Intermolecular Interactions between Halogen-Substituted p-Benzoquinones and Halide Anions: Anion-π Complexes versus Halogen Bonding

Article abstract of DOI:10.1002/cplu.202000012

Intermolecular interactions between halo-substituted p-benzoquinones (BQ) and halide anions were examined in solution, solid-state and/or in silico. While X-ray crystallography revealed only halogen bonding (XB) between tetraiodo-p-benzoquinone (I₄

Full text of DOI:10.1002/cplu.202000012

Accepted Article
Title: Intermolecular Interactions between Halogen-Substituted p-
Benzoquinones and Halide Anions: Anion-π Complexes versus
Halogen Bonding
Authors: Sergiy Rosokha, Almaz Jalilov, Muath Albukhari, Spencer
Deats, and Matthias Zeller
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Intermolecular Interactions between Halogen-Substituted p-
Benzoquinones and Halide Anions: Anion- Complexes versus
Halogen Bonding
Almaz Jalilov,[b] Spencer Deats,[a] Muath Albukhari,[b] Matthias Zeller[c] and Sergiy V. Rosokha*[a]
[a]
Spencer Deats, Prof. Dr. Sergiy V. Rosokha
Department of Chemistry
Ball State University
Muncie IN USA 47306
E-mail: svrosokha@bsu.edu
Prof. Dr. Almaz Jalilov, Muath Albukhari
Department of Chemistry
King Fahd University of Petroleum and Minerals
Dhahran, Saudi Arabia 31261
Dr. Matthias Zeller
[b]
[c]
Department of Chemistry
Purdue University
West Lafayette IN USA 47907
Supporting information for this article is given via a link at the end of the document
Abstract: Intermolecular interactions between halo-substituted p-
benzoquinones (BQ) and halide anions were examined in solution,
solid-state and/or in silico. While X-ray crystallography revealed only
halogen bonding (XB) between tetraiodo-p-benzoquinone (I4Q) and
halides, the results of a UV-Vis study in solutions were consistent with
the formation of 1:1 anion- complexes. DFT computations showed
that the anion- complexes of halides with most halo-substituted BQ
molecules were more stable (by 2 - 7 kcal/mol) than their XB
analogues, but the stabilities of different complexes of I4Q were
essentially the same. Thus, the structural features of the co-crystals
with I4Q were related to multicenter XB interactions between BQs and
halides leading to the formation of 3D-networks. The observation of
anion- complexes in solutions was attributed to their higher molar
absorptivity (by more than an order of magnitude) than that of their XB
analogues. Overall, the stabilities of anion- and XB complexes
between BQs and halides were well correlated with the values of
highest electrostatic potentials on the surfaces of BQ molecules when
their polarizations were taken into account.
substituents may potentially participate in both anion- and
halogen bonding.[6-8] Accordingly, interpretations of intermolecular
interactions in chemical and biochemical systems require
characterization of the co-existing modes of bonding, and
identification of factors determining the prevailing type of inter-
action. Yet, experimental observations of competition and/or
cooperation of anion- and halogen bonding are rare. Previous
studies of the systems capable of both types of bonding were
focused mostly on the computational analysis of different modes
of interaction of halo-substituted aromatic molecules.[8-10] There
are also a number of X-ray structural studies of co-crystals formed
by haloperfluorosubstituted benzenes with the salts of different
anions.[11] These works showed that XB is usually a dominant
mode of interaction of iodo- or bromo-substituted species, and
anion- interactions prevail in complexes of fluoro- or chloro-
substituted molecules.
p-Benzoquinones (BQ) are important redox-reagents, and their
applications range from organic synthesis to mediating electron
transfer in biochemical systems and solar cells.[12,13] The surface
electrostatic potential (ESP) of halo-substituted p-benzoquinones
shows areas of positive potentials along the extension of carbon-
halogen and carbon-hydrogen bonds, as well as over the face of
the benzoquinone’s ring (Figure 1). As such, they represent
another type of molecules capable of both anion- and XB
interactions (as well as HB, if hydrogen substituents are present).
Introduction
New types of supramolecular interactions have emerged in recent
years alongside the well-known hydrogen bonding (HB) as driving
forces of molecular associations in chemical and biochemical
systems.[1-3] Halogen bonding (XB), an attraction between
electrophilic halogen atoms and electron-rich centers, and anion‐
π bonding, an attraction of anions to the electron-deficient
aromatic or olefinic -systems, are the most prominent examples
of these newly recognized interactions.[2,3] Indeed, while appli-
cations of many new supramolecular interactions (e.g. pnictogen
or tetrel bonding) are still relatively rare,[1] XB and anion- bonding
became valuable tools for molecular recognition, crystal
engineering, catalysis, anion transport, etc.[2-5] Both these bonding
types are commonly related to the presence of areas of positive
potentials on the surfaces of -systems or covalently-bonded
halogen atoms, referred to as - or -holes, respectively.[6-9] Due
to the presence of such areas, many -acceptors with halogen
Figure 1. Surface electrostatic potential of halo-substituted BQ molecule (2,6-
diiodo-p-benzoquinone) showing areas of positive potentials along the C-X and
H
C-H bonds, and on the face of the ring (with maximum potentials VmaxX, Vmax
and Vmax, respectively).
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However, in contrast to a number of reports addressing
intermolecular interactions of halo-substituted benzenes, there
are only a few studies of intermolecular associates between
anions and p-benzoquinones.[14-16] Also, while earlier studies of
halo-substituted benzenes explored mostly halogen bonding,
previous publications describing supramolecular bonding of p-
benzoquinones were focused on the anion- interactions. For
example, an anion- complex between tetrachloro-p-benzo-
quinone and the Br- anion was observed in acetonitrile solution
earlier.[14] The most recent characterization of anion- complexes
of halides with various halo-substituted p-benzoquinones
revealed that variation of electron donor and/or acceptor strength
of the reactants leads to a switch from the formation of persistent
complexes to redox processes.[15] Short anion- contacts were
observed by Molčanov et al. in the X-ray structures of co-crystals
of tetrachloro- and tetrabromo-p-benzoquinones with iodide salts
of substituted N-methylpyridinium cations.[16] These co-crystals
comprised also halogen bonds between bromine or chlorine
substituents of p-benzoquinones and I- anions. Noticeably, the
previous works described intermolecular interactions of fluoro-,
chloro- and bromo-substituted benzoquinones, while the data for
their iodo-substituted analogues were absent. Also, the compa-
rison of the characteristics of the XB and anion- complexes of p-
benzoquinones or the analysis of the factors that determine the
prevalence of a certain mode of interaction were lacking.
represented a distorted rectangular cuboid, in which four I4Q
molecules (occupying cuboid’s faces) are bonded to eight I-
anions, which occupy vertices (Figure 2 and Figure S1 in the ESI).
Figure 2. 3D network formed by 4:4 halogen bonding between I4Q and I- anions
in 3I4Q·2((Bu4N) I) co-crystals. Blue lines show contacts shorter than van der
Waals separations. Counter-ions are omitted for clarity.
Intermolecular C-I∙∙∙I separations and C-I∙∙∙I- angles in these co-
crystals (Table 1 and Table S1 in the ESI) are typical for halogen
bonding.[2]
In order to examine anion- and halogen bonding or p-benzo-
quinones, we carried out experimental measurements of the inter-
molecular association of halide anions with tetraiodo-p-
benzoquinone in the solid state and in solutions.[17] Together with
the results of computational analyses of anion-, XB and HB
bonding of a variety of p-benzoquinones, illustrated in Chart 1, the
study established similarities and distinctions of different modes
of supramolecular bonding involving these molecules and factors
which affect the strength of these interactions.
Table 1. Characteristics of XB in the co-crystals of I4Q with Alk4NX- salts.[a]
X-
Br-
Br-
I-
Alk4N+
XB mode[b]
dI…X-, Å[c]
R[d]
0.88
0.85
0.87
0.87
Pr4N+
4/4
4/2
4/4
4/4
3.321 - 3.432
3.216 - 3.353
3.561 - 3.711
3.556 - 3.732
Pr4N+[e]
Pr4N+
I-
Bu4N+
[a] See Table S1 in the ESI for all crystallographically independent XB length
and angles. [b] Number of XB formed by each BQ and X-. [c] Range of XB
length.[d] R = dI∙∙∙X-/(rI+ rX-), where dI…X is an average I∙∙∙X separation, and rI and
rX- are van der Waals radii of iodine and X- anion.[19] [e] CH2Cl2 solvate.
Scheme 1. Structures and notations of halo-substituted p-
benzoquinones, with X = I, Br, Cl or F.
The cuboid cells in the XB layers are occupied by the
additional (enclosed) BQ moieties. Carbonyl oxygens of the
enclosed I4Q molecules form short contacts (2.90 - 2.98 Å) with
carbonyl carbons of BQ moieties from the opposite faces of the
cell. Three iodine substituents of each enclosed BQ molecule
form contacts shorter than the van der Waals radii with carbons
and iodines of p-benzoquinones on the remaining two faces of the
cuboid cell. These contacts can be interpreted as halogen
bonding between the iodine substituent of the enclosed benzo-
quinone (XB donor), and iodine or C-I bonds of the benzo-
quinones from the side of the cell (XB acceptor). Most notable,
however, is the fact that the co-crystals contain no short anion-
contacts.
The co-crystals formed in the dichloromethane solutions of I4Q
and Pr4NBr showed somewhat similar patterns of bonding. They
comprised benzoquinone molecules and Br- anions in a 1:1 ratio
and CH2Cl2 solvate molecules. Each I4Q molecule formed
halogen bonds via its iodine substituents with four Br- anions. In
turn, each Br- anion coordinates four benzoquinone molecules
(Figure 3). As a result, halogen-bonded I4Q molecules and
Results and Discussion
1. Intermolecular bonding in the co-crystals of p-benzo-
quinones with halide anions.
Diffusion of hexane into solutions containing I4Q and tetraalkyl-
ammonium salts of iodide or bromide in dichloromethane or
evaporation of acetone solutions containing similar mixtures
resulted in the formation of co-crystals comprising benzoquinones
and halide anions (see Experimental and the ESI for details, note
that co-crystallization of I4Q with Cl- anions was hindered by
substitution reactions).[18] In particular, co-crystals of I4Q with
Pr4NI or Bu4NI salts show the same (2:3) I- to BQ stoichiometry
and a similar pattern of intermolecular bonding. Each I4Q
molecule formed halogen bonds via its iodine substituents with
four I- anions and vice versa. Such 4:4 bonding produced layers
formed by iodides and p-benzoquinones and separated by
tetraalkylammonium counter-ions. Each cell in these layers
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Figure 4. Spectra of acetonitrile solutions with a constant concentration of I4Q
and variable concentrations of Pr4NBr. The dashed line shows the spectrum of
individual I4Q. Insert: Spectra of [I4Q, Br-] complexes obtained by subtraction
of absorption of components from the absorption of the mixture.
The changes in the intensities of absorption at λ = 440 nm with
concentration of Br- anions (at constant concentrations of I4Q)
were well-modelled using a 1:1 binding isotherm (Figure S3 in the
ESI), indicating formation of complexes according to eq 1:
Figure 3. Infinite networks formed by multicener halogen bonding between I4Q
and Br- in Pr4NBr∙I4Q∙3.5CH2Cl2 (A) and I4Q·2(Pr4N)Br (B). Counter-ions are
omitted for clarity, halogen bonds are shown as blue lines.
bromides form an infinite 3D network with channels filled with
Pr4N+ cations and CH2Cl2 solvates (Figure S2 in the ESI). The
I∙∙∙Br distances were about 12 % shorter than van der Waals
separations and C-I∙∙∙B angles were in the 167 o – 173 o range.
Evaporation of an acetone solution containing Pr4NBr and I4Q
resulted in the formation of crystals with a 2:1 Br-:BQ ratio. Each
I4Q molecule in these crystals formed XB (via each of its
substituents) with four Br- anions, and each bromide was bonded
with two p-benzoquinones. Such bonding resulted in the
formation of zigzag layers (Figure 3B) separated by the Pr4N+
counter-ions. The I∙∙∙Br distances in these co-crystals were, on
average, about 0.1 Å shorter than those in the 1:1 co-crystals in
which each bromide serves as an acceptor of four halogen bonds
(Table 1). Most notably, similar to the I4Q co-crystals with the
iodide salts, the solid-state associates between I4Q and Pr4NBr
showed no short anion- contacts. Thus, while the reported earlier
structures comprised either only anion- interactions (in co-
crystals between F4Q with Br- or I- , or between Cl2(CN)2Q and
Cl-),[15] or both anion- and XB (between Br4Q or Cl4Q and I-),[16]
all solid-state associates of I4Q showed only XB interactions
between BQ molecules and halides.
Keff
I4Q + Br-
[I4Q, Br-]
(1)
Fitting of the absorption data afforded the effective formation
constant of the [I4Q,Br-] complex of Keff = 7.5  1.0 M-1 and the
effective extinction coefficient of eff = 1.5 103 M-1 cm-1.[20]
The absorption peak maxima of the [I4Q,Br-] complex at  =
440 nm was close to that reported recently for the anion-
complexes of Br- with other X4Q molecules (i.e., 438 nm, 460 nm
and 450 nm for the complexes with F4Q, Cl4Q and Br4Q, respec-
tively[15]). Moreover, the energy of this band fits the Mulliken
dependence reported earlier for anion- complexes of halides
with various -acceptors (Figure 5). This suggests that either
2.
Complex formation between p-benzoquinones and
halide anions in solutions.
UV-Vis spectra of the solutions of I4Q in acetonitrile or
dichloromethane are characterized by an absorption band at max
= 328 nm ( = 7100 M-1 cm-1) and a shoulder at 400 nm (dashed
line in Figure 4). Addition of Br- anions to such a solution resulted
in an increase of the intensity of absorption in the 400 – 500 nm
range and some decrease of the intensity at the absorption
maximum or I4Q at 328 nm. Subtraction of the absorbance of the
components from the absorption of their mixtures revealed that
the increase of intensity is related to the appearance of a new
band (insert in Figure 4) with a maximum at max = 440 nm. In the
solutions with a constant concentration of I4Q, the absorption
intensity grew with the increase of the concentration of Br- anions.
Figure 5. Mulliken dependence between the energies of absorption bands of
anion- complexes formed by various -acceptors, A, with halides and
differences of the oxidation potential of halides and reduction potentials of A
(see Table S5 in the ESI for details).
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complexes which are observed in solutions containing I4Q and Br-
anions are formed via anion- interaction between these species
or that XB and anion- complexes have similar UV-Vis spectral
characteristics. To clarify the preferred modes of supramolecular
interactions of p-benzoquinones, we turned to computational
analysis of their complexes with halide anions, as follows.
3. Computational analysis of complexes of p-benzoquinones
with halide anions.
Geometries of the BQ∙X- associates were fully optimized via DFT
calculations using the M062X functional and def2tzvpp basis
set.[21] Previous works demonstrated that this method produces
reliable characteristics of XB and anion- complexes at a
reasonable computational cost.[15,22] To facilitate comparison with
experimental data, the following discussion will focus on the
results obtained via computations using a PCM solvation model
in acetonitrile.[23] To establish distinctions and similarities of the
anion-, XB and HB complexes of p-benzoquinones, these
calculations included associates formed by Cl-, Br- or I- anions
with tetrahalo-substituted p-benzoquinones, X4Q, as well as their
X2Q and X2(CN)2Q analogues (Chart 1).
Figure 7. The E values for anion- and XB complexes (as indicated) of halide
anions with Cl2Q (1), Br2Q (2), I2Q (3), F4Q (4), Cl4Q (5), Br4Q (6), I4Q (7),
Cl2(CN)2Q (8), Br2(CN)2Q (9), and I2(CN)2Q (10).
Optimizations resulted in multiple energy minima for each
BQ∙X- pairs. In agreement with the locations of -and -holes on
the surfaces of halo-substituted p-benzoquinones (Figure 1), they
represent anion-, HB and XB complexes (Figure 6).
First, the Cl- anions form the most stable associates, and I-
anions form the least stable associates among the analogous
complexes (i.e. either anion- or XB) of the same BQ molecule
(Figure 7).
Second, the stability of the anion- complexes is increasing in
the order X2Q < X4Q < X2(CN)2Q. The variations of E values are
well-correlated with the electron acceptor strengths of the BQ
molecules (the reduction potentials are about -0.2 V for X2Q,
Figure 6. Typical optimized geometries of the anion- (A), XB (B) and HB (C)
complexes between Br- anions and Cl2(CN)2Q (A), Br4Q (B) and 2,5-dichloro-p-ben-
zoquinone showing critical points (small yellow spheres) and bond paths (orange
lines) from AIM analysis and the blue-green discs indicating intermolecular attractive
interactions resulting from the NCI treatments (s = 0.3 au isosurfaces, a color scale of
-0.035 (blue) <  < 0.02 (red) au).
about 0.0
V for X4Q and about 0.55 V for X2(CN)2Q
molecules).[15,26]
Third, the E values for the series of the anion- complexes of
X4Q molecules with the same anion are practically constants. The
series of the anion- complexes of X2Q or X2(CN)2Q) molecules
show similar independence of the E values on the nature of the
halogen substituents in the BQ molecules.
Fourth, in contrast to the anion- complexes, the E values for
the XB complexes show substantial variations with the nature of
substituents in the BQ molecules. The iodine-substituted BQ
molecules form the most stable XB complexes and the chlorine-
substituted BQ form the least stable XB complexes (computations
of F4Q∙X- pairs did not produce a potential energy minimum
corresponding to a XB complex). Such variations were commonly
observed with various XB donors and they were related to an
increase of polarizabilities of the halogen atoms in the order F <
Cl < Br < I.[2,6]
The principal geometric characteristics of these complexes are
listed in the ESI. Analyses of these complexes using Quantum
Theory of Atoms in Molecules (AIM)[24] showed the presence of (3,
-1) critical points between XB, HB and anion- bonded species
(Figure 6). Characteristics of these critical points (electron density,
Laplacian of electron density, and the total energy density) in
different types of complexes between the same reactants are
comparable and they are consistent with non-covalent
intermolecular bonding between BQ and halides (Table S7 in the
ESI). Interestingly, in contrast to the linear bond path connecting
XB and HB bonded atoms, the anion- complex showed a curved
bond path. It is practically linear (and directed toward the midpoint
of the C-C bond) between the anion and the CP. However, the
bond path toward carbonyl carbon is bend as it approaches the
BQ rim (Figure 6, left). The Non-Covalent Interaction (NCI)
Indexes treatment[25] showed characteristic green-blue discs at
the critical points’ positions, confirming bonding interaction in all
these complexes.
Fifth, for the series of XB complexes of BQ molecules with the
same type of substituent (e.g. Br2Q, Br4Q and Br2(CN)2Q), the
stability is increasing in the order X2Q < X4Q < X2(CN)2Q. This
trend is similar to that observed for the anion- complexes, but
the variation of the magnitudes of E for the series of XB
complexes are smaller.
On the whole, the XB complexes of chloro-substituted p-benzo-
quinones are much weaker than the corresponding anion-
complexes. Halogen bonding of Br2Q, Br4Q, Br2(CN)2Q and
I2(CN)2Q molecules with halides are also weaker than the
corresponding anion- bonding, but the differences in the E
values are smaller. Finally, the stabilities of XB complexes of I2Q
and I4Q molecules are very close to those of the corresponding
anion- complexes (Figures 7, note that similarly to the anion-
Stabilization energies of anion-, XB and HB complexes were
evaluated as the difference E = Ec - EBQ - EX, where EC is the
energy of the (anion-, XB or HB) complex, EBQ is the energy of
the benzoquinone in the optimized geometry, and EX is the energy
of the anion. The E values for complexes of various BQ
molecules are illustrated in Figure 7. The analysis of these values
revealed several distinct trends.
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complexes, HB complexes of various X2Q molecules with the
same halide are characterized by essentially the same E values,
see the ESI). Thus, to resolve the distinctions in the results of the
solid-state and solution-phase studies, we scrutinized the X-ray
structural and UV-Vis spectral data.
also forming in such solutions (as suggested by the close
calculated E values and the results of the X-ray structural
analysis), their absorption is weak. As a result, it is overshadowed
by the absorption of the BQ molecules and anion- complexes.
The trends in the variations of the E values in Figure 7 (vide
supra) are consistent with the experimental values of formation
A close look at the X-ray structural data revealed that the co-
crystals of I4Q with halides salts comprise 3D or 2D networks
formed via multicenter (mostly 4:4) interactions between BQ
molecules and anions (Figures 2 and 3, Table 1). Such interaction
is facilitated by the availability of four relatively independent
halogen substituents in each BQ molecule. On the other hand,
these molecules may bind two halides at most via anion-
interaction. In fact, the reported structures of the co-crystals of
different -acceptors and various anions comprised frequently
alternating quasi-1D stacks formed via anion- interaction.[29]
Apparently, the propensity of the BQ molecules for multicenter XB
with halides (resulting in the formation of 3D networks) determines
the domination of this interaction in their co-crystals.
To clarify the mode of interaction between BQ and halides in
solution, we caried out time dependent (TD) DFT calculations of
the UV-Vis spectral characteristics of their intermolecular
associates. The absorption band energies and extinction
coefficients of the anion-, XB and HB complexes of BQ
molecules with Br- anions resultant from these computations are
listed in Table 2 (see Table S6 in the ESI for the characteristics of
complexes with Cl- and I- anions, as well as HB complexes).
constants of the anion- complexes with various BQ molecules[15]
.
These trends also agree with the results of the X-ray structural
analysis, which showed only anion- complexes of halides with
F4Q or Cl2(CN)2Q molecules, both XB and anion- complexes
with Cl4Q and Br4Q and only XB complexes with I4Q. Yet, different
types of complexes showed distinct dependencies of the E
values on the nature of substituents in BQ molecules. To find if
they could be uniformly accounted for by the same parameter, we
turn to their relation with the magnitudes of the electrostatic
potentials, ESP, on the surface of the BQ molecules.
The maximum ESP values on the surfaces of halogen and
X
hydrogen (if any) substituents, and the face the BQ ring (Vmax
VmaxH and Vmax, respectively, see Figure 1) are listed in Table 3.
,
X

Table 3. Maximum electrostatic potentials Vmax and Vmax on the surfaces
of the individual (Ind) and polarized (Pol)BQ molecules.
VmaxX, a.u.
Vmaxa.u.
Ind
0.046
0.033
0.024
0.050
0.038
0.030
-0.003
0.067
0.053
0.044
Pol[a]
0.135
0.099
0.076
0.148
0.110
0.085
Ind
Pol[a]
0.128
0.127
0.125
0.142
0.144
0.141
0.142
0.182
0.183
0.182
I2Q
0.046
0.050
0.051
0.048
0.056
0.059
0.071
0.082
0.087
0.090
Br2Q
Table 2. Spectral (calculated and experimental) characteristics of the XB and
anion- complexes of Br- anions with BQ molecules.[a]
Cl2Q
Exp[b]
Anion- (calc)[b]
I4Q
XB (calc)
h, eV
Br4Q
BQ
I4Q
log 
2.32
2.20
2.11
-
h, eV
2.84[c]
2.82
log 
3.50[c]
3.54
h, eV
log 
3.24[c]
3.08
3.34
3.41
3.73
3.23
3.34
2.82[c]
2.74
2.70
2.83
2.57
2.89
2.89
Cl4Q
2.97
3.13
3.23
-
[b]
F4Q
-
Br4Q
Cl4Q
F4Q
I2(CN)2Q
Br2(CN)2Q
Cl2(CN)2Q
0.167
0.127
0.102
2.85
3.57
2.97
3.63
DDQ
Cl2Q
25Cl2Q

2.73
3.53
3.53
-0.42[d]
2.30
2.15
2.00
1.15[e]
2.69
3.85
[a] Polarization induced by the 1e negative charge located at the position of XB
or anion- bonded Br- anion in the corresponding complex. See Table S5 in the
ESI for the values resulted from the polarizations by charges placed in the
3.05
3.54
2.98
3.48
-0.11[d]
-0.25[e]
H
location of Cl- and I- anions in the corresponding complexes. Note that Vmax
are (in a.u.) 0.051 (Cl2Q), 0.051 (Br2Q) and 0.047 (I2Q) for the individual and
0.109 (Cl2Q), 0.110 (Br2Q) and 0.112 (I2Q) for the HB-polarized molecules. [b]
No XB complex was found.
[a] See Table S6 in the ESI for the analogous data for the complexes with
chloride and iodide anions. [b] From ref. [15], if not noted otherwise. [c] Current
work. [d]  = 1/n (hcalc - hexp). [e]  = 1/n (log calc – log exp).
Within the series of X2Q, X4Q or X2(CN)2Q molecules, the
values of VmaxX are decreasing in the expected order I > Br > Cl >
F (the potential on the surface of fluorine substituent in F4Q is
negative). For the BQ molecules with the same substituents (e.g.,
chlorine), the values of both VmaxX and Vmax increase in the order
X2Q < X4Q < X2(CN)2Q. These trends resemble the changes of
The calculated energies of absorption bands of the anion-
complexes are much closer to the experimental data than those
of the XB complexes (the mean deviations are 0.11 and 0.42 for
the anion- and XB complexes respectively). Similarly, the
intensities of the calculated bands of the anion- complexes are
reasonably close to the experimental observations (with the
average difference of log  of  = 0.25). In comparison, the
intensities of the calculated bands of the XB complexes are more
than an order of magnitude lower ( = 1.15). These results agree
with the predominant formation of anion- complexes in solution.
The preferable formation of anion- complexes is consistent with
the higher stability of these associates for the majority of the
BQ•X- pairs resulting from the calculations. It also suggests the
anion- nature of the complex that is observed upon mixing of Br-
anions with I4Q in solution. While it is likely that XB complexes are

E in Figure 7. However, Vmax are also substituent-dependent.
For example, values of Vmax for X4Q molecules decrease in the
order F > Cl > Br > I. Yet, the E values for the complexes of these
molecules with the same anion are essentially constant (Figure 7).
Thus, the correlation between E and corresponding Vmax values
for all complexes is weak (R2 = 0.75, see Figure S4 in the ESI).
It is well-established, however, that the interacting species in
supramolecular complexes substantially polarize each other, and
their electrostatic attraction is determined by the values of Vmax on
5
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10.1002/cplu.202000012
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the surfaces of the polarized molecules.[27] The effect of
polarization is especially pronounced in the complexes involving
charged species. For example, we have shown earlier that the
formation constants of XB and HB complexes of trihalomethanes
(CHX3) with halide anions are highly correlated with the values of
maximum potentials on the surfaces of halogen and hydrogen
atoms of CHX3 only if polarizations of these molecules in XB and
HB complexes were taken into account.[28] In a similar way, the
ESPs of BQ molecules are substantially altered in the presence
of neighboring anions and the magnitudes of changes are
determined by the position of the latter. Specifically, the XB anions
increase the potential on the surfaces of the bonded halogen
substituents, while the -bonded halides increase Vmax on the
force in formation of all types of intermolecular complexes of BQ
molecules with halide anions. Yet, while the XB and HB
complexes follows essentially the same trend line, the points
representing anion- complexes are located somewhat higher.
Specifically, the anion- complexes are 0.5 - 1.0 kcal/mol more
stable than the XB/HB complexes, if the values of Vmax  are about
the same as that of VmaxX or VmaxH. This suggests that the larger
contribution of molecular-orbital interactions in the anion-
complexes between BQ molecules and halides, which is
responsible for the increased intensities of their charge-transfer
bands (Table 2), provides additional ground-state stabilizations of
these associates.
X
H
surfaces of the BQ rings facing this anion. The Vmax Vmax and
,

Vmax values for the BQ molecules polarized by a negative unity
Conclusion
charge located at the position of XB, HB, or anion- bonded
bromides, respectively, are listed in Table 2. The Cl- or I- anions
induced a similar increase of the Vmax values as compared to that
of individual molecules (see Table S5 in the ESI). The magnitudes
of these changes were consistently larger when the charges were
located at the positions of Cl- anions (since they were closer to
the BQ molecule), while the polarization by charges at iodide
positions were smaller.
Computational analysis of intermolecular interactions between
important redox reagents, halo-substituted p-benzoquinones, and
halide anions revealed that stabilities of their anion-, XB and HB
complexes are well correlated (regardless of the mode of
interaction) with the values of the maximum electrostatic
potentials provided that polarization of the BQ molecules by
neighboring anions are taken into account. For the majority of the
BQ molecules, anion- interactions led to the most stable
complexes. For the I4Q molecule, the calculated stabilities of the
anion- and XB complexes were close. Thus, the predominance
of halogen bonding in the co-crystals of I4Q with halide salts is
most likely related to the fact that it is better suited for the
multicenter interactions between BQ molecules and anions
leading to the formation of stable solid-state 3D networks. On the
other hand, XB (and HB) complexes between BQ molecules and
halides are characterized by much lower molar absorptivity’s than
their anion- bonded counterparts. Therefore, only the latter were
apparently observed in the UV-Vis studies in solution.
The effects of polarization on VmaxXB and Vmax values increase
from chloro- to bromo- and iodo-substituted BQ molecules (i.e.,
the iodo-substituted molecules showed the largest increase).
X
Since the Vmax values of individual molecules rise in the same
X
order, the polarization increases distinctions between Vmax
values in chloro-, bromo- and iodosubstituted molecules. In

contrast, due to the opposite trends of Vmax of individual
molecules and the effects of polarization, the valuesofVmax in the
series of X2Q, X4Q or X2(CN)2Q molecules are essentially
independent on the halogen substituent. The latter agrees well
with practically constant values of E in such series. Most
importantly, Figure 8 demonstrates good correlation between the
E and Vma values for all complexes regardless of nature of
x
bonding (anion-, XB and HB) and halide anion (Cl-, Br- or I-).
Experimental Section
Commercially available halide salts (Bu4NBr, Pr4NBr, Bu4NI, and Pr4NI)
were purified by recrystallization. Tetraiodo-p-benzoquinone was prepared
according to a procedure reported earlier.[30] In brief, tetrabromo-p-
benzoquinone was treated with 2 equivalents of KI and subsequently with
2 equivalents of NaI in ethanol at room temperature. It afforded, after re-
crystallization from ethyl acetate, dark-brown crystals in quantitative yield.
CH2Cl2 and CH3CN (HPLC grade) were freshly distilled over P2O5 under
dry argon.
UV-Vis measurements were performed under Ar atmosphere. Formation
constants and extinction coefficients of the intermolecular complexes were
evaluated via UV-Vis titrations (by an increase of concentration of halide
salts in the solutions with constant concentrations of BQ (see the ESI for
the details).
Single crystals of I4Q·2((Pr4N)Br) and 3I4Q·2((Bu4N)I) suitable for the X-
ray structural measurements were obtained by evaporation of the solutions
containing 1:1 mixtures of BQ and halide salts in acetone. Single crystals
of I4Q·((Pr4N)Br)·3.5CH2Cl2 and 4.5I4Q·3((Pr4N)I)·CH2Cl2 were prepared
by diffusion of hexane into dichloromethane solutions containing similar
mixtures of p-benzoquinone and a halide salt at low temperatures (-70o C).
These crystals were coated with oil and quickly transferred to the
goniometer head of a Bruker AXS D8 Quest CMOS diffractometer with a
fixed chi angle and a molybdenum radiation (Mo K) sealed tube fine focus
X-ray tube (λ = 0.71073 Å). Reflections were indexed, processed, scaled
and corrected for absorption using APEX3.[31,32] The space groups were
assigned and the structures were solved by direct methods using XPREP
within the SHELXTL suite of programs and refined by full matrix least
Figure 8. Relationship between Vmax values in the polarized BQ molecules and
E values for their XB, HB and anion- complexes (as indicated) with Cl-, Br- or
I- anion.
The almost uniform relationship between E and Vmax values in
Figure 8 indicates that electrostatic attraction represents the main
6
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10.1002/cplu.202000012
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FULL PAPER
squares against F2 with all reflections using Shelxl2016[33] using the
graphical interface Shelxle.[34] Crystallographic, data collection and
structure refinement details are presented in Table S2 in the ESI.
Complete crystallographic data, in CIF format, have been deposited with
the Cambridge Crystallographic Data Centre. CCDC 1975219
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(I4Q·((Pr4N)Br)·3.5CH2Cl2),
1975220
(I4Q·2((Pr4N)Br)),
1975221
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supplementary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.
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Geometries of the complexes were optimized without constraints via DFT
calculations with the M06-2X functional[21] and def2tzvpp basis set using
the Gaussian 09 suite of programs.[35] The absence of imaginary
frequencies confirmed that the optimized structures represent true
minima. .Calculations in CH3CN were carried out using a polarizable
continuum model.[23] Values of E were determined as: E = Ec – (EBQ
+
EX), where Ec, EBQ and EX are sums of the electronic and ZPE of the
optimized complex, BQ and anion.[36] UV-Vis spectra were obtained via
TD-DFT calculations. Electrostatic potentials were calculated on 0.001
electrons bohr-3 molecular surfaces. AIM and NCI analyses were
performed with Multiwfn using wfn files generated by Gaussian 09.[37]
Characteristics of the complexes are listed in the ESI.
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Supporting Information. Details of calculations of formation constants,
crystallographic analysis and computations. This material is available free
of charge via the Internet at http:.
Acknowledgments
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We thank the National Science Foundation for the financial
support of this work (grant CHE-1607746). X-ray structural
measurements were supported by the National Science
Foundation through the Major Research Instrumentation Program
under Grant No. CHE-1625543 (funding for the single crystal X-
ray diffractometer).
Keywords: Anion- interaction • Halogen bonding • Halides •
Quinones • Supramolecular chemistry
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with chloride anions. For example, X-ray structural analysis showed that
crystallization from the solution of I4Q with Bu4NCl resulted in substitution
of iodine and formation of co-crystals of I4Q with I- anions identical to that
in Figure 2 and unidentified oil by-products, Crystallization from the
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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, J. Tomasi, M. Cossi,
N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C.
Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G.
Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D.
8
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10.1002/cplu.202000012
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The dichotomy between experimental observations of halogen bonding in the solid state and anion- complexes in solutions
containing tetraiodo-p-benzoquinone and halides salts is reported. Factors determining strengths of different modes of intermolecular
interactions between halo-substituted p-benzoquinones and halides, in general, are elucidated.
9
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