Extended Interactions between Metal Ions

4 Intermolecular Magnetic Exchange Interactions in Molecular Crystals J. M .

GROW

and H .

H.

WICKMAN

Downloaded by EMORY UNIV on March 3, 2016 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/bk-1974-0005.ch004

O r e g o n State U n i v e r s i t y , C o r v a l l i s , O r e . 97331

Abstract Intermolecular exchange interactions have been studied in a representative, homologous series of metal complexes: iron(III) bis (dithiocarbamates) . Several of these molecular crystals exhibit cooperative m a g n e t i c order at t e m p e r a t u r e s b e l o w approximately 4 °K. Exchange paths include weak interactions between formally non-bonded molecular units. The m a g n e t i c phenomena a r e sensitive to zero-field splittings within the orbital-singlet, spin-quartet ground term o f iron(III). Discussion Molecules containing a single paramagnetic ion s u r r o u n d e d by b u l k y , organic ligands often form magnetically dilute molecular crystals. For metalm e t a l d i s t a n c e s g r e a t e r t h a n a b o u t 7 A, t h e d i p o l a r f i e l d s a r e s i g n i f i c a n t a s an o r i g i n o f m a g n e t i c o r d e r o n l y a t t e m p e r a t u r e s b e l o w -0.01 °K. Hence, most such m e t a l complexes remain s i m p l e paramagnets t o q u i t e low temperatures. However, s e v e r a l homologues o f t h e iron(III) halo-bis-dithiocarbamatesorder magnetically a t t e m p e r a t u r e s i n t h e r a n g e 1-4 K (_l-_3) . We summarize h e r e c e r t a i n s t r u c t u r a l o r c r y s t a l l o g r a p h i c f e a t u r e s o f t h e s e c o m p l e x e s w h i c h a p p e a r t o be i m p o r t a n t d e t e r m i n a n t s f o r exchange paths i n these molecular crystals. I n a d d i t i o n , we n o t e t h a t v a r i a b l e ground term c r y s t a l f i e l d interactions may enhance o r depress the c o o p e r a t i v e t r a n s i t i o n s f o r a f i x e d c r y s t a l s t r u c t u r e o r s e t o f exchange p a t h s . The s t r u c t u r a l f o r m u l a f o r an i r o n ( I I I ) b i s - d t c i s g i v e n i n F i g . 1. Much o f t h e c r y s t a l l o g r a p h i c i n f o r m a t i o n on b i s - d t c s has b e e n r e p o r t e d by H o s k i n s , e

34

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


Figure

1.

Structural

formula

of a halobisdithiocarbamatoiron(IH)


complex

36

EXTENDED

INTERACTIONS B E T W E E N

METAL

IONS

M a r t i n , W h i t e , a n d c o - w o r k e r s (4_-6_) . F o r example, the dimensions g i v e n i n the F i g u r e are f o r the d i e t h y l chloro derivative = e t h y l , i = l , 4 ; X = CI) (_5) . The i r o n ( I I I ) i s s i t u a t e d n e a r t h e c e n t r o i d o f a r e c t a n g u l a r p y r a m i d w h o s e a p e x may be o c c u p i e d b y a m o n o v a l e n t a n i o n ( h a l i d e , SCN", e t c . ) and whose b a s e c o n s i s t s o f 2 p a i r o f s u l f u r atoms f r o m t h e d t c ligands. T h i s s y m m e t r y l e a d s t o an o r b i t a l - s i n g l e t , s p i n - q u a r t e t g r o u n d t e r m (£) . The f o u r - f o l d s p i n d e g e n e r a c y i s removed by s p i n - o r b i t e f f e c t s , t o g e t h e r w i t h m e t a l - l i g a n d i n t e r a c t i o n s (7_-8_) , a n d two K r a m e r s d o u b l e t s , s e p a r a t e d b y ~2 t o 20 cm" , a r e commonly observed (9). The f o r e g o i n g l e v e l s t r u c t u r e i s a t t r a c t i v e f o r magnetism s t u d i e s . F i r s t , t h e low-symmetry e n v i r o n m e n t i s f e l t w i t h i n t h e S = 3/2 m a n i f o l d a s a s e c o n d r a n k c r y s t a l f i e l d p o t e n t i a l , d e s c r i b e d by o n l y two spin-Hamiltonian c o e f f i c i e n t s . Second, the magnetic e x c h a n g e i s r e p r e s e n t e d by an i s o t r o p i c Heisenberg i n t e r a c t i o n , owing t o the s m a l l o r b i t a l c h a r a c t e r o f the ground term. These i n t e r a c t i o n s o f t e n t u r n out t o be s m a l l c o m p a r e d w i t h t h e c r y s t a l f i e l d s p l i t t i n g s ; t h e r e s u l t i s t h a t t h e m a g n e t i c p r o p e r t i e s may be d i s c u s s e d w i t h i n a s i n g l e ground Kramers d o u b l e t . The well-known Hamiltonian f o r the s p i n - q u a r t e t m a n i f o l d is


1

The the

p a r a m e t e r s D and Ε d e s c r i b e t h e c r y s t a l f i e l d and remaining term r e p r e s e n t s magnetic exchange. B o t h f e r r o m a g n e t i c , c h l o r o - b i s - d i e t h y l d t c (1) , and a n t i f e r r o m a g n e t i c , i o d o - b i s - d i e t h y l d t c ( 3 ) , h o m o l o g u e s a r e known. The t r a n s i t i o n t e m p e r a t u r e s a r e 2.48 °K a n d 1.9 °K, r e s p e c t i v e l y . M o r e r e c e n t l y we have o b s e r v e d c o o p e r a t i v e t r a n s i t i o n s i n the f o l l o w i n g complexes: Fe(morphylyldtc) I, Fe(morphylyldtc)2Br, Fe(morphylyldtc) C1, Fe(pyrrolidyldtc) C1, F e ( p y r i o l i d y l d t c ) I , and F e ( p y r i d y l d t c ) C 1 . In these complexes, t h e m a g n e t i c o r d e r has been o b s e r v e d by Mossbauer spectroscopy (1^,2^) . A l l t r a n s i t i o n s occur b e l o w 4 °K. C r y s t a l s t r u c t u r e d a t a p r o v i d e s a means f o r i d e n t i f y i n g exchange p a t h s between atoms i n a d j a c e n t molecules. O n l y a few s t r u c t u r e s h a v e b e e n r e p o r t e d , b u t i n c l u d e d a r e t h e f e r r o m a g n e t i c and a n t i f e r r o m a g n e t i c d e r i v a t i v e s m e n t i o n e d a b o v e (5^-6^) . A d e t a i l e d d i s c u s s i o n of the o r i e n t a t i o n s o f the m o l e c u l e s i n c o n n e c t i o n w i t h exchange paths i s 2

2

2

2

2


4.

GROW A N D

/ntcrmolccular

WICK Μ A Ν

37

Exchange

h

a

v

e

a v a i l a b l e e l s e w h e r e O'^'É.) · We recently obtained s t r u c t u r a l i n f o r m a t i o n on t h e SCN-bis-diethyldtc, b r o m o - b i s - d i e t h y l d t c , and isodo-bis-morpholyldtctoluene adduct. The l a t t e r e x h i b i t s a m a g n e t i c t r a n s i t i o n at approximately 2 °K. The f o l l o w i n g p i c t u r e e m e r g e s f r o m a s t u d y o f t h e structural information. As e x p e c t e d , i r o n - i r o n d i s t a n c e s i n a l l complexes are i n e x c e s s o f 7 Â. In t h o s e c o m p l e x e s w h o s e s t r u c t u r e i s known a n d w h i c h show a m a g n e t i c t r a n s i t i o n , o r e x c h a n g e b r o a d e n e d e p r spectra, there e x i s t s a j u x t a p o s i t i o n of molecules s u c h t h a t t h e r e i s an i n t e r m o l e c u l a r S » - S l i n k a g e . T h a t i s , we may w r i t e t h e i n t e r m o l e c u l a r i n t e r a c t i o n a s F e - S » S - F e ^ w i t h t h e Fe-S u n i t s b e l o n g i n g t o d i s t i n c t molecules. The S***S d i s t a n c e i s t y p i c a l l y 3.6 to 4.5 A. The e x c h a n g e o f t e n p r o p a g a t e s t h r o u g h t h e l a t t i c e i n rather complicated paths: however, simple d i m e r i n t e r a c t i o n s may also occur. Because of the packing c h a r a c t e r i s t i c s of the p a r t i c u l a r l a t t i c e d e t e r m i n e d t h u s f a r , we h a v e n o t o b s e r v e d m o l e c u l e s a r r a n g e d so t h a t the p o t e n t i a l s t r o n g exchange p a t h Fe-X-^Fe-X i s favored. We h a v e a l s o n o t o b s e r v e d p a c k i n g w h i c h p r o d u c e s s a n d w i c h e d d t c l i g a n d s , s u c h as overlapping aromatic r i n g s t r u c t u r e s . O u r pmr shifts (3) show a d e r e a l i z a t i o n o f t h e i r o n s p i n d e n s i t y t o the d t c p e r i p h e r y , but the major d e r e a l i z a t i o n i s t o the immediate i r o n l i g a n d s . This i s of course cons i s t e n t w i t h the Fe-S- »S-Fe type exchange paths. I t i s i n t e r e s t i n g t h a t the e x i s t e n c e o f subs t a n t i a l exchange i n t e r a c t i o n s i s a necessary but not s u f f i c i e n t c o n d i t i o n f o r magnetic o r d e r i n the t e m p e r a t u r e r a n g e o f o u r e x p e r i m e n t s , Τ > 1°K. This may be s e e n b y c o n s i d e r i n g t h e e f f e c t o f g r o u n d K r a m e r s l e v e l a n i s o t r o p i e s on the c o l l e c t i v e m a g n e t i c p r o p e r t i e s of the complexes. T h a t i s , we c o n s i d e r a s i t u a t i o n where a f i x e d i n t e r m o l e c u l a r exchange e x i s t s b e t w e e n m o l e c u l e s i n two i s o m o r p h o n s l a t t i c e s . The m o l e c u l a r l a t t i c e s a r e assumed t o d i f f e r o n l y i n t h e type of s p l i t t i n g t h a t occurs i n the ground S = 3/2 manifold. I n Eq. ( 1 ) , i f D i s l a r g e i n m a g n i t u d e and d o m i n a t e s t h e r h o m b i c c r y s t a l f i e l d and t h e exchange t e r m , we may r e w r i t e the exchange i n t e r a c t i o n w i t h i n t h e g r o u n d K r a m e r s d o u b l e t u s i n g an e f f e c t i v e s p i n S = 1/2 (low t e m p e r a t u r e a p p r o x i m a t i o n ) . This i l l u s t r a t e s the magnetic a n i s o t r o p y o f the o r i g i n a l IS = 3/2, M = ± 3/2> Kramers d o u b l e t . The r e s u l t i s e


e e

e

g

TC

I

=

-2

I

9J

S S i j iz iz


(2)

38

E X T E N D E D INTERACTIONS

B E T W E E N M E T A L IONS

This i s the usual Ising Hamiltonian (E i s n e g l e c t e d ) . U p o n c h a n g i n g t h e s i g n o f D, b u t m a i n t a i n i n g t h e r e l a t i v e magnitudes o f the i n t e r a c t i o n s , the a l t e r n a t i v e ground doublet y i e l d s the a n i s o t r o p i c Heisenberg interaction "K* = -2

Τ

J..[S.

S. + 4 ( S . S. +S.

S. ) ] .

(3)


I t i s e a s i l y shown (10) t h a t i n t h e m o l e c u l a r f i e l d approximation the r a t i o o f magnetic t r a n s i t i o n temperatures w i t h and w i t h o u t t h e c r y s t a l f i e l d D term (exchange f o r c e s c o n s t a n t ) , i s g i v e n by t h e expression Τ

( D ) / T (0) = 3 < S

2 z

>

/S(S+1)

T

(4)

c I n E q . (4) , IJ» i s the i o n i c expectation value S a t t h e magfietië t r a n s i t i o n t e m p e r a t u r e . For D-*+

Extended Interactions between Metal Ions

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