ORGANOMETAT,T,TC CHEMISTRY OF URANIUM

A thesis submitted

by

R1TN R. SIGURDSON, B.Sc.

for the

DEGREE of DOCTOR of PHILOSOPHY

of the

UNIVERSITY of LONDON

Royal College of Science

Imperial College of Science and Technology

London, SW7 ?AY August 1976

TO MY PARENTS

3

ACKNOWLEDGEMENTS

I would like to express my gratitude to Professor Geoffrey

Wilkinson, F.R.S. for his guidance and enthusiastic support

throughout the course of this work.

Many thanks are also extended to Drs. Dick Andersen, Ernesto

Carmona-Guzman and David Cole-Hamilton for their suggestionS,

encouragement and advice, and to Dr. Kostas Mertis for his patient

help during the first months.

I am indebted to the Canadian Research Council of Canada for

financial support during the past three years.

4

CONTENTS

ABSTRACT

6

INTRODUCTION

I. The Chemistry of Uranium(IV)

8

.II. The Chemistry of Uranium(V) 15

III. The Chemistry of Uranium(VI)

16

CHAPTER I. DILITHIUMHEXAALKYLURANATE(IV) COMPLEXES

I. Introduction 19

II. Results and Discussion 27

III. Experimental

35

CHAPTER II. TRILITHIUMOCTAALKYLURANATE(V) COMPLEXES

I. Introduction 54

II. Results and Discussion 55

III. Experimental 60

CHAPTER III. ADDITION COMPOUNDS OF URANIUM(VI) HEXAISO-PROPDXIDE

WITH LITHIUM, MAGNESIUM AND ALUMINIUM ALKYLS

I. Introduction 70

II. Results and Discussion 71

III. Experimental 77

CHAPTER IV. ORGANOMETALLIC CHEMISTRY OF ADAMANTANE

I. Introduction 84

II. Results and Discussion 85

III. Experimental 87

REFERENCES 92

5

ABBREVIATIONS

Me - methyl

Et - ethyl

Prn- normal-propyl

Pri- iso-propyl

Bun- normal-butyl

But- iso-butyl

But- tertiary-butyl

Ph - phenyl

CP cyclopentadienyl DME - dimethoxyethane

tmed - N,N,NI,N'-tetramethylethylenediamine

pmdt - N,N,Nt,N",N"-pentamethyldiethylenetriamine

g.l.c. - gas-liquid chromatography

i.r. - infrared

s - strong

m - medium

w - weak

sh- shoulder

n.m.r. - nuclear magnetic resonance

s - singlet

d - doublet

t - triplet

q - quartet

h - heptet

br s - broad singlet

6

ABSTRACT

Organouranium compounds with six or eight uranium-to-carbon

a-bonds have been synthesized for the first time. The interaction

of uranium tetrachloride with lithium alkyls in diethyl ether

leads to the isolation of unstable lithium alkyluranate(IV)

compounds of stoichiometry Li2UR6.8Et20, R = CH3, CH2SiMe3, C6H5,

o-Me2NCH2C6H4. The lithium salts are also obtained as

tetrahydrofuran and N,N,NI ,N1-tetramethylethylenediamine solvates.

From uranium(V) pentaethoxide similar lithium salts of stoichiometry

Li3UR8.3dioxan, R = GH3, CH2SiMe3, CH2CMe3, are obtained. The

interaction of uranium(VI) hexaiso-propoxide with lithium,

magnesium or aluminium alkyls does not give compounds containing

U-C bonds, but addition compounds of stoichiometry (RLi)3U(OPri)6,

(R2Mg)3U(OPri)6 and (R3A1)6U(OPri)6 respectively, that appear to

be adducts in which the oxygen atom of the iso-propoxide group

bound to uranium is acting as a donor.

Attempts to prepare binary transition metal adamantyl

complexes are described.

INTRODUCTION

8

INTRODUCTION

I. THE CHEMISTRY OF URANIUM(IV)

The organic chemistry of uranium(IV) has been comprehensively

reviewed1-4

. The first organouranium compound, Cp3UC15, was

prepared by Reynolds and Wilkinson in 1956 by the reaction of

uranium tetrachloride with sodium cyclopentadienide. The proposed

115-coordination of the cyclopentadienyl rings was verified by

single crystal X-ray analysis6. Anderson and Crisler7 in 1969

employed thallium cyclopentadienide in an improved synthesis of

Cp3UC1 from UC14. The compound is decomposed by air and water

but is very thermally stable, subliming in good yield at 120-130°

in vacuo. A tetrahydrofuran solution of Cp3UC1 is weakly conducting

and its reaction with silver perchlorate indicates that the U-Cl

bond has ionic character. Wong et al. proposed that the long

U-Cl bond (2.559 A) indicates approximately 50% ionic character6.

Unlike the ionic Cp3Ln (Ln = a lanthanide) complexes, Cp3UC1 does

not react with FeCl2 to give ferrocene".

Fischer et al. l0

reportedthe first synthesis of Cp4U from

UC14 and excess KCp in benzene; a lengthy extraction with n-heptane

gives the product in low yield. A better synthesi,1 employing excess

NaCp in THE increased the yield to 20%7. The dark red complex

sublimes at 200° without substantial decomposition but reacts

rapidly with air and moisture. A complete structural determination

from a single crystal X-ray diffraction study11showed the planar

C5 rings u-bonded to the uranium atom in a regular tetrahedral

array. The bondinc7 probably has some covalent character and

involves use of the 5f orbitals.

9

Tricyclopentadienyluranium can be prepared from UC13 and

KCp in benzene or from reactive uranium metal and UCp412. The

bronze pyrophoric solid is not very thermally stable, decomposing

without sublimation at 120°. The Lewis acid properties of U(III)

are indicated by its reaction with such bases as cyclohexylisonitrile

and 1-nicotine.

The reaction of UC14 and T1Cp in a 1:2 ratio in DME gives a

green-brown solid which can also be prepared by reacting equivalent

amounts of UC14 and UCI34 in DME. Recent results have shown that

the product is probably [Cp3U]2[UC16].2DME 13, not the reported

Cp2UCl2. However, if the two cyclopentadienyl rings are joined

together the following reaction occurs14:

L U Clb Li THE

CI °- Cl'o U U

A C( Ct

'Li" THF

/ \ THF

X = CH 2 , CH2CH2CH2, Me2Si

It has been reported that these compounds react with organolithium

reagents to form stable alkyls but experimental details have not

yet been published.

Isoelectronic with these dicyclopentadienyl derivatives, but

having a bonding face of two carbon and three baron atoms, is the

dicarbollide derivative, [Li(THF)02[U(C2B0-1102C12]. The 1.

structure of the highly distorted tetrahedral molecule has been

communicated15.

Cyclopentadienyluranium trichloride is -relpared as a dimethoxy-

ethane adduct, CpUC13.DME, by the reaction c T1Cp with UC14 in

10

-16 .

Since the Manhattan project began, a great deal of interest

has surrounded the preparation of o-bonded uranium alkyls and,

indeed, the first alkyl derivative to be synthesized, Cp3UPh,

was reported independently by three research groups17-19

Subsequently a series of alkyl derivatives was prepared by the

following reactions:

Cp3UC1 + RLi THE Cp3UR + LiC1

18-19 ru t 18 r 18 R = Me18-19, Bun ' 2' IL

ru .5, 5 I

cis-2-butenyl18

, trans-2-butenyl18

, Pri 18

,

t 18 18 Bu , vinyl , Pha-7..0.

Cp 3UC1 + RMgX --THF ----> Cp 3UR + Mgkl

R = n-xyly119, allyl18

, p-to1y120.

The complexes are thermally stable, decomposing at elevated

temperatures with the liberation of alkane quantitatively18. The

alkyl group abstracts the proton from the cyclopentadienyl ring,

not from the solvent, and evidence excludes a free radical

decomposition pathway.

A comparable series of alkyl complexes was prepared from

Cp3ThC121. Thermolysi.s of Cp3ThR occvz.s in a manner similar to

that of Cp3UR and, in the case of Cp3Th(Bun), the thorium-con-

taining residue has been identified as the (715::1)-cyclopentadienyl-

thorium dimer (I)22

Variable temperature n.m.r. studies of

igally1)18and

Cp3Th(allyl)21indicate that both contai:: 111-allyl

group . A single crystal X-ray study23of the

(I)

Th

11

uranium derivative confirmed that the allyl group is a-bonded in

the solid state although the carbon-carbon double bond is unusually

long. An X-ray study of a single crystal of Cp3U(CmCPh)24showed

the a-nature of the bond between uranium and the phenylethynyl

group.

Kinetic studies of the thermolysis of Cp3UR in toluene indicated

that the thermal stability is in the order primary > secondary >

tertiary due to increasing steric crowding around the metal as

the size of the alkyl group increases. Restricted rotation due

to steric effects is observed about the U-R bond in Cp3UPri 18

Coordinative saturation is invoked to explain why compounds contain-

ing ligands which are not 0-elimination-stabilized can be prepared

and are, in fact, quite stable.

Addition of methanol to solutions of Cp3UR yields Cp3UOMe

and alkane, indicating that the uranium-to-carbon a-bond is

considerably more ionic than the U-Cp n-bond. Other alkoxides

are prepared by the metatheticl:_ reaction of Cp3UC1 with NaOR ,

R = Me, Et, Pri, Bun, But, in :efluxing benzene25. An attempted

preparation of UCp3 from UC13 NaCp in THF gave the ether-

cleavage product, Cp3U(0Bun),3:; the only L:'- able compound26.

The n-butoxide derivative -:=In also be pr., 1-ed in good yield

12

by the following method26:

UC14 + Na0Bun [UC13(0Bun)] + NaC1

[UC13(0Bun)] + 3NaCp ---> Cp3U(0Bun) + 3NaCl

Sodium borohydride reacts with Cp3UC1 in THE to give the

volatile, red-orange Cp3UBH47. Infrared evidence indicates that

the complex has a triply-hydrogen-bridged structure. The boro-

hydride complex reacts with trialkylboranes in benzene to give

Cp3U(H 3BR) which is also believed to contain a U(µ-H)3B group27.

Cyolopentadienyluranium amides are prepared by the reaction

of uranium(IV) diethylamide with two or three equivalents of

cyclopentadiene in pentane, giving Cp2U(NEt2)2 and Cp3UNEt2

respectively

Four unusual multinuclear complexes have been reported. The

reaction of Cp3UC1 with dilithioacetylide and p-dilithiobenzene

affords Cp3U-CFiC-UCp3 and R.-(Cp3U)2(C6114), respectively29.

Similarly, Cp3UC1 reacts with the mono- and dilithio salts of

ferrocene to give (II) and (III) respectively30. These are the

Fe Fe

(II) (III)

only uranium complexes which contain a a-bonded cyclopentadienyl

ring.

The organic chemistry of uranium(IV) has been dominated by

cyclopentadienyl derivatives but in an effort to improve the

und.:!rstanding of bonding involving f-electrons other r-bonded

13

derivatives have been prepared. Triindeny1-31 and tris(benzylcyclo-

pentadienyl)uranium chioride32 have been examined crystallographically.

Both exhibit nearly tetrahedral coordination of the uranium atom -

with the latter unambiguously having 715-bonded cyclopentadienyl

rings. The former, however, has three carbon atoms of the C5-ring

marginally closer to the metal suggesting that 1,2,3-trihapto-

indenyl bonding may be possible.

The novel "sandwich" compound, bis(cyclooctatetraenyl)uranium,

U(COT)233, is isolated from the reaction between U014 and the

dianion of cyclooctatetraene in THE at -30°. It can also be

prepared by reacting finely divided uranium metal with cycloocta-

tetraene34. Single crystal X-ray analysis confirmed that the

r-sandwich compound has D8h symmetry35.

The crystal structure of bis(1,3,5,7-tetramethylcycloocta-

tetraenyl)uranium shows two crystallographically independent

molecules, with one conformation having the methyl groups

approximately staggered and the other rotamer having approximately

eclipsed methyl groups36, suggesting that the rotational barrier

is small.

The thermally unstable allylic complexes, tetra(allyl)uranium37

and tetra(methallyl)uranium38, are prepared by the reaction of

UC14 with the appropriate Grignard. Both are spontaneously

inflammable in air and hydrolytically unstable.

A unique r-arene derivative was prepared from UC14 and A1C13

39 in benzene 39. The structure of (CO 6)11A13Cl12 (Iv) has been

confirmed by a single crystal X-ray structure.

Tetrakis(2,5-dimethylpyrrolyl)uranium4° is of particular

interest because the pyrrolyl group can bond to the metal Ln

a a- (through N) or 7-manner. N.m.r. evidence indicates th2.t. in

CI u, ,c1-----,ci i"

c--- i

CI

AI

CI

CI

CI CI

11+

(IV)

solution the uranium atom adopts a distorted tetrahedral geometry

with one pyrrolyl group 7-bonded and the remaining three a-bonded,

indicating a preferential coordination number, in this case, of

six.

The only report of binary alkyls of uranium(IV) was significant

in that none were sufficiently stable to be isolated41. Reacting

four equivalents of RLi with UC14, the thermal decomposition of

the proposed uranium tetraalkyls was examined (vide infra).

Amides, alkoxides and mercaptides are not formally

organometallic compounds, but the chemical relationship' between them

and metal alkyls warrants their inclusion in this introduction.

The low-melting, green uranium(IV) diethylamide42, and the partially

characterized diiso-propylamide43 are unstable to air and water and

have very limited thermal stability.

Compounds of the type U(OR)4 are known where R = Me, Et, But 42.

Although thermally stable, they are very easily hydrolyzed and

oxidized.

The mercaptides, U(SR)4, R = Et, Bun 42, are thermally unstable

solids which react immediately with moisture and inflame in air.

15

Finally the crystal structure of an unusual uranium complex

with a coordination cumber of fourteen, U(BH4)4, has been reported44.

The X-ray diffraction study of the borohydrido derivative failed to

locate the hydride positions in the helical polymeric solid, but

_ a neutron diffraction study44showed that, of the six BNR, ions

surrounding each uranium atom, four are attached through two bridging

hydrogen atoms and two through three bridging hydrogen atoms.

II. THE CHEMISTRY OF URANIUM(V)

Due to the ease with which uranium(V) disproportionates to

uranium(IV) and (VI) and oxidizes to uranium(VI), the chemistry

of uranium(V) has been limited. A review of the inorganic chemistry

of uranium(V) appeared in 196945. Until now no organometallic

compounds of uranium(V) had been prepared.

The largest class o2 uranium(V) compounds known is the

alkoxides. The pentaalkoxides can be prepared by treating uranium

pentachloride with the alcohol in the presence of a base such as

NaOR, but the difficulty of preparing U20110 generally precludes

this method46,47. Oxidation of uranium(IV) tetraethoxide with

bromine in the presence of sodium ethoxide gives uranium penta-

ethoxide in good yield48'49'50. The methoxide derivative can be

prepared in the same manner46'47,51. The higher alkoxides are

obtained by alcohol exchange with uranium pentaethoxide46'47'51.

Solutions of uranium(IV) tetraalkoxides when exposed to air yield

small amounts of uranium(V) pentaalkoxide46'48

The methoxide and the branched-chain alkoxides are high-

melting solids which sublime readil-' in vacuo46'47'51; the linear

alkoxy-derivatives are brown distil ,:=ble liquids46 Uranium(V)

16

alkoxides are readily hydrolyzed but have good thermal stability.

In benzene, the pentaalkoxides, U(OR)5 where R = Et, Prn, Pri,

Bun, are dimeric, but uranium pentamethoxide is trimeric.

Finally, Na[U(0E-06], Ca[U(OEt)02 and A1[U(OEt)03 have

been prepared49. The aluminium derivative is a green distillable

liquid, the calcium complex is a sublimable green solid and the

sodium salt an involatile green solid.

III. THE CHEMISTRY OF URANIUM(VI)

The chemistry of uranium(VI) is dominated by uranyl complexes

which are generally prepared in aqueous solution. These include

such compounds as UO2(02CMe)250, NaUO2(02CMe)352, UO2(S2CNEt2)53

and UO2(ROCS2)2 (R = Me, Et, Prn, Pr1, Hun, Eiu1)54, the only

actinide xanthates known.

The largest class of binary uranium(VI) complexes is, as with

uranium(V), the alkoxides. Treatment of anhydrous uranyl chloride

with sodium or lithium alkoxide in alcohol gives UO2(0R)2 (R = Me,

Et, Pr", Bu1, But and iso-penty1)55'56. The higher linear alkoxides

can also be prepared by alcohol-exchange with uranyl methoxide.

With some branched-chain alcohols a novel solvolytic disproportion-

ation occurs when the alcohol-exchange technique is employed,

yielding UO(OR)4.ROH and U205(0R)2.2ROH. The former when heated

in vacuo yields U(OR)6 (R Pr1, Bus, But)55. Other routes to the

hexaalkoxides involve treatment of NaU(OR)6 (R = Me, Et, Pr1, Prn)

with benzoyl peroxide, or alcohol exchange of uranium(VI) ethoxide56.

Like the pentaalkomic:es, the hexaalkoxides are readily soluble

in most common organic solv,mts but decompose in water. They

sublimr.: readily in vacuo a7, moderate temperatures but are less

17

thermally stable than uranium(V) alkoxides. They are generally

monomeric in benzene although there is some evidence to suggest

that U(0E06 is partially associated.

There are no known organometallic complexes of uranium(VI).

CHAPTER I

DILITHIUMEIE)CkALKYLURANAIE(IV) COMPLEXES

19

CHAPT1R I

DILITHIUNHMALKYLURANATE(IV) COMPLEUS

I. INTRODUCTION

Binary alkyl transition metal compounds constitute the simplest

class of organometallic compounds but until the rapid expansion of

the field in the 1970's, understanding of their chemical and physical

properties was limited. Failure to isolate any stable binary alkyls

led Parshall and Mrowca in 1968 to write, "By any criterion, simple

transition metal alkyls are unstable."57.

However during the next few years a large number of stable

transition metal compounds containing alkyl and aryl groups were

synthesized (see Table 1). Unfortunately many reported observations

did not satisfactorily distinguish between oxidative, hydrolytic

and thermal stabilities. The "relative stability" of a compound

was often inferred from the ease with which it could be prepared

and occasionally the apparent non-existence of a complex was

explicitly invoked as evidence of its instability.

Recent results of a study of the mean bond dissociation energy

values, D(M-X) (Table 2)83'84, and the bond energy term values,

Table 2.

Mean bond dissociation energ.y values,,

Bond Compound b (kcal'mo1 1) Ref.

Ti-C Ti(GH2But)4 50 83

Ti-C Ti(CH2Ph)4 54 83

Ti-N Ti(NMe2)4 77 83

Ti-0 Ti(OPri)4 110 83

Ta-C TaMe5 62 84

W-C WMe6 38 84

W-C w(Co)6 42 84

TABLE 1 NEUTRAL BINARY TRANSITION METAL ALKYLS

r .f,(,101..),,

TiMe4 ‘

Ti(CH2But14

Ti(CH2SiMe3)4

TiPh4

(58)

(59)

(61)

(63)

(66)

V(nor)4

V(CH2SiMe5)4

V(CH(SiMe3)2)3

V(mesityl)4

(58)

(64)

(71)

(72)

Cr(nor)4 (58)

CrMe4 (60)

Cr(CH2But)4 (60)

Cr(CH2SiMe3) 4 (64)

cr(cH2cPhme2 )4 (60)

Mn(nor)4 .

Mn(CH2But)2

Mh(CH2SiMe3)2

Mn(CH2CPhMe2)2

(58)

(62)

(62)

(62)

Fe(nor)4

00(nor)4

plicH2simc3jn

(58)

(58)

(65)

TiPh2 (67) Cr(CH(SiMe3)2)3(71)

Ti(CH2Ph)4 (68) Cr(But)4 (73) scPh3 ()

Ti(cH2Ph)2 (69) YPh3

TiAd4 (70)

Zr(nor)4 (58) M02(CH2But) 6 (61)

Zr(CH2But) 4 (61) M02(CH2S2.Me 3) 6 (77)

Zr(CH2SiMe3)4 (63) 1402(CH2Ph)6 (77)

Zr(CH2Ph)4 • (68)

Hf(nor)4 (58) TaMe5 (78) WMe 6 (79) ReMe 6 (79)

Hf(CH2But)4 (80) W2(CH,SiMe5)6 (64)

Hf(CH2SiMe3) 4 (63)

Hf(CH2Ph)4 (68) .

21

1(4-X) (Table 3)83 of some transition metal binary alkyls prove that

the metal-carbon c-bond is not inherently weak. Indeed the strength

of the W-C bond in the very thermally stable W(C0)6 is only slightly

greater than that of the thermall 84

Table 3. Bond energy term values, T(M-X)

Bond Compound E (kcal.mo1-1)

Ti-Cl TiC14 103

Ti-C Ti(CH2SiMe3)4 64

Ti-C Ti(cH2But)4 44

Ti-C Ti(CH2Ph)4 63

Ti-N Ti(NMe2)4 81

Ti-N . Ti(NEt2)4 81

Ti-0 Ti(OPr1)4 115

Zr-Cl ZrCl4 117

Zr-C Zr(CH2SiMe3)4 75 Zr-C Zr(CH2But)4 54 Zr-C zr(c1-12Ph)4 74

Zr-N Zr(NMe2)4 91

Zr-N Zr(NEt2)4 89

Zr-O Zr(OPr1)4 132

Hf-Cl HfC14 119

Hf-C Hf(CH2But)4 58

Hf-N Hf(NEt2)4 95 Hf-0 Hf(0Pr1)4 137

inability so far to prepare a particular complex must in no way be

considered as evidence s to the particular stability of that

M-C bond. Kinetic stability, rather than thermodynamic stability, j

is the main principle governing the thermal stability of metal

alkyls.

There are many modes by which a metal alkyl can decompose85,86

Y unstable WMe6 . Therefore, the

22

but the best documented pathway is "0-elimination"; i.e. n-hydrogen

abstraction by the metal, where the coordination number of the

metal in the transition state is formally increased by one:

[H-CH ;1RIt I 1 GER

M-CH2CH2R -CH2 M-H + CH2=CHR

CH2

Recognition that many metal alkyls decompose in this manner

has had substantial practical consequences; the judicious choice

of ligands which are s-elimination stabilized resulted in the

synthesis of a large series of stable binary alkyls (Table 1).

Decomposition by 0-elimination is generally thought to be a

first-order intramolecular process involving a four-centred

transition state. If very bulky ligands render a molecule too

sterically crowded to increase easily its coordination number, the

four-centred transition state becomes energetically inaccessible.

The thermally stable Cr(But)473

(dec. 80°), which has thirty-six

P-hydrogens, is an elegant example of this concept, and one of the

few transition-metal-t-butyl complexes known.

Because of the rapid expansion of the field of binary metal

alkyls following the recognition that thermally stable complexes

could be prepared by use of 0-elimination stabilized alky- groups,

great stress has been laid on the 0-elimination decomposition

pathway. In fact, it is only one of a number of pathways, although

it is generally the preferred decomposition mode for complexes of

simple alkyls which contain a 0-hydrogen.

Other possible decomposition modes include a, y, or reductive

elimination and M-C bond homolysis. With the exception of main

group and mercury alkyls, homolysis is relatively uncommon. The

conceptually simpl- eductive elimination (or coupling) is not well

23

established except for some methyl derivatives of coinage metals.

The loss of alkane through reductive elimination from a hydrido-

alkyl complex produced as an intermediate during metal alkyl

decomposition is likely. A substantial amount of information

indicates that a- (and possibly y-) elimination is a significant

decomposition mode for transition metal alkyls. As this process

likely involves an increase (by one) of the formal coordination

number of the metal, decomposition by this mode can generally be

discouraged by blocking vacant coordination positions.

Lewis bases can be used to fill coordination sites and thereby

stabilize compounds. This is particularly true in the lower valent

transition metal compounds and historically led to the incorrect

idea that transition metal-carbon bonds are weak, since the simple

alkyls at the right-hand side of the periodic table are only stable

in the presence of donor ligands. Pentamethyltantalum decomposes

autacatalytically at 25° but (Me2PCH2CH2PMe2)TaMe5 is stable to

110° 87; (Me3P)2TiMe4 is more stable than TiMe4, and both complexes

are less stable than (Me2PCH2GH2PMe2)TiMe488

due to the well-known

chelate effect.

Coordination sites can also be filled by formation of complex

anions, which yield compounds where the coordination number is greater

than the valence of the metal atom (see Table 4). For example,

TiMe4 decomposes at -40° but [TiMe5] is marginally stable at room

temperature 89

; hexamethyltungsten decomposes slowly at 25° while

DIMe(02 is stable at that temperature 79'91. Many anionic metal ue-e4- 2 alkyls such as [MnMe4]2 62, j 90, ECoMe02 91, [NiMe02.92,

N o(CH2SiMe3)02 91, [ZrMe02- 103, Emo2me 04 104

[AuMe2]- 110

(AuMe4] 110

have been prepared although the corresponding

;..ral derivatives have eluded isolation.

TABLE 2 ANIONIC BINARY TRANSITION METAL ALKYLS

[TiMe 5] (89) EVPh 6J 4 [VPh 3]

Cl/Ph 73 5

(95)

(98) (98)

C0rMe 6j 3 .

C0r 2Me a] 4

[0rPh4] 2

(93) (94) (96)

EMIlMe43 2 EMn(CH 2SiMe 3) 43 2

(62) (62)

[CoMe 4] 2 - (91) No(CH 2SiMe 3) 43 2- (91 )

.

[0r. 2Ph 6J 2 (99) [Fele 02 (90) [NiMe4]2- (92) [0rPh 6] 3 (100) [FePh5] 5 (97) [NiPh 1.] 2 - (92) NrPb53 2 (100) [Ni (01I 2Ph) 43 2 (92) [CrPh4 ] (101)

E0r2( 04H 8) 04 (102)

[ZrMe 6]27 (103) [NbPh 6] it- (105) [Mo 2Me 8] it- (101+) ENbPh 73 3 (107) EMOPh 63 3 (106)

- [TaPh 6] (111) [WMe 8] 2 (91) [rteMe 8] 2- _ (108) [AuMe2] (110)

_ [TaPh 6] it (112) [Re 2Me 8] 2 (109) [AuMe4] (110 ) [TaPh 6] 3 (112)

. .

25

In spite of this surge of interest in the 1970's in binary

alkyls of the transition metals, the field of binary rare earth

and actinide alkyls has remained relatively dormant. The high

thermal stability of cyclopentadienyl derivatives such as Cp2LnR

113-116 (lin= a lanthanide)1 and Cp3AcR (Ac = Th21, U17-20 ) suggests

that an inherent instability of these f-element-carbon a-bonds is

unlikely. Indeed, from thermochemical trends observed in the

transition metal alkyls, hexamethyluranium (if it could be prepared)

would be expected to be more stable than hexamethyltungsten84.

Initial interest in binary uranium alkyls dates back to the

Manhattan project and the search for volatile derivatives to be

used in uranium isotope separation. Attempts to synthesize compounds

such as tetramethyl- and tetraethyluranium were unsuccessful.

More recently, attention has centred around the use of organouranium

complexes as organometallic reagents and catalysts1,3.

- The possible f-orbital participation in bonding, together with

the high coordination numbers and unusual coordination geometries

exhibited by the actinides, but rarely found in transition metal

chemistry, suggest that they may display unusual chemical properties.

Indeed, preliminary reports have shown that the a-bonded organo-

actinide complexes, Cp3UR (R = Me, Ph, PhCH2) can act as catalysts

in the oligomerization of olefinsla

.

The only well characterized binary alkyl complex of an

f-element is tetrabenzylthorium117

. Spectral evidence of this

thermally unstable compound suggests that, like tetrabenzylzirconium

and hafnium, a multihapto metal-benzyl interaction is likely.

Trimethyllanthanum is reported to exist in solution but has not

been isolated

More success has been for'hcomir in the preparation of anionic

26

binary f-element alkyls. The reaction between phenyllithium and

Ln013 (Ln = La, Pr) gave the corresponding Li[LnPh4] 118. The

crystal structure of [Li(THP)OELu(2,6-Me2C6H3)4j has been

reported119; the metal exhibits approximately tetrahedral geometry.

Although the corresponding ytterbium complex was prepared, and is

isostructural, attempts to prepare the xylyl derivatives of the

earlier, larger lanthanides were unsuccessful.

There are no known binary alkyls of uranium although many

attempts have been made in this laboratory120 to prepare neutral

alkyluranium complexes. A study of the thermal decomposition of

uranium(IV) alkyls generated in situ by the metathesis of LI-RLi with

UC14 in diethyl ether or hydrocarbon solvents has been reported41.

The thermally unstable reactive species was presumed to be "URI,"

and it was stated that "a discrete four-connected, monomeric

geometry involving uranium seems reasonable...". The solutions

were unstable, decomposing at low temperatures to give the following

organic thermolysis products:

(i) comparable quantities of alkane and alkene when R

contained a 0-hydrogen, and

(ii) only alkane when no 0-hydrogen was present.

In view of the unusually high coordination numbers and resultant

geometries generally observed in stable uranium complexes, together

with the consistent failure to isolate uranium(IV) tetraalkyls, it

was thought that only highly congested uranium alkyls would be

kinetically stable. Of the many ways to increase steric crowding

about a metal (vide supra) the most obvious involves the use of

Lewis bases. Unfortunately, in this laboratory120

and others

attempts to prepare UR4.Lx or UR4.(L-L) have been unsuccessful.

However it has been possible, despite the very large effective ionic

27

radius of uranium(IV) (0.93 A) to prepare kinetically stable alkyls

of uranium(IV) by the formation cf complex anions. This chapter

describes the synthesis and chemistry of some hexaalkyluranate(IV)

complexes.

II. RESULTS AND DISCUSSION

Uranium tetrachloride reacts with alkyl- and aryllithium

reagents in diethyl ether or a diethyl ether - tetrahydrofuran

mixture at low temperature to afford thermally unstable hexaalkyl-

or -aryluranate(IV) complexes of the stoichiometry Li2UR6.8Et20

and Li2UR6.8THF respectively. Addition of N,N,NI ,Nt-tetramethyl-

ethylenediamine (tmed) to diethyl ether solutions of the complexes

precipitates the dianions as the adducts, Li2UR6.7tmed. Details

are given in the Experimental Section. The compounds and some of

their properties are listed in Table 5.

Table 5. Physical Properties of Uranium(IV) Compounds

Compound Colour M.p. (dec.)

Li2UMe6.8Et20 olive green -20°

Li2UMe6.8THF olive green -15°

Li2UMe6.7tmed dark green _5o

Li2U(GH2SiMe 3) 6.8L 20 light green 30°

Li2U(CH2SiMe3)6.8THF light green 30°

Li2U(CH2SiMe3)6.7tm,-d dark green 350

Li2UPh6.8Et20 ' red 5o

Li2U(o-Me2NCH2C6H4)6.3Et20 red CP

Li2U(0-Me2NCH2C6H4)6.7tmed red 10°

The methyl compounds are the least stable, decomposing with gas

evolution above -20°. Uranium tetrachloride, when treated with four

to eight equivalents of methyllithium, gives only dilithiumhexa-

methyluranate(IV). The reaction, described by Marks and co-worker41,

(UC14 and 4 Ma in diethyl ether) in their study of the thermal

decomposition of "URI," was repeated and Li2UR6.8Et20 was isolated.

Clearly the species studied by Marks were, in fact, these anionic

complexes rather than the neutral "UR4" species. This is born out

by his observation that unreacted UC1,, was present in the residue

after complete decomposition. Because of the insolubility of UCli,

in the reaction solvents, the solutions always contain an excess of

Ma, thus enhancing the probability that maximum alkylation, through

complex anion formation, would occur.

The improved thermal stability of coordinatively saturated

molecules over complexes with vacant coordination sites has been

disCussed (vide supra) and is pertinent here. While the six

coordinate EUMe6j2 is only stable to -20°, Cp3UMe, which is formally

ten coordinate, has a half-life of 6300 hours in toluene at 97° 18

Many factors, such as the charge of the metal and the size of the

ligand, undoubtedly influence the thermal stability of a compound,

but the direct effect of steric crowding which, in turn is related

to the effective ionic radius of the metal, can be seen from the

following table:

Table 6

Complex (Ref) Effective Ionic Radius,A (mn )

WMe6 (79) 0.68 (0+)

[CrMe03 (93) 0.69 (Cr 3+)

[ZrMe6J 2 (103) 0.80 (Zr')

Lume02— 0.97 (01-)

Decomposit: Temperature

. 25° 25° 0°

-20°

29

The green trimethylsilylmethyl and red phenyl and 2-(dimethyl-

aminomethyl)phenyl compounds are more thermally stable than the

corresponding hexamethyluranate(IV) complexes; the solutions

decompose rapidly at room temperature but the solids can be handled

briefly at that temperature without substantial decomposition.

They inflame in air and should be handled with great care (Note:

we have, on one occasion, experienced a violent explosion when

handling solid Li2UPh6 at room temperature). The solids are soluble

in polar solvents such as diethyl ether, tetrahydrofuran and pyridine,

slightly soluble in toluene and insoluble in petroleum. They are

decomposed by trace amounts of oxygen in solvents and by water,

dilute acids and bases. They do not sublime.

Most of the knowledge of the magnetic behaviour of uranium(IV)

compounds is consistent with that expected for an ion with a 5f2

ground-state configuration16 , and indeed, the solution magnetic

moments of the hexaalkyluranate(IV ) complexes (Table 7) are close

to the spin-only moment of 2.83 BM for two unpaired electrons.

Table 7. Solution Magnetic Moments of Uranium(IV) Alkyl Anions.

Compound 1.1, BM (°C)

Li2UMe6.8THE 2.9 (-30°)

Li2UMe6.7tmed 2.9 (-30° ) Li2U(CH2SiMe3)6.8THF 2.7 (0° )

Li2U(GH2SiMe3)6.7tmed 2.3 (0° )

Li2UPh6.8Et20 2.7 (-10° )

Li 2U(04-Me 2NCH2C 6H4) 6.7tmed 2.7 (-10° )

The somewhat higher values exhibited by th.: hexamethyluranate(IV)

30

complexes may be valid, but due to the difficulty in handling the

very thermally unstable complexes, a significant error (of at least

+ 0.1 BM) is incurred. Therefore, within error, the magnetic

moments of all these uranium(IV) complexes should be considered to

be the same. The magnetic moments of the thermally stable solid

complexes, Li2U(CH2SiMe3)6.8THF (2.72 BM) and Li2U(CH2SiMe3)6.7tmed

(2.77 BM)are in good agreement with the solution moments. The

thermal instability of the methyl, phenyl and [(dimethylamino)-

methyl]phenyl complexes precluded magnetic studies of the solids.

The i.r. spectra of the hexakis(trimethylsilylmethyl)uranate(IV)

complexes have the expected bands due to solvent and trimethylsilyl-

methyl vibrations. Significantly, the region between 2600 and

2850 cm-1, where low C-H stretches indicative of Li-H3C interactions

occur62, was clear, suggesting that the solids exist as discrete

ions in the solid state and that they may be formulated as

ELiS4]2PR6], where S is a solvent molecule. The thermal instability

of the other complexes precludes the use of i.r. spectroscopy as a

diagnostic tool for interactions.

It is useful to compare these complexes with the anionic binary

lanthanide [Li(THF)4][LuR4] (R = 2,6-dimethylphenyl), and the

isostructural ytterbium compound119

. A single crystal X-ray study

shows that the anion consists of four 2,6-dimethylphenyl groups in

an approximately tetrahedral c'-bonded array about lutetium. The

cation and anion exist as discrete ions with no reported interaction

between the lithium ion, which is presumably tetrahedrally coordinated

by the. oxygen atoms of tetrahydrofuran, and the aryl ligands. The

crystal structure of the first actinide metallocarbaborane complex,

[Li(THF)02[U(C2B9E 11) 2C12] shows the complex anion to have a distorted

tetrahedral geometry with pE.7.tahapto-1:- dicarbollide ligands15.

31

The lithium counterions exist as discrete [Li(THF)Xunits in

which the THE' molecules are tetrahedrally coordinated around the

lithium atom. In the absence of any structural data, it is

impossible to make a definitive statement concerning the structure

of the complexes Li2UR6.8S, S = Et20 or THF, but it is reasonable

to propose that they may be formulated as ELiSO2[U146] (V).

R

R.

RP.TR

2

S

S

-S Li

(V)

Unless N,N,N',N'-tetramethylethylenediamine acts as a unidentate

ligand, the complexes of stoichiometry Li2UR6.7tmed would require

a formulation [Li(TMED)2]2[UR6(TMED)3], thus invoking a complex

anion with twelve-coordinate uranium. Structures such as (VI)

or (VII) are possible.

N \iN

(VI)

High coordination numbers for uranium(IV ) are not unusual.

Uranium(IV) borohydride, containing a fourteen-coordinate metal

atom, is the only example of tetradecacoordination although Th(BH4)4

N,

R7

32

is thought to be isostructural44. Tetracyclopentadienyluranium(IV),

with four planar C5 rings pentahapto-bonded to the uranium atom in a

tetrahedral array, is formally twelve-coordinate10

, while both

Cp3UBH4 and Cp3UH3BR probably involve dodeca-coordinated uranium(IV)

(based on spectroscopic experiments)27. Finally, Cp3U(allyl) is

ten coordinate in the solid state, having a simple c-bonded allyl

group, but n.m.r. evidence indicates that a twelve-coordinate

n-bonded intermediate exists in solution18.

Furthermore, uranium(IV) appears to have a higher affinity

towards N-donors than 0-donors. Uranium tetrachloride reacts with

nitrogenous bases to give complexes such as UC4.8NH3, uc14.4pip

and UC14.4en, but with oxygen donors to give only UC14.3= and

UC14.4DMS0121.

The bond between the amine and the complex anion in Li2UR6.7tmed

must be quite labile as the thermal stabilities of the tmed complexes

are not significantly greater than those of the ether complexes. In

absence of crystallographic information it is not possible to make an

unequivocal statement about the bonding of the tmed to the metal,

but the possibility that the amine was trapped in the solid lattice

33

during crystallization cannot be ruled out.

The n.m.r. spectra (Table 8) were recorded at low temperature in

pyridine or pyridine-d5. In almost every case some signal broadening

occurs and on rare occasion is so severe as to wash out the multiplet

structure arising from spin-spin splitting. However due to the very

simple nature of the ligands in the uranium(IV) alkyl anions, the

assignments could be made on relative intensities and chemical

shifts. In general the signals assigned to protons on the a-carbon

of the alkyl groups are shifted upfield and broadened substantially

(width at half-peak-height 10-20 Hz) while protons further removed

from the paramagnetic centre exhibit very small isotropic shifts and

very little broadening. Almost no isotropic shift is observed in

the aryl compounds. The chemical shifts of the Lewis bases are

very close to their free resonance positions. The sharpness of the

peaks are likely due to the absence of a-protons but the lack of an

isotropic shift implies that the base-is not bound to uranium, though

this may be caused by the solvent (pyridine) preferentially occupying

the coordination sites. The spectra do not change on cooling or

heating (to just below the decomposition temperature of the compound).

Although it is likely that neutral binary alkyls of uranium(IV),

"UR4", are too thermally unstable to be isolated, it is possible

that uranium(VI) hexaalkyls, MI6, would have some thermal stability

either alone or in the presence of a Lewis base. To this end,

numerous attempts were made to oxidize the complex anions, either in

diethyl ether or a diethyl ether - tmed mixture, at low temperature,

with air, hydrogen peroxide, benzoyl peroxide, t-butyl peroxide,

chlorine, bromine and iodine, but none were successful. I.r.

evidence, indicates that the reaction with the halides produces

either UX4 or L12UX6122. The compounds react with acetic acid

Table 8. Nuclear magnetic resonance spectra of uranium(IV) compounds in pyridine or pyridine-d5

Compound

,

Temp

. •

Alkyl Group .

Lewis Base

T Multplcty

,

Intensity

,

Assign. T

, .

Multplcty Intensity

,

Assign.

• ,

Li2UMe6.8Et20 -40° 10.51 br s 9 CH3 6.64 q 16 OCH2 8.84 t 24 CH3

14i 2UMe6.8THF -40° 10.44 br s 9 CH3 6.32 br s 16 OCH2 8.22 br s 16 CH3

Li2UMe6.7tmed -40° 11.20 br s 9 CH3 7.71 s 14 NCH2 . 7.91 s 42 NCH3

Li2U(CH2SiMe3)6.8Et20 0° 10.80 8 27 SiCH3. 6.64 q 16 OCH2 17.82 br s 6 sicH2 8.98 t 24 CH3

Li 2U( CH 2SiMe 3) 6. 8THE 0° 11.22 s . 27 SiCH3. 6.51 s 16 OCH2 19.63 br s 6 sicH2 8.25 s 6 CH2

Li2U(CH2SiMe3)6.7tmed 0° 10.50 s 27 SiCH3. 7.53 s 14 NCH2 17.56 br s 6 sicH2 7.75 s 42 NCH

Id2UPh6.8Et20 -10° 2.90 M 15 CH 6.8o q 16 OCH2 9.03 t 24 CH3

Li2U(o-Me2NCH2C6114)6.8Et20 -10° 3.05 m 24 CH . 6.64 q 32 OCH2 7.13 s 12 NCH2 8.95 t 48 CH3 8.12 s 36 NCH,

14 2U ( o-Me 2NCH2C 6Hy ) 6. 7tmed 0° 2.98 m 24 CH - 7.48 s 28 NCH2 7.10 s 12 NCH2 7.70 s 84 NCH3 8.20 a 36 NCH3

35

(containing a little acetic anhydride) at -70P giving alkane and

uranium(IV) acetate. Lithiumhexamethyluranate(IV) reacts with

carbon dioxide at -70° and 1 atm affording a small amount of uranium(IV)

acetate although substantial decomposition, as evidenced by gas

evolution and concomitant metal deposition,occurs. I.r. evidence

indicates that CS2 also inserts into the U-Me bond, but the complex.

could not be isolated. The anions react with protic solvents such

as methanol, diethylamine and acetylacetone giving uranium(1V)

methoxide42, diethylamide42, and acetylacetonate123 respectively.

Lithiumhexamethyluranate(IV) reacts with carbon monoxide, nitric

oxide, nitrosyl chloride, ethylene and tetramethylphosphonium

chloride affording brown or black intractable solids from which

no uranium complexes can be isolated. No reaction occurs when the

hexaalkyluranate(IV) complexes are treated with the corresponding

alkyllithium reagent.

Uranium tetrachloride reacts with a large excess of allyl-

lithium to afford the known U(ally1)437 which, when treated further

with allyllithium, does not react. The reactions of UC14 with

neopentyllithium, norbornyllithium, mesityllithium and lithium

phenylacetylide afford brown to black solutions from which no pure

complexes can be isolated.

III. EXPERIMENTAL

Analyses

Microanalyses of thermally stable compounds were by Butterworth

Microanalytical Consultancy Ltd., Middlesex. Due to the thermal

instability of the hexamethyluranate(IV) complexes, it was not possible

to obtain direct analyses. The following procedure was employed to

36

determine the lithium-to-uranium-to-hydrolyzable methyl ratio:

Approximately 3-5 ml of a ca 0.05M solution of the complex in

diethyl ether was cooled to -700. Dry iso-propanol (1 ml) was added

slowly to the solution and the gas evolution measured with a calibrated

gas burette. The residue was hydrolyzed with 1M HC1, the volatile

organic matter removed in vacuo and the aqueous solution diluted to

25 ml. The concentration of lithium and uranium was determined by

Mr. John Gaulton, Imperial College, using a radio-frequency

inductively coupled plasma emission spectrometer.

Except for the hexamethyluranate(IV) complexes, the thermally

unstable complex anions could be handled for a short period of time

at room temperature, and for these derivatives the following procedure

was employed:

Approximately 100 mg of the solid was transferred cold.(i.e.

10-20° below decomposition temperature) into a preweighed Schlenk

tube. The Schlenk was warmed quickly to room temperature and

weighed as rapidly as possible. The solid was cooled to -70° and

1M HC1 added under N2. After warming to room temperature the solution

was diluted to 25 ml. Occasionally, hydrolysis produced a black

solid which would not dissolve in concentrated HC1. In these cases

it was necessary to treat the residue with hot concentrated sulphuric

acid. The solutions were analyzed for lithium and uranium as described

above.

Approximately 10-20 mg of the same solid were suspended in

toluene-d 8 at -70°, 0.1 ml H2O added and the solution warmed to room

temperature. From the n.m.r. of the non-aqueous layer, the ratio of

alkyl to base was determined.

Analyses of the compounds for C and H by standard microanalytical

techniques were consistently low by 10-25% due to incomplete combustion.

37

This phenomenon has been observed for other organoactinides117

and lanthanides and appears to be due to carbide formation.

Analytical data for the uranium(IV) compounds is given in

Table 9.

Spectroscopic Instruments

1 H-n.m.r. spectra were recorded on a Perkin-Elmer R12A (60 MHz)

spectrometer. Signal positions are reported on the T-scale using

hexamethyldisiloxane or benzene as the internal standard. Infrared

spectra were obtained on Perkin-Elmer model 257 or 457 spectrometers

using KBr plates.

Procedure

All preparations and other operations were carried out under

oxygen-free nitrogen, argon, or in vacuum, unless otherwise stated,

using conventional Schlenk techniques124. Petroleum, toluene,

diethyl ether, and tetrahydrofuran were distilled under nitrogen

from sodium-benzophenone. Unless otherwise stated, the term

'petroleum" refers to the low boiling fraction of the petroleum

distillate (bp ca 40-60°). Iso-propanol was refluxed overnight over

Mg - Mg(OPr1)2 and distilled just prior to use. Amires, such as

diethylamine and N,N,N',N'-tetramethylethylenediamine were distilled

from, and stored over, sodium under nitrogen. Pyridine was distilled

twice from KOH and stored under nitrogen over molecular sieves.

For spectroscopic work, dry pyridine was trap-to-trap distilled

in vacuo from KOH onto the liquid nitrogen-cooled solid.

Solvents were deoxygenated by purging with nitrogen for 5-10

min before use or, for spectroscopic work, by the freeze-pump-thaw

method.

38

Reagents

Starting materials were prepared, with some noted modifications,

by literature methods. Methyllithium was prepared as an ca 1M

solution in diethyl ether125; the method of Lewis and Brown for the

preparation of neopentyl- and trimethylsilylmethyllithium in hexane

was adapted to lithium pieces by using a high-speed stirrer126;

phenyllithium was prepared in diethyl ether from lithium metal and

bromobenzene127; allyllithium was prepared from (allyl)triphenyltin

and phenyllithium128; 2-[(dimethylamino)methyl]phenyllithium was

prepared by the metallation of N,N-dimethylbenzylamine with n-butyl-

lithium129; insoluble (diethyl ether) mesityllithium was prepared

from bromomesitylene and lithium chips in diethyl ether

metallation of phenylacetylene with n-butyllithium produced the

insoluble (diethyl ether) lithium phenylacetylide131; norbornyl-

lithium was prepared in 55% yield from norbornyl chloride and lithium

dispersion in hexane58; n-butyllithium was purchased from Maybridge

Chemical Co. Ltd., and was used as received. The concentration of

the soluble lithium reagents was estimated by the double titration

method of Gilman and Cartledge using dibromoethanel32.

Uranium tetrachloride was prepared from UO3 and hexachloro-

propene133 or purchased from U.K.A.E.A., Harwell.

Magnetic Moments

Magnetic moments on solid samples were measured on a magnetic

balance constructed by Dr. D. F. Evans.

Solution magnetic moments were obtained by the Evan's n.m.r.

method134 in standard solutions of pyridine with 3% cyclohexane

or 3% hexamethyldisiloxane. The compound was cooled to -70° and

2 ml of the standard solution added. The solution was warmed to

39

-30° and ca 0.5 ml transferred via stainless steel tubing into the

inner tube of a double n.m.r. tube cooled to -70°. The outer tube

was filled with the standard solution and from the n.m.r. spectra,

the paramagnetic shift was measured.

Dilithiumhexamethyluranate(IV) diethyl ether. (a) Methyl-

lithium(6.7 ml of a 0.90M solution, 6.0 mmol) was added dropwise to

a suspension of UC14 (0.38 g, 1.0 mmol) in diethyl ether (7 ml) at

-70°. The mixture was warmed to -25° and stirred for 2 h. After

filtration the green solution was reduced to ca 10 ml and cooled

to -70° for 24-48 h to precipitate any unreacted methyllithium.

Chilled petroleum was added to the solution at -30° yielding a green

powder. The product was washed with cold petroleUm (2 x 5 ml) and

dried in vacuo at -70° (yield ca 70%).

(b) The reaction between methyllithium (4.5 ml of a 0.90M

solution, 4.0 mmol) and UC14 (0.38 g, 1.0 mmol) afforded, after

filtration from unreacted UC14 (ca 50%) and precipitation, a green

powder (yield ca 45%). These are the conditions used by Marks41

and it is clear that he was dealing with the lithium salts.

Dilithiumhexamethyluranate(IV) tetrahydrofuran. Methyllithium

(6.7 ml of a 0.90M solution, 6.0 mmol) was added dropwise to a

solution of UC14 (0.38 g, 1.0 mmol) in tetrahydrofuran (7 ml) at

-70°. After warming to -25° and stirring for 1 h the solution was

filtered and the green filtrate evaporated at -20°. After extraction

with chilled diethyl ether (3 x 10 ml), the ether solution was

reduced to 10 ml and cooled to -70° for 24-48 h. The small amount

of methyllithium which separated was removed and chilled petroleum

added to the green solution at -30°. The green solid was collected

washed with cold diethyl ether (2 ml) and petroleum (2 x 5 ml) and

40

dried in vacuo at -70° (yield ca 65%).

Dilithiumhexamethiluranate(IV) tmed. (a) N,N,N!,N'-tetramethyl-

ethylenediamine was added to an ethereal solution of dilithium-

hexamethyluranate(IV) at -70°. The green solution was warmed slowly

to -20° and stirred for 2 h. Concentration of the solution, filtration

at -70° (to remove most of the unreacted tmed), and slow addition of

chilled petroleum yielded a dark green powder. The solid was washed

with cold petroleum and dried in vacuo at -70°.

(b) A diethyl ether solution of methyllithium (2.9 ml of a

0.90M solution, 26 mmol) was evaporated to dryness and heated in

vacuo at 100° for 16 h. The solid was suspended in petroleum (40 ml)

and tmed (4 ml) at -70° and UC14 (0.25 g, 6.6 mmol) added. As the

mixture was warmed to -20° a black oil formed. The oil was washed

with chilled petroleum (2 x 10 ml) and then extracted into chilled

diethyl ether. Addition of chilled petroleum to the ether extracts

at -40° gave the dark green powder (yield ca 40% based on UC14).

Reactions of dilithiumhexamethyluranate(IV).

A solution of dilithiumhexamethyluranate(IV) was prepared from

methyllithium (67 ml of a 0.90M solution, 60 mmol) and UC14

(3.80 g, 10 mmol) in diethyl ether (33 ml). Unreacted methyllithium

was removed by crystallization and cold tmed (10 ml) added to the

filtered solution at -30°. The stock solution was stored at -70°

and aliquots of 2 - 5 ml used in the following reactions.

(a) Reaction with air. Compressed air was bubbled through the

solution at -70° affording a black intractable solid which, from i.r.

evidence, did not contain me-...71 groups.

(b) Reaction with hydr - 7en peroxide. A 5% solution of hydrogen

peroxide (0.1 ml) was added to the solution at -70° and the reaction

41

was warmed to room temperature. The i.r. spectrum of the resulting

green solid contained hydroxyl bands but no bands due to C-H stretches.

(c) Reaction with benzoyl peroxide. Solid benzoyl peroxide

was added to the solution at -70°. No visible change occurred until

the compound decomposed without reaction at -20°.

(d) Reaction with chlorine. The solution was stirred at -70°

under one atmosphere of chlorine. There was an immediate gas

evolutic: and a light green solid precipitated. Appearance, solubility

and its i.r. (v 250 cm 1) suggest that the solid was a mixture of

UC14 and LiC1 or Li2UC16122.

(e) Reaction with bromine. Bromine (0.5 ml of a 1M solution

in benzene, 0.5 mmol) was added to an aliquot (5m1, ca 0.5mmol) of

the diethyl ether solution at -70°. Gas was evolved and a black

intractable solid precipitated, which was not further investigated.

(f) Reaction with iodine. Addition of iodine (1.0 ml of a

0.5M solution in benzene, 0.5 mmol) to an aliquot (5 ml, ca 0.5 mmol)

of the diethyl ether solution at -70° resulted in an evolution of

gas and the precipitation of a black intractable solid which was not

investigated further.

(g) Reaction with two equivalents of acetic acid. A toluene

solution of glacial acetic acid dried with acetic anhydride (10 ml

of a 0.1M solution, 1 mmol) was added slowly to an aliquot (5 ml,

ca 0.5 mmol) of the diethyl ether solution at -70°. Gas evolution

occurred immediately and a dark green solid precipitated. Appearance

and i.r. evidence (v 1564, 1518 cm-1

) suggested a mixture of uranium

metal and uranium(IV) acetate.

(h) Reaction with excess acetic acid. Addition of acetic acid

dried with acetic anhydride to the ether solution at -70° afforded

uranium(IV) acetate quantitatively. Lithium acetate was removed by

42

washing the green product with water.

(i) Reaction with carbon dioxide. A dark green intractable

solid precipitated when carbon dioxide was bubbled through the

solution at -70°. I.r. evidence indicated that carbon dioxide

inserted into the metal-carbon bond (v 1560, 1515 cm 1) but the

product could not be purified.

(j) Reaction with carbon disulfide. Addition of carbon

disulfide to the solution at -70° yielded a dark intractable solid

and some gas evolution. The i.r. spectrum showed bands at 1138 and

630 cm-1 and no absorptions near 1500 cm1 (C=S).

(k) Reaction with carbon monoxide, nitric oxide, nitrosyl

chloride and ethylene. Treatment of aliquots of the solution at

-70P with one atmosphere of carbon monoxide, nitric oxide, nitrosyl

chloride, and ethylene afforded black intractable products from which

no isolable uranium complexes could be characterized.

(1) Reaction with methyllithium. Methyllithium was added to

the ether solution at -70° with no visible reaction. The solution

was warmed slowly and at -20P gas evolution commenced. Workup in

the usual manner yielded Li2UMe6.7tmed.

(m) Reaction with tetramethylphosphonium chloride. No visible

reaction occurred when solid tetramethylphosphonium chloride was

added to the ether solution at -70°. On warming, the solution

decomposea without apparent reaction.

(n) Reaction with methanol. Methanol was added slowly to the

ether solution at -70°. A gas was evolved and the green solid

which precipitated was washed with methanol and dried in vacuo at

room temperature. The i.r. spectrum was identical to that of an

authentic sample of uranium(IV) methoxide42; i.r. (Nujol mull)

1165sh, 1110s, 1018vs, 717w, 552sh, 515m and L.77m cm-1.

43

(0) Reaction with diethylamine. The ether solution reacted

immediately with diethylamine with evolution of a gas to give a

green solution. The solvent was evaporated at room temperature

and the residue extracted with petroleum. The extracts were

evaporated to dryness and the product identified by its m.p. (35°)42

and i.r. spectrum43 as tetrakis(diethylamido)uranium(IV).

(p) Reaction with acetylacetone. Treatment of the solution at

-70° with an excess of acetylacetone gave a dark green solution and

gas evolution. Solvent was evaporated at room temperature and the

residue extracted with benzene to give a dark green solid. Crystall-

ization from diethyl ether yielded dark olive-green crystals of

uranium(IV) acetylacetonate, m.p. 176 123

Dilithiumhexakis(trimethylsilylmethyl)uranate(IV) diethylether.

Trimethylsilylmethyllithium (40 ml of a 0.93M solution, 37 mmol)

was added dropwise to a suspension of UC14 (2.31 g, 6.10 mmol) in

diethyl ether (40 ml) at -70°. After warming to 0° and stirring for

lh the solution was cooled to -70° and filtered. The solvent was

removed in vacuo at -10° and the product extracted with a minimum

amount of diethyl ether. Addition of chilled petroleum gave a green

solid which was washed thoroughly with chilled petroleum and dried

in vacuo at -20° (yield ca 80%).

Dilithiumhexakis(trimethylsilylmethyl)uranate(IV) tetrahydrofuran.

'Trimethylsilylmethyllithium (15 ml of a 0.95M solution, 14 mmol)

was added dropwise to a solution of UC14 (0.55 g, 1.45 mmol) in

tetrahydrofuran (15 ml) at -70°. With rapid stirring, the mixture

was warmed to 0° and stirred for 1 h. The solvent was removed in

vacuo at 0° and the residue extracted with a minimum of chilled

diethyl ether. Addition of chilled petroleum to the green solution

1+4

at -70° yielded a dark green solid which was washed with chilled

petroleum and dried in vacuo at 0° (yield ca 65%). I.r. (Nujol mull)

1250s, 1235sh, 1078w, 1070sh, 1020s, 860s, 740w, 731m and 668w cm-1.

The magnetic moment of the solid, as determined by the Guoy method,

was Peff (296K) 2.72 BM.

Dilithiumhexakis(trimethylsilylmethyl)uranate(IV) tmed. (a)

An ethereal solution of hexakis(trimethylsilylmethyl)uranate(IV) was

prepared as described above, from UC1,, (1.37 g, 3.60 mmol) and

trimethylsilylmethyllithium (24 ml of a 0.95M solution, 22mmol).

The solution was cooled to -70° and tmed (4 ml) added slowly. The

reaction was warmed to 0° and the volume reduced to ca 10 ml.

Addition of chilled petroleum produced a dark green solid which was

washed with chilled petroleum and dried in vacuo at 0°.

(b) Trimethylsilylmethyllithium (4.2 ml of a 0.95M solution,

4.0 mmol) was added slowly to a rapidly stirred suspension of U014

(0.38 g, 1.0 mmol) and tmed (1 ml) in petroleum(20 ml) at -70°.

The suspension was warmed to 0° and stirred for 1 h to yield a

heavy black oil. The solvent was decanted, the oil washed with chilled

petroleum (4 x 10 ml) and the resulting green product extracted into

chilled diethyl ether. Some unreacted UC14 remained. Addition of

chilled petroleum to the ether extracts at 0° precipitated the dark

green solid (yield ca 55%). I.r. (Nujol mull) 1270sh, 1248s, 1045w,

1030s, 845s and 735w cm-1. The magnetic moment of the solid (Guoy

method) was Peff (296K) 2.77 BM.

Reactions of dilithiumhexakis(trimethylsilylmethvl)uranate(IV).

A stock solution of dilithiumhexakis(trimethylsilylmethyl)-

uranate(IV) was prepared from trimethylsilylmethyllithium (63 ml of

a 0.95M solution, 60 mmol) and UC14 (3.80 g, 10.0 mmol) in diethyl

45

ether (50 ml). The solid was precipitated, washed thoroughly with

petroleum to remove unreacted trimethylsilylmethyllithium, and

redissolved in diethyl ether (90 ml) and tmed (10 ml). The stock

solution was stored at -70° and aliquots of 2 - 5 ml removed for the

following reactions.

(a) Reaction with air. The green solution reacted immediately

with air at -70° precipitating a black intractable solid which was

not further investigated.

(b) Reaction with hydrogen peroxide. Hydrogen peroxide (3%

solution) was added to the solution at -70°, affording a green

.precipitate. The i.r. spectrum showed absorptions due to M-OH and

M-OH2 vibrations only.

(c) Reaction with benzoyl peroxide. No immediate reaction

occurred when solid benzoyl peroxide was added to the solution at

-70°. On warming to 0° the solution turned brown and deposited a

black intractable solid lacking any trimethylsilylmethyl ligands, as

shown by its i.r. spectrum.

(d) Reaction with t-butyl peroxide. t-Butyl peroxide reacted

immediately with the solution at -70° affording a black intractable

solid devoid of trimethylsilylmethyl ligands, as shown by its i.r.

spectrum.

(e) Reaction with iodine. Addition of iodine (1.0 ml of a 0.5M

solution in benzene, 0.5 mmol) to an aliquot of the solution (5 ml,

ca 0.5 mmol) at -70° and warming to room temperature afforded a black

precipitate. The solid gave a strong, positive halide test and

absorptions due to trimethylsilylmethyl vibrations were absent in

its i.r. spectrum.

(f) Reaction with two equivalents of acetic acid. Addition of

acetic acid dried with acetic anhydride (10 ml of a 0.1M toluene

46

solution, 1 mmol) to an aliquot of the diethyl ether solution (5 ml,

ca 0.5 mmol) at -70° precipitated a black solid which was free of

trimethylsilylmethyl groups. After washing with water to remove

lithium acetate, the i.r. spectrum of the residue still contained

weak acetate absorptions. From appearance, solubility, and spectral

evidence (v 1557, 1512 cm-1). the solid was probably a mixture of

uranium(IV) acetate and uranium metal.

(g) Reaction with methanol. A green solid precipitated from the

ether solution when methanol was added at -70°. After washing with

methanol, the solid was identified by its i.r. spectrum as uranium(IV)

methoxide (vide supra).

(h) Reaction with trimethylsilylmethyllithium. No visible

reaction occurred when trimethylsilylmethyllithium was added to the

solution and the reaction warmed to 0°. Workup in the usual manner

afforded dilithiumhexakis(trimethylsilylmethyl)uranate(IV) tmed.

• (i) PolaroRraphy. The solvent was removed from a 5 ml aliquot

of the ether solution and the solid dissolved in 50 ml chilled

tetrahydrofuran. With a dropping mercury (working) electrode and a

platinum (reference) electrode, the polarograph showed a two-step

oxidation at +0.34v and +0.28v vs. a standard calomel electrode.

Dilithiumhexaphenyluranate(IV) diethyl ether. Phenyllithium

(35 ml of a 0.97M solution, 34 mmol) was added dropwise to a suspension

of UC14 (2.2 g, 5.8 mmol) in diethyl ether (20 ml) at -70°. The

mixture was warmed to and stirred at -5° for 4 h. After filtration

the volume was reduced to ca 15 ml and stored at -70° for 24 - 48 h

to precipitate unreacted phenyllithium. Addition of chilled petroleum

to the ether filtrate yielded a red solid, which was washed with

chilled petroleum and dried in vacuo at -20° (yield ca 455).

47

Reactions of dilithiumhexaphenyluranate(IV).

A standard solution of dilithiumhexaphenyluranate(IV) was

prepared from phenyllithium (62 ml of a 0.97M solution, 60 mmol) and

UC14 (3.80 g, 10.0 mmol) in diethyl ether (38 ml). Unreacted phenyl-

lithium was removed by crystallization and the filtered solution

stored at -70°. Aliquots of 2 - 5 ml were used in the following

reactions.

(a) Reaction with air. The solution reacted at -70° with air

to yield a black intractable solid. There were no aromatic vibrations

in the i.r. of the solid.

(b) Reaction with hydrogen peroxide. Addition of a 3% solution

of hydrogen peroxide to the red ethereal solution at -70P afforded

a green, insoluble solid, which was not further investigated.

(c) Reaction with benzoyl peroxide. Solid benzoyl peroxide was

added to the solution at -70° and the reaction warmed to 0°. No

readtion occurred and with further warming, the compound decomposed.

(d) Reaction with iodine. Addition of iodine (1.0 ml of a 0.50M

solution in benzene, 0.5 mmol) at -70P and warming to room temperature

afforded a dark oil. The i.r. spectrum of the oil was devoid of

aromatic absorptions and a silver nitrate test was positive.

(e) Reaction with two equivalents of acetic acid. A toluene

solution of acetic acid with acetic anhydride (10 ml of a 0.1M solution,

1 mmol) was added to an aliquot of the solution (5 ml, ca 0.5 mmol) at

-70°. On warming a heavy black oil precipitated which was washed with

ether and alcohol until it solidified. The solid showed only acetate

absorptions and, from appearance, was probably a mixture of uranium(IV)

acetate and uranium metal.

(f) Reaction with methanol. A dark green solid precipitated

when metl—nol was added to the solution at -70°. The product was

48

washed with methanol, dried in vacuo and identified by its i.r.

spectrum as uranium(IV) methoxide (vide supra).

(g) Reaction with phenyllithium. No apparent reaction occurred

when phenyllithium was added to the ether solution at -70°. The

reaction was warmed to 0° and workup in the usual manner afforded

dilithiumhexaphenyluranate(IV ) diethyl ether.

Dilithiumhexakist2-E(dimethylamino)methyl]phenylluranate(IV)

diethyl ether. n-Butyllithium (3.85 ml of a 2.6M solution, 10 mmol)

was added to a solution of benzyldimethylamine (1.35 g, 10 mmol) in

diethyl ether (20 ml). After stirring overnight, the product was

filtered and the lithium reagent suspended in diethyl ether (30 ml).

Uranium tetrachloride (0.62 g, 1.6 mmol) was added slowly to the

rapidly stirred suspension at -70°. The mixture was warmed to 0°

and stirred for 4 h. The solution was filtered, the volume reduced to

ca 10 ml and chilled petroleum added. The red solid was filtered,

washed with chilled petroleum and dried in vacuo at 0° (yield ca 65%).

Dilithiumhexakis(2-[(dimethylamino)methyl]phenyl1uranate(IV) tmed.

A solution of hexakis{2-[(dimethylamino)methyl]phenylluranate(IV)

in diethyl ether at -70° was treated with an excess of tmed. The

solution was warmed to 0° and the volume was reduced. Addition of

chilled petroleum precipitated a red solid which was washed with

chilled petroleum and dried in vacuo at 0°(yield ca 70%).

Reactions of dilithiumhexakis{2-[(dimethylamino)methyl]phenylk-

uranate(IV). A standard solution of dilithiumhexakisf2-[(dimethylamino)-

methyl]phenylluranate(IV) was prepared from N,N-dimethylbenzylamine

(8.1 g, 60 mmol), n-butyllithium (23 ml of a 2.6M solution, 60 mmol)

and UC14 (3.80 g, 10.0 mmol). The solution was filtered, tmed

(10 ml) added and the solution stored at -70°. Aliquots of 2 - 5 ml

49

were used for the following reactions.

(a) Reaction with air. A black intractable solid precipitated

when air was bubbled through the solution at -70°. It was not further

investigated.

(b) Reaction with hydrogen peroxide. Addition of a 3% solution

of hydrogen peroxide to the solution at -70° yielded an insoluble

green solid. The i.r. spectrum exhibited only absorptions which

could be assigned to M-OH and M-OH2 vibrations.

(c) Reaction with benzoyl peroxide. No reaction occurred with

solid benzoyl peroxide at -70°. As the reaction was warmed to room

temperature the complex decomposed to a black intractable solid which

was not further investigated.

(d) Reaction with iodine. Addition of iodine (1.0 ml of a

0.5M solution in benzene, 0.5 mmol) to an aliquot of the solution

(5 ml, ca 0.5 mmol) at -70° and warming to room temperature afforded

a black oil. A silver nitrate test of the oil was positive and the

i.r. spectrum was devoid of benzyldimethylamino vibrations.

(e) Reaction with two equivalents of acetic acid. A dark green

solid precipitated when acetic acid, dried with acetic anhydride,

(10 ml of a 0.1M toluene solution, 1 mmol) was added to an aliquot

(5 ml, ca 0.5 mmol) of the solution at -70°. From appearance,

solubilities, and i.r. spectrum, the solid appeared to be a mixture of

uranium(IV) acetate and uranium metal.

(0 Reaction with methanol. Uranium(IV) methoxide38 was isolated,

as the only uranium-containing product, from the reaction between the

ether solution and methanol at -70°.

Reaction of allyllithium with uranium(IV) tetrachloride.

Allyllithium (20 ml of a 0.30M solution, 6.0 mmol) was added slowly

to a suspension of UC14 (0.38 g, 1.0 mc.11) in diethyl ether (30 ml)

5o

at -70°. The reaction was warmed to -25° and was stirred for 1 h.

'At temperatures higher than -20° decomposition, as evidenced by gas

evolution, occurred. The solvent was removed in vacuo at -300 and the

residue extracted with cold petroleum. Reduction of the volume and

cooling to -70° afforded a red crystalline solid. The product was

identified by its n.m.r. spectrum (toluene-d 8) at -25° as U(ally1)4la.

No visible reaction occurred when allyllithium was added to an

ethereal solution of tetraallyluranium(IV) at -30°. Removal of the

solvent at -30° in vacuo and extraction into cold petroleum gave

U(ally1)4 ( 1H-n.m.r.) as the only uranium-containing complex.

Reaction of neopentyllithium with uranium(IV) tetrachloride.

Neopentyllithium (13.9 ml of a 0.76M solution, 10.5 mmol) was added

dropwise to a suspension of UC14 (0.50 g, 1.3 mmol) in diethyl ether

(20 ml) at -70°. A homogeneous brown solution formed immediately and

no further visible reaction occurred while the solution was warmed

to room temperature. Removal of the solvent in vacuo gave a dark

brown oil which would not sublime or chromatograph. The oil was

washed with petroleum to remove any unreacted neopentyllithium.

Attempts to crystallize the oil were unsuccessful. The black oil

gave a positive lithium and chloride test.

Reaction of norbornyllithium with uranium(IV) tetrachloride.

Norbornyllithium (6.0 ml of a 0.51M solution, 3.0 mmol) was added

slowly to a suspension of UC14 (0.19 g, 0. 50 mmol) in petroleum at

-70°. No immediate r::action occurred and the mixture was warmed

slowly to room temperature and stirred for 4 wks. The solution

gradually became dark brown as the UC14 disappeared and a white solid

precipitated. Filtration and evaporation of the solvent in vacuo

gave a brown oil, which would not chromatograph, sublime, nor crystallize

51

and, due to its high.solubility in petroleum, could not be separated

from the excess norbornyllithium nor any bisnorbornyl which was

present. The oil gave a positive lithium and chloride test.

Reaction of lithium hen lacet lide with uranium(IV) tetra-

chloride. n-Butyllithium (3.3 ml of a 2.48M solution, 8.2 mmol)

was added dropwise to a solution of phenylacetylene (0.90 ml, 8.2 mmol)

in petroleum (5 ml). After stirring for 10 min, the white precipitate

was filtered, washed with petroleum and dried in vacuo. The phenyl-

acetylide was dissolved in diethyl ether (20 ml), cooled to -70°

and UC14 (0.25 g, 0.65 mmol) added. No reaction occurred until 0°

when the solution began to turn brown and eventually an intractable

black solid precipitated. No complex could be isolated.

Reaction of mesityllithium with uranium(IV) tetrachloride.

Mesityllithium was prepared by refluxing mesitylbromide (2.4 ml,

16 mmol) and lithium chips (0.22 g, 32 mmol) in diethyl ether (40 ml)

for 48 h. The white slurry was cooled to -70° and UClk (1.1 g,

2.9 mmol) added. The mixture was warmed slowly to 0° and stirred

for 4 h. The solution was filtered and the solvent removed to yield

a dark brown oil which would not chromatograph, sublime, nor

crystallize. The oil gave a positive lithium and bromide/chloride

test.

52

Table 9. Analyses of Uranium(IV1 Compounds

Compound

Li ,

U - Li:U:R

Found Req. Found Req or R:L

Li2UMe6.8Et20 2.08:1:5.88

Li2UMe6.8THF 2.02:1:5.93

Li2UMe6.7tmed 2.03:1:5.99

Li2U(CH2SiMe3)6.8Et20 0.98 1.02 17.6 17.4 6:8

Li2U(CH2SiMe3)6.8TET 0.99 1.04 17.9 17.6 6:8

Li2U(cH2SiMe3)6.7tmed 0.81 0.87 14.5 15.0 6:7

Li2UFh6.8Et20 0.95 1.07 18.3 18.2 6:8

Li2U(0-Me2NCH2C61-10 6.8Et20 0.81 0.85 13.8 14.4 6:8

Li2U(0-Me2NCH2C6H06.7tmed 0.81 0.87 14.4 14.9 6:7

, .

CHAPTER II

TRILITHIUMOCTAALKYLURANATE( V ) COMPLEXES

54

CHAPTER II

TRILITHIUMOCTAALKYLURANATE(V) COMPLEXES

I. INTRODUCTION

As the available literature is devoid of any reports concerning

the organometallic chemistry of uranium(V), it was of interest to

try to extend the study of binary alkyls of uranium(IV) to uranium(V).

The meagre amount of information available concerning the

chemistry of uranium(V) in contrast to the vast number of reports on

uranium(III), (IV), and (VI), is not surprising; uranium(V) readily

disproportionates to uranium(IV) and uranium(VI) and is easily

oxidized to uranium(VI).

The great instability of uranium(V) may, at first glance seem an

undesirable complication to add to the problem of synthesizing

binary alkyls which have already been'shown to possess poor thermal

stability. However, many stable binary alkyls of transition metals,

where the metal is in an "unusual" oxidation state, have been

prepared. For example, the inorganic and coordination chemistry of

chromium is dominated by Cr(III), but almost all stable binary alkyls

involve Cr(IV)58'60. Thus, despite the general scarcity and

instability of uranium(V) inorganic and coordination compounds, it

is not unreasonable to expect that binary alkyls of uranium(V)

would be at least as stable as those of uranium(IV).

Because of the instability of most uranium(V) compounds it was

difficult to find a suitably stable starting material. • The only

well-documented compounds of uranium(V) are the oxides (U205) and

oxo-compounds (e.g. UOX3 and UO2X), the halides (e.c. UX5, MUX6,

55

M3UX8) and the alkoxides (e.g. U(OR)5 ). Only two oxo-metal alkyl

compounds have been prepared from the corresponding axo-halide, but

in each case advantitious amounts of oxygen were required to obtain

the final product. Thus, when Re0C14 was treated with methyllithium,

ReOMe,, could not be isolated unless trace amounts of oxygen were

present in the solvents135. Similarly reduction occurred when

V0C15 was treated with excess trimethylsilylmethylmagnesium chloride

affording VR4. However chromatography of VR4 on cellulose yields

VOR564. in addition to these difficulties compounds of the type UOX3 decompose readily in oxygen-containing solvents.

Uranium pentachloride is extremely sensitive to atmospheric

oxygen and even trace amounts of water vapour. Furthermore it

possesses very limited thermal stability, decomposing even at room

temperature to UC14 and 012.

The only complexes of uranium(V) which possess reasonable

thermal stability and do not rapidly reduce, oxidize, disproportionate

or react with common organic solvents are the alkoxides.

Dimeric uranium(V) pentaethoxide48 can be easily prepared in

large quantities and stored under nitrogen at room temperature for

an indefinite period of time. As a liquid with infinite solubility

in petroleum and diethyl ether, it is an eminently suitable reagent

for a study of uranium(V) chemistry. In this chapter, the reaction

between alkyllithium reagents and urani'.m pentaethoxide is

examined.

II. RESULTS AND DISCUSSION

•

The interactions of uranium(V) ethoxide, U2(OEt)10, with excess

alkyllithium reagents in petroleum or petroleum - diethyl ether

• 56

mixtures afford, on addition of dioxan, the thermally stable green

octaalkyluranate(V ) complexes, Li3UR8.3dioxan, R = Me, CR2But and

CH2SiMe3. The compounds and some of their properties are listed in

Table 10.

Table 10. Physical Properties of Uranium(V) Complexes

Compound Colour M.p. (dec)

Li3UMe 8.3dioxan pale green 265 - 268°

Li3U(CH2But )8.3dioxan olive green 120 - 122°

Li3U(GH2SiMe3)8.3dioxan olive green 150 _ 154°

The thermal stability of the complexes decreases in the order

methyl > trimethylsilylmethyl > neopentyl whereas in most binary

transition metal alkyls, the reverse trend is observed, with the

neopentyl and trimethylsilylmethyl complexes being substantially

more thermally stable than the corresponding methyl complexes.

This trend reflects the increasing steric crowding around the metal

as the size of the alkyl group increases and indicates that not

only is the uranium atom coordinatively saturated, but that the alkyl

groups are so crowded that they interact. As a result, the neopentyl

and trimethylsilylmethyl complexes lre destabilized relative to the

methyl complexes. In comparison, the six-coordinate uranium(IV)

derivatives, EUR02 , are thermally unstable principally due to

coordinative unsaturation, although other factors such as oxidation

state, geometry, U-C bond lengths, etc., obviously contribute to the

thermal stability or instability of a complex.

5?

Despite the larger effective ionic radius of uranium(IV) compared

with uranium(V), [UR02 does not react further with the alkyllithium

reagent to form [UR7]3 or DIR834 . This may be explained by

assuming that addition of a seventh alkyl group to [UR6]2 will

reduce the effective positive nuclear charge experienced by each

ligand and hence reduce the U-C bond energy. The energy gained by

forming a seventh or eighth U-C bond must therefore be less than the

energy lost in breaking the Me-Li bond. Other factors, such as

geometry and bond lengths, undoubtedly contribute to the difference

between the reactivity of uranium(IV) and (V).

The uranium(V) compounds do not sublime and cannot be

chromatographed. They are readily soluble in polar solvents such as

diethyl ether, tetrahydrofuran and pyridine, but are insoluble in

toluene and petroleum. Trace amounts of oxygen in the solvents

decompose the compounds and the solids are slowly turned red by the

small amount of oxygen in ordinary "oxygen-free" nitrogen. They

inflame in air and are decomposed by water, alcohol, dilute acids

and bases.

The compounds react with acetic acid (containing a little acetic

anhydride) giving an insoluble green solid which analyzes as

U0(020Me)3. The same material is produced from the reaction between

uranium(V) pentaethoxide and acetic acid and, after hydrolysis, from

the reaction between carbon dioxide and trilithiumoctamethyluranate(V).

Carbon disulphide also inserts into the uranium-methyl bond, but the

product could not be purified. The anions react with ethanol affording

lithium hexaethoxyuranate(V) which, when treated with one equivalent

of hydrogen chloride, gives uranium(V) pentaethoxide49. The reaction

with acetylacetone yields, after hydrolysis, uranyl(VI) acetyl-

acetonate136

. The reactions of the anions with hydrogen peroxide,

58

benzoyl peroxide, t-butyl peroxide, carbon monoxide, sulphur dioxide,

ethylene, ethanethiol. diethylamine, 1,1,1,3,3,3-hexamethyldisilazane

and tetramethylphosphonium chloride afford brown or black intractable

solids from which no isolable uranium complexes could be isolated.

The l.s.. spectra (Table 11) of the solid octaalkyluranate(V)

complexes contain the expected absorptions due to alkyl and dioxan

Table 11. Infrared Spectra of Uranium(V) Compounds

[Li(dioxan)]3.

Lime 8] [Li(dioxan)]3.

[u(cH2But) 8] [Li(dioxan)]3.

Eu(cH2sime 3 ) 81 (a)

U 2 (Oa ) 1 0

2710w 2705w 2695w 2960s

2598w 2600w 2600w 2884s

1852w 1355s 1246s 1438w

1155sh 1238vs 1103s 1367m

1095s 1105s 1055s 1348m

1050s 1052s . 884m 1120sh

965w 930m 875sh 1088s

880m 882m 858m 1045s

825w 753s 836m 1018m

718m 719m 898s

691m 872m

(a) Film of pure liquid.

vibrations. In addition, two weak absorptions are present in all the

spectra at ca 2700 and 2600 cm-1. These are the low assymmetric

and symmetric C-H stretches indicative of Li-H3C interactions`'- .

A similar situation is found in some lithium alkyls. Thus (MeLi)4

exhibits absorptions at 2840 and 2780 cm-1 137 and (ButLi)4 at

cm-1 138. 2810 and 2730 This shift is thought to be due to

59

interactions. A very short intra-clusterLi...0-H distance, ca 0.1 A

greater than that found in LiH, in the hexameric cyclohexyllithium

confirms this view; the i.r. of the solid hexamer shows absorptions

at 2800 and 2720 cm-1 139. Similar low C-H stretches (between 2710

and 2810 cm-1) are observed in the i.r. spectrum of Li3CrMe6.3dioxan93.

The crystal structure of the octahedrally coordinated chromium

complex shows that each of the lithium atoms is tetrahedrally

surrounded by two oxygen atoms belonging to two different dioxan

molecules and by two methyl groups (VIII). The phenomenon has been

Li -Me —n\ / z

(D--\ (VIII)

observed in other transition metal alkyl anions, such as Li2MnMe4.2THF

and Li2MnMe4.2tmed62 although the Li-H3C interaction has not been

confirmed crystallographically.

On the basis of the i.r. spectra of the Li3UR E1.3dioxan complexes

a molecular structure with eight alkyl groups surrounding the

uranium atom in a dodecahedral or bicapped trigonal prism or anti-

prism geometry with each lithium atom complexed by one dioxan and

bridging a face, is proposed. (For example, see Figure IX).

As with the uranium(IV) complexes, substantial broadening of

60

R

R,

/RN. .,......■• R,...:,,,,

-1.i

R IR'

R

(IX) For clarity, only one Li(dioxan) unit has been shown.

the a-protons of the alkyl group is observed in the n.m.r. spectra

(Table 12). In each case the dioxan resonance was very sharp

confirming that the dioxan is not attached to the uranium atom.

Unlike uranium(IV), the isotropic shift for the uranium(V) complexes

is downfield.

III. IaPERIMENTAL

•

The following changes and additions to the experimental procedure

as described in Chapter I are noted.

Microanalyses and metal analyses were by Butterworth

Microanalytical Consultancy Ltd.. Uranium(V) pentaethoxide was

prepared by bromine oxidation of uranium(IV) tetraethoxide in the

presence of sodium ethoxide48 Dioxan was refluxed over sodium and

distilled prior to use.

Analytical results are recorded in Table 13.

Trilithiumoctamethyluranate(V) dioxan. Methyllithium (30 ml

of a 0.98M solution, 30 mmol) was added dropwise to a petroleum

(30 ml) solution of uranium pentaethoxide (0.54 ml, 2.0 mmol) at

-700. The mixture was warmed to room temperature and stirred for 1 h.

Dioxan (5 ml) was added dropwise to the green solution, precipitating

Table 12. Nuclear Magnetic Resonance Spectra of Uranium(V) Compounds

Compound Solvent Position Multiplicity Intensity Assignment

Li BUMe8.3dioxan pyridine 8.70 br s 1 CH3 6.66 s 1 OCH2

Li3U(CH2BUt)8.3dioxan pyridine 7.85 br s 2 GH2 9.32 • s 9 GH3 6.68 s 3 OCH2

Li3U(CH2SiMe)8.3dioxan pyridine 8.56 br s 2 CH2 10.20 s 9 CH3 6.66 s 3 OCH2

U2(0Et)10 benzene -4.54 br .s 2 CH2 7.99 br s 3 ex,

62

a light green solid which was filtered, washed with petroleum

(2 x 20 ml) and extracted with diethyl ether (2 x 20 ml). The combined

extracts were concentrated to ca 10 ml and cooled to -70°. The -

light green crystals were collected, washed with cold diethyl ether

(2 x 5 ml) and dried in vacuo (yield 0.52 g, 81%).

Reactions of trilithiumoctamethyluranate(V). A 0.1M solution of

trilithiumoctamethyluranate(V) dioxan was prepared by dissolving the

compound (3.2 g, 5.0 mmol) in diethyl ether (50 ml). Aliquots of

2 - 5 ml of the green solution were used in the following reactions.

(a) Reaction with air. When treated with compressed air, the

solution initially turned red and finally deposited a yellow solid.

Appearance, solubilities and i.r. evidence suggest that the yellow

solid was a mixture of lithium and uranium oxides. If exposed only

briefly to dry air, the reaction ceases before the yellow solid

precipitates. The red product is soluble in petroleum although a

lithium test of the petroleum extracts was positive. The oil does

not sublime before decomposition, chromatograph or crystallize.

The n.m.r. spectrum in benzene-d6 exhibited a multitude of singlets

in the region of T 6 - 11.

(b) Reaction with hydrogen peroxide. A 3% solution of hydrogen

peroxide (1 ml) reacted violently with the solution at room temperature.

A gas evolved and a green solid precipitated. The i.r. spectrum

of the solid contained hydroxyl bands but C-H vibrations were absent.

(c) Reaction with t-butyl peroxide. A black intractable solid

precipitated when t-butyl peroxide (0.1 ml) was added to the solution

at -70°. The i.r. spectrum of the solid was devoid of C-H vibrations

and the product was not investigated further.

(d) Reaction with three -,,-:-.1ivalents of a7?tic acid. A toluene

solution of glacial acetic acid dried with acetic anhydride (15 ml

63

of a 0.1M solution, 1.5 mmol) was added slowly to an aliquot (5 ml,

0.5 mmol) of the diethyl ether solution at -70°. Gas evolution

occurred immediately and a dark solid precipitated. The solid was

insoluble in common organic solvents and, after washing thoroughly

with water to remove lithium acetate, the i.r. spectrum of the solid

exhibited only very weak acetate absorptions.

(e) Reaction with carbon monoxide, nitric oxide, sulphur dioxide

and ethylene. Treatment of aliquots of the ether solution at room

temperature with one atmosphere of carbon monoxide, nitric oxide,

sulphur dioxide and ethylene afforded a black precipitate. In each

case, the solid was insoluble in common organic solvents and water.

Appearance, solubilities and its featureless i.r. spectrum suggest

that the product was uranium metal.

(f) Reaction with ethanethiol, diethylamine and 1,1.1113,3,3-

hexamethyldisilazane. Ethanethiol (1 ml), diethylamine (1 ml) and

1,1,1,3,3,3-hexamethyldisilazane were added to aliquots of the diethyl

ether solution at room temperature and the reactions stirred for two

hours. Slowly a black solid precipitated from solution and appearance,

solubilities and i.r. spectrum (no absorptions between 4000 and

600 cm1) suggest that the solid was uranium metal.

(g) Reaction with tetramethylphosphonium chloride. No immediate

reaction occurred when solid tetramethylphosphonium chloride was

added to the diethyl ether solution at room temperature. After

stirring for 16 h a black solid precipitated which, from spectral and

solubility evidence, was probably uranium metal.

(h) Reaction with ethanol. The ether solution reacted

immediately with ethanol wicli evolution of a gas to yield a bright

green solution. The product was readily soluble in diethyl ether and

ethanol, and reacted rapidly with dry hydrogen chloride in diethyl

64

ether to give a brown, petroleum-soluble liquid which was identified

by its n.m.r. and i.r. spectra as uranium(V) pentaethoxide.

(i) Reaction with acetylacetone. Acetylacetone reacted

immediately with the diethyl ether solution to yield, with gas

evolution, a yellow-green solution. The solvent was removed, the

residue extracted with benzene and the combined extracts were washed

with water and dried over anhydrous magnesium sulphate. Addition of

petroleum to the filtered extracts afforded a yellow solid which was

identified by its i.r. and m.p. (225-250°) as uranyl acetylacetonate136.

(j) Reaction with carbon disulphide. Although solid trilithium-

octamethyluranate(V) reacts violently with carbon disulphide to afford

uranium metal, the diethyl ether solution of the complex at room

temperature reacted cleanly, giving a light green precipitate. The

product was insoluble in common organic solvents, but dissolved with

reaction in methanol. The i.r. spectrum exhibited two strong

, absorptions at 1143 cm

-1 0,asym CS2) and 632 cm-1 (vsYm

CS2), but

the crude product was contaminated by lithium and could not be

purified.

(k) Reaction with carbon dioxide. A green solid precipitated

when carbon dioxide was bubbled through the solution at room

temperature. The solid was washed with water and dried in vacuo

at 110° overnight. I.r. (Nujol mull) 1700w, 1560s, 1520s, 1405m,

1020m, 925s and 679m cm-1. Anal. Calcd. for U07C6H9: U, 55.22.

Found: U, 56.1.

(1) Reaction with excess acetic acid. Addition of acetic acid

dried with acetic anhydride to the diethyl ether solution at room

temperature gave U0(02CMe)3, as above.

(m) Reaction of uranium pentaethoxide with acetic acid. A

diethyl ether solution of uranium pentaethoxide reacted immediately

65

with dry acetic acid to give a green solid which was identified from

its i.r. spectrum as U0(020Me)3.

Trilithiumoctakis(neopentyl)uranate(V) dioxan. Neopentyllithium

(19 ml of a 0.84M solution, 16 mmol) was added slowly to a petroleum

(30 ml) solution of uranium pentaethoxide (0.54 ml, 2.0 mmol) at -70°.

After warming to room temperature the solution was stirred for 24 h.

The reaction was filtered and addition of dioxan (3 ml) precipitated

a green solid which was filtered, washed with petroleum (2 x 10 ml)

and dried in vacuo. Recrystallization from tetrahydrofuran/petroleum

afforded a light green solid (yield 1.4 g, 65%).

Reactions of trilithiumoctakis(neopentyl)uranate(V). Aliquots of

2 - 5 ml of a diethyl ether (50 ml) solution of trilithiumoctakis-

(neopentyl)uranate(V) dioxan (2.72 g, 2.5 mmol) were used in the

following reactions.

. (a) Reaction with air. The green diethyl ether solution slowly

turned red when exposed to one atmosphere of dry air. The red oil,

soluble in common organic solvents, was extracted into petroleum,

but all attempts to sublime, chromatograph or crystallize the extracts

were unsuccessful. The n.m.r. spectrum of the oil in benzene-d6

contained a large number of very broad singlets between T 5 and 11.

A lithium test of the oil was positive.

(b) Reaction with t-butyl peroxide. Addition of t-butyl peroxide

to the diethyl ether solution at -70° precipitated a black intractable

solid. The i.r. spectrum of the solid was devoid of absorptions due

to neopentyl vibrations and the product was not further investigated.

(c) Reaction with three equivalents of acetic acid. A black

solid precipitated when a toluene solution of acetic acid dried with

acetic anhydride (7.5 ml of a 0.1M solution, 0.75 mmol) was added

66

slowly to an aliquot (5 ml, 0.25 mmol) of the diethyl ether solution.

The solid was insoluble in common organic solvents, and its i.r.

spectrum was devoid of C-H stretches. The product was not investigated

further. The reaction was repeated in the presence of tmed (0.2 ml)

with the same results.

(d) Reaction with ethanol. Ethanol (1 ml) was added to the

diethyl ether solution at room temperature. No visible reaction

occurred but the solution reacted with dry hydrogen chloride in diethyl

ether to afford a brown solution. The solvent was evaporated and the

brown oil extracted into petroleum and identified by its i.r. and

n.m.r. spectra as uranium(V) pentaethoxide.

Trilithiumoctakis(trimethylsilylmethyl)uranate(V) dioxan.

Trimethylsilylmethyllithium (21 ml of a 0.75M solution, 16 mmol) was

added slowly to a solution of uranium pentaethoxide (0.54 ml, 2.0 mmol)

in petroleum (30 ml) at -70°. The reaction was warmed to room

temperature and stirred for 16 h. Dioxan (6 ml) was added to the

brown solution at room temperature, precipitating a green solid.

After filtration, the solid was extracted with tetrahydrofuran

(2 x 20 ml), the combined extracts reduced in volume to ca 5 ml and the

solid precipitated by addition of petroleum (20 ml). The light green

solid was washed with diethyl ether (10 ml) and petroleum (2 x 20 ml)

and dried in vacuo (yield 1.8 g, 74%).

Reactions of trilithiumoctakis(trimethylsilylmethyl)uranate(V). A

0.025n solution of trilithiumoctakis(trimethylsilylmethyl)uranate(V)

dioxan was prepared by dissolving the complex (1.52 g, 1.25 mmol) in

diethyl ether (50 ml). Aliquots of 2 - 5 ml were used in the following

reactions.

(a) Reaction with air. A yellow solid precipitated when compressed

67

air was bubbled through the solution for 5 min. Solubility and i.r.

evidence suggest that the solid was a mixture of lithium and uranium

oxides.

(b) Reaction with t-butyl peroxide. A black intractable solid

precipitated when t-butyl peroxide was added to the diethyl ether

solution at -70°. The solid was not soluble in common organic solvents

and its i.r. spectrum was clear of absorptions due to C-H stretches.

(c) Reaction with three equivalents of acetic acid. Addition of

a toluene solution of acetic acid dried with acetic anhydride (7.5 ml

of a 0.1M solution, 0.75 mmol) to the ether solution (10 ml, 0.25 mmol)

at -70° afforded a dark solid which was insoluble in common organic

solvents and water. After washing with diethyl ether and water, the

solid gave a negative lithium test. The i.r. spectrum of the solid

showed only very weak acetate absorptions.

(d) Reaction with ethanol. Treatment of the diethyl ether

solution with ethanol and subsequent addition of a small amount of

diethyl ether saturated with dry hydrogen chloride yielded a dark

brown solution. The solvent was removed in vacuo, the oil extracted

with petroleum, and the brown petroleum-soluble oil identified by its

i.r. and n.m.r. spectra as uranium(V) pentaethoxide.

68

Table 13. Analyses of Uranium(V) Compounds.

Compound Li U

Found Required Found Required

Li 3UMe 8( C lill EP2) 3 3.3 3.3 37.4 37.0

Li 3u(0H2But ) 8(04H 6102) 3 1.2 1.3 21.6 22.0

Li 3U( CH2SiMe 3) El( C 41-1 ,p2) 3 1.6 1.7 19.3 19.5

CHAPTER III

ADDITION COMPOUNDS OF URANIUM(VI) HEXAISO-PROPDXIDE

WITH LITHIUM, MAGNESIUM AND ALUMINIUM ALKYLS

70

CHAPTER III

ADDITION COMPOUNDS OF URANIUM(VI) HEXAISO-PROPDXIDE

WITH LITHIUM, MAGNESIUM AND ALUMINIUM ALKYLS

I. INTRODUCTION

No organometallic uranium(VI) compounds have been reported in

the literature but, having had some success with the preparation of

uranium(IV) and (V) alkyls, an examination of the reaction of alkylating

agents with uranium(VI) compounds was undertaken.

The greater proportion of uranium(VI) compounds known are

inorganic uranyl derivatives, such as U02012. The reaction between

metal oxyhalides and organolithium and Orignard reagents is generally

more involved than the simple, and ordinarily desired, metathesis of

the organic group and the halide. The complications have been

discussed and the few known oxo-metal alkyls described (see Chapter II).

The geometry of uranyl complexes generally involves a linear

0-U-0 group with the ligands arranged in a plane perpendicular to the

UO2 axis. Very high coordination numbers involving pentagonal- or

hexagonal-bipyramidal geometries are generally favoured140. While the

reaction between alkylating agents and U02012 would clearly lead to

highly unsaturated and likely unstable complexes, these reactions in

the presence of chelating Lewis bases were examined and are discussed

in this chapter.

Although derivatives of the dioxo-cation, U022+, dominate the

chemistry of uranium(VI), two groups of binary uranium(VI ) compounds

are known. The first, and most reactive, the hexahalides, are the

only non-oxygenated actinide(VI) compounds known. Uranium(VI)

140 hexafluoride possesses good thermal stability but if a trace of

71

moisture is present, complete hydrolysis (by the chain-propagating

reaction of HF with the quartz or Pyrex vessel) occurs. It is readily

reduced by organic solvents at room temperature, and is an unsuitable

starting reagent.

The only actinide hexachloride known is UO16140

. It is very

volatile and not readily hydrolyzed if handled under nitrogen by

ordinary Schlenk techniques. However, like UF6, it is very reactive,

being stable in petroleum but reduced by most organic solvents, even

diethyl ether. Very weakly reducing alkylating agents such as

trimethylaluminium reduce UC16 to uranium metal at low temperature141.

The uranium hexaalkoxides55,56 form the larger and more stable

class of binary uranium(VI) compounds reported. They possess good

thermal stability, are soluble in most common organic solvents

(being monomeric in benzene) and do not reduce as readily as the

hexahalides. The hexaalkoxides were therefore chosen as the most

suitable starting material for the preparation of uranium(VI) alkyls.

In this chapter, the reactions of uranium(VI) hexa(iso-propoxide)

with lithium, magnesium and aluminium alkyls are examined.

II. RESULTS AND DISCUSSION

Reactions of Uranyl Chloride. In view of the high thermal

stability of eight coordinate CURE33 , the moderate thermal stability

of six coordinate PR6]2 and the presumed thermal instability of "UR4",

it was intuitively felt that four coordinate uranyl alkyls, UO2R2,

would be too unstable to isolate and this was found to be true.

Uranyl chloride reacts with a variety of alkylating agents such

as trimethylsilylmethyllithium, bis(trimethylsilylmethyl)-, bis-

(neopentyl)-, and dimethylmagnesium, affording an insoluble green-

72

brown solid after one or two days. The product is completely insoluble

in common organic solvents, its n.m.r. spectrum is devoid of

absorptions due to organic ligands, and appearance suggests that the

solid is uranium dioxide.

Uranyl chloride could be reduced to UO2 in two ways. The first

pathway invokes the transient existence of UO2R2 before reductive

elimination of the alkyl groups:

2R 201 R-R

U02012 [UO2R2] U0 2

However, as complexes such as [UR02 have been shown to decompose

thermally with loss of alkane by proton-abstraction from the solvent41,

giving uranium metal, a similar mechanism may be involved here in

either a one-step:

U02012

2R 201 2H 2RH

[UO2R2] \ / UO 2

or two-step reduction.

R Cl H RH

U02012 [UO2R01] ›[1102C1]

R Cl- H RH

[U0201] [UO2R] > UO2

As the favoured geometries of uranyl complexes are pentagonal,

or hexagc.nal bipyramidal, the alkylation of U02012 in the presence of

N,N,NI ,N1-tetramethylethylenediamine, tmed, and N,N,N',N",N"-penta-

methyldiethylenetriamine, pmdt, in order to block the coordination

sites and form complexes of the type (X) or (XI), was examined.

0 0 R-4--R

N

0 00 (xi)

73

However these reactions also give uranium dioxide.

Reactions of Uranium(VI) iso-Propoxide. The preparation of

binary metal alkyls, such as R2Zn142, R3B142 , R4Cr

60 etc., from the

corresponding metal alkoxides is well established and it was proposed

that uranium(VI) iso-propoxide would react with lithium, magnesium

or aluminium alkyls to give the neutral binary alkyls, UR6, or with

an excess of lithium alkyl, to give the eight coordinate complex

anion, [UR 0 2 •

Neither reaction occurs. Instead, solid ether-free methyllithium

readily dissolves in a petroleum solution of uranium(VI) iso-propoxide

and forms the 3:1 complex, (MeLi)3U(OPri)6. Addition of three

equivalents of trimethylsilylmethyl- or neopentyllithium to uranium(VI)

iso-propoxide affords similar green oils, which do not react with

excess alkyllithium, as is shown by the appearance of free Pali in

the n.m.r. spectrum. Similarly, dimethyl-, bis(neopentyl)-, and

bis(trimethylsilylmethyl)magnesium react with uranium(VI) iso-

propoxide to form (R2Mg)3U(OPri)6, while trimethylaluminium forms a

6:1 adduct, (Me3A1)6U(OPri)6. The compounds are thermally stable

oils which cannot be chromatographed or sublimed. They are very

soluble in common organic solvents such as petroleum, benzene and

diethyl ether and dissolve without reaction in Lewis bases such as

pyridine, tmed, dioxan and dimethoxyethane. While not pyrophoric,

they decompose in air and react rapidly with water, alcohol,

dilute acids and bases.

Very few mixed alkyl-alkoxide bimetallic complexes appear to

be known. The reaction of sodium t-butoxide with diethylberyllium

yields the dimeric [NaBeEt20But)]2143 while beryllium t-butoxide

reacts with trimethylaluminium giving (Me3A1)[Be(alut)2] 3144

which is monomeric in benzene. Complexes of this type are not known

Me Me

MN Al \ M

Me Me

74

for the lanthanides and actinides but some covalent bimetallic

alkoxides, La[A1(0Pri) 433145, Pr[A1(0Pri)43145 U[A1(0Pri)4] 4146,

Ca[U(0E06]249 and Al[U(0E-00349 have been reported. The complexes

are covalent, highly soluble, volatile solids containing bridging

alkoxy-groups. For example, the structure proposed for La[A1(0Pri)04

is (XII). Pro OPr;

Pri Pri0 A 0 0 CAI 0 Pr

. if

/orb Pd . . Prly PrI

Al

Pk)/ Pri

(XII)

A series of bimetallic alkyls, Cp2LnAlMe4 and Cp2LnA1Et4

(Ln = Sc, Y, Gd, Dy, Ho, Er, Tin and Yb) has been prepared147 and the

preliminary crystal structure of Cp2M(Me2A1Me2) (M = Y, Yb) (XIII)

reported.

The n.m.r. spectrum (Table 14) of the methyllithium complex

exhibits a single sharp methyl resonance at 10.9 and the heptet and

Table 14. Nuclear Magnetic Resonance Spectra of Uranium(VI) Compounds.

Compound

OPri

Position Multplcty Intensity Assignment Position Multplcty Intensity Assignment

U(OPr1)6 1.98 h J=10Hz 1 CH

8.33 d " 6 CH3

(meld)3u(OPr1)6 5.94 h J=9Hz 2 CH 10.9 s 3 CH3

8.48 a " 12. CH3

(Me 2Mg) 3U(OPr1)6 6.26 h J:lOHz 1 CH 10.7 s 3 CH3

8.74 d " 6 CH3

[(ButCH2)2Mg]3U(OPr1)6 6.38 h J=10Hz 1 CH 9.65 s 2 CH2

8.65 d " 6 CH3 8.60 s 9 CH3

[(Me 3SiCH2)2Mg]3U(OPr1)6 6.24 h J=10Hz 1 CH 10.1 s 2 CH2

8.45 a " 6 CH3 9.75 5 s 9 CH3

(Me,A1)6U(OPr1)6 6.74 h J=10Hz 1 CH 10.4 s 9 CH3

8.91 a " 6 CH3

76

doublet of the iso-propyl group at 5.94 and 8.48 respectively. The

position of the methyl resonance is slightly downfield from the

resonance position of methyllithium in diethyl ether (11.8), which

suggests that a structure (XIV) in which lithium is solvated by

oxygen donors is reasonable.

Me

Li 0 ///'

.n 13,1 ) • , U —0Prl

' Pr OP')

Li

Me

(XIV)

The alkyl resonances of the dialkylmagnesium complexes,

(R2Mg)3U(OPri)6, are also just downfield from the position of the

corresponding dialkylmagnesium in diethyl ether [Me2Mg 11.4;

(ButCH2)2Mg 9.70,8.55; (Me3SiCH2)2Mg 11.2, 9.60], indicating that

the alkyl groups are probably bound to a magnesium atom which is

solvated by two oxygen-donors. The structure (XV) seems likely for

these compounds. R R

Pr! PrR— Mg 0 \/ (3--+ R

- PrO U

\ OPri

A

(XV)

77

The fact that uranium(VI) iso-propoxide requires six equivalents

of trimethylaluminium for complete reaction reflects the tendency of

aluminium to achieve four coordination. Therefore it seems reasonable

to assume that each trimethylaluminium group is bound by an oxygen

lone pair of one of the iso-propoxy groups. Free trimethylaluminium

is not formed even when the oil is heated to 200° in vacuo. As

the n.m.r. spectrum shows only one methyl resonance, it is unlikely

that any of the methyl groups has transferred to uranium, and

structure (XVI) is therefore proposed.

Pr Pl!

Me3A1.,„,_ Me3 0 0 \

Me3A1-, U Pr 00"."-C) ° 'Naval Me3

0'

1471 I F4 AIMe3 - Al Me3

(XVI)

III. EXPERIMENTAL

The following additions to the experimental procedure described

in Chapter I are noted. For reaction with diaikylmagnesium reagents,

uranyl chloride, UO2C12, was prepared by treating uranium tetrachloride

with oxygen at 300° for 7 h148. For the preparation of uranium(VI)

iso-propoxide, the yellow-green uranyl chloride trihydrate was stored

in vacuo over phosphorus pentoxide for 24 h and dry hydrogen chloride

passed over the monohydrate in a platinum boat heated to 250 - 280°

78

for 36 h. The preparation of uranium(VI) iso-propoxide from uranyl

chloride and iso-propanol has been described55. The synthesis of the

dialkylmagnesium reagents, dimethyl-, bis(neopenty1)-, and bis(tri-

methylsilylmethyl)magnesium, has been reported159. Trimethylaluminium

was purchased from Ethyl Corporation.

Analytical results are reported in Table 15.

Reaction of bis(trimethylsilylmethyl)magnesium with uranyl chloride.

(a) Reaction in diethyl ether. Bis(trimethylsilylmethyl)magnesium

(4.0 ml of a 1.0M solution, 4.0 mmol) was added to a suspension of

uranyl chloride (0.34 g, 1.0 mmol) in diethyl ether (10 ml) at -70°.

The reaction was warmed to room temperature with vigorous stirring.

Over the course of two days the bright yellow starting material

became green-brown and the supernatant liquid remained colourless.

The solid was filtered and washed with petroleum and tetrahydrofuran

and dried in vacuo. The solid was insoluble in common organic solvents

and its i.r. spectrum was devoid of absorptions due to trimethyl-

silylmethyl vibrations.

(b) Reaction in diethyl ether containing tmed or pmdt. Uranyl

chloride (0.21 g, 0.62 mmol) was suspended in diethyl ether (10 ml)

and tmed or pmdt (2 ml) at -70° and bis(trimethylsilylmethyl)-

magnesium (2.5 ml of a 1.0M solution, 2.5 mmol) added. The reaction

was warmed to room temperature and stirred for two days. The

supernatant liquid was colourless and the same green-brown solid

described above had precipitated.

Reaction of dimethylmagnesium with uranyl chloride. The reaction

between dimethylmagnesium (2.8 ml of a 0.75M solution, 2.1 mmol) and

uranyl chloride (0.18 g, 0.53 mmol) in diethyl ether (10 ml) and

tmed or pmdt (2 ml) was carried out under the same conditions

79

described above. No visible gas evolution occurred but after two

days at room temperature the green-brown solid described above was

isolated. The i.r. spectrum of the solid was free of methyl

vibrational modes.

Reaction of bis(neopentyl)magnesium with uranyl chloride. The green-

brown solid was also isolated from the reaction between bis(neopenty1)-

magnesium (2.6 ml of a 1.1M solution, 2.8 mmol) and uranyl chloride

(0.24 g, 0.70 mmol) in diethyl ether (10 ml) and tmed or pmdt (2 ml)

under the conditions described above. Absorptions characteristic of

a neopentyl group were not observed in the i.r. spectrum.

Reaction of trimethylsilylmethyllithium with uranyl chloride.

Trimethylsilylmethyllithium (3.0 ml of a 1.0M solution, 3.0 mmol)

was added dropwise to a suspension of uranyl chloride (0.17 g, 0.5

mmol) in diethyl ether (10 ml) and tmed or pmdt (2 ml) at -70°.

The Teaction was warmed to room temperature and stirred for 1 h to

afford the same insoluble green-brown sold.

Uranium(VI) hexa(iso-propoxide)tris(methyllithium). (a) Methyl-

lithium (2.8 ml of a 0.90M solution, 2.5 mmol) was evaporated to

dryness and heated to 150° in vacuo for 24 h to remove the coordinated

diethyl ether. The unsolvated methyllithium was suspended in petroleum

(5 ml) at -70° and a petroleum (5 ml) solution of uranium(VI) iso-

propoxide (0.50 g, 0.85 mmol) added dropwise. The reaction was warmed

to room temperature and stirred overnight to afford a green solution

and a white solid. (The white sod was identified by n.m.r. as

unreacted methyllithium.) The solution was filtered and the solvent

removed to yield a green oil.

(b) A large excess of methyllithium in diethyl ether was added

80

to an ethereal solution of uranium(VI) iso-propoxide. After careful

drying to remove all diethyl ether, the same product was isolated by

extracting the residue with petroleum.

Reaction of trimethylsilylmethyl- and neopentyllithium with uranium(VI)

iso-propoxide. Both trimethylsilylmethyl- and neopentyllithium in a

three to eight-fold excess, react with uranium(VI) iso-propoxide in

petroleum to afford a clear green solution. In neither case does

lithium iso-propoxide precipitate. Because of the solubility of these

alkyllithium reagents in petroleum, it was not possible to isolate the

product from free alkyllithium (as evidenced by n.m.r.), although

fortuitous addition of only three equivalents of alkyllithium gave

n.m.r. spectra which were consistent.

Uranium(VI) hexa(iso-propoxide)tris(dimethylmagnesium).•

(a) Dimethylmagnesium (1.1 ml of a 0.75M solution, 0.82 mmol) was

added dropwise to a solution of uranium(VI) iso-propoxide (0.16 g,

0.27 mmol) in petroleum (10 ml) at -70°. The reaction was warmed to

room temperature and stirred for 1 h to afford a clear green solution

and no precipitate. Removal of the solvent in vacuo for 5 - 6 h

afforded a green oil free of diethyl ether. The product was extracted

with petroleum (2 x 5 ml) and the combined extracts evaporated to

yield a green oil.

(b) An identical product was obtained when the addition was

reversed, i.e. when a diethyl ether solution of uranium(VI) iso-

propoxide was added to a diethyl ether solution of dimethylmagnesium

at -70°.

(c) Solid dimethylmagnesium was heated to 150° under high vacuum

for 16 h and suspended in petroleum at -70°. Addition of a petroleum

solution of uranium(VI) iso-propoxide afforded the same product

81

described in (a).

Uranium(VI) hexa(iso-propoxide)tris[bis(neopentyl)magnesiuml.

(a) Bis(neopentyl)magnesium (2.3 ml of a 1.1M solution, 2.5 mmol)

was added slowly to a petroleum (50 ml) solution of uranium(VI)

iso-propoxide (0.50 g, 0.85 mmol) at -70°. The reaction was warmed

Slowly to room temperature and stirred for 16 h. The solvent was

evaporated and the green oil dried at room temperature in vacuo for

5 - 6 h. The product was extracted into petroleum and removal of

solvent afforded a green oil.

(b) Addition of a diethyl ether solution of uranium(VI) iso-

propoxide to bis(neopentyl)magnesium at -70° afforded the green oil

described in (a).

(c) Unsolvated bis(neopentyl)nagnesium in petroleum was cooled

to -70° and a petroleum solution of uranium(VI) iso-propoxide added.

The reaction was warmed to room temperature to give the same green

oil.

Uranium(VI) hexa(iso-propoxide)tris[bis(trimethylsilylmethyl)-

magnesiuml. (a) Bis(trimethylsilyimethyl)magnesium (1.0 ml of a 1.2M

solution, 1.2 mmol) was added slowly to a petroleum solution of

uranium(VI) iso-propoxide (0.23 g, 0.39 mmol). The reaction was

warmed to room temperature and stirred overnight. Removal of solvent

and extraction with petroleum gave a green oil.

(b) Unsolvated bis(trimethylsilylmethyl)magnesium was prepared

by heating the diethyl etherate in vacuo at 140°. The white solid was

suspended in petroleum at -70° and a petroleum solution of uranium(VI)

iso-propoxide added slowly. Warming to room temperature afforded the

product described in (a).

(c) Reverse ad-'ition of the same reactants in diethyl ether also

:fielded the green oil described in (a).

82

Uranium(VI) hexa(iso-propoxide)hexakis(trimethylaluminium).

Trimethylaluminium (5.4 ml of a 0.50M solution in petroleum, 2.7 mmol)

was added slowly to a petroleum (20 ml) solution of uranium(VI) iso-

propoxide at -70°. No visible reaction occurred at -70° but warming

to room temperature afforded a clear green solution. The solvent

was removed and the product heated to 50° under high vacuum for

4 - 5 h, yielding a green oil.

Table 15. Analyses of Uranium(VI) Compounds.

Compound

U

,

Id, Mg or Al

Found Required Found Required

(MeLi)3U(OPri)6 36.4 36.2 3.3 3.2

(Me2Mg)3U(OPri)6 30.0 31.5 10.3 9.7

[(ButCH2)2Mg]3U(OPri)6 22.0 21.8 7.0 6.7

[ (Me 3SiCH2)2Mg]p(OPri)6 19.8 20.1 6.2 6.1

(Me3A1)6U(OPri)6 30.3 29.5 10.4 10.0 _ .

CHAPTER IV

ORGA NOMA T.LT C CHEMISTRY OF A DA MA NM NE

84

CHAPTER IV

ORGANOMEMALLTC CHEMISTRY OF ADAMANTANE

I. INTRODUCTION

Transition metal norbornyl compounds form an extensive class of

binary metal alkyls, all the tetranorbornyls of the first row metals

from titanium to cobalt being known58. The 1-norbornyl ligand is both

a- and n-elimination stabilized and thus decomposition by pathways

followed by many metal alkyls is prevented. In addition, the bulky,

rigid bicyclic structure protects the metal-carbon bond from attack

by hydrolyzing or oxidizing agents.

Adamantane (tricyclo[5.3.1.13'7]decane) (XVII) is a large

tricyclic hydrocarbon with a rigid framework which possesses all the

(XVII)

qualities of norbornane as a ligand for stable binary transition

metal alkyls and, in addition, is substantially larger. Until very

recently the organometallic chemistry of adamantane has received

little attention.

Lansbury and Sidler149 proposed that an equilibrium results when

t-butyllithium reacts with 1-iodoadamantane at -70P in a diethyl

ether - petroleum mixture:

2ButLi + AdI ButI + AdLi + ButLi

The reaction mixture, when quenched with benzaldehyde, yields

85

60 - 80% of an equimolar mixture of t-butylphenylcarbinol and

adamantylphenylcarbincl. When one equivalent of t-butyllithium is

used, only 1,1'-bisadamantyl is isolated. The isolation of the

coupled product from the reactions, even at -70°, reflects the

stability of the adamantyl radical.

Because of the difficulty in preparing 1-adamantyllithium,

adamantyl complexes are more readily prepared by reaction of organo-

metallic anions with 1-haloadamantane. Adamantyltrimethyltin150,151

is prepared from sodium trimethylstannate(IV) and 1-bromoadamantane,

while the reaction between potassium pentacyanocobaltate(II)

K3[Co(CN)5], and 1-iodoadamantane yields potassium 1-adamantyl-

pentacyanocobaltate(M), K3[AdCo(CN) 5]152, which contains a cobalt-

carbon a-bond. The synthesis of 1-adamantyl-V-cyclopentadienyl-

dicarbonyiron(II) by the decarbonylation of 1-AdC0Fe(C0)2(115-05H5)

has been reported

The tetra-l-adamantyl derivatives of silicon, germanium, tin and

titanium have been prepared in 18 - 26% yield by the Wurtz-Fittig

reaction154. They are stable to air, hydrogen peroxide, concentrated

acids and bases.

The high stability of bridgehead adamantyl compounds prompted

an investigation of the synthesis of adamantyllithium and Grignard,

and their use as reagents for preparing binary transition metal

adamantyl compounds.

II. RESULTS AND DISCUSSION

The reaction between 1-haloadamantane and magnesium in refluxing

diethyl ether does not afford the Grignard reagent. Instead

1-chloroadamantane is converted slowly (and 1-iodoadamantane rapidly)

to the coupled product, 1,11-bisadamantyl. Studies of highly reactive

86

magnesium metal155, prepared from magnesium chloride and potassium,

have shown that Grignard reagents of bicyclic bridgehead halides,

which are not obtained by standard methods, can be prepared. However,

when a tetrahydrofuran solution of 1-chloro-or 1-bromoadamantane is

added to a refluxing suspension of freshly prepared magnesium, the

only product is 1,11-bisadamantyl. 1-Chloroadamantane is also slowly

converted to the coupled product by reactive magnesium in tetrahydro-

furan at room temperature, but a very dilute solution (ca 0.03M)

of 1-adamantyl magnesium bromide in tetrahydrofuran is obtained from

the reaction of 1-bromoadamantane and reactive magnesium at room

temperature (approximately 70% of the starting material is converted

to 1,11-bisadamanty1).

Hydrolysis of the Grignard solution affords adamantane in

approximately 25% yield, but the solution does not react with gaseous

carbon dioxide. Titanium tetrachloride was completely reduced by the

Grignard solution at low temperature whereas the reaction with

chromium(IV) t-butoxide gives a green insoluble solid which appears

to be chromium(III) bromide.

Adamantane is not metallated by n-butyllithium-tmed, but reacts

with n-butyllithium in the presence of potassium t-butoxide in

petroleum giving an orange precipitate. This solid does not react

with lithium bromide in diethyl ether, and addition of titanium

tetrachloride or chromium(IV) butoxide results in reduction.

1-Lithioadamantane, prepared in situ by the low temperature

exchange of 1-iodoadamantane with two equivalents of t-butyllithium,

reacts with chromium(IV) t-butoxide at -70°, affording, together with

substantial rE—Action, a brown petroleum-soluble oil. Chromatography

on basic alumina yields dark red tetra-t-butylchromium(IV) in 15%

yield. No adamantylchromium complexes were isolated.

87

Titanium tetrachloride is reduced by a petroleum - diethyl ether

solution of adamantyl- and t-butyllithium at -70°. Only two petroleum-

soluble products are formed during the reaction: 1,1'-bisadamantyl

and tetraadamantyltitanium. After repeated recrystallization,

tetraadamantyltitanium is isolated in 8% yield.

III. EXPERIMENTAL

The experimental procedure has been described in Chapter I. The

following changes and additions are noted. The preparation of

1-chloro-, 1-bromo-, and 1-iodoadamantane156, t-butyllithium157

and chromium(IV) t-butoxide158 have been described. The preparation

of highly reactive magnesium metal155 has been reported.

1-Haloadamantane and Activated Magnesium. (a) A suspension of

freshly cut potassium (1.00 g, 25.6 mmol), finely powdered magnesium

chloride (1.36 g, 14.3 mmol) and potassium iodide (1.06 g, 6.40 mmol)

in tetrahydrofuran (35 ml) was refluxed for 2.5 h. A solution of

1-bromoadamantane (1.38 g, 6.42 mmol) in tetrahydrofuran (10 ml)

was injected into the refluxing mixture and the reaction monitored by

g.l.c. of a hydrolyzed aliquot. After refluxing for one hour, the

adamantyl bromide had been completely converted to bisadamantyl.

(b) A suspension of active magnesium in tetrahydrofuran was

prepared as described above and cooled, with stirring, to room

temperature. A tetrahydrofuran solution of adamantyl bromide was

added and the reaction stirred overnight at room temperature. The

suspension was centrifuged and the dark brown-solution filtered.

G.1.c. of a hydrolyi ,d aliquot showed a trace amount of unreacted

adamantyl bromide, 1,1'-bisadamantyl (ca 60%) and adamantane (ca 40%).

The concentration of the solution, determined by back titration of

88

an acidified aliquot, was 0.032M (yield ca 23%).

(c) Addition of 1-chloroadamantane (1.09 g, 6.42 mmol) to a

refluxing suspension of active magnesium gave only 1,1,-bisadamantyl

in 2h.

(d) Adamantyl chloride (1.09 g, 6.42 mmol) did not react

immediately with a rapidly stirred suspension of freshly prepared

active magnesium at room temperature, but after stirring overnight

the chloride was completely converted to the Wurtz-coupled product.

Reactions of adamantyl magnesium bromide.

A 0.03M tetrahydrofuran solution of adamantyl magnesium bromide

was used in the following reactions.

(a) Reaction with carbon dioxide. No visible reaction occurred

when dry carbon dioxide was bubbled through the solution for three

hours. The mixture was hydrolyzed with dilute HC1 solution and the

solution extracted with diethyl ether. The extracts contained only

adamantane and l,l'-bisadamantyl. NO adamantane carboxylic acid

was produced.

(b) Reaction with titanium tetrachloride. Titanium tetrachloride

(4.0 ml of a 0.1M solution in diethyl ether, 0.4 mmol) was added

slowly to the Grignard solution (55 ml, 1.6 mmol) at -70°. A fine

black solid precipitated as the solution was warmed slowly to room

temperature and stirred for two hours. The dark brown colour of

the Grignard was completely discharged. The solvent was removed in

vacuo and the solid extracted with petroleum (2 x 20 ml). G.l.c.

of the petroleum extracts showed only one product which was identified

by m.p. and i.r. as 1,1!-bisadamantyl. The i.r. spectrum of the

residue was devoid of adamantyl vibrations.

(c) Reaction with Chromium(IV) t-butoxide. Freshly sublimed

chromium t-butoxide (0.15 g, 0.44 mmol) in petroleu: ml) was

89 ••

added to the Grignard (60 ml, 1.8 mmol) at -70°. The reaction was

warmed to room temperature and stirred for three days, slowly

precipitating an olive-green solid as the only chromium-containing

species. It was insoluble in common organic solvents and gave a

positive halogen test. Its i.r. spectrum did not contain absorptions

due to C-H stretches.

Adamantane and n-Butyllithium-tmed. n-Butyllithium (10.5 ml of a

2.6M solution, 27.6 mmol) was added dropwise to adamantane (3.77 g,

27.6 mmol) in petroleum (40 ml) and tmed (4.2 ml, 28 mmol) at room

temperature and the reaction stirred for 16 h. Benzaldehyde (2.8 ml,

28 mmol) was added to the reaction and stirring was continued for

two hours. The solution was hydrolyzed with dilute HC1 and the

products identified by g.l.c. as trace amounts of benzaldehyde and

approximately equal quantities of adamantane and n-butylphenyl-

carbinol.

Adamantane and n-Butyllithium with Potassium t-Butoxide. n-Butyl-

lithium (9.50 ml of a 2.63M solution, 25.0 mmol) was added to a

adamantane (3.40 g, 25.0 mmol) and potassium t-butoxide (2.80 g,

25.0 mmol) suspended in petroleum (50 ml) and the solution stirred

for two hours. An orange solid precipitated and was centrifuged,

washed with petroleum (2 x 20 ml) and dried in vacuo.

(a) Reaction with titanium tetrachloride. The orange solid

was suspended in petroleum (20 ml) and cooled to -70°. Titanium

tetrachloride (0.2 ml, 1.8 mmol) in petroleum (2 ml) was added

slowly and the reaction warmed to room temperature affording an

insoluble black solid. The i.r. spectrum of the solid was devoid

of C-H vibrations.

(b) Reaction with chromium(IV) t-butoxide. :roleum (10 ml)

90

solution of chromium t-butoxide (0.68 g, 2.0 mmol) was added to a

suspension of the oran,s-e solid, prepared as above, in petroleum

(20 ml) at -70°. The reaction was warmed to room temperature,

precipitating a black insoluble solid. The i.r. spectrum of the

solid was clear in the 4000 to 600 cm-1 region.

1-Iodoadamantane and t-Butyllithium. t-Butyllithium (11.1 ml of a

0.90M solution, 10 mmol) was added slowly to 1-iodoadamantane

(2.6 g, 10 mmol) in diethyl ether (5 ml) at -70°. The solution was

stirred for 5 min at -70° and immediately used in the following

reactions.

(a) Reaction with chromium(IV) t-butoxide. Chromium t-butoxide

(0.69 g, 2.0 mmol) in petroleum (15 ml) was added slowly to the above

solution at -70° and warmed to room temperature yielding a brown

solution and a dark solid. The petroleum solution was chromatographed

on basic alumina affording a dark red petroleum eluant. Removal of

the solvent in vacuo yielded a red solid which sublimed at 55 - 60°

under high vacuum. The product was identified by i.r. and m.p.

(80°) as tetra-t-butylchromium(IV) (yield 0.08 g, 15%).

(b) Reaction with titanium tetrachloride. Titanium tetrachloride

(0.22m1, 2.0 mmol) in petroleum (5 ml) was added slowly to the above

solution at -70° and the reaction warmed to room temperature affording

a colourless solution and a substantial amount of insoluble black

solid. The solution was filtered and the solvent evaporated to give

a white sc id. Repeated recrystallization from CC14/ethanol afforded

pure tetraadamanty:titanium, m.p. 233° (yield 0.09 g, 8%).

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