A porphyrin with two coordination sites: The biquinoline ligand as a new potential external chelate

Article abstract of DOI:10.1002/ejoc.201100413

A highly symmetrical doubly fused porphyrin was obtained after two successive intramolecular cyclizations.1 The first supplementary ring was closed by using the Cadogan reaction, involving the generation of an intermediate nitrene from the starting nitro

Full text of DOI:10.1002/ejoc.201100413

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201100413
A Porphyrin with Two Coordination Sites: The Biquinoline Ligand as a New
Potential External Chelate
Christophe Jeandon[a] and Romain Ruppert*[a]
Keywords: Porphyrinoids / N ligands / Macrocycles / Cyclization
A highly symmetrical doubly fused porphyrin was obtained
after two successive intramolecular cyclizations.[1] The first
supplementary ring was closed by using the Cadogan reac-
tion, involving the generation of an intermediate nitrene
from the starting nitro function. By trying to use the same
methodology for the second ring closure, we found that the
Cadogan reaction could be used but that cyclization yields
remained low. We showed, by taking into account unusual
mass spectroscopy results, that the cyclization of the nitro
functionality with the neighboring meso-phenyl took place
in the absence of the oxygen abstracting phosphite used by
Cadogan. The bis-biquinolino-fused porphyrin, bearing an
internal and an external coordination site, was obtained in
very good yield and showed strong absorbance between 700
and 800 nm.
Therefore, the synthesis of new porphyrins bearing external
coordination sites is also a topic of great interest today.
Herein the synthesis of a porphyrin bearing an external bi-
quinoline is reported. This molecule shows an extension of
conjugation (λmax up to 775 nm) and the presence of an
external chelating site (biquinoline) that might allow the
synthesis of coordinatively linked porphyrin dimers.
Introduction
Tetrapyrrolic macrocycles (porphyrins, porphyrazines,
and phthalocyanines) are extremely useful precursors for
the synthesis of chromophores that might be used in the
field of imaging or in photodynamic therapy.[2] To obtain
dyes with the desired large absorbances in the 650–1000 nm
region, it is generally necessary to extend the already large
aromatic ring of these molecules. This can be done more or
less easily by fusing other aromatic systems to the por-
phyrin macrocycle. Alternatively, it is also possible to use
already present meso aryl substituents and fuse them to the
internal core of the porphyrin, thus leading to a much
larger aromatic system. In the last two decades, numerous
examples have been described.[3–7]
In some cases, functional groups are used as links be-
tween the meso aryl groups and the neighboring pyrrole,
and heteroatoms can be introduced at the periphery of the
porphyrinic macrocycle. These heteroatoms can then be
used as external coordination sites for metal ions. Several
different external sites are known already and have been
used to build porphyrinic dimers or oligomers in which the
macrocycles are linked by metal ions (Scheme 1). Among
the most promising are the external coordination sites con-
jugated with the macrocycle, because dimers and oligomers
linked by metal ions might lead to strong interactions be-
tween the individual porphyrins. In these compounds, elec-
tron delocalization over the whole molecule was described.
Scheme 1. Extended porphyrins that have been used to build por-
phyrin dimers linked by metal ions.[8]
Results and Discussion
We previously reported that doubly fused nickel por-
phyrin 3 was easily accessible from readily available nickel
enaminoporphyrin 1 by successive formylation and cycliza-
tion (Scheme 2).[9] The very good yield obtained for the
Vilsmeier–Haack formylation and the chemoselectivity for
the enamine carbon prompted us to try other electrophilic
aromatic substitution reactions, like nitration.
After introduction of the nitro functionality at the β-pyr-
rolic position, we expected to use the Cadogan reaction (ni-
trene generation from nitro groups with triethyl phosphite)
for a second intramolecular cyclization. This cyclization
had been used earlier for the synthesis of 1 (Scheme 3).[6a]
[a] UMR 7177 du CNRS, Institut de Chimie, Université de Stras-
bourg,
1 rue Blaise Pascal, 67000 Strasbourg, France
E-mail: rruppert@unistra.fr
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100413.
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 4098–4102

A Porphyrin with Two Coordination Sites
Scheme 2. Synthetic route leading to doubly fused nickel porphyrin
3.
Scheme 3. Synthesis of nickel enaminoporphyrin 1 from nickel 2-
nitroporphyrin.
However, all attempts to nitrate directly compound 1
failed, leading always to intractable polar materials. There-
fore, the nitrogen was first protected (Scheme 4). The en-
amine nitrogen was deprotonated by sodium hydride and
Scheme 4. Synthetic route leading to free-base porphyrin 7.
then treated with freshly distilled isobutylchloroformate to tion (leading to 1) usually proceeded with a yield of 75%.
afford 4 cleanly and in good yield. Looking at the mass spectrum, we realized that the mo-
Selective nitration of the enamine pyrrolic position was lecular mass of 6 (m/z = 694) was the same value as that
obtained by using a mild nitrating method (lithium nitrate/ observed in the mass spectrum of starting nitro derivative
acetic anhydride) at room temperature. The yield (only 5. We concluded that the desired cyclization occurred dur-
64%) obtained after purification did not reflect the clean ing the ionization process (electronic impact with heating)
reaction seen on TLC while following the reaction, and al- inside the mass spectrometer even in the absence of the oxy-
1
though the H NMR spectrum showed that the compound gen scavenger triethylphosphite. Therefore, compound 5
was pure, the mass spectrum gave unusual results. Next to was heated at reflux in 1,2-dichlorobenzene for 30 min to
the small expected molecular mass peak of 5 (m/z = 828), afford compound 6 in 80% yield. Finally, we ran the reac-
the major peak was found at m/z = 694 (see Supporting tion sequence without purifying intermediate nitrated com-
Information). This value did not correspond to an expected pound 5. Simple neutralization, followed by washings with
fragment of 5.
water, evaporation of the solvents, and heating the crude
As envisaged after introduction of the nitro functionality reaction mixture at reflux in 1,2-dichlorobenzene gave com-
on the β-pyrrolic position, we tried Cadogan’s reaction con- pound 6 in 57% yield (from 4).
ditions (triethylphosphite, 155 °C) to see if the second cycli-
The nickel porphyrin was easily demetalated by following
zation might take place. Indeed, doubly fused nickel por- standard procedures and free base 7 was isolated in almost
phyrin 6 was detected and isolated in pure form but in a quantitative yield. At room temperature, the 1H NMR spec-
very poor yield (less than 10%) after tedious chromatog- trum of this compound was rather unusual compared to the
raphy, because the reaction led to numerous products. The spectra of “normal” free-base porphyrins (Figure 1, spec-
1
compound was easily identified by H NMR spectroscopy trum at 30 °C). No pyrrolic protons (appearing around
and mass spectrometry. The high symmetry of 6 was re- 9 ppm as doublets with J ≈ 5 Hz) were found after a first
flected in a very simple aromatic proton spectrum with only look at the spectrum.
1
six pyrrolic protons and the specific pattern of cyclized
By running the H NMR spectrum at higher tempera-
phenyl for two meso phenyl groups. The very high ab- ture, the signals could be assigned. The β-pyrrolic protons
sorbance up to 764 nm (ε = 22600 m–1 cm–1) was also a clear were located at δ = 8.37 and 7.76 ppm (two doublets) and
indication for extension of the aromatic ring. The very poor at δ = 7.30 ppm (singlet) for the two β-pyrrolic protons op-
yield was quite disappointing, as the first Cadogan cycliza- posite the pentacyclic aromatic ring. In porphyrins, β-pyr-
Eur. J. Org. Chem. 2011, 4098–4102
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C. Jeandon, R. Ruppert
SHORT COMMUNICATION
Now, comparison between similar doubly fused por-
phyrins can be made. With two naphthalene units fused,
the nickel compound prepared by Smith et al. absorbs at
629 nm.[4b] With one naphthalene and one quinoline fused
(Ni porphyrin 3, Scheme 2), the λmax was found at
730 nm.[9] Finally, in nickel porphyrin 6 with two fused
quinolines the lowest energy band was found at 765 nm.
Surprisingly, the absorbances we measured (average values
from several measurements) are two times as strong as
those reported recently for the same compounds.[1]
Conclusions
We have developed a new synthetic route to doubly fused
porphyrins. Due to the good availability of the starting ma-
terial (the nickel meso-tetraaryl-2-nitroporphyrins are ac-
cessible in gram quantities) and the high-yielding synthetic
route, these compounds are potentially good photosensitiz-
ers. The free base represents a new example of a porphyrin
bearing the classical internal coordination site of por-
phyrins as well as a potential external chelating coordina-
tion site (the biquinoline ligand is a well-known chelate).
Work is in progress to obtain polymetallic complexes with
this kind of ligand.
Figure 1. 1H NMR spectra (in deuterated tetrachloroethane, aro-
matics) of free-base 7 as a function of temperature.
rolic protons usually appear around 9 ppm, and in pyrrole
they appear at δ = 6.05 ppm. The β-pyrrolic protons found
at δ = 7.30 ppm are therefore observed at a chemical shift
between those of pyrrole and porphyrins. This might be a
good indication that the macrocycle is highly distorted and
that these protons are located out of the plane of the por-
phyrin ring current. Similar unusual chemical shifts were
observed for the previously published free-base derivative
of Ni-3 (X-ray structure of this free base confirmed the dis-
torted structure).[9]
Experimental Section
General Procedures: Known starting compound nickel enamino-
porphyrin 1 was prepared according to previously published pro-
cedures.[6a] Chromatographic separations were obtained using
Merck 9385 silica gel or Merck 1097 alumina.
Nickel Porphyrin 4: To a degassed solution of green nickel enami-
noporphyrin 1 (429 mg, 627 μmol) in THF (60 mL) was added so-
dium hydride (60% dispersion, 50 mg, 1.25 mmol). After 30 min of
stirring at room temperature, freshly distilled isobutyl chloro-
formate (500 mg, 3.6 mmol) was added. After 1 h, the reaction mix-
ture was neutralized with water. After addition of dichloromethane
(150 mL), the organic phase was washed with water (3ϫ) and dried
with sodium sulfate. After evaporation of the solvents, the residue
was purified by chromatography (silica gel; cyclohexane/dichloro-
methane, 3:1). After crystallization (dichloromethane/methanol),
red compound 4 was obtained in 81% yield (397 mg, 506 μmol).
1H NMR (400 MHz, CDCl3, 25 °C): δ = 9.49 and 8.89 (2 d, J =
4.8 Hz, 2 H, pyrrole), 8.84 (s, 1 H, pyrrole), 8.70 and 8.67 (2 d, J
= 4.8 Hz, 2 H, pyrrole), 8.65 (s, 2 H, pyrrole), 8.53 (dd, J = 8,
1.2 Hz, 1 H, cycl. Ph), 8.28 (dd, J = 8, 1.2 Hz, 1 H, cycl. Ph), 8.00
(br. m, 6 H, o-Ph), 7.65–7.70 (m, 9 H, m+p-Ph), 7.62 (ddd, J = 8,
1.2, 1.2 Hz, 1 H, cycl. Ph), 7.53 (ddd, J = 8, 1.2, 1.2 Hz, 1 H, cycl.
Ph), 4.38 (d, J = 6 Hz, 2 H, CH2), 2.14 (m, 1 H, CH), 1.03 (d, J =
6.4 Hz, 6 H, CH3) ppm. UV/Vis (CH2Cl2): λmax (ε, m–1 cm–1) = 434
(190000), 552 (17800), 600 (17000) nm. C49H35N5NiO2 (784.56):
calcd. C 75.02, H 4.50, N 8.93; found C 75.05, H 4.47, N 9.45.
The electronic spectra of free-base 7 and that of nickel
porphyrin 6 are shown in Figure 2. Both compounds ab-
sorb with very high extinction coefficients (from 15000 to
22000 m–1 cm–1 in the case of metalated porphyrin Ni-6) in
the 650–800 nm window that is useful for optimal light
penetration of tissue.[2] With different metal ions (ZnII, PdII,
...), these new chromophores might be potentially good
photosensitizers for PDT. However, the toxicity and solubil-
ity issues for the new compounds must be studied.
Nickel Porphyrin 5: A solution containing nickel porphyrin 4
(160 mg, 204 μmol), acetic acid (20 mL), acetic anhydride (20 mL),
and lithium nitrate (102 mg, 1.5 mmol) in chloroform (100 mL) was
stirred at room temperature for 90 min. The nitrating agent was
then neutralized by stirring this solution in the presence of a satu-
rated aqueous sodium hydrogenocarbonate solution (100 mL) for
2 h. After washing the organic phase with water (3ϫ) and drying
Figure 2. Electronic spectra of porphyrin free-base 7 (normal line)
and nickel porphyrin 6 (bold line).
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A Porphyrin with Two Coordination Sites
with sodium sulfate, the solid residue obtained after evaporation of
the solvents was purified by chromatography (silica gel; cyclohex-
ane/dichloromethane, 1:1). After crystallization (dichloromethane/
methanol), compound 5 was obtained in 64% yield (109 mg,
Centre National de la Recherche Scientifique (CNRS) and the Uni-
versity of Strasbourg for financial support.
1
131 μmol). H NMR (400 MHz, CDCl3, 25 °C): δ = 9.45 and 8.84
[1]
[2]
[3]
During the course of this work, the same compounds 6 and 7,
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See for example: K. M. Kadish, K. M. Smith, R. Guilard
(Eds.), Handbook of Porphyrin Science Vol. 4: Phototherapy,
Radioimmunotherapy and Imaging, World Scientific, Singapore,
2010.
(2 d, J = 4.8 Hz, 2 H, pyrrole), 8.86 (dd, J = 8, 1.2 Hz, 1 H, cycl.
Ph), 8.65 and 8.57 (2 d, J = 4.8 Hz, 2 H, pyrrole), 8.61 and 8.34 (2
d, J = 4.8 Hz, 2 H, pyrrole), 8.10 (dd, J = 8, 1.2 Hz, 1 H, cycl. Ph),
7.94–7.99 (br. m, 6 H, o-Ph), 7.62–7.74 (m, 10 H, m+p-Ph + cycl.
Ph), 7.61 (ddd, J = 8, 1.2, 1.2 Hz, 1 H, cycl. Ph), 4.15 (d, J = 7 Hz,
2 H, CH2), 1.92 (m, 1 H, CH), 0.78 (d, J = 6.8 Hz, 6 H, CH3) ppm.
UV/Vis (CH2Cl2): λmax (ε, m–1 cm–1) = 452 (148000), 566 (17700),
606 (12500) nm. C49H34N6NiO4 (829.56): calcd. C 70.95, H 4.13,
N 10.13; found C 71.14, H 4.17, N 10.39.
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Nickel Porphyrin 6: A solution of nickel porphyrin 5 (75 mg,
90 μmol) in 1,2-dichlorobenzene (5 mL) was heated at reflux for
30 min. The completion of the reaction was checked by TLC. The
reaction mixture was then poured directly onto a silica gel column.
Elution with dichloromethane (to remove trace amounts of starting
material and apolar side products) followed by dichloromethane/
ethyl acetate (9:1) gave clean nickel porphyrin 6. After crystalli-
zation (dichloromethane/methanol), green compound 6 was iso-
1
lated in 80% yield (50 mg, 72 μmol). H NMR (400 MHz, CDCl3,
25 °C): δ = 8.72 (d, J = 8 Hz, 2 H, cycl. Ph), 8.68 (d, J = 8 Hz, 2
H, cycl. Ph), 8.67 and 8.11 (2 d, J = 4.8 Hz, 4 H, pyrrole), 7.80–
7.86 (m, 4 H, cycl. Ph), 7.76–7.79 (m, 4 H, o-Ph), 7.67 (s, 2 H,
pyrrole), 7.58–7.65 (m, 6 H, m+p-Ph) ppm. UV/Vis (CH2Cl2): λmax
(ε, m–1 cm–1) = 403 (69700), 474 (52700), 695 (16400), 729 (20000),
765 (22600) nm. C44H24N6Ni (695.42): calcd. C 76.00, H 3.48, N
12.09; found C 75.60, H 3.51, N 12.44. Alternatively, starting from
compound 4 (79 mg, 100 μmol), and running the nitration reaction
as before, but skipping the purification of 5 (no chromatography
after the neutralization and washings), simple heating of the crude
reaction mixture at reflux in 1,2-dichloromethane gave compound
6 in 57% yield after filtration of the solution on silica gel (dichloro-
methane followed by 10% ethyl acetate in dichloromethane) and
recrystallization.
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Porphyrin Free-Base 7: A solution of nickel porphyrin 6 (58 mg,
84 μmol) in trifluoroacetic acid (5 mL) and sulfuric acid (5 mL)
was stirred at room temperature for 3 h. Then the reaction mixture
was poured on ice, and after addition of dichloromethane
(100 mL), neutralized with a saturated sodium hydrogenocarbonate
aqueous solution. The organic phase was washed with water (3ϫ),
dried, and concentrated. Crystallization (dichloromethane/meth-
anol) gave the free base in almost quantitative yield (53 mg,
1
83 μmol). H NMR (400 MHz, C2D2Cl4, 70 °C): δ = 8.98 (m, 2 H,
cycl. Ph), 8.85 (m, 2 H, cycl. Ph), 8.37 (d, J = 4.4 Hz, 2 H, pyrrole),
7.96 (m, 4 H, cycl. Ph), 7.85 (m, 4 H, o-Ph), 7.76 (d, J = 4.4 Hz, 2
H, pyrrole), 7.67 (m, 6 H, m+p-Ph), 7.30 (s, 2 H, pyrrole) ppm.
UV/Vis (CH2Cl2): λmax (ε, m–1 cm–1) = 394 (81000), 435 (49000, sh.),
514 (12600), 619 (17800), 676 (20700), 775 (11800) nm.
C44H26N6·CH2Cl2 (723.65): calcd. C 74.69, H 3.90, N 11.61; found
C 75.13, H 4.09, N 12.30 (due to drying, the solid sample contained
a little bit less than one crystallization molecule of dichloromethane
per porphyrin molecule).
[5]
Supporting Information (see footnote on the first page of this arti-
cle): Detailed experimental procedures with spectroscopic data for
all new compounds (1H NMR, electronic, and mass spectra).
Acknowledgments
We thank Alain Hazemann for elemental analyses, Estelle Motsch
for EI mass spectra, Jennifer Wytko for helpful comments, and the
Eur. J. Org. Chem. 2011, 4098–4102
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C. Jeandon, R. Ruppert
SHORT COMMUNICATION
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Received: March 25, 2011
Published Online: June 1, 2011
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Eur. J. Org. Chem. 2011, 4098–4102