Isolable zwitterionic pyridinio-semiquinone pi-radicals. Mild and efficient single-step access to stable radicals.

Abstract

A rational design based on the proton-coupled electron transfer (PCET) concept allows us to structurally characterize for the first time isolable, air- and moisture-stable semiquinone radicals in a zwitterionic neutral form. The presence of an alkoxy and the bulky pyridinio substituents causes only a minor perturbation of either the redox potentials or the spectral UV-vis characteristics of the semiquinone core but significantly stabilizes the new radicals.

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Isolable Zwitterionic
Pyridinio-semiquinone π-Radicals. Mild
and Efficient Single-Step Access to
Stable Radicals
Chenyi Yi,†Carmen Blum,†Shi-Xia Liu,*,†Tony D. Keene,†Gabriela Frei,†
Antonia Neels,‡and Silvio Decurtins†
Departement fu ¨r Chemie und Biochemie, UniVersita ¨t Bern, Freiestrasse 3, CH-3012 Bern,
Switzerland, and XRD Application LAB, CSEM Centre Suisse d’Electronique et de
Microtechnique SA, Jaquet-Droz 1, Case postale, CH-2002 Neucha ˆtel, Switzerland
liu@iac.unibe.ch
Received March 17, 2009
ABSTRACT
Arationaldesignbasedontheproton-coupledelectrontransfer(PCET)conceptallowsustostructurallycharacterizeforthefirsttimeisolable,
air- and moisture-stable semiquinone radicals in a zwitterionic neutral form. The presence of an alkoxy and the bulky pyridinio substituents
causesonlyaminorperturbationofeithertheredoxpotentialsorthespectralUV-vischaracteristicsofthesemiquinonecorebutsignificantly
stabilizes the newradicals.
Quinones and their reduced forms, semiquinones and dihy-
droquinones, represent prototypical examples of organic
redox systems. Besides a profound chemical interest, it is
widely accepted that quinone-based redox couples play key
biological functions as electron-proton transfer agents in
bioenergetic processes such as respiration and photosynthesis,
which are essential for life.1Since hydrogen bonding and
protonation are decisive factors controlling potentials and
†Universita ¨t Bern.
‡CSEM Centre Suisse d’Electronique et de Microtechnique SA.
(1) (a) The Photosynthetic Reaction Center; Deisenhofer, J., Norris, J. R.,
Eds.; Academic Press: San Diego, CA, 1993. (b) Kaim, W.; Schwederski,
B. Bioinorganic Chemistry: Inorganic Elements in the Chemistry of Life; John
Wiley & Sons: Chichester, U.K., 1994. (c) Ferguson-Miller, S.; Babcock,
G. T. Chem. ReV. 1996, 96, 2889. (d) The Chemistry of the Quinonoid
Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1988.
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Vol. 11, No. 11
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mechanisms in the reduction of quinones, the proton-coupled
electron-transfer (PCET) concept is far-reaching. In particu-
lar, the promotion effects of the proton on ET are often
modulated by Brønsted bases (:B).2As a case in point,
consider the protonated amino acid promoted ET reduction
of a quinone by hydrogen-bond formation with the resulting
semiquinone radical anion.3Such semiquinone intermediates
have sufficiently long lifetimes to be observed during
mechanistic studies by conventional spectroscopic meth-
ods.3,4However, they have not been isolated as pure
compounds: they are transient organic radicals. Ideally, one
would like to study discrete and reversible one-proton/one-
electron PCET processes which, however, generally lead to
the formation of highly reactive radicals.
The present study reports the preparation and characteriza-
tion of isolable semiquinone radicals, namely zwitterionic
pyridinio-semiquinone π-radicals of the type 2,3-dicyano-
1-hydroxy-5-alkoxy-4-oxo-6-pyridiniocyclohexa-2,5-dien-1-
yl ion5(Scheme 1). Their stability arises not only from
π-conjugation which offers the possibility of the free spin
to be delocalized over any part of the conjugated π-system
and from the zwitterionic structure (thermodynamic stabiliza-
tion) but also from the steric protection of the substituents
(kinetic stabilization). A more detailed discussion on the
above-mentioned criteria for the stabilization of radicals is
given in a recent review by Hicks.6
The reaction used to prepare the stable radical compounds
is intriguing because of its simplicity and robustness. Thus,
2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) was dissolved
in an alcohol ROH and treated with an excess of pyridine to
generate a deep red solution (Scheme 1). By slow evapora-
tion, air- and moisture-stable zwitterionic pyridinio-semi-
quinone π-radicals 1 were obtained as dark-colored single
crystals suitable for X-ray crystal-structure analysis.
Although the work presented here does not address a
detailed reaction mechanism directly, a plausible PCET
process is involved as depicted in Scheme 1. Thereby, DDQ
initially undergoes two nucleophilic substitution reactions
giving rise to a weakly basic quinone intermediate. The
following step may be discussed in terms of the demonstra-
tion that positive shifts in the reduction potentials of different
quinones in the presence of an alcohol are ascribed to quite
specific hydrogen-bonding equilibria involving mono- and
dianions.7Consequently, this PCET-related effect will
facilitate the reduction of the quinone derivative while
stabilizing the reduced species.
For a representative structural characterization of the
radical compounds 1a-c, the 5-propoxy-substituted derivative
1b will be chosen. This radical crystallizes in the monoclinic
crystal system, space group P21/c. The ORTEP drawing
along with selected geometrical parameters is shown in
Figure 1.
The neutral molecule consists of a negatively charged
semiquinone moiety which is tetrasubstituted with two cyano,
one propoxy, and one pyridinio group. The molecule,
disregarding the pyridinio and the propyl groups, exhibits a
planar geometry (rms deviation from a least-squares plane
amounts to only 0.06 Å; maximum deviation for O1: 0.177(2)
Å). The bond lengths within the semiquinoid moiety are in
the range observed for semiquinones in charge-transfer (CT)
salts;8however, the actual asymmetric substitution pattern
and the zwitterionic electronic structure is reflected, to some
extent, in bond-length distortions of the benzenoid ring. The
(2) (a) Stowell, M. H. B.; McPhillips, T.; Rees, D. C.; Soltis, S. M.;
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M. A.; Miller, L. A.; Tiernan, C.; Clavier, G.; Audebert, P.; Rotello, V. M.
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J. Am. Chem. Soc. 1997, 119, 9230.
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1830. (c) Song, Y.; Buettner, G. R.; Parkin, S.; Wagner, B. A.; Robertson,
L. W.; Lehmler, H.-J. J. Org. Chem. 2008, 73, 8296. (d) Fukuzumi, S.;
Okamoto, K.; Yoshida, Y.; Imahori, H.; Araki, Y.; Ito, O. J. Am. Chem.
Soc. 2003, 125, 1007.
(5) Bu ¨nzli-Trepp, U. Systematic Nomenclature of Organic, Organome-
tallic and Coordination Chemistry; EPFL: Lausanne, 2007.
(6) Hicks, R. G. Org. Biomol. Chem. 2007, 5, 1321.
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Linschitz, H. J. Am. Chem. Soc. 1997, 119, 6384.
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Kurmoo, M. J. Chem. Soc., Dalton Trans. 1995, 2509.
Scheme 1. Synthetic Route to 1 (1a R ) Methyl; 1b R )
Propyl; 1c R ) 2-Methoxyethyl)
Figure 1. X-ray crystal structure of 1b (ORTEP; thermal ellipsoids
set at the 50% probability level). Selected bond lengths (Å): O1-C1
1.256(4), C1-C2 1.470(5), C2-C3 1.363(4), C3-C4 1.447(5),
C4-O3 1.238(4), C4-C5 1.453(5), C5-C6 1.387(4), and C6-C1
1.444(5). Hydrogen atoms are omitted for clarity.
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Org. Lett., Vol. 11, No. 11, 2009

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pyridinium and benzenoid planes exhibit a dihedral angle
of 65.63(9)°.
The characteristic feature of the crystal packing is the
columnar stacking of the semiquinoid rings along the a-axis
(Figure S3, Supporting Information). Thereby, within these
one-dimensional chains, two adjacent molecules form cen-
trosymmetrically related π-stacked dimers with a trans-
cofacial mode of association; hence their electric dipoles are
pointing in opposite directions, and their semiquinoid rings
are partially slipped. Within these closely packed dimers,
the short interplanar separation amounts to only 2.931(4) Å
which is substantially less than the van der Waals separation
(3.70 Å). The interplanar separation between the dimers is
3.702(4) Å.
As the single-crystal X-ray analysis reveals, the radicals
associate as spin-paired dimers in the solid state, thus the
short intradimer contacts render the crystalline material
diamagnetic, which was confirmed by magnetic susceptibility
(SQUID) measurements.
The electrochemical properties of 1a-c in CH3CN were
investigated by cyclic voltammetry (Figure 2 and Figures
S6 and S7, Supporting Information). They show almost the
same redox behavior. As shown in Figure 2, 1a exhibits two
well-separated and reversible one-electron redox waves for
the oxidation and reduction of the semiquinone radical at
0.73 and -0.05 V vs Ag/AgCl, respectively. Under the same
conditions, DDQ shows two reversible one-electron reduction
waves at 0.66 and -0.16 V. Consequently, the neutral radical
1a exhibits a fairly wide electrochemical stability range of
0.78 V and, moreover, the reversibility of the two redox
processes still remains.
Compounds 1a-c strongly absorb in the UV-vis spectral
region as evidenced by their dark color (Figure S8, Sup-
porting Information). The intense absorption bands in the
UV and blue part of the spectrum are characteristic for π-π*
transitions, specifically of the pyridinium and semiquinone
subunits. More importantly, the π-radical is characterized
by diagnostic absorption bands at 340 nm (29410 cm-1), 420
nm (23810 cm-1), 445 nm (22470 cm-1), 550 nm (18180
cm-1), and 590 nm (16950 cm-1), as illustrated in Figure 3
for 1a, which are well comparable to those of the anion
radical DDQ•-in CT salts.9The presence of π-stacked
radical dimers 1a·1a within the crystalline solid is cor-
roborated by an absorption band at 755 nm (13250 cm-1)
(Figure S9, Supporting Information). However, this π-stack-
ing association process10is not noticeable in solution at room
temperature, since no long-wavelength electronic transition
was observed, even at high concentrations and in polar
solvents.
The resulting paramagnetic characteristic in solution causes
compounds 1a-c to be1H NMR silent. This observation is
also manifested in an isotropic EPR signal (giso) 2.002) in
DMF solution (Figures S10-S12, Supporting Information).
The main IR stretching frequencies ν(CtN) and ν(CdO)
of 1a at 2208 and 1627 cm-1, respectively, represent a low
energy shift in comparison to DDQ (2232 and 1674 cm-1).
These data point consistently to the presence of the semi-
quinone unit.
From an unrestricted DFT calculation,11,12the SOMO of
the π-radical 1a is found to be as illustrated in Figure 4 and
the spin-density distribution as in Figure S13, Supporting
Information.
It should be noted that about 80% of the odd-electron
density lies on the semiquinoid unit, and some lower densities
are found on the oxygen atom of the methoxy group and on
(9) Salman, H. M. A.; Mahmoud, M. R.; Abou-El-Wafa, M. H. M.;
Rabie, U. M.; Crabtree, R. H. Inorg. Chem. Commun. 2004, 7, 1209.
(10) (a) Yamagishi, A. Bull. Chem. Soc. Jpn. 1975, 48, 2440. (b) Lu ¨,
J.-M.; Rosokha, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2003, 125, 12161.
(11) The DFT method was employed with the B3LYP functional and
the TZVP (valence triple-? plus polarization) basis set. All calculations were
done with the TURBOMOLE V5.10 program package.
(12) (a) Treutler, O.; Ahlrichs, R. J. Chem. Phys. 1995, 102, 346. (b)
Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor. Chem. Acc.
1997, 97, 119.
Figure 2. Cyclic voltammograms of 1a (red line, 5 × 10-4M) and
DDQ (black line, 5 × 10-4M) in CH3CN, supporting electrolyte
0.1 M (Bu4N)PF6, scan rate 100 mV s-1.
Figure 3. Electronic absorption spectrum of 1a in CH3CN solution
at room temperature.
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the nitrogen atoms of the two cyano groups. This π-type
SOMO bears a nodal plane passing through the two shortest
carbon-carbon bonds of the benzenoid ring. Not unexpect-
edly for this “inner-salt”-type molecule, the calculated
electric-dipole (ED) moment in the ground state (D0) amounts
to a considerably high value of 17.8 D. The ED vector points,
as one may expect, in the direction of the pyridinio group
(δ+pole).
In summary, we have directed attention toward the
preparation of isolable, air- and moisture-stable semiquinone
radicals in a zwitterionic neutral form, which adds a valuable
molecule family to the class of radical compounds.6,13The
presence of an alkoxy and the bulky pyridinio substituents
causes only a minor perturbation of either the redox potentials
or the spectral UV-vis characteristics of the semiquinone
core. However, most significantly, these substituents render
the new radicals quite stable, in anology to the recently
reported imino-semiquione radicals.13In light of these results,
further investigations on this PCET reaction of DDQ in the
presence of a variety of nucleophiles using a broader range
of alcohols as solvents are underway. On the one hand, it is
intended to explore how the basicity of nucleophiles affects
the occurrence of the PCET pathway. On the other hand,
the properties and functionalities of the resulting radicals can
be largely modified, thus it opens the possibility of exploring
their potential application in the field of single-component
conductors and multifunctional materials.14As predicted,
these molecules also bear the potential for the study of
interesting photochemical reactions based on their radical
nature.
Acknowledgment. This work was supported by the Swiss
National Science Foundation (Grant No. 200020-116003).
Supporting Information Available: General experimental
details and characterization data for compounds 1a-c; CIF
files for 1a-c and other additional figures. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL900559P
(13) Carter, S. M.; Sia, A.; Shaw, M. J.; Heyduk, A. F. J. Am. Chem.
Soc. 2008, 130, 5838.
(14) (a) Miller, J. S.; Epstein, A. J. Angew. Chem., Int. Ed. Engl. 1994,
33, 385. (b) Miyasaka, H.; Izawa, T.; Takahashi, N.; Yamashita, M.; Dunbar,
K. R. J. Am. Chem. Soc. 2006, 128, 11358.
Figure 4. SOMO of 1a. The isosurface is shown at a cutoff value
of 0.05 e Å-3.
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