (N, N' -dimethylamino)meth.yl]phenylcadmium(II) - p h e n y l b (46) B is £2- (N, N- dime thy laminome thy 1) pheny lcadrni umCII) ^ • . b

VI. Some Ph^ShMCl adducts (M = 2n, Cd, Hg)

(47) Ph^fcZnCl. TMED a (48) Ph 3SnCdCl.TMED a (49) Ph 3SnCdC. bipy a (50 ) Ph 3SnH gCl. TMED a

VII. Group IIB metal-dialkylaTTri des (51) £Bis (isopropylamino)]zinc(II) .bipy a / (52) £Bis (is opropy lamino) ] cadmium(II) bipy b

(53) £5 i s ( 2 , 2 , 6 , 6-tetram ethylpiperidiho)j 2inc(II)bipy b

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. -1 5 9 -

(.54) lBis(2,-2,6,6-tetramethylpiperidino)Jcadm±-umIIbipy b

C55) { B is ( 2 , 2 , 6 , 6-tetramethylpiperidino) Jmercury (II) • b

• b i p y (56) [Bis Qiexa-methyldisily lamino) ]zinc(II) a

C57) [Bis.(hexa-metbyldisily lamino) 3 cadmiumCII) a

Note: a * Reported in the literature and prepared by other conventional methods 1 b = Not reported in the literature.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. -1 6 0 -

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203'. E. C. Ashby and R. Kovar, J. Organometal. Chem., 22, C34 C1970). 204. D. C. Bradley and K. J. Fischer-, J. Amer. Chem. Soc‘. 93, 2058, 1971. 205. *.S. Inoue and T. Yamada, J. Organometal. Chem. 25, , 1, C1970).

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VITA a u c to r is

Name A k h ta r Osman

Date and Pl-ace 'of Birth. 10th, December 1949, Hyderabad (DN) I n d ia Marital Status single

University Education

! ' ■ University of Karachi j 1967-1970 Karachi, Pakistan B.Sc. (Chemistry)

University of Karachi 1970-1972 • Karachi, Pakistan " M.Sc. (Organic Chemistry)

Middle East Technical University 1973-1975 • Ankara, Turkey M .Sc..(Polymer Chemistry)

University of Windsor 1975-1979 Windsor, Ontario, Canada

Position Held Research Assistant ... 19.72-1973 (Organic Chemistry) * Teaching Assistant 1973-1975 (Organic Chemistry) Teaching Assistant 1975-1979 (General Chemistry) ( and ) (Organic Chemistry)

Society Chemical Institute of Canada (Student Member)

American Chemical Society (Member)

Awards and Scholarships •

Directorate of Education 1968-1970 Merit Scholarship

Diredtorate of Education* and 1970-1972 Karachi Municipal Corporation Post Graduate Merit Scholarships

_x Regional Cooperative Development 1973-1975 Scholarship

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National Research Council of Canada . 1975-1975 (Research grant through Dr. D.G. Tuck)

Publications (a) Identification of Some Structural Properties of Bis-DimethylAluminumOxide, . M.Sc. Thesis, Middle East Technical University, ' 1 975.

(b) Direct Electrochemical Synthesis of Neutral and Anionic Organocadmium H alides. J . Chem. S on. (Chem. Commun) 380, 1976.

(c) The .Direct Electrochemical Synthesis of Anionic Qrganodihalocadmate(II) Complex. J. Organometal. Chem.; 169 255 (1979)., (d) The Electrochemical Synthesis of Some Ph^SnMCl Adduct (M = Zn, Cd, Hg) . Inorg. Chim. Acta 35_ 105 (1979) .

(e)' The Electrochemical Synthesis of Neutral and Anionic Organozinc H alides. e - • Accepted for publication.

(£X Electrochem ical and Chemical-Synthesis "of . Arylcadmium Compounds Containing (N,N,-Dimethylamino)methyl Group at the Aryl Nucleus. (in preparation) (g) The Direct Electrochemical Synthesis of Group IIB Metal Dialkylamide. (in preparation)

■ ' Y . .

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