Recent Investigations of Spin Crossover - Advances in Chemistry

19 Recent Investigations of Spin Crossover P. G Ü T L I C H

Mössbauer Spectroscopy and Its Chemical Applications Downloaded from pubs.acs.org by EMORY UNIV on 03/02/16. For personal use only.

Institut für Anorganische und Analytische Chemie, Johannes GutenbergUniversität, D-6500 Mainz, West Germany A brief introduction of the phenomenon of spin crossover in transition metal complexes is followed by a discussion of the results from Mössbauer effect measurements on the metal dilution effect in the solid solutions Cl • EtOH (M — Zn, Co) and 2

[Fe M (2-pic) ] x

1-x

3

[Fe M (phen) (NCS) ] x

1-x

2

2

(M

— Mn, Co, Ni). It is shown that the spin transition behavior changes markedly with the iron concentration. It is also demonstrated that the nature of the crystal solvent molecules as well as the method of sample preparation influence the spin crossover behavior. A presentation of some examples for structural changes accompanying spin crossover concludes this review.

r

T * h e p h e n o m e n o n o f s p i n crossover, o t h e r w i s e c a l l e d m a g n e t i c crossover v

J

- o r h i g h - s p i n ( H S ) ^± l o w - s p i n ( L S ) t r a n s i t i o n , o b s e r v e d i n c e r t a i n

first-row

t r a n s i t i o n m e t a l complexes has b e e n d e s c r i b e d extensively a n d

r e v i e w e d i n a n u m b e r of articles

(1-6).

I n terms of l i g a n d field theory, s p i n crossover occurs i n t r a n s i t i o n metal complexes

with

d -d 4

8

electron c o n f i g u r a t i o n , i f t h e difference

b e t w e e n t h e n e t l i g a n d field s t r e n g t h a n d t h e m e a n s p i n p a i r i n g energy, after t a k i n g i n t o a c c o u n t a l l k i n d s of r e l e v a n t p e r t u r b a t i o n s s u c h as l o w - s y m m e t r y field components, s p i n - o r b i t i n t e r a c t i o n , c o n f i g u r a t i o n i n t e r action, a n d covalency k T. B

effects, becomes c o m p a r a b l e to t h e r m a l e n e r g y

O n t h e r m o d y n a m i c g r o u n d s , s p i n crossover is a n t i c i p a t e d i f t h e

difference i n t h e G i b b s free energy ( G ) of t h e t w o s p i n states i n v o l v e d is o n t h e o r d e r of k T:AG B

=

G(HS) -

G(LS) =

AH-

TAS ~

k T. B

T h e e n t h a l p y c h a n g e A H — H ( H S ) — H ( L S ) is p o s i t i v e g o i n g f r o m t h e l o w - s p i n to t h e h i g h - s p i n state, a n d reflects essentially t h e difference i n the e l e c t r o n i c energies of t h e t w o s p i n states. T h e e n t r o p y c h a n g e A S = S(HS)

— S ( L S ) is also p o s i t i v e f o r t h e c o n v e r s i o n f r o m l o w - s p i n to

high-spin.

I t has b e e n f o u n d i n v a r i o u s instances (7,8)

©

0065-2393/81 /0194-0405$ 12.00/0 1981 American Chemical Society

that o n l y a

406

MOSSBAUER SPECTROSCOPY A N D ITS C H E M I C A L APPLICATIONS

s m a l l e r f r a c t i o n of t h e t o t a l e n t r o p y c h a n g e arises f r o m t h e s p i n m u l t i plicity

change, namely, A S i e

=

R[ln(2S+l) s H

ln(2S+l)

L S

].

The

m a j o r p a r t of A S is d u e t o c o n t r i b u t i o n s f r o m t h e changes i n b o t h i n t r a m o l e c u l a r a n d i n t e r m o l e c u l a r v i b r a t i o n s . T h e r e is a c r i t i c a l t e m p e r a t u r e , called transition temperature T ,

where A H =

c

T A S a n d A G — 0, a n d

the t w o s p i n states coexist i n e q u a l a m o u n t s (see F i g u r e 1 ) . S p i n crossover has b e e n o b s e r v e d m o s t l y i n t h e s o l i d state, b u t also has

been found i n liquids.

I n p a r t i c u l a r , n u m e r o u s e x a m p l e s of

crossover h a v e b e c o m e k n o w n f o r


donor ligands (5).

iron (II)

spin

complexes w i t h nitrogen

M a n y s p i n crossover systems also h a v e b e e n f o u n d

i n t h e c o m p l e x c h e m i s t r y o f i r o n ( I I I ) , s u c h as t h e d i t h i o c a r b a m a t e s ( 9 ) , monothio-/?-diketonates w i t h [ F e S 0 ] c h r o m o p h o r e (10-13),

and mono-

t h i o c a r b a m a t o c o m p l e x e s w i t h [ F e S 0 ] c h r o m o p h o r e (14),

to n a m e a

3

3

3

few.

3

T h e p h e n o m e n o n also has b e e n e s t a b l i s h e d f o r a g o o d n u m b e r of

c o b a l t ( I I ) c o m p l e x e s (3,15), (3,15),

to a lesser extent f o r n i c k e l ( I I ) c o m p l e x e s

a n d i n a f e w cases f o r m a n g a n e s e ( I I ) c o m p o u n d s (16).

r e c e n t l y , the first r e p o r t o n s p i n crossover i n a c o b a l t ( I I I ) (3d ) 6

Quite complex,

i n t h e s o l i d state as w e l l as i n s o l u t i o n , has a p p e a r e d i n t h e l i t e r a t u r e (17,18).

(a)

0

T

(b) Figure 1. Schematic of various types of spin crossover behavior. The XHB(T) term is the fraction of highspin molecules as a function of temperature, (a) Gradual and abrupt spin transition, respectively; (b) incomplete spin transition (RP is the residual fraction of high-spin molecules); (c) spin transition with hysteresis (T < and T > are the transition temperatures in the cooling and heating modes, respectively). e

XHS 1.0-

0.5

0 -

c

T

19.

GUTLiCH

Spin

407

Crossover

T h e m e t h o d most c o m m o n l y u s e d to d e t e c t s p i n crossover

is t h e

m e a s u r e m e n t of the m a g n e t i c s u s c e p t i b i l i t y , w h i c h reflects the a n o m a l o u s m a g n e t i c b e h a v i o r as a f u n c t i o n of t e m p e r a t u r e . F o r crossover of i r o n ,

5 7

systems

F e M o s s b a u e r spectroscopy has p r o v e n to b e a p o w e r f u l t e c h

n i q u e ; i t enables one to f o l l o w d i r e c t l y t h e changes of the concentrations of t h e c o e x i s t i n g s p i n states w i t h t e m p e r a t u r e ( 1 9 ) .

O t h e r techniques

h a v e b e e n e m p l o y e d successfully; for e x a m p l e , v i b r a t i o n a l spectroscopy to f o l l o w the changes i n the r e l a t i v e intensities of c h a r a c t e r i s t i c v i b r a t i o n a l m o d e s ( s u c h as the m e t a l - l i g a n d s t r e t c h i n g , f o r w h i c h v ( H S ) Mössbauer Spectroscopy and Its Chemical Applications Downloaded from pubs.acs.org by EMORY UNIV on 03/02/16. For personal use only.

v(LS)

u p o n spin conversion

(7,8,20,21,22,50);

uv/vis

(see F i g u r e l c ) , examples of w h i c h are r e p o r t e d i n R e f s . (39 a n d

c

40).

T h e s p i n crossover b e h a v i o r m a y b e h i g h l y s u s c e p t i b l e to v a r i o u s influences, s u c h as i n t r a l i g a n d s u b s t i t u t i o n , l i g a n d r e p l a c e m e n t , n a t u r e of the u n c o o r d i n a t e d a n i o n a n d the c r y s t a l solvent m o l e c u l e , d e u t e r a t i o n , m e t a l d i l u t i o n , a n d m e t h o d of s a m p l e p r e p a r a t i o n . T o l e a r n m o r e a b o u t the d r i v i n g force a n d the m e c h a n i s m of the s p i n t r a n s i t i o n i n solids, p a r t i c u l a r l y to test the i d e a of S o r a i a n d S e k i of t h e s p i n t r a n s i t i o n t a k i n g place i n a cooperative

manner through a coupling between

state a n d the l a t t i c e v i b r a t i o n a l m o d e s

(7,8),

a t t e n t i o n o n t h e effect of m e t a l d i l u t i o n (41-47), t h e c r y s t a l solvent m o l e c u l e (40), a n d c r y s t a l i m p e r f e c t i o n s (36)

w e have

the spin

focused

our

t h e effect of c h a n g i n g

a n d the influence of d e u t e r a t i o n

(48)

i n i r o n ( I I ) s p i n crossover systems.

The

essential results w i l l b e r e v i e w e d here.

408


Effect of Metal Dilution Solid

Solutions

MEASUREMENTS.

of

2-pic) ] C l

[Fe^Zni

3

•C H O H .

2

2

MOSSBAUER

5

S y s t e m a t i c i n v e s t i g a t i o n s of the effect of m e t a l d i l u t i o n

o n t h e s p i n t r a n s i t i o n b e h a v i o r b y means of M o s s b a u e r s p e c t r o s c o p y w e r e i n i t i a t e d i n o u r l a b o r a t o r y some years ago, h o p i n g to find e x p e r i m e n t a l s u p p o r t for t h e c o o p e r a t i v e d o m a i n m o d e l of S o r a i a n d S e k i ( 7 , 8 ) . started w i t h the s p i n crossover system [ F e ( 2 - p i c ) ] C l 3

=

2-picolylamine),

A (O )


1

1

h

•C H O H

2

2

for w h i c h the t e m p e r a t u r e - d e p e n d e n t

We

(2-pic

5

T (O )

^±

t r a n s i t i o n has b e e n e s t a b l i s h e d b y R e n o v i t c h a n d B a k e r

(49).

5

2

h

T h i s system a p p e a r e d to be p a r t i c u l a r l y s u i t e d f o r t h e m e t a l d i l u t i o n w o r k for t w o

reasons:

(1)

The

M o s s b a u e r s p e c t r a as a f u n c t i o n

of

t e m p e r a t u r e e x h i b i t t w o w e l l - r e s o l v e d q u a d r u p o l e d o u b l e t s for the h i g h a n d l o w - s p i n states of i r o n ( I I ) , r e s p e c t i v e l y

(see

Figure 2),

demon

s t r a t i n g t h a t the t w o s p i n states coexist i n c h e m i c a l e q u i l i b r i u m a n d h a v e l i f e t i m e s of ^ 1 0 " s, the q u a d r u p o l e precession t i m e of the e x c i t e d n u c l e a r 8

state.

(2)

The compound

forms s o l i d solutions w i t h z i n c a n d

r e s p e c t i v e l y , o v e r the entire c o n c e n t r a t i o n range.

result of the measurements o n the series [ F e Z n . . ( 2 - p i c ) ] C 1 ar

(1.0 ^

x ^

0.0009) (41,42)

1

cobalt,

T h e most important

a

3

2

-CyrlsOH

b e c o m e s a p p a r e n t f r o m the f o u r r e p r e s e n t a

t i v e M o s s b a u e r spectra s h o w n i n F i g u r e 3: A t a g i v e n t e m p e r a t u r e

(101

K i n the present case of F i g u r e 3 ) , the r e l a t i v e i n t e n s i t y of the q u a d r u p o l e d o u b l e t of the i r o n ( I I ) h i g h - s p i n state, T ( O ) , increases w i t h decreas 5

i n g iron concentration.

2

h

F i g u r e 4 shows t h e t e m p e r a t u r e d e p e n d e n c e of

the area f r a c t i o n of the h i g h - s p i n q u a d r u p o l e d o u b l e t x , e v a l u a t e d f r o m H

the spectra of [ F e ^ Z n i . * ( 2 - p i c ) ] C 1 3

2

• C H O H w i t h different i r o n c o n 2

5

centrations x. F i g u r e 4 also i n d i c a t e s that the slope of t h e s p i n c o n v e r s i o n c u r v e at the s p i n t r a n s i t i o n t e m p e r a t u r e T

c

diminishes w i t h dilution, a n d

t h a t the s p i n t r a n s i t i o n is c o m p l e t e at the l o w - a n d h i g h - t e m p e r a t u r e ends f o r a l l concentrations. of

the

F i g u r e 6 demonstrates the n e a r l y l i n e a r decrease

spin transition temperature

T (x) c

with

decreasing

iron

con

centration. N o s u b s t a n t i a l differences

h a v e b e e n o b s e r v e d i n the i s o m e r shift,

apart f r o m t h e shift d u e to t h e second-order

D o p p l e r effect, or i n the

q u a d r u p o l e s p l i t t i n g of the h i g h - s p i n state i n the s o l i d solutions v a r i a b l e * at a g i v e n t e m p e r a t u r e .

with

T h u s the e l e c t r o n i c structure of the

i r o n ( I I ) i o n is not a l t e r e d s i g n i f i c a n t l y b y p a r t i a l s u b s t i t u t i o n of i r o n ( I I ) by zinc (II). DRIVING F O R C E A N D POSSIBLE M E C H A N I S M O F T H E SPIN TRANSITION.

A s has b e e n p o i n t e d out e a r l i e r (7,8,41),

a d i s c u s s i o n of the r e l a t i v e

s t a b i l i t y of the s p i n states i n v o l v e d i n s p i n crossover s h o u l d b e b a s e d o n the free energy G =

H — TS a n d its c h a n g e A G =

A H — T A S accom

p a n y i n g the c h a n g e i n s p i n state. T h e A H a n d A S terms i n c l u d e c o n t r i b u -

GUTLiCH

Spin

409

Crossover


19.

4 . 0

Velocity (mms

)

Figure 2. The Fe Mossbauer spectra of [Fe(2-pic) ]Cl • C H OH (2-pic = 2-picolylamine) at various temperatures (41). The inner two lines refer to the quadrupole doublet of the low-spin state A (O ), the outer two lines to that of the high-spin state T (Oh) of iron(II). 57

3

2

2

1

5

2

1

h

5

410

SPECTROSCOPY A N D ITS C H E M I C A L

APPLICATIONS


MOSSBAUER

Velocity ( m m s " ) 1

Figure

3.

Concentration dependence of Fe [FeJfai.JZpicMCk • C H OHat 57

s

5

Mossbauer spectra 101 K(41)

of

19.

Spin

GUTLICH

411

Crossover

1JOO

x

.80

1

.6 0


.40

.20

0

300

200

100

0

T

I K

Figure 4. Temperature dependence of the area fraction x spin quadrupole doublet in Fe Mossbauer spectra of [Fe Zn Cl ' C H OH with variable iron concentration x (41, 42): (&) ( Q ; x = 0.029; C) x = 0.2; ((3) x = 0.6; (€) x = 0.8; (O) H

57

2

2

x

x

5

of the high_ (2-pic) ]x = 0.0009; x = 1.0. x

s

tions f r o m a l l degrees of f r e e d o m ; the m o s t i m p o r t a n t ones to a c c o u n t f o r the t e m p e r a t u r e d e p e n d e n c e

of

A G stem f r o m

changes

i n electronic

structure a n d v i b r a t i o n a l modes. F o r a A (O ) 1

lg

s p i n crossover s y s t e m at a b s o l u t e z e r o

^± T (O ) 5

h

2g

h

t e m p e r a t u r e , t h e e l e c t r o n i c g r o u n d state of i r o n ( I I ) is s e p a r a t e d b y c E( T ) 5

2g

-

f r o m t h e h i g h - s p i n state (cf. F i g u r e 8 ) ; c «

E( A ) l

lg

p • A V is sufficiently s m a l l .

F r o m infrared (7,8)

t r o s c o p y ( 4 0 ) , t h e e n e r g y difference c ^

=

A H , if

and Mossbauer

spec

A H of the i r o n a t o m has b e e n

c o n f i r m e d to b e n e a r l y c o n s t a n t o v e r a w i d e t e m p e r a t u r e r a n g e . A t finite temperatures,

the

entropy

term

T A S plays

a n important role.

The

p r e d o m i n a n t c o n t r i b u t i o n s to t h e t o t a l e n t r o p y c h a n g e arise f r o m changes i n electronic structure, A S i — e

vibrational frequencies, A S contributions AS i and A S e

v i b

v i b

R[ln(2S+l)ns — ln(2S+l)

L S

],

, m a i n l y of i n t r a m o l e c u l a r m o d e s .

and in As both

a r e p o s i t i v e g o i n g f r o m t h e l o w - s p i n to t h e

h i g h - s p i n state, i t is c o n c e i v a b l e t h a t t h e s p i n t r a n s i t i o n o c c u r s e v e n i f t h e e n t h a l p y c h a n g e is essentially constant. m u s t be

considered

the m a i n

driving

T h u s , the gain i n entropy

force responsible for

the

spin

t r a n s i t i o n f r o m t h e l o w - s p i n state to t h e h i g h - s p i n state. T h e m e c h a n i s m of the s p i n t r a n s i t i o n i n t h e c r y s t a l l i n e state is s t i l l m u c h i n t h e d a r k , a n d t h e f o l l o w i n g q u a l i t a t i v e d e s c r i p t i o n is n o t m o r e t h a n a h y p o t h e t i c a l p i c t u r e , s u p p o r t e d , h o w e v e r , b y t h e results of

our

412

MOSSBAUER

S P E C T R O S C O P Y A N D ITS C H E M I C A L A P P L I C A T I O N S

m e t a l d i l u t i o n w o r k as w e l l as b y i n f r a r e d (7, 8) a n d x - r a y c r y s t a l s t r u c t u r e investigations.

S o r a i a n d S e k i (7,8)

o r i g i n a l l y suggested t h a t the s p i n

t r a n s i t i o n i n a c r y s t a l l i n e s p i n crossover system takes p l a c e i n a c o o p e r a t i v e m a n n e r i n v o l v i n g a significant c o u p l i n g b e t w e e n t h e e l e c t r o n i c t h a t changes s p i n , a n d t h e p h o n o n system e n c o m p a s s i n g ular and intermolecular region.

state

the intramolec

T h e g r o u p of m o l e c u l e s that changes

s p i n i n d u c e d b y a " p r i m a r y " s p i n c h a n g e is c a l l e d a c o o p e r a t i v e d o m a i n . A t sufficiently l o w t e m p e r a t u r e s , a l l the m o l e c u l e s r e s i d e i n t h e l o w - s p i n state w i t h c h a r a c t e r i s t i c n o r m a l m o d e s of v i b r a t i o n s .

I f the

temperature


rises, a c e r t a i n n u m b e r of m o l e c u l e s c h a n g e f r o m l o w - s p i n to h i g h - s p i n d u e to t h e r m a l e x c i t a t i o n .

A s a c o n s e q u e n c e , some n o r m a l m o d e s

v i b r a t i o n s w i l l b e m o d u l a t e d to some extent.

I t is k n o w n f r o m

t u r e - d e p e n d e n t i n f r a r e d s p e c t r o s c o p y (7,8,21,22,23)

t h a t the f r e q u e n c i e s

of the m e t a l - l i g a n d s t r e t c h i n g v i b r a t i o n s are m a r k e d l y r e d u c e d high-spin

state

as c o m p a r e d

to

the

low-spin

of

tempera

state;

the

i n the

reason

is

a

significant r e d u c t i o n i n t h e extent of 7 r - b a c k b o n d i n g w h i c h decreases t h e m e t a l - l i g a n d b o n d strength.

T h i s decrease is p a r a l l e l e d b y a n

increase

i n the b o n d l e n g t h b y some 0.2 A , as has b e e n o b s e r v e d r e c e n t l y i n a v a r i a b l e - t e m p e r a t u r e x - r a y c r y s t a l s t r u c t u r e analysis of t h e [ F e ( 2 - p i c ) ] 3

Cl

2

• C H O H (30). 2

T h r o u g h the m o d u l a t e d n o r m a l m o d e s the i n f o r m a

5

t i o n of a " p r i m a r y " s p i n c h a n g e is c o m m u n i c a t e d to n e i g h b o r i n g c o m p l e x modules.

T h e i r vibrational modes w i l l be

changed by

the

incoming

p e r t u r b a t i o n w a v e ; at a c e r t a i n t h r e s h o l d t h e y m a y c h a n g e o v e r to high-spin modulates

state.

This "secondary" spin change i n turn

the

subsequently

t h e v i b r a t i o n a l m o d e s f u r t h e r a n d carries t h e s p i n

change

i n f o r m a t i o n f u r t h e r i n t o the l a t t i c e of a c o o p e r a t i v e d o m a i n . O n the basis of this p i c t u r e , the effect of s u b s t i t u t i n g i r o n ( I I ) z i n c ( I I ) i n the [Fe Zni. (2-pic)3]Cl a ;

i P

2

2

5

t o r y : T h e z i n c ( I I ) ions, h a v i n g a c l o s e d d

configuration, cannot change

10

spin and have

different

for

• C H O H system is s e l f - e x p l a n a

b o n d properties

than i r o n complex molecules

( t h i s s h o u l d influence t h e s p i n t r a n s i t i o n b e h a v i o r ) . A r e m a r k a b l e step f o r w a r d i n t h e efforts to g a i n a d e e p e r i n s i g h t into

the

achieved

mechanism by

of

the

Mikami, Konno,

c r y s t a l s t r u c t u r e of

spin phase transition recently and

[Fe(2-pic) ]Cl 3

Saito 2

(30).

They

•C H O H

at 298,

2

5

t h a t is, a b o v e a n d b e l o w t h e t r a n s i t i o n t e m p e r a t u r e . f o u n d to b e m o n o c l i n i c , P 2 i / c w i t h Z =

has

been

determined 150,

a n d 90

the K,

T h e crystals w e r e

4 i n the t w o s p i n states. T h e y also

f o u n d e m i n e n t changes i n i r o n - n i t r o g e n b o n d lengths, n a m e l y , 2.195

A

f o r t h e h i g h - s p i n state a n d 2.013 A f o r the l o w - s p i n state o n t h e average. M o s t i m p o r t a n t i n t h e context of the c o o p e r a t i v e d o m a i n m o d e l is t h e i r o b s e r v a t i o n t h a t a l l the a m i n o n i t r o g e n atoms of the c a t i o n i c c o m p l e x e s o n t h e one side, a n d the e t h a n o l m o l e c u l e o n the other, are b o n d e d to C I " ions, f o r m i n g a t w o - d i m e n s i o n a l h y d r o g e n b o n d

hydrogen network

19.

Spin

GUTLICH

413

Crossover

a l o n g w h i c h t h e s p i n c h a n g e i n f o r m a t i o n is l i k e l y to t r a v e l .

The

t y p e of t w o - d i m e n s i o n a l h y d r o g e n - b o n d i n g n e t w o r k has b e e n f o r the m e t h a n o l a t e [ F e ( 2 - p i c ) ] C 1 3

s p i n crossover

•CH OH

2

3

same

observed

( 3 2 ) , w h i c h also e x h i b i t s

(40).

M o s t i n t e r e s t i n g is t h e o b s e r v a t i o n of M i k a m i et a l . ( 3 0 ) e t h a n o l m o l e c u l e s i n [ F e ( 2-pic ) ] C 1 3

2

•C H O H 2

t h a t the

are d i s o r d e r e d i n t h e

5

high-temperature h i g h - s p i n phase o c c u p y i n g three orientational positions. T h e p o p u l a t i o n i n t h e three sites changes w i t h t e m p e r a t u r e , w h e r e b y a g r a d u a l o r d e r i n g o c c u r s o n l o w e r i n g t h e t e m p e r a t u r e . O n e of the t h r e e


sites is f a v o r e d o v e r t h e o t h e r t w o a n d has 1 0 0 %

occupancy

at 90 K ,

t h a t i s , i n the s t r u c t u r e of the l o w - s p i n state. T h e a u t h o r s c o n c l u d e t h a t t h e c o o p e r a t i v e n a t u r e of the s p i n t r a n s i t i o n m a y b e a c c o u n t e d f o r i n terms of a n i n t e r a c t i o n of t h e s p i n p h a s e t r a n s i t i o n a n d t h e o r d e r - d i s o r d e r t r a n s i t i o n of t h e e t h a n o l m o l e c u l e t h r o u g h h y d r o g e n b o n d i n g .

Further

s u p p o r t f o r this c o n c l u s i o n has b e e n o b t a i n e d i n studies of t h e d e u t e r i u m i s o t o p e effect i n t h e d e u t e r a t e d solvates

[Fe(2-pic) ]C1 3

2

• Sol (Sol

—

C H O D , C H O D ) , w h i c h w i l l be discussed i n a later section. 2

5

3

INTERPRETATION

MODEL.

USING

A

PHENOMENOLOGICAL

THERMODYNAMIC

F o l l o w i n g t h e s u g g e s t i o n of S o r a i a n d S e k i ( 7 , 8 )

t h a t the s p i n

t r a n s i t i o n is c o o p e r a t i v e i n n a t u r e , w e h a v e a t t e m p t e d to i n t e r p r e t t h e results of the m e t a l d i l u t i o n studies o n [Fe .Zni. .(2-pic ) ] C 1 a

3

a

• C H OH

2

2

o n t h e basis of a p h e n o m e n o l o g i c a l t h e r m o d y n a m i c m o d e l assume

that a " p r i m a r y " spin

change

is f o l l o w e d

5

(43).

We

spontaneously

by

" s e c o n d a r y " s p i n changes i n n-1 s u r r o u n d i n g m o l e c u l e s . T h e n m o l e c u l e s of l i k e s p i n are c o n s i d e r e d to f o r m a " c o o p e r a t i v e " d o m a i n . T h e d o m a i n size n m a y b e t a k e n as a m e a s u r e of t h e c o o p e r a t i v e i n t e r a c t i o n s t r e n g t h . For

s i m p l i c i t y , w e assume t h e n u m b e r n to b e the same f o r h i g h - a n d

l o w - s p i n d o m a i n s i n a g i v e n s y s t e m , b u t to v a r y w i t h t h e i r o n c o n c e n t r a t i o n , n(x).

I n t e r a c t i o n s b e t w e e n t h e d o m a i n s , i r r e s p e c t i v e of t h e s p i n

state, are c o n s i d e r e d to b e n e g l i g i b l e . T h e systems Cl

[Fe Zni. .(2-pic) ]a

a?

3

• C H O H are t r e a t e d as t e m p e r a t u r e - d e p e n d e n t c h e m i c a l e q u i l i b r i a

2

2

5

b e t w e e n h i g h - a n d l o w - s p i n d o m a i n s , f o r w h i c h t h e effective

enthalpy

and

—

e n t r o p y changes, A r 7

AS°(x)

(AH°(X)

e f f

(x)

=

n(x)AH°(x)

and. A S « ( x ) e

a n d A S ° ( x ) are t h e r e s p e c t i v e changes f o r one m o l e of

the complex molecules), m a y be evaluated f r o m straightforward A r r h e n i u s p l o t s , In K — In x

H

/(l-x

H

)

— f( ) T

(43);

cf. F i g u r e 5. T h e v a l u e s for

AHeff a n d A S f f thus o b t a i n e d v a r y b e t w e e n a b o u t 13 k j m o l " a n d 110 J 1

e

mol"

1

K " , respectively, for the p u r e i r o n c o m p o u n d , a n d between 1

about

2.5 k j m o l " a n d 30 J m o l " K ' , r e s p e c t i v e l y , f o r t h e h i g h l y d i l u t e d (x 1

1

=

1

0.0009) s y s t e m . T h e t r a n s i t i o n t e m p e r a t u r e as a f u n c t i o n of x, T (x) c

=

AH (x)/ ett

ASeff(x), takes o n v a l u e s b e t w e e n 120 K f o r t h e p u r e i r o n c o m p o u n d a n d 7 7 K f o r t h e system w i t h x =

0.0009; t h e a g r e e m e n t w i t h e x p e r i m e n t a l


414


1

2

r /io K

1

Figure 5. Plot of In K = In x /(l — x ) vs. 1 / T for various iron concentrations x in the solid solutions [FeJLn (2-pic) ]Cl ' C H OH (43): (%) = 0.0009; (D)x = 0.029; (A) x = 0.2; (X)x = 0.6; (O) x = 1.0. H

H

t

x

s

2

2

5

x

d a t a is v e r y g o o d . T h e d e p e n d e n c e of T

T

c

c

on the iron concentration

x m a y be d e s c r i b e d b y t h e expression _ i

c

A f f ° (x) AS°(x)

W

The Aff°

F e

and AS°

Aff ° AS°

_

F e

F e

+

Aff °

+ AS°

c o o p

c o o p

(x)

0r)

Aff ° AS°

_

F e

F e

+ Ax« + £*

( 3

t i o n s , a n d are c o n s i d e r e d COO

o

a >

to b e i n d e p e n d e n t

p terms r e f e r to changes

molecular electron-phonon AH

C 0 0

p and A S

o and p

w

e

obtain from

E q u a t i o n 5 for a h y d r o g e n a t e d ( H ) solvate

A E ( H ) - Ac + -| h VT/^L {Vk7s - V ^ W

(6)

w h e r e Ac stands for t h e differences i n t h e e l e c t r o n i c energies. A s s u m i n g n o c h a n g e i n t h e s y m m e t r y a n d t h a t Ac r e m a i n s constant o n d e u t e r a t i o n , w e find a n analogous expression f o r t h e d e u t e r a t e d solvate.

T h e differ

ence i n A E b e t w e e n the d e u t e r a t e d ( D ) a n d the h y d r o g e n a t e d

solvate

is t h e n 8(AE) =

AE(D) -

AE(H)

= \h(VV^-

VTT^H")

(V~fcn7-

V~fcL7)

(7)

440

MOSSBAUER

As 1 / / A

d

H


445

Crossover

185K

176K

,

'c L.

o -Q

o

/

174K

J

165 K

in -»-» c D

O u

155K

JL

20

22

24

26

28

Bragg angle 2 0 Figure 3 1 . Peak profiles of x-ray powder diffraction of [Fe(phen) (NCS) ] Sample B (extracted), as a function of temperature in the transition region (36) 2

2

446

MOSSBAUER

v e r y intense 2 4 ° p e a k .


S i m u l t a n e o u s l y , one p e a k of t h e g r o u p of t h r e e

peaks a r o u n d 2 2 ° - 2 4 ° d i s a p p e a r s b e l o w T . T h e p e a k profiles i n t h e case c

of S a m p l e A are b r o a d e r t h a n i n t h e case of S a m p l e B , w h i c h is m o s t p r o b a b l y d u e t o t h e f a c t t h a t S a m p l e A is m o r e

finely

dispersed a n d

of l o w e r c r y s t a l q u a l i t y t h a n S a m p l e B . T h e o v e r a l l features of t h e x - r a y diffractometry

s p e c t r a of t h e h i g h - t e m p e r a t u r e

(high-spin)

and low-

t e m p e r a t u r e ( l o w - s p i n ) phases are n o t too different, w h i c h i m p l i e s t h a t t h e s t r u c t u r a l c h a r a c t e r i s t i c s of t h e t w o phases a r e q u i t e s i m i l a r .

Never

theless, i n v i e w of t h e m a r k e d changes i n t h e M o s s b a u e r l i n e w i d t h a n d the q u a d r u p o l e s p l i t t i n g of t h e h i g h - s p i n p h a s e n e a r T , together Mössbauer Spectroscopy and Its Chemical Applications Downloaded from pubs.acs.org by EMORY UNIV on 03/02/16. For personal use only.

c

with

t h e differences i n t h e x - r a y d i f f r a c t o m e t r y p a t t e r n b e t w e e n t h e h i g h - a n d l o w - t e m p e r a t u r e phases, some k i n d o f s t r u c t u r a l r e o r g a n i z a t i o n , b e y o n d the c h a n g e i n i r o n - l i g a n d b o n d l e n g t h i n d u c e d b y t h e s p i n t r a n s i t i o n , can no longer be excluded.

W e believe that a n orientational d i s o r d e r -

order transition, possibly w i t h the N C S groups preferring certain rota tional

sites, m a y b e a n effective

trigger for the spin transition i n

124.0 K 3

CO

c

CD

122.8 K

f

>

o

0>

or 5 121.9 K

f

3

5.0 5.5 6.0 6.5 Bragg angle 9 ^ (T

C

= 122.9 K) (66)

Chemical Physics Letters


19.

Spin

GUTLICH

L90

447

Crossover

5.15

5.40

Bragg angle 6

—

Figure 33. Peak profiles of x-ray powder diffraction on [Fe(bt) (NCS) ] in the temperature region of the spin transition (39) 2

2

Inorganica Chimica Acta

[Fe(phen) (NCS) ],

s i m i l a r to t h e suggestion of M i k a m i et a l . (30)

[Fe(2-pic) ]Cl

5

2

2

3

2

•C H O H . 2

for

A s i n g l e - c r y s t a l x - r a y s t u d y is b a d l y n e e d e d

to c l a r i f y this. V e r y fine v a r i a b l e - t e m p e r a t u r e x - r a y d i f f r a c t i o n w o r k i n c o n n e c t i o n w i t h M o s s b a u e r spectroscopy o n the p o l y c r y s t a l l i n e s p i n crossover systems [Fe(4,7-(CH ) -phen) (NCS) ] 3

2

2

2

and [ F e ( b t ) ( N C S ) ] 2

2

( b t — 2,2'-bi-2-

t h i a z o l i n e ) has b e e n p u b l i s h e d r e c e n t l y b y K o n i g et a l . (39,66).

Distinct

a n d different x - r a y p e a k profiles w e r e f o u n d f o r the h i g h - a n d l o w - s p i n phases i n b o t h systems o n p a s s i n g t h r o u g h T , i n d i c a t i n g t h a t a c r y s t a l l o c

g r a p h y phase c h a n g e is associated w i t h the s p i n t r a n s f o r m a t i o n F i g u r e s 32 a n d 3 3 ) . T h e intensities of t h e x - r a y peaks of phen) (NCS) ] 2

2

(see

[Fe(4,7-(CH ) 3

s h o w the same t e m p e r a t u r e d e p e n d e n c e a n d the

2

same

hysteresis b e h a v i o r as t h e fractions o f the h i g h - a n d l o w - s p i n

species

d e t e r m i n e d f r o m the M o s s b a u e r s p e c t r a (see F i g u r e s 34 a n d 3 5 ) .

Similar

observations w e r e m a d e o n t h e [ F e ( b t ) ( N C S ) ]

T h i s is

2

2

system (39).

c l e a r e v i d e n c e f o r a s i m u l t a n e o u s c h a n g e of the e l e c t r o n i c s p i n state a n d the crystallographic properties.

448

MOSSBAUER

S P E C T R O S C O P Y A N D ITS C H E M I C A L

APPLICATIONS

1.00

0.75

-


0.50

0.2 5

125

130 T IK)

Chemical Physics Letters

Figure 34. Temperature dependence of the relative intensity of the x-ray diffraction lines for the high-spin state in [Fe(4,7-(CH ) (phen) (NCS) ] near T (66; s 2

2

2

c

1.00

n 0.75 -

0.50

0.25 -

110

115

120

125

130

T [KJ Chemical Physics Letters

Figure 35. Temperature dependence of the high-spin t fraction in (4,7-(CH ) (NCS) ] evaluated from the spectra near _T . . . .Mossbauer . . . 3 2

2

s

d

c

[Fe(66) ....

19.

Spin

GUTLICH

449

Crossover

Acknowledgments


I w i s h to express m y sincere t h a n k s to m y students a n d r e s e a r c h associates K . B o d e , J . E n s l i n g , J . F l e i s c h , P . G a n g u l i , K . M . H a s s e l b a c h , H . K o p p e n , R . L i n k , E . W . M u l l e r , I. Sanner, M . S o r a i , H . S p i e r i n g , a n d H . G . Steinhauser, w h o h a v e c o l l a b o r a t e d w i t h m e o n s p i n crossover p r o b l e m s w i t h great e n t h u s i a s m . F i n a n c i a l s u p p o r t b y t h e D e u t s c h e Forschungsgemeinschaft, the F o n d s der C h e m i s c h e n Industrie, a n d the A l e x a n d e r v o n H u m b o l d t S t i f t u n g is g r a t e f u l l y a c k n o w l e d g e d .

Glossary of Symbols HS

H i g h - s p i n g r o u n d state of the c e n t r a l m e t a l i o n , T ( O ) i n 5

2

h

case of a n i r o n ( I I ) c o m p l e x m o l e c u l e i n t h e a p p r o x i m a t i o n of O LS

h

symmetry

L o w - s p i n g r o u n d state of the c e n t r a l m e t a l i o n , A ( O ) 1

i

h

in

case of a n i r o n ( I I ) c o m p l e x m o l e c u l e i n the a p p r o x i m a t i o n of O G

h

symmetry

G i b b s free e n e r g y

fc

B o l t z m a n n factor

H

Enthalpy

B

S

Entropy

R

G a s constant

T

c

T r a n s i t i o n t e m p e r a t u r e , f o r m a l l y d e f i n e d as t h e t e m p e r a t u r e of 5 0 % s p i n c o n v e r s i o n

* H ( * H S )

F r a c t i o n of c o m p l e x m o l e c u l e s i n the h i g h - s p i n g r o u n d state, t a k e n here as a p p r o x i m a t e l y e q u a l to the area f r a c t i o n of t h e h i g h - s p i n q u a d r u p o l e d o u b l e t of the M o s s b a u e r spectra

RP

R e s i d u a l p a r a m a g n e t i s m ( r e s i d u a l f r a c t i o n of h i g h - s p i n m o l e

RD

R e s i d u a l d i a m a g n e t i s m ( r e s i d u a l f r a c t i o n of l o w - s p i n m o l e

cules at l o w t e m p e r a t u r e s ) cules at h i g h t e m p e r a t u r e s ) I r o n c o n c e n t r a t i o n of m i x e d crystals

x n

N u m b e r of c o m p l e x molecules of l i k e s p i n state i n a d o m a i n

K

E q u i l i b r i u m constant

h

P l a n c k ' s constant

c

V e l o c i t y of l i g h t

c

E n e r g y difference b e t w e e n T ( O ) a n d A i ( O ) states 5

8 et Ao t

h

Tetragonal distortion parameter F r e e i o n s p i n o r b i t c o u p l i n g constant

R

S t e r n h e i m e r s h i e l d i n g factor

rj

Asymmetry parameter

V F k fi

C r y s t a l field p o t e n t i a l V i b r a t i o n a l force constant R e d u c e d mass

C

2

1

h

450

MOSSBAUER



Literature Cited 1. Barefield, E . K.; Busch, D. H.; Nelson, S. M. Quart. Rev. 1968, 22, 457. 2. Martin, R. L.; White, A. H. "Transition Metal Chemistry"; Carlin, R. L., Ed.; Dekker: New York, 1968; Vol. 4, p. 113. 3. Sacconi, L. "Proceedings of XIIIth I.C.C.C., Cracow/Zakopane (Poland) 1970"; Butterworths: London, 1971. 4. König, E . Ber. Bunsenges. Phys. Chem. 1972, 76, 975. 5. Goodwin, H. A. Coord. Chem. Rev. 1976, 18, 293. 6. Gütlich, P. J. Phys. (Paris) 1979, 40(3), Colloque C2, C2-378. 7. Sorai, M.; Seki, S. J. Phys. Chem. Solids 1974, 35, 555. 8. Sorai, M.; Seki, S. J. Phys. Soc. Jpn. 1972, 33, 575. 9. Ewald, A. H.; Martin, R. L.; Ross, I. G.; White, A. H. Proc. R.Soc.,London, Ser. A 1964, 280, 235. 10. Ho, R. K. Y.; Livingstone, S. E . Chem. Commun. 1968, 217. 11. Cox, M.; Darken, J.; Fitzsimmons, B. W.; Smith, A. W.; Larkworthy, L. F.; Rogers, K. A. Chem. Commun. 1970, 105. 12. Cox, M.; Darken, J.; Fitzsimmons, B. W.; Smith, A. W.; Larkworthy, L. F.; Rogers, K. A. J. Chem.Soc.,Dalton Trans. 1973, 1192. 13. Dose, E . V.; Murphy, K. M. M.; Wilson, L. J. Inorg. Chem. 1976, 15, 2622. 14. Kunze, K.R.;Perry, D. L.; Wilson, L. J. Inorg. Chem. 1977, 16, 594. 15. Morassi, R.; Bertini, I.; Sacconi, L. Coord. Chem. Rev. 1973, 11, 343. 16. Ammeter, J. J. Am. Chem. Soc. 1974, 96, 7833. 17. El Murr, N.; Chaloyard, A.; Kläui, W. Inorg. Chem. 1979, 18, 1010. 18. Gütlich, P.; Kläui, W.; McGarvey, B. R. Inorg. Chem. 1980, 19, 3704. 19. König, E . ; Ritter, G. "Mössbauer Effect Methodology"; Gruverman, I. J.; Seidel, C. W.; Dieterly, D. K., Eds.; Plenum: New York, 1974; Vol. 9, p. 3. 20. Baker, W. A., Jr.; Long, G. J. Chem. Commun. 1965, 368. 21. Takemoto, J. H.; Hutchinson, B. Inorg. Nucl. Chem. Lett. 1972, 8, 769. 22. Takemoto, J. H.; Hutchinson, B. Inorg. Chem. 1973, 12, 705. 23. Hall, G. R. Hendrickson, D. N. Inorg. Chem. 1976, 15, 607. 24. Ewald, A. H.; Martin, R. L.; Sinn, E.; White, A. H. Inorg. Chem. 1969, 8, 1837. 25. Tweedle, M. F.; Wilson, L. J. J. Am. Chem. Soc. 1976, 98, 4824. 26. Tsipis, C. A.; Hadjikostas, C. C.; Manoussakis, G. E . Inorg. Chim. Acta 1977, 23, 163. 27. Leipoldt, J. G.; Coppens, P. Inorg. Chem. 1973, 12, 2269. 28. Cukauskas, E . J.; Deaver, B. S., Jr.; Sinn, E . Inorg. Nucl. Chem. Lett. 1977, 13, 283. 29. Albertsson, J.; Oskarsson, A. Acta Crystallogr., Sect. B 1977, 33, 1871. 30. Mikami, M.; Konno, M.; Saito, Y. Chem. Phys. Lett. 1979, 63, 566. 31. Greenaway, A. M.; Sinn, E. J. Am. Chem. Soc. 1978, 100, 8080. 32. Katz, B. A.; Strouse, C. E. J. Am. Chem. Soc. 1979, 101, 6214. 33. Dose, E . V.; Hoselton, M. A.; Sutin, N.; Tweedle, M. F.; Wilson, L. J. J. Am. Chem. Soc. 1978, 100, 1141. 34. Binstead, R. A.; Beattie, J. K.; Dose, E . V.; Tweedle, M. F.; Wilson, L. J. J. Am. Chem. Soc. 1978, 100, 5609. 35. Baker, W. A., Jr.; Bobonich, H. M. Inorg. Chem. 1964, 3, 1184. 36. Ganguli, P.; Gütlich, P.; Irler, W.; Müller, E . W. J. Chem.Soc.,Dalton, Trans. 1981, 441. 37. Goodwin, H. A.; Sylva, R. N. Aust. J. Chem. 1968, 21, 83. 38. Fleisch, J.; Gütlich, P.; Hasselbach, K. M.; Müller, E . W. Inorg. Chem. 1976, 15, 958. 39. König, E . ; Ritter, G.; Irler, W.; Nelson, S. M. Inorg. Chim. Acta 1979, 37, 169. ;


19.

GUTLICH

Spin

Crossover

451

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RECEIVED June 27, 1980.

Recent Investigations of Spin Crossover - Advances in Chemistry

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