Oxidative electrochemistry of electropolymerized metalloprotoporphyrin films

J Electroanal Chem, 163 (1984) 223-236

223

Elsevier Sequota S A , Lausanne - Pnnted in The Netherlands

OXIDATIVE E L E C T R O C H E M I S T R Y OF E L E C T R O P O L Y M E R I Z E D M E T A L L O P R O T O P O R P H Y R I N FILMS

K A M A C O R and T.G SPIRO

Department of Chemistry, Prmceton Unwerstty, Princeton, NJ 08544 (U S A ) (Received 2nd February 1983; in revised form 1st September 1983)

ABSTRACT The course of electrochemical oxidation has been monitored by cychc voltammetry, ring-disk voltammetry, and absorption spectroscopy for metalloporphyrln electrode films prepared by electropolymerlzatlon of Zn u and F e m protoporphynn IX (PP) complexes. In C H 2C12 both films show stable cyclic voltammograms (CV's) over the first oxidation wave, assigned to porphyrln radical cation formation. Addition of 2% H 2 0 to the electrolyte Is without effect on the first oxadatlon of ZnUpp, but contact with aqueous buffer (pH 6.9) enhances the anodlc current six-fold, decreases the cathodic current and abohshes subsequent electroactwlty; the inactivated film contains modified porphyrln species, as shown by its absorpuon spectrum on SnO2. It is suggested that oxidation proceeds xaa nucleophlhc attack of H 2 0 on the Z n n PP+ radical caUons, leading eventually to the sLx-electron product dloxoporphomethene. The same product is indicated for the FeIIIpp film wluch also shows a six-fold anodlc current enhancement m contact with aqueous buffer, and subsequent mact~vatmn But m tlns case a large negative shift in the anodlc peak potentml suggests (Fe I v = 0)PP formation, and the lnactwatlon pathway is suggested to revolve O atom transfer to the porphyrln nngs. For the F e I n p p f, lm 2% H 2 0 already produces a slgmficant negatwe shift and broademng of the anodtc peak, and a decrease in the cathodic current. Rang-disk voltammetry demonstrates some 02 productmn accompanying oradatlon of the FeIIIpp film, but not the ZnIIpp film, m contact with aqueous buffer, the yield accounts for only 2% of the oxtdatlon current Addition of (CIPh)3P, intended to intercept the O atoms of the (FeIV=o)PP umts, results only in accelerated film inactwated, probably via a radical process.

INTRODUCTION

There is much current interest in attaching electroactive molecules to electrodes in order to catalyze useful redox reactions [1-9]. Metalloporphyrins (Fig. 1) are attractive in this context [1-4] because in addition to being effective electron transfer agents, thanks to the extended porphyrln or system, they have axial hgatlon sites available, with which the redox chemistry of coordinated ligands can potentially be controlled. The stabilization of reactive entities, such as H and O atoms, by ligation to metalloporphyrins may provide mechanisms of effective catalysis of energy storing reactions, especially the electrolysis or photoelectrolysis of H20. Being effective absorbers of visible light, metalloporphyrins can act as photosensitizers or photocatalysts [10-12]. 0022-0728/84/$03.00

© 1984 Elsevier Sequoia S.A.

224

~ Hz

CH~CH

~ ~ 1 Ha COOCCH~,CH2/

2

l~-'~ CH CH2 \CH3

Fig 1 Structure of metalloprotoporphynn d]methyl ester

We have shown that stable electroactive metalloporphyrin films of considerable thickness ( - 1000 monolayer equivalents) can be formed by the simple method of cycling an electrode suspended in a nonaqueous (methylene chloride or butyronitrile) solution of a metalloprotoporphyrin through its oxidation waves [13]. These films have been characterized by cyclic voltammetry, absorption spectroscopy on transparent SnO 2 electrodes, and resonance Raman spectroscopy, the latter indicating that one of the two vinyl groups is saturated on most of porphyrin units. Deposition continues for several minutes after the current is interrupted, and no film is formed if the metal, rather than the ring, is oxidized. This evidence is consistent with a mechanism involving electroinitiated cationic polymerization of the protoporphyrin (PP) peripheral vinyl groups (Fig. 1). Cyclic voltammetry and absorption spectroscopy shows that the film formed from (FemPP)20 oxidation contains monomer rather than dlmer units; the other films contain the intact parent porphyrin. The porphyrin sites are accessible to small ions as shown by chloride coordination of ZnPP film upon soaking in chloride solution. In the present study the oxidative chemistry of zinc and iron PP films is characterized. For the Zn H film, the porphyrin radical cation is attacked by water, resulting in rapid oxidative deactivation of the film. For the Fe In film there is evidence for FeIV=o formation in the presence of water, and oxidative film deactivation at lower water activity. Ring-disk voltammetry shows very little release of oxidizable or reducible products from either film, but for the Fem film a small amount ( - 2% of the film oxidative current) of O 2 is detected in coincidence with the film oxidation wave associated wiih FeW=O formation. Addition of tns(p-chlorophenyl)phosphine, intended to abstract O atoms from the film, led to even faster film deactivation, probably via a radical mechanism. Thus although the iron film appears capable of producing stabilized O atoms via water oxidation, the susceptibility of the porphyrin to oxidative attack is a serious limitation on the film utility.

225 EXPERIMENTAL

Metalloporphyrm films Zinc and iron protoporphyrin &methyl ester complexes were prepared by the method of Adler [14]. /~-Oxoiron protoporphyrin &methyl ester was formed by shaking a CH2C12 solution of (FePP)C1 with 1 M K O H several times until the characteristic charge transfer band of the monomer at 643 nm disappeared. Films were formed on Pt, glassy carbon (Normar Ind., Anaheim, CA) and SnO 2 (PPG, Pittsburgh, PA) electrodes by electrochemical cycling through the metalloprotoporphyrin ring oxidation waves in 0.5 m M ZnPP in 0.1 M tetrabutylammonium perchlorate (TBAP)/CH2C12 or 0.5 m M (FePP)20 in 0.1 M tetrabutylammonium hexafluorophosphate (TBAH)/CH2C12 [13]. The electrodes were removed and rinsed with CH2C12- Glassy carbon disks for rotating ring-disk experiments were mounted in a cylindrical Pt holder for film formauon.

Equipment Cychc voltammetry was carried out with a PAR model 173 Potentiostat, Model 175 Programmer, and Model 179 Coulometer. Voltammograms were recorded on an X - Y recorder. A saturated calomel electrode was used as the reference electrode. Rotating ring-disk experiments were conducted with a Pine Inst. Co. (Grove City, PA) double potentiostat, rotator, and electrode. The ring-disk electrode had a 0.250 cm diameter hole in which glassy carbon disks were easily mounted. Ohmic contact to the tuner shaft was made with a small spring. The collection efficiency of the glassy carbon ring was measured to be 0.37. Absorption spectra of SnO 2 electrodes were recorded on a Cary 118 spectrophotometer. RESULTS

Zn porphyrin film Figure 2 shows cyclic voltammograms for a ZnItpp film plated onto a Pt electrode to a thickness of - 2 0 0 equivalent monolayers (assumed to contain 2 × 10 -8 m o l / c m *), in contact with 0.1 M tetrabutylammonium perchlorate (TBAP) in CH2C12, 0.1 M TBAP in CH3CN contaimng 2% H20, and in 0.2 M aqueous N a H 2 P O 4 / K O H buffer at p H 6.9. In CHzC12 the film shows a well-behaved oxidative wave at 0.75 V vs. SCE, which can be cycled repeatedly without deterioration, associated with the formation of radical cation. There is a second wave at 1.08 V vs. SCE due to porphyrin dication formation and cycling over this wave also

* A s s u r m n g t h e m e t a l l o p o r p h y r i n u m t s h e flat a n d o c c u p y 1 7 n m 2 o f s u r f a c e a r e a

226

o

C

o c o

~g o$

/cm 2

I

1.20

6

I

I, 0 0.80

I

0.60

I 7 5 pA/cm 2

I

1,20

I

I

I

I00

0.80 0.60

E/V

vs SCE

ll I

ISOpA/crn

I

I

I

1.20 1,00 0.80 0.60

Fig 2. Cychc voltammograms of Z n n P P fdm on a Pt electrode in (a) 0 1 M TBAP/CH2C12, (b) 0.1 M T B A P / 2 % H 2 0 - C H 3 C N , and (c) 0 2 M K O H / N a H 2 P O 4 buffer (pH 6.9) at 100 mV/s Cross marks mdlcate zero current.

indicates a reversible process. Addition of a small amount of water caused no change in the cychc voltammetric behavior of the ZnUpp fdm at the first wave. However, cycling over the second wave resulted in a large anodic current and a smaller, negatively shifted cathodic peak due to reduction of a modified p o r p h y n n species. In the presence of aqueous buffer the first anodic wave is about six times larger than it is in CH:C12 (the background current of the bare Pt electrode was neglegible, < 5%); the cathodic wave is diminished, and electroactivity is lost after a single scan. Nevertheless the electrode retains a visible colored film. Similar results were obtained with glassy carbon and SnO 2 electrodes. Figure 3 shows ring-disk voltammograms, with ZnUpp film plated onto the disk. As the disk potential is scanned to positive potentials, the disk current grows at the radical cation oxidation potential, then peaks out and declines, reflecting rapxd inactivation of the film, which however remains visible on the disk. With the ring set at - 0 . 9 0 V vs. SCE, a potential at which 02 and H202 are reduced, no cathodic current ~s detected throughout the scan. With the ring set at the highly oxidizing potential of 1.25 V vs. SCE, superimposed on the rising background current is a small amount of anodic ring current which first appears at the potential of the disk

227 3pA

i,~~q_

I

13~A

I

130~A

1

C

--Jr B

iel"

I A

I

1.00

I

I

(I.25 V)

r,ng (-0.90 V)

disk

I

0.60

Disk Potentiol

ring

0.20

E/V vs SCE

Fig 3 Rotating ring-disk current-potentlal curves for ZnIIpp film on glassy carbon disk in 0 2 M KOH/NaH2PO 4 (pH 6 9) buffer. Rotation rate = 1000 rpm, disk potential scanned at 1.0 V/Bun (A) disk current, (B) glassy carbon ring current at -0.90 V vs. SCE, (C) glassy carbon ring current at 1 25 V vs SCE Cross marks indicate zero current

c u r r e n t peak. This is n o t due to H202, since at - 0 . 9 0 V the ring current is flat at these disk potentials, b u t to some o t h e r o x i d i z a b l e species; we conjecture that it is d u e to C O [15], p r o d u c e d b y oxadative d e g r a d a t i o n of the Z n I I p p film. P o r p h y r i n radical cat]ons are well k n o w n to be subject to nucleophilic a t t a c k [16-18]. F o r n a t u r a l l y occurring p o r p h y r i n s , with H a t o m s at the m e t h i n e bridges, the m e t h i n e b r i d g e is the site of nucleophilic attack, with release of H+:

N. + H - - C + <

----,'--

/C<

+

H÷

N followed b y electron transfer to a n o t h e r radical or to the electrode, leaving a

meso-substituted p o r p h y r i n :

-e-

<

228 When the nucleophile is H 2 0 the product is a meso-hydroxyporphynn:

HO--C< which has a lower oxidation potential than the original porphyrin e.g. El~ 2 = 0.63 and 0.24 V for ZnIIOEP and ZnII(HO-OEP) [19,20] (OEP = octaethylporphyrin). Thus the process is readily repeated at another methine bridge, and the resulting dihydroxyporphyrin can readily be oxadlzed, in another two-electron step to a quinoidal dioxoporphomethene (Fig. 4). This dioxo species, which represents a six-electron oxidation product of the porphyrin, is fairly resistant to further oxidation (El/2 - 0.9 for Zn 1I (a,y-dioxo OEP) [20]). It is therefore possible that oxidation of the ZnIIpp film in contact with water leads to rapid formation of the dioxoporphomethene, via the scheme shown in Fig. 4, accounting for the roughly six-fold enhancement of the anodic current on the initial cycle, and the subsequent loss of electroactivity. That the film still contains porphyrin-like material is shown in Fig. 5, which compares the absorption spectrum

(a)

(b)

l-e-

Zn(~,'r - dloxoporphomethene) -2e_2H +

I (o)-(c)

OH-

H

-e-, -H+

(c)

Fig 4. Schemefor oxldaUonof ZnnPP film in the~presenceof H20.

229

UJ Z

O3 ~K

o O0 GO

I

I

400

i

500

I

I

600

i

[

700

WAVELENGTH/nm Fig. 5 Absorption spectrum of ZnI1pp film on transparent SnO2 before oxadatlon (solid curve) and after oxldauon and electromactwatlon m H20 (dashed curve)

of a ZnlIpp film on a transparent SnO 2 electrode, before and after anodic scanning in H20. The absorption spectrum is very similar to the unoxidized z n n p p film. The absorption spectrum of Zn H (a,'t-dioxo OEP) is also sinular to that of ZnIIOEP, [21], except that the Soret band loses intensity and is red-shifted. There is a small red-shift of the z n n p p film Soret band after oxidation and inactivation in water; however, since the z n n p p film Soret band is already appreciably red-shifted and broadened relative to solution ZnMP [13] (MP = mesoporphyrln dlmethyl ester) it is difficult to quantitatively predict Soret band positions for a modified Z n ' P P film. Some porphyrin ring reaction is indicated by the detection of a small amount of diffusible oxidizable product in the ring-disk voltammogram. The suggestion that it may be CO arises from the fact that CO is a product of heme degradation, both in vivo, [22], and upon treatment of ferroheme with peroxide, or ferrlheme with ascorbic acid and 02 in pyridine [21,22].

Iron porphyrm film Figure 6 shows voltammograms for a film ( - 200 equivalent monolayers) formed by oxidative cycling of a Pt electrode in a CHEC12 solution of (Fenlpp)20. It has

230

4"

il

c

I

I

0.90

I

0.70

0

I 5 pA/cm 2

50 pA/cm~ '

I

0.50

I

0.30

i 090

I

i

i

0.70 0.50 E/V

vs.

0.30

I 0.90

I

i

0.70 0.50

i 0.30

SCE

Fig. 6 Cychc voltammograms of FeIIIpp film in (a) 0.1 M TBAP/CH2CI 2 at 20 mV/s, (b) 0 1 M TBAP/2% HzO-CHaCN at 30 mV/s and (c) 0.2 M KOH/NaH2PO4 buffer (pH = 6 9) at 100 mV/s Cross marks md~catezero current been shown that the #-oxo dlmer breaks up in the process of forming the film, which contains only monomeric high-spin [FemPP] + units [13]. As with ZnIIpp, the F e m P P electrode gives a well-behaved , first oxidation wave at 1.08 V vs. SCE, which can be cycled repeatedly and a second anodlc wave at 1.40 V, the scanning of which destroys electroactivity unlike the ZnnPP film. While there has been controversy over the site of oxidation, metal vs. ring, of F e Ill porphyrins [23-31] recent evidence implicates predominant ring oxadation, [25-31], and this is consistent with the rapid electrooxidative film formation observed for (FemPP)20 [13]. The first oxidation wave of the film in contact with CH2C12 is therefore attributed to formation of radical cation and the second wave to the dlcation, as in the case of ZnlIpp. Upon oxidation in H20, however, the first oxidation wave of the FeUIpp film is shifted substantially to lower potentials, in contrast to the behavior of the ZnIIpp film, for which the potential shift is small. The large shift in FeIHpp oxidauon

231 potential is evidence for the formation of a new species, of lower free energy than the radical cation, which is associated with the interaction with H20. We infer that this new species is a ferryl porphyrin, (Fe I v = O)PP, obtained by oxidation of F e m P P with axaally bound H 2 0 (or O H - ) . There is chemical precedent for the formation of ferryl porphyrins, e.g. as an intermediate in the oxidation of Fe ~I porphyrins by 02 [32], and much evidence implicates (Fe w = O)PP as an intermediate (compound II) of the reaction of peroxidases and catalase with peroxide [33]. It is plausible that the bound O 2- would greatly stabilize Fe TM, allowing it to compete with porphyrin nng oxidation when an electron is removed from FeUIpp. We have observed [13] that (Cr TM= O)PP, a stable molecule, does not support film formation in its oxadation wave, which we attribute to [(Crv = O)PP] + (analogous to the known [(Crv = O)TPP] + [34,35] (TPP = tetraphenylporphine)), although a film is formed at the same potential on oxidizing (CrmPP)2 O. When the contacting solution is 2% H 2 0 in CH3CN, the voltammogram shows a broadened anodic peak (which is not observed for the z n n p p film), suggesting overlapping waves due to both FeItIPP + and (Fe I v = O)PP + formation, plausibly due to incomplete coordination by H 2 0 of the various F e m P P units in the film; the cathodic current is substantially diminished. In contact with aqueous buffer the anodic peak is at 0.96 V, 0.2 V lower than in CH2C12, and the amplitude is - 6 times higher; the cathodic peak is abolished. Electroactivity is lost, although the film

Ld (_~ Z O0 n~ 0 O9

400

5 0 O0 WAVELENGTH/nm

700

Fig. 7 Absorption spectrum of FemPP fdm on transparent SnO2 before oxldauon (sohd curve) and after oxadatlon and electrolnactlvatlon(dashed curve)

232 remains visible, after a single scan, indicating, as in the case of ZnI1pp, that the porphyrin units in the film are rapidly oxidized to species which are electro-inert. The absorption spectrum of the inactivated film (Fig. 7) is again suggestwe of a modified porphyrin. The six-fold anodic current increase in the presence of aqueous buffer is suggestive of dioxoporphomethene as the oxidation product, as in the case of ZnIIpp. However, since the current is passed at a potential below that required for porphyrin radical formation, a different mechanism is implicated. A n attractive candidate is oxygen a t o m transfer from the ferryl group to the methine bridge of the same or an adjacent porphyrin, with insertion in the methine C - H b o n d to form a meso-hydroxyporphyrin directly. After two such insertions in a given porphyrin, oxidation to the d i o x o p o r p h o m e t h e n e would be facile. F e - p o r p h y r i n catalysis of oxygen atom insertion into C - H bonds is a central feature of cytochrome P450 chemistry [36], and is p r o b a b l y involved in the biodegradation of heme by heme oxygenase [22,37].

I 3pA {- C ring (125 V)

I 3FA + B ring (-0.90 V)

I rCO

(..)

I

f A disk

1 6o/JA

I

1.40

I

I

1.00 Disk Potential

I

!

i

0.60

i

020

E / V vs. SCE

Fig 8. Rotatmg ring-disk current-potential curves for FemPP film on glassy carbon dask in 0 2 M KOH/NaH2PO4 (pH 6.9) buffer Rotation rate =1000 rpm, disk potential scanned at 1 0 V/finn. (A) disk current, (b) glassy carbon ring current at -" 0 90 V vs SCE, (C) glassy carbon ring current at 1.25 V vs. SCE. Cross marks indicate zero current.

233 Ring-disk voltammetry in aqueous buffer (Fig. 8) again shows a peaked anodic current for Fen~PP film near the first oxidative peak of the cyclic voltammogram, and a small anodic ring current when the ring lS held at 1.25 V, again suggested to be due to CO produced via porphyrin degradation. In add]tion, however, there is a clear cathodic ring peak, coincident with the disk peak, when the ring is held at - 0 . 9 0 V, which is diagnostic for 02, the only plausible diffusible species which can be reduced at this potential. No ring current from 02 was detected with a bare glassy carbon disk at 0.96 V, the potential of the Fe film oxidation. Although H202 is also reduced at - 0 . 9 0 V, anodic ring current is not observed at the same disk potential when the ring is held at 1.25 V, and since H202 would by oxidized at 1.25 V it is excluded as the product. Consequently there is clear evidence that some 02 is produced when the film is oxidized in contact with H20. We presume this to happen by the coupling of ferryl groups that happen to be in proxirmty: (FelV=o)PP + (FelV=o)PP --~ 02 + 2 FelIpp This is the reverse of the reaction occurnng when 02 interacts with Fe ~j porphyrins [32], and would be driven to the right by reoxidation of the F e n P P units at the applied potential. However the amount of 02 detected at the ring accounts for only 2% of the disk current. It is evident that O atom transfer to the porphyrin rings in the film is much more probable than is coupling to another O atom. In an effort to see whether the ferryl O atoms would transfer to an acceptor, -

b

f

o

£ E O

/,

N

o

o

,L ] i

1 "~'~~o..""

/

/

J

i;, ,

I

~

I

I

I

I

f

I

0.z0

I. 0

0.80

0.60

1.20

I.oo

0.8o

0.60

E/V

vs.

SCE

Fig. 9 Cyclicvoltammogramsof Felnpp film on Pt electrode in (a) 0 1 M TBAP/CH2CI 2 (dashed curve) and 0.002 M (PhC1)aP In 0.1 M TBAP/CH2C12 (solid curve) and (b) 0.1 M TBAP/2% H20-CH3CN (dashed curve) and 0 002 M (PhC1)3P in 0.1 M TBAP/2% H20 , 4% CH2CI2-CH3CN (solid curve) at 30 mV/s. Cross marks indicate zero current

234

tris(p-chlorophenyl)phosphine [(C1Ph)3P], which is not rapidly oxidized at the bare electrode in the relevant potential range, was added to the electrolyte. In dry CH2C12, (C1Ph)3P was found to dimimsh the anodic current in the oxidative cyclic voltammogram of F e m P P film (Fig. 9) and abohsh the cathodic current. After the first scan all electroactivity was abolished. Thus (C1Ph)3P quickly Inactivates the oxidized film, presumably by acting as a nucleophlle, attacking the porphyrin radical to form an electro-inert product. When (C1Ph)3P was added to CH3CN containing 2% H 2 0 , the anodic peak shifted to lower potentials, the anodic current was inhibited further at higher potentials, and the cathodic peak and subsequent electroactivity was abolished. Thus (C1Ph)3 P accelerates film inactivation in the presence of water, thereby obscuring the possibihty that O atom transfer might be taking place. DISCUSSION

There has recently been intensive study of O atom transfer reactions from oxo-metalloporphyrins to substrates [34,38-43], inspired by the example of cytochrome P450, a ubiquitous class of monooxygenase enzymes, which hydroxylate a variety of organic molecules via an O atom transfer mechanism [37]. Groves and coworkers [38,42] have shown that O atoms can be transferred to olefins, or other acceptors, from donors such as iodosylbenzene, in the presence of (FemTPP)C1 as catalyst. The intermediacy of an oxo-Fe-porphyrin adduct, presumably [(Fe v--O)TPP] ÷ or [(Fe w = O)TPP] ÷, was demonstrated. The species (Fe i v = O)TPP has been shown to transfer its O atom to Ph3P [40]. We therefore expected that the electrode-bound (Fe TM = O)PP units, which are indicated by the voltammograms to be formed when the FeIIIpp film is oxidized in the presence of water, might transfer some of their O atoms to (C1Ph)3P in solution, in competition with O atom transfer to the porphyrin rings which takes place in the absence of (C1Ph)3P. Instead addition of (C1Ph)3P was observed to accelerate film inactivation. We infer that the difference between this behavior and the cited O atom transfer from (Fe TM = O)TPP to Ph3P [39] is that the latter reaction was carried out in the absence of water. The availability of the protons from H 2 0 can alter the reaction mechanism, viz.: Fe TM = 0 + P(P = phosphine) -o Fe n + OP

(1)

VS.."

Fe TM = O + P + H ÷ ~ F e m O H + P.+

(2)

Electron transfer with radical formation becomes more favorable when a proton is available to stabilize the resulting F e l n - O 2-. It is relevant that the mechanism of cytochrome P450 hydoxylation has been demonstrated in some instances [36] to involve H atom abstraction from the substrate, followed by attack of the resulting carbon radical on the bound hydroxide: "FeV=O" + H - C - ~

FelV-OH + • C - ~ Fe fIx + H O C -

In the present case, the proposed phosphine radical is free to attack the porphyrin rings, producing electro-inactive products.

235 W e h a d h o p e d t h a t t h e p r o x i m i t y o f the F e I I I p p u m t s in the f i l m m i g h t l e a d to e f f i c i e n t 0 2 f o r m a t i o n u p o n o x i d a t i o n in the p r e s e n c e of water, b y the c o u p l i n g of a d j a c e n t P F e TM= O g r o u p s . T h e e f f i c a c y o f b i n u c l e a r sites for 0 2 r e d u c t i o n to H 2 0 has b e e n d e m o n s t r a t e d b y C o l l m a n a n d c o w o r k e r s [3], u s i n g a c o f a c i a l l y l i n k e d C o II p o r p h y r i n , a n d t h e r e v e r s e r e a c t i o n m i g h t h a v e b e e n a t t a i n e d in t h e F e m P P f i l m via a statistical d i s t r i b u t i o n of a p p r o p r i a t e sites. W h i l e s o m e 0 2 p r o d u c t i o n was d e m o n s t r a t e d b y r i n g - d i s k v o l t a m m e t r y , this r e a c t i o n is s w a m p e d b y r a p i d p o r p h y r i n o x i d a t i o n l e a d i n g to i n a c t i v e film, p r e s u m a b l y via O a t o m t r a n s f e r to the s a m e o r a d j a c e n t p o r p h y r i n rings. C l e a r l y this r e a c t i o n wxll h a v e to b e s u p p r e s s e d if m e t a l l o p o r p h y r i n s are to b e u s e d as o x i d a t i o n catalysts. ACKNOWLEDGEMENT T h i s w o r k was s u p p o r t e d D e p a r t m e n t of E n e r g y .

by Grant

DOE-ACO2-81ER10861

f r o m t h e U.S.

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