FIRST COORDINATION SPHERE AND SPIN-STATE IN SPIN CROSS-OVER IRON AND COBALT COMPLEXES

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FIRST COORDINATION SPHERE AND SPIN-STATE IN SPIN CROSS-OVER IRON AND COBALT

COMPLEXES

C. Cartier, Pierre Thuéry, M. Verdaguer, J. Zarembowitch, A. Michalowicz

To cite this version:

C. Cartier, Pierre Thuéry, M. Verdaguer, J. Zarembowitch, A. Michalowicz. FIRST COORDINATION

SPHERE AND SPIN-STATE IN SPIN CROSS-OVER IRON AND COBALT COMPLEXES. Journal

de Physique Colloques, 1986, 47 (C8), pp.C8-563-C8-568. �10.1051/jphyscol:19868105�. �jpa-00226001�

JOURNAL DE PHYSIQUE

Colloque C8, s u p p l k m e n t au n o 1 2 , Tome 47, d k c e m b r e 1 9 8 6

F I R S T COORDINATION SPHERE AND SPIN-STATE I N S P I N CROSS-OVER IRON AND COBALT COMPLEXES

C. CARTIER* , P .

^THUERY*

^, M . VERDAGUER*

,

* * ,

J.

ZAREMBOWITCH* a n d

A . MICHALOWICZ***

* ~ a b o r a t o i r e SET, (CNRS

-

UA 4201, BStiment 420, F-91405 Orsay Cedex, France

X

* Ecole Normale Superieure, Le Parc, F-92211 Saint-Cloud, France

rl Z

* Laboratoire de Physico-Chimie Structurale, Universit6 de Paris

X I I ,

Creteil, and LURE (CNRS, MEN, CEA),

F-91405 Orsay Cedex, France

RESUME

Nous e t u d i o n s par

spectroscopic

d ' a b s o r p t i o n d e s rayons

X

les t r a n s i t i o n s

de

s p i n d e complexes du c o b a l t ( I 1 ) . du f e r

(

1 1 ) e t du f e r ( I I 1 ) .

La

c o n t r a c t i o n d e la premigre sphgre d e c o o r d i n a t i o n l o r s du passage haut-spin

->

bas-spin e s t mesur6e par

ExApS

p o u r t o u s les complexes. Le

XANES

d e s complexes du f e r m e t e n e v i d e n c e les v a r i a t i o n s de s t r u c t u r e e l e c t r o n i q u e l o r s d e la t r a n s i t i o n .

L a

v a r i a t i o n d ' o r d r e l o c a l e s t c o n f r o n t h e a u x r 6 s u l t a t s d ' a u t r e s t e c h n i q u e s pour d i s c u t e r l e c a r a c t T r e c o o g r a t i f du p h e n M n e .

ABSTRACT

We s t u d y

by

X-Ray a b s o r p t i o n s p e c t r o s c o p y t h e s p i n crwssover f o r C o ( I I ) , Fe(

I1 )

and Fe(

I11 )

complexes. EXAFS a l l o w s t o measure t h e f i r s t c o o r d i n a t i o n s p h e r e c o n t r a c t i o n through t h e t h e r m a l l y induced high-spin

->

low-spin t r a n s i t i o n f o r a l l t h e complexes. The

XANES

o f t h e i r o n s p e c t r a is modified w i t h t h e changes i n

the

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

The

m o d i f i c a t i o n o f t h e s h o r t r a n g e o r d e r is

compared t o t h e r e s u l t s of

o t h e r t e c h n i q u e s t o d i s c u s s t h e c o o p e r a t i v e c h a r a c t e r o f t h e phenomenon.

I r n r n I O N

The complexes o f t r a n s i t i o n m e t a l i o n s b e l o n g i n g t o d W 7 e l e c t r o n i c c o n f i g u r a t i o n s c a n e x i s t under high-spin and low-spin forms. I n t h e few cases where

the

mean p a i r i n g energy 5 is o f t h e same o r d e r o f magnitude as t h e molecular o r b i t a l s p l i t t i n g

( A,

i n t h e case o f an o c t a h e d r a l complex), a t h e r m a l l y induced t r a n s i t i o n c a n o c c u r from h i g h s p i n t o low s p i n s s a t e when d e c r e a s i n g t h e t e m p e r a t u r e . ? ' h e c o n d i t i o n

can be

expressed as In. - ^P )wm.

A

l o t o f s y n t h e t i c , e x p e r i m e n t a l and t h e o r e t i c a l work

has

been performed t o o b t a i n new materials

(1-6),

t o f u l l y c h a r a c t e r i z e t h e t r a n s i t i o n and t o understand its real n a t u r e , l o c a l or more o r less cooperative.

The s p i n c r o s s o v e r ( w i t h a

A S

change i n s p i n ) is associated w i t h a b r u p t o r s m o t h changes i n magnetic, s p e c t r o s c o p i c and s t r u c t u r a l properties. A v a i l a b l e X-Ray d i f f r a c t i o n d a t a d i s p l a y a c o n t r a c t i o n DE o f t h e mean c o o r d i n a t i o n r a d i u s upon t r a n s i t i o n from h i g h t o low-spin s t a t e . T h i s e v o l u t i o n mainly depends o n t h e s p i n v a r i a t i o n

A S ,

on t h e n a t u r e o f t h e f i r s t neighbours ( f o r example f o r oxygen ( O ) , n i t r o g e n ( N ) , and phosphorus (PI, b E ( 0 )

c

aii ( N )

c

aR

(P) ),

and o n steric f a c t o r s .

The a b r u p t o r smooth c h a r a c t e r o f t h e s p i n t r a n s i t i o n seems t o depend on

the

e x i s t e n c e

of

a concomitant c r y s t a l l o g r a p h i c phase t r a n s i t i o n

(1)

w i t h a m o d i f i c a t i o n

AV

o f

the

c e l l volume upon

the

t r a n s i t i o n .

W e

undertook t h i s s t u d y ( 1 ) t o p u t i n e v i d e n c e

^by

EXAPS t h e A E v a r i a t i o n d u r i n g t h e s p i n t r a n s i t i o n and t o measure

it

i n complexes where no s i n g l e crystals c a n

be

o b t a i n e d ;

( 2 )

t o c h a r a c t e r i z e

by XANES

t h e e l e c t r o n i c s t r u c t u r a l change, o t h e r t h i n g s b e i n g equal (number o f e l e c t r o n s , o x i d a t i o n state, n a t u r e o f t h e n e i g h b o w ) i n view o f a better u n d e r s t a n d i n g o f t h e

XANES

o f c o o r d i n a t i o n c o m p l e ~ e s .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868105

JOURNAL DE PHYSIQUE

B(APS OF

COBALT

( 1 1 ) COMPLEXES

The compounds a r e of t h e g e n e r a l type Co

(

H2fsa2en

) L2,

where Mfsa2en is t h e b i c h e l a t i n g Schiff base N,N1-ethylene

bis ( 3-carboxy

s a l i c y l a l d i m i n e

)

( F i g . 1 ) -

Fig.

1:

Co(FiZfsa2en)l2 F i g . 2

The complexes f o r which L=H20

(

4 ) , and Wpyridine

( 3 ),

e x h i b i t AS

= 1,

*3/2

c-D

*1/2 s p i n interconversions. These t r a n s i t i o n s are t h e most abrupt ones e v e r observed f o r c o b a l t ( 1 1 ) complexes. The

mnp

v s T p l o t

(Xm

is t h e molar magnetic s u s c e p t i b i l i t y ) f o r

Co (

B2fsa2en

)

(H20)2 is given i n Fig.

2.

Up t o n w , it h a s been impossible t o grow c r y s t a l s of t h e s e compounds;

so,we turned t o D5PS t o o b t a i n s t r u c t u r a l informations. The a n a l y s i s of t h e s p e c t r a recorded a t room temperature and a t 40K allows us t o e v a l u a t e t h e modification o f t h e c o b a l t ( I 1 ) l o c a l environment upon t h e high-spin

^{< - D}

low-spin conversion

( 5 ) .

For camparison,

we

a l s o s t u d i e d t h e compound f o r which -3-methylpyridine

( 3 - M e p y )

( 6 ) . which r e t a i n s t h e high-spin form i n t h e whole temperature range, and c o b a l t

( I1 )

and

( 111 )

compounds whose c r y s t a l l o g r a p h i c s t r u c t u r e s are

knom

( 5 ) .

The s p e c t r a

are

analysed

by

using t h e t a b u l a t e d phase parameters o f

Tea

and

Lee ( 7 )

and Beni and Lee c r i t e r i o n ( 8 ) . The EXAPS

k . X ( k )

v s

k

s p e c t r a and t h e corresponding F o u r i e r transforms f o r t h e

L =

A20 complex a t 295 and 40K are given i n Pig.

3.

The v a l u e s of t h e mean metal-ligand bond d i s t a n c e s found f o r

model

compounds

are i n good agreement with c r y s t a l l o g r a p h i c d a t a , w i t h i n experimental

error ( 2 0.02 i).

For both s p i n crossover compounds, 0 E is found t o be 0.09 f o r a camplete s p i n conversion (T&le

I ).

TABLE

I r

EXRFS r e s u l t s f o r Cobalt camplexes

L E( 2 9 5 ~

)/i K( 40K )/A

4Z/i

( a )

... ----

H20 1.96 1.88 0.09

----

PY 1.98 1.89 0.09

- ---- - ---- ---

3-Mew

2.04 2.06

NO

( a ) c a l c u l a t e d f o r 100% conversion

The respective variation of axial and equatorial bond lengths cannot

be

distinguished. However, owing to the difference in the population of the d(x2-yZ) orbital between the high spin and low spin forms (the population of the de2 orbital being unchanged). A W (eq) is expected to be much larger thanbR (ax).

, ,

! i a , ,

- .

^,

- .

\/

L t. L A a. 0. 1. I.

l lAffilllDtl4 8.

I IAffiIIIIyI

Pig. 3

:

E X W S and Fourier transform of C O ( H Z £ S ~ ~ ~ ~ ) ( H ~ O ) ~ E X W S OF IRON COMPLEXES

We choose here tris(diethy1dithiocarbamate

)

Fe( I11

),

Fe(dtc)3, the structure of which is known at low and room temperature

( 9 ) ,

even if the transition

(

S

=

5/2

<--,

S

=

1/2;AS

=

2

)

is incomplete

(

10-11

),

and cis bis

(

bphenantroline

)

bis

(

thiocyanato

)

Fe( I1

),

Pe( phen )2(NCS )2, for which single crystal X-Ray diffraction data are lacking and an abrupt and complete transition occurs

(

S

=

2

<-->

S

=

0; A S

=

2

)

(12-14).

We used tabulated phases and amplitudes of Teo

and

Lee

(

7

)

. For Pe(dtc )3, the results differ slightly from crystallographic ones at both temperaturns, due probably to the non transferability of Pe-S amplitude and phase parameters.

An extraction of amplitudes and phases from the structural data at low temperature (low spin form) to find from them the distances in the room temperature form, which is a mixture of

lw

and high-spin forms is underway.

Nevertheless the variation A R of the mean coordination sphere is practically the same

by

E X W S and by diffraction (AH

=

0.05 A, i.e.

h = 0.10

h for a complete transition. The results are gathered in Table 11.

TABLE 11

:

E G W S results for Iron complexes

~ e (

111 )(

dtc )3 290 2.30 0.002 0.6

30 2.25 0.10 0.010 3.0

Fe(II)(phen)P(NCS)P 290 2.15 0.007 1.8

30 1.91 0.24 0.012 2.4

(a) Calculated for 100% conversion; (b) Debye-Waller factor*

(c) Pit agreement factor

C8-566 JOURNAL DE PHYSIQUE

For Fe(phet1)2(NCS)2,~ t h e r e s u l t s a r e i n Table

11

and i n F i g .

4.

The c o n t r a c t i o n AR

=

0.024

A,

determined f o r t h e first time, is s t r i k i n g .

It

is i n l i n e w i t h t h e a b r u p t c h a r a c t e r o f t h e t r a n s i t i o n , w i t h t h e important v a r i a t i o n o f t h e IR spectrum ( s t r e t c h i n g Fe-N o r N-C) (12

),

&ssbauer

spectra

( 1 4 ) . T h i s s t e e p n e s s i m p l i e s a

large

v a l u e o f

the

change i n molecular v i b r a t k o n a l entropyAg ( v i b mol).

F i g . 4

On t h e o t h e r hand, X-Ray powder d i f f r a c t i o n r e s u l t s show l i t t l e t h r e e - d i m e n s i o n a l changes d u r i n g t h e c r o s s o v e r ( 1 4 ) . Comparison between s h o r t

and

long range o r d e r measurements c l e a r l y a e m o n s t r a t e s , t h e r e f o r e , t h a t a n important ' A and i n t r a m o l e c u l a r rearrangement does n o t imply a l a r g e v a r i a t i o n o f

AV.

T h i s r e e u l t p o i n t s o u t t h e complementarity between s h o r t range

and

l o n g r a n g e o r d e r t e c h n i q u e s f o r a f u l l c h a r a c t e r i z a t i o n o f t h e s t r u c t u r a l phenomenon and t h e d e t e r m i n a t i o n o f t h e r e s p e c t i v e i n f l u e n c e s o f s i n g l e t - t r i p l e t energy g a p and o f c o o p e r a t i v i t y .

An

o t h e r p o i n t

of

i n t e r e s t is t h e i n c r e a s e o f t h e Debye-Waller f a c t o r i n t h e low-spin form, o b s e r v a b l e also i n t h e F e ( I 1 ) compound. Although t h e thermal part is d e c r e a s i n g w i t h t e m p e r a t u r e , t h e o v e r a l l i n c r e a s e is a n i n d i c e o f enhanced static d i s o r d e r i n

the first

c o o r d i n a t i o n s p h e r e .

The

comparison o f

the

r e s u l t s o f Co( I I ) , Fe(11) and F e ( I I 1 ) complexes is n o t e a s y s i n c e

the

c o o r d i n a t i o n s p h e r e s CoN402, FeN6 and FeS6 are d i f f e r e n t . N e v e r t h e l e s s , f o r t h e

^two

conrpounds w i t h t 2 g f u l l o r b i t a l s i n t h e low s p i n form, i n CON402

( A

S

= 1 ),

t h e

m e a n

c o n t r a c f i o n

( A = 0.09

i) is less t h a n t h e h a l f o f t h e one i n FeN6

( A S =

2 ; ~ z

=

0.24

A,

which is c o n s i s t e n t w i t h t h e h a l f s p i n number change and w i t h t h e p r e s e n c e o f two oxygen l i g a n d s i n s t e a d o f n i t r o g e n ones. I n s p i t e o f t h e i r h a l f

AS,

t h e s p i n c r o s s o v e r Co( 1 1 ) e n t i t i e s h a v e a A a comparable i n magnitude t o t h e one

of

P e ( I I I ) S 6 , which

can be

e x p l a i n e d

by

t h e p r e s e n c e o f a n u n p a i r e d e l e c t r o n i n t h e t 2 g o r b i t a l o f t h e F e ( I 1 I ) species and t o t h e

soft

c h a x a c t e r o f s u l f u r i n t h e d i t h i o c a r b a m a t e l i g a n d a , w i t h o u t

d-lW

i n t e r a c t i o n s , which g e n e r a l l y i n c r e a s e

the

b z

( 1 )

.

1(ZWEs

OF IRON COMPLEXES

The XANES o f b o t h complexes at 300K and 3OKis shown i n F i g .

5.

The e n e r g i e s

of

t h e t r a n s i t i o n s are given i n Table

111.

TABLE III :

XANES E n e r g i e s ( a )

-- ^--

COMmUND Temp.

/K

ENERGY

/ ev

->

--- ---

Fe(

I11 )(

d t c )3 290 7106.8 7113.4 7119.0 7122 7136

30 7107.8 7112.4 7117.6 7123 7138

7128 7142

F e ( I 1 )(phen)2(NCS)2 290 7106.2 7110.5 7115 7121 7130 7137 30 7106.6 7111.4 7114.6 7121.6 7130

7117.4 7126.3 7137

---

(a) C a l i b r a t e d at t h e

f i r s t maximtrm

of t h e d e r i v a t i v e o f t h e m e t a l l i c

i r o n

spectrum (7111.2

eV)

Fig.

5 : XANES

s p e c t r a and second d e r i v a t i v e of Fe( d t c

)3

and Fe( phen

)2(NCS )2 As

expected, t h e most s p e c t a c u l a r e v o l u t i o n is f o r

the

F e ( I 1 ) conlplex, b u t Close examination o f t h e second d e r i v a t i v e s i n d i c a t e s important changes i n b o t h Compounds. The main c o n c l u s i o n s o b t a i n e d from

XANES are the

following:

( i ) new t r a n s i t i o n s appear i n b o t h s p e c t r a a t l o w

^T.

I n p a r t i c u l a r , t r a n s i t i o n s

i n t h e

edge

o f low s p i n s p e c i e s , which e x i s t also i n low s p i n ferro and

f e r r i c y a n i d e s , are probably a t t r i b u t a b l e t o s p i n allowed e l e c t r o n i c t r a n s i t i o n s

towards

the

states d e r i v e d

from

t h e

higher

terms a v a i l a b l e i n

the

s t r o n g - f i e l d

C8-568 JOURNAL DE PHYSIQUE

case,

&g, ' ~ 2 g ) , ? ~ 2 g , ' ~ g ) , (?AZg, ' ~ g ) f o r P e ( I I I ) , and t r o 4 T l g f o r F e ( I 1 ) u s i n g f o r m u l a t i o n s d e r i v e d from Oh p o i n t group;

an exact

f o r m u l a t i o n would have t o t a k e i n t o account d e p a r t u r e s from o c t a h e d r a l symnnetry).

( i i ) o t h e r t r a n s i t i o n s

are

s h i f t e d , i n p a r t i c u l a r t h e preedge one which c o r r e s p o n d s

t o a

t r a n s i t i o n towards t h e h i g h e r e q o r b i t a l s i n t h e low-spin

case

and t h e o n e

at

t h e t o p o f t h e edge: t h e phenomenon

is real

b u t d i f f i c u l t t o c h a r a c t e r i z e q u a n t i t a t i ~ e l y i n F e ( d t c ) 3 , g i v e n t h e s o f t Fe-S6 c o o r d i n a t i o n s p h e r e and t h e s m a l l AR; t h e t r e n d

is

c l e a r l y demonstrated i n Fe(phen)2(NCS)2:

d i s a p p e a r a n c e of t h e w h i t e l i n e

at

low t e m p e r a t u r e r e l a t e d t o t h e f i r s t c o o r d i n a t i o n s p h e r e d i s t o r t i o n , s h i f t t o h i g h e r energy o f t h e t o p o f t h e edge c o r r e l a t e d t o t h e c o n t r a c t i o n o f t h e Fe-N d i s t a n c e s .

Q u a n t i t a t i v e i n t e r p r e t a t i o n and s a f e a t t r i b u t i o n o f t h e observed t r a n s i t i o n s imply more e x p e r i m e n t a l and t h e o r e t i c a l work ( m o l e c u l a r o r b i t a l approach f o r preedge and edge, m u l t i p l e s c a t t e r i n g f o r edge and post-edge) and w e

are

engaged i n such a way.

CONCLUSION

S p i n c r o s s o v e r

is a

remarkable t h e r m a l l y induced t r a n s i t i o n phenomenon p r e s e n t i n some

rare

t r a n s i t i o n

m e t a l

complexes which c a n e x h i b i t b i s t a b i l i t y . The s e a r c h of such bistable systems is one o f t h e c h a l l e n g e s i n t h e developnent o f molecular e l e c t r o n i c s d e v i c e s . W e tried

t o

show t h a t X-Ray a b s o r p t i o n s p e c t r o s c o p y c a n be u s e f u l i n t h i s

area:

on t h e one hand, X-Ray a b s o r p t i o n

can

h e l p t o understand what

i s

g o i n g on i n t h e s p i n t r a n s i t i o n and

can

be used

as an

e f f i c i e n t t o o l t o c h a r a c t e r i z e q u a n t i t a t i v e l y s t r u c t u r a l s h o r t r a n g e o r d e r e v o l u t i o n s w i t h EXAPS and t o p u t i n evidence e l e c t r o n i c and s t e r e o c h e m i c a l changes w i t h XRNES.

On t h e o t h e r hand, s t u d y o f well-known c h a r a c t e r i z e d s p i n c r o s s o v e r cwaple*s

can

g i v e new i n s i g h t s t o b e t t e r understand t h e XANES part o f t h e spectrum i n t h e wsimple"

case

when t h e oxydation

state

o f t h e c e n t r a l i o n and t h e n a t u r e o f t h e l i q a n d s remain unchanged.

EXPERIMENTAL SECTION.

The e x p e r i m e n t a l d a t a

w e r e

recorded

a t LURE,

on EXAFS I s p e c t r o m e t e r w i t h channel-cut monochromators ( S i 220, 2 m e n t r a n c e

slit

f o r EXAFS and S i 400, 0 , 5 m e n t r a n c e

slit

f o r XANES), w i t h

a

s t o r a g e r i n g e n e r g y o f 1.72

GeV

and a n i n t e n s i t y o f 100-250

aA.

EXAPS

d a t a p r o c e s s i n g used

classical

p l a n e

wave

s i n g l e e l e c t r o n s i n g l e s c a t t e r i n g a n a l y s i s , w i t h u s e o f Beni

and

Lee c r i t e r i o n f o r C o ( I 1 ) complexes ( 8 ) . and f i l t e r e d m u l t i p l e s h e l l s f i t i n t h e k-space f o r Fe complexes. Samples were

w e l l

pounded m i c r o c r y s t a l l i n e powders o f c a l c u l a t e d t h i c k n e s s . The t e m p e r a t u r e was f i x e d b y a n home-made H e l i u m flow c r y o s t a t .

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P.

Giitlich, S t r u c t u r e and Bonding, 44,85 (1981)

( 3 ) J. Zarembowitch, 0 . Kahn, I n o r g . Chem., 23, 589 (1984)

( 4 ) J. Zarembowitch,

R.

Claude, 0 . Kahn, Inorg.

Chem.

24,1576 (1985)

( 5 ) J. Zarembowitch, P. Thubry,

A.

Michalowicz, 0 . Kahn,

submitted

t o InOrg.

(hem.

( 6 )

P.

Thuiiry, J. Zarembowitch, Inorg.

Chem.,

25, 2001 ( 1 9 8 6 ) ( 7 ) B.K. Teo,

P.A.

Lee, Am. Chem. Soc., 101, 2815, (1979) ( 8 )

P.A. Lee,

G. Beni, Phys. Rev., 815, 2862 (1977) ( 9 ) G. L e i p o l d t ,

P.

Coppens, I n o r g . Chem. 12,2269 (1973) (10) G.R. H a l l , D.N. Hendricson, Inorg.

Chem.,

15, 607 ( 1976) ( 1 1 ) K.

~ t I L h l ,

I. m n , Acta Chem. Scand. A37, 729 (1983) ( 1 2 )

M.

S o r a i , S . S e k i ,

J.

Phys. Chem. S o l i d s , 35, 555 (1974)

( 1 3 )

P.

Ganguli,

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( 1 4 ) E. Konig, K. Madeja, 'Inorg. Chem., 6, 48 (1967)