Isolation and crystal structures of two singlet bis(triarylamine) dications with nonquinoidal geometries

Article abstract of DOI:10.1021/ja0541534

We report the first structural data for bis(diarylamine) "bipolarons": we have isolated and crystallographically characterized salts of the dications obtained by two-electron oxidation of E-4,4′-bis-[di(p-anisyl)amino]stilbene and E,E-2,5-bis{4-[di(p-anis

Full text of DOI:10.1021/ja0541534

Published on Web 01/21/2006
Isolation and Crystal Structures of Two Singlet
Bis(Triarylamine) Dications with Nonquinoidal Geometries
Shijun Zheng,†,‡ Stephen Barlow,*,†,‡ Chad Risko,† Tiffany L. Kinnibrugh,§
Viktor N. Khrustalev,¶ Simon C. Jones,† Mikhail Yu. Antipin,§,¶ Neil M. Tucker,‡
Tatiana V. Timofeeva,§ Veaceslav Coropceanu,† Jean-Luc Bre´das,† and
Seth R. Marder†,‡
Contribution from the School of Chemistry and Biochemistry and Center for Organic Photonics
and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA;
Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721, USA; Department of
Natural Sciences, New Mexico Highlands UniVersity, Las Vegas, New Mexico 87701, USA;
Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow, Russia
Received June 23, 2005; E-mail: stephen.barlow@chemistry.gatech.edu
Abstract: We report the first structural data for bis(diarylamine) “bipolarons”: we have isolated and
crystallographically characterized salts of the dications obtained by two-electron oxidation of E-4,4′-bis-
[di(p-anisyl)amino]stilbene and E,E-2,5-bis{4-[di(p-anisyl)amino]styryl}-3,4-di(n-butoxy)thiophene, [1]2+ and
[2]2+ respectively. ESR, NMR, and magnetometry suggest both species have singlet ground states. X-ray
structures, together with 1H NMR coupling constants for [2]2+, indicate geometries in which the bond lengths
are shifted toward a quinoidal pattern relative to that in the neutral species, but not to a fully quinoidal
extent. In particular, the bond-length alternations across the vinylene bridging groups approach zero. DFT
calculations with closed-shell singlet configurations reproduce the observed structures well. Our results
indicate that singlet species for which one might expect quinoidal geometries (with differences of ca. 0.1
Å between formally single and double bonds) on the basis of a limiting valence-bond representation of the
structure can, in fact, show structures with significantly different patterns of bond lengths.
small molecules with redox-active end groups,12-14 the excess
positive charges are, at least formally, pinned at the termini of
Introduction
There has been considerable interest in “bipolarons” derived
from two-electron oxidation of organic conjugated molecules
and polymers. Bipolarons play important roles in many conduct-
ing polymers:1-10 the excess charge of “traditional” bipolarons,
such as those found in many conducting polymers (and
presumably in appropriate oligomeric model compounds8,11,12),
is concentrated in the middle of the bipolaron and diminishes
as one moves away from this center. In other cases, particularly
the molecule. In both types of system, the creation of bipolarons
can lead to enhanced nonlinear optical properties.14-16
Bis(triarylamine) derivatives can be oxidized to dications that
have been described as “bipolarons” and there have been several
studies of these species in solution;17-19 for example, the creation
of highly absorbing bis(diarylamine) dications through photo-
induced charge transfer between amino-stilbene dendrimers and
electron acceptors has been shown to form the basis of a reverse
saturable absorber system.17
Generally, molecularly based “bipolaron” species have been
generated in solution and studied by electronic or vibrational
spectroscopy; there is a scarcity of structural information from
either NMR or crystallography, although crystal structures have
been reported for a doubly oxidized oligoaniline20 and for several
oligothiophene dications,21,22 and NMR spectra have been
† Georgia Institute of Technology.
‡ University of Arizona.
§ New Mexico Highlands University.
¶ Russian Academy of Sciences.
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9
1812
J. AM. CHEM. SOC. 2006, 128, 1812-1817
10.1021/ja0541534 CCC: $33.50 © 2006 American Chemical Society

Singlet Bis(Triarylamine) Dications
A R T I C L E S
as shown in Figure 1a. This picture is supported by crystal
structures of p-phenylene-bridged species (Figure 1, n ) 1) with
a variety of end groups, X, including O,29,30 O/NOH,31 NR,32
and CPh2,33 where bond-length alternations (BLAs) of >0.1 Å
are observed between the formally double and single bonds in
the ring. Indeed, this pattern of “quinoidal” BLA has been
considered a signature of a closed-shell configuration; the
reduced BLA found in the crystal structure of Chichibabin’s
hydrocarbon (Figure 1, X ) CPh2, n ) 2) has been interpreted
as evidence for some biradical character.33 Here we show that
although [1]2+ and [2]2+ are diamagnetic, they show patterns
of BLA falling well short of the ideal “quinoidal” limiting
resonance forms.34
Figure 1. p-Phenylene-bridged species with (a) “quinoidal” and (b)
biradical structures shown. X ) O, NR, NR2+, N2R3+, CPh2 etc.
Chart 1
Experimental Section
35
Synthesis. Compound 126 and Fe(C5H4Ac)2‚AgBF4 were
synthesized by literature procedures. Compound 2 was synthe-
sized using the Horner reaction of p-(diarylmino)benzyl phos-
phonate and a thiophene dialdehyde36 in a fashion similar to a
closely related compound described in ref 37 (see Supporting
Information for details). Other materials were obtained from
commercial sources and used without further purification. In
addition to synthesizing the dications as isolable salts (described
below), we also generated the dications of 1 and 2 in CH2Cl2
solution for optical spectroscopic characterization using in situ
oxidation with two equivalents of [(p-BrC6H4)3N]+[SbCl6]-
(E1/2 ) +0.70 V vs FeCp2+/0).38
reported for a stabilized derivative of the fluorene dication.23
In particular, bis(triarylamine) dications have not been charac-
terized crystallographically or by NMR, although the X-ray
structures of several bis(triarylamine) radical monocations have
been recently reported.24-26 Here we report for the first time
the isolation and crystal structures of two bis(triarylamine)
[1]2+([SbCl6]-)2. Antimony pentachloride (0.9 mL of a 1.0
M solution in dichloromethane, 0.9 mmol) was added by syringe
to a solution of 1 (175 mg, 0.258 mmol) in dry dichloromethane
(3 mL). The solution instantly darkened and some precipitation
occurred. After 5 min, dry diethyl ether (20 mL) was added to
complete precipitation of the product; the supernatant was
removed by filter cannula and the resulting powder was washed
with dry diethyl ether (2 × 20 mL) and dried under reduced
pressure. The product was crystallized by layering a filtered
solution in dichloromethane (20 mL) with diethyl ether (100
mL); when the solvents had mixed fully, the supernatant was
removed by cannula and the black microcrystalline material was
washed with ether (3 × 10 mL) and dried under reduced
pressure (266 mg, 0.204 mmol, 79%). Single crystals were
obtained by a second dichloromethane/diethyl ether layering.
Anal. Calcd. for C42H38Cl12N2O4Sb2: C, 38.69; H, 2.94; N, 2.15.
dications, those of 1 and 2 (Chart 1).
For species in which two radical centers, such as -NAr2
•+
,
are linked by an oligo-p-phenylene, oligo-p-phenylene-vinylene,
or similar conjugated bridges, i.e., for species such as [1]2+ and
[2]2+, one can envisage two extreme types of structure: closed-
shell singlet “quinoidal” (Figure 1a) and open-shell biradical,
which may have either singlet (diamagnetic) or triplet (para-
magnetic) ground states (Figure 1b). Closed-shell structures are
expected if coupling through the bridge is strong, i.e., when
the bridge is short and/or when its frontier orbitals are close in
energy to the SOMOs of the radical end groups, X. For example,
quinones (X ) O) with short bridges (n ) 1, 2, 3) are
diamagnetic, while for n ) 4 there is an equilibrium between
biradical and closed-shell states;27 steric effects can also lead
to biradical structures, for example, in n ) 3 quinones with
bulky substituents27 and in tetramethyl-p-phenylene bridged
dihydrazine dications (n ) 1, X ) N2R3+).28 Closed-shell
structures are often represented with four single and two double
bonds within the rings and with double bonds exo to the ring,
(29) Staab, H. A.; Jo¨rns, M.; Krieger, C.; Rentzea, M. Chem. Ber. 1985, 118,
796.
(30) Irngartinger, H.; Herpich, R. Eur. J. Org. Chem. 1998, 595.
(31) Sardone, N.; Carugo, O.; Charalambous, J.; Raghvani, D. V. Acta
Crystallogr. 1996, C52, 3202.
(32) Siri, O.; Braunstein, P. Chem. Commun. 2000, 2223.
(33) Montgomery, L. K.; Huffman, J. C.; Jurczak, E. A.; Grendze, M. A. J.
Am. Chem. Soc. 1986, 108, 6004.
(34) During the review process for our paper, another paper has appeared
describing a molecule with some similarities to our dications, the neutral
“extended viologen” obtained by two-electron reduction of 4,4′-bis(1-n-
octylpyridinium-4-yl)biphenyl (Porter, W. M.; Vaid, T. P.; Rheingold, A.
L. J. Am. Chem. Soc. 2005, 127, 16559). In contrast to our dications, there
is significant population of the triplet in solution at room temperature;
however, the authors consider this less likely in the crystal and, as with
our dications, both the crystal structure and DFT results for a closed-shell
singlet configuration show quinoidal BLAs of ca. 0.06 Å in the bridging
phenylene groups. The phenylene-phenylene and phenylene-pyridinium
bonds are also considerably longer than typical double bonds according to
both X-ray and DFT data.
(21) Kozaki, M.; Tonezawa, Y.; Okada, K. Org. Lett. 2002, 4, 4535.
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Soc. 2004, 126, 3163.
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B. Chem. Eur. J. 2004, 10, 83.
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Soc., Perkin Trans. 2 1997, 1615.
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9
J. AM. CHEM. SOC. VOL. 128, NO. 6, 2006 1813

A R T I C L E S
Zheng et al.
Table 1. Electrochemical Potentials (V vs [FeCp2]+/0), with
Found: C, 39.09; H, 2.96; N, 2.19. UV-vis-NIR (CH2Cl2,
generated in situ) λmax (ꢀmax) 900 (129 000) nm (M-1cm-1). IR
(KBr) ν 1602 (s), 1587 (s, br), 1498, 1460, 1438, 1307, 1262
(s, br), 1191 (w), 1158 (s), 1116, 1020, 925 (w), 834, 804, 647,
Relative Peak Heights in Parentheses, for Compounds 1 and 2 in
CH2Cl2 / 0.1 M [nBu4N]+[PF6]-
+/
0
2+/+
3+/
2+
4+/3+
E1
/
E1
/
E1
/
E1
/2
2
2
2
1
2
+0.08(1)
+0.22(1)
-
-
578, 524 (w), 505 (w) cm-1
.
+0.05(2)a
+0.70(1)
+1.05(1)
[2]2+([SbF6]-)2. To a solution of compound 2 (84 mg, 0.095
mmol) in dichloromethane (5 mL) was added AgSbF6 (65 mg,
0.19 mmol) at room temperature under nitrogen atmosphere.
The resulting solution was stirred for 5 min, then filtered into
a vial and layered with pentane under nitrogen atmosphere. After
a Two overlapping one-electron processes.
di-p-anisylamine;46,47 full details are provided in the Supporting
Information of ref 26. Compound 2 was synthesized using the
Horner reaction48,49 of a diformylthiophene36 with the p-
(diarylamino)benzyl phosphonate, which was itself prepared
using a general method we have recently described; 37 full details
are provided in the Supporting Information. Electrochemical data
(see Supporting Information for details) for 1 and 2 are
summarized in Table 1; both compounds may be oxidized
readily and reversibly to the corresponding dications; the
potentials are close to those for other bis(di-p-anisylamino)
systems suggesting these oxidations are principally triarylamine-
centered,18,50-52 consistent with calculations of charge distribu-
tion (Vide infra) and with the ESR spectrum of [1]+.26 In the
case of 2, oxidations to tri- and tetracationic species are also
observed and are presumably associated with the bridging
1
2 days, filtration gave dark brown crystals (100 mg, 78%). H
NMR (CD2Cl2, 400 MHz, -85 °C, with Fe(C5H4Ac)2‚AgBF4
added to remove traces of the monocation) δ 7.95 (d, J ) 8.8
Hz, 2H), 7.76 (d, J ) 13.5 Hz, 2H), 7.58 (m, 4H), 7.18 (bs,
8H), 7.11 (d, J ) 8.7 Hz, 2H), 7.07 (d, J ) 8.5 Hz, 2H), 6.98
(m, 8H), 4.21 (bs, 4H), 3.82 (s, 12H), 1.72 (bs, 4H), 1.41 (bs,
4H), 0.77 (bt, J ) 7.6 Hz, 6H). HRMS (FAB+) calcd for
C56H58N2O6S (M+-2SbF6): 886.4005; Found: 866.3992. Anal.
Calcd for C56H58F12N2O6SSb2: C, 49.51; H, 4.30; N, 2.06.
Found: C, 49.66, H, 4.33, N, 1.97. UV-vis-NIR (CH2Cl2,
generated in situ) λmax (ꢀmax) 1072 (85 900) nm (M-1cm-1).
X-ray Crystal Structures. All data were acquired at 120(2)
K using Mo-KR radiation (λ ) 0.71073 Å). Structures were
refined using full-matrix least squares against F2. Parameters
relating to the crystal structure data collection and refinements
are tabulated in the Supporting Information; CIF files are also
included in the Supporting Information. The experimental data
indicated positional static disorder in some of the terminal
groups of [1]2+([SbCl6]-)2 and [2]2+([SbF6]-)2. To model the
disorder in cation [1]2+, two possible sets of atomic positions
were included in the refinement for one of the anisyl fragments
(C16-C21, O2); populations of the two positions were obtained
in ratio 0.7:0.3. In cation [2]2+ two possible positions were
considered for carbon atoms in two of the methoxy groups (C26
and C49) and for one of the n-butyl groups (C27-C30). The
ratios of occupancies after refinement were 0.55:0.45 (atoms
C34/C34′), 0.65:0.53 (atoms C48/C48′) and 0.50:0.50 (atoms
C53-C56/C53′-C56′, isotropic refinement only). In the figures
the positions of less populated sites in the disordered fragments
are omitted for clarity.
Computational Details. Geometries for the neutral, radical-
cation, and dication states of 1 and 2, and for the neutral
Chichibabin hydrocarbon were obtained using Density Func-
tional Theory (DFT). In the cases of [1]2+, [2]2+, and Chi-
chibabin’s hydrocarbon, closed-shell singlet, biradical (broken
symmetry), and triplet configurations were taken into consid-
eration. The DFT calculations were carried-out using the B3LYP
functional, where Becke’s three-parameter hybrid exchange
functional is combined with the Lee-Yang-Parr correlation
functional,39-41 with a 6-31G* split valence plus polarization
basis set. All DFT calculations were performed with Gaussian98
(Rev. A.11).42
dialkoxythiophene unit. Analytically pure crystalline [1]2+
-
([SbCl6]-)2 was isolated after oxidation of neutral 1 with ca.
three equivalents of SbCl5 in dichloromethane, followed by
precipitation with diethyl ether and by recrystallization by
layering a dichloromethane solution with diethyl ether. Analyti-
cally pure [2]2+([SbF6]-)2 was isolated after oxidation of neutral
2 with two equivalents of AgSbF6 in dichloromethane, followed
by filtration and layering with pentane.
NMR. Both dications were found to have diamagnetic ground
states. We found a solid sample of [2]2+([SbF6]-)2 to be weakly
paramagnetic; SQUID magnetometry shows Curie behavior
corresponding to µeff ≈ 0.02 µB, indicating this paramagnetism
is likely to be due to an impurity, presumably the corresponding
monocation. NMR studies of the dication are, therefore,
potentially complicated by electron exchange with traces of the
monocation; moreover, we also encountered complications due
to restricted rotation. However, we were able to obtain reason-
ably well-resolved 1H NMR spectra of [2]2+ at -85 °C (where
the restricted rotation is in the slow re´gime) in the presence of
35
a small amount of Fe(C5H4Ac)2‚AgBF4 (we found this
oxidizing agent removed monocation impurities while not over-
1
oxidizing to create [2]3+) and to assign it using H-1H COSY
and NOESY experiments: the COSY spectrum is shown in
Figure 2; additional NMR spectra of [2]2+ are given in the
Supporting Information. In the case of [1]2+ we were unable to
acquire useful NMR spectra due to paramagnetism. However,
Evans’ method NMR53 (room temperature, CD2Cl2, 400 MHz;
(44) Wolfe, J. P.; Wagaw, S.; Buchwald, S. L. J. Am. Chem. Soc. 1996, 118,
7215.
(45) Baumgarten, M.; Yuksel, T. Phys. Chem. Chem. Phys. 1999, 1, 1699.
(46) Zhao, H.; Tanjutco, C.; Thayumanavan, S. Tetrahedron Lett. 2001, 42, 4421.
(47) Kauffman, J. M.; Moyna, G. J. Org. Chem. 2003, 68, 839.
(48) Horner, L. Chem. Ber. 1958, 83, 733.
(49) Wadsworth, W. S.; Emmons, W. D. J. Am. Chem. Soc. 1961, 83, 1733.
(50) Seo, E. T.; Nelson, R. F.; Fritsch, J. M.; Marcoux, L. S.; Leedy, D. W.;
Adams, R. N. J. Am. Chem. Soc. 1966, 88, 3498.
(51) Jones, S. C.; Coropceanu, V.; Barlow, S.; Kinnibrugh, T.; Timofeeva, T.;
Bre´das, J.-L.; Marder, S. R. J. Am. Chem. Soc. 2004, 126, 11782.
(52) Lambert, C.; Risko, C.; Coropceanu, V.; Schelter, J.; Amthor, S.; Gruhn,
N. E.; Durivage, J. C.; Bre´das, J. L. J. Am. Chem. Soc. 2005, 127, 8508.
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Results and Discussion
Synthesis. Compound 1 was synthesized using the palladium-
catalyzed CsN coupling43,44 of E-4,4′-dibromostilbene45 with
(39) Becke, A. D. Phys. ReV. 1988, A38, 3098.
(40) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
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9
1814 J. AM. CHEM. SOC. VOL. 128, NO. 6, 2006

Singlet Bis(Triarylamine) Dications
A R T I C L E S
Figure 2. 1H-1H COSY NMR spectrum of the aromatic and vinylic region of [2]2+([SbF6]-)2 at -85 °C in CD2Cl2.
no measurable contact shift, indicating µeff < 0.3 µB), ESR
(CH2Cl2, room temperature, weak signal with integration
1
corresponding to ca. 6% S ) /2 molecules54), and SQUID
magnetometry (solid state, Curie behavior, µeff < 0.1 µB)
together support an impurity origin for this paramagnetism.
These data are, therefore, all consistent with singlet ground states
for both species with negligible population of any high-spin
states at room temperature.
1H NMR spectra of [2]2+ at -85 °C indicate that rotation
around the C6H4-CH bond is slow (as indicated by the
inequivalence of H1 and H4 and of H2 and H5, as seen in Figure
2, and by the observation of an NOE between H3 and H7, but
not between H6 and H7), indicating some multiple bond
Figure 3. View of [1]2+ in the crystal structure of its [SbCl6]- salt. Atoms
“A” are generated by Ci symmetry. Thermal ellipsoids are at 50% probability
and all H atoms have been omitted.
character. However, the coupling constant between the two
vinylene protons (H3 and H6) is ca. 13.5 Hz, falling in the range
found for symmetric polymethines (bond order ) 1.5),55,56 rather
than the ca. 10 Hz or ca. 16 Hz expected for trans -CHdCH-
and dCH-CH) entities, respectively.57 Thus NMR indicates
that [2]2+ has neither a benzenoid structure similar to that of
the neutral species nor the structure suggested by a limiting
quinoidal resonance structure, but lies between these extremes.
the previously reported structures of 1 and [1]+[SbF6]-.26 The
[1]n+ system is a rare example of a dinuclear mixed-valence
system where all three oxidation states are structurally charac-
terized; the only previous example of which we are aware has
dimolybdenum redox centers and, as in the [1]n+ system, the
monocation is a Robin-Day class-III58 mixed-valence compound
and the dication is a singlet.59 The data for the [1]n+ system
show that successive oxidation of 1 results in increased
shortening of the NsC6H4 bond and increased BLA with a
quinoidal sense in the bridging phenylene groups. Interestingly,
however, the BLA in the dication falls well short of the “ideal”
quinoidal BLA of ca. 0.1 Å that we originally expected based
on the alternating single and double bonds of the limiting
resonance structure.34 The vinylene bridge of [1]2+ retains the
same sense of BLA as the neutral species, although with a
greatly reduced magnitude. The experimental geometry of [1]2+
Crystal Structures and Calculated Geometries. We deter-
mined the structures of [1]2+([SbCl6]-)2 (Figure 3), 2, and
[2]2+([SbF6]-)2 (Figure 4) by X-ray crystallography; some
parameters are compared in Table 2 with analogous values from
(54) We observed a three-line spectrum (g ) 2.004; AN ) ca. 9 G) characteristic
of a localized triarylaminium cation (Seo, E. T.; Nelson, R. F.; Fritsch, J.
M.; Marcoux, L. S.; Leedy, D. W.; Adams, R. N. J. Am. Chem. Soc. 1966,
88, 3498; Bamberger, S.; Hellwinkel, D.; Neugebauer, F. A. Chem. Ber.
1975, 108, 2416), unlike the five-line spectrum we see for [1]+ or the
featureless spectra we see for other bis(diarylamino) phenylene-vinylene
dications (Barlow, S.; Risko, C.; Chung, S.-J.; Tucker, N. M.; Coropceanu,
V.; Jones, S. C.; Levi, Z.; Bre´das, J. L.; Marder, S. R. J. Am. Chem. Soc.
2005, 127, 16900).
(55) Tolbert, L. M.; Zhao, X. J. Am. Chem. Soc. 1997, 119, 3253.
(56) Barlow, S.; Henling, L. M.; Day, M. W.; Schaefer, W. P.; Green, J. C.;
Hascall, T.; Marder, S. R. J. Am. Chem. Soc. 2002, 124, 6285.
(57) Pretsch, E.; Bu¨hlmann, P.; Affolter, C. Structure Determination of Organic
Compounds; Springer-Verlag: Berlin, 2000.
(58) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247.
(59) Cotton, F. A.; Liu, C. Y.; Murillo, C. A.; Villagra´n, D.; Wang, X. J. Am.
Chem. Soc. 2004, 126, 14882.
9
J. AM. CHEM. SOC. VOL. 128, NO. 6, 2006 1815

A R T I C L E S
Zheng et al.
Figure 4. View of [2]2+ in the crystal structure of its [SbF6]- salt showing only one of the possible orientations for the disordered C34, C48, C55 and C56
atoms. Thermal ellipsoids are at 50% probability and all H atoms have been omitted.
Table 2. Selected Bond Lengths (Å) for 1 and 2 and Their Monocations and Dications from X-ray and DFTa(in italics)
BLA(C6 H4)b
C6H4−CH
CH
d
CHc
CH−Cthi
(Cd
C)thi
(C−C)thi
c
c
c
An
−N
N−C6H4
1d
1.435(3)
1.425
1.431(3)
1.429
1.424(2)
1.423
1.414
1.389(2)
1.413
1.369(8)
1.388
1.369(5)
1.379
1.407
0.021(3)
0.021
0.034(14)
0.042
0.058(3)
0.056
0.033
1.447(3)
1.461
1.473(16)
1.436
1.417(5)
1.420
1.450
1.343(4)
1.352
1.316(6)
1.370
1.370(7)
1.385
1.362
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
[1]+d
[1]2+
Singlet
BS
Triplet
1.410
1.419
0.024
1.460
1.354
2
1.423(4)
1.425
1.430(7)
1.430
1.409(4)
1.412
1.384(2)
1.373
0.010(3)
0.022
0.023(8)
0.058
1.456(3)
1.457
1.404(2)
1.414
1.327(6)
1.356
1.387
1.391
1.375
1.440(3)
1.438
1.376(4)
1.397
1.368(2)
1.385
1.406(4)
1.423
1.417(3)
1.427
1.382(11)
1.397
[2]2+
Singlet
BS
1.422
1.391
0.044
1.432
1.414
1.408
1.412
Triplet
1.417
1.405
0.035
1.445
1.365
1.426
1.398
1.423
a DFT values for dications are for the closed-shell singlet, broken-symmetry “biradical singlet” (BS), and biradical triplet configurations. b Difference
between average CipsosCo and CmsCp and average CosCm bond lengths; i.e., bonds expected to be long and short respectively in a quinoidal structure.
c Double or single bonds indicate the formal order of bonds in the neutral structures shown in Chart 1; “thi” denotes the thiophene unit of [2]n+ d Ref 26.
.
is consistent with that obtained from B3LYP/6-31G* DFT
calculations (also in Table 2) for a closed-shell singlet config-
uration; calculations for a triplet configuration show a rather
different geometry characterized by more-or-less equal shorten-
ing of all Cs N bonds on oxidation and by retention of stronger
BLA in the vinylene bridge, while the results obtained by a
broken-symmetry (BS) DFT method60 suggest that the geometry
of the open-shell singlet (“singlet biradical”) configuration is
intermediate between that of the closed-shell singlet and the
triplet biradical. Thus, comparison of the X-ray data and DFT
results strongly support a closed-shell singlet configuration for
However, the pattern suggested by both crystallography and
DFT is similar to that for [1]2+; i.e., oxidation shifts the bond
length pattern in the quinoidal direction so that the bonds across
the vinylene groups are all approximately equal, consistent with
the 1H NMR data. A similar pattern of less than “fully quinoidal”
geometry is found within the thiophene ring; previous crystal
structures of “fully quinoidal” thiophene derivatives62-64 reveal
differences of ca. 0.06 Å between long and short bonds, i.e.,
greater than that seen or calculated here.
It is worth emphasizing again that BLAs of ca. 0.1 Å have
been taken as characteristic of closed-shell structures,33 yet here
we find considerably reduced BLA in both dications.34 More-
over, we have also revisited the geometry of Chichibabin’s
hydrocarbon (Figure 1, n ) 2, X ) CPh2), for various electronic
configurations (see Supporting Information); this species is
isoelectronic with 4,4′-bis(diarylamino)biphenyl dications. The
electronic structure of this compound has been controversial,
in part because its poor solubility has hampered purification
and led to complications in spectroscopic investigations;
however, an X-ray structure showing less than quinoidal BLA
has been taken as evidence of biradical character.33 While in
[1]2+ 61
. Moreover, the X-ray and DFT results both indicate that
closed-shell singlet structures such as [1]2+, for which one might
expect a canonical “quinoidal” pattern of BLA, can, in fact,
show significant deviation from this pattern.34
Our crystals of [2]2+ were of poorer quality than those of
[1]2+, and refinement was more complicated by disorder;
consequently some of the geometric parameters are somewhat
less precisely determined than those of [1]2+ and, in particular,
the BLA pattern in the phenylene groups is less well-defined.
(60) The broken-symmetry unrestricted DFT method does not produce a well-
defined spin state, but rather a mixture of singlet and triplet states. Open-
shell singlet states generally require a multi-configurational self-consistent-
field treatment.
(61) The DFT method does not allow us to treat possible configuration interaction
between closed- and open-shell singlet configurations; however, comparison
of the X-ray geometry and the DFT results strongly suggest that the closed-
shell configuration is dominant.
(62) Pappenfus, T. M.; Raff, J. D.; Hukkanen, E. J.; Burney, J. R.; Casado, J.;
Drew, S. M.; Miller, L. L.; Mann, K. R. J. Org. Chem. 2002, 67, 6015.
(63) Murakami, F.; Sasaki, S.; Yoshifuji, M. Angew. Chem., Int. Ed. Engl. 2002,
41, 2574.
(64) Casado, J.; Pappenfus, T. M.; Mann, K. R.; Ort´ı, E.; Viruela, P. M.; Milia´n,
B.; Herna´ndez, V.; Navarreye, J. T. L. ChemPhysChem 2004, 5, 529.
9
1816 J. AM. CHEM. SOC. VOL. 128, NO. 6, 2006

Singlet Bis(Triarylamine) Dications
A R T I C L E S
this case the geometry of the broken-symmetry state is closest
to the experimental X-ray geometry,33 it is interesting that the
calculated closed-shell singlet geometry is characterized by a
less than fully quinoidal BLA (0.076 Å) and a rather long
C6H4sC6H4 bond (1.420 Å).
Crystallographic (and, for [2]2+, NMR) data indicate oxidation
results in a shift in the bond lengths toward a quinoidal structure,
but not to the “fully quinoidal” extent one might expect based
upon the limiting valence-bond representations of the structures.
DFT calculations for closed-shell singlet configurations give
similar structures, suggesting that significant open-shell character
need not be invoked to explain these geometries. Thus, we have
shown that in molecules of the type generalized in Figure 3
one cannot necessarily anticipate the same patterns of quinoidal
BLA that are seen in “typical” quinones, even for species with
closed-shell singlet configurations; the bond-length and charge-
distribution patterns described here are in fact reminiscent of
those proposed for the “polaron lattice” configuration of
conducting polyaniline.
Charge Distribution. Finally we return to the issue of the
distribution of the excess charge in “bipolarons”. As a simple
indicator of the charge distributions in the positively charged
species, we have calculated the change in Mulliken charges,
∆q, associated with oxidation from neutral to monocations and
closed-shell singlet dications for 1 and 2, and also for one-
electron oxidation of the model compound N,N-di-p-anisyla-
niline. The patterns of ∆q are very similar for both mono- and
dications, with the magnitude of ∆q for each atom in the dication
approximately twice that of the corresponding atom in the
monocation, consistent with the observation that the monocation
geometries are intermediate between the neutral and dication
geometries. The ∆q on nitrogen atoms of the dications (+0.065
and +0.051 electronic charges for [1]2+ and [2]2+ respectively)
is only a little smaller than that for the N,N-di-p-anisylaniline
cation (+0.076), and there is a similar pattern of ∆q in the
terminal p-anisyl groups; these findings suggest that the
oxidations are principally triarylamine-based, broadly consistent
with the charge distribution implied by simple resonance
structures. However, significant ∆q are found throughout the
π-systems of both molecules (∆q ) +0.054 per vinylene carbon
of [1]2+; average ∆q per vinylene atom of [2]2+ ) +0.034;
average ∆q per dialkoxythiophene S, O and sp2-C atom of [2]2+
) +0.032), thus indicating the true charge distribution in these
species is less clear-cut than that implied by quinoidal resonance
structures. This finding is consistent with the type of charge
distributions proposed for the “polaron lattice” configuration
of conducting polyaniline (emeraldine salt).65,66
Acknowledgment. This material is based upon work sup-
ported in part by the STC Program of the National Science
Foundation under Agreement Number DMR-0120967. We also
thank the NSF for funding through grant CHE-0342321, the
Defense Advanced Research Projects Agency for funding
through the MORPH program, and the National Institute of
Health for funding through the RIMI program (award number
1P20MD001104-01). We thank Man Han and Prof Z. John
Zhang for assistance with SQUID measurements.
Supporting Information Available: Complete ref 25, 26 and
1
41; details of physical measurements and synthesis of 2; H
and NOESY NMR spectra for [2]2+; additional crystallographic
experimental details including figure showing X-ray structure
of 2; tables of calculated geometries for 1, [1]+, [1]2+, 2, [2]+,
[2]2+ and the Chichibabin hydrocarbon; and full xyz coordinates
for calculated structures (pdf format). X-ray data (CIF format).
This material is available free of charge via the Internet at http://
pubs.acs.org. See any current masthead page for ordering
information and Web access instructions.
Summary
We have reported the first crystal structures of bis(triaryl-
amine) dications, both of which are ground-state singlets.
(65) Stafstro¨m, S.; Bre´das, J. l.; Epstein, A. J.; Woo, H. S.; Tanner, D. B.; Huang,
W. S.; MacDiarmid, A. G. Phys. ReV. Lett. 1987, 59, 1464.
(66) Libert, J.; Bre´das, J. L.; Epstein, A. J. Phys. ReV. B 1995, 51, 5711.
JA0541534
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