The substituent effects on the 13C chemical shifts of the azomethine carbon atom of N-(substituted phenyl)salicylaldimines

Article abstract of DOI:10.2298/JSC120319033D

Hammett correlations between the ¹³C-NMR chemical shifts of the azomethine carbon atom and the corresponding substituent constants were established for thirteen Schiff bases. The successful correlation of the chemical shifts with the electrophilic substituent constants σ⁺ indicate a significant resonance interaction of the substituents on the aniline ring with the azomethine carbon atom in the examined series of imines. The demand for electrons in the investigated compounds was compared to that of the N-benzylideneanilines and N-(substituted phenyl)pyridinealdimines. The manner of transmission of the substituent effects is discussed and they were separated into resonance and inductive effects. The inductive effects prevail over the resonance effects.

Full text of DOI:10.2298/JSC120319033D

J. Serb. Chem. Soc. 77 (8) 993–1001 (2012)
JSCS–4327
UDC 547.022+547.79+547.53.024+
547.828:54.081
Original scientific paper
The substituent effects on the 13C chemical shifts of the
azomethine carbon atom of N-(substituted
phenyl)salicylaldimines
1
#
*
1#
1#
SAŠA Ž. DRMANIĆ , ALEKSANDAR D. MARINKOVIĆ , JASMINA B. NIKOLIĆ
and BRATISLAV Ž. JOVANOVIĆ2#
1Department of Organic Chemistry, Faculty of Technology and Metallurgy, University of
Belgrade, Karnegijeva 4, Belgrade and 2Institute of Chemistry, Technology and Metallurgy,
University of Belgrade, Njegoševa 12, Belgrade, Serbia
(Received 19 March, revised 3 April 2012)
Abstract: Hammett correlations between the 13C-NMR chemical shifts of the
azomethine carbon atom and the corresponding substituent constants were
established for thirteen Schiff bases. The successful correlation of the chemical
shifts with the electrophilic substituent constants σ+ indicate a significant reso-
nance interaction of the substituents on the aniline ring with the azomethine
carbon atom in the examined series of imines. The demand for electrons in the
investigated compounds was compared to that of the N-benzylideneanilines and
N-(substituted phenyl)pyridinealdimines. The manner of transmission of the
substituent effects is discussed and they were separated into resonance and in-
ductive effects. The inductive effects prevail over the resonance effects.
Keywords: 13C-NMR chemical shifts; Hammett Equation; substituent cons-
tants; N-(substituted phenyl)salicylaldimines.
INTRODUCTION
N-(Substituted phenyl)pyridine aldimines have been in the field of interest
1,2
for years. They are important intermediates for the synthesis of aminoalkylpy-
3
4,5
ridines and pharmacologically active triazoles and triazolines. In this latter
context, it has also been established that substituents both from the aniline ring
and from the aryl group at the azomethine carbon can influence the yields of the
4
triazole products.
Schiff bases of salicylaldehydes with anilines, aminopyridines and benzyl-
amines were also investigated because of their photochromic and thermochromic
*Corresponding author. E-mail: drmana@tmf.bg.ac.rs
#Serbian Chemical Society member.
doi: 10.2298/JSC120319033D
993
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994
DRMANIĆ et al.
6
7
properties, as well as the role they play in biological systems, in addition to
8
some other applications.
Numerous previous investigations of the molecular structures of N-benzyl-
9–11
ideneanilines,
showed that the substituents in the aniline ring have a signifi-
cant influence on the deflection of the angle θ between the rings by their electro-
nic and/or steric effects (the general formula is given in Fig. 1) and consequently
determine the conformation of the corresponding molecules.
The general formula of the investigated N-(substituted phenyl)salicylaldi-
mines is shown in Fig. 1.
Fig 1. The general formula of N-(substituted phenyl)salicylaldimines.
The intention of the present investigation was to study the effects of the sub-
stituents at the aniline ring and of the 2′-hydroxyphenyl group present on the azo-
13
methine carbon (C-7) on the C-NMR chemical shifts of the azomethine carbon,
as a continuation of previous studies of 2-, 3- and 4-N-(substituted phenyl)py-
1,2
ridine aldimines. Based on the studied effects, it was possible to draw con-
clusions about the conformations of the investigated molecules.
EXPERIMENTAL
The N-(substituted phenyl)salicylaldimines used in this study were synthesized by pro-
cedures described in the literature.4,5,12,13 Equimolar amounts of salicylaldehyde and
substituted anilines in ethanol were heated to 60–70 °C under constant stirring for 2–3 h. The
products were recrystallized from analytical grade ethanol to constant melting point. The
purity of the obtained compounds was confirmed by their melting points, elemental analyses
and NMR spectra, which were in agreement with literature data.4,5,12,13
The 13C-NMR chemical shifts data of the investigated compound were obtained using a
Bruker AC 250 spectrometer at 62.896 MHz. The spectra were recorded at room temperature
in deuterated dimethyl sulfoxide (DMSO-d6) in 5 mm tubes. The chemical shifts are expres-
sed in ppm values referenced to the residual solvent signal at 39.5 ppm.
RESULTS AND DISCUSSION
13
The C-NMR chemical shifts of the azomethine carbon atom (δ ) in the
C
solvent DMSO-d6 and the differences in the chemical shifts (Δδ ) together with
C
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13C CHEMICAL SHIFTS IN N-(SUBSTITUTED PHENYL) SALICYLALDIMINES
995
14
+15
the corresponding σ and σ
constants for substituents in the m- and p-posi-
tion of the aniline ring are given in Table I.
TABLE I. 13C-NMR chemical shifts of the azomethine carbon atoms of N-(substituted phe-
nyl)salicylaldimines in DMSO-d6 solution and the corresponding substituent constants
+
Substituent
X
δC-7 / ppm
ΔδC-7 / ppm
σm or σp
σm+ or σp
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
H
p-CH3
p-Br
162.60
161.66
162.98
162.93
160.43
157.55
165.35
164.18
164.72
164.55
163.71
164.54
164.59
0.00
–0.94
0.37
0.32
–2.17
–5.05
2.75
1.52
2.13
1.94
1.11
1.93
1.98
0.00
–0.17
0.23
0.00
–0.31
0.15
p-Cl
0.23
0.11
p-OCH3
p-N(CH3)2
p-NO2
p-COCH3
2N
–0.27
–0.83
0.78
0.50
0.73
–0.78
–1.70
0.79
0.50
0.72
(9)
(10)
(11)
(12)
(13)
3N
0.62
0.78
2N, 4CH3
3N, 5CH3
3,5-diCl
0.56a
0.67a
0.74b
0.41a
0.65a
0.78b
aσ and σ+ values were calculated as the sum of the σ or σ+ substituent constants of a methyl group in the m- or
p-position and the corresponding constant for the aza-substituent; bσ and σ+ values were calculated as double
values of the σ or σ+ constants of a chlorine atom in the m- position.
13
The values of Δδ are the differences between the C-NMR chemical shifts
C
13
of the substituted imines and the C-NMR chemical shift of the unsubstituted
imine:
Δδ = δ
–δ
(1)
C
C(subst.) C(unsubst.)
The general conclusion derived from the data in Table I is that all the substi-
tuents on the aniline ring of the imine influence via their electronic effects the va-
13
lue of the C-NMR chemical shifts of the azomethine carbon atom. For electron
donor substituents, these were upfield shifts, while downfield shifts were regis-
tered for electron acceptor substituents.
13
For a quantitative assessment of the effects of substituents on the C-NMR
chemical shifts, the Hammett Equation of the type:
Δδ = ρσ
(2)
C
13
was applied, where ρ is the reaction constant reflecting the sensitivity of the C-
-NMR chemical shift to the substituent effects.
Using the data from Table I, the following correlation equation for N-(sub-
stituted phenyl) salicylaldimines was obtained:
Δδ = (4.31 0.26)σ
– 0.47
(3)
C-7
m/p
(r = 0.981; s = 0.44; n = 13)
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996
DRMANIĆ et al.
where r is the correlation coefficient, s the standard deviation and n the number
of data included in the correlation.
In order to examine the influence of the electron-donor and the electron-ac-
ceptor substituents, the obtained data were separated according to their electronic
effects, as shown in Eqs. (4) and (5):
For electron-donors:
Δδ = (6.05 0.56)σ – 0.12
(4)
(5)
C-7
p
(r = 0.992; s = 0.35; n = 4)
For electron-acceptors:
Δδ = (3.31 0.35)σ
– 0.47
m/p
C-7
(r = 0.961; s = 0.28; n = 10)
From these correlations, it could be noticed that the electron-donor substi-
tuents influence the shifts of the azomethine carbon atom more than electron-ac-
ceptor subsituents.
+
When σ , the value of the electrophilic substituent constants, are used in-
stead of σ (Eq. (3)), an even more precise correlation for N-(substituted phenyl)-
salicylaldimines was obtained:
+
Δδ = (2.91 0.08)σ
– 0.02
m/p
(6)
C-7
(r = 0.996; s = 0.20; n = 13)
This correlation is presented in Fig. 2.
For electron-donor subsitutents, the following correlation equation was ob-
tained:
+
Δδ = (2.96 0.07)σ + 0.02
(7)
C-7
p
(r = 0.999; s = 0.09; n = 4)
while for electron-acceptor subsitutents, the following was obtained:
+
Δδ = (2.89 0.25)σ
– 0.02
m/p
(8)
C-7
(r = 0.971; s = 0.23; n = 10)
The better correlation coefficients obtained when the electrophilic substi-
+
tuent constant σ was employed for both electron-acceptor and electron-donor
substituents undoubtedly shows a considerable resonance interaction of the sub-
stituents on the aniline ring with the azomethine carbon atom.
Based on the obtained results and the correlation (6), it could be concluded
13
that the electronic effect of substituents influence the C chemical shifts of azo-
methine carbon atom, and that this influence depends on the position of the sub-
stituent in the aniline part of the molecule. The fact that the majority of substi-
tuted aldimine molecules are non-planar comes from the deviation of the phenyl
ring from the molecular plane, which creates a special geometry of the molecule.
The change of the angle between the rings (θ) is the consequence of the presence
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13C CHEMICAL SHIFTS IN N-(SUBSTITUTED PHENYL) SALICYLALDIMINES
997
of substituents. The electron-acceptor substituents cause an increase in the angle
between the rings, while electron-donor substituents decrease this angle, tending
to make the molecular structure more planar.
Fig. 2. ΔδC-7 as a function of the substituent constant σ+.
+
It was deemed of interest to compare the obtained values of ρ = 2.96, Eq.
(7), for N-(substituted phenyl)salicylaldimines with those found for 2N, 3N, and
+
4N N-(substituted phenyl)pyridinealdimines, ρ = 3.06, 3.01 and 3.40, respecti-
16
+
vely, and with that for substituted N-benzylidenanilines, ρ = 2.81 carrying the
same substituents: H, p-CH , p-Cl, p-OCH and p-N(CH ) . In the case of both
3
3
3 2
N-(substituted phenyl)salicylaldimines and the various N-(substituted phenyl)py-
ridinealdimines, there was an increased demand for the electrons of the azome-
thine carbon atom relative to the demand of the corresponding carbon in N-ben-
zylideneanilines. Most probably, this could be attributed to the effect of the 2-
hydroxyphenyl and the substituted pyridine groups bound to the azomethine car-
bon atom. It is evident that the electron donating effect of the substituents on the
aniline ring of the azomethine carbon atom for N-(substituted phenyl)salicylal-
dimines lies between the same effect for the N-benzylideneanilines and the
N-(substituted phenyl)pyridinealdimines.
In addition, it was considered very interesting to compare the values of the
+
reaction constant ρ for the series of substituted N-(substituted phenyl)salicylal-
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998
DRMANIĆ et al.
dimines with those for N-benzylideneanilines and isomeric pyridinealdimines in-
1,2
vestigated in previous studies, with electron donating substituents in the p-
position of the aniline ring (p-CH , p-OCH and p-N(CH ) ), where these sub-
3
3
3 2
stituents probably allow a nearly planar conformation of the investigated com-
+
pounds. The calculated ρ values for the previously examined compounds, with
1
the same substituents, are as follow: 2.81 for N-benzylideneanilines; 3.36 for 4-
1
pyridinealdimines; 3.05 for 2-pyridinealdimines and 2.92 for 3-pyridinealdi-
2
mines and 2.96 for the N-(substituted phenyl)salicylaldimines, giving the fol-
+
lowing order for ρ :
phenyl < 3-pyridine < 2-hydroxyphenyl < 2-pyridine < 4-pyridine.
According to this comparison, it could be concluded that the inductive and
resonance effects of the nitrogen atom in the pyridine ring affect the degree of re-
sonance interaction through the molecule as a whole.
1,16,17
13
When the literature values
of the C-NMR chemical shifts of N-ben-
zylideneanilines (H, 160.31; p-CH , 159.45; p-OCH , 158.36; p-NO , 162.59;
3
3
2
13
p-N(CH ) , 155.52 and p-Cl, 160.69) are correlated with the C-NMR chemical
3 2
shifts from both investigated series with the same substituents in the aniline ring,
the following relationships were obtained:
δ
= (1.09 0.03)δ
(r = 0.999; s = 0.153; n = 6)
– 12.51
(9)
C-7(2OH,X)
C(H,X)
The high correlation coefficient obtained, as well as those reported earlier
1,2
for pyridinealdimines, proved that the mechanism of transmission of polar sub-
stituent effects to the azomethine carbon atom is the same as in the case of the
substituted N-benzylideneanilines. This also implies that the dihedral angle for
these two cases is not significantly different. However, the above intercorrelation
includes both electron acceptor and electron donor substituents.
In order to separate the inductive and resonance effects of the substituents in
13
the aniline ring and to observe their individual influence on the C-NMR che-
mical shifts of the azomethine carbon atom, it is useful to apply the Taft equation
in the form:
+
+
Δδ = ρ σ + ρ
σ
(10)
If the substituents on the aniline ring with dominantly electron-donor pro-
perties (p-CH , p-OCH and p-N(CH ) ) are taken into consideration, using
C-7
I I
R
R
3
3
3 2
18
+
literature values of σ and σ (substituent constants describing the inductive
and the positive resonance effects, respectively) for N-(substituted phenyl)sali-
I
R
cylaldimines, the following correlation is obtained:
+
Δδ = (3.51 0.01)σ + (3.00 0.03)σ – 0.03
(11)
C-7
I
R
(r = 0.999; s = 0.04; n = 4)
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13C CHEMICAL SHIFTS IN N-(SUBSTITUTED PHENYL) SALICYLALDIMINES
999
+
Based on the obtained ratio ρ /ρ , which was 3.00/3.51=0.86 for the
R
I
N-(substituted phenyl)salicylaldimines, it may be concluded that the inductive
effect prevails over the positive resonance effect of the substituents. However,
the same approach for the N-(substituted phenyl)pyridine-4-aldimines, with the
+
1
same substituents on the aniline ring, gives the result ρ /ρ of 3.35/2.98 = 1.12,
R
I
meaning that the influence of the resonance effect was considerably higher for
these previously examined compounds than for N-(substituted phenyl)salicylald-
imines.
The observation that the positive resonance effect was less pronounced than
the inductive on can be explained by the fact that the hydroxy group in position 2'
decreases the possibility for any resonance interaction in the molecule. This is
probably due to of the existence of a hydrogen bond between it and the nitrogen
atom (Fig. 3), which causes an increase in the demand for electrons of the azo-
methine carbon atom. The lone electron pair on the nitrogen is blocked by the
described hydrogen bond, which prevents the transmission of the resonance ef-
fect of the subsitutents to the aniline ring.
Fig 3. Hydrogen bond between the 2’-hydroxy group
and nitrogen in the molecule of N-(substituted phe-
nyl)salicylaldimine.
The observation that the 2’-hydroxy group causes a decrease in the positive
resonance effects, compared to pyridinealdimines, can also be explained by the
deflection from planarity of the molecule, caused by repulsion between the lone
electron pair of the hydroxylic oxygen and the π-electrons from the azomethine
bond.
CONCLUSIONS
It has been shown that linear free energy relationships can be applied to the
study of the transmission of substituent effects in N-(substituted phenyl)salicyl-
13
aldimines using the data from C-NMR chemical shifts. It can be seen from the
applied linear free energy relationships that both inductive and resonance effects
influence the electron demand of the azomethine carbon.
Reliable correlations with the Hammett σ constants for N-(substituted phe-
+
nyl)salicylaldimines, and the excellent correlation coefficients obtained with σ
+
and σ
for predominantly electron donating substituents in the aniline ring,
indicate extensive delocalization and significant resonance interactions in these
R
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1000
DRMANIĆ et al.
systems. Therefore, it is probably true that as long as electron-donating substi-
tuents are present in the aniline ring, there is a substantial decrease in the θ angle,
so the molecule attains nearly planar conformation and the resonance interaction
increases. This is in accordance with the conclusions reported in the literature
that in special cases, strong electron donor substituents present in the aniline ring
and strong electron acceptor substituents on the benzylildene ring can cause near-
planar conformation of the corresponding molecules due to the complete delo-
16,17
+
calization through the whole molecule.
Considering the ρ /ρ ratio, it is
R I
seen that the positive resonance effect decreases in the presence of the 2’-hydroxy
group in the molecule of N-(substituted phenyl)salicylaldimine in comparison to
the N-(substituted phenyl)pyridine-4-aldimine. This fact could be tentatively ex-
plained by the formation of a hydrogen bond between the 2’-hydroxy group and
the nitrogen or by the repulsion of the electrons of the azomethine bond and the
hydroxylic oxygen. Both factors could increase the angle (θ) between the phenyl
rings, thereby decreasing the planarity of the molecule and the resonance effects
of the substituents on the aniline ring on the azomethine carbon atom.
Acknowledgement. Authors are grateful to the Ministry of Education, Science and Tech-
nological Development of the Republic of Serbia for financial support (Project 172013).
И З В О Д
ЕФЕКАТ СУПСТИТУЕНАТА НА 13C ХЕМИЈСКА ПОМЕРАЊА АЗОМЕТИНСКОГ
УГЉЕНИКА N-(СУПСТИТУИСАНИ ФЕНИЛ)САЛИЦИЛАДИМИНА
САША Ж. ДРМАНИЋ1, АЛЕКСАНДАР Д. МАРИНКОВИЋ1, ЈАСМИНА Б. НИКОЛИЋ1
и БРАТИСЛАВ Ж. ЈОВАНОВИЋ2
1
Катедра за органску хемију, Технолошко–металуршки факултет, Универзитет у Београду,
2
Карнегијева 4, Београд и Институт за хемију, технологију и металругију,
Универзитет у Београду, Његошева 12, Београд
13C-NMR хемијска померања 13 Шифових база корелисана су са константама суп-
ституената коришћењем Хаметове једначине. Добра корелација хемијских померања
азометинског угљеника са електрофилним константама супституената (σ+) показује да
постоји изражена резонантна интеракција супституената на анилинском прстену са азо-
метинским угљеником. Испитиван је пренос електронских ефеката супституената и они
су раздвојени на индуктивне и резонантне ефекте, а покaзано је да преовлађују инду-
ктивни ефекти.
(Примљено 13. марта, ревидирано 3. априла 2012)
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