Electrochemistry of Nickel and Copper â-Octahalogeno-meso-tetraarylporphyrins. Evidencefor Important Role Played by Saddling-Induced Metal(dx2-y2)-Porphyrin(“a 2u”) OrbitalInteractions

Abhik Ghosh,*,† Ingar Halvorsen,† Henning J. Nilsen,† Erik Steene,† Tebikie Wondimagegn,†Renate Lie,† Eric van Caemelbecke,‡ Ning Guo,‡ Zhongping Ou,‡ and Karl M. Kadish* ,‡

Institute of Chemistry, Faculty of Science, UniVersity of Tromsø, N-9037 Tromsø, Norway, andDepartment of Chemistry, UniVersity of Houston, Houston, Texas 77204-5641

ReceiVed: May 22, 2001; In Final Form: June 13, 2001

This is an electrochemical study of the Ni(II) and Cu(II) complexes of a series ofâ-octahalogeno-meso-tetrakis(p-X-phenyl)porphyrins, M[Y8T(p-X-P)P] (M ) Ni, Cu; Y ) Cl, Br; X ) CH3, H, F, Br, COOMe,CF3, NO2). We have analyzed the oxidation and reduction potentials by means of the Hammett equation. TheNi and Cu series exhibit significant differences in electrochemical behavior in terms of the absolute valuesof the redox potentials and substituent effects. DFT calculations suggest that these differences may be ascribedto differences in saddling-induced metal(dx2-y2)-porphyrin(a2u) orbital interaction between the Ni and Cuporphyrins.

Introduction

A number of considerations prompted us to undertake anelectrochemical study of substituent effects in Ni(II) and Cu-(II) complexes of a series ofâ-octahalogeno-meso-tetrakis(p-X-phenyl)porphyrins (X) CH3, H, F, Br, COOMe, COOEt,CF3, NO2; Figure 1).1

Porphyrins are unique among aromatic compounds in havinga number of readily available substituted derivatives with avariety of conformations such as planar, ruffled, and saddled.2

This provides a unique opportunity for comparing the transmis-sion of electronic effects of substituents through planar vsnonplanar aromatic frameworks.

Second, electron-deficient metalloporphyrins rank among themost effective catalysts for hydrocarbon functionalization3 andit is possible that a better understanding of substituent effectsin these compounds might lead to an improved understandingof their reactivity.

Third, nonplanar porphyrins are also of interest from the pointof view of exploring metal(d)-porphyrin(π) orbital interactions.In planarD4h metalloporphyrins, the metal d orbitals and theporphyrin a1u and a2u HOMOs are orthogonal. However,nonplanarity makes certain metal(d)-porphyrin(π) orbital in-teractions symmetry-allowed. For example, ruffling switches onthe metal(dxy)-porphyrin(a2u) orbital interaction while saddlingallows the metal(dx2-y2)-porphyrin(a2u) orbital interaction to beturned on.4 The former interaction has been fairly wellcharacterized by NMR studies of certain low-spin six-coordinateiron(III) porphyrins4 as well as by theoretical means.5 Althougha detailed theoretical description of the metal(dx2-y2)-porphyrin-(a2u) orbital interaction has not yet been given, certain experi-mental facts demonstrate its importance. For example, thisorbital interaction accounts for the diamagnetism of the{CuII-[OETPP•]}+ cation.6 Another example involves{NiII[OETPP•]-(benzothiadiazole)2}+.7 In this case the EPR spectrum indicates

that the unpaired electron resides in a metal(dz2) orbital as intypical Ni(III) porphyrins. The Ni-N bond lengths, however,are more characteristic of high-spin Ni(II) porphyrins. This ledFajer to describe the electronic structure of this compound asinvolving a high-spin Ni(II) center antiferromagnetically coupledto a porphyrin a2u radical.7,8

Here we report that the electrochemical9 properties of Ni andCu â-octahalogeno-meso-tetrakis(p-X-phenyl)porphyrins arerather significantly different. With assistance of some prelimi-

† University of Tromsø.‡ University of Houston.

Figure 1. Metalloporphyrins studied in this work.

8120 J. Phys. Chem. B2001,105,8120-8124

10.1021/jp011984x CCC: $20.00 © 2001 American Chemical SocietyPublished on Web 08/01/2001

nary density functional theoretical calculations,10,11we proposethat saddling-induced metal(d)-porphyrin(π) orbital interactionsare a dominant factor influencing the divergent electrochemicalbehavior of the Ni and Cu complexes.

Results

Table 1 presents the oxidation and reduction potentials ofthe nickel and copper porphyrins studied.12 Typical examplesof cyclic voltammograms are shown in Figure 2. The redoxpotentials were analyzed by means of the Hammett equation (F) 1/4 dE1/2/dσ). Figure 3 presents the Hammett plots foroxidation and reduction of the various series of Cu and Nitetrakis(p-X-phenyl)porphyrins. Table 2 summarizes the variousF values. The results lead to the following conclusions andgeneralizations.

(1) The oxidation potentials of the Cu complexes are lowerthan those of the corresponding Ni complexes (Table 1).

(2) Observe from Table 2 that meso-substituent effects areextremely low for oxidation of saddled Ni porphyrins (F ) 27mV for the octabrominated series andF ) 28 mV for theoctachlorinated series). ThisF is significantly smaller than theF’s for the oxidation of planar Niâ-unsubstituted tetrakis(p-X-phenyl)porphyrins and of saddled Cu porphyrins.

(3) The HammettF’s for reduction of the saddled Ni and Cuporphyrins and of the planar Ni porphyrins are all in the samenarrow range of 39-54 mV. This suggests that nonplanarityper se does not exert a particularly strong influence on theelectronic effects of peripheral substituents on reduction po-tentials.

(4) The cyclic voltammograms shown in Figure 2 show thatthe saddled Ni porphyrins exhibit apparent two-electron oxida-tions whereas the saddled Cu porphyrins exhibit stepwise one-electron oxidations.

DiscussionTo gain some insight into the above electrochemical results

as well as to pursue our general interest in the electronicstructure of porphyrins, we have carried out some DFT13,14,15

calculations on selected saddled metalloporphyrins. In particular,

full geometry optimizations have been carried out on CuII[Br8-TPP], CuII[Br8T(p-CN-P)P], NiII[Br8TPP] and NiII[Br8T(p-CN-P)P], and the two lowest vertical ionization potentials16 (IPs,corresponding to ionization from the “a1u” and “a2u”-typeHOMOs) have been computed (Table 3). Although the calcu-lated results are somewhat preliminary (in the sense that theyneed to be rounded off with a wider range of calculations, whichwill be presented in depth elsewhere), we feel that it isnonetheless justified to invoke them here for a better discussionof the electrochemical results, which is presented below.

(1) We propose that the generally lower oxidation potentialsof the saddled Cu porphyrins, relative to the saddled Niporphyrins, originates, at least partially, from an antibondinginteraction between the porphyrin “a2u” HOMO (b2 for D2d

saddled porphyrins) and the metal(dx2-y2) orbital. Thus, the open-shell MO (Figure 4) and spin density profile of CuII[Br8TPP]clearly indicates a significant metal(dx2-y2)-porphyrins(a2u)orbital interaction. For CuII[Br8TPP], the atomic spin populationsare 42.4% Cu, 9.9% N, and 4.9% Cmeso, which may be compared

TABLE 1: Half-Wave Potentials (V vs SCE) forNiII [Y8T(p-X-P)P] (Y ) Cl, Br) and CuII [Br 8T(p-X-P)P] inCH2Cl2 Containing 0.1 M TBAP

oxidation reduction

compound

parasubstituent

(X) 4σb first second first second third

NiII[Br8T(p-X-P)P]c CH3 -0.68 1.17 -0.87a -1.26a

H 0.00 1.18 -0.82 -1.14F 0.24 1.19 -0.80 -1.12Br 0.92 1.21 -0.80 -1.10COOMe 1.80 1.23 -0.75 -0.94 -1.43a

COOEt 1.80 1.23 -0.74 -0.94 -1.43a

CF3 2.16 1.25 -0.73 -1.03 -1.60a

NO2 3.12 1.27 -0.54 -1.04

NiII[Cl8T(p-X-P)P]c CH3 -0.68 1.16 -0.83 -1.15 -1.56a

H 0.00 1.17 -0.82 -1.17F 0.24 1.18 -0.79 -1.14COOEt 1.80 1.23 -0.73 -0.99 -1.39CF3 2.16 1.24 -0.69 -1.01 -1.55a

NO2 3.12 1.26 -0.55 -1.03a

CuII[Br8T(p-X-P)P] CH3 -0.68 0.94 1.45 -0.83 -1.09H 0.00 0.96 1.45 -0.82 -1.07F 0.24 0.99 1.48 -0.74 -1.05CF3 2.16 1.10 1.49 -0.69 -0.96NO2 3.12 1.18 1.55 -0.58 -0.85

a Scan rate) 0.1 V/s. b Hansch, C.; Lea, A.; Taft, R. W.Chem.ReV. 1991, 91, 165. c These compounds exhibit apparent two-electronoxidations.

Figure 2. Cyclic voltammograms (CVs) of NiII[Br8T(p-CF3-P)P] andCuII[Br8T(p-CF3-P)P]. Note that for NiII[Br8T(p-CF3-P)P], the firstoxidation wave has twice the height of the two first reduction waves,which suggests a two-electron oxidation process.

TABLE 2: Hammett Analysis, G ) ∆E1/2/∆(4σ) in mV

compound Fox Fred

NiII[T(p-X-P)P] 77 54NiII[Br8T(p-X-P)P] 27 39NiII[Cl8T(p-X-P)P] 28 48CuII[T(p-X-P)P] 38 36CuII[Br8T(p-X-P)P] 65 53

TABLE 3: Vertical DFT(PW91/TZP) IPs (eV) of D2dSaddled Porphyrins for Ionized “a1u” and “a 2u” RadicalFinal States

compound a1u radical a2u radical

NiII[Br8TPP] 6.89 (2B1) 6.77 (2B2)NiII[Br8T(p-CN-P)P] 7.50 (2B1) 7.60 (2B2)CuII[Br8TPP] 6.88 (3A2) 6.10 (1A1)CuII[Br8T(p-CN-P)P] 7.58 (3A2) 6.96 (1A1)

Ni and Cuâ-Octahalogeno-meso-tetraarylporphyrins J. Phys. Chem. B, Vol. 105, No. 34, 20018121

to the following values for Cu(II) porphine: 50.7% Cu, 10.8%N, and 0.2% Cmeso.

(2) Why are the meso-substituent effects so very low for theoxidation potentials of the saddled Ni porphyrins studied? Aplausible explanation is that for saddled Ni porphyrins, electron-withdrawing substituents result in a switch of the HOMO from“a2u” to “a1u” but that this does not happen in the Cu case.Qualitatively, in the Ni case, saddling is expected to preferen-tially stabilize the “a2u” HOMO, relative to the “a1u” HOMO,because of a porphyrin(“a2u”)-Ni(dx2-y2) bonding interactionthat becomes symmetry-allowed under saddling. Thus, althoughfor planar Ni tetraarylporphyrins, the HOMO may be generallyexpected to be “a2u”, this is far less generally true for saddledNi porphyrins. Indeed, Table 3 shows that the a1u/a2u HOMOordering switches between NiII[Br8TPP] and NiII[Br8T(p-CN-

P)P]. In contrast, in the Cu case (Table 3), there is no questionof HOMO reordering. The b2 SOMO of saddled Cu porphyrinsis so severely destabilized by the Cu(dx2-y2)-porphyrin(a2u)antibonding interaction that the lowest IP of saddled Cuporphyrins is well-separated energetically from all other ioniza-tion potentials and always corresponds to ionization from the“a2u-like” b2 SOMO, which explains the higher meso-substituenteffects in the Cu case.17

Resonance Raman spectroscopy (Figure 5)18 appears toprovide some support for some of the arguments made above.The Soret-excited RR spectra of the Ni saddled porphyrinsstudied here reveal an almost complete suppression of theν1-(Cmeso-Cphenyl stretch), which may indicate a low level ofelectronic communication between the meso aryl groups andthe porphyrin macrocycle in these compounds. This may be aconsequence of the fact that the a2u HOMO in these compoundsis preferentially polarized toward the Ni dx2-y2 orbital rather thantoward the meso aryl groups. In contrast, this band is quite strongin the RR spectra of the Cu(II) and high-spin six-coordinateNi(II) derivatives of the saddled porphyrins, suggesting that theintensity of this band is somehow related to the occupancy ofthe metal dx2-y2 orbital.

(3) The fact that the HammettF’s for reduction are verysimilar across the various families of metalloporphyrins studied,whether saddled or not, suggests that nonplanarity per se doesnot exert a particularly strong influence on the electronic effectsof peripheral substituents on reduction potentials.

(4) Finally, we address the issue of apparent two-electronoxidations that we observe for the saddled Ni porphyrins. Thisis a relatively uncommon phenomenon for metalloporphyrinsbut has previously been observed by Gray and co-workers forZnII[Y8TPFPP], for Y) Cl, Br, and Me.12a These researchersrationalized the electrochemical double oxidations as follows.Based on EPR evidence that their compounds yield a1u-typecation radicals, they proposed that the double oxidations reflectsaddling-induced destabilization of the a1u HOMO. They further

Figure 3. Hammett plots ofE1/2 (V) versus 4σ. The slopes are equal to theF values in V.

Figure 4. Singly occupied b2 (a2u-type) HOMO of CuII[Br8TPP] (D2d).

8122 J. Phys. Chem. B, Vol. 105, No. 34, 2001 Ghosh et al.

showed that the a1u-type cation radicals are unstable with respectto disproportionation to the neutral and dicationic metallopor-phyrins.

It is relatively simple to understand why saddled Cu porphy-rins undergo stepwise, one-electron oxidations. As shown inTable 3, the two lowest IPs, corresponding to ionization fromthe b2 SOMO and the “a1u”-type HOMO of the saddled Cuporphyrins, are very different, consistent with the stepwise one-electron oxidations undergone by these compounds. For thesaddled Ni porphyrins (Table 3), the two lowest IPs are veryclose. The energies of the2B2 (“2A2u”) and 2B1 (2A1u) states of{NiII[Br8TPP•]}+ are very close, 6.77 and 6.89 eV, relative toneutral NiII[Br8TPP]. In contrast, for ZnII[Br8TPP], whichexhibits normal, successive one-electron oxidations, the twocalculated lowest IPs are not not near-degenerate. These resultssuggest that the instability of metalloporphyrin cation radicalswith respect to disproportionation to the neutral and dicationic

metalloporphyrins is somehow related to the near-degeneracyof the A1u- and A2u-type cationic states. At this point, we havenot investigated why this might be so but we offer the followingspeculative remark. If the near-degeneracy of the a1u and a2u

HOMOs leads to a pseudo-Jahn-Teller (pJT) distortion in themetalloporphyrin cation radical, the pJT distortion, and theenergy gain associated with it, may be expected to be even morepronounced for metalloporphyrin dications (i.e., species in whichtwo electrons are drained from either the a1u or the a2u HOMO).This may provide an explanation for the instability of certainmetalloporphyrinπ-cation radicals with respect to dispropor-tionation.

In summary, a study of substituent effects in saddled Ni andCu porphyrins has revealed a variety of interesting differencesbetween the two families of compounds. DFT calculationssuggest that saddling-induced metal(dx2-y2)-porphyrin(“a2u”)orbital interactions lie at the root of these differences and areindeed a key determinant of electrochemical and other electronicproperties of relevant saddled metalloporphyrins.

Acknowledgment. We acknowledge the Norwegian Re-search Council and the VISTA program of Statoil (Norway)for financial support.

References and Notes

(1) Free-base tetraarylporphyrins with different substituents in the parapositions of the aryl groups were purchased from Mid-Century Chemicals.Nickel(II) and copper(II) complexes were prepared by reaction of the freebases with metal acetates in DMF as solvent (Adler, A. D.; Longo, F. R.;Kampas, F.; Kim, J.J. Inorg. Nucl. Chem. 1970, 32, 2443-2445).â-Octachlorination and octabromination of the Ni tetraarylporphyrins wereperformed, respectively, by use ofN-chlorosuccinimide andN-bromosuc-cinimide ino-dichlorobenzene as solvent at 140°C, as described by Dolphinand co-workers. (Dolphin, D.; et al., United States Patent, Patent Number:4,892,941; Jan. 9, 1990). Bromination of the Cu tetraarylporphyrins werecarried out by use of Br2 according to the procedure of Bhyrappa andKrishnan. (Bhyrappa, P.; Krishnan, V.Inorg. Chem., 1991; 30, 239-245).All compounds studied were characterized by UV-visible absorptionspectroscopy, MALDI-TOF mass spectrometry, and elemental analysis. Thenickel complexes were also characterized by1H NMR. Within each familyof compounds studied, the UV-visible absorption maxima were relativelysubstituent-independent and may be summarized as follows. Ni[Br8T(p-X-P)P]: 447( 3 (Soret), 560( 2 (Q), 590( 3 nm (Q). Ni[Cl8T(p-X-P)P]:438 ( 2 (Soret), 554( 1 (Q), 590( 3 nm (Q). Cu[Br8T(p-X-P)P]: 446(451 for the nitro compound) and 467( 2 (two-humped Soret), 580( 1(Q).

(2) Shelnutt, J. A.; Song, X.-Z.; Ma, J.-G.; Jia, S.-L.; Jentzen, W.;Medforth, C.J. Chem. Soc. ReV. 1998, 27, 31.

(3) Grinstaff, M. W.; Hill, M. G.; Labinger, J. A.; Gray, H. B.Science1994, 264, 1311.

(4) (a) Simonneaux, G.; Schunemann, V.; Morice, C.; Carel, L.; Toupet,L.; Winkler, H.; Trautwein, A. X.; Walker, F. A.J. Am. Chem. Soc.2000,122, 4366. (b) Safo, M. K.; Nesset, M. J. M.; Walker, F. A.; DeBrunner,P. G.; Scheidt, W. R.J. Am. Chem. Soc.1997, 119,9438. (c) Walker, F.A.; Nasri, H.; Turowskatyrk, I.; Mohanrao, K.; Watson, C. T.; Shokhirev,N. V.; Debrunner, P. G.; Scheidt, W. RJ. Am. Chem. Soc.1996, 118, 12109.(d) Cheesman, M. R.; Walker, F. A.J. Am. Chem. Soc.1996, 118, 8. 7373.(e) Safo, M. K.; Walker, F. A.; Raitsimring, A. M.; Walters, W. P.; DolataD. P.; Debrunner, P. G.; Scheidt W. R.J. Am. Chem. Soc.1994, 116, 7760.(f) Safo, M. K.; Gupta, G. P.; Watson, C. T.; Simonis, U.; Walker, F. A.;Scheidt, W. R.J. Am. Chem. Soc. 1992, 114, 7066.

(5) Ghosh, A.; Gonzalez, E.; Vangberg, T.J. Phys. Chem. B1999,103, 1363.

(6) Renner, M. W.; Barkigia, K. M.; Zhang, Y.; Medforth, C. J.; Smith,K. M.; Fajer, J.J. Am. Chem. Soc.1994, 116, 8582.

(7) Renner, M. W.; Barkigia, K. M.; Melamed, D.; Smith, K. M.; Fajer,J. Inorg. Chem. 1996, 35, 5120.

(8) Ghosh, A.; Wondimagegn, T.; Gonzalez, E.; Halvorsen, I.J. Inorg.Biochem. 2000, 78, 79.

(9) Cyclic voltammetry was carried out using an EG&G Model 263APotensiostat with a three-electrode system consisting of a glassy carbonworking electrode, a platinum wire counter electrode and a saturated calomelreference electrode (SCE). Tetra-n-butylammonium perchlorate (TBAP)recrystallized from ethanol and dried in a vacuum for at least 1 week wasused as supporting electrolyte. Dichloromethane puriss. distilled and keptover molecular sieves was used as solvent. The reference electrode was

Figure 5. Soret-excited RR spectra (high-frequency region) of (a)NiII[Br8TPP], (b) CuII[Br8TPP], and (c) NiII[Br8T(p-CF3-P)P](pyridine)2.λex ) 457.9 nm.

Ni and Cuâ-Octahalogeno-meso-tetraarylporphyrins J. Phys. Chem. B, Vol. 105, No. 34, 20018123

separated from the bulk solution by a fritted-glass bridge filled with thesolvent/supporting electrolyte mixture, and all potentials were referencedto the SCE. Pure nitrogen was bubbled though solutions containing themetalloporphyrins for at least 2 min prior to running the experiments, andthe solutions were also protected from air by a nitrogen blanket during theexperiment.

(10) DFT calculations on saddled metalloporphyrins were carried outwith Slater-type valence triple-ú plus polarization basis sets, the VWN localfunctional, the Perdew-Wang 1991 gradient corrections, a fine mesh fornumerical integration of matrix elements, full geometry optimizations withD2d symmetry constraints, and the ADF11 program system. Ionizationpotentials (for ionized final states of different symmetries and orbitaloccupancies) were calculated via a∆SCF procedure.

(11) The ADF program system was obtained from Scientific Computingand Modeling, Department of Theoretical Chemistry, Vrije Universiteit,1081 HV Amsterdam, The Netherlands. For details of basis sets, grids fornumerical integration, etc., the reader is referred to the program manualobtainable from this source.

(12) For other work on electrochemistry ofâ-halogenated and saddledporphyrins, see, e.g.: (a) Hodge, J. A.; Hill, M. G.; Gray, H. B.Inorg.Chem. 1995, 34, 809. (b) Wijesekera, T.; Matsumoto, A.; Dolphin, D.; Lexa,D. Angew. Chem., Int. Ed. Engl.1990, 29, 1028. (c) Duval, H.; Bulach,V.; Fischer, J.; Renner, M. W.; Fajer, J.; Weiss, R.J. Biol. Inorg. Chem.1997, 2, 662. (d) Ochsenbein, P.; Ayougou, K.; Mandon, D.; Fischer, J.;Weiss, R.; Austin, R. N.; Jayaraj, K.; Gold, A.; Terner, J.; Fajer, J. Angew.Chem., Int. Ed. Engl. 1994, 33, 348. (e) Kadish, K. M.; Van Caemelbecke,E.; D’Souza, F.; Lin, M.; Nurco, D. J.; Medforth, C. J.; Forsyth, T. P.;Krattinger, B.; Smith, K. M.; Fukuzumi, S.; Nakanishi, I.; Shelnutt, J. A.Inorg. Chem. 1999, 38, 2188. (f) D’Souza, F.; Zandler, M. E.; Tagliatesta,P.; Ou, Z. P.; Shao, J. G.; VanCaemelbecke, E.; Kadish, K. M.Inorg. Chem.1998, 37, 4567.

(13) For reviews of DFT calculations on porphyrins, see: (a) Ghosh,

A. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R.,Eds.; Academic: San Diego, 2000; Vol. 7, Chapter 47, p 1. (b) Ghosh, A.Acc. Chem. ReV. 1931, 31, 189.

(14) For DFT calculations on nonplanar porphyrin-type molecules, see,in addition to refs 5 and ref 8: (a) Wondimagegn, T.; Ghosh, A.J. Phys.Chem. B2000, 104, 10858. (b) Parusel, A. B. J.; Wondimagegn, T.; Ghosh,A. J. Am. Chem. Soc.2000, 122, 6371. (c) Wondimagegn, T.; Ghosh, A.J.Am. Chem. Soc.2000, 122, 6375. (d) Wondimagegn, T.; Ghosh, A.J. Phys.Chem. A2000, 104, 4606. (e) Vangberg, T.; Ghosh, A.J. Am. Chem. Soc.1999, 121, 12154.

(15) For additional references to DFT calculations on transition metalporphyrins see: (a) Ghosh, A.; Gonzalez, E.Isr. J. Chem.2000, 40, 1. (b)Ghosh, A.; Wondimagegn, T.J. Am. Chem. Soc.2000, 122, 8101. (c) Ghosh,A.; Wondimagegn, T.; Parusel, A. B. J.J. Am. Chem. Soc. 2000, 122, 5100.(d) Vangberg, T.; Bocian, D. F.; Ghosh, A. J. Biol. Inorg. Chem.1997, 2,526. (e) Ghosh, A.; Gonzalez, E.; Vangberg, T.; Taylor, P.J. PorphyrinsPhthalocyanines2001, 5, 345.

(16) DFT calculations yield extremely precise results for the lower IPsof porphyrins and related molecules, as calibrated against gas-phasephotoelectron spectra: (a) Ghosh, A.J. Am. Chem. Soc.1995, 117, 4691.(b) Ghosh, A.; Vangberg, T.Theor. Chem. Acc. 1997, 97, 143.

(17) It may appear as a contradiction that the SOMO of the Cuporphyrins is significantly localized on the Cu ion and that we still describeoxidized Cu porphyrins such as{CuII[Br8TPP•]}+ as porphyrin a2u-typeradicals. The key to this paradox is that the electronic structures of theoxidized Cu porphyrins is best described as involving antiferromagneticcoupling of a Cu(II) ion and a porphyrin a2u radical. A Cu(III) descriptionappears to be precluded on the ground that the optimized Cu-N bonddistances are very similar for CuII[Br8TPP] and{CuII[Br8TPP•]}+.

(18) Halvorsen, I.; Steene, E.; Ghosh, A.J. Porphyrins Phthalocyanines,in press.

8124 J. Phys. Chem. B, Vol. 105, No. 34, 2001 Ghosh et al.