Polymeric, Mixed-Valence Transition Metal Compounds - ACS

7 Polymeric, Mixed-Valence Transition Metal

Downloaded by FUDAN UNIV on December 20, 2016 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/bk-1974-0005.ch007

Compounds GILBERT

M.

THOMAS

J.

BROWN, MEYER,

ROBERT

and

W.

TOM

CALLAHAN,

RAY

EUGENE

C.

JOHNSON,

WEAVER

U n i v e r s i t y o f N o r t h C a r o l i n a , C h a p e l Hill, N . C . 27514

In our work we have prepared and c h a r a c t e r i z e d m a t e r i a l s i n which metal atoms or ions are held i n c l o s e proximity by chemical l i n k a g e s . One o f our goals has been to e s t a b l i s h the chemical and p h y s i c a l p r o p e r t i e s which a r i s e from metal-metal i n t e r a c t i o n s i n such systems and to modify the p r o p e r t i e s i n a c o n t r o l l e d way by d i r e c t e d chemical s y n t h e s i s . Three d i f f e r e n t kinds o f systems have been studied which d i f f e r with regard to the nature o f the metal-metal i n t e r a c t i o n : 1. Compounds i n which there i s s t r o n g , d i r e c t metal-metal bonding. 2. Cases where there are strong metal-metal i n t e r a c t i o n s through b r i d g i n g ligands. 3. Cases where there are weak metal-metal i n t e r a c t i o n s through bridging l i g a n d s . In compounds c o n t a i n i n g strong metal-metal bonds, the e f f e c t o f the metal-metal bond(s) i s t o modify s t r o n g l y the chemical and e l e c t r o n i c p r o p e r t i e s o f the compounds when compared to the component monomers.(1) One o f the i n t r i g u i n g p r o p e r t i e s of such systems i s that if c e r t a i n bonding and/or s t r u c t u r a l features are p r e s e n t , they can e x i s t i n a v a r i e t y o f molecular oxidation states, e.g., [(π-C H )Fe(CO)] (2), {[(π-C H ) 5

Fe(CO)] (Ph P(CH ) PPh )} / / 2

2

2 3

[(π-C H )Fe(CO)]+4 5

5

2

2+

and

+

2+/+/0/-

5

4

5

5

(Ph is phenyl) (3). Ions l i k e

0

{[(π-C H )Fe(C0)] (Ph P(CH ) PPh )} 5

5

2

2

2 3

2

+

are

formally mixed-valence c a s e s , but evidence i s now being obtained which i n d i c a t e s that the metal atoms are s t r o n g l y coupled and that o x i d a t i o n - r e d u c t i o n processes involve d e l o c a l i z e d molecular orbitals. S t r o n g , C h e m i c a l l y - S i g n i f i c a n t I n t e r a c t i o n s Through a_ Bridging Ligand. μ-οχο-Bridged Complexes o f Ruthenium ( I I I ) . We have prepared several μ-οχο-bridged complexes o f ruthenium (III): [(AA)XRu-0-RuX(AA) ] *(AA i s 2 , 2 » - b i p y r i d i n e (bipy) o r 2

1,10-phenanthroline

2

(phen); X i s CI o r N 0 ) . The complexes 9

66

Interrante; Extended Interactions between Metal Ions ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

7.

BROWX

Polymeric

E TAL.

67

Compounds

have been c h a r a c t e r i z e d by elemental a n a l y s i s (as hexaflurophosphate s a l t s ) , s o l u t i o n c o n d u c t i v i t y measurements, e l e c t r o chemical measurements, and spectrophotometric t i t r a t i o n s using Cr as reductant. Although x - r a y c r y s t a l l o g r a p h i c data i s not y e t a v a i l a b l e , i n the complexes the X groups are almost c e r t a i n l y c i s to the b r i d g i n g oxide ion(4·) and molecular models i n d i c a t e that d i r e c t , through-space Ru-Ru i n t e r a c t i o n s are probably not p o s s i b l e . The p r o p e r t i e s o f the μ-οχο-bridged dimers are unusual when compared to c l o s e l y r e l a t e d b i s ( 2 , 2 ' - b i p y r i d i n e ) complexes of ruthenium (II) and ruthenium ( I I I ) . From electrochemical studies i n a c e t o n i t r i l e , the system L(bipy) ClRu-0-RuCl(bipy) ] 2 +


2

2

+

2

a l s o e x i s t s as mixed-valence +3 (Ru(III)-Ru(IV)) and +1 ( R u ( I I ) Ru(111)) i o n s . The +1 i o n i s chemically unstable on time s c a l e s longer than the c y c l i c voltammetry time s c a l e (seconds). The e l e c t r o n i c spectra o f the +2 and +3 ions i n a c e t o n i t r i l e include h i g h l y intense bands i n the v i s i b l e : for [ ( b i p y ) 2

ClRu-0-RuCl(bipy) ] ,

x 668nm(e 17,000); f o r [ ( b i p y ) C ! R u (Ru

i n

)

n

+ η Ce(III)

A s e r i e s of both f u l l y o x i d i z e d and p a r t l y o x i d i z e d (mixedvalence) ions have been i s o l a t e d as C10,~ s a l t s . Recently, work has begun on the low temperature magnetic p r o p e r t i e s o f the s a l t s i n c o l l a b o r a t i o n with Professor W. E . H a t f i e l d o f the U n i v e r s i t y of North C a r o l i n a . Work has a l s o begun on the s o l i d s t a t e e l e c t r i c a l c o n d u c t i v i t y p r o p e r t i e s o f the s a l t s . It may prove p o s s i b l e , to some e x t e n t , to vary s y s t e m a t i c a l l y the c o n d u c t i v i t y p r o p e r t i e s o f a s e r i e s of polymeric ions in the s o l i d s t a t e . The s y n t h e t i c chemistry involved i s extremely v e r s a t i l e and the polymeric ions can be modified i n a c o n t r o l l e d way with regard to such features a s : 1. The number of u n i t s i n the polymeric c h a i n . 2. The r a t i o o f Ru(II) to Ru(III) s i t e s . 3. The pattern of bridging l i g a n d s . 4. The non-bridging l i g a n d s . 5. The i n t r o d u c t i o n of metal ions d i f f e r e n t from ruthenium. For the 1 , 1 ' - p o l y f e r r o c e n e compounds, electrochemical measurements have shown that each ferrocenyl group (-Fc) can be o x i d i z e d to ferrocenium ( - F c ) . For example, f o r 1 , 1 ' - t e r f e r r o c e n e , there are three voltammetric waves a t w e l l separated p o t e n t i a l s . The voltammetric waves correspond to the electrode r e a c t i o n s : +


7.

BROWN

Polymeric

E T AL.

Compounds

73

E

(Fc-FoFc)

+

e

>(Fc-Fc-Fc)

(Fc-Fc-Fc)

2 +

+

e

»(Fc-Fc-Fc)

(FoFc-Fc)

3 +

+

e

>(Fc-Fc-Fc)

+

^ >

V

0.22 0.44

+

2 +

0.82

The EL. values are half-wave p o t e n t i a l s v s . the saturated sodiurfi c h l o r i d e calomel e l e c t r o d e i n 1:1 v/v dichloromethanea c e t o n i t r i l e a t 25+2°C. S o l u t i o n s c o n t a i n i n g mixed-valence ions such as ( F c - F c - F c ) and (Fc-Fc-Fc)2+ can be prepared by c o n t r o l l e d p o t e n t i a l electrolysis. For the mixed-valence biferrocenium i o n (Fc-Fc) , i t has been concluded that d i s c r e t e F e ( I I ) and F e ( I I I ) s i t e s e x i s t and that e l e c t r o n i c d e r e a l i z a t i o n between the two s i t e s i s small.(lO)Because of the s i m i l a r i t i e s i n s p e c t r a l and redox propertiesHBetween the biferrocenium i o n and the mixed-valence polyferrocene i o n s , i t appears t h a t the mixed-valence p o l y ferrocene ions a l s o contain weakly i n t e r a c t i n g but d i s c r e t e Fe(II) ana! F e ( I I I ) s i t e s . For the 1 , 1 ' - p o l y f e r r o c e n e compounds there are c h e m i c a l l y d i f f e r e n t s i t e s ( F c - and - F c - ) i n the polymeric c h a i n s . Upon o x i d a t i o n to the mixed-valence ions more than one o x i d a t i o n s t a t e isomer can e x i s t . The o x i d a t i o n s t a t e isomers d i f f e r with regard to the s i t e ( s ) o f o x i d a t i o n . For example, f o r the ion ( F c - F c - F c ) there are two e n e r g e t i c a l l y equivalent isomers — F c - F c - F c and F c - F c - F c — and one e n e r g e t i c a l l y nonequivalent isomer - - F c - F c - F c . From the e f f e c t of the f e r r o c e n y l group as a s u b s t i t u e n t , i t can be estimated that the isomers F c * - F c - F c and F c - F c - F c are of s i m i l a r energy. However, f o r other mixed-valence ions the d i f f e r e n c e i n f r e e energy between isomers can be s i g n i f i c a n t . From E% data and the f e r r o c e n y l s u b s t i t u e n t e f f e c t i t i s p o s s i b l e to estimate that f o r the r e a c t i o n +


+

+

+

+

+

+

Fc -Fc-Fc +

+

>

Fc -Fc -Fc +

+

AG * 2.8 kcal/mole. The p r o p e r t i e s of Intervalence T r a n s f e r (IT) bands are influenced by o x i d a t i o n s t a t e isomerism. IT bands f o r several of the mixed-valence ions are given i n Table I I . For some o f the ions the IT x i s s h i f t e d s i g n i f i c a n t l y from Xm^ f o r Fc -Fc. The band s h i f t s are expected when i t i s r e a l i z e d that the products o f l i g h t - i n d u c e d e l e c t r o n t r a n s f e r are not symmetric f o r the mixed-valence polyferrocene i o n s . In some cases the products of l i g h t - i n d u c e d e l e c t r o n t r a n s f e r are e n e r g e t i c a l l y unfavorable o x i d a t i o n s t a t e isomers, f o r example, m a x

x

+

F c

+

-Fc-Fc

+

—~—>

(Fcî F c - F c ) * +

As described by Hush(7)the IT t r a n s i t i o n energy i n such a case



a

I n 1:1

+

2

2

hv

hv

hv

hv

->

->

3

Transition

+

+

+

(Fc -Fc -Fc-Fc)*

+

(Fc -Fc -Fc)*

+

(Fc -Fc-Fc)*

+

(Fc-Fc )*

4

( v / v ) C H C l - C H C N , 0.1M i n (n-Bu) NPFg.

+

Fc -Fc

Fc-Fc

+

Fc -Fc

Fc -Fc

+

II

1790

1990

1900

nm

5590

max

1670

x

5990

5020

5260

kK

1720

1080

1560

760

ε

Intervalence T r a n s f e r Bands f o r Mixed-Valence 1 , 1 ' - P o l y f e r r o c e n e

Table


Ions

7.

BROW

x

Polymeric

E T AL.

75

Compounds

w i l l i n c l u d e the d i f f e r e n c e i n ground s t a t e energies between the isomers F c - F c - F c and F c - F c - F c i n a d d i t i o n to the usual Franck-Condon energy b a r r i e r . An equation has been derived by Hush which r e l a t e s the energy of IT absorption to the rate of i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r i n mixed-valence compounds.(7jThe equation allows estimates to be made for processes l i i c e : +

Fc -Fc +

+

>Fc-Fc

+

+

k * 3x10

+

Fc -Fc-Fc—>Fc-Fc -Fc+ +

k * IxlO

+

1 0

sec"

1

sec"

1

The rate of e l e c t r o n hopping between f e r r o c e n y l and f e r r i cenium groups i s slower i n F c - F c - F c than i n F c - F c , mainly because o f the e n e r g e t i c a l l y d i f f e r e n t chemical s i t e s i n Fc -Fc-Fc . The r a t e d i f f e r e n c e s involved are s m a l l , but s i g n i f i c a n t , since they i n d i c a t e that i n l i g a n d - b r i d g e d complexes, rates of i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r can be v a r i e d s y s t e m a t i c a l l y by changing the chemical environments of the c o n s t i t u e n t i o n s . +


10

+

+

+

+

Acknowledgements. Acknowledgements are made to the M a t e r i a l s Research the U n i v e r s i t y of North C a r o l i n a under grant number with DARPA and to the Army Research O f f i c e - Durham grant number DA-AR0-D-31-124-73-G104 f o r support of research.

Center of GH33632 under this

Literature Cited 1. 2. 3. 4. 5.

6. 7. 8. 9. 10.

Meyer, T . J., P r o g r . I n o r g . Chem., i n p r e s s . Ferguson, J. A. and Meyer, T . J., J. Amer. Chem. Soc., (1972), 94, 3409. Ferguson, J. Α . , and Meyer, T . J., Chem. Commun., (1971), 1544. Godwin, J. B. and Meyer, T . J., Inorg. Chem., (1971). 10, 471. The magnetic data was obtained i n c o l l a b o r a t i o n with Professor W. E . H a t f i e l d and h i s research group at the U n i v e r s i t y of North C a r o l i n a . Bleaney, B. and Bowers, K . , Proc. Royal Soc. (London), (1952), 214A, 451. Hush, N. S., Progr. Inorg. Chem., (1967), 8, 391. C r e u t z , C. and Taube, H . , J. Amer. Chem. Soc., (1973), 95, 1006. Adeyemi, S. Α . ; Johnson, E. C.; Miller, F. J.; and Meyer, T . J.; Inorg. Chem. (1973), 12, 2371. Cowan, D. O.; Le Vanda, C.; Park, J.; and Kaufman, F.; Accounts Chem. R e s . , (1973), 6, 1.


Polymeric, Mixed-Valence Transition Metal Compounds - ACS

Recommend Documents