Intermediate species generated from halogenated manganese porphyrins electrochemically and in homogeneous catalysis of alkane oxidation

Applied Catalysis A: General 308 (2006) 172–181 www.elsevier.com/locate/apcata

Intermediate species generated from halogenated manganese porphyrins electrochemically and in homogeneous catalysis of alkane oxidation Geraldo Roberto Friedermann a, Matilte Halma a, Kelly Aparecida Dias de Freitas Castro a, Fla´vio Luiz Benedito a, Fa´bio Gorzoni Doro b, Sueli Maria Drechsel a, Antonio Salvio Mangrich a, Marilda das Dores Assis b, Shirley Nakagaki a,* a

Laborato´rio de Bioinorgaˆnica e Cata´lise, Departamento de Quı´mica, Universidade Federal do Parana´ (UFPR), P.O. Box 19081, 81531-990 Curitiba, PR, Brazil b Universidade de Sa˜o Paulo-Ribeira˜o Preto (F.F.C.L-RP/USP), SP, Brazil Received 14 December 2005; received in revised form 17 April 2006; accepted 18 April 2006

Abstract This paper reports the intermediate species of six different manganese porphyrins of first, second and third generation obtained by electrolysis and homogeneous chemical oxidation catalysis of cyclohexane. In order to understand some of the electrochemical processes and transitions involved, studies focused on coupling techniques involving electro-generated species and UV–vis and EPR spectroscopy methods applied to characterize the species formed in situ. Homogenous catalysis for cyclohexane oxidation showed that Mn-second-generation porphyrins are better catalysts than the other classes, and also have the most catalyst recovery, with lower destruction of the catalyst during the run. For all porphyrins, it was possible to observe various intermediate species such as Mn(II), Mn(IV) and radicals. Only for the third-generation porphyrin was it possible to observe a radical species during electro-reductive procedures with SEC–EPR spectroscopy. # 2006 Elsevier B.V. All rights reserved. Keywords: Porphyrin; EPR; Electrochemistry; Oxidation; Catalysis

1. Introduction A number of synthetic metallomacrocycles such as manganese and iron(III) complexes of porphyrins, tetraazaannulenes and phtalocyanines have been studied as catalysts for oxidation reactions of organic substrates [1–3]. At first, the similarity between the synthetic metallotetraaza compounds and the active center of many natural enzymes, along with the high reactivity and selectivity of the latter species toward oxidation of organic substrates, stimulated the use of these metallocomplexes as models for different heme biological systems such as lignin peroxidase horseradish peroxidase, cytochrome P-450 and other heme proteins [2–4]. In this context, different metalloporphyrins, such as the socalled first-, second- and third-generation of these complex families, have been synthesized and used as catalysts in

* Corresponding author. Tel.: +55 41 361 3180; fax: +55 41 361 3186. E-mail address: [email protected] (S. Nakagaki). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.04.021

oxidation reactions for a large number of organic substrates showing selectivity and efficiency for producing alcohol from alkane and epoxide from olefin under mild conditions [3,5]. Different generations of porphyrin macrocycles were developed in order to prevent m-oxo-dimer formation and to create robust catalysts. These resulted from the presence of electronwithdrawing and/or bulky substituents on the aryl groups in the meso position of the porphyrin ring (second-generation) and attached to tetrapyrrolic b-macrocyclic position ring (thirdgeneration). These modified porphyrins are able to resist the harsh oxidative conditions that the metallocomplex experiences during the catalysis reaction [2]. The understanding of the robustness of porphyrin against oxidative degradation and consequently loss of catalytic activity is related to the oxidation potentials of these molecules in the absence of a substrate [6]. The study of the intermediate species, observed during the catalytic process can help in understanding the mechanism of the biological process and also allow researchers to model and build new and more efficient catalysts by knowing the

G.R. Friedermann et al. / Applied Catalysis A: General 308 (2006) 172–181

Fig. 1. Representative structure of the manganese porphyrins (MnPor) used in this work. (1) Mn(III)(TPP)Cl (Rb = H, R1–4 groups = P = phenyl); (2) Mn(III)(TDCPP) acetate (Rb = H, R1–4 groups = DCP = 2,6-dichlorophenyl); (3) Mn(III)(TCFPP)acetate (Rb = H, R1–4 groups = CFP = 2-chloro,6-fluorophenyl); (4) Mn(III)(TDFPP)acetate (Rb = H, R1–4 groups = DFP = 2,6-difluorophenyl); (5) Mn(III)(TPFPP) acetate (Rb = H, R1–4 groups = PFP = pentafluoro-phenyl) and (6) Mn(II)(PFPTDCPCl8P) (Rb = Cl and R1– 3 = DCP = 2,6-dichlorophenyl and R4 = PFP = pentafluoro-phenyl).

correlation between the modified porphyrin structures and the physicochemical properties. In this paper the species formed during the chemical oxidation processes of cyclohexane and also during the electrochemical redox processes of six different manganese porphyrins from the first-, Mn(TPP), second-, Mn(TDCPP), Mn(TDFPP), Mn(TCFPP) Mn(TPFPP) and third-generation, Mn(PFPTDCPCl8P), were studied by the association of UV– vis, EPR and electrochemical techniques (Fig. 1). 2. Experimental 2.1. Chemicals All reagents and solvents were of analytical grade and used without further purification. 2.2. Syntheses 2.2.1. Iodosylbenzene (PhIO) The oxidant was prepared as previously described [7–9]. It was obtained through the hydrolysis of iodosylbenzenediacetate following the methods described by Sharefkin and Saltzmann [10,11]. The purity was measured by iodometric assay.

173

2.2.2. Porphyrins and manganese porphyrins The manganese porphyrins 1–5 were synthesized and characterized as previously communicated according to procedures from the literature [12,13]. The metal insertion was carried out according to the method of Adler using chloride and acetate manganese II salts [14]. The metallopophyrins: (1) Mn(TPP)(5,10,15,20-tetraphenylporphyrinato) manganese(III) chloride; (2) Mn(TDCPP) [5,10,15,20-tetra(2,6-dichlorophenyl) porphyrinato] manganese(III) acetate; (3) Mn(TCFPP) [5,10,15,20-tetra(2-chloro,6-fluorophenyl) porphyrinato] manganese(III) acetate; (4) Mn(TDFPP) [5,10, 15,20-tetra(2,6difluorophenyl)porphyrinato]manganese(III) acetate, and (5) Mn(TPFPP) (5,10,15,20-tetrapentafluorophenylporphyrinato) manganese(III) acetate were characterized by UV–vis (acetonitrile) and infrared spectroscopy. (1) UV–vis data: l, nm (e, 10
3 mol
1 L cm
1) 372 (47), 474 (83), 580 (8.7), 616 (9.7); FTIR data cm
1 (attribution) 3056 (phenyl CH str), 2922 (pyrrole CH asy-str), 2852 (pyrrole CH sym-str), 1620 (ring str), 1597 (CH CH str), 1487 (Ring str), 1441 (R0 CH2R00 def), 1342 (CN str), 1202, 1072 and 1009 (CH bend in ring plane), 802 (CH CH trans out-of-plane def), 752 (CR0 R00 CHR CH out-of-plane def), 702 (CH CH cis CH out-of-plane def), 664 and 453 (phenyl CH def) 521 (pyrrole CH def). (2) UV–vis data: l, nm (e, 10
3 mol
1 L cm
1) 366 (36), 468 (92), 572 (8.4); FTIR data cm
1 (attribution) 3080 (phenyl CH str), 2922 (pyrrole CH asystr), 2851 (pyrrole CH sym-str), 1653 (ring str) 1558 (CH CH str), 1429 (R0 CH2R00 def), 1335 (CN str), 1192, 1074 and 1011 (CH bend in ring plane), 806 (CH CH trans out-of-plane def), 779 (CR0 R00 CHR CH out-of-plane def), 721 (CH CH cis CH out-of-plane def), 669 and 434 (phenyl CH def) 484 (pyrrole CH def). (3) UV–vis data: l, nm (e, 10
3 mol
1 L cm
1) 366 (43), 464 (55), 562 (10); FTIR data cm
1(attribution) 3091 (phenyl CH str), 2924 (pyrrole CH asy-str), 2854 (pyrrole CH sym-str), 1653 (ring str) 1568 (CH CH str), 1447 (R0 CH2R00 def), 1337 (CN str), 1206, 1076 and 1009 (CH bend in ring plane), 837 (CH CH trans out-of-plane def), 783 (CR0 R00 CHR CH out-ofplane def), 721 (CH CH cis CH out-of-plane def), 663 and 422 (phenyl CH def) 472 (pyrrole CH def). (4) UV–vis data: l, nm (e, 10
3 L
1 mol
3 cm
1) 362 (44), 464 (85), 566 (9.2), 596 (3.7); FTIR data cm
1 (attribution) 3094 (phenyl CH str), 2922 (pyrrole CH asy-str), 2851 (pyrrole CH sym-str), 1624 (ring str) 1584 (CH CH str), 1464 (R0 CH2R00 def), 1339 (CN str), 1206, 1078 and 999 (CH bend in ring plane), 837 (CH CH trans outof-plane def), 783 (CR0 R00 CHR CH out-of-plane def), 716 (CH CH cis CH out-of-plane def), 669 and 426 (phenyl CH def) 521 (pyrrole CH def). (5) UV–vis data: l, nm (e, 10
3 mol
1 L cm
1) 360 (41), 472 (58), 572 (8.5). FTIR data cm
1(attribution) 3090 (phenyl CH str), 2926 (pyrrole CH asystr), 2855 (pyrrole CH sym-str), 1651 (ring str) 1516 (CH CH str), 1493 (R0 CH2R00 def), 1339 (CN str), 1211, 1082, 1057 and 989 (CH bend in ring plane), 839 (CH CH trans out-of-plane def), 808 (CR0 R00 CHR CH out-of-plane def), 760 (CH CH cis CH out-of-plane def), 521 (pyrrole CH def). The third generation perhalogenated manganese porphyrin (6) Mn(PFPTDCPCl8P)(manganese(II) 2,3,7,8,12,13,17,18-octachloro-5-(pentafluorophenyl-10,15,20-tris(2,6-dichlorophenyl) porphyrin), was synthesized and characterized as previously reported [15].

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2.3. Electrochemical measurements and experiments 2.3.1. Cyclic voltammetry The electrochemical measurements were carried out with a Princeton Applied Research PAR 263 potentiostat-galvanostat apparatus in a 10 mL capacity cell using the conventional threeelectrode system: a glassy carbon electrode (MF-2012 Bioanalytical Inc.), an Ag/AgCl/1 mol L
1 KCl electrode and a platinum wire as working, reference and counter electrode, respectively. The potential measurements were carried out under nitrogen atmosphere. All measured potentials were corrected using ferrocene as internal standard reference and converted to NHE reference. Tetrabutylammonium hexafluorophosphate (0.1 mol L
1 TBAPF6) was employed as support electrolyte in an acetonitrile solution. 2.3.2. Chronoamperommetry In the chronoamperommetric experiments (associated with UV–vis and EPR techniques in spectroelectrochemistry experiments) the applied potentials were determined by the redox behavior of each Manganese porphyrin studied, and were applied by a Microquı´mica MQPG-1 potentiostat-galvanostat apparatus. 2.3.3. UV–vis spectroelectrochemistry (SEC-UV–vis) The UV–vis analyses associated with the electrochemistry technique were carried out with Hewlett Packard-8452-A diode array spectrophotometer using a two-compartment homemade cell, built up with two glass plates (3 cm width  6 cm high) separated by a silicon rubber spacer (15 cm length). The silicon rubber was placed forming a ‘‘U’’ shape. Another silicon rubber piece (4 cm length) was placed in the center to separate the cell thus formed into two chambers shaped like a ‘‘W’’. This set was joined over a wood support with a window, by four screws. Three platinum wires were used as working, quasi-reference and auxiliary electrodes. Working and quasi-reference electrodes were placed in parallel into the same compartment to avoid high ohmic resistance and the auxiliary electrode was placed in the other compartment. This separation avoids the mixture of species formed around working and auxiliary electrodes during the experiments. Each experiment was conducted using approximately 1.3 mL of acetonitrile solution (5  10
5 mol L
1) of each manganese porphyrin and TBAPF6 (0.1 mol L
1). The selected potentials were applied according to the redox processes previously determined by cyclic voltammetry. A continuous process of spectra acquisition was maintained after the potential application in order to observe the stability of the species formed. In general 12 min of acquiring spectra with the specific potential on, and 8 min with the potential off were used for most experiments. The UV–vis spectrophotometer was programmed to record one complete spectrum each 10 s (Kinetic mode). 2.3.4. EPR spectroelectrochemistry (SEC–EPR) The EPR analyses associated with the electrochemistry technique were carried out with a Bruker ESP 300E spectrometer at X-band (ca 9.5 GHz) at 293 K, using a Wilmad WG-

810-A quartz-flat cell. Each experiment was conducted using approximately 0.4 mL of acetonitrile solution (5  10
4 mol L
1) of each manganese porphyrin and TBAPF6 (0.1 mol L
1). The corresponding potential was applied to generate each specific species, as described for the UV–vis experiments mentioned above. 2.4. Oxidation of cyclohexane by iodosylbenzene (PhIO) catalyzed by manganese porphyrins 2.4.1. Classical homogenous catalysis A typical chemical catalytic oxidation reaction (classical) was carried out in a 2 mL thermostatted glass reactor equipped with a magnetic stirrer inside a dark chamber. In a standard experiment within the reactor, solid catalyst and iodosylbenzene (MnPor:PhIO molar ratio: 1:10 or 1:100) were suspended in 0.350 mL of solvent (dichloromethane– acetonitrile 1:1 mixture v/v) and degassed with argon during 15 min. The substrate cyclohexane (MnPor:substrate molar ratio = 1:1000) was added and the oxidation reaction was carried out for 1 h, under magnetic stirring. To eliminate the excess of iodosylbenzene, sodium sulfite was added. The products of the reaction were analyzed by capillary gas chromatography using a Shimadzu CG-14B gas chromatograph (flame ionization detector) with a DB-WAX capillary column (J&W Scientific). Yields were determined by GC by comparison with authentic samples using calibration curves with n-octanol as an internal standard. No reaction occurred in the absence of the catalysts in reactions performed under identical conditions. 2.4.2. Catalytic oxidation reaction monitored by UV–vis spectroscopy Catalytic oxidation reactions in acetonitrile were carried out in a 0.200 cm pathway quartz-cell, using 400 mL of catalyst (5  10
5 mol L
1), cyclohexane (substrate) and, after the record of the first UV–vis spectra, 40 mL of a fresh suspension of iodosylbenzene in acetonitrile (1  10
2 mol L
1) dispersed by ultrasound bath. Alternative experiments were conducted using the same volume of the suspension of iodosylbenzene in acetonitrile added to the reaction solution in three similar portions (13 mL each) with intervals of 20– 30 min. The spectrophotometer was programmed to acquire six spectra per minute (Kinetic Mode) during the experiment time. After the UV–vis monitoring, the products from organic oxidation were determined using a GC method in a procedure similar to the one described above. 3. Results and discussion 3.1. Electrochemistry study of the manganese porphyrins in acetonitrile Table 1 summarizes the redox potential results from halfwave potentials recorded during the electrochemical studies of the six manganese porphyrins in acetonitrile solvent. Representative cyclic voltammograms are shown in Fig. 2.

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175

Table 1 Manganese porphyrin redox potentialsa Compound

Second ligand reduction

First ligand reduction

Mn(III)/Mn(II)

Mn(III)/Mn(IV)

(1) (2) (3) (4) (5) (6)


1.90
1.56
1.49
1.59
1.47
1.03


1.33
1.26
1.15
1.19
1.04
0.72


0.03 0.07 0.09 0.09 0.05 0.61

1.49 1.70 1.71 1.72 1.75 1.86

a

Mn(TPP) Mn(TDCPP) Mn(TCFPP) Mn(TDFPP) Mn(TPFPP) Mn(PFPTDCPCl8P)

Volts vs. NHE, scan rate 0.1 V s
1, 0.1 mol L, TBAPF6 in acetonitrile.

The free base porphyrins 1–5 were metallated with manganese(II) salt resulting in manganese(III) porphyrin. However, the synthesis with ligand 6 produces a mixture of both the manganese(II) and manganese(III) porphyrin. The presence of a large number of electron-withdrawing chlorines at the periphery of porphyrin 6 is able to stabilize the manganese(II) oxidation state in air [15–17] in spite of the fact that the ionic radius of manganese II (83 pm), in contrast with manganese III (60 pm), is too large and does not have the proper size to fit ‘‘in-plane’’ with the porphyrin ring cavity size [4]. The solid and freshly prepared solutions of this manganese porphyrin have shown spectral properties characteristic of manganese(II) porphyrin that was slowly oxidized in air after hours (blueshifted Soret band [18–19]). The manganese(III) porphyrins (1–5) and manganese(II) porphyrin (6) undergo two quasi-reversible reduction processes centered on the metal and two reversible reductions centered on the ligand in acetonitrile [20–21]. Assignments of the metal oxidation state and the site of electron transfer are made on the

Fig. 2. Cyclic voltammogram in acetonitrile solution, support electrolyte TBAPF6 (0.1 mol L), scan rate 0.1 V/s. (a) (1) Mn(TPP), (b) (2) Mn(TDCPP), (c) (4) Mn(TDFPP) and (d) (5) Mn(TPFPP).

basis of EPR discussed below and literature data for Mn(TPP) and related porphyrin derivatives [22–23]. The first ring reduction potential for the second (2, 3, 4, 5) and third-generation manganese porphyrin (6) are anodically shifted if compared to the potential value for the first-generation porphyrin Mn(TPP) (Table 1). This is expected, based on the electron-withdrawing nature of the halogen groups present in complexes 2–6 which decreases the electron density at the ring leading to less negative potentials of reduction. Among the manganese porphyrins studied, complex 6 has shown the least negative reduction potential as expected for a third porphyrin generation. It is well known that the half-potentials for oxidation or reduction of a given metalloporphyrin depend on the central metal ion, the basicity and planarity of the porphyrin ring [24– 28]. The inductive effect generated by the introduction of halogen groups at the four meso-substituted phenyl rings of the TPP macrocycle also affects the redox potential but the largest substituents effect on the E values occurs for the third-generation macrocycles as observed for macrocycle 6 [29–30]. The introduction of bulky groups (such as chlorine atoms) onto the b-pyrrole positions of the porphyrin ring, to obtain the thirdgeneration of porphyrins, leads to a non-planar conformation of the ring affecting the ring electron density. The influence of the macrocycle distortion affecting the reductive potential is also observed for the asymmetric porphyrin 3. The presence of different substituents at the ortho-position of the phenyl groups could be provoking a distortion of the porphyrin ring which affects the potential in the same way as observed for the porphyrin 6. The presence of different halogen groups at the four meso-substituted phenyl ring (causing distortion and electron-withdrawing inductive effects) and halogens at b-pyrrole positions (causing distortion) gives rise to a sequence of more negative reduction potential Mn(TPP) > Mn(TDCPP) > Mn(TDFPP) > Mn(TCFPP) > Mn(TPFPP) > Mn(PFPTDCPC8P) for the first ligand reduction and Mn(TPP) > Mn(TDFPP) > Mn(TDCPP) > Mn(TCFPP) > Mn(TPFPP) > Mn(PFPTDCPC8P) for the second one. The Mn(III)/Mn(II) reductive process had an unpredictable behavior based on the electron-withdrawing dependence only. It was observed that 6, having a more asymmetrical structure, displayed the metal reductive potential that contrasted the most with the others (Table 1). This dependence is known in the literature [28,31–34]. The presence of halogen groups at bpyrrole positions has a direct and large effect on these transition potentials [32,33,35] with a large stabilization of the Mn(II) oxidation state.

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Table 2 Manganese porphyrins EPR signals and UV–vis Soret band wavelength characteristic observed during the spectroeletrochemical analysesa Compound

(1) (2) (3) (4) (5) (6)

Initial Soret band

MnIV EPR data

l (nm)

Aiso (G)

giso

E (V)b MnIII/MnIV

474

93.5

2.01

+1.6

468 464 464 472 462

88.3 93.1 91.9 94.7 93.4

2.01 2.01 2.01 2.01 2.01

Soret band at reduction potential l (nm)

Ec (V)

l (nm)

Ec (V)

436 (
1.0)

442 (
1.6)

434 (
1.2)

446 (
1.7)

430 (
1.1)

436 (
1.8)

428 (
0.5)

440 (
1.6)

428 (
0.8)

444 (
1.4)

416 (
0.5)

416 (
1.2)e

+2.0 +1.7 +2.0 +1.8 +2.0

MnII EPR Data DHpp (G)d

giso

E (V)b MnIII/MnII

–

–

183

2.01

–

–

168

2.00

–

–

–

800, 450f

2.10, 5.23f

–

–
1.0 –
1.0

a (1) Mn(TPP), (2) Mn(TDCPP), (2) Mn(TDCPP), (3) Mn(TCFPP), (4) Mn(TDFPP), (5) Mn(TPFPP), (6) Mn(PFPTDCPCl8P); experimental condition: 0.1 mol L
1 TBAPF6 in acetonitrile. b Applied potential (Volts vs. NHE). c Soret band observed in the SEC-UV–vis experiments at the oxired potential. d DHpp is a peak-to-peak width in Gauss. e Envelop Soret band. f In that case, the analysis was made in the solid sample. g? = 5.23, DHpp = 450 G and g// = 2.10, DHpp = 800 G at 77 K.

The manganese porphyrins (1–6) undergo irreversible or quasi-reversible oxidations in acetonitrile. These processes were attributed to metal oxidations Mn(III) $ Mn(IV), based on the results obtained by the SEC–EPR experiments presented below. The anodic shift (positive) in E for the oxidation process was observed from 1 to 6 manganese porphyrins following the sequence: Mn(PFPTDCPC8P) > Mn(TPFPP) > Mn(TDFPP)  Mn(TCFPP) > Mn(TDCPP) > Mn(TPP). The increase of the non-planar conformation of the porphyrin macrocycles caused by the introduction of bulky groups at b-pyrrole positions added to the inductive effect because the addition of halogen groups at the phenyl substituents affects the highest occupied and lowest unoccupied molecular orbital energies (HOMO and LUMO). The distortion will affect mainly the HOMO while inductive effects influence HOMO and LUMO energies [33,36] leading to an anodic shift of E. It was also observed that the magnitude of the DE depends on the type and the number of halogens. For example, the more halogenated and distorted complex 6 displayed the highest DE value. 3.2. EPR spectroelectrochemistry study of the manganese porphyrins Porphyrins containing redox inactive metals typically undergo two one-electron oxidation processes and two oneelectron reductions [24,37,38]. Chemical or electrochemical oxidations at the metal center are observed for some metal ions such as manganese, cobalt and iron [39–43]. A Manganese IV porphyrin complex was obtained and the crystal and molecular structure determined by X-ray crystallographic methods as described in the literature [44]. In order to confirm and better understand some of the electrochemical processes and transitions involved, studies focused on the coupling of techniques, especially those

involving electro-generated species. For that reason a systematic study was performed to observe whether the effect of an increased halogenation on the porphyrin rings, which produces a quasi-predictable shift on its oxired potentials, could be causing similar effects, when observed by other techniques, such as EPR or UV–vis. Before starting the spectro-electrochemical studies, all complexes (1–6) were characterized by EPR (solid and solution samples, at room temperature and 77 K). EPR experiments of the solid samples of complexes 1–5 have shown silent EPR spectra as expected from integer-spin (non-Kramers) Mn(III) S = 2 porphyrin compound [45–48]. On the other hand, the solid complex 6 displayed a typical EPR signal for a Mn(II) species [15] showing a characteristic spectrum with g? = 5.23, DHpp = 450 G (DHpp is a peak-topeak line width in Gauss) and g// = 2.10, DHpp = 800 G at 77 K(Table 2) [49]. Table 2 displays the EPR parameters for the species observed for manganese complexes 1–6 detected through SEC– EPR experiments in oxired conditions and Fig. 3 exemplifies the SEC–EPR results observed for complexes 4 and 6. Fig. 3a–d correspond to the oxidative and reductive SEC– EPR experiments for complex 4. Fig. 3a shows the initial EPR spectrum of the Mn(TDFPP) and the silent spectrum is characteristic of Mn(III) species from integer-spin (nonKramers) Mn(III) S = 2 porphyrin compound [45–48]. Fig. 3b shows EPR spectrum after a potential +1.8 V was applied over 5 min. Their six lines are characteristic of Mn(IV) species [48]. The inversion of the potential to
0.5 V applied for 20 min caused the reduction of the metal center and a broad EPR signal characteristic of Mn(II) was observed (Fig. 3c) [48–51]. When the +1.8 V potential was applied again, all Mn(II) species were converted to Mn(IV) species (Fig. 3d).

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177

6 (Fig. 3e). The oxidation of the solution of 6 (Fig. 3f) leads to a spectrum showing a typical signal for Mn(IV) species. The application of a more positive potential (+2.2 V, Fig. 3g) increased the Mn(IV) signal showing that no other oxidized species was present. At a cathodic potential of
1.0 V applied for 10 min a new signal appeared at g 2.0 (giso = 2.006 DHpp = 5.4 G) that is attributed to a Mn(II) radical species (Fig. 3h) suggesting that two reductions (di-anion) over the porphyrin ring took place (a non integer-spin species). A Mn(II) with one ring reduction would also produce an integerspin species (S = 3, considering a high spin complex), which also would be EPR silent [39]. Applying potentials from
0.6 to
1.2 V, EPR silent spectra (figure not shown) were observed. With more negative potential, for complex 6, a typical EPR signal for a radical species was observed suggesting the capacity of these complexes to better stabilize radical species compared to the other complexes. At oxidative potentials (+1.6 to +2.0 V) complexes (1–6) displayed an active species EPR with a pronounced six-line hyperfine splitting AMn  90 G (Aiso) by the 55Mn (I = 5/2) nucleus in the g = 2.0 region (giso) (Fig. 3b). This spectrum is indicative of a monomeric Mn(IV)porphyrin species, d3, S = 3/ 2 and is of the same type displayed by the monomeric Mn(IV) complex reported in the literature [40,41,52]. Oxidative experiments with potential lower than +1.6 V showed silent EPR spectra. The anisotropic spectrum of Mn(IV) porphyrin complexes is frequently observed as a broad resonance at g? = 4.0 region and a six line hyperfine pattern at g// = 2.0 region and is indicative of an axially symmetric field that has a large value for the zero field splitting parameter [26]. It is not possible to observe this characteristic here, for room temperature analyses [39]. Fig. 3. SEC–EPR spectra Mn(TDFPP) (4) (5  10
4 mol L) and Mn(PFPTDCPCl8P) (6) (4  10
4 mol L
1) acetonitrile solution. Complex 4: (a) initial spectrum of Mn(III) species (no potential was applied); (b) Mn(IV) species, after +1.8 V for 5 min; (c) Mn(II) species, after
0.5 V for 20 min; (d) Mn(IV) species, after +1.8 V during 10 min. Complex 6: (e) initial spectrum (no potential was applied); (f) Mn(IV) species, after +1.8 V for 10 min; (g) after +2.2 V for 10 min; (h) Mn(II)-anion radical species after
1.0 V for 10 min.

Reductive experiments with potentials in range from 0.0 to
0.6 V produce, on the spectra of the solution of complexes 2 and 4 solutions, a broad signal that is attributed to Mn(II) species [49] (Fig. 3c). Complexes 1, 3 and 5 did not show this signal, presenting an EPR silent spectrum indicating that the reduced species are not stable under these experimental conditions. The EPR spectrum of the manganese porphyrin solution Mn(PFPTDCPCl8P) (6), prepared and analyzed after a brief period of time, does not present characteristics of manganese(II) (Fig. 3e–h) confirming that this manganese porphyrin (in solution) is air sensible and oxidized to Mn(III) that is EPR silent (Fig. 3e). The difficulty to stabilize Mn(II) species in an oxygencontaining medium and at room temperature hinders the observation of the transition process Mn(II)/Mn(III) to complex

3.3. UV–vis spectroelectrochemistry study of the manganese porphyrins (SEC-UV–vis) The UV–vis Soret band data from the six investigated manganese porphyrins are summarized on Table 2. The spectrophotometer was programmed to acquire a UV–vis spectrum every 10 s. However, only the significant results are presented in what follows. Fig. 4a and b shows the spectra of the Mn(TDFPP) (4) at the beginning and at the end of a SEC-UV–vis experiment when+1.7 V were applied during 12 min, plus 8 min without applying potential to observe the stability of the compound formed. Fig. 4c shows the last spectrum of a run applying +2.0 V. The behavior of the more important bands of 4 in oxidative procedures were also observed for 1, 2, 3 and 6, where the Soret band displayed a small loss of intensity at the same position (variation of 2 or 4 nm high) suggesting that the oxidized manganese porphyrins were adsorbed at the electrode surface. In contrast with the others, complex 5 displayed a blue shifted Soret band with small intensification at oxidative potentials. These characteristics [44,53,54] are expected when a metal-centered oxidation occurs suggesting the Mn(IV)porphyrin formation, as observed from the EPR results under the same conditions.

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In reductive experiments at
0.5 V (Fig. 4e), an intermediate species was observed for compound 4 with the Soret band at 428 nm suggesting the metal reduction process. In the same conditions SEC–EPR experiments showed a broad EPR signal (giso = 2.00, Aiso = 168 G) for 2 and 4 suggesting the presence of Mn(II)porphyrin species. The metal centered reduction process (MnIII/MnII) is characterized by a blue Soret band shifted in comparison to Mn(III)porphyrin, as observed here for complexes 4 and for the other porphyrins (1 to 5) [19,44,54]. At more negative potentials complexes 1–5 (
1.2 V, Fig. 4f or
1.6 V Fig. 4g) displayed a Soret band at 440–450 nm with an estimated molar absortivity that was about 1.4–2.4 times more intense (in comparison to the molar absortivity observed for the initial manganese porphyrin Soret band) suggesting the formation of mono and di-anion radical species as also suggested from voltammetric data for the same conditions. At reductive potentials complex 6 has only displayed an enlarged Soret band region with a small intensity envelope at 416 nm which is not conclusive but which also suggests the metal reduction. 3.4. Oxidative catalysis

Fig. 4. UV–vis spectra from (4) Mn(TDFPP) (5  10
5 mol L) experiments. Oxidative and reductive experiments were conducted in acetonitrile solution with TBAPF6 (0.1 mol L). The potential applied for each experiment is inset in the graph legend. Catalytic reaction was conducted in acetonitrile solution of (4) Mn(TDFPP):PhIO:ciclohexane (molar ratio of 1:20:2000) for 50 min.

Another possible species expected at oxidative potential is a cation radical Mn(III) porphyrin. This species would be characterized by a typical and well known characteristic spectrum [53] with a less intense Soret band and a visible region Q-band with low band definition but with high intensity in comparison to the initial Mn(III) porphyrin spectrum. For compounds 1–6 no spectral characteristics for the radical species were observed.

Manganese porphyrin complexes have been shown to be versatile synthetic catalysts for the oxidation of a wide variety of organic substances, using different oxygen donors [2,15,55,56]. The manganese porphyrins 1–5 were comparatively studied as oxidation catalysts for the inert cyclohexene and the species generated during the oxidation reaction were monitored by UV–vis analyses. Addition of PhIO to a reaction solution containing manganese(III) porphyrin caused immediate generation of a new species A with a strong blue-shifted Soret band from the initial Soret band observed for each one of the five manganese porphyrins studied (Table 3). The formation of A species was also observed by others with a number of different oxidants such as H2O2, KHSO5, NaOCl and m-CPBA [18,43,55,57–59]. A typical UV–vis observed for A species is shown in Fig. 4i for complex 4. Similar spectral behavior was observed for complexes 1, 2, 3 and 5 (Table 3). The observed spectrum characteristics were similar to those observed by Hill and coworkers [44], Groves and Stern [60] and recently Newcomb and co-workers [61], who attributed spectral characteristics to (Porphyrin)MnIV(O) intermediate catalytic species. The consumption of A species in the oxidation reaction by the cyclohexane substrate was observed by the disappearance of the UV–vis typical characteristic for each manganese porphyrin with the concomitant recovery of the typical Soret Band of the resting state manganese(III) porphyrin. The disappearance of A species suggested that these adducts were the oxidant active species consumed by the substrate or that they decay to more stable species, as suggested by Newcomb and co-workers [61] and returned gradually to the resting manganese porphyrin. In fact, quantitative GC analyses of the reaction solution after the recovery of the manganese(III) porphyrin after 50 min of

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179

Table 3 Soret bands of manganese porphyrin intermediate species observed during catalytic experimentsa Compound

(1) Mn(TPP) (2) Mn(TDCPP) (3) Mn(TCFPP) (4) Mn(TDFPP) (5) Mn(TPFPP) (6)Mn(PFPTDCPCl8P)

Initial Soret band

Intermediate species A Soret band

l (nm)

l (nm)

474 468 464 464 472 462

422 422 422 418 422 372, 454e

Alcohol (yield%)b

20 28 25 30 27 24

Alcohol (yield%) classical catalysisc 1:10

1:100

19 43 32 42 38 –f

5 10 11 11 11 –f

%Catalyst destructiond

18 8 7 4 8 –g

a 400 mL of MnPor/acetonitrile solution (5.0  10
mol L
1) in a quartz cell (0.200 cm optical pathway); cyclohexane as substrate (1:3000 molar ratio); PhIO/ acetonitrile suspension as oxidant (1:20 molar ratio). b Alcohol production obtained on the reaction monitoring by UV–vis experiments. The yield% was calculated based on the PhIO amount added. c Classical catalysis means the catalytic reaction occurred without UV–vis monitoring (see experimental part). The manganese porphyrin/iodosylbenzene molar ratios used were 1:10 and 1:100. The yield% was calculated based on the PhIO amount added. d The %destruction of the catalyst in acetonitrile solution was determined by the comparison of the initial and final absorbance of the characteristic Soret band, and considering the volume correction. e Envelope band with high intensity. f Complete catalytic results with the complex 6 is displayed in ref. [15] in different condition used in this work. g The final species was different from the initial preventing the comparison of the molar absortivity of the Soret band and consequently the %catalyst destruction estimative.

reaction (with the recovery of the typical Soret band) indicated the production of cyclohexanol as majority product (e.g. complex 4 = 30% alcohol yield, the ketone correspondent was also observed) (fourth column results on Table 3). The UV–vis feature of the A species generated by the reaction of manganese porphyrins and iodosylbenzene in acetonitrile, and its consumption by the substrate recovering the initial manganese(III) porphyrin suggests that A species is the intermediate catalytic active oxo manganese porphyrin. The formation of an oxo manganese(V) porphyrin compound as the catalytically active species was also reviewed by Meunier et al. [62]. Groves and co-worker [43] and Nam et al. [59] supplied spectroscopic evidence of the high valence manganese species. Complex 6 has displayed a different behavior from that observed for complexes 1–5. It was observed that the Soret band characteristic of A species is a band envelope at 372 and 454 nm showing an increase in the molar absortivity. After A was consumed in the reaction, the final spectrum displayed a Soret band as an envelope shape at 454 and 492 nm in comparison with the one observed for the initial A material (454 nm). After the addition of more PhIO/acetonitrile solution, at the end of the first experiment, it was observed that A species appeared again, suggesting that complex 6 is resistant to destructive oxidative process and was recovered at the end of the catalytic reaction as a Mn(III) species. The constant observation of the band at 454 nm suggested that some residual Mn(III)porphyrin remained during the entire oxidation reaction time. The average lifetime of oxidant species determined from all porphyrins in acetonitrile solution was around 3–5 min (species A disappears when all the oxidant is consumed). Fig. 5 exemplifies the time measurement for complex 4 (Fig. 5a and b). This period of time was strongly increased for all complexes (about 10 times) when a methanol/acetonitrile (1:10) solution of PhIO was used instead of only acetonitrile. The Soret band of

Fig. 5. (a) UV–vis spectra of a reaction solution of (4) Mn(TDFPP):PhIO:cyclohexane (molar ratio of 1:7:2000) in acetonitrile solvent for 40 min (first oxidant addition); (b) Main UV–vis bands behavior of the complete catalytic experiment (three additions of oxidant PhIO/acetonitrile).

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the final species, recovered after the catalytic reaction, was redshifted probably due the axial or coordinate ligand changes [53] caused by the methanol presence. In that solvent mixture no significant change was observed in the alcohol yields for the different catalysts. On the other hand, the recovery of catalysts decreased very much suggesting the increase of the percentage of catalyst destruction. In fact for 1 the catalyst destruction increased about three times (18% in acetonitrile, Table 3, versus 53% in methanol/acetonitrile mixture). The A species is more susceptible to destruction when it is originated from a nonactivated porphyrin, e.g. complex 1, as expected for a firstgeneration porphyrin when compared to second-generation porphyrin. In fact, complexes 2–5 displayed 4–8% of destruction in acetonitrile versus 10–14% in methanol/acetonitrile mixture. Classical catalytic reaction experiments varying MnPor/ PhIO molar ratio from 1:5 to 1:20 had no significant effects on the catalyst destruction and yields of products (Table 3, fifth column results). The decrease of the product yields and catalyst recovery was observed only with large molar ratio (MnPor/ PhIO = 1:100), suggesting that in the large excess of oxidant, the first- and second-generation porphyrins probably are significantly destroyed. The efficiency of the manganese porphyrins studied towards alcohol yields based on monitoring catalytic reactions by UV–vis can be organized as follows: Mn(TDFPP) > Mn(TDCPP)  Mn(TPFPP) > Mn(TCFPP)  Mn(PFPTDCPCl8P) > Mn(TPP). The resistance of the catalysts to destruction in acetonitrile solution showed the same tendency: Mn(TDFPP) > Mn(TDCPP) ffi Mn(TPFPP) ffi Mn(TCFPP)  Mn(TPP). A similar sequence can be observed with the results obtained in the classical catalysis results for 1:20 MnPor:PhIO molar ratio (Table 3). Complex 6 could not be evaluated because the final species after the catalysis experiments displayed spectral differences from the initial one making a comparison of the absorbance more difficult. From the results on Table 3 it was observed that complex 6, the perhalogenated manganese porphyrin (third-generation) is not the best catalyst for the cyclohexane oxidation showing modest alcohol yield in comparison to complexes 2–5. It was observed from the electrochemistry data (Table 1) that the halogenation of the porphyrin ring meso position substituents (complexes 2–5) and perhalogenation of the b-pyrrole porphyrin position (complex 6) resulted in a positive shift of the redox potential. Probably this progressive positive stabilization makes them more resistant to destructive processes and, consequently, also improves the lifetime of the catalytic active species, by the high electrophilicity of the macrocycle, due to the presence of high number of halogen substituents, causing the increase of the alcohol yield in comparison to Mn(TPP) On the other hand, this stabilization makes complex 6 hard to oxidize from the Mn(II) species to form the catalytic active species and the catalytic results are modest [15]. 4. Conclusions It was observed, by means of electrochemical comparative studies for complexes 1–6, that the halogenation gave a positive

shift of the redox potentials as expected and observed for other families of metalloporphyrins [17,30,47,63]. It was observed that this behavior is the result of a sum of the electronic distortion factors caused by the kind, position and number of halogens on the porphyrin ring. The UV–vis and EPR spectroscopy for electro-generated species from the manganese porphyrins studied indicate that they produce high valency Mn(IV)porphyrins for the oxidative potentials and Mn(II) for the reductive potentials under the conditions observed for most of complexes. No conclusive results were obtained regarding the presence and stabilization of the radical species for all complexes. Compounds 2 and 4 were the only compounds that showed a Mn(II) EPR signal when submitted to SEC–EPR experiments, emphasizing that the ortho substituents play a special effect over these compounds. Compound 3, even with ortho substituents, is different because, the halogens are not the same, and these produce a low symmetry compound, when compared with 2 and 4. The cyclohexane oxidation reactions catalyzed by compound 2–5 showed similar alcohol yields and percentage of destruction suggesting high resistance to oxidative conditions of the catalytic reaction as expected by the tendency displayed by the oxired potentials investigated. Complex 6, a third-generation porphyrin, displayed a catalysis performance comparable to the first generation porphyrin, as a result of the highest positive shift of the redox potentials, which prevents its oxidation to a catalytic high valency oxo species. Acknowledgements The authors are grateful to the Conselho Nacional de Desenvolvimento Cientifico e Tecnolo´gico (CNPq), Coordenac¸a˜o de Aperfeic¸oamento de Pessoal de Nı´vel Superior (CAPES), Fundac¸a˜o Arauca´ria, Fundac¸a˜o da Universidade Federal do Parana´ (FUNPAR), Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP) and Universidade Federal do Parana´ (UFPR) for the financial support and facilities. They also gratefully acknowledge helpful suggestions from Dr. Daniel Lottis while preparing the English manuscript. References [1] [2] [3] [4] [5] [6]

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