Redox-switchable 20π-, 19π-, and 18π-electron 5,10,15,20-tetraaryl-5,15-diazaporphyrinoid nickel(II) complexes

Article abstract of DOI:10.1002/anie.201510734

The first examples of air-stable 20π-electron 5,10,15,20-tetraaryl-5,15-diaza-5,15-dihydroporphyrins, their 18π-electron dications, and the 19π-electron radical cation were prepared through metal-templated annulation of nickel(II) bis(5-arylamino-3-chloro-8-mesityldipyrrin) complexes followed by oxidation. The neutral 20π-electron derivatives are antiaromatic and the cationic 18π-electron derivatives are aromatic in terms of the magnetic criterion of aromaticity. The meso N atoms in these diazaporphyrinoids give rise to characteristic redox and optical properties for the compounds that are not typical of isoelectronic 5,10,15,20-tetraarylporphyrins. N makes all the difference: Redox-switchable 20π-, 19π-, and 18π-electron 5,10,15,20-tetraaryl-5,15-diazaporphyrinoids are prepared through metal-templated annulation of Ni^II bis(dipyrrin) complexes followed by oxidation. The meso N atoms in these diazaporphyrinoids give rise to characteristic redox and optical properties for the compounds that are not typical of isoelectronic 5,10,15,20-tetraarylporphyrins.

Full text of DOI:10.1002/anie.201510734

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201510734
German Edition: DOI: 10.1002/ange.201510734
Porphyrinoids
Redox-Switchable 20p-, 19p-, and 18p-Electron 5,10,15,20-Tetraaryl-
,15-diazaporphyrinoid Nickel(II) Complexes
5
Takaharu Satoh, Mao Minoura, Haruyuki Nakano, Ko Furukawa, and Yoshihiro Matano*
[
9]
Abstract: The first examples of air-stable 20p-electron
nine, and Chen et al. isolated a free base of isophlorin by
appending electron-withdrawing trifluoromethyl substituents
5
1
,10,15,20-tetraaryl-5,15-diaza-5,15-dihydroporphyrins, their
8p-electron dications, and the 19p-electron radical cation
[
10]
on its periphery. However, the number of air-stable 20p-
electron porphyrinoids is still quite limited, so it is therefore
important for both basic and applied research to develop
a conceptually new approach to precisely tune the redox
properties of porphyrin-based 20p-electron systems.
The redox and optical properties of 5,15-diazaporphyrins
(DAP) have been shown to differ substantially from those of
their porphyrin counterparts.
ties of DAP arise from the electronic effects of the two meso
N atoms, which not only stabilize the HOMO and LUMO
levels but also split both the HOMO/HOMO-1 and LUMO/
LUMO + 1 energy levels. We envisioned that replacement of
two of the meso C atoms of TPP with two N atoms would be
a promising strategy to obtain a new DAP-based 20p-electron
system because unshared electron pairs on the meso N atoms
should be involved in the p circuit. This type of meso
modification should also alter the net charge of the p circuit;
5,10,15,20-tetraaryl-5,15-diaza-5,15-dihydroporphyrins
were prepared through metal-templated annulation of nickel-
II) bis(5-arylamino-3-chloro-8-mesityldipyrrin) complexes
(
followed by oxidation. The neutral 20p-electron derivatives
are antiaromatic and the cationic 18p-electron derivatives are
aromatic in terms of the magnetic criterion of aromaticity. The
meso N atoms in these diazaporphyrinoids give rise to
characteristic redox and optical properties for the compounds
that are not typical of isoelectronic 5,10,15,20-tetraarylpor-
phyrins.
[11–14]
The characteristic proper-
P
orphyrins are redox-active, 18p-electron aromatic macro-
cycles. The redox properties of porphyrins have been widely
investigated, as have the aromatic and optical properties of
[1–3]
the resulting oxidized/reduced macrocyclic p systems.
The
two-electron reduction of a porphyrin ring produces a dianion,
which shows antiaromatic character resulting from its 20p-
[
2]
electron circuit. This kind of porphyrin dianion is highly
reactive because of its high-energy HOMOs, and thus
elaborate molecular designs to prepare and isolate 20p-
electron porphyrinoids in the neutral form have been
reported. Vogel and co-workers used core modification for
this purpose; they obtained N,N’,N’’,N’’’-tetramethyliso-
+
(TADAP), their radical cation (TADAPC ), and dications
(TADAP )
2
+ [15]
are isoelectronic forms of the 20p-electron
TPP dianion, 19p-electron TPP radical anion, and neutral
18p-electron TPP, respectively (Scheme 1). Herein, we report
the first examples of 20p-, 19p-, and 18p-electron TADAP
derivatives, which were all isolated as air-stable solids. The
structures, aromaticity, and optical/electrochemical properties
of these diazaporphyrinoids were investigated.
[
4]
phlorin and tetraoxaisophlorin as [20]annulenes. Core
modification has also been used to synthesize other
[5]
[6]
[7]
N-alkyl-, S-, and P, S-containing isophlorins. Meanwhile,
Vaid et al. and Leznoff et al. used the characteristic coordi-
nation properties of silicon(IV), germanium(IV), and
niobium(V) to obtain metal complexes of 20p-electron
Scheme 2 shows the syntheses of nickel(II) complexes of
2
+
TADAP and TADAP . Treatment of nickel(II) acetate with
two equivalents of 3-chloro-5-phenylamino-8-mesityldipyrrin
1a (for the synthesis of 1a,b, see the Supporting Information;
mesityl = 2,4,6-trimethylphenyl) gave nickel(II) bis(dipyrrin)
complex 2a. In the presence of K CO , 2a underwent
intramolecular double nucleophilic substitution at 1108C in
DMF to afford 10,20-dimesityl-5,15-diphenyl-5,15-diaza-5,15-
dihydroporphyrinato nickel(II) ([Ni(TADAP)]) 3a as an air-
stable reddish-brown solid. Oxidation of 3a with two equiv-
alents of tris(4-bromophenyl)ammoniumyl hexachloroantim-
[
8]
5
,10,15,20-tetraphenylporphyrin (TPP)
and phthalocya-
2
3
[
*] T. Satoh, Prof. Dr. Y. Matano
Department of Chemistry
Faculty of Science, Niigata University
Nishi-ku, Niigata 950-2181 (Japan)
E-mail: matano@chem.sc.niigata-u.ac.jp
Prof. Dr. M. Minoura
Department of Chemistry
College of Science, Rikkyo University
Toshima-ku, Tokyo 171-8501 (Japan)
Prof. Dr. H. Nakano
Department of Chemistry
Graduate School of Science, Kyushu University
Nishi-ku, Fukuoka 819-0395 (Japan)
Prof. Dr. K. Furukawa
Center for Instrumental Analysis, Institute for Research Promotion
Niigata University, Nishi-ku, Niigata 950-2181 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201510734.
2
+
.
Scheme 1. Interconversion between TADAP and TADAP
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Figure 2. Perspective views of molecules (left) and top views of DAP
rings (right) for a) 3b and b) 4a-Sb. Thermal ellipsoids set at 50%
probability. Hydrogen atoms and solvent molecules are omitted for
clarity. Bond lengths [Š] except the standard deviations (0.002–
2
+
.
Scheme 2. Synthesis of [Ni(TADAP)] and [Ni(TADAP)]
0
.003 Š) are shown.
onate (Magic Blue) in CH Cl2 gave dication ([Ni-
2
2
+
(
TADAP)] ) 4a-Sb as an air-stable green solid. Because 3a
was sparingly soluble in common organic solvents, more
soluble derivatives 3b and 4b-Sb bearing 4-(tert-butyl)phenyl
groups were prepared from 1b according to similar proce-
and 4a-Sb were 0.035 and 0.012 Š, respectively. In 4a-Sb, two
À
of the chlorine atoms of one counter anion (SbCl )
6
coordinate to the nickel center with a NiÀCl distance of
dures. When AgPF was used instead of Magic Blue in the
3.18 Š, which is within the sum of their van der Waals radii
(3.4 Š). It is therefore likely that the counter anion interacts
with the nickel center. The meso N-aryl groups of 3b and 4a-
Sb are almost perpendicular to the DAP ring (dihedral
angles = 85.3–86.18), suggesting that resonance effects of
these aryl groups on the DAP p system would be small. The
C4ÀN2 and C5ÀN2 bonds (1.368(2) and 1.375(2) Š for 3b,
6
oxidation of 3a,b, dications 4a-Sb and 4b-P were obtained.
Conversely, treatment of 4b-Sb with excess NaBH in THF
4
regenerated 3b. Therefore, the chemical redox reactions
between 3 and 4 are reversible. To discuss the electronic
effects of the meso N atoms on the redox properties of the
TPP-type p system, porphyrin 5 (Figure 1) was prepared as
a reference.
Compounds 3 and 4 were characterized by NMR spec-
troscopy and high-resolution mass spectrometry (HRMS). In
the HRMS data for 4, intense signals consistent with m/z =
1.347(3) and 1.348(3) Š for 4a-Sb) are much shorter than the
C10ÀN2 bond (1.445(2) Š for 3b, 1.465(2) Š for 4a-Sb).
These data imply that unshared electron pairs in the p orbitals
of the meso N atoms effectively conjugate with the p orbitals
in the adjacent pyrrole rings. It should be noted that the C4/
C5ÀN2 bonds in 4a-Sb are appreciably shorter than those in
2
+
[
Ni(TADAP)] (z = 1, 2) were clearly detected. In the
3
1
À
P NMR spectra of 4a,b-P, a septet attributable to the PF6
counterion was detected (coupling constant JPF = 711–
13 Hz). The structures of 3b and 4a-Sb were unambiguously
elucidated by X-ray crystallography (Figure 2; Figure S1 and
Table S1 in the Supporting Information). In the crystalline
state, these complexes have almost planar DAP p planes; the
calculated root-mean-square deviations of the p-planes of 3b
3b; removing two p electrons from the TADAP ring
strengthens the p conjugation between the pyrrolic b carbon
and meso-N atoms, which increases the double-bond charac-
ter of the C4/C5ÀN2 bonds. The change of the C4/C5ÀN2
bond lengths is reflected in the shortening of the NiÀN bonds;
7
[16]
the central N coordination sphere of 4a-Sb is smaller than
4
that of 3b. Furthermore, the difference in some bond lengths
between 3 and 4 (labelled b/d and e/f in Table S1) can be
explained by the resonance structures of these p systems.
Considering that the two unshared electron pairs on the
meso N atoms are involved in their p circuits, 3 and 4 are
2À
isoelectronic with 20p-electron TPP and 18p-electron TPP,
respectively. Therefore, 3 and 4 are appropriate model
compounds for discussing the aromaticity of diazaporphyr-
1
inoids. The H NMR spectra of 3a and 4a-Sb are shown in
Figure 3. The peripheral b and meso-aryl protons were
1
1
assigned by H– H COSY and NOESY experiments. Reso-
Figure 1. Porphyrin 5 and model compounds 3-m, 4-m, 5-m, and 6-m.
nance signals for the b protons (d and e) of 3a were detected
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1
Figure 3. H NMR spectra (700 MHz) of a) 3a in C D and b) 4a-Sb in
6
6
CD Cl . Asterisks indicate residual solvent signals.
2
2
Figure 4. a) UV/Vis/NIR absorption spectra of 3b, 4b-P, and 6b-P in
CH Cl . b) EPR spectrum of 6b-P in toluene. The simulated EPR
spectrum is shown in Figure S6.
2
2
at d = 4.61 and 3.30 ppm (both doublets, 4H), whereas those
for the b protons of 4a-Sb were detected at d = 8.72 and
8
.33 ppm (both d, 4H), respectively. These upfield and
downfield shifts of the peripheral protons indicate paratropic
20p-electron) and diatropic (18p-electron) properties of 3a
orbitals because of the electronic effect of the two meso-
N atoms. It is clear that both the HOMO and LUMO levels of
4-m are appreciably stabilized compared with those of
isoelectronic 5-m. By comparison of the experimental and
theoretical results, the long-wavelength absorption bands
around l = 530 nm detected for 3 were assigned to a combi-
nation of the HOMO to LUMO + 1 and HOMO-1to LUMO
transitions. The TD-DFT calculations also indicated that the
HOMO to LUMO transition of 3-m is symmetrically
forbidden (oscillator strength (f) = 0). In contrast, the intense,
long-wavelength absorption bands around l = 630 nm
detected for 4 were assigned to the HOMO to LUMO
transition (f = 0.356 for 4-m).
(
and 4a-Sb, respectively. The chemical shifts of the b protons
of 4a-Sb are close to the corresponding values of reference
1
8p-electron porphyrin 5 (d = 8.58 and 8.71 ppm in CD Cl ).
2 2
The b protons of 3a are detected downfield compared with
[
2d]
those of a [Zn(TPP)] dianion (d = À0.9 ppm in [D ]THF),
8
which may be partly caused by the different number of
electrons of their entire p systems. The ortho, meta, and para-
protons of the meso-aryl groups (c, f, g, and h) of 3a and 4a-Sb
also receive the opposite ring-current effects from each other.
The spectral features of 3b and other dications 4 (see the
Supporting Information) are similar to those of 3a and 4a-Sb,
respectively. Based on these observations, it can be concluded
that [Ni(TADAP)] 3 and [Ni(TADAP)] 4 have antiaromatic
and aromatic characters, respectively, according to the
magnetic criterion of aromaticity.
Redox potentials of 3b, 4b-P, and 5 were measured in
CH Cl or THF with Bu NPF as the supporting electrolyte
2+
2
2
4
6
(Figure S5). [Ni(TADAP)] 3b was oxidized reversibly in one-
electron steps with separated potentials of À0.39 (20p/
19p electrons) and + 0.30 V (19p/18p-electrons), whereas
To compare the optical properties of the newly prepared
diazaporphyrinoids with those of reference porphyrin 5, UV/
Vis absorption spectra of 3 and 4 were measured. As shown in
Figure 4a, 3b displayed two intense absorption bands at
lmax = 446 and 532 nm, whereas 4b-P exhibited two bands at
lmax = 393 and 627 nm in CH Cl . The spectral features of 3a
2+
[Ni(TADAP)] 4b-P was reduced reversibly with potentials
of + 0.29 (18p/19p-electrons) and À0.39 V (19p/20p elec-
+
trons) versus the Ag/Ag redox couple (in CH Cl ). As
2
2
expected, the redox potentials detected for 3b and 4b-P are
identical, within experimental error. The 20p/19p-electron
redox process of 3b occurs at a much more positive potential
than that of isoelectronic porphyrin 5 (E < À2 V versus Ag/
2
2
and other dications 4a-Sb, 4b-Sb, and 4a-P (Figure S3) are
similar to those of 3b and 4b-P, respectively. The Q bands of 4
+
(lmax = 626–630 nm) are considerably red-shifted compared
Ag ), which explains the remarkably high air stability of
with those of isoelectronic porphyrin 5 (lmax = 526 nm; Fig-
[Ni(TADAP)] compared with that of the TPP-type porphyrin
[2d]
ure S3a) and 10,20-dimesityl-5,15-diazaporphyrinato nickel-
dianion. The electrochemical HOMO–LUMO gap of 3b
was determined to be 1.62 eV (Figure S5e). It should also be
noted that the electron-accepting ability of 4b-P is consid-
erably higher than that of 5 [E(18p/19p) = À1.69 V versus Ag/
[12]
(
II) (lmax = 571 nm).
To obtain deeper insight into the nature of these optical
properties, we performed time-dependent density functional
theory (TD-DFT) calculations on models 3-m, 4-m, and 5-m
+
Ag (in THF)]. The measured redox potentials suggest that
(
Figure 1). The results are summarized in Table S1–S3 and
single electron transfer (SET) from 3b to 4b-P is energeti-
cally favorable. Indeed, a greenish-yellow solution with an
intense absorption band at lmax = 860 nm formed immediately
Figure S4. Both 3-m and 4-m possess C symmetry and
nondegenerate HOMO/HOMO-1 and LUMO/LUMO + 1
s
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after mixing equimolar amounts of 3b and 4b-P in CH Cl .
Hashizume, F. Iwasaki, A. Muranaka, M. Uchiyama, P. Chen,
K. M. Kadish, N. Kobayashi, J. Am. Chem. Soc. 2010, 132,
2
2
Reprecipitation of the product of this reaction from a mixture
of CH Cl :hexane gave an air-stable greenish-yellow solid
1
2627 – 12638; d) T. Kakui, S. Sugawara, Y. Hirata, S. Kojima, Y.
2
2
1
Yamamoto, Chem. Eur. J. 2011, 17, 7768 – 7771; e) S. Sugawara,
M. Kodama, Y. Hirata, S. Kojima, Y. Yamamoto, J. Porphyrins
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Sugawara, S. Kojima, Y. Yamamoto, J. Porphyrins Phthalocya-
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that showed no H NMR resonance signal for the peripheral
31
À
protons in CD Cl but a P resonance signal for the PF6
2
2
counterion at d = À144.5 ppm (septet, JPF = 711 Hz). These
results imply that SEToccurred from 3b to 4b-P, as expected,
to give the 19p-electron radical cation 6b-P [Eq. (1)]. The
UV/Vis/NIR absorption spectrum of 6b-P is presented in
Figure 4a. The same species was generated by the reaction of
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Closs, J. Am. Chem. Soc. 1963, 85, 818 – 819; b) J. W. Buchler, L.
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Academic Press, San Diego, 2000, pp. 1 – 114, and references
therein.
3
b with one equivalent of AgPF in CH Cl . In toluene, 6b-P
6 2 2
displayed an EPR signal at g = 1.9981 with the fine structure
14
1
derived from two meso- N and four b- H atoms (Figure 4b).
The spin distribution of its model 6-m (Figure 1) supported
the measured fine structure (Figure S6). Redox processes for
6
b-P in CH Cl were measured at À0.39 (19p/20p electrons)
2
2
+
and + 0.28 V (19p/18p electrons) versus Ag/Ag (Fig-
ure S5c), which are in good agreement with the corresponding
redox potentials for 3b and 4b-P.
[
3] For a 19p-electron tetraazaporphyrin, see: T. Yoshida, W. Zhou,
T. Furuyama, D. B. Leznoff, N. Kobayashi, J. Am. Chem. Soc.
2
015, 137, 9258 – 9261.
[
4] a) E. Vogel, P. Rçhrig, M. Sicken, B. Knipp, A. Herrmann, M.
b þ 4 b-P ! 2 ½NiðTADAPފCþ ðPF6ÞÀ ð6 b-PÞ
3
ð1Þ
Pohl, H. Schmickler, J. Lex, Angew. Chem. Int. Ed. Engl. 1989,
2
8, 1651 – 1655; Angew. Chem. 1989, 101, 1683 – 1687; b) M.
In summary, we have synthesized the first examples of
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TADAP compounds, their dications, and a radical cation as
air-stable solids by metal-templated annulation and subse-
quent oxidation. The antiaromatic, aromatic, and radical
[
2+
+
character of TADAP, [Ni(TADAP)] , and [Ni(TADAP)]C ,
respectively, were confirmed by NMR, UV/Vis/NIR absorp-
tion, and EPR spectroscopies. As a result of the high electron
[6] a) J. S. Reddy, V. G. Anand, J. Am. Chem. Soc. 2008, 130, 3718 –
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3
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[
affinity of nitrogen at the meso positions, 20p-electron [Ni-
+
(
TADAP)] and 19p-electron [Ni(TADAP)]C are resistant to
2+
oxidation in air, whereas 18p-electron [Ni(TADAP)] is
strongly electron accepting. These properties are difficult to
obtain with isoelectronic 5,10,15,20-tetraarylporphyrins. It is
also noteworthy that both chemical and electrochemical
redox processes between the 18p- and 20p-electron systems
proceed reversibly in one-electron steps via the 19p-electron
system. These newly constructed tetraaryldiazaporphyrinoids
can be used not only for a fundamental study of p-conjugated
azamacrocycles but also for applied research on redox-active
sensitizers.
[
9] a) J. A. Cissell, T. P. Vaid, A. G. DiPasquale, A. L. Rheingold,
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[
[
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Acknowledgements
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This work was supported by JSPS KAKENHI (15H00931 to
Y.M., 15H00962 to M.M., and 24109008 and 25287091 to
K.F.).
[
[
Keywords: annulenes · aromaticity · heterocyclic compounds ·
[
[
15] Stuzhin and co-workers studied acid forms of a copper(II)
complex of DAP in proton-donor media. For example, see: O. G.
Khelevina, N. V. Chizhova, P. A. Stuzhin, A. S. Semeikin, B. D.
Berezin, Koord. Khim. 1996, 22, 866 – 869.
16] CCDC 1437727 (3b) and 1437728 (4a-Sb) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre.
porphyrinoids · redox chemistry
How to cite: Angew. Chem. Int. Ed. 2016, 55, 2235–2238
Angew. Chem. 2016, 128, 2275–2278
[
1] For 16p-electron porphyrins, see: a) Y. Yamamoto, A. Yama-
moto, S.-Y. Furuta, M. Horie, M. Kodama, W. Sato, K.-Y. Akiba,
S. Tsuzuki, T. Uchimaru, D. Hashizume, F. Iwasaki, J. Am. Chem.
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Yap, Org. Lett. 2006, 8, 2401 – 2404; c) Y. Yamamoto, Y. Hirata,
M. Kodama, T. Yamaguchi, S. Matsukawa, K. Akiba, D.
Received: November 19, 2015
Published online: January 8, 2016
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