Complete charge separation provoked by full cation encapsulation in the radical mono- and di-anions of 5,6:11,12-di-: O -phenylene-tetracene

Article abstract of DOI:10.1039/c8dt01285g

Herein, we report the synthesis and molecular structure of the mono- and dianionic aromatic molecules [(B15C5-κ⁵O)₂K⁺](L_DOPT^-) (1) and [(B15C5-κ⁵O)₂K⁺]₂(L_DOPT^2-)THF_solv (2) derived from the parent aromatic polyhydrocarbon 5,6:11,12-di-o-phenylenetetracene (DOPT, L_DOPT) by a controlled stepwise one and two electron chemical reduction. The effect of single and double electron charge transfer to a polycondensed aromatic hydrocarbon (PAH) without any disturbing influence of an associated metal cation has been demonstrated. This was achieved by fully sandwiching the cationic K⁺ counterions between two benzo-15-crown-5-ether (B15C5) ligands resulting in a fully encapsulating (κ¹⁰O) geometry which ensures a complete separation of the K⁺ counterions and the bare anionic PAH species [L_DOPT^-] and [L_DOPT^2-]. The structural changes accompanied by the stepwise reduction from L_DOPT to [L_DOPT^-] to [L_DOPT^2-] are discussed and compared to earlier predictions based on density functional theory (DFT) as well as the results of previous studies of alkaline metal cationic PAH anion interactions of DOPT in which only a partial metal cation encapsulation has been achieved so far.

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Complete charge separation provoked by full
cation encapsulation in the radical mono- and
Cite this: DOI: 10.1039/c8dt01285g
di-anions of 5,6:11,12-di-o-phenylene-tetracene†
Tobias Wombacher, a Richard Goddard,b Christian W. Lehmannb and
a
Jörg J. Schneider
*
Herein, we report the synthesis and molecular structure of the mono- and dianionic aromatic molecules
[(B15C5-κ5O)2K+](LDOPT•−) (1) and [(B15C5-κ5O)2K+]2(LDOPT2−)THFsolv (2) derived from the parent aromatic
polyhydrocarbon 5,6:11,12-di-o-phenylenetetracene (DOPT, LDOPT) by a controlled stepwise one and two
electron chemical reduction. The effect of single and double electron charge transfer to a polycondensed
aromatic hydrocarbon (PAH) without any disturbing influence of an associated metal cation has been
demonstrated. This was achieved by fully sandwiching the cationic K+ counterions between two benzo-
15-crown-5-ether (B15C5) ligands resulting in a fully encapsulating (κ10O) geometry which ensures a
complete separation of the K+ counterions and the bare anionic PAH species [LDOPT•−] and [LDOPT2−]. The
structural changes accompanied by the stepwise reduction from LDOPT to [LDOPT•−] to [LDOPT2−] are dis-
cussed and compared to earlier predictions based on density functional theory (DFT) as well as the results
of previous studies of alkaline metal cationic PAH anion interactions of DOPT in which only a partial metal
cation encapsulation has been achieved so far.
Received 2nd April 2018,
Accepted 24th May 2018
DOI: 10.1039/c8dt01285g
rsc.li/dalton
crystal structures of the charged and neutral compounds rep-
resent the species detectable in solution.9 Recently, our group
Introduction
During the last decade, a variety of alternant polynuclear aro- has been interested in the isolation of crystalline PAH anions
matic hydrocarbon (PAH) species and derivatives thereof have of the newly accessible compound 5,6:11,12-di-o-phenylene-
been synthesized and studied in devices such as biosensors1,2 tetracene (DOPT, LDOPT) for which we have derived a scaled up
or transistors.2–4 Their remarkable capability of coordinating multigram method for its synthesis.10 This synthetic approach
transition metals onto the π-perimeter framework makes them has allowed us to further unravel its so far unknown material
interesting molecules for deployment in the field of catalysis properties4 and reaction behavior.11,12 In highly conjugated
or as building blocks in synthetic chemistry.5 Electronic pro- π-systems, any modification of the charge density (e.g. coordi-
cesses play an important role in both functional processes nation, oxidation or reduction) generally influences the
incorporating these in electronic devices4,6,7 and chemical pro- bonding situation and consequently affects the structural
cesses,8 whereby the electron redistribution and the reactivity shape of the resulting molecule compared to its parent neutral
of the compounds are strongly influenced by the changing state.13 The degree of perturbation essentially depends on the
electron density distribution along the π-perimeter. A compari- type of PAH and is most distinctive for (i) structurally flexible
son of structural data of various PAH systems in the solid state π-compounds exhibiting adjacent alkyl or phenyl groups, such
with experimental results obtained from NMR, ESR or UV-vis as 1,2-diphenylbenzene14 or rubrene15 and/or (ii) when an
measurements has revealed that in many cases the single intrinsic contortion of the aromatic framework occurs, as in
the case of half bowl-shaped corannulene16 or nanobelts.17
Since these factors represent feasible changes in shape,
aFachbereich Chemie, Eduard-Zintl-Institut für Anorganische und Physikalische
Chemie, Alarich-Weiss Str. 12, 64287 Darmstadt, Germany.
E-mail: joerg.schneider@ac.chemie.tu-darmstadt.de
bMax-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1,
45470 Mülheim an der Ruhr, Germany
they strongly contribute to the observed structural deformation
upon reduction.13,18 Despite the importance of redox changes
in the overall structure of charged PAHs, such distortions are
often solely discussed in terms of ion-pairs showing a strong
(iii) electrostatic interaction.19 In order to distinguish these
effects, the groups of Bock et al.20–22 and Petrukhina
et al.16,23–25 described solvent-separated PAH anions by
†Electronic supplementary information (ESI) available. CCDC 1820791 (1) and
1820792 (2). For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c8dt01285g
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masking the counter-ion partly with suitable open-chain poly- Synthesis and molecular structure of the potassium
ethers or encapsulating crown-ethers. Examples of structural compound [(B15C5-κ5O)2K+](LDOPT•−) (1)
changes in the mono and dianions of the PAHs decacyclene
[(B15C5-κ5O)2K+](LDOPT•−) (1) was obtained by a reaction of
(LDec), pentacene (LPent) and perylene (LPery) are revealed in the
LDOPT with one equivalent of potassium metal in dry THF at
molecular structures [(triglyme-κ4O)3Cs+](LDec•−),26 [(DME-
ambient temperature. The addition of two equivalents of dried
2− 26
κ2O)3Na+]2(LPent
)
and
[(diglyme-κ3O)2Na+]2(LPery2−).27
B15C5 yielded exclusively 1 as dark-green rhomboids in high
yield, suitable for X-ray analysis. Crystals of 1 are highly sensi-
tive to air and moisture which makes a stringent pre-drying of
all gases and solvents used absolutely necessary. So far, this
sensitivity has precluded any spectroscopic characterisation.
The solvent-separated ion-pair crystallizes in the triclinic space
Remarkably and to the best of our knowledge, no systematic
studies on the influence of electrostatic interactions in a
family of PAH anions in solvent-separated ion-pairs were
undertaken until our recent report12 in which we presented
dianionic structures featuring the DOPT dianion framework.
In this investigation, the significant and sole influence of the
counter cationic charge on the shape of the charged PAH was
exemplified in the solvent-separated naked ion pairs [(DME-
ˉ
group P1 (no. 2) with two formula units in the cell (Fig. 2 and
Table 1). Notably, 1 is one of the rare cases of a structurally
characterized PAH monoanion. PAH monoanions mainly
occur as half-solvated contact-ion-pairs. Among those are, for
example, the radical monoanions of 1,3,5-triphenylbenzene,34
κ2O)3Li+]2(LDOPT
) ) (4) and
(3), [(DME-κ2O)3Na+]2(LDOPT
2−
2−
[(diglyme-κ3O)2Na+]2(LDOPT2−) (5). With an increase in the size
of the alkaline metal cation and its surrounding polyether
shell, the distance between the countercharges increased and
deformation of the dianionic π-perimeter was progressively
naphthalene35,36
and
(9,10-diphenyl)anthracene,35,37,38
dibenzo-[a,e]pentalene,39 biphenylene,40 fluoranthene,41 pery-
lene,9 pyrene42 and rubrene,13 originating from the research
reduced going from 3 to 5. Nonetheless, a significant struc-
groups of Bock9,18,35,37,38,40,43–45 and Petrukhina13,16,46
.
2−
tural perturbation within [LDOPT
] was still observable,
accompanied by a significant deviation from the planar sym-
metry of the parent neutral DOPT. In completion of this work,
we now present our investigations into the fully naked PAH
anions of DOPT obtained by complete encapsulation and thus
shielding of the respective metal cation whereby a complete
suppression of anion/cation electrostatic interaction has been
brought about by strict separation of the individual
countercharges.
Results and discussion
The prototype of a naked anionic aromatic ligand is the free
cyclopentadienide (LCP) anion, which has been crystallized as
its
tetraethylammonium,
methyltriphenylphosphonium,
dimethyl-diphenylphosphonium and tetraethylphosphonium
salts,28 tetra-n-butylammonium salt,29 its tetraphenylphospho-
nium salt30 and its potassium salt [(15C5-κ5O)2K+](LCP−) by uti-
lizing the oxygen macrocycle 15-crown-5-ether (15C5) to encap-
sulate and isolate the potassium cation.31 In this work, we
have systematically decreased the electrostatic interaction
between the anion and cation ([LDOPTn•x−][xK+]; n = 0, 1; x =
1, 2) by coordinating the alkali metal to the benzo-oxygen
15-crown-5-ether oxygen macrocycle. We found that as a result
of the large distance between the anion and the cation, the
electrostatic interaction between the cationic countercharges is
almost completely suppressed, thus allowing the fully naked
[LDOPTn•x−] molecule to be isolated. Correlation of the crystallo-
graphic data of neutral and anionic DOPT with DFT calcu-
Fig. 1 (a) ORTEP47 plot of the molecular structure of [(B15C5-κ5O)2K+]
lations of their frontier orbitals provides insight into the (LDOPT•−) (1) drawn at the 50% probability level. (b) View of the unit cell
of 1. Each unit cell contains two molecules of [LDOPT•−] (one complete
molecule in the center and four fragments on the edges of the cell) and
two [(B15C5-κ5O)2K+] molecules. Within the [(B15C5-κ5O)2K+] molecule,
the 1,2-phenylene-groups of each B15C5 ligand adopt a staggered
arrangement around the K atom. Hydrogen atoms are omitted for
concept of complete charge separation within such ion
couples (see ESI†).32 Due to the strong affinity of the poly-O-
macrocycle benzo-15-crown-5-ether (B15C5) for K+ as shown
for the crystalline ion pair {[(B15C5-κ5O)2K+](I−)},33 we chose
this crown-ether to fully complex the metal.
clarity.
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Fig. 2 Illustration of the structural motifs found in compound 1. (a, c)
Full planarity is found for [LDOPT•−], (b) orientation of the encapsulated
K+ atop and below the central naphthacenic unit. Long-range distances
d(K⋯Cπ) (red) to the naphthacenic core are d(K⋯Cπ) = 625–640 pm for
C48–C49/C48’–C49’. (d) Space-filling model showing the dense
packing motif. The small inclination by ca. ∠13° of both encapsulating
B15C5 ligands within a [(B15C5-κ5O)2K+] unit is highlighted in light grey.
(e) Isolated [LDOPT•−] units adopt a herringbone type arrangement with
an inclination angle of ∠20° between I and II. Closest distances of the
charged π-perimeter to nearby counterions are given in blue: d(K⋯Cπ) =
582/585 pm for C51/C51’.
Fig. 3 (a) ORTEP47 plot of the molecular structure of [(B15C5-
κ5O)2K+]2(LDOPT2−)THFsolv (2) drawn at the 50% probability level showing
the major component of the slightly disordered THF molecule. (b) View
of the unit cell of 2. Each aromatic molecule of [LDOPT2−] is positioned
along the center of the (b–c) plane and sandwiched by two cationic
molecules of [(B15C5-κ5O)2K+] within the (a–b) plane. THF occupies free
Recently, Petrukhina et al. obtained the first solvent-separated
radical monoanions of bowl-shaped corannulene (LCora) in the
form
of
{[(18C6-κ6O)Na+](LCora•−)},16
{[(18C6-κ6O)(THF-
κ1O)2Na+](LCora•−)}16 and {[(DME-κ2O)3Na+](LCora•−)}46 and cor-
lattice positions between the separated ion-pairs along
a corridor
onene (LCoro) in {[(DME-κ2O)3Na+](LCoro•−)}.46
through the center of the (a–c) plane. The 1,2-phenylene groups of
B15C5 are staggered by 140°, deviating strongly from conformation
adopted by the radical monoanion 1: 50°; Δ = 90° vs. [(B15C5-κ5O)2K+]
I−:29 0°; Δ = 140°. Each unit cell contains one molecule of [LDOPT2−] (2 ×
1LDOPT2−), two molecules of [(B15C5-κ5O)2K+] (4 × 1[(B15C5-κ5O)2K+])
The crown-ether encapsulated potassium ions of 1 are
located above and below the central Cπ–Cπ-bond (C51–C51′) of
[LDOPT•−] with d(M⋯Cπ)1 = 582–640 pm (Fig. 2b). The comple-
tely planar geometry observed in the parent neutral DOPT
molecule is fully conserved in its radical monoanion (see
Fig. 2a and c).
2
2
and one THF molecule (2 × 21THF). Hydrogen atoms are omitted for
clarity.
Within the [(B15C5-κ5O)2K+] capsules, both B15C5 ligands
are tilted by ca. ∠13° (Fig. 2d) with respect to the 1,2-phenyl-
[(B15C5-κ5O)2K+]2(LDOPT2−)THFsolv (2), this time as highly air
sensitive deep red rhombic crystals. Notably, the pronounced
color change from blue via green to deep red (LDOPT → 1 → 2)
clearly reflects the ongoing reduction reaction, which is clearly
visible to the naked eye. 2 crystallizes in the triclinic space
ˉ
ene ring planes. A metal-to-oxygen distance of d(M–O)1 = 289
pm is observed for all oxygen atoms of the two polyether
ligands indicating that they contribute equally to the overall
shielding of the +1 charge. The individual anion molecules
[LDOPT•−] are oriented in a herringbone type arrangement with
an inclination angle of ∠20° between the mean planes of the
radicals (Fig. 2e). This motif results in the formation of infinite
ˉ
group P1 (no. 2) with one formula unit in the cell (Fig. 3 and
Table 1).
The dianionic π-perimeter [LDOPT2−] is as far as can be
ascertained planar with a negligible twist of the peripheric
phenylene rings (Pnperi) (Fig. 4a and c). Each [LDOPT2−] dianion
is accompanied by two [(B15C5-κ5O)2K+] capsules, one above
and one below the five membered ring positions of the major
stacks with the sequence [LDOPT•−]I⋯[(B15C5-κ5O)2K+]⋯
•−
[LDOPT
]
II
oriented in parallel along the diagonal of the unit
ˉ ˉ
cell along the [111] direction.
Synthesis and molecular structure of the dipotassium
component [87.7(3)%] with d(M⋯Cπ)2
= 646–670 pm.
compound [(B15C5-κ5O)2K+]2(LDOPT2−)THFsolv (2)
Presumably, owing to packing effects in the crystal lattice,
Employing the same procedure but using two equivalents of these distances are slightly larger than those observed for the
potassium metal followed by addition of four equivalents of radical monoanion 1 (d(M⋯Cπ)1 = 582–640 pm), whereas the
ˉ
ˉ
B15C5 affords the almost quantitative precipitation again as average metal to oxygen distance of d(M–O)2 = 289 pm = d(M–O)1
highly air and moisture sensitive solvent-separated ion-pair remains the same. This suggests that the shielding of the
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Fig. 5 (a) Characteristic structural details of solvent separated mole-
cular structures of all anionic DOPT containing [LDOPTn•x−] molecules
with n = 0, 1; x = 0, 1, 2. (†) A trend towards the planar fully naked DOPT
anion is observed using potassium as a reducing agent in combination
with two encapsulating B15C5 ligands in the (κ10O)-geometry. Metal to
carbon distance (d(M⋯Cπ)), median metal to oxygen distance (d¯(M–O))
and median diameter of the ligand shell (d¯(κyO); y = 6, 10) steadily
increase along the series Li+ < Na+ < K+ and from DME → diglyme →
B15C5 and from κ6O → κ10O. (b) The corresponding Rb+ and Cs+
Fig. 4 Detailed representation of the structural motifs found in 2.
(a, c) The plane of [LDOPT2−] is planar, (b) with both [(B15C5-κ5O)2K+] units
located above and below the centers of the five-membered rings with
d(K⋯Cπ) = 646–670 pm towards C41 (blue) and C48 (red). (d, e) The 1,2-
phenylene rings of each B15C5 ligand are inclined by only ca. ∠2° within
a [(B15C5-κ5O)2K+] unit. The stacking of both B15C5 ligands by 140°
results in an oval shape of the [(B15C5-κ5O)2K+] units and in the for-
mation of free cavities in the crystal lattice occupied by isolated solvent
molecules of THF (red). (e) Large surface area interaction between
cation and anion in 2.
compounds [((18C6-κ6O)Rb+)2-µ-(η6:η6-LDOPT2−)][((18C6-κ6O)Rb+)2-µ-
2−
(η4:η4-LDOPT2−)](THFsolv 2
)
(6)11 and [µ-(η4:η4-LDOPT 0.5((tetraglyme-κ4O)
)
2−
(tetraglyme-κ2O)Cs+)2-µ-(η2:η2-LDOPT
)
]
(7)11 are listed for compari-
0.5 n
son. Both exhibit d(M–Cπ) distances well below 360 pm and overall
longer d¯(M–O) distances. (‡) A similar deformation of the π-perimeter is
observed for the CsI compound 7 as for the solvent-separated NaI-
diglyme compound 5, but with a strongly different perturbation of the
electron distribution along the π-plane (see ESI†).
potassium ion is similar in 1 and 2. Within the [(B15C5-
κ5O)2K+] molecules, the centroids of the 1,2-phenylene groups
make a torsion angle about the centroids of the two pairs of O
atoms of 135°, resulting in an oval shape of the enclosing
ligand shell for 2, enabling a large surface area interaction
between the ions. THF molecules arranged along the [010]
direction fill the voids between the ion triads.
of the alkaline metal cations (Li+ → K+), the short metal to
carbon distances d(M⋯Cπ) increases steadily ranging from
535–577 pm for Li+ up to 646–670 pm for K+. This trend is also
The planar geometry that is observed in the parent neutral
DOPT is fully conserved in both anionic molecules [(B15C5-
κ5O)2K+](LDOPT•−) (1) and [(B15C5-κ5O)2K+]2(LDOPT2−)THFsolv (2).
Remarkably, this is in stark contrast to the previously
y
ˉ
mirrored in the increasing size of the ether ligand shell d(κ O)
distances (550 pm for Li+@DME → 725 pm with K+@B15C5).
Subsequently performed DFT calculations of the electrostatic
potential surfaces of the different alkaline metal capsules
confirm the results from the X-ray data (Fig. 6).
presented ion pairs [(DME-κ2O)3Li+]2(LDOPT2−) (3), [(DME-
2−
κ2O)3Na+]2(LDOPT
)
(4) and {[(diglyme-κ3O)2Na+]2(LDOPT2−)}
(5)12 (crystallographic data for DOPT and compounds 1–512 are
The molecular surface electrostatic potentials were all calcu-
lated at the B3LYP/6-31G** level using 0.002 e au−3 as the
contour of the electron density. Median values of the mole-
cular surface electrostatic potentials were taken from 10 points
given in Table 1 for comparison).
Structural comparison to recently synthesized compounds
within the alkali metal series of DOPT anions
along the periphery and then averaged to Vs(r).48,49 Indeed, the
ˉ
A detailed summary of the extent of the π-deformation in the predicted electrostatic potential along the periphery of the
alkali metal salts of DOPT (Li+ → Cs+) known so far is given in alkaline metal capsules is significantly diminished in going
Fig. 5. For completeness, this also includes the recently from [(DME-κ2O)3Li+] in 3 to [(B15C5-κ5O)2K+] in 1 and 2, in
reported heavier congeners containing Rb+ and Cs+ [((18C6- accordance with the increased softness of K+ compared to Li+.
κ6O)Rb+)2-µ-(η6:η6-LDOPT2−)][((18C6-κ6O)Rb+)2-µ-(η4:η4-LDOPT2−)]
Thus, the observed perturbation of the carbon skeleton
(6)11 and [µ-(η4:η4-LDOPT
)
0.5((tetraglyme-κ4O)(tetra- decreases to a minimum for 1 and 2 containing K+ (see † in
2−
(THFsolv 2
)
2−
glyme-κ2O)Cs+)2-µ-(η2:η2-LDOPT
)
]
(7).11 A comparison of Fig. 5a). This trend is also reflected in the average metal-to-
0.5 n
ˉ
the structural parameters of all the so far structurally charac- oxygen distance d(M–O) which increases from 214 pm to 289
terized solvent-separated ion-pairs of DOPT (1–5) reveals pm and correlates with the increasing size and softness of the
different trends, which substantiate the view that there is a potassium ion (r(Li+) = 76 pm ≪ r(K+) = 138 pm (ref. 51)).
maximum in charge separation for 1 and 2 compared to the However, it is worth noting that the corresponding solvent
dianions 3–5 (Fig. 5a). Increasing size and electronic softness shared ion-pairs containing Rb+ and Cs+ in 6 and 7 exhibit
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only a slight deformation of the [LDOPT2−] carbon skeletons
(Fig. 6b). Nonetheless, a strong electronic exchange between
the countercharges predominantly determines the local elec-
tron concentration on the charged [LDOPT2−] ligand of these
solvent-shared ion-pairs. To the best of our knowledge, 1 and 2
can be regarded as the first examples of a PAH containing fully
naked anions.
Despite slight disorder of one of the [LDOPT•−] radical
anions [11.5(7)%] in the crystal structure of 1 and slight dis-
order of the [LDOPT2−] dianion in 2 [12.3(3)%], both anionic
units of [LDOPT•−] in 1 and [LDOPT2−] in 2 exhibit structural
changes Δdc–c (pm) upon reduction when compared to the
parent neutral state (Fig. 5). Fig. 7 summarizes the observed
changes Δdc–c (pm) in the geometries of [LDOPT•−] in 1 and
[LDOPT2−] in 2 upon electron consumption compared to the
parent neutral state (Fig. 7).
Reduction of DOPT to [LDOPT•−] and subsequently to
[LDOPT2−] transfers one, and two electrons respectively into the
LUMO of DOPT while keeping the planarity of the π-system
intact (Fig. 7a and b).4 As a consequence, a major increase in
Fig. 7 Schematic overview of the structural changes occurring by one
electron reduction of DOPT. Its occupied molecular orbital (MO) is
depicted in transparent mode (left: HOMO of LDOPT; middle and right:
LUMO of DOPT; calculated at the B3LYP/6-31G** level4,52). The thicker
Cπ–Cπ bonds represent increased electron density in the bonding orbi-
tals i.e., decreasing bond length values and vice versa (red and blue
color decode for regions of ψ2 of opposite sign). All MOs are plotted at
the 90% probability level of the corresponding MO electron density (ψ2).
The encircled position denotes the area of pronounced change and
major electron concentration in [LDOPT•−] and [LDOPT2−]. (b) Deviations
Δdc–c values in pm are given for [LDOPT•−] (top value†) and [LDOPT2−] (in
bold‡) compared to LDOPT (C2/c polymorph4) together with the areas of
decreasing (δ+) and increasing (δ−) electron density; ψ2 is illustrated
exclusively atop the π-plane for clarity.
electron density can be proposed for the isolated mono- and
dianions of LDOPT on the 5-membered ring positions (denoted
as I) as well as on the tetracenic phenylenes (denoted as II,
Fig. 7). A drastic decrease in bond lengths is observed
Fig. 6 Electrostatic potential surfaces (kJ mol−1
) calculated at the
B3LYP/6-31G** level of (a) the alkali metal capsules [(DME-κ2O)3Li+] of
3, [(DME-κ2O)3Na+] of 4, [(diglyme-κ3O)2Na+] of 5 and [(B15C5-κ5O)2K+] (denoted as δ−, Fig. 7b), matching the calculated positions of
of 1 and 2, (b) the bare metal ions and (c) its hypothetical metal-free
up to −4.1|−4.4 pm for the peripheric Cπ–Cπ distances (a,
Fig. 7b) of the five membered rings. For the tetracenic ring
state, together with the median values of the molecular surface electro-
static potential (V¯s(r)) (red = low/negative potential, blue = high/positive
system, Pntet, a maximum of −1.2|−2.2 pm for the peripheric
inner Cπ–Cπ bonds is observed (b, Fig. 7b). In contrast, an
potential according to Feldmann50). The differences in the electrostatic
potential for the corresponding ligand capsules (V¯s(r)a) are given for
comparison with the free ions (V¯s(r)I) and the empty polyether shell increase (denoted as δ+, Fig. 7b) by +4.1 pm in bond lengths
(V¯s(r)II), respectively. A decrease in V¯s(r) is observed towards the (κ10O)-
geometry in compounds 1 and 2. Notably, the major impact on the
diminished electrostatic potential in [(B15C5-κ5O)2K+] arises from the
increasing size of the crown-ether capsule and the decreasing potential
of the encapsulated ion. Starting coordinates for the calculations were
taken from the corresponding crystal structure data of 1–5.
when going from [LDOPT•−] in 1 to [LDOPT2−] in 2 is observed
for the inner Cπ–Cπ bonds of Pnperi (I) (i, Fig. 7b) and the outer
periphery of Pntet (II) by +3.6|+4.0 pm (h, Fig. 7b). The periphe-
ric tetracene positions (g, Fig. 7b) are altered by a maximum of
+3.5|+3.7 pm and the inner naphthalenic bonds by ca.
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+1.1|+2.9 pm (f, Fig. 7b). The outer peripheric phenylene degassed perfluorinated polyether. Structures were solved by
bonds (I) are affected to a lesser extent. As a consequence, direct methods (SHELXS) and refined by full-matrix least-
these bonds mainly increase by values of up to +2.3 pm. squares (SHELXL).57 Graphics of the molecules and unit cells
Twofold reduction of DOPT to the dianion [LDOPT2−] in 2 were generated with the latest version of PLATON58 and
(Fig. 7a, right) provokes an even more pronounced elongation, Diamond59 software. All atomic displacement ellipsoids of the
and contraction of the C–C bonds compared to its monoanion ORTEP47 plots were drawn at the 50% probability level.
counterpart [LDOPT•−] in 1 respectively (Fig. 7a, middle).
Additional graphic details are given in the ESI.†
Due to the proven potent carcinogenic activity of some
polyarenes, an uncontrolled release of PAHs constitutes a
serious environmental contamination which is potentially
hazardous for health.60 Thus, handling and synthesis of PAHs
Experimental details
All syntheses were performed under an inert gas atmosphere has to be done with great care and any exposure should be pre-
of pretreated high purity argon (4.8, 99.998%) piped through a cluded. In general, the use of any silicon based grease should
4-column series arrangement filled with: (a) activated charcoal; be avoided due to the proven chance of irreversible incorpor-
(b) an activated BTS-copper catalyst (Sigma-Aldrich®); (c) acti- ation of (H3C)2Si groups into the π-perimeter via reductive
vated molecular sieves, 3 Å; (d) SICAPENT® with an indicator reaction with an alkaline metal.40 Nonetheless, we never
(Merck). Hygroscopic benzo-15-crown-5-ether (B15C5) was syn- observed any competing reaction for DOPT during synthesis or
thesized from diethylene glycol dimethanesulfonate (DEGMS) significant disturbance of the reaction solution impeding
with a modification of the literature protocol.53,54 (*) For appli- crystal growth. The distillation of technical THF over active
cation purpose, B15C5 was dissolved in dry THF (B15C5: solvent-drying agents (e.g., Na/K alloy, NaH, Na/benzophe-
2.38 g in 40.0 mL; 0.222 mol L−1), degassed and stored static none) is mandatory owing to the competing formation of
over activated molecular sieves (3 Å) for at least 1 week.
adducts with the admixed stabilizer 2,6-di(tert-butyl)-4-methyl-
DOPT was synthesized as reported earlier by our group and phenol, as has been shown well by Bock et al.55
was subsequently recrystallized from a hot THF solution.4,10
Note: The very limited solubility of the compounds (e.g., in
All other chemicals were purchased and used as received. 1,2- THF) together with their extremely high air and temperature
Benzenediol (≥99%), cesium carbonate (99.5%), triethylamine sensitivity prevented their full spectroscopic characterization.
(99%) and methanesulfonylchloride (99.50%) were obtained
from Acros Organics; potassium (98%) from Alfa Aesar and di-
ethylene glycol (≥99%) from Carl Roth. Tetrahydrofuran (THF),
Diethylene glycol dimethanesulfonate (DEGMS)
N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), Triethylamine (43.9 mL, 32.0 g, 316 mmol; 1.5 eq.) was added
1,4-dioxane, acetonitrile and dichloromethane were of techni- to a mixture of diethylene glycol (20 mL, 22.4 g, 211 mmol;
cal grade and predried (1,4-dioxane and THF:55 distilled over 1 eq.) in CH2Cl2 (1.06 L; 0.2 M) at 0 °C. Methanesulfonylchloride
Na/benzophenone; DMF and DMSO: distilled over CaH2; and (35.9 mL, 53.2 g, 464 mmol; 2.2 eq.) was added in small por-
acetonitrile and CH2Cl2: distilled over P4O10), degassed and tions within 10 min and the reaction mixture was allowed to
stored static over activated molecular sieves (3 Å/4 Å (CH2Cl2, warm up to room temperature. After extraction with iced water
DMSO)) prior to use. Pyridine was purchased from Merck (2 × 400 mL), aqueous HCl (10%, 2 × 400 mL), saturated
(dried, SeccoSolv®, ≥99.9%). Deuterated THF-d8 (99.50%) was Na2CO3 (2 × 300 mL), brine (1 × 300 mL) and water, the
purchased from Euriso-top®, dried on a potassium mirror organic layer was separated and dried with MgSO4. The solvent
under argon and then vacuum transferred.
was removed in vacuo and the oily residue so obtained was soli-
Deuterated CHCl3-d1 (99.8%) was purchased from Aldrich dified at −30 °C overnight. Subsequent recrystallization from
and used as received. Celite® was purchased from Sigma-Aldrich CH2Cl2 yielded pure and slightly hygroscopic dimesylate
and predried at 300 °C in under high vacuum (<10−3 mbar) for DEGMS (26.0 g, 99.1 mmol; 47% yield) as colorless prismatic
2 h prior to use. Suitable crystals for structural analysis were crystals suitable for X-ray structural analysis. Atomic mass cal-
obtained from a saturated THF solution under argon at −30 °C culated for C6H14S2O7: 262.29 u; MS (70 eV): m/z (%): (EI)
in sealed H-shaped flasks (see ESI†). DEGMS was crystallized 263(1) [M+], 153(25) [M+ − C2H5O3S], 123(100) [C3H7SO3+],
+
from CH2Cl2 at ambient temperature.
79(55) [CH3SO2 ]; analysis calculated for C6H14S2O7: C, 27.45;
The X-ray diffraction data for DEGMS were collected on a H, 5.30. Found: C, 27.62; H 5.24. IR (ATR): ˜ν = 3030 (m), 2939 +
STOE STADI IV 4-circle single-crystal diffractometer at ambient 2888 + 2832 (w; ν(Calkyl–H)), 1459 (m), 1330 (s; ν(CH3–SO2–
temperature. The crystal structure of DEGMS is reported in a OR)), 1169 (s; ν(CH3–SO2–OR)), 1134 + 1115 + 1073 (s + m + m;
private communication56 in the Cambridge Structural ν(C–O–C)), 1014 (s), 974 (s), 935 (s), 914 (s), 810 (s), 733 (m),
Database, Refcode: ZIVRON; CCDC 968551. The X-ray diffrac- 555 (w), 525 (s), 480 (w), 460 (w), 447 (w) cm−1 1H-NMR
.
tion data for the fully naked anions 1 and 2 were collected on, (500 MHz, CHCl3-d1, 25 °C): δ = 4.34 (t, 3J(H2,H3) = 4.5 Hz,
3
respectively, a Bruker AXS X8 Proteum diffractometer equipped H2/H2′), 3.76 (dd, J(H3,H2) = 4.5 Hz, 4H, H3/H3′), 3.00 (s, 6H,
with a Nonius FR591 copper rotating anode and a Bruker AXS H1/H1′) ppm. 13C-NMR (500 MHz, CHCl3-d1, 25 °C): δ = 69.02
Mach3 equipped with an Incoatec IµS X-ray source at 100(2) (CH2, 2C, C3/C3′), 68.88 (CH2, 2C, C2/C2′), 37.46 (CH3, 2C,
K. The crystals were mounted on a MiTiGen loop using C1/C1′) ppm.
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Benzo-15-crown-5-ether (B15C5)
the dianionic state [LDOPT2−] of DOPT. The encapsulating
agent B15C5 (285 mg, 1.06 mmol; 4 eq. in 4.8 mL THF) was
injected under vigorous stirring accompanied by a slight
change in color from deep-red to light-red. Finally, the reaction
flask was stored free of vibration at −30 °C for several days.
Red rhombic crystals suitable for X-ray structural analysis were
separated from the pale-red solution, indicative of an almost
quantitative formation of the solvent-separated ion triad 2.
Dry cesium carbonate (36.69 g, 112.6 mmol; 2.5 eq.) was
added to 1,2-benzenediol (4.96 g, 45.0 mmol; 1 eq.) in
degassed acetonitrile (1.0 L; 0.045 M), heated to reflux and
stirred for 3 h. After cooling down to ambient temperature, a
solution of the as-prepared DEGMS (11.80 g, 45.0 mmol; 1 eq.)
in degassed acetonitrile (0.5 L; 0.09 M) was added in small por-
tions (3 mL h−1). Subsequently, the mixture was refluxed for
48 h, cooled down and filtered over predried Celite®. The fil-
tration residue was washed three times with additional CH2Cl2
(each 300 mL) and the collected organic phase was concen- Conclusions
trated in vacuo. The resulting residue was redissolved in fresh
The structural results of the radical monoanion {[(B15C5-
CH2Cl2 and extracted with diluted aqueous HCl (1 M; 2 ×
30 mL), brine (2 × 30 mL) and water (2 × 30 mL). The organic
phase was dried with MgSO4 and the solvent was removed
in vacuo. Subsequent recrystallization from dry CH2Cl2 under
argon led to the precipitation of pure B15C5 as white hygro-
scopic needles (7.10 g, 26.5 mmol; 59% yield). IR (ATR): ˜ν =
3058 (w; ν(Caryl–H)), 2940 + 2919 + 2856 (s; ν(Calkyl–H)), 1591
(m), 1505 (s), 1450 (s), 1409 (w), 1360 (w), 1333 (m), 1297 (m),
1251 + 1228 (s; ν(Caryl–O–C)), 1120 + 1091 + 1075 (s + s + m;
ν(C–O–C)), 1041 (s; ν(Caryl–O–C)), 980 (w), 931 (s), 907 (m), 849
(m), 819 (w), 780 (m), 738 (s; 1,2-disubst.), 601 (w), 537 (w), 514
κ5O)2K+](LDOPT•−)} (1) (Fig. 1) and the dianion {[(B15C5-
κ5O)2K+]2(LDOPT2−)}THFsolv (2) (Fig. 3) of 5,6:11,12-di-o-phenyle-
netetracene (DOPT, LDOPT; Fig. 7) extended our work on the
reduction behavior of the newly accessible polycyclic aromatic
hydrocarbon DOPT. Reduction of DOPT with elemental
potassium completed our studies on the systematic influence
of the hard- and softness of the employed reducing agent (M =
Li, Na, K) on the shape of a charged carbon skeleton in the
solvent-separated ion-pairs of this ligand. By using the poly-O-
macrocycle, benzo-15-crown-5-ether (B15C5) full cation/anion
charge separation has been achieved, whereby encapsulation
of the K+ into the more rigid cage of [(B15C5-κ5O)2K+] results
in a (κ10O) coordination geometry. With respect to the recently
reported dianions of DOPT, containing separated (κ6O)-cap-
sules of [(DME-κ2O)3Li+], [(DME-κ2O)3Na+] and [(diglyme-
κ3O)2Na+], the full sandwich coordination in [(B15C5-κ5O)2K+]
described herein proves an exceptionally high donor capability
of the DOPT π-perimeter which becomes possible under full
charge separation. For all (κ6O)-geometries observed, the struc-
tural perturbation of the planar DOPT is distinct and can be
attributed to electrostatic long range forces between the separ-
ated opposite charges (Fig. 6).
1
(w), 468 (w) cm−1. H-NMR (500 MHz, DMSO-d6, 25 °C) = 6.96
(dd, 2H, H2/H2′), 6.89 (dd, 2H, H1/H1′), 4.04 (t, 4H, H3/H3′),
3.78 (t, 4H, H4/H4′), 3.64 (t, 8H, H5/H5′/H6/H6′) ppm.
13C-NMR (500 MHz, DMSO-d6, 25 °C) = 148.70 (Cq, 2C, C2a),
121.23 (CH, 2C, C1/C1′), 114.19 (CH, 2C, C2/C2′), 70.44 (CH2,
2C, C6/C6′), 70.31 (CH2, 2C, C5/C5′), 69.04 (CH2, 2C, C4/C4′),
68.90 (CH2, 2C, C3/C3′) ppm.
Potassium bis(benzo-15-crown-5-ether) {[(B15C5-κ5O)2K+]
(LDOPT•−)} (1)
A solution of DOPT (100 mg, 0.266 mmol; 1 eq.) in dry THF
(20 mL) was reduced on an excess of finely vacuum-deposited
potassium mirror (104 mg, 2.66 mmol; 10 eq.) for 2 hours at
0 °C. Ultrasonication supported the conversion. The reaction
progress was accompanied by a quick color change from deep-
blue to deep-red indicating the formation of the dianionic
state [LDOPT2−] of DOPT. The solution was filtered under argon
over a thin layer of predried Celite® to remove finely dispersed
potassium particles which are proven to be incompatible with
the crown-ether.61–63 An equimolar solution of additional
neutral DOPT (100 mg, 0.266 mmol) was transferred to the fil-
trate. The reaction mixture was stirred for 1 h to ensure equili-
bration to the deep-green radical monoanionic state [LDOPT•−].
Addition of the predried solution (*) of B15C5 (143 mg,
0.53 mmol; 2 eq.) in THF (2.4 mL; 0.222 mol L−1 in THF), and
storage at −30 °C for 7 days yielded dark-green rhombic crys-
tals suitable for X-ray structural analysis.
Conflicts of interest
There are no conflicts to declare.
Notes and references
1 J. E. Anthony, Angew. Chem., Int. Ed., 2008, 47, 452–483.
2 L. Torsi, M. Magliulo, K. Manoli and G. Palazzo, Chem. Soc.
Rev., 2013, 42, 8612–8628.
3 G. Dai, J. Chang, W. Zhang, S. Bai, K.-W. Huang, J. Xu and
C. Chi, Chem. Commun., 2015, 51, 503–506.
4 T. Wombacher, A. Gassmann, S. Foro, H. von Seggern and
J. J. Schneider, Angew. Chem., Int. Ed., 2016, 55, 6041–6046.
5 Z. Xu, Coord. Chem. Rev., 2006, 250, 2745–2757.
6 M. M. Ahmida and S. H. Eichhorn, ECS Trans., 2010, 25, 1–
10.
Di-potassium bis(benzo-15-crown-5-ether) {[(B15C5-
κ5O)2K+]2(LDOPT2−)} (2)
Analogous to the above procedure, addition of the B15C5-solu-
tion was carried out directly to the filtered deep-red solution of
7 L. Muccioli, C. Zannoni and D. Beljonne, Chem. Mater.,
2011, 23, 591–609.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Dalton Transactions
Paper
8 C. Melero, R. P. Herrera, A. Guijarro and M. Yus, Chem. – 33 P. R. Mallinson and M. R. Truter, J. Chem. Soc., Perkin
Eur. J., 2007, 13, 10096–10107. Trans. 2, 1972, 12, 1818–1823.
9 C. Näther, H. Bock, Z. Havlas and T. Hauck, 34 S. Krieck, H. Görls and M. Westerhausen, Organometallics,
Organometallics, 1998, 17, 4707–4715. 2010, 29, 6790–6800.
10 T. Wombacher, S. Foro and J. J. Schneider, Eur. J. Org. 35 H. Bock, C. Arad, C. Nather and Z. Havlas, J. Chem. Soc.,
Chem., 2016, 569–578. Chem. Commun., 1995, 2393–2394.
11 T. Wombacher, R. Goddard, C. W. Lehmann and 36 T. Scott, B. Ooro, D. J. Collins, M. Shatruk, A. Yakovenko,
J. J. Schneider, Chem. Commun., 2017, 53, 7030–7033.
12 T. Wombacher, R. Goddard, C. W. Lehmann and
J. J. Schneider, Dalton Trans., 2017, 46, 14122–14129.
K. R. Dunbar and H.-C. Zhou, Chem. Commun., 2009, 65–
67.
37 H. Bock, K. Gharagozloo-Hubmann, M. Sievert and
T. Prisner, Nat., 2000, 404, 267–269.
13 A. V. Zabula, N. J. Sumner, A. S. Filatov, S. N. Spisak,
V. M. Grigoryants and M. A. Petrukhina, Eur. J. Inorg. 38 H. Bock, A. John, C. Näther, Z. Havlas and E. Mihokova,
Chem., 2012, 4675–4683. Helv. Chim. Acta, 1994, 77, 41–50.
14 H. Bock, Z. Havlas, K. Gharagozloo-Hubmann, 39 M. Saito, Y. Hashimoto, T. Tajima, K. Ishimura, S. Nagase
S. Holl and M. Sievert, Angew. Chem., Int. Ed., 2003, 42,
4385–4389.
15 A. S. Paraskar, A. R. Reddy, A. Patra, Y. H. Wijsboom,
and M. Minoura, Chem. – Asian J., 2012, 7, 480–483.
40 H. Bock, M. Sievert, C. L. Bogdan and B. O. Kolbesen,
Organometallics, 1999, 18, 2387–2389.
O. Gidron, L. J. W. Shimon, G. Leitus and M. Bendikov, 41 I. L. Fedushkin, A. N. Lukoyanov, S. Dechert and
Chem. – Eur. J., 2008, 14, 10639–10647. H. Schumann, Eur. J. Inorg. Chem., 2004, 2421–2424.
16 A. V. Zabula, S. N. Spisak, A. S. Filatov, V. M. Grigoryants 42 W. Jost, M. Adam, V. Enkelmann and K. Müllen, Angew.
and M. A. Petrukhina, Chem. – Eur. J., 2012, 18, 6476–
Chem., 1992, 104, 883–884.
6484.
43 H. Bock, Nachr. Chem., 2001, 49, 18–21.
17 A. V. Zabula, A. S. Filatov, J. Xia, R. Jasti and 44 N. L. Nock, D. Tang, A. Rundle, C. Neslund-Dudas,
M. A. Petrukhina, Angew. Chem., Int. Ed., 2013, 52, 5033–
5036.
18 H. Bock, S. Holl and M. Sievert, Z. Naturforsch., B: Chem.
Sci., 2000, 55, 1163–1178.
19 D. A. Walker, B. Kowalczyk, O. De and B. A. Grzybowski,
Nanoscale, 2011, 3, 1316–1344.
20 H. Bock, R. F. C. Claridge, C. Bogdan, M. Sievert and
V. Krenzel, Helv. Chim. Acta, 2001, 84, 1227–1242.
21 H. Bock, M. Ansari, N. Nagel and R. F. C. Claridge,
J. Organomet. Chem., 1996, 521, 51–63.
A. T. Savera, C. H. Bock, K. G. Monaghan, A. Koprowski,
N. Mitrache, J. J. Yang and B. A. Rybicki, Cancer Epidemiol.
Biomarkers Prev., 2007, 16, 1236–1245.
45 H. Bock, K. Ruppert, C. Nather, Z. Havlas, H. Herrmann,
C. Arad, I. Gobel, A. John, J. Meuret, S. Nick,
A. Rauschenbach, W. Seitz and T. Vaupel, Angew. Chem.,
1992, 104, 564–595.
46 A. S. Filatov, N. J. Sumner, S. N. Spisak, A. V. Zabula,
A. Y. Rogachev and M. A. Petrukhina, Chem. – Eur. J., 2012,
18, 15753–15760.
22 H. Bock, M. Ansari, N. Nagel and R. F. C. Claridge, 47 C. K. Johnson, Oak Ridge National Laboratory, Report
J. Organomet. Chem., 1995, 501, 53–60. ORNL-5138, O, 1976.
23 A. V. Zabula, S. N. Spisak, A. S. Filatov, A. Y. Rogachev, 48 M. Bartošková and Z. Friedl, Cent. Eur. J. Energ. Mater.,
R. Clérac and M. A. Petrukhina, Chem. Sci., 2016, 7, 1954–
1961.
24 S. N. Spisak, A. V. Zabula, M. V. Ferguson, A. S. Filatov and
M. A. Petrukhina, Organometallics, 2013, 32, 538–543.
25 S. N. Spisak, N. J. Sumner, A. V. Zabula, A. S. Filatov,
2013, 10, 103–112.
49 R. F. W. Bader, M. T. Carroll, J. R. Cheeseman and
C. Chang, J. Am. Chem. Soc., 1987, 109(26), 7968–7979.
50 R. J. Feldmann, D. H. Bingt, B. C. Furief and B. Furief, Proc.
Natl. Acad. Sci. U. S. A., 1978, 75, 5409–5412.
A. Y. Rogachev and M. A. Petrukhina, Organometallics, 51 R. D. Shannon, Acta Crystallogr., 1976, 32, 751–767.
2013, 32, 3773–3779.
26 K. Gharagozloo-Hubmann, Johann Wolfgang Goethe-
Universität Frankfurt am Main, 2000.
27 H. Bock, C. Nather and Z. Havlaslc, J. Am. Chem. Soc., 1995,
117, 3869–3870.
28 H. Sjoerd, Chem. – Eur. J., 1999, 5, 1852–1861.
52 C. M. Murata, Y. Sugano, A. Wakamiya and Y. Murata,
Angew. Chem., Int. Ed., 2015, 54, 9308–9312.
53 R. E. Hanes, C. Lee, S. N. Ivy, A. Palka and R. A. Bartsch,
Arch. Org. Chem., 2010, 7, 238–248.
54 J. S. Bradshaw and P. E. Stott, Heterocycles, 1981, 75, 179–
182.
29 M. T. Reetz, S. Hutte, R. Goddard and U. Minet, J. Chem. 55 H. Bock, R. Dienelt, C. Näther and Z. Havlas, Chem. Ber.,
Soc., Chem. Commun., 1995, 275–277. 1997, 130, 1533–1537.
30 S. Harder, M. H. Prosenc and U. Rief, Organometallics, 56 J. W. Nail, R. D. Gandour and F. R. Fronczek, 2014.
1996, 15, 118–122.
31 M. L. Cole, C. Jones and P. C. Junk, J. Chem. Soc., Dalton
Trans., 2002, 896–905.
32 D. Eisenberg and R. Shenhar, Wiley Interdiscip. Rev.:
Comput. Mol. Sci., 2012, 2, 525–547.
57 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found.
Crystallogr., 2008, 112–122.
58 A. L. Spek, Acta Crystallogr., Sect. D: Biol. Crystallogr., 2009,
65, 148–155.
59 W. T. Pennington, J. Appl. Crystallogr., 1999, 32, 1028–1029.
This journal is © The Royal Society of Chemistry 2018
Dalton Trans.

View Article Online
Paper
Dalton Transactions
60 C. A. Menzie, B. B. Potocki and J. Santodonato, Environ. Sci. 62 Z. Grobelny, A. Stolarzewicz and A. Maercker, J. Organomet.
Technol., 1992, 26, 1278–1284. Chem., 2001, 628, 65–70.
61 Z. Grobelny, A. Stolarzewicz, M. Szczepanski and M. Sokol, 63 Z. Grobelny, A. Stolarzewicz, M. Sokol, J. Grobelny and
Curr. Org. Chem., 2008, 12, 1040–1049. H. Janeczek, J. Phys. Chem., 1992, 96, 5193–5196.
Dalton Trans.
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