β-Pyrrole brominated meso -tetraphenylporphyrins: synthesis, spectral...

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Copyright © 2004 Society of Porphyrins & Phthalocyanines

β-Pyrrole brominated meso-tetraphenylporphyrins: syn-thesis, spectral and electrochemical properties

Zhongping Oua, Jianguo Shaoa, Francis D’Souza*b, Pietro Tagliatesta*c and Karl M. Kadish*a

a Department of Chemistry, University of Houston, Houston, TX 77204-5003, USA b Department of Chemistry, Wichita State University, 1845 Fairmount, Wichita, KS 67260-0051, USA c Dipartimento di Scienze e Tecnologie Chimiche, Università degli Studi di Roma-Tor Vergata, 00133 Roma, Italy

Dedicated to Professor Hisanobu Ogoshi on the occasion of his 70th birthday

Received 24 February 2004Accepted 25 March 2004

ABSTRACT: The synthesis, catalytic properties, UV-visible spectra and electrochemistry of β-pyrrole brominated porphyrins are summarized in this brief review. The effect of the Br substituents of the porphyrin ring on the redox behavior, and on axial CO or pyridine binding to the porphyrins is also discussed. Copyright © 2004 Society of Porphyrins & Phthalocyanines.

KEYWORD: β-pyrrole brominated porphyrins, synthesis, catalysis, electrochemistry and spectroelectrochemistry, binding constants.

INTRODUCTIONA number of β-pyrrole brominated meso-

tetraarylmetalloporphyrins derivatives have been synthesized and characterized as to their spectral and electrochemical properties [1]. β-Halogenated porphyrins have a higher catalytic efficiency as compared to non-halogenated porphyrins having the same macrocyclic structure and this result has been often attributed to the presence of electron-withdrawing substituents on the macrocycle. This fact makes the porphyrin π-ring system harder to oxidize, thus stabilizing it against oxidative degradation. In addition, the highly halogenated porphyrins are often severely distorted [1a] and this structural aspect has been invoked as the major contribution to the remarkable catalytic activity of such macrocycles since the non-planarity of the ring hinders formation

of μ-oxo dimers which usually inhibit the catalytic properties of iron porphyrins. Thus, one aim of many previous studies on these types of compounds has been to gain insights into the role of macrocycle distortion [2-26] in biological systems containing porphyrins or related macrocycles [27, 28]. Another purpose for studying these compounds has been to determine which factors influence the robustness of such catalysts in the oxidation of organic substrates [29-49]. The spectral and electrochemical properties of β-halogenated porphyrins have been explained in terms of a combination of two effects; one is the electron-withdrawing properties of the halogen substituents at the β-pyrrole positions of the macrocycle and the other is the nonplanarity of the porphyrin macrocycle, due to steric hindrance of the peripheral groups [50-55]. In order to separate these two factors and to obtain an estimate of their magnitude, our laboratories have over the last ten years examined the spectral, chemical and electrochemical properties of different series of β-pyrrole brominated porphyrins, some of which are water-soluble [9-13, 24-26, 56, 57]. These compounds have been examined with respect to:

*Correspondence to: Karl M. Kadish, email: [email protected], fax: +1 713-743-2745, Francis DʼSouza, email: [email protected], fax: +1 316-978-3431 and Pietro Tagliatesta, email: [email protected], fax: +39 06-72594754

Journal of Porphyrins and Phthalocyanines Published at http://www.u-bourgogne.fr/jpp/

J. Porphyrins Phthalocyanines 2004; 8: 201-214

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Copyright © 2004 Society of Porphyrins & Phthalocyanines J. Porphyrins Phthalocyanines 2004; 8: 201-214

Z. OU ET AL.202

(i) the synthetic routes needed to obtain porphyrins bearing different numbers of β-pyrrole bromo substituents, (ii) their spectral behavior, (iii) their redox potentials and HOMO-LUMO energy gaps, (iv) the role played by macrocycle distortion on the site of electron transfer as determined by UV-visible and IR spectroelectrochemistry, (v) the ring basicity, (vi) the aggregation of the porphyrins, (vii) the axial ligation properties and (viii) the electrocatalytic behavior.

Three types of β-pyrrole brominated porphyrins have been reported in the literature. The first is represented as (TPPBrx)M where TPPBrx is the dianion of β-pyrrole brominated tetraphenylporphyrin, x = 0 to 8 and M = 2H, Zn, Co, Fe or Mn [9-13] (see Fig. 1). The second is (TMPyPBr8)M where TMPyPBr8 is the dianion of the tetrachloro salt of octa-β-pyrrole brominated N-methylpyridylporphyrin, M = 2H, Zn, Co, Mn or Cu and the third is (TPPSBr8)M where TPPSBr8 is the dianion of the tetrasodium salt of octa-β-pyrrole brominated (4-sulfonatophenyl)-porphyrin, M = 2H, Zn or Co [24-26] (Fig. 2). The TPPBrx derivatives have been studied in order to ascertain how the distortion of the porphyrin ring will quantitatively affect the redox and spectral properties of the porphyrin as a function of increasing the number of halogen substituents from 0 to 8.

The second series of compounds, which are water soluble, can provide information as to how the porphyrin ring basicity, aggregation, redox properties, spectroscopic properties and axial ligand binding reactions of these compounds vary as the number of Br groups at the β-pyrrole positions is increased from 0 to 8 [26, 57]. It was also observed that cobalt(II) derivatives of these water soluble porphyrins catalyze the two-electron reduction of dioxygen to hydrogen peroxide in aqueous solutions [24, 25]. Studies have also shown that manganese derivatives of the same water soluble β-pyrrole brominated porphyrins behave as potent superoxide dismutase mimics [58,

59]. The above results are summarized in the present

paper. The relationship between structure and reactivity of these highly substituted non-planar porphyrins and the effect of Br groups on CO or pyridine binding to the porphyrins are also discussed.

RESULTS AND DISCUSSION

Synthesis and characterization of β-pyrrole brominated meso-tetraphenylporphyrins

The first approach to the synthesis of β-pyrrole brominated meso-tetraphenylporphyrins was reported by Callot [60] in the case of the TPP macrocycle. This study used the free-base porphyrin as a substrate and increasing amounts of N-bromosuccinimide (NBS) as the halogen donor. The reaction, which is described on the following pages, was performed in boiling chloroform and used atmospheric oxygen as a radical initiator (see Scheme 1).

Fig. 1. Structural formula and numbering of the investigated β-pyrrole tetraphenylporphyrins

Fig. 2. Structural formula of β-pyrrole brominated water soluble porphyrins

Br

Br

Br

Br

Br

Br

N

NN

N

Br

Br

N+

N+

+N

N+

CH3

CH3

H3C

CH3

Cl-M

Cl-

Cl-

Br

Br

Br

Br

Br

N

NN

N

Br

Br

SO3-Na+

SO3-Na+

SO3-Na+

Na+ -O3S M

(TPPSBr8)M, M = 2H, Zn, Co

Cl-

(TMPyPBr8)M, M = 2H, Zn, Co, Mn or Cu

Br

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β-PYRROLE BROMINATED MESO-TETRAPHENYLPORPHYRINS 203

After chromatographic separation and purification, the mono-, bis-(two isomers), tri- and tetra β-pyrrole brominated derivatives were isolated, characterized and also used as starting materials for preparation of other β-substituted compounds and their metal derivatives [21, 61]. The β-pyrrole brominated compounds all exhibit a red shift of the Soret band of approximately 6 nm for every added bromine atom [60].

The stereochemistry of β-pyrrole brominated porphyrins was demonstrated by Crossley et al. [62] using 1H NMR spectroscopy. They showed that the two double bonds which are not directly part of the aromatic conjugation can be completely tetrabrominated because they have a higher electron density than that of the other positions in the aromatic conjugation (Scheme 1). Molecular structures of several tetrahalogenated porphyrins have also been obtained and compared to those of the octahalogenated compounds [2]. The first report on a successful β-pyrrole perbromination reaction using a tetraphenylporphyrin complex was by Traylor and Tsuchiya [14] who showed the possibility to access a new class of compounds having a lower electron density on the macrocycle rings.

The porphyrin (TDCPP)Zn, where TDCPP = the dianion of meso-tetra(2,6-dichlorophenyl)porphyrin, was claimed to undergo bromination at the eight β-pyrrole positions of the macrocycle using NBS in CCl4 (Scheme 2). The reproducibility of such a reaction was very difficult to obtain because of the low solubility of the starting compound in CCl4 and better yields (70-75%) were obtained when chloroform or tetrachloroethene were used as the reaction solvent [63].

Several papers have reported the synthesis of zinc porphyrins completely halogenated at the β-pyrrole positions, using NBS in chlorinated solvents or methanol [6, 39, 49, 64]. In some cases, different synthetic procedures using Lewis acids were proposed in order to enhance the halogenation in chlorinated solvents. The choice of procedure was based on the different mechanisms, radical or ionic, operating in polar or non-polar solvents [39, 49].

A complete bromination of (TPP)Cu was successfully made by Bhyrappa and Krishnan [4] using liquid bromine in CHCl3/CCl4 (Scheme 3). The resulting porphyrin was demetallated in perchloric acid, giving the desired free-base porphyrin in 75% yield. Similar experimental procedures, using liquid

Scheme 2.

Cl

Cl

Zn

(TDCPP)Zn

Cl

Cl

Cl

Cl

Cl

Zn

Cl

Cl

Cl

Cl

Cl

BrBr

BrBr

Cl

Cl

Cl

Br

Br

Cl

NBS

Br

Br

CCl4

(TDCPPBr8)Zn

N

N

N

N

N

N

N

N

Scheme 1.

H

(TPPBr4)H2

H

Br

HH

H

H

H

H

(TPP)H2

H

H

H

HNBS

Br

BrH

HH

H

Br

CHCl3N

N

N

N

N

N

N

N

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Z. OU ET AL.204

bromine as a reagent [34], led to other completely β-pyrrole brominated porphyrins containing iron as a central metal ion.

The partial bromination of (TPP)Zn has been achieved using increasing amounts of NBS in CCl4 [9, 12]. Chromatographic separation of the reaction products afforded the penta-, hexa-, and heptabromo derivatives. Characterization of the above free-base complexes by positive ion FAB-mass spectrometry showed the occurrence of a dehalogenation process during formation of the π-radical cation [63] and this reaction was attributed to the relative acidity of the matrix. A similar reaction was also observed during electroreduction of (TPPBrx)Co and (TPPBrx)FeCl [9, 12].

Recently, different protocols were reported for the regioselective syntheses of partially brominated porphyrins. Tse et. al. [65] utilized 5,10,15,20-tetrakis-(trifluoromethyl)-porphyrin zinc complex I, and showed the occurrence of a monobromination reaction to yield compound II, and regioselective dibromination reaction to yield compounds III and IV (Scheme 4). Subsequently, the free-base β-pyrrole bromoporphyrins were converted to aryl porphyrins through Suzuki cross-coupling reaction. Bhyrappa and coworkers [66] reported an improved protocol for regioselective synthesis of 2,3,12,13-tetrabromo-5,10,15,20-tetraphenylporphyrin, (TPPBr4)H2.

Synthesis of water soluble β-pyrrole brominated porphyrins

Water soluble β-pyrrole porphyrins can be synthesized by either direct bromination of functionalized water soluble porphyrins or by introducing ionic groups at the tetraphenyl positions of the brominated porphyrin. (TMPyPBr8)Zn was prepared by direct bromination of (TMPyP)Zn using liquid bromine [22]. The (TMPyPBr8)Co derivative was synthesized by metathesis of (TMPyPBr8)Zn with cobalt(II) chloride [25].

The free-base porphyrin was isolated in its protonated form, (TMPyPBr8)H3

+, by demetallating the zinc(II) derivative, (TMPyPBr8)Zn, and then isolating the product as a hexafluorophosphate salt [22, 24].

(TPPSBr8)Zn was synthesized by direct bromination of (TPPS)Zn using an approach similar to the one adopted for the synthesis of (TMPyPBr8)Zn [25]. The free-base derivative, (TPPSBr8)H2, was obtained by demetallating (TPPSBr8)Zn in excess sulfuric acid at 0 °C, followed by the slow addition and neutralization of the reaction product with 1 M NaOH [26]. Alternatively, Tabata et al. [23] reported the synthesis of (TPPSBr8)H2 by bromination of (TPP)H2 using N-bromosuccinamide, followed by sulfonation using concentrated sulfuric acid at 90 °C for 2 days. Here, the product was isolated in the tetrasulfonic acid form.

Two cationic Mn(II) and Cu(II) metal derivatives of octabrominated 5,10,15,20-tetrakis-(N-methyl-pyridinium-4-yl) porphyrins, (TMPyPBr8)MnII, and (TMPyPBr8)CuII, were prepared, [58] using the literature procedure for bromination of a water soluble porphyrin [22]. Unlike the TMPyP derivative, where Mn3+ is stable, the (TMPyPBr8)Mn derivative was isolated in its Mn2+ form. Another anionic porphyrin, Mn(III) β-octabromo-meso-tetrakis(4-carboxyphenyl)porphyrin, (TBAPBr8)MnIII, was prepared starting from the methyl ester derivative of the free-base β-octabromo-meso-tetrakis(4-carboxyphenyl)porphyrin, TBAPBr8, with an overall yield of 50% [59].

Scheme 3.

H

H

H

(TPP)Cu

H

H

HH

Cu

H

HClO4

Br

Br

Br

Br

(TPPBr8)H2

HH

Br

Br

Br

Br

N

N

N

N

N

N

N

NBr2/CCl4/CHCl3

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Oxidation of organic substrates mediated by manganese and iron β-pyrrole brominated tetraphenylporphyrins

The so-called third generation of metalloporphyrin catalysts for oxygenation of organic substrates, the β-pyrrole halogenated complexes, has received much interest due to the pioneering work of several groups around the world [14, 39, 49, 62, 63]. Several structures of this kind of tetrapyrrolic macrocycle have also been elucidated by X-ray diffraction studies [2, 6] showing, in the fully halogenated compounds, a severe distortion of the ring due to steric interactions between the halogens and the phenyl groups. The saddle-shaped conformations have been related to a non-linear dependence of the oxidation potentials on the number of the halogen atoms [2, 6, 11].

The first report on the catalytic activity of β-pyrrole brominated porphyrins was by Traylor and Tsuchiya [14] who showed an improved yield for the hydroxylation reactions and a good chemical stability of the catalyst, which was (TDCPPBr8)FeIIICl, where TDCPPBr8 is the dianion of 2,3,7,8,12,13,17,18-octabromo-meso- tetra(2,6-dichlorophenyl)-porphyrin. However, (TMPBr8)MnIIICl, where TMPBr8 is the dianion of 2,3,7,8,12,13,17,18-octabromo-meso-tetra-(2,4,6-trimethylphenyl)-porphyrin, was found to be a bad catalyst for the oxidation of organic substrates when H2O2 was used but the compound gave good results using KHSO5 or magnesium monoperoxyphtalate as the oxygen source [63]. On the other hand, (TDCPBr8)MnIIICl, when used for the oxidation of aromatic substrates like anisole by H2O2, showed better results. The final products of the reaction are 2 and/or 4-methoxy phenol [20].

Boschi and coworkers [67] showed that β-

tetrabromination of the porphyrin rings increased yields of the H2O2 dependent oxidation of cyclooctene and adamantane with respect to the octabrominated ones. They attributed this observation to a better heterolytic cleavage of the peroxyl bond during preliminary formation of the reactive metal-oxo complex.

Some iron porphyrins of the third generation are also able to catalyze the O2 dependent oxidation of alkanes, even at room temperature [34, 68]. (TF5PPBr8)FeIIICl, where TF5PPBr8 is the dianion of 2,3,7,8,12,13,17,18-octabromo, meso-tetra(2,3,4,5,6-pentafluorophenyl)-porphyrin, is able to oxidize isobutane to t-BuOH with 91% selectivity and 890 turnovers.

Effect of the Br substituents on the reductions of (TPPBrx)MCl and (TPPBrx)M (where M = Fe(III), Mn(III), Co(II) or Zn(II))

The reductions of (TPPBrx)MCl and (TPPBrx)M, where x = 0–8, TPPBrx = the dianion of β-pyrrole brominated tetraphenylporphyrin and M = Fe, Mn, Co or Zn, have been investigated in benzonitrile containing 0.1 M tetra-n-butylammonium perchlorate as supporting electrolyte. The first reduction and first oxidation potentials of these compounds are summarized in Table 1. Further information on the redox processes and electron-transfer mechanisms can be found in the original references [9, 11, 12, 69, 70].

The first reduction of (TPPBrx)FeCl, (TPPBrx)MnCl and (TPPBrx)Co are all metal-centered, but the first reduction of (TPPBrx)Zn is porphyrin ring-centered. In each case, the half wave potentials for these reductions shift in a positive direction with increase in the number of Br substituents and a single linear free-energy relationship can be observed between

Table 1. First oxidation and first reduction potentials (E1/2, V vs SCE) of (TPPBrx)M (where M = FeCl, MnCl, Co or Zn) in PhCN containing 0.1 M tetra-n-butylammonium perchlorate (TBAP)

Macrocycle (P)

Oxidation Reduction

(P)FeCl (P)MnCla (P)Cob (P)Zn (P)FeCl (P)MnCl (P)Co (P)Zn

TPPBr0 1.20 1.17 0.90 0.82 -0.29 -0.29 -0.84 -1.32

TPPBr1 1.24 -- 0.91 0.88 -0.26 -- -0.79 -1.25

TPPBr2 1.28 1.26 0.92 0.91 -0.18 -0.20 -0.73 -1.19

TPPBr3 1.27 1.29 0.94 0.93 -0.13 -0.09 -0.67 -1.10

TPPBr4 1.26 1.30 0.95 0.95 -0.07 -0.06 -0.61 -1.03

TPPBr5 1.26 -- 0.99 0.96 -0.02 -- -0.52 -0.97

TPPBr6 1.24 1.37 0.99 0.96 0.04 0.02 -0.46 -0.93

TPPBr7 1.21 1.36 0.97 0.97 0.06 0.05 -0.42 -0.92

TPPBr8 1.19 1.38 0.98 0.96 0.10 0.09 -0.37 -0.82

a In CH2Cl2, 0.1 M TBAP. b In CH2Cl2, 0.2 M TBAPF6

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Z. OU ET AL.206

E1/2 and the number of Br groups. The slope of the plot of E1/2 versus number of Br groups is 51 mV for (TPPBrx)FeCl, 47 mV for (TPPBrx)MnCl and 61 mV for both (TPPBrx)Co and (TPPBrx)Zn (see Fig. 3). The slopes of these plots for each complex in different solvents are summarized in Table 2. Theory predicts that the porphyrin ring deformation should have a minimum effect on the reduction potentials [11, 12] and hence, the observed linear trend for all four series of complexes in Fig. 3 can be simply explained as due to an inductive effect of the electron-withdrawing halogen substituents.

The first reduction of (TPPBrx)FeCl in PhCN results in an equilibrium mixture of [(TPPBrx)FeIICl]- and (TPPBrx)FeII [12, 71]. The UV-visible spectral data suggest that a dissociation of the Cl- ligand from [(TPPBrx)FeIICl]- is favored for compounds with zero to four Br groups and occurs rapidly for complexes with 0 or 1 Br group. However, the dominant species in solution is [(TPPBrx)FeIICl]- for compounds with six to eight Br groups. Spectroelectrochemical and ESR data indicate that the second reduction of (TPPBrx)FeCl leads to a loss of the chloride axial ligand and [(TPPBrx)FeI]- is then the only porphyrin present in solution [12]. The third electroreduction of (TPPBrx)FeCl has been assigned as conversion of an iron(I) porphyrins to give an Fe(I) porphyrin π-anion radical [12].

The first reductions of (TPPBrx)MnIIICl are all

reversible. Spectroelectrochemical data for these compounds indicates that Mn(II) porphyrins are formed after the first reduction in PhCN.

The first reduction of (TPPBrx)CoII is reversible and [(TPPBrx)CoI]- is generated after the first reduction. A further reduction leads to an elimination of Br-, thus giving a series of [(TPPBrx)CoI]- complexes with smaller and smaller values of x and, ultimately (TPP)CoI as the final porphyrin product in PhCN. These results were confirmed by experiments carried out at a rotating ring disk electrode and thin-layer spectroelectrochemistry [9].

Two well-defined reductions are observed for (TPPBrx)Zn. The potentials for both reductions are all progressively shifted in a more positive direction with increase in the number of Br substituents. A linear relationship is seen between E1/2 for the first reduction and the number of Br groups, but two linear segments are seen in the case of the second reduction [69]. The absolute potential difference between the two reductions of (TPPBrx)Zn in PhCN decreases from 0.42 to 0.32 V with increase of the number of Br groups.

Effect of the Br substituents on oxidations

The (TPPBrx)FeCl [11], (TPPBrx)MnCl and (TPPBrx)Zn [69] derivatives all undergo two rever-sible one-electron oxidations while the (TPPBrx)Co [9, 70] complexes undergo three one-electron oxidations in PhCN or CH2Cl2 containing 0.1 M TBAP. The porphyrin ring distortion of the brominated compounds results in a decreased stability of the

Table 2. Slope of first reduction potentials vs number of Br groups for β-pyrrole brominated porphyrins in different solvents

Compound #Br Solvent ΔE1/2/ΔBr (mV)

(TPPBrx)Co 0 - 8 CH2Cl2 64

(TPPBrx)Co 0 - 8 PhCN 61

(TPPBrx)Co 0 - 8 py 59

(TPPBrx)Co(Ph)a 0, 4, 8 PhCN 61

(TPPBrx)Zn 0 - 8 CH2Cl2 59

(TPPBrx)Zn 0 - 8 PhCN 61

(TPPBrx)Zn 0 - 8 py 69

(TF5PPBrx)Znb 0, 8 CH2Cl2 59

(TPPBrx)FeClc 0 - 8 PhCN 51

(TMPBrx)FeCl 0, 8 CH2Cl2 50

(TPPBrx)MnCla 0 - 8 PhCN 55

(TPPBrx)MnCla 0 - 8 CH2Cl2 47

(TPPBrx)MnCla 0, 8 py 46

a Unpublished results of Kadishʼs lab. b Value calculated according to the data taken from Hodge JA, Hill MG and Gray HB. Inorg. Chem. 1995; 34: 809. c Reference 11.

Fig. 3. Plots of the first reduction potentials versus the number of Br groups on the macrocycle of (TPPBrx)FeCl (O), (TPPBrx)M, where M = Co (Δ) or Zn (x) in PhCN and (TPPBrx)MnCl (•) in CH2Cl2 containing 0.1 M TBAP

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HOMO level and has a large effect on the oxidations and thus a nonlinear relationship is observed between E1/2 for the first oxidation and the number of Br groups on these compounds (see Fig. 4).

Each (TPPBrx)FeCl complex undergoes two reversible one-electron oxidations [12] leading stepwise to an iron(III) π-cation radical and dication in PhCN containing 0.1 M TBAP (see reference 12 and Table 1 for the first oxidation potential). The first oxidation potential initially shifts in the expected anodic direction and reaches a maximum positive value for (TPPBr2)FeCl, after which E1/2 changes direction and then shifts negatively upon going from x = 3 to x = 8. In contrast, the second oxidation of compounds in the (TPPBrx)FeCl series

shifts negatively upon going from x = 0 to x = 2 and then shifts positively from x = 3 to x = 8; thus, the absolute potential difference (ΔE1/2) between the two ring-centered oxidations varies with the number of Br groups on the macrocycle and increases linearly from 230 mV in the case of (TPPBr2)FeCl to 450 mV in the case of (TPPBr8)FeCl [12].

(TPP)MnCl undergoes two one-electron oxidations in CH2Cl2 or PhCN, 0.1 M TBAP. The first oxidation of (TPP)MnCl leads to formation of a Mn(III) π-cation radical at room temperature [1b], but Mn(IV) is formed at low temperature [72]. The same assignment is proposed for each (TPPBrx)MnCl complex, based on UV-vis and ESR spectra of the singly oxidized species. Figure 5 shows ESR spectra of (TPPBr3)MnCl and (TPPBr7)MnCl after bulk electrolysis at -70 °C in CH2Cl2 containing 0.1 M TBAP. The g value suggests formation of a Mn(IV) porphyrin after the first oxidation of (TPPBrx)MnCl under the given experimental conditions. The first oxidation potential of (TPPBrx)MnCl in CH2Cl2, 0.1 M TBAP is not linearly related to the number of Br groups but the first oxidation shifts positively upon going from (TPP)MnCl to (TPPBr8)MnCl as shown in Fig. 4.

Three one-electron oxidations are observed for (TPPBrx)Co in PhCN containing 0.1 M TBAP. The first oxidation of cobalt(II) porphyrins can occur at either the metal center or the conjugated macrocycle depending upon the strength of the counteranion

Fig. 4. Plots of the first oxidation potentials vs the number of Br groups on the macrocycle of (TPPBrx)FeCl, (TPPBrx)Zn in PhCN , 0.1 M TBAP, (TPPBrx)MnCl in CH2Cl2, 0.1 M TBAP and (TPPBrx)Co in CH2Cl2, 0.2 M TBAPF6

Fig. 5. ESR spectra obtained at 130 K after the first one-electron oxidation of a) (TPPBr3)MnCl at 1.30 V and b) (TPPBr7)MnCl at 1.40 V in CH2Cl2, 0.1 M TBAP at -70 °C

a) (TPPBr3)MnCl

b) (TPPBr7)MnCl

g = 4.25

g = 4.28

500 1500 2500 3500 4500 5500Magnetic Field (G)

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and/or the presence of any coordinating ligands in solution [73] and a differentiation between these two processes can be made on the basis of IR or UV-visible spectroscopic data [74-80]. A Co(III) complex is formed after the first oxidation of (TPP)Co in PhCN [73] and this is also the case after the abstraction of one electron from (TPPBrx)Co in the same solvent. The second and third oxidations are macrocycle-centered, leading to a Co(III) porphyrin π-cation radical and dication [9].

Three oxidations are also seen for (TPPBrx)Co in CH2Cl2 containing 0.2 M TBAPF6 [70]. The first oxidation linearly shifts in a positive direction for compounds with x = 0-5 but shifts in a slightly negative direction for compounds with 6-8 Br groups. The second and third oxidations both shift in a positive direction as the number of Br groups on the macrocycle are increased and there is an increased substituent effect for compounds with 6 to 8 Br groups. Consequently, the absolute potential difference between the first two oxidations of (TPPBrx)Co remains virtually the same upon going from (TPP)Co to (TPPBr5)Co but increases by 120 mV upon going from (TPPBr5)Co to (TPPBr8)Co. Likewise, the potential separation between the second and third oxidations of (TPPBrx)Co decreases from 200 mV in the case of (TPP)Co to 140 mV in the case of (TPPBr8)Co.

Two linear segments are seen in plots of E1/2 for the first oxidation of (TPPBrx)Co in CH2Cl2, 0.2 M TBAPF6 vs the number of Br groups on the porphyrin (Fig. 4). The slope of the first segment is 17 mV (for (TPPBrx)Co when x = 0-5) while that of the second is -5 mV (x = 5-8). The electrochemical data suggest that cobalt porphyrins with >5 Br groups are oxidized at the porphyrin π-ring system; hence, the same substituent effect is observed for oxidation of (TPPBrx)FeCl with x = 2-5 and (TPPBrx)Co with x = 6-8. The spectroelectrochemical data [70] are consistent with the electrochemical data and suggest that the initial site of electron transfer for the first oxidation of (TPPBrx)Co in CH2Cl2 containing 0.2 M TBAPF6 shifts from the central metal to the macrocycle as x is increased above 5.

The shift in E1/2 as a function of Br groups for first oxidation of (TPPBrx)Zn in PhCN, 0.1 M TBAP also consists of two segments (Fig. 4); the first comprises compounds with 0-4 Br groups and second those with 4-8 Br groups. This result parallels the spectroscopic data (see next section) and is similar for Fe, Mn and Co porphyrins which have the same set of β-pyrrole Br-substituted macrocycles. The two one-electron oxidations of (TPPBrx)Zn are all porphyrin ring- centered and lead to formation of π-cation radicals and dications. The second oxidation of (TPPBrx)Zn shifts in negative direction with increase in the number of Br substituents but this reaction exhibits

a quite small substituent effect of the Br groups. The absolute potential difference between the two oxidations of (TPPBrx)Zn in PhCN decreases with increase in the number of Br groups [69]. The result is similar to what is observed for (TPPBrx)Co in the same solvent containing 0.1 M TBAP, where the potential separation between the second and third oxidations were shown to decrease with increase in the number of Br substituents [9]. The HOMO-LUMO gap of (TPPBrx)Zn also decreases with increase of the number of Br groups. The HOMO-LUMO gap in PhCN is 2.14 V when x = 0 but decreases to 1.78 V when x = 8 [69]. The electrochemical results parallel data from theoretical calculations using the AM1-CI method and are also consistent with earlier calculations performed by other groups on nonplanar porphyrins [81-83].

Effect of the Br substituents on the UV-visible spectra

The Soret band position of each Fe, Mn, Co and Zn β-pyrrole brominated porphyrin in different solvents is given in Table 3. The Soret bands in each series of compounds are all red-shifted with increase in the number of Br groups on the macrocycle and this has been interpreted in terms of configurational interactions [4, 9].

The Soret band of singly oxidized (TPPBrx)FeCl in PhCN decreases in intensity upon the first oxidation as compared to neutral (TPPBrx)FeCl and there is also a blue shift of 1-14 nm in the bands, with the exact magnitude depending upon the number of Br groups on the macrocycle. This result indicates that the first oxidation of each (TPPBrx)FeCl complex is porphyrin ring-centered. The Soret band of reduced (TPPBrx)FeCl is red-shifted as compared to the neutral complexes. The magnitude of the shift ranges from 13 nm for (TPP)FeCl to 22 nm for (TPPBr7)FeCl. There is also an increase in intensity of the Soret band upon the first reduction of (TPPBrx)FeCl to give an Fe(II) complex. The Soret band of neutral (TPPBrx)FeCl in PhCN shifts by 44 nm as the number of Br groups is increased from 0 to 7. However, the Soret bands of reduced and oxidized (TPPBrx)FeCl shift to the red by 53 and 29 nm, respectively [12].

These results

indicate that the effect of Br substituents on the spectral data depends upon the charge of the porphyrin and follows the order: (TPPBrx)FeII > (TPPBrx)FeIIICl > [(TPPBrx)FeIIICl]+..

Plots of the Soret band energy vs the number of Br substituents on (TPPBrx)Co in three solvents are given in Fig. 6. As seen in this figure, the Soret band energy of (TPPBrx)Co decreased, meaning a red shift upon increasing the number of Br groups in the complex.

The Soret band of the singly reduced or singly

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oxidized cobalt is also red-shifted upon going from (TPP)Co to (TPPBrx)Co in PhCN containing 0.1 M TBAP. However, the magnitude of the Br substituent effect depends upon the oxidation state of the cobalt ion. For Co(II) porphyrins, the Soret band shifts by 41 nm upon going from the TPP to TPPBr8 as the macrocycle, but the Soret bands of the Co(I) and Co(III) porphyrins shift by 36 and 39 nm, respectively, when the number of Br groups is increased from 0 to 8 [9].

The UV-visible spectrum of singly oxidized (TPPBrx)Co (x = 6-8) in CH2Cl2 containing 0.2 M TBAPF6 differs from the spectrum of the same singly oxidized compounds in PhCN containing 0.1 M TBAP [70]. The spectra of the singly oxidized

products in CH2Cl2 containing 0.2 M TBAPF6 exhibit broad, low-intensity Soret bands along with broad visible bands. These kind of spectra have been assigned as representing Co(II) porphyrin π-cation radicals. However, the spectra in PhCN containing 0.1 M TBAP are characterized by a sharp Soret band with well-defined visible bands and were assigned to a cobalt(III) complex with an uncharged porphyrin ring [9].

Semiempirical AM1-CI calculations [69] suggest that the Soret and visible absorption bands of (TPPBrx)Zn are highly sensitive to distortion of the porphyrin caused by shifts in energy of the Soret band as a function of the number of Br groups on the macrocycle in three different solvents. Plots between the energy of the Soret band and the number of Br groups show two linear segments with different slopes in CH2Cl2, PhCN or pyridine (Fig. 7). The energy change per added Br substituent is larger for those compounds with x = 4-8 than for those with x = 0-4 (in PhCN) and this result agrees with the predictions of AM1-CI = 4 calculations [69].

Electrochemical behavior of water soluble β-pyrrole brominated porphyrins

The metal-centered redox reactions of (TMPyP)-MnIII and (TMPyPBr8)MnII are reversible while the majority of the porphyrin ring-centered redox reactions for the free-base, Cu and Mn TMPyPBr8 derivatives are irreversible [58]. The metal centered redox reaction of (TMPyPBr8)MnII is anodically shifted by ~420 mV compared to (TMPyP)MnIII as a result of β-pyrrole bromination (Table 4). The metal-centered reduction of the negatively charged water soluble (TBAPBr8)MnIII is located at 0.13 V vs SHE and can be compared with a value of -0.19 V vs SHE of (TBAP)MnIII [59]. Thus a shift of ~320 mV is observed as a result of the eight bromine substituents

Table 3. Location of the Soret band (λmax, nm) of (TPPBrx)MCl and (TPPBrx)M complexes in different solvents

#BrFeCl Co Zn MnCl

CHCl3 CH2Cl2 CH2Cl2 PhCN py CH2Cl2 PhCN py PhCN

0 417 417 410 416 412 419 427 430 479

1 418 418 413 419 416 421 430 433 --

2 424 422 415 421 418 423 434 435 479

3 427 428 419 427 428 435 438 439 489

4 432 432 426 434 433 438 439 442 494

5 436 432 430 436 437 -- 446 446 --

6 446 448 437 446 446 458 461 462 --

7 451 452 441 450 450 -- 471 472 502

8 461 460 445 453 453 476 477 481 505

Fig. 6. Plots of the energy of Soret bands vs the number of Br groups on the macrocycle of (TPPBrx)Co in CH2Cl2 (•), in pyridine (O) and in PhCN (Δ)

(TPPBrx)Co

Number of Br

Ener

gy o

f Sor

et B

and

(eV

)

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Z. OU ET AL.210

at the β-pyrrole positions of the porphyrin macrocycle. Interestingly, the reduction potential of (TBAPBr8)MnIII is slightly lower than that reported for the positively charged (TMPyPBr8)MnII (Table 4).

A detailed investigation of the electrochemical and spectroelectrochemical characterization of two water-soluble cobalt porphyrins, (TMPyPBr8)CoII and (TPPSBr8)CoII, was carried out under different pH conditions [57]. As predicted, the investigated nonplanar porphyrins were found to be electron-deficient owing to the presence of the eight Br groups at the β-pyrrole positions. The redox potentials corresponding to the first oxidation and first reduction were all shifted positively as compared with the redox potentials of their respective unbrominated porphyrin derivatives (Table 5) and such changes were found to be larger for the reduction process, a trend that generally agrees well with the other TPPBr8 derivatives studied in nonaqueous solutions. The peak potentials of the

first oxidation of both examined compounds were pH-dependent (Fig. 8).

Spectroelectrochemical studies revealed the formation of a cobalt(III) complex during the first oxidation. Interestingly, the first reduction of (TPPSBr8)CoII resulted in the formation of a cobalt(I) complex [57], while involvement of the peripheral N-methylpyridinium groups was suggested in the case of (TMPyPBr8)CoII [57]. A debromination of the β-pyrrole Br substituents of (TMPyPBr8)Co was observed at more negative potentials [57]. The UV-visible spectrum obtained after bulk electrolysis of (TMPyPBr8)Co at -1.0 V vs Ag/AgCl followed by reoxidation at 0.2 V to neutralize the reduced product indicated complete elimination of the Br substituents from the porphyrin periphery, thus giving (TMPyP)Co as a final product. This observation, along with that discussed earlier for (TPPBrx)Co in benzonitrile [9], collectively suggest that the brominated cobalt porphyrins undergo debromination reactions upon adding a second electron to the porphyrin moiety.

Effect of the Br substituents on CO or pyridine binding to Fe(II), Co(III) or Co(II) complexes

Figure 9a shows the FTIR spectra obtained after the

Table 4. Redox potentials (V vs NHE) for reductions of water soluble porphyrins in aqueous media

PorphyrinRing reduction Metal centered

ReferenceEpc1 Epc2 E1/2

(TMPyP)H2 -0.33 -1.14 -- [58]

(TMPyPBr8)H2 -0.01 -1.09 -- [58]

(TMPyP)MnIII -0.51 -1.19 0.06 [58]

(TMPyPBr8)MnII -0.36 -1.11 0.48 [58]

(TMPyP)CuII -0.48 -0.63 -- [58]

(TMPyPBr8)CuII -0.13 -0.35 -- [58]

(TBAP)MnIII -- -- -0.19 [84]

(TBAPBr8)MnIII -- -- 0.13 [59, 84]

Fig. 7. Plots of Soret band energy (eV) vs the number of Br groups of (TPPBrx)Zn in three different solvents

(TPPBrx)Zn

a) in CH2Cl2

c) in pyridine

b) in PhCN

3.0

2.8

2.6

3.0

2.8

2.6

3.0

2.8

2.6

0 1 2 3 4 5 6 7 8Number of Br

Ener

gy o

f Sor

et B

and

(eV

)

Table 5. Redox potentials (V vs Ag/AgCl) for the first oxidation and first reduction of brominated cobalt porphyrins in phosphate buffer solution (pH = 7) containing 0.1 M NaCla

Porphyrin Oxidationb Reduction

(TMPyP)CoII 0.45 -0.68

(TMPyPBr8)CoII 0.53 -0.31

(TPPS)CoII 0.35 -0.90

(TPPSBr8)CoII 0.50 -0.49

a From ref 57. b Epa at a scan rate of 0.1 V/s.

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first reduction of (TPPBrx)FeCl (x = 0-8) in CH2Cl2 containing 0.2 M TBAP under a CO atmosphere. The spectra of the compounds with zero to six Br groups show two IR bands at 1962-2043 cm-1 whose relative intensity depends on the number of Br groups. Under the same solution conditions, the spectrum of the reduced (TPPBr7)FeCl exhibits only a single band at 1980 cm-1 and no band is seen for the spectrum of reduced (TPPBr8)FeCl. The bands located between 1962 and 1980 cm-1 in Fig. 9a arise from the νCO of a mono-CO adduct while the higher energy bands, located between 2028 and 2043 cm-1, are assigned to the asymmetric stretches of a bis-CO adduct. The peak intensity corresponding to the bis-CO adduct first decreases in intensity and vanishes completely for Br7 and Br8 porphyrin derivatives. For the Br8 derivative, no evidence for either a mono- or bis-CO adduct was observed. This result indicates that both distortion and electron deficiency of the porphyrin macrocycle caused by the increase of the number Br substituents on the complex will affect the CO binding reactions [12].

Figure 9b illustrates the FTIR spectra of [(TPPBrx)CoIII]+ in CH2Cl2 containing 0.2 M TBAPF6 [70]. The [(TPPBrx)CoIII]+ derivatives with zero to five Br groups on the macrocycle show two IR bands between 2109 to 2148 cm-1. The higher energy band at 2144-2148 cm-1 is assigned to the bis-CO adduct, [(TPPBrx)Co(CO)2]+, while that at 2109-2112 cm-1 is assigned to the mono-CO adduct, [(TPPBrx)Co(CO)]+. The derivatives having six to eight Br groups show no IR bands at all between 2100-2200 cm-1 (Fig. 9b). This result indicates that axial ligand binding of CO is affected by the nonplanarity of the macrocycle which, in this case, switches the site of electron transfer from the metal ion to the porphyrin ring.

A pyridine titration was carried out for (TPPBrx)Co and (TPPBrx)Zn in CH2Cl2. The first was monitored by electrochemical methods and the second by UV-visible spectroscopy [85]. Only a mono-pyridine adduct was formed for both series of complexes under our experimental conditions and the measured pyridine binding constants are summarized in Table 6. The log K values for the (TPPBrx)Co complexes suggest that porphyrin ring distortion affects the pyridine binding reactions. From Fig. 10, it is seen that the pyridine binding constants for (TPPBrx)Co show a non-linear relationship with the number of Br groups on the macrocycle and this is consistent Fig. 8. Cyclic voltammograms at different pH, indicated

above each curve, for (TPPSBr8)Co (~0.25 mM). Scan rate = 0.1 V/s

Fig. 9. FTIR spectra of (TPPBrx)FeCl [12] after first reduction in CH2Cl2 containing 0.1 M TBAP and (TPPBrx)Co [70] after first oxidation in CH2Cl2 containing 0.2 M TBAPF6 under a CO atmosphere

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Z. OU ET AL.212

with the non-linear relationship between the first oxidation potential of (TPPBrx)Co and the number of Br groups on the macrocycle (see Fig. 4). However, a linear relationship is seen between log K for pyridine binding and the number of Br groups on the macrocycle of (TPPBrx)Zn (Fig. 10) [85]. These results indicate that distortion of the porphyrin ring has almost no effect on the pyridine binding constants of the Zn(II) porphyrins.

Acknowledgements

The support of the Robert A. Welch Foundation

(Grand E-680, K. M. K.), the Petroleum Research Funds administered by the American Chemical Society and National Institutes of Health (to FD) is gratefully acknowledged. The authors also thank Dr. Eric Van Caemelbecke for his helpful comments.

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Table 6. The data of CO stretching frequencies and pyridine binding constants for (TPPBrx)FeCl, (TPPBrx)Co or (TPPBrx)Zn in CH2Cl2

#BrCO binding, νco (cm-1) py binding (log K)

Fe(II) Co(III) Co(II)a Zn(II)b

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8 no band no band 3.54 5.56

a Data obtained by electrochemical titration method. b Data obtained by UV-vis spectral titration method.

Fig. 10. Plots of pyridine binding constants vs the number of Br groups on the macrocycle of (TPPBrx)Co in CH2Cl2 containing 0.1 M TBAP and (TPPBrx) in CH2Cl2

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β-Pyrrole brominated meso -tetraphenylporphyrins: synthesis, spectral...

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