A ubiquinol-based charge-transfer complex obtained from a solvent-free approach

Article abstract of DOI:10.1002/hlca.200800280

Quinones (=cyclohexa-2,5-diene-1,4-diones) and hydroquinones (=benzene-1,4-diols) belong to species that are balanced between their redox character and their ability to build supramolecular complexes. Considering the ubiquinol 2,3-dimethoxy-5-methyl-1,4-dihydroquinone (=2,3-dimethoxy-5- methylbenzene-1,4-diol; 1), the tendency to undergo an oxidation side reaction was overcome by combining this electron-donating species 1 with a nonreactive partner, benzene-1,2,4,5-tetracarbonitrile (TCNB; 3), to yield a 2 : 1 charge-transfer (CT) complex 4. This work illustrates how very convenient the solvent-free techniques are to access intermolecular species. X-Ray diffraction studies revealed that pure ubiquinol 1 (structure included) crystallizes in two enantiomeric conformations, while the triads 4 formed with TCNB (3) exist as meso forms assembled via H-bonds in zigzag-chains patterns.

Full text of DOI:10.1002/hlca.200800280

Helvetica Chimica Acta – Vol. 92 (2009)
453
A Ubiquinol-Based Charge-Transfer Complex Obtained from a Solvent-Free
Approach
a
b
b
b
by Marcel Bouvet* ), Bernard Malezieux ), Patrick Herson ), and FranÅoise Villain )
a) Institut de Chimie Mol e´ culaire de lꢀUniversit e´ de Bourgogne, UMR CNRS 5260, 9 avenue A. Savary,
BP 47870, F-21078 Dijon Cedex (phone: þ 33-38-0396086; e-mail: marcel.bouvet@u-bourgogne.fr)
b
)
Laboratoire de Chimie Inorganique et Mat e´ riaux Mol e´ culaires, Universit e´ Pierre et Marie Curie –
Paris 6, UMR CNRS 7071, 4 place Jussieu, Case courrier 42, F-75252 Paris Cedex 05
Quinones (¼cyclohexa-2,5-diene-1,4-diones) and hydroquinones (¼ benzene-1,4-diols) belong to
species that are balanced between their redox character and their ability to build supramolecular
complexes. Considering the ubiquinol 2,3-dimethoxy-5-methyl-1,4-dihydroquinone (¼2,3-dimethoxy-5-
methylbenzene-1,4-diol; 1), the tendency to undergo an oxidation side reaction was overcome by
combining this electron-donating species 1 with a nonreactive partner, benzene-1,2,4,5-tetracarbonitrile
(
TCNB; 3), to yield a 2 :1 charge-transfer (CT) complex 4. This work illustrates how very convenient the
solvent-free techniques are to access intermolecular species. X-Ray diffraction studies revealed that pure
ubiquinol 1 (structure included) crystallizes in two enantiomeric conformations, while the triads 4 formed
with TCNB (3) exist as meso forms assembled via H-bonds in zigzag-chains patterns.
1. Introduction. – Ubiquinones, i.e., 6-isoprenoid-substituted 2,3-dimethoxy-5-
methylcyclohexa-2,5-diene-1,4-diones and ubihydroquinones are involved in a wide
range of biological processes. Thus, the bacterial reaction centre, which is a membrane-
bound bacteriochlorophyll – protein complex responsible for the light-induced electron
transfer and associated proton-uptake reactions in bacterial photosynthesis, contains
two ubiquinone molecules [1]. Recently, the critical role of a thiolate – ubiquinone
charge-transfer (CT) complex in disulfide-bond generation by the inner-membrane
protein DsbB, which is present in Escherichia coli, has been reported [2]. Other authors
proposed a quinhydrone-type complex as being responsible for the absorption
properties observed during the disulfide-bond formation [3] (quinhydrone ¼ qui-
none/hydroquinone 1:1 ¼ cyclohexa-2,5-diene-1,4-dione/benzene-1,4-diol 1:1). Else-
where, p-stacking interactions involving phylloquinone a 3-isoprenoid-substituted 2-
methylnaphthalene-1,4-dione, i.e., another substituted quinone, were also assumed to
take part in the photosystem I activity [4].
The control of self-organization via intermolecular interactions is currently of much
interest in chemistry, materials science, and biological systems [5 – 7]. Among CT
complexes, quinhydrones are often cited to illustrate how supramolecular complexes
can be built up around the subtle balance between noncovalent interactions, such as H-
bonding and p – p stacking [8 – 10]. Quinhydrones are commonly described as a tandem
of an electron-donating hydroquinone moiety associated through p-stacking inter-
actions to an electron-withdrawing quinone entity. However, less is known about their
ꢁ
2009 Verlag Helvetica Chimica Acta AG, Zꢂrich

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Helvetica Chimica Acta – Vol. 92 (2009)
exact structure even though they are claimed to exist on the basis of their strong
absorption band in the VIS region, signature of an induced CT process. As a matter of
fact, only a restricted number of resolved X-ray structures of quinhydrones obtained
from substituted quinone or hydroquinone derivatives are available in the literature.
Although the acceptor/donor 1:1 stoichiometry is the most commonly encountered,
other examples exist with 2 :1 [11 – 13] or 1:2 [14][15] ratios. Furthermore, besides the
stacked pattern of these complexes, strong associations via H-bonding lead to chains
formation [16] or real polar planes [17].
Recently, we reported a convenient approach to a quinone/hydroquinone complex
starting from the solid forms under vacuum to overcome the solvation-energy effect
involved in the quinhydrone crystallization process [17]. Following the outcomes
obtained with 2-methoxy-1,4-quinone/2-methoxy-1,4-hydroquinone, we broadened the
range of tested molecules in a systematic way. Thus, we successfully prepared single
crystals of a new series of quinhydrones obtained from 1,4-quinones, 2-methoxy-1,4-
quinones, and 2,6-dimethyl-1,4-quinones associated with 1,4-dihydroquinones, 2-me-
thoxy-1,4-hydroquinones, and 2,3,5,6-tetrafluoro-1,4-hydroquinones as starting species
[
18]. Dealing with methoxy derivatives, these stimulating results have prompted us to
test the noninnocent ubiquinol 2,3-dimethoxy-6-methyl-1,4-dihydroquinone (CoH Q ;
2
0
1
), a reduced coenzyme Q without an isoprenoid side chain.
From an experimental point of view, the co-grinding of a solid quinone in a mortar
with its corresponding solid hydroquinone proved to be the simplest and most efficient
method of obtaining quinhydrones [19]. It also constitutes a preliminary test to readily
check the feasibility of the expected match and allows a subsequent UV/VIS analysis in
KBr pellets. However, if the two starting components are differently substituted, a
redox side reaction can occur that theoretically leads to the formation of four different
acceptor/donor pairs. In the case of the ubiquinone 2,3-dimethoxy-5-methyl-1,4-
quinone (CoQ ; 2) and ubiquinol 1, this method does not lead to any characteristic
0
expected colored product. Moreover, the co-grinding of 1 and 2-methoxy-1,4-quinone
produced a purple powder that was associated with the symmetrical 2-methoxyquinhy-
drone [17]. Owing to their substituents, combining mesomeric and inductive effects,
ubiquinone 2 and ubiquinol 1 were found to be particularly reactive species. An
ubiquinone acts as an oxidizing reagent, while an ubiquinol is rather sensitive to a
reoxidation process. It is reasonable to believe that these behaviors provide in vivo fast
redox reactions through easily switchable species.

Helvetica Chimica Acta – Vol. 92 (2009)
455
To ensure the CT-complex generation, a tuning has to be developed by the subtle
substitution of the aromatic rings, thus modulating the acceptor/donor character of the
selected quinone/hydroquinone pair. Therefore, the underlying redox activity (URA)
plays a key role in such supramolecular assemblies.
By analogy with the situation encountered with a calix[4]arenequinone [20], we
finally concluded that neither the co-grinding nor the solvent-free approach would
provide any ubiquinhydrone formation in the case of 1 and 2. This also holds for the 2,6-
dimethoxy-1,4-quinone and its hydroquinone from which we have been unable to
generate a quinhydrone derivative. As established previously, the underlying redox
activity imposes a restricted choice of acceptor– donor pairs. For example, in the case of
the pair 2-methoxy-1,4-hydroquinone/1,4-benzoquinone, the obtained quinhydrone
resulted from a preliminary redox reaction consisted of the assembly of two molecules
of 2-methoxy-1,4-quinone with one molecule of 1,4-hydroquinone [13].
Since, up to now, attempts to apply the above-mentioned solvent-free processes to
obtain an ubiquinhydrone from 1 and 2 have failed, we chose to associate ubiquinol 1 to
another type of acceptor molecule, namely benzene-1,2,4,5-tetracarbonitrile (TCNB;
3
), which is known for its ability to form CT complexes [21 – 24]. Among the desirable
features compared to quinones, TCNB presents a rather weak volatility and cannot
play a role in redox reactions, since this molecule constitutes a relative inert species.
Here, we report the first crystal structure of a CT complex involving an ubiquinol.
2
. Results and Discussion. – 2.1. Solvent-Free Reactions. Interestingly, the co-
grinding of solid TCNB (3) with CoH Q (1) yielded a beetroot-red powder; its VIS
2
0
spectrum (KBr) showed the characteristic CT band at 535 nm. Thus, heating 1 and 3
under primary vacuum in a two-separated-compartments home-made sublimator
(
Fig. 1) [17] led, after 3 h, to the formation of dark red single crystals of 4. TCNB (3)
was heated at 1008 in the central compartment, whereas ubiquinol 1 was heated at 458
in the side part of the sublimator.
Fig. 1. Home-made sublimator with two separated
compartments: production of single crystals of 4
from 1 and 3
2
.2. Structural Studies. Crystal Structure of CoH Q (1). Among the ubiquinones,
2 0
CoQ (2) is the unique representative whose crystal structure is known [25]. CoH Q
0
2
0
(1), as obtained by sublimation under vacuum, crystallizes in the noncentered
monoclinic space group Cc. Remarkably, in the X-ray crystal structure of 1 the C-atoms
of the MeO groups lie outside the mean plane by 1.385 (C(8)) and 1.165 (C(9)) ꢃ,

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Helvetica Chimica Acta – Vol. 92 (2009)
providing a chiral entity. In the cell, the two enantiomeric conformations correspond to
each other by a translating mirror plane.
For clarification purposes, we considered useful to mention the assignment of the
conformations evidenced in the X-ray crystal structure of 1. The chirality (R) or (S) is
defined by the position of the two MeO groups relative to the aromatic ring; one can
consider the rotation direction from bigger to smaller groups, as seen from the MeO
group above the aromatic ring (¼ reference group; Fig. 2). Another nomenclature uses
the stereodescriptors (P) or (M) following the helical geometry. Thus, the two
enantiomeric conformations of 1 adopt (M) or (P) configurations, associated to (S) or
(
R) configurations, respectively. It should be noticed that the presence of the Me group
does not affect the attribution because it belongs to the plane of chirality.
Fig. 2. Assignment of the two enantiomeric conformations of 1: (M) and (P) geometries (top) and (S)
conformation in the (R)/(S) denomination (bottom)
In pure 1, no p – p interactions occur (angle between mean planes: 56.078), and the
crystalline arrangement results from strong H-bonds providing a pattern of interlocked
molecules,
H(4)ꢀO(1) ¼ 165.028 and O(4) ··· H(1) ¼ 2.264 ꢃ, angle O(1)ꢀH(1) ··· O(4) ¼ 157.528
for atom numbering, see below, Fig. 3,a, left). Additionally, short intramolecular
O(1) ··· H(4) ¼ 2.130 ꢃ,
O(1) ··· O(4) ¼ 2.996 ꢃ,
angle
O(4) ···
(
distances exist, namely O(1) ··· O(6) ¼ 2.755 ꢃ, O(4) ··· O(5) ¼ 2.793 ꢃ, and O(5) ···
O(6) ¼ 2.858 ꢃ.
Crystal Structure of (CoH Q ) · TCNB (4). The asymmetric unit of 4 is composed
2
0 2
of one molecule of 1 and half a molecule of 3 (Fig. 3). As a matter of fact, 4 is a 2 :1
complex of 1 and 3, as it is the case in only few examples of quinhydrones. The molecule
3
lies in the center of the unit cell in addition to the eight corner points, but not forming
a body-centered lattice since they are not related by translation only. The aromatic
moieties of 1 and 3 are tilted by 14.528 as defined by their mean planes. The distance
between their centroids is 4.302 ꢃ. The distance from the centroid of unit 3 to the mean
planes of units 1 is 3.20 ꢃ, as defined from the projection along the normal to these

Helvetica Chimica Acta – Vol. 92 (2009)
457
Fig. 3. X-Ray crystal structure: a) ORTEP representations (30% probability level) of the asymmetric
units for 1 (left) and 4 (right), resp. (arbitrary atom numbering; bond lengths in ꢃ), b) projection of 4 on
the (010) plane: view of a DAD triad (80% van der Waals radii), and c) view of the molecules interacting
by H-bonds (dotted lines) showing a chain in the (110) plane (H-atoms are omitted for clarity)

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Helvetica Chimica Acta – Vol. 92 (2009)
planes, indicating strong p – p interactions. So, the structure is composed of donor–
acceptor– donor (DAD) triads.
The displacement of a triad allows an O-atom of each ubiquinol unit 1 to lie
immediately above the C(10)ꢀC(11) bond, leading to the shortest intermolecular
distances between the donor and acceptor units at 3.120 ꢃ (C(11) ··· O(1)) and 3.231 ꢃ
(C(10) ··· O(1)). Each unit 1 is connected to its mirror image by an inversion center
through two H-bonds (O(4) ··· O(5’) ¼ 2.864 ꢃ (¼O(5) ··· O(4’)) and angle O(4) ···
H(4)ꢀO(5’) ¼ 142.718), and also to a unit 3 via an H-bond that involves an N-atom
(
O(1) ··· N(14) ¼ 2.981 ꢃ, angle O(1)ꢀH(1) ··· N(14) ¼ 153.538) (Fig. 3). This provides
a zigzag chain orientation in the (110) plane with a periodicity of 20.565 ꢃ, as deter-
mined from the distance between the centroids of two consecutive TCNB (3) molecules.
The chains are connected to each other through p – p interactions, defining sheets
parallel to the (110) plane. Chains including units 3 at the center of the cell make an
angle of 828 with chains incorporating units 3 in the knots. Therefore, the structure can
be described as layers parallel to the (110) plane, with an alternated orientation of the
chains from one layer to the next. It is noteworthy that, contrarily to quinhydrone
examples, the molecules do not arrange in infinite columns. Additionally, the ArH bond
(
C(3)ꢀH(31)) of 1 is oriented towards the center of the aromatic ring of units 1
belonging to a neighboring layer, with H(31) ··· centroid (C(1)C(2)C(3)C(4)C(5)C(6))
¼
2.79 ꢃ (the distance from this atom to the mean plane is 2.756 ꢃ). Whereas, the units
make an angle of 69.598 between their mean planes, these interactions contribute to
1
the interlayer cohesion.
As in compound 1, the two MeO groups adopt a helical pattern in both sides of the
aromatic ring, with the corresponding C-atoms 1.116 (C(8)) and 1.159 (C(9)) ꢃ outside
the mean plane, providing a chiral entity. In the structure, the two enantiomers of 1
correspond to each other in each DAD triad by an inversion center.
Since the molecules 1 are not symmetrically substituted, their involvement in a
stack conformation leads to a discrimination between the two face orientations and
provides de facto chiral species. This situation remains strictly similar to that
encountered with ferrocene derivatives tetrasubstituted by two different groups, with
each of these groups positioned correspondingly at each cyclopentadienyl moiety [26].
In these compounds, if the two cyclopentadienyl moieties are identically functionalized,
a meso form exists when an inversion center is located at the Fe-atom.
2
.3. Spectroscopic Studies. Infrared Spectroscopy. The IR spectrum of complex 4
corresponds to a superimposition of the spectra of 1 and 3, except for the OH and C ꢁ
ꢀ1
N vibrations. Complex 4 exhibits two C ꢁ N vibrations at 2247 and 2257 cm , whereas
ꢀ1
the starting TCNB (3) shows only one C ꢁ N vibration at 2247 cm . This is in
accordance with the existence of H-bonds in 4 that involve only two of the four C ꢁ N
moieties, as shown in the crystal structure. The two apparently degenerate modes (b
2u
and b ) in the D initial space group [27] become discriminated in the C space group
3u
2h
2h
(
Fig. 4). Both vibrations are b modes, but correspond to elongations towards the H-
u
bonds direction and along the direction of C ꢁ N groups not involved in H-bonds,
ꢀ
1
ꢀ1
respectively. The OH vibration of 4 appears at 3456 cm , vs. 3423 cm for pure 1. In
addition, this band is very sharp compared to that of pure 1, the full width at half
ꢀ1
ꢀ1
maximum (FWHM) being 25 cm vs. 100 cm , indicating that all the H-bonds are of
the same strength in 4, in contrast to what occurs in 1.

Helvetica Chimica Acta – Vol. 92 (2009)
459
Fig. 4. Schematic view of active vibration modes for the cyano groups of TCNB in infrared (IR) and
Raman (R) spectroscopies, in D2h and C2h space groups. The mirror planes are shown for the two
symmetries as well as the H-bonds for the C2h symmetry.
Raman Spectroscopy. In contrast to the IR spectrum, the Raman spectrum of TCNB
ꢀ1
(
3) shows two peaks for nitrile vibrations at 2239 and 2249 cm , attributed to a and b
g 1g
modes. These modes lead to two a modes in C symmetry (Fig. 4). However, the
g
2h
ꢀ1
Raman spectrum of 4 exhibits an additional peak at 2255 cm , showing that the local
symmetry is lower than that deduced from X-ray studies, due to the existence of strong
p – p interactions.
UV/VIS Spectroscopy. Whereas the starting materials are white compounds, the
UV/VIS spectrum of 4 exhibits a strong and broad absorption band centered at 535 nm
(crystals diluted in KBr). Besides, co-grinding of solid 1 and 3 leads to a reddish powder
that exhibits the same UV/VIS and IR spectra as 4. This suggests a solid-state reactivity
similar to that encountered with quinone/hydroquinone pairs [13][17]. CT Complex 4
1
does not exist in solution. Consequently, the H-NMR spectrum of 4 in CD CN remains
3
as a simple superimposition of the spectra of 1 and 3.
Theoretical Calculations (MOPAC/MNDO). To appreciate the effect of the charge
transfer, the dipole moments of 1 as a pure compound and as present in 4 were
TM
calculated by Chem3D Pro software with the MNDO [28] method and the
geometrical data collected from the single-crystal X-ray structures of 1 and 4 without
geometrical optimization. In 1, the dipole moment, oriented parallel to the C(1)ꢀO(6)
direction, is 1.5 D, compared to 1.1 for the optimized geometry. In 4, it is 2.0 D, also
oriented quasi perpendicularly to the OHꢀOH axis, towards the C(1)ꢀO(6) direction.
Thus, the calculated dipole moment for the ubiquinol 1 turns out to be higher in the CT
complex than in the pure compound in which molecules are not involved in strong
intermolecular interactions.
The authors thank the Universit e´ Pierre et Marie Curie (UPMC Paris 6) and the Centre National de
La Recherche Scientifique (CNRS) for supporting this work. M. B. is grateful to the Ecole Sup e´ rieure de
Physique et de Chimie Industrielles (ESPCI) for financial support. The authors are indebted to Aude
Henneville for the fabrication of new glassware designed for that work and to Phillip Morony for helpful
discussions.

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Helvetica Chimica Acta – Vol. 92 (2009)
Experimental Part
2,3-Dimethoxy-5-methylbenzene-1,4-diol (1). Diol 1 was synthesized from 2 by reduction with
NaBH in EtOH under Ar. Crystals of 1 were obtained by sublimation under primary vacuum.
4
1
Crystal Data of 1 ): C H O , M ¼ 184.19, colorless parallelepiped (0.09 ꢂ 0.12 ꢂ 0.16 mm),
9
12
4
r
3
monoclinic, Cc; a ¼ 13.0399(15), b ¼ 8.1531(10), c ¼ 8.8900(9) ꢃ, b ¼ 96.652(10)8; V¼ 938.78(19) ꢃ ,
ꢀ
3
Z ¼ 4, D ¼ 1.30 g cm . KappaCCD-Enraf-Nonius diffractometer, l (MoK ) 0.71073 ꢃ, m (MoK ) ¼
c
a
a
ꢀ1
1
.14 cm ; 5059 reflections (T 250 K, 3 < q < 298); number of independent data collected 1364, number
2
2
of independent data used for refinement 1143 ((F ) > 1.5s (F ) ; merging R ¼ 0.0336, R ¼ S jj F jꢀj F jj /
o
o
o
c
2
2
1/2
ꢀ3
S j F j¼ 0.0352, Rw* ¼ [Sw(j j F jꢀj F jj ) /SwF ] ¼ 0.0419; ꢀ 0.15 < D1 < 0.26 e · ꢃ
.
o
o
c
o
Benzene-1,2,4,5-tetracarbonitrile Compound with 2,3-Dimethoxy-5-methylbenzene-1,2-diol (1:2) (4).
Crystals of 4 were synthesized from 1 (50 mg, 0.27 mmol) and 3 (50 mg, 0.28 mmol) which were placed in
two separated compartments of a home-made sublimator (Fig. 1) and both heated gradually up to 458
ꢀ
1
and 1008, resp. The vacuum (10 Torr) was continuously maintained, and the cold trap filled with water.
IR (attenuated total reflection (ATR); main peaks): 3455 (OH); 3109, 3043 (CH, TCNB); 2989, 2947,
2
848 (CH, CoH Q ); 2257, 2247 (CN); 1493, 1475, 1190, 1178, 1113, 1064, 991, 946. Raman (785 nm beam
2 0
of a laser diode in a microRaman set-up on a single crystal; main peaks): 2255, 2249, 2239 (CN); 1605,
1
1
595, 1540, 1257, 1249, 719, 562, 407, 225, 145. UV/VIS (KBr): 535 nm. H-NMR (300 MHz, crystals
dissolved in CD CN; d in ppm, J in Hz): 2.10 (d, J(arom. H,Me) ¼ 0.9, Me); 3.82 (s, MeO); 3.85 (s, MeO);
3
6
.16 (br. s, 2 OH); 6.40 (q, J(arom. H,Me) ¼ 0.9, 1 arom. H (1)); 8.5 (s, 1 arom. H (3)).
1
Crystal Data for 4 ): C H N O , M 546.54; burgundy-red parallelepiped (0.08 ꢂ 0.11 ꢂ 0.15 mm),
28
26
4
8
r
3
monoclinic, P21/n, a ¼ 13.9986(13), b ¼ 7.5329(9), c ¼ 14.2498(19) ꢃ; b ¼ 112.043(7); V¼ 1392.8(3) ꢃ ,
ꢀ3
Z ¼ 2, D
c
1
¼ 1.30 g cm ; KappaCCD-Enraf-Nonius diffractometer, l (MoK
a
) 0.71073 ꢃ, m (MoK
a
) ¼
ꢀ
1.14 cm ; 15165 reflections (T 250 K, 1 < q < 308); number of independent data collected 4052, number
2
2
of independent data used for refinement 1697 ((F ) > 1.5s (F ) ; merging R ¼ 0.0692, R ¼ S jj F jꢀj F jj /
o
o
o
c
2
2
1/2
ꢀ3
S j F j¼ 0.0485, Rw* ¼ [Sw(j j F jꢀj F jj ) /SwF ) ¼ 0.0413; ꢀ 0.31 < D1 < 0.23 e · ꢃ
.
o
o
c
o
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Received July 21, 2008