Specific Sequestering Agents for the Actinides - American Chemical

7 Specific Sequestering Agents for the Actinides 1

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 6, 2016 | http://pubs.acs.org Publication Date: September 23, 1980 | doi: 10.1021/bk-1980-0131.ch007

KENNETH N. RAYMOND, WILLIAM L. SMITH, FREDERICK L. WEITL, PATRICIA W. DURBIN, E. SARAH JONES, KAMAL ABU-DARI, STEPHEN R. SOFEN, and STEPHEN R. COOPER Department of Chemistry and Divisions of Materials and Molecular Research and Biology and Medicine, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720 Abstract This paper summarizes the current s t a t u s o f a c o n t i n u i n g project d i r e c t e d toward the s y n t h e s i s and c h a r a c t e r i z a t i o n of chelating agents which are specific f o r a c t i n i d e ions — especially Pu(IV). A biomimetic approach has been used that relies on the observation that Pu(IV) and F e ( I I I ) have marked similarities that i n c l u d e t h e i r biological t r a n s p o r t and distribution i n mammals. Since the n a t u r a l l y - o c c u r r i n g F e ( I I I ) sequestering agents produced by microbes commonly c o n t a i n hydroxamate o r c a t e c h o l a t e f u n c t i o n a l groups, these groups should a l s o complex the a c t i n i d e s s t r o n g l y , and s e v e r a l m a c r o c y c l i c l i g a n d s i n c o r p o r a t i n g these moieties have been prepared. We have reported the isolation and s t r u c t u r e analysis of an isostructural s e r i e s o f t e t r a k i s ( c a t e c h o l a t o ) complexes w i t h the general s t o i c h i o m e t r y Na [ M ( C H 4 O 2 ) 4 ] •21 H O (M = Th, U, Ce, H f ) . These complexes are s t r u c t u r a l archetypes f o r the c a v i t y that must be formed if an actinide-specific sequestering agent i s to conform ideally t o the c o o r d i n a t i o n requirements o f the c e n t r a l metal i o n . The [ M ( c a t ) ] 4 - complexes have the D d symmetry of the t r i g o n a l - f a c e d dodecahedron. The complexes Th[R'C(O)N(O)R] have been prepared where R = i s o p r o p y l and R' = t-butyl or neopentyl. The neopentyl d e r i v a t i v e is a l s o relatively c l o s e t o an idealized D d dodecahedron, w h i l e the sterically more hindered t - b u t y l compound is d i s t o r t e d toward a cubic geometry. A s e r i e s o f 2,3-dihydroxybenzoyl amide d e r i v a t i v e s o f linear and cyclic tetraazaand diazaalkanes have been prepared. S u l f o n a t i o n o f these compounds improves the metal complexation and e x c r e t i o n o f plutonium by t e s t animals. A t low dose levels, these results substantially exceed the capabilities o f compounds p r e s e n t l y used f o r the t h e r a p e u t i c treatment o f a c t i n i d e contamination. 4

6

4

2

2

4

2

1To whom correspondence should be addressed at the Department of Chemistry, University of California.

0-8412-0568-X/80/47-131-143$07.50/0 © 1980 A m e r i c a n C h e m i c a l Society Edelstein; Lanthanide and Actinide Chemistry and Spectroscopy ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

144

LANTHANIDE

AND

ACTINIDE

CHEMISTRY AND

SPECTROSCOPY


Introduction With the commercial development of nuclear r e a c t o r s , the a c t i n i d e s have become important i n d u s t r i a l elements. A major concern of the nuclear i n d u s t r y i s the b i o l o g i c a l hazard a s s o c i a t e d with nuclear f u e l s and t h e i r wastes 01, 2). In a d d i t i o n to t h e i r chemical t o x i c i t y , the high s p e c i f i c a c t i v i t y of alpha emission e x h i b i t e d by the common isotopes of the transuranium elements make these elements potent carcinogens (3, ^5, j6, 7). U n l i k e organic poisons, b i o l o g i c a l systems are unable to d e t o x i f y metal ions by metabolic degradation. Instead, unwanted metal ions are excreted or immobilized (8). Unfortunately, only a small p o r t i o n of absorbed t e t r a - or t r i v a l e n t a c t i n i d e i s e l i m i n a t e d from a mammalian body during i t s l i f e t i m e . The remaining a c t i n i d e i s d i s t r i b u t e d throughout the body but i s e s p e c i a l l y found f i x e d i n the l i v e r and i n the skeleton (5, 7_, 9-12). While the a b i l i t y of some metals to do damage i s g r e a t l y reduced by i m m o b i l i z a t i o n , l o c a l high concent r a t i o n s of r a d i o a c t i v i t y are produced by immobilized a c t i n i d e s — thereby i n c r e a s i n g the absorbed r a d i a t i o n dose and c a r c i n o g e n i c p o t e n t i a l . Removal of a c t i n i d e s from the body i s t h e r e f o r e an e s s e n t i a l component of treatment f o r a c t i n i d e contamination. Conventional c h e l a t i n g agents as d i e t h y l e n e t r i a m i n e p e n t a a c e t i c a c i d , DTPA (Figure 1), remove much of the s o l u b l e a c t i n i d e present i n body f l u i d s , but are almost t o t a l l y i n e f f e c t i v e i n removing the a c t i n i d e a f t e r i t has l e f t the c i r c u l a t i o n or a f t e r h y d r o l y s i s of the metal to form c o l l o i d s and polymers (13, 14, 15). The i n a b i l i t y of DTPA to completely coordinate the t e t r a valent a c t i n i d e s i s shown by the easy formation of ternary complexes between Th(DTPA) and many b i d e n t a t e l i g a n d s (16, 17_, 18). The h y d r o l y s i s of Th(IV) and U(IV) DTPA complexes at pH near 8 i s explained by the d i s s o c i a t i o n of H from a coordinated water molecule (19, 20, 21, 22). In a d d i t i o n , the polyaminocarboxylic a c i d s are t o x i c because they i n d i s c r i m i n a t e l y complex and remove b i o l o g i c a l l y important metals, e s p e c i a l l y z i n c (23, 24, 25, 26). Thus there i s a need to develop new and powerful c h e l a t i n g agents h i g h l y s p e c i f i c f o r t e t r a v a l e n t a c t i n i d e s , p a r t i c u l a r l y Pu(IV). While not the most t o x i c , plutonium i s the most l i k e l y t r a n s uranium element to be encountered. Plutonium commonly e x i s t s i n aqueous s o l u t i o n i n each of the o x i d a t i o n s t a t e s from I I I to VI. However, under b i o l o g i c a l c o n d i t i o n s , redox p o t e n t i a l s , complexat i o n , and h y d r o l y s i s s t r o n g l y favor Pu(IV) as the dominant species (27, 28). I t i s remarkable that there are many s i m i l a r i t i e s between Pu(IV) and F e ( I I I ) (Table I ) . These i n c l u d e the s i m i l a r charge per i o n i c - r a d i u s r a t i o s f o r F e ( I I I ) and Pu(IV) (4.6 and 4.2 e/A r e s p e c t i v e l y ) , the formation of h i g h l y i n s o l u b l e hydroxides, and s i m i l a r t r a n s p o r t p r o p e r t i e s i n mammals. The m a j o r i t y of s o l u b l e Pu(IV) present i n body f l u i d s i s r a p i d l y bound by the i r o n t r a n s p o r t p r o t e i n t r a n s f e r r i n at the s i t e which normally binds F e ( I I I ) . In l i v e r c e l l s , deposited plutonium i s i n i t i a l l y bound to the i r o n storage p r o t e i n f e r r i t i n and +

Edelstein; Lanthanide and Actinide Chemistry and Spectroscopy ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


a

2

4 +

2

4 +

3+

+ 40H

+ H 0 -* P u ( O H )

4

2+

+ 30H

+ H 0 -> F e ( O H )

3 +

a

+ H

+ H

+

+

Pu

;

1

0.96

1

2

-

4

S i m i l a r i t i e s of P u

4 +

3 +

=4

3

8

(10

5

5

(10

1

4

3

per OH")

per OH )

4

K = 0.031 ( i n HC10 )

K Z 10

K = 0.0009

K ~ 10

1

6

> oil -

Fe3+

and F e

3 +

3 +

4+ Pu i s transported i n the blood plasma o f mammals as a complex of t r a n s f e r r i n , the normal F e transport agent. The Pu*"* binds at the same s i t e as F e .

Pu

R e f . 74.

3)

3 +

Pu(OH) -> P u

Fe

Fe(0H ) + F e

2)

3

Charge Ionic r a d i u s

1)

Table I.


146

LANTHANIDE

AND

ACTINIDE CHEMISTRY

AND

SPECTROSCOPY

e v e n t u a l l y becomes a s s o c i a t e d with hemosiderin and other long term i r o n storage p r o t e i n s (9, 29_ 30). These s i m i l a r i t i e s of Pu(IV) and F e ( I I I ) suggested to us a biomimetic approach to the design of Pu(IV) sequestering agents modeled a f t e r the very e f f i c i e n t and h i g h l y s p e c i f i c i r o n sequestering agents, siderophores, which were developed by b a c t e r i a and other microorganisms to o b t a i n F e ( I I I ) from the environment (31, 32, 33). The siderophores (Figure 2) t y p i c a l l y contain hydroxamate or catecholate f u n c t i o n a l groups which are arranged to form an o c t a hedral c a v i t y the exact s i z e of a f e r r i c i o n . Catechol, 2,3-dihydroxybenzene, and the hydroxamic a c i d s , N-hydroxyamides, are very weak a c i d s that i o n i z e to form "hard" oxygen anions, which bind s t r o n g l y to strong Lewis a c i d s such as F e ( I I I ) and Pu(IV). Complexation by these groups forms five-membered c h e l a t e r i n g s , which s u b s t a n t i a l l y increases the s t a b i l i t y compared to complexat i o n by lone oxygen anions (34). That the hydroxamic a c i d s s t r o n g l y coordinate t e t r a v a l e n t a c t i n i d e s i s supported by the formation constants presented i n Table I I . Due to i t s higher charge and strong b a s i c i t y , the catecholate group forms even stronger complexes with the t e t r a v a l e n t a c t i n i d e s than the hydroxamic a c i d s . Thus our goal has been the i n c o r p o r a t i o n of hydroxamate or catecholate f u n c t i o n a l groups i n t o multidentate c h e l a t i n g agents that s p e c i f i c a l l y encapsulate t e t r a v a l e n t a c t i n i d e s . The s i m i l a r i t y between F e ( I I I ) and the a c t i n i d e ( I V ) ions ends with t h e i r c o o r d i n a t i o n numbers. Because of the l a r g e r i o n i c r a d i i of the a c t i n i d e ( I V ) i o n s , t h e i r p r e f e r r e d c o o r d i n a t i o n number found i n complexes with bidentate c h e l a t i n g agents i s e i g h t . O c c a s i o n a l l y higher c o o r d i n a t i o n numbers are encountered with very small l i g a n d s or by the i n c o r p o r a t i o n of a solvent molecule (43, 44). T h e o r e t i c a l c a l c u l a t i o n s i n d i c a t e that e i t h e r the square a n t i p r i s m (D4d) or the t r i g o n a l faced dodecahedron (D d) i s the expected geometry f o r an e i g h t - c o o r d i n a t e complex. The coulombic energy d i f f e r e n c e s between these polyhedra (Figure 3) i s very small and the p r e f e r r e d geometry i s l a r g e l y determined by s t e r i c requirements and l i g a n d f i e l d e f f e c t s . Cubic coordinat i o n l i e s at higher energy, but may be s t a b i l i z e d i f f - o r b i t a l i n t e r a c t i o n s were important. Another important e i g h t - c o o r d i n a t e polyhedron, the bicapped t r i g o n a l prism ( C v ) , corresponds to an energy minimum along the transformation pathway between the square a n t i p r i s m and the dodecahedron (45-50). As seen i n Table I I I , a l l four of the above geometries are found i n e i g h t - c o o r d i n a t e complexes of t e t r a v a l e n t a c t i n i d e s with bidentate l i g a n d s . However, the mmmm isomer of the t r i g o n a l faced dodecahedron i s the most prevalent i n the s o l i d s t a t e .


y

2

2

Actinide

Catecholates

Two fundamental questions i n the design of an a c t i n i d e - s p e c i f i c sequestering agent are the c o o r d i n a t i o n number and geometry a c t u a l l y p r e f e r r e d by the metal i o n with a given l i g a n d . The


7.

RAYMOND ET AL.

HOOCCH

Η

Sequestering

^ 2°

2

0 0 Η

Agents for


0

2

X

2

, DiethmenetriaminepentaaceUc acid (DTPA) 7

\

0

147

JSH COOH

jvHCHoteNiCHoîpN / HOOCCH

Actinides

CH COOH 2

j Figure 1. / 6

0

Figure 2.

Representative siderophores


148

LANTHANIDE

Table I I . Metal

A N D ACTINIDE

CHEMISTRY

A N D SPECTROSCOPY

Formation Constants f o r Some A c t i n i d e ( I V ) Hydroxamates Temp, °C

l o g 0^

log

3

2

log

3

3

log

6

4

Ref.


Benzohydroxamaic a c i d , Ph-C(O)-•N(0H)-H U(IV)

25

9.89

18.00

26.32

Th(IV)

25

9.60

19.81

28.76

Pu(IV)

25

12.73

N-phenylbenzohydroxamaic

32.94

35 35

21

a c i d , Ph-C(O)--N(0H)-Ph

Th(IV)

20

37.70

38

Th(IV)

25

37.80

36

Th(IV)

30

37.76

37

Pu(IV)

22

41.35

39

45.72

40

11.50

N-phenylcinnamohydroxamic Th(IV)

21.95

31.81

a c i d , Ph-C=C--C(0)-N(0H)-Ph

20

12.76

30

17.72

24.70

35.72

Catechol Th(IV)

41

4-Nitrocatechol Th(IV)

a

log 3

25

n

14.96

= [ML ]/[M][L] n

n

27.78

f o r the r e a c t i o n M

36.71

40.61

+ nL •> ML^ where L

i s the hydroxamate anion or the catecholate d i a n i o n .


42

Edelstein; Lanthanide and Actinide Chemistry and Spectroscopy ACS Symposium Series; American Chemical Society: Washington, DC, 1980. 57 59 58

BTP mmmm--DD gggg--DD

U Th, U

M(IV)(diisobutrylmethanate)^

M(IV)(hexafluoroacetonylpyrazolide)

61,62 mmmm--DD

Th, U Th, u, Pu, Ce

M(IV)(salicylaldehydrate)^

M(IV)(thenoyltrifluoroacetylacetonate)^

T h o r i u m ( t r i f l u o r o a c e t y l a c e t o n a t e ) ^ was o r i g i n a l l y d e s c r i b e d as a 1111-SA (Ref. 64, but a r e i n v e s t i g a t i o n e s t a b l i s h e d the presence of a coordinated water molecule forming a n i n e - c o o r d i n a t e complex (Ref. 65).

b

BTP = cicapped t r i g o n a l prism, DD = t r i g o n a l faced dodecahedron, SA = square a n t i p r i s m . The isomer n o t a t i o n i s taken from Ref. and 48 and corresponds to the edges l a b e l l e d i n Figure 3.

a

60 mmmm--DD

Am,

[M(III)(hexafluoroacetylacetonate)

Y, Eu

56

52>.55

mmmm--DD mmmm--DD

Th

M(IV)(N,N-diethyldithiocarbamate)

63

mmiTim--DD

Np

Th, u, Ce

M(IV)(dibenzoylmethanate)^

51,52

54

2

_

ssss--Cube

2

_

51,53

h

a

) P -BTP l l* ssss--SA

h

Idealized Geometry

Ligands

[M(III) (NjN-diethyldithiocarbamate)^]""

U

MCbipyridyl)^

4

Th, u, Np, Ce

B-M(IV)(acetylacetonate)^

\/r j- i Metals Th, u, Ce

0

— i b Complex

Geometry of Monomeric Eight-Coordinate A c t i n i d e Complexes with Bidentate

a-M(IV)(acetylacetonate)^

Table I I I .


150

LANTHANIDE

AND

ACTINIDE CHEMISTRY

SPECTROSCOPY

complexes formed by Th(IV) or U(IV) and c a t e c h o l , i n which the s t e r i c r e s t r a i n t s of a macrochelate are absent, serve as s t r u c t u r a l archetypes f o r designing the optimum a c t i n i d e ( I V ) sequestering agent. Thus the s t r u c t u r e s of an i s o e l e c t r o n i c , isomorphous s e r i e s of t e t r a k i s - c a t e c h o l a t o s a l t s , Na [M(C H 0 ) n ] • 21H 0; M = Th(IV), U(IV), Ce(IV), and H f ( I V ) , were determined by s i n g l e c r y s t a l X-ray d i f f r a c t i o n . S u i t a b l e c r y s t a l s were i s o l a t e d from the r e a c t i o n of the metal c h l o r i d e s or n i t r a t e s and the disodium s a l t of c a t e c h o l i n aqueous s o l u t i o n under an i n e r t atmosphere (66, 67). Measurement of magnetic s u s c e p t i b i l i t y and e l e c t r o n i c spect r a of the cerium and uranium complexes v e r i f i e d the presence of the +4 o x i d a t i o n s t a t e . I t was somewhat s u r p r i s i n g that the s t r o n g l y o x i d i z i n g Ce(IV) i o n ( E = + 1.70 V) (68) d i d not r e a c t with the c a t e c h o l d i a n i o n , a f a c i l e reducing agent (69). The a b i l i t y of c a t e c h o l to c o o r d i nate without r e d u c t i o n of o x i d i z i n g ions as Ce(IV), F e ( I I I ) (70), V(V) (71), and Mn(III) (72) i s a r e f l e c t i o n of i t s impressive coordinating a b i l i t y . The Ce(IV) complex was found by c y c l i c voltammetry to undergo a q u a s i - r e v e r s i b l e one-electron r e d u c t i o n i n s t r o n g l y b a s i c s o l u t i o n i n the presence of excess c a t e c h o l (Figure 4). Using the Nernst equation (73) and the measured p o t e n t i a l of the C e ( I V ) / C e ( I I I ) ( c a t e c h o l ) couple of - 448 mV vs NHE, the formation constant of the t e t r a k i s Ce(IV) complex was found to be greater than the corresponding Ce(III) complex by a f a c t o r of 1 0 . This enormous s h i f t of the redox p o t e n t i a l of the Ce(IV)/Ce(III) couple i s dramatic evidence of the enormous a f f i n i t y of the catecholate anion f o r the t e t r a v a l e n t lanthanides and actinides. The c r y s t a l s t r u c t u r e of t h i s i s o s t r u c t u r a l s e r i e s of c a t e chol complexes c o n s i s t s of d i s c r e t e [M(catechol) i+] " dodecahedra, a hydrogen bonded network of 21 waters of c r y s t a l l i z a t i o n and sodium i o n s , each of which i s bonded to two c a t e c h o l a t e oxygens and four water oxygens. Of the p o s s i b l e eight coordinate p o l y hedra, only the cube and the dodecahedron allow the presence of the c r y s t a l l o g r a p h i c 4 a x i s on which the metal i o n s i t s . As depicted i n Figure 5 and v e r i f i e d by the shape parameters i n Table IV, the t e t r a k i s ( c a t e c h o l a t o ) complexes n e a r l y d i s p l a y the i d e a l T> d molecular symmetry of the mmmm isomer of the t r i g o n a l faced dodecahedron. The symmetry of the dodecahedron, which can be regarded as the i n t e r s e c t i o n of one elongated and one compressed tetrahedron, allows f o r d i f f e r e n t M-0^ and M-Og bond lengths. As seen i n Table V, the experimental M-0 bond lengths are equal i n the thorium and cerium complexes. However, the M-Og bond length i s s i g n i f i c a n t l y shorter than the M - O A bond length i n the uranium and hafnium complexes. The much smaller i o n i c r a d i u s of the hafnium p u l l s the catecholate l i g a n d s i n s u f f i c i e n t l y so that i n t e r l i g a n d contacts become s i g n i f i c a n t ; the short oxygen-oxygen distance between A s i t e s of 2.550 A, n e a r l y 0.3 A l e s s than that f o r the cerium s a l t , i s w e l l w i t h i n the van der Waals contact distance of 2.8 A (75). 4


AND

6

t+

2

2

Q

4

3 6

t+

2



RAYMOND

ET AL.

Sequestering

Agents for

151

Actinides

/ \

Square Antiprism 4d D

Bicapped Trigonal Prism 2v c

Figure 3. Eight-coordinate polyhedra. The principal axes are vertical labels are taken from Refs. 45 and 48.


Edge

152

LANTHANIDE

Table IV.

ACTINIDE

CHEMISTRY AND

Shape Parameters (deg.) f o r [ M C O ^ H ^ r Ce, U, Th, Complexes 3

Metal


AND

e

A

6

SPECTROSCOPY

, M = Hf,

6

B

Th

37.9

75.4

3.6

31.3

U

37.1

75.2

3.0

31.1

Ce

36.8

74.9

2.1

32.0

Hf

35.2

73.3

0.4

32.2

Dodecahedron^

36.9

69.5

0.0

29.5

54.7

54.7

0.0

0.0

Cube

b

See Ref. 45 and 49 f o r d e f i n i t i o n s of shape parameters. C a l c u l a t e d using the Hard Sphere Model.

Table V. Ionic Radius

Metal

A

a

S t r u c t u r a l Parameters f o r Na [M(0 C H ) ]•21H 0 Complexes M-0

A

A

o A

M-0 o A

B

V°A k

V -°B M

deg

Th

1.05

2. 421(3)

2.418(3)

2.972(6)

66.8(1)

U

1.00

2. 389(4)

2.362(4)

2.883(7)

67.7(1)

Ce

0.97

2. 362(4)

2.357(4)

2.831(7)

68.3(1)

Hf

0.83

2. 220(3)

2.194(3)

2.554(5)

71.5(1)

R e f . 74.


7.

RAYMOND

ET

AL.

Sequestering

Agents for

153

Actinides

This lengthens the M-O^ bond of the hafnium complex r e l a t i v e to the others. However, s i n c e the i o n i c r a d i u s of uranium l i e s between those of cerium and thorium i t i s u n l i k e l y that the metal s i z e e x p l a i n s the d i s t o r t i o n i n the uranium complex. As a l l four complexes are i d e n t i c a l i n a l l r e s p e c t s except f o r the metal i o n , the lengthening of the M-O^ bond i n the uranium complex i s a t t r i b uted to a l i g a n d f i e l d e f f e c t from the f - e l e c t r o n s . A l i g a n d f i e l d of D d symmetry w i l l s p l i t the Hi+ ground term f o r the 5 f c o n f i g u r a t i o n of U(IV) i n t o seven l e v e l s , two of which are doubly degenerate. The observed temperature-independent magnetic susc e p t i b i l i t y of 870 x 10~~ cgs mol"" i s c o n s i s t e n t with a nondegenerate ground s t a t e (76). A q u a l i t a t i v e c r y s t a l f i e l d t r e a t ment of the D j complex p r e d i c t s a nondegenerate ground s t a t e a r i s i n g from e i t h e r the f or f metal o r b i t a l . Thus from e l e c t r o n r e p u l s i o n arguments, one expects the l i g a n d oxygen that i s c l o s e r to the z a x i s , 0^, to i n t e r a c t more with the f i l l e d metal o r b i t a l r e s u l t i n g i n the observed lengthening of the M-0^ bond. 3

2

2


6

1

2c

x

y

z

z

3

A c t i n i d e Hydroxamates As with the a c t i n i d e c a t e c h o l a t e s , we are i n t e r e s t e d i n determining the optimum s t r u c t u r e of a c t i n i d e hydroxamates f o r use i n the design of an octadentate a c t i n i d e sequestering agent. Thus the s t r u c t u r e s of t e t r a k i s ( N - i s o p r o p y l - 3 , 3 - d i m e t h y l b u t a n o and -2,2-dimethylpropano)hydroxamatothorium(IV) have been d e t e r mined by s i n g l e c r y s t a l X-ray d i f f r a c t i o n (77). Keeping the pH as low as p o s s i b l e , these compounds p r e c i p i t a t e upon the a d d i t i o n of an aqueous s o l u t i o n of thorium t e t r a c h l o r i d e to an aqueous s o l u t i o n of the sodium s a l t of the hydroxamic a c i d . The analogous uranium(IV) complexes were prepared s i m i l a r l y under an i n e r t atmosphere using deaerated s o l v e n t s . In a d d i t i o n to t h e i r hydrocarbon s o l u b i l i t y , the bulky a l k y l s u b s t i t u e n t s impart other i n t e r e s t i n g p r o p e r t i e s to these complexes. They melt at 127-8 and 116-7°C and, under a vacuum of 10" t o r r , sublime at 95 and 100°C, respectively! The a l k y l s u b s t i t u e n t s are a l s o very important i n determining the s t r u c t u r e s of the thorium hydroxamates. As i n the t e t r a c a t e c h o l a t e s ^ the metal i o n i n the t-Bu complex s i t s on a c r y s t a l l o graphic 4 a x i s , which l i m i t s the p o s s i b l e eight coordinate p o l y hedra to the dodecahedron and the cube (or t e t r a g o n a l prism). In order to minimize s t e r i c i n t e r a c t i o n s , the t - b u t y l groups s i t u a t e themselves on the corner of a tetrahedron, r e s u l t i n g i n the d i s t o r t e d cubic geometry of the complex shown i n F i g u r e 6. This s t e r i c s t r a i n a l s o manifests i t s e l f i n the C(=0)-C(t-Bu) bond l e n g t h of 1.547(5) A, which i s s i g n i f i c a n t l y longer than 1.506(5) A, the length normally found f o r an s p - s p C-C bond (78). Because the hydroxamate anion i s an unsymmetrical l i g a n d with most of the charge l o c a l i z e d on the n i t r o g e n oxygen, the Th-0^ bond, 2.357(3) A, i s 0.14 A shorter than the Th-0 bond, 2.492(3) A. 3

2

3

c



LANTHANIDE

Figure 5.

A N D ACTINIDE CHEMISTRY

A N D SPECTROSCOPY

4

The [U(catechol)J ' (M = Hf, Ce Th, and U) anion viewed along the mirror plane with the taxis vertical

Figure 6. Th[i-Pr-N(0)-C(0)'t-Bu] viewed down the Taxis. In this figure and in Figure 8, the substituent carbon atoms are drawn at 1/5 scale, the hydrogen atoms are omitted for clarity, and the nitrogen and nitrogen oxygen atoms are shaded. k


7.

RAYMOND

ET AL.

Sequestering

155

Actinides

The a v e r a g e Th-0 bond, 2.425 A, i s v e r y c l o s e t o t h e a v e r a g e Th-0 bond f o u n d i n [ T h ( c a t e c h o l ) ] * " , 2.420 A. The 0 - M - 0 ( o r b i t e ) a n g l e o b s e r v e d i n t h e t - B u c o m p l e x o f 62.3(1)° i s s m a l l e r t h a n t h e v a l u e r e q u i r e d t o s u c c e s s f u l l y s p a n an edge o f a c u b e , 70.53°, c a l c u l a t e d u s i n g a h a r d - s p h e r e model. The d i s p a r i t y i n Th-0 bond l e n g t h s and o b s e r v e d b i t e a n g l e c a u s e a d i s t o r t i o n t o w a r d s t h e gggg-isomer o f a t r i g o n a l - f a c e d dodecahedron, accompanied by a 10.3° t w i s t i n t h e BAAB t r a p e z o i d ( s e e F i g u r e 4 f o r t h e s e d e f i n i t i o n s ) . A s e x p e c t e d t h e o r e t i c a l l y ( 4 5 , 4 6 ) , t h e more n e g a t i v e l y charged n i t r o g e n oxygens a r e l o c a t e d a t t h e B s i t e s o f t h e dodecah e d r o n , b u t t h i s c o u l d a l s o be a s t e r i c e f f e c t o f t h e t - b u t y l groups. The r e l a t i o n s h i p o f t h e cube and t h e d o d e c a h e d r o n t o t h e c o o r d i n a t i o n p o l y h e d r o n o f t h e t - B u c o m p l e x i s shown i n F i g u r e 7 and a d e t a i l e d shape p a r a m e t e r a n a l y s i s i s p r e s e n t e d i n T a b l e V I . The s i m i l a r i t y o f t h i s c o m p l e x t o a cube i s shown b y t h e e q u a l edge l e n g t h s o f t h o s e n o t spanned b y t h e l i g a n d s , t h e m and g e d g e s , and t h e d i h e d r a l a n g l e s , 6, w h i c h a r e c l o s e t o 90° a b o u t t h e m a n d g e d g e s . The a a n d b edges a r e f a c e d i a g o n a l s i n t h e cube a n d t h e d i h e d r a l a n g l e s a b o u t t h e s e edges measure t h e d i s t o r t i o n t o w a r d s the dodecahedron. S t e r i c r e p u l s i o n s dominate (45, 4 6 ) , s i n c e t h e b u l k y a l k y l s u b s t i t u e n t s d i r e c t t h e geometry o f t h e complex t o w a r d s a cube. B e c a u s e t h e l i g a n d s s p a n a l t e r n a t e edges o f two p a r a l l e l s q u a r e f a c e s , t h e c o m p l e x i s b e s t d e s i g n a t e d as t h e s s s s i s o m e r o f a cube [ a f t e r t h e d e s i g n a t i o n s f o r a s q u a r e - a n t i p r i s m made b y H o a r d a n d S i l v e r t o n (45) ] w i t h t h e o v e r a l l symmetry o f t h e S p o i n t group. The i n f l u e n c e o f t h e a l k y l s u b s t i t u e n t i n d e t e r m i n i n g s t r u c t u r e i s g r e a t l y r e d u c e d b y t h e i n t r o d u c t i o n o f a m e t h y l e n e group between t h e c a r b o n y l c a r b o n and t h e t - b u t y l g r o u p . C o n t r a r y t o t h e p r e v i o u s c o m p l e x , t h e n e o p e n t y l d e r i v a t i v e (shown i n F i g u r e s 8 and 9) i s c l o s e t o t h e mmmm-dodecahedron f o u n d i n t h e t e t r a k i s ( c a t e c h o l a t o ) t h o r i u m and t h e m a j o r i t y o f o t h e r e i g h t - c o o r d i n a t e a c t i n i d e complexes w i t h b i d e n t a t e l i g a n d s (Table I I I ) . While the l a c k o f c r y s t a l l o g r a p h i c symmetry w o u l d a l l o w s t r u c t u r e s o t h e r than t h e dodecahedron (such as t h e square a n t i p r i s m o r b i c a p p e d t r i g o n a l p r i s m ) t h e s m a l l e s t d i h e d r a l a n g l e i s 35.5° and t h i s p r e c l u d e s t h e presence o f any square f a c e s i n t h e c o o r d i n a t i o n p o l y hedron ( f o r w h i c h 6 = 0 ) . As seen i n T a b l e V I t h e complex i s , however, d i s t o r t e d f r o m a n i d e a l d o d e c a h e d r o n . The b i t e o f t h e l i g a n d s , which governs t h e l e n g t h o f t h e m edges, i s s m a l l e r t h a n t h e l e n g t h o f a n i d e a l d o d e c a h e d r a l m edge. This results i n the f l a t t e n i n g o f t h e B t e t r a h e d r o n as evidenced by t h e i n c r e a s e d a n g l e between t h e Th-Og v e c t o r a n d t h e pseudo 4 a x i s , 6 g , a n d b y t h e l e n g t h e n e d g e d g e s . The b e n d i n g o f t h e l i g a n d s s e e n i n F i g u r e 8 i s due t o s t e r i c i n t e r a c t i o n s o f m o l e c u l a r p a c k i n g . A s b e f o r e , t h e T h - 0 bond [ave = 2.36(2) A], i s s h o r t e r t h a n t h e T h - 0 bond [ave = 2.46(4) A]. T h e r e i s no s i t e p r e f e r e n c e f o r t h e c h a r g e d o x y g e n a s t h e 0^ a n d 0Q a r e e q u a l l y d i s t r i b u t e d o v e r t h e A and B s i t e s o f t h e d o d e c a h e d r o n , r e s u l t i n g i n a mmmm-dodecahedron w i t h C i symmetry. k


Agents for

N

C

T

4

N


c


Figure 7.

k

The coordination polyhedron of Th[i-Pr-N(0)-C(0)-t-Bu] a dodecahedron

compared with a cube and


*ϋ

Ο Ο

C/3

Ο

«

> α

M

> Θ

>

F

1

hοι Œ>


1.48

1.58

1.07

1.26

1.27

b/r

g/r

g'/r

m/r

1.03,1.04,1.04,1.04

e

1.23,1.27,1.29,1.32,1.34,1.37,1.39,1.40

1.32,1.36,1.41,1.60

1.11,1.16

1.63 1.63 1.16 1.16 1.16

1.20 1.50 1.20 1.20 1.20

0

1.00

54.7

54.7

90.0

90.0

90.0

0.0

0.0

0.0

Cube

1.00

69.5

36.9

51.3

62.5

62.5

29.5

51.3

0.0

Dodecahedron

C a l c u l a t e d u s i n g t h e Hard Sphere Model.

?

The d o d e c a h e d r a l g e d g e s a r e d i v i d e d i n t o e d g e s spanned d e s i g n a t e d g and g respectively.

d r

by t h e

distance.

and t h o s e w h i c h a r e = a v e r a g e M-0

ligands

not,

The s h a p e p a r a m e t e r s a r e d e f i n e d i n Ref. 45 and 49; (j) i s t h e t w i s t i n t h e BAAB t r a p e z o i d , 0 i s t h e a n g l e b e t w e e n t h e M-0 v e c t o r and t h e p r i n c i p a l a x i s ,

Specific Sequestering Agents for the Actinides - American Chemical

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