57Fe- MÖSSBAUER EMISSION SPECTROSCOPY OF AN IRON (II) COMPLEX WITH TEMPERATURE DEPENDENT SPIN STATE

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57Fe- MÖSSBAUER EMISSION SPECTROSCOPY OF AN IRON (II) COMPLEX WITH TEMPERATURE

DEPENDENT SPIN STATE

P. Gutlich, H. Köppen

To cite this version:

P. Gutlich, H. Köppen. 57Fe- MÖSSBAUER EMISSION SPECTROSCOPY OF AN IRON (II)

COMPLEX WITH TEMPERATURE DEPENDENT SPIN STATE. Journal de Physique Colloques,

1980, 41 (C1), pp.C1-311-C1-312. �10.1051/jphyscol:19801113�. �jpa-00219595�

JOURNAL DE PHYSIQUE

Colloque C1, supplhent au n

^O

1, Tome 41, janvier 1980,

page

C1-3 1 1

P. G u t l i c h , H. Ktippen

I n s t i t u t

fUr

A n o r g k s c h e

und

AnaZytische Chemie, Johannes Gutenberg, Universitt?t,

0-6500

Mainz.

1. I n t r o d u c t i o n . - Mossbauer emission spectroscopy h a s been widely used t o i n v e s t i g a t e t h e chemical a f t e r - e f f e c t s a s s o c i a t e d w i t h t h e n u c l e a r t r a n s f o r - mation of 5 7 ~ o 57 Fe i n v a r i o u s k i n d s o f h o s t s ,

p a r t i c u l a r l y i n 57~o-doped m e t a l o r g a n i c compounds.

Two a s p e c t s have been o f major i n t e r e s t :

( i ) The change o f o x i d a t i o n s t a t e w i t h r e s p e c t t o t h e 5 7 ~ o p a r e n t atom /1,2/.

(ii) The change of s p i n s t a t e with r e s p e c t t o t h e e l e c t r o n i c s t r u c t u r e of t h e i r o n atom o b t a i n e d from t h e corresponding MGssbauer a b s o r p t i o n experiment / 3 , 4 , 5 , 6 / .

For t h e i n t e r p r e t a t i o n of t h e a f t e r - e f f e c t s , one h a s used two models:

(i) A u t o r a d i o l y s i s of c o o r d i n a t e d l i g a n d s , l e a d i n g t o t h e formation of r a d i c a l s with subsequent redox r e a c t i o n s i n v o l v i n g t h e nucleogenic i r o n atom /5,6

/

(ii) Fragmentation of t h e complex molecule ("ex- p l o s i o n model") / 7 , 8 / .

The phenomenon o f a u t o r a d i o l y s i s i s n o t r e s t r i c t e d t o p a r t i c u l a r k i n d s o f l i g a n d s . E s s e n t i a l f a c t o r s determining t h e e l e c t r o n i c s t r u c t u r e of t h e nucleo- g e n i c i r o n atom on t h e Mossbauer time s c a l e ( I O - ~ S )

a r e t h e r e d o x p o t e n t i a l of t h e l i g a n d r a d i c a l and t h a t of t h e c e n t r a l m e t a l i o n on t h e one hand, and t h e l i f e t i m e of t h e l i g a n d r a d i c a l on t h e o t h e r hand. However, t h e e x p l o s i o n model h a s been sug- g e s t e d t o apply p r e f e r e n t i a l l y t o systems which do n o t c o n t a i n resonance s t a b i l i z e d l i g a n d s , such a s p h e n a n t h r o l i n e and s i m i l a r d i i m i n e s / 7 / . I n t h i s s e n s e , t h e l i g a n d 2-aminomethylpyridine (2-pic) i s expected t o be l e s s s t a b l e . I n o r d e r t o t e s t t h e v a l i d i t y o f t h i s p r e d i c t i o n , we recorded Kossbauer emission s p e c t r a of t h e complex compound

5 7 ~ e ( 2 - i i ~ ) ~ C 1 2 - E t O H a t temperatures between 10 and 300 K. The M6ssbauer a b s o r p t i o n s p e c t r a o f t h i s complex a r e t y p i c a l o f a 5~ *'A s p i n - e q u i l i b r i - urn / 9 / . Hence, t h e Mdssbauer parameters o f b o t h s p i n s t a t e s a r e w e l l known.

2. Experimental

.-

The s o l i d s o l u t i o n

[Fexcol -x (2-pic) 3] C ~ ~ - E ~ O H (x = 0.006) was prepared and i t s Mossbauer a b s o r p t i o n s p e c t r a were recorded and used a s r e f e r e m e t o compare with t h e emission s p e c t r a of t h e l a b e l e d compounds

[

5 7 ~ o (2-pic)

Jc12 -

-EtOH and [Fe (57Co) (2-pic) 3]~12 ' E ~ O H . The prepara- t i o n and c h a r a c t e r i z a t i o n of t h e s o u r c e compounds

F i g . a ) Mossbauer a b s o r p t i o n s p e c t r a o f

[

~ e ~ . ~994 ~ (2-pic) 3]~1Z ~ C o ~ .'EtOH b) MBssbauer emission s p e c t r a o f

[

5 7 ~ o (2-pic) d c l 2 . E t 0 ~

C ) Mbssbauer emission s p e c t r a o f [ ~ e ( 5 7 ~ o ) (2-pic) J c ~ ~ - E ~ o H

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801113

C1-312

^{JOURNAL DE PHYSIQUE}

w i l l be r e p o r t e d elsewhere. A l l isomer s h i f t s a r e r e l a t e d t o n a t u r a l i r o n .

3. R e s u l t s and Discussion.- Two c h a r a c t e r i s t i c Mossbauer a b s o r p t i o n s p e c t r a of

[ F ~ O . 006~00.994 (2-pic) ] c ~ ~ . E ~ o H a r e shown on t h e 3

l e f t of t h e f i g u r e ( a )

.

The spectrum r e c o r d e d a t 5 K i s t y p i c a l o f a F e ( I 1 ) low-spin s p e c i e s .

5 7

The emission s p e c t r a o f

[

Co(2-pic) C 1 2 ' E t O H

3

(s.

Fig. b ) mainly c o n s i s t of a F e ( I 1 ) high-spin dou- b l e t ( A ) , even a t low t e m p e r a t u r e s (I.S. a t 11 K:

-1.031 mms-l, r e v e r s e s i g n t o compare with absorp- t i o n s p e c t r a ; bE a t 1 1 K: 2.50 inins-'). The p a r a -

Q

m e t e r s of t h i s d o u b l e t a r e i n very good agreement with t h e corresponding parameters o f t h e F e ( I 1 ) high-spin d o u b l e t of

[

Feo.006C00. 994 (2-pic) 3] C12 .EtOH The second d o u b l e t (B) seems t o be due t o a F e ( I I 1 ) low-spin impurity.

The emission s p e c t r a o f [ ~ e ( 5 7 ~ o ) ( 2 - p i c ) 4Cl2.EtOE (s. Fig. c ) a r e s i m i l a r t o t h o s e of

[ 5 7 ~ o (2-pic) 3 ] ~ 1 2 ' ~ t ~ ~ i n t h a t t h e Fe (11) high-spin d o u b l e t dominates a t a l l t e m p e r a t u r e s . They d i f f e r , however, i n view o f t h e d o u b l e t C , which may b e a t t - r i b u t e d e i t h e r t o a F e ( I I 1 ) high-spin o r a Fe (11) low-spin (I.S. a t 230 K: -0.35 mms-l, AE a t 230 K:

Q

0.63 mms-l), b u t t h e parameters d i f f e r from t h o s e of t h e low-spin d o u b l e t i n t h e a b s o r p t i o n s p e c t r a of l F e ( 2 - p i c ) ] c ~ ~ . E ~ o H . The r e l a t i v e amount o f

3

t h i s d o u b l e t i n c r e a s e s w i t h i n c r e a s i n g temperature.

The emission s p e c t r a of t h e complex

[ 5 7 ~ o (2-pic)

1~1

'EtOH c l e a r l y ,demonstrate, t h a t

3 2

t h e complex molecule i s n o t fragmentated a s a con- sequence of t h e n u c l e a r e v e n t . Independent of tem- p e r a t u r e , t h e F e ( I 1 ) i o n r e s i d e s i n t h e high-spin s t a t e a t a b o u t I O - ~ S a f t e r t h e e l e c t r o n c a p t u r e p r o c e s s , whereas t h e normal ground s t a t e o f i r o n ( I 1 ) i n t h e l a t t i c e s i t e changes s p i n on c o o l i n g .

The emission s p e c t r a of [ F ~ ( ~ ~ c o ) (2-pic) J c ~ ~ - E ~ o H

i n d i c a t e a remarkable i n f l u e n c e of t h e h o s t l a t t i c e . I t appears t h a t t h e redox p r o p e r t i e s of t h e s u r - rounding molecules p l a y a c r u c i a l r o l e i n d e t e r - mining t h e l i f e t i m e o f t h e r a d i c a l s formed by t h e a u t o r a d i o l y s i s p r o c e s s .

Acknowledgement.- We thank t h e Deutsche Forschungs- gemeinschaft, t h e Bundesministerium f u r Forschung und Technologie, and t h e Fonds d e r Chemischen I n d u s t r i e f o r f i n a n c i a l s u p p o r t .

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