Substituent effects on the redox properties and structure of substituted triphenylamines. An experimental and computational study

Article abstract of DOI:10.1016/j.tet.2009.01.023

The peak oxidation potentials of a series of polysubstituted triphenylamines were found to be highly linearly correlated (R=0.995) with their ionization potentials as computed by density functional theory. The computations, as well as confirming previous

Full text of DOI:10.1016/j.tet.2009.01.023

Tetrahedron 65 (2009) 2408–2414
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Substituent effects on the redox properties and structure of substituted
triphenylamines. An experimental and computational study
,
Xin Wu a, Anthony P. Davis a, Peter C. Lambert a, L. Kraig Steffen b, Ozan Toy a, Albert J. Fry a
*
a Chemistry Department, Wesleyan University, Middletown, CT 06459, United States
b Chemistry Department, Fairfield University, Fairfield, CT 06824, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 August 2008
Received in revised form 22 December 2008
Accepted 6 January 2009
Available online 14 January 2009
The peak oxidation potentials of a series of polysubstituted triphenylamines were found to be highly
linearly correlated (R¼0.995) with their ionization potentials as computed by density functional theory.
The computations, as well as confirming previous experiments demonstrating the planar geometry of
such substances around the central nitrogen atom, also demonstrate substantial resonance interactions
between the central nitrogen atom and a nitro group located para or ortho to it; no such interaction is
involved in the corresponding amine cation radicals. On the other hand, a methoxy group located para to
the central nitrogen atom interacts strongly with the central nitrogen in the cation radical, but not in the
neutral amine.
Keywords:
Substituent effects
Triphenylamines
Oxidation potentials
Ionization potentials
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
it. The traditional way to do this would be to construct a plot of the
experimental oxidation potentials of a series of substituted tri-
Steckhan and co-workers reported the synthesis of a series of
brominated triphenylamines and showed that these substances are
excellent electrocatalysts for oxidative transformation of a variety
of organic substrates.1 Our interest was aroused recently in this
class of substances in connection with our studies on the anodic
oxidation of electronegatively substituted alkenes2 and other such
substrates bearing one or more strong electron-withdrawing sub-
stituents. We have found that the polybromotriphenylamines of
Steckhan and co-workers1,9 are not useful electrocatalysts for an-
odic oxidation of electronegatively substituted substrates.2,4 This is
apparently because the differences between the oxidation poten-
tials of the catalyst and that of such substrates are too great. Our
aim recently has therefore been to develop triarylamine electro-
catalysts of substantially higher oxidation potential and to explore
the use of these substances as electrocatalysts for the oxidation of
difficultly oxidized organic substrates.3,4 In connection with this
goal, we felt that computation could provide a valuable adjunct to
these studies by guiding the design of promising new triarylamine
electrocatalysts. That is, it would be useful to be able to predict in
advance whether a triphenylamine containing a given combination
of ring substituents is likely to have a high enough oxidation po-
tential to be useful before embarking upon a proposed synthesis of
phenylamines versus the sum of the Hammett constants of the
substituents. If the data proved to be well correlated (correlation
coefficient close to unity), then the oxidation potential of an un-
known triphenylamine could be predicted with reasonable accu-
racy using the Hammett values of the substituents on it. Linear
correlations of this type with excellent correlation coefficients have
been observed for the reduction of many classes of substituted
arenes.5 A number of attempts to construct such correlations for
oxidative processes have also been reported.6–9 However, we re-
cently reported a better procedure for correlating and predicting
the effects of ring substituents upon reduction potential.10 The
experimental reduction potentials of a set of 100 benzylideneace-
tophenones (so-called ‘chalcones’), bearing anywhere from one to
four ring substituents, were measured by linear sweep voltam-
metry (LSV). Simultaneously, their electron affinities were com-
puted by density functional theory (DFT). A plot of reduction
potential versus electron affinity for the 100 chalcones was found to
afford an excellent linear correlation (R¼0.969), with a standard
deviation of 22 mV between the experimental and computed re-
duction potentials.10 A plot of reduction potentials versus Hammett
substituent constants afforded a linear correlation, but with greater
scatter of points about the line (R¼0.953). Similar attempts to
construct correlations between Hammett constants and the oxi-
dation potentials of a series of related compounds have been
reported.6–9 If we could show a good correlation between the ox-
idation potentials of a series of known substituted triphenylamines
* Corresponding author. Tel.: þ1 860 685 2622; fax: þ1 860 685 2211.
E-mail address: afry@wesleyan.edu (A.J. Fry).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.01.023

X. Wu et al. / Tetrahedron 65 (2009) 2408–2414
2409
and their computed ionization potentials, the correlation line could
be used to point our synthetic efforts in the direction of promising
candidates. We wish to report the results of our attempts to develop
such a correlation and, at the same time, to extend our previous
computational efforts on cathodic systems10 into the anodic arena.
The ionization potentials of a series of substituted triphenylamines
(1a–u) were computed by DFT11 and their oxidation potentials
were measured by LSV in acetonitrile. The quality of the correlation
between the two properties was assessed, together, for comparison,
with that of a similar correlation of the oxidation potentials with
Hammett substituent constants.5,10 At the same time, instructive
information on the effects of substituents upon the triarylamine
structure was found by examining the optimized geometries of
both the neutral and cationic species.
course of the present work (1f, p–u). Compounds 1v–x were ex-
amined computationally but were not synthesized.
2.2.1. 2-Nitrotriphenylamine (1f)
4-Nitrotriphenylamine (1g) can be prepared by treatment of
triphenylamine (1a) with a limited quantity of Cu(NO3)2$2.5H2O in
acetic anhydride.3,12 We have since found that 1g is accompanied
by a small amount of the 2-isomer (1f), which can be separated by
repeated chromatography: mp 103–105 ꢀC (lit.13 102–103 ꢀC); 1H
NMR (300 MHz, CD3CN):
d
¼7.82 (dd, J¼8.1 and 1.2 Hz, 1H), 7.60 (dt,
J¼8.0 and 1.5 Hz, 1H), 7.31 (t, J¼7.8 Hz, 1H), 7.2–7.4 (m, 5H), 7.05 (t,
J¼7.5 Hz, 2H), 6.96 (d, J¼7.5 Hz, 4H); 13C NMR (300 MHz, CDCl3):
d
¼146.7, 145.8, 141.3, 133.7, 129.9, 129.6, 126.2, 124.4, 123.8, 123.4;
MS: m/z (%): 291 (20), 290 (100), 244 (43), 243 (42), 242 (25), 241
(25), 166 (17). We have also prepared 1f in low yield by the copper-
catalyzed Ullmann reaction of 2-bromonitrobenzene with
diphenylamine.13
Z
X
Y'
X'
2.2.2. 4,40,400-Trichlorotriphenylamine (1p)
N
This compound was prepared using a procedure described by
Y"
Y
Patil and
co-workers.14 1-Chloro-4-iodobenzene (2.73 g,
11.5 mmol) and 4-chloroaniline (0.696 g, 5.5 mmol) were dis-
solved in dry toluene (35 mL). Copper(I) iodide (38.1 mg,
3.6 mol %) and 2,20-bipyridyl (31.5 mg, 3.6 mol %) were added to
the solution, and the reaction vessel was placed in an oil bath
heated to 115 ꢀC. Potassium tert-butoxide (1.85 g, 16.5 mmol) was
added, and then the vessel was sealed and purged with N2. After
5 h, the reaction was allowed to cool to rt. Solids were removed
by vacuum filtration, and solvent was removed by rotary evapo-
ration. The product was dissolved in CH2Cl2 (30 mL) and washed
with 0.1 M aqueous NaHSO3 (2ꢁ30 mL), followed with distilled
H2O (1ꢁ30 mL). Purification by flash chromatography using silica
with hexane as the eluent, followed by recrystallization from 95%
EtOH, yielded 1p (853 mg; 44%): mp 144–146 ꢀC (lit.15 148–
X"
1a, X = X' = X" = Y = Z = H
b, X = X' = X" = OMe
c, X = X' = X" = Me
d, X = X' = X" = Br
e, Z = NO2
f, Y = NO2
g, X = NO2
h, X = X' = X" = Me; Y = Y' = NO2
i, X = X' = NO2
j, X = Y' = NO2; X' = X" = Br
k, X = X' = X" = Me; Y = Y' = Y" = NO2
l, X = X' = NO2; X" = Y" = Br
m, X = X' = X" = NO2
n, X = X' = Me; X" = Y = Y' = NO2
o, X = X" = Y" = Br; X' = Y = NO2
p, X = X' = X" = Cl
q, X = X' = X" = Cl; Y = NO2
r, X = X' = X" = Cl; Y = Y' = NO2
s, X = X' = X" = Cl; Y = Y' = Y" = NO2
t, X = X' =X'' = SCH2CH2CH3
u, X = X' =X'' = SO2CH2CH2CH3
v, X = OCH3
149 ꢀC); 1H NMR (300 MHz, CD3CN):
d
¼7.28 (d, J¼9.0 Hz, 6H), 7.01
(d, J¼8.7 Hz, 6H); 13C NMR (300 MHz, CDCl3):
d
¼145.9, 129.8,
128.6, 125.4; MS: m/z (%): 353 (3), 351 (33), 349 (100), 347 (100),
311 (14), 277 (21), 75 (11). Anal. Calcd for C18H12Cl3N: C, 62.01; H,
3.47. Found: C, 61.86; H, 3.35.
2.2.3. 4,40,400-Trichloro-2-nitrotriphenylamine (1q)
Cu(NO3)2$2.5H2O (0.25 g) was added to a solution of 4,40,400-
trichlorotriphenylamine (1p) (200 mg, 0.574 mmol) in acetic
anhydride (40 mL), and the mixture was stirred for 3 h. The re-
action was quenched with distilled water (75 mL), and extracted
with dichloromethane (3ꢁ10 mL). After washing with saturated
aqueous NaHCO3 (150 mL), the organic layer was separated,
washed with H2O (10 mL), and dried over Na2SO4. Purification by
flash chromatography using silica with 2:1 hexanes/benzene as
the eluent, followed by recrystallization from 95% EtOH, yielded
1q (87 mg; 33%): mp 102–104 ꢀC; 1H NMR (300 MHz, CD3CN):
w, X = Br
x, X = X' = Br
2. Experimental
2.1. General
1H and 13C NMR spectra were measured on a Varian Mercury Vx
300 MHz pulsed field gradient spectrometer. Chemical shifts for the
NMR spectra are reported in parts per million with the solvent (1H:
d
¼7.89 (d, J¼2.7 Hz, 1H), 7.61 (dd, J¼8.7 and 2.4 Hz, 1H), 7.30 (d,
J¼8.7 Hz, 1H), 7.27 (d, J¼8.7 Hz, 4H), 6.96 (d, J¼8.7 Hz, 4H); 13C
NMR (300 MHz, CDCl3):
d
¼145.7, 144.6, 139.1, 134.2, 130.9, 130.3,
CD3CN,
d d 77.23(3)) as the internal standard.
1.94 (5); 13C: CDCl3,
130.0, 129.6, 126.4, 124.5; MS m/z (%): 398 (5), 396 (36), 394
(100), 392 (100), 345 (21), 311 (48), 275 (19), 241 (15), 75 (10).
Anal. Calcd for C18H11Cl3N2O2: C, 54.92; H, 2.82. Found: C, 54.68;
H, 2.80.
An Agilent Technologies 6890N Gas Chromatograph with an HP-5
column coupled to a 5973N MS Detector was used for measure-
ment of mass spectra. Microanalyses were carried out by Atlantic
Microlab, Inc., Norcross, GA. Cyclic voltammetry with electronic iR
compensation was carried out using a CH Instruments (Austin, TX)
Model 650-A electrochemistry system.
2.2.4. 4,40,400-Trichloro-2,20-dinitrotriphenylamine (1r)
Cu(NO3)2$2.5H2O (0.50 g) was added to a solution of 4,40,400-
trichlorotriphenylamine (1p) (200 mg, 0.574 mmol) in acetic an-
hydride (40 mL), and the mixture was stirred overnight. The
reaction was quenched with distilled water (75 mL), and
extracted with dichloromethane (3ꢁ10 mL). The organic solution
was washed with saturated aqueous NaHCO3 (150 mL), washed
2.2. Sources of triphenylamines
All compounds are either commercially available (1a–e), were
described in earlier papers (1g–o),3,4,12,13 or were synthesized in the

2410
X. Wu et al. / Tetrahedron 65 (2009) 2408–2414
once with H2O (10 mL), and dried over Na2SO4. Purification by
flash chromatography using silica with 9:1 hexanes/ethyl acetate
as the eluent, followed by recrystallization from 95% EtOH,
yielded 1r (92 mg; 36%): mp 168–170 ꢀC; 1H NMR (300 MHz,
0.98 (t, J¼7.4 Hz, 3H); 13C NMR (300 MHz, CDCl3):
¼150.5, 135.2,
d
130.3, 124.7, 58.2, 16.7, 13.2. Anal. Calcd for C27H33NS3O6: C, 57.52;
H, 5.90. Found: C, 57.49; H, 5.88.
CD3CN):
7.26 (d, J¼8.7 Hz, 2H), 7.17 (d, J¼9.3 Hz, 2H), 6.73 (d, J¼8.7 Hz,
2H); 13C NMR (300 MHz, CDCl3):
131.5, 130.9, 130.3, 130.1, 126.2, 123.0; MS: m/z (%): 441 (32), 439
(100), 437 (100), 359 (20), 347 (14), 309 (27), 275 (25), 239 (10),
75 (9). Anal. Calcd for C18H10Cl3N3O4: C, 49.29; H, 2.30. Found: C,
48.95; H, 2.19.
d
¼7.80 (d, J¼2.4 Hz, 2H), 7.52 (dd, J¼8.7 and 2.4 Hz, 2H),
2.3. Voltammetry
d¼145.1, 143.4, 137.6, 134.5,
Cyclic voltammograms of compounds 1a–u were measured at
a scan rate of 0.1 V/s at a glassy carbon working anode and platinum
wire counter electrode in acetonitrile containing 0.1 M lithium
perchlorate. Potentials were measured versus a Ag/0.1 M AgNO3
reference electrode. The potential of this electrode is þ0.350 V
versus SCE and ꢂ0.030 V versus Fc/Fcþ. Adams and co-workers
showed that the cation radicals of triphenylamines containing at
least one para-unsubstituted site are unstable on the cyclic vol-
tammetric time scale because they rapidly dimerize.8,18 Such
compounds therefore exhibit irreversible voltammetric behavior.
For this reason, anodic peak potentials were recorded for all sub-
stances (Table 1).
2.2.5. 4,40,400-Trichloro-2,20,200-trinitrotriphenylamine (1s)
Nitration was carried out by the procedure of Uemura and co-
workers.16 Sodium nitrite (625 mg, 9 mmol) was added to a solu-
tion of 4,40,400-trichlorotriphenylamine (1p) (349 mg, 1 mmol) in
trifluoroacetic acid (100 mL), upon which the solution turned deep
blue and then green in color. The mixture was sealed with a rubber
septum and stirred overnight. After 20 h, H2O (100 mL) was added
to the orange-brown solution and the reaction mixture was then
cooled in ice. While stirring, NaOH pellets (50þ g) were added very
slowly (exothermic) until the solution turned red litmus paper blue.
The product was extracted with CH2Cl2 (3ꢁ10 mL) and washed
thoroughly with H2O to yield 1s (350 mg; 72%): mp 302–303 ꢀC; 1H
2.4. Computational method
The energies of both the neutral species and the cation radicals
of substances 1a–x were computed11 by density functional theory
employing the B3LYP hybrid functional19 and the 6-31G(d) basis
set, which includes d-orbital polarization functions on all second
row and higher atoms (C, N, O, S, Cl, Br). This procedure was
chosen in an effort to achieve both reasonable accuracy and ac-
ceptable computation times. The geometries of both the neutral
amine and its corresponding cation radical were optimized at the
B3LYP/6-31G(d) level separately; this was necessary because
certain aspects of the geometry change upon one-electron oxi-
dation. Each structure was verified as a true minimum by
NMR (300 MHz, CD3CN):
d
¼7.96 (d, J¼2.4 Hz, 3H), 7.61 (dd, J¼8.7
and 2.4 Hz, 3H), 7.22 (d, J¼9.0 Hz, 3H); 13C NMR (300 MHz, CDCl3):
d¼144.1, 136.9, 134.5, 132.3, 129.7, 126.6; MS: m/z (%): 486 (33), 484
(96), 482 (100), 345 (24), 311 (52), 309 (74), 289 (34), 287 (49), 264
(35), 238 (25), 69 (28). Anal. Calcd for C18H9Cl3N4O6: C, 44.70; H,
1.88. Found: C, 44.81; H, 1.94.
2.2.6. 4,40,400-Tripropanesulfonyltriphenylamine (1u)
4,40,400-Tris[propylthio]triphenylamine (1t) was prepared by the
method described by Chen and co-workers for 4,40,400-tris[me-
thylthio]triphenylamine.17a The propyl analog was prepared to
improve solubility properties. Potassium propylmercaptide was
prepared by adding 1-propanethiol (734 mg, 9.6 mmol) to a solu-
tion of KOt-Bu (1.081 g, 9.6 mmol) in MeOH (17 mL). The solvent
was removed by rotary evaporation and heated until dry. The
mercaptide and 4,40,400-tribromotriphenylamine (1d) (516 mg,
1.1 mmol) were placed in a reaction vessel sealed with a rubber
septum and purged with N2. DMF (20 mL) was injected into the
reaction vessel and the solution was heated to 125 ꢀC and stirred
overnight. The solvent was removed by rotary evaporation. The
residue was dissolved in a 50:50 mixture of CH2Cl2 and hexane and
the solution was washed with saturated aqueous NH4Cl and then
successively with water and brine, and dried over Na2SO4. Purifi-
cation by flash chromatography using silica with 3:1 hexanes/
CH2Cl2 as the eluent yielded 1t (183 mg; 35%): mp 47–49 ꢀC; 1H
Table 1
Substituent effects on oxidation and ionization potentials of substituted
triphenylamines
Compound
Eox
I.P.b
Ssc
Ssnd
Ssþe
a
1a
1b
1c
1d
1e
1f
1g
1h
1i
0.54
0.25
0.40
0.78
0.84
0.83
0.88
0.96
1.08
1.17
4.77
4.35
4.61
5.02
5.03
5.07
5.16
5.35
5.45
5.54
5.61
5.73
5.78
5.72
5.80
5.02
5.29
5.60
6.94
4.49
5.58
4.61
4.85
4.93
0
0
0
ꢂ0.804
ꢂ0.510
0.696
0.710
0.778
0.778
1.05
1.56
2.09
1.82
2.02
2.33
1.99
2.25
0.681
1.46
2.24
3.11
0.00
2.04
ꢂ0.45
ꢂ0.51
0.78
0.778
0.81
0.81
1.11
1.62
2.14
1.92
2.14
2.43
1.92
2.4
0.72
1.53
2.34
3.15
0.15
2.19
ꢂ0.15
0.26
0.52
ꢂ1.94
ꢂ0.768
0.0750
0.674
0.740
0.740
1.15
1.35
1.53
1.45
1.58
2.22
1.71
1.55
0.105
0.845
1.59
1j
1k
1l
1.28
1.31
1.33
1.33
1.37
0.79
1.00
1.25
1.56
0.36
1.15
NMf
NMf
NMf
NMR (300 MHz, CD3CN):
d
¼7.25 (d, J¼8.1 Hz, 2H), 6.95 (d, J¼7.8 Hz,
1m
1n
1o
1p
1q
1r
1s
1t
1u
1v
1w
1x
2H), 2.87 (t, J¼7.1 Hz, 2H), 1.62 (sextet, J¼7.3 Hz, 2H), 1.00 (t,
J¼7.5 Hz, 3H); 13C NMR (300 MHz, CDCl3):
¼145.9, 131.2, 130.4,
d
124.6, 36.9, 22.9, 13.6; MS: m/z (%): 467 (Mþ,100), 424 (39). Puri-
fication of 1t to analytical purity was difficult because of its high
solubility and low mp, hence it was used directly in the next step.
Oxidation of 1t was carried out by the procedure of Trost and
Curran.17b To a solution of OXONEÒ (2.368 g, 7.7 mmol) in EtOH
(70 mL) and H2O (30 mL) was added 0.2 mmol of 1t (100 mg), and
the mixture was stirred overnight. The mixture was diluted with
water and extracted with dichloromethane (3ꢁ50 mL). The com-
bined organic layers were washed with water and brine and then
dried over Na2SO4. Purification by flash chromatography with 19:1
dichloromethane/ethyl ether as eluent, followed by re-
crystallization from 95% EtOH, yielded 1u (81 mg, 81%): mp 210–
2.33
ꢂ0.492
2.24
ꢂ0.268
0.232
0.464
ꢂ0.648
0.025
ꢂ0.050
a
Oxidation potential in volts versus Ag/0.1 M AgNO3 in CH3CN/0.1 M LiClO4.
Ionization potential in eV computed at the density functional B3LYP/6-31G(d)
b
level.
c
Sum of Hammett constants of ring substituents.
n constants of ring substituents.22
d Sum of van Bekkum, Verkade, and Wepster
s
212 ꢀC; 1H NMR (300 MHz, CD3CN):
(d, J¼9.0 Hz, 2H), 3.13 (t, J¼7.7 Hz, 2H), 1.68 (sextet, J¼7.5 Hz, 2H),
d
¼7.80 (d, J¼8.4 Hz, 2H), 7.30
e
Sum of
s constants of ring substituents.
f
Not measured.

X. Wu et al. / Tetrahedron 65 (2009) 2408–2414
2411
ensuring the absence of imaginary vibrational frequencies. The
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
ionization potential (I.P.) (in eV) is then, by definition, the energy
of the cation radical minus that of the neutral species. The sol-
vation energies of both neutral amine and cation in acetonitrile,
computed using the polarized continuum (PCM) method,20 were
included in the computations. Even though ionization potentials
are normally measured in the vapor phase, the correlation be-
tween the computed value and the experimental (solution) oxi-
dation potential was expected to provide
a more realistic
comparison with the experiment when the I.P. computation in-
cludes solvation effects, since the cation radicals should be
strongly solvated by the donor solvent (acetonitrile). In this
connection, it is important to note that we are not attempting to
compute ionization potentials per se; rather, we simply need
a conveniently computable parameter that correlates well with
the experimental oxidation potentials.
3. Results and discussion
3.1. Substituent effects on oxidation potential
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
A plot of the experimental voltammetric peak oxidation po-
tentials (Ep) versus the computed I.P.’s exhibited an excellent
linear correlation between these properties (R¼0.995) (Fig. 1).
Figure 2. Plot of experimental oxidation potentials of a series of substituted triphe-
nylamines versus the sum of the Hammett constants (Ss) of the substituents.
Analogous plots of Ep versus Hammett s 21 sn 21a,22 and sþ21a
,
,
showed somewhat more scatter (Figs. 2–4), and thus the cor-
relations exhibited were not as good as the correlation with
computed I.P., suggesting the presence of resonance effects with
some of the triphenylamines. The correlation between the ex-
perimental data and each of the four parameters can be
expressed as Eqs. 1–4, together with their correlation co-
efficients (R).
X
X
Epox ¼ 0:512 þ 0:347
Epox ¼ 0:692 þ 0:317
sn R ¼ 0:973
sþ R ¼ 0:945
(3)
(4)
The decrease in R as one proceeds through Eqs. 1–4 confirms the
visual impression of increasing scatter as one proceeds through
Figures 1–4. Our previous study of substituent effects on the vol-
tammetric reduction potentials of a series of 100 benzylideneace-
tophenones (chalcones) produced the same result, i.e., the best
correlation (R¼0.962) was observed between computed electron
Epox ¼ ꢂ3:190 þ 0:789I:P: R ¼ 0:995
(1)
(2)
X
Epox ¼ 0:542 þ 0:346
s
R ¼ 0:981
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
4.2
4.4
4.6
4.8
5.0
5.2
Ionization Potential (eV)
5.4
5.6
5.8
6.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Figure 1. Plot of experimental oxidation potentials of a series of substituted triphe-
nylamines versus their ionization potentials computed by density functional theory at
the B3LYP/6-31G(d) level.
Figure 3. Plot of experimental oxidation potentials of a series of substituted triphe-
nylamines versus the sum of the sþ constants (Ssþ) of the substituents.

2412
X. Wu et al. / Tetrahedron 65 (2009) 2408–2414
triphenylamine (1a) the three phenyl groups are equivalent and
1.6
oriented in a propeller-type geometry, i.e., they are rotated by 48ꢀ
from the hypothetical planar structure. In general, the central ni-
trogen atom is almost planar, allowing for maximum orbital over-
lap with the pi-system.
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Certain substituents in 1a-x, i.e., nitro and methoxy, are capable
of interacting strongly by resonance with pi-systems to which they
are attached. Such substituents on the triarylamine core structure
may affect the geometry of the system as a result of resonance
interaction with the central nitrogen atom when they are located
either ortho or para to it. This effect should be particularly impor-
tant in (a) neutral triphenylamines bearing a nitro group at the 4- or
2-position and (b) triphenylamine cation radicals bearing
a methoxy group in the 4- or 2-position. 4- and 2-Nitrosubstituted
triphenylamines are highly colored, which strongly indicates an
electronic interaction of the nitro group with the central nitrogen
atom. The geometry optimizations carried out in the course of the
I.P. computations provide a way to assess the magnitude of these
effects upon structure. Tables 2 and 3 present illustrative data for
several triphenylamines in which resonance effects are operative.
-2
-1
0
1
2
3
3.3. Effect of the nitro group
Figure 4. Plot of experimental oxidation potentials of a series of substituted triphe-
nylamines versus the sum of the sn constants (Ssn) of the substituents.
Comparison of columns 2 and 3 of Table 2 reveals that in-
troduction of a nitro group at position 3 (meta to the central ni-
trogen) has little effect upon the geometry of the molecule.
However, introduction of the nitro group at the 4-position (para)
has a very significant effect. The quinoid structure (2b) should be
a significant contributor to the resonance hybrid in triphenyl-
amines substituted at the 4-position by the strongly electronega-
tive nitro group (2a). Evidence for this is readily found in the
computed geometries of these substances; 4-nitrotriphenylamine
(1g) itself is exemplary. To the extent that the polar quinoid
structure makes a contribution to the overall structure of the
neutral compound, bonds a and c of 1g should be shorter than
a0 and c0 of the unsubstituted rings, and bonds b and d should be
longer than b0 and d0. Indeed, as shown in Table 2, bond a is shorter
than a0 by 0.047 Å while c is shorter than c0 by 0.030 Å; at the same
time b and d are lengthened by 0.016 and 0.004 Å, respectively.
Further, bond e should be shorter than the corresponding bond in
3-nitrotriphenylamine (1e) and bond f should be longer. The
computations support all of these expectations. Bond alternation
causes the range of ring bond lengths from shortest to longest to be
much greater (0.039 Å) in 1g than in the parent compound (1a)
(0.012 Å). The 2-nitro analog (1f) exhibits bond alternation but to
a lesser degree than 1g. The computed structure shows the reason
for this: the nitro group is twisted 35ꢀ out of the plane of the ring to
which it is attached, apparently because of unfavorable steric in-
teractions with the nearby unsubstituted ring, thus reducing its
resonance interaction with the central nitrogen atom. It is in-
teresting to note the effect of increasing nitro group substitution
upon structure: bond lengths a–d in the trinitro derivative (1m) are
almost the same as in the parent (1a), with greatly reduced bond
alternation. This presumably is because all three of the nitro groups
are competing for the electron pair on the central nitrogen atom, so
that each ring exhibits only a share of the resonance effect.
affinities, followed by Hammett
s values and the BVW s
n parame-
ters.10 In general, the reason for the poorer correlation between
Hammett values and any type of voltammetric data is the fact that
the experimental redox potential is related to the free energy dif-
ference between the neutral species and its corresponding anion or
cation radical, whereas the substituent constants
rived from measurements on the neutral compound. Similarly,
a
and
s
are de-
s
þ is
derived from measurements of the effect of the substituent on
a charged species. In contrast, the computation of either electron
affinity or ionization potential involves computing the energy of
both the neutral substance and its charged counterpart; the effect
of the substituent on both species is incorporated into the final
values. This procedure therefore emulates the experimental mea-
surement more realistically. However, to gain this advantage, it is
necessary to carry out the computation at a sufficiently rigorous
level that one can be assured that the computed energies of both
species are reliable. We showed in our earlier study that compu-
tations at the density functional B3LYP/6-31G(d) level satisfy this
criterion; the semi-empirical AM1 and PM3 methods were not as
good as Hammett values at correlating reduction potentials.10 As
noted above, the same premise underlies our inclusion of solvation
into the computation in the present work, because the cation
radicals should be particularly responsive to solvent effects. Sol-
vation energies of mono-charged ions in polar solvents are typically
in the range 35–45 kcal/mol, whereas those of neutral species are
generally less than 10 kcal/mol.23 The poorer correlation with sn
probably arises from resonance effects upon the structures of the
neutral and/or cation radical (see Section 3.2).
3.2. Substituent effects on molecular geometry
O
O
An electron diffraction study has shown that the three rings in
triphenylamine are equivalent in the gas phase at 160 ꢀC.24 Quan-
N+
N+
d
c
d'
c'
f
O
e
O
tum chemical computations at the HF/6-31G
level,24 X-ray crys-
*
b'
b
a
N
tallographic25 measurements, and both IR and Raman spectra26
agree that the molecule is planar or nearly so at the central nitrogen
atom. Advantage has been taken of this in the past to study the
dynamics of interconversion of the various conformations by var-
iable temperature measurements.27 Our computations agree with
the experimental data in showing that in the parent
a'
N
C6H5
C6H5
2b
2a
The radical cations (3) corresponding to these nitro compounds
present a different situation. The radical cation is produced by

X. Wu et al. / Tetrahedron 65 (2009) 2408–2414
2413
Table 2
Substituent effects on bond lengths (in Å) of substituted neutral triphenylamines
Bonda
1a
1e
1f
1g
1i
1m
1w
1x
1d
1v
1b
a
b
c
d
e
f
1.423
1.405
1.393
1.396
NAb
1.407
1.404
1.393
1.395
1.460c
1.230c
1.429
1.428
1.404
1.403
1.395
1.395
1.397
1.397
NAb
1.408
1.415
1.401
1.388
1.460d
1.235d
1.424
1.427
1.404
1.403
1.395
1.395
1.398
1.398
NAb
1.388
1.418
1.382
1.402
1.439
1.241
1.435
1.436
1.402
1.402
1.395
1.396
1.398
1.398
NAb
1.408
1.411
1.386
1.399
1.439
1.237
1.407
1.440
1.411
1.400
1.386
1.396
1.399
1.398
1.450
NAb
1.417
1.407
1.388
1.397
1.456
1.235
1.417
1.417
1.408
1.407
1.387
1.388
1.396
1.397
1.456
1.456
1.235
1.235
1.414
1.407
1.393
1.394
1.922
NAb
1.418
1.406
1.394
1.394
1.921
NAb
1.421
1.405
1.394
1.394
1.920
NAb
1.426
1.403
1.393
1.402
1.369
NAb
1.423
1.411
1.393
1.401
1.370
NAb
NAb
a0
a00
b0
b00
c0
c00
d0
d00
e0
e00
f0
1.423
1.423
1.400
1.405
1.395
1.395
1.398
1.398
NAb
1.426
1.426
1.404
1.404
1.395
1.395
1.398
1.398
NAb
1.418
1.428
1.405
1.404
1.394
1.395
1.394
1.398
1.920
NAb
1.421
1.421
1.405
1.405
1.394
1.394
1.394
1.394
1.920
1.920
NAb
1.419
1.419
1.405
1.405
1.394
1.394
1.398
1.398
NAb
1.423
1.423
1.411
1.411
1.393
1.393
1.401
1.401
1.370
1.370
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
1.237
NAb
NAb
NAb
NAb
f00
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
a
Letters refer to bond identifiers in structures 2 and 4.
Not applicable.
Nitro group is meta to central nitrogen atom.
Nitro group is ortho to central nitrogen atom.
b
c
d
O
removal of an electron from the lone pair on the central nitrogen
atom. This should reduce the contribution from the quinoid
structure to the resonance hybrid. In fact, a quinoid contributing
structure (3b) would have to involve donation of an electron in the
opposite sense, i.e., from an oxygen atom of the nitro group to the
central nitrogen, which should be a relatively minor but probably
real28 contributor to the resonance hybrid due to the higher elec-
tronegativity of oxygen. Thus the bond lengths in the ring bearing
the nitro group should be close to those in the unsubstituted ring.
The computations bear out this expectation remarkably well: (a)
the respective computed bond lengths in the radical cations of 4-
nitro- and 3-nitrotriphenylamine are essentially identical (Table 3);
(b) the range of computed ring bond lengths is only 0.012 Å, and (c)
the Mulliken charges are almost identical in the neutral (ꢂ0.46) and
cation (ꢂ0.41), demonstrating that structure 3b does not contribute
to the structure of 4-nitrotriphenylamine cation radical. Further-
more, the Mulliken charge on the central nitrogen atom of the
neutral amine (ꢂ0.66) is reduced compared to that on the cation
radical (ꢂ0.55), demonstrating that the electron is lost from an
orbital delocalized over the entire molecule but with the principal
contributor having the unpaired electron on nitrogen.
O
N+
e
N+
d
c
d'
c'
f
O
O
b'
b
a
+
+
N
a'
N
C6H5
C6H5
3b
3a
3.4. Effect of the methoxy group
The methoxy group interacts strongly with electron-with-
drawing groups located ortho and para to it on an aromatic ring, as
evidenced by its large sþ value.21 Oxidation of a 4-methoxy-
triphenylamine to the cation radical should therefore result in
substantial electron donation from the unshared electron pair on
oxygen (see structure 4b). It will be noted, therefore, that 4b is
isoelectronic with a benzyl radical and that quinoidal structure 4c
will be a significant contributor to the hybrid. This should result in
substantial bond alternation, with bonds c and e becoming shorter
and bonds b and d longer than in the neutral species. Once again,
the computations verify this expectation (structure 1v, Tables 2 and
Table 3
Substituent effects on bond lengths (in Å) of substituted triphenylamine cation radicals
Bonda
1a
1e
1f
1g
1i
1m
1w
1x
1d
1v
1b
a
b
c
d
e
f
1.413
1.411
1.389
1.401
NAb
1.420
1.403
1.388
1.395
1.47c
1.231c
1.410
1.409
1.413
1.413
1.388
1.388
1.402
1.402
NAb
1.408
1.415
1.401
1.388
1.46d
1.23d
1.424
1.427
1.404
1.403
1.395
1.395
1.398
1.398
NAb
1.419
1.408
1.388
1.396
1.469
1.231
1.409
1.409
1.413
1.413
1.388
1.388
1.402
1.402
NAb
1.419
1.408
1.388
1.396
1.471
1.231
1.418
1.400
1.408
1.418
1.388
1.388
1.396
1.404
1.231
NAb
1.411
1.411
1.386
1.397
1.473
1.231
1.412
1.412
1.412
1.412
1.386
1.386
1.395
1.404
1.473
1.470
1.231
1.231
1.407
1.408
1.389
1.396
1.897
NAb
1.418
1.405
1.393
1.394
1.921
NAb
1.413
1.407
1.389
1.397
1.894
NAb
1.396
1.418
1.379
1.413
1.341
NAb
1.410
1.411
1.384
1.409
1.349
NAb
1.410
1.410
1.411
1.411
1.384
1.384
1.409
1.409
1.349
1.349
NA
NAb
a0
a00
b0
b00
c0
c00
d0
d00
e0
e00
f0
1.414
1.414
1.411
1.411
1.389
1.389
1.401
1.401
NAb
1.418
1.418
1.405
1.405
1.390
1.390
1.399
1.399
NAb
1.418
1.428
1.405
1.403
1.394
1.395
1.393
1.398
1.921
NAb
1.413
1.413
1.407
1.407
1.389
1.389
1.397
1.397
1.894
1.894
NAb
1.421
1.421
1.407
1.407
1.391
1.391
1.399
1.399
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
f0
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NAb
NA
a
Letters refer to bond identifiers in structures 2 and 4.
Not applicable.
Nitro group is meta to central nitrogen atom.
Nitro group is ortho to central nitrogen atom.
b
c
d

2414
X. Wu et al. / Tetrahedron 65 (2009) 2408–2414
3. Mayers, B. T.; Fry, A. J. Org. Lett. 2006, 8, 411.
4. Wu, X.; Davis, A. P.; Fry, A. J. Org. Lett. 2007, 9, 5633.
3). In contrast to 1g, bond a of the 4-methoxy derivative 1v is ac-
tually slightly longer than bond a0 of the neutral compound, prob-
ably because of repulsion between the two electron pairs para to
each other on the nitrogen and oxygen atoms. Otherwise the bond
lengths in the substituted ring differ very little from those in the
unsubstituted ring or in the parent compound 1a. But in the cation
radical of 1v, bond alternation is evident: e.g., bonds a and c are
much shorter than either bonds a0 and c0 or the corresponding
bonds of 1a, and bonds b and d are longer than the same bonds in
the unsubstituted rings and in 1a cation radical. Because of bond
alternation, the range of computed ring bond lengths is much larger
(0.039 Å) in the substituted ring of the cation radical than in the
same ring of the neutral form (0.010 Å).
5. Zuman, P. Substituent Effects in Organic Polarography; Plenum: New York, NY,
1967.
6. Pysh, E. S.; Yang, N. C. J. Am. Chem. Soc. 1963, 85, 2124.
7. Amthor, S.; Noller, B.; Lambert, C. Chem. Phys. 2005, 316, 141.
8. 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.
9. Dapperheld, S.;Steckhan, E.;Brinkhaus, K. H. G.; Esch, T. Chem. Ber.1991,124, 2557.
10. Hicks, L. D.; Fry, A. J.; Kurzweil, V. C. Electrochim. Acta 2004, 50, 1039.
11. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.;
Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.;
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.;
Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.;
Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
Revision D.01; Gaussian,: Wallingford, CT, 2004.
+
MeO
MeO
MeO
d'
c'
d
c
e
b'
b
N
a
a'
N
N
C6H5
4a
C6H5
4b
C6H5
4c
12. Wu, X.; Dube, M. A.; Fry, A. J. Tetrahedron Lett. 2006, 47, 7667.
13. Forrest, J. J. Chem. Soc. 1960, 581.
14. Patil, N. M.; Kelkar, A. A.; Chaudhari, R. V. J. Mol. Catal. A 2004, 223, 45.
15. Walter, R. I. J. Am. Chem. Soc. 1955, 77, 5999.
16. Uemura, S.; Toshimitsu, A.; Okano, M. J. Chem. Soc., Perkin Trans. 1 1978,
1076.
4. Summary
17. (a) Chen, B.; Lee, S.; Venkatamaran, D.; DiSalvo, F.; Lobkovsky, E.; Nakayama, M.
Cryst. Growth Des. 2002, 2, 101; (b) Trost, B. M.; Curran, D. P. Tetrahedron Lett.
1981, 22, 1287.
18. Nelson, R. F.; Adams, R. N. J. Am. Chem. Soc. 1968, 90, 3925.
19. Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
The density functional computations reveal a strong resonance
interaction between the central nitrogen atom of triphenylamines
with nitro groups located para and ortho to it in the neutral amine and
with methoxy groups located para to it in the amine cation radical.
The experimental oxidation potentials are highly correlated (R¼0.995)
with ionization potentials computed by density functional theory.
20. Tomasi, J.; Persico, M. Chem. Rev. 1994, 94, 2027.
21. (a) Gordon, A. J.; Ford, R. A. The Chemist’s Companion; Wiley: New York, NY,
1972; pp 145–153; (b) Exner, O. In Correlation Analysis in Chemistry; Chapman,
N. B., Shorter, J., Eds.; Plenum: New York, NY, 1978; pp 439–540.
22. van Bekkum, H.; Verkade, P. E.; Wepster, B. M. Recl. Trav. Chim. Pays-Bas 1959,
78, 815.
Acknowledgements
23. Fry, A. J. Tetrahedron 2006, 62, 6558.
24. Naumov, V. A.; Samdal, S.; Naumov, A. V.; Gundersen, S.; Volden, H. V. Russ. J.
Gen. Chem. 2005, 75, 1956.
25. Sobolev, A. N.; Belsky, V. K.; Romm, I. P.; Chernikova, N. Y.; Gurianova, E. N. Acta
Crystallogr., C 1985, 41, 967.
26. Rodionov, A. N.; Rucheva, N. I.; Rogozhin, K. L.; Shigorin, D. N. J. Appl. Spectrosc.
1974, 20, 409.
Financial support was provided by the National Science Foun-
dation under grant #CHE-0438474 and by Wesleyan University.
L.K.S. was the recipient of a sabbatical leave from Fairfield University.
27. Hellwinkel, D.; Melan, M.; Degel, C. R. Tetrahedron 1973, 29, 1895.
28. The value of the sþ substituent constant of the nitro group is slightly smaller
References and notes
(0.740) than its
s value (0.778), demonstrating a slight mitigation of its pow-
erful inductive electron-withdrawing effect by resonance when it is para to an
atom bearing a positive charge and able to conjugate with it as in 3a.
1. Schmidt, W.; Steckhan, E. Chem. Ber. 1980, 113, 577.
2. Halas, S.; Okyne, K.; Fry, A. J. Electrochim. Acta 2003, 48, 1837.