Synthesis, characterization, and electrochemical studies of new π-Extended metalloporphyrins

Article abstract of DOI:10.1002/ejoc.200900856

A doubly fused porphyrin has been synthesized by two successive cyclization reactions. The first meso-phenyl group was fused, by an intramolecular Cadogan reaction leading to an enamine-functionalized porphyrin. After Vilsmeier-Haack formylation of the ni

Full text of DOI:10.1002/ejoc.200900856

FULL PAPER
DOI: 10.1002/ejoc.200900856
Synthesis, Characterization, and Electrochemical Studies of New π-Extended
Metalloporphyrins
Angel J. Jimenez,[a][‡] Christophe Jeandon,[a] Jean-Paul Gisselbrecht,[a] and
Romain Ruppert*[a]
Keywords: Porphyrinoids / Oxidation / Cyclic voltammetry / Conjugation
A doubly fused porphyrin has been synthesized by two suc-
cessive cyclization reactions. The first meso-phenyl group
was fused by an intramolecular Cadogan reaction leading to
an enamine-functionalized porphyrin. After Vilsmeier–
Haack formylation of the nickel enaminoporphyrin, an intra-
molecular Friedel–Crafts reaction followed by dehydration of
the intermediate alcohol afforded the doubly fused aromatic
nickel porphyrin, which could be demetalated. The free base
and the copper and palladium porphyrins show extremely
high absorbances in the 700–800 nm range. The palladium
complex was a good catalyst for the oxidation of cholesterol
with singlet oxygen, which makes this chromophore a poten-
tial candidate for photodynamic therapy.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
Introduction
Results and Discussion
Starting from the easily available nickel 2-nitro-meso-
tetraphenylporphyrin Ni-1, the green nickel porphyrin Ni-2
was obtained by reaction with triethyl phosphite.[7a] The Ni-
2 porphyrin is functionalized with an enamine that connects
the porphyrin aromatic core to a neighboring meso-phenyl
group. Formylation of the enamine under Vilsmeier–Haack
conditions led to the enamino aldehyde Ni-3. This nickel
porphyrin is now fitted with an external chelating site,
which allows the coordination of metal ions (see Scheme 1).
This compound and various analogues have been used to
build porphyrin dimers connected by divalent metal ions.[6a]
Strong electronic coupling through the connecting metal
ions was observed in these compounds, as in similar por-
phyrin oligomers published previously.[8]
The synthesis of extended aromatics or π-conjugated
molecules is a very active field of research.[1] Molecules of
this type are expected to be used in materials science due to
their unusual electronic properties. Among these aromatics,
porphyrin tapes, described by Tsuda and Osuka, are the
most spectacular achievement in this research area.[2]
In the particular case of porphyrins, extension of the aro-
matic core might also be useful for medical applications like
photodynamic therapy, for which sensitizers that absorb
preferentially in the 650–900 nm range are desired.[3] Sev-
eral synthetic strategies might be envisaged. One of the
most promising approaches is the fusion of the meso-aryl
group to the aromatic core of the porphyrin. Compounds
in which benzene, naphthalene, anthracene, pyrene, and
azulene have been fused across the meso and β positions or
with a porphyrinic pyrrole have been synthesized.[4,5]
Various fused heterocycles (e.g., pyridine, imidazole, pyr-
role) have also been described.[6] Herein the synthesis, char-
acterization, and electrochemical study of a doubly fused
(naphthalene and quinoline) extended porphyrin are re-
ported.
[a] UMR 7177 du CNRS, Institut de Chimie, Université de Stras-
bourg,
Scheme 1. Synthesis of enamino aldehyde Ni-3.
1 rue Blaise Pascal, 67000 Strasbourg, France
E-mail: rruppert@unistra.fr
[‡] Present address: Departamento de Quimica Organica, Uni-
versidad Autonoma de Madrid,
It is also well known that under acidic conditions, the
nickel 2-formylporphyrin Ni-4 leads to cyclized species like
the ketone Ni-5 (see Scheme 2).[4a,4b]
Cantoblanco, 28049 Madrid, Spain
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200900856.
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A. J. Jimenez, C. Jeandon, J.-P. Gisselbrecht, R. Ruppert
FULL PAPER
of singly fused meso-tetraarylporphyrins, a set of four in-
equivalent coupled protons are generally observed in ad-
dition to the usual meso-phenyl signals. Sometimes, the
neighboring pyrrole protons are also highly deshielded due
to the steric hindrance of the cyclized phenyl. The 1H NMR
spectrum (300 MHz in CDCl3 at 45 °C) of Ni-6 revealed
two sets of four inequivalent and coupled phenyl protons
in addition to one aromatic proton appearing as a broad
singlet at low field (δ = 8.98 ppm, attributed to the isolated
naphthyl proton). Three sets of doublets (two at low field
and one at higher field) were observed for the six remaining
pyrrolic protons. Ni-6 was demetalated by following the
standard procedure for removing the nickel ion (sulfuric
acid in trifluoroacetic acid, see Scheme 4).
Scheme 2. Cyclization of formylporphyrin Ni-4.
This prompted us to try to cyclize the enamino aldehyde
Ni-3 under acidic conditions. Under the experimental con-
ditions used to cyclize Ni-4 (room temperature, trifluoro-
acetic acid), no reaction occurred. However, by using more
drastic conditions (10 equiv. p-toluenesulfonic acid in re-
fluxing chlorobenzene), complete conversion of the starting
material took place after 1 h (Scheme 3).
Scheme 4. Demetalation of nickel porphyrin Ni-6.
The free base H2-6 was characterized by the usual spec-
1
troscopic techniques. The H NMR signals were satisfacto-
Scheme 3. Cyclization of enamino aldehyde Ni-3.
rily assigned after a variable-temperature study (between 25
and 75 °C). The β-pyrrolic protons were broad at room
temperature due to N–H proton exchange between the dif-
ferent protonation sites of the molecule (four pyrrolic and
one pyridine nitrogen atoms). An X-ray structure of suit-
able crystals confirmed the doubly fused nature of the com-
pound (see Figure 1).
However, instead of a ketone or methylene linkage be-
tween the β-pyrrolic position and the neighboring meso-
phenyl group (expected after dismutation of an intermedi-
ate alcohol), two major products were isolated and charac-
terized after chromatography of the reaction mixture. The
first compound to elute from the column proved to be Ni-
2 (isolated in 28% yield), which was clearly the product of
acid-catalyzed thermal decarbonylation of Ni-3. Decarbon-
ylation of aromatic aldehydes (and especially of porphyrinic
aldehydes) is a known reaction, but generally requires much
more drastic conditions.[9] The second major product was
the doubly fused Ni-6 (52% yield). This compound was ob-
tained after dehydration of the intermediate alcohol. This
pathway was highly favored in this cyclization reaction be-
cause the dehydration resulted in the aromatization of the
fused pentacyclic entity (this cannot occur if only one fused
meso-phenyl group is involved). To avoid decarbonylation,
the experimental conditions were improved by decreasing
the number of acid equivalents and/or lowering the reaction
temperature. By using only 2 equiv. of p-toluenesulfonic
acid in refluxing chlorobenzene (5 h were then required to
obtain a 95% conversion) Ni-6 was isolated in 60% yield.
This yield was further improved by lowering the reaction
temperature (48 h in toluene at reflux in the presence of
2 equiv. of p-toluenesulfonic acid). Under these conditions,
Ni-6 was obtained in a satisfactory 72% yield. Even under
these relatively mild conditions, decarbonylation was still
observed (Ni-2 was isolated in 8% yield). The doubly fused
Figure 1. X-ray crystal structure of the free base H2-6. Top and
1
nature of Ni-6 could be easily deduced from the H NMR
side views showing the ruffled nature of the porphyrin macrocycle
and HRMS spectra (m/z = 693.1534 [M + H]+). In the case and the flat pentacyclic part.
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New π-Extended Metalloporphyrins
The unit cell contains two molecules of the free base H2- tion bands are ideally located for photodynamic studies, for
6 and four benzene molecules (solvent used for the crystalli- which the therapeutic window starts from approximately
zation). The inner pyrrolic N–H hydrogen atoms of the por- 650 up to 900 nm. To check if these compounds might be
phyrin are located (at least in the solid state, not in solution) useful as sensitizers, their ability to oxidize cholesterol was
on the two opposite pyrroles not involved in the fused verified under irradiation.[11] The efficiency of the newly ob-
pentacyclic aromatic system. The porphyrin core itself is tained sensitizers was compared with the well-known tet-
ruffled. In contrast, the pentacyclic fused aromatic (naph- raphenylporphyrin (H2TPP) and all the results are summa-
thalene–pyrrole–quinoline) appears to be almost flat. The rized in Table 1 (see the Supporting Information for de-
fused quinoline and naphthalene groups cannot be distin- tails).
guished, each group half-occupying the positions in the X-
ray structure. The peri interactions between the fused naph-
thyl (or quinolyl) and the neighboring pyrroles are the cause
of the ruffled nature of the macrocycle. The two hydrogen
atoms (β-pyrrolic and naphthyl or β-pyrrolic and quinolyl)
are in close proximity (d = 2.11 and 2.13 Å, respectively).
The β–βЈ pyrrolic C–C bonds are also markedly different in
this porphyrin. Whereas the three isolated pyrroles present
bond lengths (1.358, 1.349, and 1.361 Å) in accordance with
the bond length found in porphyrin bases, the β–βЈ pyrrolic
C–C bond of the fourth pyrrole fused to the naphthyl and
quinolyl groups is significantly longer (1.445 Å). In the X-
ray structures of chlorines, these bond lengths are close to
1.36 Å for the normal pyrroles and 1.52 Å for the reduced
β–βЈ bond.[10] In this porphyrin, one of the β–βЈ pyrrolic
C–C bonds seems to have lost part of its double bond (or
aromatic) character.
This loss together with the extension of the conjugation
might explain in part the unusual electronic spectra of the
porphyrin free base and the metal complexes (see Figure 2).
Typical of simple meso-tetraarylporphyrins is a very intense
Soret band in the 400–500 nm area with extinction coeffi-
cients usually much greater than 105 –1 cm–1. In our case,
the Soret band is split into at least two bands between 400
and 470 nm (with extinction coefficients well below
105 –1 cm–1).
Table 1. Photochemical oxidation of cholesterol: efficiency of dif-
ferent sensitizers.[a]
Sensitizer[a]
λmax [nm], ε [–1 cm–1]
Turnover number
H2TPP
H2-6
Ni-6
Cu-6
Pd-6
648, 3400
751, 17500
734, 36000
732, 42000
710, 43000
16
11.5
0
5
52
[a] Experimental conditions: benzene (20 mL), cholesterol
(2ϫ10–3 ), sensitizer (10–5 ), 4 h irradiation (400 W slide projec-
tor, λ Ͼ 510 nm), cholesterol/sensitizer = 200.
Because these metalloporphyrins have an external basic
site (the nitrogen atom of the fused quinoline), their elec-
tronic spectra were expected to change in the presence of
acid. This was indeed observed, as is shown in Figure 3 for
the nickel complex Ni-6. After the addition of a drop of
trifluoroacetic acid, the spectrum was shifted to the NIR.
Figure 3. Electronic spectra of the nickel porphyrin Ni-6 before
(normal line) and after the addition of trifluoroacetic acid (bold
line).
The electrochemistry of these molecules was studied by
cyclic (CV) and rotating disk voltammetry (RDV). The elec-
trochemistry of metalloporphyrins is well established and
for simple metalloporphyrins like the one used in our stud-
ies (nonelectroactive metal ions), two reversible oxidation
and two reduction waves are usually described.[12] As ex-
pected, two well-resolved reversible one-electron reductions
Figure 2. Electronic spectra of the free base H2-6 (normal line) and
of the palladium complex Pd-6 (bold line).
The lowest-energy band of the free base was found at were observed for all of the compounds (see Table 2 for all
751 nm. Moreover, the same band was observed at wave- potential values). The RDV studies showed two oxidation
lengths higher than 700 nm (with very large extinction coef- steps, with amplitudes similar to those measured in the re-
ficients) for the nickel (λ = 734 nm, ε = 36000 –1 cm–1), the ductions (see the Supporting Information). However, the
copper (λ = 732 nm, ε = 42000 –1 cm–1), and the palladium first oxidation step was spread out over the potential axis.
complexes (λ = 710 nm, ε = 43000 –1 cm–1). These absorp- Wave analyses for Ni-6, Cu-6, and Pd-6 RDV studies dem-
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A. J. Jimenez, C. Jeandon, J.-P. Gisselbrecht, R. Ruppert
FULL PAPER
onstrated that these spread-out waves correspond to the rich” neutral molecule should be highly favored if the two
overlap of two waves of equal amplitude. Assuming a one- species are large flat aromatics. In the case of Pd-6 and Cu-
electron transfer for each reduction step, these spread-out 6, an additional third nonreversible oxidation step was ob-
oxidation steps involve two 0.5 electron steps.
served at +1.15 and 1.09 V, respectively.
Table 2. Cyclic voltammetry data for M-6.[a]
Conclusions
Species
Er°ed2
Er°ed1
Eo°x1
Eo°x2
Eo°x3
Epa
A doubly fused porphyrin has been synthesized. Starting
from the cheapest metalloporphyrin (nickel meso-tetraphen-
ylporphyrin), new chromophores were obtained in a few
steps in good yields. Successive nitration (95% yield), Cado-
gan cyclization (75% yield), Vilsmeier–Haack formylation
(90% yield), and Friedel–Crafts cyclization (72% yield) of
the nickel porphyrin gave these new chromophores in very
good overall yields. The reaction sequence is composed of
well-known and widely used reactions that all use very
cheap reagents. The nickel complex could be easily demet-
alated. Based on the preparation of the copper and palla-
dium complexes, it seems that the porphyrin free base might
be complexed to many other metal ions. All of these com-
pounds showed high absorbances in the therapeutic win-
dow and some of them were able to generate singlet oxygen
efficiently, which makes these sensitizers good candidates
for photodynamic therapy.
Owing to their large flat aromatic pentacyclic substruc-
ture, which is similar to some other natural alkaloids (cryp-
tolepine), these compounds might also be potential intercal-
ators of DNA bases or G-Quadruplex stabilizing ligands
and therefore potential inhibitors of telomerases.[15]
Finally, the presence of the pyridine might be used to
complex metal ions or metal complexes at this nitrogen co-
ordinating site. Because this pyridine is fused to the aro-
matic core of the porphyrin one might expect substantial
interactions between the metal ions located inside and out-
side of the porphyrin ring.
H2-6
–1.46
(60)
–1.60
(60)
–1.58
(60)
–1.55
(60)
–1.11
(60)
–1.20
(70)
–1.18
(60)
–1.17
(60)
+0.53
(110)
+0.42
(75)
+0.44
(60)
+0.83
(60)
+1.21
Epa
+1.03
Epa
+0.81
(60)
Ni-6
Pd-6
Cu-6
+0.58
(75)
+0.68
(60)
+0.61
(60)
+1.15
Epa
+1.09
Epa
+0.38
(60)
[a] Potentials are given in Volt vs. the ferrocene/ferrocenium couple
used as an internal standard. Values indicated in parentheses corre-
spond to the peak potential differences (∆Ep = Epa – Epc in mV).
See the Exp. Sect. for details of the conditions.
Cyclic voltammetry experiments confirmed the RDV re-
sults. Two well-resolved, reversible, one-electron reduction
waves were observed for all compounds, which were, as ex-
pected for large aromatic molecules, separated by approxi-
mately 400 mV. The “first” oxidation is clearly split into two
steps for the nickel, palladium, and copper porphyrins (see
Figure 4).
Experimental Section
General Procedures: The known starting compounds nickel enami-
noporphyrin 2 and nickel enamino aldehyde 3 were prepared ac-
cording to our previously published procedures.[7a] Chromato-
graphic separations were performed by using Merck 9385 silica gel
or Merck 1097 alumina.
Nickel Porphyrin 6: A degassed solution of nickel enamino alde-
hyde 3 (127 mg, 178 µmol) and p-toluenesulfonic acid (79 mg,
415 µmol) in toluene (45 mL) was heated at reflux under argon for
48 h. After cooling, the reaction mixture was neutralized with a
saturated aqueous solution of sodium hydrogen carbonate. The or-
ganic phase was then washed with water and dried with sodium
sulfate. After evaporation of the solvent, the residue was added
to a chromatography column [silica gel, eluent: from cyclohexane/
dichloromethane (1:1) to dichloromethane/ethyl acetate (3:1)]. Af-
ter separation and crystallization (dichloromethane/methanol),
nickel enamine 2 and nickel porphyrin 6 were obtained in 8 (10 mg,
14 µmol) and 72% (89 mg, 128 µmol) yields, respectively; m.p.
Figure 4. Cyclic voltammograms (v = 100 mVs–1) of the free base
H2-6 and of the nickel complex Ni-6 in the presence of ferrocene.
Similar behavior has been observed for other π-extended
diketoporphyrins.[13] After being generated at the anode, the
porphyrinic radical cation associates strongly with the re-
maining neutral molecule to give a dimeric radical cationic
species, which is then oxidized at a different and more an-
odic potential. Similar electrochemical results have been de-
scribed for other large flat aromatic molecules (eilatin, for
example) or even for their metal complexes.[14] Association
1
Ͼ260 °C. H NMR (300 MHz, CDCl3, 45 °C): δ = 8.63 and 8.09
(2 d, J = 5 Hz, 2 H, pyrrole), 8.61 and 8.10 (2 d, J = 5 Hz, 2 H,
between an electron-poor radical cation and an “electron- pyrrole), 7.78 and 7.72 (2 d, J = 4.8 Hz, 2 H, pyrrole), 8.98 (s, 1
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New π-Extended Metalloporphyrins
H, Hnaphth), 8.53, 8.13, 7.72, and 7.54 (4 m, 4 H, Hnaphth), 8.68,
CCDC-741936 contains the supplementary crystallographic data
8.45, 7.74, and 7.73 (4 m, 4 H, Hquin), 7.78–7.83 (m, 4 H, Hortho), for H2-6. These data can be obtained free of charge from The Cam-
7.57–7.63 (m, 6 H, Hmeta+para) ppm. UV/Vis (CH2Cl2): λmax (ε) =
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
401 (57000), 469 (85000), 668 (20000), 696 (24000), 734
(36000 –1 cm–1) nm. HRMS: calcd. for C45H25N5Ni
+
H+
Supporting Information (see also the footnote on the first page of
694.1536; found 694.1534.
this article): Experimental details for cholesterol photochemical
1
oxidation, H NMR and electronic spectra, RDV and CV experi-
Free Base H2-6: A solution of nickel porphyrin 6 (305 mg,
439 µmol) in a mixture of trifluoroacetic acid and sulfuric acid (25
and 12 mL, respectively) was stirred at room temperature for 2 h.
Then the reaction mixture was poured onto ice and after addition
of dichloromethane (100 mL), the aqueous phase was neutralized
with sodium hydrogen carbonate. The organic phase was dried and
after crystallization (dichloromethane/methanol) the green free-
base H2-6 was obtained in 95% yield (266 mg, 417 µmol); m.p.
Ͼ260 °C.1H NMR (300 MHz, CDCl3, 50 °C): δ = 8.43 and 7.80 (2
d, J ≈ 5 Hz, 2 H, pyrrole), 8.42 and 7.79 (2 d, J ≈ 5 Hz, 2 H,
pyrrole), 7.47 and 7.41 (2 d, J = 4.5 Hz, 2 H, pyrrole), 9.46 (s, 1
H, Hnaphth), 8.82, 8.34, 7.82, and 7.65 (4 m, 4 H, Hnaphth), 8.97,
8.67, 7.86, and 7.86 (4 m, 4 H, Hquin), 7.80–7.88 (m, 4 H, Hortho),
7.59–7.67 (m, 6 H, Hmeta+para) ppm. UV/Vis (CH2Cl2): λmax (ε) =
398 (61000), 443 (52000), 505 (14500), 679 (19500), 751
(17500 –1 cm–1). HRMS: calcd. for C45H27N5 + H+ 638.2339;
found 638.2331.
ments, unit cell view of the crystal structure of H2-6.
Acknowledgments
We thank A. Decian (Service Commun de Rayons X, Université
Louis Pasteur) for solving the structure, J.-M. Strub for recording
the HRMS mass spectra, and H. J. Callot and J. Wytko for helpful
comments. A. J. J. thanks the European Student Exchange Program
(ERASMUS) for a fellowship.
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Cu-6: M.p. Ͼ260 °C. UV/Vis (CH2Cl2): λmax (ε) = 401 (62000), 467
(73000), 665 (21000), 699 (27000), 732 (42000 –1 cm–1). HRMS:
calcd. for C45H25N5Cu + H+ 699.1479; found 699.1474.
Pd-6: M.p. Ͼ260 °C. 1H NMR (300 MHz, CDCl3, 45 °C): δ = 8.82
and 8.14 (2 d, J = 4.8 Hz, 2 H, pyrrole), 8.76 and 8.13 (2 d, J =
4.8 Hz, 2 H, pyrrole), 7.96 and 7.92 (2 d, J = 4.8 Hz, 2 H, pyrrole),
9.09 (s, 1 H, Hnaphth), 8.76, 8.23, 7.81, and 7.64 (4 m, 4 H, Hnaphth),
8.89, 8.57, 7.83, and 7.82 (4 m, 4 H, Hquin), 7.88–7.93 (m, 4 H,
Hortho), 7.65–7.75 (m,
6 H, Hmeta and Hpara) ppm. UV/Vis
(CH2Cl2): λmax (ε) = 402 (50000), 467 (72000), 647 (18000), 681
(27000), 710 (43000 –1 cm–1). HRMS: calcd. for C45H25N5Pd +
H+ 742.1218; found 742.1195.
Electrochemical Studies: Dichloromethane was purchased spectro-
scopic grade from Merck, dried with molecular sieves (4 Å), and
stored under argon prior to use. NBu4PF6 was purchased electro-
chemical grade from Fluka and used as received. The electrochemi-
cal experiments were carried out at room temperature in dichloro-
methane containing 0.1  NBu4PF6 in a classical three-electrode
cell by cyclic voltammetry (CV) or rotating disk voltammetry
(RDV). The working electrode was a glassy carbon disk (3 mm in
diameter), the auxiliary electrode a Pt wire, and the pseudo-refer-
ence electrode a Pt wire. All potentials are given vs. Fc/Fc+ used
as an internal standard and are uncorrected from the ohmic drop.
Crystal Data for Compound H2-6: C57H39N5, C45H27N5·2C6H6, M
¯
=
793.98, triclinic, space group P1, a = 10.8368(10), b =
11.6205(10), c = 18.234(2) Å, α = 107.42(2)°, β = 95.33(2)°, γ =
107.32(2)°, V = 2049.3(3) Å3, Z = 2, and D = 1.29 gcm–3. A total of
11945 reflections were collected from a green crystal of dimensions
0.20ϫ0.12ϫ0.08 mm3 using a KappaCCD diffractometer, graph-
ite-monochromated Mo-Kα irradiation, 1.19ϽθϽ30.07°, and a of
temperature 173 K. A total of 6427 unique reflections having
IϾ2σ(I) were used to determine and refine the structure. Final re-
sults: R = 0.0694, wR2 = 0.1907, GOF = 0.984, and final largest
difference peak: 0.274 eÅ–3.
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Received: July 28, 2009
Published Online: September 28, 2009
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2009, 5725–5730