Ex situ and in situ monitoring of the syntheses of thermochromic Schiff bases

Article abstract of DOI:10.1039/c3ce42583e

Schiff bases derived from o-hydroxy aldehydes were obtained by mechanochemical synthetic methods with a liquid or a paste present as an intermediate phase. The effectiveness of this procedure was monitored by means of ex situ PXRD and DSC, respectively. In the case of four Schiff bases, in situ PXRD studies were also conducted in order to monitor the synthesis of the product formed by merely putting the amine and the aldehyde in close contact. One of the o-hydroxy Schiff bases appears in two polymorphic forms. The monoclinic polymorph has been reported previously in the literature (herein obtained by the solution based method using methanol) while the synthesis and characterisation of the triclinic form is now reported (obtained by neat grinding or using acetonitrile during recrystallization). The reversible thermochromic properties in the solid state were also investigated in the temperature range from 298 to 110 K. The structural reasons of such behaviour were thoroughly reasoned. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3ce42583e

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Ex situ and in situ monitoring of the syntheses of
thermochromic Schiff bases†
Cite this: CrystEngComm, 2014, 16,
4162
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M. Zbačnik and B. Kaitner
Schiff bases derived from o-hydroxy aldehydes were obtained by mechanochemical synthetic methods
with a liquid or a paste present as an intermediate phase. The effectiveness of this procedure was moni-
tored by means of ex situ PXRD and DSC, respectively. In the case of four Schiff bases, in situ PXRD stud-
ies were also conducted in order to monitor the synthesis of the product formed by merely putting the
amine and the aldehyde in close contact. One of the o-hydroxy Schiff bases appears in two polymorphic
forms. The monoclinic polymorph has been reported previously in the literature (herein obtained by the
solution based method using methanol) while the synthesis and characterisation of the triclinic form is
now reported (obtained by neat grinding or using acetonitrile during recrystallization). The reversible
thermochromic properties in the solid state were also investigated in the temperature range from 298 to
110 K. The structural reasons of such behaviour were thoroughly reasoned.
Received 19th December 2013,
Accepted 17th February 2014
DOI: 10.1039/c3ce42583e
www.rsc.org/crystengcomm
ex situ PXRD study recently revealed that the relative humid-
ity (RH) plays an important role in the formation of Schiff
Introduction
Schiff bases or imines can be effortlessly obtained by conden-
sation of aldehydes (or ketones) and primary amines1 and
thereafter can be used as ligands in the coordination chemistry
of transition metals.2 Besides, imines (and their coordination
compounds) are widely recognised compounds that possess
biological and pharmacological properties3 and, as was shown
recently, they can be used as anionic receptors.4 However, the
research of successful methods of synthesis, such as mechano-
chemical, provides new insights into faster and ecologically
and economically more acceptable ways to prepare new but
also already known compounds.5 Various Schiff bases have
been obtained in such a manner by neat (NG), liquid-assisted
(LAG) and seeding-assisted (SEAG) grinding.2,6 There are three
general mechanisms of the mechanochemical synthesis of
organic compounds: (i) molecular transport across surfaces,
through vapour phase or through the bulk of a crystal,6a
(ii) formation of liquid eutectic intermediate phases6c,7 and
(iii) reaction via an amorphous intermediate phase.8 An
bases in the solid state. One could assume that with lower
RH the conversion of the aldehyde and amine into an imine
and water (Scheme 1) will be favoured. However, a study by
Cinčić et al. showed that this is not so and that moisture
actually acts as a catalyst in this solid state reaction.9
Perhaps one of the most beautiful aspects of chemistry is
the colour change of a reaction mixture upon aging. Even
more unusual and at the same time intriguing are the
changes in the colour of synthesized substances by altering
the physical parameters of the environment: temperature,
pH, solvent polarity, ionic strength, etc.10 Among the organic
compounds that exhibit reversible photo- and/or thermo-
chromic properties in the solid state, Schiff bases are particu-
larly important.11–14 Generally speaking, there are roughly
three (mutually dependent) reasons for the thermochromic
behaviour of imines in the solid state: (i) proton transfer via
intramolecular O⋯N hydrogen bonds and consequently the
change of the tautomeric form,11 (ii) at temperatures below
room temperature, the colour change of thermochromic
crystals is dominated by the contribution of fluorescence and
not only by the contribution of light absorption, which is due
to the shift of the tautomeric equilibrium13 and (iii) the
Laboratory of General and Inorganic Chemistry, Department of Chemistry,
Faculty of Science, University of Zagreb, Horvatovac 102a, HR-10002 Zagreb,
Croatia. E-mail: mzbacnik@chem.pmf.hr; Fax: +385 1 4606 341; Tel: +385 1 4606 379
† Electronic supplementary information (ESI) available: Crystallographic data
for the structural analysis have been deposited with the Cambridge
Crystallographic Data Centre, CCDC no. 973331 and 973332 for compound 2 at
LT and RT, CCDC no. 973333 and 973334 for compound 3a at LT and RT,
CCDC no. 973335 and 973336 for compound 5 at LT and RT and CCDC no.
973337 and 973338 for compound 6 at LT and RT, respectively. Photographs of
the grinding syntheses in the agate mortar, FT-IR spectra, 1H and 13C NMR
spectra, DSC and TG thermograms and PXRD patterns for all relevant materials
are listed in the ESI. See DOI: 10.1039/c3ce42583e
Scheme 1 General procedure of aromatic Schiff base synthesis.
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stereochemistry of the molecule, i.e. its planarity or amount
of deviation from planarity in correlation with the crystal
packing.11,12 For items (i) to (iii) it is also crucial that the
imine is derived from an o-hydroxy aldehyde so that an intra-
molecular O⋯N hydrogen bond is present.11–14
According to the literature, for the imine to be
thermochromic in the solid state, the dihedral angle closed
by the best planes in which the aromatic systems of the alde-
hyde and amine moiety lie should be Φ < 25°. The molecules
are then closely packed in their crystal lattice due to π⋯π
and/or C–H⋯π interactions.14a,b However, the first examples
of controversy regarding the correlation of the dihedral angle
and the chromic phenomena were two non-planar thermo-
chromic Schiff bases derived from benzylamine in 1987.12a
In addition, it was considered up until then that thermo-
and photochromism are mutually exclusive.11 The example of
N-4-methoxy-salicyilidenebenzylamine was the first one in
this class of compounds to show both properties and to put
the planarity/nonplanarity hypothesis into revision.12a Further-
more, more recent studies on Schiff bases derived from amino-
pyridines14a and aminomethylpyridines14b confirm that the
planarity of the molecule is an independent structure para-
meter that should not be taken into consideration when
predicting the chromic properties of o-hydroxy Schiff bases.
This all indicates that studies of the thermo- and photochro-
mism of Schiff bases in the solid state should be continued.
Herein, we report the study of the mechanochemical
methods of synthesis of Schiff bases and, to the best of our
knowledge, the first in situ powder X-ray diffraction (PXRD)
monitoring of their formation. The thermochromic properties
of the prepared compounds were also thoroughly investigated.
Scheme 2 Molecular schemes of the syntheses of compounds 1 to 7.
Results and discussion
The syntheses and in situ and ex situ monitoring of their
advancement by PXRD
Compounds 1 to 7 (the IUPAC names are listed in Table S1†)
were obtained by condensation of o-hydroxy aldehyde and
amine (Scheme 2) by mechanochemical methods except for
3b15 which was obtained by the solvochemical method. Due
to different colouration, and in some cases aggregation states
of the reactants and the product, the advancement of the
synthesis can be readily monitored by the naked eye or a
camera, as can be seen from Fig. 1.
The required grinding time in the agate mortar was deter-
mined empirically when the colour of the reaction mixture
stopped changing. The results of the PXRD experiments
imply that compounds 1, 3a, 4 and 6 were successfully
synthesized by neat grinding in the agate mortar (NGam)
(Fig. S15 to S21†). The traces of the reactants are present
in the NGam powder product of the synthesis of 2 and 5.
Compound 2 was therefore synthesized by means of neat
grinding in a ball mill (NGbm).
Fig. 1 NG syntheses of (a) 5, (b) 6 and (c) LAG synthesis of 7 in a mortar.
The reactants used in the syntheses of 1–7 differ in the
physical state. Nevertheless, all products were obtained as
solid powders. In the synthesis of 1 and 7, one of the reac-
tants is a solid and the other is a liquid, and in obtaining 6,
both reactants were liquids. In these three cases the role of
grinding is to mix the reactants so that the passive product
surface cannot slow or prevent the advancement of the
reaction. In the case of 7 a catalytic amount of ethanol was
added in order to accelerate the solidification of the reaction
mixture after seven minutes of grinding. In all other synthe-
ses both reactants were solids. At the same time, in all seven
cases a liquid paste was present as an intermediate phase
(Fig. S1 to S7†).
Neither NGam nor NGbm were successful enough to
obtain pure 5, so liquid-assisted grinding in the agate mortar
(LAGam) was used (Fig. 2).
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(Fig. 1 (a)) while a liquid is formed after about 1 minute in
the preparation of 3 (Fig. 3 (b)). However, there are crucial
differences between the results of grinding and “close
contact” syntheses.
The first diffraction maximum of 2 at approx. 25° 2θ
appears 9 minutes after the reactants were gently mixed with
a spatula and put into the sample holder (Fig. 3 (d)). The dif-
fraction maximum of o-van at approx. 26° 2θ disappears after
13 minutes. Nevertheless, the maxima of 2an are present in
the pattern even after one day and can be attributed to the
white needle-like crystals that can be seen on the surface of
the red product in the sample holder. The intensity of the
maxima of the reactants o-OHsal and 1an at 8.5°, 8.8° and
8.9° 2θ deteriorate severely in first 10 minutes after they were
put into close contact but remain present even after 3 days
and can be attributed to the pale purple and light red
domains that can be seen on the surface of the dark red
product in the sample holder. The first diffraction maximum
of 4 appears after about 30 minutes. It is not easy to provide
details for the contact reaction of p-OHsal and 1an since
there is only one diffraction maximum (at ca. 12° 2θ) that
does not overlap with the maxima of the reactants (Fig. S23†).
The performed in situ experiments for compound 5 indicate
that the maximum at 12.2° 2θ is present even in the first dif-
fraction pattern taken in the first minute and increases
slightly with time (Fig. S28†). The overall crystallinities dur-
ing the formation of 2, 4 and 5 are more-or-less preserved
implying a difference in the mechanism of the formation by
grinding and by only putting the reactants in close contact.
The appearance of the paste-like intermediate phase in the
grinding experiments indicates a mechanism of the formation
of a liquid (eutectic) interphase while the “close contact”
reaction more probably goes via diffusion of the molecules of
the reactants through the reaction mixture.6a–c,7
Fig. 2 The PXRD patterns of the reactants (p-OHsal and 1an) and the
powder products obtained by means of neat grinding in the agate
mortar (NGam) and in the ball mill (NGbm), respectively, and by liquid-
assisted grinding in the agate mortar (LAGam) compared to the
patterns of the recrystallized material and the calculated pattern.
Since the reactants have low melting points (or are liquids
at room temperature) the possible mechanism of the forma-
tion of these was proposed earlier.7 In this case, grinding acts
to create a high interfacial area between the starting solid
phases at which the liquid phase can form.
The shear induced by the grinding and the contact
between the liquid or paste-like intermediate phase and the
residual solid surfaces then induces the nucleation and
growth of the product from the liquid phase.7c The inter-
mediate phase is metastable and it solidifies by further usage
of mechanical force,14c by changing the temperature,7a,b,e
or by adding a catalytic amount of liquid as in the case of
compound 7 or a very small crystal of the desired product or
polymorph that acts as a nucleus of the crystallization
(solidification).6a,16
In the case of the contact reaction of o-van and 1an the
amorphization of the reaction mixture starts almost immedi-
ately (Fig. 3 (e)) and the reaction mixture is completely amor-
phous after 47 minutes. This is in good agreement with the
formation of the liquid in the grinding experiment and with
In situ PXRD monitoring was performed for 2, 3, 4 and 5
because the reactants for their synthesis are solids. As can be
seen from Fig. 3, a paste is formed after about 2 minutes in
the NG preparation of 2 (Fig. 3 (a)), 4 (Fig. 3 (c)) and 5
Fig. 3 The changes during the synthesis of (a) 2, (b) 3a and (c) 4 by NG. The PXRD patterns observed by in situ monitoring of the formation of
(d) 2, (e) 3b and (f) 4. [Time headed (d) and (f) from and (e) towards the reader for better visibility.]
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the proposed mechanism (via an intermediate liquid or
paste-like intermediate).7 The reaction mixture solidifies in
2 days but the pattern changes slightly during nine days.
Diffraction maxima in Fig. 3 (e) could not be definitely attrib-
uted to either triclinic 3a or monoclinic polymorph 3b due to
the fact that the PXRD pattern in 2θ range 5° to 45° was
recorded and the patterns were compared to the patterns of
the recrystallized material from acn and MeOH (Fig. S26†).
The only conclusion that can be made is that 3b is the more
probable product in this simple contact reaction. The fact
that goes in favour of this statement is the diffraction
maximum at 2θ ≈ 5.5°. Thus, 3a can be obtained by neat
grinding and/or (re)crystallization from acetonitrile while 3b
can be obtained from methanol or probably by a simple
contact reaction of the reactants.
Fig. 4 δF maps calculated in the domain of N1–C7–C1–C2–O1 chelate
ring of (a) 2 and (b) 6 at RT (left column) and LT (right column).
The broadening of the diffraction peaks can be noticed in
the in situ PXRD patterns and attributed to the formation of
particles smaller in size than the size of the particles of reac-
tants used and/or microstrains caused by the migration of
the molecules in the crystal.17
be used for tautomer selection. The data obtained by means
of NMR spectroscopy indicate that all seven compounds are
enol–imines in DMSO solution regardless of the tautomeric
form in the solid state (Fig. S43 to S46 and Tables S13
and S14†).
The formation of supramolecular motifs in all compounds
is dominated by C–H⋯O, C–H⋯π and π⋯π interactions.
However, an O–H⋯O type H-bond is also present in 4 and 5.
The molecules of 5 and 7 form 1D-helices along the y and z
crystallographic axis, respectively. The molecules of 2 are
interconnected by means of three C–H⋯O interactions
forming a 1D-helix along the y crystallographic axis. These
1D-helices are interconnected by an additional C–H⋯O
H-bond into a 2D supramolecular network. The O–H⋯O and
C–H⋯O interactions put the molecules of 4 in close contact
in such a manner that pairs of dimers are formed. The
dimers form 1D-chains along the x crystallographic axis by
C–H⋯O and π⋯π interactions. The molecules of 3a form a
3D supramolecular network through weak C–H⋯O type
hydrogen bonds. There is no special supramolecular motif in
1 and 6. Therefore, the molecules simply repeat in 3D space
by means of the space group symmetry elements. The packing
diagrams of the compounds reported within this publication
for the first time are presented in Fig. S15 to S18.†
The appealing thermal behaviour of 2, 3a, 3b, 4, 6 and 7
can be observed from the photographic pictures of the crystals
shown in Fig. S47 to S54.† In addition, compound 6 shows
negative thermochromism, a phenomenon of deepening of
the colour upon cooling which is rarer than the positive one
(Fig. 5). Compounds 1 and 5 do not change their colour upon
cooling or the colour change cannot be observed by the
human eye in the temperature range used.
Crystal and molecular structure v.s. thermochromism
in the solid state
The reversible thermochromic properties of the prepared
imines were studied in the temperature range 298 to 110 K.
The tautomer present in the solid state was determined
according to: (i) either C2–O1 and C7–N1 bond length crite-
rion18 or (ii) the position of the hydrogen atom (H1) located
from the electron density map (Fig. S8 to S14 b†)) or both (i)
and (ii) simultaneously. Bond length values were calculated
using the SCXRD structural data from this work and data
deposited in the CSD.19 The values of O1–C2 and N1–C7
bond lengths and of the dihedral angles between aromatic
rings for 1–7 are contained in Table S9.† The geometric
parameters of the intermolecular contacts are listed in
Table S10.†
Compound 1,20 3a (all four crystallographically indepen-
dent molecules), 5 and 7 are enol–imines at both tempera-
tures used. The diffused electron density in the region of the
chelate ring intra-molecular O⋯N hydrogen bond of 621
implies the presence of a keto–enol tautomeric equilibrium
at both temperatures (Fig. 4 (b)). Nevertheless, the existence
of the enol–imine tautomer only in the solid state of 6 is con-
firmed by the limiting values of C2–O1 and C7–N1 bond
lengths. The change from a pure tautomer to the keto–enol
tautomeric equilibrium and vice versa is observed in 2 and
4.22 Compound 2 is an intrinsic enol–imine at 298 K. At
110 K both forms are present in the tautomeric equilibrium
with a 0.74: 0.26 ratio in favour of enol–imine in 2 (Fig. 4 (a)).
The keto–enol tautomeric equilibrium in 4 with a prevalence of
the keto–amine form (0.67 : 0.33) at 298 K disappears entirely
at 110 K in favour of the keto–amine tautomer. The results of
the FT-IR spectroscopic experiments (Fig. S39 to S42 and Table
S12†) point to probable co-existence of both tautomers in the
solid state in all cases. These results could not unambiguously
The stereochemistry of aromatic Schiff bases (i.e. their
planarity defined through the value of angle between best
planes of aldehyde and amine ring moieties, the dihedral
angle Φ) and their thermochromic behaviour are in great
discrepancy with the hypothesis that Schiff bases with non
planar molecules cannot show such a phenomenon.11 Thus,
based on the increasing evidence on the molecular structure
of Schiff bases through the years, the dihedral angle has been
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originates from an o-hydroxy aldehyde and two phenol groups
that originate from the amine. Therefore, the negative
thermochromic properties of 6 and positive thermochromism
of 7 are evidence that Schiff bases with fairly non planar
molecules (Φ > 60°) can be thermochromic in the solid
state. The degree of non-planarity of the molecules of 1–7
can be easily perceived in Fig. 6 showing molecular overlap.
Ex situ DSC monitoring of the efficacy of mechanosynthesis
Fig. 5 Photographs at (a) RT and (b) LT showing negative solid state
thermochromism of 6.
The results of ex situ DSC and PXRD experiments performed
on the powder product of the conducted method of synthesis
and on the recrystallized material are mostly in good agree-
ment. The differences are in the case of 4 and 7. In the DSC
thermogram of the NGam powder product in the case of the
synthesis of 4, one can see a weak peak with an onset at
111 °C that originates from the melting of the traces of
unreacted o-OHsal. Furthermore, such a peak with an onset
at 78 °C implies traces of ethanol used in the synthesis in
the case of 7. These traces certainly cannot be found in the
ex situ PXRD diffraction patterns. The DSC and TGA thermo-
grams are presented in Fig. S28 to S38.†
shown to be an independent factor that should not be taken
into consideration when discussing the chromic behaviour of
Schiff bases. It has been reported previously that the value of
the dihedral angle should be Φ < 25° for a Schiff base to
be thermochromic.14a,b The mentioned angle in 1 is around
8° (Table S11†) but 1 does not change its colour with temper-
ature. The value of the angle in 2 is 14° and it is thermo-
chromic. The average value of the dihedral angles (four
crystallographically independent molecules in the asymmetric
unit) in 3a is around 25° (going from ca. 22° to 33° at RT) and
the compound is thermochromic in the solid state. The angle
is around 10° in the monoclinic polymorph 3b15 and it changes
its colour with temperature although not so dramatically as 3a.
The aromatic subunits of the molecules of 4 are mutually
inclined at an angle of 5° and thermochromism is present.
Compound 5 does not show thermochromic properties and the
angle is 55°, way beyond 25°. Hitherto, the thermochromic
properties of Schiff bases derived from adpm have not been
reported. These molecules cannot be planar due to the exis-
tence of a CH spacer group between the aromatic ring that
Experimental
Synthesis
The mechanochemical syntheses were performed by grinding
equimolar quantities of aldehyde and amine (1 mmol :
1 mmol) in an agate mortar (Fig. S1 to S7†) and the efficacy
was monitored ex situ by PXRD as described in the
literature.14c The IUPAC names of 1–7 are listed in Table S1.†
The syntheses of 2 and 5 were repeated in a Retsch MM200
grinder mill operating at 25 Hz frequency for 30 minutes.
Fig. 6 Overlapping pictures of (a) 1 and 2, (b) 3a, 4 and 5, and (c) 6 and 7. In all drawings the plane of aromatic amine moiety was overlapped.
In the case of compounds 6 and 7 the planes of both phenyl rings are overlapped.
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The monoclinic polymorph15 of compound 3 was obtained by
solution (methanol) based synthesis.
Conclusions
In order to monitor the advancement of the reaction using
in situ PXRD merely by putting the reactants in close contact,
equimolar quantities of aldehyde and amine (1 mmol :
1 mmol) were gently mixed in a mortar with a spatula to
provide homogeneity of the reaction mixture and to ensure
no (or minimal) mechanic force was used. The reaction
mixture was transferred into an aluminium holder, evened and
then put into the instrument.
The single crystals for single crystal X-ray diffraction
experiments (SCXRD) were obtained by recrystallization and
evaporation from a small amount of solvent [compounds 120
and 621 – acetone (ace), 3a and 5 – acetonitrile (acn), 2 –
tetrahydrofurane (thf), 3b15 – methanol (MeOH), 422 – chloro-
form (chl) and 7 – ethanol (EtOH)].
The first in situ PXRD study of the synthesis of Schiff bases is
reported.
All reactants used have melting points below 120 °C, thus
the reaction mixtures obtained by grinding could provide
liquid intermediate phases seen in all seven cases reported
herein. This indicates that the role of grinding in these seven
cases is to remove the passive surface of the product from the
reactants, to ensure the improvement of the contact between
the unreacted reactants.7e Ex situ PXRD monitoring revealed
that four compounds were successfully obtained by neat grind-
ing in an agate mortar (1, 3a, 4 and 6). More time and/or
mechanic force is necessary to obtain 2 without traces of reac-
tants in the powder product so neat ball-milling was used. To
obtain 7 and pure 5 it is essential to add a very small amount
of solvent while grinding in order to speed up the procedure
and to improve the contact between the particles of the reac-
tants as proposed by the mechanism. The ex situ DSC experi-
ments are mostly in good agreement with the ones obtained
by ex situ PXRD.
The in situ PXRD monitoring of the “close contact” forma-
tion of 2 showed that the overall crystallinity of the reaction
mixture is preserved during the procedure pointing to the
formation of the product via the diffusion mechanism.7c,d
This implies that the mechanism of the formation of 2, 4
and 5 by grinding is not the same as in the “close contact”
reaction.7a,b,e The in situ monitoring of the synthesis of com-
pound 3 by “close contact” revealed that the amorphization
of the reaction mixture is almost immediate and the mono-
clinic polymorph 3b is the more probable product.
Compounds 2, 3a, 3b, 4, 6 and 7 show vivid reversible
thermochromic behaviour in the 289 to 110 K temperature
range while 1 and 5 do not change their colour upon cooling
or the colour change cannot be observed by the human eye
in the temperature range used. In addition, compound 6
shows negative thermochromism, a rarer phenomenon than
positive thermochromism. The conducted crystallographic
study revealed that the molecules of the prepared Schiff bases
are mostly organized in the crystal by C–H⋯O, C–H⋯π and
π⋯π interactions. Also, we have once again showed that
the degree of planarity of the molecule is an independent
parameter and should not be taken into consideration when
discussing thermochromism of Schiff bases.11–14 Nevertheless,
the keto–enol tautomerism should also be revised as a reason
for such behaviour.11,12 The change in the position of the
tautomeric equilibrium with changing temperature was proven
only in the case of 2 and 4 where the ratio of the keto–amine
tautomeric form increases upon cooling. In the case of all other
compounds the molecules are in the enol–imine form.
X-ray diffraction measurements
Crystal and molecular structures were determined at 298 and
110 K using SCXRD on an Oxford Diffraction Xcalibur Kappa
CCD X-ray diffractometer with graphite-monochromated
MoKα (λ = 0.71073 Å) radiation (for details see ESI†).23 The
data concerning the results of the crystallographic experi-
ments are listed in Tables S2 to S8.† Further details are avail-
able from the Cambridge Crystallographic Centre.24
In order to monitor the efficacy of the synthetic method
used and to qualitatively identify the products (Fig. S19 to
S25†) powder X-ray diffraction (PXRD) experiments were
performed on a PHILIPS PW 1840 X-ray diffractometer with
CuKα1 (1.54056 Å) radiation at 40 mA and 40 kV. The scattered
intensities were measured with a scintillation counter. The
angular range (2θ) was from 5 to 45° with steps of 0.02°,
and the measuring time was 0.5 s per step in the case of the
ex situ experiments.
In the case of the in situ monitoring of the contact reaction
of o-vanillin (o-van) and 2-aminonaphthalene (2an), the angu-
lar range (2θ) was selected according to the reactants diffrac-
tion maxima from 23.0 to 27.0° with steps of 0.02°, and the
measuring time was 0.5 s per step. The measurements were
repeated in 40 cycles. In the case of the in situ monitoring of
the contact reaction of o-van and 1-aminonaphthalene (1an),
the angular range (2θ ) was also selected according to the reac-
tants diffraction maxima from 11.5 to 14.5° with steps of 0.02°,
and the measuring time was 0.5 s per step. The measurements
were repeated in 70 cycles. In the case of the in situ monitoring
of the contact reaction of o-hydroxysalicylaldehyde (o-OHsal)
and 1an, the angular range (2θ) was from 6.5 to 9.5° with steps
of 0.02°, and the measuring time was 0.5 s per step. The mea-
surements were repeated in 195 cycles. In the case of the in situ
monitoring of the contact reaction of p-hydroxysalicylaldehyde
(p-OHsal) and 1an, the angular range (2θ) was from 11.0 to
14.0° with steps of 0.02°, and the measuring time was 0.5 s
per step. The measurements were repeated in 200 cycles.
The data collection and analysis were performed using the
program package Philips X'Pert.25
Acknowledgements
This research was supported by the Ministry of Science,
Education and Sport of the Republic of Croatia (grant no.
119-1193079-3069).
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4168 | CrystEngComm, 2014, 16, 4162–4168
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