Reduction of π-Expanded Cyclooctatetraene with Lithium: Stabilization of the Tetra-Anion through Internal Li+ Coordination

Article abstract of DOI:10.1002/anie.202013353

The chemical reduction of a π-expanded polycyclic framework comprising a cyclooctatetraene moiety, octaphenyltetrabenzocyclooctatetraene, with lithium metal readily affords the corresponding tetra-anion instead of the expected aromatic dianion. As reveale

Full text of DOI:10.1002/anie.202013353

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Accepted Article
Title: Reduction of π-Expanded Cyclooctatetraene with Lithium:
Stabilization of the Tetraanion through Internal Li+ Coordination
Authors: Zheng Zhou, Yikun Zhu, Zheng Wei, John Bergner, Christian
Neiß, Susanne Doloczki, Andreas Görling, Milan Kivala, and
Marina A. Petrukhina
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Reduction of π-Expanded Cyclooctatetraene with Lithium:
Stabilization of the Tetraanion through Internal Li+ Coordination
Zheng Zhou,[a] Yikun Zhu,[a] Zheng Wei,[a] John Bergner,[b,c] Christian Neiß,[d] Susanne Doloczki,[e]
Andreas Görling,[d] Milan Kivala,*[b,c] and Marina A. Petrukhina*[a]
[a]
Dr. Z. Zhou, Y. Zhu, Dr. Z. Wei, Prof. Dr. M. A. Petrukhina
Department of Chemistry, University at Albany, State University of New York
1400 Washington Ave., Albany, NY12222, USA
E-mail: mpetrukhina@albany.edu
[b]
J. Bergner, Prof. Dr. M. Kivala
Organisch-Chemisches Institut
Ruprecht-Karls-Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
E-mail: milan.kivala@oci.uni-heidelberg.de
[c]
[d]
[e]
J. Bergner, Prof. Dr. M. Kivala
Centre for Advanced Materials
Ruprecht-Karls-Universität Heidelberg
Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
Dr. Christian Neiß, Prof. Dr. Andreas Görling
Department of Chemistry and Pharmacy, Chair of Theoretical Chemistry
Friedrich-Alexander-Universität Erlangen-Nürnberg
Egerlandstr. 3, 91058 Erlangen, Germany
Susanne Doloczki
Department of Chemistry and Pharmacy, Chair of Organic Chemistry I
Friedrich-Alexander-Universität Erlangen-Nürnberg
Nikolaus-Fiebiger-Str. 10, D-91058 Erlangen, Germany
Supporting information for this article is given via a link at the end of the document.
Abstract: The chemical reduction of a π-expanded polycyclic
framework comprising cyclooctatetraene moiety,
corresponding COT2– dianion upon treatment with potassium
metal in THF. The reduction resulted in a concomitant downfield
shift of the 1H NMR signal, suggesting the occurrence of a
diamagnetic ring current in the doubly-reduced species.[5]
Consequently, planarization of the 8-membered scaffold was
proposed and later confirmed through X-ray crystallographic
analysis of a dianion derived from 1,3,5,7-tetramethyl-substituted
COT derivative.[6,7] Although further reduction beyond dianions
have been observed for larger annulenes,[8,9] tetraanionic COT4–
remains a theoretical consideration.[10]
The possibility to render polycyclic aromatic hydrocarbons
(PAHs) as electron acceptors through incorporation of suitable
non-benzenoid moieties is particularly attractive and has been
successfully demonstrated for cyclopentannulated systems[11]
with corannulene and related geodesic PAHs being the most
paradigmatic examples.[12] Such PAHs form stable carbanions
upon reduction[13] and can act as n-type semiconductors when
appropriately functionalized.[14] However, related efforts involving
PAHs with embedded 8-membered COT rings remain
comparably scarce.[2d,15] For example, π-expanded COT
derivatives such tetrabenzocyclooctatetraene (TBCOT)[16,17] and
octaphenyltetrabenzocyclooctatetraene (OPTBCOT; 1)[18] have
been reported and studied. The latter was obtained in a Diels-
Alder cycloaddition between 1,2,3,4-tetraphenylcyclopenta-1,3-
dienone and 5,6,11,12-tetradehydrodibenzo[a,e]cyclooctene and
subjected to oxidative cyclodehydrogenation under different
conditions.[18] Furthermore, the saddle-shaped COT moiety is an
important structural element for the construction of negatively
curved carbon-rich compounds and carbon allotropes.[19,20]
Recently, a unique COT-containing molecular precursor towards
a
octaphenyltetrabenzocyclooctatetraene, with lithium metal readily
affords the corresponding tetraanion instead of the expected aromatic
dianion. As revealed by X-ray crystallography, the highly contorted
tetraanion is stabilized by coordination of two internally bound Li+
cations, while two external cations remain solvent-separated. The
variable-temperature 7Li NMR spectra in THF confirm the presence of
three types of Li+ ions and clearly differentiate internal binding,
consistently with the crystal structure. Density functional theory
calculations suggest that the formation of the highly charged
tetrareduced carbanion is stabilized through Li+ coordination under
the applied experimental conditions.
Cyclooctatetraene (COT), an 8-membered representative of the
family of [n]annulenes,[1,2] was initially synthesized back in 1911.[3]
Due to its analogous stoichiometric composition to 6-membered
π-system benzene, COT was originally expected to feature
aromaticity, which was, however, contradicted by its divergent
chemical behavior. In 1947, X-ray diffraction analysis of COT
revealed a boat-like conformation of the 8-membered ring with
alternating bond lengths and D2d symmetry.[4] While the eight π-
electrons in COT formally suggest its antiaromaticity, the
scaffolds avoid such destabilization through deviation from
planarity, preventing electronic communication between the
individual p-orbitals.
According to Hückel's rule planar, π-conjugated cycles with
10 π-electrons are expected to be aromatic, which motivated
experiments towards aromatization of COT via two-electron
reduction. In 1960, COT was successfully reduced to the
1
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negatively curved nanocarbon has been reported,[21] along with a
remarkable contorted chiral PAH with three embedded COT
moieties having the shape of a monkey saddle.[22] However, the
multi-electron accepting properties of π-expanded COT-
containing PAHs have not yet been investigated.
distortion is also observed in 1-NS (Figure S13) with the average
torsion angles at the above positions increased by 13.4°. The
dihedral angles of the peripheral phenyl rings remain unchanged,
but the dihedral angles I/R and K/R in 1-THF and 1-NS are
increased by 10.4° and 9.2° (Table S3), illustrating added twist of
the tetraphenylene core in comparison with 1-CS2. The observed
structural variations realized under different crystallization
conditions illustrate a sufficient core flexibility of 1.
Scheme 1. Consecutive π-expansion of COT.
The crystal structure of OPTBCOT (C72H48, 1) was first
reported back in 1997.[23] This PAH was crystallized as
C72H48·CS2 (1-CS2) with one interstitial CS2 molecule being fully
wrapped by the peripheral phenyl rings of 1 (Figure S12). Using
slow evaporation of THF solution of 1, colorless plate-shaped
crystals were obtained in this work (Yield: ~93%) and confirmed
by X-ray diffraction to be C72H48·THF (1-THF) (Figure 1).[24] To
exclude the effect of solvents, the gas phase sublimation of 1 in
vacuo at 300 °C was also used for crystal growth affording a new
non-solvated (NS) polymorph (Yield: ~84%), 1-NS (Figure S13).
Figure 2. Solid-state structure of 1-THF: (a) 1D column and (b) 3D network,
capped-stick models. Two conformations are shown in gray and black. THF
molecules are omitted. C–H···π and π···π interactions are shown in blue and
red, respectively.
To gain insight into the electron accepting properties of
OPTBCOT comprising a COT moiety embedded into a large
contorted polycyclic framework, we investigated chemical
reduction of 1 with lithium metal in THF. The reaction mixture was
stirred at room temperature for 24 hours, and the product was
successfully isolated in the form of bulk single crystals using slow
diffusion of hexanes into the THF solution (Yield: ~75%, see the
Supporting Information for more details). The X-ray diffraction
analysis confirmed the formation of [{Li+(THF)4}2{Li+ (14–)}] (2),
2
which was crystallized with two interstitial THF molecules as
2·2THF. There are two independent molecules in the unit cell
(Figure S19), but since their geometric parameters are very close,
only one is discussed below (see the Supporting Information for
average values).
Figure 1. Crystal structure of 1-THF, (a) front view (ball-and-stick model, H-
atoms are omitted) and (b) side view (space-filling model).
In the solid-state structure of 1-THF, the OPTBCOT
molecules of the same conformation are stacked into a 1D column
held by C–H···π (2.682(5)2.762(5) Å) and π···π (3.128(5) Å)
interactions between the peripheral phenyl rings (Figure 2a). The
1D columns are further assembled into a 3D network through C–
H···π interactions, with the distances ranging from 2.777(5) Å to
2.836(5) Å (Figure 2b). Similar solid-state structure is observed in
1-NS (Figure S18), although the packing is slightly tighter with the
volume per C-atom reduced to 17.6 Å3 from 18.2 Å3 in 1-THF.
The conformational flexibility of 1 can be illustrated by
evaluation of the tetraphenylene core changes in three different
polymorphs. In 1-CS2, the linear solvent molecule perfectly fits in
the internal cavity of 1 (Figure S12), allowing the host to be
symmetric. In contrast, the internal void space in 1-THF is filled
by the twisted peripheral phenyl rings (Figure 1), thus increasing
the asymmetry of the tetraphenylene core. Compared to 1-CS2,
the central 8-membered ring in 1-THF becomes asymmetrically
distorted, as can be seen from the increase of torsion angles at
the positions β, γ, ζ, and η by about 13.7° (Table S4). Similar
In the crystal structure of 2 (Figure 3), there are two
internally coordinated Li+ ions with the Li–Li separation of
3.351(11) Å. Specifically, Li1 is sandwiched by two peripheral
phenyl rings of 14– in an asymmetric η6-fashion, with the Li–C
distances ranging over 2.261(9)‒2.584(9) Å (Figure 4). Li2 is
inserted deeply into the internal cavity formed by the highly
twisted core of 14– (Figure 3b). Eight Li2–C distances fall into the
range of 2.303(9)‒2.544(9) Å with four additional contacts being
much longer (3.095(9)–3.150(9) Å). The short Li–C contacts in 2
are close to those reported for internal Li+ ions in the sandwich
formed by tetrareduced corannulene, [Li+ (C20H104–)]3.[13] In 2, two
5
additional ions, Li3 and Li4, are capped by four THF molecules
each and remain solvent-separated from the [Li+ (14–)]2 core, with
2
all Li–OTHF distances (1.914(8)‒1.958(8) and 1.894(8)‒1.943(8) Å,
respectively) being close to those previously reported.[13a, 25]
2
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having different coordination environment can be clearly
7
differentiated based on the characteristic Li NMR signal shifts.
We have previously seen the tendency for the 7Li NMR signals to
exhibit high-negative values for deeply embedded Li+ ions in
sandwich-type assemblies formed by the tetrareduced
corannulene.[13a,13c,26] In addition, 1H NMR spectroscopy
illustrates that the 14– anion can be reversibly re-oxidized back to
the neutral state by exposure to air (Figure S9).
Figure 3. (a) Crystal structure of 2, ball-and-stick model. H-atoms are omitted.
(b) Side view of [Li+2(14–)]2, space-filling model.
Figure 5. The cores of 1-THF and [Li+2(14–)]2 in 2, (a) ball-and-stick models with
rings and torsion angles labeled, (b) space-filling models.
Figure 4. Metal coordination in [Li+2(14–)]2, along with a table of Li–C distances.
Non-coordinated phenyl rings are omitted.
The geometry of the 14– anion has been analyzed by
comparing the dihedral and torsion angles with those in neutral
parent, 1-THF, as both were crystallized from THF solution. Unlike
1-THF, the central pocket in 14– is occupied by the rings A and E
that are forced to rotate and fold in order to sandwich two internal
Li+ ions (Figure 5). This is accompanied by a significant
asymmetric distortion of the tetraphenylene core reflected by the
change of the characteristic torsion angle between the rings A and
E measured at 26.5° (vs. 138.8° in 1-THF). Compared to 1-THF,
an increasing distortion is observed at positions β and ζ in the
central 8-membered ring of 14– (Table S4), with the average value
increased by 6.9°.
Figure 6. 7Li NMR spectrum of 2 in THF-d8 at –80 ºC.
The somewhat surprising formation of [Li+ (14–)]2 instead of
2
the expected dianionic species can be rationalized by looking at
the energetics of the equilibrium between these two forms, which
was calculated on the DFT level of theory; solvent effects were
taken into account by a conductor-like continuum model (see the
Supporting Information for details). According to our computations,
the dianion 12 is mainly coordinated by one Li+ cation in solution.
Therefore, we consider the equilibrium (neglecting the
concentrations of lower/higher coordinated species):
The solution behavior of 2 was investigated using variable
temperature 7Li NMR spectroscopy. At room temperature (Figure
S5), the only 7Li NMR resonance signal at –0.64 ppm corresponds
to the solvated [Li+(THF)n] cations. Upon cooling, this peak slightly
7
upshifts and two new signals appear in the low temperature Li
2 [Li+(12–)]
1 + [Li+ (14–)]2
2
NMR spectra (Figure S5). The 7Li resonance signal at –1.88 ppm
(Figure 6) can be assigned to the sandwiched Li+ ion (Li1) while
the signal at –5.08 ppm represents the deeply embedded Li+ ion
(Li2). Importantly, the integration ratio of these three peaks of
2:1:1 is fully consistent with the three types of lithium ions present
in the crystal structure. Remarkably, the internal lithium ions
The calculated free energy change ∆G° of this reaction is
0.6 kcal/mol, i.e., the equilibrium constant K of this reaction is in
the order of 1, and the actual concentration of the tetrareduced
species will crucially depend on the concentration of the neutral
3
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Financial and instrumentational support of this work from the
National Science Foundation (CHE-2003411 and MRI-1726724)
is acknowledged (M. A. P.). NSF's ChemMatCARS Sector 15 is
principally supported by the Divisions of Chemistry (CHE) and
Materials Research (DMR), National Science Foundation, under
grant number NSF/CHE-1834750. The use of the Advanced
Photon Source, an Office of Science User Facility operated for the
U.S. Department of Energy (DOE) Office of Science by Argonne
National Laboratory, was supported by the U.S. DOE under
Contract No. DE-AC02-06CH11357. The generous funding by the
Deutsche Forschungsgemeinschaft (DFG) – Project number
182849149 – SFB 953 and project number 401247651 – KI
1662/3-1 is acknowledged (M. K.).
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Keywords: electron acceptors • cyclooctatetraene • chemical
reduction
diffraction
• density functional-theory calculations • X-ray
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4
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Entry for the Table of Contents
Insert graphic for Table of Contents here.
COT-Based Four Electron Acceptor: A highly contorted π-expanded polycyclic tetraanion comprising a cyclooctatetraene moiety is
readily formed upon reduction of the neutral parent with lithium metal. The highly charged carbanion crystallizes with four lithium
counter-ions, two of which stabilize the contorted structure through internal coordination. Density functional theory calculations
rationalize the unexpected formation of the tetraanion and provide insights into its electronic properties.
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