Unexpected formation of the nickel seco-tribenzoporphyrazine with a tribenzotetraazachlorin-type absorption spectrum

Article abstract of DOI:10.1039/c2cc30760j

Minor products in the reaction between substituted 1,3-diiminoisoindolines and 2,5-diamino-3,4-dicyanothiophene were identified as the nickel seco-tribenzoporphyrazines 4 and 5, which have been characterized by UV-vis, MCD, NMR, and mass spectroscopy. Experimentally observed tetraazachlorin-type UV-vis spectra of new seco-tribenzoporphyrazines were explained on the basis of DFT and TDDFT calculations.

Full text of DOI:10.1039/c2cc30760j

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Unexpected formation of the nickel seco-tribenzoporphyrazine with a
tribenzotetraazachlorin-type absorption spectrumwz
Victor N. Nemykin,*a Anna E. Polshyna,a Elena A. Makarova,ab Nagao Kobayashi*c and
Evgeny A. Lukyanets*b
Received 2nd February 2012, Accepted 22nd February 2012
DOI: 10.1039/c2cc30760j
Minor products in the reaction between substituted 1,3-diiminoiso-
indolines and 2,5-diamino-3,4-dicyanothiophene were identified as
the nickel seco-tribenzoporphyrazines 4 and 5, which have been
characterized by UV-vis, MCD, NMR, and mass spectroscopy.
Experimentally observed tetraazachlorin-type UV-vis spectra of
new seco-tribenzoporphyrazines were explained on the basis of
DFT and TDDFT calculations.
To the best of our knowledge, all of the synthetic strategies
reported so far for preparation of seco-porphyrazines4,5 and their
close analogues seco-porphyrins6 require oxidative cleavage of the
pyrrole ring(s). In this communication, we describe an unexpected
formation of the simplest nickel seco-tribenzoporphyrazines in
the reaction between substituted 1,3-diiminoisoindolines and
2,5-diamino-3,4-dicyanothiophene, which represent an earlier
unknown class of seco-tribenzoporphyrazines.
In an attempt to prepare functionalized low-symmetry
thiophene analogues of nickel phthalocyanine, we have investigated
cross-condensation between 5-tert-butyl-1,3-diiminoisoindoline
1 and 2,5-diamino-3,4-dicyanothiophene 3 using nickel ions as
the template. After 24 h of refluxing the reaction mixture in
N,N-dimethylaminoethanol, however, we were able to isolate
only symmetric nickel phthalocyanine 6 (68% yield) as the
major reaction product (ESIz). In addition, during purification
workup, we noticed reproducible formation of the very minor
(0.5% yield) blue colored reaction by-product, which, based on
the spectroscopic and theoretical data presented below, was
assigned to the new nickel seco-porphyrazine complex 4
(Scheme 1). It should also be noted that no other phthalocyanine-
type or expanded phthalocyanine-type7 compounds were isolated
from or identified in the reaction mixture. The formation of
complex 4 is very sensitive to the solvent. Indeed, we were not
able to isolate any seco-porphyrazine products when the reaction
was conducted in bromonaphthalene, trichlorobenzene, butanol,
or DMF and thus the role of the solvent in the formation of
complex 4 remains unclear.
Phthalocyanines and their analogues were studied because of
their industrial importance as dyes, pigments, and catalysts as
well as their ability to mimic biologically relevant electron-
and atom-transfer reactions.1 During the last several decades,
however, in addition to the traditional areas of applications,
these chemically and thermally robust systems with well-
defined photophysical, coordination, and redox properties
were intensely investigated as promising photosensitizers for
photodynamic therapy of cancer, charge carriers for copiers
and printers, redox-driven fluorescence markers and materials
for molecular electronics, light-harvesting components in dye
sensitized solar cells, and materials for gas sensors.2 In many
cases, such high-tech applications require preparation of the
low-symmetry phthalocyanine analogues with the target sub-
stituents located at specific positions of the macrocyclic core. As
a result of this requirement, many strategies were proposed for
synthesis of low-symmetry phthalocyanines and their analogues.3
Seco-porphyrazines represent a rare class of low-symmetry
phthalocyanine analogues in which one or several a–b or b–b
pyrrolic carbon–carbon bonds are cleaved and transformed into
the different functional groups.4,5 It has been shown that these
macrocycles exhibit enhanced optical and fluorescence properties,
which make them potential candidates for biomedical applications.
In the absence of X-ray data, the structure of the new seco-
porphyrazine 4 was established on the basis of spectroscopic
and theoretical methods. First, we found that the retention
time in size-exclusion experiments for seco-porphyrazine 4 was
longer compared to that for the symmetric phthalocyanine 6
suggesting a smaller molecular weight for minor product 4.
In agreement with this observation, both FAB and APCI mass
spectra of seco-porphyrazine 4 are dominated by the peak with
m/z = 665, which corresponds to the [M + 1]+ ion (ESIz).
In addition, low intensity peaks with m/z = 682 and 649,
which correspond to the [M + H2O]+ and [M ꢀ CH3]+ ions were
observed in APCI and FAB spectra (ESIz). Further APCI MS/MS
experiments on seco-porphyrazine 4 suggested a loss of the
peripheral substituents8 (i.e. methyl and tert-butyl groups in 4;
ESIz) that is typical for porphyrinoids. In agreement with the
a Department of Chemistry & Biochemistry, University of Minnesota
Duluth, Duluth, MN 55812, USA. E-mail: vnemykin@d.umn.edu;
Fax: +1 218 726 7394; Tel: +1 218 726 6729
b Organic Intermediates & Dyes Institute, Moscow 123995, Russia.
E-mail: rmeluk@niopik.ru
c Department of Chemistry, Graduate School of Science,
Tohoku University, Sendai, 980-8578, Japan.
E-mail: nagaok@m.tohoku.ac.jp
w This article is part of the ChemComm ‘Porphyrins and phthalo-
cyanines’ web themed issue.
z Electronic supplementary information (ESI) available: Selected
spectroscopic properties, DFT data, and detailed experimental proce-
dure for the preparation of 4 and 5. See DOI: 10.1039/c2cc30760j
c
3650 Chem. Commun., 2012, 48, 3650–3652
This journal is The Royal Society of Chemistry 2012

Scheme 1 Synthesis of seco-porphyrazines 4 and 5.
1
structure of seco-porphyrazine 4, its H NMR spectrum has
signals originated from protons in seco-fragments of the
molecule observed between 9.9 and 10.2 ppm (ESIz), which
are close to the chemical shifts for C–H protons in the earlier
reported seco-porphyrazines.5a The complex pattern observed
Fig. 1 MCD, UV-Vis, and BP86/6-31G(d) PCM-TDDFT predicted
UV-vis spectra of seco-porphyrazine 4 in DCM.
1
in the H NMR spectrum for seco-protons as well as benzene
It can be speculated that the p-system in new seco-porphyrazine
4 resembles the p-system in nickel tribenzotetraazachlorins.11
Indeed, UV-vis and MCD spectra of seco-porphyrazine 4 on
the one hand, and nickel tetramethyltribenzotetraazachlorin11a
or b-oxatetraazachlorins11b are very close to each other with
respect to the transition energies as well as relative band intensities.
In order to clarify the nature of these similarities, we explored the
electronic structure and calculated vertical excitation energies in 4
using PCM-DFT and PCM-TDDFT approaches, which are
proven to provide reliable electronic structures and spectroscopic
signatures in numerous phthalocyanines and their analogues.12
The PCM-DFT predicted energies and frontier orbitals are
shown in Fig. 2, while MO compositions are given in ESI.z
The HOMO in seco-porphyrazine 4 has p-character and
resembles an a1u-type (D4h notation) HOMO in transition-
metal phthalocyanines. This MO is well-separated (B0.5 eV)
from the closely spaced predominantly nickel-centered
HOMO ꢀ 1 (dxz), HOMO ꢀ 2 (dz2), and HOMO ꢀ 3 (dyz)
with the HOMO ꢀ 2 having significant contribution from the
seco-fragment of the macrocycle. The HOMO ꢀ 4 is a mixture
of the n-orbital localized at meso nitrogen atoms and the
nickel-centered dxy orbital (Fig. 2). Similar to the other
phthalocyanine compounds,12 this MO has higher energy than
the second occupied macrocycle-centered p-orbital (HOMO ꢀ 5).
PCM-DFT predicts that the virtual, nickel-centered MO is a
LUMO + 1, while both LUMO and LUMO + 2 are centered
at the seco-porphyrazine core. The presence of the nickel-centered
MOs in the frontier orbital region should result in numerous
transitions with predominant MLCT or LMCT character. Indeed,
PCM-TDDFT calculations predict the numerous MLCT and
LMCT transitions over the large energy envelop (ESIz). In
general, PCM-TDDFT predicted and experimental data for
and tert-butyl protons is characteristic for the positional
isomers expected for seco-porphyrazine 4 (ESIz). It is important
to note that the seco-protons in 4 are not exchangeable with
D2O thus eliminating any possibility of the presence of –OH or
–NH groups in the macrocycle. In order to gain further insight
in the spectroscopy of individual isomers of seco-porphyrazine
4, we attempted a separation of the individual isomers using
high-performance TLC approaches, which resulted in isolation
of one individual isomer of 4 (ESIz). The 1H NMR spectrum of
this isomer is significantly easier to interpret. Indeed, it is indicative
of two slightly different seco-protons and three different tert-butyl
groups with two of them being in close chemical environments,
which was interpreted as the characteristic pattern for isomer 4a or
4b (ESIz). Because of the well-known1 negligible influence of the
tert-butyl group on the absorption spectra of phthalocyanines, it
was not surprising to see that the UV-vis spectra of the isomer
mixture and individual isomers of seco-porphyrazine 4 are
virtually the same and consist of intense Qx and Qy bands centered
at 723 and 593 nm as well as B-band observed at 331 nm (Fig. 1).
In addition, a relatively weak band at 412 nm was also observed in
the UV-vis spectrum of seco-porphyrazine 4. The MCD spectrum
of the seco-porphyrazine 4 reflects its low-symmetry nature.
Indeed, all of the most intense bands observed in the UV-vis
spectrum of 4 (Qx, Qy, and B bands) correspond to three intense
Faraday B-terms, centered at 727, 595, and 338 nm (Fig. 1).
The observed negative sign of the low energy Qx band and
positive sign for higher energy Qy band are characteristic of
phthalocyanine-type compounds9 and indicative of the smaller
energy difference between the porphyrazine core-centered LUMO
and LUMO + 1 p* MOs (DLUMO) than between the porphyr-
azine core-centered HOMO and HOMO ꢀ 1 p MOs (DHOMO).10
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3650–3652 3651

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The dashed line designates the HOMO–LUMO frontier.
seco-porphyrazine 4 are in excellent agreement (Fig. 1).
Specifically, it is predicted that the Q-band region is dominated
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the HOMO - LUMO and HOMO - LUMO + 2 excitations.
In addition, MLCT and LMCT transitions contribute to the
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In order to explore the applicability of the new route for the
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Similar to the previous case, we were able to purify a small amount
of seco-porphyrazine 5 (0.3% yield), which has UV-vis and MCD
spectra that are very close to those of 4 (ESIz). In addition, the
mass and 1H NMR spectra of 5 are in a good agreement with the
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Generous support from the NSF CHE-1110455 and Minnesota
Supercomputing Institute to VN is greatly appreciated. EM and
EL were partially supported by the Moscow CityGovernment for
this work. This work was partly supported by a Grant-in-Aid For
Scientific Research on Innovative areas (no. 20108007, ‘‘pi-Space’’),
and one for Scientific Research (B) (no. 23350095, NK) from
the Ministry of Education, Culture, Sports, Science, and
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Notes and references
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K. M. Kadish, K. M. Smith and R. Guilard, World Scientific,
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2 (a) B. Grimm, A. Hausmann, A. Kahnt, W. Seitz, F. Spanig and
D. M. Guldi, in Handbook of Porphyrin Science, ed. K. M. Kadish,
c
3652 Chem. Commun., 2012, 48, 3650–3652
This journal is The Royal Society of Chemistry 2012