Doubly Strapped Redox-Switchable 5,10,15,20-Tetraaryl-5,15-diazaporphyrinoids: Promising Platforms for the Evaluation of Paratropic and Diatropic Ring-Current Effects

Article abstract of DOI:10.1021/acs.joc.0c02433

This paper presents a novel series of chemically stable and redox-switchable 20π, 19π, and 18π5,10,15,20-tetraaryl-5,15-diazaporphyrinoids (TADAPs) that have two alkyl-chain straps above and below the diazaporphyrin ring. Three types of doubly strapped TA

Full text of DOI:10.1021/acs.joc.0c02433

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Article
Doubly Strapped Redox-Switchable 5,10,15,20-Tetraaryl-5,15-
diazaporphyrinoids: Promising Platforms for the Evaluation of
Paratropic and Diatropic Ring-Current Effects
Hikari Ochiai, Ko Furukawa, Haruyuki Nakano, and Yoshihiro Matano*
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ABSTRACT: This paper presents a novel series of chemically stable and redox-switchable 20π, 19π, and 18π 5,10,15,20-tetraaryl-
5,15-diazaporphyrinoids (TADAPs) that have two alkyl-chain straps above and below the diazaporphyrin ring. Three types of doubly
strapped TADAPs were prepared as nickel(II) complexes using meso-N-(2,6-dihydroxyphenyl)-substituted TADAP and the
corresponding aliphatic diacids as precursors. Theoretical calculations revealed that regardless of their oxidation states, all strapped
TADAPs had essentially flat π-planes. It was found that the alkyl-chain straps slightly affected the optical and electrochemical
1
properties of the DAP rings, particularly in the oxidized forms. H NMR spectroscopy was used to evaluate the antiaromatic
character of the 20π TADAPs and the aromatic character of the 18π TADAP dications, and it was observed that they displayed
paratropic and diatropic ring-current effects, respectively, on the chemical shifts of methylene protons in the spatially separated alkyl
chains. The degree of shielding and deshielding depended on the position of the methylene units; it decreased with increase in
separation from the π-plane and central axis of the porphyrin ring. The NMR experiments also revealed that the degree of the
diatropic ring currents was clearly related to the π-electron density of the porphyrin ring; the ring-current effects decreased as the
charge increased from 0 to +2. These findings are also qualitatively supported by the nucleus-independent chemical shifts.
characterize under ambient conditions because of their
extreme instability toward air and moisture. To overcome
this limitation, several elaborate approaches, such as N-
alkylation,15−17 core modification,18−20 peripheral functional-
ization,21 and metal insertion,22−24 have been developed to
isolate 20π porphyrins in their neutral state. In many cases,
however, the attachment of an excessive number of
substituents causes the porphyrin ring to be severely distorted,
thus impeding the experimental evaluation of the magnitudes
of aromaticity and antiaromaticity associated with the 18π−
20π redox processes. Therefore, the construction of a stable
INTRODUCTION
■
Aromatic−antiaromatic switching of π-conjugated organic
compounds has received considerable attention in the fields
of structural and synthetic chemistry.1−6 Porphyrins have been
widely used as reliable platforms for identifying the aromaticity
of (4n + 2)π and 4nπ macrocycles because their redox
properties can be finely and systematically tuned either by
chemical modification of the periphery or core-nitrogen atoms
or by ring expansion.7−12 In particular, significant attention has
been dedicated toward the antiaromatic character of doubly
reduced, 20π porphyrins.10,11 The magnetic criteria of
aromaticity are generally evaluated by NMR spectroscopy.13
Received: October 14, 2020
Published: January 7, 2021
For example, Mullen et al. reported on the NMR spectra of
̈
potassium and lithium salts of 20π porphyrin dianions, wherein
the peripheral pyrrolic-β and meso protons were observed at
significantly higher fields than those of the 18π counterparts,
with a difference of 9.9−10.4 ppm in the chemical shifts.14
However, such anionic porphyrins are difficult to isolate and
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Scheme 1. Interconversion of TADAPs
and flat porphyrin platform that is interconvertible between the
18π and 20π electron systems remains a challenge for the
evaluation of porphyrin-induced paratropic and diatropic ring
currents.
ester linkages. This paper presents the first examples of doubly
strapped TADAPs and discusses their aromaticity.
RESULTS AND DISCUSSION
■
Partial replacement of the meso units of a porphyrin ring
with nitrogen atoms lowers its molecular symmetry and
stabilizes the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) of its π-electron
system.25−28 For example, the redox potentials of 5,15-
diazaporphyrins (DAPs) differ substantially from those of
their porphyrin counterparts.28 Taking this into consideration,
we recently reported on a new approach to stabilize 20π
porphyrins by replacing two meso carbon atoms with two
nitrogen atoms in 5,10,15,20-tetraarylporphyrin, thus altering
the net charge of the porphyrin ring by two electrons.29
Remarkably, several metal complexes and freebases of
5,10,15,20-tetraaryl-5,15-diazaporphyrinoids (TADAPs),
namely, P1, P2, and P3 (Scheme 1; M = Ni, Cu, Zn, 2H),
were obtained as air-stable solids.29−31 Most importantly, these
TADAPs maintain highly flat π-planes and involve the same π-
conjugation pathway, as described in Scheme 1. In this regard,
P1 is fairly different from the previously reported neutral 20π
porphyrins that contain formal isophlorin frameworks passing
through an outer 20π molecular perimeter.32
Synthesis. Scheme 2 illustrates the synthesis of oligo-
methylene-strapped TADAPs (Cn-strapped TADAPs; n = 7, 8,
9) as nickel(II) complexes in the 20π (7), 19π (8), and 18π
(9) forms. Metal-templated cyclization of bis[1-chloro-9-(2,6-
dimethoxyphenyl)amino-5-mesityldipyrrin]−nickel(II) com-
plex 1 (mesityl = 2,4,6-trimethylphenyl) was performed in
the presence of K2CO3 in N,N-dimethylformamide (DMF) at
110 °C to produce 20π TADAP 2. O-Demethylation of 2 with
excess BBr3 in CH2Cl2 afforded meso-N-2,6-dihydroxyphenyl
derivative 3 with 19π electrons. O-Acylation of 3 with a
mixture of excess acetic anhydride and pyridine afforded meso-
N-2,6-diacetoxyphenyl derivative 4, a 19π TADAP radical
cation, which was then quantitatively converted to the 20π
TADAP 5 and the 18π TADAP dication 6 by treatment with
cobaltocene (CoCp2) and AgPF6, respectively. To increase its
solubility, 3 was oxidized to the 18π structure with AgPF6.
Subsequently, it was condensed with 2 equiv of nonanedioic
acid in the presence of N,N′-diisopropylcarbodiimide (DIC)
and 4-dimethylaminopyridine (DMAP) in tetrahydrofuran
(THF) to produce C7-strapped 20π TADAP 7-C7 that has
two −OOC(CH2)7COO− bridges above and below the
TADAP skeleton in 22% yield. Treatment of 7-C7 with
AgPF6 in CH2Cl2 yielded the 19π TADAP radical cation 8-C7,
which was further converted to the 18π TADAP dication 9-C7
by the sequential one-electron oxidation reaction with AgPF6.
C8-strapped and C9-strapped TADAPs (7/8/9-C8 and 7/8/
9-C9, respectively) were prepared from 3 and the correspond-
ing diacids by following a similar procedure (Scheme 2). All
18π TADAPs were reversibly converted to the corresponding
20π TADAPs by treatment with CoCp2 (checked by UV−vis
absorption spectroscopy). Both nonstrapped and strapped
TADAPs were isolated as air-stable solids by column
chromatography and/or recrystallization from the appropriate
solvents.34
1H NMR spectroscopy was used to evaluate the antiaromatic
and aromatic characters of the reversibly redox-switchable
TADAPs, P1 and P2. It was observed that the chemical shifts
of pyrrolic-β protons in P1 and P2 appeared at high and low
fields, respectively, and the difference between these chemical
shifts was approximately 4.0−5.0 ppm (for M = Ni). However,
care should be taken when using chemical shifts of the
peripheral protons as aromaticity indices because they are
associated not only with the ring-current effects but also with
the through-bond inductive/resonance effects. Presumably, the
extent of the additional through-bond effects would be lower
for the alkyl-chain straps located above and below a DAP ring.
Thus, the quantification of the magnitude of aromaticity, based
on the global ring-current effects arising from the 20π and 18π
DAP rings, would be more precise if spatially separated alkyl
chains are used as indicators.33 On the basis of this
presumption, we designed a series of novel TADAPs with
alkyl-chain straps connected to the meso-N-aryl substituents via
Scheme 3 illustrates the synthesis of the reference
porphyrins. The nickel(II) complex of 5,15-bis(2,6-dihydrox-
yphenyl)-10,20-dimesitylporphyrin 11 was prepared by the
metallation of freebase 1035 with Ni(OAc)2·4H2O. Non-
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Scheme 2. Synthesis of Strapped TADAPs
strapped 18π porphyrin 12 and strapped 18π porphyrins 13-
Cn (n = 7, 8, 9) were obtained by the acylation of 11 with
acetic anhydride and the corresponding diacid dichlorides,
respectively. Diphenyl diesters 14-Cn (n = 7, 8, 9; Figure 4)
were also prepared as references from the corresponding
aliphatic diacid chlorides and phenol (see the Experimental
Section).
Structures. The structures of the novel TADAPs and the
reference compounds were characterized by NMR and IR
spectroscopies and high-resolution electrospray ionization
(ESI) mass spectrometry. The ESI mass spectra revealed that
the base peaks corresponded to either the molecular ions (for
7-Cn) or the fragment cations (for 8-Cn and 9-Cn). The IR
spectra displayed the peaks corresponding to the CO
stretching bands of 7-Cn, 9-Cn, and 13-Cn at higher
frequencies (1758−1770 cm−1) than those of 14-Cn (1745−
1749 cm−1). X-ray crystallography was used to elucidate the
crystal structures of 7-C8 and 7-C9 although the quality of
their crystallographic data is not at a publishable level.36 The
currently available data (not reported herein) revealed the
following structural features. (i) Both 7-C8 and 7-C9 have flat
DAP π-planes, as observed for the previously reported 20π
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along with some selected orbital diagrams, are shown in
Figures S1−S4 in the Supporting Information (SI). The bond
lengths, bond angles, and torsion angles are summarized in
Tables S1 and S2 in the SI.
Scheme 3. Synthesis of Strapped Porphyrins
The structural information obtained by the DFT calculations
is as follows. (i) Regardless of the oxidation state, alkyl-chain
length, and molecular symmetry, the DAP rings in the strapped
derivatives are essentially flat. (ii) The in-plane C−C/C−N
bond lengths and C−Nmeso−C/C−Cmeso−C bond angles of 7-
Cn, 9-Cn, and 13-Cn are in good accordance with those of 5,
6, and 12, respectively. (iii) In the bridging alkyl chains, the
two adjacent methylene units adopt anti- or gauche-staggered
conformation. (iv) The carbonyl groups of the ortho-ester
linkages are twisted to maintain the staggered conformation of
the alkyl chains. This result is consistent with the previously
mentioned IR observations as the distortion of the ortho-ester
moieties may weaken the resonance interactions between the
carbonyl and phenoxy groups. (v) The meso-mesityl rings are
perpendicular to the DAP/porphyrin rings, whereas the meso-
2,6-diacyloxyphenyl groups in 6 and the C7-strapped
derivatives are inclined toward the rings. The C7-strapped
derivatives show the most significant inclination in each
oxidation state. (vi) Depending on the oxidation states and
charges of the porphyrin rings, the positions of the methylene
units differ slightly in 7-Cn, 9-Cn, and 13-Cn, possessing the
same alkyl-chain straps (Table S2 and Figure S5 in the SI).
These structural features are significantly related to the
magnitude of ring-current effects caused by the DAP/
porphyrin rings (vide infra).
Optical and Electrochemical Properties. The optical
and electrochemical properties of the TADAPs and the
reference porphyrins were investigated by ultraviolet/visible/
near-infrared (UV−vis−NIR) absorption spectroscopy (Figure
2) and cyclic voltammetry (Figure 3), and the data are
summarized in Table 1. The UV−vis−NIR spectra of the
strapped TADAPs 7-Cn, 8-Cn, and 9-Cn were similar to those
of the nonstrapped TADAPs 5, 4, and 6, respectively. The 20π
TADAPs 7-Cn exhibited two intense absorption bands with
the absorption maxima (λmax) of 443−444 and 522−523 nm,
whereas the 18π dications 9-Cn exhibited two intense bands
with λmax of 394−397 and 631−651 nm. The Q-like bands of
9-Cn were red-shifted by 1670−2100 cm−1 relative to the
corresponding Q bands of 13-Cn (λmax = 558 nm), indicating
that the optical HOMO−LUMO gaps of the 18π TADAP
dications were substantially smaller than those of the
isoelectronic porphyrins. The main characteristics of the
π−π* electronic excitations in the 20π, 19π, and 18π TADAPs
were previously assigned by the time-dependent DFT
calculations.30 The lowest-energy absorption bands of 8-C7
(SOMO−1-to-SOMO excitation; SOMO: singly occupied
molecular orbital) and 9-C7 (HOMO-to-LUMO excitation)
are slightly broader and red-shifted compared with those of 8-
C8/8-C9 and 9-C8/9-C9, respectively. Electrochemical data,
discussed in the following section, suggest that shorter alkyl
chains may perturb the orbital energies of the cationic DAP
rings, indicating that the excitation energies and spectral shapes
of the strapped TADAP cations are influenced by the alkyl-
chain lengths.
TADAP P1 (M = Ni; 5,15-Ar = p-tBuC6H4; 10,20-Ar =
mesityl).28 (ii) The meso-aryl rings are almost perpendicular to
the DAP ring, suggesting that there is an insignificant amount
of π-conjugative interaction between the meso-aryl groups and
the DAP ring. (iii) The O−CO units of the ortho-ester
groups are twisted away from the adjacent benzene rings. (iv)
The bridging alkyl chains have essentially anti- or gauche-
staggered conformation.
To further examine the structures of TADAPs 5−9 and
porphyrins 12 and 13, density functional theory (DFT)
calculations were performed at the B3LYP/6−311G(d,p) level
of theory. The structural parameters of 7-C8 and 7-C9
obtained by X-ray crystallographic analysis were used for
setting their initial structures. The optimized structures of 7-
C8 and 7-C9 were further used as models for examining the
C8- and C9-strapped derivatives. The structures of the C7-
strapped and nonstrapped derivatives were optimized by
various global symmetries (Ci, C2, and C1). Although several
metastable structures with small differences in energy (<1 kcal
mol−1) were predicted by DFT calculations, the bond
parameters of their porphyrin rings were almost identical to
each other. The optimized structures of 7-Cn with their point
groups are presented in Figure 1, and all of the structures,
Redox potentials of 2, 4, 7-Cn, 12, and 13-Cn were
measured in CH2Cl2 by cyclic voltammetry using Bu4NPF6 as
the supporting electrolyte. As shown in Figure 3, TADAPs 4
and 7-Cn displayed two reversible redox processes with a
difference in potentials of 0.64−0.87 V. The 20π/19π half-
Figure 1. Optimized structures of (a) 7-C7 (Ci), (b) 7-C8 (Ci), and
(c) 7-C9 (C1) calculated by the DFT method. Front views (left) and
side views (right).
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Table 1. Optical and Electrochemical Data for TADAPs and
a
Porphyrins
b
c
compd
λmax/nm (log ε)
E/V
2
4
5
445 (5.01), 531 (4.99)
380 (4.72), 439 (4.97), 903 (4.54)
448 (n.d.), 535 (n.d.)
−0.68, +0.08
−0.46, +0.19
n.d.
6
392 (4.81), 627 (4.51)
n.d.
7-C7
7-C8
7-C9
8-C7
8-C8
8-C9
9-C7
9-C8
9-C9
12
444 (5.00), 522 (4.97)
444 (5.14), 522 (4.95)
443 (4.96), 523 (4.94)
380 (4.61), 439 (4.90), 914 (4.37)
379 (4.61), 438 (4.88), 895 (4.50)
379 (4.60), 438 (4.87), 894 (4.52)
397 (n.d.), 651 (n.d.)
394 (n.d.), 634 (n.d.)
397 (n.d.), 631 (n.d.)
412 (5.57), 527 (4.43), 559 (4.05)
410 (5.55), 524 (4.42), 558 (4.12)
411 (5.51), 524 (4.40), 558 (4.08)
411 (5.61), 524 (4.45), 558 (4.01)
−0.46, +0.18
−0.46, +0.41
−0.47, +0.34
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
+0.60, +0.82
+0.62
+0.63
+0.60
d
de
,
13-C7
13-C8
13-C9
de
,
de
,
a
b
Measured in CH2Cl2. n.d., not determined. The values of λmax
>
c
350 nm and log ε > 4 are listed. Half-wave potentials (vs Fc/Fc+)
measured by cyclic voltammetry with Bu4NPF6 as the electrolyte.
d
e
The oxidation processes are listed. The quasi-reversible or
irreversible oxidation peaks were observed at Epa = ca. +1.2−1.35 V
vs Fc/Fc+.
have a negligible impact on the energies of the HOMO in their
20π DAP rings. However, it was observed that the 19π/18π
half-wave potentials differ from each other; the substantial
reduction potential of 9-C7 (E1/2 = +0.18 V) is similar to that
of 4 (E1/2 = +0.19 V), but those potentials are negatively
shifted compared with those of 9-C8 (E1/2 = +0.41 V) and 9-
C9 (E1/2 = +0.34 V). These findings suggest that the energy
levels of the LUMOs in 9-Cn are sensitive toward the alkyl-
chain length although the reason behind this has not been
clearly understood. In contrast, the influence of the alkyl-chain
length on the 18π/17π redox processes of porphyrins 12 and
13-Cn is almost insignificant (E1/2 = +0.60 to +0.63 V vs Fc/
Fc+).
Figure 2. Normalized UV−vis−NIR absorption spectra of (a) 4, 5, 6,
and 12, (b) 7-C7, 8-C7, 9-C7, 13-C7, (c) 7-C8, 8-C8, 9-C8, 13-C8,
and (d) 7-C9, 8-C9, 9-C9, 13-C9 in CH2Cl2.
Antiaromaticity and Aromaticity. TADAPs 7-Cn and 9-
Cn are the ideal candidates for identifying antiaromatic and
aromatic characters of the 20π and 18π porphyrinoids,
respectively, as the two unshared electron pairs on their
meso-nitrogen atoms are involved in their π-networks. As
mentioned earlier, precise information regarding the ring
currents above and below the DAP π-planes can be obtained
by analyzing the chemical shifts of the methylene protons in
the bridging alkyl chains of 7/9. The assignment of the CH2
signals was verified by H−1H COSY. The structures and
1
numbering of the CH2 units of TADAPs 7/9, porphyrins 13,
and the reference diesters 14 are illustrated in Figure 4. The 1H
NMR spectra of the C8-strapped derivatives are shown in
Figure 5, and those of the C7- and C9-strapped derivatives are
shown in Figures S6 and S7, respectively, in the SI. On an
average, 7, 9, and 13 have symmetric structures, as indicated by
their NMR spectra. They exhibited four (for 7/9/13-C7 and
7/9/13-C8) or five (for 7/9/13-C9) signals corresponding to
the bridging methylene protons and two signals corresponding
to the pyrrolic-β protons, implying that the conformational
changes of their alkyl chains are rapid on the NMR time scale.
The chemical shifts (δ) of the bridging methylene and pyrrolic-
Figure 3. Cyclic voltammograms of 4, 7-C7, 7-C8, and 7-C9.
Measured in CH2Cl2 with Bu4NPF6 as a supporting electrolyte. Scan
rate was 60 mV s−1.
wave potentials of 4, 7-C7, 7-C8, and 7-C9 were almost
identical to each other (E1/2 = −0.46 to −0.47 V vs ferrocene/
ferrocenium; Fc/Fc+), indicating that the alkyl-chain straps
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Figure 4. Partial structures of (a) 7, 8, and 9 (z denotes the charge number) and (b) 13. Ar1 = mesityl. (c) Structures of 14.
β protons are detailed in Table S4 in the SI. The signals of the
pyrrolic-β protons (f and g) of 7-Cn appeared at 3.59−3.61
and 4.50−4.52 ppm, whereas those of 9-Cn appeared at 8.77−
8.86 and 8.58−8.69 ppm, respectively. The high-field and low-
field appearances of these peripheral protons confirm the
paratropic (20π) and diatropic (18π) ring-current effects in 7
and 9, respectively. The differences between the chemical shifts
(Δδ) of the methylene-proton signals (a−e) of TADAPs/
porphyrins (7-Cn, 9-Cn, and 13-Cn) and those of the
corresponding reference diesters 14-Cn are plotted in Figure
6. As expected, the Δδ values are mainly related to the global
paratropic and diatropic ring-current effects. An analysis of the
plot reveals the following information. (i) The absolute Δδ
values observed for the C8- and C9-strapped derivatives
increase in the order a < b < c < d (<e), indicating that the
inner methylene protons experience stronger shielding/
deshielding effects arising from the ring currents. (ii) The
absolute Δδ values of the C7-strapped TADAPs show irregular
orders, a < c < b < d for 7-C7 and a < c < d < b for 9-C7. These
results may reflect the low flexibility of the bridging C7-alkyl
chains. (iii) The absolute Δδ values of the central methylene
units of the C7-strapped derivatives (d) are larger than those of
the C9-strapped derivatives (e) for all three π-electron systems.
(iv) For most methylene protons, the absolute Δδ values of the
TADAP dications 9-Cn are smaller than those of the
corresponding porphyrin counterparts 13-Cn, thus reflecting
the difference in the net charge between these two 18π-
electron systems.
To further investigate the ring-current effects, the nucleus-
independent chemical shifts (NICS)37−41 were evaluated at 73
× 73 lattice positions (from −3.6 to +3.6 Å for x- and y-axes
with a pitch of 0.1 Å and from 1 to 5 Å for the z-axis with a
pitch of 1 Å) above the π-planes of the zinc(II) complexes of
5,15-dihydro-5,15-diazaporphyrinoids (DHDAP) 15 (20π)
and 16 (18π dication) and the 18π porphine 17. These
compounds were selected to exclude the excessive electronic
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antiaromatic molecules, respectively.42,43 The NICS(n)zz maps
provided the following information on the ring-current effects
observed in 7, 9, and 13. (1) The absolute NICS(n)zz values
of 15, 16, and 17 gradually decrease with an increase in the
vertical distance from the π plane and horizontal distance from
the central axis, thus qualitatively supporting the experimental
results. (2) The contour maps of 15 indicate that the global
paratropic ring currents are dominant around the central axis at
heights of 2−5 Å, where some bridging methylene protons may
be present. For 15, the local diatropic ring currents are merged
above the four pyrrole rings. (3) The absolute NICS(n)zz
values of 16 are considerably smaller than those of 17 at each
height (n = 1−5), suggesting that the neutral porphyrin ring in
13 exhibits larger diatropic ring-current effects on the bridging
alkyl chains than those of the cationic TADAP ring in 9. This
explicitly reflects a difference in the π-electron density between
17 (13) and 16 (9).44,45 (4) The irregular orders observed for
7-C7 and 9-C7 (Figure 6) are noteworthy. The absolute Δδ
values of CH2-b for these C7-strapped derivatives are
considerably larger than those of the others, suggesting that
the CH2-b units retain their average positions near the DAP
rings (Figure S5 in the SI). This is probably because the
relatively short alkyl chains cause them to be less flexible. Thus,
it can be concluded that the alkyl-chain straps can be used as
promising indicators for the experimental identification of the
paratropic and diatropic ring currents arising from the 20π-
and 18π-electron systems, respectively, in DAP/porphyrin
rings.
EPR and NMR spectra of 19π TADAPs. Electron
paramagnetic resonance (EPR) spectroscopy revealed that an
unshared electron spin of TADAP radical cation 8 was
completely delocalized over the DAP ring (Figure S9 in the
SI). The g values and the hyperfine coupling (hfc) constants of
8-C8 are very similar to those of 4, and the calculated spin
distribution of 8-C8 supports the observed fine structure. The
g and hfc values of 8-C7 and 8-C9 are almost identical to those
of 8-C8. These results indicate that the alkyl-chain straps have
little influence on the delocalization of the unshared electron
spin of the strapped 19π TADAP radical.
Figure 5. 1H NMR spectra of (a) 7-C8 in C6D6, (b) 9-C8 in CD2Cl2,
and (c) 13-C8 in CD2Cl2. Asterisks indicate residual solvent peaks.
Assignments of a−d are indicated in Figure 4.
In general, NMR spectroscopy is not used for evaluating the
ring-current effects of porphyrin π-radicals because a strong
nuclear−electron spin−spin coupling occurs, which induces
fast electronic relaxations and causes significant line-broad-
ening of the peripheral proton peaks.46 This behavior was
observed for P3, which did not show any detectable NMR
1
signals for the pyrrolic-β protons. In contrast, the H NMR
spectra of 8-Cn exhibited broadened signals in the range of 1−
2.6 ppm due to the bridging methylene protons (Figure 8).
Other than these, no assignable 1H NMR signals were
detected. Therefore, it is likely that compared with the
pyrrolic-β and meso-aryl protons, the spatially separated alkyl-
strap methylene protons of 8-Cn experience a considerably
weaker ring-current effect due to the electron−nucleus spin−
spin interactions. It is apparent that the 19π DAP rings in 8-Cn
produce negligible ring currents, and thus, they do not alter the
chemical shifts of the bridging methylene protons. These
findings demonstrate that the strapped TADAPs can also be
used as probes to experimentally evaluate the ring-current
effects caused by porphyrin-based 19π-electron systems.
Figure 6. Differences in 1H NMR chemical shifts (Δδ) of the bridging
CH2 protons of 7, 9, and 13 vs 14. For the numbering of the CH2
groups (a−e), see Figure 4.
effects caused by a nickel atom from the NICS calculations.
The isotropic NICS(n) and their out-of-plane components,
NICS(n)zz, are plotted as contour maps in Figure S8 in the SI
and Figure 7, respectively. The NICS-scan method reported
herein was found to be significantly more coherent than the
evaluation using a single NICS value for identifying
diamagnetic and paramagnetic ring currents of aromatic and
CONCLUSIONS
■
Doubly strapped TADAPs in the three different oxidation
states, 18π, 19π, and 20π, were successfully designed and
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Figure 7. Contour maps of the NICS(n)zz values (n = 1, 2, 3, 4, 5) at the 73 × 73 lattice positions (lattice pitch = 0.1 Å for x- and y-axes; the zinc
center is placed at x = y = 0) calculated by the Hartree−Fock method: (a) 15 (D2h), (b) 16 (D2h), and (c) 17 (D4h).
strapped 18π porphyrins were prepared. The DFT calculations,
as well as the preliminary X-ray crystallographic analyses,
revealed that regardless of the oxidation states, all strapped
TADAPs possess flat π-planes. The bridging alkyl chains adopt
staggered conformations, and depending on the number of the
methylene units, these chains occupy different spaces above
and below the porphyrin π-plane. The flexibility of the straps
may be somewhat restricted in the cationic TADAPs with
short alkyl-chain straps. NMR spectroscopy of the strapped
TADAPs clearly confirmed the antiaromatic and aromatic
characters of 20π and 18π TADAPs, respectively. Both the
paratropic and diatropic ring-current effects were experimen-
tally evaluated based on the differences in chemical shifts (Δδ)
of the bridging methylene protons, which show an insignificant
amount of through-bond electronic effects. The observed Δδ
values implied that the ring-current effects decreased with
increase in separation from the porphyrin π-plane and central
axis of the porphyrin ring. This was in accordance with the
NICS(n)zz values calculated for their bare models, 20π and
18π 5,15-dihydro-5,15-diazaporphyrinoids and 18π porphine.
It is also noteworthy that the charge, i.e., the π-electron
densities of the 18π porphyrin rings, impacted the ring
currents; the TADAP dications generated weaker diatropic
ring currents than those generated by the porphyrin counter-
Figure 8. 1H NMR spectra of (a) 8-C7 in CDCl3, (b) 8-C8 in
CD2Cl2, and (c) 8-C9 in CDCl3. Asterisks indicate residual solvent
peaks.
synthesized as platforms for the identification of the ring
currents arising from these porphyrinoids. In addition,
reference compounds, nonstrapped TADAPs, and similarly
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136.2, 133.3, 129.7, 127.2, 116.6, 104.8, 102.1, 79.7, 55.6, 21.0, 19.4.
HRMS (ESI) m/z: [M]+ calcd for C52H48N6NiO4 878.3091; found
878.3093. UV−vis (CH2Cl2): λmax (log ε) 445 (5.01), 531 nm (4.99).
Synthesis of 3. To a solution of 2 (193 mg, 0.220 mmol) in
CH2Cl2 was added BBr3 (7.6 mL, 7.6 mmol), and the mixture was
stirred for 16 h at room temperature. After being quenched with
water, the aqueous layer was extracted with CH2Cl2, and the
combined organic extracts were washed with an aqueous NaHCO3
solution, dried over Na2SO4, and evaporated under reduced pressure.
The residue was then reprecipitated from MeOH−Et2O to give crude
3 as a green solid (190 mg). Although we did not characterize the
counteranion of 3, bromide or bicarbonate is plausible. In the
following experiments, the molar quantity of 3 was calculated
assuming that the counteranion is bromide. Compound 3 was kept
as a solid until the following condensation reactions; HRMS (ESI) m/
z: [M − X]+ calcd for C48H40N6NiO4 822.2459; found 822.2457.
UV−vis−NIR (CH2Cl2): λmax (relative intensity) 388 (1), 443 (0.90),
parts. This paper provides detailed experimental information
on the ring-current effects arising from flat 20π and 18π
porphyrins, which have the same π-conjugation pathway.
Further studies on the development of three-dimensionally
functionalized diazaporphyrinoids are being conducted.
EXPERIMENTAL SECTION
General Remarks. All melting points were recorded on a micro
melting point apparatus and are uncorrected. NMR spectra were
■
1
recorded on 700 MHz and/or 400 MHz spectrometers. The H and
13C chemical shifts are reported in ppm as relative values vs
tetramethylsilane (in CDCl3 and CD2Cl2) or a solvent residual signal
(δH 7.16 ppm in C6D6), and the 31P chemical shifts are reported in
ppm vs H3PO4. High-resolution mass spectra (HRMS) were
measured using an electron spray ionization (ESI)−quadrupole
method. UV−vis−NIR absorption spectra were recorded in the
range of 300−1000 nm. For some compounds, the relative intensities
are reported instead of molar extinction coefficients because a trace
amount of oxidized/reduced species was generated during the
measurement of sample solutions. IR spectra were obtained using
an attenuated total reflection (ATR) method. Electrochemical
measurements were performed using a glassy carbon working
electrode, a platinum wire counter electrode, and a Ag/Ag+ [0.01
M AgNO3, 0.1 M Bu4NPF6 (MeCN)] reference electrode. Chemicals
and solvents were of reagent grade quality and used without further
purification unless otherwise noted. Thin-layer chromatography and
preparative column chromatography were performed using silica gel
(neutral). All reactions were performed under an argon or nitrogen
atmosphere unless otherwise noted.
Synthesis and Characterization of New Compounds. Syn-
thesis of 1. (i) A mixture of 1,9-dichloro-5-mesityldipyrromethene
(908 mg, 2.74 mmol), 2,6-dimethoxyaniline (1.01 g, 6.58 mmol), and
MeCN (30 mL) was stirred for 19 h at room temperature. After being
quenched with water, the aqueous layer was extracted with CH2Cl2,
and the combined organic extracts were dried over Na2SO4 and
evaporated under reduced pressure. The residue was subjected to
silica-gel column chromatography (hexane/AcOEt = 5/1). The
fraction of Rf = 0.4 (hexane/AcOEt = 5/1) was collected and
evaporated to give 1-chloro-9-(2,6-dimethoxyphenyl)amino-5-mesi-
tyldipyrromethene 18 (the structure is shown in Figure S10 in the SI)
as a red solid (1.267 g, 100%). Mp 82−84 °C. 1H NMR (CDCl3, 400
MHz): δ 7.18 (d, 1H, J = 8.4 Hz), 6.89 (s, 2H), 6.68 (d, 2H, J = 8.4
Hz), 6.58 (s, 1H), 6.44 (d, 1H, J = 4.6 Hz), 6.04 (d, 1H, J = 4.6 Hz),
5.95 (d, 1H, J = 4.0 Hz), 5.83 (d, 1H, J = 4.0 Hz), 3.91 (s, 6H), 2.33
(s, 3H), 2.10 (s, 6H). The pyrrolic NH proton was not clearly
observed. 13C{1H} NMR (CDCl3, 100 MHz): δ 163.1, 153.8, 144.3,
137.6, 136.9, 134.2, 133.3, 133.0, 128.7, 127.7, 125.9, 123.1, 117.2,
117.1, 116.4, 109.1, 104.3, 55.9, 21.1, 20.0. HRMS (ESI) m/z: [M +
H]+ calcd for C26H27ClN3O2 448.1792; found 448.1774. (ii) A
mixture of 18 (392 mg, 1.18 mmol), Ni(OAc)2·4H2O (110 mg, 0.444
mmol), MeOH (10 mL), and CH2Cl2 (10 mL) was stirred for 16 h at
room temperature. The mixture was concentrated under reduced
pressure to leave a solid residue, which was reprecipitated from
MeOH to give 1 as a gray solid (360 mg, 86%). Rf = 0.4 (hexane/
AcOEt = 5/1). Mp 168−170 °C. HRMS (ESI): m/z [M]+ calcd for
C52H50Cl2N6NiO4 950.2619; found 950.2634. The 1H and 13C NMR
data/spectra of 1 are not reported because of its paramagnetic
character.
1
880 nm (0.30). The H and 13C NMR data/spectra of 3 are not
reported because of its paramagnetic character.
Synthesis of 4. A mixture of 3 (31 mg, ca. 0.034 mmol), acetic
anhydride (0.10 mL, 1.1 mmol), pyridine (50 μL, 0.96 mmol), and
CH2Cl2 (15 mL) was stirred at room temperature. After 27 h, an
aqueous 1 M HCl solution was added, and the separated organic
phase was washed with an aqueous NaHCO3 solution. To the washed
organic phase was added an aqueous KPF6 solution, and the resulting
mixture was vigorously stirred for 20 min. The organic phase was
separated, dried over Na2SO4, and evaporated under reduced
pressure. The residue was subjected to silica-gel column chromatog-
raphy (CH2Cl2/MeOH = 40/1). The fraction of Rf = 0.4 (CH2Cl2/
MeOH = 20/1) was collected and evaporated to leave a solid residue,
which was reprecipitated from hexane−CH2Cl2 to give 4 as a gray
solid (20 mg, 48%). Mp > 300 °C. 31P{1H} NMR (CDCl3, 162
MHz): δ −143.7 (septet, JP‑F = 716.2 Hz). HRMS (ESI) m/z: [M −
PF6]+ calcd for C56H48N6NiO8 990.2882; found 990.2879. IR (ATR):
ν 1778 (CO), 838 (P−F) cm−1. UV−vis−NIR (CH2Cl2): λmax
(log ε) 380 (4.72), 439 (4.97), 903 nm (4.54). The 1H and 13C NMR
data/spectra of 4 are not reported because of its paramagnetic
character.
Synthesis of 5. A mixture of 4 (6.7 mg, 5.3 μmol), cobaltocene
(1.0 mg, 5.4 μmol), and toluene (3 mL) in a 50 mL flask was
sonicated for 5 min at room temperature. The resulting mixture was
filtered through a thin silica-gel bed, and the filtrate was concentrated
under reduced pressure. The solid residue was reprecipitated from
hexane to give 5 as an orange solid (4.5 mg, 86%). Mp > 300 °C. 1H
NMR (C6D6, 400 MHz): δ 6.80 (d, 4H, J = 8.3 Hz), 6.65 (t, 2H, J =
8.3 Hz), 6.52 (s, 4H), 4.77 (d, 4H, J = 4.4 Hz), 3.86 (d, 4H, J = 4.4
Hz), 2.43 (s, 12H), 2.30 (s, 12H), 1.99 (s, 6H). Prolonged standing of
5 in solution caused the generation of a small amount of the 19π
radical, which hampered in obtaining the high-quality 13C NMR
spectrum of 5. HRMS (ESI) m/z: [M]+ calcd for C56H48N6NiO8
990.2887; found 990.2887. IR (ATR): ν 1769 (CO) cm−1. UV−vis
(CH2Cl2): λmax (relative intensity) 448 (1), 535 nm (0.79).
Synthesis of 6. A mixture of 4 (4.8 mg, 4.2 μmol), AgPF6 (1.0 mg,
4.0 μmol), and CH2Cl2 (10 mL) was stirred for 5 min at room
temperature. The resulting mixture was filtered through a thin silica-
gel bed, and the filtrate was concentrated under reduced pressure. The
solid residue was reprecipitated from hexane to give 6 as a green solid
1
(4.2 mg, 78%). Mp > 300 °C. H NMR (CD2Cl2, 400 MHz): δ 8.67
(d, 4H, J = 5.2 Hz), 8.39 (d, 4H, J = 5.2 Hz), 8.11 (t, 2H, J = 8.4 Hz),
7.67 (d, 4H, J = 8.4 Hz), 7.28 (s, 4H), 2.56 (s, 6H), 1.87 (s, 12H),
1.49 (s, 12H). Prolonged standing of 6 in solution caused the
generation of a small amount of the 19π radical, which hampered in
obtaining the high-quality 13C NMR spectrum of 5. HRMS (ESI) m/
z: [M − 2 PF6]z+ calcd for C56H48N6NiO8 990.2882 (z = 1), 495.1438
(z = 2); found 990.2882 (z = 1), 495.1437 (z = 2). IR (ATR): ν 1777
(CO), 834 (P−F) cm−1. UV−vis (CH2Cl2): λmax (log ε) 392
(4.81), 627 nm (4.51).
Synthesis of 2. A mixture of 1 (351 mg, 0.368 mmol), K2CO3
(1.05 g, 7.57 mmol), and DMF (20 mL) was stirred for 31 h 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 reprecipitated from CH2Cl2−MeOH to give 2 as a
reddish-brown solid (245 mg, 76%). Rf = 0.3 (hexane/AcOEt = 5/1).
Mp > 300 °C. 1H NMR (C6D6, 400 MHz): δ 6.78 (t, 2H, J = 8.4 Hz),
6.52 (s, 4H), 6.04 (d, 4H, J = 8.4 Hz), 4.85 (d, 4H, J = 4.4 Hz), 3.78
(d, 4H, J = 4.4 Hz), 3.30 (s, 12H), 2.34 (s, 12H), 1.93 (s, 6H).
13C{1H} NMR (C6D6, 100 MHz): δ 160.8, 158.0, 140.5, 137.0, 136.5,
Synthesis of 7. Typical procedure: To a CH2Cl2 solution of 3 (16.7
mg, ca. 0.019 mmol) was added AgPF6 (3.8 mg, 0.015 mmol), and the
resulting solution was filtered through a thin Celite bed. The
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formation of an 18π species was confirmed by HRMS spectrometry
[m/z [M − 2 PF6]z+ calcd for C48H40N6NiO4 822.2459 (z = 1),
411.1227 (z = 2); found 822.2457 (z = 1), 411.1227 (z = 2)] and
UV−vis absorption spectroscopy (λmax = 386 and 641 nm in CH2Cl2).
The filtrate was concentrated in vacuo, and the residue was dissolved
in THF (50 mL), followed by addition of decanedioic acid (7.2 mg,
0.038 mmol), N,N′-diisopropylcarbodiimide (DIC; 0.12 mL, 0.078
mmol), and 4-dimethylaminopyridine (DMAP; 11 mg, 0.090 mmol).
The resulting solution was stirred for 91 h at room temperature. The
mixture was filtered through a paper filter, and the filtrate was
concentrated under reduced pressure. The residue was dissolved in
toluene, and the resulting solution was washed with brine several
times and then concentrated under reduced pressure. The residue was
subjected to silica-gel column chromatography (hexane/AcOEt = 5/
1). The fraction of Rf = 0.3 (hexane/AcOEt = 5/1) was collected and
evaporated to give 7-C8 as an orange solid (5.0 mg, 22%). 7-C7 and
7-C9 were similarly prepared by condensation reactions of 3 with
nonanedioic acid and undecanedioic acid, respectively. 7-C7: Orange
1182.4760; found 1182.4757. IR (ATR): ν 1771 (CO), 838 (P−F)
cm−1. UV−vis−NIR (CH2Cl2): λmax (log ε) 379 (4.60), 438 (4.87),
894 nm (4.52). The 1H NMR spectra (400 MHz) of 8-Cn are shown
in Figure 8.
Synthesis of 9. Typical procedure: A mixture of 8-C7 (5.0 mg, 4.0
μmol), AgPF6 (1.1 mg, 4.4 mmol), and CH2Cl2 (1 mL) was stirred
for 2 min at room temperature. The mixture was passed through a
thin silica-gel bed, and the eluent was concentrated under reduced
pressure. The residue was reprecipitated from CH2Cl2−hexane to give
9-C7 as a green solid (4.6 mg, 83%). 9-C8 and 9-C9 were similarly
prepared from 8-C8 and 8-C9, respectively. 9-C8 could also be
prepared by oxidation of 7-C8 with two equivalents of AgPF6. 9-C7:
1
Mp > 300 °C. H NMR (CD2Cl2, 700 MHz): δ 8.85 (d, 4H, J = 4.8
Hz), 8.69 (d, 4H, J = 4.8 Hz), 8.26 (t, 2H, J = 8.2 Hz), 7.80 (d, 4H, J
= 8.2 Hz), 7.38 (s, 4H), 2.61 (s, 6H), 1.89 (s, 12H), 1.76 (t, 8H, J =
7.6 Hz), 0.26 (tt, 8H, J = 7.6, 7.6 Hz), −0.51 (quint, 4H, J = 7.6 Hz),
−1.60 to −1.70 (m, 8H). HRMS (ESI) m/z: [M − 2 PF6 + H2O]z+
calcd for C66H66N6NiO9 1126.4134 (z = 1), 572.7156 (z = 2); found
1126.4126 (z = 1), 572.7145 (z = 2). IR (ATR): ν 1767 (CO), 837
(P−F) cm−1. UV−vis (CH2Cl2): λmax (relative intensity) 397 (1), 651
1
solid, 8.4 mg (20%). Mp > 300 °C. H NMR (C6D6, 400 MHz): δ
6.80 (d, 4H, J = 8.3 Hz), 6.61 (t, 2H, J = 8.3 Hz), 6.50 (s, 4H), 5.72
(quint, 4H, J = 7.8 Hz), 4.52 (d, 4H, J = 4.6 Hz), 3.94 (tt, 8H, J = 8.2,
7.8 Hz), 3.59 (d, 4H, J = 4.6 Hz), 3.36 (tt, 8H, J = 8.2, 7.8 Hz), 3.11
(t, 8H, J = 8.0 Hz), 2.37 (s, 12H), 1.96 (s, 6H). HRMS (ESI) m/z:
[M]+ calcd for C66H64N6NiO8 1126.4139; found 1126.4135. IR
(ATR) ν 1769 (CO) cm−1. UV−vis (CH2Cl2): λmax (log ε) 444
1
nm (0.45). 9-C8: Green solid, 3.7 mg (63%). Mp 252−254 °C. H
NMR (CD2Cl2, 400 MHz): δ 8.86 (d, 4H, J = 5.2 Hz), 8.63 (d, 4H, J
= 5.2 Hz), 8.21 (t, 2H, J = 8.4 Hz), 7.75 (d, 4H, J = 8.4 Hz), 7.34 (s,
4H), 2.58 (s, 6H), 1.92 (t, 8H, J = 7.2 Hz), 1.83 (s, 12H), 0.06 (tt,
8H, J = 7.6, 7.2 Hz), −0.61 to −0.64 (m, 8H), −0.73 to −0.82 (m,
1
(5.00), 522 nm (4.97). 7-C8: Mp > 300 °C. H NMR (C6D6, 400
8H). 31P{1H} NMR (CDCl3, 162 MHz): δ −144.9 (septet, JP‑F
=
MHz): δ 6.74 (d, 4H, J = 7.6 Hz), 6.63 (t, 2H, J = 7.6 Hz), 6.51 (s,
4H), 4.75−4.65 (m, 8H), 4.50 (d, 4H, J = 4.4 Hz), 3.60 (d, 4H, J =
4.4 Hz), 3.41−3.26 (m, 8H), 3.15 (tt, 8H, J = 7.7, 7.1 Hz), 2.85 (t,
8H, J = 6.8 Hz), 2.39 (s, 12H), 1.97 (s, 6H). HRMS (ESI) m/z: [M]+
calcd for C68H68N6NiO8 1154.4447; found 1154.4436. IR (ATR): ν
1766 (CO) cm−1. UV−vis (CH2Cl2): λmax (log ε) 444 (5.14), 522
708.3 Hz). HRMS (ESI) m/z: [M − 2 PF6]z+ calcd for
C68H68N6NiO8 1154.4447 (z = 1), 577.2221 (z = 2); found
1154.4456 (z = 1), 577.2207 (z = 2). IR (ATR): ν 1769 (CO),
837 (P−F) cm−1. UV−vis (CH2Cl2): λmax (relative intensity) 394 (1),
1
634 nm (0.46). 9-C9: Green solid, 4.2 mg (86%). Mp > 300 °C. H
NMR (CD2Cl2, 700 MHz): δ 8.77 (d, 4H, J = 5.3 Hz), 8.58 (d, 4H, J
= 5.3 Hz), 8.15 (t, 2H, J = 8.6 Hz), 7.69 (d, 4H, J = 8.6 Hz), 7.32 (s,
4H), 2.58 (s, 6H), 1.89 (t, 8H, J = 7.0 Hz), 1.83 (s, 12H), 0.82 (tt,
8H, J = 7.7, 7.0 Hz), −0.14 (tt, 8H, J = 7.7, 7.7 Hz), −0.24 (tt, 8H, J =
7.7, 7.7 Hz), −0.29 (quint, 4H, J = 7.7 Hz). HRMS (ESI) m/z: [M −
2 PF6]z+ calcd for C70H72N6NiO8 1182.4760 (z = 1), 592.2455 (z =
2); found 1182.4757 (z = 1), 593.2371 (z = 2). IR (ATR): ν 1770
(CO), 835 (P−F) cm−1. UV−vis (CH2Cl2): λmax (relative
intensity) 397 (1), 631 nm (0.56). Prolonged standing of 9-Cn (n
= 7, 8, 9) in solution caused the generation of a small amount of 19π
radicals, which hampered in obtaining high-quality 13C NMR spectra
of 9-Cn.
1
nm (4.95). 7-C9: Orange solid, 6.9 mg (12%). Mp > 300 °C. H
NMR (C6D6, 400 MHz): δ 6.80 (d, 4H, J = 8.3 Hz), 6.64 (t, 2H, J =
8.3 Hz), 6.51 (s, 4H), 4.50 (d, 4H, J = 4.2 Hz), 4.23 (quint, 4H, J =
7.9 Hz), 3.61 (d, 4H, J = 4.2 Hz), 3.51 (tt, 8H, J = 7.9, 7.0 Hz), 2.96
(t, 8H, J = 7.0 Hz), 2.88 (tt, 8H, J = 7.7, 7.0 Hz), 2.66 (tt, 8H, J = 7.7,
7.0 Hz), 2.36 (s, 12H), 1.97 (s, 6H). HRMS (ESI) m/z: [M]+ calcd
for C70H72N6NiO8 1182.4765; found 1182.4757. IR (ATR): ν 1766
(CO) cm−1. UV−vis (CH2Cl2): λmax (log ε) 443 (4.96), 523 nm
(4.94). Prolonged standing of 7-Cn (n = 7, 8, 9) in solution caused
the generation of a small amount of 19π radicals, which hampered in
obtaining high-quality 13C NMR spectra of 7-Cn.
Synthesis of 8. Typical procedure: A mixture of 7-C7 (11.2 mg,
0.0105 mmol), AgPF6 (2.5 mg, 0.010 mmol), and CH2Cl2 (6 mL)
was stirred for 2 min at room temperature. The mixture was
concentrated under reduced pressure. The residue was subjected to
silica-gel column chromatography (CH2Cl2/MeOH = 40/1). The
fraction of Rf = 0.3 (CH2Cl2/MeOH = 20/1) was collected and
evaporated to give 8-C7 as a dark-greenish-yellow solid (6.9 mg,
51%). 8-C8 and 8-C9 were similarly prepared from 7-C8 and 7-C9,
Synthesis of 10. This compound was prepared from 2,6-
dimethoxybenzaldehyde and pyrrole according to the reported
procedure.35 1H NMR (CDCl3, 400 MHz): δ 8.92 (d, 4H, J = 4.6
Hz), 8.79 (d, 4H, J = 4.6 Hz), 7.63 (t, 2H, J = 8.4 Hz), 7.30 (s, 4H),
7.00 (d, 4H, J = 8.4 Hz), 4.70 (s, 4H), 2.64 (s, 6H), 1.84 (s, 12H),
−2.61 (br-s, 2H).
Synthesis of 11. A mixture of 10 (110 mg, 0.144 mmol),
Ni(OAc)2·4H2O (111 mg, 0.444 mmol), and DMF (5 mL) was
stirred for 19 h at 140 °C. After being quenched with water, the
precipitated solid was filtered, and the residue was washed with water
(100 mL × 8) and dissolved in CH2Cl2, and the solution was
evaporated under reduced pressure. The residue was then
reprecipitated from CH2Cl2−hexane to give 11 as a pink solid (104
1
respectively. 8-C7: Mp > 300 °C. H NMR (CDCl3, 400 MHz): δ
1
2.7−2.3 (br-s, 8H), 2.0−1.2 (br-s, 20H). The other H signals were
not clearly detected. 31P{1H} NMR (CDCl3, 162 MHz): δ −144.3
(septet, JP‑F = 712.2 Hz). HRMS (ESI) m/z: [M − PF6]+ calcd for
C66H64N6NiO8 1126.4134; found 1126.4126. IR (ATR): ν 1771
(CO), 839 (P−F) cm−1. UV−vis−NIR (CH2Cl2): λmax (log ε) 380
(4.61), 439 (4.90) 914 nm (4.37). 8-C8: Dark-greenish-yellow solid,
1
mg, 88%). Mp > 300 °C. H NMR (CDCl3, 400 MHz): δ 8.82 (d,
4H, J = 5.0 Hz), 8.69 (d, 4H, J = 5.0 Hz), 7.56 (t, 2H, J = 8.0 Hz),
7.23 (s, 4H), 6.93 (d, 4H, J = 8.0 Hz), 4.67 (br-s, 4H), 2.59 (s, 6H),
1.81 (s, 12H). 13C{1H} NMR (CDCl3, 100 MHz): δ 155.9, 143.6,
143.1, 138.8, 138.1, 136.3, 133.1, 131.9, 131.1, 127.9, 118.6, 114.3,
107.8, 104.4, 21.4. HRMS (ESI) m/z: [M]+ calcd for C50H40N4NiO4
818.2398; found 818.2403. UV−vis (CH2Cl2): λmax (log ε) 411
(5.53), 525 (4.54), 556 nm (4.08).
1
3.4 mg (60%). Mp > 300 °C. H NMR (CD2Cl2, 400 MHz): δ 2.6−
2.4 (br-s, 8H), 2.1−1.9 (br-s, 8H), 1.8−1.6 (br-s, 8H), 1.5−1.4 (br-s,
1
8H). The other H signals were not clearly detected. 31P{1H} NMR
(CDCl3, 162 MHz): δ −144.4 (septet, JP‑F = 712.8 Hz). HRMS (ESI)
m/z: [M − PF6]+ calcd for C68H68N6NiO8 1154.4447; found
1154.4436. IR (ATR): ν 1766 (CO), 837 (P−F) cm−1. UV−
vis−NIR (CH2Cl2): λmax (log ε) 379 (4.61), 438 (4.88), 895 nm
(4.50). 8-C9: Dark-greenish-yellow solid, 7.7 mg (74%). Mp > 300
°C. 1H NMR (CDCl3, 400 MHz): δ 2.7−2.3 (br-s, 8H), 2.1−1.9 (br-
Synthesis of 12. A mixture of 11 (30.0 mg, 0.0392 mmol), acetic
anhydride (0.10 mL, 1.1 mmol), pyridine (50 μL, 0.96 mmol), and
CH2Cl2 (3 mL) was stirred for 24 h at room temperature. After being
quenched with water, the aqueous layer was extracted with CH2Cl2,
and the combined organic extracts were subsequently washed with
aqueous HCl and NaHCO3 solutions, dried over Na2SO4, and
1
s, 4H), 1.8−1.2 (br-s, 24H). The other H signals were not clearly
detected. 31P{1H} NMR (CDCl3, 162 MHz): δ −144.3 (septet, JP‑F
=
712.2 Hz). HRMS (ESI) m/z: [M − PF6]+ calcd for C70H72N6NiO8
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evaporated under reduced pressure. The residue was subjected to
silica-gel column chromatography (hexane/EtOAc = 10/1). The
fraction of Rf = 0.3 (hexane/EtOAc = 10/1) was collected and
evaporated to leave a solid residue, which was reprecipitated from
hexane−CH2Cl2 to give 12 as a purple solid (27.6 mg, 71%). Mp >
triethylamine (0.40 mL, 2.9 mmol), and CH2Cl2 (35 mL) was stirred
for 16 h at room temperature. After being quenched with ice-water,
the aqueous layer was extracted with Et2O, 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 = 5/1). The fraction of Rf =
0.6 (hexane/AcOEt = 5/1) was collected and evaporated to give 14-
C8 as a colorless solid (78 mg, 45%). 14-C7 and 14-C9 were similarly
prepared by reactions of phenol with nonanedioyl dichloride and
undecanedioyl dichloride, respectively. 14-C7: Colorless solid. 78.1
1
300 °C. H NMR (CDCl3, 400 MHz): δ 8.65 (d, 4H, J = 4.8 Hz),
8.58 (d, 4H, J = 4.8 Hz), 7.82 (t, 2H, J = 8.3 Hz), 7.46 (d, 4H, J = 8.3
Hz), 7.22 (s, 4H), 2.59 (s, 6H), 1.79 (s, 12H), 1.14 (s, 12H).
13C{1H} NMR (CDCl3, 100 MHz): δ 168.6, 151.3, 143.2, 143.0,
138.7, 137.9, 137.1, 131.6, 131.1, 129.7, 127.8, 127.5, 120.5, 117.6,
108.2, 21.4, 21.3, 20.1. HRMS (ESI) m/z: [M]+ calcd for
C58H48N4NiO8 986.2820; found 986.2815. IR (ATR): ν 1761 (C
O) cm−1. UV−vis (CH2Cl2): λmax (log ε) 412 (5.57), 527 (4.43), 559
nm (4.05).
1
mg (45%). Mp 69−71 °C. H NMR (CDCl3, 400 MHz): δ 7.39 (t,
4H, J = 7.8 Hz), 7.23 (t, 2H, J = 7.8 Hz), 7.08 (d, 4H, J = 7.8 Hz),
2.58 (t, 4H, J = 7.6 Hz), 1.79 (tt, 4H, J = 7.6, 7.6 Hz), 1.49−1.45 (m,
6H). 1H NMR (C6D6, 700 MHz): δ 7.12 (d, 4H, J = 8.1 Hz), 7.08 (t,
4H, J = 8.1 Hz), 6.92 (t, 2H, J = 8.1 Hz), 2.24 (t, 4H, J = 7.4 Hz),
1.55 (tt, 4H, J = 7.7, 7.4 Hz), 1.14 (tt, 4H, J = 7.7, 7.7 Hz), 1.10
(quint, 2H, J = 7.7 Hz). 1H NMR (CD2Cl2, 700 MHz): δ 7.39 (t, 4H,
J = 7.8 Hz), 7.24 (t, 2H, J = 7.8 Hz), 7.08 (d, 4H, J = 7.8 Hz), 2.56 (t,
4H, J = 7.6 Hz), 1.75 (tt, 4H, J = 7.6 Hz), 1.46−1.42 (m, 6H).
13C{1H} NMR (C6D6, 100 MHz): δ 171.8, 151.9, 129.9, 126.1, 122.3,
34.7, 29.5, 29.4, 25.4. HRMS (ESI) m/z: [M + H]+ calcd for
C21H25O4 341.1747; found 341.1747. IR (ATR): ν 1749 (CO)
cm−1. 14-C8: Mp 68−70 °C. 1H NMR (700 MHz, CDCl3): δ 7.38 (t,
4H, J = 8.1 Hz), 7.23 (t, 2H, J = 8.1 Hz), 7.09 (d, 4H, J = 8.1 Hz),
2.57 (t, 4H, J = 7.0 Hz), 1.78 (tt, 4H, J = 7.7, 7.0 Hz), 1.45−1.43 (m,
Synthesis of 13. Typical procedure: A mixture of 11 (31.7 mg,
0.0387 mmol), decanedioyl dichloride (17.9 mg, 0.0749 mmol),
triethylamine (0.03 mL, 0.2 mmol), and CH2Cl2 (110 mL) was stirred
for 39 h at room temperature. The mixture was then passed through a
paper filter, and the filtrate was concentrated under reduced pressure.
The residue was subjected to silica-gel column chromatography
(hexane/AcOEt = 5/1). The fraction of Rf = 0.3 (hexane/AcOEt = 5/
1) was collected and reprecipitated from CH2Cl2−hexane to give 13-
C8 as a purple solid (7.0 mg, 16%). 13-C7 and 13-C9 were similarly
prepared by acylation of 11 with two equivalents of nonanedioyl
dichloride and undecanedioyl dichloride, respectively. 13-C7: Purple
solid, 24.1 mg (23%). Mp > 300 °C. 1H NMR (CDCl3, 400 MHz): δ
8.65 (d, 4H, J = 5.0 Hz), 8.58 (d, 4H, J = 5.0 Hz), 7.84 (t, 2H, J = 8.0
Hz), 7.47 (d, 4H, J = 8.0 Hz), 7.23 (s, 4H), 2.59 (s, 6H), 1.78 (s,
12H), 1.20 (t, 8H, J = 7.6 Hz), −1.18 (tt, 8H, J = 7.6, 7.6 Hz), −1.39
1
4H), 1.41−1.40 (m, 4H). H NMR (C6D6, 700 MHz): δ 7.12−7.10
(d, 4H, J = 8.1 Hz), 7.09−7.06 (t, 4H, J = 7.0 Hz), 6.92 (t, 2H, J = 7.7
Hz), 2.26 (t, 4H, J = 7.0 Hz), 1.56 (tt, 4H, J = 7.7, 7.0 Hz), 1.20−1.16
1
(m, 4H), 1.14−1.12 (m, 4H). H NMR (CD2Cl2, 700 MHz): δ 7.38
1
(tt, 8H, J = 7.2, 7.6 Hz), −1.74 (quint, 4H, J = 7.2 Hz). H NMR
(t, 4H, J = 8.1 Hz), 7.23 (t, 2H, J = 8.1 Hz), 7.07 (d, 4H, J = 8.1 Hz),
2.55 (t, 4H, J = 7.0, 7.7 Hz), 1.74 (tt, 4H, J = 7.0, 7.7 Hz), 1.44−1.42
(m, 4H), 1.40−1.38 (m, 4H). 13C{1H} NMR (C6D6, 100 MHz): δ
171.8, 151.9, 129.9, 126.0, 122.3, 34.7, 29.7, 29.6, 25.5. HRMS (ESI)
m/z: [M + H]+ calcd for C22H27O4 355.1904; found 355.1899. IR
(ATR): ν 1745 (CO) cm−1. 14-C9: Colorless solid. 71.4 mg
(38%). Mp 68−70 °C. 1H NMR (CDCl3, 400 MHz): δ 7.39 (t, 4H, J
= 8.1 Hz), 7.23 (t, 2H, J = 8.1 Hz), 7.09 (d, 4H, J = 8.1 Hz), 2.57 (t,
4H, J = 7.4 Hz), 1.77 (tt, 4H, J = 7.7, 7.4 Hz), 1.50−1.34 (m, 10H).
1H NMR (C6D6, 700 MHz): δ 7.11 (d, 4H, J = 7.8 Hz), 7.08 (t, 4H, J
= 7.8 Hz), 6.92 (t, 2H, J = 7.8 Hz), 2.27 (t, 4H, J = 7.7 Hz), 1.61 (tt,
(CD2Cl2, 700 MHz): δ 8.61 (d, 4H, J = 4.9 Hz), 8.55 (d, 4H, J = 4.9
Hz), 7.86 (t, 2H, J = 8.4 Hz), 7.47 (d, 4H, J = 8.4 Hz), 7.26 (s, 4H),
2.59 (s, 6H), 1.79 (s, 12H), 1.16 (t, 8H, J = 7.9 Hz), −1.07 (tt, 8H, J
= 8.1, 7.9 Hz), −1.57 (tt, 8H, J = 8.1, 8.1 Hz), −1.67 (quint, 4H, J =
8.1 Hz). 13C{1H} NMR (CDCl3, 100 MHz): δ 171.2, 151.9, 143.3,
143.1, 138.5, 137.8, 137.3, 132.0, 130.9, 130.7, 129.1, 127.8, 120.5,
117.4, 108.4, 33.5, 29.6, 27.1, 24.3, 21.4, 21.3. HRMS (ESI) m/z:
[M]+ calcd for C68H64N4NiO8 1122.4072; found 1122.4077. IR
(ATR): ν 1762 (CO) cm−1. UV−vis (CH2Cl2): λmax (log ε) 410
1
(5.55), 524 (4.42), 558 nm (4.12). 13-C8: Mp > 300 °C. H NMR
(CDCl3, 400 MHz): δ 8.70 (d, 4H, J = 4.8 Hz), 8.57 (d, 4H, J = 4.8
Hz), 7.83 (t, 2H, J = 8.3 Hz), 7.42 (d, 4H, J = 8.3 Hz), 7.22 (s, 4H),
2.59 (s, 6H), 1.79 (s, 12H), 1.68 (t, 8H, J = 6.8 Hz), −0.57 (tt, 8H, J
= 6.8, 7.6 Hz), −1.47 to −1.58 (m, 8H), −1.70 to −1.80 (m, 8H). 1H
NMR (CD2Cl2, 400 MHz): δ 8.66 (d, 4H, J = 4.8 Hz), 8.56 (d, 4H, J
= 4.8 Hz), 7.85 (t, 2H, J = 8.3 Hz), 7.42 (d, 4H, J = 8.3 Hz), 7.26 (s,
4H), 2.59 (s, 6H), 1.79 (s, 12H), 1.66 (t, 8H, J = 6.9 Hz), −0.56 (tt,
8H, J = 7.6, 6.9 Hz), −1.50 to −1.60 (m, 8H), −1.64 to −1.77 (m,
8H). 13C{1H} NMR (CDCl3, 100 MHz): δ 171.3, 151.5, 143.3, 143.1,
138.7, 137.7, 137.4, 131.9, 130.8, 130.0, 128.6, 127.8, 120.5, 117.6,
108.0, 34.8, 27.5, 27.2, 23.8, 21.5, 21.4. HRMS (ESI) m/z: [M]+ calcd
for C70H68N4NiO8 1150.4385; found 1150.4382. IR (ATR): ν 1758
(CO) cm−1. UV−vis (CH2Cl2): λmax (log ε) 411 (5.51), 524
(4.40), 558 nm (4.08). 13-C9: Purple solid, 10.6 mg (24%). Mp >
1
4H, J = 7.7, 7.4 Hz), 1.23−1.19 (m, 4H), 1.18−1.14 (m, 6H). H
NMR (CD2Cl2, 700 MHz): δ 7.39 (t, 4H, J = 8.1 Hz), 7.24 (t, 2H, J =
8.1 Hz), 7.07 (d, 4H, J = 8.1 Hz), 2.55 (t, 4H, J = 7.2 Hz), 1.74 (tt,
4H, J = 7.7, 7.2 Hz), 1.44−1.39 (m, 4H), 1.38−1.33 (m, 6H).
13C{1H} NMR (C6D6, 100 MHz): δ 172.0, 152.2, 130.2, 126.3, 122.6,
35.0, 30.2, 30.1, 29.9, 25.8. HRMS (ESI) m/z: [M + H]+ calcd for
C23H29O4 369.2060; found 369.2060. IR (ATR): ν 1748 (CO)
cm−1.
Density Functional Theory Calculations. The geometries were
optimized with the density functional theory (DFT) method (for
details, see the main text). The basis sets used for the optimization
were the 6−311G(d,p) basis set47 for H, C, and N and the Wachters−
Hay all-electron basis set48 supplemented with one f-function
(exponent: 1.29 for Ni, 1.62 for Zn). The functional of DFT was
the Becke, three-parameter, Lee−Yang−Parr (B3LYP) exchange-
correlation functional.49 The optimized geometries were confirmed to
be minima by vibrational analysis. The Cartesian coordinates and
computed total energies are summarized in Table S3 in the SI. The
nucleus-independent chemical shift (NICS) was calculated at the
Hartree−Fock level with gauge-including atomic orbitals (GIAOs) at
the DFT-optimized geometries. The basis set used for the NICS
calculations was 6-31+G(d).50 All of the calculations were carried out
using the Gaussian 16 suite of programs.51
Electron Paramagnetic Resonance Measurements. The EPR
spectra of 19π TADAPs 4 and 8-Cn were measured at room
temperature. All samples were prepared as a 0.1 mM solution in
CH2Cl2. After three freeze−pump−thaw cycles, the solution sample
in a quartz tube was sealed by frame. Spectral simulation was
performed using EasySpin,52 which is a MATLAB toolbox meant for
1
300 °C. H NMR (CDCl3, 400 MHz): δ 8.69 (d, 4H, J = 4.8 Hz),
8.57 (d, 4H, J = 4.8 Hz), 7.81 (t, 2H, J = 8.4 Hz), 7.42 (d, 4H, J = 8.3
Hz), 7.21 (s, 4H), 2.59 (s, 6H), 1.78 (s, 12H), 1.63 (t, 8H, J = 7.0
Hz), 0.07−0.04 (m, 8H), −0.98 − −1.14 (m, 16H), −1.45 (quint,
4H, J = 6.4 Hz). 1H NMR (CD2Cl2, 700 MHz): δ 8.65 (d, 4H, J = 4.9
Hz), 8.54 (d, 4H, J = 4.9 Hz), 7.82 (t, 2H, J = 9.0 Hz), 7.42 (d, 4H, J
= 9.0 Hz), 7.24 (s, 4H), 2.57 (s, 6H), 1.78 (s, 12H), 1.60 (t, 8H, J =
6.3 Hz), 0.08 (tt, 8H, J = 7.0, 6.3 Hz), −1.00 to −1.12 (m, 16H),
−1.42 (quint, 4H, J = 6.4 Hz). 13C{1H} NMR (CDCl3, 100 MHz): δ
171.4, 151.5, 143.0, 142.8, 138.7, 137.7, 137.4, 131.9, 130.9, 129.7,
128.0, 127.8, 120.4, 117.4, 108.1, 33.2, 26.3, 25.6, 23.9, 23.1, 21.5,
21.4. HRMS (ESI) m/z: [M]+ calcd for C72H72N4NiO8 1178.4698;
found 1178.4711. IR (ATR) ν 1759 (CO) cm−1. UV−vis
(CH2Cl2): λmax (log ε) 411 (5.61), 524 (4.45), 558 nm (4.01).
Synthesis of 14. Typical procedure: A mixture of phenol (174 mg,
1.85 mmol), decanedioyl dichloride (218 mg, 0.913 mmol),
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this. The static magnetic field and microwave frequency were
measured by a gauss meter.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.joc.0c02433.
■
sı
NMR and EPR spectra, data of DFT calculations, and
(12) (a) Peeks, M. D.; Claridge, T. D. W.; Anderson, H. L. Aromatic
and Antiaromatic Ring Currents in A Molecular Nanoring. Nature
2017, 541, 200−203. (b) Peeks, M. D.; Jirasek, M.; Claridge, T. D.
W.; Anderson, H. L. Global Aromaticity and Antiaromaticity in
Porphyrin Nanoring Anions. Angew. Chem., Int. Ed. 2019, 58, 15717−
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additional figures and tables (PDF)
AUTHOR INFORMATION
Corresponding Author
Yoshihiro Matano − Department of Chemistry, Faculty of
Science, Niigata University, Nishi-ku, Niigata 950-2181,
Japan; orcid.org/0000-0003-3377-0004;
Email: matano@chem.sc.niigata-u.ac.jp
■
(13) (a) Gershoni-Poranne, R.; Stanger, A. Magnetic Criteria of
Aromaticity. Chem. Soc. Rev. 2015, 44, 6597−6615. (b) Mitchell, R.
H.; Iyer, V. S.; Khalifa, N.; Mahadevan, R.; Venugopalan, S.;
Weerawarna, S. A.; Zhou, P. An Experimental Estimation of
Aromaticity Relative to That of Benzene. The Synthesis and NMR
Properties of a Series of Highly Annelated Dimethyldihydropyrenes:
Bridged Benzannulenes. J. Am. Chem. Soc. 1995, 117, 1514−1532.
(c) Mitchell, R. H. Measuring Aromaticity by NMR. Chem. Rev. 2001,
101, 1301−1316.
Authors
Hikari Ochiai − Department of Chemistry, Graduate School of
Science and Technology, Niigata University, Nishi-ku,
Niigata 950-2181, Japan
Ko Furukawa − Center for Coordination of Research Facilities,
Institute for Research Promotion, Niigata University, Nishi-
ku, Niigata 950-2181, Japan; Institute for Molecular Science,
Okazaki 444-8585, Japan
Haruyuki Nakano − Department of Chemistry, Graduate
School of Science, Kyushu University, Fukuoka 819-0395,
Japan; orcid.org/0000-0002-7008-0312
(14) Cosmo, R.; Kautz, C.; Meerholz, K.; Heinze, J.; Mullen, K.
̈
Highly Reduced Porphyrins. Angew. Chem., Int. Ed. 1989, 28, 604−
607.
(15) Pohl, M.; Schmickler, H.; Lex, J.; Vogel, E. Isophlorins:
Molecules at the Crossroads of Porphyrin and Annulene Chemistry.
Angew. Chem., Int. Ed. 1991, 30, 1693−1697.
(16) Setsune, J.; Kashihara, K.; Wada, K.; Shiozaki, H. Photo-
reduction of N,N′-Bridged Porphyrins to 20π Antiaromatic
Isophlorins. Chem. Lett. 1999, 28, 847−848.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.joc.0c02433
(17) Vaid, T. P. A Porphyrin with a C=C Unit at Its Center. J. Am.
Chem. Soc. 2011, 133, 15838−15841.
Notes
(18) Kon-no, M.; Mack, J.; Kobayashi, N.; Suenaga, M.; Yoza, K.;
Shinmyozu, T. Synthesis, Optical Properties, and Electronic
Structures of Fully Core-modified Porphyrin Dications and
Isophlorins. Chem. − Eur. J. 2012, 18, 13361−13371.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(19) (a) Reddy, J. S.; Anand, V. G. Planar Meso Pentafluorophenyl
Core Modified Isophlorins. J. Am. Chem. Soc. 2008, 130, 3718−3719.
(b) Reddy, B. K.; Gadekar, S. C.; Anand, V. G. Non-covalent
composites of antiaromatic isophlorin−fullerene. Chem. Commun.
2015, 51, 8276−8279. (c) Panchal, S. P.; Gadekar, S. C.; Anand, V. G.
Controlled Core-Modification of a Porphyrin into an Antiaromatic
Isophlorin. Angew. Chem., Int. Ed. 2016, 55, 7797−7800.
(20) Nakabuchi, T.; Nakashima, M.; Fujishige, S.; Nakano, H.;
Matano, Y.; Imahori, H. Synthesis and Reactions of Phosphaporphyr-
ins: Reconstruction of π-Skeleton Triggered by Oxygenation of a
Core Phosphorus Atom. J. Org. Chem. 2010, 75, 375−389.
(21) Liu, C.; Shen, D.-M.; Chen, Q.-Y. Synthesis and Reactions of
20π-Electron β-Tetrakis(trifluoromethyl)-meso-tetraphenylporphyr-
ins. J. Am. Chem. Soc. 2007, 129, 5814−5815.
(22) (a) Cissell, J. A.; Vaid, T. P.; Rheingold, A. L. An Antiaromatic
Porphyrin Complex: Tetraphenylporphyrinato(silicon)(L)2 (L =
THF or Pyridine). J. Am. Chem. Soc. 2005, 127, 12212−12213.
(b) Cissell, J. A.; Vaid, T. P.; Yap, G. P. A. Reversible Oxidation State
Change in Germanium(tetraphenylporphyrin) Induced by a Dative
Ligand: Aromatic GeII(TPP) and Antiaromatic GeIV(TPP)-
(pyridine)2. J. Am. Chem. Soc. 2007, 129, 7841−7847.
The authors gratefully thank Prof. Mao Minoura (Rikkyo
University) for his kind and continuous support regarding X-
ray crystallographic analyses. This work was financially
supported by JSPS KAKENHI (18H01961 to Y.M.,
18K05036 to H.N.) and Union Tool foundation (Y.M.).
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