DOI: 10.1002/chem.201303141
Full Paper
& Analytical Chemistry
Planar and Nonplanar Free-Base Tetraarylporphyrins: b-Pyrrole Substituents and Geometric Effects on Electrochemistry, Spectroelectrochemistry, and Protonation/Deprotonation Reactions in Nonaqueous Media Yuanyuan Fang,[a] P. Bhyrappa,[a, b] Zhongping Ou,[a, c] and Karl M. Kadish*[a]
Abstract: A series of planar and nonplanar free-base b-pyrrole substituted meso-tetraarylporphyrins were characterized by electrochemistry, spectroelectrochemistry, and protonation or deprotonation reactions in neutral, acidic, and basic solutions of CH2Cl2. The neutral compounds are represented as H2(P), in which P represents a porphyrin dianion with one of several different sets of electron-withdrawing or -donating substituents at the messo and/or b-pyrrole positions of the macrocycle. The conversion of H2(P) to [H4(P)]2 + in CH2Cl2 was accomplished by titration of the neutral porphyrin with trifluoroacetic acid (TFA) while the progress of the protonation was monitored by UV/Vis spectroscopy, which was also used to calculate logb2 for proton addition to the core nitrogen atoms of the macrocycle. Cyclic voltammetry was performed after each addition of TFA or TBAOH to CH2Cl2 solutions of the porphyrin and half-wave potentials for reduction
were evaluated as a function of the added acid or base concentration. Thin-layer spectroelectrochemistry was used to obtain UV/Vis spectra of the neutral and protonated or deprotonated porphyrins under the application of an applied reducing potential. The magnitude of the protonation constants, the positions of lmax in the UV/Vis spectra and the half-wave or peak potentials for reduction are then related to the electronic properties of the porphyrin and the data evaluated as a function of the planarity or nonplanarity of the porphyrin macrocycle. Surprisingly, the electroreduction of the diprotonated nonplanar porphyrins in acid media leads to H2(P), whereas the nonplanar H2(P) derivatives are reduced to [(P)]2 in CH2Cl2 containing 0.1 m tetra-n-butylammonium perchlorate (TBAP). Thus, in both cases an electrochemically initiated deprotonation is observed.
Introduction
been interested in how the above factors will affect the electrochemical and spectroscopic properties of porphyrins in various oxidation states and have recently turned our attention towards elucidating the effect of nonplanarity as it applies to the redox reactivity of neutral and protonated free-base (metalfree) porphyrins in nonaqueous media. A number of articles have reported the chemical, structural, and optical properties of planar and nonplanar free-base porphyrins in their neutral and diprotic forms.[4–20] A nonplanar conformation of the porphyrin macrocycle will result from repulsive interactions among the peripheral substituents that also induces dramatic redshifts of the electronic absorption spectral bands.[10, 21] The energy of the HOMO and the LUMO are both influenced by the nature of the peripheral substituents and the nonplanarity of the porphyrin macrocycle. For example, the increased steric crowding of bromine atoms in the free-base-brominated tetraphenylporphyrins H2TPPBrn (n = 1–8) leads to an increasing nonplanar distortion of the ring with increase in the number of bromine atoms. The increased bromination produces a stabilization of the LUMO and a destabilization of the HOMO energy levels, which is then reflected in a change of redox potentials for both oxidation and reduction.[22–24]
Porphyrins and porphyrin analogues have been employed as model systems for a number of biological tetrapyrrole pigments. They have also been investigated as essential components in a number of applications.[1, 2] The redox properties of natural and synthetic porphyrins will depend upon a number of factors, examples of which are the core structure, the type and location of electron-donating or -withdrawing substituents, the type and oxidation state of the central metal ion, and the type and number of bound axial ligands.[3] We have long [a] Y. Fang, Prof. P. Bhyrappa, Prof. Z. Ou, Prof. K. M. Kadish Department of Chemistry, University of Houston Houston, TX 77204-5003 (USA) Fax: (+ 1) 713-743-2745 E-mail: [email protected] [b] Prof. P. Bhyrappa Department of Chemistry Indian Institute of Technology Madras Chennai 600 036 (India) [c] Prof. Z. Ou School of Chemistry and Chemical Engineering Jiangsu University, Zhenjiang 212013 (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303141. Chem. Eur. J. 2014, 20, 524 – 532
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Full Paper Protonation of the central nitrogen atoms of a free-base porphyrin will also induce a nonplanar distortion of the macrocycle.[25] Under some conditions, the addition of two protons can occur in separate steps as shown in Equations (1) and (2),[8, 26–29] but in most cases the experimentally observed process actually occurs in a single step as illustrated in Equation (3).[8, 26–31] H2 ðPÞ þ Hþ Ð ½H3 ðPÞþ
ð1Þ
½H3 ðPÞþ þ Hþ Ð ½H4 ðPÞ2þ
ð2Þ
H2 ðPÞ þ 2 Hþ Ð ½H4 ðPÞ2þ
ð3Þ
the solvent, the strength of the added base, and the planarity of the macrocycle. The deprotonation of H2(P) to give [H(P)] and [(P)]2 has been examined by UV/Vis spectroscopy in nonaqueous media[27, 30, 37–40] but, to our knowledge, there has been no electrochemistry reported for the deprotonated porphyrins under these solution conditions. Thus, an electrochemical and spectroelectrochemical study of the protonated and deprotonated compounds 1–10 in CH2Cl2 should provide new information on redox potentials and UV/Vis spectra for a series of planar and nonplanar free-base porphyrins.
Results and Discussion
There are several reports in the literature on the redox reactions of protonated free-base porphyrins in aqueous[32–34] and nonaqueous[35, 36] media, but the combined effect of macrocyclic substituents and geometry on the electrochemistry of protonated and deprotonated porphyrins in nonaqueous media has been largely unexamined. There have also been no published studies that have elucidated the spectroscopic properties of reduced protonated porphyrins in nonaqueous media. This is investigated in the present study for the compounds shown in Scheme 1, each of which was investigated by electrochemical and spectroelectrochemical methods.
Electrochemistry in CH2Cl2, 0.1 m tetra-n-butylammonium perchlorate (TBAP): Two types of electrochemical behavior are observed for the free-base porphyrins in CH2Cl2 containing 0.1 m TBAP. The porphyrins with planar macrocycles exhibit two reversible one-electron reductions as shown in Figure 1a for compounds 1, 3, and 5 while the nonplanar porphyrins 7– 9 show multiple irreversible reductions with coupled chemical reactions (Figure 1b).
Figure 1. Cyclic voltammograms illustrating reductions of a) the planar porphyrins 1, 3, and 5 and b) nonplanar porphyrins 7, 8, and 9 in CH2Cl2, 0.1 m TBAP.
Scheme 1. Molecular structures of the investigated planar and nonplanar free-base porphyrins.
As seen Figure 1a, the first reduction of the planar porphyrins occurs at E1/2 = 1.19 V for 1, E1/2 = 1.16 V for 3, and E1/2 = 0.82 V for 5 to give a porphyrin p-anion radical,[3] whereas the second reduction generates a porphyrin dianion[3] at E1/2 = 1.53 V for 1, E1/2 = 1.33 V for 3, and E1/2 = 0.91 V for 5. The potential separation between these processes is not constant but varies from a DE1/2 of 340 mV in the case of 1 and to a smaller potential separation of 90 mV in the case of 5, for which the two reductions are almost overlapped. The singly and doubly reduced planar free-base porphyrins are both stable in CH2Cl2 containing 0.1 m TBAP and no other redox processes are observed up to the negative potential limit of the solvent, which is 2.0 V in the case of CH2Cl2. In contrast to the planar free-base porphyrins, significantly different electrochemical behavior is exhibited for the nonpla-
The assignment of macrocycle planarity to the neutral compounds in Scheme 1 is based in part on structural data in the solid state and in part on the compound’s UV/Vis spectra that exhibit diagnostic patterns that indicate planarity or nonplanarity of the macrocycle, as described later in the manuscript. All of the porphyrins in Scheme 1 are anticipated to have a nonplanar configuration after the addition of two more protons to the core nitrogen atoms, as was demonstrated for tetraphenylporphyrin [H4TPP]2 + , octaethyltetraphenylporphyrin [H4OETPP]2 + , and octabromotetraphenylporphyrin [H4TPPBr8]2 + which were structurally characterized as having a saddle-type distortion (D2d symmetry).[25] A loss of protons from free-base porphyrins can occur in nonaqueous media containing added base, with the degree of deprotonation depending upon the porphyrin substituents, Chem. Eur. J. 2014, 20, 524 – 532
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Full Paper nar derivatives 7–9 as shown by the cyclic voltammograms in and three for compounds 7, 8, and 9. More importantly, the Figure 1b. The initial reduction of each compound again innonplanar porphyrins, 7–10, have a much larger redshift in the volves a one-electron transfer but the shape of the current– Soret and Q bands than the planar porphyrins 2–6, as comvoltage curves are consistent with a chemical reaction followpared to the unsubstituted H2TPP, compound 1. This redshift ing the first electron addition (an EC mechanism) to give new in lmax can be ascribed to both an inductive and resonance inelectroactive product that is then reduced at more negative teraction of the macrocycle substituents with the porphyrin ppotentials of 1.09 to 1.28 V and also reoxidized on the ring system as well as to a distortion of the porphyrin macroreturn sweep at Ep = 0.25 to 0.52 V. The most reactive eleccycle. Protonation at the porphyrin central nitrogen atoms will trogenerated porphyrin monoanions are 8 and 9, as judged by readily occur upon addition of acid to the compounds in the total lack of a coupled reoxidation peak after the first reCH2Cl2 solution. Changes in the UV/Vis spectra of H2TPP (1), duction and the significantly decreased peak current for the second reduction that would correspond to formation of a porH2TPP(Ph4) (3), and H2TPP(Ph4)Br4 (7) in CH2Cl2 containing inphyrin dianion from the porphyrin p-anion radical. Attempts creasing concentrations of TFA are shown in Figure 3 along were made to stabilize the singly reduced nonlanar porphyrins with the corresponding log–log plots used to analyze the data. by lowering the temperature but the first reduction remained H2TPP (1), which is planar,[41] is taken as a reference standard irreversible at all temperatures. while H2TPP(Ph4) (3) and H2TPP(Ph4)Br4 (7) were selected as The difference in electrochemistry between compounds 1–6 representative planar and nonplanar porphyrins, respectively. and 7–9 is consistent with what has been reported in the literSimilar types of spectral changes are seen for the three porature for a large number of free-base porphyrins,[3] some of phyrins in Figure 3 upon going from H2(P) to [H4(P)]2 + . The which exhibit well-defined stepwise one-electron reductions neutral compounds 1, 3, and 7 have Soret bands at 417, 434, with relatively stable electroreduction products, while others and 471 nm, respectively, while after formation of [H4(P)]2 + these bands are redshifted to 438, 463, and 494 nm. As the show irreversible processes with multiple chemical reactions coupled to the electron transfers. Porphyrins 1, 2, 3, and 5 Table 1. Half-wave potentials (V vs SCE) for electroreduction of free-base porphyrins 1–10 in CH2Cl2, 0.1 m have an almost planar geometry TBAP before and after addition of 2.0 equivalents TFA. as revealed by the small displacement of the b-pyrrole H2(P) [H4P]2 + Ref. DE[c] Porphyrin DCb [][a] carbon atoms from the plane of 1st 2nd DE(1-2)[b] 1st the macrocycle (DCb = 0.10– H2TPP (1) 0.14 [41] 1.19 1.53 0.34 0.44[d] 0.75 0.14 ) whereas 7, 9, and 10 0.12 [42] 1.28 1.60 0.32 0.46 0.82 H2TPP(CH3)4 2) 0.10 [43] 1.16 1.33 0.17 0.30 0.86 H2TPP(Ph)4 (3) show much larger displace0.58 [44] 0.99 1.10 0.11 0.22 0.77 H2TPP(2-thienyl)4 (4) ments of 1.26 and 1.30 and 0.12 [45] 0.82 0.91 0.09 0.06 0.76 H2TPPBr4 (5) are nonplanar. The data in NA – 0.21 0.51 0.30 0.17[e] 0.04 H2TPP(CN)4 (6) Table 1 and Figure 1 suggest H2TPP(Ph)4Br4 (7) 1.30 [46] 0.88 0.97 0.09 0.20 0.68 that the difference in reversibili1.34 [47] 0.68[e] 0.78 0.10 0.03 0.65 H2TPPCl8 (8) ty of electroreductions between 1.26 [48] 0.72[e] 0.81 0.09 0.03 0.69 H2TPPBr8 (9) the two series of compounds is 1.26 [49] 0.70 0.98 0.18 0.10 0.80 H2T(Ph*)PBr16 (10) related not to the specific elec[a] DCb = displacement of the b-pyrrole carbon atom of the porphyrin macrocycle. [b] Potential difference betron-donating or -withdrawing tween the first and second reductions of H2(P). [c] Potential difference between the first reductions of H2(P) substituents but rather to the and [H4(P)]2 + . [d] Measured at 25 8C. [e] Irreversible peak potential at a scan rate of 0.1 V s1. N/A = not available. planarity of the macrocycle, a point that has not previously been considered. UV/Vis spectra of neutral and protonated porphyrins in CH2Cl2 : UV/Vis spectral data for the investigated porphyrins in CH2Cl2 are summarized in Table 2 and examples of spectra are shown in Figure 2 for compounds 1, 3, and 5 and 7, 9, and 10. One difference between spectra of the planar and nonplanar porphyrins is in the number of Q bands, which are four for compounds 1–6 and 10 Chem. Eur. J. 2014, 20, 524 – 532
Table 2. UV/Vis spectral data, lmax [nm] (loge) of neutral planar and nonplanar free-base porphyrins in CH2Cl2. Porphyrin
Soret region
Q region
H2TPP (1) H2TPP(CH3)4 (2) H2TPP(Ph)4 (3) H2TPP(2-thienyl)4 (4) H2TPPBr4 (5) H2TPP(CN)4 (6)
417 420 434 445 436 439
(5.56) (5.69) (5.34) (5.26) (5.75) (4.74)[a]
515 520 528 541 535 554
(4.23) (4.50) (4.14) (4.27) (4.54) (3.39)
549 562 565 588 576 600
(3.97) (4.17)[b] (3.17)[b] (3.98)[b] (3.78)[b] (3.64)
590 588 599 633 617 660
(3.85) (4.18) (3.67) (3.75) (3.75) (3.33)
646 641 677 700 682 728
H2TPP(Ph)4Br4 (7) H2TPPCl8 (8) H2TPPBr8 (9) H2T(Ph*)PBr8 (10)
471 452 468 467
(5.31) (5.32) (5.19) (5.25)
574 551 566 563
(4.14) (4.10) (3.82) (4.23)
630 601 627 603
(4.12) (4.13) (4.03) (3.90)
733 718 741 648
(3.87) (3.95) (3.93) (3.72)
719 (3.49)
(3.64) (4.11) (3.37) (3.95) (4.18) (3.73)
[a] Additional Soret peak at 449 (4.76). [b] Shoulder peak.
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Full Paper protonation proceeds, the multiple Q bands of the neutral compound disappear and a single broad band grows in at 652, 695, and 738 nm, respectively, for the three [H4(P)]2 + derivatives. Well-defined isobestic points are located at 426 for 1, 447 for 3, and 481 nm for 7 during the three titrations, which indicates the lack of a spectroscopically detectable intermediate, such as the monoprotonated porphyrin [H3(P)] + . Changes in the Soret and/or Q bands intensities were analyzed as a function of the added TFA concentration by using Equation (4) and the corresponding log–log plots show that two protons are added in a single step (n = 2.0) for all three compounds as described in Equation (3). A complete conversion of H2(P) to [H4(P)]2 + was accomplished for seven of the ten investigated porphyrins upon addition of 2–5 equivalents of TFA, after which no further spectral changes were observed in solutions containing up to 0.1 m TFA. Compounds 6 and 10 are harder to protonate than the other investigated free-base porphyrins and higher concentrations of TFA were needed for conversion of 106 m H2(P) to [H4(P)]2 + , after which no further spectral changes were observed in CH2Cl2 solutions containing up to 0.1 m TFA. The spectra of the planar compounds 1, 3, and 5 and the nonplanar derivatives 7, 9 and 10 in CH2Cl2 after addition of 0.1 m TFA to solution are shown in Figure 4 and a summary of the UV/Vis data for the diprotonated porphyrins is given in Table 3 which also lists the calculated logb2 values.
Figure 2. UV/Vis spectra of a) neutral planar porphyrins 1, 3, and 5 and b) nonplanar porphyrins 7, 9, and 10 in CH2Cl2.
log½ðAi A0 Þ=ðA0 Af Þ ¼ logb2 þ nlog ½Hþ ð4Þ The spectral data in Tables 2 and 3 indicate that nine of the ten investigated diprotonated porphyrins have redshifted Soret and Q bands in CH2Cl2 containing TFA as compared to the same unprotonated freebase derivatives in CH2Cl2 before addition of TFA. The one exception to this trend is compound 10, which undergoes a redshift of the Soret band from 467 to 494 nm and a concommitant blueshift in the longest wavelength band from 719 (Table 2) to 700 nm (Table 3).
Figure 3. UV/Vis spectral changes during titration of free-base porphyrins 1 (a), 3 (b), and 7 (c) with TFA in CH2Cl2 and analysis of data for calculation of proton addition. Chem. Eur. J. 2014, 20, 524 – 532
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The magnitude of the shift in electronic absorption bands upon protonation of 1–10 depends upon the electron-donating/-withdrawing properties of the substituents as well as upon the geometry of the protonated
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Full Paper Electrochemistry in CH2Cl2-containing TFA: The conversion of H2(P) to [H4(P)]2 + was monitored by cyclic voltammetry in CH2Cl2 as increasing concentrations of acid were added to the solution, an example of which is given in Figure 5 for the case of H2TPP(Ph)4 (3). As seen in this figure, the peak current for the second reduction of H2TPP(Ph)4 at E1/2 = 1.33 V initially decreases in magnitude during the titration as that for reduc-
Figure 4. UV/Vis spectra of the diprotonated porphyrins in CH2Cl2 with added TFA.
Table 3. UV/Vis spectral data, lmax [nm] (loge) of diprotonated porphyrins in CH2Cl2 containing added TFA. Geometry planar
nonplanar
Compound[a] 2+
1, [H4TPP] 2, [H4TPP(CH3)4]2 + 3, [H4TPP(Ph)4]2 + 4, [H4TPP(2-thienyl)4]2 + 5, [H4TPPBr4]2 + 6, [H4TPP(CN)4]2 + 7, [H4TPP(Ph)4Br4]2 + 8, [H4TPPCl8]2 + 9, [H4TPPBr8]2 + 10, [H4T(Ph*)PBr8]2 +
Soret region
Q region
logb2[b]
438 449 463 457 464 482 494 483 494 494
652 665 695 726 704 770 738 732 749 700
9.96 9.60 9.90 9.90 9.03 3.10 10.7 9.95 10.1 6.89
(5.40) (5.68) (5.24) (5.09) (5.67) (4.73) (5.17) (5.43) (5.33) (5.30)
(4.42) (4.69) (4.35) (4.46) (4.84) (4.00) (4.17) (4.67) (4.63) (4.07)[c]
Figure 5. Cyclic voltammograms of H2TPP(Ph)4 (3) in CH2Cl2 containing 0.1 m TBAP with different equivalents of added TFA. Scan rate = 0.1 V s1. * = reduction of generated unprotonated porphyrin.
tion of the chemically generated [H4TPP(Ph)4)]2 + (at E1/2 = 0.30 V) increases. In contrast to these changes, the peak current for the first reduction of unprotonated H2TPP(Ph)4 (at E1/2 = 1.16 V) begins to decrease after 0.5 equivalents of H + have been added to solution, despite the fact that [H4TPP(Ph)4]2 + begins to form upon addition of TFA. Only when the added TFA is greater than 0.5 equivalents will the current for reduction of H2(P) begin to decrease and, in CH2Cl2 containing 1.0 equivalent TFA, the peak current for formation of [H2(P)] will have decreased to almost half of its original value. Under these conditions, the second reduction at 1.33 V is no longer present and the E1/2 for reduction of H2(P) has shifted from 1.16 to 1.14 V.
[a] Diprotonated form of the compound. [b] See Equation (3). [c] A peak at 643 nm (4.09) can also be seen.
porphyrins, but this is not reflected in the magnitude of the protonation constants. As seen in Figure 3 and Table 3, eight of the ten porphyrins have almost identical logb2 values, the two exceptions being 6 and 10. The largest logb2 value is for compound 7 which also has the largest DCb value. This is consistent with the fact that a large out-of plane distortion of the macrocycle will orient the nitrogen atoms to lie out of the plane of the porphyrin ring, thus providing an easy access for the addition of protons. The much smaller protonation constant of 6 (logb2 = 3.10) as compared to all of the other investigated porphyrins may be attributed to the combined electronic effects of planarity and the four electron-withdrawing CN groups. In the case of 10, which is nonplanar, the small logb2 of 6.89 may be due to a steric effect of the b-bromo and o-dibromo groups as well as to electronic effects of the Br groups. Chem. Eur. J. 2014, 20, 524 – 532
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After addition of 1.5 equivalents TFA to solution, both oneelectron reductions of the initial H2TPP(Ph)4 compound have totally disappeared while the single process for reduction of [H4TPP(Ph)4]2 + at E1/2 = 0.30 V is well-defined and reversible. Finally, when the TFA concentration is further increased above 3.0 equivalents, the reduction of [H4TPP(Ph)4]2 + becomes irreversible, consistent with a rapid chemical reaction following
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Full Paper electron transfer (an EC mechanism). A proposed mechanism is given later in the manuscript. Spectroscopic evidence for the formation of [H3(P)] + has been given in the literature[8, 19, 20, 50–58] and we would like to suggest that the peak at Ep = 0.66 V (labeled by an asterisk in Figure 5) can be assigned to reduction of the singly protonated porphyrin [H3TPP(Ph)4] + . The peak at Ep = 1.18 V (also labeled by an asterisk in Figure 5) is then assigned to reduction of the unprotonated porphyrin that is generated in a chemical reaction following the first electron transfer. With the exception of 6, similar electrochemistry is seen for all of the investigated porphyrins in acid media and selected cyclic voltammograms of the in situ generated [H4(P)]2 + species in CH2Cl2 containing 0.1 m TBAP and 2.0 equivalents TFA are shown in Figure 6. Each doubly protonated porphyrin is significantly easier to reduce than the initial H2(P) derivative with the same macrocycle, consistent with the two additional
A 680 to 800 mV separation in potentials is seen between E1/2 values for the first reversible two-electron reduction of the protonated nonplanar porphyrins 7–10 in solutions containing 2.0 equivalents TFA and the Epc for the corresponding irreversible reduction of the unprotonated nonplanar porphyrin in the absence of acid. This separation is also given in Table 1. Spectroelectrochemistry: Spectroelectrochemistry was used to spectrally characterize the reduction products of H2(P) and [H4(P)]2 + in CH2Cl2. Three different types of electron-transfer mechanisms were observed, depending upon the porphyrin planarity and the presence or absence of added acid in the solution. The first mechanism, labeled as Scheme 2a, occurred only for the planar porphyrins in CH2Cl2 without added acid and involved formation of a stable p-anion radical after a rever-
Scheme 2. Proposed reduction mechanisms of the neutral and diprotonated porphyrins in CH2Cl2.
sible one-electron addition. Examples of the spectral changes during this process are shown in Figure 7a for reduction of H2TPP(CH3)4 2 at 1.40 V in the thin-layer cell. The second type of mechanism is labeled as Scheme 2b and occurred only for the nonplanar porphyrins in CH2Cl2, 0.1 m TBAP. An example of these spectral changes is shown in Figure 7a for H2TPPBr8 9 and gives a final product characteristic of a deprotonated porphyrin in its unreduced form, either [H(P)] or [(P)]2.
Figure 6. Cyclic voltammograms illustrating reduction of investigated a) planar porphyrins 1, 3, and 5 and b) nonplanar porphyrins 7–9 in CH2Cl2 containing 2.0 equivalents TFA, 0.1 m TBAP.
The third type of reaction mechanism occurs for all of the protonated free-base porphyrins and involves a two-electron transfer followed by a chemical reaction leading to H2(P) as a final product. Two examples of these spectral changes are shown in Figure 7b, one for an initially planar porphyrin and the other for a porphyrin that is nonplanar in its neutral form. As seen in the figure, the product of electroreduced H2TPP(CH3)4 (2) has a Soret band at 420 nm and that for electroreduced H2TPPBr8 (9) has a band at 468 nm. Both absorption bands of the electroreduction products are exactly the same as for the neutral compounds in CH2Cl2. Thus, the combined electrochemical and spectral data indicate an initial reversible formation of doubly reduced [H4(P)]0 from [H4(P)]2 + in solutions containing 2.0 equivalents TFA but the final UV/Vis spectrum in each case indicates the ultimate loss of two protons on the spectroelectrochemistry timescale with regeneration of H2(P) at the electrode surface. This overall process is labeled as Scheme 2c.
positive charges on [H4(P)]2 + . The peak currents for reduction of [H4(P)]2 + in CH2Cl2 containing 2.0 equivalents TFA are also approximately double those for reduction of H2(P) in CH2Cl2, 0.1 m TBAP. This is shown in Figure S1 (Supporting Information) and is consistent with an overlapping of two one-electron transfer processes and a direct conversion of [H4(P)]2 + to a transient H4(P) species as shown in Equation (5). ½H4 ðPÞ2þ þ 2e ! H4 ðPÞ
ð5Þ
The difference in potential between the reversible two-electron reduction of the doubly protonated compounds 1–5 in solutions containing 2.0 equivalents TFA and the reversible one-electron reduction of the related planar H2(P) derivative in CH2Cl2 without added acid is given in the last column of Table 1 and ranges from 0.86 V for compound 3 to 0.76 V for compound 5. The first reduction of the diprotonated compound 6 is irreversible in CH2Cl2 containing 2.0 equivalents TFA and the potential separation between Ep of [H4(P)]2 + and E1/2 of H2(P) amounts to just 0.04 V as seen in Figure S2 (Supporting Information). Chem. Eur. J. 2014, 20, 524 – 532
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Electrochemistry and spectroelectrochemistry in CH2Cl2 containing TBAOH: To further identify the deprotonated products of electroreduction in mechanism Scheme 2b, electrochemistry 529
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Full Paper The above described sequence of coupled electrochemical and chemical steps is simplified in CH2Cl2 at low temperature where no reoxidation peaks can be observed (Figure 8b). It is also simplified in CH2Cl2 containing added TBAOH in which the cyclic votammogram exhibits only one major reduction and one major reoxidation, which are labeled as Processes III and IIc, respectively, in Figure 8c. The UV/Vis spectra of the deprotonated product generated by addition of TBAOH to solutions of compounds 8 and 9 are shown in Figure 9 and are characterized by two major absorption bands, one at 491–502 nm Figure 7. UV/Vis spectra of the planar H2TPP(CH3)4 (2) and nonplanar H2TPPBr8 (9) before (c) and after (a) and the other at 715–733 nm. controlled potential reduction in a) CH2Cl2 and b) CH2Cl2 containing 0.1 m TBAP and 2.0 equivalents added TFA. This type of spectrum was previously characterized in the litand spectroelectrochemistry experiments were carried out on erature[8, 30] as a deprotonated porphyrin dianion, [(P)]2, which the nonplanar compounds 7–10 in CH2Cl2 containing added suggests the loss of two protons as shown in Equation (7). TBAOH. The addition of TBAOH to H2(P) should lead stepwise More importantly as seen in Figure 9, the UV/Vis spectrum of 2 to [H(P)] and [(P)] as shown in Equations (6a) and (6b), but chemical generated [(P)]2 is virtually identical to the UV/Vis under our experimental conditions, only the doubly deprotonspectrum of the product formed after the two-electron reducated porphyrins were formed upon electroreduction of H2(P) tion of 8 and 9 in the thin-layer cell. This result suggests that or after addition of TBAOH to the neutral porphyrin in CH2Cl2 the same deprotonated porphyrin product is formed from as shown in Equation (7). H2 ðPÞ þ OH Ð ½HðPÞ þ H2 O
ð6aÞ
H2 ðPÞ þ OH Ð ½ðPÞ2 þ H2 O
ð6bÞ
H2 ðPÞ þ 2 OH Ð ½ðPÞ2 þ 2 H2 O
ð7Þ
Evidence for the formation of [(P)]2 is given by the cyclic voltammograms in Figure 8 and by the UV/Vis spectrum of the product formed after controlled potential reduction in a thinlayer cell (Figure 9). The first and second reductions of H2TPPCl8 (8) (Processes I and II in Figure 8) are overlapped and followed by a rapid chemical reaction leading to a new electroactive species that is reduced via Process III. Processes I and II occur at potentials at 0.68 and 0.78 V for compound 8, while Process III is located at Ep = 1.08 V for a scan rate of 0.1 V s1. Two reoxidation peaks are then seen on the reverse potential sweep, the first is labeled as Process II and the second as Process I’ in Figure 8a. The identity of the species involved in the reoxidation Process I’ was not characterized but is proposed to be involved in a form of the free-base p-anion radical with planarity different than that of the transient electrogenerated monoanion. Similar processes are observed for compounds 7–10 as seen in Figure 1b and Table S1 (Supporting Information), which lists the potentials for each redox process of these compounds. Chem. Eur. J. 2014, 20, 524 – 532
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Figure 8. Cyclic voltammograms illustrating reductions of H2TPPCl8 (8) in CH2Cl2, 0.1 m TBAP a) at room temperature, b) at 60 8C, and c) with 2.0 equivalents TBAOH at room temperature. The peak I’ marked by an asterisk is associated with an unidentified product generated following the first electron transfer. The process as II is labeled as IIa (anodic) and IIc (cathodic).
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Full Paper corded on a Hewlett–Packard model 8453 diode array spectrophotometer. 1H NMR spectra of porphyrins was recorded on a Jeol 400 MHz (or 500 MHz) spectrometer by using CDCl3 with TMS as the internal reference at 298 K. Mass spectral measurements of the samples were carried out using electrospray ionization (ESI) mass spectrometer model Micromass Q-TOF Micro using 10 % formic acid in CHCl3 as the solvent medium. Chemicals: Absolute dichloromethane (CH2Cl2) was purchased from Aldrich and used as received without further purification. TBAP was procured from Fluka and used as received. TFA and 5,10,15,20-tetraphenylporphyrin, H2TPP (1) purchased from Aldrich was used without further purification. The other free-base porphyrins, 2–10, were synthesized according to literature procedures[44, 59–64] and characterized by UV/Vis, 1H NMR spectroscopy, and mass spectroscopic methods. Calculation of protonation constants: Changes in the UV/Vis spectra of 1–10 in CH2Cl2 were monitored during a titration with TFA and the resulting spectral data then 2 Figure 9. UV/Vis spectra of H2(P) (c) and [(P)] (a) generated a) by addition of used to calculate formation constants for proton addiTBAOH and b) electroreduction of H2TPPCl8 and H2TPPBr8 in CH2Cl2. tion by using the Hill equation[65] (Equation 4) in which Ai is the absorbance in solutions with a specific concentration of added protons [H + ], A0 is the initial absorbH2(P) by an electrochemically initiated process or by addition + ance in which [H ] = 0.0 and Af is the final absorbance of the fully of base to the same porphyrin in CH2Cl2. Under these condiprotonated porphyrin. The slope of the log [(AiA0)/(A0Af)] versus tions the overall reduction and reoxidation mechanism is given log[H+] plot gives n, the number of protons added to the core niby Scheme 3. trogen atoms, and the value of logb2 is evaluated from the intercept of the line at log[(AiA0)/(A0Af)] = 0.0. In the present study, the logb2 values were evaluated by using a minimum of two waveConclusion lengths and an average value of n and logb2 is reported.
In summary, electroreduction of the diprotonated planar and
Acknowledgements Support of the Robert A. Welch Foundation (K.M.K., Grant E680) and Natural Science Foundation of China (Grant 21071067) is gratefully acknowledged. Scheme 3. Proposed redox mechanism of the nonplanar porphyrins in CH2Cl2.
Keywords: cyclic voltammetry · deprotonation · porphyrins · protonation · spectroelectrochemistry
nonplanar porphyrins [H4(P)]2 + in acid media leads to H2(P) while the nonplanar H2(P) derivatives are reduced to [(P)]2 in CH2Cl2 containing 0.1 m TBAP. Thus, in both cases, an electrochemically initiated deprotonation is observed.
[1] K. M. Kadish, K. M. Smith, R. Guilard, The Porphyrin Handbook, Academic Press, New York, 2000. [2] K. M. Kadish, K. M. Smith, R. Guilard, Handbook of Porphyrin Science World Scientific, Singapore, 2010. [3] K. M. Kadish, K. M. Smith, R. Guilard in The Porphyrin Handbook, Vol. 8 (Eds.: K. M. Kadish, E. Van Caemelbecke, G. Royal), Academic Press, New York, 2000, pp. 1 – 114. [4] M. A. Castriciano, M. Samperi, S. Camiolo, A. Romeo, S. L. Monsu, Chem. Eur. J. 2013, 19, 12160 – 12168. [5] P. K. Goldberg, T. J. Pundsack, K. E. Splan, J. Phys. Chem. A 2011, 115, 10452 – 10460. [6] I. H. Wasbotten, J. Conradie, A. Ghosh, J. Phys. Chem. B 2003, 107, 3613 – 3623. [7] J. R. Weinkauf, S. W. Cooper, A. Schweiger, C. C. Wamser, J. Phys. Chem. A 2003, 107, 3486 – 3496. [8] D. G. Luca, A. Romeo, L. M. Scolaro, G. Ricciardi, A. Rosa, Inorg. Chem. 2007, 46, 5979 – 5988. [9] T. Kojima, K. Hanabusa, K. Ohkubo, M. Shiro, S. Fukuzumi, Chem. Commun. 2008, 6513 – 6515. [10] K. M. Barkigia, L. Chantranupong, K. M. Smith, J. Fajer, J. Am. Chem. Soc. 1988, 110, 7566 – 7567.
Experimental Section Instrumentation: Cyclic voltammetry was carried out by using an EG&G Princeton Applied Research 173 potentiostat. A homemade three-electrode cell was used for cyclic voltammetry measurements and consisted of a glassy carbon working electrode, a platinum wire counter-electrode, and a homemade saturated calomel reference electrode (SCE). The SCE electrode was separated from the bulk solution by a fritted-glass bridge of low porosity, which contained the solvent/supporting electrolyte mixture. UV/Vis spectroelectrochemical experiments were performed with a home-built thin-layer cell that had a light transparent platinum net working electrode. Potentials were applied and monitored with an EG&G PAR model 173 potentiostat. Time-resolved UV/Vis spectra were reChem. Eur. J. 2014, 20, 524 – 532
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Full Paper [38] H. Guo, J. Jiang, Y. Shi, Y. Wang, Y. Wang, S. Dong, J. Phys. Chem. B 2006, 110, 587 – 594. [39] E. C. A. Ojadi, H. Linschitz, M. Gouterman, R. I. Walter, J. S. Lindsey, R. W. Wagner, P. R. Droupadi, W. Wang, J. Phys. Chem. 1993, 97, 13192 – 13197. [40] M. Meot-Ner, A. D. Adler, J. Am. Chem. Soc. 1975, 97, 5107 – 5111. [41] M. J. Hamor, T. A. Hamor, J. L. Hoard, J. Am. Chem. Soc. 1964, 86, 1938 – 1942. [42] P. Bhyrappa, K. Karunanithi, B. Varghese, Acta Crystallogr. Sect. E 2007, 63, o4755/4751 – o4755/4758. [43] K. S. Chan, X. Zhou, B. S. Luo, T. C. W. Mak, J. Chem. Soc. Chem. Commun. 1994, 271 – 272. [44] P. Bhyrappa, V. Velkannan, Inorg. Chim. Acta 2012, 387, 64 – 73. [45] J.-Z. Zou, Z. Xu, M. Li, X.-Z. You, H.-Q. Wang, Acta Crystallogr. Sect. C 1995, 51, 760 – 761. [46] P. Bhyrappa, C. Arunkumar, B. Varghese, Inorg. Chem. 2009, 48, 3954 – 3965. [47] M. Gruden, S. Grubisic, A. G. Coutsolelos, S. R. Niketic, J. Mol. Struct. 2001, 595, 209 – 224. [48] P. Bhyrappa, M. Nethaji, V. Krishnan, Chem. Lett. 1993, 869 – 872. [49] P. Bhyrappa, C. Arunkumar, B. Varghese, J. Porphyrins Phthalocyanines 2007, 11, 795 – 804. [50] T. Honda, T. Kojima, S. Fukuzumi, Chem. Commun. 2009, 4994 – 4996. [51] R. Harada, T. Kojima, Chem. Commun. 2005, 716 – 718. [52] T. Kojima, T. Nakanishi, R. Harada, K. Ohkubo, S. Yamauchi, S. Fukuzumi, Chem. Eur. J. 2007, 13, 8714 – 8725. [53] T. Nakanishi, T. Kojima, K. Ohkubo, T. Hasobe, K.-i. Nakayama, S. Fukuzumi, Chem. Mater. 2008, 20, 7492 – 7500. [54] T. Ishizuka, M. Sankar, Y. Yamada, S. Fukuzumi, T. Kojima, Chem. Commun. 2012, 48, 6481 – 6483. [55] S. Aronoff, J. Phys. Chem. 1958, 62, 428 – 431. [56] A. B. Rudine, B. D. DelFatti, C. C. Wamser, J. Org. Chem. 2013, 78, 6040 – 6049. [57] E. Austin, M. Gouterman, Bioinorg. Chem. 1978, 9, 281 – 298. [58] R. J. Abraham, G. E. Hawkes, K. M. Smith, Tetrahedron Lett. 1974, 15, 71 – 74. [59] P. Bhyrappa, M. Sankar, B. Varghese, Inorg. Chem. 2006, 45, 4136 – 4149. [60] P. K. Kumar, P. Bhyrappa, B. Varghese, Tetrahedron Lett. 2003, 44, 4849 – 4851. [61] H. J. Callot, Bull. Soc. Chim. Fr. 1974, 7 – 8, Pt. 2, 1492 – 1496. [62] T. Wijesekera, A. Matsumoto, D. Dolphin, D. Lexa, Angew. Chem. 1990, 102, 1073 – 1074; Angew. Chem. Int. Ed. 1990, 29, 1028 – 1030. [63] P. Bhyrappa, V. Krishnan, Inorg. Chem. 1991, 30, 239 – 245. [64] P. Bhyrappa, B. Purushothaman, J. J. Vittal, J. Porphyrins Phthalocyanines 2003, 7, 682 – 692. [65] P. Ellis, J. Linard, T. Szymanski, R. Jones, J. Budge, F. Basolo, J. Am. Chem. Soc. 1980, 102, 1889 – 1896.
[11] D. Mandon, P. Ochenbein, J. Fischer, R. Weiss, K. Jayaraj, R. N. Austin, A. Gold, P. S. White, O. Brigaud, P. Battioni, D. Mansuy, Inorg. Chem. 1992, 31, 2044 – 2049. [12] J. A. Shelnutt, X. Song, J. Ma, S. Jia, W. Jentzen, C. J. Medforth, Chem. Soc. Rev. 1998, 27, 31 – 42. [13] M. O. Senge in The Porphyrin Handbook, Vol. 1, (Eds.: K. M. Kadish, K. M. Smith, R. Guilard), Academic Press, New York, 2000, pp. 239 – 335. [14] M. W. Renner, L. R. Furenlid, K. M. Barkigia, A. Forman, H. K. Shim, D. J. Simpson, K. M. Smith, J. Fajer, J. Am. Chem. Soc. 1991, 113, 6891 – 6898. [15] K. M. Barkigia, M. W. Renner, L. R. Furenlid, C. J. Medforth, K. M. Smith, J. Fajer, J. Am. Chem. Soc. 1993, 115, 3627 – 3635. [16] L. D. Sparks, C. J. Medforth, M. S. Park, J. R. Chamberlain, M. R. Ondrias, M. O. Senge, K. M. Smith, J. A. Shelnutt, J. Am. Chem. Soc. 1993, 115, 581 – 592. [17] J. A. Shelnutt, C. J. Medforth, M. D. Berber, K. M. Barkigia, K. M. Smith, J. Am. Chem. Soc. 1991, 113, 4077 – 4087. [18] M. W. Renner, K. M. Barkigia, Y. Zhang, C. J. Medforth, K. M. Smith, J. Fajer, J. Am. Chem. Soc. 1994, 116, 8582 – 8592. [19] O. S. Finikova, A. V. Cheprakov, P. J. Carroll, S. Dalosto, S. A. Vinogradov, Inorg. Chem. 2002, 41, 6944 – 6946. [20] S. Thyagarajan, T. Leiding, S. P. Arskold, A. V. Cheprakov, S. A. Vinogradov, Inorg. Chem. 2010, 49, 9909 – 9920. [21] R. E. Haddad, S. Gazeau, J. Pecaut, J.-C. Marchon, C. J. Medforth, J. A. Shelnutt, J. Am. Chem. Soc. 2003, 125, 1253 – 1268. [22] F. D’Souza, A. Villard, C. E. Van, M. Franzen, T. Boschi, P. Tagliatesta, K. M. Kadish, Inorg. Chem. 1993, 32, 4042 – 4048. [23] K. M. Kadish, F. D’Souza, A. Villard, M. Autret, C. E. Van, P. Bianco, A. Antonini, P. Tagliatesta, Inorg. Chem. 1994, 33, 5169 – 5170. [24] F. D’Souza, M. E. Zandler, P. Tagliatesta, Z. Ou, J. Shao, C. E. Van, K. M. Kadish, Inorg. Chem. 1998, 37, 4567 – 4572. [25] M. Senge, T. Forsyth, L. Nguyen, K. Smith, Angew. Chem. 1994, 106, 2554 – 2557; Angew. Chem. Int. Ed. Engl. 1994, 33, 2485 – 2487. [26] R. Karaman, T. C. Bruice, Inorg. Chem. 1992, 31, 2455 – 2459. [27] Y. Girard, M. Kondo, K. Yoshizawa, Chem. Phys. 2006, 327, 77 – 84. [28] Y. Zhang, M. X. Li, M. Y. Lue, R. H. Yang, F. Liu, K. A. Li, J. Phys. Chem. A 2005, 109, 7442 – 7448. [29] O. Almarsson, A. Blasko, T. C. Bruice, Tetrahedron 1993, 49, 10239 – 10252. [30] G. Lu, X. Zhang, X. Cai, Y. Fang, M. Zhu, W. Zhu, Z. Ou, K. M. Kadish, J. Porphyrins Phthalocyanines 2013, 17, 10.1142/S1088424613500557. [31] S. Okada, H. Segawa, J. Am. Chem. Soc. 2003, 125, 2792 – 2796. [32] B. P. Neri, G. S. Wilson, Anal. Chem. 1972, 44, 1002 – 1009. [33] D. L. Langhus, G. S. Wilson, Anal. Chem. 1979, 51, 1139 – 1144. [34] B. P. Neri, G. S. Wilson, Anal. Chem. 1973, 45, 442 – 445. [35] M. Sankar, T. Ishizuka, Z. Wang, T. Ma, M. Shiro, T. Kojima, J. Porphyrins Phthalocyanines 2011, 15, 421 – 432. [36] T. Honda, T. Nakanishi, K. Ohkubo, T. Kojima, S. Fukuzumi, J. Phys. Chem. C 2010, 114, 14290 – 14299. [37] H. Guo, J. Jiang, Y. Shi, Y. Wang, J. Liu, S. Dong, J. Phys. Chem. B 2004, 108, 10185 – 10191.
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Received: August 8, 2013 Published online on December 2, 2013
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Full Paper
& Analytical Chemistry
Planar and Nonplanar Free-Base Tetraarylporphyrins: b-Pyrrole Substituents and Geometric Effects on Electrochemistry, Spectroelectrochemistry, and Protonation/Deprotonation Reactions in Nonaqueous Media Yuanyuan Fang,[a] P. Bhyrappa,[a, b] Zhongping Ou,[a, c] and Karl M. Kadish*[a]
Abstract: A series of planar and nonplanar free-base b-pyrrole substituted meso-tetraarylporphyrins were characterized by electrochemistry, spectroelectrochemistry, and protonation or deprotonation reactions in neutral, acidic, and basic solutions of CH2Cl2. The neutral compounds are represented as H2(P), in which P represents a porphyrin dianion with one of several different sets of electron-withdrawing or -donating substituents at the messo and/or b-pyrrole positions of the macrocycle. The conversion of H2(P) to [H4(P)]2 + in CH2Cl2 was accomplished by titration of the neutral porphyrin with trifluoroacetic acid (TFA) while the progress of the protonation was monitored by UV/Vis spectroscopy, which was also used to calculate logb2 for proton addition to the core nitrogen atoms of the macrocycle. Cyclic voltammetry was performed after each addition of TFA or TBAOH to CH2Cl2 solutions of the porphyrin and half-wave potentials for reduction
were evaluated as a function of the added acid or base concentration. Thin-layer spectroelectrochemistry was used to obtain UV/Vis spectra of the neutral and protonated or deprotonated porphyrins under the application of an applied reducing potential. The magnitude of the protonation constants, the positions of lmax in the UV/Vis spectra and the half-wave or peak potentials for reduction are then related to the electronic properties of the porphyrin and the data evaluated as a function of the planarity or nonplanarity of the porphyrin macrocycle. Surprisingly, the electroreduction of the diprotonated nonplanar porphyrins in acid media leads to H2(P), whereas the nonplanar H2(P) derivatives are reduced to [(P)]2 in CH2Cl2 containing 0.1 m tetra-n-butylammonium perchlorate (TBAP). Thus, in both cases an electrochemically initiated deprotonation is observed.
Introduction
been interested in how the above factors will affect the electrochemical and spectroscopic properties of porphyrins in various oxidation states and have recently turned our attention towards elucidating the effect of nonplanarity as it applies to the redox reactivity of neutral and protonated free-base (metalfree) porphyrins in nonaqueous media. A number of articles have reported the chemical, structural, and optical properties of planar and nonplanar free-base porphyrins in their neutral and diprotic forms.[4–20] A nonplanar conformation of the porphyrin macrocycle will result from repulsive interactions among the peripheral substituents that also induces dramatic redshifts of the electronic absorption spectral bands.[10, 21] The energy of the HOMO and the LUMO are both influenced by the nature of the peripheral substituents and the nonplanarity of the porphyrin macrocycle. For example, the increased steric crowding of bromine atoms in the free-base-brominated tetraphenylporphyrins H2TPPBrn (n = 1–8) leads to an increasing nonplanar distortion of the ring with increase in the number of bromine atoms. The increased bromination produces a stabilization of the LUMO and a destabilization of the HOMO energy levels, which is then reflected in a change of redox potentials for both oxidation and reduction.[22–24]
Porphyrins and porphyrin analogues have been employed as model systems for a number of biological tetrapyrrole pigments. They have also been investigated as essential components in a number of applications.[1, 2] The redox properties of natural and synthetic porphyrins will depend upon a number of factors, examples of which are the core structure, the type and location of electron-donating or -withdrawing substituents, the type and oxidation state of the central metal ion, and the type and number of bound axial ligands.[3] We have long [a] Y. Fang, Prof. P. Bhyrappa, Prof. Z. Ou, Prof. K. M. Kadish Department of Chemistry, University of Houston Houston, TX 77204-5003 (USA) Fax: (+ 1) 713-743-2745 E-mail: [email protected] [b] Prof. P. Bhyrappa Department of Chemistry Indian Institute of Technology Madras Chennai 600 036 (India) [c] Prof. Z. Ou School of Chemistry and Chemical Engineering Jiangsu University, Zhenjiang 212013 (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201303141. Chem. Eur. J. 2014, 20, 524 – 532
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Full Paper Protonation of the central nitrogen atoms of a free-base porphyrin will also induce a nonplanar distortion of the macrocycle.[25] Under some conditions, the addition of two protons can occur in separate steps as shown in Equations (1) and (2),[8, 26–29] but in most cases the experimentally observed process actually occurs in a single step as illustrated in Equation (3).[8, 26–31] H2 ðPÞ þ Hþ Ð ½H3 ðPÞþ
ð1Þ
½H3 ðPÞþ þ Hþ Ð ½H4 ðPÞ2þ
ð2Þ
H2 ðPÞ þ 2 Hþ Ð ½H4 ðPÞ2þ
ð3Þ
the solvent, the strength of the added base, and the planarity of the macrocycle. The deprotonation of H2(P) to give [H(P)] and [(P)]2 has been examined by UV/Vis spectroscopy in nonaqueous media[27, 30, 37–40] but, to our knowledge, there has been no electrochemistry reported for the deprotonated porphyrins under these solution conditions. Thus, an electrochemical and spectroelectrochemical study of the protonated and deprotonated compounds 1–10 in CH2Cl2 should provide new information on redox potentials and UV/Vis spectra for a series of planar and nonplanar free-base porphyrins.
Results and Discussion
There are several reports in the literature on the redox reactions of protonated free-base porphyrins in aqueous[32–34] and nonaqueous[35, 36] media, but the combined effect of macrocyclic substituents and geometry on the electrochemistry of protonated and deprotonated porphyrins in nonaqueous media has been largely unexamined. There have also been no published studies that have elucidated the spectroscopic properties of reduced protonated porphyrins in nonaqueous media. This is investigated in the present study for the compounds shown in Scheme 1, each of which was investigated by electrochemical and spectroelectrochemical methods.
Electrochemistry in CH2Cl2, 0.1 m tetra-n-butylammonium perchlorate (TBAP): Two types of electrochemical behavior are observed for the free-base porphyrins in CH2Cl2 containing 0.1 m TBAP. The porphyrins with planar macrocycles exhibit two reversible one-electron reductions as shown in Figure 1a for compounds 1, 3, and 5 while the nonplanar porphyrins 7– 9 show multiple irreversible reductions with coupled chemical reactions (Figure 1b).
Figure 1. Cyclic voltammograms illustrating reductions of a) the planar porphyrins 1, 3, and 5 and b) nonplanar porphyrins 7, 8, and 9 in CH2Cl2, 0.1 m TBAP.
Scheme 1. Molecular structures of the investigated planar and nonplanar free-base porphyrins.
As seen Figure 1a, the first reduction of the planar porphyrins occurs at E1/2 = 1.19 V for 1, E1/2 = 1.16 V for 3, and E1/2 = 0.82 V for 5 to give a porphyrin p-anion radical,[3] whereas the second reduction generates a porphyrin dianion[3] at E1/2 = 1.53 V for 1, E1/2 = 1.33 V for 3, and E1/2 = 0.91 V for 5. The potential separation between these processes is not constant but varies from a DE1/2 of 340 mV in the case of 1 and to a smaller potential separation of 90 mV in the case of 5, for which the two reductions are almost overlapped. The singly and doubly reduced planar free-base porphyrins are both stable in CH2Cl2 containing 0.1 m TBAP and no other redox processes are observed up to the negative potential limit of the solvent, which is 2.0 V in the case of CH2Cl2. In contrast to the planar free-base porphyrins, significantly different electrochemical behavior is exhibited for the nonpla-
The assignment of macrocycle planarity to the neutral compounds in Scheme 1 is based in part on structural data in the solid state and in part on the compound’s UV/Vis spectra that exhibit diagnostic patterns that indicate planarity or nonplanarity of the macrocycle, as described later in the manuscript. All of the porphyrins in Scheme 1 are anticipated to have a nonplanar configuration after the addition of two more protons to the core nitrogen atoms, as was demonstrated for tetraphenylporphyrin [H4TPP]2 + , octaethyltetraphenylporphyrin [H4OETPP]2 + , and octabromotetraphenylporphyrin [H4TPPBr8]2 + which were structurally characterized as having a saddle-type distortion (D2d symmetry).[25] A loss of protons from free-base porphyrins can occur in nonaqueous media containing added base, with the degree of deprotonation depending upon the porphyrin substituents, Chem. Eur. J. 2014, 20, 524 – 532
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Full Paper nar derivatives 7–9 as shown by the cyclic voltammograms in and three for compounds 7, 8, and 9. More importantly, the Figure 1b. The initial reduction of each compound again innonplanar porphyrins, 7–10, have a much larger redshift in the volves a one-electron transfer but the shape of the current– Soret and Q bands than the planar porphyrins 2–6, as comvoltage curves are consistent with a chemical reaction followpared to the unsubstituted H2TPP, compound 1. This redshift ing the first electron addition (an EC mechanism) to give new in lmax can be ascribed to both an inductive and resonance inelectroactive product that is then reduced at more negative teraction of the macrocycle substituents with the porphyrin ppotentials of 1.09 to 1.28 V and also reoxidized on the ring system as well as to a distortion of the porphyrin macroreturn sweep at Ep = 0.25 to 0.52 V. The most reactive eleccycle. Protonation at the porphyrin central nitrogen atoms will trogenerated porphyrin monoanions are 8 and 9, as judged by readily occur upon addition of acid to the compounds in the total lack of a coupled reoxidation peak after the first reCH2Cl2 solution. Changes in the UV/Vis spectra of H2TPP (1), duction and the significantly decreased peak current for the second reduction that would correspond to formation of a porH2TPP(Ph4) (3), and H2TPP(Ph4)Br4 (7) in CH2Cl2 containing inphyrin dianion from the porphyrin p-anion radical. Attempts creasing concentrations of TFA are shown in Figure 3 along were made to stabilize the singly reduced nonlanar porphyrins with the corresponding log–log plots used to analyze the data. by lowering the temperature but the first reduction remained H2TPP (1), which is planar,[41] is taken as a reference standard irreversible at all temperatures. while H2TPP(Ph4) (3) and H2TPP(Ph4)Br4 (7) were selected as The difference in electrochemistry between compounds 1–6 representative planar and nonplanar porphyrins, respectively. and 7–9 is consistent with what has been reported in the literSimilar types of spectral changes are seen for the three porature for a large number of free-base porphyrins,[3] some of phyrins in Figure 3 upon going from H2(P) to [H4(P)]2 + . The which exhibit well-defined stepwise one-electron reductions neutral compounds 1, 3, and 7 have Soret bands at 417, 434, with relatively stable electroreduction products, while others and 471 nm, respectively, while after formation of [H4(P)]2 + these bands are redshifted to 438, 463, and 494 nm. As the show irreversible processes with multiple chemical reactions coupled to the electron transfers. Porphyrins 1, 2, 3, and 5 Table 1. Half-wave potentials (V vs SCE) for electroreduction of free-base porphyrins 1–10 in CH2Cl2, 0.1 m have an almost planar geometry TBAP before and after addition of 2.0 equivalents TFA. as revealed by the small displacement of the b-pyrrole H2(P) [H4P]2 + Ref. DE[c] Porphyrin DCb [][a] carbon atoms from the plane of 1st 2nd DE(1-2)[b] 1st the macrocycle (DCb = 0.10– H2TPP (1) 0.14 [41] 1.19 1.53 0.34 0.44[d] 0.75 0.14 ) whereas 7, 9, and 10 0.12 [42] 1.28 1.60 0.32 0.46 0.82 H2TPP(CH3)4 2) 0.10 [43] 1.16 1.33 0.17 0.30 0.86 H2TPP(Ph)4 (3) show much larger displace0.58 [44] 0.99 1.10 0.11 0.22 0.77 H2TPP(2-thienyl)4 (4) ments of 1.26 and 1.30 and 0.12 [45] 0.82 0.91 0.09 0.06 0.76 H2TPPBr4 (5) are nonplanar. The data in NA – 0.21 0.51 0.30 0.17[e] 0.04 H2TPP(CN)4 (6) Table 1 and Figure 1 suggest H2TPP(Ph)4Br4 (7) 1.30 [46] 0.88 0.97 0.09 0.20 0.68 that the difference in reversibili1.34 [47] 0.68[e] 0.78 0.10 0.03 0.65 H2TPPCl8 (8) ty of electroreductions between 1.26 [48] 0.72[e] 0.81 0.09 0.03 0.69 H2TPPBr8 (9) the two series of compounds is 1.26 [49] 0.70 0.98 0.18 0.10 0.80 H2T(Ph*)PBr16 (10) related not to the specific elec[a] DCb = displacement of the b-pyrrole carbon atom of the porphyrin macrocycle. [b] Potential difference betron-donating or -withdrawing tween the first and second reductions of H2(P). [c] Potential difference between the first reductions of H2(P) substituents but rather to the and [H4(P)]2 + . [d] Measured at 25 8C. [e] Irreversible peak potential at a scan rate of 0.1 V s1. N/A = not available. planarity of the macrocycle, a point that has not previously been considered. UV/Vis spectra of neutral and protonated porphyrins in CH2Cl2 : UV/Vis spectral data for the investigated porphyrins in CH2Cl2 are summarized in Table 2 and examples of spectra are shown in Figure 2 for compounds 1, 3, and 5 and 7, 9, and 10. One difference between spectra of the planar and nonplanar porphyrins is in the number of Q bands, which are four for compounds 1–6 and 10 Chem. Eur. J. 2014, 20, 524 – 532
Table 2. UV/Vis spectral data, lmax [nm] (loge) of neutral planar and nonplanar free-base porphyrins in CH2Cl2. Porphyrin
Soret region
Q region
H2TPP (1) H2TPP(CH3)4 (2) H2TPP(Ph)4 (3) H2TPP(2-thienyl)4 (4) H2TPPBr4 (5) H2TPP(CN)4 (6)
417 420 434 445 436 439
(5.56) (5.69) (5.34) (5.26) (5.75) (4.74)[a]
515 520 528 541 535 554
(4.23) (4.50) (4.14) (4.27) (4.54) (3.39)
549 562 565 588 576 600
(3.97) (4.17)[b] (3.17)[b] (3.98)[b] (3.78)[b] (3.64)
590 588 599 633 617 660
(3.85) (4.18) (3.67) (3.75) (3.75) (3.33)
646 641 677 700 682 728
H2TPP(Ph)4Br4 (7) H2TPPCl8 (8) H2TPPBr8 (9) H2T(Ph*)PBr8 (10)
471 452 468 467
(5.31) (5.32) (5.19) (5.25)
574 551 566 563
(4.14) (4.10) (3.82) (4.23)
630 601 627 603
(4.12) (4.13) (4.03) (3.90)
733 718 741 648
(3.87) (3.95) (3.93) (3.72)
719 (3.49)
(3.64) (4.11) (3.37) (3.95) (4.18) (3.73)
[a] Additional Soret peak at 449 (4.76). [b] Shoulder peak.
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Full Paper protonation proceeds, the multiple Q bands of the neutral compound disappear and a single broad band grows in at 652, 695, and 738 nm, respectively, for the three [H4(P)]2 + derivatives. Well-defined isobestic points are located at 426 for 1, 447 for 3, and 481 nm for 7 during the three titrations, which indicates the lack of a spectroscopically detectable intermediate, such as the monoprotonated porphyrin [H3(P)] + . Changes in the Soret and/or Q bands intensities were analyzed as a function of the added TFA concentration by using Equation (4) and the corresponding log–log plots show that two protons are added in a single step (n = 2.0) for all three compounds as described in Equation (3). A complete conversion of H2(P) to [H4(P)]2 + was accomplished for seven of the ten investigated porphyrins upon addition of 2–5 equivalents of TFA, after which no further spectral changes were observed in solutions containing up to 0.1 m TFA. Compounds 6 and 10 are harder to protonate than the other investigated free-base porphyrins and higher concentrations of TFA were needed for conversion of 106 m H2(P) to [H4(P)]2 + , after which no further spectral changes were observed in CH2Cl2 solutions containing up to 0.1 m TFA. The spectra of the planar compounds 1, 3, and 5 and the nonplanar derivatives 7, 9 and 10 in CH2Cl2 after addition of 0.1 m TFA to solution are shown in Figure 4 and a summary of the UV/Vis data for the diprotonated porphyrins is given in Table 3 which also lists the calculated logb2 values.
Figure 2. UV/Vis spectra of a) neutral planar porphyrins 1, 3, and 5 and b) nonplanar porphyrins 7, 9, and 10 in CH2Cl2.
log½ðAi A0 Þ=ðA0 Af Þ ¼ logb2 þ nlog ½Hþ ð4Þ The spectral data in Tables 2 and 3 indicate that nine of the ten investigated diprotonated porphyrins have redshifted Soret and Q bands in CH2Cl2 containing TFA as compared to the same unprotonated freebase derivatives in CH2Cl2 before addition of TFA. The one exception to this trend is compound 10, which undergoes a redshift of the Soret band from 467 to 494 nm and a concommitant blueshift in the longest wavelength band from 719 (Table 2) to 700 nm (Table 3).
Figure 3. UV/Vis spectral changes during titration of free-base porphyrins 1 (a), 3 (b), and 7 (c) with TFA in CH2Cl2 and analysis of data for calculation of proton addition. Chem. Eur. J. 2014, 20, 524 – 532
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The magnitude of the shift in electronic absorption bands upon protonation of 1–10 depends upon the electron-donating/-withdrawing properties of the substituents as well as upon the geometry of the protonated
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Full Paper Electrochemistry in CH2Cl2-containing TFA: The conversion of H2(P) to [H4(P)]2 + was monitored by cyclic voltammetry in CH2Cl2 as increasing concentrations of acid were added to the solution, an example of which is given in Figure 5 for the case of H2TPP(Ph)4 (3). As seen in this figure, the peak current for the second reduction of H2TPP(Ph)4 at E1/2 = 1.33 V initially decreases in magnitude during the titration as that for reduc-
Figure 4. UV/Vis spectra of the diprotonated porphyrins in CH2Cl2 with added TFA.
Table 3. UV/Vis spectral data, lmax [nm] (loge) of diprotonated porphyrins in CH2Cl2 containing added TFA. Geometry planar
nonplanar
Compound[a] 2+
1, [H4TPP] 2, [H4TPP(CH3)4]2 + 3, [H4TPP(Ph)4]2 + 4, [H4TPP(2-thienyl)4]2 + 5, [H4TPPBr4]2 + 6, [H4TPP(CN)4]2 + 7, [H4TPP(Ph)4Br4]2 + 8, [H4TPPCl8]2 + 9, [H4TPPBr8]2 + 10, [H4T(Ph*)PBr8]2 +
Soret region
Q region
logb2[b]
438 449 463 457 464 482 494 483 494 494
652 665 695 726 704 770 738 732 749 700
9.96 9.60 9.90 9.90 9.03 3.10 10.7 9.95 10.1 6.89
(5.40) (5.68) (5.24) (5.09) (5.67) (4.73) (5.17) (5.43) (5.33) (5.30)
(4.42) (4.69) (4.35) (4.46) (4.84) (4.00) (4.17) (4.67) (4.63) (4.07)[c]
Figure 5. Cyclic voltammograms of H2TPP(Ph)4 (3) in CH2Cl2 containing 0.1 m TBAP with different equivalents of added TFA. Scan rate = 0.1 V s1. * = reduction of generated unprotonated porphyrin.
tion of the chemically generated [H4TPP(Ph)4)]2 + (at E1/2 = 0.30 V) increases. In contrast to these changes, the peak current for the first reduction of unprotonated H2TPP(Ph)4 (at E1/2 = 1.16 V) begins to decrease after 0.5 equivalents of H + have been added to solution, despite the fact that [H4TPP(Ph)4]2 + begins to form upon addition of TFA. Only when the added TFA is greater than 0.5 equivalents will the current for reduction of H2(P) begin to decrease and, in CH2Cl2 containing 1.0 equivalent TFA, the peak current for formation of [H2(P)] will have decreased to almost half of its original value. Under these conditions, the second reduction at 1.33 V is no longer present and the E1/2 for reduction of H2(P) has shifted from 1.16 to 1.14 V.
[a] Diprotonated form of the compound. [b] See Equation (3). [c] A peak at 643 nm (4.09) can also be seen.
porphyrins, but this is not reflected in the magnitude of the protonation constants. As seen in Figure 3 and Table 3, eight of the ten porphyrins have almost identical logb2 values, the two exceptions being 6 and 10. The largest logb2 value is for compound 7 which also has the largest DCb value. This is consistent with the fact that a large out-of plane distortion of the macrocycle will orient the nitrogen atoms to lie out of the plane of the porphyrin ring, thus providing an easy access for the addition of protons. The much smaller protonation constant of 6 (logb2 = 3.10) as compared to all of the other investigated porphyrins may be attributed to the combined electronic effects of planarity and the four electron-withdrawing CN groups. In the case of 10, which is nonplanar, the small logb2 of 6.89 may be due to a steric effect of the b-bromo and o-dibromo groups as well as to electronic effects of the Br groups. Chem. Eur. J. 2014, 20, 524 – 532
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After addition of 1.5 equivalents TFA to solution, both oneelectron reductions of the initial H2TPP(Ph)4 compound have totally disappeared while the single process for reduction of [H4TPP(Ph)4]2 + at E1/2 = 0.30 V is well-defined and reversible. Finally, when the TFA concentration is further increased above 3.0 equivalents, the reduction of [H4TPP(Ph)4]2 + becomes irreversible, consistent with a rapid chemical reaction following
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Full Paper electron transfer (an EC mechanism). A proposed mechanism is given later in the manuscript. Spectroscopic evidence for the formation of [H3(P)] + has been given in the literature[8, 19, 20, 50–58] and we would like to suggest that the peak at Ep = 0.66 V (labeled by an asterisk in Figure 5) can be assigned to reduction of the singly protonated porphyrin [H3TPP(Ph)4] + . The peak at Ep = 1.18 V (also labeled by an asterisk in Figure 5) is then assigned to reduction of the unprotonated porphyrin that is generated in a chemical reaction following the first electron transfer. With the exception of 6, similar electrochemistry is seen for all of the investigated porphyrins in acid media and selected cyclic voltammograms of the in situ generated [H4(P)]2 + species in CH2Cl2 containing 0.1 m TBAP and 2.0 equivalents TFA are shown in Figure 6. Each doubly protonated porphyrin is significantly easier to reduce than the initial H2(P) derivative with the same macrocycle, consistent with the two additional
A 680 to 800 mV separation in potentials is seen between E1/2 values for the first reversible two-electron reduction of the protonated nonplanar porphyrins 7–10 in solutions containing 2.0 equivalents TFA and the Epc for the corresponding irreversible reduction of the unprotonated nonplanar porphyrin in the absence of acid. This separation is also given in Table 1. Spectroelectrochemistry: Spectroelectrochemistry was used to spectrally characterize the reduction products of H2(P) and [H4(P)]2 + in CH2Cl2. Three different types of electron-transfer mechanisms were observed, depending upon the porphyrin planarity and the presence or absence of added acid in the solution. The first mechanism, labeled as Scheme 2a, occurred only for the planar porphyrins in CH2Cl2 without added acid and involved formation of a stable p-anion radical after a rever-
Scheme 2. Proposed reduction mechanisms of the neutral and diprotonated porphyrins in CH2Cl2.
sible one-electron addition. Examples of the spectral changes during this process are shown in Figure 7a for reduction of H2TPP(CH3)4 2 at 1.40 V in the thin-layer cell. The second type of mechanism is labeled as Scheme 2b and occurred only for the nonplanar porphyrins in CH2Cl2, 0.1 m TBAP. An example of these spectral changes is shown in Figure 7a for H2TPPBr8 9 and gives a final product characteristic of a deprotonated porphyrin in its unreduced form, either [H(P)] or [(P)]2.
Figure 6. Cyclic voltammograms illustrating reduction of investigated a) planar porphyrins 1, 3, and 5 and b) nonplanar porphyrins 7–9 in CH2Cl2 containing 2.0 equivalents TFA, 0.1 m TBAP.
The third type of reaction mechanism occurs for all of the protonated free-base porphyrins and involves a two-electron transfer followed by a chemical reaction leading to H2(P) as a final product. Two examples of these spectral changes are shown in Figure 7b, one for an initially planar porphyrin and the other for a porphyrin that is nonplanar in its neutral form. As seen in the figure, the product of electroreduced H2TPP(CH3)4 (2) has a Soret band at 420 nm and that for electroreduced H2TPPBr8 (9) has a band at 468 nm. Both absorption bands of the electroreduction products are exactly the same as for the neutral compounds in CH2Cl2. Thus, the combined electrochemical and spectral data indicate an initial reversible formation of doubly reduced [H4(P)]0 from [H4(P)]2 + in solutions containing 2.0 equivalents TFA but the final UV/Vis spectrum in each case indicates the ultimate loss of two protons on the spectroelectrochemistry timescale with regeneration of H2(P) at the electrode surface. This overall process is labeled as Scheme 2c.
positive charges on [H4(P)]2 + . The peak currents for reduction of [H4(P)]2 + in CH2Cl2 containing 2.0 equivalents TFA are also approximately double those for reduction of H2(P) in CH2Cl2, 0.1 m TBAP. This is shown in Figure S1 (Supporting Information) and is consistent with an overlapping of two one-electron transfer processes and a direct conversion of [H4(P)]2 + to a transient H4(P) species as shown in Equation (5). ½H4 ðPÞ2þ þ 2e ! H4 ðPÞ
ð5Þ
The difference in potential between the reversible two-electron reduction of the doubly protonated compounds 1–5 in solutions containing 2.0 equivalents TFA and the reversible one-electron reduction of the related planar H2(P) derivative in CH2Cl2 without added acid is given in the last column of Table 1 and ranges from 0.86 V for compound 3 to 0.76 V for compound 5. The first reduction of the diprotonated compound 6 is irreversible in CH2Cl2 containing 2.0 equivalents TFA and the potential separation between Ep of [H4(P)]2 + and E1/2 of H2(P) amounts to just 0.04 V as seen in Figure S2 (Supporting Information). Chem. Eur. J. 2014, 20, 524 – 532
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Electrochemistry and spectroelectrochemistry in CH2Cl2 containing TBAOH: To further identify the deprotonated products of electroreduction in mechanism Scheme 2b, electrochemistry 529
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Full Paper The above described sequence of coupled electrochemical and chemical steps is simplified in CH2Cl2 at low temperature where no reoxidation peaks can be observed (Figure 8b). It is also simplified in CH2Cl2 containing added TBAOH in which the cyclic votammogram exhibits only one major reduction and one major reoxidation, which are labeled as Processes III and IIc, respectively, in Figure 8c. The UV/Vis spectra of the deprotonated product generated by addition of TBAOH to solutions of compounds 8 and 9 are shown in Figure 9 and are characterized by two major absorption bands, one at 491–502 nm Figure 7. UV/Vis spectra of the planar H2TPP(CH3)4 (2) and nonplanar H2TPPBr8 (9) before (c) and after (a) and the other at 715–733 nm. controlled potential reduction in a) CH2Cl2 and b) CH2Cl2 containing 0.1 m TBAP and 2.0 equivalents added TFA. This type of spectrum was previously characterized in the litand spectroelectrochemistry experiments were carried out on erature[8, 30] as a deprotonated porphyrin dianion, [(P)]2, which the nonplanar compounds 7–10 in CH2Cl2 containing added suggests the loss of two protons as shown in Equation (7). TBAOH. The addition of TBAOH to H2(P) should lead stepwise More importantly as seen in Figure 9, the UV/Vis spectrum of 2 to [H(P)] and [(P)] as shown in Equations (6a) and (6b), but chemical generated [(P)]2 is virtually identical to the UV/Vis under our experimental conditions, only the doubly deprotonspectrum of the product formed after the two-electron reducated porphyrins were formed upon electroreduction of H2(P) tion of 8 and 9 in the thin-layer cell. This result suggests that or after addition of TBAOH to the neutral porphyrin in CH2Cl2 the same deprotonated porphyrin product is formed from as shown in Equation (7). H2 ðPÞ þ OH Ð ½HðPÞ þ H2 O
ð6aÞ
H2 ðPÞ þ OH Ð ½ðPÞ2 þ H2 O
ð6bÞ
H2 ðPÞ þ 2 OH Ð ½ðPÞ2 þ 2 H2 O
ð7Þ
Evidence for the formation of [(P)]2 is given by the cyclic voltammograms in Figure 8 and by the UV/Vis spectrum of the product formed after controlled potential reduction in a thinlayer cell (Figure 9). The first and second reductions of H2TPPCl8 (8) (Processes I and II in Figure 8) are overlapped and followed by a rapid chemical reaction leading to a new electroactive species that is reduced via Process III. Processes I and II occur at potentials at 0.68 and 0.78 V for compound 8, while Process III is located at Ep = 1.08 V for a scan rate of 0.1 V s1. Two reoxidation peaks are then seen on the reverse potential sweep, the first is labeled as Process II and the second as Process I’ in Figure 8a. The identity of the species involved in the reoxidation Process I’ was not characterized but is proposed to be involved in a form of the free-base p-anion radical with planarity different than that of the transient electrogenerated monoanion. Similar processes are observed for compounds 7–10 as seen in Figure 1b and Table S1 (Supporting Information), which lists the potentials for each redox process of these compounds. Chem. Eur. J. 2014, 20, 524 – 532
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Figure 8. Cyclic voltammograms illustrating reductions of H2TPPCl8 (8) in CH2Cl2, 0.1 m TBAP a) at room temperature, b) at 60 8C, and c) with 2.0 equivalents TBAOH at room temperature. The peak I’ marked by an asterisk is associated with an unidentified product generated following the first electron transfer. The process as II is labeled as IIa (anodic) and IIc (cathodic).
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Full Paper corded on a Hewlett–Packard model 8453 diode array spectrophotometer. 1H NMR spectra of porphyrins was recorded on a Jeol 400 MHz (or 500 MHz) spectrometer by using CDCl3 with TMS as the internal reference at 298 K. Mass spectral measurements of the samples were carried out using electrospray ionization (ESI) mass spectrometer model Micromass Q-TOF Micro using 10 % formic acid in CHCl3 as the solvent medium. Chemicals: Absolute dichloromethane (CH2Cl2) was purchased from Aldrich and used as received without further purification. TBAP was procured from Fluka and used as received. TFA and 5,10,15,20-tetraphenylporphyrin, H2TPP (1) purchased from Aldrich was used without further purification. The other free-base porphyrins, 2–10, were synthesized according to literature procedures[44, 59–64] and characterized by UV/Vis, 1H NMR spectroscopy, and mass spectroscopic methods. Calculation of protonation constants: Changes in the UV/Vis spectra of 1–10 in CH2Cl2 were monitored during a titration with TFA and the resulting spectral data then 2 Figure 9. UV/Vis spectra of H2(P) (c) and [(P)] (a) generated a) by addition of used to calculate formation constants for proton addiTBAOH and b) electroreduction of H2TPPCl8 and H2TPPBr8 in CH2Cl2. tion by using the Hill equation[65] (Equation 4) in which Ai is the absorbance in solutions with a specific concentration of added protons [H + ], A0 is the initial absorbH2(P) by an electrochemically initiated process or by addition + ance in which [H ] = 0.0 and Af is the final absorbance of the fully of base to the same porphyrin in CH2Cl2. Under these condiprotonated porphyrin. The slope of the log [(AiA0)/(A0Af)] versus tions the overall reduction and reoxidation mechanism is given log[H+] plot gives n, the number of protons added to the core niby Scheme 3. trogen atoms, and the value of logb2 is evaluated from the intercept of the line at log[(AiA0)/(A0Af)] = 0.0. In the present study, the logb2 values were evaluated by using a minimum of two waveConclusion lengths and an average value of n and logb2 is reported.
In summary, electroreduction of the diprotonated planar and
Acknowledgements Support of the Robert A. Welch Foundation (K.M.K., Grant E680) and Natural Science Foundation of China (Grant 21071067) is gratefully acknowledged. Scheme 3. Proposed redox mechanism of the nonplanar porphyrins in CH2Cl2.
Keywords: cyclic voltammetry · deprotonation · porphyrins · protonation · spectroelectrochemistry
nonplanar porphyrins [H4(P)]2 + in acid media leads to H2(P) while the nonplanar H2(P) derivatives are reduced to [(P)]2 in CH2Cl2 containing 0.1 m TBAP. Thus, in both cases, an electrochemically initiated deprotonation is observed.
[1] K. M. Kadish, K. M. Smith, R. Guilard, The Porphyrin Handbook, Academic Press, New York, 2000. [2] K. M. Kadish, K. M. Smith, R. Guilard, Handbook of Porphyrin Science World Scientific, Singapore, 2010. [3] K. M. Kadish, K. M. Smith, R. Guilard in The Porphyrin Handbook, Vol. 8 (Eds.: K. M. Kadish, E. Van Caemelbecke, G. Royal), Academic Press, New York, 2000, pp. 1 – 114. [4] M. A. Castriciano, M. Samperi, S. Camiolo, A. Romeo, S. L. Monsu, Chem. Eur. J. 2013, 19, 12160 – 12168. [5] P. K. Goldberg, T. J. Pundsack, K. E. Splan, J. Phys. Chem. A 2011, 115, 10452 – 10460. [6] I. H. Wasbotten, J. Conradie, A. Ghosh, J. Phys. Chem. B 2003, 107, 3613 – 3623. [7] J. R. Weinkauf, S. W. Cooper, A. Schweiger, C. C. Wamser, J. Phys. Chem. A 2003, 107, 3486 – 3496. [8] D. G. Luca, A. Romeo, L. M. Scolaro, G. Ricciardi, A. Rosa, Inorg. Chem. 2007, 46, 5979 – 5988. [9] T. Kojima, K. Hanabusa, K. Ohkubo, M. Shiro, S. Fukuzumi, Chem. Commun. 2008, 6513 – 6515. [10] K. M. Barkigia, L. Chantranupong, K. M. Smith, J. Fajer, J. Am. Chem. Soc. 1988, 110, 7566 – 7567.
Experimental Section Instrumentation: Cyclic voltammetry was carried out by using an EG&G Princeton Applied Research 173 potentiostat. A homemade three-electrode cell was used for cyclic voltammetry measurements and consisted of a glassy carbon working electrode, a platinum wire counter-electrode, and a homemade saturated calomel reference electrode (SCE). The SCE electrode was separated from the bulk solution by a fritted-glass bridge of low porosity, which contained the solvent/supporting electrolyte mixture. UV/Vis spectroelectrochemical experiments were performed with a home-built thin-layer cell that had a light transparent platinum net working electrode. Potentials were applied and monitored with an EG&G PAR model 173 potentiostat. Time-resolved UV/Vis spectra were reChem. Eur. J. 2014, 20, 524 – 532
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Full Paper [38] H. Guo, J. Jiang, Y. Shi, Y. Wang, Y. Wang, S. Dong, J. Phys. Chem. B 2006, 110, 587 – 594. [39] E. C. A. Ojadi, H. Linschitz, M. Gouterman, R. I. Walter, J. S. Lindsey, R. W. Wagner, P. R. Droupadi, W. Wang, J. Phys. Chem. 1993, 97, 13192 – 13197. [40] M. Meot-Ner, A. D. Adler, J. Am. Chem. Soc. 1975, 97, 5107 – 5111. [41] M. J. Hamor, T. A. Hamor, J. L. Hoard, J. Am. Chem. Soc. 1964, 86, 1938 – 1942. [42] P. Bhyrappa, K. Karunanithi, B. Varghese, Acta Crystallogr. Sect. E 2007, 63, o4755/4751 – o4755/4758. [43] K. S. Chan, X. Zhou, B. S. Luo, T. C. W. Mak, J. Chem. Soc. Chem. Commun. 1994, 271 – 272. [44] P. Bhyrappa, V. Velkannan, Inorg. Chim. Acta 2012, 387, 64 – 73. [45] J.-Z. Zou, Z. Xu, M. Li, X.-Z. You, H.-Q. Wang, Acta Crystallogr. Sect. C 1995, 51, 760 – 761. [46] P. Bhyrappa, C. Arunkumar, B. Varghese, Inorg. Chem. 2009, 48, 3954 – 3965. [47] M. Gruden, S. Grubisic, A. G. Coutsolelos, S. R. Niketic, J. Mol. Struct. 2001, 595, 209 – 224. [48] P. Bhyrappa, M. Nethaji, V. Krishnan, Chem. Lett. 1993, 869 – 872. [49] P. Bhyrappa, C. Arunkumar, B. Varghese, J. Porphyrins Phthalocyanines 2007, 11, 795 – 804. [50] T. Honda, T. Kojima, S. Fukuzumi, Chem. Commun. 2009, 4994 – 4996. [51] R. Harada, T. Kojima, Chem. Commun. 2005, 716 – 718. [52] T. Kojima, T. Nakanishi, R. Harada, K. Ohkubo, S. Yamauchi, S. Fukuzumi, Chem. Eur. J. 2007, 13, 8714 – 8725. [53] T. Nakanishi, T. Kojima, K. Ohkubo, T. Hasobe, K.-i. Nakayama, S. Fukuzumi, Chem. Mater. 2008, 20, 7492 – 7500. [54] T. Ishizuka, M. Sankar, Y. Yamada, S. Fukuzumi, T. Kojima, Chem. Commun. 2012, 48, 6481 – 6483. [55] S. Aronoff, J. Phys. Chem. 1958, 62, 428 – 431. [56] A. B. Rudine, B. D. DelFatti, C. C. Wamser, J. Org. Chem. 2013, 78, 6040 – 6049. [57] E. Austin, M. Gouterman, Bioinorg. Chem. 1978, 9, 281 – 298. [58] R. J. Abraham, G. E. Hawkes, K. M. Smith, Tetrahedron Lett. 1974, 15, 71 – 74. [59] P. Bhyrappa, M. Sankar, B. Varghese, Inorg. Chem. 2006, 45, 4136 – 4149. [60] P. K. Kumar, P. Bhyrappa, B. Varghese, Tetrahedron Lett. 2003, 44, 4849 – 4851. [61] H. J. Callot, Bull. Soc. Chim. Fr. 1974, 7 – 8, Pt. 2, 1492 – 1496. [62] T. Wijesekera, A. Matsumoto, D. Dolphin, D. Lexa, Angew. Chem. 1990, 102, 1073 – 1074; Angew. Chem. Int. Ed. 1990, 29, 1028 – 1030. [63] P. Bhyrappa, V. Krishnan, Inorg. Chem. 1991, 30, 239 – 245. [64] P. Bhyrappa, B. Purushothaman, J. J. Vittal, J. Porphyrins Phthalocyanines 2003, 7, 682 – 692. [65] P. Ellis, J. Linard, T. Szymanski, R. Jones, J. Budge, F. Basolo, J. Am. Chem. Soc. 1980, 102, 1889 – 1896.
[11] D. Mandon, P. Ochenbein, J. Fischer, R. Weiss, K. Jayaraj, R. N. Austin, A. Gold, P. S. White, O. Brigaud, P. Battioni, D. Mansuy, Inorg. Chem. 1992, 31, 2044 – 2049. [12] J. A. Shelnutt, X. Song, J. Ma, S. Jia, W. Jentzen, C. J. Medforth, Chem. Soc. Rev. 1998, 27, 31 – 42. [13] M. O. Senge in The Porphyrin Handbook, Vol. 1, (Eds.: K. M. Kadish, K. M. Smith, R. Guilard), Academic Press, New York, 2000, pp. 239 – 335. [14] M. W. Renner, L. R. Furenlid, K. M. Barkigia, A. Forman, H. K. Shim, D. J. Simpson, K. M. Smith, J. Fajer, J. Am. Chem. Soc. 1991, 113, 6891 – 6898. [15] K. M. Barkigia, M. W. Renner, L. R. Furenlid, C. J. Medforth, K. M. Smith, J. Fajer, J. Am. Chem. Soc. 1993, 115, 3627 – 3635. [16] L. D. Sparks, C. J. Medforth, M. S. Park, J. R. Chamberlain, M. R. Ondrias, M. O. Senge, K. M. Smith, J. A. Shelnutt, J. Am. Chem. Soc. 1993, 115, 581 – 592. [17] J. A. Shelnutt, C. J. Medforth, M. D. Berber, K. M. Barkigia, K. M. Smith, J. Am. Chem. Soc. 1991, 113, 4077 – 4087. [18] M. W. Renner, K. M. Barkigia, Y. Zhang, C. J. Medforth, K. M. Smith, J. Fajer, J. Am. Chem. Soc. 1994, 116, 8582 – 8592. [19] O. S. Finikova, A. V. Cheprakov, P. J. Carroll, S. Dalosto, S. A. Vinogradov, Inorg. Chem. 2002, 41, 6944 – 6946. [20] S. Thyagarajan, T. Leiding, S. P. Arskold, A. V. Cheprakov, S. A. Vinogradov, Inorg. Chem. 2010, 49, 9909 – 9920. [21] R. E. Haddad, S. Gazeau, J. Pecaut, J.-C. Marchon, C. J. Medforth, J. A. Shelnutt, J. Am. Chem. Soc. 2003, 125, 1253 – 1268. [22] F. D’Souza, A. Villard, C. E. Van, M. Franzen, T. Boschi, P. Tagliatesta, K. M. Kadish, Inorg. Chem. 1993, 32, 4042 – 4048. [23] K. M. Kadish, F. D’Souza, A. Villard, M. Autret, C. E. Van, P. Bianco, A. Antonini, P. Tagliatesta, Inorg. Chem. 1994, 33, 5169 – 5170. [24] F. D’Souza, M. E. Zandler, P. Tagliatesta, Z. Ou, J. Shao, C. E. Van, K. M. Kadish, Inorg. Chem. 1998, 37, 4567 – 4572. [25] M. Senge, T. Forsyth, L. Nguyen, K. Smith, Angew. Chem. 1994, 106, 2554 – 2557; Angew. Chem. Int. Ed. Engl. 1994, 33, 2485 – 2487. [26] R. Karaman, T. C. Bruice, Inorg. Chem. 1992, 31, 2455 – 2459. [27] Y. Girard, M. Kondo, K. Yoshizawa, Chem. Phys. 2006, 327, 77 – 84. [28] Y. Zhang, M. X. Li, M. Y. Lue, R. H. Yang, F. Liu, K. A. Li, J. Phys. Chem. A 2005, 109, 7442 – 7448. [29] O. Almarsson, A. Blasko, T. C. Bruice, Tetrahedron 1993, 49, 10239 – 10252. [30] G. Lu, X. Zhang, X. Cai, Y. Fang, M. Zhu, W. Zhu, Z. Ou, K. M. Kadish, J. Porphyrins Phthalocyanines 2013, 17, 10.1142/S1088424613500557. [31] S. Okada, H. Segawa, J. Am. Chem. Soc. 2003, 125, 2792 – 2796. [32] B. P. Neri, G. S. Wilson, Anal. Chem. 1972, 44, 1002 – 1009. [33] D. L. Langhus, G. S. Wilson, Anal. Chem. 1979, 51, 1139 – 1144. [34] B. P. Neri, G. S. Wilson, Anal. Chem. 1973, 45, 442 – 445. [35] M. Sankar, T. Ishizuka, Z. Wang, T. Ma, M. Shiro, T. Kojima, J. Porphyrins Phthalocyanines 2011, 15, 421 – 432. [36] T. Honda, T. Nakanishi, K. Ohkubo, T. Kojima, S. Fukuzumi, J. Phys. Chem. C 2010, 114, 14290 – 14299. [37] H. Guo, J. Jiang, Y. Shi, Y. Wang, J. Liu, S. Dong, J. Phys. Chem. B 2004, 108, 10185 – 10191.
Chem. Eur. J. 2014, 20, 524 – 532
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