Deprotonation Reactions and Electrochemistry of Substituted Open

Article pubs.acs.org/JPCB

Deprotonation Reactions and Electrochemistry of Substituted OpenChain Pentapyrroles and Sapphyrins in Basic Nonaqueous Media Zhongping Ou,*,† Deying Meng,† Mingzhu Yuan,† Wenhao Huang,† Yuanyuan Fang,‡ and Karl M. Kadish*,‡ †

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States

‡

ABSTRACT: Electrochemical and spectroelectrochemical properties of three open-chain pentapyrroles and the corresponding sapphyrins were examined in pyridine containing 0.1 M tetra-n-butylammonium perchlorate and dichloromethane (CH2Cl2) or benzonitrile (PhCN) containing tetra-n-butylammonium hydroxide (TBAOH). The investigated compounds are represented as (Ar)4PPyH3 and (Ar)4SH3, where Ar is a F− or Cl− substituted phenyl group, PPy is a trianion of the open-chain pentapyrrole, and S is a trianion of the sapphyrin. The pentapyrroles, (Ar)4PPyH3, undergo two reversible one-electron reductions in pyridine, while the structurally related sapphyrins exhibit four reductions in this solvent, the first two of which are irreversible due to coupled chemical reactions following the electron transfers. Both series of neutral compounds could be deprotonated in CH2Cl2 or PhCN by addition of TBAOH to solution, and the progress of these reactions was monitored as a function of the base concentration by cyclic voltammetry and UV−vis spectroscopy. The neutral pentapyrroles were spectroscopically shown to undergo a loss of two protons in a single step to generate the [(Ar)4PPyH]2− dianion while the sapphyrins could only be monodeprotonated, leading to formation of the [(Ar)4SH2]− monoanion under the same solution conditions. The deprotonation constants were measured for each series of compounds in benzonitrile, and oxidation−reduction mechanisms are examined as a function of the solution ‘basicity’.

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INTRODUCTION

The two series of investigated compounds are represented as (Ar)4PPyH3 and (Ar)4SH3, where Ar is a F− or Cl− substituted phenyl group, PPy is a trianion of the open-chain pentapyrrole, and S is a trianion of the sapphyrin (Chart 1). The NH protons remain associated to the neutral compounds in pyridine but not in CH2Cl2 or PhCN containing added tetra-n-butylammonium hydroxide (TBAOH), which induces a loss of one or two protons as shown in eqs 1 and 2.

Sapphyrins, which are pentapyrrolic macrocycles containing one direct link and four bridging methane groups between five pyrrolic subunits,1 have attracted a great deal of research interest over the past decade,2−12 in part because of their structural similarity to porphyrins and corroles, and in part because of their potential applications as photosensitizers for photodynamic therapy, magnetic resonance imaging contrasting agents,11−17 and macrocyclic receptors for the transport of neutral and anionic substrates.18−25 The most often characterized sapphyrins have contained core-modified macrocycles with O, S, or Se heteroatoms26−42 but N-fused and N-confused macrocycles43−46 as well as β-pyrrole substituted47−50 and meso-substituted sapphyrin macrocycles have also been described in the literature.51−54 We recently reported the synthesis, electrochemistry, spectroelectrochemistry, and protonation reactions of mesotetraaryl substituted N-5 sapphyrins and the related open-chain pentapyrroles in dichloromethane and benzonitrile.55 In the present paper, we have expanded our studies to include spectral and electrochemical properties of the same two series of compounds in basic nonaqueous media in order to better understand the electron transfer reactions of these compounds in their deprotonated anionic forms. © 2013 American Chemical Society

K

(Ar)4 SH3 + OH− ⇌ [(Ar)4 SH 2]− + H 2O

(1)

log β2

(Ar)4 PPyH3 + 2OH− HooooI [(Ar)4 PPyH]2 − + 2H 2O

(2)

The progress of the deprotonation reactions for the compounds in Chart 1 was monitored in the present study by cyclic voltammetry and UV−vis spectroscopy, which was also used to characterize the final deprotonated forms of the compounds. Received: September 5, 2013 Revised: September 25, 2013 Published: September 26, 2013 13646

dx.doi.org/10.1021/jp408905a | J. Phys. Chem. B 2013, 117, 13646−13657

The Journal of Physical Chemistry B

Article

solution by a fritted glass bridge of low porosity which contained the solvent/supporting electrolyte mixture. Thin-layer UV−visible spectroelectrochemical experiments were performed with a home-built thin-layer cell, which has a light transparent platinum net working electrode. Potentials were applied and monitored with an EG&G PAR Model 173 potentiostat. Time-resolved UV−visible spectra were recorded with a Hewlett-Packard Model 8453 diode array spectrophotometer. High-purity N2 from Trigas was used to deoxygenate the solution and kept over the solution during each electrochemical and spectroelectrochemical experiment. Determination of Deprotonation Equilibrium Constants. A series of PhCN solutions containing TBAOH in different concentrations was prepared and used as the basetitration reagent. Microliter quantities of TBAOH in PhCN were added gradually to a 5.5 mL PhCN solution of the openchain pentapyrroles and sapphyrins in a homemade 1.0 cm cell and the spectral changes monitored after each addition. Changes in UV−visible spectra during the titration were analyzed as a function of the concentration of added base, using the molar ratio method56,57 to calculate equilibrium constants for deprotonation in the nonaqueous solvent. Chemicals. The investigated open-chain pentapyrroles and sapphyrins were synthesized according to procedures described in the literature.55 Absolute dichloromethane (CH2Cl2) and pyridine (py) were purchased from Sigma-Aldrich and used as received for electrochemistry and spectroelectrochemistry measurements. Benzonitrile (PhCN) was purchased from Sigma-Aldrich and distilled over P2O5 under vacuum prior to use. Tetra-n-butylammonium hydroxide (TBAOH, 1.0 M in methanol) was purchased from Sigma-Aldrich, and tetra-n-

Chart 1. Structures of Investigated Open-Chain Pentapyrroles and Sapphyrins

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EXPERIMENTAL SECTION Instrumentation. Cyclic voltammetry was carried out at 298 K using an EG&G Princeton Applied Research (PAR) 173 potentiostat/galvanostat. 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 was separated from the bulk of the

Figure 1. UV−visible spectra of open-chain pentapyrroles 1, 2, 3 and sapphyrins 4, 5, 6 in pyridine containing 0.1 M TBAP. 13647



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Table 1. UV−Visible Spectral Data of Open-Chain Pentapyrroles in Pyridine, PhCN, and CH2Cl2 Containing 0.1 M TBAP compound (FPh)4PPyH3 1

pyridine PhCNa CH2Cl2a pyridine PhCNa CH2Cl2a pyridine PhCNa CH2Cl2a

(ClPh)4PPyH3 2

(Cl2Ph)4PPyH3 3

a

λmax, nm (ε × 10−4 M−1 cm−1)

solvent 385 382 371 386 384 376 370 369 369

(2.8) (3.0) (3.3) (4.3) (3.4) (3.8) (2.5) (3.4) (3.9)

501 501 498 505 505 502 502 502 499

(3.1) (3.4) (3.6) (5.0) (3.7) (4.2) (3.2) (4.0) (4.5)

812 825 810 814 830 815 793 802 794

(0.4) (0.5) (0.6) (0.8) (0.6) (0.7) (0.4) (0.6) (0.6)

893 903 896 899 907 896 879 881 866

(0.4) (0.6) (0.7) (0.9) (0.7) (0.8) (0.3) (0.6) (0.6)

Data taken from reference 55.

Table 2. UV−Visible Spectral Data, λmax, nm (ε×10−4 M−1 cm−1) of Sapphyrins in Pyridine, PhCN, and CH2Cl2 Containing 0.1 M TBAP compound (FPh)4SH3 4

(ClPh)4SH3 5

(Cl2Ph)4SH3 6

a

solvent pyridine PhCNb CH2Cl2b pyridine PhCNb CH2Cl2b pyridine PhCNb CH2Cl2b

Soret region 491 493 489 495 498 493 491 494 488

(7.4) (6.6) (9.6) (9.5) (9.4) ((9.3) (17.1) (15.6) (16.1)

518 520 514 521 524 518 514 517 511

visible region (4.6) (4.3) (5.7) (6.2) (6.3) (5.9) (10.3) (10.1) (9.7)

645 642 635 651 650 649 636 637 629

(0.3) (0.4) (0.5) (0.5) (0.6) (0.5) (0.9) (0.8) (0.8)

714 701 694 715 706 704 702 695 687

(0.8)a (0.8) (0.9) (1.2)a (1.2) (1.0) (1.5)a (1.4) (1.3)

721 (0.8) 722 (0.8) 722 (1.2) 724 (0.8) 711 (1.3) 714 (1.0)

796 797 795 798 800 800 783 787 787

(0.5) (0.5) (0.5) (0.7) (0.7) (0.5) (0.6) (0.5) (0.4)

A broad two-overlapped absorption band. bData taken from reference 55.

Figure 2. UV−visible spectral changes and diagnostic molar ratio plot of (a) (Cl2Ph)4PPyH3 3 and (b) (ClPh)4SH3 5 during a titration with TBAOH in PhCN.

thus indicating a negligible solvent effect on the absorption spectra. UV−vis spectra of the related sapphyrins 4−6 in pyridine are also shown in Figure 1. Each compound is characterized by a split Soret band between 491 and 521 nm and three Q bands between 636 and 798 nm. Again, the UV−visible spectra of the sapphyrins in pyridine are similar to the spectra in CH2Cl2 or PhCN55 (Table 2), although four Q bands are seen in the later two solvents, two of which are overlapped in pyridine. It is well-known that the bi-, ter-, and quarter-pyrroles can be deprotonated in nonaqueous media to form the corresponding anions and dianions after addition of a strong base to

butylammonium perchlorate (TBAP) was purchased from Fluka Chemika, recrystallized from ethyl alcohol, and dried under vacuum at 40 °C for at least 1 week prior to use.

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RESULTS AND DISCUSSION

UV−Visible Spectra. In pyridine, the open-chain pentapyrroles 1−3 exhibit two major Soret-like bands at 370 to 505 nm and two broad less intense Q bands between 793 and 899 nm as seen in Figure 1. The UV−visible spectra of the pentapyrroles in this solvent are almost identical to spectra for the same series of compounds in CH2Cl2 or PhCN55 (Table 1), 13648



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Table 3. UV−Visible Spectral Data (λmax, nm) of Deprotonated Pentapyrroles 1−3 and Sapphyrins 4−6 in PhCN with Added TBAOH and Equilibrium Constants of the Deprotonation Reactions cpd 1 2 3 4 5 6

λmax, nm

deprotonated form 2‑

[(FPh)4PPyH] [(ClPh)4PPyH]2‑ [(Cl2Ph)4PPyH]2‑ [(FPh)4SH2]− [(ClPh)4SH2]− [(Cl2Ph)4SH2]−

524 544 547 508 515 517

883 900(sh) 939 730 732 720

1000(sh) 993 1020(sh) 801 805 784

na

equilibrium constantb

2.2 2.1 2.0 1.0 1.1 1.1

log β2 = 6.4 log β2 = 5.2 log β2 = 5.1 log K = 6.2 logK = 6.3 log K = 6.1

sh = shoulder peak. aNumber of protons lost after addition of TBAOH in PhCN. bSee eq 1 for compounds 4−6 and eq 2 for compounds 1−3.

Table 4. Half-Wave Potentials (V vs SCE) of Pentapyrroles in Pyridine, PhCN, and CH2Cl2, 0.1 M TBAP oxidation compound (4σa) (FPh)4PPyH3 1 (0.24)

(ClPh)4PPyH3 2 (0.92)

(Cl2Ph)4PPyH3 3 (1.72)

solvent (AN)b pyridine (14.2) PhCN (15.5)f CH2Cl2 (20.4)f Ppyridine (14.2) PhCN (15.5)f CH2Cl2 (20.4)f pyridine (14.2) PhCN (15.5)f CH2Cl2 (20.4)f

third 0.65 0.64 0.64 0.63 0.80 1.40

second 0.48

e

0.47e 0.86e 0.58e 1.23e 0.93

reduction first e

0.38 0.58e 0.37 0.38e 0.68e 0.40 0.50e 1.02e 0.54

first

second

ΔE1/2(1r-2r)c

ΔE (V)d

−0.83 −0.88 −0.95 −0.80 −0.83 −0.91 −0.75 −0.77 −0.85

−1.13 −1.17 −1.19 −1.10 −1.13 −1.14 −1.11 −1.15 −1.17

0.30 0.29 0.24 0.30 0.30 0.23 0.36 0.38 0.32

1.21 1.27 1.32 1.18 1.24 1.31 1.25 1.21 1.39

a

Hammett substituent constant; see reference 63. bAcceptor Number of the solvent (AN) shown in parentheses taken from reference 62. cPotential difference between the first and second reductions. dThe potential difference between the first oxidation and first reduction. eIrreversible peak potential at a scan rate of 0.10 V/s. fData taken from reference 55.

solution.58−60 The lack of a solvent effect on the sapphyrin and open-chain pentapyrrole UV−vis spectra is consistent with identical structures existing for each derivative in the two series of compounds in all three solvents, and it also indicates the lack of a deprotonation reaction in the basic pyridine solvent. In contrast, deprotonation of the currently investigated openchain pentapyrroles and sapphyrins could be accomplished by titration of the compounds with TBAOH in PhCN. Examples of the spectral changes which occur for the open-chain pentapyrrole are shown in Figure 2a for (Cl2Ph)4PPyH3 3, which also includes the diagnostic plot used to calculate the number of protons lost and the deprotonation constant. Two linear segments are observed in the plot of absorbance vs [TBAOH]/[3] with an intercept at 2.0, thus indicating that compound 3 is dideprotonated in a single step to give the dianion, [(Cl2Ph)4PPyH]2−. The deprotonation equilibrium constant was calculated as log β2 = 5.1 using the molar ratio method and standard equations.56,57 Similar deprotonation reactions occurred for compounds 1 and 2, with log β2 values calculated as 6.4 and 5.2, respectively. The UV−vis spectral data of the deprotonated open-chain pentapyrroles and the values of log β2 are summarized in Table 3. The investigated sapphyrins are also easily deprotonated in PhCN containing added TBAOH. An example of the spectral changes and diagnostic plot for analysis of the data is given in Figure 2b for compound 5 in PhCN during a titration with TBAOH. One proton is lost in the titration, and the calculated log K value is 6.3 for compound 5. This value is given in Table 3 along with equilibrium constants for the deprotonation reaction of the other two sapphyrins. Table 3 also includes the UV−visible spectral data for the monodeprotonated forms of the compounds, [(Ar)4SH2]−, in PhCN. Electrochemistry Pentapyrroles in Pyridine. As previously reported, the open-chain pentapyrroles 1−3 undergo

two reversible one-electron reductions and two reversible oneelectron oxidations in CH2Cl2, 0.1 M TBAP.55 Two reversible one-electron reductions are also observed in pyridine (Table 4), but “more complicated” behavior occurs upon oxidation in this solvent as seen by a comparison of the cyclic voltammograms in Figure 3a,b for (ClPh)4PPyH3 2. The oxidations are reversible in CH2Cl2 but irreversible in pyridine due to one or more coupled chemical reactions following electron transfer, where two overlapping processes occur at Epa = 0.38 and 0.47 V for a scan rate of 0.10 V/s. These oxidations are then followed by a reversible electron transfer at E1/2 = 0.64 V, exactly the same value (within experimental error) for the second reduction of compound 2 in CH2Cl2. The shape of the current−voltage curves and the potentials for oxidation of 2 are independent of the initial scan direction (negative from 0.0 V as shown in Figure 3b or positive from 0.0 V as shown in Figure 3c), but scanning first in a positive direction leads to a marked difference in the peaks seen upon reduction at negative potentials. This is illustrated in Figure 3b,c. The two reductions in Figure 3b are reversible and located at E1/2 = −0.80 and −1.10 V, while those in Figure 3c are irreversible and located at Epc = −0.53 and −1.38 V for a scan rate of 0.10 V/s. The peak potential at Epc = −0.53 V in Figure 3c is close to that for reduction of pyH+ in pyridine61 while the second reduction process at Epc = −1.38 V in this solvent is close to the potential for reduction of [(ClPh)4PPyH2]− after addition of TBAOH to solution (see following section). The two irreversible oxidations at 0.38 and 0.47 V in Figure 3b,c, when coupled with the reversible process at 0.64 V, are consistent with an electrochemical ECEC mechanism of the type shown in Scheme 1 where the first chemical reaction (C1) involves addition of pyridine to the singly oxidized pentapyrrole, and the second chemical reaction (C2) involves the loss of py+ from [(Ar)4PPyH3(py)]2+ with regeneration of 13649



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further reduction of this monoanion at Epc = −1.38 V then generates [(Ar)4PPyH2]2‑ according to eq 4. +e +e

[(Ar)4 PPyH3(py)]+ → → [(Ar)4 PPyH2]− + py + 1/2H 2 (3) −

2−

[(Ar)4 PPyH2] + e ⇌ [(Ar)4 PPyH2]

(4)

Evidence for the number of electrons transferred in eq 3 is given by the height of the peak current at −0.53 V, which is double that for the single electron addition when initially scanning in a negative direction, while assignment of a liberated pyridine molecule in eq 3 is consistent with the negative potential shift of the reduction peak as a function of pyridine concentration. The experimentally measured shift in peak potential with increasing pyridine concentration is illustrated in Figure 4, where E pc = −0.30 V for reduction of

Figure 3. Cyclic voltammograms of (ClPh)4PPyH3 2 (a) in CH2Cl2, (b) in pyridine when scanning initially from 0.0 to −2.0 V, (c) in pyridine when scanning initially from 0.0 to 1.0 V, and (d) in pyridine showing the second scan for oxidation of the compound containing 0.1 M TBAP.

Scheme 1. Proposed Oxidation Mechanism of Open-Chain Pentapyrroles in Pyridine Containing 0.1 M TBAPa

a

The redox potentials in the scheme are given for compound 2.

[(Ar)4PPyH3]+ at the electrode surface. The conversion of py to py+ via reactions C1 and C2 reduces the concentration of pyridine at the electrode surface and, under these conditions, the second oxidation of (Ar)4PPyH3 readily occurs, unhindered by the chemical reaction C1 involving the singly oxidized pentapyrrole. This is demonstrated by the second scan cyclic voltammogram in Figure 3d where two well-defined oneelectron oxidations are obtained. The same [(Ar)4PPyH3]+• species is formed as product of the second chemical reaction (C2) or upon the one electron reduction of (Ar)4PPyH3. This radical anion can be reversibly oxidized by one-electron at 0.64 V or irreversibly reduced at −0.53 V, as seen in Figure 3c. The reduction at Epc = −0.53 V in Figure 3c is proposed to involve a two electron addition to [(Ar)4PPyH3(py)]+ to give the singly reduced and monodeprotonated pentapyrrole [(Ar)4PPyH2]− as shown in eq 3. A

Figure 4. Cyclic voltammograms of 5.1 × 10−4 M (ClPh)4PPyH3 2 in CH2Cl2 containing 0.1 M TBAP and 0−100 equiv TBAOH. Scan rate of 0.10 V/s.

[(Ar)4PPyH3(py)]+ in CH2Cl2 containing 2 equiv of pyridine, −0.42 V in the same solvent containing 24 equiv of pyridine, and −0.48 V in CH2Cl2 containing 100 equiv pyridine. As predicted by eq 3, the higher the pyridine concentration in solution, the harder will be the reduction of [(Ar)4PPyH3(py)]+, i.e., the more negative the reduction potential. Electrochemistry of Pentapyrroles in CH2Cl2 Containing Added TBAOH. Electrochemistry of the open-chain pentapyrroles was also carried out in CH2Cl2 containing 0.1 M 13650



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formation of an electroactive monodeprotonated species on the cyclic voltammetry time scale, namely [(Ar)4PPyH2]−, which is reversibly reduced at E1/2 = −1.19 V in CH2Cl2 for compound 1. The cyclic voltammograms of (FPh)4PPyH3 in CH2Cl2 containing 2−6 equiv TBAOH are consistent with an equilibrium between [(FPh)4PPyH]2− and [(FPh)4PPyH2]− in solution where the dianion is oxidized at E1/2 = −0.62 V and the monoanion is oxidized at E1/2 = −0.42 V (see Figure 5). No reduction peaks are observed for the doubly deprotonated pentapyrrole, [(FPh)4PPyH]2− which exists in the bulk of the solution (as spectroscopically determined) but is rapidly converted to the more easily reduced [(Ar)4PPyH2]− under the application of an applied potential on the negative potential sweep of the cyclic votammogram. Additional evidence for an equilibrium involving [(FPh)4PPyH2]− and [(FPh)4PPyH]2‑ is given by the sum of the peak currents for the two oxidations at −0.62 and −0.42 V, which is equal to the peak current for the single reduction at E1/2 = −1.18 V in solutions containing 2−6 equiv TBAOH. In summary, the neutral (FPh)4PPyH3 can be converted almost exclusively to its monodeprotonated form, either in the presence of low TBAOH concentration (1.0 equivalent) or under the application of a controlled reducing potential in CH2Cl2 solutions containing 2−6 equivalents TBAOH. The monoanionic species can be reduced at E1/2 = −1.18 V and oxidized at E1/2 = −0.42 V to generate [(FPh)4PPyH2]2‑ and (FPh)4PPyH2, respectively. An equilibrium exists between [(FPh)4PPyH2]− and [(FPh)4PPyH]2− containing 2−6 equiv of TBAOH, and the oxidation of doubly deprotonated [(FPh)4PPyH]2− is then observed at E1/2 = −0.62 V. Similar electrochemical behavior is seen for the other two pentapyrroles and the overall redox mechanism is proposed to occur as shown in Scheme 2, where the potentials correspond to the reactions of compound 1, and the three electrochemical reductions in the top line involve what occur for the compound in CH2Cl2 prior to the addition of TBAOH. Electrochemistry of Sapphyrins in Pyridine. The electrochemistry of compounds 4−6 was previously reported in PhCN, 0.1 M TBAP, and is now examined in pyridine and CH2Cl2 containing added base. Examples of cyclic voltammograms for compound 6 in pyridine, 0.1 M TBAP are shown in Figure 6. The electrochemical results in this figure indicate that chemical reactions are coupled with the first two electron transfers leading to the monodeprotonated sapphyrin in its singly and doubly reduced forms, i.e., [(Ar)4SH2]− and

TBAP and added TBAOH. An example of the cyclic voltammograms obtained in CH2Cl2 containing 1.0 to 6.0 equiv TBAOH is shown in Figure 5 for (FPh)4PPyH3 1.

Figure 5. Cyclic voltammograms of 5.2 × 10−4 M (FPh)4PPyH3 1 in CH2Cl2 containing 0.1 M TBAP and added TBAOH.

Although pyridine is not a strong enough base to deprotonate the investigated pentapyrroles (see earlier discussion), this is not the case when OH− is added to the CH2Cl2 solution in the form of TBAOH. As described in an earler section of the manuscript, two protons are easily removed from the (Ar)4PPyH 3 compounds which are quantitatively converted to [(Ar)4PPyH]2− as determined spectroscopically (see eq 2). A monodeprotonated pentapyrrole intermediate was not detected by UV−visible spectra in PhCN, but the electrochemical data in CH2Cl2 solutions containing 1−6 equiv TBAOH (Figure 5) suggest the

Scheme 2. Proposed Reduction/Oxidation Mechanism of Open-Chain Pentapyrroles in CH2Cl2 Containing 0.1 M TBAP and Added TBAOHa

a

The potentials are measured for compound 1. 13651



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Scheme 3. Proposed Mechanism of Sapphyrin Reductions in Pyridine Containing 0.1 M TBAP

shown in Figure 7a for compound 6, where a new Q-band grows in at 783 nm. The final spectrum obtained after the first

Figure 6. Cyclic voltammograms showing the reductions of (Cl2Ph)4SH3 6 in pyridine containing 0.1 M TBAP.

[(Ar)4SH2]2‑. The first two reductions (I and II) correspond to reactions of the initial neutral compound (Ar)4SH3, while the last two (III and IV) are assigned as a stepwise reduction of the monodeprotonated sapphyrin, [(Ar)4SH2]−, at the electrode surface. The chemical reaction following the first reduction of (Ar)4SH3 is sufficiently slow so as to allow a further reduction of any remaining [(Ar)4SH3]− at the electrode surface. This electrode reaction occurs at the conjugated sapphyrin macrocycle and is located at Epc = −1.27 V for compound 6 (Rxn II in Figure 6). The concentration of [(Ar)4SH3]− at the electrode surface is smaller than that of the neutral compound due to the formation of [(Ar)4SH2]−, and therefore the current for the second reduction peak (at −1.27 V in the case of 6) is much smaller than that for reduction of the initial compound (at −1.01 V in Figure 6). Similar electrochemical behavior is seen for all three sapphyrins as well as for a series of free-base triarylcorroles,61 which were earlier examined under the same solution conditions. A summary of reduction potentials for 4−6 is given in Table 5. The proposed mechanism for electron transfer of the sapphyrin is shown in Scheme 3 and is similar to what was reported earlier for the same three compounds in PhCN.55 This proposal was confirmed in the present study by thin-layer spectroelectrochemical measurements. The Soret and Q bands of the neutral sapphyrin decrease in intensity during the first controlled potential reduction. This is

Figure 7. UV−visible spectral changes of (Cl2Ph)4SH3 6 in pyridine containing 0.1 M TBAP during controlled potential reduction and oxidation in a thin-layer cell.

reduction of 6 has a Soret band at 515 nm and two Q bands at 720 and 783 nm. This spectrum is almost identical to that of the monodeprotonated form of the compound in PhCN with added TBAOH (see Table 3). The initial UV visible spectrum

Table 5. Half-Wave Potentials (V vs SCE) of Substituted Sapphyrins in Pyridine and PhCN, 0.1 M TBAP oxidation compound (FPh)4SH3 4 (ClPh)4SH3 5 (Cl2Ph)4SH3 6

solvent pyridine PhCNd pyridine PhCNd pyridine PhCNd

third

second

first b

0.70 1.36 1.27

0.92 0.96

[(Ar)4SH2]− red

(Ar)4SH3 red

0.47 0.45 0.50b 0.49 0.62b 0.57

ΔE(2o-1o)

I

0.25 0.43 0.39

−1.02 −1.08c −1.00b −0.87b −1.01b −1.02b

a

II b

−1.25

b

−1.24b −1.27b

III

IV

ΔE (V)c

−1.44 −1.52b −1.41 −1.43b −1.42 −1.50b

−1.82 −1.68b −1.82 −1.60b −1.81

1.49 1.53 1.50 1.36 1.63 1.59

Potential difference between the second and first oxidations. bIrreversible peak potential at a scan rate of 0.10 V/s. cPotential difference between the first oxidation (1st ox) and first reduction (Rxn I). dData taken from reference 55.

a

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be rereduced via another electrochemical EC type mechanism to give the transient [(Ar)4SH2]−, which reacts with pyH+ to regenerate the neutral sapphyrin and pyridine as shown in Scheme 4. Electrochemistry of Sapphyrins in CH2Cl2 Containing Added TBAOH. The electrochemistry of compounds 4−6 was also carried out in CH2Cl2 containing added TBAOH, and an example of the cyclic voltammograms for compound 6 is shown in Figure 9. The addition of TBAOH to a PhCN solution of

could not be recovered after the potential was set back to 0.0 V. These results suggest that (Cl2Ph)4SH3 initially accepts an electron at −1.01 V (Rxn I in Figure 6) to electrogenerate [(Cl2Ph)4SH3]−•, which is then converted to [(Cl2Ph)4SH2]− as shown in Scheme 3. It is worth noting that the UV−visible spectrum of 6 undergoes only very minor changes during the second controlled potential reduction at −1.35 V (Figure 7b). This is consistent with the smaller peak current for the second reduction in Figure 6. Thin-layer UV−visible spectral changes of the sapphyrins were also measured during controlled-potential oxidation of each sapphyrin in pyridine containing 0.1 M TBAP. As seen in Figure 7d, the Soret band and Q bands of compound 6 are both decreased in intensity upon oxidation. The spectral changes are irreversible in pyridine, consistent with the irreversible cyclic voltammogram in Figure 8. The peak potential for the first one-

Figure 9. Cyclic voltammograms of (Cl2Ph)4SH3 6 in CH2Cl2 containing 0.1 M TBAP before and after addition of TBAOH to solution. Peaks indicated by an asterisk are unidentified side products.

(Ar)4SH3 leads to [(Ar)4SH2]−, as shown by the spectroscopically monitored base titrations described in Figure 2b for compound 5. Evidence for the formation of [(Ar)4SH2]− is also given by the cyclic voltammograms in Figure 9 for 4 and by the UV−visible spectrum of the product formed after the first controlled potential reduction of 4 in a thin-layer cell (Figure 10a) or after adding TBAOH into the CH2Cl2 solution. The first reduction of the neutral compound 6 at Epc = −1.02 V in CH2Cl2 (Processes I in Figure 9) disappears while a reversible oxidation (Process IV) appears at E1/2 = 0.07−0.09 V after adding 1−2 equivalents of TBAOH to solution. This result is consistent with a deprotonation of (Cl2Ph)4SH3 to give [(Cl2Ph)4SH2]− under the given solution conditions. The chemically generated monoanion undergoes two reductions in CH2Cl2 either with or without added TBAOH. The potentials for these processes are almost identical to the reduction potentials in pyridine for the same compound (see Figure 6). The above-described sequence of coupled electrochemical and chemical steps is simplified in CH2Cl2 containing 2 equiv of TBAOH at low temperature (−10 °C), where only one reduction and no major reoxidation peaks can be observed over a range of potential from −0.40 to −2.0 V vs SCE (Figure 9). The formation of a monodeprotonated sapphyrin product was confirmed by thin-layer UV−visible spectroelectrochemistry in CH2Cl2 containing 0.1 M TBAP in the absence of TBAOH. As seen in Figure 10a, the product generated after one-electron reduction of compound 4 at −1.30 V is

Figure 8. Cyclic voltammograms showing the first oxidation and rereduction of substituted sapphyrins in pyridine containing 0.1 M TBAP.

electron abstraction is located at 0.47 V for 4, 0.50 V for 5 and 0.62 V for 6, while the rereduction peaks are found at −0.32, −0.28, and −0.24 V for compounds 4−6. The mechanism for the oxidation and rereduction is described by the ‘square scheme’ illustrated in Scheme 4. The first oxidation of the sapphyrin is quasi-reversible and involves the conversion of (Ar)4SH3 to [(Ar)4SH3]+•, followed by a chemical reaction with pyridine to generate (•Ar)4SH2 and pyH+ (an electrochemical EC mechanism). (•Ar)4SH2 can then Scheme 4. Proposed Mechanism for Oxidation of the Sapphyrins in Pyridine Containing 0.1 M TBAP

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Figure 10. (a) UV−visible spectra of the neutral (FPh)4SH3 4 and [(FPh)4SH2]− generated after the first controlled potential reductions at −1.30 V in CH2Cl2 containing 0.1 M TBAP and (b) in CH2Cl2 with added TBAOH.

characterized by a Soret-like band at 500 nm and two major Q bands at 727 and 795 nm. The UV−visible spectrum of electrogenerated [(FPh)4SH2]− in the thin-layer cell is virtually identical to the UV−visible spectrum of the product formed after adding 2 equiv TBAOH to 4 in CH2Cl2 (see Figure 10b). This result suggests that the same monodeprotonated sapphyrin product, [(FPh)4SH2]−, is generated by an electrochemically initiated process or by addition of base to the sapphyrin in CH2Cl2. On the basis of the electrochemical and spectral data, the reaction mechanism shown in Scheme 5 is proposed to occur for the sapphyrins 4−6.

Figure 11. Plots of E1/2 for the first reversible reductions of pentapyrroles 1−3 with (a) the solvent Acceptor Number (AN) and (b) sum of the Hammett substituent constants (4σ).

of the substituents63 (Figure 11b). The slope of the correlation in the plots is defined by the equation ΔE1/2 = Σσρ, where Σσ represents the sum of the substituent constants, and ρ is the reaction constant that measures the effect of the substituent on the redox potential.63 The larger the interaction between the substituent and the site of electron transfer, the larger will be the value of ρ. For example, in the case of substituted free-base tetraphenylporphyrins (TPP), electron-donating or electronwithdrawing substituents at the β-pyrrole positions of the macrocycle will have a larger interaction with the electrogenerated π-anion radical (and a larger ρ value) than when the substituents are located on the four phenyl groups of the TPP macrocycle.64 The values of ρ for the first reduction of the pentapyrroles were calculated from the slope of the plots in Figure 11b as 67 mV in pyridine, 68 mV in CH2Cl2 and 74 mV in PhCN. Similar values of ρ have been reported for free-base porphyrins64 and corroles.61 The absolute potential difference between the first reduction and first oxidation of 1−3 (the HOMO−LUMO gap) ranged from 1.18 to 1.39 V in the three solvents. This separation is much smaller than the average HOMO−LUMO gap (2.25 ± 0.15 V) for ring-centered reactions of free-base porphyrins.64 Comparisons are also possible between the electrochemically measured HOMO−LUMO gap of the pentapyrroles 1−3, the sapphyrins 4−6, and the structurally related free-base corroles. This comparison is shown graphically in Scheme 6 for compounds 3, 6 and ((CF 3 ) 2 Ph) 3 CorH 3 . The largest HOMO−LUMO gap is obtained for the neutral and monoanionic free-base corroles (1.73 and 1.81 V, respectively), and the smallest for the neutral and monoanionic pentapyrroles (1.39 and 0.93 V). In each series of compounds, the monoanion is easier to oxidize and harder to reduce than the neutral species, with the negative shift in reduction potential amounting to 320 mV in the case of the pentapyrrole 3, 450

Scheme 5. Proposed Mechanism for Reduction and Oxidation for the Sapphyrins 4−6 in CH2Cl2 Containing 0.1 M TBAP and Added TBAOH

Correlations of Redox Potentials with Solvent and Substituent Effects. As seen in Table 4, the difference in halfwave potentials between the first and second reversible oneelectron reductions of the pentapyrroles 1−3 ranges from 0.23 to 0.38 V depending upon the specific solvent and specific electron-withdrawing substituent on the compound. The largest ΔE1/2(1r‑2r) values are observed in pyridine, and the smallest in CH2Cl2. As discussed above, changes in the solvent have almost no effect on the UV−visible spectra of the pentapyrroles, but it does effect the redox potentials as shown in Figure 11a where E1/2 for the first reduction of 1−3 is linearly related to the acceptor number (AN)62 of the solvent. The reductions are easier in pyridine than in PhCN or CH2Cl2 due to solvation of the compound under the given solution conditions. A linear relationship is also observed between the measured E1/2 for the first reduction of 1−3 and the Hammett constants 13654



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Scheme 6. Comparisons of Potentials for the First Reduction and First Oxidation of Structurally Related Free-Base (a) Open-Chain Pentapyrroles, (b) Sapphyrins, and (c) Corroles in CH2Cl2a

REFERENCES

(1) Bauer, V. J.; Clive, D. L. J.; Dolphin, D.; Paine, J. B., III; Harris, F. L.; King, M. M.; Loder, J.; Wang, S. W. C.; Woodward, R. B. Sapphyrins: Novel Aromatic Pentapyrrolic Macrocycles. J. Am. Chem. Soc. 1983, 105, 6429−6436. (2) Sessler, J. L.; Seidel, D. Synthetic Expanded Porphyrin Chemistry. Angew. Chem., Int. Ed. 2003, 42, 5134−5175. (3) Misra, R.; Chandrashekar, T. K. Structural Diversity in Expanded Porphyrins. Acc. Chem. Res. 2008, 41, 265−279. (4) Ghosh, A. A Perspective of One-pot Pyrrole-aldehyde Condensations as Versatile Self-assembly Processes. Angew. Chem., Int. Ed. 2004, 43, 1918−1931. (5) Sessler, J. L.; Camiolo, S.; Gale, P. A. Pyrrolic and Polypyrrolic Anion Binding Agents. Coord. Chem. Rev. 2003, 240, 17−55. (6) Pushpan, S. K.; Chandrashekar, T. K. Aromatic Core-modified Expanded Porphyrinoids with meso-aryl Substituents. Pure Appl. Chem. 2002, 74, 2045−2055. (7) Sessler, J. L.; Andrievsky, A.; Genge, J. W. Anion Binding by Sapphyrins. Adv. Supramol. Chem. 1997, 4, 97−142. (8) Ravi, K. M.; Chandrashekar, T. K. Molecular Receptors Based on Expanded Porphyrins. J. Inclusion Phenom. Macrocyclic Chem. 1999, 35, 553−582. (9) Sessler, J. L.; Allen, W. E. Anion Carriers: New Tools for Crossing Membranes. CHEMTECH 1999, 29, 16−24. (10) Pareek, Y.; Ravikanth, M.; Chandrashekar, T. K. Smaragdyrins: Emeralds of Expanded Porphyrin Family. Acc. Chem. Res. 2012, 45, 1801−1816. (11) Shin, J.-Y.; Kim, K. S.; Yoon, M.-C.; Lim, J. M.; Yoon, Z. S.; Osuka, A.; Kim, D. Aromaticity and Photophysical Properties of Various Topology-controlled Expanded porphyrins. Chem. Soc. Rev. 2010, 39, 2751−2767. (12) Sessler, J. L.; Davis, J. M. Sapphyrins: Versatile Anion Binding Agents. Acc. Chem. Res. 2001, 34, 989−997. (13) Saito, S.; Osuka, A. Expanded Porphyrins: Intriguing Structures, Electronic Properties, and Reactivities. Angew. Chem., Int. Ed. 2011, 50, 4342−4373. (14) Sessler, J. L.; Weghorn, S. J. Expanded, Contracted & Isomeric Porphyrins, Tetrahedron Organic Chemistry Series; Pergamon Press: New York, 1997; Vol. 15. (15) Jasat, A.; Dolphin, D. Expanded Porphyrins and Their Heterologs. Chem. Rev. (Washington, D. C.) 1997, 97, 2267−2340. (16) Bonnett, R. Photosensitizers of The Porphyrin and Phthalocyanine Series for Photodynamic Therapy. Chem. Soc. Rev. 1995, 24, 19−33. (17) Naumovski, L.; Sirisawad, M.; Lecane, P.; Chen, J.; Ramos, J.; Wang, Z.; Cortez, C.; Magda, D.; Thiemann, P.; Boswell, G.; et al. Tumor Localization and Antitumor Efficacy of Novel Sapphyrin Compounds. Mol. Cancer Ther. 2006, 5, 2798−2805. (18) Furuta, H.; Cyr, M. J.; Sessler, J. L. Phosphate Anion Binding: Enhanced Transport of Nucleotide Monophosphates Using a Sapphyrin Carrier. J. Am. Chem. Soc. 1991, 113, 6677−6678. (19) Kral, V.; Sessler, J. L.; Furuta, H. Synthetic Sapphyrin-cytosine Conjugates: Carriers for Selective Nucleotide Transport at Neutral pH. J. Am. Chem. Soc. 1992, 114, 8704−8705. (20) Sessler, J.; Brucker, E. A. The First “Crowned” Expanded Porphyrin. Tetrahedron Lett. 1995, 36, 1175−1176. (21) Shionoya, M.; Furuta, H.; Lynch, V.; Harriman, A.; Sessler, J. L. Diprotonated Sapphyrin: A Fluoride Selective Halide Anion Receptor. J. Am. Chem. Soc. 1992, 114, 5714−5722. (22) Sessler, J. L.; Ford, D. A.; Cyr, M. J.; Furuta, H. Enhanced Transport of Fluoride Anion Effected Using Protonated Sapphyrin as a Carrier. J. Chem. Soc., Chem. Commun. 1991, 1733−1735. (23) Sessler, J. L.; Cyr, M. J.; Burrell, A. K. Sapphyrins: New Life for an Old “Expanded Porphyrin”. Synlett 1991, 127−134. (24) Sessler, J. L.; Cyr, M. J.; Lynch, V.; McGhee, E.; Ibers, J. A. Synthetic and Structural Studies of Sapphyrin, a 22-p-electron Pentapyrrolic “Expanded Porphyrin”. J. Am. Chem. Soc. 1990, 112, 2810−2813.

a

Data for the corroles was measured in PhCN and taken from reference 61.

mV in the case of the sapphyrin 6 and 570 mV in the case of the corrole ((CF3)2Ph)3CorH3. By contrast, the negative shift of the first oxidation potential for the monoanion (as compared to the related neutral compound) amounts to 780 mV in the case of 3, 520 mV in the case of 6, and 490 mV in the case of the free-base corrole ((CF3)2Ph)3CorH3.

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CONCLUSIONS Almost identical UV−visible spectra were observed for the examined open-chain pentapyrroles in pyridine, benzonitrile, and dichloromethane, indicating that these compounds are stable and deprotonation does not occur in the weak pyridine base. The same is true for the investigated sapphyrins which do not deprotonate in pyridine. However, deprotonation can be easily accomplished in PhCN or CH2Cl2 by addition of TBAOH to solution. This generates the di-deprotonated pentapyrroles and monodeprotonated sapphyrins, which were spectroscopically and electrochemically characterized. The neutral pentapyrroles undergo two reversible one-electron reductions in pyridine, while the sapphyrins undergo four reductions, the first two of which correspond to the neutral compound, and the last to the monoanion generated in solution after the first or second reductions.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the Natural Science Foundation of China (Grant 21071067) and the Robert A. Welch Foundation (KMK, Grant E-680). 13655



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(25) Iverson, B. L.; Shreder, K.; Kral, V.; Sessler, J. L. Phosphate Recognition by Sapphyrin. A New Approach to DNA Binding. J. Am. Chem. Soc. 1993, 115, 11022−11023. (26) Rajeswara, R. M.; Ravikanth, M. Synthesis and Studies of Covalently Linked Porphyrin-Expanded Heteroporphyrin Dyads. Eur. J. Org. Chem. 2011, 1335−1345. (27) Pareek, Y.; Ravikanth, M. Synthesis of Mono-Functionalized Core-Modified Expanded Porphyrin Building Blocks and Covalently Linked Expanded Porphyrin Dyads. Eur. J. Org. Chem. 2011, 2011, 5390−5399. (28) Shetti, V. S.; Ravikanth, M. Supramolecular Tetrads Containing Sn(IV) Porphyrin, Ru(II) Porphyrin, and Expanded Porphyrins Assembled Using Complementary Metal−Ligand Interactions. Inorg. Chem. 2011, 50, 1713−1722. (29) Srinivasan, A.; Pushpan, S. K.; Kumar, M. R.; Mahajan, S.; Chandrashekar, T. K.; Roy, R.; Ramamurthy, P. meso-Aryl Sapphyrins with Heteroatoms; Synthesis, Characterization, Spectral and Electrochemical Properties. J. Chem. Soc., Perkin Trans. 2 1999, 961−968. (30) Szterenberg, L.; Latos-Grazynski, L. Geometry and Tautomerism of 26,28-Dioxasapphyrin and 26,28-Dithiasapphyrin: DFT Studies. J. Phys. Chem. A 1999, 103, 3302−3309. (31) Narayanan, S. J.; Sridevi, B.; Chandrashekar, T. K.; Vij, A.; Roy, R. Sapphyrin Supramolecules Through C−H···S and C−H···Se Hydrogen BondsFirst Structural Characterization of Meso-arylsapphyrins Bearing Heteroatoms. Angew. Chem., Int. Ed. 1998, 37, 3394− 3397. (32) Narayanan, S. J.; Sridevi, B.; Chandrashekar, T. K.; Vij, A.; Roy, R. Novel Core-Modified Expanded Porphyrins with meso-Aryl Substituents: Synthesis, Spectral and Structural Characterization. J. Am. Chem. Soc. 1999, 121, 9053−9068. (33) Hooker, J. D.; Nguyen, V. H.; Taylor, V. M.; Cedeno, D. L.; Lash, T. D.; Jones, M. A.; Robledo, S. M.; Velez, I. D. New Application for Expanded Porphyrins: Sapphyrin and Heterosapphyrins as Inhibitors of Leishmania Parasites. Photochem. Photobiol. 2012, 88, 194−200. (34) Malakalapalli, R. R.; Mangalampalli, R. Synthesis of Functionalized Core-modified Sapphyrins and Covalently Linked Porphyrinsapphyrin Dyads. Tetrahedron 2012, 68, 1306−1314. (35) Lim, J. M.; Gupta, I.; Furuta, H.; Kim, D. Comparative Photophysics of Sapphyrin Derivatives: Effects of Confused and Fused Pyrrole Rings. J. Porphyrins Phthalocyanines 2011, 15, 858−864. (36) Cho, D.-G.; Plitt, P.; Kim, S. K.; Lynch, V.; Hong, S.-J.; Lee, C.H.; Sessler, J. L. Dioxabenzosapphyrin: A New Benzodifuran-Derived Sapphyrin Analogue. J. Am. Chem. Soc. 2008, 130, 10502−10503. (37) Jeong, S.-D.; Sessler, J. L.; Lynch, V.; Lee, C.-H. Dithiabenzisapphyrin: A Core-Modified Sapphyrin Bearing Exocyclic Double Bonds at the meso-Positions. J. Am. Chem. Soc. 2008, 130, 390−391. (38) Kang, S.; Hayashi, H.; Umeyama, T.; Matano, Y.; Tkachenko, N. V.; Lemmetyinen, H.; Imahori, H. meso-3,5-bis(trifluoromethyl)phenyl-substituted Expanded Porphyrins: Synthesis, Characterization, and Optical, Electrochemical and Photophysical Properties. Chem. Asian J. 2008, 3, 2065−2074. (39) Lopez, R.; Menendez, M. I.; Santander-Nelli, M.; CardenasJiron, G. I. A Theoretical Investigation on the Spectroscopic Properties and Photosensitizing Capability of 5,10,15,20-tetraphenylsapphyrin and 5,10,15,20-tetraphenyl-26,28-diheterosapphyrins with Two O, S, or Se Atoms. Theor. Chem. Acc. 2010, 127, 475−484. (40) Ishida, M.; Karasawa, S.; Uno, H.; Tani, F.; Naruta, Y. Protonation-Induced Formation of a Stable Singlet Biradicaloid Derived from a Modified Sapphyrin Analogue. Angew. Chem., Int. Ed. 2010, 49 (5906−5909), S5906/1−S5906/22. (41) Sathyamoorthy, B.; Axelrod, A.; Farwell, V.; Bennett, S. M.; Calitree, B. D.; Benedict, J. B.; Sukumaran, D. K.; Detty, M. R. Novel 21,23-Ditelluraporphyrins and the First 26,28-Ditellurasapphyrin and 30,33-Ditellurarubyrin. Organometallics 2010, 29, 3431−3441. (42) Nakabuchi, T.; Matano, Y.; Imahori, H. Remarkable Effects of PPerfluorophenyl Group on the Synthesis of Core-Modified Phosphaporphyrinoids and Phosphadithiasapphyrin, Org. Lett., 12, 1112-1115.

(43) Pushpan, S. K.; Srinivasan, A.; Anand, V. G.; Venkatraman, S.; Chandrashekar, T. K.; Joshi, B. S.; Roy, R.; Furuta, H. N-Confused Expanded Porphyrin: First Example of a Modified Sapphyrin with an Inverted N-Confused Pyrrole Ring. J. Am. Chem. Soc. 2001, 123, 5138−5139. (44) Kee, S.-Y.; Lim, J. M.; Kim, S.-J.; Yoo, J.; Park, J.-S.; Sarma, T.; Lynch, V. M.; Panda, P. K.; Sessler, J. L.; Kim, D.; et al. Conformational and Spectroscopic Properties of p-extended, Bipyrrole-fused Rubyrin and Sapphyrin Derivatives. Chem. Commun. (Cambridge, U. K.) 2011, 47, 6813−6815. (45) Gupta, I.; Srinivasan, A.; Morimoto, T.; Toganoh, M.; Furuta, H. N-confused and N-fused meso-aryl Sapphyrins. Angew. Chem., Int. Ed. 2008, 47, 4563−4567. (46) Sessler, J. L.; Cho, D.-G.; Stepien, M.; Lynch, V.; Waluk, J.; Yoon, Z. S.; Kim, D. Inverted Sapphyrin: A New Family of Doubly NConfused Expanded Porphyrins. J. Am. Chem. Soc. 2006, 128, 12640− 12641. (47) Sessler, J. L.; Lisowski, J.; Boudreaux, K. A.; Lynch, V.; Barry, J.; Kodadek, T. J. Synthesis and Characterization of Diaryl Sapphyrins Prepared under Lindsey-type Conditions. J. Org. Chem. 1995, 60, 5975−5978. (48) Yoon, Z. S.; Cho, D.-G.; Kim, K. S.; Sessler, J. L.; Kim, D. Nonlinear Optical Properties as a Guide to Aromaticity in Congeneric Pentapyrrolic Expanded Porphyrins: Pentaphyrin, Sapphyrin, Isosmaragdyrin, and Orangarin. J. Am. Chem. Soc. 2008, 130, 6930−6931. (49) Boul, P. J.; Cho, D.-G.; Rahman, G. M. A.; Marquez, M.; Ou, Z.; Kadish, K. M.; Guldi, D. M.; Sessler, J. L. Sapphyrin-Nanotube Assemblies. J. Am. Chem. Soc. 2007, 129, 5683−5687. (50) Panda, P. K.; Kang, Y.-J.; Lee, C.-H. A Benzodipyrrole-derived Sapphyrin. Angew. Chem., Int. Ed. 2005, 44, 4053−4055. (51) Chmielewski, P. J.; Latos-Grazynski, L.; Rachlewicz, K. 5,10,15,20-Tetraphenylsapphyrin. Identification of a Pentapyrrolic Expanded Porphyrin in the Rothemund Synthesis. Chem.Eur. J. 1995, 1, 68−73. (52) Narayanan, S. J.; Sridevi, B.; Srinivasan, A.; Chandrashekar, T. K.; Roy, R. One Step Synthesis of Sapphyrin and N-confused Porphyrin using Dipyrromethane. Tetrahedron Lett. 1998, 39, 7389− 7392. (53) Simkhovich, L.; Rosenberg, S.; Gross, Z. Facile Synthesis of a Novel Sapphyrin and Its Rhodium(I) Complex. Tetrahedron Lett. 2001, 42, 4929−4931. (54) Koszarna, B.; Gryko, D. T. Efficient Synthesis of mesoSubstituted Corroles in a H2O-MeOH Mixture. J. Org. Chem. 2006, 71, 3707−3717. (55) Yuan, M.; Ou, Z.; Fang, Y.; Huang, S.; Xue, Z.; Lu, G.; Kadish, K. M. Synthesis, Characterization, Protonation Reactions, and Electrochemistry of Substituted Open-Chain Pentapyrroles and Sapphyrins in Nonaqueous Media. Inorg. Chem. 2013, 52, 6664−6673. (56) Budesinsky, B. W. Limits of the Molar-ratio Method. Talanta 1974, 21, 323−326. (57) Momoki, K.; Sekino, J.; Sato, H.; Yamaguchi, N. Theory of Curved Molar Ratio Plots and a New Linear Plotting Method. Anal. Chem. 1969, 41, 1286−1299. (58) Fu, Z.; Zhang, M.; Zhu, W.; Karnas, E.; Mase, K.; Ohkubo, K.; Sessler, J. L.; Fukuzumi, S.; Kadish, K. M. Electroreduction and Acid− Base Properties of Dipyrrolylquinoxalines. J. Phys. Chem. A 2012, 116, 10063−10073. (59) Zhang, M.; E, W.; Ohkubo, K.; Sanchez-Garcia, D.; Yoon, D.W.; Sessler, J. L.; Fukuzumi, S.; Kadish, K. M. Electron-Transfer and Acid−Base Properties of a Two-Electron Oxidized Form of Quaterpyrrole that Acts as Both an Electron Donor and an Acceptor. J. Phys. Chem. A 2008, 112, 1633−1642. (60) E, W.; Ohkubo, K.; Sanchez-Garcia, D.; Zhang, M.; Sessler, J. L.; Fukuzumi, S.; Kadish, K. M. Electron-Transfer Oxidation Properties of Substituted Bi-, Ter-, and Quaterpyrroles. J. Phys. Chem. B 2007, 111, 4320−4326. (61) Shen, J.; Shao, J.; Ou, Z.; E, W.; Koszarna, B.; Gryko, D. T.; Kadish, K. M. Electrochemistry and Spectroelectrochemistry of mesoSubstituted Free-Base Corroles in Nonaqueous Media: Reactions of 13656



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(Cor)H3, [(Cor)H4]+, and [(Cor)H2]. Inorg. Chem. 2006, 45, 2251− 2265. (62) Gutmann, V. The Donor−Acceptor Approach to Molecular Interactions; Plenum Press: New York, 1978. (63) Zuman, P. Substituents Effects in Organic Polarography; Plenum Press: New York, 1967. (64) Kadish, K. M.; Van Caemelbecke, E.; Royal, G. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 8, pp 1−144.

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