Spectroscopic studies of the intramolecular hydrogen bonding in o-hydroxy Schiff bases, derived from diaminomaleonitrile, and their deprotonation reaction products

Article abstract of DOI:10.1016/j.saa.2017.08.028

The structural study of five Schiff bases derived from diaminomaleonitrile (DAMN) and 2-hydroxy carbonyl compounds was performed using ¹H, ¹³C and ¹⁵N NMR methods in solution and in the solid state as well. ATR-FTIR and X-Ray spectroscopies were used for confirmation of the results obtained by NMR method. The imine obtained from DAMN and benzaldehyde was synthesized as a model compound which lacks intramolecular hydrogen bond. Deprotonation of all synthesized compounds was done by treating with tetramethylguanidine (TMG). NMR data revealed that salicylidene Schiff bases in DMSO solution exist as OH forms without intramolecular hydrogen bonds and independent on the substituents in aromatic ring. In the case of 2-hydroxy naphthyl derivative, the OH proton is engaged into weak intramolecular hydrogen bond. Two of imines (salDAMN and 5-BrsalDAMN) exist in DMSO solution as equilibrium mixtures of two isomers (A and B). The structures of equilibrium mixture in the solid state have been studied by NMR, ATR-FTIR and X-Ray methods. The deprotonation of three studied compounds (salDAMN, 5-BrsalDAMN, and 5-CH₃salDAMN) proceeded in two different ways: deprotonation of oxygen atom (X form) or of nitrogen atom of free primary amine group of DAMN moiety (Y form). For 5-NO₂salDAMN and naphDAMN only one form (X) was observed.

Full text of DOI:10.1016/j.saa.2017.08.028

Accepted Manuscript
Spectroscopic studies of the intramolecular hydrogen bonding in
o-hydroxy Schiff bases, derived from diaminomaleonitrile, and
their deprotonation reaction products
Anna Szady-Chełmieniecka, Beata Kołodziej, Maja Morawiak,
Bohdan Kamieński, Wojciech Schilf
PII:
DOI:
Reference:
S1386-1425(17)30657-1
doi: 10.1016/j.saa.2017.08.028
SAA 15385
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
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Revised date:
Accepted date:
13 March 2017
3 August 2017
9 August 2017
Please cite this article as: Anna Szady-Chełmieniecka, Beata Kołodziej, Maja Morawiak,
Bohdan Kamieński, Wojciech Schilf , Spectroscopic studies of the intramolecular
hydrogen bonding in o-hydroxy Schiff bases, derived from diaminomaleonitrile, and
their deprotonation reaction products, Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy (2017), doi: 10.1016/j.saa.2017.08.028
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ACCEPTED MANUSCRIPT
Spectroscopic studies of the intramolecular hydrogen bonding in o-hydroxy
Schiff bases, derived from diaminomaleonitrile, and their deprotonation
reaction products
Anna Szady-Chełmienieckaa, Beata Kołodzieja*, Maja Morawiakb, Bohdan Kamieńskib,c, and
Wojciech Schilfb
a- West Pomeranian University of Technology, Szczecin; Faculty of Chemical
Technology and Engineering; Department of Inorganic and Analytical Chemistry, Al.
Piastów 42, 71-065 Szczecin, Poland
b- Institute of Organic Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-
224 Warsaw, Poland
c- Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-
224 Warsaw, Poland
*- corresponding author
Key words
diaminomaleonitrile, Schiff bases, 1H, 13C, and 15N NMR, ATR-FTIR, X-Ray, intramolecular
hydrogen bond, deprotonation.
Abstract
The structural study of five Schiff bases derived from diaminomaleonitrile (DAMN)
1
13
15
and 2-hydroxy carbonyl compounds was performed using H, C and N NMR methods in
solution and in the solid state as well. ATR-FTIR and X-Ray spectroscopies were used for
confirmation of the results obtained by NMR method. The imine obtained from DAMN and
benzaldehyde was synthesized as a model compound which lacks intramolecular hydrogen
bond. Deprotonation of all synthesized compounds was done by treating with
tetramethylguanidine (TMG). NMR data revealed that salicylidene Schiff bases in DMSO
solution exist as OH forms without intramolecular hydrogen bonds and independent on the
substituents in aromatic ring. In the case of 2-hydroxy naphthyl derivative, the OH proton is
engaged into weak intramolecular hydrogen bond. Two of imines (salDAMN and 5-
BrsalDAMN) exist in DMSO solution as equilibrium mixtures of two isomers (A and B). The
structures of equilibrium mixture in the solid state have been studied by NMR, ATR-FTIR and
X-Ray methods.
The deprotonation of three studied compounds (salDAMN, 5-BrsalDAMN, and 5-
CH3salDAMN) proceeded in two different ways: deprotonation of oxygen atom (X form) or of

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nitrogen atom of free primary amine group of DAMN moiety (Y form). For 5-NO2salDAMN
and naphDAMN only one form (X) was observed.
Introduction
Imines (known as Schiff bases), derived from aromatic o-hydroxyaldehydes and o-
hydroxyketones and primary amines, may have intramolecular hydrogen bonds between
phenolic oxygen and formal imino nitrogen atom, and therefore can form dynamic equilibrium
mixture of imine and enamine tautomeric forms (Fig. 1) [1-4].
Figure 1
If the proton is almost localized in the central position between the proton donor and the
proton acceptor, we call such bonds as low barrier hydrogen bonds [5].
The OH···N intramolecular hydrogen bonds present in Schiff bases are specific and
have important interactions, which determine chemical and physicochemical properties of
investigated compounds [6-8]. The remarkable changes caused by the proton transfer from the
hydroxyl oxygen atom to imine nitrogen atom are connected with the color changes known as
thermochromism and photochromism [9]. These features make Schiff bases as good potential
elements for constructing optical switches, molecular electronics and computing [10], and
optical memory devices [11].
Our previous investigations [5, 7-8, 12] showed that position of proton in intramolecular
hydrogen bond is determined by:
(i) type of substituent present in salicylaldehydes
- when acidity of OH group is increased (introducing of substituents, which are electron
acceptors, for example –NO2), the proton transfer from oxygen atom to nitrogen atom is
promoted,
- when acidity of OH group is decreased (introducing of electron donor substituents, for
example alkyl groups), the enol-imine form is promoted,
(ii) type of amine
- when an aliphatic amine is used, the proton is more effectively transferred to
nitrogen atom comparing with aromatic amine,
- when an aromatic amine is used and the basicity of amine is increased (introducing of
substituents, which are donors of electron, for example –NH2), the proton transfer to
nitrogen atom is more effective,

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(iii) the phase (solution or solid)
- generally, solid phase gives rise of keto-amine form.
Substituents in salicylaldehyde ring influence not only the acidity of the phenolic group
but can also strongly stabilize the polar structure of the respective compounds. This problem
was intensively discussed for N-(5-nitrosalicylidene)-ethylamine in solid and in solutions as
well as for other Schiff bases derived from aromatic ortho-hydroxyaldehydes [5].
We have decided to expand our research on diaminomaleonitrile (DAMN), organic π–
conjugated compound with electron donor (-NH2) and electron acceptor (-CN) substituents in
molecule (Fig. 2) with high electron affinity, which features a strong intramolecular charge
transfer (ICT) interaction [13-15]. Sometimes this compound is treated as a tetramer of
hydrogen cyanide [16-19].
Figure 2
The π–conjugated system, present in DAMN molecule, is responsible for changes in
reactivity of amine group [20]. In the most of aliphatic diamines, both amine groups react with
carbonyls easily [21-25]. When we use the same conditions (methanolic or ethanolic solution,
room or a little higher temperature, time of reaction – 1h) for reaction of DAMN with carbonyl,
we obtain only mono Schiff base, even in presence of excess of carbonyl [16-17].
Diaminomaleonitrile (DAMN) is a low-cost organic compound with a high nitrogen
content [26] and it is often used as versatile precursor of heterocycles [27], for example
imidazoles, triazoles, porphyrazines, pyrimidines, pyrazines, diazepines, and purines. DAMN
can also be used as a synthetic precursor of fluorescent dyes, jet-printing inks, hair dyes, and
biologically active compounds such as insecticides and anticancer agents. [18].
Schiff base ligands derived from DAMN and aromatic aldehydes have been recently
used in various fields, for example pharmacology, thermostable optical material industry, or
synthesis of conjugated linear polymers [17]. Schiff base ligands based on DAMN and
salicylaldehydes are taken into account for applications in Cu2+ and Pt2+ sensing [28].
It is well known that metal chelates of salicylaldehydes and diaminomaleonitrile
(DAMN) derivatives are the efficient materials as an electron transporter [29].
According to the literature data the nitrogen NMR spectroscopy seems to be one of the
best methods for investigations of proton position in hydrogen bonded structures [7-8, 30]. This
method is very attractive for these studies because of:
(i) the sensitivity of the nitrogen chemical shift of imine atom on proton position in H-
bond (from -50 to -60 ppm for pure imine form to about -240 ppm for the structure
with proton transfer),

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(ii) the insensitivity of the nitrogen chemical shift of formally imine atom on the
substitution effects connected with different substituents in salicylaldehyde unit [7].
The general rule, used in analysis of Schiff bases having intramolecular H-bond, is: the
most upfield shift of the signal of the nitrogen atom means the strongest proton transfer from
oxygen atom to nitrogen site. For aliphatic derivatives without H-bonds this chemical shift is
about -60 ppm, relative weak H-bond can shift this signal to about -100 ppm, and for
compounds with strong proton transfer nitrogen signal can be shifted to about -240 ppm [8].
Full protonation of an imine nitrogen atom in Schiff bases gives rise to a shielding increase of
about 180 ppm (about -60 ppm for pure imine form and about -230 ppm for proton transferred
structure) [30].
The second parameter suitable for the structure elucidation is carbon chemical shift of
formal C-OH atoms. In this case we observe opposite effect - strong proton transfer to nitrogen
atom causes the downfield shift of this signal. This effect is much weaker than that observed
for nitrogen chemical shift. For compounds without H-bonding the C-2 signal is located close
to 155 ppm. Proton transfer process can shift it to about 175 ppm. In contrast to imine nitrogen
chemical shifts, the C-2 chemical shifts are less diagnostic, since they are also affected by
substituents present in phenyl ring [8].
FTIR spectroscopy is used as a suitable method as an additional source of information
confirming molecule structure proposed on the base of results obtained from NMR studies.
Particular attention for this class of compounds is directed to the two ranges: 3500-3300 cm-1
and 1770-1550 cm-1, where characteristic bands are present.
In the present work we report the synthesis and structural studies in both DMSO
solution and in the solid state of six known mono Schiff bases obtained from DAMN and
aromatic aldehydes: benzaldehyde (benz), salicylaldehyde (sal), 5-methylsalicylaldehyde (5-
CH3sal), 5-bromosalicylaldehyde (5-Brsal), 5-nitrosalicylaldehyde (5-NO2sal), and 2-
hydroxynaphtaldehyde (naph). In previous studies [16, 17, 31-34] the structures of Schiff bases
obtained from DAMN were investigated by IR, UV-VIS and 1H, and 13C NMR methods. Since
the possibility of different types of isomers (rotamers or geometrical cis-trans isomers) should
be considered [35], we applied the full set of NMR measurements together with ATR-FTIR
and X-Ray experiments to describe structures of obtained compounds in details. Additionally,
we performed deprotonation reactions of mentioned Schiff bases using tetramethylguanidine
(TMG) as the strong base reagent. The molecular structures of Schiff bases and deprotonated
species were analyzed by spectroscopic methods.

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Experimental
Synthesis of Schiff bases
All compounds were obtained according to the literature method [17] slightly modified
for the present reaction.
To an ethanolic solution of suitable aldehyde (2,3 mmol) an ethanolic solution of
DAMN (2,3 mmol) was added.
In the case of 5-BrsalDAMN, 5-NO2salDAMN, and benzDAMN, precipitate was
formed after a while and the mixture was allowed to stand for 24 h.
In the case of naphDAMN, salDAMN, and 5-CH3salDAMN the mixture was stirred and
refluxed for 1 h, and then it was cooled to room temperature and allowed to stand for 24 h.
After 24 h the obtained precipitate was filtered out and washed by cold ethanol and
dried in the air.
The authenticity and purity of obtained compounds were examined by proton NMR
measurements.
Deprotonation of obtained imines
Deprotonation reactions were performed in NMR tubes by adding some excess of pure
TMG to DMSO solution of appropriate Schiff base.
NMR measurements
All solution state NMR experiments were performed on Varian-Agilent 600 MHz
VNMRS instrument using 5 mm inverse probehead equipped with Z-gradient coil. For signal
assignment 1D proton and carbon spectra were recorded as well as 2D GHSQC, GCOSY, and
GHMBC experiments. For data acquisition and spectra processing standard Varian-Agilent
software was used. The proton and carbon spectra were referenced with respect to internal
TMS as a standard. For nitrogen-15 external nitromethane was applied as a standard. In some
cases, due to dynamic character of investigated structures no correlation for carbon atoms of
CN groups was found, so suggested signal assignment was done only on the base of
comparison with other compounds.
The solid state NMR measurements were run on a Bruker Avance 500 MHz
spectrometer. The solid state spectra were done using 4 mm CPMAS Bruker probehead. The
typical spectral condition for natural abundance nitrogen CPMAS NMR spectra were: spectral
width 28 kHz, acquisition time 40ms, spin rate 6-12 kHz, contact time for spin-lock 5ms,
relaxation delay 10 to 120 s depending on relaxation condition for particular sample, estimated

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from carbon measurement. The typical experimental condition for carbon CPMAS were:
spectral width 31 kHz, acquisition time 20 ms, contact time 2 ms, spin rate 12 kHz. To
distinguish protonated and quaternary carbon atoms, the short contact time spectra with contact
time 40s were done. In SCT experiments only protonated carbon atom signals are displayed.
Originally, the solid state spectra were referenced to the solid glycine sample and then the
obtained chemical shifts of appropriate signals were recalculated to TMS scale for carbon, and
nitromethane for nitrogen measurements, respectively.
ATR-FTIR measurements
ATR-FTIR spectra were recorded on a Bruker Alpha Fourier Transform IR (FTIR)
spectrometer equipped with a platinum ATR single reflection diamond-sampling module
(Bruker Optics).
The infrared spectra were collected as an average of 64 scans per sample between the
wavenumber range of 4000–360 cm-1 at a resolution of 4 cm-1, controlled by Optics User
Software (OPUS) version 7.5 (Bruker Optics). Air was used as reference background spectra.
The ATR diamond surface was cleaned with acetone before each sample was scanned.
X-ray structure analysis
Yellow crystals of compounds 5-CH3salDAMNa, 5-CH3salDAMNb, and brown crystals
of compound benzDAMN were obtained from ethanol. Crystal data for these compounds were
collected at room temperature, on Bruker X8 APEXII diffractometer using Cu-Kα radiation (λ
= 1.54178 Å). Frames were integrated with the Bruker SAINT [36] software package using a
narrow-frame algorithm. The structure was solved and refined using the Bruker SHELXTL
Software Package [37]. Data were corrected for absorption effects using the face-indexed
numerical method (SADABS) [38]. Hydroxyl group hydrogens were found from the difference
electron density maps and refined with an anisotropic thermal motion model. The structure was
solved by direct methods SHELXS-2014 [39] and refined with full-matrix least-squares
calculations on F2 using SHELX-2014 [39]. All non-hydrogen atoms were refined
anisotropically. The hydrogen atom positions were geometrically idealized and allowed to ride
on their parent atoms.
Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre, 12
Union Road, 129 Cambridge CB2 1EZ, UK, and copies can be obtained on request, free of
charge, by quoting the publication citation and the deposition number CCDC 1528338 for 5-
CH3salDAMNa, CCDC 1528337 for 5-CH3salDAMNb and CCDC 1528336 for benzDAMN.

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Results and discussion
Five Schiff bases (four imines obtained from salicylaldehyde and its derivatives, and
one obtained from 2-hydroxynaphthaldehyde) and diaminomaleonitrile (DAMN) (Fig. 3 a, b)
have been investigated by NMR methods in DMSO solution. The sixth Schiff base (Fig. 3 c),
derivative of DAMN and benzaldehyde, was obtained in order to determine the NMR spectral
data for compound, which does not have possibility to form intramolecular hydrogen bond.
Figure 3
The main subject of the study is structure investigation of possible intramolecular
hydrogen bonds between formal OH group in position 2 and imine moiety. The results of NMR
measurements of investigated Schiff bases are collected in Table 1. The first observation made
on those data is that two compounds: 5-CH3salDAMN and 5-NO2salDAMN form well defined,
single-component systems, whereas the other two compounds: 5-BrsalDAMN and salDAMN
form two-component equilibrium systems. The molecular structures of obtained Schiff bases
can be determined mainly by analysis of chemical shifts of OH, C-2, C-7, =N- and NH2 groups.
The proton chemical shift of hydroxyl group is typical for phenols, but it is not shifted enough
to lowfield for intramolecular hydrogen bonds. The carbon chemical shifts for C-2 atoms are
also in the range that expected for C-OH groups. For C=O groups those signals should be
shifted to lowfield (170-180 ppm range).
Table 1
The analysis of nitrogen chemical shifts of imine groups is much more difficult. The
typical nitrogen chemical shift value for imine without hydrogen bond is about -50 – -60 ppm.
The observed δ values (-83.9 to -95.8 ppm) (Table 1) suggest that some weak hydrogen
bonding should exist. Comparison of those values with nitrogen chemical shift of benzaldehyde
derivative (δN = -83,6 ppm), which cannot form hydrogen bond (Table 2), leads to the
conclusion that in the all investigated compounds derived from salicylaldehydes no hydrogen
bond was found. In the case of 2-hydroxy naphthyl derivative the imine nitrogen chemical shift
is -104,7 ppm (Table 3). This value suggests that naphDAMN exists in the form with weak
intramolecular hydrogen bond (O-H∙∙∙N). The upfield shift of imine nitrogen signal is due to
substitution effect introduced by the rest of diaminomaleonitrile substituent.
Table 2
Table 3
The structure differences between forms A and B which are observed for 5-
BrsalDAMN and salDAMN are the next problem. Since the all NMR data for those forms are

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very similar, one can conclude that the differences are connected rather with geometry than
functionality. Theoretically there are at least two possible groups of rotamers obtained by
rotation around C1-C7 bond or around N-C1’ bond. Some of them are presented on Figure 4
(upper trace a-c). Additionally, due to two CN and one NH2, strongly electron-withdrawing
substituents in the molecules, we have to consider the presence of cis-trans isomers (lower
trace d, e).
Figure 4
Unfortunately, the observed spectral differences are so small and unspecific so we could
not assign them with certainty to appropriate structures. Considering, usually lower energy
activation for rotamers transformation comparing with cis-trans isomerization the last process
seems to be more expected, but on the base of acquired data we cannot make final
differentiation. The dynamic character of A and B systems was verified by NOESY spectra,
where exchange cross peaks between appropriate positions were found, so from the NMR time
scale they are slow exchange processes.
The next step of our investigations was deprotonation reaction using 1,1,3,3-
tetramethylguanidine (TMG) as a strong base. The deprotonation reactions were done in NMR
tube by adding slight excess of TMG and reaction products were monitored by proton spectra.
The first symptom of deprotonation was the loss of well-separated, relatively sharp OH, H2O
(from solvent) and NH2 signals. Instead of them very broad dynamic averaged signal in the
range from 2 to 6 ppm was found. It means that deprotonation reaction and proton exchange
processes take place in both proton reach sites: OH and NH2 substituents. Additionally,
residual water from the solvent and excess of TMG participate in the exchange processes,
leading to dynamically averaged one broad signal.
The 5-NO2salDAMN (Table 4), naphDAMN (Table 3), and benzDAMN (Table 2) after
deprotonation reaction form single component systems, whereas the three remaining Schiff
bases (salDAMN, 5-BrsalDAMN, and 5-CH3salDAMN (Table 4)) in such conditions form
two-component systems. The benzDAMN (Table 2) was used as a model compound, where
only one position is capable for the deprotonation.
Table 4
In contrast to pure Schiff bases, the deprotonated species X and Y observed for
salDAMN, 5-BrsalDAMN, and 5-CH3salDAMN show significantly different spectral
properties, which suggest that the structural differences between X and Y are larger than those
found for pure Schiff bases (Table 4).

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Some problem can be with the interpretation of NMR data for deprotonation of 5-
BrsalDAMN. In this case the minor product has very small, about 12 % abundance, and
additionally, proton signals are broadened, so only GHSQC correlation was obtained.
Consequently we could not record quaternary carbon atoms signals. But, based on available
data we could distinguish and interpret structures of both forms by comparing with spectral
data of remaining compounds.
For the X forms of deprotonated species a few characteristic spectral effects were
found:
1. C-2 atom signals are shifted downfield after deprotonation
2. C-5 signals are shifted upfield
3. C-7 signals are close to parent Schiff bases
4. Imine N signals are upfield shifted
5. C-1’ signals are weakly upfield or downfield shifted.
All those effects lead to the conclusion that X forms are created by removing protons
from OH group located on C-2 (Fig. 5; Tables 1 and 4).
Figure 5
The contribution of quinon (C=O) structure changes the carbon chemical shifts in positions
2 and 5, whereas positions of C-7 signals are unchanged, since in both resonance forms (C-O-
and C=O) the electron structures of C-7 atoms are very similar to the parent imine. The imine
nitrogen atom is slightly upfield shifted probably due to some possible weak hydrogen bonding
with amine group in position 2’.
The carbon and nitrogen spectral data for Y forms show different effects than in X forms:
1. C-2 carbon signals are close to the parent imines
2. C-4 signals are upfield shifted after deprotonation reaction
3. C-5 carbon signals are close to the parent imines
4. C-7 carbon signals have strong upfield effect
5. Imine nitrogen atom signals are weakly downfield shifted
6. C-1’ carbon signals have strong upfield shifts

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Since the mentioned above effects are different comparing with those observed for X form
we can conclude that in the case of form Y protons were removed from amine group (Fig. 6,
Tables 1 and 4).
Figure 6
The negative charge created originally on nitrogen atom in position 2’ can be
transferred via coupled double bonds system to aromatic ring, modifying chemical shifts of
entire molecules. Some small, but significant downfield shift of imine nitrogen signals to the
value of -88.9 ppm and -89.3 ppm for 5-CH3salDAMN+TMG and 5-BrsalDAMN+TMG
respectively, close to those found for benzDAMN+TMG (Table 2), where only NH2 to -NH
transformation is possible, supports above conclusion of molecular structure of form Y.
For naphDAMN (Table 3) the spectral effects are a little different comparing with
previously discussed. This could be due to possible delocalization of negative charge connected
with quinone (C=O) structure in whole condensed rings aromatic system. In this case the
upfield shift effect observed in para position for single ring aromatic compounds is transferred
and spread out into whole condensed naphthalene system. The other effects observed after
deprotonation of naphDAMN (downfield effects found on C-2, C-11 and C-1’) suggest that in
this case proton was removed from oxygen atom, similarly to 5-NO2salDAMN.
The problem of isomers existence was investigated also in the solid state by CPMAS
measurements. The results of those experiments were collected in Table 5.
Table 5
The first observation made on the base of those results is that three compounds: 5-
CH3salDAMN, salDAMN and benzDAMN in the solid state exist as the mixture of two forms.
The analysis of chemical shifts of both forms leads to the similar conclusion as those in the
DMSO solution: both forms are either rotamers or geometrical isomers. Since the differences in
chemical shifts of appropriate position are small we are not able to decide what kind of isomers
exists. The only one significant difference between structure in solution and the solid state is
proton position in intramolecular hydrogen bridge. The upfield shift of imine nitrogen atoms of
compounds capable to form hydrogen bonds indicates that in the solid state protons are more
shifted, comparing with DMSO solution, to nitrogen site forming typical six-membered
intramolecular unsymmetrical hydrogen bridges. For the other three compounds (5-
BrsalDAMN, 5-NO2salDAMN, and naphDAMN) only one set of signals for each compound
was detected, so one can assume that in those cases no isomerization takes place.
To confirm those statements, the ATR-FTIR spectra of all Schiff bases were recorded.
To verify structure conclusions obtained from NMR data, two spectral ranges were considered.

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In the first range (3600-3000 cm-1) we should observe weak or medium intensity bands
assigned to stretching vibrations of N-H bonds in primary amino group (doublet). In this range
broad bands which come from stretching vibrations of O-H group engaged in intramolecular
hydrogen bonding could be seen but they probably overlap with bands assigned to ν (N-H) and
consequently are difficult to identify. For DAMN spectrum (Suppl.Mat. IA blue line) in this
range five bands are observed (3436, 3366, 3344, 3299, and 3199 cm-1). In benzDAMN
spectrum (Suppl.Mat. IA pink line) also five bands were recorded: two of them (3408 and 3300
cm-1) are sharp and intensive, whereas the other bands (3191, 3177, and 3161 cm-1) are weak
15
and unresolved. It can be considered as a confirmation of results obtained from N CPMAS
experiment, where two signals assigned to primary amino groups were found (Table 5). The
salDAMN spectrum (Suppl.Mat. IA red line) contains only three intensive bands (3415, 3303,
and 3189 cm-1) and one very weak band (3373cm-1). It is in good agreement with CPMAS data,
where carbon spectra indicate presence of isomer mixtures but in the nitrogen spectrum of
salDAMN only one NH2 signal was found. For 5-CH3salDAMN sample (Suppl.Mat. IA green
line) in this range four intensive bands were recorded (3415, 3393, 3305, and 3204 cm-1) which
suggests presence of two non-equivalent amine groups. Comparing this observation with NMR
and X-ray data we can conclude that this compound exists as the mixture of two geometrical
isomers, probably cis and trans.
On the Fig. IB the ATR-FTIR spectra of remaining compounds (5-BrsalDAMN, 5-
NO2salDAMN and naphDAMN) are presented. On the base of NMR data (only one isomer
detected for each compound) one can expect fewer number of bands in this region. In fact, for
all those compounds only sets of three intensive bands were recorded (5-BrsalDAMN: 3396,
3297, and 3195 cm-1; 5-NO2salDAMN: 3402, 3303, and 3202 cm-1; naphDAMN: 3466, 3337,
and 3191 cm-1).
The second interesting region (1680-1600 cm-1) is characteristic for strong bands caused
by stretching vibrations of double bonds: C=C, C=N and strong or medium intensity bands of
scissoring vibrations single bond N-H. The IR spectra (Suppl. Mat. IIA and IIB) have strong
absorption bands (1630-1603 cm-1) which can be assigned to following vibration:
1. C=N stretching vibrations of the Schiff base functional group (1690-1590 cm-1[40]),
2. C=C stretching vibrations for cis isomers (1680-1600 cm-1 [41]),
3. N-H scissoring vibrations close to 1615 cm-1 [41],
4. C=C skeletal vibrations (1650-1430 cm-1 [41].
It is very likely that some of those bands can overlap [40]. The band assignment of this
region was done on the base of literature data [29, 31, 34, 42-49] and is presented in Table 6.

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Table 6
The 1680-1600 cm-1 range for DAMN derivatives usually is assigned to C=N vibration,
however the strong band close to 1625 cm-1 (5-BrsalDAMN, 5-NO2salDAMN, 5-
CH3salDAMN, salDAMN) can be assigned to stretching vibration of aliphatic cis C=C bond
coupled with aromatic ring. For isomer trans those vibrations are in the same region but are less
intensive [50]. So, those bands are not diagnostic to distinguish cis and trans isomers in the
mixture.
For benzDAMN and naphDAMN compounds strong bands at 1607 and 1603 cm-1
respectively, were recorded. They can be assigned to stretching vibrations of imine C=N bonds
and to stretching vibrations of C=C bonds in aromatic rings. In the naphDAMN spectrum the
additional 1615 cm-1 band was found, which was assigned to scissoring vibration of non-
bonded amino group [41].
From the 13C CPMAS NMR (Table 5) measurements we know that salDAMN
derivative exists as the mixture of two isomers (double signals of aliphatic carbon atoms). In
the IR spectrum of this compound in this range is the band of 1623 cm-1 with the shoulder on
the left side, which suggests overlapping of two signals from different isomers. In the IR
spectra of remaining compounds we do not observe such shoulders.
Crystal data and selected geometrical parameters for the X-Ray investigated crystal
structures of 5-CH3salDAMNa, 5-CH3salDAMNb, and benzDAMN are summarized in Tables
7-8.
Table 7
Table 8
The X-Ray structure analysis of 5-CH3salDAMNb and benzDAMN revealed, that the
asymmetric part of the unit cell contains two independent molecules I and II.
View of the molecules with numbering of the atoms is shown in Figure 7.
Figure 7
The bond lengths and angles in benzDAMN are identical to (Z)-2-Amino-3-[(E)-
benzylideneamino]but-2-enedinitrile [51], while 5-CH3salDAMNa and 5-CH3salDAMNb are
close related to structures of 2-Amino-3-[(E)-(2-hydroxy-3-methylbenzylidene)amino]but-2-
enedinitrile [52] and (2Z)-2-Amino-3-{[(1E)-(4-hydroxyphenyl)methylene]-amino}but-2-
enedinitrile [53]. However, in both cases the observed geometry of the nitrile groups is
identical to that which we can see in compounds 5-CH3salDAMNb and benzDAMN. The
arrangement of the nitrile groups observed in 5-CH3salDAMNa was not previously described in

ACCEPTED MANUSCRIPT
the literature. This is example of polymorphism. Two different conformations were observed in
crystals from the same sample. 5-CH3salDAMN crystallized in two different shapes as a needle
and plate.
There are two types of intramolecular hydrogen bonds in Schiff bases, which may be
stabilized either in keto-amine (N—H···O hydrogen bond) [54] or phenol-imine (N···H— O
hydrogen bond) tautomeric forms [55-56]. The present X-Ray investigation shows that the 5-
CH3salDAMNa and 5-CH3salDAMNb are Schiff bases which exist in the phenol-imine form in
the solid-state.
An intramolecular O1—H1···N1 hydrogen bond, a characteristic hydrogen bond for Schiff
bases, leads to the formation of a S(6) six-membered ring (Figure 8). Details of the hydrogen
bonds are given in Table 9.
Figure 8
Table 9
In case of benzDAMN in both independent molecules the benzene rings and
diaminomaleonitrile moieties are anti with respect to azomethine C=N bonds. In the crystal
structure, intermolecular N—H···N hydrogen bonds (Table 9) connect the molecules into
ribbons in one direction (Figure 9).
Figure 9
The differences between conformations of molecules 5-CH3salDAMNa, 5-CH3salDAMNb and
benzDAMN are illustrated in Figure 10, showing the overlay of all molecules by fitting of C=N
in azomethine. The conformations of these molecules are described the torsion angles φ2 = C2–
C1–C7–N1 and φ3 = C7–N1–C8–C9, which are of 2.6(3)° for 5-CH3salDAMNa, -1.8(5)° and -
1.2(5)° for 5-CH3salDAMNb, 0.9(2)° and 2.5(2)° for benzDAMN and 2.2(3)° for 5-
CH3salDAMNa, 0.7(5)° and -0.4(4)° for 5-CH3salDAMNb, 5.6(2)° and -2.9(2)° for
benzDAMN, respectively.
Figure 10

ACCEPTED MANUSCRIPT
Conclusions
On the basis of proton, carbon and nitrogen NMR data it was found that all investigated
compounds obtained from salicylaldehydes exist in DMSO solution as OH forms without
intramolecular hydrogen bonds. In the case of 2-hydroxy naphthyl derivative, the proton was
engaged into weak intramolecular hydrogen bond. The upfield shifts of imine nitrogen atoms
observed in the solid state indicated that in this phase unsymmetrical intramolecular hydrogen
bonds are present.
Additionally, in some imines derived from DAMN (salDAMN and 5-BrsalDAMN), it is
observed that two-component (A and B) are in equilibrium mixtures.
All the investigated imines were deprotonated by reaction with tetramethylguanidine
(TMG). Deprotonation reaction of imine derivatives of DAMN occurs in two different ways.
For three DAMN derivatives (salDAMN, 5-CH3salDAMN, and 5-BrsalDAMN) deprotonation
products form two-component (X and Y) mixtures. In contrast to parent imines (equilibrium
mixture of cis and trans isomers), structures of deprotonated forms (X and Y) differ
significantly. In form X, deprotonation takes place on oxygen atom, whereas in form Y, proton
is subtracted from NH2 group of diaminomaleonitrile residue. Two other derivatives (5-
NO2salDAMN and naphDAMN) form only one deprotonation product, where the proton is
removed from OH group.
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Table 1. Results of NMR measurements of Schiff bases in DMSO solution.
5-CH3salDAMN 5-BrsalDAMN
salDAMN
5-NO2salDAMN
compound
A 72%
B 18%
A 80%
B 20%
1H
13C/15N
1H
13C/15N
1H
13C/15N
1H
13C/15N
1H
13C/15N 1H
13C/15N
position
C-1
C-2
C-3
C-4
C-5
C-6
C-7
OH
-
-
121.5
156.7
116.8
134.4
128.5
129.1
153.2
-
-93.8
104.0
126.3
114.4
115.0
-292.4
91Hz
20.5
-
-
124.4
157.8
119.1
135.6
112.0
130.4
150.2
-
-87.0
103.7
127.2
114.3
114.9
-290.3
91Hz
-
-
-
122.9
157.5
-
-
121.7
158.6
116.85
133.7
119.8
129.3
153.2
-
-92.9
103.9
126.4
115.0
114.4
-
-
-
120.4
-
-
122.6
163.6
117.5
128.3
140.8
124.3
149.8
-
-83.9
103.6
127.8
114.3
114.9
-288.0
91Hz
158.5
116.9
132.9
120.0
130.7
153.5
-
-95.8
103.95
126.4
113.7
112.4
-
6.80
7.11
-
7.83
8.52
10.1
-
-
-
-
-
6.86
7.41
-
8.29
8.47
10.6
-
6.87 119.3
7.43 135.0
-
7.82 131.5
8.40 150.7
11.1
6.90
7.29
6.85
8.01
8.56
10.4
6.90
7.29
6.85
7.62
8.42
11.1
7.08
8.15
-
8.92
8.52
11.9
-
111.2
-
N
-89.1
103.9
128.4
114.2*
113.6*
-292.9
92Hz
-
C-1’
C-2’
1’-CN
2’-CN
NH2
-
7.80
8.00
-
8.1
7.81
8.00
8.17
CH3
2.2
*- assignment can be reversed
For NH2 nitrogen signals the line widths are specified in second row

ACCEPTED MANUSCRIPT
Table 2. NMR data for benzDAMN and deprotonation reaction product in DMSO solution.
benzDAMN
13C/15N
benzDAMN + TMG
13C/15N
1H
1H
-
compound
position
C-1
C-2
C-3
-
135.9
129.6
129.2
139.9
8.00
7.88
7.39
ov.
7.39
ov.
-
128.6
129.1
7.45 ov.
C-4
7.45
ov.
-
-
131.9
130.8
C-5
C-6
-
-
-
-
-
C-7
OH
8.23
-
155.5
-
8.1
-
151.1
-
N
-83.6
103.1
127.4
114.8
114.2
-290.8
91.8Hz
-80.2
99.6
126.1
117.1*
116.8*
-
C-1’
C-2’
1’-CN
2’-CN
NH2
7.92
-
ov.- overlapped signals
*- assignment can be reversed

ACCEPTED MANUSCRIPT
Table 3. NMR data for naphDAMN and its deprotonation product in DMSO solution.
naphDAMN
13C/15N
naphDAMN + TMG
1H
1H
13C/15N
compound
position
C-1
-
-
110.8
160.3
119.0
122.5
129.4
129.1
124.5
135.7
132.2
128.4
155.5
104.4
125.6
114.6
115.2
-
-
110.9
179.1
128.6
135.8
128.2
127.9
119.9
124.4
135.4
120.0
159.2
111.8
124.9
115.7*
117.9*
-
C-2
C-3
C-4
C-5
C-6
C-7
C-8
C-9
C-10
C-11
C-1’
C-2’
1’-CN
2’-CN
OH
-
7.20
8.60
7.85
7.59
7.40
7.97
-
6.41
7.34
7.33
7.24
6.88
9.20
-
-
-
9.22
-
9.11
-
-
-
11.9
=N-
NH2
-104.7
-292.3
(91.3Hz)
-127.5
-
7.88
-
*- assignment can be reversed

ACCEPTED MANUSCRIPT
Table 4. NMR data for deprotonation products in DMSO solution.
5-CH3salDAMN + TMG 5-BrsalDAMN + TMG
salDAMN + TMG
5-
compound
NO2salDAMN
+TMG
X 65%
13C/15N 1H
Y 35%
13C/15N 1H
X 88%
13C/15N 1H
Y 12%
13C/15N 1H
X 66%
13C/15N 1H
Y 34%
1H
-
-
6.57
6.90
-
7.53
8.45
13C/15N
122.2
157.6
118.8
128.6
116.4
128.5
139.1
1H
13C/15N
122.5
179.3
123.1
128.8
130.5
126.2
155.6
position
C-1
C-2
C-3
C-4
C-5
C-6
C-7
121.75
-
121.6
-
124.6
173.5
125.9
135.6
102.2
129.7
155.2
124.6
-
122.3
166.6
120.0
132.4
114.5
128.6
152.0
-
-
-
-
162.2
118.7
133.6
124.3
128.2
151.6
-
155.45
116.2
129.5
127.3
128.8
139.8
-
-
-
6.62
6.84
-
7.02
7.74
6.10
6.84
-
7.75
8.50
6.50
7.01
120.3
131.3
106.8
129.5
6.52
7.0
6.40
7.64
8.56
6.70
7.01
6.70
7.2
7.82
6.11
7.74
-
8.64
8.58
7.39
7.8
br.
-
OH
N
C-1’
-
-
-
-
-
-
-
-
-
-
-
-
-
-101.4
101.4
-88.9
93.2
-113.2
106.6
-105.1
101.6
br.
-89.3
92.8
-102.1
106.0
C-2’
127.3
116.1*
117.5*
-
129.3
116.6*
117.0*
-
123.4
116.0*
115.2*
-
133.3
117.2*
116.9*
-
123.4
117.9*
116.7*
-
124.2
115.0*
115.7*
-
1’-CN
2’-CN
NH2
-
-
-
-
-
-
CH3
2.13
20.56
2.16
20.59
*- assignment can be reversed
br.- broad signal

ACCEPTED MANUSCRIPT
Table 5. 13C and 15N NMR data for imines derived from DAMN in the solid state.
5-CH3salDAMN benzDAM
N
5-
5-
naphDAMN
salDAMN
NO2salDAMN BrsalDAMN
C-1
C-2
134.2 ov.
157.4
136.5
127.1 ov.
139.0 ov.
164.6
125.7 ov.
157.0
106.8
159.4
124.8 ov.
158.1
157.5
C-3
C-4
C-5
117.7
134.2 ov.
128.8
127.1 ov.
132.1
-
118.5
132.3*
139.0 ov.
119.2 ov.
137.3 br.
119.2 ov.
119.0
122.6
134.0*
115.9
127.5
C-6
C-7
134.2 ov.
161.5
159.7
104.4
-
127.8 ov.
158.9
134.5 br.
160.3
135.0*
161.7
159.2
105.0
105.9
124.8 ov.
117.9
117.2
112.6
113.0
156.4 br.
153.3
105.0
C-1’
106.1 br.
104.7
105.2
C-2’
1’-
CN
2’-
CN
CH3
NH2
127.1ov.
113.3 br.
127.8 ov.
116.1*
125.7 ov.
114.2*
111.8 br.
115.3*
114.4*
113.3 br.
112.9*
113.3*
20.9
-
-
-
-
-
-298.7
-301.2
-122.4*
-120.0*
-116.3*
-113.1*
-105.2*
-302.7
-304.1
-121.8*
-119.5*
-111.8*
-115.5*
-86.1
-295.9
-117.7*
-111.5*
-109.1
-294.5
-119.6*
-115.2*
-107.9
-302.5
-301.2
-119.6
-114.8
-108.3
CN
CN
-128.2*
-116.8*
-106.8*
-
C=N-
-88.0
*-assignment can be changed
?-different position numbers in naphthalene rings with respect to benzene ring

ACCEPTED MANUSCRIPT
In naphtyl derivative assignment of other aromatic and C-2’carbon atoms was impossible,
136.2 (CH), 132.1 (q), 130. (CH), 128.6 (CH), 127.1 (q), 123.3 (CH)

ACCEPTED MANUSCRIPT
Table 6. ATR-FTIR spectral data assignments of DAMN and its derivatives.
compound
type
DAMN
benzDAMN
salDAMN
5-CH3salDAMN
5-BrsalDAMN
5-NO2salDAMN
naphDAMN
of vibrations
 N-H (-NH2)
3436, 3366,
3344, 3299
3408, 3300,
3177, 3161
3415, 3373,
3303
3415, 3393,
3305
3396, 3297
3402, 3303
3466, 3337
 C=N*
-
1607
1623
1629
1630
1626
1603
ν C≡N
2210, 2167
2239, 2204
2234, 2204
2240, 2212
2245, 2209
2238, 2219
2239, 2200
 C=C*
1644
1616
-
1607
a
1623
a
1629
a
1630
1615
1626
a
1607
1615
(aliphatic part)
δ N-H (-NH2)
 C=C
1607, 1579,
1556, 1449
1600, 1562,
1487
1599, 1566, 1489 1599, 1547, 1474 1599, 1556, 1478 1603, 1556, 1468
(aromatic ring)
a – probably the bands from δ N-H vibrations are overlaped with the other strong bands present in this region.
* - overlaping of  C=N and  C=C (aliphatic part) vibrations should be considered.

ACCEPTED MANUSCRIPT
Table 7. Crystal data and structure refinement for compounds 5-CH3salDAMNa, 5-CH3salDAMNb and benzDAMN
5-CH3salDAMNa 5-CH3salDAMNb
benzDAMN
C11H8N4
196.21
Brown needle
0.11 x 0.19 x 0.40
Monoclinic
P21/c
Chemical formula
Formula weight
Crystal appearance
Crystal size [mm]
Crystal system
Space group
Unit cell parameters
a [Å]
C12H10N4O
226.23
C12H10N4O
226.23
Yellow needle
0.03 x 0.18 x 0.33
Monoclinic
P21/n
Yellow plate
0.23 x 0.25 x 0.53
Monoclinic
P21/n
7.1040(4)
14.7515(7)
11.0890(5)
90
6.9836(11)
24.727(4)
13.700(2)
90
6.9481(2)
22.7995(5)
13.5132(3)
90
b [Å]
c [Å]
α [°]
β [°]
101.038(3)
90
1140.57(10)
4
1.318
472
102.593(9)
90
2308.9(6)
8
1.302
944
100.938(2)
90
2101.78(9)
8
1.240
816
γ [°]
V [Å3]
Z
Dcalc. [g/cm3]
F(000)
θ range [°]
Absorption coefficient
µ [mm-1]
Absorption correction
Max. and min.
transmission
5.05 to 68.26
3.76 to 66.58
3.86 to 68.63
0.730
numerical, SADABS3
0.721
numerical, SADABS3
0.642
multi-scan, SADABS3
0.9790 and 0.7960
0.8530 and 0.7010
0.9330 and 0.7810
-8 ≤ h ≤ 7
-8 ≤ h ≤ 8
-8 ≤ h ≤ 7
Index ranges
-17 ≤ k ≤ 17
-12 ≤ l ≤ 13
-29 ≤ k ≤ 29
-12 ≤ l ≤ 16
-27 ≤ k ≤ 27
-16 ≤ l ≤ 16
No. of measured
reflections
32921
29652
41348
No. of independent
reflections
Completeness
Refinement method
Final R indices: R1, wR2
Goodness-of-fit on F2
2069 (Rint = 0.0745)
98.3 %
3873 (Rint = 0.0673)
94.9 %
3856 (Rint = 0.0573)
99.1 %
Full-matrix least-squares on F2
0.0428, 0.1006
1.062
0.0640, 0.1428
1.064
0.0423, 0.1045
0.984
Data/restraints/parameters 2069/0/175
R indices (all data): 0.0715, 0.1162
3873/0/357
0.1126, 0.1670
3856/0/336
0.0601, 0.1172

ACCEPTED MANUSCRIPT
R1, wR2
Excitation coefficient
Δ/σmax
None
0.002
None
0.028
0.0021(2)
0.001
Absolute structure
parameter
-
-
0.0(3)
Largest diff peak and hole
0.199 and -0.147
0.228 and -0.224
0.194 and -0.193
[eÅ-3]

ACCEPTED MANUSCRIPT
Table 8. Selected bond lengths (Å) and angles (°) for 5-CH3salDAMNa, 5-CH3salDAMNb and benzDAMN.
5-CH3salDAMNa 5-CH3salDAMNb _I 5-CH3salDAMNb _II
benzDAMN _I
benzDAMN _II
O1–C2
N1–C7
N1–C8
N2–C9
C8–C10
N3–C10
N4–C11
C1–C7–N1
C7–N1–C8
N1–C8–C9
N1–C8–C10
C8–C10–N3
C8–C10–C11
N3–C10–C11
C7–C1–C2–O1
C2–C1–C7–N1
C7–N1–C8–C9
N1–C8–C10–N3
C9–C8–C10–N3
1.363(2)
1.286(2)
1.388(2)
1.140(2)
1.363(2)
1.341(2)
1.138(2)
121.36(17)
122.95(15)
120.64(15)
118.37(16)
127.71(17)
116.28(16)
115.99(16)
0.9(3)
1.356(4)
1.292(4)
1.398(3)
1.138(4)
1.361(4)
1.337(4)
1.138(4)
123.0(3)
119.2(3)
120.3(3)
120.6(3)
126.3(3)
118.6(3)
115.1(3)
4.5(5)
1.355(4)
1.289(4)
1.400(3)
1.142(4)
1.365(4)
1.338(4)
1.136(4)
123.1(3)
119.2(3)
120.9(3)
119.9(3)
125.7(3)
118.5(3)
115.7(3)
-1.5(5)
-
-
1.2738(17)
1.3894(17)
1.144(2)
1.3587(19)
1.345(2)
1.1360(19)
121.70(14)
121.06(13)
122.04(12)
117.88(12)
124.09(13)
119.79(13)
116.12(13)
-
0.9(2)
5.6(2)
-1.9(2)
177.12(15)
178.73(13)
-2.2(2)
1.2763(18)
1.3878(17)
1.142(2)
1.3617(19)
1.348(2)
1.136(2)
121.04(15)
121.37(13)
122.67(13)
117.43(13)
124.13(13)
119.37(14)
116.45(14)
-
2.5(2)
-2.9(2)
0.8(2)
-178.83(15)
178.38(14)
-1.3(2)
2.6(3)
2.2(3)
-176.45(17)
1.6(3)
-1.8(5)
0.7(5)
-1.7(5)
179.2(3)
175.0(3)
-4.1(5)
-1.2(5)
-0.4(4)
-2.9(5)
177.0(3)
179.0(3)
-1.1(5)
N1–C8–C10–C11 1.8(3)
C9–C8–C10–C11 179.84(17)

ACCEPTED MANUSCRIPT
Table 9. Hydrogen bond distances (Å) and angles (°) for compound 5-CH3salDAMNa, 5-CH3salDAMNb and benzDAMN
D-H ···A
D-H
H···A
D···A
5-CH3salDAMNa
2.597(2)
Angle
Symmetry codes
O1–H1···N1
N3–H3A···N2
N3–H3B···O1
C3–H3···N4
0.95(2)
0.86
0.86
1.72(2)
2.24
2.28
152(2)
169.7
169.7
3.094(2)
3.126(2)
[-x + 2, -y, -z + 3]
[x + ½, -y - ½, z + ½]
[x - ½, -y - ½, z - ½]
0.96(2)
2.64(2)
3.532(3)
154.7(18)
5-CH3salDAMNb
2.692(3)
O1–H1···N1
O2–H2···N5
N3–H3A···N2
N3–H3B···N8
N7–H7A···N6
N7–H7B···N4
1.00(5)
1.00(5)
0.86
0.86
0.89(4)
0.87(4)
1.82(5)
1.85(5)
2.15
2.27
2.21(4)
2.20(4)
144(4)
143(4)
146.8
168.5
147(3)
168(3)
2.719(3)
2.910(4)
3.119(4)
2.998(4)
[x – 1, y, z]
[-x + ½, y - ½, -z + ½]
[x +1, y, z]
3.063(4)
[-x + , y + ½, -z + ½]
benzDAMN
3.099(2)
3.226(2)
3.095(2)
3.065(2)
N3–H3A···N2
N7–H7A···N6
N3–H3B···N8
N7–H7B···N4
0.93(2)
0.95(2)
0.89(2)
0.89(2)
2.25(2)
2.34(2)
2.22(2)
2.20(2)
151.9(16)
156.1(15)
168.8(18)
166(2)
[x + 1, y, z]
[x – 1, y, z]
[-x + 1, y - ½, -z + ½]
[-x, y + ½, -z + ½]

ACCEPTED MANUSCRIPT
R
N
R
N
H
O
H
O
enol-imine form
keto-amine form
Figure 1. Tautomeric forms of Schiff bases.