The intramolecular hydrogen bonds in some Schiff bases derived from cyclopropyl-, cyclobutyl- and cyclopentylamine

Article abstract of DOI:10.1016/j.molstruc.2005.02.035

Eight Schiff bases (N-(R-salicylidene)-cycloalkilamines, R=H, 5-NO ₂ and 5-Br) obtained from cyclopropyl, cyclobutyl- and cyclopentylamine were investigated in terms of intramolecular hydrogen bond formation in chloroform solution and in the so

Full text of DOI:10.1016/j.molstruc.2005.02.035

Journal of Molecular Structure 743 (2005) 237–241
www.elsevier.com/locate/molstruc
The intramolecular hydrogen bonds in some Schiff bases derived
from cyclopropyl-, cyclobutyl- and cyclopentylamine
a,
a
W. Schilf , B. Kamienski , A. Szady-Chełmieniecka , E. Grech
b
b
*
´
aInstitute of Organic Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warsaw, Poland
´
Department of Inorganic and Analytical Chemistry, Technical University of Szczecin, Al. Piastow 42, 71-065 Szczecin, Poland
b
Received 13 January 2005; revised 14 February 2005; accepted 24 February 2005
Available online 13 April 2005
Abstract
Eight Schiff bases (N-(R-salicylidene)–cycloalkilamines, RZH, 5-NO2 and 5-Br) obtained from cyclopropyl, cyclobutyl- and
cyclopentylamine were investigated in terms of intramolecular hydrogen bond formation in chloroform solution and in the solid state. The
relationship between cycloalkyl ring size and proton position was discussed. For the compounds which exist as OH form (salicylaldehyde and
5-bromosalicylaldehyde derivatives), no correlation between ring size and proton position in hydrogen bond was found. For 5-nitro
derivatives, the structure of hydrogen bridge strongly depends on the cycloaliphatic ring size.
q 2005 Elsevier B.V. All rights reserved.
Keywords: 1H, 15N, 13C NMR; 13C and 15N CPMAS; Schiff base; Hydrogen bonds; Cyclopropyl; Cyclobutyl; Cyclopentyl rings
1. Introduction
the nitrogen atom, it is necessary to increase acidity of OH
group by appropriate substitution of phenyl ring or by
increasing basicity of imine moiety using specific sub-
stituted aniline derivatives for condensation (for example,
N-diMe substituent in position 4) [17,18]. In the present
paper, we would like to study the effect of presence of small
aliphatic ring located on nitrogen atom on tautomeric
equilibrium in some Schiff bases obtained from salicylalde-
hyde derivatives.
The Schiff bases derived from substituted salicylalde-
hyde are very interesting and useful compounds in practical
and theoretical considerations [1,2]. Many of the Schiff
bases show tautomeric properties and have been studied by
UV-VIS techniques [3–5], and X-ray [6–10], and DFT
calculation [11], and both using NMR in the liquid and in
solid state [12–16].
In the previous papers [12–16], we have investigated the
influence of substituents in phenyl ring and amine used for
condensation on the structure of intramolecular hydrogen
bond formed in some Schiff bases. The first conclusion was
as follows: all substituents in phenyl ring, which increase
acidity of OH group in applied salicylaldehyde derivatives,
promote the proton transfer from oxygen atom to nitrogen
and vice versa. The second conclusion is, the imine
derivatives of aliphatic amines can form intramolecular
bridge in which proton is more effectively transferred to
nitrogen atom comparing with aromatic amine derivatives.
To obtain aromatic derivatives with proton transferred to
Generally for Schiff bases obtained from aliphatic
amines, the nitro group in position 5 in benzene ring
promotes the proton transfer from oxygen to nitrogen site
leading to large upfield shift of nitrogen NMR imine atom
signal to about K100.0 ppm at room temperature and to
K180.2 ppm at 195 K [16]. In the solid state, we observed
even stronger effect, the upfield shift to K200.9 ppm
indicating even more effective proton transfer in this
condition. After replacing ethyl substituent on nitrogen
atom by cyclopropane ring, the structure of hydrogen bridge
is completely different. In both phases, the nitrogen signals
of imine moiety are in the same range: K70.9 ppm in
chloroform solution at room temperature, K74.5 ppm at
223 K and K71.0 ppm in the solid state [15]. It means that
in all applied conditions, the OH form with weak hydrogen
bond is present. This is in contrast to ethyl derivative, where
even at room temperature, the tautomeric equilibrium is
*
Corresponding author. Tel.: C 48 22 632 0904; fax: 48 22 632 6681.
E-mail address: schilf@icho.edu.pl (W. Schilf).
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.02.035

238
W. Schilf et al. / Journal of Molecular Structure 743 (2005) 237–241
(303 K), about 5 mg of samples were dissolved in CDCl3. The
H
R
H
R
O
N
nitrogen and carbon chemical shifts were obtained and
assigned by 2D GHMBC and GHSQC methods. In some
cases to assign proton signals, the GCOSY homonuclear
correlations were necessary. The nitrogen chemical shifts are
reported with respect to external nitromethane. The typical
spectral condition for natural abundance nitrogen CPMAS
NMR spectra were: spectral width 28 kHz, acquisition time
40 ms, spin rate 6–12 kHz, contact time for spin-lock 5 ms,
relaxation delay 10–120 s depending on relaxation condition
for particular sample, estimated 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 ms 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.
O
N
2
7
3
4
1
6
X
5
X
X = none, 5-Br, 5-NO2
R = cyclopropyl, cyclobutyl, cyclopentyl
Fig. 1.
shifted to NH form, at low temperature and in the solid state
the NH tautomeric form is a predominant one. To verify this
observation, we extend now this investigations on eight new
derivatives containing different substituents in phenyl ring
and three different aliphatic small rings on nitrogen atom
(Fig. 1).
2. Experimental
All investigated compounds have been synthesized by
direct condensation of appropriate substituted salicylalde-
hydes with cycloalkyl amines in methanol solution. The
mixtures of aldehyde and amine in proportion 1:1 were
refluxed in methanol about 4 h and then after removing of
the solvent were crystallized from ethanol. The yield of
reactions were approximately 40–60% for oils (1A, 1B, 1C
and 2B) and 60–80% for solids (2A, 2C, 3A, 3B and 3C).
Only in the case of reaction of 2-hydroxy-5-nitrobenzalde-
hyde, the product precipitates right after mixing of both
substrates.
3. Results and discussion
All results of NMR measurements of eight new
compounds and one taken from literature, in both phases,
if possible, are collected in Tables 1–3. In contrast to the
other compounds, the cyclobutyl and cyclopentyl derivative
of 5-nitrosalicylaldehyde (3B and 3C) were measured in
chloroform solution only at 253 K. At lower temperatures,
the severe broadening of signals appeared making any
nitrogen NMR measurements impossible.
The NMR measurements were run on a Bruker Avance
DRX 500 spectrometer. For solutions, the triple resonance
inverse 5 mm probehead with Z-gradient was applied. The
solid state spectra were done using 4 mm CPMAS Bruker
probehead. For solution experiments run at room temperature
For the intramolecular hydrogen bond investigation,
three spectral parameters have practical significance. The
most important one is the nitrogen chemical shift of imine
atom. This parameter provides quantitative estimation of
Table 1
Proton, carbon and nitrogen chemical shifts of cycloalkylamine derivatives of salicylaldehyde in chloroform solution
Atom position
Solution
N-salicylidene-cyclopropylamine, 1A
N-salicylidene-cyclobutylamine. 1B
N-salicylidene-cyclopentylamine, 1C
1H
13C/15
N
1H
13C/15
N
1H
13C/15
N
1
–
–
119.1
160.4
116.8
131.6
118.6
130.7
161.9
–
–
–
118.7
161.7
117.2
132.2
118.4
131.4
162.0
–
–
–
118.9
161.2
117.0
131.9
118.4
131.1
162.3
–
2
3
6.91
6.93
6.98
4
7.25
6.85
7.20
8.46
12.71
–
7.27
6.83
7.20
8.20
13.13
–
7.26
6.83
5
6
7.20
7
8.30
OH
13.65
–
N 303 K
K72.9
K73.5a
40.3
–75.0
K87.7
61.3
K70.0
K72.3
69.9
N 223 K
–
–
–
10
20
30
2.95
0.97, 0.93
–
4.10
2.35, 2.18
1.85
3.74
9.3
30.6
1.21, 1.70
1.83, 1.67
34.6
–
15.7
24.3
a
Weak correlation with OH proton.

W. Schilf et al. / Journal of Molecular Structure 743 (2005) 237–241
239
Table 2
Proton, carbon and nitrogen chemical shifts of cycloalkylamine derivatives of 5-bromosalicylaldehyde in chloroform solution and in solid state
Atom position N-(5-bromosalicylidene)-cyclopropylamine, 2A
N-(5-bromosalicylidene)-cyclo-
butylamine, 2B
N-(5-bromosalicylidene)-cyclopentylamine, 2C
Solution
1H
CPMAS
13C/15
Solution
Solution
1H
CPMAS
13C/15
13C/15
N
N
1H
13C/15
N
13C/15
N
N
1
–
120.5
159.4
118.7
134.1
110.2
132.7
160.6
–
122.0
160.8
119.1
133.1a
–
–
120.1
160.4
119.2
135.0
109.8
133.4
160.7
–
–
120.2
160.4
119.1
134.7
109.8
133.2
160.9
–
121.0
161.3
119.9
133.7a
2
–
–
3
6.82
6.84
6.83
7.35
–
4
7.33
7.34
–
b
b
5
–
–
–
6
7.32
133.1a
161.9
–
7.31
8.13
13.65
–
7.33
8.25
13.65
–
133.7a
163.2
–
7
8.39
OH
12.7
N 303 K
–
K69.3
K70.3
40.2
K66.2
–
K74.2
K73.0
61.5
K68.4
K69.1
69.9
K69.6
–
N 223 K
–
–
–
10
20
30
2.98
40.4
9.6
4.12
2.36, 2.18
1.86
3.78
1.95, 1.69
1.87, 1.69
70.3
34.9
24.7
1.01, 0.96
–
9.7
30.4
34.6
–
–
15.7
24.3
a
Specific assignment impossible because of signals overlapping.
Missing signal probably due to large broadening.
b
proton position. The typical value of this parameter for pure
imine structure is about K50 ppm [19]. Intramolecular
hydrogen bond or proton transfer shift this signal upfield to
K240 ppm [13]. The one bond coupling constants are even
better for such estimation but only for the compounds where
the tautomeric equilibrium is shifted to NH form [16]. For
the tautomeric systems where the equilibrium is shifted to
OH form, measurement of this coupling is impossible. For
the tautomeric equilibrium estimations, but only on
qualitative level, the carbon chemical shifts of atoms in
position 2 can be applied. In this case, the downfield shift of
C-2 signal to about 170–180 ppm is typical for the structure
with proton transfer from oxygen to nitrogen atom [16].
The analysis of nitrogen chemical shifts collected in
Tables 1 and 2 leads to the conclusion that both series of
compounds have a very similar structure in both solvent and
solid state. The dN values in the range from K66 to K87 ppm
show that in all cases the OH form with hydrogen bond is
present. At low temperature, usually the tautomeric equili-
brium position is shifted slightly more to NH form, but this
effect for the investigated compounds is very weak, except
for the compound 1B, where observed difference suggests
some higher contribution of form NH. However, the
differences between nitrogen chemical shifts for compounds
1A–1C, 2A–2C and 3A are small to make some reliable
conclusion in terms of hydrogen bond strengths. We have to
Table 3
Proton, carbon and nitrogen chemical shifts of cycloalkylamine derivatives of 5-nitrosalicylaldehyde in chloroform solution and in solid state
Atom pos-
ition
N-(5-nitrosalicylidene)-cyclopropylamine,
N-(5-nitrosalicylidene)-cyclobutylamine, 3B N-(5-nitrosalicylidene)-cyclopentylamine,
3C
3A
Solution
1H
CPMAS
13C/15
Solution
1H
CPMAS
13C/15
Solution
1H
CPMAS
13C/15
13C/15
N
N
13C/15
N
N
13C/15
N
N
1
–
118.1
166.4
118.2
126.8
139.7
126.7
160.5
–
K70.9a
K74.5a
40.1
118.6
166.2a
117.5
126.7
138.9
126.7
160.6
–
–
116.3
170.4
119.5
128.5
138.5
128.6
161.2
–
112.9
180.4
122.7
130.4
132.3
135.9
164.5
–
116.3
170.7
119.5
128.2
138.4
128.3
161.8
–
113.0
180.4
123.0
130.2
132.2
135.0
163.7
–
2
–
–
–
3
6.98
8.17
–
6.95
8.18
–
6.98
8.18
–
4
5
6
8.21
8.54
13.85
–
8.23
8.25
14.4
–
8.24
8.36
14.4
–
7
OH
N 303 K
K71.0a
–
K100.8
K130.7b
59.8
K198.4
–
K101.7
K129.6b
68.0
K197.2
–
N 223 K
–
–
–
3.07
1.02, 1.09
–
40.2
9.8
4.23
2.45, 2.25
1.90
54.2
3.54
62.3c
10
20
30
9.9
30.3
31.9, 25.0 2.05, 1.75
14.5 1.85, 1.70
34.2
33.1, 28.0
22.9, 21.9
–
–
15.6
24.1
a
Data taken from literature [15].
253 K.
b
c
Two close overlapped signals.

240
W. Schilf et al. / Journal of Molecular Structure 743 (2005) 237–241
remember that nitrogen chemical shift depends not only
on hydrogen bond structure but also on substitution effects on
nitrogen atom. In our case we should expect few long-range
substitution effects whose values are difficult to estimate
because in literature there are no data concerning this type
of interaction in small aliphatic rings. Usually such
substitution increments are about a few parts per million
upfield or downfield which makes any estimation
very difficult. Taking into account these facts, we can state
that in all discussing cases the hydrogen bonds are weak and
similar to each other. Examination of 13C NMR data for
compounds 1A, 1B, 1C and 2A, 2B, 2C leads to the
same conclusion. It is known that only carbon chemical shift
of atom 2 provides valuable structural information. The
signals of those atoms for the discussed compounds are
located very close to 160 ppm which is typical for OH weakly
hydrogen bonded structure and they do not change at low
temperature or in solid state. The number of 13C CPMAS
signals for 3B and 3C in aliphatic region suggests that in
the solid state, the four and five membered rings are non-
planar and in this condition two non-equivalent forms are
present (two signals for C-20 and two for C-30 positions).
This differentiation also leads to two overlapped signals of
C-10 atoms.
pKaZ9.10 of cyclopropylamine [20] comparing with
cyclobutylamine (pKZ10.04) and cyclopentylamine
(pKZ10.65) can be explained by the partially olefinic
character of cyclopropane ring and by the differences in
stability of the cycloalkanes with respect of ring size [21].
These differences are not big enough to change structure of
hydrogen bonds in salicylaldehyde and 5-bromosalicylalde-
hyde derivatives (Tables 1 and 2). The proton positions in
those compounds are determined by relatively low acidity of
OH group. The nitro group in position 5 increases the
acidity of OH and makes possible the proton transfer to
nitrogen atom. The electron donating character of chain
alkyls and cyclobutyl and cyclopentyl rings, which
increases basicity of imine group, make this transfer very
effective.
Cyclopropyl ring does not possess such property and as a
consequence, the imine group is not enough basic to allow
such transfer. From this point of view, the cyclopropane ring
reacts very similar to substituted benzene ring (for example,
pK value for aniline is 9.42).
4. Conclusion
Quite different situation was found for 5-nitro deriva-
tives. Previously, three N-(5-nitrosalicylidene)-alkylamines
[16] have been investigated: N–Et, N–iPr and N–tBu. All of
those compounds, at room temperature, show a large upfield
shift of nitrogen signal indicating a strong intramolecular
hydrogen bond (for 5-nitrosalicylaldehyde derivative with
EtNH2 at 254 K dNZK131.3 ppm). The lowering of
temperature causes further proton transfer from oxygen to
nitrogen site. As a result of this, the nitrogen signals are
shifted to K180 ppm region at 195 K. In the solid state, this
process is even stronger, shifting nitrogen signals from
K180 to K200 ppm range. The cyclobutyl 3B and
cyclopentyl 3C derivatives provide very similar spectral
picture, which allow to conclude that there are no significant
differences between the structure in intramolecular hydro-
gen bonds of derivatives with four and five membered
cycloaliphatic rings. In contrast to this, cyclopropyl
derivative 3A in all applied conditions exists as OH
structure with relatively weak hydrogen bond. This effect
was observed previously for cyclopropyl derivative of
5-nitrosalicylaldehyde [15]. Since in all 5-nitro derivatives,
acidity of OH group determined by nitro substituent should
be very similar, then the difference in molecular structure
between 3A and 3B, 3C must be caused by property of
aliphatic rings bonded to nitrogen atom. Comparing the data
in Table 3 with the other aliphatic derivatives available, we
can state that the cyclobutyl and cyclopentyl rings have very
similar proton donating properties as the chain aliphatic
groups. In contrast to this, the electron donating properties
of cyclopropyl ring must be much weaker and it means
that cyclopropylamine must be significantly weaker base
than cyclobutyl- and cyclopentylamine. The low value of
The proton position in intramolecular hydrogen bridge
formed by Schiff bases obtained from salicylaldehyde
derivatives is very sensitive to both substituents present in
phenyl ring and amine used for condensation. The nitro
substituent in position 5 in phenyl ring increases acidity of
OH group so effectively that, for aliphatic derivatives the
proton transfer to nitrogen atom becomes possible,
especially at low temperature and in the solid state [16].
Similar effect was found for cyclobutyl and cyclopentyl
derivatives. The cyclopropyl derivative behaves completely
differently. In all experimental conditions, chloroform
solution and in the solid state, only a OH structure with a
weak hydrogen bond was observed. It means that electron-
donating effect created by four and five membered aliphatic
ring is strong enough to increase basicity of imine nitrogen
atom to make proton transfer effective. For derivative with
cyclopropane ring, this effect is much smaller or even
absent.
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