Coordination Chemistry - American Chemical Society

Chapter 32

Coordination Chemistry in the Solvent Extraction of Metals

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 25, 2015 | http://pubs.acs.org Publication Date: November 4, 1994 | doi: 10.1021/bk-1994-0565.ch032

Developments from Russian Laboratories Yu. A. Zolotov Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow 117907, Russia

Extractive separation of metals i s usually ba sed on complex formation with inorganic and organic ligands. Therefore the use of the ide as, approaches, and methods of coordination chemistry has always been a most f r u i t f u l ap proach to the extraction of the elements. His tory shows that many problems of s e l e c t i v i t y of separation or enhanced isolation have been successively solved by the rational applica tion of coordination chemistry, e.g., the con cept of hard and soft acids and bases. The ef ficiency of extraction depends on, inter a l i a , the r a t i o of charge and coordination number of metal ion. Study of this effect permitted the development of ways to improve separation due to changes in hydration of the species to be extracted. General requirements f o r e x t r a c t i o n of i n o r g a n i c species from an aqueous s o l u t i o n i n t o an i m m i s c i b l e o r g a n i c phase a r e known. T h e s e i n c l u d e e l e c t r o n e u t r a l i t y , h i g h e r s o l u b i l i t y i n the organic solvent than i n water, bulky mole c u l e s , a n d s t a b i l i t y , i . e . , a h i g h enough s t a b i l i t y c o n stant . T h e m a i n t y p e s o f i n o r g a n i c compounds u s e d f o r e x t r a c t i v e separation of metal ions include (1): N e u t r a l Compounds 1. 2. 3.

C o o r d i n a t i v e l y n o n s o l v a t e d n e u t r a l compounds, s u c h H g C l 2 , Asl3, o r 0s04. Metal chelates. C o o r d i n a t i v e l y s o l v a t e d (mixed) n e u t r a l c o m p o u n d s , s u c h a s A u C l 3 L , U02(N03)2L2, where L i s a n e u t r a l

0097-6156/94/0565-0395$08.00/0 © 1994 American Chemical Society In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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e x t r a c t a n t w i t h i n the complex. Ion

inner coordination

sphere of

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4.

C o o r d i n a t i v e l y nonsolvated ion associates of the type Cat*An" where Cat* and An" a r e a l a r g e and h y d r o p h o b i c c a t i o n and a n i o n , r e s p e c t i v e l y , w i t h p r e d o m i n a n t l y e l e c t r o s t a t i c b o n d i n g between them. 5. S t r o n g m i n e r a l a c i d s . 6. Complex ( m e t a l - c o n t a i n i n g ) s t r o n g a c i d s , such as HGaCl4, HAuBr4, o r HNbFe; h e t e r o p o l y a c i d s a r e a l s o i n c l u d e d i n t h i s group. 7. V a r i o u s compounds n o t i n c l u d e d i n g r o u p s 1-6 f o r one reason or another. T h i s l i s t c o n s i s t s o f two l a r g e g r o u p s : n e u t r a l comp o u n d s and i o n a s s o c i a t e s . Evidently, with rare exceptions, extracted substanc e s a r e c o m p l e x compounds. Complex f o r m a t i o n i n a q u e o u s and n o n a q u e o u s s o l u t i o n s i s i n d e e d a d e c i s i v e f a c t o r f o r the e f f i c i e n c y of solvent e x t r a c t i o n . Correspondingly, one o f t h e most f r u i t f u l d e v e l o p m e n t s i n t h e t h e o r y o f e x t r a c t i o n has been t h e i n c o r p o r a t i o n o f t h e a c h i e v e ments o f c o o r d i n a t i o n c h e m i s t r y . T h i s i s e s p e c i a l l y imp o r t a n t f o r r e s o l v i n g s e l e c t i v i t y problems — the main problem i n t h i s f i e l d . Other problems are: a p r i o r y evaluation of the conditions required to e x t r a c t a spec i f i c metal, purposeful synthesis of extractants, the u s e o f modern t e c h n i q u e s f o r e v a l u a t i n g t h e r e a l s t a t e of metals i n the phases, e t c . C o m p l e x a t i o n i n e x t r a c t i o n systems has been t h e subj e c t o f e x t e n s i v e r e s e a r c h . The r e s u l t s o b t a i n e d helped t o e s t a b l i s h t h e mechanisms o f many e x t r a c t i o n p r o c e s s e s , t o p r e d i c t t h e t y p e o f compounds t h a t must be f o r m e d i n t h e s y s t e m , and t o e v a l u a t e t h e c o n d i t i o n s f o r s e l e c t i v e or group e x t r a c t i o n . T h i s review c o n c e n t r a t e s p r i m a r i l y on r e s u l t s o b t a i n e d i n o u r l a b o r a t o r i e s i n Moscow. Halide

Complexes

One f i e l d o f u s i n g c o o r d i n a t i o n c h e m i s t r y a p p r o a c h e s i s t h e s o l v e n t e x t r a c t i o n of h a l i d e complexes o f m e t a l s . The c l a s s i c a l example i s t h e e x t r a c t i o n o f F e ( I I I ) f r o m HC1 s o l u t i o n w i t h o x y g e n - c o n t a i n i n g s o l v e n t s s u c h a s d i e t h y l ether. I t i s of i n t e r e s t to consider h i s t o r i c a l l y how t h e c h e m i s t r y o f t h i s e x t r a c t i o n was s t u d i e d (2) . The p r a c t i c a l method was d e v e l o p e d a s e a r l y a s a c e n t u r y ago (1892) ( 3 ) . A t t h e e a r l i e s t s t a g e o f d e v e l o p m e n t t h e r e was no i n f o r m a t i o n on t h e n a t u r e o f t h e compound e x t r a c t e d . Then, t h e p r o p o s a l was made t h a t F e C l 3 i s e x t r a c t e d . L a t e r , i t was c o n s i d e r e d (4-6) t h a t t h e e x t r a c t e d f o r m i s HFeCl4. However, N e k r a s o v and Ov-

In Coordination Chemistry; Kauffman, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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s y a n k i n a (7) made c o n c l u s i o n t h a t s e v e r a l c h l o r o comp l e x e s a r e e x t r a c t e d , m o s t l y t h e h i g h e r o n e s (FeClï, FeCls"" ,FeCl6~) . F i n a l l y , i t was f o u n d (8-10) t h a t t h e o n l y e x t r a c t e d f o r m i s FeClï (as HFeCl4) . A n o t h e r f i e l d o f s t u d y was s o l v a t i o n and h y d r a t i o n o f t h i s s p e c i e s H S F e C l 4 7 t h e n {H(H20)xSy} FeClï, b u t t h i s i s n o t t h e t o p i c o f t h i s p a p e r . Now we know t h a t e x t r a c t a b l e h a l i d e s and r e l a t e d c o m p l e x e s a r e , i n p r i n c i p l e , c o o r d i n a t i v e l y u n s o l v a t e d s p e c i e s MXm (M™* i s t h e m e t a l i o n ; X" i s a h a l i d e o r s i m i l a r a n i o n ) ; c o o r d i n a t i v e l y s o l v a t e d compounds MXmLn (L i s t h e e x t r a c t a n t ) ; o r c o m p l e x a n i o n s Mx5+n i n c o r p o r a t e d as i o n - a s s o c i a t i o n compounds. I t h a s b e e n shown (11,12) t h a t i t i s p o s s i b l e t o e x p l a i n why and w h i c h c o m p l e x e s o f t h e s e t y p e s a r e f o r m e d by u s i n g t h e a p p r o a c h o f h a r d and s o f t a c i d s and b a s e s (HSAB). F o r i n s t a n c e , t h e f o l l o w i n g f i e l d s were s t u d i e d : - c o m p a r i s o n o f t h e s t a b i l i t y o f c o m p l e x e s and t h e i r e x traction; - e l u c i d a t i o n o f t h e r e a s o n s and c o n d i t i o n s f o r e x t r a c t i o n o f complexes o f d i f f e r e n t t y p e s ; - study of coordination hydration. C o o r d i n a t i v e l y s a t u r a t e d n e u t r a l c o m p l e x e s a r e weak a c c e p t o r s ( o n l y t o σ-antibonding m o l e c u l a r o r b i t a l s o r t h e v a c a n t 4d a t o m i c o r b i t a l o f germanium) s u c h a s t h e h a l i d e s o f Ge (GeX4, where X i s C I , B r , o r I) , Hg, As, Sn, o r Sb. E l e m e n t s w h i c h f o r m s u c h c o m p l e x e s , whose e x t r a c t i o n i s v e r y s e l e c t i v e , a r e s i t u a t e d i n one a r e a o f the p e r i o d i c table — a t t h e end o f t h e l o n g p e r i o d s (13). F o r h a l i d e complexes of s o f t m e t a l s , i t i s p o s s i b l e t o p r e d i c t t h e change o f e x t r a c t i o n i n a s e r i e s : c h l o r i des - bromides - i o d i d e s , because s i z e , s t a b i l i t y , and weakness o f h y d r a t i o n of complexes a r e changed i n a s i n g l e d i r e c t i o n . I t i s d i f f i c u l t t o p r e d i c t such behavior for h a l i d e s of hard metal ions. MC13 MBra MI3 Soft ions Stability size +

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t i m a t i o n o f t h e p o s s i b l e c o o r d i n a t i o n numbers o f t h e met a l atom and t h e i r i n v o l v e m e n t i n e x t r a c t i o n i s often u s e f u l . For instance, c o o r d i n a t i o n aquation of i n o r g a n i c c o m p l e x e s h a s b e e n s t u d i e d (14,15). The r e s u l t s i n d i c a t e d t h e p o s s i b i l i t y o f f o r m a t i o n o f i n t e r m e d i a t e comp l e x e s , h y d r a t e d i n aqueous s o l u t i o n s , e s p e c i a l l y f o r l a n t h a n i d e s and t r i v a l e n t a c t i n i d e s . Thus l a n t h a n i d e s are e x t r a c t e d from c h l o r i d e s o l u t i o n s by l o n g - c h a i n amine s a l t s a s s i n g l y and d o u b l y c h a r g e d t e t r a - and p e n t a c h l o r o y b o m p l e x e s (MClï, MCLs"), r e s p e c t i v e l y (16). B e c a u s e t h e c o o r d i n a t i o n numbers o f t h e l a n t h a n i d e s a r e h i g h , i t was p o s s i b l e t o e x p e c t t h a t c o m p l e x e s w i l l be h y d r a t e d i n t h e i n n e r c o o r d i n a t i o n s p h e r e . I f t h i s i s t r u e , t h e d e g r e e o f e x t r a c t i o n c a n be i n c r e a s e d by a d d i n g t o t h e s y s t e m a s e c o n d e x t r a c t a n t , c a p a b l e of r e p l a c i n g water i n the i n n e r c o o r d i n a t i o n s p h e r e . An example o f s u c h a compound i s t r i b u t y l p h o s p h a t e . T h i s p r e d i c t i o n was t e s t e d i n t h e 1970s (17,18), and i t was f o u n d t h a t a m i x t u r e o f a c a t i o n i c and a n e u t r a l e x t r a c t a n t produced a c o n s i d e r a b l e s y n e r g i s t i c e f f e c t , and t h e d i s t r i b u t i o n c o e f f i c i e n t s w e r e i n c r e a s e d . T h i s t y p e o f s y n e r g i s t i c e f f e c t was e m p l o y e d t o i m p r o v e t h e e f f i c i e n c y o f l a n t h a n i d e and a c t i n i d e e x t r a c t i o n f r o m c h l o r i d e and n i t r a t e s o l u t i o n s . The same a p p r o a c h was u s e d t o i n c r e a s e e x t r a c t i o n o f i o n a s s o c i a t e s o f complex m e t a l a n i o n s w i t h c a t i o n i c d y e s a s i n t h e c a s e o f cadmium i o d i d e c o m p l e x e s C d l 3 w i t h such dyes, used i n photometric a n a l y s i s . Cdl3(TBP)~ i s e x t r a c t e d r e a d i l y by b e n z e n e ( 1 9 ) . I t was a l s o p o s s i b l e t o e x p l a i n t h e known f a c t t h a t t h e i n d i u m c h l o r i d e complex i s p u r e l y e x t r a c t e d i n comp a r i s o n w i t h t h e g a l l i u m and t h a l l i u m c h l o r i d e c o m p l e x es. T1C1Ï InCli GaClï Stability Size Softness Hydration Extraction

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p a r a m e t e r s , v i z , , t h e c h a r g e on t h e c e n t r a l m e t a l i o n , t h e maximum c o o r d i n a t i o n number o f t h e m e t a l i o n , t h e number o f i o n o g e n i c g r o u p s i n t h e r e a g e n t m o l e c u l e , a n d the coordination capacity of t h i s molecule. The e x t r a c t i o n o f c h e l a t e s was known f r o m t h e b e g i n ning of the century (20).At the beginning of 60s i t was f o u n d t h a t i n a d d i t i o n t o c l a s s i c a l c h e l a t e s , c o o r d i n a t i v e l y u n s a t u r a t e d c h e l a t e s a r e f o r m e d when t h e c o o r d i n a t i o n number o f t h e m e t a l i o n e x c e e d s i t s d o u b l e c h a r g e ( f o r d i d e n t a t e l i g a n d s ) . Water m o l e c u l e s e n t e r the i n n e r c o o r d i n a t i o n sphere i n t h i s case, and e x t r a c t i o n c a n be i n c r e a s e d i f c o o r d i n a t i v e l y a c t i v e o r g a n i c s o l v e n t s a r e used. Instead of such s o l v e n t s , a mixture of nonpolar organic solvents with s p e c i a l a c t i v e a d d i t i v e s c a n be employed, a n d i n t h i s c a s e s y n e r g i s t i c e f fects result (increasing the d i s t r i b u t i o n c o e f f i c i e n t s when a m i x t u r e i s u s e d i n c o m p a r i s o n w i t h d i s t r i b u t i o n c o e f f i c i e n t s f o r i n d i v i d u a l e x t r a c t a n t s ; more e x a c t l y — nonadditive values of d i s t r i b u t i o n c o e f f i c i e n t s ) . The HSAB a p p r o a c h h e l p e d t o f i n d new e f f e c t i v e e x t r a c t a n t s such as 2-octylaminopyridine f o r s o f t platinum metals (22), t o s e l e c t s y n e r g i s t i c a d d i t i v e s l i k e t h e

s o f t triphenylphosphine (but not t h e hard t r i p h e n y l p h o s phate) f o r s i l v e r c h e l a t e s — a c e t y l a c e t o n a t e , thenoylt r i f l u o r o a c e t o n a t e , or 8-hydroxyquinolinate. Parameters m e n t i o n e d e a r l i e r were u s e d t o f o r m u l a t e c o n d i t i o n s f o r the formation o f charged c h e l a t e s . C a t i o n i c c h e l a t e s a r e f o r m e d , f o r i n s t a n c e , when t h e c o o r d i n a t i o n number i s l e s s t h a n d o u b l e t h e c h a r g e o f a c e n t r a l atom, u s i n g d i d e n t a t e r e a g e n t s . T i n ( I V ) b i s ( h y d r o x y q u i n o l i n a t e ) , g o l d ( I I I ) b i s ( d i t h i z o n a t e ) , o r bor o n ( I I I ) b i s ( a c e t y l a c e t o n a t e ) a r e examples o f s u c h m e t a l c h e l a t e s . Another reason f o r the formation o f c a t i o n i c c h e l a t e s i s s t e r i c hindrance as i n t h e cases o f alumin urn b i s ( 2 - m e t h y l - 8 - h y d r o x i q u i n o l i n a t e ) , or cobalt(III) bis(dimethylglyoximate) (23,24). A number o f s e p a r a t i o n p r o c e d u r e s f o r t h e e x t r a c t i o n o f m e t a l s by u s i n g c a t i o n i c a n d a n i o n i c c o m p l e x e s h a v e b e e n d e v e l o p e d . T h e s e c o m p l e x e s c a n be e x t r a c t e d b y a d d i n g hydrophobic counter ions t o t h e system. R e t u r n i n g t o e x t r a c t a n t s , we n e e d t o e m p h a s i z e t h a t t h e main problem i s t o look f o r s e l e c t i v e e x t r a c t a n t s . The g e n e r a l p r i n c i p l e t h a t was u s e d a s t h e b a s i s f o r d e s i g n i n g s e l e c t i v e e x t r a c t a n t s i s maximum c o m p a t i b i l i t y o f t h e p r o p e r t i e s o f t h e m e t a l i o n and t h e e x t r a c t a n t molec u l e . T h i s i s t h e o l d a p p r o a c h w h i c h i s v a l i d f o r many d i f f e r e n t f i e l d s o f a p p l i c a t i o n o f c o o r d i n a t i o n comp o u n d s . However, t h e a p p r o a c h i s t o o g e n e r a l . I n e a c h



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c a s e many s p e c i f i c f a c t o r s s h o u l d be t a k e n i n t o c o n s i deration. F o r example, f o r m a c r o c y c l i c e x t r a c t a n t s ( 2 5 ) , t h e r e a r e s e v e r a l s u c h f a c t o r s , v i z . . t h e number a n d p o s i t i o n o f t h e d o n o r atoms, t h e c a v i t y s i z e , t h e p r e s e n c e o f f l e x i b l e c h a i n s , t h e r i g i d i t y o f t h e m o l e c u l e s a s a who l e , and t h e p o s s i b i l i t y f o r h y d r a t i o n o f t h e c e n t r a l atom. T h e e f f e c t o f c a v i t y s i z e a n d r i g i d i t y o f t h e mo l e c u l e on e x t r a c t i o n s e l e c t i v i t y h a s been s t u d i e d i n more d e t a i l . I n t h e c a s e o f t h e r i g i d , s m a l l - c a v i t y n i t r o g e n - c o n t a i n i n g m a c r o c y c l e s ( I - I I I ) o n l y one e l e m e n t (silver) i s extracted. I f a f l e x i b l e small-cavity l i g a n d ( I V a n d V) i s u s e d , t h r e e o r f o u r e l e m e n t s a r e e x t r a c t e d . W i t h a f l e x i b l e l a r g e - c a v i t y l i g a n d (VI) t h e r e i s no s e l e c t i v i t y . I t i s d e s i r a b l e t o m e n t i o n two i n t e r e s t i n g a n d u n u s u a l groups o f e x t r a c t a n t s which have been s u g g e s t e d , s t u d i e d , and used i n our l a b o r a t o r y . The f i r s t g r o u p i s o r g a n o t i n compounds, e . g . , d i n o n y l t i n d i n i t r a t e . These a r e e x c e l l e n t , perhaps t h e b e s t , e x t r a c t a n t s f o r o x y g e n - c o n t a i n i n g a n i o n s s u c h a s ΡΟ4" o r As04~ (26) . F o r m a l l y , t h e e x t r a c t i o n p r o c e s s c a n b e c o n s i d e r e d a s an i o n exchange, b u t complex f o r m a t i o n a c t u a l l y t a k e s p l a c e : t h e o x y g e n atom o f , f o r i n s t a n c e , t h e phosphate enters i n t o t h e inner c o o r d i n a t i o n sphere o f t h e t i n atom. The s e c o n d g r o u p c o n s i s t s o f s p i n - l a b e l e d c h e l a t e f o r m i n g r e a g e n t s (27). E x t r a c t a n t s V I I - V I I I c a n be u s e d as examples.

I

II

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401 Coordination Chemistry in Solvent Extraction of Metah


32. ZOLOTOV


402


U s i n g t h e s e r e a g e n t s made i t p o s s i b l e t o e m p l o y ESR t h e d e t e r m i n a t i o n o f a l m o s t any m e t a l i o n , n o t o n l y paramagnetic ones. A s t o methods f o r t h e i n v e s t i g a t i o n o f t h e c o o r d i n a t i o n c h e m i s t r y o f e x t r a c t e d compounds, Raman s p e c t r o s c o py was shown t o be an e x c e l l e n t t e c h n i q u e (28,29). It is p o s s i b l e t o study both phases of the e x t r a c t i o n system, and i t i s n o t n e c e s s a r y t o u s e h i g h c o n c e n t r a t i o n o f t h e e x t r a c t e d m e t a l . R e s u l t s f o r bismuth c h l o r i d e complexes e x t r a c t e d by t r i b u t y l p h o s p h a t e a r e e x a m p l e s (28). Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 25, 2015 | http://pubs.acs.org Publication Date: November 4, 1994 | doi: 10.1021/bk-1994-0565.ch032

for the

Conclusion E v e n t h i s s h o r t r e v i e w o f some s o l v e n t w o r k s made m o s t ly i n author's laboratories (in V.I.Vernadskii Institute o f G e o c h e m i s t r y and A n a l y t i c a l C h e m i s t r y a n d M . V . L o m o n o s o v Moscow S t a t e U n i v e r s i t y i n 1950-1980s) shows t h e i m portance of the c o o r d i n a t i o n chemistry f o r e x t r a c t i v e s e p a r a t i o n o f m e t a l i o n s . Of c o u r s e , t h e c o n s i d e r a t i o n was n o t c o m p r e h e n s i v e ; t h u s , t h e r o l e o f d i f f e r e n c e s i n f o r m a t i o n c o n s t a n t s on s e p a r a t i o n o r e n t r o p y o f c h e l a t e f o r m a t i o n and o t h e r s i g n i f i c a n t a s p e c t s o f t h e p r o b l e m were n o t c o n s i d e r e d .

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Coordination Chemistry - American Chemical Society

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