DOI: 10.1002/chem.201405570

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& Porphyrinoids

Effect of Solvent and Protonation/Deprotonation on Electrochemistry, Spectroelectrochemistry and Electron-Transfer Mechanisms of N-Confused Tetraarylporphyrins in Nonaqueous Media Songlin Xue,[a] Zhongping Ou,*[a] Lina Ye,[a] Guifen Lu,[a] Yuanyuan Fang,[b] Xiaoqin Jiang,[b] and Karl M. Kadish*[b]

system. Each porphyrin undergoes multiple irreversible reductions and oxidations. The first one-electron addition and first one-electron abstraction are located on the porphyrin p-ring system to give p-anion and p-cation radicals with a potential separation of 1.52 to 1.65 V between the two processes, but both electrogenerated products are unstable and undergo a rapid chemical reaction to give new electroactive species, which were characterized in the present study. The effect of the solvent and protonation/deprotonation reactions on the UV/Vis spectra, redox potentials and reduction/oxidation mechanisms is discussed with comparisons made to data and mechanisms for the structurally related free-base corroles and porphyrins.

Abstract: A series of N-confused free-base meso-substituted tetraarylporphyrins was investigated by electrochemistry and spectroelectrochemistry in nonaqueous media containing 0.1 m tetra-n-butylammonium perchlorate (TBAP) and added acid or base. The investigated compounds are represented as (XPh)4NcpH2, in which “Ncp” is the N-confused porphyrin macrocycle and X is a OCH3, CH3, H, or Cl substituent on the para position of each meso-phenyl ring of the macrocycle. Two distinct types of UV/Vis spectra are initially observed depending upon solvent, one corresponding to an inner-2H form and the other to an inner-3H form of the porphyrin. Both forms have an inverted pyrrole with a carbon inside the cavity and a nitrogen on the periphery of the p-

Introduction N-Confused porphyrins have attracted a great deal interest[1–9] since the first synthesis of these compounds was independently reported by the groups of Furuta[10] and Latos-Grazynski[11] in 1994. N-Confused porphyrins are structurally similar to “normal” porphyrins, but have an inverted pyrrole with a carbon inside the cavity and a nitrogen on the periphery of the p-system as shown in Scheme 1. Furthermore, the N-confused free-base porphyrins (NcpH2) can exist in two tautomeric forms in solution, the exact composition of which will depend upon the proton-accepting ability of the solvent.[12–18] The inner-2H form of the porphyrin is mainly observed in strongly proton-accepting nonaqueous solvents, such as N,N’-dimethylformamide (DMF), dimethylacetamide (DMAc), and dimethyl-

Scheme 1. Structures of a) N-confused porphyrins and b) normal free-base porphyrins.

sulfoxide (DMSO), while the inner-3H form is exclusively observed in aprotic solvents such as dichloromethane (CH2Cl2), chloroform (CHCl3), and toluene.[1] N-Confused porphyrins have unique spectral and coordination properties as compared to porphyrins[1–9, 19–22] and have possible applications in the fields of catalysts, functional materials, molecular switches, and ion transducers as well as in the areas of coordination and supramolecular chemistry.[23–32] The electrochemistry of “normal” free-base porphyrins has been well documented in the literature,[33] but this is not the case for N-confused porphyrins for which the solution electrochemistry is characterized by multiple irreversible processes

[a] S. Xue, Prof. Z. Ou, L. Ye, G. Lu School of Chemistry and Chemical Engineering Jiangsu University, Zhenjiang 212013 (China) E-mail: [email protected] [b] Y. Fang, X. Jiang, Prof. K. M. Kadish Department of Chemistry University of Houston, Houston, Texas 77204-5003 (USA) Fax: (+ 1) 713-743-2745 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405570. Chem. Eur. J. 2014, 20, 1 – 12

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Full Paper and only a few free-base derivatives have been electrochemically examined in DMF or CH2Cl2.[16, 25, 34] There have been no systematic studies on how changes in solvent, porphyrin ring substituents, and protonation/deprotonation reactions will affect the electrochemistry and oxidation/reduction products of N-confused free-base porphyrins in nonaqueous media, and very little is known about the spectral properties of the oxidized or reduced compounds as a function of solvent or applied oxidation/reduction potential. These points are addressed in the present work where a series of N-confused free-base tetraarylporphyrins is electrochemically examined in dichloromethane (CH2Cl2), benzonitrile (PhCN), and N,N’-dimethylformamide (DMF) containing 0.1 m tetra-n-butylammonium perchlorate. Each neutral, oxidized, and reduced form of the porphyrin was also characterized by spectroelectrochemistry. The effect of the solvent, phenyl ring substituents, and protonation/deprotonation reactions on the redox potentials and reaction mechanisms is discussed, with comparisons made to the structurally related free-base corroles and porphyrins. The investigated compounds are shown in Scheme 2 in their inner2H form and are represented as (XPh)4NcpH2, in which “Ncp” is the N-confused porphyrin macrocycle and X is a OCH3, CH3, H or Cl substituent on the para position of each meso-phenyl ring of the macrocycle.

Scheme 2. Structures of examined N-confused free-base porphyrins shown in their inner-2H form.

Results and Discussion Synthesis and characterization The first N-confused free-base porphyrins were initially isolated in very low yield as a byproduct during synthesis of a porphyrin in 1994.[10, 11] Five years later, Lindsey and co-workers reported an improved synthetic route for N-confused free-base tetraphenylporphyrin and obtained a product yield of 39 % using DDQ as an oxidation reagent.[35] A similar method was followed in the present paper for synthesis of 1–4, but p-chloranil was Figure 1. 1H NMR spectra of (ClPh)4NcpH2 4 in a) [D7]DMF, b) CDCl3, and c) CDCl3 with 30 equivalents added TFA at utilized as the oxidation reagent 298 K. instead of DDQ and the yield ranged from 12 to 44 %, depending upon the specific phenyl ring substituent. CDCl3, compound 4 exists in its inner-3H form, which has a The identity of the tautomer obtained, inner-2H or inner-3H resonance at 2.56 ppm for the two inner NH protons and as seen in Scheme 1, was determined by 1H NMR measureanother resonance at 5.00 ppm for the inner CH proton (Figure 1 b). A similar assignment was earlier made for ments at room temperature and examples of the spectra are il(Ph)4NcpH2 in CDCl3.[22] lustrated in Figure 1 for (ClPh)4NcpH2 4. In [D7]DMF, compound 4 exists in its inner-2H form as previously shown for Further confirmation for synthesis of the free-base N-con(Ph)4NcpH2 in DMF.[22] Resonances are seen for the one outer fused porphyrins was obtained by examining the NMR spectrum of 4 in CDCl3 containing 30 equivalents of added acid. An and one inner NH proton at 13.46 and 2.39 pm, while the single inner CH proton is observed at 0.85 ppm (Figure 1 a). In example of this spectrum is shown in Figure 1 c in which the &

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Full Paper inner CH and outer NH protons of the inverted pyrrole are characterized by sharp absorptions at 1.37 and 11.28 ppm, respectively. The other three inner NH protons of the porphyrin have resonances between 0.65 and 2.49 ppm. This result is consistent with the formation of [(ClPh)4NcpH4]2 + in solution and with the initial compound having an N-confused porphyrin framework.[10–12]

as the strongest band, which also has the longest wavelength and is located at 690 nm in DMF and 704 nm in pyridine (Figure 2 b). Similar solvent-dependent spectra are observed for compounds 1, 2 and 3 as seen from the data in Table S1 and Figures S1 and S2 in the Supporting Information.

Electrochemistry Each N-confused free-base porphyrin was electrochemically examined in DMF, CH2Cl2, and PhCN containing 0.1 m tetra-nbutylammonium perchlorate (TBAP). The cyclic voltammograms of 1–4 are characterized by multiple irreversible reduction and oxidation peaks in the three solvents and these are described below along with UV/Vis spectra of the oxidation/reduction products and the prevailing reduction and oxidation mechanisms. The first reduction and first oxidation are in each case located at the conjugated porphyrin macrocycle, but the generated radicals are highly reactive and rapidly convert to new porphyrin oxidation/reduction products as shown in Scheme 3. This is discussed separately for reduction and then oxidation on the following pages.

UV/Vis spectra Two different types of UV/Vis spectra were obtained for (XPh)4NcpH2 1–4 depending upon the solvent utilized. One type of spectrum is assigned to an inner-3H form of the porphyrin and the other to an inner-2H form (Scheme 1). The spectral data of each porphyrin in ten different nonaqueous solvents are summarized in Table S1 in the Supporting Information, in which spectra of the inner-3H forms are listed as Type I and those of the inner-2H form as Type II. Examples of spectra between 300–800 nm are given in Figure 2 and Figures S1 and S2 in the Supporting Information.

Scheme 3. Proposed mechanism of compounds 1–4 in DMF, CH2Cl2 and PhCN. The neutral initial porphyrin is shown in bold print.

The potentials listed for reduction or oxidation of the initial (XPh)4NcpH2 compounds 1–4 in Scheme 3 range from 0.82 to 1.07 V for reduction and 0.55 to 0.75 V for oxidation, with the exact value in each case depending upon the solvent and the electron-donating or electron-withdrawing substituent on the compound. These potentials are summarized in Table 1 as ox1 and red1 under the heading of “initial (XPh)4NcpH2”, which also includes the absolute separation between the peak potential for the one-electron abstraction and one-electron addition, DE(o1r1), corresponding to an approximate HOMO–LUMO gap of 1.52 to 1.66 V.

Figure 2. UV/Vis spectra of (ClPh)4NcpH2 4 in its a) Type I inner-3H form in CH2Cl2 and toluene and b) Type II inner-2H form in DMF and pyridine.

The difference between the two types of spectra can be seen in Figure 2 which compares the Type I spectrum of compound 4 in CH2Cl2 and toluene to the Type II spectrum of 4 in DMF and pyridine. Both sets of spectra have a sharp Soret band and four bands in Q-band region labeled as Q1, Q2, Q3, and Q4. The Type II (inner-2H form) spectra in Figure 2 have a 4–8 nm red-shift in the position of the Soret band as compared to the Type I spectra. There is also a difference in the ratio of intensities for the four bands in the two types of spectra. The strongest band of 4 in the Type I spectra is Q2 at 584 nm in CH2Cl2 and at 587 nm in toluene, while the weakest band is the Q3 located at 645 and 651 nm in these two solvents (Figure 2 a). In contrast, the Type II spectra of 4 have Q4 Chem. Eur. J. 2014, 20, 1 – 12

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Reduction Figure 3 illustrates the first two reductions of (XPh)4NcpH2 1–4 in CH2Cl2 containing 0.1 m TBAP. As seen in this figure, the potential of the first irreversible reduction ranges from Epc = 1.07 (for 1) to 0.90 V (for 4), while the second process ranges from Epc = 1.69 to 1.58 V for a scan rate of 0.1 V s1. A third irreversible reduction, not shown in the figure, occurs at the edge of solvent limit and is located at Epc = 1.85 to 2.06 V for a scan rate of 0.1 V s1. The first one-electron addition to (XPh)4NcpH2 is irreversible and generates a porphyrin p-anion radical, [(XPh)4NcpH2]C , 3

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Full Paper Table 1. Redox potentials (V vs SCE) of compounds 1–4 in CH2Cl2, PhCN and DMF Containing 0.1 m TBAP. Solvent

Compound

4s[a] 3rd

CH2Cl2

PhCN

DMF

1-(OCH3) 2-(CH3) 3-(H) 4-(Cl) 1-(OCH3) 2-(CH3) 3-(H) 4-(Cl) 1-(OCH3) 2-(CH3) 3-(H) 4-(Cl)

1.08 0.68 0.00 0.92 1.08 0.68 0.00 0.92 1.08 0.68 0.00 0.92

Oxidation product 2nd 1st

1.55 1.46 1.52 1.53 1.54[b] 1.58[b] 1.56[b] 1.56[b]

1.36 1.08 1.19 1.24 1.14[c] 1.39[d] 1.20 1.24 0.92[b]

ox1 0.59[b] 0.64[b] 0.69[b] 0.70[b] 0.55[b] 0.58[b] 0.57[b] 0.70[b] 0.55[b] 0.60[b] 0.65[b] 0.72[b]

0.78 0.89 0.95 0.97 0.85[b] 0.76[b] 1.00[b] 1.08[b] 0.80[b] 1.07[b] 1.09[b] 0.94[b]

Initial (XPh)4NcpH2 red1 DE(o1-r1) 1.07[b] 1.04[b] 0.97[b] 0.90[b] 1.04[b] 1.01[b] 0.87[b] 0.86[b] 0.93[b] 0.92[b] 0.89[b] 0.82[b]

1.66 1.68 1.66 1.60 1.59 1.59 1.44 1.56 1.48 1.52 1.54 1.54

Reduction product 1st 2nd 1.69[b] 1.69 1.54 1.58[b] 1.75[b] 1.71[b] 1.62[b] 1.58[b] 1.72[b] 1.63[b] 1.59[b] 1.53[b]

– – 2.06[b] 1.98[b] – – 2.00[b] 1.95[b] – 1.95[b] 1.85[b] 1.96[b]

[a] Values of s taken from reference [40]. [b] Irreversible peak potential at a scan rate of 0.10 V s1. [c] An irreversible broad peak can also be seen at 1.00 V. [d] An irreversible peak can also be seen at 0.93 V.

Scheme 4. Proposed reduction mechanism of compounds 1–4 (potential given for 4 in PhCN). The initial neutral porphyrin is shown in bold print.

bined electrochemical and spectroelectrochemical experiments carried out in PhCN containing 0.1 m TBAP and added tetrabutylammonium hydroxide (TBAOH), and also by comparison with previously published spectra for deprotonated N-confused free-base tetraphenylporphyrin in CH2Cl2, DMF, and toluene.[14] Under our experimental conditions, the UV/Vis spectrum of the product formed after the first reduction of 4 in the thin-layer cell is characterized by a Soret band at 457 and four visible bands at 549, 591, 641 and 691 nm (Figure 4, top right). This spectrum compares well with the published spectrum of deprotonated N-confused free-base tetraphenylporphyrin[14, 36] Figure 3. Cyclic voltammograms illustrated first two reductions of compounds 1–4 in CH2Cl2 containing 0.1 m TBAP. Scan rate = 0.1 V s1. The first reduction corresponds to a reaction of the initial compound and the second to the mono-deprotonated reduction product.

and this is followed by a rapid chemical reaction (an electrochemical (EC) mechanism) to give a species which can then be further reduced in two steps, the first of which occurs at a peak potential of 1.58 to 1.69 V as shown in Figure 3 and Figure S3 in the Supporting Information. Similar reduction behavior is seen for 1–4 in DMF and PhCN and a proposed reduction mechanism is shown in Scheme 4, in which the initial neutral porphyrin is shown in bold and the electroactive product of the first reduction and following chemical reaction is assigned as the mono-deprotonated porphyrin [(XPh)4NcpH] . Evidence for the deprotonated species [(XPh)4NcpH] as the final product of the first one-electron addition is given by com&

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Figure 4. UV/Vis spectra of neutral (ClPh)4NcpH2 4 and its mono-deprotonated product formed after a one-electron reduction in PhCN or by addition of TBAOH to the solution.

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Full Paper and it is also virtually identical to the UV/Vis spectrum of the product formed by adding 1.0 equivalents of TBAOH to a solution of compound 4 in PhCN (Figure 4, bottom right). The reduction mechanism shown in Scheme 4 and the identity of the chemically generated product of the first electron transfer was further confirmed by a comparison of cyclic voltammograms for 4 in PhCN with and without added TBAOH. These cyclic voltammograms are illustrated in Figure 5. Compound 4 undergoes three reductions located at 0.86, 1.58, and 1.95 V in PhCN, before addition of TBAOH (Figure 5 a), but after 1.0 equivalent of TBAOH has been added to the solution only the last two redox processes remain at 1.60 and

Oxidation As indicated in Scheme 3, the first oxidation of 1–4 is followed by a rapid chemical reaction to give one or more products which can be further oxidized at more positive potentials. The first one-electron abstraction to give a p-cation radical is located at Ep = 0.55 to 0.72 V, and this is followed by a second and third redox process at 0.76 to 1.07 V and 0.92 to 1.39 V, with the exact potentials depending upon the solvent and specific electron-donating or electron-withdrawing porphyrin ring substituent. Examples of cyclic voltammograms are shown in Figure 6 for compound 4 and a summary of potentials are

Figure 5. Cyclic voltammograms illustrating the reductions of (ClPh)4NcpH2 4 a) in PhCN containing 0.1 m TBAP and b) after addition of 1.0 equivalent of TBAOH to the solution. Scan rate = 0.1 V s1.

1.98 V, with the first reduction at 0.86 V having totally disappeared as seen in Figure 5 b. This result is consistent with [(ClPh)4NcpH] being generated as the final product in the first reduction and it is also consistent with the second and third reductions at 1.60 and 1.98 V in Figure 5 being assigned to electron transfers involving the mono-deprotonated porphyrin to give [(ClPh)4NcpH]2 and [(ClPh)4NcpH]3, respectively. In summary, our results unambiguously indicate that the same species is formed from (ClPh)4NcpH2 by a one-electron reduction in PhCN or by addition of base to the same porphyrin in this solvent. This spectrum is assigned as the mono-deprotonated porphyrin anion [(ClPh)4NcpH] in Scheme 4 and this species will undergo a further reduction at 1.58 and 1.95 V in PhCN without TBAOH. The mechanism shown in Scheme 4 is proposed to occur for each compound in the three solvents in which the first reduction involves electron addition to the neutral N-confused porphyrin and the next two reductions involve electron addition to an anionic deprotonated N-confused porphyrin. Potentials for each reduction process are given in Table 1 and labeled as corresponding to reduction of the initial neutral porphyrin or to reduction of the (deprotonated) reduction product.

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Figure 6. Cyclic voltammograms showing the oxidations of (ClPh)4NcpH2 4 in a) DMF, b) CH2Cl2 and c) PhCN containing 0.1 m TBAP at a scan rate of 0.1 V s1. The same reactions are seen at a glassy carbon electrode (GCE) and a platinum working electrode (PtE) as illustrated in part c).

listed in Table 1, in which oxidation of the neutral porphyrins to its p-cation radical form is labeled as ox1 under the heading “initial (XPh)4NcpH2” and that for oxidation of the product resulting from the chemical reaction of the p-cation radical is labeled as 1st, 2nd, and 3rd (oxidation product). Oxidation potentials were measured in PhCN using both glassy carbon and platinum electrodes to rule out an earlier suggestion[16] that the initial two oxidations observed for freebase N-confused porphyrins in DMF and toluene actually involved a surface process of hydroquinone type groups at the glassy carbon electrode and not an electron transfer of the porphyrin itself (spectroelectrochemical data discussed on the following pages also clearly rules out this possibility). Previous electrochemical studies on the oxidation of freebase corroles (Cor)H3 and free-base porphyrins (Por)H2 in nona5

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Full Paper queous media showed that a protonated form of the oxidation product, [(Cor)H4] + or [(Por)H4]2 + , was generated in solution by means of a chemical reaction following electron transfer.[33, 37, 38] This is also the case for the currently examined N-confused free-base porphyrins, in which mono- and bis-protonated porphyrins, [(ClPh)4NcpH3] + and [(ClPh)4NcpH4]2 + , are generated stepwise as the products of oxidation. The one-electron oxidations and following chemical reactions are represented as E1 or E2 and C1 or C2, respectively, in Equations (1) and (2). ðClPhÞ4 NcpH2 G

e E1

þ þ H½ðClPhÞ4 NcpH2  ƒ!½ðClPhÞ 4 NcpH3  C1

-e 2þ ½ðClPhÞ4 NcpH3 þ G H½ðClPhÞ4 NcpH3 2þ ƒ!½ðClPhÞ 4 NcpH4  C2

Figure 7 illustrates cyclic voltammograms for the oxidation of compound 4 in PhCN before and after addition acid to solution. In the absence of acid, two oxidations are observed. The first, at 0.70 V, is irreversible and coupled to a re-reduction at Epc = 0.30 V on the reverse scan, while the second, at Epa = 1.08 V, is coupled to a re-reduction at Epc = 0.50 V upon reversing the potential sweep. The new peaks are labeled as I’ and II’ in Figure 7 and correspond to the reduction of the mono- and bis-protonated N-confused free-base porphyrins as described on the following pages. The effect of acid on the oxidation of 4 in PhCN is shown in Figure 7 b and 7 c, in which the first oxidation peak at 0.70 V (labeled as rxn I) significantly decreases in current when 0.5 equivalents TFA are added to solution and totally disappears when 1.0 equivalents TFA have been added (Figure 7 c). The second oxidation of the porphyrin at 1.08 V (labeled as rxn II) is unaffected by the addition of acid, but the reduction peak labeled as process II’ remains and the irreversible re-reduction at Epc = 0.30 V in neat PhCN (Figure 7 a) becomes reversible in a PhCN solution with > 0.5 equivalents of added TFA (Figure 7 b and 7 c). Similar electrooxidation behavior is observed for the other free-base porphyrins in PhCN, CH2Cl2, and DMF and a proposed mechanism for the first two oxidations of 1–4 can be derived by comparison of data from the spectroelectrochemical measurements and TFA titrations of the neutral compounds in PhCN in which stepwise protonations are observed [Eqs. (3) and (4)]. The proposed mechanism for conversion of (ClPh)4NcpH2 to [(ClPh)4NcpH3] + in the first step and [(ClPh)4NcpH3] + to [(ClPh)4NcpH4]2 + in the second is shown in Scheme 5 in which the first and second one-electron abstractions are described as EC mechanisms (a reversible electron transfer followed by an irreversible chemical reaction).

ð1Þ ð2Þ

E2

and Evidence for formation of [(ClPh)4NcpH3] + 2+ [(ClPh)4NcpH4] as electrooxidation products is given in part by comparison with published spectral data for N-confused porphyrins in their protonated form,[14, 36, 37] in part by cyclic voltammograms taken in PhCN solutions with and without added trifluoroacetic acid (TFA; Figure 7) and in part by comparisons

Figure 7. Cyclic voltammograms of (ClPh)4NcpH2 4 in PhCN a) without TFA, b) with 0.5 equivalents of TFA and c) with 1.0 equivalent of TFA containing 0.1 m TBAP.

Scheme 5. Proposed mechanism for the first two oxidations of compounds 1–4.

Mono-protonated porphyrins are obtained as the final product of the first oxidation in both PhCN and DMF, but the chemical reaction following electron transfer is slower in DMF than in PhCN and a transient p-cation radical spectrum of [(ClPh)4NcpH2]C + can be detected on the spectroelectrochemical timescale. The chemical reaction following the second electron transfer is also slower in DMF than in PhCN and in

of the UV/Vis spectra obtained by spectroelectrochemical monitoring of the oxidations and that obtained after protonationof the neutral porphyrins 1–4 as shown by Equations (3) and (4).

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ðClPhÞ4 NcpH2 þ Hþ Ð ½ðClPhÞ4 NcpH3 þ

ð3Þ

½ðClPhÞ4 NcpH3 þ þ Hþ Ð ½ðClPhÞ4 NcpH4 2þ

ð4Þ

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Full Paper electron transfer is slowed down compared to what is observed in PhCN. This results in a stepwise conversion of (ClPh)4NcpH2 to [(ClPh)4NcpH3] + with evidence for a transient unprotonated pcation radical seen on the spectroelectrochemical timescale. This is shown in Figure 9 a, in which the Soret band and four visible bands of the neutral compound decrease in intensity as a broad 820 nm band grows in from 0 to 193 s. These spectral changes are consistent with an initial formation of the porphyrin p-cation radical [(ClPh)4NcpH2]C + in the initial stages of the oxidation. However, the spectra continue to change with time and the final spectrum after 314 s is characterized by a 457 nm Soret band and two broad Q bands at 772 and 830 nm. This spectrum is shown in the middle of FigFigure 8. Thin-layer UV/Vis spectral changes of (ClPh)4NcpH2 4 a) during the first one-electron oxidation in PhCN ure 9 a and assigned as due to containing 0.1 m TBAP or with a TFA titration of the neutral compound to give the mono-protonated product and [(ClPh)4NcpH3] + . Again, the same b) during the second one-electron oxidation or with TFA titration of the mono-protonated porphyrin in PhCN. mono-protonated spectrum is obtained as the product in a titration of (ClPh)4NcpH2 with TFA in DMF (Figure 9 b). this case only the radical cation of the oxidized mono-cationic species is seen, that is, [(XPh)4NcpH3]2 + . Further oxidation of the mono-protonated porphyrin at 1.10 V in DMF leads to the spectral changes shown in the Evidence in support of the above statements is given by the bottom of Figure 9 a. The Soret band at 457 nm and Q bands data in Figures 8 (in PhCN) and 9 (in DMF) in which the UV/Vis at 772 and 830 nm all decrease in intensity and no new bands spectra obtained during electrooxidation in a thin-layer cell are are observed. This process is assigned to a ring-centered compared to the spectral changes obtained upon addition of reduction of the mono-protonated porphyrin to give a monoTFA into a solution of 4 in PhCN or DMF. For example, the protonated N-confused porphyrin p-cation radical, the specspectral product after oxidation of 4 in PhCN has a well-detrum of which differs from the spectrum of the unoxidized bisfined Soret band at 457 nm and a Q band at 799 nm. Almost protonated porphyrin generated in an acid titration according the same absorptions were earlier reported for unoxidized Nto Equation (4). This last spectrum is shown in bottom of confused free-base tetraphenylporphyrin in its protonated Figure 9 b. In summary, a stable [(ClPh)4NcpH3]2 + dication is form.[14] More importantly, however, is the fact that exactly the same bands are seen after the first protonation of the neutral generated after the second oxidation in DMF and a following compound by Equation (3) to form the mono-protonated chemical reaction to give the bis-protonated porphyrin is [(ClPh)4NcpH3] + (bottom spectrum in Figure 8 a). Likewise, the not observed in this solvent on the spectroelectrochemical timescale. spectrum obtained after the second controlled potential oxidation of 4 at 1.20 V has a Soret band at 464 nm and two Q bands at 668 and 826 nm (top spectrum in Figure 8 b). Again, Substituent effects on potentials and electrochemical almost the same spectrum with absorption bands at 467, 668 HOMO–LUMO gap and 828 nm is observed for the doubly protonated porphyrin [(ClPh)4NcpH4]2 + , which is generated according to Equation (4) In general, an easier reduction and harder oxidation can be observed for porphyrins or corroles with electron-withdrawing (bottom spectrum in Figure 8 b). These results unambiguously substituents, while a harder reduction and easier oxidation can indicate that the mono- and bis-protonated porphyrins, be seen for compounds with electron-donating groups on the [(ClPh)4NcpH3] + and [(ClPh)4NcpH4]2 + , are the porphyrinic macrocycles.[33] This is also the case for the currently examined species generated after the first and second one-electron oxidations of the N-confused porphyrin in PhCN. N-confused porphyrins. Compound 4, which contains four elecA different pattern of spectral changes is observed during tron-withdrawing Cl groups, is easiest to reduce and hardest oxidation in DMF in which the chemical reaction following to oxidize, while compound 1, with four electron-donating Chem. Eur. J. 2014, 20, 1 – 12

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Full Paper The difference in potentials between the first reduction to give a p-anion radical and the first oxidation to give a p-cation radical averages 1.52 V in DMF and 1.65 V in CH2Cl2, as seen in Figure S4 in the Supporting Information and also in Table 1, which lists this DE value for each individual porphyrin in the three solvents. When the electrochemical reactions are reversible, DE1/2 can be taken as a measure of the electrochemical HOMO– LUMO gap, which has been reported to average (2.25  0.15) V for the case of meso-substituted tetraarylporphyrins[33, 41] and 2.26 V for (TPP)H2 in CH2Cl2. DFT calculations at the PCMB3LYP/LanL2DZ/6-31G(d) level show that the energy of the HOMO orbital for compounds 1– 4 follows the order at 4.61 (1) > 4.72 (2) > 4.79 (3) > Figure 9. Thin-layer UV/Vis spectral changes of (ClPh)4NcpH2 4 a) during the first and second controlled potential 4.92 eV (4), while the LUMO oroxidations in DMF containing 0.1 m TBAP and b) during protonation of 4 in DMF with TFA. The first oxidation is bital energy follows the order at characterized by two sets of spectral changes, one at 0–193 s and the second at 194–314 s of applied potential. 2.41 (1) > 2.46 (2) > 2.50 (3) > 2.63 eV (4). The calculated HOMO–LUMO gap is 2.20 V for 1, 2.26 V for 2, and 2.29 V for 3 and 4 and the average calculatOCH3 substituents, is hardest to reduce and easiest to oxidize ed HOMO–LUMO gap for the four N-confused free-base tetamong the four examined compounds (see potentials in raarylporphyrins is 2.26 V (see Figure 10). These separations are Table 1). exactly the same as for the “normal” free-base tetraphenylporThe shift in oxidation and reduction potentials can be quanphyrins. However, the experimentally measured potential sepatitated by the linear free-energy relationship E = Ss1,[33, 40] in ration (DE) between the first oxidation and first reduction of which E is the measured half-wave or peak potential for the compound, s is the Hammett substituent constant and 1, measured in volt, represents the sensitivity of the given redox reaction to the change of substituent. Figure S4 in the Supporting Information illustrates how potentials for the first oxidation and first reduction of the (XPh)4NcpH2 compounds vary as a function of the phenyl ring substituent, X. Linear relationships are observed between the peak potentials for oxidation or reduction and the sum of the Hammett substituent constants (4s), with the slope of the straight line (1) ranging from 53 to 98 mV in the different solvents. Similar 1 values of 50–100 mV have been reported for a number of meso-substituted tetraarylporphyrins[41] and mesosubstituted triarylcorroles.[38, 42, 43] Although there is no doubt that the first oxidation of 1–4 involves the N-confused porphyrin macrocycle, the positive linear shift of potential for the first oxidation with increasing electron-withdrawing properties of the phenyl ring substituents clearly rules out this process as being due to a non-porphyrin-based surface reaction of the glassy carbon electrode, as was incorrectly suggested in an Figure 10. Frontier molecular orbitals of the HOMO, HOMO1 and LUMO, earlier publication.[16] LUMO + 1 energy (eV) levels for the optimized structure of (XPh)4NcpH2 calculated at the B3LYP/6-31G* level.

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1–4 range from 1.52 to 1.65 V (see Table 1), which is much smaller than values from the theoretical calculations. This difference might be attributed to the fact that the DFT calculation at the B3LYP level may overestimate the HOMO–LUMO energy gap of the porphyrin-like compounds.[44] It should also be pointed out that the shift of potentials for the electrode reactions of N-confused porphyrins as compared to the corresponding TPP complexes is greater for oxidations than for reductions. This is shown in Figure S5 in the Supporting Information which compares cyclic voltammograms of (TPP)H2 and (Ph)4NcpH2 in CH2Cl2 with 0.1 m TBAP. Comparisons of the experimentally measured DE values for the Ncp compounds in their neutral, deprotonated and protonated forms can also be made to the values of DE for free-base corroles; which have been characterized in their protonated and deprotonated forms.[38] This is shown in Figure 11 for (ClPh)4NcpH2 and (Cor)H3 ; for which a decrease in DE is observed with increase in the number of protons on the porphyrin, the potential separations ranging from 1.83 to 1.38 V for the currently investigated Ncp derivatives and from 1.83 to 1.50 V for the related corroles.[38] Thus, the decreasing “non-thermodynamic” HOMO–LUMO gap of the N-confused free-base porphyrins with increase in the degree of protonation is consistent with what was previously reported for free-base triarylcorroles, which follow the order: mono-deprotonated corrole [(Cor)H2] > neutral corrole (Cor)H3 > monoprotonated corrole [(Cor)H4] + .[38] Again, a thermodynamically meaningful DE will depend upon obtaining reversible oxidation and reduction potentials and attempts are now underway with different solvents and experimental conditions to try and stabilize the electrogenerated Ncp radicals.

In summary, the electrochemistry of N-confused free-base porphyrins in nonaqueous media is strongly influenced by protonation or deprotonation reactions that occur after the initial electron transfers. Radical anions and radical cations are generated in an initial one-electron addition and one-electron abstraction, respectively, but both of these species are highly unstable on the electrochemical and spectroelectrochemical timescale and only the deprotonated and protonated N-confused porphyrins are obtained as the final products of electroreduction and electrooxidation, respectively. Similar deprotonation and protonation reactions have been reported in the case of free-base corroles[38, 45, 46] and free-base tetraarylporphyrins after reduction and oxidation,[39, 47] suggesting that extreme care must be taken in correctly assigning spectra to a specific product of electron transfer upon oxidation or reduction of a given N-confused free-base porphyrin.

Experimental Section Instrumentation Electrochemical measurements were carried out at 298 K using an EG&G Princeton Applied Research 173 potentiostat/galvanostat or a CHI-730C Electrochemistry Workstation. A homemade three-electrode cell was used for cyclic voltammetric measurements and consisted of a platinum button or glassy carbon working electrode, a platinum counter electrode and a homemade saturated calomel reference electrode (SCE). The SCE is separated from the bulk of the solution by a fritted glass bridge of low porosity, which contained the solvent/supporting electrolyte mixture. Thin-layer UV/Vis spectroelectrochemical experiments were performed with a home-built thin-layer cell, which has a light

Figure 11. Cyclic voltammograms showing the first oxidation and first reduction as well as the HOMO–LUMO gap of a) N-confused free-base tetraarylporphyrins and b) free-base triarylcorroles in their mono-deprotonated, neutral and mono-protonated forms in PhCN containing 0.1 m TBAP. Data for the corroles was taken from reference [36]. Chem. Eur. J. 2014, 20, 1 – 12

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Full Paper Data for (Ph4)NcpH2 (3): Yield: 300 mg, 40 %; UV/Vis (CH2Cl2): lmax (log e) = 438 (5.18), 540 (3.96), 582 (4.05), 726 nm (4.02); UV/Vis (DMF): lmax (log e) = 360 (4.58), 442 (5.13), 591(3.90), 643 (4.03), 696 nm (4.13); 1H NMR (400 MHz, CDCl3, 298 K): d = 8.99 (d, J = 4 Hz, 1 H), 8.92 (d, J = 4 Hz, 1 H), 8.76 (s, 1 H), 8.56 (m, 4 H), 8.38 (m, 4 H), 8.17 (m, 4 H), 7.87 (m, 12 H), 2.38 (s, 2 H), 4.96 ppm (s,1 H); MS (MALDI-TOF): m/z calcd for C44H30N4 614.736; found: 615.577. Data for (ClPh)4NcpH2 (4): Yield: 112 mg, 12 %; UV/Vis (CH2Cl2): lmax (log e) = 440 (5.27), 541(3.99), 584 (4.12), 726 nm (4.09); UV/Vis (DMF): lmax (log e) = 340 (4.50), 448 (5.13), 474 (4.84).592 (4.00), 640 (4.07), 690 nm (4.16); 1H NMR (400 MHz, CDCl3, 298 K): d = 8.95 (d, J = 4 Hz, 1 H), 8.91 (d, J = 4 Hz, 1 H), 8.70 (s, 1 H), 8.58 (m, 4 H), 8.28 (m, 4 H), 8.08 (m, 4 H), 7.87 (m, 4 H), 7.75(d, J = 4 Hz, 4 H), 2.61 (s, 2 H), 5.01 ppm (s, 1 H); MS (MALDI-TOF): m/z calcd for C48H26Cl4N4 752.516; found: 753.650.

transparent platinum networking electrode. Potentials were applied and monitored with an EG&G PAR Model 173 potentiostat or BiStat electrochemistry station. Time-resolved UV/Vis spectra were recorded with a Hewlett–Packard Model 8453 diode array spectrophotometer. High purity N2 were used to deoxygenate the solution and kept over the solution during each electrochemical and spectroelectrochemical experiment. 1 H NMR spectra were recorded on a Bruker Avanc II 400 MHz instrument. Chemical shifts (d in ppm) were determined with TMS as the internal reference. MALDI-TOF mass spectra were taken on a Bruker BIFLEX III ultra-high resolution instrument using a-cyano-4-hydroxycinnamic acid as the matrix.

Theoretical calculations Density-functional theory (DFT) calculations were performed were implemented in Gaussian 09, Revision D.01.[48] Geometry optimizations were carried out in CH2Cl2 solution (dielectric constant, e = 8.93) at the PCM-B3LYP/LanL2DZ functional and 6-31G(d) basis set.[49–56] The structures were confirmed to be minima by vibrational frequency analyses. Graphical outputs of the computational results were generated with the GaussView, version 5.09.

Acknowledgements We gratefully acknowledge support from the Natural Science Foundation of China (Grant Nos. 21071067, 21001054) and the Robert A. Welch Foundation (K.M.K., E-680).

Chemicals Dichloromethane (CH2Cl2) and N,N’-dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co. or Sigma–Aldrich Chemical Co. and used as received. Benzonitrile (PhCN) was purchased from Sigma–Aldrich Chemical Co. and distilled over P2O5 under vacuum prior to use. Tetra-n-butylammonium perchlorate (TBAP) was purchased from Sigma Chemical or Fluka Chemika Co., recrystallized from ethyl alcohol, and dried under vacuum at 40 8C for at least one week prior to use.

Keywords: deprotonation · electrochemistry · porphyrinoids · protonation · spectroelectrochemistry [1] M. Toganoh, H. Furuta, Chem. Commun. 2012, 48, 937 – 954. [2] X. F. Li, H. C. Liu, A. T. Zheng, X. Y. Yu, P. G. Yi, Youji Huaxue 2011, 31, 166 – 175. [3] J. D. Harvey, C. J. Ziegler, J. Inorg. Biochem. 2006, 100, 869 – 880. [4] H. Maeda, H. Furuta, Pure Appl. Chem. 2006, 78, 29 – 44. [5] P. J. Chmielewski, L. Latos-Graz˙yn´ski, Coord. Chem. Rev. 2005, 249, 2510 – 2533. [6] A. Srinivasan, H. Furuta, Acc. Chem. Res. 2005, 38, 10 – 20. [7] J. D. Harvey, C. J. Ziegler, Coord. Chem. Rev. 2003, 247, 1 – 19. [8] H. Furuta, H. Maeda, A. Osuka, Chem. Commun. 2002, 1795 – 1804. [9] M. Toganoh, H. Furuta in Handbook of Porphyrin Science, Vol. 2 (Eds.: K. M. Kadish, K. M. Smith, R. Guilard), World Scientific, New Jersey, 2010, pp. 295 – 367. [10] H. Furuta, T. Asano, T. Ogawa, J. Am. Chem. Soc. 1994, 116, 767 – 768. [11] P. J. Chmielewski, L. Latos-Graz˙yn´ski, K. Rachlewicz, T. Glowiak, Angew. Chem. Int. Ed. 1994, 33, 779 – 781; Angew. Chem. 1994, 106, 805 – 808. [12] H. Furuta, T. Ishizuka, A. Osuka, H. Dejima, H. Nakagawa, Y. Ishikawa, J. Am. Chem. Soc. 2001, 123, 6207 – 6208. [13] A. Ghosh, Angew. Chem. Int. Ed. Engl. 1995, 34, 1028 – 1030; Angew. Chem. 1995, 107, 1117 – 1119. [14] C. J. Ziegler, N. R. Erickson, M. R. Dahlby, V. N. Nemykin, J. Phys. Chem. A 2013, 117, 11499 – 11508. [15] E. A. Alemn, C. S. Rajesh, C. J. Ziegler, D. A. Modarelli, J. Phys. Chem. A 2006, 110, 8605 – 8612. [16] E. A. Alemn, J. Manrquez Rocha, W. Wongwitwichote, L. A. Godnez Mora-Tovar, D. A. Modarelli, J. Phys. Chem. A 2011, 115, 6456 – 6471. [17] J. P. Belair, C. J. Ziegler, C. S. Rajesh, D. A. Modarelli, J. Phys. Chem. A 2002, 106, 6445 – 6451. [18] H. Furuta, T. Morimoto, A. Osuka, Org. Lett. 2003, 5, 1427 – 1430. [19] R. Acharya, L. Paudel, J. Joseph, C. E. McCarthy, V. R. Dudipala, J. M. Modarelli, D. A. Modarelli, J. Org. Chem. 2012, 77, 6043 – 6050. [20] S. Vyas, C. M. Hadad, D. A. Modarelli, J. Phys. Chem. A 2008, 112, 6533 – 6549. [21] J. L. Shaw, S. A. Garrison, E. A. Aleman, C. J. Ziegler, D. A. Modarelli, J. Org. Chem. 2004, 69, 7423 – 7427. [22] S. A. Wolff, E. A. Aleman, D. Banerjee, P. L. Rinaldi, D. A. Modarelli, J. Org. Chem. 2004, 69, 4571 – 4576. [23] H. Maeda, A. Osuka, Y. Ishikawa, I. Aritome, Y. Hisaeda, H. Furuta, Org. Lett. 2003, 5, 1293 – 1296.

Synthesis and characterization Synthesis of (XPh)4NcpH2 : Pyrrole (335.5 mg, 5.0 mmol) and the corresponding substituted benzoaldehyde (5.0 mmol) were added into a flask (1000 mL) containing dichloromethane (300 mL) and 4methanesulfonic acid (MSA, 602.7 mg, 3.5 mmol) was then added to the mixture. The reaction mixture was stirred for 30 min while the flask was protected from light at room temperature. p-Chloranil (1081.9 mg, 4.4 mmol) was added and the mixture was allowed to stir for 1 min, after which the acid was quenched by addition of triethylamine (2 mL). After stirring for 5 min, the solvent was evaporated under vacuum and the residue was purified on basic alumina using a hexane/CH2Cl2 mixture as eluent (v/v = 1:1). The yellow-green portion was collected and evaporated to dryness. Data for (CH3OPh)4NcpH2 (1): Yield: 348 mg, 38 %; UV/Vis (CH2Cl2): lmax (log e) = 442 (4.40), 544 (3.28), 590 (3.48), 734 nm (3.33); UV/Vis (DMF): lmax = 375, 446, 600, 656, 709 nm; 1H NMR (400 MHz; CDCl3, 298 K): d = 8.93 (d, J = 4 Hz, 1 H), 8.87 (d, J = 8 Hz, 1 H), 8.67 (s, 1 H), 8.52 (m, 4 H), 8.31 (d, J = 8 Hz, 2 H), 8.25 (d, J = 8 Hz, 2 H), 8.04 (m, 4 H), 7.39 (d, J = 8 Hz, 4 H), 7.27 (m, 4 H), 4.07 (m, 12 H), 2.29 (s, 2 H), 4.78 ppm (s, 1 H); MS (MALDI-TOF): m/z calcd for C48H38N4O4 734.480; found: 737.057. Data for (CH3Ph)4NcpH2 (2): Yield: 369 mg, 44 %; UV/Vis (CH2Cl2): lmax (log e) = 440 (5.33), 543 (3.99), 584 (4.23), 730 nm (4.17); UV/Vis (DMF): lmax (log e) = 362 (4.63), 442 (5.23), 594 (3.97), 646 (4.11), 702 nm (4.21); 1H NMR (400 MHz, CDCl3, 298 K): d = 8.97 (d, J = 4 Hz, 1 H), 8.92 (d, J = 4 Hz, 1 H), 8.72 (s, 1 H), 8.60 (m, 4 H), 8.27 (m, 4 H), 8.05 (m, 4 H), 7.67 (d, J = 8 Hz, 4 H), 7.56 (m, 4 H), 2.70 (m, 12 H), 2.38 (s, 2 H), 4.92 ppm (s, 1 H); MS (MALDI-TOF): m/z calcd for C48H38N4 670.842; found: 671.779.

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FULL PAPER & Porphyrinoids S. Xue, Z. Ou,* L. Ye, G. Lu, Y. Fang, X. Jiang, K. M. Kadish* && – && Effect of Solvent and Protonation/ Deprotonation on Electrochemistry, Spectroelectrochemistry and ElectronTransfer Mechanisms of N-Confused Tetraarylporphyrins in Nonaqueous Media

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Are you confused? A series of Nconfused meso-substituted free-base tetraarylporphyrins was synthesized and characterized as to their electrochemistry and spectroelectrochemistry in CH2Cl2, PhCN, and DMF containing 0.1 m tetra-n-butylammonium perchlorate. The effect of the solvent and

protonation/deprotonation reactions on the UV/Vis spectra, redox potentials and reduction/oxidation mechanisms is discussed with comparisons made to data and mechanisms for the structurally related free-base corroles and porphyrins (see figure).

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deprotonation on electrochemistry, spectroelectrochemistry and electron-transfer mechanisms of N-confused tetraarylporphyrins in nonaqueous media.

A series of N-confused free-base meso-substituted tetraarylporphyrins was investigated by electrochemistry and spectroelectrochemistry in nonaqueous m...
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