On the tuning of metalloporphyrin redox potentials

Bioelectrochemistry and Bioenergetics, 8 (1981) 213--222 A section of J. Electroanal. Chem., and constituting Vol. 128 (1981)

213

Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

417 -- ON THE TUNING OF METALLOPORPHYRIN

REDOX POTENTIALS

*

K.M. KADISH, L.A. BOTTOMLEY, S. KELLY, D. SCHAEPER and L.R. SHIUE

The Department of Chemistry, University of Houston, Houston, TX 77004 (U.S.A.) (Manuscript received October 13th 1980)

SUMMARY Comparisons are made between Co, Cr, Fe and Zn metalloporphyrin redox potentials. For TPPCoC104, TPPFeC104, and TPPMnC104, the redox potentials for the M(III) ~ M(II) reduction shift in a negative direction as the coordinating ability of the solvent increases. Comparisons are made between potentials for a series of TPPFeX and OEPFeX complexes. Changing the counter ion from C10~ to Cl-, Br-, N~, or F - reverses the direction of the potential shift with coordinating ability of the solvent. The half-wave potentials for each metal-centered and ring-centered metalloporphyrin reaction also shift upon axial binding of nitrogenous bases. Eleven substituted pyridines were utilized as axial ligands. For Cr, Fe and Zn, U 1/2 shifts in a negative direction as the pK a increases, while Mn shows U1/2 which shifts positively with the pK a of an axially complexed ligand. Redox tuning of metalloporphyrin reactivity is discussed as a function of axial complexation by solvent, counter-ion and/or nitrogenous base molecules.

INTRODUCTION

The electron transfer properties of synthetic porphyrin complexes have been extensively reported in the literature [1--4]. The thermodynamic half-wave potential, the electrochemical reversibility, as well as the overall electrode mechanism, are dependent on a number of factors: the type of electrode reaction (i.e. ring- or metal-centered), the solvent composition, the degree of axial ligation and the porphyrin basicity. Electrode reactions of over 30 different metalloporphyrins have been characterized, with the reactions of iron, cobalt and manganese porphyrins being the most extensively studied. Several papers have been published which suggest a relationship between potentials for metalloporphyrin oxidation and dioxygen binding ability of the 5- or 6-coordinate transition metal complex [ 5--7]. For synthetic porphyrins containing Fe, Cr or Co, oxidation potentials for the metal(II) ~ metal(III) reaction vary in the order Cr(II) > Fe(II) > Co(II) [8]. This parallels the order of 02 binding by the metal ion [5]. Mn(II) has an oxidation potential between Fe(II) and Cr(II), but yields a log Ko2 several orders of magnitude less than that predicted from the half-wave potentials [6]. In addition, half-wave potentials for the Co(III)-Co(II) and the Fe(III)--Fe(II) couple vary with the magnitude * Presented at the Fifth International Symposium on Bioelectrochemistry, 3--8 September 1979, Weimar (G.D.R.). 0302-4598/81/0000--0000/$02.50, © 1981, Elsevier Sequoia S.A.

214 o f the formation constants for nitrogenous base addition and with the coordinating ability of the solvent [9--14]. For this reason, a more complete understanding of the relationship between redox potentials, stability constants for axial ligand addition and solvent binding by synthetic metalloporphyrins is of interest. In a recent series of papers, we reported the effect of axial ligand binding on half-wave potentials and electron transfer kinetics for oxidation of Fe(II) [10-13], Co(II) [10], and Mn(II) [15,16] metalloporphyrins. This manuscript concentrates on the redox tuning of half-wave potentials for metalloporphyrin reactions by axial coordination with substituted pyridines, halide ion and/or solvent molecules. The main electrode reactions described in this paper can be written as follows: [PM(III)] ÷ -~- [PM(II)] °

(1)

where P represents octaethylporphyrin, OEP 2-, or tetraphenylporphyrin, TPP 2-, and M is Fe, Mn, Co or Cr. In addition, we have also investigated the porphyrin ring-centered reactions of TPPZn as a function of solvent and nitrogenous base complexation. For the TPPZn complexes, the electrooxidation studied was [17] [TPPZn(L)]-~-'[TPPZn(L)] ÷

(2)

EXPERIMENTAL Chemicals TPPCo, TPPZn and (TPPFe)20 were used as received from Strem Chemical Company. TPPFeC104 was prepared by acid hydrolysis of (TPPFe)20 [ 18]. TPPCrC1 and TPPMnC1 and TPPMnC104 were prepared by the m e t h o d of Adler et al. [ 19]. Technical grade dichloromethane (CH2C12) (Fisher Scientific) was distilled from P205 prior to use. Tetrabutylammonium perchlorate (TBAP) (Eastman) was used as the supporting electrolyte and was vacuum-dried prior to use. All ligands used in this study were purchased from Aldrich Chemicals and, unless noted, were used as received. 3-Picoline and 4-picoline were distilled from KOH pellets and stored in the dark. 3-Cyanopyridine and 4-cyanopyridine were recrystallized from benzene and dried in vacuo at 40°C. Methods Cyclic voltammograms were obtained with an EG&G Princeton Applied Research (PAR) Model 174 Polarographic Analyzer in conjunction with a H o u s t o n Omnigraphic 2000 X--Y recorder. A three-electrode system, consisting of Pt b u t t o n working, Pt wire c o u n t e r electrodes and a commercial saturated calomel reference electrode (s.c.e.) was utilized. Differential pulse polarographic measurements were made with the PAR Model 174 Polarographic Analyzer utilizing the X--Y recorder. A conventional three-electrode system consisting of a dropping mercury electrode (d.m.e.), platinum wire counter electrode and s.c.e, was used. The instrument parameters of the d.m.eo were a scan rate of 10 mV/s, pulse interval of 0.5 s and a pulse amplitude, AU, of 25 inV.

215 For all electrochemical measurements, the s.c.e, was separated from the bulk o f the solution by a bridge filled with solvent and supporting electrolyte. The solution in the bridge was changed periodically to avoid aqueous contamination entering the cell from the s.c.e. All solutions were 1.0 mM in p o r p h y r i n and 0.1 M in TBAP. The substituted pyridine solutions were 1.0 M ligand in CH2C12. Oxygen was removed from all solutions by passing solvent-saturated prepurified nitrogen through the solution for 10 min prior to obtaining cyclic voltammograms or differential pulse polarograms. After d e o x y g e n a t i o n , a blanket o f nitrogen was maintained over the solution. The U1/2 values were measured as the potential halfway between the oxidation and r e d u c t i o n peaks for a given couple, (Upc + Upa)/2 from cyclic voltammetric data, or as Up + (AU/2) f r om differential pulse polarographic data. For reactions th at do n o t exhibit a reversible couple, the peak potential, Up, is r ep o r ted at a scan rate of 0.1 V/s. All potentials are r e p o r t e d vs. the s.c.e. RESULTS AND DISCUSSION

Electron transfers o f TPPFeX and OEPFeX Table 1 lists half-wave potentials for each M(III) ~ M(II) electrode reaction o f TPPFeX, and is grouped by solvent. In CH2C12, a totally non-coordinating solvent, the ultimate p r o d u c t of the electron transfer is TPPFe or OEPFe, a four-coordinate, intermediate spin com plex [20]. However, five c o o r d i n a t e complexes are observed depending on the nature of the c o u n t e r ion, X- [21]. The reactant, T PPFeX or OEPFeX, is postulated to be intermediate spin for TABLE I Comparison of half-wave potentials for reduction of OEPFeX and TPPFeX Solvent a

Counter ion

OEPFe ÷

TPPFe÷

AU1/2 b

CH2C12

C10~ BrC1N~ FC10~ BrC1N~ FC104 BrC1N3 F-

0.10 --0.34 --0.42 --0.52 -0.63 --0.18 --0.22 -0.34 --0.45 -0.64 -0.04 -0.03 --0.03 -0.03 (-0.52) c -0.04 (-0.57) c

0.22 --0.21 --0.30 --0.42 -0.50 -0.05 --0.05 -0.18 ---0.25 -0.40 0.17 0.17 0.16 (--0.27) c 0.18 (-0.33) c 0.16 (--0.54) c

0.12 0.13 0.12 0.10 0.13 0.13 0.17 0.16 0.20 0.24 0.21 0.20 0.19 0.21 0.20

DMF

Py

a 0.1 M TBAP. b AUI/2 = U 1 / 2 ( T P P F e + ) _ _ U1/2(OEPFe+)" c Up c of second cathodic process.

216

X = C10~ [22] and high spin for the remainder of the complexes [23]. As seen from Table 1, the shift of potentials observed in CHIC12 is substantial. For b o t h OEPFeX and TPPFeX, a cathodic potential shift greater than 700 mV is observed on going from X = C10~ to X = F-. The Fe(III) porphyrin-counter ion binding strength increases in the order C10~ < Br- < C1- < N~ < F-, and this is reflected in the half-wave potentials. A parallel trend is observed between the OEPFeX and TPPFeX complexes. The absolute difference of potentials between TPPFeX and OEPFeX for a given X is a constant 0.12 + 0.01 V in CH2C12. On changing from the non-bonding solvent, CH2C12, to the weakly coordinating solvent, DMF, the cathodic shift in potentials due to X- decreases and the potential difference ( A U , n ) between the TPPFeX and OEPFeX increases from 0.13 to 0.24 V as a function of counter ion. It is not clear if the change in AU~/2 between CH2C12 and DMF is due to changes in axial ligation, changes in metal spin state or changes in iron--porphyrin plane distance. All three of these are possible. In pyridine, all TPPFeX potentials for the first reduction are identical and are approximately 0.20 V positive of the OEPFeX potentials. The Fe(II) product is axially complexed by two pyridine molecules. However, spectroscopic studies [34] of the oxidized complexes show that, for X = CI-, N~ or F-, some PFeX remains in equilibrium with PFe(py)~. This assignment was confirmed by the observation of two cathodic peaks on the negative potential sweep with cyclic voltammetry (see Table 1).

Solvent effect on half-wave potentials The effect of solvent on half-wave potentials for the metal-centered reactions of TPPCo [14,24] and TPPFeC1 [13] has been discussed in the literature. In CH2C12,:the Co(II) ~ Co(III) reaction is observed at 0.69 V. Changing to an aprotic solvent of higher coordinating ability results in a cathodic shift of potential for the metal reaction [14]. In many cases the coordinating ability of aprotic solvents can be correlated to half-wave potentials of organometallic complexes by use of the Gutmann donor number (DN) [25]. In the case of metalloporphyrin reactions, solvent effects may be similar to ligand effects in that shifts of potential will be either positive or negative, depending on the relative porphyrin--solvent interaction. If the solvent molecule binds only to the oxidized form of the complex, a negative potential shift with increasing donor number will be observed. In contrast, binding of the solvent to the reduced form, but n o t the oxidized form, will yield a positive shift in potential with increase in donor number. Finally, if b o t h the oxidized and the reduced forms are complexed by the solvent, the direction of potential shift will depend on the ratio of stability constants of each oxidation state. In this study, a plot of U,/2 for the reactions TPPM(III)--TPPM(II) and TPPZn(II)--[TPPZn(II)] ÷ (referenced against the ferrocene--ferrocinium ion couple) vs. DN has been constructed (Fig. 1). For TPPCo, TPPFe and TPPMn, the potentials shift in a negative direction with increasing donor number. This is consistent with a stabilization of the higher oxidation state over the lower oxidation state, and agrees with data obtained for numerous simple inorganic

217

0.4

o

0,0 -0.2

": •

Fe

-0.4 -0.6

I

M° Donor number

- 0.8

I

0

,

I

10

J

I

2O

i ~

I

,

30

Fig. 1. S o l v e n t effects o n half-wave p o t e n t i a l s for t h e m e t a l - c e n t e r e d o x i d a t i o n s o f T P P M ( I I ) w h e r e M = Co, F e a n d M n a n d t h e ring o x i d a t i o n o f T P P Z n ( I I ) . AU1/2 = U1/2 - - U1/2 ferrocene.

systems [25]. In contrast, however, the positive shift observed for the reactions of TPPZn--[TPPZn] ÷ indicate that the lower oxidation state is stabilized relative to the higher state, which for this complex, is a cation radical. The sensitivity of U1/2 to solvent binding varies in the following order for the metal-centered reactions: TPPMn < TPPFe < TPPCo. In the case of TPPCo, the slope of the plot AU1/2/ADN = --22 mV/DN. The magnitude and direction of the potential shift with solvent indicates a substantially stronger axial coordination of solvent by Co(III) than by Co(II). Stability constant measurements for formation of TPPCo(S)~ and TPPCo(S) confirm this. Values of log 32 for his ligand addition to [TPPCo(III)] ÷ in CHIC12 are 15.6 for py, 9.2 for DMSO, and 5.7 for DMF [10]. Log 31 for addition of py and DMSO to TPPCo is 2.9 and 2.7 respectively [10]. Py does not fit the Uu2--DN relationship. The substantial negative deviation in potential is greater than that which can be expected by the slight increase in donor number between DMSO and py. The deviation may be related to changes of enthalpy or entropy between DMSO and py addition to TPPCo(II). In the former solvent, the stoichiometry of [TPPCo(III)] + and TPPCo involves complexes of two and one axial solvent molecules respectively. In neat py, both Co(III) and Co(II) may be his ligated. All Co(III) and Co(II) porphyrin complexes are low spin in every solvent system investigated. Similar to TPPCo, when the reactant was TPPFeC104, the most positive potentials for Fe(III) ~_ Fe(II) were observed in non-coordinating solvents, while the most negative (with the exception of pyridine) are in higher donor solvents. In this case, however, the deviation for py is opposite in direction to that observed for TPPCo. Because py deviates in an opposite direction for Fe with respect to Co, this rules out the possibility of uncorrected liquid junction potential causing the observed deviation. The slope of Uu2 vs. DN is linear between acetonitrile and DMSO and is parallel to the TPPCo plot. However, in low donor number solvents Uu2 is independent of donor number.

218

In CH2C12 both Fe(III) and Fe(II) are postulated to exist as intermediate spin complexes [20,22]. In pyridine, both Fe(III) and Fe(II) are low spin. We do n o t know the spin state in EtC12 or CH3NO2 b u t would postulate that, in these solvents, the complexes are intermediate spin. In all solvents between donor numbers 15 and 25 the reaction probably involves a high-spin five-coordinate reactant and high-spin five-coordinate product. Previous data, taken from several laboratories, have shown that the reactions of TPPMnX are virtually independent of solvent and only moderately dependent on counter ion (when compared to potential shifts observed for TPPCoX and TPPFeX). The half-wave potential for TPPMnC1 reduction has been reported as --0.23 V in acetonitrile [28], DMSO [29], and py [30], and --0.30 V in CH2C12 [16]. TPPMnC104 has a reduction potential of --0.19 V in EtCl~ [ 30]. These values were not corrected for liquid junction potential. The plot of U,n for reduction of TPPMnC104 vs. D N (when corrected for liquid junction potential) is shown in Fig. 1. The difference in U,n between the complexes in all solvents is < 200 mV. Least-squares analysis of the data yields a negative shift of potential with increasing donor number. Both CH2C12 and EtC12 do n o t fit the relationship shown in Fig. 1. For the case of TPPMnC1, the slope of AU, n / A D N approached zero, indicating equal stabilization of Mn(III) and Mn(II) b y solvent. Finally, for comparison with the metal-centered reactions, we have elucidated solvent effects on the ring-centered oxidation of TPPZn. In the presence of strong axial ligands such as pyridine, Zn(II) exists in solution as the five-coordinate complex TPPZn(L) [31]. Electrooxidation to yield the cation radical preserves the nature of the axial coordination [17]. The extent of axial coordination by other solvent molecules, S, to the cation radical is unclear at this time. Thus, the overall mechanism for all possible electrode reactions can be shown as follows:

TPPZn <

e-- " ~ [TPPZn] ÷ ~

s

) TPPZn(S)

IIe-

S c h e m e I.

[TPPZn(S)] +

All neutral TPPZn complexes are four-coordinate in non-complexing media. Axial complexation by a solvent molecule yields the five-coordinate complex, TPPZn(S), which upon electrooxidation may yield the five-coordinate [TPPZn(S)] ÷, or the complex may dissociate to yield the four-coordinate radical [TPPZn] ÷ and S. Alternatively, the electron transfer sequence may involve reactions of TPPZn and [TPPZn(S)] ÷. Pyridine does n o t fit the U,n --DN relationship and is shifted toward a negative potential, indicating stabilization of the complexed radical cation more than any other solvent. Based on this deviation, and the positive slope of the U,n --DN plot, it is postulated that the cation radical does n o t bind any of the solvents investigated other than pyridine, and that the electrode reaction in these solvents is as follows:

219 e-

TPPZn(S) ~- [TPPZn] + + S

(3)

The inability of most solvent molecules to bind to [TPPZn] + can be explained on the basis of a competition of C10~ for the single axial position on the radical. Halides such as C10~ have been shown to complex the zinc cation radical strongly. An X-ray structure of [TPPZn]+C10~ has also been obtained

[33]. Counter ion--solvent interactions Based on the potential shifts shown in Fig. 1, it can be stated that the porphyrin--Co(III) oxidation state is preferentially stabilized over the Co(II) oxidation state by axial ligands (in this case solvent molecules) and that, in a similax manner, the porphyrin--Fe(IIDoxidation state is also stabilized over the Fe(II) oxidation state by complexation with solvent. For complexes of iron, however, this is not always the case! By switching the counter ion from C10~ to C1- we are able to reverse the direction of A U l n / D N with respect to the trend observed for TPPFeC104. The magnitude and direction of the half-wave potential shift with solvent indicate, for both TPPFeC1 and OEPFeC1, a strong solvent stabilization of Fe(II) relative to Fe(III). Measurements of formation constants for addition of solvent molecules to TPPFe and TPPFeC1 bear this out. Log ~2 for axial ligand binding to Fe(II) in CH2C12 is 7.45 [26] to 7.8 [12] for py and 0.53 for DMF [26]. In contrast, TPPFeC1 binds py only w e a k l y [10,27] and DMF n o t at all [ 21]. Similar stabilizations of Fe(II) over Fe(III) by solvent binding was also observed for TPPFeX when X was Br-, N~ or F-. This is shown in Table 1, and has been presented in detail in a separate publication [21].

Linear free-enthalpy relationships of half-wave potentials It is well known that half-wave potentials for metalloporphyrin reactions shift upon axial binding by nitrogenous bases [9--12]. In this study similar shifts of potential are presented for the metal-centered reactions of TPPFe(L)~, TPPMn(L) ÷, TPPCr(L)~ and TPPCo(L)~, as well as for the cation radical reactions involving TPPZn(L) as reactant. Ligands utilized are listed in Table 2. Figure 2 illustrates plots of U~n vs. pKa for the TPP complexes of Fe(II), Mn(II), Cr(II) and Zn(II). Oxidation of the first three species yield the corre sponding M(IH) complex, while oxidation of Zn(II) yields a Zn(II) cation radical. The largest shift of Uln is observed for the electrode reactions of Fe, while the smallest is for the reactions involving Mn. Product stoichiometries have been determined for each of the complexes presented in Fig. 2. Both Fe(III) and Fe(II), as well as Cr(III) and Cr(II), form six-coordinate, bis-pyridine complexes in CH2C12 solutions containing 1.0 M ligand. In contrast, the reactants and products of the Zn and Mn complexes are five-coordinate monoligated species [ 17 ]. The electrode reactions of TPPFe(L)~ have been extensively discussed in a previous publication [ 12]. In CHIC12 containing substituted pyridines of pK, > 5.4, an identical Fe(III)--Fe(II) reduction potential is observed independent of the starting material, in this case either TPPFeC104 or TPPFeC1. For these complexes, the higher Fe(III) oxidation state is m o r e stabilized by o

220

0.8

"0"-0,,~,...,.0....,..__. Z n ( i i )i Z n ( i i ) +

0.6 0.4 0.2

"o .o "~

0~0~

0.0

F e(ll)/Fe(IH) +

'O-oo O ~

D>

-0.2

_%-~

Mn(ll)/Mn(llll +

- -A~

~h~.A~

&-&'--~'~&

"IA'A~

-0.4 -0.6 -0.8 ,

t

,

I

#Ka~

I

,

I

2 4 6 8 Fig. 2. E f f e c t o f a x i a l l y c o m p l e x e d ligands on half-wave p o t e n t i a l s f o r m e t a l - c e n t e r e d oxidations of TPPM(II), where M = Cr, Fe, Mn, and the ring oxidation of TPPZn(II).

bonding than the lower Fe(II) state, and this results in a negative shift of potential with increasing pKa of the ligand. Similarly, for oxidation of TPPZn(L), or reduction of TPPCr(L)~, the higher oxidation state is stabilized to a larger degree by o bonding when compared to the lower state. There is almost no effect of ligand pK a on U 1/2 for reduction of TPPMnClO4. This invariance with axial ligand implies an equal o stabilization of Mn(III) and Mn(II). This has been suggested in an earlier study for reactions of TPPMn(L)+C1 - where L is a substituted pyridine [ 16] and, in this study, for reactions of TPPMn(S) + C10~ where S is a solvent molecule and x is 0, I or 2. We were unable to obtain corresponding Uln--pKa plots for the reactions of TPPCo(L)~ in CH2C12. This was due to the complicated nature of the reaction Co(III)--Co(II) in CH2C12 containing 1.0 M ligand. Although reversible waves are obtained in neat pyridine, large separations are obtained between the reduction and the oxidation peak in CH2C12 containing 1.0 M ligand. This is due either to irreversibility of the electron transfer step proper, or to an E.C. mechanism, which probably involves a transient TPPCo(L)2 species. For comparison, we looked at the reduction peak potentials obtained by differential pulse polarography at a platinum electrode. The difference in reduction potentials between TPPCo(L)~ where L was 3,5-dichloropyridine (ligand 1) and L was 4-N,N-dimethylaminopyridine (ligand 11) is 659 inV. Plots of Upc vs. pK a were linear and gave a slope of --87 mV/pKa. Based on this plot, the order of sensitivity to o effects on the oxidized and reduced forms of the investigated complexes can be listed as TPPCo > TPPFe > TPPCr > TPPZn ~ TPPMn. It should be stressed that this order refers only to the specific M(III)--M(II) and Zn(II) radical--Zn(II) couple investigated, and that substantially different effects will be observed for different electrode reactions of the same metal.

221

Redox tuning of metalloporphyrin reactivity In general, redox tuning of metalloporphyrin half-wave potentials may be accomplished by axial binding of ligands, solvent molecules or counter ions. This was clearly demonstrated with TPPFeX, for which the greatest shift in half-wave potentials was obtained. In the non-bonding solvent CH2C12, shifts of up to 720 mV may be obtained by changing the nature of X-. Maintaining a constant X- and varying the solvent also resulted in a shift of potential, either positive or negative depending on the nature of X-. When X = CLOY, a negative shift of up to 270 mV may be obtained on going from CH2C12 to DMF while, when X = Br-, CI-, N~ or F-, a positive shift of up to 160 mV may be obtained. Finally, complexation of a substituted pyridine, such as one of those listed in Table 2, will result in an up to 480 mV shift of potential depending on the pKa of the ligand. This difference of potentials correspond to a 108 difference of redox stability between the Fe(III) and the Fe(II) center as a function of complexed ligand [ 12]. Metalloporphyrin redox tuning also depends on the specific metal center and its degree of axial interaction. This was demonstrated by comparison of Fe potentials with those of Cr, Co and Mn. Similar anodic or cathodic shifts of potentials are observed for reactions of TPPCr(L)~ and TPPCo(L)~ with solvent and/or other axial ligands. However, very little effect is observed for the reactions of TPPMnC1 or TPPMnC104. For these complexes an almost equal o stabilization of the oxidized and reduced forms results, and there is virtually no potential shift with either solvent DN or ligand pKa. The degree of tuning also depends on the type of electrode reaction and the specific porphyrin involved. Modifications in electrochemical reactivity may involve either the higher or the lower oxidation state in order to achieve the same result, a desired potential. However, modification in structure of the reactant or product does not have to be at the redox center. This is shown by the reactions of TPPZn. In this complex the electron is abstracted from the porTABLE 2 Substituted pyridines utilized No.

Ligand

pK a a

1 2 3 4 5 6

3,5-diehloropyridine 3-cyanopyridine 4-cyanopyridine 3-chloropyridine 3-bromopyridine 4-acetylpyridine

0.67 1.40 1.86 2.81 2.84 3.51

7

pyridine

5.28

3-pieoline 4-picoline 3,4-1utidine 4-N ,N-dimethylaminopyridine

5.79 5.98 6.46 9.71

8 9 10 11

0 b

0.74 0.56 0.66 0.373 0.391 0.50 0.00 --0.069 -0.170 -0.239 -0.44

a K. Schoefield, Hetero-Aromatic Nitrogen Compounds, Plenum Press, New York, 1967, p. 146. b John A. Dean, Lange's Handbook of Chemistry, 79th edn., McGraw-Hill, New York, i973.

222 phyrin 7r system. Shifts of U l n for the radical formation reaction may be accomplished by binding either a solvent molecule or nitrogenous base axially to the non-electroactive zinc center. Even greater effects may be accomplished by complexation of [TPPZn] ÷ or TPPZn with different halides. This will be presented in a later study. ACKNOWLEDGEMENTS

The attthors wish to thank the National Institutes of Health (Grant GM 2517-02), the R o b e r t A. Welch Foundation (Grant E-680) and the National Science Foundation (CHE-7921536) for support of this work. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 3~ 31 32 33 34

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