Magnetic Resonance of Oxidized Metalloporphyrins - Advances in

16 Magnetic Resonance of Oxidized Metalloporphyrins 1

HAROLD M. GOFF , MARTIN A. PHILLIPPI, ARDEN D. BOERSMA, and ANDREW P. HANSEN University of Iowa, Department of Chemistry, Iowa City, IA 52242

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0201.ch016

Iron(III) porphyrin anion complexes undergo reversi ble one-electron oxidation at platinum electrodes in apo lar media. In favorable cases, oxidized products have been isolated from supporting electrolyte in analytically pure form. Although earlier reports favored electron abstraction from a metal-centered molecular orbital to yield an iron(IV) species, the results cited here for weak-field anion complexes of iron(III) tetraphenylpor phyrins(TPP) favor predominant porphyrin-centered oxidation. This result is demonstrated by the fact that NMR hyperfine coupling constants for phenyl protons of TPPFe(Cl)(ClO ) are equivalent to those determined by ESR of known π-radical species. Mössbauer results are also consistent with little perturbation of charge at the iron center on oxidation. Unique IR bands observed for both oxidized TPPFe species and TPPZn π-cation radicals may serve to distinguish the site of oxidation. Analogous results are reported for oxidized manganese porphyrins. Low spin diimidazole adducts of oxidized iron porphyrins are spectroscopically observed at low temperature. Porphyrin radical character is also appar ent in these derivatives. The chemistry of electrochemi cally oxidized metalloporphyrins and other reported iron(IV) species is reviewed in the context of known hemoprotein oxidation reactions. 4

2+

"D

+

e v e r s i b l e o n e - or two-electron oxidation o f the iron(III) p o r p h y r i n prosthetic group occurs d u r i n g the catalytic c y c l e o f peroxidases 1

Author to whom correspondence should be addressed. 0065-2393/82/0201-0357$06.00/0 © 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.

BIOLOGICAL REDOX COMPONENTS

358

a n d c a t a l a s e ( 1 - 3 ) . S u c h o x i d a t i o n i s a l s o p o s t u l a t e d for r e d o x p r o cesses o f o t h e r h e m o p r o t e i n s .

T h e best characterized

o x i d a t i o n se

q u e n c e s are t h o s e for h o r s e r a d i s h p e r o x i d a s e ( H R P ) , for w h i c h h y d r o gen peroxide or organic peroxides p r e s u m a b l y abstract two

electrons

from the h e m e group to g i v e C o m p o u n d I : PFe(III)

+

+

+ H 0 -> 2

PFe(IV)=0 + H 0

2

2

I


O n e - e l e c t r o n substrate reduction y i e l d s C o m p o u n d II: PFe(IV)=0

+

+ S-» PFe(IV)=0 + S

+

II I r o n ( I V ) p o r p h y r i n π - e a t i o n r a d i c a l f o r m u l a t i o n s for C o m p o u n d I o f H R P a n d c a t a l a s e w e r e s u g g e s t e d p r e v i o u s l y (4). C u r r e n t s u p p o r t for this u n u s u a l r e d o x s t r u c t u r e is b a s e d o n e l e c t r o n i c , E S R , a n d M ô s s b a u e r spectral measurements. Thus, the electronic spectrum o f H R P C o m p o u n d I exhibits a b r o a d Soret b a n d w i t h d i m i n i s h e d inten sity, a n d b r o a d , n o n d i s t i n c t i v e v i s i b l e r e g i o n b a n d s l i k e those o b served for π - e a t i o n r a d i c a l m e t a l l o p o r p h y r i n s . D e t e c t i o n o f a v e r y b r o a d g = 2 E S R s i g n a l for C o m p o u n d I a l s o s u p p o r t s a 7r-cation r a d i c a l state ( 5 ) . T h e s t r o n g e s t e v i d e n c e f a v o r i n g m e t a l - c e n t e r e d o x i d a t i o n i n b o t h C o m p o u n d s I a n d I I is f o u n d i n M ô s s b a u e r i s o m e r s h i f t v a l u e s o f v e r y n e a r 0.0 m m / s ( 5 - 7 ) . O x i d a t i o n o f aromatic or sulfur-based a m i n o a c i d residues appears to o c c u r for c y t o c h r o m e c p e r o x i d a s e ( C C P ) ( 8 ) . T w o - e l e c t r o n o x i d a t i o n o f C C P y i e l d s a s p e c i e s s p e c t r a l l y e q u i v a l e n t to H R P C o m p o u n d II, but e x h i b i t i n g a sharp E S R signal a n d E N D O R resonances inter p r e t e d as i n v o l v i n g a m e t h i o n i n e r e d o x site (9). B a s e d o n N M R r e s u l t s , s o m e r e s e a r c h e r s (10-12) s u g g e s t e d a n a n a l o g o u s f o r m u l a t i o n for H R P C o m p o u n d I i n w h i c h a n a m i n o a c i d r e s i d u e is o x i d i z e d a n d t h e i r o n ( I V ) p o r p h y r i n is i n t h e h i g h s p i n state. L a r g e i s o t r o p i c N M R shifts m a y b e e x p l a i n e d b y the w i d e l y a c c e p t e d iron(IV) p o r p h y r i n π - c a t i o n r a d i c a l e l e c t r o n i c s t r u c t u r e (13,14). A d d i t i o n a l c o m m e n t o n t h i s p o i n t is m a d e ( v i d e i n f r a ) w i t h r e s p e c t t o o u r r e s u l t s f o r o x i d i z e d m o d e l c o m p o u n d s . E v e n m o r e c o n v i n c i n g a r g u m e n t s for t h e i r o n ( I V ) π - c a t i o n r a d i c a l formulation for H R P C o m p o u n d I are f o u n d i n E N D O R spectra, w h i c h reveal radical c o u p l i n g w i t h r i n g m e t h y l - m e t h y l e n e protons a n d p y r r o l e n i t r o g e n atoms (15). H i g h o x i d a t i o n state i n t e r m e d i a t e s w e r e a l s o p o s t u l a t e d for c y t o c h r o m e P - 4 5 0 (16,17) a n d c y t o c h r o m e o x i d a s e (18 ). T h e n e t r e a c t i o n for a p o r t i o n o f t h e c y t o c h r o m e P - 4 5 0 c y c l e c a n b e s u m m a r i z e d b y : PFe(II) + 0

2

+ e" + 2 H

+

- >

PFe(IV)=0 + H 0 2

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

16.

GOFF ET AL.

Resonance of Oxidized Metalloporphyrins

359

T h e C o m p o u n d I a n a l o g w a s n o t i d e n t i f i e d , b u t its p r e s e n c e is i m p l i c a t e d t h r o u g h g e n e r a t i o n f r o m t h e i r o n ( I I I ) state b y h y d r o g e n p e r o x i d e and

by

similar

peroxidases.

substrate

reactions

of

cytochrome

A n i r o n ( I V ) p o r p h y r i n state w a s

means o f e x p l a i n i n g the " s i l e n t " E S R c o m p o n e n t c h r o m e oxidase (18). H o w e v e r , m o r e r e c e n t

P-450

also postulated

and as

a

of o x i d i z e d cyto

findings

p r o v i d e no sup

p o r t for a h i g h l y o x i d i z e d h e m e d u r i n g t h e c y t o c h r o m e o x i d a s e c y c l e . Systematic generation a n d p h y s i c a l examination o f o x i d i z e d iron porphyrin intermediates i n isolated iron porphyrins might be parallel i n s i g n i f i c a n c e to t h e d e m o n s t r a b l y p r o d u c t i v e o x y g e n b i n d i n g s t u d i e s


of iron porphyrin m o d e l compounds

( 1 9 , 20).

I n this regard

chemistry of isolated iron(IV) porphyrin compounds

the

is c r i t i c a l l y r e

v i e w e d , w i t h e m p h a s i s o n r e c e n t r e s u l t s f r o m t h i s l a b o r a t o r y for b o t h oxidized iron and manganese porphyrin complexes. T h e

following

h i g h o x i d a t i o n state s p e c i e s a n d r e a c t i o n s w i l l b e c o n s i d e r e d : (1) i r o n p o r p h y r i n μ , - n i t r i d o d i m e r i c c o m p l e x e s , (2) c a r b e n e a n d v i n y l i d e n e c o m p l e x e s , (3) i o d o s y l b e n z e n e o x i d a t i o n s , (4) μ - p e r o x o i r o n p o r p h y r i n reactions,

(5) o t h e r

c h e m i c a l oxidations, a n d

(6) e l e c t r o c h e m i c a l l y

o x i d i z e d m e t a l l o p o r p h y r i n c o m p o u n d s , to i n c l u d e t h o s e e x t e n s i v e l y s t u d i e d b y this research

group.

Iron Porphyrin-μ-Nitrido

Dimeric Complexes

I n a x y l e n e reflux, n i t r o g e n was e l i m i n a t e d from the a z i d e c o m p l e x , T P P F e N , to y i e l d t h e μ - n i t r i d o d i m e r , T P P F e - N - F e T P P (21). A n x - r a y s t r u c t u r e d e t e r m i n a t i o n c o n f i r m e d t h e f o r m u l a t i o n (22). A s s i g n i n g the b r i d g i n g nitrogen atom a - 3 charge puts formal charges o f + 3 a n d + 4 , o r a n a v e r a g e o f + 3 . 5 , o n t h e i r o n c e n t e r s . I r o n a t o m s are e q u i v a l e n t on the M ô s s b a u e r t i m e scale a n d increased charge ç n i r o n c e n t e r s is r e f l e c t e d i n a n i s o m e r s h i f t v a l u e o f + 0 . 1 0 m m / s (vs. 0 . 2 9 m m / s for T P P F e - O - F e T P P ) (21). 3

T h e μ - n i t r i d o d i m e r is o x i d i z e d c h e m i c a l l y or e l e c t r o c h e m i c a l l y ( + 0 . 1 7 V , S C E ) to y i e l d t h e m o n o c a t i o n d i m e r (23). T h e p r o d u c t is formally an iron(IV)-iron(IV) species. Solution characterization b y p r o t o n N M R r e v e a l s that, u n l i k e t h e o d d - s p i n p a r e n t d i m e r , t h e o x i d i z e d c o m p l e x exhibits little paramagnetic character, p r e s u m a b l y as a c o n s e q u e n c e o f s t r o n g c o u p l i n g t h r o u g h t h e μ , - n i t r i d o b r i d g e . T h i s finding is e v i d e n t i n l i n e w i d t h a n d s h i f t d i f f e r e n c e s for s p e c t r a i n F i g u r e s l a a n d l c . A d d i t i o n o f p y r i d i n e to T P P F e - N - F e T P P ( F i g u r e l b ) y i e l d s n o s p e c t r a l c h a n g e s o t h e r t h a n a p p e a r a n c e o f free p y r i d i n e res o n a n c e s . I n c o n t r a s t , a d d i t i o n o f p y r i d i n e to t h e o x i d i z e d d i m e r ( F i g ure Id) i n d u c e s c h a n g e s i n p o r p h y r i n resonances, a n d p y r i d i n e reso n a n c e s (not s h o w n ) are s h i f t e d u p f i e l d b y p o r p h y r i n r i n g c u r r e n t s . B o t h m o n o p y r i d i n e a n d d i p y r i d i n e l i g a t i o n are d e t e c t e d , a n d p y r i d i n e e x c h a n g e is r a p i d o n t h e N M R t i m e s c a l e at a m b i e n t t e m p e r a t u r e . T h e



360

h i g h e r o x i d a t i o n state i r o n c e n t e r s e e m i n g l y e x h i b i t s a h i g h e r b i n d i n g affinity for n i t r o g e n o u s bases t h a n does that o f the parent, n e u t r a l dimer.

Carbene and Vinylidene Complexes Combination of iron porphyrins and chlorinated hydrocarbons i n the presence o f a strong r e d u c i n g agent y i e l d s isolable iron p o r p h y r i n carbene

complexes

(24-27).

Aromatic

substituents

m a y further

stabilize the a d d u c t i n the form o f a v i n y l i d e n e c o m p l e x (25): TPPFe(II) + C l C C H A r Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0201.ch016

3

TPPFeC=CAr

2

2

-2C1-

T h i s p r o d u c t is q u i t e r e m a r k a b l y air-stable, b u t m a y b e o x i d i z e d b y a g e n t s s u c h as f e r r i c c h l o r i d e o r c u p r i c c h l o r i d e . B a s e d o n s p e c t r a l similarity o f the o x i d i z e d v i n y l i d e n e c o m p l e x a n d the s p e c t r u m o f H R P C o m p o u n d I, t h e f o l l o w i n g p r e d o m i n a n t resonance

form was

o r i g i n a l l y i n v o k e d (26, 27): TPPFe(IV)=C=CAr

2

A l t h o u g h t h e m a g n e t i c m o m e n t (μ^ = 3 . 7 B . M . ) i s c o n s i s t e n t w i t h s u c h a f o r m u l a t i o n , the u n u s u a l E S R g = 4.4 v a l u e suggests the inter m e d i a t e s p i n , S = 3/2, state o f i r o n ( I I I ) p o r p h y r i n s (28-30). T h i s i n formation a n d analogous reactions o f carbenes (generated from d i a z o a l k e n e s ) w i t h c o b a l t (31-33) a n d n i c k e l (34) p o r p h y r i n s s u g gested a n i r o n - c a r b o n - p y r r o l e n i t r o g e n - b r i d g e d a d d u c t o f the type s h o w n i n F i g u r e 2 (35). S i m u l t a n e o u s x-ray c r y s t a l l o g r a p h i c w o r k a n d N M R solution measurements demonstrated the v a l i d i t y of this b r i d g e d structure (36, 37). H e n c e , t h e u n u s u a l spectroscopic properties o f o x i d i z e d v i n y l i d e n e - i r o n p o r p h y r i n complexes result from p o r p h y r i n m o d i f i c a t i o n r e a c t i o n s r a t h e r t h a n f r o m o x i d a t i o n t o a n i r o n ( I V ) state. R e c e n t i t e r a t i v e H i i c k e l c a l c u l a t i o n s for u n o x i d i z e d v i n y l i d e n e c o m p l e x e s are inconsistent w i t h i r o n ( I V ) character (3).

lodosylbenzene Oxidations I o d o s y l b e n z e n e (38-42) [or 2 - i o d o s o - m - x y l e n e (43)] w a s a n effec t i v e o x i d i z i n g a g e n t for c o n v e r s i o n o f a l k a n e s t o a l c o h o l s a n d a l k e n e s to e p o x i d e s i n t h e p r e s e n c e o f m e t a l l o p o r p h y r i n c a t a l y s t . A 5 0 % c o n v e r s i o n o f c y c l o h e x a n e to c y c l o h e x y l a l c o h o l w a s p o s s i b l e b a s e d o n t h e i o d o s y l b e n z e n e c o n s u m e d , u s i n g T P P M n C l (40). S i m i l a r r e s u l t s w e r e r e p o r t e d i n R e f e r e n c e 41 as w a s t h e f o r m a t i o n o f s i g n i f i c a n t c y c l o h e x y l c h l o r i d e or other c y c l o h e x y l adducts o f the o r i g i n a l m a n g a n e s e ( I I I ) a n i o n i c l i g a n d . T h e s e results are p a r t i c u l a r l y n o t e w o r t h y i n v i e w o f the n e e d for m i l d - c o n d i t i o n C H b o n d a c t i v a t i o n c a t a l y s t s , a n d i n t e r m s o f


16.

GOFF ET A L .


361


PYRR

solvent, 26°C, 5 m M dimer, referenced to TMS.

partial s i m u l a t i o n o f the monooxygenase action o f c y t o c h r o m e P-450. T h e p o s t u l a t e d a c t i v e o x i d a n t i n t h e s e r e a c t i o n s is t h e m e t a l - o x o c o m p l e x , w h i c h c a n b e d e s c r i b e d w i t h t h e s a m e r e s o n a n c e s t r u c t u r e as t h a t o f H R P C o m p o u n d I:

TPPM(III)X +

^ Q ^ — I P ->

1

·TPPM(IV)=0

+

+ X "+

(O^

S p e c t r o s c o p i c o b s e r v a t i o n s at r e d u c e d t e m p e r a t u r e (43) or w i t h i n s e c o n d s o f m i x i n g T P P F e C l (44) a n d i o d o s y l b e n z e n e r e v e a l a b s o r p t i o n b a n d s r e s e m b l i n g t h o s e o f H R P C o m p o u n d I. T h e c o r r e s p o n d i n g m a n g a n e s e c o m p o u n d w a s g e n e r a t e d a n d p a r t i a l l y c h a r a c t e r i z e d at l o w t e m p e r a t u r e (40-42). A m a n g a n e s e ( V ) a s s i g n m e n t b a s e d o n t h e μ ff v a l u e o f 2.9 B . M . (40) m u s t b e c o n s i d e r e d as a f o r m a l i s m r a t h e r t h a n a d e s c r i p t i o n o f the e l e c t r o n i c structure. E n h a n c e d s t a b i l i t y o f the analogous o x i d i z e d T P P C r X c o m p l e x permitted isolation and room t e m p e r a t u r e c h a r a c t e r i z a t i o n (39). A c h r o m i u m ( V ) o x i d a t i o n state w a s e



362

Ar^Ar II


Figure 2. Alternate structure for the oxidized iron porphyrin vinylidene complex.

i n f e r r e d from the 2 . 0 5 - B . M . m a g n e t i c m o m e n t v a l u e . I R measure ments demonstrated the presence o f a C r = 0 l i n k a g e associated w i t h a 1 0 2 6 - c m " b a n d . G e n e r a t i o n o f the o x i d i z e d c h r o m i u m c o m p l e x was also p o s s i b l e u s i n g m - c h l o r o p e r o x y b e n z o i c a c i d or s o d i u m h y p o chlorite. 1

T h e o x i d i z e d i r o n p o r p h y r i n species generated i n situ was further c h a r a c t e r i z e d u s i n g i o d o s y l b e n z e n e or m - c h l o r o p e r o x y b e n z o i c a c i d as o x i d i z i n g a g e n t s (45). Reaction o f iron(III) tetramesitylporphyrin chloride ( T M P F e C l ) w i t h two-electron equivalents o f o x i d i z i n g agent y i e l d s s o l u t i o n s p e c i e s a d e q u a t e l y s t a b l e at l o w t e m p e r a t u r e s for e x amination b y N M R , v i s i b l e - U V , and M ô s s b a u e r spectroscopy. C h e m i cal properties o f o x i d i z e d products d e p e n d o n the o x i d i z i n g agent ( T a b l e I). O x i d a t i o n b y m - c h l o r o p e r o x y b e n z o i c a c i d p r o d u c e s gross c h a n g e s i n t h e p r o t o n N M R s p e c t r u m o f T M P F e C l . T h e far d o w n f i e l d p y r r o l e proton N M R signal o f the parent c o m p o u n d can be contrasted w i t h a s i g n a l u p f i e l d f r o m T M S i n t h e o x i d i z e d f o r m . T h i s p a t t e r n is consistent w i t h electron abstraction from the d - orbital. T h e surpris i n g l y l a r g e m e s i t y l p r o t o n N M R shifts a r e b e s t e x p l a i n e d b y s i g n i f i c a n t u n p a i r e d s p i n d e n s i t y at t h e m e t h i n e - c a r b o n a t o m o f a π - c a t i o n r a d i c a l iron porphyrin. Iodosylbenzene oxidation yields a similar upfield pyr r o l e p r o t o n resonance, b u t the m e s i t y l p r o t o n signals are v i r t u a l l y u n shifted from d i a m a g n e t i c positions. T h i s latter observation does not r u l e o u t r a d i c a l c h a r a c t e r , h o w e v e r , as t h e a radical type exhibits l i t t l e s p i n d e n s i t y at t h e m e t h i n e - c a r b o n a t o m (3). A n a n a l o g y is f o u n d i n o x i d i z e d m a n g a n e s e p o r p h y r i n s ( v i d e i n f r a ) , w h i c h a r e c l a s s i f i e d as π-cation radicals, but w h i c h show no significant p h e n y l proton N M R s h i f t s . M a g n e t i c m o m e n t v a l u e s a n d v i s i b l e s p e c t r a ( c o l o r ) for t h e t w o c o m p o u n d s g e n e r a t e d b y i o d o s y l b e n z e n e or m - c h l o r o p e r o x y b e n z o i c a c i d o x i d a t i o n d i f f e r s i g n i f i c a n t l y ( T a b l e I). T h a t t h e s e d i f f e r e n c e s are d u e to s p i n a n d / o r l i g a t i o n state r a t h e r t h a n o x i d a t i o n state is d e m o n strated b y i n t e r c o n v e r s i o n o f the t w o species t h r o u g h a d d i t i o n o f a c i d o r b a s e (45). Very l o w M ô s s b a u e r i s o m e r s h i f t v a l u e s c o n f i r m t h e h i g h o x i d a t i o n state o f t h e i r o n a t o m . F u r t h e r c h a r a c t e r i z a t i o n o f t h e s e s p e c i e s s h o u l d s e r v e to b e t t e r d e f i n e t h e r e a c t i o n c h e m i s t r y o f o x i d i z e d iron porphyrins. x2

y2

iu


Table I.


363

GOFF ET AL.

16.


Properties o f O x i d i z e d Iron Porphyrins

Iron Te trame si t y lporphyrin Chloride a

Property

Iron Te train-toly lporphyrin Chloride

Iron Tetraphenylporphyrin Chloride

b

+

R SbCl (SbCl salt)

0

Electrochem. ( C 1 0 - salt)

Oxidizing agent

ArCOOOH

Electrons transferred Proton N M R (temp.) pyrrole-H o-m ethyl m-H p-methyl

2

2

1

1

1

(-77°C)

(-73°C)

(-73°C)

(26°C)

(29°C)

- 2 7 ppm 24, 26 68 11.1

-33.5 2.4 7.6 2.8

Visible-UV

406, 645 n m (red) (green) 4.2 B . M . 2.9

p y r r o l e - H 5 66 37.6, 34.4 o-H 9 -12.4 m-H 8 29.5 p-U 8 m-methyl 3 - 4 2 0 , - 5 6 0 , 397, 530, 600, 820 -585 5.5 2.9

ArlO

0

2

(+1-Me Imidazole)

6

6

4

e

Magnetic moment Môssbauer l.S. Q.S. Reactivity toward olefins 9

0.05 mm/s 1.49 mm/s more re active

/

0.1 Ρ -0.03 1.24 2.13 less re not deter mined active Λ

0.45 1.27

66 39 -15.1 31 387, 533, 615, 750 5.1

0.40 0.55 not d e t e r m i n e d

a

Ref. 45. Ref. 48. Ref. 57. Ref. 53. Referenced to TMS, downfield shifts are given a positive sign. 'For TPP(p-OCH )Fe(Cl)(C10 ). Referenced to iron metal. Ref. 50.

b c

d

e

3

4

9

ft



364

μ-Peroxo Iron Porphyrin Reactions A μ - p e r o x o d i m e r i c iron(III) p o r p h y r i n species was generated l o w t e m p e r a t u r e s t h r o u g h d i o x y g e n attack o n iron(II) p o r p h y r i n s

at (46,

47): 2PFe(II) + 0

2

PFe(III)-0-0-Fe(III)P

A n t i f e r r o m a g n e t i c c o u p l i n g t h r o u g h t h e p e r o x o b r i d g e is s i g n i f i c a n t , w i t h 2J = - 2 6 5 K . U p o n w a r m i n g to r o o m t e m p e r a t u r e t h e i r o n ( I I I ) μ-οχο

d i m e r is f o r m e d :


2PFe(III)-0-0-Fe(III)P ^

2 PFe(III)-0-Fe(III)P +

0

2

A d d i t i o n o f t w o e q u i v a l e n t s o f n i t r o g e n o u s b a s e to t h e μ - p e r o x o d i m e r at l o w t e m p e r a t u r e g e n e r a t e s a n e w s p e c t r o s c o p i c s p e c i e s

(48):

P F e ( I I I ) - 0 - 0 - F e ( I I I ) P + 2 Β - » 2 B P F e O or B P F e - O - O - F e P B T h e u n i q u e p r o t o n N M R s p e c t r u m o f t h i s b a s e a d d u c t is s h o w n i n F i g u r e 3 a n d v a r i o u s p r o p e r t i e s are d e s c r i b e d i n T a b l e I . C u r i e l a w b e h a v i o r o f p r o t o n N M R shift values i m p l i e s that antiferromagnetic c o u p l i n g t h r o u g h a b r i d g i n g l i g a n d w o u l d h a v e to b e v e r y s m a l l ; h e n c e , t h e m o n o m e r i c s t r u c t u r e is f a v o r e d . S u c h a s t r u c t u r e is i s o e l e c tronic w i t h H R P C o m p o u n d II, B P F e ( I V ) = 0 . W i t h a magnetic mo m e n t o f 2.9 B . M . p e r i r o n a t o m the species m u s t b e i n the l o w s p i n state. R a p i d r e a c t i o n w i t h i r o n ( I I ) p o r p h y r i n s a n d o x y g e n t r a n s f e r r e a c t i o n w i t h t r i p h e n y l p h o s p h i n e (49) t o y i e l d t h e o x i d e f u r t h e r s u p p o r t t h e m o n o m e r i c f o r m u l a t i o n . T h e p a r a m a g n e t i c N M R shifts for t h i s c o m p o u n d are v e r y s m a l l , a n d m a y b e contrasted w i t h c o n s i d e r a b l y l a r g e r v a l u e s for m - c h l o r o p e r o x y b e n z o i c a c i d a n d i o d o s y l b e n z e n e o x i d a t i o n p r o d u c t s . R e l a t i v e l y s m a l l p a r a m a g n e t i c shifts a r e o b s e r v e d , h o w e v e r , for H R P C o m p o u n d I I (11 -14), a n d are r a t i o n a l i z e d b y t h e o r e t i c a l treatments that p l a c e s i z a b l e u n p a i r e d s p i n d e n s i t y o n the axial o x y g e n l i g a n d (3). M ô s s b a u e r s p e c t r a l s t u d i e s (50) f u r t h e r s u p p o r t a n i r o n ( I V ) , S = 1, c o n f i g u r a t i o n for t h e 1 - m e t h y l i m i d a z o l e a d d u c t g e n e r ated from the μ - p e r o x o species.

Other Chemical Oxidations A l t h o u g h h a l o g e n s a n d h a l o g e n d e r i v a t i v e s are a d e q u a t e o x i d a n t s for v a r i o u s d i v a l e n t m e t a l l o p o r p h y r i n s ( 5 1 , 5 2 ) , o n l y r e c e n t l y h a s a n o x i d i z i n g a g e n t o f g e n e r a l u t i l i t y b e e n r e p o r t e d for i r o n ( I I I ) a n d o t h e r t r i v a l e n t p o r p h y r i n s ( 5 3 ) . T h e s t a b l e p h e n o x a t h i i n c a t i o n r a d i c a l , as t h e S b C l " salt, p r o v i d e s a d e q u a t e r e d o x p o t e n t i a l for s i n g l e - e l e c t r o n oxidation o f a l l c o m m o n iron(III) p o r p h y r i n s . F u r t h e r details o f the p r e p a r a t i o n a n d c h a r a c t e r i z a t i o n o f o x i d i z e d i r o n p o r p h y r i n s are p r o 6



16.

GOFF ET AL.

Resonance

I

I

ι

-2β

-15

-IB

of Oxidized

Metalloporphyrins

365

'

ι

I

-5

0

5

SHIFT IN PPM, FROM TMS Figure 3. Proton NMR spectrum of TPP(m-CH )Fe-0-0-FeTPP(mCH ) with excess l-methylimidazole in toluene-d at 200 K. Key: a = o-phenyl-H, h = m - and Ό-phenyl-H, c = uncoordinated l-methyl imidazole, d = pyrrole-H, and e = m-methyl-H. (Reproduced from Ref 48. Copyright 1980, American Chemical Society.) 3

3

8

v i d e d e l s e w h e r e (50). T h e c h e m i c a l oxidant generates o x i d i z e d i r o n p o r p h y r i n s w i t h e l e c t r o n i c s t r u c t u r e s e q u i v a l e n t to t h o s e p r e p a r e d b y e l e c t r o c h e m i c a l o x i d a t i o n ( v i d e infra). A l t h o u g h the r e a g e n t has b e e n u s e d to a l i m i t e d e x t e n t , z i n c p o r p h y r i n o x i d a t i o n s o c c u r w i t h t h e c a t i o n r a d i c a l p e r c h l o r a t e o f d i b e n z o d i o x i n (54). I n s p e c i a l cases t w o a d d i t i o n a l c h e m i c a l o x i d a n t s are o f v a l u e . E l e c t r o c h e m i c a l l y o x i d i z e d i r o n p o r p h y r i n s w e r e u t i l i z e d as c h e m i c a l o x i d a n t s for i r o n p o r p h y r i n s (55) or o t h e r m e t a l l o p o r p h y r i n s (52) w i t h lower oxidation potential. T h e doubly o x i d i z e d [ ( T P P F e ) 0 ] ( C 1 0 ) s p e c i e s is e s p e c i a l l y u s e f u l i n t h i s r e g a r d (55). A n a d d i t i o n a l c h e m i c a l o x i d a n t s y s t e m is v i a b l e for m e t a l l o p o r p h y r i n s w i t h l o w e r o x i d a t i o n potentials (^1.0 V , S C E ) . Thus, one equivalent o f iodine (in 2


4

2


366

c h l o r o f o r m ) a d d e d to a m e t h y l e n e c h l o r i d e s o l u t i o n o f t h e m e t a l l o p o r p h y r i n , f o l l o w e d b y a s u i t a b l e s i l v e r salt ( A g C 1 0 a c e t o n e , or A g N 0

3

or A g C F S 0

4

3

3

in

i n acetonitrile) o x i d i z e s the m e t a l l o p o r p h y r i n . T h i s

m e t h o d w a s u s e d for z i n c p o r p h y r i n s (54, 56), a n d m o r e r e c e n t l y for v a r i o u s i r o n p o r p h y r i n d e r i v a t i v e s (23,

57).

A report o f air o x i d a t i o n o f μ - ο χ ο iron(III) p o r p h y r i n d i m e r s i n the presence of strong L e w i s acids ( B F , E t O · B F , H B F , a n d H P F ) 3

z

3

4

6

was refuted b y subsequent electrochemical measurements

(58)

(59).


Electrochemical Oxidations Literature Results. W o l b e r g a n d M a n a s s e n f o u n d t h a t e l e c trochemical oxidation o f various metalloporphyrins y i e l d e d species a d e q u a t e l y s t a b l e for i n s i t u s p e c t r o s c o p i c e x a m i n a t i o n (60). I n p a r ticular, one-electron oxidation o f T P P F e ( I I I ) X gave a product w i t h a b r o a d e n e d Soret b a n d , no detectable E S R signals, a n d a solution m a g n e t i c m o m e n t o f 2.7 B . M . B a s e d o n these observations, the o x i d i z e d p r o d u c t w a s f o r m u l a t e d as a n i r o n ( I I I ) p o r p h y r i n π - c a t i o n r a d i c a l s p e c i e s . B e n z o n i t r i l e w a s e m p l o y e d as a p r e p a r a t i v e e l e c t r o l y tic solvent, u n l i k e subsequent studies where m e t h y l e n e c h l o r i d e was u t i l i z e d . T h i s solvent difference a n d the p o s s i b i l i t y o f other c o m p e t i n g reactions m a y e x p l a i n differences i n m a g n e t i c m o m e n t values ob t a i n e d b y other workers. S i m i l a r preparative scale e l e c t r o c h e m i c a l oxidations e m p l o y e d t e t r a p r o p y l a m m o n i u m p e r c h l o r a t e as a s u p p o r t i n g e l e c t r o l y t e i n m e t h y l e n e c h l o r i d e s o l v e n t (61, 62). I r o n ( I I I ) o c t a e t h y l p o r p h y r i n w a s e x a m i n e d a l o n g w i t h T P P F e C l a n d the μ - ο χ ο d i m e r forms o f b o t h synthetic iron(III) porphyrins. The singly oxidized species, ( T P P F e ) O C 1 0 , was isolated following evaporation of solvent and a h o t w a t e r w a s h to r e m o v e s u p p o r t i n g e l e c t r o l y t e salts. R e v e r s i b i l i t y o f oxidations was c o n f i r m e d v i a b o t h e l e c t r o c h e m i c a l a n d i o d i d e i o n re d u c t i o n to t h e p a r e n t i r o n ( I I I ) s p e c i e s . T h e s i n g l y o x i d i z e d μ - ο χ ο d i mers e x h i b i t e d an E S R s i g n a l n e a r g = 2, a n d a r e d u c e d m a g n e t i c m o m e n t o f 2 . 9 B . M . ( 4 0 ° C ) as a c o n s e q u e n c e o f F e - F e a n t i f e r r o m a g n e t i c c o u p l i n g . A s o l u t i o n m a g n e t i c m o m e n t o f 5.1 B . M . w a s o b t a i n e d for T P P F e C l " " . B a s e d l a r g e l y o n N M R s p e c t r a l m e a s u r e m e n t s , t h e o x i d i z e d d e r i v a t i v e s w e r e f o r m u l a t e d as i r o n ( I V ) c o m p o u n d s . T h i s i n terpretation f o l l o w e d from the r e l a t i v e l y s m a l l ( 4 - 7 p p m ) isotropic shifts for p h e n y l p r o t o n s o f T P P F e C l a n d ( T P P F e ) 0 . 2

4

1

+

+

2

Recent Results for Oxidized Iron Porphyrins.

Chemical and

electrochemical oxidation o f h i g h s p i n iron(III) porphyrins was evalu a t e d i n d e t a i l i n t h i s l a b o r a t o r y . T o s t a b i l i z e t h e o x i d i z e d state, v a r i o u s anionic ligands were synthetically incorporated i n place of usual c h l o r i d e ligands. T h e c o m p l e t e list i n c l u d e s X = F " , C l " , B r ~ , I",


16.

GOFF ET A L .

ClOr,


367

S O / , NO3- N " , N C S " O P h " , Ο Ac", p - C H C e H S 0 - , H S 0 ~ , a n d C F 3 S O 3 " ( 6 3 ) . Q u i t e s u r p r i s i n g l y , t h e first o x i d a t i o n p o t e n t i a l for t h e T P P F e X c o m p l e x e s w a s i n v a r i a n t at 1.10 ± 0 . 0 2 V ( S C E , 0.1 M B u N C 1 0 , C H C 1 ) . T h i s o b s e r v a t i o n is c o n s i s t e n t w i t h ( b u t d o e s n o t p r o v e that) e l e c t r o n a b s t r a c t i o n is f r o m a p o r p h y r i n - b a s e d rather than a metal-centered m o l e c u l a r orbital. -

s

3

4

3

4


4

4

2

2

O x i d a t i o n p o t e n t i a l s are d i m i n i s h e d a n d s t a b i l i t y o f t h e o x i d i z e d p r o d u c t is c o n s i d e r a b l y e n h a n c e d v i a i n c o r p o r a t i o n o f e l e c t r o n r e l e a s i n g substituents i n the p o r p h y r i n structure. T h e o x i d i z e d i r o n ρ - m e t h o x y t e t r a p h e n y l p o r p h y r i n derivatives are m o r e r e a d i l y isolated a n d s h o w m o r e f a v o r a b l e s t a b i l i t y w i t h r e s p e c t to a u t o r e d u c t i o n . M o d i f i c a t i o n o f t h e s y n t h e t i c p r o c e d u r e s i n R e f e r e n c e s 61 a n d 62 p e r mitted isolation of o x i d i z e d c o m p o u n d s of analytical purity i n favorable cases ( 5 7 ) . O x i d i z e d i r o n t e t r a p h e n y l p o r p h y r i n , o c t a e t h y l p o r p h y r i n , etioporphyrin, and natural-derivative porphyrin compounds were p r e p a r e d i n b o t h t h e m o n o m e r i c a n d μ-οχο d i m e r i c * f o r m s . I s o l a b l e t w o - e l e c t r o n o x i d i z e d μ-οχο d i m e r c o m p o u n d s w e r e r e a d i l y g e n e r a t e d . C h a r a c t e r i z a t i o n o f o n e s u c h s p e c i e s is d e m o n s t r a t e d i n F i g u r e 4 , through N M R m o n i t o r i n g o f a titration o f ( T P P F e ) 0 into a solution o f (TPPFe) 0 (55). U p o n a d d i t i o n o f one e q u i v a l e n t o f ( T P P F e ) 0 , the s p e c t r u m for t h e s i n g l y o x i d i z e d c o m p l e x ( w h i c h m a y b e p r e p a r e d d i r e c t l y ) is o b t a i n e d . C o n t i n u a l a d d i t i o n o f ( T P P F e ) 0 y i e l d s s p e c t r a a p p r o a c h i n g that o f the parent ( T P P F e ) 0 . S u c h m e a s u r e m e n t s d e m onstrate b o t h r a p i d i n t r a m o l e c u l a r a n d i n t e r m o l e c u l a r e l e c t r o n trans fer, as w e l l as t h e r e v e r s i b i l i t y o f e l e c t r o c h e m i c a l o x i d a t i o n s . P e a k a s s i g n m e n t s are a l s o f a c i l i t a t e d b y s e q u e n t i a l s p e c t r a l o b s e r v a t i o n . 2

2 +

2

2

2

2

Proton N M R spectra are l i k e w i s e o b t a i n e d for oxidized m o n o m e r i c c o m p l e x e s , a n d s p e c t r a l assignments are l i s t e d i n T a b l e I ( 5 3 , 5 7 ) . L a r g e p h e n y l p r o t o n i s o t r o p i c shifts d i f f e r f r o m t h o s e r e p o r t e d e a r l i e r ( 6 1 , 6 2 ) , p e r h a p s as a c o n s e q u e n c e o f e l e c t r o n e x c h a n g e w i t h c o n s i d e r a b l e amounts o f r e d u c e d m a t e r i a l . A strong case c a n b e m a d e for π - c a t i o n r a d i c a l c h a r a c t e r o f t h e o x i d i z e d i r o n p o r p h y r i n b a s e d o n the alternation i n d i r e c t i o n a n d m a g n i t u d e o f p h e n y l p r o t o n shifts. T h u s , t h e E S R c o u p l i n g c o n s t a n t o f 0 . 3 2 G a u s s for t h e p h e n y l p r o t o n s o f T P P Z n C 1 0 π - c a t i o n r a d i c a l (64) m a y b e t r a n s l a t e d as a n N M R i s o t r o p i c s h i f t o f 2 3 p p m , or as o b s e r v e d c h e m i c a l s h i f t v a l u e s o f 3 1 p p m d o w n f i e l d or - 1 5 p p m u p f i e l d from T M S . C o r r e s p o n d e n c e o f t h e s e v a l u e s e x p e c t e d for a p o r p h y r i n r a d i c a l a n d t h o s e m e a s u r e d for T P P F e ( C l ) ( C 1 0 ) is c o m p e l l i n g e v i d e n c e for t h e i r o n ( I I I ) π - c a t i o n r a d ical nature of oxidized iron p o r p h y r i n complexes containing weakfield a n i o n i c l i g a n d s . T h e s a m e c o n c l u s i o n is r e a c h e d for t h e c h e m i c a l l y o x i d i z e d T P P F e C l species (53). 4

4

O f t h e t w o p o s s i b l e p o r p h y r i n r a d i c a l states ( 3 ) , t h e T P P F e ( C l ) ( C 1 0 ) c o m p l e x m u s t b e representative o f the a type. This radical 4

2u




368

Figure 4. Proton NMR titration of [(TPPFe) 0](Cl0 ) by (TPPFe) 0; oxidized dimer originally 5 m M in 0.5 mL of CD Cl , 26°C. Moles of(TPPFe) 0 added ( χ 10 ): a, 0.0; h, 1.05; c, 2.50; d, 3.91; and e, 5.00. (Reproduced from Ref. 55.Copyright 1979, American Chemical Society.) 2

4 2

2

2

2

6

2

t y p e is e x p e c t e d to e x h i b i t l a r g e u n p a i r e d r a d i c a l s p i n d e n s i t y at m e t h i n e carbon positions. T h e O E P F e ( C l ) ( C 1 0 ) d e r i v a t i v e , on the o t h e r h a n d , is b e t t e r d e s c r i b e d as a n a r a d i c a l t y p e . A l t h o u g h c a l c u l a t i o n s p r e d i c t n o u n p a i r e d s p i n d e n s i t y at m e t h i n e c a r b o n a t o m s for t h i s radical, E S R m e a s u r e m e n t s r e v e a l a m e t h i n e proton c o u p l i n g constant o f 1.48 G a u s s for O E P M g C 1 0 ( 3 ) . A s h i f t o f t h e O E P F e C l m e t h i n e p r o t o n N M R s i g n a l f r o m —54 to —18 p p m u p o n o x i d a t i o n ( 5 7 ) is n o t r a t i o n a l i z e d b y the 1.48-Gauss c o u p l i n g constant o f k n o w n a radicals. H o w e v e r , a d i r e c t c o r r e s p o n d e n c e is n o t n e c e s s a r i l y e x p e c t e d , b e cause the m e t h i n e proton experiences a large a n d v a r i a b l e s p i n d e n s i t y transfer f r o m the i r o n center (65). A h i g h s p i n c o n f i g u r a t i o n 4

lu

4

lu


16.

GOFF ET AL.

369


is i n d i c a t e d b y s o l u t i o n m a g n e t i c

moments

o f 5.5 B . M . for b o t h

T P P ( p - O C H ) F e ( C l ) ( C 1 0 ) a n d O E P F e ( C l ) ( C 1 0 ) (57). 3

Môssbauer

4

results

4

for b o t h

chemically a n d electrochemically

o x i d i z e d c h l o r o c o m p l e x e s also s h o w l i t t l e p e r t u r b a t i o n o f c h a r g e at t h e i r o n c e n t e r as c o m p a r e d w i t h p a r e n t s p e c i e s ( T a b l e I). I s o m e r s h i f t v a l u e s o f 0.4 m m / s f o r p a r e n t i r o n p o r p h y r i n c h l o r o c o m p l e x e s a r e c h a n g e d b y n o m o r e t h a n 0 . 0 5 m m / s u p o n o x i d a t i o n (53, 57), i n c o n trast t o t h e a p p e a r a n c e o f v e r y l o w i s o m e r s h i f t v a l u e s for m o n o m e r i c c o m p l e x e s p r e s u m a b l y c o n t a i n i n g a n oxo l i g a n d . I R s p e c t r o s c o p y a l s o p r o v i d e s a d i a g n o s t i c t o o l for d e t e c t i n g p o r p h y r i n 7r-cation r a d i c a l c h a r a c t e r (66).

For k n o w n zinc and cobalt T P P


- 1

radical complexes a n intense n e w b a n d appears i n the 1 2 8 0 - c m re g i o n . T h i s b a n d is a l s o p r e s e n t i n b o t h o x i d i z e d m o n o m e r i c a n d d i m e r i c T P P F e s p e c i e s , as w e l l as i n o x i d i z e d T P P M n C l

complexes

d i s c u s s e d later. L i k e w i s e , o x i d a t i o n o f O E P c o m p l e x e s o f i r o n ( I I I ) , c o b a l t ( I I I ) , a n d z i n c ( I I ) is a s s o c i a t e d w i t h t h e a p p e a r a n c e o f a d i a g n o s tic b a n d i n the 1 5 5 0 - c m

- 1

region.

Various reactions o f o x i d i z e d iron p o r p h y r i n s w e r e

investigated

(67). F o r e x a m p l e , s t o i c h i o m e t r i c a c i d c l e a v a g e o f t h e o x i d i z e d μ - ο χ ο dimers yields expected (oxidized) monomeric derivatives. Addition o f n i t r o g e n o u s b a s e s at r o o m t e m p e r a t u r e r e d u c e s t h e i r o n p o r p h y r i n . A t - 5 0 ° C , on the other h a n d , N M R spectral m o n i t o r i n g o f i m i d a z o l e a d d i tion to o x i d i z e d m o n o m e r i c iron porphyrins demonstrates diligation o f t h e i m i d a z o l e g r o u p as a p r e f e r r e d e q u i l i b r i u m c o n d i t i o n . A l a r g e u p field

a n d d o w n f i e l d s p r e a d o f p h e n y l p r o t o n s i g n a l s is c o n s i s t e n t w i t h a

l o w s p i n i r o n ( I I I ) π - c a t i o n r a d i c a l f o r m u l a t i o n , as d e s c r i b e d for t h e h i g h s p i n iron(III) p o r p h y r i n radicals. A solution magnetic

moment

v a l u e o f 2.8 B . M . s u p p o r t s a l o w s p i n S = 1 (or S = I + S = i) c o n f i g u r a t i o n . π - C a t i o n r a d i c a l c h a r a c t e r is s t r o n g l y i n d i c a t e d b y t h e s p r e a d o f TPPFe

p h e n y l proton signals, b u t n o significant

antiferromagnetic

c o u p l i n g b e t w e e n r a d i c a l a n d i r o n ( I I I ) is a p p a r e n t . T h i s s i t u a t i o n i s a n a l o g o u s t o t h a t o f H R P C o m p o u n d I for w h i c h

antiferromagnetic

c o u p l i n g b e t w e e n i r o n ( I V ) a n d r a d i c a l s p i n s is v e r y s m a l l . A p o r p h y r i n r i n g m e t h y l s i g n a l a t 1 3 3 p p m ( 2 2 2 K ) for t h e d i i m i d a z o l e c o m p l e x o f o x i d i z e d i r o n e t i o p o r p h y r i n (67) e x p l a i n s t h e l a r g e r i n g m e t h y l shifts o f H R P C o m p o u n d I as b e i n g d u e t o r a d i c a l s p i n d e r e a l i z a t i o n r a t h e r t h a n t h e s u g g e s t e d h i g h s p i n i r o n ( I V ) f o r m u l a t i o n (10-12). Oxidized

Manganese

benzene-oxidized

species

Porphyrins. discussed

Aside

from

previously,

the

iodosyl-

manganese(IV)

p o r p h y r i n s have b e e n p r e p a r e d i n basic a q u e o u s solution t h r o u g h the a c t i o n o f h y d r o g e n p e r o x i d e o r s o d i u m h y p o c h l o r i t e (68-73). T h e d i o x y g e n - m a n g a n e s e ( I I ) a d d u c t h a s b e e n f o r m u l a t e d as a p e r o x o m a n g a n e s e ( I V ) c o m p l e x (74-76). W e p e r f o r m e d p r e p a r a t i v e - s c a l e

elec

t r o c h e m i c a l o x i d a t i o n r e a c t i o n s for m a n g a n e s e ( I I I ) p o r p h y r i n s as d e -



370


s c r i b e d for i r o n p o r p h y r i n s . S i m i l a r o x i d a t i o n p o t e n t i a l s a n d s o l ubilities permit analogous oxidation a n d isolation procedures. O x i d a tions w e r e t y p i c a l l y p e r f o r m e d u s i n g a p l a t i n u m w o r k i n g e l e c t r o d e i n m e t h y l e n e c h l o r i d e s o l u t i o n c o n t a i n i n g 1.0 m g o f m a n g a n e s e p o r p h y r i n / m L a n d 0.1 M i n t e t r a p r o p y l a m m o n i u m p e r c h l o r a t e s u p porting electrolyte. T h e counterelectrode a n d reference electrode w e r e s e p a r a t e d f r o m b u l k s o l u t i o n b y fine glass frits. T h e c u r r e n t flow w a s a l l o w e d to d r o p to 1% o f t h e o r i g i n a l v a l u e at w h i c h t i m e t h e electrolysis was stopped. Oxidation was m o n i t o r e d b y U V - v i s i b l e s p e c t r a l m e a s u r e m e n t s , as s h o w n i n F i g u r e 5. T h e o x i d i z e d m a t e r i a l m a y b e r e v e r s i b l y r e d u c e d b y t e t r a b u t y l a m m o n i u m i o d i d e s a l t (a l a r g e e x c e s s m u s t b e a v o i d e d or t h e m a n g a n e s e ( I I I ) i o d i d e c o m p l e x is formed). Oxidation potentials measured b y cyclic voltammetry approxi m a t e t h o s e for c o r r e s p o n d i n g i r o n ( I I I ) p o r p h y r i n s (1.1 V v s . S C E for T P P M n C l ) . C y c l i c v o l t a m m e t r i c scans o f the o x i d i z e d p r o d u c t s w e r e e q u i v a l e n t to t h o s e for t h e i n i t i a l c o m p l e x , t h u s i n d i c a t i n g n o i r r e v e r -

0.8h

0.6h

nm Figure 5. Electronic spectra of manganese porphyrins, approx. 9 x J O " M - Key: a, TPPMnCl, reduced product of in spectrum b and b, TPPMn(Cl)(Cl0 ). 5

CH Cl , material 2

4


2

GOFF ET A L .

16.

Resonance

of Oxidized

Metalloporphyrins

371

sible modification o f the p o r p h y r i n r i n g . Isolation o f o x i d i z e d p r o d u c t w a s a c c o m p l i s h e d b y first e v a p o r a t i n g t h e m e t h y l e n e c h l o r i d e s o l u tion, w i t h slow addition o f benzene. T e t r a p r o p y l a m m o n i u m perchlorate is i n s o l u b l e i n b e n z e n e a n d w a s s e p a r a t e d b y zene

solution containing oxidized manganese

filtration.

The ben

p o r p h y r i n was

then

s u b j e c t e d to s l o w r o t a r y e v a p o r a t i o n w i t h a d d i t i o n o f h e p t a n e .

The

solid product m a y be separated room temperature.

by

filtration

Oxidized compounds

a n d v a c u u m d r i e d at

prepared

i n this

manner

include TPPMn(Cl)(C10 ), TPP(p-OCH )Mn(Cl)(C10 ), and O E P M n 4

3

4

(C1)(C10 ). 4


P r o t o n N M R s p e c t r a for m a n g a n e s e ( I I I ) p o r p h y r i n s w e r e r e p o r t e d p r e v i o u s l y ( 7 7 ) . T h e s p e c t r u m for T P P M n C l is f o u n d i n F i g u r e 6 a .

CDHCh

PYRROLE

PPM

-20

•40

•60

Figure 6. Proton NMR spectra of manganese porphyrins, CD Cl solvent, 29°C, TMS reference. Key: a, TPPMnCl and b, TPPMn(Cl)(Cl0 ); X = salt and benzene impurities. 2

4


2


372

T h i s s p e c i e s is i s o e l e c t r o n i c w i t h a n i r o n ( I V ) p o r p h y r i n , b u t t h e p y r r o l e p r o t o n r e s o n a n c e is far u p f i e l d r a t h e r t h a n d o w n f i e l d as is o x i d i z e d T P P F e ( C l ) ( C 1 0 ) . T h i s o b s e r v a t i o n f u r t h e r s u p p o r t s t h e 7r-cation r a d i c a l f o r m u l a t i o n for o x i d i z e d i r o n p o r p h y r i n s . A p r o t o n N M R s p e c t r u m for T P P M n ( C l ) ( C 1 0 ) is s h o w n i n F i g u r e 6 b . N o gross s p e c t r a l c h a n g e s are n o t e d u p o n o x i d a t i o n , a n d the large s p l i t t i n g o f p h e n y l signals does n o t o c c u r . T h e m a j o r s p e c t r a l c h a n g e is f o u n d i n b r o a d e n i n g a n d u p field s h i f t o f t h e p y r r o l e p r o t o n r e s o n a n c e . C h e m i c a l s h i f t v a l u e s for O E P M n ( C l ) ( C 1 0 ) a r e as f o l l o w s ( w i t h t h o s e for O E P M n C l l i s t e d i n p a r e n t h e s e s ) : m e t h i n e , 7 0 P P M (52); C H , 16.3 (22.8); a n d C H , 2.7 (2.6). 4

4

4


2

3

E l e c t r o n s p i n r e s o n a n c e s p e c t r a a r e o b s e r v e d for t h e o d d - s p i n , o x i d i z e d m a n g a n e s e c o m p l e x e s , as m a y b e s e e n i n F i g u r e 7. A b r o a d g = 2 . 0 f e a t u r e is c o m m o n t o a l l s p e c i e s , w i t h l i n e w i d t h s r a n g i n g f r o m 3 0 0 G a u s s for O E P M n ( C l ) ( C 1 0 ) t o 5 0 0 G a u s s for T P P ( p - O C H ) M n ( C l ) ( C 1 0 ) . A w e a k e r g = 3.0 c o m p o n e n t is a p p a r e n t for O E P M n (C1)(C10 ). V a r i a b l e appearance o f this s e c o n d signal a n d different l i n e w i d t h s m a y r e f l e c t d i f f e r e n t m a g n i t u d e s a n d m e c h a n i s m s for r a d i c a l - m a n g a n e s e s p i n - s p i n interactions a m o n g diverse p o r p h y r i n s t r u c t u r a l types. T h e s e spectra differ from those o f k n o w n or s u g g e s t e d m a n g a n e s e ( I V ) c o m p o u n d s (73-75, 78) i n e x h i b i t i n g m u c h b r o a d e r a b s o r p t i o n s l a c k i n g a n y h y p e r f i n e s t r u c t u r e . T h e b e s t e x p l a n a t i o n is b a s e d o n t h e m a n g a n e s e ( I I I ) 7r-cation r a d i c a l f o r m u l a t i o n . T h e g = 2 s i g n a l is m o s t l i k e l y d e r i v e d f r o m t h e r a d i c a l , w i t h c o n s i d e r a b l e e l e c tronic relaxation from the paramagnetic metal center. T h e m a g n e t i c m o m e n t v a l u e o f 4.7 B . M . for T P P M n ( C l ) ( C 1 0 ) is i n v a r i a n t w i t h t e m perature ( - 5 0 ° to 2 9 ° C ) , i n d i c a t i n g that a n y antiferromagnetic b e h a v i o r is e i t h e r v e r y s t r o n g o r v e r y w e a k . T h e m a n g a n e s e m u s t b e h i g h s p i n , b u t t h e m a g n e t i c m o m e n t v a l u e is n o t o t h e r w i s e p a r t i c u l a r l y elucidating i n terms of possible " s p i n - o n l y " formulations. 4

3

4

4

4

A s s i g n m e n t o f the manganese(III) π - c a t i o n r a d i c a l electronic s t r u c t u r e is c o n s i s t e n t w i t h t h e b r o a d e n i n g a n d i n t e n s i t y loss o f S o r e t bands, a n d a general increase i n intensity i n the l o n g w a v e l e n g t h s p e c t r a l r e g i o n . A b s e n c e o f l a r g e i s o t r o p i c p h e n y l p r o t o n N M R shifts is i n d i c a t i v e o f l i t t l e u n p a i r e d s p i n d e n s i t y at t h e m e t h i n e p o s i t i o n , a n d is r e a d i l y e x p l a i n e d b y a s s u m i n g that an a t y p e r a d i c a l is f o r m e d . O x i d i z e d m a n g a n e s e p o r p h y r i n s are i s o e l e c t r o n i c w i t h H R P C o m p o u n d I , b u t a d m i t t e d l y d i f f e r i n s p i n state. Xu

Conclusion A variety o f p h y s i c a l e v i d e n c e demonstrates the r a d i c a l character of electrochemically oxidized iron and manganese porphyrins. This formulation m u s t b e q u a l i f i e d , h o w e v e r , b y n o t i n g that these species


GOFF ET AL.

373 Resonance

of Oxidized

Metalloporphyrins


16.

Figure 7. ESR spectra of oxidized manganese porphyrins, 2 raM in 1:1 methylene chloride-toluene, 77 K, 50-5050 Gauss sweep. Key: a, OEPMn(Cl)(Cl0 ) and b, TPP(p-OCH )Mn(Cl)(Cl0 ). 4

3

4


374


are associated with only weak-field anionic ligands. The wellrecognized role of oxo ligands in stabilizing high oxidation state metal ions seems to apply for the oxidized metalloporphyrins generated by iodosylbenzene oxidation or μ,-peroxo dimer cleavage reactions. The role of a frans-imidazole or similar strong-field ligand in dictating and/or stabilizing metal-centered oxidation remains to be elucidated.


Acknowledgments Support from N S F Grant C H E 79-10305 and N I H Grant G M 28831-01 is gratefully acknowledged. We thank R. E . Cofrman of this Department for assistance with E S R determinations, and D . N . Hendrickson of the University of Illinois for providing Môssbauer mea surements. Literature

Cited

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