Iron(I) and Iron(IV) Porphyrins - Advances in Chemistry (ACS...
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15 Iron(I) and Iron(IV) Porphyrins
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CHRISTOPHER A. R E E D University of Southern California, Chemistry Department, Los Angeles, CA 90007
Critical review of the literature on iron(I) and iron(IV) porphyrins reveals many ambiguities in the assigned spin and oxidation states. The question of whether a redox reaction has occurred at the iron atom or at the porphyrin ring in highly reduced or highly oxidized complexes is not semantic and can usually be decided collectively from UV-VIS spectroscopy, Mössbauer spectroscopy, and structural parameters derived from x-ray crystallography. The low spin iron(I) state has considerable validity in iron porphyrin complexes that are reduced below the iron(II) level. Complexes that are oxidized above the iron(III) level give rise to either iron(IV) porphyrins or, more often, iron(III) radical cations. The site of redox is largely dictated by the nature of the axial ligation. Highly reduced and highly oxidized iron porphyrin complexes provide a basis for understanding intermediates in the reactions of cytochrome P-450 and the peroxidases.
T r o n porphyrin complexes have been studied intensively i n their c o m m o n l y o c c u r r i n g o x i d a t i o n states, i r o n ( I I ) a n d i r o n ( I I I ) . A l l p o s s i b l e s p i n states a r e k n o w n a n d f r o m x - r a y c r y s t a l s t r u c t u r e d e t e r m i n a tions a consistent a n d r e l i a b l y p r e d i c t i v e r e l a t i o n s h i p b e t w e e n s p i n state a n d s t r u c t u r e h a s e v o l v e d (J ). A l o n g w i t h v a r i o u s m a g n e t i c c r i t e r i a , b o t h M ô s s b a u e r s p e c t r a (2) a n d p r o t o n N M R s p e c t r a (3) a r e n o t a b l y d i a g n o s t i c o f s p i n a n d o x i d a t i o n states. T h e s y s t e m a t i c s a r e s u f f i c i e n t l y w e l l - e s t a b l i s h e d s u c h that most properties o f iron(II) a n d iron(III) p o r p h y r i n c o m p l e x e s are u n d e r s t o o d a n d c a n b e p r e d i c t e d w i t h a g o o d degree o f metrical accuracy a n d confidence. O n the other h a n d , c o m p a r a b l e definitive characterization o f h i g h l y r e d u c e d o r h i g h l y o x i d i z e d i r o n p o r p h y r i n s is l a c k i n g . T h e l i t erature contains m a n y inconsistencies o n iron(I) porphyrins a n d m a n y 0065-2393/82/0201-0333$07.00/0 © 1982 A m e r i c a n C h e m i c a l Society In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
BIOLOGICAL REDOX COMPONENTS
334
a m b i g u i t i e s i n the characterization o f s o - c a l l e d i r o n ( I V ) p o r p h y r i n s . T h i s situation s h o u l d be clarified because s u c h porphyrins m a y have c o n s i d e r a b l e r e l e v a n c e to u n d e r s t a n d i n g
the r e d o x m e c h a n i s m s
w h i c h the h e m o p r o t e i n s p e r o x i d a s e , catalase, a n d c y t o c h r o m e
by
P-450
carry out t h e i r c h e m i s t r y . O f t e n , the characterization o f a n i r o n p o r p h y r i n c o m p l e x c a n n o t b e c o n s i d e r e d d e f i n i t i v e u n t i l its x - r a y c r y s t a l Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 5, 2015 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0201.ch015
structure has b e e n d e t e r m i n e d a n d a fairly c o m p l e t e battery o f spec t r o s c o p i c p r o b e s has b e e n a p p l i e d . E a r l y s o l u t i o n w o r k o n spectroscopy,
electrochemistry,
and
magnetic
UV-VIS
measurements
has
t u r n e d o u t to b e less t h a n a d e q u a t e . I m p r o v e d s y n t h e s e s a n d a m o r e extensive use o f M ô s s b a u e r a n d N M R spectroscopy a n d x-ray crystal s t r u c t u r e d e t e r m i n a t i o n has r e c e n t l y a l l o w e d t h e r e s o l u t i o n o f m a n y ambiguities.
Iron(l) Porphyrins R e d u c t i o n o f an i r o n p o r p h y r i n c o m p l e x b e l o w the iron(II) o x i d a t i o n state w a s first r e p o r t e d i n 1971 (4). S p e c t r o s c o p i c p r o p e r t i e s o f a r e d c o m p l e x p r e s u m e d to b e [ F e ( T P P ) ] ~ w e r e d e t e r m i n e d . T h i s c o m p l e x was p r e p a r e d b y s o d i u m a m a l g a m reduction o f the μ - ο χ ο iron(III) c o m p l e x F e ( T P P ) 0 i n tetrahydrofuran ( T H F ) s o l u t i o n ( T P P = tetraphenylporphyrinate). A distinctive l o w temperature E P R spectrum w i t h g± = 2 . 3 0 a n d gn = 1.93 w a s o b s e r v e d . S u c h a n i s o t r o p y is c o n sistent w i t h a s i n g l e , m e t a l - l o c a l i z e d u n p a i r e d e l e c t r o n a n d b y c o m parison w i t h rf -cobalt(II) p o r p h y r i n c o m p l e x e s , a l o w s p i n iron(I) formulation was assigned. H o w e v e r , an iron(II) p o r p h y r i n radical a n i o n f o r m u l a t i o n , w h e r e a n S = 1 i r o n is s t r o n g l y s p i n - c o u p l e d to t h e S = i r a d i c a l g i v i n g a n o v e r a l l S = i state, w o u l d b e e q u a l l y consistent w i t h t h e E P R s p e c t r u m . B u t , t h e m a g n e t i c m o m e n t , d e t e r m i n e d at h i g h e r t e m p e r a t u r e s , w a s c o n s i d e r a b l y i n e x c e s s o f t h a t e x p e c t e d for a n S = \ i r o n ( I ) c e n t e r . T h i s finding l e d to t h e p r o p o s a l t h a t t h e s p i n state c h a n g e d to a n S = f state o n t h a w i n g o f t h e f r o z e n t e t r a h y d r o f u r a n solution. S p i n state-structure relationships i n iron porphyrins sug g e s t t h a t s u c h a h i g h s p i n i r o n ( I ) s p e c i e s is v e r y u n l i k e l y b e c a u s e u n r e a l i s t i c c o r e e x p a n s i o n or o u t - o f - p l a n e i r o n d i s p l a c e m e n t [ g r e a t e r t h a n for h i g h - s p i n i r o n ( I I ) ] w o u l d b e r e q u i r e d ( J ) . M o r e o v e r , t h e V I S s p e c t r a l m a x i m a at 5 4 0 a n d 6 0 5 n m r e p o r t e d for [ F e ( T P P ) ] (4) are u n c o m f o r t a b l y c l o s e to those o f the r e d iron(II) c o m p l e x F e ( T P P ) ( T H F ) ( 5 3 9 a n d 6 1 0 n m ) , w h i c h is a n S = 2 h i g h s p i n c o m p l e x ( 5 ) . M o s t l i k e l y t h e r e p o r t e d m a g n e t i c m o m e n t (μ = 5.2 B M ) is d u e n o t t o [ F e ( T P P ) ] " b u t to a m i x t u r e c o n t a i n i n g m o s t l y h i g h s p i n i r o n ( I I ) . B e c a u s e iron(II) p o r p h y r i n s are E P R silent a n d the E P R t e c h n i q u e was not a p p l i e d q u a n t i t a t i v e l y , o n l y s m a l l a m o u n t s o f authentic [ F e ( T P P ) ] " w o u l d b e n e c e s s a r y to m a t c h t h e o b s e r v a t i o n s . 2
2
7
-
2
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
15.
335
REED
Iron(I) and Iron(IV) Porphyrins
A subsequent electrochemical study o f [Fe(TPP)]~ and related i r o n p o r p h y r i n s (6) s t r e n g t h e n e d t h e l o w s p i n a s s i g n m e n t i n f r o z e n solution, a n d further E P R studies w e r e c o n d u c t e d . [ F e ( T P P ) ] ~ m a y b e d e v o i d o f a x i a l l i g a t i o n , e v e n i n t h e p r e s e n c e o f g o o d d o n o r s s u c h as i m i d a z o l e or p y r i d i n e . T h e s l i g h t s o l v e n t d e p e n d e n c e o f t h e E P R s p e c t r u m w a s i n t e r p r e t e d i n t e r m s o f a (d 2) c o n f i g u r a t i o n b y c o m p a r i s o n to c o b a l t ( I I ) p o r p h y r i n s . B o t h t h e (d^) c o n f i g u r a t i o n a n d t h e o v e r a l l n e g a t i v e c h a r g e p r o v i d e a r a t i o n a l e for t h e l a c k o f a x i a l c o o r d i n a t i o n b u t t h e g r o u n d state is b y n o m e a n s c e r t a i n . A (d ) c o n f i g u r a t i o n is equally viable. 1
Z
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1
1
xz
A t h i r d s t u d y (7) r e p o r t e d a s y n t h e t i c r o u t e t h a t , for t h e first t i m e , a l l o w e d i s o l a t i o n o f a c r y s t a l l i n e s o d i u m salt, N a [ F e ( T P P ) ] . Na
+
naphthalide" + Fe(II)(TPP) ™
Na[Fe(TPP)] +
naphthalene
A e r o b i c sensitivity created difficulties i n o b t a i n i n g e l e m e n t a l analyses, b u t a t o m r a t i o s w e r e s a t i s f a c t o r y for N a [ F e ( T P P ) ] , e x c e p t for c a r b o n . T h i s result o c c u r r e d p r o b a b l y b e c a u s e T H F solvation was not taken into account. U n e x p l a i n e d , h o w e v e r , was the h i g h m a g n e t i c m o m e n t (μ = 5.0 B M ) . A l t h o u g h s u c h a v a l u e is s i m i l a r to t h a t o b t a i n e d i n t h e o r i g i n a l s o l u t i o n w o r k , i t is a g a i n i n c o n s i s t e n t w i t h a l o w s p i n S = i i r o n ( I ) c o m p l e x . T h e l i k e l y o r i g i n o f t h i s h i g h m a g n e t i c m o m e n t is a e r o b i c u p t a k e o f o x y g e n b y [ F e ( T P P ) ] ~ to form the h i g h s p i n i r o n ( I I I ) species [ F e ( 0 ) ( T P P ) ] " . 2
W h e n the synthesis a n d characterization are c a r r i e d out u n d e r s c r u p u l o u s l y anaerobic conditions, p u r p l e crystals o f formulation [ N a ( T H F ) ] [ F e ( T P P ) ] a r e p r o d u c e d (8). [Anal C a l c u l a t e d for C 5 6 H 0 3 N F e N a : C , 74.08; H , 5.77; N , 6.17. F o u n d : C , 74.34; H , 5.94; N , 6.06. Q u a n t i t a t i v e G L C a n a l y s i s for T H F : c a l c u l a t e d : 2 3 . 8 % . F o u n d : 25.4%] T h i s isolated product had a room-temperature magne t i c m o m e n t o f 2 . 3 B M , w h i c h is m u c h c l o s e r to t h a t e x p e c t e d o f a n S = i s y s t e m . T h e c o m p l e x reacts i m m e d i a t e l y w i t h o x y g e n i n the s o l i d state w i t h a c o n c o m i t a n t i n c r e a s e i n m a g n e t i c m o m e n t , a p p r o a c h i n g 5.9 B M (8). 3
5 2
4
T h e e a r l y s y n t h e t i c s t u d y (7) a l s o r e p o r t e d t h a t [ F e ( T P P ) ] " b i n d s c a r b o n m o n o x i d e to g i v e a n u n u s u a l " h y p e r " t y p e U V - V I S s p e c t r u m r e l a t e d to c y t o c h r o m e P - 4 5 0 . T h i s o b s e r v a t i o n is at o d d s w i t h t h e ear l i e r r e p o r t e d i n a b i l i t y o f [ F e ( T P P ) l ~ to b i n d a x i a l l i g a n d s . R e p e a t i n g t h i s e x p e r i m e n t (8) i n d i c a t e d t h a t C O has n o effect o n t h e U V - V I S s p e c t r u m o f [ F e ( T P P ) r i n T H F (392 Soret,424 Soret,512,576sh, 6 0 5 s h , a n d 6 7 4 n m ) a n d t h a t t h e s p e c t r u m r e p o r t e d for t h e p r o p o s e d C O a d d u c t ( 4 5 5 n m S o r e t ) c a n b e a c c o u n t e d for e i t h e r b y t h e p r e s e n c e o f t h e d i a n i o n [ F e ( T P P ) ] (358 a n d 4 4 8 n m S o r e t ) o r b y t h e p r e s e n c e of a n a p p a r e n t l y different form o f [ F e ( T P P ) ] ~ , the s o - c a l l e d form A (457 n m Soret). 2 -
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
BIOLOGICAL REDOX COMPONENTS
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336
T h u s , a l t e r n a t i v e e x p l a n a t i o n s c a n b e g i v e n for a l l t h e a n o m a l o u s p r o p e r t i e s r e p o r t e d for s o - c a l l e d i r o n ( I ) p o r p h y r i n s . B e c a u s e t h e s p i n state o f a n i r o n p o r p h y r i n is a l w a y s r e f l e c t e d i n its s t r u c t u r e ( I ), w e d e c i d e d t o d e t e r m i n e t h e x - r a y c r y s t a l s t r u c t u r e o f [ F e ( T P P ) ] ~ to c h e c k for c o n s i s t e n c y w i t h t h e s p i n a n d a x i a l l i g a t i o n states d e d u c e d . C r y s t a l s s u i t a b l e for x - r a y w e r e o b t a i n e d f r o m a p y r i d i n e - T H F s o l u t i o n o f N a [ F e ( T P P ) ] i n the p r e s e n c e o f d i b e n z o - 1 8 - c r o w n - 6 (8). T h e p r o d u c t has the f o r m u l a t i o n [ N a ( c r o w n ) ( T H F ) ] [ F e ( T P P ) ] · T H F . P y r i d i n e l i g a t i o n w a s a b s e n t , d e s p i t e its u s e as c o s o l v e n t . T h e a n i o n is t e t r a c o o r d i n a t e d a n d p l a n a r w i t h a n a v e r a g e F e - N d i s t a n c e o f 1.988 (1) Â ( 9 ) . T h i s d i s t a n c e is e n t i r e l y c o n s i s t e n t w i t h t h e l a c k o f d&^p o c c u p a t i o n i n t h e l o w s p i n state. It is s l i g h t l y l o n g e r t h a n t h a t o f t h e i n t e r m e d i a t e s p i n i r o n ( I I ) c o m p l e x F e ( T P P ) ( 1 . 9 7 Â ) as e x p e c t e d f r o m v a l e n c y c h a r g e c o n s i d e r a t i o n s . It is s l i g h t l y s h o r t e r t h a n t h o s e o f l o w s p i n h e x a c o o r d i n a t e d i r o n ( I I ) c o m p l e x e s (—2.00 Â ) a n d is far r e m o v e d f r o m t h o s e o f h i g h s p i n i r o n ( I I ) c o m p l e x e s (—2.08 Â ) . 2
F i n a l l y , the M o s s b a u e r spectrum o f crystalline [ N a ( T H F ) ] [ F e ( T P P ) ] w a s d e t e r m i n e d a n d t h e i s o m e r s h i f t (δ) r e l a t i v e to m e t a l l i c i r o n is 0 . 6 5 m m / s (JO). T h i s v a l u e is v e r y h i g h for a n i r o n p o r p h y r i n a n d is q u i t e c o n s i s t e n t w i t h t h e h i g h e l e c t r o n d e n s i t y at t h e n u c l e u s o f i r o n i n t h e f o r m a l l y u n i v a l e n t o x i d a t i o n state. T a b l e I l i s t s s o m e representative examples that illustrate the u s u a l t r e n d o f increas i n g δ w i t h d e c r e a s i n g o x i d a t i o n state. H o w e v e r , t h e apparently s t r a i g h t f o r w a r d t r e n d i n δ is d e c e p t i v e . E x c e p t i o n s c a n e a s i l y b e f o u n d . I n t h e a b s e n c e o f m a g n e t i c field i n v e s t i g a t i o n s ( w h i c h d o n o t h e l p i n d i a m a g n e t i c c o m p o u n d s ) a s i m p l e t r e n d i n isomer shift s h o u l d b e a c c e p t e d w i t h c a u t i o n . S i m i l a r l y , the q u a d r u p o l e shift m a y defy a s i m p l e e x p l a n a t i o n . I n t h e p r e s e n t c a s e t h e l a r g e v a l u e (ΔΕη = 2 . 2 3 m m / s ) is c o n s i s t e n t w i t h t h e l a r g e c h a r g e a s y m m e t r y e x p e c t e d o f t h i s tetragonal ion. 3
I r o n (I) p o r p h y r i n s m i g h t a p p e a r to b e w e l l - b e h a v e d s y s t e m s w i t h all properties consistent w i t h an S = i d system. O n close inspection, h o w e v e r , t w o p r o p e r t i e s are s o m e w h a t o u t o f l i n e w i t h t h e i r o n ( I ) p o r p h y r i n f o r m u l a t i o n a n d are m o r e c o n s i s t e n t w i t h a n i r o n ( I I ) p o r p h y r i n r a d i c a l a n i o n formulation, [ F e ( T P P - ) ] ~ - F i r s t , the U V - V I S spec t r u m is q u i t e l i k e t h a t o f a p o r p h y r i n r a d i c a l a n i o n h a v i n g b r o a d , l o w intensity Soret m a x i m a a n d b r o a d m a x i m a i n the α , β - r e g i o n c o m p a r e d w i t h , for e x a m p l e , F e ( T P P ) i n T H F (see F i g u r e 1). A p a r t f r o m s u c h b r o a d e n i n g , r e d - s h i f t e d Soret b a n d s also are c o n s i d e r e d d i a g n o s t i c o f p o r p h y r i n r a d i c a l a n i o n s (J9). H o w e v e r , t h e m a j o r S o r e t b a n d s o f F e ( T P P ) a n d o f [ F e ( T P P ) ] ~ are essentially the s a m e w a v e l e n g t h i n T H F . S e c o n d , i n s p e c t i o n o f the b o n d lengths a n d angles o f the porp h i n a t o core i n [ F e ( T P P ) ] ~ s h o w trends that m a y b e consistent w i t h o c c u p a t i o n o f the p o r p h y r i n l o w e s t u n o c c u p i e d m o l e c u l a r o r b i t a l 7
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
2
+
c
6
a
Γ Γ II II III III IV IV IV IV IV
Oxidation State
S= 1
s s = i,s
=
S = i S = 0 S = 1 S = 0 S =1 S = 2 S = 1 = 1,S = S = 1 = 1,S =
S =i low S = 1 S = 0 S = f S = f S = 1 S = 1 S = 1 S = 1
s
Overall Spin State
Iron Spin State
Reference 10 10 11 11 12 13 14 15 16 17 18
AEq 2.23 1.29 1.51 1.44 0.46-0.70 0.56 1.24 1.55 1.61 1.49 1.25
(rel. Fe) 0.65 0.48 0.52 0.51 0.41-0.47 0.41 0.11 0.05 0.03 0.05 0.08
δ
T r e n d i n M ô s s b a u e r P a r a m e t e r s ( m m / s at 4 . 2 K ) w i t h O x i d a t i o n S t a t e o f I r o n
Probably in resonance with Fe(II). See text. The S = i radical is protein located and does not interact significantly with the S = 1 iron atom. Weakly interacting.
[FeCl(TPP-)] FeO(TPP)U-Melm) Compd E Sof C C P Compd II of H R P FeO(TMP)Cl Compd I of H R P
2
[Fe(TPP)][Fe(TPP)] Fe(TPP) Fe(TPP)(pip) FeX(TPP)
Compound
Table I.
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Co CO
κ, ο
BIOLOGICAL REDOX COMPONENTS
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338
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
15.
REED
339
Iron(I) and Iron(IV) Porphyrins
( L U M O ) . I n F i g u r e 2 t h r e e s t r u c t u r e s are c o m p a r e d : F e ( T P P ) ( T H T )
2
as a t y p i c a l l o w s p i n i r o n ( I I ) p o r p h y r i n o f w e l l - d e f i n e d v a l e n c y (20); [Fe(TPP)]"
as
its
sodium
crown-THF-solvated
salt
2
[ F e ( T P P ) ] " , the further r e d u c e d species that q u i t e
(9);
and
unambiguously
h a s p o r p h y r i n r a d i c a l a n i o n c h a r a c t e r ( d i s c u s s e d later) (9). T h e b o n d d i s t a n c e s are l i s t e d i n o r d e r for t h e s e t h r e e c o m p o u n d s i n t h e
upper
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r i g h t q u a d r a n t o f F i g u r e 2 a n d t h e l a b e l i n g s c h e m e is i n t h e u p p e r left quadrant. T h e lengthening
o f the
and C
a
- N bonds and
the
s h o r t e n i n g o f the C - C & b o n d i n [ F e ( T P P ) ] " r e l a t i v e to those o f a a
n o r m a l p l a n a r p o r p h y r i n are c o n s i s t e n t w i t h s o m e p o p u l a t i o n o f t h e porphyrin L U M O , an e
g
C -C b
b
and C
a
o r b i t a l t h a t is a n t i b o n d i n g w i t h r e s p e c t to
- N a n d b o n d i n g w i t h r e s p e c t to C - C a
structures o f u n a m b i g u o u s
(21).
6
radical anion complexes
M o r e x-ray
are n e e d e d
c l a r i f y t h e f u l l effects o f L U M O p o p u l a t i o n .
124.4(2) Figure 2. Porphyrin core dimensions for Fe(TPP)(THT) , [Fe(TPP)]~, and [Na(THF) ] [Fe(TPP)] (9). Each group of digits gives the dimensions for these three species in their respective order. 2
3
2
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
to
BIOLOGICAL REDOX COMPONENTS
340
T h e c o n c l u s i o n is t h a t s o m e c o n t r i b u t i o n o f a s p i n - c o u p l e d i r o n ( I I ) p o r p h y r i n r a d i c a l a n i o n formulation to the iron(I) p o r p h y r i n formula tion m a y b e necessary to r a t i o n a l i z e f u l l y the properties o f [ F e ( T P P ) ] " . R e s o n a n c e forms c a n b e w r i t t e n w i t h iron(I) as the major c o n t r i b u t o r :
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[Fe(I)(TPP)]- ~ [Fe(II)(TPP · )]"
S u c h resonance suggests that i n the f o r m a l i z e d iron(I) c o n t r i b u t o r the u n p a i r e d e l e c t r o n m u s t r e s i d e i n a n o r b i t a l o f t h e s a m e s y m m e t r y as t h e p o r p h y r i n L U M O . B e c a u s e t h e p o r p h y r i n L U M O has e s y m m e t r y , g
the s i n g l y o c c u p i e d iron(I) orbital must be d
xz
or d . yz
1
T h i s o b s e r v a t i o n is i n c o n f l i c t w i t h t h e (d^) c o n f i g u r a t i o n s u g gested b y E P R results, w h i c h c o m p a r e d iron(I) to t h e isoelectronic 1
C o ( I I ) ( T P P ) (6). T h i s i s s u e r e m a i n s to b e r e s o l v e d , b u t t h e (d )
spin
xz
c o n f i g u r a t i o n for i r o n ( I ) w o u l d b e m o r e i n k e e p i n g w i t h t h e
2
(d ,d ) xz
yz
configuration o f F e ( I I ) ( T P P ) w h e r e the d 2 orbital is l o w e r i n energy Z
than
d ,d . xz
yz
I n s u m m a r y , t h e l o w s p i n i r o n ( I ) f o r m u l a t i o n for [ F e ( T P P ) ] " is a n adequate assignment
for m o s t p r o p e r t i e s , b u t s o m e r e s o n a n c e
con
t r i b u t i o n f r o m a s p i n - c o u p l e d i r o n ( I I ) r a d i c a l f o r m u l a t i o n is n e e d e d to r a t i o n a l i z e the broadness o f the U V - V I S s p e c t r u m a n d the s u b t l e dif ferences i n core b o n d lengths b e t w e e n [ F e ( T P P ) ] " a n d a n o r m a l i r o n p o r p h y r i n . F o r t h e first t i m e , a s i n g l e , d i s c r e t e v a l e n c y is i n a d e q u a t e for d e s c r i b i n g a m e t a l l o p o r p h y r i n c o m p l e x . T y p i c a l l y , a p o r p h y r i n c a n b e c o n s i d e r e d a " h a r d , " i n n o c e n t l i g a n d a n d the issue o f assigning o x i d a t i o n states is n o t s e m a n t i c . F o r e x a m p l e , b o t h v a l e n c e i s o m e r s ô f an oxidized nickel porphyrin were
observed. T h e metal
oxidized
+
[ N i ( I I I ) ( T P P ) ] c a n b e c o n v e r t e d to t h e r i n g o x i d i z e d [ N i ( I I ) ( T P P )]
+
s i m p l y b y m a k i n g m i n o r alterations i n the solvation e n v i r o n m e n t o f the c o m p l e x (22), s u g g e s t i n g t h a t b o t h N i a n d T P P m a i n t a i n d i s c r e t e v a l encies i n a c h e m i c a l e q u i l i b r i u m :
+
[ N i ( I I I ) ( T P P ) ] 5± [ N i ( I I ) ( T P P · ) ]
+
W h e t h e r a true e q u i l i b r i u m exists has not b e e n i n v e s t i g a t e d . H o w e v e r , t h i s i n t e r p r e t a t i o n is to b e c o n t r a s t e d w i t h t h e r e s o n a n c e b e t w e e n v a l e n c y h y b r i d s j u s t s u g g e s t e d for i r o n ( I ) . I f r e s o n a n c e c o n t r i b u t o r s a r e n e e d e d w i t h r a d i c a l a n i o n s t r u c t u r e s b u t n o t for r a d i c a l c a t i o n s t r u c tures, a rationale m a y l i e i n orbital o v e r l a p s y m m e t r i e s . A l t h o u g h the e L U M O s o f a p o r p h y r i n h a v e t h e c o r r e c t s y m m e t r y for o v e r l a p w i t h t h e (d ,d ) m e t a l o r b i t a l s , n e i t h e r t h e a n o r t h e α% H O M O s ( h i g h e s t
g
xz
yz
2u
η
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
15.
341
REED
lron(l)
and Iron(IV)
Porphyrins
o c c u p i e d m o l e c u l a r o r b i t a l s ) h a v e t h e c o r r e c t s y m m e t r y for o v e r l a p w i t h a n y o f the m e t a l d-orbitals. H o w e v e r , l a c k o f o v e r l a p does not r u l e out s p i n c o u p l i n g i n r a d i c a l cations.
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C h o i c e o f a n i n t e g r a l v a l e n c y d e p e n d s on the p a r t i c u l a r p h y s i c a l p r o p e r t y that forms the basis o f the assignment. T h e f o r e g o i n g analysis demonstrates that M o s s b a u e r spectroscopy c a n be a g o o d p r o b e o f the i r o n o x i d a t i o n state, U V - V I S s p e c t r o s c o p y m a y b e t h e b e s t p r o b e o f the oxidation l e v e l o f the p o r p h y r i n , a n d a consistency c h e c k w i t h an x - r a y c r y s t a l s t r u c t u r e is h i g h l y d e s i r a b l e . M a g n e t i c c r i t e r i a a r e fre q u e n t l y a m b i g u o u s . E l e c t r o c h e m i c a l c r i t e r i a for d i s t i n g u i s h i n g r i n g redox from m e t a l r e d o x i n c l u d e the ~ 2 . 2 5 - V difference i n p o t e n t i a l b e t w e e n t h e first r i n g o x i d a t i o n a n d t h e first r i n g r e d u c t i o n (23). T h i s c r i t e r i o n is n o t p a r t i c u l a r l y h e l p f u l for i r o n p o r p h y r i n s . A n o t h e r e l e c t r o c h e m i c a l c r i t e r i o n for i d e n t i f y i n g t h e s i t e o f r e d o x i n v o l v e s c o m p a r i n g t h e r e d u c t i o n p o t e n t i a l s for M ( T P P ) ( M = m e t a l ) c o m p l e x e s w i t h those o f the tetracyano-substituted t e t r a p h e n y l p o r p h y r i n c o m p l e x e s M ( T P P - C N ) (24). D i f f e r e n c e s o f 1.0 ± 0.1 V i n d i c a t e r i n g r e d o x , w h e r e a s m e t a l c e n t e r e d r e d o x u s u a l l y has m u c h s m a l l e r d i f f e r e n c e s . F o r t h e i r o n ( I I I - I I ) r e d u c t i o n t h e d i f f e r e n c e is o n l y 0 . 0 5 V b u t for t h e n e x t r e d u c t i o n , t h e d i f f e r e n c e is 0.8 V . T h i s r e s u l t s u p p o r t s t h e i r o n ( I ) a s s i g n m e n t b u t a l s o i n d i c a t e s t h a t t h e i r o n ( I - I I ) p o t e n t i a l is c l o s e to the T P P - T P P - potential. 4
O n e p u z z l i n g feature o f the e l e c t r o c h e m i c a l p r o d u c t i o n o f iron(I) p o r p h y r i n s r e m a i n s . B o t h e l e c t r o c h e m i c a l s t u d i e s (6, 25) r e p o r t t w o forms o f [ F e ( T P P ) ] " , l a b e l e d A a n d B . T h e y are d i s t i n g u i s h e d b y U V - V I S s p e c t r a a n d f o r m Β c o r r e s p o n d s to t h e s y n t h e t i c a l l y a c c e s s i b l e s p e c i e s ( 3 9 0 a n d 4 2 4 n m S o r e t ) a l r e a d y d i s c u s s e d . F o r m A is p r o d u c e d w h e n r e d u c t i o n is c a r r i e d o u t at a m o r e n e g a t i v e p o t e n t i a l . I n l i g h t o f t h e c h e m i s t r y n o w e s t a b l i s h e d for [ F e ( T P P ) ] ~ , i t is n o t c l e a r h o w t w o n o n i n t e r c o n v e r t i b l e forms c o u l d exist. T h e i n v o l v e m e n t o f h y d r o x i d e w a s d i s c u s s e d (25) b u t t h e r e d u c t i o n l e v e l o f A ( 3 6 2 a n d 4 5 7 n m Soret) was not u n a m b i g u o u s l y d e t e r m i n e d . T h e air sensitivity o f these h i g h l y r e d u c e d species makes this w o r k e x p e r i m e n t a l l y v e r y d e m a n d i n g , b u t m o r e s t u d i e s a r e n e e d e d to a s s i g n a f o r m u l a a n d s t r u c ture to the s o - c a l l e d form A o f [ F e ( T P P ) ] " .
Utility of lron(l) Porphyrins: A New Type of Dioxygen Adduct I r o n ( I ) p o r p h y r i n s d o n o t a p p e a r to b e b i o l o g i c a l l y i m p o r t a n t b e c a u s e t h e v e r y n e g a t i v e i r o n ( I - I I ) r e d o x p o t e n t i a l is i n a c c e s s i b l e w i t h t y p i c a l b i o l o g i c a l r e d u c i n g agents. H o w e v e r , the one-electron
reduc
tion o f the o x y g e n a d d u c t d e r i v e d from ferrous c y t o c h r o m e
P-450
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
342
BIOLOGICAL REDOX COMPONENTS
[ f o r m a l l y a n F e ( I I I ) s u p e r o x o c o m p l e x (26)] is t h e k e y s t e p t h a t i n i tiates m o n o o x y g e n a s e c h e m i s t r y :
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Fe(III)J
substrate b o u n d ,
r e s t i n g state
oxygenated P-450
ferric P - 4 5 0
+ ROH+ H 0 2
I n the presence o f organic substrate ( R H ) a n d a source o f protons, the o n e - e l e c t r o n r e d u c e d o x y g e n a d d u c t reacts r a p i d l y , w i t h o u t i s o l a b l e i n t e r m e d i a t e s , t o g i v e o x y g e n a t e d s u b s t r a t e ( R O H ) a n d H 0 (27). C o n c e p t u a l l y , t h e o n e - e l e c t r o n r e d u c t i o n o f a n o x y g e n a d d u c t is t h e s a m e as a d d i n g 0 to a n i r o n ( I ) p o r p h y r i n . It is a l s o c o n c e p t u a l l y t h e s a m e as a d d i n g s u p e r o x i d e to a n i r o n ( I I ) p o r p h y r i n (28). 2
2
Fe(0 )(porph)
+
[Fe(I)(porph)]-
+
0
2
Fe(II)(porph)
+
0
2
2
I g n o r i n g t h e effect o f a x i a l l i g a t i o n , t h e r e a c t i o n o f i r o n ( I ) p o r p h y r i n w i t h oxygen should y i e l d a n e w type of reduced oxygen complex, p o s s i b l y r e l a t e d to the k e y r e d u c t i o n s t e p i n the c y t o c h r o m e P - 4 5 0 cycle. T r e a t m e n t o f [ F e ( T P P ) ] " w i t h d r y o x y g e n i n T H F s o l u t i o n at l o w t e m p e r a t u r e s or as a s o l i d at r o o m t e m p e r a t u r e p r o d u c e s [ F e ( 0 ) ( T P P ) ] ~ as its s o d i u m s a l t (8). It is s p e c t r o s c o p i c a l l y i d e n t i c a l to the species p r o d u c e d b y treatment o f F e ( T P P ) w i t h superoxide i o n (28, 2 9 ) . It is c h a r a c t e r i z e d b y U V - V I S s p e c t r o s c o p y ( 4 3 3 , 5 6 4 , a n d 6 0 7 n m i n T H F ) , Ε P R s p e c t r o s c o p y (g = 8, 4 . 2 2 , 1.98 i n f r o z e n D M S O ) , M ô s s b a u e r s p e c t r o s c o p y [ δ = 0 . 5 7 m m / s , AEq = 1 . 0 m m / s at 4 . 2 Κ i n t h e s o l i d state (10)], a n d v i b r a t i o n a l s p e c t r o s c o p y o f t h e o c taethylporphyrinate ( O E P ) analogue [ K ) - 0 = 806 c m ( 2 8 ) ] . T h e ef fects o f t e m p e r a t u r e a n d a p p l i e d m a g n e t i c field o n t h e M ô s s b a u e r s p e c t r u m are r e a d i l y i n t e r p r é t a b l e i n terms o f a h i g h - s p i n S = f i r o n ( I I I ) state (20). T a k e n a l l t o g e t h e r t h e s e d a t a s u g g e s t a p e r o x o i r o n ( I I I ) f o r m u l a t i o n for [ F e ( 0 ) ( T P P ) ] ~ w i t h t r i a n g u l a r d i h a p t o c o o r d i n a t i o n l i k e T i ( 0 ) ( O E P ) (30): 2
- 1
2
2
Ο
Ο
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
15.
REED
343
Iron(I) and Iron(IV) Porphyrins
A l t h o u g h [ F e ( I I I ) ( 0 ) ( T P P ) ] ~ lacks the axial thiolate c o o r d i n a t i o n 2
o f c y t o c h r o m e P-450, t h i s w o r k s u g g e s t s t h a t a p e r o x o
intermediate
c o u l d f o r m at t h e d i o x y g e n a d d u c t r e d u c t i o n s t e p i n t h e P-450 c a t a l y t i c c y c l e , p o s s i b l y w i t h t r a n s i t o r y a x i a l - l i g a n d d i s s o c i a t i o n . M o r e w o r k is n e e d e d to v a l i d a t e s u c h s p e c u l a t i o n , h o w e v e r . T h u s , a l t h o u g h iron(I) p o r p h y r i n s t h e m s e l v e s are u n l i k e l y inter Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 5, 2015 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0201.ch015
mediates i n hemoprotein biochemistry, their reaction chemistry may l e a d to u s e f u l m o d e l c o m p o u n d s .
Further Reduced Iron Porphyrins E l e c t r o c h e m i c a l p r o d u c t i o n o f a s p e c i e s t h a t is t w o - e l e c t r o n r e d u c e d b e l o w i r o n ( I I ) w a s i d e n t i f i e d b y U V - V I S s p e c t r o s c o p y (6, 25) (see F i g u r e 1) a n d a s s i g n e d t h e f o r m u l a [ F e ( T P P ) ] " . T h i s g r e e n s p e c i e s is p r e s u m a b l y t h e s a m e as t h a t s e e n as t h e final p r o d u c t o f N a ( H g ) r e d u c t i o n o f F e ( T P P ) 0 (4), a n d o n t h e b a s i s o f t h e b r o a d m a x i m a a n d t h e r e d - s h i f t e d S o r e t b a n d w a s f o r m u l a t e d as a r a d i c a l a n i o n o f i r o n ( I ) , [ F e ( I ) ( T P P - ) ] ~ - A s y n t h e s i s for t h i s d i a n i o n w a s d e v e l o p e d b y treating F e C l ( T P P ) w i t h three equivalents of s o d i u m ant h r a c e n i d e i n T H F s o l u t i o n (8). D a r k g r e e n c r y s t a l s o f t h e d i s o d i u m s a l t w e r e i s o l a t e d a n d a n a l y z e d for [ N a ( T H F ) ] [ F e ( T P P ) ] [Anal C a l c u l a t e d for C 8 H 0 6 N 4 F e N a : C/71.19; H,6,68; N,4.88. F o u n d : C,71.42; H , 6 . 7 8 ; N,5.08.] T h i s s p e c i e s is e x t r e m e l y a i r s e n s i t i v e , d e c o m p o s i n g a e r o b i c a l l y to [ F e ( T P P ) ] 0 . It is d i a m a g n e t i c a n d s h o w s n o E P R s i g n a l w h e n p u r e . A s u s u a l , m a g n e t i c d a t a d o n o t a l l o w o n e to d i s t i n g u i s h b e t w e e n t h r e e p o s s i b l e , b u t n o t e x c l u s i v e , f o r m u l a t i o n s : (1) a n iron(O) rf -porphyrin, [ F e ( 0 ) ( T P P ) ] " , (2) a s p i n - c o u p l e d S = i i r o n ( I ) p o r p h y r i n r a d i c a l a n i o n , [ F e ( I ) ( T P P - ) ] ~ , a n d (3) a s p i n - c o u p l e d S = 1 iron(II) p o r p h y r i n d i r a d i c a l dianion, [Fe(II)(TPP:)] ~. A s m e n t i o n e d e a r l i e r , the g r e e n c o l o r a n d U V - V I S s p e c t r a l features are c o n s i s t e n t w i t h r a d i c a l a n i o n c h a r a c t e r o f t h e p o r p h y r i n r i n g (see F i g u r e 1) s u g g e s t i n g that the iron(I) and/or the iron(II) formulations are m o s t s u i t a b l e . H o w e v e r , M ô s s b a u e r spectroscopy a n d a n x-ray crystal struc t u r e w e r e d e s i r a b l e to m a k e a firmer a s s i g n m e n t . 2
2
2
2
3
6
7 6
2
2
2
8
2
2
2
T h e x-ray crystal structure o f [ N a ( T H F ) ] [ F e ( T P P ) ] w a s deter m i n e d (9) a n d r e v e a l s t h e 2 : 1 i o n - p a i r e d s t r u c t u r e i l l u s t r a t e d i n F i g u r e 3. T h e a v e r a g e F e - N d i s t a n c e is 1.968 (2) Â , w h i c h is c o n s i s t e n t w i t h a l o w s p i n state. I n F i g u r e 2 t h e r i n g b o n d l e n g t h s a r e c o m p a r e d w i t h those o f an iron(II) l o w s p i n p o r p h y r i n a n d w i t h [ F e ( T P P ) ] ~ . A g a i n , the l e n g t h e n i n g o f CÔ-C& a n d C - N a n d the s h o r t e n i n g o f C - C f t s u g g e s t s p o p u l a t i o n o f t h e p o r p h y r i n L U M O , m o r e so t h a n w i t h [ F e ( T P P ) ] " . T h e structural results therefore support the iron(I) r a d i c a l formulation b u t d o not r e a d i l y d i s t i n g u i s h it from the iron(II) d i r a d i c a l formulation. 3
2
a
a
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
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344
BIOLOGICAL REDOX COMPONENTS
Figure 3.
The crystal
structure
of [Na(THF) ] [Fe(TPP)] 3
2
(9).
T h e somewhat surprising result comes from the M ô s s b a u e r spec t r u m . T h e i s o m e r s h i f t o f [ N a ( T H F ) ] [ F e ( T P P ) ] is 0 . 4 8 m m / s (JO), w h i c h is c o m p a r a b l e w i t h i r o n ( I I ) p o r p h y r i n s n o t i r o n ( I ) (see T a b l e I). A v a l u e s i m i l a r to i r o n ( I ) or e v e n h i g h e r m i g h t h a v e b e e n e x p e c t e d b u t n o t l o w e r . T w o p o s s i b l e e x p l a n a t i o n s for t h i s r e s u l t are (1) t h e i o n p a i r i n g w i t h s o d i u m v i a the porphinato nitrogen atoms m a y decrease the s-electron density seen b y the i r o n n u c l e u s , m a k i n g the d i a n i o n a p p e a r to b e m o r e o x i d i z e d t h a n t h e m o n o a n i o n [ F e ( T P P ) ] " ( δ = 0 . 6 5 m m / s ) a n d (2) a n u n e x p e c t e d l y l a r g e c o n t r i b u t i o n c o u l d c o m e f r o m t h e i r o n ( I I ) d i r a d i c a l s t r u c t u r e t o t h e f o r m u l a t i o n . I n fact, a l l t h e d a t a o n [ F e ( T P P ) ] can be rationalized b y the s p i n - c o u p l e d iron(II) d i r a d i c a l f o r m u l a t i o n . H o w e v e r , a n u n e x p l a i n e d fact is t h a t t h e t w o - e l e c t r o n r e d u c t i o n o f iron(II) retains l a r g e l y iron(II) character w h i l e the oneelectron r e d u c t i o n gives l a r g e l y iron(I) character. T h u s , a resonance h y b r i d o f i r o n ( I ) a n d i r o n ( I I ) is f a v o r e d . 3
2
2 -
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
REED
15.
345
Iron(I) and Iron(IV) Porphyrins 2
2
[Fe(I)(TPP-)] ~ ~ [Fe(II)(TPP:)] -
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C o n s i d e r e d i n d i v i d u a l l y , b o t h o f these contributors m u s t b e c o m p l e t e l y s p i n - c o u p l e d b e c a u s e these formulations are i n resonance, not i n e q u i l i b r i u m , a n d t h e c o m p l e x is d i a m a g n e t i c . M e t a t h e s i s w i t h a l k y l a m m o n i u m salts t o r e m o v e t h e s o d i u m i o n - p a i r i n g is b e i n g at t e m p t e d to r e s o l v e t h e a m b i g u i t y i n t h e M ô s s b a u e r i n t e r p r e t a t i o n ( 8 ) .
[FeCl(TPP)]+: Iron(IV) Porphyrin or Iron(III) Radical Cation? I n t h e first r e p o r t (31) o f a n i r o n p o r p h y r i n o x i d i z e d a b o v e
the
i r o n ( I I I ) l e v e l , F e ( O A c ) ( T P P ) was 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 to a species written [ F e ( T P P ) ]
2 +
, a n d o n t h e b a s i s o f its U V - V I S
spectrum
( b r o a d , b l u e - s h i f t e d , m o d e r a t e l y l o w i n t e n s i t y S o r e t at 3 9 6 n m broad
and
α,/3-region) assigned an iron(III) radical cation formulation,
[Fe(III)(TPP)]
2 +
. A m a g n e t i c m o m e n t o f 2 . 7 1 B M w a s i n t e r p r e t e d as
an overall S = 1 system h a v i n g a l o w s p i n S = i iron(III) a n d an S = i p o r p h y r i n radical w i t h no s p i n - c o u p l i n g . I n v i e w o f subsequent mea surements
on
the
very
closely related
S = 2
chloride
complex
+
[ F e C l ( T P P ) ] , this m a g n e t i c s u s c e p t i b i l i t y m e a s u r e m e n t most l i k e l y w a s i n e r r o r . W e t s o l v e n t s p r o b a b l y g a v e r i s e to [ F e ( T P P ) ] 2 0 a n d its oxidation products. These antiferromagnetically c o u p l e d systems have l o w m a g n e t i c m o m e n t s a n d the p u b l i s h e d U V - V I S s p e c t r a are consis t e n t w i t h t h e i r p r e s e n c e as m a j o r c o m p o n e n t s . Solutions
of [FeCl(TPP)]
+
were
prepared
by
electrochemical
m e t h o d s (32, 33). T h e c o m p l e x w a s a s s i g n e d a h i g h s p i n i r o n ( I V ) for m u l a t i o n b a s e d o n t h e S = 2 s p i n state, U V - V I S a n d p r o t o n N M R spectroscopy, and molecular orbital calculations. T h e closely related [Fe(p-methoxyTPP)Cl]
+
s h o w s s i m i l a r s p e c t r o s c o p i c p r o p e r t i e s to t h e
a n a l o g o u s p r o t o p o r p h y r i n I X d i m e t h y l e s t e r c o m p l e x (34). H o w e v e r , n o n e o f these data u n a m b i g u o u s l y d i s t i n g u i s h b e t w e e n the alternate formulations o f [ F e C l ( T P P ) ]
+
as (1) a h i g h s p i n i r o n ( I V ) p o r p h y r i n o r
(2) a h i g h s p i n i r o n ( I I I ) r a d i c a l c a t i o n w h e r e t h e S = i i r o n is s p i n c o u p l e d t o t h e S = i p o r p h y r i n r a d i c a l to g i v e a n o v e r a l l S = 2 state (19). I n k e e p i n g w i t h t h e v i e w s t h a t M ô s s b a u e r s p e c t r o s c o p y is o n e o f t h e b e s t c r i t e r i a o f o x i d a t i o n a n d s p i n states o f t h e i r o n a n d t h a t a synthetic m e t h o d that a l l o w e d isolation o f [ F e C l ( T P P ) ]
+
for x - r a y
a n a l y s i s w a s d e s i r a b l e to r e s o l v e t h i s p r o b l e m , t h i s s y s t e m w a s r e i n v e s t i g a t e d (13). u s i n g the
A convenient synthetic method was d e v e l o p e d b y
hexachloroantimonate
+
salts o f o r g a n i c r a d i c a l s ( R · )
as
clean, one-electron oxidants of k n o w n potential. +
FeCl(TPP) + R · SbCl " ^ > 6
[FeCl(TPP)][SbCl ] + R 6
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
346
BIOLOGICAL REDOX COMPONENTS
I n m a n y respects these reagents represent the synthetic e q u i v a l e n t o f controlled potential electrochemistry. T h e product [FeCl(TPP)][ S b C l ] is i s o l a b l e as b l a c k c r y s t a l s a n d h a s a U V - V I S s p e c t r u m i n d i c h l o r o m e t h a n e i d e n t i c a l t o t h a t r e p o r t e d for [ F e C l ( T P P ) ] p r o d u c e d e l e c t r o c h e m i c a l l y . T h e m a g n e t i c m o m e n t (μ = 5.1 B M ) is i n d i c a t i v e of four u n p a i r e d electrons a n d C u r i e - W e i s s l a w behavior ( 5 - 3 0 0 K ) shows it to b e a w e l l - b e h a v e d S = 2 system. T h i s magnetic m o m e n t a l s o r u l e s o u t t h e u n l i k e l y f o r m u l a t i o n o f [ F e C l ( T P P ) ] as a n u n c o u p l e d p o r p h y r i n radical cation o f intermediate s p i n iron(III) b e c a u s e s o m e c o u p l i n g o f a d j a c e n t S = f a n d S= i s y s t e m s w o u l d b e e x p e c t e d at 5 K . N o t a b l y , t h e M ô s s b a u e r s p e c t r u m o f [ F e C l ( T P P ) ] [ S b C l e ] is v e r y m u c h l i k e h i g h s p i n i r o n ( I I I ) , n o t o n l y i n t e r m s o f t h e i s o m e r s h i f t a n d q u a d r u p o l e s p l i t t i n g (see T a b l e I ) , b u t a l s o i n t h e a s y m m e t r y o f t h e d o u b l e t as a f u n c t i o n o f t e m p e r a t u r e ( 2 3 ) . T h i s e v i d e n c e is p a r t i c u l a r l y c o m p e l l i n g f o r a h i g h s p i n i r o n ( I I I ) r a d i c a l for m u l a t i o n . A l l the spectroscopic data p r e v i o u s l y u s e d to support the alternate iron(IV) assignment (32, 3 3 ) c a n b e reinterpreted. T h e U V V I S spectrum o f [ F e C l ( T P P ) ] has most o f the characteristics o f a r a d i c a l cation i n c l u d i n g a b r o a d α , β - r e g i o n a n d a Soret b a n d (396 n m ) t h a t is b r o a d e n e d , b l u e - s h i f t e d , a n d l o w e r e d i n i n t e n s i t y c o m p a r e d to F e C l ( T P P ) (see F i g u r e 4 ) . T h e s e c h a r a c t e r i s t i c s w e r e p r o b a b l y o v e r l o o k e d at first b e c a u s e t h e r a d i c a l c h a r a c t e r i s t i c s a r e n o t as d r a m a t i c as i n a n i d e a l c a s e s u c h as [ Z n ( T P P ) ] w h e r e m e t a l a n d l i g a n d orbital energies are p r e s u m a b l y well-separated. T h e proton N M R s p e c t r u m has the /3-pyrrole protons m a r k e d l y s h i f t e d d o w n f i e l d (-63.8 ppm) i n a manner reminiscent o f h i g h s p i n iron(III) [ - 7 9 p p m in F e C l ( T P P ) ] (3). T h e value o f - 6 9 p p m originally reported (32) 6
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+
+
+
+
1
p r o b a b l y arose from s i g n a l a v e r a g i n g from a m i x t u r e o f [FeCl(TPP)]" " a n d F e C l ( T P P ) . T h i s l a r g e d o w n f i e l d s h i f t is p r o b a b l y d u e t o s p i n d e n s i t y i n t h e d 2_ o r b i t a l a n d i t is n o t a b l y u n l i k e t h a t o f t h e h i g h s p i n d - c o m p l e x M n C l ( T P P ) [ + 2 0 p p m ( 3 ) ] , w h i c h h a s d 2_ unoc c u p i e d . B e c a u s e M n C l ( T P P ) is i s o e l e c t r o n i c w i t h h i g h s p i n i r o n ( I V ) , i t is p r o b a b l y a g o o d N M R m o d e l . T h e c h e m i c a l shifts o f t h e p h e n y l p r o t o n s a r e a n a l y z e d i n m o r e d e t a i l e l s e w h e r e (34). T h e s i n g l e o c c u pation o f the p o r p h y r i n H O M O i n T P P - m i g h t b e e x p e c t e d to shift various ring vibrational modes. Interestingly, [ F e C l ( T P P ) ] [ S b C l ] h a s a s t r o n g b a n d at 1 2 9 0 c m i n t h e I R s p e c t r u m t h a t d o e s n o t a p p e a r i n F e C l ( T P P ) . T h i s b a n d d o e s n o t a r i s e f r o m t h e a n i o n a n d is s e e n i n other T P P r a d i c a l cation species ( 3 5 ) . T h i s b a n d appears to b e a u n i q u e l y d i a g n o s t i c criterion o f r a d i c a l cations o f T P P . X
y2
4
X
y2
+
6
- 1
T h e final a r b i t e r o f t h i s f o r m u l a t i o n q u e s t i o n w i l l b e t h e x - r a y crystal structure because a h i g h s p i n iron(III) atom i n [ F e C l ( T P P ) ] s h o u l d have c o o r d i n a t i o n g r o u p parameters e n t i r e l y different from a n i r o n ( I V ) a t o m . W i t h d^- i o c c u p i e d , h i g h s p i n i r o n ( I I I ) , i f p e n t a c o o r +
y
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
Figure
4.
UV-VIS
6
spectra of Fe(III)Cl(TPP) (—) and [Fe(III)Cl(TPP)][SbCl ] (->) methane (35). These essentially are identical to those of Ref. 33.
in
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dichloro-
348
BIOLOGICAL REDOX COMPONENTS
d i n a t e d s h o u l d h a v e l o n g F e - N d i s t a n c e s (—2.05 Â ) a n d s h o u l d b e d i s p l a c e d c o n s i d e r a b l y (—0.5 Â ) f r o m t h e p o r p h y r i n p l a n e . O n t h e o t h e r h a n d , h i g h s p i n i r o n ( I V ) , h a v i n g άχ2- 2 u n o c c u p i e d , is e x p e c t e d to s h o w short F e - N distances ( ^ 2 . 0 0 À) a n d c o o r d i n a t i o n g r o u p pa rameters s i m i l a r to, b u t s l i g h t l y c o n t r a c t e d f r o m , M n C l ( T P P ) .
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υ
T h e e l e c t r o c h e m i c a l c r i t e r i a for d i s t i n g u i s h i n g m e t a l o x i d a t i o n f r o m r i n g o x i d a t i o n are at b e s t a m b i g u o u s a n d at w o r s t m i s l e a d i n g for i r o n . T h e 2 . 2 5 ± 0 . 1 5 - V d i f f e r e n c e b e t w e e n t h e first r i n g o x i d a t i o n a n d first r i n g r e d u c t i o n o f m o s t o c t a e t h y l p o r p h y r i n a t e ( O E P ) c o m p l e x e s l e a d s to a m b i g u i t y i n F e ( O E P ) ( O H ) (23). F o r F e ( T P P ) d e r i v a t i v e s t h e [ F e ( I ) ( T P P ) ] - / [ F e ( I I I ) C l ( T P P ) ] d i f f e r e n c e is - 2 . 1 V a n d t h e [ F e ( T P P ) ] 7 F e ( I I I ) C l ( T P P - ) ] r i n g d i f f e r e n c e is - 2 . 7 V . C l e a r l y , t h e 2 . 2 5 - V c r i t e r i o n is n o t a p p l i c a b l e i n t h i s c a s e . T h e d i f f e r e n c e b e t w e e n t h e first a n d s e c o n d r i n g o x i d a t i o n s is f r e q u e n t l y 0 . 2 9 ± 0 . 0 5 V , s u g g e s t i n g a n o t h e r c r i t e r i o n for a s s i g n i n g r e d o x s t e p s ( 1 9 ) . I n e t i o p o r p h y r i n I iron(III) chloride a n d tetra-p-tolylporphyrin iron(III) chlor i d e d i f f e r e n c e s o f 0 . 5 4 a n d 0 . 4 3 V , r e s p e c t i v e l y , w e r e u s e d to s u b stantiate the i r o n ( I V ) assignment (33). I n v i e w o f the c o n c l u s i o n that [ F e C l ( T P P ) ] is n o t i r o n ( I V ) a n d t h e l i k e l i h o o d t h a t t h e e t i o p o r p h y r i n I a n d t o l y l p o r p h y r i n a n a l o g u e s are l i k e w i s e not i r o n ( I V ) , this e l e c t r o c h e m i c a l c r i t e r i o n is a p p a r e n t l y n o t a l w a y s r e l i a b l e . O f s u b s t a n t i a l l y m o r e r e l i a b i l i t y is t h e c r i t e r i o n o f i n s e n s i t i v i t y o f r i n g o x i d a t i o n t o a x i a l l i g a t i o n c h a n g e (36). T h i s study l e n d s support to the r i n g o x i d a tion o f F e C l ( T P P ) . T h u s , a l t h o u g h i n favorable cases e l e c t r o c h e m i c a l c r i t e r i a c a n s u p p o r t the a s s i g n m e n t o f a site o f r e d o x , they s h o u l d not b e r e l i e d o n i n cases w h e r e m e t a l r e d o x p o t e n t i a l s m a y b e c l o s e to ring potentials. +
2
+
+
I n s u m m a r y , t h e w e i g h t o f e v i d e n c e is h e a v i l y a g a i n s t a n i r o n ( I V ) f o r m u l a t i o n for [ F e C l ( T P P ) ] a n d b e c a u s e a n x - r a y s i n g l e - c r y s t a l s t u d y is i n p r o g r e s s ( 3 7 ) a n d e x t e n d e d x - r a y a b s o r p t i o n fine s t r u c t u r e ( E X A F S ) m e a s u r e m e n t s a r e b e i n g m a d e ( 3 8 ) , t h e final r e s o l u t i o n o f this question c a n b e a n t i c i p a t e d i n the near future. M ô s s b a u e r spec t r o s c o p y h a s p r o v e d to b e a d e c i s i v e p r o b e for t h e o x i d a t i o n s t a t e o f iron. Furthermore, although U V - V I S spectroscopy remains a useful p r o b e o f t h e o x i d a t i o n l e v e l o f t h e p o r p h y r i n , i t is n o t a l w a y s c l e a r c u t . S p e c t r a l p a t t e r n s t h a t i d e n t i f y p o r p h y r i n r a d i c a l s (19, 39) t e n d t o p o r t r a y i d e a l cases. F o r i r o n , w h e r e t h e d - o r b i t a l e n e r g i e s a r e c l o s e t o t h o s e o f t h e p o r p h y r i n H O M O or L U M O , less p r e d i c t a b l e s p e c t r a a r i s e . P o s s i b l y N M R a n d I R c r i t e r i a w i l l e v e n t u a l l y p r o v e to b e o f greater u t i l i t y than U V - V I S i n i d e n t i f y i n g r a d i c a l character i n a por phyrin. +
S o m e c o m m e n t m u s t b e m a d e o n the nature o f the lar antiferromagnetic c o u p l i n g b e t w e e n a p a r a m a g n e t i c and a p o r p h y r i n radical cation. T h e overall S = 2 [ F e ( I I I ) C l ( T P P ) ] i n d i c a t e s t h a t c o u p l i n g is v e r y l a r g e . +
intramolecu metal center character o f This finding
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
REED
15.
349
Iron(I) and Iron(IV) Porphyrins
is i n m a r k e d c o n t r a s t w i t h C o m p o u n d I o f h o r s e r a d i s h
peroxidase
1
where a very small c o u p l i n g ( - J — 2 c m " ) b e t w e e n the S = 1 iron(IV) a n d t h e p o r p h y r i n r a d i c a l is o b s e r v e d (18). S i m i l a r l y , t h e [ C u ( I I ) ( P - ) ]
+
s p e c i e s a p p a r e n t l y h a s n e g l i g i b l e c o u p l i n g , g i v i n g r i s e to t w o u n p a i r e d e l e c t r o n s (31, 40).
O n e report, h o w e v e r , f o u n d d i a m a g n e t i s m
(39). C l e a r l y , t h e r e are t o o f e w u n a m b i g u o u s d a t a t o r e v e a l a n y s y s Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 5, 2015 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0201.ch015
t e m a t i c trends. B e c a u s e a n t i f e r r o m a g n e t i c interactions are g e n e r a l l y d i f f i c u l t to r a t i o n a l i z e , i t is o f l i t t l e u s e t o s p e c u l a t e o n w h y c o u p l i n g is +
strong i n [ F e ( I I I ) C l ( T P P - ) ] b u t not i n other c o m p l e x e s . P o p u l a t i o n o f 5
a l l the d-orbitals i n h i g h s p i n
rf -iron(III)
certainly m a x i m i z e s the op
p o r t u n i t y for c o u p l i n g .
Oxidized μ-Οχο Ferric Dimers T h e two-electron electrochemical oxidation o f [ F e ( T P P ) ] 0 oc c u r s i n d i s c r e t e s t e p s s e p a r a t e d b y ~ 0 . 3 V (33). T h i s s i t u a t i o n h a s a l l o w e d study o f b o t h the one-electron o x i d i z e d " m i x e d - v a l e n c e " c o m p o u n d , [ F e ( T P P ) 0 ] a n d the two-electron o x i d i z e d p r o d u c t , [ F e ( T P P ) 0 ] . T h e F e ( O E P ) 0 complex apparently behaves i n a very similar manner. 2
+
2
2
2 +
2
2
2
2
+
T h e singly o x i d i z e d species [ F e ( T P P ) 0 ] was characterized b o t h i n s o l u t i o n a n d i n t h e s o l i d state (33, 34). T h e U V - V I S s p e c t r u m is n o t c o m p e l l i n g l y r a d i c a l c a t i o n - l i k e b u t is s u g g e s t i v e o f o n e (see F i g u r e 5). It s h o w s a r e d u c e d i n t e n s i t y S o r e t b a n d a n d a b r o a d e n e d α , β - r e g i o n b u t no b l u e - s h i f t i n g o f the Soret. T h e l o w m a g n e t i c m o m e n t (μ - 2 . 9 at 4 0 ° C ) w a s t e n t a t i v e l y i n t e r p r e t e d as a m i x t u r e o f S = \ a n d S = f states i n e q u i l i b r i u m (33) b u t is m o r e l i k e l y to a r i s e f r o m a n t i f e r 2
2
r o m a g n e t i c c o u p l i n g . T h e E S R s p e c t r u m (g = 2 . 0 5 9 , 1.993) is c o n s i s t e n t w i t h m e t a l - l o c a l i z e d u n p a i r e d s p i n (33), b u t i t d o e s n o t r u l e o u t a p o r p h y r i n r a d i c a l b e c a u s e its s p i n w o u l d a l m o s t c e r t a i n l y b e c o u p l e d to t h e i r o n as i n [ F e C l ( T P P - ) ] . T h e p r o t o n N M R a s s i g n m e n t s h a v e b e e n p u t o n a firm b a s i s (34) a n d t e m p e r a t u r e d e p e n d e n c e s t u d i e s s u p p o r t t h e p r e s e n c e o f a n t i f e r r o m a g n e t i s m . T h e b e s t e v i d e n c e for a p o r p h y r i n r a d i c a l formulation comes from the appearance o f the d i a g n o s t i c 1 2 8 0 - c m " I R b a n d (35). A r e p o r t o f a s y n t h e t i c m e t h o d o f i s o l a t i n g [ F e ( T P P ) 0 ] as a B F " o r P F " s a l t a n d t h e d e t e r m i n a t i o n o f m a g n e t i c , M ô s s b a u e r , E P R , a n d I R c h a r a c t e r i s t i c s (41) s h o u l d p r o b a b l y b e e n t i r e l y d i s c o u n t e d b e c a u s e t h e p r e s e n c e o f fluoro-bridged d i m e r s p r o v i d e s a m o r e a c c e p t a b l e r a t i o n a l e for t h e o b s e r v a t i o n s (42). C o m p a r e d to [ F e C l ( T P P ) ] , t h e p r e s e n c e o f a n o x o l i g a n d s h o u l d i m p r o v e t h e a c c e s s i b i l i t y o f i r o n ( I V ) b u t t h e w e i g h t o f e v i d e n c e is t o w a r d a r a d i c a l c a t i o n f o r m u l a t i o n for [ F e ( T P P ) 0 ] r a t h e r t h a n a m i x e d - v a l e n c e i r o n ( I I I ) - ( I V ) s p e c i e s . T h e d e t e r m i n a t i o n o f its M ô s s b a u e r s p e c t r u m a n d x-ray c r y s t a l structure s h o u l d l e a d to a confi dent assignment. +
1
+
2
2
4
6
+
+
2
2
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
Figure
5. 2
e2
2
+
600 nm
e2
2
2+
UV-VIS spectra of Fe^TPP) 0 (—), [F (TPP) 0] SbClf (•••), and [F (TPP) 0] (- • -) in dichloromethane (35). These essentially are identical to those of Ref. 33.
1
500
1
400
1
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 5, 2015 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0201.ch015
2ShClï
REED
15.
351
Iron(I) and Iron(IV) Porphyrins +
A c o m p a r i s o n m a y be d r a w n b e t w e e n [ F e ( T P P ) 0 ] a n d the i s o e l e c t r o n i c n i t r i d e F e ( T P P ) N (43). T h i s s p e c i e s , h o w e v e r , d e f i e s s t r a i g h t f o r w a r d f o r m u l a t i o n d e s p i t e e x t e n s i v e s t u d y o f its p h y s i c a l p r o p e r t i e s . M a g n e t i c c r i t e r i a s u c h as s u s c e p t i b i l i t y m e a s u r e m e n t s a n d E S R s p e c t r o s c o p y c l e a r l y s h o w t h a t t h e d i m e r is a n o v e r a l l S = i s y s t e m w i t h t h e u n p a i r e d e l e c t r o n l o c a l i z e d o n t h e F e N F e m o i e t y (43, 44). A l l e x p e r i m e n t s s h o w t h e t w o i r o n a t o m s to b e i d e n t i c a l so t h a t a n a v e r a g e d v a l e n c e f o r m a l i s m is r e q u i r e d (43). B e c a u s e t h e U V - V I S s p e c t r u m s h o w s n o p o r p h y r i n r a d i c a l cation characteristics, the metals c a n b e a s s i g n e d a n a v e r a g e d i r o n ( I I I ) - ( I V ) v a l e n c y . T h e c o m p o u n d is a s o - c a l l e d C l a s s I I m i x e d - v a l e n c e c o m p o u n d a n d c a n b e v i e w e d as h a v i n g a n i r o n ( I I I 1) v a l e n c y , r e p r e s e n t e d b y e q u a l c o n t r i b u t i o n f r o m t w o resonance forms: 2
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2
2
2
F e ( I I I ) — N — F e ( I V ) F e ( I V ) = N — F e ( I I I ) ' T h e u n a n s w e r e d q u e s t i o n is t h a t o f the s p i n state(s). T h e M ô s s b a u e r s p e c t r a w e r e i n t e r p r e t e d as i n d i c a t i v e o f h i g h s p i n (43); t h e x - r a y c r y s t a l s t r u c t u r e is m o r e i n a c c o r d w i t h a n a v e r a g e o f l o w s p i n i r o n ( I I I ) a n d h i g h , s p i n i r o n ( I V ) (44); t h e X P S s p e c t r a r e q u i r e a l o w s p i n a s s i g n m e n t (45), a n d t h e r e s o n a n c e R a m a n s u g g e s t a h i g h s p i n state (46). S o , u n l i k e a l l o f t h e m o n o m e r i c i r o n p o r p h y r i n s d i s c u s s e d p r e v i o u s l y , t h e a s s i g n m e n t o f s p i n states to i r o n i n F e ( T P P ) N a p p e a r s to b e at b e s t , p e r m a n e n t l y a m b i g u o u s a n d at w o r s t , o n e o f t h o s e n u i sance questions o f inorganic chemistry. 2
2
I n a s o m e w h a t r e l a t e d m a n n e r , the a m b i g u o u s assignment o f o x i d a t i o n a n d s p i n states i n c a r b e n e c o m p l e x e s o f the type F e ( C R ) ( p o r p h ) p o i n t s to t h e n o n i n n o c e n t b e h a v i o r o f a c a r b e n e ' l i g a n d 2
(47): Fe(IV)=CR
2
F e ( I I ) ^ : C R
2
S o m e i r o n ( I V ) c h a r a c t e r c a n b e i n v o k e d to r a t i o n a l i z e t h e s i m i l a r i t y o f the U V - V I S spectra o f v i n y l i d e n e c o m p l e x e s w i t h C o m p o u n d II o f p e r o x i d a s e s (48). I n a d d i t i o n to t h e s i n g l y o x i d i z e d μ-οχο d i m e r t h e r e is a l s o t h e d o u b l y o x i d i z e d s p e c i e s [ F e ( T P P ) 0 ] a n d its O E P a n a l o g u e . F i r s t p r o d u c e d e l e c t r o c h e m i c a l l y (32), t h e d i p e r c h l o r a t e s a l t n o w h a s b e e n i s o l a t e d (34). T h e b i s h e x a c h l o r a n t i m o n a t e s a l t c a n a l s o b e s y n t h e s i z e d u s i n g R · S b C l ~ r e a g e n t s (35). T h e v i s i b l e s p e c t r u m o f [Fe (TPP) 0] is r a t h e r l i k e a c a t i o n r a d i c a l h a v i n g a b r o a d e n e d b l u e - s h i f t e d S o r e t b a n d at 3 9 6 n m a n d a b r o a d e n e d α , β - r e g i o n (see F i g u r e 5). T h i s s p e c t r u m s u g g e s t s a n i r o n ( I I I ) r a d i c a l f o r m u l a t i o n b u t is b y n o m e a n s c o n c l u s i v e w i t h o u t M ô s s b a u e r a n d s t r u c t u r a l c o r r o b o r a 2 +
2
2
+
6
2 +
2
2
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
BIOLOGICAL REDOX COMPONENTS
352
t i o n . S t r o n g 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 v i a t h e o x o b r i d g e is s u g g e s t e d b y a n a n a l y s i s o f t h e p r o t o n N M R s p e c t r u m (34). T h e a p p e a r a n c e o f a strong 1 2 9 0 - c m
- 1
b a n d i n t h e I R s p e c t r u m o f [ F e ( T P P ) 0 ] [ S b C l e ] 2 is 2
2
d i a g n o s t i c o f p o r p h y r i n r a d i c a l f o r m u l a t i o n (35), a n d o t h e r s (34) f a v o r s u c h a n assignment over the iron(IV) formulation p r e v i o u s l y assigned
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(33).
Monomeric Iron(lV) Porphyrins T h e s o l u t i o n c h a r a c t e r i z a t i o n o f w h a t is a p p a r e n t l y t h e first a u thentic
iron(IV) porphyrin
was reported
recently
(49).
μ-Peroxo
i r o n ( I I I ) d i m e r s c a n b e c l e a v e d h o m o l y t i c a l l y w i t h l i g a n d s ( L ) s u c h as 1-methylimidazole
( l - M e l m ) , p y r i d i n e , or p i p e r i d i n e to y i e l d o x o
iron(IV) porphyrins [FeO(porph)L]: (porph)Fe-O-O-Fe(porph) + 2 l - M e l m
> 2 FeO(porph)L toluene
T h e r e a c t i o n m u s t b e c a r r i e d o u t at l o w t e m p e r a t u r e s u n d e r s c r u p u l o u s l y a p r o t i c c o n d i t i o n s , a n d t h e p r o d u c t is u n s t a b l e i n s o l u t i o n a b o v e - 3 0 ° C . T h e U V - V I S spectrum of F e O ( T m T P ) ( l - M e I m ) ( T m T P = t e t r a - m - t o l y l p o r p h i n a t o ) is t y p i c a l l y m e t a l l o p o r p h y r i n - l i k e a n d s h o w s n o c a t i o n r a d i c a l c h a r a c t e r . T h e m a g n e t i c s u s c e p t i b i l i t y is 2 . 9 B M at - 4 0 ° C , a n d t h e C u r i e l a w b e h a v i o r o f t h e p o r p h y r i n shifts i n t h e p r o ton N M R s p e c t r u m suggests a w e l l - b e h a v e d m o n o m e r i c S = 1 system. T h e s e data support an intermediate s p i n iron(IV) formulation a n d the close similarity o f the s u s c e p t i b i l i t y a n d electronic spectra to those o f C o m p o u n d I I o f p e r o x i d a s e s is v e r y g r a t i f y i n g . T h e c e n t r a l i m p o r t a n c e o f t h i s c o m p l e x to i r o n ( I V ) p o r p h y r i n c h e m i s t r y a n d its r e l e v a n c e to peroxidase c h e m i s t r y m a k e d e f i n i t i v e structural characterization a paramount objective. T h e thermal insta b i l i t y o f F e O ( T P P ) ( l - M e I m ) means this characterization w i l l not b e easy, b u t i n the absence o f a n x-ray structure, M ô s s b a u e r s p e c t r o s c o p y c a n p r o v i d e a d d i t i o n a l c h a r a c t e r i z a t i o n . S p e c t r a w e r e o b t a i n e d for F e O ( T P P ) ( l - M e I m ) (14) a n d t h e y g i v e s t r o n g s u p p o r t t o t h e i r o n ( I V ) a s s i g n m e n t . T h e i s o m e r s h i f t ( δ ) is 0 . 1 1 m m / s at 4 . 2 Κ a n d t h e q u a d r u p o l e s p l i t t i n g is 1.24 m m / s . S u c h a l o w i s o m e r shift r e f l e c t s a h i g h o x i d a t i o n state, a n d δ p r o g r e s s e s f r o m a h i g h o f 0 . 6 5 i n t h e iron(I) p o r p h y r i n to a l o w o f 0.03 i n i r o n ( I V ) h e m o p r o t e i n s ( T a b l e I). T h e t e m p e r a t u r e a n d a p p l i e d m a g n e t i c field d e p e n d e n c i e s o f F e ( 0 ) ( T P P ) ( l - M e I m ) a r e i n a c c o r d w i t h a n S = 1 state a n d c l o s e l y m i m i c the spectra o f C o m p o u n d E . S . o f c y t o c h r o m e c peroxidase ( C C P ) (14, 15). The comparison of F e ( I V ) 0 ( T P P ) ( l - M e I m ) and [Fe(III)Cl( T P P ) ] d e m o n s t r a t e s h o w t h e site o f r e d o x is d i c t a t e d b y t h e a x i a l +
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
15.
REED
353
Iron(I) and Iron(IV) Porphyrins
ligand. T h e ferryl moiety ( F e = 0 )
p r o v i d e s the o n l y axial ligation
capable of stabilizing iron(IV) i n mononuclear species. T h e nitrido m o i e t y ( F e N F e ) a l s o s e e m s to s h o w t h i s c a p a c i t y i n d i m e r s .
The
u n i q u e a b i l i t y o f o x o l i g a n d s t o s t a b i l i z e h i g h o x i d a t i o n states o f t r a n sition metals
is w e l l - k n o w n a n d o n l y
fluoride,
hydroxide, and alk-
o x i d e a x i a l l i g a t i o n r e m a i n as u n t e s t e d b u t v i a b l e a l t e r n a t i v e s i n t h e Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 5, 2015 | http://pubs.acs.org Publication Date: June 1, 1982 | doi: 10.1021/ba-1982-0201.ch015
s e a r c h for o t h e r i r o n ( I V ) p o r p h y r i n c o m p l e x e s . T h e c i r c u m s t a n t i a l e v i d e n c e for o x o l i g a t i o n i n i r o n ( I V ) h e m o p r o t e i n s is n o w q u i t e
strong,
a l t h o u g h h y d r o x o l i g a t i o n cannot b e r u l e d out.
Further Oxidized Iron Porphyrins I r o n p o r p h y r i n s t h a t are t w o - e l e c t r o n o x i d i z e d a b o v e
iron(III)
h a v e b e e n r e f e r r e d to as i r o n ( V ) p o r p h y r i n s p u r e l y as a f o r m a l i s m o f c o n v e n i e n c e . A p p a r e n t l y , h o w e v e r , the o n e - e l e c t r o n o x i d a t i o n p r o d ucts o f S = 1 i r o n ( I V ) p o r p h y r i n s are a c t u a l l y i r o n ( I V ) r a d i c a l cations. S y n t h e t i c a l l y , t h e y w e r e a p p r o a c h e d b y reaction o f i o d o s o b e n z e n e or p e r a c i d reagents w i t h iron(III) porphyrins. A green intermediate o f possible composition F e O ( O E P ) C l was reported
i n the interaction o f F e ( O E P ) C l w i t h i o d o s y l x y l e n e (50).
S o m e U V - V I S s p e c t r a l s i m i l a r i t y w i t h C o m p o u n d I o f c a t a l a s e ( C a t I) l e n d s s u p p o r t to the a s s i g n m e n t o f a n i r o n ( I V ) r a d i c a l c a t i o n . S u c h s p e c t r a l s i m i l a r i t y is n o t , h o w e v e r , m u c h c l o s e r t o C a t I t h a n o x i d i z e d c a r b e n e c o m p l e x is to H R P I (48).
an
C l e a r l y , structural assign
ments based upon modest spectral similarity should be made
with
c o n s i d e r a b l e c a u t i o n . A m a g n e t i c m o m e n t o f 4.8 B M w a s r e p o r t e d a n d a h i g h s p i n i r o n ( I V ) a s s i g n m e n t s u g g e s t e d . T h i s r e p o r t is i n c o n s i s t e n t w i t h expectations based on H R P I a n d leaves some doubt about
the
i d e n t i t y o f t h i s c o m p l e x . T h e h i g h - s p i n state o f i r o n ( I V ) , i f i t e x i s t s , r e m a i n s to b e e s t a b l i s h e d i n a d e f i n i t i v e l y c h a r a c t e r i z e d p o r p h y r i n complex. A more definitive characterization o f the p r o d u c t o f iron(III) por p h y r i n s w i t h " o x y g e n a t o m " r e a g e n t s w a s p u b l i s h e d (17).
Treatment
of
m-chloro-
FeCl(TMP)
( T M P = tetramesitylporphyrinate)
with
peroxybenzoic a c i d gives a green intermediate, w h i c h can be studied s p e c t r o s c o p i c a l l y at l o w e r e d t e m p e r a t u r e s . Its M ô s s b a u e r (see
spectrum
T a b l e I) is s i m i l a r to t h e S = 1 i r o n ( I V ) s p e c i e s F e ( 0 ) ( T P P ) ( l -
M e l m ) . Its m a g n e t i c m o m e n t o f 4 . 2 B M is s l i g h t l y l a r g e r t h a n e x p e c t e d for a n S = f s y s t e m b u t is c o n s i s t e n t w i t h a n e s s e n t i a l l y n o n c o u p l e d S = 1 iron(IV) a n d a p o r p h y r i n radical cation. T h e proton N M R spec t r u m s h o w s p y r r o l e shifts ( - 2 7 p p m ) t y p i c a l o f a r a d i c a l c a t i o n , w h i c h , together w i t h the distinctly radical cation-like U V - V I S spectrum leads to t h e a s s i g n m e n t o f a n i r o n ( I V ) r a d i c a l c a t i o n f o r m u l a t i o n : F e ( I I I ) C l ( T M P ) + R C O O O H -> F e ( I V ) 0 ( T M P )C1 +
RCOOH
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
BIOLOGICAL REDOX COMPONENTS
354
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T h e product's U V - V I S s p e c t r u m s o m e w h a t resembles that o f C o m p o u n d I o f H R P , w h i c h is a l s o g r e e n a n d is w i d e l y a c c e p t e d t o b e a n i r o n ( I V ) r a d i c a l c a t i o n (18, 51). A l t h o u g h o x o a x i a l l i g a t i o n is p r o p o s e d i n b o t h F e ( 0 ) ( T M P )C1 a n d H R P I , t h e c o n j u g a t e a c i d , n a m e l y h y d r o x i d e , cannot b e r u l e d out. A l s o , the presence or absence o f other a x i a l l i g a t i o n s u c h as c h l o r i d e o r s o l v e n t ( m e t h a n o l ) is u n k n o w n . C h e m i c a l l y r e l a t e d to this green c o m p l e x is a r e d c o m p l e x that c a n be generated either b y treatment o f the green c o m p l e x w i t h base ( O H ~ ) o r b y t r e a t m e n t o f F e C l ( T M P ) w i t h i o d o s o b e n z e n e (17). Its t r u e n a t u r e is q u i t e p u z z l i n g . Its M ô s s b a u e r s p e c t r u m ( δ = - 0 . 0 3 p p m , Δ Ε = 2 . 1 3 m m / s ) s u g g e s t s o f e i t h e r i r o n ( I V ) o r i r o n ( V ) b u t its m a g n e t i c m o m e n t (2.9 B M ) is i n d i c a t i v e o f a C u r i e l a w b e h a v e d o v e r a l l S = 1 s y s t e m . N o strong E P R signals are noted, b u t the p r o t o n N M R has the β - p y r r o l e r e s o n a n c e i n t h e d o w n f i e l d p o s i t i o n (δ - 3 3 . 5 p p m ) u s u a l l y associated w i t h a radical cation. W o r k o n t h e s e h i g h l y o x i d i z e d i r o n p o r p h y r i n c o m p l e x e s is v e r y d e m a n d i n g experimentally. T h e m u c h needed information o n their s t r u c t u r e s a n d t h e r e s o l u t i o n o f u n c e r t a i n t i e s i n t h e i r p r o p e r t i e s is greatly h a m p e r e d b y their h i g h reactivity toward hydrocarbons. T h e i n t e g r i t y o f t h e f e r r y l l i g a t i o n r e m a i n s t o b e e s t a b l i s h e d , at l e a s t b y I R s p e c t r o s c o p y , a n d c l o s e r m o d e l s for H R P I a n d C a t I n e e d t o b e sought. F i n a l l y , F e ( I V ) ( 0 ) ( P ) X s p e c i e s d o n o t s h o w m u c h e v i d e n c e for s p i n - c o u p l i n g b e t w e e n t h e S = 1 m e t a l a n d S = i r i n g . T h i s finding is i n c o n t r a s t to [ F e ( I I I ) C l ( T P P ) ] w h e r e c o u p l i n g is c o m p l e t e . F o r H R P I the c o u p l i n g constant has b e e n estimated from M ô s s b a u e r a n d E P R s p e c t r o s c o p y t o b e - J -2 c m " (18) w h e r e a s i n [ F e C l ( T P P - ) ] | - J | m u s t b e » 2 0 0 c m . A l m o s t n o t h i n g is u n d e r s t o o d a b o u t t h e mechanism(s) a n d magnitude o f antiferromagnetic c o u p l i n g i n metalloporphyrin radical species. I n s u m m a r y , authentic iron(IV) porphyrins are accessible w h e n a n o x o l i g a n d is p r e s e n t . C o m p l e x e s o f t h e t y p e F e ( I V ) 0 ( T P P ) L a r e g o o d m o d e l s for C o m p o u n d I I o f H R P a n d c o m p o u n d s o f t h e t y p e F e ( I V ) 0 ( T M P - )C1 a r e s a t i s f a c t o r y m o d e l s for C o m p o u n d I . T h e i r r e a c t i v i t y is c u r r e n t l y r e c e i v i n g c o n s i d e r a b l e a t t e n t i o n ( 2 7 , 5 2 ) a n d is l e a d i n g to a l o n g a w a i t e d insight into the c h e m i c a l m e c h a n i s m s b y w h i c h the peroxidases a n d the c y t o c h r o m e s P - 4 5 0 function. +
1
+
- 1
Acknowledgments M u c h o f the present u n d e r s t a n d i n g o f the iron cussed i n this r e v i e w has arisen from c o l l a b o r a t i v e R o b e r t Scheidt ( U n i v e r s i t y o f N o t r e D a m e ) , G e o r g e v a n i a State U n i v e r s i t y ) , a n d J e a n - C l a u d e M a r c h o n
porphyrins dis studies w i t h W . L a n g (Pennsyl (Centre Etudes
In Electrochemical and Spectrochemical Studies of Biological Redox Components; Kadish, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1982.
15.
REED
Iron(I) and Iron(IV) Porphyrins
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N u c l é a i r e s de Grenoble). I particularly thank them, their students, and my students who are cited in the references. I also thank John T. Groves and Harold Goff for discussion of results prior to publication. I sincerely hope that the authors whose work I reinterpret will grant me the liberty of critical scientific enquiry and the license of alternative interpretation.
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