Inorganic Compounds with Unusual PropertiesâII - American

15 Binuclear Metal Complexes of Cofacial Diporphyrins C. K. CHANG

Downloaded by UNIV LAVAL on September 14, 2015 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch015

Michigan State University, East Lansing, MI 48823

A group of novel binuclear metal ligands composed of two alkyl porphyrins covalently linked in a cofacial configuration has been synthesized. The interplanar distance of the diporphyrins can be varied from 6.4 to 4.2 Å by changing the length of the linkage. The presence of exciton interaction in these dimers was evidenced by a substantial blue shift (10-30 nm) of the Soret peak. Both homo- and hetero-dimetalloporphyrins have been prepared, e.g., Fe-Fe, C u Cu, Mg-Mg, Co-Co, and Fe-Cu. Dioxygen was found to form both 1:1 and 2:1 complexes with Co(II) and Fe(II) diporphyrins. The ring separation played a major role in determining the metal-to-oxygen ratio. Implications of the dimer studies on biological oxygen reduction and the "special-pair" chlorophylls are discussed.

e r e c e n t l y h a v e b e e n d e v e l o p i n g t h e c h e m i s t r y of a g r o u p of n o v e l * * binuclear metal ligands composed of t w o porphyrins

covalently

l i n k e d i n a true parallel configuration. These cofacial diporphyrins have great significance i n m a n y b r a n c h e s of c h e m i s t r y . A s o r g a n i c m o l e c u l e s , i n a d d i t i o n to b e i n g c h a l l e n g i n g s y n t h e t i c targets, t h e y c a n present

a

m u l t i t u d e of p r o p e r t i e s b y t h e m e r e t o k e n of t h e i r size a n d b y t h e r e s u l t i n g i n t e r a c t i o n of t h e t w o 18 7r-electron p o r p h y r i n r i n g s . A s i n o r g a n i c c o m p o u n d s , they h a v e t h e u n u s u a l c a p a b i l i t y of c o n s t r a i n i n g t w o m e t a l ions at selected distances a n d thus c a n d i s p l a y i n t e r e s t i n g p r o p e r t i e s a r i s i n g f r o m m e t a l - m e t a l interactions.

F u r t h e r m o r e , f r o m t h e p o i n t o f v i e w of

b i o c h e m i s t r y , t h e y represent a class of e l a b o r a t e l y d e s i g n e d b i o i n o r g a n i c m o d e l s f o r m a n y essential b i o l o g i c a l systems; a m o n g these w e c i t e :

(1)

"special p a i r " chlorophyll m o d e l i n photosynthetic unit; ( 2 ) chlorophyll 0-8412-0429-2/79/33-173-162$05.0070 © 1979 American Chemical Society In Inorganic Compounds with Unusual Properties—II; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

15.

Cofacial

CHANG

Diporphyrin

163

Complexes

aggregates m o d e l f o r s t u d y i n g e x c i t a t i o n e n e r g y transfer processes;

(3)

c y t o c h r o m e oxidase m o d e l c a p a b l e of m u l t i e l e c t r o n r e d u c t i o n of o x y g e n ; ( 4 ) monooxygenase m o d e l b y w h i c h molecular oxygen c a n be "activated" v i a t w o - e l e c t r o n transfer; a n d ( 5 ) p o l y n u c l e a r complexes w i t h

certain

c a t a l y t i c a c t i v i t y . I t is n o d o u b t that t h e s t u d y of c o f a c i a l d i p o r p h y r i n s a n d t h e i r m e t a l complexes w i l l l e a d to some v e r y i n t e r e s t i n g c h e m i s t r y . I n this c h a p t e r o u r v e r y recent w o r k i n this area is s u m m a r i z e d , a n d some i n i t i a l findings c o n c e r n i n g t h e i r b i o l o g i c a l i m p l i c a t i o n s are p r e s e n t e d .


Synthesis T h e c o m p o u n d s to b e d i s c u s s e d i n c l u d e t h e f o l l o w i n g : Hex

1

R = - C H C H C O N (n-Bu) C H C H C H -

d — 6.4 A

2

R — -CH CON(n-Bu) C H C H C H -

d — 5.4 A

3

R — -CH CON(n-Bu)CH CH -

d — 4.2 A

2

2

2

2

2

2

2

2

2

2

2

2

w h e r e t h e t w o M s c a n b e i d e n t i c a l o r different.

A r a t i o n a l synthesis o f

these s t a c k e d m a c r o c y c l i c c o m p o u n d s w o u l d b e b a s e d o n a n u m b e r o f premises:

(1) a porphyrin monomer carrying diagonally functionalized

s i d e chains (C h s y m m e t r y ) s h o u l d b e p r e p a r e d i n l a r g e q u a n t i t y ; ( 2 ) t h e 2

c o u p l i n g of t h e t w o p o r p h y r i n s s h o u l d b e i n s u c h a m a n n e r that t h e resultant d i m e r c a n consist of d i s s i m i l a r p o r p h y r i n s a n d / o r hetero m e t a l ions; a n d ( 3 ) the composite diporphyrins should have relatively h i g h s o l u b i l i t y a n d g o o d c h e m i c a l s t a b i l i t y so that a w i d e range of experiments can be performed i n solution. Since porphyrins w i t h C h symmetry can2

not b e obtained b y m o d i f y i n g the naturally occurring type I X porphyrins, our objective at t h e outset o f this w o r k w a s to find a p r a c t i c a l synthesis f o r porphyrins h a v i n g the desired functional groups. It w i l l b e noticed that t e t r a a r y l p o r p h y r i n ( T P P ) d e r i v a t i v e s h a v e n o t b e e n selected b e c a u s e o f

In Inorganic Compounds with Unusual Properties—II; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

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the p o s s i b l e steric h i n d r a n c e associated w i t h the v e r t i c a l b e n z e n e rings w h i c h c a n p r e v e n t the f o r m a t i o n of a t i g h t l y s p a c e d d i m e r . W e h a v e d i s c o v e r e d that the c a r b o x y d i p y r r o m e t h e n e

6, w h i c h is

m o r e c o n v e n i e n t l y p r e p a r e d t h a n the u n s u b s t i t u t e d a n a l o g 7, c a n b e b r o m i n a t i v e l y d e c a r b o x y l a t e d a n d c y c l i z e d w i t h o u t i s o l a t i o n to p o r p h y r i n s w i t h v e r y g o o d y i e l d s ( 1 , 2 ) . T h i s m e t h o d is e s p e c i a l l y attractive i n that the c y c l i z a t i o n c a n b e p e r f o r m e d i n l a r g e scale (0.2 m o l ) a n d suffers n o decrease i n y i e l d . T h i s feature c o m b i n e d w i t h the f a c t that most d i p y r r o m e t h e n e precursors also c a n b e p r e p a r e d i n large quantities f r o m r e a d i l y


a v a i l a b l e materials have m a d e it p o s s i b l e f o r the first t i m e f o r p o r p h y r i n s w i t h e l a b o r a t e d s u b s t i t u t i o n patterns to b e r e a l i s t i c a l l y p r e p a r e d i n a l a r g e scale ( S c h e m e 1 ) . Scheme 1


15.

Cofacial

CHANG

L i

2

2

CH SO Cl 3

> R = CH CH CH OH x

2

2

2

a

> R = CH CH CH OS0 CH x

2

2

2

2

or C H C 0 M e

orCH CH OH

or C H C H O S 0 C H

R = hcxyl

R = hexyl

R = hexyl

2

2

2


165

Complexes

AlH*

8 Rx = C H C H C 0 M c 2

Diporphyrin

2

2

2

2

2

2

;

3

2

40-60% 10 or 11 T h e p o r p h y r i n ester 8 w a s t h e n r e d u c e d a n d c o n v e r t e d to t h e secondary amine b y Reaction 1 ( 3 ) . T h e c o u p l i n g of t w o porphyrins v i a a m i d e f o r m a t i o n ( R e a c t i o n 2 ) w a s effected b y a h i g h d i l u t i o n , s l o w m i x i n g procedure that used a syringe p u m p ( I , 3 ) . T h e resulting diporp h y r i n c a n consist o f t w o diastereoisomers, 12 a n d 1 3 ; each, i n t u r n , c a n exist as a p a i r of enantiomers. A t t e m p t s to separate t h e m b y H P L C h a v e not b e e n successful.



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13 M e t a l ions c a n b e i n s e r t e d to the ( d i p o r p h y r i n l i g a n d s b y s t a n d a r d procedures.

I f a m e t a l c o m p l e x of t h e d i a m i n e 10 o r 11 w a s c o u p l e d

w i t h a f r e e b a s e p o r p h y r i n , the r e s u l t a n t d i m e r w o u l d h a v e o n l y o n e m e t a l i o n w h i l e s u b s e q u e n t m e t a l i n s e r t i o n c o u l d l e a d to a m i x e d d i m e t a l system.

U s i n g this a p p r o a c h , w e h a v e s u c c e s s f u l l y p r e p a r e d

Cu-H , 2

F e - C u , M g - H , a n d F e - M g d i p o r p h y r i n s . T h e s t a b i l i t y of m o s t of t h e 2

d i m e t a l c o m p l e x e s appears r a t h e r s i m i l a r to t h e m o n o m e r s . system, h o w e v e r , is m o r e sensitive to a c i d .

The

Mg-Mg

U p o n washing with dilute

a c i d s , o n e M g is e x p e l l e d to g i v e a m o r e stable M g - H

2

dimer.

T h e N M R s p e c t r a of t h e f r e e b a s e d i p o r p h y r i n s r e v e a l e d v e r y s m a l l shifts f o r t h e p e r i p h e r a l substituents.

T h i s is e x p e c t e d since the p s u b -

stituents of o n e p o r p h y r i n f e l l i n the b l a n k r e g i o n of t h e

anisotropic

s h i e l d i n g of t h e s e c o n d r i n g . T h e i n n e r n i t r o g e n p r o t o n s , h o w e v e r , w e r e s h i f t e d d r a s t i c a l l y to h i g h

field,

p r e s u m a b l y b e c a u s e of e n h a n c e d Table I. NMR

Compound 8 M e ester

Soret

Band

N-H

(CDCls)

a

ring

Characteristic D a t a Fluorescence* Q(0fl)

398 n m (169.9 m M )

-3.8 8

619

nm

1 Dimer-7

383 n m (191)

-6.2 8

628

nm

2 Dimer-6

381 n m (209)

-6.6 8

630

nm

3 Dimer-5

373 n m (201)

-8.5 8

630

nm

c

° Absorption spectra recorded in C H C l 2 at 23°C. 77°K in toluene glass. • Fluorescence quantum yields assuming yield for etioporphyrin I = 0.09. 2

6


15.

CHANG

Cofacial

c u r r e n t (3,4).

Diporphyrin

167

Complexes

T h e m a g n i t u d e of the shift d e p e n d s o n the r i n g separa-

t i o n . T h i s i n t e r p l a n a r distance c a n b e e s t i m a t e d b y s t u d y i n g the d i p o l a r i n t e r a c t i o n of t w o p a r a m a g n e t i c m e t a l ions ( 5 )

s u c h as C u

2 +

(I =

3/2).

T h e E P R s p e c t r u m of the C u - C u d i p o r p h y r i n 3 is s h o w n i n F i g u r e 1. T h e t w o u n p a i r e d c o p p e r electrons are l o c a l i z e d l a r g e l y i n the p o r p h y r i n p l a n a n d c a n n o t p a i r ; o n e c a n c l e a r l y see the t r i p l e t s p e c t r u m w i t h the seven-line h y p e r f i n e splittings as w e l l as the intense signals i n the n o r mally forbidden half-field region ( A M — ± 2 ) S

ent z e r o - f i e l d s p l i t t i n g ( D )

(6,7,8).

F r o m the a p p a r -

o b t a i n e d f r o m t h e f u l l - f i e l d lines, one

can


estimate the separation b e t w e e n the coppers to b e 4.2 A . E P R parameters f o r other C u - C u d i p o r p h y r i n s are l i s t e d i n T a b l e I.

Exciton

Interaction

T h e a b s o r p t i o n m a x i m a of the d i p o r p h y r i n s ( T a b l e I a n d F i g u r e 2 ) d i f f e r e d f r o m those of m o n o m e r i c p o r p h y r i n s i n t h a t : ( 1 ) the Soret b a n d of d i p o r p h y r i n s s h i f t e d to the b l u e ; ( 2 ) the v i s i b l e b a n d s shift s l i g h t l y to the r e d ; a n d ( 3 )

t h e Soret b a n d has a p r o m i n e n t r e d t a i l e x t e n d i n g o u t

to the 500 n m r e g i o n . T h e s e s p e c t r a l a b n o r m a l i t i e s c a n b e u n d e r s t o o d i n terms of e x c i t o n c o u p l i n g (4).

If a p a i r of d e g e n e r a t e d i p o l a r states

( X , Y ) o n p o r p h y r i n A is to interact w i t h a s i m i l a r p a i r ( X ' , Y ' ) o n p o r p h y r i n B , the nature of the e x c i t o n c o u p l i n g w i l l d e p e n d o n the d i m e r geometry. The.energy

l e v e l d i a g r a m i n F i g u r e 3 shows t h e r e l a t i o n s h i p

be-

t w e e n the energy of a p a r t i c u l a r t r a n s i t i o n f o r a n i s o l a t e d m o l e c u l e , A E , 0

a n d the e n e r g y of t h e a l l o w e d c o m p o n e n t of the s a m e t r a n s i t i o n i n the

of Cofacial Diporphyrins CUr-Cu Zero-Field Splitting*

Interplanar^ Separation

0.094 6.4

A

0.0205 cm"

5.4

A

0.0415 c m '

4.2

A

0.035

0.011

0.021 0.007

cm"

E P R spectra were recorded on a Varian E-4 spectrometer, 77 °K in CH2CI2/ toluene. • Calculated according to Ref. 7. d


In Inorganic Compounds with Unusual Properties—II; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1979. 2

2

Figure 1. EPR spectrum of the di-copper complex of diporphyrin 3 in CH Cl /toluene at 77° Κ (4)


I

w

a

w w

ο

•

C/î

ο

§

η η ο

I

ο

§

M

σ> oo

Cofacial Diporphyrin Complexes

CHANG

169


15.

350

400

450

500

WAVELENGTH (nm)

550

600

650

Journal of Heterocyclic Chemistry

Figure 2. (A) Diporphyrin 1 (solid line) vs. monomer 8 dipropionic acid ester (dotted line). (B) Diporphyrin 3 (solid line) vs. monomer 8 diacetic acid ester (dotted line) (A).


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Dan DIMER MONOMER

A B


tt

AE

0) c UJ

0

AE= AE +AD+V 0

Figure 3. Exciton splitting in a dimer d i m e r . T h e A D t e r m is t h e " s o l v e n t p a r a m e t e r " w h i c h represents t h e d i f f e r e n c e i n t h e effect of s o l v a t i o n o n t h e e n e r g y of t h e g r o u n d a n d e x c i t e d states. T h e e x c i t o n c o u p l i n g b e t w e e n t w o p a r a l l e l t r a n s i t i o n d i p o l e s is b a s i c a l l y set b y E q u a t i o n 3, w h e r e /* a n d R a r e t h e d i p o l e m o m e n t a n d u

V =

2

(3)

—G

t h e p e r p e n d i c u l a r d i s t a n c e b e t w e e n t h e t w o d i p o l e s , r e s p e c t i v e l y . G is a geometric factor related to the orientation of the t w o p o r p h y r i n planes. I n t h e case o f t h e i d e a l i z e d g e o m e t r y w i t h

symmetry (no tilting

a n d s l i d i n g of t h e t w o r i n g s ) , there is a h i g h e r energy, degenerate, a l l o w e d p a i r o f e x c i t o n states ( X + X ' a n d Y + Y ' ) a n d a l o w e r energy, f o r b i d d e n p a i r o f states ( X — X ' a n d Y — Y ' ) , w h o s e n e t t r a n s i t i o n d i p o l e m o m e n t s are e q u a l t o z e r o . C o n s e q u e n t l y o n l y o n e l i n e w i l l b e o b s e r v e d i n t h e a b s o r p t i o n s p e c t r u m , a n d i t is s h i f t e d t o t h e b l u e w i t h respect t o t h e a b s o r p t i o n of t h e m o n o m e r . I n s u c h case G equals u n i t y , a n d f r o m a v a i l a b l e g e o m e t r i c a l d a t a o n e c a n estimate a r a n g e o f m a g n i t u d e s f o r t h e exciton coupling term V i n the diporphyrins. H o w e v e r , if the dimer g e o m e t r y deviates f r o m t h e p o r p h y r i n planes w o u l d cause

symmetry, the tilting a n d sliding of the fluctuation

o f t h e h i g h e r e x c i t o n levels as

w e l l as d e v e l o p i n g i n t e n s i t y i n the l o w e r e x c i t o n states ( 9 ) .

Although

t h e i n t e n s i t y of t h e l o w e r e x c i t o n t r a n s i t i o n ( A E ~ ) c a n n o t c o m p a r e w i t h t h a t of t h e A E , A E ~ c a n b e r e s p o n s i b l e f o r t h e a p p e a r a n c e of t h e "Soret +

t a i l " i n the 450-nm region.


15.

CHANG

Cofacial

Diporphyrin

171

Complexes

2000 c m " < V < 6000 crrf 1

(4)

6.4 A > R > 4.2 A

T h e a b o v e e s t i m a t i o n is s u p p o r t e d f u r t h e r b y s p e c t r a l i n f o r m a t i o n s h o w n i n F i g u r e 2. L e t us first c o n s i d e r the b l u e shift of the Soret b a n d ( B s t a t e ) . T o first a p p r o x i m a t i o n , t h e B


A#

B

+

+

b a n d s h o u l d o c c u r at:

= A# ° + A D + B

(5)

V

w h e r e A E ° is the Soret a b s o r p t i o n of m o n o m e r . A D u s u a l l y causes a r e d B

shift b u t its v a l u e is d i f f i c u l t to estimate.

I n d i p o r p h y r i n 1, f r o m the A E

at 384 n m a n d t h e r e d t a i l w h e r e the c e n t e r of AE ~ B

B

+

b a n d p r o b a b l y oc-

c u r r e d a r o u n d 480 n m , one c a n set a n u p p e r l i m i t f o r the e x c i t o n c o u p l i n g as h a l f of this energy gap or V =

2500 c m * , c o m p a r a b l e w i t h the V o b 1

t a i n e d b y E q u a t i o n 3. I n d i p o r p h y r i n 3, w i t h a s h o r t e n e d R, V is e n h a n c e d m o r e t h a n t h r e e f o l d , a n d this w a s reflected b y the w i d e r s e p a r a t i o n of the b l u e m a x i m u m a n d the r e d t a i l . I n the v i s i b l e r e g i o n ( Q state) since t h e t r a n s i t i o n d i p o l e strength is m u c h w e a k e r t h a n the B b a n d , the e x c i t o n c o u p l i n g is m u c h smaller.

H e r e the solvent r e d shift t e r m A D c a n

be

c o m p a r a b l e w i t h V i n m a g n i t u d e ; therefore, the slight r e d shift a n d the m o r e d i f f u s e d b a n d shape p o s s i b l y is a result of b o t h the m a n i f e s t a t i o n of A D a n d i n h o m o g e n e o u s solvent b r o a d e n i n g

(10).

T h e Soret b l u e shift appears to b e a c o m m o n feature of most c o f a c i a l m e t a l d i p o r p h y r i n s as w e l l , a l t h o u g h the m a g n i t u d e of the shift varies f r o m m e t a l to m e t a l . C o l l m a n et a l . ( I I )

have synthesized a T P P type,

face-to-face b i n a r y p o r p h y r i n w i t h 6.5 A s e p a r a t i o n a n d r e p o r t e d a 15-nm b l u e shift f o r t h e Soret peak.

T h e characteristic s p e c t r a l shifts also h a v e

b e e n o b s e r v e d i n z i n c p o r p h y r i n aggregates ( 1 2 ) , /x-oxo s c a n d i u m d i m e r s (10),

a n d sometimes e v e n i n n o n r i g i d l y h e l d " c l a m s h e l l " d i m e r s

I n the past year, h o w e v e r , t w o other g r o u p s (14,15)

have

(13).

described

b r i e f l y the synthesis of s i m i l a r t y p e d i p o r p h y r i n s , p r e s u m a b l y w i t h a r i n g s e p a r a t i o n i n the 6 A r a n g e a n d h a v e r e p o r t e d the absence of the Soret b l u e shift.

I n v i e w of the l a r g e b o d y of e v i d e n c e d e v e l o p e d f r o m o u r

w o r k , m e a n i n g f u l d i s c u s s i o n of the d i s c r e p a n c y m u s t a w a i t u n a m b i g u o u s p r o o f of t r u e c o f a c i a l g e o m e t r y i n the latter t w o d i p o r p h y r i n s . Fluorescence

e m i s s i o n d a t a of the d i p o r p h y r i n s also are g i v e n i n

T a b l e I : the Q ( 0 , 0 ) b a n d is r e d s h i f t e d a n d creased.

A l t h o u g h some r e d u c t i o n i n &

F

fluorescence

y i e l d is d e -

is p r e d i c t e d b y the

exciton

m o d e l ( 9 ) , the extent of q u e n c h i n g f o u n d i n the d i p o r p h y r i n s is c e r t a i n l y i m p r e s s i v e . It is l i k e l y that the e n o r m o u s r e d t a i l of the Soret b a n d f u r t h e r enhances the self q u e n c h i n g of the d i m e r

fluorescence

yield.


172

INORGANIC

Cation Radicals of the M g — Mg Electrolysis or ( C H ) N P F 4

9

4

of M g - M g 6

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

II

Diporphyrin diporphyrins i n C H C 1 2

[(CJIo^NCK^

2

as electrolyte] y i e l d e d v a r i o u s o x i d a t i o n p r o d u c t s

(16).

C y c l i c v o l t a m m e t r y s h o w e d that the first o x i d a t i o n p e a k f o r ( M g - M g ) * ± (Mg-Mg)

+ +

took p l a c e at a b o u t + 0 . 6 V ( v s . A g / A g C l e l e c t r o d e ) a n d

that t h e s e c o n d o x i d a t i o n f o r ( M g - M g )

+ +

*± ( M g - M g )

o c c u r r e d at

4 +

a b o u t + 1-0 V . P a r t i a l electrolysis of t h e M g - M g d i p o r p h y r i n 1 i n d e gassed C H C 1 2

2

at + 0 . 5 3 V ( s t o p p e d w h e n c o u l o m e t e r i n d i c a t e d that

a b o u t 0.2 m o l a r e q u i v a l e n t s of t h e d i m e r h a d b e e n o x i d i z e d ) y i e l d e d a Downloaded by UNIV LAVAL on September 14, 2015 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch015

v i o l e t s o l u t i o n w i t h a n a b s o r p t i o n p e a k at 670 n m , b e l i e v e d to c o n t a i n (Mg-Mg)

monocation radicals.

+

E P R measurements

o n this s o l u t i o n

s h o w e d a s i g n a l , g = 2.003, w h i c h has a p e a k - t o - p e a k s e p a r a t i o n of o n l y 1.05G

(Figure 4).

U n d e r identical conditions, M g O E P

f o u n d to h a v e a l i n e w i d t h of 2.5 G (17).

+

radicals were

T h e extremely narrow l i n e w i d t h

of the d i m e r is s u r p r i s i n g , a n d w e s p e c u l a t e that o l i g o m e r i z a t i o n of t h e monocation radicals m a y have taken place i n solution. Nevertheless, the n a r r o w i n g i n E P R l i n e w i d t h c l e a r l y i n d i c a t e s that there is extensive elect r o n e x c h a n g e b e t w e e n the t w o r i n g s , s i m i l a r to t h e p r o p e r t i e s e x h i b i t e d b y i n v i v o s p e c i a l p a i r ( b a c t e r i o ) c h l o r o p h y l l s P 7 0 0 (18)

a n d P870

(19).

F u r t h e r studies o n the p r o p e r t i e s of the i o n r a d i c a l species as a f u n c t i o n of the r i n g s e p a r a t i o n are b e i n g c a r r i e d o u t . W e also h a v e p r e p a r e d several M g - H

2

d i p o r p h y r i n s ; i t is e x p e c t e d that p h o t o l y s i s w o u l d p r o d u c e

charge-separated

Mg -H " +

2

species s i m i l a r to t h e ( P A « ± P A " ) +

donor-

a c c e p t o r b a c t e r i a p h o t o s y n t h e t i c units ( 2 0 ) .

Figure 4. EPR spectra of cation radicals of Mg-Mg-7 (1) and MgOEP. Electrolyses were carried out at + 0.53 V (vs. Ag/AgCl) in CH Cl with Bu NPF as electrolyte. Spectra were recorded on a Varian E-4 instrument at 23°C. 2

2


A

6


15.

Cofacial

CHANG

Figure

Multielectron

Diporphyrin

173

Complexes

5. Midpoint reduction potentials of oxygen cording to Wang (25)) and multielectron transfer

Reduction of Molecular

(ac-

Oyxgen

R e d u c t i o n of m o l e c u l a r o x y g e n is d i f f i c u l t f r o m the c h e m i c a l p o i n t of v i e w because one n o t o n l y m u s t c o m e u p w i t h a m e c h a n i s m to react w i t h the t r i p l e t g r o u n d state o x y g e n b u t also has to a v o i d the f o r m a t i o n of toxic, h i g h e n e r g y intermediates

s u c h as s u p e r o x i d e ( F i g u r e 5 )

(21).

N e v e r t h e l e s s , n a t u r e has s o l v e d this p r o b l e m m a r v e l l o u s l y b y u s i n g m u l t i n u c l e a r m e t a l catalysts.

T h e best e x a m p l e is p r o v i d e d b y

cytochrome

oxidase w h i c h contains t w o hemes a n d t w o c o p p e r ions a n d carries o u t a f o u r - e l e c t r o n r e d u c t i o n of o x y g e n to h y d r o g e n (22).

T h e i d e a of m u l t i -

e l e c t r o n r e d u c t i o n of o x y g e n c a n b e r e a l i z e d i n m e t a l d i p o r p h y r i n s i f o x y g e n c a n f o r m a s a n d w i c h e d c o m p l e x w i t h metals a n d receive f r o m t h e m . I n d e e d o x y g e n w a s f o u n d to f o r m either 1:1 or 2 : 1

electrons complexes

w i t h C o ( I I ) a n d F e ( I I ) d i p o r p h y r i n s , d e p e n d i n g o n the m e t a l s e p a r a t i o n . W h e n a b u l k y ligand, 1-triphenylmethylimidazole, was m i x e d w i t h C o ( I I ) - C o ( I I ) d i p o r p h y r i n 1 (6.5 A g a p ) a n d e x p o s e d to o x y g e n , b o t h v i s i b l e a n d E P R spectra d o c u m e n t e d the f o r m a t i o n of a d o u b l e Co—0

2

complex.

T h e oxygenation can be shown reversible:

(1:1)

evacuation

r e s u l t e d i n e l i m i n a t i n g the superoxo c o m p l e x a n d r e s t o r i n g the C o ( I I ) s i g n a l ( F i g u r e 6 A ) . C o ( I I ) - C o ( I I ) d i p o r p h y r i n 3 (4.2 A g a p ) , o n the other h a n d , r e a c t e d c o m p l e t e l y i n a different w a y . A d d i t i o n of o x y g e n to the [ < £ C I m - C o ( I I ) ] 3

2

c o m p l e x at r o o m t e m p e r a t u r e instantaneously p r o -

d u c e d a species consistent w i t h the f o r m u l a t i o n of 2 C o / 0

2

Co:0

2

2

=

(gasometry

1 : 0 . 5 5 ) . T h i s c o m p l e x , w r i t t e n as ju-peroxo [ C o - 0 - C o ] , is d i a -

m a g n e t i c a n d has n o E P R signals. W h e n a trace of I

2

w a s a d d e d to this

solution, a well-defined isotropic spectrum was obtained c o n s i s t i n g of 15 lines ( g

i s o

=

2.024 | A | = Co

10 G ) .

(Figure

6B)

Such a spectrum



174

INORGANIC COMPOUNDS W I T H UNUSUAL PROPERTIES

II

Figure 6. EPR spectra of oxygen-containing samples of [ CImCo(II)] diporphyrins. (A) Upper trace: complex of 1 with 1 atm of 0 at 77°K; bottom trace: after sample was evacuated at —20°C and recorded at 77°K. (B) Room ternperature spectrum of the dioxygen adduct of cobalt complex of 3 after addition of small amount of I . This is a typical binuclear fi-superoxo dicobalt spectrum. All experiments were carried out in CH Cl /toluene mixtures. s

2

2

2

2

2

w o u l d b e e x p e c t e d i f t h e //,-peroxo d i c o b a l t c o m p l e x b e c a m e o x i d i z e d t o a ft-superoxo d i c o b a l t c o m p l e x i n w h i c h t h e t w o e q u i v a l e n t

5 9

C o nuclei

w o u l d g i v e a t o t a l o f ( 2 X 2 X 7/2) + 1 = 15 lines ( 2 3 ) . T h e /*-supero x o f o r m u l a t i o n has f u r t h e r b e e n s u b s t a n t i a t e d b y

0 experiments

1 7

(24).

B e h a v i o r of t h e i r o n c o m p l e x of d i p o r p h y r i n s i n g e n e r a l p a r a l l e l s that of t h e c o b a l t system.

Oxygenation of

five-coordinate

c o m p l e x e s of 1 a n d 2 at — 4 5 ° C i n D M F or C H C 1 2

s p e c t r u m (a = 562 n m ,

2

[ CIm-Fe(II)] 3

2

resulted i n a visible

= 529 n m ) s i m i l a r to that of t h e o x y g e n a d d u c t

of t h e " m y o g l o b i n a c t i v e site m o d e l " ( 2 6 , 2 7 , 2 8 ) o r t h e " c r o w n e d h e m e " (1).

A d d i t i o n of o x y g e n to t h e same c o m p l e x of 3 nevertheless r e s u l t e d

i n instantaneous

o x i d a t i o n of t h e h e m e , e v e n at — 4 5 ° C .

k i n e t i c measurements

Preliminary

i n d i c a t e d that t h e rate of a u t o x i d a t i o n of this

b i s - h e m e is at least 1 0 times faster t h a n t h e m o n o m e r i c m y o g l o b i n site 3


15.

Cofacial

CHANG

Diporphyrin

T h i s is consistent w i t h the scheme

(29).

175

Complexes

that d i o x y g e n adds to the

b i s - h e m e to g i v e a ju-peroxo d i m e r [ F e - 0 - F e ] w h i c h t h e n 2

to y i e l d other F e ( I I I ) species (30).

decomposes

W i t h m o n o m e r i c hemes, f o r m a t i o n

of t h e /x-peroxo c o m p l e x is u s u a l l y the r a t e l i m i t i n g step (31,32).

In

F e - F e 3, since the t w o i r o n ions are p o s i t i o n e d at f a v o r a b l e distances, t h e e n e r g y b a r r i e r l e a d i n g to t h e s a n d w i c h e d o x y g e n c o m p l e x is l o w e r e d , t h e r e b y r e s u l t i n g i n a faster o x i d a t i o n rate. T h e e x c e e d i n g l y fast a u t o x i d a t i o n rate e x h i b i t e d b y some of t h e m e t a l d i p o r p h y r i n s suggests that these complexes c a n b e u s e d as a n efficient


catalyst f o r t h e e l e c t r o c h e m i c a l r e d u c t i o n of o x y g e n . S e v e r a l studies 34,35)

(33,

h a v e suggested the u s e of m e t a l p h t h a l o c y a n i n e s o r p o r p h y r i n s

s t r o n g l y a b s o r b e d o n v a r i o u s g r a p h i t e surfaces as catalysts.

Although a

r e d u c t i o n i n the o x y g e n o v e r p o t e n t i a l g e n e r a l l y has b e e n o b s e r v e d u s i n g these a b s o r b e d substances,

problems i n reproducibility, stability, a n d

a c t i v i t y over a w i d e range of p H s h a v e b e e n a c k n o w l e d g e d . W e h a v e e x a m i n e d the r e d o x b e h a v i o r of a n u m b e r of m e t a l , d i p o r p h y r i n - c o a t e d g r a p h i t e electrodes. P r e l i m i n a r y results i n d i c a t e d that t h e c u r r e n t - p o t e n t i a l (i-E)

curves generated b y l i n e a r s w e e p v o l t a m m e t r y i n o x y g e n -

s a t u r a t e d solutions w e r e rather s i m i l a r to those of m o n o m e r i c m e t a l l o p o r p h y r i n s ( F i g u r e 7 ) (36).

N o e x p l a n a t i o n c a n b e offered at present

100/z.A

(D

/

F e - F e - 7 /

(3) Fe-Fe-5 D

-02

/

\ N

/ / -0.4

-0.6

-OB

1 -1.0

1 -12 V

Figure 7. Linear sweep voltammograms of oxygen reduction by iron porphyrins deposited on carbon electrode in O . I N KOH, oxygen-saturated solution; 25°C, 20 mV'/sec


176

INORGANIC

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

II

b e c a u s e of t h e c o m p l e x i t y i n a n a l y z i n g t h e s u r f a c e - c o u p l e d o x y g e n r e d u c tion mechanism. catalyst

H o w e v e r , experiments

c o v a l e n t l y t o electrode

a r e u n d e r w a y t o attach t h e

surface so as t o m i n i m i z e t h e hetero-

g e n i t y of t h e surface. Prospect V a r i o u s lines f o r f u t u r e d e v e l o p m e n t o f t h e c h e m i s t r y o f c o f a c i a l d i p o r p h y r i n s c a n b e r e c o g n i z e d . A p a r t f r o m t h e f e w areas w e h a v e d i s cussed i n this chapter, several other examples

of investigation c a n b e


s e l e c t e d : (1) t h e c o o r d i n a t i o n a n d r e d u c t i o n o f m o l e c u l a r n i t r o g e n a n d u n s a t u r a t e d o r g a n i c c o m p o u n d s s u c h as R u - N - R u a n d R h - C H = C H - R h ; 2

( 2 ) t h e e l e c t r o c h e m i c a l a n d p h o t o c h e m i c a l reactions o f M n - M n d i p o r p h y r i n s w i t h w a t e r , since b i n u c l e a r m a n g a n e s e complexes a r e b e l i e v e d t o p l a y i m p o r t a n t r o l e i n the o x i d a t i o n of w a t e r to g i v e o x y g e n i n green p l a n t photosynthesis; ( 3 ) t h e synthesis of m e t a l p o r p h y r i n s o f n o v e l g e o m e t r y . M a n y 2 : 1 porphyrin-metal compounds can be prepared using diporp h y r i n as l i g a n d . W e b e l i e v e t h a t t h e s t u d y o f d i p o r p h y r i n s c a n l e a d t o m a n y c h a l l e n g i n g research areas w h i c h w i l l h a v e significant i m p a c t o n b o t h f u n d a m e n t a l as w e l l as a p p l i e d research. Acknowledgment I w o u l d l i k e t o t h a n k R e s e a r c h C o r p o r a t i o n a n d t h e donors o f t h e P e t r o l e u m R e s e a r c h F u n d , a d m i n i s t e r e d b y t h e A m e r i c a n C h e m i c a l Society, f o r p a r t i a l s u p p o r t of this research a n d t o C . B . W a n g f o r s y n t h e t i c effort. Literature Cited 1. Chang, C. K.,J,Am. Chem. Soc. (1977) 99, 2819. 2. Wang, C. B., Chang, C. K., unpublished data. 3. Chang, C. K., Kuo, M. S., Wang, C. B.,J.Heterocycl. Chem. (1977) 14, 943. 4. Chang, C. K., J. Heterocycl. Chem. (1977) 14, 1285. 5. Boyd, P. D. W., Smith, T. D., Price, J. H., Pilbrow, J. R., J. Chem. Phys. (1972) 56, 1253. 6. Blumberg, W. E., Peisach, J.,J.Biol. Chem. (1965) 240, 870. 7. Chasteen, N. D., Belford, R. L., Inorg. Chem. (1970) 9, 169. 8. Mengersen, C., Subramanian, J., Fuhrhop, J.-H., Mol. Phys. (1976) 32, 893. 9. Kasha, M., Rawls, H . R., El-Bayoumi, M. A., Pure Appl. Chem. (1965) 11, 371. 10. Gouterman, M., Holten, D., Lieberman, E., J. Chem. Phys. (1978). 11. Collman, J. P., Elliott, C. M., Halbert, T. R., Tovrog, B. S., Proc. Natl. Acad. Sci. USA (1977) 74, 18. 12. Zachariasse, K. A., Whitten, D. G., Chem. Phys. Lett. (1973) 22, 527. 13. Ichimura, K., Chem. Lett. (1977) 641.



15.

CHANG

Cofacial

Diporphyrin

Complexes

177

14. Ogoshi, H., Sugimoto, S., Yoshida, Z., Tetrahedron Lett. (1977) 169. 15. Kagan, N. E., Mauzerall, D., Merrifield, R. B., J. Am. Chem. Soc. (1977) 99, 5484. 16. Fuhrhop, J.-H., Kadish, K. M., Davis, D. G., J. Am. Chem. Soc. (1973) 95, 5140. 17. Felton, R. H., Dolphin, D., Borg, D. C., Fajer, J., J. Am. Chem. Soc. (1969) 91, 196. 18. Warden, J. T., Bolton, J. R., Acc. Chem. Res. (1974) 7, 189. 19. Corker, G. A., Photochem. Photobiol. (1976) 24, 617. 20. Clayton, R. K., Proc. Natl. Acad. Sci. USA (1972)69,44. 21. Hamilton, G. A., Prog. Bioorg. Chem. (1971) 1, 83. 22. Caughey, W. S., Wallace, W. J., Volpe, J. A., Yoshikawa, S., "The Enzymes," P. D. Boyer, Ed., 3rd ed., Vol. XIII, p. 299, Academic, New York, 1976. 23. Weil, J. A., Kinnaird, J. K.,J.Phys. Chem. (1967) 71, 3341. 24. Chang, C. K.,J.Chem.Soc.,Chem. Comm. (1977) 800. 25. Wang, J. H., Acc. Chem. Res. (1970) 3, 90. 26. Chang, C. K., Traylor, T. G., Proc. Natl. Acad. Sci. USA (1973) 70, 2647. 27. Chang, C. K., Traylor, T.G.,J.Am. Chem. Soc. (1973) 95, 5819. 28. Brinigar, W. S., Chang, C. K.,J.Am. Chem. Soc. (1974) 96, 5595. 29. Chang, C. K., Powell, D., Traylor, T. G., Croat. Chem. Acta (1977) 49, 295. 30. Chin, D.-H., Gaudio, J. D., La Mar, G. N., Balch, A. L.,J.Am. Chem. Soc. (1977) 99, 5486. 31. Cohen, I. A., Caughey, W. S., Biochemistry (1968) 7, 636. 32. Ochiai, E.-I., Inorg. Nucl. Chem. Lett. (1974) 10, 453. 33. Kozawa, A., Zilionis, V. E., Brodd, R. J.,J.Electrochem. Soc. (1970) 117, 1470. 34. Zagal, J., Sen, R. K., Yeager, E., Electroanal. Chem. Interfacial Electrochem. (1977) 83, 207. 35. Kuwana, T., Fujihara, M., Sunakwa, K., Osa, T., Electroanal. Chem. Interfacial Electrochem. (1978)88,299. 36. Deborski,G.,Armstrong, N., Chang, C. K., unpublished data. RECEIVED February 22, 1978.


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