1,2,4-Triazole-based molecular switches: Crystal structures, Hirshfeld surface analysis and optical properties

Article abstract of DOI:10.1039/c6ce00749j

We have studied a series of eight closely related N-salicylidene-4-amino-1,2,4-triazole molecules 1-8, obtained by condensation of the corresponding aldehyde with 4-amino-4H-1,2,4-triazole. ¹H NMR spectroscopy in solution revealed the presence

Full text of DOI:10.1039/c6ce00749j

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1,2,4-Triazole-based molecular switches: crystal
structures, Hirshfeld surface analysis and optical
properties†
Cite this: DOI: 10.1039/c6ce00749j
*
Damir A. Safin, Koen Robeyns and Yann Garcia
We have studied a series of eight closely related N-salicylidene-4-amino-1,2,4-triazole molecules 1–8,
obtained by condensation of the corresponding aldehyde with 4-amino-4H-1,2,4-triazole. 1H NMR
spectroscopy in solution revealed the presence of a single structure at least in DMSO-d6. According to
single-crystal X-ray diffraction, it was established that the crystal structures of 5–8 each are stabilized by a
linear intramolecular hydrogen bond of the O–H⋯N type, formed between the o-OH hydrogen atom of
the phenolic ring and the imine nitrogen atom. The same o-OH function in the crystal structures of 1–4
was found to be involved in the intermolecular hydrogen bonds with one of the triazole nitrogen atoms of
the adjacent molecule. The overall geometry of each molecule in the structures of 1–8 was found to be al-
most planar or slightly deviated from planarity. Hirshfeld surface analysis showed that the structures of all
compounds are mainly characterized by H⋯H, H⋯C, H⋯N and H⋯O contacts but some contribution
from C⋯C and C⋯N contacts is also clearly observed. Diffuse reflectance spectroscopy reveals the exclu-
sive presence of the enol form in the solid state at room temperature for 1–6 and 8, while a mixture of
dominant enol and cis-keto forms were found for 7. All the studied molecules 1–8 were not photo-
switchable, while 7 was found to be thermochromic from coloured to colourless upon cooling.
Received 3rd April 2016,
Accepted 20th May 2016
DOI: 10.1039/c6ce00749j
www.rsc.org/crystengcomm
The solid state thermochromic properties of N-salicylidene
aniline derivatives were first considered to result from the
Introduction
Optical devices are of great importance due to their consider-
able and leading role in both daily life and instrumentation.1
Chromism-based molecular switches seem to be the most at-
tractive ones because of the smart and controlled change of
their chromic properties. Among them solid state thermo-
and photochromic compounds are considered the most rele-
vant working mass for smart devices.2 N-Salicylidene aniline
derivatives (Scheme 1), as well as their N-heterocyclic ana-
logues, dominate over other classes of molecules, exhibiting
thermo- and photochromic molecules in the crystalline state.3
This is explained by both their colour panel and their accessi-
ble forms (Scheme 1) as well as ease of synthesis due to Schiff
base condensation.4 Another advantage of N-salicylidene ani-
line derivatives, making them attractive from a synthetic
point of view, is the possibility of being included into various
matrices to form hybrid materials5 and blends.6
planarity of the molecule and the formation of a “close-
packed crystal structure” (dihedral angle between the aro-
matic rings ϕ <25°), whereas photochromic behaviour is
caused by the significant rotation of aromatic rings and the
formation of an “open structure” (ϕ >25°).4,5 Thermo- and
photochromic properties were stated over the years to be
mutually exclusive.4b A number of contradicting examples
showing both thermo- and photochromic properties have,
Institute of Condensed Matter and Nanosciences, Molecules, Solids and Reactivity
(IMCN/MOST), Université catholique de Louvain, Place L. Pasteur 1, 1348
Louvain-la-Neuve, Belgium. E-mail: yann.garcia@uclouvain.be;
Fax: +32 1047 2330; Tel: +32 1047 2831
† Electronic supplementary information (ESI) available: Additional data, Fig. S1–
S8 and Tables S1–S3. CCDC 1451196–1451202. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c6ce00749j
Scheme 1
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however, been recently discovered.5b,7,8 All these newly
obtained examples demonstrated that it is not possible to ex-
plain the thermo- and photochromism of N-salicylidene ani-
line derivatives solely based on their crystal structures, but
energy differences between ground and excited states need
also to be considered.5b Furthermore, a complete and de-
tailed crystal structure analysis, including ϕ, crystal packing
and the available free space around the switching unit in ad-
dition to the flexibility of the nearby environment, is neces-
Although both the crystal structure and optical properties
of 1 were described recently,7a it is also discussed herein for
a better comparison within the whole family of the closely re-
lated compounds. To examine and discuss the contribution
and influence of intermolecular interactions responsible for
the crystal packing, Hirshfeld surface analysis17 and associ-
ated 2D fingerprint plots,18 obtained using the
CrystalExplorer 3.1 software,19 as well as the enrichment ra-
tios,20 derived as the decomposition of the crystal contact
surface between pairs of interacting chemical species, have
been performed for the listed compounds. It should be noted
that this contribution is only the second one which reports
the study of noncovalent interactions by means of Hirshfeld
surface analysis in the structures of N-salicylidene aniline
derivatives.10t
sary.8 All this is obviously dictated by
a diversity of
noncovalent intermolecular interactions responsible for the
overall crystal packing of molecules.9
The thermo- and photochromic properties of a great num-
ber of mono-N-salicylidene aniline derivatives as well as
several bisIJsalicylidene) and trisIJsalicylidene) derivatives have
been thoroughly studied.4–8,10 In continuation of our compre-
hensive studies of N-salicylidene aniline derivatives and with
the aim of understanding their structural features, which in-
fluence their optical properties, we have directed our atten-
tion to a series of eight closely related N-salicylidene-4-amino-
1,2,4-triazole-based molecules modified by their N-salicylidene
fragment (Chart 1). Among them only the crystal structure
and optical properties of N-salicylidene-4-amino-1,2,4-triazole,
1 (Chart 1), have been studied so far.7a This is really surpris-
ing given that ironIJII) complexes containing 4-substituted-
1,2,4-triazole usually display spin crossover properties, which
could be used in molecular-based memory devices,11 dis-
plays,12 and sensors.13 For instance, the dinuclear ironIJII)
complex [Fe2(1)5(NCS)4]·4MeOH exhibits both temperature-
dependent spin crossover and fluorescence properties.5d Fur-
thermore, this complex affords a platform for photomagnetic
studies, including the light-induced excited-spin-state trap-
ping (LIESST) and ligand-driven light-induced spin change
(LD-LISC) effects,14 the latter being potentially accessible
through the formation of the trans-keto form of 1. 1 is also
interesting in its deprotonated form, being able to afford
Schiff base mononuclear15 and polynuclear complexes.16
Results and discussion
The N-salicylidene-4-amino-1,2,4-triazole-based molecules 1–8
were synthesized by reacting 4-amino-4H-1,2,4-triazole with
the corresponding salicylaldehyde in ethanol (Scheme 2). The
as-synthesized compounds form thin needle-like crystals,
which are soluble in most polar solvents and are insoluble in
n-hexane and diethyl ether. The obtained compounds were
divided into two groups, shown in the top and bottom rows
in Chart 1, based on the orientation of the o-OH group of the
N-salicylidene fragment as found in their crystal structures
(see X-ray description section below).
1
The H NMR spectra of 1–8 in DMSO-d6 each reveal a sin-
gle set of signals, which testifies to the presence of a single
structure in solution. The signals for the benzene (in 1–7)
and naphthalene (in 8) protons were found at 6.36–8.84 ppm,
while two singlet signals for the arylCHN and triazole protons
were observed at 8.97–9.65 and 9.07–9.29 ppm, respectively.
The signal of the o-OH protons in the spectra of the com-
pounds was shown at 9.77–11.36 ppm. In addition, the spec-
trum of 7 contains a singlet signal for the second OH proton
at 9.13 ppm, while the protons of the two OH fragments in
the spectrum of 3 were shown as one singlet at 10.36 ppm.
The methoxy protons in the spectrum of 4 were observed as a
singlet at 3.85 ppm.
The crystal structures of 1–8 were elucidated by single-
crystal X-ray diffraction. Compounds 1–3, 5, 6 and 8 crystallize
in the monoclinic space groups P21/n, Pc, P21/c and P21, re-
spectively, while the structures of compounds 4 and 7 were
refined in the tetragonal space group P42/n and trigonal
space group P31, respectively. Compounds 1 and 7 contain
two and three independent molecules, respectively, in the
asymmetric unit, namely 1-I, 1-II, 7-I, 7-II and 7-III.
Chart 1
Scheme 2
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Molecules in all the structures were found in the
enolimine form (Fig. 1–5). The bond lengths of C–O, with re-
spect to the moieties marked in bold in Scheme 2, are about
1.35 Å and those of C–C are about 1.45 Å (Tables 1 and S1 in
the ESI†), which indicates single bonds, whereas a double
bond of about 1.28 Å is revealed for C–N (Tables 1 and S1 in
the ESI†). The bond angles C–C–N and N–N–C of 119.6IJ2)–
Fig. 3 Top and side views of the molecular structure of 3 (top), and
stick (middle) and spacefill (bottom) hydrogen bonded 1D chain of
molecules in the structure of 3. Colour code: H = grey, C = black, N =
blue, O = red.
122.1IJ9)° and 115.60IJ12)–119.4IJ17)°, respectively, indicate the
sp2-hybridization of both the carbon and the nitrogen atoms
of the imine fragment, further supporting the enolimine
form (Tables 1 and S1 in the ESI†). The crucial difference be-
tween the structures 1–8 consists of the dihedral angle ϕ be-
tween phenol and triazole rings. While in 2 the two rings are
at 0.2°, the dihedral angle is larger in 1-II (6.1°), 7-II (6.6°), 7-
III (2.0°) and 8 (9.9°) and is significantly larger in 1-I (16.4°),
3 (19.2°), 4 (15.1°), 5 (16.1°), 6 (29.4°) and 7-I (13.5°). Notably,
a remarkable change in the dihedral angle between the
aromatic rings is also distinguished for the independent
Fig. 1 Top and side views of the molecule structure of 1-I (top), and
stick (middle) and spacefill (bottom) hydrogen bonded 1D zigzag chain
of molecules in the structure of 1. Colour code: H = grey, C = black,
N = blue, O = red.
Fig. 2 Top and side views of the molecular structure of 2 (top), and
stick (middle) and spacefill (bottom) hydrogen bonded 1D zigzag chain
of molecules in the structure of 2. Colour code: H = grey, C = black,
N = blue, O = red.
Fig. 4 Top and side views of the molecule structure of 4 (top), and
stick (middle) and spacefill (bottom) hydrogen bonded 1D zigzag chain
of molecules in the structure of 4. Colour code: H = grey, C = black,
N = blue, O = red.
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analysed by the program TOPOS,21 has been determined con-
sidering molecules as nodes connected through inter-
molecular hydrogen bonds. The resulting topology is a
4-connected uninodal network with a point Schläfli symbol of
{44·62}. This network is identified by an sql/Shubnikov tetrag-
onal plane net topological type in the RCSR database.
Thus, the presence of the triazole ring in the structure of
N-salicylidene aniline derivatives is crucial and indeed plays a
strong structure-directing role, through efficient inter-
molecular hydrogen bonds, that drives molecules to leave
their typical molecular geometry.
The presence of the 1,2,4-triazole ring is of great impor-
tance but not enough to break the normal molecular geome-
try of N-salicylidene aniline derivatives. Crystal packing influ-
ence has also to be considered. This was clearly defined from
the crystal structures of 5–8, all molecules of which are stabi-
lized by the typical intramolecular hydrogen bond between
the hydroxyl hydrogen atom and the imine nitrogen atom
(Fig. 5 and Table S2 in the ESI†). Notably, the second m-OH
function in the crystal structure of 7 is involved in the inter-
molecular hydrogen bond with the triazole nitrogen atom
(Fig. 5 and Table S2 in the ESI†) similar to those in the struc-
tures of 1–4 (Fig. 1–4). However, as a result a 1D supramolec-
ular linear instead of a zigzag chain is formed.
Fig. 5 Top and side views of the molecular structure of 5 (top left), 6
(top right), 7 (bottom left) and 8 (bottom right). Colour code: H = grey,
C = black, N = blue, O = red.
molecules in the structures of 1 and 7 (Tables 1 and S1 in the
ESI†). Both independent molecules in the crystal structure of
1 each develop a new type of molecular geometry where the
intramolecular hydrogen bond between the hydroxyl hydro-
gen atom and the imine nitrogen atom is broken by rotation
of the phenolic ring (Chart 1 and Fig. 1) and replaced by a
number of intermolecular interactions.7a In particular, the
formation of an intermolecular hydrogen bond between a ni-
trogen atom of the triazole ring and the phenolic hydroxyl
function is observed (Fig. 1 and Table S2 in the ESI†). As a re-
sult, a 1D supramolecular zigzag chain is formed (Fig. 1).
In the frame of this work, we also report three more struc-
tures 2–4 with the same type of molecular geometry (Chart 1)
as well as with the same type of intermolecular hydrogen
bond (Fig. 2–4 and Table S2 in the ESI†). Furthermore, while
molecules in the structures of both 2 and 4 also form a 1D
supramolecular zigzag chain (Fig. 2 and 4), the second p-OH
function in the crystal structure of 3 is also involved in the
intermolecular hydrogen bond with the triazole nitrogen
atom of the third neighbouring molecule (Fig. 3 and Table S2
in the ESI†). As a result, an interlocked double layered 2D
sheet is formed (Fig. 3). Notably, the underlying net of 3, as
In order to examine the interactions in the crystal struc-
tures of 1–8, the Hirshfeld surface analysis17 and associated
2D fingerprint plots18 were obtained using CrystalExplorer
3.1.19
Unlike other molecular volumes and surfaces (e.g. van der
Waals volumes, solvent-accessible surfaces, solvent-excluded
surfaces), Hirshfeld surfaces are not simply a function of the
molecular geometry but are only defined within the crystal.
Consequently, Hirshfeld surfaces reflect the interplay be-
tween different atomic sizes and intermolecular contacts in
the crystal (condensed phase). Hirshfeld surfaces and vol-
umes are much larger than conventional ones, generally fill-
ing at least 95% of the crystal volume,22 compared with more
conventional packing coefficients of between 0.65 and 0.80.23
Hirshfeld surfaces obviously pack very tightly in the crystal,
at most touching and never overlapping. However, quite
Table 1 Selected bond lengths (Å) and angles (°) for 1-I, 2–6, 7-I and 8a
1-Ib
2
3
4
5
6
7-Ic
8
Bond lengths
C–N
C–C
C–O
N–N
1.279(2)
1.458(2)
1.3510(19)
1.4000(19)
1.272(4)
1.454(5)
1.336(4)
1.398(4)
1.281(15)
1.468(16)
1.370(12)
1.394(13)
1.270(3)
1.464(4)
1.343(3)
1.404(3)
1.30(2)
1.42(2)
1.340(16)
1.38(2)
1.284(9)
1.460(9)
1.363(9)
1.403(8)
1.313(16)
1.436(19)
1.360(17)
1.421(14)
1.287(5)
1.445(5)
1.361(5)
1.394(5)
Bond angles
C–C–N
N–N–C
121.00(13)
115.60(12)
120.8(3)
117.0(3)
121.1(10)
117.4(9)
119.6(2)
116.7(2)
120.5(17)
119.4(17)
121.5(6)
115.8(5)
120.8(10)
116.6(9)
121.7(3)
117.7(3)
Torsion angles
C–C–N–N
Dihedral angles (ϕ)
Phenol–triazole
−175.52IJ14)
−179.5IJ3)
177.4(9)
19.2
−176.9IJ2)
177.4(15)
16.1
−178.2IJ6)
177.5(9)
13.5
179.3(3)
9.9
16.4
0.2
15.1
29.4
a
b
c
Values with respect to the moieties marked in bold in Scheme 2. Data for 1-II are similar to those of 1-I and are given in the ESI. Data for
7-II and 7-III are similar to those of 7-I and are given in the ESI.
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unlike any other partitioning or packing scheme, they leave
small intermolecular voids, which can be regarded as regions
where the crystalline electron density is very low and is not
dominated by any single molecule. The use of Hirshfeld sur-
faces for the analysis of molecular crystal structures encour-
ages the adoption of a whole structure view of intermolecular
interactions, rather than concentrating exclusively on as-
sumed interactions. While the discussion of crystal structures
in terms of individual interatom contacts is unavoidable and
certainly valuable, a broader picture of intermolecular inter-
actions in the crystal is increasingly desirable; such a picture
is available from the Hirshfeld surface. The size and shape of
the Hirshfeld surface are intimately related to the chemical
environment surrounding the molecule, making it ideal for
use in comparing different crystal structures incorporating
the same molecule.
than for a direct two-atom contact.17 The structure of 1-I is
also dominated by H⋯C and H⋯N contacts, comprising
21.9% and 34.8%, respectively, while the same contacts in
the structure of 1-II are remarkably less and occupy 17.9%
and 25.1%, respectively, of the total Hirshfeld surface areas
(Tables 2 and S3 in the ESI†).
The H∙∙∙C contacts in fingerprint plots are shown in the
form of “wings” (Fig. 6 and Fig. S1 in the ESI†), with the
shortest de + di ≈ 2.6–2.8 Å. These contacts are recognized as
characteristic of C–H⋯π nature.17 It is worth adding that the
fingerprint plot of 1-I exhibits a significant number of points
at large de and di, shown as tails at the top right of the plot
The 2D fingerprint plots not only clearly identify each type
of intermolecular contact but also enable the analysis of very
small differences in these patterns and as such represent an
entirely new way of summarizing the major intermolecular
contacts of an entire crystal structure in a single 2D colour
picture. Fingerprint plots have been shown to be particularly
suited to comparing the crystal structures of closely related
molecules (e.g. polymorphs, different co-crystals, structures
with Z′ >1.0).
According to the Hirshfeld surface analysis, for both inde-
pendent molecules of 1 the intermolecular H⋯H contacts,
comprising 30.1% and 34.7% of the total number of contacts,
are major contributors to the crystal packing (Tables 2 and
S3 in the ESI†). The shortest H⋯H contacts are shown in the
fingerprint plots of 1-I and 1-II as characteristic broad spikes
at de + di ≈ 2.2 Å (Fig. 6 and S1 in the ESI†). Furthermore, a
subtle feature is evident in the fingerprint plot of 1-I. There
is a splitting of the short H⋯H fingerprint. This splitting oc-
curs when the shortest contact is between three atoms, rather
Fig. 6 2D fingerprint plots of observed contacts for 1-I, 2–6, 7-I and 8.
Table 2 Hirshfeld contact surfaces and derived “random contacts” and “enrichment ratios” for 1-I, 2, 3 and 4a
1-I
2
3
4
H
C
N
O
H
C
N
O
H
C
N
O
H
C
N
O
Contacts (C, %)b
H
C
N
O
30.1
21.9
34.8
1.2
—
—
—
1.1
1.2
—
—
—
0.0
13.7
21.5
20.1
21.2
—
—
—
2.4
9.3
—
—
—
2.0
29.6
11.3
24.2
16.8
—
—
—
0.6
0.5
—
—
—
0.1
39.4
15.7
24.3
4.6
—
—
—
0.5
3.4
—
—
—
0.1
2.4
3.5
2.7
0.3
5.4
3.8
5.2
10.1
1.6
2.2
4.5
5.3
Surface (S, %)
59.1
16.5
20.9
2.6
45.1
15.7
19.8
19.2
55.8
16.7
18.0
9.6
61.7
15.0
16.6
6.8
Random contacts (R, %)
H
C
N
O
34.9
19.5
24.7
3.1
—
—
—
4.4
1.1
—
—
—
0.1
20.3
14.2
17.9
17.3
—
—
—
3.9
7.6
—
—
—
3.7
31.1
18.6
20.1
10.7
—
—
—
3.2
3.5
—
—
—
0.9
38.1
18.5
20.5
8.4
—
—
—
2.8
2.3
—
—
—
0.5
2.7
1.1
0.9
2.5
1.0
6.0
2.8
1.0
3.2
2.3
0.7
2.0
Enrichment (E)c
H
C
N
O
0.86
1.12
1.41
0.39
—
—
—
0.25
1.09
—
—
—
—
0.67
—
—
—
0.62
1.22
—
—
—
0.54
0.95
—
—
—
0.19
0.14
—
—
—
0.11
1.03
—
0.96
—
—
—
0.18
1.48
—
—
—
—
0.89
3.18
3.00
1.51
1.12
1.23
0.12
5.40
0.63
0.61
1.20
1.57
1.86
10.10
0.50
0.85
1.19
0.55
2.65
a
b
Data for 1-II are similar to those of 1-I and are given in the ESI. Values are obtained from CrystalExplorer 3.1.19 c The enrichment ratios
were not computed when the “random contacts” were lower than 0.9%, as they are not meaningful.20
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(Fig. 6 and S1 in the ESI†). These points, similar to those ob-
served in the fingerprint plot of benzene17 and phenyl-
containing compounds,24 correspond to regions on the
Hirshfeld surface without any close contacts to nuclei in adja-
cent molecules. The H⋯N contacts in the fingerprint plots of
both independent molecules are shown as a pair of sharp
spikes at de + di ≈ 1.7–1.8 Å (Fig. 6 and S1 in the ESI†). These
contacts correspond to the N–H⋯O hydrogen bonds (Fig. 1
and Table S1 in the ESI†). The fourth main contribution to
intermolecular interactions in the structure of 1-II arises
from H⋯O contacts, comprising 7.4% of the total Hirshfeld
surface area (Table S3 in the ESI†) and are shown in the fin-
gerprint plot as a pair of spikes at de + di ≈ 2.4 Å (Fig. S1 in
the ESI†). The same contacts in the structure of 1-I are very
negligible and only 1.2% (Tables 2 and S3 in the ESI†). The
structures of both molecules of 1 are also described by C⋯C
and C⋯N contacts. However, these contacts comprise a negli-
gible proportion of the total Hirshfeld surface area and of
2.4% and 3.5% for 1-I, and 4.3% and 6.4% for 1-II, respec-
tively. Moreover, these contacts are shown on the fingerprint
plots as the area on the diagonal at de = di ≈ 1.7–2.0 Å (Fig. 6
and S1 in the ESI†). These contacts are evidence for π⋯π
stacking interactions. Close inspection of other inter-
molecular contacts in the structures of 1-I and 1-II also re-
vealed a proportion of C⋯O (1.7–2.7%), N⋯N (1.1–1.7%) and
N⋯O (0.7–1.2%) contacts (Tables 2 and S3 in the ESI†).
We have also determined the enrichment ratios (E)20 of
the intermolecular contacts for both independent molecules
of 1 to study the propensity of two chemical species to be in
contact. The enrichment ratio, derived from the Hirshfeld
surface analysis, is defined as the ratio between the propor-
tion of actual contacts in the crystal and the theoretical pro-
portion of random contacts. E is larger than unity for a pair
of elements with a higher propensity to form contacts, while
pairs which tend to avoid contacts yield an E value lower than
unity.
The H⋯H contacts are favoured in the structure of 1-II
since the enrichment ratio EHH is close to unity (0.97) and
generate a majority (34.7%) of the interaction surface (Table
S3 in the ESI†). Contrarily, the H⋯H contacts are less
favoured in the structure of 1-I (EHH = 0.86). This is explained
by a significantly lower proportion (30.1%) of H⋯H contacts
of the total Hirshfeld surface area in 1-I, although its struc-
ture contains almost the same amount of random contacts as
in the structure of 1-II (Table S3 in the ESI†). The vice versa
trend is observed for H⋯C contacts, which show an in-
creased propensity to form (EHC = 1.12) in the structure of 1-
I, and only slightly favoured (EHC = 0.86) in 1-II. This is due
to a higher amount of H⋯C contacts of the total Hirshfeld
surface area in 1-I compared to that in 1-II, despite both
structures being characterised by almost the same values of
the SH proportion and random contacts RHC (Tables 2 and S3
in the ESI†). The EHO values are larger than unity (1.41 and
1.18) for both molecules of 1, indicating that H⋯O contacts
have an increased propensity to form, with close random
contacts. It should be noted that H⋯N contacts are highly
favoured in the structure of 1-II since the enrichment ratio
EHN is higher than unity (1.25), while the same contacts in
the structure of 1-I are significantly impoverished (EHN
=
0.39). Remarkably, the structure of 1-I is further characterized
by favoured C⋯N contacts (ECN = 3.18), while the same con-
tacts in the structure of 1-II are even much more favoured
(ECN = 5.82). This is due to a negligible amount of random
contacts RCN (1.1), despite both molecules being
characterised by a small amount of C⋯N contacts of the total
Hirshfeld surface area (Tables 2 and S3 in the ESI†). How-
ever, while the ECO value is equal to unity for the structure of
1-II, the same contacts are much more favoured in the struc-
ture of 1-I (ECO = 3.00), which is explained by a higher
amount of the C⋯O contacts on the molecular surface to-
gether with a lower amount of random contacts RCO for 1-II
compared to those of 1-II (Tables 2 and S3 in the ESI†). Inter-
estingly, the C⋯C contacts in the structure of 1-II are remark-
ably enriched (ECC = 1.43), which is due to a relatively high
value of their proportion of the total Hirshfeld surface area.
Although the Sc value and random contacts proportion RCC of
the structure of 1-I are almost the same as those for 1-II, its
structure is characterized by fewer C⋯C contacts (ECC
=
0.89). This is explained by a smaller amount of C⋯C contacts
on the molecular surface of 1-I (Table 2). The structure of 1-I
is also enriched by N⋯O contacts (ENO = 1.09), while the
same contacts in 1-II are significantly impoverished. Finally,
the structures of both molecules are described by impover-
ished (0.53 for 1-II) or very impoverished (0.25 1-I) N⋯N
contacts, which is due to a significantly higher proportion of
random contacts RNN compared to a proportion of C⋯C con-
tacts on the molecular surface (Tables 2 and S3 in the ESI†).
By introduction of the NO2 group into the phenolic frag-
ment of 1, yielding 2, the contribution of each contact to the
total Hirshfeld surface area is changed considerably. In par-
ticular, while the proportion of the H⋯C contacts on the mo-
lecular surface of 2 is almost the same as for 1-I, a proportion
of both H⋯H and H⋯N contacts decreased significantly up
to 13.7% and 20.1%, respectively, with the simultaneous sud-
den jump of the amount of H⋯O contacts up to 21.2%, the
highest value among all the discussed compounds 1–8
(Tables 2 and S3 in the ESI†). This is not surprising and is
obviously explained by a terminal position of the NO2 group
on the periphery of a molecule (Fig. 2). The same trend is ob-
served for the corresponding enrichment ratios. While the
enrichment ratios EHC and EHN are notably higher than unity
and similar to those in the structure of 1-I, the EHH value de-
creased significantly, accompanied by a pronounced increase
in the EHO value (Tables 2 and S3 in the ESI†), which is
higher than unity. This indicates that the H⋯O contacts, de-
spite a highly increased proportion of the random contacts
RHO (Table 1), are favoured in the structure of 2. The shortest
H⋯H contacts are shown in the fingerprint plot of 2 at de +
di ≈ 2.3–2.4 Å (Fig. 6 and S2 in the ESI†). The H⋯C contacts
in the fingerprint plot are shown at de + di > 3.1 Å and an
overwhelming majority of points are at larger de and di,
shown as tails at the top right of the plot (Fig. 6 and S2 in
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the ESI†), which is a characteristic region on the Hirshfeld
surface without any close contacts to nuclei in adjacent mole-
cules.17,19 The H⋯N contacts in the fingerprint plot of 2 are
shown as a pair of sharp spikes at de + di ≈ 1.7 Å (Fig. 6 and
S1 in the ESI†), similar to those in the fingerprint plots of 1,
corresponding to the N–H⋯O hydrogen bonds (Fig. 2 and Ta-
ble S1 in the ESI†). Moreover, the H⋯O contacts in the fin-
gerprint plot of 2 are also shown as a pair of sharp spikes at
de + di ≈ 2.3 Å (Fig. 6 and S1 in the ESI†). Although the struc-
ture of 2 is also described by a relatively small proportion of
the C⋯N contacts (5.4%) on the total Hirshfeld surface, they
are highly favourable as evidenced from the corresponding
enrichment ratio (ECN = 5.40). At the same time, the enrich-
ment ratios of C⋯O and C⋯C contacts are impoverished or
very impoverished compared to those in the structure of 1
(Tables 2 and S3 in the ESI†). This is due to remarkably
higher proportions of the random contacts RCO and RCC com-
pared to the corresponding proportions of these contacts on
the molecular surface. Other intermolecular contacts in the
structure of 2 also revealed highly favoured N⋯O (9.3%), and
impoverished N⋯N (2.4%) and O⋯O (2.0%) contacts (Table 2).
Moving from 2 to 3, containing a p-OH function in the
phenolic fragment, and 4, containing a m-OMe group in the
phenolic part of a molecule, a significant increase in the pro-
portion of the intermolecular H⋯H contacts, comprising
29.6% and 39.4% of the total number of contacts, is observed
(Table 2). The proportion of these contacts in the structure of
4 is the most abundant among all the studied compounds
1–8, which is due to the presence of the methyl fragment
(Fig. 4). The shortest H⋯H contacts are shown on the finger-
print plots of 3 and 4 at de + di ≈ 2.3 Å and as a sharp spike
at de + di ≈ 2.1 Å, respectively (Fig. 6 and S1 and S4 in the
ESI†). Although the H⋯H contacts occupy a remarkably
higher proportion of the molecular surface as well as produce
a higher SH value in the structure of 4 compared to those of
3, both compounds are characterised by almost the same en-
richment ratio EHH very close to unity (Table 2). This is
caused by a proportional increase of the random contacts
RHH in the structure of 4. Furthermore, the H⋯N contacts in
the structure of both 3 and 4 are favoured, occupying the
same amount of the total Hirshfeld surface (∼24%), and in
the fingerprint plot of both compounds are shown as a pair
of sharp spikes at de + di ≈ 1.8 Å (Fig. 6 and S3 and S4 in the
ESI†), corresponding to the N–H⋯O hydrogen bonds
(Fig. 3 and 4 and Table S1 in the ESI†). Both structures are
further dominated by H⋯C contacts, comprising 11.3% and
15.7%, respectively, while the random contacts RHC are the
same. All this explains a notable difference in the enrichment
ratios EHC between 3 and 4. This value is 0.85 for 4, charac-
teristic for the less favourable contacts, and 0.61 for 3, indi-
cating an impoverished enrichment. Furthermore, the H⋯C
contacts in the fingerprint plot of 4 are shown in the form of
clearly pronounced “wings” (Fig. 6 and S4 in the ESI†), with
the shortest de + di ≈ 2.7 Å and being characteristic of a C–
H⋯π nature.15 The H⋯O contacts in the structure of 3 also
occupy a significant proportion of the molecular surface and
on the corresponding fingerprint plot are shown as a pair of
sharp spikes at de + di ≈ 2.5 Å (Fig. 6 and S3 in the ESI†),
while the same contacts are negligible in the structure of 4
(Table 1). This is also reflected in the corresponding enrich-
ment ratio EHO, which is 1.57 for 3 and 0.55 for 4. Thus, the
former structure is highly enriched by the H⋯O contacts,
while this type of intermolecular contacts is impoverished in
the latter structure. The structures of both 3 and 4 are also
described by C⋯C and C⋯N contacts. However, these con-
tacts comprise a negligible proportion of the total Hirshfeld
surface area in the structure of 4, but remarkably higher in
the structure of 3 (Table 2). These contacts are shown on the
fingerprint plots as the characteristic area on the diagonal at
de = di ≈ 1.7–2.0 Å (Fig. 6 and S3 and S4 in the ESI†) and cor-
respond to π⋯π stacking interactions. Notably, while the
C⋯C contacts are favourable in the structure of 4, as
evidenced from the corresponding enrichment ratio ECC be-
ing close to unity, the structure of 3 is more enriched by the
same contacts as well as characterised by an extremely
enriched value, ECN = 10.10 (Table 2). Two more significant
differences in the total Hirshfeld surface areas of 3 and 4 are
concealed in the C⋯O and N⋯O intermolecular contacts.
These contacts are very negligible in the structure of 3 (1.6%
and 0.5%, respectively), while their proportion is remarkably
higher in the structure of 4 (5.3% and 3.4%, respectively). All
this, together with a higher proportion of the corresponding
random contacts RCO and RNO in the structure of 3 compared
to those of 4, explains the extremely high and poor enrich-
ment ratios of these contacts on their molecular surfaces
(Table 2).
The second group of studied compounds (4–8) is charac-
terized by the blocked o-OH group of the phenolic fragment
due to the formation of the intramolecular hydrogen bond
with the imine nitrogen atom (Chart 1 and Fig. 5).
The distribution of main intermolecular contacts, viz.
H⋯H (19.2% and 17.5%), H⋯N (24.9% and 22.5%), H⋯O
(8.8% and 7.3%), C⋯C (9.0% and 7.6%) and H⋯Cl/Br
(20.1% and 21.9%), on the total Hirshfeld surface of both 5
and 6 is similar and only minor differences are observed
(Table 3). Furthermore, enrichment ratios, corresponding to
the listed contacts, are also very similar for both structures.
In particular, H⋯N (1.44 and 1.48), H⋯O (1.73 and 1.43)
and H⋯Cl/Br (1.73 and 1.62) contacts are highly favoured,
and C⋯C (3.21 and 2.92) contacts are even much more
favoured. The C⋯C contacts are shown on the fingerprint
plots of 5 and 6 as the characteristic pale blue/green area,
mixed with yellow and red points, on the diagonal at de = di
≈ 1.7–2.0 Å (Fig. 6 and S5 and S6 in the ESI†), and corre-
spond to the formation of strong π⋯π stacking interactions.
The H⋯N and H⋯O contacts each are shown on the finger-
print plots of 5 and 6 as a pair of sharp spikes at de + di ≈
2.2–2.3 Å and de + di ≈ 2.3–2.5 Å, respectively (Fig. 6 and S5
and S6 in the ESI†). Interestingly, the H⋯Cl/Br contacts are
shown on the fingerprint plots of 5 and 6 as two broad spikes
containing pale blue/green points, with the shortest de + di ≈
2.9 and 3.0 Å, respectively (Fig. 6 and S5 and S6 in the ESI†).
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Table 3 Hirshfeld contact surfaces and derived “random contacts” and “enrichment ratios” for 5, 6, 7-I and 8a
5
6
7-I
8
H
C
N
O
Cl
H
C
N
O
Br
H
C
N
O
H
C
N
O
Contacts (C, %)b
H
C
N
O
19.2
6.4
24.9
8.8
—
—
—
1.3
0.0
0.0
—
—
—
0.0
0.6
—
—
—
—
0.6
17.5
11.7
22.5
7.3
—
—
—
2.1
0.0
2.1
—
—
—
0.3
0.1
—
—
—
—
1.5
27.0
14.1
23.7
18.8
—
—
—
—
0.9
1.0
—
—
—
—
0.5
—
29.6
27.6
22.6
9.2
—
—
—
0.4
0.1
—
—
—
—
0.0
—
9.0
6.5
0.9
1.7
7.6
2.9
2.3
0.3
5.2
8.1
0.7
—
3.7
6.7
0.1
—
Cl/Br
20.1
21.9
—
Surface (S, %)
49.3
16.8
17.0
5.2
11.8
49.2
16.2
15.9
5.2
13.7
55.3
16.7
17.3
10.8
59.3
20.9
15.1
4.7
Random contacts (R, %)
H
C
N
O
24.3
16.6
16.8
5.1
—
—
—
2.9
1.8
4.0
—
—
—
0.3
1.2
—
—
—
—
1.4
24.2
15.9
15.6
5.1
—
—
—
2.5
1.7
4.3
—
—
—
0.3
1.4
—
—
—
—
1.9
30.6
18.5
19.1
11.9
—
—
—
—
3.0
3.7
—
—
—
—
1.2
—
35.2
24.8
17.9
5.6
—
—
—
2.3
1.4
—
—
—
—
0.2
—
2.8
5.7
1.7
4.0
2.6
5.2
1.7
4.4
2.8
1.0
3.6
—
4.4
1.3
2.0
—
Cl/Br
11.6
13.5
—
Enrichment (E)c
H
C
N
O
0.79
0.39
1.48
1.73
1.73
—
—
—
0.45
0.00
0.00
—
—
—
—
—
—
—
—
0.72
—
—
—
0.84
0.00
0.49
—
—
—
—
—
—
—
—
0.88
0.76
1.24
1.58
—
—
—
—
0.30
0.27
—
—
—
—
0.42
—
0.84
1.11
1.26
1.64
—
—
—
—
0.17
0.07
—
—
—
—
—
—
3.21
1.14
0.53
0.43
0.74
1.44
1.43
1.62
2.92
0.56
1.35
0.07
1.86
8.10
0.19
—
0.84
5.15
0.05
—
Cl/Br
0.50
0.43
0.07
0.79
a
b
Data for 7-II and 7-III are similar to those of 7-I and are given in the ESI. Values are obtained from CrystalExplorer 3.1.19 c The enrichment
ratios were not computed when the “random contacts” were lower than 0.9%, as they are not meaningful.20
The H⋯H contacts in both structures are less favoured (EHH
= 0.79 and 0.72, respectively), which is due to relatively
higher values of the random contacts RHH compared to the
proportion of these contacts on the molecular surface.
surprisingly, close to values found for the intermolecular con-
tacts on the total Hirshfeld surface of 3 (Tables 2 and S3 in
the ESI†). Thus, the position of the second OH group in the
phenolic fragment as well as the fact that the o-OH group is
involved in intermolecular hydrogen bonding in the structure
of 3 and that the same function is blocked by the intramolec-
ular hydrogen bonding in the structures of all independent
molecules of 7, almost have no influence on the type, distri-
bution and enrichment ratios of contacts on the molecular
surfaces of 3 and 7. The main difference, however, arises
from the distribution of the H⋯O contacts on the fingerprint
plots. These contacts are shown on the fingerprint plots of
each independent molecule of 7 as two sharp spikes at much
shorter de + di ≈ 2.2 Å compared to that of 3 (Fig. 6 and S3
and S7 in the ESI†).
According to the Hirshfeld surface analysis, for 8 the inter-
molecular H∙∙∙H contacts, comprising 29.6% of the total num-
ber of contacts, are major contributors to the crystal packing
(Table 3). The shortest H⋯H contacts are shown in the fin-
gerprint plot as a sharp spike at de + di ≈ 2.4 Å (Fig. 6 and S8
in the ESI†). The structure of 8 is also dominated by H⋯C,
H⋯N and H⋯O contacts, comprising 27.6%, 22.6% and
9.2%, respectively, of the total Hirshfeld surface areas
(Table 2). The former contacts in the fingerprint plot are
shown in the form of clearly pronounced “wings” (Fig. 6 and
S8 in the ESI†), with the shortest de + di ≈ 2.7 Å, correspond-
ing to C–H⋯π interactions.17 The latter two contacts are
shown on the fingerprint plot of 8 as two sharp, for H⋯N, or
broad, for H⋯O, spikes with the shortest de + di ≈ 2.3 and
2.6 Å, respectively (Fig. 6 and S8 in the ESI†). The H⋯C,
H⋯N and H⋯O contacts are highly favoured in the structure
of 8 since the corresponding enrichment ratios EHH are
The H⋯C intermolecular contacts occupy 6.4% of the mo-
lecular surface in the structure of 5, while the same contacts
are about two times more in the structure of 6 (Table 3).
However, the proportion of the random contacts RHC is simi-
lar (∼16%) and relatively high for both structures, yielding
less favourable for enrichment ratios 6 and impoverished for
5 (Table 2). The H⋯C contacts in the fingerprint plots of 5
and 6 are shown as diffusely spread points (Fig. 6 and S5 and
S6 in the ESI†), with the shortest de + di ≈ 3.2 Å. The vice
versa trend is observed for C⋯N contacts, which show an in-
creased propensity to form (ECN = 1.14) in the structure of 5,
and impoverished (ECN = 0.56) in 6. This is due to about two
times higher amount of C⋯N contacts of the total Hirshfeld
surface area in 5 compared to that in 6, despite both struc-
tures being characterised by almost the same values of SC
and SN as well as random contacts RCN (Table 3). Contrarily
to the C⋯N contacts, the structure of 5 contains a negligible
amount of the C⋯O contacts (0.9%), while the proportion of
the same contacts is two and a half times more (2.3%) on the
total Hirshfeld surface area of 6. All this, together with the
same value of the random contacts RCO for both structures,
show that the molecular surface of 6 is highly enriched with
the C⋯O contacts, while they are impoverished on the sur-
face of 5 (Table 3). Some other negligible intermolecular con-
tacts were found in the structures of 5 and 6 (Table 3).
The distribution of intermolecular contacts, as well as
their corresponding enrichment ratios, is very similar for all
the three independent molecules in the structure of 7 and,
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higher than unity (1.11–1.64). Contrarily, the H⋯H contacts
are less favoured (EHH = 0.84) even though they generate a
majority of the interaction surface. This is explained by a rel-
atively higher proportion of the random contacts RHH
(Table 3). The last two moderate contributors into the molec-
ular surface in the structure of 8 are C⋯C and C⋯N con-
tacts. Contrary to expectations, based on the presence of the
naphthalene fragment, the former contacts occupy only 3.7%
of the total Hirshfeld surface, while the proportion of the lat-
ter ones is about two times more and of 6.7%. Furthermore,
the molecular surface of 8 is highly enriched with the C⋯N
contacts, as evidenced from the corresponding enrichment
ratio, ECN = 5.15, while the C⋯C contacts are significantly
less favourable (ECN = 0.84). Other intermolecular contacts in
the structure of 8 occupy a very negligible proportion of the
molecular surface (0.6%) and are very impoverished (Table 3).
Compounds 1–8 were analyzed by diffuse reflectance
spectroscopy (DRS) as pure solid powders to avoid matrix
and environment effects that are known to intensively modify
the optical properties of N-salicylidene aniline derivatives.5b
The diffuse reflectance spectra of 1, 3 and 4, for which a
Kubelka–Munk (KM) treatment was applied, each exhibit
bands exclusively in the UV region (Fig. 7 and Table 4), which
is typical of the enol form. In the spectra of 5 and 6 the enol
band is slightly shifted to lower energies, which is caused by
the presence of m-Cl/Br substituents in the phenolic frag-
ment, while in the spectrum of 8 this shift is more remark-
able due to the naphthalene fragment (Fig. 7 and Table 4).
The presence of the second m-OH group in the phenolic frag-
ment of 7 also resulted in the bathochromic shift of the enol
band; however, its spectrum is also characterised by a broad
shoulder centred at about 500 nm and ranging up to 600 nm
(Fig. 7 and Table 4). This band in the visible range of the
spectrum is responsible for the orange colour of the crystals
of 7 and is typical of the cis-keto form of the molecule. A sim-
ilar low intense shoulder, but up to about 500 nm, is also
present in the spectrum of 2 (Fig. 7 and Table 4) and is re-
sponsible for the pale yellow colour of its crystals. However,
in this case the band in the visible range of the spectrum is
mainly due to the presence of the chromophoric nitro group.
All the studied compounds 1–8 were observed to be non
photochromic regardless of the irradiation wavelength (λ =
254, 365, 450 and 546 nm) and time, while 7 was found to be
thermochromic from coloured to colourless upon cooling
with liquid nitrogen (Fig. 7).
Fig. 7 Normalised KM spectra of 1–8 at 23 °C and photographs
showing thermochromism for 7.
H⋯π interactions are highly favourable to facilitating the
generation of the cis-keto form. These two situations can also
influence the energy differences between ground and excited
states, which, in turn, can favour the formation of the
cis-keto form. However, to check this hypothesis detailed and
in-depth theoretical studies are needed.
Regarding the relationship between the intermolecular in-
teractions and the existence of the enol or cis-keto forms of
1–8, one must keep in mind that the Hirshfeld surface analy-
sis can only be run after the crystal structure is obtained
from X-ray analysis. Since no crystal structures of the cis-keto
forms of 1–8 were obtained, such a relationship for the enol
and cis-keto forms cannot be discussed yet. Notably, if the
same form of a molecule results in two or several polymorphs,
different Hirshfeld surfaces with different contributions of
Among the structures of 5–8, which potentially can exhibit
chromic properties due to the intramolecular hydrogen N–
H⋯O bond, only 7 exhibits thermochromic properties. This
might be explained by the presence of highly favourable
C⋯C and even much more favourable C⋯N contacts and
slightly impoverished H⋯C contacts together with the ab-
sence of highly favourable H⋯Hal contacts, as in the struc-
tures of halogen-containing compounds 5 and 6, on the mo-
lecular surface of 7, while the opposite trend is observed for
5, 6 and 8 (Table 3). In other words, the presence of π⋯π
intermolecular interactions accompanied by the decreased C–
Table 4 λmax of the bands in diffuse reflectance spectra of 1–8
1
2
3
4
5
6
7
8
248, 318, 372
239, 301, 381, 430 (sh.)
241, 264, 354 (sh.), 373 (sh.)
223, 262, 306 (sh.), 331, 379
235, 262, 306 (sh.), 345, 371 (sh.)
242 (sh.), 261, 336, 381 (sh.)
236, 272, 344, 400 (sh.), 490 (sh., cis-keto)
230 (sh.), 257, 307, 325 (sh.), 348, 380 (sh.), 406
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certain contacts are obviously produced. This can be compli-
cated if different solvomorphs of the same molecule can be
obtained too. Furthermore, since the enol to cis-keto transfor-
mation is exclusively an intramolecular process, where the
Hirshfeld surface analysis is completely blind, it might mean
that different forms (enol and cis-keto) could generate the
same Hirshfeld surface keeping the overall geometry of a
molecule.
pressure mercury arc lamp (LSN261). Elemental analyses
were performed on a CHNS HEKAtech EuroEA 3000 analyzer.
Hirshfeld surface analysis
The Hirshfeld molecular surfaces17 and their associated 2D
fingerprint plots18 were generated using the CrystalExplorer
3.1 software19 on the basis of crystal structures. The dnorm
(normalized contact distance) surface and the breakdown of
the 2D fingerprint plots were used for decoding and quantify-
ing the intermolecular interactions in the crystal lattice. The
dnorm is a symmetric function of distances to the surface
from the nuclei inside (di) and outside (de) the Hirshfeld sur-
face, relative to their respective van der Waals radii. 2D fin-
gerprint plots were generated using di and de in the trans-
lated 0.4–3.0 Å range and including reciprocal contacts as a
pair of coordinates in 2D histograms. A colour gradient in
the fingerprint plots ranging from blue to red is used to visu-
alize the proportional contribution of contact pairs in the
global surface.
Conclusions
In summary, a series of eight closely related N-salicylidene-4-
amino-1,2,4-triazole compounds 1–8 has successfully been
prepared by the condensation reaction of the corresponding
aldehyde with 4-amino-4H-1,2,4-triazole. All the obtained com-
pounds were studied by means of 1H NMR spectroscopy in
solution, revealing the presence of a single structure at least
in DMSO-d6.
According to single-crystal X-ray diffraction, it was
established that the crystal structures of 5–8 each are stabi-
lized by a linear intramolecular hydrogen bond of the O–
H⋯N type, formed between the o-OH hydrogen atom of the
phenolic ring and the imine nitrogen atom. The same o-OH
function in the crystal structures of 1–4 was found to be in-
volved in intermolecular hydrogen bonds with one of the
triazole nitrogen atoms of the adjacent molecule. The overall
geometry of each molecule in the structures of 1–8 was found
to be almost planar or slightly deviated from planarity.
Hirshfeld surface analysis showed that the structures of
all compounds are mainly characterized by H⋯H, H⋯C,
H⋯N and H⋯O contacts as well as some contribution from
C⋯C and C⋯N contacts is clearly observed.
Enrichment ratio
The enrichment ratio (E)20 of a pair of elements (X,Y) is the
ratio between the proportion of actual contacts in the crystal
and the theoretical proportion of random contacts. E is larger
than unity for pairs of elements which have a high propensity
to form contacts in crystals, while pairs which tend to avoid
contacts with each other yield an E value lower than unity. E
values are calculated from the percentage of contacts, which,
in turn, are given by the CrystalExplorer 3.1 software,19 be-
tween one type or two types of chemical elements in a crystal
packing.
Diffuse reflectance spectroscopy reveals the exclusive pres-
ence of the enol form in the solid state at room temperature
for 1–6 and 8, while a mixture of dominant enol and cis-keto
forms was found for 7. All the studied compounds 1–8 were
observed to be non photochromic, while 7 was found to be
thermochromic from coloured to colourless upon cooling.
This work, being an attempt to study noncovalent interac-
tions in a series of structures of N-salicylidene aniline deriva-
tives by means of Hirshfeld surface analysis, might stimulate
further crystal engineering of the described compounds.
Synthesis of 1–8
All compounds were synthesized according to the previously
described procedure.7a
A
solution of salicylaldehyde,
5-nitrosalicylaldehyde, β-resorcylaldehyde, o-vanilin, 5-chloro-
salicylaldehyde, 5-bromosalicylaldehyde, gentisaldehyde or
2-hydroxy-1-naphthaldehyde (10 mmol; 1.221, 1.671, 1.381,
1.522, 1.566, 2.010, 1.381 and 1.722 g, respectively) dissolved
in ethanol (20 mL) was added to a solution of 4-amino-4H-
1,2,4-triazole (10 mmol, 0.841 g) in the same solvent (20 mL).
The mixture was stirred for 30 min and, afterwards, heated at
reflux for 1 h. The resulting solution was allowed to cool to
room temperature to give crystals of 1–8.
Experimental
Physical measurements
1. 1H NMR: δ = 6.94 (t. d, JH,H = 7.3 Hz, JH,H = 0.9 Hz,
1H, m-H, C6H4), 6.99 (d. d, JH,H = 8.3 Hz, JH,H = 0.9 Hz, 1H,
3
4
3
4
1H NMR spectra in DMSO-d6 were obtained on a Bruker AC
300 MHz spectrometer at 25 °C. Diffuse reflectance spectra
were obtained with a Varian Cary 5E spectrometer using poly-
tetrafluoroethylene (PTFE) as a reference. Spectra were mea-
sured on pure solids to avoid matrix effects. Eventual distor-
tions in the Kubelka–Munk spectra that could result from the
study of pure compounds have not been considered because
no comparison with absorption spectra was necessary. Light
irradiations were carried out with a LOT-ORIEL 200 W high-
m-H, C6H4), 7.41 (t. d, JH,H = 7.3 Hz, JH,H = 1.7 Hz, 1H, p-H,
3
4
3
4
C6H4), 7.78 (d. d, JH,H = 7.8 Hz, JH,H = 1.7 Hz, 1H, o-H,
C6H4), 9.15 (s, 1H, arylCHN), 9.17 (s, 2H, triazole), 10.49 (br.
s, 1H, OH) ppm. Calc. for C9H8N4O (188.19): C 57.44, H 4.28;
N 29.77. Found: C 57.49, H 4.31, N 29.81.
2. 1H NMR: δ = 7.05 (d, JH,H = 8.8 Hz, 1H, m-H, C6H3),
3
3
4
7.49 (d. d, JH,H = 8.8 Hz, JH,H = 2.6 Hz, 1H, p-H, C6H3), 7.82
4
(d, JH,H = 2.6 Hz, 1H, o-H, C6H3), 9.06 (s, 1H, arylCHN), 9.22
(s, 2H, triazole), 10.84 (br. s, 1H, OH) ppm. Calc. for
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C9H7N5O3 (233.19): C 46.36, H 3.03; N 30.03. Found: C 46.41,
H 2.99, N 30.09.
completed against an HKLF5 formatted reflection file. For
all, structure data were recorded up to 0.83 Å. Data cut-offs
were, however, made during integration/refinement based on
I/σ, and Rint values for the outer resolution shells are 1, 1.04
and 0.95 Å for 3, 5 and 7, respectively. Figures were generated
using the program Mercury.29
3. 1H NMR: δ = 6.36–6.43 (m, 2H, m-H, C6H3), 7.60 (d,
3JH,H = 6.6 Hz, 1H, o-H, C6H3), 8.97 (s, 1H, arylCHN), 9.07 (s,
2H, triazole), 10.36 (br. s, 2H, OH) ppm. Calc. for C9H8N4O2
(204.19): C 52.94, H 3.95; N 27.44. Found: C 52.98, H 3.91, N
27.48.
Crystal data for 17a. C9H8N4O, Mr = 188.19 g mol−1, mono-
clinic, space group P21/n, a = 5.101(2), b = 28.807(9), c =
12.029(5) Å, β = 93.20(2)°, V = 1764.8(11) Å3, T = 120(2) K, Z =
8, ρ = 1.417 g cm−3, μ(Mo-Kα) = 0.099 mm−1, reflections:
25 383 collected, 3490 unique, Rint = 0.047, R1(all) = 0.0486,
wR2(all) = 0.1143.
1
3
4. H NMR: δ = 3.85 (s, 3H, MeO), 6.90 (t, JH,H = 8.1 Hz,
3
4
1H, m-H, C6H3), 7.15 (d. d, JH,H = 8.1 Hz, JH,H = 1.5 Hz, 1H,
3
4
p-H, C6H3), 7.38 (d. d, JH,H = 8.1 Hz, JH,H = 1.5 Hz, 1H, o-H,
C6H3), 9.17 (s, 3H, arylCHN + triazole), 9.87 (br. s, 1H, OH)
ppm. Calc. for C10H10N4O2 (218.22): C 55.04, H 4.62; N 25.68.
Found: C 55.10, H 4.67, N 25.62.
Crystal data for 2. C9H7N5O3, Mr = 233.20 g mol−1, mono-
clinic, space group P21/n, a = 5.3288(4), b = 18.3034(13), c =
10.3561(8) Å, β = 94.819(7)°, V = 1006.51(13) Å3, T = 150(2) K,
Z = 4, ρ = 1.539 g cm−3, μ(Mo-Kα) = 0.121 mm−1, reflections:
6795 collected, 1826 unique, Rint = 0.146, R1(all) = 0.1234,
wR2(all) = 0.1670.
5. 1H NMR: δ = 7.01 (d, JH,H = 8.8 Hz, 1H, m-H, C6H3),
3
3
4
7.43 (d. d, JH,H = 8.8 Hz, JH,H = 2.6 Hz, 1H, p-H, C6H3), 7.75
(d, JH,H = 2.6 Hz, 1H, o-H, C6H3), 9.09 (s, 1H, arylCHN), 9.18
4
(s, 2H, triazole), 10.81 (br. s, 1H, OH) ppm. Calc. for
C9H7ClN4O (222.63): C 48.55, H 3.17; N 25.17. Found: C
48.62, H 3.12, N 25.14.
Crystal data for 3. C9H8N4O2, Mr = 204.19 g mol−1, mono-
clinic, space group P21/n, a = 5.8474(12), b = 21.040(4), c =
7.5567(18) Å, β = 107.25(2)°, V = 887.9(3) Å3, T = 298(2) K, Z =
4, ρ = 1.528 g cm−3, μ(Mo-Kα) = 0.113 mm−1, reflections: 1626
collected, 1626 unique, Rint = 0.000, R1(all) = 0.1607, wR2(all)
= 0.1998.
3
6. 1H NMR: δ = 6.96 (d, JH,H = 8.8 Hz, 1H, m-H, C6H3),
3
4
7.54 (d. d, JH,H = 8.8 Hz, JH,H = 2.6 Hz, 1H, p-H, C6H3), 7.89
4
(d, JH,H = 2.6 Hz, 1H, o-H, C6H3), 9.09 (s, 1H, arylCHN), 9.19
(s, 2H, triazole), 10.81 (s, 1H, OH) ppm. Calc. for C9H7BrN4O
(267.08): C 40.47, H 2.64; N 20.98. Found: C 40.51, H 2.68, N
21.03.
Crystal data for 4. C10H10N4O2, Mr = 218.22 g mol−1, tetrag-
onal, space group P42/n, a = 19.9241(13), b = 19.9241(13), c =
5.1153(5) Å, V = 2030.6(3) Å3, T = 298(2) K, Z = 8, ρ = 1.428 g
cm−3, μ(Mo-Kα) = 0.104 mm−1, reflections: 11 261 collected,
1798 unique, Rint = 0.106, R1(all) = 0.1008, wR2(all) = 0.1221.
Crystal data for 5. C9H7ClN4O, Mr = 222.64 g mol−1, mono-
clinic, space group Pc, a = 3.7737(14), b = 14.259(5), c =
8.993(3) Å, β = 101.23(4)°, V = 474.6(3) Å3, T = 150(2) K, Z = 2,
ρ = 1.558 g cm−3, μ(Mo-Kα) = 0.378 mm−1, reflections: 849 col-
lected, 849 unique, Rint = 0.000, R1(all) = 0.0831, wR2(all) = 0.2008.
Crystal data for 6. C9H7BrN4O, Mr = 267.10 g mol−1, mono-
clinic, space group P21/c, a = 13.4933(16), b = 10.4263(13), c =
7.2548(13) Å, β = 93.731(13)°, V = 1018.5(3) Å3, T = 180(2) K, Z
= 4, ρ = 1.742 g cm−3, μ(Mo-Kα) = 4.013 mm−1, reflections:
2757 collected, 2757 unique, Rint = 0.000, R1(all) = 0.0704,
wR2(all) = 0.1573.
3
7. 1H NMR: δ = 6.82 (d, JH,H = 8.8 Hz, 1H, m-H, C6H3),
3
4
6.87 (d. d, JH,H = 8.8 Hz, JH,H = 2.6 Hz, 1H, p-H, C6H3), 7.18
4
(d, JH,H = 2.6 Hz, 1H, o-H, C6H3), 9.06 (s, 1H, arylCHN), 9.13
(s, 1H, m-OH), 9.15 (s, 2H, triazole), 9.77 (s, 1H, o-OH) ppm.
Calc. for C9H8N4O2 (204.19): C 52.94, H 3.95; N 27.44. Found:
C 52.87, H 3.99, N 27.50.
3
8. 1H NMR: δ = 7.27 (d, JH,H = 9.2 Hz, 1H, 3-H, C10H6),
3
3
7.43 (t, JH,H = 7.5 Hz, 1H, 6-H, C10H6), 7.61 (t, JH,H = 7.5 Hz,
3
1H, 7-H, C10H6), 7.90 (d, JH,H = 7.7 Hz, 1H, 5-H, C10H6), 8.04
3
3
(d, JH,H = 8.8 Hz, 1H, 4-H, C10H6), 8.84 (d, JH,H = 8.4 Hz, 1H,
8-H, C10H6), 9.29 (s, 2H, triazole), 9.65 (s, 1H, arylCHN), 11.36
(br. s, 1H, OH) ppm. Calc. for C13H10N4O (238.25): C 65.54, H
4.23; N 23.52. Found: C 65.62, H 4.29, N 23.46.
Crystal data for 7. C9H8N4O2, Mr = 204.19 g mol−1, trigonal,
space group P31, a = 11.7895(5), b = 11.7895(5), c = 17.6053(8)
Å, γ = 120°, V = 2119.2(2) Å3, T = 298(2) K, Z = 9, ρ = 1.440 g
cm−3, μ(Mo-Kα) = 0.107 mm−1, reflections: 3464 collected,
3464 unique, Rint = 0.000, R1(all) = 0.0669, wR2(all) = 0.1121.
Crystal data for 8. C13H10N4O, Mr = 238.25 g mol−1, mono-
clinic, space group P21, a = 8.7625(6), b = 5.8854(4), c =
11.0081(12) Å, β = 91.833(7)°, V = 567.41(8) Å3, T = 295(2) K, Z
= 2, ρ = 1.395 g cm−3, μ(Mo-Kα) = 0.094 mm−1, reflections:
4774 collected, 2045 unique, Rint = 0.045, R1(all) = 0.0618,
wR2(all) = 0.1587.
Single-crystal X-ray diffraction study
X-ray data collection from 2–8 was performed on a Mar345
image plate detector using Mo-Kα radiation (Rigaku UltraX
18S generator, Xenocs Fox3D mirror). The data were inte-
grated using the CrysAlisPro software.25 The implemented
empirical absorption correction was applied. All structures
were solved by direct methods using the SHELXS97 pro-
gram26 and refined by full-matrix least squares on |F2| using
SHELXL2014/7.27 Non-hydrogen atoms were anisotropically
refined and the hydrogen atoms were placed on calculated
positions in riding mode with temperature factors fixed at
1.2 times Ueq of the parent atoms and 1.5 times Ueq for the
methyl hydrogens. The crystals of 3 and 6 were identified as
non-merohedral twins and integrated as such. For 5 the
twinrotmat algorithm in Platon28 was applied to separate
three twin domains. Refinement of these three structures was
Acknowledgements
This work was funded by the Fonds National de la Recherche
Scientifique (FNRS) (PDR T.0102.15). We also thank the F.R.
This journal is © The Royal Society of Chemistry 2016
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S.-FNRS (Belgium) for a post-doctoral position allocated to D.
A. S. We thank the COST action MP1202.
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