Carbene spacers in bis-porphyrinic scaffolds

Article abstract of DOI:10.1039/c2cc36800e

The preparation of two meso-imidazolylporphyrins was achieved in good yield by Ullmann coupling between a meso-bromotriarylporphyrin and imidazoles. Subsequent alkylation afforded the corresponding imidazolium functionalized porphyrins as direct precursors of the carbene. The complexation of these azolium salts with palladium led to trans-anti bis-porphyrin carbenic complexes.

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Carbene spacers in bis-porphyrinic scaffolds
Julien Haumesser, Jean-Paul Gisselbrecht, Jean Weiss* and Romain Ruppert*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
presence of copper salts and proline12 to afford the 5-(1-
50 imidazolyl)-10,15,20-triarylnickelporphyrin in 50% yield.
Changing the catalytic system for a recently described one
(CuI, Cs2CO3, phenylbenzoylhydrazine in DMSO at 110 °C)13
gave even more satisfying yields. Starting from the cheaper
and easily accessible brominated derivative 2, the meso-(1-
55 imidazolyl) functionalized porphyrin 3a could be prepared in
92% yield (see scheme 1).
5
The preparation of two meso-imidazolylporphyrins was
achieved in good yield by Ullmann coupling between a meso-
bromotriarylporphyrin
and
imidazoles.
Subsequent
alkylation afforded the corresponding imidazolium
functionalized porphyrins as direct precursors for the
10 carbene. The complexation of these azolium salts with
palladium led to trans-anti bis-porphyrin carbenic complexes.
In Nature, imidazole is one of the most encountered ligand for
metalloporphyrins. In synthetic inorganic chemistry, the
coordinating properties of imidazole have led to the design of
15 several meso-2-imidazolyl functionalized porphyrins as
precursors of axially linked multiporphyrinic linear or circular
arrays.1,2 In some cases, peralkylation of meso-2-imidazolyl
porphyrins afforded polycationic water soluble species which
were used as catalysts for different oxidation reactions.3 The
20 recent introduction of two imidazolyl groups through a C-C
coupling reaction at two borylated beta-pyrrolic positions has
opened access to exocyclic chelation of palladium at the
periphery of the porphyrin.4 Imidazolyl groups have also been
fused to the pyrrole ring of the porphyrins and been used as
25 precursors of N-heterocyclic carbenes and porphyrinic dimers
Tol
Tol
N
N
N
N
N
N
N
N
Ni
Ni
Br
MeO
An
NBS
Tol
Tol
1
2
Imidazole, CuI
Cs2CO3, L
DMSO
Tol
Ni
N
N
N
N
N
N
An
Tol
Tol
linked by
a
palladium ion.5 This strategy to organize
3a
Bu
N
BuI
porphyrins around metal ions was used earlier with porphyrins
bearing external coordination sites such as pyridines,
phosphines, enamino(thio)ketones, or enamino(thio)aldehy-
N
PdI2
Tol
Pd(OAc)2
tBuOK
N
30 des.6
N
N
N
N
N
Ni
THF
N
Ni
N
N
An
Of the C-X coupling reactions possible, the Buchwald-
Hartwig aminations are among the most studied for the
peripheral functionalizations of porphyrins.7 However, to the
best of our knowledge, the introduction of imidazolyl groups
35 linked to a meso position by a nitrogen atom has been much
less studied. In an early report, Smith used imidazole as a
nucleophile that reacted with the porphyrin radical cation to
introduce the imidazolyl group at the meso position, but in
very poor yields (7%).8 Aromatic nucleophilic substitution
40 was used more recently to introduce good nitrogen
nucleophiles (like alkyl amines or azides) at the brominated
meso positions of porphyrins.9
The meso positions of porphyrins are sterically hindered and
imidazole is not the best nucleophile. Therefore, we tested if
45 the Ullmann coupling was a feasible method for this C-N
coupling.10 Modern improvements of this old reaction are now
available.11 In our first attempts, 5-iodo-10,15,20-
triarylnickelporphyrins was reacted with imidazole in the
N
Tol
I
An
2
Tol
5a
4a
Scheme 1. Preparation of imidazolium porphyrins (3a, 4a, 5a for
imidazole or 3b, 4b, 5b for benzimidazole functionalized compounds)..
60 Compound 3a was characterized by MS (Maldi-TOF: m/z =
719.2) and 1H NMR (3 additional aromatic signals at 8.37,
7.98, and 7.53 ppm). Alkylation was carried out in neat
butyliodide at 110 °C overnight. Again the butylated
imidazolium derivative 4a was easily characterized by MS
1
65 (Maldi-TOF : m/z = 775.2) and by H NMR (the acidic proton
located between the two nitrogen atoms was found at 10.30
ppm). In contrast to most of the imidazolyl functionalized
porphyrins, where the carbon between the two nitrogen was
not available, in compound 3a the NHC carbene can now be
70 generated from imidazolium 4a, like in the fused exemple of
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Richeter and coll.5
The two benzimidazole moieties were almost coplanar and
Therefore 4a was reacted with palladium(II) acetate in the 45 orthogonal to the two porphyrin cores.
presence of a base (tBuOK) at room temperature overnight.
Several products were expected: the cis and trans isomers of a
palladium(II) bis(NHC carbene) complex and also the syn or
View Online
5
anti isomers, which were the most probable structures.
Experimentally, the starting imidazolium porphyrin was fully
consumed and only one major compound was isolated. The
MS study showed that this compound was indeed a porphyrin
50
55
60
65
70
10 dimer 5a (MaldiTOF : m/z = 1783.3) that corresponds to the
formula of bis(iodo)bis(carbene)Pd(II). At room temperature,
1
the H NMR spectrum was quite simple showing that the two
porphyrins were equivalent. However, several expected
signals were missing. At higher temperature (85 °C in
15 C2D2Cl4), these signals could be detected and assigned,
indicating that their absence was due to dynamic phenomena,
namely rotation of different aromatic groups with the
coalescence occuring at room temperature. The two remaining
imidazolyl proton signals were observed as two doublets at
20 7.83 and 6.75 ppm. The butyl proton signals were located
around or even below zero ppm (respectively 2.67, 0.07, -
0.63, and -0.72 ppm), thus indicating clearly that these groups
were exposed to the ring current and sitting on top of a
porphyrin ring. According to all spectroscopic data, the trans-
25 anti isomer was almost certainly the correct structure. No
single crystals suitable for X-ray structure determination
could be obtained to confirm these observations.
Therefore, the same reaction sequence was repeated, replacing
the imidazolyl group by the benzimidazolyl group. The same
30 chemistry led to similar yields for the benzimidazolyl
derivatives 3b and 4b and the final isolated dimer 5b
presented very similar spectroscopic characteristics similar to
Figure 1. Two orthogonal views of the X-ray structure of 5b.
The electrochemical data of the two azolyl substituted
75 nickelporphyrins 3a and 3b were very similar and typical of
nickelporphyrins, with two oxidation and two reduction
steps.However, especially for the benzimidazolyl derivative,
the first oxidation was spread out over a large potential range,
denoting either a slow electron transfer or other electrode
80 surface problems (see SI for cyclic voltammetry curves of
monomeric porphyrins). The first reduction step was clearly
reversible for both compounds and the other two steps showed
less reversible behaviour.
those of 5a. Single crystals suitable for
a structure
determination were grown from dichloromethane and ethanol.
35 The bis-carbene palladium complex was indeed the trans-anti
isomer, which was not surprising from a steric hindrance point
of view (see Figure 1). The coordination around the
palladium(II) ion was square planar. The Pd-C and Pd-I
distances are respectively 2.007 and 2.596 Å. The two nickel
40 porphyrins are ruffled and the nickel(II) coordination is
square planar with Ni-N distances around 1.92-1.93 Å. The
distance between the two nickel ions is 11.41 Å.
85
Table 1 Electrochemical data of two monomeric nickelporphyrins 3a and 3b and two nickelporphyrin dimmers 5a and 5b.a
Compound
NiP 3a
NiP 3b
Dimer 5a
Dimer 5b
ERed3
-2.33
-2.33
ERed2
-2.21
-2.18
-2.37
ERed1
-1.64
E°Ox1
0.53
0.59
0.57
0.58
E°Ox2
0.77
0.72
0.65
0.65
E°Ox3
E°Ox4
EpOx
-1.64
0.82
0.87
0.94
-1.77 (4e-)
-1.78 (4e-)
0.96
1.00
1.15
1.20
a Cyclic voltammetry data (V vs Fc/Fc+) recorded in dichloromethane with Bu4NPF6 (0.1 M) as supporting electrolyte (see SI for experimental details).
The electrochemical study of the two dimers was quite 95 one electron steps were found for the two dimers. This
90 interesting. On the reduction side, a first irreversible step was
found at -1.77 V and a second one was close to the electrode
discharge. The first reduction is probably the sum of the two
porphyrin ring reductions (2 e-) and the linking palladium(II)
behaviour is quite surprising because splitting of the oxidation
waves was observed earlier for conjugated porphyrin dimers.15
This is not the case here, but the first nickelporphyrin
oxidation steps were split by 80 and 70 mV, respectively for
ion reduction (2 e-).14 On the oxidation side, four reversible 100 5a and 5b. Even the second nickelporphyrin oxidation steps
2
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60 16.311(3) Å, b = 26.899(5) Å, c = 13.696(2) Å, α = 90.00, β =
were split with similar separations between the waves. Several
109.885(4), γ = 90.00, V = 5651.1(17) Å3, Z = 2, T = 173 K, MoKα =
0.71073, 1.75 < θ < 27.83, 13185 reflections measured, 6059 unique
reflections, R1 = 0.0887, wR2 = 0.2348, GoF = 0.958.
years ago, the splitting value of the first oxidation steps was
considered as a measure of the electronic communication
between two conjugated porphyrins.15 The values reported
here can be compared with the 80 mV splitting reported by
Higuchi for an ethylenyl linked NiOEP dimer.16 Typical
splitting values for the first oxidation potentials in porphyrin
conjugated dimers are respectively 80 mV, 240 mV, or 400
mV for the ethylenyl and ethynyl linked dimers or for Osuka’s
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5
65 Notes and references
1
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V. Rauch, J. A. Wytko, M. Takahashi, Y. Kikkawa, M.
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85
Figure 2. Cyclic voltammetry of 5a and deconvoluted oxidation curve.
90
6
See for example: S. Richeter, C. Jeandon, R. Ruppert, H. J.
Callot, Chem. Commun., 2002, 266. S. Richeter, C. Jeandon,
J.-P. Gisselbrecht, R. Ruppert, H. J. Callot, J. Am. Chem. Soc.,
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At this stage, intramolecular coulombic effects (between two
radical cations and then between two dications) seems the
30 most reasonable explanation for the clear splitting observed
here (the shortest distance between the two porphyrins, C-
meso to C-meso is 6.85 Å). However, in porphyrin dimers
with similar but even shorter palladium(II) linkages, no
splitting was observed.5 Extremely important splitting values,
35 as high as 240 mV, were observed for cofacial
nickelporphyrins, but the intermediate formation of the
neutral-radical cation dimer was invoked for this high value.18
Further studies of similar compounds containing identical or
different connections as well as calculations might be needed
40 to fully understand the interactions between the two
macrocycles.
In conclusion, the modern Ullmann coupling reaction allowed
the introduction of an imidazolyl group at the meso positions
of porphyrins. These derivatives were used to generate new
45 NHC carbenic species and palladium(II) linked porphyrin
dimers. Preliminary tests using the same coupling reaction
twice with 5,15-dihalogenated porphyrins led to meso-
bis(imidazolyl)porphyrins. Work in progress should lead to
larger porphyrin scaffolds.
95
7
K. B. Fields, J. V. Ruppel, N. L. Snyder, X. P. Zhang, in the
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R. Guilard Eds, World Scientific (Singapore), 2010, Vol.3,
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50
Institut de Chimie, UMR 7177 du CNRS, Université de Strasbourg, 1 rue
Blaise Pascal, 67008 Strasbourg cedex, France. Tel: ++(33) 368851700;
E-mail: rruppert@unistra.fr, jweiss@unistra.fr.
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† Electronic Supplementary Information (ESI) available: [detailed
55 experimental part
: preparation and characterization of the new
compounds, additional electrochemical data]. CCDC reference number
898396. See DOI: 10.1039/b000000x/
‡ Crystal data for 5b. From CH2Cl2/EtOH, C104H84N12Ni2O2Pd, M =
2011.45, 0.40×0.28×0.05 red prisms, monoclinic, space group P 21/c, a =
18 G. Pognon, C. Boudon, K. J. Schenk, M. Bonin, B. Bach, J.
Weiss, J. Am. Chem. Soc., 2006, 128, 3488.
This journal is © TheRoyal Society of Chemistry [year]
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