Isolation and characterization of phenyl viologen as a radical cation and neutral molecule

Article abstract of DOI:10.1021/jo050328g

The chemical synthesis, isolation, and characterization of phenyl viologen (PV) as a dication, radical cation, and neutral species are described. Single-crystal X-ray diffraction of PV²⁺2Cl^-H ₂O and PV^.+PF₆^-.pyridine reveals the expected differences in bond lengths and also a structural change from two coplanar central rings in PV^.+ to a twist of 36° between the two central rings in PV²⁺. The phenyl viologen radical cation exhibits behavior characteristic of many radical cations, including weak π-dimerization in the solid state and reversible π-dimerization in solution. Electrical conductivity measurements of neutral phenyl viologen, the first such measurements of a neutral viologen, reveal that it is a significantly better conductor than the radical cation. Differences in geometric relaxation during charge transfer offer a possible explanation for the higher conductivity of the neutral viologen.

Full text of DOI:10.1021/jo050328g

Isolation and Characterization of Phenyl Viologen as a Radical
Cation and Neutral Molecule
William W. Porter, III and Thomas P. Vaid*
Center for Materials Innovation and Department of Chemistry, Washington University,
St. Louis, Missouri 63130
vaid@wustl.edu
Received February 21, 2005
The chemical synthesis, isolation, and characterization of phenyl viologen (PV) as a dication, radical
2
+
-
cation, and neutral species are described. Single-crystal X-ray diffraction of PV 2Cl ‚2H
2
O and
•
+
-
PV PF ‚pyridine reveals the expected differences in bond lengths and also a structural change
from two coplanar central rings in PV to a twist of 36° between the two central rings in PV
6
•
+
2+
.
The phenyl viologen radical cation exhibits behavior characteristic of many radical cations, including
weak π-dimerization in the solid state and reversible π-dimerization in solution. Electrical
conductivity measurements of neutral phenyl viologen, the first such measurements of a neutral
viologen, reveal that it is a significantly better conductor than the radical cation. Differences in
geometric relaxation during charge transfer offer a possible explanation for the higher conductivity
of the neutral viologen.
Introduction
The viologens1 (1,1′-disubstituted 4,4′-bipyridylium
dications, Figure 1) are a well-known class of redox
couples that undergo two reversible, one-electron reduc-
2
tions to a radical cation and the neutral form. Many of
the 1,1′-dialkyl-substituted versions are commercially
available as the dications and have been used extensively
in electrochemical experiments. Viologens have been
thoroughly examined in the solid state as electrochromic
3
materials and more recently as components of supramo-
lecular systems in the solution phase, solid state, and
4
on surfaces.
FIGURE 1. Viologens in their dication, radical cation, and
neutral form. In phenyl viologen, R ) phenyl.
*
To whom correspondence should be addressed. Phone: 314-935-
243. Fax: 314-935-4481.
1) Monk, P. M. S. The Viologens; John Wiley and Sons: New York,
8
1
(
Viologen radical cations have been produced by chemi-
998.
5
cal or electrochemical reduction, photolysis, and even
(
2) Bird, C. L.; Kuhn, A. T. Chem. Soc. Rev. 1981, 10, 49-82.
3) Monk, P. M. S.; Mortimer, R. J.; Rosseinsky, D. R. Electro-
chromism: Fundamentals and Applications; VCH: Weinheim, 1995.
6
sublimation of viologen dications. Most of the charac-
(
terization of viologen radical cations has been spectro-
10.1021/jo050328g CCC: $30.25 © 2005 American Chemical Society
5028
J. Org. Chem. 2005, 70, 5028-5035
Published on Web 05/20/2005

PV as a Radical Cation and Neutral Molecule
scopic: only the methyl viologen radical cation has been
No electrical conductivity measurements of any neutral
viologen have been reported.
7
crystallographically characterized. Several of the alkyl
5
,8-12
viologen radical cations have been studied by ESR,
UV-vis, and IR
In the course of our studies of isostructural dopants
7
,8
6,7,13
23
spectroscopy. Many of those in-
for molecular semiconductors, we have investigated the
vestigations addressed the reversible π-dimerization of
the radical cations in solution. ESR and ENDOR inves-
tigations of aryl viologens have addressed both the
neutral form of phenyl viologen as an isostructural
n-dopant for p-quaterphenyl. Herein we report the syn-
thesis, isolation, and characterization of both neutral
phenyl viologen and its radical cation, including the X-ray
crystal structure of both the phenyl viologen dication and
radical cation and solid-state electrical conductivity
measurements of neutral phenyl viologen and the phenyl
viologen radical cation.
12,14
interpretation of the spectra
and the thermodynamics
of the monomer-dimer equilibrium.1
5,16
A variety of
organic π-radicals are known to undergo a reversible
π-dimerization in solution.17
The radical cations of viologens, because they are odd-
electron species, might be expected to be good electrical
conductors as solid, crystalline materials. Several salts
of viologens with the 7,7,8,8-tetracyanoquinodimethanide
Experimental Section
General Procedures and Materials. All manipulations
were carried out using Schlenk line or glovebox techniques
unless otherwise noted. Reagents were purchased from com-
mercial suppliers and purified and dried by the following
methods. Aniline was distilled twice prior to use, tetrabutyl-
ammonium hexafluorophosphate was recrystallized twice from
ethanol, and p-quaterphenyl was recrystallized from xylene.
Acetonitrile was distilled from phosphorus pentoxide and
vacuum transferred from activated 3 Å molecular sieves
immediately prior to use. Dimethyl sulfoxide was distilled from
sodium hydroxide and stored over activated 3 Å molecular
(
TCNQ) anion have been synthesized and their solid-state
18-20
electrical conductivity measured,
state of the viologen in these compounds is probably best
considered to be close to 2 , although the oxidation state
but the oxidation
+
is somewhat ambiguous because TCNQ can exist in
either the anionic or neutral form. In any case, the
majority of the conductivity in these compounds is likely
due to the TCNQ, as indicated by the fact that the
conductivity is highest when the TCNQ molecules are
present as infinite stacks and are of mixed valence.
Viologen radical cations with a well-defined oxidation
state (in other words, salts of an anion with a well-defined
charge) have rarely been isolated in the solid state. Solid-
state conductivity measurements of only one well-defined
viologen radical cation, 1,1′-bis(p-cyanophenyl)-4,4′-bi-
pyridylium (as the Cl , Br , BF , and ClO salts), have
4 4
been reported.
The neutral form of the viologens has been investigated
to a much smaller extent; the only neutral viologen that
has been isolated and characterized is methyl viologen.
2
sieves. Absolute ethanol was distilled from Mg/I and vacuum
transferred from 3 Å molecular sieves immediately prior to
use. Dimethyl sulfoxide-d was dried over 3 Å molecular sieves
6
3
and stored in a nitrogen-filled glovebox. Acetonitrile-d was
vacuum transferred onto 3 Å molecular sieves from phosphorus
pentoxide and stored in a nitrogen-filled glovebox. Xylene and
hexane were distilled from a purple sodium benzophenone
solution with added tetraglyme. Pyridine was distilled from
potassium hydroxide and vacuum transferred from 3 Å mo-
-
-
-
-
2
1
lecular sieves immediately prior to use.
1
H NMR (300 MHz) and 13C NMR (75 MHz) spectra were
7,22
recorded as solutions in the solvents as noted below. Infrared
spectra were obtained as Nujol mulls (made with a small
mortar and pestle) on NaCl plates. The ESR spectrum was
obtained on an X-band spectrometer with 0.10 G modulation.
(4) Jang, S. S.; Jang, Y. H.; Kim, Y.-H.; Goddard, W. A. I.; Flood, A.
H.; Laursen, B. W.; Tseng, H.-R.; Stoddart, J. F.; Jeppesen, J. O.; Choi,
2
+
-
1
,1′-Diphenyl-4,4′-bipyridinium dichloride (PV 2Cl )
J. W.; Steuerman, D. W.; DeIonno, E.; Heath, J. R. J. Am. Chem. Soc.
24
2
005, 127, 1563-1575 (and references therein).
was prepared by a published procedure; the intermediate
(5) Johnson, C. S.; Gutowsky, H. S. J. Chem. Phys. 1963, 39, 58-
1,1′-bis(2,4-dinitrophenyl)-4,4′-bipyridiinium dichloride was
6
2.
(
2+
-
recrystallized from acetone/water. Data for orange PV 2Cl .
6) Poizat, O.; Sourisseau, C.; Mathey, Y. J. Chem. Soc., Faraday
1
Mp: 306-308 °C (under N
DMSO-d at 2.50 ppm): δ 9.76 (d, 4H, J ) 6.9 Hz), 9.16 (d,
H, J ) 6.9 Hz), 8.01 (m, 4H), 7.81 (m, 6H). C NMR (DMSO-
, ref DMSO-d at 39.51 ppm): δ 148.9, 146.0, 142.3, 131.6,
2 6
, dec). H NMR (DMSO-d , ref
Trans. 1 1984, 80, 3257-3274.
7) Bockman, T. M.; Kochi, J. K. J. Org. Chem. 1990, 55, 4127-
4
5
9
5
(
1
3
4
135.
(
8) Kosower, E. M.; Cotter, J. L. J. Am. Chem. Soc. 1964, 86, 5524-
d
6
6
527.
1
3
(
30.3, 126.8, 124.9. UV-vis (DMSO, 5.8 × 10 M): λmax
-5
)
(
9) Evans, A. G.; Evans, J. C.; Baker, M. W. J. Am. Chem. Soc. 1977,
4
-1
-1
-1
14 nm, ꢀ ) 2.1 × 10 L‚mol ‚cm . IR (Nujol, cm ): 1628
9, 5882-5884.
m), 1242 (w), 1032 (vw), 998 (vw), 854 (w), 764 (s), 691 (m).
,1′-Diphenyl-4,4′-bipyridinium Bis(hexafluorophos-
(10) Evans, A. G.; Evans, J. C.; Baker, M. W. J. Chem. Soc., Perkin
1
Trans. 2 1977, 1787-1789.
11) Guerin-Ouler, D.; Nicollin, C.; Sieiro, C.; Lamy, C. Mol. Phys.
977, 34, 161-170.
12) Clack, D. W.; Evans, J. C.; Obaid, A. Y.; Rowlands, C. C.
2
+
-
2+
-
(
phate) (PV 2PF
dissolved in a minimal amount of hot H
PF (6.143 g, 37.7 mmol) in 15 mL of H
6
). PV 2Cl (1.175 g, 3.08 mmol) was
O. A solution of NH
O was added. The
1
4
-
2
(
6
2
Tetrahedron 1983, 39, 3615-3620.
resulting pale yellow precipitate was collected via filtration
and recrystallized from ethanol/water to give 1.791 g (96%) of
orange crystals.
(
13) Hester, R. E.; Suzuki, S. J. Phys. Chem. 1982, 86, 4626-4630.
14) Clack, D. W.; Evans, J. C.; Obaid, A. Y.; Rowlands, C. C. J.
(
Chem. Soc., Perkin Trans. 2 1985, 1653-1657.
0
(
15) Evans, J. C.; Nouri-Sorkhabi, M. H.; Rowlands, C. C. Tetrahe-
dron 1982, 38, 2581-2584.
16) Evans, J. C.; Evans, A. G.; Nouri-Sorkhabi, N. H.; Obaid, A.
Y.; Rowlands, C. C. J. Chem. Soc., Perkin Trans. 2 1985, 315-318.
17) L u¨ , J.-M.; Rosokha, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2003,
Neutral Phenyl Viologen (PV ). Ethanol (50 mL),
2+
-
PV 2Cl (1.181 g, 3.10 mmol), and zinc dust (0.801 g, 12.2
mmol) were combined in a 100-mL flask, and the stirred
(
2
suspension was brought to reflux under N for 96 h. The
(
1
25, 12161-12171.
resulting slurry was cooled to room temperature, filtered, and
washed with ethanol. The mixture of an insoluble red product
and excess zinc was dried under vacuum and separated by
(
18) Ashwell, G. J.; Allen, J. G. J. Phys. 1983, 44 (C3), 1261-1264.
19) Ashwell, G. J.; Allen, J. G.; Goodings, E. P.; Nowell, I. W. Phys.
(
Status Solidi A 1984, 82, 301-306.
(
20) Ashwell, G. J.; Cross, G. H.; Kennedy, D. A.; Nowell, I. W.;
Allan, J. G. J. Chem. Soc., Perkin Trans. 2 1983, 1787-1791.
21) Rosseinsky, D. R.; Monk, P. M. S. J. Chem. Soc., Faraday Trans.
994, 90, 1127-1131.
22) Mohammad, M. J. Org. Chem. 1987, 52, 2779-2782.
(23) Vaid, T. P.; Lytton-Jean, A. K.; Barnes, B. C. Chem. Mater.
(
2003, 15, 4292-4299.
1
(24) Kamogawa, H.; Sato, S. Bull. Chem. Soc. Jpn. 1991, 64, 321-
(
323.
J. Org. Chem, Vol. 70, No. 13, 2005 5029

Porter and Vaid
sublimation under high vacuum at 250 °C for 50 h to yield
0
1
0
.670 g (70%) of red PV . Mp: 359-361 °C. H NMR (DMSO-
, 90 °C): δ 7.34 (t, 4H, J ) 7.8 Hz), 7.17 (d, 4H, J ) 7.8 Hz),
.02 (t, 2H, J ) 7.3 Hz), 6.60 (d, 4H, J ) 8.2 Hz), 5.78 (d, 4H,
d
6
7
1
3
J ) 8.2 Hz). C NMR (DMSO-d
29.1, 125.4, 121.9, 116.2, 109.0. UV-vis in THF, saturated:
max ) 442 nm; 462 nm (overlapping peaks, fit to two Gaussian
peaks). IR (Nujol, cm ): 1660 (m), 1609 (w), 1554 (w), 1519
m), 1306 (s), 1232 (w), 1019 (w), 984 (m), 805 (m), 744 (m).
HREI MS: C22 310.147 calculated, 310.148 found. Anal.
Calcd for C22 : C, 85.13; H, 5.35; N, 9.02. Found: C, 85.45;
H, 5.35; N, 8.83.
Phenyl Viologen Radical Cation Hexafluorophos-
6
, 90 °C, partial spectrum): δ
1
λ
-
1
(
18 2
H N
H N
18 2
•
+
-
2+
-
phate (PV PF
6
). A mixture of PV 2PF
mmol) and PV (97 mg, 0.313 mmol) in 15 mL of acetonitrile
was brought to reflux under N for 16 h. After cooling of the
6
(188 mg, 0.313
0
2
•
+
-
dark green solution to room temperature, the acetonitrile was
slowly removed under vacuum to give 0.268 g (96%) of dark
FIGURE 2. ESR spectrum of PV PF
6
in THF.
-
5
purple microcrystals. UV-vis (CH
3
CN, 4.5 × 10 M) nm, ꢀ
diffracted X-rays. However, in this case the crystals were large
plates that were too thin to obtain a structure.
-1
-1
(
L‚mol ‚cm ) (many overlapping peaks; fit to a set of Gauss-
3
3
3
ian peaks): 250, 5.4 × 10 ; 318, 3.7 × 10 ; 377, 4.2 × 10 ; 437,
4
4
3
3
2
.4 × 10 ; 627, 1.1 × 10 ; 644, 2.2 × 10 ; 664, 3.7 × 10 ; 713,
3 -1
Results and Discussion
7
.3 × 10 . IR (Nujol, cm ): 1634 (s), 1587 (m), 1572 (m), 1509
(
w), 1489 (s), 1343 (w), 1266 (s), 1197 (s), 1022 (m), 1015 (m),
85 (w), 843 (s), 759 (m), 695 (w), 604 (w), 557 (w).
Synthesis. 1,1′-Diphenyl-4,4′-bipyridinium dichloride
(PV 2Cl ) was prepared by a published procedure.
Reduction of PV 2Cl by zinc in ethanol gave neutral
phenyl viologen (PV ), which has very little solubility
in any solvent. Sublimation of PV under vacuum at
250 °C yielded red, thin, plate-shaped crystals of PV .
Because PV 2Cl has poor solubility in most organic
solvents, it was converted to the hexafluorophosphate
9
2+
-
24
0
Cocrystallization of p-Quaterphenyl and PV . A mix-
2+
-
0
ture of 197 mg of p-quaterphenyl, 20 mg of PV , and 90 mL of
p-xylene was heated to 135 °C to form a pale red solution. The
solution was allowed to cool to room temperature with stirring
over a period of 4 h. Metallic red plates were collected by
filtration, washed with three 20-mL portions of ether, and
dried under vacuum at 50 °C. Isolated yield: 160 mg.
0
0
0
2+
-
•
+
-
Magnetic Susceptibility of PV PF
susceptibility was measured as a solution in CD
Evans method as described previously. Solid-state suscep-
tibility measurements were performed at room temperature
with a Johnson Matthey Mark I magnetic susceptibility
balance.
6
. Solution-phase
2+
-
(
PV 2PF
by addition of a solution of NH
radical cation hexafluorophosphate, PV PF
6
) by precipitation from an aqueous solution
3
CN by the
2
3
4
PF . The phenyl viologen
6
•
+
-
6
, was pre-
pared by conproportionation of PV2 2PF
+
-
and PV in
0
6
acetonitrile. The radical cation forms a deep green
solution in acetonitrile but is dark purple in the solid
state.
Spectroscopy. UV-vis spectra of PV and PV PF
in THF and CH CN, respectively, were similar to re-
ported spectra of the analogous methyl viologen com-
Electrochemistry and Solid-State Conductivity. Cyclic
voltammetry and solid-state conductivity measurements were
performed with an EG&G/PAR 263A potentiostat with electri-
cal connection to the inside of a nitrogen-filled drybox, where
all materials and solutions were maintained during the
measurements. Cyclic voltammetry was performed on satu-
rated solutions of the analyte in anhydrous THF with 0.10 M
0
•+
-
6
3
0
•+
- 7
6
pounds, MV and MV PF . Because there were over-
[
Bu
internal standard. Scans were run at a rate of 50 mV/s with a
.3 Hz low-pass filter. The working and pseudoreference
4
N][PF
6
] supporting electrolyte and 0.0013 M ferrocene
lapping peaks in each spectrum, they were both fit to sets
of Gaussian peaks to obtain the wavelengths and ab-
sorptivities reported in the Experimental Section.
5
electrodes were 0.50 mm diameter platinum disks and the
counter electrode was a 2.5 mm diameter platinum disk. Solid-
state conductivity of pressed powders was measured in a two-
electrode configuration on a home-built apparatus that consists
of a Delrin block with a 6.35 mm diameter cylindrical hole
and two 6.35 mm diameter copper cylindrical contacts. The
material to be studied was inserted between the two contacts
within the Delrin block and a mass of 3.85 kg was placed on
the top contact, thus applying 12 bar pressure.
•+
-
An ESR spectrum of a dilute solution of PV PF
6
in
THF (Figure 2) consists of a series of lines spaced by
approximately 0.266 G (determined by Fourier transform
of the spectrum), in agreement with a simulated spec-
1
2
trum produced with published hyperfine couplings.
2
+
-
Crystal Structures of PV 2Cl ‚2H O and
2
•+
-
PV PF
6
‚Pyridine.
Crystallographic
data
for
2+
-
•+
-
PV 2Cl ‚2H
2
O and PV PF
6
‚pyridine and CIF files
X-ray Crystallography. Without the exclusion of air or
are provided in the Supporting Information. The struc-
2+
-
water, orange prisms of PV 2Cl ‚2H
2
O were grown by cooling
2+
-
2+
ture of PV 2Cl ‚2H
2
O consists of stacks of PV cations
a hot solution in 1:1 ethyl acetate/acetone. Purple needles of
-
•
+
-
with well-separated Cl anions. The two chlorides and
two waters of crystallization form a hydrogen-bonded
square with chlorides and water molecules at alternate
corners, each water donating two hydrogens and each
chloride accepting two. Crystals of PV PF
consist of infinite stacks of PV radical cations and
separate columns of partly disordered PF
one partly disordered pyridine of crystallization per
formula unit.
6
PV PF ‚(pyridine) were grown by slow diffusion of hexane
into a pyridine solution. Data collection was performed at the
University of Houston on a Siemens SMART platform diffrac-
tometer equipped with a 1 K CCD area detector. Crystals were
mounted in mineral oil and data were collected at 223 K. An
empirical absorption correction was applied. Structures were
solved with SHELXS and refined with SHELXL.
•+
-
6
‚pyridine
•+
-
6
anions with
0
0
Attempted Crystal Structure of PV . Crystals of PV
grown by slow sublimation under vacuum diffracted X-rays
well. However, in all attempts the crystals were conglomerates
and a single-crystal structure was unattainable. Crystals of
2
+
Figure 3 shows the solid-state structures of PV in
0
2+
-
•+
•+
-
PV grown by controlled cooling of a p-xylene solution also
2 6
PV 2Cl ‚2H O (top) and PV in PV PF ‚pyridine
5030 J. Org. Chem., Vol. 70, No. 13, 2005

PV as a Radical Cation and Neutral Molecule
TABLE 2. Representative Bond Lengths in PV2 and
+
•
+
PV
distance (Å)
distance (Å)
PV2+ PV•+
1.3755(19) 1.348(4)
+
PV•+
bond
PV2
bond
C(1)-C(2) 1.386(2)
C(2)-C(3) 1.386(2)
C(3)-C(4) 1.383(2)
C(4)-C(5) 1.388(2)
C(5)-C(6) 1.385(2)
C(6)-C(1) 1.380(2)
1.374(5) C(7)-C(8)
1.380(5) C(8)-C(9)
1.388(5) C(9)-C(10) 1.396(2)
1.384(4) C(10)-C(11) 1.374(2)
1.385(5) C(11)-N(1) 1.3590(18) 1.375(4)
1.382(5) C(9)-C(9) 1.485(3) 1.427(4)
1.391(2)
1.422(4)
1.424(4)
1.349(4)
C(4)-N(1) 1.4580(17) 1.438(4) C(9)-C(12)
N(1)-C(7) 1.3520(18) 1.375(4)
FIGURE 3. Solid-state structure of PV in PV2+2Cl ‚2H
2
+
-
2
O
•
+
•+
-
to PV2+) and the bonds C(7)-N(1), C(11)-N(1), C(8)-
C(9), and C(9)-C(10) have increased in length, as ex-
pected for a molecule with a valence-bond representation
that is halfway between that of the aromatic dication and
the quinoid neutral viologen (see Figure 1). The most
pronounced change is seen at the central C(9)-C(12)
bond, where the difference in bond length is 0.057 Å.
(
top) and PV in PV PF
6
‚pyridine (bottom).
TABLE 1. Representative Dihedral Angles in PV2+ and
•
+
PV
angle (deg)
+
PV•+
dihedral
PV2
C(3)-C(4)-N(1)-C(11)
C(10)-C(9)-C(9)-C(8)
C(10)-C(9)-C(12)-C(13)
C(11)-N(1)-C(4)-C(3)
C(15)-N(2)-C(17)-C(22)
37.4(2)
37.7
32.0(4)
1.0(5)
There are hundreds of published X-ray crystal struc-
tures containing 1,1′-dialkyl viologen dications, most
commonly as methyl viologen salts or as dialkyl viologens
that are part of a larger supramolecular system. Surpris-
ingly, in the majority of the dications the two aromatic
rings appear to be coplanar. It is possible that in many
cases the apparent coplanarity of the rings is due to a
dynamic solid-state structure in which the rings are
twisting relative to each other, with two energy minima
on either side of the coplanar structure, resulting in an
averaged X-ray structure that appears to have coplanar
rings. This phenomenon has been thoroughly studied in
37.4(2)
33.4(4)
(
bottom). The most obvious difference between the two
structures is the alternating twist in the planes of the
aromatic rings that is present between all the rings in
PV , whereas in PV the two central rings are nearly
coplanar. Table 1 lists the dihedral angles for representa-
2
+
•+
2+
tive sets of atoms in the two ions. In PV , the inter-ring
•
+
twists are consistently near 37°. For PV , the dihedral
angles between the outer phenyl groups and the viologen
core are about 33°, while the two central rings are nearly
coplanar with a twist of 1°. The driving force for the
observed twists between rings is the steric interaction
between hydrogens on the ortho carbons of adjacent
rings. A low-temperature crystal structure of p-quater-
27,28
crystals of biphenyl.
It is also possible that in many of the viologen dications
the rings are in fact coplanar and static in that config-
uration. Partial charge transfer from the counteranions
to the viologen dications, resulting in some contribution
of the radical cation electronic structure in the dications,
could provide the driving force for adoption of the
coplanar structure. For example, in the methyl viologen
dihalides (dichloride, dibromide, and diiodide) the violo-
2+
phenyl, which is isoelectronic with PV , indicates similar
inter-ring twists of 17-22°,25 although there is significant
2
6
torsional motion in the crystals at room temperature.
The coplanarity of the two central rings of PV•+ is due to
conjugation between the rings, as illustrated in Figure
29
gen dications are planar. There are close contacts
between the halide anions and the methyl viologen
cations in each of the crystals, and there is spectroscopic
evidence for a charge-transfer interaction, at least in the
1
. The bond order of the C(9)-C(12) bond is approxi-
mately 1.5, and twisting the two central rings relative
to each other would break the partial π-bond. The
published crystal structure of the methyl viologen radical
cation contains two independent cations with inter-ring
30
cases of the iodide and bromide. Another interesting
2-
n-
set of methyl viologen salts occur with CoCl
4
, [CuCl
2
]
n
,
2-
and PdCl
is evidence of charge transfer, CoCl
4
as the anions. In the two salts in which there
7
torsional angles of 7° and 11°, somewhat larger than
30
2-
n-
4
and [CuCl
2
]
n
,
might have been expected.
31
2-
the methyl viologen ions are planar. In the PdCl
4
salt,
The bond lengths of PV2 and PV•+ are given in Table
. There are some slight, insignificant differences in the
+
where there is no evidence of charge transfer and the
cation and anion are well separated, the methyl viologen
cation’s two rings are twisted 50° relative to each other.
The authors attribute the differences in structure to
2
carbon-carbon bond lengths of the phenyl groups of the
two cations. The inner two rings of the two cations, on
the other hand, display the expected variations in bond
lengths due to their different electronic structures. In
31
crystal packing effects, but the distinct difference in
2
+
PV , the two central rings are aromatic and the bond
lengths are similar to those in previously reported
(
27) Cailleau, H.; Baudour, J. L.; Zeyen, C. M. E. Acta Crystallogr.
1
979, B35, 426-432.
•
+
viologen dications. In PV the bonds C(7)-C(8), C(10)-
C(11), and C(9)-C(12) have decreased in length (relative
(28) Corish, J.; Morton-Blake, D. A.; O’Donoghue, F.; Baudour, J.
L.; Beniere, F.; Toudic, B. Theochem 1995, 358, 29-38.
(
29) Russell, J. H.; Wallwork, S. C. Acta Crystallogr. B 1972, B28,
1
527-1533.
(
25) Baudour, J. L.; Delugeard, Y.; Rivet, P. Acta Crystallogr. B
978, B34, 625-628.
26) Delugeard, Y.; Desuche, J.; Baudour, J. L. Acta Crystallogr. B
976, 32, 702-705.
(30) Macfarlane, A. J.; Williams, R. J. P. J. Chem. Soc. (A) 1969,
1
1
1517-1520.
(
(31) Prout, C. K.; Murray-Rust, P. J. Chem. Soc. (A) 1969, 1520-
1525.
J. Org. Chem, Vol. 70, No. 13, 2005 5031

Porter and Vaid
TABLE 3. Magnetic Susceptibility of PV•+PF6- in
a
CD3CN Solution Determined by the Evans Method
concn
M)
apparent
µ (µB)
concn
(M)
apparent
µ (µB)
(
T (°C)
T (°C)
0
0
0
0
0
0
.0665
20.6
21.6
21.7
21.0
20.6
21.0
1.02
1.15
1.26
1.41
1.58
1.75
0.0397
0.0389
0.0384
0.0379
0.0375
21.6
32.0
42.0
49.0
58.0
1.15
1.24
1.29
1.32
1.34
.0397
.01976
.00988
.00494
.00247
a
The left side of the table shows data for a range of concentra-
tions and the right side of the table shows data for a range of
temperatures.
FIGURE 4. Stacking of PV•+ radical cations in PV PF -‚
•+
6
and repacked between measurements, and a total of six
measurements were taken on each sample. The molar
susceptibility was calculated and a correction for dia-
pyridine. Distances are as follows (in Å): a ) 3.973, b ) 4.003,
c ) 3.955, d ) 3.852, e ) 3.502, f ) 3.339.
magnetism3
2,33
of 2.72 × 10 emu/mol was applied. The
-4
structure between our PV2+ dication and PV•+ radical
cation lend credence to the idea that electronic effects
may be important.
There are two published structures of 1,1′-diaryl vi-
ologen dications (the p-fluorophenyl and p-cyanophe-
nyl derivatives), and in both structures the viologen
cation has two coplanar central rings and two outer rings
that are twisted relative to the central rings. In both
compounds TCNQ is the counterion, so it is likely that
the charge on the cations is not a full 2+, resulting in
magnetic moments calculated from the data of the two
B
samples were 1.55(4) and 1.58(3) µ . Both are less than
the ideal value of 1.73 µ for one unpaired electron,
B
19
presumably because of spin pairing in the dimers seen
in the crystal structure.
2
0
(
ii) Solution Phase. An Evans method34 determina-
•
+
-
tion of the magnetic susceptibility of PV PF
solution in CD CN indicated a magnetic moment of only
about 1.2 µ per radical cation, much less than the ideal
value of 1.73 µ . The reversible π-dimerization of viologen
radical cations to diamagnetic dimers has been observed
6
as a
-
3
B
B
the observed structures.
•
+
-
The structure of PV PF ‚pyridine exhibits one other
6
previously in solution,9
,10,15,16
so an investigation of the
•
+
notable feature: the stacks of PV consist of pairs of
closely spaced radical cations. Figure 4 shows four radical
cations with labeled interatomic distances. The closest
intermolecular contact, labeled f, is 3.34 Å between two
central carbons. The intermolecular distance between
nitrogens on the same two molecules is 3.50 Å, indicating
some bending of the radical cations. A contact of 3.34 Å
concentration and temperature dependence of the mag-
netic moment was undertaken. Table 3 presents the
•
+
-
magnetic moment of PV PF
6 3
in CD CN over a range of
concentrations and temperatures. At room temperature,
concentrations from 0.0665 to 0.00247 M were examined
and displayed apparent magnetic moments of 1.02 µ
B
to
1
.75 µ , increasing with decreasing concentration, as
B
is at the high end of the range of distances of shortest
contact observed in π-dimers in the solid state.17 Still,
expected for a monomer-dimer equilibrium shifting
toward monomer. Furthermore, Table 3 shows the in-
crease in apparent magnetic moment as the temperature
of a solution is raised from 22 to 58 °C, once again
consistent with a shift in the equilibrium from dimer to
monomer. (The small decrease in concentration with
increasing temperature is due to thermal expansion of
the solvent.) The data are not accurate enough to extract
thermodynamic parameters for the monomer-dimer
equilibrium. It is interesting that in the more concen-
trated solutions the apparent magnetic moment is lower
than it is in the solid state. It is possible that the packing
the dimerization might lead to some spin pairing and a
paramagnetic susceptibility less than that expected from
one unpaired electron per radical cation, so the magnetic
•
+
-
susceptibility of PV PF
solid state and in solution.
Magnetic Susceptibility.
i) Solid State. Our intent was to examine the solid-
6
‚pyridine was examined in the
(
•+
-
state properties of PV PF ‚pyridine so that a correlation
between its solid-state properties and the observed X-ray
structure could be examined, but we found it difficult to
6
•
+
-
isolate PV PF
dine in the crystals. Oxidation of samples of PV PF
pyridine) with I in DMSO-d to the dication and
6
‚pyridine with a full equivalent of pyri-
•+
•+
-
•
+
-
6
of PV in PV PF ‚pyridine is not ideal for π-dimeriza-
6
‚
tion and the radical cations are able to find a more
favorable orientation in solution.
(
x
2
6
1
integration of the H NMR spectrum revealed that a
sample that had been under vacuum had the stoichio-
metry PV PF ‚(pyridine)0.12. However, this sample
was still visibly crystalline and had a powder X-ray
diffraction pattern in reasonable agreement with a
powder pattern simulated with the single-crystal data
Solid-State Conductivity of PV and PV PF
6
-‚
0
•+
•
+
-
(Pyridine)0.43. Polycrystalline samples were pressed
between two freshly cleaned copper contacts in the
apparatus described in the Experimental Section. Be-
cause these were simple two-point measurements, a
series of sample masses for each substance were mea-
sured to examine the influence of contact resistance. The
6
•
+
-
of PV PF ‚(pyridine)1.0. Therefore, it is likely that
the relevant infinite stacks of PV are maintained in
6
•
+
•
+
-
PV PF
crystals.
The magnetic susceptibility of microcrystalline
6
‚(pyridine)
x
when some pyridine has left the
(32) Carlin, R. L. Magnetochemistry; Springer-Verlag: New York,
1
6
986.
(
33) Weast, R. C.; Ed. CRC Handbook of Chemistry and Physics,
•
+
-
PV PF
6
‚(pyridine)0.12 was measured at room tempera-
ture on two separate samples. Each sample was shaken
7th ed.; CRC Press: Boca Raton, FL, 1986.
(34) Evans, D. F. J. Chem. Soc. 1959, 2003-2005.
5032 J. Org. Chem., Vol. 70, No. 13, 2005

PV as a Radical Cation and Neutral Molecule
FIGURE 5. Solid-state electrical conductivity measurements
FIGURE 6. Solid-state electrical conductivity measurements
0
of PV . The top figure is current-voltage plots for a series of
•+
-
6
of PV PF ‚(pyridine)0.43. The top figure is current-voltage
sample masses (8, 16, 24, 32, 40, 48 mg). The bottom figure is
the resistance of the samples plotted versus their mass.
Extrapolation to zero mass yields the contact resistance of 54
kΩ.
plots for a series of sample masses (8.6, 16.1, 24.3, 31.8 mg).
The bottom figure is the resistance of the samples plotted
versus their mass. Extrapolation to zero mass yields the
contact resistance of 650 MΩ.
cross-sectional area of the pressed pellets in the ap-
paratus is constant, so the thickness of the sample, and
therefore its measured resistance, will increase in direct
proportion to the sample mass if the contact resistance
is not significant. At each mass, the same sample was
repacked and measured for a total of four measurements.
Current was measured as a function of applied potential
from 0 to +4 to -4 to 0 V. Each current-voltage plot in
Figures 5 and 6 is the average of the four runs at each
mass.
Figure 6 shows the results of conductivity measure-
•+
-
ments of PV PF
in the same manner as those for PV . In this case, the
6
‚(pyridine)0.43. The data were treated
0
contact resistance is significant, 650 MΩ. The density
3
calculated from the crystal structure (1.46 g/cm ) was
used in the calculation of the resistivity. An average
-10
-10
-1
-1
conductivity of 2.3 × 10
((0.7 × 10 ) Ω cm was
calculated from 16 measurements. (Similar results were
obtained for a sample of PV PF ‚(pyridine)0.12.) There
•+
-
6
is one previous report of the conductivity of an aryl
viologen radical cation: p-cyanophenyl viologen radical
cation as a pressed powder has a conductivity between
0
Figure 5 shows the results for PV . The current-
voltage plots for several masses are shown in the top
graph and all of them are linear, indicating Ohmic
behavior. The inverse of the slope of each plot (obtained
by linear regression) is the resistance. The resistance as
a function of mass is shown in the bottom graph. The
horizontal error bars represent the estimated weighing
error, and the vertical error bars are the standard
deviation over the four measurements at each mass
-
9
-7
-1
-1
7.1 × 10 and 5.0 × 10 Ω cm , depending upon the
21
anion and preparation method. The lower resistivity of
that material may be intrinsic but may be due in part to
the much higher pressures applied to the samples in
those experiments.
0
It is somewhat surprising that the closed-shell PV has
an electrical conductivity 3 orders of magnitude higher
•+
-
(
representing, essentially, the variation due to sample
than the radical PV PF ‚(pyridine) . The difference may
6
x
packing). Extrapolation of a least-squares-fit line to zero
mass yields the contact resistance, which is a relatively
small 54 kΩ. The conductivity of PV was calculated by
assuming a density of 1.25 g/cm and using the known
cross-sectional area of the sample disk (0.317 cm ) to
be partly due to differences in crystal packing and
intermolecular contacts in the solid state. But if charge
transport through the crystal occurs by a hopping mech-
anism, which is most likely the best description of charge
transport in these systems, then the geometric transfor-
mation that accompanies charge transfer from one mol-
ecule to the next is another important factor affecting
0
3
2
calculate the sample thickness. The contact resistance
of 54 kΩ was subtracted from all measurements. An
-
7
-7
-1
-1
0
•+
average conductivity of 3.1 × 10 ((0.6 × 10 ) Ω cm
charge mobility. Both PV and PV are easily oxidized
0
•+
-
was calculated from 24 measurements (four times each
on six samples).
species, so in both PV and PV PF
state charge carriers are almost certainly holes. When a
6
x
‚(pyridine) the solid-
J. Org. Chem, Vol. 70, No. 13, 2005 5033

Porter and Vaid
0
hole moves through a crystal of PV , the two species
0
•+
involved in charge transfer are PV and PV . There is
0
•+
some change in bond lengths between PV and PV , but
the overall molecular structure, with two coplanar central
rings and two outer rings twisted relative to those (an
0
assumption for PV ), is the same. In contrast, the
•
+
-
relevant species for charge transport in PV PF ‚
6
•
+
2+
(pyridine)
x
are PV and PV , which have coplanar and
twisted central rings, respectively. Charge transport in
•
+
-
PV PF
6
‚(pyridine)
x
therefore involves a major molecular
rearrangement with each hop (of course, it may be
difficult for PV within a crystal of PV PF
2+
•+
-
6
‚(pyridine)
x
to actually adopt its preferred geometry). A recent paper
reported theoretical calculations that showed that, in
molecular conductors, hole mobilities are determined in
large part by the reorganization energy that accompanies
charge transfer and changes in inter-ring dihedral angles
are a significant component of the reorganization en-
3
5
ergy.
Electrochemistry. Our original motivation for isolat-
0
ing PV was to investigate its behavior as an isostructural
2
3
n-dopant for p-quaterphenyl. One requirement for iso-
structural n-doping is that the dopant molecule donates
an electron to the host organic semiconductor in the solid
state. Solution-phase electrochemistry provides an esti-
mate of the electronic energy levels in the solid state. For
0
/+
PV and p-quaterphenyl, for example, if the PV poten-
FIGURE 7. Cyclic voltammograms of p-quaterphenyl and
0/-
tial is negative of the p-quaterphenyl potential, then
PV should spontaneously transfer an electron to neutral
2+
6 2
PV [PF ] as saturated solutions in THF.
0
p-quaterphenyl in the solution phase and possibly in the
it appears that a significant fraction of PV0 can be
solid state. Although the electrochemical reduction po-
substitutionally cocrystallized with p-quaterphenyl and
the cocrystals maintain the crystal structure of p-quater-
phenyl. However, the cocrystals displayed no significant
electrical conductivity at room temperature.
2
tentials of phenyl viologen in water and p-quaterphenyl
in dimethylamine36 have been reported, it is necessary
to compare their electrochemistry under identical condi-
tions. Cyclic voltammograms of p-quaterphenyl and
2
+
-
PV 2PF
electrolyte are shown in Figure 7. An internal reference
6 4 6
in THF with 0.10 M [Bu N][PF ] supporting
Conclusions
0/+
of ferrocene was set to 0 V. Two reversible, one-electron
2+
Phenyl viologen has been synthesized, isolated, and
characterized as a dication, radical cation, and neutral
species. X-ray crystallography revealed a structural
dichotomy between the dication and radical cations
twisted versus coplanar central ringssthat had not been
clearly demonstrated previously in viologens. The phenyl
viologen radical cation exhibits behavior characteristic
of many radical cations, including π-dimerization in the
solid state and reversible π-dimerization in solution. The
solid-state electrical conductivity of neutral phenyl vi-
ologen is about 3 orders of magnitude larger than that
of the radical cation. A possible explanation for the higher
conductivity of the neutral viologen is the fact that during
hole transfer there is a significant geometric change
between the radical cation and dication (the relevant
species for conduction by the radical cation), whereas the
neutral molecule and radical cation (the relevant species
for conduction by neutral phenyl viologen) have similar
structures.
reductions of PV occur at -0.68 and -0.94 V. Two
reversible, one-electron reductions of p-quaterphenyl
occur at -2.81 and -2.99 V. In a separate experiment, a
nonreversible oxidation of p-quaterphenyl was observed
at approximately 1.27 V. The combined data indicate
that p-quaterphenyl has a band gap of approximately
.1 eV and the donor level of PV0 in p-quaterphenyl
4
lies approximately 1.9 eV from the conduction band
0
edge, so there is little chance that PV will act as a
donor in p-quaterphenyl. Nevertheless, we examined
their cocrystallization and the conductivity of the coc-
rystals.
0
Cocrystallization of PV and Quaterphenyl. Slow
cooling of a hot p-xylene solution of p-quaterphenyl yields
thin, colorless, plate-shaped crystals. Powder X-ray dif-
fraction agreed with the unit cell of a reported single-
26
crystal structure. Cooling a 9:1 mixture of p-quater-
0
phenyl and PV under the same conditions results in thin,
red, plate-shaped crystals. Powder diffraction showed
that the unit cell was unchanged and no other crystalline
0
Acknowledgment. We thank Jun Lang for as-
sistance in acquiring the ESR spectrum and James Korp
of the University of Houston for collecting single crystal
X-ray diffraction data and assistance in solving the
structures. Mass spectrometry was provided by the
Washington University Mass Spectrometry Resource
phases were present, including crystals of pure PV . Thus
(
35) Hutchison, G. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem.
Soc. 2005, 127, 2339-2350.
36) Meerholz, K.; Juergen, H. J. Am. Chem. Soc. 1989, 111, 2325-
326.
(
2
5034 J. Org. Chem., Vol. 70, No. 13, 2005

PV as a Radical Cation and Neutral Molecule
with support from the NIH National Center for Re-
search Resources (Grant P41RR0954). Funding was
provided by the National Science Foundation grant CHE
Supporting Information Available: X-ray crystallo-
2+
-
2
graphic information and data in CIF format for PV 2Cl ‚2H O
•+
-
6
and PV PF ‚pyridine are provided. This material is available
0
133068, Research Corp. grant RI0697, and by the
free of charge via the Internet at http://pubs.acs.org.
JO050328G
Department of Education (Graduate Assistance in Areas
of National Need, Fund P200A000217).
J. Org. Chem, Vol. 70, No. 13, 2005 5035