Intermolecular interactions in bromo-, methyl-, and cyanoimidazole derivatives

Article abstract of DOI:10.1021/cg400273p

Materials containing bistable N-H...N hydrogen bonds, such as imidazole crystals, show promise for applications in electronics. Herein, we examine the effect of imidazole functionalization upon structural parameters relating to proton transfer, molecular

Full text of DOI:10.1021/cg400273p

Article
pubs.acs.org/crystal
Intermolecular Interactions in Bromo‑, Methyl‑, and Cyanoimidazole
Derivatives
Christopher J. Serpell*,† and Paul D. Beer
Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford, OX1 3TA, U.K.
S
* Supporting Information
ABSTRACT: Materials containing bistable N−H···N hydrogen bonds, such as
imidazole crystals, show promise for applications in electronics. Herein, we
examine the effect of imidazole functionalization upon structural parameters
relating to proton transfer, molecular rotation, and order−disorder transitions.
Three different substituents are studied: methyl-, bromo-, and cyano-, resulting in
steric, electronic, and supramolecular modification of the imidazole core.
aterials containing switchable bistable hydrogen bonds
halogen bonding observed in neutral imidazole derivatives
Mare of current interest for their potential ferroelectric could serve to facilitate the design of new XB motifs, a field that
is sparsely populated compared with the number of established
hydrogen-bonding units available to supramolecular chemists.
We present herein crystal structures of methyl- and
bromoimidazole derivatives with different substitution patterns
and assess the intermolecular forces displayed therein. We also
discuss the effect of replacing the electron-donating methyl
groups with electron-withdrawing nitrile functionalities and the
resulting increase in polarization of the halogen atom.
and relaxor properties: ordering of hydrogen-bonded chains can
result in permanent dipole moments (either in the bulk or in
nanodomains), which may then be reversed under an externally
applied electric field. Although most reported ferroelectric
materials are inorganics, simple organic N−H···N hydrogen-
bonded materials, such as monoprotonated 1,4-diazabicyclo-
[2.2.2]octane1,2 or imidazole,3 are now receiving attention.
Furthermore, the presence of bistable hydrogen bonds with a
low barrier to proton transfer can give rise to proton
conductivity, for which imidazole is also of noted interest.4
However, in order to maintain a continuous current,
reorganization of individual molecules must occur, against
which there is a high energetic penalty in imidazole itself.5 Both
ferroelectricity and proton conductivity could be exploited to
make smaller, greener, and cheaper electronic devices.
Recent studies have worked to improve proton transfer
properties of crystalline imidazole and benzimidazole using
either high-pressure6,7 or cocrystallization8,9 techniques. Proton
transport, molecular rotation, and order−disorder transitions
could be also addressed through chemical modification, and
hence the crystal structures of small functionalized imidazoles
that may display altered packing are of interest in this field.
Very recently, imidazolium salts substituted on the ring with
a halogen have come to the fore as a major halogen-bonding
(XB) motif10−14 due to the large covalent contribution15 and
overall positive charge. However, the inductive and mesomeric
electron-withdrawing effect of the nitrogen atoms in neutral
imidazole compounds may also permit a degree of halogen
bonding, and therefore, it is possible that the packing of
nonquaternized halo-imidazole crystals could be influenced in
new ways by halogen-bonding interactions. Furthermore, any
EXPERIMENTAL PROCEDURES
■
Synthesis. The syntheses of 4,5-dimethyl-, 2-bromo-4,5-dimethyl-,
and 4,5-dibromo-2-methylimidazole have been reported previously.15
1H and 13C NMR spectra were recorded on a Varian Mercury
VX300 spectrometer. Mass spectrometry was performed on a Bruker
microTOF (ESI) spectrometer.
2-Bromo-4,5-dicyanoimidazole.16 4,5-Dicyanoimidazole (1.00 g,
8.47 mmol) was added to 1 M NaOH (aq) (25 mL) and stirred until
complete dissolution had been achieved. Elemental bromine (1.5 mL,
29.6 mmol) was added carefully, and the resultant mixture was stirred
under a nitrogen atmosphere overnight, producing a voluminous white
precipitate. The mixture was extracted with ethyl acetate (3 × 25 mL),
which was dried over MgSO4, and filtered, giving 2-bromo-4,5-
dicyanoimidazole as a white crystalline solid upon solvent removal
1
(1.42 g, 85%). H NMR (300 MHz, DMSO-d6) δ (ppm) no peaks;
13C NMR (125 MHz, DMSO-d6) δ (ppm) 126.7, 118.4, 112.1; ESMS
m/z calcd for [M + H]+ 196.9, found 197.1; mp 84 °C.
N-Benzyl-2-bromo-4,5-dicyanoimidazole. Benzyl bromide (0.97 g,
5.70 mmol), 2-bromo-4,5-dicyanoimidazole (1.02 g, 5.17 mmol), and
K2CO3 (7.16 g, 51.7 mmol) were stirred overnight in DMF (50 mL) at
Received: February 17, 2013
Revised: May 20, 2013
© XXXX American Chemical Society
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Table 1. Selected Crystallographic Data for Structures Discussed Herein
room temperature. After filtration and washing of the solids with
RESULTS AND DISCUSSION
Bromo- and Methylimidazoles. The primary packing
motif in imidazole-based crystals consists of N−H···N hydro-
gen-bonded chains of heterocycles, with adjacent molecules
oriented in an approximately antiparallel arrangement (Figure
1). Such a structure permits an extended network of linear N−
■
acetone (10 mL), the solvent was removed in vacuo, leaving the crude
product. Purification was achieved by addition of CH2Cl2 (50 mL) and
subsequent washing with water (3 × 50 mL), followed by drying over
MgSO4, filtration, and solvent removal, giving the product as a white
crystalline solid (0.51 g, 34%). 1H NMR (200 MHz, CDCl3) δ (ppm)
7.30−7.14 (5H, m, ArH), 5.19 (2H, s, CH2); 13C NMR (125 MHz,
CDCl3) δ (ppm) 132.1, 129.6, 129.5, 127.8, 126.4, 123.5, 114.5, 110.6,
107.4, 52.2; HR-ESMS m/z calcd for [M + Na]+ 308.9746, found
308.9744; mp 76 °C.
Crystallography. Crystals were obtained in each case by slow
evaporation of organic solvents, as specified in the main text (see
Table 1). Single-crystal diffraction data were collected using a Nonius
Kappa CCD equipped with a Cryostream N2 open-flow cooling
device17 operating at 150(2) K using graphite monochromated Mo Kα
radiation (λ = 0.71073 Å). Series of ω-scans were performed in such a
way as to collect every independent reflection to a maximum
resolution of 0.77 Å, aiming for 99.5% completeness. Cell parameters
and intensity data (including interframe scaling) were processed with
DENZO-SMN.18 Structures were solved with SIR9219 or SuperFlip20
and refined using the CRYSTALS package.21−23 All non-hydrogen
atoms were refined with anisotropic displacement parameters. The H
atoms were usually visible in the difference map, but those attached to
carbon atoms were repositioned geometrically. Protic H atoms that
could not be located in the difference map were positioned to satisfy
hydrogen-bonding requirements. The H atoms were initially refined
with soft restraints on the bond lengths and angles to regularize their
geometry (C−H in the range of 0.93−0.98 Å, N−H in the range of
0.86−0.89 Å, and O−H = 0.82 Å, and isotropic displacement factors in
the range of 1.2−1.5 times Ueq of the parent atom), after which the
positions were refined with riding constraints.
Figure 1. Archetypal hydrogen-bonded packing of imidazoles into
chains.
H···N hydrogen bonds with a good degree of close-packing.
However, depending upon the substitution pattern of the
heterocycle, the hydrogen-bonding distances (and hence
proton transfer barriers) and geometry of the chains may be
significantly affected by additional interactions, which are the
features of note in this study.
In the reports of the crystal structure of imidazole itself24−28
(Figure 2), adjacent molecules in the chains are rotated by
approximately 120° with respect to each other along the axis of
the hydrogen-bonded chain, a distortion that permits the
approach of the relatively electron-poor C2 proton to the
nitrogen lone pair, thus linking the 1D chains into 2D layers
(Figure 2): this can be considered an optimization of the
archetypal chain motif that provides an additional electrostatic
interaction. The rotation of molecules dominates over π-
stacked dispersion interactions, which would be possible in a
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Figure 2. X-ray crystal structure of imidazole (CCDC code
IMAZOL03) showing (left) strong N−H (blue) and polar C−H···N
(gray) contact, and (right) rotation of imidazole molecules within N−
H···H bonded chains. Thermal ellipsoids shown at 50% probability.
purely planar array due to the electron-rich nature of the
heterocycle. The reported N(H)···N distance generally varies
between 2.842 and 2.869 Å.
Unexpectedly, 4,5-dimethylimidazole crystallized from dieth-
yl ether with three molecules in the asymmetric unit (we have
previously reported this structure,15 but without analysis)
(Figure 3); it can be seen that the steric demands of the methyl
Figure 4. X-ray crystal structure of 2-methylimidazole showing (top)
hydrogen bonds (blue) and C−H···π interactions (gray), and
(bottom) formation of a tetrad of hydrogen-bonded chains by C−
H···π interactions. Thermal ellipsoids shown at 50% probability.
the δ− π system. Again, a short N(H)···N distance is observed
(2.824(4) Å).
Figure 3. X-ray crystal structure of 4,5-dimethylimidazole, showing
hydrogen bonds (blue) and C−H···π interactions (gray). Thermal
ellipsoids shown at 50% probability.
Thus, we can see that, in imidazole itself, and C-substituted
methyl derivatives, the N−H···N hydrogen-bonded chains are
supplemented by C−H···π interactions over and above π−π
stacking, which would be permitted in alternative packing
arrangements with the rings coplanar. A survey of simple
imidazole compounds in the Cambridge Structural Database
(CSD) found no examples of π−π stacked rings, indicating that
comparatively strong electrostatic attractions between the δ+
protons and the electron-rich π surface outweigh π−π stacking,
which only arises primarily due to weak dispersion forces.
In both 2-bromo-4,5-dimethylimidazole and 4,5-dibromo-2-
methylimidazole, the location of the N−H proton could not be
determined, the two sides of the imidazole being crystallo-
graphically indistinguishable, even with respect to bond lengths
within the ring. It is likely that the donor−acceptor polarity of
the hydrogen-bonded chains varies throughout the crystal,
resulting in nano- or microdomains, which may indicate
ferroelectric or relaxor properties. The mildly electron-with-
drawing effect of the bromine atoms may be responsible for the
disorder by increasing the acid−base self-dissociation constant.
The crystal structure of 2-bromo-4,5-dimethylimidazole,
grown from chloroform (Figure 5), displayed the greatest
similarity to the classical hydrogen-bonded chains, exhibiting no
other close contacts. In this case, the N(H)···N distance is long
at 2.920(5) Å, presumably due to the increased steric bulk,
indicating a higher kinetic barrier to proton transfer than in
imidazole.The size of the bromine atom appears to be
complementary to the steric demands of the two methyl
groups, allowing an almost perfectly antiparallel arrangement,
the angle between the planes of adjacent imidazole rings being
groups interfere with the optimal close-packing pattern for
unsubstituted imidazole, although the hydrogen-bonded chain
is preserved. In this case, two of the molecules are mutually
antiparallel (173.5° rotation between the two rings), but the
third is distinctly out of plane (rotated by 72.7° and 67.8°
compared to the adjacent imidazoles), and takes part in an
interchain C−H···π interaction, electrostatically driven by the
electron-rich nature of the ring compared to the protons.
Overall, each hydrogen-bonded chain has become helical, with
both hands being represented in the extended structure due to
the centrosymmetric space group (P21/c). The variety of
orientations in this structure suggests that molecular reorgan-
ization may have a lower energetic barrier than in imidazole
itself, an observation consistent with the lower density (1.112
compared with 1.287 g mL−1). At 2.822(5), 2.834(5), and
2.861(5) Å, the N(H)···N distances include two significantly
shorter than those in imidazole itself, indicating a reduced
barrier to proton transfer. 4,5-Dimethylimidazole is, therefore,
an interesting candidate for future proton conductivity studies.
The structure of 2-methylimidazole has been reported
once,29 but was measured again in this study (Figure 4).
Again, hydrogen-bonded chains were observed, but in this case,
the hydrogen-bond-linked imidazoles are rotated 168° with
respect to each other. The C5 proton approaches the center of
a nearby imidazole ring, in a C−H···π contact likely to be
driven by electrostatic differences between the δ+ proton and
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adding methyl groups alone can reduce N(H)···N distances,
which may improve proton conductivity, attachment of
bromines results in disorder, which may indicate ferroelectric
properties.
2-Bromo-4,5-dicyanoimidazoles. To assess the effect of
the polarization of the bromine atom in these neutral
imidazoles, the methyl groups were replaced with strongly
electron-withdrawing cyano groups (Scheme 1). 4,5-Dicyanoi-
Figure 5. X-ray crystal structure of 2-bromo-4,5-dimethylimidazole.
Proton disorder omitted for clarity. Thermal ellipsoids shown at 50%
probability.
Scheme 1. Synthesis of 2-Bromo-4,5-dicyanoimidazoles
just 25.3°. This structure highlights the cause of the reduction
in symmetry in 4,5-dimethylimidazole: without the bromine,
the steric demands of the methyl groups on adjacent-but-one
units cause a clash that is resolved by the emergence of helicity.
In 4,5-dibromo-2-methylimidazole (Figure 6), a long N-
(H)···N distance is again observed (2.916(11) Å), suggesting a
midazole was treated with elemental bromine and aqueous
sodium hydroxide to give the 2-bromo derivative in 85% yield.
Since haloimidazolium salts are of current interest, we also
attempted the stepwise N-alkylation, successfully at first, using
potassium carbonate and benzyl bromide in DMF, giving N-
benzyl-2-bromo-4,5-dicyanoimidazole. However, the second N-
benzylation could not be achieved even under forcing
conditions, such as a pressurized microwave reactor with
benzyl bromide acting as the solvent, presumably due to the
electron-poor nature of the ring depleting the nucleophilicity of
the nitrogen atom.
Despite the difficulty in obtaining positively charged 4,5-
dicyano-2-halo-imidazolium compounds, given the results
discussed above, it appeared likely that the neutral heterocycle
could be sufficiently electron-withdrawing to induce a σ hole on
the polarizable halogen, and thus provide a new uncharged
motif for halogen bond formation. Such interactions have been
observed in both 2- and 5-chloro-1-methyl-4-nitroimidazole,35
which is, in principle, less favorably disposed to halogen bond
formation than 2-bromo-4,5-dicyanoimidazole, possessing
fewer electron-withdrawing groups and a less polarizable
halogen. Single crystals suitable for X-ray diffraction were
obtained for 2-bromo-4,5-dicyanoimidazole to examine its
hydrogen bonding behavior, and for its N-benzyl derivative to
provide a second perspective on any halogen-bonding proper-
ties.
Figure 6. X-ray crystal structure of 4,5-dibromo-2-methylimidazole
showing (left) hydrogen bonds (blue) and Br···Br interactions
(brown), and (right) side view of Br···Br interactions. Thermal
ellipsoids shown at 50% probability.
comparatively high kinetic barrier to proton transfer, while the
disorder also indicates the presence of polarity domains.
While no Br···Br contacts were seen in 2-bromo-4,5-
dimethylimidazole, by contrast, in 4,5-dibromo-2-methylimida-
zole, each bromine atom has close contacts (3.552(4) Å, RBrBr
=
0.9630) with two others, one almost linear (162.5(15)°), and
another that is bent (122.7(17)°). The geometry implies that
this is due to the difference in charge between the δ+ tip of the
halogen (the σ hole31), and the δ− belt around its meridian,
leading to formation of weak halogen bonds in which each
bromine atom acts both as a donor and as an acceptor.
Presumably, the interaction is facilitated by a combination of
the electronic effects of the ring substituents, and the geometric
factors that give rise to this packing pattern. Although such
amphiphilic halogen bonding has rarely been discussed for
bromine32 (as opposed to iodine33,34), a search of the
Cambridge Structural Database for sub-van der Waals C−
Br···Br−C contacts with angles of 90° and 180° ( 10°) gave
227 hits. This figure represents more than half of the 410
results returned without specifying the angles, indicating that
these interactions are frequently overlooked.
2-Bromo-4,5-dicyanoimidazole was found to crystallize from
evaporating acetone with two molecules of water per molecule
(Figure 7). Strong hydrogen bonds enforced the crystal
packing, with both nitrogen atoms and water molecules acting
as donors and acceptors. While the O−H···N(nitrile) hydrogen
bonds were always present, the location of the protons involved
in hydrogen bonding with the heterocycle core displayed long-
range disorder. As with the bromomethylimidazoliums
discussed above, it is likely that nano- or microdomains of
donor−acceptor polarity are formed. Although the bromine
Despite the preservation of the primary supramolecular motif
(the hydrogen-bonded donor−acceptor chain of hermaphro-
ditic imidazole molecules), these structures display a great
variety among them. CH−π interactions are observed, as well
as simultaneous donor/acceptor halogen bonding. Interestingly,
no conventional face-to-face π−π contacts are seen. Although
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the point of contact closer to the centroid of the ring). This is
consistent with the delocalized aromatic electron density acting
as a halogen bond acceptor, analogous to the complex of
benzene and dibromine with a C···Br distance of 3.18 Å.37 Such
halogen-bonding contacts with π systems have also been
documented with the CN bonds of cyanide ligands attached
to metal centers,38,39 and to the π systems of aryl ligands for
metals,40 and are perhaps more common still.41 Furthermore,
significant lone pair−π interactions are seen between the
imidazole nitrogen and the electron-deficient carbons of the
nitrile groups on an adjacent molecule (3.073(5), 3.128(5),
3.072(5), 3.244(5) Å; RCN = 0.95−0.99). The Lewis basic
nitrogen bridges the dicyano system, sitting almost equidistant
from each carbon. The intermolecular forces at play in this
crystal serve, therefore, to highlight the variety of intermo-
lecular interactions that can occur using simple molecules, and
have yet to be employed in the strategic fabrication of self-
assembled supramolecular constructs.
Figure 7. X-ray crystal structure of 2-bromo-4,5-dicyanoimidazole·
2H2O, showing disordered (black) and ordered (gray) hydrogen
bonds. Pentagonal supramolecules indicated with dotted red lines.
Thermal ellipsoids shown at 50% probability.
CONCLUSIONS
■
We have reported six new crystal structures of imidazole
derivatives, which display a variety of intermolecular
interactions. From a fundamentals perspective, the (albeit
weak) halogen bonds formed despite the lack of quaternization
(i.e., positive charge on the heterocycle) are of particular note:
amphiphilic Br···Br halogen bonds in the case of 4,5-dibromo-
2-methylimidazole and Br···π in N-benzyl-2-bromo-4,5-dicya-
noimidazole. Such interactions could be used for further crystal
engineering applications, or taken beyond into broader
supramolecular assembly and host−guest chemistry. Viewed
through the lens of proton-transfer materials, the shorter
N(H)···N distances combined with lower densities in
methylimidazoles are of interest for proton conduction studies,
whereas the disorder seen in the bromo derivatives may result
in interesting ferroelectric and relaxor properties.
atom is directed toward an electron-rich end of an adjacent
molecule (i.e., dipoles are aligned), no halogen short contacts
occur.
Interestingly, if the two water molecules hydrogen-bonded
through the ring nitrogen atoms are considered as part of a
single supramolecular entity with the heterocycle, forming a
pseudo-5-fold symmetric tile (indicated by the dashed red
lines), the arrangement constitutes the optimal 2D close-
packing for regular pentagons, which has only recently been
reported in a highly engineered crystal structure.36 More
intriguingly in this case, the inclusion of water from an
(undried) organic solvent means that the packed pentagonal
structure has been favored spontaneously by a strong
thermodynamic driving force here, rather by human design.
The crystal structure of N-benzyl-2-bromo-4,5-dicyanoimi-
dazole (Figure 8), containing two molecules in the asymmetric
unit, displayed a number of interesting features. While no
classical halogen bonding was observed, close contacts
(3.365(4) and 3.317(4) Å, RBrC = 0.95, 0.93) were observed
between the bromine atoms and the π system of the benzene
ring, with nearly linear geometry (165.68(15) and
162.00(16)°) to an aryl carbon atom (exactly 180° would put
ASSOCIATED CONTENT
* Supporting Information
Crystal structures in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: christopher.serpell@mcgill.ca.
■
Present Address
†Department of Chemistry, McGill University, Room 400, 801
Sherbrooke St. West, Montreal, Quebec, H3A 0B8, Canada.
Author Contributions
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Oxford University Chemical Crystallography for use
of the instruments, and Amber L. Thompson for technical
expertise and comments on this manuscript. C.J.S. thanks
Johnson Matthey and the EPSRC for a CASE studentship and
the EPSRC for a PhD+ award.
Figure 8. X-ray crystal structure of N-benzyl-2-bromo-4,5-dicyanoi-
midazole showing nonclassical halogen bonds (brown) and N−C
interactions (blue). The black- and white-bonded molecules are not
symmetry-related. Thermal ellipsoids shown at 50% probability.
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