Isolation and X-ray structural characterization of a dicationic homotrimer of 2,3,6,7-tetramethoxy-9,10-dimethylanthracene cation radical

Article abstract of DOI:10.1016/j.tetlet.2009.09.081

Electrochemical oxidation of 2,3,6,7-tetramethoxy-9,10-dimethylanthracene (1) showed that it undergoes a highly reversible electrochemical oxidation (E_ox = 0.81 V vs SCE) and forms a modestly stable cation-radical salt in solution. X-ray crystallography established that 1^{+{radical dot}}SbCl₆^- crystallizes as a (centrosymmetric) dicationic homotrimer via a close cofacial association of a pair of cationic and one neutral molecule of 1 with an interplanar separation of ～3.2 ?. The structure of the dicationic homotrimer was also reproduced by DFT calculations. Furthermore, the structure of a dicationic spiro adduct, formed by a slow decomposition of a solution of 1^{+{radical dot}}SbCl₆^-, was also established by X-ray crystallography.

Full text of DOI:10.1016/j.tetlet.2009.09.081

Tetrahedron Letters 50 (2009) 6687–6690
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Tetrahedron Letters
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Isolation and X-ray structural characterization of a dicationic homotrimer
of 2,3,6,7-tetramethoxy-9,10-dimethylanthracene cation radical
*
Matthew J. Modjewski, Ruchi Shukla, Sergey V. Lindeman, Rajendra Rathore
Department of Chemistry, Marquette University, PO Box 1881, Milwaukee, WI 53201, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Electrochemical oxidation of 2,3,6,7-tetramethoxy-9,10-dimethylanthracene (1) showed that it under-
Received 5 August 2009
Revised 13 September 2009
Accepted 15 September 2009
Available online 19 September 2009
goes a highly reversible electrochemical oxidation (Eox = 0.81 V vs SCE) and forms a modestly stable cat-
ion-radical salt in solution. X-ray crystallography established that 1+Å SbCl6 crystallizes as
a
ꢀ
(centrosymmetric) dicationic homotrimer via a close cofacial association of a pair of cationic and one
neutral molecule of 1 with an interplanar separation of ꢁ3.2 Å. The structure of the dicationic homotri-
mer was also reproduced by DFT calculations. Furthermore, the structure of a dicationic spiro adduct,
formed by a slow decomposition of a solution of 1+Å SbCl6ꢀ, was also established by X-ray crystallography.
Ó 2009 Elsevier Ltd. All rights reserved.
The stable organic cation radicals are not only critical reaction
intermediates when (poly)aromatic electron donors are exposed
to various oxidants or subjected to different electrochemical, pho-
toinduced, and radiolytic (activation) methodologies,1–3 but they
also pertain directly to the contemporary interest in organic mate-
rials science for molecular devices such as electrical and photocon-
ductors, ferromagnets, sensors, and optical and electrochemical
switches.4–7
Our continued interest in the design and synthesis of various
(poly)aromatic hydrocarbons (such as substituted benzenes, naph-
thalenes, anthracenes, pyrenes, poly-p-phenylenes, and hexa-peri-
hexabenzocoronenes),7 which form stable cation radicals (or hole
carriers) prompted us to examine the possibility of isolation and
X-ray crystallographic characterization of the cation radical of
2,3,6,7-tetramethoxy-9,10-dimethylanthracene (1),8 whose deriv-
atives have been extensively explored for various modern materi-
als9 owing to the potential applications in the emerging areas of
molecular electronics and nanotechnology.10
at room temperature, whose structure was also determined by X-
ray crystallography. The details of these preliminary findings are
discussed herein.
The tetramethoxydimethylanthracene 1 was readily obtained by
a simple condensation of 1,2-dimethoxybenzene with acetaldehyde
in a mixture of sulfuric acid and acetic acid at 0 °C, that is, Eq. 1.8
O
O
O
O
O
O
O
H2SO4
+
ð1Þ
H
AcOH
0 oC/20 h
1
The structure of anthracene 1 was established by 1H/13C NMR
spectroscopy and was further confirmed by X-ray crystallography
(see Fig. 1). In the crystals, the molecules of anthracene 1 occupy
a crystallographic inversion center and have an ideal planar geom-
etry. Moreover, the molecules of 1 form layers along the crystallo-
graphic ‘ab’ plane, and within these layers, neighboring parallel
anthracene moieties are separated at van der Waals distances of
Herein, we now report that anthracene 1 can be quantitatively
oxidized to its cation radical using either a stable aromatic oxi-
ꢁ3.4 Å. Based on the observation of the limited
the molecules of 1 in the layers, it is suggested that the crystal
contacts (see Fig. 1).
p,p-overlap of
12
dant11 or an inorganic oxidant such as NO+ SbCl6
.
The cation
ꢀ
packing is largely dominated by C–Hꢂ ꢂ ꢂ
p
radical of 1 was found to be stable at low temperatures and allows
the isolation of single crystals of a unique dicationic homotrimer,
formally represented as a sandwich of a neutral molecule of 1 be-
tween the two cationic molecules of 1+Å, as established by X-ray
crystallography and corroborated by DFT calculations. Moreover,
it is shown that the cation radical of anthracene 1 undergoes a slow
multi-step transformation to a novel dicationic spiro product (52+Å),
The electron donor strength and the initial indication of the cat-
ion radical stability of 1 were evaluated by electrochemical oxida-
tion at
a
platinum electrode as
a
1 ꢃ 10ꢀ3 M solution in
dichloromethane containing 0.1 M n-Bu4NPF6 as the supporting
electrolyte. The cyclic voltammograms of 1 (Fig. 2) consistently
met the reversibility criteria at various scan rates of 50–600 mV/
s, as they all showed cathodic/anodic peak current ratios of ia/
ic = 1.0 (theoretical) as well as the differences between anodic
and cathodic peak potentials of Epa–Epc = 70 mV at 22 °C. The
reversible oxidation potential of 1 (Eox = 0.81 V vs SCE) was
* Corresponding author. Tel./fax: +1 414 288 2076.
E-mail address: Rajendra.Rathore@marquette.edu (R. Rathore).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.081

6688
M. J. Modjewski et al. / Tetrahedron Letters 50 (2009) 6687–6690
Figure 3. Spectral changes observed upon the reduction of 3.4 ꢃ 10ꢀ5 M MA+Å (red
line) by addition of sub-stoichiometric increments of 1.0 ꢃ 10ꢀ3 M 1 to its radical
cation (gray lines) in anhydrous dichloromethane at 22 °C. The final plot (green
line) of 1+Å obtained after the addition of 1 equiv of 1 which remained unchanged
upon further addition of neutral 1.
Ered = 1.11 V vs SCE) as a one-electron oxidant.11 Thus Figure 3
shows the spectral changes attendant upon an incremental addi-
tion of sub-stoichiometric amounts of 1 to a 3.4 ꢃ 10ꢀ5 M MA+Å
[kmax (log e) = 518 nm (3.86)] in dichloromethane at 22 °C. It is
noted that the formation of green-colored 1+Å (i.e., increase in the
absorbance at 700 nm) and concomitant disappearance of MA+Å
(i.e., decrease in the absorbance at 518 nm) was complete after
the addition of 1 equiv of 1; and the resulting highly structured
absorption spectrum of 1+Å [kmax = 277, 387, 409 (log
e = 4.61),
Figure 1. The unit cell of anthracene 1 showing the limited
the molecules of 1 (top) and its extended packing arrangement in the crystals
p,p-overlap between
497, 473, 627, and 706 nm] remained unchanged upon further
addition of neutral 1 (i.e., Eq. 2). Furthermore, the presence of
(multiple) well-defined isosbestic points (i.e., k = 294, 334, 503,
and 508 nm) in Figure 3 attest to an uncluttered character of elec-
tron transfer from 1 to MA+Å (i.e., Eq. 2).
(bottom) largely dominated by CH- contacts.
p
O
O
O
O
O
.
1+
ð2Þ
+
+ MA
+
.
1
MA+
O
It is also noted that the 1+Å did not show either the self-aggrega-
tion (i.e., pimer formation)13 or the formation of dimer cation rad-
ical [i.e., 1+1Mð1Þ2þꢂ] in the presence of excess neutral 1 in
dichloromethane solutions, as judged by the singular absence of
any new absorption band in the near infrared region.13 The dichlo-
romethane solution of the cation radical of anthracene 1 showed
modest stability at ambient temperatures but was stable for sev-
eral days at ꢀ10 °C, as discerned by the periodic monitoring of
the solutions of 1+Å by UV–vis spectroscopy.
In order to isolate crystalline salts of 1+Å, a solution of sufficient
amounts of 1+Å was prepared by chemical oxidation using nitroso-
nium hexachloroantimonate12 as a 1-eꢀ oxidant according to the
stoichiometry in Eq. 3.
Figure 2. Cyclic voltammograms of 1 ꢃ 10ꢀ3 M 1 in CH2Cl2 containing 0.1 M tetra-
n-butylammonium hexafluorophosphate [(n-Bu)4N PF6] at 22 °C at scan rates
between 50 and 600 mV sꢀ1
.
CH2Cl2
+ NO+SbCl6
ð3Þ
-
-
1
1+
SbCl6 + NO
calibrated with added ferrocene (Eox = 0.45 V vs SCE) as an internal
standard. It is also noted that under similar conditions as men-
tioned above, the parent 9,10-dimethylanthracene undergoes an
electrochemical oxidation at Eox = 1.16 V versus SCE owing to the
absence of 4 electron-donating methoxy groups.
0 o
C
Thus, a solution of 1 in anhydrous dichloromethane was added
to crystalline NOþSbCl6 under an argon atmosphere at ꢁ0 °C. The
gaseous nitric oxide produced was entrained by bubbling argon
through the solution to yield a dark green solution, which upon
spectro-photometric analysis indicated the formation of 1+Å
ꢀ
The electrochemical reversibility and relatively low oxidation
potential of 1, prompted us to generate its cation radical by chem-
ical oxidation using a stable aromatic cation-radical (MA+Å SbCl6
;
SbCl6 (see Fig. 3). Repeated attempts to isolate single crystals of
ꢀ
ꢀ

M. J. Modjewski et al. / Tetrahedron Letters 50 (2009) 6687–6690
6689
1
+Å SbCl6ꢀ by a slow diffusion of toluene or hexane into the solution
of 1+Å in dichloromethane, during a period of 4 days at ꢀ10 °C, did
not result into suitable single crystals. However, a solution of a 1:1
mixture of neutral 1 and 1+Å SbCl6 in dichloromethane afforded
ꢀ
dark-colored crystals, suitable for X-ray crystallographic studies,
by a slow diffusion of hexanes at ꢀ10 °C.
The crystal structure of 2,3,6,7-tetramethoxy-9,10-dimethylan-
thracene (1) cation radical revealed that it forms isolated (centro-
symmetric) dicationic homotrimers resulting from a close cofacial
association of a pair of cationic 1+Å SbCl6 and one neutral molecule
ꢀ
of 1 (see Fig. 4) with an interplanar distance of ꢁ3.2 Å, which is
considerably shorter than the van der Waal’s contact (ꢁ3.4 Å).
Within a homotrimer, the central anthracene ring is found to be
completely planar whereas the two outer anthracene molecules
are bent inward by ꢁ7°. (see Fig. 4). Unfortunately, the limited pre-
cision of the structure of the dicationic homotrimer (i.e.,
esd = 1 pm) did not allow an accurate estimation of the distribu-
tion of the cationic charges onto the three anthracene molecules.
However, a pair of counter anions (SbCl6ꢀ) associated with each
homotrimer are located in proximity to the outer anthracene
molecules than to the central anthracene molecule (see Fig. 4A),
and thus suggest that the charge distribution may not be similar
amongst the three anthracene moieties in the dicationic homotri-
mer. Calculation of the molecular structure of the dicationic (trip-
Figure 5. ORTEP (left) and stick (right) diagrams of a (doublet) dicationic spiro
adduct [52+Å ðSbCl6ꢀÞ2] formed via the decomposition of a CH2Cl2 solution of 1+Å
ꢀ
SbCl6 at 22 °C. The thermal ellipsoids are shown in 30% probability and the
hydrogens and solvent molecules (CH2Cl2) are omitted for the sake of clarity.
The X-ray structure in Figure 5 showed that the decomposition
of a dichloromethane solution of 1+Å SbCl6 at 22 °C produces a
ꢀ
(doublet) dicationic spiro adduct [52+Å ðSbCl6ꢀÞ2] via a multi-step
transformation. The decomposition of 1+Å SbCl6 to the dicationic
ꢀ
spiro adduct in Figure 5 can be reconciled by a sequence of trans-
formations as elucidated in Scheme 1.
The 1-eꢀ oxidation of a methylbenzene to its cation radical is
known to enhance the acidity of methyl protons by several orders
of magnitude.15 Thus, a loss of H+ from 1+Å generates a benzyl-type
radical 2Å which undergoes self dimerization to produce an elec-
tron-rich dianthrylethane 3.16 The 1-eꢀ oxidation of 3 with 1+Å af-
fords 3+Å which undergoes an efficient intramolecular Friedel–
Crafts-type alkylation to form a distonic cation radical17 4+Å. A facile
loss of a proton and a pair of electrons then furnishes the dicationic
Spiro adduct [5+Å ðSbCl6ꢀÞ2] shown in Figure 5.18
*
let) homotrimer using DFT calculations at the B3LYP/6-31G level
reproduced a similar arrangement of the three anthracene mole-
cules. Furthermore, an examination of the bond length changes
in various anthracene molecules (see Fig. S1 and Table S1A–C in
the Supplementary data) showed that the cationic charge was lar-
gely (ꢁ70%) localized onto the outer (bent) anthracene molecules
whereas the central (planar) anthracene molecule contained only
a partial cationic charge (ꢁ30%).14
In summary, we have demonstrated that the readily available
2,3,6,7-tetramethoxy-9,10-dimethylanthracene (1) cation radical
The instability of 1+Å SbCl6 at ambient temperatures was fur-
ꢀ
ther probed by allowing its dichloromethane solution to stand
for a period of 1–2 days at 22 °C. After which time, the solution
deposited shiny dark-colored needles which were analyzed by X-
ray crystallography as follows.
crystallizes as a dicationic homotrimer with the stoichiometry
2þꢂ
[ð1Þ3
ðSbCl6ꢀÞ2] as established by X-ray crystallography. The
molecular structure of the dicationic homotrimer was reproduced
O
O
O
O
O
O
O
O
-H+
+
CH2
+
1
Radical
Dimerization
2
O
O
O
O
O
O
O
O
+
-e-
+
3
3
O
O
O
O
O
O
O
O
Friedel-Crafts
Alkylation
O
O
-e-
O
-H+
-e-
+
O
+
2
O
5
O
O
O
O
O
H
O
O
O
O
+
O
+
Figure 4. Crystal structure of the tetramethoxydimethylanthracene cation radical
with the packing diagram (A) showing that it crystallizes as a centrosymmetric
homotrimer (B and C) with a pair of cationic charges ½ðSbCl6ꢀÞ2ꢄ. The thermal
ellipsoids are shown in 50% probability and the hydrogens and solvent molecules
(CH2Cl2) are omitted for the sake of clarity.
+
O
4
A "doublet" dication
Scheme 1. Proposed mechanism for the formation of dicationic spiro adduct (52+Å
)
by a decomposition of a CH2Cl2 solution of 1+Å SbCl6 at 22 °C.
ꢀ

6690
M. J. Modjewski et al. / Tetrahedron Letters 50 (2009) 6687–6690
*
by DFT calculations at B3LYP/6-31G level which provided evi-
dence that the charge distribution is dissimilar amongst the three
anthracene moieties and the central anthracene molecule bears
only a partial charge in the dicationic homotrimer. It was also
References and notes
1. Eberson, L. Electron-Transfer Reactions in Organic Chemistry; Springer: New York,
1987.
2. Yoshida, K. Electrooxidation in Organic Chemistry. The Role of Cation Radicals as
Synthetic Intermediates; Wiley: New York, 1984.
3. Rathore, R.; Kochi, J. K. Adv. Phys. Org. Chem. 2000, 35, 193. and references cited
therein.
4. (a) Shirota, Y.; Kageyama, H. Chem. Rev. 2007, 107, 953; (b) Carroll, R. L.;
Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378.
shown that prolonged storage of 1+ÅSbCl6 at 22 °C leads to its
ꢀ
decomposition to a dicationic spiro adduct [52+Å ðSbCl6ꢀÞ2] as estab-
lished by X-ray crystal structure analysis. The formation of a stabi-
lized dicationic homotrimer from 1 in solid state is unique in the
light of the fact that generally aromatic cation radicals crystallize
as dimeric cation radical where a single charge is evenly delocal-
ized over both the aromatic rings.7,14 Studies are underway for a
more comprehensive investigation of the structure modulation of
the anthracene ring system for the preparation and study of a cova-
lently linked homotrimers and their higher homologues.
5. Miller, J. S.; Epstein, A. J.; Reiff, W. M. Science 1988, 240, 40. and references cited
therein.
6. (a) Rovira, C. Chem. Rev. 2004, 104, 5289; (b) Segura, J. L.; Martın, N.; Guldi, D.
M. Chem. Soc. Rev. 2005, 34, 31; (c) Chebny, V. J.; Shukla, R.; Lindeman, S. V.;
Rathore, R. Org. Lett. 2009, 11, 1939. and references cited therein.
7. (a) Banerjee, M.; Shukla, R.; Rathore, R. J. Am. Chem. Soc. 2009, 131, 1780; (b)
Chebny, V. J.; Navale, T. S.; Shukla, R.; Lindeman, S. V.; Rathore, R. Org. Lett.
2009, 11, 2253. and references cited therein.
8. Davidson, I. M.; Musgrave, O. C. J. Chem. Soc. 1963, 3154.
9. (a) Zhu, X.-Z.; Chen, C.-F. J. Am. Chem. Soc. 2005, 127, 13158; (b) Zhu, X.-Z.;
Chen, C.-F. J. Org. Chem. 2005, 70, 917. and references cited therein.
10. (a)Introduction to Molecular Electronics; Petty, M. C., Bryce, M. R., Bloor, D., Eds.;
Oxford Univ. Press: New York, 1995; (b)Organic Electronics; Klauk, H., Ed.;
Wiley-VCH: Weinheim, 2006.
11. Rathore, R.; Burns, C. L.; Deselnicu, M. I. Org. Synth. 2005, 82, 1.
12. Rathore, R.; Burns, C. L.; Deselnicu, M. I. Org. Lett. 2001, 3, 2887.
13. It should be noted that the addition of up to 10 equiv of neutral 1 to the
solution of 1+Å or increasing the concentration of 1+Å by 10-fold did not show
any change in its absorption spectrum.7
Acknowledgment
We thank the National Science Foundation for the financial
support.
Supplementary data
Synthetic details of 1, spectral data, and DFT calculation data for
the dicationic homotrimer are available. Crystallographic data
(excluding structure factors) for 1, [ð1Þ32þꢂðSbCl6ꢀÞ2], [52+Å
ðSbCl6ꢀÞ2] have been deposited with the Cambridge Crystallo-
graphic Data Centre CCDC 747382, 747384, and 747384. Copies
of the data can be obtained, free of charge, on application to CCDC,
12 Union Road, Cambridge CB2 1EZ, UK, (fax: +44-(0)1223-336033
or email: deposit@ccdc.cam.ac.uk). Supplementary data associated
with this article can be found, in the online version, at doi:10.1016/
j.tetlet.2009.09.081.
14. Compare: (a) Le Magueres, P.; Lindeman, S. V.; Kochi, J. K. Org. Lett. 2000, 2,
3567; (b) Lee, S.; Chen, B.; Fredrickson, D. C.; DiSalvo, F. J.; Lobkovsky, E.;
Adams, J. A. Chem. Mater. 2003, 15, 1420.
15. Schlesener, C. J.; Amatore, C.; Kochi, J. K. J. Am. Chem. Soc. 1984, 106, 7472.
16. Note that the 9,10-dimethylanthracene cation radical is known to produce the
corresponding dianthrylethane via a radical intermediate akin to 2Å. See: (a)
Parker, V. D.; Eberson, L. Tetrahedron Lett. 1969, 2839; (b) Parker, V. D.;
Eberson, L. Acta Chem. Scand. 1970, 24, 3542.
17. Wadumethrige, S. H.; Rathore, R. Org. Lett. 2008, 10, 5139. and references cited
therein.
18. Compare: Becker, H. D.; Sanchez, D. J. Org. Chem. 1979, 44, 1787.