Control of Redox Potentials in Mononuclear and Dinuclear Copper

5 Control of Redox Potentials in Mononuclear and Dinuclear Copper Cryptates J. P. GISSELBRECHT and M. GROSS

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0201.ch005

Université Louis Pasteur, Laboratoire d'Electrochimie et de Chimie Physique du Corps Solide, Equipe de Recherches Associée au CNRS (n° 468), Institut Le Bel, 67000 Strasbourg, France

The electrochemical behavior of mononuclear and dinuclear macrocyclic copper complexes was studied on solid electrodes in water and in organic media, and the factors controlling the redox properties of the complexes were identified. In mononuclear complexes, the formal redox potential of the copper(II)/copper(I) system ranged from -0.10 to +0.49 V vs. SCE in water. This potential was shifted to more positive values by introducing thioether groups in the macrocycle and by increasing the size of the N-substituents. In dinuclear copper complexes, symmetrical cryptates exhibited a single, reversible dielectronic interconversion between the dicopper(II) and the dicopper(I) cryptate. On the other hand, one nonsymmetrical cryptate exhibited two successive and distinct monoelectronic reduction steps, as a consequence of the large difference between the two coordination sites in the ligand. Incremental stabilization of copper(I) relative to copper(II) was observed with changes in chemical composition of the ligand and coordination geometry of the copper ions for all dinuclear macrocyclic copper complexes studied.

T n r e c e n t y e a r s , s t r o n g efforts w e r e d i r e c t e d t o w a r d t h e u n d e r s t a n d i n g A

a n d r e p r o d u c i b i l i t y o f experimental data d e a l i n g w i t h the p h y s i c o c h e m i c a l properties o f copper i n copper proteins a n d enzymes through appropriate synthetic coordinating ligands ( J , 2). O f s p e c i a l relevance i n this context are the spectral a n d redox properties o f the copper(II)/copper(I) system i n natural molecules, w h i c h a r e c h a r a c t e r i z e d b y r e v e r s i b l e e l e c t r o n transfers h a v i n g f o r m a l 0065-2593/82/0201-0109$08.25/0 © 1982 A m e r i c a n C h e m i c a l Society Kadish; Electrochemical and Spectrochemical Studies of Biological Redox Components Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

110

BO ILOGC IAL REDOX COMPONENTS

p o t e n t i a l s r a n g i n g f r o m + 0 . 2 to + 0 . 8 V v s . N H E . E l e c t r o c h e m i c a l results on synthetic m o d e l s ( 3 - 6 ) gave clear-cut e v i d e n c e that appro p r i a t e d e s i g n o f l i g a n d s c o o r d i n a t e d t o c o p p e r m a y h a v e m a r k e d ef fects o n the r e d o x p o t e n t i a l o f the copper(II)/copper(I) c o u p l e a n d o n the spectral properties o f the c u p r i c c o m p l e x . S y n t h e t i c m o n o n u c l e a r c h e l a t e s w e r e d e s c r i b e d (6) w h o s e r e d o x p r o p e r t i e s ( o n e - e l e c t r o n r e v e r s i b l e t r a n s f e r at m a r k e d l y p o s i t i v e p o t e n t i a l s ) a n d s p e c t r a l c h a r a c teristics (intense b a n d a r o u n d 600 nm) m i m i c those o f b l u e m o n o n u clear copper proteins (type 1 copper). R e c e n t l y (7), e x p e r i m e n t a l r e p r o d u c i b i l i t y o f the spectral a n d redox properties o f type 3 copper were a c h i e v e d i n a m o d e l , although l o w - m o l e c u l a r weight copper complexes, w h i c h reproduce separately some properties of type 3 copper, were previously reported (8-18). In t h i s c h a p t e r w e p r e s e n t a s y s t e m a t i c s t u d y o f t h e effects o f t y p i c a l m a c r o c y c l i c l i g a n d characteristics o n the r e d o x b e h a v i o r o f the copper(II)/copper(I) c o u p l e , i n m o n o n u c l e a r a n d i n d i n u c l e a r chelates. M o n o c y c l i c , bicyclic, and tricyclic ligands were used; their typical p r o p e r t i e s w e r e p r e s e n t e d e l s e w h e r e (19).


1

Experimental A l l measurements were carried out at 2 5 ° C u s i n g p l a t i n u m , glassy carbon, or g o l d electrodes. Unless otherwise noted, a l l reported measurements were taken u s i n g p l a t i n u m electrodes. W i t h some complexes the current-voltage curves were distorted on p l a t i n u m due to adsorption effects a n d i n these cases either g o l d or glassy carbon electrodes were u t i l i z e d . These electrodes s h o w e d less surface effects, and thus better defined current-voltage curves c o u l d be observed. N o differences i n potentials were observed between the three types of electrode material. These three electrodes were also used as rotating disk electrodes ( R D E ) w i t h rotation rates from 250 to 5000 r e v / m i n . T h e measure ments were performed w i t h an electrochemical d e v i c e consisting of a potentiostat ( S O L E A Tacussel P R T 20 X ) , a voltage pilot unit ( S O L E A Tacussel G S T P 2), a current-potential converter ( S O L E A Tacussel A D T P 1), a n d a potentiometric X Y - r e c o r d e r ( I F E L E C I F 3802). T h i s d e v i c e also was used for c y c l i c voltammetric measurements at scan rates up to 1 V / s . ; for higher scan rates, the signal was stored i n a two-channel transient recorder ( B R Y A N S 512 A ) , a n d afterwards plotted on the X Y - p o t e n t i o m e t r i c recorder. Potentiostatic coulometry p r o v i d e d the amount o f Faradays exchanged per m o l e o f cryptate. T h r o u g h o u t the measurements, a c a l o m e l electrode was used as reference electrode, i n a saturated aqueous solution o f potassium chloride ( S C E ) . T h i s electrode was connected electrically to the studied solution b y a b r i d g e filled w i t h the solvent plus the background electrolyte used i n the c e l l . E x p e r i m e n t s were performed i n propylene carbonate ( P C ) c o n t a i n i n g 0.1 M tetraethylammonium perchlorate ( T E A P ) , d i m e t h y l sulfoxide ( D M S O ) con t a i n i n g 0.1 M T E A P , i n 0.1 M KC1 aqueous solutions for those complexes that were soluble i n water. 1

These characteristics of type 3 copper are (a) Cu(II)/Cu(I) formal redox potential about 0.4 to 0.5 V vs. NHE, (b) reversible two-electron transfer, (c) very large antiferromagnetic coupling constant and lack of EPR signal in both the oxidized and reduced states of the complex, (d) electronic absorption band near 330 nm.

Kadish; Electrochemical and Spectrochemical Studies of Biological Redox Components Advances in Chemistry; American Chemical Society: Washington, DC, 1982.

5.

GissELBRECHT

AND GROSS

111

Copper Cryptâtes

We investigated e l e v e n mononuclear complexes w i t h L i g a n d s l a - 5 a n d seven d i n u c l e a r complexes w i t h L i g a n d s 6 - 1 2 . T h e syntheses of these c o m plexes was reported p r e v i o u s l y (16, 20, 21).


N / ' X

V

^ X

>

/ N

X-N

N-X

la: X = Y = 0

2a:

lb: X = 0 ; Y = S

2b: X = C H

lc: X = Y = S

2c: X = C H C H

Χ—Ν

5

Ν—Χ

4a: Χ = Η 4b: Χ = C H

3

NH

5

e

Χ—Ν

3a: Χ = Η

ΗΝ

3

2

Ν—Χ

3b: X = C H

X=H

ΗΝ

3

NH

6



η οο

ο ο

o

Ο ss

ϋ

U 10

ss

u 11

.s s

^ 12


y

5.

GissELBRECHT

AND GROSS

113

Copper Cryptâtes

Mononuclear Copper Chelates P h y s i c o c h e m i c a l data o n the copper(II) c o m p l e x e s w i t h l a - 5 have b e e n v e r y scarce. S e v e r a l stability constants w e r e m e a s u r e d a n d indicate a h i g h stability o f the o x i d i z e d form i n solution. Also, structural analysis o f the c u p r i c c o m p l e x w i t h 5 r e v e a l e d that c o p p e r ( I I ) is o c t a h e d r a l l y c o o r d i n a t e d w i t h f o u r o u t o f t h e s i x b o n d s b e l o n g i n g t o t h e m a c r o c y c l e 5 (24, 25), w h e r e a s a n a p p r o x i m a t e l y t e t r a g o n a l p y r a m i d is o b s e r v e d i n t h e c u p r i c c o m p l e x w i t h 2 f r o m t h e k n o w n structure (J 7) o f the c o r r e s p o n d i n g d i n u c l e a r c o m p l e x w i t h 10, w h i c h contains t w o subunits o f m a c r o c y c l e 2. Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0201.ch005

2 , 3

Results Redox measurements on the copper complexes w i t h l a - 5 (Table I) r e v e a l e d t h a t a l l o f t h e c o p p e r ( I I ) c o m p l e x e s — e x c e p t for t h o s e w i t h l a a n d 5 — w e r e r e d u c e d at a R D E i n t w o s i n g l e , o n e - e l e c t r o n s t e p s . T h e first s t e p w a s t h e r e v e r s i b l e r e d u c t i o n f r o m a c o p p e r ( I I ) t o a c o p p e r ( I ) c o m p l e x ; t h e s e c o n d s t e p , at m o r e n e g a t i v e p o t e n t i a l s , w a s t h e r e d u c t i o n f r o m a c o p p e r ( I ) c o m p l e x t o n o n v a l e n t c o p p e r a n d free u n a l t e r e d l i g a n d i n s o l u t i o n . F i g u r e 1 is g i v e n as a n e x a m p l e d o c u m e n t i n g t h e t w o o n e - e l e c t r o n t r a n s f e r r e d u c t i o n m e c h a n i s m for t h e c u p r i c c o m p l e x w i t h 3 a a n d also illustrates the important difference b e t w e e n t h e p o t e n t i a l s o f t h e t w o r e d u c t i o n s t e p s . F i g u r e 2 , for t h e s a m e c o m plex, shows clearly the reversibility o f the c u p r i c - c u p r o u s system. T h e o b t a i n i n g o f a n o n v a l e n t c o p p e r after t h e s e c o n d s t e p w a s a s c e r t a i n e d b y a copper deposit o n the p l a t i n u m R D E . T h e second step [ C u ( I ) —> C u ( 0 ) ] m a y b e o b s e r v e d o n l y i n s o l v e n t s t h a t h a v e a l a r g e c a t h o d i c e l e c t r o a c t i v i t y r a n g e , s u c h as P C . I n c o n t r a s t , i n P C t h e c o p per(II) c o m p l e x e s w i t h l a a n d 5 w e r e r e d u c e d to n o n v a l e n t c o p p e r a n d free l i g a n d i n a s i n g l e t w o - e l e c t r o n s t e p [ w i t h l a , C u ( I I ) r e d u c e s to C u ( 0 ) at - 1 . 7 V v s . S C E , a n d w i t h 5 at - 0 . 9 5 V v s . S C E ] . Stationary V o l t a m m e t r y . F o r those c u p r i c c o m p l e x e s r e d u c e d i n t w o distinct one-electron steps (Table I), the stationary v o l t a m m e t r y c u r v e s at a R D E i n d i c a t e d t h a t t h e C u ( I I ) —» C u ( I ) r e d u c t i o n i s d i f f u s i o n c o n t r o l l e d . T h i s a n a l y s i s is b a s e d o n b o t h t h e p r o p o r t i o n a l i t y b e t w e e n t h e c a t h o d i c l i m i t i n g c u r r e n t (Iu ) a n d t h e a n a l y t i c a l c o n c e n t r a t i o n o f t h e c o m p l e x as w e l l as f r o m L e v i c h ' s (26) l i n e a r p l o t o f l / i n = / ( 1 / ω ) . L o g a r i t h m i c analysis [log ( / / / „ „ - / ) = / ( £ ) ] o f the C u ( I I ) -> C u ( I ) c a t h o d i c curves (Tables I I a n d III) c o r r e s p o n d e d to r e v e r s i b l e or quasi-reversible processes. B e c a u s e the s t u d i e d solutions d i d not c o n m

m

1 / 2

2

Log Κ [Cu(II)-L] in H O + 0.1 M TEAP 25°C with ligands la (6.81) and 5 (6.18) z

(22). 3

Log Κ [Cu(II)-L] in H 0 + 0.1 M TEAP 25°C with ligands 2a (8.44), 2b (12.75), 3a (13.04) (23). 2


114

BO ILOGC IAL REDOX COMPONENTS Table I.

Redox Measurements on Mononuclear Copper Complexes

Ligand

log K (

la, lb lc 2a, 2b, 2c

C l < ( / / )

6.81

8.44 12.75 13.04 — 6.18

_

Reduction steps

a L )

b

o n e , di-e two, mono-e t w o , mono-e

c


c

t w o , mono-e t w o , mono-e t w o , mono-e o n e , di-e

c

3a, 3b 4a, 4b 5

ft

a

Copper complexation with ligand L is represented by C u - L . Experiments were performed in water plus 0.1 M TEAP at 25°C. Log Κ [Cu(II)-L] in H 0 + 0.1 M TEAP 25°C with ligands la (6.81) and 5 (6.18) (22). Log Κ [Cu(II)-L] in H 0 + 0.1M TEAP 25°C with ligands 2a (8.44), 2b (12.75), and 3a (13.04) (23). b

2

c

2

—

-

CuUcu

1

• Ε V/SCE

0.5

0

- Q 5

-1

-1.5

"2

Figure 1. The mono electronic reduction waves of the Cu(II) com plex with 3a: in PC + 0.1 M TEAP, on a platinum RDE (2000 revlmin), [Cu(Il)J = 1.9 10- M . 4


Copper


5. GS ISELBRECHT AND GROSS

Cryptâtes

115

Figure 2. Cyclic voltammogram of the first reduction step [Cu(II) —* Cu(I)] in Cu(II)-3a (ο = 1.9·10~ M). Electrode: Pt Solution: PC + 0.1 M TEAP. Scan rates: 1,10 mV/s; 2, 20 mV/s; 3, 50 mV/s. 4

t a i n free l i g a n d i n e x c e s s [ a l l s o l u t i o n s w e r e p r e p a r e d f r o m c r y s t a l l i z e d copper(II) c o m p l e x e s o f s t o i c h i o m e t r y 1 : 1 ] , the o b s e r v e d l i n e a r i t y o f t h e p l o t s [ l o g (I/Inm ~I) = / ( £ ) ] c o n f i r m e d t h a t t h e n u m b e r o f l i g a n d s c o o r d i n a t e d to c o p p e r r e m a i n e d c o n s t a n t t h r o u g h o u t t h e r e d u c t i o n ( 2 7 ) . T h u s , t h e r e d u c t i o n p r o d u c t o f c o p p e r ( I I ) is a l w a y s a c o p p e r ( I ) c o m p l e x o f i d e n t i c a l s t o i c h i o m e t r y . O n the other h a n d , further r e d u c t i o n o f t h e c o m p l e x f r o m c o p p e r ( I ) to c o p p e r ( O ) m a y b e c a r r i e d o u t o n l y at r a t h e r n e g a t i v e p o t e n t i a l , i n d i c a t i v e o f t h e h i g h s t a b i l i t y o f t h e c o p p e r ( I ) c o m p l e x g e n e r a t e d b y t h e first r e d u c t i o n s t e p . C y c l i c V o l t a m m e t r y . M e a s u r e m e n t s o n t h e first r e d u c t i o n s t e p [ C u ( I I ) —» C u ( I ) ] p r o v i d e c l e a r - c u t i n f o r m a t i o n o n t h e r e v e r s i b i l i t y o f the e l e c t r o d e processes (Tables I I a n d III). S o m e c o m p l e x e s are re d u c e d r e v e r s i b l y as i n d i c a t e d b y a n i n v a r i a n t a n d c l o s e t o 5 8 - m V p e a k separation Δ £ ( Δ Ε = E - E ) at l o w s c a n rates (v < 0.1 V / s ) . F o r o t h e r c o m p l e x e s , o n l y q u a s i - r e v e r s i b l e r e d u c t i o n o c c u r s , w h i c h is a l s o ρ

Ρ

Pa

Pc



-0.10 -0.02 -+0.14 +0.14 -0.10 +0.28 (composite) -+0.18 + 0.49 (composite)

m

E Cu(II)/Cu(I) (V vs. SCE) a

m

— 65

66 90

— 56

61 58

-0.50

— — — — —

-0.67

Slope (mV) E Cu(I)/Cu(0) [ E vs. log (I/l ~ I)] (V vs. SCE)

RDE Ρ

+0.145 -0.10 +0.31

70 66 100

+0.49

-0.10 -0.025

— 65

0 + 0.1 M K C 1

-+0.42 +0.73

+0.14 +0.22 -+0.38 +0.38 +0.14 +0.55

E ° ' C u ( I I ) / C u ( I ) Ε Cu(II)/Cu(I) (V vs. NHE) (V vs. S C E )

70 70

a t h

Δ Ε = Eg" - E£ (mV)

2

Cyclic Voltammetry

R e d o x Characteristics o f the Cu(II)/Cu(I) C o u p l e C o m p l e x e d i n L i g a n d s 1 to 4, i n H

Note: RDE experiments and cyclic voltammograms were carried out on Pt.

lb lc

2a 2b 2c 3b 4a 4b

Ligand

Table II.


Copper

Cryptâtes

GS ISELBRECHT AND GROSS ο

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118


seen from stationary curves on the R D E [Tables I I a n d I I I , c o m p l e x e s w i t h L i g a n d 4b i n water a n d L i g a n d s 4b and l c i n P C ] .T h e E ° ' values given i n Tables II and III were calculated from E and E [(E + E ) / 2 ] a n d a r e a l m o s t i d e n t i c a l to t h e E o b t a i n e d at a R D E . T h i s c a l c u l a t i o n a l s o w a s u s e d for t h e r e d u c t i o n s t e p s t h a t w e r e a l m o s t reversible [Tables II a n d III, L i g a n d s 4b a n d l c ] . Pa

P c

Pc

P a

1/2

F o r a l l o f t h e c o m p l e x e s s t u d i e d ( e x c e p t for C u ( I I ) i n 2 c a n d l b i n a q u e o u s s o l u t i o n s ) t h e h e t e r o g e n e o u s rate c o n s t a n t s w e r e e s t i m a t e d at t h e f o r m a l p o t e n t i a l E°' C u ( I I ) / C u ( I ) (28 ); a l l c a l c u l a t e d k v a l u e s r a n g e d f r o m 1 0 " t o 1 0 " c m / s a n d n o s i g n i f i c a n t effect w a s d e t e c t e d from t h e n i t r o g e n substituents nor f r o m the solvents ( H 0 or P C ) . P e c u l i a r B e h a v i o r O b s e r v e d for S o m e C r y p t â t e s . REDUCTION O F COPPER(II) IN COMPOUNDS 2 c , 4 b , A N D l c IN W A T E R . I n w a t e r , t h e c o m p l e x o f copper(II) w i t h 2c does not s h o w any w a v e b y stationary v o l t a m m e t r y o n a R D E . I n s t e a d , t h e v o l t a m m e t r i c c u r v e s are p e a k s h a p e d , w h i c h m a y b e a s c r i b e d to a p a s s i v a t i n g film g e n e r a t e d at t h e e l e c t r o d e surface d u r i n g the r e d u c t i o n process. Therefore, the g i v e n r e d u c t i o n p o t e n t i a l ( + 0 . 1 4 V v s . S C E ) is o n l y a p p r o x i m a t e . F u r t h e r a n o d i c p o l a r i z a t i o n o f t h e e l e c t r o d e at + 0 . 2 5 V v s . S C E e l i m i n a t e s t h e film. O n t h e o t h e r h a n d , for s o l u t i o n s o f 1 : 1 c u p r i c c o m p l e x e s w i t h 4 b a n d l c , m i x e d a n o d i c - c a t h o d i c w a v e s are o b s e r v e d for 4 b a n d e n t i r e l y a n o d i c w a v e s for l c , t h u s i n d i c a t i n g t h a t a s i g n i f i c a n t ( w i t h 4 b ) o r quantitative (with l c ) spontaneous homogeneous redox reaction oc c u r s , w h i c h c o n v e r t s a c o p p e r ( I I ) c o m p l e x to c o p p e r ( I ) c o m p l e x i n these solutions. 3

2


2

PC.

REDUCTION O F COPPER(II) IN COMPOUNDS 4 a , 4 b , A N D l c IN I n P C , r e s u l t s w e r e o b t a i n e d for t h e r e d u c t i o n o f c o p p e r ( I I )

c o m p l e x e s w i t h 4 b a n d l c [ w h i c h is s p o n t a n e o u s l y r e d u c e d c h e m i c a l l y to C u ( I ) c o m p l e x e s , see f o l l o w i n g s e c t i o n ] t h a t w e r e v e r y s i m i l a r to t h o s e j u s t r e p o r t e d for t h e s a m e c o m p l e x e s i n w a t e r . F o r c o p p e r ( I I ) c o m p l e x e s w i t h 4a a n d 4b (Table III), stationary v o l t a m m o g r a m s o n R D E , a p p e a r e d less r e v e r s i b l e t h a n c y c l i c v o l t a m m o g r a m s . T h i s d r a w n - o u t c h a r a c t e r o f t h e s t a t i o n a r y c u r v e s m a y l i k e l y b e a s c r i b e d to a s l o w c h e m i c a l process o c c u r r i n g i n the solution o f c o m p l e x e s i n v o l v i n g 4 b a n d 4a; on the shorter t i m e scale o f c y c l i c v o l t a m m e t r y , these processes m a y b e p r e v e n t e d from i n t e r f e r i n g w i t h the e l e c t r o n transfer kinetics. T h u s , w h e n c r y p t a t e d i n l c o r 4 b , d i v a l e n t c o p p e r is s p o n t a n e o u s l y r e d u c e d to t h e m o n o v a l e n t c o m p l e x i n w a t e r as w e l l as i n P C . A l s o , after c o r r e c t i o n for t h e l i q u i d j u n c t i o n p o t e n t i a l b e t w e e n P C a n d H 0 (29): 2

ESCE i n P C +

0.1 M T E A P - E

S C E

i n H 0 + 0.1 M KC1 = 0.13 V 2


119

Copper Cryptâtes


a n d for t h e p o t e n t i a l o f t h e S C E r e f e r r e d to t h e N H E ( 0 . 2 4 2 V ) , c u p r i c c o m p l e x e s w h o s e h a l f - w a v e r e d u c t i o n p o t e n t i a l s are m o r e c a t h o d i c than +0.50 V vs. N H E r e m a i n stable i n both solvents, whereas c u p r i c c o m p l e x e s w h o s e p o t e n t i a l s are m o r e a n o d i c t h a n + 0 . 5 0 V v s . N H E are r e d u c e d s p o n t a n e o u s l y t h r o u g h h o m o g e n e o u s r e d o x p r o c e s s e s , partially [ C u ( I I ) - 4 b ] or q u a n t i t a t i v e l y [ C u ( I I ) - l c ] . S u c h b e h a v i o r i m p l i e s that the reductant i n v o l v e d e x h i b i t s a standard redox p o t e n t i a l c l o s e to, a n d not m u c h m o r e a n o d i c t h a n , + 0 . 5 0 V vs. N H E .


Discussion R e l a t i v e S t a b i l i t y o f Copper(II) a n d C o p p e r ( I ) i n C o m p l e x e s . W h e n c o p p e r ( I I ) is c o m p i e x e d b y l i g a n d s c o n t a i n i n g o n l y n i t r o g e n a n d o x y g e n h e t e r o a t o m s ( L i g a n d s l a a n d 5), r e d u c t i o n o f c o p p e r ( I I ) occurs i n a s i n g l e t w o - e l e c t r o n transfer step. I n other c o m p l e x e s w h e r e c o p p e r is c o o r d i n a t e d t o s u l f u r h e t e r o a t o m s , t h e s t a n d a r d r e d o x p o t e n t i a l o f c o p p e r ( I I ) / c o p p e r ( I ) shifts w i t h t h e r e l a t i v e s t a b i l i t y o f c o p p e r ( I I ) a n d c o p p e r ( I ) t h a t are c o m p i e x e d b y t h e s a m e l i g a n d s . D e p e n d i n g o n t h i s r e l a t i v e s t a b i l i t y , t h e r e s u l t i n g p o t e n t i a l is e x p e c t e d to b e m o r e c a t h o d i c or m o r e a n o d i c t h a n t h e s t a n d a r d r e d o x p o t e n t i a l o f t h e u n c o m p l e x e d c o p p e r ( I I ) / c o p p e r ( I ) s y s t e m , a c c o r d i n g to t h e k n o w n r e l a t i o n s h i p ( 3 0 , 31) (at 2 5 ° C ) : [E

1 / 2

Cu(II)/Cu(I)] compiexed

= [E

1 / 2

Cu(II)/Cu(I)]

u n c o m p l e x e ( i

- 0.059 I o g

2

1 0

p ^

(1)

*Ved

w h e r e Ko a n d Kred are t h e s t a b i l i t y c o n s t a n t s o f t h e c u p r i c a n d c u p r o u s complexes, respectively. Several E Cu(II)/Cu(I) were determined for t h e c o m p l e x e s i n T a b l e s I I a n d I I I . M o s t h a l f - w a v e p o t e n t i a l s o f t h e c o m p i e x e d copper(II)/copper(I) species are m o r e p o s i t i v e t h a n the standard potential of u n c o m p l e x e d copper(II)/copper(I). T h e value u s e d i n E q u a t i o n 1, i n t h e a b s e n c e o f c o m p l e x a t i o n , w a s t h e standard redox potential o f copper(II)/copper(I) i n water. T h i s poten t i a l w a s i n i t i a l l y g i v e n as + 0 . 1 5 3 V v s . N H E (32). L a t e r d a t a g a v e a r a n g e o f a b o u t + 0 . 1 6 to + 0 . 1 7 V v s . N H E (33-35). H o w e v e r , t h e q u e s t i o n a b l e e l e m e n t i n t h e d a t a l e a d i n g to E° = + 0 . 1 5 3 V v s . N H E for t h e a q u e o u s c o p p e r ( I I ) / c o p p e r ( I ) c o u p l e w a s t h e h y d r a t i o n free e n e r g y o f c o p p e r ( I ) , w h e r e a s r e l i a b l e d a t a w e r e a v a i l a b l e for d i v a l e n t X

4

1 / 2

4

[E Cu(Il)/Cu(I)]ii2/NHE have been calculated, for the studied complexes, from experimental [E Cu(II)/Cu(I)] /SCE through the following relationship: [E Cu(II)/ Cu(I)] complex/NHE = [E Cu(II)/Cu(I)]i complex/SCE + Ε SCE/NHE - E where E = 0 in water, and +0.13 (29) in PC. 1/2

1/2

/2

t

}


120


c o p p e r [ £ ° C u ( I I ) / C u ( 0 ) i n water = +0.34 V vs. N H E ] (32). R e c e n t findings ( 3 6 ) c o n f i r m e d b o t h t h e r e l i a b i l i t y o f [ C u ( I I ) / C u ( 0 ) ] d a t a ( E ° = + 0 . 3 4 V vs. N H E ) a n d the q u e s t i o n a b i l i t y o f [ C u ( I I ) / C u ( I ) ] poten tial ( + 0 . 1 5 3 V v s . N H E ) . H o w e v e r , f r o m the dissociation constant o f c u p r o u s i o n s ( 3 7 ) e x p e r i m e n t a l l y o b t a i n e d i n w a t e r at 2 5 ° C , a n d w i t h the a s s u m p t i o n , b a s e d o n these arguments, that E ° [ C u ( I I ) / C u ( 0 ) ] = + 0 . 3 4 V v s . N H E is i n d e e d c o r r e c t , Ε ° [ C u ( I ) / C u ( 0 ) ] m a y b e c a l c u l a t e d e q u a l to + 0 . 5 2 V v s . N H E . T h e r e s u l t i n g E ° c a l c u l a t e d for C u ( I I ) / C u ( I ) u s i n g t h e s e v a l u e s for C u ( I ) / C u ( 0 ) a n d C u ( I I ) / C u ( 0 ) is then E ° [ C u ( I I ) / C u ( I ) ] = +0.16 V vs. N H E . T h i s value, based on r e l i a b l e e x p e r i m e n t a l d a t a , is q u i t e c l o s e to t h e v a l u e r e p o r t e d i n 1 9 5 2 (32). a q

a q

a Q

a q


a q

T h u s , a s £ ° [ C u ( I I ) / C u ( I ) ] = +0.16 V vs. N H E , i t m a y b e c o n c l u d e d from E q u a t i o n 1 a n d Tables II a n d I I I that most o f the copper(I) c o m p l e x e s are m o r e s t a b l e t h a n t h e c o p p e r ( I I ) c o m p l e x e s w i t h t h e ligands used. a q

For complexes o f L i g a n d s 2a a n d 2b w i t h copper(II) the l o g a r i t h m s o f the s t a b i l i t y constants are k n o w n i n w a t e r a n d the corre s p o n d i n g v a l u e s c a n b e c a l c u l a t e d for c o p p e r ( I ) f r o m t h e e l e c t r o c h e m ical half-wave potentials: l o g K = 8.1 w i t h 2 a [ C u ( I ) - 2 a ] a n d l o g Kstab = 13.8 w i t h 2 b [ C u ( I ) - 2 b ] . S u c h h i g h s t a b i l i t y o f c o p p e r ( I ) c o m p l e x e s is q u i t e u n c o m m o n i n w a t e r . T h i s h i g h s t a b i l i t y is c o n s i s t e n t w i t h the rather negative potential ( E = - 0 . 6 7 V vs. S C E ) observed for t h e r e d u c t i o n o f [ C u ( I ) - 2 a ] i n w a t e r . 5

s t a b

1 / 2

C o m m o n Effect o f L i g a n d S i z e a n d o f the N u m b e r o f Sulfur H e t e r o a t o m s o n the R e d o x P o t e n t i a l o f Copper(II)/Copper(I) i n the M a c r o c y c l e s . T o c h a r a c t e r i z e t h i s effect, m a c r o c y c l i c l i g a n d s o f d i f ferent size w e r e u s e d w i t h i d e n t i c a l substituents o n the nitrogens. T h e redox behavior o f copper(II) complexes was e x a m i n e d i n water a n d i n p r o p y l e n e c a r b o n a t e for t h e t w e l v e - , fifteen-, a n d e i g h t e e n - m e m b e r e d m o n o c y c l i c l i g a n d s . I n t h e first series o f t h e s e m a c r o c y c l i c l i g a n d s , t h e s u b s t i t u e n t o n the nitrogens w a s h y d r o g e n a n d i n the s e c o n d series it was m e t h y l (Table IV). A s a g e n e r a l t r e n d i n e i t h e r s e r i e s , t h e f o r m a l r e d o x p o t e n t i a l for t h e c o u p l e C u ( I I ) / C u ( I ) shifts a n o d i c a l l y w i t h i n c r e a s i n g s i z e o f t h e m a c r o c y c l e a n d w i t h increasing n u m b e r o f sulfur heteroatoms. O n e e x c e p t i o n is o b s e r v e d for 4 a . T h e s e r e s u l t s w e r e q u a l i t a t i v e l y e x p e c t e d f r o m t h e k n o w n effect o f t h e n u m b e r o f s u l f u r d o n o r a t o m s i n a m o n o c y c l i c l i g a n d , b e c a u s e p r e v i o u s e x p e r i m e n t s (6) r e v e a l e d that c o o r d i n a t i n g c o p p e r to s u l f u r s t r o n g l y s t a b i l i z e s m o n o v a l e n t c o p p e r m o r e t h a n d i v a l e n t copper. F u r t h e r m o r e , these results r e v e a l e d that s o l v e n t effects are m u c h m o r e i m p o r t a n t o n s m a l l c y c l e s t h a n o n l a r g e cycles.


Copper

Cryptâtes


121


Effect o f N i t r o g e n Substituents on the R e d o x P o t e n t i a l o f Copper(II)/Copper(I). I n this study, three groups o f c o m p l e x e s w e r e c o m p a r e d ( T a b l e V). I n a g i v e n g r o u p , l i g a n d s d i f f e r f r o m e a c h o t h e r o n l y b y the nature o f the substituent o n the n i t r o g e n . P o t e n t i a l s s h o w n i n T a b l e V s h o w t h a t , for a g i v e n m a c r o c y c l e , t h e f o r m a l r e d o x p o t e n t i a l o f C u ( I I ) / C u ( I ) is a n o d i c a l l y s h i f t e d as t h e s u b s t i t u e n t o n t h e n i t r o gen becomes more b u l k y a n d exhibits better electron donor properties. H o w e v e r , t h e l a t t e r c h a r a c t e r i s t i c s c a n n o t a c c o u n t for t h e o b s e r v e d s h i f t , b e c a u s e i n c r e a s i n g e l e c t r o n d o n o r effect w o u l d i n d u c e c a t h o d i c shifts o f t h e p o t e n t i a l (38-40), w h e r e a s a n a n o d i c s h i f t is o b s e r v e d

T a b l e IV. E f f e c t o f L i g a n d S i z e a n d N u m b e r o f S u l f u r H e t e r o a t o m s o n t h e R e d o x P o t e n t i a l o f Cu(II)/Cu(I) i n M a c r o c y c l e s 2, 3, a n d 4 E ° ' Cu(II)ICu(I) (H 0) (V vs. SCE) 2

Ligand

2a

-0.10 — -0.10 -0.025 +0.14 +0.31

3a

4a 2b

3b 4b α

E ° ' Cu(II)/Cu(I) (V vs. SCE) +0.01 +0.05 -0.09 +0.10 +0.19 +0.30

Corrected for liquid junction potential

T a b l e V. E f f e c t o f N i t r o g e n S u b s t i t u e n t s o n t h e R e d o x Potential o f the C o m p i e x e d Cu(II)/Cu(I) C o u p l e E°' Ligand

2a 2b 2c 3a 3b 4a 4b

a

Cu(U)ICu(l) (V vs. SCE) c

- 0 . 1 0 (0.5) - 0 . 0 2 (11)" + 0 . 1 4 (5.7 x 10 ) 3

—

3

+ 0 . 1 4 (5.7 x 10 ) - 0 . 1 0 (0.5) + 0 . 3 1 (4.4 x 10 ) 6

b

E°' Cu(II)/Cu(I) (V vs. SCE)

+ 0 . 1 3 (25) + 0 . 2 2 (8 x 1 0 ) + 0 . 3 0 (1.8 x 1 0 ) + 0 . 1 7 (115) + 0 . 3 1 (2.7 x 1 0 ) + 0 . 0 2 4 (0.4) + 0 . 4 4 (4.5 x 10 ) 2

4

4

6

Note: Values in parentheses are the ratio of the stability con stant of Cu(I) in L to the stability constant of Cu(II) in L. I n H O + 0.1M KC1. In PC + 0.1 M TEAP. As log K for Cu(II)L is 8.44, it may be calculated that log K for Cu(I)L is 8.1. Same comment: log K for Cu(II)L is 12.75, thus log K for Cu(I)L is 13.8. a

2

δ

c

stab

stab

d

stab

stab


(PC)


122


h e r e . T h e t r e n d o b s e r v e d w i t h t h e s e c o m p l e x e s is r e m i n i s c e n t o f t h a t o b s e r v e d for other c o m p l e x e s (5), w h e r e a n a n o d i c shift o f t h e p o t e n tial p a r a l l e l e d the change o f nitrogen substituents from h y d r o g e n to b u t y l . T h i s shift w a s r a t i o n a l i z e d (5) i n terms o f l i g a n d steric c o n straints d u e to substituents that favored n o n p l a n a r copper(I) c o m p l e x e s . S i m i l a r a r g u m e n t s m a y b e d e v e l o p e d t o a c c o u n t for t h e p r e s e n t r e s u l t s . A l s o , t h e v i s i b l e s p e c t r a o f c o p p e r ( I I ) c o m p l e x e s r e v e a l , for a g i v e n m a c r o c y c l e , a r e d s h i f t o f t h e d-d b a n d w h e n t h e n i t r o g e n s u b s t i t u e n t s c h a n g e f r o m h y d r o g e n to m e t h y l a n d a f u r t h e r s h i f t u p o n c h a n g i n g t o b e n z y l ( T a b l e V I ) . S u c h a s h i f t is i n d i c a t i v e o f c o n f o r m a t i o n a l c h a n g e s i n t h e m a c r o c y c l e (41-43) c o r r e s p o n d i n g to a closer tetrahedral coordination o f copper(II) i n the presence o f the b e n z y l group than i n the presence o f hydrogen. N u m b e r o f Sulfur Heteroatoms i n the M a c r o c y c l e a n d the Stan d a r d P o t e n t i a l o f C o p p e r ( I I ) / C o p p e r ( I ) . MONOCYCLIC LIGANDS 2 τ ο 5. C u p r i c c o m p l e x e s o f t h e s e l i g a n d s e x h i b i t r e d o x p r o p e r t i e s t h a t d e p e n d critically on the l i g a n d c h e m i c a l composition. Thus, i n P C + 0.1 M T E A P t h e c o p p e r ( I I ) c o m p l e x w i t h 5 u n d e r g o e s a t w o - e l e c t r o n i r r e v e r s i b l e r e d u c t i o n at E = -0.95 V vs. S C E , whereas i n the same s o l v e n t t h e c u p r i c c o m p l e x w i t h 4 a is r e v e r s i b l y r e d u c e d f r o m c o p p e r ( I I ) t o c o p p e r ( I ) at +0.03 ± 0.01 V v s . S C E . A s d o c u m e n t e d i n Tables I I - I V , the presence o f sulfur heteroatoms i n the m a c r o c y c l i c ligands c l e a r l y stabilizes monovalent copper w i t h respect to divalent c o p p e r . T h i s r e s u l t is c o n s i s t e n t w i t h P e a r s o n ' s h a r d a n d soft a c i d a n d b a s e ( H S A B ) p r i n c i p l e (44): a m o n g t h e s i x h e t e r o a t o m s i n t h e l i g a n d 4a t h a t m a y c o o r d i n a t e t o c o p p e r ( I ) (soft a c i d ) , t h e f o u r s u l f u r a t o m s are soft b a s e s . I n c o n t r a s t , t h e m a c r o c y c l e 5 c o n s i s t s o f f o u r o x y g e n s ( h a r d bases) a n d t w o n i t r o g e n s t h a t a r e i n t e r m e d i a t e b a s e s . C r y s t a l l o g r a p h i c analysis o f the copper(II) c o m p l e x w i t h 5 r e v e a l e d (24) that d i v a l e n t c o p p e r ( i n t e r m e d i a t e a c i d ) is c o o r d i n a t e d t o t h e t w o n i t r o g e n s a n d t o t w o o x y g e n s , as e x p e c t e d f r o m t h e p r e f e r r e d c o o r d i n a t i o n o f copper(II) to n i t r o g e n p r e d i c t e d b y H S A B theory. B a s e d o n the m

Table V I .

Ligand 2a 2b 2c 4a 4b a

S p e c t r a l a n d R e d o x Shifts from N i t r o g e n Substituents i n P C + 0.1 M T E A P Cu(II) d - d (nm)

σ—» Cu (nm)

σ—* Cu

595 600 620 610 670

370 375 375 385 415

310 325 325 310 340

(nm)

E°'

Cu(II)/Cu(I)

(V vs.

a

SCE)

+0.01 +0.10 +0.18 -0.09 +0.31

Corrected for liquid junction potential.


Copper Cryptâtes

123

5. GS ISELBRECHT AND GROSS same arguments

o f Pearson, it m a y b e

e x p e c t e d that

monovalent

c o p p e r w o u l d coordinate i n 5 o n l y w i t h the t w o nitrogens, thus l e a d i n g to u n s t a b l e l i n e a r g e o m e t r y (45) i n t h i s m a c r o c y c l e 5. BICYCLIC LIGANDS l a , l b , A N D l c .

A s observed with monocyclic

ligands, sulfur heteroatoms m a r k e d l y stabilize monovalent vs. divalent c o p p e r , a n d t h i s effect i n c r e a s e s w i t h t h e n u m b e r o f s u l f u r s i n t h e c y c l e . T h e effect is c l e a r l y o b s e r v a b l e f r o m l b to l c ( T a b l e s I I a n d I I I ) — r e d o x p o t e n t i a l d i f f e r e n c e o f 0.3 V — a n d i t m a y b e a s c r i b e d to t h e m a c r o b i c y c l i c structure o f the l i g a n d .


Dinuclear Copper Cryptâtes I n this series o f c o m p l e x e s ( 6 - 1 2 ) , a l l l i g a n d s e x h i b i t t w o d i s t i n c t c o o r d i n a t i o n sites t h a t are a b l e to a c c o m m o d a t e c o p p e r ( I I ) a n d c o p per(I).

I n this respect,

three salient characteristics

o f the

ligands

should be noted: 1. I n L i g a n d s 6 a n d 7, e a c h c o o r d i n a t i o n site is t r i d e n t a t e , the c o o r d i n a t i n g L e w i s bases b e i n g t w o sulfur a n d one n i t r o g e n i n 6, a n d t h r e e n i t r o g e n s i n 7 . 2. L i g a n d 8 e x h i b i t s the s p e c i a l feature o f h a v i n g t w o v e r y d i f f e r e n t c o o r d i n a t i o n sites, o n e s i t e b e i n g q u i t e s i m i l a r to t h a t i n l i g a n d 7 , t h e o t h e r s i t e b e i n g m u c h l i k e t h e site o b s e r v e d i n t h e m o n o n u c l e a r c h e l a t e s o f t h e t w e l v e - m e m b e r e d L i g a n d s 2 a - 2 c a n d also s i m i l a r to those o f L i g a n d 10. 3. I n t h e m a c r o t r i c y c l i c s e r i e s 9 t o 1 2 , t h e o x y g e n h e t e r o a t o m s i n t h e t w e l v e - m e m b e r e d m o n o c y c l e s are s u b s t i t u t e d b y s u l f u r f r o m 9 to 1 0 , 11 a n d 1 2 . A l s o , u s i n g t w o i d e n t i c a l m o n o c y c l i c s u b u n i t s , the c h e m i c a l c o m p o s i t i o n o f the s i d e - b r a n c h l i n k i n g the t w o m o n o c y c l e s w a s changed i n 10-12. N o n e o f t h e d i n u c l e a r c o m p l e x e s , e x c e p t L i g a n d 6, e x h i b i t e d s i g n i f i c a n t C u - C u c o u p l i n g , as c o n s i s t e n t l y v e r i f i e d e x p e r i m e n t a l l y (18) a n d e x p e c t e d t h e o r e t i c a l l y (46). T h e r e d o x b e h a v i o r o f t h e c o p p e r ( I I ) / c o p p e r ( I ) c o u p l e i n t h e s e l i g a n d s is e x p e c t e d to p r o v i d e s i g n i f i c a n t i n s i g h t i n t o the parameters a l l o w i n g c o n t r o l o f the t h e r m o d y n a m i c s o f t h e e l e c t r o n transfers f r o m d i n u c l e a r c o p p e r ( I I ) t o d i n u c l e a r c o p p e r ( I ) moieties.

Results T h e r e d o x characteristics o f the d i n u c l e a r c o p p e r chelates

with

L i g a n d s 6 - 1 2 are g i v e n i n T a b l e V I I . D i n u c l e a r C o p p e r C o m p l e x w i t h 6. lized complex of 6 with two C u ( N ) 3

2

T h e E S R - s i l e n t (47) c r y s t a l

g r o u p s is o n l y v e r y s l i g h t l y s o l u -



light blue violet

green

brown

11

12

yellow-brown light blue green

Color

2

2

2

PC H 0 PC H 0 PC H 0 PC

DMSO PC PC

Solvent

Cyitr Au Au

Pt Pt Pt Au

Pt Hg Hg

Elec trode

1 1 1 1 1 1 1

1 1 2 2 2 2 2 2 2 2

2 2 1/1

Num ber of reduc η tion (F/ steps mol)

+0.10 -0.20 +0.31 -0.17 +0.28 +0.20 +0.27 +0.08 +0.22 +0.29 +0.48

El/2 ( V vs. SCE)

Cu(II)/Cu(I)

revers. 57 80 62 82 89 63

—

1

2 1

1 1

mV/u log 90 73

d

of reduc tion steps

slope Uog (11 i - m

Num ber

-1.50

-0.70 -0.65 -0.61 -1.0 -0.75

El/2 ( V vs. SCE)

2

2

2 2 1/1

η (F/ mole)

Ligand)

Cu(I)/Cu(0)

R e d o x Characteristics o fD i n u c l e a r C o p p e r Chelates (Stoichiometry: T w o C o p p e r s / O n e

9 10

7 8

6

land

Table V I I .


ο w

ho

ο

χ Ω

r ο ο η > r » w d ο

Ο

to

Copper 5. GS ISELBRECHT AND GROSS

Cryptâtes

125

b l e i n several organic solvents, but it dissolves reasonably,

although

s l o w l y , i n D M S O r e s u l t i n g i n a y e l l o w - b r o w n s o l u t i o n (47) tion

bands

at

280

(e = 6 0 0 0 M "

nm

1

1

cm" ), 398

[absorp

(e = 3 7 5 0 ) ,

738

(e = 2 2 5 0 ) ] . T h i s s o l u t i o n is n o t t h e r m o d y n a m i c a l l y s t a b l e a n d c o l o r t u r n s to g r e e n - b r o w n

after

24 h , then

to g r e e n

after

the

48

h,

c o r r e s p o n d i n g to t h e d e c o m p l e x a t i o n o f o n e o f t h e t w o c o m p i e x e d c o p p e r ( I I ) a t o m s , as e x p e c t e d f r o m t h e h i g h e r s t a b i l i t y o f t h e m o n o n u c l e a r copper(II) c o m p l e x t h a n that o f the d i n u c l e a r copper(II) 5

(48 ) . H o w e v e r , c o n s i d e r i n g t h e t i m e s c a l e o f t h e r e d o x

species

experiments,

this p a r t i a l d e c o m p l e x a t i o n m a y b e n e g l e c t e d i f fresh solutions o f the


d i n u c l e a r c o p p e r ( I I ) c o m p l e x are u s e d i n t h e e l e c t r o c h e m i c a l

mea

s u r e m e n t s , as i n t h i s s t u d y . U s i n g fresh solutions o f the c o m p l e x C u ( N ) 2 - 6 , r e d o x studies re 3

v e a l e d (7) t h r e e r e a c t i o n s o n p l a t i n u m i n t h e e l e c t r o a c t i v i t y r a n g e o f D M S O + 0.1 M T E A P ( + 1 . 2 to - 1 . 9 V v s . S C E ) . 1. T h e first s i g n a l w a s a n o x i d a t i o n at E = +0.88 V vs. S C E , c o r r e s p o n d i n g to t h e o x i d a t i o n o f a z i d e s N i " r e l e a s e d i n s o l u t i o n after d i s s o l u t i o n o f t h e i n i t i a l c o m p l e x . S u b s e q u e n t analyses demonstrated that this o x i d a t i o n i n v o l v e d t w o out o f the four azides i n i t i a l l y i n c l u d e d i n each d i n u c l e a r copper(II) c o m p l e x . 1 / 2

2 . A s e c o n d s i g n a l , at E = +0.10 V vs. S C E , corresponds to t h e r e v e r s i b l e ( 7 ) d i e l e c t r o n i c r e d u c t i o n f r o m [ C u ( I I ) , C u ( I I ) ] to [ C u ( I ) , C u ( I ) ] , w h i c h a c t u a l l y r e s u l t s f r o m t h e m e r g i n g o f t w o m o n o e l e c t r o n i c s u b s e q u e n t steps, re spectively 1 / 2

[Cu(II), Cu(II)]/[Cu(II), Cu(I)] ( Ε ° ' = +0.12 V vs. S C E ) and [Cu(II), Cu(I)]/[Cu(I), Cu(I)] ( Ε ° ' = +0.06 V vs. S C E )

3 . T h e l a s t s t e p ( E i = - 0 . 7 0 V v s . S C E ) is d i e l e c t r o n i c a n d c o r r e s p o n d s to t h e r e d u c t i o n o f [ C u ( I ) , C u ( I ) ] t o free, u n a l t e r e d L i g a n d 6 a n d to u n c o m p l e x e d C u ( 0 ) . /2

D i n u c l e a r C o m p l e x o f C u ( C 1 0 ) w i t h 7. I n p r o p y l e n e c a r b o n a t e (+0.1 M T E A P ) de polarograms on a D r o p p i n g M e r c u r y E l e c t r o d e ( D M E ) r e v e a l e d t w o r e d u c t i o n w a v e s at E = - 0 . 2 0 ± 0.01 V v s . S C E ( l o g a r i t h m i c s l o p e = 7 3 m V / l o g ) a n d at E = - 0 . 6 5 ± 0.01 V v s . S C E ( l o g a r i t h m i c s l o p e = 90 m V / l o g ) ( F i g u r e 3). T h e u n e q u a l heights o f the two reduction waves result from a slight adsorption w a v e m e r g e d w i t h 4

2

1 / 2

1 / 2

5

2+

4+

( C u L ) : log β = 9.84. ( C u L ) : log β = 12.88. 2


126

BO ILOGC IAL REDOX COMPONENTS 0.75

I

'c (uA)

0.50

0.25 -

I


Ε V/SCE I -0.5

ι -1

ι -1.5

m

0.25

® 0.25H Ε V/SCE

©

Q25h

0.50

0.75k

Figure 3. dc Folarograms of a dinuclear Cu(II) complex with 7 (anion: CIO4). Solvent: PC +0.1 M TEAP, Key: a, reduction of the dinuclear complex [Cu(II)] -7; h, polarogram recorded after ex haustive coulometry at -0.400 V vs. SCE; and c, polarogram recorded after exhaustive coulometry at -0.900 V vs. SCE. 2


5.

Copper Cryptâtes

GissELBRECHT A N D GROSS

127

t h e first r e d u c t i o n s t e p . P o t e n t i o s t a t i c c o u l o m e t r y o f t h i s first w a v e g i v e s 2 F / m o l , t h e c o l o r o f t h e s o l u t i o n t u r n i n g f r o m l i g h t b l u e to yellow. A f t e r e x h a u s t i v e c o u l o m e t r y p e r f o r m e d o n t h e first r e d u c t i o n s t e p , t h e r e c o r d e d p o l a r o g r a m s h o w s ( F i g u r e 3 b ) o n e o x i d a t i o n w a v e at E = - 0 . 2 2 ± 0.1 V v s . S C E a n d , as e x p e c t e d , t h e r e m a i n i n g u n a l ll2

t e r e d s e c o n d r e d u c t i o n w a v e at E = - 0 . 6 5 V vs. S C E . Exhaustive c o u l o m e t r y o n t h i s s e c o n d w a v e at - 0 . 9 V v s . S C E i n d i c a t e s 1.9 F / m o l , w h e r e a s t h e s o l u t i o n b e c o m e s c o l o r l e s s . P o l a r o g r a m s r e c o r d e d after t h i s s e c o n d c o u l o m e t r y e x h i b i t o n l y t h e o x i d a t i o n w a v e at - 0 . 2 0 V v s . S C E ( F i g u r e 3c), w h i c h was also o b s e r v e d i n solutions c o n t a i n i n g o n l y t h e free L i g a n d 7 . T h e o x i d a t i o n at - 0 . 2 0 V v s . S C E w a s a u t h e n t i c a t e d as t h e l i g a n d - a s s i s t e d o x i d a t i o n o f t h e m e r c u r y e l e c t r o d e , w h i c h w a s s t u d i e d a l r e a d y (49) w i t h n i t r o g e n - c o n t a i n i n g m a c r o c y c l i c l i g a n d s . T h u s , t h e first r e d u c t i o n s t e p o f t h e c o m p l e x c o r r e s p o n d s to t h e d i e l e c t r o n i c r e d u c t i o n o f [ C u ( I I ) , C u ( I I ) ] - 7 to [ C u ( I ) , C u ( I ) ] - 7 , a n d t h e s e c o n d s t e p l e a d s , t h r o u g h t h e r e d u c t i o n o f b o t h C u ( I ) a t o m s , to u n c o m p l e x e d n o n v a l e n t c o p p e r a n d free, u n a l t e r e d L i g a n d 7 . T h e first s t e p (two-electron reduction) may be reversed, a n d c y c l e d many times b y successive cathodic a n d anodic exhaustive coulometries.


m

D i n u c l e a r C o m p l e x o f C u ( C 1 0 ) w i t h 8. P o l a r o g r a m s o n m e r c u r y r e v e a l e d f o u r d i s c r e t e r e d u c t i o n s t e p s i n P C + 0.1 M T E A P . H o w ever, the E a n d the g e o m e t r y o f these four waves w e r e s u c h that t h e y p r e v e n t e d any straightforward analysis o f the four steps. T h e r e f o r e , the reduction potentials were obtained w i t h a precision o f ± 0 . 0 1 V b y d i f f e r e n t i a l p u l s e p o l a r o g r a p h y ( F i g u r e 4) a n d g a v e , r e s p e c t i v e l y , values o f +0.31, - 0 . 1 7 , - 0 . 6 1 , a n d - 1 . 0 0 V vs. S C E . T h e current of the l a s t s t e p (at - 1 . 0 0 V v s . S C E ) w a s m u c h s m a l l e r t h a n t h a t o f t h e t h r e e others. O w i n g to the poor resolution o f the waves, the lack o f d e t a i l e d i n f o r m a t i o n l e d us to a s s i g n t h e w a v e s b y a c o m p a r i s o n o f t h e i r p o t e n tials w i t h those o f analogous b i n u c l e a r s y m m e t r i c a l c o m p l e x e s ( w i t h 7 a n d 10) a n d a l s o , w i t h t h o s e o f m o n o n u c l e a r c o r r e s p o n d i n g c h e l a t e s w i t h L i g a n d s 2. 4

2

1 / 2

T h e first r e d u c t i o n , at E = + 0 . 3 1 V v s . S C E , is m o n o e l e c t r o n i c a n d w a s o b s e r v e d at i d e n t i c a l p e a k p o t e n t i a l s o n g l a s s y c a r b o n o r o n m e r c u r y e l e c t r o d e s . T h i s finding i n d i c a t e s t h a t t h i s s t e p d o e s n o t i n v o l v e c h e m i c a l reaction o f the e l e c t r o d e m a t e r i a l . A s the m o n o n u c l e a r c u p r i c c o m p l e x w i t h t w e l v e - m e m b e r e d L i g a n d 2c was s h o w n (Table I I I ) t o b e r e d u c e d to t h e c u p r o u s c o m p l e x at + 0 . 3 0 V v s . S C E i n t h i s s o l v e n t ( P C ) , t h e p r e s e n t s t e p at + 0 . 3 1 V v s . S C E m a y b e a s c r i b e d to t h e r e d u c t i o n o f c o p p e r ( I I ) to c o p p e r ( I ) i n t h e t w e l v e - m e m b e r e d m o n o c y c l i c s u b u n i t o f 8, w h e r e f o u r L e w i s b a s e s ( t w o s u l f u r s a n d t w o nitrogens) are a v a i l a b l e to c o o r d i n a t e the c o p p e r . p


b i o l o g i c a l redox components

128


I nA

0

0.5

-0.5

-1

-1.5

Figure 4. Differential pulse polarogram of dinuclear Cu(H) complex with asymmetrical Ligand 8. Electrode: Hg, Medium: PC + 0.1 M TEAP.

T h e s e c o n d r e d u c t i o n is a l s o m o n o e l e c t r o n i c a n d is o b s e r v e d at a p o t e n t i a l ( E = - 0 . 1 7 V v s . S C E ) c l o s e to t h a t ( E = - 0 . 2 0 V vs. S C E ) c o r r e s p o n d i n g to t h e r e d u c t i o n o f t h e t w o c o p p e r s i n s y m m e t r i c a l L i g a n d 7 . T h e r e f o r e , t h i s r e a c t i o n m a y b e a s c r i b e d to t h e r e d u c t i o n o f c o p p e r ( I I ) [to C u ( I ) ] , w h i c h is c o o r d i n a t e d to t h e t r i d e n t a t e s i t e i n v o l v i n g t h r e e n i t r o g e n s i n d i s s y m m e t r i c a l L i g a n d 8. U s i n g s i m i l a r arguments just m e n t i o n e d , the t h i r d redox process (E = - 0 . 6 1 V v s . S C E ) i s a t t r i b u t e d to t h e r e d u c t i o n o f o n e Cii(I) t o C u ( 0 ) i n t h e t r i d e n t a t e site o f 8, i n v o l v i n g a s i m i l a r c o n f o r m a t i o n as i n 7. T h e f o u r t h s t e p (E = - 1 . 0 0 V v s . S C E ) w o u l d b e e x p e c t e d to c o r r e s p o n d to t h e r e d u c t i o n o f C u ( I ) to C u ( 0 ) i n t h e t w e l v e - m e m b e r e d c y c l e i n v o l v i n g t w o nitrogens a n d t w o sulfurs. H o w e v e r , the s m a l l a m p l i t u d e o f t h e s i g n a l as c o m p a r e d w i t h t h o s e o f t h e first t h r e e r e m a i n s u n e x p l a i n e d . O n e p o s s i b i l i t y is t h a t a c h e m i c a l r e a c t i o n f o l l o w s e l e c t r o n transfer, for i n s t a n c e , p a r t i a l d i s s o c i a t i o n o f t h e m o n o n u c l e a r c u p r o u s c o m p l e x g e n e r a t e d after t h e first t h r e e o n e - e l e c t r o n r e ductions. p

1 / 2

P

P

T h u s , the d i n u c l e a r c o m p l e x o f copper(II) w i t h 8 e x h i b i t s remark a b l e r e d o x p r o p e r t i e s ; as a r e s u l t o f t h e m a r k e d d i f f e r e n c e s i n t h e t w o c o o r d i n a t i o n sites, t h e r e d o x p o t e n t i a l s o f C u ( I I ) / C u ( I ) as w e l l as o f C u ( I ) / C u ( 0 ) c o u p l e s are s i g n i f i c a n t l y s h i f t e d . I n a d d i t i o n , t h e r e d u c t i o n p o t e n t i a l s c o r r e s p o n d i n g to C u ( I I ) -> C u ( I ) r e m a i n a l m o s t u n c h a n g e d i n a g i v e n c o o r d i n a t i o n s i t e , as i n d i c a t e d b y a c o m p a r i s o n o f t h e v a l u e s


5.


129

Copper Cryptâtes

o b t a i n e d i n 8 w i t h t h e v a l u e s g i v e n for s y m m e t r i c a l L i g a n d s 7 o r 10 or L i g a n d 2c i n T a b l e I I I ( F i g u r e 5). T h e s e r e s u l t s i n d i c a t e t h a t a n a p p r o p r i a t e c o n t r o l o f t h e c o o r d i n a t i o n sites a r o u n d t h e c o p p e r s i n p o l y n u c l e a r c o m p l e x e s m a y b e u s e d as a n e f f i c i e n t t o o l t o d e s i g n p o l y n u c l e a r c o m p l e x e s i n w h i c h c a s c a d e - l i k e p o l y e l e c t r o n processes m a y b e t u n e d a n d operated efficiently.

Reduction of the Dinuclear Complex of Cu(C10 ) with 9. 4

2

In PC,


t h i s c o p p e r ( I I ) c o m p l e x is r e d u c e d i n t w o d i e l e c t r o n i c s t e p s at Ε °' = + 0 . 2 5 V v s . S C E { [ C u ( I I ) , C u ( I I ) ] t o [ C u ( I ) , C u ( I ) ] } a n d at E°' = - 0 . 7 5 V vs. S C E { [ C u ( I ) , C u ( I ) ] to n o n v a l e n t u n c o m p l e x e d c o p p e r a n d free L i g a n d 9}. C y c l i c v o l t a m m o g r a m s i n d i c a t e t h a t t h e first c a t h o d i c s t e p { [ C u ( I I ) , C u ( I I ) ] t o [ C u ( I ) , C u ( I ) ] } is r e v e r s i b l e ( p e a k p o t e n t i a l s i n d e p e n d e n t o f s c a n r a t e u p to 0.1 V / s ) . F r o m a p r e v i o u s l y d e s c r i b e d m e t h o d ( 5 0 ) , Δ Ε ° ' = 6 0 m V w a s c a l c u l a t e d as t h e d i f f e r e n c e b e t w e e n t h e f o r m a l p o t e n t i a l s ( £ ? ' a n d E l ' ) c o r r e s p o n d i n g , r e s p e c t i v e l y , to t h e first a n d t h e s e c o n d e l e c t r o n t r a n s f e r . O n t h e s a m e t i m e s c a l e o f t h e e l e c t r o c h e m i c a l m e t h o d s , b o t h El' a n d El' a r e m e r g e d i n t h e s i n g l e t w o - e l e c t r o n s t e p at + 0 . 2 5 V v s . S C E . T h u s , Ε\' = + 0 . 2 8 V v s . S C E a n d Ε 1 ' = + 0 . 2 2 V v s . S C E . A l s o , the d i n u c l e a r c u p r o u s c o m p l e x w i t h 9, w h e n e l e c t r o g e n e r a t e d b y e x h a u s t i v e r e d u c t i o n c o r r e s p o n d i n g to t h e first r e d u c t i o n s t e p , is u n s t a b l e a n d u n d e r g o e s s l o w c h e m i c a l d e g radation i n the solution.

Dinuclear Complexes of Copper(II) with 10,11, and 12 in Aque ous 0.1 M KC1. I n t h e e l e c t r o a c t i v i t y r a n g e c o r r e s p o n d i n g t o t h i s s o l v e n t o n a r o t a t i n g d i s k e l e c t r o d e ( f r o m + 1 . 2 to —0.9 V v s . S C E ) a u n i q u e t w o - e l e c t r o n r e d u c t i o n w a v e is o b s e r v e d for t h e s e d i n u c l e a r copper(II) complexes. T h e o b s e r v e d w a v e corresponds to the r e d u c t i o n from [Cu(II), C u ( I I ) ] t o [ C u ( I ) , C u ( I ) ] , as c o n f i r m e d b y e l e c t r o n i c a b s o r p t i o n a n d E S R s p e c t r a r e c o r d e d i n t h e s o l u t i o n b e f o r e a n d after e x h a u s t i v e r e d u c t i o n at 2 F / m o l ( 1 5 ) . W i t h t h e d i n u c l e a r c u p r i c c o m p l e x e s o f 10 a n d 11, t h e l i m i t i n g c a t h o d i c c u r r e n t s a r e d i f f u s i o n c o n t r o l l e d , as i n d i c a t e d f r o m t h e p l o t 1/iiim v s . 1 / ω (linear t h r o u g h origin) (26). C y c l i c v o l t a m m o g r a m s o f t h e s e t w o c o m p l e x e s are i n d i c a t i v e o f a r e v e r s i b l e r e d o x p r o c e s s at t h e e l e c t r o d e , at l o w s c a n rates ( < 1 0 0 m / s ) , w i t h a d i f f e r e n c e Δ Ε b e t w e e n t h e a n o d i c a n d c a t h o d i c p e a k p o t e n t i a l s b e i n g 6 5 m V ( w i t h 10) a n d 8 0 m V ( w i t h 11). A t h i g h e r s c a n rates, a n a l y s i s o f t h e v o l t a m m o g r a m s w a s o b s c u r e d for 11 b y a c h e m i c a l r e a c t i o n s u b s e q u e n t t o t h e e l e c t r o n t r a n s f e r , w h e r e a s , i n t h e c o m p l e x w i t h 10, t h e e x c e l l e n t s t a b i l i t y o f t h e i n i t i a l reactant a n d o f the r e d u c e d p r o d u c t a l l o w e d c o m p l e t e analysis o f t h e v o l t a m m o g r a m s . I n t h e c o m p l e x w i t h 10, t h e d i f f e r e n c e b e t w e e n the f o r m a l p o t e n t i a l s o f the t w o o n e - e l e c t r o n steps c o r r e s p o n d i n g to 1 / 2

Ρ



st

n

=

i

2

E/ =-1,35

1 Q

= 2 ο ι ο

" '

m2 ?

J

=

2

1

Ey =-1,45 η =1

n

t

\Ks CuL

|E/2..a3ofK^=l810. J (

.4Λ

4\

CH>P

Figure 5. Comparison of the reduction potentials of the dinuclear Cu(II) complex with disymmetrical Ligand 8 (medium: PC + 0.1 M TEAP). All Ε1/2 in volts vs. SCE. Values in parentheses are the ratio of the stability constants of, respectively, the reduced to the oxidized conjugated form; Ο is Cu(II) or Cu(l).

2

2

Ust^-Cu^L

E^-0,17/Kst(cJ) L

n= 1

-4

E/ 2 =-0,61

0 6 3 χ )

η =2

VKstlCu^L

1

st ( C u ^ L

U

Ει/ =-065

η =2

o

Ey =-020/K (CuV_

I

sX

/Ε^«+0,31 /K [c£-çJ)L


5.

Copper Cryptâtes


131

t h e t w o - e l e c t r o n r e d o x p r o c e s s w a s c a l c u l a t e d a c c o r d i n g to k n o w n p r o c e d u r e s ( 5 0 ) a n d w a s ( 1 5 ) e q u a l to Δ Ε ° = 3 0 ± 5 m V , w h i c h is q u i t e c l o s e to t h e v a l u e 3 5 . 6 m V c o r r e s p o n d i n g t o t h e r e d u c t i o n o f t w o i n d e pendent redox centers (51, 52). A s for t h e d i n u c l e a r c u p r i c c o m p l e x w i t h 12, r e d o x m e a s u r e m e n t s s h o w t h a t i t u n d e r g o e s , i n p a r t , a s p o n t a n e o u s c h e m i c a l r e d u c t i o n to [Cu(I), Cu(I)] i n solution. F o r instance, stationary v o l t a m m o g r a m s o n a g o l d R D E r e m a i n e d c h a r a c t e r i z e d b y a constant E

1/2

at + 0 . 2 9 V v s .

S C E , a l t h o u g h the e n t i r e w a v e s h i f t e d w i t h t i m e a l o n g the c u r r e n t axis from the c a t h o d i c t o w a r d the a n o d i c side, r e s u l t i n g

finally

in a mixed


a n o d i c - c a t h o d i c w a v e . T h e a n o d i c l i m i t i n g current was diffusion c o n t r o l l e d , whereas the cathodic l i m i t i n g current i n v o l v e d a c h e m i c a l step i n a d d i t i o n to t h e e l e c t r o n transfer. T h i s c h e m i c a l s t e p b e c o m e s

unde

t e c t a b l e w h e n t h e r e d o x p r o c e s s is s t u d i e d i n P C + 0.1 M T E A P , as documented

later.

D i n u c l e a r C o m p l e x e s o f C o p p e r ( I I ) w i t h 10, 11, a n d 12, i n P C + 0.1 M T E A P .

W h e n studied on solid electrodes i n P C where i n h i b i t

ing

are a b s e n t , a l l t h r e e c o m p l e x e s e x h i b i t q u a l i t a t i v e l y

processes

a n a l o g o u s b e h a v i o r . T h e c o n v e r s i o n o f [ C u ( I I ) , C u ( I I ) ] to [ C u ( I ) , C u ( I ) ] is d e t e c t a b l e as a s i n g l e t w o - e l e c t r o n w a v e o n t h e r o t a t i n g d i s k e l e c t r o d e . T h e l i m i t i n g c a t h o d i c c u r r e n t w a s c h e c k e d as b e i n g d i f f u s i o n controlled, a n d spectroscopic e v i d e n c e was obtained (absorption a n d E S R s p e c t r a ) t h a t t h e t w o e l e c t r o n s t r a n s f e r r e d to [ C u ( I I ) , C u ( I I ) ] i n deed generate [Cu(I), Cu(I)]. T h e c h a r a c t e r i s t i c s o f the r e d o x c o u p l e [ C u ( I I ) , C u ( I I ) ] / [ C u ( I ) , C u ( I ) ] are g i v e n i n T a b l e V I I . I n c y c l i c v o l t a m m e t r y , the a n o d i c a n d c a t h o d i c p e a k currents

are

e q u a l at a l l s c a n rates. H o w e v e r , as t h e r e d u c t i o n w a s q u a s i - r e v e r s i b l e i n all three complexes, Δ Ε consequence,

Ρ

increases

Δ Ε ' ° (50) b e t w e e n

the

w i t h t h e s c a n r a t e a n d , as a two

successive

one-electron

transfers c o u l d n o t b e c a l c u l a t e d . H o w e v e r , as Δ Ε is less t h a n 1 0 0 m V Ρ

at l o w s c a n rates, Δ Ε '° s t i l l c a n b e e s t i m a t e d to b e less t h a n 6 0 m V .

Discussion T h e results o b t a i n e d on the redox b e h a v i o r o f d i n u c l e a r c o p p e r chelates (Table V I I ) p r o v i d e general trends o n the s t r u c t u r e - r e d o x r e a c t i v i t y r e l a t i o n s h i p i n the s t u d i e d c o m p l e x e s . T h e data c o l l e c t e d c l e a r l y demonstrate that a l l of the l i g a n d s stud i e d s t a b i l i z e copper(I) vs. copper(II), a n d that the copper(I) s t a b i l i z a tion increases w i t h the n u m b e r o f sulfur heteroatoms. T h i s c o n c l u s i o n is c o n s i s t e n t w i t h p r e v i o u s o b s e r v a t i o n s o n o t h e r l i g a n d s ( 5 , 6 ) . I n a n y o f t h e s e d i n u c l e a r c o m p l e x e s , s t a b i l i z a t i o n o f c o p p e r ( I ) is d o c u m e n t e d b y the a n o d i c shift o f the E c o r r e s p o n d i n g to C u ( I I ) / C u ( I ) ( i n t h e 1 / 2


132

BIOLOGICAL REDOX COMPONENTS

s t r i c t e s t s e n s e t h i s s h i f t i l l u s t r a t e s t h e s t a b i l i z a t i o n o f C u ( I ) r e l a t i v e to C u ( I I ) , i n the l i g a n d ) a n d also b y the c a t h o d i c shift o f E correspond i n g to C u ( I ) / C u ( 0 ) , i n t h e l i g a n d . I n t h i s r e s p e c t , t h e r e s u l t s o b s e r v e d for t h e r e d u c t i o n o f d i n u c l e a r c u p r i c c o m p l e x e s i n 9 l e a d to t h e f o l l o w i n g observations: m

1. E [ C u ( I I ) , C u ( I I ) ] - » [ C u ( I ) , C u ( I ) ] i n 9 is v e r y c l o s e to t h e Ei/2 m e a s u r e d i n t h e h o m o l o g o u s L i g a n d 10 ( a l t h o u g h 10 i n v o l v e s s u l f u r s a n d 9 d o e s not). 2. E [ C u ( I ) , C u ( I ) ] - » 2 C u ( 0 ) is m u c h m o r e n e g a t i v e i n 10 (-1.50 V vs. S C E ) than i n 9 ( - 0 . 7 5 V vs. S C E ) . 1 / 2


1 / 2

T h e first o b s e r v a t i o n is a c l e a r i n d i c a t i o n t h a t t h e r a t i o o f t h e s t a b i l i t y c o n s t a n t s o f t h e c u p r o u s to t h e c u p r i c d i n u c l e a r c o m p l e x e s is a l m o s t i d e n t i c a l i n L i g a n d s 9 a n d 10. H o w e v e r , t h e s e c o n d o b s e r v a t i o n i n d i cates t h a t t h e c u p r o u s c o m p l e x w i t h 10 is m u c h m o r e s t a b l e t h a n w i t h 9. T h i s r e s u l t is f u l l y c o n s i s t e n t w i t h o u r o b s e r v a t i o n t h a t t h e C u ( I ) d i n u c l e a r c o m p l e x i n 9 is c h e m i c a l l y u n s t a b l e , as m e n t i o n e d . T h e c o m p l e x w i t h 9 is t h e first r e p o r t e d i n w h i c h r e v e r s i b l e , d i e l e c t r o n i c i n t e r c o n v e r s i o n o f [ C u ( I I ) , C u ( I I ) ] / [ C u ( I ) , C u ( I ) ] o c c u r s at v e r y p o s i t i v e potentials (about +0.5 V vs. N H E ) i n the presence o f o n l y nitrogen a n d o x y g e n as L e w i s b a s e h e t e r o a t o m s (5). F u r t h e r m o r e , r e p l a c e m e n t o f t h e o x y g e n h e t e r o a t o m s b y s u l f u r (9-10) b a s i c a l l y s t a b i l i z e s c o p p e r ( I ) . H o w e v e r , t h e a b i l i t y o f a g i v e n d i n u c l e a r c o p p e r c o m p l e x to a c t as a t w o - e l e c t r o n a c c e p t o r / d o n o r at p o s i t i v e p o t e n t i a l s d e p e n d s p r i m a r i l y on the relative stability o f copper(II) a n d copper(I) i n the m a c r o c y c l i c l i g a n d . T h e r e d o x b e h a v i o r o f s u c h c o m p l e x e s w i t h 9 a n d 10 s h o w s t h a t t h e r e l a t i v e s t a b i l i t y o f c o p p e r ( I ) c o m p a r e d to c o p p e r ( I I ) i n e a c h c o m p l e x d e p e n d s c r i t i c a l l y o n the c o o r d i n a t i o n g e o m e t r y m u c h more than o n the presence o f o x y g e n or sulfur i n the c y c l e . I n terms o f s e l e c t i v e l y shifting the redox potential o f a g i v e n c o u p l e [ h e r e C u ( I I ) / C u ( I ) ] , r e s u l t s o b t a i n e d w i t h the d i n u c l e a r c u p r i c / c u p r o u s c o m p l e x e s i n 7 , 8, a n d 10 ( T a b l e V I I ) p r o v i d e f u r t h e r u n a m b i g u o u s e v i d e n c e that c o n t r o l l i n g the c o o r d i n a t i o n stereochemis t r y is t a n t a m o u n t to c o n t r o l l i n g t h e r e d o x p o t e n t i a l o f C u ( I I ) / C u ( I ) , as b o t h o f t h e r e d o x sites a r e n o n i n t e r a c t i n g i n t h e c o m p l e x e s . I n a d d i t i o n , the d i n u c l e a r c o m p l e x w i t h 8 illustrates that the ex c e l l e n t selectivity operating on the redox potentials t h r o u g h the coor d i n a t i o n g e o m e t r y p r o v i d e s a b a s i s for the r a t i o n a l s y n t h e s i s o f m i x e d v a l e n c e c o m p l e x e s o f p r e d i c t a b l e s t a b i l i t y ( F i g u r e 5). A s a n e x a m p l e d o c u m e n t i n g this c o n c l u s i o n , the coproportionation constant K o f t h e c o m p l e x w i t h a s y m m e t r i c a l L i g a n d 8, w h o s e e q u a t i o n is: c o p r

[Cu(II), Cu(II)]-8 + [Cu(I), C u ( I ) ] - 8 ^ 2 [ C u ( I I ) , Cu(I)]-8


5.

G I S S E L B R E C H T A N D GROSS

Copper Cryptâtes

133

8

w a s c a l c u l a t e d as e q u a l t o 1.3 x 1 0 at 2 5 ° C i n P C + 0.1 M T E A P (assuming E = E ° , a n d the r e d o x reactions b e i n g reversible).


m

T h e s e c o n c l u s i o n s on d i n u c l e a r c o p p e r c o m p l e x e s m a y b e ex t e n d e d to o t h e r d i m e t a l l i c h o m o n u c l e a r a n d h e t e r o n u c l e a r c o m p l e x e s ; w o r k is i n p r o g r e s s o n t h i s p o i n t ( 5 3 ) . F i n a l l y , the s y m m e t r i c a l d i n u c l e a r complexes o f copper w i t h L i g a n d s 1 0 , 1 1 , a n d 12 w e r e u s e d to c l a r i f y t h e effect o f t h e s i d e c h a i n s l i n k i n g t h e t w o m o n o c y c l i c s u b u n i t s to t h e C u ( I I ) / C u ( I ) r e d o x p o t e n tials. A s spectral results r e v e a l e d that the t w o coppers i n a g i v e n c o m p l e x d o n o t i n t e r a c t s i g n i f i c a n t l y (18), e a c h c o m p l e x m a y b e c o n s i d e r e d as t h e d u p l i c a t i o n o f t w o m o n o n u c l e a r s u b u n i t s o f i d e n t i c a l structure a n d redox potential, each subunit i n v o l v i n g one copper c o o r d i n a t e d to a m o n o c y c l e b e a r i n g o n e - h a l f l a t e r a l c h a i n o n e a c h n i t r o g e n . T h e results on m o n o n u c l e a r chelates r e v e a l e d that b u l k y s u b s t i t u e n t s o n n i t r o g e n s i n d u c e a n o d i c shifts o f the C u ( I I ) / C u ( I ) p o t e n t i a l , t h u s s t a b i l i z i n g c o p p e r ( I ) w i t h r e s p e c t to c o p p e r ( I I ) , a n d , as a l r e a d y o b s e r v e d i n o t h e r m o n o n u c l e a r c o m p l e x e s ( 5 ) , t h i s effect m a y b e a s c r i b e d to a n i n c r e a s e d distortion o f the c o o r d i n a t i o n sphere— from p l a n a r to tetrahedral-distorted—as the s i z e o f the n i t r o g e n s u b s t i tuents increases. P r o b a b l y , then, i n the d i n u c l e a r c o m p l e x e s w i t h 10, 1 1 , a n d 1 2 , t h e s a m e q u a l i t a t i v e a r g u m e n t s h o l d . H o w e v e r , at v a r i a n c e w i t h m o n o n u c l e a r chelates, i n l i g a n d s l i k e 10, 11, a n d 12, other c o n f o r m a t i o n a l e l e m e n t s c o m e i n t o p l a y , so t h a t t h e w h o l e flexibility o f the l i g a n d , rather than just the flexibility o f the side chains, m a y be c o n s i d e r e d as a c r u c i a l p a r a m e t e r i n t h e p r e f e r e n t i a l s t a b i l i z a t i o n o f a g i v e n f o r m , o x o r r e d . H o w e v e r , t h e less flexible t h e l a t e r a l c h a i n (flex i b i l i t y d e c r e a s i n g f r o m 11 to 1 0 , a n d t o 12), t h e m o r e c o p p e r ( I ) is s t a b i l i z e d t o w a r d copper(II), p r o b a b l y d u e to c o r r e l a t i v e i n c r e a s i n g distor tion o f e a c h t w o m o n o c y c l e s (two nitrogens, t w o sulfurs). Q u a n t i t a t i v e c o r r e l a t i o n i n t h e s e r i e s is p r e v e n t e d b y t h e i n t e r a c t i o n o f c o p p e r w i t h o x y g e n i n t h e c o m p l e x w i t h 10 (17). See F i g u r e 6. O n the other h a n d , o f a l l the d i n u c l e a r c o m p l e x e s , the c o m p l e x w i t h 6 , 2 [ C u ( I I ) ( N ) ] - 6 , is u n i q u e i n b e i n g a b l e t o s i m u l t a n e o u s l y r e p l i c a t e the r e d o x properties (reversible t w o - e l e c t r o n acceptor/donor at r a t h e r p o s i t i v e p o t e n t i a l ) a n d t h e m a g n e t i c c h a r a c t e r i s t i c s ( E S R s i l e n t ) o f t y p e 3 c o p p e r p a i r s i n c o p p e r e n z y m e s (7). 3

2

T h e redox a n d spectral properties o f the d i n u c l e a r copper c o m p l e x w i t h 6 a r e m u c h a k i n to t h o s e r e p o r t e d o n t r i k e t o n a t e d i c o p p e r c o m p l e x e s (14,15), w i t h t h e a d d i t i o n a l f e a t u r e t h a t t h e s t a n d a r d r e d o x p o t e n t i a l o f t h e c o m p l e x w i t h 6 is p o s i t i v e a n d is c l o s e to t h o s e r e p o r t e d for t y p e 3 c o p p e r p r o t e i n s , at v a r i a n c e w i t h o t h e r m o d e l s 1

(14, 15). I n a d d i t i o n , the results o b t a i n e d o n d i n u c l e a r c o p p e r c o m p l e x e s o t h e r t h a n t h o s e w i t h l i g a n d 6 p r o v i d e t h e a n s w e r t o a q u e s t i o n ( 14), as


Kadish; Electrochemical and Spectrochemical Studies of Biological Redox Components Advances in Chemistry; American Chemical Society: Washington, DC, 1982. 2

^CH ) 4

2

8tab

2

3

(57.10 )

ligand 5 : E ^ - + 0 2 7 ; n = 2

3

2

7

ll2

^ ^ ^ ^ ^ ^ ^ ^ ^

7

(2.0 1 0 )

ligand 7 : E ^ = + 0 4 8 ; n = 2

8tab

Figure 6. Effect of the side chain on the redox potential [Cu(II), Cu(II)]/[Cu(I), Cu(I)] and on the relative stability of cuprous to cupric dinuclear complexes with Ligands 10, 11, and 12. All E are in Table IL Values of {K of [Cu(I)] L/K of [Cu(II)] L} are 5.7 x I 0 , 8 χ J O , and 2.0 x 10 for 10,11, and 12, respectively. Medium: PC + 0.1 M TEAP.

(C»(l

(8102j

;

ligand 6 : E-j^ = + 0.22 n=2


5.


Copper Cryptâtes

135

they demonstrate c l e a r l y that the t w o adjacent coppers(II) i n a c o m p l e x d o n o t n e e d to b e s t r o n g l y i n t e r a c t i n g t o a l l o w a r e v e r s i b l e * t w o e l e c t r o n c a t h o d i c transfer, as a l r e a d y d i s c u s s e d i n a p r e v i o u s

paper

d e v o t e d t o t h e d i n u c l e a r c o m p l e x w i t h 10 ( 1 5 ) .

Conclusion F i r s t , t h e m o n o n u c l e a r c o m p l e x e s s t u d i e d r e v e a l e d t h a t t h e for mal

redox

copper(I)

potential

o f C u ( I I ) / C u ( I ) is a n o d i c a l l y s h i f t e d , t h a t i s ,

stabilized vs. copper(II),

by

increasing the

number

of

s u l f u r a t o m s i n t h e m a c r o c y c l i c l i g a n d s . T h i s t r e n d is v e r i f i e d for a Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0201.ch005

g i v e n l i g a n d s t r u c t u r e (for i n s t a n c e f r o m l a t o l c , f r o m 5 to 4a) a n d a l s o w h e n t h i o e t h e r g r o u p s are i n c r e a s i n g l y i n t r o d u c e d i n t o t h e m a c r o c y c y l i c l i g a n d , for e x a m p l e , f r o m 2 to 3 a n d 4. O x y g e n h e t e r o a t o m s i n a l i g a n d are u n f a v o r a b l e to t h e c o n v e r s i o n o f c o p p e r ( I I ) t o c o p p e r ( I ) i n these m o n o n u c l e a r c o m p l e x e s . T h e s e results are consistent w i t h , a n d to s o m e e x t e n t p r e d i c t a b l e f r o m p r e v i o u s o b s e r v a t i o n s o n o t h e r c o m plexes

( 5 , 6)

as w e l l

as f r o m H S A B

considerations

(44).

Second,

t h e f o r m a l r e d o x p o t e n t i a l o f C u ( I I ) / C u ( I ) a l s o is s h i f t e d a n o d i c a l l y w h e n steric strains are e x e r t e d o n the c o m p l e x i n g m a c r o c y c l e s u c h that conformational changes o c c u r that alter the preferred c o o r d i n a t i o n o f copper(II). S u c h changes m a y very w e l l result from the substitution o f b u l k y g r o u p s ( l i k e m e t h y l or b e n z y l ) for h y d r o g e n o n t h e n i t r o g e n s . W i t h d i n u c l e a r c r y p t â t e s , the results o b t a i n e d are consistent w i t h the c o n c l u s i o n s d r a w n f r o m the results o n m o n o n u c l e a r c r y p t â t e s , a n d they provide additional information: • T h e C u ( I I ) / C u ( I ) potential d e p e n d s almost a l w a y s on the coordinat i n g s i t e , so t h a t t w o i d e n t i c a l sites i n d u c e a s i n g l e t w o - e l e c t r o n t r a n s fer, w h e r e a s n o n i d e n t i c a l sites (see

L i g a n d 8) i n d u c e t w o d i s t i n c t ,

one-electron steps. • S p e c i f i c effects o f t h e b i c y c l i c s t r u c t u r e o f t h e l i g a n d a r e o b s e r v a b l e , as for i n s t a n c e w i t h L i g a n d 9 w h e r e E° C u ( I I ) / C u ( I ) is q u i t e s i m i l a r to t h e p o t e n t i a l o b s e r v e d i n t h e p r e s e n c e o f t h i o e t h e r s ( L i g a n d s 1 0 , 11, a n d

12), a n d is at v a r i a n c e w i t h t h e r e s u l t s i n c o r r e s p o n d i n g

m o n o n u c l e a r c r y p t â t e s ( L i g a n d s 4 a n d 5). • T h e o c c u r r e n c e o f a r e v e r s i b l e , t w o - e l e c t r o n t r a n s f e r is i n d e p e n d e n t o f the c o o r d i n a t i o n o f b r i d g i n g units b e t w e e n the t w o coppers

(see

c o m p l e x w i t h 6 a n d 10). T h u s , u n a m b i g u o u s i n f o r m a t i o n is n o w m a d e a v a i l a b l e o n t h e l i g a n d d e s i g n r e q u i r e d to f o r m stable copper(II) a n d copper(I) c o m p l e x e s w h o s e redox properties m i m i c satisfactorily those o f c o p p e r proteins. M o r e g e n e r a l l y , the a b o v e results o n the redox properties o f mononuclear a n d d i n u c l e a r copper complexes p r o v i d e rational ele-



136

BIOLOGICAL REDOX COMPONENTS

ments in understanding the control of redox potentials in natural multielectron metalloorganic redox mediators. Until now wide efforts had been directed to the control of redox potentials of models. Convincing evidence exists that such control may indeed be achieved efficiently through appropriate substitutions in the ligand(s) and/or coordination changes on the metal ion. Typi cally, the redox potential of a given couple in such systems may be shifted by an interval as large as 1 V (54). However, in many circum stances the shift of the redox potentials through such means is rather questionable in living systems. The results presented in this chapter provide reasonable clues to the possibility of achieving the control of redox potentials mainly through the conformation of the ligand surrounding the redox active metal, in addition to the effects on the potentials of the chemical com position of the ligand.

Acknowledgments We are indebted to S. Sullivan, B. Dietrich, A. Alberts (Laboratoire de Chimie Organique Physique) and J. M . Lehn (Univer sité Louis Pasteur, Strasbourg) for the syntheses of the ligands.

Literature Cited 1. Gray, H. B. In "Bioinorganic Chemistry," Dessy, R.; Dillard, J.; Taylor, L., Eds.; ACS ADVANCES IN CHEMISTRY SERIES, No. 100, ACS: Washington, D.C., 1971; p. 365. 2. Fee, J. A. Struct. Bonding 1975, 23, 1-60. 3. James, B. R. Williams, J. P. J. Chem. Soc. 1961, 2007. 4. Hawkins, C. J.; Perrin, D. D. J. Chem. Soc. 1962, 1351. 5. Patterson, G. S.; Holm, R. H. Bioinorg. Chem. 1975, 4, 257. 6. Dockal, E. R.; Jones, T. E.; Sokol, W. F.; Engerer, R. J.; Rorabacher, D. B.; Ochrymowycz, L. A. J. Am. Chem. Soc. 1976, 98, 4322. 7. Louis, R.; Agnus, Y.; Weiss, R.; Gisselbrecht, J. P.; Gross, M. Nouv. J. de Chimie, 1981, 5 (2), 71. 8. Harris, C. M.; Hoskins, B. F.; Martin, R. L. J. Chem. Soc. 1959, 3728. 9. Majumbar, A. K.; Saka, S. C. J. Indian Chem. Soc. 1973, 50, 697. 10. Corbett, M.; Hoskins, B. F.; McLeod, N. J.; O'Day, B. P. Acta Crystallogr., Sect. A 1972, 28, 576. 11. Gupta, S.; Kalia, K. C.; Chatravorty, A. Inorg. Chem. 1971, 10, 1534. 12. de Courcy, J. S.; Waters, T. N.; Curtis, N. F. J. Chem. Soc., Chem. Comm. 1977, 572. 13. Fenton, D. E.; Schroeder, R. R.; Lintvedt, R. L. J. Am. Chem. Soc. 1978, 100, 1931. 14. Fenton, D. E.; Lintvedt, R. L. J. Am. Chem. Soc. 1978, 100, 6367. 15. Gisselbrecht, J. P.; Gross, M.; Alberts, A. H.; Lehn, J. M. Inorg. Chem. 1980, 19, 1386. 16. Alberts, A. H.; Annunziata, R.; Lehn, J. M. J. Am. Chem. Soc. 1977, 99, 8502. 17. Louis, R.; Agnus, Y.; Weiss, R. J. Am. Chem. Soc. 1978, 100, 3604. ;


5.

GISSELBRECHT AND GROSS

Copper Cryptates

137


18. Kahn, O.; Morgenstern-Badaray, I.; Audière, J. P.; Lehn, J. M.; Sullivan, S. J. Am. Chem. Soc. 1980, 102, 5935. 19. Lehn, J. M. Struct. and Bonding 1973, 16, 1. 20. Dietrich, B.; Lehn, J. M.; Sauvage, J. P. Tetrahedron Lett. 1969, 34, 2885. 21. Dietrich, B.; Lehn, J. M.; Sauvage, J. P. Chem. Commun. 1970, 1055. 22. Arnaud-Neu, F.; Spiess, B.; Schwing-Weill, M. J. Helv. Chim. Acta 1977, 60, 263. 23. Sullivan, S.; Lehn, J. M., personal communication. 24. Herceg, M.; Weiss, R. Inorg. Nucl. Chem. Lett. 1970, 6, 435. 25. Herceg, M.; Weiss, R. Acta Crystallogr., Sect. Β 1973, 29, 542. 26. Levich, V. G. "Physicochemical Hydrodynamics"; Prentice-Hall: New York, 1962. 27. Butler, C. G.; Kaye, R. C.J.Electroanal. Chem. 1964, 8, 463. 28. Nicholson, R. S. Anal. Chem. 1965, 37, 1351. 29. Diggle, J. W.; Parker, A.J.Aust.J.Chem. 1974, 27, 1617. 30. Kolthoff, I. M.; Lingane, J. J. "Polarography"; Wiley: New York, 1952; p. 217. 31. Heyrovsky, J.; Kuta, J. "Principles of Polarography"; Academic: New York, 1966; p. 157. 32. Latimer, W. "Oxidation Potentials"; Prentice-Hall: New York, 1952. 33. Kolthoff, I. M.; Coetzee,J.F.J.Am. Chem. Soc. 1957, 79, 1852. 34. Yves, D. G.J.;Janz, G. J. "Reference Electrodes"; Academic: New York, 1961; p. 333. 35. Dobos, D. "Electrochemical Data"; Am. Elsevier: New York, 1975; p. 248. 36. Khomutov, N. E. Russ.J.Phys. Chem. 1962, 36, 1095; 1964, 38, 681; 1966, 40, 315. 37. Tindall, G. W.; Bruckenstein, S. Anal. Chem. 1968, 40, 1402. 38. Bossu, F. P.; Chellappa, K. L.; Margerum, D. W.J.Am. Chem. Soc. 1977, 99, 2195. 39. Grimshaw, J.; Hamilton, R.J.Electroanal. Chem. 1980, 106, 339. 40. Addison, A. W.; Stenhouse, J. H. Inorg. Chem. 1978, 17, 2161. 41. Bjerrum, J.; Ballhausen, C. J.; Jorgensen, C. K. Acta Chem. Scand. 1954, 8, 1245. 42. Furlani, C.; Morpurgo, G. Theor. Chim. Acta 1963, 1, 102. 43. Sacconi, L.; Giampolini, M.J.Chem. Soc. 1964, 276. 44. Pearson, R. G.J.Chem. Educ. 1968, 45, 581, 643. 45. Cotton, F. Α.; Wilkinson, G. "Advanced Inorganic Chemistry," 3rd ed.; Wiley: New York, 1972; p. 904. 46. Mehrotra, P. K.; Hoffmann, R. Inorg. Chem. 1978, 17, 239. 47. Agnus, Y.; Louis, R.; Weiss, R.J.Am. Chem. Soc. 1979, 101, 3381. 48. Arnaud-Neu, F.; Schwing-Weill, M.J.,unpublished data. 49. Peter, F.; Gross, M.; Pospisil, L.; Kuta,J.J.Electroanal. Chem. 1978, 90, 239. 50. Myers, R. L.; Shain, I. Anal. Chem. 1969, 41, 980. 51. Ammar, F.; Savéant, J. M.J.Electroanal. Chem. 1973, 47, 215. 52. Flanagan, J. B.; Margel, S.; Bard, A.J.;Anson, F.J.Am. Chem. Soc. 1978, 100, 4248. 53. Gisselbrecht, J. P.; Gross, M., unpublished data. 54. Giraudeau, Α.; Callot, H.J.;Jordan,J.;Ezahr, I.; Gross, M.J.Am. Chem. Soc. 1979, 101, 3857. RECEIVED for review June 30, 1981. Accepted November 30, 1981.


Control of Redox Potentials in Mononuclear and Dinuclear Copper

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