Substituent effects on the redox reactions of tetraphenylporphyrins

BIOINORGANIC

CHEMISTRY 7,107-

115 (1377)

107

Substituent Effects on the Redox Reactions of Tetraphenylporphyrins

KARL M. KADISH* and MARK M. MORRISON

Department of Chemistry. California State University, * Fullerton, Fullerton. California 92634

ABSTRACT

The effect of substituents on r7 radical reactions of parasubstituted tetraphenylporph*s was investigated by cyclic voltammetry in methylene chloride. In all cases electron donating substituents produced a more difficult reduction and an easier oxidation_ Plots of El,2 vs. u yielded Hammett linear free energy relationships fox cation radical and &cation formation and anion radical and dianion formation. An average reaction constant of p = 0.07 2 0.01 V was obtained. This was true for tetraphenylporphyrins containing the central metals VO, Mn, Fe, Co, Ni, Cu, and Zn, as well as the free base H2 @-X)TPP. The value of p appears not to be directly affected by the central meta! oxidation state or the overall charge on the complex.

INTRODUCTION In nonaqueous media rnetalloporphyrins may be oxidized in single electron transfer steps to yield R cation radicals and dications [l-3] , or reduced to yield n anion radicals and dianions r4,5] _ Half wave potentials of these reversible reactions have been shown to depend on the electronegativity and oxidation state of the central metal [6,7], dimer formation [8] _ number of coordinated axial ligands [9,10], and overall basicity of the porphyrin ring [l l] _ Generally, as the porphyrin ring basicity increases, the reductions become more difficult and the oxidations more facile. This change in electrochemical reactivity, induced by electron-donating or electron-withdrawing substituents attached to the porphyrin ring, can be quantitated by the Hammett linear free energy relationship [ 121 A&,,

= OP.

(1)

*Author to whom inquiries should be sent; present address: Department of Chemistry, University of Houston, Houston, Texas 77004.

K. M. KADISH

108

AND M. M_ MORRISON

5E1,a is the difference in half wave potential between the substituted derivitive and the hydrogen derivative, u is the Hammett substituent constant, and p, the “reaction constant”, given in volts, measures the effect of substituents on the electrode reaction. Recently, linear free energy relationships have been used to investigate solvent and substituent effects on half wave potenti& of free base porphyrins [13] and also to differentiate metal center redox reactions of metalloporphyrins from electron transfer reactions at the porphyrin ring [ 141. Comparisons between substituent effects on redox reactions of cobalt(H) porphyrins and axial base or dioxygen addition have also been reported [ 15]_ The effect of electron-withdrawing substituents on porphyrin electrode reactions to form P anion radicals and dianions has been investigated by polarographic techniques for several metal complexes of polycyanoporphyrins [16] _ In this paper we report the effect of substituents on both n cation and n anion electron transfer reactions of vanadyl, manganese, iron, cobalt, nickel, copper, and zinc para-substituted

tetraphenylporphyrins,

structure I.

To date, this is the only comparison which has been made involving substituent effects on both cation and anion radical reactions of the same complex for a similar series of +l, +2, +3, and +4 metal porphyrins. The electrode reactions can be summarized as M@-X)ll?P M(p-x)Tpp-

+ e- =+M@-X)lPl+.

anion radical;

(2)

+ e-

* M@-x)ZIPF2,

dianion;

(3)

* M@-x)”

cation radical;

(4)

dication;

(5)

M@-x)Ti?P

M(p-JQiPP

+ em,

=+ M(p-X)2F~2

+ e-,

PARA-SUBSTITUTED

TETRAPHENYLPORPI-IYRINS

109

where M represents the central metal V*4, Mn+3, Fei4, Fe+l, CO+~, Ni+2, c&2 252 or H *2, and @-X)TPF represents the para-substituted tetraphen$porph~rin. Szbstituents Include -OCHa, CHs, -H, -F, -Cl, COOCH3, CN, and -NOa. All reactions reported in this paper have been well characterized by spectroscopic techniques [l-6,1 7]_ EXPERIMENTAL Burdick and Jackson, Inc.) was used Methylene chloride (“Distilled-in-glass”, as received_ Tetrabutlyammonium perchlorate (TBAP) (Southwestern Analytical, Inc.), used as supporting electroly?e, was recrystallized from absolute methanol, and dried at reduced pressure over P,Olo at room temperature_ Para-substituted free base tetraphenylporphyrins, H2(pX)TPP, and most of the vanadyl, manganese, iron, cobalt, and nickel derivatives were the generous gift of Dr. F. Ann Walker. Others, including the copper and zinc derivatives, were synthesized from the para-substituted free base tetraphenylporphyrins by the method of Adler et al. [ 18]_ Cyclic voltammograms were obtained usin g a PAR model 174 Polarographic Analyzer, in conjunction with either an MFE model 1015 or Houston Instruments model 2000 X-Y recorder. The standard three electrode system included platinum working and auxiliary electrodes, and a commercial type saturated KCl calomel electrode (SCE), which was separated from the bulk of the solution by a medium porosity fritted disk. Solutions were degassed with high purity nitrogen or argon for 10 min prior to analysis, and blanketed with nitrogen or argon during determinations_ For the volatile methylene chloride solutions, the Na or Ar was presaturated with solvent before entering the cell in order to avoid changes in concentration due to evaporation_ Solutions were maintained at 20 +_ 1 “C. Potentials are reported vs. the saturated calomel electrode (SCE). Half-wave potentials were measured as the average of the cathodic and anodic peak potentials. Slopes of El,, vs. 40 plots were analyzed by a least squares best fit program to calculate the reaction constant, p_ Since all four phenyl rings of tetraphenylporphyrin are substituted, 40 was used as the sum of the substituent constant. McDaniel and Brown substituents constants [ 19],40, used in this paper are: (P-H) = 0.00, @-OCHa) = -1.072, @CH3) = -0.68, W-F) = 0.248, @-Cl) = 0.908, @-COOCHa) = 1.240, (p-CN) = 2.640, and @-NOa) = 3.112. RESULTS

AND DISCUSSION

A typical cyclic voltammogram of VOTPP in methylene chloride is shown in Fig. 1_ The four peaks correspond to reactions (2) and (3) at -1.13 V and -1.51 V to produce the anion radical and dianion, and reactions (4) and (5) at

110

M. M. MORRISON

K.

.

50pA

b

1 1.50

I 1.00

I 0.50

VOLTS

I 0.00

VS.

I -0.50

I -1.00

I

-1.50

S.C.E.

FIG_ 1. Cyclic vohammogram of 0.5 mM VOTPP at a platinum electrode in CH2C12, 0.1 M TBAP. Scan rate = 0.1 Vlsec. The reaction numbers correspond to reactions given in the text. 1 .I3

V and 1.35 V to form the cation radical and dication, respectively [ 17]_ state is observed and the potential difference between reactions (2) and (4) agrees with the previously characterized value of 225 I 0.15 V for Metallo-octaethylporphyrins [6,7] _ Cyclic voltarnmograms of all VO@-X)TPP complexes were similar in shape and reversibility, but shifted along the potential axis due to the electron-denoting or electronwithdrawing character of the substituent X. For complexes containing electrondonating substituents, the half wave potential for each of the four electrode reactions was shifted cathodically, so that the reductions become more difficult and the oxidations, easier. In contrast, when X was an electron-withdrawing substituent, the reductions were easier and the oxidations harder. This is shown in Fig. 2, where the half wave potentials, f& ,2, of each reaction of VO@-X)Z’PY’ are plotted vs. 40. As seen from this figure, a linear relationship is obtained indicating a constant mechanism throughout each series. Utilization of Eq. (I), gave a p of 0.06, 0.07,0.05, and 0.09 V for reactions (2) to (5), respectively. These are summarized iu Table 1, along with values of El,, and p for all ‘ITradical reactions of H&-X)TPP and A@-X)TPP, where

No change in metal oxidation

M = VO, Mn, Fe, Co, Ni, Cu, and Zn. The sign of p is defied as positive when electron-withdrawing substituents shift a redox reaction to more anodic potentials.

PAM-SUBSTITUTED

TETRAPKENYLPORPIIYRINS

3

1.40

1.30

1.20

111

C

l-3

1.10

ui

_

1.00

cj

:

vi

pQCH3

.

d

T

PCN

0

-l_OO

ii -1.10 2 !z

-1.20 _ cv

WI-

PCN

-1.30

I

-i-4(3

I-

-isa

,-

0

-

40

1.00

2.00

FIG. 2. Plot of EIIP vs. 40 for the four porphyrin ring reactions TPP in CHzClz, 0.1 M TBAP. The reaction numbers correspond given in the text and in Fig. 1_

of VO(p-X) to reactions

For all H&+X)TPP and Mb-x)TPP complexes studied, the same trends were noted. When complexes contained electron-donating substituents (u < 0), the redox potentials were shifted cathodicahy from the nnsubstituted complex, while complexes containing electron-withdrawing substituents (a > 0), gave half wave potentials shifted anodically from the unsubstituted complex_ The magnitude of the shift was similar for all porphyrin ring reactions, and in CKzCl2 gave an average p = 0.06 + 0.01 V and p = 0.07 + 0.01 V for reactions (2) and (3) to yield the anion radical and dianion, respectively. In the same solvent a p = 0.07 i- 0.01 V was obtained for both reaction (4) and reaction (5), to yield

112

K. M. KADISH AND M. M_ MORRISON

the cation radical and dication, respectively. This was true for all of the metalloporphyrins in Table 1 and was apparently independent of changes in metal oxidation state. Central metal oxidation states of the complexes in Table 1 which react to form the n anion radical have been characterized as Fe(I) [8], V(W) [l?] , Ni(I1) [.5], Cu(I1) [S], and Zn(I1) [Sl- AU complexes, except iron(I) which carriers a negative charge, are neutral as the reactant in reaction (2) Central metal oxidation states of the complexes which react to form the rr cation radical have been characterized as Fe(W) 131, V(N) 1171, Co(II1) ]1,2] , Ni(II) [2b, species carries a 14,201, Cu(I1) [ 1,3b] and Zn(I1) [l-3] _ Of these; the iron +2 charge as reactant in reaction (4), the cobalt and manganese complexes carry a +1 charge, and the zinc, nickel, and copper complexes are neutral. As seen in Table 1, a similar p was obtained for the reduction of negatively charged complexes containing a +l metal (Fe’l) and neutral complexes containing a +2 A similar p was metal (Ni’2i Cu +-2 &+a), a +4 metal (VO*a), or Ha@-X)TR? also obtained for oxidation of neutral complexes containing a 5-2 metal (Cu+*, ZrP2, Ni+*), positive complexes containing a +3 metal (Fe+*, W4), or neutral Ha@-X)TPP Thus, it appears that neither the overall charge on the complex nor the central metal oxidation state significantly influences substituent effects on rr radical electron transfer reactions of metalloporphyrins. Of special interest is a comparison between p for rr cation and rr anion radical formation. Electrochemical studies of Co(px)Z??P in several solvents have shown that porphyrin ring oxidation to yield B cation radicals was more sensitive to substituents than ring reduction to yield rr anion radicals, and that formation of the dication was the most sensitive to substituents, as defined by the magnitude of p [15] _ This does not generally appear to be true for all metalloporphyrins and no general trends may be noted from Table 1. Vanadyl and cobalt complexes show the largest substituent effect for dication formation (reaction 5) but, under the same conditions, copper and zinc complexes show the smallest substituent effect for the same reaction @ = 0.03 and 0.05 V, respectively). Nickel and copper complexes give slightly larger reaction constants for cation radical formation (reaction 4) than for anion radical formation (reaction 2) agreeing with the reported results of Co@-X)TPP [15] _ In contrast however, vanadyl, iron, and zinc as well as free base tetraphenylporphyrin show approximately equal sensitivity to cation and anion radical formation, so that when the average value is taken, no difference appears to exist_ It might be expected that the magnitude of p for complexes in this study should increase with maximum conjugation between the porphyrin ring and the four phenyl groups of meso-tetraphenylporphyrin. X-ray diffraction results indicate that the four phenyl rings are tilted at an angle to the plane of the porphyrin ring [21,22] . This angle has been measured at 61°-63O in H2TPP [23],but is closer to 90” for many metalloporphyrins [21] _ The cation radical

-1.63 k) (11)

-1.28 -1.32 -1.32

Fe Mn cot

Nif cu 211 0.05 0.06 0,05

OS07 k) 01)

0.07 0.06

(volts)

PC

0.06 k 0.0 1

(2)

(II) -1.74 -1,71

(11) (6) (11)

-1.55 -1.51

(volts)

El,Zb

(II) 0) 0,07

01) (6) 01)

0,07 0.07

&ts)

C

0.07 k 0.01

(3) h/2b

1#OS 1,oo 0878

1,40 11,13 ,oo

I,02 1,13

(volts)

(4)

0.07 f 0.01

0.09 0.08 0.06

0.05 0.07 0.09

0.06 0.05

(volts)

P"

(11) 1.25 I ,09

. (II) W) 1.20

1.27 1.35

(volts)

EllZb

Ring Oxidationa

n Reactions correspond to those given in text, b Hydrogen substituted derivative,J!(jhH)TPP. c Calculated from Eq, ( l), d Ref. [13]. c Ref. [151, , f Ref, [ 141. R Irreversiblereaction. I*All or almosl all para-substitutedcon~plcxcsreacted beyond polcntial range of solvent.

Averagep

-1.20 -1.13

(volts)

El,Zb

I-l2d vo

Central Metal

Ring Reductiona

M(pX)?‘Ip in CHzClz,0.1 M TBAP

(5)

5

F E

F! 4

2 6

2 3

z $

E 3 E 0.07 ?: 0,Ol 8

Ul) 0.03 0.05

(11) 00 0.09

0.07 0.09

(volts)

PC

Summary of Hammett Reaction Constants,p, for Oxidation or Reduction of Para-SubstitutedTetrapllenylporpllyrin,

TABLE 1

114

.

K. M. KADISH

AND M.

M. MORRISON

ZnTPP ClC4- has been measured to have an angle of 50”-68” [24], while other estimates of 44.5”-81” have been made for complexes of cobaIt(II) 125, 261 and iron(II1) 1271 porphyrins_ It has been suggested that, in solution, the tetraphenyl~orphyrin molecule approaches closer to a planar configuration_ Calculations by Wolberg demonstrate that in solution I-IaTP! assumes a structure closer to coplanarity by 17” from the solid state, while in metallotetraphenylporphyrins the tendency towards coplanarity is 40” greater than in the solid state [25] _ Thus, it is especiahy interesting to note that within experimental error, all complexes gave approximately the same reaction constant of 0.07 V in methylene chloride, suggesting that a similar II overlap occurs for all complexes of free base and metallotetraphenylporphyrins. In conclusion, it appears that substituent effects on cation and anion radical reactions of para-substituted tetraphenylporphyrins are only slightly influenced by changes in the central metal or overall oxidation state of the reacting species. In methylene chloride a similar substituent effect was measured for both n cation and z anion reactions of para-substituted tetraphenylporphyri independent of whether the reacting species had a +2, +1 , 0, or -1 overall charge. This was true for seven first row transition metal complexes of oxidation state +l , l2, +3, or +4, suggesting that a similar electron transfer mechanism occurred throughout the metalloporphyrin series, and that for all complexes investigated, similar coplanarity exists between the porphyrin plane and the four phenyl rings. The support of Research Corporation is gratefiIiy acknowledged_ We would like to thankDr. F. Ann Walkerfor donatingmany of the subsMuted metalloporphyrins.

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Received

8 June 19 76; revised 7 JuZy I9 76.

Chem

Sot. 94,2066 (1972).