Synthesis of 2,7,12,17-tetraaryl-3,8,13,18-tetranitroporphyrins; electronic effects on aggregation properties of porphyrins

Article abstract of DOI:10.1039/a805471a

Novel porphyrins substituted with aryl and nitro substituents at the β-position were prepared by the tetramerization of 2-hydroxymethyl-3-aryl-4-nitropyrroles. Aggregation properties of these porphyrins are investigated by means of UV-VIS, ¹H-NMR and EPR spectroscopy. Porphyrin 4a substituted with 4-methoxyphenyl and nitro groups was found to form a strong cofacial aggregate in solution (K = 900-1300 dm³ mol^-1). The extent of aggregation decreases in the following order: porphyrin 4a (nitro and 4-methoxyphenyl substituents) > 4b (nitro and phenyl substituents) > TMTP (methyl and phenyl substituents) > OEP (ethyl substituents). EPR spectra of Cu-4a support the dimer structure of 4a, and the distance between two porphyrins is estimated to be about 4 A.

Full text of DOI:10.1039/a805471a

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Synthesis of 2,7,12,17-tetraaryl-3,8,13,18-tetranitroporphyrins;
electronic effects on aggregation properties of porphyrins
Noboru Ono,*a Emiko Muratani,a Yumiko Fumoto,a Takuji Ogawaa and Kunihiko Tazimab
a Department of Chemistry, Faculty of Science, Ehime University, Matsuyama 790, Japan
b Department of Polymer Science and Technology, Kyoto Institute of Technology, Matsugasaki,
Kyoto 606, Japan
Received (in Cambridge) 14th July 1998, Accepted 21st September 1998
Novel porphyrins substituted with aryl and nitro substituents at the β-position were prepared by the tetramerization
of 2-hydroxymethyl-3-aryl-4-nitropyrroles. Aggregation properties of these porphyrins are investigated by means
of UV–VIS, 1H-NMR and EPR spectroscopy. Porphyrin 4a substituted with 4-methoxyphenyl and nitro groups
was found to form a strong cofacial aggregate in solution (K = 900–1300 dm3 molϪ1). The extent of aggregation
decreases in the following order: porphyrin 4a (nitro and 4-methoxyphenyl substituents) > 4b (nitro and phenyl
substituents) > TMTP (methyl and phenyl substituents) > OEP (ethyl substituents). EPR spectra of Cu–4a support
the dimer structure of 4a, and the distance between two porphyrins is estimated to be about 4 Å.
Nitroporphyrins are important because of their reactivity,1,2
altered electronic properties,3 aggregation properties,4 and use
as non-linear optical materials.5 For example, Crossley and
his co-workers have reported that the reaction of nitroporph-
yrins with alkoxides, amines or Grignard reagents provides a
new method for the preparation of alkoxy-, amino- or alkyl-
substituted porphyrins.1 The reaction of nitroporphyrins with
the carbanion derived from active methylene compounds such
as malononitrile or isocyanoacetate provides a new synthetic
route to dihydroporphyrins or pyrroloporphyrins.2 The elec-
tronic properties of porphyrins are effectively altered by the
presence of a strong electron withdrawing nitro group.3 In an
extreme case, zinc and nickel β-heptanitroporphyrins form
air-stable π-anion radicals by electrochemical reduction.3
Aggregation effects have been observed in porphyrins and
related compounds, and aggregation appears to be most prom-
inent in porphyrins bearing strongly electron withdrawing
groups at either the meso- or β-positions. Thus, nitroporphyrins
may provide important models for the study of porphyrin
aggregation.4 Although a number of nitroporphyrins have been
prepared by the nitration of porphyrins,6 this method is severely
limited by the lack of regioselective control in the production of
the desired nitroporphyrin. This paper reports a new approach
to meso-unsubstituted-β-nitroporphyrins 4 by tetramerization
of β-nitropyrroles, which cannot be prepared by the ordinary
nitration method. Porphyrins 4 are interesting because they
have four nitro and four aryl groups at alternate positions.
Aggregation of such porphyrins may provide an important
strategy for making supramolecular porphyrin arrays via self-
assembly of porphyrins. Therefore, the aggregation properties
O2N
Ar
O2N
Ar
Ar
i
ii
NO2
CHO
N
H
N
H
2
1
Ar
O2N
O2N
Ar
NO2
Ar
iv
N
iii
N
N
H
CH2OH
N
H
H
N
3
O2N
Ar
Ar
NO2
MeO
Ar =
4a
4b
Ar = Ph
Scheme 1 Reagents and conditions: i, TosMIC, NaH; ii, POCl3, DMF;
iii, NaBH4; iv, p-TsOH, Zn(OAc)2, chloranil; then HCl.
80 ЊC followed by oxidation gave a mixture of regioisomers of
tetraaryl-tetranitroporphyrin in about 1% yield. The meso-
1
proton portion of the H-NMR spectrum of the mixture of
nitroporphyrins thus obtained is shown in Fig. 1(b), where
complicated resonances are observed. This is explained by the
presence of four regioisomers. The reaction of 1 with aromatic
aldehydes did not give the desired porphyrins due to the low
reactivity of 1. Therefore, we employed another tetramerization
reaction developed by Ogoshi and our group for the synthesis
of porphyrins with electron withdrawing groups.8 Formylation
of 1 followed by reduction with NaBH4 gave 2-hydroxymethyl-
pyrroles 3, which were reactive enough to give porphyrins 4.
Porphyrin 4a (Ar = p-MeOC6H4) and 4b (Ar = Ph) were pre-
pared in 5–10% yield by treatment with p-TsOH and sub-
sequent oxidation in the presence of zinc acetate. Zinc acetate
acts as an effective template to improve the yield of Zn–4a, and
the zinc metal is finally removed on treatment with acid to give
1
of nitroporphyrins 4 were studied by means of UV–VIS, H-
NMR and EPR spectroscopy and were compared with those of
other related nitro-free porphyrins.
Results and discussion
(1) Synthesis
The route used for the preparation of 2,7,12,17-tetraaryl-
3,8,13,18-tetranitroporphyrins 4 is shown in Scheme 1. The
requisite β-nitropyrroles (1) were prepared by the reaction of
nitrostyrenes with tosylmethyl isocyanide (TosMIC).8 The reac-
tion of 1 with dimethoxymethane and p-TsOH in benzene at
1
4a. The H-NMR spectrum of the meso protons of the porph-
yrin prepared by this method is shown in Fig. 1(a), where the
meso protons are observed as a singlet. Type 1 porphyrins 4 as
J. Chem. Soc., Perkin Trans. 1, 1998, 3819–3824
3819

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Table 1 Absorption spectra of 4a, 4b and other porphyrins
Porphyrina
Solvent
λmax (ε/104 dm3 molϪ1 cmϪ1
)
w1/2/nmb
4a
CHCl3
DMF
CHCl3
CHCl3
DMF
CHCl3
CHCl3
CHCl3
470 (5.6) 551 (1.9) 592 (0.68) 616 (0.63) 681 (0.35)
477 (5.5) 568 (1.2) 627 (1.2)
464 (5.6) 575 (1.1) 620 (1.2)
457 (11) 546 (3.6) 591 (1.1) 622 (1.0) 682 (0.75)
453 (11) 570 (2.2) 627 (2.0)
453 (1.2) 567 (2.1) 608 (2.0)
78
66
54
33
42
33
15
10
Cu–4a
Cu–4a
4b
Cu–4b
Cu–4b
TMTP
OEP
406 (20) 502 (1.7) 536 (1.2) 570 (0.87) 622 (0.57)
400 (22) 500 (1.8) 532 (1.4) 565 (0.91) 612 (0.63)
a TMTP: 2,7,12,17-Tetramethyl-3,8,13,18-tetraphenylporphyrin; OEP: 2,3,7,8,12,13,17,18-octaethylporphyrin. The concentration of the porphyrins
was held at 1.0 × 10Ϫ5 mol dmϪ3 b w1/2 = The half width of the Soret band.
.
Table 2 Redox properties of metal porphyrins 4a
ox
1/2
ox
1/2
re
1/2
re
1/2
Porphyrin
E
(1)
E
(2)
E
(1)
E
(2)
∆E1/2(1)
Cu–4a
1.19
1.30
0.80
0.66
Ϫ0.61
Ϫ0.58
Ϫ1.50
Ϫ1.71
Ϫ1.10
Ϫ1.06
1.80
1.88
2.30
2.37
Cu–4b
CuTMTP
CuOEP
1.20
1.18
a E/V vs. SCE in CH2Cl2, 0.2 M TBAP.
Fig. 1 1H-NMR spectra of the meso protons of 4a; (a) preparation
according to Scheme 1, (b) reaction of 1 with CH2(OMe)2.
illustrated in Scheme 1 are formed selectively via tetrameriz-
ation of 3 as shown in Fig. 1, isomeric purity is estimated by
NMR analysis and is not greater than 95%.
The absorption spectra of these porphyrins and their metal
complexes are shown in Fig. 2. Absorption maxima (λmax) and
molar extinction coefficients (ε) are summarized in Table 1. The
Soret and Q bands of 4 are shifted to the longer wavelengths
and are very broad compared to those of 2,7,12,17-tetramethyl-
3,8,13,18-tetraphenylporphyrin (TMTP)9 and 2,3,7,8,12,13,
17,18-octaethylporphyrin (OEP). The broad spectra suggest
that 4a and 4b form aggregates even at a low concentration of
10Ϫ5 mol dmϪ3. Reasonably good linear relationships exist
between the interplanar distance between the two porphyrin
rings and the width of the UV–VIS Soret band.10 The half-
width of the Soret band (w1/2) is a simple estimation of porph-
yrin aggregation. According to this estimation, the extent of
aggregation is in the order of 4a > 4b > TMTP. This is further
confirmed by means of 1H-NMR and EPR experiments.
The redox potentials of the copper porphyrins were meas-
ured by the cyclic voltammetry (CV) method. The CV studies
Fig. 2 Absorption spectra of porphyrins 4.
were performed on a Pt electrode using CH2Cl2 as the solvent
and 0.2 M tetrabutylammonium perchlorate (TBAP) as the
supporting electrolyte. The redox potential data of Cu–4 are
summarized in Table 2. The one-electron redox potentials of
porphyrins reflect the energy levels of their HOMO and
LUMO. The difference (∆E1/2) reflects the energy difference
between HOMO and LUMO, which is normally 2.2–2.3 V for
usual porphyrins. The introduction of four nitro groups lowers
the LUMO and HOMO levels by 0.9–1.0 V and 0.5 V, respect-
ively. These values are in good agreement with those of known
nitroporphyrins.3 Thus, ∆E, namely the π–π* gap, for tetranitro-
3820
J. Chem. Soc., Perkin Trans. 1, 1998, 3819–3824

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Fig. 3 Plot of 1H chemical shift versus concentrations for the meso-H
resonances of 4a.
porphyrins 4 is smaller than that of other porphyrins which
brings about the red shift of the UV–VIS spectra. This is due
either to π-conjugation of the nitro group with the porphyrin
rings or to aggregation of 4. Since electron deficient porphyrins
are important because they are excellent catalysts in metal-
loporphyrin catalyzed oxygenation reactions,4 many electron
deficient porphyrins have been prepared. Perfluoroalkylated
porphyrins behave differently from 4. Namely, the π–π* gap of
perfluoroalkylated porphyrins is not affected by the introduc-
tion of the perfluoroalkyl substituents.11
Fig. 4 EPR spectra in toluene at 77 K.
hyperfine splittings are well-resolved for Cu–TMTP which are
typical of spectra for non-aggregated Cu–porphyrins.14 On the
other hand, EPR spectra of Cu–4a are different from those of
Cu–TMTP, where two strong perpendicular transitions, with-
out nitrogen hyperfine splittings are observed in the g = 2
region. This is typical of a triplet (S = 1) state with axial sym-
metry. In the case of Cu–4b, mixed spectra derived from
monomer and dimer were observed. In the triplet system, the
electron spin is delocalized over both Cu() centers, causing a
decrease in the Cu and N hyperfine splitting. The zero-field
splitting parameter, D, was used to calculate the distance
R between the Cu() centers.14,15 The observed Cu–Cu distance
of 4.3 Å is in good agreement with the reported value of the
covalent linked dimers14 as well as those derived from non-
covalent dimers.15 The geometry of the porphyrin–porphyrin
interaction is also the subject of a recent study. In general por-
phyrins prefer to stack in an offset manner rather than with one
molecule directly over another, and several types of interaction
have been proposed.16 However, the aggregates of 4a may form
cofacial dimers rather than slipped ones due to the symmetrical
substituents of 4a.
A new route to nitroporpyrins 4 has been found, and they
and their metal complexes form cofacial aggregates in solution.
As biologically active porphyrins usually occur as dimers, syn-
thetic porphyrin dimers or aggregated dimers of porphyrins
have been extensively studied. They are built up by a covalent
bond,17 self assembly via electrostatic interaction between posi-
tively and negatively charged porphyrins15,18 or a hydrogen
bonding interaction.19 They are the focus of much interest as
model enzymes, model photosynthetic systems, and oxygen
activators.20 In contrast to the cofacial aggregations of porph-
yrins studied so far, the cofacial aggregation of 4 takes place
by spontaneous association of neutral porphyrins.
(2) Aggregation properties
The aggregation properties of porphyrins are of central
importance to understanding the chemistry of chlorophylls and
metalloporphyrins, especially in relation to their important bio-
logical functions such as efficient electron transfer in the photo-
synthetic reaction center.12 The aggregation of porphyrins has
been extensively studied in solution by NMR spectroscopy.13
1
The H-NMR spectra of 4a showed large complexation shifts
as shown in Fig. 3 and Table 3. Similar shifts were also observed
for 4b. 1H-NMR measurements at different concentrations can
be used to determine the dimerization constant, K, using the
reported procedure.12 The chemical shifts of the monomers (δm)
and of the dimers (δd) are estimated, and they are summarized
in Table 4. By this method, K was found to be 900 (from meso-
H change)–1300 (from NH change) and 90 (from meso-H
change)–130 (from NH change) dm3 molϪ1 for 4a and 4b,
respectively. The effect of the concentration of TMTP and OEP
was also studied, and K was evaluated from these experiments.
Some chemical shifts at certain concentrations are shown in the
Experimental section. Interestingly, the effect of concentration
1
on the H-NMR chemical shifts was very small in these cases.
Although K for TMTP could be estimated to be 16 (from meso-
H change)–10 (from NH change) dm3 molϪ1, no evidence of
self-association was observed for OEP. The NH proton signals
are shifted upfield for the cofacial dimers relative to the corres-
ponding monomers. The magnitude of this shift (∆δ) is correl-
ated with the distance between the two porphyrins. Namely, P–
P = 6.46 ϩ 0.88∆δ Å.11 According to this equation, the distance
between monomers in the dimers derived from 4a is estimated
to be about 4 Å. Aggregation of alkyl substituted porphyrins
such as OEP is negligible when they are present as free bases.
Metal complexes of OEP and their radical cations form aggre-
gates.14 TMTP shows greater aggregation than OEP but less
than 4a and 4b. Thus, aggregation is strongly enhanced by
the introduction of a nitro group. As 4a has a symmetrical
structure, cofacial dimers should be favored for the dimer
structure.
Experimental
1H-NMR spectra were recorded on a JEOL-JNM-GSX 270
spectrometer using tetramethylsilane as an internal standard.
IR and UV–VIS spectra were obtained with Hitachi 270-30 and
Shimadzu-2200 spectrometers, respectively. EPR spectra were
recorded on a JES-ME-3X X-band spectrometer. FAB mass
spectra of porphyrins were measured with a JEOL JMS-DX-
300 spectrometer; samples were dissolved in CHCl3 and m-
nitrobenzyl alcohol was used as the matrix.
EPR spectra of the copper complexes of these porphyrins
were measured at 77 K (Fig. 4), and EPR parameters are sum-
marized in Table 5. Both copper (I = 3/2) and nitrogen (I = 1)
J. Chem. Soc., Perkin Trans. 1, 1998, 3819–3824
3821

Table 3 Observed chemical shifts [δ (ppm)] for 4a versus concentration (10Ϫ3 mol dmϪ3
)
Concentration/10Ϫ3 mol dmϪ3
Proton
9.84
8.20
7.03
6.15
5.57
4.92
4.10
3.51
3.07
2.73
2.46
1.64
1.34
1.13
0.92
0.74
0.53
0.41
0.082
meso
Ha
Hb
OCH3
NH
9.99
7.81
7.19
4.13
Ϫ6.19
9.99
7.82
7.20
4.13
Ϫ6.15
10.02
7.86
7.20
4.13
Ϫ6.10
10.03
7.84
7.22
4.13
Ϫ6.05
10.05
7.88
7.22
4.13
Ϫ6.01
10.06
7.86
7.23
4.13
Ϫ5.94
10.08
7.87
7.24
4.13
Ϫ5.89
10.09
7.88
7.24
4.13
Ϫ5.83
10.11
7.88
7.25
4.13
Ϫ5.79
10.12
7.89
7.25
4.13
5.81
10.13
7.89
7.26
4.13
Ϫ5.64
10.17
7.94
7.27
4.12
Ϫ5.56
10.19
7.93
7.27
4.12
Ϫ5.59
10.20
7.95
7.28
4.12
Ϫ5.32
10.22
7.94
7.28
4.12
Ϫ5.31
10.24
7.93
7.29
4.12
Ϫ5.11
10.27
7.94
7.29
4.11
Ϫ4.95
10.30
7.95
7.30
4.11
Ϫ4.84
10.45
8.00
7.32
4.09
Ϫ4.14

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Table 4 Calculated equilibrium constants (K) and monomer and dimer shifts (δm, δd)
meso-H
NH
Porphyrin
K/dm3 molϪ1 δm
δd
∆δ
K/dm3 molϪ1
δm
δd
∆δ
4a
900
90
16
0
10.49
9.83
9.84
9.95
0.66
0.91
0.14
0
1300
130
10
Ϫ3.70
Ϫ2.70
Ϫ3.51
Ϫ3.75
Ϫ6.75
Ϫ6.32
Ϫ4.10
Ϫ3.75
3.05
3.62
0.59
0
4b
10.75
10.09
10.11
TMTP
OEP
10.11
0
Table 5 EPR parameters
Monomer
Dimer
a
a
a
g||
g⊥
A||
A⊥
g||
g⊥
A||
Da
Rb
Cu–4a
2.198
2.222
2.144
2.058
1.712
2.073
3.88
3.93
4.3
4.3
Cu–4b
2.193
2.187
2.096
2.088
2.072
2.152
0.392
0.304
Cu–TMTP
a Hz. b 10Ϫ2 cm. c Å.
Table 6 Observed proton chemical shifts versus concentration
3-Aryl-2-formyl-4-nitropyrroles
They were prepared according to the procedure given in the
literature.7
Concentration/
Porphyrin
10Ϫ3 mol dmϪ3
meso-H δ (ppm)
NH δ (ppm)
Ϫ3.77
Preparation of porphyrin 4a
4b
2.36
1.69
1.05
0.75
0.58
0.33
17.51
8.01
5.09
2.91
1.09
0.68
0.37
19.82
9.91
4.95
0.47
10.53
10.57
10.62
10.65
10.67
10.71
10.05
10.06
10.07
10.07
10.07
10.09
10.09
10.11
10.10
10.11
10.11
Ϫ3.61
Ϫ3.38
Ϫ3.21
Ϫ3.14
Ϫ2.97
Ϫ3.63
Ϫ3.59
Ϫ3.51
Ϫ3.54
Ϫ3.47
Ϫ3.54
Ϫ3.54
Ϫ3.74
Ϫ3.75
Ϫ3.75
Ϫ3.75
A mixture of 2-formyl-3-(4-methoxyphenyl)-4-nitropyrrole (2.5
g, 10 mmol) and NaBH4 (10 mmol) in THF (25 ml) was stirred
at room temperature for 8 h, poured into water and extracted
with CH2Cl2 (100 ml × 3). To the combined extracts were added
toluene-p-sulfonic acid (0.05 g) and zinc acetate dihydrate (4.49
g, 20 mmol), and the resulting solution was stirred for 12 h at
room temperature and then chloranil (2.5 g, 10 mmol) was
added. The mixture was refluxed for 8 h, and then washed with
5% sodium hydrogen carbonate, and the organic layer was dried
with anhydrous sodium carbonate. After evaporation, col-
umn chromatography of the residue (silica gel, CH2Cl2) and
recrystallization from CH2Cl2–MeOH gave Zn–4a which was
treated with 6 M HCl (10 ml) in CH2Cl2 (20 ml) at room
temperature. The organic layer was evaporated to give 4a which
was further purified by recrystallization from CH2Cl2–MeOH
to give 4a (0.23 g, 10% yield). The copper complex of 4a
was prepared by treatment with copper acetate in CH2Cl2–
MeOH. Porphyrin 4b was prepared in 8% yield by the same
procedure.
TMTP
OEP
from these data using the same procedure as given in the liter-
ature.13 The results are summarized in Table 4.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports and
Culture, Japan.
4a: At a concentration of 9.84 × 10Ϫ3 mol dmϪ3, δH(CDCl3)
9.99 (s, meso-H), 7.81 (d, 8 H), 7.19 (d, 8 H), 4.13 (s, 12 H),
Ϫ6.19 (s, NH); these chemical shifts are changed to δH(CDCl3)
10.45, 8.00, 7.32, 4.09, Ϫ4.14 at a concentration of 0.082 × 10Ϫ3
mol dmϪ3; νmax(KBr)/cmϪ1 3420, 1510, 1330; m/z (FAB) 914.8
(Mϩ, C48H34N8O12 requires 914.843), 1829 (2Mϩ) (Found: C,
62.67; H, 3.83; N, 3.53. Calc. for C48H34N8O12: C, 63.02; H,
3.75; N, 12.25%). Cu–4a: m/z (FAB) 976 (Mϩ, C48H32N8O12Cu
requires 976.366), 1953 (2Mϩ).
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N, 14.10%). Cu–4b: m/z (FAB) 856.14 (Mϩ, C44H24N8O8Cu
requires 856.27).
1H-NMR measurement
Chemical shifts were determined at various concentrations of
the porphyrins. The data for 4a are shown in Table 3. Selected
data for the meso and NH protons of 4b, TMTP and OEP are
shown in Table 6. Equilibrium constants (K) were determined
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3823

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