Synthesis and crystal structure of 3-(4′-nitrophenyl)iminocoumarin, and cyclization reaction mechanism based on DFT calculation

Article abstract of DOI:10.1016/j.molstruc.2015.01.051

A convenient method was reported to synthesize 3-(4′-nitrophenyl)iminocoumarin by a cyclization reaction following a Knoevenagel reaction of 2-hydroxybenzaldehyde with 4-nitrophenylacenitrile in an ethanol solution. Piperidine or piperazine was employed r

Full text of DOI:10.1016/j.molstruc.2015.01.051

Accepted Manuscript
Synthesis and crystal structure of 3-(4’-nitrophenyl)iminocoumarin, and cyli-
zation reaction mechanism based on DFT calculation
Chao-Zhi Zhang, Shi-Juan Li, Hui Cao, Ming-Xia Song, Qing-Gang Kong
PII:
S0022-2860(15)00070-8
DOI:
http://dx.doi.org/10.1016/j.molstruc.2015.01.051
MOLSTR 21296
Reference:
Journal of Molecular Structure
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Received Date:
Revised Date:
Accepted Date:
27 July 2014
27 December 2014
30 January 2015
Please cite this article as: C-Z. Zhang, S-J. Li, H. Cao, M-X. Song, Q-G. Kong, Synthesis and crystal structure of
3-(4’-nitrophenyl)iminocoumarin, and cylization reaction mechanism based on DFT calculation, Journal of
Molecular Structure (2015), doi: http://dx.doi.org/10.1016/j.molstruc.2015.01.051
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Synthesis and crystal structure of 3-(4’-nitrophenyl)iminocoumarin, and
cylization reaction mechanism based on DFT calculation
Chao-Zhi Zhang*, Shi-Juan Li, Hui Cao, Ming-Xia Song, Qing-Gang Kong
Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of
Environmental Science & Engineering, Nanjing University of Information Science & Technology,
Nanjing 210044, PR China, E-mail: chzhzhang@sohu.com, Fax: +86-25-58731221
Abstract
A convenient method was reported to synthesize 3-(4’-nitrophenyl)iminocoumarin by a
cyclization reaction following a Knoevenagel reaction of 2-hydroxybenzaldehyde with
4-nitrophenylacenitrile in an ethanol solution. Piperidine or piperazine was employed respectively
as catalyst. Crystal structure of 3-(4’-nitrophenyl)iminocoumarin shows that the molecules are
H-aggregation due to π-π stacking and hydrogen bonds between adjacent molecules, as a result,
electrons would transfer easily from a molecule to an adjacent molecule. Based on theoretical
calculations of the electronic structures and thermodynamic parameters of the reactive
intermediates in these cyclization reactions, the reaction mechanisms were postulated. Data of
single crystal and spectrum of UV-vis absorption show that 3-(4’-nitrophenyl)iminocoumarin is
good π-conjugated compound and would be potentially employed as donor-acceptor polymer units
for developing bulk heterojunction solar cell. This paper suggests a convenient and effective
method for synthesizing ring-locked D-A copolymer units for developing solar cell materials.
1

Keywords: Crystal structure; Cyclization reaction; DFT calculation; Reaction mechanism;
Iminocoumarin; Bulk heterojunction solar cell.
Introduction
Opto-electronic properties of compounds usually depend on its geometric structure. [1-3]
Syntheses of organic or polymeric compounds with desired geometric structures is a fatal
procedure for developing bulk heterojunction (BHJ) solar cells with high power change efficient
(PCE).[2-6] Generally, a desired organic compound or donor-acceptor (D-A) copolymer unit
should be π-conjugated well in order that donor materials would exhibit high mobility, because
π-conjugated compounds usually aggregate to ordered structure in quenching procedure of making
solar cell due to π-π stacking and hydrogen bonds.[4-8] Suzuki reaction and Stille reaction were
developed for syntheses of π-conjugated copolymer containing D-A units.[9-10] However, the
reaction systems must be oxygen free. Catalysts of these reactions are expensive so that
synthesized polymers are too expensive to be industrialized for developing organic solar cell
devices. Therefore, it would be interesting that a convenient and effective method would be
developed to synthesize copolymer containing D-A units.[1,11-12] Knoevenagel reaction is high
performance for forming double bond C=C under a convenient condition.[13-14] A compound
containing two aryl rings connected with a double bond C=C isn’t π-conjugated well. However, a
compound containing two aryl rings connected with a ring-locked double bond C=C is usually
π-conjugated well.[15-16] Therefore, it is useful that a ring-locked reaction would take place
following a Knoevenagel reaction of an aryl compound containing electron-withdrawing substitute
group with an aryl compound containing electron-pushing substitute group.
2

4-Nitrophenylacenitrile reacted with 2-hydroxylbenzaldehyde in an ethanol solution to acquire
3-(4’-nitrophenyl)iminocoumarin. The designed compound is a well π-conjugated molecule so
that it could be employed as D-A copolymer units for developing organic solar cell. Organic solar
cell materials containing this kind of polymer units could exhibit a good mobility, because the π
electrons would be better delocalized in the whole molecule.[17-18] Synthetic method of
iminocoumarin derivatives was seldom researched.[19-20] Complex synthetic method that resin
was used as catalyst and polymeric solid support results that the product was too expensive to be
widely used as an opto-electron materials. Therefore, it is of great interest that a convenient and
environmentally benign method would be developed to synthesize the compound. In this paper,
3-(4’-nitrophenyl)iminocoumarin was synthesized by a cyclization reaction following a
Knoevenagel reaction of 2-hydroxybenzaldehyde with 4-nitrophenylacenitrile in an ethanol
solution. Its crystal structure was determination by X-ray analysis.[21] The reaction mechanism
was supposed based on time dependent density functional theory (TD-DFT) calculations of the
electronic structures and thermodynamic parameters of the reaction intermediates at the
B3LYP/6-31G(d) level.[22-23]. It would suggest a convenient and effective method for
synthesizing
polymers
containing
ring-locked
D-A
units,
similar
to
3-(4’-nitrophenyl)iminocoumarin, used as organic solar cell materials.
Results and discussion
1. Synthetic method and catalysis activity
3-(4’-Nitrophenyl)iminocoumarin was synthesized by a cyclization reaction following a
Knoevenagel reaction of 2-hydroxybenzaldehyde with 4-nitrophenylacenitrile in an ethanol
3

solution. Piperidine or piperazine was employed respectively as catalyst. Acetic acid was used to
control pH value of reaction solution in the range from 9 to 11. A dehydration agent, anhydrous
Na2SO4, was added into the reactive solution to remove water given from cyclization reaction for
improving yield of 3-(4’-nitrophenyl)iminocoumarin. The reactions were shown in Scheme 1.
Scheme 1. Sythesis of 3-(4’-nitrophenyl)iminocoumarin.
Scheme 1 shows that the yields of 3-(4’-nitrophenyl)iminocoumarin were 84.2% and 92.5 %
when piperidine or piperazine was employed as a catalyst, respectively. Piperazine is a better
catalyst than piperidine due to nucleophilicity of lone pair electrons.
Scheme 2. The structure of piperazine or piperidine optimized by DFT calculations and atomic
charges (unit: atomic unit) of N atoms.
The structures of piperazine and piperidine were optimized by TD-DFT calculations using
Gaussian03 software at the B3LYP/6-31G(d) level.[22-23] Nucleophilicity of lone pair electrons
in piperazine was stronger than that of piperidine because negative charge (Mulliken atomic
charges: -0.555 A.U.) of nitrogen atom in piperazine was denser than that (Mulliken atomic
charges: -0.545 A.U.) in piperidine. [13-14] Carbon atom in the carbonyl group would be attacked
4

more easily by nitrogen atom in piperazine to form 2-hydroxybenzaldehyde dipiperazine
acetal.[13-14] Therefore, piperazine was a better catalyst than piperidine. Using piperazine as
catalyst, the yield is higher.
2. Mechanism of reaction
Density Functional Theory (DFT) calculation at B3LYP/6-31G (d) have been used to suggest
reaction mechanism.[24-25] The suggested mechanisms have well predicted experimental
results.[24-25] Therefore, based on TD-DFT calculations on the electronic structures and
thermodynamic parameters of the reaction intermediates and resultants in the cyclization reaction,
the mechanisms of the reactions were postulated.[22-23]
Scheme 3. Mechanism of the cyclization reaction.
The mechanism was proposed for the reactions of 4-nitrophenylacenitrile with
2-hydroxybenzaldehyde to acquire 3-(4’-nitrophenyl)iminocoumarin in an ethanol solution of
alkaloid. (Scheme 3) At first, there was a Knoevenagel reaction of 4-nitrophenylacenitrile with
2-hydroxybenzaldehyde to acquire intermediate 1. The hydroxyl in intermediate 1 was
deprotonated to give a phenolic anion derivative (2) in an ethanol solution of alkaloid. Charge of
the phenolic anion in intermediate 2 transferred to carbon atom of nitrile to form a single bond
O-C. Charge of triple bond of nitrile group transferred to nitrogen atom of nitrile group to form a
double bond C=N, at the same time, a proton was seized by nitrogen atom of nitrile to form a
5

single bond N-H. As a result, 3-(4’-nitrophenyl)iminocoumarin was synthesized.
Enthalpies, entropies, energies and free energies of reactants, resultants and intermediates are
summarized in Table 1.
Table 1. Thermodynamic parameters of reactants, resultants and intermediates (Unit: KJ/mol).
No.
Comp.
Ha
Sa
Ea
Ga
ΔG
1
2
3
4
5
Resultant
-2395349.9 0.5035 -2395352.4 -2395500.1 -76.0b
-1104130.7 0.3428 -1104133.2 -1104232.9
-1491362.7 0.3776 -1491365.1 -1491475.3
2-Hydroxybenzaldehyde
4-Nitrophenylacenitrile
Water
-199811.1
0.1896 -199813.6
-199867.6
3-(2-Hydroxyphenyl)-2-
(4-nitrophenyl)acrylonitrile
-2395268.9 0.5204 -2395271.4 -2395424.1 -416.5c
a H, S, E and G are sum of electronic and thermal enthalpies, entropies, energies and free energies.
b Change of free energy in procedure from compound 5 to compound 1 is calculated by following
equation, ΔG1 = G1 – G5 = (H1 – H5) – T(S1 – S5).
c Change of free energy in the reaction of compound 2 with compound 3 to produce compound 5
is calculated by following equation, ΔG5 = (G5 + G4) – (G2 + G3) = (H5 + H4 – H2 – H3) - T(S5 +
S4 – S2 – S3).
In Table 1, ΔG5 (- 416.5 KJ/mol) is a big negative value in 298.15 K, therefore,
2-hydroxybenzaldehyde would react completely
with 4-nitrophenylacenitrile to produce
3-(2-hydroxyphenyl)-2-(4-nitrophenyl)acrylonitrile. In the reaction procedure, change of enthalpy
is - 413.4 KJ/mol. Moreover, change of entropy is -0.0104 KJ/mol. Therefore, the reaction would
carry on spontaneously. In 298.15 K, ΔG1 (-76 KJ/mol) is also a big negative value, therefore,
3-(2-hydroxyphenyl)-2-(4-nitrophenyl)acrylonitrile would change to title compound. In the
procedure, change of enthalpy is - 81 KJ/mol. Moreover, change of entropy (- 0.0169 KJ/mol) is
very small. Therefore, the reaction would carry on spontaneously. In addition, the energy of
6

compound 5 (-2395271.4 KJ/mol) is higher than that of compound 1 (-2395352.4 KJ/mol). It
suggests that compound 5 trends to transfer to compound 1.
3. Computational details
The cyclization reaction mechanism was suggested based on theoretical calculation of the
reaction intermediates. Geometries were optimized by TD-DFT calculations using Gaussian03
software at the B3LYP/6-31G(d) level.[22-23] All results support the suggested mechanism. The
results of the calculations are shown as following.
Figure 1. The structure of intermediate 1 optimized by TD-DFT and atomic charges (unit:
atomic unit).
Figure 1 shows clearly that H30 (Positive charge: 0.420 A.U.) was attacked easily by
electron doublet of nitrogen atom at alkaloid to give the deprotonated intermediate 2.
Figure 2. The structure of intermediate 2 optimized by DFT calculations and atomic charges
(unit: atomic unit).
7

Figure 2 shows that negative charge delocalization from the O- in the intermediates 2 to the
carbon atom in nitrile group results the bond between O28 and C13. Charge of triple bond of
nitrile group transferred to N29, at the same time, N29 was attacked easily by proton to give
3-(4’-nitrophenyl)iminocoumarin.
Figure 3. The structure of intermediate 3 optimized by DFT calculations, atomic charges and
bond length between C13 and O27 (unit: atomic unit).
Charge of O28, N29, C13 and H30 are -0.513, -0.605, 0.451 and 0.298 (Unit: A.U.). Length
of bond between C13 and O28 being 1.386 Ǻ shows that the bond is a normal singer bond. The
length of bond (1.26 Ǻ) between C13 and N25 is a double bond.
4. Crystal structure
The geometry of title compound was studied by summarizing crystallographic data.[21] Part
A of Figure 4 shows that the molecule is π-conjugated well because pyran ring and benzene ring
(C1-C2-C3-C4-C5-C9) are coplanar. Nitro group is in a plane of benzene ring containing C10.
Part B of Figure 4 shows clearly that title molecules are H-aggregation through π-π stacking. This
organic crystalline materials exhibits a short interplanar distance (3.493 Ǻ). As a result,
π-electrons would transfer easily from a molecule to an adjacent molecule in the lamellar π-π
stacked structural materials.[26-30] Therefore, the planar compounds are significant for many
applications. Hydrogen bond was formed between a hydrogen atom of hydroxyl group and an
8

oxygen atom of nitro group at an adjacent molecule. Hydrogen bond was also formed between a
hydrogen atom of nitrobenzene ring and an oxygen atom of pyran ring at an adjacent molecule
(Part B of Figure 4). These hydrogen bonds and π-π stacking affections result that the title
compounds were stacked up to crystal, which is useful property for developing BHJ solar cell with
high PCE. [8, 31-35]
Figure 4. Crystal of 3-(4’-nitrophenyl)iminocoumarin (Part A: Structure geometry of a molecule
3-(4’-nitrophenyl)iminocoumarin. Part B: The crystal structures viewed along the a-axis. Part C:
The crystal structures viewed along the b-axis.).
Table 2 Crystal data and summary of structure refinement for title compound.
CCDC deposit no.
Empirical formula
Formula weight
982576
C15 H10 N2 O3
266.25
Temperature
296(2) K
Wavelength
0.71073 Å
Crystal system, space group
Unit cell dimensions
Orthorhombic, P2(1)2(1)2(1)
a = 3.7837 Å alpha = 90 deg.
b = 14.7712 Å beta = 90 deg.
c = 21.858 Å gamma = 90 deg.
9

Volume
1221.6(3) Å3
4, 1448 Kg/m3
0.103 mm-1
Z, Calculated density
Absorption coefficient
F(000)
552
Crystal size
0.40 x 0.38 x 0.34 mm
Theta range for data collection 2.754 to 22.783 deg.
-4<=h<=4, -17<=k<=17,
Limiting indices
-26<=l<=23
Reflections collected / unique
8703 / 2171 [R(int) = 0.0347]
Completeness to theta = 25.01 99.7 %
Absorption correction
Multi-sacn
Max. and min. transmission
Refinement method
0.9657 and 0.9599
Full-matrix least-squares on F2
Data / restraints / parameters
Goodness-of-fit on F2
2171 / 0 / 181
1.047
Final R indices [I>2sigma(I)]
R indices (all data)
R1 = 0.0388, wR2 = 0.0855
R1 = 0.0570, wR2 = 0.0971
0.111 and -0.159 e. Å -3
Largest diff. peak and hole
Table 2 shows that single crystal is orthorhombic crystal system, P2(1) 2(1) 2(1) space group.
The molecules aggregated face to face to form a big single crystal (0.40 x 0.38 x 0.34 mm) due to
π-conjugated structure of molecule and the π-π stacking affection.
Table 3. Selected interatomic distances and dihedral angles
Dihedral angle ( o )
Interatomic distance (Å)
N1-C6 1.256(3) C6-C7
1.468(3)
1.439(3)
1.390(3)
1.463(3)
1.387(2)
C6-O1-C9-C1
N1-C6-C7-C8
C4-C5-C8-C7
179.1(2)
178.0(2)
179.5(2)
C7-C8
C5-C9
1.340(3) C5-C8
1.385(3) C2-C3
C7-C15 1.484(3) N2-C12
O1-C6 1.383(2) O1-C9
C8-C7-C15-C10 132.6(2)
O2-N2-C12-C13 178.7(2)
O3-N2-C12-C11 -178.8(2)
N1-H1 0.8600
C10-H10 0.9300
1.218(3)
O3-N2 1.222(3) O2-N2
N1-C6-C7-C15
-2.1(4)
Bond lengths of N1-C6, C7-C8, C5-C9, C2-C3 are 1.256 Å, 1.340 Å, 1.385 Å, 1.390 Å,
respectively. They are double bond. Bond lengths of single bonds C6-C7 (1.468 Å) and C5-C8
(1.439 Å) are shorter than that of C7-C15 (1.484 Å) due to π-electrons delocalized in pyran ring
10

O1-C6-C7-C8-C5-C9. Bond lengths of single bonds O1-C6 (1.383 Å) and O1-C9 (1.387 Å) are
similar. Bond lengths of O3-N2 (1.222 Å) and O2-N2 (1.218 Å) are similar due to π-electrons
delocalized in nitro group. Bond lengths of N1-H1 (0.860 Å) are shorter than that of C10-H10
(0.930 Å) because electronegativity of nitrogen atom are bigger than that of carbon atom. [36]
Dihedral angle N1-C6-C7-C8 (178.0o) and C4-C5-C8-C7 (179.5o) show that pyran ring is in a
plane. Dihedral angle C6-O1-C9-C1 (179.1o) shows that pyran ring and benzene ring C1-C2-
C3-C4-C5-C9 are coplanar. Dihedral angle O2-N2-C12-C13 (178.7o) and O3-N2-C12-C11
(-178.8o) show that nitro group is in a plane of benzene ring C10-C11-C12-C13-C14-C15.
Dihedral angle N1-C6-C7-C15 (-2.1o) shows atoms N1 and C15 are in a plane of the pyran ring.
Dihedral angle C8-C7-C15-C10 (132.6o) shows that angle between C1-C2-C3-C4-C13-C14 plane
and C10-C11-C12-C13-C14-C15 plane is 47.4o due to steric repulse between hydrogen atoms H14
and H8. If thiophene derivatives instead of benzene derivatives were used to synthesize
benzopyran derivatives, designed compounds could be more suitable to be used as D-A copolymer
units of donor materials for BHJ solar cell than title compound.
5. UV-vis absorption spectrum of 3-(4’-nitrophenyl)iminocoumarin
Figure 5. UV-vis absorption spectrum of title compound in CH2Cl2 (λmax of the compound in
11

CH2Cl2 was marked on curves.).
UV-vis absorption spectrum of title compound in CH2Cl2 was determined by using a lambda
25 Perkin-Elmer spectrophotometer. Figure 5 shows that there are three absorption peaks in the
UV-vis absorption spectrum. The solution of title compound in CH2Cl2 can effectively absorb light
which wavelength is shorter than 400 nm. Therefore, UV-vis absorption spectrum shows that
3-(4’-nitrophenyl)iminocoumarin is good π-conjugated compound and would be suitable to be
employed as D-A polymer units in donor materials of BHJ solar cells.[7, 18]
Conclusion
A conveniently method was reported to synthesize 3-(4’-nitrophenyl)iminocoumarin by a
cyclization reaction following a Knoevenagel reaction of 2-hydroxybenzaldehyde with
4-nitrophenylacenitrile in an ethanol solution containing alkaloid in the pH range from 9 to 11.
Piperidine or piperazine was employed respectively as catalyst. Anhydrous Na2SO4 was added
into the reactive solution as a dehydration reagent. Crystal structure of 3-(4’-nitrophenyl)imino-
coumarin shows that the molecules are H-aggregation due to π-π stacking and hydrogen bonds
between adjacent molecules, as a result, electrons would transfer easily from a molecule to an
adjacent molecule. Geometries, electronic structure, energies, enthalpies, entropies and free
energies of reactants, resultants and intermediates were calculated using TD-DFT at the
B3LYP/6-31G(d) level. Results of calculations show that title compound is a planar molecule and
piperazine is a better catalyst. Based on results of theoretical calculations and experiments, the
mechanisms of the cyclization reactions were postulated. Data of single crystal and spectrum of
UV-vis absorption show that 3-(4’-nitrophenyl)iminocoumarin is good π-conjugated compound
and would be potentially employed as D-A polymer units for developing BHJ solar cells. This
12

paper would suggest a convenient and effective method for synthesizing polymer containing
ring-locked D-A units, similar as 3-(4’-nitrophenyl)iminocoumarin, for developing BHJ solar cells
with high PCE.
Experimental
Melting points were determined on a Yanaco micro melting point apparatus (uncorrected). 1H
NMR were recorded on Bruker AM 300 (Germany), and δ were given in ppm (relative to TMS) and
coupling constants (J) in Hz. Mass spectra were recorded by a GCMS-QP2010 gas chromatogram
mass spectrometer under GC/MS mode. UV-visible spectra were recorded on a Lambda 25
Perkin-Elmer spectrophotometer. IR spectra were recorded on
a Brucker Vector 22
spectrophotometer, in which samples were embedded in KBr thin films. Precoated silica gel plates
(GF254) were used for analytical TLC. All solvents were purified by standard procedures. All
chemicals were purchased from Sigma or Aldrich.
3-(4’-Nitrophenyl)iminocoumarin was obtained as yellow crystal. The crystal structure was
determined by Bruker CCD X-ray diffraction.[21] The analytical data of the compound (1H NMR,
13C NMR, IR, MS) were in accord with the assigned structures, respectively.
Synthesis of 3-(4’-nitrophenyl)iminocoumarin. The 2-hydroxybenzaldehyde (2.44 g, 20.0
mmol), 4-nitrophenylacenitrile (4.14 g, 18 mmol) and anhydrous Na2SO4 (2.84g, 20 mmol) were
added into anhydrous ethanol (50 mL) under a N2 atmosphere. The mixture was heated, stirred
and refluxed for 20 minutes. Then, the catalysts, piperidine (0.34g, 4 mmol) or piperazine (0.34g,
4 mmol) and acetic acid (0.12 g, 2 mmol), were added. A yellow precipitate was produced as soon
as the catalyst was added. The reaction mixture was refluxed for 6 hours, cooled to room
temperature. The reaction mixture was filtrated, washed by ethanol (20 mL × 3) and dried to give
13

a yellow solid. The solid was recrystallized from acetnitril (25 mL) to give the pure compound
3-(4’-nitrophenyl)iminocoumarin as a yellow crystal (Piperazine used as catalyst: 4.43 g, 92.5 %;
or Piperidine used as catalyst: 4.04 g, 84.2 %.).
1H NMR (300 MHz, CDCl3, 25oC, TMS) δ: 8.28 (d, 2J = 7.5 Hz, 2 H in nitrophenyl)，7.87 (d,
2J = 7.5 Hz, 2 H in nitrophenyl)，7.70 (s, br, 1 H in imino)，7.60-7.40 (m, 2 H in benzopyran)，
7.33 (s, 1 H in alkene)，7.22-7.05 (m, 2 H in benzopyran).
13C NMR (300 MHz, CDCl3) δ: 115.49, 119.51, 123.58, 123.81, 128.42, 128.73，129.78，
131.39, 132.59, 135.25, 142.97, 147.61, 153.49.
MS (+CESI) m/z: 267.42, calc. for C15H10N2O3 [M+1]+: 267.25.
UV-Vis (HCCl3) λmax: 352 nm (ε = 7605).
IR (KBr) υ: 3370.7 (br, N-H) cm-1, 1656.7 (C=N).[19-20]
Acknowledge
This work was supported by Jiangsu Specially-Appointed Professor Program (R2012T01),
Foundation for special project on the Integration of Industry, Education and Research of Jiangsu
Province (BY2012028) and Scientific Research Foundation for Returned Scholars from Ministry
of Education of China (2013S010).
14

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17

Graphical Abstract

Highlight:
1.
A
convenient
and
novel
cylization
reaction
for
synthesizing
3-(4’-nitrophenyl)iminocoumarin is reported.
2. Its crystal structure shows that title molecules aggregate to lamellar π-π stacked structural
materials.
3. The mechanisms of the cyclization reactions were postulated based on TD-DFT
calculations.
18