Formation and Isolation of a Four-Electron-Reduced Porphyrin Derivative by Reduction of a Stable 20π Isophlorin

Article abstract of DOI:10.1002/anie.201711058

The two-electron reduction of a diprotonated dodecaphenylporphyrin derivative by Na₂S₂O₄ gave a corresponding isophlorin (Iph) selectively. Formation of Iph was confirmed by spectroscopic measurements and the isolation of tetramethylated Iph. Further reduction of Iph proceeded to form an unprecedented four-electron-reduced porphyrin (IphH₂), which was fully characterized by spectroscopic and X-ray crystallographic analysis. IphH₂, with a unique conformation, could be oxidized to reproduce the starting porphyrin, resulting in a proton-coupled four-electron reversible redox system.

Full text of DOI:10.1002/anie.201711058

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Accepted Article
Title: Formation and Isolation of a Four-Electron Reduced Porphyrin
Derivative via Further Reduction of a Stable 20πꢀ Isophlorin
Authors: Takahiko Kojima, Wataru Suzuki, Hiroaki Kotani, Tomoya
Ishizuka, Yoshihito Shiota, and Kazunari Yoshizawa
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201711058
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Formation and Isolation of a Four-Electron Reduced Porphyrin
Derivative via Further Reduction of a Stable 20π Isophlorin
Wataru Suzuki,[a] Hiroaki Kotani,[a] Tomoya Ishizuka,[a] Yoshihito Shiota,[b] Kazunari Yoshizawa,[b] and
Takahiko Kojima*[a]
Abstract: The two-electron reduction of
a
diprotonated
We have focused on a diprotonated saddle-distorted
dodecaphenylporphyrin derivative by Na2S2O4 gave a corresponding
isophlorin (Iph) selectively. Formation of Iph was confirmed by
spectroscopic measurements and the isolation of tetra-methylated
Iph. Further reduction of Iph proceeded to form an unprecedented
four-electron reduced porphyrin (IphH2), which is fully characterized
by spectroscopic and X-ray crystallographic analysis. IphH2 with a
unique conformation is capable of performing a reversibility to
reproduce the starting porphyrin by the chemical oxidation, which
acts as a proton-coupled four-electron reversible redox system.
dodecaphenylporphyrin (H4DPP2+)[8] as an appropriate precursor
for the formation of a corresponding isophlorin derivative, since
H4DPP2+ shows
a
high reduction potential to induce
disproportionation of one-electron reduced species of H4DPP2+
(H4DPP•+), affording two-electron reduced species.[8b] Herein, we
would like to report the selective formation of an isophlorin
derivative (Iph) from H4DPP2+ having electron-withdrawing
trifluoromethyl (CF3) groups at the para-position of meso phenyl
groups (H4P(Cl)2). Interestingly, it was revealed that Iph could
be further reduced to form a unique four-electron reduced
porphyrin (IphH2), which was fully characterized for the first time.
In addition, IphH2 undergoes the four-electron oxidation to afford
the starting porphyrin, which acts as a reversible multi-redox
system (Scheme 1).
Porphyrins are widely known to be 18π conjugated
aromatic macrocycles and redox-active molecules showing
reversible multi-redox processes. So far, the interesting redox
properties of porphyrins have been investigated mainly through
electrochemical measurements for several decades.[1] In
addition, porphyrins with a phenolic moiety as a redox-active site
at the periphery have been reported to show reversible
multielectron redox behavior by external stimuli.[2] As an
alternative candidate for a porphyrinoid showing multi-electron
redox behavior without redox-active moieties at the periphery, is
considered
a
multi-electron reduced porphyrin such as
porphyrinogen and isophlorin, which have been proposed as
intermediates in the synthetic pathway of porphyrins.[3] However,
there is no report on the reversible redox system as a “multi-
electron pool” based on the porphyrin framework without redox-
active sites because of the instability of multi-electron reduced
porphyrinoids.
Among various species of reduced porphyrinoids,
isophlorin, which is one of the two-electron reduced form of a
porphyrin in a 20π conjugated system, has attracted much
attention due to the fact that isophlorin undergoes the two-
electron oxidation to afford porphyrin.[3-7] So far, some isophlorin
derivatives have been isolated through the reduction of
corresponding porphyrins having highly positive central ions
(Si(IV), Ge(IV))[4] or core modified porphyrins[5] such as
tetraoxaporphyrin, showing 20π antiaromatic character. However,
no investigation on further reduction of isophlorin has been
reported, in spite of the fact that a porphyrin-based redox system
is expected to act as a reversible multi-electron-redox system.
Scheme 1. The multi-electron-redox cycle composed of a four-
electron-reduced porphyrin (IphH2) via the selective formation of
Iph from H4P(Cl)2.
A diprotonated dodecaphenylporphyrin derivative with four
CF3 groups, H4P(Cl)2, was synthesized based on a literature
method.[9] Characterization of H4P(Cl)2 was conducted by
spectroscopic measurements, elemental analysis, and X-ray
crystallography. In the molecular structure of H4P(Cl)2 (Figure
S1), H4P(Cl)2 shows a large saddle distortion (Δrms = 0.81 Å)[10]
,
which is similar to that of H4DPP2+ (Δrms = 0.87 Å).[8a] Cyclic
voltammetry (CV) and differential pulse voltammetry (DPV) of
H4P(Cl)2 showed reversible two-electron reduction waves of
H4P(Cl)2 at –0.67 V vs Fc/Fc+ in DMSO at 298 K containing 0.1
M [N(n-butyl)4]PF6 (TBAPF6) as an electrolyte (Figure S2a). The
reduction potential of H4P(Cl)2 (–0.67 V) is slightly more positive
than that of H4DPP2+ (–0.73 V) due to the electron-withdrawing
CF3 groups (Figure S2b).
[a]
W. Suzuki, Dr. H. Kotani, Dr. T. Ishizuka, Prof. Dr. T. Kojima
Department of Chemistry
Upon addition of 4 equiv. Na2S2O4 in H2O to a DMSO
solution containing H4P(Cl)2 under Ar, UV-Vis spectral changes
were observed as shown in Figure S3. The Soret band at 495
nm and the Q band at 706 nm due to H4P(Cl)2 disappeared
completely with a concomitant appearance of new bands at 476
nm and 567 nm (Figure S3). When the reduction reaction of
H4P(Cl)2 by Na2S2O4 in DMSO-d6 was monitored by 1H NMR
spectroscopy, 1H NMR signals due to H4P(Cl)2 disappeared
together with an appearance of new signals at 13.99 ppm and
Graduate School of Pure and Applied Sciences
University of Tsukuba
1-1-1 Tennoudai Tsukuba, Ibaraki 305-8571 (Japan)
E-mail: kojima@chem.tsukuba.ac.jp
[b]
Prof. Dr. Y. Shiota, Prof. Dr. K. Yoshizawa
Institute for Materials Chemistry and Engineering
Kyushu University
Motooka, Nishi-Ku, Fukuoka 819-0395 (Japan)
Supporting information for this article is given via a link at the end of the
document.
1
7.1 – 6.5 ppm (Figure 1). The H NMR signal at 13.99 ppm was
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assigned as NH protons of pyrrole rings, which is confirmed by
the disappearance through H/D exchange in the presence of
D2O (Figure S4). The NH proton signal of the pyrrole rings at a
low magnetic field indicates the loss of aromaticity of H4P(Cl)2 by
calculations were conducted. As
a
result, Iph was
thermodynamically disfavored rather than Phl (–6.3 kcal mol–1
relative to Iph) and Pdm (–15.3 kcal mol–1 relative to Iph),
respectively (Figure S8). However, the similarity of saddle-
distorted structures between H4P(Cl)2 and Iph is assumed to
induce kinetically favorable formation of Iph as described above.
In the case of DMSO, Iph was stabilized by the formation of
hydrogen bonding with DMSO molecules rather than Phl or
Pdm.[13,14] Therefore, the electron-transfer reduction of H4P(Cl)2
in polar aprotic solvents is essential for the selective formation of
Iph.
1
the chemical reduction. In addition, simple H NMR signals due
to the β-Ph and meso-Ph groups and one singlet signal in the
19F NMR spectrum were indicative of the symmetrical structure
due to the reduced derivative of H4P(Cl)2. Therefore, these
spectroscopic data suggested the selective formation of Iph as
the two-electron reduced product of H4P(Cl)2 as seen in the
previous report for the formation of an isophlorin derivative.[6] It
should be noted that no formation of Iph isomers such as a
phlorin derivative (Phl) and a 5,15-porphodimethene derivative
(Pdm) was observed because of no singlet signals due to their
methine protons around 6 ppm.[11]
Unfortunately, crystallization of Iph was not achieved due
to the limitation of reaction conditions described above. As an
alternative way for characterization of Iph, methylation of inner-
pyrrole nitrogen atoms was employed to characterize Iph
derivatives by X-ray crystallography. As shown in Scheme 2,
tetra-methylation of Iph was accomplished sequentially in a one-
pot reaction in DMSO by two-electron reduction of H2P with
CoII(cp)2 and methylation by iodomethane (MeI) in the presence
of sodium hydride (NaH) as a base. The tetra-methylated Iph
derivative (Me4Iph) was characterized by MALDI-TOF-MS and
1H NMR (Figure S10). The observed 1H NMR signal at 5.24 ppm
due to an inner methyl group under air suggested the formation
of non-aromatic air-stable Me4Iph (Figure S10b). The UV-vis
spectrum of Me4Iph in acetone shows
a characteristic
absorption band at 475 nm due to Me4Iph, which is similar to
that of non-aromatic Iph (Figure S11).[6,7a] Then, a molecular
structure of Me4Iph was revealed as shown in Figure 2 by X-ray
crystallography. Single crystals of Me4Iph were obtained by
vapor diffusion of 2-propanol to a CH2Cl2 solution containing
Me4Iph. As shown in Figure 2, Me4Iph shows a large saddle
distortion (Δrms = 0.83 Å), which is comparable with that of Iph
(Δrms = 0.96 Å) estimated by DFT calculations. No inclusion of
counter anions in the structure indicates that the macrocycle
should be chargeless and maintains the two-electron-reduced
form after methylation of Iph, in contrast to the case of H4P(Cl)2
that includes two chloride ions as the counter anions (Figure
S1).[15]
Figure 1. 1H NMR spectra of a) H4P(Cl)2, and b) H4P(Cl)2 in the
presence of 4 equiv Na2S2O4aq in DMSO-d6 at 298 K. *:CHCl3 in
crystals of H4P(Cl)2.
To evaluate the aromaticity of Iph, the nucleus
independent chemical shift (NICS(0))[12] was calculated at the
B3LYP/6-31G** level of theory (Figure S5). The estimated
NICS(0) value (+2.97 ppm) at the ring center of Iph was
comparable with that of the previously reported non-aromatic
isophlorin (+2.25 ppm).[6] In the case of planar antiaromatic
isophlorins, the estimated NICS(0) values were estimated to be
above +20 ppm due to the paratropic ring current effect of a 4nπ
macrocycle.[4,5] Thus, the aromaticity of Iph is interpreted as a
non-aromatic character.
Scheme 2. Synthesis of Me4Iph via formation of Iph.
The requirements for reaction conditions to form Iph were
investigated by changing reductants and solvents (Table S1).
Formation of Iph was also confirmed by using cobaltocene
(CoII(cp)2) or zinc powder (Zn) as an electron donor instead of
Na2S2O4, whereas the reduction of H4P(Cl)2 by NaBH4
unsuccessfully afforded unidentified species. In addition, the
formation of Iph was confirmed in polar aprotic solvents such as
DMSO and DMF (Figure S6): Iph is stable in these solvents for
several days under deaerated conditions at room temperature.
In contrast, mixtures of reduced products were produced in other
solvents such as methanol, acetone, and acetonitrile, although
the formation of Iph was detected at the beginning of the
Figure 2. ORTEP drawings of Me4Iph a) side view, and b) top
view. Hydrogen atoms were omitted for clarity.
1
reaction by H NMR measurements in each solvent. In the case
Next, we investigated the reductive transformation of Iph in
of methanol, formation of Pdm as one of the reduced products
was confirmed by X-ray crystallography (Figure S7). To compare
the thermodynamic stability of Iph, Phl, and Pdm, DFT
the presence of excess Na2S2O4 in DMSO. It was revealed that
Iph was further reduced to form
a four-electron reduced
1
porphyrin (IphH2) as seen in Figure 3. The H NMR signals due
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to Iph were completely converted to those due to IphH2 with
lower symmetry as seen in three kinds of NH protons around 10
ppm. A characteristic singlet signal was also observed at 5.99
ppm due to the two protons attached to the meso carbons in the
reduced porphyrin ring. MALDI-TOF-MS showed a peak cluster
at m/z = 1499.67, which was assigned to [IphH2 + H]+ as a four-
electron reduced H4P(Cl)2 (Figure S13). UV-Vis spectral
changes from Iph to IphH2 were observed with isosbestic points
(519 nm, and 580 nm) as shown in Figure S3b. The single
crystals of IphH2 were successfully obtained by vapor diffusion
Syn should be the most stable isomer of the four-electron-
reduced species.
of EtOH to
a DMF solution containing IphH2. Then, the
molecular structure of IphH2 was unambiguously revealed by X-
ray crystallography as shown in Figure 4. In the structure, the
5,10-meso-carbons were reduced to be sp3 configurations in a
“syn” geometry for the two adjacent 4-CF3-phenyl groups
attached to the carbon atoms, forming a unique conformation of
the porphyrinoid core: The left half part shows an isophlorin-like
conformation and the right half part shows a porphodimethene-
like conformation. As shown in Figure 4c, the C-C bond-length
alternation was observed in the left part (green colored) of IphH2,
as seen in the case of Me4Iph;[15] in the right part (red colored),
the C-C bond lengths around the meso-carbons were
determined to be 1.508(5) Å, 1.507(5) Å, 1.495(5) Å and
1.500(5) Å. This C-C bond elongation and the detection of
Figure 4. ORTEP drawings of IphH2 a) side view, and b) top view.
Hydrogen atoms were omitted for clarity except for protons of
nitrogen atoms and protons attached to the meso carbons. c)
The C-C bond lengths (Å) of the macrocycle in IphH2.
1
methine protons in H NMR measurements clearly indicate that
the further reduction and protonation occur at the meso-carbons.
Figure 5. Comparison of thermodynamic stability of 5,10-Syn
(IphH2) with structural isomers (5,10-Anti and 5,15-Syn) based
on DFT calculations.
Finally, the thermodynamic stability and redox behavior of
IphH2 were investigated to ascertain that IphH2 could act as a
reversible multi-redox system. Since no 1H NMR spectral
change was observed for IphH2 in DMSO-d6 under deaerated
conditions at 353 K for two days (Figure S15), the compound
should be thermodynamically stable. In contrast, IphH2 was
smoothly oxidized by excess amount of 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ) to produce H4P(Cl)2 (Figure
S16), which should be derived from protonation of H2P formed
as the oxidized product. The yield of H4P(Cl)2 was determined to
be 96% by H NMR measurements. This result clearly indicates
that the reversible four-electron redox cycle between H4P(Cl)2
and IphH2 was established (Scheme 1).
In summary, we have established a methodology for the
selective formation of an isophlorin derivative (Iph) by using a
saddle-distorted diprotonated dodecaphenylporphyrin derivative
through the reduction with electron donors in polar aprotic
Figure 3. 1H NMR spectrum of IphH2 in DMSO-d6 at 298 K.
To compare the thermodynamic stability of IphH2 with
other structural isomers, DFT optimized structures of three
possible isomers of four-electron reduced porphyrins were
calculated at the B3LYP/6-31G** level of theory. We considered
5,10-Anti and 5,15-Syn of a triplet biradical form as the
structural isomers of IphH2 (5,10-Syn, Figure 5, S14). As shown
in Figure 5, the stability of 5,15-Syn is thermodynamically
disfavored compared with that of 5,10-Syn (+20.7 kcal mol–1
relative to 5,10-Syn) due to the unstable open-shell structure.
5,10-Anti was destabilized by 7.2 kcal mol–1 in comparison to
5,10-Syn because of steric repulsion between a meso-aryl
group and a β-phenyl group. Therefore, it was clarified that 5,10-
1
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Maerker, A. Dransfeld, H. Jiao, N. J. R. v. E. Hommes, J. Am. Chem.
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electron reduction of Iph by Na2S2O4 afforded a novel four-
electron reduced porphyrinoid (IphH2), showing
conformation. Finally, we have demonstrated a four-electron
reversible redox system by re-oxidation of IphH2 to reproduce
the starting diprotonated dodecaphenylporphyrin, H4P(Cl)2.
[13]
[14]
a unique
We conducted DFT calculations of Iph and other isomers in the
presence of two DMSO molecules. As a result, the difference of
energy between Iph and Phl or Pdm decreased to 1.2 kcal mol–1 and
1.7 kcal mol–1
, respectively as shown in Figure S9. This result
suggested the stabilization of Iph through hydrogen bonds with a
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[15]
The core structure of Me4Iph (Figure S12) also suggests the
preservation of a 20π-conjugated isophlorin skeleton (see ref 6).
Acknowledgements
This work was supported by Grants-in-Aid (Nos 24245011 and
17H03027) from the Japan Society of Promotion of Science
(JSPS, MEXT) of Japan and a grant from Yazaki Memorial
Foundation for Science and Technology. We also appreciate Dr.
Tatsuhiro Kojima (Osaka University) for helpful guidance in X-
ray crystallography.
Keywords: Saddle-distorted porphyrin•Porphyrinoids•Isophlorin
•Electron-transfer reduction•Electron pool
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Entry for the Table of Contents
Layout 2:
COMMUNICATION
Wataru Suzuki, Hiroaki Kotani, Tomoya
Ishizuka, Yoshihito Shiota, Kazunari
Yoshizawa, and Takahiko Kojima*
Page No. – Page No.
Formation and Isolation of a Four-
Electron Reduced Porphyrin
Derivative via Further Reduction of a
Stable 20π Isophlorin
A four-electron reduced porphyrin (IphH2) was fully characterized by X-ray
crystallography for the first time. IphH2 shows a unique conformation. Additionally,
IphH2 was oxidized to afford the starting porphyrin, which acts as a proton-coupled
four-electron reversible redox system.
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