Synthesis and properties of redox-switchable zinc complexes of 10,15,20-triaryl-15-aza-5-oxaporphyrin

Article abstract of DOI:10.1002/hc.21456

The synthesis, aromaticity, and optical and electrochemical properties of zinc(II) complexes of 10,15,20-triaryl-15-aza-5-oxaporphyrin (TriAAOP) were investigated. Metal-templated cyclization of a zinc(II) 1,19-dichloro-5,10,15-triaryl-10-azatetrapyrrin c

Full text of DOI:10.1002/hc.21456

Received: 1 October 2018
DOI: 10.1002/hc.21456
Revised: 27 October 2018
Accepted: 29 October 2018
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S P E C I A L I S S U E : A T R I B U T E T O P R O F E S S O R
N A O M I C H I F U R U K AWA O N T H E O C C A S I O N O F
H I S 8 2 N D B I R T H D AY - B Y I N V I T AT I O N O N LY
Synthesis and properties of redox-switchable zinc complexes of
10,15,20-triaryl-15-aza-5-oxaporphyrin
Keisuke Sudoh1
Ko Furukawa2
Haruyuki Nakano3
Soji Shimizu4
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Yoshihiro Matano5
1Department of Fundamental
Sciences, Graduate School of Science and
Technology, Niigata University, Niigata,
Japan
2Center for Coordination of Research
Facilities, Institute for Research
Promotion, Niigata University, Niigata,
Japan
3Department of Chemistry, Graduate School
of Science, Kyushu University, Fukuoka,
Japan
4Department of Chemistry and
Biochemistry, Graduate School of
Engineering, Kyushu University, Fukuoka,
Japan
Abstract
The synthesis, aromaticity, and optical and electrochemical properties of zinc(II)
complexes of 10,15,20-triaryl-15-aza-5-oxaporphyrin (TriAAOP) were investi-
gated. Metal-templated cyclization of a zinc(II) 1,19-dichloro-5,10,15-triaryl-10-
azatetrapyrrin complex with an oxygen source afforded 20π TriAAOP in the neutral
form. Oxidation of 20π TriAAOP with silver (I) hexafluorophosphate generated the
19π radical cation or 18π dication depending on the content of oxidant used. The
interconversion between the three oxidation states (18π, 19π, and 20π) resulted in
distinctchangesinthearomaticityandopticalpropertiesofthe15-aza-5-oxaporphyrin
π-system. Nuclear magnetic resonance spectroscopy of 20π TriAAOP revealed its
antiaromatic character, whereas that of the 18π TriAAOP dication showed its aro-
matic character. The combined effect of the two meso-heteroatoms was directly
reflected in the redox properties of the porphyrin ring; TriAAOP was reduced more
easily and more difficult to oxidize than the zinc(II) complex of 5,10,15,20-tetraar
yl-5,15-diaz-aporphyrin (TADAP). In the ultraviolet-visible-near–infrared spectra
of the materials, the lowest-energy electronic excitations of the 19π and 18π
TriAAOP derivatives were considerably red-shifted compared with those of the
isoelectronic TADAP derivatives. Based on the results of density functional theory
calculations, it was concluded that the observed differences between TriAAOP and
TADAP would arise from the high electronegativity of oxygen; specific frontier
orbitals of the TriAAOP π-systems were energetically stabilized relative to those of
the TADAP π-system. The present findings corroborate that the meso-modification
of a porphyrin rings with different kinds of heteroatoms is a promising strategy to
fine tune their light-response properties that are switchable by reversible single
electron transfer processes.
5Department of Chemistry, Faculty of
Science, Niigata University, Niigata, Japan
Correspondence
Yoshihiro Matano, Department of
Chemistry, Faculty of Science, Niigata
University, Niigata, Japan.
Email: matano@chem.sc.niigata-u.ac.jp
Funding information
Japan Society for the Promotion of
Science, Grant/Award Number: 18H01961,
15K05392, 18K05036
Dedicated to Prof. Naomichi Furukawa on the occasion of his 82nd birthday.
Contract grant sponsor: JSPS KAKENHI
Contract grant number: 18H01961
Contract grant number: 15K05392
Contract grant number: 18K05036
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Heteroatom Chemistry. 2018;e21456.
https://doi.org/10.1002/hc.21456
wileyonlinelibrary.com/journal/hc
© 2018 Wiley Periodicals, Inc.
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,15-diazaporphyrin (TADAP)[23,24] and 5,10,20-triaryl-5,15
-diazaporphyrin (TriADAP)[25] were isolated in the neutral
or cationic form. Most importantly, these novel diazapor-
phyrin derivatives have very flat π-planes and are extremely
air-stable compared with the isoelectronic 20π porphyrin
dianions and 19π porphyrin radical anions. Furthermore,
the oxidation states of TADAP and TriADAP are switchable
through reversible redox processes (Scheme 1), which en-
abled us to investigate the correlation between the oxidation
state and optical and magnetic properties of the porphyrin
π-circuit in detail.
In addition to diazaporphyrin derivatives, 20π porphyrins
modified with chalcogen atoms at the 5 and 15 positions have
received considerable attention from both porphyrin and hetero-
atom chemists.[26–30] Recently, Shimizu et al.[28] and Shinokubo
et al.[29,30] independently reported the nickel(II) complex of 10
,20-diaryl-5,15-dioxaporphyrin (DOP) and several metal com-
plexes and the free base of 10,20-diaryl-5,15-dithiaporphyrin
(DTP), respectively (Chart 1). It is worth noting that the struc-
ture and aromaticity of these heteroporphyrins varied markedly
depending on the meso-chalcogen atoms. The DOP ring was
flat and exhibited 20π antiaromatic character, whereas the DTP
rings (M = Ni, Zn) showed considerable bending at the two
sulfur atoms and exhibited nonaromatic character. Interestingly,
electrochemical oxidation of the 20π DOP did not give the ex-
pected 18π dication but its dimer and oligomers via a 19π radical
1
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INTRODUCTION
Porphyrins are well known as redox-switchable macrocycles
with optical and electrochemical properties that vary widely
depending on the peripheral substituents and/or central metal.
In general, two-electron- and one-electron-reduced porphy-
rins (20π and 19π porphyrins, respectively) are extremely
unstable under atmospheric conditions because of their high-
energy highest occupied molecular orbital (HOMO) and
singly occupied molecular orbital (SOMO), respectively.[1]
Several approaches have been proposed to isolate 20π and
19π porphyrins in neutral forms, which basically rely on the
concept of lowering the HOMO or SOMO energy levels.[2–22]
However, most of the reported neutral 20π and 19π por-
phyrins are highly distorted and/or still air-sensitive. Little
information about the aromaticity and optical properties ac-
companying redox conversion of the π-systems of neutral 20π
and 19π porphyrins is available. It is therefore important to
establish a reliable approach to provide reduced forms of por-
phyrins with flat π-planes and air stability.
Recently, we reported a new approach to synthesize neu-
tral 20π and 19π porphyrins by replacing meso-methine units
of a porphyrin ring with nitrene units, which was based on the
concept of macrocyclic π-conjugation involving the unshared
electrons in the p-orbitals of meso-nitrogen atoms.[23–25] As
expected, 20π and 19π derivatives of 5,10,15,20-tetraaryl-5
R2
R2
N
R2
N
+
2+
•
N
5
N
20
N
N
N
N
N
N
– e–
+ e–
– e–
+ e–
N
N
N
N
R1
R1
R1
R1
R1
R1
10
M
M
M
N
15
N
N
R2
N
R2
R2
TADAP (20π)
TADAP•+ (19π)
TADAP2+ (18π)
R2
R2
N
+
•
N
5
N
N
N
– e–
+ e–
N
N
N
N
R1
R1
R1
R1
20
10
Ni
Ni
N
15
N
N
TriADAP• (19π)
TriADAP+ (18π)
This Work
R2
N
R2
N
R2
N
+
•
2+
R1
15
N
20
N
N
N
N
N
N
N
N
N
N
– e–
+ e–
– e–
+ e–
R1
R1
R1
R1
Zn
R1
Zn
10
Zn
N
5
O
O
O
SCHEME 1 Redox reactions of
TriAAOP (20π)
TriAAOP•+ (19π)
TriAAOP2+ (18π)
TADAP, TriADAP, and TriAAOP

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generated 19π radical cation 8 or 18π dication 9 depending
on the content of oxidant used. The quantitative formation
of 8 and 9 in solution was confirmed by ultraviolet-visible-
near–infrared (UV-vis-NIR) absorption spectroscopy (vide
infra). However, it was difficult to isolate the solid samples of
8 and 9 with high purity because they gradually decomposed
in concentrated solutions under atmospheric conditions. This
behavior of 8 and 9 was in sharp contrast to the remarkable
stability observed for the corresponding TADAP derivatives
P1 and P2, although the reason for this difference is not yet
clear.
CHART 1 DOP, DTP, TADAPs (P0, P1, P2), and models (P0-
m, P1-m, P2-m)
Compounds 7, 8, and 9 were characterized by high-
resolution electrospray ionization (ESI) mass spectrometry
1
and H NMR spectroscopy (for 7 and 9). In the ESI mass
spectra, 7 exhibited the parent ion peak (M+), whereas 8
and 9 exhibited intense fragment ion peaks ([M − nPF6]n+;
n = 1 and 2, respectively). The 1H NMR spectral features of
7 and 9 will be discussed in the following section. Electron
paramagnetic resonance (EPR) spectroscopy of 8 showed
a signal at g = 1.9955, although hyperfine structure was
not clearly observed (Figure S1). All attempts to grow sin-
gle crystals of the TriAAOP derivatives were unsuccessful.
Therefore, we carried out density functional theory (DFT)
calculations on their model compounds, 7-m, 8-m, and 9-
m, in which the meso-substituents were replaced with phe-
nyl groups (Figure 1). For the optimized structures, the bond
lengths between the meso-nitrogen/oxygen and adjacent α-
carbon atoms gradually decreased from 7-m (1.394/1.358 Å)
to 8-m (1.381/1.347 Å) to 9-m (1.366/1.336 Å), indicat-
ing that the oxidation of the 15-aza-5-oxaporphyrin (AOP)
ring strengthened the resonance interactions between the p-
orbitals of the meso-heteroatoms and α-carbon atoms. With
this oxidation-induced X–C bond shortening (X = O, N) at
the periphery, the porphyrin core is contracted to provide a
narrower coordination environment for the central metal in
the order 7-m > 8-m > 9-m, as represented by their Zn–N
bond lengths of 2.021/2.029 Å for 7-m, 2.010/2.021 Å for 8-
m, and 2.008/2.013 Å for 9-m.
cation intermediate. With these results in mind, we decided to
investigate the cooperative effect of an oxygen atom and nitrene
unit placed at the 5 and 15 positions, respectively, on the funda-
mental properties of a porphyrin ring. Herein, we report the first
examples of redox-switchable zinc(II) complexes of 10,15,20-tr
iaryl-15-aza-5-oxaporphyrin (TriAAOP, Scheme 1) and discuss
their aromatic character and optical properties determined from
experimental and theoretical analyses.
2
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RESULTS AND DISCUSSION
2.1 | Synthesis
Scheme 2 illustrates the synthesis of TriAAOP.
Treatment of zinc(II) acetate with a mixture of equimo-
lar amounts of 1,9-dichloro-5-mesityldipyrrin 1 (mesi-
tyl = 2,4,6-trimethylphenyl)[31] and 1-chloro-9-(4-methox
yphenylamino)-5-mesityldipyrrin 2[23,24] in the presence of
excess triethylamine in CH2Cl2/MeOH at room temperature
afforded three kinds of zinc(II)–bis(dipyrrin) complexes, 3,
4, and 5, which were successfully separated from one another
by column chromatography on silica gel. The unsymmetri-
cally substituted zinc complex 3 was isolated as a reddish
brown solid, and the symmetrically substituted zinc com-
plexes 4 and 5 were isolated as reddish brown and orange
solids, respectively. Heating a mixture of 3 and potassium
carbonate in N,N-dimethylformamide (DMF) at 110°C for
1 hour afforded the zinc(II) complex of 1,19-dichloro-5,15
-dimesityl-10-(4-methoxyphenyl)-10-azatetrapyrrin 6 as a
green solid in quantitative yield.
Attempts to synthesize the target compound TriAAOP
by sequential SN2 reactions of 6 with several oxygen sources
(Me4NOH, KOH, Ag2O) under various conditions were
unsuccessful. However, we eventually found that the ring-
closure reaction of 6 with excess 4-methoxybenzaldoxime/
NaH, which had been used to synthesize DOP,[28] afforded
20π TriAAOP 7 as a yellow solid. Treatment of 7 with sil-
ver(I) hexafluorophosphate in CH2Cl2 instantaneously
2.2 | Aromaticity
The NMR data for 7 and 9 presented below were collected
1
from their freshly prepared samples. The H NMR spec-
trum of 7 in CD2Cl2 displayed peaks from the β-pyrrolic,
ortho-methyl, and para-methyl protons at δ 3.8-4.9 (total
8H), 2.30, and 2.17 ppm, respectively, whereas that of 9 in
CD2Cl2 showed the corresponding protons at δ 7.98-8.66
(total 8H), 2.56, and 1.92 ppm, respectively (Figure 2).
The chemical shifts of the β-pyrrolic as well as ortho/para-
methyl protons of 7 basically reflect the paratropic ring-
current effect arising from the 20π-electron antiaromatic
conjugation and are considerably shifted downfield com-
pared with those of P0 (δ 3.02 and 4.07 ppm in C6D6).[24]

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positions in 9-m were −12.30, −12.31, and −13.42 ppm,
respectively, which are apparently affected by diatropic
ring currents arising from the 18π circuit. The absolute
NICS(0) values calculated for 7-m and 9-m are smaller
than those calculated for the corresponding models of 20π
TADAP P0-m (+8.13 and +9.32 ppm for a and b, respec-
tively) and 18π TADAP P2-m (−17.72 and −17.11 ppm for
a and b, respectively).[24] Based on these results, it can be
safely concluded that 20π (18π) TriAAOP exhibited weaker
antiaromatic (aromatic) character than that of 20π (18π)
TADAP in terms of ring-current effects. In both 7-m and 9-
m, the negative NICS(0) values (from −6.14 to −8.27 ppm)
at positions d and e imply that the pyrrole rings have local
diatropic ring currents. The NICS(0) values of 9-m are less
negative than those of a model for the zinc(II) complex
of 10,20-dimesityl-5,15-diazaporphyrin (DAP) (−18.93
and −19.22 ppm at the corresponding positions).[33] This
indicates that the macrocyclic ring-current effect of 18π
TriAAOP is also weaker than that of 18π DAP.
2.3 | Optical and redox properties
The optical properties of the three oxidation states of
TriAAOP were investigated by UV-vis-NIR absorption
spectroscopy. Table 1 summarizes the experimentally
observed data for the TriAAOP, TADAP, and porphyrin/
DAP reference materials, and Figure 3A shows the UV-
vis-NIR absorption spectra of 7, 8, and 9 in CH2Cl2. The
20π TriAAOP 7 exhibited two intense absorption bands
with absorption maxima (λmax) of 437 and 488 nm, which
are blue-shifted compared with those of P0 (λmax = 444 and
519 nm).[24] In contrast, the lowest-energy band of the 19π
TriAAOP 8 (λmax = 1034 nm) was red-shifted compared
with that of P1 (λmax = 890 nm). In addition, an intense
band was observed for 18π TriAAOP 9 (λmax = 671 nm) at
much longer wavelength than those of the related 18π de-
rivatives, P2 (λmax = 634 nm)[24] and zinc(II) complexes of
DAP (λmax = 584 nm)[33] and 5,10,15-triphenylporphyrin
(λmax = 543 nm).[34] It is now evident that the concurrent
replacement of two meso-methine units of a porphyrin
ring with both a nitrene unit (arylimino group) and oxygen
atom as well as the addition of positive charge(s) effec-
tively lowered the excitation energies for the generation
of the lowest excited states of the 19π and 18π porphyrins.
The successive and quantitative formation of the
19π radical cation and 18π dication was monitored by
spectrometric titration; the stepwise addition of tris(4-
bromophenyl)ammoniumyl hexachloroantimonate (magic
blue) to a CH2Cl2 solution of 7 (yellow) revealed continu-
ous spectral changes to 8* (smoky yellow; 19π radical cat-
ion, λmax = 1043 nm) and then to 9* (green; 18π dication,
λmax = 673 nm) with several isosbestic points (Figure 3B).
SCHEME 2 Synthesis of 7, 8, and 9
This suggests that the 20π-electron circuit of 7 is less an-
tiaromatic in character than that of P0. A similar trend was
observed in the 1H NMR spectra of the nickel(II) complexes
of DOP and 20π TADAP, where the former meso-modified
heteroporphyrin exhibited a smaller paratropic ring-current
effect than the latter. Probably, the greater electronegativ-
ity of oxygen than that of nitrogen hampers global delo-
calization of its lone-pair electrons in both DOP and 20π
TriAAOP. In contrast, the diatropic ring-current effect on
the β-pyrrolic protons of 9 was slightly weaker than that of
P2 (δ 8.20 and 8.54 ppm in CDCl3).[24] It appears that the
18π-electron circuit of 9 was less aromatic in character than
that of P2, although deshielding caused by the inductive ef-
fect through the heteroatom-carbon σ bonds should also be
considered.
To obtain more insight into the ring-current effects,
we calculated nucleus-independent chemical shift (NICS)
values[32] at five positions in the π-planes of 7-m and 9-m
(Figure 1). The NICS(0) values at the midpoints between
the two adjacent pyrrole rings (a, b, and c) in 7-m were
+5.79, +7.11, and +6.11 ppm, respectively, indicating
that there are global paratropic ring currents in its 20π cir-
cuit. In contrast, the NICS(0) values at the corresponding

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FIGURE 1 Optimized structures, selected bond lengths, dihedral angles, and NICS(0) values calculated by the DFT method with the solvent
effect (B3LYP/6-311G(d,p) and Wachters-Hay(f) with PCM, CH2Cl2): A, 7-m, B, 8-m, C, 9-m. NICS(0) values at the five positions (a-e) were
calculated at HF-GIAO/6-31G+(d) level
This result strongly supported that the single electron trans-
fer from the TriAAOP π-system to the oxidant occurred
stepwise and quantitatively.
observed for 9 was attributed to the symmetry-allowed
HOMO-to-LUMO electronic transition (state 1 for 9-m in
Table 2). The highly electronegative oxygen atom at the
meso position in the 18π AOP ring (9-m) energetically
stabilized the LUMO more efficiently than the HOMO
relative to the corresponding orbitals of the 18π TADAP
ring (P2-m) because their LUMOs have moderate orbital
coefficients at the meso-heteroatoms. This readily ex-
plains the red-shifted HOMO-to-LUMO excitation of 9
(λmax = 671 nm) relative to that of P2 (λmax = 634 nm).
The lowest-energy excitation calculated for 8-m was the
HOMO−1-to-SOMO electronic transition, although the
calculated excitation energy did not match well with the
observed value.
The redox potentials of 7 were measured in CH2Cl2 by
using cyclic voltammetry with tetrabutylammonium hexa-
fluorophosphate (Bu4NPF6) as a supporting electrolyte.
As shown in Figure 5, TriAAOP exhibited two separate
and reversible redox processes at −0.35 and +0.28 V (vs
ferrocene/ferrocenium; Fc/Fc+), which were attributed to
To understand the origin of the π–π* electronic transitions
of the TriAAOP π-systems, we carried out time-dependent
DFT (TD-DFT) calculations on 7-m, 8-m, and 9-m using the
B3LYP functional (for the results of P0-m and P2-m, see
ref.[24]). Selected Kohn-Sham orbitals of 7-m, 9-m, P0-m,
and P2-m are depicted in Figure 4. The orbital features of
the HOMO of 7-m largely correspond to those of the lowest
unoccupied molecular orbital (LUMO) of 9-m, implying that
the two electrons in the HOMO of 7 were removed in the
chemical oxidation process to form 9.
As summarized in Table 2, the lowest-energy excitation
for 7-m was the symmetry-forbidden HOMO-to-LUMO
electronic transition, which was not clearly observed in
the spectrum of 7. The intense bands observed for 7 were
assigned to combinations of the HOMO−1-to-LUMO and
HOMO-to-LUMO+1 electronic transitions (states 2 and 3
for 7-m in Table 2). In contrast, the lowest-energy band

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FIGURE 2 1H NMR spectra of A, 7 in CD2Cl2, B, P0 in C6D6, C, 9 in CD2Cl2, and D, P2 in CDCl3. Asterisks indicate residual solvent
peaks. 700 MHz for 7 and 9; 400 MHz for P0 and P2
20π/19π and 19π/18π redox processes, respectively. The
redox potentials of TriAAOP were considerably shifted to
the positive side (ΔE = 0.34-0.36 V), compared with the
corresponding potentials of TADAP. The observed results
are in good accordance with the theoretically calculated
results for their models (Figure 4); the HOMO of 7-m
(E = −4.56 eV) and LUMO of 9-m (E = −5.49 eV) are en-
ergetically stabilized compared with the corresponding or-
bitals of P0-m (E = −4.26 eV) and P2-m (E = −5.16 eV),
respectively, at the same level of theory.[25] These molecular
orbitals have relatively large orbital coefficients at the
meso-heteroatoms (vide supra) and therefore are markedly
stabilized upon introducing the highly electronegative ox-
ygen atom.
3
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CONCLUSION
We prepared TriAAOPs using metal-templated annulation
and redox reactions. The magnetic criteria of aromaticity
for the 20π and 18π TriAAOP derivatives were evaluated by
NMR spectroscopy and NICS calculations, which revealed
that the replacement of the meso-methine units with a ni-
trene unit and oxygen atom decreased both the paratropic
and diatropic ring-current effects arising from the porphyrin
π-circuits. In addition, the optical and redox properties of
TriAAOP were studied using UV-vis-NIR absorption spec-
troscopy, cyclic voltammetry, and DFT calculations. The
meso-modification energetically stabilized specific molecu-
lar orbitals and, as a result, decreased the electronic excita-
tion energies generating the lowest excited states of the 18π
and 19π porphyrins. The observed results were rationally
explained by considering the combined effect of the nitrene
unit and oxygen atom, which was not observed for the related
TADAP and DOP derivatives. The present results highlight
the potential of meso-heteroatom-modified porphyrins as
redox-switchable catalysts and sensitizers; such applications
will be studied in due course.
TABLE 1 Optical data for 7, 8, 9, and related zinc(II)–porphyrin
derivativesa
Compounds
λmax/nm
7
437, 488
8
9
P0
P1
P2
ZnDAP
ZnPor
363, 424, 886, 977, 1034
381, 671
444, 519b
384, 448, 890b
389, 634b
394, 552, 584c
418, 543d
aMeasured in CH2Cl2 unless otherwise noted.
bData from ref.[23]
cZnDAP = 10,20-dimesityl-5,15-diazaporphyrinatozinc(II),
CH2Cl2–MeOH; data from ref.[33]
measured
in
dZnPor = 5,10,15-triphenylporphyrinatozinc(II), measured in toluene; data from
ref.[34]

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4.1 | Compounds 3 and 4
A mixture of 1 (455.0 mg, 1.374 mmol), 2 (577.2 mg,
1.384 mmol), Zn(OAc)2 (303.6 mg, 1.654 mmol), triethyl-
amine (1 mL), CH2Cl2 (50 mL), and MeOH (50 mL) was
stirred for 18 hours at room temperature. The mixture was
concentrated under reduced pressure. The residue was then
subjected to silica-gel column chromatography (hexane/
AcOEt = 10/1). TLC indicated three fractions (Rf = 0.57,
0.72, 0.74; hexane/AcOEt = 5/1), of which the second frac-
tion (Rf = 0.72) was collected, concentrated, and recrystallized
from MeOH to give 3 as a reddish brown solid (622.4 mg,
56%). The other fractions were also collected and recrystal-
lized. The first (Rf = 0.74) and third (Rf = 0.57) fractions were
evaporated to give 4 (201.1 mg, 20%, orange solid) and 5
(128 mg, 20%, reddish brown solid), respectively. Compound
5 was characterized based on the reported spectral data.[24]
1
3: Rf (hexane/AcOEt = 5/1) = 0.72. Mp 163-164°C H
NMR (400 MHz, CDCl3): δ 1.95 (s, 3H, para-Me), 2.14
(s, 3H, para-Me), 2.17 (s, 6H, ortho-Me), 2.36 (s, 3H, or-
tho-Me), 2.37 (s, 3H, ortho-Me), 3.77 (s, 3H, OMe), 6.08
(d, 1H, J = 3.8 Hz, pyrrole-β), 6.16 (d, 1H, J = 3.8 Hz,
pyrrole-β), 6.19 (d, 1H, J = 4.4 Hz, pyrrole-β), 6.30 (d,
1H, J = 4.4 Hz, pyrrole-β), 6.31 (s, 1H, NH), 6.57 (d, 1H,
J = 4.4 Hz, pyrrole-β), 6.59 (d, 1H, J = 4.4 Hz, pyrrole-β),
6.76-6.85 (m, 4H, Ar), 6.88 (s, 1H, meta-Mes), 6.93 (s, 2H,
meta-Mes), 6.94 (s, 1H, meta-Mes); 13C NMR (100 MHz,
CDCl3): δ 19.52, 19.56, 21.04, 21.07, 55.28, 109.82,
110.04, 114.43, 116.59, 122.96, 124.03, 127.52, 127.57,
127.87, 131.92, 132.84, 133.06, 134.64, 135.15, 135.58,
136.05, 136.26, 136.74, 136.77, 136.93, 137.20, 137.28,
137.94, 137.70, 145.52, 147.15, 156.45, 161.98; HRMS
(ESI): m/z calcd for C43H38Cl3N5OZn: 810.1428; found:
810.1393 ([M]+).
FIGURE 3 A, Normalized UV-vis-NIR absorption spectra of 7
(solid line), 8 (dotted line), and 9 (dashed line) in CH2Cl2. B, Spectral
changes during the stepwise addition of Magic blue to a CH2Cl2
solution of 7 (ca. 1 × 10−5 M); the first oxidation from 7 to 8* (upper),
and the second oxidation from 8* to 9* (lower). The counter anion of
8* and 9* was hexachloroantimonate
4: Rf (hexane/AcOEt = 5/1) = 0.74. Mp > 300°C; 1H
NMR (400 MHz, CDCl3): δ 2.14 (s, 12H, ortho-Me), 2.38
(s, 6H, para-Me), 6.24 (d, 4H, J = 4.0 Hz, pyrrole-β), 6.54
(d, 2H, J = 4.0 Hz, pyrrole-β), 6.95 (s, 4H, Mes-meta); 13C
NMR (100 MHz, CDCl3): δ 19.63, 21.14, 115.92, 127.68,
132.58, 133.46, 136.82, 137.50, 137.60, 145.73, 146.14;
HRMS (ESI): m/z calcd for C36H30Cl4N4Zn: 722.0511;
found: 722.0511 ([M]+).
4
|
EXPERIMENTAL
NMR spectra were recorded on 700 MHz (Agilent) and/or
400 MHz (Agilent or JEOL JNM-ECP) spectrometers. The
1H chemical shifts are reported in ppm as relative values vs
tetramethylsilane. High-resolution mass (HRMS) spectra
were measured on a Thermo Fisher Scientific EXACTIVE
spectrometer (electron spray–quadrupole). UV-Vis-NIR
absorption spectra were measured on a JASCO V-530
spectrometer. UV-vis fluorescence spectrum and quantum
yield of 6 were measured on a JASCO FP-8300 spectrom-
eter and a Hamamatsu Photonics Quantaurus-QY spec-
trometer, respectively. Thin-layer chromatography was
performed with Kieselgel 60 F254, and preparative col-
umn chromatography was performed using Silica Gel 60
spherical, neutrality. All reactions were performed under
an argon or nitrogen atmosphere unless otherwise noted.
Compounds 1[31] and 2[23,24] were prepared according to the
reported procedures.
4.2 | Compound 6
A mixture of 5 (621 mg, 0.764 mmol), K2CO3 (876 mg,
6.34 mmol), and DMF (40 mL) was stirred for 1 hour at
110°C. After being quenched with water, the aqueous layer
was extracted with toluene, and the combined organic extracts
were washed with brine, dried over Na2SO4, and evaporated
under reduced pressure. The residue was then subjected to
silica-gel column chromatography (hexane/AcOEt = 10/1).
The green fraction (Rf = 0.44; hexane/AcOEt = 5/1) was

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FIGURE 4 Selected Kohn-Sham orbitals and their energies (in eV) of A, 7-m and P0-m and B, 9-m and P2-m calculated by the DFT
method with the solvent effect (B3LYP/6-311G(d,p) and Wachters-Hay(f) with PCM, CH2Cl2). H = HOMO; L = LUMO. Data for P0-m and P2-m
are taken from ref.[24]
collected and evaporated to give 6 as a green solid (622 mg,
was heated at 70°C for 30 minutes. 4-Methoxybenzaldoxime
(317.5 mg, 2.10 mmol) was added, and the mixture was
stirred at the same temperature. After 1 hour, 6 (104.7 mg,
0.135 mmol) was added and the resulting mixture was heated
at 100°C for 1.5 hour. After being quenched with water, the
aqueous layer was extracted with toluene, and the combined
organic extracts were dried over Na2SO4 and evaporated
under reduced pressure. The residue was then subjected to
silica-gel column chromatography (hexane/AcOEt = 5/1).
The yellow fraction (Rf = 0.51; hexane/AcOEt = 5/1)
was collected and evaporated to give 7 as a yellow solid
1
quant). Mp 210-213°C; H NMR (400 MHz, CDCl3): δ 2.11
(s, 6H, ortho-Mes), 2.20 (s, 6H, ortho-Mes), 2.36 (s, 6H, para-
Mes), 3.89 (s, 3H, OMe), 5.61 (d, 2H, J = 4.4 Hz, pyrrole-β),
6.32 (d, 2H, J = 3.8 Hz, pyrrole-β), 6.33 (d, 2H, J = 3.8 Hz,
pyrrole-β), 6.53 (d, 2H, J = 4.4 Hz, pyrrole-β), 6.93 (s, 4H,
Mes-meta), 7.02-7.07 (m, 2H, Ar), 7.36-7.41 (m, 2H, Ar);
13C NMR (100 MHz, CDCl3): δ 19.58, 19.78, 21.08, 55.44,
112.25, 114.13, 115.06, 125.19, 126.77, 127.57, 127.59,
130.16, 132.43, 133.32, 133.96, 136.90, 137.06, 137.30,
138.30, 138.59, 139.33, 140.37, 158.01, 159.85; HRMS (ESI):
m/z calcd for C43H37Cl2N5OZn: 773.1661, found: 773.1663
[M+]; UV/Vis (CH2Cl2): λmax (log ε) 452 (4.89), 669 (3.90)
nm. In CH2Cl2, 6 emitted fluorescence (λex = 450 nm) with
the emission maximum at 736 nm, and fluorescence quantum
yield was determined to be 1.3%. The UV-vis absorption and
fluorescence spectra of 6 are summarized in Figure S1.
1
(13.2 mg, 14%). H NMR (700 MHz, CD2Cl2): δ = 2.17 (s,
6H, para-Me), 2.30 (s, 12H, ortho-Me), 3.73 (s, 3H, OMe),
3.84 (broad-s, 2H, pyrrole-β), 4.38 (broad-s, 2H, pyrrole-β),
4.84 (broad-s, 4H, pyrrole-β), 6.71 (s, 4H, Mes-meta), 6.86
(broad-s, 2H, Ar), 6.98 (broad-s, 2H, Ar); HRMS (ESI): m/z
calcd for C43H37N5O2Zn: 719.2233, found: 719.2229 ([M+]);
UV/Vis (CH2Cl2): λmax = 437, 488 nm.
4.3 | Compound 7
Sodium hydride (60% dispersion in a mineral oil, 53.6 mg,
1.34 mmol) was added to a mixture of well-dried DMSO
(20 mL) and dioxane (20 mL), and the resulting mixture
4.4 | Compound 8
This compound was found to gradually decompose in
a concentrated solution and on silica gel. Therefore, we

SUDOH et al.
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TABLE 2 Excitation energies and
oscillator strengths of 7-m, 8-m, and 9-m
calculated by the TD-DFT methoda
Excitation energy
Oscillator
strength
State
(eV)
(nm)
Excitation (weight, %)
7-m
1
2
3
4
1.44
2.50
3.21
3.25
860
497
386
382
0.013
0.110
1.196
0.199
H → L (100.0)
H → L + 1 (71.9), H–1 → L (28.1)
H–1 → L (59.1), H → L + 1 (22.8)
H → L + 2 (83.4)
8-m
1
2
10
19
9-m
1
2
6
14
15
18
1.39
1.49
2.63
3.19
892
831
472
389
0.156
0.111
0.062
0.203
H–1 → S (71.8), S → L (25.4)
S → L (70.8), H–1 → S (25.4)
S → L + 1 (31.5)
H–7 → L (18.4)
1.81
1.87
2.27
3.04
3.14
3.41
685
664
547
407
395
364
0.029
0.346
0.105
0.502
0.137
1.072
H → L (95.9)
H → L (69.5), H–4 → L (28.4)
H–5 → L (85.3)
H–1 → L + 1 (94.7)
H-4 → L + 1 (61.6)
H → L + 1 (33.8)
aB3LYP/6-311G(d,p) and Wachters-Hay(f) (PCM, CH2Cl2) at the optimized structures. H: HOMO; L: LUMO;
S: SOMO. Except for the lowest-energy excited states of 7-m and 9-m, the states whose oscillator strengths are
less than 0.1 are not included.
characterized it in the dissolved state by using UV-vis spec-
troscopy and HR-ESI mass spectrometry. Silver(I) hex-
afluorophosphate (4.6 mg, 18 μmol) was added to a CH2Cl2
solution (20 mL) of 7 (13 mg, 18 μmol). The resulting solu-
tion was passed through a Celite bed, and the filtrate was
immediately characterized by the abovementioned methods.
HRMS (ESI): m/z calcd for C43H37N5O2Zn: 719.2255, found:
719.2261 [M − (PF6)+]; UV/Vis (CH2Cl2): λmax = 363, 424,
886, 977, 1034 nm.
4.5 | Compound 9
This compound was found to gradually decompose in a
FIGURE 5 Cyclic voltammograms for TriAAOP 7 (upper)
and TADAP P1 (lower) measured in CH2Cl2 with Bu4NPF6 as the
supporting electrolyte: Scan rate = 60 mV/s
concentrated solution. Therefore, we characterized it in the
1
dissolved state by using H NMR as well as UV-vis spec-
troscopy and HR-ESI mass spectrometry. Silver(I) hexafluo-
rophosphate (ca. 2 mg, ca. 8 μmol) was added to a CD2Cl2
solution (4 mL) of 7 (ca. 2 mg, ca. 3 μmol). The color of the
solution quickly turned to green, and the product was im-
mediately characterized by the abovementioned methods.
1H NMR (700 MHz, CD2Cl2): δ = 1.92 (s, 6H, para-Me),
2.56 (s, 12H, ortho-Me), 4.06 (s, 3H, OMe) 7.29 (s, 4H, Mes-
meta), 7.34 (d, 2H, J = 8.0 Hz, Ar), 7.98 (d, 2H, J = 4.9 Hz,
pyrrole-β), 8.07 (d, 2H, J = 8.0 Hz, Ar), 8.20 (d, 2H,
J = 4.9 Hz, pyrrole-β), 8.49 (d, 2H, J = 4.9 Hz, pyrrole-β),
8.66 (d, 2H, J = 4.9 Hz, pyrrole-β); HRMS (ESI): m/z calcd
for C43H37N5OZn: 359.6114 (z = 2), 719.2233 (z = 1),
found: 359.6110 (z = 2), 719.2227 (z = 1) [M − 2(PF6)z+];
UV/Vis (CH2Cl2): λmax = 381, 671 nm.
4.6 | Electrochemical measurements
Redox potentials of 7 in CH2Cl2 were measure by cyclic
voltammetry at room temperature on a CH Instruments
model 660A electrochemical workstation using a glassy

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SUDOH et al.
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carbon working electrode, a platinum wire counter elec-
trode, and an Ag/Ag+ [0.01 M AgNO3, 0.1 M Bu4NPF6
(MeCN)] reference electrode. The scan rate was 60 mV/s.
The potentials were calibrated to a ferrocene/ferrocenium
(Fc/Fc+) couple.
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ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI (Grant Number:
18H01961 to YM, 15K05392 and 18K05036 to HN).
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Chem. Eur. J. 2012, 18, 16129.
ORCID
Yoshihiro Matano
https://orcid.org/0000-0003-3377-0004
[30] N. Wachi, T. Kondo, S. Ito, S. Hiroto, J.-Y. Shin, H. Shinokubo, J.
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SUPPORTING INFORMATION
Additional supporting information may be found online in
the Supporting Information section at the end of the article.
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How to cite this article: Sudoh K, Furukawa K,
Nakano H, Shimizu S, Matano Y. Synthesis and
properties of redox-switchable zinc complexes of
10,15,20-triaryl-15-aza-5-oxaporphyrin. Heteroatom
Chem. 2018;e21456. https://doi.org/10.1002/hc.21456
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Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.
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