Journal of Inorganic Biochemistry xxx (2014) xxx–xxx

JIB-09444; No of Pages 10

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Journal of Inorganic Biochemistry

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Cobalt triarylcorroles containing one, two or three nitro groups. Effect ofNO2 substitution on electrochemical properties and catalytic activity forreduction of molecular oxygen in acid media

Bihong Li a, Zhongping Ou a,⁎, Deying Meng a, Jijun Tang a, Yuanyuan Fang b, Rui Liu a, Karl M. Kadish b,⁎⁎a School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR Chinab Department of Chemistry, University of Houston, Houston, TX 77204-5003, USA

⁎ Corresponding author. Tel.: +86 511 88791800.⁎⁎ Corresponding author. Tel.: +1 713 7432740.

E-mail addresses: zpou2003@yahoo.com (Z. Ou), kkad

0162-0134/$ – see front matter © 2014 Elsevier Inc. All rihttp://dx.doi.org/10.1016/j.jinorgbio.2013.12.014

Please cite this article as: B. Li, et al., Cobalt trproperties and catalytic..., J. Inorg. Biochem.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 September 2013Received in revised form 24 December 2013Accepted 24 December 2013Available online xxxx

Keywords:Cobalt corrolesNitro-substitutionSynthesisElectrochemistryDioxygen reduction

Cobalt(III) triarylcorroles containing 0–3 nitro groups on the para-position of the threemeso-phenyl rings of themacrocycle were synthesized and characterized by electrochemistry, mass spectrometry, (UV–vis) and 1H NMRspectroscopy. The examined compounds are represented as (NO2Ph)nPh3 − nCorCo(PPh3), where n varies from 0to 3 and Cor represents the core of the corrole. Each compound can undergo two metal-centered one-electronreductions leading to formation of Co(II) and Co(I) derivatives in CH2Cl2 or pyridine containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). A stepwise two electron reduction of each NO2Ph group of the compoundis also observed. The first is reversible and occurs in a single overlapping step at the same potential which in-volves an overall one-, two- or three-electron transfer process for compounds 2–4, respectively. This indicatesthe lack of an interaction between these redox active sites on the corroles. The second reduction of the NO2Phgroups is irreversible and located at a potential which overlaps the Co(II)/Co(I) process of the compounds.Thin-layer UV–visible spectroelectrochemical measurements in CH2Cl2, 0.1 M TBAP demonstrate the occurrenceof an equilibrium between a Co(III) π-anion radical and a Co(II) derivative with an uncharged macrocycle afterthe first controlled potential reduction of the nitro-substituted corroles. All four cobalt corroles were also exam-ined as catalysts for the electroreduction of O2 when coated on an edge-plane pyrrolytic graphite electrode in 1.0MHClO4. This study indicates that the larger the number of nitro-substituents on the cobalt corrole, the better thecompound acts as a catalyst.

© 2014 Elsevier Inc. All rights reserved.

1. Introduction

Cobalt corroles are one of the most frequently studied groups ofmetallocorroles because of their unique spectral and electrochemicalproperties [1–22], as well as their potential applications as catalystsfor a variety of reactions [23–32]. In this regard, part of our own researchefforts have been directed toward the synthesis [4,7–9,28–30,33,34]and characterization of substituted cobalt corroles as catalysts for theelectroreduction of O2 in acid media [30,33–36].

It is known that the addition of one or more highly electron-withdrawing NO2 groups to a corrole will significantlymodify its chem-ical and physical properties, with the type and magnitude of the effectbeing dependent upon the position of the substituents. Three types ofnitro-substituted corroles have been reported in the literature. Theseare: (i) meso-substituted nitrocorroles [37], (ii) β-pyrrole substitutednitrocorroles [38–45] and (iii) meso-phenyl substituted nitrocorroles[38,46–50].

ish@uh.edu (K.M. Kadish).

ghts reserved.

iarylcorroles containing one, t(2014), http://dx.doi.org/10.1

Previous electrochemical characterization of nitrocorroles hasshown that the presence of one or more electron-withdrawing NO2

groups at the meso-phenyl or β-pyrrole positions of the conjugatedmacrocycle will have a large effect on the E1/2 for electroreduction,shifting the potentials in a positive direction by up to 200 mV pernitro group in the case of β-pyrrole substituted (TPC)M where M =Cu or Fe, TPC = triphenylcorrole. This compares to a 70–80 mV shiftin potentials in the case ofmeso-nitrophenyl substituted corroles [38]. Adirect electroreduction of the NO2 group does not occur in nonaqueousmediawhen this electron-withdrawing group is attached at theβ-pyrroleposition of a porphyrin or corrole, but this is not the case when the NO2

group is located on the phenyl ring of a TPC type complex. Under theseconditions a reversible one-electron reduction is observed at a potentialof about−1.1 V vs SCE (saturated calomel electrode) [38,50].

We have earlier demonstrated that the type of substituents locatedat the meso-phenyl positions of a triarylcorrole or tetraarylporphyrinnot only will affect the redox potentials, but also significantly influencethe ability of the compound to act as an electrocatalyst in the reductionof O2when adsorbed at an electrode surface in acidmedia [21,36]. How-ever, no studies of these types have been carried out with corroles orporphyrins having NO2 substituents and it is therefore not knownhow the addition of one or more strongly electron-withdrawing NO2

wo or three nitro groups. Effect of NO2 substitution on electrochemical016/j.jinorgbio.2013.12.014

2 B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx

groups on themacrocyclewill affect the compounds' electrocatalytic ac-tivity for the reduction of dioxygen. This is examined in the presentstudy where a series of cobalt corroles containing zero, one, two orthree para-nitrophenyl substituents were synthesized and then charac-terized by electrochemistry and spectroscopic techniques. The investi-gated compounds are represented as (NO2Ph)nPh3 − nCorCo(PPh3),where n varies from 0 to 3 and Cor is the core of the corrole and thestructure of compounds is shown in Chart 1.

Reduction/oxidation potentials and UV-visible (UV-vis) spectra ofeach neutral, oxidized and reduced species were measured in dichloro-methane (CH2Cl2) containing 0.1M tetra-n-butylammoniumperchlorate(TBAP). The effect of theNO2 substituents on redoxpotentials and the siteof electron transfer are examined and an overall electroreduction/electrooxidation mechanism is proposed.

The catalytic activity of the corroles in Chart 1 was also examined asto their ability to reduce O2 when adsorbed on an electrode surface. Thenumbers of electrons transferred and the percentage of H2O2 producedin the catalytic reduction of oxygenwere calculated from cyclic and lin-ear sweep voltammetry measurements coupled with measurements ata rotating disk electrode (RDE).

2. Experimental

2.1. Chemicals

CH2Cl2 was purchased from Sinopharm Chemical Reagent Co. orAldrich Chemical Co. and used as received for electrochemistry andspectroelectrochemistry experiments. TBAP was purchased fromSigma Chemical or Fluka Chemika Co., recrystallized from ethyl alcohol,and dried under vacuum at 40 °C for at least one week prior to use.

2.2. Synthesis of A3 type compounds 1 and 4 [51]

Aldehyde (2.5 mmol) and pyrrole (0.35 mL, 5 mmol) were dis-solved in MeOH (100 mL) followed by H2O (100 mL) and HCl (36%,2.1 mL) being added to solution. After stirring at room temperaturefor 3 h the compound was extracted with CHCl3. The organic layerwas washed twice with H2O, then dried with MgSO4, filtered and dilut-ed to 300 mL with CHCl3. After the addition of p-chloranil (615 mg,2.5 mmol) themixture was refluxed for 1 h and then chromatographedon a silica columnusing CH2Cl2/hexane as eluent. The free-base corroleswere obtained by evaporating the solvent, after which ~20 mg of thecompound was dissolved in 30 mL MeOH containing Co(OAc)2·4H2Oand triphenylphosphine. The mixture was heated to reflux for 1 h andthe progress of the reaction monitored by thin-layer chromatographyuntil the starting free-base corrole was consumed. The sample wasevaporated to dryness and flash chromatographed (neutral alumina,200–300 mesh, CH2Cl2/hexane as eluent). The red fraction was collect-ed and evaporated to dryness to yield thedesired cobalt corrole product.

2.3. Synthesis of A2B type compounds 2 and 3 [52,53]

Dipyrromethane (1 mmol) and aldehyde (0.5 mmol) were dissolvedin MeOH (50 mL). HCl (36%, 2.5 mL) in H2O (50 mL) was added and the

Chart 1. Structure of investigated cobalt meso-substituted

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solution stirred at room temperature for 1 h. The same procedure wasthen followed as described above for the A3 type compounds.

2.3.1. Ph3CorCo(PPh3)22 mg 68.7% yield. UV–vis λmax/nm in CH2Cl2: 387, 562. 1H NMR

(400 MHz CDCl3) 8.63 (s, 2H, β-H), 8.34 (s, 2H, β-H), 8.12–8.06(m, 7H, β and Ph-H), 7.65 (s, 11H, Ph-H), 7.42 (s,1H, Ph-H),7.08(s, 3H, p-H of PPh3), 6.73(s, 6H, m-H of PPh3), 4.76 (t, 6H, o-H ofPPh3).m/z observed 582.52 [M-PPh3]+, calcd 582.54 for C37H23N4Co.

2.3.2. (NO2Ph)Ph2CorCo(PPh3)21 mg 67.5% yield. UV–vis λmax/nm in CH2Cl2: 384, 559. 1H NMR

(400 MHz CDCl3) 8.70 (s, 2H, β-H), 8.44 (m, 4H, β-H), 8.19 (s, 1H, β-H),8.09–8.05 (m, 6H, β and Ph-H), 7.67 (s, 8H, Ph-H), 7.45 (s, 1H, Ph-H),7.09 (s, 3H, p-H of PPh3), 6.73 (s, 6H, m-H of PPh3), 4.69 (t, 6H, o-H ofPPh3).m/z observed 628.36 [M-PPh3]+, ca1cd 629.13 for C37H24N5O2Co.

2.3.3. (NO2Ph)2PhCorCo(PPh3)18 mg 59.4% yield. UV–vis λmax/nm in CH2Cl2: 375,576. 1H NMR

(400 MHz CDCl3) 8.77 (s, 2H, β-H), 8.51 (s, 4H, β-H), 8.36–8.25(m, 5H, β and Ph-H), 8.04–7.99 (m, Ph-H), 7.69–7.89 (m, 6H, Ph-H),7.39 (s, 1H, Ph-H), 7.11 (s, 3H, p-H of PPh3), 6.73 (s, 6H, m-H of PPh3),4.65 (t, 6H, o-H of PPh3). m/z observed 673.43 [M-PPh3]+, calcd674.11 for C37H23N6O4Co.

2.3.4. (NO2Ph)3CorCo(PPh3)23 mg 67.6% yield. UV–vis λmax/nm in CH2Cl2: 372, 575. 1H NMR

(400 MHz CDCl3) 8.84 (s, 2H, β-H), 8.54–8.44 (m, 8H, β and Ph-H),8.28–8.08 (m, 7H, Ph-H), 7.72 (s, 2H, Ph-H), 7.45 (s, 1H, Ph-H), 7.13(s,3H, p-H of PPh3), 6.73 (s, 6H, m-H of PPh3), 4.61 (t, 6H, o-H ofPPh3). m/z observed 718.62 [M-PPh3]+ cacld 719.10 for C37H22N7O6Co.

2.4. Instrumentation

Thin-layer UV–visible spectroelectrochemical experiments wereperformed with a home-built thin-layer cell which has a light trans-parent platinum net working electrode. Potentials were applied andmonitored with an EG&G PAR Model 173 potentiostat or a BiStatelectrochemistry station. Time-resolved UV–visible spectra were re-corded with a Hewlett-Packard Model 8453 diode array spectropho-tometer. High purity N2 from Trigas was used to deoxygenate thesolution and kept over the solution during each electrochemical andspectroelectrochemical experiment.

All reported 1HNMR spectra were recorded on a Bruker Avanc II 400MHz instrument. Chemical shifts (δ ppm) were determined with TMS(tetramethylsilane) as the internal reference. MALDI-TOF (matrix-assisted laser desorption/ionization) mass spectra were taken on aBruker BIFLEX III ultra-high resolution instrument using alpha-cyano-4-hydroxy-cinnamic acid as the matrix.

All electrochemicalmeasurementswere carried out at 298 K using anEG&G Princeton Applied Research (PAR) 173 potentiostat/galvanostat ora Chi-730CElectrochemistryWork Station. A three-electrode systemwasused and consisted of a graphite working electrode (Model MT134, PineInstrument Co.) for cyclic voltammetry and rotating disk voltammetry. A

triphenylcorroles, where the L is the PPh3 axial ligand.

wo or three nitro groups. Effect of NO2 substitution on electrochemical016/j.jinorgbio.2013.12.014

3B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx

platinum wire served as the auxiliary electrode and a home-madeSCE as the reference electrode, which was separated from thebulk of the solution by means of a salt bridge. The RRDE was pur-chased from Pine Instrument Co. and consisted of a platinum ringand a removable edge-plane pyrolytic graphite (EPPG) disk (A =0.196 cm2). A Pine Instrument MSR speed controller was used forthe rotating disk electrode (RDE) and rotating ring disk electrode(RRDE) experiments. The Pt ring was first polished with 0.05 μmα-alumina powder and then rinsed successively with water and ac-etone before being activated by cycling the potential between 1.20

a) Route for A3 type compounds 1 and 4

b) Route for A2B type compounds 2 and 3

Scheme 1. Synthesis route

Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, tproperties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1

and−0.20 V in 1.0 M HClO4 until reproducible voltammograms areobtained [54,55].

The corrole catalysts were irreversibly adsorbed on the elec-trode surface by means of a dip-coating procedure described inthe literature [56]. The freshly polished electrode was dipped in a1.0 mM catalyst solution of CH2Cl2 for 5 s, transferred rapidly topure CH2Cl2 for 1–2 s, and then exposed to air where the adheringsolvent rapidly evaporated leaving the corrole catalyst adsorbed onthe electrode surface. All experiments were carried out at roomtemperature.

s for compounds 1–4.

wo or three nitro groups. Effect of NO2 substitution on electrochemical016/j.jinorgbio.2013.12.014

a) (Ph)3CorCo(PPh3)

b) (NO2Ph)(Ph)2CorCo(PPh3)

c) (NO2Ph)2(Ph)CorCo(PPh3)

d) (NO2Ph)3CorCo(PPh3)

Fig. 1. Cyclic voltammograms of compounds 1–4 in CH2Cl2 containing 0.1 M TBAP at a scan rate of 0.1 V/s.

Table 1Half-wave potential (V vs SCE) of (4-NO2Ph)xPh3 − xCorCo(PPh3) 1–4 in CH2Cl2 containing0.1 M TBAP.

cpd #NO2

groupsOxidation Reduction

E1/2 (2) E1/2 (1) ΔE (2− 1) Ep (1) E1/2 (NO2Ph) Ep (2)

1 None 0.94 0.51 0.40 −0.73a – −1.62a

2 One 0.99 0.57 0.42 −0.65a −1.10 −1.63a,b

3 Two 1.03 0.62 0.41 −0.63a −1.09c −1.68a,b

4 Three 1.05 0.67 0.38 −0.56a −1.06d −1.71a,b

a Irreversible peak potential at a scan rate of 0.1 V/s.b Multiple overlapping electron transfer processes.c Two overlapping one-electron processes.d Three overlapping one-electron processes.

4 B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx

3. Results and discussions

3.1. Synthesis

The A3 type of compounds (Ph)3CorCo(PPh3) 1 and (NO2Ph)3CorCo(PPh3) 4 were synthesized according to the synthetic route de-scribed in Scheme 1a which follows a procedure reported in the litera-ture [51]. The A2B type of compounds, (NO2Ph) Ph2CorCo(PPh3) 2 and(NO2Ph)2PhCorCo(PPh3) 3, were synthesized according to the routeshown in Scheme1b [52,53]. The formula of the compoundwas verifiedby 1H NMR and mass spectrometry.

3.2. Electrochemistry in CH2Cl2

The effect of nitration on the overall oxidation/reductionmechanismand the site of each electron transfer were investigated by cyclicvoltammetry and thin-layer UV–visible spectroelectrochemistry. Theaddition of one or more NO2 groups on the three meso-phenyl rings ofthe triarylcorrole would be expected to shift all redox potentials towardmore positive values [57,58], but it was not clear what would be themagnitude of the potential shift for the addition of one, two or threeelectron-withdrawing NO2 groups on the three meso-phenyl rings ofthe cobalt corroles, nor was it clear how the addition of multiple NO2

groups might effect the site of electron transfer which could occur atthe centralmetal ion, at the conjugatedπ-ring systemof themacrocycle,and/or at the NO2Ph groups of compounds 2, 3 and 4.

Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, tproperties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1

In order to elucidate these points, electrochemistry and spectro-electrochemistry experiments were carried out in the nonbondingsolvent CH2Cl2. Cyclic voltammograms of compounds 1–4 in CH2Cl2 areshown in Fig. 1 and a summary of measured half-wave and peak poten-tials is given in Table 1.

As seen from the cyclic voltammograms and table of potentials, Ph3-CorCo(PPh3) 1, which lacks anNO2 substituent, undergoes two irrevers-ible reductions at Epc=−0.73 and−1.62 V in CH2Cl2 containing 0.1MTBAP. These two reductions have been shown to generate the Co(II) andCo(I) forms of the corrole, respectively [34]. There are also twomacrocycle-centered oxidations at E1/2 = 0.51 and 0.91 V. Similar

wo or three nitro groups. Effect of NO2 substitution on electrochemical016/j.jinorgbio.2013.12.014

-2

-1.5

-1

-0.5

0

0.5

1

1.5

0 1 2 3

Fig. 2. Plots of redox potentials of the cobalt corroles 1–4 in CH2Cl2 containing 0.1M TBAPvs the number of NO2 groups.

5B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx

redox processes are seen for compounds 2–4 but with the potentialsshifted positively due to the electron-withdrawingnitro groups. In addi-tion, two new reduction processes are observed for compounds 2, 3 and4which involve the electroactiveNO2Ph groups on themeso positions ofthe triarylcorrole.

The reduction of nitrobenzene in CH2Cl2 occurs in two steps, the firstof which is reversible and located at E1/2 = −1.08 V [59]. As seen inFig. 1, almost identical half-wave potentials are seen for reduction ofthe meso-NO2Ph groups on compounds 2 (E1/2 = −1.10 V), 3 (E1/2 =−1.09 V) and 4 (E1/2 = −1.06 V) under the same solution conditions.The peak current for the NO2Ph reduction of compound 2 at −1.10 Vis the same as that of the preceding CoIII/CoII reaction at Epc =−0.65 V, consistent with a one-electron transfer in each step, whilethe peak current for reduction of NO2Ph in compound 3 is located atEpc = −1.09 V and is twice high as that of the first reduction of thesame compound at Epc = −0.63 V. Likewise, the process for reductionof NO2Ph in compound 4 is located at Epc = −1.06 V and is threetimes as high as the current for the first reduction at Epc = −0.56 V(see voltammograms in Fig. 1). These results are consistent with atwo-electron addition to compound 3 and a three-electron addition tocompound 4 in the second reduction step. A singlemultielectron reduc-tion of meso-NO2Ph groups has previously been reported for iron [38]and copper triarylcorroles [50] as well as for tetraphenylporphyrinshaving NO2 substituents at the four meso-positions of the macrocycle[60]. The fact that only a single overlapping process is seen for thesecond reduction of compounds 3 and 4 is consistent with a lack ofany interaction between the meso-NO2Ph groups of the corrole, eachof which is reduced at essentially the same potential.

As indicated above, nitrobenzene undergoes two one-electronreductions in CH2Cl2 [59] and the same is seen for the NO2Ph groupsof compounds 2, 3 and 4. The second reduction of theNO2Ph group is ir-reversible as seen in Fig. 1 and located at a peak potential of −1.63 V(cpd 2) to −1.71 V (cpd 4). These redox processes are overlappedwith the Co(II)/Co(I) reduction of the corrole as evidenced by the rela-tive peak currents for the three reductions of the nitrocorroles.

Fig. 3.Plots of the redox potentials of cobalt corroles containingOMe,Me, H, F andCl (solidpoints) or NO2 groups (open points or asterisk) on the para-positions of the phenyl ringsof themacrocycle vs the sum of Hammett substituent constants (Σσ). The potentials weremeasured in CH2Cl2 containing 0.1 M TBAP and are summarized in Table 1. The Hammettsubstituent constants are taken from Ref. [62].

3.3. Substituent effect on the redox potentials and the site ofelectron transfer

The addition of one to three electron-withdrawing nitro groups tothe phenyl rings of the macrocycles leads to a systematic positive shiftin potential for the first two reductions and first two oxidations of com-pounds 2–4. For example, the CoIII/CoII transition of 1 is located at Epc =−0.73 V in CH2Cl2, 0.1 M TBAP and the potential shifts to −0.65 V for(NO2Ph)Ph2CorCo(PPh3) 2, −0.63 V for (NO2Ph)2PhCorCo(PPh3) 3and −0.56 V for (NO2Ph)3CorCo(PPh3) 4 (see Table 1 and Fig. 1). Plotsof E1/2 or Ep for each redox reaction of 1–4 vs the number of NO2 groupson the compound were constructed and give a linear relationship foreach electron addition and electron abstraction as shown in Fig. 2. Thefirst and second oxidations in the plots of Fig. 2 have a slope of +53and +46 mV per each added NO2 group while the slopes of ΔEp/Δ#NO2 for the reductions are +53, +20 and −32 mV, respectively.The change from a positive slope for the first two reductions to a nega-tive slope for the third reduction is perhaps expected, because of thenegative charge on the reaction site after the first one-electron reduc-tion of the NO2Ph group.

It is worth noting that the average ~50 mV positive shift in redoxpotentials of the first reduction and first two oxidations for eachadded NO2 group is similar to the shift of potential for iron corroleswith the same meso-NO2Ph substituents [38]. It is however, lessthan the ~200 mV shift in E1/2 observed when NO2-substituents areadded to the β-pyrrole positions of a triarylcorrole [38,41,42]. Thesmaller effect of the meso-phenyl substituents as compared to theβ-pyrrole substituents is consistent with data reported in the literaturefor other corroles, porphyrins and related macrocycles [39,58–61].

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Correlations were also made between redox potentials of thecurrently investigated nitrocorroles and five previously examinedtriarylcorroles containing electron-withdrawing or electron-donatingsubstituents at the meso-phenyl positions of the macrocycle [34]. Thiscomparison is shown in Fig. 3 which plots E1/2 or Epc vs the sum of theHammett substituent constants (Σσ) taken from Ref. [63].

Linear relationships are seen for the first and second oxidations ofall the compounds, with the slopes being +58 and +63 mV, respec-tively (see Fig. 3). This suggests that the three nitro-substitutedcorroles examined in the present study and the five previously inves-tigated non-nitro-substituted corroles all undergo the same twostepwise one-electron macrocycle-centered oxidations as described

wo or three nitro groups. Effect of NO2 substitution on electrochemical016/j.jinorgbio.2013.12.014

6 B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx

in the literature [34]. In contrast, two linear plots with differentslopes are observed for the three reductions in the plots in Fig. 3and this suggests that compounds in the series of non-nitrocorrolesand the nitrocorroles might undergo two different electron transfermechanisms or, alternately, two different sites of electron transfer.It is known that in the case of non-nitrocorroles, the first reductionof the compound is a metal-centered electron transfer to give aCo(II) derivative in CH2Cl2 [34]. Thus, the nitrocorroles might under-go a macrocycle-centered reduction to generate a Co(III) π-anionradical under the same solution conditions.

The site of electron transfer for each redox reaction was confirmedby thin-layer UV–visible spectroelectrochemistry. Examples of thespectral changes obtained for (Ph)3CorCoIII(PPh3) 1 and (NO2Ph)Ph2CorCoIII(PPh3) 2 during reduction in CH2Cl2, 0.1 M TBAP are illus-trated in Fig. 4 and the final spectrum of each electrogenerated Co(II)and Co(I) complex is shown in Fig. 5 which also includes the UV–vis-ible spectra of compounds 1–4 in their neutral form.

The Soret (378 nm) and Q bands (562 nm) of the neutral corrole 1are both red-shifted during the first controlled potential reduction at−1.20 V to give a final product with an intense Soret band at 421 nmand a weak Q band at 575 nm. This type of spectrum has previouslybeen assigned to a Co(II) corrole [34]. The UV–visible spectrum ofunreduced 2 is almost identical to that of unreduced 1 (see Table 2and Fig. 5a), but the spectral changes for 2 during the first one-electron reduction at −0.95 V in CH2Cl2 differ from that of 1 (Fig. 4b).In the case of 2, the Soret band of the neutral corrole at 384 nm only

a) (Ph)3CorCo(PPh3)

Fig. 4. UV–vis spectral changes during reduction of compounds 1 and 2 in CH2Cl2 containing 0

Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, tproperties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1

slightly decreases in intensity and a newly formed band of the prod-uct at 414 nm is present only as a shoulder. Interestingly, the spectralchanges of 2 during the second reduction at−1.40 V (Fig. 4b) are al-most the same as the spectral changes obtained for 1 during the firstreduction of this compound. This result suggests that the second re-duction of 2 involves a complete CoIII/CoII conversion after the addi-tion of two electrons, one of which is added to the NO2Ph substituent.Similar spectroelectrochemical behavior is seen for compounds 3and 4 and the electron transfer mechanism for the first two reduc-tions of the three nitrocorroles is proposed to occur as shown inScheme 2.

The final UV–visible spectra after the second one-electron reductionof 1 at−1.80 V and the third two-electron reduction of 2 at−1.70 V arequite similar. Both spectra are characterized by a single Soret band at414–415 nm and a single Q band at 575–580 nm (see Figs. 4 and 5).These spectra are assigned to a Co(I) corrole in each case.

3.4. Electrocatalytic reduction of O2

Fig. 6 illustrates the examples of cyclic voltammograms for 1 and 4adsorbed on an EPPG disk electrode in 1.0 M HClO4 under N2 (dashedline) and under air (solid line). A surface reaction is seen for bothcorroles at about 0.4 V, but no peaks assigned to oxidation or reductionof the cobalt corrole can be observed from 0.60 to −0.20 V under N2.The lack of a Co(III) corrole reduction at the electrode surface is consis-tent with the fact that PPh3 coordination to Co(III) shifts the half-wave

b) (NO2Ph)(Ph)2CorCo(PPh3)

.1 M TBAP. Cyclic voltammograms under the same solution conditions are shown in Fig. 1.

wo or three nitro groups. Effect of NO2 substitution on electrochemical016/j.jinorgbio.2013.12.014

a) neutral form b) CoII form c) CoI form

Fig. 5. UV–vis spectra of compounds 1–4 in CH2Cl2 containing 0.1 M TBAP under the following conditions: (a) in their neutral Co(III) form, (b) after reduction to Co(II) and (c) after re-duction to Co(I) in a thin-layer cell. Examples of the spectral changes as a function of time under a controlled reducing potential are shown in Fig. 4.

7B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx

potential for the CoIII/CoII process to potential located more negativethan −0.20 V under the given experimental conditions [56]. A similarlack of a cobalt corrole redox process was earlier reported for otherfive-coordinate cobalt corroles containing a bound PPh3 in acid mediaunder N2 [32,36].

A well-defined current–voltage curve is seen when the corrole coat-ed electrode is placed in 1.0 M HClO4 under air. This is shown by thesolid line in Fig. 6 where an irreversible cathodic reduction peak isseen at Epc = 0.13 V (1) and 0.17 V (4) for a scan rate of 50 mV/s. Com-pounds 2 and 3 also exhibit a similar reduction peak at 0.15 and 0.14 V,respectively (see Table 3). As will be demonstrated, the irreversible

Table 2UV–visible spectral data, λmax, nm (ε × 10−4, cm−1 M−1) of compounds (Cpd) 1–4 inCH2Cl2.

Compound Soret region Q region

Ph3CorCo(PPh3) 1 387 (4.3) 562 (0.8)(NO2Ph)Ph2CorCo(PPh3) 2 384 (4.6) 559 (1.0)(NO2Ph)2PhCorCo(PPh3) 3 375 (4.3) 417 (2.9)a 576 (1.3)(NO2Ph)3CorCo(PPh3) 4 372 (4.2) 419 (3.1)a 575 (1.1)

a Shoulder peak.

Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, tproperties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1

peaks obtained in HClO4 under air correspond to the catalytic reductionof dissolved O2 to give a mixture of H2O2 and H2O.

It should be pointed out that when the corresponding free-basecorrole was used as the catalyst under the same solution conditions(data not shown), a reduction peak with very a small current was ob-served at a more negative potential. The more negative potential andthe much reduced current indicates that cobalt ions are necessary forthe catalytic activity of the cobalt corrole to electroreduce oxygen inacid solution.

Peak potentials for the catalytic reduction of oxygen at the corrolecoated electrode are similar for compounds 1 to 4, which indicatesthat the potential for this reaction is not strongly influenced by differ-ences in the number of NO2 substituents on the corrole. The O2 reduc-tion peak varies from Ep = 0.13 for 1 to 0.17 V for 4 (see Table 3) anda similar potential (~0.14 V) has been reported when other cobaltcorroles with a bound PPh3 axial ligand are used as catalysts for theelectroreduction of O2 [32,36]. These peak potentials are all signifi-cantly lower than the potential reported for O2 reduction using aβ-pyrrole substituted mono-cobalt corrole such as 7,8,12,13-tetramethyl-2,3,10,17,18-pental- phenylcorrole cobalt(III), [(Me4Ph5Cor)Co] (0.38 V) [30] or 2,3,7,8,12,13,17,18-octalbromine-5,10,15-trispentafluorophenylcorrole cobalt(III), [(Br8TF5PCor)Co(0.56 V)] [25] as the catalyst. The O2 reduction potentials in

wo or three nitro groups. Effect of NO2 substitution on electrochemical016/j.jinorgbio.2013.12.014

Scheme 2. Proposed reduction mechanism of Co(III) corroles 2–4 in CH2Cl2 containing 0.1 M TBAP.

8 B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx

Fig. 6 are also lower than when the catalyst is a hangman corrole(0.43–0.48 V) [26] or a meso-substituted corrole, such as 5,10,15-trispentafluorophenyl corrole cobalt(III), [(TPFCor)Co], 10-pentafluorophenyl-5,15-dimesitylcorrole cobalt(III), [(F5PhMes2Cor)Co] or 5,10,15-trismesitylcorrole cobalt(III), [(Mes3Cor)Co] [28].

Measurements were also performed at a rotating disk electrode(RDE) to calculate the number of electrons transferred in the catalyticelectroreduction of dioxygen. The RDE response was similar for allfour corroles in air-saturated 1.0 M HClO4 (see Fig. 7a) and is character-ized by a half-wave potential located at 0.28 to 0.29 V for 1–4, whereimax is the limiting currentmeasured at 400 rpm and E1/2 is the potentialwhen i = 0.5imax.

The number of electrons transferred during oxygen reduction wascalculated from the magnitude of the steady-state limiting currentswhich were taken at a fixed potential on the catalytic wave plateau ofthe current–voltage curves in Fig. 7b (−0.10 V). When the amount of

a) (Ph)3CorCo(PPh3)

Fig. 6. Cyclic voltammograms of 1 and 4 adsorbed on EPPG electrode in 1.0 M HClO4 sat

Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, tproperties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1

O2 reduction at the corrole modified electrode is controlled by masstransport alone, the relationship between the limiting current and rota-tion rate should obey the Levich equation given in Eq. (1) [63].

jlev ¼ 0:62nFAD2=3cv−1=6ω1=2 ð1Þ

where n is the number of electrons transferred in the overall electrodereaction, F is the Faraday constant (96,485 C mol−1), A is the electrodearea (cm2), D is the dioxygen diffusion coefficient (cm2·s−1), c is thebulk concentration of O2 in 1.0 M HClO4, v is the kinematic viscosity ofthe solution andω is the angular rotation rate of the electrode (rad·s−1).

When the reciprocal of the limiting current density is plotted againstthe reciprocal of the square root of the rotation rate, the resultingstraight line (see Fig. 7) obeys the Koutecky–Levich equation [64].

1= j ¼ 1= jlev þ 1= jk ð2Þ

b) (NO2Ph)3CorCo(PPh3)

urated with nitrogen (———) and saturated with air (____) at a scan rate of 50 mV/s.

wo or three nitro groups. Effect of NO2 substitution on electrochemical016/j.jinorgbio.2013.12.014

Table 3Data of catalytic reduction of O2 in 1.0 M HClO4.

catalyst Ep with O2 E1/2a n H2O2%

(Ph)3CorCo(PPh3) 1 0.13 0.28 2.6 70(NO2Ph)Ph2CorCo(PPh3) 2 0.15 0.28 2.7 65(NO2Ph)2PhCorCo(PPh3) 3 0.14 0.29 2.9 55(NO2Ph)3CorCo(PPh3) 4 0.17 0.28 3.0 50

a Half-wave potential at i = 0.5imax, where imax is the limiting current measured at400 rpm of the RDE.

9B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx

where j is the measured limiting current density (mA·cm−2), jlev is theLevich current, and jk is the kinetic current which can be obtained ex-perimentally from the intercept of the Koutecky–Levich line in Fig. 7b.

jk ¼ 103knFGc: ð3Þ

In Eq. (3), the value of k (M−1 s−1) is the second-order rate constantof the reactionwhich limits the plateau current and Γ (mol·cm−2) is thesurface concentration of corroles. The other terms have their usualsignificance, as described above.

The slope of a plot in Fig. 7 obtained by linear regression can then beused to estimate the average number of electrons (n) involved in thecatalytic reduction of O2. This analysis was carried out and the numberof electrons transferred per dioxygen molecule (n) during the catalyticreduction of O2 by corroles 1–4 was calculated. These values aresummarized in Table 3 which also includes the reduction potentials for1–4 in air saturated HClO4. The Koutecky–Levich plots for 1–4 indicatethat the number of electrons transferred to O2 in the electroreductionprocess ranges from 2.6 to 3.0 (Table 3) with the largest value beingobtained for 4 (n = 3.0) and the smallest for 1 (n= 2.6).

a) (Ph)3CorCo(PPh3)

Fig. 7. Current–voltage curve and the Koutecky–Levich plots for catalytic reduction of O2 in 1.0MElectrode rotating rates (ω) are indicated on each curve. Potential scan rate = 50 mV/s.

Please cite this article as: B. Li, et al., Cobalt triarylcorroles containing one, tproperties and catalytic..., J. Inorg. Biochem. (2014), http://dx.doi.org/10.1

Dioxygen can be cathodically reduced by 4e− to give H2O or via 2e−

to give H2O2 after which the resulting H2O2might be further reduced toH2O, catalytically decomposed on the electrode surface or removed intothe bulk of the solution [25,32].When the products of O2 reduction con-sist of a mixture of H2O2 and H2O, values of n between 2 and 4 will becalculated. In the current study, the Koutecky–Levich plots show thatthe number of electrons transferred (n) ranges from 2.6 to 3.0 for com-pounds 1–4, indicating that the catalytic electroreduction of O2 is nei-ther a simple 2e− transfer process to give H2O2 nor a 4e− process toproduce H2O, but consistent with the formation of both H2O and H2O2

using compounds 1–4 as catalysts in acidic media.As seen in Table 3, compound 1, which doesn't contain any substitu-

ent on the phenyl rings, produces the largest amount of H2O2 as indicat-ed by the lower number of electron transferred (n = 2.6), whilecompound 4, which contains three strong electron-withdrawing NO2

groups, produced the smallest amount ofH2O2 andmoreH2Oas indicatedby the higher n values of 3.0, under the same experimental conditions.These results indicate that cobalt corroles substituted with the stronglyelectron-withdrawing NO2 groups may have a higher dioxygen-bindingability as was proposed for other substituted cobalt corroles and porphy-rins having different structures [65].

4. Conclusion

Thin-layer UV-vis spectroelectrochemistry indicate that the first one-electron reduction of the nitro-substituted cobalt(III) triarylcorroles isdelocalized, leading to the formation of amixture of a Co(III)π-anion rad-ical and a Co(II) corrole in CH2Cl2 containing 0.1 M TBAP. Each NO2Phgroup of the nitrocorrole can be reduced at the same potential before aCo(I) corrole is generated at a more negative potential. The cobaltcorroles may catalyze the reduction of O2 to produce a mixture of H2O

b) (NO2Ph)3CorCo(PPh3)

HClO4 saturatedwith air at a rotating EPGdisk electrode coatedwith compounds 1 and 4.

wo or three nitro groups. Effect of NO2 substitution on electrochemical016/j.jinorgbio.2013.12.014

10 B. Li et al. / Journal of Inorganic Biochemistry xxx (2014) xxx–xxx

and H2O2 in 1.0 MHClO4. The larger the number of nitro-substituents onthe corrole, the better the cobalt compound acts as a catalyst for thereduction of molecular oxygen in acid media.

Acknowledgments

This work was supported by grants from the Natural Science Foun-dation of China (Grant 21071067) and the Robert A. Welch Foundation(KMK, Grant E-680).

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