Novel "Potentially Antiaromatic", Acidichromic Quinonediimines with Tunable Delocalization of Their 6π-Electron Subunits

Article abstract of DOI:10.1021/ja035463u

We present a novel family of "potentially antiaromatic" alkyl-substituted p-benzoquinonediimine pH-dependent chromophores. It appears from the structural data that these overall 12π-electron molecules should be better considered as constituted by two chem

Full text of DOI:10.1021/ja035463u

Published on Web 10/16/2003
Novel “Potentially Antiaromatic”, Acidichromic
Quinonediimines with Tunable Delocalization of Their
6π-Electron Subunits
Olivier Siri,† Pierre Braunstein,*,† Marie-Madeleine Rohmer,‡ Marc Be´nard,‡ and
Richard Welter§
Contribution from the Laboratoire de Chimie de Coordination, UMR 7513 CNRS,
UniVersite´ Louis Pasteur, 4, rue Blaise Pascal, 67070 Strasbourg Cedex, France,
the Laboratoire de Chimie Quantique, UMR 7551 CNRS, UniVersite´ Louis Pasteur,
4, rue Blaise Pascal, F-67070 Strasbourg Cedex, France, and the Laboratoire DECMET,
UMR 7513 CNRS, UniVersite´ Louis Pasteur, 4, rue Blaise Pascal,
F-67070 Strasbourg Cedex, France
Received April 4, 2003; E-mail: braunst@chimie.u-strasbg.fr
Abstract: We present a novel family of “potentially antiaromatic” alkyl-substituted p-benzoquinonediimine
pH-dependent chromophores. It appears from the structural data that these overall 12π-electron molecules
should be better considered as constituted by two chemically connected but electronically not conjugated
6π-electron subunits. Molecule 5 appears to be the first example of two separated, conjugated, and localized
6π-electron systems that can be tuned by reversible protonation to become delocalized. The mono- and
diprotonated derivatives have been characterized by spectroscopic methods and X-ray diffraction. These
systems develop supramolecular interactions in the solid state that clearly reflect the degree of protonation
and depend on the nature of the counterion. These compounds constitute new chromophores for which
the color can be tuned depending on the degree of protonation, going in solution from yellow for 5 to red
for 5‚HCl and blue for 5‚2HCl. Theoretical calculations have provided a deeper insight into the electronic
structure of these molecules and allowed an assignment of the experimental UV-vis spectra. The visible
and near-UV spectrum of the neutral and protonated benzoquinonediimines can be classically assigned
from the coupling of two 6π-electron polymethine units. TD-DFT calculations confirm the observed red
shift of the two lowest π f π* transitions of the benzoquinonediimines upon protonation and relate it to the
moderate energy lowering of the HOMO f LUMO transition induced by the delocalization of the polymethine
π system.
Introduction
quinonoid structure constitute a large and important class of
organic compounds which display interesting colors and are
Research on the relation between color and structure in
organic compounds has received a great deal of attention for
decades.1-3 In 1926, the polymethine structure was recognized
by Ko¨nig as the true origin of color.4 Designing molecules with
controlled colors may result from fine-tuning of the photophysi-
cal characteristics of a given family of organic chromophores.5
For example, conjugated oligomers and polymers permit color
tuning through chemical or structural modifications.6 A simple
color test of enantiomeric excess for an amino acid ester has
also been recently reported.7 Smaller molecules exhibiting a
endowed with very rich chemical and physical properties.8-10
For years, studies on structure/color relationships have involved
N-substituted benzoquinonediimines of type 1 (Chart 1) which
were only accessible by self-condensation of aniline and/or 1,4-
diaminobenzene,11-13 and this limited considerably the nature
of the N-substituents on both types of nitrogen atoms. A related
system A has been envisaged from a theoretical standpoint to
contain two 6π-trimethine cyanine subunits linked via two C-C
single bonds (Chart 1).2 However, no experimental data were
available to support this description. We recently described a
related system B which is a rare example of zwitterion being
† Laboratoire de Chimie de Coordination.
‡ Laboratoire de Chimie de Quantique.
§ Laboratoire DECMET.
(7) van Delden, R. A.; Feringa, B. L. Chem. Commun. 2002, 174.
(8) Siri, O.; Braunstein, P. Chem. Commun. 2002, 208.
(9) Bock, H.; Ruppert, K.; Na¨ther, C.; Havlas, Z. Angew. Chem., Int. Ed. Engl.
1991, 30, 1180.
(1) Christie, R. M. Colour Chemistry; Royal Society of Chemistry: Cambridge,
2001.
(2) Da¨hne, S.; Leupold, D. Angew. Chem., Int. Ed. Engl. 1966, 5, 984.
(3) Da¨hne, S.; Leupold, D.; Radeglia, R. J. Prakt. Chem. 1972, 314, 525.
(4) Ko¨nig, W. J. Prakt. Chem. 1926, 112, 1.
(10) Pata¨ı, S.; Rappoport, Z. The Chemistry of the Quinonoid Compounds; Wiley
and Sons: New York, 1988; Vols. 1 and 2.
(5) Joshi, H. S.; Jamshidi, R.; Tor, Y. Angew. Chem., Int. Ed. 1999, 38, 2721.
(6) (a) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed.
1998, 37, 403. (b) Noda, T.; Ogawa, H.; Noma, N.; Shirota, Y. AdV. Mater.
1997, 9, 720.
(11) Bandrowski, E. Monatsh. Chem. 1889, 10, 123.
(12) Bandrowski, E. Chem. Ber. 1894, 27, 480.
(13) (a) Fischer, O.; Hepp, E. Ber. Dtsch. Chem. Ges. 1888, 21, 676. (b) Kimish,
C. Ber. Dtsch. Chem. Ges. 1875, 8, 1026.
9
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J. AM. CHEM. SOC. 2003, 125, 13793-13803
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A R T I C L E S
Siri et al.
Chart 1
Scheme 1. Synthesis of Compounds 5-7: (i) RC(O)Cl/NEt3/MeCN, (ii) LiAlH4/THF, (iii) Aerobic Workup
more stable than its canonical forms and which is best described
as constituted of two chemically connected but electronically
not conjugated 6π-electron subunits.8,14 A single π f π*
excitation in this 4n π system is sufficient to fully restore π
delocalization and antiaromaticity and this has led us to call
such systems “potentially antiaromatic”.14 By analogy, we will
term 4n π systems such as A, B, and to some extent 1
“potentially antiaromatic” (see below). In view of the potential
of such molecules in color chemistry15 and as ligands in
coordination and organometallic chemistry, we became inter-
ested in a simpler and more versatile access to molecules of
type 1 bearing N-alkyl substituents (instead of the usual and
limited aryl groups) whose nature could be more easily varied.16
Figure 1. Suggested supramolecular network based on 2.
Chart 2
Herein, we report an experimental and theoretical study on a
novel family of 12π-electron chromophores based on the 2,5-
diamino-1,4-benzoquinonediimine skeleton. These molecules
will provide the first experimental evidence for their description
as two coupled trimethine cyanine subunits involving 6π
electrons each. In this family, the color will be shown to be
controlled by successive protonation and depending on the pH,
the conjugated localized π electron systems can be tuned to
become delocalized. Their electronic structure has been inves-
tigated by density functional theory (DFT) calculations. Con-
sidering that noncovalent interactions such as hydrogen bonding
are essential for the creation of molecular architectures by self-
assembly and mutual recognition,17 our molecules will be shown
to have interesting potential as tunable building blocks in
supramolecular chemistry for the construction of different types
of hydrogen-bonded molecular networks.
zene and an appropriate acyl chloride in MeCN and in the
presence of excess NEt3 (Scheme 1).
We have recently suggested that the striking insolubility of
2, which limited its characterization to the solid state,16 resulted
from a H-bonded supramolecular network involving the amido
functions,18,19 as represented in Figure 1. The fact that the
diester-diamido analogue (Chart 2) is highly soluble in most
organic solvents8 supports the view that the four amido groups
of 2 are involved in this H-bonded network.
The situation is different in the derivatives 3 or 4 which could
be dissolved in [d6]-dmso for NMR characterization. Reduction
of 2-4 with LiAlH4 in THF followed by aerobic workup
afforded the expected p-benzoquinonediimine ligands 5-7,
respectively. Although the preparation of 3 was mentioned in
1960, its full characterization was not reported.20 All compounds
are stable in the solid state and in solution for several days at
room temperature and in the presence of light. Having shown
Results and Discussion
Synthesis and Characterization. The preparation of N,N′,-
N′′,N′′′-tetralkyl-p-benzoquinonediimine compounds 5-7 re-
quired the synthesis of the tetraamido derivatives 2-4, which
was achieved by condensation reaction between tetraaminoben-
(14) Braunstein, P.; Siri, O.; Taquet, J.-P.; Rohmer, M.-M.; Be´nard, M.; Welter,
R. J. Am. Chem. Soc. 2003, 125, 12246.
(18) Hamuro, Y.; Geib, S. J.; Hamilton, A. D. J. Am. Chem. Soc. 1996, 118,
7529.
(15) Corbett, J. F. Hair Colorants: Chemistry and Toxicology; Micelle Press:
Dorset, England, 1998.
(19) Hamuro, Y.; Geib, S. J.; Hamilton, A. D. J. Am. Chem. Soc. 1997, 119,
10587.
(16) Siri, O.; Braunstein, P. Chem. Commun. 2000, 2223.
(17) Lehn, J. M. Supramolecular Chemistry; VCH: Weinheim, 1995.
(20) Arient, J.; Marhan, J.; Ta¨ublova, H. Collect. Czech. Chem. Commun. 1960,
25, 1602.
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Novel Antiaromatic, Acidichromic Quinonediimines
A R T I C L E S
Scheme 2. Protonation Reactions of 5 with HCl
Figure 2. Color changes resulting from mono- and diprotonation of
compound 5.
with 5-7 that a general access to N-alkyl derivatives of 1 is
now available, we will concentrate our detailed studies on 5.
Figure 3. View of the crystal structure of 5‚HCl.
Influence of pH on Color. The mono- and dicationic
compounds derived from 5 were prepared to study the pH color
dependence. The successive protonations of 5 were carried out
in THF at room temperature with HCl (Scheme 2).
known 5.16 Since crystallographic data of 5‚HBF4 are very
similar to those of 5‚HCl, they will not be discussed in detail
and are given in the Supporting Information.
A remarkable color change occurs in solution from yellow
for 5 to red for the monoprotonated form 5‚HCl and to blue for
the diprotonated form 5‚2HCl (Figure 2).
Single crystals of 5‚HCl were isolated from a solution of
chloroform/n-hexane (Figure 3). Examination of the bond
distances within the N(1)-C(1)-C(6)-C(5)-N(4) moiety of
5‚HCl reveals an alternating succession of single and double
bonds, whereas the C-C and C-N distances of the N(2)-
C(2)-C(3)-C(4)-N(3) moiety show a bond equalization (Table
2). The C(1)-C(2) and C(4)-C(5) distances of 1.502(4) and
1.493(4) Å, respectively, correspond to a single bond and
indicate the lack of conjugation between the two halves of the
molecule. Therefore, 5‚HCl contains in the solid state two
different conjugated 6π systems, one with and the other without
delocalization. For the former, the positive charge is shared
between the two nitrogen atoms. The intramolecular N(3)H‚‚‚N(4)
hydrogen-bonding distance of 2.036 Å (associated with a C(4)-
N(3)-H angle of only 108.5°) is shorter than the N(1)H‚‚‚N(2)
and N(2)H‚‚‚N(1) distances of 2.412 and 2.335 Å, consistent
with the presence of the chloride ion at a distance of 2.265 and
2.172 Å from N(1)H and N(2)H, respectively.
The UV-visible data will be discussed below. Note that in
the solid state, 5‚2HCl is green. These dramatic color changes
can be explained by the different electronic situations resulting
from protonation at the nitrogen atoms of the imino groups
which allows the resulting positive charge to be stabilized by
delocalization between the two nitrogen atoms (Scheme 2).
Mixing equimolar amounts of 5 and 5‚2HCl in THF afforded
quantitatively 5‚HCl. The protonation reaction is reversible upon
addition of water or base. All compounds have been fully
characterized, including by X-ray diffraction.
Crystal Structures. The molecular structures of compounds
5‚HCl (Figure 3), 5‚2HCl‚3CHCl3 (Figure 4), and 5‚HBF4 have
been elucidated by X-ray crystallography. Crystal data and
selected bond lengths for 5‚HCl and 5‚2HCl‚3CHCl3 are listed
in Tables 1 and 2, respectively. The experimental values are
compared with those calculated for model compounds in which
the neopentyl group has been replaced with H atoms. The results
of the theoretical calculations are discussed below. For com-
parison, we also give in Table 2 relevant structural data for
Single crystals of 5‚2HCl‚3CHCl3 were isolated from a
solution of chloroform/n-hexane, and the X-ray analysis revealed
(Figure 4) full delocalization of the conjugated π systems as
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A R T I C L E S
Siri et al.
Table 1. X-ray Crystallographic Data for 5‚HCl and
5‚2HCl‚3CHCl3
5‚HCl
5‚2HCl‚3CHCl3
formula
formula weight
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C26H49ClN4
453.14
P21
C29H53Cl11N4
847.70
P21/n
11.239(5)
10.953(5)
12.182(5)
90.000(5)
105.337(5)
90.000(5)
1446.2(11)
2
15.060(2)
14.970(3)
19.749(3)
90.000(5)
108.537(1)
90.000(5)
4221.71(11)
4
Z
µ (mm-1
)
0.150
1.041
0.749
1.334
F (g cm-3
)
θ limits (deg)
hkl limits
1 < θ < 30
-15 e h e 15
-11 e k e15
0 e l e 17
8722
1 < θ < 27.5
-19 e h e 18
0 e k e 19
0e l e 25
9686
collected reflections
independent reflections (Rint
)
7376
9685
observed reflections (I>2σ(I))
5253
6336
Ra
0.069
0.071
b
Rw
0.178c
0.199d
GOFe
1.052
-0.38
0.46
1.058
-1.47
1.77
∆Fmin(e Å-3
)
∆Fmax(e Å-3
)
a
2
2
2
R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) {∑[w(Fo - Fc )2]/∑[w(Fo )2]}1/2
.
c w ) 1/[σ2(Fo ) + (0.1115P)2 + 0.0000P] where P ) (Fo + 2Fc )/3.
2
2
2
2
2
2
d w ) 1/[σ2(Fo ) + (0.1042P)2 + 2.7829P] where P ) (Fo + 2Fc )/3.
2
2
e GOF ) S ) {∑[w(Fo - Fc )2]/(n - p)}1/2 where n is the number of
reflections and p is the total number of refined parameters.
zine22,23 backbones, respectively, except that in the latter, the
bond lengths corresponding to C(1)-C(2) and C(4)-C(5) did
not represent a “pure” single bond. Therefore, our molecules
constitute the first class of cyanine-type chromophores in which
the two “face to face” π systems are fully independent.
Compounds 5‚HCl and 5‚2HCl should actually be described as
constituted of two chemically connected and interacting, but
electronically not conjugated, 6π-electron subunits (see Theo-
retical Calculations). As established for the related 6π + 6π
zwitterion B of Chart 1, a single π f π* excitation from the
HOMO to the LUMO would restore the communication between
the two π subsystems along with delocalization and antiaro-
maticity.14 We have therefore termed such systems “potentially
antiaromatic.” Since the X-ray analysis of 5 established its
p-benzoquinonediimine form with no delocalization of the π
systems,16 5 appears to be the first example of two separated,
conjugated, but localized 6π-electron systems that can be tuned
by reversible protonation to become delocalized.
Supramolecular Arrangements. Numerous close intermo-
lecular contacts are revealed upon inspection of the crystal lattice
in 5.16 Two intermolecular C(1)‚‚‚N(1) distances of 3.998 Å
are consistent with intermolecular π-π interactions (Figure 5).24
Depending on the degree of protonation of 5, different
supramolecular architectures are obtained. Thus, 5‚HCl forms
a 1-D supramolecular network involving the chloride ion Cl(1)
as an intermolecular connector between two molecules. It
interacts with the two N-H protons of its associated cation (see
Figure 4. View of the crystal structure of 5‚2HCl in 5‚2HCl‚3CHCl3 (top),
view of 5‚2HCl‚3CHCl3 (bottom).
shown by the narrow range of C-C distances from 1.388(5) to
1.395(5) Å and of C-N distances from 1.316(4) to 1.329(4) Å
(Table 2).
The C(1)-C(2) and C(4)-C(5) bond lengths of 1.509(5) and
1.503(5) Å represent typical single bonds and are again
indicative of the lack of conjugation between the two halves of
the ligand. Delocalization of the π system is therefore confined
to the upper and lower parts of the ligand and generates a
dicationic benzoquinonediimine-type system with delocalization
of each positive charge between the nitrogen atoms. As a result,
the dication in 5‚2HCl‚3CHCl3 can be regarded as constituted
by two cyanine-type chromophores which are mutually con-
nected by two C-C single bonds. The intramolecular N(1)H‚‚‚
N(2) and N(2)H‚‚‚N(1) hydrogen-bonding distances of 2.528
and 2.374 Å are comparable, respectively, to the N(4)H‚‚‚N(3)
and N(3)H‚‚‚N(4) distances of 2.447 and 2.367 Å. The Cl(1)
chloride ion is at distances of 2.274 and 2.364 Å from N(3)H
and N(4)H, respectively, and Cl(2) is at distances of 2.293 and
2.422 Å from N(2)H and N(1)H, respectively. In addition, three
CHCl3 molecules cocrystallized and interact with 5‚2HCl by
hydrogen bonding via C-H‚‚‚Cl interactions involving the
Cl(5), Cl(7), Cl(8), and Cl(9) atoms (Figure 4).
(21) Wudl, F.; Koutentis, P. A.; Weitz, A.; Ma, B.; Strassner, T.; Houk, K. N.;
Khan, S. I. Pure Appl. Chem. 1999, 71, 295.
(22) Ghosh, A. M.; Mitra, K. N.; Mostafa, G.; Goswami, S. Eur. J. Inorg. Chem.
2000, 1961.
(23) Rychlewska, U.; Broom, M. B. H.; Eggleteon, D. S.; Hodgson, D. J. J.
Am. Chem. Soc. 1985, 107, 4768.
The bond distances in 5‚HCl and 5‚2HCl‚3CHCl3 are similar
to those in analogues of quinoxalinophenazine21 and phena-
(24) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525.
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Novel Antiaromatic, Acidichromic Quinonediimines
A R T I C L E S
+
Table 2. Selected Bond Lengths (Å) Observed for 5,16 5‚HCl, and 5‚2HCl‚3CHCl3 and Calculateda for Their Respective Models 5H, 5H
(optimal and planar), and 5H and for the Isolated Fragments HN-(CH)3-NH2 (11) and [H2N-(CH)3-(NH2)]+ (12+). Labeling of the
++
Atoms: for 5‚HCl and 5H+, N(2) Is Assumed To Be the Protonated Nitrogen:
5
5H
5‚HCl
5H+ optimal
5H+ planar
5‚2HCl‚3CHCl3
5H2+
11 planar
12+
C(1)-C(2)
C(1)-C(6)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(4)-N(3)
C(1)-N(1)
C(2)-N(2)
C(5)-N(4)
1.508(3)
1.358(4)
1.437(3)
1.358(4)
1.508(3)
1.437(3)
1.346(3)
1.346(3)
1.286(3)
1.286(3)
1.502
1.370
1.440
1.370
1.502
1.440
1.358
1.358
1.303
1.303
1.502(4)
1.361(4)
1.388(4)
1.383(4)
1.493(4)
1.438(4)
1.338(5)
1.368(4)
1.321(3)
1.290(4)
1.484
1.364
1.396
1.394
1.508
1.446
1.326
1.387
1.343
1.291
1.488
1.372
1.396
1.394
1.506
1.437
1.325
1.364
1.344
1.294
1.509(5)
1.395(5)
1.393(5)
1.388(5)
1.503(5)
1.395(5)
1.318(4)
1.316(4)
1.323(4)
1.329(4)
1.504
1.395
1.395
1.395
1.504
1.395
1.332
1.332
1.332
1.332
1.360
1.390
1.390
1.448
1.366
1.292
1.326
1.326
a With ADF/BP86.
Figure 5. Crystalmaker side view of the stacking of 5 in the solid state.
Color coding: nitrogen, blue; hydrogen, red.
above) and a C-H hydrogen of two CH2 groups of another
monocationic molecule, as indicated by the C(17)‚‚‚Cl(1) and
C(12)‚‚‚Cl(1) distances of 3.779 and 3.641 Å, respectively
(Figure 6). Therefore, each 5‚HCl unit interacts with two other
units. The existence of C-H‚‚‚Cl hydrogen bonding is now well
recognized.25 A side view of 5‚HCl shows the wavelike
arrangement of the H-bonded network (Figure 6, bottom).
In order to study the influence of the nature of the counterion
(geometry and/or electronegativity) on the solid-state structure,
we prepared 5‚HBF4 and observed the formation of a new
H-bonded arrangement (Figure 7). Each monocation interacts
with four BF4 anions (three BF4 anions with F‚‚‚CH2(t-Bu)
separations in the range 3.391-3.594 Å and one BF4 with
F‚‚‚HN separations of 2.071 and 2.193 Å) which themselves
interact with four monocations. Therefore, 5‚HBF4 forms in the
solid state a 3-D supramolecular network resulting from the
tetrahedral geometry of BF4-. The bonding parameters are very
similar to those for 5‚HCl and are given in the Supporting
Information.
Figure 6. Crystalmaker top and side views of the cation-anion associations
in 5‚HCl. Color coding: nitrogen, blue; hydrogen, red; chloride, green.
In 5‚2HCl‚3CHCl3, a further step of intermolecular associa-
tion compared to that of 5‚HCl is achieved through the presence
of a second chloride ion on the opposite side of the dication
(Figure 4) which generates a 2-D supramolecular network based
on C-H‚‚‚Cl bonding interactions involving now one 5‚2HCl
unit in interaction with four other units (Figure 8). Each chloride
interacts again with two N-H protons (see above) and a C-H
hydrogen of two CH2 groups (four C‚‚‚Cl distances in the range
3.759-3.845 Å). The Cl anions shown in Figure 8 are coplanar
and sandwiched between the planes containing the organic
cations, labeled A and B for clarity.
(25) Aakero¨y, C. B.; Evans, T. A.; Seddon, K. R.; Pa´linko´, I. New J. Chem.
1999, 145.
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A R T I C L E S
Siri et al.
Figure 8. Crystalmaker top and side views of the supramolecular
arrangement of 5‚2HCl in the crystal of 5‚2HCl‚3CHCl3, illustrating the
interactions of each chloride ion with two dications and the almost coplanar
arrangement of the chlorides between the A and B layers. Molecules of
CHCl3 are omitted for clarity.
a
Table 3. Comparative 1H NMR Data in CDCl3
Figure 7. Crystalmaker view of monocation of 5‚HBF4 interacting with
four BF4- (top) and BF4- interacting with four monocations (bottom, t-Bu
groups omitted for clarity). Color coding: nitrogen, blue; hydrogen, red;
fluoride, green.
solvent
516
5‚HCl
5‚2HCl
CDCl3
Ha
Hb
1.00
1.01(s)
1.09(s)
3.07(s)
3.23(d)
5.38
1.12
2.95
3.37
Compound 5 can therefore be considered as the parent
molecule to a series of salts whose supramolecular architectures
in the solid state can be very easily modulated and fine-tuned
as a function of the degree of protonation and of the counterion.
Hc
NH
5.16
6.75
5.63
11.92
8.20
9.79
NMR Spectroscopy. The 1H NMR data for 516 and its
chloride salts are summarized in Table 3. Compound 5 showed
only three C-H signals16 consistent with a fast intramolecular
double proton transfer involving two degenerate tautomers
generating in solution a structure of higher symmetry (Scheme
3).26 The detailed mechanism of the related double proton
transfer observed in the case of azophenine has been established
by kinetic NMR studies.27
a H codes: Ha (t-Bu protons), Hb (CH2 protons), Hc (olefinic protons).
Scheme 3. Tautomeric Equilibrium of 5 in Solution
5‚HCl revealed a C2 symmetry in solution which could be
Scheme 4. Tautomeric Equilibrium of 5‚HCl in Solution
explained similarly by a proton transfer involving two tautomers
1
(Scheme 4). It must be rapid on the H NMR timescale since
NMR studies carried out at low temperatures did not allow the
observation of the two species in solution. Two 1H NMR N-H
resonances are observed at δ ) 9.79 and 8.20 ppm in a 2:1
ratio, and the two pairs of chemically different CH2 groups
appear as a doublet at δ ) 3.23 ppm and a singlet at δ ) 3.07
ppm, respectively. The doublet collapses to a singlet upon
irradiation of the signal at δ ) 9.79 ppm, showing that these
protons are mutually coupled and that the resonance at δ )
9.79 ppm corresponds to the N-H not involved in a dynamic
process.
(26) For recent results on double proton transfer in N-H‚‚‚O and N‚‚‚H-O
systems, see: Osmialowski, B.; Kolehmainen, E.; Gawinecki, R. Chem.
Eur. J. 2003, 9, 2710.
(27) Rumpel, H.; Limbach, H.-H. J. Am. Chem. Soc. 1989, 111, 5429.
9
13798 J. AM. CHEM. SOC. VOL. 125, NO. 45, 2003

Novel Antiaromatic, Acidichromic Quinonediimines
A R T I C L E S
Table 4. Spectroscopic Data in CH2Cl2 of 5 and Its Protonated
Forms (relative intensities)
5
5‚HCl
5‚2HCl
λ (nm)
340(br) (1.0)
372(br) (1.0)
374(br) (1.0)
426(br) (<10-3
)
527(br) (<10-3
)
564(br) (<10-3
)
Figure 9. UV-vis spectra in CH2Cl2 of 5(- - -), 5‚HCl (s), and 5‚2HCl
(- -).
The cation of 5‚2HCl shows a D2h symmetry in solution
which is consistent with the protonation of both imino groups
of 5 inducing the electronic delocalization of the two positive
charges between the nitrogen atoms (Scheme 2). The four N-H
protons are shifted downfield (δ ) 11.92 ppm) with respect to
5‚HCl because of the lower electronic density due to the
presence of two positive charges.
Figure 10. Orbital interaction diagram obtained for the model of 5‚2HCl
by means of EHT calculations.
As expected, the NMR resonances for the protonated forms
are shifted downfield from those for the neutral parent 5. The
olefinic protons shift from δ ) 5.16 for 5 to 5.38 ppm for 5‚HCl
and to 5.63 ppm for 5‚2HCl in CDCl3, which is consistent with
the increase of the positive charge density on the corresponding
C-H carbon atom.
UV Absorption Spectroscopy. Compounds 5-7 show
intense absorption bands approximately at 340 nm that are
assigned to the intraquinone transitions of the ligands.10 The
absorption wavelength is therefore only slightly influenced by
the nature of R, at least when R is an alkyl group. In contrast,
the color can be modulated by exogenous additives such as a
metal ion. Indeed, we recently reported the color change from
yellow to green in the diplatinum derivative 10,16 and this
prompted us to study the influence of the simplest electrophile,
the proton, on the color change (Figure 2 and Table 4).
To gain more insight into the relation between the electronic
spectrum and the π-electron delocalization (or not) of our
systems, theoretical calculations were performed on 5, 5‚HCl,
and 5‚2HCl.
Theoretical Calculations. Geometry optimizations have been
carried out at the DFT level on models of 5, 5‚HCl, and 5‚
++
2HCl, respectively referred to as 5H, 5H+, and 5H in which
the four neopentyl substituents have been replaced by hydrogens.
The calculated distances are displayed in Table 2. The maximal
discrepancy with respect to the distances observed in the real
molecules is equal to 0.022 Å (C(2)-N(2) distance in the
monoprotonated system). The neutral molecule has a fully
quinonic structure with a marked alternation of short and long
bonds throughout both N-C-C-C-N moieties. Conversely,
both moieties are fully delocalized in the diprotonated molecule,
but the communication between the two 6π subsystems is
interrupted by two long C(1)-C(2) and C(4)-C(5) bonds, both
longer than 1.5 Å, with very little π character. In fact, the π
MOs of 5H and its protonated derivatives can be deduced from
an interaction diagram between the π orbitals of two [H2N-
(CH)3-NH2]n+ fragments (n ) 0,1), as displayed in Figure 10
for 5H++. Since the two π subsystems are forced to approach
each other because of the σ framework, the orbitals do overlap
and split into a stabilized bonding level and its antibonding
counterpart. But despite this interaction, the two π subsystems
basically remain separated in the molecular ground state, since
both the bonding and the antibonding terms of each interacting
couple are either doubly occupied or unoccupied. Each π-bond-
ing contribution is therefore canceled by its antibonding
Compounds 5‚HCl and 5‚2HCl display a broad, intense
absorption band at 372 and 374 nm, respectively, which can be
assigned to the intraquinone transition (Figure 9).
Each compound revealed an additional, very weak absorption
(Table 4).
The pronounced red shift of the intraquinone transition
absorption of 5‚HCl and 5‚2HCl with respect to 5 (λmax
)
340 nm) may result from the delocalization of the conjugated
π system. Similarly, organic conjugated chromophores with
more π delocalization exhibit a more cyanine-like character.28,29
(28) Bourhill, G.; Bre´das, J.-L.; Cheng, L.-T.; Marder, S. R.; Meyers, F.; Perry,
J. W.; Tiemann, B. G. J. Am. Chem. Soc. 1994, 116, 2619.
(29) Gorman, C. B.; Marder, S. R. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 11297.
9
J. AM. CHEM. SOC. VOL. 125, NO. 45, 2003 13799

A R T I C L E S
Siri et al.
counterpart.30 The diagram of Figure 10 summarizes the
“coupling principles” formulated 35 years ago by Da¨hne and
Leupold to assign the visible spectrum of organic dyes,2 and
the Kohn-Sham orbitals obtained for the ground state of the
12π-electron systems studied in the present work match these
principles. Qualitatively similar diagrams involving one or two
nonsymmetric fragments could indeed be obtained for 5H+ and
for 5H, respectively. In the monoprotonated molecule, the N(2)-
C(2)-C(3)-C(4)-N(3) moiety appears quite delocalized, whereas
the neutral fragment displays bond length alternation. An
important difference between the protonated molecules consid-
ered isolated or in the crystal concerns the pyramidality at
nitrogen N(1) (Figure 3). The sum of the angles at N(1) in the
optimized geometry of the 5H+ model amounts to 342°, which
corresponds to a pronounced pyramidality. For comparaison,
the sum of the angles calculated at N(2) and N(3), in the
delocalized part, is close to 357°, with experimental values of
359.8 and 359.5°, respectively. Since no such trend toward
pyramidality is calculated either at the NH2 groups of 5H (sum
of the experimental angles around N(1) in 5: 357.5°) or at that
of the isolated fragment HN-(CH)3-NH2 (11), the weakening
of the CdN double bond character in the quinonic structure is
probably not sufficient to induce an umbrella folding at N. We
expect the H‚‚‚H nonbonding contact (2.12 Å) between the
closest hydrogens of the neighboring NH2 groups to activate
the trend toward pyramidality of the weakly conjugated nitrogen.
This trend is, however, not observed in the crystal structure of
5‚HCl where deviations from planarity do not exceed 0.2 Å
(sum of the angles at N(1) ) 359.8°). This difference could be
due to the influence of the adjacent counterion (Figure 3) or to
the presence of the alkyl substituent in the real molecule. An
and a destabilized, out-of-phase MO. A good overview of the
π MOs in 5‚2HCl can therefore be obtained from an orbital
interaction diagram between the π levels of two [H2N-(CH)3-
NH2)]+ fragments. This diagram obtained from extended Hu¨ckel
calculations is displayed in Figure 10.
The orbital splitting will be mainly controlled by the
coefficient of the π orbital on the terminal carbons of the
fragment. This coefficient is large in the two fragment π-unoc-
cupied orbitals, which generates an important splitting. It is
much reduced in the set of three occupied fragment orbitals;
the splitting is relatively small and all of the six resulting MOs
are close in energy. That is why the two π f π* transitions of
the visible spectrum and some of those in the near UV result
from a transition to the molecular LUMO. The molecular levels
have been labeled according to point group D2h, to which
belongs 5H++, the model of the dication. According to the
symmetry selection rules, the HOMO-LUMO transition, la-
beled 1 in Figure 10, should be forbidden, whereas transitions
labeled 2-5 should be allowed.
Although the diagram of Figure 10 generates the orbitals of
the fully symmetric 5H++ dication, it can also be used to assign
the π f π* transitions of 5 and 5‚HCl. The π orbitals of the
neutral HN-(CH)3-NH2 fragment are basically similar in shape
and in energy to those of [H2N-(CH)3-NH2]+ despite a slight
deviation from C2V symmetry. The symmetry of the resulting
molecule is therefore lowered to Ci for 5H (allowing for a slight
out-of-plane distortion of (0.12 Å on hydrogens), and to Cs
for 5H+, assumed planar. In the latter case only, the HOMO f
LUMO transition becomes formally allowed, but the oscillator
strength remains weak, as can be expected (Table 5).
The lowest excitation energies of 5H, 5H+, and 5H++ and those
of fragments 11 and 12+ (see Table 2) have been calculated
using the TD-DFT formalism31-34 as implemented in ADF and
in Gaussian 98 (See Computational Details). The computed
energies and oscillator strengths are displayed in Table 5. It is
important to recall that the assignment of the π f π* excitations
in terms of the orbital-to-orbital scheme of Figure 10 represents
an approximation, since each excited state should be properly
described in terms of a combination of occupied-to-virtual
contributions of same symmetry. The proposed assignment refers
to the contributions with highest weight.
An interesting trend can be deduced from the HOMO f
LUMO excitation energy in fragments 11 and 12+. In the
nonsymmetric, neutral fragment, this lowest π f π* transition
has its highest weight (55%) in an excited-state calculated at
5.40 eV (230 nm). The energy of the lowest π f π* excited
state in the cationic fragment, which mainly corresponds to the
same HOMO f LUMO excitation, is significantly decreased
at 5.21 eV (238 nm). The replacement of 11 by 12+ which
occurs once for 5H+ and twice for 5H++ therefore explains the
bathochromic trend observed upon protonation for the two
lowest π f π* excitations (Table 5).
+
optimization of 5H with a constraint of complete planarity
yielded a destabilization of 1.9 kcal.mol-1. In the planar form,
the calculated C(1)-N(1) distance is reduced from 1.387 to
1.364 Å, in better agreement with experiment (1.368 Å), and
the amplitude of the bond alternation in the quinonic fragment
is somewhat decreased (Table 2), with practically no change in
the delocalized part. Note that the amplitude of the C-C bond
alternation in the localized moiety of planar 5H+ (1.372 Å/1.437
Å) is slightly lower than in the isolated fragment HN-(CH)3-
NH2 (11) (1.360 Å/1.448 Å), suggesting a residual influence
of the delocalized fragment on the π system.
The coupling principles formulated first by Da¨hne and
Leupold can also be used to assign the π f π* transitions of 5,
5‚HCl, and 5‚2HCl. According to Da¨hne and Leupold,2 organic
dyes are composed of several polymethine structural units of
the type X-(CR)n-X′, with n odd and X and X′ being
heterosubstituents. The π system of such units is generally
composed of p + 1 (sometimes p - 1) electrons, p being the
number of chain atoms. Compounds 5, 5‚HCl, and 5‚2HCl meet
this definition since they are composed of two polymethine units
corresponding to n ) 3, p ) 5, and generating each 6π electrons.
Although each polymethine unit remains basically unaltered
because of the lack of interunit π delocalization, the π orbitals
of the isolated units are split in the resulting molecule in
proportion to their overlap. Each level of the isolated 6π system
generates in 5H or in its cations a stabilized, in-phase orbital
Excitation energies calculated with the hybrid B3LYP
functional are larger by 0.25-0.8 eV than those calculated with
BP86, and oscillator strengths are systematically lower. How-
(31) ADF user’s guide; Theoretical Chemistry Department, Vrije Universiteit,
Amsterdam, The Netherlands, 1999.
(32) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 41.
(33) Te Velde, G.; Baerends, E. J. J. Comput. Phys. 1992, 99, 84.
(34) Fonseca-Guerra, C.; Visser, O.; Snijders, J. G.; Te Velde, G.; Baerends, E.
J. In Methods and Techniques in Computational Chemistry: METECC-95;
Clementi E.; Corongiu, G., Eds.; STEF: Cagliari, Italy, 1995; p 305.
(30) The interaction between the π subsystems can be compared to that occurring
between two atoms of rare gas forced to approach each other. Atomic levels
are split by the interaction generating Pauli repulsion, but with no bond
and no electron delocalization.
9
13800 J. AM. CHEM. SOC. VOL. 125, NO. 45, 2003

Novel Antiaromatic, Acidichromic Quinonediimines
A R T I C L E S
Table 5. Relative Energies (nm) and Oscillator Strengths (f,
dimensionless) Calculateda for the Lowest-Excited States of 11,
HOMO f LUMO+1 transitions (Table 5). It should be noted
that these two π f π* excitations are not internal to the set of
four levels stemming from the splitting of the frontier orbitals
in fragments 11 and 12+ (Figure 10). The second excitation
allowed by symmetry within this orbital set, namely the
HOMO f LUMO+2 is the main contributor to a very intense
band predicted to appear around 200 nm.
++ b
12+, 5H, 5H+, and 5H
ADF/BP86
sym energy
A′ 230 0.47
12+ (C2V) B2 238 0.71
H (Ci) Ag 511
Au 353 0.20
Ag 331
Gaussian/B3LYP
energy
assignment
experimentc
fd
f
type
weighte
55
λmax (nm)
11 (Cs)
220 0.10
231 0.10
π f π*
π f π*
98
5
0
427
307 0.04
289
0
1
2
98
81
426
340
Conclusion
0
0
n f π* 100
Au 318 0.003 277 0.0006 n f π*
Au 251 0.14
99
43
34
In this article, we have presented a novel family of “poten-
tially antiaromatic” alkyl-substituted p-benzoquinonediimine
ligands of type 1. We have shown that these 12π-electron
molecules should be better considered as constituted by two
chemically connected but electronically not conjugated 6π-
electron subunits. Molecule 5 appears to be the first example
of two separated, conjugated, but localized, 6π-electron sys-
tems that can be tuned by reversible protonation to become
delocalized. The mono- and diprotonated derivatives have been
characterized by spectroscopic methods and X-ray diffraction.
These systems form supramolecular arrangements in the solid
state that clearly reflect the degree of protonation and depend
on the nature of the counterion. These compounds constitute
new chromophores for which the color can be tuned depending
on the degree of protonation, going in solution from yellow for
5 to red for 5‚HCl and blue for 5‚2HCl. Theoretical calculations
have provided a deeper insight into the electronic structure of
these molecules and allowed an assignment of the experimental
UV-vis spectra. The visible and near-UV spectrum of the
neutral and protonated benzoquinonediimines can be classically
assigned from the coupling of two 6π-electron polymethine
units. TD-DFT calculations confirm the observed red shift of
the two lowest π f π* transitions of the benzoquinonediimines
upon protonation and relate it to the moderate energy lowering
of the HOMO f LUMO transition induced by the delocalization
of the polymethine π system. The protonation-dependent
behavior of these molecules has prompted us to examine the
use of such molecules as sensors in the detection of protons,
including in biocompatible media, and the results will be
presented elsewhere.35 The obvious potential of these new
ligands in inorganic chemistry and homogeneous catalysis is
also being explored.
214 0.03
4
3
5H+ (Cs)f
A′ 613 0.01
A′′ 407 0.001 352 0.0002 n f π* 100
519 0.001
1
97
527
372
A′
A′
359 0.17
265 0.09
316 0.03
228 0.02
2
4
3
3
4
5
86
45
41
50
37
66
A′
A′
245 0.03
206 0.59
217 0.005
194 0.13
5H++ (D2h) B1g 682
0
602
351 0.03
217 0.04
0
1
2
3
4
3
4
5
98
83
66
31
31
65
83
564
374
B3u 396 0.18
B2u 247 0.17
B2u 230 0.004 207 0.0002
B3u 208 1.02 198 0.17
a With respect to the ground-state energy. The reported results have been
obtained using the TD-DFT formalism with ADF/BP86 and Gaussian/
B3LYP. b Assignments are proposed on the basis of the occupied-to-virtual
excitation with highest weight. The π f π* excited states follow the
numbering of Figure 10. c In CH2Cl2 (See Table 4). d A forbidden transition
is indicated by 0. e In %, from ADF calculations. f Assumed to be planar.
ever, the sequence of states and the energy differences are quite
similar and allow a reliable assignment of the experimental
spectrum. In 5H as in the two cationic systems, the lowest excited
state originates from the HOMO f LUMO excitation, which
++
is forbidden by symmetry in 5H and 5H and allowed with a
weak oscillator strength in 5H+. The red shift calculated from
+
++
5H to 5H (∼100 nm) and then to 5H (70-80 nm) parallels
the observed displacement of the weak band from λmax ) 426
nm for 5 to λmax ) 564 nm for 5‚2HCl (Table 5). The intense
band of the experimental spectra is assigned to a π f π*
excitation, the major component of which is the HOMO-1 f
LUMO transition. A significant red shift is also observed for
this band upon protonation (34 nm from 5 to 5‚2HCl), but the
calculated shift is more pronounced for the second protonation
process, at variance with experiment. The two nitrogen lone
pairs of 5 and the single one remaining in 5‚HCl should generate
as many low-energy n f π* transitions. Calculations locate
these transitions about 20 and 30-35 nm beyond the intense
band for 5H, but one-electron protonation shifts the n f π*
Experimental Section
Commercial analytical-grade reagents were obtained from com-
mercial suppliers and were used directly without further purification.
Solvents were distilled under argon prior to use and dried by standard
methods. 1H NMR spectra were recorded in CDCl3 and [d6]-dmso with
1
a AC300 Bruker spectrometer, operating at 300 MHz for H spectra.
Chemical shifts are reported in δ units, in parts per million (ppm)
relative to the singlet at δ ) 7.26 for CDCl3. Splitting patterns are
designated as s, singlet; d, doublet; t, triplet; q, quartet; sext, sextuplet;
m, multiplet; br, broad. Elemental analyses were performed by the
service de Microanalyse de l’Institut de Chimie, Strasbourg. FAB mass
spectral analyses were recorded on an autospec HF mass spectrometer,
and EI mass spectral analyses were recorded on a Finnigan TSQ 700.
+
transition to low energies by ∼70 nm, leading for 5H to an
inversion of the relative ordering with respect to the intense
π f π* transition. When allowed by symmetry, the n f π*
transitions are characterized by a very low oscillator strength,
and their detection in this part of the spectrum should prove
difficult. The occurrence of two π f π* excitations is then
predicted in the region 250-230 nm. These excitations are
allowed, with an intensity globally similar to that of the band
observed in the region 370-340 nm. Both excitations are
assigned to a mixture between HOMO-2 f LUMO and
Synthesis. 1,2,4,5-Tetraacetamidobenzene (3). Similarly to the
procedure described for the synthesis of 216 but using THF as a solvent,
tetraminobenzene tetrahydrochloride (500 mg, 1.76 mmol) was reacted
(35) Elhabiri, M.; Siri, O.; Sornosa-Tent, A.; Albrecht-Gary, A.-M.; Braunstein,
P. Chem. Eur. J., in press.
9
J. AM. CHEM. SOC. VOL. 125, NO. 45, 2003 13801

A R T I C L E S
Siri et al.
with acetyl chloride (553 mg, 7.04 mmol), and 3 was obtained as a
white solid (110 mg, 20%). 1H NMR (300 MHz, [d6]-dmso): δ )
2.05 (s, 12 H, CH3), 7.74 (s, 2 H, Harom), 9.32 (br s, 4 H, NH);
MS (40 eV, EI): m/z ) 306.2 [M]+. Anal. Calcd for C14H18N4O4: C,
54.89; H, 5.92; N, 18.29. Found: C, 54.09; H, 5.88; N, 18.05.
Computational Details. DFT calculations have been carried out on
models of 5, 5‚HCl, and 5.2HCl in which the four neopentyl substituents
have been replaced by hydrogens. These model molecules, respectively
referred to as 5H, 5H+, and 5H++, have been optimized within the
framework of the Generalized Gradient Approximation (GGA), as
implemented in the ADF program31-34 with the so-called BP86
exchange-correlation functional.36,37 The 1s shell of carbon and nitrogen
was frozen and described by a single Slater function. The valence shells
were described by triple-ú Slater orbitals and supplemented with one
polarization function.38,39 Molecular bonding energies are reported with
respect to an assembly of neutral atoms assumed isolated and in their
1,2,4,5-Tetrapropylamidobenzene (4). Similarly, tetraminobenzene
tetrahydrochloride (500 mg, 1.76 mmol) was reacted with propanoyl
chloride (651 mg, 7.04 mmol), and 4 was obtained as a white solid
(223 mg, 35%). 1H NMR (300 MHz, [d6]-dmso): δ ) 1.08 (t, 3JHH
)
3
7.5 Hz, 12 H, CH3), 2.33 (q, JHH ) 7.5 Hz, 8 H, CH2), 7.70 (s, 2 H,
H
arom), 9.24 (s, 4 H, NH); MS (40 eV, EI): m/z ) 362.3 [M]+. Anal.
ground state. Geometry optimizations have been carried out with the
Calcd for C18H26N4O4: C, 59.65; H, 7.23; N, 15.46. Found: C, 59.42;
H, 7.16; N, 15.39.
++
following symmetry constraints: C2h for 5H, C1 for 5H+, C2V for 5H
.
Planarity was then assumed for 5H+ and 5H++. The optimization cycles
were continued until all of the three following convergence criteria
were fulfilled: (i) the difference in the total energy between two
successive cycles is less than 0.001 hartree; (ii) the difference in the
norm of the gradient between two successive cycles is less than 0.001
hartree‚Å-1; (iii) the maximal difference in the Cartesian coordinates
between two successive cycles is less than 0.01 Å. The energies of the
lowest excited states have then been calculated using the TD-DFT
formalism,40 as implemented in ADF and using the same BP86
exchange-correlation functional. To test the influence of the exchange-
correlation functional on the calculated excitation energies, the TD-
DFT formalism was applied again to all three model systems using
the hybrid B3LYP functional, the all-electron 6-31G** set of basis
functions for all atoms, and reoptimized geometries. These calculations
were carried out with Gaussian 98.41-43
General Procedure for the Synthesis of 5‚HX. To a solution of 5
dissolved in THF (50 mL) were added a few drops of diluted HX (50/
50 v/v) until the color changed from yellow to deep red. The solution
was stirred at room temperature for 10 min. After evaporation to dryness
under reduced pressure, the residue was suspended in Et2O. Filtration
of the insoluble red solid afforded 5‚HX.
Synthesis of 5‚HCl. Route A. As described above in the general
procedure, 5‚HCl was obtained as a red solid (0.121 g, 75%). 1H NMR
(300 MHz, CDCl3): δ ) 1.01 (s, 18 H, CH3), 1.09 (s, 18 H, CH3),
3
3.07 (s, 4 H, CH2), 3.23 (d, JHH ) 6.0 Hz, 4 H, CH2), 5.38 (s, 2 H,
Holefinic), 8.20 (br s, 1 H, NH), 9.79 (br s, 2 H, NH). Anal. Calcd for
C26H49ClN4: C, 68.91; H, 10.90; N, 12.36. Found: C, 68.46; H, 11.18;
N, 12.08.
Route B. To a blue solution of 5‚2HCl (prepared as detailed below)
dissolved in THF (50 mL) was added a yellow solution of 5 dissolved
in THF (20 mL). The color of the solution turned red. The solution
was stirred at room temperature, and after 10 min, the solvent was
evaporated under reduced pressure. The residue was taken up in Et2O,
and the insoluble red suspension was filtered, affording quantitatively
5‚HCl.
X-ray Data. Selected crystals were mounted on a Nonius Kappa-
CCD area detector diffractometer (Mo KR, λ ) 0.71073 Å). The
complete conditions of data collection (Denzo software) and structure
refinements are given in Table 1. The cell parameters were determined
from reflections taken from one set of 10 frames (1.0° steps in phi
angle), each at 20 s exposure. The structures were solved using direct
methods (SIR97) and refined against F2 using the SHELXL97 software.
The absorption was not corrected. All nonhydrogen atoms were refined
anisotropically. Hydrogen atoms were generated according to stereo-
chemistry and refined using a riding model in SHELXL97.44
Synthesis of 5‚HBF4. Using the general procedure, we similarly
1
obtained 5‚HBF4 as a red solid (0.148 g, 68%). H NMR (300 MHz,
CDCl3): δ ) 1.03 (s, 18 H, CH3), 1.06 (s, 18 H, CH3), 3.14 (s, 4 H,
3
CH2), 3.20 (d, JHH ) 5.6 Hz, 4 H, CH2), 5.50 (s, 2 H, Holefinic), 7.06
(br s, 2 H, NH), 8.29 (br s, 1 H, NH). Anal. Calcd for C26H49BF4N4:
C, 61.90; H, 9.79; N 11.11. Found: C, 61.43; H, 9.77; N, 11.09.
Acknowledgment. We gratefully acknowledge support of this
research by the Centre National de la Recherche Scientifique
Synthesis of 5‚2HCl. To a solution of 5 dissolved in THF (50 mL)
was added dropwise a large excess of HCl 12N (0.25 mL). The solution
was stirred at room temperature for 10 min. The precipitate was then
(36) Perdew, J. P. Phys. ReV. 1986, B34, 7406.
(37) Becke, A. D. Phys. ReV. 1988, A38, 3098.
1
collected by filtration as a blue solid (0.133 g, 66%). H NMR (300
(38) Snijders, J. G.; Baerends, E. J.; Vernooijs, P. At. Nucl. Tables 1982, 26,
483.
MHz, CDCl3): δ ) 1.12 (s, 36 H, CH3), 3.37 (s, 8 H, CH2), 5.63 (s,
2 H, Holefinic), 11.92 (br s, 4 H, NH). Anal. Calcd for C26H50Cl2N4: C,
63.78; H, 10.29; N, 11.44. Found: C, 62.66; H, 10.36; N, 11.24.
(39) Vernooijs, P.; Snijders, J. G.; Baerends, E. J. Slater type basis functions
for the whole periodic system (Internal Report); Free University of
Amsterdam: The Netherlands, 1981.
(40) Casida M. E. In Recent DeVelopments of Modern Density Functional
Theory; Seminario, J. M., Ed.; Theoretical and Computational Chemistry,
Vol. 4; Elsevier: Oxford, 1996, p 391.
(41) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys. 1988, 109,
8218.
N,N′,N′′,N′′′-Tetraethyl-p-benzoquinonediimine (6). Similarly to
the procedure described for the synthesis of 5,16 3 (110 mg, 0.359 mmol)
was reduced with LiAlH4, and 6 was obtained as a yellow solid (35
1
3
mg, 39%). H NMR (300 MHz, CDCl3): δ ) 1.28 (t, JHH ) 7.5 Hz,
12 H, CH3), 3.27 (q, 3JHH ) 7.5 Hz, 8 H, CH2), 5.22 (s, 2 H, Holefinic),
6.75 (br s, 2 H, NH); MS (40 eV, EI): m/z ) 248.3 [M]+. Anal. Calcd
for C14H24N4: C, 67.70; H, 9.74; N, 22.56. Found: C, 67.12; H, 9.64;
N, 22.41. UV-vis (CH2Cl2): λmax ) 339 nm [ꢀ ) 26400 mol-1 dm3
cm-1].
(42) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.6; Gaussian,
Inc.: Pittsburgh, PA, 1998.
(43) For a critical and systematic evaluation of TD-DFT for the calculation of
excitation energies in organic chromophores with the hybrid B3LYP
functional, see Fabian, J.; Diaz, L. A.; Seifert, G.; Niehaus, T. J. Mol.
Struct.: THEOCHEM 2002, 594, 41.
(44) (a) KappaCCD Operation Manual; Bruker Nonius BV: Delft, The
Netherlands, 1997. (b) Sheldrick, G. M. SHELXL97, Program for the
Refinement of Crystal Structures; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
N,N′,N′′,N′′′-Tetrapropyl-p-benzoquinonediimine (7). Similarly,
4 (223 mg, 0.615 mmol) was reduced with LiAlH4, and 7 was obtained
as an orange solid (109 mg, 58%). 1H NMR (300 MHz, CDCl3): δ )
3
3
0.99 (t, JHH ) 7.5 Hz, 12 H, CH3), 1.70 (sext, JHH ) 7.5 Hz, 8 H,
CH3-CH2), 3.20 (br s, 8 H, N-CH2), 5.22 (s, 2 H, Holefinic), 6.35 (br
s, 2 H, NH); MS (40 eV, EI): m/z ) 304.5 [M]+. Anal. Calcd for
C18H32N4: C, 71.01; H, 10.59; N, 18.40. Found: C, 70.42; H, 10.65;
N, 17.94. UV-vis (CH2Cl2): λmax ) 339 nm [ꢀ ) 26800 mol-1 dm3
cm-1].
9
13802 J. AM. CHEM. SOC. VOL. 125, NO. 45, 2003

Novel Antiaromatic, Acidichromic Quinonediimines
A R T I C L E S
(CNRS) and the Ministe`re de l’Education Nationale, de la
Recherche et de la Technologie.
their optimal geometries (DFT/BP86) (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
The crystallographic material can also be obtained from the
CCDC, the deposition numbers being CCDC 218698-218700.
Supporting Information Available: X-ray crystallographic
data for structure determinations of 5‚HCl, 5.2HCl.3CHCl3, and
++
5‚HBF4 (CIF). Cartesian coordinates of 5H, 5H+, and 5H in
JA035463U
9
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