Face-to-face stacking of dianionic quinoid rings in crystals of alkali salts of 2,5-dihydroxyquinone in view of π-system polarization

Article abstract of DOI:10.1039/c2ce26119g

A series of alkali (M⁺ = K⁺, NH₄ ⁺, Rb⁺ and Cs⁺) salts of 2,5-dihydroxyquinone [M₂⁺ (C₆H₄O₄) ^2-] was prepared and their structures were determined by X-ray structure analysis. A less-common type of π-stacking of dianionic quinoid rings in face-to-face fashion (no offset) is observed. However, in the potassium salt of this series the stacking motif of the dianionic quinoid rings is in a herring-bone pattern, usually observed in the stacking of aromatics. The different types of crystal structures, induced by varied cations, are discussed in terms of the relative energies calculated by the DFT method. The structural features observed were reproduced by the calculations. The stacking of the quinoid rings is assisted by dispersion interactions. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2ce26119g

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PAPER
Face-to-face stacking of dianionic quinoid rings in
crystals of alkali salts of 2,5-dihydroxyquinone in view
of p-system polarization3
Cite this: CrystEngComm, 2013, 15,
135
a
a
ab
Kresimir Molcanov,* Biserka Kojic-Prodic, Darko Babic and Jernej Starec
ˇ
ˇ
´
´
´
A series of alkali (M+ = K+, NH4+, Rb+ and Cs+) salts of 2,5-dihydroxyquinone [M2+ (C6H4O4)22] was prepared
and their structures were determined by X-ray structure analysis. A less-common type of p-stacking of
dianionic quinoid rings in face-to-face fashion (no offset) is observed. However, in the potassium salt of
this series the stacking motif of the dianionic quinoid rings is in a herring-bone pattern, usually observed in
the stacking of aromatics. The different types of crystal structures, induced by varied cations, are discussed
in terms of the relative energies calculated by the DFT method. The structural features observed were
reproduced by the calculations. The stacking of the quinoid rings is assisted by dispersion interactions.
Received 10th July 2012,
Accepted 17th October 2012
DOI: 10.1039/c2ce26119g
www.rsc.org/crystengcomm
parallel arrangement (no offset).10,12,13 Dianionic rings of
chloranilates and bromanilates have a more pronounced
electron delocalisation and favour offset stacking (Fig. 1c).11–13
The observed stacking can be understood as the overlapping of
electron-rich and electron-deficient units between the polarised
p-systems.
Introduction
The interactions between stacked aromatic rings are exten-
…
sively studied and characterised as p-stacking forces or p
p
interactions.1–8 According to the most recent analysis of these
forces based on experimental and theoretical findings
Martinez and Iverson9 advocate for reconsideration of the
term ‘p-stacking’. They provide evidence that these interac-
tions should be viewed as ones occurring between polarised
p-systems, i.e. interactions of electron rich vs. electron
deficient regions.
Forced face-to-face stacking of aromatic rings has
a
potential use in the design of organic semiconducting
materials.16,17 A synthetic approach has been used to control
However, the stacking of quinoid rings has been little
studied. It was documented only recently in the crystals of
some simple alkali salts of chloranilic acid (3,6-dichloro-2,5-
dihydroxyquinone)10–12 and bromanilic acid (3,6-dibromo-2,5-
dihydroxyquinone).13 The two types of planar rings, aromatic
and quinoid, have different electronic structures:14 the former
have delocalised p electrons and all C–C bonds almost equal
(Fig. 1a), whereas quinoid rings reveal well-distinguishable
single and double bonds (Fig. 1b). Thus, stacking interactions
between these rings are different to those in aromatics.
Energetically the most favoured arrangement of aromatic
rings is either offset parallel or T-shaped,7,15 however, neutral
and monoanionic quinoid rings can appear in face-to-face
a
ˇ
´
ˇ
Rudjer Boskovic Institute, Bijenicka 54, HR-10000, Zagreb, Croatia.
E-mail: kmolcano@irb.hr; Tel: +385 1 4561025
bDepartment of Chemistry, Faculty of Science, Horvatovac 102a, HR-10000, Zagreb,
Croatia
cNational Institute of Chemistry, Hajdrihova 19, SI-1001, Ljubljana, Slovenia
Fig. 1 The distribution of p-electrons in (a) aromatic rings, (b) neutral quinoid
rings and (c) DHQ22 anions. While aromatics comprise a delocalised
(conjugated) system over the whole ring, in neutral quinoid rings p-bonds are
essentially localised. DHQ22 (and similar chloranilate and bromanilate dianions)
comprise two delocalised systems separated by single C–C bonds.
Electronic supplementary information (ESI) available: Experimental details on
3
redetermination of Li2DHQ?2H2O, ORTEP drawings of DHQ22 anions and cation
coordination spheres; parameters of DFT-optimised unit cells and
computational details. CCDC 889125–889129. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2ce26119g
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symmetry D2h. The crystal structure of Li2DHQ?2H2O was
published by Xiang et al.18 during our research on the herein
presented series. The structure of the lithium–homologue was
the organisation of aromatic rings of semiconductor mole-
cules in the solid state supporting face-to-face p-stacking
where p-orbital overlap is reliably enforced by stronger
hydrogen bonds.16,17 However, quinoid rings can self-assem-
ble into face-to-face p-stacks without directed chemical
modifications.10,12,13
The present study is dealing with an unsubstituted
analogue of chloranilic and bromanilic acid, 2,5-dihydroxy-
quinone (DHQ, Scheme 1). The main difference between DHQ
and its halogen analogues is the lack of bulky, electronegative
substituents. Electron density in the DHQ quinoid ring is
therefore slightly higher than in its halogenated analogues,
whereas the polarity of C–H bonds is reversed compared to C–
Cl and C–Br bonds.
redetermined by us (ESI ) and it is briefly discussed and
3
compared to other members of the series. There are two
symmetry-independent anions in Li2DHQ?2H2O. According to
the bond lengths (Table 1, Fig. 2) dianions comprise two
delocalised systems separated by single C–C bonds (Scheme 1,
Table 1). This structure is analogous to the dianion of
chloranilic acid (3,6-dichloro-2,5-dihydroxybenzoquinone)11,19
and bromanilic acid (3,6-dibromo-2,5-dihydroxybenzoqui-
none).13 The DHQ22 in Cs2DHQ is disordered: atoms C1 and
O1 are located on a twofold axis, therefore atoms O2 and H of
the quinoid ring are disordered over two positions (Fig. 3).
Crystal packing and p-stacking of 2,5-dihydroxybenzoquinone
dianions (DHQ22
)
Results and discussion
The salts of alkali cations (K+, NH4 , Rb+, and Cs+) and 2,5-
The striking feature of the crystal packing is face-to-face
p-stacking of the dianionic rings with no offset (Table 2, Fig. 4–
7). The space group symmetry and location of the quinoid
rings on special positions correspond to strictly planar and
parallel stacks with no offset. An exception is the packing of
K2DHQ, where rings stack in an aromatic-like fashion with a
large centroid distance and a considerable offset resulting in a
herring-bone pattern (Fig. 8). In our previous studies of similar
compounds, alkali chloranilates10–12 and bromanilates,13 we
observed the stacking of neutral and monoanionic rings with
no offset, while dianions were arranged in offset stacks.11–13
A qualitative explanation can be based on the electronic
structures of the species involved (Scheme 1, Fig. 1): neutral
molecules and dianions comprise well-defined single and
double bonds; therefore, they can avoid repulsion between
electron-rich regions and maximise attraction between elec-
tron-rich and electron-deficient regions simply by stacking
without offset.10,12 In halogen-substituted dianions, compris-
ing two delocalised systems, repulsion between electron-rich
regions cannot be avoided, and therefore halogen-derivatives
form offset stacks.11–13 Moreover, the repulsion of two doubly
negatively charged rings is much stronger. Therefore, the face-
to-face stacking of dianionic DHQ22 rings came as a surprise.
While the interplanar distances in halogen derivatives10–13 are
about 3.2 Å, considerably shorter than the sum of the van der
+
dihydroxybenzoquinonate dianions were prepared and their
crystal structures determined and discussed in view of their
dianion stacking and the interactions responsible for the
motifs detected. Their crystal packings are not isostructural.
Crystal packings of all studied salts exhibit a 3D-network via
cation–dianion interactions. The two homologous salts,
(NH4)2DHQ and Rb2DHQ crystallise in the orthorhombic
space group Ibam, whereas K2DHQ appears in the monoclinic
space group P21/c, and Cs2DHQ in the orthorhombic space
group Cmmm (see Experimental section). The crystal structures
of Li2DHQ?2H2O18 and (NH4)2DHQ, both with similar packing
densities, exhibit extensive hydrogen bonds using the proton
donor functionalities of crystal water molecules and of
ammonium cations, respectively.
Molecular structure and symmetry of 2,5-
dihydroxybenzoquinone anion (DHQ22
)
Molecular symmetry of the DHQ22 dianion is D2h. In crystal
structures, the crystallographic symmetry of the DHQ22 can be
different. In the series studied the anions possess a crystal-
lographic symmetry: Ci in K2DHQ; C2h in Li2DHQ?2H2O,18
(NH4)2DHQ, and Rb2DHQ; in Cs2DHQ the anion is disordered
and located in a special position with the crystallographic
Table 1 Bond lengths in the DHQ22 dianion.a Bond distances for the known
structure of the Li-homologue18 are listed for comparison
Li2DHQ?2H2O
A ring
B ring
K2DHQ
Rb2DHQ
(NH4)2DHQ
C1–C2 1.537(2)
C2–C3 1.391(3)
1.536(2)
1.397(2)
1.543(2)
1.398(2)
1.541(5)
1.381(7)
1.529(4)
1.395(4)
C1–C3b 1.396(3)i 1.394(3)ii 1.402(2)iii 1.407(5)iv 1.392(4)iv
C1–O1 1.261(2)
C2–O2 1.264(2)
C3–H3 1.00(3)
1.264(2)
1.254(2)
0.99(3)
1.252(2)
1.258(2)
0.97(2)
1.257(7)
1.263(6)
1.12(6)
1.253(3)
1.271(3)
1.04(4)
a
The geometry of the anion in Cs2DHQ is not realistic due to
b
disorder and is not included. The symmetry operators given in the
Scheme 1 Dissociation of 2,5-dihydroxybenzoquinone (DHQ). DHQ22 has two
table are: (i) 2x, y, 1 2 z; (ii) 1 2 x, y, 1 2 z; (iii) 21 2 x, 1 2 y, 1 2
z; (iv) 2x, 2y, z.
delocalised systems separated by the two single C–C bonds.
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chloranilates and bromanilates in staggered orientation allow
attractive interactions of the quinoids only (Fig. 9a and b).
However, such enhanced attractions are only a part of the
picture and alone can not overcome the significant repulsion
of doubly charged rings. The most important contribution is
probably the compensation of the negative charges by cation–
anion interactions.
Unlike previously described chloranilates with an extensive
network of hydrogen bonds,10–12 in the DHQ salts only
hydrogen bonds are present in Li2DHQ?2H2O and
(NH4)2DHQ where water molecules and ammonium cations
are proton donors and dianion oxygen atoms are acceptors. In
…
other structures studied here there are C–H O interactions
between DHQ22, only (Table 3). Thus, no competition exists
between hydrogen bonding and stacking. Since similar types
of stacks are observed in the structures with extensive
hydrogen bonding [Li2DHQ?2H2O, (NH4)2DHQ, chlorani-
lates10,12 and bromanilates13] and in those lacking hydrogen
bonding (Rb2DHQ, Cs2DHQ), one can conclude that an impact
of hydrogen bonds over the stacking arrangements is not of
primary influence. Steric factors appear to have a crucial role
in stabilising or destabilising the stacks. The different
environments of DHQ22 in the structures of potassium and
rubidium salts (Fig. 10 and 11, respectively) illustrate the
impact of cation size on anion packing in crystals. In the
potassium salt with a herring-bone motif each dianion is
surrounded with ten cations: each ortho-positioned O1 and O2
function as bidentate bridging groups to five K+ (Fig. 10 and
Fig. 2 ORTEP-320 drawing of a DHQ22 in K2DHQ. The centroid of the ring
coincides with a crystallographic inversion centre. The atomic numbering
scheme is applied to all structures discussed, except that of Cs2DHQ.
Displacement ellipsoids are drawn at the probability level of 50% and hydrogen
atoms are depicted as spheres of arbitrary radii.
Waals radii for carbon (3.5 Å), interplanar separation between
DHQ22 rings is greater than 3.5 Å, indicating much stronger
repulsion.
DHQ, lacking large partially negatively charged halogen
atoms, is less sterically hindered (Fig. 9) than its halogen
analogues. Therefore, it can pack more closely. While
contiguous hydrogen chloranilate and hydrogen bromanilate
(Fig. 9a and b) rings are in staggered orientations, the DHQ22
rings are almost perfectly eclipsed (Fig. 9c). Such an arrange-
ment allows maximised attractions between electron-rich and
electron-deficient regions not only of the quinoid core, but
also of C–O and C–H bonds (Fig. 9c). The additional attraction
between the antiparallel dipoles of C–O and C–H bonds also
contributes to the total interaction. On the other hand, stacked
ESI, Fig. S6). However, in the crystal structure of the rubidium
3
salt each of the stacked dianions is embedded into the
environment of twelve Rb+. Each pair of ortho-positioned O1
and O2 of a dianion acts as a bidentate bridging group to six
cations.
In (NH4)2DHQ, Rb2DHQ and Cs2DHQ the centroid separa-
tion distances are proportionally larger with the increased
cation size (Table 2). The stacks of quinoid rings in these three
structures reveal no offset but they have larger intrastack
distances than in their halogenated homologues.10–13 The
enlarged separation distance between stacked rings reduces
the inter-ring repulsions.
However, Li2DHQ?2H2O crystallises as a dihydrate where
hydrogen bonds stabilise the crystal packing; probably, the
small size of the lithium cation could not be sufficient to
stabilise the crystal packing and anhydrous crystals were not
obtained. In its crystal packing stacks of dianions the shortest
…
Cg Cg separation distance is 3.2209(1) Å (no offset, Table 2).
Comparison of stacking types in the crystal structures of
M2DHQ salts by DFT calculations
We tried to explain the differences in stacking types of DHQ22
in the crystal structures of M2DHQ salts by DFT calculations.
Initially, we focused onto the stacking types displayed in the
crystal structures of K2DHQ – hereafter referred to as type A,
and Rb2DHQ – referred to as type B. We assumed that each salt
can form stable structures of both types and that the more
stable one – with lower energy – occurs in reality. The
structures of M2DHQ, M = K, Rb, Cs, were optimized starting
from the experimental structures of K2DHQ and Rb2DHQ as
the initial geometry templates, with M+ ions replacing K+ and
Fig. 3 Nature of the disorder of the DHQ22 ring in Cs2DHQ. The crystallographic
symmetry of the DHQ22 ring: the centroid of the ring is at (1/2, 0, 1/2), a special
position with symmetry mmm; atoms C1 and O1 are located on a crystal-
lographic twofold axis (site symmetry m2m), while atoms C2 and O2 are located
in a mirror plane (site symmetry m). Therefore, C2, O2 and the hydrogen atom
(which could not be located due to disorder) are disordered about a twofold
axis: at each position of O2 (with population parameters for O2 and H of 0.25).
ORTEP drawing of the disordered molecule is deposited into ESI.3
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…
Table 2 Geometric parameters of the p p interactions. Symmetry operators: (i) 1 2 x, y, 1 2 z; (ii) 21 2 x, 12y, 12z; (iii) 2x, 2y, z; (iv) x, 2y, 2z
a
ab
bc
Cg plane (Cg2)/Å
Offset/Å
Symm. op. on Cg2
…
…
…
p
p
Cg Cg/Å
Li2DHQ?2H2O
C1A A C3A C1B A C3B
i
…
3.2209(1)
3.9251(9)
3.5130(6)
3.6154(2)
3.888(5)
0
0
0
0
0
0
3.2209(1)
3.4229(6)
3.5129(6)
3.6154(2)
0
21/2 + x, 21/2 + y, z
22 2 x, 1 2 y, 1 2 z
x, 2y, 21/2 2 z
K2DHQ
C1 A C3ii C1 A C3
29.30
1.921
ii
…
(NH4)2DHQ
C1 A C3iii C1 A C3
0
0
0
0
0
0
iii
…
Rb2DHQ
C1 A C3iii C1 A C3
x, 2y, 21/2 2 z
iii
…
Cs2DHQ
C1 A C2vi C1 A C2
3.888(5)
1/2 + x, 1/2 + y, z
iv
…
a
b
c
…
Cg = centre of gravity of the aromatic ring. a = angle between planes of two aromatic rings. b = angle between Cg Cg line and normal to
the plane of the first aromatic ring.
Rb+, respectively. The optimization of Cs2DHQ that was
initiated from the structure type A ended up in a third
structure – hereafter referred to as type C. It is similar to the
type B inasmuch as having also face-to-face stacks of DHQ22
anions. In contrast to the B-type, in which the orientation of
the C–H bonds along the stacked anions strictly alternates, the
stacked anions in C are all parallel. The type C can be
produced from type B by shifting the alternate layers of
coplanar DHQ22 anions along the vector [0.5, 0.5, 0]. The
optimization of K2DHQ and Rb2DHQ structures was repeated
starting with the initial geometry of type C as well.
As the pure Na2DHQ could not be prepared, we tried to
predict its hypothetical structure as the most stable among the
Fig. 4 Crystal packing of Li2DHQ?2H2O18 viewed in the directions (a) [010] and
(b) [001]. Lithium cations are depicted as spheres of arbitrary radii. DHQ22
anions are located on a crystallographic mirror plane and are therefore parallel.
Fig. 5 Crystal packing of Rb2DHQ viewed in the directions: (a) [001] and (b)
[100]. Rubidium cations are depicted as spheres of arbitrary radii. DHQ22 anions
are located on a crystallographic mirror plane and are therefore strictly parallel.
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Fig. 6 Crystal packing of (NH4)2DHQ viewed in the direction [001]. DHQ22
anions are located on a crystallographic mirror plane and are therefore parallel.
A, B and C types. On the other hand, the above procedure did
not involve the NH4 cation since the quasi-octahedral
coordination of oxygen atoms around the cation in the
potassium salt is obviously unsuitable for the ammonium
ion. In contrast, the octa-coordination polyhedron in Rb2DHQ
(ESI, Fig. S7) has 4 out of 8 apices in almost tetrahedral
3
arrangement, favourable for hydrogen bonding to NH4+, and
in agreement with the experimentally found structure of
(NH4)2DHQ. This was considered sufficient to explain the
structure of (NH4)2DHQ.
Fig. 8 Crystal packing of K2DHQ viewed in the directions: (a) [100] and (b) [001].
Offset dianions form a herringbone pattern. Potassium cations are depicted as
spheres of arbitrary radii.
The calculated parameters of the unit cells are in good
agreement with the experimental data (ESI, Table S2). The
3
Rb2DHQ, the energy of the type B structure was 10.6 kJ
mol21 lower than the type A structure. These results mean that
the experimental structures of K2DHQ and Rb2DHQ are
successfully predicted by calculation. The energies of type B
and type C structures of Cs2DHQ are quite close, with the
energy of the C-structure only 2.3 kJ mol21 lower than the
B-structure. Taking into account the structural similarity of
types B and C and ease of transformation between them, the
energy of a mixed structure with consecutive pairs of DHQ
layers randomly being of B- or C-type, should not be
significantly different from the fully ordered types B and C.
This agrees with precisely the same character of the
experimentally found disorder in the Cs2DHQ structure.
Besides, optimisation of Rb2DHQ, initiated from the C-type,
resulted in energy that was 10.1 kJ mol21 higher than the
B-type, in agreement with the fully ordered experimental
structure. Similarly, the structure of K2DHQ initiated from the
C-type was higher in energy by 27.2 kJ mol21 than the A-type.
energy of the K2DHQ structure obtained from the experimental
geometry (type A) is 27.3 kJ mol21 lower than the alternative
structure derived from Rb2DHQ (type B). Similarly for
Interaction energy of the stacked pair of DHQ22 dianions and
evaluation of the dispersion component
The interaction energy was calculated by using various types of
model chemistry and for three distinct models – the isolated
pair of dianions, the same pair in the field of explicit point
charges located at the positions of cations and anions in the
Fig. 7 Crystal packing of Cs2DHQ viewed in the direction [001]. Caesium cations
are depicted as spheres of arbitrary radii. Disordered DHQ22 anions are located
on a crystallographic mirror plane and are therefore parallel.
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Fig. 10 Environment of the DHQ22 dianion in K2DHQ. Symmetry operators: (i)
2x, 2y, 2z; ii) x, 2y + 3/2, z + 1/2; (iii) 2x, y + 1/2, 2z + 1/2; (iv) 2x, 2y + 1, 2z
+ 1; (v) 2x + 1, y + 1/2, 2z + 3/2; (vi) x + 1, y, z.
and Grimme’s B97D offer improved support for the dispersion
component; the same is true of the ab initio MP2 methodology.
The inclusion of explicit charges at their crystallographic
positions influences the calculated interaction energy, making
it even more repulsive. The sensitivity of the energy to the size
of the array of surrounding charges is considerable when the
surrounding shell of charges is relatively small, but the 100 Å
cluster of charges provides practically convergent values of the
interaction energy. Interestingly, the difference in interaction
energy between the dispersion-deficient and dispersion-sup-
ported methodologies remains virtually identical, the latter
yielding by about 7 kcal mol21 less repulsive interaction. The
insensitivity of the relative values supports the view that the
origin of difference between the two groups lies in dispersion
rather than in electrostatics or polarization.
Fig. 9 Pairs of contiguous quinoid rings in face-to-face stacks: (a) hydrogen
bromanilate monoanions13 in a staggered arrangement, (b) hydrogen chlor-
anilate monoanions10 in a partially staggered arrangement and (c) DHQ dianions
in an eclipsed arrangement. To illustrate the bulkiness of halogen substituents,
models of the anions using van der Waals spheres are shown in the upper row.
crystal structure and, finally, the pair of neutral DHQ
molecules. Calculated energies are listed in Table 4.
The interaction between the stacked pair of DHQ22 dianions
is largely repulsive. Evidently, the interaction is dominated by
the repulsive electrostatic force between the negatively charged
DHQ22 entities. However, the most interesting result is the
subtle, yet significant difference between the HF and B3LYP
methodologies on one side and M06, B97D and MP2 on the
other. Namely, interaction energies calculated with the latter
set are consistently (by 5–9 kcal mol21) less repulsive than with
the former. The difference can likely be attributed to the
dispersion component of the interaction. While the HF and
traditional DFT methods such as B3LYP are notorious for their
lack of dispersion, the novel functionals such as Truhlar’s M06
Finally, in order to remove the predominant electrostatic
component, we considered a pair of neutral DHQ molecules.
While the HF and B3LYP methods suggest a slightly repulsive
interaction, it becomes attractive with the methods with
enhanced dispersion. Again, the differences are in the range
between 6.5 and 10 kcal mol21, suggesting that the dispersion
Table 3 Geometric parameters of hydrogen bonds
…
…
…
D–H/Å
H
A/Å
D
A/Å
D–H A/u
Symm. op. on A
Li2DHQ?2H2O
…
O5–H5A O1A
0.94(3)
0.94(3)
0.90(2)
2.06(3)
2.33(3)
1.95(2)
2.8209(17)
3.1701(16)
2.8280(17)
137(2)
148(2)
164(2)
1/2 + x, 21/2 + y, z
1/2 + x, 21/2 + y, z
1 2 x, y, 1 2 z
…
O5–H5A O2A
…
O5–H5B O2A
K2DHQ
C2–H2 O1
…
0.97(2)
2.59(2)
3.565(2)
178.3(13)
x, 3/2 2 y, 1/2 + z
(NH4)2DHQ
…
N1–H1A O2
0.88(3)
0.85(3)
1.04(4)
2.05(3)
2.12(3)
2.29(4)
2.915(2)
2.938(2)
3.323(3)
166(2)
161(3)
168(3)
x, y, z
…
N1–H1B O1
1/2 2 x, 1/2 2 y, 1/2 + z
21/2 + x, 1/2 2 y, 2z
…
C3–H3 O2
Rb2DHQ
C3–H3 O2
…
1.12(6)
2.22(7)
3.323(6)
171(5)
21/2 + x, 1/2 2 y, 2z
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DHQ22 (dianions) are related to their electronic structures and
the size, and polarisability of cations. The eclipsed orientation
of DHQ22 rings in a stack (Fig. 9c) allows maximised
attractions between electron-rich and electron-deficient
regions of the quinoid ring, but also of C–O and C–H bonds
(Fig. 9c). The additional attraction between antiparallel dipoles
of C–O and C–H bonds also contributes to the total
interaction. The strong attractions between C–O and C–H
bonds of the two contiguous rings significantly contribute to
the stability of the face-to-face arrangement, whereas the
repulsions in these structures can be minimised by enlarge-
ment of ring separation distance within the stack, which is
controlled by the size of NH4+, Rb+ and Cs+ (Table 2).
Calculations by methodologies with different support for the
dispersion component indicate that the attractive dispersion
interaction between the stacked quinoid rings is in the range
between 5 and 10 kcal mol21, enhancing the stacking of the
otherwise ionic entities at a close distance. For the salt
structure with the relatively small K+, the face-to-face stacking
is not optimal and a herring-bone type of stacking occurs. The
relative energies of the crystal packings (including stacking
types observed with different alkali cations), calculated by DFT
method, revealed the lower values for the X-ray-structures.
Other notable features in the crystal structures, like structural
disorder in Cs2DHQ, were also explained by computational
results.
Fig. 11 Environment of the DHQ22 dianion in Rb2DHQ. Symmetry operators: (i)
x, y, 2z; (ii) x + 1/2, y + 1/2, z + 1/2; (iii) 2x + 1/2, 2y + 1/2, z + 1/2; (iv) 2x + 1/
2, 2y + 1/2, 2z + 1/2; (v) x + 1/2, y + 1/2, 2z + 1/2.
forces can provide a decisive shift in the interactions that
govern the alignment of DHQ rings.
The presented results provide evidence that, while the main
interaction in the crystal is electrostatic, there is a relatively
small but noticeable amount of dispersion component in the
intermolecular interaction between the DHQ22 rings.
Dispersion probably assists the observed face-to-face stacking
of the rings. While quantitative determination of the disper-
sion component is probably beyond the reach and the scope of
the present work, the present calculations clearly suggest that
the attractive dispersion component is likely significant
enough for the observed alignment of the rings.
The self-organisation of 2,5-dihydroxyquinone dianions
through dispersion interactions, leading to stacking, can be
used in crystal engineering, and in the design of functional
materials.
Experimental
All samples were prepared by slow evaporation of an aqueous
solution containing 2,5-DHQ (Sigma-Algdich, p.a. grade) and
an equivalent amount of alkali carbonate (Sigma-Aldrich,
Merck, Kemika, all p.a. grade).
Conclusions
A series of alkali (M+ = K+, NH4 , Rb+ and Cs+) salts of 2,5-
dihydroxyquinone [M2+(C6H4O4)22] was prepared and their
structures determined by X-ray structure analysis. A novel type
of face-to-face p-stacking without the offset of dianionic
quinoid rings is observed. The factors affecting stacking of
+
Single crystal measurements were performed on an Oxford
Diffraction Xcalibur Nova R (microfocus Cu tube) at room
temperature. Program package CrysAlis PRO21 was used for
data reduction. The structures were solved using SHELXS9722
and refined with SHELXL97.22 The models were refined using
the full-matrix least squares refinement; all non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were
located in a difference Fourier map and refined as free
entities. Molecular geometry calculations were performed by
PLATON,23 and molecular graphics were prepared using
ORTEP-3,20 and CCDC-Mercury.24 Crystallographic and refine-
ment data for the structures reported in this paper are shown
in Table 5.
The computations were performed with the Gaussian 09.25
Electronic structure was calculated by the DFT method, and
the exchange and correlation energies were evaluated with the
PBE26 functional. Optimization was performed without any
constraints, including all the molecular and the cell para-
Table 4 Calculated interaction energies (kcal mol21) of a stacked pair of quinoid
rings. Repulsive interactions are denoted by the positive sign while the attractive
ones are characterized by negative values of energy
(DHQ22
69 858 point
charges
)
2
(DHQ22
isolated
)
(DHQ)2
isolated
2
Method
HF
244.3
242.5
237.6
235.3
235.9
276.9
274.4
264.4
266.4
267.9
5.5
3.7
22.8
24.6
24.4
B3LYP
M06
B97D
MP2
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Table 5 Crystallographic, data collection and structure refinement details
Compound
K2DHQ
(NH4)2DHQ
Rb2DHQ
Cs2DHQ
Empirical formula
C6H2K2O4
216.28
C6H10N2O4
174.16
C6H2Rb2O4
309.02
C6H2Cs2O4
403.89
Formula wt/g mol21
Crystal dimensions/mm
Space group
0.25 6 0.04 6 0.03
P21/c
0.21 6 0.07 6 0.06
Ibam
0.23 6 0.16 6 0.04
Ibam
0.15 6 0.12 6 0.10
Cmmm
a/Å
b/Å
c/Å
3.92510(10)
11.2358(3)
8.3337(2)
90
102.757(3)
90
8.2507(9)
12.4085(13)
7.0259(12)
90
90
90
8.1716(4)
12.4680(5)
7.2307(3)
90
90
90
8.2553(11)
12.8490(16)
3.8880(4)
90
90
90
a (u)
b (u)
c (u)
Z
2
4
4
2
V/Å3
358.457(16)
2.004
11.470
719.30(16)
1.608
1.172
736.69(6)
2.786
17.029
412.41(9)
3.252
68.746
Dcalc/g cm23
m/mm21
H range (u)
T/K
Radiation wavelength
Diffractometer type
Range of h, k, l
6.72–76.09
293 (2)
6.44–75.41
293 (2)
6.48–75.48
293 (2)
6.37–76.18
293 (2)
1.54179 (Cu Ka)
Xcalibur Nova
24 , h , 4;
214 , k , 11;
210 , l , 10
2524
737
700
Multi-scan
0.0273
1.54179 (Cu Ka)
Xcalibur Nova
27 , h , 10;
212 , k , 15;
28 , l , 8
772
386
324
Multi-scan
0.0225
1.54179 (Cu Ka)
Xcalibur Nova
29 , h , 10;
213 , k , 15;
24 , l , 9
850
407
333
Multi-scan
0.0213
1.54179 (Cu Ka)
Xcalibur Nova
210 , h , 8;
215 , k , 16;
24 , l , 4
1318
274
253
Multi-scan
0.0972
Reflections collected
Independent reflections
Observed reflections (I ¢ 2s)
Absorption correction
Rint
R(F)
0.0294
0.0845
0.0569
0.1873
0.0433
0.1216
0.0550
0.1534
Rw(F2)
Goodness of fit
H atom treatment
No. of parameters
No. of restraints
1.119
Free
59
0
1.122
None
47
3
1.130
Free
40
0
1.102
None
25
0
Drmax, Drmin (eÅ23
)
0.352, 20.274
0.338, 20.496
0.789, 21.057
2.168, 21.912
meters. In order to reduce computational cost, the optimiza-
tions were performed with primitive cells. The computational
parameters for periodic boundary conditions were automati-
cally set up by the program,25 with numbers of k-points
ranging between 112 and 148 and the number of replicated
cells between 279 and 423.
The choice of the basis sets was quite important since the
standard basis sets ordinarily used for molecules resulted in
computational failures.27 It was remedied by using all-electron
basis sets published on the Crystal Resources webpage.27 The
basis consisted28 of contracted Gaussian-type functions of the
form s(8)s(2)s(1)p(1) for element H, s(6)sp(1)sp(1)sp(1)sp(1)d(1)
for element C, s(6)sp(2)sp(1)d(1) for elements N and O,
s(8)sp(5)sp(1)sp(1) for element Na, s(8)sp(6)sp(5)sp(1)sp(1)d(3)
for element K, s(9)sp(7)sp(6)d(5)d(1)sp(3)sp(1) for element Rb
and s(9)sp(7)sp(6)d(6)sp(3)d(3)d(1)sp(3)sp(1)sp(1) for element
Cs, where the letters give shell-type and the number in brackets
gives the number of primitive Gaussians in each shell
contraction. The exponents and contraction coefficients are
B3LYP,29,30 Grimme’s B97D31 and Truhlar’s M06,32 the latter
two being improved for dispersion. In addition, the plain
Hartree–Fock methodology and the perturbational correlation
method MP233,34 were also used. The models included two
neighbouring DHQ22 rings taken from the experimental
geometry of (NH4)2DHQ. Effects of surroundings were mod-
elled by an array of point charges of +1.0 and 22.0,
+
corresponding to the positions of centroids of NH4 and
DHQ22 entities in the crystal lattice, respectively, within the
nearest 100 Å from the stacked dimer (a total number of
69 858 point charges). In addition to the deprotonated DHQ22
ions, stacked pairs of neutral DHQ molecules were also
considered. The neutral model was created by appending two
hydrogen atoms to each of the DHQ22 anions. The interaction
energies were corrected for the basis set superposition error by
the standard counterpoise protocol, as implemented in
Gaussian 09.
reported in the Computational details in ESI.
3
Acknowledgements
Calculations of interaction energy of an isolated stacked
pair of dianionic quinoid rings were performed by the
Gaussian 09 program using the 6-31++G(d,p) basis set and
various quantum methodologies, including the functionals
This work was supported by Ministry of Science, Education
and Sports of Croatia (grants nos. 098-1191344-2943 and 098-
0982915-2942), Slovenian Research agency (program group P1-
142 | CrystEngComm, 2013, 15, 135–143
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Paper
23 A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7–13.
0012) and Slovenian–Croatian bilateral collaboration (grant for
2012/2013).
24 C. F. Macrae, P. R. Edgington, P. McCabe, E. Pidcock, G.
P. Shields, R. Taylor, M. Towler and J. van de Streek, J. Appl.
Crystallogr., 2006, 39, 453–457.
25 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M.
A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J.
L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery, Jr.,J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin,
V. N. Staroverov, R. Kobayashi, J. Normand,
K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
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E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
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