Organometallic Electron-Transfer Salts with Tetracyanoethylene

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with Tetracyanoethylene Exhibiting Ferromagnetic Coupling Joel S. Miller and Arthur J. Epstein 1

2

Central Research and Development, Ε. I. du Pont de Nemours and Company, Experimental Station-E328, Wilmington, D E 19880-0328 Department of Physics and Department of Chemistry, The Ohio State University, Columbus, O H 43210-1106

1

2

Some molecular organic solids comprising linear chains of alter nating total spin angular momentum quantum number S = / metallocenium donors, D, and cyanocarbon acceptors, A (i.e., • • • D · + A · - D · + A · - • • • ) exhibit cooperative magnetic phenom ena (i.e., ferro-, antiferro-, ferri-, and metamagnetism). For [FeIII(C Me ) ]• [TCNE]• (Me is methyl; TCNE is tetracyanoeth ylene), bulk ferromagnetic behavior is observed below the Curie tem perature of 4.8 K. Replacement of Fe with Cr , Ni , andFe leads to complexes with antiferromagnetic coupling, ferrimagnetic behav ior, and almost no magnetic interaction, respectively. These results are consistent with a model of configuration mixing of the lowest charge-transfer excited state with the ground state developed earlier to understand the magnetic coupling of such systems. The model, which predicts the magnetic coupling as a function of electron con figuration and direction of charge transfer, is a useful guide in the design of new organic and organometallic complexes with cooperative magnetic coupling. New TCNE-based electron-transfer salts were prepared to test the model and identify new materials with ferro magneticcoupling. 1

5

5

2

+

2

-

III

III

III

II

L O L E C U L A R A N D ORGANIC F E R R O M A G N E T I C C O M P O U N D S , a l t h o u g h p o s

t u l a t e d i n the 1960s, have o n l y recently b e e n s y n t h e s i z e d a n d c h a r a c t e r i z e d 0065-2393/90/0226-0419$06.00/0 © 1990 American Chemical Society

In Electron Transfer in Biology and the Solid State; Johnson, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

420

E L E C T R O N TRANSFER IN BIOLOGY A N D T H E SOLID STATE

(1-5). T h i s d e v e l o p m e n t , w h i c h parallels t h e discovery o f m o l e c u l a r a n d organic-based superconductors, extends the study o f cooperative p h e n o m e n a i n m o l e c u l a r organic materials. T h e b r o a d range o f p h e n o m e n a i n t h e m o lecular organic s o l i d state, c o m b i n e d w i t h t h e a n t i c i p a t e d modification o f p h y s i c a l properties v i a c o n v e n t i o n a l synthetic organic c h e m i s t r y a n d t h e ease o f fabrication enjoyed b y soluble materials, m a y u l t i m a t e l y l e a d to t h e i r use i n future generations o f e l e c t r o n i c a n d p h o t o n i c devices. T h i s chapter summarizes t h e configuration m i x i n g o f a v i r t u a l t r i p l e t

Downloaded by UCSF LIB CKM RSCS MGMT on August 20, 2014 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0226.ch022

excited state w i t h the g r o u n d state for an a l t e r n a t i n g d o n o r - a c c e p t o r

(D-A)

o n e - d i m e n s i o n a l c h a i n m o d e l for the stabilization o f ferromagnetic c o u p l i n g of this electron-transfer c o m p o u n d (1-6). A discussion o f the c o m m o n i d e a l i z e d magnetic behaviors e x p e c t e d i n materials (2) a n d a m o r e c o m p r e h e n s i v e discussion o f several models for ferromagnetic c o u p l i n g i n m o l e c u l a r p o l y m e r i c materials can b e f o u n d i n p u b l i s h e d reviews (1-5).

Stabilization of Ferromagnetic Coupling by Configuration Mixing S p i n a l i g n m e n t t h r o u g h o u t t h e s o l i d is necessary for b u l k f e r r o m a g n e t i s m . S e v e r a l m e c h a n i s m s ( J - 5 ) have b e e n p r o p o s e d for the p a i r w i s e stabilization o f ferromagnetic c o u p l i n g a m o n g spins. H o w e v e r , these schemes are insuf ficient

to account for t h r e e - d i m e n s i o n a l ferromagnetic behavior. A m e c h a

n i s m to account for this t h r e e - d i m e n s i o n a l interaction is d e s c r i b e d i n this chapter. T h e m o d e l o f configuration m i x i n g o f a v i r t u a l t r i p l e t charge-transfer excited

state

with

the ground

state

for a · · · 0 * Α * " 0 * Α * ~ Ο +

+

β

+

Α * · · · c h a i n to stabilize ferromagnetic c o u p l i n g was o r i g i n a l l y i n t r o d u c e d b y M c C o n n e l l (6)> F o r a D * A " ~ p a i r w i t h a half-occupied nondegenerate +

highest o c c u p i e d m o l e c u l a r o r b i t a l ( H O M O ) , t h e spins c o u p l e antiferromagnetically ( F i g u r e 1, la). ( P r e s u m a b l y , the v i r t u a l charge transfer involves o n l y t h e highest-energy partially o c c u p i e d m o l e c u l a r o r b i t a l ( P O M O ) . C i r cumstances i n w h i c h v i r t u a l excitation from a l o w e r - l y i n g filled (or to a h i g h e r l y i n g filled) o r b i t a l dominate t h e a d m i x i n g e x c i t i n g state are conceivable, a n d t h e o r b i t a l degeneracy a n d s y m m e t r y restrictions are relaxed.) A d m i x t u r e o f the h i g h e r e n e r g y charge-transfer states w i t h t h e g r o u n d state lowers the total electronic energy a n d stabilizes antiferromagnetic c o u p l i n g . F i g u r e 1 ( l i b a n d l i e ) illustrates ai* e l e c t r o n b e i n g d e l o c a l i z e d onto an adjacent site. T h i s energy r e d u c t i o n a n d d e r e a l i z a t i o n does n o t occur w h e n the two e l e c t r o n spins are p a r a l l e l (ferromagnetically aligned), i n accord w i t h t h e P a u l i exclusion p r i n c i p l e . T h u s , antiferromagnetic a n d (for o t h e r electron configurations) ferromagnetic c o u p l i n g can b e a c h i e v e d along a n d b e t w e e n chains (2, 5, 7, 8).


22.

M I L L E R & EPSTEIN

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Organometallic Electron-Transfer Salts

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Figure 1. Schematic illustration of stabilization of antiferromagnetic or fer romagnetic coupling. If both the D and A have a half-filled nondegenerate POMO (s ) (la), then the A*~D (or D«-A) charge-transfer excited state (lb or equivalently Ic) stabilizes antiferromagnetic coupling. If either D or A has a non-half-filled degenerate POMO (e.g., d , assumed here to be the D) (Ha), then the D w i t h tetracyanoethylene, 2 ( M e is m e t h y l ; T C N E n

5

5


CH

3

is tetracyanoethylene) (9, 10). T h i s electron-transfer salt possesses b o t h the alternating · · · D * A * ~ D A "~D * A *~ · · · ( F i g u r e 2) crystal a n d e l e c t r o n i c structures p r e s c r i b e d b y the configuration m i x i n g m e c h a n i s m already d e s c r i b e d (9). +

β +

+

T h e h i g h - t e m p e r a t u r e susceptibility of [ F e ( C M e ) r [ T C N E ] - fits the C u r i e - W e i s s expression w i t h θ = + 3 0 Κ ( F i g u r e 3) a n d indicates d o m inant ferromagnetic interactions (9). T h e s u s c e p t i b i l i t y a n d saturation m a g netization calculated as the s u m of the contributions from [ F e ( C H ) 2 ] * p a r a l l e l to the C m o l e c u l a r axis a n d [ T C N E ] is 6.46 m i l l i e l e c t r o m a g n e t i c u n i t s p e r m o l e ( m e m u / m o l ) at 2 9 0 Κ a n d 1 6 . 7 e l e c t r o m a g n e t i c u n i t s * k i l o g r a m p e r m o l e ( e m u k G / m o l ) , respectively. T h e s e values are i n excellent agreement w i t h the o b s e r v e d values of 6.67 m e m u / m o l a n d 16.3 e m u k G / m o l for single crystals a l i g n e d p a r a l l e l to the c h a i n axis (10). m

5

5

2

+

m

5

+

5

5

A spontaneous magnetization is o b s e r v e d for p o l y c r y s t a l l i n e samples b e l o w 4.8 Κ i n the E a r t h ' s magnetic field (9). T h e magnetization of these crystals is 3 6 % greater than that of i r o n m e t a l o n a p e r - i r o n basis, a n d it agrees w i t h the calculated saturation m o m e n t for ferromagnetic a l i g n m e n t of the d o n o r a n d the acceptor spins. T h e c r i t i c a l (Curie) t e m p e r a t u r e , T , is 4.8 K , a n d hysteresis loops characteristic of ferromagnetic materials are o b s e r v e d (1-5). A large coercive f i e l d of 1 k G is r e c o r d e d at 2 Κ (10). T h e p h y s i c a l properties are s u m m a r i z e d i n T a b l e I. Single-crystal susceptibility can be c o m p a r e d w i t h different p h y s i c a l c



Organometallic Electron-Transfer Salts

423


22.

Figure 2. Alternating donor-acceptor, · · · D ' A · · · ), linear chain structure of [Fe (C Me ) ]' [A]'(A is TCNQ, TCNE, DDQ, or C (CN) ), [Fe (C5H ) ][TCNE], and [Fe (C Me ) ]' [C (CN) ]'-. The structure shows adjacent out-of-registry chains for TCNE. +

m

lI

5

5

2

5

2

+

4

m

5

5

2

+

3

6

5

m o d e l s to a i d the u n d e r s t a n d i n g of microscopic s p i n interactions. F o r s a m ples o r i e n t e d p a r a l l e l to the field, the susceptibility above 16 Κ fits a o n e d i m e n s i o n a l H e i s e n b e r g m o d e l w i t h a ferromagnetic exchange, / , o f 19 c m " (JO). V a r i a t i o n o f the l o w - f i e l d magnetic susceptibility w i t h t e m p e r a t u r e for an u n u s u a l l y b r o a d temperature range above T [χ oc (Τ - T ) ~ ] , m a g n e t i 1

c

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800 -

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400

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300

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200

DC

100

ο. ο φ

[Fe (C5Me5)2]-+[TCNEK m

θ s +30 Κ

300

Temperature, Τ, Κ Figure 3. Reciprocal susceptibility, x~ , extrapolated from the high-tempera ture data for [M (C Me ) ]' [TCNE]-[M is Fe (ferromagnetic: θ = 30 Κ), Ni (antiferromagnetic: θ = - 1 0 Κ), and Co (paramagnetic: θ = -1 Κ)]. J

UI

5

5

2

+

zation w i t h t e m p e r a t u r e b e l o w T [ M °c ( T - T ) " ] , a n d t h e magnetization w i t h magnetic field at T (Μ « Η ) e n a b l e d t h e estimation o f the β , 7, a n d δ c r i t i c a l exponents. T h e values o f 1.2, ~ 0 . 5 , a n d 4.4, r e s p e c t i v e l y , w e r e d e t e r m i n e d for t h e magnetic field p a r a l l e l to t h e c h a i n axis. T h e s e values are consistent w i t h a m e a n - f i e l d - l i k e t h r e e - d i m e n s i o n a l b e h a v i o r . T h u s , above 16 K , o n e - d i m e n s i o n a l nearest-neighbor s p i n interactions are sufficient c

c

c

p

1 / δ


57

Additional magnetic behaviors observed:

+

26

30

4

[TCNE]'"

+

1

1

+

+

C H N Fe l - D - · - D ' A - - D - A - - D Α · · · · Chains Conventional organic solvents 4.8 Κ + 30 Κ Yes, in zero applied field 0.00667 emu/mol (obs, 290 K) 0.00640 emu/mol (calc, 290 K) 0.00180 emu/mol (obs, 290 K) 0.00177 emu/mol (calc, 290 K) 16,300 emuG/mol (calc 16,700 emuG/mol); 36% greater than iron (iron basis) 6,000 emuG/mol (calc 8,800 emuG/mol) 27.4 Κ (19 cm' ) 8.1 Κ (5.6 c m ) Yes (1000 G coercive field; cf. 1 G for iron metal) 0.5 (cf. 0.38 for iron metal) 1.22, 1.19 (cf. 1.33 for iron metal) 4.4 (cf. unknown for iron metal) Yes; neutron diffraction studies on polycrystalline deuterated samples Yes; in zero applied field [large internal field: 424,000 G (4.2K)] New mechanism for ferromagnetism appears to be operative; predictive model based upon configurational mixing of the lowest charge transfer excited state developed Chemical modification leads to meta-, antiferro-, ferri-, para-, and diamagnetic behavior

5

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S"

C/3

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1

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Ο

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MIL

Formula Structure Solubility Critical/ Curie temperature Curie-Weiss θ constant Spontaneous magnetization Magnetic susceptibility (|| to l - D chains) Magnetic susceptibility (|| to l - D chains) Magnetic susceptibility ( 1 to l - D chains) Magnetic susceptibility ( 1 to l - D chains) Saturation magnetization (|| to l - D chains) Saturation magnetization (X to l - D chains) Intrachain exchange interaction (|| to l - D chains) Intrachain exchange interaction (X to l - D chains) Hysteresis curves β Critical constant y Critical constant (|| and X to l - D chains) δ Critical constant Ferromagnetic ordering Fe Mossbauer Zeeman splitting Physical model

5

Table I. Physical Properties of [ F e ( C M e ) J ·


22.


426

E L E C T R O N T R A N S F E R IN B I O L O G Y A N D T H E SOLID STATE

to u n d e r s t a n d t h e magnetic c o u p l i n g , b u t near T

C

three-dimensional spin

interactions are d o m i n a n t (10). The

5 7

F e M o s s b a u e r spectra o f t h e T C N E

Fe(C Me ) 5

5

2

electron-transfer salt o f

are informative. A t y p i c a l six-line Z e e m a n split spectra are o b

s e r v e d i n zero a p p l i e d magnetic field at l o w t e m p e r a t u r e as the radical anions p r o v i d e a n i n t e r n a l d i p o l a r field. F o r e x a m p l e , a Z e e m a n split s p e c t r u m w i t h a n i n t e r n a l field o f 424 k G (4.2 K ) is o b s e r v e d for t h e [ T C N E ] " " (9) salt. T h e i n t e r n a l fields are substantially greater than the expectation o f 110


kG/spin/Fe. S i m i l a r l y s t r u c t u r e d electron-transfer complexes based o n T C N E a n d organometallic donors w e r e investigated i n an effort to u n d e r s t a n d the s t r u c t u r a l features necessary to stabilize t h e i r b u l k ferromagnetic b e h a v i o r . O u r study i n c l u d e d complexes w i t h s u b s t i t u t i o n o f the M e groups o n t h e c y c l o pentadienide ring w i t h H , increasing the ring size to six b y u s i n g bis(arene)chromium, a n d substitution o f F e w i t h R u a n d O s . Stable radicals are n e e d e d to f o r m ferromagnetically c o u p l e d chains. T h u s , e l e c t r o n transfer m u s t occur to enable c l o s e d - s h e l l donors a n d accep tors to b e candidates for magnetic materials. T h e o n e - e l e c t r o n s o l u t i o n r e v e r s i b l e r e d u c t i o n p o t e n t i a l , E°, provides a means to gauge w h e t h e r o r n o t e l e c t r o n transfer m i g h t occur for a s o l i d . F o r e x a m p l e , ferrocene is m o r e difficult to oxidize (by 0.5 V ) than decamethylferrocene, a n d i t is u n a b l e to r e d u c e T C N E (11-15). N e v e r t h e l e s s , t h e diamagnetic ferrocene analog o f [ F e ( C M e ) r [ T C N E ] * ~ (i.e., [ F e " ( C M e ) ] [ T C N E ] ) forms (12-14) a n d m

5

5

2

+

5

5

2

possesses t h e i d e n t i c a l structural m o t i f (16-18) ( F i g u r e 3). E i t h e r a t e m p e r a t u r e - o r p r e s s u r e - i n d u c e d " n e u t r a l - i o n i c " transition (19-22) m i g h t b e sufficient to l e a d to t h e stabilization o f ferromagnetic behavior. H o w e v e r , above 2 Κ at a m b i e n t p r e s s u r e , o n l y F e " is o b s e r v e d v i a M o s s b a u e r spec troscopy, a n d n o d i s c o n t i n u i t y is o b s e r v e d i n t h e s u s c e p t i b i l i t y data (15). The C o "

1

analog, [ C o

(C Me ) ]

i n

5

5

2

, +

[ T C N E ] - , has b e e n p r e p a r e d a n d

exhibits essentially the C u r i e s u s c e p t i b i l i t y anticipated for S = V2 [ T C N E ] (Θ = - 1 . 0 K ) (9). Because t h e cation is diamagnetic, t h e electron-transfer c o m p l e x has o n l y o n e s p i n p e r f o r m u l a u n i t . It appears that the · · · ϋ * Α * ~ +

D " A * " » · · structure type w i t h b o t h S ^ V2O a n d S ^ V2 A is necessary, +

b u t insufficient, for stabilizing cooperative h i g h l y magnetic behavior. A t tempts to p r e p a r e [ M ( C M e ) r m

5

5

2

+

( M is R u , Os) salts o f [ T C N E ] ' " have

yet to l e a d to suitable c o m p o u n d s for c o m p a r i s o n w i t h the h i g h l y magnetic Fe

phase (23). F o r m a t i o n o f [ R u ( C M e 5 ) ] '

m

n i

p o r t i o n a t i o n to R u " ( C M e ) 5

Os

1 1 1

5

2

5

and [ R u

2

I V

is c o m p l i c a t e d b y d i s p r o

+

(C Me )(C Me CH )] 5

5

5

4

2

+

(24). T h e

analog l e d to t h e p r e p a r a t i o n o f a salt w i t h T C N E ; h o w e v e r , l o w sus

c e p t i b i l i t i e s a n d crystals unsuitable for single-crystal X - r a y studies (23) have

hampered

progress

i n this

[ F e ( C M e ) r [ T C N E ] - with N i m

5

5

2

+

area. m

Replacement

(S = Vfc) o r C r

m

of

Fe

1 1 1

in

(S = %) leads to

c o m p o u n d s e x h i b i t i n g cooperative magnetic properties (25). T h e m o t i v a t i o n for s t u d y i n g these complexes emanated from the m o d e l


22.


Organometallic Electron-Transfer

427

Salts

for the stabilization o f ferromagnetic c o u p l i n g i n m o l e c u l a r solids (26). A n tiferromagnetic c o u p l i n g is p r e d i c t e d for d - s J

complexes w i t h s

s y m m e t r y , S >: 1/2 radidals w i t h a degenerate P O M O are r e q u i r e d . It is a challenge to the synthetic c h e m i s t to p r e p a r e radicals that have nondegenerate P O M O s and do not u n d e r g o a J a h n - T e l l e r d i s t o r t i o n , w h i c h w o u l d e l i m i n a t e the d e s i r e d electronic configuration. I n a d d i t i o n to p r e p a r a t i o n of the d e s i r e d radicals, t h e i r secondary a n d tertiary solid-state structures m u s t b e a c h i e v e d . F i n a l l y , single crystals large e n o u g h for the study of t h e i r anisotropic m a g netic properties m u s t b e p r e p a r e d . 2d

3

Acknowledgments T h e authors gratefully acknowledge partial support b y the D e p a r t m e n t of E n e r g y D i v i s i o n of M a t e r i a l s Science ( G r a n t N o . D E - F G 0 2 - 8 6 E R


432


45271.A000). W e d e e p l y thank o u r co-workers (R. W . B i g e l o w , J . C . C a l abrese, S. C h i t t i p e d d i , A . C h a k r a b o r t y , K . M i n g - C h i , K . R . C r o m a c k , D . A . D i x o n , P. J . K r u s i c , V . L . G o e d k e n , D . M . O ' H a r e , W . M . Reiff, H . R o m m e l m a n n , C . V a z q u e z , M . D . W a r d , D . W i p f , a n d J . H . Zhang) for t h e i m p o r t a n t c o n t r i b u t i o n s t h e y have m a d e t o w a r d t h e success o f t h e w o r k reported herein.


References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

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RECEIVED for review May 1, 1989. A C C E P T E D revised manuscript October 10, 1989.


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