Synthesis, characterization, protonation reactions, and electrochemistry of substituted open-chain pentapyrroles and sapphyrins in nonaqueous media

Article abstract of DOI:10.1021/ic4006955

Open-chain pentapyrroles were isolated as side-products from the synthesis of triaryl-corroles and then converted to the corresponding sapphyrins by catalytic oxidation in acidic media. The investigated compounds were characterized by UV-vis and ¹H NMR spectroscopy, mass spectrometry, electrochemistry, and spectroelectrochemistry and 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. Cyclic voltammetry and thin-layer UV-vis spectroelectrochemistry measurements were carried out in PhCN and CH₂Cl₂ containing 0.1 M tetra-n-butylammonium perchlorate. The open-chain pentapyrroles undergo two reversible one-electron reductions and two reversible one-electron oxidations to generate [(Ar)PPyH₃]^-, [(Ar)PPyH₃] ^2-, [(Ar)PPyH₃]⁺, and [(Ar)PPyH ₃]²⁺ which were spectroscopically characterized. The corresponding sapphyrins exhibit two or three reversible one-electron oxidations in PhCN, but the reductions of these compounds are irreversible because of coupled chemical reactions following electron transfer. Comparisons are made between redox potentials and spectral properties of the open-chain pentapyrroles, sapphyrins, and structurally related corroles. Protonation of the open-chain pentapyrroles and sapphyrins was also carried out in CH ₂Cl₂, and equilibrium constants were calculated by monitoring the spectral changes during titrations with trifluoroacetic acid. The pentapyrroles undergo a simultaneous two-proton addition to generate [(Ar) ₄PPyH₅]²⁺ while the sapphyrins undergo two stepwise single proton additions to give [(Ar)₄SH₄] ⁺ and [(Ar)₄SH₅]²⁺, respectively.

Full text of DOI:10.1021/ic4006955

Article
pubs.acs.org/IC
Synthesis, Characterization, Protonation Reactions, and
Electrochemistry of Substituted Open-Chain Pentapyrroles and
Sapphyrins in Nonaqueous Media
Mingzhu Yuan,† Zhongping Ou,*,† Yuanyuan Fang,‡ Shi Huang,† Zhaoli Xue,† Guifen Lu,†
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
S
* Supporting Information
ABSTRACT: Open-chain pentapyrroles were isolated as side-products
from the synthesis of triaryl-corroles and then converted to the
corresponding sapphyrins by catalytic oxidation in acidic media. The
1
investigated compounds were characterized by UV−vis and H NMR
spectroscopy, mass spectrometry, electrochemistry, and spectroelec-
trochemistry and 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. Cyclic
voltammetry and thin-layer UV−vis spectroelectrochemistry measure-
ments were carried out in PhCN and CH2Cl2 containing 0.1 M tetra-n-
butylammonium perchlorate. The open-chain pentapyrroles undergo two reversible one-electron reductions and two reversible
one-electron oxidations to generate [(Ar)PPyH3]−, [(Ar)PPyH3]2−, [(Ar)PPyH3]+, and [(Ar)PPyH3]2+ which were
spectroscopically characterized. The corresponding sapphyrins exhibit two or three reversible one-electron oxidations in
PhCN, but the reductions of these compounds are irreversible because of coupled chemical reactions following electron transfer.
Comparisons are made between redox potentials and spectral properties of the open-chain pentapyrroles, sapphyrins, and
structurally related corroles. Protonation of the open-chain pentapyrroles and sapphyrins was also carried out in CH2Cl2, and
equilibrium constants were calculated by monitoring the spectral changes during titrations with trifluoroacetic acid. The
pentapyrroles undergo a simultaneous two-proton addition to generate [(Ar)4PPyH5]2+ while the sapphyrins undergo two
stepwise single proton additions to give [(Ar)4SH4]+ and [(Ar)4SH5]2+, respectively.
Knowledge of the spectral and electrochemical properties of
sapphyrins is important with respect to expanding the potential
applications of these compounds in the field of biomedical and
functional materials.1−8 Although sapphyrins have been studied
in great detail, there have been few reports on the
electrochemical properties of N-5 meso-tetraaryl substituted
sapphyrins. There is also very little known about the
electrochemistry of open-chain pentapyrroles.
In the present paper, three substituted open-chain
pentapyrroles were isolated as side-products from the synthesis
of corroles after which the corresponding sapphyrins were
synthesized by catalytic oxidation under acid conditions. Each
pentapyrrole and sapphyrin was then characterized by UV−
visible and 1H NMR spectroscopy, mass spectrometry,
electrochemistry, and spectroelectrochemistry. 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. Protonation of the open-chain pentapyrroles and
INTRODUCTION
■
Sapphyrins have attracted a great deal of interest over the past
decade1−11 because of their similarity to porphyrins and their
potential applications in many fields, including their use as
functional materials, photosensitizers for photodynamic therapy
(PDT) or magnetic resonance imaging (MRI) contrasting
agents,1,7,12−16 and macrocyclic receptors for the transport of
neutral and anionic substrates.17−24
Sapphyrins are pentapyrrolic macrocycles containing one
direct link and four bridging methane groups between five
pyrrolic subunits.25 These compounds have been described as
β-pyrrole substituted macrocycles,26−29 as N-fused or N-
confused macrocycles30−33 or as core-modified macrocycles
with O, S, or Se heteroatoms.34−50 A few N-5 meso-tetraaryl-
substituted sapphyrins have also been reported51−54 after
isolation as a side-product from a Rothemund reaction in the
synthesis of porphyrins51,52 or from the one-step acid catalyzed
self-coupling reaction of dipyrromethane.53 An N-5 sapphyrin
can also be synthesized by transformation of an open-chain
pentapyrrole,54 which is a side-product in the synthesis of
corroles.53−56
Received: March 20, 2013
© XXXX American Chemical Society
A
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a
Scheme 1. Synthetic Route for Obtaining Open-Chain Pentapyrroles
a
Ar is one of the substituted phenyl groups.
alcohol, and dried under vacuum at 40 °C for at least one week prior
to use.
sapphyrins was monitored by UV−vis spectroscopy in CH2Cl2
during a titration with trifluoroacetic acid (TFA), and the
resulting spectroscopic data used to calculate equilibrium
constants for these reactions.
Synthesis of Open-Chain Pentapyrroles. The examined open-
chain pentapyrroles were obtained according to the synthetic route
shown in Scheme 1. Substituted aldehyde (10 mmol) and pyrrole (30
mmol 2.4 mL) were dissolved in a mixed solvent of CH3OH/H2O
(30/30 mL) and after addition of 2.5 mL of HCl (36%), the solution
was stirred at room temperature for 2.5 h. The reaction product was
extracted with CHCl3, the organic layer washed twice with H2O, dried
with Na2SO4, filtered, and then diluted to 150 mL with CHCl3. p-
Chloranil (2.46 mg, 10 mmol) was added to the solution which was
then refluxed for 4 h. The reaction mixture was passed over a silica
column using CH2Cl2 as eluent. The first collected red-color fraction
contains the corrole, and the following fractions contain the
pentapyrrole and other side-products. The latter fractions were
combined and evaporated via vacuum distillation, and the pure
product was obtained by chromatography of the concentrated mixture
on Al2O3 with CH2Cl2/hexane (5:1) as eluent.
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 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.
(FPh)4PPyH3 1. Yield 3%. UV−vis (PhCN): λmax, nm (ε × 10−4
M−1 cm−1) 382 (3.04), 446 (1.82), 501 (3.38), 825 (0.52), 903 (0.62).
1H NMR (400 MHz, CD3COCD3) δ, ppm: 12.84 (br, s, 3H), 7.55−
7.51 (m, 4H), 7.35−7.24 (m, 8H), 7.16 (t, J = 8.9 Hz, 4H), 6.71 (s,
2H), 6.62 (s, 2H), 6.58 (d, J = 4.6 Hz, 2H), 6.44 (d, J = 4.5 Hz, 2H),
6.06 (s, 4H). MS (MALDI-TOF): calcd. for C48H31F4N5, m/z
753.787, found [M+2H]2+ 755.710.
Determination of Protonation Equilibrium Constants. A
series of CH2Cl2 solutions containing trifluoroacetic acid (TFA) in
different concentrations was prepared and used as the acid-titration
reagent. Microliter quantities of TFA in CH2Cl2 were added gradually
to a 5.5 mL CH2Cl2 solution of the open-chain pentapyrroles and
sapphyrins in a homemade 1.0 cm cell, and the spectral changes
monitored after each addition.
(ClPh)4PPyH3 2. Yield 5%. UV−vis (PhCN): λmax, nm (ε × 10−4
M−1 cm−1) 384 (3.36), 450 (1.79), 505 (3.70), 830 (0.59), 907(0.73).
1H NMR (400 MHz, CD3COCD3): δ, ppm: 12.77 (br, s, 3H), 7.56−
7.51 (m, 8H), 7.45−7.41 (m, 4H), 7.33−7.28 (m, 4H), 6.79−6.77 (m,
2H), 6.63−6.57 (m, 4H), 6.49−6.46 (m, 2H), 6.11−6.04 (m, 4H). MS
(MALDI-TOF): calcd. for C48H31Cl4N5, m/z 819.605, found [M
+2H]2+ 821.059.
The prevailing protonation reaction is given by eq 1 for the open-
chain pentapyrroles and eqs 2−4 for the sapphyrins.
(Cl2Ph)4PPyH3 3. Yield. 5%. UV−vis (PhCN): λmax, nm (ε × 10−4
M−1 cm−1) 369 (3.45), 441 (1.45), 501 (4.00), 802 (0.62), 881(0.65).
1H NMR (400 MHz, CD3COCD3) δ, ppm: 12.93 (br, 3H, NH),
7.72−7.64 (m, 2H), 7.55−7.45 (m, 6H), 7.42−7.40 (m, 2H), 7.18−
7.08 (m, 2H), 6.83−6.71 (m, 2H), 6.45−6.36 (m, 4H), 6.31−6.17 (m,
2H), 6.13−6.10 (m, 2H), 5.95 (br, 2H). MS (MALDI-TOF): calcd.
for C48H27Cl8N5, m/z 957.385, found [M+2H]2+ 959.722.
Synthesis of Sapphyrins 4, 5, and 6. Synthesis of the sapphyrins
4−6 was carried out as shown in Scheme 2. The pentapyrrole (20 mg)
was dissolved in 20 mL of toluene and 0.2 mL of TFA and 9 mg of I2
was added. The mixture was stirred for 7 h at 38 °C. The pure
sapphyrin was obtained by chromatography, first on silica gel with
CH3OH/CH2Cl2 (98.5/1.5%) as eluent and then on an Al2O3 column
with CH2Cl2 as eluent.
β2
(Ar)4PPyH + 2H+ ⇌ [(Ar)4PPyH ]2+
(1)
(2)
(3)
(4)
3
5
K1=β1
(Ar)4SH3 + H+ HoooooI [(Ar)4SH4]+
K2
[(Ar)4SH4]+ + H+ ⇌ [(Ar)4SH5]2+
K1K2=β2
(Ar)4SH3 + 2H+ HooooooooI [(Ar)4SH5]2+
Changes in UV−visible spectra during the titration with TFA were
analyzed as a function of the concentration of added acid, using both
the mole ratio method and the Hill equation57,58 to calculate
equilibrium constants for proton addition in the nonaqueous solvent,
CH2Cl2.
Chemicals. Absolute dichloromethane (CH2Cl2) was purchased
from Aldrich Co. and used as received for electrochemistry and
spectroelectrochemistry. Benzonitrile (PhCN) was purchased from
Aldrich Co. and distilled over phosphorus (P2O5) under vacuum prior
to use. Tetra-n-butylammonium perchlorate (TBAP) was purchased
from Sigma Chemical or Fluka Chemika Co., recrystallized from ethyl
5,10,15,20-Tetrafluorophenylsapphyrin, (FPh)4SH3 4. Yield 19%.
UV−vis (PhCN): λmax, nm (ε × 10−4 M−1 cm−1) 309 (1.03), 493
1
(6.57), 520 (4.29), 642 (0.41), 701 (0.76), 721(0.77), 797 (0.48). H
NMR (500 MHz, CD2Cl2) δ, ppm: 11.83 (br, 1H, NH), 10.10 (d, J =
4.6 Hz, 2H), 9.12 (d, J = 4.5 Hz, 2H), 9.05 (d, J = 4.4 Hz, 2H), 8.94 (t,
J = 8.8 Hz, 2H), 8.35−8.27 (m, 6H), 7.56−7.53 (m, 4H), 7.46 (d, J =
8.3 Hz, 2H), 7.37 (m, 2H), 7.15−7.12 (m, 2H), −1.29 (m, 2H). MS
B
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a
Scheme 2. Synthesis Route for Obtaining Sapphyrins
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The open-chain penta-
pyrroles were isolated as side-products in the synthesis of
structurally related triarylcorroles (Scheme 1) and then
converted to sapphyrins via cyclization in acidic media (Scheme
2) which gave the final product in yields ranging from 19 to
53%. Both series of compounds were characterized by UV−vis
1
and H NMR spectroscopy, mass spectrometry, and electro-
chemistry.
1H NMR spectra of (FPh)4PPyH3 1, (FPh)4SH3 4, and
[(FPh)4SH5]2+ are shown in Figure 1. The spectrum of
(FPh)4PPyH3 is consistent with an open cycle molecular
framework. The three NH protons appear as a broad peak
located at 12.85 ppm while the phenyl and β-CH protons are
located between 7.60 and 6.00 ppm. There are no resonances
of the open-chain pentapyrrole 1 in the high field region of the
spectrum (Figure 1a), but this is not the case for the related
sapphyrin 4 (Figure 1b). Upon conversion of 1 to 4, one of the
three NH resonances is shifted upfield and appears as a broad
and weak absorption located at 11.83 ppm. The other two NH
resonances for 4 are too weak to be observed because of a rapid
tautomeric equilibrium of the inner core NH.59 The phenyl and
β-CH protons both shift downfield upon conversion of 1 to 4.
The β-CH protons appear as four doublet signals at 10.10, 9.12,
9.05, and 8.94 ppm and a singlet signal at −1.29 ppm. These
results indicate that two adjacent pyrrole rings of 4 are linked
after oxidation, with one of the pyrrole rings being inverted to
the outside of the molecule as shown by the structure in Figure
1b.
a
Ar is one of the substituted phenyl groups shown in Scheme 1.
(MALDI-TOF): calcd. for C48H29F4N5, m/z 751.771, found [M
+2H]2+ 753.701.
5,10,15,20-Tetrachlorophenylsapphyrin, (ClPh)4SH3 5. Yield 25%.
UV−vis (PhCN): λmax, nm (ε × 10−4 M−1 cm−1) 317(1.60),
498(9.37), 524 (6.33), 650 (0.59), 706 (1.19), 722 (1.22), 800 (0.68).
1H NMR (500 MHz, CD2Cl2) δ, 11.84 (s, 1H), 10.12 (d, J = 4.9
Hz, 2H), 9.12 (d, J = 4.9 Hz, 2H), 9.05 (d, J = 4.9 Hz, 2H), 8.94 (d, J
= 4.9 Hz, 2H), 8.31−8.24 (m, 4H), 7.85−7.82 (m, 6H), 7.46−7.37 (m,
4H), 7.15−7.12 (m, 2H), −1.26 (s, 2H). MS (MALDI-TOF): calcd.
for C48H29Cl4N5, m/z 817.589, found [M+H]+ 818.859.
5,10,15,20-Tetra-2,4-dichlorophenylsapphyrin, (Cl2Ph)4SH3 6.
Yield 53%. UV−vis (PhCN): λmax, nm (ε × 10−4 M−1 cm−1) 316
(1.59), 494 (15.59), 517 (10.08), 637 (0.77), 695 (1.36), 711 (1.26),
1
787 (0.48). H NMR (500 MHz, CD2Cl2) δ 11.45−11.38 (m, 1H,
NH), 10.15−10.08 (m, 2H), 9.27−9.24 (m, 1H), 9.02−8.97 (m, 2H),
8.86−8.71 (m, 3H), 8.30−8.07 (m, 3H), 7.99−7.90 (m, 2H), 7.78−
7.71 (m, 2H), 7.63−7.61 (m, 0.5H), 7.48−7.37 (m, 2H), 7.15−6.95
(m, 2H), 6.88−6.86 (m, 0.5H), −1.30 to −1.49 (m, 2H). MS
(MALDI-TOF): calcd. for C48H25Cl8N5, m/z 955.370, found [M
+2H]2+ 957.011.
The NMR spectrum of protonated (FPh)4SH3 was generated
by addition of trifluoroacetic acid to a CD2Cl2 solution (1:20
molar ratio) to give [(FPh)4SH5]2+. The spectrum after
Figure 1. 1H NMR spectra of (a) (FPh)4PPyH3 1 in CD3COCD3, (b) (FPh)4SH3 4, and (c) [(FPh)4SH5]2+ in CD2Cl2 at 298 K. *TFA and impurity
peaks, Ar = FPh.
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protonation is shown in Figure 1c and indicates that the
molecule maintains its inverted structure. For instance, the
outer NH proton together with the two inner β-CH protons
are both shifted downfield and located at 14.27 ppm for the
NH proton and −0.55 ppm for the inner β-CH protons. The
other NH protons are located at −2.52 to −2.75 ppm (see
Figure 1c). The same phenomenon has been reported by Latos-
UV−vis spectra of the related sapphyrins are shown in Figure
2b and differ significantly from the corresponding open-chain
pentapyrroles under the same solution conditions. For example,
sapphyrin 4 is characterized by a split Soren band at 493 and
520 nm and four Q bands at 642, 701, 721, and 797 nm in
PhCN as compared to the pentapyrrole 1 which exhibits major
bands at 382 and 501 nm and two broad overlapping bands at
825 and 903 nm. The spectrum of sapphyrins 5 and 6 are quite
similar to that of 4 (see Figure 2b) and only a several nm
difference can be observed in the Soret and Q-band positions of
the three sapphyrins, indicating a weak effect of substituents on
the UV−visible spectra under the given solution conditions.
The UV−visible spectra of each sapphyrin are similar in
CH2Cl2 (Supporting Information, Figure S1b) as in PhCN
(Figure 2b), consistent with identical structures existing in
these two solvents.
Grazyn
́
̇
UV−Visible Spectra. UV−visible spectra of the open-chain
pentapyrroles 1−3 and sapphyrins 4−6 were measured in
CH2Cl2 and PhCN containing 0.1 M TBAP. The absorption
spectra in PhCN containing 0.1 M TBAP are illustrated in
Figure 2, and the λmax values of each compound are summarized
The UV−visible spectra of the sapphyrins indicate
compounds with a polypyrrolic aromatic macrocycle.50 The
Soret and Q bands of the sapphyrins are red-shifted from bands
of the corroles and porphyrins containing identical substituents.
For example, (ClPh)4SH3 4 in CH2Cl2 (Supporting Informa-
tion, Figure S1b) exhibits a split Soret band at 493, 518 nm and
four Q bands at 649, 704, 724, and 800 nm as compared to the
spectrum of meso-chlorophenyl substituted free-base corrole
(ClPh)4CorH3 which has an unsplit Soret band at 417 nm and
three Q bands at 578, 616, and 649 nm in the same solvent.56
It is well-known that the bi-, ter-, and quaterpyrroles can be
protonated to give the corresponding cations and dications
upon addition of acid to solutions of the compound,60−62 and
the currently investigated open-chain pentapyrroles can also be
protonated as shown by eq 1. An example of the spectral
changes which occur during titration of (Cl2Ph)4PPyH3 3 with
trifluoroacetic acid in CH2Cl2 as solvent is shown in Figure 3a.
The final spectrum of the protonated pentapyrrole is
characterized by bands at 403, 514, and 876 nm (Figure 3a).
The slope (2.0) from the Hill plot, shown as an inset in Figure
3a, indicates a single step, bis-protonation reaction to generate
the dication, [(Cl2Ph)4PPyH5]2+. The protonation constant was
calculated as log β2 = 8.1 for compound 3 in CH2Cl2, and
similar values of log β2 were calculated for the bis-protonated
compounds 1 and 2 under the same solution conditions (Table
3). UV−vis spectra of the three bis-protonated open-chain
pentapyrroles and the calculated log β2 values are summarized
in Table 3.
Figure 2. UV−visible spectra of neutral (a) pentapyrroles and (b)
sapphyrins in PhCN containing 0.1 M TBAP.
Sapphyrins can also be protonated in acidic solutions, leading
to mono and bis-protonated species with different UV−visible
spectra from that of the neutral compound.51,59 An example of
the spectral changes which occur for compound 4 during a
titration with TFA in CH2Cl2 is shown in Figure 3b. As seen
from this figure, two sets of well-defined spectral changes are
observed during the titration, indicating two stepwise
in Tables 1 and 2. As seen in Figure 2a, the pentapyrrole 1
exhibits an intense split absorption band around 382 and 501
nm together with two moderate intensity absorption bands at
825 and 903 nm. Similar UV−visible spectra for the other two
pentapyrroles are observed in PhCN and CH2Cl2 (see
Supporting Information, Figure S1a and Table 1).
Table 1. UV-Visible Spectral Data of Open-Chain Pentapyrroles 1−3 in PhCN and CH2Cl2 Containing 0.1 M TBAP
solvent
PhCN
compound
λmax, nm (ε × 10−4 M−1 cm−1
)
(FPh)4PPyH3 1
(ClPh)4PPyH3 2
(Cl2Ph)4PPyH3 3
382 (3.04)
384 (3.36)
369 (3.45)
501 (3.38)
505 (3.70)
501 (4.00)
825 (0.52)
830 (0.59)
802 (0.62)
903 (0.62)
907 (0.73)
881 (0.65)
CH2Cl2
(FPh)4PPyH3 1
(ClPh)4PPyH3 2
(Cl2Ph)4PPyH3 3
371 (3.28)
376 (3.75)
369 (3.90)
498 (3.61)
502 (4.15)
499 (4.47)
810 (0.57)
815 (0.68)
794 (0.61)
896 (0.71)
896 (0.81)
866 (0.63)
D
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Table 2. UV-Visible Spectral Data of Sapphyrins 4−6 in PhCN and CH2Cl2 Containing 0.1 M TBAP
solvent
PhCN
compound
λmax, nm (ε × 10−4 M−1 cm−1
)
(FPh)4SH3 4
(ClPh)4SH3 5
(Cl2Ph)4SH3 6
493 (6.57)
498 (9.37)
494 (15.59)
520 (4.29)
524 (6.33)
517 (10.08)
642 (0.41)
650 (0.59)
637 (0.77)
701 (0.76)
721 (0.77)
722 (1.22)
711 (1.26)
797 (0.48)
800 (0.68)
787 (0.48)
706 (1.19)
695 (1.36)
CH2Cl2
(FPh)4SH3 4
(ClPh)4SH3 5
(Cl2Ph)4SH3 6
489 (9.55)
493 ((9.28)
488 (16.12)
514 (5.71)
518 (5.86)
511 (9.70)
635 (0.48)
649 (0.47)
629 (0.84)
694 (0.94)
704 (0.97)
687 (1.28)
722 (0.78)
724 (0.83)
714 (1.02)
795 (0.51)
800 (0.50)
787 (0.45)
Electrochemistry of the Open-Chain Pentapyrroles.
Electrochemical properties of the open-chain pentapyrroles 1−
3 were evaluated in CH2Cl2 and PhCN containing 0.1 M TBAP
as supporting electrolyte. Examples of cyclic voltammograms
are shown in Figure 4 (CH2Cl2) and Supporting Information,
Figure S2 (PhCN) while the reduction/oxidation potentials are
summarized in Table 4.
Each open-chain pentapyrrole exhibits two reversible one-
electron reductions and two reversible one-electron oxidations.
The difference in half-wave potentials between the first and the
second reduction is given in Table 4 as ΔE1/2(1r‑2r) and ranges
from 0.23 to 0.32 V. The potential difference between the first
and second oxidation, ΔE(2o‑1o), ranges from 0.23 to 0.39 V and
is also given in Table 4. Both separations are similar to ΔE1/2
values often reported for ring-centered reductions and
oxidations of free-base porphyrins.63
As expected, the measured E1/2 values for oxidation and
reduction of 1−3 vary with the specific electron-withdrawing
substituent at the four phenyl rings of the compound, and a
linear relationship is observed between the E1/2 and the
Hammett constants of the substituents64 as shown in Figure 5.
The slope of the correlation in the plots of Figure 5 are defined
by the equation ΔE1/2 = ∑σρ, where ∑σ represents the sum of
the substituent constants and ρ is the reaction constant.64 The
values of ρ for the first reduction and first oxidation of the
open-chain pentapyrroles were calculated as 117 and 68 mV,
respectively and are similar to values of reaction constants
obtained in the case of free-base porphyrins.63 The absolute
potential differences between the first reduction and first
oxidation (the gap between the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO)) ranged from 1.31 to 1.39 V in CH2Cl2 (Table 4)
with an average value of 1.34 0.04 V as shown in the figure.
This HOMO−LUMO gap is much smaller than that of
porphyrins which generally averages 2.25 0.15 V for the ring-
centered reactions of free-base porphyrins.63
The UV−visible spectra of each pentapyrrole in CH2Cl2 and
PhCN were obtained after electroreduction or electrooxidation
in a thin-layer cell. An example of the time-resolved spectral
changes obtained during the first and second controlled-
potential reductions of (Cl2Ph)4PPyH3 3 in CH2Cl2 is shown in
Figure 6a. Two well-defined sets of spectral changes are seen,
corresponding to the stepwise formation of [(Cl2Ph)4PPyH3]−
and [(Cl2Ph)4PPyH3]2− under the application of an applied
reducing potential. Almost identical spectral changes were
observed for the other open-chain pentapyrroles upon
controlled potential reductions in the same solvent. The
electrode reactions are described by eqs 5 and 6 where the
stepwise reduction products are assigned as a π-anion radical
and dianion, respectively.
Figure 3. UV−visible spectral changes of (Cl2Ph)4PPyH3 3 and
(FPh)4SH3 4 during a titration with TFA in CH2Cl2 (insets are the
molar ratio plot and Hill plot for determination of the number of
protons and protonation equilibrium constants).
protonation reactions to give [(FPh)4SH4]+ and
[(FPh)4SH5]2+, respectively. The relevant reactions are given
by eqs 2 and 3, and the calculated equilibrium constants of log
K1 = 9.8 and log K2 = 2.4 are listed in Table 3 which also
includes the UV−visible spectral data for [(Ar)4SH4]+ and
[(Ar)4SH5]2+ in CH2Cl2.
(Ar)4PPyH + e ⇌ [(Ar)4PPyH ]−
(5)
3
3
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Table 3. UV-Visible Spectral Data (λmax, nm) of Protonated Open-Chain Pentapyrroles 1−3 and Sapphyrins 4−6 in CH2Cl2
with Added TFA and the Protonation Constants
a
cpd
final protonated form
λmax, nm
protonation constant
1
2
3
[(FPh)4PPyH5]2+
[(ClPh)4PPyH5]2+
[(Cl2Ph)4PPyH5]2+
400
404
403
524
526
514
915
914
876
log β2 = 11.0
log β2 = 9.1
log β2 = 8.1
4
5
6
4
5
6
[(FPh)4SH4]+
[(ClPh)4SH4]+
[(Cl2Ph)4SH4]+
[(FPh)4SH5]2+
[(ClPh)4SH5]2+
[(Cl2Ph)4SH5]2+
483
488
481
488
492
499
655
657
651
586
596
586
747
752
748
655
659
659
log K1 = 9.8
log K1 = 9.2
log K1 = 7.5
log K2 = 2.4
log K2 = 2.7
log K2 = 1.3
731
737
767
795
795
a
See eq 1 for compounds 1−3 and eqs 2−4 for compounds 4−6.
Figure 5. Plots of half-wave potentials for the first reduction and first
oxidation of compounds 1−3 in CH2Cl2 containing 0.1 M TBAP vs
the Hammett substituent constants (∑σ). Values of E1/2 and ∑σ are
given in Table 4.
Figure 4. Cyclic voltammograms of open-chain pentapyrroles 1−3 in
CH2Cl2 containing 0.1 M TBAP.
spectra also resemble spectra of the doubly protonated open-
chain pentapyrroles, [(FPh)4PPyH5]2+ and [(ClPh)4PPyH5]2+.
The fact that very similar UV−visible spectra are seen after
controlled potential oxidation of 1−3 in CH2Cl2 and after
addition of TFA to the neutral compounds in CH2Cl2 suggests
that a protonation is occurring in both cases. Oxidation-induced
protonation reactions have previously been reported for other
open chain oligopyrrole systems61,62 as well as for oxidized free-
base corroles65,66 and oxidized free-base porphyrins.67
Although the time-resolved spectra during the first oxidation
of (Cl2Ph)4PPyH3 3 at 0.70 V pass through two well-defined
isosbestic points (at 395 and 515 nm), isosbestic behavior is
not observed during the second oxidation at a controlled
potential of 1.10 V. These spectral changes are shown in the
[(Ar)4PPyH ]− + e ⇌ [(Ar)4PPyH ]2−
(6)
3
3
The thin-layer spectral changes obtained during controlled-
potential oxidation of (Cl2Ph)4PPyH3 3 in CH2Cl2 are
illustrated in Figure 6b. The first oxidation at an applied
potential of 0.70 V leads to a spectrum with bands at 406, 530,
and 889 nm. This spectrum is very similar to the UV−visible
spectrum of the bis-protonated form of compound 3,
[(Cl2Ph)4PPyH5]2+, which has bands at 403, 514, and 876
nm (Figure 3a). Similar UV−visible spectra are also seen after
the one-electron oxidation of compounds 1 and 2, and these
Table 4. Half-Wave Potentials (V vs SCE) of Substituted Pentapyrroles in CH2Cl2 and PhCN, 0.1 M TBAP
oxidation
reduction
second
a
c
d
e
solvent
CH2Cl2
compound
4σ
third
1.40
second
first
ΔE(2o-1o)
first
ΔE1/2(1r-2r)
HOMO−LUMO gap (V)
(FPh)4PPyH3 1
(ClPh)4PPyH3 2
(Cl2Ph)4PPyH3 3
(FPh)4PPyH3 1
(ClPh)4PPyH3 2
(Cl2Ph)4PPyH3 3
0.24
0.92
1.72
0.24
0.92
1.72
0.64
0.63
0.93
0.58
0.68
0.78
0.37
0.40
0.54
0.39
0.41
0.53
0.27
0.23
0.39
0.19
0.27
0.25
−0.95
−0.91
−0.85
−0.88
−0.83
−0.77
−1.19
−1.14
−1.17
−1.17
−1.13
−1.15
0.24
0.23
0.32
0.29
0.30
0.38
1.32
1.31
1.39
1.27
1.24
1.30
b
b
b
PhCN
b
0.86
1.02
b
a
b
c
Hammett substituent constant, see ref 64. Irreversible peak potential at a scan rate of 0.10 V/s. Potential difference between the second and first
d
e
oxidations. Potential difference between the first and second reductions. The potential difference between the first oxidation and first reduction.
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Figure 6. Thin-layer UV−visible spectral changes of (Cl2Ph)4PPyH3 3 during the controlled potential (a) reductions and (b) oxidations in CH2Cl2
containing 0.1 M TBAP.
lower part of Figure 6b and were not investigated further in the
present study.
and formation of the deprotonated species [(Ar)4SH2]− as a
final product of the initial one-electron reduction. The anionic
product of this electrochemical EC mechanism can then be
oxidized by one-electron on a return scan to give (Ar)4SH2 or it
can be stepwise reduced by two electrons to give [(Ar)4SH2]2−
and of [(Ar)4SH2]3− at more negative potentials. The
electrogenerated products, [(Ar)4SH2]2− and [(Ar)4SH2]3−,
are only moderately stable by cyclic voltammetry and show
several anodic oxidation peaks on the return scan. This is
illustrated in Figure 8 for the case of (ClPh)4SH3 5. This
proposed mechanism is shown in Scheme 3 and is similar to
what has been reported for the reduction of free-base corroles
under similar solution conditions.66
Electroreduction of Sapphyrins. The redox properties of
sapphyrins 4−6 were also examined in PhCN containing 0.1 M
TBAP. Cyclic voltammograms of these compounds are shown
in Figure 7, and the half-wave potentials are summarized in
Table 5.
Evidence for the formation of [(Ar)4SH2]− after the addition
of one electron to (Ar)4SH3 comes from thin-layer
spectroelectrochemistry of the type shown in Figure 9a for
(ClPh)4SH3 5. As seen in this figure, the Soret and Q bands of
5 decrease in intensity during controlled potential reduction at
−1.00 V as new bands grow in at 514, 732, and 805 nm. The
final spectrum of this reduction is almost exactly the same as
the spectrum of the unreduced compound obtained in PhCN
containing 2.0 equiv of added TBAOH (see Figure 10). This
result is consistent with the electrochemistry and the
conversion of (ClPh)4SH3 to [(ClPh)4SH2]− by addition of
base or by electroreduction. Similar UV−visible spectra are also
seen for the other two sapphyrins after reduction or after
addition of base to solutions of the neutral compound (see
Figure 10).
The spectral changes which occur during the second
controlled potential reduction of (ClPh)4SH3 5 at −1.50 V
actually correspond to a conversion of [(ClPh)4SH2]− to
[(ClPh)4SH2]2− (Process II in Scheme 3).
Figure 7. Cyclic voltammograms of sapphyrins 4−6 in PhCN
containing 0.1 M TBAP. Scan rate = 0.10 V/s.
Electrooxidation of Sapphyrins. Up to three reversible
one-electron oxidations are observed for each sapphyrin in
PhCN. The first oxidation ranges from E1/2 = 0.45 V for
compound 4 to 0.57 V for compound 6 while the second
oxidation ranges from E1/2 = 0.70 V for 4 to 0.96 V for 6 (see
Table 5). The difference in half-wave potentials between the
first and the second oxidations are listed in Table 5 and
amounts to 0.25 V for 4, 0.43 V for 5 and 0.39 V for 6,
While the open-chain pentapyrroles exhibit reversible
reductions in PhCN (Supporting Information, Figure S2), the
reductions of the sapphyrins are irreversible under the same
solution conditions (Figure 7). The first reduction occurs at Epc
= −0.87 to −1.08 V (depending upon the specific compound)
while the second and third reductions are located between
−1.43 and −1.68 V vs SCE. As will be demonstrated, the first
reduction of (Ar)4SH3 involves a following chemical reaction
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Table 5. Half-Wave Potentials (V vs SCE) of Substituted Sapphyrins in PhCN, 0.1 M TBAP
oxidation
reduction
second
a
b
d
compound
4σ
third
second
first
ΔE(2o-1o)
first
third
HOMO−LUMO gap (V)
c
c
c
c
c
c
(FPh)4SH3 4
(ClPh)4SH3 5
(Cl2Ph)4SH3 6
0.24
0.92
1.72
0.70
0.92
0.96
0.45
0.49
0.57
0.25
0.43
0.39
−1.08
−0.87
−1.02
−1.52
−1.43
−1.68
−1.60
1.35
1.36
1.59
c
1.36
1.27
c
−1.50
a
b
c
Hammett substituent constant, see ref 64. Potential difference between the second and first oxidations. Irreversible peak potential at a scan rate of
d
0.10 V/s. The potential difference between the first oxidation and first reduction.
Figure 9. Thin layer UV−visible spectral changes of (ClPh)4SH3 5
during controlled potential (a) reductions and (b) oxidations in PhCN
containing 0.1 M TBAP.
Figure 8. Cyclic voltammograms illustrating the reductions of
(ClPh)4SH3 5 in PhCN containing 0.1 M TBAP. Scan rate = 0.10
V/s.
Scheme 3. Reduction Mechanism of Sapphyrins in PhCN
Containing 0.1 M TBAP
related porphyrins with similar substituents. A true thermody-
namic HOMO−LUMO gap cannot be determined for the
sapphyrins because of the irreversible first reduction. However,
it is still important to point out that the measured separation
between the first reduction and the first oxidation of the
sapphyrins 4 and 5 (1.35 and 1.36 V, respectively) is almost
identical to the electrochemically measured HOMO−LUMO
gap of the open-chain pentapyrroles 1 and 2 (1.32 and 1.31 V).
All four values are substantially less than for free-base
porphyrins where the HOMO−LUMO gap averages about
2.25 V in nonaqueous media.63
In conclusion, the open-chain pentapyrroles undergo two
reversible one-electron reductions and two reversible one-
electron oxidations to generate [(Ar)PPyH3]−, [(Ar)PPyH3]2−,
[(Ar)PPyH3]+, and [(Ar)PPyH3]2+ in nonaqueous media. The
corresponding sapphyrins exhibit two or three reversible one-
electron oxidations in PhCN, but the reductions of these
compounds are irreversible because of coupled chemical
reactions following electron transfer. Very similar UV−visible
spectra are seen after controlled potential oxidation of the
open-chain pentapyrroles in CH2Cl2 and after addition of TFA
to the neutral compounds in the same solvent, suggesting that
protonation reactions are occurring in both cases. These kinds
of oxidation-induced protonation reactions have previously
been reported for other open chain oligopyrrole systems as well
as for oxidized free-base corroles and oxidized free-base
porphyrins.
respectively. Similar values of ΔE1/2 were measured for two of
the three related open-chain pentapyrroles, namely, 1 (ΔE1/2
=
0.27 V) and 3 (ΔE1/2 = 0.39 V) (Table 4). A bigger difference
occurs between compound 2 and 5 where ΔE1/2 = 0.23 and
0.42 V, respectively.
Thin-layer UV−visible spectral changes of each sapphyrin
were measured during the first and second controlled-potential
oxidations in PhCN containing 0.1 M TBAP. As seen in Figure
9b for compound 5, the Soret and Q bands both decrease in
intensity during the two stepwise oxidations, suggesting
formation of a π-cation radical and dication, respectively. The
spectral changes in each oxidation step are reversible, and
consistent with a stable oxidation product being formed under
the given experimental conditions.
It should be pointed out that the investigated sapphyrins are
both easier to reduce and easier to oxidize than structurally
H
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ASSOCIATED CONTENT
* Supporting Information
■
S
Neutral UV−visible spectra of open-chain pentapyrroles 1−3
and sapphyrins 4−6 in CH2Cl2 and cyclic voltammograms of
the compounds 1−3 in PhCN containing 0.1 M TBAP. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(28) Boul, P. J.; Cho, D.-G.; Rahman, G. M. A.; Marquez, M.; Ou, Z.;
Kadish, K. M.; Guldi, D. M.; Sessler, J. L. J. Am. Chem. Soc. 2007, 129,
5683−5687.
(29) Panda, P. K.; Kang, Y.-J.; Lee, C.-H. Angew. Chem., Int. Ed. 2005,
44, 4053−4055.
AUTHOR INFORMATION
Corresponding Author
*E-mail: zpou2003@yahoo.com (Z.O.), kkadish@uh.edu
(K.M.K.).
■
(30) Pushpan, S. K.; Srinivasan, A.; Anand, V. G.; Venkatraman, S.;
Chandrashekar, T. K.; Joshi, B. S.; Roy, R.; Furuta, H. J. Am. Chem. Soc.
2001, 123, 5138−5139.
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Lynch, V. M.; Panda, P. K.; Sessler, J. L.; Kim, D. Chem. Commun.
2011, 47, 6813−6815.
Notes
The authors declare no competing financial interest.
(32) Gupta, I.; Srinivasan, A.; Morimoto, T.; Toganoh, M.; Furuta,
H. Angew. Chem., Int. Ed. 2008, 47, 4563−4567.
ACKNOWLEDGMENTS
■
(33) Sessler, J. L.; Cho, D.-G.; Stepien, M.; Lynch, V.; Waluk, J.;
Yoon, Z. S.; Kim, D. J. Am. Chem. Soc. 2006, 128, 12640−12641.
(34) Rao, M. R.; Ravikanth, M. Eur. J. Org. Chem. 2011, 1335−1345.
(35) Pareek, Y.; Ravikanth, M. Eur. J. Org. Chem. 2011, 5390−5399.
(36) Shetti, V. S.; Ravikanth, M. Inorg. Chem. 2011, 50, 1713−1722.
(37) Srinivasan, A.; Pushpan, S. K.; Kumar, M. R.; Mahajan, S.;
Chandrashekar, T. K.; Roy, R.; Ramamurthy, P. J. Chem. Soc., Perkin
Trans. 2 1999, 961−968.
This work was supported by grants from the Natural Science
Foundation of China (Grants 21071067 and 21001054) and
the Robert A. Welch Foundation (K.M.K., Grant E-680).
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