Synthesis and characterization of ion pairs between alkaline metal ions and anionic anti-aromatic and aromatic hydrocarbons with π-Conjugated central seven- And eight-membered rings

Article abstract of DOI:10.3390/molecules25204742

The synthesis, isolation and full characterization of ion pairs between alkaline metal ions (Li⁺, Na⁺, K⁺) and mono-anions and dianions obtained from 5H-dibenzo[a,d]cycloheptenyl (C₁₅H₁₁ = trop) is reported. According to Nuclear Magnetic Resonance (NMR) spectroscopy, single crystal X-ray analysis and Density Functional Theory (DFT) calculations, the trop^- and trop²?? anions show anti-aromatic properties which are dependent on the counter cation M⁺ and solvent molecules serving as co-ligands. For comparison, the disodium and dipotassium salt of the dianion of dibenzo[a,e]cyclooctatetraene (C₁₆H₁₂ = dbcot) were prepared, which show classical aromatic character. A d⁸-Rh(I) complex of trop^? was prepared and the structure shows a distortion of the C₁₅H₁₁ ligand into a conjugated 10π -benzo pentadienide unit—to which the Rh(I) center is coordinated—and an aromatic 6π electron benzo group which is non-coordinated. Electron transfer reactions between neutral and anionic trop and dbcot species show that the anti-aromatic compounds obtained from trop are significantly stronger reductants.

Full text of DOI:10.3390/molecules25204742

molecules
Article
Synthesis and Characterization of Ion Pairs between
Alkaline Metal Ions and Anionic Anti-Aromatic and
Aromatic Hydrocarbons with π-Conjugated Central
Seven- and Eight-Membered Rings
Jan Bloch 1, Stefan Kradolfer 1 , Thomas L. Gianetti 2 , Detlev Ostendorf 1, Subal Dey 1
,
Victor Mougel 1 and Hansjörg Grützmacher 1,
*
1
Department of Chemistry and Applied Biosciences, ETH Zürich, 8093 Zürich, Switzerland;
bloch@inorg.chem.ethz.ch (J.B.); krastefa@inorg.chem.ethz.ch (S.K.); detos@gmx.de (D.O.);
deys@ethz.ch (S.D.); mougel@inorg.chem.ethz.ch (V.M.)
Department of Chemistry & Biochemistry, 1306 E. University Blvd., Tucson, AZ 85719, USA;
tgianetti@email.arizona.edu
2
*
Correspondence: hgruetzmacher@ethz.ch
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: Yves Canac
Received: 21 September 2020; Accepted: 4 October 2020; Published: 15 October 2020
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: The synthesis, isolation and full characterization of ion pairs between alkaline metal
ions (Li+, Na+, K+) and mono-anions and dianions obtained from 5H-dibenzo[a,d]cycloheptenyl
(C15H11 = trop) is reported. According to Nuclear Magnetic Resonance (NMR) spectroscopy, single
crystal X-ray analysis and Density Functional Theory (DFT) calculations, the trop- and trop2−•
anions show anti-aromatic properties which are dependent on the counter cation M+ and solvent
molecules serving as co-ligands. For comparison, the disodium and dipotassium salt of the dianion
of dibenzo[a,e]cyclooctatetraene (C16H12 = dbcot) were prepared, which show classical aromatic
character. A d8-Rh(I) complex of trop− was prepared and the structure shows a distortion of the C15H11
ligand into a conjugated 10
π -benzo pentadienide unit—to which the Rh(I) center is coordinated—and
an aromatic 6 electron benzo group which is non-coordinated. Electron transfer reactions between
π
neutral and anionic trop and dbcot species show that the anti-aromatic compounds obtained from
trop are significantly stronger reductants.
Keywords: aromaticity; ion pairs; alkali metals; tropylidenyl ions; cyclooctatetraene ions;
rhodium; electron paramagnetic resonance (EPR) spectroscopy; density functional theory
(DFT); electrochemistry
1. Introduction
Transition metal complexes with 5H-dibenzo[a,d]cyclohepten-5-yl units (trivial name
tropylidenyl = trop, see Scheme 1, right) as ligands are well established in the literature [
Their special properties give rise to complexes with extraordinary catalytic activities. For example,
bis(trop)amine as ligand in d8-Rh(I) complex
(Scheme 1, top) provokes an unusual butterfly-type
structure for tetracoordinated sixteen electron configured transition metal complexes and can act
as a cooperating ligand [ ]. Both factors contribute to the high activities in the hydrogenation of
ketone derivatives [ ] or the dehydrogenative coupling of alcohols [ ] under very mild reaction
conditions. In another example, complexes of type with the redox and chemically non-innocent
cooperative diazadiene ligand trop2dad (dad = diazadiene) [10] show an unprecedented high efficiency
1–4].
A
5
6
,7
8,9
B
in dehydrogenation reactions of methanol or formaldehyde [10
–
12]. The olefinic double bond in
Molecules 2020, 25, 4742; doi:10.3390/molecules25204742
www.mdpi.com/journal/molecules

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the trop moiety acts as an electronically flexible ligand [13] and allows to stabilize low-valent metal
centers [10].
Scheme 1. Complexes
A
and
B
with trop amines as ligands. (
a
) and (
b
): In situ generation of the tro−p
anion trop- by deprotonation of tropH or photochemical CO2 extrusion from the carboxylate tropCO2
.
(
c) In situ synthesis of Na2trop. (
d
) Synthesis of the trop- complex [K(18-crown-6)][Cr(trop)(CO)3].
(e) Synthesis of Mdbcot and M2dbcot.
On the other hand, compounds with main group element metals and trop-type ligands are very
scarce [14]. We became especially interested in compounds which contain an alkaline metal ion
and an anionic trop moiety because of their potential as building blocks for the synthesis of new
trop-type ligands. Furthermore, these species, (M+)n[trop]n−, may form various forms of ion pairs
with fascinating properties, and related species with reduced arenes as anions are interesting to explore
on their own [15–17].
To the best of our knowledge, salts containing anionic 5H-dibenzo[a,d]cycloheptenide−s,
[trop]n− (n = 1, 2) have never been isolated. Based on the 4n-Hückel-rule, the trop anion, C15H11
,

Molecules 2020, 25, 4742
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with its central 8
π
electron system flanked by two annulated benzo groups (giving a 16
π
electron
system in total), should show anti-aromatic character. Indeed, when the trop anion is generated in situ
in liquid ammonia by deprotonation of the neutral hydrocarbon suberene (tropH) ((a) in Scheme 1) [18],
strongly shielded signals are observed in the 1H NMR spectrum, as expected for an anti-aromatic
compound [19]. Wan et al. suggested that the photodecarboxylation of the trop carboxylate anion
−
tropCO2
[20,21] and the photochemical hydrogen–deuterium exchange of tropH [22,23] both proceed
via the trop anion, trop-, as intermediate ((b) in Scheme 1). This anion was also generated by
deprotonation of suberene with n-butyllithium and characterized in situ by Ultraviolet/Visible (UV/Vis)
spectroscopy [22,23]. In addition, the reduction of trop methoxide with elemental sodium was reported
to give the trop dianion radical, C15H112−• ((c) in Scheme 1), which was characterized in situ by Electron
Paramagnetic Resonance (EPR) spectroscopy [24].
The only compound we are aware of in which trop− acts as a ligand to a transition metal was
reported by Venzo et al., which showed that in complex
significantly decreases upon coordination to a tricarbonylchromium group [25]. Based on 1H and 13
NMR spectroscopy, the authors suggested that the anti-aromatic 16 -electron structure is split into a
-electron system located on one of the benzo groups which coordinates to the Cr(CO)3 fragment and
C, the paratropic character of the trop anion
C
π
6
π
an uncoordinated 10π-electron system, as shown in (d) in Scheme 1 [25,26].
Cyclooctatetraene, C8H8 (cot), is a textbook example in which the anti-aromatic character
of a hypothetical planar molecule with an 8π-electron system leads to a severe structural
distortion into a tub conformation. The same is true for its tub-shaped benzoannulated
derivative dibenzo[a,e]cyclooctatetraene, dbcot, with alternating single and double bonds
(
∆C = C = 0.1170(10) Å) [27]. Reduction of dbcot with alkaline metals Li, Na and K ((e) in Scheme 1),
proceeds stepwise to first give Mdbcot containing the dbcot−• anion radical and then to M2dbcot
with the dbcot dianion (vide infra) [28 29]. The EPR spectra of Mdbcot suggest that there are
contacts between the cations M+ (M = Li, Na, K) and dbcot−•
28 30]. Only the structure of the ion
,
[
,
triple [Li(tmeda)]2[dbcot] (tmeda = tetramethylethylenediamine) was determined. In this ion triple,
the dianion adopts a planar structure with nearly equal C-C bond lengths in the central ring (the average
bond length variation amounts to ∆C = C = 0.024(2) Å). The Li(tmeda)+ units are 8-bound above
η
and below the center of the dianion [31]. The 1H NMR spectrum of the dianion shows that the proton
resonances are significantly de-shielded. For example, the olefinic resonances in the central C8 ring are
shifted from 6.72 ppm in dbcot [32] to 7.08 ppm in Li2dbcot [28]. In combination with the structural
data, the dbcot2− dianion is therefore considered to be aromatic [19].
Herein, we report a new simple and clean synthesis of Mtrop and M2trop which allows the isolation
of various close and separated ion pairs which contain the trop mono- and di-anion. For comparison,
the disodium and dipotassium salts of dbcot2− were also prepared and structurally characterized.
Their anti-aromatic or aromatic character is assessed experimentally based on structural and magnetic
criteria. DFT-calculations on simplified model compounds, specifically calculation of the Nuclear
Independent Chemical Shifts (NICS), were performed in order to bolster the conclusions derived from
the experimental data.
2. Results
In order to obtain the trop anion trop– in pure form, the silylated trop derivatives 1a (R = H) [
1b (R = Me) (
) were prepared by reacting tropCl [33] with an excess of Li and quenching the trop- as
appearing intermediate with HMe2SiCl or Me3SiCl ((a) in Scheme 2). Both 1a and 1b can be easily
purified and obtained in pure form as crystalline solids [ ]. The trop silanes were subsequently treated
with various alkali metal tert-butoxides to give the ion pairs Litrop Natrop and Ktrop respectively,
with trop- as the anion, as pure crystalline solids in moderate to good yields ((a) in Scheme 2).
3] or
2
3
,

Molecules 2020, 25, 4742
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Scheme 2.
(M = Li, Na, K). (
Ktrop. (
) Synthesis of the dbcot2- dianion containing ion triples M2dbcot (M = Na, K) by reduction of
dbcot. (
) Synthesis of the organometallic d8-Rh(I) complex [Rh(trop)(cod)]. r.t. = room temperature;
(a
) Synthesis of the silylated tropylidenes 1a, b and the trop- containing ion pairs Mtrop
) Synthesis of trop2−• dianion radical containing ion triple K2trop by reduction of
b
c
d
DME: 1,2-dimethoxyethane; Et2O: diethyl ether; THF: tetrahydrofuran.
Compound Ktrop can be further reduced with potassium graphite to give the ion triple K2trop
,
which, apart from two K+ ions, contains the trop dianion radical trop2−• ((b) in Scheme 2). K2trop was
isolated as a dark green, micro-crystalline solid. We failed to prepare and isolate the dilithium and
the disodium analogue so far. The synthesis of the disodium and the dipotassium derivatives of the
dbcot2− dianion, Na2dbcot or K2dbcot, is achieved in a straightforward manner by simply exposing
dibenzo[a,e]cyclooctatetraene (dbcot) to elementary sodium or potassium graphite in anhydrous
tetrahydrofuran ((c) in Scheme 2) as was likewise reported for the preparation of Li2dbcot [28,31].
The trop anion can be used as a ligand in transition metal complexes, and reaction with half an
equivalent of [Rh2Cl2(cod)2] gives [Rh(trop)(cod)] as a dark red crystalline compound in moderate
isolated yield (63%) ((d) in Scheme 2). In this complex, the d8-valence electron configured Rh(I) center
is exclusively coordinated to hydrocarbon ligands and represents to the best of our knowledge the
first fully characterized mononuclear Rh(I) heptatrienide species. Only very few related compounds
such as bimetallic Rh complexes [34,35], a Rh(I) azulene complex [36] and a Rh(III) cycloheptatrienide
complex have been reported [37]. In contrast, the reaction with the dianion radical containing salt
K2trop did not yield any identifiable compound, likely due to the strongly reducing properties of the
trop2−• (vide infra).

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2.1. Characterization by NMR, EPR and X-ray Diffraction Methods
All diamagnetic compounds were fully characterized by NMR spectroscopy. The paramagnetic
compound K2trop was analyzed by EPR spectroscopy. Single crystals of Litrop Natrop Ktrop
K2trop Na2dbcot K2dbcot and [Rh trop)(cod were grown and investigated with X-ray diffraction
methods in order to determine the structures experimentally. In addition, the structures of compounds
Mtrop (M = Li K) and [Rh trop)(cod and their degree of anti-aromatic or aromatic character was
,
,
,
,
,
(
)]
−
(
)]
evaluated by calculating the Nuclear Independent Chemical Shifts (NICSs) (DFT, PBE [38,39] and
6-311+G (df, pd) [39] in a continuum solvation model (THF) [40,41]).
All compounds containing the trop- anion show strongly shielded signals for all protons in the
1H NMR spectra. In the dimer of the trop radical [23], namely C15H11
−
C15H11 = trop2 [42] (entry 1,
Table 1), which has an electronic structure reminiscent of the conjugated hydrocarbon cis-stilbene,
the olefinic protons 1Hol and the benzylic proton 1Hbz attached to the central seven-membered ring are
observed in the normal range at
are significantly shifted to lower frequencies to
δ
= 7.06 ppm and
δ
= 4.73 ppm. In the ion pairs, Mtrop, these resonances
(1Hol) < 2.6 ppm and (1Hbz) < 0.56 ppm (M = Li,
δ
δ
Na, K, entries 2–4, Table 1). Also, the protons of the annulated benzo groups are strongly shifted to
lower frequencies by about 3 ppm.
Table 1. Characteristic properties of the trop anions, the dbcot dianions and the reference compounds
trop2 and dbcot. Abbreviations:
bz: benzylic, ol: olefinic, 1JHC: coupling constant (Hz]) between the benzylic proton and carbon atom.
δ δ
1H and 13C: 1H and 13C NMR chemical shifts (ppm) respectively,
13
13
δ 1Hol
6.90
2.64
2.60
2.00
5.16
6.72
7.08
7.17
7.17
δ
C
ol
δ 1Hbz
4.90
δ
C
1JHC
bz
trop2
Litrop
131.43
139.03
138.79
139.26
97.52
52.18
83.67
81.56
89.44
55.84
0.50
139.8
142.9
144.5
144.6
Natrop
0.56
Ktrop
0.06
[Rh(trop)(cod)]
dbcot [32]
Li2dbcot [28]
Na2dbcot
K2dbcot
2.88
126.76
93.37
95.78
The experimental structures of the contact ions pairs Lithftrop
,
Lidmetrop, Nadmetrop Kthftrop
,
and K18c6trop were determined with X-ray diffraction methods using a suitable single crystal of every
compound and are depicted in Figure 1. Selected bond distances and angles are listed in Tables 2 and 3.
The most remarkable feature is the bending of the central seven-membered ring in the trop- moiety
expressed by the fold angles Θ1 and Θ2 (Table 3). These angles vary strongly with the nature of
the counter cation but also with the nature of the solvent molecules acting as co-ligands to M+.
In Lithftrop, the [Li(thf)3]+ cation binds predominantly to C1 [Li-C1 2.279(3) Å] which provokes a rather
strong bending (Θ1 = 24.2◦; Θ2 = 14.1◦), about half of the one observed in the hydrocarbon trop2
(
Θ1 = 49.4◦; Θ2 = 22.5◦). Encapsulation of Li+ by three dimethoxyethane (dme) solvent molecules
leads to [Li (dme)3]+[trop−] (Lidmetrop), in which no close contact between cation and anion occurs
(shortest Li–C contact = 6.79 Å) and the trop- anion adopts an almost flat structure (Θ1 = 6◦; Θ2 = 3◦).
Likewise, in the structure of Natrop, which contains [Na(dme)3]+, the contact between cation and
anion is long (the distance between Na+ and the centroid (cnt) of the central C7 ring is 5.25 Å) and the
trop- anion is flat (Θ1 = 6.5◦; Θ2 = 4.7◦).

Molecules 2020, 25, 4742
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Figure 1. ORTEP plots Lithftrop
50% ellipsoid probability (hydrogen atoms are omitted for clarity). Lidmetrop: only one of the two
distorted trop moieties is shown. [Rh trop)(cod : A co-crystallized toluene-molecule is omitted for
, , , , (trop)(cod)] at
Lidmetrop Nadmetrop Kthftrop K18c6trop and [Rh
(
)]
clarity. Selected Rh-C bond distances [Å]: Rh1–C1 2.0912(16), Rh1–C2 2.3427(16), Rh1–C3 2.3995(16),
Rh1–C4 2.2396(17), Rh1–C5 2.4044(18). Further selected structural parameters of all compounds are
given in Tables 2 and 3.
Table 2. Selected bond lengths (Å) for the trop anions (Figure 1). Abbreviation: Cnt: centroid.
K18c6trop [Rh(trop)(cod)]
Lithftrop
1.440(2)
1.423(2)
1.468(2)
1.338(3)
1.464(3)
1.423(2)
1.434(2)
2.279(3)
3.715(4)
3.783(3)
Lidmetrop a
Nadmetrop Kthftrop
C1–C2
C2–C3
1.430(15) 1.420(16) 1.4265(19)
1.422(14) 1.435(17) 1.4342(19)
1.428(3)
1.433(2)
1.429(3)
1.430(3)
1.461(3)
1.339(3)
1.464(3)
1.434(3)
1.431(3)
3.0213(18)
4.937(3)
4.694(2)
3.8117(18)
3.303(2)
1.470(2)
1.435(2)
C3–C4
1.40(2)
1.325(14) 1.324(14)
1.47(3) 1.46(3)
1.54(2)
1.455(3)
1.308(3)
1.463(3)
1.458(3)
C4–C5
1.332(3)
1.380(3)
C5–C6
1.414(3)
1.467(3)
1.482(3)
C6–C7
1.422(16) 1.412(17)
1.422(13) 1.414(15)
1.419(2)
1.430(3)
1.398(2)
C7–C1
1.419(2)
1.425(3)
1.492(2)
3.0977(17) b
3.4072(19)
3.346(2)
2.8223(4) c
2.9708(4) d
C1–M1
C4–M1
C5–M1
M1–Cnt1
M1–Cnt2
M1–Cnt3
M1–Cnt4
6.788(17)
7.63(2)
7.91(2)
7.50(2)
6.64(2)
7.34(2)
5.3528(15)
5.4943(17)
5.5599(19)
5.2530(15)
2.0912(16)
2.2397(17)
2.4044(18)
2.2600(16)
2.2186(18)
1.9682(19)
2.1139(19)
8.35(2)
7.282(17)
a
b
The asymmetric unit contains two distorted trop moieties: a and b. Distance to benzylic C of neighboring trop
anion: C10–K1: 3.0171(17) (see Kthftrop in Figure 1 for clarification). Centroid of C1, C2, C5, C6, C7. Allylic
c
d
centroid of C10, C20 and C80.

Molecules 2020, 25, 4742
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The most planar structure for a trop- anion is observed in Kthftrop
(
Θ
1 = 1.7◦;
Θ
2 = 2.4◦), which is
best described as a close ion pair with a rather short distance between K1 and the centroid cnt1 of the
C7 ring of 2.82 Å. The K+ is coordinated to the C7 ring in a slightly asymmetric manner as indicated by
the unequal K-C1 (3.098 Å) and K-C4/C5 distances (3.407 Å; 3.346 Å). Furthermore, the potassium ion
has an additional contact at about 2.97 Å to another trop- unit, such that K+ . . . trop− . . .
K
+ . . . .trop−
chains are formed in the solid state. Remarkably, in K18c6trop, the trop− anion shows larger fold angles
Θ1 = 12.4◦; Θ2 = 12.3◦). In this ion pair, K+ is rather asymmetrically bound and the distance to the
(
centroid of to the central C7 ring is large (3.812 Å). Instead, K+ binds closer to the allylic fragment C1,
C7, C15 (K1-cnt2 3.303 Å) with a rather short contact to the benzylic carbon center C1 (3.02 Å) and
longer ones to C7 (3.263 Å) and C15 (3.490 Å). It is highly speculative to which extent these features
of the solid-state structures are retained in solution. But, the coupling constants 1JHC of the benzylic
13
1
carbon nucleus, Cbz, to the benzylic proton, Hbz, can be empirically correlated to the s-orbital
character in the Cbz–H bond. In a bent trop anion, the Cbz–H bond will be closer to a sp3 configuration
(25% s, smaller 1JHC) that in a flat structure, where the Cbz–H is closer to a sp2 configuration (33% s,
larger 1JHC). Indeed, 1JHC obtained in deuterated tetrahydrofuran, [D8]THF, as solvent (Table 1) is
larger for the potassium salt than in the bent lithium salts, indicating that the solid-state structures are
retained to a certain degree in solution.
At this point, it is interesting to compare these main group metal complexes of trop− with
a “classical” organometallic complex such as [Rh
(
trop)(cod
)
]
. In contrast to Mtrop, which shows
1
1
strongly shielded H NMR resonances for all protons, the signals for the H nuclei at the benzo
groups in [Rh
trop2 (δ
1H: 6.58–7.25 ppm). Also, the chemical shifts of the benzo 13C nuclei are in the normal
(trop)(cod)] are in the same region (δ
1H: 6.82–7.01 ppm) as in the reference compound
range for arenes in between
δ
= 127 and 137 ppm. Only the olefinic and benzylic 1H and 13C NMR
13
13
signals (
δ
1Hol,
δ
1Hbz,
δ
C ,
ol
δ
C ) are significantly shifted to lower frequencies, indicating an
bz
interaction with the metal center, c.f.
(13Col) ~139 ppm and (13Cbz) = 81.6–89.4 ppm in Mtrop, see Table 1). In solution, [Rh
is seemingly C2v-symmetric which is not in agreement with the structure in solid state but can be
explained by a dynamic phenomenon (vide infra). Complex [Rh trop)(cod crystallizes as racemic
δ
(13Col) = 97.6 ppm,
δ
(13Cbz) = 56.5 ppm in [Rh
(
trop)(cod)] vs.
δ
δ
(trop)(cod)]
(
)]
mixture in the space group P21/c. The structure of one of the enantiomers is depicted in Figure 1 and is
very different from the Mtrop compounds. The Rh(I)(cod) fragment binds to C1, C2, C3, C4 and C5 at
distances between 2.09 to 2.40 Å (see legend to Figure 1) of the central seven-membered ring in a way
which is also observed in other metal [43
,
44] and specifically, Rh(I) pentadienide complexes [43
,44].
This pentadienide-type interaction donates 6
π
electrons to the Rh(I)(cod) fragment which thereby
reaches an 18-electron configuration. Metal-to-ligand electron back donation leads to a slight elongation
of all involved C–C bonds by about 0.03 Å with respect to the distances in the trop anion in the Mtrop
contact pairs (M = Li, Na, K). The most notable structural feature is the strongly bent conformation of the
trop- unit: the intersection angle between the non-coordinated benzo group and the benzopentadienide
unit is θ3 = 49.9(1)◦ (cf. Figure 1). The η5 coordination mode and non-symmetric structure observed in
the solid state is in contrast with the apparent C2v symmetry of the complex in solution. We therefore
assume that in solution, [Rh(trop)(cod)] underlies a dynamic phenomenon by which the rhodium
center rapidly exchanges between the two possible η5 binding sites of the central seven-membered
ring, as shown in Scheme 2d. This process must have a very low activation barrier because even at low
temperatures, this process is not frozen out on the NMR time scale. This coordination mode is in stark
contrast to the one of the only other dibenzo[a,d]cycloheptenide metal complex
Here, NMR data indicate that the Cr(0)(CO)3 fragment binds in a
6 fashion to one of the annulated
benzo groups of the trop moiety which leads to C1 symmetric structure. The 13C chemical shifts of
this coordinated benzo group are characteristically shifted to lower frequencies [
(13C) = 70–100 ppm].
That is, in this case, the anti-aromatic
-electron system of trop- is localized in a different way from the
one in [Rh trop)(cod , namely into a 6 -electron system at a terminal arene bound to the metal and an
uncoordinated 10π-electron system.
C (Scheme 1d) [25,26].
η
δ
π
(
)]
π

Molecules 2020, 25, 4742
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The structures of the ion triples K2trop
,
Na2dbcot and K2dbcot with the trop2-• dianion radical
or the dbcot2- dianion respectively, are shown in Figure 2. Selected bond lengths and angles are listed
in Tables 3 and 4.
Figure 2. ORTEP plots of dianions at 50% ellipsoid probability (hydrogen atoms are omitted for
clarity). K2trop: A co-crystallized DME-molecule is omitted for clarity. Na2dbot, K2dbot: The second
chemically identical but crystallographically unique compound in the asymmetric unit is omitted for
clarity. Selected bond lengths (Å) and angles (◦) are shown in Tables 3 and 4.
Table 3. Data from solid-state structures (Figure 1, Figure 2, and Figure 4). For comparison, trop2
trop+, and Li2dbcot are listed as well. Abbreviations: C = C: average bond length in the central ring
(Å), ∆C = C: average bond length difference in the central ring (Å), θ1 and θ2: dihedral angles in the
,
trop scaffold (◦), ϕ1 and ϕ2: dihedral angles in the dbcot scaffold (◦), K18c6trop
:
Ktrop with 18-crown-6.
Central Ring
C=C ∆C=C
1.4451(7) 0.0802(10) 49.37(12)
1.4134(11) 0.0283(15) 1.6(2)
1.4271(9) 0.0537(12) 24.16(19)
1.421(10) 0.060(14) 6(4)
1.4108(9) 0.0420(13) 6.49(19)
Trop Angles
Dbcot Angles
Compound
trop2 [42]
trop+ [45],a
Lithftrop
θ1
θ2
ϕ1
ϕ2
22.46(9)
3.44(14)
14.13(14)
2.9(25)
Lidmetrop
Natrop
4.66(12)
2.37(14)
12.35(17)
2.30(13)
32.29(13)
Kthftrop
K18c6trop
1.4253(11) 0.0491(15)
1.4269(11) 0.0449(16)
1.4346(10) 0.0337(14)
1.7(2)
12.4(3)
1.3(2)
K2trop
[Rh(trop)(cod)]
Li2dbcot [31]
Na2dbcot
K2dbcot
1.4450(9) 0.0626(13) 55.49(18)
0 b,c
1.421(2)
0.024(2)
1.4281(5)
0.0234(7)
5.38(3)
4.88(3)
1.40(12) b
1.4279(7) 0.0263(10)
a
b
The counter ion is tetrakis(3,5-bis(trifluoromethyl)phenyl)borate. The respective point group leads to pairwise
c
identical dihedral angles. The localization of dbcot on a crystallographic mirror plane leads to the perfect
planarity [31].

Molecules 2020, 25, 4742
9 of 23
Table 4. Selected bond lengths (Å) for ion pairs containing the trop2- and dbcot2- dianions (Figure 2).
Abbreviation: Cnt: centroid.
K2dbcot a,b
K2trop
1.426(3)
1.463(2)
1.434(3)
1.385(3)
1.440(3)
1.466(2)
Na2dbcot
1.4121(15)
1.4254(14)
1.4555(14)
1.4223(14)
1.4094(15)
1.4209(14)
1.4596(14)
C1–C2
C2–C3
C3–C4
C4–C5
C5–C6
C6–C7
C7–C8
C4–C1
C7–C1
C8–C1
C1–M1
C2–M1
C4–M1
C5–M1
C6–M1
C1–M2
C2–M2
C4–M2
C5–M2
C6–M2
M1–Cnt1
M2–Cnt1
1.409(2)
1.409(2)
1.418(2)
1.463(2)
1.426(2)
1.460(2)
1.420(2)
1.418(2)
1.428(2)
1.4199(14)
3.2455(18)
2.6792(11) 3.0482(15)
2.7258(11) 3.0093(15)
3.0905(14)
3.0728(14)
3.2607(18)
3.1092(17)
2.6971(11)
2.8896(11)
2.7507(11)
2.6557(11)
3.0902(17)
3.3042(18)
3.1789(18)
2.9216(11)
2.7689(11)
2.7876(18)
2.7495(17)
166.507(12)
2.0924(11) 2.4669(15)
2.1348(11)
2.5385(14)
180.00(4)
K1-Cnt1-K2
174.05(4)
180.00(5)
a
b
Two chemically identical, but crystallographically unique compounds are in the asymmetric unit. The space
group P21/n leads to pairwise identical C–C bond lengths.
As expected, the reduction of Ktrop to the radical dianion K2trop leads to an overall elongation
of all bonds within the central seven-membered ring (see increased average bond length C = C in
Table 3). The average bond length difference (∆C = C in Table 3) becomes smaller and the C3–C4,
C4–C5 and C5–C6 bond lengths’ alternation is slightly diminished. The plots and structural data for
Na2dbcot and K2dbcot are given in Figure 2 and Table 4 and are very similar to the ones reported for
Li2dbcot (Scheme 1 and Table 3) [31]. The dbcot2- dianions in K2dbcot and Li2dbcot adopt a rather
flat conformation while the one in Na2dbcot shows a slightly twisted conformation likely because of
crystal packing forces (see dihedral angles ϕ1 and ϕ2 in Table 3). All C–C bond lengths are longer than
1.4 Å and there is very little bond length variation (see ∆C = C in Table 3).
The paramagnetic compound K2trop was further characterized by EPR spectroscopy.
In tetrahydrofuran (THF) as solvent, the EPR spectrum of K2trop shows a rather complex hyperfine
coupling pattern (Figure 3, line a) which can be simplified by the addition of two equivalents of
18-crown-6. This indicates that complexation of K+ by 18-crown-6 significantly elongates the distances
between the cation and dianion radical, which leads to much smaller and hence non-detectable
39K-hyperfine splittings. Indirect proof comes from a comparison of the structures K2trop, where one
18-crown-6 binds to every K+ ion, and K2dbcot, where each K+ ion is coordinated by four THF
molecules. In the former the K+-Cnt distance is 2.79 Å while in the latter a shorter distance of 2.47 Å is

Molecules 2020, 25, 4742
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observed (Cnt = centroid of the central ring). The less complex spectrum (Figure 3, line b) was used
to simulate the hyperfine pattern (Figure 3, line c). The experimentally determined and calculated
hyperfine coupling constants (HFCs) indicate delocalization of the unpaired electron over the whole
dianion (see spin density plot in Figure 3). These data and the corresponding g-factor of 2.0064 are in
fairly good agreement with the ones of the reported sodium analogue Na2trop
ago (g = 2.0027 and HFCs shown in Figure 3).
[24] more than 50 years
a
b
c
337.0
337.5
338.0
B / mT
338.5
339.0
339.5
Figure 3. Top left: Reported hyperfine splittings (mT) of Na2trop. Top middle: Calculated hyperfine
coupling constants (HFCs) (mT) from the simulated spectrum of K2trop. Top right: Spin density plot
of K2trop (isovalue = 0.0009, positive spin densities in blue and negative spin densities in green).
DFT calculation: PBE [38] 6-311+G(df, pd) [39] in a continuum solvation model (THF). Bottom (
EPR spectrum of K2trop (0.5 mM) in THF, g = 2.0064. ( ): EPR spectrum of K2trop (0.5 mM) and
18-crown-6 (2 equivalents) in THF, g = 2.0067. (c): Simulated spectrum of b.
a):
b
The 1H NMR spectra of the contact ion pairs M2dbcot (M = Na, K) show, in comparison to neutral
dbcot, which is best described as a cyclic conjugated polyolefin, a diatopic ring current which leads to
significantly de-shielded olefinic resonances, 1Hol > 7 ppm (Table 1).
2.2. Evaluation of Anti-Aromaticity and Aromaticity by Calculation of NICS Values
Table 5 lists the calculated nuclear independent chemical shifts in the center of the rings, NICS(0),
and 1 Å above and below the central plane of these, NICS(1) (DFT, PBE [38] 6-311+G(df, pd) [39]).
The data for both six-membered rings A and C and the central seven-membered ring B is given.
For comparison, also the data for the tropylium cation trop+ are listed. As expected for an aromatic
molecule with a 14 π .
-electron configuration, the NICSs data are strongly negative for all rings in trop+
On the other end of the scale, cyclobutadiene (CBD) is listed for comparison which is considered to be an
archetypical anti-aromatic molecule with a 4-electron system and shows positive NICS values ((NICS(0)
= 27.49, NICS(1) = 18.03). The values are largely exceeded by the NICS values of the central ring B in
the flat and counter cation-free trop anion trop- which amount to 45.00 and 34.54. Even the flanking

Molecules 2020, 25, 4742
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rings A and C in trop- reach positive values in the range of CBD. For simplicity, the NICS data were
calculated for simplified model compounds of the contact ion pairs Mtropm (M = Li, Na, K; m indicates
neglecting coordinated solvent molecules at M+) and K2tropm, where the solvent molecules bound to
M+ were not considered. Nevertheless, the calculated
π–π* transitions of the calculated UV/Vis spectra
correspond well to the recorded UV/Vis spectra (Supplementary Figures S1–S4)—going from Li to K,
the observed bathochromic shift is for example well reproduced by the model compounds. In addition,
the overall structural agreement between these model compounds and the experimentally determined
structures given in Table 3 is very good (e.g., the calculated distances C5-M are listed in Supplementary
Table S1) and specifically, the decrease of the bent angles in the order Litropm
well reproduced. The NICS values of the contact ion pairs are very sensitive to the bending of the central
seven-membered ring. The stronger the ring B is bent, as in Litropm 1 = 26.2◦, 2 = 13.5◦), the smaller
> >
Natropm Ktropm is
(θ
θ
the NICS values are and the less pronounced the anti-aromatic character. On the contrary, when the
bending becomes small such as in Ktropm, the NICS values reach the ones of flat trop-. While it may
be debated whether CBD is an anti-aromatic compound because of its rectangular distortion [46],
Ktropm can be truly considered as an anti-aromatic species which in the form of Kthftrop can be
isolated as a substance. This claim is further bolstered by the fact that the averaged experimental
C=C bond distances listed in Table 3 do not differ much between trop+ and Kthftrop and the C=C
C=C is modest (0.028 Å in trop+, vs. 0.049 Å in Kthftrop
bond length variation ∆ ), indicating only
small structural distortions. This is in contrast to CBD, where this variation is significant (C-C 1.576 Å,
C=C 1.332 Å, ∆C = C = 0.244 Å) [46].
Table 5. Calculated isotropic NICS(0) and NICS(1) values for trop+
(M = Li, Na, K) and K2tropm
[Rh trop)(cod and cyclobutadiene (CBD). The model compounds
do not contain solvent molecules attached to the alkali cations. Computational details: PBE [38
,
trop-, model compounds Mtropm
,
(
)]
]
6-311+G(df, pd) [39] in a continuum solvation model (THF). Abbreviations: θ1 and θ2: dihedral angles
of the optimized structures (◦), CBD: 1,3-cyclobutadiene.
1
4
1
2
3
7 B 2
A
C
5
θ2
θ1
6
3
4
7
6
5
A
B
C
θ1
θ2
NICS(0)
NICS(1)
NICS(0)
NICS(1)
NICS(0)
NICS(1) a
NICS(1) b
NICS(0)
NICS(1) a
NICS(1) b
NICS(0)
NICS(1) a
NICS(1) b
−8.87
−11.76
21.05
15.12
6.41
−4.71
−7.56
45.00
34.54
25.23
18.82
17.36
30.78
23.32
22.04
42.39
32.38
32.86
−8.87
−11.76
21.02
15.10
6.66
trop+
trop–
0.00
0.00
0.01
0.01
26.19
13.54
11.00
1.77
Litropm
Natropm
Ktropm
3.85
3.89
0.95
1.67
10.23
7.00
10.23
7.00
19.99
7.41
4.40
4.41
17.02
12.03
11.18
17.02
12.02
11.18

Molecules 2020, 25, 4742
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Table 5. Cont.
1
4
1
2
3
7 B 2
A
C
5
θ2
θ1
6
5
3
4
7
6
A
B
C
θ1
θ2
NICS(0)
NICS(1) a
NICS(1) b
NICS(0)
NICS(1)
−6.04
−7.41
−8.47
4.77
−4.33
−1.38
−26.89
12.87
8.78
−5.79
−7.90
−8.53
4.77
58.20
34.54
0.02
[Rh(trop)(cod)]
K2tropm
0.02
1.93
1.93
NICS(0)
NICS(1)
27.49
18.03
CBD
a
b
NICS(1) opposite to the metal center. NICS(1) on the same side as the metal center. CBD = cyclobutadiene, C4H4.
As already stated above, the trop- anion as a ligand in a transition metal complex behaves very
differently. The NICS data of the annulated benzo rings A and C in [Rh(trop)(cod)] are negative and
indicate modest magnetic aromatic character.
The NICS data for the ion triple containing paramagnetic dianion radical trop2-• are much smaller
than for the mono-anions and indicate diminished anti-aromatic character in accord with conclusions
derived from a comparison of the experimental structural data (the C=C bond lengths variation is
slightly diminished in the less anti-aromatic dianion radical, see Table 3).
For completeness, the NICS(0) and NICS(1) data for the free dianion dbcot2- and the model
compounds M2dbcotm (M = Li, Na, K) are listed in Table 6. As expected, all rings A, B, and C show
negative NICS values indicating aromatic character. Note that the structural parameters and the UV/Vis
spectra of the ion triples are accurately reproduced by DFT-calculations (Supplementary Figures S5–S7).
Table 6. Calculated isotropic NICS(0) and NICS(1) values (ppm) for dianionic dbcot compounds.
Computational details: PBE [38] 6-311+G(df, pd) [39] in a continuum solvation model (THF).
Abbreviations: ϕ1 and ϕ2: dihedral angles of the optimized structures (◦).
A
B
C
A
B
C
ϕ1
ϕ2
NICS(0)
NICS(1)
NICS(0)
NICS(1)
NICS(1)
NICS(0)
NICS(1)
NICS(0)
NICS(1)
NICS(0)
NICS(1)
−6.60
−7.39
−4.42
−6.51
−5.31
−5.59
−7.06
−5.41
−6.93
−17.20
−15.13
−15.27
−13.49
−15.04
−14.57
−14.15
−17.48
−16.96
−7.68
−6.60
−7.39
−6.76
−8.45
−7.77
−5.59
−7.06
−5.42
−6.94
0.07
0.07
dbcot2-
3.73
2.50
Li2dbcotm
Na2dbcotm
K2dbcotm
Benzene
0.03
0.03
0.05
0.04
−10.09

Molecules 2020, 25, 4742
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2.3. Electrochemistry
The electrochemical properties of trop2
,
Ktrop and dbcot were investigated to provide some
understanding of the reactivity, especially with respect to mutual electron exchange processes.
As previously reported, the cyclic voltammogram of dbcot shows two close lying reduction events
at E◦ =
−
2.24 V and E◦ = 2.33 V vs. Fc/Fc+, resulting from two consecutive one-electron transfer
−
steps to give a planar dbcot2− dianion (Fc = ferrocene, Fc+ = ferrocenium) [28 29]. The dbcot-• anion
,
radical was detected by EPR at low temperatures but cannot be isolated. The formation of the trop2−•
dianion radical using Ktrop as starting material was investigated electrochemically and the cyclic
voltammogram in a 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) solution in THF is
presented in Figure 4. It shows two well-defined waves at
−
3.04 and
3.37 V vs. Fc/Fc+. The redox
−
process at 3.04 V is fully reversible, as indicated by the linear dependence of the anodic and cathodic
−
peak heights as function of ν1/2 (Supplementary Figure S8). This reduction was determined to be a
one-electron reduction and attributed to the reduction of trop- to the trop2−• dianion radical. The second
process at
even at high scan rates (this could correspond to the formation of the trianion which was reported for
[C7H7]3- 1/2 is below
47]). The dependence of the cathodic peak current height of this process with
−
3.37 V is irreversible and no anodic wave is recorded upon reversal of the potential scan„
[
ν
the theoretical curve for a one electron Nernstian reaction (Supplementary Figure S9). In addition,
the cathodic peak current decreases upon multiple cycling, while a new oxidation wave at –0.72 V
appears, increasing upon multiple cycling (Supplementary Figure S10). This behavior is expected
for a redox process coupled to a chemical reaction (EC mechanism) resulting in the deposition of the
electrochemically generated species on the electrode. Indeed, a blue film formed after electrolysis
on the electrode surface. The identity of that deposit could so far not be determined. The neutral
hydrocarbon trop2 shows a comparable behavior. Two consecutive redox waves at –2.86 and –3.29 V
vs. Fc/Fc+ (Figure 4) are observed in its cyclic voltammogram. The first occurs at a significantly more
anodic potential than for the potassium salt Ktrop
fully reversible whereas the second redox wave at
linear sweep voltammetry data indicate that the first reduction process at
(see Supplementary for details, Figures S11–S12 and Table S2). In contrast to the reduction of trop–,
the variation of the cathodic peak current height of this process with ν1/2 follows the theoretical
curve for a two-electron Nernstian reaction at low scan rates and a one-electron Nernstian reaction at
high scan rates, suggesting an electron transfer-chemical reaction-electron transfer (ECE) mechanism
(
−
∆
E = 180 mV). The redox process at
3.29 V is irreversible. Chronoamperometry and
2.86 V is a 1 e– reduction
−
2.86 V is
−
(Supplementary Figure S9). The reversibility of the first reduction process at 2.86 V suggests therefore
−
that the singly reduced radical trop2–• has a certain lifetime under electrochemical conditions. The ECE
2-
mechanism observed for the second reduction to the dianion trop2 suggests the cleavage of the C–C
bond of trop22-, forming two equivalents of the trop– mono-anion, which are further reduced at these
potentials to give the dianion radical trop2−•. Hence, the electrochemical behavior of trop2 can be
summarized with the steps (1)–(4) indicated below:
-•
trop2 + 1e- → trop2
(1)
-•
2−
trop2 + 1e- → trop2
(2)
(3)
(4)
trop2 → 2 trop-
2−
2 trop- + 2 e- → 2 trop2−•

Molecules 2020, 25, 4742
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Figure 4. Cyclic voltammogram of trop2 (1 mM) in dry tetrahydrofuran at ambient temperature with
scan rates ranging from 50 to 400 mV/s and referenced vs. Standard Calomel Electrode (SCE) [48].
[(nBu)4N][PF6] (100 mM) was used as a supporting electrolyte.
Interestingly, cyclic voltammetry studies of trop2 in the presence of 10 equivalents of KPF6 showed
anodically shifted redox waves, indicating that potassium ions contribute in stabilizing the trop anions
and in facilitating the C–C bond cleavage of trop2 (Supplementary Figures S13–S14).
The electrochemical behavior of trop2 and Ktrop is reflected in the chemical reactivity of these
species. First, the neutral hydrocarbon trop2 is cleanly converted with KC8 under cleavage of the
central C–C bond to give Ktrop (step (a) in Scheme 3). This reduction is reversible and addition
of the ferrocenium salt FcPF6 oxidizes Ktrop to reform trop2 (step (b) in Scheme 3). As already
shown in Scheme 2, Ktrop is further reduced to K2trop with potassium graphite (see (c) in Scheme 3).
The anti-aromatic radical dianion salt K2trop is a strong reductant (E◦ = –3.04 V). Addition of half an
equivalent trop2 oxidizes K2trop to give the mono-anion Ktrop, although heating to 60 ◦C is required
((d) in Scheme 3). Upon addition of half an equivalent of dbcot to K2trop, the former is fully reduced
to the aromatic dianion K2dbcot ((e) in Scheme 3).
Scheme 3. Interconversions reactions between and within the trop and the dbcot scaffold. FePF6:
ferrocenium hexafluorophosphate.

Molecules 2020, 25, 4742
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When one equivalent of dbcot is used in the oxidation of the dianion radical trop2-•, a mixture
of K2dbcot (0.65 equivalents), aside 0.64 equiv. Ktrop and 0.16 equiv. of trop2 is obtained according
to 1H NMR spectroscopy. A featureless EPR signal (g = 2.00485) suggests that also K+[dbcot-•] may
be present [28
,
30
,
49]. Considering the stoichiometry of the reaction, we suspect that K+[dbcot-•]
0.3 equiv.)
(≈
0.06 equiv.) is in a rapid equilibrium with the remaining neutral dbcot molecules (
≈
1
which we could not detect by H NMR spectroscopy. We also reacted two equivalents of the
anti-aromatic mono-anionic ion pair Ktrop with dbcot. Also, in this reaction, a product mixture
consisting of K2dbcot (0.5 equiv.), unreacted Ktrop (0.9 equiv.) and trop2 (0.5 equiv.) is obtained.
EPR spectroscopy indicates that, very likely, K+[dbcot-•] (g = 2.00565) is formed in this reaction as well
(Supplementary Scheme S1).
Finally, the oxidation of the dianion salts K2trop and K2dbcot with dry oxygen was investigated
at low temperature. Remarkably, the oxidation of the paramagnetic K2trop gives the hydrocarbon
C15H12 (suberene = tropH) as a major product (95%) and not as expected Ktrop or trop2, which is
formed in only 5% yield ((f) in Scheme 3). On contrast, the reaction between oxygen and K2dbcot
,
which contains the diamagnetic and aromatic dbcot2- dianion, is very clean and gives the neutral
hydrocarbon dbcot in almost quantitative yield ((g) in Scheme 3).
3. Materials and Methods
3.1. General Comments
All experiments were performed under an argon atmosphere using standard Schlenk and
vacuum-line techniques or in a MBraun inert-atmosphere drybox (argon atmosphere). All reagents
were used as received from commercial suppliers unless otherwise stated. The following compounds
were synthesized according to literature procedure: KC8 [50], dbcot [32] and [Rh2Cl2(cod)2] [51]. THF,
DME, diethyl ether, toluene and n-hexane were purified using an Innovative Technologies PureSolv
system and stored over 4 Å molecular sieves. THF-d8, C6D6 were distilled from sodium benzophenone
ketyl. CDCl3 was distilled from CaH2. Solution NMR spectra were recorded on Bruker Avance 500,
400, 300, 250 and 200 MHz spectrometers. The chemical shifts (δ) are expressed in ppm relative to
SiMe4 for 1H and 13C respectively. Coupling constants (J) are given in Hertz (Hz) as absolute values.
The multiplicity of the signals is indicated as s, d, t, q, or m for singlets, doublets, triplets, quartets,
or multiplets. If the data allowed it, the assignment was based on the IUPAC recommendations for
fused polycyclic hydrocarbons (Scheme 4) [52]. Further abbreviations were used in the assignment:
br for broadened signals, Ar for aromatic signals, quart for quaternary 13C signals.
4
6
5
6
4
7
5
3
7
3
2
8
9
4a 6a
12a 10a
4a 5a
2
8
11a 9a
1
9
1
10
12
11
11
10
Scheme 4. The assignment in the NMR spectra are based on these numbering schemes.
EPR spectra were recorded by Dr. Reinhard Kissner on an X-band (9.50 GHz) Magnettech
Miniscope 5000 EPR spectrometer with liquid nitrogen cooling. EPR spectra were simulated with
Matlab R2016b, using EasySpin-5.2.11 package. UV/Vis spectra were recorded on a UV/Vis/NIR
Lambda-19 spectrometer in a cell with a 2 mm path length. IR spectra were collected on a PerkinElmer
Spectrum 2000 FT-IR-Raman spectrometer. Absorption bands are described as w, m or s for weak,
medium or strong. Elemental analyses were performed by Peter Kälin in the Mikrolabor of the
ETH Zürich. Melting points were determined with a Büchi melting-point apparatus and are not
corrected. X-ray diffraction was performed at 100 K on an Oxford Xcalibur or Venture diffractometer
with a CCD area detector and a molybdenum X-ray tube (0.71073 A). Using Olex2 [53], the structure
was solved by direct methods (SHELXS [54] or SHELXT [55]), followed by least-squares refinement

Molecules 2020, 25, 4742
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against full matrix (versus F2) with SHELXL [54]. All non-hydrogen atoms were refined anisotropically.
The contribution of some hydrogen atoms, in their calculated positions, was included in the refinement
using a riding model.
3.2. Synthesis
trop–Si(H)Me2 (1a): 5-Chloro-5H-dibenzo[a,d]cycloheptene (10.02 g, 44.20 mmol, 1.0 equivalent)
and elementary lithium (0.71 g, 102.31 mmol, 2.3 equivalents) were reacted in dry tetrahydrofuran
(40 mL) at a.T. overnight. The resulting dark red liquid was added dropwise to a solution of
chlorodimethylsilane (5.4 mL, 4.60 g, 48.63 mmol, 1.1 equivalents) in dry tetrahydrofuran (40 mL) over
an ice bath. The resulting yellow liquid was allowed to warm to a.T. and stirred overnight. The turbid
suspension was concentrated under reduced pressure and subsequently distilled in high vacuum (only
one fraction was observed from 90 to 115 ◦C). The obtained yellow liquid was placed in the fridge to
give a white, crystalline solid after three days, which was triturated with dry diethyl ether (10.0 mL)
and dried in high vacuum (8.328 g, 33.26 mmol, yield: 75.3%).
1H NMR (300 MHz, CDCl3, jb071.1.4)
2H, H-4,6), 6.74 (s, 2H, H-10,11), 4.18–4.07 (m, 1H, SiH), 3.49 (d, 3JHH = 4.1 Hz, 1H, H-5),
= 3.6 Hz, 6H, SiCH3) ppm. 13C NMR (75 MHz, CDCl3, jb071.1.4) 140.83 (s, 2C, Cquart), 135.05 (s, 2C,
Cquart), 132.59 (s, 2C, C-10,11), 129.59 (s, 2C, CAr), 129.13 (s, 2C, C-4,6), 128.96 (s 2C, CAr), 125.71(s, 2C,
δ
7.27–7.09 (m, 6H, HAr), 7.06 (dd, 3JHH = 7.4 Hz, 4JHH = 1.4 Hz,
−
0.09 (d, 3JHH
δ
CAr), 47.60 (s, 1C, C-5), −1: 3015 (w, =C–H st), 2955 (w, –C–H st),
3.71 (s, 2C, Si(CH3)2) ppm. ATR IR: λ
−
2143 (m, Si–H st), 868 (s), 796 (s), 726 (s), 446 (s) cm−1. Elemental analysis: C (81.29%, calc.: 81.54%);
H (7.37%, calc.: 7.24%).
trop–SiMe3 (1b): 5-Chloro-5H-dibenzo[a,d]cycloheptene (4.32 g, 19.06 mmol, 1.0 equivalent)
and elementary lithium (0.33 g, 47.55 mmol, 2.5 equivalents) were reacted in dry tetrahydrofuran
(30 mL) at a.T. overnight. The resulting deep red liquid was added dropwise to a colorless solution of
trimethylsilyl chloride (3.2 mL, 2.74 g, 25.21 mmol, 1.3 equivalents) in dry tetrahydrofuran (35 mL).
The reaction mixture was allowed to warm to a.T. and stirred overnight. The solvent of the orange
solution was removed under reduced pressure to give an orange solid. The crude product was purified
by sublimation (up to 150◦C, HV) to give a yellow, crystalline solid (4.20 g, 15.88 mmol, yield: 83.3%).
1H NMR (300 MHz, C6D6, jb121.1.3)
2H, H-3,7), 7.06–6.92 (m, 4H, H-1,2,8,9), 6.86 (d, 3JHH = 7.4 Hz, 2H, H-4,6), 6.48 (s, 2H, H-10,11), 3.41
(s, 1H, H-5), 140.91 (s, 2C, C-4a,5a),
0.06 (s, 9H, SiCH3) ppm. 13C NMR (75 MHz, C6D6, jb121.1.3)
135.49 (s, 2C, C-9a,11a), 132.86 (s, 2C, C-10,11), 129.95 (s, 2C, C-1,9), 129.73 (s, 2C, C-4,6), 129.07 (s, 2C,
δ
7.08 (ddd, 3JHH = 7.5 Hz, 3JHH = 6.1 Hz, 4JHH = 2.7 Hz,
−
δ
C-3,7), 125.71 (s, 2C, C-2,8), 50.24 (s, 1C, C-5), 0.39 (s, 3C, SiCH3) ppm. Elemental analysis: C (81.70%,
−
calc.: 81.76%); H (7.74%, calc.: 7.62%).
Litrop: Method A: Lithium sand (0.348 g, 50.20 mmol, 2.3 equivalents) was suspended
in dry tetrahydrofuran (10 mL) and placed in a cooling bath (propan-2-ol and dry ice).
5-Chloro-5H-dibenzo[a,d]cyclo-heptene (5.00 g, 22.06 mmol, 1.0 equivalent) was dissolved in dry
tetrahydrofuran (30 mL). The resulting light yellow solution was added dropwise (during 25 min) to
the brown suspension. The reaction mixture was allowed to warm to a.T. and a color change to dark
red was observed under heat development. After two hours, the dark red liquid was concentrated to
approximately 25 mL and filtered over a plug of celite. The resulting red solution was layered with dry
n-hexane (71 mL) and stored in the freezer (
23 ◦C, 6 days). A first crop of green, crystalline needles
−
was obtained (0.479 g, 1.26 mmol, yield: 2.5%; according to 1H NMR spectroscopy, the compound
contained 2.5 equivalents of THF). The supernatant was layered with dry n-hexane (30 mL) and stored
in the freezer (
23 ◦C, 1 day) and a second crop was obtained (7.448 g, 19.68 mmol,1 yield: 94.0%,
−
overall yield: 95.0%).
Method B: (5H-Dibenzo[a,d]cycloheptene-5-yl)-dimethyl-silane (1a, 0.159 g, 0.64 mmol,
1.0 equivalent) was dissolved in dry 1,2-dimethoxyethane (2.0 mL). The resulting slightly yellow
solution was added to a white suspension of lithium tert-butoxide (0.051 g, 0.64 mmol, 1.0 equivalent)
in dry 1,2-dimethoxyethane (2.0 mL) (in a solvent with lower polarity (such as diethyl ether or

Molecules 2020, 25, 4742
17 of 23
tetrahydrofuran), a significantly lower yield was obtained). A swift color change to dark red was
observed and the reaction mixture was stirred at a.T. (3 h). The dark red liquid was filtered over
glass filter paper and celite, layered with dry n-hexane (10.0 mL) and stored in the freezer (
overnight). The light yellow supernatant was discarded and the obtained green crystals were washed
with dry n-hexane (3 1 mL) and dried under a stream of argon (0.1170 g, 0.35 mmol, yield: 55.2%;
according to 1H NMR spectroscopy, the compound contained 1.5 equivalents of dme).
−
30 ◦C,
×
1H NMR (300 MHz, C6D6/THF-d8, jb070.1.6)
δ
5.70 (td, 3JHH = 7.5 Hz, 4JHH = 1.6 Hz, 2H, H-3,7),
5.15 (td, 3JHH = 7.1 Hz, 4JHH = 1.3 Hz, 2H, H-2,8), 4.87 (dd, 3JHH = 7.1 Hz, 4JHH = 1.6 Hz, 2H, H-1,9),
4.59 (dd, 3JHH = 7.9 Hz, 4JHH = 1.3 Hz, 2H, H-4,6), 3.92 (s, 2H, H-10,11), 1.54 (s, 1H, H-5) ppm. H NMR
1
(400 MHz, THF-d8, jb070.1.7)
H-1,9), 3.48–3.42 (m, 2H, H-4,6), 2.64 (s, 2H, H-10,11), 0.50 (s, 1H, H-5) ppm. 13C NMR (101 MHz, THF-d8,
jb070.1.7) 163.60 (s, 2C, C-4a,5a), 139.03 (s, 2C, C-10,11), 138.27 (s, 2C, C-9a,11a), 132.65 (s, 2C, C-3,7),
132.05 (s, 2C, C-1,9), 120.67 (s, 2C, C-4,6), 112.44 (s, 2C, C-2,8), 83.67 (s, 1C, C-5) ppm. UV/Vis (THF):
233 (
: 1276(321) m2mol−1), 287 ( : 2984(340) m2mol−1), 443 ( : 631(102) m2mol−1), 513 (shoulder,
: 411(65) m2mol−1) nm. ATR IR:
δ 4.80–4.73 (m, 2H, H-3,7), 4.12–4.05 (m, 2H, H-2,8), 3.73–3.66 (m, 2H,
δ
λ
max
ε
ε
ε
ε
λ
−1: 2985 (w, –C–H st), 2926 (w, –C–H st), 1364 (s), 1068 (s), 912 (s),
724 (s), 436 (s) cm−1. Elemental Analysis for Litrop(dme)1.7: C (74.79%, calc.: 75.07%); H (7.78%, calc.:
7.95%). MP: 62 ◦C (decomp.).
Natrop
:
(5H-Dibenzo[a,d]cycloheptene-5-yl)-dimethyl-silane (1a, 2.005 g, 8.01 mmol,
1.0 equivalent) was dissolved in dry diethyl ether (30 mL). The resulting colorless solution was
added to a white suspension of sodium tert-butoxide (0.769 g, 8.00 mmol, 1.0 equivalent) in dry
diethyl ether (60 mL) over an ice bath. After 30 min, the reaction mixture was allowed to warm to
a.T. and a dark red liquid was obtained. The solvent was removed under reduced pressure and dry
1,2-dimethoxyethane (30 mL) was added to the red residue. The liquid was filtered over a plug of
celite, layered with dry n-hexane (80 mL) and stored in the freezer (
23 ◦C, 2 days). The light yellow
−
supernatant was discarded and the obtained dark green crystals were washed with dry n-hexane
1
(2
×
10 mL) and briefly dried in vacuo (1.931 g, 6.16 mmol, yield: 77.0%; according to H NMR
spectroscopy the compound contained 1.1 equivalents of dme).
1H NMR (300 MHz, THF-d8, jb075.3) 4.76 (ddd, 3JHH = 7.8 Hz, 3JHH = 7.1 Hz, 4JHH = 1.6 Hz, 2H,
δ
H-3,7), 4.08 (td, 3JHH = 7.1 Hz, 4JHH = 1.2 Hz, 2H, H-2,8), 3.68 (dd, 3JHH = 7.1 Hz, 4JHH = 1.6 Hz, 2H,
H-1,9), 3.43 (d, 3JHH
C6D6/THF-d8, krastefa.50)
≈
6.9 Hz§, 2H, H-4,6), 2.60 (s, 2H, H-10,11), 0.56 (s, 1H, H-5) ppm. H NMR (400 MHz,
1
δ
5.18 (td, 3JHH = 7.5 Hz, 4JHH = 1.6 Hz, 2H, H-3,7), 4.54 (td, 3JHH = 7.1 Hz,
4JHH = 1.3 Hz, 2H, H-2,8), 4.11 (dd, 3JHH = 7.1 Hz, 4JHH = 1.6 Hz, 2H, H-1,9), 3.93 (dd, 3JHH = 7.9 Hz,
3JHH = 1.4 Hz, 2H, H-4,6), 2.98 (s, 1H, H-10,11), 1.10 (s, 1H, H-5) ppm. 13C NMR (101 MHz, C6D6/THF-d8,
krastefa.51)
C-3,7), 132.57 (s, 2C, C-1,9), 120.12 (s, 2C, C-4,6), 113.22 (s, 2C, C-2,8), 81.56 (s, 1C, C-5) ppm. UV/Vis
(THF): max 285 (
: 3074(349) m2mol−1), 459 ( : 803(158) m2mol−1) nm. ATR IR: −1: 3039 (w, =C–H
δ 161.95 (s, 2C, C-4a,5a), 138.79 (s, 2C, C-10,11), 136.39 (s, 2C, C-9a,11a), 133.16 (s, 2C,
λ
ε
ε
λ
st), 3003 (w, =C–H st), 2978 (w, –C–H st), 2932 (w, –C–H st), 2913 (w, –C–H st), 2869 (w, –C–H st),
2823 (w, –C–H st), 1553 (s), 1443 (s), 1362 (s), 1118 (s), 1082 (s), 1028 (s), 734 (s) cm−1. Elemental Analysis
for Natrop(dme)1.1: C (74.20%, calc.: 74.36%); H (6.92%, calc.: 7.08%). MP: 45 ◦C (decomp.).
§
The upper part of the multiplet lies underneath the signal of the solvent DME.
Ktrop: Method A: (5H-Dibenzo[a,d]cyclo-hepten-5-yl)-dimethyl-silane (1a, 1.500 g, 5.99 mmol,
1.0 equivalent) and potassium tert-butoxide (0.693 g, 6.18 mmol, 1.0 equivalent) were placed under
argon. Dry tetrahydrofuran (20 mL) was added and a dark red liquid was obtained immediately.
The reaction mixture was stirred at a.T. (10 hr) and filtered over a plug of celite. The dark red filtrate
◦
was layered with dry n-hexane (68 mL) and stored in the freezer (
−
23 C, 4 days). A first crop of
red crystals was isolated (0.624 g, 2.34 mmol, yield: 39.1%; according to 1H NMR spectroscopy the
compound contained 0.5 equivalent of THF). The supernatant was layered with dry n-hexane (60 mL),
stored in the freezer (
overall yield: 71.5%).
−
23 ◦C, 3 days) and a second crop was obtained (0.461 g, 2.00 mmol, yield: 32.4%,

Molecules 2020, 25, 4742
18 of 23
Method B: (5H-Dibenzo[a,d]cycloheptene-5-yl)-trimethyl-silane (1b, 4.909 g, 18.57 mmol,
1.0 equivalent) and potassium tert-butoxide (2.085 g, 18.58 mmol, 1.0 equivalent) were placed under
argon. Dry tetrahydrofuran (35 mL) was added over an ice bath. The yellow solution was allowed to
warm to a.T. and a gradual color change to dark red was observed. The reaction mixture was stirred
overnight. The solvent was removed under reduced pressure. The red residue was triturated with
dry diethyl ether (2
×
20 mL) and then washed with dry diethyl ether (40 mL). The resulting red solid
was briefly dried in vacuo (3.552 g, 15.42 mmol, yield: 83.1%). The red solid (0.271 g, 1.18 mmol) was
recrystallized in dry tetrahydrofuran (5.0 mL), filtered over glass filter paper and celite, layered with
dry n-hexane (10.0 mL) and stored in the freezer (
30 ◦C, 2.5 days). The long, crystalline needles were
−
washed with dry n-hexane (4
recrystallization: 63%).
×
1.5 mL) and dried under a stream of argon (0.171 g, 0.74 mmol, yield of
1H NMR (300 MHz, THF-d8, jb073.1.1EA) 4.47 (ddd, 3JHH = 7.9 Hz, 3JHH = 7.1 Hz, 4JHH = 1.6 Hz,
δ
2H, H-3,7), 3.74 (td, 3JHH = 7.1 Hz, 4JHH = 1.2 Hz, 2H, H-2,8), 3.26 (dd, 3JHH = 7.1 Hz, 4JHH = 1.6 Hz,
2H, H-1,9), 2.97 (dd, 3JHH = 8.0 Hz, 4JHH = 1.3 Hz, 2H, H-4,6), 2.00 (s, 2H, H-10,11), 0.06 (s, 1H, H-5)
1
ppm. H NMR (400 MHz, THF-d8, krastefa.11, T = 273 K)
δ
4.40 (ddd, 3JHH = 8.2 Hz, 3JHH = 7.1 Hz,
4JHH = 1.6 Hz, 2H, H-3,7), 3.65 (td, 3JHH = 7.0 Hz, 4JHH = 1.3 Hz, 2H, H-2,8), 3.18 (dd, 3JHH = 7.1 Hz,
4JHH = 1.6 Hz, 2H, H-1,9), 2.89 (dd, 3JHH = 8.0 Hz, 4JHH = 1.3 Hz, 2H, H-4,6), 1.91 (s, 2H, H-11,10),
−
0.01 (s, 1H, H-5) ppm. 13C NMR (101 MHz, THF-d8, krastefa.12, T = 273 K)
δ
162.96 (s, 2C, C-4a,5a),
139.26 (s, 2C, C-10,11), 138.42 (s, 2C, C-9a,11a), 133.95 (s, 2C, C-3,7), 132.89 (s, 2C, C-1,9), 120.17 (s, 2C,
C-4,6), 111.87 (s, 2C, C-2,8), 89.44 (s, 1C, C-5) ppm. UV/Vis (THF): 286 (
: 3343(566) m2mol−1),
−1: 3006 (w, =C–H st),
λ
max
ε
296 (shoulder, ε ε λ
: 3086(595) m2mol−1), 478 ( : 1025(277) m2mol−1) nm. ATR IR:
2985 (w, –C–H st), 2939 (w, –C–H st), 2924 (w, –C–H st), 1366 (m), 1121 (s), 1039 (m), 914 (m), 797 (m),
740 (s), 436 (m) cm−1. Elemental Analysis: C (78.25%, calc.: 78.21%); H (4.94%, calc.: 4.81%). MP:
240 ◦C (decomp.).
K2trop: Ktrop (0.461 g, 2.00 mmol, 1.0 equivalent) was dissolved in dry tetrahydrofuran (5.0 mL).
The resulting dark red solution was added to a golden suspension of potassium graphite (0.355 g,
2.62 mmol, 1.3 equivalents) in dry tetrahydrofuran (5.0 mL) and stirred at a.T. (3 days). The dark
red suspension was filtered over a plug of celite and the dark red filter cake was washed with dry
tetrahydrofuran (3.5 mL). The dark red filtrate was split into two portions and each was layered with
dry n-hexane (6.0 mL) and stored in the freezer (
of both fractions was washed with dry n-hexane (2
0.77 mmol, yield: 38.3%). The supernatant of both fractions was concentrated under reduced pressure
and the dark residue was recrystallized again from dry tetrahydrofuran (3.0 mL) and dry n-hexane
(6.0 mL) at −30 ◦C to give a second crop of dark red, microcrystalline solid (0.060 g, 0.12 mmol, yield:
6.2%, overall yield: 44.5%).
−
30 ◦C, 3 days). The dark red, microcrystalline solid
2 mL each) and briefly dried in vacuo (0.372 g,
×
UV/Vis (THF):
λ
286 (
ε
ε
: 4098(190) m2mol−1), 297 (shoulder,
ε
: 3857(204) m2mol−1), 361 (
λ
−1: 3020 (w, =C–H st), 2970 (w, –C–H
ε:
max
1988(237) m2mol−1), 482 (
: 1633(37) m2mol−1) nm. ATR IR:
st), 2868 (w, –C–H st), 1324 (m), 1015 (m), 736 (m), 686 (m), 433 (m) cm−1. Elemental Analysis for
K2trop(thf)(n-hexane)0.2: C (67.95%, calc.: 67.62%); H (5.92%, calc.: 6.12%). MP: >250 ◦C.
Na2dbcot: Dibenzo[a,e]cyclooctatetraene (0.507 g, 2.48 mmol, 1.0 equivalent) was dissolved in
dry tetrahydrofuran (10 mL). The colorless solution was added to a Schlenk tube with a sodium mirror
(0.55 g, 23.92 mmol, 9.6 equivalents). A swift color change to dark red was observed. The reaction
mixture was stirred at a.T. (1 day). The solvent was removed under reduced pressure. The purple
residue was suspended in dry tetrahydrofuran (7 mL) and filtered over a plug of celite to give a dark
red filtrate (a) and a dark red filter cake (b). (a) The dark red filtrate was layered with dry n-hexane
(10 mL) and stored at a.T. (7 days). The obtained dark crystals were washed with dry n-hexane (2 mL)
and dried under a stream of argon (0.145 g, 0.34 mmol, yield: 13.6%). (b) The filter cake was extracted
with dry 1,2-dimethoxyethane (6 mL). The dark red filtrate was layered with dry n-hexane (8 mL) and
stored in the freezer (
−
30 ◦C, 6 days). The obtained dark crystals were washed with dry n-hexane
(2 × 2 mL) and dried under a stream of argon (0.256 g, 0.45 mmol, yield: 18.3%, overall yield: 31.9%).

Molecules 2020, 25, 4742
19 of 23
1H NMR (300 MHz, THF-d8, jb105.1.2)
δ
7.87 (dd, 3JHH = 6.6 Hz, 4JHH = 3.3 Hz, 4H, H-1,4,7,10),
7.17 (s, 4H, H-5,6,11,12), 6.21 (dd, 3JHH = 6.6 Hz, 4JHH = 3.4 Hz, 4H, H-2,3,8,9) ppm. 13C NMR (75 MHz,
THF-d8, jb105.1.2)
93.37 (s, 4C, C-5,6,11,12) ppm. UV/Vis (THF):
m2mol−1), 328 ( : 5293(512) m2mol−1), 392 ( : 853(87) m2mol−1), 514 (
m2mol−1), 597 (ε: 76(9) m2mol−1) nm.
δ
135.26 (s, 4C, C-1,4,7,10), 108.90 (s, 4C, C-2,3,8,9), 107.43 (s, 4C, C-4a,6a,10a,12a),
max 238 ( : 1842(132)
: 2523(2) m2mol−1), 261 (shoulder,
: 94(12) m2mol−1), 553 (
: 106(13)
λ
ε
ε
ε
ε
ε
ε
K2dbcot: Dibenzo[a,e]cyclooctatetraene (0.301 g, 1.47 mmol, 1.0 equivalent) was dissolved in
dry tetrahydrofuran (12 mL). Potassium graphite (0.526 g, 3.89 mmol, 2.6 equivalents) was added.
The resulting dark red suspension was allowed to stir at a.T. (1 day). The reaction mixture was filtered
over a plug of celite. The filter cake was washed with dry tetrahydrofuran (3 mL). The filtrate was split
into two portions and each layered with dry n-hexane (8 mL) and stored in the freezer (
−
30 ◦C, 3 days).
The obtained dark green crystals were washed with dry n-hexane (2 1 mL) and briefly dried in vacuo
×
(0.450 g, 1.15 mmol, yield: 78.3%). The supernatant was concentrated under reduced pressure and the
black residue was recrystallized in the same manner to give a second crop (0.047 g, 0.12 mmol, yield:
8.2%, overall yield: 86.6%).
1H NMR (300 MHz, THF-d8, jb085.1.1) 7.87 (dd, 3JHH = 6.7 Hz, 4JHH = 3.5 Hz, 4H, H-1,4,7,10),
δ
7.17 (s, 4H, H-5,6,11,12), 6.19 (dd, 3JHH = 6.7 Hz, 4JHH = 3.3 Hz, 4H, H-2,3,8,9) ppm. 13C NMR (75 MHz,
THF-d8, jb085.1.1) 135.72 (s, 4C, C-1,4,7,10), 109.36 (s, 4C, C-4a,6a,10a,12a), 108.91 (s, 4C, C-2,3,8,9),
95.78 (s, 4C, C-5,6,11,12) ppm. UV/Vis (THF): 235 (
: 3997(184) m2mol−1), 261 (shoulder,
1053(87) m2mol−1), 326 ( : 1959(338) m2mol−1), 388 ( : 334(53) m2mol−1), 502 ( : 38(6) m2mol−1),
544 (ε: 42(7) m2mol−1), 587 (ε: 31(5) m2mol−1) nm. MP: >250 ◦C.
[Rh trop)(cod : Cyclooctadiene rhodium(I) chloride dimer (0.050 g, 0.10 mmol, 1.0 equivalent)
δ
λ
max
ε
ε:
ε
ε
ε
(
)]
and Ktrop (0.047 g, 0.20 mmol, 2.0 equivalents) were stirred in dry toluene (6 mL) at a.T. (12 hr).
The dark red liquid was filtered over a plug of celite, layered with dry n-hexane (6 mL) and stored in
the freezer (
and dried under a stream of argon (0.052 g, 0.13 mmol, yield: 63.0%).
1H NMR (400 MHz, C6D6, jb083.8A) 7.01–6.88 (m, 6H, HAr), 6.85 (dd, 3JHH = 7.1 Hz, 1JHH = 1.7 Hz,
2H, H-4,6), 5.16 (d, 2JRhH = 1.2 Hz, 2H, H-10,11), 4.09 (sbr, 2H, CHCOD), 3.59 (sbr, 2H, CHCOD), 2.88 (s, 1H,
−
30 ◦C, 13 days). The obtained dark red crystals were washed with dry n-hexane (0.5 mL)
δ
H-5), 2.21 (sbr, 4H, CH2COD), 1.76 (sbr, 4H, CH2COD) ppm. 13C NMR (101 MHz, C6D6, jb083.8A)
δ 137.43
(s, 2C, C-4a,5a), 132.63 (s, 2C, C-9a,11a), 128.06 (s, 2C, C-1,9/3,7§), 126.86 (s, 2C, C-4,6), 126.66 (s, 2C,
C-3,7/1,9), 126.13 (s, 2C, C-2,8), 97.52 (d, 1JRhC = 5.4 Hz, 2C, C-10,11), 91.23 (s, 2C, CHCOD), 70.69 (s, 2C,
CHCOD), 55.84 (d, 1JRhC = 13.2 Hz, 1C, C-5), 35.22 (s, 2C, CH2COD), 29.91 (s, 2C, CH2COD) ppm.
§
The peak is buried under the signal for C6D6.
1H NMR (500 MHz, THF-d8, jb083.8.2A, 40 ◦C) 7.10–6.99 (m, 6H, HAr), 6.89–6.84 (m, 2H, H-4,6),
δ
−
5.46 (s, 2H, H-10,11), 3.92 (mbr, 2H, CHCOD), 3.70 (mbr, 2H, CHCOD), 2.81 (s, 1H, H-5), 2.30 (m, 4H,
CH2COD), 1.76 (m, 4H, CH2COD) ppm. 13C NMR (126 MHz, THF-d8, jb083.8.2A, 40 ◦C)
137.75 (s,
−
δ
2C, C-4a,5a), 133.25 (s, 2C, C-9a,11a), 128.54 (s, 2C, C-1,9/3,7), 127.42 (s, 2C, C-3,7/1,9), 127.24 (s, 2C,
C-4,6), 126.78 (s, 2C, C-2,8), 97.57 (d, 1JRhC = 5.0 Hz, 2C, C-10,11), 90.61 (d, 1JRhC = 6.9 Hz, 2C, CHCOD),
70.35 (d, JRhC = 15.6 Hz, 2C, CHCOD), 56.45 (d, JRhC = 13.0 Hz, 1C, C-5), 35.09 (s, 2C, CH2COD),
1
1
30.46 (s, 2C, CH2COD) ppm.
4. Conclusions
A desilylation reaction using silyl trop derivative which proceeds by addition of KOtBu allowed
the very clean synthesis of larger amounts of the salts of the trop anion, trop-, which contain alkali metal
counter cations M+ = Li, Na, K. With these at hand, an in-depth study on the electronic configuration
of these conjugated hydrocarbons exclusively composed from sp2-valence electron hybridized carbon
centers could be performed. As the Hückel counting rules imply, the 16π-electron configuration of
trop- predicts this to be anti-aromatic. This is indeed the case as NMR data and as well as calculated
NICS data clearly show. A number of contact ion pairs, Mtrop, could be structurally characterized by
X-ray diffraction methods. In these, ethereal molecules complete the coordination sphere around the

Molecules 2020, 25, 4742
20 of 23
M+ cation. From tetrahydrofuran (THF), close ion pairs are obtained which show an intimate contact
between cation and trop- anion. In these, the trop- anion is bent and its anti-aromaticity lowered.
On the contrast, crystallization from solvents, which have a higher solvation energy for alkali cations,
lead to separated ion pairs in which the trop- anion is flatter and consequently, more anti-aromatic.
Moreover, Li+ shows a higher tendency to form contact ion pairs than the larger potassium cation with
a significantly reduced charge density. This culminates in the isolation of [K(thf) ][trop] , which forms
∞
2
a one-dimensional coordination polymer in the solid state with long K+ trop- distances. This compound
contains a flat trop- anion which shows very little variation of the C=C bond lengths, strongly shielded
NMR signals, strongly positive NICS values and as such, fulfils all formal criteria requested for an
anti-aromatic compound. In this light, the ease of synthesis from trop silyl ethers and alcoholates is
somewhat surprising, indicating that Mtrop salts are remarkably stable. The trop- monoanion can be
further reduced to give salts M2trop with a paramagnetic dianion radical, trop2-•, which was isolated
and fully characterized as K2trop. In this species, the anti-aromaticity is reduced compared to the
mono-anion. In a reaction between a Rh(I) halide complex and trop-, the complex [Rh
(trop)(cod)]
was prepared. Not unexpected, this pure organometallic 18 electron complex shows a very different
structure in which the trop unit—especially its annulated benzo groups—is aromatized. This reaction
also shows that trop anions may have potential as ligands for transition metal complexes and as
a building block for main group element compounds. It remains to be seen in how far trop-type anions
can be used as electron carriers related to the well-established use of arenes for that purpose [15].
Supplementary Materials: The following are available online: 1. DFT-Calculations; Table S1: Selected bond
lengths [Å] for the calculated trop mono- and dianions; Table S2: Key parameters for the determination of the
number of electrons; Figure S1: Bottom: Recorded (red) vs. calculated (blue) spectrum of Litrop in THF; Figure S2:
Bottom: Recorded (red) vs. calculated (blue) spectrum of Natrop in THF; Figure S3: Bottom: Recorded (red)
vs. calculated (blue) spectrum of Ktrop in THF; Figure S4: Stacked spectra of Litrop, Natrop and Ktrop in THF;
Figure S5: Bottom: Recorded (red) vs. calculated (blue) spectrum of K2trop in THF; Figure S6: Recorded (red)
vs. calculated (blue) spectrum of Na2dbcot in THF; Figure S7: Recorded (red) vs. calculated (blue) spectrum of
K2dbcot in THF; 2. Electrochemical data for Ktrop and trop2 on GC electrode; Figure S8: Scan rate dependence
of the 1st reduction process for Ktrop and trop2 in 0.1 M TBAPF6 solution in THF on glassy carbon electrode;
1-/2-
Figure S10: Overlay of the Ip currents vs. scan rate1/2 plot of Ktrop2-/3− and trop2
redox processes with
theoretical 1e- and 2e− plot slopes; Figure S9: Overlay of the successive CV scans for 1 mM of Ktrop solution in
THF (0.1 M TBAPF6) at 100 mVs−1; Figure S11: I vs. t−1/2 plot of the chronoamperometry data obtained from
a 10 mM trop2 solution at
3.07 V vs. Fc/Fc+ and a 10 mM ferrocene solution at 0.23 V vs. Fc/Fc+ in 0.1 M TBAPF6
−
in THF; Figure S12: Overlay of linear sweep voltammograms of a 10 mM trop2 solution and a 10 mM ferrocene in
a 0.1 M TBAPF6 solution in THF obtained in stationary regime at a carbon microelectrode (5 mVs-1 scan rate);
Figure S13: Overlay of the CVs recorded for Ktrop, trop2, and trop2 in presence of 10 mM KPF6; Figure S14:
Overlay of the cyclic voltammograms at various scan rates with 1 mM trop2 in 0.1 M TBAPF6 solution in THF on
glassy carbon electrode; Scheme S1: Selected interconversion reactions between the trop and the dbcot scaffold.
Author Contributions: Conceptualization, H.G., T.L.G., and V.M.; investigation, J.B, S.K., D.O., S.D.;
writing—original draft preparation, J.B. and H.G.; writing—review and editing, T.L.G., V.M., H.G.; supervision,
H.G.; funding acquisition, H.G. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Swiss National Science Foundation (SNF) through grant
number 2-77199-18.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the
study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to
publish the results.
References
1.
Thomaier, J.; Boulmaâz, S.; Schönberg, H.; Rüegger, H.; Currao, A.; Grützmacher, H.; Hillebrecht, H.;
Pritzkow, H. Dibenzotropylidene Phosphanes (TROPPs): Synthesis and Coinage Metal Complexes.
New J. Chem. 1998, 22, 947–958. [CrossRef]
2.
3.
Schönberg, H.; Boulmaâz, S.; Wörle, M.; Liesum, L.; Schweiger, A.; Grützmacher, H. A Monomeric
d9-Rhodium(0) Complex. Angew. Chem. Int. Ed. 1998, 37, 1423–1426. [CrossRef]
Ostendorf, D.; Landis, C.; Grützmacher, H. Trigonal Pyramids: Alternative Ground-State Structures for
Sixteen-Electron Complexes. Angew. Chem. Int. Ed. 2006, 45, 5169–5173. [CrossRef] [PubMed]

Molecules 2020, 25, 4742
21 of 23
4.
Lichtenberg, C.; Viciu, L.; Adelhardt, M.; Sutter, J.; Meyer, K.; de Bruin, B.; Grützmacher, H. Low-Valent
Iron(I) Amido Olefin Complexes as Promotors for Dehydrogenation Reactions. Angew. Chem. Int. Ed. 2015
54, 5766–5771. [CrossRef]
,
5.
6.
Grützmacher, H. Cooperating Ligands in Catalysis. Angew. Chem. Int. Ed. 2008, 47, 1814–1818. [CrossRef]
Maire, P.; Büttner, T.; Breher, F.; Le Floch, P.; Grützmacher, H. Heterolytic Splitting of Hydrogen with
Rhodium(I) Amides. Angew. Chem. Int. Ed. 2005, 44, 6318–6323. [CrossRef]
7.
8.
9.
Zweifel, T.; Naubron, J.-V.; Büttner, T.; Ott, T.; Grützmacher, H. Ethanol as Hydrogen Donor: Highly Efficient
Transfer Hydrogenations with Rhodium(I) Amides. Angew. Chem. Int. Ed. 2008, 47, 3245–3249. [CrossRef]
Zweifel, T.; Naubron, J.-V.; Grützmacher, H. Catalyzed Dehydrogenative Coupling of Primary Alcohols with
Water, Methanol, or Amines. Angew. Chem. Int. Ed. 2009, 48, 559–563. [CrossRef]
Gianetti, T.L.; Annen, S.P.; Santiso-Quinones, G.; Reiher, M.; Driess, M.; Grützmacher, H. Nitrous Oxide
as a Hydrogen Acceptor for the Dehydrogenative Coupling of Alcohols. Angew. Chem. Int. Ed. 2016
55, 1854–1858. [CrossRef] [PubMed]
,
10. Rodríguez-Lugo, R.E.; Trincado, M.; Vogt, M.; Tewes, F.; Santiso-Quinones, G.; Grützmacher, H.
A Homogeneous Transition Metal Complex for Clean Hydrogen Production From Methanol–Water Mixtures.
Nat. Chem. 2013, 5, 342. [CrossRef]
11. Trincado, M.; Sinha, V.; Rodriguez-Lugo, R.E.; Pribanic, B.; de Bruin, B.; Grützmacher, H. Homogeneously
Catalysed Conversion of Aqueous Formaldehyde to H2 and Carbonate. Nat. Commun. 2017, 8, 14990.
[CrossRef] [PubMed]
12. Sinha, V.; Pribanic, B.; de Bruin, B.; Trincado, M.; Grützmacher, H. Ligand- and Metal-Based Reactivity of a
Neutral Ruthenium Diolefin Diazadiene Complex: The Innocent, the Guilty and the Suspicious. Chem. Eur. J.
2018, 24, 5513–5521. [CrossRef] [PubMed]
13. Defieber, C.; Grützmacher, H.; Carreira, E.M. Chiral Olefins as Steering Ligands in Asymmetric Catalysis.
Angew. Chem. Int. Ed. 2008, 47, 4482–4502. [CrossRef] [PubMed]
14. Harder, S.; Martin, J.; Langer, J.; Wiesinger, M.; Elsen, H. Dibenzotropylidene Substituted Ligands for Early
Main Group Metal-Alkene Bonding. Eur. J. Inorg. Chem. 2020, 2020, 2582–2595. [CrossRef]
15. Bock, H.; Ruppert, K.; Näther, C.; Havlas, Z.; Herrmann, H.-F.; Arad, C.; Göbel, I.; John, A.; Meuret, J.;
Nick, S.; et al. Distorted Molecules: Perturbation Design, Preparation and Structures. Angew. Chem. Int. Ed.
1992, 31, 550–581. [CrossRef]
16. Bock, H.; Gharagozloo-Hubmann, K.; Sievert, M.; Prisner, T.; Havlas, Z. Single Crystals of an Ionic Anthracene
Aggregate with a Triplet Ground State. Nature 2000, 404, 267–269. [CrossRef]
17. Müllen, K. Reduction and Oxidation of Annulenes. Chem. Rev. 1984, 84, 603–646. [CrossRef]
18. Vos, H.W.; Bakker, Y.W.; Maclean, C.; Velthorst, N.H. Paramagnetic Ring Currents in the Carbanion of
5H-dibenzo[a,d]cycloheptene and the Nitranion of 5H-dibenz[b,f]azepine. Chem. Phys. Lett. 1974, 25, 80–83.
[CrossRef]
19. von Schleyer, P.R.; Jiao, H. What is Aromaticity? Pure Appl. Chem. 1996, 68, 209–218. [CrossRef]
20. McAuley, I.; Krogh, E.; Wan, P. Carbanion Intermediates in the Photodecarboxylation of Benzannelated
Acetic Acids in Aqueous Solution. J. Am. Chem. Soc. 1988, 110, 600–602. [CrossRef]
21. Krogh, E.;
Wan, P. Photodecarboxylation of Diarylacetic Acids in Aqueous Solution:
Electron Carbanions. J. Am. Chem. Soc. 1992
Enhanced Photogeneration of Cyclically Conjugated Eight
π
,
114, 705–712. [CrossRef]
22. Wan, P.; Krogh, E.; Chak, B. Enhanced Formation of 8π(4n) Conjugated Cyclic Carbanions in the Excited
State: First Example of Photochemical C–H Bond Heterolysis in Photoexcited Suberene. J. Am. Chem. Soc.
1988, 110, 4073–4074. [CrossRef]
23. Budac, D.; Wan, P. Excited-State Carbon Acids. Facile Benzylic Carbon-Hydrogen Bond Heterolysis of
Suberene on Photolysis in Aqueous Solution: A Photogenerated Cyclically Conjugated Eight
π Electron
Carbanion. J. Org. Chem. 1992, 57, 887–894. [CrossRef]
24. Bauld, N.L.; Brown, M.S. Dianion Radicals. II. Tropenide Systems. J. Am. Chem. Soc. 1967, 89, 5417–5421.
[CrossRef]
25. Ceccon, A.; Gambaro, A.; Pizzato, L.; Romanin, A.; Venzo, A. Metal Stabilized Carbanions. Kinetic Acidity
and 1H NMR Spectrum of the
Chem. Commun. 1982, 16, 907–908. [CrossRef]
π-(Tricarbonylchromium)-5H-dibenzo[a,d]cycloheptenyl Anion. J. Chem. Soc.

Molecules 2020, 25, 4742
22 of 23
26. Ceccon, A.; Gambaro, A.; Venzo, A. Metal-Stabilized Carbanions. J. Organomet. Chem. 1984, 275, 209–222.
[CrossRef]
27. Irngartinger,
H.;
Reibel,
W.R.K.
Structures
of
Dibenzo[a,e]cyclooctatetraene
and
Tetrabenzo[a,c,e,g]cyclooctatetraene (o-Tetraphenylene). Acta Cryst. B 1981, 37, 1724–1728. [CrossRef]
28. Katz, T.J.; Yoshida, M.; Siew, L.C. The sym-Dibenzcyclooctatetraene Anion Radical and Dianion. J. Am.
Chem. Soc. 1965, 87, 4516–4520. [CrossRef]
29. Kojima, H.; Bard, A.J.; Wong, H.N.C.; Sondheimer, F. Electrochemical Reduction of sym-
Dibenzocyclooctatetraene, sym-Dibenzo-1,5-cyclooctadiene-3,7-diyne, and sym-Dibenzo-1,3,5-cyclooctatrien
-7-yne. J. Am. Chem. Soc. 1976, 98, 5560–5565. [CrossRef]
30. Gerson, F.; Martin, W.B., Jr.; Plattner, G.; Sondheimer, F.; Wong, H.N.C. ESR. Spectra and Structures of Radical
Anions in the Dibenzo[a,e]cyclooctene Series. Helv. Chim. Acta 1976, 59, 2038–2048. [CrossRef]
31. Sygula, A.; Fronczek, F.R.; Rabideau, P.W. The First Example of η8 Coordination of Lithium Cations
with a Cyclooctatetraene Dianion: Crystal Structure of Li2(dibenzo[a,e]cyclooctatetraene)(TMEDA)2.
J. Organomet. Chem. 1996, 526, 389–391. [CrossRef]
32. Franck, G.; Brill, M.; Helmchen, G. Dibenzo[a,e]cyclooctene: Multi-gram Synthesis of a Bidentate Ligand.
Org. Synth. 2012, 89, 55–65.
33. Berti, G. Communications - Dibenzo[a,e]tropylium and 5-Phenyldibenzo[a,e]tropylium Cations. J. Org. Chem.
1957, 22, 230. [CrossRef]
34. Ball, R.G.; Edelmann, F.; Kiel, G.Y.; Takats, J.; Drews, R. Cycloheptatrienyl-Bridged Heterobimetallic
Complexes: Facile Phosphine Substitution Reactions of (µ-C7H7)Fe(CO)3Rh(CO)2. Organometallics
1986, 5, 829–839. [CrossRef]
35. Dickson, R.S.; Jenkins, S.M.; Skelton, B.W.; White, A.H. The Addition of Small Molecules to
-C5H5)2Rh2(CO)(CF3C2CF3)—IX. C–H Bond Activation in the Reactions with Dienes, Polyenes and
(η
Arenes; the Crystal and Molecular Structure of [(η-C5H5)2Rh2{C(CF3)=C(CF3)H}]2(C6H4). Polyhedron 1988,
7, 859–870. [CrossRef]
36. Oro, L.A.; Valderrama, M.; Cifuentes, P.; Foces-Foces, C.; Cano, F.H. Azulene as a Ligand in Cationic Rhodium
and Iridium Complexes. Crystal Structure of [Rh(TFB)(az)]PF6. J. Organomet. Chem. 1984, 276, 67–77.
[CrossRef]
37. Kunkely, H.; Vogler, A. Ligand-to-Ligand Charge Transfer in [(η (η
-C5Me5)RhIII -C7H7)]3+: Absorption and
Emission. Inorg. Chem. Commun. 2004, 7, 650–653. [CrossRef]
38. Perdew, J.P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett.
1996, 77, 3865, Erratum in 1997, 78, 1396–1396. [CrossRef]
39. Krishnan, R.; Binkley, J.S.; Seeger, R.; Pople, J.A. Self-Consistent Molecular Orbital Methods. XX. A Basis Set
for Correlated Wave Functions. J. Chem. Phys. 1980, 72, 650–654. [CrossRef]
40. von Schleyer, P.R.; Maerker, C.; Dransfeld, A.; Jiao, H.; van Eikema Hommes, N.J.R. Nucleus-Independent
Chemical Shifts: A Simple and Efficient Aromaticity Probe. J. Am. Chem. Soc. 1996, 118, 6317–6318.
[CrossRef]
41. Chen, Z.; Wannere, C.S.; Corminboeuf, C.; Puchta, R.; von Schleyer, P.R. Nucleus-Independent Chemical
Shifts (NICS) as an Aromaticity Criterion. Chem. Rev. 2005, 105, 3842–3888. [CrossRef] [PubMed]
42. Lichtenberg, C.; Prokopchuk, D.E.; Adelhardt, M.; Viciu, L.; Meyer, K.; Grützmacher, H. Reactivity of an
All-Ferrous Iron–Nitrogen Heterocubane under Reductive and Oxidative Conditions. Chem. Eur. J. 2015
21, 15797–15805. [CrossRef] [PubMed]
,
43. Zhong, H.M.; Rawal, V.H. 1,3-Pentadiene. In Encyclopedia of Reagents for Organic Synthesis; John Wiley & Sons,
Ltd.: New York, NY, USA, 2001.
44. Ernst, R.D. Structural and Reactivity Patterns in Transition-Metal-Pentadienyl Chemistry. Chem. Rev. 1988
88, 1255–1291. [CrossRef]
,
45. Cordoneanu, A.; Drewitt, M.J.; Bavarian, N.; Baird, M.C. Synthesis and Characterization of Weakly
Coordinating Anion Salts of a New, Stable Carbocationic Reagent, the Dibenzosuberenyl (Dibenzotropylium)
ion. New J. Chem. 2008, 32, 1890–1898. [CrossRef]
46. Wu, J.I.C.; Mo, Y.; Evangelista, F.A.; von Ragué Schleyer, P. Is Cyclobutadiene really highly Destabilized by
Antiaromaticity? Chem. Commun. 2012, 48, 8437–8439. [CrossRef]
47. Bahl, J.J.; Bates, R.B.; Beavers, W.A.; Launer, C.R. Cycloheptatrienyl and Heptatrienyl Trianions. J. Am.
Chem. Soc. 1977, 99, 6126–6127. [CrossRef]

Molecules 2020, 25, 4742
23 of 23
48. Connelly, N.G.; Geiger, W.E. Chemical Redox Agents for Organometallic Chemistry. Chem. Rev. 1996
96, 877–910. [CrossRef]
,
49. Salcedo, R.; Sansores, L.E.; Fomina, L. Theoretical Study about the Radicals and Anions of [8]annulenes.
J. Mol. Struct. 1997, 397, 159–166. [CrossRef]
50. Viculis, L.M.; Mack, J.J.; Mayer, O.M.; Hahn, H.T.; Kaner, R.B. Intercalation and Exfoliation Routes to Graphite
Nanoplatelets. J. Mater. Chem. 2005, 15, 974–978. [CrossRef]
51. Giordano, G.; Crabtree, R.H.; Heintz, R.M.; Forster, D.; Morris, D.E. Di-µ -
-Chloro-Bis(η4
1,5-Cyclooctadiene)-Dirhodium(I). In Inorganic Syntheses; Angelici, R.J., Ed.; John Wiley & Sons, Ltd.:
New York NY, USA, 2007.
52. IUPAC. Section A -Hydrocarbons. In Nomenclature of Organic Chemistry; Pergamon Press: Oxford, UK, 1979.
53. Dolomanov, O.V.; Bourhis, L.J.; Gildea, R.J.; Howard, J.A.K.; Puschmann, H. OLEX2: A Complete Structure
Solution, Refinement and Analysis Program. J. Appl. Crystallogr. 2009, 42, 339–341. [CrossRef]
54. Sheldrick, G. A Short History of SHELX. Acta Cryst. A 2008, 64, 112–122. [CrossRef] [PubMed]
55. Sheldrick, G. SHELXT-Integrated Space-Group and Crystal-Structure Determination. Acta Cryst. A 2015
71, 3–8. [CrossRef] [PubMed]
,
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