From hexaoxy-[6]pericyclynes to carbo-cyclohexadienes, carbo-benzenes, and dihydro-carbo-benzenes: Synthesis, structure, and chromophoric and redox properties

Article abstract of DOI:10.1002/chem.201102993

When targeting the quadrupolar p-dianisyltetraphenyl-carbo-benzene by reductive treatment of a hexaoxy-[6]pericyclyne precursor 3 with SnCl ₂/HCl, a strict control of the conditions allowed for the isolation of three C₁₈-macrocyclic products: the targeted aromatic carbo-benzene 1, a sub-reduced non-aromatic carbo-cyclohexadiene 4 A, and an over-reduced aromatic dihydro-carbo-benzene 5 A. Each of them was fully characterized by its absorption and NMR spectra, which were interpreted by comparison with calculated spectra from static structures optimized at the DFT level. According to the nucleus-independent chemical shift (NICS) value (NICS≈-13 ppm), the macrocyclic aromaticity of 5 A is indicated to be equivalent to that of 1. This is confirmed by the strong NMR spectroscopic deshielding of the ortho-CH protons of the aryl substituents, but also by the strong shielding of the internal proton of the endocyclic trans-CH=CH double bond that results from the hydrogenation of one of the C≡C bonds of 3. Both the aromatics 1 and 5 A exhibit a high crystallinity, revealed by SEM and TEM images, which allowed for a structural determination by using an X-ray microsource. A good agreement with calculated molecular structures was found, and columnar assemblies of the C₁₈ macrocycles were evidenced in the crystal packing. The non-aromatic carbo-cyclohexadiene 4 A is shown to be an intermediate in the formation of 1 from 3. It exhibits a remarkable dichromism in solution, which is related to the occurrence of two intense bands in the visible region of its UV/Vis spectrum. These properties could be attributed to the dibutatrienylacetylene (DBA) unit that occurs in the three chromophores, but which is not involved in a macrocyclic π-delocalization in 4 A only. A versatile redox behavior of the carbo-chromophores is evidenced by cyclic voltammetry and was analyzed by calculation of the ionization potential, electron affinity, and frontier molecular orbitals. Copyright

Full text of DOI:10.1002/chem.201102993

DOI: 10.1002/chem.201102993
From Hexaoxy-[6]Pericyclynes to Carbo-Cyclohexadienes, Carbo-Benzenes,
and Dihydro-Carbo-Benzenes: Synthesis, Structure, and Chromophoric and
Redox Properties
Lꢀo Leroyer,[a, b] Christine Lepetit,[a, b] Arnaud Rives,[a, b] Valꢀrie Maraval,*[a, b]
Nathalie Saffon-Merceron,[c] Dmytro Kandaskalov,[a, b] David Kieffer,[a, b] and
Remi Chauvin*[a, b]
Abstract: When targeting the quadru-
polar p-dianisyltetraphenyl-carbo-ben-
zene by reductive treatment of a hex-
aoxy-[6]pericyclyne precursor 3 with
SnCl2/HCl, a strict control of the condi-
tions allowed for the isolation of three
C18-macrocyclic products: the targeted
aromatic carbo-benzene 1, a sub-re-
duced non-aromatic carbo-cyclohexa-
diene 4A, and an over-reduced aro-
matic dihydro-carbo-benzene 5A. Each
of them was fully characterized by its
absorption and NMR spectra, which
were interpreted by comparison with
calculated spectra from static structures
optimized at the DFT level. According
to the nucleus-independent chemical
shift (NICS) value (NICSꢃꢀ13 ppm),
the macrocyclic aromaticity of 5A is
indicated to be equivalent to that of 1.
This is confirmed by the strong NMR
spectroscopic deshielding of the ortho-
CH protons of the aryl substituents,
but also by the strong shielding of the
internal proton of the endocyclic trans-
CH=CH double bond that results from
evidenced in the crystal packing. The
non-aromatic carbo-cyclohexadiene 4A
is shown to be an intermediate in the
formation of 1 from 3. It exhibits a re-
markable dichromism in solution,
which is related to the occurrence of
two intense bands in the visible region
of its UV/Vis spectrum. These proper-
ties could be attributed to the dibuta-
trienylacetylene (DBA) unit that
occurs in the three chromophores, but
which is not involved in a macrocyclic
p-delocalization in 4A only. A versatile
redox behavior of the carbo-chromo-
phores is evidenced by cyclic voltam-
metry and was analyzed by calculation
of the ionization potential, electron af-
finity, and frontier molecular orbitals.
ꢁ
the hydrogenation of one of the C C
bonds of 3. Both the aromatics 1 and
5A exhibit a high crystallinity, revealed
by SEM and TEM images, which al-
lowed for a structural determination by
using an X-ray microsource. A good
agreement with calculated molecular
structures was found, and columnar as-
semblies of the C18 macrocycles were
Keywords: alkynes · aromaticity ·
carbo-mers
·
dichromism
·
reduction
Introduction
The definition of carbok-merization, that is, the systematic
insertion of C2k units and corresponding 4k reactive p elec-
trons into all the bonds of a Lewis structure,[1] raises the
basic question of their chemical stability, even for the first
generation (k=1) that is henceforth addressed.[2] Three
types of C2 units that differ in intrinsic stability can be dis-
[a] Dr. L. Leroyer, Dr. C. Lepetit, Dr. A. Rives, Dr. V. Maraval,
Dr. D. Kandaskalov, D. Kieffer, Prof. R. Chauvin
CNRS, LCC (Laboratoire de Chimie de Coordination)
205 Route de Narbonne, 31077 Toulouse (France)
Fax : (+33)561-55-30-03
ꢀ ꢁ ꢀ
E-mail: vmaraval@lcc-toulouse.fr
tinguished: the acetylene type (A C C B), the butatriene
chauvin@lcc-toulouse.fr
ꢁ ꢀ ꢁ
type (A=C=C=B), and the diacetylene type (A C C B). In
the cyclic series,[3] the so-called “ring carbo-mers” of the
first kind (with acetylenic C2 units only) were exemplified
early on by Scottꢀs [n]pericyclynes, the ring carbo-mers of
[n]cycloalkanes,[4] which were later on generalized to oxy-
functional versions:[5,6] in spite of the vicinity of the nC2
units, the relative stability of small [n]pericyclynes (nꢂ4)
can be explained by a relaxative bending at the -sp-C- cen-
ters, and by steric insulation of the triple bonds between the
propargylic substituents.[1b,7] Compared to alkynes, buta-
trienic molecules are less available (butatriene itself is less
stable than its butenyne isomer),[8] and carbo-mers of the
[b] Dr. L. Leroyer, Dr. C. Lepetit, Dr. A. Rives, Dr. V. Maraval,
Dr. D. Kandaskalov, D. Kieffer, Prof. R. Chauvin
Universitꢁ de Toulouse, UPS, INPT, LCC
31077 Toulouse (France)
[c] Dr. N. Saffon-Merceron
Universitꢁ de Toulouse, UPS
Institut de Chimie de Toulouse, ICT-FR 2599
118 Route de Narbonne, 31062 Toulouse (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102993. It includes experi-
mental procedures and characterization data for 1, 3, 4A, and 5A.
Calculated structures and absorption spectra and NMR spectroscopic
data for 1, 4A, 4B, and 5A. X-ray diffraction analysis data for 5A.
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FULL PAPER
second kind (which contain butatrienic C2 units) are mainly
found in the family of the ring carbo-mers of benzene (rou-
tinely called “carbo-benzenes”),[9] in which the butatriene
edges are stabilized by resonance of the two equivalent
Kekulꢃ structures of the C18 carbo-benzene ring.[1b,5,9] This
macrocycle indeed satisfies the Hꢄckel rule just as the
parent C6 benzene ring does and proved to be definitely aro-
matic according to various calculated structural, magnetic,
and energetic indices.[10] In the dozen substituted carbo-ben-
zene representatives synthesized to date, the aromaticity is
revealed by their stability, by the planarity and symmetry of
give the expected pericyclynediol 3 as a mixture of diaste-
reoisomers (14 in theory) in 76% yield (Scheme 2).
1
Scheme 2. Preparation of [6]pericyclynediol 3 (mixture of stereoisomers).
the C18 ring, and by a strong NMR signal of peripheral H
nuclei.[6d,11] All these carbo-benzenes have been obtained by
reductive aromatization of hexaoxy-[6]pericyclynes.[5,6d,11]
An optimal route to the subfamily of centrosymmetric 1,10-
disubstituted carbo-benzenes, such as p-dianisyltetraphenyl-
carbo-benzene (1), involves the [6]pericyclynedione key-in-
termediate 2, the synthesis of which has been recently opti-
mized (Scheme 1).[12] The target 1 was selected for its strong
This mixture was then submitted to the reducing system
SnCl2/HCl.[8] To minimize undesired transformations of the
p-electron-rich system of 3 (in particular, polymerization),
the reduction was carried out at low temperature in diethyl
ether, in which the acidity of HCl is moderate. The solution
turned to green at ꢀ608C, and then reversibly to dark red at
ꢀ508C. TLC monitoring (petroleum ether/CHCl3 2:8) dis-
played several highly colored spots: green (Rf ꢃ0.25 and
0.30), purple (Rf ꢃ0.62), and orange-red (Rf ꢃ0.69). After
3 h at ꢀ508C, the medium was treated with 1m aqueous
sodium hydroxide, then extracted with diethyl ether, and the
organic layer was evaporated to dryness. The dark green
oily residue was purified by two successive column chroma-
tography runs over silica gel (eluted with chloroform and
chloroform/heptane 7:3), followed by a separation using
semi-preparative HPLC techniques. Three fractions of pure
products 1, 4, and 5 were isolated, the two latter initially
presenting structural ambiguity (Scheme 3). The determina-
tion of their exact structures (more or less reduced with re-
spect to the initial target 1) and properties are detailed
below.
Scheme 1. Targeted quadrupolar carbo-benzenic chromophore from
a
[6]pericyclynedione.[12,13]
quadrupolar character (with a calculated maximal diagonal
element Qzz of the quadrupole tensor of ꢀ340 versus
ꢀ317 D.ꢅ for hexaphenyl-carbo-benzene) and anticipated
chromophoric properties (in particular in nonlinear optics),
and an unusual outcome of the classical methodology envis-
aged for its synthesis was mentioned.[13] Among the side
products, indeed a ring carbo-mer of cyclohexadiene stands
as the first example of non-aromatic ring carbo-mer of the
second kind. Full results are reported hereafter, in which
the characterization and physicochemical properties of three
types of unprecedented chromophores are compared and
discussed on the basis of joint experimental and computa-
tional studies.
Characterization of 1: The product of the less polar fraction
(Rf ꢃ0.69) was obtained as an air-stable green-gold solid
that turned red in solution. MALDI-TOF MS analysis indi-
cated a single molecular peak at m/z 738.26 uma that corre-
sponded to the targeted carbo-benzene 1, which was thus
isolated in 8% yield. In a solution of 1 in CDCl3, two
1H NMR doublets at d=9.47 and 9.42 ppm in a 2:1 integra-
tion ratio were assigned to the ortho-CH protons of the re-
spective phenyl and anisyl substituents that experienced the
characteristic deshielding by the strong diatropic ring cur-
rent of the C18 ring. In spite of a weak solubility, a consistent
13C NMR spectrum could be recorded by using [Cr
ACHTUNGTRENNUNG(acac)3]
Results and Discussion
(acac=acetylacetonate) as a relaxation agent, thereby al-
lowing for the detection of the requested number of quater-
nary carbon signals.
As X-ray diffraction analysis of crystals of 1 appeared
first impracticable (see below), the structure of 1 was inves-
tigated at the B3PW91/6-31G** level of calculation. Several
conformations close in energy were obtained on the singlet
spin-state potential energy surface (the lowest triplet spin
state of 1 was found to be 26.16 kcalmolꢀ1 higher in energy,
Synthesis of 1 and derivatives thereof: The known [6]pericy-
clynedione 2 was prepared in nine steps and 7% overall
yield from trans-1,2-dibenzoylethene and was obtained as a
quasi-statistical mixture of its five diastereoisomers, which
could be partly resolved by semi-preparative HPLC tech-
niques.[12] The unresolved mixture underwent double nucleo-
philic attack by p-anisylmagnesium bromide (2 equiv) to
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3227

R. Chauvin, V. Maraval et al.
Scheme 3. Outcome of a reductive treatment (SnCl2/HCl) of pericyclynediol 3.
with
a
zero-point energy (ZPE)-corrected value of
24.00 kcalmolꢀ1). The totally planar structure of C2h symme-
try is a seventh-order saddle point that lies 6.34 kcalmolꢀ1
higher in energy than the C2-symmetric minimum that ex-
hibits aryl substituents tilted by around (18.5ꢄ1.3)8 with re-
spect to the quasi-planar C18 macrocycle in a helical way
(see Figure S4 in the Supporting Information).
The C2-symmetric conformation is degenerated in energy
(DE= +0.03 kcalmolꢀ1) with another dipolar conformation
with “syn” methoxy groups, hereafter referred to as C1-syn.
The C2-symmetric global minimum was calculated to be
quasi-isoenergetic with a Ci-symmetric conformer (DE= +
0.88 kcalmolꢀ1) that exhibits anisyl substituents almost co-
planar with the C18 macrocycle and parallel phenyl substitu-
ents tilted by 218. This latter quadrupolar conformation is
almost identical to the experimental conformation finally
found in the crystal state (see below).
Figure 1. Experimental and calculated 1H and 13C chemical shifts [ppm]
of 1 (B3PW91/6-31+G** level).[15]
As observed in other phenyl-substituted carbo-ben-
zenes,[11] whereas the o-1H nuclei of the phenyl substituents
of 1 undergo a strong deshielding by the macrocyclic ring
current (d=9.47 versus 7.59 ppm in biphenyl), the ipso-13C
nuclei do not (d=140.4 ppm in 1 versus d=140.8 ppm in bi-
phenyl). Further insight into the magnetic aromaticity of the
C18 ring and the role played by the phenyl substituents was
appraised from the nucleus-independent chemical shift
(NICS) at the geometric center of the macrocycle. The cal-
culated value for 1 (C18An2Ph4), NICS=ꢀ13.5 ppm, is iden-
tical to the corresponding value for hexaphenyl-carbo-ben-
zene (C18Ph6), but is significantly lower than the values
found for unsubstituted carbo-benzene (C18H6: NICS=
ꢀ17.9 ppm) or unsubstituted p-dianisyl-carbo-benzene
(C18An2H4: NICS=ꢀ15.9 ppm) at the same B3PW91/6-31+
G** level of calculation.[11,13] The present results confirm
that the magnetic aromaticity of the carbo-benzene ring is
The calculated structure of 1 (Figure S4 in the Supporting
Information) allowed for further calculations of spectroscop-
ic properties and confrontation with experimental data.
With regards to NMR spectroscopy, an assignment of all the
1H and 13C NMR spectroscopic signals of 1 could be thus
1
proposed on the basis of H-1H COSY-45, 13C-1H heteronu-
clear single-quantum correlation (HSQC), and heteronu-
clear multiple-bond correlation (HMBC) experiments
(Figure 1). This assignment was confirmed by comparison
1
with H and 13C NMR spectroscopic chemical shifts calculat-
ed at the B3PW91/6-31+G** level by using the gauge-inde-
pendent atomic orbital (GIAO) methodology.[14] As previ-
ously observed for other carbo-benzene derivatives,[11] the
agreement is remarkable, in particular for all the 13C chemi-
cal shifts (Figure 1).[15]
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Hexaoxy-[6]Pericyclynes
FULL PAPER
highly sensitive to aryl substitution, and indicate that anisyl
and phenyl substituents have similar effects. In a symmetric
manner, and rather unexpectedly, the NICS values calculat-
ed at the geometric center of the phenyl (NICS=ꢀ5.4 ppm)
or anisyl (NICS=ꢀ6.1 ppm) substituents of 1 are also signif-
icantly lowered relative to benzene (NICS=ꢀ8.0 ppm).
The absorption spectrum of 1 in CHCl3 displays an in-
tense band at lmax =482 nm (e=222112 Lmolꢀ1 cmꢀ1), a
weaker band at l’=525 nm (with a shoulder at 540 nm), and
a very weak band at l’’=580 nm (Figure S1 in the Support-
ing Information). By analogy with the Soret and Q bands of
porphyrins, these main patterns and ranges of wavelengths
constitute a signature of the carbo-benzene macrocycle,[16]
and the present results confirm previous observations that
the lmax value is poorly sensitive to the substituents (phenyl,
4-anisyl, 4-pyridyl) of hexaaryl-carbo-benzenes (472 nm for
C18Ph6).[5,11] The spectrum of 1 was calculated in the gas
phase for the C2-symmetric geometry of minimum energy.
Both ZINDO and TDB3PW91 calculations are in good
agreement with the experimental data (Figure 2), in particu-
Supporting Information) from the ground state jS0 > to sin-
glet excited states jS3 > and jS4 >, respectively (Table S2 in
the Supporting Information). As in the Gouterman four-or-
bital model invoked for the explanation of the electronic
spectra of porphyrins,[18] these transitions involve mainly two
one-electron excitations from one of the two HOMOs to
one of the two LUMOs of 1 in the C2-symmetric geometry.
As these orbitals are mostly localized over the C18 ring,
these excitations do not exhibit any charge-transfer charac-
ter and the corresponding transition densities are confined
inside the macrocycle (Figure S6 in the Supporting Informa-
tion).
The weak experimental absorption bands at 525 nm
(2.3615 eV) and 580 nm (2.1376 eV) do not show up in the
calculated spectra of 1 at either the TDDFT or the ZINDO
level (Figure 2). The transitions from the ground state to the
first singlet excited states jS1 > and jS2 >, calculated at
731.5 and 630.1 nm, respectively, at the TDB3PW91/6-
31G** level, are indeed out of range and exhibit very small
oscillator strengths (Table S2 in the Supporting Informa-
tion). These weak absorption bands cannot be ascribed to
the dynamic behavior of 1 because the calculated absorption
spectra are insensitive to the conformation of 1.
The first singlet excited states, jS1 > and jS4 > on one
hand, and jS2 > and jS3 > on the other, are described by
combinations of the same one-electron excitations, adding
or canceling each other (Table S2 in the Supporting Infor-
mation). This picture actually fits with the Gouterman four-
orbital model for porphyrins, which predicts strongly al-
lowed transitions to the higher excited states (Soret band)
and weakly allowed transitions to the lower excited states
(Q bands). One possible interpretation for the failure of the
calculation to reproduce the weak experimental bands
would be that they correspond to the Q bands of porphyrins,
that is, to transitions from the ground state jS0 > to the low-
lying excited states jS1 > and jS2 >, but that TDDFT fails to
predict their range. Multireference configuration interaction
calculations are underway to refine this interpretation.
As mentioned above, crystals of 1 were repeatedly ob-
tained from various solvent mixtures, but their minute size
and low diffracting power long prevented any structural de-
termination by X-ray crystallography. The high crystallinity
of 1 was, however, illustrated by SEM images of golden
flakelike crystals obtained by slow diffusion of methanol
into a chloroform solution of 1 (Figure 3). The regular form
of the microcrystals (mostly smooth and thin rectangles or
squares) suggested their single-crystal nature and encour-
aged further efforts in X-ray crystallography.
Figure 2. Absorption spectra (absorbance versus energy) of 1 calculated
at the semi-empirical ZINDO and TDB3PW91/6-31G** levels. Simula-
tion with Gaussian-shaped bands of 0.09919 eV half bandwidth. The ref-
erence experimental spectrum of 1 (experimental (EXPTL), in CHCl3) is
also given (see Figure S1a in the Supporting Information).
lar with regards to the absolute lmax value for the time-de-
pendent DFT (TDDFT) calculation (482 nm). It is also
noteworthy that TDDFT calculations performed on alterna-
tive conformations of 1 (with C1-syn, Ci, or C2 symmetry),
which are likely to be also present in the experimental solu-
tion, afford virtually the same spectrum.
The calculated transition energy of largest oscillator
strength (2.5683 eV) is in good agreement with that of the
strongest experimental band (2.5722 eV) and is compatible
with the mean error range of TDDFT calculations (0.3–
0.4 eV).[17] This absorption band is actually related to two
quasi-degenerate vertical transitions (see Figure S5 in the
After many unsuccessful attempts, the X-ray crystal struc-
ture of 1 could finally be determined with a satisfactory reli-
ability factor (R1 (I>2s(I))=0.0481) by using an X-ray mi-
crosource (l=0.71073 ꢅ) on single crystals deposited from
a 1:1 CHCl3/C2H4Cl2 solution (Figure 4). Within the estimat-
ed standard deviation (ESD) errors for interatomic distan-
ces and angles, the structural data in the crystal state are
almost identical with the DFT-calculated ones in the gas
phase (Figure S4 in the Supporting Information). The mac-
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3229

R. Chauvin, V. Maraval et al.
occur in the calculated structure of C2 symmetry (Figure S4
in the Supporting Information). Likewise, the experimental
torsion angles between the C18 macrocycle and aryl substitu-
ents (ꢀ18.4, 0.3, 17.18) differ from those of the calculated
C2-symmetric structure by at most ꢄ178 for the anisyl sub-
stituents, and by at most ꢄ38 for the phenyl substituents.
In the crystal, the macrocycles of 1 stack in regular hexag-
onal columns (the axis of a column being aligned with the
normal of the macrocycles), in which two successive C18 sec-
tions are separated by 7.34 ꢅ. In a given column, phenyl
substituents of two neighboring columns intercalate by weak
p-stacking as indicated by short contacts between ortho-
carbon atoms and C2 units of two successive C18 rings. The
crystal packing involves disordered 1,2-dichloroethane mole-
cules outside the columns and thus differs from previously
reported hexagonal packing of other carbo-benzenic or
[6]pericyclynic derivatives, in which chlorinated solvent mol-
ecules (CHCl3, CH2Cl2) intercalate inside columns of C18
macrocycles.[5b,6d,11,19]
Characterization
of
1,10-dianisyldimethoxytetraphenyl-
carbo-cyclohexadiene (4): The 1H NMR spectrum of the
more polar fraction that corresponds to the most polar
green TLC spot (Rf ꢃ0.25 and 0.30) displayed two pairs of
OCH3 signals in a 55:45 ratio in the region of the methoxy
substituents of the [6]pericyclynic precursor 3, namely, at
d=3.65 and 3.77 ppm, respectively. A partial reduction of 3
was therefore anticipated, and indeed the molecular peak at
m/z 800.39 uma of the MALDI-TOF spectrum was compati-
ble with the doubly reduced species 4A or 4B, which occur
as pairs of diastereoisomers (Scheme 4). The doubly reduced
products 4C and 4D (in which at least one of the anisyl sub-
stituents is not conjugated with a butatriene edge of the
macrocycle) could thus be ruled out, but the 1H and
13C NMR spectra did not allow us to discriminate between
the regioisomers 4A and 4B.
Figure 3. SEM images of crystals of 1.
As many attempts failed to produce single crystals from
the mixture of stereoisomers of 4, the structures of both the
possible diastereoisomers of 4A and 4B were calculated at
the B3PW91/6-31G** level. The four isomers were found to
accommodate quasi-isoenergetic twist, boat or chair confor-
mations of the C18 macrocycle, with the most stable con-
formers ranking in the order trans-4A (0.0)<cis-4B (0.4)<
trans-4B (1.1)<cis-4 A (1.5), in which the relative energy
values given in brackets are in kcalmolꢀ1. Whereas the boat
conformation of cis-4B and trans-4B allows for the trans-an-
nular coplanarity of the non-conjugated C4 butatriene edges,
the twisted conformation of cis-4A and trans-4A results in a
dihedral angle between the conjugated butatriene edges of
18.0 and 19.28, respectively (Figure 5).
The absorption spectrum in chloroform of the product 4
(Scheme 3) displays two strong absorption bands at
437.4 nm and lmax =602.3 nm (e=41943 molLꢀ1 cmꢀ1; Fig-
ure S1 in the Supporting Information). The absorption spec-
tra of the cis- or trans-diastereoisomers of the regioisomers
4A and 4B were also calculated at the ZINDO and
TDDFT levels in the gas phase (Figure 6a and b).
Figure 4. Molecular view of the X-ray crystal structure of 1, with thermal
ellipsoids drawn at the 50% probability level (for clarity, the hydrogen
atoms are omitted). Space group: P21/c, R1 (I>2s(I))=0.0481 (Table S1
ꢀ
in the Supporting Information). Selected bond lengths [ꢅ]: C1 C2=
ꢀ
ꢀ
ꢀ
ꢀ
1.389(4), C2 C3=1.387(4), C2 C10=1.470(4), C3 C4=1.223(4), C4
ꢀ
ꢀ
ꢀ
C5=1.384(4), C5 C6=1.380(4), C5 C17=1.477(4), C6 C7=1.226(4),
ꢀ
ꢀ
ꢀ
C7 C8=1.383(4), C8 9=1.388(4), C8 C23=1.479(4). Selected bond
angles [8]: C13-O1-C16=117.5(2), C9A-C1-C2=177.2(3), C3-C2-C1=
117.5(2), C3-C2-C10=120.6(2), C1-C2-C10=121.9(2), C4-C3-C2=
176.8(3),
119.5(2),
C3-C4-C5=176.9(3), C6-C5-C4=119.7(2), C6-C5-C17=
C4-C5-C17=120.8(2), C7-C6-C5=174.9(3), C6-C7-C8=
177.4(3), C7-C8-C9=119.7(2).
ꢀ
rocycle is indeed found to be quasi-planar, and the C C
bond lengths differ by at most ꢄ0.01 ꢅ from those that
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Hexaoxy-[6]Pericyclynes
FULL PAPER
Scheme 4. A priori possible bis-butatrienic structures for product 4 result-
ing from partial reduction of the pericyclynic precursor 3 (Scheme 3).
Each of them corresponds to cis and trans diastereoisomers. Only the re-
gioisomers 4A and 4B, in which both the anisyl substituents are conju-
gated with
a butatriene edge, were shown to be compatible with
MALDI-TOF analysis (m/z 800.39 uma).
At the TDB3PW91 level, the spectra of the 4A-type re-
gioisomers exhibit two strong bands of comparable oscillator
strength, the transition energies of which are close to the ex-
perimental ones (f=0.9209, 2.8505 eV=435.0 nm versus
2.8345 eV=437.4 nm;
f=0.7497,
1.8481 eV=670.9 nm
versus 2.0584 eV=602.3 nm). The spectra of the cis- and
trans-diastereoisomers are almost superimposable. These
transitions involve excitations from the ground state jS0 > to
the first excited state jS1 >(0.71 H!L) and third excited
state jS3 >(0.54H!L+1+0.43Hꢀ1!L). Similar results
were obtained at the ZINDO level, with an even better fit
for the bathochromic absorption (see Table S3 in Supporting
Information).
Figure 5. Side view of the calculated structures of the cis and trans diaste-
reoisomers of regioisomers 4A and 4B (see Scheme 4). Front views are
given in Figure S7 in the Supporting Information. B3PW91/6-31G** level
of calculation. Relative energies are given in kcalmolꢀ1
.
singlet excited state (0.56H!L+1ꢀ0.43Hꢀ1!L; see
Table S4 in the Supporting Information).
On the other hand, the TDDFT spectra of the 4B-type re-
gioisomers exhibit only one strong absorption band at
449 nm (similar values are calculated at the TDB3PW91/6-
31G** level: f=1.7396, 2.7448 eV=451.7 nm, and at the
ZINDO level: f=2.5204, 2.7779 eV=446.3 nm). It corre-
sponds to an excitation from the ground state to the jS4 >
The striking difference between the calculated UV/Vis
spectra of the regioisomers 4A and 4B, and the remarkable
fit of the experimental pattern with the two-band pattern of
4A, allows us to assign the regiochemistry of the product 4
to that of 4A, which corresponds to the largest conjugation
extent (in which the two butatriene edges are conjugated
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3231

R. Chauvin, V. Maraval et al.
room temperature were averaged, as was done for the
carbo-benzene 1 (see above, Figure 1).[15] Both sets of results
are, however, compatible with the experimental data within
the error range (see Figure 7 and Figure S8 in the Support-
Figure 7. Experimental and calculated 1H and 13C chemical shifts [ppm]
of the carbo-cyclohexadienes trans-4A and cis-4A (B3PW91/6-31+G**).
The averaging treatment is the one used for carbo-benzene
1
Figure 6. Calculated absorption spectra (absorbance versus energy) of the
carbo-cyclohexadienes 4, 4A, and 4B. a) TDB3PW91/6-31G** level for
the four isomers trans-4A, cis-4A, trans-4B, and cis-4B in the gas phase.
b) ZINDO level for the cis diastereoisomers of the regioisomers of 4A
and 4B in the gas phase. Simulation with Gaussian-shaped bands of
0.09919 eV half bandwidth. The reference experimental spectrum of 4=
4A (experiment, in CHCl3) is also given (see Figure S1b in the Support-
ing Information).
(Figure 1).[15]
ing Information). The only noticeable difference concerns
the 13C chemical shifts of the sp-carbon atoms of the buta-
triene moieties, which are predicted to occur at slightly
lower field in 4B (d=148.5, 149.8, 147.2, and 150.8 ppm for
trans-4B; see Figure S8 in the Supporting Information) than
in 4A (d=141.1 and 147.7 ppm for trans-4A; d=142.7 and
147.5 ppm for cis-4A; see Figure 7). The experimental data
(d=144.7, 144.8, 147.1, and 147.2 ppm) are closer to the cal-
culated values for 4A than to the calculated values for 4B.
Although assignment of the experimental regioisomer to the
structure 4A (instead of 4B) would be presumptuous on the
sole basis of such small differences in chemical shifts (dꢃ3–
6 ppm), the NMR spectroscopic results tend to confirm the
assignment 4=4A on the basis of the comparison of the cal-
culated absorption spectra of 4A and 4B.
through a butyne edge and conjugated with both the anisyl
substituents). This illustrates how theoretical calculations
can be useful for the determination of molecular structures
in the absence of X-ray diffraction data.
1
1
The H and 13C NMR and H-13C HMBC spectra of the
mixture of stereoisomers of 4 are fully consistent with the
regioisomer 4A, but also with the regioisomer 4B (see the
Experimental Section in the Supporting Information). To
confirm the above assignment 4=4A, the H and 13C NMR
1
spectroscopic chemical shifts of the cis- and trans-diastereo-
isomers of both 4A and 4B were calculated at the B3PW91/
6-31+G** level, and those of the nuclei that became equiv-
alent on the experimental NMR spectroscopic timescale at
Finally, semi-preparative HPLC separation of mixture 4
afforded pure fractions of the diastereoisomers trans-4A
and cis-4A, the NMR spectra of which could thus be record-
3232
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Hexaoxy-[6]Pericyclynes
FULL PAPER
ed separately (Figure S2 in the
Supporting Information). Al-
though the stereoisomers trans-
4A and cis-4A could not be un-
ambiguously assigned, their re-
spective signature is given by
ꢀ
the two types of PhC OCH3
and phenyl o-CH 1H chemical
shifts. The two sets of experi-
Scheme 5. Quantitative reductive aromatization of carbo-cyclohexadiene 4A (mixture of stereoisomers) into
carbo-benzene 1, as monitored in situ by H NMR spectroscopy.
1
mental
7.90 ppm}
values
and
{d=3.65,
{d=3.77,
7.83 ppm} are tentatively assigned on the basis of the corre-
sponding calculated H chemical shifts to trans-4A (d=3.39,
8.40 ppm) and cis-4A (d=3.70, 8.25 ppm), respectively
(Figure 7). On the basis of these results, the product of the
less (respectively more) polar fraction of 4 is therefore ten-
tatively assigned to trans-4A (respectively cis-4A).
regions of the visible spectrum (Figure 6). This unusual spec-
troscopic property is also to be related with an unusual
chromophoric property of 4A, namely, a remarkable di-
chromism when dissolved in most classical organic solvents
(CHCl3, CH2Cl2, Et2O, toluene, and so on).[20] Such diluted
solutions indeed appear turquoise-blue through short optical
paths, and deep purple through longer optical paths
(Figure 8). In spite of its fascinating visual effect, and likely
because of its rareness,[21] the solution dichromism of pure
organic molecules has been sporadically addressed in the lit-
erature.[20] Although the visual effect might also have been a
priori explained by a fluorescence phenomenon (possibly
overcoming the absorption of incident light at long optical
path in the orthogonal direction), the emission spectrum of
4A evidenced such a weak fluorescence (in the green region
with a Stokes shift of 116 nm) that it cannot be invoked for
the phenomenon (Figure 8).[22] The dichromic property of
4A is actually explained by the intensity and sharpness of
the absorption bands at 437 and 602 nm: it is correlated
with the relative position of the absorption wavelengths of
4A and the region of high sensitivity of the human eye in
the chromatic diagram (Figure 8).[21b]
1
It is finally noteworthy that the o-CH nuclei of the phenyl
and anisyl substituents of the dibutatrienylacetylene (DBA)
unit resonate at higher field in 4A (d=7.90 and 7.76 ppm,
respectively) than in 1 (d=9.47 and 9.42 ppm, respectively).
This is the anticipated consequence of the non-aromatic
character of the macrocycle of 4A, which is confirmed by
the calculation of a quasi-vanishing NICS value of d= +
1.15 ppm for trans-4A (versus d=ꢀ13.5 ppm for 1).
The carbo-cyclohexadienes 4A that result from a four-
electron reduction of the pericyclynediol 3 are likely inter-
mediates in the six-electron reductive aromatization that
leads to the carbo-benzene 1. The reactivity of 4A in the
prolonged presence of the SnCl2/HCl reducing system was
1
monitored by H NMR spectroscopy. A solution of 4A in
CDCl3 was thus treated with SnCl2 and DCl in D2O directly
in an NMR spectroscopy tube for one night at room temper-
ature. The initial dark green of the organic layer turned
1
slowly to deep red, and the H NMR spectrum of the crude
Characterization of p-dianisyltetraphenyldihydro-carbo-ben-
zene (5): The third fraction isolated by HPLC (Scheme 3)
and that corresponds to the violet TLC spot of intermediate
polarity (Rf =0.62) afforded a dark violet stable crystalline
solid 5 (Scheme 3) soluble in most organic solvents. The
MALDI-TOF mass spectrum of 5 indicated a molecular
mass of 740.2 versus 738.2 uma for the carbo-benzene 1,
mixture confirmed the total conversion of the carbo-cyclo-
hexadiene 4A to carbo-benzene 1 (Scheme 5).
The highly conjugated but non-aromatic pseudo-quadru-
polar character of the carbo-cyclohexadiene 4A is to be re-
lated to the unusual occurrence of two strong absorption
bands of similar intensities in complementary blue and red
Figure 8. Illustration of the dichromism of a solution of dianisyl-carbo-cyclohexadiene 4A in chloroform appearing turquoise-blue at short optical path
(left) and deep purple at long optical path (right). The corresponding schematic interpretations, namely, “soustractive” (perception of a complementary
color at short optical path) or “additive” (perception of the superimposition of pure chromatic colors at long optical path), depends on the regions of the
human eye sensitivity (middle).[20b]
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3233

R. Chauvin, V. Maraval et al.
thereby suggesting that 5 is a
dihydrogenated derivative of 1,
namely,
a dihydro-carbo-ben-
zene, which was thus isolated in
6% yield. The 1H NMR spec-
trum of 5 showed highly de-
shielded signals in the range
d=8.50–9.20 ppm for ortho pro-
tons of phenyl and anisyl sub-
stituents, thus evidencing that
the C18 macrocycle is as aromat-
ic as in the carbo-benzene 1. At
even
lower
field
(d=
10.31 ppm), a doublet could be
attributed to a proton directly
linked to the central macrocycle
(by comparison, the protons di-
rectly linked to the macrocycle
of the “ortho” and “para” iso-
mers of tetraphenyl-carbo-ben-
zenes C18Ph4H2 resonate at d=
9.70 and 9.87 ppm, respective-
ly).[11] This proton is actually
coupled with another proton
that resonates as a doublet at
the opposite side of the spec-
Scheme 6. A priori possible structures for dihydro-carbo-benzene 5 (Scheme 3) on the basis of crude MS and
1H NMR spectroscopic data. Refined NMR spectroscopic analyses reduced the possibilities to the three trans
isomers, and finally to isomer 5A (or its mesomeric form 5A’).
3
trum (d=ꢀ3.46 ppm). The coupling constant of J
A
energy than 5B and 5C) are comparable to those calculated
for 1 at the same level (Figure 9 and Figure S4 in the Sup-
13 Hz suggests the presence of a trans-disubstituted double
bond, which would be compatible with the extreme shield-
ing and deshielding of the -CH=CH- protons that reside out-
side and inside the macrocycle, respectively. Among the five
regio- or stereoisomeric structures a priori possible for 5,
the compatible structures were thus reduced to the three
trans isomers 5A, 5B, and 5C (Scheme 6). Finally, 1H
NOESY experiments evidenced that the double bond is lo-
cated between two phenyl-substituted vertices, and thus al-
lowed to assign the structure of 5 to the isomer 5A, which
features a trans-substituted but-2-ene edge. It is noteworthy
that the resonance form 5A’ suggested by standard meso-
merism is thus forced to retain the transoid configuration of
the corresponding butadiene edge of 5A, at least on the
NMR spectroscopic timescale at RT (Scheme 6). Although
both forms may be equally representative (see below), the
structure of the product 5 is henceforth represented by the
form 5A, which is more descriptive of the rigid trans/trans-
oid geometry.
ꢀ
porting Information). It is noteworthy that the three C C
bond lengths of the hydrogenated edge of 5A are almost
equal (1.397–1.399 ꢅ), thereby suggesting that the Kekulꢃ-
type resonance forms 5A and 5A’ have identical weights
(Scheme 6). This indicates that 5A and 1 sustain the same
perfect cyclic delocalization of their pz-electron systems, and
exhibit similar structural aromaticity. Likewise, the NICS
values calculated at the B3PW91/6-31+G** level at the
geometrical center of the C18 macrocycle are equal to
NICS=ꢀ12.8 ppm for 5A and NICS=ꢀ13.5 ppm for 1,
As in the cases of 1 and 4A (see above), accurate struc-
tural data could not be gained by X-ray diffraction analysis
of crystals of 5A (see below). The structure of 5A and
those of its isomers 5B–E were thus calculated at the
B3PW91/6-31G** level. All the trans-isomers 5A–C were
found to be quasi-isoenergetic and exhibit a planar C18 mac-
rocycle. In contrast, very distorted nonplanar structures
were calculated for the cis-isomers 5D and 5E, that are re-
spectively 17.5 and 28.6 kcalmolꢀ1 higher in energy than
5A. The calculated bond lengths of the non-hydrogenated
edges of the C18 ring of 5A (only 0.2 kcalmolꢀ1 lower in
Figure 9. Calculated structure of dihydro-carbo-benzene 5A at the
B3PW91/6-31G** level. Bond lengths given in ꢅ, dihedral angles in de-
grees. For comparison with 1, see Figure S4 in the Supporting Informa-
tion.
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Hexaoxy-[6]Pericyclynes
FULL PAPER
thus showing that both carbo-benzenes and trans-dihydro-
carbo-benzenes exhibit similar magnetic aromaticity. In
agreement with the Hꢄckel rule, indeed, both the carbo-
benzene 1 and dihydro-carbo-benzene 5A satisfy an 18-pz-
electron count over the planar C18 macrocycle.
The similarity of the p-electron delocalization in the dihy-
dro-carbo-benzene 5A and carbo-benzene 1 is also apprais-
ed by comparison of their absorption spectra. In the visible
region, 5A indeed presents a strong absorption band at
lmax =467 nm (e=107679 Lmolꢀ1 cmꢀ1) and weaker bands at
526 and 588 nm (Figure S1c in the Supporting Information),
which may be related to the bands of 1 at 482 (e=
222112 Lmolꢀ1 cmꢀ1), 525 and 580 nm, respectively (Figure 2
and Figure S1a in the Supporting Information). The main
absorption band of 5A is, however, blue-shifted with respect
to 1, thereby suggesting a slightly weaker “p-delocalization”
2
2
ꢀ
through the sp -C sp -C bond of 5A than through the sp-
ꢀ
C sp-C bond of 1, as crudely indicated, in the dynamic con-
text, by their relative NICS values (NICS=ꢀ12.8 versus
ꢀ13.5 ppm; see above). The lmax value of 5A is thoroughly
reproduced at the TDB3PW91/6-31G** level of theory by
two transitions that occur at 470.1 nm (2.6377 eV, f=1.326)
and 465.7 nm (2.6623 eV, f=1.898). The description of the
corresponding third and fourth singlet excited states is how-
ever different from the one of 1 (jS3 > =0.42jHꢀ1!L> +
0.38jH!L+1>ꢀ0.36jHꢀ1!L+1>
and
jS4 > =0.46j
Hꢀ1!L+1> +0.30jHꢀ1!L>). Secondary excitations of
much weaker oscillator strengths, which exhibit a larger dis-
crepancy with the observed secondary bands, are also calcu-
lated (see Table S5 in the Supporting Information).
Figure 10. SEM images of crystals of dihydro-carbo-benzene 5A, showing
long needles of regular hexagonal section (left), with possible tubular
shapes (right).
A complete NMR spectroscopic study of 5A, including
1
COSY-45, H-13C HSQC, and HMBC experiments, allowed
1
us to assign all the H signals and most of the 13C signals.
The complete attribution of the macrocyclic quaternary 13C
nuclei was, however, not possible due to the absence of
quite unusual.[23] The formation of such tubular auto-assem-
blies indeed generally requires very particular conditions
(addition of surfactants,[24] liquid crystals,[25] or mixtures of
solvents[26]). In the case of the dihydro-carbo-benzene 5A,
tubular architectures are formed in the absence of any tem-
plate upon quick evaporation of various usual organic sol-
vents.
To gain insights into the way the hexagonal hollow tubes
are formed, TEM analysis was performed from a rapidly
evaporated drop of a diluted solution of 5A in chloroform.
The TEM images evidenced a fast auto-organization of the
dihydro-carbo-benzene molecules in sheets in which parallel
alignments are visible (Figure 11a). This structural periodici-
ty was confirmed by the presence and the spacing of diffrac-
tion spots in the picture generated by Fourier transform of
the TEM image, thereby revealing that the alignments are
spaced out by approximately 15.5 ꢅ (Figure 11b). The inter-
pretation of these alignments is addressed below.
long-range coupling (4J
(C,H) were not detected at
500 MHz). The H and 13C chemical shifts of 5A were thus
calculated at the B3PW91/6-31+G** level with the GIAO
method, from the static optimized geometry (Figure 9) and
using the same averaging method for nuclei that become
equivalent at the experimental NMR spectroscopic time-
scale at room temperature as the one used for the carbo-
benzene 1.[15] The results are in good agreement with avail-
able experimental data and allowed us to refine the assign-
ment of the 13C chemical shifts of the C18 macrocycle (see
Figure S9 in the Supporting Information).
1
The dihydro-carbo-benzene 5A spontaneously crystallizes
by evaporation of solutions in most organic solvents and
mixtures thereof to give very thin needles, which did not
appear suitable for X-ray diffraction analysis. The shape of
these microcrystals could, however, be observed by SEM,
after deposition on an aluminum support and coating with a
conducing platinum film (Figure 10). The needles were
found to be several hundreds of micrometers long, with a di-
ameter of 15–20 mm. Most of them exhibited a hexagonal
section, and some of them were found to be tubular. Crys-
tallization of organic compounds in tubular forms is actually
After several unsuccessful attempts, the X-ray crystal
structure of 5A could finally be determined using the X-ray
microsource on microcrystals deposited from a 9:1 CHCl3/
CH3CN solution (Figure 12, Table S1 in the Supporting In-
formation). Due to the weak diffraction of the crystals and
to a high disorder of two non-equivalent molecular motifs
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3235

R. Chauvin, V. Maraval et al.
are the normals to their centroids and oriented along the
unit-cell axis that corresponds to a=7.118 ꢅ. They are con-
stituted by the two types of alternating molecular motifs
separated by approximately 3.56 ꢅ (see the Supporting In-
formation). This is the shortest distance ever observed be-
tween two C18 carbo-meric macrocycles in the crystal state,
and thus the first evidence of a weak p-stacking between
such macrocycles. In the same series, for example, the
stacked C18 macrocycles of the carbo-benzene 1 were found
separated by twice the distance (7.34 ꢅ) through weak p-
stacking with intercalated phenyl rings (see above). In con-
trast to all previous observations, no solvent is involved in
the crystal packing of 5A, neither inside the columns[5b,11]
nor outside (as in the case of 1 described above).
Figure 11. TEM image of dihydro-carbo-benzene 5A (top), and corre-
sponding Fourier transform (bottom) revealing the presence of long-
range alignments with a periodicity of 15.5 ꢅ.
In the same crystal of 5A, the distance between two near-
est columns is equal to 17.917 ꢅ, and the distance between
two next-nearest columns is very close to this value and
equal to the length of the unit-cell edge b=18.865 ꢅ
(Figure 13). A set of tightly neighboring columns defines the
plane of their parallel axes along the direction of a. Two
series of such planes are thus defined: the series parallel to
the b axis (in magenta in Figure 13), and the series parallel
to the diagonal of the (b,c) rectangle (in blue in Figure 13).
The two series are characterized by interplane distances of
1
2
2
=
2
x=bc/ACTHNURGTNE(UNG b +c ) =16.0 ꢅ and y=c/2=15.2 ꢅ, respectively.
Both distances are very close (within a 3% error) to the pe-
riodicity of the alignments (15.5 ꢅ) observed in the TEM
image of 5A (Figure 11).[27]
Figure 12. Molecular view of the X-ray crystal structure of dihydro-
carbo-benzene 5A. Thermal ellipsoids drawn at the 50% probability
level (for clarity, the hydrogen atoms are omitted). Space group: P21/n,
R1 (I>2s(I))=0.0872).
(Figure 13 and Figure S10 in
the Supporting Information),
the poor quality of the resolu-
tion prevents a detailed discus-
sion of quantitative structural
data. Nonetheless, inspection of
the less disordered motif
(Figure 12) confirms the gener-
al features of the calculated
structure in the gas phase
(Figure 9): quasi-planarity of
the macrocycle, relative order
of magnitude of the bond
lengths, bond angles and torsion
angles, and so on. The only no-
ticeable difference is a slight
bending of the planes of the
anisyl substituents out of the
Figure 13. Wireframe view of the unit cell (monoclinic, Z=4) along the a axis, revealing the regular hexagonal
columnar packing of macrocycles 5A. The projected global disorder results from the superimposition of two
types of alternating molecular motifs with different specific disorders along the columns. Two series of planes
containing the axes of tightly neighboring columns (either nearest columns equidistant by 17.917 ꢅ, or next-
nearest columns equidistant by b=18.465 ꢅ) are defined: the series parallel to the b axis (in magenta), and
plane of the macrocycle (by
around 78 in the less disordered
motif and by 3.58 in the most
disordered one).
the series parallel to the diagonal of the (b,c) rectangle (in blue). Each series is characterized by an interplane
1
The macrocycles of 5A stack
2
distance x=bc/
N
in columns, the axes of which 15.5 ꢅ of the TEM image of 5A (Figure 11).
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Hexaoxy-[6]Pericyclynes
FULL PAPER
Table 1. CV and SWV data for 1, 4A, and 5A in CH2Cl2, and corresponding energetic data in the gas phase calculated at the B3PW91/6-31G** level of
theory. Electrochemical conditions: supporting electrolyte: [nBu4N]ACHTNUGTRNEUNG[PF6] (0.1m); working electrode: Pt; reference electrode: saturated calomel electrode
(SCE, 0.242 V versus the hydrogen electrode); scan rate 0.1 Vsꢀ1 unless otherwise noted.
Reduction
Oxidation
Er1ed [V][a]
DErped [mV][b]
Erped [V][c]
ꢀ1.70
EA [eV][d]
1.997
eLUMO [eV][e]
ꢀ2.83433
Eo1 x [V][a]
DEopx [mV][b]
Eopx [V][c]
IP [eV][f]
5.841
eHOMO [eV][g]
ꢀ5.02975
=
2
=
2
1
ꢀ0.77
ꢀ1.19
ꢀ0.78
60
60
58
0.90[i]
4A[h]
5A
ꢀ1.07
ꢀ1.27
ꢀ1.22
ꢀ1.48
2.085
1.907
ꢀ2.90824
ꢀ2.75597
1.00
5.612
5.718
ꢀ4.80863
ꢀ4.89913
ꢀ0.85
69
0.92[j]
68
1.16
[a] Half-wave potential. [b] Peak potential difference at v=0.1 Vsꢀ1. [c] Peak potential for irreversible electron transfer. [d] Adiabatic electron affinity.
[e] LUMO eigenvalue in the neutral ground state. [f] Adiabatic ionization potential. [g] HOMO eigenvalue in the neutral ground state. [h] Electrochemi-
cal measurements performed on the mixture of the cis and trans isomers of 4A, calculation performed on trans-4A. [i] The oxidized product deposited
on the electrode. [j] The oxidation is irreversible at v=0.1 Vsꢀ1 but becomes reversible at v=1.0 Vsꢀ1 (see Figure S3 in the Supporting Information).
Comparative study of the redox properties of the dianisyl-
carbo-chromophores 1, 4A, and 5A: The electrochemical
properties of 1, 4A, and 5A were studied by cyclic voltam-
metry (CV) and square-wave voltammetry (SWV) in di-
chloromethane (Table 1). Each of them exhibits three reduc-
tion waves corresponding to one-electron transfers accord-
ing the half-height widths of the SWV peaks (Figure S3 in
the Supporting Information).
approximation, might be regarded as the half-filled LUMOs
of the neutral species. It is incidentally observed that
ꢀeLUMO correlates increasingly (but not linearly) with Epred2
,
+
red
av
and finally that eLUMO varies accurately as E =(Epred1
Erped2)/2 (in which Epred1 is denoted as E1red in Table 1).[28]
=
2
In spite of their electron-rich character, the three chromo-
phores 1, 4A, and 5A can thus undergo sequential reduc-
tion steps, the first one at least being reversible. Their oxida-
tion is more difficult, and only the dihydro-carbo-benzene
In all cases, the first reduction step is reversible and
occurs at similar potentials
(Er1ed =ꢀ0.77 V for 1, ꢀ0.78 V
=
for2 4A, and ꢀ0.85 V for 5A).
This suggests that this first re-
duction occurs at a common
conjugated structural unit, and
indeed the LUMO of 1, 4A,
and 5A is spread out over the
dibutatrienylacetylene (DBA)
units substituted by the same
pairs of anisyl, phenyl, and al-
kynyl substituents (Figure 14).
Nevertheless, the first reduction
potential Er1ed does not correlate
=
2
with the LUMO eigenvalue
(eLUMO), which, however, accu-
rately varies as the calculated
adiabatic electron affinity (EA)
in the gas phase (EA=ꢀ1.1688
eLUMO
ꢀ
1.3146 eV, R=1:
Table 1).
The second reduction step is
reversible for
1
at Er1ed2
=
=
2
ꢀ1.19 V (Figure S3 in the Sup-
porting Information) and irre-
versible for 4A and 5A at
Erped2 =ꢀ1.07 and ꢀ1.22 V, re-
spectively (Table 1). The corre-
sponding electron transfer is
anticipated to target the
SOMOs of the intermediate
cations, which, in a very crude Figure 14. Frontier and near-frontier orbitals of 1, trans-4A, and 5A at the B3PW91/6-31G** level.
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3237

R. Chauvin, V. Maraval et al.
5A can be reversibly oxidized
at Eo1 x =0.92 V, albeit at scan
=
2
rates higher than 1.0 Vsꢀ1 (Fig-
ure S3 in the Supporting Infor-
mation). The carbo-benzene
and carbo-cyclohexadiene coun-
terparts 1 and 4A exhibited
one oxidation step at similar
Eopx potentials of 0.90 and
1.00 V, respectively, but in the
case of 1, the formation of a
conductive film on the elec-
trode was observed (Table 1).
Although the HOMO eigenval-
ue eHOMO varies as the adiabatic
ionization
potential
(IP=
ꢀ1.029 eHOMO +0.66873 eV, R=
0.998: Table 1), Eopx does not
properly correlate with eHOMO
.
Scheme 7. Summary of the versatile reactivity of [6]pericyclynediol 3 under reducing acidic conditions (SnCl2/
HCl) in diethyl ether at low temperature: in addition to the “classical” carbo-benzene 1, the sub-reduced
carbo-cyclohexadiene 4A and the over-reduced dihydro-carbo-benzene 5A are produced.
As in the case of Erped1 versus
eLUMO (see above), the odd cor-
relation might be first explained
by the influence of the solvent
and temperature (the DFT cal-
culations were performed in the gas phase, and total Gibbs
free energies were not considered) or by a possible multi-
configurational character of the ionized species (and calcula-
tion at higher level would thus be required). Nevertheless,
the redox flexibility of the carbo-chromophores 1, 4A, and
5A is clearly indicated by both electrochemical measure-
ments and theoretical calculations. It accounts a posteriori
for the versatile outcome of the reduction of the pericycly-
nediol 3 (Scheme 3).
It is finally noteworthy that the HOMO and two LUMOs
exhibit similar localization along the series (Figure 14),
whereas the strict HOMO–LUMO gap increases in the
order 4A<5A<1. As the formal p conjugation is less ex-
tended in 4A than in 1 and 5A, this observation reminds
that the definition of p delocalization depends on the select-
ed criterion, and the HOMO–LUMO gap is just one among
other.
redox properties of these carbo-chromophores have been in-
vestigated in detail with the complementary tools of experi-
mental measurements and theoretical calculations at the
DFT level. Their intriguing properties (aromaticity, di-
chromism, crystallinity, columnar auto-assembling, redox
flexibility, and so on) are promising indicators of possible
applications. In particular, some of these properties could be
attributed to the common DBA unit, and the synthesis and
physicochemical studies of other DBA derivatives deserve
further investigation. Finally, the strongly quadrupolar or
pseudo-quadrupolar character induced by the anisyl sub-
stituents of 1, 4A, and 5A also encourages investigation of
their nonlinear optical properties, and in particular their
two-photon absorption (TPA) properties. Prior to TPA
cross-section measurements, preliminary theoretical studies
are in current progress. Results will be communicated in
due course.
Conclusion
Acknowledgements
Under controlled conditions of temperature and time, re-
ductive treatment of the hexaoxy-[6]pericyclynediol 3 with
SnCl2/HCl in diethyl ether has been shown to produce two
unprecedented C18 “carbo-chromophores” beside the initial-
ly targeted macro-aromatic carbo-benzene 1: the sub-re-
duced non-macro-aromatic carbo-cyclohexadiene 4A and
the over-reduced macroaromatic dihydro-carbo-benzene
5A. Whereas the former is an intermediate in the formation
of 1, the latter does not result from the reduction of 1, and
its endo-macrocyclic double bond should thus result from an
early reduction of one of the triple bonds of 3 (Scheme 7).
The structure and the spectroscopic, chromophoric, and
D.K. thanks the French Embassy in Kiev, Ukraine, for financial support.
Theoretical investigations were partly performed within the framework
of the “groupement franco-ukrainien en chimie molꢁculaire” (GDRI) co-
directed by Prof. Zioa Voitenko, and the advisory participation of Prof.
Oleg Shishkin. The authors would like to thank Dr. Jean-Louis Heully
for his help and fruitful discussions, Alix Saquet for electrochemical
measurements, Vincent Colliere for electronic microscopy, and Chantal
Zedde for HPLC separations. The authors also wish to thank Prof.
Leonid Patsenker for recording the fluorescence spectrum of the com-
pound 4A, and Prof. Heinz Gornitzka for his advice in X-ray crystallog-
raphy studies of the compounds 1 and 5A. The authors are also grateful
to Prof. Jean-Pierre Launay, Dr. Suzanne Ferry-Forgues, and Prof.
Leonid Patsenker for their interest and valuable suggestions in the inter-
pretation of the dichromic properties of the compound 4A. The compu-
3238
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ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 3226 – 3240

Hexaoxy-[6]Pericyclynes
FULL PAPER
tational studies were performed using HPC resources from CALMIP
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1
1
2
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(b2+c2) = =17.9 and b=18.9 ꢅ), yet being different. The
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1
scheme below) would entail c/b=3 = =1.732 (it may be incidentally
2
remarked that the largest basis (b,c) of the experimental unit cell is
1
=
2
a golden rectangle: c/b=30.456/18.865=1.615ꢃ(1+5 )/2=1.618).
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1
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ꢀ
ꢀ
[28] This observation is completely empirical. However, by using the
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[16] More qualitatively, the strongest absorption band of 1 may be relat-
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red
av
duced species, E appears as the average one-electron reduction
potential through one-electron and two-electron transfer processes,
Chem. Eur. J. 2012, 18, 3226 – 3240
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3239

R. Chauvin, V. Maraval et al.
and might thus be also be regarded as a rough measure of the
anyway non-observable LUMO energy.
Received: September 23, 2011
Published online: February 9, 2012
3240
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Chem. Eur. J. 2012, 18, 3226 – 3240