Synthesis and structural characterisation of the first N-heterocyclic carbene ligand fused to a porphyrin

Article abstract of DOI:10.1039/b704681b

The functionalisation of two neighboring β-pyrrolic positions of a porphyrin by a fused N-heterocyclic carbene ligand, the subsequent metallation of this external coordination site by palladium(ii) and the structural characterisation of the resulting comp

Full text of DOI:10.1039/b704681b

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Synthesis and structural characterisation of the first N-heterocyclic
carbene ligand fused to a porphyrin{
Se´bastien Richeter,*a Aure´lie Hadj-A¨ıssa,a Ce´line Taffin,a Arie van der Leeb and Dominique Leclercqa
Received (in Cambridge, UK) 28th March 2007, Accepted 19th April 2007
First published as an Advance Article on the web 1st May 2007
DOI: 10.1039/b704681b
The functionalisation of two neighboring b-pyrrolic positions of
a porphyrin by a fused N-heterocyclic carbene ligand, the
subsequent metallation of this external coordination site by
palladium(II) and the structural characterisation of the resulting
compounds are presented.
The synthesis of multiporphyrin systems is of great interest because
of their wide potential applications in catalysis,1 molecular
materials2 and medicine.3 A lot of synthetic strategies have been
developed to build multiporphyrin systems through covalent4 and
non-covalent bonds.5 The use of coordination bonds to link
porphyrins together opened exciting opportunities to build systems
with different shapes, such as linear,6 cyclic,7 2-dimensional8 and
3-dimensional9 geometries. The classical building blocks employed
Fig. 1 Structures of the complexes 1–7.
for this strategy are porphyrins bearing one or more peripheral
29-arylimidazole ring. Herein, we report another synthetic
coordination sites, as is well illustrated by the examples of
porphyrins linked to pyridyl derivatives.8,10
approach to obtain the fused porphyrin-imidazole 1 with a non
functionalised 29-imidazole carbon (Scheme 1).
Surprisingly, the number of examples of porphyrins bearing
The synthetic porphyrin used as starting material was the
N-heterocyclic carbene (NHC) ligands as external coordination site
complex 4.18 First, a nitration reaction was realized according to
the literature procedure to yield the b-nitroporphyrin 5.19 The
electron withdrawing group NO2 switched the reactivity of the
neighboring pyrrolic b-carbon to an electrophilic center. Callot
and co-workers showed that the 4-amino-4H-1,2,4-triazole is a
powerful amination reagent for a,b-unsaturated ketones and
aldehydes conjugated with the porphyrin core.20 This can also be
applied to 5 and it was possible to get the compound 6 in good
yield (82%). The b-NH2 1H NMR signal of 6 could be detected as
a broad singlet at d = 6.55 ppm in CDCl3. Its UV-visible
absorption bands were bathochromically shifted compared to 5.
The lowest energy absorption band of 6 shifted to 600 nm (587 nm
for 5). The diaminoporphyrin 7 was obtained by the reduction of
remains limited11 and, to our knowledge, there is no example of
NHC conjugated with the aromatic core of the porphyrin. Stable
N-heterocyclic carbenes (NHCs), first isolated by Arduengo et al.,12
are versatile ligands for transition metal complexes and remain
very important in the field of organometallic chemistry and
homogeneous catalysis.13 Beyond catalysis, NHCs are emerging as
relevant components in materials science.14 It is obvious that there
is a considerable potential for NHCs containing functional or
tunable groups in conjugation with the carbene moiety.15 Thus, we
focused our attention on the design of the porphyrin 1 containing
an imidazole ring fused across a b,b9-pyrrolic position and fully
conjugated to the macrocycle. The formation of the imidazolium
salt 2 gives a synthetic access to the porphyrin 3 fused to an NHC
(Fig. 1).
The peripheral functionalisation of porphyrins is of great
interest in order to access to new chromophores or to new
multiporphyrin systems.16 Crossley and co-workers prepared fused
porphyrin-imidazole systems by the condensation of porphyrin-
2,3-diones with an arylaldehyde in the presence of ammonia.17
Following this procedure, the compounds obtained have a
aInstitut Charles Gerhardt, UM2, CNRS, e´quipe CMOS, CC 007,
Universite´ Montpellier 2, Place Euge`ne Bataillon, 34095, Montpellier
Cedex 05, France. E-mail: Sebastien.Richeter@univ-montp2.fr
bInstitut Europe´en des Membranes, UM2, CNRS, ENSCM, CC 045,
Universite´ Montpellier 2, Place Euge`ne Bataillon, 34095, Montpellier
Cedex 05, France
{ Electronic supplementary information (ESI) available: Synthesis and
characterisation of the compounds 1, 2, 4–6 and 10. See DOI: 10.1039/
b704681b
Scheme 1 Synthesis of the imidazolium salt 2.
2148 | Chem. Commun., 2007, 2148–2150
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Scheme 3
Scheme 2
the iodine and the porphyrin-NHC. The UV-visible absorption
bands of 10 are slightly broadened and red-shifted compared to
those of the imidazolium 2. This result suggests that there is
negligible ground state electronic communication between the
prophyrins through the palladium center.
the NO2 group of 6 with NaBH4 in the presence of methanol and
Pd/C.21 The porphyrin 7 was not isolated but used immediately for
the formation of the imidazole ring.
The cyclisation reaction with formic acid was more challenging
and realized in two steps compared to the classical procedure
described for 2,3-diaminobenzene derivatives (one quantitative
step).22 First, the monoformamide 8 was obtained by the complete
condensation reaction of 7 with formic acid in refluxing toluene
(1 : 1 mixture) in less than 10 min, accompanied by two other
compounds which appeared to be the imidazole 1 and the
diformamide 9 (Scheme 2). The monoformamide 8 was stable
enough to be purified by silica gel chromatography. The structure
of 8 was clearly established by the usual characterisation
The single-crystal X-ray analysis{ unambiguously establishes the
structure of the complex 10 and shows that the palladium atom lies
on an inversion centre which ensures a trans-planar coordination
(Fig. 2). The two porphyrin moieties show a combination of saddle
and ruffle distorsions.24 A maximum displacement of 0.909 A (for
˚
˚
Cb) and an average displacement of 0.386 A of the core atoms
from the porphyrin mean plane (based on a least-squares plane
calculated for the 24 core atoms) are observed. The pyrrole rings
exhibit twist angles of 16.3, 19.6, 21.2 and 22.6u (with respect to the
porphyrin mean plan). Despite this distorted geometry of the
aromatic core of the porphyrin, the additional imidazoylidene ring
and the adjacent pyrrole are coplanar. The nickel(II) was found in
a slightly distorted square planar geometry: the four Ni–N
distances are almost equivalent (1.899(10), 1.907(10), 1.924(9),
1
techniques. In the H NMR spectrum of 8 in CDCl3, the two
signals observed at d = 4.59 and 6.75 ppm correspond respectively
to b-NH2 and b-NHCO. In the IR spectrum of 8 (KBr disc), the
n
NH2 absorption bands are located at 3457 and 3389 cm21, and the
nCLO is located at 1691 cm21. The right mass peak for 8 was
obtained by FAB+ mass spectrometry (m/z = 953). In the
conditions described above, the intramolecular cyclisation of 8 to
give the imidazole 1 occurred in competition with the formation of
the diformamide 9 (Scheme 2). To prevent the formation of the
undesirable diformamide 9, it was preferable to remove the formic
acid and to change the reaction conditions after the formation of
the monoformamide 8. The imidazole 1 was preferentially
obtained by the slow addition of TFA to a refluxing solution of
8 in toluene. The overall yield from 6 to 1 was 48%. The 1H NMR
spectrum of 1 in CDCl3 showed the broad signals due to the
˚
and 1.960(10) A) and the four N–Ni–N cis-angles are close to 90u
(88.5(4), 90.2(4), 91.1(4), and 91.2(4)u). In this square-planar trans
palladium(II) complex, the Pd–C distances which are consistent
˚
with Pd–C single bonds (2.035(13) A) are shorter than the Pd–I
distances (2.5910(13) A).23 The two imidazoylidene rings are close
˚
to coplanarity and are tilted y82u from the square planar central
plane PdI2C2. This tilted conformation minimises the steric
interactions between the methyl groups and the iodines linked to
the palladium(II).
To conclude, we have presented here a new synthetic procedure
to obtain a porphyrin with an additional imidazole ring fused
across a b,b9-pyrrolic position. We have shown that it is possible to
1
imidazole tautomerism that is slow on the H NMR timescale.17
The double N-alkylation of the imidazole ring of 1 with
iodomethane yielded the corresponding imidazolium salt 2 in
81% yield. A sharp singlet at d = 11.50 ppm corresponding to the
iminium CH was observed in the 1H NMR spectrum (C6D6) of 2.
This downfield chemical shift shows the highly electron-with-
drawing nature of the metalloporphyrin macrocycle.
Then, we investigated the possibility of generating the carbene
fused to the porphyrin 3. As a proof of its existence, we performed
the direct metallation of 2 with the basic metal precursor Pd(OAc)2
in refluxing toluene.23 This reaction proceeded smoothly leading to
the formation of the complex 10 which was purified by
chromatography on silica gel (Scheme 3). No signal corresponding
1
to the iminium proton was observed by H NMR spectroscopy
(CDCl3) showing the formation of the NHC–Pd(II) bond. The
evidence of the formation of the complex 10 was also provided by
FAB+ mass spectrometry. The molecular peak corresponding to
the dimer was observed at m/z = 2289. Two fragments were
observed at m/z = 2162 and 1324 and correspond to the fragments
of the complex 10 in which one ligand was removed, respectively
Fig. 2 Two views of the X-ray structure of the complex 10 (solvent
molecules have been omitted for clarity). (a) t-Butyl groups and Hs
omitted for clarity. (b) meso-Aryl groups and Hs omitted for clarity.
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(NHC)porphyrins are also under investigation.
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{ Red-purple crystals have been obtained by vapour diffusion of acetone to
a solution of the complex 10 in chloroform. Crystallographic data were
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˚
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˚
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For crystallographic data in CIF or other electronic format see DOI:
10.1039/b704681b
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This journal is ß The Royal Society of Chemistry 2007