Evaluation of viability of halogen...O2N interactions: Insight from crystal packing in a series of isomeric halo and nitro substituted triaryl compounds with modular positioning of halogen and NO2 groups

Article abstract of DOI:10.1039/c3ce40192h

A series of isomeric triaryl compounds with a modular positioning of the halogen and NO₂ groups have been synthesized by the azide-alkyne click reaction and have been characterized by single crystal X-ray structure analysis. This isomeric series has provided an opportunity for understanding the efficiency of the bifurcated halogen...NO₂ synthon in the organization of the molecules in the crystalline lattice. The changes in molecular conformation, crystal packing and supramolecular aggregation due to the change in the relative positioning of the complementary groups, halogen atom and the NO₂ group on ring A and ring C respectively, have been discussed. All the isomers synthesized are crystalline and establish the triazole as a reliable linker for crystal engineering oriented molecular synthesis. The 2-NO₂ derivatives display in general, a helical architecture and 3-NO₂ derivatives exhibit a centrosymmetric dimeric assembly via the complementary C-H...O interactions leading to either a helical or a 2-dimensional sheet pattern. The molecular organization in 4-NO₂ derivatives revealed in general a 2D sheet pattern.

Full text of DOI:10.1039/c3ce40192h

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Evaluation of viability of halogen O2N interactions:
Insight from crystal packing in a series of isomeric halo
and nitro substituted triaryl compounds with modular
positioning of halogen and NO2 groups3
Cite this: CrystEngComm, 2013, 15,
5283
C. V. Ramana,*a Yogesh Goriya,a Kulbhushan A. Durugkar,a Soumitra Chatterjee,a
Shobhana Krishnaswamyb and Rajesh G. Gonnade*b
A series of isomeric triaryl compounds with a modular positioning of the halogen and NO2 groups have
been synthesized by the azide–alkyne ‘‘click reaction’’ and have been characterized by single crystal X-ray
structure analysis. This isomeric series has provided an opportunity for understanding the efficiency of the
…
bifurcated halogen NO2 synthon in the organization of the molecules in the crystalline lattice. The
changes in molecular conformation, crystal packing and supramolecular aggregation due to the change in
the relative positioning of the complementary groups, halogen atom and the NO2 group on ring A and
ring C respectively, have been discussed. All the isomers synthesized are crystalline and establish the
triazole as a reliable linker for crystal engineering oriented molecular synthesis. The 2-NO2 derivatives
display in general, a helical architecture and 3-NO2 derivatives exhibit a centrosymmetric dimeric assembly
Received 29th January 2013,
Accepted 26th April 2013
DOI: 10.1039/c3ce40192h
…
via the complementary C–H O interactions leading to either a helical or a 2-dimensional sheet pattern.
www.rsc.org/crystengcomm
The molecular organization in 4-NO2 derivatives revealed in general a 2D sheet pattern.
recognize the remarkable properties of halogen bonding
interaction in the self-organization of molecular complexes
Introduction
Crystal engineering deals with the understanding and exploi-
tation of intermolecular interactions in order to design
functional organic solids which can find applications in
material chemistry.1 Among the various strong and weak
non-covalent interactions such as conventional hydrogen
of Br2 with several other organic compounds 50–60 years ago,
its real scope and potential was successfully explored largely by
Metrangolo and Resnati from Politecnico di Milano in recent
years.2g The current rejuvenation in the studies of halogen
bonding interaction led to the construction of new functional
solids,5 desired supramolecular assemblies.6 The role of
halogen bonds has also been identified in ligand binding
and molecular folding in biological systems such as protein
kinase complexes with halogenated ligands,7 in the formation
of molecular conductors and supramolecular liquid crystals,8
in enantiomer separation9 and selective binding of small
molecules to receptors10 as well as in polymorphic behavior
and stability and promotion of crystal growth.11 This wide-
spread recognition of halogen bonding interaction encouraged
the International Union of Pure and Applied Chemistry
(IUPAC) in 2010, to set up the task force (IUPAC project
2009-032-1-100, see www.halogenbonding.eu) for framing its
modern definition covering all its experimental and theoretical
aspects. The task group recently recommended the definition
…
…
bonds (X–H Y; X/Y = O, N, F), weak hydrogen bonds C–H
X
…
(X = O, N, S, halogens, p), p p interactions and van der Waal’s
interactions that govern the process of nucleation and crystal
growth, ‘‘halogen bonding’’ (C–X Y, X = halogens; Y = N, O, S,
halogens; except X Y halogen bond of Type II; X/Y = halogens;
…
…
see ESI, Fig. S1) interactions have gained importance in recent
3
years because of their selective and directional nature for the
construction of the desired crystal lattice.2 Halogen bonding
interactions are non-covalent interactions having properties
parallel to classical hydrogen bonding interactions in terms of
directionality and strength.3 Although, Hassel4 was the first to
aDivision of Organic Chemistry, National Chemical Laboratory (CSIR-NCL), Pune,
India. E-mail: vr.chepurix@ncl.res.in; Fax: +91 20 2590 2629; Tel: +91 20 2590 2277
bCenter for Materials Characterization, National Chemical Laboratory (CSIR-NCL),
Pune, India. E-mail: rg.gonnade@ncl.res.in; Fax: +91 20 2590 2642; Tel: +91 20
2590 2225
…
that, ‘A halogen bond R–X Y–Z occurs when there is evidence
of a net attractive interaction between an electrophilic region
on a halogen atom X belonging to a molecule or a molecular
fragment R–X (where R can be another atom, including X, or a
group of atoms) and a nucleophilic region of a molecule, or
Electronic supplementary information (ESI) available: 1H and 13C NMR for all
3
new compounds. ORTEPS with labelling scheme of all the compounds. CCDC
731506–731514 and 914828–914836. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3ce40192h
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molecular fragment, Y–Z’.12 Thus, the term ‘‘halogen bond-
ing’’ encompasses any non-covalent interaction between
polarizable halogen atoms (Cl, Br and I) as electrophilic
species and atoms possessing electron lone pairs like N, O and
S.13 This interaction is due to the anisotropic electron density
distribution around the halogen atom nucleus exhibiting
electrophilic (d+) character along the axis of the C–X bond and
nucleophilic (d2) character perpendicular to the C–X bond (see
ESI, Fig. S2). Therefore, electrophiles in general tend to
Scheme 1 Reagents and conditions: (A) 1.2 eq. NaN3, CuSO4 (20 mol%), Na-
ascorbate (15 mol%), L-proline (20 mol%), Na2CO3 (20 mol%), DMSO–H2O (9 :
1), 65-70 uC, 24 h; (B) same conditions as in (A) but without NaN3.
3
approach halogens at angles of 90u and nucleophiles at 180u.14
Recent experimental X-ray charge density analysis in combina-
tion with periodic quantum calculations using the B3LYP
6-31G(d,p) level of theory confirmed the attractive nature of
halogen bonding interactions.15
…
(Type III), with the Cl O–N angle approaching linearity (145–
172u). This may be a reason why the reports dealing with the
Among the various halogen bonding interactions known so
far, we were interested in the assessment of the occurrence of
…
NO2 Cl interactions are limited, especially for cases probing
…
its occurrence and nature across a set of isomers.19 In contrast,
the three centered bifurcated NO2 X (X = Cl, Br, I) halogen
bond, earlier identified by Desiraju and co-workers16 by
more systematic studies are available with the other two
…
halogen –(Br and I) O2N interactions, which indeed have
designing the molecules comprising both interacting groups.
…
been identified as ‘‘discriminatory synthons’’.20,21 However, its
predictability, when examined over a range of isomeric
compounds, revealed certain reservations. Glidewell and co-
The three centered bifurcated NO2 X synthon was categorized
by Allen et al. into three types in accordance with their
geometries.17 The three different motifs obtained by the close
approach of X atom to the NO2 are: (I) symmetric bifurcated
motif, (II) the asymmetric bifurcated motif and the (III) motif
in which halogen (X) forms mono-coordinate interactions with
one nitro O atom. In the first two categories, the X atom
approaches both nitro O atoms in a bifurcated manner and
has a trans relationship to the C–N bond. The tendency to form
such bifurcated motifs increases in the order Cl , Br , I. In
…
workers have examined the occurrence of the I NO2 synthon
employing diaryl compounds connected through a variety of
polar functional groups and by varying the I and NO2 group
positions systematically.21 It led to the conclusion that this
…
three center iodo nitro interaction appears to behave pre-
dictably for certain isomers and in remaining isomers it is not
seen.
…
…
With this background we reasoned that such X NO2
the last category the X O is in a cis relationship to the nitro C
interactions could be ideally examined in the family of crystal
structures bearing both halogen atom (X) and NO2 groups by
substituent about the N–O bond. The results obtained from
the ab initio calculations using intermolecular perturbation
theory (IMPT at 6-31G* level) were in agreement with crystal
structure geometries.17,18
Earlier studies on the evaluation of the occurrence of these
…
…
interaction motifs revealed that iodo nitro and bromo nitro
interactions, in general, tend to form the bifurcated three-
…
…
centered NO2 X synthon (Type I or II), however NO2 Cl
association, was in general, found to be mono-coordinated
…
Fig. 1 Schematic diagram representing the geometric features of halo nitro
Chart 1 All possible isomers of halo-nitro-1,4-diaryltriazole
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interactions.
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modular positioning of these two groups from ortho to meta
and para in their respective rings. To this end, we designed a
collection of halogen and NO2 substituted isomeric triaryl
compounds containing a 1,2,3-triazole ring flanked by phenyl
ring A (at position 1) carrying the NO2 group and the other
phenyl ring C (at position 4) possessing the halogen atom (X =
Cl, Br and I, Fig. 1). A detailed structural analysis of all these
isomeric compounds would address the issues related to the
steric and geometrical preferences for the occurrence of the
dealing with the synthesis of four series (X = H, Cl, Br, I)
comprising 30 compounds and their structural analysis.
Experimental
General methods
All solvents were dried according to standard methods before
use. Commercial reagents were used without purification.
Column chromatography was carried out by using
Spectrochem silica gel (60–120, 100–200, 230–400 mesh). All
the compounds were characterized by 1H and 13C NMR
spectroscopy using Bruker AC 200 MHz or Bruker DRX 400
and Bruker DRX 500 MHz spectrometers, and TMS was used as
an internal standard. 1H and 13C NMR chemical shifts are
reported in ppm downfield from chloroform-d (d = 7.25) or
TMS and coupling constants (J) are reported in Hertz (Hz). The
elemental analysis data were obtained on a Thermo Finnigan
Flash EA 1112 Series CHNS Analyser.
…
molecular aggregation through X NO2 interactions, in gen-
eral, and its competition with the other well-studied weak
…
…
intermolecular interactions such as C–H O/N, C–H p, C–
H
…
… … …
X, X X, C–X p and p p and their effect on the molecular
organization in the solid state. Furthermore, this study will
also shed more light on the preferable choice for the selection
of halogen atom and its favorable position with respect to the
nitro group for designing of supramolecular assemblies
…
through the X NO2 synthon. Scheme 1 saliently describes
the structures of the designed compounds and a retrosynthetic
scheme for their synthesis which is founded upon the Cu-
mediated azide–alkyne cycloaddition (CuAAC) reaction. Our
preliminary investigations dealing with the synthesis of two
series of compounds where X = H or Br have been disclosed.22
Herein we describe the complete details of investigations
Synthesis
As shown in Scheme 1, 1,4-substituted triazoles have been
synthesized by two different routes. For the 2,4-nitro series of
isomers we have followed the one-pot two-step sequence
involving a nucleophilic aromatic substitution (SNAr) of 2-/4-
fluorobenzenes with azide nucleophile and the in situ Huisgen
Fig. 2 Molecular organization in (a) 2N, (b) 2N2Cl, and (c) 2N2I; molecular organization in 2N2Br is similar to 2N2Cl.
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cycloaddition of the resulting aryl azides with alkynes.22–24 For
the 3-nitro series, 3-azidonitrobenzene has been used directly
for the CuAAc reaction. All the products were isolated in good
yield (Chart 1). The solid residue obtained was crystallized
from appropriate solvent systems to produce colorless to
the hydrogen atoms for compounds 2N2Cl, 4N2Cl, 4N4Cl were
located in difference Fourier map and refined isotropically
whereas they were placed in idealized positions for com-
pounds 2N3Cl, 2N4Cl, 4N3Cl (C–H = 0.93 Å) and 2N3I, 2N4I,
3N2I, 3N3I, 3N4I, 4N2I, 4N3I, 4N4I (C–H = 0.95 Å) and
constrained to ride on their parent atoms [Uiso(H) = 1.2Ueq(C)].
All H-atoms in 3N2Cl (except H1 and H5) and 3N4Cl (except
H12), while H1, H5, H12 in 3N3Cl, were obtained from
difference Fourier and refined isotropically whereas the other
H-atoms in these structures were placed in geometrically
idealized positions (C–H = 0.93 Å) and constrained to ride on
their parent atoms [Uiso(H) = 1.2Ueq(C)]. In the crystal structure
of 4N4I, both nitro group and iodine atom showed an
orientational disorder such that the positions of nitro and
iodo groups were exchanged with occupancies 0.9 and 0.1
respectively. Details of crystallographic data of these com-
pounds are summarized in Table 1. Crystal structure of 12
compounds (3-non-halo and 9-bromo derivatives, Chart 1)
discussed here have already been reported earlier.22 They have
been directly used for the analysis without further refinement
yellow crystals (see ESI for details).
3
Crystallographic details
Crystals of chloro and iodo compounds were stable at room
temperature and the diffraction data measurements were
carried out on a Bruker SMART APEX CCD diffractometer with
graphite-monochromatized (Mo Ka = 0.71073Å) radiation. The
X-ray generator was operated at 50 kV and 30 mA. Data were
collected with v scan width of 0.3u at different settings of w (0u,
90u, 180u and 270u) keeping the sample-to-detector distance
fixed at 6.145 cm and the detector position (2h) fixed at 228u.
The X-ray data collection was monitored by SMART program
(Bruker, 2003).25 All the data were corrected for Lorentzian,
polarization and absorption effects using SAINT and SADABS
programs (Bruker, 2003). SHELX-97 was used for structure
solution and full matrix least-squares refinement on F2.26 All
(see ESI for their crystal structure data).
3
Table 2 Geometrical parameters of the intermolecular interactions for 2-NO2 series
…
…
…
…
2-NO2 series
Entry
D–H
A
D–H (Å)
H
A (Å)
D
A (Å)
D–H A (u)
Symmetry codes
…
2N
1
2
3
4
5
6
7
8
C1–H1 O1
0.94(3)
0.97(3)
0.97(3)
0.90(2)
0.88(2)
0.91(2)
0.93
0.93
0.93
0.95
0.95
2.087(5)
0.95
0.93
0.93
0.93
0.93
0.93
0.93
0.93
1.893(3)
0.95
0.95
2.094(3)
0.93
0.93
0.93
0.93
0.93
0.93
2.47(3)
2.71(3)
2.59(3)
2.58(2)
2.70(2)
2.89(2)
2.65
2.68
2.92
2.44
2.73
3.618(2)
2.64
2.68
2.71
2.64
2.94
2.92
2.32
2.54
3.696(1)
2.27
2.52
3.903(1)
2.63
2.65
2.71
2.55
2.49
2.50
3.270(3)
3.642(4)
3.334(4)
3.397(3)
3.385(3)
3.652(3)
3.451(6)
3.482(6)
3.690(6)
3.357(6)
3.581(6)
143(2)
162(2)
133(2)
150(2)
135(2)
142(2)
141
145
141
163
149
157.6(1)
124
168
168
137
132
128
150
147
141(1)
153
148
144(1)
122
140
147
163
143
169
3/2 2 x, 21/2 + y, 1/2 2 z
x, 21 + y, z
1 2 x, 2y, 1 2 z
1/2 2 x, 1/2 + y, 1/2 2 z
1 2 x, 2 2 y, 2z
x, 21 + y, z
3/2 2 x, 1/2 + y, 3/2 2 z
1 2 x, 2 2 y, 2 2 z
x, 21 + y, z
1 + x, y, z
1 + x, y, z
2x, 1/2 + y, 1 2 z
2x, 21/2 + y, 1 2 z
2 2 x, 1/2 + y, 1 2 z
1 + x, y, z
…
C11–H11 N2
…
C13–H13 O2
…
2N2Cl
2N2Br
2N2I
C1–H1 O1
…
C13–H13 O2
…
C5–H5 Cl1
…
C1–H1 O1
…
C13–H13 O2
…
9
C5–H5 Br1
…
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
32
33
34
35
36
37
39
40
C1–H1 N3
…
C1–H1 N2
…
C8–I1 Cg
…
C7–H7 O1
3.263(6)
3.591(6)
3.621(6)
3.382(6)
3.629(5)
3.563(5)
3.153(4)
3.360(4)
…
…
2N3Cl
C13–H13 N2
C11–H11 N2
…
C5–H5 O2
x, y, 1 + z
1 + x, y, z
…
C4–H4 Cl1
…
C1–H1 Cg
2 2 x, 1/2 + y, 2 2 z
1/2 2 x, 21/2 + y, 1/2 2 z
x, 21 + y, z
21 2 x, 1 2 y, 2z
3/2 2 x, 1/2 + y, 1/2 2 z
x, 1 + y, z
…
2N3Br
2N3I
C1–H1 O1
…
C11–H11 N2
…
C7–Br1 Br1
…
C1–H1 O1
3.152(4)
3.365(4)
…
C11–H11 N2
…
C7–I1 I1
2x, 2 2 y, 2z
…
2N4Cl
C10C–H10C O2B
C1C–H1C O2B
C5C–H5C O1A
C1B–H1B N3C
C1B–H1B N2C
C12B–H12B O1C
C5A–H5A N2B
3.221(4)
3.416(4)
3.530(4)
3.445(4)
3.282(4)
3.413(4)
3.629(4)
1 2 x, 21/2 + y, 1/2 2 z
1 2 x, 21/2 + y, 1/2 2 z
1 2 x, 21/2 + y, 1/2 2 z
2 2 x, 1/2 + y, 1/2 2 z
2 2 x, 1/2 + y, 1/2 2 z
x, 1 + y, z
…
…
…
…
…
…
…
0.93
2.74
160
x, y, z
C6A–Cl1A O2A
…
2N4Br
2N4I
C1–H1 N2
0.86(3)
1.903(3)
0.93
0.95
0.95
2.107(4)
0.95
0.95
2.66(3)
3.473(1)
3.14
2.54
2.51
3.805(2)
2.64
2.93
3.447(4)
152(2)
155(1)
148
156
157
162(1)
120
150
1 2 x, y, z
…
C6–Br1 Br1
2x, 1 2 y, 2 2 z
x, 1/2 2 y, 21/2 + z
1 + x, y, z
…
C13–H13 Cg1 (Cg1–triazole ring)
3.958
3.425(4)
3.402(5)
…
C1–H1 N3
…
C1–H1 N2
1 + x, y, z
…
C6–I1 I1
1 2 x, 1 2 y, 2z
x, 3/2 2 y, 1/2 + z
x, 3/2 2 y, 1/2 + z
…
…
C13–H13 O1
C13–H13 Cg1 (Cg1–triazole ring)
3.219(3)
3.779(4)
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…
screw axis via C–H O interactions involving the triazole ring
Results and discussion
and the nitro group (Fig. 2a and 2b, Fig. S3 (ESI ), entries 1, 4
3
As indicated in Chart 1, the resulting 30 compounds were
classified according to the position of the nitro group as 2NO2,
3NO2 and 4NO2 families. Detailed crystal structure analyses of
all the compounds have been performed and all the inherent
structural features present in these compounds are described
below.
and 7 in Table 2). This helical association also brings the unit-
translated molecules closer along the helical assembly to
…
generate C–H N interactions in 2N (Table 2, entry 2) and C–
…
H
X (X = Cl and Br) contacts in 2N2Cl and 2N2Br (Table 2,
entries 6 and 9). The neighboring helices along the ac diagonal
…
are bridged via centrosymmetric C–H O contact thus enga-
ging the nitrobenzene group across the inversion center
2NO2 family
(Table 2, entries 3, 5, 8). Molecules in 2N2I also form a helical
…
Crystal structures of all the 10 compounds in this family
contain one molecule in the asymmetric unit (Z9 = 1) except in
2N4Cl, the asymmetric unit of which contains three symmetry
independent molecules (Z9 = 3). Structures of 2N, 2N2Cl and
2N2Br are isostructural and crystallized in the centrosym-
metric monoclinic space group, P21/n. Similarly, the structures
of 2N3Br and 2N3I, as well as 2N4Br and 2N4I, are respectively
isostructural and crystallize in the centrosymmetric mono-
clinic space group P21/n and P21/c respectively. The remaining
three structures in this family belong to chiral space groups;
2N2I and 2N3Cl crystallized in monoclinic, P21 and 2N4Cl in
orthorhombic space group P212121.
architecture along the b-axis exclusively via linear C–I
p
halogen bonding interactions (Table 2, entry 12) comprising
adjacent iodobenzene rings along the twofold screw axis. This
linear driving force seems to be responsible for guiding chiral
molecular packing in crystals of 2N2I.27 This arrangement is
…
also supplemented by long and non-linear C–H O contact
(Table 2, entry 13). The adjacent helices along the a-axis are
stitched together in parallel fashion via edge-to-edge linear
…
bifurcated C–H N contacts (Table 2, entries 10, 11) to
generate an overall helical architecture in the ab plane
(Fig. 2c).
2N3X series. In all the three structures of this series the
halobenzene ring is slightly (11–15u) out of plane, while the
nitrobenzene ring showed a marked difference (47–52u) from
2N2X series. Both phenyl rings (nitro as well as halo) in this
series of compounds showed significant deviation from the
planarity of the central triazole ring. Molecules in their
the least square plane of the triazole ring moiety. Similar to
…
…
crystals, in general, are associated via C–H O and/or C–
2N2X series, molecules in this series are also linked by C–H
O
…
…
…
H
N, C–H X and C–X p interactions (X = halogens) and do
…
not assemble via any of the three possible X NO2 synthon.
Molecules in the isostructural crystals of 2N, 2N2Cl and
2N2Br assemble helically across the crystallographic twofold
Fig. 3 Molecular organization in (a) 2N3Cl, and (c) 2N3I; molecular organiza-
tion in 2N3Br is similar to 2N3Cl.
Fig. 4 Molecular organization in (a) 2N4Cl and (b) 2N4I; 2N4Br is isostructural
with 2N4I.
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and/or C–H N, C–H X and halogen halogen interactions of
CrystEngComm
…
…
…
…
Cl NO2 synthon is noticed in 2N4Cl although having the
mono-coordinated Type III motif. In crystals of 2N4Br and
2N4I halogen halogen and C–H p contacts seem to dictate
the molecular packing.
Type I (see ESI, Fig. S1) category but do not pack in their
3
…
…
…
crystals via any of the three possible X NO2 synthons.
Molecules in the chiral crystals of 2N3Cl arranged helically
along the crystallographic 21-screw axis forming two separate
In compound 2N4Cl, molecules B and C form a composite
helix (blue–red) wherein each molecule is distributed alter-
…
helical patterns via edge-to-edge C–H N and edge-to-face off-
…
…
centered C–H p interactions between the C–H of the triazole
nately along the helical a-axis via bifurcated C–H O and edge-
…
ring and the p electron cloud of chlorobenzene ring (Table 2,
to-edge C–H N contacts (Table 2, entries 25, 26, 28, 29). The
entries 14, 17). These two intercalated helices are connected
along the c-axis via C–H O contact (Table 2, entry 16) to
adjacent helices are packed in antiparallel style via another
short and linear C–H O interaction (Table 2, entry 30).
…
…
generate a concave crystal packing on the bc plane (Fig. 3a).
Molecules in isostructural crystals of 2N3I and 2N3Br are also
Interestingly, the third molecule A also forms its own helical
architecture (green) which is sandwiched between the com-
plementary helices of molecules B and C along the c-axis
(Fig. 4a). The successive molecules along the helix of A adhere
arranged helically (Fig. 3b and Fig. S4 (ESI )) like in crystals of
3
2N, 2N2Cl and 2N2Br but here the helices are packed much
…
more discretely providing well-guided channels. The succes-
…
to the composite helix of molecules B and C via C–H O and
…
sive molecules along the helix are connected via short C–H
and C–H N contacts (Table 2, entries 19, 20, 22, 23) and the
O
C–H N contacts (Table 2, entries 27, 31). Molecular packing
…
viewed down the helical axis (a-axis) revealed a layered
…
adjacent helices are assembled in antiparallel mode via a
arrangement of molecules B and C through C–H O contacts
16b
…
centrosymmetric halogen halogen contact of Type I motif.
2N4X series. Compound 2N4Cl crystallized in chiral space
group P212121 with Z9 = 3. Out of the three symmetry
independent molecules, molecules B and C had a similar
conformation whereas the third molecule A showed a slight
deviation from other two. In all three molecules the
nitrobenzene ring showed a significant deviation (range 48–
65u) from the least square plane of the triazole ring than the
chlorobenzene ring (2–9u). A similar trend is also observed for
crystals of 2N4Br and 2N4I for the nitrobenzene ring; however
the halobenzene ring showed more out of plane deviation (16–
20u) than 2N4Cl. In this series also, molecular packing is
encapsulating the layer of molecule A (Fig. S5, ESI ). Molecules
3
of A are held to each other along the layer through linear
…
mono-coordinated Cl NO2 halogen bonding contact of Type
III motif. In contrast, molecules in isostructural crystals of
2N4Br and 2N4I have a quite different arrangement forming a
one-dimensional chain structure along the a-axis via bifur-
…
cated C–H N interactions (Table 2, entries 33, 36, 37). The
neighboring 1D chains are held together via off-centered C–
…
…
H
p (p cloud of the triazole ring) and C–H O contacts to
generate a cone shape structure down the a-axis (Table 2,
entries 35, 39, 40). Such contiguous cone structures are linked
to each other along the b-axis via short centrosymmetric
…
…
…
N
halogen halogen contacts (Table 2, entries 34, 38) generating
mostly governed by C–H
O
and C–H
interactions.
the helical architecture (Fig. 4b).
However, this is the first series where the existence of the
Fig. 5 Molecular organization in (a) 3N2Cl and (b) 3N2Br; 3N2I (c) has a similar association but forms wavy chains, while in 3N2Br it forms a 2D sheet pattern.
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Table 3 Geometrical parameters of the intermolecular interactions for the 3-NO2 series
…
…
…
…
D–H A (u) Symmetry codes
Entry D–H
A
D–H (Å)
H
A (Å)
D
A (Å)
…
3N
1
2
3
4
5
6
7
8
9
C1–H1 O1
0.93
0.93
0.93
0.93
0.93
0.93
0.94(5)
0.95(4)
2.53
2.48
2.70
2.57
2.50
2.77
2.66(6)
2.58(4)
3.847(3)
2.67
3.409(3)
3.408(3)
3.593(3)
3.372(3)
3.358(3)
3.249(6)
3.499(6)
3.328(6)
Dihedral angle 0u
3.57(2)
158
176
160
144
154
113
148(4)
135(3)
2x, 2 2 y, 1 2 z
2x, 2 2 y, 1 2 z
2x, 2 2 y, 1 2 z
x, 3/2 2 y, 1/2 + z
1 2 x, 2y, 1 2 z
2 2 x, 21/2 + y, 3/2 2 z
21 + x, 3/2 2 y, 21/2 + z
21 + x, 1/2 2 y, 21/2 + z
1 2 x, 1 2 y, 1 2 z
1/2 + x, 5/2 2 y, 1/2 + z
1/2 + x, 5/2 2 y, 1/2 + z
1/2 + x, 3/2 2 y, 1/2 + z
1/2 + x, 3/2 2 y, 1/2 + z
1/2 + x, 5/2 2 y, 1/2 + z
1 2 x, 1/2 + y, 1/2 2 z
1 2 x, 1/2 + y, 1/2 2 z
1 2 x, 1/2 + y, 1/2 2 z
x, 1 + y, z
x, 1 + y, z
2 2 x, 1 2 y, 2 2 z
x, 1 + y, z
x, 1 + y, z
1 2 x, 2 2 y, 2z
x, y, z
x, y, z
21 + x,21 + y, 21 + z
x, y, z
x, y, z
x, y, z
…
C14–H14 O1
…
C8–H8 O2
…
C6–H6 O2
…
C10–H10 N2
…
3N2Cl
C1–H1 O1
…
C6–H6 O1
…
C7–H7 O2
…
…
Cg2 Cg2 (p p), Cg2–chlorobenzene ring
C10–H10 O2
…
3N2Br 10
0.93
0.93
0.93
0.93
165
125
161
135
144(1)
169
127
137(1)
157(4)
162
161(4)
171(3)
164
169
173
146
161
167
151
173
172
…
11
12
13
14
15
16
17
C10–H10 O1
2.72
2.71
2.73
3.34(2)
3.61(2)
3.446
…
C7–H7 N3
…
C7–H7 N2
…
C8–Br1 O2
1.913(2) 3.317(17)
0.95
0.95
…
…
3N2I
C10–H10 O2
2.48
2.69
3.414(4)
3.500(6)
C11–H11 O1
…
C8–I1 O2
2.103(3) 3.482(3)
…
3N3Cl 18
C5–H5 N2
0.93(5)
0.93(5)
0.94(2)
0.98(4)
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
0.93
1.905
1.903
0.95
2.60(5)
2.82
2.52(3)
2.26(4)
2.72
3.04
2.47
2.67
3.04
2.44
2.55
2.66
2.98
2.73
3.433
3.403
2.37
3.470(6)
3.715
…
19
20
21
22
C5–H5 N3
…
C12–H12 O2
3.418(6)
3.235(5)
3.623(5)
3.961(5)
3.395(3)
3.478(4)
3.926(3)
3.351(4)
3.391(4)
3.584(4)
3.908(3)
3.636
…
C1–H1 O1
…
C10–H10 O1
…
23
C6–H6 Cl1
…
3N3Br 24
C1–H1 O19
…
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
C4–H4 O29
…
C8–H8 Br19
…
C19–H19 O1
…
C49–H49 O2
…
C149–H149 O1
…
C89–H89 Br1
1 + x, 1 + y, 1 + z
x, y, z
…
…
C14–H14 O19
166
C79– Br19 N3
160.9
163.7
173
149
161(1)
173
1 + x, 1 + y, 1 + z
21 + x, 21 + y, 21 + z
2x, 2 2 y, 1 2 z
2x, 2 2 y, 1 2 z
1 2 x, 2 2 y, 21 2 z
2x, 2 2 y, 1 2 z
1 2 x, 2 2 y, 1 2 z
21 + x, 21 + y, 1 + z
1 2 x, 2 2 y, 1 2 z
2x, 2y, 1 2 z
…
C7– Br1 N39
…
3N3I
C1–H1 O1
3.314(4)
3.437(5)
…
C4–H4 O2
0.95
2.59
…
C7–I1 N3
2.105(4) 3.354(3)
0.95
0.95
0.95
0.88(3)
0.88(3)
0.89(3)
0.89(3)
…
C14–H14 O1
2.65
3.149
3.248
2.59(3)
2.51(3)
2.64(4)
2.53(3)
3.598(4)
4.074(3)
3.964(3)
3.338(5)
3.385(4)
3.460(5)
3.399(5)
…
C8–H8 I1
165
134
…
…
…
C12–H12 I1
C10–H10 N2
3N4Cl 40
143(3)
174(3)
153(3)
165(3)
156(1)
161(3)
153(3)
177(3)
142(3)
158(1)
168
41
42
43
44
C14–H14 O1
…
C8–H8 O2
2x, 2y, 1 2 z
2x, 2y, 1 2 z
…
C1–H1 O1
…
C6–Cl1 O2
1.730(4) 3.087(3)
21/2 + x, 1/2 2 y, 21/2 + z
1 2 x, 2y, 1 2 z
1 2 x, 2y, 1 2 z
1 2 x, 2y, 1 2 z
2 2 x, 2 2 y, 1 2 z
21/2 + x, 1/2 2 y, 21/2 + z
x, 1 + y, z
x, 1 + y, z
x, 1 + y, z
x, 1 + y, z
1 + x, 2 + y, 1 + z
…
3N4Br 45
C1–H1 O1
0.89(3)
0.86(4)
0.92(3)
0.93(4)
2.56(3)
2.70(4)
2.49(3)
2.56(4)
3.413(5)
3.482(5)
3.404(4)
3.342(5)
…
46
47
48
49
50
51
52
53
54
C8–H8 O2
…
…
C14–H14 O1
C10–H10 N2
…
C6–Br1 O2
1.896(4) 3.169(3)
…
3N4I
C1–H1 O1
0.95
0.95
0.95
0.95
2.31
2.57
2.75
2.60
3.251(6)
3.428(7)
3.667(6)
3.526(6)
…
C7–H7 N2
151
163
164
159(1)
…
C7–H7 N3
…
C10–H10 O1
…
C6–I1 O2
2.105(5) 3.300(5)
tion. Similarly, the halo group in 3N2Cl, 2N2Br, 3N2I, 3N3Br,
3N3I C–H group of triazole have cis-conformation whereas
both groups acquire anti-conformation in 3N3Cl. The struc-
tures of 3N, 3N4Cl, 3N4Br are isostructural, and the same is
the case with 3N2Br, 3N2I and 3N3Br, 3N3I respectively.
3N2X series. Halobenzene and nitrobenzene rings in 3N2Cl
and 3N2I slightly deviate from the least square plane of the
triazole moiety, whereas in 3N2Br all three rings are in plane.
Molecules in the crystals of this series are associated mostly
3NO2 family
Crystals of all the 10 compounds in this family crystallized in
the centrosymmetric space group with Z9 = 1 except in 3N3Br,
where Z9 = 2. In all cases, the three rings, the triazole,
nitrobenzene and halobenzene have different orientations
with respect to each other. In 3N, 3N3Cl, 3N3Br, 3N3I, 3N4Cl,
3N4Br, the C–H group of the triazole ring and the nitro group
are in cis-conformation whereas in 3N2Cl, 3N2Br, 3N2I and
3N4I, the C–H group and the NO2 group have anti-conforma-
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Fig. 6 Molecular organization in 3N3Cl (a, b) and 3N3I (c, d); 3N3Br has similar organization as 3N3I.
…
…
via C–H O and C–H N interactions. In addition, mono-
Molecules in crystals of 3N2Cl assemble in helical fashion
…
…
coordinated X NO2 synthon is also observed in crystals of
across the 21-screw axis through long and non-linear C–H
O
3N2Br and 3N2I.
contacts (Fig. 5a, Table 3, entry 6). The neighboring helices
Fig. 7 Molecular organization in (a) 3N, (b) 3N4Br and (c), (d) 3N4I; 3N4Cl has a similar molecular architecture as 3N4Br.
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Table 4 Geometrical parameters of the intermolecular interactions for 4-NO2 series
…
…
…
…
Entry
D–H
A
D–H (Å)
H
A (Å)
D
A (Å)
D–H A (u)
Symmetry codes
…
4N
1
2
3
4
5
6
7
8
C6–H6 O1
0.956(17)
0.956(17)
0.97(3)
0.93(3)
0.92(3)
0.91(3)
0.93
2.575(17)
2.660(17)
2.54(3)
2.51(3)
2.93(3)
2.57(3)
2.62
3.342(2)
3.333(2)
3.486(4)
3.377(4)
3.487(3)
3.367(4)
3.321(5)
3.231(4)
137(1)
128(1)
166(3)
155(2)
120(2)
147(2)
132
x, 1 + y, 1 + z
1 + x, 1 + y, 1 + z
x, 1 + y, 21 + z
x, 1 + y, z
x, 1 + y, z
…
C6–H6 O2
…
4N2Cl
C6–H6 O2
…
C14–H14 O2
…
C5–H5 Cl1
…
C11–H11 N2
x, 21 + y, z
…
4N2Br
4N2I
C6–H6 O1
1/2 + x, 1/2 2 y, 1/2 + z
x, 1 2 y, 21/2 + z
1/2 2 x, 1/2 2 y, 2z
1/2 2 x, 1/2 2 y, 2z
1 2 x, 21/2 + y, 5/2 2 z
x, 21 + y, z
1 2 x, 21/2 + y, 5/2 2 z
2x, 21/2 + y, 3/2 2 z
1 2 x, 21/2 + y, 1/2 2 z
1 2 x, 21/2 + y, 1/2 2 z
2 2 x, 21/2 + y, 21/2 2 z
2 2 x, 21/2 + y, 3/2 2 z
2 2 x, 21/2 + y, 3/2 2 z
2 2 x, 21/2 + y, 3/2 2 z
1 2 x, 21/2 + y, 1/2 2 z
x, 21 + y, z
…
C11–H11 N2
0.93
2.56
130
…
9
C8–Br1 O2
1.914(4)
1.914(4)
0.95
2.103(3)
0.95
0.95
0.93
0.93
0.93
0.93
0.93
0.93
0.93
1.897(3)
0.95
2.111(3)
2.111(3)
0.95
3.268(4)
4.413(3)
2.60
3.496(3)
2.70
2.69
2.36
2.63
2.58
2.33
2.66
2.56
2.52
3.292(3)
2.43
3.454(3)
3.406(3)
2.64
145(1)
165(1)
152
144(1)
118
143
175
143
153
169
161
148
155
160(1)
161
164(1)
160(1)
131
…
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
C8–Br1 O1
…
C10–H10 O2
3.465(4)
…
C8–I1 O1
…
C11–H11 O1
3.263
3.500
…
C5–H5 N2
…
4N3Cl
4N3Br
C1–H1 O2
3.283(6)
3.423(7)
3.438(7)
3.247(3)
3.556(4)
3.387(4)
3.387(4)
…
C10–H10 O1
…
C5–H5 N2
…
C1–H1 O2
…
C8–H8 O2
…
C10–H10 O1
…
C5–H5 N2
…
C7–Br1 O1
…
4N3I
C6–H6 O1
3.341(4)
1 + x, 3/2 2 y, 1/2 + z
1 + x, 1/2 2 y, 1/2 + z
1 + x, 1/2 2 y, 1/2 + z
x, 21 + y, z
…
C7–I1 O1
…
C7–I1 O2
…
…
C11–H11 N2
3.342
3.432
C14–H14 O2
0.95
2.71
134
x, 1 + y, z
…
4N4Cl
4N4Br
C1–H1 O2
0.86(2)
0.94(2)
0.92(3)
0.93
0.93
0.93
0.93
0.93
0.93
0.95
2.43(3)
2.62(3)
2.74(3)
2.36
2.60
2.31
2.71
2.65
2.60
2.59
3.288(3)
3.521(4)
3.622(4)
3.279(5)
3.523(5)
3.230(5)
3.620(5)
3.401(5)
3.334(5)
3.515(5)
3.557(5)
174(2)
162(1)
161(1)
171
174
168
165
139
137
166
1 2 x, 21/2 + y, 3/2 2 z
1 2 x, 21/2 + y, 3/2 2 z
2x, 21/2 + y, 5/2 2 z
x, y, z
x, y, z
21 + x, y, 1 + z
21 + x, y, 1 + z
2x, 2y, 2z
1 2 x, 1 2 y, 2 2 z
x, 1 + y, z
x, 1 + y, z
…
C10–H10 O2
…
C5–H5 N2
…
C1–H1 O29
…
C10–H10 O29
…
C19–H19 O2
…
C109–H109 O2
…
C4–H4 N3
…
C49–H49 N39
…
4N4I
C1–H1 N3
…
C1–H1 N2
0.95
2.103(6)
2.103(6)
2.64
3.520(6)
3.320(6)
162
153(1)
167(1)
…
C6–I1 O1
1/2 + x, 5/2 2 y, 21/2 + z
1/2 + x, 5/2 2 y, 21/2 + z
…
C6–I1 O2
along the ac diagonal are packed in an antiparallel manner via
two moderate C–H O interactions, engaging both oxygens of
structures of all the compounds in this series are, in general,
packed by the C–H O, C–H N and C–H halogen interac-
…
…
…
…
the nitro group and phenyl H-atoms (Table 3, entries 7, 8).
This arrangement also brings the chlorobenzene rings of
adjacent helices in stacking mode to form a centrosymmetric
tions. In addition, molecules in 3N3Br and 3N3I make almost
linear C–X N halogen bonding interactions.
In 3N3Cl, molecules form a 1D molecular tape connecting
…
…
aromatic p p interaction, thus supporting the tight binding of
the unit-translated molecules along the b-axis via bifurcated
…
…
the helices. Molecules in the isostructural crystal of 3N2Br and
3N2I form 2D layered structures wherein closely associated
C–H N and C–H O interactions (Fig. 6a, Table 3 entries 18,
19, 21, 22). The adjoining antiparallel 1D molecular tapes are
…
…
molecules are linked via C–H O and mono-coordinated
linked centrosymmetrically along the c-axis via C–H
O
…
X
NO2 Type III synthon (Table 3, entries 10, 11, 14–17) that
interactions to form a dimeric tape structure bringing the
nitrobenzene moieties in close association across the inver-
sion centre (Table 3, entry 20). These dimeric tapes are further
resemble flat helical ribbons in 3N2Br and wavy layers in 3N2I
(Fig. 5b and 5c). These helical layers are weakly associated via
…
bifurcated C–H N contacts in 3N2Br forming a planar sheet
linked centrosymmetrically along the same direction via C–
…
structure, while it generates periodic wavy layers in 3N2I
H
Cl contacts to form a 2D molecular sheet pattern (Table 3,
(Table 3, entries 10, 11, 14–17). In both cases the neighbouring
…
entry 23). Molecules in isostructural crystals of 3N3Br and
3N3I create a zero dimensional face-to-face centrosymmetric
dimeric assembly via C–H O interactions (Fig. 6c and S6
layers create a multiple stack structure through aromatic p
p
…
interactions.
(ESI ), Table 3, entries 24, 25, 27–29, 31, 34, 35, 37). These
3N3X series. Halobenzene and nitrobenzene rings are
almost coplanar with the central triazole ring in 3N3Cl
whereas in 3N3Br and 3N3I, they deviate slightly. The crystal
3
dimers extended along the c-axis through centrosymmetric C–
…
…
H
X (X = halogen) and C–X N halogen bonding contacts
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…
connected via centrosymmetric short C–H N interactions
(Table 3, entries 5, 40, 48) forming a 2D sheet structure in the
ab plane whereas in 3N3Br and 3N3I they are linked via short
…
C–H H–C contacts. These successive sheets along the c-axis
…
are perpendicularly stitched through the C–H O interaction
in 3N (Fig. 7a, Table 3, entry 4) whereas the same interaction is
…
replaced by mono-coordinated X NO2 synthon of Type III
motif in 3N4Cl and 3N4Br, forming a square grid type network
(Fig. 7b and S7 (ESI ), Table 4, entries 44, 48). The generation
3
of face-to-face dimeric assembly in 3N4I is precluded due to
the entirely opposite orientation of nitro group. Instead, the
molecules in 3N4I generate a 1D sheet structure by linking
…
…
unit-translated molecules via bifurcated C–H O and C–H
N
interactions (Table 3, entries 50–53). The nearest sheets along
the c-axis are linked via Type III short and linear mono-
…
coordinated C–I O halogen bonding synthon, thus forming a
2D sheet structure (Fig. 7c, Table 3, entry 54). The adjoining
2D sheets along the third dimension create a stacking
…
assembly via p p interactions.
4NO2 family
Crystals of all the 10 compounds in this family crystallized in
centrosymmetric space groups as Z9 = 1 except in 4N4Br, where
Z9 = 2. In all the structures, the triazole, nitrobenzene and
halobenzene groups adopts diverse orientations with respect
to each other. However the structures of 4N2I, 4N3Cl, 4N3Br
are found to be isostructural.
In 4N, benzene and nitrobenzene rings showed significant
deviation from the planarity of triazole moiety. Molecules form
…
a corrugated sheet pattern linked via bifurcated weak C–H
O
interactions (Fig. 8a, Table 4, entries 1, 2). The adjacent sheets
generate multiple stack layers through various weak interac-
tions (Fig. 8b).
Fig. 8 Molecular organization in 4N (a) corrugated sheet formed by association
…
of molecules via chains of C–H O interactions, (b) arrangement of the sheets in
4N2X series. Molecules in the crystals of this series are
the ab plane.
…
…
…
mostly associated via C–H O, C–H N and C–H halogen
…
interactions. The existence of C–X O interaction of mono and
bifurcated motifs is also observed (Table 4). All the three rings
displayed dissimilar conformations resulting in different
packing arrangements.
(Table 3, entries 26, 30, 32, 33, 36, 38, 39), whereas they are
loosely stitched along the ab diagonal via hydrophobic edge-to-
14
…
edge short C–H H–C contacts forming a 2D sheet pattern
(Fig. 6c). The 2D sheets in all cases generates a multiple stack
In 4N2Cl and 4N2Br, the halobenzene ring is almost
coplanar whereas the nitrobenzene ring showed a noticeable
difference in conformation from the least square plane of
triazole ring. However, both the phenyl rings adopt a similar
orientation in 4N2I but deviated from the plane of the triazole
ring by y24u. In 4N2Cl, molecules form a flat 1D molecular
chain connecting the unit-translated molecules along the
…
structure down the a-axis through aromatic p p interactions
(Fig. 6b and 6d).
3N4X series. Nitro group in 3N, 3N4Cl and 3N4Br displays a
similar orientation with respect to the triazole ring while its
orientation is exactly opposite in 3N4I. Moreover, both phenyl
rings in 3N, 3N4Cl and 3N4Br showed slight divergence from
the least square plane of triazole moiety, however these rings
are almost coplanar with the triazole ring in 3N4I. These
conformational features at the single molecular level led
isostructurality in 3N, 3N4Cl and 3N4Br while 3N4I revealed a
different molecular architecture. Molecules in all these cases
…
…
…
b-axis via C–H O, C–H N and C–H Cl interactions (Fig. 9a,
Table 4, entries 4–6). Arrangement of these nearest 1D chains
…
through C–H O interactions generate a 2D corrugated sheet
structure in the bc plane (Table 4, entry 3). The closest
…
neighbouring sheets are connected via Cl Cl (3.482 Å)
contacts of Type I geometry to generate a ladder type network,
…
…
…
are arranged mostly via C–H O, C–H N and mono-coordi-
wherein the Cl Cl association act as rungs and the sheet as
…
nated C–X O halogen bonding synthon in their crystals.
rails. These parallel ladders are loosely packed through van der
Waal’s interactions (Fig. 9b). Molecules in crystals of 4N2Br,
displays the first occurrence of the asymmetric bifurcated Type
II motif where two centrosymmetrically related molecules
Molecules in 3N, 3N4Cl and 3N4Br form a zero dimensional
…
face-to-face centrosymmetric dimeric assembly via C–H
O
interactions similar to that of 3N3Br and 3N3I (Table 3, entries
1–3, 41–43, 45–47). The difference lies only in the stitching of
these dimers in 2D. The dimers in 3N, 3N4Cl and 3N4Br are
…
associated via three centered Br O2N synthons (Fig. 9c). The
nitro group oxygen O2 is closer to the halogen atom Br1 than
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Fig. 9 Molecular organization in 4N2Cl (a) and (b), 4N2Br (c) and (d) and 4N2I (e) and (f).
the oxygen O1 (Table 4, entries 9, 10). This dimeric motif
contacts (Table 4). Molecules in 4N3Br form a flat helical
assembly (similar to 4N2I) along the crystallographic two-fold
screw axis via three C–H O and a mono-coordinated Br NO2
halogen bonded synthon of Type III geometry (Table 4, entries
18–20, 22) whereas in 4N3Cl the molecular arrangement
slightly altered from the global planarity keeping the two C–
acquired the double convex spindle shape28 down the c-axis;
…
…
…
…
which are further connected through C–H O and C–H
N
interactions to generate a 2D pattern of an intercalated helix
(Fig. 9d, Table 4, entries 7, 8). Crystal structure of 4N2I
revealed a flat helical self-assembly of molecules across the
…
…
…
crystallographic two-fold screw axis through two C–H O and a
H
O contacts intact (Table 4, entries 15, 16) but lost a C–H
…
O
…
mono-coordinated I NO2 halogen bonding synthon of Type
and a Cl NO2 contacts (Fig. 10a and 10c). These flat helices
are connected via C–H N interactions along the c-axis to
…
III motif (Table 4, entries 11–13). The adjoining helices along
…
the c-axis are bridged via C–H N contacts forming a 2D sheet
produce a 2D sheet structure in 4N3Br (Fig. 10d) and a
periodic wavy structure in 4N3Cl (Fig. 10b). These 2D sheets
and wavy layers are packed in antiparallel fashion via van der
Waal’s forces.
structure (Fig. 9e, Table 4, entry 14). Viewing down the ac
diagonal revealed a periodic wavy structure running along the
b-axis. The antiparallel wavy sheets are stacked over each other
…
by centrosymmetric p p interactions engaging triazole rings
In 4N3I, the out of plane orientation of the iodobenzene ring
could be due to its engagement in symmetric bifurcated three
across the inversion center (Cg-centroid of the ring, Fig. 9f).
4N3X series. In 4N3Cl and 4N3Br, both benzene rings are
coplanar with the central triazole ring while in 4N3I
nitrobenzene and triazole ring showed a similar conformation
whereas the iodobenzene ring revealed a noticeable difference
in conformation with respect to the former. The similar
conformation of all the three rings led to isostructurality in
crystals of 4N3Cl and 4N3Br whereas 4N3I adopts a different
molecular packing. Molecules in all the structures are
…
centered I O2N halogen bond generation, which appeared to
be the driving force in molecular organization. Notably, this is
…
the first case where symmetric X O2N interaction (Type I) is
observed among all the crystal structures studied so far. The
two oxygen atoms of the nitro group are at a similar distance
from the iodine atom and the angle of approach is comparable
…
(Table 4, entries 24, 25). This symmetric I O2N interaction
motif generates a flat helical structure by closely associating
the c-glide related molecules. Such unit-translated nearest
…
…
…
associated via C–H O, C–H N and X O2N halogen bonding
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Fig. 10 Molecular organization in the structures of 4N3Br (a) and (b), 4N3Cl (c) and (d) and 4N3I (e) and (f).
helices along the b-axis are connected by short and linear C–
fashion through van der Waals forces (Fig. 11b). Similarly in
…
H
O interactions to make a 2D sheet structure (Fig. 10e,
crystals of 4N4Br molecules creates a 2D sheet network
…
…
Table 4, entry 23). Molecular association within the 2D sheet
through C–H
N
and C–H
O
contacts (Fig. 11c). Both
…
…
structure is further supported by C–H O and C–H N contacts
symmetry independent molecules forms centrosymmetric
…
dimers via C–H N contacts with their respective pairs
(Table 4, entries 26, 27). The adjoining 2D sheets are stacked
…
in antiparallel fashion via centrosymmetric aromatic p
p
(Table 4, entries 35, 36). The neighboring dimers are linked
…
interactions engaging two nitrobenzene rings and other van
der Waal’s forces (Fig. 10f).
through bifurcated C–H O contacts (Table 4, entries 31–34),
forming a 2D flat sheet structure. The adjoining sheets
generate
a multiple stack structure by aligning in an
4N4X series. All the three rings differed slightly in their
orientation. However, the difference is more significant in
4N4I. The two symmetry independent molecules in the
asymmetric unit of 4N4Br deviate by a small dihedral angle
between the triazole and bromobenzene groups while the
difference is more observable in the nitrobenzene ring and the
triazole ring.
antiparallel manner through van der Waals forces (Fig. 11d).
Closely associated molecules in 4N4I form a 1D molecular
chain along the b-axis through bifurcated C–H N interactions
(Table 4, entries 37, 38). The adjoining 1D chains are stitched
…
…
via unsymmetrical bifurcated three centered I O2N halogen
bonding interactions of Type II motif along the c-axis
generating 2D corrugated sheet pattern (Fig. 11e). Two oxygen
atoms of the NO2 group are at dissimilar distances from the
iodine atom and same is the case with their respective
approach angle (Table 4, entries 39, 40). The adjoining sheets
The crystal structure of 4N4Cl is grossly similar to the crystal
structures of 4N2I, 4N3Cl and 4N4Br. Molecules in 4N4Cl form
a flat helical assembly around the crystallographic two-fold
…
screw axis through short and linear bifurcated C–H
O
…
are linked in anti-parallel fashion via C–H p interactions
(Fig. 11f).
contacts (Table 4, entries 28, 29). These flat helices are
…
bridged along the c-axis via short and linear C–H
N
A comprehensive compilation of various intermolecular
interactions observed in all the crystal structures that led to
interactions forming a 2D sheet structure (Fig. 11a, Table 4,
entry 30). The adjacent sheets are stacked in antiparallel
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Fig. 11 Association of molecules in the structures of 4N4Cl (a) and (b) and 4N4Br (c) and (d) and 4N4I (e) and (f).
…
different molecular packing revealed that in the absence of
conventional H-bonding interactions, the weaker interactions
like C–H O, C–H N, C–H p, p p, C–H X, X X and X O
structures (halogen isomers), we found X O2N contacts in a
total of 12 structures, of which 9 are of Type III motif (2N4Cl,
3N2Br, 3N2I, 3N4Br, 3N4Cl, 3N4I, 4N2I, 4N3Cl and 4N3Br),
two are of Type II (4N4I and 4N2Br) and only one structure i.e.
…
…
…
…
…
…
…
played a dominant role in the outcome of the solid structures.
Moreover, the conformation of both the phenyl rings had a
significant effect on the organization of the molecules, leading
to different molecular architectures. Molecules in the 2-NO2
family largely pack in helical fashion due to the non-
…
4N3I showed bifurcated X O2N contacts (Type I motif) in its
molecular association. We also observed centrosymmetric
Type I X X halogen bond contact stabilizing the molecular
packing with support from C–H O and C–H N contacts in
some of the structures. The presence of edge-to-edge bifur-
…
…
…
…
coplanarity of all the three rings supported by C–H O, C–
…
…
…
…
H
N, X X interactions but not by X O2N halogen bonding
cated C–H N interactions in the molecular aggregation
contacts. Molecular association in 3NO2 and 4NO2 family
generates either a helical assembly or a 2D sheet pattern
depending on the conformation of all three rings. A slight
deviation from planarity led to helical packing or to the
corrugated 2D sheet pattern, whereas the planar structure
generates the 2D sheet pattern. Furthermore, it has been
directing lateral self assembly into 2D sheet structures or
wavy structures are also noticed.29 Hydrophobic forces such as
…
p
p and other van der Waal’s interactions also played a
significant role in the stacking of the 2D sheets with the
interplanar distance lying in the range y3.4–3.5 Å leading to
their subsequent stabilization. It is quite interesting to note
…
…
observed that X O2N interactions are found to be present in
that molecules in only one structure utilize the C–X p halogen
the 2D sheet or corrugated sheet structure. Out of a total of 27
bond in their association, which greatly contributed to its
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helical architecture (2N2I). As often observed,30 the crystals of
halo derivatives are mostly isomorphous; however in these
series of isomeric compounds very few crystal systems revealed
an isostructural arrangement whereas in most cases different
packing arrangements are noticed.
Notes and references
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The modular positioning of halogen (X) and NO2 functional
groups on the series of isomeric triaryl compounds for the
evaluation of the occurrence of the X NO2 synthon is
…
demonstrated. The structures are classified as 2-NO2, 3-NO2
and 4-NO2 families representing the position of nitro groups
on ortho, meta and para respectively each comprising the
structures with and without halo groups; the halogen atoms
positions are also varied from ortho and meta to para positions.
In all the crystal structures, the molecules either assembled
helically or formed the 2D sheet structure depending on the
conformations of all the three rings; irregular geometry
produced a helical architecture and planar conformations
…
displayed a 2D flat assembly. The possibility of the X NO2
synthon in the helical arrangement appeared to be remote and
revealed only associations via mono-coordinated Cl NO2
…
halogen bonds of Type III motif. However, the prospect of
…
X
NO2 halogen bond in a planar structure seemed positive
although few in number. Type I and Type II motifs have
materialized during head to tail association of molecules in
heavier halogen derivatives i.e. bromo and iodo as observed
earlier20,21 whereas the Type III pattern does not seem to have
any such preference. This indicates that the lighter halogen
atom i.e. Cl, does not seem to be suitable for the generation of
3 P. Metrangolo, H. Neukirch, T. Pilati and G. Resnati, Acc.
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…
three centered bifurcated X NO2 halogen bonding synthons
whereas the other two heavier halogens have a tendency to
engage in Type II and Type III motifs although it again
depends on their positions with respect to the nitro groups.
Also the very few hits observed for Type II and Type III motifs
even for the bromo and iodo derivatives could be due to the
existence of other equally energetic and competitive interac-
tions during crystal growth which made things more complex
to predict the patterns of supramolecular aggregation. This
showed that even in simple cases of such isomeric series of
compounds the pattern of supramolecular arrangement and
even the nature of interaction involved could differ signifi-
cantly. Aggregation patterns of molecules in isomeric struc-
tures could not be readily predicted even from a knowledge of
the patterns in similar structures.21
7 (a) P. Auffinger, F. A. Hays, E. Westhof and P. S. Ho, Proc.
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Acknowledgements
The authors would like to acknowledge the CSIR (India) for
funding this project under ORIGIN program of 12FYP and for
financial assistance in the form of research fellowships to YG,
KAD, SC and SK.
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…
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20 Selected references that deal with X NO2 3-centered
…
bifurcated halo O2N synthon see: (a) J. M. A. Robinson,
D. Philp, K. D. M. Harris and B. M. Kariuki, New J. Chem.,
2000, 24, 799; (b) P. K. Thallapally, G. R. Desiraju,
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S. Thallapally, S. Basavoju, G. R. Desiraju, M. Bagieu-
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…
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…
19 For selected papers that deal with the Cl NO2 synthon see:
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1983, 56, 1541; (b) M. Kubicki, J. Mol. Struct., 2004, 698, 67;
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