Superazaporphyrins: Meso-pentaazapentaphyrins and one of their low-symmetry derivatives

Article abstract of DOI:10.1002/anie.201203191

Supersized: Three pentaazapentaphyrin derivatives, that is, the superazaporphyrins (SAzPs), as well as a superphthalocyanine (SPc) and a mixed low-symmetry derivative have been prepared and characterized. Decaaryl SAzPs have a distorted (4n+2) πstructure and show the Q bands at about=840-880 nm. These compounds are relatively air stable. Copyright

Full text of DOI:10.1002/anie.201203191

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DOI: 10.1002/anie.201203191
Expanded Phthalocyanines
Superazaporphyrins: Meso-Pentaazapentaphyrins and One of Their
Low-Symmetry Derivatives**
Taniyuki Furuyama, Yosuke Ogura, Kenji Yoza, and Nagao Kobayashi*
The chemistry of tetrapyrrolic tetraazaporphyrin (TAP),
phthalocyanine (Pc), naphthalocyanine (Nc), and anthraco-
cyanine (Ac) has been intensively explored by many
researchers in the last decade.[1] Their properties, such as
color, depend, for example, on the size of the p system, the
size, position, number, and type of substituent, the type,
oxidation state, and spin state of the central metal, and the
mode of stacking in the solid state.[2] These compounds are
also important in materials science because of their potential
application in various leading-edge fields such as organic
electronic devices, deodorants, and photodynamic therapy.[3]
As ring-contracted tri-pyrrolic TAP, Pc, and Nc derivatives,
the compounds called subporphyrazines, have attracted
increasing attention since 1990.[4] A common feature of
these compounds is that they contain boron in the center, and
despite many attempts, its removal has not yet been achieved.
Since the p system of these tri-pyrrolic subcompounds is
smaller than the corresponding tetrapyrrolic congeners, the
main absorption band, called the Q band, of the former
appears at shorter wavelengths than that of the latter in the
visible region. In contrast, as a ring-expanded pentapyrrolic
congener, only the compound called superphthalocyanine
(SPc), consisting of five isoindole units, is known.[5] The first
SPc was reported more than 40 years ago,[5a] with only four
papers appearing since then.[5b–e] SPcs are attractive in
modern technology in that they show a Q band in the near-
IR region beyond l = 900 nm. To date, researchers in the
fields of, for example, solar cells and photodynamic therapy,
have tried to obtain stable compounds having strong absorp-
tion in the near-IR region. This absorption may be attained by
going from smaller to larger compounds, since the absorption
bands generally shift to longer wavelengths (reduction of the
HOMO–LUMO gap).[6]
However, when easily prepared Ncs and Acs are used, the
problem has been the instability of the larger compounds as
they generally decompose within a few days.[7] Herein, we
report the synthesis and properties of the so-called super-
azaporphyrins (SAzP) which consist of five pyrrole units. In
particular, we show why they are stable even though their
Q band appears beyond l = 800 nm. After SPc, SAzP is the
second example of a compound consisting of five pyrrolic
rings and meso-nitrogen atoms.
= =
An unsubstituted SPc containing a linear and rigid O U
O[8] central core (1) was obtained by condensation of 2 in the
presence of UO2(OAc)2·2DMF[9] in DMF in 23% yield. The
SAzPs 5 were obtained by uranium-templated cyclization of
five molecules of 4 which were synthesized from the
fumaronitrile 3 (Scheme 1). The reaction is generally appli-
cable for aryl-substituted precursors equipped either with
donor (5b) or acceptor (5c) groups, although SPc 1 is known
[*] Dr. T. Furuyama, Y. Ogura, Prof. Dr. N. Kobayashi
Department of Chemistry
Graduate School of Science Tohoku University
Aoba-ku, Sendai 980-8578 (Japan)
E-mail: nagaok@m.tohoku.ac.jp
Dr. K. Yoza
Bruker AXS K. K., Yokohama, Kanagawa 221-0022 (Japan)
[**] This work was partly supported by a Grant-in-Aid for Scientific
Research on Innovative areas (No. 20108007, “pi-Space”), Scientific
Research (B) (No. 23350095), and Young Scientist (B) (No.
24750031) from the Ministry of Education, Culture, Sports, Science,
and Technology (MEXT). The authors thank Prof. Takeaki Iwamoto,
Dr. Shintaro Ishida, and Dr. Soji Shimizu at Tohoku University for X-
ray measurements. Some of the calculations were performed using
supercomputing resources at Cyberscience Center, Tohoku Univer-
sity.
Scheme 1. Synthesis of the superphthalocyanine (SPc) 1, superaza-
porphyrins (SAzPs) 5a–c, and the low-symmetry SAzP 6. Reagents and
conditions: a) Na, NH3 (gas), nC5H11OH, 1058C, 3 h; b) UO2-
(OAc)2·2DMF, 1908C, 1 h. DMF=N,N’-dimethylformamide.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203191.
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to be obtained practically only when substituent-free 2 is
used. The incorporation of a bromo substituent in the
macrocycle (5c) also allowed further synthetic modification
utilizing Pd0- or Ni0-catalyzed reactions. In the case of the low-
symmetry 6, which contains one isoindole and four pyrroline
rings, a mixed condensation was carried out using 2 and 4a at
a mole ratio of 1:1 at 1908C for 1 hour. From the reaction
mixture, analytically pure 6 was obtained as a brown powder
after chromatography. All of the substituted SAzPs can be
stored as solids in air under ambient light.
core structure of 5b (see Figure S1 in the Supporting
Information), as would be anticipated for a nonplanar
heteroaromatic p system.
Figure 2 shows the electronic absorption and magnetic
circular dichroism (MCD) spectra of compounds 1, 5a, and 6.
The SPc 1 exhibited Q- and Soret-like bands at l = 915 and
Formation of the compounds was first confirmed by HR/
MALDI-FT-ICR/MS and 1H NMR spectroscopy. Compound
1 showed only two 1H NMR signals assignable to the a and b
protons of the isoindoles (d = 9.15 and 8.09 ppm), whereas
5a–c showed only two sets of signals arising from the ortho
and meta protons of the phenyl groups, thus suggesting
a highly symmetrical structure in solution. The compound 6
showed both a and b protons of the isoindoles and ortho and
meta protons of the phenyl groups in the expected ratio
(1:1:8:8), all at different positions, thus reflecting the low
symmetry of the molecule.
Single crystals of 5b suitable for X-ray crystallography
were grown by slow diffusion of n-hexane into a toluene
solution of 5b (Figure 1). The crystal structure of 5b[10] reveals
that the macrocycle comprises five pyrroline rings linked by
five nitrogen atoms, with the UVI ion sitting in the center of
the SAzP 5N mean plane (D5N < 0.005 ꢀ). 5b is not planar,
but has a severely saddled structure, as reported for the
crystal structure of SPc.[5c,d] The peripheral aryl moieties are
oriented essentially perpendicularly to the macrocycle, thus
suggesting only a weak electronic interaction between the
substituents and the macrocycle. No significant bond-length
À
À
alternation was observed for the C N and C C bonds of the
Figure 2. Magnetic circular dichroism (top) and electronic absorption
(middle) spectra of 1 (dotted lines), 5a (solid lines), and 6 (dotted
broken lines). Inset: Magnified MCD spectrum of 5a. Theoretical
absorption spectra (bottom); the solid lines are those calculated by
the TD-DFT method. Calculations were carried out for the free-base
dianion species without tert-butyl groups at the B3LYP/6-31G* level of
theory.
424 nm, respectively, which are at longer wavelengths than
those of normal Pcs (ca. l = 650–700 nm and l = 330–350 nm,
respectively), and the corresponding dispersion-type, Faraday
A MCD curves, thus experimentally supporting the fact that
the excited state is orbitally degenerate. The SAzP 5a
exhibited these bands at l = 878 and 437 nm. Therefore, in
going from 1 to 5a, the Q band shifted to a shorter wave-
length, while the Soret band shifted to a longer wavelength,
both of which are phenomena similar to those observed in
going from Pc to octaaryltetraazaporphyrin.[3b] A weak
absorption on the longer-wavelength side of the Soret band,
at about l = 500–600 nm, is characteristic of AzPs having
many phenyl groups.[11] The substitution effect at the aryl
position is relatively weak so that the positions of the Q bands
of 5a–c are close together (see Figure S2 in the Supporting
Figure 1. The molecular structure of 5b. The thermal ellipsoids are
shown at 50% probability. a) Top view and b) side view (peripheral
substituents omitted). Hydrogen atoms and solvent molecules are
omitted for clarity.
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Information). These results imply that the peripheral aryl
groups can be used as anchoring groups for further function-
alization. The low-symmetry 6 showed a Q band at l =
884 nm, which is between that of 5a (l = 878 nm) and 1 (l =
915 nm), thus suggesting indeed that 6 is an intermediate
compound between 1 and 5a. The dispersion-type MCD
curves corresponding to the Q bands of 5a and 6 are
theoretically Faraday A and pseudo Faraday A terms,
respectively.[12] In the case of 6, it is thought that the splitting
of the Q band is so small that the superimposition of two
oppositely-signed Faraday B terms give seemingly A-term-
like MCD signals. A larger MCD intensity in the Q band than
in the Soret band also indicates that the angular momentum
change in the Q transitions is larger than that in the Soret
transitions. As shown later in the section on molecular orbital
(MO) calculations, the transition of the Q band corresponds
to an angular orbital momentum change of Æ 11. Further-
more, a minus-to-plus MCD sign change in ascending energy
suggests that the energy difference between the HOMO and
HOMOÀ1 (DHOMO) is larger than that between the LUMO
and LUMO + 1 (DLUMO).[13]
To enhance the interpretation of the electronic absorption
spectra, MO calculations have been performed for unsubsti-
tuted and pyrrole proton-deprotonated Pc, uranium ion-free
and pyrrole proton-deprotonated 1, 5a, and 6 without tert-
butyl groups, that is, [Pc]2À, [1]2À, [5’]2À, and [6’]2À, at the DFT
level. This analysis omitting the metal is due to the fact that it
is difficult to evaluate accurately the electron density of f-
block metals such as uranium. The uranyl dication in this
study acts only as a template for stabilizing the macrocycle.
Accordingly, it appears possible to use these structures as
model structures for the calculations. Calculated stick
absorption spectra are attached at the bottom of Figure 2,
with obtained data summarized in Table S1 of the Supporting
Information, and the MO energy levels and isosurface plots of
some frontier MOs shown in Figure 3. The optimized
structures of [1]2À, [5’]2À, and [6’]2À are also distorted, thus
agreeing well with the crystal structures of the reported
SPc[6c,d] and 5b, which, therefore, suggests that the influence
of the uranyl cation on the structure is indeed marginal. The
calculated Q bands, which are transitions mainly from the
HOMO to LUMO and LUMO + 1, were estimated at
wavelengths close to the experimental values for the com-
pounds. In addition, as can be judged from the configuration
in Table S1, the contribution of the four frontier orbitals is
large in both the Q- and Soret-band regions, thus indicating
that these bands can be explained using Goutermanꢁs four-
orbital model.[14] The calculated Q-band intensity increases on
going from [5’]2À to [1]2À, that is, with increasing size of the
p system, as generally observed among tetraazaporphyrins of
varying sizes.[15]
Figure 3. Energies and some frontier orbitals of [Pc]2À, [1]2À, [5a’]2À
,
and [6’]2À. Calculations were performed at the B3LYP/6-31G* level of
theory.
a similar change as observed when the cyclic tetramers Pc and
Nc are compared. In going from Pc to Nc, with increasing p-
molecular size by fusion of the benzo rings, both the HOMO
and LUMO destabilize and the Q band shifts to a longer
wavelength. However, even the LUMOs and HOMO of [1]2À
(SPc 1) lie lower in energy than those of Pcs. This lower
energy of the HOMO appears to be the reason for high
stability of the SAzPs 5 and SPc 1. It is well known that Pcs are
susceptible to oxidation because the meso positions are prone
to oxidation depending on the extent of the destabilization of
the HOMO. Although the SAzPs 5 and SPc 1 in this study
have their Q bands at around l = 840–880 nm and l = 915 nm,
respectively, they appear to be much more stable than Ncs.[7a]
The cyclic p-electron systems can also be interpreted
using the (4n + 2) electron perimeter model.[13] A 22-electron
[20]-annulene ([C20H20]2À) was calculated as a model of the
inner ring of 1, and it was found that the number of nodal
planes of the frontier MOs of [1]2À is the same as that of
[C20H20]2À (see Figure S3 in the Supporting Information).
Thus, from the number of nodes, it is clear that the
compounds 1 and 5 are aromatic compounds satisfying n = 5
in (4n + 2), that is, Hꢂckelꢁs aromaticity rule. The angular
momentum change in the Q and Soret bands are therefore
Æ 11 and Æ 1, respectively.
As shown in Figure 3, the LUMO and LUMO + 1 of [1]2À
and [5’]2À are degenerate, similar to [Pc]2À, whereas the
degeneracy is lifted slightly in the case of [6’]2À. In going from
[Pc]2À to [1]2À, that is, from cyclic tetramer (Pc) to pentamer
(1), the energy of the four frontier orbitals is stabilized, but
since the stabilization of the LUMO is larger than that of the
HOMO, the Q band shifts to a longer wavelength. Compar-
ison between the cyclic pentamer [5’]2À and [1]2À reveals
Cyclic voltammograms of the tetra-tert-butyl-substituted
H2Pc (tBu4H2Pc), SPc 1, and 5a were measured in o-
dichlorobenzene (o-DCB; Figure 4). The tBu4H2Pc showed
redox couples at 0.26, À1.44, and À1.79 V (E1oxÀE1red
=
1.70 V), whereas the SPc 1 exhibited couples at 0.24, À0.94,
À1.29, À1.69 V (E1oxÀE1red = 1.18 V). Although the oxidation
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symmetry superphthalocyanines and to develop novel func-
tional expanded Pcs absorbing in the near-IR region.
Experimental Section
For full experimental details, spectroscopic data, and calculation
procedures, see the Supporting Information. X-ray crystal data is
available from the Cambridge Structural Database.
Synthesis of 5a: A mixture of 3,4-bis (4-tert-butylphenyl)pyrro-
line-2,5-dimine (4a, 70 mg, 0.20 mmol), and UO2(OAc)2·2DMF
(105 mg, 0.20 mmol) was dissolved in DMF (0.50 mL). The solution
was stirred at 1908C for 1 h and concentrated. The product was
purified by silica gel column chromatography (CHCl3/n-hexane =
1:1). The target compound was obtained (7 mg, 6% over two steps
based on compound 3a) as a brown solid. Mp: > 2808C. 500 MHz
1H NMR (CDCl3): d = 7.87 (d, 20H, J = 9.0 Hz), 7.42 (d, 20H, J =
9.0 Hz), 1.44 ppm (s, 90H). HRMS-MALDI: Calcd for
Figure 4. Cyclic voltammograms for 1.0 mm tBuH2Pc (bottom),
1 (middle), and 5a (top) in o-DCB containing 0.1m nBu4NClO4.
Ferrocene was used as an internal standard and set to 0 V.
C
120H130N10O2U [M]+: 1981.0881. Found: 1981.0855. UV/Vis
(CHCl3) (e ꢃ 10À4): lmax = 878 nm (3.4), 437 nm (3.7).
potentials are similar, the reduction of the SPc 1 is much
easier (by DE = 0.50 V) than that of tBu4H2Pc, thus indicating
marked stabilization of the LUMOs of the SPc 1. This result
implies that the origin of the narrow HOMO–LUMO gap of
1 is attributable to the low-lying LUMOs. The SAzP 5a
showed redox couples at 0.48, À0.85, and À1.18 V
(E1oxÀE1red = 1.33 V), thus revealing that the frontier orbitals
of 5a are further stabilized compared with those of the SPc 1,
and provides support for the predicted MO energy level.
Thus, the longer-wavelength shift of the Q bands of the SAzP
5 and SPc 1 is attributable to the stabilization of the frontier
orbitals, particularly the LUMOs. The position of the Q band
of general Pcs (ca. l = 650–700 nm), 5, and 1 is also in accord
with the order of the (E1oxÀE1red) values.
Received: April 25, 2012
Revised: August 9, 2012
Published online: October 4, 2012
Keywords: dyes/pigments · magnetic circular dichroism ·
.
porphyrinoids · structure elucidation · uranium
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In conclusion, we have synthesized superazaporphyrins
(SAzPs) consisting of five pyrroline-2,5-diimine units and
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by using pyrroline-2,5-diimine as a precursor. We have also
reinvestigated superphthalocyanine (SPc) using methods
which were not used 35–40 years ago. X-ray crystallography
revealed a distorted macrocyclic structure for the super-
azaporphyrins. Decaaryl SAzPs showed Q bands at approx-
imately l = 840–880 nm, and the spectra could be interpreted
by Goutermanꢁs four-orbital model. Both SAzPs and SPcs are
aromatic and can be explained by the (4n + 2) aromaticity
rule, where n = 5. MCD spectroscopy and MO calculations
strongly suggest that the Q-excited state is doubly degenerate.
Although the Q bands of SAzPs and SPcs appear at longer
wavelengths than those of Ncs, electrochemical measure-
ments and MO calculations indicated that they are relatively
air stable, since the energy of both the LUMOs and HOMO is
lower than those of Pcs and Ncs. These molecules can be new
candidates for functional near-IR dyes and produce new
insights into the chemistry of expanded phthalocyanines.
Further work is currently underway to prepare a series of low-
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ꢀ
1998.00, triclinic, space group P1 (No. 2), a = 10.2931(11) ꢀ,
b = 19.270(2) ꢀ, c = 24.443(3) ꢀ, a = 69.593(1)8, b = 84.516(1)8,
g = 89.051(1)8, V= 4522.4(9) ꢀ3, Z = 2, 1calcd = 1.424 gcmÀ3, T=
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