Inorganic Compounds with Unusual Properties

8 Electron Exchange between Pairs of Vanadium Atoms in Novel Geometric

Downloaded by UNIV OF BATH on July 3, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch008

Isomers of Heteropoly Tungstates MICHAEL

T.

POPE,

STEPHEN

E.

O'DONNELL,

and

RONALD

A.

PRADOS

Georgetown University, Washington, D.C. 20057

The heteropoly anions PV Mo O405- and PV W O 05- are each shown to exist as mixtures of the five possible stereo isomers by P NMR spectroscopy. Controlled potential electrolytic reduction of H PV W O 0 yields PV V W 006- (I) and PV2IVW10O407- (II) which were isolated as potassium salts. The ESR spectrum of anion I consists of superimposed 8- and 15-line components with = 1.952 and = 104.5 and 53 G. The relative intensities of the 8- and 15-line spectra are in quantitative agreement with the assumption that electron exchange between neighboring vanadium atoms (V-O-V) is rapid, but that in isomers with remote vanadium atoms (V-O-W-O-V, etc.) the electron is effectively trapped on a single vanadium. The ESR spec trum of anion II has a normal 15-line pattern arising from the triplet state. The intervalence optical transition in anion I occurs at 8.8 kK. 2

10

5

2

2

10

4

31

IV

10

V

4

1

O4

Τ T e t e r o p o l y t r a n s i t i o n m e t a l oxocomplexes g e n e r a l l y h a v e structures based on edge- and corner-shared M 0

6

o c t a h e d r a ( J , 2).

These

structures c o n s e q u e n t l y r e s e m b l e discrete fragments of c l o s e - p a c k e d m e t a l o x i d e lattices, a n d h e t e r o p o l y complexes

frequently exhibit properties

associated w i t h electron d e r e a l i z a t i o n a n d m a g n e t i c e x c h a n g e t h a t are f o u n d i n m e t a l oxides. T h e s o - c a l l e d h e t e r o p o l y blues for e x a m p l e , are heteropoly

anions i n w h i c h some of

the d° m e t a l atoms

m o l y b d e n u m , or v a n a d i u m ) h a v e b e e n r e d u c e d to the d (3).

1

(tungsten,

o x i d a t i o n state

I n s u c h c o m p o u n d s , the extra electrons a p p e a r to b e d e l o c a l i z e d

over a l l s t r u c t u r a l l y a n d c h e m i c a l l y 85 e q u i v a l e n t m e t a l atoms b y a h o p p i n g process (4)

semiconduction i n m i x e d valence metal oxides). King; Inorganic Compounds with Unusual Properties Advances in Chemistry; American Chemical Society: Washington, DC, 1976. (cf.

86

INORGANIC

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

T h e present w o r k w a s u n d e r t a k e n i n o r d e r to define m o r e e x a c t l y the processes of

electron d e r e a l i z a t i o n i n m i x e d valence

heteropoly

anions. T h e w o r k has also p r o v e d the existence of g e o m e t r i c isomers t h a t r e s u l t f r o m the v a r i o u s w a y s of a r r a n g i n g t w o k i n d s of m e t a l atoms w i t h i n a given structure T h e complexes

(5,6).

studied have the general formula

w h e r e Ζ is M o or W , a n d χ =

1 a n d 2.

ΈΥ Ζ .χΟ^ ' χ

T h e s t r u c t u r e of these anions

( F i g u r e 1) is a n a r r a n g e m e n t of 12 e d g e - a n d c o r n e r - s h a r e d Z O hedra surrounding a central P 0 m e t a l a t o m is a p p r o x i m a t e l y C that the P V W n O

4 0

4

+α>)

12

e

octa-

4

t e t r a h e d r o n . T h e site s y m m e t r y of e a c h

4 v

. W e d e m o n s t r a t e d e l s e w h e r e (7, 8, 9 ) ,

" and P V M o n O

4 0

4

" complexes undergo

one-electron


r e d u c t i o n s to g i v e complexes t h a t are i n t e n s e l y c o l o r e d b u t t h a t n e v e r theless h a v e t r a p p e d valences ( i.e., t h e y are P V O

4 0

5

" ) a c c o r d i n g to E S R spectroscopy.

I V

WnO

4 0

5

" and P V

I V

Mou-

W h e n m o r e t h a n one t u n g s t e n or

m o l y b d e n u m a t o m has b e e n r e p l a c e d b y v a n a d i u m , the p o s s i b i l i t y of i s o m e r i s m arises. S i n c e the 12 m e t a l atoms i n the s t r u c t u r e d e p i c t e d i n F i g u r e 1 ( t h e K e g g i n s t r u c t u r e ) are e q u i v a l e n t , there are 1 2 ! / x ! ( 1 2 — x)\

Figure 1. Keggin structure for MZ O ~ heteropoly anions showing arrangement of ZO octahedra around the central MO tetrahedron 12

u

It0

n

h

w a y s of a r r a n g i n g the v a n a d i u m atoms i n Ρ ν * Ζ ι _ α . 0 ο 2

4

(3+Λ?)

'.

The total

n u m b e r of d i s t i n g u i s h a b l e isomers t h a t r e s u l t has b e e n c o m p u t e d for e a c h p o s s i b l e v a l u e of χ (5). F o r the case χ = 2, there are five isomers w i t h r e l a t i v e statistical a b u n d a n c e s of 6, 12, 12, 12, a n d 24 (see

Figures 2 and 3).

Experimental P r e p a r a t i o n of C o m p o u n d s . D e c a m o l y b d o d i v a n a d o p h o s p h o r i c a c i d w a s p r e p a r e d b y t h e m e t h o d of T s i g d i n o s a n d H a l l a d a (10). A n a l , for H P V M o 0 o · 1 7 H 0 : c a l c d : V 4.99, M o 46.95; f o u n d : V 4.91, M o 47.43, M o / V 5.13. Decatungstodivanadophosphoric a c i d was prepared b y Kokorin's m e t h o d as d e s c r i b e d b y S m i t h a n d P o p e (7). A n a l , for H P V W i o 0 * 1 2 H 0 : c a l c d : V 3.60, W 64.90; f o u n d : V 3.73, W 65.10, W / V 4.83. 5

2

1 0

4

2

5

2

2

King; Inorganic Compounds with Unusual Properties Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

4 0


α

b

Figure 2. (a) Unfolded Keggin structure showing numbering of Z-atoms; (b) simplified version of (a) in which only edgeshared octahedra are depicted as linked

A

A

AVA

AVA

AVA

1.4 (12*)

1,5 (24*)

1,2 (12)

Figure

3.

A

A AVA

A AVA

1,6 (12)

1.11 (6)

The five isomers of ?V Z O ~ with their re spective degeneracies (in parentheses) Species hbelled with an asterisk are dissymmetric, i.e. they have non-superimposable mirror images. Both enantiomorphs are included in the degeneracy count of such species. 2

10

5

U0

V o l t a m m o g r a m s of solutions o f this a c i d a g r e e d w i t h those r e p o r t e d previously (7,8). Potassium 10-tungstodivanado(IV,V) phosphate was prepared b y c o n t r o l l e d p o t e n t i a l r e d u c t i o n of a s o l u t i o n of t h e o x i d i z e d c o m p l e x i n a n acetate buffer, p H 5. A g r a p h i t e c l o t h c a t h o d e ( U n i o n C a r b i d e C o r p . )


88

INORGANIC

WITH

UNUSUAL

PROPERTIES

at + 0 . 2 9 V vs. S C E w a s u s e d . A s l i g h t excess of p o t a s s i u m c h l o r i d e w a s a d d e d to the r e d u c e d s o l u t i o n . D a r k g r e e n crystals of the p r o d u c t s e p a r a t e d after t h e s o l u t i o n h a d b e e n s t o r e d o v e r n i g h t i n t h e r e f r i g e r a t o r . A n a l , for K P V W O 4 0 · 1 2 H 0 : c a l c d : V 3.33, W 60.05, e q u i v . w t 3062.0; f o u n d : V 3.33, W 59.92, e q u i v . w t ( c o u l o m e t r i c a l l y ) 3066, W / V 4.98. Potassium 10-tungstodivanado( I V ) phosphate was prepared b y cont r o l l e d p o t e n t i a l r e d u c t i o n at —0.10 V of the v a n a d i u m ( I V , V ) c o m p l e x at p H 9 or 10. T h e d a r k b r o w n p o t a s s i u m salt w a s i s o l a t e d f r o m the r e d u c e d s o l u t i o n as d e s c r i b e d a b o v e . T h e p r o d u c t gave satisfactory I R spectra a n d voltammograms. P h y s i c a l M e a s u r e m e n t s . F o r the electrolyses, a W e n k i n g p o t e n t i o s t a t m o d e l 7 0 T S 1 a n d a K o s l o w S c i e n t i f i c c o u l o m e t e r m o d e l 541 w e r e u s e d . V o l t a m m e t r y w i t h w a x - i m p r e g n a t e d g r a p h i t e a n d r o t a t i n g p l a t i n u m electrodes was p e r f o r m e d as d e s c r i b e d e l s e w h e r e ( 7 , 8 ) . I R a n d e l e c t r o n i c s p e c t r a w e r e m e a s u r e d o n P e r k i n - E l m e r 225 a n d C a r y 14 i n s t r u m e n t s . X - b a n d E S R s p e c t r a w e r e r e c o r d e d at r o o m t e m p e r a t u r e o n a J E O L M E S - 3 X spectrometer. P h o s p h o r u s - 3 1 N M R s p e c t r a w e r e r e c o r d e d i n t h e p u l s e m o d e o n a V a r i a n X L - 1 0 0 i n s t r u m e n t at 40.5 M H z u s i n g a d e u t e r i u m l o c k , or o n a B r u k e r H F X - 9 0 i n s t r u m e n t at 36.43 M H z u s i n g a fluorine l o c k . 6


COMPOUNDS

Results and

2

1 0

2

Discussion

N M R Spectroscopy. Moio0

4 0

The

3 1

P N M R s p e c t r a of solutions of

H PV 5

2

a n d H P V W i o 0 o are p r e s e n t e d i n F i g u r e s 4 a n d 5. T h e p e a k s 5

2

4

at 3.09 a n d 3.79 p p m i n F i g u r e 4 h a v e i n t e g r a t e d intensities i n the r a t i o 3:8, a n d t h e y are consistent w i t h the presence of t h e five isomers w i t h relative

abundances

of

( 6 + 12) : (24 +

12 + 1 2 ) .

The

spectrum

of

H P V 2 W O 4 0 ( F i g u r e 5 ) has a s i m i l a r p a t t e r n of resonances at h i g h e r 5

field,

1 0

13.90,14.20, a n d 14.27 p p m . T h e s e p e a k s h a v e i n t e g r a t e d intensities

i n the r a t i o 1 : 3 : 1 , a n d t h e y are consistent w i t h t h e presence of f o u r of the e x p e c t e d isomers w i t h r e l a t i v e a b u n d a n c e s of 1 2 : ( 2 4 + fifth

1 2 ) : 12.

The

( 1,11 ) i s o m e r , w i t h a r e l a t i v e a b u n d a n c e of 6, m a y b e i n d i c a t e d b y

the resonance at 12.37 p p m .

A m o r e c o m p l e t e d i s c u s s i o n of the N M R

s p e c t r a of m i x e d h e t e r o p o l y anions is g i v e n e l s e w h e r e ( I I ,

12).

R e d u c t i o n of P V W i 0 o " . A c c o r d i n g to p r e v i o u s l y r e p o r t e d v o l t 5

2

0

4

ammetric measurements (7, 8 ) , the P V W i O 2

0

4 0

5

' a n i o n is stable i n s o l u -

t i o n u p to p H 5. A t t h a t p H , a v o l t a m m o g r a m i n d i c a t e s t w o w e l l - d e f i n e d , r e v e r s i b l e o n e - e l e c t r o n r e d u c t i o n s at ca. + 0 . 3 a n d 0.0 V . I n m o r e a c i d i c solutions, the s e c o n d

r e d u c t i o n shifts to m o r e p o s i t i v e p o t e n t i a l s a n d

o v e r l a p s w i t h the first ( 7 , 8 ) .

C o n t r o l l e d p o t e n t i a l electrolysis at + 0 . 2 9 V

of a s o l u t i o n of the a n i o n at p H 5 c o n s u m e d one f a r a d a y p e r m o l e of heteropoly

complex.

The

reduced

solution was

brown-green,

and

a

r o t a t i n g p l a t i n u m e l e c t r o d e v o l t a m m o g r a m of this s o l u t i o n d e m o n s t r a t e d t h a t the first w a v e w a s t o t a l l y a n o d i c . T h e E S R s p e c t r u m of the r e d u c e d solution

(Figure 6).

consists of

o v e r l a p p i n g eight- a n d 15-line


com-

8.

POPE E T A L .

Heteropoly

89

Tungstates

p o n e n t s . T h e r e l a t i v e intensities o f a l l t h e features o f t h e E S R s p e c t r u m d i d n o t c h a n g e d u r i n g t h e electrolysis

although the overall

S PV Mo


2

I

I

1

Figure

4.

P

31

12

Figure

5.

3 I

3 X 1 0 / s e c ) to g i v e a 15-line 8

spectrum.

T h e r e m a i n i n g isomers ( 1,4, 1,5, a n d 1,11 ) c o n t a i n v a n a d i u m

atoms that are separated b y one or m o r e tungsten atoms (i.e.

they have

V - O - W - O - V s e q u e n c e s ) , a n d t h e e l e c t r o n is effectively t r a p p e d

(life-

t i m e > ca. 10" sec) o n a single v a n a d i u m a t o m thus g i v i n g a n e i g h t - l i n e 8

spectrum

( t h e e x p l a n a t i o n is analogous to that p r o p o s e d for the p a r a -


8.

Heteropoly

POPE E T A L .

93

Tungstates

c y c l o p h a n e anions i n Réf. 14).

T h e r e l a t i v e intensities of t h e 15- a n d

e i g h t - l i n e s p e c t r a s h o u l d therefore b e ( 1 2 +

1 2 ) : ( 2 4 + 12 + 6 ) o r 4:7.

H o w e v e r , there is n o c o n v i n c i n g e v i d e n c e i n t h e N M R s p e c t r u m t h a t t h e 1,11 i s o m e r is i n d e e d f o r m e d i n the case of the tungstate anions. I t c o u l d b e a r g u e d t h a t t h e p e a k at 12.37 p p m i n F i g u r e 5 is c a u s e d b y traces of impurities.

I f this w e r e so, t h e r e l a t i v e intensities of the 15- a n d 8-line

s p e c t r a w o u l d b e ( 12 + 12) : (24 + 12) or 4:6 ( i n s t e a d of 4 : 7 ) . A l t h o u g h i t is n o t p o s s i b l e to d i s t i n g u i s h b e t w e e n these t w o ratios b y s i m u l a t i o n , the E S R

findings

p r o v i d e s t r o n g e v i d e n c e f o r the p r e s e n c e of s e v e r a l

isomers as w e l l as f o r the e l e c t r o n h o p p i n g process of d e r e a l i z a t i o n i n m i x e d v a l e n c e p o l y a n i o n s of this t y p e .


T h e E S R s p e c t r u m of the f u l l y r e d u c e d a n i o n , P V 7 ) , has the 15-line p a t t e r n ((g) —

2

W i o 0 o ' (Figure

I V

7

4

1.95, (a) — 53 G ) e x p e c t e d f o r the i n t e r -

a c t i o n of the u n p a i r e d electrons w i t h t w o v a n a d i u m n u c l e i . O b s e r v a t i o n of t h e h a l f - f i e l d ( A m = 8

2 ) t r a n s i t i o n i n the s p e c t r u m of the s o l i d p o t a s -

s i u m salt confirms t h e t r i p l e t state of the species r e s p o n s i b l e f o r s p e c t r u m i n F i g u r e 7.

the

I t seems v e r y l i k e l y that those isomers (1,2 a n d

1,6 w i t h V - O - V g r o u p s ) w h i c h are r e s p o n s i b l e for the 1 5 - l i n e s p e c t r u m of the m i x e d v a l e n c e c o m p l e x w o u l d h a v e a n t i f e r r o m a g n e t i c a l l y c o u p l e d spins i n the f u l l y r e d u c e d a n i o n . T h e r e m o t e isomers w i t h v a n a d i u m atoms s e p a r a t e d b y O - W - O

(1,4

a n d 1,5)

or O - W - O - W - 0

(1,11)

sequences are t h e r e f o r e p r o b a b l y r e s p o n s i b l e for the t r i p l e t state E S R spectrum.

F u r t h e r E S R a n d magnetic susceptibility measurements

are

p l a n n e d o n i n d i v i d u a l isomers. E l e c t r o n i c s p e c t r a l d a t a f o r the t w o r e d u c e d anions are c o m p a r e d i n T a b l e I w i t h those r e p o r t e d f o r P V

I V

WuO

4 0

' (7,8,

5

T h e assign-

15).

T a b l e I . E l e c t r o n i c T r a n s i t i o n s of R e d u c e d Tungstovanadophosphate Anions ργινα

2

8.8 (290) 12 sh,b (260) 15.5 (360) 19 s h (510)

12 sh,b ( 1 3 0 ) 14.5 sh,b (300) 20 (750) 25 s h (600)

6

Tentative Assignment

P7 iv

ργινγγ

V 11 sh,b (150) 15.5 s h (430) 19.5 sh (950) 26.5 s h (1430)

c

V V

I

I

I

V

V

-> V d-d d-d W -> W

V

apyiv PVWiiCUo - at p H 2 (15); P V V - P V V W i O 4 0 - at p H 5; P V P V W i o 0 o - at p H 10. Energy in k K (molar absorbance, M^cm" ), sh: shoulder, b: broad. Obscured by intense Ο —• V charge transfer absorption. 6

β

2

IV

4

I V

V

I V

v

0

e

2

v

V I

V I

I V

«

7

6

1

c

v

ments l i s t e d i n T a b l e I are b a s e d o n c o n s i d e r a t i o n s d i s c u s s e d p r e v i o u s l y (7, 8, 15,16).

M o s t of t h e s p e c t r a l features are b r o a d , a n d t h e energies

g i v e n are subject to u n c e r t a i n t i e s of ± 0 . 5 k K .

F u r t h e r d i s c u s s i o n of t h e


94

INORGANIC COMPOUNDS W I T H UNUSUAL PROPERTIES

e l e c t r o n i c structures o f these h e t e r o p o l y anions m u s t a w a i t m e a s u r e m e n t s o n s i n g l e isomers. Acknowledgments W e t h a n k E . S o k o l o s k i ( N a t i o n a l Institutes of H e a l t h ) a n d A . E n g l i s h ( Ε . I . d u P o n t d e N e m o u r s & C o . , I n c . ) f o r assistance i n o b t a i n i n g the N M R s p e c t r a , a n d S u s a n n e R a y n o r f o r assistance i n p r o g r a m m i n g the s p e c t r a s i m u l a t i o n .


Literature Cited 1. 2. 3. 4.

Evans, Jr., H. T., Perspect. Struct. Chem. (1971) 4, 1. Weakley, T. J. R., Struct. Bonding Berlin (1974) 18, 131. Pope, M. T., Inorg. Chem. (1972) 11, 1973. Prados, R. Α., Meiklejohn, P. T., Pope, M. T.,J.Amer. Chem. Soc. (1974) 96, 1261. 5. Pope, M. T., Scully, T. F., Inorg. Chem. (1975) 14, 953. 6. Pope, M. T., O'Donnell, S. E., Prados, R. Α., J. Chem. Soc. Chem. Commun. (1975) 22. 7. Smith, D. P., Pope, M. T., Inorg. Chem. (1973) 12, 331. 8. Smith, D. P., So, H., Bender, J., Pope, M. T., Inorg. Chem. (1973) 12, 685. 9. Altenau, J. J., Pope, M. T., Prados, R. Α., So, H., Inorg. Chem. (1975) 14, 417. 10. Tsigdinos, G. Α., Hallada, C. J., Inorg. Chem. (1968) 7, 437. 11. O'Donnell, S. E., Ph.D. Dissertation, Georgetown University, 1975. 12. O'Donnell, S. E., Pope, M. T., manuscript in preparation. 13. Lange, G., Hahn, H., Dehnicke, Κ., Ζ. Naturforsch. Teil Β (1969) 24, 1498. 14. Weissman, S. I.,J.Amer. Chem. Soc. (1958) 80, 6462. 15. Flynn, C. M., Jr., Pope, M. T., Inorg. Chem. (1973) 12, 1626. 16. So,H.,Pope, M. T., Inorg. Chem. (1972) 11, 1441. RECEIVED January 24, 1975. Work supported by NSF grants GP-10538 and -4099IX. The ESR spectrometer was purchased with the aid of NSF equip ment grant GP-29184.


Inorganic Compounds with Unusual Properties

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