Synthesis, characterization, and electronic properties of metalloporphyrins annulated to exocyclic imidazole and imidazolium rings

Article abstract of DOI:10.1002/ejoc.200901310

meso-Tetraarylporphyrin complexes (M = Ni, Cu, Zn, H₂) fused to an imidazole ring across two neighboring β-pyrrolic positions were synthesized through a cyclization reaction between β,β'- diaminoporphyrins and formic acid or trimethyl orthoformate under acidic conditions. Two synthetic procedures were used to obtain the corresponding porphyrin N,N'-dimethylimidazolium salts derivatives: alkylation of the imidazole nitrogen atoms with iodomethane and the cyclization reaction between the porphyrin bearing two neighboring β-N-methyl groups and trimethyl orthoformate in the presence of ammonium hexafluorophosphate. The electrochemical properties of these porphyrins annulated to an imidazole/imidazolium ring have been investigated by cyclic and rotating disk voltammetry.

Full text of DOI:10.1002/ejoc.200901310

FULL PAPER
DOI: 10.1002/ejoc.200901310
Synthesis, Characterization, and Electronic Properties of Metalloporphyrins
Annulated to Exocyclic Imidazole and Imidazolium Rings
Jean-François Lefebvre,[a] Dominique Leclercq,[a] Jean-Paul Gisselbrecht,[b] and
Sébastien Richeter*[a]
Keywords: Porphyrinoids / Cyclization / Alkylation / Fused-ring systems / Electrochemistry
meso-Tetraarylporphyrin complexes (M = Ni, Cu, Zn, H2)
fused to an imidazole ring across two neighboring β-pyrrolic
positions were synthesized through a cyclization reaction be-
tween β,βЈ-diaminoporphyrins and formic acid or trimethyl
orthoformate under acidic conditions. Two synthetic pro-
cedures were used to obtain the corresponding porphyrin
N,NЈ-dimethylimidazolium salts derivatives: alkylation of the
imidazole nitrogen atoms with iodomethane and the cycliza-
tion reaction between the porphyrin bearing two neigh-
boring β-N-methyl groups and trimethyl orthoformate in the
presence of ammonium hexafluorophosphate. The electro-
chemical properties of these porphyrins annulated to an
imidazole/imidazolium ring have been investigated by cyclic
and rotating disk voltammetry.
the corresponding imidazolium salts are the key precursors
of porphyrin–NHC complexes.[10] In this paper, we describe
our investigations on the synthesis, characterization, and
electrochemical studies of various metalloporphyrins fused
to imidazole and imidazolium rings.
Introduction
Tetrapyrrolic macrocycles have been found to be of great
interest in various areas such as molecular recognition,[1]
molecular sensing,[2] and medicine.[3] Tetrapyrrolic chromo-
phores are widely studied because they are known to play
several important biological roles such as catalysis,[4] energy
and electron transfer,[5] and light harvesting.[6] These mole-
cules are also promising candidates to be used in the devel-
opment of new molecular materials with improved photo-
chemical and/or electrochemical properties. The function-
alization of tetrapyrrolic macrocycles such as metallopor-
phyrins is crucial to access new chromophores with ad-
ditional properties, like optoelectronic properties. Several
synthetic procedures have been developed for the prepara-
tion of new porphyrins with extended π system.[7] An ele-
gant pathway to extend the π systems of porphyrins is to
introduce exocyclic fused rings (benzene, naphthalene, an-
thracene, pyrene, azulene, quinoline, or pyridine) onto the
aromatic core of the macrocycle.[8] Several porphyrins fused
to five-membered N-heterocycles (e.g., triazole, pyrrazole,
imidazole, or pyrrole) have also been reported.[9] We re-
cently launched a study of porphyrins conjugated with a
fused peripheral N-heterocyclic carbene (NHC) group, and
Results and Discussion
Porphyrin 1 (Figure 1) fused across the β,βЈ-pyrrolic po-
sitions to a 2Ј-arylimidazole ring has been described pre-
viously by Crossley and co-workers, and it was obtained
by the condensation of porphyrin-2,3-dione with an aryl
aldehyde in the presence of ammonia.[9f] However, the aryl
group of the 2Јarylimidazole ring was not suitable to gener-
ate the NHC. Thus, we reported another synthetic ap-
proach to obtain porphyrin–imidazole derivatives 2 with a
nonfunctionalized 2Ј-imidazole carbon atom. The prepara-
tion of porphyrin 3 annulated to an N,NЈ-dimethylimid-
azolium salt has been realized. Deprotonation of the imid-
azolium salt allowed the formation of carbene 4 and the
anchoring of a metal cation such as palladium(II) at the
periphery of the porphyrin to obtain porphyrin dimeric spe-
cies like complex 5 (Figure 1).[10]
To increase the solubility of the different compounds, we
[a] Institut Charles Gerhardt Montpellier, UMR 5253 CNRS- synthesized porphyrin derivatives bearing four meso-4-tert-
UM2-ENSCM-UM1, équipe CMOS, Université Montpellier 2,
butylphenyl groups. Condensation of p-tert-butylbenzalde-
bâtiment 17, cc 1701,
hyde and pyrrole under Adler–Longo conditions afforded
free-base porphyrin 6 in 21% yield, which could be quanti-
tatively metalated with nickel(II) or copper(II).[11] The clas-
sical method to prepare imidazole compounds is the con-
densation reaction between a 1,2-diamine and a carboxylic
acid, especially formic acid, to obtain imidazoles containing
a nonfunctionalized 2Ј-carbon atom.[12] Thus, we first con-
Place Eugène Bataillon, 34095, Montpellier Cedex 05, France
Fax: +33-4-67-14-38-52
E-mail: sricheter@univ-montp2.fr
[b] Laboratoire d’Electrochimie et de Chimie Physique du Corps
Solide, Institut de Chimie, UMR 7177 CNRS-Université de
Strasbourg,
4 rue Blaise Pascal, 67000 Strasbourg, France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200901310.
1912
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 1912–1920

Metalloporphyrins Annulated to Exocyclic Imidazole and Imidazolium Rings
Figure 1. Structures of 2Ј-arylimidazole 1, imidazole 2, imid-
azolium salt 3, N-heterocyclic carbene 4, and palladium complex
5.
sidered a synthetic route involving the reaction of 2,3-di-
amino-meso-tetraarylporphyrin with formic acid. It is
known that 2,3-diamino-meso-tetraarylporphyrin deriva-
tives are air sensitive.[13] On the contrary, 2-amino-3-nitro-
porphyrins appeared to be air stable and can be stored
without particular care. β-Nitroporphyrins 9 and 10 were
easily obtained in high yield by treating nickel complex 7
and copper complex 8 with lithium nitrate in CHCl3/Ac2O/
AcOH at 45 °C.[14] The electron-withdrawing NO2 function
of 9 and 10 switched the reactivity of the neighboring pyr-
rolic β-carbon to an electrophilic center. Thus, this β-car-
bon can be aminated with 4-amino-4H-1,2,4-triazole under
basic conditions. This reagent was previously used by Callot
and co-workers to prepare porphyrins conjugated to en-
amino ketone and enamino aldehyde functions.[15] Com-
plexes 11 and 12 were obtained in 82 and 90% yield, respec-
tively (Scheme 1).
Scheme 1. Synthetic route to 2,3-diamino-meso-tetraarylporphyr-
ins.
bis(formamide) 16. At this stage, formic acid was eliminated
and monoformamide 17 was treated with trifluoroacetic
acid in toluene to favor the intramolecular cyclization reac-
tion. Following this procedure, the yield of imidazole 15
was increased to 55%.
The nitro function of complexes 11 and 12 could be
quantitatively reduced with sodium borohydride and 10%
Pd/C in a dichloromethane/methanol mixture to obtain 2,3-
diaminoporphyrins 13 and 14 (Scheme 1).[16] Imidazole
rings with a nonfunctionalized 2Ј-carbon atom are easily
obtained in high yield by a cyclization reaction between a
1,2-diamine and an excess amount of formic acid, which is
usually used as the solvent.[12] In our case, toluene was
added to the reaction mixture, as the porphyrin derivatives
were not soluble enough in formic acid. The cyclization re-
action was first performed between 2,3-diaminoporphyrin
13 and formic acid in toluene (50%) for 2 h (Scheme 2).
Two major compounds were obtained: imidazole 15 and
bis(formamide) 16, respectively, in 25 and 45% yield. Both
compounds 15 and 16 are issued from two competitive reac-
tions on the same intermediate, which is monoformamide
17. The low yield obtained for imidazole 15 indicates that
the intramolecular cyclization process was more difficult to
realize than the formation of the second amide bond, which
was favored by the excess amount of formic acid. The yield
Scheme 2. Cyclization reaction with formic acid.
However, there are two major drawbacks for the forma-
of imidazole 15 can be increased by decreasing the concen- tion of the imidazole ring with formic acid: (1) Quite low
tration of formic acid (5%). After heating at reflux for 2 h, yields were obtained. (2) Undesired bis(formamide) 16 was
TLC revealed that monoformamide 17 was the major prod- always formed (Scheme 2). Thus, we explored another syn-
uct, accompanied by trace amounts of imidazole 15 and thetic procedure excluding the use of formic acid as reagent.
Eur. J. Org. Chem. 2010, 1912–1920
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1913

J.-F. Lefebvre, D. Leclercq, J.-P. Gisselbrecht, S. Richeter
FULL PAPER
After the reduction of the nitro function of 11, HC(OMe)3
was added to an acidified (HCl) solution of 2,3-diaminopo-
rphyrin 13 in toluene, and the mixture was heated to
90 °C.[17] Following this simple procedure, imidazole 15 was
obtained in 69% yield (Scheme 3). The formation of the
imidazole ring could be easily deduced from the FAB+ MS
(m/z = 963.46 [M + H]+) data and 1H NMR spectra. Broad
signals were observed at room temperature, showing slow
equilibration of the tautomerization on the 1H NMR times-
cale.[9f] Sharp signals were observed at 55 °C in CDCl3 and
the C(2Ј)-H signal was clearly observed as a singlet at δ =
7.90 ppm.
Scheme 4. Demetalation/remetalation sequence for the synthesis of
Zn and Cu complexes 19 and 20.
were the most polar fractions and were eluted with 5%
Scheme 3. Cyclization reaction with HC(OMe)3 under acidic condi-
tions.
methanol/dichloromethane. Imidazolium salts 21–24 have
1
similar and simple H NMR spectra because of the high
symmetry of the molecules. The 1H NMR spectrum of free-
base imidazolium salt 24 in CDCl3 (25 °C) is presented in
Figure 2. It clearly shows the downfield signal of the elec-
tron-deficient C(2Ј)–H at δ = 10.77 ppm and the upfield
signal of the inner N–H at δ = –2.94 ppm. The chemical
shift of the signals corresponding to the iminium proton is
due to the charged hydrogen bond between the electron-
deficient imidazolium ring of porphyrin 24 and the I–
anion.[18]
These reactions were performed on porphyrins contain-
ing nickel(II) as the inner metal. It rapidly appeared that it
was the best choice, as it was not possible to prepare the
copper(II) porphyrin fused to the imidazole ring starting
from copper(II) complex 14. However, we could easily ob-
tain other metalloporphyrins annulated to the imidazole,
since nickel complex 15 could be demetalated with a TFA/
H2SO4 (9:1) mixture, and free-base porphyrin 18 was ob-
1
tained in 95% yield (Scheme 4). The typical H NMR sig-
nal due to the internal NH of the free-base porphyrin was
observed as a singlet at δ = –2.87 ppm, and the broad sig-
nals due to slow imidazole tautomerization were observed
by 1H NMR spectroscopy, showing that the macrocycle and
the exocyclic imidazole ring did not suffer from the acidic
conditions. Remetalation reactions with other metals such
as zinc(II) or copper(II) were realized to obtain porphyrin
complexes 19 and 20 in 86 and 91% yield, respectively
(Scheme 4).
We recently reported the synthesis of porphyrins with a
fused peripheral NHC group. The corresponding imid-
azolium salts are the key precursors of such porphyrin–
NHC systems.[10] Our primary focus was on the synthesis of
porphyrins fused to N,NЈ–dimethylimidazolium salts, which
were obtained by two different synthetic procedures. The
first one was the alkylation of the nitrogen atoms of the
imidazole ring with an excess amount of iodomethane un-
der basic conditions. Imidazolium salts 21 (M = Ni), 22 (M
= Zn), 23 (M = Cu), and 24 (M = H2) were obtained by
following this procedure, respectively, from imidazole 15,
19, 20, and 18 in similar yields in the range of ca. 80%
(Scheme 5). Purification of these imidazolium salts by col-
umn chromatography on SiO2 was feasible and allowed the
separation of the trace amounts of the corresponding unre-
acted imidazole and monomethylated imidazole derivatives
Scheme 5. Synthesis of porphyrins annulated to imidazolium salts
21–24.
Figure 2. 1H NMR spectrum (200 MHz, CDCl3, 25 °C) of free-
(both eluted with dichloromethane). The imidazolium salts base porphyrin imidazolium salt 24.
1914 Eur. J. Org. Chem. 2010, 1912–1920
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Metalloporphyrins Annulated to Exocyclic Imidazole and Imidazolium Rings
Scheme 6. Synthesis of a porphyrin annulated to imidazolium salt 26.
The second synthetic strategy was based on a cyclization
reaction between N,NЈ-dimethylaminoporphyrin 25 bearing
two neighboring β N–CH3 functions and HC(OMe)3 in the
presence of NH4PF6 (Scheme 6).[19] The synthesis of N,NЈ-
dimethylaminoporphyrin 25 was realized through the re-
duction of the two carbonyl functions of bis(formamide) 16
with the neat borane dimethyl sulfide (DMS) complex.[20]
Porphyrin 25 was not isolated, but used immediately for
Scheme 7. Synthesis of porphyrins annulated to imidazolium rings
containing two different N-alkyl groups 28 and 29.
the cyclization reaction with HC(OMe)3 in the presence of
NH4PF6. Imidazolium salt 26 was obtained in 65% yield.
1
The H NMR (200 MHz) spectra of imidazolium salts 26
as, for example, meso-tetraarylporphyrin complex 6 and
imidazole 18 have similar UV/Vis spectra. The absorption
spectra of imidazole porphyrins have the typical features of
porphyrins involving a sharp Soret band and two Q-bands
(four Q-bands for free-base porphyrin 18, Figure 3). Ba-
thochromic shifts (5–20 nm for the lowest-energy Q-bands)
of the absorption bands were observed upon alkylation of
the imidazole ring as it is illustrated by nickel complexes 15
(imidazole, 568 nm), 27 (N-methylimidazole, 573 nm), and
21 (imidazolium salt, 575 nm).
and 21 in CDCl3 at 25 °C are similar (some signals are
broad for 26), and the signal of the iminium C(2Ј)–H ap-
pears at δ = 8.90 and 11.07 ppm for 26 and 21, respectively.
The significantly large difference of ∆δ ca. 2 ppm in the
chemical shifts of the signals corresponding to the iminium
–
proton is due to the fact that PF6 is a weakly complexing
anion in comparison to I–.[21]
The synthesis of porphyrins annulated to imidazolium
rings containing two different N-alkyl groups was also real-
ized. During the alkylation of porphyrin imidazole 15 with
iodomethane, we observed by TLC the formation of inter-
mediate monomethylated imidazole 27 (Scheme 7), which
can be isolated after column chromatography separation.
However, this procedure was not suitable for the prepara-
tion of monomethylated imidazole 27 in high yield, as it
was not possible to prevent the formation of imidazolium
salt 21 and to selectively obtain monomethylated com-
pound 27. Thus, we applied a synthetic procedure pre-
viously described by Shieh et al. for the N-methylation of
benzimidazole derivatives with dimethyl carbonate (DMC)
and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) under con-
ventional thermal conditions (Scheme 7).[22] Following this
procedure (60 h in toluene at 95 °C), we obtained monome-
thylated porphyrin imidazole 27 in very high yield (95%),
Table 1. UV/Vis data for imidazoles 15, 18–20, and 27 and imid-
azolium salts 21–24 and 26.
Compound
λmax Soret band (nm) λmax Q-band (nm)
15 Ni–imidazole
27 Ni–methylimidazole
19 Zn–imidazole
419
422
424
417
420
425
426
432
425
428
531, 568
536, 573
551, 588
20 Cu–imidazole
18 H2–imidazole
538, 575
516, 552, 587, 644
537, 575
21 Ni–imidazolium I–
–
26 Ni–imidazolium PF6
22 Zn–imidazolium I–
23 Cu–imidazolium I–
24 H2–imidazolium I–
537, 575
563, 608
547, 584
528, 568, 595, 657
The electrochemical properties of the imidazole and
and more interestingly, it was not necessary to purify this imidazolium porphyrins were investigated by cyclic voltam-
compound by column chromatography. Well-resolved sig- metry (CV; Table 2 and Supporting Information) and rotat-
1
nals were observed by H NMR spectroscopy, showing the ing-disc voltammetry (RDV, Supporting Information). Re-
absence of slow tautomerism equilibrium for 27. The sec- producible results were obtained on freshly polished elec-
ond nitrogen atom of the monomethyl imidazole ring could trodes. Redox potential for the nickel, copper, and zinc
be alkylated with an excess amount of other reagents such imidazoles 15, 18–20, and 27 and imidazolium derivatives
as iodobutane and benzyl bromide to obtain the corre- 21–24 and 26 are reported in Table 2. As a general trend,
sponding porphyrin imidazolium salts 28 and 29.
two reversible one-electron oxidations as well as two revers-
UV/Vis spectroscopic data are summarized in Table 1. ible one-electron reductions occurred for the porphyrins
UV/Vis spectra of 18 and 24 are represented in Figure 3. fused to an imidazole ring as observed for porphyrins.[23]
The annulation of the porphyrin to an exocyclic imidazole Electrode deposit and/or inhibition at low scan rates were
ring across two neighboring β-pyrrolic positions has a observed in some cases like for free-base imidazole 18. For
minor effect on the electronic properties of the porphyrin, the latter, only the first reduction step and the first oxi-
Eur. J. Org. Chem. 2010, 1912–1920
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1915

J.-F. Lefebvre, D. Leclercq, J.-P. Gisselbrecht, S. Richeter
FULL PAPER
For the imidazolium salts, several reduction and oxi-
dation potentials were also observed. For nickel and zinc
complexes 21 and 22, three reductions are observed at low
scan rates, and the first and third ones are reversible elec-
tron transfers, whereas the second reduction was irrevers-
ible. Increasing the scan rates up to 10 Vs–1 showed that
the second reduction became reversible for scan rates higher
than 1 Vs–1 and that, at the same time, the third reduction
step decreased in amplitude and vanished. Such an evol-
ution is in agreement with an electrochemical chemical (EC)
electron-transfer process for the second reduction step. The
generated species, being electroactive, is reduced reversibly
at –2.18 V for 21 and –2.24 V for 22. Up to now we were
unable to identify the generated species obtained after the
second reductive electron transfer. For copper complex 23,
the generated dianion is much more stable and did not un-
dergo a follow up chemical reaction in contrast to 21 and
22. It has to be mentioned that the imidazolium ring is a
reducible substituent,[24] but its reduction occurs at rather
negative potentials. Therefore, none of the electron transfers
observed for the imidazolium derivatives involve the imid-
azolium substituent. Only reduction steps occurring on the
porphyrin were observed. For imidazolium salts 21–24 with
I– as the anion, the two first oxidations behave as irrevers-
ible processes at around 0 and +0.30 V and are attributed
to the oxidation of iodide (dissociation of the iodide salt)
occurring in two steps, namely, a first oxidation corre-
Figure 3. UV/Vis spectra of imidazole 18 (dashed line) and imid-
azolium salt 24 (solid line) in CH2Cl2.
dation step are well resolved, whereas further reduction and
oxidation steps were unresolved. However, well-resolved
signals were obtained at higher scan rates (Ͼ1 Vs–1). The
potentials are dependent on the electronegativity (EN) of
the central metal, and therefore, the potential of zinc com-
plex 19 (EN = 1.65) is slightly shifted to more negative po-
tentials relative to that of nickel (EN = 1.91) and copper
(EN = 1.90) complexes 15 and 20.
–
sponding to 3 I– Ǟ I3– + 2 e– and a second oxidation of I3
Ǟ 3/2 I2 + e–.[25] As expected, these two first oxidation steps
–
of iodide were not observed for imidazolium 26 with PF6
as the anion. The electrochemical properties of the imid-
azolium salts are drastically different compared to those of
the corresponding imidazoles, as it is clearly illustrated by
comparing nickel complexes 15 (imidazole) and 21 (imid-
azolium salt). The redox potentials of nickel imidazolium
salt 21 are shifted to more positive potentials (200 mV for
the first oxidation and the first reduction) compared to the
redox potentials of nickel imidazole 15. This is due to the
electron-accepting character of the imidazolium ring. In
oxidation, a drastic change was also observed, as two re-
versible one-electron oxidations occurred for imidazole 15
(+0.44 and +0.65 V vs. Fc+/Fc), whereas the two one-elec-
tron oxidations merged into a single two-electron step for
imidazolium salt 21 (+0.64 V vs. Fc+/Fc). The data ob-
tained also showed that the influence of the imidazolium
ring on the shifts of the redox potentials was dependent on
the nature of the inner metal of the porphyrin. For example,
the redox potentials of nickel complex 21 are shifted to
more positive potentials by 200 mV for the first oxidation
and the first reduction, whereas those of zinc complex 22
are shifted to more positive potentials by 100 mV for the
first oxidation and 150 mV for the first reduction. Thus,
although identical structural modifications have been
brought to the porphyrin core, that is, the methylation of
the nitrogen atoms of the imidazole ring, the redox poten-
tials were strongly dependant of the inner metal of the por-
phyrin when going from the imidazoles to the imidazolium
salts.
Table 2. Cyclic voltammetry data for imidazoles 15, 18–20, and 27
and imidazolium salts 21–24 and 26.[a]
[b]
E°red2
E°red1
E°ox1
E°ox2
Ep
15
27
19
20
18
21
–2.27
(80)
–2.22
(90)
–2.15
(110)
–2.14
(90)
–1.73
(90)
–1.76
(70)
–1.81
(60)
–1.74
(60)
–1.65
(60)
–1.54
(60)
+0.44
(100)
+0.43
(70)
+0.28
(90)
+0.65
(100)
+0.73
(90)
+0.61
(90)
+0.44
(60)
+0.77
(90)
+047
(60)
+0.69[c]
(170)
–2.05
–2.18
(80)
+0.64[d]
(70)
–2.03
–0.01
+0.33
26
22
–2.01
(60)
–2.24
(90)
–1.56
(60)
–1.69
(80)
+0.66[d]
(100)
+0.36
(60)
+0.66
(60)
–1.93
–0.04
+0.28
+0.00
+0.35
+0.00
+0.38
23
24
–1.99
(60)
–1.73
(70)
–1.59
(60)
–1.48
(60)
+0.54
(100)
+0.57
(60)
+0.90
(60)
+0.75
(60)
[a] Potentials are given in volt vs. the Fc+/Fc couple used as an
internal standard and measured at a scan rate of 0.1 Vs–1. Values
indicated in parentheses correspond to the peak potential differ-
ences (∆Ep = Epa – Epc in mV). See the Experimental Section for
details of the conditions. [b] Ep = irreversible peak potentials.
[c] Reversible electron transfer observed at a scan rate of 2 Vs–1.
[d] Two-electron process.
1916
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 1912–1920

Metalloporphyrins Annulated to Exocyclic Imidazole and Imidazolium Rings
1.12ϫ10–2 mol) in a mixture of toluene (200 mL) and ethanol
(10 mL) was heated under reflux for 1 h. After cooling and evapo-
Several porphyrins annulated to imidazole and imid- ration, dichloromethane was added, and the organic phase was
Conclusions
then washed with water (3ϫ) and dried with sodium sulfate.
Chromatography on silica gel (dichloromethane/pentane, 7:3) and
crystallization from dichloromethane/methanol afforded 11 in 82%
azolium rings were synthesized starting from a meso-tetra-
arylporphyrin, which can be obtained at the gram scale. The
functionalization of two neighboring β-carbon atoms with
amino groups was achieved in three steps, namely, the ni-
tration of one β-carbon followed by the amination of the
electrophilic neighboring β-carbon, and the reduction of
the NO2 group. Several cyclization reactions were explored
to obtain porphyrins fused to an imidazole ring: we found
that the better one was the cyclization reaction between the
1
yield (0.83 g). H NMR (200 MHz, CDCl3, 25 °C): δ = 8.71 (d, J
= 5.0 Hz, 1 H, pyrrole), 8.67 (d, J = 5.0 Hz, 1 H, pyrrole), 8.57 (d,
J = 5.0 Hz, 1 H, pyrrole), 8.56 (s, 2 H, pyrrole), 8.47 (d, J = 5.0 Hz,
1 H, pyrrole), 8.01–7.89 (m, 8 H, Arortho), 7.87–7.63 (m, 8 H,
Armeta), 6.55 (br. s, 2 H, NH2), 1.58 (s, 36 H, tBu) ppm. UV/Vis
(CH2Cl2): λ (ε, Lmol–1 cm–1) = 444 (131600), 558 (12600), 601
(8900) nm. MS (FAB+): m/z
=
954 [NiC60H61N6O2+].
nickel 2,3-diaminoporphyrin complex and trimethyl ortho- C60H60N6NiO2·H2O (973.87): calcd. C 74.00, H 6.42, N 8.63; found
C 74.03, H 6.39, N 8.58.
formate under acidic conditions (around 70% yield). The
nickel complex can be demetalated and remetalated with
other metal ions. Alkylation of the nitrogen atoms of these
imidazoles with iodomethane afforded the corresponding
N,NЈ-dimethylimidazolium salts. Imidazolium salts bearing
two different N-alkyl groups were also obtained. The elec-
trochemical properties of the imidazolium salts are different
than those of the corresponding imidazoles. Moreover, the
influence of the imidazolium ring on the shifts of the redox
potentials was strongly dependant of the inner metal of the
porphyrin. To conclude, we now have access to a wide panel
of ligands by varying (i) the inner metal of the porphyrin
and (ii) the groups on the nitrogen atoms. We are currently
investigating the coordination properties of these molecules,
especially the properties of metal complexes that can be an-
chored at the periphery of the porphyrin through the NHC
ligand issued from the imidazolium salts.
Copper 2-Amino-3-nitroporphyrin 12: Same procedure as for 11.
Copper complex 12 was obtained in 90% yield. UV/Vis (CH2Cl2):
λ (ε, Lmol–1 cm–1) = 443 (170100), 564 (14500), 601 (10600) nm.
MS (ESI+): m/z = 960.6 [CuC60H61N6O2+]. CuC60H60N6O2·H2O
(978.72): calcd. C 73.63, H 6.39, N 8.59; found C 73.80, H 6.32, N
8.69.
Synthesis of Nickel Imidazole 15 and Nickel Bis(formamide) 16
Method A – Cyclization with Formic Acid: A solution of 11
(500 mg, 5.23ϫ10–4 mol) and palladium on activated carbon (10%,
20 mg, 1.88ϫ10–4 mol) in a mixture of dichloromethane (125 mL)
and methanol (15 mL) was prepared and degassed. Sodium
borohydride (500 mg, 1.32ϫ10–2 mol) was added, and the mixture
was stirred under an atmosphere of argon for 1 h. Completion of
the reduction was verified by silica gel TLC (one red spot corre-
sponding to 2,3-diaminoporphyrin 13). After evaporation of the
solvents, formic acid (25 mL) and toluene (25 mL) were added to
the residue, and the reaction mixture was heated at reflux under
an atmosphere of argon for 2 h. After cooling, the mixture was
neutralized with K2CO3, washed with water (3ϫ), and concen-
trated. The residual mixture was purified by silica gel column
chromatography (CH2Cl2). The first fraction containing a mixture
of brown nonpolar unidentified compounds was eliminated. The
second fraction contained red-pink 15, and the third more polar
fraction eluted with CH2Cl2/MeOH (95:5) contained 16. After
evaporation of the solvents and crystallization from CH2Cl2/
MeOH, compounds 15 and 16 were obtained as purple solids in 25
(125 mg) and 45% (231 mg) yield, respectively.
Experimental Section
Experimental Details: 1H NMR spectra were recorded with a
Bruker DPX-200 spectrometer and referenced to SiMe4 in ppm.
Abbreviations for 1H NMR spectra used are as follows: s, singlet; d,
doublet; m, multiplet. UV/Vis spectra were recorded with a Perkin–
Elmer Lambda 35 spectrophotometer in quartz cuvettes and
CH2Cl2 was used as the solvent. IR spectra were recorded with an
Avatar 320 FTIR spectrometer as KBr discs. FAB mass spectra
were recorded with a SX 102 Jeol 1993 MS instrument and 3-ni-
trobenzyl alcohol was used as matrix. ESI mass spectra were re-
corded with a Q-Tof Waters 2001 MS instrument and CH2Cl2 was
used as solvent. Elemental analyses were realized at the Service
Central d’Analyse du Centre National de la Recherche Scientifique
of Solaize (France). All reagents and solvents were of commercial
reagent grade and used without further purification except where
noted. Dry CH2Cl2 was obtained by distilling from CaH2. Dry tol-
uene was obtained by distilling from Na. Dry THF was obtained
by distilling from CaH2, then Na/benzophenone. Preparative sepa-
rations were performed by silica gel flash column chromatography
(Baeckeroot-Labo 60M).
Method B – Cyclization with HC(OMe)3]: A solution of nickel(II)
porphyrin 11 (500 mg, 5.23ϫ10–4 mol) and palladium on 10% acti-
vated carbon (20 mg, 1.88ϫ10–4 mol) in a mixture of dichloro-
methane (125 mL) and methanol (15 mL) was prepared and de-
gassed. Sodium borohydride (500 mg, 1.32ϫ10–2 mol) was added,
and the mixture was stirred under an atmosphere of argon for 1 h.
Completion of the reduction was verified by silica gel TLC (one
red spot corresponding to 2,3-diaminoporphyrin 13). After evapo-
ration of the solvents, HC(OMe)3 (7 mL, 6.4ϫ10–2 mol), toluene
(25 mL), and hydrochloric acid (0.5 mL) were added to the residue,
and the mixture was degassed. The solution was heated at 90 °C
for 1 h under an atmosphere of argon. After cooling, chloroform
(40 mL) and water (80 mL) were added. The mixture was neutral-
ized with K2CO3, washed with water (3ϫ), and concentrated. After
purification by silica gel column chromatography (CH2Cl2),
crystallization from CH2Cl2/MeOH afforded 15 in 69% yield
(338 mg).
Synthesis of the Starting Compounds: Porphyrins 6–8, β-nitropor-
phyrins 9 (M = Ni) and 10 (M = Cu), and 2-amino-3-nitropor-
phyrins 11 (M = Ni) and 12 (M = Cu) were prepared according to
our previously published procedures.[10]
Nickel 2-Amino-3-Nitroporphyrin 11: A solution of β-nitropor-
Nickel Imidazole 15: 1H NMR (200 MHz, CDCl3, 25 °C): δ = 8.90–
8.70 (br. s, 4 H, pyrrole), 8.79 (s, 2 H, pyrrole), 8.52 (br. s, 1 H,
phyrin
9
(1 g, 1.06ϫ10–3 mol), sodium hydroxide (0.2 g,
5ϫ10–3 mol),
and
4-amino-4H-1,2,4-triazole
(0.943 g, NH imidazole), 8.10–7.90 (m, 8 H, Arortho), 7.94 (s, 1 H, CH imid-
Eur. J. Org. Chem. 2010, 1912–1920
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1917

J.-F. Lefebvre, D. Leclercq, J.-P. Gisselbrecht, S. Richeter
FULL PAPER
azole), 7.90–7.75 (br. s, 4 H, Armeta), 7.72 (d, J = 8.2 Hz, 4 H,
Armeta), 1.62 (br. s, 18 H, tBu), 1.59 (s, 18 H, tBu) ppm. UV/Vis
(CH2Cl2): λ (ε, Lmol–1 cm–1) = 419 (248600), 531 (14500), 568
pentane afforded 19 in 86% yield (171 mg). 1H NMR (200 MHz,
CDCl3 + 10% CD3OD, 25 °C): δ = 8.86 (d, J = 4.6 Hz, 2 H, pyr-
role), 8.83 (s, 2 H, pyrrole), 8.71 (d, J = 4.6 Hz, 2 H, pyrrole), 8.03
(7700) nm. MS (FAB+): m/z
=
935 [NiC61H61N6+]. (d, J = 8.1 Hz, 4 H, Arortho), 7.95 (d, J = 8.0 Hz, 4 H, Arortho), 7.64
C61H60N6Ni·H2O (953.88): calcd. C 76.81, H 6.55, N 8.81; found
C 77.13, H 6.48, N 8.90.
(d, J = 8.0 Hz, 4 H, Armeta), 7.61 (d, J = 8.1 Hz, 4 H, Armeta), δ =
7.51 (s, 1 H, CH imidazole), 1.53 (s, 36 H, tBu) pm ppm. UV/Vis
(CH2Cl2): λ (ε, Lmol–1 cm–1) = 424 (554000), 551 (20900), 590
Nickel Bis(formamide) 16: 1H NMR (200 MHz, CDCl3, 25 °C): δ
= 8.75 (s, 2 H, pyrrole), 8.73 (s, 4 H, pyrrole), 7.94 (d, J = 7.9 Hz,
4 H, Arortho), 7.87 (m, 4 H, Arortho), 7.75 (br. s, 2 H, CHO), 7.71
(d, J = 7.5 Hz, 8 H, Armeta), 7.09 (br. s, 2 H, NH), 1.47 (s, 18 H,
tBu), 1.46 (s, 18 H, tBu) ppm. UV/Vis (CH2Cl2): λ (ε, Lmol–1 cm–1)
(9800) nm. MS (ESI+): m/z
=
941.5 [ZnC61H61N6+].
C61H60N6Zn·H2O (960.59): calcd. C 76.27, H 6.51, N 8.75; found
C 76.46, H 6.43, N 8.90.
Copper Imidazole 20: Free-base imidazole 18 (400 mg,
4.55ϫ10–4 mol) was mixed with Cu(OAc)2·2H2O (220 mg,
1.10ϫ10–3 mol) in CHCl3/methanol (40/5 mL). The mixture was
heated at reflux for 12 h. After solvent evaporation and purification
by silica gel column chromatography (CH2Cl2), crystallization from
CH2Cl2/methanol afforded 20 in 91% yield (390 mg). UV/Vis
(CH2Cl2): λ (ε, Lmol–1 cm–1) = 417 (418300), 538 (19100), 575
= 424 (232100), 538 (17400) nm. IR (KBr): ν = 1704 (CO) cm–1.
˜
MS (ESI+): m/z
=
981.3 [NiC62H63N6O2+]. C62H62N6NiO2
(981.89): calcd. C 75.84, H 6.36, N 8.56; found C 75.73, H 6.41, N
8.50.
Nickel Monoformamide 17:
A
solution of 11 (500 mg,
5.23ϫ10–4 mol) and palladium on activated carbon (10%, 20 mg,
1.88ϫ10–4 mol) in a mixture of dichloromethane (125 mL) and
methanol (15 mL) was prepared and degassed. Sodium borohyd-
ride (500 mg, 1.32ϫ10–2 mol) was added, and the solution was
stirred under an atmosphere of argon for 1 h. Completion of the
reduction was verified by silica gel TLC (one red spot correspond-
ing to 2,3-diaminoporphyrin 13). After evaporation of the solvents,
formic acid (2.5 mL) and toluene (47.5 mL) were added to the resi-
due, and the reaction mixture was heated at reflux under an atmo-
sphere of argon for 2 h. After cooling, the mixture was neutralized
with K2CO3, washed with water (3ϫ), and concentrated. The resid-
ual mixture was purified by silica gel column chromatography
(CH2Cl2). The first fraction containing a mixture of brown nonpo-
lar unidentified compounds was eliminated. The second fraction
contained red 17, followed by trace amounts of 15. Trace amounts
of bis(formamide) can be eluted with CH2Cl2/MeOH, 95:5. After
evaporation of the solvents and crystallization from CH2Cl2/
MeOH, monoformamide 17 was obtained as a purple solid in 58%
(8300) nm. MS (ESI+): m/z
=
940.0 [CuC61H61N6+].
CuC61H60N6·2H2O (976.75): calcd. C 75.01, H 6.60, N 8.60; found
C 74.94, H 6.52, N 8.66.
Nickel Imidazolium Salt 21 (Alkylation with Iodomethane): Nickel
imidazole 15 (125 mg, 1.33ϫ10–4 mol) was dissolved in acetone
(30 mL). Iodomethane (5 mL) and K2CO3 (0.5 g) were added, and
the solution was stirred at 40 °C under an atmosphere of argon for
24 h. Completion of the alkylation was verified by silica gel TLC,
and the solvent was evaporated. The residue was purified by silica
gel column chromatography (CH2Cl2 to CH2Cl2/MeOH, 95:5).
Nickel imidazolium salt 15 was the most polar compound.
Crystallization from pentane afforded 21 in 81% yield (116 mg).
1H NMR (200 MHz, CDCl3, 25 °C): δ = 11.07 (s, 1 H, CH imid-
azolium), 8.78 (s, 2 H, pyrrole), 8.75 (d, J = 5.0 Hz, 2 H, pyrrole),
8.70 (d, J = 5.0 Hz, 2 H, pyrrole), 8.09 (d, J = 8.3 Hz, 4 H, Arortho),
7.99 (d, J = 8.3 Hz, 4 H, Arortho), 7.84 (d, J = 8.3 Hz, 4 H, Armeta),
7.76 (d, J = 8.3 Hz, 4 H, Armeta), 3.30 (s, 6 H, methyl), 1.60 (s, 18
H, tBu), 1.59 (s, 18 H, tBu) ppm. UV/Vis (CH2Cl2): λ (ε,
Lmol–1 cm–1) = 425 (276100), 537 (18000), 575 (sh., 3800) nm. MS
(FAB+): m/z = 963 [NiC63H65N6+]. C63H65IN6Ni·H2O (1109.84):
calcd. C 68.18, H 6.08, I 11.43, N 7.57; found C 67.48, H 6.05, I
11.19, N 7.55.
1
yield (290 mg). H NMR (200 MHz, CDCl3, 25 °C): δ = 8.75 (m,
6 H, pyrrole), 7.90 (m, 8 H, Arortho), 7.85 (br. s, 1 H, CHO), 7.77
(m, 8 H, Armeta), 7.07 (br. s, 1 H, NH), 6.99 (br. s, 2 H, NH2), 1.58
(s, 36H tBu), 1.57 (s, 36 H, tBu) ppm. λ (ε, Lmol–1 cm–1) = 419
(211000), 538 (14600), 580 (7500) nm. IR (KBr): ν = 1704 (CO),
˜
3389, 3457 (NH2) cm–1. MS (ESI+): m/z = 953.0 [NiC61H63N6O+].
C61H62N6NiO (953.88): calcd. C 76.81, H 6.55, N 8.81; found C
75.07, H 6.63, N 8.26.
Zinc Imidazolium Salt 22: Same procedure as for 21. Zinc imid-
azolium salt 22 was obtained in 80% yield. 1H NMR (200 MHz,
CDCl3, 25 °C): δ = 10.17 (br. s, 1 H, CH imidazolium), 8.93 (s, 2
H, pyrrole), 8.92 (d, J = 4.8 Hz, 2 H, pyrrole), 8.81 (d, J = 4.7 Hz,
2 H, pyrrole), 8.29 (d, J = 8.2 Hz, 4 H, Arortho), 8.13 (d, J = 8.1 Hz,
4 H, Arortho), 7.85 (d, J = 8.2 Hz, 4 H, Armeta), 7.77 (d, J = 7.9 Hz,
4 H, Armeta), 3.22 (s, 6 H, methyl), 1.63 (s, 18 H, tBu), 1.62 (s, 18
H, tBu) ppm. UV/Vis (CH2Cl2): λ (ε, Lmol–1 cm–1) = 432 (463900),
Free-Base Imidazole 18: Nickel imidazole 15 (460 mg,
4.91ϫ10–4 mol) was dissolved in TFA/H2SO4 (16/4 mL), and the
solution was stirred at room temperature for 30 min. It was then
poured on ice, diluted with chloroform (200 mL), neutralized with
saturated K2CO3, washed with water, dried (Na2SO4), and concen-
trated. Crystallization from CH2Cl2/MeOH afforded 18 in 95%
yield (410 mg). 1H NMR (200 MHz, CDCl3, 25 °C): δ = 9.00 (d, J
= 4.8 Hz, 2 H, pyrrole), 8.88 (d, J = 4.8 Hz, 2 H, pyrrole), 8.85 (s,
2 H, pyrrole), 8.22 (d, J = 8.3 Hz, 4 H, Arortho), 8.20 (d, J = 8.3 Hz,
4 H, Arortho), 8.01 (s, 1 H, CH imidazole), 7.90 (d, J = 8.2 Hz, 4
H, Armeta), 7.80 (d, J = 8.3 Hz, 4 H, Armeta), 1.69 (s, 18 H, tBu),
1.65 (s, 18 H, tBu), –2.89 (s, 2 H, internal NH) ppm. UV/Vis
(CH2Cl2): λ (ε, Lmol–1 cm–1) = 420 (350800), 516 (16200), 552
(9400), 587 (7100), 644 (3180) nm. MS (ESI+): m/z = 879.5
[C61H63N6+].
563 (18250), 608 (7800) nm. MS (ESI+): m/z
= 969.5
[ZnC63H65N6+]. C63H65IN6Zn·H2O (1116.56): calcd. C 67.77, H
6.05, N 7.53; found C 67.41, H 6.02, N 7.54.
Copper Imidazolium Salt 23: Same procedure as for 21. Copper
imidazolium salt 23 was obtained in 60% yield. UV/Vis (CH2Cl2):
λ (ε, Lmol–1 cm–1) = 425 (331200), 547 (17700), 584 (4200) nm. MS
(ESI+): m/z = 968.5 [CuC63H65N6+]. C63H65CuIN6 (1096.68): calcd.
C 69.00, H 5.97, N 7.66; found C 68.89, H 6.38, N 7.67.
Free-Base Imidazolium Salt 24: Same procedure as for 21. Free-
base imidazolium salt 24 was obtained in 83% yield. 1H NMR
Zinc Imidazole 19: Free-base imidazole 18 (185 mg, (200 MHz, CDCl3, 25 °C): δ = 10.77 (s, 1 H, CH imidazolium),
2.10ϫ10–4 mol) was mixed with Zn(OAc)2·2H2O (100 mg, 8.98 (s, 4 H, pyrrole), 8.79 (s, 2 H, pyrrole), 8.43 (d, J = 8.2 Hz, 4
4.46ϫ10–4 mol) in toluene (25 mL). The mixture was heated at re-
flux for 2 h. After solvent evaporation and purification by silica gel
column chromatography (CH2Cl2), crystallization from CH2Cl2/
H, Arortho), 8.23 (d, J = 8.2 Hz, 4 H, Arortho), 7.97 (d, J = 8.3 Hz,
4 H, Armeta), 7.86 (d, J = 8.3 Hz, 4 H, Armeta), 3.27 (s, 6 H, methyl),
1.66 (s, 18 H, tBu), 1.64 (s, 18 H, tBu), –2.94 (s, 2 H, internal NH)
1918
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 1912–1920

Metalloporphyrins Annulated to Exocyclic Imidazole and Imidazolium Rings
ppm. UV/Vis (CH2Cl2): λ (ε, Lmol–1 cm–1) = 428 (421500), 528 (d, J = 8.4 Hz, 2 H, Arortho), 7.84 (d, J = 8.4 Hz, 2 H, Arortho), 7.77
(16300), 568 (9900), 595 (6100), 657 (7600) nm. MS (ESI+): m/z = (d, J = 8.4 Hz, 2 H, Arortho), 3.38 (s, 3 H, methyl), 3.19 (t, J =
907.4 [C63H67N6+]. C63H67IN6·2H2O (1071.18): calcd. C 70.64, H
6.68, N 7.85; found C 70.56, H 6.58, N 7.79.
7.5 Hz, 2 H, CH2CH2CH2CH3), 1.23 (m, 2 H, CH2CH2CH2CH3),
0.86 (m, 2 H, CH2CH2CH2CH3), 0.58 (t, J = 7.2 Hz, 2 H,
CH2CH2CH2CH3), 1.60 (s, 18 H, tBu), 1.59 (s, 18 H, tBu) ppm.
UV/Vis (CH2Cl2): λ (ε, Lmol–1 cm–1) = 427 (244000), 539 (17900),
Nickel Imidazolium Salt 26: A solution of nickel bis(formamide)
16 (260 mg, 2.65ϫ10–4 mol) in anhydrous THF was prepared and
degassed. BH3·DMS (2  in THF, 0.9 mL, 1.8ϫ10–3 mol) was
added under an atmosphere of argon, and the solution was stirred
at 65 °C for 2 h. Completion of the reduction was verified by silica
gel TLC. Then, methanol (1 mL) was added to neutralize the excess
amount of the borane/dimethyl sulfide complex. After evaporation
of the solvents, HC(OMe)3 (10 mL, 9.1ϫ10–2 mol) and NH4PF6
(50 mg, 3.07ϫ10–4 mol) were added to the residue, and the reaction
mixture was degassed and heated at reflux for 16 h under an atmo-
sphere of argon. After cooling, the mixture was poured into a satu-
rated aqueous solution of NH4PF6. The obtained solid was filtered,
dried, and purified by alumina column chromatography (CH2Cl2
to CH2Cl2/EtOH, 95:5). Nickel imidazolium salt 26 was the most
polar compound. Crystallization from pentane afforded 26 in 65%
573 (5400) nm. MS (ESI+): m/z
=
1005.6 [NiC63H66N6+].
C66H71IN6Ni·4H2O (1586.68): calcd. C 65.73, H 6.60, N 6.97;
found C 65.71, H 6.22, N 6.96.
Nickel Imidazolium Salt 29 [R = (CH2Ph)]: A solution of N-methyl-
imidazole 27 (150 mg, 1.58ϫ10–4 mol) and benzyl bromide
(15 mL, 0.126 mol) in chloroform (30 mL) was prepared. The mix-
ture was degassed and heated at 70 °C under an atmosphere of
argon for 96 h. Completion of the reaction was verified by silica
gel TLC, and the solvent was then evaporated. The residue was
purified by silica gel column chromatography (CH2Cl2 to CH2Cl2/
MeOH, 95:5). Nickel imidazolium salt 29 was the most polar com-
pound. Crystallization from dichloromethane/pentane afforded 29
in 46% yield (81 mg). 1H NMR (200 MHz, CDCl3, 25 °C): δ =
11.38 (s, 1 H, CH imidazolium), 8.89, (d, J = 5.0 Hz, 1 H, pyrrole),
8.84 (d, J = 5.0 Hz, 1 H, pyrrole), 8.79 (d, J = 5.0 Hz, 1 H, pyrrole),
8.70 (d, J = 5.0 Hz, 1 H, pyrrole), 8.68 (d, J = 5.0 Hz, 1 H, pyrrole),
8.51 (d, J = 5.0 Hz, 1 H, pyrrole), 8.08 (d, J = 8.4 Hz, 2 H, Arortho),
8.03 (d, J = 8.4 Hz, 2 H, Arortho), 8.00 (d, J = 8.4 Hz, 2 H, Arortho),
7.98 (d, J = 8.4 Hz, 2 H, Arortho), 7.83 (d, J = 8.4 Hz, 2 H, Armeta),
7.82 (d, J = 8.4 Hz, 2 H, Armeta), 7.77 (2 d, J = 8.4 Hz, 2 H, Armeta),
6.92 (m, 3 H, Ph), 6.72 (m, 2 H, Ph), 4.43 (s, 2 H, CH2), 3.38 (s, 3
H, methyl), 1.59 (s, 18 H, tBu), 1.56 (s, 18 H, tBu) ppm. UV/Vis
(CH2Cl2): λ (ε, Lmol–1 cm–1) = 428 (280000), 539 (17500), 572
1
yield (191 mg). H NMR (200 MHz, CDCl3, 25 °C): δ = 8.90 (s, 1
H, CH imidazolium), 8.76 (s, 2 H, pyrrole), 8.73 (d, J = 5.1 Hz, 2
H, pyrrole), 8.70 (d, J = 5.1 Hz, 2 H, pyrrole), 8.09 (d, J = 8.3 Hz,
4 H, Arortho), 7.98 (d, J = 8.3 Hz, 4 H, Arortho), 7.83 (d, J = 8.3 Hz,
4 H, Armeta), 7.75 (d, J = 8.3 Hz, 4 H, Armeta), 3.10 (s, 6 H, methyl),
1.58 (s, 18 H, tBu), 1.56 (s, 18 H, tBu) ppm. UV/Vis (CH2Cl2): λ
(ε, Lmol–1 cm–1) = 426 (234900), 537 (19400), 575 (sh., 4100) nm.
MS (FAB+): m/z = 963 [NiC63H65N6+]. C63H65F6N6NiP·H2O
(1127.90): calcd. C 67.09, H 5.99, F 10.11, N 7.45, P 2.75; found
C 67.25, H 5.96, F 10.79, N 7.53, P 3.01.
(5300) nm. MS (ESI+): m/z
=
1039.6 [NiC69H69N6]+.
Nickel N-Methylimidazole 27: A solution of nickel imidazole 15
(1 g, 1.07ϫ10–3 mol) in toluene (200 mL) was prepared. Dimethyl
carbonate (50 mL, 0.594 mol) and 1,8-diazabicyclo[5.4.0]undec-7-
ene (2 g, 1.31ϫ10–2 mol) were added, and the reaction mixture was
degassed and heated at 95 °C for 72 h. After evaporation of tolu-
ene, 27 was precipitated by adding methanol to the residue. After
filtration, 27 was obtained in 95% yield (965 mg). 1H NMR
(200 MHz, CDCl3, 25 °C): δ = 8.77 (s, 2 H, pyrrole), 8.76 (s, 2 H,
pyrrole), 8.75 (d, J = 4.6 Hz, 1 H, pyrrole), 8.62, (d, J = 4.6 Hz, 1
H, pyrrole), 8.05 (d, J = 8.2 Hz, 4 H, Arortho), 7.97 (d, J = 8.4 Hz,
4 H, Arortho), 7.80 (s, 1 H, CH imidazole), 7.73 (d, J = 8.2 Hz, 4
H, Armeta), 7.71 (d, J = 8.4 Hz, 4 H, Armeta), 2.99 (s, 3 H, methyl),
1.60 (s, 9 H, tBu), 1.59 (s, 18 H, tBu), 1.56 (s, 9 H, tBu) ppm. UV/
Vis (CH2Cl2): λ (ε, Lmol–1 cm–1) = 422 (243400), 536 (14800), 573
C69H69BrN6Ni·2H2O (1156.95): calcd. C 71.63, H 6.36, N 7.26;
found C 71.65, H 6.25, N 7.25.
Electrochemical Studies: Spectroscopic-grade dichloromethane was
purchased from Merck, dried with molecular sieves (4 Å), and
stored under an atmosphere of argon prior to use. Electrochemical-
grade Bu4NPF6 was purchased from Fluka and used as received.
Electrochemical measurements were carried out at room tempera-
ture in CH2Cl2 containing 0.1  Bu4NPF6 in a classical three-elec-
trode cell by cyclic voltammetry (CV) and rotating-disk voltamme-
try (RDV). The working electrode was a glassy carbon disk (3 mm
in diameter), the auxiliary electrode a Pt wire, and the reference
electrode a Pt wire used as pseudoreference electrode. The elec-
trodes were polished before each measurement, otherwise irreprod-
ucible CVs. were obtained due to insulating film formation either
during oxidation or reduction. The cell was connected to an Auto-
lab PGSTAT30 potentiostat (Eco Chemie, Holland) driven by a
GPSE software running on a personal computer. All potentials are
given vs. Fc+/Fc used as internal reference and uncorrected from
ohmic drop.
(7700) nm. MS (ESI+): m/z
=
949.3 [NiC62H63N6+].
C62H62N6Ni·H2O (967.90): calcd. C 76.94, H 6.66, N 8.68; found
C 76.82, H 6.65, N 8.73.
Nickel Imidazolium Salt 28 [R = (CH2)3CH3]: A solution of N-
methylimidazole 27 (120 mg, 1.27ϫ10–4 mol) and 1-iodobutane
(4 mL, 0.035 mol) in chloroform (30 mL) was prepared. The mix-
ture was degassed and heated at 70 °C under an atmosphere of
argon for 72 h. Completion of the reaction was verified by silica
gel TLC, and the solvent was then evaporated. The residue was
purified by silica gel column chromatography (CH2Cl2 to CH2Cl2/
MeOH, 95:5). Nickel imidazolium salt 28 was the most polar com-
pound. After evaporation of the solvent, crystallization from pen-
tane afforded 28 in 36% yield (51 mg). 1H NMR (200 MHz,
CDCl3, 25 °C): δ = 11.10 (s, 1 H, CH imidazolium), 8.87 (d, J =
5.0 Hz, 1 H, pyrrole), 8.83 (d, J = 5.0 Hz, 1 H, pyrrole), 8.79 (d, J
= 5.0 Hz, 1 H, pyrrole), 8.71 (d, J = 5.0 Hz, 1 H, pyrrole), 8.70 (d,
J = 5.0 Hz, 1 H, pyrrole), 8.56 (d, J = 5.0 Hz, 1 H, pyrrole), 8.11
(d, J = 8.4 Hz, 2 H, Arortho), 8.09 (d, J = 8.4 Hz, 2 H, Arortho), 8.01
(d, J = 8.4 Hz, 2 H, Arortho), 8.00 (d, J = 8.4 Hz, 2 H, Arortho), 7.87
Supporting Information (see footnote on the first page of this arti-
1
cle): H NMR spectra of the compounds, rotating disk data, and
cyclic voltammograms.
Acknowledgments
We are grateful to the Centre National de la Recherche Scientifique
(CNRS), the French Ministry of Research, and the Agence
National de la Recherche (ANR) for financial support of the re-
search project ANR-09-JCJC-0089-01.
[1] J.-Y. Zheng, K. Konishi, T. Aida, Tetrahedron 1997, 53, 9115–
9122.
Eur. J. Org. Chem. 2010, 1912–1920
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1919

J.-F. Lefebvre, D. Leclercq, J.-P. Gisselbrecht, S. Richeter
[2] a) C. Lee, D. H. Lee, J.-I. Hong, Tetrahedron Lett. 2001, 42, [9] a) C. M. A. Alonso, M. G. P. M. S. Neves, A. M. S. Silva,
FULL PAPER
8665–8668; b) S. D. Starnes, S. Arungundram, C. H. Saunders,
Tetrahedron Lett. 2002, 43, 7785–7788.
[3] a) X. Chen, L. Hui, D. A. Foster, C. M. Drain, Biochemistry
2004, 43, 10918–10929; b) H. Gu, K. Xu, Z. Yang, C. K.
Chang, B. J. Xu, Chem. Commun. 2005, 4270–4272.
[4] J.-P. Collman, P. S. Wagenknecht, J. E. Hutchison, Angew.
Chem. Int. Ed. Engl. 1994, 33, 1537–1554.
[5] a) D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 1993,
26, 198–205; b) M. R. Wasielewski, Chem. Rev. 1992, 92, 435–
461.
J. A. S. Cavaleiro, H. K. Hombrecher, Tetrahedron Lett. 1997,
38,2757–2758; b) C. K. Johnson, D. Dolphin, Tetrahedron Lett.
1998, 39, 4753–4756; c) C. M. A. Alonso, M. G. P. M. S. Neves,
A. C. Tome, A. M. S. Silva, J. A. S. Cavaleiro, Eur. J. Org.
Chem. 2004, 3233–3239; d) C. M. A. Alonso, V. I. V. Serra,
M. G. P. M. S. Neves, A. C. Tome, A. M. S. Silva, F. A. A. Paz,
J. A. S. Cavaleiro, Org. Lett. 2007, 9, 2305–2308; e) P. S. S. La-
cerda, A. M. G. Silva, A. C. Tome, M. G. P. M. S. Neves,
A. M. S. Silva, J. A. S. Cavaleiro, A. L. Llamas-Saiz, Angew.
Chem. Int. Ed. 2006, 45, 5487–5491; f) M. G. Crossley, J. A.
McDonald, J. Chem. Soc. Perkin Trans. 1 1999, 2429–2431; g)
K. Jayaraj, A. Gold, L. M. Ball, P. S. White, Inorg. Chem. 2000,
39, 3652–3664; h) T. D. Lash, V. Gandhi, J. Org. Chem. 2000,
65, 8020–8026; i) S. Tokuji, Y. Takahashi, H. Shinmori, H. Shi-
nokubo, A. Osuka, Chem. Commun. 2009, 1028–1030.
[6] D. Gust, Nature 1997, 386, 21–22.
[7] A. Tsuda, A. Osuka, Science 2001, 293, 79–82.
[8] a) K. Henrick, P. G. Owston, R. Peters, P. A. Tasker, A. Dell,
Inorg. Chim. Acta 1980, 45, L161–L163; b) H. J. Callot, E.
Schaeffer, R. Cromer, F. Metz, Tetrahedron 1990, 46, 5253–
5262; c) N. Kobayashi, M. Numao, R. Kondo, S. I. Nakajima,
T. Osa, Inorg. Chem. 1991, 30, 2241–2244; d) R. W. Doyle, D.
Dolphin, J. Chem. Soc., Chem. Commun. 1994, 2463–2464; e)
L. Barloy, D. Dolphin, D. Dupre, T. P. Wijesekera, J. Org.
[10] S. Richeter, A. Hadj-Aïssa, C. Taffin, A. van der Lee, D. Le-
clercq, Chem. Commun. 2007, 2148–2150.
[11] A. D. Adler, F. R. Longo, J. D. Finarelli, J. Goldmacher, J. Ass-
our, L. Korsakoff, J. Org. Chem. 1967, 32, 476.
Chem. 1994, 59, 7976–7985; f) M. A. Faustino, M. G. P. M. S. [12] E. C. Wagner, W. H. Millett, Org. Synth. 1939, 19, 12–14.
Neves, M. G. H. Vicente, A. M. S. Silva, J. A. S. Cavaleiro, Tet-
rahedron Lett. 1995, 36, 5977–5978; g) B. Krattinger, D. J.
[13] M. J. Crossley, L. G. King, I. A. Newsom, C. S. Sheehan, J.
Chem. Soc. Perkin Trans. 1 1996, 2675–2684.
Nurco, K. M. Smith, Chem. Commun. 1998, 757–758; h) H. [14] a) A. Giraudeau, H. J. Callot, J. Jordan, I. Ezahr, M. Gross, J.
Aihara, L. Jaquinod, D. J. Nurco, K. M. Smith, Angew. Chem.
Int. Ed. 2001, 40, 3439–3441; i) Y. V. Ishkov, Russ. J. Org.
Chem. 2001, 37, 288–290; j) S. Richeter, C. Jeandon, R. Rup-
pert, H. J. Callot, Tetrahedron Lett. 2001, 42, 2103–2106; k) D.
Sengupta, B. C. Robinson, Tetrahedron 2002, 58, 5497–5502; l)
M. Nath, J. C. Huffman, J. M. Zaleski, Chem. Commun. 2003,
858–859; M. Nath, J. C. Huffman, J. M. Zaleski, J. Am. Chem.
Soc. 2003, 125, 11484–11485; m) H. S. Gill, M. Harmjanz, J.
Santamaria, I. Finger, M. J. Scott, Angew. Chem. Int. Ed. 2004,
43, 485–490; n) O. Yamane, K. I. Sugiura, H. Miyasaka, K.
Nakamura, T. Fujimoto, K. Nakamuara, T. Kaneda, Y. Sak-
ata, M. Yamashita, Chem. Lett. 2004, 33, 40–41; o) S. Fox,
R. W. Boyle, Chem. Commun. 2004, 1322–1323; p) H. J. Callot,
R. Ruppert, C. Jeandon, S. Richeter, J. Porphyrins Phthalocy-
anines 2004, 8, 111–119; q) J. R. McCarthy, M. A. Hyland, C.
Brückner, Org. Biomol. Chem. 2004, 2, 1484–1491; r) A. N.
Cammidge, P. J. Scaife, G. Berber, D. L. Hughes, Org. Lett.
2005, 7, 3413–3416; s) D.-M. Shen, C. Liu, Q.-Y. Chen, Chem.
Commun. 2005, 4982–4984; D.-M. Shen, C. Liu, Q.-Y. Chen, J.
Org. Chem. 2006, 71, 6508–6511; t) K. Kurotobi, K. S. Kim,
S. B. Noh, D. Kim, A. Osuka, Angew. Chem. Int. Ed. 2006, 45,
3944–3947; u) E. Hao, F. R. Fronczek, M. G. H. Vicente, J.
Org. Chem. 2006, 71, 1233–1236; v) S. Jasinsky, E. A. Ermilov,
N. Jux, B. Röder, Eur. J. Org. Chem. 2007, 1075–1084; w) M.
Tanaka, S. Hayashi, S. Eu, T. Umeyama, Y. Matano, H. Im-
ahori, Chem. Commun. 2007, 2069–2071; x) N. K. S. Davis, M.
Pawlicki, H. L. Anderson, Org. Lett. 2008, 10, 3945–3947; y)
Y. Cao, A. F. Gill, D. W. Dixon, Tetrahedron Lett. 2009, 50,
4358–4360; z) P. J. Chmielewski, J. Maciolek, L. Szterenberg,
Eur. J. Org. Chem. 2009, 3930–3939.
Am. Chem. Soc. 1979, 101, 3857–3862; b) A. W. Johnson, D. J.
Oldfield, J. Chem. Soc. 1965, 4303–4312.
[15] a) S. Richeter, C. Jeandon, R. Ruppert, H. J. Callot, Chem.
Commun. 2002, 266–267; b) S. Richeter, C. Jeandon, J.-P. Gis-
selbrecht, R. Ruppert, H. J. Callot, J. Am. Chem. Soc. 2002,
124, 6168–6179; c) S. Richeter, C. Jeandon, J.-P. Gisselbrecht,
R. Graff, R. Ruppert, H. J. Callot, Inorg. Chem. 2004, 43, 251–
263.
[16] a) J. E. Baldwin, M. J. Crossley, J. DeBernardis, Tetrahedron
1982, 38, 685–692; b) V. Promarak, P. L. Burn, J. Chem. Soc.
Perkin Trans. 1 2001, 14–20.
[17] J. P. Collman, Y.-P. Yan, J. Lei, P. H. Dinolfo, Org. Lett. 2006,
8, 923–926.
[18] a) K. Sato, S. Arai, T. Yamagishi, Tetrahedron Lett. 1999, 40,
5219–5222; b) J.-L. Thomas, J. Howarth, K. Hanlon, D.
McGuirk, Tetrahedron Lett. 2000, 41, 413–416.
[19] M. Scholl, S. Ding, C. W. Lee, R. H. Grubbs, Org. Lett. 1999,
1, 953–956.
[20] S. B. Herzon, J. F. Hartwig, J. Am. Chem. Soc. 2007, 129, 6690–
6691.
[21] A. D. Headley, N. M. Jackson, J. Phys. Org. Chem. 2002, 15,
52–55.
[22] W.-C. Shieh, S. Dell, O. Repicˇ, Org. Lett. 2001, 3, 4279–4281.
[23] K. M. Kadish in Progress in Inorganic Chemistry (Ed.: S. J. Lip-
pard), Wiley, New York, 1986, vol. 34, pp. 435–605.
[24] B. Gorodetsky, T. Ramnial, N. R. Branda, J. A. C. Clyburne,
Chem. Commun. 2004, 1972–1973.
[25] Y. Zhang, J. B. Zheng, Electrochim. Acta 2007, 52, 4082–4086.
Received: November 16, 2009
Published Online: February 15, 2010
1920
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 1912–1920