An EPR analysis of β-dimerization in α-blocked pyrroles in oxidant conditions

Article abstract of DOI:10.1002/mrc.2710

Electron paramagnetic resonance (EPR) analysis of neutral and acidic solutions of 2,5-dimethyl-1-phenylpyrrol (1) and meta-, para-, and ortho-bis(2,5-dimethylpyrrol-1-yl)benzenes (4-6) in the presence of Tl(III) trifluoroacetate as oxidant reveals the poor stability of their generated monomeric radical cations which dimerize through C(β) i￡C(β) bond formation. EPR spectra of the monomeric radical cations 4^?+, 5^?+, and 6 ^?+ coincide with that of 1^?+, suggesting that the unpaired electron in these charged species is confined in one of the pyrrolic rings. The very twisted angles between pyrrolic and phenyl planes due to steric hindrance in the X-ray analysis of the molecular structure of 4 confirm the absence of extended conjugation in the π-system. Copyright

Full text of DOI:10.1002/mrc.2710

Research Article
Received: 29 March 2010
Revised: 31 August 2010
Accepted: 12 November 2010
Published online in Wiley Online Library: 3 January 2011
(wileyonlinelibrary.com) DOI 10.1002/mrc.2710
An EPR analysis of β-dimerization in α-blocked
pyrroles in oxidant conditions
a∗
b
b
Luis Juli a` , Jordi Rius and Xavier Torrelles
Electron paramagnetic resonance (EPR) analysis of neutral and acidic solutions of 2,5-dimethyl-1-phenylpyrrol (1) and meta-,
para-, and ortho-bis(2,5-dimethylpyrrol-1-yl)benzenes (4–6) in the presence of Tl(III) trifluoroacetate as oxidant reveals the
poor stability of their generated monomeric radical cations which dimerize through C(β)–C(β) bond formation. EPR spectra
•+
•+
•+
•+
of the monomeric radical cations 4 , 5 , and 6 coincide with that of 1 , suggesting that the unpaired electron in these
charged species is confined in one of the pyrrolic rings. The very twisted angles between pyrrolic and phenyl planes due to steric
hindrance in the X-ray analysis of the molecular structure of 4 confirm the absence of extended conjugation in the π-system.
Copyright ꢀc 2011 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: pyrroles; EPR; radical cations; X-ray analysis molecular structure
Polypyrroles and polythiophenes are among the most exten-
sively studied conducting polymers in the last two decades.[1–3]
While polypyrroles have been mainly synthesized by electrochem-
ical procedures, chemical methods have also been used in the
g = 2.0022 was performed by using the following hfs constants:
a(6H) = 7.25 G, a(6H) = 4.62 G, and a(2H) ∼ a(2N) ≤ 0.425 G,
with a peak-to-peak linewidth ꢀHpp = 0.38 G (Fig. 1(e)). A similar
behavior was showed in neutral solution when pyrrole 1 dissolved
in1,1,1,3,3,3-hexafluoropropan-2-olwastreatedinthesamecavity
of the spectrometer with a solution of Tl(III) in the same solvent;
[
3]
preparation of polythiophenes. It is well known that electro-
chemical polymerization of pyrroles proceed via dimerization of
radicalcations.[ TheoxidationoftheseheterocyclesbyTl(III)triflu-
oroacetate in different media such as trifluoroacetic (TFA) acid, or
mixtures of TFA acid and CH2Cl2, or 1,1,1,3,3,3-hexafluoropropan-
4]
•+
initially, a strong signal of the radical cation 1 appeared in the
spectrum which gave rise in a very few minutes to a new and
weak multiplet, identical with that detected in acidic conditions.
This fact suggested the presence of a dimeric radical cation in the
2
-ol containing small amounts of TFA acid is a general and very
•
+
useful method to generate their monomeric radical cations in
solution. In this context, many radical cations conveniently substi-
tuted in both α-positions, to make them sufficiently stable, have
been detected by EPR, and the distribution of the spin density in
solution whose assignment is given by 2 (a similar behavior was
advanced by Davies et al.[ in the EPR analysis of the oxidation
of 2,5-dialkylthiophenes in similar conditions upon oxidation with
5b]
ꢁ
thallium (III) trifluoroacetate and confirmed as the β,β -dimer later
on by Eberson et al. ) (Scheme 1). In attempts to obtain more in-
formation on the structure of this β,β -dimer, a solution of pyrrole
[
5]
[5f]
the SOMO orbital has been analyzed. However, in spite of the
presence of alkyl groups blocking the α-carbons in the monomeric
charged pyrrolic species, they are still very short-lived displaying
very weak EPR spectra which disappear within few minutes.
To go more thoroughly into these species, now we report some
resultsconcerningtheEPRofradicalsgeneratedinthecourseofthe
chemical oxidation of 2,5-dimethyl-1-phenylpyrrol and the three
isomeric bis(2,5-dimethylpyrrol-1-yl)benzenes either in acidic or
neutral conditions. There are very few reports in the literature
ꢁ
1 in TFA acid treated with Tl(III) trifluoroacetate was quenched
with water. A GC–MS analysis of the reaction product showed the
+
presence of the dimer (M , 340) as the main compound.
The detection of this dimeric species in the neutral and acidic
conditionsanditsgrowthwiththedecayofthemonomericspecies
•+
•+
1
suggested that it derived from 1 . A possible mechanism for
its formation may involve a radical coupling reaction between two
radical cations followed by deprotonation to a neutral bipyrrole
and consecutive oxidation to radical cation as reported for the
oxidative polymerization of pyrrole[ (Scheme 2) although an
alternative mechanism consisting of a nucleophilic attack of a
ꢁ
[6]
concerning the EPR spectra of radical cations of 2,2 -bipyrroles
and, as far as we know, the spectrum of the radical cation of a
8]
ꢁ
3
,3 -bipyrrole is reported for the first time.
The addition of Tl(III) to a solution of 2,5-dimethyl-1-
−
2
−1
◦
phenylpyrrole (1) (10 to 10 M) in TFA acid at 25 C led to
the development of a weak EPR spectrum of the corresponding
monomeric radical cation 1 with a well-resolved pattern, a(6H)
•+
∗
Correspondence to: Luis Juli a` , Departament de Química Biol o` gica i Mod-
elitzaci o´ Molecular, Institut de Química Avan c¸ ada de Catalunya (CSIC), Jordi
Girona 18-26, 08034 Barcelona, Spain. E-mail: ljbmoh@cid.csic.es
=
16.6 G, a(2H) = 3.60 G, and a(N) = 4.40 G, and a g value of
.0023.[ This spectrum disappeared within few minutes while
5b]
2
a new one slowly increased (Fig. 1(a)–(c)). In more concentrated
a Departament de Química Biol o` gica i Modelitzaci o´ Molecular, Institut de
Química Avan c¸ ada de Catalunya (CSIC), Jordi Girona 18-26, 08034 Barcelona,
Spain
−
1
acidic solutions of pyrrole 1 (10 to 1 M), the spectrum of the
second-generated radical species appeared with stronger inten-
sity (Fig. 1(d)). The simulation (the simulation was performed by
b Institut de Ci e` ncia de Materials (CSIC), Campus de la U. A. B., 08193 Bellaterra,
Spain
[
7]
using the WINSIM program) of this new multiplet centered at
Magn. Reson. Chem. 2011, 49, 164–167
Copyright ꢀc 2011 John Wiley & Sons, Ltd.

An EPR analysis of β-dimerization in α-blocked pyrroles in oxidant conditions
CH3
H
CH3
Ar
N
2
H C
3
N
CH3
N
H
Ar
Ar
H3C
H C
3
1
-
2 H
-
1e
2
2
•+
Scheme 2. Mechanism of the generation of 2
.
CH3
H
CH3
Ar
+
1
N
H C
N
CH3
N
3
H
Ar
H C
3
Ar
H C
3
3
1
-
-
H
1e
CH3
CH3
Ar
-
1e
N
2
N
H
Ar
H C
3
H C
3
•+
Scheme 3. Alternative mechanism of the generation of 2
.
H C
N
CH3
3
Figure 1. (a–c) Evolution of the EPR spectra during the oxidation of 2,5-
dimethyl-1-phenylpyrrol (1) in TFA acid with thallium(III) trifluoroacetate
at 298 K; (a) 0 min; (b) 15 min; (c) 45 min (modulation of amplitude, MA =
CH3
4
5
: m-
: p-
N
0
.5); (d) final spectrum (MA = 0.32); (e) computer simulation of the second
radical species using the values given in the text.
6: o-
H C
3
Scheme 4. Molecular structures of compounds 4, 5 and 6.
CH3
CH3
CH3
CH3
Ar
Ar
N
N
hfs constants, a(6H) = 7.47 G, a(6H) = 4.45 G, and a(2H) ∼ a(2N) ≤
0.45 G, with ꢀHpp = 0.26 G. Attempts to corroborate these results
with the third isomer, bis(2,5-dimethyl-1-pyrrolyl)-(1,2-phenylene)
N
N
Ar
Ar
H C
H C
3
3
H C
H C
3
3
2
(6)(Scheme 4),wereunsuccessful.Eventhoughthesamespectrum
•
+
of the monomeric radical cation initially appeared when fresh
solutions of 6 and thallium (III) in TFA acid were introduced into
the cavity of the spectrometer, in no case the radical cation of the
Scheme 1. Two of the canonical forms of the dimeric radical cation 2
.
•
+
ꢁ
neutral molecule of 1 in the β-position of radical cation 1 to
β,β -dimer was generated.
•+
•+
give 3 which yields 2 by consecutive deprotonation and
Initially, the detection in the EPR of radical cations with
practically the same parameter values than those of radical cation
[
9]
oxidation is also supported in the literature (Scheme 3). This
last mechanism may be favored due to the presence of the
electron-donating methyl substituents.
Bis(2,5-dimethyl-1-pyrrolyl)-(1,3-phenylene) (4) [compound 4
was prepared by the same method of synthesis of its isomer
•+
1
, when bis(2,5-dimethylpyrrol-1-yl)benzenes 4, 5, and 6 were
oxidized with Tl(III) trifluoroacetate denotes that the unpaired
electronisexclusivelyconfinedtoonepyrrolylunitofthemolecule
and not delocalized in both pyrrolyl units. This phenomenon was
[
5d]
◦
1
[5d]
5
.
Characterization: mp 112–114 C (from hexane); H NMR
previously observed by Davies et al. in the isomer 5 which was
(
200 MHz, CDCL3) δ, 2.08 (s, 12H), 5.92 (s, 4H), 7.59–7.08 (m, 4H);
accounted for by the non-planarity of the molecule (the twisted
anglesbetweenpyrrolesandphenylringinthemolecularstructure
1
3
C NMR (50 MHz, CDCl3) δ, 13.03, 106.08, 127.44, 128.02, 128.61,
[
5d]
◦
[10]
1
29.49, 139.62] and its isomer -(1,4-phenylene) (5)
(Scheme 4)
of 5 are 90 in both the cases).
We have also confirmed this
showed similar behavior when TFA acid solutions of them in the
presence of oxidant were analyzed by EPR. Initially, a very similar
spectrum as that derived from radical cation 1 appeared and
absence of conjugation in the electronic structure of 4 by X-ray
diffraction. Isomer 4 crystallizes from hexane in crystals of good
quality suitable for X-ray analysis [crystal data of 4: Molecular
formula C18H20N2. Molecular weight 264.36. Monoclinic P21/a, No.
14. Cell dimensions: a = 9.308(3), b = 14.957(2), c = 11.425(2) ꢀ,
•+
next replaced by a weak multiplet as shown in Fig. 2 assigned to
ꢁ
[7]
the β,β -dimer (g = 2.0024) which was simulated by using the
Magn. Reson. Chem. 2011, 49, 164–167
Copyright ꢀc 2011 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc

L. Juli a` , J. Rius and X. Torrelles
Figure 3. An ORTEP drawing of the molecular structure of bis(2,5-
dimethyl-1-pyrrolyl)-(1,3-phenylene) (4) (hydrogens have been removed
◦
for clarity). Selected interatomic distances (Å) and angles ( ): N1–C16
1
1
.387(2), N1–C13 1.390(2), N1–C6 1.428(2), N2–C10 1.379(2), N2–C7
.388(2), N2–C2 1.429(2), C1–C2 1.382(2), C1–C6 1.385(2), C2–C3 1.386(2),
C3–C4 1.375(2), C4–C5 1.370(2), C5–C6 1.386(2), C7–C8 1.351(2), C7–C12
1
1
1
1
1
1
1
1
1
.491(3), C8–C9 1.394(3), C9–C10 1.352(2), C10–C11 1.475(3), C13–C14
.353(2),C13–C181.488(2),C14–C151.406(2),C15–C161.361(2),C16–C17
.471(2); C16–N1–C13 109.56(12), C16–N1–C6 125.18(12), C13–N1–C6
25.16(12), C10–N2–C7 109.80(13), C10–N2–C2 125.99(12), C7–N2–C2
24.05(14), C2–C1–C6 120.04(12), C1–C2–C3 119.94(12), C1–C2–N2
20.31(12), C3–C2–N2 119.75(12), C4–C3–C2 119.57(13), C5–C4–C3
20.89(13), C4–C5–C6 119.86(13), C1–C6–C5 119.67(12), C1–C6–N1
20.19(11), C5–C6–N1 120.14(12), C8–C7–N2 106.4(2), C8–C7–C12
31.7(2), N2–C7–C12 121.9(2), C7–C8–C9 108.6(2), C10–C9–C8 109.0(2),
Figure 2. (a) EPR spectrum of a solution of bis(2,5-dimethyl-1-pyrrolyl)-
1,3-phenylene) (4) in TFA acid in the presence of thallium(III) trifluoroac-
etate at 298 K. (b) Computer simulation using the values given in the
text.
(
C9–C10–N2 106.3(2), C9–C10–C11 130.8(2), N2–C10–C11 122.85(14),
C14–C13–N1 106.76(14), C14–C13–C18 131.2(2), N1–C13–C18
1
21.92(13), C13–C14–C15 108.67(15), C16–C15–C14 108.40(14),
C15–C16–N1 106.62(14), C15–C16–C17 130.82(15), N1–C16–C17
22.54(13).
1
◦
◦
◦
3
α = 90.00(1) , β = 102.34(2) , γ = 90.00(1) , V = 1553.7(6) ꢀ ,
3
−1
Z = 4, F(000) = 568, Dc = 1.13 g/cm , µ = 0.067 mm , crystal
dimensions 0.72 mm × 0.38 mm × 0.14 mm. Data collection:
The diffractometer was an Enraf Nonius CAD4 with a graphite
monochromated MoKα radiation, λ = 0.71073 ꢀ. Cell parameters
were determined from refinement of 15 reflections using the
Inthiscontext,itisworthtosaythatabinitiocalculationsperformed
in 1 and its radical cation 1 predict that the minimum energy
conformation in both the structures correspond to a torsional
angle of 90 .
This case of β-dimerization in pyrrole radical cations, reported
in the literature in the case of electrochemical oxidation of 1,2,5-
trimethylpyrrole,[ confirms the reactivity of the β-carbon atoms
during the oxidative processes to polypyrroles and this is the
reason why chemical or electrochemical growing polypyrroles are
•+
◦
[5i]
[
11]
CAD4 Express software.
For thousand one hundred fifty-two
◦
reflections were measured with 2.28 ≤ θ ≤ 28.44 and index
4c]
ranges −12 ≤ H ≤ 0, 0 ≤ K ≤ 20, −14 ≤ L ≤ 15 in ω/2θ mode,
0
6
.55 + 0.65 tan(θ) scan width and maximum final scan time of
0 s. Three standard reflections were measured every 3600 s to
ꢁ
check for the intensity variation and three more standards were
measured every 55 reflections to check the crystal orientation.
Intensity gain was 0.00% and was not corrected. Absorption
correction was made using seven psi-scans, Tmax = 98.98% and
Tmin = 92.49%. Resolution and refinement: The structure was
not exclusively α,α -bonded, causing some crosslinking of the
polymers. These defects are practically eliminated by blocking the
β-positions with alkyl groups.
[
12]
2
solved by direct methods using the SumF-TF and refined on F
Experimental Section
with 4648 unique reflections and 316 parameters. Final R factors:
[
0
I > 2σ(I)] R1 = 0.0498 and wR2 = 0.1342; R factors (all data) R1 =
Melting points were obtained by using a K o¨ fler microscope
Reichert and are uncorrected. NMR spectra were determined
at 200 MHz with Varian Gemini 200 HC spectrometer. TFA acid was
distilled before use.
EPR spectra were recorded using a Varian E 109 spectrometer
working in the X band. To detect the radical cations, solutions of
substrates in TFA acid or in 1,1,1,3,3,3-hexafluoropropan-2-ol in a
quartz tube (internal diameter 2 mm) were degassed by passing a
stream of dry argon through the solutions to remove oxygen, and
then a small excess of thallium trifluoroacetate was added in the
same tube.
2
.1125 and wR2 = 0.1459. Goodness-of-fit on F = 0.985, where
2
2
2 2
2 2 1/2
R = ꢁ||F0| − |Fc||/ꢁ|F0|, wR = [ꢁw(F0 − Fc ) /ꢁw(F0 ) ]
,
2
2
2
2
2
w = 1/[ꢁδ(F0 ) +(0.0800P) +0.00P], P = [Max(F0 , 0)+2Fc ]/3,
2
2
1/2
GooF = [(ꢁw(F0 − Fc ))/(n − p)] (n, number of reflections; p,
number of parameters). Final shifts/esd were <0.000 in the last
cycle (with convergence to 0), and the maximum and minimum
residual electron density in the final Fourier difference was 0.12
−
3
[13]
and −0.18eꢀ , respectively. A SHELXL-93 program was used
[
14]
for refinement and plots were made with PLUTON programs ].
An ORTEP drawing of this molecule is shown in Fig. 3. The
dihedral angles between the pyrrolic rings and the benzene are
◦ ◦
6
7.25(0.05) and 61.56(0.05) . This torsion from planarity avoids
Acknowledgements
the steric interaction between methyl substituents and hydrogens
in benzene. It is reasonable to suggest that these angles will not
changeinagreatextensionongoingtotheradicalcationstructure.
Financial support from the MeI (Spain) through project AGL2009-
12374-C03-03/ALI is gratefully acknowledged. We also thank the
wileyonlinelibrary.com/journal/mrc
Copyright ꢀc 2011 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2011, 49, 164–167

An EPR analysis of β-dimerization in α-blocked pyrroles in oxidant conditions
EPR service of the Centre d’Investigaci o´ I Desenvolupament (CSIC)
for recording the spectra.
Trans. 2 1989, 239; (c) D. N. Avila, A. G. Davies, J. Chem. Soc. Faraday
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L. Juli a´ , J. Org. Chem. 1997, 62, 878; (f) F. Barbosa, L. Eberson,
G. Gescheidt, S. Gronowitz, A.-B. H o¨ rnfeldt, L. Juli a´ , O. Persson, Acta
Chem. Scand. 1998, 52, 1275; (g) L. l. Fajarí, E. Brillas, C. Alem a´ n,
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C. Alem a´ n, L. Juli a´ , J. Chem. Soc. Perkin Trans. 2 2000, 905; (i)
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Supporting information
Supporting information may be found in the online version of this
article.
6
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Magn. Reson. Chem. 2011, 49, 164–167
Copyright ꢀc 2011 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc