Substituent effects in solid-state assembly of activated benzotriazoles

Article abstract of DOI:10.1039/c8ce01757c

Aromatic donor-acceptor stacking involving electron-rich π-donors and electron-deficient π-acceptors has been utilized in a broad spectrum of diverse applications to great effect. We report the discovery of unprecedented donor-acceptor stacking from a non

Full text of DOI:10.1039/c8ce01757c

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Substituent effects in solid-state assembly of
activated benzotriazoles†
Cite this: DOI: 10.1039/c8ce01757c
Daniel S. Wenholz,a Mohan Bhadbhade,b Hakan Kandemir,ac Junming Ho,a
a
a
*
Naresh Kumar
and David StC. Black
Aromatic donor–acceptor stacking involving electron-rich π-donors and electron-deficient π-acceptors
has been utilized in a broad spectrum of diverse applications to great effect. We report the discovery of un-
precedented donor–acceptor stacking from a non-mixed activated benzotriazole scaffold to give cofacial
π-stacking in 2D sheets. We further report the effects of altering the substituents of the bicyclic aromatic
system. It was found that any alteration made to the substituents disrupted the cofacial stacking, however,
both aromatic donor–acceptor stacking and 2D sheet formation was observed in the solid-state assembly
of the analogues. Furthermore, interactions that satisfied both electrostatic and direct-substituent
π-stacking models were observed.
Received 15th October 2018,
Accepted 19th December 2018
DOI: 10.1039/c8ce01757c
rsc.li/crystengcomm
role in the degree of cofacial overlap of stacked ring systems.
However, there has been limited discussion on how this “di-
Introduction
Noncovalent aromatic interactions such as π-stacking play
fundamental roles within supramolecular chemistry and bio-
logical recognition.1,2 These interactions are of key impor-
tance in both synthetic applications, such as self-assembly ar-
chitectures,3 to naturally occurring phenomena including
DNA base–base interactions.4–6 Despite this prominence,
π-interactions remain perhaps the least understood non-
covalent interaction.
In the 1990s a model explanation was postulated by
Hunter and Sanders in which electrostatics brought about by
the quadrupole moment of each ring play a dominant role in
π–π interactions. This “electrostatic model” was used to ex-
plain a perceived preference for electron deficient aromatic
systems to stack with electron rich aromatics in a face-
centered manner.7 By this explanation, the main attractive
force was due to the differential polarization between the two
aromatic systems.8 The resultant alternated stacking between
electron rich and electron deficient aromatics has since been
referred to as “aromatic donor–acceptor” stacking or interac-
tions.9 More recent experimental work by Rashkin and Wa-
ters10 and Snyder et al.,11 along with numerous computa-
tional studies,12–16 have shown that direct substituent–
substituent and substituent–ring interactions play a major
rect interaction model” applies to aromatic donor–acceptor
stacking.
Since their inception donor–acceptor interactions have
been utilized to great effect in molecular architectures,17–21
self-assembly,22–26 self-healing polymers,27,28 molecular
recognition29–32 and catalysis.33 Stoddart and co-workers have
used a variety of aromatic donor–acceptor systems in the self-
assembly of molecular architectures of catenanes and
rotaxanes.34–39 These works have included the use of electron
deficient bipyridinium residues as π-acceptors and electron
rich 1,5-dialkoxynaphthalene (DAN), hydroquinone, and resor-
cinol residues as π-donors. Iverson and co-workers have un-
dertaken extensive work on DAN donor and 1,4,5,8-
a School of Chemistry, UNSW Sydney, Australia. E-mail: d.black@unsw.edu.au
b Mark Wainwright Analytical Centre, UNSW Sydney, Australia
c Department of Chemistry, Faculty of Art and Science, Namik Kemal University,
Tekirag, Turkey
† Electronic supplementary information (ESI) available: Experimental proce-
dures, characterisation data, and X-ray crystal data. CCDC 1856989–1856995. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8ce01757c
Scheme 1 Reagents and conditions; i) Ac2O, 0 °C, 2 h, 94%; ii) HNO3/
Ac2O, 0 °C, 30 min, 59%; iii) N2H4·H2O, Pd/C, EtOH, reflux, 2 h, 98%;
iv) NaNO2/HCl, 0 °C, 10 min, 87%.
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Fig. 1 X-ray crystal structure of activated benzotriazole 1 with hydrogen bonds displayed as blue dotted lines; a) anti-parallel continuous stacking
with perpendicular stacking distance displayed as black dotted line, distance measured in angstroms; b) overlap of ring systems as viewed along
b-axis, ellipsoids are shown at 50% probability; c) 2D sheets viewed along a-axis; d) single layer of 2D sheets viewed along b-axis.
naphthalenetetracarboxylic diimide (NDI) acceptor systems in
numerous applications including foldamers,40 the self-
assembly of aggregates41 and macromolecules,42 artificial
DNA bases,43 thermochromic materials,44 and mesophases.45
Although reports of a limited number of DAN–NDI and re-
lated donor–acceptor system cocrystals exist in the
literature,20,21,31,44–46 there is a distinct lack of crystal struc-
tures exhibiting donor–acceptor stacking from a homo-
molecular or non-mixed system. Whilst such homo-molecular
interactions have been observed in dyads developed for
organic-based devices, these typically involve very large aro-
matic polycycles including perylenes and hexabenzocoron-
enes.47,48 Based upon the DAN–NDI system, Peebles et al. de-
veloped a series of significantly smaller donor–acceptor
solvent. Crystal structures for similar MAN–NI dyads devel-
oped by Benanti et al. showed only donor–donor and accep-
tor–acceptor stacking, although it should be noted that this
stacking is beneficial to an organic-based device.50
The formation of cofacial π-stacked aromatic ring systems
in 2D sheets has been sought after to improve charge-carrier
transport efficiency due to increased overlap in molecular or-
bitals.51,52 In this study we report the rather serendipitous
discovery of homo-molecular stacking from substituted
benzotriazoles possessing both π-donor and π-acceptor moie-
ties and its use to study substituent effects on aromatic
stacking.
Results and discussion
dyads
consisting
of
covalently
linked
mono-
alkoxynaphthalene (MAN) and naphthalimide (NI) units.49
Whilst non-continuous face-to-face donor–acceptor interac-
tions were observed between neighbouring molecules in
some dyad crystal structures, the only evidence for continu-
ous alternating donor–acceptor stacking came from powder
XRD analysis of dyad crystals grown from fast evaporating
Activated benzotriazole 1 was originally developed as an
electron rich reagent similar to other activated heterocycles
made previously by our group.53,54 It was prepared by first
protecting 3,5-dimethoxyaniline 2 with acetic anhydride at
0 °C, followed by nitration of acetamide 3 with nitric acid in
acetic anhydride to give 2-nitro-3,5-dimethoxyacetamide 4.
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an unexpectedly remarkable stacking structure. Most notice-
ably, the crystal structure showed a continuous anti-parallel
head-to-tail stacking of benzotriazole molecules, resulting in
heterogeneous cofacial π-stacking between the benzene and
triazole rings of neighbouring molecules with considerable
overlap (Fig. 1a and b). Our interest was even further aroused
when we observed that the molecules had also arranged into
perfectly flat 2D sheets to form a network of coplanar weak
carbon–hydrogen bonds (Fig. 1c and d). In addition to these
atypical features, the interplanar stacking distance was mea-
sured to be considerably short at 3.325 0.001 Å.7,55
We were keenly interested to understand the underlying
phenomena behind this curious crystal structure. A search of
compounds with a benzo[1,2,3]triazole nucleus in the Cam-
bridge Crystallographic Data Centre (CCDC) showed that out
of over 500 such compounds only two exhibited both contin-
uous anti-parallel cofacial stacking and formation of 2D
sheets.56,57 Whilst this demonstrated the rarity of this type of
solid state assembly, it was notable that both database struc-
tures possessed a cationic charge at N3. Therefore, the head-
to-tail interaction in these structures was likely influenced by
favorable π–cation interactions. Although activated benzo-
triazole 1 lacked any such charge, we speculated that its
triazole ring was likewise preferentially stacking with the ben-
zene due to an attractive electrostatic interaction.
Fig. 2 Electronic resonance pathways of activated benzotriazole 1.
The preference for the methoxy-substituted ring to overlap
with the acyl-substituted ring bore a striking resemblance to
the aromatic donor–acceptor DAN–NDI electrostatic attraction
interaction. We reasoned that orbitals of N1 of activated benzo-
triazole 1 were immensely electron-deficient from a combina-
tion of electron withdrawing effects of its acetyl substitution
and resonance associated with the permanent dipole of N2 and
N3, as shown in Fig. 2 (resonance pathways 1 and 2, respec-
tively).58 Although electron-deficient N1 was covalently bound
to the electron rich benzene ring, its meta-positioning to the
electron donating methoxy groups meant that the donated
electrons were instead channelled to N2 and N3 (resonance
pathway 3). Furthermore, the adjacent acyl α-carbon would
likewise be electron deficient due to the inductive effect of the
carbonyl oxygen (resonance pathway 1).
We therefore hypothesised that the association of this
electron-deficient portion of the 5-membered ring with the
comparatively electron-rich 6-membered benzene was due to
electrostatic attraction. Although this reasoning for an aro-
matic–aromatic interaction largely followed Hunter and
Sanders' electrostatic model, a short contact between the
electron-rich benzene ring and electron-deficient acetyl
α-carbon indicated that the substituent was also involved in
the π interaction. This substituent–aromatic interaction was
more in line with the direct interaction model, however, it
could not be said which was the more influential factor. A
very similar substituent–aromatic interaction can be ob-
served in the majority of the previously mentioned donor–
acceptor co-crystals, where the electron-rich naphthalene
ring significantly overlaps the non-aromatic imide substitu-
ent ring.20,21,31,45,46
Scheme 2 Reagents and conditions; i) KOH, MeOH, rt, 30 min, then
10% H3PO4, 86%; ii) acetic formyl anhydride, −5 °C, 1 h, 66%; iii) AlCl3,
PhMe, 150 °C, 1.5 h, 31%; iv) BTC, Et3N, THF, rt, 1 h, then MeOH, 65%;
v) chloroacetyl chloride, K2CO3, acetone, 0 °C then rt, 3 h, 86%; vi)
acetyl chloride, Et3N, DCM, 0 °C, 30 min, 78%.
Palladium-catalysed hydrazine reduction of the introduced
nitro group yielded 2-acetamidoaniline 5, which was subse-
quently diazotized and cyclised in one step to the novel
substituted N-acetylbenzotriazole 1 by treatment with sodium
nitrite in cooled hydrochloric acid (Scheme 1).
Originally developed solely for structure confirmation, the
single crystal structure of activated benzotriazole 1 showed
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Table 1 Crystal structure packing details
Compound
N1 substituent
4,6-Substitution
Continuous π–π stacking
Alignment
Packing mode
Interplanar distancec (Å)
1
6
8
9
10
11
12
COCH3
H
COCH3
COCH3
CHO
OMe
OMe
H
Yesa
Noa
Yes
Anti-parallel
Anti-parallel
Parallel
Anti-parallel
Parallel
Cofacial
3.325 0.001
3.373 0.004 (3.410 0.002)
3.326 0.001
3.324 0.003 (3.212 0.002)
3.252 0.004
3.257 0.003
Brick layer
Herringbone
Slipped
Herringbone
Herringbone
Herringbone
OH
Yesa,b
Yes
OMe
OMe
OMe
COOMe
COCH2Cl
Yes
Yes
Parallel
Parallel
3.368 0.009
a
b
2D sheet formation. Asymmetric π-stacking. c Measurement uncertainties listed as estimated standard deviations.
Fig. 3 X-ray crystal structures of parent benzotriazole 6 (a–d) and unactivated benzotriazole 8 (e–h), ellipsoids are shown at 50% probability,
hydrogen bonds, CH–π short contacts and perpendicular stacking distances are displayed as blue, orange and black dotted lines, respectively, all
distances measured in angstroms; a) stacking interaction viewed along c-axis; b) side view of non-continuous stacking and interplanar distance; c)
2D sheets viewed along b-axis; d) single layer of 2D sheets viewed along c-axis; e) ring-overlap of continuous stacking; f) side-view of parallel con-
tinuous stacking; g) herringbone array viewed along b-axis; h) co-planar hydrogen bonding.
The ring overlap in the stacking interaction of activated
benzotriazole 1 appeared to be further stabilized by a car-
bon–hydrogen bond between the 6-methoxy substituent and
N3 of the neighbouring molecule (Fig. 1a).
Whilst it was clear that the ring substituents were playing
roles in the alternated stacking, it was unclear if they played
any major role in the formation of the perfectly flat 2D
sheets. We therefore set out to not only determine if the
cofacial stacking could be influenced by altering the electron
withdrawing/donating character of the substituents, but also
whether any changes to the scaffold would affect 2D sheet
formation.
The simplest way to examine if the substituents were
playing a role was their removal from the scaffold. Likewise,
the degree to which the 6-methoxy to N3 carbon–hydrogen
bond contributed to ring overlap could be determined by de-
methylating the substituent so that it lacked the capability to
form the interaction. Lastly, by varying the electron withdraw-
ing ability of the N1 substituent, changes in the stacking in-
teractions could be observed with varying electrostatic land-
scapes of the analogues.
The N-acetyl group from benzotriazole 1 was easily re-
moved using potassium hydroxide in methanol followed by
acidification with 10% phosphoric acid, yielding the parent
4,5-dimethoxybenzotriazole 6 in good yield. Benzotriazole 7
was treated with acetyl chloride to obtain the unsubstituted
N-acetylbenzotriazole 8. Demethylation of the methoxy groups
of benzotriazole 1 was achieved by treatment with aluminium
chloride, yielding N-acetyl-4,6-dihydroxybenzotriazole 9.
Parent 4,5-dimethoxybenzotriazole 6 allowed easy access to
changes in substitution at N1 with groups of varying electron
withdrawing character. Firstly, parent benzotriazole 6 was
reacted with acetic formic anhydride, which was formed im-
mediately prior from acetic anhydride and formic acid. Fol-
lowing observations from Pasqua et al.,59 the reaction mix-
ture was kept cooled in a salt ice slurry to favour the
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heterocyclic and non-heterocyclic rings, but rather required
the acetyl group. This also showed that the polarization of N1
by N2 and N3 alone was not enough to elicit donor–acceptor
stacking.
Almost antithetical to the packing of parent benzotriazole
6, N-acetylbenzotriazole 8 lacked 2D sheet formation yet still
showed continuous stacking at a perpendicular distance of
3.326 0.001 Å (Fig. 3f and h). The packing was also quite
distinct from that of activated benzotriazole 1, exhibiting a
parallel orientation with minimal ring overlap that resulted
in a herringbone packing mode (Fig. 3e and g). Additionally,
this overlap occurred with N3 rather than N1, possibly due to
a now relatively electron-deficient benzene ring interacting fa-
vorably with the electron rich heterocyclic atom. However, it
should be noted that this positioning allowed a substituent–
substituent carbon hydrogen bonding between acetyl groups
(Fig. 3f). Although it could not be determined which was the
driving force in the stacking, it was apparent that the removal
of the electron donating group had drastically decreased ring
overlap. In combination with the results of parent benzo-
triazole 6, it was exceedingly clear that only through a combi-
nation of the electron donating methoxy and electron with-
drawing acetyl substituents could the donor–acceptor
stacking of activated benzotriazole 1 be observed. These re-
sults also demonstrated that formation of 2D sheets did not
tolerate the removal of the two methoxy substituents.
Fig. 4 X-ray crystal structure of dihydroxybenzotriazole 9, hydrogen
bonds are displayed as blue dotted lines; a) overlap in asymmetrical
stacked neighbors with hydrogen bonding pair (blue) and lone pair–π
interacting pair (orange), ellipsoids are shown at 50% probability; b)
side view of asymmetrical anti-parallel continuous stacking; c) 2D
sheet formation; d) top-down view of single 2D sheet.
Demethylation of the methoxy groups to give dihydroxy-
benzotriazole 9 resulted in a very similar crystal structure to
that of activated benzotriazole 1. This compound gave contin-
uous anti-parallel stacking and likewise formed 2D sheets,
however these were found not to be perfectly flat with the ar-
omatic rings lying slightly out of plane by approximately one
degree (Fig. 4c). The stacking was also observed to be closer
to slipped stacking than cofacial.60 Peculiarly, this slipped
stacking was observed to be asymmetrical on either face of
each molecule. On one face, the two molecules were aligned
in a very similar manner to the donor–acceptor stacking ob-
served for activated benzotriazole 1 with an almost identical
N-formyl product 10 over the N-acetyl product 1. Secondly,
treatment of parent benzotriazole 6 with triphosgene under
basic conditions followed by quenching with methanol
afforded the methyl ester derivative 11. Lastly, chloroacetyl
derivative 12 was obtained by the addition of 2-chloroacetyl
chloride to a solution of parent benzotriazole 6 in anhydrous
acetone (Scheme 2). The single crystal structure for each of
these compounds was subsequently analyzed and relevant
data are summarized in Table 1.
Although still forming 2D sheets, parent benzotriazole 6
showed a complete loss of continuous cofacial stacking
(Fig. 3a and c). In fact, the only observed π-interactions were
between slightly offset dimethoxybenzene rings, which
appeared to be stabilized by carbon–hydrogen bonds between
the 4-methoxy group of each molecule and occurred over a
slightly larger distance of 3.373 0.004 Å. Further diverging
from activated benzotriazole 1, this interaction was non-
continuous as it was not present on the opposite face, in-
stead forming a CH–π bond with a 6-methoxy substituent
perpendicular stacking distance of 3.324
0.003
Å
(Fig. 4a and b in blue). However, the rings were almost
completely offset with N1 and its acetyl substituent only just
overlapping with the benzene ring. Although the substituent–
aromatic interaction with N3 was no longer present, the 6-hy-
droxy group acted now as a hydrogen bond acceptor, forming
a carbon–hydrogen bond with the acetyl substituent of the
neighbouring molecule. These results indicated that the
substituent-ring interplanar carbon–hydrogen bond of acti-
vated benzotriazole 1 was non-essential for the aromatic do-
nor–acceptor stacking between the triazole and benzene ring.
On the opposite face, the anti-parallel interaction occurred in
resulting in an interplanar distance of 3.410
0.002 Å
(Fig. 3b). This resulted in a packing arrangement more akin
to brick-work stacking.60 The 2D sheets lacked the lattice of
carbon–hydrogen bonds, but were rather observed to contain
distinct methoxy and triazole domains (Fig. 3d). No short
contacts were observed within the methoxy domain, indicat-
ing that this arrangement was most likely due to steric ef-
fects. It was clear from this crystal structure that the triazole
ring–benzene ring stacking in activated benzotriazole 1 was
not simply due to a tendency for mixed interactions between
a
considerably different manner. The two molecules
overlapped one another by a far greater extent over a much
shorter distance of 3.212 0.002 Å (Fig. 4a and b in orange).
This position resulted in the acetyl group undergoing a sub-
stituent–substituent π–lone pair interaction with the 4-hy-
droxy group of the adjacent molecule.
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Fig. 5 X-ray crystal structures of formyl derivative 10 (a–c), methyl ester derivative 11 (d–f) and chloroacetyl derivative 12 (g–i); a, d and g) overlap
between head–tail stacked molecules, ellipsoids are shown at 50% probability; b, e and h) continuous parallel stacking with hydrogen bonds are
displayed as blue dotted lines and plane of bottom molecule in red; c, f and i) herringbone arrays viewed along b-axis.
Notably, the 2D sheets formed by dihydroxybenzotriazole
9 contained a network of much stronger conventional OH⋯O
and OH⋯N hydrogen bonds (Fig. 4d). Although introducing
a small amount of torsion to the ring systems, this result in
combination with that of N-acetylbenzotriazole 8 showed that
possession of hydrogen bond accepting oxygens on the ben-
zene ring was crucial to 2D sheet formation.
The single crystal structures of all three N1-substituent an-
alogues 10–12 showed continuous parallel aligned stacking
in a herringbone array similar to unactivated benzotriazole 8
(Fig. 5). However, they each differed from the unsubstituted
benzotriazole in that they all showed significantly more ring
overlap in a donor–acceptor fashion. This overlap was in a
similar manner to that of activated benzotriazole 1, involving
both N1 and the α-carbon of each acyl substituent. Whilst do-
nor–acceptor stacking was maintained by the three N1-
substituent analogues 10–12, it was clear that changing the
electron withdrawing character of the N1 substituent had lit-
tle effect on the degree of cofacial overlap. This may have
been due to direct substituent interactions, such as the car-
bon hydrogen bond between the 6-methoxy and N-acyl groups
in all three structures (Fig. 5b, e and f), or due to other con-
tributing forces to crystal growth. Although the formyl deriva-
tive 10 and methyl ester derivative 11 both showed short per-
pendicular stacking distances of 3.283 0.004 Å and 3.292
0.003 Å respectively, chloroacetyl derivative 12 displayed a
larger perpendicular distance of 3.352
0.009 Å. However,
this was likely due simply to steric effects from the increased
bulk of the chlorine atom.
Due to their similarity in structure to activated benzo-
triazole 1 it was particularly surprising that none of the three
analogues exhibited 2D sheet formation. This result therefore
demonstrated the fine-tuned balance achieved by benzo-
triazole 1 between both electrostatic character and hydrogen
bonding capability to exhibit the cofacial stacking that leads
to perfectly flat sheets.
Conclusions
In conclusion, the notable crystal packing structure of activated
benzotriazole 1 with completely flat sheet formation and anti-
parallel cofacial stacking was found to be extremely uncommon
among benzotriazoles. Furthermore, the unique solid-state as-
sembly has revealed the N-acetyl-4,6-dimethoxy-1H-benzo-
ij1,2,3]triazole scaffold to be a highly novel aromatic donor–ac-
ceptor stacking system that provides both electron rich and
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electron deficient areas on a simple bicyclic ring structure. By
studying the solid-state assembly of its analogues, we have ob-
served that the electron deficient portions of the triazole ring
and acyl substituent associate with a relatively electron rich
benzene ring under a variety of substituent conditions. How-
ever, this donor–acceptor stacking only occurred with the pres-
ence of both electron donating and electron withdrawing
groups substituted onto the ring system. Although the interac-
tion was still observed with the use of different electron-
donating and electron-withdrawing groups, cofacial stacking
was lost in all examples. Similarly, 2D sheet formation was
found to be lost with either removal of benzene substituents or
modification of the N-acyl group. Overall, these results empha-
sise that the cofacial stacking assembly of activated benzo-
triazole 1 is extremely sensitive and have been achieved by the
molecule only through a sensitive balance of both electrostatic
and hydrogen bonding character. Moreover, the observation of
substituent–substituent and substituent–aromatic interactions
in the cofacially π-stacked molecule is in agreement with the di-
rect substituent model for π-interactions.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work was supported by a Project Grant from the National
Health and Medical Research Council (NHMRC) Australia to
N. K. [NHMRC Grant APP1008014]. We are thankful to the
Nuclear Magnetic Resonance Facility and the Bioanalytical
Mass Spectrometry Facility (BMSF) in the Mark Wainwright
Analytical Centre at UNSW Sydney for the opportunity of
using their instruments.
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