Tridecacyclene: A Cyclic Tetramer of Acenaphthylene

Article abstract of DOI:10.1002/chem.201600165

In this manuscript, we describe the single-step preparation of a cyclic tetramer of acenaphthylene through a Lewis acid-catalyzed aldol cyclization of 1-acenaphthenone. The previously unexplored cyclic tetramer material differs from the better-known cyclic trimer, decacyclene, due to the presence of a central eight-membered ring. This ring not only forces the molecule to distort significantly from planarity, but is also responsible for its unique electronic properties, including a decrease in the reduction potential (by about 0.4 eV) and optical gap (by about 0.73 eV), compared to the more planar decacyclene. The synthesized compound crystallizes into a unique packing structure with significant π-stacking observed between adjacent molecules. Furthermore, due to its saddle-like shape, the cyclic tetramer is able to form shape-complementary interactions between its concave surface and the convex outer surface of buckminsterfullerene to generate cocrystalline supramolecular assemblies. What a catch! A cyclic tetramer of acenaphthylene was synthesized in a single step. The compound exhibits interesting electronic and structural properties when compared to the better-known cyclic trimer due to the presence of a central eight-membered ring. The saddle-like shape of cyclic tetramer allows it to form cocrystalline supramolecular assemblies with C₆₀ in the solid state (see scheme).

Full text of DOI:10.1002/chem.201600165

DOI: 10.1002/chem.201600165
Communication
&
Fused-Ring Systems
Tridecacyclene: A Cyclic Tetramer of Acenaphthylene
Daniel P. Sumy,[a] Nicholas J. Dodge,[a] Chloe M. Harrison,[a] Aaron D. Finke,[b] and
Adam C. Whalley*[a]
though this reaction has been used almost exclusively for the
synthesis of cyclic trimers, a number of groups have reported
the presence of an analogous tetrameric species in mass spec-
tra of crude reaction mixtures.[8b,9a,11] Surprisingly, despite the
interesting structural and electronic properties that have re-
cently been observed as a result of the incorporation of eight-
membered rings into PAHs,[12] only a single report mentions
the isolation of a cyclic tetramer from one of these reactions
and the physical properties of these species have yet to be ex-
plored.[11a] Herein, we describe the synthesis and properties of
1.2;3.4;5.6;7.8-tetra[(1.8)-naphthylene]-1,3,5,7-cyclooctatetraene
(2), which following the nomenclature used for decacyclene
(ten cycles) we have appropriately named tridecacyclene (thir-
teen cycles).
Abstract: In this manuscript, we describe the single-step
preparation of cyclic tetramer of acenaphthylene
a
through a Lewis acid-catalyzed aldol cyclization of 1-ace-
naphthenone. The previously unexplored cyclic tetramer
material differs from the better-known cyclic trimer, deca-
cyclene, due to the presence of a central eight-membered
ring. This ring not only forces the molecule to distort sig-
nificantly from planarity, but is also responsible for its
unique electronic properties, including a decrease in the
reduction potential (by about 0.4 eV) and optical gap (by
about 0.73 eV), compared to the more planar decacyclene.
The synthesized compound crystallizes into a unique
packing structure with significant p-stacking observed be-
tween adjacent molecules. Furthermore, due to its saddle-
like shape, the cyclic tetramer is able to form shape-com-
plementary interactions between its concave surface and
the convex outer surface of buckminsterfullerene to gen-
erate cocrystalline supramolecular assemblies.
Decacyclene (1), a polycyclic aromatic hydrocarbon (PAH) ex-
hibiting a three-fold symmetric propeller-like structure, was
[1]
´
first reported by Dziewonski in 1903. Over the past century,
1 has generated considerable interest and has found utility in
a range of disciplines, including surface modification,[2] organo-
metallic complexes,[3] and liquid crystals.[4] Furthermore, the
ability of 1 to reversibly accept up to four electrons has provid-
ed motivation for its study as an n-type semiconductor,[5] its
use in organic photovoltaics,[6] and investigations of its ferro-
magnetism.[7] Significant efforts have been made to develop
functionalized derivatives of 1 that can be used as precursors
for the synthesis of highly strained PAHs—the most notable
being Scott’s syntheses of both circumtrindene[8] and buckmin-
sterfullerene (C60).[9] Throughout these studies, obtaining the
necessary precursors has relied heavily on the Lewis acid-cata-
lyzed aldol cyclization of 1-acenaphthenone derivatives.[10] Al-
Although all reported cyclic tetramers have been derived
from substituted 1-acenaphthenone derivatives, our investiga-
tions focused on unfunctionalized 1-acenaphthenone due to
its availability, both commercially and synthetically, and the
simplicity of the products that would be produced. To our sur-
prise, subjecting 1-acenaphthenone to the most commonly
used aldol cyclotrimerization conditions (TiCl4 in 1,2-dichloro-
benzene at 1808C) produced two major products, which were
distinguishable by thin layer chromatography (TLC)—a yellow
fluorescent spot with spectroscopic data matching that of 1
(<1% yield) and a brown spot of similar polarity that we have
identified as 2 (21% yield).
Compound 2 is a stable brown solid that unlike 1 readily dis-
1
solves in a range of organic solvents. The H NMR spectrum of
[a] D. P. Sumy, N. J. Dodge, C. M. Harrison, Prof. A. C. Whalley
Department of Chemistry
2 in CDCl3 gives three peaks of equal integration at d=7.87,
7.67, and 7.56 ppm. Similarly, the 13C NMR spectrum displays
seven unique chemical environments for the 48 carbon atoms
in the structure, indicating a high degree of symmetry. Further-
more, the 1H NMR shifts provide a strong indication that 2
adopts a nonplanar topology. The 1H NMR of 1 exhibits a down-
field resonance at d=8.83 ppm, which corresponds to the
proton in the bay region of the molecule. The significant
downfield shift is a result of the fact that each of these protons
University of Vermont
Burlington, Vermont 05405 (USA)
E-mail: Adam.Whalley@uvm.edu
[b] Dr. A. D. Finke
Swiss Light Source
Paul Scherrer Institute
5232 Villigen PSI (Switzerland)
Supporting information and ORCID from one of the authors for this article
is available on the WWW under http://dx.doi.org/10.1002/chem.201600165.
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feels the ring current of both the aromatic rings that they are
attached to as well as the adjacent aromatic ring, a direct
result of the molecule being primarily planar in geometry. In 2,
the proton is found about 1 ppm further upfield, indicating
much less of a contribution from the adjacent ring, and there-
fore, a strong indication that the compound is not planar.
Dark brown crystals of 2·1,2-DCE, grown from the slow diffu-
sion of hexane into 1,2-dichloroethane, were used to validate
the structure through single-crystal x-ray diffraction (SC-XRD)
studies.[13] The solid-state structure (Figure 1A) matches that
predicted from density functional theory (DFT) calculations,[14]
and unlike 1, the structure distorts significantly from planarity
(with the average atom in the eight-membered ring deviating
from the mean plane of the ring by 0.37 Š), producing an alter-
nating up-down-up-down arrangement of the acenaphthylene
units.
of each molecule forming p-stacking interactions (about 3.4 Š
apart) with two adjacent molecules, resulting in a zigzag pat-
tern throughout the crystal. The structure is further stabilized
by an abundance of intermolecular CÀH/p-interactions.
The aromaticity of each of the rings of 2 was calculated
using the harmonic oscillator model of aromaticity (HOMA),[15]
and the results were found to be consistent to those of 1. In
each case, the six-membered rings of the naphthylene units
are highly aromatic (HOMA values of 0.89 and 0.90 for 1 and
0.84 and 0.90 for 2) and the five-membered rings are predomi-
nantly non-aromatic (HOMA values of 0.11 and À0.07 for
1 and 2, respectively), although a small degree of antiaromatic-
ity is present in 1.
Despite the fact that similar bonding patterns exist in 1 and
2, the optoelectronic properties of the two are considerably
different. In the UV/Vis spectra (Figure 2A), we observe that
Figure 1. A) The solid-state structure of 2; hydrogen atoms have been omit-
ted for clarity and the ellipsoids are shown at 80% probability. B) Represen-
tative bond lengths (Š) calculated from the solid-state structure of 2; a full
list of bond lengths is provided in the Supporting Information. C) Crystal
packing of 2, illustrating the p-stacking interactions that exist between adja-
cent molecules.
Figure 2. A) Absorption spectra of 1 (red) and 2 (blue) in CH2Cl2. B) Cyclic
voltammetry of 1 (red) and 2 (blue) in 0.05m tetrabutylammonium hexa-
fluorophosphate in MeCN with a Pt working electrode, Pt counter electrode,
and Ag/AgCl reference electrode. Scan rate: 50 mVsÀ1. Ferrocene was added
as an internal standard and referenced to 0 V.
the lmax undergoes a hypsochromic shift when converting
from 1 (377 nm) to 2 (360 nm) with a slight reduction in molar
absorptivity, likely as a result of more efficient orbital overlap
for the more planar structure. Surprisingly, the solution-based
optical gaps display the opposite trend, as judged by the ab-
sorption onsets, with the optical gap of 2 being considerably
lower in energy than that of 1. The cyclic voltammograms (Fig-
ure 2B) support this observation because 2 is considerably
easier to reduce than 1 (by about 0.4 eV), primarily as a result
of a significant decrease in the energy of the lowest unoccu-
pied molecular orbital (LUMO). This observation is directly re-
lated to the presence of the eight-membered cyclooctate-
traene core, because aromaticity is known to develop in such
The bond lengths obtained from the x-ray structure (Fig-
ure 1B) of 2 are consistent with those of acenaphthylene as
substantial bond alternation is observed in the naphthalene
subunits and significant double-bond character exists in the
bond shared by the five- and eight-membered ring. Bond alter-
nation is also observed in the central ring, with the bonds con-
necting the acenaphthylene units displaying significantly more
single-bond character than those within the five-membered
rings. This is in contrast to what has been observed for 1, in
which the bond alternation places the shorter bonds between
the acenaphthylene units. The solid-state packing of 2 (Fig-
ure 1C) is unique, with two orthogonal naphthalene subunits
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rings through two-electron reduction processes.[16] Interesting-
ly, this occurs through two one-electron processes rather than
a single two-electron process, indicating that the radical anion
generated from the first reduction is a stable species under the
experimental conditions. The highest occupied molecular orbi-
tal (HOMO) energies are similar between the two compounds
with the HOMO of 2 being 0.16 eV higher in energy than that
of 1 (Table 1).
Table 1. Summary of optoelectronic properties of 1 and 2.[a]
1
2
Eox/onset
0.73
0.57
Ered/onset
À1.90
À5.53
À2.90
2.63
À1.54
À5.37
À3.26
2.11
HOMO [eV]
LUMO [eV]
E-chem gap [eV]
lmax [nm]
377
360
optical gap [eV]
2.57
1.84
[a] Measurements taken at sample concentrations of 0.2 mm and poten-
tials measured relative to a ferrocenium/ferrocene redox couple used as
an internal standard (Figure 2B). Eox/onset is the onset of the oxidation po-
tential and Ered/onset is the onset of the reduction potential. HOMO and
LUMO values were calculated on the basis of the oxidation of the ferro-
cene reference in vacuum (4.8 eV).
Figure 3. A) The solid-state cocrystalline supramolecular assembly of 2 (red)
and C60 (blue). Hydrogen atoms and solvent molecules have been omitted
for clarity and the ellipsoids are shown at 80% probability. The C60 molecule
on the left is related to the C60 molecule on the right by the symmetry oper-
ation (Àx+1/2, y+1/2, Àz+1/2). B) The crystal packing of 2 and two sepa-
rate units of C60 (blue and green), illustrating the zigzag arrangement of
components as well as the p-stacking interactions that exist between 2 and
one of the C60 units (blue) and the CÀH/p-interactions that exist between
adjacent molecules of 2. The unit cell is indicated with a doted blue line.
In view of the concave surfaces of 2, we were interested to
see if the compound would form supramolecular interactions
with the convex surface of buckminsterfullerene (C60). Al-
though such interactions are prevalent in the literature,[17] the
ability to complex a fullerene on two opposing concave surfa-
ces is quite unique and such assemblies have only been re-
ported in the literature on two occasions.[18] Such complexes
are inherently applicable to the area of photovoltaics due to
their potential to act as p–n junctions. It is important to note,
however, that in the two previous examples of such com-
plexes, the host molecules were more difficult to prepare than
2.
When saturated chlorobenzene solutions of 2 and C60 were
combined, crystals of the supramolecular assembly (Figure 3)
suitable for SC-XRD were produced.[12] The crystal is composed
of C60, 2, and chlorobenzene (asymmetric unit ratio of 2:1:3, re-
spectively), wherein 2 and C60 form a repeating zigzag pattern
throughout the solid-state structure. As predicted, each mole-
cule of 2 interacts (“interaction” here means atom–atom con-
tacts less than the sum of their van der Waals radii) with two
symmetry-related C60 molecules, one on each of its opposing
concave faces. On the other hand, the two C60 molecules
occupy two distinctly different environments. One C60 unit
(blue) interacts with half of two molecules of 2 in one hemi-
sphere, whereas the other hemisphere interacts with the
second C60 unit. The other C60 unit (green) interacts with one
acenaphthylene moiety of two different molecules of 2, three
molecules of the first C60 unit, and two molecules of chloro-
benzene. The complexes between 2 and C60 exhibit strong p–
p interactions, wherein the shortest distance from an atom of
C60 to the plane of an interacting naphthalene moiety is
3.00 Š. This distance is nearly identical to those observed in
the previously reported systems.
In conclusion, we have isolated a saddle-shaped cyclic tetra-
mer of acenaphthylene as the major product from a single-
step reaction that has historically been used extensively for
cyclic trimer formation. This compound exhibits a range of
beneficial properties when compared to its trimeric analog, in-
cluding increased solubility, a significant decrease in reduction
potential (and a decrease in the corresponding optically and
electrochemically measured HOMO-LUMO gap), and the ability
to form supramolecular assemblies with fullerenes. Further
tuning of such properties for a host of applications is currently
ongoing in our laboratory.
Acknowledgements
We would like to thank Bruce O’Rourke for assistance with
HRMS data and Kyle Plunkett for assistance with cyclic voltam-
metry. C.M.H thanks the UVM Summer Research Program for
Women in Science for funding. We thank the Small Molecule
Crystallography Center (SMoCC), ETH-Zurich, for collecting the
X-ray data of 2. A.C.W. would also like to thank the University
of Vermont for generous start-up funding.
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Tobe, Angew. Chem. Int. Ed. 2015, 54, 2090–2094; Angew. Chem. 2015,
127, 2118–2122.
Keywords: arenes
· buckminsterfullerene · Lewis acids ·
[13] CCDC 1440430 (2) and 1440429 (2·2(C60)·3(C6H5Cl)) contain the supple-
mentary crystallographic data for this paper. These data are provided
free of charge by The Cambridge Crystallographic Data Centre. Crystal-
lographic data: 2: triclinic, C48H24, Mr =600.67, a=11.361(3), b=
11.595(3), c=15.309(4) Š, V=1822.1(8) Š3, T=100 K, space group P-1,
polycyclic aromatic hydrocarbons · supramolecular chemistry
´
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, , R1 =0.0540,
m=0.062 mmÀ1
wR2 =0.1582 for I> 2s(I). C60 Complex (2·2(C60)·3(C6H5Cl)): monoclinic,
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Received: January 14, 2016
Published online on February 17, 2016
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