Synthesis of stable free base secochlorins and their corresponding metal complexes from meso-tetraarylporphyrin derivatives

Article abstract of DOI:10.1039/c2cc17951b

Cleavage reactions of 2,3-diamino-meso-tetraarylporphyrins and meso-tetraarylporphyrins fused to imidazole rings afforded secochlorins including stable free base derivatives.

Full text of DOI:10.1039/c2cc17951b

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COMMUNICATION
Synthesis of stable free base secochlorins and their corresponding metal
complexes from meso-tetraarylporphyrin derivativeswz
a
a
b
Mamadou Lo,a Jean-Franc
¸ ois Lefebvre, Nathalie Marcotte, Claire Tonnele,
b
´
David Beljonne, Roberto Lazzaroni, Sebastien Clement and Sebastien Richeter*
b
a
a
´
´
´
Received 20th December 2011, Accepted 3rd February 2012
DOI: 10.1039/c2cc17951b
Cleavage reactions of 2,3-diamino-meso-tetraarylporphyrins
and meso-tetraarylporphyrins fused to imidazole rings afforded
secochlorins including stable free base derivatives.
Here, we report some new procedures to obtain secochlorins,
including stable free base ones. Hoffman and coworkers showed
that amino groups are electron-rich groups activating b,b0-double
bonds of porphyrazines and that cleavage reactions afforded
secoporphyrazines like 4-Ni and 4-H2. Surprisingly, 2,3-diamino-
porphyrins have never been considered as starting material for
the synthesis of secochlorins. Recently, Telvekar and Bachhav
reported the synthetic utility of hypervalent iodine reagent
(diacetoxyiodo)benzene (DIB) for diaminoaryl carbon–carbon
cleavage reaction (Scheme 2).7 We noticed that 2,3-diamino-
porphyrins react in the same way with DIB as observed for
diaminoaryl derivatives (Scheme 3).
Secochlorins are chlorin derivatives containing one cleaved
b,b0-bond and possessing maximum wavelength absorption
bands at around 650 nm. They were first described almost
twenty years ago and were initially prepared by the thermolysis
of (octadehydrocorrinato)nickel(II) chloride salts.1 Then, several
secochlorins2–5 and secoporphyrazines6 were reported in
the literature. Some representative structures of secochlorins
(1-Ni, 1-H2, 2-H2 and 3-H2) and secoporphyrazines (4-Ni and 4-H2)
are displayed in Scheme 1. The oxidative pyrrole ring opening of
porphyrins and porphyrazines involves activated b,b0-double
bonds by the presence of electron-rich substituents. Surprisingly,
few examples of free base secochlorins are known in the
literature. Secochlorins 2-H2 and 3-H2 (Scheme 1) synthesized
by Sessler and coworkers are rare examples of stable free base
secochlorins.4 Indeed, free base secochlorin 1-H2, which was
2,3-Diaminoporphyrin 6-Ni was easily obtained from the
2-amino-3-nitroporphyrin 5-Ni through the reduction of the
nitro function. Then, two equivalents of DIB were added to a
solution of 6-Ni in CH2Cl2. Within a few minutes, the colour
of the reaction mixture evolved from brown to green. The
compound exhibited a less polar character than the starting
reactant, as evidenced by TLC. The major green compound
corresponded to the symmetrical 2,3-dicyano-2,3-secochlorin
7-Ni, which could be easily purified by column chromatography
on SiO2 (eluent: CH2Cl2). It was obtained in 75% yield after
this two-step procedure (reduction and cleavage reactions).
No reaction occurred when DIB was added to a solution of
synthesized by Bruckner and coworkers, appeared to be
¨
unstable due to its high reactivity.3
Scheme 2 Cleavage reaction of 1,2-diaminobenzene with DIB.
Scheme 1 Structures of secochlorins 1-Ni, 1-H2, 2-H2 and 3-H2, and
secoporphyrazines 4-Ni and 4-H2.
a Institut Charles Gerhardt Montpellier ICGM, UMR 5253
´
´
CNRS-ENSCM-UM2-UM1, equipe CMOS, Universite Montpellier 2,
`
Place Eugene Bataillon, Batiment 17, CC1701,
ˆ
34095 Montpellier Cedex 5, France. E-mail: sricheter@univ-montp2.fr;
Fax: +33 4 67 14 38 52; Tel: +33 4 67 14 45 28
b Service de Chimie des Mate´riaux Nouveaux, Universite´ de Mons
(UMONS), Place du Parc 20, 7000 Mons, Belgium
w This article is part of the ChemComm ‘Porphyrins and Phthalocyanines’
web themed issue.
z Electronic supplementary information (ESI) available: Synthetic
procedures, characterisation data and computational studies. See
DOI: 10.1039/c2cc17951b
Scheme 3 Synthesis of secochlorins 7-Ni and 7-Zn.
c
3460 Chem. Commun., 2012, 48, 3460–3462
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Fig. 2 Top (left) and side (right) views of the optimized structure of
7-Ni. Corresponding views of 7-H2 are displayed in Fig. S1 in ESI.z
Fig. 1 UV-visible spectra of 7-Ni (dashed), and 7-Zn (dotted) and
7-H2 (full) in CH2Cl2.
compared to complexes 7-Ni and 7-Zn. The UV-visible spectrum
of the 7-H2 displayed a broadened Soret band centered at
435 nm and Q-bands at 560 nm, 619 nm and 685 nm (Fig. 1).
Molecular modeling was used to gain insight into the
possible conformations of those molecules and to help the
interpretation of their optical absorption spectra. DFT and
TD-DFT methods were applied at the B3LYP/6-31g(d) level
to optimise the molecular structure of 7-Ni, 7-Zn and 7-H2,
and simulate their respective optical absorption spectra. All
optimised geometries present a ruffled-like conformation induced
by the steric interaction between the cyano groups (Fig. 2, left).
This is best appreciated from the dihedral angle between the two
opposite pyrrole rings along the diagonal direction.8 This angle is
231 in 7-H2, 221 in 7-Zn and 391 in 7-Ni (Fig. 2, right). Thus,
while substitution of the internal H with Zn2+ has a vanishingly
small effect on the conformation of the core, complexation of
Ni2+ leads to a strongly distorted structure. This is consistent
with the smaller size of the nickel ion, combined with the large
flexibility of the secochlorin ring allowed by the b,b0-bond
cleavage.9 The simulated absorption spectra are chlorin-like
and in good agreement with the experimental ones (see ESIz,
Fig. S2): they predict strong Soret B bands and weak Q bands as
expected from the simple Gouterman’s four-orbital model.10
Recently, we were also interested in the synthesis of por-
phyrins fused to imidazole and imidazolium rings,11 and we
showed that free base porphyrins fused to imidazole rings are
precursors of secochlorins. We noticed that the free base
porphyrin 8-H2 (R = H) was slowly converted into a green
compound both in solution and solid state in the presence of
air and light (Scheme 5). Mass spectrometry analysis of this
green compound showed that it corresponds to an oxidized
species of the free base of imidazole 8-H2 (m/z = 911.5 compared
to m/z = 879.5 [M + H+] for 8-H2). According to 1H NMR and
mass spectra, this oxidized species corresponds to unsymmetrical
2-cyano-3-N-formylamide-2,3-secochlorins 9-H2 (R = H,
Scheme 5). In the FT-IR spectrum of 9-H2, a cyano stretching
frequency appeared at n = 2203 cmꢀ1, and two carbonyl
2-amino-3-nitroporphyrin 5-Ni in CH2Cl2, showing the importance
to have two neighboring electron-rich b-amino groups. The
mass peak corresponding to the mono protonated form of
7-Ni was observed by HR-MS (m/z = 921.4166 [M + H]+).
The UV-visible spectrum showed the Soret absorption band at
l = 451 nm and one Q band at l = 661 nm (Fig. 1). In the
FT-IR spectrum of 7-Ni, the cyano stretching frequencies
appeared as a single band at n = 2211 cmꢀ1 1H NMR
.
spectroscopy shows the 2-fold symmetry of secochlorin 7-Ni,
since signals corresponding to b-hydrogen atoms exhibit the
expected splitting pattern: a pair of doublets at d = 7.92 and
8.35 ppm and one singlet at d = 8.13 ppm.
Then, we turned our attention to the formation of the
corresponding free base secochlorin. The demetallation of
7-Ni by using standard acidic conditions (H2SO4 20% in
TFA at 25 1C) failed (a complex mixture of several compounds
was obtained). Since Zn2+ is easier to remove from the inner core
of the macrocycle compared to Ni2+, we synthesized 7-Zn and
obtained it in 78% yield following the procedure described for
1
the synthesis of 7-Ni (Scheme 3). H NMR and mass spectra
are in accordance with the proposed structure for 7-Zn
(see ESIz). The electronic absorption spectrum of 7-Zn
displayed a Soret band at l = 443 nm and an intense Q band
at l = 646 nm (Fig. 1). In the FT-IR spectrum of 7-Zn, the
cyano stretching frequencies appeared as a single band at
n = 2208 cmꢀ1. Free base secochlorin 7-H2 was then successfully
obtained in 95% yield by treating 7-Zn with TFA in CH2Cl2
at room temperature (Scheme 4). HR-MS showed a peak at
m/z = 865.4967 ([M + H]+) corresponding to the mono
protonated form of 7-H2. The 1H NMR signal due to the
internal NH of the free base secochlorin was observed at
d = ꢀ0.38 ppm. The presence of the cyano groups was confirmed
by FT-IR spectroscopy and their stretching frequencies were
observed as a single band at n = 2202 cmꢀ1, a lower value
stretching frequencies appeared at n = 1683 and 1733 cmꢀ1
.
The 1H NMR spectrum of 9-H2 confirmed the absence of
2-fold symmetry of the molecule. The broad doublet at
d = 9.22 ppm (J = 9.7 Hz) is the characteristic signal of the
N-formyl group CHO.12 1H NMR COSY experiments showed
the NOE correlation with the neighboring NH group at
d = 8.41 ppm (see ESIz). 13C{1H} NMR spectroscopy also
Scheme 4 Synthesis of secochlorin 7-H2.
c
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Chem. Commun., 2012, 48, 3460–3462 3461

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only (the perfect nontoxic reagents) or hypervalent iodine
reagent DIB, which generates iodobenzene and acetic acid as
the side compounds (less toxic than osmium and lead complexes).
Current investigations are now devoted to the chemical trans-
formations of the cyano groups to obtain more elaborated
chromophores.
We are grateful to the CNRS and the Agence Nationale de
la Recherche for financial support of the research project
ANR-09-JCJC-0089-01. Research in Mons is supported by
FNRS/FRFC, BELSPO (PAI 6/27) and Region Wallonne
´
(Programme d’excellence OPTI2MAT). DB is FNRS Research
Director.
Notes and references
Scheme 5 Synthesis of secochlorins 9-H2 and 11-H2.
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confirmed the presence of two CQO groups at d = 168.1 and
161.9 ppm. UV-visible spectrum of 9-H2 is very similar to that
of secochlorin 7-H2 with a Soret band at l = 435 and Q-bands
at l = 559, 601 and 684 nm. The same photoxidation reaction
carried out with porphyrin 10-H2 (R = Me) afforded the
corresponding secochlorin 11-H2 (R = Me, Scheme 5). As
expected for 11-H2, the signal due to the CHO group is a
broad singlet at d = 9.75 ppm due to the absence of the
neighboring NH group. Absorption bands of 11-H2 are
slightly hypsochromically shifted compared to secochlorin
9-H2: the Soret band is observed at l = 429 nm, and three
Q-bands are observed at l = 547, 597 and 673 nm.
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We presume that singlet oxygen (1O2) generated by free base
macrocycles under ambient light is the dominant oxidant.
Preliminary experiments showed that irradiation of diluted
solution of 8-H2 and 10-H2 in CH2Cl2 during 15 hours with a
slide projector (250 W) dramatically increased the yields of
secochlorins 9-H2 (24%) and 11-H2 (72%). This can also
explain why nickel(II) complexes 8-Ni and 10-Ni which are
1
not able to generate O2 are not converted into secochlorins
(Scheme 5). A possible mechanism to explain the formation of
secochlorins 9-H2 and 11-H2 is the formation of unstable
endoperoxide intermediates, which were previously described
by Foote and Ryang for imidazole derivatives (Scheme 6).13
Interestingly, free base porphyrin 12-H2 fused to one imidazolium
ring is stable and did not undergo cleavage reaction of the
b,b0-double bond of the fused pyrrole (Scheme 5): this is consistent
with the fact that imidazolium rings are electron-deficient and
disfavor the [4+2] cycloaddition of the singlet oxygen.
11 S. Richeter, A. Hadj-Aıssa, C. Taffin, A. van der Lee and
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To summarize, we describe here for the first time some new
synthetic pathways to obtain secochlorins, including stable
free base derivatives. The interesting aspect of this work is the
porphyrin - secochlorin transformation with light and oxygen
13 H.-S. Ryang and C. S. Foote, J. Am. Chem. Soc., 1979, 101,
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3462 Chem. Commun., 2012, 48, 3460–3462
This journal is The Royal Society of Chemistry 2012