Ladder oligo(m -aniline)s: Derivatives of azaacenes with cross-conjugated π-systems

Article abstract of DOI:10.1021/jo2025948

We describe the synthesis and electronic properties of ladder oligomers of poly(m-aniline) that may be considered as derivatives of azaacenes with cross-conjugated π-systems. Syntheses of ladder oligo(m-aniline)s with 9 and 13 collinearly fused six-member

Full text of DOI:10.1021/jo2025948

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Ladder Oligo(m-aniline)s: Derivatives of Azaacenes with
Cross-Conjugated π-Systems
Andrzej Rajca,*,† Przemysław J. Boratynski,†,§ Arnon Olankitwanit,† Kouichi Shiraishi,† Maren Pink,‡
́
and Suchada Rajca†
†Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304, United States
‡IUMSC, Department of Chemistry, Indiana University, Bloomington, Indiana 47405-7102, United States
S
* Supporting Information
ABSTRACT: We describe the synthesis and electronic proper-
ties of ladder oligomers of poly(m-aniline) that may be
considered as derivatives of azaacenes with cross-conjugated
π-systems. Syntheses of ladder oligo(m-aniline)s with 9 and 13
collinearly fused six-membered rings employed Pd-catalyzed
aminations and Friedel−Crafts-based ring closures. Structures
were confirmed by either X-ray crystallography or correlations between DFT-computed and experimental spectroscopic data
such as 1H, 13C, and 15N NMR chemical shifts and electronic absorption spectra. All compounds have planar “azaacene” moieties.
The experimental band gaps Eg ≈ 3.5−3.65 eV, determined by the UV−vis absorption onsets, were in agreement with the
TD-DFT-computed vertical excitation energies to the S1 state. Fluorescence quantum yields of up to 20% were found.
Electrochemically estimated HOMO energies of −4.8 eV suggested propensity for a facile one-electron oxidation and just
sufficient environmental stability toward oxygen (O2). For two oligomers with “tetraazanonacene” moieties, potentials of
E4+/3+ ≈ 1.6−1.7 V vs SCE were determined for four-electron oxidation to the corresponding tetraradical tetracations.
1. INTRODUCTION
π-Conjugated systems with ladder connectivity are of recent
interest because of their potential for the development of organic
materials.1−6 Ring-fused polycyclic structures may provide
superior electronic properties2−8 as well as an opportunity to
obtain molecules with well-defined, robust conformation, e.g.,
planar or helical.8−21 Examples are molecules that may be viewed
as fragments of graphene, or other conjugated carbon allotropes,
and their heterocyclic analogues.8−11,17−21
Acenes are ring-fused polycyclic structures with “collinearly-
“diazapentacene” backbones.
fused” benzene rings, such as in pentacene, which possesses
Figure 1. Triplet ground state nitroxide and aminyl diradicals with
favorable p-type semiconducting properties.22,23 The search for
improved semiconducting and materials properties, as well as their
interesting biradical electronic structures,24 have driven recent
synthetic effort in the preparation and studies of extended acenes
with up to nine fused rings,25,26 and acenes with solubilizing
pendants.27 This effort includes azaacenes and hydroazaacenes for
development of n-type semiconductors.28−32 In general, the
extension of acene conjugated π-system beyond five fused rings
faces a trade-off with lowered stability at ambient conditions.
Recently, we reported the isolation of a triplet (S = 1) ground-
state nitroxide and aminyl diradicals, derivatives of diazapenta-
cene (Figure 1).33−36
These diradicals illustrated the potential of the azaacene-
based backbone as building block for high-spin aminyl
polyradicals. In particular for aminyl diradical, the planar
“diazapentacene” backbone demonstrated the balance between
maintaining an effective 2pπ−2pπ overlap between the radicals,
which enhances electron spin−spin interactions (exchange
coupling) and provides an adequate protection for the atoms
with significant spin densities that is required for isolation.35
We plan to investigate higher homologues of the aminyl
diradical derived from diazapentacene and have carried out the
synthesis of ladder oligo(m-aniline)s 2−4 (Figure 2),37 which
we expect will facilitate the preparation of ladder high-spin poly-
radicals,38 relevant to the design of organic magnetic materials.8,39−42
While pursuing the synthesis of 2−4, we recognized their
interesting electronic structure, including that of the previously
reported diamine 1, which may be relevant to materials
applications.
Oligoanilines and related heterocyclic compounds with a
two-dimensional ladder-like structural motif are of interest for
electronic materials. The ladder polyhydro-azaborines with
oligo(m-aniline)-like connectivity (and up to seven fused rings)
have been reported. One of the polyhydro-azaborines, a
Received: December 26, 2011
Published: February 13, 2012
© 2012 American Chemical Society
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Figure 2. Ladder oligo(m-aniline)s: ladder oligoamines 1−4.
derivative of tetrahydro-5,7-diaza-12,14-diborapentacene, pos-
sessed fluorescence quantum yields of about 98% in cyclo-
hexane at room temperature.43 Analogous structures with five
fused rings based upon oxygen−nitrogen- and sulfur−nitrogen-
containing heterocycles have been studied as hole injection
materials in light-emitting diodes. For example, emitting devices
constructed from 5,7,12,14-tetrahydro-5,7-bis(1-naphthyl)-5,7-
diaza-12,14-dithiopentacene showed a long half-life of 90 h
at high current densities (50 mA/cm2) and a high emitting
efficiency (2.6 lm/W).44 The ladder oligo(p-aniline) structures
containing indolocarbazole units with up to nine fused rings
have been synthesized,45 and an oligomer with five fused rings
was reported to provide semiconductors for p-type field-effect
transistors with promising mobilities (0.2 cm2 V−1 s−1) and
on/off current ratios (>106).46
From another point of view, we consider oligo(m-aniline)s
1−4 as derivatives of azaacenes (hydroazaacenes) with cross-
conjugated π-systems.47 The corresponding hydroazaacene
backbones, tetrahydrodiazapentacene, octahydrotetraazanon-
acene, and dodecahydrohexaazatridecacene, with up to 13
fused rings, are illustrated in Figure 3. Cross-conjugation is
expected to improve environmental stability of hydroazaacenes
and result in a larger band gap with increased transparency in
the visible region, compared to acenes.19,48,49 Cross-conjugated
π-systems are being explored for electron transport in
molecular junctions, optical materials, and sensors.50,51
Figure 3. Ladder oligo(m-aniline)s: hydroazaacenes with cross-
conjugated π-systems.
fused six-membered rings (Figure 2). Ladder oligoamines 1−4
were characterized by UV−vis absorption and fluorescence
spectroscopy and voltammetry. Both experiments and compu-
tations (DFT and TD-DFT) indicate that within the series of 1,
3, and 4, HOMO energies and band gaps show negligible
change between tetraamine 3 and hexamine 4.
2. RESULTS AND DISCUSSION
2.1. Synthesis of Ladder Tetraamine 2. Our synthetic
route to ladder tetraamine 2 is based upon the palladium-
catalyzed C−N coupling reactions (aminations), followed by
We report efficient syntheses of the extended derivatives of
hydroazacenes, ladder oligoamines 2−4 with up to 13 collinearly
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annelation forming six-membered rings (Scheme 1). In the first
step, amination of 4,6-diisopropenyl-1,3-dibromobenzene36
Scheme 2. Synthesis of Building Blocks 11 and 14 for
Tetraamine 3 and Hexaamine 4
Scheme 1. Synthesis of Ladder Tetraamines 2 and 5
with 4-tert-butylaniline using Pd(OAc)2/1,1′-bis(diphenyl-
phosphino)ferrocene (DPPF) as catalyst52 provides mono-
amine 6. In the subsequent C−N coupling reactions of 6 with
2,6-diaminotoluene and 1,3-diaminobenzene, a more reactive
catalyst such as Pd(OAc)2/P(t-Bu)3 is used37a,53,54 to provide
tetraamines 7 and 8, respectively. Quadruple ring closure of 7
in 85% phosphoric acid, via Friedel−Crafts-type electrophilic
aromatic substitution,33a,36,55 gives ladder tetraamine 2 with
nine collinearly fused rings in very good yields (90−95% per
ring closure); from the analogous reaction of 8 in phosphoric
acid (or 1:1 phosphoric/sulfuric acid mixture), only an
“angular” tetraamine 5 is isolated in low yields.56 These results
suggest that the “blocking” group, such as methyl group in 2, is
essential for the synthesis of collinearly fused oligo(m-anilines).
2.2. Synthesis of Ladder Tetraamine 3 and Hexaamine
4. Because the ladder tetraamine 2 (and 5) is poorly soluble in
common organic solvents, we sought to modify the synthesis by
introducing solubilizing groups, such as 4-tert-butyl-phenyl
pendants34 and tert-undecyl chains.35 This can be accomplished
by the preparation of building blocks 11 and 14, which are
analogous to 6 and 2,6-diaminotoluene in the synthesis of 2. As
shown in Scheme 2, building block 11 and 14 are prepared from
2-methyl-2-phenyldecane35 and 1,3-dibromo-2-iodobenzene.57
The C−N coupling reaction of 11 with 14 in a 2.2:1 molar
ratio gives tetraamine 15 in 45−62% isolated yields (Scheme 3).
Hexaamine 17, is obtained via two consecutive C−N coupling
reactions: (1) 11 and 14 in the 1:1 molar ratio provide triamine
16 in 34−43% isolated yields and (2) triamine 16 and 4,6-
diisopropenyl-1,3-dibromobenzene in a 2:1 molar ratio give 17
in 41−48% isolated yields.
Ladder tetraamine 3 and hexaamine 4 are obtained from 15
and 17, respectively, by Friedel−Crafts-type ring closures in a
1:1 mixture of 85% phosphoric acid and 98% sulfuric acid at
1
55−60 °C for about 15 min under nitrogen atmosphere. H
NMR spectra of selected crude reaction mixtures indicate
excellent yields for 3 and 4, specifically almost quantitative
yields for 3 (Figures S77, S78, and S83, Supporting
Information). Even though small amounts of starting material
and byproduct are observed in some other crude reaction
mixtures, this result indicates an extraordinary efficiency for
the closures of four and six rings in 3 and 4, respectively.
Purification of the crude reaction mixtures by flash chromatog-
raphy at 0 °C on silica deactivated with triethylamine provides a
range of isolated yields of 3 and 4 (Scheme 3), perhaps due to
the sensitivity of the ladder oligoamines to acidic environments
such as silica, even after deactivation with triethylamine.58 In
the case of 4, another complicating factor in chromatographic
purification is its modest solubility in typical organic solvents.
2.3. Structure of Ladder Oligoamines. X-ray Structure
of 2. The structure of tetraamine 2 is established by X-ray
crystallography using synchrotron radiation (Figure 4). The
nine collinearly fused six-membered rings form an approx-
imately planar π-system; the mean deviation from a calculated
least-squares plane (C1−C34, N1−N4) is 0.0559 Å, which may
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Scheme 3. Synthesis of Tetraamine 3 and Hexaamine 4
1
by the assignment of their experimental H and 13C NMR
spectra in acetone-d6 or benzene-d6 using standard 2D NMR spectros-
1
1
copy, including H−13C HSQC, long-range H−13C HMBC,
1H−1H NOESY, and 1H−15N HSQC (Figures S36−S47,
Supporting Information).59 Diamine 1, for which the X-ray
structure has been reported,34 is subjected to similar NMR
experiments and serves as a reference for 3 and 4.
Additional evidence in support of ladder structures 3 and 4 is
obtained from the correlation between the DFT-calculated and
experimental H and 13C NMR chemical shifts. Diamine 1, as
1
Figure 4. Molecular structure and conformation for 2: (A) top view of
2; (B) side view of 2. Carbon and nitrogen atoms are depicted with
thermal ellipsoids set at the 50% probability level; disorder is omitted
for clarity. In the side view, hydrogen atoms are omitted. Further details
are reported in Table S1 and Figures S1−S3, Supporting Information.
well as the simplified structures 3a and 4a corresponding to 3
and 4, in which the tert-undecyl groups are replaced with tert-
butyl groups, are studied (Figure 5). Geometries of 1, 3a, 4a, and
tetramethylsilane were optimized at the B3LYP/6-31G(d,p) level
of theory and confirmed as minima by vibrational analyses.60
In all structures, the “azaacene” moieties are planar, similar to that
found in the X-ray structures of 134 and 2 (Figure 4). Also, the
dihedral angle of 75.82(4)° between the planes of the azaacene
moiety and the tert-butylphenyl pendant in the X-ray structure of
1 is in good agreement with the corresponding torsion angles of
about 75.1° in the DFT-optimized geometry of 1.
be compared to the mean deviation of 0.0763 Å found for the
analogous plane of five collinearly fused six-membered rings
in diamine 1.34 The computed structure of 2 at the B3LYP/
6-31G(d,p) level also provides a similar planar geometry, though
the out-of-plane vibrational mode with the lowest frequency is in
the range of about 10 cm−1 (Table S4, Supporting Information),
indicating considerable flexibility for the “tetraazanonacene” moiety.
Interestingly, tetraamine 2 shows in-plane bending of the
ladder structure. This bending may be associated with the
presence of gem-dimethyl (C(Me)2) bridges with eight C(sp3)−
C(sp2) bonds of about 1.5 Å that are longer than C(sp2)−
C(sp2) and C−N bond lengths of about 1.4 Å (for further
analyses, see Figure 6 and Table 2).
1
Using these DFT-optimized geometries, H and 13C NMR
isotropic shieldings for 1, 3a, 4a, and tetramethylsilane are
computed at the GIAO/B3LYP/6-31G(d,p) level of theory with
the IEF-PCM-UA0 solvent model for acetone or benzene
(Gaussian 03),60−62 to provide calculated H and 13C NMR
1
chemical shifts δDFT(1H) and δDFT(13C) for direct comparison
with the experimental NMR spectra of 1 and 3 in acetone-d6 and
4 in benzene-d6.33c The correlations between DFT-computed
and experimental 13C NMR chemical shifts give correlation
coefficients (R2) that are expected for correctly assigned
Determination of Structures 3 and 4 Using NMR
Spectroscopy and DFT-Computed NMR Chemical Shifts.
The structures of tetraamine 3 and hexaamine 4 are confirmed
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Figure 5. (A) Correlations between calculated (δDFT) and experimental (δexpt
)
13C NMR chemical shifts for 1, 3a, 4a and 1, 3, 4, respectively. Solid
lines are the best fit to δDFT = a + b δexpt. (B−D) Difference between scaled (δscaled = (δDFT − a)/b) and experimental (δexpt)
13C NMR chemical
shifts; highlighted differences correspond to carbon atoms on the “outer rim” of the azaacene moieties. Statistical parameters are summarized in
Table 1. Further details are reported in Table S6, Supporting Information.
structures of medium sized organic molecules (Figure 5A and
Table 1).62,63
The relationship between computed and experimental NMR
chemical shifts is further illustrated by applying the correlations
to scale linearly δDFT(13C), to provide δscaled(13C), and then by
plots of the differences between the scaled and experimental
NMR chemical shifts for each distinct carbon atom in the
structure (Figures 5B−D).33c,62 These plots confirm the good
agreement between the theory and experiment, and specifically
the low values of statistical parameters for 13C NMR chemical
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Table 1. Statistical Parameters for the Correlations of 13C
a
1
and H NMR Chemical Shifts
a
b
R2
MaxErr
CMAE
1 (13C)
5.317
5.404
0.9230
0.9204
0.9178
1.0212
1.0206
1.0125
0.9966
0.9962
0.9957
0.9966
0.9912
0.9942
6.08
6.22
6.77
0.40
0.73
0.39
2.106
2.211
2.206
0.112
0.169
0.172
3/3a (13C)
4/4a (13C)
1 (1H)
5.339
−0.026
−0.072
−0.075
Figure 6. B3LYP/6-31G(d,p) geometry of octaamine 18 and
definition of radius (R) for “bent” ladder.
3/3a (1H)
4/4a (1H)
a
a and b are the intercept and slope of the linear fit to δDFT = a + b
the distance between terminal carbon atoms of the inner rim
(or chord of the circle) and α is the segment CCC angle defined
in Figure 6.
Analyses of DFT-optimized geometries for homologous di-,
tetra-, hexa-, and octaamines, as well as X-ray structures of 1
and 2, suggest that the bending of the ladder corresponds to
a circle radius of 50−60 Å (Table 2). A complete circle would
δexpt, and R2 is its correlation coefficient. MaxErr is the maximum
corrected absolute error with respect to the linear fit (|δscaled − δexpt|).
CMAE is the corrected mean absolute error for n chemical shifts
((∑|δscaled − δexpt|/n).
shifts such as MaxErr and CMAE in Table 1, as expected for
correctly assigned structures. Furthermore, the plots reveal the
presence of similar systematic errors in the structures of diamine,
tetraamine, and hexaamine. For example, 13C NMR chemical
shifts for carbon atoms on the “outer rim” of the azaacene
moieties are consistently overestimated by DFT by about
4−7 ppm (chemical shift differences highlighted in Figures 5B−
D). Because the structure of diamine 1 was established by X-ray
crystallography, the 13C NMR chemical shift correlations that
include 1 confirm structures of the higher homologues 3 and 4.
Similar analyses of the relationship between computed and
experimental 1H NMR chemical shifts are summarized in Table 1
and illustrated in the Supporting Information (Figures S27−S32).
As the IEF-PCM solvent model does not properly account for
specific solvent−solute interactions such as hydrogen bonding,64
the protons of NH moieties are not included in the analyses.
As DFT computations of 1H NMR chemical shifts tend to be less
accurate, overall agreement between theory and experiment is less
Table 2. “Bent” Ladder Geometry of X-Ray Structures and
DFT-Optimized Geometries at the B3LYP/6-31G(d,p) Level
d (Å)
α (deg)
X-ray
R (Å)
compd
X-ray
DFT
DFT
X-ray
DFT
1
9.617
9.627
19.13
19.14
28.48
37.58
174.93
171.11
175.6
168.7
169.5
163.4
158.3
54.4
62.1
62.3
47.1
52.3
50.0
50.9
2
19.185
3a
4a
18
require about 130 fused rings, corresponding to hexaamine 4
extended 10 times.
These results are consistent with the radii obtained for the
outer rims of the ladders. For tetra-, hexa- and octaamines, radii
for the outer rim corrected for the rim-to-rim distance have the
values within 2 Å of the corresponding radii for the inner rim in
Table 2; diamine 1 shows larger deviations of about 5 Å.
2.4. Electronic Structure of Oligoamines 1−4. UV−vis
Absorption and Fluorescence Spectroscopy. Absorption
spectra for 1−4 in cyclohexane show similar spectral envelopes
with λmax = 275−278 nm and εmax increasing with the number
of diarylamine moieties (Figure 7). Onsets of absorption (λonset),
determined by the tangent method (Figure S9, Supporting
Information), are dependent both on the length of the ladder
and the pendant group (methyl vs 4-tert-butylphenyl); values
of λonset appear to saturate for 3 and 4, the cross-conjugated
oligo(m-aniline)s ladders with 4-tert-butylphenyl pendants
(Table 3). The band gap, Eg ≈ 3.5 eV, is estimated for such
ladder oligomers.
Fluorescence spectra of 1, 3, and 4 are nearly identical,
showing featureless bands with λmax ≈ 400 nm and large Stokes
shifts of over 120 nm. Quantum yields of fluorescence, ϕf, are
on the order of 1% only. These findings suggest that the
emitting excited state has a relatively low energy and possesses
geometry that is significantly different from the ground state.
Most likely, torsional angles of the biphenyl moieties are
decreased in the excited state. Such fluorescence properties and
changes in geometry of excited states are well-known for
biphenyls, including N,N-diethyl-2-aminobiphenyl.65
33c,61
satisfactory for H, compared to 13C NMR chemical shifts.
1
1H−15N HSQC spectra for 1, 3, and 4 show the expected
number of cross-peaks between H and 15N confirming the
1
presence of one, two, and three nonequivalent NH groups,
respectively (Figures S33, S36, and S42, Supporting Informa-
tion). The 15N NMR chemical shifts are clustered around δ =
−286 ppm (relative to nitromethane at 0.0 ppm), which is within
the typical range for NH groups in amines; also, DFT-calculated
15N NMR chemical shifts for these NH groups are within 2 ppm
of the experimental values (Table S7, Supporting Information).
For angularly annelated ladder tetraamine 5, characterization
by NMR spectroscopy was hindered by its air-sensitivity, poor
solubility, and limited availability. 1H NMR spectra (1D and 2D
COSY) of 5 indicate the presence of two AMX (or ABX) spin
coupling systems for 4-tert-butyl-substituted benzene rings and
AX (or AB) set of doublets for the center benzene ring; the other
two benzene rings appear as four 1-proton singlets. Another set
of four H/D exchangeable 1-proton singlets are assigned to the
NH groups. In the aliphatic region, two distinct gem-dimethyl
moieties with diastereotopic methyl groups (4 different methyl
groups) and two distinct tert-butyl groups are found.
Bent Ladder Geometry. As illustrated by the X-ray structure
of tetraamine 2 (Figure 2), analogous ladder oligoamines with
C(Me2) bridges are expected to show in-plane bending of the
ladder. For each bent ladder oligoamine, the carbon and
nitrogen atoms at the inner rim may be viewed as forming an
arc, a part of circumference of a circle, as illustrated for the DFT-
computed structure of octaamine 18 in Figure 6. Specifically, the
circle is defined by the radius, R = d/(2sin(180 − α)), where d is
For comparison, the fluorescence band of 2 with pendant
methyl group shows vibrational structure and considerably
smaller Stokes shift (∼80 nm). The fluorescence quantum yield,
ϕf = 23%, is significantly increased. The intersection between
the normalized fluorescence spectrum and either normalized
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Figure 7. UV−vis electronic absorption and fluorescence spectra for
ladder diamine 1, tetraamines 2 and 3, and hexaamine 4 in cyclohexane.
Spectral parameters are summarized in Table 3. Excitation spectra are
plotted in Figures S5−S8, Supporting Information.
absorption or excitation spectrum is at a wavelength of 340 nm, the
same as the onset of absorption, corresponding to Eg = 3.65 eV.
The band gaps of ∼3.5 and 3.65 eV may be compared to the
experimental excitation energies of 4.00 and 3.31 eV for
naphthalene and anthracene.66
Figure 8. Cyclic voltammograms (CV, 50 mV s−1) and square wave
voltammograms (SWV, frequency 10 Hz, pulse height 25 mV) for ladder
diamine 1, tetraamines 2 and 3, and hexaamine 4 in 0.1 M
[n-Bu4N]+[PF6]− in dichloromethane, and internally referenced to
Cp*2Fe+/0 (−0.130 V vs SCE).73 Voltammograms including deca-
methylferrocene reference are shown in Figures S10−S16, Supporting
Information. Oxidation potentials are summarized in Table 4.
Electrochemistry. Cyclic voltammograms indicate reversible
oxidations in the 0.5−1.1 V range corresponding to the removal
of up to two electrons for 1, three electrons for 2 and 3, and
four electrons for 4 (Figure 8). The first oxidation potentials
E1+/0 (and onsets of oxidation) are similar for 1−4, E1+/0
≈
0.6 V for 1 and 0.5 V for 2 − 4 (Table 4), because it is expected
that the aminium radical cations formed by one-electron
oxidation of cross-conjugated oligoamines will be localized on a
single diarylaminium moiety. Such charge/spin localization was
reported in the isoelectronic radical anions formed by oxidation
of cross-conjugated carbopolyanions.67 The first oxidation
potentials E1+/0 ≈ +0.5 V (vs SCE) of oligoamines 2−4 is low
enough for facile oxidation and just high enough for environ-
mental stability at ambient conditions with respect to oxidation
by O2.68
The potential separation between the first two one-electron
oxidations decreases from 0.56 V in diamine 1, to 0.10−0.14 V
in tetraamines 2 and 3, and it becomes unresolvable in
hexaamine 4. The subsequent one-electron oxidations in 2−4
become more difficult with potentials increasing in 0.4−0.5 V
increments. For example, the fourth oxidation potentials of 2
and 3, corresponding to the formation of tetraradical tetracat-
ions, are much more positive, E4+/3+ ≈ 1.6−1.7 V (Table 4),
and could be measured with square wave voltammetry
(Figures S17 and S18, Supporting Information).72 These trends
in oxidation potentials may be ascribed to stronger and weaker
repulsive electrostatic interactions between aminium radical
cations localized on the nearest neighbor and non-nearest
neighbor diarylaminium moieties, respectively. For example, in
the diradical dication of 2, two aminium radical cations may be
The onsets of oxidation suggest that the HOMO energy is
about −4.9 eV in diamine 1 and −4.8 eV in the tetraamines
and the hexaamine (Table 4).69,70 These energies are similar to
those found in EDOT/benzo[c]thiophene terthiophene71
[EDOT = 3,4-ethylenedioxythiophene] and they are higher
than −5.1 eV for pentacene.27b
a
Table 3. Optical Properties for Ladder Oligoamines 1−4
absorption
fluorescence
a
b
c
d
εmax
λonset
band gap Eg
excitation spectrum λmax
ϕf
Stokes shift
(nm)
λmax (nm)
277
(104 L mol−1 cm−1
)
(nm)
(eV)
(nm)
λexc (nm)
λmax (nm)
397
(%)
1
2
3
4
4.2
333
3.72
3.65
3.52
274
276
274
275
283
1.4
23
123
∼80
127
124
275, 300 (sh)
9.7, 4.3 (sh)
8.6
340
275, 300 351, 358, 365
275, 320 401
276
278
352.5
351.5
1.6
1.0
19.7
3.53
277, 313 399
a
b
Band gap estimated form the onset of absorption (λonset). Excitation spectra observed at the fluorescence maximum (Figures S5−S8 Supporting
c
Information). Fluorescence quantum yield in cyclohexane calculated with quinine sulfate as standard; average from two excitation wavelengths for
2−4. Stokes shift estimated from excitation and fluorescence spectra λmax
d
.
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a
(Table 5). The computed energy of the LUMO is about
0.3 eV higher for methyl-substituted tetraamine 2 and unsub-
stituted derivatives 19 and 20 (Chart 1) than that for 4-tert-
Table 4. Oxidation Potentials for Ladder Oligoamines 1−4
b
oxidation
potentials
CV EHOMO
ox
ox
compd
CV E1/2 (V)
SWV Ep (V)
(eV)
1
E+/0
E2+/+
E+/0
E2+/+
E3+/2+
E4+/3+
E+/0
E2+/+
E3+/2+
E4+/3+
E2+/0
E3+/2+
E4+/3+
0.601 0.004
1.158 0.004
0.494 0.014
0.628 0.014
1.087 0.014
0.600 0.004
1.160 0.004
0.496 0.002
0.638 0.002
1.084 0.004
1.663 0.002
0.525 0.008
0.622 0.004
1.098 0.006
1.654 0.026
−4.9
Chart 1. Structures of Diamine 19 and Tetraamine 20
2
3
4
−4.8
0.516 0.008
0.612 0.002
1.092 0.004
−4.8
−4.8
0.544 0.006
0.743 0.020
1.092 0.016
butylphenyl-substituted 1, 3a, and 4a; consequently, the
HOMO−LUMO gap (Δε) for 2 (as well as 19 and 20) is
correspondingly higher than the “converged” Δε ≈ 4 eV for 3a,
4a, and “homologous” octaamine 18. These Δε overestimate
experimental band gaps (Eg) by about 0.5 eV for 1, 3, and 4 and
by 0.7−0.8 eV for 2.
TD-DFT computed UV−vis spectra for 1, 2, 3a, and 4a are
in agreement with the experimental spectra for 1−4; slight
red-shift for the Gaussian envelope of the TD-DFT spectra
obtained with B3LYP functional is observed (Figure 9).74,75 In
contrast to Δε, TD-DFT computed vertical excitation energies
to the lowest excited state (ΔE(S0−S1)) reproduce the experi-
mental Eg quite well.75 Specifically, values of ΔE(S0−S1) are
within about 0.2 eV of experimental Eg for 1 and 2; for 3a and
4a, values of ΔE(S0−S1) are nearly coincident with Eg for 3 and
4. Values of ΔE(S0−S1) for 2, 19, and 20 are within 0.02 eV,
suggesting that the effect of the methyl group in 2 on the band
gap is very small (Table 5).
a
Potentials (vs SCE) from cyclic voltammetry (CV) and square wave
voltammetry (SWV) based on average values from multiple
voltammograms with scan rates of 20−500 or 50−500 mV s−1 (and
with frequency of 10 Hz) in 0.1 M [n-Bu4N]+[PF6]− in dichloro-
methane; 100 μm Pt-disk as working electrode. The potentials are
calibrated with decamethylferrocene as the internal standard (−0.130
V vs SCE for Cp*2Fe+/0 in dichloromethane).73 Calculated using the
b
empirical relationship EHOMO = −(Eoxonset + 4.4);69,70 further details in
Figures S10−S16, Supporting Information.
localized on non-nearest neighbor diarylaminium moieties, however,
in the triradical trication one strong repulsive interaction to the
nearest neighbor diarylaminium moieties is added, and in
tetraradical tetracation two more such interactions are added.
Square wave voltammograms of 2 and 3 (and cyclic voltammo-
grams) show that the onsets of reduction of tetraamines are at
potentials that are more negative than about −2.2 V (vs SCE),
which is approximately the potential limit for our supporting
electrolyte, [n-Bu4N]+[PF6]− in dichloromethane, and Pt-
electrode (Figures S19−S22, Supporting Information).72 Because
onsets of oxidation are at about +0.4 V (vs SCE), these results
indicate that the band gap Eg > 2.6 V, in qualitative agreement
with Eg = 3.65 eV and Eg = 3.52 eV obtained from optical data for
2 and 3, respectively.
Computed Band Gaps. DFT energies of frontier MOs and
time-dependent DFT (TD-DFT) vertical excitations were
calculated at the B3LYP/6-31G(d,p) level of theory with the
IEF-PCM-UA0 solvent model for cyclohexane using B3LYP/
6-31G(d,p)-optimized geometries in the gas phase.60
DFT energies of HOMOs for 1, 2, 3a, and 4a show a similar
trend for the experimental values of HOMOs for 1−4 obtained
from electrochemical experiments, though computations using
a relatively small double-ξ basis set overestimate the
experimental values in dichloromethane by 0.4−0.5 eV
3. CONCLUSION
Ladder oligomers of poly(m-aniline), with up to 13 collinearly
fused six-membered rings were prepared by Pd-catalyzed amina-
tions and Friedel−Crafts-based ring closures. Such oligomers
may be considered as derivatives of hydroazaacenes with cross-
conjugated π-systems. The experimental Eg ≈ 3.5 eV for
tetraamine 3 and hexaamine 4 (with 4-tert-butylphenyl
pendants), determined by the UV−vis absorption onsets, is in
excellent agreement with the TD-DFT-computed vertical
excitation energies to the S1 state (3.5 eV) for the analogous
model tetraamine and hexaamine. In comparison, the experi-
mental Eg for tetraamine 2 (with methyl pendant) is somewhat
higher, Eg ≈ 3.65 eV, with a TD-DFT excitation energy of 3.87 eV.
Among the ladder oligoamines 1−4 (Figure 2), the optical
Table 5. Energies of Frontier Molecular Orbitals (εHOMO and εLUMO), HOMO−LUMO Gaps (Δε), and Vertical Excitation
a
Energies to the S1 State (ΔE(S0−S1), in eV) for Ladder Oligoamines in Cyclohexane
compd
εHOMO
εHOMO (expt)
εLUMO
Δε
ΔE(S0−S1)
Eg (expt)
diamine 1
−4.537 (−4.496)
−4.489 (−4.489)
−4.450 (−4.424)
−4.411 (−4.364)
−4.394 (−4.337)
−4.561 (−4.582)
−4.466 (−4.490)
−4.9
−4.8
−4.8
−4.8
−0.371 (−0.425)
−0.082 (−0.079)
−0.388 (−0.460)
−0.387 (−0.454)
−0.412 (−0.471)
−0.059 (−0.069)
−0.068 (−0.081)
4.17 (4.07)
4.41 (4.41)
4.06 (3.96)
4.02 (3.91)
3.98 (3.87)
4.50 (4.51)
4.40 (4.41)
3.56
3.87
3.50
3.49
3.46
3.88
3.86
3.72
3.65
3.52
3.53
tetraamine 2
tetraamine 3a
hexaamine 4a
octaamine 18
diamine 19
tetraamine 20
a
B3LYP/6-31G(d,p) values in the gas phase are given in parentheses.
2114
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(nitromethane) = 0 ppm scale,76 using the relation: δ 15N (urea) =
δ
15N (nitromethane) + 304.7 ppm.77
IR spectra were obtained using an instrument equipped with an
ATR sampling accessory. For HR-FABMS, 3-nitrobenzylalcohol
(3-NBA) and 1-(2-nitrophenoxy)octane (o-nitrophenyl octyl ether =
ONPOE) were used as matrices.
DFT and TD-DFT computations were carried out using an 8-CPU
workstation running Gaussian 03 and/or Gaussian 09.60 All optimized
geometries had rms forces in Cartesian coordinates of about 1.3 × 10−6
a.u or less, except for hexaamine 4a, for which the rms forces were
3.5 × 10−6 au. All reported computed structures are minima on the
B3LYP/6-31G(d,p) gas phase potential energy surface, as determined
by vibrational analyzes (Table S4, Supporting Information).
X-ray Crystallography. Crystals of 2 for X-ray studies were
prepared by sublimation under high vacuum. Data collections were
performed at the Advanced Photon Source, Argonne National
Laboratory in Chicago, using synchrotron radiation (λ = 0.49595 Å).
Final cell constants were calculated from the xyz centroids of strong
reflections from the actual data collection after integration (SAINT);78
intensity data were corrected for absorption (SADABS).79 The space
group C2 was determined based on intensity statistics and systematic
absences. The structure was solved with direct methods using Sir200480
and refined with full-matrix least-squares/difference Fourier cycles
using SHELXL-97.81 All non-hydrogen atoms were refined with
anisotropic displacement parameters. The hydrogen atoms were placed
in ideal positions and refined as riding atoms with relative isotropic
displacement parameters. Disorder was refined for both tert-butyl
groups. Crystal and structure refinement data for 2 are in the
Supporting Information and the accompanying file in CIF format.
UV−vis and Fluorescence Spectroscopy. UV−vis absorption
and fluorescence spectra were obtained in cyclohexane. Fluorescence
quantum yields (ϕf) were determined in reference to quinine sulfate
solution in 0.5 N H2SO4 (ϕf = 0.546) at 297 K.
Electrochemistry. Cyclic voltammetry and square wave voltam-
metry data were obtained under argon atmosphere using a commercial
potentiostat/galvanostat. About 1 mg of 1−4 was dissolved in the
supporting electrolyte solution (2.4 mL), 0.1 M tetrabutylammonium
hexafluorophosphate (n-Bu4NPF6) in anhydrous DCM, and then
transferred to a custom-made cell equipped with quasi-reference
(Ag wire), counter (Pt foil), and working (100 μm Pt-disk) electrodes.
The redox potentials were referenced to SCE using an internal
decamethylferrocene (−0.130 V vs SCE for Cp*2Fe/Cp*2Fe+ in
CH2Cl2).73 For each compound, plots of peak current vs square root
of scan rate (50−500 or 20−500 mV/s) in cyclic voltammetry were
linear. A detailed description of the voltammetry may be found in the
Supporting Information.
Synthesis. Standard techniques for synthesis under inert atmo-
sphere (argon or nitrogen), using custom-made Schlenk glassware,
custom-made double manifold high vacuum lines, and argon-filled
Vacuum Atmospheres gloveboxes, were employed. Chromatographic
separations were carried out using normal phase silica gel. For selected
separations, chromatography was carried out at 0 °C and/or silica gel
was deactivated by treatment with 2−5% of triethylamine in pentane or
hexane.58
General Procedures for Pd-Catalyzed Aminations Leading
to Bromoamines 6 and 11 (Partial Br→NH Exchange). 1,3-
Dibromo-4,6-diisopropenyl-benzene (1.5−2.0 mmol), sodium tert-
butoxide (1.5 equiv), Pd(OAc)2 (5−10 mol %), and DPPF (15−17
mol %) were charged to a Schlenk vessel under N2. 4-tert-Alkylaniline
(0.6−1.3 equiv) in toluene (7.5 mL) was added, and the reaction
mixture was stirred for 1−3 d at 90 °C. Then the mixture was filtered
through a plug of silica gel with pentane/benzene 9:1, evaporated, and
purified by column chromatography.
Figure 9. UV−vis electronic absorption spectra computed at the TD-
B3LYP/6-31G(d,p) level with the IEF-PCM-UA0 solvent model for
cyclohexane. Gaussian-function convolutions (fwhm = 0.1 eV) and
vertical scaling are applied to match experimental spectra in
cyclohexane. Oscillator strengths, f, for vertical transitions are shown
as stick plots.
properties of 2 such as large Eg and fluorescence quantum yield
of about 20% are most representative of parent hydroazaacenes
with cross-conjugated π-systems (Figure 3). Electrochemically
estimated HOMO energies of −4.8 eV for 2−4 suggest
propensity for a facile one-electron oxidation and just sufficient
environmental stability toward oxygen (O2).
4. EXPERIMENTAL SECTION
NMR and IR Spectroscopy, Mass Spectrometry, and
Computations. NMR spectra (1H, 400, 500, and 600 MHz) were
obtained using benzene-d6, acetone-d6, and chloroform-d (CDCl3) as
solvent. The 500 MHz instrument was equipped with a cryoprobe.
The chemical shift references were as follows: (1H) benzene-d5,
7.15 ppm; (13C) benzene-d5, 128.39 ppm (benzene-d6), (1H) acetone-
d5, 2.05 ppm; (13C) acetone-d5, 29.92 ppm (acetone-d6), (1H)
chloroform, 7.26 ppm; (13C) CDCl3, 77.0 ppm (CDCl3). Natural
abundance 1H−15N HSQC spectra were externally referenced to
15N2-labeled urea (98+% 15N, 0.7 mg in 0.4 mL of acetone-d6). The
reported 15N chemical shifts were converted to the δ 15N
Preparation of Bromoamine 6. According to the general pro-
cedure, 1,3-dibromo-4,6-diisopropenylbenzene (0.472 g, 1.49 mmol)
and 4-tert-butylaniline (0.32 mL, 2.00 mmol) provided 0.288 g (55%)
of 6 as a colorless oil in 50% yield after chromatography (deactivated
1
silica gel, pentane/benzene, 96:4). H NMR (400 MHz, acetone-d6):
δ 7.358 (s, 1H), 7.343 (d, J = 8.8 Hz, 2H), 7.077 (d, J = 8.4 Hz,
2H), 7.005 (s, 1H), ∼6.58 (br. s, 1H), 5.262 (m, 1H), 5.192 (m, 1H),
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1
5.072 (m, 1H), 4.911 (m, 1H), 2.069 (dd, J = 1.6, 0.8 Hz, 3H), 2.051
(dd, J = ∼1.5, ∼1 Hz, 3H), 1.302 (s, 9H). 13C NMR (100 MHz,
acetone-d6): δ 146.3, 145.3, 143.9, 142.1, 141.3, 136.7, 133.5, 130.7,
127.0, 120.72, 120.67, 120.0, 117.2, 116.4, 34.8, 31.8, 24.1, 23.5. IR
(ZnSe): 3414, 3073, 2964, 2909, 1590, 1514, 1371, 1323, 1258, 1191
cm−1. HRMS (FAB): calcd for [C22H26N79Br]+• 383.1243, found
383.1259; calcd for [C22H26N81Br]+• 385.1223, found 385.1230.
Preparation of Bromoamine 11. According to the general pro-
cedure 1,3-dibromo-4,6-diisopropenylbenzene (1.15 g, 3.67 mmol, 1.85
equiv) and 4-tert-undecylaniline (10) (0.490 g, 1.98 mmol) provided
391 mg (41%) of 11 as light reddish brown liquid after chromatography
(ethyl acetate/hexane gradient 1:20, 1:10, and 1:5). Rf = 0.41 (Et2O/
hexane, 1:20). 1H NMR (400 MHz, acetone-d6): δ 7.349 (s, 1H), 7.292
(d, J = 8.8 Hz, 2H), 7.076 (d, J = 8.8 Hz, 2H), 7.002 (s, 1H), 6.55 (br.
s, 1H, NH), 5.25−5.28 (m, 1H), 5.18−5.20 (m, 1H), 5.06−5.08 (m,
1H), 2.070 (dd, J = 1.5, 1.0 Hz, 3H), 2.05−2.06 (m, 3H), 1.58−1.64
(m, 2H), 1.282 (s, 6H), 1.19−1.31 (m, 10H), 1.06−1.14 (m, 2H),
0.855 (t, J = 7.0 Hz, 3H). 13C NMR (100 MHz, acetone-d6): 146.3,
143.8, 142.0, 141.0, 136.5, 133.2, 130.6, 127.5, 120.7, 120.6, 119.9,
117.2, 116.4, 45.4, 37.8, 32.6, 31.1, 30.3, 30.1, 29.6, 25.5, 24.1, 23.6,
23.4, 14.5. IR (ZnSe): 3404 (m), 2957 (s), 2925 (s), 2853 (s), 1592
(s), 1555 (m), 1513 (s), 1489 (s), 1367 (m), 1314 (s), 897 (s), 826
(m) cm−1. HRMS (EI): calcd for [C29H40N79Br]+• 481.2339, found
481.2334; calcd for [C29H40N81Br]+• 483.2318, found 483.2320.
General Procedures for Pd-Catalyzed Aminations Leading to
Oligoamines 7, 8, 15, 16, and 17 (Complete Br→NH Exchange).
Under nitrogen atmosphere, mixtures of bromo and amino
components, palladium(II) acetate (2−30 mol %), and sodium tert-
butoxide (1.5 equiv) were placed in a resealable Schlenk vessel and
dissolved/suspended in dry degassed toluene (10 mL/mmol). A
solution of tri-tert-butylphosphine in toluene (0.3 M, 6−90 mol %,
3 equiv for Pd) was added, the vessel was sealed, and the mixture was
stirred for 8−72 h at 90−100 °C. The mixture was subsequently
cooled to room temperature and diluted with benzene, washed with
brine, and dried over Na2SO4. Chromatography on deactivated (3%
Et3N) silica gel delivered the respective products.
Rf: 0.34 (Et2O/hexane, 1:20). H NMR (400 MHz, acetone-d6): δ
7.590 (d, J = 8.5 Hz, 2H), 7.254 (d, J = 8.8 Hz, 2H), 7.211 (s, 2H),
7.157 (d, J = 8.5 Hz, 2H), 7.128 (t, J = 8.2 Hz, 1H), 7.062 (d, J = 8.8
Hz, 2H), 6.909 (d, J = 8.2, 2H), 6.828 (s, 2H), 6.43 (br. s 2H, NH),
5.76 (br. s 2H, NH), 5.152 (m, 2H), 4.990 (m, 2H), 4.876 (m, 2H),
4.416 (m, 2H), 2.022 (m, 6H), 1.803 (m, 6H), 1.621 (m, 2H), 1.368
(s, 9H), 1.285 (s 12H), 1.07 − 1.26 (m, 28H), 0.820 (t, J = 6.8 Hz,
6 H). 13C NMR (126 MHz, acetone-d6): δ 152.1, 144.7, 142.4, 142.3,
141.0, 139.7, 132.7, 131.1, 129.6, 128.8, 127.7, 127.3, 126.3, 125.9,
122.2, 118.9, 115.66, 115.54, 109.9, 106.0, 45.5, 37.8, 35.3, 32.7, 31.7,
31.3, 30.4, 30.2, 29.7, 25.6, 24.4, 24.0, 23.4, 14.5. IR (ZnSe): 3676 (m),
2989 (s), 2972 (s), 2901 (s), 1582 (m), 1513 (m), 1466 (m), 1406
(m), 1394 (m), 1384 (m), 1254 (m), 1231 (m), 1076 (s), 1066 (s),
1057 (s), 897 (m) cm−1. HRMS (FAB): calcd for [C74H99N4]+•
1042.7786, found 1042.7784.
Preparation of Triamine 16. According to the general procedure,
bromoamine 11 (0.128 g, 0.279 mmol), 2,6-diamino-4′-tert-butylbi-
phenyl (14) (63.2 mg, 0.263 mmol), and Pd(OAc)2 (6.0 mg, 12 mol %)
were stirred for 72 h at 100 °C to provide 49.7 mg (34%) of tetraamine
15 and 69.0 mg (43%) of triamine 16 as light brown oil after
chromatography (diethyl ether/pentane 1:8). Rf: 0.49 (Et2O/hexane,
1
1:5). H NMR (400 MHz, acetone-d6): δ 7.557 (d, J = 8.5 Hz, 2H),
7.248 (d, J = 8.8 Hz, 2H), 7.241 (s, 1H), 7.172 (d, J = 8.5 Hz, 2H),
7.049 (d, J = 8.8 Hz, 2H), 6.985 (t, J = 8.0 Hz, 1H), 6.809 (s, 1H), 6.716
(dd, J = 8.0 Hz, 0.8 Hz, 1H), 6.396 (s, 1H), 6.384 (dd, J = 8.0 Hz, 0.8
Hz, 1H), 5.667 (s, 1H), 5.143 (m, 1H), 4.986 (m, 1H), 4.888 (m, 1H),
4.393 (m, 1H), 4.055 (s, 1H), 2.018 (m, 3H), 1.807 (m, 3H), 1.58−1.64
(m, 2H), 1.358 (s, 9H), 1.273 (s, 6H), 1.20−1.30 (m, 10H), 1.07−1.14
(m, 2H), 0.860 (t, J = 6.9 Hz, 3H). 13C NMR (100 MHz, acetone-d6): δ
151.4, 147.1, 144.7, 144.0, 142.29, 142.24, 141.7, 140.8, 140.4, 133.6,
131.1, 129.4, 129.0, 127.4, 127.3, 125.6, 125.4, 118.7, 118.1, 115.51,
115.45, 108.8, 107.5, 105.4, 45.4, 37.7, 35.2, 32.7, 31.7, 31.2, 30.4, 30.1,
29.6, 25.6, 24.5, 24.0, 23.4, 14.5. IR (ZnSe): 3395 (w), 2960 (m), 2923
(m), 2852 (m), 1608 (m), 1581 (s), 1513 (s), 1464 (s), 1254 (m), 898
(m), 835 (m), 729 (m) cm−1. HRMS (FAB): calcd for [C45H59N3]+•
641.4704, found 641.4708.
Preparation of Tetraamine 7. According to the general procedure,
6 (0.322 g, 0.83 mmol), 2,6-diaminotoluene (47.6 mg, 0.39 mmol),
and Pd(OAc)2 (2.6 mg, 3 mol %) were stirred for 8 h at 90 °C to
provide 0.192 g (67%) of 7 as yellow powder after chromatography
(pentane/benzene, 1:1). Mp: 83−87 °C. 1H NMR (400 MHz,
acetone-d6): δ 7.198 (d, J = 8.8 Hz, 4H), 7.035 (t, J = 8.0 Hz, 1H),
6.973 (s, 2H), 6.936 (d, J = 8.8 Hz, 4H), 6.794, (d, J = 8.0 Hz, 2H),
6.776 (s, 2H), ∼6.4 (br. s, 2H), ∼6.1 (br. s, 2H), 5.228 (m, 2H), 5.167
(m, 2H), 5.083 (m, 2H), 5.013 (m, 2H), 2.121 (s, 3H), 2.073 (s, 6H),
2.039 (br. s, 6H), 1.219 (s, 18H). 13C NMR (100 MHz, acetone-d6): δ
144.84, 144.79, 143.62, 143.51, 142.6, 141.9, 140.8, 129.9, 127.2,
126.68, 126.63, 126.2, 122.3, 118.1, 115.81, 115.67, 115.60, 107.7,
34.6, 31.9, 24.23, 24.08, 12.4. IR (ZnSe): 3407, 3073, 2961, 1583,
1513, 1471, 1391, 1370, 1327, 1256, 1193 cm−1. HRMS (FAB): calcd
for [C51H60N4]+• 728.4812, found 728.4835.
Preparation of Hexaamine 17. According to the general pro-
cedure, triamine 16 (38.2 mg, 59.5 μmol), 1,3-dibromo-4,6-diisopro-
penylbenzene (30.0 μmol), and Pd(OAc)2 (1.6 mg, 24 mol %) were
stirred for 168 h at 90−100 °C to give 20.4 mg (48%) of hexaamine 17
as light brown oil after chromatography (Et2O/pentane 1:15). Rf =
1
0.58 (ethyl acetate/hexane, 1:10). H NMR (400 MHz, acetone-d6): δ
7.574 (d, J = 8.3 Hz, 4H), 7.295 (s, 1H), 7.278 (d, J = 8.6 Hz, 4H),
7.252 (s, 2H), 7.218 (t, J = 8.0 Hz, 2H), 7.158 (d, J = 8.3 Hz, 4H), 7.088
(d, J = 8.6 Hz, 4H), 6.990 (d, J = 8.0 Hz, 2 H), 6.978 (d, J = 8.0 Hz, 2
H), 6.847 (s, 2H), 6.686 (s, 1H), 6.413 (s, 2H), 5.774 (s, 2H), 5.724 (s,
2H), 5.168 (m, 2H), 5.013 (m, 2H), 4.892 (m, 2H), 4.861 (m, 2H),
4.440 (m, 2H), 4.392 (m, 2H), 2.040 (m, 6H), 1.820 (m, 6H), 1.771
(m, 6H), 1.59−1.65 (m, 4H), 1.356 (s, 18H), 1.288 (s, 12H), 1.19−1.28
(m, 20H), 1.07−1.17 (m, 4H), 0.813 (t, J = 6.5 Hz, 6H). 13C NMR
(100 MHz, acetone-d6): δ 152.1, 144.7, 144.10, 144.08, 142.6, 142.5,
142.4, 142.2, 141.1, 141.0, 139.8, 139.7, 132.8, 131.1, 129.6, 129.1, 128.9,
127.7, 127.4, 126.2, 125.9, 125.3, 122.5, 119.13, 119.06, 115.7, 115.55,
115.50, 110.4, 110.1, 45.5, 37.8, 35.1, 32.7, 31.7, 31.3, 30.4, 30.2, 25.7,
24.4, 24.3, 24.0, 23.4, 14.5. IR (ZnSe): 3676 (m), 2989 (s), 2901 (s),
1454 (m), 1405 (m), 1394 (m), 1383 (m), 1250 (m), 1241 (m), 1230
(m), 1066 (s), 1058 (s), 1028 (m), 878 (m) cm−1. HRMS (ESI): calcd
for [C102H128N6 + H]+ 1438.0273, found 1438.0276.
General Procedure for Friedel−Crafts Cyclizations Leading
to Oligoamines 3, 4 and 5. Amine (approximately 0.05 mmol) was
placed in a pear-shaped flask under nitrogen flow and a degassed mixture
of 98% sulfuric and 85% phosphoric acids was added (1:1 v/v, 0.5 mL).
The mixture was continuously agitated with a glass rod for 15 min at
60 °C, cooled in an ice bath, and transferred into an ice-cold 10%
aqueous NaOH solution. The precipitate was carefully extracted with
benzene. The combined extracts were dried over Na2SO4 and evaporated.
Preparation of Tetraamine 3. According to the general procedure,
15 (71.0 mg) provided 66.8 mg (94%) of tetraamine 3 as a white solid.
Mp (under argon): 230−232 °C. 1H NMR (500 MHz, acetone-d6): δ
7.721 (d, J = 8.1 Hz, 2H), 7.622 (s, 2H), 7.501 (s, 1H), 7.417 (s, 2H),
Preparation of Tetraamine 8. According to the general procedure, 6
(85.9 mg, 0.223 mmol), m-phenylenediamine (11.8 mg, 0.109 mmol),
and Pd(OAc)2 (1.0 mg, 2 mol %) were stirred for 12 h at 90 °C to give
72.5 mg (93%) of 8 as yellow powder after chromatography (pentane/
1
benzene, 7:3). Mp: 92−93 °C. H NMR (400 MHz, acetone-d6): δ
7.197 (s, 2H), 7.183 (d, J = 8.8 Hz, 4H), 7.048 (t, J = 8.0 Hz, 1H), 7.001
(s, 2H), 6.988 (d, J = 8.8 Hz, 4H), 6.904 (t, J = 2.2 Hz, 1H), 6.536 (dd,
J = 2.2, 8.0 Hz, 2H), 6.489 (br. s, 2H), 6.351 (br. s, 2H), 5.195 (m, 2H),
5.171 (m, 2H), 5.032 (m, 4H), 2.063 (s, 6H), 2.029 (s, 6H), 1.205 (s,
18H). 13C NMR (100 MHz, acetone-d6): δ 146.5, 144.8, 144.7, 143.9,
141.9, 140.9, 130.5, 130.2, 127.5, 127.3, 126.8, 118.8, 115.8, 115.9, 115.7,
109.7, 109.0, 105.2, 34.6, 31.9, 24.1, 24.0. IR (ZnSe): 3414, 2964, 2860,
1590, 1513, 1391, 1370, 1323, 1257, 1191, 899 cm−1. HRMS (FAB):
calcd for [C50H58N4]+• 714.4656, found 714.4683.
Preparation of Tetraamine 15. According to the general
procedure, 11 (0.425 g, 0.881 mmol), 2,6-diamino-4′-tert-butylbiphen-
yl (14) (96.4 mg, 0.401 mmol), and Pd(OAc)2 (29.6 mg, 33 mol %)
were stirred for 72 h at 97 °C to provide 0.261 g (62%) of 15
as light brown oil after chromatography (ethyl acetate/pentane 1:20).
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7.369 (d, J = 8.1 Hz, 2H), 7.359 (d, J = 2.0 Hz, 2H), 6.996 (dd, J = 8.2,
2.0 Hz, 2H), 6.655 (d, J = 8.2 Hz, 2H), 6.063 (s, 2H), 5.899 (s, 2H),
1.617 (s, 12H), 1.57−1.61 (m, 4H), 1.588 (s, 12H), 1.467 (s, 9H),
1.282 (s, 12H), 1.16−1.26 (m, 20 H), 1.05−1.11 (m, 4H), 0.835 (t, J =
6.9 Hz, 6H). 13C NMR (126 MHz, acetone-d6): δ 152.0, 141.1, 138.7,
138.3, 137.6, 135.5, 132.1, 131.9, 129.3, 129.3, 127.9, 124.7, 123.6,
123.3, 122.5, 122.3, 122.2, 113.8, 110.9, 98.7, 45.4, 37.9, 36.82, 36.79,
35.4, 32.6, 31.98, 31.90, 31.87, 31.81, 31.1, 25.5, 23.4, 14.4. IR (ZnSe):
3412 (m), 2958 (s), 2926 (s), 2856 (s), 1630 (m), 1607 (s), 1501 (s),
1465 (s), 1419 (m), 1294 (s), 1181(m), 809 (m) cm−1. HRMS
(FAB): calcd for [C74H98N4]+• 1042.7786, found 1042.7773, calcd for
[C74H98N4 + H]+ 1043.7864, found 1043.7866.
for [C51H60N4]+• 728.4812, found 728.4807, calcd for [C51H60N4 + H]+
729.4891, found 729.4872.
4-Nitro-(1,1-dimethylnonyl)benzene (9). A mixture of concen-
trated nitric acid (4.1 mL) and concentrated sulfuric acid (2.6 mL) was
cooled to 5 °C and then added to 2-methyl-2-phenyldecane (4.19 g,
18.1 mmol) and stirred vigorously in an ice−water bath. The
heterogeneous mixture was allowed to attain room temperature. After
being stirred at room temperature for 16 h, the reaction mixture
was poured onto ice, Na2CO3 was added until the pH turned alkaline,
and the mixture was extracted with hexane and dried over Na2SO4.
Column chromatography on silica gel with CH2Cl2/hexane (1:20 to
1:13) gave 9 as light yellow liquid (3.61 g, 88%). Rf = 0.27 (benzene/
1
hexane, 1:25). H NMR (400 MHz, CDCl3): δ 8.151 (d, J = 8.9 Hz,
Preparation of Hexaamine 4. According to the general procedure,
16 (16.9 mg, 0.0117 mmol) provided 11.2 mg (66%) of hexaamine 4
as a white solid after chromatography on deactivated silicagel at 0 °C
(ethyl acetate/benzene/pentane, 2:1:20). Mp (under argon): 272 °C
2H), 7.473 (d, J = 8.9 Hz, 2H), 1.60 − 1.65 (m, 2H), 1.328 (s, 6H),
1.15−1.29 (m, 10H), 0.96−1.04 (m, 2H), 0.853 (t, J = 7.0 Hz, 3H).
13C NMR (100 MHz, CDCl3): δ 157.8, 145.9, 126.7, 123.2, 44.4, 38.5,
31.8, 30.2, 29.4, 29.2, 28.7, 24.6, 22.6, 14.0. IR (ZnSe): 2925 (m), 2854
(m), 1598 (m), 1516 (s), 1465 (m), 1340 (s), 1109 (m), 851 (s), 758
(m), 700 (s) cm−1.
1
dec. Rf = 0.31 (pentane/ethyl acetate, 12:1). H NMR (500 MHz,
acetone-d6): δ 7.663 (d, J = 8.2 Hz, 4H), 7.635 (s, 2H), 7.484 (s, 2H),
7.417 (s, 1H), 7.404 (s, 2H), 7.351 (d, J = 2.3 Hz, 2H), 7.314 (d, J =
8.2 Hz, 4H), 6.992 (dd, J = 8.3, 2.3 Hz, 2H), 6.650 (d, J = 8.3 Hz, 2H),
6.088 (s, 2H), 5.981 (s, 2H), 5.868 (s, 2H), 5.646 (s, 1H), 1.605 (s,
12H), 1.597 (s, 12H), 1.57−1.60 (m, 4H), 1.555 (s, 12H), 1.425 (s,
18H), 1.17−1.31 (m, 20H), 1.256 (s, 12H), 1.04−1.10 (m, 4H), 0.829
(t, J = 6.9 Hz, 6H). 1H NMR (600 MHz, benzene-d6): δ 7.713 (s, 2H),
7.586 (s, 1H), 7.545 (d, J = 2.0 Hz, 2H), 7.513 (s, 2H), 7.477 (d, J =
8.3 Hz, 4H), 7.353 (d, J = 8.3 Hz, 4H), 7.211 (dd, J = 8.2, 2.0 Hz, 2H),
6.527 (d, J = 8.2 Hz, 2H), 5.788 (s, 2H), 5.660 (s, 2H), 5.182 (s, 2H),
4.792 (s, 2H), 4.187 (s, 1H), 1.852 (s, 12H), 1.837 (s, 12H), 1.69−
1.73 (m, 4H), 1.700 (s, 12H), 1.404 (s, 12H), 1.303 (s, 18H), 1.19−
1.30 (m, 24H), 0.878 (t, J = 7.0 Hz, 6H). 13C NMR (151 MHz,
benzene-d6): δ 151.9, 141.8, 137.98, 137.83, 137.84, 137.7, 137.2,
135.7, 135.4, 133.0, 132.4, 129.8, 127.9, 124.6, 123.6, 123.05, 122.96,
122.85, 122.81, 122.76, 122.74, 122.5, 113.6, 110.5, 99.0, 98.6, 45.6,
38.0, 36.93, 36.90, 36.88, 36.2, 32.8, 32.6, 32.3, 31.9, 31.3, 30.3, 30.1,
30.0, 23.4, 25.7, 14.9. IR (ZnSe): 3673 (m), 2989 (s), 2972 (s), 2900
(s), 1405 (m), 1394 (m), 1383 (m), 1250 (m), 1241 (m), 1230 (m),
1076 (s), 1066 (s), 1058 (s), 1028 (m), 879 (m) cm−1. HRMS (FAB):
calcd for [C102H128N6]+• 1437.0195, found 1437.0223; calcd for
[C102H128N6 + H]+ 1438.0273, found 1438.0257.
4-(1,1-Dimethylnonyl)aniline (10). 4-Nitro(1,1-dimethylnonyl)-
benzene (9) (1.698 g, 6.12 mmol) and acetic acid (60 mL) were
placed in a round-bottom flask equipped with a reflux condenser. Iron
powder (325 mesh, 6.72 g, 121 mmol) was added, and the suspension
was stirred vigorously at room temperature. (Initial raise in temperature
was observed due to an exothermic reaction.) After 16 h, the mixture
was evaporated and the residue was treated with powdered KOH, the
product was taken up in benzene and filtered through a pad of Celite.
Removal of solvents afforded 1.445 g (96%) of pure 10 as light orange
liquid. 1H NMR (400 MHz, CDCl3): δ 7.119 (d, J = 8.3 Hz, 2H), 6.646
(d, J = 8.3 Hz, 2H), 3.6 (br. 2H, NH2), 1.50−1.56 (m, 2H), 1.241 (s,
6H), 1.12−1.31 (m, 10H), 1.00−1.09 (m, 2H), 0.866 (t, J = 6.9 Hz,
3H). 13C NMR (100 MHz, CDCl3): δ 143.5, 140.1, 126.6, 114.9, 44.7,
36.8, 31.9, 30.4, 29.5, 29.3, 29.1, 24.7, 22.6, 14.1. IR (ZnSe): 3360 (w),
2955 (m), 2926 (s), 2853 (m), 1621 (s), 1516 (s), 1466 (m), 1278
(m), 1189 (m), 912 (m), 825 (s), 736 (s) cm−1. HRMS (CI): calcd for
[C17H29N]+• 247.2295 found 247.2292, calcd for [C17H29N + H]+
248.2373 found 248.2388.
4′-tert-Butyl-2,6-dibromobiphenyl (12). 2-Iodo-1,3-dibromoben-
zene (2.10 g, 5.82 mmol), 4-tert-butylphenylboronic acid (1.16 g,
6.52 mmol, 1.12 equiv), and tetrakis(triphenylphosphino)palladium
(0) (69.4 mg, 0.060 mmol, 1.0 mol %) were charged to a Schlenk
vessel. Then, a mixture of degassed benzene (40 mL), ethanol (10 mL),
and aqueous Na2CO3 solution (2 M, 10 mL) was added. The mixture
was stirred vigorously at 85−95 °C for 168 h. After attaining room
temperature, the reaction mixture was extracted with benzene, washed
with NaOH solution (10%), dried over Na2SO4, and concentrated.
Chromatography on silica gel with hexane gave 1.74 g (81%) of 12 as
Preparation of Tetraamine 5. According to the general procedure
8 (17.6 mg, 0.0246 mmol) provided 3.9 mg of 5 (22%) as brown
solid after PTLC (3% deactivated silica gel, benzene). Mp (under air):
1
200 °C dec. Mp (under argon): >320 °C dec, 350 °C (liquid). H
NMR (400 MHz, acetone-d6): δ 7.781 (br. s, 1H, exch D2O), 7.762
(br. s, 1H, exch D2O), 7.694 (br. s, 1H, exch D2O), 7.439 (br. s, 3H),
7.408 (s, 1H), 7.107 (d, J = 8.4 Hz, 1H), 7.082 (dd, J = 2.0, 8.0 Hz,
1H), 7.069 (dd, J = 2.0, 8.0 Hz, 1H), 6.764 (br. s, 1H, exch D2O),
6.717 (d, J = 8.4 Hz, 1H), 6.685 (d, J = 8.4 Hz, 1H), 6.433 (s, 1H),
6.295 (d, J = 8.0 Hz, 1H), 6.084 (s, 1H), 1.930 (s, 6H), 1.595 (s, 6H),
1.584 (s, 6H), 1.504 (s, 6H), 1.297 (s, 18H). IR (ZnSe): 3406 (m),
2962 (s), 2924(s), 2856 (s), 1630 (m), 1608 (s), 1501 (s), 1462 (s),
1415 (m), 1302 (m) cm−1. HRMS (ESI): calcd for [C50H58N4]+•
714.4656, found 714.4683.
Preparation of Tetraamine 2. Phosphoric acid (85%, 2.0 mL) was
added to tetraamine 7 (83.0 mg, 0.113 mmol) under N2. After 6 h
at 90 °C, the reaction mixture was poured into H2O. The precipitate
was collected and dissolved in THF. The obtained red solution was
poured into aqueous KOH (0.02 M) and MeOH, which gave a yellow
precipitate that was triturated with MeOH and MeOH/KOH and
dried under vacuum to give a yellow powder. Precipitation from THF
and aqueous KOH gave tetraamine 2 as light brown solid (54.0 mg,
65%). Mp (under Ar): >430 °C dec, 448 °C (liquid). 1H NMR
(400 MHz, acetone-d6,): δ ∼7.71 (br s, 2H, exch D2O), 7.431 (d, J =
2.0 Hz, 2H), 7.406 (s, 2H), 7.345 (s, 1H), ∼7.11 (br. s, 2H, exch
D2O), 7.059 (dd, J = 2.0, 8.4 Hz, 2H), 6.676 (d, J = 8.4 Hz, 2H), 6.401
(s, 2H), 2.184 (s, 3H), 1.577 (s, 12H), 1.569 (s, 12H), 1.291 (s, 18H).
IR (ZnSe): 3673 (m), 2989 (s), 2971 (s), 2901 (s), 1630 (w), 1613
(m), 1504 (m), 1491 (m), 1406 (m), 1394 (m), 1383 (m), 1250 (m),
1231 (m), 1066 (s), 1057 (s), 1028 (m) cm−1. HRMS (FAB): calcd
1
colorless oil. Rf = 0.58 (benzene/hexane, 1:25). H NMR (400 MHz,
CDCl3): δ 7.626 (d, J = 8.0 Hz, 2H), 7.465 (d, J = 8.1 Hz, 2H), 7.143
(d, J = 8.1 Hz, 2H) 7.050 (t, J = 8.0 Hz, 1H), 1.383 (s, 9H). 13C NMR
(100 MHz, CDCl3): δ 150.9, 143.1, 138.1, 131.8, 129.6, 128.7, 125.0,
124.8, 34.7, 31.4. IR (ZnSe): 3085 (w), 3052 (w), 2962 (m), 2905 (m),
2864 (m), 1546 (s), 1476 (m), 1461 (m), 1431 (s), 1420 (s), 1397
(m), 1362 (m), 1268 (m), 1188 (s), 1114 (m), 1026 (m), 832 (s),
771 (s), 717 (s) cm−1. HRMS (EI): calcd for [C16H1679Br2]+• 365.9613,
found 365.9610.
2,6-Bis(N-benzylamino)-4′-tert-butylbiphenyl (13). Dibromo com-
pound 12 (0.743 g, 2.02 mmol) was evacuated in a Schlenk vessel.
Then, in a glovebag, palladium(II) acetate (35.6 mg, 0.16 mmol,
7.9 mol %), BINAP (212.8 mg, 0.342 mmol 17 mol %), and sodium
tert-butoxide (0.770 g, 8.02 mmol, 3.97 equiv) were added. Dry
degassed toluene (33 mL) and benzylamine (8.8 mL, 81 mmol,
40 equiv) were added. The reaction mixture was stirred at 120 °C for
25 h. After attaining room temperature, the reaction mixture was
diluted with ethyl acetate, washed with brine, dried over Na2SO4, and
concentrated. The residual benzylamine was removed in high vacuum.
Flash chromatography on deactivated silica gel (4% Et3N) with ethyl
acetate/benzene/hexane (5:2:100) afforded yellowish solid that was
washed with pentane (2 × 0.6 mL) to give 0.686 g (81%) of pure 13 as
a white crystalline solid. Mp: 130−133 °C. 1H NMR (400 MHz,
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CDCl3): δ 7.574 (d, J = 8.3 Hz, 2H), 7.364 (d, J = 8.3 Hz, 2H), 7.22 −
7.38 (m, 10H), 7.035 (t, J = 8.2 Hz, 1H), 6.149 (d, J = 8.2 Hz, 2H),
4.330 (d, J = 5.9 Hz, 4H), 3.876 (t, J = 5.9 Hz, 2H, NH), 1.401 (s,
9H). 13C NMR (100 MHz, CDCl3): δ 150.7, 145.9, 140.1, 132.0,
130.9, 129.0, 128.4, 127.0, 126.8, 126.7, 112.3, 101.0, 47.9, 34.6, 31.3.
IR (ZnSe): 3436 (m), 2963 (m), 2863 (m), 1580 (s), 1574 (s), 1495
(s), 1473 (s), 1453 (m), 1237 (m), 840 (m), 740 (s), 697 (s) cm−1.
HRMS (FAB): calcd for [C30H32N2]+• 420.2560, found 420.2559.
2,6-Diamino-4′-tert-butylbiphenyl (14). Diamine 13 (0.810 g,
1.92 mmol) was suspended in ethanol (80 mL) in a round-bottom
flask equipped with a reflux condenser. A small amount of recry-
stallized ammonium formate (∼0.01 g) was added, followed by 10%
palladium on charcoal (72 mg, 0.068 mmol, 3.5 mol %). The mixture
was stirred under reflux, and recrystallized ammonium formate was
added in portions (total 1.44 g, 22.9 mmol, 11.9 equiv) over 3 h. The
reflux was continued for 46 h, and the mixture was cooled to room
temperature, filtered through Celite, and concentrated. The residue
was dissolved in ethyl acetate and washed with brine. The aqueous
phase was extracted with ethyl acetate, and the combined organic
extracts were dried over Na2SO4 and filtered. Evaporation afforded
0.442 g (96%) of pure 14 as a white solid. Mp: 179−180 °C. 1H NMR
(400 MHz, CDCl3): δ 7.505 (d, J = 8.6 Hz, 2H), 7.286 (d, J = 8.6 Hz,
2H), 6.951 (t, J = 8.0 Hz, 1H), 6.233 (d, J = 8.0 Hz, 2H), 3.45 (br. 4H,
NH2), 1.359 (s, 9H). 1H NMR (400 MHz, acetone-d6): δ 7.544 (d, J =
8.6 Hz, 2H), 7.228 (d, J = 8.6 Hz, 2H), 6.784 (t, J = 7.9 Hz, 1H), 6.132
(d, J = 7.9 Hz, 2H), 3.87/3.84 (br. 4H, NH2), 1.355 (s, 9H). 13C NMR
(100 MHz, acetone-d6): δ 150.8, 146.68, 146.65, 134.5, 131.2, 129.2,
127.2, 105.4, 35.9, 31.7. IR (ZnSe): 3464 (m), 3373 (m), 2962 (m),
2949 (m), 1612 (s), 1598 (m), 1581 (m), 1464 (s), 1270 (m), 1114
(m), 1001 (m), 840 (s), 779 (s), 726 (s) cm−1. HRMS (FAB): calcd
for [C16H20N2]+• 240.621, found 240.1633.
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ASSOCIATED CONTENT
■
S
* Supporting Information
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General procedures and materials, additional experimental
details, X-ray crystallographic file for 2 (CIF), and complete
ref 60. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: arajca1@unl.edu.
Notes
§On leave from Wrocław University of Technology, Poland.
(22) Bendikov, M.; Wudl, F. Chem. Rev. 2004, 104, 4891.
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(24) Extended acenes have open-shell singlet ground states:
Bendikov, M.; Houk, K. N.; Duong, H. M.; Starkey, K.; Carter, E.
A.; Wudl, F. J. Am. Chem. Soc. 2004, 126, 7416−7417;Correction:
J. Am. Chem. Soc. 2004, 126, 10493.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation for support of this
research through Grant Nos. CHE-0718117 and CHE-1012578.
ChemMatCARS Sector 15 is principally supported by the
National Science Foundation/Department of Energy under
Grant No. NSF/CHE-0822838. Use of the Advanced Photon
Source was supported by the U.S. Department of Energy, Office
of Science, Office of Basic Energy Sciences, under Contract No.
DE-AC02-06CH11357. We thank Bridget Foley for preliminary
synthesis of derivatives of building block 14.
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Soc. 2009, 131, 3424−3425.
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