Superexchange Interactions in Copper(II) Complexes - American

10 Superexchange Interactions in Copper(II) Complexes

Downloaded by NORTH CAROLINA STATE UNIV on November 24, 2012 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/bk-1974-0005.ch010

WILLIAM E. HATFIELD U n i v e r s i t y of N o r t h C a r o l i n a , Chapel Hill, N . C . 27514

This paper represents a b r i e f survey of superexchange i n t e r a c t i o n s (1) i n copper(II) complexes with s p e c i a l emphasis on research at the U n i v e r s i t y of North C a r o l i n a , which i s princi pally concerned with s t r u c t u r a l , magnetic s u s c e p t i b i l i t y , and EPR measurements. The systems to be discussed i n c l u d e (I) a s e r i e s of hydroxo-bridged complexes of the general formula [CuL(OH)] X ·nH O, where L i s a bidentate amine such as 2,2'b i p y r i d i n e or an N-substituted 2-(2-aminoethyl)pyridine; (II) a s e r i e s of chloro-bridged dimers i n c l u d i n g [Co(en) ] [Cu Cl ]Cl · 2H O, [Cu (guaninium) Cl ], [Cu (α-picoline) Cl ], and [Cu (di methylglyoxime) Cl ]; and ( I I I ) the compound [Cu(pyrazine)(NO ) ] and r e l a t e d chains. Since most, if not all, of the copper(II) complexes to be considered have orbitally nondegenerate s i n g l e - i o n ground s t a t e s the Hamiltonian appropriate f o r the problem i s 2

2

2

3

2

2

2

2

6

2

4

2

4

2

8

2

2

4

3

H = -2JE S.-î. (1) K j For those cases i n which antisymmetric exchange and a n i s o t r o p i c exchange become important the f o l l o w i n g terms may be added to (1): 1

-> D

J

-> -> · S.xS. + S.' J 1

Γ..*

1

=

1 J

-> S. J

where D^. i s the antisymmetric vector coupling constant and r ^ . i s the a A i s o t r o p i c coupling tensor. For o r b i t a l s i n g l e t single** ions undergoing exchange, Moriya (2.) has estimated that D

^(Ag/g)J

and 2

—(Ag/g) J ij where Ag = |g-2|. For many copper compounds Ag i s approximately 108

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

2

n

10.

HATFIELD

Superexchange

109

Interactions

0. 1, so | D | ~ 5 cm" f o r J = 100 cm~l. There i s no l i m i t on the magnitude of D and Γ f o r o r b i t a l l y degenerate s i n g l e i o n s t a t e s undergoing exchange, and a suggestion has been made that t h i s may be an a p p r o p r i a t e approach f o r the r a t i o n a l i z a t i o n of the magnetic p r o p e r t i e s of the tetramers [Cu^OXi+L^ ]. (3) These pro blems w i l l be discussed here. 1


1. Di-y-Hydroxo-bridged

Copper(II) Complexes

I t has been known f o r some time that copper (II) forms com plexes of the type [CuL0H]2X2 > where L i s a bidentate amine and X~ i s an a p p r o p r i a t e counterion. (4-8) These complex ions may be described as two planar or t e t r a g o n a l pyramidal u n i t s sharing an edge which i s defined by two b r i d g i n g hydroxo oxygen atoms. The magnetic p r o p e r t i e s of the compounds g i v e evidence f o r exchange i n t e r a c t i o n s which d i f f e r widely. From a c l o s e examination of the s t r u c t u r a l and magnetic data f o r s i x of these compounds i t has been p o s s i b l e to i d e n t i f y some of the f a c t o r s which i n f l u e n c e the exchange i n t e r a c t i o n s , and the r e s u l t s of that study w i l l be reviewed here. The chemical and s t r u c t u r a l f e a t u r e s which w i l l be examined i n c l u d e 1) the nature of the c h e l a t i n g amine. 2) the copper-oxygen (hydroxo) bond d i s t a n c e . 3) the nature of any out-of-plane c o o r d i n a t i o n . 4) the geometry of the b a s a l plane. 5) the s i n g l e i o n ground s t a t e . 6) hydrogen bonding by the hydroxo-bridge hydrogen atom. 7) the Cu-Cu s e p a r a t i o n . 8) the Cu-O-Cu bridge angle. Magnetic parameters have been obtained from analyses of EPR s p e c t r a , and from the temperature v a r i a t i o n of the magnetic sus c e p t i b i l i t y . The l a t t e r i s c h a r a c t e r i s t i c of exchange coupled copper(II) p a i r s and the s i n g l e t - t r i p l e t s p l i t t i n g s have been determined by f i t t i n g the data to the Van Vleck equation X

m

-

{ l + ~ exp(-2J/kT)}'

1

+ Να

(2)

where the symbols have t h e i r usual meaning, Να i s the temperature independent paramagnetism, and the equation as w r i t t e n gives the s u s c e p t i b i l i t y per copper i o n . In some cases Τ has been replaced by (T-9) to account f o r interdimer i n t e r a c t i o n s . A. S t r u c t u r a l and Magnetic

Data.

f

1. Di-y-hydroxobis[N,N,N ^ ' - t e t r a m e t h y l e t h y l e n e d i a m i n e copper(II)] bromide. The f i r s t compound of t h i s type to be c h a r a c t e r i z e d by an X-ray c r y s t a l s t r u c t u r e determination was di-y-hydroxobis[N,N,N jN'-tetramethylethylenediaminecopper(II)] bromide, [Cu(tmen)0H]2Br .(9) The s t r u c t u r e of the formula u n i t viewed along the b-axis i s shown i n F i g u r e 1 along with some of f

2



110

EXTENDED

Figure

1.

Structure

INTERACTIONS

BETWEEN

of [Cu(tmen)OH] * viewed along the b axis from Ref. 9) 2

2

METAL

(adapted


IONS

10.

HATFIELD

Superexchange

111

Interactions

the important s t r u c t u r a l parameters. The s t r u c t u r a l data are a l s o c o l l e c t e d f o r a l l compounds of t h i s type i n Table I. The Cu-N bond d i s t a n c e of 2.030 A, the Cu-0 bond d i s t a n c e of 1.902 A, and the N-Cu-N angle of 86.7° are a l l normal f o r s u b s t i t u t e d e t h y l enediamine complexes. The c o o r d i n a t i o n about the copper i s square planar with the n i t r o g e n atoms being 0.14 A out of the plane of the CU2O2 unit.The bromide i o n i s involved i n hydrogen bonding with the hydroxo-bridge, s i n c e O-Br d i s t a n c e of 3.366 A i s com parable to the O-Br d i s t a n c e s of 3.39 and 3.37 A, r e s p e c t i v e l y , i n the hydrogen bonded systems M n B ^ ^ ^ O and CoBr *2H20. The i n f r a r e d spectrum a l s o i n d i c a t e s the presence of hydrogen bond ing i n that there i s a strong OH s t r e t c h i n g band at 3410 cm" , a value which i s approximately 200 cm" lower than that of a f r e e OH group.(10-12) The magnetic s u s c e p t i b i l i t y has been measured i n the range 77-300°K both on a powdered sample and on a s i n g l e c r y s t a l along the a,b, and c c r y s t a l l o g r a p h i c axes. (13) The data were f i t t e d to the Van Vleck equation (2) y i e l d i n g the magnetic parameters 2J = -509 cm" , g = 2.0, and Να = ~150xl0"6 cgs u n i t s . Within the p r e c i s i o n of the experimental measurement the c r y s t a l sus c e p t i b i l i t i e s were i s o t r o p i c . e


o

2

1

1

1

2. D i - y - h y d r o x o b i s t N ^ N * ,Ν'-tetraethylethylenediamine copper(II)] p e r c h l o r a t e . As a part of an i n v e s t i g a t i o n of the thermochromic p r o p e r t i e s of N - a l k y l s u b s t i t u t e d ethylenediamine complexes of c o p p e r ( I I ) , H a t f i e l d , P i p e r , and Klabunde(6) reported, i n 1963, the temperature v a r i a t i o n of the magnetic s u s c e p t i b i l i t i e s of the Ν,Ν,Ν ,Ν'-tetraethylethylenediamine (teen) and NjN-diethyl-N'-methylethylenediamine complexes of the general formula [Cu(diamine)OH] (CIO1J2. The data f o r the l a t t e r com pound were f i t t e d to Equation (2) f o r the determination of the magnetic parameters, while 2J f o r the teen compound was d e t e r mined from the expression 2J = -1.11 T where T i s the temperature at which the magnetic s u s c e p t i b i l i t y a t t a i n s the max imum v a l u e , and the constant has u n i t s of cm" deg" . The s t r u c t u r e of [Cu(teen)0H] (C10i )2 has been completed only r e c e n t l y . ( 1 4 ) The s t r u c t u r e c o n s i s t s of [ C u ( t e e n ) 0 H ] 2 u n i t s and d i s c r e t e ClOi*" anions. (While t h i s geometry at the copper(II) i o n i s comparable to the s i t u a t i o n described above(9) f o r [Cu(tmen)0H]2Br2, i t i s i n marked contrast to the geometry of the s t r u c t u r e s of other compounds i n t h i s s e r i e s with oxyanions, v i d e post.) The best l e a s t squares plane of the N Cu0 CuN2 u n i t c a l l s a t t e n t i o n to the s l i g h t d i s t o r t i o n i n the molecule, the oxygen and n i t r o g e n atoms are approximately 0.15 Â out of the plane. The CU2O2 u n i t i s planar owing to the i n v e r s i o n center. The s t r u c t u r a l f e a t u r e s , which are given i n Table I, are very s i m i l a r to those determined f o r the tmen compound with the s i g n i f i c a n t exception being the decrease i n Cu-O-Cu angle from 104.1° i n [Cu(tmen)OH]2Br2 to 103.0° i n [ C u ( t e e n ) 0 H ] ( C l O ^ ) . Apparently there i s a hydrogen bonding i n t e r a c t i o n between 1

2

m a x

m a x

1

2

1

+

2+

2

2

2


2


2

4

2

4

3

pyridine nitrogen

*

4

2

2

80.6(1)° 95.6(1)

2

2

97° 97°

96.3(5)° 98.8(3)° 93.0(5)° 99.5(2)°

[Cu(bipy)OH] (N0 )

2

81°

4

100.4(1)°

87.8(2)° 103.0(2)°

[Cu(bipy)0H] S0 -5H 0

2

[Cu(EAEP)0H] (C10 )

2

β-[Cu(DMAEP)0H] (C10 )

2

[Cu(teen)0H] (C10 )

2

[Cu(tmen)0H] Br

Cu-O-Cu

ο

in-plane

ο

1.99 2.00 2.00 2.02

4

2.379(2)

2

2.917(5) -130

2.847

+172

2.21(S0 ") 2.893(2) +48 2.24(H 0)

1.981(8)* 2.562(10) 1.998(10) 2.618(9) 2.001(8)* 2.053(10)

1.920(2) 1.998(2) 1.923(1) 2.000(2)

1.92 1.94 1.95 1.95

1.895(7) 1.913(7) 1.927(2) 1.930(8)

2.935(1) -201

-509

2.978(2) -410

x

-

2

-i cm

-

ο Cu-Cu,A 3.000

out-ofplane Cu-0,A

1.900(3) 2.003(3)* 2.721(4) 1.919(3) 2.066(3)

1.899(4) 2.013(5) 1.907(4) 2.024(15)

Cu-0,A Cu-N,A 86.7(8)° 104.08(.17)° 1.902(3) 2.030(10)

N-Cu-N

S t r u c t u r a l and Magnetic P r o p e r t i e s of [Cu(diamine)OH]

Table I


29,30

25,26,27, 28

17,20,21

15,17

6,14

8,9,13

References

10.

HATFIELD

Superexchange

Interactions

113


the hydroxo group and the p e r c h l o r a t e i o n , s i n c e the average Cl-0 bond digtance f o r the three oxygens which are not i n v o l v e d i s 1.389(5) A, while the Cl-0 bond d i s t a n c e f o r the oxygen which i s probably hydrogen bonded i s 1.434(5) Â . 3. 3-Di-y-hydroxobis[2-(2-dimethylaminoethyl)pyridinecopper( I I ) ] p e r c h l o r a t e . Two forms of the compound [Cu(DMAEP)0H]2" ( C l O i ^ can be i s o l a t e d . (15,16) The 3-form i s obtained, essent i a l l y uncontaminated with the α-form, by mixing equimolar q u a n t i t i e s of copper(II) p e r c h l o r a t e hexahydrate and DMAEP i n ethanol/ether while both forms may be found i f methanol/ether i s used. U h l i g and co-workers (17) had reported only one form of t h i s compound i n t h e i r study of the c o o r d i n a t i o n chemistry of Ns u b s t i t u t e d 2-(2-aminoethyl)pyridine, and from a comparison of the magnetic p r o p e r t i e s of the two isomers, i t i s evident that they had the t r i c l i n i c α-form. The α-form has, i n a d d i t i o n to the two expected hydroxo b r i d g e s , two p e r c h l o r a t e bridges and i t w i l l not be considered f u r t h e r here.(16) The 3-form has two hydroxo bridges (15) and, as shown i n F i g u r e 2, the copper ions are i n a t e t r a g o n a l pyramidal environment with oxygen atoms from p e r c h l o r ate ions occupying the a x i a l p o s i t i o n s . The N2CUO2CUN2 u n i t i s e s s e n t i a l l y planar with the l a r g e s t d e v i a t i o n from the best l e a s t squares plane being 0.09 A. An unusual f e a t u r e s obtains here i n that the copper ions are not d i s p l a c e d out of the plane toward the a x i a l l i g a n d as has been observed i n many t e t r a g o n a l pyramidal copper(II) complexes. Presumably t h i s absence of d i s placement i s a r e f l e c t i o n of the weak nature of the p e r c h l o r a t e c o o r d i n a t i o n . The bond d i s t a n c e s and angles are comparable to those observed i n other s i m i l a r complexes. The s t r u c t u r a l para meters p e r t i n e n t to t h i s d i s c u s s i o n are l i s t e d i n Table I where i t may be seen that the Cu-pyridine n i t r o g e n bond d i s t a n c e i s somewhat shorter than the Cu-amine n i t r o g e n bond. The Cu-O-Cu angle i s 100.4(1).° Hydrogen bonding i n v o l v i n g the hydroxo b r i d g i n g group i s i n d i c a t e d by the oxygen-oxygen s e p a r a t i o n of 2.993 A, which i s l e s s than twice the van der Waals r a d i u s of oxygen as given by Bondi (3.02 A) (18) but s l i g h t l y greater than the corresponding value given by P a u l i n g (2.80 Â ) . (19) That the bonding i s weak i s evident from the s t r o n g , sharp 0-H s t r e t c h i n g band at 3580 cm~l, a value which i s only 40 cm" l e s s than the value u s u a l l y asc r i b e d to f r e e hydroxyl groups. The temperature v a r i a t i o n of the magnetic s u s c e p t i b i l i t y of 3-[Cu(DMAEP)0H] (C10 ) from 50-300°K i s shown i n F i g u r e 3.(15) There i s a very broad maximum i n the magnetic s u s c e p t i b i l i t y at about 175°K, and the data may be f i t t e d to expression f o r ex-_^ change coupled p a i r s of copper(II) ions y i e l d i n g 2J = -195 cm and g = 2.00, where the c r i t e r i o n f o r the best f i t was the minimization of the f u n c t i o n 1

2

A

B F

l+

2

= Z[{ (exptl).X

X

(calcd) }T ] i

2

i


EXTENDED INTERACTIONS


114

BETWEEN M E T A L IONS

3h

01 50

I

1 100

1

1 150

1

J — 200

1

1

250

300

β

Τ ( Κ) Inorganic Chemistry

Figure 3. Susceptibility of fi-[Cu(DMAEP)OH] (ClO, )2 per Cu atom as a function of temperature. Solid line represents values calculated from equation (2) with g = 2.03 and 2] = 201 cm (15). 2

t

1


10.

HATFIELD

Superexchonge

115

Interactions

4

For t h i s set of parameters A = 2 . 1 x l 0 ~ . Since t h i s best f i t g-value i s somewhat lower than the average g-value (2.03) found from the EPR spectrum taken at 77°K, a second c a l c u l a t i o n was made i n which the g-value was held constant and only 2J was v a r i e d . The somewhat poorer f i t ( A B F " 5.7x10"^) gave 2J = -201 cm"l. In view of these r e s u l t s a value of = 2.03 i s probably accurate w i t h i n 2% and 2J - -200110cm" . B F


1

4. Du-y-hydroxobis[2-(2-ethylaminoethyl)pyridinecopper(II)]p e r c h l o r a t e . The s t r u c t u r e of the complex [Cu(EAEP)0H]2~ (010^)2 and c o o r d i n a t i o n geometry around copper i s very s i m i l a r to that described above f o r [Cu(DMAEP)OH] (CIO1J2 · The p e r t i n e n t s t r u c t u r a l d e t a i l s are tabulated i n Table I. In t h i s compound the copper ions are d i s p l a c e d approximately 0.12 A from the best least-squares b a s a l plane which i s formed by the n i t r o g e n atoms and the two b r i d g i n g oxygen atoms. I t i s f u r t h e r noteworthy that the copper-oxygen(perchlorate) a x i a l i n t e r n u c l e a r separations are s i g n i f i c a n t l y shorter i n [Cu(EAEP)0H]2(CIO1J2 than i n the c o r r e s ponding complex [Cu(DMAEP)0H]2 (C10i*)2 where the copper ions are not d i s p l a c e d from the b a s a l plane. The two b a s a l planes are n e a r l y coplanar, with the angle between them being 1.4°. The average of the two Cu-O-Cu angles i s 99.2(3)° and the Cu-Cu s e p a r a t i o n i s 2.917(5) Â . The s t r u c t u r a l data i n d i c a t e that any hydrogen bonding i n v o l v i n g the b r i d g i n g hydroxo hydrogen atom i s very weak. The short oxygen(bridge)-oxygen(perchlorate) separations are 2.89 and 2.95 A, values which are l e s s than twice the van der Waals r a d i u s of oxygen as given by Bondi but greater than that given by P a u l i n g . The i n f r a r e d spectra shows a strong, sharp band at 3580 cm"" . The magnetic s u s c e p t i b i l i t y data (21) f o r [Cu(EAEP)0H] (010^)2 show a broad maximum at ~ 1 2 0 ° K and when the data are f i t t e d to Equation (2) the parameters 2J = -130cm~ and g = 2.04 result. 2

c

1

2

1

5. D i - y - h y d r o x o b i s [ 2 , 2 * - b i p y r i d i n e c o p p e r ( I I ) ] s u l f a t e pentahydrate. The complexes of the general formula [Cu(diamine)OH]2X "nH20 which are formed by 1,10-phenanthroline and 2 , 2 ' - b i p y r i d i n e are of c o n s i d e r a b l e i n t e r e s t s i n c e a v a r i e t y of counter ions may be used and the magnetic p r o p e r t i e s depend on the i d e n t i t y of the counter ion.(22-25) The f i r s t compound of t h i s type to be c h a r a c t e r i z e d f u l l y was [Cu(bipy)0H] S0i · 5H 0 (25-28). The s t r u c t u r a l d e t a i l s necessary f o r t h i s d i s c u s s i o n are l i s t e d i n Table I. In t h i s compound the copper ions are found i n d i s t o r t e d t e t r a g o n a l pyramidal environments with a water molecule c o o r d i n ated to one copper and the s u l f a t e i o n coordinated to the other coppeç. The copper ions are d i s p l a c e d approximately 0.18 and 0.23 A from the b a s a l planes formed by the n i t r o g e n atoms and the b r i d g i n g oxygens. I t i s i n t e r e s t i n g to note that the greater 2

2


+

2

116

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

displacement of 0.23 Â occurs i n the p o r t i o n of the molecule i n which the s u l f a t e i o n i s coordinated to copper and that that Cu-0 ( a x i a l ) bond d i s t a n c e i s the shorter of the two. The d i h e d r a l angle between the b a s a l planes i s 7.9°. There i s extensive hydrogen bonding with an 0(hydroxo)-0(sulfate) separation of 2.77 A. The magnetic s u s c e p t i b i l i t y of t h i s compound has been measured as a f u n c t i o n of temperature over the range 4.2-300°K, (26-28) and the χ vs.Τ p l o t , Figure 4, c l e a r l y i n d i c a t e s a d e v i a t i o n at low temperature from the Curie-Weiss behavior e x h i b i t e d at higher temperatures. The measured s u s c e p t i b i l i t i e s deviate i n the manner expect f o r an exchange coupled p a i r of copper ions with a p o s i t i v e 2J value i n d i c a t i n g a ferromagnetic i n t e r a c t i o n and a t r i p l e t ground s t a t e with a low l y i n g s i n g l e t s t a t e . The best l e a s t square f i t of the data y i e l d 2J = +48(±10) cm"" and g = 2.2. The presence of the t r i p l e t s t a t e i s confirmed by the EPR spectrum shown i n Figure 5. The spectrum may be described using the spin Hamiltonian


- 1

1

H , = g J |BH S s

z

where the resonance \(z>

f

2

z

+ gj3(S^ H + s J H ) + D(S ' -2/3) y

f i e l d s are

= (g| | 3 )

- 1

^ ν -D| 1

H ( z ) = (g| j B ) " ^

+D)

2

1

Η ( χ , ) = (g^3)" [hv(hv - D ) ] χ

z

1 / 2

Υ

1

H (x,y) = (gjB)" [hv(hv + D ) ]

1 / 2

2

H ^ f o r b ) = ( 2 g | |Β)"\ν H ( f o r b ) = (2gj3) 2

-12 2 2 1/2 ( h V -D ) A

Z

± / Z

Here the s u b s c r i p t 1 designates the low f i e l d t r a n s i t i o n s and the s u b s c r i p t 2 designates the high f i e l d t r a n s i t i o n s , while the ΔΜ = 2 t r a n s i t i o n s are l a b e l e d forbidden. 3

1

6. D i - y - h y d r o x o b i s [ 2 , 2 - b i p y r i d i n e c o p p e r ( I I ) ] n i t r a t e . The s t r u c t u r e of [Cu(bipy)0H]2(N03) i s very s i m i l a r to that of the s u l f a t e s a l t with the d i f f e r e n c e being that n i t r a t e s are coor dinated i n the a x i a l p o s i t i o n s of the t e t r a g o n a l pyramids. (29) The copper(II) ions are d i s p l a c e d from the b a s a l planes toward the oxygen atom of the coordinated n i t r a t e where the Cu-0 ( a x i a l ) bond i s r e l a t i v e l y s h o r t , being 2.379(2) Â. S t r u c t u r a l d e t a i l s are l i s t e d i n Table I . The magnetic s u s c e p t i b i l i t y has been measured as a f u n c t i o n of temperature i n the range 1.6-300°K and can be f i t to Equation (2) y i e l d i n g 2J = 172 cm"! and g = 2.10. (30) The estimated 2


HATFIELD


10.

Superexchonge

Interactions

117

20

40

60

80

300

Temperature (°K) Inorganic Chemistry

Figure 4. Inverse of magnetic susceptibility per cop per atom of [(hipy)Cu(OH) Cu(bipy)]SO · 511*0 as a function of temperature. Solid line represents valuescalculated from the Van Vleck equation; dashed line is extrapolation of Curie-Weiss law. Observed values are black dots. Data above 80K from Ref. 26 (28). 2

Figure

5.

X-band EPR spectrum of [Cu(bipy)OH] of/,Ο at 77°K, 0-10.0 kG

lf

,SO, ·


118

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

standard d e v i a t i o n on t h i s 2J value i s very l a r g e s i n c e the equa t i o n i s very i n s e n s i t i v e to v a r i a t i o n s i n l a r g e p o s i t i v e 2Jvalues. B. C o r r e l a t i o n of S t r u c t u r a l and Magnetic P r o p e r t i e s . 1. The nature of the c h e l a t i n g amine. Inspection of the data i n Table I r e v e a l s that the two complexes with the aromatic d i a mine, 2 , 2 - b i p y r i d i n e , have t r i p l e t ground s t a t e s , the two com plexes with a l i p h a t i c diamines, tmen and teen, have s i n g l e t ground s t a t e s with l a r g e | 2 j | values, and that the two complexes with the mixed a l i p h a t i c / a r o m a t i c diamines, DMAEP and EAEP, have s i n g l e t ground s t a t e s with intermediate |2j| v a l u e s . There i s no c o r r e l a t i o n with the N-Cu-N angle, s i n c e these angles increase i n the order aromatic < a l i p h a t i c < mixed. Furthermore, Casey (25) has shown that [Cu(bipy)OH] (NCS) *H 0, [Cu(bipy)OH] (NCSe) ·Η 0, and [ C u ( b i p y ) 0 H ] C l - 3 H 0 h a v e ^ s i n g l e t ground s t a t e s with 2J values of -6, -34, and -39 cm. , r e s p e c t i v e l y . Since the N-Cu-N angle i s expected to be f a i r l y constant f o r a l l of the 2,2 b i p y r i d i n e compounds, and s i n c e there i s no apparent c o r r e l a t i o n of t h i s angle with 2J, then i t i s reasonable to conclude that changes i n t h i s angle are of secondary importance .However, the chemical nature of the bidentate amine i s probably important s i n c e as of yet there are no known overlaps of 2J values between the groups of compounds formed by the aromatic, mixed aromatic/ a l i p h a t i c , and a l i p h a t i c diamines.


f

2

2

2

2

2

2

2

2

2

f

2. The copper-oxygen(hydroxo) bond d i s t a n c e . The copperb r i d g i n g oxygen bond d i s t a n c e s are a l l comparable and f a l l i n the range 1.9 to 1.95 A with the average bond d i s t a n c e being 1.915 A. For a l l p r a c t i c a l purposes the copper-oxygen bond d i s t a n c e i s constant i n t h i s s e r i e s of compounds. 3. The nature of any out-of-plane c o o r d i n a t i o n . The two com plexes with the a l i p h a t i c diamines, [Cu(tmen)OH] Br and [Cu(teen)OH] (ClO^) , are formed by two planar u n i t s sharing an edge. The c l o s e s t out-of-plane contacts to copper i n [Cu(tmen)O H ] B r are to bromide ions which are centered over the f i v e member c h e l a t e r i n g , these d i s t a n c e s are 4.778 and 4.933 Â , and are considered to be too long even foç semi-coordination. There are no out-of-plane atoms w i t h i n 4.0 A of copper(II) i n [Cu(teen)0H] (C10i ) . (31) The other four compounds are composed of t e t r a g o n a l pyramidal u n i t s sharing an edge with a p i c a l l i g a n d s on opposite s i d e s of the j o i n e d b a s a l planes. There i s an important trend here; the out-of-plane copper-oxygen bond d i s t a n c e s decrease with an i n c r e a s e i n the displacement of the copper i o n from the b a s a l plane toward the a p i c a l l i g a n d , v i z . , 2

2

2

2

2

2

2

+

2


10.

Superexchonge

HATFIELD

compound

Cu-0,

3-[Cu(DMAEP)OH] (C10 ) 2

4

[Cu(EAEP)OH] (C10 ) 2

4

[Cu(bipy)OH] (N0 ) Downloaded by NORTH CAROLINA STATE UNIV on November 24, 2012 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/bk-1974-0005.ch010

2

3

4

ο

ο

displacement, A

A

none

2.721(4)

2

2

2

2.618(9) 2.562(10) 2.379(2)

0.11 0.13 0.16

2

0.23 0.18

2.21(SO, ") 2.24(H 0)

[Cu(bipy)OH] S0 -5H 0 2

119

Interactions

2

2

There i s a general c o r r e l a t i o n of J with the copper-out-of-plane l i g a n d d i s t a n c e . Except f o r [Cu(bipy)0H] S0i 5H 0, the exchange coupling constant becomes more p o s i t i v e as the out-of-plane bond d i s t a n c e decreases. e

2

+

2

4. The geometry of the b a s a l planes. To a good approximation the bases of the t e t r a g o n a l pyramids are a l l planar with d e v i a t i o n s f r o m the best l e a s t squares plane being on the order of 0.15 A or l e s s . The two b a s a l planes [or the c o o r d i n a t i o n planes i n the case of [Cu(tmen)0H] Br and [Cu(teen)0H] (C10i ) ]are f r e q u e n t l y coplanar, with the l a r g e s t d e v i a t i o n from c o p l a n a r i t y being a 7.9° d i h e d r a l angle between these planes i n [ C u ( b i p y ) 0 H ] SO^'5^0. Consequently, except f o r t h i s l a t t e r compound, the geo metry of the b a s a l plane remains constant throughout the s e r i e s . o

2

2

2

+

2

2

5. The s i n g l e i o n ground s t a t e s . I t i s very w e l l e s t a b l i s h e d that the unpaired e l e c t r o n i s i n the σ* o r b i t a l f o r square planar copper(II) complexes,(32) Although s p i n - o r b i t c o u p l i n g and the low symmetry c r y s t a l f i e l d components permit the mixing i n of other s t a t e s , to a good approximation the exchange mechanisms can be given i n terms of t h i s e l e c t r o n i c c o n f i g u r a t i o n . Although the displacement of the copper(II) i o n from the b a s a l plane toward the a x i a l l i g a n d i n the t e t r a g o n a l pyramidal complexes complicates even f u r t h e r the nature of the s i n g l e i o n ground s t a t e , i t w i l l be assumed that the unpaired e l e c t r o n i s i n the d >cr*, o r b i t a l i n these complexes. This assumption i s a d m i t t e d l y ïess tenable f o r [Cu(bipy)0H] S0i '5H 0 and [ C u ( b i p y ) 0 H ] ( N 0 ) where the d i s p l a c e ments are considerable and the a x i a l bond d i s t a n c e s are rather short. P r e c i s e d e s c r i p t i o n s of the ground s t a t e s must await the completion of d e t a i l e d EPR i n v e s t i g a t i o n s which are p r e s e n t l y underway. (33) 2

2

X

2

t

2

2

3

2

6. Hydrogen bonding by the h y d r o x o - b r i d g e h y d r o g e n atoms.The nature of the hydrogen bonding spans a wide range i n these compounds. A s u i t a b l e working hypothesis would suggest that the e l e c t r o n d e n s i t y on the b r i d g i n g oxygen atom should increase with the s t r e n g t h of the hydrogen bond and that t h i s v a r i a t i o n i n e l e c t r o n d e n s i t y should have a s i g n i f i c a n t e f f e c t on the exchange


120

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS


coupling. I f e i t h e r the 0···Χ i n t e r n u c l e a r separation or Δν(ΟΗ) (the d e v i a t i o n from the " f r e e " hydroxyl s t r e t c h i n g energy) are taken as gauges of the hydrogen bond, then i t i s c l e a r from the data i n Table I I that there i s no simple c o r r e l a t i o n between e i t h e r of these parameters and the exchange coupling constant. Thus i t i s reasonable to conclude that hydrogen bonding i s of secondary importance i n determining the nature of the exchange coupling. 7. The copper-copper s e p a r a t i o n . There i s an i n t e r e s t i n g c o r r e l a t i o n between the Cu-Cu separation and the s i n g l e t - t r i p l e t s p l i t t i n g . As the Cu-Cu s e p a r a t i o n increases from 2.847 A i n [Cu(bipy)0H] (N0 ) to 3.000 Â i n [Cu(tmen)0H] Br ,_Jhe s i n g l e t t r i p l e t s p l i t t i n g changes from +172 cm" to -509 cm . I f the data f o r [Cu(bipy)0H] S0^·5H 0 are omitted, s i n c e the b a s a l planes i n the compound are not coplanar, then the best l i n e through the f i v e a v a i l a b l e v a l u e s o f the copper-copper separation and^2J has a slope of -4545 c n r V A and an i n t e r c e p t of 13,130 cm 2

3

2

2

2

1

2

2

0

8. The copper-oxygen-copper bridge angle. Since the copDeroxygen (bridge) bond d i s t a n c e s are n e a r l y constant at 1.915 A, and s i n c e the C u 0 u n i t s are a l l n e a r l y planar there i s a s i m i l a r s t r i k i n g c o r r e l a t i o n between the 2J values and the Cu-0Cu bridge angle. T h i s l i n e a r c o r r e l a t i o n ( i n the range 95.6° < φ < 104.1°) i s i l l u s t r a t e d i n Figure 6, where the slope i s -79.5 cm" deg" and the i n t e r c e p t i s 7790 cm" . 2

1

2

1

1

C. R a t i o n a l i z a t i o n i n Terms of a Molecular O r b i t a l Model. The c o r r e l a t i o n s which have been observed can be understood i n terms of a simple molecular o r b i t a l model, which i s most appro p r i a t e f o r the two joined-planar compounds, but which i s s t i l l a good approximation f o r the j o i n e d t e t r a g o n a l pyramidal compounds. (34) I t i s necessary to consider the two oxygen o r b i t a l s , p , Py, and the two copper in-plane o r b i t a l s d _ , d . I f the f o l l o w ing coordinate system i n D i s adopted x

2

v 2

x v

2 n

01

Cui X

i t can be seen that the o r b i t a l s transform as A

d

2

x -y

2

+

d

2

x -y

2

Yl " Ύ2



2

2

4

2

4

ο

" f r e e " hydroxyl

4

3

2

2

3620

2.877

2

2

-

2.89

2.993

3.00

3.366

X, A

[Cu(bipy)OH] (N0 )

3580

0

2.77

2

3580

1

-

4

2

3410

v(O-H),cnf

[Cu(bipy)OH] S0 '5H 0

2

[Cu(EAEP)OH] (C10 )

2

β-[Cu(DMAEP)] (C10 )

2

[Cu(teen)OH] (C10 )

[Cu(tmen)OH] Br

Compound

3

2

0N0 "

3

oso "

3

ocio ""

3

ocio "*

ocio "

Br"

X

Hydrogen Bonding Parameters f o r the B r i d g i n g Hydroxo Group.

Table I I

2

2

ο Bondi (3.02) Pauling (2.80)

MnBr ·2Η 0(3.39 A) CoBr -2H 0(3.37 A) ?

Comments


122

EXTENDED

B, lg

:

d x

B„ : 2u

l

d

"

xy


Yl

B

0

3u

+ d

xy

x

INTERACTIONS

BETWEEN

METAL

IONS

xy

2

- d

xy

+ Ύ2

: d 2 2 x -y z

^ 2

z

Χχ + x

2

2

where x^ and y^ designate the oxygen ρ c a l l y these symmetry r e l a t i o n s h i p s are

χ

and p

v

orbitals.

Symboli

67

J

3u

Inspection of these w i l l show that the A and B^ molecular o r b i t a l s w i l l have i d e n t i c a l energies as w i l l ^ t h e B j ^ a n d u



10.

Superexchange

HATFIELD

200 -

\

*

Interactions

123

bipy NO,

100 ^bipy

S0

4

0

-100 τ

-

IAEP

ε ^

X.

-200

-300

-

-400

-

-500

-

-DM ΑΙ Ρ

\o

t

e

e

n

Ν.

1 95

Figure

$

1

6*. Correlation

1

! 97

of singlet-triplet

I 99

1

splittings

1 101

1—

i 103

—

with the Ctt-O-Cu

tmen

λ „

1 105

bridge


angle

124

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

o r b i t a l s . With t h i s information the molecular o r b i t a l diagram f o r t h i s fourteen e l e c t r o n system may be constructed; i t i s given i n Figure 7a. As the Cu-O-Cu angle increases the overlap of the A and B combinations increase r e l a t i v e to that of the B3 and 2u a t i o n s . At a s u f f i c i e n t l y l a r g e enough angle i t may be a n t i c i p a t e d that the separation between the B2 * and B i g * mole c u l a r o r b i t a l s would exceed the p a i r i n g energy and the s i n g l e t s t a t e with c o n f i g u r a t i o n ( b 2 * ) would r e s u l t as the ground s t a t e . The t r i p l e t s t a t e from the c o n f i g u r a t i o n ( b * ) ( b i * ) would be the low l y i n g paramagnetic s t a t e . g

l g

B

U

c o m D i n

U

2

u


2 u

II.Di-y-Chloro-bridged

g

Copper(II) Complexes

The p r o p e r t i e s of four d i - y - c h l o r o - b r i d g e d copper(II) com plexes w i l l be described here. These include (Class I) [Co(en)3] [Cu2Cl8]Cl2'2H 0 and [Cu(guaninium)Cl3]2 , where the copper(II) ions are i n d i s t o r t e d t r i g o n a l bipyramids which share an equator-to-apex edge, (35-40) and (Class II) [Cu(2-methyl2

2

p y r i d i n e ) 2 C l 2 l 2 and [Cu(dimethylglyoxime)Cl2]2»where the coordina t i o n about copper i s d i s t o r t e d t e t r a g o n a l pyramidal and the d i meric s t r u c t u r e i s formed by the sharing of the base-to-apex edge. (41-44) Unlike the hydroxo-bridged copper(II) complexes described i n Section I where the Cu-0 (bridge) bond lengths are constant, the chloro-bridged dimers have g r e a t l y d i f f e r e n t Cu-Cl (bridge) d i s t a n c e s . I t w i l l be shown here that the s i n g l e t t r i p l e t s p l i t t i n g s are dependent on the b r i d g i n g bond lengths as w e l l as the Cu-Cl-Cu b r i d g i n g angle. A. S t r u c t u r a l and Magnetic Data. 1. T r i s ( e t h y l e n e d i a m i n e ) c o b a l t ( I I I ) D i - u - c h l o r o b i s [ t r i c h l o r o c u p r a t e ( I I ) ] D i c h l o r i d e Dihydrate. The unusual spec t r a l p r o p e r t i e s (45) of the compound Co(en)3CUCI5Ή2Ο, as f o r mulated by Kurnakowin 1898, l e d to an X-ray c r y s t a l s t r u c t u r e examination (35,36) which revealed that the new and unusual [C^Cls] *"" ion was present. The compound c r y s t a l l i z e s i n the orthorhombic space group Pbca with four molecules i n a u n i t c e l l of dimensions a = 13.560(9), b = 14.569(9), and c = 17.885(12) A. The b r i d g i n g Cu-Cl distances are 2.325(5) and 2.703(5) A, the Cu-Cu separa t i o n i s 3.722(5) A, and the angle at the bridge i s 95.2(1)°. The exchange i n t e r a c t i o n (37) i n [ C u ( e n ) 3 ] [ C u C l 8 ] C l 2 ' 2 H 0 has been p r e c i s e l y c h a r a c t e r i z e d by s i n g l e c r y s t a l magnetic s u s c e p t i b i l i t y measurements. (38) Data were c o l l e c t e d i n the temperature range 4.2-80°K on a c r y s t a l ( 5 . 2 x 2 . 9 x 2 . 5 mm) with the magnetic f i e l d a p p l i e d along the c r y s t a l l o g r a p h i c axes. The data are given i n Figure 8, where i t may be seen that the s u s c e p t i b i l i t y maximizes at 12.9°K, and the usual g - f a c t o r a n i s o tropy i s observed ( x > x > χ ^ ) . The data may be f i t t e d to 1

2

c

2

a


2

10.

HATFIELD

Superexchange

Interactions

125

», *

2u

a.

4 -


\

\

\

d 2 χ

-v

\

2

w W

>

Λ-

\ X \ \ \

W e l e c t rons metal

molecular

ion orbitals

ligand

orbitals

orbitals

b.

/

/

/

/

Λ

*

Η—-χ. κ *

/

/

Figure 7. (a) Molecular orbital diagram for the in-plane orbitals in the CuO, ring, D symmetry, 90° Cu-O-Cu angle, (b) Antibonding molecular orbitals for Cu-O-Cu angle different from 90° (adapted from Ref. 34). 2ll


126

EXTENDED

INTERACTIONS

METAL

IONS

Equation (2) y i e l d i n g J / k = -10.7±.2° and g = 2.07±.02; J / k = -10.8±.2° and g = 2.03±.02; and J / k = -10.6±.2° and g = 2.18±.02. The data f i t the t h e o r e t i c a l curve very w e l l . For example, of the 44 data p o i n t s c o l l e c t e d with the f i e l d a p p l i e d along the c - a x i s , only f i v e deviated from the c a l c u l a t e d values by more than 1.5% while 34 of the p o i n t s d i f f e r e d by l e s s than 1%. No s i g n i f i c a n t improvement of the f i t s were observed when interdimer i n t e r a c t i o n s were included. Thus w i t h i n experimental e r r o r J = J = J = J , and = (1/3)(g +g +g ) = 2.09, a value which i s i n e x c e l l e n t agreement with the g value obtained from powder data c o l l e c t e d i n the temperature range 130-235°K. Here g was c a l c u l a t e d from the C u r i e constant using the formula a

a

b

a


BETWEEN

b

b

c

c

c

a

g

2

b

c

2

= 3kC/N3 S(S+l)

The exchange i n t e r a c t i o n observed along the c - a x i s , which i s almost c o l i n e a r with the copper(II)-copper(II) v e c t o r , and the exchange i n t e r a c t i o n observed along the a and b-axes, which are almost perpendicular to the copper(II)-copper(II) v e c t o r , are equal i n magnitude. A l s o , there i s no s i g n i f i c a n t long range interdimer exchange present i n the system. This r e s u l t i s not unexpected i n view of the l a r g e interdimer separation. The l a r g e c o p p e r ( I l ) - c o p p e r ( I I ) separation of 3.722 A precludes any through-space i n t e r a c t i o n s s i n c e no s i g n i f i c a n t o r b i t a l overlap can occur over t h i s d i s t a n c e , and d i p o l e - d i p o l e i n t e r a c t i o n s could not produce a s p l i t t i n g of the observed magnitude. Hence, i t seems reasonable to conclude that the i n t e r a c t i o n occurs v i a superexchange through the c h l o r i d e b r i d g e s . 0

2. D i - y - c h l o r o b i s [ d i c h l o r o ( g u a n i n i u m ) c o p p e r ( I I ) ] d i h y d r a t e . In 1970 Carrabine and Sundaralingam (39a) reported the s t r u c t u r e of d i - y - c h l o r o b i s [ d i c h l o r o ( g u a n i n i u m ) c o p p e r ( I I ) ] dihydrate, where the guaninium l i g a n d i s the c a t i o n formed by monoprotonating guanine, one of the bases bonded to the sugar residues i n the backbone of d i o x y r i b o n u c l e i c a c i d (DNA). The same authors presented a more complete s t r u c t u r a l a n a l y s i s the f o l l o w i n g year, (39b) and the s t r u c t u r e was then confirmed by Declercq, Debbaudt, and Van Meerssche (39c) i n an independent i n v e s t i g a t i o n . Both research groups reported the s t r u c t u r e to be that of a dimer c o n s i s t i n g of chloro-bridged, t r i g o n a l - b i p y r a m i d a l l y coordinated copper(II) ions, as shown i n Figure 9. The monoprotonation was shown to occur at the imidazole n i t r o g e n , N(7), of the purine r i n g system, and binding to the copper i o n , at N£9). The b r i n g ing Cu-Cl distances were determined to be 2.447 A and 2.288 A, with a Cu-Cl-Cu b r i d g i n g angle of 98° and a Cu-Cu separation of 3.575 A. The temperature v a r i a t i o n of the inverse s u s c e p t i b i l i t y ( c a l c u l a t e d per copper(II) ion) of a powdered sample i n the temperature region 1.6 to 255°K i s represented by the black data p o i n t s i n Figure 10. (40) The maximum i n the curve occuring at 0


Superexchange

HATFIELD

Interactions


10.

Figure

9.

Structure

of

[Cu(guaninium)Cl. ]j {

(adapted

from

Ref.


39a)

128

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

approximately 15°Κ i s probably due t o a small percentage of mono mer i c impurity which d i d not a f f e c t the percentage composition of the elemental a n a l y s i s . To c o r r e c t f o r t h i s impurity the data p o i n t s from 1 . 6 t o 11.2°K were f i t to the Curie-Weiss law


χ = C/(T - Θ) Values of 7.86 χ 10 and -3.22° were obtained f o r the constants C and θ, r e s p e c t i v e l y . In t h i s temperature r e g i o n the s u s c e p t i b i l i t y of the dimer i s n e g l i g i b l y small (vide p o s t ) . A l l the data p o i n t s were then c o r r e c t e d f o r the c o n t r i b u t i o n of the impurity to the observed s u s c e p t i b i l i t y , and the c o r r e c t e d p o i n t s a r e a l s o p l o t t e d i n F i g u r e 10 as the u n - f i l l e d c i r c l e s . The impurity was estimated to be present to the extent of 1% i n the f o l l o w i n g way: The s u s c e p t i b i l i t y f o r an assumed monomer having a molecular weight equal t o one-half that of the dimer was c a l c u l a t e d from the expression χ - N3 y /3kT 2

2

1/2 where μ = g$[S(S + 1) ] , a t a s e l e c t e d temperature, and the c a l c u l a t e d s u s c e p t i b i l i t y was compared with the experimental sus c e p t i b i l i t y a t that temperature. The s o l i d l i n e i n F i g u r e 10 i s the best f i t of the c o r r e c t e d data to Equation ( 2 ) , which y i e l d s the parameters 2 J « - 8 2 . 6 ± 1 . 0 cm" and g - 2.12±0.02. 3. D i - y - c h l o r o b i s [ c h l o r o ( d i m e t h y l g l y o x i m e ) c o p p e r ( I I ) ] . The s t r u c t u r e (43) of [Cu(DMG)Cl2l2 i s shown schematically i n Figure 11 with p e r t i n a n t molecular dimensions i n d i c a t e d thereon. The copper atom i s f i v e - c o o r d i n a t e d i n a square-based pyramidal a r rangement c o n s i s t i n g of a n e a r l y square planar arrangement of two n i t r o g e n atoms and two t i g h t l y bound c h l o r i n e atoms with a c h l o r i n e atom from an adjacent u n i t i n the a p i c a l p o s i t i o n . The magnetic data (44) can be described by the s i n g l e t t r i p l e t equation ( 2 ) with 2 J = 6 . 3 cm" , g = 2 . 0 6 , and θ = - 1 . 7 ° . A d d i t i o n a l and convincing evidence f o r the t r i p l e t ground s t a t e i s provided by the magnetization s t u d i e s . The magnetization curves f o r S' = 1/2 and S' » 1 were evaluated from the B r i l l o u i n function (46) 1

1

1

τ>ίγ\ - ^ S ' * „ .. B(X) , coth n#

2 s

2S'+1

1 χ - 2s' v

c o t h

2S*

where X • (H/T)(S g3/k) and S i s the e f f e c t i v e s p i n . To account f o r interdimer i n t e r a c t i o n s the f i e l d H was s e t equal t o the e x t e r n a l f i e l d p l u s a molecular f i e l d , H , where i t was assumed that Hm was p r o p o r t i o n a l to the magnetization. Thus, 1 ^ = N N3
where = gS'B(X) and = (3ke )/[Ng 3 S (S +1)].(47) A best f i t ( l e a s t squares) of the experimental data was used to s e l e c t a value of -1.2 f o r Θ thereby i n d i c a t i n g an a n t i f e r r o f

f

m

f

2

2

f

f

W

1



10.

Superexchange

HATFIELD

129

Interactions

TEMPERATURE Inorganic Chemistry

Figure 10. Temperature vs. inverse susceptibility for the complex [(guaninium)CuClg]* · 2H 0 (38). Experimental points, · ; experimental points corrected for monomeric impurity, Q; Van Vleck equation best fit, . 2


130

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IONS

1


magnetic interdimer i n t e r a c t i o n . The Θ term i n the magnetization s t u d i e s i s very s i m i l a r to the θ term i n Equation 2, but i n view of the approximate nature of the theory, i d e n t i c a l values f o r these interdimer i n t e r a c t i o n parameters are not expected. 4. D i - p - c h l o r o b i s [ c h l o r o b i s ( 2 - m e t h y l p y r i d i n e ) c o p p e r ( I I ) ] . In recent years many copper(II) complexes of the type CUL2X2» where X i s c h l o r i d e or bromide and L i s p y r i d i n e or s u b s t i t u t e d p y r i dine, have been prepared and c h a r a c t e r i z e d . (48) These complexes are mainly polymeric, having s i x c o o r d i n a t i o n about the copper ion with h a l i d e l i g a n d s from adjacent molecules occupying the o u t of-plane c o o r d i n a t i o n p o s i t i o n s . However, the complexes of 2methylpyridine were found to have p r o p e r t i e s somewhat d i f f e r e n t to those found f o r the analogous p y r i d i n e complexes, (49)and i t was p o r t u l a t e d that the methyl group i n the 2 - p o s i t i o n provides s t e r i c hindrance to the u s u a l o c t a h e d r a l c o o r d i n a t i o n . Subse quently, Duckworth and Stephenson (41) determined that such was the case f o r d i c h l o r o b i s ( 2 - m e t h y l p y r i d i n e ) c o p p e r ( I I ) . The coor d i n a t i o n about copper i n t h i s complex i s t e t r a g o n a l pyramidal with the f i f t h p o s i t i o n (out-of-plane) occupied by a c h l o r i d e l i g a n d from an adjacent planar moiety, and the s i x t h p o s i t i o n i s e f f e c t i v e l y blocked by the methyl groups of the p y r i d i n e l i g a n d s . The s t r u c t u r a l d e t a i l s are summarized i n Table I I I . As shown i n F i g u r e 12, the magnetic s u s c e p t i b i l i t y (42) data f o r Cu(2-methylpyridine)2CI2 obey the Curie-Weiss law, χ = C / ( T - 9 ) , i n the range 295°K to approximately 30°K. For the c h l o r o complex. the C u r i e constant C = 0.394, θ = 1°K. and y f f 2.828C ' i s 1.78 B.M. However, a t the low temperature l i m i t i t i s apparent that the Curie-Weiss law f a i l s . There i s a d i s t i n c t minimum i n the χ"~^ versus Τ p l o t a t approximately 7°K. The data obey the Van V l e c k equation (2) f o r m a g n e t i c a l l y coupled p a i r s of copper ions y i e l d i n g g = 2.15 and -2J = 7.4 cm"^. =

e

1

2

B. C o r r e l a t i o n of S t r u c t u r a l and Magnetic P r o p e r t i e s . I t i s of i n t e r e s t to compare the magnetic parameters and s t r u c t u r a l data f o r the s t r u c t u r a l l y - and m a g n e t i c a l l y - c h a r a c t e r i z e d c h l o r o bridged b i m e t a l l i c copper(II) complexes discussed here. These data a r e compiled i n Table I I I . Both the guaninium complex and the [Cu2Cl3]^""anion are made up of t r i g o n a l bipyramids sharing equatorial-to-apex edges, w h i l e the other two complexes l i s t e d i n the t a b l e are square-based pyramids sharing base-to-apex edges. In the t r i g o n a l - b i p y r a m i d a l complexes i t i s l i k e l y that the un p a i r e d e l e c t r o n s a r e i n the d o r b i t a l s of the copper(II) ions i n the ground s t a t e , whereas i n the square-pyramidal complexes i t i s l i k e l y that they a r e i n the d _ 2 o r b i t a l s . A q u a n t i t a t i v e comparison of the exchange energies of the four complexes cannot be based on s t r u c t u r a l data because of the d i f f e r e n t o r b i t a l s involved i n superexchange, but a q u a l i t a t i v e comparison i s pos sible. The s t r u c t u r a l parameters of the guaninium complex and the z 2

x 2

y



2

2

4-

2

2

1

DMG = dime thy lgloxime

2-mepy = 2-methylpyridine

2

[(DMG)CuCl ]

2

2

anion

[(2-mepy) CuCl ]

2

[Cu Clg] 2.26

2.24

101.4

88

squarepyramidal squarepyramidal

+6.3

2.70

2.45

43,44

41,42 3.37

2.70

35,36, 37,38

39,40

REF.

2.33

2.29

Cu-Cl BOND Cu-Cl BOND in-plane out-of-plane (A) (A)

-7.4

95.2

98

Cu-Cl-Cu ANGLE (degrees)

trigonalbipyramidal

trigonalbipyramidal

STRUCTURE

-14.6

-82.6

2

[(guaninium)CuCl ] ·2H 0

3

2J (cm )

COMPLEX X

Table I I I . Magnetic and s t r u c t u r a l data f o r c h l o r o - b r i d g e d copper(II) dimers.



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IONS

[ C i r c l e ] ^ " a n i o n are compared by superimposition i n Figure 13 with the s o l i d l i n e representing the guaninium complex. The smaller s i n g l e t - t r i p l e t s p l i t t i n g f o r the [ C ^ C l e l anion i n comparison to the guaninium complex^accompanies an increase i n the Cu-Cl-Cu bond angle from 95° to 98° and a decrease i n the b r i d g i n g bond lengths. Although i t has been demonstrated that the b r i d g i n g angle i s important i n determining the s i g n and magnitude of the s p l i t t i n g parameter 2J i n the s e r i e s of hydroxobridged copper(II) complexes i t i s u n l i k e l y t h i s e f f e c t can be presented as the s o l e explanation i n t h i s comparison because the b r i d g i n g bond lengths of these two chloro-bridged species are q u i t e d i f f e r e n t , ranging between 2.3 and 2.7 A, whereas they arg n e a r l y constant i n the hydroxo-bridged species at 1.90 to 1.95 A. (See Table I ) . The complexes [ ( 2 - m e t h y l p y r i d i n e ) 2 C u C l 2 ] and [(dimethylg l y o x i m e ) C u C l 2 l 2 have d i f f e r e n t s t r u c t u r e s and presumably a d i f f e r e n t exchange coupling mechanism from that described above. In comparing these two square-pyramidal complexes i t should be noted that there i s a change i n ground s t a t e m u l t i p l i c i t y . The b r i d g i n g bond length i n [(2-mepy)2CUCI2]2 0.67 A longer than the comparable bond i n [(DMG)CuCl2]2 and angle at the b r i d ging c h l o r i d e i s 13.4° l a r g e r i n the 2-methylpyridine complex than i n the dimethylglyoxime complex. I t i s , t h e r e f o r e , not p o s s i b l e to a t t r i b u t e the change i n the exchange coupling constant only to bridge angle changes. C l e a r l y a number of a d d i t i o n a l chloro-bridged copper(II) dimers of both s t r u c t u r a l forms must be studied before the bridge-angle e f f e c t on 2J can be separated from the b r i d g i n g bond-length e f f e c t . 2

i s

t n e

I I I . The Polymeric Compound [Cu(pyrazine)(N0q)?] Chains.

n

and

Related

The magnetic p r o p e r t i e s of the 1:1 copper(II) n i t r a t e pyrazine complex, [ C u i C i ^ H ^ ) ( ^ 3 ) 2 ] , r e f l e c t an exchange coupl i n g between the copper ions although as shown i n Figure 14 the copper(II) ions are separated by 6.712 A.(50) While exchange coupling across bidentate h e t e r o c y c l i c amine l i g a n d s had been suggested p r e v i o u s l y , (51) the p r e l i m i n a r y i n v e s t i g a t i o n (52) provided the f i r s t demonstration of an antiferromagnetic i n t e r a c t i o n i n a system which has been c h a r a c t e r i z e d by s t r u c t u r a l s t u d i e s , magnetic measurements c o l l e c t e d over a wide temperature range and EPR measurements. The c r y s t a l s t r u c t u r e of t h i s compound r e v e a l s a chemical chain p a r a l l e l to the a a x i s i n the orthorhombic c r y s t a l . Copper atoms are bridged along t h i s a x i s by the aromatic h e t e r o c y c l i c bidentate amine, pyrazine. Coordinated oxygen atoms from the n i t r a t e ions complete the h i g h l y d i s t o r t e d octahedron around each copper atom. The magnetic s u s c e p t i b i l i t y data c o l l e c t e d using a powdered n


10.

HATFIELD

Superexchange

133

Interactions


800h

0

10

30

50

100 200 3 0 0

T e m p e r a t u r e , °K Figure 12. Temperature variation imental inverse susceptibility of pyridine) Clt~\ t

of the exper [Cu(2-methyl-

g

Ν (CI)

Inorganic Chemistry

Figure 13. Comparison of the structural parameters for ninium)CuCl ] · 2H 0 and [Cu Cl ywith the solid line senting the guaninium complex (40) 3 2

£

â

8


[(guarepre-

134

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INTERACTIONS

BETWEEN

METAL

IONS

sample were d e s c r i b e d (52) by the I s i n g model f o r l i n e a r a n t i ferromagnetic i n t e r a c t i o n s i n chains using equations (3a,3b) which was developed by F i s h e r . (53) The equations are 2 2 XJL - ^ f j — [tanh ( φ Η φ sech (^)] (3a) 2

2

and 2 2 Downloaded by NORTH CAROLINA STATE UNIV on November 24, 2012 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/bk-1974-0005.ch010

XII - ^ f i J —

exp

(2J/kT)

(3b)

where 1 s

^

2

3 XII +3

(4)

XI

The parameters which g i v e the best f i t to the experimental data are = 2.22 and J • -6.04 cm" . The best f i t value i s to be compared with the EPR r e s u l t s of Kokosζka and Reimann, (54) who reported g = 2.295, g = 2.054, and g = 2.070 ( = 2.133). In an attempt to determine more p r e c i s e l y the magnetic model which i s a p p r o p r i a t e f o r the d e s c r i p t i o n of the p r o p e r t i e s of t h i s compound and to c l a r i f y the exchange pathway, measurements have been performed on s i n g l e c r y s t a l s of t h i s m a t e r i a l between 1.7° and 60°K. (55) Reasonably l a r g e needle-shaped s i n g l e c r y s t a l s of Cu(pyrazine)(N03)2 were grown by slow-evaporation of an aqueous 1:1 s o l u t i o n of copper (II) n i t r a t e and pyrazine (Ci+Hi^). Because of the c r y s t a l morphology s e v e r a l of the l a r g e s t c r y s t a l s were s e l e c t e d and c a r e f u l l y o r i e n t e d under a microscope so that a l l a axes of the c r y s t a l s were c o l l i n e a r . The s u s c e p t i b i l i t i e s from 1.7° to 60°K both p a r a l l e l and perpendicular to the & a x i s are shown i n F i g u r e 15. A c l e a r maximum i s observed at approximately 6.8°K i n each d i r e c t i o n . These measurements r e v e a l t y p i c a l i s o l a t e d antiferromagnetic l i n e a r chain behavior down to the lowest temperature achieved i n t h i s experiment. The I s i n g chain equations can not be used to f i t these data. Bonner and F i s h e r (56) have performed machine c a l c u l a t i o n s on f i n i t e Heisenberg r i n g s and have been able to estimate the l i m i t i n g behavior f o r an i n f i n i t e r i n g . Applying these r e s u l t s to the experimental data i n both d i r e c t i o n s J/k equals -5.30 (±0.05) °K where J i s as d e f i n e d by the term -2JS-j/Sj i n the Hamiltonian described by Bonner and F i s h e r . The g value g i v i n g the best f i t to theory f o r the perpendicular d i r e c t i o n was 2.10 (±0.01) and 2.03 (±0.01) f o r the p a r a l l e l d i r e c t i o n . As can be seen i n F i g u r e 15, the f i t f o r both d i r e c t i o n s down to 1.7°K i s i n q u i t e good agree ment with experiment. Commonly "4+2" t e t r a g o n a l l y d i s t o r t e d copper complexes e x h i b i t two small g values ( g ) and one l a r g e g value (S, J)· (57) I f i t i s assumed that the smallest g value l i e s along the a^ a x i s , corresponding to the s h o r t e s t bond d i s t a n c e , then one of the other small g values and the l a r g e s t g value l i e i n the plane perpendicular to t h i s a x i s . The s u s c e p t i b i l i t y perpendicular 1

z

x

y



10.

HATFIELD

Superexchange

135

Interactions

Journal of the American Chemical Society

Figure

14.

Structure

(52)

of [CufC H N XN0 ) ']n 4

h

i

8

i

Tempera tu re *K

Journal of Chemical Physics

Figure 15. Best fit of the corrected magnetic susceptibility to the one-dimensional Heisenberg model with T/k = -5.30° for both directions and g == 2.03 and gj_ — 2.10 (55) n


136

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INTERACTIONS

BETWEEN

IONS

to the a a x i s should then r e f l e c t a g value intermediate between the two extremes detected by the EPR measurements i f one assumes a random o r i e n t a t i o n of the b and c axes. T h i s i s c o n s i s t e n t with the experimental o b s e r v a t i o n . The pathway of the exchange i n t e r a c t i o n must now be i d e n t i f i e d . N i t r a t e ions bridge copper(II) ions i n Cu(N0 ) *2.5H 0 forming a crooked c h a i n . Even though t h i s chain i s present the magnetic s u s c e p t i b i l i t y , which d i s p l a y s a rounded maximum at 3.2 °K, has been adequately d e s c r i b e d as a r i s i n g from antiferromagn e t i c a l l y exchange coupled p a i r s (58,59) with the predominant mode of exchange thought to occur between the chemical chains.(60) The s h o r t e s t Cu-Cu s e p a r a t i o n (^4.7 A) i s between copper atoms i n the crooked c h a i n . Much weaker a n t i f err omagne t i c interdimer ex change e f f e c t s were i n c l u d e d to improve the f i t at low tempera t u r e s . As shown i n F i g u r e 16 a p o s s i b i l i t y f o r t h i s same n i t r a t e b r i d g e e x i s t s i n the b-c glane i n Cu(pyrazine)(N03)2 where the Cu-Cu s e p a r a t i o n i s 5.1 A. Because of these s t r u c t u r a l s i m i l a r i t i e s dimeric s u s c e p t i b i l i t y behavior was explored i n Cu(pyrazine)(Νθ3)2· When the dimer s u s c e p t i b i l i t y equation was used to f i t the data a value of J/k » -5.4°K r e s u l t s when the g v a l u e was r e s t r i c t e d to the range 2.00 to 2.20. However, the c a l c u l a t e d s u s c e p t i b i l i t i e s are c o n s i s t e n t l y lower than the experimental values with the d i f f e r e n c e i n c r e a s i n g as the temperature de creases. 3


METAL

2

2

Although an exchange pathway through the n i t r a t e i o n cannot be completely r u l e d out i n Cu(pyrazine)(N03)2> the c l e a r l i n e a r c h a i n behavior i n t h i s compound as compared to the p a i r i n t e r a c t i o n i n Cu(N03)2·2.5H20 argues s t r o n g l y a g a i n s t t h i s pathway f o r exchange. The r e s u l t s with the s u b s t i t u t e d p y r i d i n e complexes to be d e s c r i b e d below o f f e r s convincing evidence against t h i s pathway, however, before that can be presented the p o s t u l a t e d mechanism of the exchange i n t e r a c t i o n through the pyrazine bridge w i l l be d e s c r i b e d . The p y r a z i n e r i n g i s p e r p e n d i c u l a r to the plane i n which the b i s ( n i t r a t o ) c o p p e r ( I I ) u n i t s l i e s , and the unpaired e l e c t r o n , i n the s i n g l e i o n approximation, i s i n the d 2 . 2 o r b i t a l . Now i t develops that the pyrazine r i n g i s canted wïth respect to the xy plane such that the highest energy occup i e d molecular o r b i t a l , the B^ π o r b i t a l , has the proper symme t r y to overlap with the d 2 „ 2 o r b i t a l . I t i s suggested here that the exchange i s propagated i n t h i s manner. To t e s t t h i s mechanism complexes have been prepared with s u b s t i t u t e d pyrazines with the expectation that the exchange c o u p l i n g would be a f f e c t e d by the e l e c t r o n i c nature of the s u b s t i t u e n t on the p y r a z i n e r i n g . Ex change by means of the a l t e r n a t e pathway through the n i t r a t e " b r i d g e " would be dependent on the s t e r i c p r o p e r t i e s of the sub s t i t u e n t and only s e c o n d a r i l y dependent on the e l e c t r o n i c nature. y

x

The experimental data which have been c o l l e c t e d (61,62) are tabulated i n Table IV. The s i m i l a r i t y of the s p e c t r a an3"tne g values can be taken as good evidence that the s t r u c t u r e s of the complexes are the same as that of the pyrazine complex. As can be



HATFIELD

Superexchange

Interactions

c

Journal of Chemical Physics

Figure 16. A view of Cu(pyrazineXNO$) in the be plane which illustrates the weak nitrate bridge between copper atoms. For clarity only copper atoms lying at (Oil) have been included (55). 2


138

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BETWEEN

METAL

IONS

seen i n Table IV there i s a s t r i k i n g c o r r e l a t i o n of the l i g a n d π«-ΗΓ* t r a n s i t i o n with 2J.There i s a l s o a c o r r e l a t i o n between 2J Table IV S p e c t r a l and Magnetic Data f o r [Cu(R-pyrazine) ( N C O J ligand


LIGAND

ΤΓ—TT* , cm

^ 2J,cm" -9.0

1

2.15

complex d-d,cm" π -KJ*cm~ 18,500 29,000

quinoxaline

42,300

pyrazine

38,460

-7.2

2.133

17,860

34,600

methyl pyrazine

37,900

-6.2

2.145

17,900

35,100

chloro pyrazine

33,700

-2.8

2.153

17,800

36,400

and the charge t r a n s f e r band i n the complex which i s most l i k e l y 7r(ligand)-*J*(metal). Since there i s such a good c o r r e l a t i o n of 2J with the e l e c t r o n i c p r o p e r t i e s of the l i g a n d , and none at a l l with the s i z e of the s u b s t i t u e n t (which should separate the chains thereby a f f e c t i n g exchange through the n i t r a t e i o n ) , i t seems c o n c l u s i v e that the exchange pathway i s v i a the pyrazine bridge. IV. Acknowledgements T h i s research has been supported by the N a t i o n a l Science Foundation through grant number GP-22887 and by the M a t e r i a l s Research Center of the U n i v e r s i t y of North C a r o l i n a through grant number GH-33632 from the N a t i o n a l Science Foundation. I am g r a t e f u l f o r t h i s c o n t i n u i n g support. T h i s research e f f o r t has b e n e f i t e d g r e a t l y from c o l l a b o r a t i o n with P r o f e s s o r D.J. Hodgson and from the work of s e v e r a l i n d u s t r i o u s graduate students and research a s s o c i a t e s , many of whom are named i n the r e f e r e n c e s . T h e i r c o n t r i b u t i o n to t h i s program has been i n v a l u a b l e . V. L i t e r a t u r e C i t e d 1. For reviews see a. R.L. M a r t i n in New Pathways in Inorganic Chemistry,Edited by E.A.V. Ebsworth, A.G. Maddock, and A.G. Sharpe, Cambridge U n i v e r s i t y Press, 1968. b. E. Sinn, Coordn. Chem. Rev., 5., 313 (1970). c. G.F. Kokoszka and G. Gordon i n T r a n s i t i o n Metal Chemistry V o l . 5, E d i t e d by R.L. C a r l i n , Marcel Dekker, Inc., New York, 1969. d. J.B. Goodenough,Magnetism and the Chemical Bond, I n t e r s c i e n c e P u b l i s h e r s , Inc., New York, 1963.



10.

HATFIELD

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Interactions

139

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140

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

31. E.D. Estes, p r i v a t e communication. 32. See, f o r example, C. Chow, K. Chang, and R.D. W i l l e t t , J . Chem. Phys., 59, 2629 (1973). 33. K.T. McGregor and W.E. H a t f i e l d , to be p u b l i s h e d . 34. D.J. Hodgson, Progress in Inorganic Chemistry, E d i t e d by S.J. L i p p a r d , W i l e y - I n t e r s c i e n c e , New York, in p r e s s . 35. D.J. Hodgson, P.K. Hale, J.A. Barnes, and W.E. H a t f i e l d , Chem. Comm., 786 (1970). 36. D.J. Hodgson, P.K. Hale, and W.E. H a t f i e l d , Inorg. Chem., 10, 1061 (1971). 37. J.A. Barnes, D.J. Hodgson, and W.E. H a t f i e l d , Chem. Phys. Letters, 7, 374 (1970). 38. K.T. McGregor, D.B. Losee, D.J. Hodgson, and W.E. H a t f i e l d , Inorg. Chem., 13, 756 (1974). 39. a. J.A. Carrabine and M. Sundaralingam, J . Amer. Chem. Soc., 92, 369 (1970);b.M. Sundaralingam and J.A. Carrabine, J. Molec. Biol., 61, 287 (1971); c. J.P. D e c l e r c q , M. Debbaudt, and M. Van Meerssche, B u l l . Soc. Chim. Belges, 80, 527 (1971). 40. R.F. Drake, V.H. Crawford, N.W. Laney, and W.E. H a t f i e l d , Inorg. Chem.,13, 1246 (1974). 41. V.F. Duckworth and N.C. Stephenson, A c t a C r y s t a l l o g r . , Sect. B, 25, 1795 (1969). 42. D.Y. J e t e r , D.J. Hodgson, and W.E. H a t f i e l d , Inorg. Chim. Acta, 5, 257 (1971). 43. D.H. Svedung, Acta Chem. Scand., 23, 2865 (1969). 44. N.T. Watkins, E.E. Dixon, V.H. Crawford, K.T. McGregor, and W.E. H a t f i e l d , Chem. Commun., 133 (1973). 45. W.E. H a t f i e l d and T.S. P i p e r , unpublished o b s e r v a t i o n s . 46. J.S. Smart, E f f e c t i v e F i e l d Theories of Magnetism, W.B. Saunders Co., P h i l a d e l p h i a , 1966. 47. For example, see J.A. Bertrand, A.P. Ginsberg, R.I. Kaplan, G.E. Kirkwood, R.L. M a r t i n , and R.C. Sherwood, Inorg. Chem., 10, 240 (1971). 48. W.E. H a t f i e l d and R. Whyman, T r a n s i t i o n Metal Chemistry, E d i t e d by R.L. C a r b i n , Marcel Dekker, Inc., New York, V o l . 5, p. 53ff (1969). 49. D.P. Graddon, R. Schulz, E.C. Watton, and D.G. Weeden, Nature, 198, 1299 (1963). 50. A. Santoro, A.D. M i g h e l l , and C.W. Reimann, A c t a C r y s t . , B26, 979 (1970). 51. See f o r example, D.E. Billing, A.E. U n d e r h i l l , D.M. Adams, and D.M. M o r r i s , J. Chem. Soc. (A), 902 (1966); M.J.M. Campbell, R. Grzeskowiak, and F.B. T a y l o r , ibid., 19 (1970). 52. J.F. Villa and W.E. H a t f i e l d , J . Amer. Chem. Soc., 93, 4081 (1971). 53. M.E. F i s h e r , J. Math Phys., 4, 124 (1963). 54. G.F. Kokoszka and C.W. Reimann,J. Inorg. Nucl. Chem., 32, 3229 (1970). 55. D.B. Losee, H.W. Richardson, and W.E. H a t f i e l d , J . Chem. Phys., 59, 3600 (1973).



10.

HATFIELD

Superexchange

Interactions

141

56. J. Bonner and M. F i s h e r , Phys. Rev. Sect. A, 135, 640 (1964). 57. E. König Magnetic P r o p e r t i e s of T r a n s i t i o n Metal Compounds., S p r i n g e r - V e r l a g , B e r l i n , 1966. 58. L. Berger, S. F r i e d b e r g , and J. Schriempf, Phys. Rev., 132, 1057 (1963). 59. B. Myers, L. Berger, and S. F r i e d b e r g , J. A p p l . Phys., 40, 1149 (1969). 60. J. Bonner, S. F r i e d b e r g , H. Kobayashi, and B. Myers, Proc. 12th I n t e r n a t i o n a l Conf. on Low Temp. P h y s i c s , Kyoto (1970). 61. H.W . Richardson and W.E. H a t f i e l d , t o be p u b l i s h e d . 62. H.W. Richardson, W.E. H a t f i e l d , H.J. S t o k l o s a , and J.R. Wasson, Inorg. Chem., 12, 2051 (1973).


Superexchange Interactions in Copper(II) Complexes - American

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