J. Phys. Chem. 1991,95,4415-4418
4415
OncEiectron Oxldation of Nickel Porphyrins. Effect of Structure and Medium on Formatlorr of Nickel( I I I) Porphyrin or Nickel( I I ) Porphyrin r-Radical Cation G.S. Nahor,l P. Neta,*il P. Hambright? and L. R. Robinson* Chemical Kinetics Division, National Institute of Standards and Technology. Gaithersburg, Maryland 20899, and Department of Chemistry, Howard University, Washington, D.C. 20059 (Received: November 2, 1990; In Final Form: January 2, I99I)
The oxidation of several nickel(I1) porphyrins by various radicals has been studied by pulse radiolysis in different media (CI2' and Br,' in aqueous systems, Br atoms in organic solvents, and peroxyl radicals in organic and aqueous/organic systems). Photochemical oxidation was also examined in some cases. The absorption spectrum of the oxidation product was monitored within several microseconds after the pulse. Two types of differential spectra were observed, a broad absorption at 640-700 nm ascribed to the *-radical cation, or a sharp absorption at 560-580 nm ascribed to nickel(II1) porphyrin. NiIITPP (tetraphenylporphyrin) in several organic solvents, protic and aprotic, was oxidized to NilrlTPP. The addition of 10% water as cosolvent or 0.1 M of electrolyte changed the route of oxidation to give the radical cation NitlTPP'+. On the other hand, NiIITSPP (tetrakis(4-~ulfonatophenyl)prphyrin), which has four negative charges, was oxidized on the porphyrin ligand by all the radicals examined, in water and in several organic solvents. Ni" bis(N-methyl-4-pyridyl)diphenylporphyrin,with a charge of +2, and Nil1 tris(4sulfonatopheny1)(N-methyl-4-pyridyl)porphyrin, with an overall charge of -2, were oxidized on the ligand in aqueous solution but on the metal in organic solvents. These and other results led to the conclusion that most radicals react with Ni"P by an inner-sphere mechanism to bind onto the metal and give the NilIIP form. However, when the porphyrin is sufficiently charged to repel the axially bound anion, and/or when the medium enhances the separation of this anion from the metal, the result is oxidation of the porphyrin *-system. In all cases, however, the one-electron-oxidation products, whether Ni"P'+ or Ni"'P, decay to yield two-electron ring oxidation products.
Introduction Oxidation of nickel(I1) porphyrin (NiIIP) may take place on the metal, to give Ni"'P, or on the ligand, to produce the *-radical cation Ni"P+. Oxidation on the metal has served as a model for studying high oxidation state metalloporphyrins of biological i n t e r e ~ t . Earlier ~ electrochemical studies, which were confined to long-lived species, reported -formation of Nil"P only a t low temperatures (frozen solutions), whereas NilIP'+ was formed a t room temperature in fluid solutions.c6 Both oxidation products have been observed with the same porphyrin and were interconverted upon changes in t e m p e r a t ~ r e . Since ~ the potentials for oxidation on the ligand and on the metal are very close? it seems that minor changes in the experimental conditions can shift the balance between the two specie^.^ In this study we examine the products of one-electron oxidation of several nickel(I1) porphyrins by pulse radiolysis and laser photolysis in several solvents a t room temperature. The identity of short-lived species was elucidated from their optical absorption spectra. We find that the product of oneelectron oxidation,NinlP or Ni"P+, depends on the nature of the porphyrin ligand and on the medium.
Experimental Section'
methyl-4-pyridyl)porphyrin), DMPyDPP (5,10-bis(N-methyl-4pyridy1)- 15,2O-diphenylporphyrin,and H M P (hematoporphyrin). Pyridine and N,N-dimethylformamide were vacuum distilled prior to use. All other solvents and reagents were of analytical grade purity and were used as received. Water was purified with a Millipore Super-Q system. Solutions containing 3-5 mg/50 mL (0.05-0.1 mM) nickel porphyrin in the medium specified in Table I were prepared freshly before use and were irradiated under air or after bubbling with the desired gas (N20 or N 2 0 / 0 2 mixtures). Pulse radiolysis experiments were carried out with the apparatus described before? Steady-state irradiation was carried out in a Gammacell 220 @Cosource with a dose rate of 135 Gy/min. The laser flash photolysis system consisted of a Lambda Physik EMG 201 MSC excimer laser, utilizing KrF for 248 nm, with a pulse duration of 20-40 ns and pulse energies of 150-500 mJ. Other details of the laser and the detection system were described before.'O
Results and Discussion Oxidation Reactions. The nickel porphyrins were oxidized in various media. In aqueous solutions, they were oxidized by C12'or Br2'- radicals produced in N20-saturated solutions containing C1- or Br-."
+ - +
The nickel porphyrins were prepared as described previou~ly.~ X- + *OH X' + OH(1) The following abbreviations are used: TPP (5,10,15,20-tetraX' x- X2.(2) phenylporphyrin), TSPP (5,10,15,20-tetrakis(4-suIphonatophenyl)porphyrin), TMPyP (5,10,15,20-tetrakis(N-methyl-4NiP 2X- (Nip)+ X2'(3) pyridy1)porphyrin). TrPPyP (5,10,15-triphenyl-2O-pyridylporphyrin), TrSPMPyP (5,10,1S-tris(4-sulfonatophenyl)-20-(N(Nip)+ represents an oxidized nickel porphyrin, without specifying the site of oxidation. In aqueous/organic or in organic solvents, Ni"P was oxidized by CC1302' or other peroxyl radicals, which (1) NIST. were produced by reduction of CC14or other haloalkanes in the (2) Howard University. presence of oxygen.12 (3) Dolphin, D.; Niem, T.; Felton, R. H.; Fujita, I. J. Am. Chem. Soc.
+
1975. 97, 5288.
(4) Johnson, E. C.; Niem, T.; Dolphin. D. Can. J. Chcm. 1978.56, 1381. ( 5 ) Wolbcrg, A.; Manassen, J. Inorg. Chcm. 1910, 10, 2365. (6) Kim, D.; Miller, L. A.; Spiro, T.0. Inorg. Chcm. 1986. 25, 2468. (7) Wolbcrg. A.; Manassen, J. J. Am. Chcm. Soc. 1970. 92, 2982. (8) The identification of m " a l equipment or material does not imply recognition or endorsement by the National Institute of Standards and Technology, nor docs it imply that the material of equipment identified is neccararily the best available for the purpose. (9) Nahor, G. S.; Neta, P.; Hambright, P.; Robinson, L. R.; Harriman, A. J. Phys. Chcm. 1990. 94, 6659.
OO22-3654/91/2095-441 5$02.50/0
CC14
+ -
+ e-
-
*CC13+ C1-
(4)
CC1302*
(5)
*CC13 O2
CC1302*+ NiP
CC1302-+ (Nip)+
(6)
(10) Huie, R. E.; Clifton, C. L. Int. J. Chem. Kiner. 1989, 21, 61 1. ( I 1) See e.g. Neta, P. J . Phys. Chem. 1981,85,3678. Morehouse, K. M.; Neta, P. J . Phys. Chem. 1984, 88, 1575.
0 1991 American Chemical Society
Nahor et al.
4416 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991
TABLE I: Oxidation products of Nickel(I1) Porpbydm in Various Media porphyrina mediumb NiTSPP(-4) HzO, pH 3, N20 H20, pH 11, N2O 2-PrOH:H20:CC14(20:5:1) MeOH MeOH:CC14 (20:l) Me0H:CCl4:pyridine (945: 1) DMFCC14 (20:l) DMFCCII:H20 (20:1: 1) H20:CH3CN(9:1), N20:02 (4:l) NiTMPyP(+4) H20, PH 3, N20 H20, pH 11, N20 H20, pH 11, N2O NiHMP(-2) CH2Cl2 N iTPP(0) CH2C12 CH2Cl2 + 0.1 M Bu~N+BF~2-PrOH:CH2C12:H20(5:5: 1) CC14 (w/wo pyridine)' CC14:2-PrOH(1:l and 1O:l) CC14:2-PrOH:H20(5:5:1) DMF:CCI4 (10~1) NiTrSPMPyP(-2) H20, pH 11, N2O DMF:CCII (2011) CH2Br2 NiDMPyDPP(+2) H20 NiTrPPyP(0) CH2C12(w/wo pyridiney 2-PrOH:CCI4 (3:2) 2-PrOH:CC14:H20(15:lO:l) benzene:CH2Br2(50:l) DMF:CCl, (2011)
radicalC Cl2'Br2'CC1302' CH30' CC1302' CCI3O2' cc130;
cc1,o; NCCH202' Cl2'Br2*Br2*hv, CH2C102' CHCIZO; CHC120; hv, CH2ClO2' CCI 0 CCt02'
cc1:o:-
CC1302' Br2*CCl302' Br' Br;CHCl 0 CCI n 2
0'
ccl:o:*
Br' CC1302'
peaks, nm 595,660 595, 660 600,680 590,670 585,670 570,670 570,670 570,670 600, 670 695 695 680, 720 570, 670 570,650 600,640 605,640 570 570 i 10 585,670 560, 660 sh 705 565 570, 660 695 560 570 580, 660 sh 560 560
species Nil1P+ Ni1lP+ ~ilIp'+ ~iIlp'+
NiiIp'+ Nil1P+ ~ilIp'+ NiIIp'+ Ni"p'+ Nii!p'+ Ni"p'+ ~iIlp'+
Ni"'P Nir1IP
~iIlp'+ ~ilIp'+
NilIIP NilIIP ~ilIp'+ NilIIP ~ilIp'+
Ni"'P N?IIP Ni"p'+ Ni"'P NilllP Ni"'P NiI1'P Ni"'P
O s e e . Experimental Section for the abbreviations of the porphyrins; the net charge is given in parentheses. bThe solution was irradiated under air, unless N20 or N20:02is specified. CTheradicals were produced by pulse radiolysis unless hv is specified to indicate a laser photolysis experiment. dThe porphyrin is oxidized by the small fraction of methoxyl radicals produced in MeOH; the main radicals formed in aerated methanol are HOCH200' which cannot oxidize this porphyrin. With or without 1% pyridine.
(12)Neta, P.;Huie, R. E.; Ross,A. B. J . Phys. Chem. ReJ Data 1990,
all cases we observed bleaching of the Q band of Ni"P upon oxidation. Typical results are given in Figure 1 and all the results are summarized in Table I. Table I shows that the nature of the initial one-electron-oxidation product of Ni"P depends on the porphyrin ligand and on the medium, but not necessarily on the oxidizing radical. Nil1TSPP, which bears four negative charges, was dissolved in water, methanol, or DMF, with or without a cosolvent, and was oxidized by a variety of radicals. The product in all cases exhibited the typical broad absorptions of the species produced by oxidation of the porphyrin n-system (Figure 1). NiIIHMP, which is negatively charged, and NiIITMPyP, which is positively charged, were also dissolved in water and oxidized by dihalide radicals, and in both cases the n-radical cations were produced.'* On the other hand, NiIITPP, which is uncharged and is soluble only in organic solvents, is found to be oxidized on the metal in most cases,whether the solvent was CH2CI2, CC14, DMF, or their mixtures with 2-PrOH. However, the addition of a small fraction (110%) of water as cosolvent, or the addition of 0.1 M electrolyte, changed the route of oxidation to give the n-radical cation of the Nil1 state (Figure 1 and Table I). It has been noted before that addition of water or Br- to a Ni"TPP solution in CH2C12stabilizes the electrochemical oxidation product of this porphyrin in the NiI1P+ rather than NilIIP form.4 Since NiIITPP and NiIITSPP are expected to exhibit very similar behavior, the observed difference in oxidation products must be due to the difference in solvent or in the charge of the porphyrin. To achieve some solubility of the same porphyrin in both aqueous and organic solvents, we prepared nickel porphyrins
(13) Emmi, S.S.;Bcggiato, G.; Casalbore-Miceli, G. Radiat. Phys. Chem. 1989,33,29.Alfassi, 2.B.;Mosseri, S.;Neta, P. J . Phys. G e m . 1989,93, 1380. (14)Neta, P.; Huie, R. E.;Mosseri, S.;Shastri, L. V.; Mittal, J. P.; Maruthamuthu, P.; Steenken, S.J . Phys. Chem. 1989,93, 4099. (15)Shoute. L. C. T.; Neta, P. J . Phys. Chem. 1990,94, 2447,7181. (16)The term r-radical cation is used for the product of one-electron oxidation of the porphyrin r-system, disregarding the positive or negative charges that preexisted on the porphyrin due to substituents.
(17)In some cases, the sharp peak at -570 nm was accompanied by a weaker absorption around 650 nm. Such absorptions were also observed previously (ref 4) and may be typical of NirirPin certain media. We cannot rule out, however, the possibility that these absorptions arc due to traceu ( IO5) in this region. (18) NiriTMPyPcould not be oxidized effciently by the peroxyl radicals because of its higher redox potential.
Reduction of CCl, took place by reaction with solvated electrons and in some cases also by reaction with organic radicals. In CH2C12as solvent, two peroxyl radicals are formed, CH2C102' and CHC120;.13 Other oxidants were the peroxyl radical formed in acetonitrile'' and bromine atom complexes formed by radiolysis of CH2Br2as solvent or as solute in b e n ~ e n e . ' ~ In the photochemical oxidation, the triplet excited state of the porphyrin undergoes rapid oxidative quenching by CC14or CH2C12 to give (Nip)+. Of the other products, C1- and 'CCl,, the latter reacts with O2to give the peroxyl radical which then oxidizes a second nickel porphyrin molecule. Oxidation Products. The nature of the species formed upon one-electron oxidation was determined from the differential absorption spectrum, monitored in the 460-760-nm region, immediately at the end of the oxidation reaction (1200ps). Specifically, we attribute broad absorptions at h 600-700 nm to the ?r-radical cationsI6 Ni"P'+, based on the previous results with nickel porphyrins'.' and in parallel with absorption spectra of other metalloporphyrin n-radical cations." Changes in the redox state of the metal in a metalloporphyrin are known to result in minor shifts of the peaks without strong absorption in the red.3,7,9J'Therefore, in the cases in which no intense broad absorptions at 600-700 nm appeared, and the differential spectrum reflected only a red shift in the Q bands (generally a sharp peak at -570 nm), we attribute the spectrum to the species oxidized on the metal, Ni1I1P.I7 In
-
19, 413. and
references cited therein.
One-Electron Oxidation of Nickel Porphyrins
The Journal of Physical Chemistry, Vol. 95, No. 11. 1991 4417
I="'
.12
.08-
U
a
.04
.08
.04
-
a 0
0-.04 0
-.04
600
500
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NiTrSPM Py P
L 500 600 700 A
nm
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-.06/
o
,
500
,
600
A
I
,
700
nm
Figure 1. Transient differential absorption spectra observed following pulse radiolytic oxidation of several Nit[ porphyrins. Upper row: NiTSPP (left) in aqueous 0.1 M KBr at pH 11 (0)and with 0.012 M NaCN added (A),(center) in 2-PrOH:H20CC14(2051); NiTMPyP in aqueous 0.1 M NaCl at pH 3. Middle row: NiTPP (left) in CC14:2-propanol(1:l) (0)and with 8.5% water added (A),(center) in CH2C12(0)and with 0.1 M Bu,N+BF,added (A),(right) laser photolysis experiment in CH2C12(0)and in CH2CI2:2-PrOH:H20( 5 5 1 ) (A). Lower row: NiTrSPMPyP in DMFCCI, (20:l) ( 0 )and in aqueous 0.1 M KBr, pH 11 (A); NiDMPyDPP in CH2Br, (0)and in aqueous 0.1 M KBr (A); NiTrPPyP in CH2CI,.
with mixed substitution a t the meso positions. Ni"TrSPMPyP, which has a net charge of -2, and Ni 'DMPyDPP, with a charge of +2, were oxidized on the ligand when dissolved in water, but on the metal when dissolved in organic solvents (Figure 1 and Table I). The other mixed porphyrin, NiI'TrPPyP, is neutral and behaved essentially like NiIITPP, except that addition of up to 4% water to the solution did not change the route of oxidation from the metal to the ligand, and it was not possible to add higher concentrations of water without precipitating the porphyrin. To examine the effect of axial ligation to the metal on the route of oxidation, CN- or pyridine was added to the solutions of several porphyrins. The spectra of these porphyrins were changed due to axial ligation. In the presence of CN- or pyridine, the spectra of the oxidation products in aqueous solutions resembled those of the *-radical cations, observed in the absence of these ligands, except that the spectra were narrower and shifted toward the lower
wavelength region (Figure 1). It was sufficiently different from that of the Ni'I'P, however, to suggest that the site of oxidation is still on the ligand. Addition of cyanide or pyridine to organic solvents, in which the Ni"P was oxidized to Ni'I'P, resulted in a decreased absorption of the oxidation product but with no change in shape or peak position. The one-electron-oxidation products monitored by pulse radiolysis were short-lived and decayed over milliseconds to seconds, depending on the porphyrin and the medium. To examine the stable products, we carried out y-radiolysis experiments of similar solutions and recorded the optical absorption spectra before and after several irradiation intervals. In all cases the peaks uf the starting material, NilIP, disappeared gradually upon irradiation and new broad absorptions were formed in the 600-800-nm region. In the case of NiTSPP (Figure 2a) and NiTMPyP in aqueous solutions, the absorptions were very broad with no clear peaks,
Nahor et al.
4418 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991
nickel(II1) porphyrin to form a neutral species, unless the medium or the porphyrin ligand enhances charge separation. If the charges are separated, the positive charge on the Ni"'P undergoes redistribution to form the *-radical cation.
11
al u
1.6
ROO--NilllP
c
-
ROO-
+ NitlP'+
(9)
Charge separation is enhanced by electrolytes and also by water and other protic polar solvents. In the latter case, the reaction may be enhanced also by donating a proton to the anion (for ROO-). ROO--Ni"'P + H+ ROOH N i W + (10)
0
-e0
-
v)
I]
4
0 500
600
500
700
A
600
700
800
nm
i 2. Spectral changes upon y-radiolytic oxidation of Nil* porphyrins (a) NiTSPP in N20-saturated aqueous solution containing 0.1 M CI-at pH 3; irradiation times were 0,0.5, 1 , 3, 5, 8, and 12 min. (b) NiTPP in aerated CCll solution; irradiation times were 0, 20, 40,60, and 80 s. F
but with NiTPP (Figure 2b) and NiTrPPyP two clear peaks were observed, a sharp peak at 608 nm and a broad one at 680 nm for NiTPP in CCI4 and two broad peaks at 620 and 675 nm for NiTrPPyP in CH2CI2. These species are clearly different from the one-electron-oxidation products observed in the pulse experiments. These products, which disappeared upon prolonged irradiation, are ascribed to species produced following two-electron ring oxidation. These results indicate that both N i P + and Nil"P disproportionate to give ultimately ring oxidized products. This is in line with our previous results on the reduction of Ni" porphyrir~,~ where both Ni"P'- and Ni'P ultimately give the twoelectron ring reduced products. Oxidation Mechanism. The dependence of the nature of one-electron-oxidation product on the porphyrin structure anu on solvent composition suggests that the mechanism of oxidation of the nickel porphyrins by the various radicals is by an inner-sphere electron transfer involving addition of the radical to the metal center, e.g.,
+ Ni"P (or Br2*-)+ Ni"P ROO'
Br'
-
ROO--NilIIP Br--NilIIP (+ Br-)
(7) (8)
The product anion, such as ROO- or Br-, remains bound to the
+
Negative charges on the porphyrin macrocycle also contribute in repelling the anion as is evident from a comparison of NiTSPP with NiTPP. NiTSPP, with a charge of -4, is oxidized on the porphyrin A-system even in DMF or alcohol, with no added water or electrolyte, whereas the neutral NiTPP gives the Ni"' state under similar conditions. This inner-sphere electron-transfer mechanism is supported by previous conclusions that the oxidation of Zn porphyrins by CClpOO' l9 and of Fe porphyrins by (CH,),C(OH)OO' 2o also occur by an inner-sphere process involving addition of the radical to the metal center. Formation of a Br adduct has been suggested for Mn porphyrins"b and for several Ni macrocyclic complexes2' upon oxidation by Br2*-. The present conclusions are in contrast with recent results on the reduction of Ni" porphyrins, where the structure of the ligand, specifically the presence of a pyridyl group at the meso position, was crucial in directing the reduction toward the ligand to give the *-radical anion Ni"P'-, whereas most other Nil' porphyrins were reduced on the metal? This delicate balance in the reduction and oxidation of N P P may be related to the proximity of the redox potentials of the metal and the ligand in these complexes,'J2 which is rare among metalloporphyrins.
Acknowledgment. This research was supported by the Office of Basic Energy Sciences of the Department of Energy and by the Howard University Faculty Research Support Program. (19) Alfassi, Z. B.; Harriman, A,; Mosseri, S.;Neta, P. Inr. J . Chem. Kiner. 1986, 18, 1315. (20) Brault, D.; Neta, P. G e m . Phys. Len. 1985, 121, 28. (21) Meyerstein, D.; Espenson, J. H.; Ryan, D. A.; Mulac, W. A. Inorg. Chem. 1979, 18. 863. Morliere, P.; Patterson, L. K. Inorg. Chem. 1982, 21, 1837. Also references cited therein. (22) Felton, R. H. In The Porphyrins; Dolphin, D., Ed.;Academic: New York, 1978; Vol. 5, Part C, p 53.
