Multiple-electron transfer in a single step. Design and synthesis of highly charged cation-radical salts

Article abstract of DOI:10.1021/ol0163474

(equation presented) Macromolecules 1c and 2c bearing multiple redox-active sites are synthesized by an efficient palladium-catalyzed coupling of 2,5-dimethoxytolylmagnesium bromide with readily available hexakis(4-bromophenyl)benzene and tetrakis(4-bromophenyl)methane. These macromolecular electron donors undergo reversible oxidation at a constant potential of 1.15 V vs SCE to yield robust, multiply charged cation radicals that are isolated in pure form using SbCl₅ as an oxidant. These nanometer-size cation-radical salts are shown to act as efficient "electron sponges" toward a variety of electron donors.

Full text of DOI:10.1021/ol0163474

ORGANIC
LETTERS
2001
Vol. 3, No. 18
2887-2890
Multiple-Electron Transfer in a Single
Step. Design and Synthesis of Highly
Charged Cation-Radical Salts
Rajendra Rathore,* Carrie L. Burns, and Mihaela I. Deselnicu
Department of Chemistry, Marquette UniVersity, PO Box 1881,
Milwaukee, Wisconsin 53201
rajendra.rathore@marquette.edu
Received June 25, 2001
ABSTRACT
Macromolecules 1c and 2c bearing multiple redox-active sites are synthesized by an efficient palladium-catalyzed coupling of 2,5-
dimethoxytolylmagnesium bromide with readily available hexakis(4-bromophenyl)benzene and tetrakis(4-bromophenyl)methane. These
macromolecular electron donors undergo reversible oxidation at a constant potential of 1.15 V vs SCE to yield robust, multiply charged cation
radicals that are isolated in pure form using SbCl5 as an oxidant. These nanometer-size cation-radical salts are shown to act as efficient
“electron sponges” toward a variety of electron donors.
Design and synthesis of nanometer-size macromolecules is
receiving considerable attention owing to their importance
as materials that can be used as molecular devices such as
sensors, switches, molecular wires, ferromagnets, and other
electronic and optoelectronic devices.1-3 Thus, construction
of organic molecules containing multiple redox-active chro-
mophores are of particular interest owing to their importance
not only for the preparation of materials with novel properties
but also as multielectron redox catalysts.4-6 Accordingly, we
now report the utilization of readily available hexaphenyl-
benzene7 and tetraphenylmethane8 as platforms for the
preparation of multicentered electron donors that are bearing
six and four redox-active centers, respectively. Moreover,
these novel macromolecular electron donors undergo multiple
(3) Keegstra, M. A.; Feyter, S. D.; Schryver, F. C. D.; Mu¨llen, K. Angew.
Chem., Int. Ed. Engl. 1996, 35, 774. (b) Oldham, W. J., Jr.; Lachicotte, R.
J.; Bazan, G. C. J. Am. Chem. Soc. 1998, 120, 2987. (c) Wang, S.; Oldham,
W. J., Jr.; Hudack, R. A., Jr.; Bazan, G. C. J. Am. Chem. Soc. 2000, 24,
5695 and references therein.
(4) (a) Fillaut, J.-L.; Linares, J.; Astruc, J. Angew. Chem., Int. Ed. Engl.
1994, 33, 2460. (b) Lambert, C.; Noll, G. Angew. Chem., Int. Ed. Engl.
1998, 37, 2107.
(5) (a) Blackstock, S. C.; Selby, T. D. J. Am. Chem. Soc. 1998, 120,
12155. (b) Bonvoisin, J.; Launay, J.-P.; Auweraer, M. V.d.; Schryver, F.
C. D. J. Phys. Chem. 1994, 98, 5052 and references therein.
(6) (a) Bauld, N. L.; Bellville, D. J. Tetrahedron Lett. 1982, 23, 825. (b)
Heo, R. W.; Somoza, F. B.; Lee, T. R. J. Am. Chem. Soc. 1998, 120, 1621.
(c) Manners, I. Angew. Chem., Int. Ed. Engl. 1996, 35, 1602 and references
therein.
(1) (a) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35,
1154. (b) Balzani, V., Campagna, S.; Denti, G.; Juris, A.; Serroni, S.;
Venturi, M. Acc. Chem. Res. 1998, 31, 26. (c) Reichert, V. R.; Mathias, L.
J. Macromolecules 1994, 27, 7015.
(2) (a) Pollak, K. W.; Leon, J. W.; Frechet, J. M. J.; Maskus, M.; Abruna,
H. D. Chem. Mater. 1998, 10, 30. (b) Gorman, C. B. AdV. Mater. 1997, 9,
1107. (c) Wilson, L. M.; Griffin, A. C. J. Mater. Chem. 1993, 3, 991.
(7) Fieser, L. F. Organic Syntheses; Wiley: New York, 1973; Collect.
Vol. V, p 605.
(8) Wassmundt, F. W.; Kiesman, W. F. J. Org. Chem. 1995, 6, 1713.
10.1021/ol0163474 CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/09/2001

electron loss in a single step (at a constant potential) to afford
highly charged cation-radical salts that are remarkably robust
and can be isolated in pure form.
The hydroquinone ether moiety, 2,5-dimethoxy-4-methyl-
benzene, was chosen as the redox-active unit to be linked to
hexakis(4-bromophenyl)benzene (1b) and tetrakis(4-bromo-
phenyl)methane (2b) platforms owing to its ability to
generate a stable cation-radical salt upon a one-electron
oxidation.9 As shown in Scheme 1, these molecular platforms
The electron donor strengths of 1c and 2c in comparison
with the model compound (4,4′-dimethyl-2,5-dimethoxy-
biphenyl 314) were evaluated by electrochemical oxidation
at a platinum electrode as a 2 × 10-3 M solution in
dichloromethane containing 0.2 M tetra-n-butylammonium
hexafluorophosphate as the supporting electrolyte. The highly
reversible cyclic voltammograms (see Figure 1) of various
Scheme 1. Synthesis of Redox-Active Donors 1c and 2c
Figure 1. Cyclic voltammograms of 2 mM 1c and 2c and 5 mM
model compound 3 (as indicated) in dichloromethane at a scan rate
of ν ) 200 mV s-1 (25 °C).
donors were consistently obtained at a scan rate of 25-200
mV s-1, and they all showed anodic/cathodic peak current
ratios of ia/ic ) 1.0 (theoretical) at ambient temperatures. It
(12) General Procedure. A 200-mL flask equipped with reflux con-
denser and a dropping funnel was charged with magnesium turnings (1.06
mg, 44 mmol) and THF (10 mL) under an argon atmosphere. A small crystal
of iodine was added, and the mixture refluxed. A solution of 4-bromo-2,5-
dimethoxytoluene (3.5 g, 22 mmol) in THF (40 mL) was added dropwise.
After the addition was complete, the reaction mixture was refluxed for
another 2 h and cooled to 22 °C. The Grignard solution was transferred via
a cannula to a Schlenk flask containing 1b (3.04 g, 3 mmol) and (PPh3)2-
PdCl2 (70 mg, 0.1 mmol), under a flow of argon. The resulting pale yellow
reaction mixture was refluxed for 12 h and cooled to room temperature. A
standard aqueous workup and recrystallization of the crude solid from a
dichloromethane/ethanol mixture afforded pure 1c in 96% yield. Compare:
Kumada, K. Pure Appl. Chem. 1980, 52, 669.
(13) Hexakis(4-methyl-3,6-dimethoxybiphenyl)benzene (1c): mp > 350
°C; 1H NMR (CDCl3) δ 2.18 (s, 18H), 3.39 (s, 18H), 3.681 (s, 18H), 6.61
(s, 6H), 6.70 (s, 6H), 7.12 (d, J ) 8.1 Hz, 12H), 6.98 (d, J ) 8.4 Hz, 12
H); 13C NMR (CDCl3) δ 16.02, 55.91, 57.47, 112.73, 117.50, 126.21,
127.49, 129.30, 131.26, 135.25, 139.36, 140.42, 150.50, 152.31. Anal. Calcd
for C96H90O12: C, 80.31; H, 6.32, O, 13.37. Found: C, 80.26, H, 6.35.
Tetrakis(4-methyl-3,6-dimethoxybiphenyl)methane (2c): mp > 320 °C; 1H
NMR (CDCl3) δ 2.29 (s, 12H), 3.79 (s, 12H), 3.84 (s, 12H), 6.83 (s, 4H),
6.89 (s, 4H), 7.12 (d, J ) 8.1 Hz, 12H), 6.98 (d, J ) 8.4 Hz, 12 H);
13C NMR (CDCl3) δ 16.79, 57.03, 57.91, 64.90, 113.68, 115.62, 126.78,
128.40, 129.00, 131.87, 136.61, 146.05, 150.87, 152.52. Anal. Calcd for
C61H60O8: C, 79.54; H, 6.57; O, 13.90. Found: C, 79.44; H, 6.48. These
structures were further confirmed by high-resolution mass spectra.
were readily prepared from hexaphenylbenzene (1a) and
tetraphenylmethane (2a) by bromination using neat bro-
mine.10 Thus, a coupling of a slight excess (5%) of
2,5-dimethoxy-4-methylphenyl Grignard reagent (derived
from 4-bromo-2,5-dimethoxytoluene11 and Mg in anhydrous
tetrahydrofuran) with 1b (or 2b) in the presence of a
palladium catalyst12 afforded the macromolecule 1c (or 2c)
in essentially quantitative yield (see Scheme 1). The struc-
tures of these highly symmetrical macromolecular donors
1
1c and 2c were readily established by H/13C NMR spec-
troscopy and correct elemental analysis.13
(9) Rathore, R.; Kumar, A. S.; Lindeman, S. V.; Kochi, J. K. J. Org.
Chem. 1998, 63, 5847 and references therein.
(10) Thus, a treatment of 5 g of 1a (or 2a) with excess Br2 (3-5 mL)
afforded 1b (or 2b) in quantitative yields by a simple trituration with chilled
(-25 °C) ethanol.
(11) McHale, D.; Mamalis, P.; Green, J.; Marcinkiewicz, S. J. Chem.
Soc. 1958, 1600.
2888
Org. Lett., Vol. 3, No. 18, 2001

is interesting to note that irrespective of the number of the
electroactive aryl (2,5-dimethoxytolyl) groups (i.e., 6 aryls
in 1c, 4 aryls in 2c), these electron donors showed only a
single reversible (CV) wave at a potential of 1.15 ( 0.01 V
vs. SCE, similar to that obtained with the model compound
3. A quantitative evaluation of the CV peaks with added
ferrocene (as an internal standard, Eox ) 0.45 V vs SCE)
revealed that the reversible cyclic voltammograms in Figure
1 correspond to the production of a monocation of 3,
tetracation of 2c, and hexacation of 1c by transfer of one,
equiv of neutral 1c in dichloromethane at 22 °C, a dramatic
color change to bright green (λmax ) 415, 550, and 1105
nm, ꢀ1105 ) 1400 M-1 cm-1) occurred immediately:
4
+• + 1/61c f 1/61c6+• + 4
(2)
(It is noteworthy that the absorption spectrum of hexacationic
1c6+• obtained above was identical to that obtained by an
oxidation of 1c using either SbCl5 or by an electrochemical
method.19]
15
four, and six electrons, respectively.
The UV-vis spectral analysis established the simultaneous
oxidation of 1c and reduction of 4+• in quantitative yields
(eq 2), and the uncluttered character of the electron transfer
was established by the presence of well-defined isosbestic
points at λmax ) 388, 520, and 714 nm when a solution of
4+• was treated with incremental amounts of 1c (see Figure
2A). Furthermore, a plot of the depletion of 4+• (i.e., decrease
of the absorbance at 672 nm) and formation of 1c6+• (i.e.,
increase in the absorbance at 1105 nm) against the increments
of added 1c in Figure 2B established that 4+• was completely
consumed after the addition of 1/6 equiv of 1c; the resulting
absorption spectrum of 1c6+• remained unchanged upon
further addition of neutral 1c (i.e., eq 2).
The highly characteristic absorption spectra of the hexa-
cationic 1c6+• and tetracationic 2c4+• are strikingly similar
to that of the model cation radical 3+• as shown in Figure
2C.20 Moreover, the spectra of partially oxidized 1c (or 2c)
were uniformly the same irrespective of the degree of
oxidation, as confirmed by the reverse addition of a solution
of naphthalene cation radical 4+• to a solution of 1c (or 2c).
As such, the strong similarity in the UV-vis absorption
spectra of 1c6+• and 2c4+• with that of the model 3+• suggests
that there are minimal interactions between the donor
moieties in 1c and 2c via the platform frameworks.21
It is interesting to note that the oxidizing strength of these
multiply charged cation radicals 1c6+• and 2c4+• (Ered ) 1.15
V vs. SCE) is similar to that of the extensively utilized tris-
4-bromophenylamminium cation radical22 (magic blue, Ered
) 1.1 V vs. SCE) as an aromatic oxidant in a variety of
The highly reversible oxidation of 1c and 2c prompted
the isolation of their multiply charged paramagnetic cation-
radical salts. Thus, treatment of a solution of 1c with
antimony pentachloride (SbCl5) in anhydrous dichloro-
methane at -78 °C immediately resulted in a bright green
solution (vide infra) from which the microcrystalline salt
-
[1c6+•(SbCl6 )6] could be isolated, by precipitation using
hexane, in nearly quantitative yield according to the stoi-
chiometry shown:16
1c + 9SbCl5 f 1c6+•(SbCl6 )6 + 3SbCl3
(1)
-
-
In a similar vein, tetracationic [2c4+•(SbCl6 )4] was prepared
in 96% yield from 2c. These highly colored cation-radical
salts are extremely robust and can be recrystallized readily
from a dichloromethane solution by a slow diffusion of
n-hexane at -30 °C. The purity of the crystalline hexa-
cationic 1c6+• and the tetracationic 2c4+• was determined by
iodometric titrations17 and was further confirmed by a
spectroscopic titration method (vide infra).
To confirm these multiple electron-transfer processes for
the formation of the hexacation and tetracation from 1c and
2c using SbCl5, we carried out their oxidation (in dichloro-
methane) using a stable hindered naphthalene cation radical
(4+•)18 as an aromatic one-electron oxidant (Ered ) 1.34 V
vs. SCE).
(16) For the stoichiometry of the oxidation of donors with SbCl5, see:
Rathore, R.; Kumar, A. S.; Lindeman, S. V.; Kochi, J. K. J. Org. Chem.
1998, 63, 5847.
(17) The purity of the isolated [1c6+•(SbCl6-)6] and [2c4+•(SbCl6-)4] was
determined by iodometric titration and was found to be greater than 98%.
For a general procedure for iodometric titrations of cation radicals, see:
Rathore, R.; Kochi, J. K. J. Org. Chem. 1995, 60, 4399 and also ref 9.
(18) Rathore, R.; Le Mague`res, P.; Lindeman, S. V.; Kochi, J. K. Angew.
Chem., Int. Ed. 2000, 39, 809. Also see: Rathore, R.; Le Mague`res, P.;
Kochi, J. K. J. Org. Chem. 2000, 65, 6826.
(19) The oxidation of 1c to its green cation radical can be carried out
electrochemically at Eox ) 1.2 V vs SCE in dichloromethane containing
0.05 M n-Bu4NPF6 at 0 °C.
When a dark blue solution of 4+• (λmax ) 672, 616, 503,
and 396 nm; ꢀ672 ) 9300 M-1 cm-1) was mixed with 1/6
(14) 4,4′-Dimethyl-3,6-dimethoxybiphenyl (3): mp 162-163 °C; 1H
NMR (CDCl3) δ 2.44 (s, 3H), 2.55 (s, 3H), 3.89 (s, 3H), 3.96 (s, 3H), 6.89
(s, 1H), 6.98 (s, 1H), 7.35 (d, 2 H), 7.55 (d, 2H); 13C NMR (CDCl3) δ
16.79, 21.76, 56.73, 57.52, 113.92, 115.88, 127.04, 127.82, 129.49, 130.02,
136.71, 137.18, 150.93, 152.63; GC-MS m/z 242 (M+), 242 calcd for
C16H18O2. Anal. Calcd for C16H18O2: C, 79.31; H, 7.49; O, 13.21. Found:
C, 79.22; H, 7.58.
(15) The reduced current response for equimolar 1c (or 2c) as compared
to the ferrocene as an added internal standard can be readily attributed to
the difference in the diffusion coefficient of 1c (or 2c) due to difference in
the size and the shape; see: Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson,
F. C. J. Am. Chem. Soc. 1978, 100, 4248. However, a preliminary cyclic
voltammetric study, which will be published elsewhere, showed a current
ratio of 1 with an equimolar solution of 1c and hexakis(4-ferrocenylphenyl)-
benzene as an internal standard: Rathore, R.; Burns, C. L., unpublished
results.
(20) UV-vis absorption data for various cation radicals in dichloro-
methane at 22 °C: [1c6+•(SbCl6-)6] λmax 415, 550, and 1105 nm, ꢀ1105
1400 M-1 cm-1; [2c4+•(SbCl6-)4] λmax 385, 570, and 1000 nm, ꢀ1000
)
)
1700 M-1 cm-1; [3+•SbCl6-] λmax 420, 640, and 1115 nm, ꢀ1115 ) 3200
M-1 cm-1
.
(21) The lack of electronic communication amongst various aryl groups
in 1c can be attributed to the propeller shape of the hexaphenylbenzene
framework. We believe that the synthesis of an analogue of 1c (in progress)
in which the six aryl groups are connected to planar hexa-peri-hexa-
benzocoronene framework will lead to extensive electronic communication
amongst aryl groups. Compare: Ito, S.; Herwig, P. T.; Bo¨hme, T.; Rabe, J.
P.; Rettig, W.; Mu¨llen, K. J. Am. Chem. Soc. 2000, 122, 7698.
Org. Lett., Vol. 3, No. 18, 2001
2889

Figure 2. (A) Spectral changes attendant upon the reduction of 1.5 × 10-4 M naphthalene cation radical 4+•
(
_ _ _) by incremental addition
of 1.2 × 10-3 M 1c to its hexacation radical 1c6+• (s) in dichloromethane at 22 °C. (B) A plot of depletion of absorbance (data from Figure
2A) of 4+• (O, monitored at 672 nm) and an increase of the absorbance of 1c6+• (b, monitored at 1100 nm) against the equivalent of added
1c showed a complete consumption of 4+• after addition of 1/6 equiv of 1c. (C) UV-vis absorption spectra of cation radicals 1c6+•
2c4+• (s), and 3+• (‚ ‚ ‚) obtained by SbCl5 oxidation in dichloromethane at 22 °C.
(
_ _ _),
electron-transfer catalyzed (ETC) organic transformations.23
For example, the oxidation of a colorless solution of
octamethylbiphenylene24 (5, Ered ) 0.8 V vs. SCE) with 1/6
in various organic and organometallic transformations and
the isolation of their single crystals for X-ray crystallography
and the study of solid-state properties.
equiv of [1c6+•(SbCl6 )6] or 1/4 equiv of [2c4+•(SbCl6 )4]
in dichloromethane immediately yielded a dark blue solution
of 5+• (λmax ) 602 nm)24 in quantitative yield as estimated
by the UV-vis spectral analysis (eq 3).
-
-
Acknowledgment. We are grateful to Professor F. A.
Khan (State University of West Georgia at Carrolton) for
the mass spectral data of macromolecular donors 1c and 2c.
OL0163474
(22) Bell, F. A.; Ledwith, A.; Sherrington, D. C. J. Chem. Soc. C 1969,
2719. Also see: Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877
and references therein.
(23) (a) Bauld, N. L.; Bellville, D. J.; Harirchian, B.; Lorenz, K. T.;
Pabon, R. A., Jr.; Reynolds, D. W.; Wirth, D. D.; Chiou, H. S.; Marsh, B.
K. Acc. Chem. Res. 1987, 20, 371. (c) Mirafzal, G. A.; Liu, J.; Bauld, N.
L. J. Am. Chem. Soc. 1993, 115, 6072. (d) Chanon, M. Bull. Soc. Chim.
Fr. 1985, 209. (e) Rathore, R.; Kochi, J. K. Acta Chem. Scand. 1998, 52,
114 and references therein.
(24) Rathore, R.; Le Mague`res, P.; Kochi, J. K. J. Org. Chem. 2000, 65,
6826.
(25) Cation radicals of various donors mentioned here have been
characterized previously; see: (a) Rathore, R.; Lindeman, S. V.; Kumar,
A. S.; Kochi, J. K. J. Am. Chem. Soc. 1998, 120, 6931. (b) Rathore, R.;
Kumar, A. S.; Lindeman, S. V.; Kochi, J. K. J. Org. Chem. 1998, 63, 5847
and references therein.
In a similar vein, a variety of aromatic and olefinic
donors25 (D) such as 2,5-dimethyl-1,4-dimethoxybenzene,
perylene, octamethylanthracene, 1,2-dianisyl-1,2-ditolyl-
ethylene, tetraanisylethylene, and 2,3-dianisylbicyclo-
[2.2.2]oct-2-ene were quantitatively oxidized to their
-
brightly colored cation radicals25 using [1c6+•(SbCl6 )6] and
-
[2c4+•(SbCl6 )4] according to the stoichiometry in eq 3.
Presently, we are actively pursuing the usage of these
highly robust cation-radical salts as electron-transfer catalysts
2890
Org. Lett., Vol. 3, No. 18, 2001