Triptycene 1,2-Quinones and Quinols: Permeable Crystalline Redox-Active Molecular Solids

Article abstract of DOI:10.1021/acs.joc.8b02706

Suitably designed quinones and quinols are promising modules for the programmed construction of ordered redox-active molecular solids. To explore this potential, we have synthesized compounds 1-4, in which multiple 1,2-benzoquinone and 1,2-quinol units are attached to a triptycene core. The resulting molecules have topologies that disfavor efficient packing, and structural studies show that they crystallize to form open networks held together by characteristic attractive intermolecular forces, including O-H···O hydrogen bonds, C-H···O interactions, π-stacking, and dipolar interactions. Remarkably, the resulting solids are permeable and can undergo reversible redox reactions without loss of crystallinity. Our work may thereby help lead to the design of robust carbon-based batteries with electrodes derived from quinones, quinols, and other redox-active molecules abundantly produced by nature.

Full text of DOI:10.1021/acs.joc.8b02706

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Article
Triptycene 1,2-Quinones and Quinols. Permeable
Crystalline Redox-Active Molecular Solids
Sophie Langis-Barsetti, Thierry Maris, and James D. Wuest
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b02706 • Publication Date (Web): 15 Nov 2018
Downloaded from http://pubs.acs.org on November 20, 2018
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Triptycene 1,2-Quinones and Quinols.
Permeable Crystalline Redox-Active Molecular Solids
Sophie Langis-Barsetti,1 Thierry Maris, and James D. Wuest*
Département de Chimie, Université de Montréal, Montréal, Québec H3C 3J7
Canada
*Author to whom correspondence may be addressed. E-mail:
james.d.wuest@umontreal.ca
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Abstract
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Suitably designed quinones and quinols are promising modules for the programmed construction
of ordered redox-active molecular solids. To explore this potential, we have synthesized
compounds 1–4, in which multiple 1,2-benzoquinone and 1,2-quinol units are attached to a
triptycene core. The resulting molecules have topologies that disfavor efficient packing, and
structural studies show that they crystallize to form open networks held together by characteristic
attractive intermolecular forces, including O-H···O hydrogen bonds, C-H···O interactions, π-
stacking, and dipolar interactions. Remarkably, the resulting solids are permeable and can
undergo reversible redox reactions without loss of crystallinity. Our work may thereby help lead
to the design of robust carbon-based batteries with electrodes derived from quinones, quinols,
and other redox-active molecules abundantly produced by nature.
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Introduction
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Quinones and hydroquinones (known historically as quinols) play a uniquely important role in
chemistry, in part because of their widespread occurrence in nature and their characteristic ability
to engage in redox reactions.2,3 In addition, these compounds take part in diverse intermolecular
interactions and thereby show strongly associative behavior, allowing them to serve as useful
modules in the programmed construction of ordered molecular materials.4 In particular, the
participation of these compounds in charge-transfer interactions and π-stacking has been used to
produce assemblies with radicals5,6 and with aromatic compounds.7–9 Quinones and quinols can
also form hydrogen-bonded networks with a variety of partners, including heterocycles,8,10
phenols,9,11–15 and other classes of compounds.16–18 The first example of the phenomenon of co-
crystallization, reported by Wöhler in 1844,19 was produced by combining 1,4-benzoquinone and
1,4-hydroquinone in a 1:1 ratio. The resulting structure, known as quinhydrone, features an open
network held together by a combination of hydrogen bonds and charge-transfer interactions.20–22
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However, surprisingly little effort has been made so far to build on this promising foundation by
designing more complex quinones and quinols for use in the modular construction of new
molecular materials.23–27 Moreover, the chemistry of 1,4-benzoquinones and hydroquinones has
been explored much more extensively than that of the isomeric 1,2-benzoquinones and their
reduced forms,28 despite the potential of all of these compounds to engage in similar
intermolecular interactions and to serve as modules in supramolecular construction.29,30 In
particular, the historic work of Wöhler provides a strong motivation for studying the co-
crystallization of 1,2-benzoquinones with catechols, but little work of this type has been
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reported.31–32 We have therefore undertaken a study to assess the utility of 1,2-benzoquinones
and the corresponding catechols in the modular assembly of complex redox-active molecular
structures held together by hydrogen bonds, charge-transfer interactions, and other forces. Of
particular interest is the possibility of using such modules to make porous crystalline solids for
use as electrodes in carbon-based batteries.33–36
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In addition to being redox-active, 1,2-benzoquinones can also take part in Diels-Alder reactions,
leading to the formation of dimers,37,38 or by undergoing sequences of hydration,
tautomerization, and autoxidation to give hydroxy-substituted 1,4-benzoquinones and other
products.39,40 To avoid these degradative pathways, we elected to study suitably substituted 1,2-
benzoquinones. Moreover, to favor the formation of open networks created by modular
assembly, we decided to attach multiple 1,2-benzoquinone and catechol units to molecular cores
with shapes designed to inhibit efficient packing. In this way, we identified
triptycene(trisquinone) 1 and reduced forms 2–4 as compounds of particular interest as modules
for supramolecular construction. Triptycenes are attractive choices because their characteristic
rigid trigonal topology interferes with close molecular packing and thereby favors the formation
of open structures with significant volume available for the inclusion of guests.41 Moreover,
triptycenes 1–4 provide relatively simple symmetric structures that incorporate multiple 1,2-
benzoquinone and catechol units, all freely exposed on the molecular periphery in a way that lets
them form characteristic intermolecular interactions without significant interference. Simpler
triptycenes with quinone and catechol moieties have been prepared by White and MacLachlan
for use in constructing molecular assemblies,42,43 but no structural analyses of the triptycenes
themselves have been reported.
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O
OH
O
OH
O
HO
O
HO
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O
O
HO
HO
HO
HO
O
O
OH
OH
OH
OH
O
O
O
O
1
2
3
4
Results and Discussion
Syntheses of Triptycenes 1–4. Ghanem et al. reported using BBr3 to convert the known
hexamethoxytriptycene 5 into triptycene(triscatechol) 4 (Scheme 1), but no characterization of
the product was reported.44 We prepared triscatechol 4 from precursor 5 by a modified procedure
in which only three equivalents of BBr3 were used, because the demethylation of ortho methoxy
substituents is known to occur via cyclic borate intermediates.45 In triptycene derivatives,
controlled stepwise oxidation of the three aromatic rings is possible because electronic effects
are communicated transannularly.46,47 Notably, Chen and collaborators reported the progressive
oxidation of an analogue of triscatechol 4 with nitric acid by varying the concentration of oxidant
and the length of contact.47 Nitric acid was also used by Han and coworkers to prepare an
analogue of trisquinone 1, but they reported only a partial characterization of the product.48 To
avoid possible side reactions involving nitration and ring opening,49 and to better control the
extent of oxidation, we decided to treat triscatechol 4 with other agents, and we achieved the
synthesis of compounds 1–3 in a selective manner, as summarized in Scheme 1.
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Scheme 1
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OMe
OH
HO
1,4-benzoquinone
3
2
1
MeO
77%
o-chloranil
79%
BBr3
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91%
HO
MeO
MeO
DDQ
OMe
OMe
OH
OH
HO
61%
5
4
Although 1,4-benzoquinones are typically less powerful oxidants than their 1,2-benzoquinone
counterparts,50,51 each catechol unit in triscatechol 4 is substituted by an electron-donating
tetrahydroxy-9,10-dihydroanthracenyl scaffold, and 1,4-benzoquinone can in fact be used to
effect a single oxidation to give monoquinone 3 in 77% yield. Oxidation to produce bisquinone 2
occurred in 79% yield when triscatechol 4 was treated with tetrachloro-1,2-benzoquinone (o-
chloranil), whereas p-chloranil was ineffective. These reactions are highly selective, and only a
single product of oxidation was observed in the crude mixtures. No significant amounts of
starting material, intermediates, or over-oxidized compounds were observed, even after extended
periods of time or in the presence of excess oxidant. Finally, 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) oxidized triscatechol 4 to give trisquinone 1 in 61% yield. Compounds 1–4
all proved to be stable in the solid state, although triscatechol 4 is slightly air-sensitive, and
quinones 1–3 decomposed slowly in solution, especially when exposed to heat or light.52 All
characterizations were therefore performed using freshly prepared solutions, and crystallizations
were carried out in the dark.
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Absorption and Emission Properties of Triptycenes 1–4. Absorption and emission spectra of
compounds 1–4 are shown in Figure 1. All four compounds absorb near 290 nm, and the band
shifts bathochromically from trisquinone 1 to triscatechol 4 as electron-donating catechol units
are added to the triptycene core. Quinones 1–3 show additional absorption near 380 nm, leading
to spectra that superimpose the characteristic features of catechol itself (λmax = 277 nm) and those
of 1,2-benzoquinone (λmax = 273, 377 nm).53,54 Monoquinone 3 also exhibits an important
charge-transfer band around 480 nm, which is typical of triptycene donor-acceptor
systems,46,47,55 and bisquinone 2 shows a band that is similar but less intense. Emission spectra
for compounds 2–4 reflect the relative positions of their absorption bands near 290 nm. Emission
from trisquinone 1 is bathochromically shifted, but spectra of compound 1 were recorded in
DMF to circumvent poor solubility in CH3CN, and the observed shift may be solvent-induced.
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Figure 1. Absorption spectra (left) and normalized emission spectra (right) of solutions of
triptycene(trisquinone) 1 (green), bisquinone 2 (blue), monoquinone 3 (red), and triscatechol 4
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(black). Compound 1 was dissolved in DMF, and compounds 2–4 were dissolved in CH3CN,
with λex = 280 nm for compounds 1 and 2, λex = 290 nm for compound 3, and λex = 300 nm for
compound 4.
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Structure of Triptycene(trisquinone) 1. Layering CH3CN on a solution of trisquinone 1 in
DMSO induced the formation of red crystals, which were found to belong to the hexagonal space
group P63. Additional crystallographic information is summarized in Table 1, and a
representation of the structure appears in Figure 2. Dipolar interactions between canted oxygen
atoms and dienes (3.03 Å for the shortest C···O distance) underlie organization of molecules of
compound 1 to form sheets (Figure 2a). Adjacent sheets are linked in an offset arrangement by
C-H···O interactions (2.46–2.48 Å) involving oxygen atoms and hydrogen atoms of the quinone
units (Figure 2b), thereby defining parallel hexagonal channels. The channels contain disordered
molecules of CH3CN, and 17% of the total volume of the crystals is accessible to guests.56–58
Direct removal of the guests under reduced pressure leads to the loss of crystallinity, and we
have not yet been able to prepare a guest-free crystalline solid constructed from trisquinone 1.
a
b
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Figure 2. Representations of the structure of crystals of triptycene(trisquinone) 1 grown from
DMSO/CH3CN. (a) View down the c-axis showing two adjacent offset sheets, one with all
molecules drawn in blue. The offset sheets define parallel hexagonal channels. (b) View down
the b-axis showing molecules in five adjacent sheets and revealing how the sheets are connected
by C-H···O interactions. The most significant interactions are represented by broken lines.
Unless otherwise indicated, atoms of carbon are shown in gray, atoms of hydrogen in white, and
atoms of oxygen in red. Included molecules of solvent are omitted.
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Table 1. Crystallographic data for triptycenes 1–4.
Compound
1 • CH3CN
2 • 1.5 dioxane
3 • 3.5 dioxane
4a
crystallization
medium
DMSO/CH3CN
dioxane/PhMe
dioxane/hexane
Me-THF
formula
crystal system
space group
a (Å)
C20H8O6 • C2H3N C20H10O6 • 1.5 C4H8O2 C20H12O6 • 3.5 C4H8O2 C20H14O6
hexagonal
P63
monoclinic
P21/c
monoclinic
I2/a
hexagonal
6
P 2c
15.7405(3)
15.7405(3)
11.9054(3)
90
17.2624(5)
8.6308(2)
16.1531(5)
90
24.0009(11)
8.5212(4)
31.560(2)
90
13.3732(7)
13.3732(7)
10.8212(7)
90
b (Å)
c (Å)
α (°)
β (°)
90
109.286(2)
90
93.151(2)
90
90
γ (°)
120
120
V (Å3)
2554.53(12)
6
2271.57(11)
4
6444.8(6)
8
1676.0(2)
2
Z
ρcalcd (g cm-3)
1.503
1.399
1.354
0.694b
T (K)
150
150
150
150
μ (mm-1)
R1, I > 2σ
R1, all data
ωR2, I > 2σ
ωR2, all data
1.503
0.574
0.560
0.276
0.0968
0.1035
0.2556
0.2804
0.0539
0.1298
0.0654
0.1397
0.0811
0.2113
0.1295
0.2532
0.0488
0.1623
0.0493
0.1635
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no.
reflections
measured
38684
3924
29785
4327
40908
37342
1279
no. independent
reflections
5526
3431
no.
reflections
2σ(I)
obs.
I
>
3184
3615
1248
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a Guests not identified unambiguously by crystallography are omitted from the composition.
b Calculated without contributions from guests.
Structure of Triptycene(bisquinone) 2. The addition of toluene, chloroform, or ether to a
saturated solution of bisquinone 2 in dioxane afforded rectangular dark orange crystals. Analysis
by X-ray diffraction established that the crystals belonged to the monoclinic space group P21/c.
Crystallographic parameters are provided in Table 1, and views of the structure appear in Figure
3. Molecules of compound 2 are connected by O-H···O hydrogen bonds (O···O distance = 2.735
Å) between quinone and catechol units to form undulating tapes. The tapes are further linked into
sheets by C-H···O interactions (H···O distance = 2.50 Å) involving the bridgehead hydrogens
and quinone moieties (Figure 3a). The sheets in turn are paired by additional C-H···O
interactions (H···O distance = 2.38 Å) and by dipolar interactions between oxygen atoms and
electron-deficient carbon atoms incorporated in the quinone units (2.90 Å for the shortest C···O
distance). It is interesting to note that the quinone units involved as donors in these dipolar
interactions are planar, whereas the acceptor quinones are not, and their adjacent carbonyl groups
form a dihedral angle of 17º. Additional cohesion within the sheets is provided by short C···O
interactions (2.77 Å) involving the oxygen atoms of well-ordered molecules of included dioxane
and electron-deficient carbon atoms in the distorted quinone units. Pairs of sheets are isolated
from one another by layers of partly ordered molecules of dioxane (Figure 3b). In total, 42% of
the volume of the crystal is accessible to guests.56
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a
b
Figure 3. Representation of the structure of crystals of triptycene(bisquinone) 2 grown from
dioxane/toluene. (a) View showing how undulating hydrogen-bonded tapes are further linked by
C-H···O interactions to form sheets. Two individual tapes are highlighted in blue, and the most
significant intermolecular interactions are represented by broken lines. (b) View down the b-axis
showing how paired sheets are isolated from one another by layers of dioxane. One individual
sheet is highlighted in blue, and all molecules of dioxane appear in red. Unless stated otherwise,
atoms of carbon are shown in gray, atoms of hydrogen in white, and atoms of oxygen in red.
Crystallization of bisquinone 2 from a mixture of THF and hexane provided a pseudopolymorph
in which the quinone units are less strikingly deformed.59 As a result, the torsion observed in the
structure of crystals grown from dioxane may not be an inherent feature of compound 2 but may
rather be a consequence of molecular packing or the result of multiple C···O interactions
involving dioxane. However, distorted 1,2-benzoquinones have been noted by other researchers;
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for example, Fukin et al. observed a torsion angle of 34º between the carbonyl groups in 4,5-
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dimethoxy-3,6-di-tert-butyl-1,2-benzoquinone.60
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Structure of Triptycene(monoquinone) 3. Allowing vapors of hexane to diffuse into a solution
of monoquinone 3 in dioxane induced the formation of dark red plates, which were found to
belong to the monoclinic space group I2/a. Additional crystallographic data are presented in
Table 1, and views of the structure are shown in Figure 4. Like bisquinone 2, monoquinone 3
crystallizes to form a structure built from sheets separated by layers of included molecules of
dioxane. Molecules of compound 3 form pairs held together by π-stacking of quinone units
(centroids separated by 3.65 Å) and by C-H···π interactions involving hydrogen atoms of
quinone units and the π-systems of catechol units (H···centroid distance = 2.38 Å). The pairs are
further linked into sheets by multiple intermolecular O-H···O hydrogen bonds and C-H···O
interactions (Figure 4). In particular, bifurcated O-H···O hydrogen bonds (O···O distances of
2.86 and 2.89 Å) are formed along the b-axis, with the two oxygen atoms of quinone units acting
as a bidentate acceptor and an OH group of catechol units acting as donor. In addition, C-H···O
interactions along the b-axis link oxygen atoms of quinone units with aromatic hydrogens and
bridgehead hydrogens (H···O distances of 2.42 and 2.63 Å). Along the a-axis, intermolecular O-
H···O hydrogen bonds between catechol units (O···O distance = 2.81 Å) and C-H···O
interactions between catechol oxygen atoms and aromatic hydrogen atoms (H···O distance =
2.56 Å) reinforce the sheets. The structure is also strengthened by interactions of monoquinone 3
with oxygen atoms of included molecules of dioxane, which form O···H-O hydrogen bonds with
catechol units (O···O distance = 2.76 Å), O···C interactions with quinone units (O···centroid
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distance = 2.78 Å), and various O···H-C interactions. Approximately 59% of the volume of the
crystal is accessible to guests.56
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Figure 4. Representation of the structure of crystals of triptycene(monoquinone) 3 grown from
dioxane/hexane. (a) View showing how π-stacked pairs are further associated by multiple O-
H···O hydrogen bonds and C-H···O interactions to form sheets. An individual pair is highlighted
in blue, and various intermolecular interactions are represented by broken lines. (b) View down
the b-axis showing how sheets are isolated from one another by layers of dioxane. One
individual sheet is highlighted in blue, and all molecules of dioxane appear in red. Unless stated
otherwise, atoms of carbon are shown in gray, atoms of hydrogen in white, and atoms of oxygen
in red.
Structure of Triptycene(triscatechol) 4. Cooling a saturated solution of triscatechol 4 in
methyltetrahydrofuran (Me-THF) yielded colorless needles that were found to belong to the
hexagonal space group P 2c. Other crystallographic data are provided in Table 1, and
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representations of the structure appear in Figure 5. Molecules of compound 4 can be considered
to be packed in sheets in the ab-plane without forming any noteworthy intra-sheet interactions
(Figure 5a), but adjacent sheets are linked along the c-axis by multiple O-H···O hydrogen bonds
(O···O distances in the range 2.71–2.73 Å). In this way, each molecule of triscatechol 4
participates in a total of six intermolecular O-H···O hydrogen bonds.
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Figure 5. Representation of the structure of crystals of triptycene(triscatechol) 4 grown from
Me-THF. (a) Side view of part of three offset sheets, showing how molecules in adjacent sheets
are connected by O-H···O hydrogen bonds. Molecules in one sheet are highlighted in blue, and
hydrogen bonds are represented by broken lines. (b) Space-filling view along the c-axis showing
the presence of trilobate channels defined by two adjacent sheets, with one sheet highlighted in
blue. Unless stated otherwise, atoms of carbon are shown in gray, atoms of hydrogen in white,
and atoms of oxygen in red. Disordered molecules of included solvent are not shown.
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Adjacent sheets are offset in a way that defines large trilobate channels (Figure 5b). The channels
are filled with disordered molecules of solvent, and 63% of the total volume of the crystals is
accessible to guests.56 Larger fractions of guest-accessible volumes have been observed in other
molecular crystals, particularly those of proteins.61–63 However, the structure of
triptycene(triscatechol) 4 is noteworthy because the molecule has a simple, open, symmetric
topology and consists of only 26 non-hydrogen atoms. Our observations underscore the inherent
difficulty of finding effective ways to pack molecules that have awkward topologies and must
simultaneously take part in multiple directional interactions. Molecules of this type are typically
programmed by their topologies and interactions to form open networks with significant volumes
available for the inclusion of guests.
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In addition to the structures described above in detail, pseudopolymorphs with similar molecular
organization were also obtained by crystallizing triscatechol 4 from dioxane/hexane and by
crystallizing bisquinone 2 and monoquinone 3 from THF/hexane.59 The additional
pseudopolymorphs differ primarily in the extent of inclusion of molecules of solvent. In
particular, structures produced by crystallization from dioxane are more highly solvated than
those obtained from THF, possibly because the extra atom of oxygen in dioxane provides an
additional site of hydrogen bonding. In all cases, molecules of triptycenes 1–4 are linked by
interactions typical of quinones and quinols, including O-H···O hydrogen bonds, C-H···O
interactions, π-stacking, and dipolar interactions.4,23 Crystals obtained under all conditions
include molecules of solvent and typically have large guest-accessible volumes in the form of
channels or layers. We attribute this behavior to the relatively rigid trigonal geometry of the
triptycenyl core of compounds 1–4, which cannot be packed efficiently in ways that allow the
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quinone and quinol units to engage in their preferred modes of association. As a result, normal
patterns of crystallization are disrupted, and molecules of compounds 1–4 are forced to create
open networks with significant fractions of volume occupied by guests. Moreover, because the
compounds cannot crystallize in ways that simultaneously satisfy the demands of topology and
directional intermolecular interactions, there is no overriding preference for a particular pattern
of molecular organization, and the compounds have significant opportunities to crystallize in
various pseudopolymorphic forms.
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The prototypical 1:1 co-crystallization of 1,4-benzoquinone and hydroquinone to give
quinhydrone led us to attempt co-crystallizations of different triptycenes 1–4. Of special interest
to us were co-crystallizations of pairs in which the numbers of complementary quinone and
quinol units are properly balanced. Unfortunately, however, attempted co-crystallization of
trisquinone 1 and triscatechol 4 was thwarted by disproportionation (Scheme 2), and pronounced
differences in solubility prevented us from co-crystallizing compounds 2 and 3.
Disproportionation also occurred in mixtures containing compounds 1 and 3 or compounds 2 and
4 (Scheme 2).
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Scheme 2
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OH
O
O
OH
HO
O
O
HO
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+
+
HO
HO
O
HO
O
OH
OH
O
OH
OH
O
O
HO
O
O
O
2
3
4
1
OH
OH
HO
O
HO
O
+
2
HO
HO
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HO
HO
OH
OH
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OH
OH
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2
4
3
O
O
OH
O
O
HO
+
2
HO
HO
O
O
OH
OH
O
O
O
O
O
O
3
2
1
Redox Properties of Triptycenes 1–4 in Solution. The highly selective syntheses of mono-,
bis-, and trisquinones 1–3 by controlled oxidation prompted us to study the electrochemical
behavior of the compounds in more detail. Cyclic voltammograms of solutions of trisquinone 1
and triscatechol 4 in DMF are presented in Figure 6. Redox potentials were assessed by square-
wave voltammetry to allow the potential of non-reversible events to be measured more
accurately. Square-wave voltammograms are shown in Figure 7, and redox potentials are
summarized in Table 2, along with the properties of 1,2-benzoquinone and catechol as model
compounds. The effect of transannular interactions on the redox properties of triptycene
derivatives of 1,4-benzoquinone is well documented.46,64–66 This behavior is mirrored in our
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redox data for related triptycene derivatives of 1,2-benzoquinone, and three distinct redox events
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were found for all compounds 1–4.
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a
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Figure 6. Cyclic voltammograms of solutions of triptycene(trisquinone) 1 (Figure 6a) and
triptycene(triscatechol) 4 (Figure 6b) in DMF (~1 mM) containing tetrabutylammonium
hexafluorophosphate (0.1 M) as supporting electrolyte. The voltammograms were recorded at a
scan rate of 100 mV/s, and potentials are reported with respect to the ferrocene/ferrocenium
couple.
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a
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Figure 7. (a) Square-wave voltammograms showing the reduction waves of solutions of
compounds 1–3 (top to bottom) in DMF containing 1% MeCOOH. (b) Square-wave
voltammograms showing the oxidation waves of compounds 2–4 from top to bottom. All
voltammograms were recorded in DMF solutions (~1 mM) containing tetrabutylammonium
hexafluorophosphate (0.1 M) as supporting electrolyte. The voltammograms were recorded at a
scan rate of 20–50 mV/s, and potentials are reported with respect to the ferrocene/ferrocenium
couple.
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Table 2. Redox potentials of triptycenes 1–4.a
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Compound
Reduction (V)
Oxidation (V)
-0.78
1,2-Benzoquinone
-
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-0.69b
-1.05, -0.79, -0.58
Trisquinone 1
-
-0.91,b -0.63,b -0.49b
Bisquinone 2
Monoquinone 3
Triscatechol 4
Catechol
-0.79,b -0.64b
-0.75b
0.48
0.35, 0.49
0.16, 0.35, 0.50
0.49
-
-
aMeasured by square-wave voltammetry in DMF solutions (~1 mM) containing
tetrabutylammonium hexafluorophosphate (0.1 M) as supporting electrolyte. Potentials are
reported with respect to the ferrocene/ferrocenium couple.
b Values recorded in DMF solutions containing 1% MeCOOH.
Quinones typically undergo one-electron reduction in aprotic media.50,51,67–69 Solutions of
triptycene(trisquinone) 1 in DMF exhibit three reversible reduction waves corresponding to the
formation and oxidation of semiquinone anions. Addition of MeCOOH shifts reduction to
slightly more positive potentials, increases the intensity of the signals, and leads to a loss of
reversibility. We attribute these changes to a transition to a two-electron process in which the
resulting catecholate dianion is rapidly protonated by MeCOOH and is therefore no longer
available for the return wave. The redox behavior of quinones and quinols is notoriously
complex, however, and more detailed studies will be necessary to confirm the origin of our
observations. In pure DMF, the reduction profiles of bisquinone 2 and monoquinone 3 appeared
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to be complicated by the effect of intra- and intermolecular proton transfers, and the addition of
MeCOOH simplifed the voltammograms. Under these conditions, an irreversible reduction wave
was observed for each quinone unit at potentials similar to those needed to reduce trisquinone 1.
In all media examined, the first reductions of triptycenequinones 1 and 2 occurred at a lower
potential than that observed for 1,2-benzoquinone.53 This behavior presumably results from the
electron-withdrawing effect of other quinone units in the compounds.
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Cyclic voltammograms of compounds 2–4 in DMF show one, two, and three irreversible
oxidation waves, respectively, and addition of MeCOOH had little effect on the oxidation
profiles. The irreversibility of the transformations is most likely due to deprotonation of the
highly unstable protonated quinones formed during the oxidation.66–69 Oxidation potentials are
consistent throughout the triptycene series, with equivalent oxidations taking place at nearly
identical potentials for each compound. The substituted 9,10-dihydroanthracene scaffold can be
seen to act as an electron-donating group, with all oxidation potentials being below those of
catechol itself.
Redox Reactions of Triptycenes 1–4 as Crystalline Solids. Our structural studies of
compounds 1–4 establish that the triptycenyl topology, along with the characteristic ability of
quinones and quinols to engage in multiple intermolecular interactions, gives rise to the
formation of open networks composed of redox-active molecules. This special combination of
features suggested that materials derived from compounds 1–4 might be able to serve as
electrodes in carbon-based batteries and led us to study their redox behavior in the solid state.
We found that exposure to vapors of H2NNH2 reduced quinones 1–3 to triscatechol 4, both in
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solution and in the solid state. Correspondingly, vapors of HNO3 (produced by 70% aqueous
acid) could be used to oxidize solid samples of catechols 2–4. Under these conditions, the redox
reactions take place without the formation of significant amounts of by-products. Moreover, the
progress of reactions can be assessed by visual inspection because the color of samples changes.
For example, when dark purple crystals of the THF solvate of monoquinone 3 (Figure 8a) were
exposed to vapors of H2NNH2, triscatechol 4 was formed as a colorless solid that retained the
morphology of the initial crystals (Figure 8b). No significant cracking was observed, and the
resulting product was shown by polarized light microscopy to retain crystallinity. X-ray powder
diffraction confirmed that the product of solid-state reduction is microcrystalline, and
comparison with the characteristic pattern produced by crystals of monoquinone 3 established
that the initial molecular organization is retained despite the change in composition (Figure 9).
As a result, solid-state reduction under these conditions produces triscatechol 4 in crystalline
form, with the molecules now arranged as they were in crystals of monoquinone 3. Notably, the
resulting structure of triscatechol 4 is not the same as that of crystals grown directly from
solution. Analogous oxidations could be carried out by treating the Me-THF solvate of
triscatechol 4 with HNO3 to form monoquinone 3. However, preventing the initial product from
undergoing further oxidation was challenging, and the product was less highly crystalline.
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Figure 8. Micrographs showing a dark purple crystal of monoquinone 3 grown from
THF/hexane (Figure 8a) and a colorless crystal of triscatechol 4 resulting from solid-state
reduction caused by exposure to vapors of H2NNH2 (Figure 8b).
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Figure 9. Comparison of the X-ray powder diffraction pattern (black trace) of a microcrystalline
sample of triscatechol 4 (as produced by the solid-state reduction of crystals of monoquinone 3)
with the simulated pattern derived from the crystal structure of the THF solvate of compound 3
(red trace).
To understand how such processes occur, we examined the structure of the THF solvate of
monoquinone 3 in detail and confirmed that it resembles the pseudopolymorphic solvate of
dioxane (Figure 4). Again, π-stacking of quinone units creates pairs that are further linked into
sheets by multiple intermolecular O-H···O hydrogen bonds and C-H···O interactions (Figure
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10a). The sheets are connected by additional O-H···O hydrogen bonds, and molecules of THF
are included between the layers (Figure 10b).
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a
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Figure 10. Representation of the structure of crystals of triptycene(monoquinone) 3 grown from
THF/hexane. (a) View showing how π-stacked pairs are further joined by O-H···O hydrogen
bonds and C-H···O interactions to form sheets. An individual π-stacked pair is highlighted in
blue, and other significant intermolecular interactions are represented by broken lines. (b) View
along the c-axis showing how sheets are connected by O-H···O hydrogen bonds and how
molecules of THF are included between the layers. One individual sheet is highlighted in blue,
and all molecules of THF appear in red. (c) View along the a-axis, showing layers in the ac-
plane in which included molecules of THF occupy parallel channels (green). Unless otherwise
stated, atoms of carbon are shown in gray, atoms of hydrogen in white, and atoms of oxygen in
red.
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Topotactic processes in which crystals of small molecules react with added agents to give new
compounds in crystalline form are very rare.70–73 Moreover, we are not aware of any previous
reports showing that transformations of this type can be carried out reversibly. Topotactic
processes accompanied by changes in chemical composition cannot be dismissed as mere
chemical curiosities; in fact, they promise to help solve key problems in emerging areas of
technology. Of particular importance is the challenge of creating a new generation of green
batteries that are cheap, rechargeable, and entirely derived from renewable carbon-based
resources. In such devices, redox reactions must take place reversibly without substantially
altering molecular organization in the electrodes.
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Previous examples of topotactic processes occurring with changes in chemical composition
suggest that molecular packing must normally be inefficient, so that external agents can penetrate
the initial crystals and by-products can exit. Moreover, the crystals must be held together by
intermolecular interactions that are strong enough to inhibit dissolution, disfavor unwanted
molecular reorientation, and maintain the integrity of the crystals despite internal stresses
generated by compositional and structural changes. In crystals of monoquinone 3, the volume
accessible to guests defines layers composed of parallel individual channels with the
approximate dimensions 4 × 6 Å2 (Figure 10c). These channels include THF and are therefore
wide enough to allow smaller molecules such as H2NNH2 and HNO3 to diffuse throughout the
crystal and to reduce or oxidize molecules of monoquinone 3.
Exposing crystals of compound 3 to vapors of H2NNH2 for periods longer than those needed to
effect complete conversion into triscatechol 4 caused fracturing along the longest axis (Figure
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S1). Indexing the crystal faces revealed that the fractures are aligned with the planes containing
channels of THF. This finding is consistent with the hypothesis that H2NNH2 penetrates the
crystals by entering the channels, replaces molecules of THF, and reduces host 3 to triscatechol
4. These changes introduce stresses that are most easily relieved by fracturing the crystals along
the planes of weakest bonding, which are those rich in included molecules of solvent.
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Processes in which single crystals of simple molecular compounds are transformed by exposure
to a suitable reagent into structurally related crystals of a different compound are highly
uncommon. Moreover, no such transformations are known to be reversible or to involve
interconversion of the constituent molecules by redox reactions. For these reasons, our results
offer important evidence that it may be possible to engineer carbon-based rechargeable batteries
analogous to the current generation of lithium-ion analogues, but with inorganic electrodes
replaced by molecular solids that can undergo reversible redox processes without disruptive
structural changes. Earlier studies have highlighted the potential of quinones and their reduced
forms to serve as components in carbon-based batteries.33–36,74,75 Our work takes a significant
further step by showing how suitable quinones and quinols can be designed so that they form
robust permeable crystals able to take part in reversible redox reactions without significant
changes in molecular organization. In future work, we hope to reveal how related materials can
be derived inexpensively from the large number of quinonoid compounds produced renewably
by nature.
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Suitably designed quinones and their reduced forms can serve as modules for the rational
construction of ordered redox-active carbon-based solids. Our study of triptycenes 1–4 has
revealed that attaching multiple quinone and quinol units to cores chosen to disfavor efficient
packing can give rise to compounds that are programmed to form open networks held together
by characteristic attractive intermolecular forces, including O-H···O hydrogen bonds, C-H···O
interactions, π-stacking, and dipolar interactions. Remarkably, the resulting materials can
undergo reversible redox reactions without losing crystallinity. In this way, our work helps
define a path leading to the design of robust rechargeable carbon-based batteries incorporating
electrodes derived from quinones, quinols, and other redox-active molecules that are abundantly
produced by nature.
Experimental Section
Anhydrous oxygen-free solvents were obtained by passage through columns packed with
activated alumina and supported Cu catalyst (Glass Contour, Irvine, CA). Reagents and all other
solvents were purchased from commercial sources and used without further purification unless
otherwise indicated. 1,2-Benzoquinone76 and hexamethoxytriptycene 544 were synthesized
according to published procedures. A full characterization of known compound 4 is presented
below to provide data not included in earlier publications.
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1
NMR spectra were recorded at 20 °C, using a spectrometer operating at 400 MHz for H NMR
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spectra and at 100 MHz for C NMR spectra. Chemical shifts are reported in parts per million
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relative to tetramethylsilane, with the signal of residual undeuterated solvent used as an internal
standard. Cyclic voltammograms were obtained using solutions in dry N2-sparged
dimethylformamide (DMF) or in DMF containing 1% acetic acid, with tetrabutylammonium
hexafluorophosphate (TBAP) as the supporting electrolyte (0.1 M). The working electrode was a
glassy carbon electrode, the counter-electrode was a Pt wire, and the pseudo-reference electrode
was an Ag wire. The concentration of the compounds examined was ~1 mM, and
ferrocene/ferrocenium was used as an internal standard.
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2,3,6,7,14,15-Hexahydroxytriptycene (4). Compound 4 was synthesized by modifying the
method reported by Ghanem et al.44 A solution of 2,3,6,7,14,15-hexamethoxytriptycene (5; 2.01
g, 4.63 mmol) in dichloromethane (100 mL) was stirred at 0 °C under N2 and treated dropwise
with a solution of BBr3 (1.0 M, 16 mL, 16 mmol) in dichloromethane. The mixture was stirred at
25 °C for 18 h, treated with water, and stirred for 15 min. The resulting precipitate was separated
by filtration, washed with water, and dried under vacuum to give 2,3,6,7,14,15-
hexahydroxytriptycene (4; 1.47 g, 4.20 mmol, 91%) as a nearly colorless solid. Further
purification could be achieved by crystallization from hot dioxane. Single crystals for analysis by
X-ray diffraction were grown by adding hexane to a saturated solution of compound 4 in dioxane
or by cooling a saturated solution in Me-THF: mp > 300 °C; FTIR (ATR) 3542, 3462, 3278,
3191, 1610, 1485, 1449 cm-1; 1H NMR (400 MHz, DMSO-d6) δ 8.40 (s, 6H), 6.71 (s, 6H), 4.88
13
(s, 2H); C{1H} NMR (100 MHz, DMSO-d6) δ 141.0, 137..8, 111.6, 51.0; UV-Vis (CH3CN)
λmax (ε) 305 (18,700); Emission (THF, λex 300 nm) λem 326 nm; SWV (DMF, NBu4PF6) E1/2 0.16,
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The Journal of Organic Chemistry
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0.35, 0.50 V (Fc/Fc+); HRMS (APCI-TOF) m/z [M - H]- calcd for C20H13O6 349.0712, found
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6,7,14,15-Tetrahydroxy-9,10-dihydro-9,10-[1,2]benzenoanthracene-2,3-dione
(3).
1,4-
Benzoquinone (0.232 g, 2.15 mmol) was added to a suspension of 2,3,6,7,14,15-
hexahydroxytriptycene (4; 0.509 g, 1.45 mmol) in THF (10 mL). The mixture was stirred at 25
°C for 20 h, and volatiles were then removed by evaporation under reduced pressure. The residue
was triturated with water, and the resulting dark purple solid was separated by filtration and dried
under vacuum to afford 6,7,14,15-tetrahydroxy-9,10-dihydro-9,10-[1,2]benzenoanthracene-2,3-
dione (3; 0.385 g, 1.11 mmol, 77%). Further purification could be achieved by crystallization
induced by allowing vapors of hexane to diffuse slowly into a saturated solution of compound 3
in THF in the dark: mp > 300 °C; FTIR (ATR) 3397, 3254, 3129, 3054, 2956, 2808, 1673, 1639,
1617, 1590, 1572, 1499, 1460 cm-1;1H NMR (400 MHz, DMSO-d6) δ 9.01 (s, 4H), 6.81 (s, 4H),
13
6.24 (s, 2H), 5.03 (s, 2H); C{1H} NMR (100 MHz, DMSO-d6) δ 180.3, 153.4, 144.8, 130.3,
119.6, 111.9, 48.9; UV-Vis (CH3CN) λmax (ε) 291 (8090), 435 (5640); Emission (CH3CN, λex 290
nm) λem 324 nm; SWV (DMF/AcOH 1%, NBu4PF6) E1/2 -0.75, 0.35, 0.49 V (Fc/Fc+); HRMS
(APPI-TOF) m/z [M + H]+ calcd for C20H13O6 349.0712, found 349.0711.
14,15-Dihydroxy-9,10-dihydro-9,10-[1,2]benzenoanthracene-2,3,6,7-tetraone
(2).
o-
Chloranil (0.352 g, 1.43 mmol) was added to a suspension of 2,3,6,7,14,15-
hexahydroxytriptycene (4; 0.198 g, 0.565 mmol) in anhydrous oxygen-free dioxane (10 mL).
The mixture was stirred at 25 °C for 20 h, and volatiles were then removed by evaporation under
reduced pressure. The residue was then triturated with dichloromethane, and the resulting dark
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