A 6π + 6π potentially antiaromatic zwitterion preferred to a quinoidal structure: Its reactivity toward organic and inorganic reagents

Article abstract of DOI:10.1021/ja0354622

A straightforward synthesis of the zwitterionic benzoquinonemonoimine 8 is reported. This molecule is a rare example of a zwitterion being more stable than its canonical forms. It is shown that 8 is best described as constituted of two chemically connecte

Full text of DOI:10.1021/ja0354622

Published on Web 09/16/2003
A 6π + 6π Potentially Antiaromatic Zwitterion Preferred to a
Quinoidal Structure: Its Reactivity Toward Organic and
Inorganic Reagents
Pierre Braunstein,*,† Olivier Siri,† Jean-philippe Taquet,† 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, F-67070 Strasbourg Cedex, France, Laboratoire
de Chimie Quantique, UMR 7551 CNRS, UniVersite´ Louis Pasteur, 4, rue Blaise Pascal,
F-67070 Strasbourg Cedex, France, and 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: A straightforward synthesis of the zwitterionic benzoquinonemonoimine 8 is reported. This
molecule is a rare example of a zwitterion being more stable than its canonical forms. It is shown that 8 is
best described as constituted of two chemically connected but electronically not conjugated 6 π electron
subunits. Its reactivity with electrophiles such as H+, CH3+, and metal salts leads to the synthesis of new
12 π electron molecules 12 (H+), 14 (CH3+), and 20 (Pd2+), respectively, in which one or both 6 π electron
subsystems localize into an alternation of single and double bonds, as established by X-ray diffraction.
The acidity of the N-H protons of 8 can be modulated by an external reagent. Dependent on the electrophile
used, the control of the π system delocalization becomes possible. When the electrophile simply adds to
the zwitterion as in 12, 14, or 15, there is no more negative charge to be delocalized and only the positive
charge remains delocalized between the nitrogen atoms. Furthermore, when a reaction with the electrophilic
reagent results in deprotonation, as in 17-21, there remains no charge in the system to be delocalized.
DFT calculations were performed on models of 8, 12, 14, 20, and on other related zwitterions 9 and 10 in
order to examine the influence of the fused cycles on the charge separation and on the singlet-triplet
energy gap. An effect of the nitrogen substituents in 8 is to significantly stabilize the singlet state. The
dipole moment of 8 was measured to be 9.7 D in dichloromethane, in agreement with calculated values.
The new ligands and complexes described in this article constitute new classes of compounds relevant to
many areas of chemistry.
with lower chemical reactivity and higher stability.7 For
example, the attempt to prepare the two electron oxidation
Introduction
More than one century ago, Kekule´ proposed the 6-fold
symmetric structure of benzene for explaining the characteristic
differences in physical properties and chemical reactivity
between benzenoid and nonbenzenoid compounds.1 During the
following decades, organic chemists were so much involved in
this field that the theory of “aromatic character” was broadened
to encompass a wider range of molecules. The concept of
“aromaticity” was then defined by the Hu¨ckel rule on the basis
of quantum-mechanical considerations.2 According to this rule,
an aromatic compound is a fully conjugated, planar monocyclic
molecule with 4n + 2 π electrons (n ) 1, 2, 3...). However,
definitions continue to evolve3 and are currently based on
experimental and theoretical data while a number of criteria
are now accepted to define aromaticity.3 Furthermore, aroma-
ticity is not confined only to planar rings but also exists in three-
dimensional systems.4-6 “Aromaticity” was often associated
product of benzene, that is, an unsubstituted antiaromatic
dication (see below), failed, which provided additional evidence
for the stability of benzene toward oxidation (eq 1).8,9
However, a strong ring current throughout the π system does
not always guarantee benzene-like reactivity and stability. This
(3) See the following reviews: (a) Minkin V. I.; Glukhovtsev M. N.; Simkin
B. Y. Aromaticity and Antiaromaticity. Electronic and Structural Aspects;
Wiley: New York, 1994. (b) von Rague´ Schleyer, P.; Jiao, H. Pure Appl.
Chem. 1996, 68, 209. (c) Special issue on aromaticity, Chem. ReV. 2001,
101, No. 5 (guest editor: P. von Rague´ Schleyer). (d) Smith, M. B.; March,
J. March’s AdVanced Organic Chemistry, 5th ed.; Wiley: New York, 2001.
† Laboratoire de Chimie de Coordination.
‡ Laboratoire de Chimie Quantique.
§ Laboratoire DECMET.
(1) Kekule´, A. Bull. Soc. Chim. Fr. (Paris) 1865, 3, 98.
(2) Hu¨ckel, E. Z. Phys. 1931, 70, 204.
9
12246
J. AM. CHEM. SOC. 2003, 125, 12246-12256
10.1021/ja0354622 CCC: $25.00 © 2003 American Chemical Society

6π + 6π Potentially Antiaromatic Zwitterion
A R T I C L E S
Chart 1
was observed in the case of substituted benzene compounds.
For example, when tetraaminobenzene was prepared by Nietzki,
the compound was found to undergo spontaneous oxidation in
air at room temperature, yielding the 2,5-diamino-1,4-benzo-
quinonediimine (eq 2).10 With 12 π electrons, this molecule is
antiaromatic according to Breslow’s definition of a planar,
conjugated 4n π electrons system.11 Since we shall return to
the question of antiaromaticity in the course of this and
following papers,12 it is appropriate here to explain what will
be understood with this term. The question was again raised
recently whether a fundamental difference exists between
nonaromatic and antiaromatic cyclic, conjugated 4n π electrons
systems.13 Although he had not considered in his review the
six-membered ring systems in this respect, Wiberg concluded
that the “smaller 4n π species are markedly destabilized if they
are planar...When the ring size becomes larger, the antiaromatic
character is decreased and is small eVen for cyclooctatet-
raene”.13 Shaik and Hiberty on the other hand reject the classical
interpretation of an electronic delocalization initiated by the π
electrons and show that the bond length equivalence in a
delocalized system is due to the σ backbone.14
The 2,5-diamino-1,4-benzoquinonediimine shown in eq 2
belongs to the large and important class of quinonoid molecules,
defined on a structural basis and displaying specific and
remarkable chemical and physical properties.17 For years, studies
on structure/color relationships have typically involved quinones
with N-substituted amino groups (Chart 1) whose synthesis
generally requires substitution at the quinonoid system.18 Indeed,
when a reasonably strong electron donor group is attached to a
benzoquinone structure, an intense absorption band in the visible
region always appears which can be attributed to an electronic
transition involving a transfer of electron density from the donor
group to the quinone moiety. For example, aminoquinones such
as 2,5-diamino-1,4-benzoquinones 1 represent a color system
for which the physicochemical properties can only be fully
understood if the molecule is regarded as constituted of a pair
of merocyanine structural units coupled via two C-C bonds
(see 2, Chart 1).19a The determination of interatomic distances
confirmed the coupling concept by revealing an equalization
of bond lengths within the W-like merocyanine subunits with
values intermediate between single and double bonds and a
stretching of the coupling bonds to the range of C-C σ bonds.19b
Similarly, 4,5-diamino-o-benzoquinones 3 reveal two coupled
merocyanine structural elements involving 6 π electrons as
indicated in 4 (Chart 1).20
Quinonemonoimines occupy a central role in many com-
mercial applications, such as color photography21 and hair-
coloring chemistry.22,23 In view of the potential of such
molecules as ligands in coordination and organometallic chem-
istry, we recently developed a versatile access that does not
require initial N-substitution of the amine before its condensation
with benzoquinone.24 This synthesis involved oxidation of
tetraaminobenzene 5 into 6 which was then treated with an acyl
chloride and water to afford 7 while 5 was acting as a reducing
agent (Scheme 1). Reduction of 7 followed by an aerobic
workup led to an intermediate A which rearranges to B by
proton migration from the oxygen atom to the more basic
Cyclobutadiene is perhaps the most typical example of
antiaromatic molecule, and it is interesting that a closely related,
all-metal antiaromatic 4 π system was recently reported with
[Al4]4- 15
Its π-HOMO is bonding along the two shorter Al-
.
Al bonds but is antibonding between the two Al2 pairs, resulting
in the rectangular structure and antiaromatic character. This
system may therefore be viewed as two 2 π electron systems
connected by two σ single bonds. With a 4n π electron count
of 8, the phenolate anion is however clearly not antiaromatic.16
Obviously, it is composed of two separate π subsystems, one
aromatic (the phenyl ring) and the other nonaromatic (the O-
ion). The resulting phenolate retains the aromatic character of
the ring. In most cases, however, the π system of exocyclic
substituents should be taken into account, since it exerts an
influence on the intracyclic delocalization. In most substituted
4n π systems, the arrangement of the substituents is compatible
with a consistent scheme of bond alternation throughout the
system. The molecule then localizes its π electrons and escapes,
through a quinoidal structure, the destabilization associated with
antiaromatic delocalization. In other, less common cases, a
partition scheme of the 4n π system not reducible to an
alternation of single and double bonds can successfully compete
with the canonical quinonoid species. At variance with quinon-
oids, a single π f π* excitation in these systems is sufficient
to fully restore π delocalization and antiaromaticity. We will
term such systems “potentially antiaromatic”.
(4) King, R. B. Chem. ReV. 2001, 101, 1119.
(5) (a) von Rague´ Schleyer, P.; Najafian, K. Inorg. Chem. 1998, 37, 3454. (b)
Balakrishnarajan, M. M.; Hoffmann, R. Angew. Chem. Int. Ed. 2003, 42,
3777.
(6) Bu¨hl, M.; Hirsch, A. Chem. ReV. 2001, 101, 1153.
(7) Hafner, K. Angew. Chem., Int. Ed. Engl. 1964, 3, 165.
(8) Pagni, R. M. Tetrahedron 1984, 40, 4161.
(17) (a) Bock, H.; Ruppert, K.; Na¨ther, C.; Havlas, Z. Angew. Chem., Int. Ed.
Engl. 1991, 30, 1180. (b) Pata¨ı, S.; Rappoport, Z. The Chemistry of the
Quinonoid Compounds; Wiley and Sons: New York, 1988; Vols. 1 and 2.
(18) (a) Kallmayer, H. J.; Tappe, C. Arch. Pharm. (Weinheim, Ger.) 1985, 318,
569. (b) Kallmayer, H. J.; Tappe, C. Arch. Pharm. (Weinheim, Ger.) 1986,
319, 421. (c) Tindale, C. R. Aust. J. Chem. 1984, 37, 611.
(9) Prakash, G. K. S.; Rawdah, T. N.; Olah, G. A. Angew. Chem., Int. Ed.
Engl. 1983, 22, 401.
(10) (a) Nietzki, R.; Hagenbach, E. Ber. Dtsch. Chem. Ges. 1887, 20, 328. (b)
Nietzki, R. Ber. Dtsch. Chem. Ges. 1887, 20, 2114.
(11) Breslow, R. Angew. Chem., Int. Ed. Engl. 1968, 7, 565.
(12) Siri, O.; Braunstein, P.; Rohmer, M.-M.; Be´nard, M.; Welter, R. J. Am.
Chem. Soc., in press.
(13) Wiberg, K. B. Chem. ReV. 2001, 101, 1317.
(14) Shaik, S.; Shurki, A.; Danovich, D.; Hiberty, P. C. Chem. ReV. 2001, 101,
1501.
(19) (a) Da¨hne, S.; Leupold, D. Angew. Chem., Int. Ed. Engl. 1966, 5, 984. (b)
Kulpe, S. Angew. Chem., Int. Ed. Engl. 1966, 5, 599.
(20) Da¨hne, S.; Leupold, D.; Radeglia, R. J. Pr. Chem. 1972, 314, 525.
(21) Tong, L. K. J. In The Theory of the Photographic Process; James, T. H.,
Ed.; New York, 1977; Chap 12.
(22) (a) Corbett, J. F. J. Soc. Cosmt. Chem. 1973, 24, 103. (b) Corbett, J. F. J.
Soc. Cosmet. Chem. 1984, 35, 297. (c) Brown, K. C.; Corbett, J. F. J. Soc.
Cosmet. Chem. 1979, 30, 191.
(23) Zviak, C. Science des traitements capillaires; Masson: Paris, 1988.
(24) Siri, O.; Braunstein, P. Chem. Commun. 2002, 208.
(15) Kuznetsov, A. E.; Birch, K. A.; Boldyrev, A. I.; Li, X.; Zhai, H.-J.; Wang,
L.-S. Science 2003, 300, 622.
(16) We thank a referee for raising this question.
9
J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003 12247

A R T I C L E S
Braunstein et al.
Scheme 1. Synthesis of Zwitterion 8 Reported in 200224
Chart 2
However, the efficiency of the original preparation of 8 was
limited by the low yield of the synthesis of 7 owing to the
difficult control of the oxidation step, thus limiting the scope
of the synthesis of new benzoquinonemonoimines such as 8
and their use as precursors in organic and inorganic chemistry.
An improved synthesis was therefore desirable, and this article
reports a new and extremely efficient synthesis of 7 and 8. The
high yields obtained with the new procedure have allowed a
detailed investigation of the reactivity of 8 and the synthesis of
new 12 π electron molecules in which either part of the π
system, or all of it, displays an alternation of single and double
bonds as a result of electron relocalization. DFT calculations
were performed on 8 and on the related zwitterions 9 and 10 in
order to examine the influence of the fused cycles on the charge
separation and on the singlet-triplet energy gap.25,26 We will
finally discuss the amount of π delocalization in 8 and related
compounds and examine whether these molecules are best
described as 12 π electron systems or as constituted of two
chemically connected but electronically not conjugated 6 π
electron subunits.
nitrogen site, leading to the first zwitterionic quinonemonoimine
8 constituted by two molecular moieties in which the conjugated
π system is fully delocalized (Scheme 1). The situation in 8
represents a novel electronic pattern for N, N, O, O systems
since previously known quadrupole merocyanine-type structures
2 and 4 display electronic delocalization exclusively along the
N-C-C-C-O chain of atoms with no occurrence of a
zwitterionic structure.
The zwitterion 8 is a planar, 12 π electron system. The
theoretical analysis reported below will lead us to envisage this
molecule as a potentially antiaromatic system constituted by
two 6 π systems facing each other and connected by two C-C
σ bonds. It is relevant that recent studies have also reported
rare cases of potentially antiaromatic compounds with zwitte-
rionic ground states, such as 9 and 10, with π electron counts
also typical of antiaromaticity (4n with n ) 4) (Chart 2).25,26 In
these heterocyclic zwitterions, the charge differenciation also
results from the splitting of the π system into two separated
subunits.
These conjugated π subsystems are composed of 6 + 10
electrons and linked by two C-C bonds of the central carbon
rings and by two single N-N or N-S bonds in the fused
heterocycles. The interruption of π delocalization along the fused
cycles therefore makes these heterocyclic compounds electroni-
cally similar to a single ring with four exocyclic substituents.
Zwitterion 8 appears to be a first choice reagent in organic
and coordination chemistry which could give access to new
ligands and complexes with a benzoquinonemonoimine-type
structure. To our knowlegde, no N, N, O, O quinones have so
far been used as ligands in the preparation of metal complexes,
although they could give access to novel classes of compounds.27
Results and Discussion
Synthesis of Zwitterion 8. We developed a new synthesis
of 8 starting from the commercially available diaminoresorcinol
11‚HCl instead of tetraaminobenzene as originally reported.24
Smooth reaction of 11‚HCl with t-BuC(O)Cl in CH3CN and
excess NEt3 afforded 7 in high yield (90%) (Scheme 2), whereas
the previously reported synthesis afforded 7 in 25% yield
maximum (Scheme 1). Interestingly, 1H NMR data of compound
7 revealed the presence of two, far apart aromatic signals (6.93
and 8.88 ppm), suggesting that, in solution, one aromatic proton
is strongly hydrogen bonded (δ ) 8.88 ppm) to the two carbonyl
oxygen atoms, while the other is not. Reduction of 7, followed
by an aerobic workup, led to the formation of 8 as a purple
solid.24
Reactivity of 8 with Organic Electrophiles. We decided
first to use the simplest electrophile and found that protonation
of one oxygen atom of 8 occurs easily upon treatment with
strong acids such as HCl, affording 12 as a red compound
(Scheme 3).
(25) (a) Beer, L.; Oakley, R. T.; Mingie, J. R.; Preuss, K. E.; Taylor, N. J. J.
Am. Chem. Soc. 2000, 122, 7602. (b) Beer, L.; Brusso, J. L.; Cordes, A.
W.; Haddon, R. C.; Itkis, M. E.; Kirschbaum, K.; MacGregor, D. S.; Oakley,
R. T.; Pinkerton, A. A.; Reed, R. W. J. Am. Chem. Soc. 2002, 124, 9498.
(26) Hutchison, K.; Srdanov, G.; Hicks, R.; Yu, H.; Wudl, F.; Strassner, T.;
Nendel, M.; Houk, K. N. J. Am. Chem. Soc. 1998, 120, 2989.
(27) Note added in proof. A dinickel complex of a benzoquinone has just been
reported: Zhang, D.; Jin, G.-X. Organometallics 2003, 22, 2851.
9
12248 J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003

6π + 6π Potentially Antiaromatic Zwitterion
Scheme 2. New Synthesis of Zwitterion 8
A R T I C L E S
Scheme 3. Reaction of 8 with H+
The 1H and 13C NMR data of 12 revealed a higher symmetry
of the molecule in solution which is explained by the presence
of an intermolecular keto-enolic equilibrium involving two
tautomers (12 and 13) in solution (Scheme 3).28 Both neopentyl
groups become then equivalent on the NMR time scale.
Furthermore, a 3JHH coupling of each CH2 group with one N-H
proton was observed, and the N-H proton signal appeared at
δ ) 8.32 ppm. As expected, both CdCsH protons are
downfield shifted with respect to those in 8. The 15N NMR
spectrum of 12 showed one signal at δ ) -261 ppm which is
in a range between amine and imine nitrogen atoms29 and
therefore consistent with the delocalization of the π system
between the two nitrogen atoms, as observed in the case of 8
(δ ) -264 ppm). Compound 12‚2CHCl3 could be isolated as
a crystalline solid by slow evaporation of a CHCl3 solution,
and the X-ray analysis confirmed the presence of an OH group.
Its structure is shown in Figure 1, and crystallographic data and
selected bond lengths are listed in Tables 1 and 2, respectively.
Examination of the bond distances within the O(1)-C(2)-
C(1)-C(6)-O(2) moiety revealed an alternation of single and
double bonds, whereas the C-C and C-N distances of the
N(1)-C(3)-C(4)-C(5)-N(2) moiety show a bond equalization
with the positive charge being delocalized between the two
nitrogen atoms (Table 2). The C(2)-C(3) and C(6)-C(5)
distances of 1.494(4) and 1.527(4) Å, respectively, are consistent
with single bonds and the lack of conjugation between the
O-C-C-C-O and N-C-C-C-N moieties. Thus, two
conjugated π systems, respectively without and with delocal-
ization, coexist in 12‚2CHCl3. In contrast, full delocalization
within each of the π subsystems occurred in 8 with delocal-
ization of the positive charge between the nitrogen atoms and
of the negative charge between the oxygen atoms (Table 2). Its
C-C distances in the range from 1.379(5) to 1.399(5) Å show
a remarkable bond equalization. In contrast, the C(2)-C(3) and
C(6)-C(5) distances of 1.516(5) and 1.523(5) Å correspond to
single bonds and indicate the lack of conjugation between the
two “halves” of the ligand (Table 2).24 The Cl(1) atom in
12‚2CHCl3 is located near the protonated site and interacts with
the O(1)-H proton by hydrogen bonding (d(O(1)-H‚‚‚‚Cl(1)
) 2.192(6) Å).
Formation of a p-benzoquinonemonoimine with localized
bonding from 12 by removal of the iminium proton did not
occur since the O-H group is more acidic than N-H and only
compound 8 was recovered upon treatment of 12 with a base
such as NEt3. We then decided to alkylate an oxygen atom of
8 in order to obtain a cationic species susceptible to be
deprotonated at the iminium nitrogen. Usual alkylating agents
such as methyl iodide did not react with 8, which is consistent
with recent reports on other zwitterions.30 The O-methylation
reaction was successful only after treatment with (R3O)BF4
reagents (R ) Me, Et) in dichloromethane at room temperature
which afforded 14 and 15 in high yields (Scheme 4). Com-
pounds 14 and 15 are highly soluble in acetone, dichlo-
romethane, or protic solvents, such as water and alcohols, but
insoluble in hydrocarbons.
1
The H NMR data of 14 revealed coupling of both CH2
groups with one N-H proton. The 15N spectrum contains two
signals at δ ) -245 and -255 ppm consistent with chemically
different nuclei, and these chemical shifts are in a range between
those of amine and imine nitrogen atoms.29 These values are
therefore in agreement with the delocalization of the π system
between the two nitrogen atoms, as observed in the case of 8
(δ ) -264 ppm). Compound 14‚CH2Cl2 could be isolated as a
crystalline solid from a solution of dichloromethane/n-hexane
and analyzed by X-ray crystallography. Its structure is shown
(28) Bonnet, A.; Barre, G.; Gilard, P. J. Chim. Phys. 1995, 92, 1823.
(29) Rabiller, C.; Ricolleau, G.; Martin, M. L.; Martin, G. J. New J. Chem.
1980, 4, 35.
(30) Kirby, A. T.; Komarov, I. V.; Bilenko, V. A.; Davies, J. E.; Rawson, J. M.
Chem. Commun. 2002, 2106.
Figure 1. Ortep view of structure of 12 in 12‚2CHCl3. Thermal ellipsoids
are drawn at the 50% level.
9
J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003 12249

A R T I C L E S
Braunstein et al.
Table 1. Crystal Data and Details of the Structure Determination for Compounds 12‚2CHCl3, 14‚CH2Cl2, and 20‚CH2Cl2
crystal data
12‚2CHCl3
14‚CH Cl2
20‚CH Cl2
2
2
formula
C16H27N2O2Cl‚2CHCl3
553.57
C17H29N2O2BF4‚CH2Cl2
465.16
C21H32N2O4Pd‚CH2Cl2
567.81
formula weight (g mol-1
)
crystal system
space group
a [Å]
b [Å]
c [Å]
R [deg]
â [deg]
γ [deg]
V [Å3]
monoclinic
P21/c
12.653(5)
11.538(5)
18.794(5)
90
97.116(5)
90
2722.6(18)
4
monoclinic
C2/c
32.358(2)
13.502(2)
24.127(2)
90
116.35(5)
90
9445.8(17)
16
triclinic
Ph1
9.483(5)
11.395(5)
12.583(5)
90.876(5)
102.760(5)
97.566(5)
1313.2(10)
2
1.436
0.938
584
Z
density (calc) [g cm-3
]
1.346
0.745
1136
1.308
0.322
3904
µ(Mo KR) [mm-1
F(000)
]
crystal size [mm3]
temperature (K)
radiation [Å]
0.08 × 0.10 × 0.13
0.13 × 0.10 × 0.02
0.15 × 0.12 × 0.10
173
173
173
Mo KR -0.710 69
2.4, 30.0
-17/17; 0/16; 0/26
13853, 7823, 0.045
4243
7823, 302
0.0861, 0.2014, 1.128
0.01, 0.00
-0.830, 1.125
Mo KR -0.710 69
2.06, 27.46
-41/37; 0/17; 0/31
35013, 10504, 0.036
4649
10504, 524
0.0925, 0.1818, 1.123
0.01, 0.00
-0.516, 0.788
Mo KR -0.710 69
2.19, 32.07
-14/14; -16/17; 0/18
15068, 9068, 0.040
7686
9068, 284
0.0365, 0.0929, 1.029
0.00, 0.00
-1.017, 0.953
θ min-max [deg]
data set [h; k; l]
tot., uniq. data, R(int)
observed data [I > 2σ(I)]
N reflections, N parameters
R, wR2, GOF
max and avg shift/error
min, max resd. dens. [e/Å3]
Table 2. Interatomic Distances (Å) Observed in 8, Its Protonated (12), Methylated (14), and Palladated (20) Derivatives, and Comparison
with the Calculateda Values in Model Compound 8H and in the Related Cationic Species 12H, 14H, and Metalated 20H
8
8H
12‚2CHCl3
12H
14‚CH Cl2
14H
20‚CH Cl2
20H
AMe
2
2
obsd
calcd
obsd
calcd
obsd
calcd
obsd
calcd
calcd
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(2)-O(1)
C(3)-N(1)
C(5)-N(2)
C(6)-O(2)
Pd-O(1)
1.379(5)
1.399(5)
1.516(5)
1.393(5)
1.389(5)
1.523(5)
1.265(4)
1.323(5)
1.320(5)
1.254(4)
1.406
1.406
1.545
1.393
1.393
1.545
1.258
1.325
1.325
1.258
1.337(4)
1.432(5)
1.494(4)
1.405(4)
1.370(4)
1.527(4)
1.330(4)
1.311(4)
1.324(4)
1.224(4)
1.356
1.450
1.490
1.397
1.390
1.538
1.350
1.332
1.322
1.228
1.334(7)
1.427(7)
1.508(7)
1.375(7)
1.382(7)
1.517(7)
1.354(6)
1.328(6)
1.314(6)
1.243(6)
1.361
1.447
1.494
1.397
1.390
1.537
1.341
1.332
1.323
1.230
1.369(2)
1.416(3)
1.500(3)
1.414(2)
1.377(2)
1.525(3)
1.297(2)
1.320(2)
1.333(2)
1.235(2)
1.9860(15)
1.9952(16)
1.383
1.425
1.497
1.418
1.378
1.534
1.309
1.321
1.340
1.250
2.055
2.020
1.364
1.436
1.503
1.431
1.380
1.529
1.334
1.305
1.344
1.245
Pd-N(1)
a Geometries optimized from DFT/BP86 calculations. The subscript H or Me indicates that the neopentyl substituents have been replaced by H or Me,
respectively.
Scheme 4. Reaction of 8 with (R3O)BF4 Reagents (R ) Me, Et)
in Figure 2, and crystallographic data and selected bond lengths
are listed in Tables 1 and 2, respectively.
(7) and 1.517(7) Å, respectively, correspond again to single
bonds and indicate a lack of conjugation between the two
“halves” of the ligand. The tetrahedral counterion interacts by
hydrogen bonding with the N(1)-H proton; the shortest distance
of 2.219(6) Å corresponds to the F(7)‚‚‚‚H-N(1) interaction.
The solid-state structure of 14‚CH2Cl2 revealed a complicated
network due to the presence of solvent molecules and π-π
Similarly to 8, examination of the bond distances within the
O(1)-C(2)-C(1)-C(6)-O(2) moiety revealed alternating single
and double bonds, whereas the C-C and C-N distances in the
N(1)-C(3)-C(4)-C(5)-N(2) moiety show a bond equalization
(Table 2). The C(2)-C(3) and C(6)-C(5) distances of 1.508-
9
12250 J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003

6π + 6π Potentially Antiaromatic Zwitterion
A R T I C L E S
except by X-ray crystallography since no single crystals suitable
for a structure determination were isolated. Formation of a σ
bond between the oxygen and the metal center and a dative
bond between the nitrogen and the metal center has occurred.
Only one isomer is present in solution, and although the cis
or trans configuration of the isomer could not be determined
from the NMR data, the cis geometry is likely to be disfavored
due to the proximity of the bulky neopentyl groups. Other square
planar nickel complexes with N, O ligands were found to adopt
1
the trans geometry (centrosymmetric).32,33 The H NMR data
revealed the presence of two far apart neopentyl CH3 signals
(δ ) 1.02 and 1.19 ppm), suggesting that one CH3 group is
closer to the nickel center. Only one N-H signal is observed
at δ ) 6.45 ppm which is consistent with an aminic proton.
Following a similar procedure, we also prepared the zinc and
copper analogues 18 and 19 by reaction of 8 with [Zn(acac)2]
and [Cu(acac)2], respectively. Compound 18 appeared to be
unstable in solution unlike its Ni and Cu analogues.
Interestingly, reaction of 8 with palladium bis(acetylacetonate)
gave in a stepwise manner the mononuclear complexes 20 and
21 (Scheme 6). The intermediate 20 could be isolated after
separation by preparative chromatography, whereas 21, which
precipitated, was recovered by filtration.
1
The H NMR data of 20 revealed the presence of only one
N-H proton. Compound 20‚CH2Cl2 could be isolated as a
crystalline solid from a solution of dichloromethane/n-hexane
and analyzed by X-ray crystallography. The structure is shown
in Figure 3, and crystallographic data and selected bond lengths
are listed in Tables 1 and 2, respectively.
The coordination geometry around the palladium center is
square planar. Examination of the bond distances within the
O(1)-C(2)-C(1)-C(6)-O(2) and N(1)-C(3)-C(4)-C(5)-
N(2) moieties reveals an alternation of single and double bonds,
which is consistent with two conjugated but localized π systems
(Table 2). The relative lengthening of the N(1)-C(3) bond is
due to N-coordination to the metal. The C(2)-C(3) and C(6)-
C(5) distances of 1.500(3) and 1.525(3) Å, respectively,
correspond again to single bonds and indicate the lack of
conjugation between the two π systems. Interestingly, the
packing in the solid state is such that two molecules form a
centrosymmetric pseudo-dimer with an intermetallic distance
of 3.536 Å.
We have found that, depending on the electrophile used, it
is possible to control the delocalization of the π system. When
the electrophile simply adds to the zwitterion as in 12, 14, or
15, there is no more negative charge to be delocalized and only
the positive charge remains delocalized between the nitrogen
atoms. Furthermore, when reaction with the electrophilic reagent
results in deprotonation, as in 17-21, there remains no charge
in the system to be delocalized.
Figure 2. Ortep view (top) of 14 in 14‚CH2Cl2 and crystalmaker perspective
views (middle and bottom) of the staking of two cations in the solid state.
Thermal ellipsoids are drawn at the 50% level.
interactions between two molecules in a face-to-face arrange-
ment, separated by 3.402 Å (Figure 2).31 There is a slippage of
0.747 Å horizontally and 1.345 Å vertically between the centers
of two molecules.
Deprotonation of 14 was achieved by treatment with NEt3 in
dicholoromethane at room temperature and afforded 16 as a
beige compound. Its 1H NMR data showed two signals at δ )
1.00 and 1.02 ppm corresponding to the methyl protons of the
two neopentyl groups. The methoxy protons appeared at δ )
3.82 ppm, and the N-H proton, at δ ) 5.41 ppm. Compound
16 is the first member of a new class of p-benzoquinonemon-
oimines.
Electronic Spectra
Reactivity of 8 with Inorganic Electrophiles. Reactions of
8 with inorganic electrophiles were examined in view of the
promising features of this ligand in coordination chemistry. Two
benzoquinonemonoimines were assembled by reaction of 8 with
[Ni(acac)2], which gave rapidly a green material 17 insoluble
in most solvents and poorly soluble in chloroform or dichlo-
romethane (Scheme 5). Compound 17 was fully characterized
The electronic spectral data are reported in Table 3. The UV-
vis absorption spectrum of 8 is characterized by two strong
bands at 350 nm (log ꢀ ) 4.49) and 343 nm (log ꢀ ) 4.45).24
Sawicka et al. have recently described the first theoretical
(32) Richeter, S.; Jeandon, C.; Ruppert, R.; Callot, H. J. Chem. Commun. 2001,
91.
(33) (a) Richeter, S.; Jeandon, C.; Ruppert, R.; Callot, H. J. Chem. Commun.
2002, 266. (b) Richeter, S.; Jeandon, C.; Gisselbrecht, J.-P.; Ruppert, R.;
Callot, H. J. J. Am. Chem. Soc. 2002, 124, 6168.
(31) Hunter, C. H.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525.
9
J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003 12251

A R T I C L E S
Braunstein et al.
Scheme 5. Reaction of 8 with M2+
Scheme 6. Reaction of 8 with Pd(acac)2
Table 3. Electronic Spectra λ (nm) in CH2Cl2
λ (log ꢀ)
analysis of 8, and they reported that the first band corresponds
to the π f π* absorption of 8 and the second one could be
assigned to a vibrational excitation on top of the electronic
transition.34a,35 Similarly to the UV-vis absorption spectrum
of 8, that of 12 is characterized by two strong absorptions at
337 and 349 nm which correspond to intraquinone charge
transfer. In addition, a weak absorption at 506 (log ꢀ ) 3.65) is
observed for 12. In contrast, 14 is characterized by only one
strong absorption at 327 nm (log ꢀ ) 4.26) for the intraquinone
charge transfer, and by the presence of one more band at 503
nm (log ꢀ ) 3.65). Similarly, 15 revealed two absorption bands
at 328 nm (log ꢀ ) 4.28) and 504 nm (log ꢀ ) 3.73).
Interestingly, the deprotonation of 14 to 16 led to a significant
blue shift of the highest energy band (∆λ ) 18 nm) and of the
lowest energy absorption band (∆λ ) 48 nm). The main
8
12
14
15
16
17
18a
19
20
21
343 (4.45), 350 (4.49)
337 (4.35), 349 (4.34), 506 (3.15)
327 (4.26), 503 (3.65)
328 (4.28), 504 (3.73)
311 (4.13), 455 (3.41)
333 (sh) (4.45), 485 (4.22), 521 (4.11)
338 (4.54), 352 (4.56)
292 (4.35), 361 (4.70), 376 (4.80)
379 (4.25), 395 (4.27), 412 (4.24), 530 (3.24)
296 (4.33), 419 (4.62), 444 (4.71)
a In CHCl3.
absorptions can be attributed to the π f π* transition of the
benzoquinone derivatives. The weak absorption band corre-
sponds to an n f π* transition originating at the nitrogen lone
pair.36
Coordination of a metal center to 8 leads to new absorption
bands corresponding to a metal-to-ligand charge transfer
(MLCT) for which the extinction coefficients are the most
intense. The UV-vis absorption spectrum of 17 shows three
bands at 333 (log ꢀ ) 4.45), 485 (log ꢀ ) 4.22), and 521 nm
(log ꢀ ) 4.11). Similarly, 21 revealed absorption bands at 296
nm (log ꢀ ) 4.33), 419 nm (log ꢀ ) 4.62), and 444 nm (log ꢀ
) 4.71). By comparison, the UV-vis absorption spectrum of
20 is characterized by three strong absorptions at 379 nm (log
ꢀ ) 4.25), 395 nm (log ꢀ ) 4.27), and 412 nm (log ꢀ ) 4.24).
We shall come back to these aspects in the theoretical analysis.
Theoretical Calculations
The first theoretical analysis of 8 has been recently pub-
lished.34 These authors carried out geometry optimizations at
the restricted Hartree-Fock level and energy calculations at the
MP2 level on the zwitterionic form of 8 and on various
tautomers displaying a “canonical” quinonoid structure. This
study was mainly focused on the N-H‚‚‚OfN‚‚‚H-O proton-
(34) (a) Sawicka, A.; Skurski, P.; Simons, J. Chem. Phys. Lett. 2002, 362, 527.
(b) Note added in proof: another theoretical analysis of 8 has been
performed by Le, H. T.; Nam, P. C.; Dao, V. L.; Veszpre´mi, T.; Nguyen,
M. T. Mol. Phys. 2003, in press.
Figure 3. Ortep view (top) of 20 in 20‚CH2Cl2 (thermal ellipso¨ıds are
drawn at the 50% level) and crystalmaker perspective view (bottom) of the
staking of two molecules in the solid state.
(35) We thank a referee for his comments.
(36) Zhang, R.; Zheng, H.; Shen, J. J. Mol. Struct. 1998, 446, 103.
9
12252 J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003

6π + 6π Potentially Antiaromatic Zwitterion
A R T I C L E S
Table 4. Properties of Hexaazaanthracene 9,
Chloro-bis-1,2,3-dithiazole 10, Models of 8 (8H and 8Me and 8Et;
Me, AEt, and A (Scheme 1)), and Pd Complex 20H Calculated at
the DFT/BP86 and (except for AMe and 20H) DFT/B3LYP Levelsa
transfer process and showed the zwitterionic form to be more
stable than the lowest canonical form (form A, see Scheme 1)
by 1.42 kcal mol-1, but the kinetic barrier to the proton transfer
A
was estimated to 13.2 kcal mol-1 34a
The attachment of an excess
.
BP86
B3LYP
electron to the various tautomers was also studied in detail.34a
The results we obtain from DFT calculations corroborate this
result, since the energy difference between models of 8 and A,
in which neopentyl substituents have been replaced by methyls,
is calculated to be 3.0 kcal mol-1 in favor of the zwitterion.
However, in the present theoretical section, we rather concentrate
on an analysis of the zwitterionic form of 8, on the origin of its
stability, and on the comparison between 8 and two series of
heterocyclic molecules displaying similar properties.
q
S−T µS µT C−C C−C
q
S−T µS µT C−C C−C
T
S
T
S
8H (0.42 25.7 7.9 6.1 1.545 1.464 (0.47 23.3 8.0 5.1 1.547 1.461
8Me (0.46 28.1 8.8 6.7 1.541 1.463 (0.49 25.7 8.6 5.6 1.545 1.462
9
(0.41 23.7 5.0 4.8 1.457 1.423 (0.35 18.9 5.0 4.6 1.461 1.421
10 -0.08 9.7 4.7 3.4 1.470 1.449 -0.61 2.4 4.4 2.8 1.478 1.446
+0.25b
+0.55b
AMe (0.03
5.1
5.8
1.503
1.529
1.497
1.534
20H -0.44
-0.11
ab initio MP234a
Indeed, several heterocyclic molecules, belonging to the
family of polyazaacenes 9 and dithiazoles 10 have been recently
characterized, by means of experiment and theory, to have a
zwitterionic ground state.25,26,37 As for the present benzoquino-
nemonoimine 8, the charge differentiation results from the
splitting of a potentially antiaromatic π system into two separate
subunits (Chart 2). These conjugated π subsystems are com-
posed of 6 + 10 electrons, in heterocyclic zwitterions, compared
to 6 + 6 electrons in 8. Another difference comes from the
linking between the π subsystems. In 8, this link is ensured
solely by the carbon-carbon bonds C(2)-C(3) and C(5)-C(6),
which display an exclusive σ character in accordance with the
observed and calculated bond lengths (Table 2). In 9, 10, and
related systems, the two C-C bonds of the central carbon rings
are supplemented as separators by two single bonds involving
heteroatoms of the adjacent cycles. The observed distances, as
in the analysis of the DFT calculations, suggest that these four
bonds retain a minor π bonding component.25,26,37 This implies
that the separation between the two π subsystems should be
more complete in 8. We report here some results obtained from
DFT/BP86 and DFT/B3LYP calculations. These calculations
have been carried out with ADF38 and with Gaussian 98,39
respectively, on 9, 10, and on two models of 8 in which the
neopentyl substituents have been replaced by H (8H) or CH3
(8Me). DFT/BP86 calculations have also been carried out on a
similar model of the Pd complex 20 (20H). Some computed
properties directly related to the zwitterionic ground states of
these compounds are listed in Table 4.
∆E
µS
8
8Et
A
0
0
10.0
10.1
4.8
+1.4
+1.0
AEt
5.1
a The dipole moments of 8Et, 8, AEt, and A in the lowest singlet state
(Debye) refer to the SCF calculations of Sawicka et al.34a ∆E (kcal mol-1
)
represents the relative energies of the corresponding tautomers, 8 and A
and 8Et and AEt.34a For the other molecules (this work; DFT/BP86 and DFT/
B3LYP levels of calculation) are reported the Mulliken charge q of the
two subsystems in the singlet state (electrons); the dipole moment of the
singlet (µS) and of the triplet state (µT) (Debye); the singlet-triplet energy
separation (S-T, kcal.mol-1); and the length of the carbon-carbon bond
separating the two π subsystems (C-CS, singlet ground state; C-CT, triplet
state; Å). ∆E(AMe) ) +3.0 kcal mol-1 with the BP86 functional. b The
charge of the Cl atom has not been taken into account.
LUMO of 8, 9, and 10 is destabilized because of antibonding
π interactions with the neighboring heteroatoms, which il-
lustrates the potential antiaromatic character of these molecules.
The lowest triplet state always results from an HOMO-LUMO
transfer. Populating the LUMO with one electron breaks the
separation between the π subsystems and provides the lateral
C-C bonds some π bonding character and a shorter bond length
(Table 2). The contraction of the lateral C-C bonds in the triplet
state is especially important in 8 due to the prominent weight
of the C-C π bonding contribution in the LUMO. The high
energy of the C-C bonding, C-X antibonding LUMO is
directly related to the relative stability of the zwitterionic ground
state. It should be emphasized that these systems in the lowest
triplet state display semioccupied orbitals with a π delocalized
character, illustrated in the spin density distributions (see
Supporting Information) rather than behaving like a standard
biradical with a localized arrangement of the unpaired electrons.
An important feature, common to 8, 9, and 10, is the π nature
of both the HOMO and the LUMO. In all three compounds,
the LUMO is delocalized over the whole molecular π system.
This orbital has a large weight, with π bonding character, on
the lateral C-C bonds of the C6 or C5N ring.37 This weight is
maximal in 8 (55%), in relation with the smaller extension of
the overall π system. Despite this C-C bonding character, the
The properties calculated for 9 and 10 at the B3LYP/6-31G**
level closely reproduce the result of previous theoretical
investigations.25,26,37 Even though the BP86 functional seems
to slightly overestimate the S-T energy gap, both series of
calculations suggest that the zwitterionic ground state calculated
for the models of 8 are most stable in the series, closely followed
by 9. The Mulliken charge separation between the two molecular
subsystems appears somewhat dependent upon the chosen basis
set, especially for 10. The zwitterionic character of the
considered molecules is more reliably illustrated by the dipole
(37) Wudl, F.; Koutentis, P. A.; Weitz, A.; Ma, B.; Strassner, T.; Houk, K. N.;
Khan, S. I. Pure Appl. Chem. 1999, 71, 295.
(38) User’s Guide, Release 1999; Chemistry Department, Vrije Universiteit:
Amsterdam, The Netherlands, 1999.
(39) 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. G., 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.
moment, which is also maximal for 8H and especially for 8Me
.
Most zwitterionic molecules are in close energetic competition
with an isomer not displaying a strong charge separation, which
is usually a biradical. Such a low energy biradical, with a
localized arrangement of the unpaired electrons, does not exist
9
J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003 12253

A R T I C L E S
Braunstein et al.
for 8 and 9. For 10, the lowest triplet is close in energy to the
ground state but displays as for 8 and 9 a π delocalized
character, illustrated in the spin density distributions (Supporting
Information). The high energy of the lowest triplet state
computed for 8 and 9, and, to a much lesser extent, for 10,
should be related to the fact that populating the LUMO with
one electron restores the communication between the two π
subsystems and therefore brings back into the molecule the
antiaromatic character that was kept away in the zwitterionic
form. As investigated by Sawicka et al.,34a a possibility to cancel
the charge separation between the molecular moieties without
introducing antiaromaticity would consist in transferring a proton
from an amino group to the closest quinonic oxygen. This
transfer generates a molecule that is assumed to be an undetected
intermediate in the synthesis of 8 (intermediate A, Scheme 1).
DFT/BP86 calculations on intermediate AMe, in which the CH2-
t-Bu substituents are modeled by CH3, show that this quinonoid
form is destabilized by 3.0 kcal mol-1. This is to be compared
with the relative energies of +1.42 and +1.03 kcal mol-1
computed by Sawicka et al. at the ab initio MP2 level for A
and AEt, respectively34a (Table 4). The present calculations point
out an important decrease of the dipole moment when going
from the zwitterionic form 8Me (8.8 D) to the quinonic tautomer
(5.1 D). The charge separation ((0.03 e) is close to zero,
compared to (0.46 e in the zwitterionic form (Table 4). These
values are in keeping with the results of Sawicka et al., showing
a decrease of the dipole moment from 10.0 D in 8 to 4.8 D in
A and from 10.1 D in 8Et to 5.1 D in AEt.34a The experimental
dipole moment was measured at 9.7 D in dichloromethane at
21 °C, which is consistent with calculated values.
Figure 4. LUMO of 20.
significant in the X-ray structure (Table 2). The LUMO of
complex 20H which represents the counterpart to this interaction
is displayed in Figure 4.
Protonation or methylation at one oxygen atom of 8 has quite
similar effects on the π electronic structure and subsequently
on the molecular geometry (Table 2). The π connection between
the alkoxide (or methoxy) substituent and the delocalized
subsystem next to it is strongly decreased. Therefore, the
remaining π subsystem tends to localize, giving rise to a strong
bond alternation quite reminiscent of the acrolein structure
(Table 2). The net charge of this molecular moiety becomes
slightly positive (+0.28 e for the protonated species, +0.30 e
for the methylated one). With an unmodified 6 π electron
system, the other molecular moiety remains fully delocalized,
with an increased positive charge (+0.72 e) and interatomic
distances practically unchanged with respect to those calculated
in 8H. The two π subsystems, localized and delocalized, remain
largely disconnected, even though the long C-C distance next
to the protonated or methylated oxygen decreases by ∼0.05 Å
to 1.49 Å (Table 2).
Conclusion
In the optimized geometry of AMe (Table 4), both lateral C-C
bonds C(2)-C(3) and C(5)-C(6) remain longer than 1.5 Å, at
variance with the same bond lengths in the lowest triplet state
of 8H and 8Me. The two π subsystems therefore remain largely
separated in A, but a marked alternation of the bond lengths is
initiated in both molecular moieties (Table 2), in agreement with
the quinonoid representation of the structure displayed in
Scheme 1. A bond alternation, restricted to the ROsCdCHs
CdO moiety, was observed in cations 12 and 14 and computed
in their models 12H and 14H. However, this alternation did not
affect the diamino system, which remained fully delocalized.
Interestingly, a similar, but somewhat weaker bond alternation
affecting the whole molecule is observed in the Pd complex 20
and computed in the model 20H.
We have found that the unusual zwitterionic benzoquinon-
emonoimine 8 is a rare example of a zwitterion being more
stable than its canonical forms and is best described as
constituted of two chemically connected but electronically not
conjugated 6 π electron subunits. Its new and straightforward
synthesis has allowed a study of its reactivity with electrophiles
such as H+, CH3+, and metal salts leading to the synthesis of
new 12 π electron molecules in which one or both 6 π
subsystems localize into an alternation of single and double
bonds. We have found that the acidity of the N-H protons of
8 can be modulated by an external reagent and that, depending
on the electrophile used, it is possible to control the delocal-
ization of the π system. When the electrophile simply adds to
the zwitterion as in 12, 14, or 15, there is no more negative
charge to be delocalized and only the positive charge remains
delocalized between the nitrogen atoms. Furthermore, when
reaction with the electrophilic reagent results in deprotonation,
as in 17-21, there remains no charge in the system to be
delocalized. DFT calculations were performed on 8 and on the
related zwitterions 9 and 10 in order to examine the influence
of the fused cycles on the charge separation and on the singlet-
triplet energy gap.25,26 An effect of the nitrogen substituents in
8 is to significantly stabilize the singlet state. The dipole moment
of 8 was measured to be 9.7 D in dichloromethane, in agreement
with calculated values. The new ligands and complexes
described in this article constitute new classes of compounds
relevant to many research areas such as organometallic chem-
istry, homogeneous catalysis, and color chemistry. The following
paper will report new antiaromatic quinonediimines whose
In complex 20, the PdII center is coordinated to the depro-
tonated form of 8. Despite the nitrogen deprotonation, which
cancels the zwitterionic character of the quinoneimine ligand,
the two π subsystems remain largely separated, as in 8. The
O-C-CH-C-O moiety retains the same negative charge as
in the isolated molecule (-0.44 e). Despite this charge
conservation, some bond alternation is observed in this part of
the molecule as in the amino-imino moiety. The geometry
changes induced by bond alternation are however less important
than for the hypothetical intermediate A, except for the C(2)-
C(3) bond whose length is reduced to 1.500(3) Å. This
shortening should be assigned to a slight back-bonding interac-
tion from the occupied orbital of Pd with appropriate symmetry
to the LUMO of the ligand which introduces some π bonding
character into the C(2)-C(3) bond and reduces the bond length
by ∼0.04 Å with respect to C(5)-C(6). This difference is also
9
12254 J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003

6π + 6π Potentially Antiaromatic Zwitterion
A R T I C L E S
1
15: 1.07 g, 90% yield. MS (FAB, 70 eV) m/z: 307 [M + 1]+. H
colors and degree of delocalization of their 6 π electron
subsystems can be tuned by protonation.12 In general, quinone-
type ligands are also of considerable interest in inorganic
chemistry owing to their often unique redox and electron-transfer
properties.40,41
NMR (300 MHz, CDCl3) δ: 1.06 (s, 9 H, CH3), 1.07 (s, 9 H, CH3),
3
3
1.65 (t, JHH ) 7.02 Hz, 3 H, CH3), 3.28 (d, JHH ) 6.66 Hz, 2 H,
CH2), 3.57 (d, 3JHH ) 7.05 Hz, 2 H, CH2), 4.27 (q, 3JHH ) 7.02 Hz, 2
H, OCH2), 5.85 (s, 1 H, NsCdCsH), 5.88 (s, 1 H, OsCdCsH),
7.60 (s br, 1 H, NsH), 8.90 (s br, 1 H, NsH). 13C NMR (50 MHz,
CDCl3) δ: 13.49 (CH3sCH2), 27.28 (CMe3), 27.44 (CMe3), 32.67
(CMe3), 34.14 (CMe3), 54.89 (CH2N), 55.7 (CH2N), 68.2 (OCH2), 85.24
(HsCdC), 102.91 (HsCdC), 149.64 (C N), 156.17 (C N), 159.84
(CsO), 178.71 (CdO). Anal Calcd. for C18H31BF4N2O2 C, 54.82; H,
7.93; N, 7.11. Found: C, 54.02; H, 7.72; N, 7.00.
Experimental Section
1
General. H NMR (300 or 400 MHz), 13C NMR (75 MHz), and
‚‚‚
‚‚‚
15N NMR (40.562 MHz) spectra were recorded on a Bruker AC-300,
DPX-400, or ARX-500 instrument. FAB mass spectral analyses were
recorded on an autospec HF mass spectrometer, and EI mass spectral
analyses were recorded on a Finnigan TSQ 700. Elemental analyses
were performed by the “Service de Microanalyse, Universite´ Louis
Pasteur (Strasbourg, France)”. Preparative chromatography was per-
formed using silica gel with an average particle size of 40 µm. Solvents
were freshly distilled under nitrogen prior to use. 4,6-Diaminoresorcinol
was purchased from Across, and trialkyloxonium tetrafluoroborate, from
Avocado. All reactions of air- or water-sensitive compounds were
performed using standard Schlenk techniques under dry argon atmo-
sphere.
Synthesis of 16. The methylated zwitterion 14 (0.20 g, 0.526 mmol)
was dissolved in anhydrous dichloromethane (50 mL), and an excess
of dry NEt3 was added to the solution at room temperature resulting
rapidly in a color change from red to yellow. After addition of water,
the aqueous phase was extracted with dichloromethane. The organic
phase was dried on magnesium sulfate and evaporated. The residue
was dissolved in pentane, and the solution was filtered and evaporated
to afford 16 as a yellow-brown powder (0.06 g, 40% yield). MS (EI,
70 eV) m/z: 292 [M+]. H NMR (300 MHz, CDCl3) δ: 1.00 (s, 9 H,
CH3), 1.02 (s, 9 H, CH3), 2.81 (d, JHH ) 6.7 Hz, 2 H, CH2), 3.46 (s,
1
3
Synthesis of the Zwitterion 8. To a suspension of 4,6-diaminore-
sorcinol 11 (1.00 g, 4.70 mmol) was added an excess of triethylamine
(4 mL) in acetonitrile (150 mL). After the mixture was stirred for 5
min at room temperature, 4 equiv of trimethylacetyl chloride (2.34 mL,
18.80 mmol) were added dropwise to the resulting solution. After the
mixture was further stirred at room temperature for 12 h, the solvent
was evaporated, the residue was taken up in water, and the insoluble
product in suspension was filtered, washed with water, and dried to
afford the diamidodiester 7 as a light brown powder (2.08 g, 92%).
This latter was reduced as previously described.24 8 was obtained as a
purple solid after crystallization from a mixture of dichloromethane
and hexane (0.84 g, 70% yield).
2 H, CH2), 3.82 (s, 3 H, OCH3), 5.41 (s br, 1 H, NsH), 5.52 (s, 1 H,
NsCdCsH), 5.77 (s, 1 H, OsCdCsH). 13C NMR (50 MHz, CDCl3)
δ: 27.80 (CMe3), 28.63 (CMe3), 32.10 (CMe3), 33.40 (CMe3), 54.13
(CH2N), 56.44 (CH2N), 64.15 (OCH3), 86.85 (HsCdC), 102.36 (Hs
CdC), 142.60 (CsN), 154.56 (CsO), 165.45 (CdN), 183.92 (CdO).
No satisfactory elemental analysis was obtained despite several attemps
owing to the moisture sensitivity of the product.
Synthesis of 17, 18, and 19. General procedure: The zwitterion 8
(0.50 g, 1.80 mmol) was dissolved in anhydrous toluene (100 mL),
and 0.5 equiv of [M(acac)2] (M ) Ni, Zn, Cu) (0.9 mmol) was added
to the solution. The mixture was then refluxed for 12 h. At room
temperature, a crystalline green (Ni) and red (Zn) powder was filtered
and washed with cold toluene to afford 17 and 18, respectively. When
M ) Cu, the purple solid which precipitated at room temperature was
first isolated by filtration and redissolved in anhydrous dichloromethane.
The solution was filtered through Celite and evaporated to dryness to
afford 19 as a purple solid.
Synthesis of 12. The zwitterion 8 (0.20 g, 0.72 mmol) was dissolved
in a mixture of anhydrous CH2Cl2-toluene (20 mL/80 mL), and diluted
aqueous HCl (ca. 0.5 mL concentrated HCl in 10 mL of toluene) was
added dropwise to the solution which resulted in an instantaneous color
change to intense violet at room temperature with precipitation of a
violet solid. After the solid was filtered and washed with toluene, 12
1
17: 0.45 g, 81% yield. MS (FAB, 70 eV) m/z: 613 [M + 1]+. H
1
was obtained quantitatively as a violet powder. H NMR (300 MHz,
NMR (400 MHz, CDCl3) δ: 1.02 (s, 18 H, CH3), 1.19 (s, 18 H, CH3),
3
CDCl3) δ: 1.05 (s, 18 H, CH3), 3.27 (d, JHH ) 6.6 Hz, 4 H, CH2),
3
2.73 (s, 4 H, CH2), 2.88 (d, JHH ) 6.3 Hz, 4 H, CH2), 5.20 (s, 2 H,
5.41 (s, 1 H, NsCdCsH), 6.26 (s br, 1 H, OsCdCsH), 8.32 (s br,
2 H, NsH). 13C NMR (50 MHz, CDCl3) δ: 27.50 (CMe3), 33.03
(CMe3), 55.13 (CH2N), 82.84 (HsCdC), 102.97 (HsCdC), 155.01
NsCdCsH), 5.31 (s, 2 H, OsCdCsH), 6.45 (t br, 2 H, NsH); 13
C
NMR (50 MHz, CDCl3) δ: 27.58 (CMe3), 27.89 (CMe3), 32.31 (CMe3),
35.85 (CMe3), 54.17 (CH2N), 57.181 (CH2N), 85.01 (HsCdC),
102.108 (HsCdC), 146.88 (CsN), 168.58 (CsO), 179.88 (CdN),
180.31 (CdO). Anal Calcd. for C32H50N4NiO4: C, 62.65; H, 8.22; N,
9.13. Found: C, 62.16; H, 8.22; N, 8.93.
‚‚‚
(C N), 170.02 (CdO). Anal Calcd. for C16H27ClN2O2‚0.5HCl: C,
57.78; H, 8.18; N, 8.42. Found: C, 58.06; H, 8.38; N, 8.25.
Synthesis of 14 and 15. General Procedure. The zwitterion 8 (0.84
g, 3.01 mmol) was dissolved in anhydrous CH2Cl2 (100 mL), and solid
trialkyloxonium tetrafluoroborate (0.45 g, 3.01 mmol) was added to
the solution. After the solution was stirred at room temperature for 3
h, the solvent was evaporated and the alkylated product was obtained
as a red powder by precipitation from a mixture of dichloromethane
and hexane.
1
18: 0.45 g, 80% yield. MS (FAB, 70 eV) m/z: 620 [M + 1]+. H
NMR (400 MHz, CDCl3) δ: 0.965 (s, 18 H, CH3), 1.02 (s, 18 H, CH3),
3
2.94 (s, 4 H, CH3), 3.32 (d, JHH ) 6.0 Hz, 4 H, CH3), 5.30 (s, 2 H,
NsCdCsH), 5.69 (s, 2 H, OsCdCsH), 6.80 (br t, 2 H, NsH); 13
C
NMR (50 MHz, CDCl3) δ: 27.65 (CMe3), 28.33 (CMe3), 32.38 (CMe3),
34.19 (CMe3), 54.18 (CH2N), 61.34 (CH2N), 83.85 (HsCdC), 102.14
(HsCdC), 149.29 (CdN), 164.26 (CsN), 173.23 (CsO), 179.69 (Cd
O). Anal Calcd. for C32H50N4O4Zn: C, 61.98; H, 8.13; N, 9.03.
Found: C, 61.67; H, 8.65; N, 8.81.
19: 0.57 g, 91% yield. MS (FAB, 70 eV) m/z: 618 [M + 1]+. Anal
Calcd. for C32H50CuN4O4‚CH2Cl2: C, 56.36; H, 7.45; N, 7.97. Found:
C, 55.87; H, 7.45; N, 7.76.
Synthesis of 20 and 21. The zwitterion 8 (0.50 g, 1.80 mmol) was
dissolved in anhydrous toluene (100 mL), and 1.5 equiv of [Pd(acac)2]
(0.82 g, 2.70 mmol) were added to the solution. The mixture was then
refluxed for 12 h. At room temperature, a crystalline brown powder
was formed, isolated by filtration, and washed with cold toluene. The
filtrate A was kept. The solid was redissolved in dichloromethane. The
solution was filtered through Celite and was evaporated to afford 21
1
14: 1.03 g, 90% yield. MS (FAB, 70 eV) m/z: 293 [M + 1]+. H
NMR (300 MHz, CDCl3) δ: 1.06 (s, 9 H, CH3), 1.07 (s, 9 H, CH3),
3.27 (d, 3JHH ) 6.7 Hz, 2 H, CH2), 3.53 (d, 3JHH ) 7.1 Hz, 2 H, CH2),
4.07 (s, 3 H, OCH3), 5.81 (s, 1 H, NsCdCsH), 5.89 (s, 1 H, Os
CdCsH), 7.41 (s br, 1 H, NsH), 8.99 (s br, 1 H, NsH). 13C NMR
(50 MHz, CDCl3) δ: 27.28 (CMe3), 27.44 (CMe3), 32.80 (CMe3), 34.15
(CMe3), 54.87 (CH2N), 55.81 (CH2N), 58.35 (OCH3), 85.12 (HsCdC),
‚‚‚
‚‚‚
102.66 (HsCdC), 149.86 (C N), 156.17 (C N), 160.77 (CsO),
178.61 (CdO). Anal Calcd. for C17H29BF4N2O2: C, 53.70; H, 7.69;
N, 7.37. Found: C, 53.41; H, 7.65; N, 7.35.
(40) Pierpont C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331.
(41) Min, K. S.; Weyhermu¨ller, T.; Wieghardt, K. J. Chem. Soc., Dalton Trans.
2003, 1126 and refernces cited.
9
J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003 12255

A R T I C L E S
Braunstein et al.
as a brown powder (0.07 g, 11.7% yield). The filtrate A was taken to
dryness. The brown residue was dissolved in dichloromethane and
purified by preparative chromatography on silica gel (eluant: CH2Cl2/
MeOH 99/1) to afford 20 as a red crystalline solid (0.36 g, 27% yield).
significantly deviate from planarity. 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
;
20: 1H NMR (300 MHz, CDCl3) δ: 0.97 (s, 9 H, CH3), 1.07 (s, 9
3
(iii) the maximal difference in the Cartesian coordinates between two
successive cycles is less than 0.01 Å. The geometries of 8H, 8Me, 9,
and 10 were then reoptimized and the energy differences recomputed
using the hybrid B3LYP exchange-correlation functional as imple-
mented in Gaussian 98.39 The all-electron 6-31G** set of basis functions
was then used for all atoms.
H, CH3), 2.00 (s, 3 H, CH3), 2.03 (s, 3 H, CH3), 2.85 (d, JHH ) 6.3
Hz, 2 H, CH2), 3.06 (s, 2 H, CH2), 5.27 (s, 1 H, NsCdCsH), 5.39 (s,
1 H, CsHacac), 5.59 (s, 1 H, OsCdCsH), 6.55 (br t, 1 H, NsH). 13
C
NMR (50 MHz, CDCl3) δ: 25.45 (CH3acac), 25.69 (CH3acac), 27.49
(CMe3), 29.00 (CMe3), 32.16 (CMe3), 35.73 (CMe3), 54.04 (CH2N),
‚‚‚
57.36 (CH2N), 84.79 (HsCdC), 101.37 (HsC
C
acac), 103.80 (Hs
Dipole Moment Measurement. The determination of the dipole
moment of 8 was achieved in a dichloromethane solution by measuring
the variation of the dielectric constant with the concentration.50 For
this purpose, an HP 16452A Liquid Test Fixture for capacitive
measurement connected to a QuadTech 7600 Precision LCR meter was
used. The measurement cell was immerged in a thermostatic bath for
temperature stabilization with a 0.1 °C precision, and the capacity is
investigated with a 1 V, 300 kHz modulated voltage.
X-ray Data. The selected crystals were mounted on a Nonius Kappa-
CCD area detector diffractometer (Mo KR, λ ) 0.710 73 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 non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were generated according to stereo-
chemistry and refined using a riding model in SHELXL97.51
CdC), 146.55 (CsN), 171.57 (CsO), 179.72 (CdN), 183.50 (CdO),
‚‚‚ ‚‚‚
185.50 (C acac), 189.14 (C
O
Oacac). Anal Calcd. for C21H32N2O4Pd:
C, 52.23; H, 6.68; N, 5.80. Found: C, 52.80; H, 6.85; N, 5.30.
1
21: MS (FAB, 70 eV) m/z: 661 [M + 1]+; H NMR (400 MHz,
3
CDCl3) δ: 0.99 (s, 18 H, CH3), 1.08 (s, 18 H, CH3), 2.87 (d, JHH
)
6.3 Hz, 4 H, CH2), 3.22 (s, 4 H, CH2), 5.24 (s, 2 H, NsCdCsH),
5.47 (s, 2 H, OsCdCsH), 6.48 (br t, 2 H, NsH). 13C NMR (50 MHz,
CDCl3) δ: 27.60 (CMe3), 28.82 (CMe3), 32.33 (CMe3), 36.04 (CMe3),
54.20 (CH2N), 61.26 (CH2N), 84.57 (HsCdC), 102.61 (HsCdC),
146.88 (CsN), 170.83 (CsO), 179.92 (CdN), 182.79 (CdO). Anal
Calcd. for C32H50N4O4Pd‚0.5CH2Cl2: C, 55.80; H, 7.31; N, 7.96.
Found: C, 55.60; H, 7.34; N, 7.93.
Computational Details. Calculations have been carried out first
using the formalism of the density functional theory (DFT) within the
generalized gradient approximation (GGA), as implemented in the ADF
program38,42-44 with the so-called BP86 exchange-correlation func-
tional.45-47 For first row atoms, the 1s shell was frozen and described
by a single Slater function. For complex 20H, the frozen core of the Pd
atom composed of the 1s to 3sp shells was also modeled by a minimal
Slater basis. For hydrogen, carbon, nitrogen, and oxygen, the Slater
basis set used for the valence shell is of triple-ú-quality, supplemented
with one p- or d-type polarization function. The 4s and 4p shells of
palladium are described by a double-ú-Slater basis; the 4d and 5s shells
by a triple-ú basis and the 5p shell by a single orbital. No polarization
function is added for Pd.48,49 Molecular bonding energies are reported
with respect to an assembly of neutral atoms assumed isolated and in
their ground state. Most geometry optimizations have been carried out
without any symmetry constraint, but the optimized structures never
Acknowledgment. We are grateful to the CNRS for support,
to the Ministe`re de la Recherche for a Ph.D. grant to J.P.T.,
and to Dr. Lo¨ıc Mager (IPCMS, Strasbourg) for his assistance
in the determination of the dipole moment of 8.
Supporting Information Available: Crystallographic details
for the structure of 12, 14, and 20 as cif files (CCDC 201315-
201317); total energies, Cartesian coordinates, and (for the triplet
states) spin density distribution of hexaazaanthracene (9), chloro-
bis-1,2,3-dithiazole (10), models of 8 (8H and 8Me), cationic
systems 12H and 14H, intermediate AMe and Pd complex 20H
optimized at the DFT/BP86 and (except for 20H) DFT/B3LYP
levels. This material is available free of charge via the Internet
at http://pubs.acs.org.
(42) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 41.
(43) Fonseca-Guerra, C.; Visser, O.; Snijders, J. G.; Te Velde, G.; Baerends, E.
J. Methods and Techniques in Computational Chemistry: METECC-95;
Clementi E., Corongiu, G., Eds.; STEF: Cagliari, Italy, 1995; p 305.
(44) Te Velde, G.; Baerends, E. J. J. Comput. Phys. 1992, 99, 84.
(45) Becke, A. D. Phys. ReV. 1988, A38, 3098.
(46) Perdew, J. P. Phys. ReV. 1986, B33, 8882.
(47) Perdew, J. P. Phys. ReV. 1986, B34, 7406.
JA0354622
(48) 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.
(50) Barriol, J. Les moments dipolaires; Gauthier-Villars: Paris, 1957.
(51) (a) Kappa CCD Operation Manual; Nonius B. V.: Delft, The Nederlands,
1997. (b) Sheldrick, G. M. SHELXL97, Program for the refinement of
crystal structures; University of Gottingen: Germany, 1997.
(49) Snijders, J. G.; Baerends, E. J.; Vernooijs, P. At. Nucl. Tables 1982, 26,
483.
9
12256 J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003